Patent ID: 8305088

Claim:
A process for differentiating objects influencing an electromagnetic alternating field having metal objects, using a coil arranged on a conveying route on which a material stream is moved in one direction at a predetermined speed, wherein the material stream comprises conductive and/or ferromagnetic objects, comprising the steps of: generating an electromagnetic alternating field by exciting the coil with a sinusoidal voltage of a constant frequency, detecting an impedance change in the coil, which has been caused by an object, by determining at least one pair of values from a peak value of a sinusoidal coil current and a phase shift between the coil current and an excitation voltage and determining the material of the object by comparing the at least one pair of values from a peak value and a phase shift of the sinusoidal coil current with reference values, wherein the peak values of the sinusoidal coil current are calculated at at least one predetermined measuring phase angle with the aid of a window comparator having a constant, adjustable, window width, wherein the time between the window inlet point and the window outlet point of the coil current is measured and the gradient of the current profile is calculated from the window width of the window comparator and the measured time and the peak value is calculated from the gradient of the sinusoidal current profile and its known frequency.