Wideband Amplitude Variation Displacement Sensor in Transmission Mode

A wideband amplitude variation-based displacement sensor with wide dynamic range is presented in this article. The sensor is designed using a microstrip transmission line loaded with a
$\pi $
-shaped capacitive network realized using two rectangular metallic patches with a gap between them. A window is etched out of the ground plane behind the capacitive patches. A movable metallic slab slides on the back of the ground plane through which the size of the window can be tuned. This creates two variable parallel plate capacitances between the rectangular patches on the top surface and the bottom ground plane. Lateral displacement of the metallic slab translates into the variation of the capacitance values that in turn changes the amplitude of the transmission coefficient (
$|{S}_{{21}}|$
). The amplitude variation can be measured at a single frequency, and due to a wideband response of the sensor, the measurement is very robust to the fluctuations of the measurement frequency. The direction of displacement can be detected using this sensor, and the design concept can be extended to a two-dimensional displacement measurement. The sensor design and operation are theoretically investigated and verified using an accurate circuit-based analysis as well as full-wave electromagnetic (EM) simulation results. The design principle is validated through the experimental measurement of a fabricated sensor prototype. The designed sensor shows a measurable displacement range of 10 mm, with an average sensitivity of 1.44 dB/mm. The measurable displacement range is customizable according to the design requirements and equals the length of the capacitive patches.