Ironless position sensor with intrinsic immunity to external magnetic fields

A novel sensing structure for linear position measurement is presented. It is based on variable inductive coupling between different coils, as in Linear Variable Differential Transformers. Nevertheless, the novelty of the concept stays in the intrinsic immunity of the proposed sensor to external DC/slowly varying magnetic fields, achieved by avoiding the use of magnetic materials (i.e. magnetic cores). Features like absence of sliding contacts, infinite resolution and magnetic field immunity make this solution very attractive for harsh environment applications. Experimental measurements have been performed on a first prototype, in order to test the working principle and the immunity in a case study (longitudinal interfering field) with constant and ramped magnetic flux density profiles. The sensor showed to be practically insensitive to external fields.