Accelerometers sense speed changes by measuring the force produced by the
change in velocity of a known mass (seismic mass), see Eq. (10.1). These devices
can be made with a cantilevered mass and a strain gauge for force measurement
or can use capacitive measurement techniques. Accelerometers are now
commercially available, made using micromachining techniques. The devices canbe as small as 500 μm × 500 μm, so that the effective loading by the accelerometer
on a measurement is very small. The device is a small cantilevered seismic mass
that uses capacitive changes to monitor the position of the mass. Piezoelectric
devices similar to the one shown in Fig. 10.3c are also used to measure acceleration.
The seismic mass produces a force on the piezoelectric element during acceleration
which causes a voltage to be developed across the element. Accelerometers
are used in industry for the measurement of changes in velocity of moving equipment,
in the automotive industry as crash sensors for air bag deployment, and in
shipping crates where battery operated recorders are used to measure shock during
the shipment of expensive and fragile equipment.
Vibration sensors typically use acceleration devices to measure vibration.
Micromachined accelerometers make good vibration sensors for frequencies up
to about 1 kHz. Piezoelectric devices make good vibration sensors with an excellent
high-frequency response for frequencies up to 100 kHz. These devices have
very low mass so that the damping effect is minimal. Vibration sensors are
used for the measurement of vibration in bearings of heavy equipment and
pressure lines.