Page 70 - The Mechatronics Handbook

Acceleration

Another commercially successful microsensor is the silicon microfabricated accelerometer, which in
various forms can measure acceleration ranges from well below one to around a thousand meters per
square second (i.e., sub-g to several hundred g’s), with resolutions of one part in 10,000. These sensors
incorporate a micromachined suspended proof mass that is subjected to an inertial force in response to an
acceleration, which causes deflection of the supporting flexures. One means of measuring the deflection is
by utilizing piezoresistive strain gages mounted on the flexures. The primary disadvantage to this approach
is the temperature sensitivity of the piezoresistive gages. An alternative to measuring the deflection of the
proof mass is via capacitive sensing. In these devices, the capacitance is measured between the proof mass
and an electrode that is rigidly mounted and parallel. Examples of this approach are those by Boxenhorn
and Greiff [40], Leuthold and Rudolf [41], and Seidel et al. [42]. Still another means of measuring the
inertial force on the proof mass is by measuring the resonant frequency of the supporting flexures. The
inertial force due to acceleration will load the flexure, which will alter its resonant frequency. The frequency
of vibration is therefore a measure of the acceleration. These types of devices utilize some form of
transduction to excite the structural resonance of the supporting flexures, and then utilize some other
measurement technique to detect the frequency of vibration. Examples of this type of device are those
by Chang et al. [43], which utilize electrostatic excitation and capacitive detection, and by Satchell and
Greenwood [44], which utilize thermal excitation and piezoresistive detection. These types of acceler-
ometers entail additional complexity, but typically offer improved measurement resolution. Still another
variation of the micro-accelerometer is the force-balanced type. This type of device measures position
of the proof mass (typically by capacitive means) and utilizes a feedback loop and electrostatic or
electromagnetic actuation to maintain zero deflection of the mass. The acceleration is then a function
of the actuation effort. These devices are characterized by a wide bandwidth and high sensitivity, but are
typically more complex and more expensive than other types. Examples of force-balanced devices are
those by Chau et al. [45], and Kuehnel and Sherman [46], both of which utilize capacitive sensing and
electrostatic actuation.

Force
Silicon microfabricated force sensors incorporate measurement approaches much like the microfabricated
pressure sensors and accelerometers. Various forms of these force sensors can measure forces ranging on
the order of millinewtons to newtons, with resolutions of one part in 10,000. Mechanical sensing typically
utilizes a beam or a flexure support which is elastically deflected by an applied force, thereby transforming
force measurement into measurement of strain or displacement, which can be accomplished by piezore-
sistive or capacitive means. An example of this type of device is that of Despont et al., which utilizes
capacitive measurement [47]. Higher resolution devices are typically of the resonating beam type, in
which the applied force loads a resonating beam in tension. Increasing the applied tensile load results in
an increase in resonant frequency. An example of this type of device is that of Blom et al. [48].

Angular Rate Sensing (Gyroscopes)
A conventional-scale gyroscope utilizes the spatial coupling of the angular momentum-based gyroscopic
effect to measure angular rate. In these devices, a disk is spun at a constant high rate about its primary
axis, so that when the disk is rotated about an axis not colinear with the primary (or spin) axis, a torque
results in an orthogonal direction that is proportional to the angular velocity. These devices are typically
mounted in gimbals with low-friction bearings, incorporate motors that maintain the spin velocity, and
utilize strain gages to measure the gyroscopic torque (and thus angular velocity). Such a design would
not be appropriate for a microsensor due to several factors, some of which include the diminishing effect
of inertia (and thus momentum) at small scales, the lack of adequate bearings, the lack of appropriate
micromotors, and the lack of an adequate three-dimensional microfabrication processes. Instead, micro-
scale angular rate sensors are of the vibratory type, which incorporate Coriolis-type effects rather than

©2002 CRC Press LLC