Metal-electrode-sandwiched quartz crystal-based oscillator as proximity sensor

A quartz crystal (QC) is a thermostable and high-quality material. The present paper proposes for the first time a QC-based oscillator as a proximity sensor. Various targets on a positioner were pushed into the oscillator. The dielectric target plate exhibited a minimal frequency shift compared with the semiconductor and other various metal plates. The metal-electrode-sandwiched QC showed a higher resonance frequency shift for target plates with higher conductivities. These results suggested that the inductive proximity effect was more dominant than the capacitive proximity effect. The dependence of the amplitude of the induced current on the gap between the target and the QC is also revealed. The QC coated with an acrylic rubber compound enhanced the frequency shift, although it may have also caused more frequency fluctuations during the measurements. A high detectable force slope may explain the higher sensitivity of the coated QC to the induced current.