MEMS Variable Reluctance Sensor Based on a Micromachined Coil

This work explores the potential of using a MEMS micromachined coil combined with an external magnet and front-end electronics as a variable-reluctance (VR) microsensor. The development of VR microsensors using MEMS technology represents a significant advance, as it could enable high spatial resolution for focused detection of small metallic/ferromagnetic target components and parts. To validate the principle, a squared micromachined coil with a side length of 2180 μm was adopted to detect the rotation of a ferromagnetic drill bit within the magnetic field generated by a neodymium magnet. The temporal variation of the associated flux due to the reluctance change induces a voltage in the micromachined coil that can be correlated with the target rotation speed. The VR microsensor signal was compared with a reference signal from an optical sensor used to monitor the drill bit rotation. Rotation speeds of up to 1551 and 1393 rpm were detected for drill bits with diameters of 5 and 8 mm, respectively, at a stand-off distance from the micromachined coil of approximately 2 mm, validating the proposed approach.