Torque ripple compensation method for joint torque sensor embedded in harmonic drive using order analysis

The main contribution of this work is an algorithmic compensation of joint torque ripple signals in harmonic drive for joint torque sensing from measuring the deformation of flex spline in the harmonic drive. When an external force is applied to a robot, the flex spline of harmonic drive undergoes an elastic deformation. However, such a deformation is not easy to measure by strain gauges on the flex spline because whose signals are corrupted by a periodic vibration which is “torque ripple” caused by reducing operation of the harmonic drive. We perform order analysis and design a filter to eliminate the torque ripple without using many strain gauges. As the order sampling normalizes the frequency characteristic, our method outperforms the filters with fixed cutoff frequency with manageable computational expense. Moreover, the effect of misalignment of the strain gauges is minimum unlike other methods. The experimental evaluation is provided using a robotic manipulator which has the proposed harmonic drive joint torque sensor.