# A MEMS Variable Reluctance Sensor for Contactless Detection of a Ferrous Rotating Target

## TL;DR

This paper introduces a MEMS-based sensor that detects the rotation of ferromagnetic objects without physical contact, using magnetic field changes.

## Contribution

The novel contribution is a MEMS variable reluctance sensor for noncontact detection of rotating ferromagnetic targets.

## Key findings

- The sensor reliably detects rotational speeds up to 1500 rpm and works with drill bits as small as 5 mm.
- Voltage waveforms vary with target geometry, proximity, and speed, enabling shape and motion detection.
- The sensor functions effectively at distances up to 8 mm from the target.

## Abstract

Variable reluctance sensors are widely adopted for robust and contactless detection of motion in harsh and space-constrained environments. This paper presents a MEMS-based variable reluctance induction sensor for the noncontact characterization of rotating ferromagnetic targets, based on a micromachined planar micro-coil coupled with an external permanent magnet. The rotation of a ferromagnetic object modulates the magnetic circuit reluctance, generating a voltage signal across the micro-coil that encodes information on the target rotational speed, proximity, and cross-sectional shape. Sensor operation is investigated through a lumped-element magnetic–electrical circuit model and finite-element magnetostatic simulations, quantifying the effects of target diameter, distance, and angular position on the linked magnetic flux. Experimental validation is performed using rotating drill bits as representative targets and a dedicated high-gain, high-input-impedance front-end circuit to amplify the induced voltage. Measured results at fixed rotation frequency show periodic voltage waveforms whose amplitude and shape vary consistently with target geometry, proximity and speed. Reliable detection is achieved for rotational speeds up to 1500 rpm, for drill bit diameters as small as 5 mm, and at sensor-to-target distances up to 8 mm. These results demonstrate the potential of MEMS variable reluctance induction sensors for compact speed sensing and target shape detection.