S. Eswar Prasad, Adjunct Professor, Department of Mechanical & Industrial Engineering, Chairman, Piemades Inc, Piemades, Inc.

Transcription

1 Lecture 1: Introduction to Smart Materials and Systems Lecture 2: Sensor technologies for smart systems and their evaluation criteria. Lecture 3: Actuator technologies for smart systems and their evaluation criteria. Lecture 4: Piezoelectric Materials and their Applications. Lecture 5: Control System Technologies. Lecture 6: Smart System Applications. S. Eswar Prasad, Adjunct Professor, Department of Mechanical & Industrial Engineering, Chairman, Piemades Inc, Piemades, Inc. 1

2 Precision sensors for measurement of strain, displacement and acceleration S. Eswar Prasad, Adjunct Professor, Department of Mechanical & Industrial Engineering, Chairman, Piemades Inc, Piemades, Inc. 2

3 Sensors for Smart Systems Sensors? Physical Principles - How they work? Selection of sensors and evaluation criteria Examples 3

4 Need for Sensors Sensors are pervasive. They are embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications. Without the use of sensors, there would be no automation! 4

5 Sensors for Smart Systems Need for sensors determine the load on the structure forces acting on the body nature of vibrational excitations magnitude of displacements to be controlled 5

6 Sensors - Definition American National Standards Institute A device which provides a usable output in response to a specified measurand. Sensors are devices that produce an output signal for the purpose of sensing a physical phenomenon. Sensors are also referred to as transducers. A transducer is a device that converts a signal from one physical form to a corresponding signal that has a different form. Quantities at the input level are different from the output level. Generally the output is in the form of an electrical signal. Sensors are used for measuring and recording a quantity. The measured quantity can be just recorded or further processed for controlling a system. 6

7 Types of Sensors : Analog, Digital, Active, Passive Analog: Output is continuous, output is a function of input. Requires ADC for interfacing. Digital: The output is in the form of a digital signal. Can be directly connected to a computer. PWM, serial, parallel, etc. Active Sensors:Need separate power source to obtain the output. Passive Sensors: These are self generating in the sense that they produce (electrical) signals when subjected to the sensed quantity. Piezoelectric, thermoelectric, radioactive,... Sensor output is generally in the form of resistance change or voltage change or capacitance change or current change when input quantity is changed. Appropriate circuit is required to measure the above changes. 7

8 Sensors - Basic Characteristics Sensitivity: It is the ability of the measuring instrument to respond to changes in a measured quantity. It is the ratio of change of the output to change of the input. The sensitivity K is defined as the rate of change of the output (O) with respect to the input (I). I Transducer O Energy Source I - input, quantity to be sensed. O - output, signal which can be recorded. Sensitivity = O/ I For a linear sensor: O/ I = k = constant For a non-linear sensor: O/ I = K a1i + a2i 2 + a3i

9 Sensor Characteristics Quality Error & Non-linearity Static and Dynamic Characteristics Types 9

10 Quality of a sensor Resolution: It is defined as the smallest increment in the measured value that can be detected. Resolution is defined as the largest change in I that can occur without a corresponding change in O.! Accuracy: It is a measure of the difference between the measured value and the actual value. Generally, it is defined as percentage of actual value. Precision: Precision is the ability of an instrument to reproduce a certain set of readings within a given deviation. Repeatability: It is the ability to reproduce the output signal exactly when the same measured quantity is applied repeatedly under the same environmental conditions. Bull s Eye Target Plate a. High precision with poor accuracy b. Good average accuracy with poor precision c. High accuracy with high precision d. Poor accuracy with poor precision 10

11 Quality of a Sensor Resolution: It is defined as the smallest increment in the measured value that can be detected. Resolution is defined as the largest change in input (I) that can occur without a corresponding change in output (O).! O IR I 11

12 Quality of a Sensor Range & span: The range of input physical signals which may be converted to electrical signals by the sensor. Signals outside of this range are expected to cause unacceptably large inaccuracy.! Span is maximum value minus the minimum value of the input. Stability (drift)-it is the ability to give the same output when a constant input is measured over a period of time. Drift is expressed as a percentage of full range output. Dead band: It is the range of input values for which there is no output. Backlash: It is defined as the maximum distance or angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part. Hysteresis: Different outputs corresponding to a single value of the input. 12

13 Error and Non-Linearity Error: The discrepancy between the instrument reading and the true value is called error. Absolute error = measured value - actual value Relative error = absolute error / true value For many transducers a linear relationship between the input and output is assumed over the working range. Few transducers, however, have a truly linear relationship and thus errors often occur as a result of the assumption of linearity. Various methods are used for the numerical expression of the non- linearity error - End-range values - Best straight line for all values - Best straight line through zero point 13

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15 Non-linearity N(I) = O(I) - OIdeal(I) = O(I) - (KI + a) Maximum Non-linearity Text NMAX% = NMAX x 100 (OMAX - OMIN) 15

16 Error Bands It is often impractical to separate and determine nonlinearity, resolution and other such effects in these cases, non ideal performance is classified by one broad term: the error band Accuracy Generally defined as the largest expected error between actual and ideal output signals. h O(I) = Oideal ± h 16

17 Sensor Characteristics Static characteristics are the values given when steady state conditions occur. Input is not varying and output is constant. Output changes only due to drift. Dynamic characteristics refer to time varying signal with corresponding time varying output. Response time: time which elapses after a step input, when the sensor reaches the output corresponding to some specified percentage of its steady state value e.g. 95%. Time constant: This is 63.2 % of the response time. Rise time: Time taken for the output to rise to some specified percentage of the steady state output. From 10% to 90%. Settling time: This is the time taken for the output to settle to within some percentage e.g. 2% of steady state value. 17

18 Sensors for Smart Systems Smart System response can fall into five categories - mechanical, electrical, magnetic, thermal or chemical. Electrical response is the easiest to monitor and analyze. Discuss types of sensors - that measure mechanical response and temperature. Scope is limited to a few types of sensors for each type. 18

19 Commonly Detectable Phenomena Temperature Mechanical motion(displacement, velocity, acceleration, etc.) Optical Electrical Chemical Electromagnetic Biological Radioactivity 19

20 Examples of Physical Principles Used Amperes s Law A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer) Curie-Weiss Law There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behaviour Faraday s Law of Induction A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer) Photoconductive Effect When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor) 20

21 Sensor Principles Parameter Sensor Principle Displacement Potentiometer LVDT Capacitance Change of Resistance Electromagnetic Induction Variation of Capacitance Strain Strain Gage Piezoelectric Fibre Optics Change of Resistance Generator of Voltage Interferometers 21

22 Sensor Principles Parameter Sensor Principle Force and Acceleration Load Cells Piezoelectric Change of Resistance Generation of Voltage Temperature Thermistors Thermocouples Fibre Optics Change of Resistance Generator of Voltage Interferometers 22

23 Sensors for Smart Systems Displacement (proximity) Strain Force and Acceleration Temperature (Flowmeters not discussed in this lecture) 23

24 Displacement Sensors Resistive Potentiometers Inductive: Linear Variable Differential Transformer Proximity Sensors Non-contact Encoders Capacitive Sensors 24

25 Displacement (Proximity) Sensors - Potentiometers Potentiometers are variable resistance sensors. A wiper contact is linked to a mechanical shaft. Linear or rotational movement. Output is proportional to resistance and thence the position. 25

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27 Displacement (Proximity) Sensors - Potentiometers 27

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31 Displacement (Proximity) Sensors - Potentiometers Celesco 31

32 Displacement (Proximity) Sensors - Linear Variable Differential Transformer Popularly known as LVDT No mechanical wear Very accurate Linear output Expensive compared to potentiometers 32

33 Displacement (Proximity) Sensors - Linear Variable Differential Transformer 33

34 Primary excitation Secondary 1 Secondary 2 Secondary 1 - Secondary

35 Displacement (Proximity) Sensors - Linear Variable Differential Transformer Schaevitz 35

36 Non-contact Sensors - Inductive (Proximity) Sensors Detection of metallic objects in front of the sensors without physical contact. Useful in wet and dirty environments. Small range. Used in traffic signals, bicycle computers, exercise machines etc. 36

37 Non-contact Sensors - Inductive (Proximity) Sensors 37

38 Non-contact Sensors - Rotary Encoders Generates square pulses using photo or infra red cell arrangement. Convert mechanical movement into electrical signal. High resolution. Used in computer hard drives, CD/DVD etc. 38

39 Non-contact Sensors - Rotary Encoders Output! = 360/n where n is the number of segments on coded disc. 39

40 Non-contact Sensors - Capacitive Sensors Measure the change in capacitance. Convert mechanical movement into electrical signal. High precision. Metrology, multi-axis measurements, out of plane measurements. 40

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44 Non-contact Sensors - Capacitive Sensors Capacitance Area x Dielectric Constant Distance 44

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48 Sensors for Smart Systems Displacement (proximity) Strain Force and Acceleration Temperature 48

49 Strain Sensors Silicon Strain Gauges Piezoelectric Strain Gauges. Fibre-optic Strain gauges. 49

50 Strain Sensors Basic definitions 50

51 Strain Sensors Strain gages convert mechanical motion into electrical signal. A change in resistance, inductance or capacitance is proportional to the strain induced. Strain can be bending, torsional or poisson. 51

52 Measurements Using Strain Gauge Elements Stress Force (by measuring the strain of a flexural element) Position Pressure (by measuring strain in a flexible diaphragm) Temperature (by measuring thermal expansion of a material) The strain Gage has a finite size and thus a measurement reflects an average of strain over a small area. 52

53 Strain Sensors The metallic foil-type strain gage consists of a grid of wire filament bonded directly to the strained surface by a thin layer of epoxy resin. When a load is applied to the surface, the electrical resistance of the foil wire varies linearly with strain. The adhesive also serves as an electrical insulator between the foil grid and the surface. 53

54 Strain Gages 54

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56 Strain Gages are generally mounted on cantilevers and diaphragms and measure the direction of these. More than one strain gage is generally used and the readout generally employs a bridge circuit. 56

57 Strain Sensors Bending Strain (also known as moment strain) is based on bending produced by the applied force. Shear Strain is based on the angular distortion produced. Torsional strain is the ratio of torsional stress to the torsional modulus of elasticity. Poisson strain is defined as the negative ratio of the strain in the transverse direction to the strain in the longitudinal direction. 57

58 Basic Principle of Measurement The resistance of the foil changes when deformed. The connected metal foil grid lines in the active portion of the gage can be approximated by a single rectangular conductor. A is the cross sectional area; A = W x h ρ is the foil metal resistivity Total resistance R is given by R= ρ. L / A [1] 58

59 Basic Principle of Measurement The gage end loops and the solder tabs have a much larger cross-section tan the foil lines and thus have a smaller effect on the gage resistance. [1] ln R = ln ρ + ln L - ln A [2] Taking the differential dr = dρ + dl - da [3]! R ρ L A but A = W. h da = W dh + h DW A w h da = dh + A h dw W 59

60 Principle of Measurement - continued. Poisson s Ratio ν = -ε transverse ε axial and for most materials ν! 0.3 ε axial = dl/l ε 1 transverse = dh/h ε 2 transverse = dw/w dh/h = ν dl/l [5] dw/w = - ν dl/l [6] [4] da/a = -2 ν dl/l = -2 ν ε axial [7] 60

61 Principle of Measurement - continued. when the conductor is elongated, ε axial > 0 and T=thus based on [7] da/a < 0 and R increases. [3] dr/r = dρ/ρ + ε axial (1 =2 ν) dr/r = (1 + 2ν)ε axial + d ρ/ρ dr/r = 1+ 2ν + dl/l ε [8] axial ε axial The last term is the effect of strain on resistivity of material (piezoresistive efffect) and is typically constant over operating range of typical strain gage metal foils 61

62 Principle of Measurement - continued. Gage Factor G = (1 + 2ν) + dl/l εaxial So dr/r = G εaxial relates to change in resistance to strain. [9] The strain is determined on the surface of the loaded component in the Z direction of the gage long axis. G is typically 2 for metal wires or foil. It can be much larger for semiconductor wires. So in order to measure strain, the resistance change needs to be measured and is based on knowing R and G in advance,. The changes in resistance are measured using a Wheatstone Bridge. 62

63 Principle of Measurement - continued. There are two modes of operation of a Wheatstone Bridge. 1. Static Balanced Mode (used initially to balance the bridge) 2. Dynamic unbalanced mode(used to measure changes in resistance) Vi is the input voltage to the bridge and VAB Voltmeter with high input impedance R1: Strain gage (measure the change in resistance) R3: Precision potentiometer R2 and R4 are precision resistors i3 i2 i4 63

64 Principle of Measurement - continued. i2 i2 i3 i4 i4 Step1: Balance the bridge by changing R3 until the Voltage VAB is zero. Step 2: When the bridge is in balanced state, i1r1 = i3r3 [10] Assuming the current going through the voltmeter is essentially zero, then, i1 = i2 = Vi/(R1+R2) [11] i3 = i4 = Vi/(R3+R4) [12] 64

65 Static Balanced Mode - continued [10] Vin. R1 = R1+R2 Vin R3+R4. R3! R1 = R1+R2! R3 R3+R4! R1(R3+R4) = R3(R1+R2)! R1R4 = R3R2! R1 = R2R3/R4 [13] So R1 can be determined based on the known values of R2, R3 and R4 such that the bridge is balanced. 65

66 Static Balanced Mode - continued It is important to note that R1 is independent of input voltage Vin. The static balanced mode of operation can be used to measure an unknown resistance but usually balancing is used only as a preliminary step to measure changes in resistance. Changes in resistance are measured using the dynamic deflection operation mode. 66

67 Dynamic Deflection Operation Amplifier must be high input impedance type (e.g. instrumentation amplifier with a gain of 1) R 1 represents the strain gage R 4 - potentiometer V 0 = (V in - R 2 i 2 ) - (V in - R 1 i 1 ) [14] Vin = (R 2 + R 3 ) i 2 = (R 1 + R 4 ) i 1 [15] [14] V 0 = - R 2 V in + R 1 V in R 2 +R 3 R 1 +R 4 V 0 = V in ( R 1 - R 2 ) [16] R 1 +R 4 R 2 +R 4 Note that when the bridge is balanced, R 1 = R 2 R 4 /R 3 or R 1 (R 2 +R 3 ) = R 2 (R 1 +R 4 ) So when the bridge is balance, V 0 = 0 and R 1 has a known value. 67

68 Dynamic Deflection Operation When R1 changes value, the bridge is not balanced and the earlier equation cannot be used to relate changes in output voltage ( V0) to the change in resistance ( R1). 0 [16]!! V0 + V0 = Vin ( R1 + R1 - R2 )!!!! [17]!!! R1 + R1 + R4 R2 + R3 V0 = 0 since this is a deviation from the balanced mode. [17]!! V0 = R1 + R1 - R2!! Vin R1 + R1 + R4 R2 + R3!! ( V0 + R2 ) ( R1 + R1 + R4 ) = R1 + R1!! Vin R2 + R3!! R1 ( V0 + R2-1) = R1 - (R1 + R4 ) ( V0 + R2 )!!! Vin R2 + R3!!!! Vin R2 + R3! 68

69 Dynamic Deflection Operation! R1 = 1 - ( 1 + R4/R1) { ( V0/Vin) + (R2/R2+R3)} R1! V0/Vin + (R2/R2+R3) -1! R1 = {1 - ( V0/Vin) - (R2/R2+R3) } - ( R4/R1) { ( V0/Vin) + (R2/R2+R3)} R1! V0/Vin + (R2/R2+R3) -1! R1 = (R4/R1) { ( V0/Vin )+ (R2/R2+R3) } - 1" " " " [18] R1" 1- V0/Vin - (R2/R2+R3) By measuring the V0, we can estimate R, and therefore, get an estimate of the axial strain axial = ( R1/R)/G 69

70 R R+ R Strain gage R R - R For balanced mode V0 = (Vin - R i2) - (Vin - (R+ R) i1) V0 = R i1 - R i2 + R i1 Vin = 2 R i2 = (R+ R) i1 + (R- R) i1 = 2 R i1 V0 = R i1 = Vin. R!!! 2 R If we use four active gages placed such that R- R R+ R It can be shown that V0 Vin. ( R/R) which leads to improved sensitivity of the bridge. R+ RR R - R R 70

71 An example of a displacement sensor strain gage cantilever-type load transducer Voltage from balanced position is given by V0 = Vin ( R+ R1 - R2 )! R+ R1+R4 R2+R3 Neutral Axis If R1 = R2 = R3 = R4 = R R1 = R and R << R from balanced mode V0 = Vin ( R+ R - R )! 2R+ R1 2R R2 R1+ R1 V0 = Vin. R + 2 R - (R + R)! 2(2R+ R ) R3 R R4 V0 = Vin R = Vin. R 2 2R+ R 4 R Gage1 Suppose now the bridge involves two active gages, then the gages are subject to equal but opposite strain. Neutral Axis Gage2 71

72 The Strain Gage Load Cell When the cylinder is compressed - 2 strain gages are in compression - 2 strain gages are subject to tensile loading This produces a signal enhancement factor of 2 ( 1+ν) where ν is the Poisson s ratio. 72

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74 Strain Sensors Text Omega Engineering Strain gage installation methods 74

75 Dynamic Deflection Operations (Effect of wire leads) R1 is the strain gage. Bridge is balanced: V0 = 0 R1 = R2 R1+R4 R2+R3 R1 changes (R1 + R1) : R4 ( V0 + R2 ) - 1 R1 Vin R2+R3 R1 = R1 1 - V0 - R2 Vin R2+R3 εaxial = R1/R1 G 75

76 2-wire connection Two wire connection makes bridge sensitive to variations in load resistance RL. This circuit is not expected to work well. V0 represents changes in R1 and the 2 RLs. 76

77 3-wire connection R R2 R1 R R3 R4 V0 = Vin ( R1 - R2 ) R = 0 R1+R4 R2+R3 V0 = Vin ( R1+R / - R2 ) (R1+ R / ) + (R4+R / ) R2+R3 R1 = R1+R / R1+R4 (R1+ R / ) + (R4+R / ) R4 = R1 V0 = Vin ( 1 - R2 )!!!!! 2 R2+R3 When R1 is a constant and R1 = R4, the addition of the three wire connection has no effect on the bridge. 77

78 What is the effect of the addition of R / on the bridge if R1 varies and becomes R1 + R1? 0 V0 + V0 = V0 = Vin ( R1 + R1+R / - R2 ) (R1+ R1+R4+2R / ) R2+R3 V0 = R1 + R1+R / - R2 Vin (R1+ R1+R4+2R / ) R2+R3 ( V0 + R2 ) = R1 + R1+R / Vin R2+R3 (R1+ R1+R4+2R / ) (R1+R4+2R / ) ( V0 + R2 ) - ( R1 +R / ) = R1 ( 1 - V0 - R2 ) R1 = When R1 = R4 R1 = Vin R2+R3 Vin R2+R3 (R1+R4+2R / ) ( V0 + R2 ) - ( R1 +R / ) Vin R2+R3 ( 1 - V0 - R2 ) Vin R2+R3 (R1+R / ) [ 2( V0 + R2 ) - 1 ] Vin R2+R3 ( 1 - V0 - R2 ) Vin R2+R3 So even for R1 = R4, R / remains in the R1 expression. Therefore R / effects the bridge measurements. R1 is selected so that it does not exceed 0.1% of the nominal gage resistance. 78

79 Effect of Temperature Temperature changes in the actual strain gage can cause large changes in resistance which would lead to errors in the measurements. To be bonded Vin To overcome this, a half-bridge circuit is used where two of the four bridge legs contain identical strain gages. The dummy gage is made of unstressed material of the same composition and at the same temperature and is identical to the active gage. The resistance changes in the two gages due to temperature will cancel since they are adjacent branches of the bridge circuit. The bridge will generate an unbalanced voltage only in response to a strain in the active gage. 79

80 Sensors for Smart Systems Displacement (proximity) Strain Force and Acceleration Temperature 80

81 Force and Acceleration Sensors The operating principles force and acceleration are very similar and most often the specifics of configuration determine the output. 81

82 Force and Acceleration Sensors What is an accelerometer? A sensor that measures acceleration based on Newton s second law of motion The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. or, mathematically, F = m a 82

83 Force and Acceleration Sensors Trossen Robotics 83

84 Force and Acceleration Sensors 84

85 Force and Acceleration Sensors - Piezoelectrics Piezoelectric elements are bi-directional transducers capable of converting stress into an electric potential and vice versa. They consist of metallized quartz or ceramic materials. One important factor to remember is that this is a dynamic effect, providing an output only when the input is changing. This means that these sensors can be used only for varying pressures 85

86 Omega Force and Acceleration Sensors - Piezoelectrics 86

87 Sensors for Smart Systems Displacement (proximity) Strain Force and Acceleration Temperature 87

88 Temperature Measurement Bimetallic Strips Fluid Expansion Devices Change-of-State Temperature Measurement Thermocouples (TCs) Resistance Temperature Devices (RTDs) Infrared Devices 88

89 Temperature Measurement - Bimetallic Strips Two sheets of metals, usually brass and steel or their alloys, with different coefficients of thermal expansion are bonded together. The resulting bimetallic strip bends when heated. This phenomenon has many applications, including thermal switches and thermometers. 89

90 Temperature Measurement - Bimetallic Strips 90

91 Temperature Measurement - Bimetallic Strips 91

92 Temperature Measurement - Fluid Expansion Devices Fluid-expansion devices, typified by the household thermometer, generally come in two main classifications: the mercury type and the organic-liquid type. Other types may also contain gas instead of liquid. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable after repeated cycling. They do not generate data that are easily recorded or transmitted, and they cannot make spot or point measurements. 92

93 Temperature Measurement - Change-of-State Measurement Devices Change-of-state temperature sensors form a broad category of sensors consisting of labels, pellets, crayons, lacquers or liquid crystals. The appearance of the surface of these devices changes once a certain temperature is reached. Typical applications are traps - when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. 93

94 Temperature Measurement - Change-of-State Measurement Devices Response time typically takes minutes, so these devices often do not respond to transient temperature changes. Accuracy is lower than with other types of sensors. The change in state is irreversible in most cases. Change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons during product shipment. 94

95 Temperature Measurement - Thermocouples A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire. 95

96 Temperature Measurement - Thermocouples The thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in The voltage is nonlinear with respect to temperature (but for small changes in temperature it is approximately linear) The voltage is given by V S T!(1) where V is the change in voltage, S is the Seebeck coefficient, and T is the change in temperature. 96

97 Temperature Measurement - Thermocouples Several standard types that are given designations according to the materials used. These thermocouples use a variety of different materials. The ones used in the thermocouples mentioned above are all forms of metal alloys: Alumel! Nickel 96%, manganese 2%, aluminum 2% Chromel Nickel 90%, chrome 10% Constantan! Copper 55%, nickel 45% Nicrosil! Nickel chrome silicon Nisil! Nickel silicon 97

98 Temperature Measurement - Thermocouples 98

99 Temperature Measurement - Thermocouples 99

100 Temperature Measurement - Thermocouples Cold-Junction Compensation in Thermocouples Thermocouples require some form of temperature reference to compensate for room temperature. The term cold junction comes from the traditional practice of holding this reference junction at 0 C in an ice bath. The National Institute of Standards and Technology (NIST) thermocouple reference tables are created with this in view. David Potter, National instruments 100

101 Temperature Measurement - Thermocouple Colour Codes 101

102 Temperature Measurement - Resistance Temperature Detectors (RTDs) A resistance-temperature detector (RTD) is a temperature sensing device whose resistance increases with temperature. An RTD consists of a wire coil or deposited film of pure metal. RTDs can be made of different metals and have different resistances, but the most popular RTD is platinum and has a nominal resistance of 100 Ω at 0 C. 102

103 Temperature Measurement - Resistance Temperature Detectors (RTDs) Why RTDs? RTDs are known for their excellent accuracy over a wide temperature range. RTDs have accuracies as high as 0.01 Ω (0.026 C) at 0 C. RTDs are also extremely stable devices. Common industrial RTDs drift less than 0.1 C/year, and some models are stable to within C/year. 103

104 Temperature Measurement - Resistance Temperature Detectors (RTDs) R t = R 0 * (1 + A* t + B*t 2 + C*(t-100)* t 3 ) Where: R t is the resistance at temperature t, R 0 is the resistance at 0 C, and A= E-3 o C -1 B = E-7 o C -2 C = E C -4 (below 0 C), or C = 0 (above 0 C) 104

105 Temperature Measurement - Non-contact Measurement Fibre optic thermometers have proven invaluable in non-contact measurement of temperatures, particularly in harsh conditions, such as high temperatures, large electric and magnetic fields etc. Typical applications are: Basic metals and glass production and initial hot forming processes for such materials. Boiler burner flames and tube temperatures Critical turbine areas in power generation operations Rolling lines in steel and other fabricated metal plants Automated welding, brazing and annealing equipment Fusion, sputtering, and crystal growth processes in the semiconductor industry. 105

106 Temperature Measurement - Non-contact Measurement Fibre-optic sensors can be used to detect heat or stress. Two types of fibre-optic sensors are used, intrinsic and extrinsic types. In the extrinsic type, fibre acts as a medium of transmission. The light exits and interacts with the environment to be analyzed and then re-enters the fibre. This is a low cost method and can use photodiodes for the operation. In the intrinsic type, one or more field parameters become modulated with the field which propagates in the fibre to allow the measurement of environmental effects. Generally these techniques involve interferometric methods and can detect both strain and temperature fluctuations. 106

107 Temperature Measurement - Non-contact Measurement Text The Mach Zehnder interferometer is a device used to determine the relative phase shift between two collimated beams from a coherent light source. It splits an optical signal into two components and directs them down two separate paths, then recombines them. By inducing a phase delay between the two optical signals, the resulting interference can cause intensity changes. 107

108 Temperature Measurement - Non-contact Measurement The Fabry Pérot interferometer makes use of multiple reflections between two closely spaced partially silvered surfaces. Part of the light is transmitted each time the light reaches the second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like the multiple slits of a diffraction grating increase its resolution. 108

109 Sensor Evaluation Criteria Sensor Characteristics Environmental Factors Economic Factors Sensitivity Range Precision Resolution Accuracy Offset Linearity Hysteresis Response Time Dynamic Linearity Size Power Consumption Ruggedness Temperature Range Corrosion Sensitivity to humidity Over range Protection Self Test / Self-calibrate Immunity to EM Interference Cost Availability Service Life Joseph Carr, John Brown, Introduction to Biomedical Equipment Technology, National instruments Sookram Sobhan, Introduction to Sensors,

110 Selection of Displacement Sensors Performance Parameter Potentiometer LVDT Sensitivity N/A 110 mv/mm Range 25 mm mm Resolution 0.1 mm N/A 110

111 Selection of Strain Sensors Performance Parameter Strain Gage Piezoelectric Fibre Optics Sensitivity High Moderate Moderate Resolution Moderate Moderate High Range Moderate High High 111

112 Selection of Acceleration Sensors Performance Parameter Load Cell Accelerometer Sensitivity N/A 110 mv/mm Range 50 g ± 50 g Resolution 0.2 fsd g pk 112

113 Selection of Temperature Sensors Performance Parameter Thermocouple J-type K-type RTD Sensitivity 61 mv/ 0 C 40 mv/ 0 C ±0.20C Range C to C C to C C to C Response Time <5 s <5 s 0.5 s 113

114 Intelligent Sensor Systems Compensation Self-diagnostics, self-calibration, adaptation Computation Signal conditioning, data reduction, detection of trigger events Communications Network protocol standardization Integration Coupling of sensing and computation at the chip level n! Micro electromechanical systems (MEMS) Others Multi-modal, multi-dimensional, multi-layer n!active, autonomous sensing 114

115 Revised Thermocouple Reference Tables TYPE J Reference Tables N.I.S.T. Monograph 175 Revised to ITS-90 Extension Grade + Iron vs. Copper-Nickel Thermocouple Grade + MAXIMUM TEMPERATURE RANGE Thermocouple Grade 32 to 1382 F 0 to 750 C Extension Grade 32 to 392 F 0 to 200 C LIMITS OF ERROR (whichever is greater) Standard: 2.2 C or 0.75% Special: 1.1 C or 0.4% COMMENTS, BARE WIRE ENVIRONMENT: Reducing, Vacuum, Inert; Limited Use in Oxidizing at High Temperatures; Not Recommended for Low Temperatures TEMPERATURE IN DEGREES C REFERENCE JUNCTION AT 0 C C C C C Thermoelectric Voltage in Millivolts C C C C Z