Inductive Sensor Based on Micromachined Coil for Conductive Target Detection

This letter presents an inductive sensor based on a micromachined coil for the detection of conductive targets. The proposed sensor consists of a planar micromachined sensing coil, a signal conditioning instrumentation, and a nearby nonferromagnetic conductive target. The working principle relies on the magnetic coupling between the sensing coil, driven by AC current, and the target. Due to the eddy currents induced in the target, the magnetic flux and, thus, the equivalent inductance of the sensing coil is altered as a function of the target position. Two configurations have been experimentally adopted to explore the proposed inductive sensor. A proximity sensor has been achieved by measuring the inductance variation while changing the distance between the coil and a brass tip with a diameter of 3 mm. A nondestructive defect detector has been achieved by sliding a copper target over the sensing coil. For the proximity sensor, a sensitivity of 6500 nH/m within the range 0–50 µm has been achieved. The resolution of the proximity sensor at one standard deviation is 470 nm. For the defect detector, a 1-mm wide and 35-µm deep cut induced in the target has been detected. Experimental results have confirmed the possibility to exploit the proposed sensor in both configurations, thus validating its working principle in good agreement with the theoretical analysis.