handbook of modern sensors

336 Force, Strain, and Tactile Sensors
possible quality factor, which means the sensor must be decoupled from the environment
and possibly should operate in vacuum. On the other hand, application of force
or pressure requires a relatively rigid structure and a substantial loading effect on the
oscillation crystal, thus reducing its quality factor.
This difficulty may be partly solved by using a more complex sensor structure.
For instance, in a photolithographically produced double- and triple-beam structures
[13,16], the so-called “string” concept is employed. The idea is to match dimensions
of the oscillating element to the acoustic quarter-wavelength (1/4λ). The total wave
reflection occurs at the supporting points through which the external force is coupled
and the loading effect on the quality factor is significantly reduced.
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