Patent ID: 7708130

Claim:
A method for testing coins which move along a runway, using a coil assembly which, on either side of the runway, has a transmission coil C, D adapted to be supplied by a transmission signal through a differential amplifier and a reception coil A, B, transmission coil C, D and reception coil A, B being located on one side of the runway on a common ferrite core (with the transmission coil C and reception coil A on one side and the transmission coil D and reception coil B on the other side of the runway), wherein the reception coil A, B being smaller in diameter than the transmission coil C, D is closer to the runway than is the transmission coil C, D, and the diameter of the transmission coil is smaller C, D is smaller than the diameter of the smallest coin to be tested, and wherein the ferrite core has arranged thereon a secondary coil E, F inductively coupled to the transmission coil so that a signal of which is connected, as a negative-feedback signal, to the input of a differential amplifier in such a way that the signal of the transmission coil C, D corresponds to the transmission signal supplied to the other input of the differential amplifier, the method comprising the steps below: A transmission signal includes harmonics and is periodically generated; during a predetermined measuring interval of periodically recurring portions of the transmission signal, amplitudes of the attenuating function are determined from the signals of a respective reception coil A, B at least three different measuring times, at least four measuring cycles are successively passed through in time; during which two reflection measurement each are performed on each side of the runway and two transmission measurements are performed each using a transmission coil and a reception coil on either side of the runway, wherein the each opposite transmission coil is connected also as a reception coil during one of the two transmission measurements, the values measured and the attenuating functions from at least one of the measuring cycles are correlated to each other and/or are compared with predetermined reference values, and coins are sorted based on said measurements.