Integrated Accelerometers

1
Integrated Accelerometers

• -Presentation for Area Exam
• Kush Gulati
• Dept. of Electrical Engineering and Computer
  Science
• Massachusetts Institute of Technology

2
Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Introduction Motivation / Goals
MOTIVATION Chip Scale integration of
accelerometers creates new markets

• GOALS
• Representation of all major accelerometer types
• (Sensors, with/ without Feedback, Surface/Bulk
  Micromachining)
• Performance Issues, trade-offs and comparison
• More focus on circuit details
• More focus on capacitive sensors (most popular
  sensor)

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Introduction Application Space

• Automotive
• Air-bag Actuation
• Vehicle Stabilization

• Space/Defense
• Inertial Guidance for
• space-ships missiles

Industrial Robotics Vibration Sensing

• Consumer
• Virtual Reality
• 3D Mouse
• Sports Equipment
• Camcorder
• Personal Navigation

Desired Specifications Airbag 3
Personal Navigation Range /-
50g /-1g Bandwidth DC-400Hz DC-10Hz Resolut
ion lt100mg lt10µg
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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Concept

• STEP 1 Acceleration to Displacement
• STEP 2 Displacement measurement
• STEP 3 Force-Feedback (Optional)

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Concept STEP 1- Acceleration to Displacement
Spring/Proof-mass System
mmass, kspring constant, ddamping constant
Example Proof-mass Suspensions
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Concept STEP 1- Acceleration to Displacement,
contd.

• Sensitivity - Bandwidth Trade-off
• Sensitivity enhanced by reducing Wr
• Bandwidth enhanced by increasing Wr
• Sensitivity proportional to m/k (mass/
  spring-constant)
• Integration gt lower mass
• Can recover lower sensitivity with more compliant
  spring

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Concept STEP 2- Displacement Measurement

• Capacitive Sensors
• Tunneling Current Sensors
• Piezoresistive Sensors

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Concept STEP 3- Force Feedback

• Advantages of Force Feedback
• A. Extended Dynamic Range
• B. Linearity
• C. Shock resistance
• D. Accuracy
• E. Extended Bandwidth
• F. Lower Brownian Noise

Closed Loop Force Feedback Model
Hxa/Wr2 Hedisplacement to voltage HcLoop
Stability Compensation HfVoltage to Force
(electrostatic Actuation)
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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Performance Parameters

• SNR
• Bandwidth
• Linearity
• Off-axis Sensitivity
• Power Consumption
• Range
• Offset
• Area
• Shock Survival

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Performance Specifications SNR
SNR amax/anoise

• Mechanical Noise
• Random collisions of gas/air molecules with mass.

• Electronic Noise
• Thermal noise of Transistors

For m0.5ug, fr10KHz, Q0.5
For fr10KHz, gm10mA/V, xo1um, Vs2V Cs500ff,
Ci500ff, NG3
200ug/rt(Hz) for air damping 1ug/rt(Hz) for
vacuum (with larger Q)
2.2ug/rt(Hz)
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Performance Specifications Bandwidth
Force Feedback allows d0 (vacuum) while
employing electronic damping
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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Capacitive Accelerometer - Concept

• Concept
• Make proof-mass one plate of one or more
  capacitors
• Proof-mass displacement causes capacitance to
  change which is then measured electrically
• Examples
• Analog Devices ADXL50 (closed-loop, surface
  micromachined, fully integrated)
• ADXL105 (open-loop, surface micromachined,
  fully integrated)
• Motorola MMA1201P (open-loop, surface-micromachin
  ed, 2 chip single package)

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Capacitive Accelerometers Physical Structures
Two main types of capacitive structures
Single ended
Differential
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Capacitive Accelerometers Physical Structures,
contd.
Differential as compared to single-ended

• Advantages
• Single ended is inherently asymmetrical needs
  offset compensation
• Differential lends itself more easily to high
  performance differential circuits
• Disadvantages
• Single ended has larger cap for given area
• Single ended has smaller fringe cap/ area cap
  ratio more sensitive
• Single-ended has greater mass for given area
  less mechanical noise

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Capacitive Accelerometers Electronics
Sensing Schemes (a) Voltage Sensing (b)
Charge Sensing Circuitry (a) Single-ended
(b) Differential
Voltage Sensing
1mggt 0.1Aº (for fr5KHz) gt5aF change in 500ff
capgt40µV
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Capacitive Accelerometers Electronics - Sensing
Schemes, contd.
Charge Sensing Parasitic Insensitive
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Capacitive Accelerometers Electronics - Circuits
Differential Circuit
Single-ended Circuit

• Advantages
• Single clock supply Even a few µV difference can
  mask the signal
• Improved noise rejection from substrate, external
  interference, power supply.
• Disadvantages
• Introduces large CM swing at opamp inputs

Differential compared to Single-ended
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Capacitive Accelerometers Force Feedback
Generic Force Feedback Model
Exact Circuit Implementation
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Capacitive Accelerometers Digital Force Feedback

• Advantages
• Intrinsically Linear Feedback
• Free A/D Conversion
• Disadvantages
• Limit Cycles
• Dead zone

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Capacitive Accelerometers Comments

• Incorrect Expressions and Numerical errors
• Input referred electronic noise
• Incorrect estimate of optimal value of Cgs for
  minimizing noise
• Offset hasnt been addressed at all
• 1-bit Digital force feedback leads to large dead
  zone (9mg in ref 7 in paper)
• Analog Feedback linearity problems can be
  addressed simply by including calibration step

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Capacitive Accelerometers Comments, contd.

• Consider following circuit

• Advantages (compared to previous approaches)
• Voltage difference between V1 and V2 has no
  impact.
• Improved noise rejection from substrate, external
  interference, power supply.
• No large CM swing at opamp inputs.

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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Tunneling Accelerometer - Concept

• Concept
• Based on Tunneling Phenomenon
• Proof-mass displacement causes change in
  tunneling current which is detected electrically.
• Commercial Examples
• None known. Research at Univ. Michigan, JPL,
  Stanford

? (ht. Of tunneling barrier)0.2eV
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Tunneling Accelerometer - Electronics
Linear forward gain
Logarithmic forward gain
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Tunneling Accelerometer - Force feedback

• Force Feedback is mandatory
• Small Distance between Tunneling electrodes
• Exponential position-to-current relationship

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Tunneling Accelerometer - Comments

• High voltage is unattractive for portable
  applications
• Additional noise source in this device - Shot
  noise
• Due to large displacement-to-current gain, this
  device less effected by electronic noise
• Tip characteristic change is very problematic

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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Piezoresistive Accelerometer - Concept

• Concept
• Place piezoresistor on high stress region of
  beam.
• Proof-mass displacement causes change in
  piezoresistance which is detected electrically.
• Commercial Examples
• Endevco

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Piezoresistive Accelerometer - Position Sense
Note The above wheatstone configuration is a
typical configuration. Paper does not report
configuration
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Piezoresistive Accelerometer - Electronics

• Typical piezoresistive bridge susceptible to
• non-linearity, offset, temperature dependent
  sensitivity and offset drift
• Electronics thrust
• Sensitivity drift correction by making power
  supply dependent on temperature
• Digital offset correction loop
• Offset drift correction using replica wheatstone
  bridge

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Piezoresistive Accelerometer - Comments

• Paper chosen for high performance device and
  integrated approach - Circuit details extremely
  unclear though
• Device will have high off-axis sensitivity
• In general Piezoresistor response highly effected
  by accurate positioning

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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Comparison

• Fabrication (cost, sensor/circuit integration,
  electronic noise)gt Capacitive most attractive
• Power Supply gt Tunneling sensor unattractive
• Offset Drift with Temperature gt piezoresistive
  sensor unattractive

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Overview

• I. Introduction
• II. Concept
• III. Performance Specifications
• IV. Accelerometer Designs
• Capacitive
• Tunneling
• Piezoresistive
• V. Comparison
• VI. Conclusions

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Conclusions

• Three promising integrated accelerometer have
  been discussed, critiqued and compared
• sensing schemes
• electronics
• force feedback
• Capacitive Sensors are promising due to
  technology advantage whereas Tunneling sensors
  are attractive due to potentially high
  performance
• Possible new capacitive sensing scheme for use
  with differential circuits is suggested

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Acknowledgements
Mathew Varghese (MIT) for his technical
assistance