Proximity sensor of a coated quartz crystal in air

Quartz crystal (QC) is one of the well-known piezoelectric materials. However, it was used as a proximity sensor in air in this work. This work stabilized the voltage output to an oscillator circuit connected with a QC. As the thickness-shear mode QC moved closer to the conductive plates from 10 mm to 5 mm while the other electrode was coated with an acrylic rubber compound to eliminate the quality factor, the resonant frequencies of the QC increased to about 0.5~0.8%. However, the QC was found to have an insignificant change on the non-conductive plates. Additionally, the QC showed a higher degree of frequency shift when the clear side faced the copper target than when the coated side faced the copper plate. The result implied that the capacitances could be different for the clear and coated sides when facing metal.