Heat treatment effects on sensitivity and hysteresis loops of magnetoelastic torque transducers

Recent experiments have indicated that a torque transducer can be construct ed based on local bands of naturally stabilized remanent circumferential ma gnetization within a hollow steel shaft. A strong coercive force along with the crystalline anisotropy combine to stabilize the circumferential magnet ization of the sensing area of the shaft. A torqued shaft has its magnetic axis tilted into a helical orientation, which generates an axial field sign al linearly proportional to the applied torque that can be sensed externall y to the shaft. Our research has shown that the transducer function can be improved by appropriate sequential heat treatments in a helium atmosphere. Subsequent to all heat treatment steps, the test shafts were measured to de termine the changes in axial and circumferential magnetic hysteresis proper ties and to track the development of the magnetic sensing area to applied t orque (sensitivity). Although little change was produced in the originally narrow axial hysteresis loops, a 10%-20% decrease was observed in both axia l coercive force and remanent magnetization, and correspondingly, a 50% inc rease was observed in sensitivity, accompanied by a higher linear saturatio n limit. Additionally, the heat treatment cycle significantly widened the o riginally broad circumferential hysteresis loops, resulting in a substantia l increase in circumferential coercive force, which improves the stability of the sensory region. (C) 2000 American Institute of Physics. [S0021-8979( 00)71008-5].