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Accelerometers

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Title: Accelerometers


1
Accelerometers
  • Cory Celestino
  • Frank Esselink
  • Joe Goode
  • ME 601

2
Outline
  • Definition
  • Applications
  • Functionality
  • Selection
  • Difficulties
  • Example

3
Definition
  • An instrument for measuring acceleration or for
    detecting and measuring vibrations
  • Every measuring device used to record an event
    is, at its most fundamental level, an
    accelerometer.
  • www.circlon.com/html/maxcar.html

4
Applications
  • Airbag sensors
  • Industrial Machinery
  • Inertial Navigation
  • Aircraft
  • Computer Joysticks
  • Subwoofer design
  • Experimental measurement equipment

5
Types
  • Potentiometric (deflection)
  • Linear Variable Differential Transformer (null
    balance)
  • Variable reluctance
  • Piezoelectric

6
Potentiometric
  • Deflection type
  • Low frequency, slowly varying accleration
  • 1g to 50g full scale
  • 12Hz to 86Hz natural frequency
  • Rely on variable resistors

7
LVDT
  • Null Balance type
  • Mid-range natural frequencies
  • Use changing magnetic fields

8
Piezoelectric
  • Null Balance type
  • Voltage generated by stressed crystal
  • Natural frequencies up to 5 KHz
  • Cannot measure constant acceleration

9
Functionality
  • Can measure velocity and position values through
    electrical integration
  • Use Newtons Law and Hookes Law
  • Fma, FkDx
  • Potentiometric most useful in low vibration
  • Piezoelectrics useful in vibration and shock
    measurements due to high natural frequency

10
Interface
  • Produce an output voltage which varies with
    acceleration
  • Calibration is required for accurate measurements
    of shock, vibration, accel
  • Response changes over the operating range
  • Algorithms needed

11
Selection Factors
  • Frequency range
  • Natural frequency
  • Damping coefficient
  • Scale factor
  • Relates output to input
  • Measurable acceleration range
  • Cost
  • Size / Weight
  • Durability / Life
  • Accuracy

12
Difficulties
  • Maintaining appropriate voltage range for full
    use of signal processing
  • Accurate calibration
  • Developing data algorithm to provide meaningful
    insight into data
  • Mass system is affected by gravity

13
Resonance
  • Natural frequency and damping couple to cause
    problems
  • At much higher frequencies, input frequency has
    little effect on output
  • At lower frequencies, input frequencies have some
    effect on output
  • Want to operate well away from the natural
    frequency to avoid resonance

14
Example Airbag
  • Deployment Process
  • Crash causes rapid deceleration
  • Tiny accelerometer etched on a silicon chip
    activates igniter
  • Igniter causes rapid release of gas into airbag
    which then inflates
  • Small holes in bag allow rapid deflation
  • Cost 10 each
  • 50 million airbag accelerometers sold a year

15
Sources
  • Measurement Systems Application and Design 4th
    Ed.
  • by Ernest O. Doebelin
  • National Instruments (zone.ni.com)
  • www.howstuffworks.com
  • www.merriamwebster.com
  • www.circlon.com
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