Truncated half-sine methods for metal detectors

摘要

This invention relates to a metal detector where a coil is used to transmit a periodic magnetic field to energize metal objects that are concealed and often buried or hidden in a matrix (ground) containing ferromagnetic minerals. There are many difficult and often simultaneous challenges, such as detecting large deep targets, detecting minutely small targets, identifying target properties, ignoring the ferromagnetic matrix, avoiding a net magnetic field which can trigger magnetic sensors in land mines, and ignoring conductive salt responses. Either time-domain or frequency domain methods have been used to address these challenges with mixed levels of success. The ability to simultaneously use time-domain and frequency-domain methods can expand detection capability. Techniques are presented for achieving these goals.

主权项

1. A metal detector comprising:
(a) one or more transmit coils for producing a cyclic transmitted magnetic field in response to a cyclic transmit current, wherein each transmit current cycle is comprised of one or more transmit current pulses, wherein one or more transmit current pulses exhibit approximately a half-sine waveform during turn-on which sinusoidally rises to a peak current, and which at or past the peak current is truncated by substantially shutting off the coil current, and which is followed by a turn-off time in which the applied transmit current is substantially zero; (b) transmit circuitry for generating the transmit current pulses applied to the one or more transmit coils comprising one or more switching networks for switching the one or more coils to appropriate drive circuitry comprised of a unipolar or bipolar power supply voltage and one or more series-connected resonant capacitors for the purpose of producing a substantially half-sine response during the turn-on time; (c) one or more receive coils for receiving a response signal created from the transmitted magnetic field and its effect on a surrounding matrix and conductive targets; (d) receive circuitry comprising a plurality of receive channels each comprised of sampling circuitry and processing circuitry, wherein a receive channel samples a received flyback response created by the truncation of the transmit current and herein called the flyback sample, and wherein a receive channel samples a received half-sine response after the peak of the transmit half-sine current but before the truncation point and herein called the ground sample, and wherein the flyback sample and the ground sample are combined and either or both samples are adjusted such that the combined response due to a ground matrix is substantially minimized.