Introduction to Procon

Process control is a branch of control engineering that deals with the operation of plants in industries such as petrochemicals, food, steel,glass, paper and energy.

The Procon 38 Series system is a complete package dealing with all aspects of Process Control. It will introduce industrial-standardequipment, and cover all aspects of equipment usage (those related to general use, and those specific to process control). Each piece ofequipment is investigated individually, so that its use is fully understood and its position in process control is appreciated.

When all the hardware has been covered, the different methods of modern process control are explored. These start at the most basic, withOn/Off Control, and will lead up to full Proportional, Integral, and Derivative Dual Loop Control.

For each type of control, a series of practical experiments will demonstrate its advantages and disadvantages, and discuss situations that itwould and would not be suitable for.

Control Systems

The diagram below shows a general control system. This diagram can be applied to all control situations.

As the assignments proceed this control arrangement will be applied again and again, with each element taking on different forms.

For example, the measurement devices will range from yourself deciding if the process is operating as it should be, to a pulse flow sensor andtransmitter arrangement automatically measuring rate of flow of fluid.

The controller will be simple logic level switching equipment or a universal microprocessor-based indicator/controller with 300 internal controlalgorithms.

Introduction To The Basic Process Rig

The Basic Process Rig (BPR) primarily consists of a low pressure flowing water circuit which is completely self contained. The followingcomponents are strategically placed within this circuit :

o Sump Tanko Dual Compartment Process Tanko Circulating Pumpo Visual Indication Flow Metero Motorised Flow Gate Valveo 3 Solenoid Valveso 5 Manual Valves

The following components are additional options for use with the BPR. This Discovery software assumes that these devices are available.

o Level Sensor Pack (38-400)o Float Switcho Pulse Flow Sensor

A photograph of the Basic Process Rig rig with these components is shown below:

The following assignments contain a number of practicals associated with the BPR, which teach the concepts of process control, from the verybasics to complex control scenarios.

Introduction To The Temperature Process Rig

The Temperature Process Rig (TPR) consists of two flow circuits, primary and secondary.

The primary flow circuit is integral to the TPR, while the secondary flow circuit is derived from one of two sources depending upon thelaboratory configuration.

If you only have the TPR then your secondary flow circuit is supplied by the optional auxiliary valve. Otherwise you have both the TPR and theBasic Process Rig (BPR), with the latter supplying the secondary flow.

Temperature Process Rig

If your laboratory configuration consists of a Temperature Process Rig only (pictured below), then the secondary flow is supplied from themains water supply which is controlled by the Optional Auxiliary Valve.

Temperature Preliminary Assignments

Before attempting any of the temperature assignments, it is assumed that a basic understanding of the equipment has been maintained fromthe previous level/flow assignments. If this is not the case it is essential that the following preliminary assignments are carried out beforecontinuing with the temperature practicals.

Introduction to ProconFlow/Level Rig FamiliarisationFlow/Level Rig CalibrationInterface FamiliarisationInterface CalibrationController FamiliarisationController CalibrationPulse Flow TransmitterProportional ControlPI and PID Flow Control

The previous assignments have been developed around the Basic Process Rig, however all the actual flow assignments can be carried outusing one of the flow transducers on the TPR.

Although it is not possible to carry out all of the previous assignments, it is suggested that the Theory, Background, Practical and Questionsare studied, as this will broaden your general understanding of the system and theoretical knowledge of process control principles.

The temperature assignments are described using the Basic Process Rig for the secondary flow, however, in your case, simply substitute forthis the Auxiliary Control Valve.

Level Flow and Temperature Rigs

If you have both the Temperature Process Rig and the Basic Process Rig in your laboratory they can be connected together. In thiscase, the secondary flow for your TPR is being provided by the BPR.

Level Flow and Temperature Preliminary Assignments

Before attempting any temperature assignments it is assumed that a basic understanding of the BPR, Process Controller (PC) and the ProcessInterface (PI) has been maintained from the previous level/flow assignments. If this is not the case it is essential that the followingpreliminary assignments are carried out before the temperature assignments are attempted.

Introduction to ProconFlow/Level Rig FamiliarisationFlow/Level Rig CalibrationInterface FamiliarisationInterface CalibrationController FamiliarisationController CalibrationPulse Flow Transmitter

Although it is essential to carry out the above assignments before proceeding onto the temperature assignments, it is suggested that all thelevel/flow assignments are carried out prior to attempting the temperature assignments, as this will broaden your general understanding ofthe system and theoretical knowledge of process control principles.

Secondary Flow Water TemperatureIf you are using the Optional Auxiliary Valve to provide the secondary flow, then the water temperature being applied to the input of theTemperature Process Rig will remain relatively constant, somewhere between approximately 8oC and 25oC.

However if you are using the Basic Process Rig to supply the secondary flow, then the very nature of the design will mean that thesecondary flow fluid, will, in certain circumstances heat up greater than the specified operating temperature of 20 - 30oC. Therefore beforeeach practical check the water temperature in the sump (for example use a mercury thermometer). If the water temperature is outside thespecified operating range, simply drain off the majority of the sump water and replace with fresh cold water.

Introduction to Process ControlProcess control is not a new discipline. A 'process' is anything from filling a bucket of water from a tap, to monitoring the performance of a carand determining the operating parameters (fuel injected, fuel mix, braking, temperature, oil, etc) which would produce an optimumperformance.

Although the latter example is obviously a complex and modern one, the former is a very simple problem but it still requires control. The tapmust be turned on to fill the bucket, and the flow rate will determine the length of time taken. When the bucket is full the tap must be turnedoff. Although these two would not be considered very similar, they both require control to operate effectively.

Process control as an aid to industry is becoming an ever increasingly important area, as it determines how well a plant is operating. With theright type of control a plant should be operated as efficiently and optimally as circumstances will allow.

The early development of process control and another discipline, servo systems, occurred side by side with very little interaction. During theSecond World War the greatest advances were made in the field of control engineering, but security prevented open publication and debateon developments.

As a result, the terminology used by process control engineers differed from that used by servo designers, despite the fact that bothdisciplines were striving for roughly similar objectives.

Process Control and Servo Systems

The one basic difference between a process control system and a servo system is that generally the emphasis in process control is on theperformance of the loop as a Regulator, i.e disturbance rejection.

In servo systems the emphasis is on how well the control system can follow changes in the reference or desired input signal.

This does not mean that process control systems are never subject to changes in reference values, or that servo systems never receivedisturbances. What is true however is that in a typical process control system the reference value will not change frequently. For instance, therequired temperature of a particular product may be constant for days.

The terminology in the process industry is influenced by the fact that quantities are often treated non-dimensionally, i.e listed as a percentagefigure. For example a valve may be 80% open, a flow is 50% of max, etc.

This simplifies the task of keeping track of the huge diversity of variables that have to be controlled, and is possible since everything withinthe plant is constrained to lie within certain limits.

The term Set Point is used to represent the reference input to the process control system. The reference represents a desired constantoperating point. The set point can be imposed by turning a knob, typing in a value, or it can be transmitted to a controller from a computer oranother controller.

The term Measured Value represents the output of the measurement system (transmitter, sensor or transducer). The measurement systemproduces a signal which is a function of the actual value of the physical process variable being controlled. The signal may be electrical,pneumatic or mechanical.

The set point is compared to the measured value to produce the deviation, which is simply the difference between the two. The controllerthen uses this deviation to make the set point and the measured value as close as possible.

Safety considerations are always paramount in process control. Therefore, safeguarding and monitoring systems must be included, to dealwith cases of equipment failure. All possible failures must be considered so that the system is prevented from failing or, if failure cannotalways be avoided, then that it will fail safely.

Instrument Classifications

There are generally two kinds of instrument used in process control: Firstly, instruments for monitoring process variables such as temperatureor pressure, give an audio or visual indication of the magnitude of the physical quantity measured. An example is a liquid-in-glassthermometer.

Secondly, instruments referred to as transmitters in process control engineering are those incorporated in an automatic control system. Theyare needed to provide (transmit) information about the plant status to the controller, and hence their output must be in a suitable form(electrical, hydraulic, pneumatic) to be accepted by the controller.

The primary component of both types of measurement instrument is a transducer, or a sensor that converts the measured physical quantityfrom one form to another.

Other possible components within the instrument are an amplifier, and an output display.

Another classification is between active and passive instruments. If the instrument output is entirely produced by the quantity beingmeasured, the instrument is termed passive. A pressure gauge is a passive instrument since the pressure of the fluid is translated intomovement of a pointer against a scale without any external power source.

A liquid tank level indicator is an active instrument, since the change in liquid level moves a potentiometer arm, in which case the outputsignal is a portion of the external voltage applied across the two ends of the potentiometer.

Greater control over measurement resolution can be obtained from an active instrument, because of the external power source. However, anactive instrument is more expensive than a passive instrument. Thus, balancing the measurement resolution requirements against cost isoften required.

A further distinction is the null versus deflection instruments.

In a deflection type of instrument the value of the quantity being measured is displayed in proportion to the displacement of a pointer.

A dead weight pressure gauge is an example of a null-type instrument. The pressure being measured displaces a piston, and weights areplaced on top of this piston until it reaches its zero position again. The value of the weights needed to reach this position represents thepressure measurement.

Null-type are more accurate than deflection type instruments because of the calibration accuracy in the former. However, deflection typeinstruments are generally more user friendly.

A final comparison is between analogue and digital instruments.

An analogue instrument gives an output signal that is a continuous function of the input signal being measured. An example of an analogueinstrument is the deflection type pressure gauge.

A digital instrument however, gives an output that varies in discrete steps. The advantage of using a digital instrument is that it can bedirectly connected to a computer so that digital control of the process can be carried out. The increasing application of digital computers inautomatic process control greatly increases the ease of computer connection.

If on the other hand an analogue instrument is used in a digital control system, an analogue-to-digital converter is needed to convert theanalogue output signal from the instrument to a digital form, for processing by the computer. This produces cost and time overheads. Notethat the extra time involved can be critical in the control of fast processes, and as a result the accuracy may be degraded.

Instrument Characteristics

Knowledge of the various instrument characteristics provides an indication of the possible degree of measurement errors that can affect theperformance of a process control system.

The characteristics of an instrument can be classified as either static or dynamic. Examples of static characteristics of an instrument areaccuracy, tolerance, precision, range, bias, linearity, sensitivity, drift, hysteresis, and resolution.

Accuracy is a measure of the deviation of a reading from the true value, and is usually quoted as a percentage of the full-scale reading ofthe instrument.

Tolerance describes the maximum deviation of a component from a specified value and can be used in place of accuracy.

Precision describes the extent to which an instrument is free from random errors, these are errors due to electrical noise, environmentalchanges etc. A large number of readings of the same quantity taken by a high precision instrument should differ very little.

Here, the clear distinction between precision and accuracy must be emphasised, in order to avoid confusion. High precision does not implyanything about measurement accuracy. A high precision instrument may have low accuracy. Low accuracy measurements from a highprecision instrument are normally caused by an offset in the instrument and can be corrected with calibration.

Range or Span defines the range of values of a quantity that an instrument is designed to measure.

Bias is a constant error that appears in every measurement made by an instrument and is caused by an offset in the device. It can beremoved with calibration.

Linearity describes an instrument whose output reading is linearly proportional to the quantity being measured, for a large number ofmeasurements.

Non-linearity is the maximum percentage deviation of any of the output readings from a straight line of best fit through all output readings,displayed graphically.

Sensitivity of Measurement is defined as the ratio of meter deflection to change in input quantity of the instrument.

The effect that environmental changes (temperature, pressure, etc) have on instruments is characterised by the 'zero drift' and 'sensitivitydrift'.

Zero Drift describes the change in the zero reading of an instrument, due to a change in ambient conditions.

Sensitivity Drift specifies any change in the 'sensitivity of measurement' caused by a change in the ambient conditions.

With reference to the figures below, the following instrument parameters can be illustrated: dead space, hysteresis, and threshold.

Dead Space can be seen below.

Dead space is the range of input values for which there is no change in the output. Dead space is also sometimes called Deadband.

The Hysteresis Curve is shown below.

It consists of two curves identical in shape. The upward and downward arrows describe the way in which the output reading varies as theinput quantity to the instrument increases and decreases respectively. We can see that the instrument has different output characteristics forlow-to-high and for high-to-low input changes. Hysteresis is the non-coincidence between these two curves.

Threshold is shown below.

Threshold is the minimum input value at which the output begins to change. If the input is less than this threshold value there will be nocorresponding output change to the input change.

Another important characteristic of an instrument is its measurement resolution, which is the minimum change in the input measured quantitythat will produce an observable change in the instrument output. The resolution is dependent on the subdivision of the output scale.

While the static characteristics of an instrument are concerned only with the steady-state reading that the instrument reaches, the dynamiccharacteristics describe the transient response of the instrument, i.e its time output response to an input signal before the output reaches thesteady state.

Dynamic parameters are the time constant, static sensitivity, undamped natural frequency, damping ratio, and steady state error.

The damping ratio controls the shape of the output response. Difficulties arise in choosing suitable values for the damping factor, since theoutput response also depends on the type of input signal applied to the instrument.

In most cases the physical quantities which instruments are required to measure are in the form of ramps of varying amplitudes and thus acompromise must be reached when choosing a damping factor for a particular input variable.

Finally, considerations of cost, durability and maintenance must be made when choosing an instrument for a particular measurement.

The Centrifugal Pump

The pump of the Basic Process Rig is a submersible, ignition protected pump, fitted at the bottom of the lower tank.

The task of the pump is to move the water from the lower tank to the upper tank through the piping network.

An electric dc motor drives the pump on or off and no intermediate value can be set. The motor is powered by the Process Interface (PI) fromthe ac (switched or continuous) power supply outputs on the rear panel of the PI. The rating of the pump is 12V, 4A.

The pump used is of the centrifugal type as opposed to the positive displacement type that includes the reciprocating and rotary pumps.

The centrifugal pump accomplishes its pressure boost by imparting kinetic energy to the fluid. A fluid at low pressure enters the pumpassembly at the base of the rotor. The fluid flows around the cavity and is drawn up through the pump by the rotor action and the cavityprofile.

A centrifugal pump is used in cases where high flows and low pressure heads are needed. If flow needs to be cut to zero, the centrifugalpump can be simply valved out by closing the manual valve MV2.

Special features of this pump include:

i. A non-airlocking mechanism, which is the number one problem of all centrifugal pumps.ii. A water cooled motor, which reduces operating temperatures and so minimises overheating, one of the major causes of pump

failure.

In this practical you will put the pump into operation and calculate the rate of flow (in litres per minute) through the interconnected pipes. Atypical performance figure of the pump supplied is 300 litres/hour full flow rate.

Note however, that the rate of flow will be much lower than the full flow performance of the pump, because of the small dimensions of thepipes, the resistance to flow incurred by the inner surface of the pipes, and the pressure head of the water in the upper tank.

To ensure full flow the servo valve should be fully open, therefore turn the Current Source control fully clockwise.

Manual Valves & Flow Meter

In this practical you will control manually the flow rate of the water supplied by the pump and the level of the water in the upper tank. Youwill do this by adjusting the various manual valves.

A manual valve is fully open when its adjusting knob is parallel to the pipe holding it, and fully closed when the knob is perpendicular to thepipe. The size of the valve is variable between these two extremes, making it possible to control the amount (or volume) of water passingthrough the valve in a given time, i.e the flow rate.

This type of control is clearly low in accuracy since it relies on human observation and reaction, but is used to demonstrate a manual controlsystem.

In this system, you are the controller, controlling some parameters (e.g level, flow) of the plant (rig), using the control elements i.e themanual valves, and information about the current values of these parameters (plant status), obtained from the measurement elements, anddisplayed on the flow-gauge and the level indicator.

The flow gauge used is a variable-area flowmeter. It provides an indication of the flow, ranging from 0.4 to 4.4 litres/minute, and is onlysuitable for water. This type of instrument gives a visual indication of flow rate, and so it is of no use in automatic control systems. However,it is reliable, cheap and used extensively throughout industry.

The instrument consists of a tapered glass tube containing a float which takes up a stable position when its submerged weight is balanced bythe upthrust of the water. The position of the float is a measure of the flow passage and hence of the flow rate. The accuracy of suchinstruments varies from +/-3 to +/-0.2 per cent.

To ensure full flow, the servo valve should be fully open, therefore turn the Current Source control fully clockwise.

The Servo Valve

A servo system is a control system, designed such that its output follows a desired input value with the minimum of error. The principle ofoperation of the servo valve is the same.

The servo valve is based on a very simple idea. It uses a gate to block the path of the liquid through the valve and since the gate is in effectlowered down on demand it can take any position between 100% open and 100% closed.

The vertical movement of the gate and stem of the valve changes the area of the port that is open. The flow rate of the fluid passing throughthe port is therefore proportioned or throttled by positioning the valve stem. The stem is in turn positioned by an actuator. This can all beseen in the following diagram.

This type of servo valve is also known as a Gate Valve.

The position of the gate is controlled by a 4-20mA signal supplied by the PI current source. At 4mA, the gate is fully lowered, thereby closingoff the flow, while a 20mA signal fully opens the valve.

The servo valve is almost linear in that the applied current is approximately proportional to the flow rate. Thus, the servo valve can be used inplace of the manual valve. It can also be used in an automatic control system, unlike the manual valve.

Like any other servo system, a servo valve is characterised by a time constant i.e it exhibits a transient response: a sudden change in the loopcurrent will take a finite time to establish a new flow rate.

An important point to bear in mind when switching off the servo valve is that the gate will be in the same position when it is next turned on.

Therefore, in order to avoid any contribution of the servo valve in future practicals, always fully open it by setting the current to 20mA, justbefore disconnecting it or switching off the PI.

The Solenoid Valves

A solenoid valve, unlike the manual or servo valve, can only be open or closed, i.e on or off. It is suitable for automation as it can becontrolled remotely. An electrical solenoid coil is the main element of the valve. The normal state of a solenoid valve is closed and it is openedby passing a current through the coil.

Since no intermediate setting is possible, no fine variable control can be accomplished, as with the servo valve, and hence only on/off controlcan be applied.

There are three solenoid valves supplied with the rig.

They are labelled SV1, SV2, and SV3 and have hole diameters of 5mm, 5mm, and 3mm respectively.

The electrical connections to these valves are on the right hand side of the rig and the power to turn them on is supplied by the ProcessInterface (PI) 24Vdc outputs, either switched (one on the front panel of the PI), or unswitched (two on the rear panel of the PI).

To ensure full flow, the servo valve should be fully open, therefore turn the Current Source control fully clockwise.

Process Instrument Calibration

In any process control system, measurements at intermediate and final stages of the production line are carried out to monitor quality-relatedprocess parameters and to inspect and test the final product.

Such measurements ensure that certain quality criteria are maintained. Thus, the accuracy of the measurements must be guaranteed by theproper and regular calibration of the instruments used.

Regular calibration is necessary because the characteristics of any instrument change (drift) over a period of time, due to the mechanicalwear, ageing of components, environmental changes, dirt, dust, etc.

Calibration consists of comparing the output of the process instrument being calibrated against the output of a standard instrument of knownaccuracy, when the same input (measured quantity) is applied to both instruments. During calibration, the process instrument is tested overits whole range, by repeating the comparison procedure for a range of inputs.

Calibration guarantees that the accuracy of the output reading in a calibrated instrument will be at a certain acceptable level, when theinstrument is used under the environmental conditions (e.g. temperature, humidity, pressure) present during the calibration process.

Outside those conditions, the characteristics of the measuring instrument may change, and hence the accuracy of the instrument will vary toa greater or a lesser extent according to its susceptibility to the modifying inputs inherent in the new environmental conditions.

However, in most measurement situations it is usually impossible to control the environmental conditions to be at the levels specified forcalibration, and so correction of the measuring instrument output reading is necessary.

Process Instrument Calibration (2)

Precautions must be taken to preserve the accuracy of the equipment used for calibration, by handling them carefully and using them only forcalibration purposes. Also, calibration equipment is often chosen to be of a much greater accuracy than the process instruments. Thecalibration equipment should form part of a calibration chain, in which they are calibrated against still more accurate standards.

This practice should be carried out so that the accuracy of all process instruments and the 'standard' equipment used to calibrate them, canbe traced back to the fundamental standards set up and maintained by a national organisation (The National Physical Laboratory in the UnitedKingdom).

When the process instrument is calibrated against a standard instrument, its accuracy will be found to be either inside, or outside thatrequired by the application measurement accuracy limits.

In the former case, the calibration results are recorded in the instrument's record sheet. If however, the instrument's accuracy is found to beoutside the acceptable measurement limits, then its characteristics should be adjusted by turning the adjustment screws (zero and span)provided, until the characteristics of the instrument are within the specified measurement limits.

However, there are cases where no adjustment is possible, or the range of possible adjustment is insufficient to bring the accuracy of theinstrument back within measurement limits. In such cases, the instrument should be either repaired, or scrapped.

Bear in mind that all calibration and measurement procedures should be documented, so that a record of the calibration history of theinstrument will always be available.

Processes involving the flow of liquids through connected pipes and vessels are common examples of industrial processes and are oftenemployed to demonstrate the different methods of operation. Therefore, everything that has been said above is applied to the Basic ProcessRig and all associated equipment. As a consequence, before every experiment is attempted, proper calibration of ALL the measurementinstruments used, should be carried out.

Measurement Errors

Measurement is important in process control. It can significantly affect the quality of the final product. In the case of a nuclear power station,for example, it can also be a critical safety issue. Thus, all process parameters must be measured to known standards of accuracy.

The main aim is to reduce the errors in instrument output readings as much as possible, and to quantify the remaining error, since it is notalways possible or at least cost-effective to remove all measurement errors.

When there are known errors present, appropriate signal processing of the measurement signals can be carried out to improve the quality ofthe measurement data. This is described in the next theory section.

In order to reduce errors to a minimum, the sources of measurement error must be considered, which is the purpose of this theory section.

There are two types of measurement error: Random and Systematic. Random errors are small differences in the output readings of aninstrument when the same quantity is measured a number of times. The magnitude and sign of the error is random, so that for a largenumber of samples, the positive errors approximately balance the negative errors and the net error is zero.

A typical example of a possible random error is when measurements are made by human observation of an analogue meter reading. Othersources of random error are electrical noise, environmental changes (temperature, pressure), dust, friction, vibration etc. As stated above,random errors can be avoided if the same measurement is repeated a large number of times and an average is taken.

However, this will only be so if the errors are truly random. That is, in the case of the analogue meter readings taken by a human, if thehuman observer is persistently reading the meter from one side only, then the error induced is not random but systematic and averaging overany number of readings will not eliminate it.

Similarly, errors due to temperature fluctuations will not be random, if instead of both positive and negative temperature variations about aconstant value, there is a net change in the temperature during the period of time that the readings are taken.

Systematic errors are errors in the output readings of a measurement instrument that are unlikely to be revealed by repeated readings, andtherefore cause greater problems.

Apart from the random and systematic errors there are errors inherent with every instrument and are quoted by the instrument manufactureras accuracy figures. Examples of these are bias and sensitivity.

Systematic ErrorsThe two main sources of systematic errors are system disturbance due to measurement and the effect of modifying inputs. Other sources ofsystematic error include the use of uncalibrated or improperly calibrated instruments, problematic wiring, and the generation of thermale.m.f.'s.

System Disturbance due to MeasurementIn nearly every measurement situation, the process of measurement disturbs the system and affects the values of the physical quantitiesbeing measured.

As an example, consider the process of measuring car tyre pressures with a common pressure gauge. Measurement is carried out by pushingone end of the pressure gauge onto the tyre valve and reading a needle deflection against a scale.

During this process, a small amount of air flows from the tyre into the gauge. This air does not return to the tyre after measurement, so thetyre has been disturbed and the air pressure inside it has been permanently reduced.

An electrical example is a common voltmeter. When applied to measure the voltage across the terminals of a circuit, it draws current, thusloading the circuit and corrupting all measurements to a degree.

In all cases better instrument design (to compensate for or prevent system disturbance) is required to minimise this kind of error. In theexample above, a very high impedance voltmeter will draw a negligible current and the effect will be insignificant.

However, bear in mind that with passive instruments, the improvement of one performance parameter deteriorates another. This is whyactive instruments such as digital voltmeters are preferred, where the inclusion of a power unit improves performance considerably.

Modifying InputsThe static and dynamic characteristics of measuring instruments are defined in the first theory section for specified working environmentalconditions. These conditions should be reproduced as closely as possible during the calibration procedures, if the measuring instruments arerequired to perform according to the specifications.

Any variation of the specified environmental conditions is described as a modifying input or disturbance to the measuring system. This isbecause an environmental change has the same effect on the system output as a change in the measured quantity.

In any measurement situation it is either impractical or impossible to control the environmental conditions surrounding a measuring system.Since it is very difficult to avoid modifying inputs, the susceptibility of the measuring instruments is restricted, or alternatively the effect ofmodifying inputs is quantified (in terms of sensitivity drift and zero drift) and corrected in the output reading.

Appropriate instruments, called secondary transducers, must be chosen to measure the relevant environmental parameters so that suitablecorrections can be made to the measurements obtained from the primary measuring instruments.

Careful instrument design is crucial in making an instrument insensitive to modifying inputs as much as possible.

The method of opposing inputs compensates for the effect of a modifying input in a measurement system by introducing an equal andopposite input which will cancel any modification out.

High gain negative feedback is also used to compensate for the effect of a disturbance in the system. The effect of a periodic noise isminimised by appropriate filtering of the output signal.

Cabling measurement instruments to equipment is another source of error. These errors are caused by ignoring the resistance andtemperature coefficient of the connecting leads. Adequate screening of the cables can minimise induced noise and interference. Often properrouting of the cables can also help to reduce noise.

Another source of measurement error is thermal e.m.f. A thermal e.m.f voltage is generated across the ends of a joint connecting twodifferent metals due to a difference in temperature of the two metals. Thus errors will occur in voltage measurements whenever thermale.m.fs are not taken into consideration.

Signal Processing of Measurement DataIn most cases it is neither possible nor cost-effective to remove all measurement errors. Signal processing techniques are used to improve thequality of the signal at the output of the measurement system.

Operations such as amplification, attenuation, linearisation, bias removal, and filtering are common methods of processing measurementsignals. The specific processing depends on the nature of the raw output signals from the measurement transducers.

For example, signal amplification is carried out when the signal output level of a measurement transducer is very low. Signal linearisation maybe required in cases where a measurement transducer has an output which is a non-linear function of the measured input quantity.

Signal filtering is used to remove a particular band of frequencies within a signal. For example, low pass filtering may be required to removethe high frequency noise component in a signal.

Signal processing can be either analogue or digital. However, some analogue signal conditioning is often necessary prior to digital signalprocessing.

The choice between analogue and digital signal processing is mainly determined by the degree of accuracy required.

Digital signal processing has a much higher degree of accuracy than analogue processing, but the cost of the processing equipment involvedis much greater and the processing time is also longer, which can be a critical factor when controlling fast processes.

In cases where a physical quantity is measured by an inaccurate transducer, it is sufficient to use analogue processing.

When digital signal processing is chosen, it will be carried out by a digital processor, implemented either in software or in hardware. Since theoutput signal of most measurement transducers is in analogue form and a digital computer accepts only digital signals, an analogue-to-digital(A/D) converter is required at the interface between analogue transducers and the digital computer.

Level - Volume correspondence

Volume measurement is required in its own right as well as a necessary component in some techniques for the measurement and calibrationof other quantities such as flow rate and viscosity. In a dimension measurement, various human-induced errors can be introduced. Checks onthe way that the human operator is using equipment are just as important as calibration checks on the instruments themselves.

The upper tank is of a regular shape, with a rectangular cross-section. This enables the volume of the tank to be calculated easily from itsdimensions.

The golden rule in dimension measurement is that the line of measurement and the line of dimension being measured should be coincident.In the case of steel rules and tapes, the greatest potential source of user-induced error is failure to position the rule squarely across thedimension being measured. Parallax error is also possible if the user does not position the rule and read it from directly above.

For a given volume of water in the tank, the depth (or level) can be obtained simply from a scale fitted up the front from the bottom to thetop of the tank. The other two dimensions, length and width, can be measured with a properly calibrated instrument such a 'Vernier Caliper'for instance.

Once the length and width of the tank have been measured, the cross section of the tank is given by:

Area = (length) (width)

Then, the volume of the water occupying the tank at a specified depth (level) is given by:

Volume = (Area) (depth)

This produces a linear relationship between volume and depth, which can be graphically represented by a straight line. In order to plot astraight line, only two points are needed and therefore only two measurements of depth need be carried out.

Visual Flow Meter Calibration

The simplest piece of equipment available for calibrating instruments measuring liquid flow rates is the calibrated tank. The process rig isequipped with a rectangular calibrated (upper) tank. The graduated scale fitted from top to bottom allows the volume of the water to bemeasured quickly, which you have carried out in the previous practical.

Flow rate calibration is performed by measuring the time taken, starting from an empty tank, for a given volume of water to flow into thetank.

Because the calibration procedure starts and ends in zero flow conditions, it is not suitable for calibrating instruments which are affected byflow acceleration and deceleration characteristics.

The technique is further limited to the calibration of low viscosity liquid flows, although lining the tank with an epoxy coating can allow thesystem to cope with higher viscosities.

If liquids of high viscosities are used, the limiting factors to calibration are the drainage characteristics of the tank, which must be such thatthe residue liquid left after draining has an insufficient volume to affect the accuracy of the next calibration.

In this practical you will measure the 'fill' times for various volumes of water, calculate in each case the flow rate in litres/min and take theaverage of all these flow rate values. Finally, you will compare this average flow rate value with the value indicated by the visual flow meter.

Note however that the resolution of the flow meter is 0.2 l/min, its range is 0.4 l/min to 4.0 l/min, and water should be used as the liquid.

Servo Valve Calibration

In the Familiarisation assignment you have familiarised yourself with the operation of the servo valve. In this practical, the characteristics ofthe servo valve will be investigated to provide a means of calibrating the valve.

The servo valve characteristic to be investigated is the relationship between the flow rate and the supplied current.

The flow rate value for a particular current is obtained from the visual flow meter, while the value of current is obtained from the DigitalDisplay Module (DDM) connected in series with the servo valve. The DDM is a simple milliammeter which will be used a great deal in laterassignments.

The aperture of the gate valve enclosed in the servo valve metal box, is controlled by the magnitude of the current supplied by the processinterface 4 - 20mA dc current source.

This gate aperture defines the amount of water that will pass through the valve in a given time, i.e the flow rate.

The graph plotted between current and flow provides the characteristics of the servo valve. Ideally, for a linear servo valve, this graph shouldbe a straight line.

Solenoid Valve Calibration

In this practical you will switch on the solenoid valves and find their size coefficients Cv by knowing only the level and flow rate. The followinggives you an outline of the calculation:

The equation governing the flow of fluid through a restriction such as a valve may be derived from the laws of fluid mechanics. For a controlvalve, the flow rate of a liquid is assumed to be given by:

f = k a [2g (h1 - h2) ]1/2

where:

f = flow rate, lt/sec

k = a flow coefficient

a = area of control valve port, m2

g = acceleration due to gravity, m/sec2

h1 - h2 = difference between upstream and downstream water levels

The flow coefficient k and port area a, are different for every style and size of control valve. Consequently, it is standard practice to combinecertain terms of the above equation into a single number Cv, termed the size coefficient:

Cv = k aThe size coefficient Cv is defined as the flow rate of water in litres per minute, provided by a pressure differential of 1kg per m2 through a fullyopen control valve.

The size of the control valve is important, because it affects the operation of the automatic controller. If the control valve is oversize, forexample, the valve must operate at low lift (position of the gate) and the minimum controllable flow is large. Alternatively, if the control valveis undersize, the maximum flow required for operation of a process may not be provided.

Circuit BreakersA circuit breaker, part of a power supply unit, is a protection device that is often used in place of a fuse. One difference between it and a fuseis that the fuse breaks permanently, whilst the circuit breaker breaks temporarily and can provide more comprehensive protection.

Another difference is that the circuit breaker section performs the functions of switching and isolation, protection against overload currentsand protection against short-circuit currents, whilst fuses are electrical safety devices that protect equipment and components only fromdamage caused by overloaded circuits.

These functions of the circuit breaker are accomplished by means of thermal and electro-magnetic protection devices. The earth faultdetection section energises a trip coil when an earth fault occurs, causing the circuit breaker to open.

Circuit breakers are produced in various sizes and specifications, depending on the intended application.

The circuit breaker protects the circuit against overload currents by automatically switching off the supply. When only a slight overload occurs,however, the opening action of the circuit breaker must be delayed.

This is to guard against the effects of voltage surges and spikes, which can cause unnecessary tripping of the device, as well as surgescaused by switching on of motor, lighting and inductive circuits.

In contrast, short circuit currents must be interrupted as quickly as possible.

In order to accomplish protection for both overload and short-circuit currents, the circuit breaker has a bi-metallic strip and anelectromagnetic unit. The bi-metallic strip provides the thermal tripping characteristics. When an overload current occurs, the strip is heatedto a temperature that is dependent on the size and duration of the overload current.

This heating will bend the strip until after a certain time, the device is switched off.

The electromagnetic section contains a coil and a moving latch. When a short circuit current flows through the coil, a magnetic field is createdthat causes the latch to be attracted to the coil.

This effect is immediate and the circuit breaker will trip instantly.

Another device, also part of the circuit breaker, protects against earth fault currents. The main function of the device is to detect anydifference between the currents that flow in the live and neutral lines. If a discrepancy is detected, the live and the neutral contacts areopened by an electromechanical device.

The tripping system used in the device is a drop out system that is rated as fail safe. This system contains a magnetic circuit, which links toan adjacent coil core when an earth fault has been detected. When not in operation, the system is kept on standby by a permanent magnet,that ensures that the striker pin is held in position.

If an earth fault occurs, it is detected by the electronic section of the device. A capacitor is then discharged and a pulse is given to the coil ofthe tripping system. As a result of the coil being energised, the coil core becomes part of the magnetic circuit. In this situation, magnetic fluxno longer exists in the striker pin, which is released, discharging a spring and so providing the energy to operate the tripping mechanism.

The test circuit includes a white test button, marked 'T', which covers a contact spring and resistor. The resistor is connected to the phasevoltage and, when the test button is pressed, is also connected to the neutral conductor situated outside the core.

By pressing the test button, an earth fault is simulated and the circuit breaker will trip.

Current Loops in Process Control

Signalling is necessary in controlled installations.

Consider for example that a controller is situated in a control room, and its transmitter and control valve are mounted locally to a tank. Inorder for the controller to get information from the transmitter, and also to be able to affect the position of the control valve (to alter the flowrate or the level in the tank for example), it is necessary for the units to be able to communicate with each other.

Signalling may either be done pneumatically (compressed air signalling), or electrically (current signalling). A great advantage with signallingis that standard signals can be used, which means that instruments can be bought from different suppliers and still remain compatible.

Electrical signals in a control system are usually DC (Direct Current) signals, and can be divided into current and voltage signals.

Current signals are used for signalling over long distances, and voltage signalling is used for shorter distances.

Nowadays, computers are increasingly taking over control room instrumentation, and there has been a corresponding drop in the use ofvoltage signalling.

Current signalling is very often used between transmitters, controllers and signal transducers.

This shows a simple signalling arrangement between a control room and its transmitter.

Current Loops in Process Control

From an electrical point of view, a transmitter can be regarded as a current generator, which in our case is powered by the Process Interface(PI), situated in a remote control room. This means that it is the transmitter that determines the current, independently of the line resistance.

However, Ohm's law still applies:

Imax = E/R

where E is the voltage supplied by the PI (in the control room) and R is the line resistance.

Industry standard current signals are 4-20 mA and 0-20 mA.

A transmitter for a 4-20 mA signalling loop works as follows: The transmitter draws about 3 mA in order to work itself. The voltage required atthe transmitter's terminals is usually of the order of 12-15 V.

This diagram shows a typical transmitter arrangement.

The sensing device is converting a physical quantity (e.g level, flow rate, pressure, etc.) into a current signal. This is compared by the currentsensor, with the outgoing current, the difference is amplified and is used to alter the setting of the current generator.

The current signal changes in proportion to the signal from the sensing device.

Signalling between the transmitter and a number of instruments sited in the control room often requires a current-to-voltage conversion, thattakes place in the instruments by passing the current through their resistors.

This multiple instrument signalling is shown below.

The voltage levels obtained are then used internally within the instruments which are based on analogue electronics.

Voltage signals also occur in computer equipment, where analogue signals are processed firstly using current-to-voltage conversion, and thenAnalogue-to-Digital (A/D) conversion.

The most common signal range is 0-10 V, but 1-5 V and 2-10 V are also used. Voltage signalling is uncommon between transmitters andcontrollers within the process industries. There are exceptions however, particularly in rotational speed control motors, where the outputsignal from the tacho-generator is a DC voltage.

The most commonly used current signals are 4-20 mA and 0-20 mA.

For signalling using a 0-20 mA loop, the following advantages and disadvantages may be listed:

Advantages

o 20 mA resolutiono Current signal is independent of lead resistance, however, Imax = E/R still applies

Disadvantages

o The transmitter must be provided with separate supply. This adds to the installation cost.o It is not possible to provide a transmitter fail-safe system.o It is difficult to calibrate the zero.

Similarly, for signalling using a 4-20 mA loop:

Advantages

o 2-wire connection system, i.e signalling and power supply in the same leads may be used.o The floating zero point (4 mA) means :

Simple to calibrate zero point because the lowest current can be reduced below 0%.Simple to provide transmitter fail-safe system.Current signal is independent of line resistance, however, Imax = E/R still applies.

Disadvantages

o Resolution of only 16 mA.o Considering the above points, the 4-20 mA current loop is used in the experiments you will be doing in this package.

Circuit Breaker, Current Loop and Connections

In this practical you will become familiar with the power, current source and process connections on the front panel of the Process Interface(PI). You will also see how the Digital Display Module (DDM) is used to display the loop current representing the process variable beingmeasured.

The power section in the top left corner of the front panel includes a V40H multi9 30mA circuit breaker by Merlin Gerin, capable of providingprotection against overcurrent and earth fault currents (earth leakage).

The V40H 30mA device protects the circuit against overload currents by automatically switching off the supply in times of fault. Also, thecircuit breaker protects against earth fault currents, by detecting any difference between the currents that flow in the live and neutral lines. Ifa discrepancy is detected, the live and neutral contacts are opened by an electromechanical device.

The above two actions of the circuit breaker are the two different tripping characteristics of the device.

To test the operation of the V40H, press the test button on the front of the device. The V40H should trip every time. Failure to do so indicateseither no supply to the V40H, or a faulty device.

Note that all electrical equipment protected by the V40H must be effectively earthed and the measured value of the earth loop impedance inOhms, must be such that the product of this value and the operating current of the V4OH should not exceed 16A. The maximum permissibleearth fault loop impedance for the V40H is 8000 Ohms.

A V40H 30mA device is not used as the sole means of protection against direct contact, but it is used to reduce the risk associated with directcontact.

Proceeding with the description of the various sections on the front panel of the PI, the current source is considered next.

Circuit Breaker, Current Loop and Connections

The internal circuitry of the current source is of no concern here. A voltage applied at the input of the current source gives a current at the +and - output terminals, proportional to the input voltage. The output current can be varied between 4-20mA, using the black knob. Thecontrols labelled span and zero will be used in the Controller Familiarisation assignment to calibrate the device.

There are four general purpose process connections on the front panel of the PI. The fifth process connection is reserved for the servo valve.

Each process connection consists of a 7-pin DIN socket and a pair of + and - terminals. These can be either input or output terminals,depending on the circuit configuration. Note that the + and - terminals of the current source are input to the servo only.

The associated 7-pin DIN lead carries the power supply and current signals to the auxiliary devices (transmitters, DDM) that can be attachedmagnetically to the process rig.

The current signal alone, representing the process variable being measured by an auxiliary device, can be obtained from the + and -terminals of the process connection and subsequently distributed to the I-V converters or to the controller.

The Digital Display Module (DDM), when included in the current loop, indicates the value of current in the loop in mA or as a percentage(%) of the maximum current. Thus, 4mA corresponds to 0% and 20mA to 100%.

In this practical, a current loop incorporating the current source, two process connections and the DDM is set up to illustrate the above.

Servo Valve

In this practical you will connect a current loop using the current source, the Digital Display Module (DDM) and the servo valve.

As described in the Flow/Level Rig Familiarisation assignment, the servo valve opening and hence the flow rate are set by the current in thecircuit, which can be varied using the black knob in the current source section on the front panel of the PI.

A 4mA setting implies a fully closed servo valve, whilst the valve is fully open when the current is at 20mA.

To operate the valve, you should connect the + and - current source output terminals to the respective + and - servo valve input terminals,and the servo valve process connection 7-pin DIN socket on the PI to the servo valve socket on the BPR using the appropriate leads. This isall shown in Patching Diagram 2.

The block diagram for the current loop is shown below.

The current from the current source enters the + and - input terminals of the servo valve connection and exits through the corresponding 7-pin DIN socket to the process rig servo valve socket.

The Digital Display Module (DDM) is connected in the loop to display the value of current in mA or as a percentage (%).

As shown previously, the flow rate is controlled by the servo valve which is set by the loop current. The servo valve opening is directlyproportional to the loop current.

This enables the DDM to be set to %, and the value that it reads can be considered the state of the servo valve, e.g. the servo valve is k%open. It is very common to describe process variables in control as percentages rather than absolute values.

The main reason for adopting this approach is the huge diversity of variables that have to be controlled, even in a small plant. It is verydifficult for operators to remember every absolute pressure, flow, temperature, and so on, but by using a non-dimensional representation ofquantities, the task of keeping track of operation is greatly simplified

Another reason is that everything in the plant is constrained to lie within certain limits. For example, a valve can only operate from fully closedto fully open, a temperature sensor is designed to give correct readings over a certain range. This makes it very simple to express quantitiesas percentages.

Current-Voltage Converters

In this practical you will incorporate the current source, process connections 1 and 2, and the Digital Display Module (DDM) in a loop, similarto the previous practicals. The new feature is the current-to-voltage (I-V) converter.

There are two I-V converters in the PI, labelled I1->V1 and I2->V2 on the front panel of the PI. These are used to convert the current in theloop to a voltage across R, a 100 Ohm resistors.

The voltages obtained can be used as inputs to the comparator or logic inputs in the on-off section on the front panel of the PI (this sectionwill be described in the Controller Familiarisation Assignment) or as inputs to the various relays of the process controller. In this way, differentmodes of control can be obtained.

The voltages produced across the terminals G and 0V, or H and 0V, are referenced with respect to 0V, whilst the voltages across thecorresponding + and - input terminals of the converters, are not. This ensures that all the voltages are commonly referenced to 0 Volts.

You will use converter 1, i.e I1->V1, to convert the current around the loop using the 100 Ohms resistor to a voltage across the terminals Gand 0V.

You will repeat this for a number of different values of current and plot the graph of voltage against current. This graph should approximateto a straight line with the gradient approximately equal to 100 Ohms.

Process Instrument Calibration

In any process control system, measurements at the intermediate and final stages of a production line are carried out to monitor quality-related process parameters, and to inspect and test the final product.

This is to ensure that the quality of the product is maintained. For this to be successful the accuracy of these measurements must bemonitored and corrected if necessary by proper calibration of the instruments used.

Regular calibration is necessary because instrument characteristics change (drift) over a period of time, due to mechanical wear, ageing ofcomponents, environmental changes, dirt, dust, etc.

When calibrating an instrument, its output is compared to the output of a standard instrument of known accuracy, when the same input(measured quantity) is applied to both instruments. During the calibration process, the process instrument is tested over its whole range, byrepeating the comparison procedure for a range of inputs.

Calibration ensures that the accuracy of a calibrated instrument will be at a certain acceptable level, when used under the environmentalconditions present during the calibration process.

Outside those conditions, the characteristics of the measuring instrument change, and its accuracy can no longer be relied upon.

However, in most situations it is usually impossible or impractical to control environmental conditions,and correction of the measuringinstrument reading is necessary.

Calibrated equipment should form part of a calibration chain, in which each link in the chain is calibrated against still more accurateequipment, ensuring reliability and accuracy.

When a process instrument is calibrated against a standard instrument, its accuracy will be found to be either inside, or outside that requiredby the application measurement accuracy limits. In the former case, the calibration results are taken down in the instrument's record sheet.

If however, the instrument's accuracy is found to be outside the acceptable measurement limits, its characteristics should be corrected untilthey lie within the specified measurement limits.

If adjustment is not possible the instrument should be either repaired, or scrapped.

Processes involving the flow of liquids through connected pipes and vessels are common examples of industrial processes and are oftenemployed to demonstrate the operation of many different kinds of processes. As a consequence, before every experiment is attempted,proper calibration of ALL the measurement instruments used, should be carried out.

ComparatorsComparator circuits are nonlinear circuits which are based on operational amplifiers, and usually produce two discrete outputs, each of whichis dependent on the input level.

Comparators are widely employed in applications involving the selection of a finite number of possible circuit conditions. Comparators areused as key elements of A/D and D/A conversion systems and also in oscillator and waveform generator applications.

Circuits in which one or more conditions of operation occur at a saturation level are referred to as saturating circuits. Saturating comparatorcircuits are relatively slow in operation and thus are limited in application. By various clamping techniques, it is possible to establish referencelevels well below saturation, and hence increase the switching speed significantly. Such circuits are referred to as nonsaturating circuits.

Comparators may also be classified as either non-inverting or inverting, according to the following: If the output is high when the input isabove a certain minimum transition level, the circuit is considered as non-inverting.

Conversely, if the output is low when the input is above a certain minimum transition level, the circuit is considered as inverting.

Open-Loop ComparatorsThe simplest comparator circuits are those that operate with no feedback at all. Such comparators are referred to as open-loop comparators.

The simplest open-loop comparator is the non-inverting saturating comparator.

The input signal is applied to the non-inverting input Vin, and the inverting input Vref is grounded. If V in > 0, the differential input voltage andthe output voltage are both positive.

Because of the very large typical open-loop voltage gain Ad, a positive voltage in the microvolt range will drive the output to positivesaturation. For example, if Vsat = 13 V and Ad = 200,000, a positive voltage as small as Vin = 65 microvolts will cause the output to reachsaturation.

If Vin < 0, the differential input voltage and the output voltage are both negative.

Therefore, the operation of the non-inverting saturating comparator can be described by the two expressions:

Vout = Vsat for Vin > 0, andVout = -Vsat for Vin < 0

An inverting saturating comparator circuit is formed by grounding the non-inverting input and applying the signal to the inverting input.

In this case the mathematical operation can be expressed as :

Vout = Vsat for Vin < 0, andVout = -Vsat for Vin > 0

The two open-loop comparator circuits considered above both have transition points at zero volts, established by the voltage level of the inputVref. By applying a dc bias voltage to either of the op-amp (operational amplifier) inputs, the transition level may be established at somearbitrary voltage level.

Schmitt Triggers

Comparators employing positive feedback are widely known as Schmitt Trigger circuits. The addition of positive feedback produces an effectcalled hysteresis.

Hysteresis, as introduced in the Familiarisation assignment is the non-coincedence of the transition curves of the output when the inputchanges from high-to-low state and from low-to-high state. The response curve to input state changes is direction-sensitive.

The advantages of the Schmitt trigger circuit are:

o The possibility of undesirable state changes due to spurious noise pickup is minimised by employing hysteresis. The change in inputmust be of a certain magnitude before it triggers a state change, with the 'certain magnitude' controlled by the level of hysteresis.

o Also the switching process is emphasised, or exaggerated, by the hysteresis effect, and this effect can be advantageous in certainwaveform generators.

Consider the inverting Schmitt trigger circuit below.

The divider network consisting of R1 and R2 establishes a voltage at the non-inverting input terminal proportional to the output voltage. Themagnitude of the voltage across R2 is defined as the threshold voltage, Vt.

This voltage is : Vt = R2Vout/(R1 + R2).

Note that :

Vref = Vt when Vout = Vsat andVref = -Vt when Vout = -Vsat

With reference to the input-output characteristic curve shown, assume initially that the circuit is in a state corresponding to the position A.

At A, Vout = Vsat, Vref = +Vt, and Vin is negative.

Since Vref = +Vt, the input voltage Vin must slightly exceed Vt to force the op-amp differential input voltage to change sign. Vin must travelalong line AC.

When Vin reaches and slightly exceeds Vt (at point C), the op-amp output starts to drop. The voltage at the non-inverting input, which isalways a constant fraction of the output voltage, will follow the output. This increases the differential voltage and accentuates the transitionprocess. The output is travelling down line CD.

The op-amp output has now changed from +Vsat to -Vsat in a short time interval, limited primarily by the maximum rate of change (the slewrate). Any further increase in Vin causes the input to move along line DE, but the output remains at -Vsat.

While Vout = -Vsat, Vref = -Vt. To return to the initial state, Vin must decrease along line EF until it is less than -Vt. At which point the switching istriggered again and the output will travel up line FB until Vout = +Vsat.

Observe how the switching level is a function of the direction of change. By choosing an appropriate value of V t, the effects of noise at thetransition points can be minimised. However, if Vt is too large, the accuracy of the crossover point may be degraded, so the threshold must becarefully selected.

The rectangle on the input-output characteristic curve is called a hysteresis loop. Note that it is necessary to label a hysteresis loop witharrows in order to identify the proper direction.

With the feedback loop connected to the non-inverting input, the input signal V{in} is always connected to the inverting input of the Schmitttrigger. This means that a Schmitt trigger is an inverting device. When its input becomes more positive than V{t}, the output switches to low(normally negative). When its input becomes more negative than -V{t}, the output switches to high (normally positive). This must always beremembered when using one of these devices.

Current Source Calibration

In this practical you will calibrate the current source of the PI. You will setup a current loop, so that the Digital Display Module (DDM) can beincluded in the loop, and indicate the value of the current source output.

You will then calibrate the current source against the DDM. In order to do that, two adjustments must be made:

i. Calibrate for zero: You will adjust, using an appropriate screwdriver, the zero control, located in the current source section onthe front panel of the PI, so that exactly 4 mA or 0% is shown on the DDM.

ii. Calibrate for span: You will use an appropriate screwdriver to adjust the span control, located in the current source section onthe front panel of the PI, so that the DDM displays exactly 20 mA or 100%.

In this practical you are only required to perform and understand the calibration process. The accuracy of your calibration is limited by theaccuracy of the instruments used in the calibration process. To perform calibrations of a very high accuracy, instruments must be used thathave been calibrated themselves to a very high level of accuracy. The more often an instrument is calibrated, the more its readings can berelied upon.

This calibration should be carried out before every practical to ensure the output of the current source is accurate. To rely on the last use ofthe current source not to have changed the calibration is very bad practice, and every effort should be made to ensure its reliability.

This does not apply only to the current source. You will be shown how to calibrate other instruments in the following assignments, and onceyou have become familiar with each procedure it should form part of a pre-practical setup routine. By calibrating each instrument you canthen be satisfied that any results produced will be up to a certain level of accuracy. A measurement is only as accurate as the least accurateinstrument involved in a practical.

Control Systems

A control system consists of a controller and a plant. A plant is the machine, vehicle, or process that is being controlled. The controller isthe system that is required to produce satisfactory results from the plant.

A manual control system is one where the controller is a person. The alternative to this is an automatic control system, where the controller isa device, usually implemented electronically, either using analogue circuits or a digital computer (microprocessor). Pneumatic and hydrauliccontrollers can also be found in industry, these are still legitimate automatic control systems.

The interface between the plant and the controller requires actuators (control elements) to provide the control action. Actuators arecommonly electric, pneumatic, or hydraulic, depending on the application and power level required.

In addition, detectors, sensors (measurement elements) and instrumentation are needed to provide information about the plant statusto the controller.

Control Systems

The diagram below shows the basic elements of a control system. The flow of information between these elements can be seen.

The information that is passed between the controller and the plant is in the form of signals. These signals can be very diverse, for exampleelectrical, pneumatic, mechanical, etc.

The term transmitter is used to describe the action of the measurement element when it sends signals to the controller, which represent themeasured values of the system.

A control system can be open-loop or closed-loop.

An open-loop control system utilises a controller or control actuator in order to obtain the desired response, without incorporating feedback.The input-output relationship of the system is only the cause and effect relationship of the output from the controller and the plant.

In contrast to an open-loop control system, a closed-loop control system utilises an additional measure of the actual output. This is thencompared to the desired output response, or reference input signal. The measure of the output is called the 'feedback signal'. A feedbackcontrol system often uses a prescribed function between the output and reference input to control the process. Often the difference betweenthe output of the process under control and the reference input is amplified and used to control the process, so that the difference iscontinually reduced

The notion of feedback exists in everyday life. For instance we use visual feedback to walk. Feedback not only gives verification of ouractions, it allows us to cope with a changing environment by adjusting our actions in the presence of unforseen events and changingconditions.

Feedback has similar advantages when applied to automatic control. Feedback gives an automatic control system the ability to deal withunexpected disturbances and changes in the plant behaviour.

A manually controlled closed-loop system for regulating the level of fluid in a tank uses negative feedback. The input is a reference level offluid that the operator is instructed to maintain. This reference is memorised by the operator. The operator views the level of fluid through aport in the side of the tank. The power amplifier is the operator and the sensor is visual. The operator compares the actual level with thedesired level and opens or closes the valve (actuator) to adjust the flow and hence maintain the desired level.

Automatic Control Systems

The control of an industrial process by automatic rather than human means is called automation.

In its modern usage, automation can be defined as a technology that uses programmed commands to operate a given process, combinedwith feedback of information to determine that the commands have been properly executed. This kind of automation is provided by the digitalPID process controller COMMANDER 300.

Modern control systems are self-organising, adaptive, robust, able to learn about a process, and can optimise control.

Automation is often used for processes that were previously operated by humans. When automated, the process can operate without humanassistance or interference. In fact, most automated systems are cabable of performing their functions with greater accuracy and precision,and in less time, than humans are able to do.

However, semi-automated (hybrid, or human-robot) processes that incorporate human workers and robots (computer controlled machines)and manually controlled systems, still exist, since some tasks are best carried out by humans.

Control systems are sometimes divided into two classes.

If the object of the control system is to maintain the physical variable at some constant value in the presence of disturbances, the system iscalled a regulator.

One example of a regulator control system is the speed-control system on the ac generators of power utility companies. The purpose of thiscontrol system is to maintain the speed of the generators at the constant value that results in the generated voltage having a frequency of 50Hz in the presence of varying electrical power loads.

Another example of a regulating process control system is the biological system that maintains the temperature of the human body atapproximately 36}o{C in an environment that usually has a different temperature.

The second class of control systems is the servomechanism (sometimes called a Kinetic control system). Although this term was originallyapplied to a system that controlled a mechanical position or motion, it is now often used to describe a control system in which a physicalvariable is required to follow, or track, some desired time function.

An example of this type of system is an automatic landing system, in which the aircraft follows a 'ramp' trajectory to the desired touchdownpoint.

A second example is the control systems of a robot, in which the robot 'hand' is made to follow some desired path in space.

Control Types

Different types of control can be carried out by the Process Controller (38-300), depending on the requirements of the process and thedesired output. All types are covered thoroughly in their own assignments, but an introduction to each is given in this theory. Each type isshown by applying it to a simple level control problem, and by doing this you will gain an insight into the types of control that are available.

A tank is holding liquid to feed a process. The process being supplied requires a constant head of liquid and so a control system is required tokeep the tank level constant. A valve is located in the tank inlet to vary the flow rate.

The diagram below illustrates the situation.

Open-loop Operation

The simplest strategy is to calibrate the inlet valve. By experimentation, a relationship between tank level and position of the handwheel canbe obtained.

If the outflow is constant, a position of the handwheel can be found that keeps the level constant. If the valve is opened a little more, so thatmore water is coming into the tank than is going out, then the level will rise. Conversely if the valve is closed a little, so that more water isgoing out than is coming in, then the level will fall.

Now, if a different level is required, the handwheel can be changed to increase or decrease the flow until the new level is reached.

This method is Open-Loop Operation. It is simple and will work well, provided there is no change in the outflow of the liquid, and all otherparameters affecting the level in the tank remain constant.

There is no electrical or mechanical feedback path, so the system is open loop, but feedback is being provided through the user. He/she isdeciding if the actual level is above or below the desired level, and adjusting the actuator accordingly.

Feedforward/Feedback Control

Feedforward Control

The major cause of disturbances affecting the tank level is likely to be changes in the tank outflow rate. An increased outflow will cause thetank level to drop. Therefore, a more reasonable approach is to produce calibration curves for a number of outflow rates.

By monitoring the outflow rate, the correct position of the handwheel can be determined by examining the calibration curve for the new flow.The handwheel is then adjusted to keep the tank at the required level. This technique is Feedforward Control, and requires a measurement ofthe outflow rate in order to calculate the change in the position of the inlet valve.

Although feedforward control is an improvement over open-loop operation, it does have disadvantages that restrict its usefulness.

One of these disadvantages is the calibration curves between the handwheel position, outflow and level. These must be accurate for theprocess to function correctly. Another is that the process may vary with time, or disturbances occur that are not included in the calibrationcurves or are not monitored. Under these circumstances, feedforward control will not be successful.

Feedforward/Feedback Control

Feedback Control

We could carry out more measurements to compensate for the errors that can occur in feedforward control. However, the obvious solution tokeep the level in the tank constant, is to monitor the level itself. If it deviates from the desired value, the inlet valve is adjusted by an amountdependent on the difference between the actual level and the desired level. This control strategy is called Feedback Control.

Feedback control is error driven in that the control effort is a function of the difference between the desired and the actual levels. Therelationship between the error and the control effort is called the control law.

Feedback level control does require a more elaborate level measurement technique, and an accurate valve actuator. It also requires a signalrelated to the actual level (i.e a level transmitter). In addition, the valve actuator must be able to hold the valve in any position, and to alsochange its position gradually and smoothly.

The diagram below illustrates how this may be implemented for the situation described earlier.

The important characteristic of feedback control is that it is capable of providing a range of control effort, that is, it can produce small as wellas large corrections. An appropriate control law must be designed or selected to produce a satisfactory performance.

The control law represents the action of the controller. Common control law types are the P-type (proportional), I-type (integral) and D-type(derivative), or a combination of these, i.e PI, PID.

Examples of feedback control systems can be found in nature, one of which is the temperature-control of the human body. This controlsystem attempts to maintain the body temperature at a constant value. Generally, the environment tends to vary the body temperature. Thebody responds to a difference in temperature by perspiring, by increasing or decreasing blood flow, by shivering, and so on.

This control system has one characteristic that control systems designed by humans do not often have : it normally operates in a satisfactorymanner for seventy years or more. Another characteristic of this system, and one that is usually present in control systems that we design, isthat if the magnitude of the error becomes too large, the system fails.

On/Off Feedback Control

A simplification of the general feedback control type is On/Off Feedback Control. The level in our example would now only have two states;either above the desired level or below it. Monitoring can now be carried out by a float switch, mounted at the desired level.

The switch produces a binary (on/off) signal that indicates whether the level is above or below the desired value. The signal can then be usedto operate the inlet valve directly.

When the level is above the reference value, the inlet valve is closed, and when below, it is opened. The control law in on/off control is keptsimple, it switches the control effort between two extremes, depending on the sign of the error.

The diagram below illustrates the control method in the context of the equipment set-up.

Whatever the cause of the change in level, provided the deviation is large enough to activate the switch, then control action will be applied tocorrect the situation.

By using on/off control our equipment requirements have been simplified. However, there are several problems associated with on/off control.

One problem concerns the abrupt fluctuations in flow as the valve switches between fully open and fully closed.

Another problem is that the precision of on/off control depends heavily on delays associated with the switch, the inlet valve and the rate ofchange of flow. With lengthy delays, overflow could occur if the valve is not shut as soon as the desired level is reached. The answer is notjust to make the switch quick and sensitive as this can lead to unnecessary switching caused by waves or ripples.

The type of control chosen for a particular situation will depend on the accuracy required, cost of equipment, maintenance (the simpler thesystem, the easier it will be to maintain), disturbances expected and the degree of disturbance rejection expected, degree of humanintervention required, health and safety (how dangerous is an overflow of the process in question?), and so on.

Serial Communication

Practical 1 will take you through the steps that must be carried out before you can attempt any practical that uses the Process Controller(38-300) and a personal computer together. It is vital that these steps are completed successfully to allow the 38-300 to communicate withyour personal computer.

Steps 1 and 2 concern the physical link between the 38-300 and your personal computer. They ensure that the cable supplied to link the twodevices is connected to the correct ports, and that the 38-300 is terminated correctly. It is possible to use more than one 38-300 if a practicaldemands it, and in such a case the controllers form a chain from the computer. This is when the termination of the serial lines is important.

Step 3 deals with the parameters that must be set up in the 38-300 to allow it to communicate with your personal computer. The parametersare the speed of communication (or Baud rate), the identity of the 38-300 (to allow more than one to be used), the type of parity checkingand the block check character enable (the last two are both error checking facilities). These four amount to the 'language' that is beingspoken, if they are set up incorrectly the 38-300 will not understand the messages being sent by your personal computer.

Practical 2 considers the 38-300 in much greater detail, and explains the control panel and how it functions, but this practical must becompleted first. Although you may not fully understand all of the steps yet they will become clear in the next practical. It is sufficient just tofollow them for now.

The reason for keeping this in a separate practical is that it can now be referenced at any time very easily. Linking the 38-300 to a computeris accomplished by completing this practical.

Navigating The Controller

The following diagram shows the configuration of the front control panel of the ABB Commander 300 controller.

There are six tactile membrane switches. Each switch enables a certain function, or a list of functions to be carried out.

The switches are outlined below:

The Page Advance key is used to move forward through the program pages.

Note : The various possible programming functions are divided up into groups, called program pages. Each program page consistsof a number of parameters that can be set up by the operator, to achieve a particular function or mode of operation.

The Parameter Advance switch is used to advance to the next parameter within a program page.

The Enter switch is used for storing the programmed parameters and values into the instrument's non-volatile memory. The same switch isalso used in some parameters to toggle between, or step through, a selection of displays/ parameters.

Note : A flashing decimal point on the upper display indicates a change to the value/parameter in the lower display. The change canbe stored in memory by pressing the Enter switch, and the flashing decimal point will disappear showing that the change has beenstored. If the Enter key is not pressed before advancing to the next parameter or page, the changed parameter will not be storedand it will revert to its original value.

The Raise switch is used to increase a parameter value, or step up through a selection of parameters.

The Lower switch is used to decrease a parameter value or step down through a selection of parameters.

Note : Continued pressure on the Raise and Lower keys, causes the rate of change of the displayed variable to increase. To makesmall adjustments, press the keys momentarily.

The Auto/Manual switch is used for selecting automatic or manual mode. In manual mode, the displays automatically revert to controloutput (bottom display) and the process variable values (top display), and the Raise and Lower keys can be used to alter the control output.When in auto mode this facility is disabled.

To return to the operating page (the first page) from any other page or parameter, press Page Advance once and then Enter once

Controller Displays

When a parameter within a program page is being viewed, the upper display shows the name of that parameter and the lower display showsthe value or setting for that parameter.

An 11 segment bar graph display indicates the deviation of the measured value from the set-point.

The LED indications are as follows :

A1 is associated with the states of alarms 'A' to 'E';A2 is associated with the states of alarms 'F' to 'K';

More specifically, if:

the LED is flashing, the relative alarm is active, but not acknowledged;the LED is on, all active alarms are acknowledged;the LED is off, all alarms are inactive;L is on, when the local set point is being used;R is on, when the remote set point is being used;L&R are both off, when the dual set point or dual fixed set points are used;ST is on while the self-tune procedure is being performed;ST flashes when the procedure is complete;M is on when the controller is in manual control mode.

Computer Initialisation

A personal computer can be used to initialise the 38-300 for an experiment, saving you a lot of time. The Commander 300 contains some 200parameters and for these to be set up by hand for every experiment is extremely time consuming. But help is available from the computer, asit can send a file of parameters to the 38-300 which will set up all necessary values and settings. By completing Practical 1 you will enable the38-300 and your computer to talk to each other, so that this file can be downloaded. Computer initialisation shall be used a great deal in thefollowing practicals and assignments.

Another advantage with initialising the 38-300 before starting a practical is that it is then placed in a known (and safe) state, no matter whatit was last used for.

Controller Documentation

The following manufacturers documentation is supplied with the controller :

o Operating Instructionso Quick Reference Guideo Serial Data Communication Supplement

It would be useful to reference these manuals when carrying out practicals involving the controller. For example the Quick Reference Guidecontains a comprehensive parameter map, which can be used for quickly finding your way around the many controller menus.

Using The Controller

In this practical you shall make use of some of the facilities of the Process Controller (38-300) that are available for different modes ofcontrol. This shall be kept simple as it is also a good way of becoming familiar with the 38-300, and introducing aspects of practical controllersthat can be seen in the 38-300. As the assignments develop, so too will the use of the 38-300 and more of its full capacity shall be seen.

Manual Control

Initially you will control the output of the 38-300 using the Raise and Lower keys on the control panel. The 38-300 will take the place of thecurrent source on the Process Interface (PI), and the Raise/Lower keys will carry out the same function as the current source control knob onthe PI. You will be the operator of the process, manually controlling its operation, and it is a very simple matter to set the level of the tank.

The manual control example.

Here the manual control effort, Um, is you, changing the output at will. The actuator is the servo valve, and the measurement section is alsoyou watching the level in the tank. Disturbance is normally included in this sort of situation to account for any fluctuations that are out of yourcontrol (environmental changes can affect the flow for example).

Bumpless Transfer between Automatic and Manual Control

When switching control modes (manual-auto or auto-manual) a problem can arise when the automatic set-point and the measured operatingpoint are not equal. Normally a controller will start in manual mode, where the automatic set-point is adjusted until it is equal to the requiredoperating point.

The controller output will then be adjusted until the deviation between set-point and actual operating point is zero, and the controllerswitched to auto mode. This ensures that the plant is operating steadily at the desired operating point at the instant of switching.

If the actual operating point is changed (using the manual controls) but the set-point is not, when the plant has reached steady state and isswitched to auto there will be a deviation (because the measured operating point is not equal to the set-point), and the automatic controllerwill attempt to correct this.

This will cause a bump and will drive the plant away from the operating point set manually by the operator. The same effect can occur whenswitching from auto to manual, if the manual output control is not equal to the actual automatic controller output.

The way to avoid this is to employ automatic bumpless transfer, which is a facility most modern controllers include. When using the 38-300, ifthere is a deviation between the desired operating point (as set by the manual controls) and the automatic set-point at the time of switchingfrom manual to auto, the plant will continue at its desired operating point (with no bump).

This is slightly anomalous as there will be a (possibly large) deviation and an incorrect set-point, but it has prevented bumps. Althoughbumpless transfer is available on modern controllers, it is good engineering practice to ensure no deviation in operating points (automatic set-point and actual measured operating point) before switching modes, rather than rely on this facility.

Alarms

The 38-300, like other industry process controllers, is capable of triggering alarms should certain predefined conditions be met. There are 10alarms available (A to K except I) and each can be programmed separately. There is an alarm priority, with A being the highest and K beingthe lowest.

On the control panel of the 38-300, LED A1 is assigned to alarms A to E, and LED A2 is assigned to alarms F to K. These LEDs will flash toshow that they have been triggered but not acknowledged, and will remain on when they have been acknowledged. The display will also flashto signal the triggering of an alarm.

Each alarm can be assigned a Type, a Trip Level and a Hysteresis setting.

The alarm Type describes the situation that the alarm is watching for and it can be one of the following;

High or Low ProcessHigh or Low OutputHigh or Low DeviationFast or Slow Rate of Change of Process Variablea Programmed EventMode Alarm

This last item is triggered by switching to a particular mode, such as manual, auto, all set-point types or even a failure of some sort.

The trip level is the level of the selected type that should trigger the alarm. For example, alarm A can have the type 'HOUT' which is HighOUTput and a trip level of 80%. If the output of the 38-300 is increased to 80% or above, A will be triggered and LED A1 will flash to showthis.

The hysteresis setting is another way of checking process parameters. The hysteresis setting is operational when an alarm is active, and it isspecified as a percentage or in engineering units.

It is best shown with the above example; alarm A has been triggered by the output of the 38-300 increasing above 80%, and the hysteresissetting is 5%. The output is lowered, but it must decrease below 75% (80% trip level - 5% hysteresis setting) before the alarm is turned off.The output must move into the safe region by an amount equal to the hysteresis setting.

When you are carrying out Practical 3, after the alarms have been set up, all of the above can be seen on the Setup Alarms page (SEtUPALAr_S). Use Page Advance to find this page and then Parameter Avance to pass through Type, Trip Level, Hysteresis Setting, and also therelay setup parameters which are covered in the next assignment.

Practical Controllers

The task of a controller is to maintain the desired system performance despite any disturbances in the system.

Controllers are usually implemented electronically, either using analogue circuits, or a digital computer (microprocessor). However pneumaticand hydraulic controllers are still in use.

In process control, it is unusual to design a specific controller for a particular plant, because the dynamics of the plant are uncertain and oftenvery dependent on operating conditions.

Therefore, a general purpose controller is normally implemented, which has a number of variable parameters that can be set to meet thestatic and dynamic requirements of the control system.

The static characteristics of a system are independent of time and the response of the system depends only on the inputs. The dynamiccharacteristics of a system depend on both time and inputs.

Various types of general purpose controllers exist, and each can be characterised by its actions and methods of controlling a system.

General Process Controllers

A block diagram of a General Process Control System is shown below:

This shows the plant, and some means of measuring a process variable. This measured variable is fed back to the controller to determine howwell the system is operating.

With the addition of the feedback loop it has now become a closed loop system. The controller will compare the measured output and thedesired output (the Set Point) to determine the control effort. Um is the manual input, and the manual/auto switch can also be seen.

With the switch in the manual position the control law has been disconnected from the process plant and the system is controlled by theoperator only (a manual control system, the type of control we have been implementing so far).

With the switch in the automatic position the control law is added to the manual input and this will determine the behaviour of the process.The process can now be controlled automatically, provided it is given desired operating levels.

An automatic controller cannot determine how to control a process, it can only carry out desired control, determined by a third party, you.

We shall be carrying out various types of feedback control in later assignments.

Digital Control Systems

The use of digital computers for the control and monitoring of processes is becoming increasingly important.

Digital computers offer improved performance, better management of a process, reliability, flexibility, reduced cost and can perform complexcalculations that could not be done by other analogue means.

Powerful software development tools (programming languages for instance) which can be run on digital computers, result in reduceddevelopment costs for computer-based control systems.

A number of digital single loop controllers can be used (an example of one of these controllers being the 38-300), each carrying out thefeedback control of a single variable in a multi-variable process. These controllers are called the slave controllers and are co-ordinated by adigital controller called the master.

A master controller sends set point information to the slave controllers and receives back information on the measured variables. This type ofcontrol is called set point control and a major benefit of this type of control is that even in the event of a master controller failure, theindividual slave controllers will continue to operate, and with these operating the process should continue to run.

The other type of computer control is called Direct Digital Control (DDC). DDC as the name implies, uses digital controllers to determine theactual control effort applied to the process or plant.

The digital controllers are microprocessor-based single loop or multi-loop controllers that control a single- or multi-variable plant, in place ofthe old analogue, mechanical or pneumatic controllers.

Digital Control Law

The digital control law of a process that determines the control effort can be obtained from two completely different design techniques.

The simple method is to approximate the analogue control law with a discrete time control law. In essence we implement the existinganalogue controller with a digital controller. This method has the advantage of familiarity with the analogue concepts and terminology.However, a major disadvantage is to restrict the vast capabilities of digital computers due to the digital approximation of the analoguecontrollers having limited capabilities.

The other method for the design of digital controllers is to design directly in discrete time. The drawback of this method is that during discretetime (i.e. during the sample times) very good control may be achieved, but between samples the controlled variable is effectively in the openloop condition and may oscillate. Design techniques must be must be accurate and thorough to encompass such eventualities.

The two block diagrams show the analogue and digital discrete time controllers and how they differ. The control law in the discrete timeexample is implemented digitally, but the plant is a continuous analogue system. A digital to analogue converter (DAC) must be includedbetween the controller output and the plant input, and a corresponding analogue to digital converter (ADC) between the plant output and thecomparator in the feedback path.

The sampling rate fs of the DAC and ADC will determine how the controller copes with oscillation and fluctuations. Consequently it must bemuch faster than the process dynamics, so that the digital approximation will produce similar results to the conventional analogue controller.

The controller you are provided with is a single loop, digital controller. The DAC and ADC are integral parts of the controller, and as such theywill not concern you through your practicals.

Chart Recorders

In this assignment you a chart recorder will be seen for the first time. The chart recorder is used to display a representation of changinganalogue signals that are presented to the controller.

This provides several facilities for you to monitor process variables in conjunction with the controller. As well as displaying variables in realtime the chart recorder has the facility to record a curve. The recorded files can then be selected and replayed using the playback practical.

Playback

When the playback menu item is selected, two options will be displayed, Playback and Comparison Playback.

Playback allows you to load and replay a process recording that has previously been made. The Comparison Playback is used to show tworecordings side by side to compare them. On the control, there are several features:

Load allows the loading of recorded data from disk:

Pause will halt the chart recorder:

Clear will clear the current curve from a recorder:

Ghost will display a faint frozen curve from the instant that the button was clicked on.

The scrollbar can be used for moving forwards and backwards through the replayed trace. The speed of the playback of the recorded tracecan also be changed using a drop-down box. The speed can be altered as the playback occurs.

Playback

When the playback menu item is selected, two options will be displayed, Playback and Comparison Playback.

Playback allows you to load and replay a process recording that has previously been made. The Comparison Playback is used to show tworecordings side by side to compare them. On the control, there are several features:

Load allows the loading of recorded data from disk:

Pause will halt the chart recorder:

Clear will clear the current curve from a recorder:

Ghost will display a faint frozen curve from the instant that the button was clicked on.

The scrollbar can be used for moving forwards and backwards through the replayed trace. The speed of the playback of the recorded tracecan also be changed using a drop-down box. The speed can be altered as the playback occurs.

38-300 Calibration

With reference to the 38-300, measurements are carried out to monitor process parameters and to determine the control effort that should beapplied when controlling a process.

Such measurements ensure the correct operation of the process system. Because of this, the accuracy of these measurements must beguaranteed by proper calibration of the controller used.

Calibration consists of comparing the measured value or level of a parameter, as shown by the controller being calibrated, with the knownvalue or level of the parameter, as measured by a standard instrument of known accuracy.

Calibration guarantees that the accuracy of the input or output reading in a calibrated controller will be at a certain acceptable level, whenused under the environmental conditions (e.g temperature, humidity, pressure) present during the calibration process.

Outside those conditions, characteristics may change, and so the accuracy of the instrument will vary to a greater or lesser extent accordingto its susceptibility to the modifying inputs inherent in the new environmental conditions.

When a process controller is calibrated against a standard instrument, its accuracy will be either inside, or outside that required by theapplication measurement accuracy limits.

If the controller's accuracy is outside the acceptable measurement limits, then its characteristics should be adjusted by the zero and spanparameters provided, until they are within the specified measurement limits. Bear in mind that all calibration and measurement proceduresshould be documented, so that a record of the calibration history of any instrument will always be available.

Calibration Check of the Process Controller

In this practical you will be shown how to check the calibration of the process variable and remote setpoint inputs of the 38-300.

There is no need to actually calibrate the controller in this practical, not because it does not need to be accurate, but because the 38-300 hasbeen calibrated to a level of accuracy much greater than most meters readily available to you. It has, stored in its memory, settings for thespan and zero parameters of its inputs, and you shall reset these to ensure the 38-300 is accurately calibrated.

As mentioned in the Interface Calibration Assignment this calibration should now be carried out before every practical that uses the 38-300.The more often it is carried out the more familiar you will become with the importance of calibration and the need to carry it out before everypractical.

Before beginning this practical, make sure that your process interface is switched on so that the 38-300 controller is powered up.

38-300 Relays

The remaining two practicals in this assignment introduce other aspects of the 38-300 that are available and made use of in laterassignments.

The alarms that the 38-300 offers will have already been met in a previous assignment, but there is a further facility that is tied in with thesealarms; namely Relays.

A Relay is essentially a voltage-controlled switch, whose state is determined electronically.

The action of a relay can be one of two types; if it is normally open (n.o.) the relay has no connection (an open circuit), and will 'close' (shortcircuit) when triggered by the input voltage. A normally closed (n.c.) relay is a short circuit until it is triggered, when the relay will break itsconnection and create an 'open' circuit.

Relays are also described by the 38-300 as Positive Action or Negative Action. Positive action is where a relay follows its n.o. or n.c. label; i.e.a positive action, n.o. relay will be open until triggered, when it will close, whereas a negative action, n.o. relay will be closed until it istriggered, when it will open (so a negative action n.o. relay is actually behaving like a positive action n.c. relay).

Relays and Alarms

In the 38-300 an alarm can be associated to a relay. When an alarm is triggered by its condition being met (High/Low Output, High/LowProcess, High/Low Deviation, etc.) the relay assigned to that alarm will register this.

A relay will be triggered by its trip condition being met, and will remain in its triggered state until that condition is no longer true. In the caseof the 38-300, when a relay has been assigned to an alarm, it will be triggered when the trip level of the alarm has been passed.

The alarms allow a hysteresis level to be set, which determines when the alarm is no longer active. This will apply to the relay assigned to analarm with a hysteresis level; the relay also has a hysteresis curve, showing that it is voltage direction dependent (its switch ON is at adifferent voltage to its switch OFF).

The above graph illustrates a characteristic curve for a relay with hysteresis action (inherited from an alarm).

The combination of relays and alarms in the 38-300 allow us to operate On/Off control. Devices can be turned on and off when specificconditions are met. For now, only the operation of the relays will be demonstrated, the On/Off control of a process that these offer will beinvestigated in a later assignment.

In this practical you shall vary the output of the 38-300 manually with the Raise and Lower keys, which will control the flow through theservo valve.

An alarm will be assigned an output trip level, and one of the relays will control a solenoid valve. The process will now cut off flow when anoutput level is reached, and open it again when the output has been decreased to a sufficiently low level.

Reading The Controller

In the Controller Familiarisation assignment you were introduced to the concept of computer initialisation of the 38-300, via the serial link.This was discussed primarily in terms of one-way communication, but it is in fact two-way.

When your personal computer sends a command to the 38-300 containing information about a particular parameter, the 38-300 willacknowledge the reception of this data by sending a signal back to the computer. The computer uses this returned signal as the cue to sendthe next command to the 38-300.

The message from your computer will contain a parameter name, and the desired value of that parameter. The reply from the 38-300 willrepeat the parameter name and value, and also attach a message acknowledged character. Your computer will check to see that the messagewas acknowledged, and if it was, will continue with the next parameter command.

The reason for checking for the acknowledge character is that the 38-300 can also attach a message not acknowledged, meaning there was amistake in the message received by the 38-300, either in transmission or in original message construction.

Rather than tell the 38-300 what a particular parameter should be, your computer can also ask what it currently is, and the 38-300 will returna value. The message sent by your computer will not contain a parameter value, only the parameter name to be read. In this way, variablesbeing monitored by the 38-300 can be displayed by your computer. This practical will demonstrate the read facility so that you will be familiarwith it for later practicals, to record and chart process variables automatically.

This practical will take the output from the current source on the PI and use it as a process variable input to the 38-300 controller. Yourpersonal computer will monitor this process variable continuously, and display its value on the virtual chart recorder. This enables you toimmediately get a graph of input to the 38-300.

You will be able to watch the virtual chart recorder display the value of the current as you vary it by hand.

As the assignments develop and the need to monitor variables increases, the computer, with its virtual instrumentation, will play a greaterand greater part.

As well as the chart recorder displaying a parameter graphically, there is also a virtual control bar facility. This enables you to change aparameter value by varying the position of a bar with your mouse. The value you set on the control bar is displayed in the small upperwindow, and this value is sent to the 38-300. The parameter will then be read back by your computer to check its value and this result isdisplayed in the small lower window. The slight delay between the number appearing in the upper and lower windows is the delay while serialcommunication is carried out.

Both of these display facilities are shown in the practical. Having completed it, you will then be familiar with a personal computer as a controltool in a process system.

The computer offers the following to the user; it can initialise the 38-300 by writing a value to all of its parameters, read any of its parameters(to be displayed in a number of ways, depending on requirements), and finally allow the user to directly alter parameters from the terminal

and observe their effect, either as a confirmation of that change and then directly observing the process or by monitoring the processvariables with virtual instrumentation.

Feedback Control

The idea of feedback was introduced in the Controller Calibration Assignment, but it was not explored theoretically or applied experimentally.

In this assignment, and the Pulse Flow Transmitter Assignment, the subject of feedback is considered fully, so that you are familiar with it forthe assignments that consider different control strategies and process control examples.

The control law, which was first considered in Controller Familarisation, is the relationship between the measured and desired values of aprocess parameter.

The addition of a feedback loop to a process allows the control law to be implemented automatically; now the control action is dependent onthe measured value. A system which employs feedback control has become error-driven. It is able to deal with unexpected disturbances.

The above diagram shows the general feedback control system. This diagram applies to all systems incorporating a feedback loop. Thediagrams met in the previous assignment (general process control system, digital and analogue control systems) were all based on thisgeneral form.

It shows the forward path, encompassing the plant, represented by G. This function includes any controller dynamics. The feedback path isrepresented by H, and this includes the measurement system.

A system can be modelled so that its behaviour can be assessed before it is physically implemented. By using a block diagram of a Feedbacksystem, the modelling of a system can be explained.

The output, C, can be described as a function of input, R, and measured value, Cm, and the transfer function G:

C = (R - Cm) G

But,

Cm = C H

So,

C = (R - C H) G or C (1 + H) = R G

By rearranging this, a function of output over input can be produced:

this is the transfer function of the whole system, and shows how output and input are related.

The control law represents the action of the controller. Common control law types are the P-type (proportional), I-type (integral) or D-type(derivative), or a combination of these, i.e PI, PID. Each of these shall be met in turn in later assignments.

An example of a regulatory biological feedback control system is the temperature-control of the human body. This control system attempts tomaintain the body temperature at a constant value. Generally, the environment tends to vary the body temperature from the desired value.The body responds to an error in temperature by perspiring, by increasing or decreasing blood flow, by shivering, and so on.

This control system has one characteristic that control systems designed by humans do not usually have : it usually operates in a satisfactorymanner for more than seventy years. Another characteristic of this system, and usually present in control systems that we design, is that ifthe magnitude of the error becomes too large, the system fails.

Control System DynamicsSystems will invariably have dynamic characteristics. Behaviour which is time dependent is inevitable, as the response of any system to aninput is not instantaneous. The greater the distance a telephone call is over, the more the dynamic behaviour becomes apparent; the time lagbetween transmission and reception increases. Computers are becoming quicker and quicker, but there will always be an 'access time' toretrieve data from its hard disk.

Modelling of dynamic systems is concerned with the cause and effect relationship between input and output. Models of dynamic behaviour fora system can be obtained by two methods. The basic equations of physics and mechanics can be applied to a system where the underlyingprinciples are clear and it is sufficiently simple, or can be broken down into simple subsystems. This is a first principle approach.

The second approach is to observe normal behaviour or introduce test signals. A model produced from this method is based on observationand input/output behaviour.

Both of these methods will produce differential equations which describe the dynamic characteristics of the system. Differential equationsinvolve time as a variable, normally in the form of derivatives and integrals with respect to time. But solving these by hand is difficult and timeconsuming, so a range of numerical methods have been developed which can be implemented on digital computers.

Models can be used to make predictions about a system, or they can be rearranged to enable the design of a system to meet a requiredperformance. A model cannot explicitly determine output at a particular time, but it gives the input/output/time relationship so that for agiven input, the output can be found.

Float Level Transmitter

The Float Level Transmitter (FLT) is a device which takes level information from the Float Level Sensor in the tank and transmits it to theProcess Interface.

The sensor is a potentiometer (pot) connected across a low voltage supply, which is turned by a floating disk.

As the level of water in the tank changes, the disk turns the potentiometer which changes the voltage across it, and this voltage is passed tothe Transmitter.

The FLT will then convert this to a current signal of the 4-20mA format, and transmit it to the PI. By converting to the 4-20mA signal format,communication is no longer restricted to very short distances, a concern when dealing with large process plants whose control rooms aresituated away from the measurement devices.

Also by converting to the 4-20mA signal format, the signals from the sensor are compatible with all other devices. The equipment has becomestandardised and the set up of the hardware is more flexible.

The sensor is the first device to actually produce information on the state of the process, which can then be used to determine the futureoperation of that process. This, and similar devices (the Pulse Flow Transmitter which is covered in the next assignment), enable feedbackcontrol to be used.

Calibrating the FLT

When using the float level sensor and transmitter combination, measurements are carried out to monitor a process parameter, namely tankfluid level. This measurement is monitored and used to determine the control effort that should be applied to control the process correctly.

Such measurements help to maintain the correct operation of the process system. Because of this, the accuracy of these measurements mustbe guaranteed by proper calibration of the instruments. The importance of calibration cannot be stressed enough.

Full calibration consists of comparing the measured value or level of a parameter, as shown by the instrument being calibrated, with theknown value or level of the parameter, as measured by a standard instrument of known accuracy.

Calibration guarantees that the accuracy of the input or output reading in a calibrated instrument will be at a certain acceptable level, whenused under the environmental conditions (e.g temperature, humidity, pressure) present during the calibration process.

Outside those conditions, characteristics may change, and so the accuracy of the instrument will vary to a greater or lesser extent accordingto its susceptibility to the modifying inputs inherent in the new environmental conditions.

Calibration of the Float Level Sensor and Transmitter

In this practical you will be shown how to calibrate the output of the two devices supplied with the float level pack. The calibration is done intwo halves, with an empty tank and with a full tank, and the method is described below.

In this practical the FLT will be calibrated to zero ('zero' being the lowest reading, i.e. 4mA) without the sensor connected. The sensor is thenconnected with the level in the tank at the zero indicator, and its screw adjusted until the DDM reads 4mA again. The zero reading of thesensor and transmitter has now been calibrated against an empty tank.

The tank is now filled until the level is at the 100 (full) indicator, this is the highest level that will be reached in the tank. At this level thesignal output should be maximum, 20mA. The span of the transmitter is adjusted so that the DDM does read 20mA at the top of the tank.

It can obviously be seen that 0 and 100 on the tank scale are not absolute values. This type of arrangement is common in industry and isknown as allowing the plant to have breathing space in the event of an emergency. For example if needed, for what ever reason, you cantake the process slightly above 100 % and slightly bellow 0 %. However, in theory the header tank should never reach absolute zero, exceptfor maintenance, ie. when the plant is shut down.

This process has calibrated the level sensor and transmitter against the upper tank, so that the 4-20mA signals necessary for control of thisprocess will be produced between empty and full. This simple procedure should now be carried out before every practical that uses the floatlevel sensor and transmitter.

Demonstration of Fluid Level Control

This practical is intended only to give you a glimpse of the sort of control that will be applied in later assignments. It is a fully functioningproportional control system, monitoring actual water level in the upper tank, comparing this to a desired level (the set point), and altering theposition of the servo valve with the aim of achieving the set point.

It is not necessary for you to understand exactly what proportional control is, and how it is being implemented at this stage. This practical isonly intended to demonstrate the fact that there is now sufficient equipment to apply feedback control to a real process control situation.

Also in this practical you will be able to observe another feature available when using a computer in a process control situation; that of themimic diagram. This is a graphical representation of the whole system, showing the flow loop and all parts of the system.

You will be able to alter the set point and see how the controller applies a controlling action to reach that set point. The actual tank level isshown and also the state of the servo valve. It will give you a qualitative 'feel' for the system, rather than the quantitative information thatthe chart recorder provides.

This method of display is widely used in industry as it enables the whole system to be observed, particulary when the control room is situateda long distance from the plant.

The next assignment introduces the last two devices that will be used extensively when applying different control methods, the Pulse FlowTransmitter and Pulse Flow Sensor. Once you have become familiar with all the equipment, the different functions that each can carry out,and the exact nature of each control method available, shall be explored.

System Model TheoryThe differential equations met in the last assignment are the basis for analysis and design of control systems. A simple water tank exampleshall be used to illustrate this.

A water tank of fixed cross section area A is filling with a flow q. The flow rate of q is equal to the rate of change of volume, so initially thesystem is;

q = dv / dt

but, where h is the level.

v = A h

So now the model becomes;

q = A dh / dt

This is a time dependent model of the tank, where the input is inflow q(t) and output is level h(t). This model can be used to discover thebehaviour of the system for a particular input. When analysing a system, assumptions will be made to simplify the mathematics. Rather thanproduce an exhaustive solution for a very specific input, a general idea of behaviour is more useful, although it may not be exact.

The pump is switched on. The servo is fully open initially, but is gradually shut over 30 seconds until fully closed. Maximum flow is 1 litre /second (0.001m3 / s). Three assumptions will be made; tank level is zero before the pump is switched on, no fluid is leaving the tank, andwhen the pump is switched on, maximum flow is achieved instantaneously.

The flow rate is now;

q = 0.001 (1 - t / 30) m3 / s

This is substituted into the model to give;

A dh / dt = 0.001 (1 - t / 30) m3 / s

Now integrating with respect to time and rearranging gives;

h(t) = 0.001 (t - t2 / 60) / A m

This describes the behaviour of the level of the tank for that particular input.

Transfer Operators

Differential equations are not an ideal way of representing dynamic systems because the system input, output and their derivatives are spreadthroughout the equation. This makes the cause and effect relationship difficult to recognise.

A way of simplifying these is to use Transfer Operators, mathematical operators which represent the process of differentiation with respect totime. They can be manipulated in differential equations to simplify the system representation, but since they are mathematical operationsthey do not have a value, and so cannot be multiplied explicitly.

The transfer operator is D, and it represents;

dx / dt -> Dx

This enables differential equations to be reduced in the following way.

The flow rate q is given by;

q = A dh / dt

as was seen in the last theory section. Using the D operator this becomes;

q = A Dh

and this can be rearranged to give;

h = q / (A D)

As mentioned earlier this could not be multiplied out explicitly since D has no value. Used on its own, the D operator does offer somesimplification, but a better way of handling differential equations is through the use of the Laplace Transform. This method of solvingdifferential equations represents functions of time, t, as functions of s, a new variable. This is covered in the next theory section.

Laplace Transforms

The Laplace Transform is a mathematical transformation that allows functions in the time domain, t, to be represented in the s domain, wheres is a new variable. The Laplace transform is given by;

This is applied to all time domain equations to produce their s domain equivalents.

A simple example of a time domain function is;

....the exponential function.

Its transformation is as follows;

The equation has been substituted into the standard form of the Laplace transform and the integration carried out.

Although the mathematics looks involved, the result is;

....which is rather straightforward.

Another example is the Laplace transformation of the derivative of a function of time;

Substituting this into the standard form as before....

....and carrying out the integration, the transformation is as follows;

x(0) is the value of x(t) at t = 0, the initial condition. Provided this is zero....

the transform reduces to the following;

....which is also rather straightforward.

From this a general result can be extracted; differentiating a function of time is equivalent to multiplying the Laplace transform by s, providedthe initial conditions are zero.

By applying the Laplace transform to some general equations, a table of common transformations can be produced....

This can be used directly for simple problems, and adapted for others.

There is an equivalence between the D operator and the Laplace operator, s, but this is only true while initial conditions are zero.

Also it must be recalled that D is an operator but s is a variable, so this equivalence must never be extended to equality.

The reason for using Laplace transforms is to simplify solving differential equations. The standard methods were to substitute an assumedsolution or to use D operators, but both of these are tedious, as a general solution is first found, with the arbitrary constants evaluated byinsertion of the initial conditions.

When using the Laplace transform, the solution is found largely by algebraic means, with the initial conditions involved from an early stage.This shortens the determination of a particular solution.

As different control methods are explored in the following assignments, Laplace transforms will be covered again with practical examples. Thisis meant as a theoretical introduction to the subject only.

Pulse Flow Transmitter

The Pulse Flow Transmitter is a device which takes rate of flow information from the Pulse Flow Sensor in the pipe network and transmits it tothe Process Interface.

The sensor is a small water wheel inside a pipe, which is turned by the flow of fluid through the pipe. The speed of the wheel is proportionalto the rate of fluid through it; the faster the fluid, the faster the wheel.

There is an infra-red sensor across the wheel detecting its movement. When the wheel turns, its blades will break the beam reducing theoutput. With the wheel spinning this produces a pulse waveform, whose frequency is proportional to rate of rotation (and so rate of flow).

This pulse train will be passed to the transmitter where it is converted into a 4-20mA current signal, whose magnitude is dependent on thewaveform frequency. This current signal is then transmitted to the PI.

By converting to the 4-20mA signal format, communication is no longer restricted to very short distances, a concern when dealing with largeprocess plants whose control rooms are situated away from the measurement devices.

Also by converting to the 4-20mA signal format, the signals from the pulse flow sensor are compatible with all other devices. The equipmenthas become standardised and the set up of the hardware is more flexible.

The pulse flow sensor is the second device to produce information on the state of the process, which can then be used to determine thefuture operation of that process. This, in a similar way to the Float Level Sensor and Transmitter met in the previous assignment, enablesfeedback control to be used.

Calibrating the PFT

When using the pulse flow sensor and transmitter combination, measurements are carried out to monitor a process parameter, namely rate offlow of fluid. This measurement is monitored and used to determine the control effort that should be applied to control the process correctly.

Such measurements help to maintain the correct operation of the process system. Because of this, the accuracy of these measurements mustbe guaranteed by proper calibration of the instruments. The importance of calibration cannot be stressed enough.

Full calibration consists of comparing the measured value or level of a parameter, as shown by the instrument being calibrated, with theknown value or level of the parameter, as measured by a standard instrument of known accuracy.

Calibration guarantees that the accuracy of the input or output reading in a calibrated instrument will be at a certain acceptable level, whenused under the environmental conditions (e.g temperature, humidity, pressure) present during the calibration process.

Outside those conditions, characteristics may change, and so the accuracy of the instrument will vary to a greater or lesser extent accordingto its susceptibility to the modifying inputs inherent in the new environmental conditions.

Calibration of the Pulse Flow Sensor and Transmitter

In this practical you will be shown how to calibrate the output of the two devices that together make up the pulse flow pack. The calibration iscarried out in two halves, with zero flow and with full flow, and the method shall be described below.

In this practical the PFT will be calibrated to zero ('zero' being the lowest reading, i.e. 4mA) without the pump switched on. This is theminimum level of flow; zero, so this should be signalled by 4mA.

The pump is then switched on, and all the manual valves are fully opened, this is now the greatest flow that will occur. At this flow rate thesignal output from the transmitter should be maximum, 20mA. The span of the transmitter is adjusted so that the DDM does read 20mA atmaximum flow rate.

This process has calibrated the flow sensor and transmitter, so that the 4-20mA signals necessary for control of this process will be produced,and they will accurately reflect the rate of flow through the pipe network. This should now form part of your calibration procedure before eachpractical. Without calibration, no weight can be placed on any measurements taken in a practical.

Demonstration of Rate of Flow Control

This practical is intended only to give you a glimpse of the sort of control that will be applied in later assignments. It is a fully functioningproportional control system, monitoring actual flow rate through the pipe network, comparing this to a desired flow rate (the set point), andaltering the position of the servo valve with the aim of achieving the set point.

It is not necessary for you to understand exactly what proportional control is, and how it is being implemented at this stage. Only toappreciate that you have now been introduced to sufficient equipment to apply feedback control to a real process control situation.

When proportional control was met earlier in the previous assignment, with the Float Level Sensor and Transmitter, you were introduced to aplant mimic diagram. In this practical you shall again be using a mimic diagram to observe the operation of the plant as a whole. In laterassignments you shall be concerned with particular variables and the chart recorder is ideal, but for this demonstration the mimic is muchmore suitable.

The system dynamics and time responses of a fluid flow process are much quicker than those of the equivalent fluid level process, and thispractical can exhibit considerable oscillation around the set point. It is this type of behaviour that shall be investigated in the laterassignments, and methods of avoiding it will be explored.

On the controller you may notice the alarm LED's (A1 & A2) flash. This is because the alarms are being used to trigger the relays which inturn allows us to switch devices on the Procon rigs. In this case we are switching the solenoid valve SV1.

Control TypesThe simplest control strategy is Open Loop Operation, which has no feedback. This is not strictly a type of control, since no 'control' of theprocess can be carried out. Although manual control is an open loop arrangement, it cannot be termed Open Loop Control because thisimplies no feedback, and there is feedback from the user.

The problem with open loop operation is that a process using this is inherently unstable. Without any fluctuations the process should operatequite happily, but fluctuations will occur. They will go unchecked and will not be suppressed in any way, simply because the controller isunaware of their presence.

Using a feedback controller without feedback will also lead to failure, as the controller is attempting to make the measured variable equal tothe set point. Without feedback, the controller will continue to apply a control effort unaware of the measured value.

This situation occurring with the Procon rig will lead to overflow or an empty tank (the two extremes), but considering process systemsgenerally a failure of some kind will be reached, which could be very serious if the plant is an office heating system, an elevator motor driveor a nuclear reactor for example.

On/Off Feedback Control

It is obvious then, that some sort of feedback is required for a system to operate under automatic control successfully. In the last twoassignments instruments were introduced that made feedback a viable option (these being the Float Level Transmitter/Sensor and the PulseFlow Transmitter/Sensor). Now, investigation of the different types of feedback control available can begin.

The first type of control is actually a simplification of the general feedback control arrangement, and this is On/Off Feedback Control. Itrequires much simpler equipment when implemented in a control situation.

When deciding which control strategy to implement in a plant situation, characteristics of each type will normally be weighed against eachother. In the case of on/off control the major opposing features are the general simplicity against its inherent binary nature.

When using on/off control, all devices are either fully on or fully off, 100% open or 100% closed. There is no middleground possible, sooscillation is often a feature of on/off control systems, as the control law is switching between extremes, driving the output between states.

Whatever the cause of the change in the measured value, if the deviation is large enough to activate the switch, then control action will beapplied to correct the situation. This means that on/off control is often subject to unnecessary switching caused by disturbances.

One such situation is illustrated below. The level of water in the header tank must be kept at a certain level in order to allow the process torun correctly. The flow of water into the tank is controlled by a servo valve placed somewhere in the inflow pipe. The opening and closing ofthis valve is, in this case, to be controlled by the float switch.

The disturbances caused by the fluid inflow are sufficient to raise and lower the float above and below the on/off switching points. This resultsin the inflow valve being driven open and closed unnecessarily. The on/off method of control is obviously unsuitable in this situation.

On/off control is suited to situations where it is only necessary to keep a process variable between two limits. For continuous processes,where the variable is required to be at a particular level it becomes impractical.

The Process Interface (PI) Comparator

Hysteresis was first introduced in the Rig Familiarisation assignment, and comparators were discussed in the Interface Calibration assignment.Now these ideas will be applied directly, and their relevance to this assignment will be made clear.

The comparator in the PI has two inputs; one non-inverting (marked with a +), and one inverting (marked with a -). The inputs to thecomparator must be voltage signals, so when using this the current/voltage converters must be incorporated. They will take the currentsignals (reference or measured values) and convert them to voltage signals suitable for the comparator.

This comparator feeds its output to a Schmitt Trigger (which is a comparator with positive feedback connected to its non-inverting input).

This arrangement is shown below.

The current source is providing the reference input to the comparator. A measured process variable is the other input, and the differencebetween these is the deviation. The size of this deviation will control the logic output C of the Schmitt trigger, which is either 0V or 5V.

Although the measured signal is shown connected to the positive (non-inverting) input, in practice it can be connected to either dependingwhich action is desired. These actions will be explored in the practicals.

The logic output C from the Schmitt trigger is used to control the switched supplies (and so the devices they are supplying).

Process Interface Comparator and Hysteresis

Recall the hysteresis loop curve:

This shows the two state-change paths that are followed, depending on direction of change of the deviation. It also shows the two voltagelevels + & - Vt which are the actual switching levels.

Initially the deviation shall be defined as large and negative, so that the operation can be explained. The output will be high (5V) as shown onthe hysteresis curve. As the measured value and the reference value become closer together, the deviation will become less negative, atsome point be zero and then become positive.

When the deviation has become more positive than Vt, the output will switch to its low state (0V). This is shown on the hysteresis curve. Thisis the control effort which will drive the measured value in the other direction. Deviation will decrease, until at some point it will be morenegative than -Vt. This will switch output back to its original state (high, 5V).

The hysteresis control determines the value of the threshold voltage Vt, and so this will control the difference between high-to-low and low-to-high switching levels. This is controlling disturbance rejection, since deviation must change considerably to cause a state change if V t is large.

As you will discover with on/off control, the measured value cannot remain constant at the reference value level, but it will lie between twolimits either side of the reference, determined by the amount of hysteresis. The greater the hysteresis, the further apart the two limits will be.

Thus, the on/off control technique is not used where precise control is required, but where a variable is required to lie between two values. Itis often only appropriate in certain applications, e.g to control the temperature in a room.

On/Off Pump Control

In this and the next practical you will become familiar with the use of the on-off control section of the Process Interface (PI).

On/off control will be used to control the water level in the tank, by automatically turning on and off the centrifugal pump (and so cutting theflow on and off).

The on-off control section of the PI includes a comparator and Schmitt trigger arrangement with variable hysteresis control, and logic inputsto control the switched power supplies (one ac, supplying the pump, and one 24V dc, both on the front panel).

The output from the Schmitt Trigger, which will be in one of two states depending on the deviation of the measured signal from the referencesignal, is controlling the supply to the centrifugal pump by providing a logic signal to input D.

The current source will supply the reference signal to the on/off control apparatus and will be connected to input B on the front of the PI. Thisreference signal will provide a desired tank water level. The measured value from the FLT will be connected to input A.

The pump should be on while the tank water level is less than the desired level, and should switch off when the measured level has passedthe desired level by an amount set by the hysteresis value. Once the measured level has dropped enough to trigger the Schmitt trigger, thepump should switch on again.

The reference signal is connected to the inverting input of the comparator, so that while the measured level is less than the desired level thedeviation is negative. When the measured value has passed the desired level, the deviation becomes positive. Considering the hysteresiscurve of a Schmitt trigger, this is an inverted action, since the output will switch from positive to negative as the deviation is moving fromnegative to positive.

Care should be taken when setting manual valve MV3, connected to the upper tank. This should be open enough for the level in the uppertank to increase when the pump is on and decrease when the pump is off.

This valve is controlling the times taken to rise and to fall between the two limits set by the hysteresis level.

On/Off Solenoid Control

In the last practical the pump was repeatedly switched on and off, to control the tank water level. This is not good practice as it will shortenthe pump's working life; this is not how a pump is designed to be used.

A better way of controlling the tank level is to use a different 2-state device; a solenoid valve. The solenoid valve is designed with repeatedswitching in mind and is much more suitable for on/off control. This shall be investigated now.

Solenoid valve SV1 could replace the pump as the device that cuts the flow on and off. The action would be the same as that for the pump;inverting. The valve must be open when the tank level is below desired level, and closed when above desired level.

But there is another way of controlling level which exhibits a non-inverting action. This is to use solenoid valve SV2, connected to the uppertank. It lets water flow from the upper to the sump tank.

Now when the actual tank level is above the desired level the valve should switch on, open, so that more water can flow out, and when theactual level is below the desired level it should switch off, close, so the water level can rise.

To accomplish this the reference signal from the current source is fed into input A (which is the non-inverting input to the comparator), andthe measured signal from the FLT is fed into input B. Now while the measured level is less than the desired level the deviation is positive, andwill become negative when the measured level is greater than desired. This is the reverse of the last practical.

Translating this to the action of the comparator and Schmitt trigger together, the output should switch from negative to positive as thedeviation changes from negative to positive. This is a non-inverting action.

Since the solenoid valve is to be controlled, the output of the Schmitt trigger will now feed the logic input E of the switched 24Vdc output toSV2.

As before care must be taken when setting the position of manual valve MV3. For the operation of this on/off control example to be asexpected, the level in the tank should go up when SV2 is closed and down when it is open. It may take a little trial and error to get this right.

The Float Switch

The float switch is a simple two-state device which is used to determine if the fluid in the tank is above or below a particular level. Itsoperation is shown below.

The stem of the switch contains a reed relay, which is normally open. The barrel of the switch, which moves up and down, contains amagnet. The magnet can open and close the relay by moving up and down the stem.

With the barrel at the bottom of the stem, the relay is open and no current will flow through the switch. This state is converted to a 4mAsignal by the transmitter. When the barrel is at the top of the stem, the relay will be closed and a current will flow through the float switch.This state is converted to a 20mA signal by the transmitter.

There is no other position of the reed relay, it is either open or closed.

The barrel floats, so that when the water is high enough the barrel is carried up the stem. The barrel can only move a short distance verticallyup or down because the stem is only short, but the stem can be positioned anywhere in the tank. This can then be set up to signal if the levelin the tank is above (20mA signal from the transmitter) or below (4mA signal from the transmitter) the level of the switch.

The float switch is binary in operation, open or closed. It can give no clue to how much over or under the desired level the water in the tankis. This makes it a device suitable for on/off control only.

It will take the place of the float level sensor, which produced a current signal proportional to the actual tank level. The solenoid SV1 shall beused to switch flow on and off. This exhibits the same action (inverted) as the pump, from the point of view of the Schmitt trigger, and its usewas discussed in the background in the previous practical.

The float switch uses the FLT to convert its signal into the 4-20mA format. The FLT, then, must be recalibrated for this new device. Thecalibration for the float switch is very simple and should be carried out as follows before beginning the practical.

The float switch is connected to the FLT with the barrel in its lowest position (this means that the reed relay is open), and the zero screw onthe FLT adjusted to read 4mA on the DDM. The barrel is then held in its highest position (this will close the relay), and the span screw on theFLT adjusted to read 20mA on the DDM.

38-300 On/Off Control

In this practical, the 38-300 shall switch devices on and off to control level. In the previous practicals it was the PI that determined the controleffort, with the comparator and Schmitt Trigger arrangement providing the control law.

Now the 38-300 shall take the measured value from the FLT as the process variable input, and use its relays to drive the process plant. The38-300 alarms were introduced in the Controller Familiarisation Assignment, and the relays introduced in the Controller CalibrationAssignment. This practical will be using these facilities for full on/off control.

The alarms allow process variable trip levels to be defined so that certain actions can be triggered at a particular level. These alarms are usedto trigger the relays, which provide the binary state signals. The alarm trip level and hysteresis value will act in a similar way to thecomparator and Schmitt trigger arrangement.

The difference is that when using the PI, a reference level was defined and the hysteresis set up trip levels either side of this reference. Whenusing the 38-300, the alarm trip level is equivalent to one of the hysteresis-set trip levels and the alarm hysteresis sets the other a distanceaway. This is shown in the diagram below.

The solenoid SV2, which allows water to flow from the upper to lower tank, shall be used to control level, with its logic input from the 38-300.An alarm shall be set up whose trip level will act as the desired tank level, and this will control the relay which is producing the logic signal byopening and closing a 5V line.

The trip level and hysteresis value of the alarm will be changeable, each with an onscreen control bar, which you can vary. This makes theuse of the 38-300 very similar to the PI in the previous practicals. A facility which is available from the 38-300, that the PI could not offer, is afailure alarm.

This will be set up to trip when the tank is approaching overflow, as a safety feature of the system. It will switch off the pump when thewater has reached this high level. To be able to do this without the 38-300, a second PI would be required, whose reference level was theoverflow limit. This is obviously much more inconvenient, and the multiple alarms and relays of the 38-300 allow great flexibility.

Proportional Control

The task of a controller is to maintain a desired system performance, coping with any system disturbances.

A simple controller is the proportional controller. The control effort is directly proportional to the deviation between the measured value andthe set point (desired value).

The following diagram shows a general proportional control system. It is similar to the general process control diagram met in the ControllerCalibration assignment.

It shows the manual input, Um, with the manual/auto switch. The output, C, is fedback along the loop, and the deviation between it and r,the set point, is found. This error, e, is passed to the control law, where a control effort is produced which is proportional to this error.

This control effort will then determine how the process reacts in the next time period.

By considering automatic proportional control, the following can be written;

U = Ke

This shows that the control effort, U, is directly proportional to error, e, where K is the gain or sensitivity of the controller.

This is the most basic equation describing this type of control, but it produces a problem; without an error, there would be no input to thesystem. This is not strictly true, if there was no error the input would in practice be at a base or quiescent level. Applying this to the aboveequation gives;

U = Ke + Um

where Um is the quiescent point.

Applying this to a practical control situation, the quiescent point, Um, would be the manual effort or input, and Ke the automatic controleffort, Uc, from the controller. In this process control example (Procon), the total flow into the tank is U, the error, e, is the differencebetween set point and measured value, the gain, K, represents how much the servo is open, and the output, C, is the tank level.

The total inflow can also be split up in the following way; Um is the water input needed to overcome any outflow, and Uc, the change ofinflow produced by a change in servo position (to counteract any deviation).

Now total inflow can be described as;

U = Uc + Um

Uc is the outcome of the control law, and for proportional control this is simply;

Uc = Ke

To establish a desired operating point, the set point is adjusted until it is the desired value, and the quiescent (manual) output of thecontroller is changed to reduce the deviation to zero. At this point U, the total output, is equal to Um, the manual output, and the controllercan be switched to automatic mode of operation. Since there is no deviation, the controller will apply no control effort, and a bumplesstransfer has been achieved.

Without reducing deviation to zero, the controller would attempt to correct the output with a control effort, and this would be seen as a bumpwhen switching modes. The process would actually be driven away from the desired operating point back.

Proportional Band

It is usual in industrial controllers to consider gain in terms of a proportional band (PB) or %PB. The proportional band represents the changein measured value (normally fractional change) that will generate 100% change in control effort. It can also be represented as the deviationthat will generate 100% change in control effort.

But this can be reduced....

Deviation is the error e divided by the measurement span, and the fractional change in control effort is the change in control effort divided bythe output span of the controller. Now PB is;

Controller gain, K, is just Uc, the control effort, divided by error, and this reduces the equation to;

The above equation shows the relationship between proportional band and controller gain, which is inverse proportionality.

A figure for gain alone is meaningless since it will be dependent on the units used. Expressing PB as a percentage does have meaning, even ifnothing is known about the process plant.

Proportional control alone is not normally used in process control, because a steady state error must always exist for any control effort to beexerted. Proportional control is a form of deviation correction, but without some deviation, no corrective action will be produced.

Increasing gain will reduce this deviation, but a large gain increases the chance of oscillation.

On/Off Verses Proportional Control

From the previous assignment, you will now appreciate the binary nature of on/off control. An upper limit and a lower limit are assigned forthe measured variable, and the controller will produce a control effort to keep the measured variable within these limits. There is no middleground, the controller will not know if the measured variable is 'near' the desired value (so a small control effort would be more appropriate),or 'far' (so a large control effort is needed).

Oscillation will invariably be a feature of on/off control, as the measured variable swings between its two limits. These limits can be made veryclose to reduce the swing, but they will always be distinct, and to keep the measured variable within very close limits the actuator and sensormust be fast acting.

Proportional control on the other hand, produces a control effort that has a direct relation to the size of the error it is correcting. The smallerthe error, the smaller the control effort to correct that error. There are no measured variable limits to keep within, only a desired value thatthe controller attempts to reach.

The disadvantage with proportional control, as explained in the previous theory section, is that to correct an error with a control effort, theerror is needed to produce the control effort. This leads to the conclusion that some error will always be present. It can be reduced byincreasing gain, but oscillation can then become a problem.

Simulation of Proportional Control

This practical aims to introduce the concepts of proportional control in a simulated environment before moving on to the Process Rig.

The practical is carried out by use of a mimic diagram. The concept of these representations will become more familiar as further assignmentsare carried out. The mimic diagram for this practical represents the Basic Process Rig when set-up to demonstrate the proportional control offluid level.

The servo valve is automatically opened and closed in order to keep the level of fluid in the tank as close as possible to the set-point. The setpoint can be altered on-screen at any time, as can the state of the solenoid valves and main pump.

The Proportional Band of this control system is also an important consideration. Altering the size of this band is equivalent to altering the gainof the controlling device. That is, the size of the control reaction induced by a deviation from the set-point.

The relationship between proportional band size and controller gain is one of inverse proportionality. A small proportional band will lead to alarge controller gain and large control reactions for small deviations. A large proportional band will lead to small control reactions even to thelargest of set-point deviations.

The control bars can be used to alter the proportional band and set-point. In this way a good initial understanding of the behaviour andparameters can be gained. Any ideas formulated can tested in the next practical using the process hardware.

P-Control of Level

This practical shows a fully functioning Proportional Control System, monitoring actual water level in the upper tank, comparing this to adesired level (the set point), and altering the position of the servo valve with the aim of achieving the set point.

Now that proportional control has been introduced, it is hoped that the operation of this system will be better understood.

The float level sensor is producing a current proportional to the water level in the upper tank, which is converted to the 4-20mA signal formatby the FLT. This is fed to the 38-300 as the process variable input, it is the measured value of the system.

The 38-300 is operating proportional control, and controlling the position of the servo valve with a 4-20mA current output signal. The exactsize of this output signal is in direct proportion to the deviation of process variable input and set point. The solenoid valve SV2 is used toensure constant outflow, this enables comparisons to be made between results obtained from different practicals.

P-Control and Offset

As explained in the theory sections, an offset will always be present when a controller applies correcting action to a process. This is becausethe control effort is directly proportional to the error only, and for there to be any control effort there must be some error.

This is the major drawback of proportional control, and it shall be shown in this practical. The control bar for the set point has been restrictedto 0-20% so that the chart recorder can zoom in on this area. Now you will see in greater detail how the measured variable approaches theset point.

By allowing the process plenty of time to settle down, the behaviour can be considered to approach its limiting steady state operation. This isthe state when no more changes are made to the inputs, and the plant is allowed to reach equilibrium.

Proportional Band

As explained in the theory, an offset will always occur when using proportional control because an error must be present for any control effortto be produced.

This offset was considered in the previous practical, by examining closely the region around the set point, and how the measured valueapproaches it. Also explained in the theory was the relationship between the proportional band and the gain of a process; this was inverseproportionality. If the proportional band is increased, the gain is decreased.

The offset can be reduced by increasing the gain of the system, but if the gain is too high, oscillation will occur. When designing a controlsystem the possibility of oscillation must be weighed against an acceptable level of offset.

In this practical the proportional band will be varied, and its effect on the offset observed. Oscillation can be induced, and by exploringdifferent values of proportional band, the region in which it starts can be found.

Oscillation and Gain

As discussed in the previous Assignment, proportional control applies a very simple control law; the control effort is directly proportional tothe deviation between the measured value and the set point (desired value).

Also met in the practical was the idea of discussing gain in terms of proportional band. As you will recall, gain and proportional band areinversely related, so that increasing gain will decrease proportional band.

Recalling the expressions for proportional control you can see that the greater the system gain, K, is, the greater the control effort, Uc, willbe.

Uc = Ke

where e is the error or deviation between measured value and desired value.

This system gain will determine how the process reacts to error, and its ability to reject disturbance. If the gain is large, the controller willapply a large control effort to correct an error. By greatly reducing the proportional band in the previous assignment, the controller was forcedto produce a large control effort.

If a large control effort is able to produce a considerable change in the measured value, it is very likely that the measured value will be drivenbeyond the set point which will invert the error. This in turn will produce a large and opposite control effort, forcing the measured value backagain. This is the oscillatory behaviour that can be exhibited.

In the Proportional Band practical of the previous assignment, the proportional band had to be very small (in the vicinity of <4%) to produceoscillation, but when dealing with flow control oscillation occurs for a much higher value of proportional band.

Proportional Band

For normal operation with an acceptable (meaning not extreme) level of gain, increasing the tank inflow will increase the tank level, but onlyby a small amount since the tank is large (a large tank will of course only rise by a small amount for a unit of water, whereas a small tank willrise much more). This means that the gain must be very large to produce a control effort that changes the inflow enough to push themeasured value beyond the set point, and so kick-start oscillation.

This does suggest that it would require a much smaller system gain value to produce oscillation if the tank were much smaller (whilst the restof the plant was unchanged). This is true, if the tank were a long tube, say, it would be much easier to swing the tank level beyond the setpoint because a unit of water would greatly increase level.

Changing flow by varying servo position will change the flow rate to a much greater extent than it will change the tank level. This means thatit takes a much smaller control effort to force the measured value of flow beyond its set point. For a given system gain producing a controleffort, the percentage change to flow will be greater than the percentage change to level.

This can be demonstrated with a few simple calculations.

Control Action Calculation

A tank, whose cross-sectional area is 40cm x 20cm (800cm2), holds 15 litres of water. The inflow and outflow are equal, both are 3 l/min.Since the volume of 1 litre of water is 10cm x 10cm x 10cm (1000cm3), 15 litres will have a volume of 150cm x 10cm x 10cm (15000cm3).

The depth of the water in the tank will be the ratio of volume of water to tank cross-sectional area;

15000cm3 / 800cm2 = 18.75cm deep

Inflow is increased by 1 l/min, and there is no change to outflow. After one minute there will be an extra litre of water, and this will haveincreased the tank level by;

1 litre = 10cm x 10cm x 10cm =1000cm3

1000cm3 / 800cm2 = 1.25cm (this is depth of 1 litre)

Tank level will have increased by 1.25cm, to 20cm. This is an increase of;

100 x (1.25 / 18.75) = 13.3%

But flow has increased from 3 l/min to 4 l/min in a minute. As a percentage increase this is;

100 x (1 / 3) = 33.3%

For a given control effort (and so a given system gain value) tank level increased by only 13.3%, but flow increased by 33.3%. This meansthat oscillation will occur much more readily when measuring and controlling flow.

Of course in a single loop process situation either flow or level would be used as the control variable, it would not be possible to measureboth. The above was used to show the differences between the two, and the factors that must be considered when deciding which to use.

When measuring and controlling flow, the system gain must be much smaller (and so proportional band must be much greater). Although thistheory section has been discussing how gain level causes oscillation, and at what level it occurs, it must be appreciated that in a processcontrol situation it is the objective to avoid oscillation.

The operation of the system must be understood so that proportional band is never made small enough to cause oscillation.

Servo P-Control

This practical shows a fully functioning proportional control system, monitoring actual rate of flow through the pipe network, comparing thisto a desired flow rate (the set point), and altering the position of the servo valve with the aim of achieving the set point.

Again, having introduced you properly to proportional control, this practical should now be better understood.

The pulse flow sensor is producing a pulse train, whose frequency is proportional to the rate of flow of water spinning the wheel inside thesensor. This pulse train is converted to the 4-20mA signal format by the PFT, and is fed to the 38-300 as the process variable input. Thissignal is the measured value from the system.

The 38-300 is operating proportional control, and controlling the position of the servo valve with a 4-20mA current output signal. The exactsize of this output signal is in direct proportion to the deviation of the measured process variable input from the set point.

When running the practical, you will be able to vary the set point of the 38-300 with an onscreen control bar, and observe its effect on theprocess. The chart recorder facility will again be used to display measured value and set point, and so indirectly also show error.

When carrying out the practical keep in mind the following simple expressions;

PB = A (1 / K)

where PB is proportional band, K is system gain, and A is the constant of proportionality, and

Uc = K e

where Uc is the control effort and e is the error. The constant of proportionality is a constant associated with the dynamics of the systemas it is incorrect to state that the proportional band is simply the reciprocal of the gain.

P-Control Offset

As explained in the theory section, the relationship between the proportional band and the system gain of a process is inverse proportionality.If the proportional band is increased, the gain is decreased.

You will have seen from the previous practical that proportional control with a large fixed gain (small proportional band) was not suitable forcontrolling the process. Therefore in this practical the initial gain value is very small (large proportional band), hence no oscillation, but a largeoffset.

It will be possible to find an optimum level whereby you have the smallest offset without oscillation occurring, this will be the objective of thepractical.

In the last assignment it was shown how the offset can be reduced by increasing the system gain (by reducing proportional band), but ifincreased too much, oscillation could occur. Now the system is stable, but a large offset is present. Therefore by reducing the proportionalband (increasing gain) the offset is reduced, but oscillation is introduced.

These two characteristics are in opposition as far as a process control system designer is concerned. When applying proportional control only,it must be decided if a large offset is acceptable so that oscillation is unlikely, or offset must be a minimum although this increases the chanceof oscillation.

In this practical the proportional band will be reduced (or varied), and its effect on the process (offset and oscillation) observed. This is themain part of the practical, and it is intended for you to have a very good grasp of proportional band, gain and control effort by the end of it.

Proportional plus Integral (PI) Control

The major problem with proportional control, as explored in the two previous assignments, is the inherent offset produced by the controller.

The control effort needed to correct an error is directly proportional to that error, and so the minimum error possible is finite. The way toremove this error is to use a control action that will produce a control effort for zero error.

This is done by introducing an extra component into the control effort which is the integral of the error. This will continue to change until theerror is zero, which should remove the error entirely.

Controllers that employ integral action are described as automatic reset controllers. They will exhibit a proportional action and an integralaction (the integral action is often termed the reset action).

Integral Control

The amount of integral action is controlled by a constant, Tr, which is the reset time. The control effort to the process is now described by;

Uc = Up + Ur

where Up is the proportional term, and Ur is the reset (integral) term.

From the previous assignments, you will already have met the expression for the proportional term;

Up = K e

The reset term, Ur, is described by;

which shows the position of the reset time constant, Tr.

so that the control effort can now be determined by the following expression;

This describes the action of an automatic reset controller.

The diagram above is the previous proportional control example, where;

Uc = K e

The diagram below shows the new control effort arrangement to produce PI control. Now you can see the two distinct elements of Uc; Ur isthe reset term and Up is the proportional term.

The reset time constant, Tr, is a very important variable as it controls the contribution of the integral action to the control effort over a givenlength of time.

If an integrator is given a step input of fixed duration, its response is a ramp. The slope of the ramp is controlled by Tr; the smaller Tr is, thesteeper the ramp. With a steep ramp, the contribution of the integral term will be large in a given time, and the time taken to reduce theerror present will be short.

Unfortunately it is not possible to keep reducing Tr, increasing the integral action, to remove all error. As with the proportional band, there willbe a minimum level of reset time constant that makes the system unstable, and this should be avoided. At this minimum level, the integralaction will be too large for the system and oscillation (our old friend) will result.

A problem that can occur when using normal (i.e. not extreme) values of Tr is Reset Windup. This is when an increase in control effort doesnot reduce error. This can be caused by a fault in the control system, such as a control valve stuck fully open. If this was to happen, thecontrol effort would build up due to the control action attempting to reduce the error, but without success.

When the fault is cleared, the error will drop rapidly because of the very high control effort. The output of the controller, on the other hand, isnot able to drop because of the long persistent reset action that was being exerted. This will remain until the measured value has been drivenbeyond the set point, producing the opposite error, for a sufficiently long time to cancel out this control effort windup.

The net result of this action is that there will be a large overshoot of the measured value, and a significant delay before the system is undercomplete control again.

Anti-reset windup is a technique incorporated in modern controllers, including the 38-300 that you are using, that limits the integratingaction as soon as the controller output saturates. This will prevent windup of the control effort, which will reduce (hopefully remove)overshoot and restore control sooner.

Integral or reset action is covered again in the next assignment, so do not worry if you have not completely grasped the subject yet. Thethree practicals in this assignment will introduce PI and PID (Proportional plus Integral plus Derivative control), and the next assignment willdevelop these ideas. The theory behind each type of control is covered again, and PI/PID control are applied to a process whose measuredvalue is flow.

Derivative Action and Proportional plus Integral (PID) control

Proportional control on its own reacts immediately to any deviation but it is insensitive to the rate of change of deviation. By adding anintegral action, the control law now removes long term errors (offset). But if the error was increasing very rapidly a very large control effortwould be desired (much larger than simple direct proportionality can provide) to halt this. PID control adds a derivative term which isproportional to the rate of change of error.

Considering the control effort in a similar way to the theory on Integral action, it can be split up into the following terms;

Uc = Up + Ur +Ud

where Up is the proportional term, Ur is the reset term, and Ud is the derivative term.

The proportional and integral terms have already been met;

The derivative term is described by the following expression;

Ud = K Td (de / dt)

where Td is the derivative time.

This derivative time, Td, is very similar to the reset time, Tr, of the integral term. It controls the contribution of the derivative term to theoverall control effort.

The control effort produced by a PID controller is as follows;

The time constant of the derivative term appears in the numerator, but the time constant of the integrating term appears in the denominator.This means that a derivative time of zero will remove any derivative action, but an infinite reset time is needed to remove all reset action.

As briefly mentioned, the derivative contribution is directly proportional to the rate of change of deviation between measured value and setpoint. As a result, the derivative term will be positive whilst the deviation is increasing, and negative whilst the deviation is decreasing.

Considering the action of the controller, while the error is increasing, the derivative term will increase the control effort, with the size of theincrease determined by the rate of change of the error. When the error is decreasing, the derivative term will reduce the control effort as therate of change of the error decreases. Coupled with the proportional action, this produces a braking effect as the measured value approachesthe set point.

The overall effect of the derivative term is to increase the speed of response, to improve damping of oscillation, and to reduce the size of theovershoot.

Derivative action will play no part in removing the offset present in proportional control. This offset is a steady state error, it has no rate ofchange since it is not time dependent, and the derivative of this will be zero.

Unfortunately derivative action cannot be applied to every control situation, as it is not suitable for systems with noisy environments. Noisysignals contain high frequency components, which are amplified by the derivative action. These amplified high frequency components willappear at the controller output, and will cause large changes in the position of the actuator.

While these may not affect the plant to a large extent (since plant dynamics will usually act as a filter to high frequencies), the rapid changeswill almost certainly shorten the life of the actuator. The high frequencies may also cause fluctuations in the power supply.

Also, it must be understood that derivative action is most successfully employed in systems with fast changing variables. The reaction speedof the level and flow variables in this system are not sufficiently fast to show-off the potential of derivative action.

This fact will be demonstrated later during the controller self-tune. The controller decides upon the most appropriate levels of proportionalband, integral and derivative to control the process connected to it. The level of derivative action required will be seen to be extremely small.

The servo valve does not allow the flow rate to change fast enough to require much of the characteristic braking action of the derivativecomponent.

However, there are certain situations where the derivative term is of great use. A servo motor is designed to respond to input signals relatingto speed to direction almost immediately. A square-wave input to a servo motor exercises the reactions of the motor to the full.

If a servo motor were being driven by a square wave of large amplitude the response of the motor would be similar to the graph below. Itcan be seen that there is a large degree of overshoot at the direction changes.

The second graph below illustrates the difference the addition of derivative action could make to the response of such a servo system. Thereis still a certain degree of overshoot, but the braking effect reduces this to a minimum.

It can be seen that the derivative action is reducing the response overshoot considerably.

Proportional plus Integral plus Derivative (PID) Control

The control effort of a PID controller is made up of three terms. Each term plays its own specific part in controlling a process, and eachrequires careful thought to operate as well as possible. All three terms have been introduced; proportional control in the previous twoassignments, integral and derivative in this assignment. During the next three practicals, and also the following assignment, the application ofthese control methods to real processes will be explored.

In this theory section, the aim is to consider three different types of error. For each type, all three control terms will be considered, todetermine what effect each is having on the control effort produced to remove that error.

To simplify the following examples, several assumptions will be made; both PB and reset time are sufficiently high to avoid oscillation, and thederivative time is sufficiently low to avoid oscillation.

The diagram below shows the cases of error that will be considered;

The curves show the pattern of each type of error, how each would continue if it could do so uncorrected. This would not strictly happensince any controller would take some course of action to remove the error, whether this is the correct action to be taken or not.

Case a), a constant error (offset): The proportional term would not be able to remove this error since it relies on the existence of error toproduce any control effort.

The reset (integral) action would be present, reducing the error. As the error reduced, so too would the reset action.

The derivative action would only be present when the reset action had started to reduce the error (since derivative is the rate of change oferror). This would reduce the control effort, braking the approach of the measured value to the setpoint and smoothing any overshoot thatmay occur.

Case b), an increasing error (gradient A): The proportional term would be attempting to remove this, but its action would only be dependenton the size of the error, not the rate of change.

For an increasing error, the reset action would be increasing exponentially, but since the other terms will be decreasing the error this will soonbecome an increase whose gradient was decreasing.

The derivative action will be the important term since it is proportional to the rate of change of the error. The combination of these terms willforce the control effort to slow down the error, until it finally changes direction, and the measured value approaches the set point. The closerit gets to the set point, the slower it will approach, since the proportional action is decreasing, reducing the control effort and so the rate ofchange of error, and so the derivative action.

Case c), an increasing error (gradient B, B >> A): This is a very similar case to the previous, but since the rate of change of error is greater,the derivative action will be much greater, increasing the control effort, and correcting the process much quicker than if there were noderivative action.

PI Control of Level

The practical will begin with reset action turned off. This will leave the process in the proportional control situation, which you have metbefore. By doing this you can familiarise yourself with proportional control again before adding the reset action.

As mentioned in the theory section, reset action approaches zero as the reset time increases towards infinity. In terms of the 38-300controller, when Tr is set to 7201 seconds the reset action is OFF. When the practical begins, your computer will set this. There will be acontrol bar onscreen to change Tr and next to this there will be an ON/OFF button. By clicking on the button the control bar will be enabledand you can introduce a reset action.

When enabled, the range of Tr will be 0 to 100 seconds, this will be sufficient to explore reset action. The full range of Tr, from 0 to 7200seconds, is of course available through the manual keys on the front of the controller.

As for several of the practicals in previous assignments, only part of the full range will be displayed by the chart recorder (0 - 50% rather than0 - 100%), so that a 'close up' of that area can be observed. This does mean that the range of the set point is effectively restricted, but if thefull range were to be shown, the level of offset could not be appreciated (this is the important difference between P and PI control).

You will be able to vary the following with onscreen control bars; the set point, the proportional band (PB), and the reset or integral time (Tr).You should already have a good understanding of PB from your previous work, but now you will be able to investigate the effect of varying Tr,and also how the two parameters tie in.

In terms of its dynamic characteristics, the level control system is slow. When changing parameters to observe their effect, you must allowplenty of time for behaviour to become apparent. It may take one or two minutes for the process to reach steady state, so you must allow forthis before considering any offsets etc.

Limitations of PI Control

This practical will be used to demonstrate the shortfalls of Proportional plus Integral Control. Unlike the previous PI Control of Level practical,the 38-300 will be initialised to begin PI control immediately, and by changing the available parameters, the drawbacks will be encountered.

As explained in the theory, the integral or reset action is used to remove constant errors or offsets. The exact contribution from the resetterm is controlled by the reset time parameter, Tr. If this is set incorrectly the control effort including the reset action can be too large, andoscillation will result. This was observed in the previous practical.

This practical will investigate the rate of change of error, and response of the system to that error. It is this lack of reaction to the rate ofchange of error by a PI controller that has produced the need for a further control method; PID control.

You will force the error to change very quickly, and observe the effect on the control effort and the response of the system, comparing it tothe response seen in the first practical. While the error is changing relatively slowly (usually, if one of the dynamics of a system is slow, thenall are slow), PI control is adequate at maintaining predefined operating conditions. With a fast rate of change of error, the controller isunable to take extra action to account for this.

Using the record and playback functions will enable you to very easily compare traces from different control algorithms, during this practicaland also between this and other practicals.

You should also attempt to produce instability/oscillation, so the level at which it occurs can be found. In an industrial application this shouldbe well known, so that it can be avoided.

When this practical begins, PB will be set to 50, which is higher than in previous practicals. This enables the effect of the integral action to beseen more clearly as Tr will have a greater influence on response. With a large PB, system gain is smaller and so the level of offset from theproportional action is large. To remove this offset, the integral action must be large, and so Tr will be small (since Tr and amount of integralcomponent are inversely proportional).

Because of this, the control bar for Tr will only go between 1% and 60%. You will of course be able to use values outside this range with the38-300's manual keys.

PID Control of Level

This practical will demonstrate to you full three term, PID control as applied to the control of fluid level in the upper tank. You will be able tovary Proportional Band (changing the gain which changes the contribution of the proportional action term), Integral Action Time (or ResetTime, determining the contribution of the reset action term), and Derivative Action Time (determining the contribution of the derivative actionterm).

This is your first introduction to full PID control, and it is a straightforward example of this type of control. In the following assignment thisalgorithm will be applied to a different process, allowing the ideas met here to be built upon and expanded.

The derivative component of the control effort enables a controller to recognise a rapidly changing error and take extra action to account forit. By applying a control effort that is not simply directly proportional to the error, the response of the plant has been improved. There is nowan element of the control action that is proportional to the rate of change of error.

This new element is a very important one in some situations. A massive increase of the inflow to a tank, caused by a failure elsewhere in aplant for example, could result in overflow. By applying a very large control effort, the time taken to reverse the direction of the system(towards failure) has been reduced. It is producing an overcompensation for the extra error to halt its progress.

But it is not only overcompensation that a derivative action offers to a system. As the measured value of a system approaches its set point,the rate of change of error will decrease as the proportional action reduces. This reducing error rate will produce a negative controlcontribution from the derivative term, reducing the control effort further. This applies a breaking effect to the control effort, and reduces thechance of overshoot.

The derivative action will pull a system away from failure by producing an overly large control effort, and slow down its approach to the setpoint with the aim of preventing overshoot.

The Reset Action of PI ControlThis theory section carries on from the previous assignment, and considers the reset action and control effort produced by theoreticaldeviation curves graphically. It will expand on your knowledge of the reset action, and enable you to have some idea of control effort forvarious forms of deviation produced in practical situations. PID control will also be considered in the same manner by the following theorysection.

Graphical representations of each element of a control effort will increase your practical understanding of the different forms of control to amuch greater extent than purely theoretical and mathematical discussion. It may also eventually enable you to make intuitive suggestions tothe shape of each element of a practical control example very quickly.

It is hoped that the link between control law block diagrams and physical response characteristics will become clearer as the practicalsprogress.

The following diagram shows the control law of a PI controller. This is the same as the one seen in the theory section on PI control in theprevious assignment.

It also shows the response to a theoretical step input deviation. The reset time constant, Tr, is the variable that is controlling the contributionof the integral action to the control effort over a given length of time.

If an integrator is given a step input of fixed duration, its response is a ramp. The slope of the ramp is controlled by Tr; the smaller Tr is, thesteeper the ramp. With a steep ramp, the contribution of the integral term will be large in a given time, and the time taken to reduce theerror present will be short.

The diagram above shows the responses for two different Tr values.

Here Tr1 > Tr2.

The diagram shows that before time t i deviation is zero, and the corresponding control effort, U, is at the quiescent point, Um (this is usually amanual control effort term). At time t i the deviation becomes a positive value, e, and the control effort increases to Ke, which is a result ofthe proportional control term (Up as introduced previously).

The control effort is then a rising slope of gradient K / Tr, due to the reset action term, Ur. The integral of a curve is a measure of the areaunder that curve, so the area under a step input is increasing at a constant rate. With a small value of Tr the slope is steep, and so thecontribution of the reset action to the control effort is large.

At time t i+j the deviation, e, becomes zero again, and this removes the proportional term from the control effort (since Up = Ke), and so thecontrol effort drops by an amount Ke. The integral term has become a constant, since the area under the curve is no longer increasing.

By considering the terms present during each time period, the following expressions describing control effort can be produced;

Before time t i, control effort: U = Um

At the instant of t i: U = Um + Up

From t i to t i+j: U = Um + Up + Ur

After t i+j: U = Um + Ur

It must be remembered that the diagram only shows the deviation, and the corresponding control effort. It is, in effect, an open loop system,since the effect of the control effort on the deviation present is not being considered. In a real application, the deviation would not be stepinput since the control effort that it produced would change it (hopefully reducing it).

The Three Components of PID Control

In a similar way to the previous theory section on the Reset Action, PID will be considered graphically, splitting up a control effort into itsconstituent parts. By breaking down a control effort into the terms that it contains, it becomes easier to understand how the final controleffort is created.

The diagram below shows the control law of a process carrying out PID control.

The enhancement and braking effects of the derivative action on the control effort are clearly visible.

The level and flow variables of interest to the following assignments do not provide a perfect example of the uses of derivative action. Thederivative method is suited best to processes whose variables change swiftly and frequently. It is in such processes that the braking effect ofthe derivative component can be observed.

As before, the control law cannot be built in isolation as it is shown in the diagram above. It must always be an element of a completefeedback loop since its input is the deviation between the measured value and set point of a system. By producing a control effort it willchange the profile of the deviation providing the input, but this is not being considered.

The control law shows the three terms of PID control, including the expressions describing the behaviour of each. The upper curve is afictional deviation profile which has two obvious regions, the first increasing with a constant rate, and the second decreasing with a constantrate. The lower curve is the corresponding control effort produced by the deviation.

The diagram below shows the three terms of the control effort, with the deviation curve at the top.

From this you can see the two definite regions of deviation, and the corresponding regions of each control component.

The important point that can be made from the control effort profile (which is the combination of all three components) is the enhancementand braking effects caused by the derivative action.

While the error is increasing, the derivative component has greatly increased the control effort, beyond mere proportionality, to halt theprogress of the deviation as quickly as possible. While the deviation is decreasing (measured value is approaching the set point), thederivative action reduces the control effort so that the rate of approach of measured value decreases, and the possibility ofovershoot/oscillation is less.

This is seen in the control effort by the 'spike' as deviation increases, and the downward drop as deviation decreases.

P + I Control of Flow

In this practical you will attempt to control flow with the Proportional plus Integral algorithm, which you have already met in the PI Control ofLevel practical within the previous assignment. It will not be as straight forward in this practical to avoid oscillation, because level and flowhave very different behaviour.

As discussed previously, the dynamic characteristics of flow are much quicker than those for level, and the control parameters must be set upaccordingly. Recalling the control law for proportional plus integral control from your previous work;

Uc = Up + Ur = K [ e + (1 / Tr) e]

...you will know that system gain K is controlling the size of the control effort applied, and that a large gain value will produce extreme controlefforts, resulting in oscillation if the error or deviation is swinging.

This is to some extent a 'runaway' situation, a small error will produce a control effort to drive the process in the opposite direction, but with alarge gain there will invariably be overshoot causing opposite deviation, producing a larger control effort and driving the process back again.This is the oscillatory behaviour that can easily occur.

System gain is inversely proportional to PB (Proportional Band), so when controlling flow (a process variable whose deviation is quick andoften alternating) PB must be large (and so gain will be small) to prevent extreme control action and so oscillation.

In practical systems there are other factors which need to be considered, such as noise. In reality no transducer is perfect, which is also trueof the flow sensor transducer. Any noise supplied by the transducer is passed on to the controller. If the noise is considerable and thecontroller has a small PB (large gain) oscillation would be the result.

From this practical you can observe, on the chart recorder, the noise level on the signal, therefore to account for this the PB has to be slightlylarger (smaller gain) than if the signal from the transducer was totally clean.

This practical will emphasise the above case; PB will initially be 100, large compared to previous practicals, and you will vary the reset time,Tr, to reduce the offset present. As you should recall, offset will be very large if PB is very large, so initially when the integral action is notswitched on the deviation between measured variable and set point will be excessive.

As the reset component is introduced, the offset will decrease at a rate controlled by the reset time value, Tr. If Tr is chosen to be too low, theintegral component is too great and oscillation will be induced, but this 'too low' figure is also determined by PB. The aim of this practical is tofurther your knowledge of integral action by changing all relevant parameters and observing their effect.

There will be control bars onscreen to change the set point and Tr, with the range for Tr 0-60 seconds. There is no need to apply integralaction with Tr any greater than 60 seconds because its effect will be negligible with PB so high. Although there is not a control bar onscreenfor PB, you are still able to change its value using the manual keys on the front of the 38-300 (PB is found on the 'ContrL PAGE').

P + I + D Control of Flow

During this practical, you will attempt to control flow using full three term PID control, as introduced in the previous assignment. Unlike the PIControl of Level practical, this will be an example of the problems associated with PID control, and will aim to show you how the choice ofcontrol method is very much situation-dependent.

You will know from earlier work how the dynamics of level and of flow are very different to each other, and so the same control systemcannot be applied to both without parameter alterations. A small change in the control effort will alter the position of the servo valve onlyslightly, but this will affect flow rate quite considerably. It can be said that the process plant has a very high gain; a small change to the input(control effort to servo position) produces a large change in the output (flow rate).

If the deviation between measured value and set point is changing rapidly, this will produce a large derivative action in the control effort. Thismakes the system very susceptible to instability, and so great care must be taken when selecting parameter values. The expression below isthat met previously for PID control law.

Uc = Up + Ur + Ud

= k [ e + ( 1 / Tr) e dt + Td (de / dt) ]

The greater PB is (so the smaller the system gain, K), the smaller the overall control effort will be for a measured deviation. This allows for agreater range of derivative contribution to the control effort before it produces oscillation.

In this practical the PID values have been set such that the best performance is achieved out of the system. You can observe the systemperformance with respect to set point changes, noting the system response on the chart recorder.

In the second part of the practical you can investigate the effect of the derivative action on the overall system. You will be able to change Td

from 0 to 5 seconds, in 1 second divisions.

Although this sounds very small, it will be enough of a variation to observe all consequences of derivative action in the control effort, bothgood and bad. You will be able to switch derivative action on and off, as you did with integral action earlier.

By providing the same deviation (changing the set point is the most convenient way of doing this) to PI and PID algorithms, you will be ableto observe the differences between them.

There will be control bars onscreen to change the set point, Tr and Td. You will not need to apply Td greater than 5 seconds as there willalready be instability at this level. Although there is not a control bar onscreen for PB again, you are still able to change it using the manualkeys on the front of the 38-300.

Tuning Process ControllersTuning process controllers is a procedure to select control effort parameters that will produce a desired system performance. The idealsolution is simulation as this will produce values before the system is operational, so that from start up it will be running with tunedparameters. Unfortunately this is not always possible, so practical methods must be available.

The desired performance of a system is entirely dependent on that system, but there are criteria for system performance that can be appliedto all systems. For example, the aim of tuning could be to reduce the overshoot of the process variable to a minimum whilst maintaining afast response

A common desired performance criteria for tuning controllers, is that the tuned settings produce 'a transient response with a decay ratio of1/4'. The transient response of a system is its reaction to a change in input over a period of time, normally the time between two steadystates. This is shown in the diagram below.

The decay ratio is the ratio between the size of successive overshoots, and this is also shown in the diagram. The decay ratio is:

OS3 / OS1 = 1/4 (for a tuned system)

During this assignment, three methods of tuning will be considered; Continuous Cycling, Reaction Curve, and Self Tuning carried out bythe controller automatically. The first two methods will be discussed theoretically and Continuous Cycling and Self Tuning will be implementedpractically. As mentioned before it is not necessary to understand the theory before attempting the practicals.

The Continuous Cycling Method of Tuning

The first method of tuning that will be discussed, and one which is widely used in industry, is the Continuous Cycling method or Zeigler-Nichols tuning. This method is based on an article published by Zeigler and Nichols in 1942 establishing a set of empirical rules for tuningcontrollers.

The aim of continuous cycling is to experimentally find the value of gain (or the value of PB to be precise) which produces marginal stability.Stability and Damping are two important characteristics of a system and they are shown in the following diagram:

Curve A shows a system whose controller gain is such that its oscillations decay. This system is over-damped and so it is stable - it will reacha steady state. Curve B shows a system whose oscillations are sustained, neither growing nor decaying, and this is considered marginallystable. Its gain is such that it is critically damped. Curve C shows a system that is unstable. Its oscillations are increasing and the gain is suchthat it is under-damped. The system gains, Ki, are as follows;

Ka < Kb < Kc

The aim is to find the value of proportional band that produces marginal stability, and the practical method is as follows;

1. Place the controller in Manual control mode, and achieve steady state with the system in its normal operating condition. Themeasured value will be equal to the set point.

2. Remove all control actions except proportional. As you should recall, reset action is removed when Tr = maximum (7201 in thiscase), and derivative action is removed when Td = 0.

3. Select a wide proportional band, so that controller gain is small.4. Switch the controller into Automatic control, this should produce no control effort (this is a bumpless transfer). Introduce a small set

point change in the region of 5%-10%.5. Observe the response of the system to this change.6. Switch the controller back to Manual operation, and restore the system back to the original stable operating condition, with

measured value equal to the set point.7. Reduce the proportional band and repeat steps 4, 5, 6 and 7 until the system exhibits sustained oscillations. At this point the

system is marginally stable.

At the point of marginal stability, the value of proportional band is recorded, this is the ultimate proportional band, PBu. Also the period ofoscillation is recorded, this is the ultimate period, Tu.

Continuous Cycling Zeigler-Nicholls Tuning

ControlAlgorithm

ControllerPB

Tr Td

P 2PBu - -

PI 2.2PBuTu/1.2 -

PID 1.7PBuTu/2 T

u/8

Using PBu, Tu and the above table of Zeigler-Nichols recommended settings, the control action contributions are calculated for each type ofcontrol algorithm.

The Reaction Curve Tuning MethodThis is derived from the same source as the Continuous Cycle Tuning method but it uses an open-loop test to determine the tunedparameters. It is more suited to slow-reacting processes, where oscillation gives rise to uncontrollable operation.

Reaction Curve tuning is very simple to carry out, and the basic method is described below:

1. With the controller in Manual mode, place the process system in a stable operating condition with the measured variable and setpoint equal.

2. Apply a small step change to the controller output, and record the response of the system.

This is called the Reaction Curve method because it uses the response, or reaction, curve of the system to determine the tuned parametervalues. An example reaction curve is shown below. This is a very typical response curve, 'S' shaped and showing the delays that occur with adynamically slow process.

The first step is to find the maximum slope of the reaction curve, N, and to draw the tangent of this point. Next the effective delay, D, isfound. This is the delay between applying the step change and the point at which the tangent of slope N crosses the line of initial controlleroutput value. Both of these can be seen on the diagram.

The fractional change in controller output is calculated, and using effective delay, D, maximum slope, N, and the table of Zeigler-Nicholsrecommended values the tuned parameters are calculated.

ControlAlgorithm

ControllerPB Tr Td

P - -

PI -

PID

u - fractional o/p change

N - maximum slope of response

D - effective delay

For both of the Zeigler-Nichols methods discussed, there are expressions for calculating all control parameters for all three control algorithms,P, PI, and PID. By considering the differences between these algorithms it becomes clearer how the level of constants in the expressions aredetermined.

Changing from P to PI, the controller gain must be reduced (PB increased) because the reset action is increasing the risk of oscillation of thesystem. Changing from PI to PID, the controller gain can be increased because the derivative action is opposing changes in the measuredvariable, reducing the chance of oscillation.

Zeigler-Nichols Tuning

This is a widely used method of controller tuning throughout industry, and it is reliable. You shall apply this tuning algorithm to the 38-300 todetermine parameter values whose decay ratio is 1/4. This method uses results from a closed-loop test to calculate the parameters.

The method has been explained thoroughly in the theory section, but the practical steps are as follows;

1. Using manual control, settle the plant into its normal operating condition.2. Remove all actions except proportional, and select a wide proportional band.3. Switch to auto and introduce a small set point change (5%-10%).4. Observe the response.5. Switch the controller back to manual mode, restore set point back to its original setting and operating conditions back to normal.6. Reduce the proportional band and repeat steps 3, 4, and 5.

When the plant exhibits sustained oscillations, neither growing nor decaying, record the value of PB, and also the period of oscillation of theplant. These are called the ultimate proportional band, PBu, and ultimate period, Tu. From these can be calculated all parameter values so thatthe controller can be tuned, using the following table;

Continuous Cycling Zeigler-Nicholls Tuning

ControlAlgorithm

ControllerPB Tr Td

P 2PBu - -

PI 2.2PBuTu/1.2 -

PID 1.7PBuTu/2 T

u/8

Using the two ultimate parameters, all control action components can be calculated and the controller tuned accordingly.

Reaction Curve Tuning

This method of tuning is also derived from the same source as the Continuous Cycling Tuning method but this uses an open-loop test tocalculate the parameter values. You shall apply this tuning algorithm to the 38-300 to determine parameter values whose decay ratio is 1/4.

As before in Practical 1, this method has already been explained thoroughly in the Theory Section, but the practical steps are as follows;

1. Using manual control, settle the plant into its normal operating condition (adjust the 38-300 output so that there is no deviation).2. Apply a small step change to the controller output and record the response.

The open loop response of the plant is called the Reaction Curve, and it is the response to a step change in the controller output. From it canbe found the maximum slope and delay needed to calculate the tuned control action parameters. The exact quantities to be measured weredescribed in the theory corresponding to this method, and you may need to refer back when you calculate parameters in the questions forthis practical.

Before recording the reaction curve, carry out the steps above several times to practice. Once you are happy with the 'S' shaped curveproduced, record the chart so that you can examine it at your leisure with the playback facility, and take all the measurements necessary.

Self Tuning The 38-300 Controller

The 38-300 controller contains a facility to automatically tune the control action parameters for all three control algorithms, with two types ofconstraints on the tuned parameters. You shall set up the controller manually to carry out self tuning, and then observe the response of thesystem using the tuned parameters with the chart recorder.

The Self Tune page contains the following parameters:

St-tYP : self tune type, either at start-up (StrtUP self tune will be applied from the initial start up or when there is a large changein set point) or at set point (AtSPt self tune will begin when the process is close to the set point).

StEP : Output step size, is a percentage of the control output range, and is the amount the output will change by while self tuning.

HYSt : hysteresis value, used by the At Set Point cycle, determining when to change the output value.

St-HI : self tune high limit, used by the At Set Point cycle, self tuning will stop if the measured (process) variable exceeds this.

St-LO : self tune low limit, used by the At Set Point cycle, self tuning will stop if the measured (process) variable drops below this.

tEr_S : control terms, P, PI, or PID.

tYPE : type of control, used by At Start-Up cycle, type A is the quickest response with a damping ratio of 1/4, type B is the quickestresponse with the minimum overshoot.

S-tunE : on or off, while the controller is self tuning, the L.E.D. marked ST will be on, when it is complete this L.E.D. will flash. Ifself tuning fails the L.E.D. will go out and an error message will be displayed at the start of the parameter page.

When self tuning is complete it will have produced parameters for the type of control action selected. These can be seen after S-tunE as thefollowing parameters: AdU.P, AdU.I, AdU.d; advisory PB, advisory integral action time and advisory derivative action time respectively. Thenext parameter will offer the chance to accept these values, if accepted they will change the ContrL PAGE accordingly.

The advisory values should be used to control the process. The performance of the tuned P,I and D terms should be noted. If experimentaltime allows, you should alter the characteristics of the process in some way and then repeat the self-tune procedure. Perhaps remove theorange bung to increase the tank capacity, or increase the rate of outflow from the tank. Observe how the advisory P, I and D values alterwith a change in the process characteristics.

Profile Programming

A profile is a curve over a period of time, which is normally rising, falling, changing. Up until now the profile of your set point has been onlysteps, as you change it with the onscreen control bars. The controller contains a facility for producing profiles containing ramps, which allowsyou to create many different types of profiles, including saw tooth wave forms. The ability of the process plant to follow a changing set pointcurve can now be considered.

The controller enables you to enter a program to produce the profile of your choice, by providing nine groups of up to 31 segments, eachgroup being an empty program. You can decide the shape of each program by defining the number of segments, the starting level of eachsegment and the length of time each segment lasts. You can then select how many times to repeat each program, and select up to fourprograms to be replayed in any order.

The profile programming facility also has a Hold feature which will pause the program when one of three conditions are met;

n-HLd manual hold - switching the controller to manual mode will hold the program,

OPErAt operator - pressing 'Enter' or a logic input signal 1 (if assigned on the setup control page) will hold the program,

H-bACK hold back - the program will hold if the process variable deviates by more than the hysteresis value assigned in the ProfileProgramming page and will resume when it returns within the hysteresis limit.

Programming a Profile

There are two pages that must be used when programming, and a third which monitors the program when it is running. The first page is theProfile Programming Page ('PrOFLE PrOGr_'), and its parameters are as follows; 'PrOFLE' (profile enable) - this enables the profileprogramming feature, and should be ON.

PrOGr_ (program select) - the program number that is to be programmed (1-9)

PG-bEG (program begin) - the starting segment (0-29)

PG-End (program end) - the finishing segment (1-30)

LEU.L x (segment x start level) - this is the set point level that the first segment is to start at, x is the number of the startingsegment (any value, 0% to 100%)

the following two parameters are set for each segment of the program (for example 'PG-bEG' = 1 and 'PG-End' = 10, so this will cyclethrough until level and time for each segment has been set).

tI_E x (segment x time period) - the length of tie segment x will last for (any value, 0.0 to 999.9 secs)

LEU.L xx (segment xx start level) - this is the set point level that the current segment (x) will finish at and the next segment (xx)will begin at. When all segments are programmed, the parameter page will continue.

rEPEAt (program repeat) - The number of times this program is repeated (1-99 times)

HYSt (program hysteresis) - If the process variable deviates beyond this value the program will hold until it returns within this limit(any value, 0%-100%).

Once the Profile Programming page is set up, the 'PrOFLE StAtES', Profile States page is programmed, as follows;

PrOGr_ (program select) - here the program numbers to be run are selected. Up to four programs can be run in one session, inany order. In the Lower display of the controller, the program number is selected and entered. If less than four programs are to beplayed, the terminator character is entered (this is three horizontal lines).

t_-dLY (time delay) - the countdown delay before the profile begins (any value, 0.0 to 999.9 mins).

StArt (start profile run) - this initiates the profile.

Once the profile has begun, the hold parameter is available, showing the state of the hold. If there is no hold, this will be OFF. There is alsothe option to reset the current profile and also skip the current segment. These are both available while the profile is running.

Also while the profile is running there is a Profile Operating Page, 'P-StAt'. This contains the following parameters;

P-StAt (profile status) - shows the current status of the profile. This can be any of the following;

StOP - waiting for operator action,

SOAK - the set point is constant through the current segment,

rA_P - (ramp) the set point is rising or falling though the current segment,

C-dO - (countdown) there is a time delay before beginning,

Hb-HLd, OP-HLd, _n-HLd - one of the hold features has been triggered,

End - the current profile has finished.

t-Strt (countdown time) - the time delay before starting, if one was set in the Profile States Page.

PrG - SEG - - current program and segment.

t-SEG - the time remaining before the current segment finishes.

rPt-Ct - program repeat count is the number of outstanding repeats for the current program.

Remote Set Point

This practical will introduce the Remote Set Point facility of the controller by allowing you to vary the process set point using the PI currentsource.

The controller will be self tuned so that it is applying what it considers to be the best control action possible, and it will follow the set point asbest it can.

Using the current source as the set point is not really the normal practical use of the remote set point, but it is enough to demonstrate its use.

The remote set point is used when two processes are not connected but the operating point of one is dependent on the operating conditionsof the other. There is an example of this in the following diagram:

There are two unconnected flows, the primary and the secondary. The secondary is controlled by the position of a servo valve which isdetermined by the controller. The flow through the primary is monitored with a flow sensor.

The primary flow is used to remove a quantity of heat from the secondary flow. The signal from the primary flow sensor is providing theRemote Set Point (RSP) for the secondary. MV is Measured Variable and OP is the process controller Output.

This arrangement allows for the following; if the flow in the primary drops, the set point for the secondary will drop and the controller willapply a control action to drop the secondary flow, ensuring that the same quantity of heat is always removed for any value of primary flow.

Profile Programming

During this practical you will set up a profile for the set point to follow, made up of programs and segments, and will initiate the profile toobserve how the system follows a changing set point.

You will self tune the controller so that it can follow the set point as well as can be expected, and then manually enter a profile for thecontroller to follow. The profile programming facility offers great scope for different profiles, and it will be up to you to decide the exact shapeof your profile. The theory section on Profile Programming discusses the construction of a profile, and also the exact steps to program thecontroller.

It is hoped that you will not treat this practical as a rigid set of instructions to be followed once and once only, but more as a starting point forexploration. The exact shape of a profile is limitless and coupled to this are the three different control algorithms, so there is plenty of roomfor experimentation.

Time Proportioned Output

Time Proportioned Output is a form of On/Off control that switches state at a rate controlled by the deviation between set point andmeasured value. The On/Off control met earlier switched state as the sign of the error changed, but Time Proportioned Output produces asquare wave shaped output where the ON and OFF times (high and low) are determined by the size of deviation and not sign.

The following diagram illustrates Time Proportioned Output control. The waveforms are imaginary but they demonstrate the differencesbetween conventional On/Off control and Time Proportioned Output control.

If the error were zero the control effort for Timed Proportioned Output would be 50%, and the output would be ON and OFF for equal lengthsof time. As the error increases above the set point, the control effort increases and the OFF time becomes greater than the ON time, until at100% control effort the ON time is zero. As the error increases below the set point, the control effort decreases and the ON time becomesgreater than the OFF time, until at 0% control effort the OFF time is zero. This is all shown graphically in the following diagram.

Time Proportioned Output has one major benefit over conventional On/Off control which makes it more attractive. When the deviation is zeroTime Proportioned Output will produce a control effort of 50%, which will switch the logic output between high and low equally, it will be aperfect square wave. This should keep the process variable from changing. With deviation at zero for On/Off control the control effort will beeither 0% or 100%, which will drive the process away from the set point, and when far enough switch state driving it back.

On/Off control can only produce oscillation at best (small with high frequency or large with low frequency), but a tuned Time ProportionedOutput control system should get much closer to a steady process. The disadvantage of Time Proportioned Output is its increased complexity.

Plant and Control Loop Characteristics

For the engineer concerned only with process control it is perhaps unfortunate that automatic controllers have, to a very large extent becomestandardised and usually are housed in attractive cases.

This coupled with the fact that controllers are often housed in a clean, well lit room, could give the impression that the automatic controller isthe most important element in the loop.

This impression is quite false. It is the plant that is the most complex part and requires the most detailed study.

In process control systems there is likely to be a longer delay in the detection of error than in position or speed control systems. The responseof the plant to changes in the controller output is also likely to be slower.

On the other hand, most plant processes possess the property of inherent regulation, which can loosely be defined as: 'the property of aprocess by which, in the absence of control, equilibrium is reached after a disturbance'. This in general terms can be stated as: the greaterthe inherent regulation of a process, the more easily automatic control can be applied to it.

Potential Temperature

The Potential Temperature is the equilibrium or limiting value of the controlled condition that tends to be attained following a particularadjustment of the controlling unit. Without inherent regulation the attainment of such an equilibrium value would be impossible.

In order to illustrate this, let us take the example of a temperature control system applied to a gas fired continuous annealing furnace. Theheat input over a given period may be calculated from the calorific value of the gas and the rate of gas flow.

The heat output consists of that taken from the furnace in the form of a heated product plus heat losses in the form of conduction, convectionand radiation. All heat losses increase as the furnace temperature increases, that due to radiation depending on the absolute temperature.

Therefore, providing the flow of material through the furnace remains constant, when gas flow increases or decreases, the furnacetemperature must increase or decrease, until that potential temperature is reached at which the heat balance is restored. It would beinteresting to consider, what would happen if heat losses were independent of temperature!

Process Reaction Rate (Capacitance and Resistance)

Process reaction rate is the maximum rate of change of the controlled condition following a specific step change at the controlling unit.

For a temperature control system, the reaction rate is determined from the region of maximum slope of the temperature/time graph followinga sudden change in heat energy input and is expressed in degrees per second.

In whatever form it takes, for any process, capacity has the effect of delaying the attainment of the potential value. In a simple processcontaining just a simple capacity, the reciprocal of the reaction rate gives the capacity lag, e.g. seconds per degree.

Resistance is a property of an electric circuit, which obviously has the relationship:

I = V / R

Also (not as obvious) conductance is the reciprocal of resistance.

In the field of heat transfer, the term thermal conductivity is widely used. The flow of heat from one side of a material to the other requires atemperature difference between the two sides, just as a potential difference is required across the ends of an electrical conductor to promotecurrent flow.

In the process control field the term resistance is used to denote opposition to flow. It is measured as the potential change required toproduce unit change in flow and its units are oC J/s.

Transfer Lag

A transfer lag occurs when energy is transferred through a resistance to or from a capacity. See the following diagram for the effect of adistance velocity lag :

In a temperature control system transfer lag results from the resistance to heat transfer from the heating element to the process and thethermal capacity of the process material and its container etc. A further example of this type of lag is provided by the compressed air systemshown below:

If P2 is initially zero air would enter the receiver at a high rate, but as pressure builds up the rate of air flow is reduced, this has the sameeffect as a simple RC circuit.

The following diagram shows the time delay introduced by transfer lag.

The change in amplitude due to the effect of resistance and capacity can be shown using a sine wave input, see the following diagram. Thetwo effects of a transfer lag on a sinusoidal input are, a phase lag and a reduction in amplitude.

Time lags in the Control Loop

To give a complete picture of the lags in a typical control process, consider the following diagram :

Within the plant itself the following can be said :

1. There is a distance velocity lag on the supply side. This is the time taken for the heating fluid to travel from the valve to the supplyinput of the heat exchanger.

2. A second lag, similar to the previous one is present on the demand side. This is the time taken for the process fluid to travel fromthe demand side outlet to the temperature transducer.

3. Due to supply capacity, time is required following a change in valve position for the supply side of the heat exchanger to heat up orcool down.

4. A similar effect occurs on the demand side, this is also due to capacity.5. A transfer lag exists between the supply and demand sides of the heat exchanger.

In addition to the previous time lags mentioned, further lags and delays occur in the loop as follows :

f. Temperature transducer lag. This is in fact a transfer lag and is due to the resistance to heat flow from the process fluid throughthe pocket and the heat transfer compound, in addition the thermistors also possess a thermal capacity. Hence the thermaltransducer's response would take the form of an exponential curve associated with transfer lag.

g. The time taken for the signal to propagate from the transducer, to the controller and the controller to initiate any control action.However in many cases this is negligible.

h. The delay for the corrective action to propagate to the control valve and for the control valve to maintain a possible new position.

It is typical for a plant to have several lags, as shown below in the typical response curve, which can be approximated by a distance velocitylag L followed by a linear reaction curve. However the greater the number of lags that are present, the more S shaped the response becomes.Therefore it becomes more difficult to decide the exact slope of the approximated reaction curve.

In practice the plant being considered by the engineer is almost always infinitely more complex than the example shown previously and inmany cases it is an enormous task to try to identify and calculate the various lags associated with the system.

Calibration of the TTT

The Thermistor Temperature Transmitter is a device which takes temperature information from the thermistors (T1 - T5) and transmits it tothe Process Interface (PI), see photograph below :

The thermistor itself is a small component which has the characteristic of resistance dependant upon temperature. The ThermistorTemperature Transmitter reads the resistance value and converts it to a 4-20mA signal with respect to actual temperature.

By converting to the 4-20mA current signal format, communication is no longer restricted to short distances, a concern when dealing withlarge process plants. Also by using this format signals and equipment become standardised, removing the need for special interfaces.

Instrument Calibration

When using the thermistor and transmitter combination, temperature measurements are carried out to monitor a process parameter. Thisparameter is monitored and used to determine the control effort that should be applied to control the process correctly.

Such measurements help to maintain the correct operation of the process system. Because of this, the accuracy of these measurements mustbe guaranteed by proper calibration of the instruments. The importance of calibration cannot be stressed enough.

Full calibration consists of comparing the measured value or level of a parameter, as shown by the instrument being calibrated, with theknown value or level of the parameter, as measured by a standard instrument of known accuracy.

Calibration guarantees that the accuracy of the input or output reading in a calibrated instrument will be at a certain acceptable level, whenused under the environmental conditions (e.g. temperature, humidity, pressure) present during the calibration process.

Outside those conditions, characteristics may change, and so the accuracy of the instrument will vary to a greater or lesser extent accordingto its susceptibility to the modifying inputs inherent in the new environmental conditions.

Calibration of the Thermistor Temperature Transmitter

In this practical you will be shown how to calibrate the output of the TTT. The TTT has a built in calibration circuit which allows the output tobe adjusted to a known value.

This is achieved by using the two buttons on the front of the TTT. As the labels indicate, the instrument is calibrated for 25oC and 80oC.

Once calibrated using this process, the TTT 4-20mA output signal will directly represent actual temperature, with the desired degree ofaccuracy.

Thermistors

A thermistor is a device whose electrical characteristics alter in a predictable way with a change of temperature. The resistance of athermistor is a function of the temperature around it, or 'ambient' temperature. This behaviour allows the thermistor to be used as anaccurate temperature measuring device.

There are five such devices included with the Temperature Process Rig. They are positioned to measure the temperature at five points aroundthe secondary and primary flows.

In the primary flow they are positioned before (T1) and after (T2) the heat exchanger. This is obviously crucial in observing the cooling effectof the heat transfer.

In the secondary flow they are also positioned before (T3) and after (T4) the heat exchanger. The fifth device is placed at the output (T5) ofthe radiator in order to show the temperature of the flow before and after cooling has taken place.

The thermistors are connected to the Thermistor Temperature Transmitter. This device converts the resistance of the device connected to itinto a 4-20mA signal representing the temperature. The device can be calibrated for 25oC and 80oC by use of the panel mounted buttons.

Taking Measurements with Thermistors

It is possible to construct a thermister based temperature device by adding a milliammeter and a voltage source. The milliammeter can becalibrated for temperature by use of boiling water (100oC) and ice (0oC). The diagram below shows a simple arrangement.

The thermistors used in the temperature process rig are known as 'NTC' thermistors. This abbreviation represents 'negative temperaturecoefficient'. This refers to the behaviour of the device resistance with respect to temperature.

An NTC thermistor has resistance which decreases with an increase in temperature. A PTC (Positive Temperature Coefficient) device behavesin the opposite way.

There are many types of thermistor. Most devices consist of a circular, flat section of resistive material connected to leads. The type in usewith the rig are known as 'beads'. They are mounted in a conductive holder and surrounded with conductive paste for good heat transfer. Thedevice is protected from the liquid flow by a thin metal film.

Thermistors are usually specified by their resistance at 25oC. Common values are 3KOhms, 5KOhms, 10KOhms and 100KOhms. Workingtemperatures range from -80oC to 150oC. Resistance tolerances are usually in the region of +/- 2% across the range.

Choosing Thermistors

A number of parameters exclusive to thermistor use must be considered when choosing a device for a specific application. Some of theseparameters are explained below.

Heat capacity: The amount of heat required to raise the temperature of a thermistor by 1oC.

Current: The maximum steady current to be passed by a thermistor for an extended period of time. Above this value damage mayoccur to the device.

Maximum operating temperature: The maximum temperature at which a thermistor will operate correctly with acceptableperformance.

Stability: The ability of a thermistor to retain its characteristics across the range of its specified environmental conditions.

Switch temperature: The temperature at which the resistance of a PTC device begins to increase very rapidly.

The aim of the practical is to provide experience of the operation of thermistors.

Their response to changes in temperature, their accuracy and their general operating characteristics can be considered.

Once having completed the practical the varied uses of these simple devices should be appreciated.

Bleeding the Secondary Flow

Domestic heating systems often consist of a series of radiators designed to extract energy from hot water being pumped through them. Thesituation sometimes occurs whereby one or more of the radiators is partly filled with air instead of water.

This does not damage the system in any way. It simply means that the system does not function as efficiently as it should. This is for tworeasons. The first is that the air is trapped in the radiator and the water is therefore not being pumped around. The second, and mostimportant reason is that air is not as good a conductor as water. The air does not transfer heat to the metal of the radiator as effectively asthe water.

This can be demonstrated by the time taken for the element of an electric kettle to become too hot in the absence of water. The air aroundthe element does not remove the energy from it fast enough to prevent overheating.

The cooling radiator supplied as part of the Temperature Process Rig can sometimes fall victim to the same problem. Air can be introducedinto the system in a number of ways through pumps and joints. This air can find itself trapped in the upper part of the cooling radiator, whereit will remain until bleeding can be carried out.

Bleeding involves the removal of air from a fluid system by whatever method. The type of domestic system mentioned earlier is usually bledfrom a small 'tap' on the offending radiator. Air is pushed out under the system pressure until water begins to be expelled. The tap is closedand the radiator is free of air.

A similar procedure can be carried out on the temperature rig if air is suspected to be present in the radiator. Air can usually be detected by a'gurgle' being emitted from the device while the water is flowing around the system. If this is the case the practical should be followed toavoid the water in the primary flow becoming too hot too quickly whilst carrying out later work.

It should be noted that this problem is not encountered only as a result of the temperature rig design. In an industrial situation bleedingwould be carried out after periodic draining and cleaning, as well as after prolonged periods of shutdown. Air can easily be introduced intoeven the most well sealed of systems.

Heat Exchanger Design OptionsA major element in the topic of process control is the heat exchanger. These devices can be found in so many configurations that a personwho has been simply introduced to the science of heat exchangers can be quite perplexed in trying to determine which of the almost limitlesstypes available, many apparently satisfying the required heat transfer duty, should be used.

For example, designs which incorporate tubes are only a subset of the many heat exchangers available. However, in spite of only being asubset, there is an organisation that sets standards for tubular heat exchangers (TEMA), actually there are several organisations which dealwith heat exchangers.

An Overview Of Heat Exchangers

Heat exchangers have been the focus of many articles, books and papers. Most of them outline the numerous design analysis techniques thathave been developed. However often the most critical step in the analysis of a heat exchanger is the determination of the overall heattransfer coefficient, U. This in turn involves the application of convection and (or) phase change correlation's to find the surface coefficients, hand uses these with the areas, A1 and A2 and wall resistance, Rw, to find the result of the following equation :

The determination of pressure drop should be evaluated, as this also is an important design aspect. Some heat exchangers that performextremely well thermally may however require a very high pumping power. Therefore it is a compromise between these constants whensatisfying the specification. It must be noted that the intention is not to go into great detail on this topic matter, but to give an insight into theheat exchange design and selection process.

Short of performing a detailed study of a range of heat exchangers, to define which is preferred for a particular application, some short cutmethods are in existence. One such method is the effective index. This technique was defined by Brown (1986) and is the overall heattransfer coefficient for the given heat exchanger divided by the cost per unit area of the heat exchanger. Therefore the higher the value ofthis index, the better the buy, as all other factors are constant.

In general there is a large amount of design information accompanying the various heat exchangers, however an inexperienced designengineer can be somewhat overwhelmed by the vast range of exchangers on the market and which type would be most suited to a particularapplication. To outline the general characteristics of certain types of heat exchangers, some categorisations of these devices follow. However,it must be noted that only the more important aspects have been discussed and not a detailed study of each type of exchanger.

Shell and Tube Heat Exchangers

These are by far the most widely applied heat exchangers in the process control, industry. They are constructed of a shell which containedone of the fluids and the tubes which contain the other fluid. The heat transfer therefore takes place between the two fluids across the tubewalls.

The shell and tube category can be further subdivided into three major sub categories : return bend (A.K.A. U-tube), fixed tube sheet andfloating head. Before describing the various configurations, following is a brief look at applications.

Tube Diameter - This should be made as small as possible, which will increase the surface area per unit volume of fluid.Limitations on pressure drop and the ability to clean the outside of the tubes may place a constraint on this parameter.

Tube Length - Generally this should be as long as possible to decrease costs.

Tube Pitch - Normally a triangular arrangement is used to decrease overall size. However it may be necessary to use otherarrangements to decrease pressure drop etc.

Shell Design - Concern must be given to the shell design, so that the shell- side fluid cannot short cut the desired path, this isaccomplished by using well defined baffles.

Shell Side/Tube Side Applications - Normally higher viscosity and lower flow rate fluids are applied to the shell side. This is dueto the fact that turbulence is more easily initiated on the shell side due to the more complicated flow path.

If a certain fluid use causes periodic cleaning of the heat transfer surfaces or has a special need (e.g. fouling, toxic, corrosive high pressurehigh temperature, etc.), it is normally flowed through the tubes. The highest heat transfer per unit of pressure drop is generally possible inthe tubes.

Counter Flow/Parallel Flow - Counter flow opposed to parallel flow offers the potential for maximum temperature change of afluid stream. Only in special circumstances is parallel flow used, where many of these circumstances are a result of the physicallayout.

In generally it can be said that for shell and tube heat exchangers, the tubes represent 60 - 70% of the total purchase cost. Hence the type ofmaterial used for the tube construction can have a dramatic effect on the cost of the heat exchanger. The following table shows the relativecost of some of the more common tube materials :

Material Approximate Relative Material Cost

Low-carbon steel 1.0

Copper 1.1

Red brass 1.2

Admiralty brass 1.3

98/10 copper nickel 1.6

Aluminium 2.0

304 stainless steel 2.5

Nickel 5.0

Titanium 12.0

Operating pressures also have a distinct impact on the cost of heat exchangers. An example of pressure effects on shell cost can be seen inthe following table:

Design Pressure(psi)

Heat ExchangerRelative Cost

300 1.0

600 1.3

750 1.6

1000 2.0

1200 2.5

Return Bend (U-Tube) Type Shell and Tube Heat ExchangerThis is one of the most common heat exchangers used in industry. The following diagram shows a simplified view of this type of heatexchanger. In practice many tubes would be used whereas only one tube is displayed in the diagram. The vertical plates can serve as bafflesto change flow direction, supports for the tubes, or for both purposes.

The return bend shell and tube heat exchanger provides the following advantages :

Large variations in fluid temperature can be tolerated, as the tubes can readily expand and contract, without the added feature ofexpansion joints, which can be found in other types of exchanger.

Very high pressures can be applied to the tube side.

This type of design is generally less expensive than many other shell and tube designs.

The tube package can usually be removed for cleaning or repair.

This type of heat exchanger is not without it's disadvantages :

Typically, the exchanger is restricted to clean fluids, as although the tubes can be removed it is often the case that this task canprove time consuming and in some cases difficult.

Many of the tubes may be impossible to replace if a failure occurs.

U-tubes always result in an even number of passes, whilst this is a limitation it is only minor.

Fixed Tubesheet Heat ExchangersThe following diagram is a simplified representation of a fixed tubesheet heat exchanger. In contrast to the U-tube exchanger, here the tubesare attached to both sides of the shell arrangement.

In many respects the fixed tubesheet heat exchanger is similar to the U-tube exchanger. Although one clearly distinguishing feature is that anend chamber is used to return the tube side flow through the second set of tubes. Hence the U portion of the heat exchanger is replaced witha plenum region in the fixed tubesheet exchanger. This can lead to some differences in possible applications with respect to the U-tube heatexchanger. Among the advantages of the fixed tubesheet are the following :

This type of exchanger can handle fouling fluids on the tube side. The straight through flow of the tubes allows mechanical cleaningon the inside.

Fixed tube exchangers generally allow configurations with odd or multiple tube passes, which was not possible with U-tubes.

A fixed tube exchanger has fewer joints than other types of straight tube exchangers.

Minimal capital expense is involved in the fixed tubesheet type in comparison with other straight tube exchangers.

These type of exchangers provide greater protection to the environment with respect to shell side fluid leakage.

This configuration can result in the minimum shell diameter of all shell and tube heat exchangers, for a given heat transfer surface.

Obviously there are some disadvantages inherent with this type of heat exchanger :

One of the most important aspects with this type of exchanger is that thermal stresses can become critical if the effects oftemperature profiles in the tubes and shell are not matched.

As the tube side can accommodate fouling fluids due to easy mechanical cleaning the shell side is restricted to clean fluids.

Floating Head Heat Exchangers

This type of heat exchanger is simply a variation of the fixed tube sheet configuration. They are designed to accommodate the movement ofthe tubes that might result from thermal expansion and contraction, see the following diagram.

Due to this movement the exchangers yield most of the advantages of the fixed types, without the concern for thermal stress factors.However this simple modification complicates the design and maintenance of the device resulting in higher capital and operation costs.Attention must be made to the designing of the floating head, such that it does not leak.

Plate Heat ExchangersThese exchangers are a relatively early development, with at least one patent dating back to the late 1870s. They are made up from speciallyformed metal plates with grooves pressed in them, as shown in the following diagram. The grooves provide two basic functions, to aid theheat transfer process and add rigidity to the overall assembly.

Heat transfer takes place between two streams across n plates in the overall assembly, with two additional plates forming the outercontainers of the device.

The plates are normally constructed from cold worked metal, often stainless steel, in the order of 1mm thick. Gaskets are used to contain thefluids within the flow channels and the plates are held in their layered format by the frame.

Plate heat exchangers are available on the market with total heat transfer areas down to a fraction of a square meter and up to over 1000m2.

In contrast with the shell and tube heat exchanger, the plate heat exchanger is best suited to liquid-liquid duty with flow rate/specific heatproduct almost the same for the two fluids. Flows with dissimilar products can be applied but with decreased effectiveness.

For high pressure duty (above 300 psi ), a shell and tube exchanger is preferred over a plate exchanger. The basic construction of the plateheat exchanger make them unsuitable for high pressures.

There is more area per unit volume of heat transfer surface available in the plate type device compared to almost all other closed typeexchangers. With this characteristic coupled with the ease of manufacture of the plate device, the plate type usually has a lower cost than anytube type exchanger.

It can be seen from this discussion that there is almost a limitless option for heat exchangers on the market. It is therefore a very importantdesign consideration for the engineer trying to satisfy a specification. All the various points mentioned previously have to be balancedtogether, with the cost (in most cases) being the leading consideration.

On/Off Heater Control

This is the most primitive type of control and is often considered by some engineers to be inferior to complex modes of control. Howeverthere are still many applications where it can provide the necessary control with the minimal cost and layout.

With this method of control the correcting element can assume one of two positions, On or Off.

In a temperature control system (Temperature Process Rig), one position of the correcting element gives a heat input which is too small tomeet the process requirements and hence the temperature falls below the desired value. In the other position the heat input is greater thanthat required by the process and the temperature rises above the desired value. Control action is discontinuous and permanent oscillation ofthe temperature is the result.

In the case of the Temperature Process Rig, the On-Off Control is applied to the heating element of the reservoir.

The following diagram shows the oscillation of potential and recorded temperature and illustrates the effect of the transfer lag. The potentialtemperature is that which would be reached if transfer lag were not present.

It is assumed that the change-over from ON to OFF takes place at the instant the actual temperature passes through the desired value, alsothe heating and cooling actions are equal. The actual temperature rises and falls above and below the set point due to transfer lag.

On/Off Heater Control (2)

As can be seen from the previous diagram, there was no overlap present. The following diagram shows the control effort with overlap present

This has the same result as the first control method except that the angle of lag is nearly 180o.

The Basic Process Rig or Optional Auxiliary Valve provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the Basic Process Rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI link the servo valve current inputs to the current source outputs.

o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, toensure that both servo valves are fully open.

On completion of the Temperature Process Rig patching diagram the temperature can be controlled manually using ON OFF controltechniques.

The Heat Exchanger

In its most basic form a heat exchanger, as the name suggests, is a process component for either heating or cooling process fluids, by meansof an isolated heat transfer fluid.

The heat exchanger on the Temperature Process Rig is a three-pass shell and tube exchanger. Its function is to transfer heat between theprimary flow and the secondary flow, of which is supplied by either the Basic Process Rig or Optional Auxiliary Valve.

As mentioned above, the heat exchanger used on the Temperature Rig is a three-pass unit. This simply means that the cooling fluid follows apath through the exchanger in the shape of an `S', allowing the fluid to effectively take three passes through the exchanger. Thisconfiguration provides better efficiency than, say a two pass exchanger.

The heat exchanger is made up from a number of cupro-nickel tubes through which the secondary flow fluid makes its three-pass flow, thusallowing heat transfer between the primary flow passing over the tubes. The body of the heat exchanger consists of a cast aluminium shellwith bronze end plates. The following diagram shows an exploded view of the device.

It can be seen that the efficiency of the heat exchanger is flow dependent. For example a `fast' primary flow would transfer more heat than a`slow' primary flow.

Therefore this shows that the heat in the primary circuit can be controlled simply by controlling the flow of the secondary circuit. In the samemanner the secondary flow heat absorption could be controlled by the primary flow rate.

Primary Circuit Pump

The pump used for the primary circuit is a basic domestic central heating pump. The pump itself is a Grundfos Selectric, as can be seen below:

The motor is a three speed squirrel cage induction wet rotor type, running in water lubricated bearings and should never be run dry. It canproduce a maximum system pressure of 10 bars and operate over a water temperature range of +15oC to 110oC.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the Basic Process Rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure that both servo valves are fully open.

This practical demonstrates the effect of the heat exchanger by allowing the cool secondary flow to absorb the heat of the primary flow, thuscausing a drop in temperature across the heat exchanger in the primary flow.

It must be noted that the very nature of this practical causes the secondary temperature to heat up very quickly. Therefore if you are usingthe Basic Process rig, the temperature in the sump could rise above 30oC. If this is the case the sump should be partly drained and refilledwith cold water to bring the sump temperature back into the reasonable operating range 20-30oC.

Operation Of The Cooler

The main reason for the cooler on the Temperature Process Rig (TPR) is to drop the temperature of the heated return fluid (secondary flow)before it re-enters the Basic Process Rig (BPR). The overall effect of this process is to prevent the secondary flow circuit (water in the tank ofthe BPR) from heating up too quickly.

This is achieved using a cooler, which consists of a radiator and a fan unit, commonly known as Airblow Water Coolers. The radiator itselfcomprises an aluminium structure of heat dissipating fins, whereby the fluid to be cooled passes behind. In order to increase the coolingefficiency, a fan is attached to the rear of the radiator to draw air through the radiator dissipating the heat from the fins.

It must be noted that coolers of this type can only reduce the temperature to a minimum degree equal to the ambient air temperature.However with respect to the TPR due to the size of the cooler this would actually take a considerable amount of time depending upon thetemperature of the BPR fluid.

It is therefore shown that the cooler is only intended to provide a degree of cooling to the BPR. However in industrial applications, a coolermay be the primary source (only source) for cooling a process, in which case its specification would be critical to the dissipation required.Coolers of this type tend to be relatively large with respect to their function.

In this particular case the cooler is switched on to demonstrate its efficiency in cooling the secondary flow before returning to the sump tankof the BPR. Therefore, carefully note the temperature drop as it will be relatively small with respect to the heat exchanger.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.

o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, toensure the servo valve is fully open.

Modelling of Dynamic Systems

In everyday life we all construct models of complicated systems in order to predict the effect our actions will have upon them. For instance,when driving a car it is possible to predict the degree of steering wheel movement required to negotiate a particular corner.

Little thought is required as to the operation of the steering rack-and pinion mechanism or to the coefficient of friction between the tyres andthe road. All we need to know is that a steering input produces the desired degree of directional change in the car.

This directional change is the output of a dynamic control system. A dynamic system takes a certain period of time to react to changes in thesystem input; the output of a dynamic system evolves with time. The diagram below illustrates the concept of a system, with its stimuli andresponses.

Engineers and scientists strive to construct mathematical models of complicated machinery and dynamic processes to allow performancepredictions to be made. The successful construction of such models can be advantageous for two reasons.

System models enable the engineer to construct ideas regarding performance without the need to expend large amounts of capital building aprototype which may or may not function. Of course, a prototype will eventually be constructed, but if a model has been proved to functionthe engineer can be confident that the same will be true of the prototype.

One perfect example is the use of super-computer modelling to predict the performance of nuclear weapons. New designs of device can bemathematically modelled using data and experience gathered from numerous past detonations. In this way a new weapon can be 'detonated'inside the processor of a computer rather than having to resort to underground tests of actual devices.

This method not only saves vast amounts of money but also protects the environment from unnecessary contamination and damage.

The second great use of system modelling is in the teaching and training of personnel for complex and potentially dangerous tasks. It ispossible to learn a task without the need to be 'on-site'. For example, a flight simulator avoids the need to have inexperienced pilots takingthe controls of real aircraft. This not only reduces the risk of accidents but also alleviates the need to have an aircraft out of service forextended lengths of (expensive) time.

Multi-processor computers used in a flight simulator are programmed with every aspect of the performance of the aircraft in question. Oneprocessor may be occupied in simulating the engine performance in heavy rain, whilst another processor may be occupied in simulatingacceleration along the runway by tipping the whole cockpit back. Every aspect must be catered for to provide a thoroughly realisticenvironment.

Modelling and Procon

It is possible to develop complex simulations and models with more modest equipment. Mathematical software such as MATLAB allowcomplex technical computations to be carried-out in within a user-friendly PC-based programming environment.

The simulation of the temperature rig presented in the following practical is by no means a comprehensive representation of the behaviour ofthe hardware. If this were the case modelling of the heat transfer characteristics of the exchanger would be required across the full range ofexpected temperatures.

A model would also be required of the effect on flow rate of the impeller in the pulse flow transducer. Modelling of the thermal characteristicsof the radiator would be required. All aspects of rig behaviour would need to be considered, even down to the friction experienced by the fluidwhile flowing along the pipe surfaces. In any case, such a simulation would render the hardware redundant.

The simulation implemented as part of this practical is designed to illustrate the basic behaviour of the temperatures throughout thetemperature rig. The heating and cooling effects in the primary and secondary flows can be observed, as can the effects of the heatexchanger and the radiator.

Once the simulation has been 'played with', the user should have formulated ideas and opinions about the expected behaviour of the real rig.

Simulation of Temperature Rig

Simulation of processes and situations is an extremely effective method of training and education. Flight simulators allow inexperienced pilotsto take control of an airliner without the risk of accidents. Process simulators allow would-be process operators to control complicated andpotentially dangerous equipment with no risk of injury.

Not only do simulations reduce the danger and expense involved in training and familiarisation, they also allow experimentation and student-led discovery. The simulation that follows in this practical has been designed for this reason.

The simulation is designed to reflect the behaviour of the temperature process rig whilst connected to the basic process rig. Temperaturesaround both the primary and secondary flows can be monitored to allow a picture of the behaviours of such components as the heatexchanger and radiator to be built-up.

It should be noted before beginning the practical that the simulation has been built- up around a discrete digital model of the system. Thereal rig is very much an analogue system, with all the infinite variations that this entails.

The behaviour of the simulation does not exactly model the rig in every detail; this kind of simulation is beyond even some of the mostpowerful computers. However, the relationships between the flow rates and temperature fluctuations are a true reflection of the real world.

During the practical you are led through some situations which display the fundamental facts which must be understood. Beyond that thesimulation can be freely experimented upon.

Primary Flow Control

This practical is intended only to give you a 'feel' of the control effort required to maintain temperature at a given set point.

You will basically be controlling the flow of the primary circuit using the primary circuit manual valve. This in turn controls the flow ratethrough the heat exchanger. Therefore if the primary flow increases, more heating fluid flows through the heat exchanger, hence transferringmore energy across into the secondary flow. The overall effect is heating the secondary flow.

Alternatively if the inverse of the above was carried out, that is, the primary flow rate reduced, the overall effect would be less heat energytransferred to the secondary. This is true to such a point where the primary flow is so low that the heat transferred is less than the coolingradiator is taking out, hence the temperature of the secondary flow decreases. Using this theory, the temperature can be controlled.

It must be noted that the secondary flow temperature at T5 is the temperature directly after passing through the cooling radiator. Therefore ifyou are using the BPR for the secondary flow it will be some time before a noticeable change will be present at T3 (secondary flow input toTPR). This is due to the lag created by the BPR sump tank.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the Basic Process Rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the Basic Process Rig PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

Once the system has 'settled down', that is the system is being maintained about the set point it can be seen that the control effort becomesmuch less.

This is because the variation in temperature between the two flows is small, hence a small control effort is all that is needed.

Secondary Flow Control

This practical is similar to the previous one except the control effort is applied to the secondary flow circuit.

It must be noted that if you are using the Basic Process Rig to supply the secondary flow, then there is a possibility that the temperature inthe sump may exceed the upper operating parameter of 30oC.

This is due to the fact that the secondary flow is being used to regulate the temperature of the primary, hence the secondary flow is beingused to absorb the heat that is being constantly applied by the heater to the primary.

If the secondary flow water temperature in the sump does exceed 30oC, then simply drain off and add fresh cold water (see the BleedingSecondary Flow Practical).

If this practical is carried out for any length of time the situation mentioned above will be inevitable. However, if this were an actual industrialprocess there would have to be an element within the process which removed the heat which was being applied. In our case this is thecooling radiator, but due to practical size limitations it is not large enough for the job. If it was replaced with a suitably sized chiller, forexample, the process would be satisfactory.

If you do not replace the Basic Process Rig sump water when it exceeds 30oC, the process will reach such a point whereby the temperature ofthe secondary will be nearing the temperature of the primary.

It is then not possible to remove heat from the primary and the whole process would become unstable with the temperature of the primaryreaching saturation at about 70oC.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

You will basically be carrying out the same function as the controller in the future assignments. Therefore strictly speaking this is not openloop control as the human element is acting as a link, closing the loop.

Once the system has `settled down', it can be seen that the control effort becomes much less. This is because the variation in temperature ofthe TPR, caused by the secondary flow is much less effective due to the fact it has `warmed up'; the efficiency of the heat exchange processis therefore reduced. This is displayed by the fact that the valve is constantly open at some particular value, instead of opening and closing, inan effort to maintain the set point.

Process Control and Mimic Diagrams

Throughout most aspects of everyday life human beings strive to represent complex situations with simplified representations and symbols.These are all designed to make the exchange of information from one point to another as smooth as possible.

It is impossible to ignore the impact of symbolism and representations on the world today. It is hard to imagine road signs consisting entirelyof text. A red circle with a black '30' printed in its centre would need to be replaced with the text 'The speed limit on this road beyond this

point until another sign is passed is thirty miles per hour unless your particular vehicle is restricted to a lesser speed. You should drive yourvehicle at this speed so long as the road conditions allow'.

The long winded definition of the law is replaced by a simple symbol understood by all road users. This idea can easily be carried over intothe field of industrial process control.

Process Control and Mimic Diagrams (2)

The control of a large industrial process requires the assimilation of many important variables relating to the operation of the equipment.These variables could be temperatures, pressures, flow rates, throughput rates or weights. Any number are possible. It is likely that each partof the process would have attached to it a device to allow monitoring.

If the process included a vat of liquid to be kept at a constant temperature a thermometer would be present on the vat to allow thetemperature of the liquid to be monitored by an operator.

It can be seen that symbolism is already being used to create a piece of instrumentation. The level of energy contained within the moleculesof the liquid is being converted, via a suitable conversion device, into an understandable quantity. This quantity is temperature, and wechoose to represent it in terms of Fahrenheit or Celsius units.

Process Control and Mimic Diagrams (3)

It is no surprise that the proliferation of computers into everyday life has had an effect on the way in which the behaviour of processvariables, including temperature, are monitored and controlled. The SCADA (Supervisory Control And Data Acquisition) standard is an exampleof the degree to which computers have eased the burden on process operators.

Almost all processes within industry utilise a system of Virtual Instrumentation and mimic diagrams in order to allow more effective monitoringand control. The idea behind virtual instrumentation is that the process operator can sit in front of a computer monitor and see a real-timevisual representation of the process as it progresses.

Many of the packages allow alteration of process set-points and some even incorporate live CCTV pictures direct from points around theprocess. The packages require that every monitoring instrument around the system output their measurements in a standard control form.

The data is collected in this standard form via a data acquisition i/o card plugged into the computer. The information is processed by thesoftware and displayed in a user- defined form on the monitor. This type of output is called a software based MMI (Man Machine Interface)

The philosophy behind this form of process management is that an operator sitting in the quiet of a control room observing his processesfrom a distance can make better judgments than if he were on the shop-floor surrounded by noise and bustle. He will also, of course, be ableto react to emergencies in a more controlled manner.

Process Control and Mimic Diagrams (4)

Manufacturers such as National Instruments, ISS and Orsi produce software which can be designed and tailored by the end-user to fit exactrequirements. In this way it is possible for the software to expand and develop as new processes and components are added.

The Industrial Process Control Mimic Diagram practical included as part of this assignment gives a feel for exactly what the operator of a largefactory might see from his control room.

The diagram shows all the components of importance to the control and monitoring of the process. All the control devices can be operateddirectly by use of the mouse with feedback instantly being obtained as readings from related transducers.

Although the workstation computer is likely to be situated close to the process rigs in this case, the 4-20 mA control signals allow the user tobe positioned far from the hardware. In some cases this may be a different building or even a different site.

The advantages of virtual instrumentation and mimic diagrams are seemingly endless, and they are set to play an ever increasing role incomplex processes where the flow of good quality, accurate and up-to-date information is essential.

Closed Loop Temperature Control

This practical is intended only to give you a glimpse of the sort of control that can be achieved with the Temperature Process Rig (TPR) andthe controller.

It is a fully functioning Proportional + Integral Control system, monitoring actual temperature, comparing this to a desired level (the setpoint), and altering the position of the servo valve controlling the secondary flow with the aim of achieving the set point.

The Set Point can be changed by the user, however it must be noted that as the temperature of the secondary flow increases, the lessefficient the exchanger becomes, until such a point that the secondary flow temperature is greater than the set point, hence the systembecomes unstable.

It is not necessary for you to understand exactly what Proportional + Integral Control is, and how it is being implemented, only to appreciatesome of the applications available with this product.

Preparing for the Practical

The Basic Process Rig (or Auxiliary Control Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Auxiliary Control Valve) to the servo valve input on the Temperature Process

Rig PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

The TPR the connections shown in the following diagram should also be carried out:

Industrial Process Control

This practical is intended only to give you a glimpse of the sort of control environment that is used in industry for modern plants.

The control used in this practical is proportional + integral which is maintained by the controller. The manual intervention is carried out by theoperator, who may change the set point, switch the pump/fan on/off and monitor a Process Variable (PV) such as temperature & output.

Information about the process is presented in the form of a mimic diagram, which follows the style of a typical industrial plant mimic.

It diagrammatically displays all the information in a visually pleasing format to the operator, this is also known as virtual instrumentation.Thus allowing him/her to provide the necessary overall observation/control of the plant by simply using the mouse to select various options.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows:

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

On the TPR the connections found in the following diagram should be carried out.

The controller will try and maintain proportional control of the Temperature Process Rig whilst the user can control the various parametersand observe the result.

Automatic On/Off Control

The aim of this practical is to automatically control the temperature of the fluid at the radiator exit (T5) by turning the primary flow heater onand off.

The first practical of the Temperature Rig Familiarisation Assignment dealt with the manual on/off control of the heater to sustain atemperature at the primary flow output from the heat exchanger.

The practical here introduces a more complicated method of control, and also clearly displays the lags associated with temperature basedsystems.

The practical makes use of the Comparator and Schmitt Trigger contained in the on-off control section of the Process Interface. The heaterwill be turned on as the temperature falls below the set-point, and then off as the temperature rises above the set-point.

Also contained in the on-off section are the variable hysteresis control and the logic inputs to the switched supplies. One of these switchedsupplies will be connected to the heater.

The comparator of the PI accepts two voltage inputs. In this case the inputs are derived from the control set-point and the temperatureoutput.

These current signals are converted into voltage form for input to the comparator. The comparator has two inputs; one inverting (+) and onenon-inverting (-). The output of the comparator is fed into the Schmitt trigger, whose output is either 0v or 5v depending on the comparatorinputs.

The output voltage range of the current to voltage convertors is 0.4 to 2 Volts. This results from the 4 to 20mA currents flowing through the100 Ohm resistors provided on the PI.

There are no standard control voltages in use in the process industry. The voltage into which control currents are converted depends entirelyon the intended purpose of the conversion. For instance, different voltages are required for different logic families.

The Schmitt trigger output will be used as input to the logic control of the switched 240V supply. It is this switching which will turn the heateron and off.

Hysteresis

The Hysteresis level determines the difference between the high-to-low and low-to-high switching levels. The greater the level of hysteresisthe greater the difference between the two levels. Hysteresis is not only able to control oscillation of processes but also to control the effectsof outside interference.

The hysteresis loop curve below displays the action of hysteresis. When the deviation becomes greater than Vt, the output switches to its lowstate. This is the control effort that will drive the measured value in the opposite direction.

This diagram shows the two change state paths dependent on the direction of the deviation from the set point. It also shows the twoswitching levels +Vt and -Vt.Imagine that the deviation is initially large and negative. The output will be high (5V) . As the variable and setpoint levels become closer together the deviation will become less negative. Eventually the deviation will become positive.

When the deviation becomes greater than +Vt, the output will switch to its low (0V) state. This is the control effort that will drive themeasured value in the opposite direction.

On/off control determines that the measured value cannot remain at the reference level. It must lie in the region between two referencelevels determined by the hysteresis value. The greater the hysteresis, the greater the distance between these levels.

On/off control is not used in situations where precise control is required. A variable must need only to lie between certain levels for on/offcontrol to be acceptable.

The perfect example of this is the control of the temperature in a room via a thermostat. People cannot detect variations of a few degreesCelsius so slight variations in temperature a perfectly acceptable.

During the practical it is possible to vary the level of hysteresis. This can be used to discover more about the way in which hysteresis levelsaffect the operation of on/off control systems.

PID Control

For a comprehensive theory and background on Proportional, Integral and Derivative control, see the PI + PID assignments for the BasicProcess Rig.

The following theory describes proportional and derivative control.

Derivative control and the use of the Temperature Process rig can be explored using practical 3, PID Control of Temperature with the integralaction term turned off.

Proportional Plus Derivative Control

In order to reduce the overshooting caused by sudden load changes, the effect of lags in a process may be counteracted by the addition of aderivative term to a proportional only controller. The position of the regulating unit will now be proportional to the deviation and to the rate ofchange of the deviation.

Therefore, in a temperature control system the effect of derivative action is to cause the control valve to be positioned by additional amountsas the rate at which the temperature is rising or falling, increases or decreases.

The relationship between proportional and derivative control action is illustrated graphically in the following diagram.

The deviation in the diagram is assumed to change at a constant rate and owing to this, the valve is immediately positioned by the amount x.As the deviation increases the valve continues to be moved by proportional action. The time taken for proportional action to increase by anamount equal to the derivative action, when the deviation is changing at a constant rate, is known as the derivative action time.

Proportional Control of Temperature

In ON/OFF control, small deviations from the desired value cause just as much movement of the correcting element as large deviations.However it is frequently convenient to arrange that the position of the correcting element is directly related to the deviation, this is known asproportional control.

The following diagram shows that within the proportional band, there is a unique correcting element position for every value of the controlledcondition

It is usually arranged that the correcting element position required to give the desired value of the controlled condition occurs at the centre ofthe proportional band, i.e. the valve is half open when the temperature is correct under normal condition of load.

The proportional band is that range of values of the controlled condition which operates the correcting unit (valve) over its full range. A verynarrow proportional band is obviously tending toward ON/OFF control, since a small change in the controlled condition would result in a largechange in controller output.

On the other hand, a very wide proportional band may result in sluggish control, and the sustained deviation (offset) of proportional controlwill be present.

Proportional Control (2)

Suppose water passing through the secondary circuit of a heat exchanger is being heated by the hot water primary circuit. If the temperatureof the secondary circuit is being controlled by regulating the primary flow circuit, via a proportional controller, the primary circuit valve wouldbe half open when the temperature is correct and the flow of water through the secondary was normal, this is point A on the followingdiagram.

Should the temperature rise or fall, then the primary valve will open or close, in an attempt to restore the temperature to the desired value. Ifhowever, the flow of water in the secondary be suddenly increased, then the primary flow must also be increased to maintain thetemperature desired value.

The primary valve will open, but, as the temperature begins to rise towards the desired value, the valve will gradually close. The only way ofensuring a greater primary flow is for the control point to settle at some new temperature below the desired value, see point B.

If a narrow proportional band is used, the offset due to any likely load changes may be small enough to be permissible without any seriousrisk to the process, the opposite of this being true for a wide proportional band.

In this practical the controller has proportional control over the TPR servo valve, with the process variable being T5. Therefore the processbeing maintained is the control of the primary flow to control the temperature of the secondary flow at T5.

Once the process settles to a new set point (obviously with an offset), the measured variable (temperature at T5) can be seen to oscillate at avery low frequency.

This is due to the hysteresis of the safety thermostat present in the TPR heater tank. For example the thermostat cuts out at 70oC, but doesnot cut back in until the temperature has fallen to 65oC. Hence this effect propagates (taking into account the system lags) through thesystem to T5.

The user has full control of the set point and proportional band to investigate the effect of varying the proportional band on the system. Theresult being displayed on the chart recorder.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the Basic Process Rig.o Connect the servo valve on the Basic Process Rig (or Optional Auxiliary Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

On the TPR the connections found in the following diagram should be carried out.

P + I Control of Temperature

Integral control produces a rate of movement of the correcting element proportional to the deviation. Therefore if integral action is added toproportional action, assuming linear operation, the position of the correcting element will be proportional to the magnitude and duration ofthe deviation.

As long as there is offset, the correcting element (valve) will continue to open or close at a rate proportional to the deviation until the desiredvalue is attained.

Although offset is eliminated, the combination of proportional with integral action brings two disadvantages:

1. The process takes longer to stabilise than with proportional control only.2. For negative deviation, integral action shifts the whole proportional band above the desired value until the controller condition

reaches it. This means that the correcting element does not start to close until it is too late to prevent over shoot.

The following diagram shows the output from a controller set to PI following a sudden deviation.

The initial step is due to proportional action. Assuming that the deviation can be held constant, integral action will cause the output toincrease at a constant rate until, at time t2, the output is double that at time t1. The time interval t2-t1 is known as the integral action time.

In other words, the integral action time is the time in minutes (or seconds) taken for the integral action of the controller to move theregulating unit by the same amount as it is moved by the proportional action of the controller when the deviation is constant.

In this practical the controller has proportional and integral control over the TPR servo valve, with the process variable being T5. Thereforethe process being maintained is the control of the primary flow to control the temperature of the secondary flow at T5.

The user has full control of the set point, proportional band and integral time to investigate the effect of varying the P+I values on thesystem. The result being displayed on the chart recorder.

Preparing for the Practical

The Basic Process Rig (or Auxiliary Control Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Auxiliary Control Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

On the TPR the connections found in the following diagram should be carried out.

PID Control of Temperature

For satisfactory control of processes having large distance-velocity and transfer lags, and where load changes may be sudden and/orsustained, the controller must incorporate P, I and D action. Such a controller is known as a three term controller .

The derivative component of the control effort enables a controller to recognise a rapidly changing error and take extra action to account forit. By applying a control effort that is not simply directly proportional to the error, the response of the plant has been improved.

This ability to recognise a rapid change in the rate of change of the error in a system is very important in many situations.

A massive increase in the core temperature of a nuclear reactor, caused by a failure elsewhere in a plant for example, could result inmeltdown. By applying a very large control effort, the time taken to reverse the direction of the system (towards failure) can be reduced. It isproducing an overcompensation for the rapidly changing error to halt its progress.

But it is not only overcompensation that a derivative action offers to a system. As the measured value of a system approaches its set point,the rate of change of error will decrease as the proportional action reduces. This reducing error rate will produce a negative controlcontribution from the derivative term, reducing the control effort further. This applies a breaking effect to the control effort, and reduces thechance of overshoot.

PID Control of Temperature (2)

The derivative action will pull a system away from failure by producing an overly large control effort, and slow down its approach to the setpoint with the aim of preventing overshoot.

However for a process with slow load changes, such as the temperature rig, the addition of Derivative Action to Proportional + Integral, doesnot produce any real advantage.

This practical will demonstrate to you full three term, PID control as applied to the control of temperature in the primary flow circuit.

You can vary Proportional Band (changing the gain which changes the contribution of the proportional action term), Integral Action Time (orReset Time, determining the contribution of the reset action term), and Derivative Action Term (determining the contribution of the derivativeaction term ).

Conclusions should be drawn from this practical as to the effect of the derivative action.

Preparing for the Practical

The Basic Process Rig (or Optional Auxiliary Valve) provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig (or Auxiliary Control Valve) to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

On the TPR the connections found in the following diagram should be carried out.

Complex Control SystemsFor various reasons adequate control of a process is not always possible using a single control loop. Complex or multiloop systems are thenemployed. In a cascade control system two controllers are used, a primary controller and a secondary controller. There are two reasons forusing cascade control, firstly to reduce the difficulties of control by reducing effective time lags and secondly to reduce the effect of supplychanges, load changes and other disturbances near to their source.

The following diagram shows a capacity and resistance process arrangement. The controlled variable in this system is to be the liquid level intank No. 4. The regulating unit is control valve A, but direct control of flow through this valve using the level in C4 is unsatisfactory due to thefour capacities and three restrictions.

A more satisfactory way of controlling this process is to use cascade control with primary and secondary controllers positioned as shown in thefollowing diagram. The secondary controller is employed to measure and control the level in capacity Cl, the set point of this beingdetermined by the primary controller connected to tank 4. The secondary controller and its control valve, together with that part of theprocess under its control, may be considered as the regulating unit for the primary controller. Two important advantages are obtained.

1) The secondary controller, tank Cl and control valve A form a single capacity control loop in which supply changes are rapidly detected andcounteracted, therefore level in tank C4 is little affected by changes on the upstream side of the control valve.

2) The primary controller has now only three capacities in its loop; time delay is reduced and control is simplified.

A more practical application of cascade control is shown in the following diagram. The controlled variable is the temperature at some suitablepoint in the fractionating tower. This temperature depends on the flow of steam and also on the composition and rate of flow of feed. Thetwo advantages of cascade control mentioned previously are both achieved, i.e. time delay due to column capacitance is reduced and theeffect of disturbances is minimised.

The following block diagram generally covers all types of cascade control systems.

The primary controller needs all the control actions necessary to achieve close regulation of the controlled variable. If a wide proportionalband is necessary for satisfactory control then integral action will be required to eliminate the offset. Also, if long time lags remain in that partof the process not under the control of the secondary controller, derivative action will also be needed.

The secondary controller may be of the simple proportional type if the part of the process under its control is simple. However, if thesecondary control loop contains a difficult part of the process, a wide proportional band may be required and integral action will then beneeded to eliminate offset.

Ratio Control

A common example of the use of two controllers, one of which varies the desired value setting of the other, is met in ratio control. Forexample, the flow of two fluids A and B may have to be proportioned so that they are mixed in the correct ratio (see the following diagram)even when the uncontrolled flow of fluid A varies over a wide range. In this case the primary controller merely acts as a transmitter producingan output directly proportional to the flow of fluid A.

The desired value setting of the secondary flow controller is adjusted so that the flow rate of fluid B is kept in the correct proportion.

Often a second measured variable is used (dual loop) to provide safety for the plant or its product, whereby the secondary controlling systemcan override the primary to keep the process in the safety region of the set point.

The next diagram shows an example of override control, whereby a high pressure fluid source supplies a number of processes, some at highpressure and others at constant lower pressure The processes supplied at high pressure are such that pressures below a certain critical valuewill result in a spoilt product. Should the demands of the low pressure users become excessive, the source may be unable to maintain therequired pressure on the high pressure side.

Both controllers are able to affect the control valve opening. The supply pressure to the secondary controller is the output pressure of primarycontroller and, if the high pressure falls below the set point of the latter, the output pressure of the secondary controller will fall because itsinput fluid pressure falls. The valve closes to maintain pressure on the upstream side and the pressure on the downstream side may fall wellbelow the set point. Conditions return to normal when the low pressure demands are reduced or the source provides a greater output.

Flow Ratio Control

Flow ratio control is used when two flows have to be proportioned so that they flow at a predefined ratio, even when the uncontrolled flowvaries over a wide range.

This can be achieved effectively by using two controllers, a primary and a secondary. The primary controller lies in the uncontrolled flow andpropagates the (changing) set point to the secondary controller, which endeavours to control the secondary flow to keep it in ratio with theuncontrolled flow.

This practical is based upon the above scenario. The aim is to maintain a pre-defined ratio between the primary and secondary flows throughthe heat exchanger, hence keeping the heat transfer constant. See the following diagram.

Fluid A is the secondary flow (BPR) and fluid B is the primary flow (TPR). The set point of fluid A can be altered by the operator, whilst theSecondary Controller automatically maintains the flow ratio of fluid B with respect to fluid A.

The primary controller (BPR) controls the flow to the given set point, whilst retransmitting the measured variable using the signal retransmitfunction to the secondary controller. In order to keep the ratio, the secondary controller reads in this measured variable on the remote setpoint input, and hence maintains the ratio.

The user has full control of the set point and integral for the secondary flow (BPR), the proportional band has been set to 120. However theproportional band can also simply be altered using the on-screen control if necessary.

On the primary flow the proportional band has also been set to 120, whilst the integral time can be changed using the virtual controls.

The chart recorder has been set up to display both measured variables, to allow investigation of the ratio control. The control outputs are alsoshown on-screen (labelled P.Output). These boxes directly show the effort being produced by the two controllers.

Preparing for the Practical

The Basic Process Rig provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

On the TPR the connections found in the following diagram should be carried out.

Use the extended dual 'banana-plug' lead to connect the signal retransmit of the BPR to the Remote Set Point Input of the TPR.

The user participation in this practical has intentionally been limited in order to focus the attention on the basics of flow ratio control. Thiscontrol method is a very important process in industry.

Dual Loop Temperature & Flow

In some process situations it is impossible to achieve automatic control with only one control loop (one controller). Therefore two loops, ordual loop control has to be employed. This is where two controllers are used, known as primary control and secondary control.

This practical is based upon the process scenario of control over temperature and flow. The control parameters are secondary flow andtemperature of secondary. Many industrial processes are based around this scenario.

The flow control of the secondary flow (BPR) is the same as the previous practical, where the flow set point and the integral time can bealtered using the on screen virtual controls. The Proportional Band is initially set to 120 but can also be altered using the on-screen controls.

The temperature of the secondary is monitored at T5 and is controlled by the heating fluid of the primary flow. Each process (flow & temp)has its own controller, primary & secondary respectively, therefore two separate loops exist.

The primary flow is controlled by the secondary controller and using the virtual controls the following parameters can be changed: set point,proportional band and integral time.

Preparing for the Practical

The Basic Process Rig provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.

Dual Loop Temperature & Level

This practical is basically the same as the previous one except, level is being controlled instead of flow. However the intention is to give thestudent the feel of a real industrial application.

In complex industrial plants the process scenario may be similar to the one in this practical, but more likely to be much more complex. Henceit would be very impractical for the plant operators to have to manually read all the various parameters from the controllers themselves.Therefore the plant control is usually based around a central control station where all the plant parameters are displayed.

This practical displays a multitude of virtual displays and controls, that are similar to industrial applications. See following diagram.

Preparing for the Practical

The Basic Process Rig provides the secondary flow to the Temperature Rig and should be set up as follows :

o Connect the 240 V switched AC output on the back of the PI to the basic process rig.o Connect the servo valve on the Basic Process Rig to the servo valve input on the PI.o On the PI, link the servo valve current inputs to the current source outputs.o Turn the current source fully clockwise opening the valve fully. This should also be carried out on the Temperature Process Rig, to

ensure the servo valve is fully open.