Displacement Measurement using Eddy Current Sensors

The eddy current measurement principle is an inductive measuring method. A coil is supplied with an alternating current, which causes a magnetic field to form around the coil. If an electrically conducting object is placed in this magnetic field, eddy currents are induced, which form an electromagnetic field according to Faraday’s Induction Law. This field acts against the field of the coil, which also causes a change in the impedance of the coil. The controller calculates the change in energy transferred from the sensor coil to the target material and converts this into a displacement measurement.

The advantages of the eddy current principle are that the inductive method can be used on all electrically conductive, ferromagnetic and non-ferromagnetic metals. The size of the sensor is relatively small compared to other technologies and the temperature range is high due to the resistance measurement of the sensor and cable. The technology is high accuracy and is immune to dirt, dust, humidity, oil, high pressures and dielectric materials in the measuring gap.

However, output and linearity depend on the electric and magnetic features of the target. Therefore, individual linearisation and calibration is required. Maximum cable length is 15m and the diameter of the sensor increases as the measuring range increases.

Custom Eddy Current Sensors

Micro-Epsilon has developed a unique technology for use with eddy current displacement sensors. New technologies and manufacturing methods enable a wide variety of custom sensors to be produced, which are not only capable of operating in higher temperatures, but which also benefit from low thermal expansion (thermal drift) and extremely compact conditioning electronics, making the sensors ideal for the aerospace sector.

The new Embedded Coil Technology (ECT) represents a technological breakthrough in eddy current sensor design and manufacture, enabling the previous limitations of using eddy current sensors to be overcome. Due to its ultra-compact design and by using new inorganic materials in its construction, the new eddyNCDT ECT sensors provide almost unlimited scope in terms of the external design and geometrical shape of the sensor. This means the sensors can be adapted to suit virtually any application requirements.

EddyNCDT ECT sensors offer extreme mechanical robustness, resulting in longer service intervals and higher temperature stability. In addition, the complete circuit electronics can now be integrated into the sensor itself, providing an even more compact measurement solution for OEMs and machine builders. The sensors are also suitable for harsh operating environments, including high vibration, impact shocks and high operating temperatures as high as 350 deg C.

Sensors have been produced with extremely low thermal drift and with temperature errors of less than 20ppm/K (parts per million per degree Kelvin).