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T.Y.B.Sc.PHYSICS : PHY 354(B)
Instrumentation II
Presented by
M.D. MahanubhavDept of Physics
JET’s Z.B. Patil College, Dhule
Introduction to Instrumentation
Functional Elements of Measurement System
Basic functional elements
Auxiliary elements
Transducer Element
o Transducer element senses and converts thedesired input in one physical form to an output inanother physical form.
o Input could be pressure, acceleration, temp., etc.
o Output may be displacement, voltage, resistance,etc. depending on type of transducer element.
o Examples: Thermocouple (Temp → voltage),
LVDT (Displacement → inductance change),
Venturimeter (flow rate → pressure),
Pirani gauge (Gas press → Resistance change)
Spring balance (Force → Displacement).
Signal conditioning element
o Amplification• Mechanical amplifying elements• Hydraulic amplifying elements• Optical amplifying elements• Electrical amplifying elements
o Signal filtration• Mechanical filters• Hydraulic filters• Electrical filters
o Other signal conditioning operators• Signal compensation/Signal linearization• Differentiation/Integration• Analog to digital conversion• Signal averaging/signal sampling
Data presentation element
o Presents the output – seen/read or store
• Visual display type: Pointer and scale, CRO
• Graphic recording type: Pen recorders, ultravioletrecorders
• Magnetic tape: data reproduced at a later date foranalysis.
New devices: CD, Pen drive etc.
o If the data is in digital form - displayed or recorded.
Calibration Element – Provides calibration facility
External Power element – Provides electrical power to other elements.
Classification of Instruments
Deflection and null types
Manually operated and automatic types
Analog and digital types
Self generating and power operated types
Contacting and non-contacting types
Dumb and intelligent types
Deflection and null types
Deflection type instruments
o In this type of instruments, the physicaleffect generated by the measurand producesan equivalent opposing effect in theinstrument such as displacement, deflection.
o Main advantages –
• Simple in construction and operation.
• Good dynamic response.
o Disadvantage - They interfere with the stateof the measurand hence error may beintroduced.
Null type instruments
o A null type instrument is either manuallyoperated or automatic type.
o It generates an equivalent opposing effect tonullify the physical effect produced by themeasurand.
o Main advantage - They do not interfere withthe measurand hence extremely accuratemeasurements are possible.
o Disadvantage - Especially those manuallyoperated are quite slow in operation and hencepoor dynamic response.
Manually operated and automatic types
o Any instruments which requires the services of human operator are manually operated instruments.
o Automatic types - The manual operation is replaced by an auxiliary device incorporated in the instrument.
o Automatic types devices preferred because of
• fast dynamic response
• low operation cost
Analog and digital types
Analog type instruments
o It represents the physical variables in the form of continuous variation with respect to time.
o Main advantages –
• Consists of simple functional elements
• They are easy to repair and maintain
• Generally cheaper
Digital type instrumentso It represents the physical variables in the digital
quantities which are discrete and vary in steps.
o Main drawback – unable to indicate the quantity which is a part of the step value
o Main advantages -
• Allows online use of digital computers for data processing – automatic control system, computer aided design, etc.
• Noise immunity during transmission.
• Several techniques of coding have been developed for digital signals.
Self generating and power operated types
Self generating types instrumentsEnergy requirements of the instruments are metentirely from the input signal e.g. exposure meter ofthe camera, Bourdon gauge, mercury-in-glassthermometer, pitot tube, tachogenerator.
Power operated types instruments
• These instruments requires some source ofauxiliary power such as compressed air,electricity, hydraulic supply, etc.
• Input signal supplies only small portion ofthe power required.
• e.g. LVDT (Displacement, force, press →change in inductance) needs external powerto energize its primary as well as twosecondary coils.
Contacting and non-contacting types
• A contacting type instrument is one that isKept in the measuring medium itself.
• e.g. Clinical thermometer.
• A non-contacting type or proximity typeinstruments measure the desire input eventhough they are not in close contact withmeasuring medium.
• e.g. Optical pyrometer.
Dumb and intelligent types
Dumb or conventional types instruments
o Input variable is measured and displayed but datais processed by the observer.
o e.g. Bourdon gauge.
Intelligent or smart instruments
o Process the data in conjunction with μP or anonline digital computer.
o Provides noise reduction, automatic calibration,drift correction, gain adjustments , etc.
o Equipped with diagnostic sub-routines with alarmgeneration in case of any malfunctioning.
Definitions Resolution: Smallest increment in the measured
value that can be detected with certainty .
• L.C. → resolution
• Expressed in absolute values or percentage of FCD
Threshold: Minimum value of the input belowwhich no output can be detected.
• Expressed in absolute values or percentage of FCD
• Causes: Friction between moving parts, play,inertia of moving parts, length of the scale, size ofthe pointer, spacing of graduations, parallax effect
Range and Span
o Range is specified by the lower and upper limits in which it isdesigned to operate for the measurement .
o Span: The algebraic difference between the upper and lowerrange values is termed as the span of the instrument. Therange can be
• unidirectional (0-10 V) or
• bidirectional (-10 to 110 oC) or
• expanded type (40-100 oC).
Hysteresis
o Magnitude of error caused in the output for a given inputwhen this value is approached from opposite directions.
• Causes: Backlash, elastic deformation, magneticcharacteristics, but mainly caused due to frictionaleffects.
Dead band: Largest change in the measurand to which the instrument does not respond.
• Fig shows output-input curve with hysteresis due to Coulomb’s friction
Backlash: Maximum distance or angle throughwhich any part of the mechanical system may bemoved in one direction without causing the motionof next part.
• Can be minimised if the components are made tovery close tolerances.
Drift: The variation of the output for a given inputcaused due to change in sensitivity of the instrumentdue to temperature changes, component instability,etc.
• For certain electronic instruments usually drift isconsiderable for 15 minute.
• Switch on such instrument at least half an hour.
Dynamic Characteristics of instrumentso Represent each instrument by its mathematical model.
o Obtain relation between its input and output.
o e.g. Relation between input & output of thermocouple
(1 + τD) xo (t) = k xi (t) ………… (1) first orderk xo(t)
----------- = --------- → transfer fun of the instrument(1 + τD) xi (t)
o For sec order instrument -
Steps essential to understand the dynamic behavior of the instruments
o To formulate the governing equationrelating dynamic input and output.
o To obtain the dynamic output response forgiven input by solution of governingequation.
o If the output response is not satisfactory,improve the same by compensation.
Formulation of system equation
Resistance thermometer connected to display unit
Where
Therefore
Governing equation → first order → first order instrument
Thermal Element
• Rate of heat flow into the thermal element
• Rate of enthalpy gain
From equations (I and (II), θi = (1 + τD) θo → First order
U-tube Manometer
Inertia force of liquid column =
Press force= pA
Gravity force = hρgA
Friction force= (Δp)A
Where Δp is the press loss in the length L
This is second order equation → second order instrument
Impedance loading and matching Impedance loading
o Any measuring instrument extract some energy → Changing the value of measured variable
o For no impedance loading (Ideal case)
EAB = E
Impedance matching
o We want to deliver power from the device to the
external load
Therefore, Zi = Z
Typical Applications of instrumentation systems
Measurement of system parameters/informations
o To determine various parameters of the system/process.
o Present the desired information about the condition ofthe system (indication or recording).
Control of a certain process or operation
o In automatic control system it provides guidance ormanipulation to maintain them at set point . e.g.thermostat.
Experimental design studies
o To design and develop new product involves trial anderror procedures – use empirical relations, handbookdata, standard practices, design codes and equations.e.g. aircraft .
To perform various manipulations
o Instruments are employed to perform applications –addition, subtraction, division, multiplications,differentiation, integration, signal linearization, signalsampling, signal averaging, etc
o To determine solutions of complex differential equation.e.g. Pocket calculator
Testing of materials, maintenance of standardsand specifications of products
o Most countries have standards organization that specifiesmaterials standards and product specifications.
o They ensure reliability.
Verification of physical phenomena/scientific theories
o Experimental data is generated to verify physicalphenomena.
o e.g. Coulomb’s law of dry friction.
Quality control in industry
o Test continuously the quality of industrial product.
o Defective components are rejected at early stages ofproduction.
o Thus final assembly is free from defects.
o This improves reliability of products.
o e.g. boiler plate undergoes various tests such as x-rayexamination, metallographic examination, strength tests,etc.
Selection of the instrument
o Selection depends on performance characteristics and
its cost.Value of useful information
o Pay-off-ratio = ----------------------------------------Necessary total cost
Pay-off ratio should be as maximum as possible
Instrument’s qualities, value guided
o Performance parameters e.g. accuracy, precision,sensitivity, linearity, hysteresis, dead band, drift, etc .
o Nature and type of data available i.e. analog, digital,continuous or sampled.
o Nature and type of read out i.e. indicating, recording, etc.
o Nature of further data computations, if required.
o Signal to noise ratio characteristics of transducer andsystem fidelity, especially when extensive datatranslation is involved.
Convenience aspect, value judged
o Suitability for given application. e.g. laboratory, field orboth.
o Adoptability to different sizes inputs. e.g. scaleexpansion, range changes.
o Easy in calibration.
o Material durability.
o Fool-proof assembly.
o Easy in maintenance and repair, local representation andsteady delivery.
o Ready self indication in case of instrument malfunction.
o Safety in use.
o Proper shape, appealing appearance and necessaryprotective envelope.
Cost, initial and cumulative total
o Initial cost of instrument, installation including variousattachments and accessories.
o Maintenance, repair, recalibration, etc.
o Running cost.
o Expected life span considering the salvage value ofcomponents.
~ ~ ~ ~ ~ ~ ~ ~
Transducer elements
o Transducer element converts the input physical variable to usable form .
Advantages of electrical signal as an output
o Inertia and frictional effects are absent.
o The electrical output can be amplified to any desired value.
o The electrical system can be operated with small power.
o The electrical output can be indicated or recorded remotely.
Classification of transducerMain classes: Analog and digital transducers
Analog transducers[1] Electromechanical types transducers
(A) Potentiomeric resistance type
(B) Inductive type
(C) Capacitive type
(D) Piezo-electric type
(E) Resistance strain gauges
(F) Ionisation transducer
(G) Mechano-electronics type
[2] Opto-electrical transducers
(A) Photo-emissive transducer
(B) Photo-conductive transducer
(C) photo-voltaic transducer
Digital transducers
[1] Frequency domain transducers
(A) Electromagnetic frequency domain transducers
(B) Opto-electrical frequency domain transducers
(C) Vibrating string transducers
[2] Digital encoders
(A) Optical encoders
(B) Resistive digital encoders
(C) Shaft encoder
Potentiomeric resistance type
o Wire wound potentiometer may be used .
o Mechanical displacement input → Electrical output.
o Potentiometer → resistive element with sliding contact (wiper).
o Motion of wiper may be translatory or rotational.
o Combination of translatory & rotational → helipots.
o Advantages
• Inexpensive
• Simple to operate
• Useful for measurement of large displacement
• Electrical efficiency is very high
• No need of amplification
o Disadvantages
• Linear potentiometer requires large force.
• Sliding contact → wear out, become misaligned and generates noise.
Inductive type transducers
o Self generating and non-self generating types
o LVDT → non-self generating type
o Eo = Es1 ~ Es2
o Advantages
• Linearity: upto 5 mm.
• High output: no need of amplification.
• High sensitivity: as high as 40 V/mm.
• Ruggedness
• Less friction: no sliding contacts.
• Low hysteresis: repeatability excellent.
• Low power consumption: less than 1 W.
o Disadvantages
• Sensitive to stray magnetic fields.
• Dynamic response is limited mechanically and electrically.
• Sensitive to temperature.
Capacitive type transducers
o Displacement sensitive transducer.
o Motion changes capacitance between two plates.
o Suitable circuit is used to generate voltage.
o C = kA/d
Piezo-electric transducers
o Electric potential appears across certain faces of a crystal if the dimensions of the crystal are changed.
o Quartz, Rochelle salts, barium titanate are examples of piezo-elecric crystals.
o Piezo-elecric effect is reversible.
o Relation between force & deformation
ΔtF = YA -------
t
Resistance strain gauges
o Conductor is stretched, its resistance will change.
o Causes: Change in length, area, resistivity .
o Types of strain gauges
• Wire strain gauges
• Foil strain gauges
• Semiconductor strain gauges
ρLR = ------
A
ΔR/Ro Gauge factor = ---------
ΔL/L
Semiconductor strain gauges
o To have high sensitivity, gauge factor should be high.
o Semiconductor strain gauges are used when a very high gauge factor is required.
o They works on the piezo-resistive effect.
o Semiconducting materials such as Si, Ge are used.
o Thickness of wafer or filament = 0.05 mm.
o Substrate = teflon
o Gold leads
o With IC OP-AMP
o Advantages
• Very high gauge factor (about 130).
• Excellent hysteresis characteristics.
• Fatigue life more than 10^7 operations.
• Very small in size.
o Disadvantages
• Very sensitive to change in temperature.
• Poor linearity.
• More expensive.
• Difficult to attach.
Ionisation transducer
o Works on principle of development of voltage across two electrodesplaced in an ionised gas.
o Magnitude of voltage depends on electrode spacing and state ofbalance.
o Fig. - Moving tube type ionisation tran.
o Glass tube contains gasat reduced pressure.
o A D.C. voltage is developedacross internal electrodes ‘A’when tube is subjected to anelectric field.
o Gas in the tube gets ionised & D.C.Voltage is produced.
o Motion ‘x’ of the tube relative to fixed ext. electrodes varies the output Voltage.
Opto-electric al/ Photo-electrical transducers
o An optical transducer is light sensor i.e. it converts light beam into an electrical signal.
o Types: Photo-emissive, photo-conductive, photo-voltaic.
Photo-emissive transducer
o When light beam strikes
photo-emissive cathode,
electrons are emitted.
o A & K are enclosed in
enclosure that is either
evacuated or filled with
inert gas.
o Current is proportional to intensity of radiation.
o High gain is possible by using photomultipliers.
o Advantages
• High frequency response.
• High sensitivity.
o Disadvantages
• Large in size.
• Need 300 V to 2500 V for their operation.
• Expensive.
Photo-conductive transducer
o Photoconductive materials – PbS, InSb changesresistance due to change in intensity of incident light.
o When a cell is in darkness, dark resistance of theorder of 10^12 Ω.
o If a cell is illuminated, its resistance decreases and hence current incircuit increases.
o Current dependson the intensity of incident light.
Photo-voltaic transducer
o Photo-voltaic transducer (Solar cell) generates voltage due to incident light.
o Sandwich construction – metal base plate 3 semiconductor material layer 2 thin transparent metal layer 1.
o When light incident on solar cell, voltage is generated due photo-voltaic effect.
Main advantages
o Ability to generate voltage without any bias.
o Fast response.
Digital transducers
o Advantages
• Digital computers can be used along with transducer for data manipulation
• Digital signals are easy to transmit without distortion and external noise.
• In pulse counting accuracy is greater.
• So many advantages in presenting digital data
Frequency domain transducers
o The output is in the form of pulses or sinusoidal waveforms.
o Frequency of pulses is measure of the magnitude of the physical variable.
o Types of frequency domain transducers
(A) Electromagnetic frequency domain transducers
(B) Opto-electrical frequency domain transducers
(C) Vibrating string transducers
Electromagnetic frequency domain transducerso Used for measurement of speed of rotation.
o Fig. – The device consists of permanent magnet.
o Gear of ferromagnetic material is attached on shaft .
o As each gear passes in front of the magnet, gap length changes.
o This changes flux density & hence voltage pulse is induced inthe coil.
o Pulse frequency is measure of speed of rotation.
Opto-electrical frequency domain transducers
o Used for measurement speed of rotation of a shaft.
o Shaft has half dark and half shining portions.
o Every time later portion is in front of the light.
o The reflected light falling on the photo-electrictransducer gives an electric pulse as an output.
o Pulse frequency is measure of speed of rotation.
Digital Encoderso By the use of digital code, it is possible to identify the position
of movable piece in terms if binary number.
o The position is converted into a train of pulses.
o Types: optical encoder and resistive digital encoders.
o Optical encoder: Consists of system of coded tracks oftransparent and opaque sections.
o Photo-electric sensor & light source is placed on the two sidesof the sector.
o The displacement is applied to the sector.
o The amount of light falling on the photo-electric sensorchanges.
o This change carries the information about the location ofthe sector.
o Number of levels in encoder determines the accuracy.
o Advantages
• It gives true digital readout.
• No mechanical contacts → no problem of wear andtear and alignment.
o Disadvantages
• Light sources burn out.
Resistive digital encodero It consists of system of coded tracks of conducting and non-
conducting sections.
o Sliding contacts are used for making the contacts.
o Circuits of sliding contacts which come in contact with theconducting areas are completed.
o Advantages
• It is relatively inexpensive.
• It can made any degree of accuracy as desired.
o Disadvantage : Wear and tear of contacts causes error.
Shaft encodero It is mechanical converter: angular position → digital no.
o It consists of glass disk mounted on a shaft with a codingpattern printed on the disk.
o The pattern is made with ink that is opaque to infrared.
o The code to be produced is generated by concentric rings.
o The disk is placed between light source and detector.
Thank You
