German patent DE 195 25 131 C2 discloses a known method. In this known method, a sensor having two measuring coils is used, which are arranged immediately one after the other perpendicular to the conducting surface, and also offset from each other. In the known method, frequencies are applied to the input side of the two measuring coils from an oscillator in each case, where the oscillator frequencies differ from each other. Depending on the respective distance of the sensor from the conducting profiled surface, there is a resultant impedance change in the measuring coils that is detected by means of differential amplifiers connected to the output side of the measuring coils.
A demodulator unit comprising two demodulators and connected to the output of the differential amplifiers, and a circuit comprising two EPROMs connected to the output of the demodulator unit, are used to generate digital measured-values that correspond to the distance of the respective coil of the sensor from the conducting profiled surface. The digital measured-values are supplied to an arithmetic unit implemented in the form of a comparator and then examined there to evaluate whether the digital measured-values of the two measuring coils agree within a certain tolerance range. If this is the case, then the distance currently detected is deemed to be measured correctly, and an OK signal is generated by the arithmetic unit.