The eddy current measuring principle is a non-contact sensor. These types of sensors are typically used in applications where fast displacement changes have to be measured, where no forces must be exerted on the measurement object, where highly sensitive surfaces do not allow any contact, or where a long sensor service life of the sensor is required.
Measuring principle
Strictly speaking, the eddy current principle should be classified as the inductive measuring principle. The effect for measuring via eddy current 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. Here, a coil is supplied with alternating current, therefore creating a magnetic field around the coil. If an electrically conducting object is present in this magnetic field, eddy currents are induced in it, according to Faraday’ s induction law. According to the Lenz Rule, the field of these eddy currents is opposed to the field of the coil, which causes a change in the coil impedance. This distance-dependent impedance change can be captured at the controller as a measurable factor using the change in the amplitude of the sensor coil.
The principle can be used for all electrically-conductive materials. As eddy current penetrates insulation materials, even metal behind an insulating layer can be used as a measuring object. A special coil winding means that very compact sensor designs are possible, which can still be used across high temperature ranges. All eddy current sensors are insensitive to dirt, dust, moisture, oil and pressure.
Nevertheless, eddy current sensors are subject to some limitations in their use. For example, individual linearity and calibration is necessary for each application. Also, the output signal is dependent on the electrical and mechanical characteristics of the measuring object.
However, these restrictions help to give the eddyNCDT measuring principle from Micro-Epsilon extremely high resolutions of a few tenths of a nanometre. Measuring ranges of less than 100µm and just under one hundred millimetres are currently available. Depending on the measuring range, installation sizes of between 2mm and 140mm can be achieved.
Practical use
Mechanical engineering without displacement sensors is very difficult to imagine. These sensors are used to control various movements, monitor fill levels, check product quality and for many other applications. Yet it is exactly this sector where sensors meet many different and harsh operating conditions, which must be overcome. Sensors often have to operate reliably in the harshest conditions. They are used in oil, hot vapours or fluctuating temperatures. Some sensors are also used on high vibrating parts, in strong electromagnetic fields or need to be located a certain distance from the target. Important application criteria include accuracy and temperature stability, resolution and cut-off frequency. For these reasons, different advantages of the various measuring principles come into play. This means that no general statement can be made about the optimum measuring principle.
Eddy current sensors are subdivided into shielded and unshielded versions. With shielded sensors, a narrower distribution of the field lines is achieved and they are not sensitive to radially adjacent metals. For unshielded sensors, the field lines emanate sideways from the sensor. The measuring range is usually larger. Correct installation is crucial for the signal quality, as neighbouring objects can strongly influence the signal.
NEW – eddyNCDT ECT
Using “Embedded Coil Technology” (ECT), the sensor is housed in an inorganic carrier material, which provides temperature and shape stability. The technology provides more freedom in the physical design of the sensors; in the case of special operating environments, the ECT sensors can be easily adapted to the conditions. The electronics are either integrated in the sensor or are housed separately. The significant benefits of ECT technology are its high operating temperatures of up to 350°C, extreme temperature stability, high mechanical load capacity and its suitable for use in strong electromagnetic fields. Due to a hermetically sealed enclosure, the sensors can also be used in ultra high vacuums.
