Magnetic flux biasing of magnetostrictive sensors

The performance of magnetostrictive materials, especially those with high initial magnetic permeability and associated low magnetic reluctance, is sensitive to not just the amount of magnetic bias but also how the bias is applied. Terfenol-D and Galfenol have been characterized under constant magnetic field and constant magnetomotive force, which require active control. The application of a magnetic flux bias utilizing permanent magnets allows for robust magnetostrictive systems that require no active control. However, this biasing configuration has not been thoroughly investigated. This study presents flux density versus stress major loops of Terfenol-D and Galfenol at various magnetic flux biases. A new piezomagnetic coefficient $d_{33}^{\varphi }$ is defined as the locally-averaged slope of flux density versus stress. Considering the materials alone, the maximum $d_{33}^{\varphi }$ is 18.42 T GPa⁻¹ and 19.53 T GPa⁻¹ for Terfenol-D and Galfenol, respectively. Compared with the peak piezomagnetic coefficient $d_{33}^{\ast }$ measured under controlled magnetic fields, the piezomagnetic coefficient $d_{33}^{\varphi }$ is 26% and 74% smaller for Terfenol-D and Galfenol, respectively. This study shows that adding parallel magnetic flux paths to low-reluctance magnetostrictive components can partially compensate for the performance loss. With a low carbon steel flux path in parallel to the Galfenol specimen, the maximum $d_{33}^{\varphi }$ increased to 28.33 T GPa⁻¹ corresponding to a 45% improvement compared with the case without a flux path. Due to its low magnetic permeability, Terfenol-D does not benefit from the addition of a parallel flux path. (paper)