Sensori capacitivi. Elettronica per Sensori e Trasduttori - Gianvito Matarrese

Transcript

1 Sensori capacitivi 1

2 Sensori capacitivi 2

3 Sensori capacitivi 3

4 Capacitive Measurement d Capacitance ε r b a Capacitance varies with Relative Dielectric Constant ε R Dimensions a b C ε 0 ε R d (ε 0 is the permittivity of free space) ε R of various Materials Air / Gases ε R 1 Solids / Liquids ε R Water ε R 81 Dielectric Constant & Dimensions vary with Temperature Q = C V capacitance C is a measure of the charge Q stored in a capacitor for a 4 given voltage V

5 Capacitance Sensors: Applications Position Sensing Moisture Sensing Level Sensing Pressure Sensing Proximity Sensing Industrial Alignment Valves SC Equip Wear-out Security Automotive Seat Belts Gear/Throttle /Seat Position Sunroofs, Hoods, Trunks Industrial Building Controls Process Control Automotive Rain Humidity Climate Control Medical Infusion Pump (Air bubble) Industrial Process Control Switch Automotive Fuel Level Oil Level Fuel Mix Ratio Industrial Smart Transmitters Automotive Occupancy Seatbelts Medical Blood Pressure Respiration Control Industrial Control Panels Dead-Man Switches Automotive PVE/RKE Park Assist Intelligent Lighting Medical Hearing Aid 5

6 Length Measurement and Position Sensing a b C ε 0 ε R d Size of capacitor changes with position Simple capacitive sensor CDC Possible compensation: 2 nd Capacitive channel or Temperature sensor Processing DATA High resolution Σ CDC Automotive & Industrial applications 6

7 Rain Sensing a b C ε 0 ε R d Dielectric constant changes with amount of water Ceramic Substrate ε R 3 Water ε R 81 Capacitive Sensor CDC Processing DATA Algorithm implemented in the host system Automotive applications Environmental appl. 7

8 Water-repellent Isolation (Plastic, glass ) Capacitive Rain Sensor Demo Components C IN C EXC CDC Digital Processing DATA Water drops Ground (Shielding) Sensor electrodes PCB 8

9 Humidity Sensing Capacitiv e Sensor Temp. Sensor Vapors exchange between Air and Ceramic Substrate a b C ε 0 ε R d CDC ADC Processing Temperature Measurement DAT A Dielectric constant changes with humidity Ceramic Substrate ε R 3 Water ε R 81 Simple capacitive sensor Temperature sensor» Temperature Compensation» Relative Humidity Calculation High resolution Σ CDC Industrial Air Conditioning Environmental 9

10 Level Sensing Capacitance changes with the effective area of electrodes, i.e., with level Capacitive Sensor Capacitive Sensor a b C ε 0 ε R d CDC CDC Processing DATA 2 nd capacitive sensor compensates dielectric constant changes ε R Changes with temperature ε R Changes with material, (e.g. type of oil) ε R Compensation 10

11 Capacitive Level Sensor Demo (EVAL-AD7746LDZ) Components (AD7746) Ground (Shielding) AD7746 Digital Processing DATA Sensor (electrodes inside PCB) C 1 Level C2 C 1 ~ Level C 2 ~ Ref 11

12 Pressure Sensor a b C ε 0 ε R d Distance in capacitor changes with pressure Pressure Simple capacitive sensor Temp. compensation (Differential capacitance) Temp. Sensor + - CDC ADC Processing DATA High resolution Σ CDC Industrial Automotive Medical Temperature Compensation 12

13 Proximity Sensing / Proximity Detection Capacitive Sensor a b C ε 0 ε R d CDC THRESHOLD TRIGGER Electric Field changes with an object proximity Simple capacitive sensor Threshold algorithm eliminating environmental changes Low Cost CDC Automotive / Industrial / Security 13

14 Sensori capacitivi 14

15 Sensori capacitivi 15

16 Sensori capacitivi 16

17 Sensori capacitivi 17

18 Sensori capacitivi 18

19 Sensori capacitivi 19

20 MEMS Sensors 22

21 Summary MEMS Technology Accelerometers Gyroscopes (Angular Rate Sensors) isensors Microphones 23

22 What is MEMS? Batch Fabricated Micro Electro Mechanical Systems Bulk Micromachining Pressure Sensors Accelerometers/Gyros Printing Nozzles Surface Micromachining Pressure Sensors Accelerometers/Gyros Optical Products 24

23 What is ADI s MEMS Technology? imems (Integrated MicroElectroMechanical Systems) Surface Micromachining SENSOR SAC. OXIDE SUBSTRATE Passivations BPSG Thox Met SENSOR Nwell Emitter Base SUBSTRATE NSD 23

24 Automotive Inertial Safety Systems Z Z ECU Rollover Z Z Yaw Roll Pitch Crash Detection System Electronic Stability Control Navigation/Driver Assistance System Body/Chassis Control Systems Adaptive Cruise Radar 24

25 Accelerometers 25

26 How do accelerometers work? Newton s Law Mass m Acceleration a Displacement x Mass m Force F = Mass m Acceleration a Force F = Spring Constant k Displacement x a = k/m x 26

27 How do accelerometers work? Realization M a s s Suspension Spring Proof Mass Motion Suspension Spring (ca ) 27

28 Capacitive Sensor Principle Differential Capacitor Tether Inertial Mass (Beam) Top View Applied Acceleration C = ε S d CS1 Fixed outer plates CS2 Sensor at rest Unit Cell CS1 = CS2 Anchor CS1 CS2 Unit Cell CS1 > CS2 Responding to an applied acceleration 28

29 Sensori capacitivi: accelerometri Applied Acceleration CS1 CS2 Unit Cell CS1 > CS2 31

30 Sensori capacitivi: accelerometri 32

31 Sensori capacitivi: accelerometri 33

32 Sensori capacitivi: accelerometri 34

33 Sensori capacitivi: accelerometri 35

34 How Do Dual Axis Accelerometers Work? Arbitrary Acceleration Finger Sets Proof Mass Finger Sets Suspension Spring Suspension Spring MA15: Elettronica per i Sensori 37

35 Sensori capacitivi: accelerometri 38

36 Sensori capacitivi: accelerometri 39

37 Sensori capacitivi: accelerometri 39

38 Sensori capacitivi This device, often found just in front of the keyboard on a laptop computer, drives a pulse in succession on each column and measures coupling to each row. By locating the coordinates of peak coupling and interpolating between adjacent rows, the location of a shielding human finger is measured to a fraction of a mm. 40

39 Sensori capacitivi 41

40 Sensori capacitivi 42

41 imems Gyro Principle of Operation What Does a Gyro Do? Measures How slow an object is turning Measures Change of Inclination or Direction The imems Gyro measures angular rate by measuring the Coriolis Force. F C = 2m ( v ω) 41

42 imems Gyro: applicazioni Misura della velocità di rotazione di un veicolo: montando un giroscopio all interno del veicolo, se la velocità di rotazione supera un certo valore, un sistema di controllo agisce in modo differenziale sui freni, per riportare la velocità di rotazione sotto controllo; Integrando la velocità angolare nel tempo si può ottenere la posizione angolare, utile per mantenere la continuità nella navigazione GPS, se il segnale del satellite viene perso per poco tempo. 42

43 imems Gyro: principio di funzionamento La misura della velocità di rotazione avviene mediante l accelerazione di Coriolis: una massa posta su 1 piattaforma in rotazione ha una velocità tangenziale proporzionale alla distanza dal centro. Se la massa si muove dal centro verso la periferia, la velocità tangenziale aumenta; la variazione di velocità tangenziale, dovuta ad una velocità radiale, è l accelerazione di Coriolis. 43

44 imems Gyro: principio di funzionamento 44

45 imems Gyro: principio di funzionamento when the resonating mass moves toward the outer edge of the rotation, it is accelerated to the right and exerts on the frame a reaction force to the left. When it moves toward the center of the rotation, it exerts a force to the right 45

46 imems Gyro: principio di funzionamento To measure the Coriolis acceleration, the frame containing the resonating mass is tethered to the substrate by springs at 90 relative to the resonating motion. The Coriolis sense fingers are used to capacitively sense displacement of the frame in response to the force exerted by the mass. The displacement is determined from the change in capacitance between the Coriolis sense fingers on the frame and those attached to the substrate. If the springs have a stiffness, K, then the displacement resulting from the reaction force will be 2 ΩvM/K. 46

47 imems Gyro Accelerometer Vibratory Mass Accelerometer Vibratory Mass 47

48 imems Gyro The ADXRS gyro electronics can resolve capacitance changes as small as 12 x farads (12 zeptofarads) from beam deflections as small as Angstroms (16 femtometers). The only way this can be utilized in a practical device is by situating the electronics, including amplifiers and filters, on the same die as the mechanical sensor. The differential signal alternates at the resonator frequency and can be extracted from the noise by correlation. 48

49 imems Gyro 49

50 imems Gyro One of the most important concerns for a gyro user is the device s ability to provide an accurate angular rate-output signal even in the presence of environmental shock and vibration. One example of such an application is automotive rollover detection, in which a gyro is used to detect whether or not a car is rolling over. Some rollover events are triggered by an impact with another object that results in a shock to the vehicle. If the shock saturates the gyro sensor, and the gyro cannot filter it out, then the airbags may not deploy. Similarly, if a bump in the road results in a shock or vibration that translates into a rotational signal, the airbags might deploy when not needed. The use of two resonators differentially senses the signals and rejects common-mode external accelerations that are unrelated to angular motion. The two resonators in Figure 3.22 are mechanically independent, and they operate anti-phase. As a result, they measure the same magnitude of rotation, but give outputs in opposite directions. Therefore, the difference between the two sensor signals is used to measure angular rate. This cancels non-rotational signals that affect both sensors. MA15: Elettronica per i Sensori 50

51 Sensori induttivi: LVDT 51

52 Sensori induttivi: LVDT 52

53 Sensori induttivi: LVDT 53

54 Sensori induttivi: LVDT 54

55 Sensori induttivi: LVDT 55

56 Sensori induttivi: LVDT 56