The sensitivity of linear variable differential transformer (LVDT) position sensors to external slowly varying magnetic fields represents a critical issue when these sensors are installed close to high-current cables or electrical motors with significant fringe fields. The resulting position error can reach several hundreds of micrometers against a specified uncertainty normally below a few micrometers. In this paper, the design of a LVDT position sensor with high rejection to external constant or slowly varying magnetic fields is addressed by exploiting the finite element method (FEM) simulator FLUX. A shield, isolated from the sensor's magnetic circuit, has been considered to reduce the effect of magnetic fields on the secondary voltages of the LVDT. In addition, a dc current is used in order to polarize the magnetic circuit to reduce the sensitivity of the sensor to external interferences.

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.

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