Page 409 - The Mechatronics Handbook

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Most micro- and nanoaccelerometers detect acceleration by measuring the relative motion between
proof mass and mounting substrate. The proof mass is suspended above the substrate by a mechanical
spring suspension. When the sensor undergoes acceleration, the proof mass tends to remain stationary
and therefore displaces with respect to the moving substrate. This displacement is measured capacitively
or by means of piezoresistive or piezoelectric methods using CMOS technology. Chip circuits provide
offset cancellation for bias stability, gain scale factor stability, zero acceleration bias stability, temperature
compensation, prefiltering, noise immune digital output, and so on.
The operational principles of some of the microaccelerometers are very similar to capacitive force-
balance or vibrating-beam accelerometers, discussed earlier. Manufacturing techniques may change from
one manufacturer to another. However, in general, vibrating-beam accelerometers are preferred because
of better air-gap properties and improved bias performance characteristics.
Vibrating-beam accelerometers, also termed resonant-beam force transducers, are made in such a way
that an acceleration along a positive input axis places the vibrating beam in tension. Thus, the resonant
frequency of the vibrating beam increases or decreases with the applied acceleration.
In DETF, an electronic oscillator capacitively couples energy into two vibrating beams to keep them
oscillating at their resonant frequency. The beams vibrate 180° out of phase to cancel reaction forces at the
ends. The dynamic cancellation effect of the DETF design prevents energy from being lost through the
ends of the beam. Hence, the dynamically balanced DETF resonator has a high Q factor, which leads to a
stable oscillator circuit. The acceleration signal is produced from the oscillator as a frequency-modulated
square wave that can be used for a digital interface.
The frequency of resonance of the system must be much higher than any input acceleration, and this
limits the measurable range. In a micromachined accelerometer, used in military applications, the fol-
lowing characteristics are given: a range of ±1200g, a sensitivity of 1.11 Hz/g, a bandwidth of 2500 Hz,
an unloaded DETF frequency of 9952 Hz. The frequency at +1200g is 11,221 Hz, the frequency at –1200g
is 8544 Hz, and the temperature sensitivity is 5 mg/°C. The accelerometer size is 6 mm diameter by
4.3 mm length, with a mass of about 9 g. It has a turn-on time of less than 60 s, the accelerometer is
powered with +9 to +16 V DC, and the nominal output is a 9000-Hz square wave.
Surface micromachining has also been used to manufacture specific application accelerometers, such
as air-bag applications in the automotive industry. In one type, a three-layer differential capacitor is
created by alternate layers of polysilicon and phosphosilicate glass (PSG) on a 0.38-mm thick, 100-mm
long wafer. A silicon wafer serves as the substrate for the mechanical structure. The trampoline-shaped
middle layer is suspended by four supporting arms. This movable structure is the seismic mass for the
accelerometer. The upper and lower polysilicon layers are fixed plates for the differential capacitors. The
glass is sacrificially etched by hydrofluoric acid (HF).

Signal Conditioning and Biasing

Common signal conditioners are appropriate for interfacing accelerometers to computers or other instru-
ments for further signal processing. Generally, the generated raw signals are amplified and filtered suitably
by the circuits within the accelerometer casing supplied by manufacturers. Nevertheless, piezoelectric
and piezoresistive transducers require special signal conditioners with certain characteristics that will be
discussed next.
Piezoelectric Accelerometers
Piezoelectric accelerometers supply small energy to the signal conditioners since they have high capacitive
source impedances. The equivalent circuit of a piezoelectric accelerometer can be regarded as an active
capacitor that charges itself when mechanically loaded. The selection of the elements of the external
signal conditioning circuit is dependent on the characteristics of the equivalent circuit. A most common
approach is the charge amplifier since the system gain and low-frequency responses of these amplifiers
are well defined. The performance of the circuit is also independent of cable length and capacitance of
the accelerometer. In many applications, noise-treated cables are necessary to avoid the triboelectric
charges occurring due to movement of cables.

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