A wide range planar coil based displacement sensor with high sensitivity

A new, non-contact, planar spiral coil based inductive displacement sensor is presented in this paper. The proposed sensor has a fixed planar coil and a movable U-shaped magnetic core. The coil is designed such that the inductance vs. displacement of the sensor has a predictable characteristic that can be easily represented. The sensor presented has a sinusoidal characteristic, which helps to retrieve the displacement information from the inductance value easily, without the need of a look-up table, which is not the case for most of the sensors that use planar coils. The inductance characteristic for the proposed is symmetric with respect to the center of the coil. A simple method to distinguish the position of the core between the two halves of the coil is proposed in the paper. This not only solves the symmetry problem, but also enables to extend the sensor range with multiple coils connected in series. A Phase-Sensitive Detector (PSD) based measurement is employed to measure the inductance. This approach provides two advantages; (a) very low sensitivity to external interferences and (b) the sensitivity (of output vs. displacement) remain constant even if multiple number of coils are connected in series to increase the range of the sensor. A finite element analysis has been conducted to design the coil and to analyze the effect of vertical movement of the core. A prototype of the proposed sensor and measurement scheme has been developed and tested. The results are presented in the paper. The low-cost sensor presented will be useful for many industrial linear position sensing applications.