Sensors & transducers


Why eddy current sensors are replacing inductive sensors and switches


Both eddy current sensors as well as inductive switches and displacement sensors each have their respective advantages when measuring position and displacement of objects in harsh environments. However, recent advances in eddy current sensor design, integration, packaging and overall cost reduction, have made these sensors a much more attractive option, particularly where high linearity, high speed measurements and high resolution are critical requirements, says Glenn Wedgbrow, business development manager at Micro-Epsilon UK.


important to first understand the operating principle of both types. The classic inductive displacement sensor


I


comprises a coil that is wound around a ferromagnetic core. When excited by an alternating current from an oscillator-based driver circuit, the coil generates a magnetic field that is concentrated around the core. The lines of flux interact with the target conductor as it approaches, creating eddy currents that are the reverse of the initial excitation current and have the effect of reducing the voltage across the oscillator. These variations in voltage due to the change in air gap distance are detected and converted to an analogue output signal, such as a 4-20mA loop, and then processed upstream to determine displacement. A proximity sensor, also called a proximity


switch, is a simplified application of the principles behind the induction effect, detecting only whether or not an object (the conductive target) is present or not. A comparator (Schmitt trigger) detects the drop in voltage and sends a signal to an amplifier. This in turn switches the output in a binary fashion. The output can be normally open (NO) or normally closed (NC), depending on the user’s choice of configuration. Because of the ferromagnetic core in an inductive displacement sensor, the output is non-linear and so


n order to appreciate the inherent advantages of eddy current sensors relative to inductive switches and displacement sensors, it is


it needs to be linearised either in the sensor electronics or mathematically using polynomials in the plant or machine control system. Along with non-linearity, another downside of


using a ferromagnetic core is the “iron losses” due to the core itself absorbing the magnetic field. These losses increase with frequency, to the extent that an inductive displacement sensor maxes out at around 50 measurements per second. A third problem with inductive displacement


sensors is poor tolerance of wide temperature variations because of the high thermal coefficient of expansion of the ferrite core material. This wide variation makes temperature compensation very difficult, usually resulting in a large thermal drift of inductive displacement sensors.


Eddy cUrrEnt sEnsors offEr iMprovEd prEcision To overcome these limitations, a certain class of inductive displacement sensor called “eddy current sensors” have been developed which rather than a ferrite core, use an air-core coil. While the eddy current sensor’s operating


principles are in line with Faraday’s Laws, it is the eddy currents’ effect on the impedance of the coil that is measured rather than the oscillator’s voltage change. The controller calculates the impedance by looking at the change in the amplitude and phase position of the sensor coil.


HiGH pErforMancE sEnsinG WitH Eddy cUrrEnt sEnsors While all the sensors mentioned above are able to detect targets such as metals and ferromagnetic and non-ferromagnetic materials in harsh, non- contact environments, the eddy current sensor’s architecture, as well as advanced electronics, manufacturing and calibration techniques, put it in a much higher category in terms of performance. These performance characteristics can be


In an inductive displacement sensor, a coil is wrapped around a ferromagnetic core and when an alternating current is passed through the coil it generates a


magnetic field. The lines of magnetic flux interact with a conductive object as it gets closer and generates opposing eddy currents, per Faraday’s Laws of


magnetic induction. The eddy currents push back against the excitation current to cause a drop in


voltage in the oscillator and it is this drop in voltage that is used to determine displacement.


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broken down into two categories: inherent characteristics and characteristics resulting from product design, manufacturing and calibration. The three most exciting inherent features are:


A very high measurement frequency of up to 5 kHz.


High resolution, down to 0.5 µm. High linearity and temperature stability.


Eddy current sensors employ the same laws of magnetic induction as inductive displacement and


proximity sensors. However, the use of an air-core coil along with advanced electronics, manufacturing and calibration techniques, puts them in a much higher performance category.


February 2022 Instrumentation Monthly High precision, robustness, linearity and


temperature tolerance allow an eddyNCDT sensor to track parameters such as the lubrication gap in a combustion engine.


Because of the use of an air-core coil versus a


ferrite core, an alternating current of up to 1MHz can be used, although the electronics in devices such as the eddyNCDT3001 and NCDT3005 provide measurement frequencies of 5 kHz. This is still 10x that of their inductive displacement counterparts which typically top out at 50 Hz (translating to 50 measurements per second). Higher-end Micro-Epsilon eddy-current sensors can reach 100 kHz. In terms of linearity and temperature, the air-core


coil does not have to deal with the flux losses or compensate for the thermal expansion of a ferrite core, so it has a 10x improvement in linearity. This linearity is also the result of Micro-Epsilon’s manufacturing and calibration process, whereby the coil is placed in an oven and cycled between -20˚ and +60˚C. The changes in the material are stored in the calibration set-up in the sensor and used to compensate for temperature fluctuations. While some inductive displacement sensors also


have compensation built into the sensor, it is typically limited to plus-or-minus three to five per cent of the full-scale output (FSO). Eddy current sensors provide compensation for the full measurement channel (± 0.025 per cent FSO.) It is also important to note that eddy-current sensors are calibrated to the target material at the factory for maximum accuracy, which puts a higher premium on these devices.


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