A Novel Inverse-Magnetostrictive Force Sensor

The change in magnetic permeability of a material under stress (inverse magnetostriction) offers the potential for a high-performance, low-cost force sensor capable of being used in harsh real-world environments. The existing force sensor technologies are limited in their use in commercial products by either cost issues or susceptibility to electromagnetic noise. Inverse Magnetostriction has been used to measure strain in controlled environments since its discovery by Joule in 1847, but not in practical applications due to a lack of data on how magnetic material properties change with environmental conditions such as temperature. Utilizing an innovative noise-reducing self-inductance design, this paper presents the basis for an inverse-magnetostrictive compressive load sensor. A lumped-parameter model for the change in sensor inductance under load, due to both mechanical and magnetic effects, is derived. The material properties of a magnetostrictive iron alloy are empirically determined over a broad range of loads and temperatures. The model and material properties are confirmed by testing a prototype force sensor. The prototype measures compressive forces from 100 to 25 000 N over a temperature range of 20 degC to 120 deg with a typical error of +/-2% (4% max). The sensor does experience significant thermal hysteresis for which the model does not currently account. This work was motivated by the need for a force sensor in an automobile electric brake system and used a single iron alloy (50% Ni), but the model and testing procedure provide a roadmap for future research to improve the performance and capabilities of such a sensor