A Wide Range Transmission Line Based Linear Displacement Sensor

Accurate displacement measurement is critical for many applications. Recently, a variety of radio-frequency-based linear displacement sensing techniques were introduced. However, the application of the previously proposed techniques was either not fully demonstrated towards realizing the sensor and/or they were limited in measurement range to less than 4 cm. This paper introduces a fully characterized wide-range radio-frequency-based contactless linear displacement sensor. The sensor consists of a short-circuited 50 Ω microstrip line and a movable current-sensing resonant probe. The probe accurately measures the sinusoidal magnetic field distribution along the short-circuited transmission line from a certain distance above it. The measured normalized field values are mapped to displacement using the inverse of the sinusoidal function in the post-processing stage. The proposed technique is comprehensively validated using simulations and measurements of a compact sensor prototype operating at 727.5 MHz. Furthermore, the merits of the proposed sensor compared to the widely-accepted linear variable differential transformer (LVDT) displacement transducer are highlighted herein. The metrological characterization of the proposed sensor shows that it offers a very wide dynamic range of 68 mm with a standard deviation of the estimation error of less than 0.09 mm (0.13% of the full range). It is also demonstrated that the proposed sensor outperforms the commercial LVDT transducer in terms of overall displacement measurement accuracy. In general, the proposed sensor is scalable and has a theoretical dynamic range of λ/2 making it suitable for a wide range of applications.