T.Y.B.Sc. PHYSICS : PHY 354(B) Instrumentation II. M.D... · o Transducer element converts the...
Transcript of T.Y.B.Sc. PHYSICS : PHY 354(B) Instrumentation II. M.D... · o Transducer element converts the...
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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
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Introduction to Instrumentation
Functional Elements of Measurement System
Basic functional elements
Auxiliary elements
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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).
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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
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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
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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.
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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 -
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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.
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Formulation of system equation
Resistance thermometer connected to display unit
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Where
Therefore
Governing equation → first order → first order instrument
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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
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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
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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
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Impedance matching
o We want to deliver power from the device to the
external load
Therefore, Zi = Z
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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 .
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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.
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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.
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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.
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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.
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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.
~ ~ ~ ~ ~ ~ ~ ~
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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.
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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
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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
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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.
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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.
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Inductive type transducers
o Self generating and non-self generating types
o LVDT → non-self generating type
o Eo = Es1 ~ Es2
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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.
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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Thank You