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