{"id":20842,"date":"2022-02-02T10:20:38","date_gmt":"2022-02-02T10:20:38","guid":{"rendered":"https:\/\/www.engineernewsnetwork.com\/blog\/?p=20842"},"modified":"2022-02-02T10:20:40","modified_gmt":"2022-02-02T10:20:40","slug":"why-eddy-current-sensors-are-replacing-inductive-sensors-and-switches-2","status":"publish","type":"post","link":"https:\/\/www.engineernewsnetwork.com\/blog\/why-eddy-current-sensors-are-replacing-inductive-sensors-and-switches-2\/","title":{"rendered":"Why eddy current sensors are replacing inductive sensors and switches"},"content":{"rendered":"\n

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<\/strong><\/p>\n\n\n\n

In order to appreciate the inherent advantages of eddy current sensors relative to inductive switches and displacement sensors, it is important to first understand the operating principle of both types.<\/p>\n\n\n\n

The classic inductive displacement sensor 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. <\/p>\n\n\n\n

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\u2019s Laws of magnetic induction. The eddy currents push back against the excitation current to cause a drop in voltage in the oscillator and it\u2019s this drop in voltage that is used to determine displacement.<\/p>\n\n\n\n

\"Why<\/a>
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\u2019s Laws of magnetic induction. The eddy currents push back against the excitation current to cause a drop in voltage in the oscillator and it\u2019s this drop in voltage that is used to determine displacement<\/figcaption><\/figure>\n\n\n\n

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\u2019s choice of configuration. <\/p>\n\n\n\n

Because of the ferromagnetic core in an inductive displacement sensor, the output is non-linear and so it needs to be linearised either in the sensor electronics or mathematically using polynomials in the plant or machine control system. <\/p>\n\n\n\n

Along with non-linearity, another downside of using a ferromagnetic core is the \u201ciron losses\u201d 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.<\/p>\n\n\n\n

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. <\/p>\n\n\n\n

Eddy current sensors offer improved precision<\/strong><\/p>\n\n\n\n

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.<\/p>\n\n\n\n

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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<\/figcaption><\/figure><\/div>\n\n\n\n

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 <\/p>\n\n\n\n

While the eddy current sensor\u2019s operating principles are in line with Faraday\u2019s Laws, it is the eddy currents\u2019 effect on the impedance of the coil that is measured rather than the oscillator\u2019s voltage change. The controller calculates the impedance by looking at the change in the amplitude and phase position of the sensor coil. <\/p>\n\n\n\n

High-performance sensing with eddy current sensors<\/strong><\/p>\n\n\n\n

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\u2019s architecture, as well as advanced electronics, manufacturing and calibration techniques, put it in a much higher category in terms of performance.<\/p>\n\n\n\n

These performance characteristics can be broken down into two categories: inherent characteristics and characteristics resulting from product design, manufacturing and calibration. The three most exciting inherent features are: <\/p>\n\n\n\n