Joint Torque Sensor Embedded in Harmonic Drive Using Order Tracking Method for Robotic Application

The main contribution of this work is the low-cost joint torque measurement method using harmonic drive characteristics. To measure the external torque, we use the strain measurement of the flexspline of the harmonic drive. The flexspline of the harmonic drive is deformed by the external torque, as well as by the velocity reducing operation, which creates torque ripple on the strain signal from the flexspline. As the frequency characteristics of such torque ripple are strongly related to the joint speed, an order tracking technique is used to compensate the torque ripple. The resampling-based order tracking data acquisition is used to rearrange the torque ripple signal into the order signal, which has independent frequency characteristic to the joint speed. Then, a simple notch filter easily cancels the torque ripple from the rearranged signal. Our algorithm switches the signal source between the order tracking and the conventional time sampling to overcome the zero velocity condition. To overcome the lack of order information at low velocity, we propose a back propagation method using trained order sampling data. The developed joint torque sensing method is applied to the harmonic drive reducer of a robot manipulator's joint and is experimentally evaluated for the torque measurement during the motion of 5th order polynomial trajectory following.