Process and device for differentiating objects influencing an electromagnetic alternating field, in particular metal objects

A device for differentiating objects (O) influencing an electromagnetic alternating field, comprising:

• a voltage source for generating a sinusoidal excitation potential (ue(t)) of a constant frequency (fM), a coil (S) which is supplied with the excitation potential (ue(t)) for generating an electromagnetic alternating field,
• calculating means for detecting an impedance change in the coil (S), which has been caused by an object (O), by determining at least one pair of values from a peak value (ÎM) of a sinusoidal coil current (iM(t)) flowing through the coil (S) and a phase shift (φM) between the coil current (iM(t)) and the excitation potential (ue(t)), and means for determining the material (Mat1, Mat2) of the object (O) by comparing the at least one pair of values from a peak value (ÎM) and a phase shift (φM) of the sinusoidal coil current (iM(t)) with reference values, wherein a window comparator has a constant, adjustable, window width (Δi) which is supplied with the coil current (iM(t)) and, respectively, a voltage signal uM(t) proportional thereto, time recording means for measuring the length of time (ΔtM) which the coil current (iM(t)) needs in order to pass through the comparator window (Δi), with the calculating means being designed for calculating the gradient of the current profile (iM(t)) from the length of time (ΔtM) and the window width (Δi) of the window comparator at least one predetermined measuring phase angle (φM) as well as for calculating the peak value (ÎM) from the gradient of the sinusoidal coil current iM(t) and its known frequency (fM).

A sorting plant comprising a conveying means on which a material stream is movable at a constant speed v in which objects O are contained which are supposed to be identified based on their conductivity and/or ferromagnetic properties and sorted out by means of an object discharging unit, at least one device for differentiating objects (O) influencing an electromagnetic alternating field, having a voltage source for generating a sinusoidal excitation potential (ue(t)) of a constant frequency (fM), a coil (S) which is supplied with the excitation potential (ue(t)) for generating an electromagnetic alternating field, calculating means for detecting an impedance change in the coil (S), which has been caused ban object (O), by determining at least one pair of values from a peak value (ÎM) of a sinusoidal coil current (iM(t)) flowing through the coil (S) and a phase shift (φM) between the coil current (iM(t)) and the excitation potential (ue(t)), and means for determining the material (Mat1, Mat2) of the object (O) by comparing the at least one pair of values from a peak value (ÎM) and a phase shift (φM) of the sinusoidal coil current (iM(t)) with reference values, having a window comparator having a constant, adjustable, window width (Δi) which is supplied with the coil current (iM(t)) and, respectively, a voltage signal uM(t) proportional thereto, time recording means for measuring the of time (ΔtM) which the coil current (iM(t)) needs in order to pass through the comparator window (Δi), with the calculating means being designed for calculating the gradient of the current profile (iM(t)) from the length of time (ΔtM) and the window width (Δi) of the window comparator at least one predetermined measuring phase angle (φM) as well as for calculating the peak value (ÎM) from the gradient of the sinusoidal coil current iM(t) and its known frequency (fM), wherein the coil (S) is arranged close to the conveying means so that objects (O) moving in the conveying direction (x-direction) of the material stream will cause a material characteristic impedance change in the coil (S), which is detected by the device, with the device activating the object discharging unit.