Page 107 - MEMS Mechanical Sensors

96 Mechanical Transduction Techniques

5.4.3 Wavelength

Wavelength-based sensing relies on the source spectrum being modulated by inter-
action with the microsensor. Normally a source with a broad spectrum is used. The
light returned from the microsensor is split into spectral segments and incident on a
photodetector for measurement of its intensity. By a prior knowledge of the poten-
tial modulation mechanism present with the microsensor, one can identify the
measurand and its magnitude. A good example of a wavelength-based sensor is one
based on the gas absorption, which is highly wavelength specific according to the
quantity of gas present.
The advantage of wavelength-based sensors is that they can be made insensitive
to intensity variation since these affect the whole spectrum in the same way. There-
fore, the measurement of a nonabsorbed wavelength can be used to reference the
absorbed wavelength, therefore compensating for intensity variations. In addition,
wavelength-based sensors often lend themselves to the measurement of multiple
parameters since the light spectrum can be divided according to the particular wave-
length corresponding to the measurand of interest.

5.4.4 Spatial Position
Figure 5.8 illustrates the principle of the modulation of special position by means of
the movement of a microsensor. This technique is often known as triangulation.
This technique is simple to implement and has the advantage of immunity to
source intensity variations. Its resolution is less then phase-based techniques.

5.4.5 Frequency
If optical radiation at a frequency f is incident upon a body moving a velocity v, then
the radiation reflected from the moving body appears to have a frequency f , where
1
f  ν 
f = ≈ f 1+ (5.19)


1 v  c 
1−
c
Reflective microstructure surface
(position 2)
Reflective microstructure surface
(position 1)
Microstructure
displacement

Position-sensitive detector
Optical source
Displacement on detector
corresponding to movement
of microstructure
Figure 5.8 An example of a spatial position measurement system.