Capacitive Sensor for Micrometer-Scale Proximity Detection with Microwaves

This paper presents a highly sensitive proximity sensor with micrometer-scale resolution. The working principle is the change in the capacitance of a movable step-impedance resonator (SIR), in relative (vertical) motion with respect to the static part, a pair of matched lines terminated with a patch capacitance and sharing the same axis. If the patches of the lines are aligned with the SIR patches, the resulting structure is a two-port device loaded with a series connected resonator with variable capacitance (caused by the SIR vertical motion). The output signal of the reported proximity sensor is the phase of the transmission coefficient at the operating frequency. Such frequency can (canonically) be the resonance frequency of the SIR when the substrate where such element is etched is in contact with the static part, but the device can be tuned to different operating frequencies, opening the possibility to further enhance the sensitivity. Thus, this transmission-mode phase-variation sensor is a single-frequency device (this simplifies the electronics required for sensor feeding and processing in situations where a vector network analyzer cannot be used). An exhaustive sensitivity analysis for sensor operation at the resonance frequency of the SIR is carried out in the paper. According to such an analysis, it is concluded that for sensitivity (and resolution) optimization, the SIR must be designed with a long narrow strip and small metal patches. Nevertheless, from a simulation-based analysis, it is shown that the sensitivity can be dramatically boosted up by device operation at the frequency where the phase of the transmission coefficient is ±180°, provided the static part substrate is adequately chosen. Three prototype proximity sensors are presented and experimentally validated to demonstrate all these aspects. The achieved maximum sensitivity, resolution, and dynamic range in one of such prototypes are 4485°/mm, 5 μm, and 0.1 mm, respectively.