Non-contact strain measurements based on inverse magnetostriction

For the detection of mechanical quantities such as strain or torque in sensor applications, inverse magnetostrictive materials are widely used. Commonly, the measurement is strongly dependent on the distance between the sensing coil and the magnetic material and the measurement range is restricted to rather low strains. A novel measurement principle to detect magnetostrictive responses based on frequency mixing is proposed that allows to overcome these limitations. For this purpose, a magnetic field consisting of two frequencies is used. The presence of a magnetic material produces new peaks in the Fast Fourier Transformation (FFT) spectra of the measurement signal that are specific to the materials non-linear magnetization curve. Since the magnetization curve is altered by strain, the amplitudes of the peaks reveal a characteristic dependence on the strain level of the material. To study the performance of this method, a nanocrystalline, soft magnetic material as inverse magnetostrictive material was investigated. The experimental data are supported by simulations based on a simple model of the inverse magnetostrictive effect. It is shown that this method allows measurements up to one percent strain and that it is insensitive to distance variations between the strained material and the sensing coil.