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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 is in the process of patenting a method for improving pulse inductive metal detector sensitivity using multiple, current steps to reduce the voltage rating of electronic components. Current reductions in one implementation is done with multiple switched resistors coupled to a voltage source and connected to the transmitter coil. The current reduction in another implementation is done with multiple switched current sources that are connected to a transmitter coil and are switched inactive one by one to reduce the total current to the transmitter coil. The current is stepped down in rapid secession with a time delay between current transitions less than the time constant of the metal target under investigation.
US 7,148,691 [MORE INFO
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