Switched Coil Receiver Antenna for 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 PIMDs. First is the high kick-back 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 of decreasing the response time or latency of a receiver coil having multiple loops operating in a pulse induction metal detector. The PMID also includes a transmitter coil. The receiver coil is broken into segments during the time period when the transmitter coil is abruptly turned off and re-connected once the transmitter coil transients have decayed to an acceptable level. This invention also discloses a PMID that includes a pulse transmitter coil for generating and transmitting a first electromagnetic signal and a receiver coil for detecting and amplifying a second electromagnetic signal emitted by a near by metal target in response to the first electromagnetic signal. The receiver coil has multiple loops of wire about its common axis. At least one switch is coupled with the coil of wire such that when the switch is on its open position the coil of wire is a broken circuit. In this condition the inductive properties of the receiver coil are minimized during abrupt transmitter coil turn off.
US 7,176,691 [MORE INFO
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