Walk-through metal detector
First Claim
1. A pulsed-field metal detector system comprising a transmitting coil for transmitting pulses and a receive coil for receiving signals derived from said transmitted pulses, means for driving the transmitting coil responsive to current pulses recurring in cyclically alternating polarity, timing means coupled to synchronize measurement cycles at an odd half-multiple of an AC power main frequency, means for amplifying received signals derived from said transmitting pulses, means for subtracting from the amplified signals those components of said received signals which are due to periodic noise signals and static metal, means responsive to the subtracting means for limiting the absolute amplitude of residual signals remaining after said subtraction, means for demodulating and averaging the residual signals, means for detecting excursions of the averaged signal above or below preset threshold values, and means responsive to detection of said signals beyond said preset threshold values for giving an alarm when said threshold is exceeded for a majority of several successive process intervals.
2 Assignments
0 Petitions
Accused Products
Abstract
A pulsed-field walk-through metal detector contains circuitry to reduce several different types of external noise, or interference due to external stationary or moving metal, and signals due to small objects, such as jewelry. A self-test fixture monitors system performance. The detector has digitally controlled timing means operated at an odd half-multiple of the power mains frequency. A gated integrator is active for periods equal to a multiple of the TV retrace frequency. A switched-capacitor FIR filter removes signals due to steady power line hum, and a subtractive means removes signals due to stationary metal, prior to averaging or lowpass filtering. A peak limiter controls transient noise bursts. A voting circuit reduces the likelihood of false alarm triggering. A simple self-test circuit, consisting of a wire loop and a switch, tests the entire signal path and has no moving parts.
-
Citations
17 Claims
- 1. A pulsed-field metal detector system comprising a transmitting coil for transmitting pulses and a receive coil for receiving signals derived from said transmitted pulses, means for driving the transmitting coil responsive to current pulses recurring in cyclically alternating polarity, timing means coupled to synchronize measurement cycles at an odd half-multiple of an AC power main frequency, means for amplifying received signals derived from said transmitting pulses, means for subtracting from the amplified signals those components of said received signals which are due to periodic noise signals and static metal, means responsive to the subtracting means for limiting the absolute amplitude of residual signals remaining after said subtraction, means for demodulating and averaging the residual signals, means for detecting excursions of the averaged signal above or below preset threshold values, and means responsive to detection of said signals beyond said preset threshold values for giving an alarm when said threshold is exceeded for a majority of several successive process intervals.
-
17. A measurement process comprising the steps of applying a nominally steady magnetic excitation field for a minimum of 300 microseconds, then removing the field and immediately beginning an integration of signals beginning at the instant when the applied field begins to collapse and continuing thereafter for 300 microseconds, with the added step of increasing the integrator sensitivity late in an integration period in order to compensate for the shortness of an interval during which measurements are taken.
Specification