Measurement of non-electrical quantities
Measurement chain analog part digital part ef bias conditioning circuitry ef µc DSP SOC... C/A sensor A/C processing 2.718 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.
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... https://www.analog.com/en/education/ education-library/ transducer-interfacing-handbook.html
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)
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)
Pressure transducer MPX2300 (Freescale) sensitivity 5 µv/v/mmhg FS = 3 mv
A weighting scale oberval mechanism (Gilles Personne de oberval, 1602 1675) sensitivity 0.5-1 mv/v/fl (full load) FL: ranges from grams to tons
emperature measurement hermocouple thermoelectric effect Seebeck effect (homas J. Seebeck, 1821) V=-S() 1 2 ( ) S ( )d V = ( SA B ) 1 2 1 2 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 -200 1200 39 E chromel/constantan 0 900 76 J iron/constantan -200 760 55 copper/constantan -200 400 45
emperature measurement hermistor a + bln + c ln = 1 Steinhart-Hart equation ( ) 3 For NC thermistors 1 1 = 0 exp B 0 30 25 B=3500 K resistance [kohm m] 20 15 10 5 0 0 10 20 30 40 50 temperature [deg C] esistive thermometer detector D (William Siemens, 1871) ( + C + C 2 3 + C +K) = 1 0 1 2 3 Pt-100 sensor 100 Ω @ 0 C; C 1 ~0.75 Ω/ C
nbalanced bridge linearity problem m + + m n n = ε 2( 2 + ε ) = n ε ( n + 1) 2 + ( n +1)ε / 0.2 0.1 0-0.1 /@ O ε=0 0.2 0.3 0.25 0.1 0.2 0 0.15-0.1 0.1 n=1 n=2 n=3-0.2-0.5-0.25 0 0.25 0.5 ε 0.05-0.2-0.5 0-0.25 2 4 0 6 0.258 0.5 10 n
Methods of bridge linearisation + - + =?? =?? + + - + - ε = + 2
Frequency Frequency measurement former gate measured signal ref counter display 1359.078 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)
ime interval/frequency measurement reference frequency time interval 2 = (N-1) 1 3 = 1 + 2 + 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
Distance measurement (ranging) 1. Pulse reflectometric method x x... s = v t p /2 acoustic waves: v 300 m/s @ 100 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 3 10 8 m/s @ λ 700 nm 430 Hz t p = 10 ns s = 1.5 m... 2.3 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 3 10 8 m/s f m = 100 MHz Φ = 1 s 4 mm...