Accelerometers

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Accelerometers

• Justin Piccirillo
• Texas Instruments Incorporated
• Sensors Controls Division
• jpiccirillo_at_ti.com

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Summary

• Definition of Acceleration
• Technologies
• Texas Instruments - Capacitive Acceleration
  Sensor (CAS)
• Terminology
• Effect of Tilt
• Typical applications
• Demonstration
• Summary
• Questions Answers

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Acceleration Fundamentals

• What is Acceleration?
• Definition the time rate ofchange of velocity
• A.K.A. the time rate of changeof the time rate
  of change of distance
• What are the units?
• Acceleration is measured in (ft/s)/s or (m/s)/s
• What is a g?
• A g is a unit of acceleration equal to Earths
  gravity at sea level
• 1 g 32.2 ft/s2 or 9.81 m/s2

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More Notes on Acceleration

• What is the time rate of change of velocity?
• When plotted on a graph, velocity is the slope of
  distance versus time
• Acceleration is the slope of velocity versus time

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Acceleration in Human Terms

• What are some g reference points?
• Description g level
• Earths gravity 1g
• Passenger car in corner 2g
• Bumps in road 2g
• Indy car driver in corner 3g
• Bobsled rider in corner 5g
• Human unconsciousness 7g
• Space shuttle 10g

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Whats the point?

• Why measure acceleration?
• Acceleration is a physical characteristic of a
  system.
• The measurement of acceleration is used as an
  input into some types of control systems.
• The control systems use the measured acceleration
  to correct for changing dynamic conditions

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Common Types of Accelerometers

• Sensor Category Key Technologies
• Capacitive -Metal beam or micromachined feature
  produces capacitance change in
  capacitance related to acceleration
• Piezoelectric -Piezoelectric crystal mounted to
  mass voltage output converted to
  acceleration
• Piezoresistive -Beam or micromachined feature
  whose resistance changes with acceleration
• Hall Effect -Motion converted to electrical
  signal by sensing of changing magnetic
  fields
• Magnetoresistive -Material resistivity changes
  in presence of magnetic field
• Heat Transfer -Location of heated mass tracked
  during acceleration by sensing
  temperature

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What Type of Acceleration Sensor Does TI Produce
and why?

• Capacitive Acceleration Sensor
• CAS

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CAS Conceptual Design
AccelerationInduced Load
100
Movable Blade
75
Output Voltage
Capacitive SensingElement
OUTPUT (Vcc)
50
25
Rigid Substrate
BearingPin
0
ACCELERATION
Electronic Calibration
Acceleration
Voltage Output Proportional to Mech. Input
Change in Capacitance
Conditioning Electronics
Mechanical Deflection
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Capacitive Sensing Element (CSE)
Blade(welded to bearing pin)
Capacitancedetect
Capacitance generated by electrical current
through parallel plates
Compliant Pins
Substrate
C capacitanceK dielectric constant of the
insulating mediumeo permittivity of free
spaceA effective aread distance between
plates (gap) function of
acceleration
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Conditioner Module
Connection to Base
Application Specific Integrated Circuit
(ASIC) Works in conjunction with discrete
electronic components (i.e. capacitors,
resistors, thermistors, etc.) to transform the
variable capacitance read at the CSE into an
output voltage given an electrical potential
(Vcc vs. GND 5V)
ASIC
Connection to CSE
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Capacitive Acceleration Sensor
Conditioner Module
Base
Blade
BearingPin
Shell
Bracket
Substrate
Cup
CompliantPins
ProtectiveSleeve
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Acceleration Sensor Terminology

• (Typical TI Convention)
• 1g Output of the sensor with the base
  connector pointed up
• 0g Output of the sensor with the base connector
  horizontal
• -1g Output of the sensor with the base
  connector pointed down
• Linearity The maximum deviation of the
  calibration curve from a straight line.

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Acceleration Sensor Terminology

• Sensitivity A measure of how much the output of
  a sensor changes as the input acceleration
  changes. Measured in Volts/g
• Vcc The voltage supplied to the input of the
  sensor
• 5.000 0.25V for CAS device
• Vcc Readings are often represented as a of
  the supply voltage. This allows for correction
  due to supply voltage variances between readings.

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Example Sensitivity Linearity
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Acceleration Sensor Terminology

• Ratiometric The output of the sensor changes
  with a change in the input voltage.
• Example
• At Vcc 5.000V, Vout at 0g 1.800V
• In terms of Vcc, this is 1.800Vout/5.000Vcc
  100 36Vcc
• Now suppose the input voltage changes Vcc
  5.010V.
• At 0g, the ratiometric device output is still 36
  Vcc.
• In terms of the output voltage, 36Vcc 5.010Vcc
  1.804Vout
• So a 0.010V change in Vcc will cause a 0.004V
  error in the 0g output if you do not evaluate the
  output as Vcc

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DC Response

• DC response means that a sensor can measure 0Hz
  (static) events.
• Static position or tilt are 0Hz events
• Some sensor types cannot measure static events.
  They need motion to give an output. This type of
  sensor rolls off as you get closer to 0Hz.
• The CAS is a DC response sensor

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Effect of Tilt

• DC response sensors measure tilt. Mounting
  errors are therefore significant
• a 1o tilt in the 0g position creates an output
  error equivalent to a 10o tilt in the 1g or -1g
  positions
• 0g is the most sensitive to mounting errors

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Why is device sensitive to tilt in the 0g
orientation?
Gx
q
GnGCos(q) g level going from 1g to some of 1g
q
Gx
G
G
q
Gn
blade
Gn
pin
Gn GSin(q) g level going from 0g to some value
CSE substrate
q
0g Orientation q 1 ? Gn 1.7x10-2G
1g Position (-1g Position uses same equation)
q 1 ? Gn 0.9998G
Conclusion at 0g orientation, change in 1 tilt
causes 57x bigger change in sensor output versus
-1g or 1g orientation
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Effect of Tilt on DC Accelerometer
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Typical Accelerometer Applications

• Tilt / Roll
• Vibration / Rough-road detection
• Can be used to isolate vibration of mechanical
  system from outside sources
• Vehicle skid detection
• Often used with systems that deploy smart
  braking to regain control of vehicle
• Impact detection
• To determine the severity of impact, or to log
  when an impact has occurred
• Input / feedback for active suspension control
  systems
• Keeps vehicle level

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Important Setup Requirements for your CAS Device

• Rigid Mounting
• Bees Wax
• Double Sided tape
• Bolt(s)
• No Loose Wires
• Loose wires can create false signals
• Secure wires firmly to mounting body
• Weight of Sensor
• Should be approximately an order of magnitude
  less than object being measured
• Example CAS 47g accelerating object should
  be more than 470g
• Dont drop the sensor!
• Extreme jarring accelerations can cause permanent
  errors in device output

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Demonstration
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Electronics Setup
Piezoelectric speaker
9V
1 channel 12 bit A/D Converter LTC1286
Serial connection to PC
CAS Sensor
Vcc
Microprocessor, RAM, EEPROM
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Example Output Tilt Data
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Vehicle Acceleration on Road Raw Data 4
points / second
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Acceleration 1 second averages
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Summary

• Acceleration is a measure of how fast the speed
  of something is changing
• It is used as an input to control systems
• Sensor voltage output should be determined as a
  percentage of voltage input for consistency
• The device is sensitive to tilt in the 0g
  position
• ?Vout for 1o tilt in 0g ?Vout for 10o of tilt
  in the 1g and -1g positions
• Application of the device must be done in a
  secure fashion and to bodies having an
  appropriate sensor-to-body weight ratio
• Think about how you can best use the data
• Sample rates (if sensor output is to be converted
  to digital)
• Averaging schemes
• Control limits

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Questions?
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EXTRA SLIDES
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Frequency Response (FR)

• Definition of FR
• A sensors ability to track a given input
  acceleration in both magnitude and time
• Why is FR important?
• Tells us how well a sensor will measure
  acceleration over a wide range of frequencies.
• Allows us to design sensors to measure only the
  quantities of interest (i.e. want to measure a
  braking event but not vibrations in the mounting
  panel.)

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Components of FR

• Magnitude or Amplitude
• Ratio of the output (CAS) / input (reference
  accel. output)
• Calculated in Decibels (dB) 20 log10(CAS
  Output Voltage / reference acceleration output
  voltage)
• Phase Angle
• A measure of the time delay between the input and
  the output (CAS)
• No time delay would have a Phase angle 0
  degrees
• Frequency
• A measure of the rate at which an event occurs
• 1Hz 1 cycle/second
• The higher the frequency the faster the event
  occurs

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Measurement of FR

• Equipment
• Shaker Table
• Signal Analyzer
• Reference Accelerometer
• CAS
• Setup
• Both the reference accelerometer and the device
  being measured (CAS) are mounted to the shaker
  table.
• The voltage outputs of both sensors are sent to
  the signal analyzer
• The signal analyzer compares the references
  output to the CAS output and produces several
  graphs.

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Key Information on FR Graphs

• Magnitude Graph
• -3dB Point (Often referred to as the roll off
  point) This is the frequency (Hz) point where
  the measured output of the sensor is equal to 70
  of the input acceleration.
• The output is attenuated beyond the -3dB point in
  order to filter out unwanted inputs
• Ex. Input 10g but the output 7g
• Calculated 20log(7/10) -3dB
• Typical -3dB points for a CAS
• 10 to 60 Hz

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Key Information on FR Graphs

• Phase Graph
• The phase angle must be less than a specified
  amount at a specified frequency. Each customer
  will specify different values.
• Examples of phase specifications
• 10o at 5Hz
• 10o at 10 Hz
• 70o at 15Hz

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FR of Your CAS Sensor
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CAS Design Characteristics Effecting FR

• Air Damping of the blade
• The CAS metal beam is critically damped.
• The higher the frequency of the event the more
  the air in the gap reduces the movement of the
  blade.
• The 0g gap has set limits to make sure that the
  sensor remains critically damped.
• If the gap is too small the sensor becomes
  over-damped and will give a reduced output.
• If the gap is too large the sensor becomes
  under-damped and will give an amplified output.

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CAS Design Characteristics Effecting FR

• Effect of Gap on Frequency Response, -3dB

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CAS Design Characteristics Effecting FR

• Electrical Filters
• Capacitors and resistors on the circuit
  electrically filter the output of the CAS
• Different capacitor and resistor values in
  combination with the mechanical air damping of
  the blade create different -3dB points
• One design needs -3dB lt 15Hz
• Another design needs -3dB 50Hz