Design consideration on the torque sensor for intelligent mechanical systems

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In order to develop a magnetostrictive torque sensor using a steel shaft, factors affecting the accuracy of the sensor, such as magnetic inhomogeneity at the surface of the shaft which causes zero-level-fluctuation with shaft rotation and hysteresis in the input-output relationship, are investigated for several promising steel shaft svailable commercially. We examined, using magnetic heads with figure of eight coils, nickel-chromium-molybdenum steel shafts such as SNCM420, SNCM616 and SNCM447, and chromium-molybdenum steel shafts SCM420. The sensitivity was highest with SNCM616 and then SNCM420/SNCM447 following this, finally SCM420 when these shafts were casehardened. The input-output hysteresis is analyzed for the magnetostrictive shaft torque sensor with phase-sensitive-detection. We describe a method of tuning operating condition of the torque sensor to obtain zero hysteresis for a class of the steel shaft. We found that the lowest hysteresis was obtained with a carburized SNCM420 shaft at the excitation condition of 60 kHz 0.3 A.The linearity measured with this shaft was as good as 0.8% (full scale=1000 Nm for a shaft of 25 mm in diameter). The zero-level-fluctuation of the torque sensor using SNCM420 carburized shaft was (]SY.+-。[)1.3% (full scale). We further established a method to select a proper steel shaft yielding zero hysteresis.We also developed a method of imaging surface anomaly using a miniaturized magnetic anisotropy probe. Using this, we found helical patterns of inhomogeneities at the surface of unheat-treated shafts which were probably induced during a final stage of steel bar manufacturing process.