Improved Hybrid Pneumatic-Electric Actuator for Robot Arms

Robot arms require actuators that are powerful, precise and safe. In this paper we present the design and implementation of a novel rotary hybrid pneumatic-electric actuator (HPEA) for use in robot arms, particularly those intended for collaborative applications. It produces 3.5 times higher torque than prior HPEAs while maintaining the low mechanical impedance and inherent safety of the HPEA approach. Its low mechanical impedance results from its low friction and inertia. It has 450 times less inertia and 15 times less static friction than an industrial robot actuator with similar maximum continuous output torque. The design features four pneumatic cylinders connected in parallel with a small DC motor. The DC motor is directly connected to the output shaft. After the mechatronic design and system model are described, the control system design consisting of an outer position control loop and inner pressure control loop is presented. Experiments were performed with the actuator prototype rotating a link and payload with a rotational inertia equivalent to a linear actuator moving a 573 kg mass. Averaged over five tests, a root-mean-square error of 0.038° was achieved for upwards vertical moves. The steady-state error (SSE) was only 0.0045°, even when the arm was under maximum gravity load, primarily due to the adaptive friction compensator employed in the outer control loop. This SSE is almost ten times smaller than the best value reported for previous HPEAs.