Study of Magnetic Interference on an LVDT: FEM Modeling and Experimental Measurements

Abstract

Static or slowly varying magnetic fields can affect the performances of linear variable differential transformer by inducing a position reading drift. The problem is barely addressed in LVDTs' datasheets, and no quantitative information on the induced error is given. An LVDT finite element model is here presented together with its experimental validation in order to propose a tool for the study of the effects of external magnetic fields on LVDTs and for the design of less sensitive devices. The LVDT model has been validated in standard working conditions and in presence of an external magnetic field by means of a complete set of experimental measurements performed on a custom prototype, manufactured following the FEM guidelines.

Electromagnetic Analysis and Validation of an Ironless Inductive Position Sensor

“…Therefore, the low-frequency components will be filtered out, and no cross effect on the main harmonics will be present, with the whole system described in the model being linear. These performances have been experimentally observed in [12] and cannot be obtained with LVDTs due to the nonlinear ferromagnetic materials, whose permeability is modulated by the external field [7], [9], [10].…”

“…The numbers of turns are 3150, 4500, and 1400 for the moving, sense, and supply coils, respectively; whereas the overall sensor's dimensions are 22 and 170 mm for the diameter and length, respectively. The moving coil has been wound with a thicker wire, in order to limit the resistance and guarantee a reasonable induced current, according to (7). For this reason, as moreover evident in Fig.…”

“…The relatively low cost and wide range (from a few millimeters up to a meter) make them a frequent choice also for common applications such as automotive and industry [4]. Nevertheless, LVDT sensors show significant measurement drifts when an external magnetic field is applied in the longitudinal direction (i.e., parallel to the sensor's axis) [6], [7]. Position drifts of some hundreds of micrometers have been observed with dc/slowly varying longitudinal magnetic fields [7].…”

“…Nevertheless, LVDT sensors show significant measurement drifts when an external magnetic field is applied in the longitudinal direction (i.e., parallel to the sensor's axis) [6], [7]. Position drifts of some hundreds of micrometers have been observed with dc/slowly varying longitudinal magnetic fields [7]. These drifts are often an unacceptable drawback in high-precision applications [8].…”

“…On the other hand, a transversal magnetic field has shown negligible influence on the LVDT position reading [6]. Analytical study of the phenomenon [9], characterizations through finite-element method (FEM) simulations [7], [10], and experimental measurement campaigns [7], [11] have demonstrated the unavoidability of this effect for typical LVDT structures due to the presence of ferromagnetic materials with a nonlinear B-H relation, which are part of the intrinsic design of an LVDT. Such nonlinearity between the field and flux densities makes even a dc or slowly varying magnetic field affect the harmonics of the sensor windings' voltages, therefore affecting the position reading.…”