A Modified Quartz Crystal Capacitance Circuit by Using Parallel Inductance and Its Application for Microdisplacement Sensing

In this article, we proposed a modified quartz crystal (QC) capacitance circuit with enhanced capacitance load performance constructed by using an inductor in parallel with the capacitor in a traditional QC capacitance circuit. The modified circuit was called as QCLC circuit for short. The QCLC circuit principle was studied by theoretical derivation and finite element simulation. Analytical expression for resonant frequency of the QCLC circuit is derived. Both theoretical calculation and finite element simulation results demonstrated that the QCLC circuit has a large capacitance (>50 pF) load capacity. Furthermore, the capacitance-frequency pulling sensitivity of the QCLC circuit can be adjusted by setting the parallel inductance value. Finally, the QCLC circuit was applied to construct a microdisplacement (MD) sensor with quasi-digital frequency output. A sensor prototype was fabricated by using a capacitive displacement detection element instead of the capacitor in the QCLC circuit. A sensitivity of 1.85 Hz/
$\mu \text{m}$
was obtained for a detecting distance of 0–1 mm. Sensing performances such as repeatability and stability were studied as well. The proposed QCLC circuit had the potential for developing capacitive sensors with quasi-digital frequency output.