A torque transducer based on local bands of naturally stabilized remanent circumferential magnetization

A simple construction for "polarized ring"-type torque transducers is described. Instead of residing within a separate magnetoelastically active ring, bands of circumferential remanent magnetization are established in the shaft itself. Coercivity, crystal anisotropy, and the closed circumferential configuration combine to stabilize the polarization. Symmetry precludes the appearance of magnetic fields from an untorqued shaft. Stress anisotropy associated with applied torque tilts the average easy axis into a helical orientation with chirality and helix angle proportional to torque direction and amplitude. Fields, associated with the divergence of the axial component of this magnetization, then arise in the space around each such band. Suitable shaft materials have high enough anisotropy and coercivity, and low enough magnetostriction to prevent these fields from significantly magnetizing proximate shaft regions. Experimental transducers, using heat treated alloy steel shafts, perform adequately for many industrial applications. (C) 1999 American Institute of Physics. [S0021-8979(99)31308-6].