2. Accelerometer
• An accelerometer is an electromechanical device that measures
acceleration forces.
• These forces may be static, like the constant force of gravity pulling at
your feet, or they could be dynamic - caused by moving or vibrating
the accelerometer.
3. What are accelerometers useful for?
• By measuring the amount of static acceleration due to gravity, we can
find out the angle the device is tilted at with respect to the earth.
• By sensing the amount of dynamic acceleration, we can analyze the
way the device is moving.
4. Common Accelerometer Types
• Resistive
• Strain Gauge
• Piezoresistive
• Micromachined
• Thin-Film
• Capacitive
• Fiber Optic
• Servo or Force Balance
• Vibrating Quartz
• Piezoelectric
7. Resistive / Capacitive Applications
• Low frequency and/or long duration events
Ride quality
o Automobile road response
o Amusement park rides
o Elevator movement
o Motion simulators
Aerospace structure modal analysis surveys
Crash dummy instrumentation
• Tilt sensors
• Airbag or automobile alarm triggering devices
8. Fiber Optic Operating Principle
• Amount of light gathered by receivers is proportional to applied acceleration
9. Servo or Force Balance Operating Principle
• Feedback force required to maintain uniform capacitance is proportional to acceleration
11. Piezoelectric Accelerometer
• Piezoelectric accelerometer is based on principle of piezoelectric
effect.
• A Piezoelectric substance is one that produces an electric charge
when a mechanical stress is applied.
• In a Piezoelectric accelerometer a mass is attached to a Piezoelectric
crystal which is in turn mounted to the case of the accelerometer.
13. Piezoelectric Accelerometer
• When the body of the accelerometer is subjected to vibration the mass
mounted on the crystal wants to stay still in space due to inertia and so
compresses and stretches the piezo electric crystal.
• This force causes a charge to be generated and due to Newton law
(F=ma) this force is in turn proportional to acceleration.
• The charge output is converted to voltage output by the use of integral
electronics (for example: in an IEPE accelerometer) or made available
as a charge output (pc /g) in a charge output Piezo electric
accelerometer.
14. Piezoelectric Materials
Piezoelectric Effect
• Word origin comes from the Greek work “piezen” which translates
“to squeeze”.
• The generation of an electrical signal by a dielectric material as it is
subjected to a mechanical stress.
+ + + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _
+
_
F
Piezoelectric Materials
15. Piezoelectric Materials
• Naturally Piezoelectric
• Rochelle Salt
• One of first materials used to
make sensors
• Tourmaline
• Sensitive to hydrostatic pressure
• Exotic, “Man-Made” Materials
• Langasite
• Lithium Niobate
• Cultured Quartz
• Artificially Polarized
• Piezofilm
• Made of a special polymer -
PVDF
• Piezoceramics
• Lead Zirconate Titanate (PZT)
• Bismuth Titanate
16. Mechanical Design
• Piezoelectric Sensing Element
• Mechanical transduction mechanism as important as piezoelectric material
selection
• The key is to design the sensor so that it only measures the parameter of interest
and minimizes the affects of any outside environmental conditions
• Types
• Compression Mode
• Flexural Mode
• Shear Mode
18. Calibration Methods
• Absolute Method
• Single channel test where the sensor is subjected to a known, accurate and reliable
measure of “a”
• Drop Test
• Gravity Inversion Test
• Handheld Shaker
Known
Measure
of “a”
Test Sensor
Amplifier,
Attenuator,
Filter, Etc...
Voltmeter,
Analyzer,
Scope, Etc...
19. Drop Test
• Accelerometer is allowed to free-fall in Earth’s gravity which varies by less than +/- 0.5%
around the globe
20. Gravity Inversion Test
Sensor is rotated 180 Degrees in the Earth’s gravity so that it
experiences a 2g (-1 g to +1 g) step function
• Requires long DTC or DC response for accurate results
• Signal Conditioning and readout device must be DC coupled
21. Applications of Accelerometers
• Used in cars to study shock and vibrations.
• Camcorders use accelerometers for image stabilization.
• Still cameras use accelerometers for anti-blur capturing.
• Used in mobile phones for multiple functions including tilt detection, motion
detection..etc.
• Process control systems and safety installations in industries.
• Used to measure seismic activity, inclination, machine vibration, dynamic distance and
speed with or without the influence of gravity.
