industrial instrumentation

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Shot Term Training Programme on

Industrial Instrumentation

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Syllabus Basic concept of measurement, Transducers and their

classification, Measurement of non-electrical quantities like Angular and Linear Displacement, Strain, Vibration, Pressure, Liquid flow, Temperature, Force and Torque. Instrumentation amplifiers, Data acquisition and conversion, Digital signal transmission and processing, Microprocessor based instrumentation. Input and output devices and Displays.

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INTRODUCTION

An instrumentation system is an assembly of various instruments and components interconnected to measure analyse and control the electrical, thermal hydraulic and other non-electrical physical quantities.

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Elements of Measurement System

The whole operation can be described in terms of three functional elements. Each functional element is made up of a distinct component or group of components, which performs required and definite steps in measurement. These may be taken as basic elements, whose scope is determined by their functioning rather than their construction.

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Elements of Measurement System

Primary sensing element Variable conversion element Data-presentation element Terminating stage Signal Conditioning Transmitting element

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Primary sensing element

The quantity under measurement makes its first contact with primary sensing element of a measurement system. Immediately after this a transducer converts measured into an analogous electrical signal. This is true in most of the cases but in many cases the measured quantity is directly converted into an electrical quantity by a transducer.

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Variable conversion element

The output of the primary sensing element may be any kind of electrical signal. It may be a voltage, a frequency or some other electrical parameter. Often this output does not suit to the system. For the instrument to perform the desired function, it may be necessary to convert this output to some other suitable form while retaining the original signal. The function of variable manipulation element is to manipulate the signal .

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Data-presentation element

The information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring control, or analysis purposes. The information conveyed must be in a form intelligible to the personnel.

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Data-presentation element cont..

These devices may be analogue or digital indicating instruments like

Ammeters Recorders like magnetic tapes, High speed camera Storage type CRT Printers Analogue and digital computers.

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Terminating stage

The final stage in a measurement system is known as terminating stage.

When a control device is used for the final measurement stage it is necessary to apply some feedback to the input signal to accomplish the control objectives.

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Signal Conditioning

Many a times one has to perform certain operations on the signal before it is transmitted further. These processes may be linear like amplification, alternation, integration, differentiation addition and subtraction. Some non-linear processes like modulation, detection, sampling filtering, chopping and clipping etc. are performed on the signal to bring to the desired form. This is called signal conditioning. In fact the element that follows the primary sensing element in any instrument or instrumentation system should be called signal-conditioning element.

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Transmitting element

When the elements of an instrument are physically separated, it becomes necessary to transmit data from one to another.

The element that performs this function is called a transmitting element.

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Typical Instrumentation System

Consists of following operation.

Monitoring processes: - An instrumentation system has an essential function of monitoring the process. It simply indicates the value or condition of parameter under the study in the process.

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Process Control

In order to control the process variables like temperature, pressure, humidity etc. these quantities must be measured at the desired location in the processing plant. They are compared with the reference or desired signal and the error signal is amplified to operate the actuator in controlling the process. The corrective action goes on till the controlled output is same as the desired output, in that case the error signal will be zero and the actuator will not operate.

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TRANSDUCERS A device, which converts one form of energy into another form of energy, is called transducer.

For the field of electrical instrumentation transducer is defined as a device, which converts a physical quantity (or a physical condition) into an electrical signal Transducer is often referred as pick-up.

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Type of Transducers

Primary Transducers Secondary Transducers

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Primary transducers Primary transducers are detectors, which sense

a physical phenomenon.

The example of primary transducer is a thermocouple, which senses the radiant energy and directly converts it into an analogous electrical output, which is a voltage.

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Secondary transducers

Secondary transducers convert the analogous output into on electrical signal.

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Classification of transducersThe transducers can be active, that is self generating type or passive, that is externally powered.

Passive transducers derive the power required for energy conversion from an external power source. Potentiometers are the examples of passive transducers.

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Active transducers

(i)    Thermoelectric,(ii)   Piezoelectric,(iii)  Photovoltaic(iv)   Magnatostrictive, (v)    Electro kinetic(vi)   Electrodynamic,(vii)  Electromagnetic,(viii) Pyroelectric,(ix) Galvanic

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Passive transducers

(i)   Resistive

(ii)   Inductive

(iii)  Capacitive

(iv)  Photoconductive

(v)   Magneto resistive

(vi)  Thermo resistive

(vii) Elastoresistive

(viii) Hall effect

(ix)   Synchro

(x)    Gyro

(xi)   Radio active absorption

(xii)  Ionic conduction

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Basic requirements of Transducers1.  Ruggedness2.  Linearity3.  Repeatability4.  Convenient instrumentation5.  High stability & reliability6.  Good dynamic response7.  Excellent mechanical characteristics8. Built-in integrated device with noise and

other defects minimized

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Types of Detectors

Electrical Transducers Pressure Transducers Piezzo-electric Transducers Oscillation transducers Inductive Transducer

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Advantages of Electrical transducer over mechanical transducers1. Minimized friction

2. Minimized mass-inertia effect

3. Easy Electrical amplification

4. Very small power electrical/electronic system controlled

5. The electrical output can be easily used, transmitted and processed for the purpose of measurement.

6. Miniaturization due to integrated circuits and use of microprocessors.

7. Easy Data transmission etc.

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MEASUREMENT OF

DISPLACEMENT

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Major electrical transducers for displacement measurement

Variable resistance-Potentiometric/strain gauge

Variable inductance-LVDT/ variable reluctance

Variable capacitance Synchros and resolvers

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Advantages of Synchros and resolvers

i.     Infinite resolutionii.    No wear of rotation, except at slip ringsiii.  System operating at much higher speediv. Relatively insensitive characteristics to stray cable capacitancesv.   High reliability and accuracyvi. Useful operating angle of 360 degree and capable of continuous rotations.

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MEASUREMENT OF

STRAIN

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Types of strain gauges

Mechanical gaugesOptical gaugesElectrical strain gauges

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Types of electrical strain gauges Wire gauges

i. Flat grid typeii.  Wrap around typeiii. Single wire gaugeiv. Woven type

Unbonded Strain gauges Foil gauges Semiconductor strain gauges Thin film gauges

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MEASUREMENT OF

PRESSURE

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Classification of transducers1. Gravitational i.e. Manometer

2. Elastic type DiaphragmsCapsuleBourdon tubeBellows

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Transduction methods: 1.      Potentiometric Device2.       Strain Gauge transducer3.       Variable reluctance Sensor4.       LVDT type transducer5.       Variable capacitance Device6.       Thin Film Pressure transducer7.       Force Balance transducer8. Piezoelectric Pressure Transducer

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MEASUREMENT OF

FLOW

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Classification of Flow meters 1. Head type flow meter based on differential pressure

measurement i.e.Orifice plate, Ventury tube, Flow nozzle and Pitot tube

2. Electromagnetic flow meter

3. Rotameter (Variable area meter)

4. Mechanical flow meter

5. Anemometer

6. Ultrasonic flow meter

7. Vortex flow meter

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MEASUREMENT OF

TEMPERATURE

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Types of Temperature sensors1.       Mechanical temperature sensors

i.    Liquid in glass thermometerii.   Liquid filled systemsiii.  Vapour pressure thermometer

2.       Resistance type temperature sensors3.       Platinum resistance thermometer4.       Thermocouples5.       Thermisters6.       Solid state sensors7.       Quartz thermometer8.       Radiation thermometer9.       Optical pyrometer

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MEASUREMENT OF

FORCE AND TORQUE

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Types of transducers 1. Force Measuring sensor – Load cell:

i.         Column type deviceii.       Proving ringsiii.      Cantilever beamiv.     Shear type load cellv.       Pressducer

2. Digital force transducer3. Force balance device4. Hydraulic load cell

5. Electronic weighing system

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Torque measuring transducers

1.       Absorption type

2.       Transmission type

3.       Stress type

4.       Deflection type

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INSTRUMENTATION AMPLIFIER

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Important features i. Selectable gain with high gain accuracy and gain linearityii. Differential input capability with high common mode rejection, even with sources having unbalance high out put impedancesiii. High stability of gain with low temperature coefficientiv.  Low DC offset and drift errors referred to inputv.  Low out put impedance