Phase-Variation Microwave Displacement Sensor With Good Linearity and Application to Breath Rate Monitoring

This article presents a novel highly linear reflective-mode displacement sensor based on planar microwave technology. The sensor consists of two parts: 1) the stator, or reader, a one-port transmission line terminated with a matched load and 2) the movable part, or tag, a dielectric slab with an electric LC (ELC) resonator etched on it. In the proposed system, the resonator is allowed to displace longitudinally along the line axis of the reader at a fixed vertical distance (air gap). Due to magnetic coupling between the line and the ELC resonator, the feeding signal (a harmonic signal tuned to the resonance frequency of the ELC resonator) is totally reflected at the resonator's position, and the phase of the reflection coefficient, the output variable of the sensor, correlates with that position, i.e., it varies roughly linearly with the distance between the resonator and the input port. A prototype example, with a dynamic range of 4.40 cm, is reported, and validated at laboratory level by means of a linear displacement system. Then, the potential of the proposed sensor to monitor the breath rate in humans is discussed, and a belt-based prototype device system that can be applied for that purpose is presented and validated. The key idea is the chest and abdomen expansion due to breathing, which leads to a periodic relative displacement between the tag and the reader at the respiration rate.