Design, Implementation and Control of an Improved Hybrid Pneumatic-Electric Actuator for Robot Arms

Actuators used in robot arms need to be powerful, precise, and safe. We present the design, implementation, and control of a novel rotary hybrid pneumatic-electric actuator (HPEA) for use in robot arms, and collaborative robots in particular (also known as “cobots”). This HPEA is capable of producing torque 3.5 times larger than existing HPEA designs while maintaining low mechanical impedance (due to low values of friction and inertia) and inherent safety. The HPEA prototype has 450 times less inertia and 15 times less static friction in comparison to a conventional robot actuator with similar maximum continuous output torque. The HPEA combines the large slow torque generated by four pneumatic cylinders, connected to the output shaft via rack and pinion gears, with the small fast torque generated by a small DC motor directly connected to the output shaft. The direct connection of the motor avoids the higher cost and lower precision caused by a gearbox or harmonic drive. The control system consists of an outer position control loop and two inner pressure control loops. High precision position tracking control is achieved due to the combination of a model-based pressure controller, model-based position controller, adaptive friction compensator, and offline payload estimator. Experiments were performed with the actuator prototype rotating a link and payload in the vertical plane. Averaged over five tests, a root-mean-square error of 0.024° and a steady-state error (SSE) of 0.0045° were achieved for a fast multi-cycloidal trajectory. This SSE is almost ten times smaller than the best value reported for previous HPEAs. An offline payload estimation algorithm is used to improve the control system's robustness. Finally, the superior safety of the HPEA is shown by modeling and simulating a constrained head-robot impact, and comparing the result with similar electric and pneumatic actuators.