Design of an inductive proximity sensor using a two-coil planar transformer

Abstract

Here we present results for a new inductive proximity sensor that makes use of two coupled coils, forming a planar transformer, fabricated on a micromachined membrane. Macroscopic proximity sensors based on eddy-current techniques are currently commercially available. However, their design is not conducive to scaling inductor. When such an inductor is scaled down and made planar, the inductance decreases while the resistance of the coil increases dramatically. This dictates that the sensor be operated at much higher frequencies for which the inductance of the coil dominates its resistance. In our new two-coil design a signal is applied across the primary coil and a significant sensitivity to the gap to the 'target' plate is seen in the phase difference between the primary and secondary output. This phase change is largely invariant with change in coil resistance (due to either temperature change or change due to scaling to IC levels), allowing operation at lower frequencies than that required for a single coil proximity sensor. By fabricating the transformer on the back side of a micromachined membrane, the coil is protected from the environment, while the deleterious effect of the silicon substrate on inductance is eliminated.