Sensors & transducers
Selecting eddy current displacement sensors
When selecting a non-contact eddy current displacement sensor, a number of factors need to be considered, including temperature of the target, calibration, target thickness and size, speed of measurements and mounting, says Chris Jones of Micro-Epsilon
A
lthough most engineers are familiar with non-contact eddy current displacement sensors, this article acts as a useful
checklist of the factors that should be considered before specifying a sensor, as well as the benefits and limitations of this measuring principle.
MEasuring prinCiplE The eddy current measuring principle occupies a unique position among inductive measuring methods. The principle is based on the extraction of energy from an oscillating circuit. This energy is required for the induction of eddy currents in electrically conductive materials. A coil is supplied with an alternating current, causing 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 a 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. This impedance can be calculated by a controller, which looks at the change in the amplitude and phase position of the sensor coil.
advantagEs 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.
50 August 2019 Instrumentation Monthly
HarsH EnvironMEnts 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.
CustoM CapabilitiEs 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.
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