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Piezoelectric (PE) accelerometers, as their name
implies, are based on a small piezoelectric bar,
Fig. 5. If the support is accelerated, the crystal
element generates an electrical charge at the ter­
minals; either charge or voltage is measured.
PEAs do not function at frequencies approaching Quartz plates
zero, but they operate to high frequencies, have Electronics
wide dynamic ranges (from a fraction of a g to
Seismic mass
several thousands gs), are small and rugged, and
are quite simple to mount mechanically. In con­
trast with other types of accelerometers that must
be supplied with power, PE accelerometers are
self-generating.

Sensor Comparison
Fig. 7—PE accelerometer with part of signal­
Relative advantages of different sensors can be conditioning electronics in the sensor. When
easily summarized: displacement and velocity pick­ this is done, problems with leakage resistance,
ups are losing favor to accelerometers. Engineers capacitance and cable noise are minimized or
eliminated.
in the vibration field generally feel that accelera­
tion better indicates internal forces due to failed
gears, bearings, or other components. Hence, ac­
celerometers are preferred to other sensors, espe­
cially for critical applications. Some other advan­
tages of accelerometers are noted. Signal Conditioning Equipment
Accelerometers have wide linear-frequency
range—often to beyond 10 ,000 Hz, Fig. 6. Many Signals from sensors are seldom suitable for
velocity pickups are linear only to about 100 Hz. direct measurement any more than, for example,
signals from microphones. Suitable amplification
Accelerometers can be very rugged; some PE
units are useful at 10,000 g, whereas few velocity (and possibly other processing) must be per­
pickups will stand 100 g. formed for the signal to swing an oscillograph
galvanometer, to move the needle of a meter, or
■ Velocity and displacement sensors (even if sensor to be recorded on tape.
response is linear) often ignore high-frequency The major difficulties with using PE accelerom­
vibration such as might be initiated by gear and eters stem from the cables that carry the signals
bearing failures.
and from the signal-conditioning amplifiers them­
■ Under conditions of varying ambient tempera­ selves. When older voltage-sensing amplifiers are
tures, PE accelerometers are superior to other types used, changes in cable length after a system is
of sensors. One manufacturer offers a line of ac­ calibrated can change overall sensitivity. Also,
celerometers that will function over a temperature high-impedance crystal accelerometers, when used
range of — 452F to +750F; another claims good with these older amplifiers, have been found to
operation to 1100F. be sensitive to resistive leakage. Moisture, salt
■ In most vibration measurements the sensor be­ —even fingerprints—can cause significant errors
comes part of the vibrating system, thus affect­ in low-frequency measurements.
ing the motion. Natural frequencies are lower, and Newer charge amplifiers minimize these diffi­
mode shapes are different from those that would culties but do little to help another problem:
occur without the pickup. A half-pound velocity “triboelectric” effects in cables, which are noise
pickup could never be used on a flimsy structure signals generated when cables are physically
such as a printed-circuit board. Many PE and PR stressed. Nor do charge amplifiers help the prob­
accelerometers are small and light, thus having lem of “ground loops” that exist when pickups
minimum effect upon the motion being measured. are fastened to points having different electrical
■ Larger sensors can be hard to mount; some re­ potentials than do the electronics grounding points.
quire a flat surface and four bolts. By contrast, These problems are less severe when strain-gage,
PE and PR accelerometers offer several mounting PR, and servo accelerometers or velocity pickups
methods because of their small size. The best way are used, all of which have low electrical imped­
for severe service is to use a machine screw or stud ance.
to compress the accelerometer and structure to­ Another advance in signal conditioning results
gether, thus preventing chattering. Other accept­ from the practice of placing part of the condition­
able mounting methods include adhesive bonding ing electronics package inside the accelerometer
and attaching with double-faced pressure-sensitive itself, Fig. 7. Accelerometer signals are amplified
adhesive tape. prior to transmittal through the cables, reducing
May 29, 1969 123