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Accurate control of joint forces is essential to achieve high performance in advanced assembly and other tasks that involve fine motion, active force control, or high speed operations. Joint force control can be substantially improved by sensory feedback. In this paper we present the design and describe the actual characteristics of a joint torque sensor for a PUMA 500. Using this sensor, a joint torque servo-mechanism has been designed and implemented. A model of the actuator-transmission-load system, including flexibility, was developed and verified using both time and frequency domain techniques. Compensators based on this model were designed and tested. Experimental results obtained from pure torque control and joint motion tracking are presented. These results demonstrate a significant reduction of the effective friction (97%), and substantial improvement in fine motion control.
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Pfeffer et al. (1986) studied this question.