On-chip eddy current sensor for proximity sensing and crack detection

摘要

In this paper, we present a new integrated eddy current sensor for proximity sensing and for the detection of microcracks on the surface of metals. The device consists of two stacked planar coils fabricated onto a glass substrate and encapsulated on one side by a Ni/Fe permalloy magnetic core. Fabrication of the device is achieved by a UV-LIGA thick photoresist lithography process, which involves the lithographic patterning of 15-25 μm thick molds using AZ-4000 series photoresist. The introduction of the permalloy core coupled with the thick conductor lines produces a high inductance, low resistance device capable of generating large magnetic fields at low driving currents. The device has been tested in the frequency range of 10-500 kHz and has been shown to work as both a proximity sensor and crack detector at input powers of 30 mW or less. When used as a proximity sensor, the unamplified output voltage on the sensing coil changes by as much as 75 mV with an aluminum target placed at a distance of 400 μm from the coil. The device has also shown the capability of clearly detecting cracks with depths of as little as 8 mil (200 μm) in both aluminum and titanium. Results show an extremely linear relation between crack depth and output signal voltage with an unamplified signal strength of several millivolts.