Method and apparatus for testing coins

Abstract

A method of testing a coin in a coin testing mechanism, comprising subjecting a coin inserted into the mechanism to an oscillating field generated by an inductor, measuring the reactance and the loss of the inductor when the coin is in the field, and determining whether the direction in the impedance plane of a displacement line, representing the displacement of a coin-present point which is defined by the measurements, relative to a coin-absent point representing the inductor reactance and loss in the absence of a coin, corresponds to a reference direction in the impedance plane. The reactance and loss measurements may be taken by a phase discrimination method. Techniques are disclosed for compensating for phase error in the phase discrimination, for measuring the direction of the displacement line relative to a different axis in order to avoid measurement errors being a consequence of any phase discrimination phase error, for applying offsets to achieve advantages in signal handling, for making the measurements thickness-sensitive, and using the change in reactance as an additional coin acceptance criterion. Some of these refinements are usable independently of the phase discrimination method. Apparatus for carrying out the methods is also disclosed.