Design, Fabrication, and Characterization of Micro Variable Reluctance Sensors

This letter highlights a significant advancement in the miniat- urization of conventional variable reluctance sensors (VRSs) using an optimized micromachined spiral coil and a cylindrical magnet. The primary objective is to create low-cost, high-performance micro-VRSs (µVRS) that can measure the position, speed, and proximity of ferrous components. The sensitivity of the µVRS was optimized through theoretical analysis, aiming to achieve a minimum noise equivalent magnetic induction. The fabrication of the µVRS involved using a cost-effective single-mask lithography process to create a planar spiral inductor on a silicon wafer. Once the fabrication was complete, the µVRS was wire-bonded to a printed circuit board (PCB). The cylindrical magnet was then affixed to the backside of the PCB, resulting in a passive µVRS. The presented µVRS successfully measured the position and rotation speed of an iron gear around the shaft of a stepper motor with high accuracy. The high sensitivity of the µVRS enabled the generation of an output electrical signal of sufficient magnitude, which could be measured using an oscilloscope or a commercial analog-to-digital converter. The distinguishing feature of the presented µVRS is its complete passivity, as it operates without any power source, unlike semiconductor-based magnetic field sensors, such as Hall effect sensors, commonly used in position and speed sensors. The passive nature, robustness, low-temperature dependency, and small size of the presented µVRS make it particularly suitable for smart energy-efficient buildings and smart manufacturing to reduce carbon footprint in the era of Artificial Intelligence of Things (AIoT).