Sensitivity analysis of eddy current sensors using computational simulation

Eddy current sensors can detect the position and movement of metal parts without direct contact. The magnetic fields of these sensors can penetrate protective metal enclosures when designed and applied appropriately. Thus particularly robust solutions for industrial applications are possible, e.g. tracking objects electrically like conductive or ferromagnetic work pieces (device currently being tested) during a treatment process under difficult production conditions. The disadvantage of a test set up where the sensor and the tested specimen are surrounded by equipment and enclosures is reduced sensor sensitivity, this combined with different test piece material properties and tolerances adversely influences the measurements. In order to evaluate the performance of an eddy current sensor, a sensitivity analysis for selected measurement frequencies are necessary. Experimental studies on the subject of sensor sensitivity under production conditions are difficult and usually not feasible due to the costs involved. Therefore, using a virtual 3D model such effects were simulated using the finite element program ANSYS. The sensor output is the phase angle between voltage and current of the sensor coil. The use of the magnetic field strength to determine this phase angle yielded results in accordance with the experimental trends. The problem is the large differences in the geometric dimensions, such as the sensor size compared to effective magnetic air gaps. Magnetic permeability and electrical conductivity are the relevant parameters for the eddy current sensors. In experimental tests, these parameters were determined, also as a function of temperature. It was followed by the simulation of the sensor characteristics by varying different parameters affecting the sensor signal. The results obtained with element type SOLID117 were highly dependent on finite element meshing. But requires significantly less computational effort than an element type SOLID236, which was used to verify selected results based on SOLID117 element type. The result of the simulations is the influence of the considered parameters on the achievable accuracy in the detection of the device under test during the process of machining, individually or in combination.