Chapter 3 Measurement Devices in Electrical Engineering

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1 Bachelor Program Chapter 3 Measurement Devices in Electrical Engineering Prof. Rong-yong Zhao (zhaorongyong@tongji.edu.cn) Second Semester,

2 3.1 Introduction Zoom in on meas. Devices; Device nature depends on meas.objectives: Objective name Aim Requirement 1,Research Enlarge human insight Large range excellent linearity, good dynamic behaviour 2,Consumed quantities measurement Record the delivered quantities(power meter) Frequent calibration 3,Safety Ensure human and environment safety(nuclear radiation level) Reliability(fail- safe) 4,Verification Product meeting specifications Regular calibration of tolerance interval, gono-go meas. 5,Process control Corrections based on meas. Dynamic behaviour 2

3 Advantages of signal processing in electronic mode 1 Electronic circuits -----easily realize(signal amplification, high sensitivity, etc.); 2 Possible to minimize the load on the object; 3 Silent, wear-free and low power consumption; 4 Processing speed can be high; 5 Flexibility(different function with different structure); 3

4 Disadvantages of signal processing in electronic mode 1 Can not process high power signals for which one needs hydraulic signals; 2 Susceptible(sensitive) to environment disturbances: temperature, humidity, nuclear radiation, etc. 4

5 3.2 Input Transducers Mechanoelectric transducers Thermoelectric transducers Magnetoelectric transducers 5

6 Methods to reduce restrictions To reduce/eliminate individual restrictions(combine non-linear systems into linear sys.); Two methods for combining individual transduction principles into one single compound transducer; 1 Balanced configuration(d 0, y=0;p134,fig,3.1) 2 Feedback configuration(to reduce non-linearity of T 1, with a negative feedback by stable T 2 p135,fig3.2) 6

Balanced configuration Two identical transducers(same transfer character): 7 ) ( ) ( ') ( ' ': ); ( : x f x f y x f y T x f y T Maybe non-linear transfer function Taylor expansion:... 2 2 ) ( ) (.

7 Balanced configuration Two identical transducers(same transfer character): 7 ) ( ) ( ') ( ' ': ); ( : x f x f y x f y T x f y T Maybe non-linear transfer function Taylor expansion: ) ( ) (... ) (... ) (... ) ( x a x a x f x f x a x a x a a x f x a x a x a a x f x a x a x a a x f If f(x) does not contain uneven terms, single transducer: non-linear, the balanced transducers: linear sys.: y=0; In order to be immune the additive disturbances if S d =S d In order to be immune the multiplicative disturbances if disturbance coefficients: C d =C d

8 Fig 3.1(b) shows balanced capacitive transducer configuration; 8 V n d n S d x n n V V C C C C n n V V x d A C x d A C o o ' ' ' 0 ) ( Supply voltage determines the sys. sensitivity number of turns in the transformer

9 Negative Feedback configuration Purpose: to convert signal x to an output y; Preconditions: In Fig.3.2 Transducer T 1 : 1 Unacceptable large non-linearity; 2 Too large sensitivity to disturbance; 3 Conclusion: unsuitable to be used Transducer T 2 : 1 Capable of reverse/conversion, x=f(y); 2 Linear; 3 Not susceptible to disturbances 4 Requirements: T1,T2,A(amplifier)----large loop gain 9

10 Accelerometer feedback configuration(fig.3.2 b) Input: acceleration x; Voice coil to move Magnetic field Counteracting effect Negative feedback config Output: Voltage 10

11 Notes about feedback config 1 When the input range of transducer is small, the solution: series connection(relay config.) of several transducers; 2 Transducer reliability is problem, T1 Solution(redundant config.): using several transducers T2 to measure the same Quantity; No less than 2 transducers of n can Tn run well, the sys. can be detective; 11

12 12 Notes about feedback config. Linear transducer s dynamic behaviour can be described by its electrical analogue(fig.3.3 d); Electrodynamic transducer: Velocity of membrane Turn number Single turn length Constant induction V m 1/ nlb F 0 0 V nlb I Voltage Output voltage is proportional to membrane velocity Can cat as both input transducer(microphone) and output transducer(loudspeaker) Exerted force is proportional to current

13 3.2.1 Mechanoelectric transducers Several operation principles for mechanical quantities transducers: Resistive displacement transducer transducers Displacement transducers Velocity transducers Acceleration transducers Capacitive displacement transducer Inductive displacement transducer Optical displacement transducer Force transducers 13

angular displacement(rotation); 1) Resistive displacement transducer

14 14 1, Displacement Transducers To meas.linear displacement(translation); To meas.angular displacement(rotation); 1) Resistive displacement transducer Potentiometer transducer is popular; Sliding potentiometer---translation; Rotating potentiometer---rotation; Advantages: good accuracy, temperature coefficient

15 Disadvantages of potentiometer 1 Finite resolution of a turning potentiometer: R=x/ x; (except metal film potentiometer ); 2 Mechanical wear, chemical corrosion, leading to lower characteristic; 3 Nonlinearity: 4 Another type: size-depending displacement,force leads length, cross area, resistivity (Fig.3.4(a)) 15

16 Strain wire Resistance R R( A, l, ) Cross section area Conductor length When conductor is mechanically strained or compressed R R R( A, l, ) ( A l A 1 ( 2! A A l l l ) Higher order terms are neglected when A / A 1; l / l 1; / 1 Curvature at the point Resistivity ) R( A, l, ) 2 R( A, l, )... R( A, l, ) is small (1) 16

17 Subtracting R R( A, l, ) from both sides of equation (1), and d R R R dr da dl A l d Functions : A A( d), d d( l), ( l) R and l relation Conductor Diameter dr dr R R A d dl A d l R d l R A R dl l A d d l R dl dl l R R l l (2) Resistance sensitivity with length R S l 17

18 Sensitivity magnitude Electrical resistance Material constant c Circular cross section Poisson constant l A R d l l A c l 2 d 4 d l S R l 2 1 R Most metal: 2 S l c Conclusion :Metal strain gauge, dr R S dl l R l 2 dl l 18

19 R 5 Semiconductor : S l 2,piezoresistive effect To meas. Strain direction----angular combination of strain gauge; 6 Creepage effect and hysteresis (due to glue and carrier cause); To reduce creepage and hysteresis: thin glue, hard glue, large Young s modulus temperature; 7 Non-zero temperature coefficient To reduce non-zero temp coefficient, use same temp.coefficient material, carrier and compensaion (R1 meas.and non-zero,r4 same disturbance without stress)(fig.3.5(a)) 19

2), Capacitive displacement transducer Capacitance function C C( d, A, ) Electrode distance Three methods to realize a capacitive displacement sensor: changing d, A, (Fig.3.

20 2), Capacitive displacement transducer Capacitance function C C( d, A, ) Electrode distance Three methods to realize a capacitive displacement sensor: changing d, A, (Fig.3.6,p144) Facing area Dielectric Permittivity To reduce the edge effects: to shield electrode(fig.3.7(a),p146) To reduce the edge effects further: to shield electrode+ variable capacitor Cn+ balance bridge(fig.3.7(b),p146) 20

21 3), Inductive displacement transducers Possible variation to meas.: 1 To vary the self-inductance of single coil; 2 To vary the mutual inductance between two coils(fig.3.8(a)); 3 To vary the magnetic resistance by air gap(between yoke and armature) changing(fig.3.8(b)); Magnetic force is considerable; Differential transformer: variation of mutual induction between two coils(fig.3.9,p148) 21

22 4),Optical displacement transducers Encoder strip to measure translation(fig.3.10(a)); Rotary encoder to measure Rotation; Encoder Strip 1, strip : transparent and opaque bars; 2,light source: narrow light beam; 3, sensor: light sensors; 4, digital code determined by bars position; Disadvantages of binary code: 1, small displacement can lead several bits change simultaneously ; 2,finite resolution leads to -----same state(code) corresponding different position ; 22

23 Moiré /mwɑ:reɪ/ effect Can realize higher resolution; Moiré Pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different

23 23 Moiré /mwɑ:reɪ/ effect Can realize higher resolution; Moiré Pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes. Cross grating+measure grating=light and dark beams pattern----meas.grating moves horizontally, leading a grating move vertically grating up/2 adjacent grating lines A moiré pattern, formed by two sets of parallel lines, one set inclined at an angle of 5 to the other

24 2. Velocity transducers Classification: translation velocity and angular velocity; 1) Electrical signal frequency can be measured accurately------velocity Frequency meas. V= x*n/t= x*f f=v/ x; 1 x: equal distance between the marks on a disc or strip; 2 n: marks number; 3 Based on optical meas. 4 Resolution decreases as the speed decreases; 24

25 To meas. Low speed, x should be small; 2) Velocity Measurement based on differentiation and integration By differentiation Advantage: easy to design electronic circuit; Disadvantage : dx( t) v( t), dt ( t) d ( t) dt 1 Output signal discontinuity leads to large glitches in velocity signal; 2 High frequency noise and other disturbance are amplified by differentiation; 25

26 By integration v( t) t 0 a( t) dt v(0), ( t) ( t) dt (0) Advantages: performed by a simple analogue electric circuit; Disadvantages : 1 Output signal change very slowly; 2 Frequency character decreases as frequency increases at a rate of -6dB/octave; 3 Noise and disturbance are suppressed more; t 0 26

27 3) Inductive velocity transducers Meas. object velocity Magnetic flux changes Induced voltage(fig 3.11,p152): Induced electrical potential in a conductor Coil turns Magnet velocity Sensitivity, but non-linear V n i1 d dx i dx dt v In Fig. 3.11(a), magnet is stationary, and coil moves electro-dynamic transducer; In Fig.3.11(b), flow rate measment:strong magnetic field B+ conductive liquid with velocity v V=BLV n i1 d dx i vk( x) Variations in flow lead to non-linear Distance between two electrodes 27

28 3. Acceleration Transducers Force and mass relation: a F / While the additional mass of transducer housing influences the acceleration meas. result Measure Force Force transducers m 28

29 4. Force Transducers Mechanical force elastic body deformation/size change measured by displace sensor; Hooke s Law: Stress on a rod: Several types of spring in Fig.3.12(P155) (a) ring type, F x; (b) bourdon helix spring, F (c) Bourdon spiral spring, F (d)membrane spring, P x F A x E x Modulus of elasticity of the material 29

Piezoelectric Force Transducers Mechanical force deformation electrical polarization(different electrical charge on the opposite surfaces)

30 Piezoelectric Force Transducers Mechanical force deformation electrical polarization(different electrical charge on the opposite surfaces) piezoelectric(stress-electrical ) effect; Two kinds of materials: Crystal lattice lacking a center of symmetry(quartz); Material with symmetry disturbed by a strong electrical field; 30

31 Reverse effect: voltage piezoelectric material deformed Charge sensitivity: Q F S q depends on crystal material+orientation; Independent on crystal size; Voltage sensitivity: V SV F S Q CV q CS v Transducer capacitance S V Depends on transducer dimensions; 31

32 3.2.2 Thermoelectric Transducers Every temperature measurement: Energy(both extracted and supplied) from object heat exchange; Heat exchange meas. Errors; Heat exchange modes: 1 Heat conduction(stationary material, constant temp. sensor); 2 Heat conversion(moving mass, gas, flow. etc.) 3 Heat radiation(by the infra-red electronagetic radiation); 32

33 Four conversion principles Temp. meas. is dependent of electrical component; Thermocouples (two metals); Heat radiation Indirect temp. meas. : temp. mechanical quantities(crystal resonance frequency) electrical quantity(oscillator frequency) used in a quartz crystal thermometer; 33

34 Resistive temperature sensor Any material resistance depends(to a certain extent) on the temperature; Types: 1 Metal thermometer(pure metal) platinum and nickel; 2 Semiconductor thermometer 34

35 Values for various popular resistance thermometers Values for various popular resistance thermometers Temperatur e in C Pt100 in Ω Pt1000 in Ω PTC in Ω NTC in Ω NTC in Ω NTC in Ω NTC in Ω NTC in Ω Typ: 404 Typ: 501 Typ: 201 Typ: 101 Typ: 102 Typ: 103 Typ: 104 Typ: ,31 803, ,29 822, ,16 921, ,12 941, ,09 960, ,04 980, , , , , , , , , , , , , , , , , , ,4 35

36 Resistance thermometer wiring configurations 36 The simplest resistance thermometer configuration uses two wires. In order to minimize the effects of the lead resistances a three wire configuration can be used. The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured.

37 37 IC temperature sensors Principle: fundamental band gap voltage of silicon depends on temperature Integrated circuit(ic): includes two bipolar transistors, close together; The silicon bandgap temperature sensor is an extremely common form of temperature sensor (thermometer) used in electronic equipment. VOUT Supply voltage:2.4v ~ 5.5V; Meas.temp.: -55 C ~ 130 C; VOUT = ( T 2 ) + ( T) V VCC GND

Junction potential depends on Metals nature+ absolute

38 Thermocouples Two metals, atomic contact to generate electrical potential difference junction potential; Junction potential depends on Metals nature+ absolute temperature; Thermocouple: two junctions connected(fig.3.16(a)); 38

39 Physical effects on output voltage 1 Seebeck effect potential difference depends on temperature desired; 2 Peltier effect: curent flow junction temp. change current direction junction warmer or cooler temp.error undesired; 3 Thomson effect: metal conductor temp. gradient generate heat or extract heat temp error undesired; 4 Joule heat: thermocouple heat-self up undesired 5 Moisture: galvanic cell voltage undesired should be waterproof 39

40 Radiation Thermometer Principle: a radiation thermometer absorbs some infra-red radiation emitted by measurement object; Application : high temp.meas. pyrometer Fig 3.18(a): a concave mirror focus the light thermal detector; 40

41 How to select the temperature sensor 41

42 3.2.3 Magnetoelectric Transducers To meas. Magnetic induction B; Dimension: magnetic induction B T(telas) Magnetic flux density Wb/m 2 1T=1 Wb/m 2 ; Induced AC voltage Rotating coil Aear V AC n d dt nb n A cos t Static magnetic field + rotating coil 42

43 Another magnetometer Principle( magnetoresistive effect ): magnetic field electrical resistance; -in 1856, W. Thomson discovered: exposing of current conductor to a magnetic field causes the changing of its resistance; Hall effect explain magnetoresistive effect occurring in semi-conductor; principle: Lorentz force solid charge carriers; F L q( v B) Deflect charge carriers Electrical field E 43

44 Opposite force: Fe qe At a certain point(fig.3.19,p170), F L F e E V B Current density J nqv Charge carries sum Charge velocity Charge V J nq E J nq B V 1 nq IB d R H IB d Hall coefficient 44

45 In semiconductor 1 P-type semiconductor Charge carrier is holes q is positive; 2 N-type semiconductor Charge carrier is electrons q is negtive; In fact, charge carriers: not same velocity collisions Hall coefficient R Hreal (0.8~1.2)R htheory To explain magnetoresistive effect: short circuit voltage terminals creat a current Hall voltage 45

46 Other explanation: voltage short electric field E=0 charge carrier deflection more collisions resistance ; To realize short circuit two ways: 1 Disc-shaper sensor(corbino disc); 2 Many Hall platelets in series connection; Magnetoresistive sensor: field plate, Gauss plate; Alternative field voltage inductive zerooffset error minimized by twisting wires; 46

47 Hall plate can measure: 1) static magnetic field; 2) suitable high frequency measurement can measure Large DC current(fig.3.22,p173) two two plates to eliminate disturbance 47

48 3.3 Signal conditioning Aim: to make signal suitable for display,record, or control; Linear signal conditioning : 1 Attenuation: gain <1 2 Amplification: gain>1 3 Compensation Non-linear signal conditioning 1 Peak value determination, 2 RMS value Signal conversion: 1 Sampling; 2 analogue-to-digital conversion ; 3 digital-to-analogue conversion; 48

Two precision laboratory calibrated 30dB RFattenuators, DC-18GHz, Agilent

49 Attenuators An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform. Two precision laboratory calibrated 30dB RFattenuators, DC-18GHz, Agilent type 8491B with N-type coaxial connectors Coaxial Dynamics 100 Watt power attenuator. 49

50 Attenuators Deal with too large signal; To prevent signal distortion and information loss; Attenuator shifts signal range to higher signal levels; Realizing solution: 1 Resistor network(resistive attenuators) 2 Inductive attenuators (transformers) 3 Capacitive attenuators Deal with large signal 50

51 Input attenuators At sys. Input, easy to attenuate signal; To attenuate Voltage signal series impendance with input impedance Zseries Z input To attenuate current signal parallel impedance with input impedance Z parallel Z input 51

52 Voltage and current attenuators Voltage transfer Vi v V o Z Z series input Z input To Filter high freq. Current transfer To eliminate freq. dependence. i I I i o Z Z parallel parallel Z input 52

53 Voltage dividers Low-impedance voltage source + highimpedance instrument input----voltage divider, potentiometer; 10 segment, 1K/segment 53

54 Kelvin-Varley voltage divider

55 Characteristic attenuators To deal with high-frequency or wide-band sys. to use characteristic attenuator; Driven by the signal; Realization : a number of cascaded T- sections(fig 3.26); Rc: characteristic resistance; Rv: source resistance; Rl: load resistance Advantage: can be immediately cascaded; 55

56 Measurement transformers High voltage + large current to use transformer; In theory, transformer relation: Turns ratio determines the voltage and current transfer V V I I n n 2 1 n n n n Turns ratio Voltages are in phase Currents are 180 out of phase 56

Note use of the grounded conductor as one leg of the primary feeder.

57 Single-phase and three phase transformer Pole-mounted single-phase transformer with center-tapped secondary. Note use of the grounded conductor as one leg of the primary feeder. Cut away view of three-phase oil-cooled transformer. The oil reservoir is visible at the top. Radiative fins aid the dissipation of heat. 57

58 Compensator networks Compensator: based on compensation network; Major advantages: without loading object, high accuracy; Human observer Non-automatic Controller/computer observer automatic 58

59 3.3.3 Measurement bridges Bridge method: to meas. or compare impedances; To generalize the impedance mechanical bridge, thermal bridge; Map: damp resistor(r=1/d), spring inductor(l=1/k), mass capacitor(c=m) 59

60 Two port network, 60 Schematic Linkages lines: Null condition: Z 1Z3 Z2Z4

3.3.4 Instrumentation amplifiers Application: signal is too small amplifier; Requirements : accurate, low-noise, lowdistortion, high sensitivity; Voltage

61 3.3.4 Instrumentation amplifiers Application: signal is too small amplifier; Requirements : accurate, low-noise, lowdistortion, high sensitivity; Voltage amplifier: R R v v A i o total R R R v v A i o total Amplifier internal Impedance: infinity current equation 2 1 R v i R v i o o i i R R v i R v i o o i i

62 Differential amplifier To select 4 resistors with relation: R2 R 1 R R ' 2 ' 1 A differential v v o i R R 2 1 R R ' 2 ' 1 62

63 3.3.5 Non-linear signal conditioning Aim: to meas. Amplitude characters of a periodic signal. 1), peak value; 2), average value; 3), RMS-value Peak detectors: based on rectifying(diode) circuit 63

64 Average amplitude detectors Average of the absolute value of AC amplitude; Realization: double-rectified circuit; Diode knee voltage:0.7 V 64

65 65 RMS amplitude detectors RMS: root of mean of the squares; Realization: a diode-resistor( function generator ) network, or by two identical thermojunctions; V output RI R n j1 V V i R aj j V output f ( V ) i To choose V j and R aj f() approaches a quadratic function f() approaches a squareroot function

66 An true RMS-to-DC converter 66

67 3.3.6 Digital-to analogue and analogue-to-digital conversion 67 Analogue output V DAC: Digital-to Analogue Converter : to map digital signal into an analogue signal; A digital value: Switch n i A V0D V0 a i 2 i0 Small, fixed, incremental voltage, Minimum step of V A D ( an, an 1,... a1a 0) Example: D=( )

68 DAC 68 n i R i R av I 1 Current sum: A IR t V Output Voltage: Resistors relation: R R R R R R R R n n n i i i n i i i t R A a V a R R V V Voltage corresponding switch a i =1

69 DA conversion by a resistor ladder network 69

70 Analogue-to-digital converter Employs automatic compensation+ DAC as the feedback to compensate the input voltage V A V c is the output Voltage from DAC V A > V c positive result switch counter up, otherwise switch down 70

71 ADC examples 71

72 3.4 Measurement Display Display: to perform meas. Result to a human observer; Information senses: sight(often), hearing(sometimes); Display definition: transducers which convert electrical signals into a visual signal; Display types: 1 Analogue/continuous display 2 Digital/discrete display; 1 Electro-mechanical device 2 Electro-optical device 72

73 Electro-mechanical displays Moving meter-convert signal into a angular rotation Principle : meter motion equation(section ) M J r 2 d dt M 2 da D M d i M d r M d dt Deflecting torque Cause pointer to deflect an angle and drive moment Md Four mechanical couples Restoring torque Damping torque Inertial torque The deflection opposing moment by spiral spring Deflection opposing moment Counteracting moment M r K r M M i da D r d dt 2 d J 2 dt To prevent pointer overloading Rotating inertia moment Angular acceleration

74 3.4.2 Electro-optic displays Mostly used; By emitting light or changing optical characters; Liquid crystal: organic substance, solid-liquid characters-three states: 1 Solid state when t T solid 2 Liquid crystal(liquid-solid) state when T liquid < t<t solid 3 liquid state when t T liquid 74

75 In liquid-solid state, Principle of LCD Molecules on the two opposite sides arranged at 90 Molecules realignment no light pass through An electrical field Molecules orientation varies Reflective

The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth. 3. Twisted nematic liquid crystal. 4.

75 75 In liquid-solid state, Principle of LCD Molecules on the two opposite sides arranged at 90 Molecules realignment no light pass through An electrical field Molecules orientation varies Reflective twisted nematic liquid crystal display. 1. Polarizing filter film with a vertical axis to polarize light as it enters. 2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth. 3. Twisted nematic liquid crystal. 4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter. 5. Polarizing filter film with a horizontal axis to block/pass light. 6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source.)

76 Light Pass or block without electrical field Add electrical field 76

77 Description of LCD The surface alignment directions at the two electrodes are perpendicular to each other; molecules arrange themselves in a helical structure, or twist; Applied voltage :liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray 77

78 Examples of LCD LCD alarm clock A general purpose alphanumeric LCD, with two lines of 16 characters. A subpixel of a color LCD 78

79 LED-Light Emitting Diode Electrons of the atoms are excited by means of recombination; A GaAs or Gap semiconductor diode will emit light when forwarded biased; Parts of an LED The inner workings of an LED 79

80 CRT-Cathode Ray Tube Four processes take place in a CRT: 1 Beam generation: heat +electrical field accelerate electrons 2 Focusing : project a small area to a dot on the CRT screen; 3 Deflection : deflect the electron beam with high freq.; 4 Conversion : electron beam to excite the phosphor layer, emit light; 80

81 CRT Structure 81

82 3.5 Recording Function : serve as a memory; Active information(energy dissipation, not suitable to store) Passive information(with energy supply, suitable to store) Solution: convert the active information to passive to store; Classes: 1 Analogue recording 2 Digital recording 1 Graphical recording( pen recorder ) 2 Magnetic recording( based on Hysteresis of magnetic layer-bh loop, tape recorder ) 3 Electro-optical recording( transient recorder ) 82

TRANSDUCERS transducer Measurand

TRANSDUCERS transducer Measurand TRANSDUCERS Transduction: transformation of one form of energy into another form. Sensing with specificity the input energy from the measurand by means of a "sensing element" and then transforming it into