Page 69 - The Mechatronics Handbook

are in essence analog measurements, while the latter is in essence a binary-type measurement, since the
sensed quantity is typically the frequency of vibration. Since the resonant-type sensors measure frequency
instead of amplitude, they are generally less susceptible to noise and thus typically provide a higher
resolution measurement. According to Guckel et al., resonant sensors provide as much as one hundred
times the resolution of analog sensors [33]. They are also, however, more complex and are typically more
difficult to fabricate.
The primary form of strain-based measurement is piezoresistive, while the primary means of displace-
ment measurement is capacitive. The resonant sensors require both a means of structural excitation as
well as a means of resonant frequency detection. Many combinations of transduction are utilized for
these purposes, including electrostatic excitation, capacitive detection, magnetic excitation and detection,
thermal excitation, and optical detection.

Strain

Many microsensors are based upon strain measurement. The primary means of measuring strain is via
piezoresistive strain gages, which is an analog form of measurement. Piezoresistive strain gages, also
known as semiconductor gages, change resistance in response to a mechanical strain. Note that piezo-
electric materials can also be utilized to measure strain. Recall that mechanical strain will induce an
electrical charge in a piezoelectric ceramic. The primary problem with using a piezoelectric material,
however, is that since measurement circuitry has limited impedance, the charge generated from a mechan-
ical strain will gradually leak through the measurement impedance. A piezoelectric material therefore
cannot provide reliable steady-state signal measurement. In constrast, the change in resistance of a
piezoresistive material is stable and easily measurable for steady-state signals. One problem with piezore-
sistive materials, however, is that they exhibit a strong strain-temperature dependence, and so must
typically be thermally compensated.
An interesting variation on the silicon piezoresistor is the resonant strain gage proposed by Ikeda et al.,
which provides a frequency-based form of measurement that is less susceptible to noise [34]. The resonant
strain gage is a beam that is suspended slightly above the strain member and attached to it at both ends.
The strain gage beam is magnetically excited with pulses, and the frequency of vibration is detected by
a magnetic detection circuit. As the beam is stretched by mechanical strain, the frequency of vibration
increases. These sensors provide higher resolution than typical piezoresistors and have a lower temper-
ature coefficient. The resonant sensors, however, require a complex three-dimensional fabrication tech-
nique, unlike the typical piezoresistors which require only planar techniques.

Pressure

One of the most commercially successful microsensor technologies is the pressure sensor. Silicon micro-
machined pressure sensors are available that measure pressure ranges from around one to several thou-
sand kPa, with resolutions as fine as one part in ten thousand. These sensors incorporate a silicon
micromachined diaphragm that is subjected to fluid (i.e., liquid or gas) pressure, which causes dilation
of the diaphragm. The simplest of these utilize piezoresistors mounted on the back of the diaphragm to
measure deformation, which is a function of the pressure. Examples of these devices are those by Fujii
et al. [35] and Mallon et al. [36]. A variation of this configuration is the device by Ikeda et al. Instead
of a piezoresistor to measure strain, an electromagnetically driven and sensed resonant strain gage, as
discussed in the previous section, is utilized [37]. Still another variation on the same theme is the
capacitive measurement approach, which measures the capacitance between the diaphragm and an
electrode that is rigidly mounted and parallel to the diaphragm. An example of this approach is by Nagata
et al. [38]. A more complex approach to pressure measurement is that by Stemme and Stemme, which
utilizes resonance of the diaphragm to detect pressure [39]. In this device, the diaphragm is capacitively
excited and optically detected. The pressure imposes a mechanical load on the diaphragm, which increases
the stiffness and, in turn, the resonant frequency.

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