Metallic article discriminator

Abstract

A multicoin tester has a coin inlet path 1 along which coins under test run edgewise past coils 2, 3 on opposite sides of the path, and through the windings of a coil 4. Electronic circuitry responsive to the inductive coupling of the coin with the coils operates a gate 5 to either reject the coin onto path 1b or to accept the coin into path 1a. As shown in FIG. 2 each of the coils 2, 3 and 4 is arranged in a parallel L-C resonant circuit 10, 11, 12 connected in the feedback path of an amplifier A1, the resonant circuit being energised sequentially by multiplexer M1. Each of the circuits 10, 11 and 12 has its own natural resonant frequency. The resonant circuits 10, 11, 12 are driven by a voltage controlled oscillator VCO. A phase locked loop including a phase comparator PS1 drives the oscillator VCO at a frequency corresponding to the natural resonant frequency of whichever of the circuits 10, 11 and 12 is connected thereto. As a coin passes say the coil 2, the resonant frequency of circuit 10 is modified by the coin and the phase locked loop changes the frequency of the VCO to maintain resonance. The resulting output at 15 varies both in amplitude and frequency. The amplitude deviation is digitized by an analogue to digital coverter ADC and compared by a microprocessor MPU with stored values in an EEPROM to determine coin acceptability and denomination; so as to operate gate 5 and provide other optional outputs.