COMPUTER‐AIDED INSTRUMENTATION

UNIT ONESensors and Transducers

SENSOR• The sensor or the sensing element is the firstelement in a measuring system and takesinformation about the variable being measuredand transforms it into a more suitable form to bemeasured.

Transducer• A transducer is a device that converts a signal inone form of energy to another form of energy. Energytypes include (but are not limitedto) electrical, mechanical, electromagnetic (includinglight), chemical, acoustic and thermal energy. Whilethe term transducer commonly implies the use of asensor/detector, any device which converts energy canbe considered a transducer. Transducers are widelyused in measuring instruments.

Transducer = Sensor + Signal conditioning circuit

Transducer = Sensor + Transmitter

• In the following sections, we will present themain features of different transducers tomeasure:

• Temperature• Displacement• Object detector sensors• Pressure• Level• Flow

Temperature sensors and transducers

• Thermometer• Resistance Thermal Detectors• Thermistors• Thermocouples• Solid state temperature sensors

Thermometer sensor• A thermometer is an instrument that measures thetemperature of a system in a quantitative way (sensingelement). The easiest way to do this is to find asubstance having a property that varies regularly withits temperature. The most direct 'regular' way is alinear one:

T(x) = ax + b,• where T is the temperature of the substance andchanges as the property x of the substance changes.The constants a and b depend on the substance usedand may be evaluated by specifying two temperaturepoints on the scale, such as 32° for the freezing point ofwater and 212° for its boiling point.

Capillary tube thermometer• The element mercury is a liquid in the temperature range

of ‐38.9° C to 356.7° C As a liquid, mercury expands (moves)as it gets warmer, its expansion rate is linear and can beaccurately calibrated.

The mercury‐in‐glass thermometer illustrated in the abovefigure contains a bulb filled with mercury that is allowed toexpand into a capillary tube. Its rate of expansion is calibratedon the glass scale. Mercury can be replaced by alcohol for lowtemperature measurement.

Filled thermal system

• The device consists of a bulb filled with expanding substanceconnected to a Bourdon tube mechanism via a capillary tube(≈30m long) as shown in figure. The pressure inside the bulbchanges as the temperature changes. Consequently, thepressure moves the pointer at the moving end of Bourdontube. This movement is marked using the temperature scale.

Bimetallic thermometers• This type of temperature sensor has the characteristics of being

relatively inaccurate, having hysteresis, having relatively slow timeof response. This sensor consists of two materials (metals) withgross different expansion coefficients and bonded together.Therefore, the temperature will make each metal to expand with adifferent length. Consequently, this effect can be used to closeswitch contacts or to actuate an on/off mechanism whentemperature increases to some operating set point.

Resistance Thermal Detectors (RTDs)• It is a temperature sensor that is based on a metalresistance increasing with temperature. Metals used inthese devices vary from platinum, which is veryrepeatable, quite sensitive, and very expensive, tonickel, which is not quite as repeatable, more sensitive,and less expensive. For pure metals, the characteristicrelationship that governs resistance is given by:

• The RTDs is generally used in a bridge circuit. The RTDs canbe found with two leads, or thee leads, or four leads forconnections. The two wires type is used for short distanceapplication, where the process is too near to the controller.The connection will be as shown in figure, where theterminal block is connected to the bridge circuit.

• Three wires RTDs are used for remote applications asshown in figure. The compensation line in the R3 leg of thebridge is required when the lead lengths are so long thatthermal gradients along the RTD leg may cause changes online resistance. These changes show up as falseinformation, suggesting changes in RTD resistance. By usingthe compensation line, the same resistance changes alsoappear on the R3 side of the bridge and cause no net shiftin the bridge null.

• Four wires RTD can be used with low current source toavoid the self heating problem in the resistance. We feedthe current to close the circuit with RTD and measure theoutput voltage across the other two terminals which isproportional to the RTD value. The terminal blockconnection is illustrated in the following figure.

Thermistors• The name thermistor is derived from thermally sensitive

resistor, sense the resistance of a thermistor varies as afunction of temperature. It is an electrical device made of asolid semiconductor with a high temperature sensitivity.When a thermistor is used as a temperature sensingelement, the relationship between resistance andtemperature can be expressed as:

• Thermistor Properties:Range of measurement, Maximum up to 300oC toavoid melting of semiconductor materials.Normally, it can be used for low temperaturerange measurements.

Response time, (0.5 seconds or faster)Sensitivity, (highly sensitive, it can be changed by 1KΩ for change in 1 C)

• Signal conditioning:Circuits with instrumentation amplifiers can be orsimple potential dividers. But, be care with thenonlinearity in the measurement circuit and tryto obtain a simple and linear overall relationship.

Thermocouples• For many years the thermocouples was the clear‐cut choice of

instrumentation and control engineers in the process industries, butin recent years the position of the thermocouple has beenincreasingly challenged by the RTD. Nevertheless, thethermocouple still is used widely especially in large range oftemperature measurement applications.

• Seebeck effectAs early as 1821, Seebeck found that bonding wires of two dissimilar

metals together to form a closed circuit caused an electric currentto flow in the circuit whenever a difference in temperature wasimposed between the end junctions.

Thermocouple consists of two different materials such as:• J‐type, Iron and Constantan, measurement range ( ‐190oC to 760oC)• K‐type, Chromel and Alumel, measurement range (‐190oC to

1260oC)• T‐type, Constantan and Copper, measurement range (‐200oC to

371oC)• E‐type, Chromel and Constantan, measurement range (‐100oC to

1260oC)• S‐type, 90%platinum+10% rhodium and platinum, measurement

range (0oC to 1482oC)• R‐type, 87%platinum+13% rhodium and platinum, measurement

range (0oC to 1482oC)

• The output voltage from the thermocouple can be presented as:

• Signal conditioning:The output from a thermocouple is in mV. Thismeans that considerable amplification will benecessary for a practical application. Inaddition, the small signal levels make thedevices susceptible to electrical noise. In mostcases the thermocouple is used with a highgain differential amplifier.

Solid state temperature sensors

• Solid state temperature sensors offer voltagesthat vary linearly with a temperature range (from‐50oC to 150 oC), such as a zener diode. Thetime constant in a good thermal contact is in therange 1 to 5 seconds. The dissipation constant isin the range from 2 to 20 mW/oC. This valuedepends on the conditions and the heat sinking.These sensors are easy to interface to controlsystems and computers, and are becomingpopular for measurements within a limited range.It can be used to provide automatic referencetemperature compensation for thermocouples.

Displacement sensors• Displacement sensors are widely used not only to measure

the distance of a moving object but also it can be embeddedin other sensors or transducer devices to measure pressure orlevel or flow as it will be shown later. Many physical variableshave the capability to produce a displacement that can beconverted into active signal. This is the basic principle in manytransducer devices as shown in figure.

Different types of mechanical sensors can be considered such as:

1‐ Potentiometer types:2‐ Capacitive and inductive types3‐ Variable reluctance types (LVDT)4‐ Ultrasonic and Laser Range finders

Potentiometer• The simplest type of displacement sensor involves theaction of displacement in moving the wiper of apotentiometer. This device then converts linear orangular motion into change of resistance. Then, asignal conditioning circuit converts the change inresistance value into voltage change.

This sensor suffers from the following drawbacks:• Limited resolution• Friction in the wiper• High electric noise

Capacitive types• The basic operation of capacitive type sensors can beseen from the familiar equation for a parallel‐platecapacitor.

There are three ways to change the capacitance:1‐ variation of the distance between the plates (d), that can be usedfor linear displacements2‐ variation of the common area (A), that can bused for angularmotion displacements.3‐ Variation of the dielectric (K), that will be shown later in liquidlevel measurements.

Inductive types• If a permeable core is inserted into an inductor asshown in figure, the net inductance is increases.

• Every new position of the core produces differentinductance. The inductor and movable core assemblycan be used as a displacement sensor. An ac bridgeor other active electronic circuit sensitive toinductance then may be employed for signalconditioning.

LVDT• In this type, the moving core changes the magnetic flux

coupling between two or more coils. A Linear VariableDifferential Transformer (LVDT) is an electro mechanicaldevice that produces an output proportional todisplacement. LVDTs offer many distinct advantages overother displacement measurement devices including:frictionless movement, infinite resolution, nullrepeatability, temperature stability, and environmentalruggedness. LVDTs can measure displacements from a fewmicrons to several feet in a wide variety of environments.

• An LVDT operates on the principal of magnetic couplingbetween a primary and two secondary windings. The primarycoil is typically energized with a 2‐5 Volt sine wave withfrequencies between 2‐10 kHz. The primary windingproduces a magnetic field that passes through the twosecondary windings. A magnetically permeable metal core(Ni‐Ir) slides through the center of the coils and provides anefficient path for the magnetic flux. The amount of Voltageinduced in the secondary windings varies with the core'sposition. In ratiometric signal conditioning, both the Voltageon coil A and coil B are used to determine the position of thecore. The following equation is computed by the ratiometricsignal conditioner to determine the position where G is thegain or sensitivity.

• The above requires that the sum of the secondary coilsremain constant for high linearity. Ratiometrically woundLVDTs provide this necessary condition. Ratiometric LVDTsignal conditioning offers several advantages over the openstyle. The ratiometric scheme is highly temperatureinsensitive. Temperature affects the LVDT signal bychanging the magnetic induction efficiency. Since bothVoltage A and Voltage B are affected equally, the net effectis high temperature immunity. The ratiometric scheme isalso insensitive to phase shifts between the primary andsecondary windings. As a result very long cables may beemployed with no loss of accuracy around the null position.In the open type, the voltage Vc is directly proportional tothe displacement of the core as shown in the followingequation where G is the gain or sensitivity.

Displacement = G Vc

Ultrasonic sensor• Ultrasonic sensors (also known as transceivers when they both send and receive, but more generally called transducers) work on a principle similar to radar or sonarwhich evaluate attributes of a target by interpreting the echoes from radio or sound waves respectively. Ultrasonic sensors generate high frequency sound waves and evaluate the echo which is received back by the sensor. Sensors calculate the time interval between sending the signal and receiving the echo to determine the distance to an object.

• Systems typically use a transducer which generates sound waves in the ultrasonic range, above 18,000 hertz, by turning electrical energy into sound, then upon receiving the echo turn the sound waves into electrical energy which can be measured and displayed.

Object detector sensors• One type of feedback frequently needed byindustrial‐control systems is the position of oneor more components of the operation beingcontrolled. Object detection sensors are devicesused to provide information on the presence orabsence of an object as shown in figure.

Object detector sensors• Limit switches• Photoelectric sensors• Inductive proximity sensors• Capacitive proximity sensors• Ultrasonic sensors

Limit Switches• A typical limit switch consists of a switch body and an operating

head. The switch body includes electrical contacts to energize anddeenergize a circuit. The operating head incorporates some type oflever arm or plunger, referred to as an actuator. The standard limitswitch is a mechanical device that uses physical contact to detectthe presence of an object (target). When the target comes incontact with the actuator, the actuator is rotated from its normalposition to the operating position. This mechanical operationactivates contacts within the switch body.

Limit Switches• One type of actuator operation is momentary. When the

target comes in contact with the actuator, it rotates theactuator from the free position, through the pre‐travel area,to the operating position. At this point, the electrical contactsin the switch body change state. A spring returns the actuatorlever and electrical contacts to their free position when theactuator is no longer in contact with the target.

Inductive proximity sensors• The sensor incorporates an electromagnetic coilwhich is used to detect the presence of a conductivemetal object. The sensor will ignore the presence ofan object if it is not metal.

• This type of sensor consists of four elements: coil,oscillator, trigger circuit, and an output. Theoscillator is an inductive capacitive tuned circuit thatcreates a radio frequency. The electromagnetic fieldproduced by the oscillator is emitted from the coilaway from the face of the sensor. The circuit has justenough feedback from the field to keep the oscillatorgoing.

Inductive proximity sensors

Inductive proximity sensors• This type of sensor consists of four elements: coil,oscillator, trigger circuit, and an output. Theoscillator is an inductive capacitive tuned circuit thatcreates a radio frequency. The electromagnetic fieldproduced by the oscillator is emitted from the coilaway from the face of the sensor. The circuit has justenough feedback from the field to keep the oscillatorgoing.

Inductive proximity sensors• When a metal target enters the field, eddy currentscirculate within the target. This causes a load on thesensor, decreasing the amplitude of theelectromagnetic field. As the target approaches thesensor the eddy currents increase which is increasingthe load on the oscillator and further decreasing theamplitude of the field. The trigger circuit monitorsthe oscillator’s amplitude and at a predeterminedlevel switches the output state of the sensor from itsnormal condition (on or off). As the target movesaway from the sensor, the oscillator’s amplitudeincreases. At a predetermined level the triggerswitches the output state of the sensor back to itsnormal condition

Capacitive proximity sensors• Capacitive proximity sensors are similar to inductiveproximity sensors. The main difference between thetwo types is that capacitive proximity sensorsproduce an electrostatic field instead of anelectromagnetic field. Capacitive proximity switcheswill sense metal as well as nonmetallic materials suchas paper, glass, liquids, and cloth.

Capacitive proximity sensors

Capacitive proximity sensors• The sensing surface of a capacitive sensor is formedby two concentrically shaped metal electrodes of anunwound capacitor. When an object nears thesensing surface it enters the electrostatic field of theelectrodes and changes the capacitance in anoscillator circuit. As a result, the oscillator beginsoscillating. The trigger circuit reads the oscillator’samplitude and when it reaches a specific level theoutput state of the sensor changes. As the targetmoves away from the sensor the oscillator’samplitude decreases, switching the sensor outputback to its original state.

Ultrasonic proximity sensors• Ultrasonic proximity sensors use a transducer tosend and receive high frequency sound signals.When a target enters the beam the sound isreflected back to the switch, causing it to energize ordeenergize the output circuit. A piezoelectric ceramicdisk is mounted in the sensor surface. It can transmitand receive high‐frequency pulses. A high frequencyvoltage is applied to the disk, causing it to vibrate atthe same frequency. The vibrating disk produceshigh‐frequency sound waves.

Ultrasonic proximity sensors• When transmitted pulses strike a sound‐reflectingobject, echoes are produced. The duration of thereflected pulse is evaluated at the transducer. Whenthe target enters the preset operating range, theoutput of the switch changes the state. When thetarget leaves the preset operating range, the outputreturns to its original state.

Ultrasonic proximity sensors• The echo can be in micro‐volts. A blind zone existsdirectly in front of the sensor. Depending on thesensor the blind zone is from 6 to 80 cm. An objectplaced in the blind zone will produce an unstableoutput.

Ultrasonic proximity sensors• The time interval between the transmitted signal and the

echo is directly proportional to the distance between theobject and sensor. The operating range can be adjustedin terms of its width and position within the sensingrange. The upper limit can be adjusted on all sensors. Thelower limit can be adjusted only with certain versions.Objects beyond the upper limit do not produce a changeat the output of the sensor. This is known as “blankingout the background”. On some sensors, a blocking rangealso exists. This is between the lower limit and the blindzone. An object in the blocking range preventsidentification of a target in the operating range. There isa signal output assigned to both the operating range andthe output range.

Ultrasonic proximity sensors• The time interval between the transmitted signal and the

echo is directly proportional to the distance between theobject and sensor. The operating range can be adjustedin terms of its width and position within the sensingrange. The upper limit can be adjusted on all sensors. Thelower limit can be adjusted only with certain versions.Objects beyond the upper limit do not produce a changeat the output of the sensor. This is known as “blankingout the background”. On some sensors, a blocking rangealso exists. This is between the lower limit and the blindzone. An object in the blocking range preventsidentification of a target in the operating range. There isa signal output assigned to both the operating range andthe output range.

Ultrasonic proximity sensors• The radiation pattern of an ultrasonic sensor consistsof a main cone and several neighboring cones. Theapproximate angle of the main cone is 5°. Soundtravel time can be affected by physical properties ofthe air. This, in turn, can affect the preset operatingdistance of the sensor.

Photoelectric sensor• A photoelectric sensor is another type of positionsensing device. Photoelectric sensors, similar to theones shown below, use a modulated light beam thatis either broken or reflected by the target. The ratedoperating voltage is 10 to 30 VDC.

Photoelectric sensor• The control consists of an emitter (light source), a

receiver to detect the emitted light, and associatedelectronics that evaluate and amplify the detected signalcausing the photoelectric output switch to change state.We are all familiar with the simple application of aphotoelectric sensor placed in the entrance of a store toalert the presence of a customer. This, of course, is onlyone possible application.

Photoelectric sensor• Modulated light increases the sensing range whilereducing the effect of ambient light. Modulated lightis pulsed at a specific frequency between 5 and 30KHz. The photoelectric sensor is able to distinguishthe modulated light from ambient light. Light sourcesused by these sensors range in the light spectrumfrom visible green to invisible infrared. Light emittingdiode (LED) sources are typically used.

Photoelectric sensorScan techniques• Thru‐Beam scan

Reflective scan

Photoelectric sensorScan techniques• Diffuse scan