Page 105 - MEMS Mechanical Sensors

94 Mechanical Transduction Techniques

where V is the supply voltage. So this arrangement provides a linear relationship
s
d
that is preserved over a range of x < and is capable of detecting displacement of a
few picometers.
Capacitor structures are relatively straightforward to fabricate, and
membrane-type devices are often used as the basis for pressure sensors and micro-
phones. More elaborate structures, such as interdigitated capacitors, are also used,
and the effects of the fringing fields cannot always be ignored. With such devices, the
simple parallel plate capacitor equation only provides a crude estimate of the
expected capacitance change.
Capacitive techniques are inherently less noisy than those based on piezoresis-
tance owing to the lack of thermal (Johnson) noise. With micromachined devices,
however, the values of capacitance are extremely small (in the range of femto- to
attofarads), and the additional noise from the interface electronic circuits often
exceeds that of a resistance-based system.
There are a variety of techniques for measuring capacitance changes including
charge amplifiers (often used with piezoelectric devices), charge balance tech-
niques, ac bridge impedance measurements, and various oscillator configurations.
There are also a variety of commercially available ICs that can be used to measure
capacitance changes of a few femtofarads in stray capacitances up to several
hundred picofarads [2].

5.4 Optical Techniques

Optical sensing techniques primarily rely on modulating the properties of an optical
frequency electromagnetic wave. In the case of optical sensors, the measurand
directly modulates the properties of the electromagnetic wave. In the case of micro-
sensors, which use optical interfacing, the miniaturized sensor interacts with the
measurand. The microsensor then modulates a property of the optical signal in
order to provide an indication of the measurand.
The following properties of the electromagnetic wave can be altered:

1. Intensity;
2. Phase;
3. Wavelength;
4. Spatial position;
5. Frequency;
6. Polarization.

The basic principles of each of these techniques will now be reviewed in turn.

5.4.1 Intensity
The primary advantage of intensity modulation is that intensity variations are simply
detected because all optical detectors (e.g., photodiodes, phototransistors) directly
respond to intensity variations. Therefore, if the microsensor can be arranged to vary
the intensity of an optical signal, these variations can then be simply observed using a