EE1320: Measurement Science Lecture 2: Sensors

Transcription

1 EE1320: Measurement Science Lecture 2: Sensors Dr. ir. Michiel Pertijs, Electronic Instrumentation Laboratory April 26, 2013 Delft University of Technology Challenge the future

2 Course program 2013 week ate topic 4.1 Tu 23/4 #1 intro measurements an meas. systems Fr 26/4 #2 sensors 4.3 Tu 7/5 #3 sensor reaout an signal conitioning 4.4 Tu 14/5 #4 instrumentation amplifiers We 15/5 intermeiate test 4.5 Tu 21/5 #5 analog-to-igital converters 4.6 We 29/5 #6 measurement instruments I 4.7 Tu 4/6 #7 measurement instruments II We 5/6 intermeiate test 4.8 Tu 11/6 tutorial 4.11 We 3/7 final exam Lecturer: r. ir. Michiel Pertijs room HB , M.A.P.Pertijs@tuelft.nl,

3 Last time Measuring = etermining the value of a certain quantity measurement requires international stanars calibration is neee for traceable, comparable measurements every measurement is subject to measurement uncertainty Measurement system: converts quantity to be measure x into usable output signal y (often electrical, igital) Characterize by transfer y = H(x) with sensitivity H (x) Deviation transfer can lea to measurement errors: non-linearity, ambiguity, cross sensitivity, finite resolution Don t forget to practice! Answers to the exercises can be foun on BlackBoar 3

4 Structure of a measurement system DATA ACQUISITION measurement object sensor signal conitioning analog-toigital conversion transuction of information from a (non-electrical) omain to the electrical omain lecture 2: sensors lecture 5: A/D convertors amplification buffering (impeance transformation) filtering signal conversion (e.g. R V) lecture 3/4: sensor reaout & instrumentation amplifiers Regtien 1.1 4

5 Toay: Sensors General properties an classification Resistive sensors Capacitive sensors Inuctive sensors Thermoelectric sensors Piezoelectric sensors Semiconuctor sensors Wenesay 1/5: EPO-2 just-in-time training capacitive sensors Wenesay 8/5: EPO-2 just-in-time training inuctive sensors 5

6 Overview stuy material General properties an classification slies Resistive sensors Regtien Capacitive sensors Inuctive sensors Thermoelectric sensors Piezoelectric sensors Semiconuctor sensors en

7 Sensors Transuction of non-electrical signals to electrical signals Ieal sensor: imposes no loa on the source oes not a noise is selective: only sensitive to relevant information 7

8 Self-generating vs. moulating self-generating moulating measure object info energy Sensor measure object info Sensor energy information an energy rawn from the measure object + minimal sources of error (e.g. no offset) loa is impose on the source information of measure object moulates energy transfer from external auxiliary source + minimal loa on source extra sources of error 8

9 Self-generating vs. moulating Photoioe, solar cell optical angle encoer 9

10 Self-generating or moulating? Pt100 10

11 Self-generating or moulating? Pt100 Moulating Moulating 11

12 Tanem transucers Transuction consisting of one or more intermeiate steps measure object input omain trans. 1 intermeiate omain trans. 2 electrical omain Tanem sensor Besies the input omain an the output omain (electrical), at least one other (non-electrical) intermeiate omain 12

13 Tanem transucers Example: raiation sensor with bi-metal an capacitive reaout Metal 2 Absorbing surface Raiation Metal 2 Absorbing surface Metal 1 Metal 1 13

14 Resistive sensors Resistance of a wire-shape conuctor: R material l = ρ [Ω] A geometry A l ρ = resistivity [Ωm] σ = 1/ρ = conuctance Regtien

15 Resistive sensors: mechanical Examples: weighing scale with strain gauges (see lecture 1) angular isplacement sensor base on a potentiometer Regtien ,

16 Reaout of a potentiometric sensor Ieal sensor: R b = 0, R L U U o i α R = R = α Say R L, but R b 0. What kin of transfer error will this yiel? Regtien

17 Resistive sensors: LDR Light epenent resistor (LDR), or photoconuctor High-resistivity semiconuctor (typically CS) Light more free charge carriers lower resistance Not very accurate, but very sensitive: ark resistance on the orer of 10 6 Ω resistance at high light intensity on the orer of 10 2 Ω Spectral sensitivity can be ajuste from UV via visible light to infrare, epening on the application Regtien

18 Capacitive sensors Parallel-plate capacitor: A C = ε A material geometry ε = ε 0 ε r = permittivity or ielectric constant [F/m] ε 0 = F/m = permittivity of vacuum Regtien

19 Capacitive isplacement sensors Lateral isplacement: x x l l C C single A = = A l x A ifferential C = 1 C 2 Regtien

20 Capacitive isplacement sensors Vertical isplacement: C = C + C / C Systematically non-linear transfer Linear approximation will quickly result in large errors! How can we make this sensor more linear??? / Regtien

21 24 Capacitive isplacement sensors Vertical isplacement ifferential: Error term scales with ( / ) 2 instea of / much smaller for small / C C + = C C = = + + = / C C C C

22 Capacitive isplacement sensors C C single enkel ifferential ifferentieel transfer single enkel ifferentieel ifferential non linearity / Generally: ifferential approaches eliminate even-orer non linearities (quaratic, fourth-orer, etc.) -0.1 / 25

23 Capacitive humiity sensor Parameter Conitions Value Nominal capacitance Nominal sensitivity Cross sensitivity to temperature Long-term reproucibility porous electroe humiitysensitive polymer What is the maximum measurement error one year after calibration in the temperature range of 10 C to 45 C? 26

24 Inuctive sensors µ A L = µ n 2 A l material l winings geometry approximation vali for a long, thin coil (iameter << length) µ = µ 0 µ r = permeability [H/m] µ 0 = 4π 10-7 H/m = permeability of vacuum Regtien

25 Inuctive isplacement sensor Ajustable coil coil with movable core µ changes Disavantage: limite linearity µ A Differential better linearity L L 1 L 2 x x L l = µ n 2 A l single ifferential Regtien

26 Inuctive sensors: LVDT LVDT = Linearly-variable ifferential transformer -- How woul that work?? primary coil V p x movable core V s1 V s2 seconary coils Regtien

27 Thermal sensors temperature epenent resistors (e.g. Pt100) pn-junction temperature sensors Metal a Metal b Metal a thermoelectric sensors Regtien ,

28 Thermocouples Measure temperature ifference between two junctions of ifferent (semi)conuctors Self-generating effect intrinsically offset-free Suitable for a broa temperature range (up to 2800 K) hot junction col junction Metal a Metal b Metal a For absolute temperature measurement, T 2 must be stabilize (e.g. at 0 C) or measure with another temperature sensor Regtien

29 Thermocouples Metal a Base on the Seebeckeffect: Fermi level E F (T ) is temperature epenent T E F U Metal b Metal a Seebeck coefficient: Thermocouple: Material combination Sensitivity MATERIAALCOMB. (at 0 C)[µV/K] GEVOELIGHEID Iron / constantan (type J) 45 (BIJ 0 C) [µv/k] Temp. range [ C] TEMP. BEREIK [ C] Copper / constantan (type T) ijzer/constantaan (type J) Chromel koper/constantaan / alumel (type K) (type T) chromel/alumel (type K) Platinum / platina/platina+rhoium platiunum+rhoium Regtien

30 Thermopile Series connection of n thermocouples n times larger sensitivity But also: n times larger impeance larger thermal conuctance (heat loss) between the hot an col junctions 34

31 Piezoelectric effect Force F Force F No loa: no polarization charge Transverse piezoelectric effect Longituinal piezoelectric effect Regtien

32 Piezoelectric sensors Piezoelectric crystal: force F polarization charge Q = S q F Charge sensitivity S q : typically pc/n Two reaout approaches: Voltage: Q on sensor capacitance C s open voltage U s = Q / C s Charge: connect sensor to charge amplifier Piezoelectric crystal Symbol Simple moel Regtien

33 Reaout of a piezoelectric accelerometer Test mass M exerts a force F = M a on a piezoelectric crystal Resulting polarization charge Q = S q F is integrate on C f U o Sq M = a C f 37

34 Photo ioe Photons lea to a photocurrent I ph Typical sensitivity of a Si photoioe Characteristic properties: spectral sensitivity R [A/W] ark current I ark quantum efficiency η [%]: electrons / photon Regtien

35 Summary Sensors allow transuction to the electrical omain self-generating / moulating irect / tanem transuction Examples: Resistive: thermistor, potentiometer, strain gauge Capacitive: isplacement, acceleration, humiity Inuctive: isplacement, istance Thermoelectric: thermocouples Piezoelectric: force, acceleration Semiconuctor: photoioe 39

36 What s next? Stuy: Regtien sections 7.2, en Practice: See exercises on Blackboar! Questions, things unclear? Let me know! M.A.P.Pertijs@tuelft.nl Next time: sensor reaout 40