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Silicon Capacitive Accelerometers

Ulf MeriheinäM.Sc. (Eng.)

Business Development ManagerVTI TECHNOLOGIES

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Measuring Acceleration

The acceleration measurement is based on Newton’s2nd law: Let the acceleration act on a known proofmass and measure the force acting on it = m * aUsually the spring acts as a force gauge: 1) capacitivemeasurement of deflection, 2) piezoresistive or 3)piezoelectric measurement of strainThe capacitive and piezoresistive principle can be usedto measure DC acceleration (inclination), whereaspiezoelectric sensors have a high pass feature

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Sensing Element Operation

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Frequency Response

0,00001

0,0001

0,001

0,01

0,1

1

10

100

0,01 0,1 1 10 100

UnderOverCritic.

Acceleration

SpeedPosition

If the sensor is under-damped, there is aresonance at somefrequency - aphenomenon, which mightnot always be desiredFor frequencies well abovethe resonance the sensormeasures its position(seismometer)In the intermediatefrequency range an over-damped sensor measuresits AC speed

For slow accelerations the deflection of the proof mass inthe sensor coordinates is directly proportional to theacceleration

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Bulk Micro Machining

(100)

(111)S √2 S

Glass Silicon

- V + V

Bulk micro machiningis using anisotropicetching (wet/dry) toform 3d structures intothe bulk of a singlecrystal siliconWafers are bondedtogether: anodic andfusion bondingLithography, oxidationand thin films(electrodes)

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Silicon Capacitive Accelerometers

The sensing elementconsists of three layersof siliconProof mass and springsin the mid waferCapacitors one on eachside of the proof massSymmetrical structureGas damping in thehermetically sealedcavityGlass insulation betweenthe electrodes

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Ideally, the directional dependence of the sensitivity off thenominal measuring axis is a cosine function of the angle ofdeviation from nominal direction: S = S0 * COS(Phi)The measuring direction may be parallel or perpendicular to themounting planeCross-axis sensitivity = maximum sensitivity in the planeperpendicular to the measuring direction relative to the sensitivityin the measuring directionIn the silicon capacitive technology the cross-axis sensitivityresults from alignment errors in the mounting

Measuring Direction &Cross-axis Sensitivity

aPhi

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3-axis Element

Four triangular spring-proof mass systems. Threeacceleration components can be calculated as linearcombinations of mass tilting angles

mass

X

Y

Z

axis of rotation

centre of gravity

Most of thecapacitance area islocated where most

of the gapdeformation is

formed

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Sensor Structure

First version dimensions 2.65mm x 2.65mm x 1.2mm.Five layer structure familiar from other VTI sensors.

contactpads

glass gridelectricalfeedthroughs

structuralwafer

capping wafersglass grid

Siliconblock

Silicon block

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Prototype Results

-180 -135 -90 -45 0 45 90 135 1803.6

3.8

4

4.2

4.4

4.6

4.8

5 3-axis accelerometer rotated in 1g field

Rotation angle [deg]

Cap

acita

nce

[pF]

E2E1E6E8E7E5E4E3

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Measuring InclinationThe accelerometer can be used as an inclinometerIt measures the combination of the force of earth’s gravityand acceleration: Signal = a + 1g * SIN(ϕ)The sensing element is strongly over-damped (f-3dB ≈ 2 …28Hz) to reduce the influence of accelerationBecause of angular mounting errors (component of cross-axis sensitivity in the plane of inclination) there is an offsetin the sinus function sin(ϕ + ϕ0)

+1g 90°0g 0°G

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Measuring PositionThe inclinometer can be used to measure position:

L

L * SIN(ϕ)

ϕ

L2

L2 * COS(ϕ2) – L1 * SIN(ϕ1)

L1

ϕ2ϕ1

L1 * COS(ϕ1) + L2 * SIN(ϕ2)

ϕ L * COS(ϕ)L

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Inclinometers and low-g accelerometers up to a fullrange of about 3g can easily be calibrated in earth’sgravitational fieldHorizontal accelerometer and inclinometer: Normalposition = Zero Position = 0g, +90° turned = +1g + 1g *SIN(ϕ0), - 90° turned = -1g + 1g * SIN(ϕ0) => Sensitivity =[(+90° turned) - (-90° turned)]/2; if the inclinometer fullrange is less than ±90° the full scale angle can be usedas calibration point for SensitivityVertical accelerometer: Normal position = Zero Position= +1g, +90° turned = +0g + 1g * SIN(ϕ0), - 90° turned = -0g - 1g * SIN(ϕ0) => Sensitivity = Normal position - [(+90°turned) + (-90° turned)]/2

Calibration

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Single crystal silicon

Capacitive sensingHermetically sealedstructures

Symmetrical structures

Customised sensors

Bulk micro machiningProof Mass and Springs in VTI’s Low-g Sensor

Why This Technology?

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Ideal elastic material: no plastic deformation,tough up to 70 000 g

Single Crystal Silicon

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Direct measurement of deflectionBased on the variation of a gap between two planarsurfacesThe capacitance or charge storage capacity of a pair ofplates only depends on gap width d and plate area A: C= ε0 * A/dOn one side a force (acceleration) decreases the gapand on the other side increases it: C1 = C01 + C11/(1 - k1 *a), C2 = C02 + C12/(1 + k2 * a)Assuming symmetry and small stray capacitance (or (k *a)2

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Reduced packaging requirementsReliability: no particles or chemicals canget into the element

Hermetically Sealed Sensor

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Symmetrical Structures

Improved accelerometer zero stability,linearity and cross-axis sensitivity

Temperature dependencewell below 1 mg/ºCNon-linearity typically below 1%Cross-axis sensitivity typicallyless than 3%

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Application specific sensitivity andfrequency responseFlexible 2-chip solution

Customised Sensors

SCA600

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Surface Micro MachinedAccelerometers

Dominating technology for high g-ranges

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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

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Large proof mass, large capacitance enablehigh performance low-g sensingZero stability and noise performance (high resolution)

Noise spectrum (1 - 100 Hz)

1

10

100

1000

10000

0,0 20,0 40,0 60,0 80,0 100,0 120,0

Frequency (Hz)

Nois

e (u

g/sq

rt(Hz

))

Noise (ADXL202)Noise (HML288)Noise(SCA61T)

Noise performance comparison

Bulk Micro Machining

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Why Two AccelerometerTechnologies?

+ IC-compatible+ Lowest cost+

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Minimise the effect of stray impedance (capacitance): usevirtual ground and fixed voltagesMinimise electrostatic forces: charge balanceCharge balance principle: Sensing element capacitors C1and C2 are charged with amplitude modulated squarewaves of opposite signs; V1 = Vref + V0 and V2 = -(Vref - V0)in such a way that the net flow of charge ∆Q = 0.Now C1 * (Vref + V0) = C2 * (Vref - V0) and V0 = (C1 - C2)/(C1 +C2) * Vref = linear measure of accelerationNet electrostatic force: ∆F = F1 - F2 ∝ (V1/d1)2 - (V2/d2)2 ∝Q12 - Q22 = 0

Capacitive Sensing Circuitry

AC1C2

V1

V2

∆Q

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Packaging

10mm

5 mm

Lowest level:Sensor as SMD

First level:Calibrated SMDcomponent

Second level:Stand-aloneaccelerometer

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Sensing Elements

+ Electronics,ASICs

Sensor Components

SCA610 / SCA600

Stand aloneaccelerometers

SCA320 (z-axis)

Product Concepts

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Available ranges (g)

SCA61T/100T ±0.5 ±1.0

SCA610 ±0.5 ±1.0 ±1.5 ±1.7SCA600 ±1.0 ±1.5 ±1.7 ±3.0

SCA620/320 ±1.5 ±3.0 ±12.0

SCA111 ±1.2 ±2.0SCA110 ±1.2

VTI Standard Products

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ABS and TCS keeps slip at max.10%level for maneuverabilityIn All Wheel Drive (AWD) vehicles allwheels may slip (no speed reference)Speed or deceleration/accelerationinformation from inertial sensor,longitudinal accelerometer(v =0∫t a dt + v0 )

Applications: ABS and TCS

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ABS and TCS are not enough in a curve ESP corrects for under- and over-steering Yaw rate (Ω) and centrifugal

acceleration (aT) from anangular rate sensor and a

lateral accelerometer are compared to those calculated from wheel speed and steering wheel angle (Ω = v/R, aT = v2/R)

Applications: VDC or ESP

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Vertical accelerometers in the vehicle cornersmeasure body accelerationShock absorbers are adjusted in real-time or inaverage for safe and smooth rideIn advanced systems wheel force is measured withwheel hub accelerometers and vehicle inclination with inclinometersIn vehicles without steering wheel angle sensor alateral accelerometer measures centrifugal force

g

gg

gApplications: ECS

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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, ...

Other Applications

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Applications: ExcavatorInclinometer: 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]

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Motor Grader with Inclinometers (a = 1g *SIN(Phi))High Speed Train Inclination and Suspension(aT = v2/R * COS(Phi) = 1g * SIN(Phi)).

Other Vehicle Applications

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Other MEMS Sensors: AbsolutePressure Sensor

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Pressure Sensor Sensing Circuitry

Voltage based interface− ∆Vout = V(S2) - [V(S1) + V(S3)]/2 = - C0/C(P) * ∆Vin− C(P) = C1/(1 – P/P0) + C2 => P = P0 * {1 – C1/C * [1 +

(C2/C) + (C2/C)2 + (C2/C)3 + …]}− Sampling before - during - and after the pulse− 3-point calibration gives excellent accuracy

-A

C(P)

C0∆Vin∆Vout

S1 S2 S3

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Time based interface− Nout = Fout * ∆T = ∆T * VDD/VH/4R * 1/C(P) = k/C(P)− P = P0 * {1 – C1/C * [1 + (C2/C) + (C2/C)2 + (C2/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

-A

C(P)

RFout

Pressure Sensor Sensing Circuitry

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Other MEMS Sensors: AngularRate Sensor

Based on the same process as the accelerometer

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Thank You!

Silicon Capacitive AccelerometersMeasuring AccelerationSensing Element OperationFrequency ResponseBulk Micro MachiningSilicon Capacitive AccelerometersMeasuring InclinationSurface Micro Machined AccelerometersTwo Accelerometer TechnologiesPackaging