Design and characteristics of contactless small-magnetic-metal detector base on tunnel magnetoresistance sensors in differential circuit

Unexpected metal pieces are significant problems for food and pharmaceutical manufacturers. Commonly, conventional coil metal detectors are used to detect these metal pieces. However, these detectors can only detect cm-size metal pieces, making it challenging to detect mm-size metal pieces. This paper presents an advanced contactless small-magnetic-metal detector based on tunnel magnetoresistance sensors (TMR). To improve the system's sensitivity, the differential circuit combined with a simple AC-bias lock-in amplifier was designed to reduce the noise of TMR sensors. The sensitivity of this designed detection system is 0.44V/mm with a good linearity of R2=0.82674. The experimental results demonstrate that the fabricated prototype can detect the 100 – μm diameter and 1-mm length metal wire. Due to high sensitivity and low-cost fabrication, this system is promising to apply on the manufacturing lines for detecting unexpected tiny metal pieces.