Large-Scale and Small-Scale Torque Sensors From a Magnetoelastic Heat-Treated 14\% Chromium Stainless Steel

We have produced a large scale (0.75 inch) and a small scale (0.25 inch) torque sensor from type ESR-420 stainless steel for torque transfer or small scale applications by polarizing two adjacent sections of the shafts with oppositely directed circumferential magnetization. The resultant field signal, found to be linear with applied torque up to 15 N-m, emanated from the domain wall formed between the two regions. A heat treatment consisting of a rapid quench to room temperature from 1038\r{ }C, followed by a slow cool from 871\r{ }C to restore desired magnetic and mechanical properties, was applied to enhance performance. The torque-load sensitivity (field signal in $\mu $G per unit applied shear stress in psi) was found to be linear and as high as 237 $\mu $G/psi, with excellent re- zeroing capability, making it ideal for small-scale applications where weak signals are usually a plaguing problem. Hysteresis properties studied prior and subsequent to the heat treatments show that the axial coercive force remained around 5-6 Oe throughout heat treatment, in correspondence with the large sensitivity values, while the circumferential coercive force remained around 25-27 Oe, which is sufficient to guarantee integrity of the magnetically polarized regions at both scale levels.