According to the Faraday's Induction Law, the induced eddy current flows in an electrical metal conductor, if the conductor is located in a time changeable magnet field. The magnet field is generated here by an aternating current flowing in a wire coil. The eddy current generates a opposite magnet field, which superimposes with the exciting magnet field. As consequence, the impedance Z of the sensor coil changes.

The impedance change depends on the distance a between the measuring target (metal plate) and sensor, and on material of the target. For a defined measuring target the change of coil impedance is a function of the distance a. Therefore, the distance can be derived by measuring impedance change. Using an Impedance-Voltage Converter (Fig. 2), the impedance change can be converted into a voltage change for further signal processing.
 

Fig. 1 Eddy current position sensor

In order to increase the measuring range of an eddy current sensor, the linearity of the output signal must be improved by signal processing. The output signal at frequencies lower than the resonance frequency approximates an exponential function. In this case one can use analog signal processing to linearize the output voltage. Fig. 4 shows an example of output signal linearization at an optimal frequency of 810kHz. The linear range of the processed signal is larger than that of the original signal. A linearity less than 0.1% is realizable after the sensor optimization.

Special sensor electronics, signal processing and self-calibration are developed for compensation of temperature and material influences. Thus, the developed distance eddy current sensors are independent on measuring objects and can be used under normal measuring conditions for precise position, distance and vibration measurements.