Development of an advanced industrial metal detector instrumentation

In the normal three-coil balanced detection system for metal contaminants in products, a centrally excited coil provides a symmetric field linking two outer detector coils which are appropriately balanced to produce a zero voltage output for the detector circuit. The product to be inspected is passed on a conveyor belt through the centre of the triple coil system and the presence of any metal causes an imbalance initially on the input coil which is eventually mirrored by an imbalance in the output coil, with the signal being appropriately identified by phase-sensitive detection circuitry. The output signal is a function of many variables, including signal frequency, permeability and resistance of the sample, geometrical position of the sample within the coil cross-sectional area, and coil spacing. The reference phase angle is a crucial adjustment factor which, inter alia, compensates for product factors such as moisture content and other parameters which affect the electrical characteristics. Utilising the work of Poritsky (1960) it was not difficult to solve the fundamental field equations and optimise the various design parameters to maximise the output signal for any coil shape or sample type, and a simple CAD program was written to automate the design process. The performance specifications of new designs could be easily generated without the need for prototype construction and testing and, in addition, the claims of competitive manufacturers could be countered. Another problem addressed was that of long-term drift in the balanced coil system caused by thermal and other effects.<>