A CMOS Hybrid Magnetic Field Sensor for Real-Time Speed Monitoring

In this article, we present a CMOS hybrid magnetic field sensor for real-time motor speed monitoring for smart energy-efficient autonomous systems. The sensor system consists of a micro Hall effect sensor ( $\mu $ HES) and a micro search coil magnetometer ( $\mu $ SCM) using Taiwan Semiconductor Manufacturing Company (TSMC) 0.18- $\mu \text{m}$ 1P6M CMOS process with a die size of $1.5\times2$ mm. The hybrid sensor is realized completely by the CMOS process without postprocessing. A wide range of rotation speeds can be measured by the presented passive $\mu $ SCM without signal amplification (>600 r/min) and the combination of the $\mu $ SCM and $\mu $ HES on the same chip allows the low rotation speed range measurement (< 600 r/min). Furthermore, the $\mu $ SCM has zero offset and very low-temperature dependency. The magnetic field of a permanent magnet on the rotating shaft of the motor is measured by the $\mu $ SCM and $\mu $ HES to determine the rotation speed. The sensitivity of the $\mu $ HES (at 5-V bias voltage) and the $\mu $ SCM inside a uniform magnetic field are 113.6 and $11.3 \mu \text{V}$ /mT at 1 Hz without amplification, respectively. The noise equivalent magnetic induction (NEMI) of the $\mu $ SCM was calculated and verified experimentally by utilizing a zero-drift instrumentation amplifier. Furthermore, a simplified equivalent circuit model for the $\mu $ SCM is presented and verified experimentally. According to the equivalent circuit model, the linear frequency response region of the $\mu $ SCM is up to 20 kHz, which suits the controlling motors in smart autonomous systems and other low-frequency applications.