Page 523 - The Mechatronics Handbook

0066_frame_C19 Page 145 Wednesday, January 9, 2002 5:32 PM

FIGURE 19.118 Schematic diagram of a bulk micromachined accelerometer.
Pressure Sensors Made of Non-Silicon Materials
For certain applications such as monitoring of internal combustion engine, pressure sensors are required
to sustain high temperature of operation. In such cases, silicon is not the optimal material because high
temperature causes doped silicon junctions to fail.
Work has also been done to implement polymer materials for pressure sensors. Though such devices
are relatively few, they represent an important development trend for future sensors.
Accelerometers
Bulk Micromachined Accelerometers
Acceleration sensors (or so-called inertial measurement units, IMU) are important for monitoring
acceleration and vibration experienced by a subject, such as an automobile, a machine, or a building.
Low-cost accelerometers used in automobile airbag deployment systems can reduce the costs and enhance
driver safety. Micromachined sensors can be made small and sufficiently low-cost to be used in smart
projectiles, for example, concrete penetrating bombs. Small, multi-axial accelerometers can also be applied
in writing instruments (smart pens) for handwriting recognition.
A representative bulk micromachined accelerometer is illustrated in Fig. 19.118. A SEM micrograph
of a prototype sensor is shown in Fig. 19.119. A silicon proof mass is attached to the end of a cantilever
beam. At the base of the cantilever beam lies a piezoresistive element. Supposing the mass of the proof
mass is m, and the magnitude of the acceleration is a, one can estimate the sensor output following a
few simple analysis steps. First, a concentrated force with a magnitude of F = ma is applied in the center
of the proof mass according to Newton’s first law. Secondly, the force translates into a torque loading at
the base of the cantilever with the magnitude being



--- =
M = F l + L  ma l + L 
---

2
2

The magnitude of the strain experienced at the surface of the cantilever beam, where the piezoresistors
are located, is
Mt
e = --------
2EI
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