Design for Joint Torque Sensors and Torque Feedback Control for Direct-Drive Arms

In a direct-drive arm, motors are directly coupled to their arm linkages without reducers. This eliminates some defects of traditional gearing such as backlash, friction and deflection, and therefore enables the drive system to have improved control accuracy. The D.C. motors used for the direct-drive arm, however, have a significant amount of torque ripples and deadband, which limits the control accuracy, particularly for compliance and force control.

In this paper, a method for the torque feedback control of a direct-drive joint is presented. The motor output torque is directly measured by a strain gage sensor built into the motor. It is found that closed loop torque control can compensate for the complicated nonlincaritics and improve the overall control accuracy.

First, the design of the torque sensor is presented. The effect of the structural vibration mode which results from the insertion of the flexible torque sensor is analysed, and a design method to place the natural frequency of this mode far above the operating frequencies is shown. Torque feedback control using the sensor is discussed. The sensor design is verified experimentally, and the control performance using direct torque feedback is evaluated.