EPRO PR6426/010-010 CON021 Eddy Current Sensor

An eddy-current sensor produces an alternating magnetic field at the probe tip. When this field is near a conductive material, the field creates (induces) small electrical eddy-currents in the material. These currents generate a magnetic field that opposes the field from the sensor. As the gap between the probe and target gets smaller, the field interaction changes. 

The sensor electronics measure this field interaction and generate an output voltage proportional to the change in the gap. Conductive Targets The eddy-currents induced in the target material are dependent on the material’s properties of permeability and resistivity. As a result, eddy-current sensors are sensitive to changes in material. There can be a drastic difference between magnetic (iron, steel) and nonmagnetic (aluminum, copper) materials. Less drastic, but still significant, is the difference between two nonmagnetic materials. 

The material and alloy must be specified with orders to assure an accurate calibration. Because the magnetic fields penetrate the surface of the target, target materials have minimum thickness requirements; this varies with material type. TechNote LT02-0011, Minimum Recommended Target Thickness details the minimum target thickness for many materials. 

The TechNote is available at www.lionprecision.com; click on Technical Library. Hostile Environments The magnetic fields of eddy-current sensors do not “see” nonconductive materials and therefore are not affected by most contaminants. This allows an eddy-current sensor to operate while immersed in liquids and in the presence of machine coolants or other liquid materials present in the sensing area.

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