Metal detector systems for identifying targets in mineralized ground

Abstract

Synchronous demodulators demodulate the signal developed in a receive coil coupled to an oscillator driven transmit coil and produce signals that include target pulses when the detector head housing the transmit and receive coils passes over a metallic object (e.g., a target) buried in the ground. The signals are processed to remove ground mineralization information and to determine the 'phase angle' of the target, which angle uniquely identifies the detected target. The tangent of the phase angle is determined using either implicit division (sometimes referred to as steepest descent division) or direct division. Regardless of which division technique is used, as the signals are processed, they are continuously filtered to remove ground mineralization information. In addition, the target component of the signals is converted from pulse form into ringing signal form. The ringing signals are processed in a manner that maintains phase correlation between the signals and produces the desired phase angle information. Regardless of how implemented the phase angle information is only determined when the detector head is passing over the target, each such pass improving the accuracy of the result. Further, if the target signals being processed reach a magnitude sufficient to saturate the amplifiers used in various circuits, phase angle determination is inhibited. In addition, the depth of the target is determined by analyzing the magnitude of the target pulse produced when the detector head passes over a target. Both the target identity and the depth signals are displayed on juxtaposed LCD bar graphs. The depth display enhances the target identity display, particularly in situations where the phase angle of undesirable targets (e.g., pull tabs) is close to the phase angle of desirable targets (e.g., $2.50, $5.00 and $10.00 gold pieces).