Unit ii transducers
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This document discusses transducers and capacitive transducers. It provides definitions and examples of transducers, noting they convert one type of energy to another, usually electrical or mechanical. Capacitive transducers are described as working by varying capacitance through changes in plate area, plate separation, or dielectric constant between parallel plates. Mathematical equations show how capacitance varies with these factors and can be used to measure linear or angular displacement. Specific capacitor circuit examples are given to demonstrate measurement of linear and angular position using changes in overlapping plate area.
Unit ii transducers
- 1. BETL 305 ELECTRONIC MEASUREMENT & INSTRUMENTATION BY PRASHANT KUMAR ASST. PROFESSOR MITS GWALIOR
- 2. UNIT II TRANSDUCERS Transducer Block Diagram A transducer will have basically two main components. They are 1. Sensing Element The physical quantity or its rate of change is sensed and responded to by this part of the transistor. 2. Transduction Element The output of the sensing element is passed on to the transduction element. This element is responsible for converting the non-electrical signal into its proportional electrical signal. • There may be cases when the transduction element performs the action of both transduction and sensing. The best example of such a transducer is a thermocouple. • There are transducers which don’t require external Excitation • measurand=quantity under measurement
- 3. TRANSDUCER Defnition of transducer: A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute (eg displacement, change in velocity) to another (generally electrical or mechanical) for various measurement purposes including measurement or information transfer (for example, pressure sensors).
- 4. STATIC CHARECTERISTICS OF TRANSDUCERS1. Linearity Its input vs output characteristics should be linear and it should produce these characteristics in balanced way. (Linearity–can be defined as the variation in the constant of proportionality between the input physical quantity and the output electrical signal. A sensor or a transducer is therefore said to be linear when the constant of proportionality has the same value within the whole measurand range (i.e. when the graph relating input to output is a straight line)) 2. Ruggedness A transducer should be capable of withstanding overload and some safety arrangements must be provided with it for overload protection. 3. Repeatability The device should reproduce the same output signal when the same input signal is applied again and again under unchanged environmental conditions, e.g., temperature, pressure, humidity, etc. 4. High Reliability and Stability The transducer should give minimum error in measurement for temperature variations, vibrations and other various changes in surroundings.
- 5. CONTINUED....... 5. High Output Signal Quality (High SNR) The quality of output signal should be good, i.e., the ratio of the signal to the noise should be high and the amplitude of the output signal should be enough. 6. No Hysteresis It should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa. 7. Residual Reformation There should not be any deformation on removal of input signal after long period of use.
- 6. Resolution is the minimal change of the input necessary to produce a detectable change at the output Precision: The capacity of a measuring instrument to give the same reading when repetitively measuring the same quantity under the same prescribed conditions Precision implies agreement between successive readings, NOT closeness to the true value Two terms closely related to precision Repeatability : The precision of a set of measurements taken over a short time interval Reproducibility : The precision of a set of measurements BUT i) taken over a long time interval or ii) Performed by different operators or iii) with different instruments or
- 7. Hystheresis: The difference between two output values that correspond to the same input depending on the trajectory followed by the sensor (i.e., magnetization in ferromagnetic materials) The basic equation is y = mX + B, where X is the scaling or multiplier, and B is the offset. Using a little basic algebra we can apply this same set up to calculate scale and offset of any linear sensor or device with a linear output
- 9. ACTIVE TRANSDUCERS Active transducers are those which do not require any power source for their operation. They work on the energy conversion principle. ACTIVE TRANSDUCER PARAMETER 1.PEIZOELCTRIC TRANSDUCER Pressure/strain/displacement to Electric potential 2.THERMOCOUPLE Temperature to electrical potential (emf) 3. Electromagnetic Change in area/velocity /displacement to Induced Emf
- 10. EXAMPLE OF PRECISION & ACCURACY (SHOOTING DART)
- 11. STRAIN GAUGE MECHANICAL STRAIN = ELECTRICAL STRAIN ELECTRICAL OUTPUT = STRAIN GAUGE l l R R Wheatston e bridge R R GENERAL WORKING OF STRAIN GAUGE STRAIN: It is defined as extension or reduction per unit length l S l
- 13. The change in the value of resistance by the application of force can be explained by the normal dimensional changes of elastic material. If a positive strain occurs, its longitudinal dimension (x-direction) will increase while there will be a reduction in the lateral dimension (y-direction). The reverse happens for a negative strain. Since the resistance of a conductor is directly proportional to its length and inversely proportional to its cross sectional area, the resistance changes. The resistivity of a conductor is also changed when strained. This property is known as piezoresistive effect.
- 14. GAUGE FACTOR 2 resistance of unstrained gauge=R = .....( ) to find R we need to differentiate R w.r.t stress S ...............( ) both side by R = we get 1 1 1 L i A dR L L A L ii dS A S A S A x L dividing A dR L A R dS L S A S 1 ...............( )iii x As we observe it from eq (iii) The effective change in resistance w.r.t strain is due to three factors Per unit change in length Per unit change in area Per unit change in resistivity (Peizoresitivity) The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied L L A A
- 15. GAUGE FACTOR….. Continued.. 2 2 2. . 4 4 1 1 2 .2. . ......( ) 4 4 this value of eq (iv) in (iii) we get 1 1 2 1 poisson's ratio strain = / D A D Area D S S A D D D iv DA S S D S substituting dR L D R dS L S D S S now from Lateral Longitudnal axia starin . D D Ll L D L D L
- 16. 1 1 2 1 when 0 it can be approximated as 1 1 2 1 2 ........( ) factor = G eq (v) both side by get 1 2 1 f f f R L L R S L S L S S S R L L R S L S L S S R L L v R L L R RGauge L L L dividing L we G L L G 2 ( ) L strain S L
- 17. Continued….. 1 1 2 1 factor = G eq (v) both side by get 1 1 2 ( ) 2 f f f dR L L R dS L S L S S R RGauge L L L dividing L we G L L L G strain S L
- 18. Desirable characteristics in a material for strain Gauge 1.High Gauge factor 2.High resistivity 3. Low temp. sensitivity (Low temp coefficient of resistance) 4. Low Hysteresis 5. Linearity (Strain Vs Resistance should be linear) 6. High yield point
- 19. STRAIN GAUGE
- 20. A bonded strain gauge will be either a wire type or a foil type as shown in the figure below. It is connected to a paper or a thick plastic film support. The measuring leads are soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is connected to the elastic member whose strain is to be measured. Unbonded Strain Gauge is used at places where the Gauge is to be detached and used again and again
- 21. BASIC STRAIN GAUGE WORKING USING WHEATSTONE BRIDGE
- 22. A quarter bridge output corresponding to the application of a force is shown below. Initially, the circuit will be balanced without the application of any force. When a downward force is applied, the length of the strain gauge increases and thus a change in resistance occurs. Thus an output is produced in the bridge corresponding to the strain.
- 23. SEMICONDUCTOR STRAIN GAUGE For Semiconductor strain gauge material used is either n-type S.C or P- type S.C It is based on Principle of Piezo-resistivity Advantages : 1. High Gauge factor (>>130) 2. Very Low hystresis (<<0.05%) Disadvandage 1. Poor linearity 2. prone to error due to high sensitivity to temp.
- 24. COMPARISON BETWEEN SEMICONDUCTOR VS METALLIC GAUGE PARAMETERS METALLIC STRAIN GAUGE SEMICONDUCTOR STRAIN GAUGE GAUGE FACTOR LOW HIGH HYSTRESIS HIGH LOW LINEARITY HIGH POOR COST LOW EXPENSIVE
- 25. STRAIN GAUGE
- 26. PRINCIPLE OF WORKING OF S.GAUGE: Resistance of a conductor changes when it is strained. This property can be used to measure Pressure, Force and Displacement. APPLICATION OF STRAIN GAUGE: 1.TO measure pressure 2. To measure Displacement 3. To measure force
- 27. THERMISTORS
- 29. APPLICATION OF THERMISTORS 1. measurement of temp.(thermistors’s large change of resistance with temp. provides good accuracy and resolution) 2. temperature compensator in in electronic circuits (because of NTC) 3. measurement of power at high frequency 4. measurement of level of liquid/ flow of liquid 5. measurement of time delay
- 30. MATHEMATICAL ANALYSIS OF THERMISTOR 0 0 0 0 1 1 R=R exp .. eq ....( ) (i) R is the resistance of thermistor at the temperature T in K R is the resistance at given temperature T in K is the material specific constant differentiat i T T wher i g e n 2 0 0 w.r.t Temp. T we get log ln ln 1 1 dR R dT T taking weget R R T T 2 1 dR R dT T 2 1 dR R dT T
- 31. CAPACITIVE TRANSDUCERS • PRINCIPLE OF WORKING : A capacitive transducer works on principle of Capacitance of Parallel plate capacitor. • Capacitance of a parallel plate capacitor is given by • Capacitive transducer works on the principle that capacitance is varied by variation in any one of following parameters I. Change in effective plate area A /(overlapping area) II. Change in sepration between parallel plates III. Variation in dielectric constant 0 r A C d r
- 32. Change in capacitance can be converted into required electrical format using capacitor equation by capacitor eqution Q=CV......(i) differentiating both side we get ( )C C dQ dV dC C V dt dt dt dV dC dQ I C V I dt dt dt
- 33. VARIABLE AREA BASED CAPACITOR TRANSDUCER Capacitance of parallel plate capacitor is directly proportional to effective overlapping area between two plates. 0 r A C d C A C A C A Used to measure Linear displacement Used to measure angular displacement
- 34. LINEAR DISPLACEMENT MEASUREMENT USNG VARIABLE AREA BASED CAPACITOR 0 0 0 of overlapping part of plates of overlapping part of plates Sensitivity= F/m r r r A wl C C d d l length w width wC l d
- 35. ANGULAR DISPLACEMENT MEASUREMENT USNG VARIABLE AREA BASED CAPACITOR 0 2 2 0 2 0 2 2 F/radian 2 r r r A C d r A r C d C r Sensitivity C d
- 36. VARIABLE DISTANCE BASED CAPACITOR TRANSDUCER It is used for linear displacement/pressure measurement. 0 0 2 1 F/meter r r A C d C d AC sensitivity S d d
- 37. VARIABLE DIELECTRIC CAPACITIVE TRANSDUCERLiquid level measuremen t using dielectric variation Displacement measurement
- 38. ADVANTAGE OF CAPACITIVE TRANSDUCER 1) High sensitivity 2) Very small power requirement 3) Good frequency response 4) High input impedence so minimum loading effect DISADVANTAGE OF CAPACITIVE TRANSDUCER 1. Proper insulation of metallic parts required 2. Non linear behaviour due due to edge effect 3. Stray capacitance between metallic wires affect performance Applications: Measurement of i) force, ii) angular and linear displacement ,iii) moisture, iv) pressure v) water level indicator
- 39. THERMOCOUPLE It is temperature to electrical (emf) energy conversion based transducer. WORKING PRINCIPLE: Themocouple is based on three effects i) Seeback effect ii) Peltier effect iii) Thompson effect Seeback effect: when the temperature difference exists between junctions of two dissimilar metals the the thermo emf is generated across terminals. Peltier effect when electric current passes junction of two dissimilar metals heat is absorbed at one junction and liberated at another junction Thompson effects: when a homogeneous electrical conductor is subjected to a temperature gradient then a corresponding voltage gradient is
- 40. Materials used in thermocouple POSITIVE WIRE NEGATIVE WIRE Temp (degree centigrade ) COPPER CONSTANTAN -253 to 400 Iron CONSTANTAN -200 to 850 platinum Platinum(87%) +Rhodium(13 %) 0-1400 2 emf generated ( ) ( ) thermal E a T b T
- 41. Adavantage: High temperature measurement range Easy calibration Fast response Low cost Disadvantage 1. Reference junction is necessary 2. Non linearity
- 42. LVDT (LINEAR VARIABLE DIFFERENTIAL AMPLIFIER) WORKING PRINCIPLE: LVDT is based on variation in inductance with differential output. Inductance can be varied by variation in these three quantities I. Magnetic permeability of material II. Number of turns III. Geometric configuration Secondary winding are connected with opposite polarity to provide differential output 0 r A L l
- 43. CONSTRUCTION LVDT consists of a cylindrical transformer where it is surrounded by one primary winding (P1) on one side and two secondary windings (S1and S2 ) on the other side The number of turns in both secondary windings are equal, but they are connected such that both have opposite polarity. The primary winding is connected to an AC source. A soft iron core moves within the hollow cylindrical core and changes the magnetic flux linked with the primary and two secondary windings.
- 44. WORKING Case 1: when iron core is at centre( null position) E0=0, (Es1=Es2) because the flux linking with both secondary windings is equal, hence equal emf are induced in them. (Es1 = Es2 ) ( E0=Es1-Es2) Case 2: When the core is moved to the left of null position (O - A) more flux links with winding S1 and less with winding S2. Hence, output voltage is Es1 is greater than Es2. The output voltage is positive and in phase with input signal. Case 3: When the core is moved to the right of null position (O- B) more flux links with winding S2 and less with winding S1 Hence, output voltage Es2 is greater than Es1.The output voltage is negative and 180o out of phase with input signal.
- 47. .. Adavntage of LVDT 1. high linearity 2. high sensitivity 3. very low hystresis 4. low power consumption Disadvantages 1.large offset(threshold) 2. easily affected by stray magnetic field Applications: 1. Linear displacement measurement 2. Measurement of pressure, tension,
- 48. PEIZOELECTRIC TRANSDUCER Working principle: The main principle of a piezoelectric transducer is that a force, when applied on the quartz crystal, produces electric charges on the crystal surface. The charge thus produced can be called as piezoelectricity. Piezo electricity can be defined as the electrical polarization produced by mechanical strain on certain class of crystals. The rate of charge produced will be proportional to the rate of change of force applied as input. As the charge produced is very small, a charge amplifier is needed so as to produce an output voltage big enough to be measured