A Reflective-Mode Phase-Variation Displacement Sensor

In this paper, a displacement sensor based on an open-ended step-impedance transmission line is reported. The sensor operates in reflection, and the output variable is the phase of the reflection coefficient. The static part of the sensor is the step-impedance transmission line, where the open-ended line section is the sensitive part (sensing line). The movable part is a dielectric slab, e.g., an uncladded microwave substrate. When such slab, located on top of the sensing line, is in relative motion to the line, in the direction of the line axis, the portion of the sensing line covered by the slab varies, and this results in a change in the phase of the reflection coefficient of the line. The step impedance discontinuity contributes to optimize the sensor sensitivity, the key parameter. A detailed analysis providing the design guidelines is carried out and used to design a prototype displacement sensor. The characterization of the fabricated device points out the potential of the approach to implement highly sensitive displacement sensors. The sensor is a one-port device and operates at a single frequency.