Measurement of non-electrical quantities

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Dwight Watson
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1 Measurement of non-electrical quantities

2 Measurement chain analog part digital part ef bias conditioning circuitry ef µc DSP SOC... C/A sensor A/C processing CLK Sensor a converter that measures a physical quantity, converting it into a signal convenient for an observer or some data acquisition system. ransducer a device, converting one form of energy (eg. mechanical, thermal, electromagnetic etc.) into other form (often of electrical nature), convenient for transmission, presentation or processing.

3 Which quantities can be measured......almost all - temperature (thermistor, termocouple) - force, tention (tensometer) - pressure (tensometer transducer) - displacement (potentiometer, LVD transformer) - flow of substance (anemometer) - speed, speed of rotation (gyroscope) - acceleration - distance - vibrations (piezoelectric effect) - humidity - chemical composition - time ECG signals - HM - gesture, position - smoke/particles detections... education-library/ transducer-interfacing-handbook.html

4 Strain gauge (gage), tensometer l S F l l S+ S F Practical realisation foil strain gauge l = ρ = l + S + ρ S l S ρ foil ~5µm Cu, Ni, Fe, Pt,... l = G f l semiconductor strain gauge 0.5 mm 1.5 mm G f ~2 source: sensormag.com G f ~-100 (Si)

5 Strain measurement (force, mass, pressure, tension...) expansion compression compression x expansion l l l/l ~ 0.4 1% = G = G f f = 1 kω l l ~ 8 20 Ω (assuming G f = 2)

6 Pressure transducer MPX2300 (Freescale) sensitivity 5 µv/v/mmhg FS = 3 mv

A weighting scale oberval mechanism (Gilles Personne de oberval, 1602

7 A weighting scale oberval mechanism (Gilles Personne de oberval, ) sensitivity mv/v/fl (full load) FL: ranges from grams to tons

8 emperature measurement hermocouple thermoelectric effect Seebeck effect (homas J. Seebeck, 1821) V=-S() 1 2 ( ) S ( )d V = ( SA B ) Standard measurement setup 1 measured temperature reference block V V voltmeter V 1 = ( S A ( ) S ( ) = E B ( ) E ( ) 1 ) d = Possible solutions cold junction, aka ice bath compensated cold junction hot junction (rare) Standard thermocouple junctions type junction range [ C] sensitivity [µv/ C] K chromel/alumel E chromel/constantan J iron/constantan copper/constantan

emperature measurement hermistor a + bln + c ln = 1

0 exp B 0 30 25 B=3500 K resistance [kohm m] 20 15 10 5

thermometer detector D (William Siemens, 1871) ( + C +

9 emperature measurement hermistor a + bln + c ln = 1 Steinhart-Hart equation ( ) 3 For NC thermistors 1 1 = 0 exp B B=3500 K resistance [kohm m] temperature [deg C] esistive thermometer detector D (William Siemens, 1871) ( + C + C C +K) = Pt-100 sensor C; C 1 ~0.75 Ω/ C

10 nbalanced bridge linearity problem m + + m n n = ε 2( 2 + ε ) = n ε ( n + 1) 2 + ( n +1)ε / /@ O ε= n=1 n=2 n= ε n

11 Methods of bridge linearisation =?? =?? ε = + 2

12 Frequency Frequency measurement former gate measured signal ref counter display Hz gating generator gating pulse measured signal counted pulses What is a true ferquency value? gating pulse correct result f m > f p (~1 excess pulse) f m < f p (~1 pulses less)

13 ime interval/frequency measurement reference frequency time interval 2 = (N-1) 1 3 = f = 1/ How to determine the tails? analog method digital vernier method (coincidence) coincidence I 1 const. I 2 const. C = Q + Δ A 1 : charging C I > 1E : discharging = 1E 2 < I1 1E 1 1 I I 1 2 Δ = P C A = ( Q + 1) -P A A < P = Q + 1 Δ = ( Q+ 1 ) ( - ) A C A Δ = B eg.: A/B = 9/10 1 Δ = ( Q+ 1) 10 ( Q + 1) 1

14 Distance measurement (ranging) 1. Pulse reflectometric method x x... s = v t p /2 acoustic waves: v khz λ 3mm = 10 µs resolution limit wavelength...but there are other problems... obstacle measurement resolution: - wavelength λ, - time interval measurement: s = 0.5v t p optical waves: v λ 700 nm 430 Hz t p = 10 ns s = 1.5 m fs resolution limit technical capabilities of time interval measurement 2. Coherent method phase shift measurement of CW signal x x s =... Φ v 2 2π f m obstacle Φ m = 1/f m ΔΦ 2π = 2s β = 2s λ m fm = 2s 2π v measurement resolution phase shift v m/s f m = 100 MHz Φ = 1 s 4 mm...

Temperature Measurements

ME 22.302 Mechanical Lab I Temperature Measurements Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-1 Copyright 2001 A transducer is a device that converts some mechanical quantity