Eddy current sensor for analyzing a test object and method of operating same

A method for operating an eddy current sensor having a measuring coil, and an evaluation circuit for determining material or geometric parameters of a test object, comprising the steps of

• positioning the test object at a first predetermined distance from the measuring coil which is greater than twice the radius of the measuring coil and measuring the impedance or the inductance value and/or the damping value of the measuring coil when supplied with an alternating voltage at a given frequency,
• positioning the test object at a second predetermined distance from the measuring coil which is less than twice the radius of the measuring coil and measuring the impedance or the inductance value and/or the damping value of the measuring coil when supplied with an alternating current at the given frequency, and
• computing in the evaluation circuit the material and geometric parameters of the test object based on the values measured at the first and second distances.

An eddy current sensor comprising

• a measuring coil and a compensation coil which are each adapted for receiving an alternating current, and with the compensation coil arranged in the vicinity of the measuring coil so that it is exposed to the thermal environmental conditions of the measuring coil,
• an evaluation circuit connected to the measuring coil and the compensation coil for determining material and/or geometric parameters of an electrically conductive test object from the impedances of the measuring coil and the compensation coil, and
• the compensation coil being designed and arranged such that the test object has substantially no influence on its impedance.