Modelling of a Magnetostrictive Torque Sensor

Existing magnetostrictive torque sensor designs typically measure the rotation of the saturation magnetization under an applied torque and their theoretical treatment revolves around the minimization of the free energy equation adapted according to the assumptions considered valid in each design. In the torque measurement design discussed in this paper, Ni-rich NiFe films have been electrodeposited on cylindrical austenitic steel rods. Contrary to existing designs, the excitation field is applied along the axial direction and is low enough to ensure that the resulting magnetization along the same direction remains in the linear region of the M(H) characteristic. Assuming homogeneous magnetization, positive magnetostriction constant 2, negligible hysteresis and demagnetizing fields, torque T may be expressed in terms of an effective uniaxial anisotropy constant Ku around 45 to the axial direction. It is shown, that for the proposed arrangement, the resulting M is the linear superposition of the effect of a torque -induced effective field and the excitation field, the applied field accounts for the vertical offset of the magnetization response and the applied torque increases the slope of the M(H) characteristic.