ECE S07 Accelerometer Types
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Accelerometer Types
� Common Accelerometer Types± Resistive
� Strain Gauge
� Piezoresistive
� Micromachined
� Thin-Film
± Capacitive
± Fiber Optic
± Servo or Force Balance
± Vibrating Quartz± Piezoelectric
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Accelerometer Types
� Resistive Operating Principle± Voltage output of resistor bridge changes proportionally with applied acceleration
+ Power - Signal+ Signal - Power
Mass
Sensing Resistor #1
Flexure
Sensing Resistor #2
Fixed Resistors
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Accelerometer Types
� Capacitive Operating Principle± Utilizes frequency modulation technique through varying capacitor bridge
Power SignalGround
Mass
Sensing Capacitor #1
Built-In ElectronicsFixed Capacitors ~
Sensing Capacitor #2
Flexure
Insulator
Insulator
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Accelerometer Types
� Resistive / Capacitive± Typical Characteristics
� Measure down to 0 Hz (DC response)
� Limited dynamic range (<80 dB = 10,000:1)
� Limited high frequency range (<10 kHz)
� Often a damped frequency response (0.7% of critical)
� Sensitivity may vary with input (mV/g/V)
� Traditionally fragile (limited shock protection)
� Operates multi-conductor cable (at least 3 wires)
� Micro-machined versions are small and lightweight� Performance matches cost ($10 to $1000 USD)
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Accelerometer Types
� Resistive / Capacitive± Applications
� Low frequency and/or long duration events
± Ride quality
� Automobile road response
� Amusement park rides
� Elevator movement
� Motion simulators
± Aerospace structure modal analysis surveys
± Crash dummy instrumentation� Tilt sensors
� Airbag or automobile alarm triggering devices
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Accelerometer Types
� Fiber Optic Operating Principle± Amount of light gathered by receivers is proportional to applied acceleration
Power SignalGround
Mass
Transmitter
Built-In Electronics
Receiver
Flexure Flexure
Receiver
Reflective Surface
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Accelerometer Types
� Fiber Optic± Similar characteristics and applications as resistive and
capacitive sensors
± Additional features
� Provision for remotely locating electronics
± High temperature operation to 1000 F (537 C)
± Cabling is transmitting only light, which consequently
eliminates the possibility of RF and EM interference in
³noisy´ environments
± Traditionally, light loss in long cables and connections
was a consideration
± Expensive sensors, cabling and signal conditioning
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Accelerometer Types
� Servo or Force Balance Operating Principle± Feedback force required to maintain uniform capacitance is proportional to acceleration
Power SignalGround
Flexure
CoilMagnetic
Mass
Capacitance Gap
Stationary
Support
Feedback Power AmplifierSensing Amplifier
Insulator
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Accelerometer Types
� Vibrating Quartz± Resonant frequency difference between elements is proportional to applied acceleration
Power SignalGround
Flexure
Frequency Tracking Amplifiers
Inverting Amplifier
Vibrating Crystal #2
Mass
Mass
Vibrating Crystal #1
Flexure
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Accelerometer Types
� Force Balance / Vibrating Quartz± Typical Characteristics
� Measure down to 0 Hz (DC response)
� Wide dynamic range (>120 dB = 1,000,000:1)
� Extremely stable over time and temperature (ppm)
� Limited high frequency range (<1 kHz)
� Poor overload survivability (<100 g¶s)
� Force balance may exhibit large magnetic sensitivity
� Very expensive (~$1000 USD)
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Accelerometer Types
� Piezoelectric± Force on self-generating crystal provides charge output proportional to acceleration
Signal/Power Ground
Base
Voltage or Charge Amplifier
Preload Ring
Mass
Piezoelectric Crystal
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Piezoelectric Materials
� Piezoelectric Effect± Word origin comes from the greek work ³piezen´ which
translates ³to squeeze´.
± The generation of an electrical signal by a dielectric material as
it is subjected to a mechanical stress.
F
F
+
-
Piezoelectric
Material+ + + + + +
- - - - - -
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Piezoelectric Materials
� Piezoelectric Materials± Naturally Piezoelectric
� Rochelle Salt
± One of first materials used to make sensors
� Tourmaline
± Sensitive to hydrostatic pressure
� Exotic, ³Man-Made´ Materials
± Langasite
± Lithium Niobate
� Cultured Quartz
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Piezoelectric Materials
� Piezoelectric Materials± Artificially Polarized
� Piezofilm
± Made of a special polymer - PVDF
� Piezoceramics
± Lead Zirconate Titanate (PZT)
± Bismuth Titanate
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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
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Mechanical Design
� Shear Mode± Most commonly utilized design based on overall performance
Preload RingSeismicMass
Center
Post
Signal (+)
Optional Built-In
Electronics
Ground (-)
Piezoelectric
Crystal
(d26-Quartz)
(d15-Piezoceramic)
-
--
-
-
--
-
+
+
+
+
+
+
+
+
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Accelerometer Types
� Piezoelectric± Typical Characteristics
� Dynamic events only (>0.2 Hz)
� Wide dynamic range (>100dB = 100,000:1)
� Wide frequency bandwidth (<1 Hz to >10 kHz)
� Solid-state (No moving parts)
� Self-generating piezoelectric elements require no power
� Operates over two conductors
� Rugged (5,000 g¶s)
� High temperature charge versions operate to 1000 F (537 C)
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Accelerometer Types
� Summary± Many different types of accelerometers are available and they
often represent an excellent choice for making vibration
measurements; however, accelerometers are not well-suited for
all applications as no single sensor can meet every vibration
requirement.± Don¶t underestimate the sensor selection process as it is easy to
generate ³bad data´ without the proper tr ansducer.
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Calibration Methods
� Absolute Method± Single channel test where the sensor is subjected to a known,
accurate and reliable measure of ³a´
� Drop Test
� Gravity Inversion Test
� Handheld Shaker
Test
Sensor
Amplifier,
Attenuator,
Filter, Etc...
Voltmeter,
Analyzer,
Scope, Etc...
Known
Measure of
³a´
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Calibration Methods
� Drop Test± Accelerometer is allowed to free-fall in Earth¶s gravity which
varies by less than +/-0.5% around the globe
Signal OutAccelerometer
MountingMass
Flexible
Monofilament Line
Elastic Suspension
Cords
Impact ForceFixed
Supports
Earth¶s Gravity
0 Deg Latitude: 9.78 m/s2
90 Deg Latitude: 9.32 m/s2
Altitude Correction: -3 mm/s2 per 1000 m above sea level
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Calibration Methods
� 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
Test Sensor
Rotation Fixture
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Calibration Methods
� Relative Method± Dual channel test where the test sensor and calibr at ed r efer ence
are subjected to the ident ical input acceler at ion. The ratio of
the output signals provides the calibration factor.
� Laser Fringe Counting (Primary Method)
� Back-to-Back Calibration (Secondary Method)
Test
Sensor
Amplifier,
Attenuator,
Filter, Etc... Voltmeter,
Analyzer,
Scope, Etc...
Input
SignalReference
Sensor
Amplifier,
Attenuator,
Filter, Etc...
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Calibration Methods
� Laser Calibration± Non-contacting measurement principle
� Structure not affected by measurement device
± Utilizes ³fringe counting´ of laser light
� This method provides primary calibration as it is based on
a constant on nature«the wavelength of light
± Expensive
± Requires relatively large accelerations at high frequencies
� 25 g¶s at 5 kHz; 50 g¶s at 10 kHz; 100 g¶s at 20 kHz
± Procedure and set-up is documented in approved ISOStandard ISO 5347-1
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Calibration Methods
� Back-To-Back Calibration± Test sensor mounts directly to a reference accelerometer which
has been previously calibrated by primary means or by a
recognized laboratory
InstrumentGrade Shaker
Reference
Accelerometer
TestAccelerometer
Controllable
Acceleration Level
Vtest
Vref