Datorbaserad mätteknik

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Viktoria Isaksson
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1 Datorbaserad mätteknik Elektromagnetisk Kompatibilitet och Filter Introduktion till multivariat dataanalys 1:52 Outline ElectroMagnetic Compatibility (EMC) Capacitive coupled disturbance Inductive coupled disturbance Coupling through common impedance Electromagnetic radiation Ground connections Filter Analog LP filters Passive and Active Industrial components 2:52 1

2 ElectroMagnetic Compatibility ElectroMagnetic Compatibility (EMC) means the ability for different electronic equipment to operate in the same electromagnetic environment. Immunity requirements means the ability for a system to resist the influence of disturbances coming from other equipment. Emission requirements means the level of allowed generated disturbances of an equipment. 3:52 ElectroMagnetic Compatibility Disturbances can be coupled from one equipment to another via: Capacitive coupling Inductive coupling Common impedance Electromagnetic radiation Currents in grounding system 4:52 2

3 Capacitive coupled disturbance d R πε 0 0 r [ F m] Ccyl = [ F / m] C 12 = / 2D ln d r πε ε R ln r 5:52 Capacitive coupled disturbance U S Z = Z + Z C U 1 6:52 3

4 Capacitive coupled disturbance - Suppression 12 U S The resulting disturbance voltage U S is efficiently suppressed if the shield is connected with low impedance Z S to ground. 7:52 Inductive coupled disturbance There is always a mutual inductance M between two wires acting like a transformer. di1 U ind = M1 dt Increasing the distance between the wires reduces the magnetic coupling and thus the mutual inductance M. 8:52 4

5 Inductive coupled disturbance The induced disturbance U ind is added to the sensor signal U = U + U in sensor ind 9:52 Inductive coupled disturbance - Suppression A current i 2 is generated in in the screen wire in opposite direction (Lenz law). Both i 1 and i 2 are coupled through mutual inductances M 1 and M 2 to the amplifier input, but with opposite polarity. If the screen- and signal wire are close enough, ideally U ind U 0, 1 + ind,2 = 10:52 5

6 Inductive coupled disturbance - Suppression A close to complete suppression can be achieved if the shield surrounds the signal wire. Important - The shield needs to be connected to ground in both ends to allow current to flow through the shield. 11:52 Coupling through common impedance U U 1 in,1 Z shield U ( i in,1 1 + i ) = 0 = U 2 1 Z shield ( i 1 + i Conclusion Signal transmissions for Sensor 1 and 2 are disturbing each other through Z shield. 2 ) 12:52 6

7 Coupling through common impedance- Suppression Using a pair of wires for each sensor solves the problem. The shield can still be used for suppression of other kinds of disturbances. 13:52 Electromagnetic radiation U e = E P L U b db = A dt V Dipole antennas are sensing either magnetic B-field or electric E-field. 14:52 7

8 Electromagnetic radiation An electromagnetic field wave generates disturbance in a measurement system. Both the E-field along the sensor signal wire as well as the B-field perpendicular to the ground loop area contributes to this disturbance. 15:52 Electromagnetic radiation - Suppression The E-field can be almost totally suppressed inside an encapsulation if it is made out conducting metal. But this is rarely a realistic solution. 16:52 8

9 Electromagnetic radiation - Suppression U b / 2 A U b db = A dt V U b / 2 U e = E P L A twisted pair of wires constitute a series of small current loop areas. The induced voltage in each current loop is suppressed by the induced voltage of opposite polarity in its neighbor loop. The E-field induces an equal voltage U e on both wires, thus a common mode signal that can be suppressed in the input amplifier. 17:52 Ground connections If there are differences in the ground potential due to a long distance, a voltage difference will be added to the sensor signal. 18:52 9

10 Ground connections Connect the ground to the shield only at one endpoint if the shield is also used as return current feeder. The shield will efficiently suppress capacitive coupled disturbances but not inductive coupled. 19:52 Ground connections Both ends of the shield can be connected to ground if an additional wire (pair) is used for return current. This means in this case that the shield suppresses both capacitive and inductive coupled disturbances. 20:52 10

11 Ground connections Single ended Differential ended Active sensor Passive sensor We need a differential ended sensor to drive a differential mode signal on a pair of wires. How can a single ended sensor be converted into a differential ended sensor? 21:52 Ground connections Differential ended 22:52 11

12 Filters Analog filters Time continuous Amplitude continuous Active or passive Sensor Amplifier Filter AD-converter Signal processing Digital filters Time discrete Amplitude discrete 23:52 Filters Band pass filter Band stop filter Low pass filter High pass filter 24:52 12

13 Första ordningens passiva lågpassfilter R U in (t) C U out (t) U in 0 ( t) = U för t <= 0 max för t > 0 U out ( t) U max 1 e = RC t τ = R C Anger en slags tröghet för hur utsignalen svarar på insignalen Driv U(t) med en stegfunktion.vi tiden t=τ har utsignalen nått 63% av maxvärdet (värde efter lång tid) 25:32 Första ordningens passiva lågpassfilter Ett exempel: R = 10kΩ C = 100nF τ = R C = = 1ms x :32 13

14 Första ordningens passiva lågpassfilter Vi kallar kretsen LågPassfilter (LP) därför att kretsen också har en tröghet och ovilja att släppa fram oscillerande signaler med hög frekvens. Från exemplet: R = 10kΩ C = 100 nf τ = R C = = 1ms Gränsfrekvensen där dämpningen av höga frekvenser börjar bli märkbar kallas gränsfrekvens 1 1 f g = = 160 Hz πτ π 27:32 Analoga filter - Amplitudkarakteristik ω g = 2 πf g =1000 rad / s f g = Hz 2π 20 db/decade 28:52 14

15 Analoga filter Aktivt LP-filter C R R in _ U in + U out Detta aktiva filter har samma frekvensegenskaper som det förra passiva filtret förutom en skalfaktor R/R in. 29:52 Analog filters Increased SNR Sinusoidal signal Sinusoidal signal with added white noise, SNR 0.2 Sinusoidal signal Sinusoidal signal with noise after single pole low pass filter SNR 10 30:52 15

16 Industriella komponenter LP filter Ref. LVDT från Applied Measurements Ltd Denna modul är tänkt att anslutas till en differentiell transformator. LVDT, Linear Variable Differential Transformer. Enheten driver (modulerar) transformatorn med en frekvens av 1-5 khz. Den demodulerade sensorsignalen på utgången filtreras med ett lågpassfilter med valbar gränsfrekvens, väljs i fasta lägen mellan Hz. Förstärkning och nollpunkt är steglöst reglerbar. 31:52 Industriella komponenter LP filter Ref. Model 4999 från Endevco Denna modul benämns 16 channel low-pass filter signal conditioner. Den förstärker signalen med 1 alternativt 10 ggr. Varje kanal är utrustat med ett fast 4:e eller 6:e ordningens lågpassfilter. Gränsfrekvenser väljs vid beställning till fasta värden från 1 till 10 khz. 32:52 16

17 Introduktion till multivariat dataanalys 33:52 Uni- och bivariat statistik Läs in data till analysprogrammet SIMCA 34:52 17

18 Medelvärde och standardavvikelse 35:52 Quick info Options 36:52 18

19 Korskorrelation 37:52 Gör 2D-plottar med regressionslinje 38:52 19

20 Gör 3D-plottar Färgkodning för att illustrera en fjärde dimension 39:52 Multi-Variat DataAnalys MVDA Varför? 40:52 20

21 Rita en banan och måttsätt När vi ska projicera och måttsätta bananen faller det sig naturligt att börja med den dimension som har längst utsträckning (variabilitet) PC1. Därefter PC2 och PC3. 41:52 42:52 21

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