Silicon Capacitive Accelerometers. Ulf Meriheinä M.Sc. (Eng.) Business Development Manager VTI TECHNOLOGIES

Transcription

1 Silicon Capacitive Accelerometers Ulf Meriheinä M.Sc. (Eng.) Business Development Manager VTI TECHNOLOGIES 1

2 Measuring Acceleration The acceleration measurement is based on Newton s 2nd law: Let the acceleration act on a known proof mass and measure the force acting on it = m * a Usually the spring acts as a force gauge: 1) capacitive measurement of deflection, 2) piezoresistive or 3) piezoelectric measurement of strain The capacitive and piezoresistive principle can be used to measure DC acceleration (inclination), whereas piezoelectric sensors have a high pass feature 2

3 Sensing Element Operation 3

4 ,1 0,01 0,001 0,0001 0,00001 Acceleration Speed 0,01 0, Frequency Response For slow accelerations the deflection of the proof mass in the sensor coordinates is directly proportional to the acceleration If the sensor is underdamped, there is a resonance at some frequency - a phenomenon, which might not always be desired Position Under Over Critic. For frequencies well above the resonance the sensor measures its position (seismometer) In the intermediate frequency range an overdamped sensor measures its AC speed 4

5 Bulk Micro Machining S 2 S (111) (100) Glass Silicon - V + V Bulk micro machining is using anisotropic etching (wet/dry) to form 3d structures into the bulk of a single crystal silicon Wafers are bonded together: anodic and fusion bonding Lithography, oxidation and thin films (electrodes) 5

6 Silicon Capacitive Accelerometers The sensing element consists of three layers of silicon Proof mass and springs in the mid wafer Capacitors one on each side of the proof mass Symmetrical structure Gas damping in the hermetically sealed cavity Glass insulation between the electrodes 6

7 Measuring Direction & Cross-axis Sensitivity Ideally, the directional dependence of the sensitivity off the nominal measuring axis is a cosine function of the angle of deviation from nominal direction: S = S 0 * COS(Phi) The measuring direction may be parallel or perpendicular to the mounting plane Cross-axis sensitivity = maximum sensitivity in the plane perpendicular to the measuring direction relative to the sensitivity in the measuring direction In the silicon capacitive technology the cross-axis sensitivity results from alignment errors in the mounting a Phi 7

8 3-axis Element Four triangular spring-proof mass systems. Three acceleration components can be calculated as linear combinations of mass tilting angles Y axis of rotation Most of the capacitance area is located where most of the gap deformation is formed mass Z X centre of gravity 8

9 Sensor Structure First version dimensions 2.65mm x 2.65mm x 1.2mm. Five layer structure familiar from other VTI sensors. Silicon block electrical feedthroughs structural wafer glass grid glass grid Silicon block capping wafers contact pads 9

10 Prototype Results Capacitance [pf] axis accelerometer rotated in 1g field Rotation angle [deg] E2 E1 E6 E8 E7 E5 E4 E3 10

11 Measuring Inclination The accelerometer can be used as an inclinometer It measures the combination of the force of earth s gravity and acceleration: Signal = a + 1g * SIN(ϕ) The sensing element is strongly over-damped (f -3dB 2 28Hz) to reduce the influence of acceleration Because of angular mounting errors (component of crossaxis sensitivity in the plane of inclination) there is an offset in the sinus function sin(ϕ + ϕ 0 ) +1g <=> 90 G 0g <=> 0 11

12 Measuring Position The inclinometer can be used to measure position: L L * SIN(ϕ) ϕ L ϕ L * COS(ϕ) L1 L2 ϕ1 ϕ2 L2 * COS(ϕ2) L1 * SIN(ϕ1) L1 * COS(ϕ1) + L2 * SIN(ϕ2) 12

13 Calibration Inclinometers and low-g accelerometers up to a full range of about 3g can easily be calibrated in earth s gravitational field Horizontal accelerometer and inclinometer: Normal position = Zero Position = 0g, +90 turned = +1g + 1g * SIN(ϕ 0 ), - 90 turned = -1g + 1g * SIN(ϕ 0 ) => Sensitivity = [(+90 turned) - (-90 turned)]/2; if the inclinometer full range is less than ±90 the full scale angle can be used as calibration point for Sensitivity Vertical accelerometer: Normal position = Zero Position = +1g, +90 turned = +0g + 1g * SIN(ϕ 0 ), - 90 turned = - 0g - 1g * SIN(ϕ 0 ) => Sensitivity = Normal position - [(+90 turned) + (-90 turned)]/2 13

14 Why This Technology? Single crystal silicon Capacitive sensing Hermetically sealed structures Symmetrical structures Customised sensors Bulk micro machining Proof Mass and Springs in VTI s Low-g Sensor 14

15 Single Crystal Silicon Ideal elastic material: no plastic deformation, tough up to g 15

16 Capacitive Sensor Direct measurement of deflection Based on the variation of a gap between two planar surfaces The capacitance or charge storage capacity of a pair of plates only depends on gap width d and plate area A: C = ε 0 * A/d On one side a force (acceleration) decreases the gap and on the other side increases it: C 1 = C 01 + C 11 /(1 - k 1 * a), C 2 = C 02 + C 12 /(1 + k 2 * a) Assuming symmetry and small stray capacitance (or (k * a) 2 << 1) one gets: (C 1 - C 2 )/(C 1 + C 2 ) a 16

17 Hermetically Sealed Sensor Reduced packaging requirements Reliability: no particles or chemicals can get into the element 17

18 Symmetrical Structures Improved accelerometer zero stability, linearity and cross-axis sensitivity Temperature dependence well below 1 mg/ºc Non-linearity typically below 1% Cross-axis sensitivity typically less than 3% 18

19 Customised Sensors Application specific sensitivity and frequency response Flexible 2-chip solution SCA600 19

20 Surface Micro Machined Accelerometers Dominating technology for high g-ranges 20

21 Two Accelerometer Technologies Ultimate limit: F= m a = k x Adhesion and electrostatic forces start to dominate at small F Surface and bulk micro machining approach each other 21

22 Bulk Micro Machining Large proof mass, large capacitance enable high performance low-g sensing Zero stability and noise performance (high resolution) Noise spectrum (1-100 Hz) Noise performance comparison Noise (u g /sqrt(hz)) Noise (ADXL202) Noise (HML288) Noise(SCA61T) 1 0,0 20,0 40,0 60,0 80,0 100,0 120,0 Frequency (Hz) 22

23 Why Two Accelerometer Technologies? Surface + IC-compatible + Lowest cost + <1mm thickness High noise Poor stability Sticking will limit scalability Wide band response Lack of flexibility Bulk + Low noise + Good stability + Scalable to low g-ranges + Flexible to customization + High damping available Higher cost Bulky size and shape 23

24 Capacitive Sensing Circuitry Minimise the effect of stray impedance (capacitance): use virtual ground and fixed voltages Minimise electrostatic forces: charge balance Charge balance principle: Sensing element capacitors C 1 and C 2 are charged with amplitude modulated square waves of opposite signs; V 1 = V ref + V 0 and V 2 = -(V ref - V 0 ) in such a way that the net flow of charge Q = 0. Now C 1 * (V ref + V 0 ) = C 2 * (V ref - V 0 ) and V 0 = (C 1 - C 2 )/(C 1 + C 2 ) * V ref = linear measure of acceleration Net electrostatic force: F = F 1 - F 2 (V 1 /d 1 ) 2 - (V 2 /d 2 ) 2 Q Q 2 2 = 0 V 1 C 1 C 2 Q A V 2 24

25 Packaging Lowest level: Sensor as SMD First level: Calibrated SMD component 10 mm 5 mm Second level: Stand-alone accelerometer 25

26 Product Concepts Sensing Elements Sensor Components SCA610 / SCA600 Stand alone accelerometers + Electronics, ASICs SCA320 (z-axis) 26

27 VTI Standard Products Available ranges (g) SCA61T/100T ±0.5 ±1.0 SCA610 ±0.5 ±1.0 ±1.5 ±1.7 SCA600 ±1.0 ±1.5 ±1.7 ±3.0 SCA620/320 ±1.5 ±3.0 ±12.0 SCA111 ±1.2 ±2.0 SCA110 ±1.2 27

28 Applications: ABS and TCS ABS and TCS keeps slip at max.10% level for maneuverability In All Wheel Drive (AWD) vehicles all wheels may slip (no speed reference) Speed or deceleration/acceleration information from inertial sensor, longitudinal accelerometer (v = 0 t a dt + v 0 ) 28

29 Applications: VDC or ESP ABS and TCS are not enough in a curve ESP corrects for under- and over-steering Yaw rate (Ω) and centrifugal acceleration (a T ) from an angular rate sensor and a lateral accelerometer are compared to those calculated from wheel speed and steering wheel angle (Ω = v/r, a T = v 2 /R) 29

30 g Applications: ECS g g g Vertical accelerometers in the vehicle corners measure body acceleration Shock absorbers are adjusted in real-time or in average for safe and smooth ride In advanced systems wheel force is measured with wheel hub accelerometers and vehicle inclination with inclinometers In vehicles without steering wheel angle sensor a lateral accelerometer measures centrifugal force 30

31 Other Applications Vehicle Tilt Monitoring and Control Digging Depth and Slope Control Train Lateral Force and Vertical Acceleration Monitoring Seismic Monitoring Platform Leveling Inclinometer Instruments Marine Applications Inertial Navigation Medical: Patient Monitoring, Cardiac Pacemakers Sports and Fitness: Motion, position, altitude, energy,... 31

32 Applications: Excavator Inclinometer: Vout = Vdd/2 * ( 1+ k * SIN(Phi)) Digging Position: 1) Z = L * SIN(Phi) & X = L * COS(Phi) [Normally horizontal arm of length L] 2) X = L * SIN(Phi) & Z = L * COS(Phi) [Normally vertical arm of length L] 32

33 Other Vehicle Applications Motor Grader with Inclinometers (a = 1g * SIN(Phi)) High Speed Train Inclination and Suspension (a T = v 2 /R * COS(Phi) = 1g * SIN(Phi)). 33

34 Other MEMS Sensors: Absolute Pressure Sensor 34

35 Pressure Sensor Sensing Circuitry Voltage based interface V out = V(S2) - [V(S1) + V(S3)]/2 = - C 0 /C(P) * V in C(P) = C 1 /(1 P/P 0 ) + C 2 => P = P 0 * {1 C 1 /C * [1 + (C 2 /C) + (C 2 /C) 2 + (C 2 /C) 3 + ]} Sampling before - during - and after the pulse 3-point calibration gives excellent accuracy C(P) S1 S2 S3 V in C 0 V out -A 35

36 Pressure Sensor Sensing Circuitry Time based interface N out = F out * T = T * V DD /V H /4R * 1/C(P) = k/c(p) P = P 0 * {1 C 1 /C * [1 + (C 2 /C) + (C 2 /C) 2 + (C 2 /C) 3 + ]} One can add one or two references by switching in constant capacitors instead of C(P) The oscillator can be switched off between measurements to reduce power C(P) R -A F out 36

37 Other MEMS Sensors: Angular Rate Sensor Based on the same process as the accelerometer 37

38 Thank You! 38