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FIGURE 19.24 Bonding of piezoresistive and piezoelectric accelerometers to the inertial systems.

of piezoresistive accelerometers may be listed to be 100 mV/g as the sensitivity, 0–750 Hz as the frequency
range, 2500 Hz in resonance frequency, 25g as the amplitude range, 2000g as the shock rating, and 0–95°C
as the temperature range, with a total mass of about 25 g.
Most contemporary piezoresistive sensors are manufactured from a single piece of silicon. This gives
better stability and less thermal mismatch between parts. In a typical monolithic sensing element a 1-mm
silicon chip incorporates the spring, mass, and four-arm bridge assembly. The elements are formed by a
pattern of dopant in the originally flat silicon. Subsequent etching of channels frees the gauges and simul-
taneously defines the masses as regions of silicon of original thickness.

Strain-Gauge Accelerometers

Strain-gauge accelerometers are based on resistance properties of electrical conductors. If a conductor
is stretched or compressed, its resistance alters due to (a) dimensional changes, and (b) the changes in
the fundamental property of material called piezoresistance. This indicates that the resistivity ρ of the
conductor depends on the mechanical strain applied onto it. The dependence is expressed as the gauge
factor:

dR/R dr/r
------------- = 1 + 2v + ------------ (19.23)
dL/L dL/L
where 1 indicates the resistance change due to length, 2v indicates resistance change due to area, and
(dρ/ρ)/(dL/L) indicates the resistance change due to piezoresistivity.
There are many types of strain-gauges: unbonded metal-wire gauges, bonded metal-wire gauges,
bonded metal-foil gauges, vacuum-deposited thin-metal-film gauges, bonded semiconductor gauges, and
diffused semiconductor gauges. However, usually bonded and unbonded metal-wire gauges find wider
applications. A section of the strain-gauge accelerometers, particularly bonded semiconductor types,
known as the piezoresistive transducers, are used, but they suffer from high temperature sensitivities,
nonlinearities, and some mounting difficulties. Nevertheless, with the recent developments of microma-
chine technology, these sensors have been improved considerably, thus finding many new applications.
Unbonded-strain-gauge accelerometers use the strain wires as the spring element and as the motion
transducer, using similar arrangements as in Fig. 19.25. They are useful for general-purpose motion and
vibration measurements from low to medium frequencies. They are available in wide ranges and char-
acteristics: typically ±5g to ±200g full scale, a natural frequency of 17–800 Hz, a 10-V excitation voltage
AC or DC, full scale output ±20 mV to ±50 mV, a resolution less than 0.1%, an inaccuracy less than 1%
full scale, and a cross-axis sensitivity less than 2%. The damping ratio (using silicone oil damping) is
0.6–0.8 at room temperature. These instruments are small and light, usually with a mass less than 25 g.

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