Target-identifying metal detector

Abstract

The target signal produced by the receive coil of an induction-balance metal detector is demodulated and filtered in matched V.sub.X and V.sub.R demodulator and filters (11, 13) driven by quadrature versions (TA/TB) of the signal that drives the transmit coil of the detector. The resultant phase quadrature signals--V.sub.X and V.sub.R --are applied to a ramp computer (15) that internally produces a ramp signal whose slope is a function of the magnitude of the sum of the V.sub.X and V.sub.R signals. The ramp computer (15) inludes a comparator that combines the ramp signal with a fixed signal whose magnitude is a function of the magnitude of the V.sub.X signal. The result is a pulse whose width is a function of the phase angle and, thus, the identity of the target that created the V.sub.X and V.sub.R signals. The pulse is integrated (17) and the results stored at a TI voltage whose magnitude is functionally related to the identity of the target. The TI voltage is used to control the state of a display (23) calibrated in terms of target identity. A controller (37) that controls the foregoing operations is subject to preventing the display from occurring when a discriminate circuit (25) determines that the TI voltage denotes a target (such as a nail) that is not of interest. The discrimination disablement of the display is subject to being overridden by a signal from a target-select circuit (27) when the TI voltage denotes a target of interest, (e.g., a nickel) falling within the phase angle range that is normally subject to discrimination. Further, the controller (37) only allows the detection sequence to operate if the target produces a signal having a magnitude above a minimal level. The controller is also subject to being overridden by the output of a deep-target circuit (35) designed to be operator-enabled when the detector is being used to locate deeply buried targets in ground having a low mineralization content.