Non-contact high-frequency measurements of mechanically loaded ferromagnetic thin films: An alternative approach for torque sensors

Abstract This study presents an alternative sensor setup for non-contact torque measurements of a mechanically loaded shaft by using thin soft ferromagnetic Fe32Co46Hf11N12 and Fe40Co37Zr11N12 films. The films possess a thermally induced in-plane uniaxial anisotropy after annealing them in a static magnetic field. They show a soft ferromagnetic behaviour with a high saturation polarisation up to around 1.35 T. The cut-off frequencies of the films are approximately 2.7 GHz. A short-circuited triplate strip line was designed, in order to detect changes in the frequency-dependent scattering parameter S11 due to mechanically induced stress in the ferromagnetic film. Compressive stress in the ferromagnetic film, which arises from the torsion of the shaft, causes an increase of the cut-off frequency by around 37 % applying a torque from 0 to 100 Nm. The dependence of the cut-off frequency on mechanical load is theoretically described and in good agreement with the experimental data.