Digital Nulling Pulse Inductive Metal Detector
Pulse induction metal detector (PIMD) antennas come in two basic types. First is a single transmit and receiver coil. The second use separate transmitter and receiver coils. Two basic problems exist with existing PIMD's. First is the high kickback voltage of the transmitter coil, which temporally blinds the receiver coil from the amplifying metal target signals near the turn-off time of the transmitter coil. Second is that the protection circuitry typically has a delay time that also temporally blinds the receiver coil during the same turn-off time of the transmitter coil.
The Johns Hopkins University Applied Physics Laboratory has developed and patented a method for improving pulse inductive metal detector sensitivity by minimizing adverse ground responses and transmitter coil transients. The method provides a calibration mode for a typical pulsed electromagnetic induction EMI metal detector. The purpose of the calibration mode is to determine and record a nulling signal representative of the transmitter coil coupling to the receiver coil and a ground response that has no metal. The nulling signal is then used during normal operation of the metal detector by combining it with the instant receiver coil signal in a difference amplifier. The difference amplifier effectively subtracts the nulling signal from the instant signal yielding a response signal that has removed the ground response that may be present in the instant signal. The metal detector can be periodically recalibrated. It may also be recalibrated upon discovery of a metal target to provide the most up to date nulling signal for the ground around the metal target.CONTACT: