Eddy current displacement sensors belong to the inductive displacement group of sensors and are well-adapted for industrial applications. Unlike conventional inductive sensors, the measuring principle for eddy current sensors enables measurements on non-ferromagnetic materials (e.g., aluminum) as well as ferromagnetic materials (e.g., steel). They are designed for non-contact and wear-free measurements of displacement, distance, position, oscillation, vibration, and thickness. Therefore, they are ideally suited to monitoring machines and systems — they can make measurements in harsh Industrial environments, even where pressure, dirt, or temperature fluctuations occur.
Typically, eddy current displacement sensors are used where high measurement accuracy is required and other sensors cannot cope with the prevailing ambient conditions. Optical sensors, for example, are influenced by dirt or dust in the measuring gap and by high temperatures. Conventional inductive displacement sensors use ferrite cores, which have a comparatively a high linearity error and a lower frequency response. Also, their measurement accuracy decreases with fluctuating ambient temperature.
Only conductive metallic targets, whether ferromagnetic or non-ferromagnetic can be measured using eddy current displacement sensors. Non-conductive materials are invisible to the eddy current measuring system and thus have no effect on the measurement results. For this reason, these sensors can measure through materials such as plastics and oil onto metallic objects. This enables applications such as oil gap measurements and distance measurements against rollers that guide plastic films.
Limitations of Conventional Inductive Displacement Sensors
The classic inductive displacement sensor uses a coil wound around a ferromagnetic core. Because of that core, the output is nonlinear, so it must either be linearized in the sensor electronics or the user must account for this nonlinearity in their plant control system.
In addition to non-linearity, other limitations include iron losses caused by the fact that the core itself absorbs the magnetic field. These losses increase with frequency, to the extent that an inductive displacement sensor reaches the limits of its performance at around 100 measurements per second.
Another issue with inductive displacement sensors is their sensitivity to extreme temperature variations due to the high thermal coefficient of expansion of the ferrite core. This makes temperature compensation challenging, normally resulting in high thermal drift.
Eddy Current vs Capacitive Displacement Sensors
Eddy current and capacitive sensors detect the distance to an electrically conductive measurement object based on changes in the electrical field. Eddy current sensors measure the distance via the change in impedance of the sensor coil. With capacitive sensing, the sensor and measurement object form the plates of a capacitor.
Both types can measure in the submicrometer range. Nevertheless, they differ considerably with respect to the operating environment. Eddy current sensors are ideally suited to harsh industrial environments that include dirt, dust, and humidity. Capacitive sensors on the other hand, require the kinds of clean surroundings that can be found in electronics production, laboratories, and clean rooms.
Eddy Current Displacement Sensors
Although eddy current sensors employ the same laws of magnetic induction as inductive displacement and proximity sensors, their air-core coil construction enables higher accuracy, measurement speed, and stability — nonlinearity and temperature drift are not problems.
Their advantages include:
Fast measurements up to 100 kHz.
High resolution down to 0.5 μm or better.
High temperature stability in fluctuating ambient temperatures, which can even be improved with embedded active temperature compensation.
Measurement using either ferromagnetic or non-ferromagnetic target materials.
They are not affected by high pressure, high temperature, dirt, vapor, or oil.
Eddy Current Sensor Operating Principle
An alternating current in the sensing coil produces a changing magnetic field. This field induces a current in the target — the eddy current. The alternating eddy current produces its own magnetic field, which opposes the sensing coil’s field, thus changing the impedance of the sensing coil. The amount of impedance change is dependent on the distance between the target and the sensing coil in the probe. Current flow in the sensing coil, which is impedance dependent, is processed to produce the output voltage, which is an indication of the position of the target relative to the probe.
Since several different eddy current sensor designs are available from Micro-Epsilon, engineers can select the optimal sensor for their particular application. For example, active temperature compensation is necessary if highly precise measurements are required. With varying temperatures there are two factors that can influence the measurement signal: mechanical changes, where the geometric dimensions of the sensor and the target change in the form of extension or contraction of the sensor and the target. And electrical effects have an even greater impact than mechanical because of changing electromagnetic characteristics.
For example, the eddyNCDT 3001 series is specially designed for applications where conventional inductive displacement sensors have often reached their performance limits. They have compact dimensions and are available in M12 and M18 housings, covering measuring ranges from to 2mm to 8mm. They are protected to IP67, and so are applicable in automation, machine building, and design. Furthermore, they are temperature-compensated up to 70 °C. They have high measurement accuracy and linearity as well as a 5-kHz frequency response rate and are factory-calibrated for ferromagnetic and non-ferromagnetic objects such as aluminum and steel.
One application for eddy current displacement sensors is in large machinery such as stone mills or telescopic installations, which often work with hydrostatic bearings. These bearing systems are continuously supplied with liquid lubricant via an external pressure supply. The lubricant is pressed between the bearing surfaces, which are therefore continuously separated from one another by a thin lubricant film. The bearing surfaces are not exposed to friction and hence operate wear-free. This enables sub-micrometer position control. Any disturbances in the hydraulics, or drops in pressure, however, can have disastrous consequences. This could result in damage to the bearings and ultimately system failure, causing high maintenance and repair costs. The oil gap in hydrostatic bearings, therefore, requires continuous reliable inspection. For this application, the sensor is mounted horizontally to the bearing shoe, so it is not directly exposed to the oil pressure. It measures through the oil film onto the opposing bearing surface.
The exact position of the piston, the piston rings, and the existing pressure conditions are essential information for manufacturers of combustion engines. Using simulation tools, this data is primarily used to make reliable predictions about wear, friction, and oil consumption. The eddy current sensor measures the piston ring and so-called piston secondary movements, at high accuracies. Here, the advantages of eddy current sensors — resistance to the high temperatures in combustion engines (up to 180°C and even higher for a short period of time) — are apparent. The prevailing vibration, pressure, oil, fuel, combustion gas, and continuous mechanical movement, do not influence the precision of the results. In addition, eddyNCDT sensors offer fast measurement speeds with small measuring ranges (0 – 0.5 mm) and extremely high resolution (less than 1μm).
Micro-Epsilon has developed a sensor that uses embedded coil technology (ECT) for miniaturization. This production technique allows almost unlimited scope in terms of the external design and geometrical shape of the sensors, while enabling the evaluation electronics to be integrated into the sensor. It is constructed by embedding a two-dimensional eddy current coil in an inorganic material, which improves the sensors’ stability, robustness, and thermal resistance. These sensors are suitable for extremely harsh applications such as ultra-high vacuums in semiconductor production.
Eddy Current Displacement Sensors — Small but Powerful Industrial Workers
These little sensors are ideal for industrial environments, where environments are harshest and most challenging, yet call for extremely precise measurements. They can be used in everything from measuring the gaps in hydrostatic bearings in large machines, the clearance between piston and cylinder, to distance measurements against rollers that guide plastic films.