An assessment of alternative soft magnetic materials in rotary variable differential transformers

Abstract

Position sensors are a key technology for controlled actuation systems, which are required to meet increasingly exacting dynamic performance specifications. Of the various sensing technologies, variable differential transformers are capable of satisfying stringent performance criteria, in terms of resolution, repeatability, and stability of output, while operating in the harshest of environments. They utilize the variation of mutual inductance which occurs between a primary and two secondary coils as a ferromagnetic core is moved by the object whose position is to be measured. The article is concerned with rotary variable differential transformers, which currently use high permeability magnetic alloys, such as nickel–iron, either solid or laminated. However, since they are being required to operate at increased excitation frequencies, up to 5 kHz, there is interest in the use of powder composite magnetic materials, which, although having a lower permeability, have a higher electrical resistivity, and hence reduced eddy current effects. The potential for such materials is investigated by steady-state ac finite element analysis, and shown to be promising.