GB1490191A - Metal detectors - Google Patents

Abstract

1490191 Metal detector PLESSEY CO Ltd 16 Jan 1975 [23 Oct 1973] 49174/73 Heading G1N A metal detector operative according to the pulsed eddy current principle comprises a probe 1, a pulse drive circuit 2 for supplying pulses to the probe, a preamplifier 5 responsive to signals received by the probe, first signal sampling circuit 7 connected in a feedback loop 6 of the preamplifier for sampling the signals fed from the preamplifier just prior to the initiation of each drive pulse, the sampled signals being used for feedback purposes to stabilize the preamplifier, and a second; signal sampling circuit 8 for sampling signals fed from the preamplifier to provide signal samples in dependence upon which an indication of the presence of metal in the vicinity of the probe is provided. The probe may include a coil for transmitting a pulse signal and a coil for receiving signals deriving from eddy currents induced in metal objects in the vicinity of the probe. Alternatively one coil may be used for both purposes. The coil may be an open frame coil or a winding or windings carried on a ferrite core. The drive pulses may be produced under the control of signals from a timing circuit 3, which may comprise an appropriate gating circuit fed from a control pulse generator 4. The sampling circuits 7, 8 are operated under the control of gating signals from timing circuit 3. The circuit 8 is arranged to sample signals at some time after the probe has been pulsed, these signals being averaged and fed to main amplifier 10, which has a feedback loop 11 and integrator 12 associated with it, the function of the latter being to stabilize the amplifier so as to reduce sensitivity to drift. Output signals from the amplifier 10 are fed to a voltage sensitive pulse generator 15 which feeds an acoustic transducer 16. The integrator may be switched to provide a number of alternative time constants. A switch 14 may be provided for battery checking purposes. Various types of probe may be used depending upon the application of the detector. For example, probes of 20 cm. length may be used for inspecting contents of mail, and probes of 1m. length may be used for searching terrain. A probe in the form of a large diameter sledge coil may be pulled or towed behind a vehicle. Smaller diameter coils may be sewn into the palms of gloves for search purposes. The detectors may be mounted on a wooden vestibule or gateway for searching personnel and luggage. The detector is stated to be an improvement or modification of that described in Specification 1390606.