Feature: Sensors


The benefits of eddy-current sensors R


option compared to inductive switches and displacement sensors – particularly where high-linearity, high-speed measurements and high resolution are of crucial importance. Both categories of devices – eddy-current sensors and inductive switches and displacement sensors – have their respective advantages and disadvantages.


By Glenn Wedgbrow, Business Development Manager, Micro-Epsilon UK creating eddy currents that are the reverse of the initial excitation current and have the effect of reducing the oscillator voltage. Tese 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


ecent advances in eddy- current sensor design, integration, packaging and cost reductions have made these sensors a much more attractive


Inductive sensors Te classic inductive displacement sensor comprises a coil wound around a ferromagnetic core. When excited by an alternating current from an oscillator- based driver circuit, the coil generates a magnetic field concentrated around the core; see Figure 1. Te lines of flux interact with the target conductor as it approaches,


22 March 2022 www.electronicsworld.co.uk


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. 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 manner – i.e., the output can be ‘normally open’ (NO) or ‘normally closed’ (NC), depending on the configuration. Because of the ferromagnetic core


in an inductive displacement sensor, the output is non-linear, so 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 fifty measurements per second. A third problem with these sensors


is their poor tolerance of wide temperature variations due to 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 the inductive displacement device.


Eddy-current sensors To overcome these limitations, a class of inductive displacement sensor called “eddy-current sensors” (Figure 2) has


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