Page 109 - MEMS Mechanical Sensors

98 Mechanical Transduction Techniques

Measurand
Frequency
output
Vibration Vibration
excitation Resonator detection
mechanism mechanism

Amplifier

Figure 5.9 Block diagram of a resonant sensor.

the sensor and this signal is also fed back to the drive mechanism via an amplifier
maintaining the structure at resonance over the entire measurand range.
In mechanical sensing applications, the most common mechanism for coupling
the resonator to the measurand is to apply a strain across the structure. When used
in such a manner the resonator effectively becomes resonant strain gauge. Coupling
to the measurand is achieved by mounting the resonator in a suitable location on a
specifically designed sensing structure that deflects due to the application of the
measurand. The resonator output can be used to monitor the deflection of the sens-
ing structure and thereby provide an indication of the magnitude of the measurand.
When used as a resonant strain gauge, the applied strain effectively increases the
stiffness of the resonator, which results in an increase in its natural frequency. This
principle is commonly applied in force sensors, pressure transducers, and acceler-
ometers (see Chapters 6 through 8 for detailed examples).
Coupling the measurand to the mass of the resonator can be achieved by
surrounding the structure by a liquid or gas, by coating the resonator in a chemically
sensitive material, or by depositing material onto the resonator. The presence of the
surrounding liquids or gases increases the effective inertia of the resonator and
lowers its resonant frequency. Density sensors and level sensors are examples of
mass coupled resonant sensors. Coating the resonator in a chemically sensitive
material is used in gas sensors. The sensitive material absorbs molecules of a particu-
lar gas, adding to the mass of the film and thereby reducing the frequency of the
resonator.
The shape coupling effect is similar to the strain effect except changes in the
measurand alter the geometry of the resonator, which leads to a shift in the resonant
frequency. This is the least commonly used coupling mechanism.

5.5.1 Vibration Excitation and Detection Mechanisms
The piezoelectric nature of GaAs and quartz materials enables straightforward exci-
tation and detection of resonant modes of vibrations [7]. Suitable electrode materi-
als must be deposited and patterned on the surface of the resonator. The location
and geometry of the electrodes should be carefully designed to maximize the electri-
cal to mechanical coupling with the desired mode of operation (drive efficiency).
Maximizing this coupling will promote the excitation of the desired mode and maxi-
mize the corresponding vibration detection signal.