Two-Dimensional Displacement Sensor Based on CPW Line Loaded by Defected Ground Structure With Two Separated Transmission Zeroes
In this paper, application of a defected ground structure in realization of 1-D and 2-D microwave displacement sensors is presented. A coplanar waveguide (CPW) line with two slots along the line that generate a transmission zero (TZ) in the transmission response is printed on a fixed substrate. A metallic patch is also printed on the bottom face of a movable substrate located on the fixed substrate. By moving the upper substrate, the length of the etched slots on the fixed underneath substrate changes, and hence, the TZ shifts proportionally. The displacement can be measured based on the frequency shift of the TZ. The 2-D displacement sensor has a similar structure with two additional slots perpendicular to the CPW line on the signal trace. The parallel and perpendicular slots generate two separated TZs in two different frequency bands. The proposed 2-D displacement sensor uses the frequency range of 2.8–4.2 GHz to measure the horizontal displacement with the high sensitivity of 0.41 GHz/1 mm and the frequency range of 5.7–8.8 GHz to measure the vertical displacement with the high sensitivity of 1.2 GHz/1 mm. In comparison with the previous works, the displacement measurement in each direction is independent of the other one. Furthermore, since TZs show narrow notch-frequency bands with high and fixed rejection levels (high $Q$ -factor resonator), independent of the measured displacement, the measurement error is minimized. Also, the dynamic range of the sensor is wide and almost has no intrinsic geometrical limitation. The 2-D sensor is fabricated and tested. The measured results show good agreement with the simulated results.