A Differential Resonant Edge Detection Sensor

Abstract

The detection of metal edges is a common problem in manufacturing of metal products including strips and tapes, as well as in alignment of metal sheets for assembly of products. The detection may be as simple as detecting the position of the edge—either qualitatively, that is, the very existence of a metal edge or may be highly quantitative, whereby the exact position of the edge is sought for the purpose of alignment or assembly. Metal strips may be very thin or relatively thick, may be ferromagnetic or nonmagnetic and in some applications, such as during production, may be at high temperatures. The present work proposes a self-compensating, highly sensitive resonant sensor suitable for the industrial environment. The method relies on two split coils one for each edge, detecting the position of one edge of the strip. To compensate for a variety of effects such as jitter, drift, and frequency locking, the two coils are switched at a fixed rate onto a single resonator. The difference in resonant frequency of the two coils is calculated and is used to locate the edges and to generate correction signals for the purpose of positioning the metal strip. The results presented here are based on tests made on a scale prototype sensor.