Fig. 1 Elements of the torque transducer and the magnetic parameters concerning with ring type torque sensor by using inverse magnetostriction. (H T =Magnetization strength, M r =Residual magnetization, s=Applied stress) Galfenol (FeGaAlZrC) alloy ring was used for torque sensing by detecting a leakage flux from the ring surface based on the inverse magnetostriction effect. This leakage flux changes depending on the magnetic parameter such as magnetostrictive strain (l), residual magnetization (M r ), applied or residual stresses(s) of the ring. Beside, the hoop stress (s h )which is one of the residual stress factor in the ring also has strong effect on torque sensing because this affects those magnetic parameters. The fitting gap between ring and shaft was four kinds of 6, 10, 20 and 30 mm and hoop stress (s h ) in each case can be estimated from the elastic formula of material mechanics. As a result, torque sensitivity showed the maximum value in the case of 10 micrometers gap and it decreased toward the gaps of 10 and 20 mm and drastically decreased in the case of minimum gap of 6 mm. This change of torque sensitivity depending fitting gap is discussed based on the magnetic domain mobility and inelastic material effect at the interface of Galfenol ring and rotating shaft.
The anisotropy constants K1 and (K2-K3) of anisotropic Nd-Fe-B magnet powders prepared by the modified HDDR process were obtained from torque measurements in magnetic fields of up to 7.2 MA/m. A value of K1 = 5.210 6 J/m 3 was obtained by Fourier analysis of an unsaturated torque curve. In the case of quasi-single crystal powders, K1 was 5.9 10 6 J/m 3 , which was closer to the results for Nd2Fe14B sintered magnets. The magnitude of the anisotropy field HA distribution in each direction for 2 kinds of anisotropic and isotropic Nd-Fe-B magnet powders prepared by the HDDR process was also determined by Flanders and Shtrikman's method. In powders prepared by the modified HDDR process, the quantity of particles with higher HA was greater than in those prepared by the ordinary HDDR process in the easy direction, and the angular distribution of HA was sharp. The easy-axis distribution of particles was less than 20 for anisotropic powders. In the case of isotropic powders, the easy axes were equally distributed in all directions. The results of experiments showed that Flanders and Shtrikman's method was appropriate for determining the magnitude of HA and the angular distributions of permanent magnets with high anisotropy.
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