Joint torque measurement of a direct-drive arm

The design and analysis of joint torque sensors for a direct-drive arm is presented. The direct-drive arm is driven by high torque motors without reducers or other transmission. It can serve as a suitable device for implementing torque control schemes. The direct coupling of motors to their loads reduces friction, eliminates backlash and increases mechanical stiffness. All these are desirable features for precision torque control. The drive system, however, still has some errors due to electric and electromagnetic nonlinearities at the motors and amplifiers. The direct measurement of torque is therefore necessary in order to compensate for these errors. First, two different types of torque measurement are described and compared; an indirect method via current measurement and a direct method using strain gages. Next, the strain gage torque sensor is analyzed. A design principle is developed in order to achieve high sensitivity torque measurement without sacrificing the high mechanical stiffness or degrading the dynamic performance of the drive system. Implementation and experiments are described for verifying the design principle and for evaluating the sensors.