Quartz force sensor and facing spiral coils for noncontact detection

A noncontact detectable force sensor is a promising technique for monitoring the defects of a structural object, such as buildings and bridges, and medical surgery devices. In this study, the noncontact detectable force sensor with an AT-cut quartz crystal resonator and a pair of spiral coils is proposed. The thickness-shear mode is excited with energy trapping excitation. The resonant frequency of this vibration mode is a function of the applied external force because of the nonlinear elastic module. Then, from the resonant frequency change of the quartz crystal, the input force can be measured. Admittance characteristics of this sensor contained series and parallel resonance. We confirmed that noncontact detection of the resonant frequency change is possible by using two facing spiral coils. The coil gap effect could be eliminated by calculating the difference between the series and parallel resonant frequencies.