There are several distinct advantages of using eddy current measurement sensors. First, measurements are made on a non-contact basis and so are wear-free. Second, the principle offers high precision, high resolution and high temperature stability. The sensors can also be used on both ferromagnetic and non-ferromagnetic materials. The sensors also perform at high speed if required, with some sensor suppliers offering measurement speeds up to 100kHz. With high measurement accuracies and frequency response times, together with an extremely robust design, eddy current sensors enable measurements to be made in tasks where conventional sensors have reached their performance limits.
Eddy current sensors also perform well in demanding, harsh industrial environments where oil, dirt, dust, high pressures and high temperatures are present. Some suppliers, for example, offer robust eddy current sensors with increased protection (to IP67) for harsh environments and pressure-resistant versions that withstand pressures up to 2,000 bar.
It is also important to look for a supplier that offers a wide range of different eddy current sensor designs, which will enable the optimal sensor to be selected for a particular application. Applications for eddy current displacement sensors are often found where the standard versions of the sensors and the controller are performing at their limits. For these special tasks, consider sensor suppliers that can modify the sensor according to your specific individual requirements. Typical modifications requested include modified sensor designs, miniature sensors (2mm to 4mm in diameter), target calibration, mounting options, cable lengths, modified measuring ranges and sensors with integrated controller.
Depending on the supplier, eddy current sensors can operate in ambient temperatures from -40°C to +200°C. However, temperature fluctuations occur during operation. As the temperature of the target material changes, its resistance changes and so the accuracy of the sensor depends on the target temperature. It is therefore important to monitor the temperature of the target and compensate for this. Micro-Epsilon, for example, provides an ‘Active Temperature Compensation’ feature on its eddy current sensors. This actively measures the temperature of the sensor, electronics, cable and controller, adjusting the measurement values accordingly.
To improve sensor performance, most eddy current sensors are factory-calibrated by the supplier to a specific target material. Some suppliers, however, offer advanced eddy current measurement systems that are able to calibrate themselves. The eddyNCDT 3300 series from Micro-Epsilon, for example, can store up to four different material types.
Mounting and installation
Eddy current sensors are grouped into shielded and unshielded sensors. With shielded sensors, the field lines run closer together due to a separate casing. These are less sensitive to radial flanking metals. With unshielded sensors, the field lines emerge at the side of the sensor normally causing an extended measuring range. Correct installation is crucial for good signal quality.