Parameters affecting inductive displacement sensors

Abstract

An investigation of the limitations of inductive displacement sensors (IDSs) was conducted with the use of electromagnetic finite element analysis (FEA). A comparison of displacement sensing technologies highlighted the advantages of EDSs in harsh industrial environments, but an understanding of the operation of IDSs showed that they are limited by the influence of target material, width and offset. It was proposed that studying the electromagnetic field around IDSs could reveal more information than was available from the simple impedance measurements employed by a commercially available IDS.A test coil sensor and signal processing system was designed and the result was a reliable system for measuring the magnetic field around the IDS. Experiments showed that the 1 MHz field had an amplitude of 5 x 10(^-6) T at the base of the IDS and two- and three-dimensional FEA models were constructed that gave closely matching central field values. The unreliability of the IDS for different target materials was demonstrated experimentally. FEA simulations showed that changing target permeability and varying target displacement both altered the whole field amplitude uniformly. This showed that it was not possible to counteract the target dependence by monitoring the field with the test coil system in this way. Further FEA simulations revealed field patterns that changed with target offset. An experiment with the test coil system confirmed that it was possible to use the change in lobe amplitude to measure the offset of the target; for example when target displacement d, = 25 mm and offset = 1.2 times the IDS coil diameter, the distance error was 3.6 %, which corresponded to a normalised test coil output of 0.54. A similar effect was found from target width FEA simulations. Hence it was possible to correct the output signal from the IDS coil to counteract the effect of an offset small target.