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 another form by a "transduction element." The sensor-transduction element combination referred to as the "transducer". Measurand relates to the quantity, property, or state that the transducer seeks to translate into an electrical output. Classified: self-generating or externally powered. Self-generating transducers: develop own voltage or current while absorbing all the energy needed from the measurand. Externally powered transducers: need power supplied from an external source, (may absorb some energy from the measurand.) 1
TRANSDUCTION MECHANISMS Capacitive Thermoelectric effects (Seebeck and Peltier) Inductive and Electromagnetic Ionization effects Resistive Resistive and thermoresistive Photoelectric effect Piezoresistive effect Photoresistive effect Hall effect Photovoltaic effect Lateral effect Acoustooptic effect Extrinsic, interferometric and Fluorescence and fluorescence evanescent effects in optical fiber quenching Magnetoresistive effect Field effect Tunneling effect Doppler effect 2
MEASURANDS Displacement Atomic and surface profiles Position Gas concentration and ph Velocity ph and partial pressure of O 2 Acceleration and CO 2 in blood Force and load Infrared radiation Strain Torque Rotation and encoding Magnetic field Vibrations Acoustic fields Flow Medical imaging Temperature Non-destructive testing Pressure Audio fields and noise Vacuum Rotation and guidance 3
SELECTION OF TRANSDUCERS Cost Output impedance Sensitivity Power requirements Range Noise Physical properties Error or accuracy Loading effect and distortion Calibration Frequency response Environment Electrical output format 4
CLASSIFICATION OF TRANSDUCERS 5
Input Transducers: o Combining individual transduction principles into one single compound transducer. o Composite methods reduce or eliminate certain restrictions associated with individual transducers. o Balanced configuration: y = f(x) f(-x) 6
f(x) = non-linear transfer function (need to be linearized). f(x)= a 0 +a 1 x+a 2 x 2 + a 3 x 3 + a 4 x 4 + a 5 x 5. Using the balance equation: y = f(x) f(-x)= 2a 1 x + 2a 3 x 3 + 2a 5 x 5 + offset a 0 and even terms a 2 x 2, a 4 x 4, disappear. 7
Difference configuration improves system linearity over a limited range of the input quantity x. Feedback configuration 8
Problem: transducer reliability Use several transducers measure the same quantity: o Take average as long as the differences are in the tolerance. o Eliminate the output which differs substantially. o n-2 can malfunction before n transducers become defective. Redundant configuration improves the system reliability (increases cost). 9
MECHANOELECTRIC TRANSDUCERS Transducers measure mechanical quantities. 1. Displacement transducers Measure either linear displacement (translation) or angular displacement (rotation). Classified according to the principle of transduction. Referred to as gauges or sensors. 10
a. Resistive displacement transducers Measuring: translation (sliding potentiometer), rotation (rotating potentiometer). Wire-wound potentiometers (finite resolution). Maximal resolution R = x/ax is equal to the number of turns on the potentiometer body. Disadvantage: mechanical wear and chemical corrosion R=R(A,l,ρ) If the conductor is mechanically strained or compressed, the parameters A, l, and ρ, and as a consequence R, will change. 11
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b. Capacitive displacement transducers C=C(d,A,ε) Non-linear characteristics Linearized by using in balanced configuration. 13
The force F(x) causes the movement of sensor with a constant voltage across the capacitor (Q=VC(x)): 14
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c. Inductive displacement sensors Vary self-inductance (single coil) or mutual inductance between two coils. Varying inductance of a coil: o vary effective number of turns, o vary magnetic resistance (reluctance) of the yoke (air gap of variable width). 16
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d. Optical displacement sensors Detected optically: o Encoding strip (translation) o Rotary encoder (rotation). 18
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2. Velocity transducers Translational and angular transducers. Measure of velocity is converted to accurate frequency measurement. Conversion is performed by a strip or disc (detection elements) have been put at equal distances x. Velocity is calculated v= x.n/t = x.f Detection performed optically, mechanically, inductively or capacitively. 20
a. Measurement of velocity by differentiation or integration: Linear velocity v(t) v (t) = dx(t) dt x(t) = linear displacement Angular velocity ω(t) (t) = d(t) dt θ(t) = rotation angle. Velocity = derivative of a displacement sensor output. Simple to design an electronic circuit for differentiating an electrical signal. 21
Another possibility: integrating of the linear acceleration a(t) v(t) t = a(t)dt + 0 v(0) or of an angular acceleration α(t): (t) t = (t)dt + 0 (0) 22
b. Inductive velocity transducers: Velocity is made to give rise to a change of magnetic flux Φ, which induces an electrical potential in a conductor. The induced voltage in turn i of the coil of this inductive velocity pick up is given by: Terminal voltage of n turns coil: d = dt d V = i i= n dt ΦI=ΦI(x), x = position of the magnet with respect to the center of the coil, x=x(t): n d n = i dx d V = v i = vk(x) i= n dt dt i= 1 dx V i Output voltage V is proportional to the velocity v of the magnet for a given value of x. n i 23
Sensitivity of the transducer is equal to k. k=k(x), the transducer is non-linear (i.e., balanced configuration.) Since the magnet is moving, the velocity sensor is referred to as a magnetodynamic transducer. l V = Blv 24
3. Acceleration transducers Measurement of the force F required to give a known mass (the seismic mass m) the same acceleration, a, as the measurement object. From the force and the mass, acceleration is determined: a=f/m The extra mass has to be kept to a minimum, especially when the measurement object is highly elastic or has a low mass (extra mass influences the measure acceleration). 25
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THERMO-ELECTRIC TRANSDUCERS 1. Resistive temperature sensors a. Metal: Electrical resistance of any material depends to a certain extend on the temperature Convert a temperature measurement from a measurement of resistance. Material used, 2 kinds of thermometers: o metal thermometers o semiconductor thermometers. Pure metals resistance: R(T) = R(T0 )[1 + (T T0 ) + (T T0 ) + (T T0 ) +...] 2 If the temperature range is not too large, the first 2 terms of the expansion will suffice, the sensor is approximately linear. 2 27
The most frequently used metals are platinum and nickel. The measurement range of a platinum sensor runs from 70K to 1000K and nickel from 200K to 500K. At T 0 =273K: α Pt =3.85 x 10-3 K -1 α Nt =6.17 x 10-3 K -1 β Pt =-5.83 x 10-7 K -2 γ Pt =-3.14 x 10-12 K -3 28
b. Semiconductors: Number of free charge carriers depends on the absolute temperature. Higher temperature, larger number of electrons which cross the band gap from the valence band into the conduction band (intrinsic) or the larger the number of activated donor and acceptor atoms (extrinsic). Number of free charge carriers: n=n 0 e -Eg/2kT o E g = energy required for the crossing the band gap o k = Boltzmann s constant (1.3804 x 10-23 J/K). Resistance of a semiconductor decreases as temperature increases Semiconductor has a Negative Temperature Coefficients (NTC-resistor). Resistance of a semiconductor: R(T)=Ae B/T Coefficients A and B also depend on the temperature R(T) = R(T 0 )e B(1/ T 1/ T 0 ) 29
Highly non-linear. Temperature coefficient: (T) = 1 dr(t) R(T) dt = B 2 T Coefficient B lies between 2700K and 5400K (300K). At 300K, temperature coefficient ranges from 3 x 10-2 K -1 to 6 x 10-2 K -1. At 300K, a semiconductor sensor is more sensitive than a metal sensor. Semiconductor temperature-sensing resistor = thermistor. Improvement in the making of thermistors yields better linear characteristic. 30
2. IC temperature sensors An alternative temperature sensor in bipolar transistor. Sensor makes use of the fundamental band gap voltage of silicon, which depends in the temperature. 2 bipolar transistors, on the same IC, are biased to different collector current. If the ratio of the (collector) current densities (the transistors may have different areas) is equal to r, the difference between the base-emitter voltages of the two transistors is equal to (kt/q)ln(r). This base-emitter voltage difference is a linear measure of absolute temperature. Addition electronic circuits amplify this voltage to provide a practical output value. Typical IC sensor specifications are: range -55 o C to 150 o C, non-linearity over the entire range of approximately 0.3K, sensitivity 10mV/K or 1uA.K, instability over 1000hr of operation ±0.08K, dissipation 1.5mW to 3mW. 31
3. Thermal couples Electrical potential difference is generated when 2 different metals are brought into atomic contact with each other. Junction potential depends only on the nature of the two metals and on the absolute temperature. Surface area of the junction has no influence. Many combinations of metals provide linearly proportional potential difference to the absolute temperature of the junction, (not too large range of temperature). Thermocouple: net thermo voltage V will result if the two junctions are at different temperatures. 32
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Thermovoltage is a measure of the temperature difference between the two junctions. Output voltage of a thermocouple: n increases, more accurate. Thermocouple characterized by its own series of temperature independent coefficients a i (i = 1,..., n). An inaccuracy of ±l% requires roughly eight coefficients (n=8) for most materials. The coefficient a 1 is referred to as the Seebeck coefficient. Large temperature range, use more than one coefficient for accuracy. 34
Physical effects contribute to thermocouple voltage: -Seebeck effect. Desired effect Arises from the temperature dependence of the junction potential difference. Originates from the difference in Fermi levels of two dissimilar metals. The higher the temperature, the larger the number of electrons with a higher energy level than the Fermi level. This causes the junction potential difference to become temperature dependent. 35
-Peltier effect. Junction temperature changes when current flows Warmer or cooler than ambient depends on the direction of the current. Electrical conduction transportation of heat. Thermal conduction, electrical conduction caused by free electrons. Undesirable effect in a thermocouple (temperature error) -Thomson effect. Occurs in uniform metal conductor Heat generated in negative temperature gradient, Heat extracted from the conductor in positive t O gradient Undesirable effect (error) 36
-Joule heat. Current flows, I 2 R joules is dissipated. Thermocouple heats up itself. Undesirable effect (error) No current may flow through for accurate measurements The measurement circuit must have high input impedance. 37
- Other considerations: Moisture creates galvanic element with both metals and generates a galvanic cell voltage (waterproof case). Measure absolute temperature, hold one of the junctions at a fixed known reference temperature (achieved by controlling the temperature of one of the junctions with a thermostat). Compensate the temperature of the reference: 38
4. Radiation thermometers Radiation thermometer absorbs a fraction of the infrared radiation emitted by the measurement object. Pyrometer: radiation thermometer for high temperatures 39
Other pyrometers are based on quantum detectors. (electrons of the material being excited by infrared radiation). E = hf = 0 0 0 Crystal thermometer based on the temperature dependence of the resonance frequency of a piezoelectric crystal. hc 40
MAGNETOELECTRIC TRANSDUCERS Magnetometers or magnetic field sensors measure induction of a magnetic field Magnetic induction B is expressed in teslas (T) (magnetic flux density). Tesla = 1 Weber per square meter 1T = 1Wb/m2 Flux Φ through the coil: Φ = B n A sin θ(t), o (θ) = instantaneous angle between the coil and B n. o θ(t) =ωt, induced ac voltage: V = n d dt = nb n cos(t) 41
Determine B n from this expression. Induction sensors always require a changing flux (arise from an alternating magnetic field in a static coil). Another type of magnetometer is based on the influence of a magnetic field on the electrical resistance of a material. Exposes a current conducting body to a magnetic field the electrical resistance changes (magnetoresistive effect) Hall effect sensors 42
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Result from Lorentz force, exerted on charge carriers in solids. F l =q(vb) V = 1 nq IB d = R H IB d R H = Hall coefficient Hall coefficient is large for semiconductors (concentration n of charge carriers is much smaller in semiconductors than in metals). 44
Transducer Selection Begin with specification of physical quantity to be measured! " #$ Cost, accuracy, duration, cyclic behavior Environment, calibration,... 45
1. MEASUREMENT REQUIREMENTS Range: A set of values to be measured Adjust to cover the whole range, may need to use more than one transducer. Input threshold Smallest detectable value Dynamic behavior How the transducer response to a changing input Impossible to have instantaneous response Specified in frequency response or response time 46
Accuracy and resolution The difference between measured and accepted values Detect small change Proportionally increases with cost Causes: fatigue life (pressure transducers), non-linearity, drifting due to aging, long term repeatability Repeatability: Repeatability and hysteresis errors o max. difference between consecutive measurement approached from the same direction Hysteresis errors: o max. difference between consecutive measurement approached from the different direction 47
2. OPERATIONAL AND ENVIRONMENTAL CONSIDERATIONS Natural hazards Ability to withstand the effects of exposure to the required environment Dust, dirt, high/low temp., humidity, chemicals,... Must present no hazard to the environment (electrical spark) Human-caused hazards Hazardous chemical, vibration,. Electrical environment (noisy) Power requirements AC, DC required? Power leads? Noise problem 48
Signal-conditioning requirements Amplification or signal conditioning may be required Signal may need to be converted into suitable formats Space requirement Physical requirements Loading effects Mechanical loading (rotating-vane flowmeter) Electrical (high Thevenin resistance transducers) Human factors Operating skill of the installers, operators, service technician,... 49
3. CALIBRATION REQUIREMENTS Calibration interval Precise prediction of the transducer response? 50
BRIDGE MEASUREMENT Circuit detect small change (Wheatstone), high sensitivity Need a stable source of supply Can be impedance Z 51
Instrumentation amplifier detects balance condition 52
Need high common mode rejection (>100dB), stable gain, low-drift instrumentation amplifier 53