Types of Accelerometers
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Accelerometer |
Theory of Operation
Types of secondary transducers, which describe how the electric signal is
generated from mechanical displacement, include:
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Piezoelectric transducers are often used in vibration-sensing accelerometers,
and sometimes in shock-sensing devices. The piezoelectric crystals (often
quartz or ceramic) produce an electric charge when a force is exerted by the
seismic mass under some acceleration. The quartz plates (two or more) are
preloaded so that a positive or negative change in the applied force on the
crystals results in a change in the electric charge. Although the sensitivity
of piezoelectric accelerometers is relatively low compared with other types of
accelerometers, they have the highest range (up to 100,000 g's) and frequency
response (over 20 kHz).
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The displacement of the spring-mass system is linked mechanically to a wiper
arm, which moves along a potentiometer. The system can use gas, viscous, or
magnetic damping to minimize acoustic noise caused by contact resistance of the
wiper arm. Potentiometric accelerometers typically have a frequency range from
zero to 20 - 60 Hz, depending on the stiffness of the spring, and have a
high-level output signal. They also have a lower frequency response than most
other accelerometers, usually between 15 - 30 Hz.
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Reluctive accelerometers use an inductance bridge, similar to that of the LVDT
(Linear Variable Differential Transducer) to produce an output voltage
proportional to the movement of the seismic mass. The displacement of the
seismic mass in inductance-bridge accelerometers causes the inductances of two
coils to vary in opposing directions. The coils act as two arms of an
inductance bridge, with resistors as the other two arms. The AC output voltage
of the bridge varies with applied acceleration. A demodulator can be used to
convert the AC signal to DC. An oscillator can be used to generate the required
AC current when a DC power supply is used, as long as the frequency of the AC
signal is far greater than that of the frequency of the acceleration.
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In servo accelerometers, acceleration causes a seismic mass "pendulum" to move.
When motion is detected by a position-sensing device, a signal is produced that
acts as the error signal in the closed-loop servo system. After the signal has
been demodulated and amplified to remove the steady-state component, the signal
is passed through a passive damping network and is applied to a torquing coil
located at the axis of rotation of the mass. The torque developed by the
torquing coil is proportional to the current applied, and counteracts the
torque acting on the seismic mass due to the acceleration, preventing further
motion of the mass. Therefore, the current through the torquing coil is
proportional to acceleration. This device can also be used to measure angular
acceleration as long as the seismic mass is balanced. Servo accelerometers
provide high accuracy and a high-level output at a relatively high cost, and
can be used for very low measuring ranges (well below 1 g).
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Strain gauge accelerometers, often called "piezoresistive" accelerometers, use
strain gauges acting as arms of a Wheatstone bridge to convert mechanical
strain to a DC output voltage. The gauges are either mounted to the spring, or
between the seismic mass and the stationary frame. In the picture, strain gauge
windings, which contribute to the spring action, are stressed (two in tension,
two in compression), and a DC output voltage is generated by the four arms of
the bridge that is proportional to the applied acceleration.
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These accelerometers can be made more sensitive with the use of semiconductor
gauges and stiffer springs, yielding a higher frequency response and output
signal amplitude. And unlike other types of accelerometers, strain gauge
accelerometers respond to steady-state accelerations.
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A change in acceleration causes a change in the space between the moving and
fixed electrodes of a capacitive accelerometer. The moving electrode is
typically a diaphragm-supported seismic mass or a flexure-supported,
disk-shaped seismic mass. The element can act as the capacitor in the LC or RC
portion of an oscillator circuit. The resulting output frequency is
proportional to the applied acceleration.
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In a vibrating element accelerometer, a very small displacement of the seismic
mass varies the tension of a tungsten wire in a permanent magnetic field. A
current through the wire in the presence of the magnetic field causes the wire
to vibrate at its resonant frequency (like a guitar string). The circuitry then
outputs a frequency modulation (deviation from a center frequency) that is
proportional to the applied acceleration. Although the precision of such a
device is high, it is quite sensitive to temperature variations and is
relatively expensive.
Last Updated: January 16, 2000, beam@bits.me.berkeley.edu
Copyright © 1993-1995, 2000, Pamela A. Eibeck and Brandon Muramatsu
Original WWW Conversion by Winston Wang, 1994
WWW ReConversion by Brandon Muramtasu, 2000
Secondary Transducers