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

Abstract

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. INDEX TERMS Collaborative robots, hybrid pneumatic-electric actuator, impact modeling, pneumatic actuators, position control, robot control.

Optimal Force Allocation and Position Control of Hybrid Pneumatic–Electric Linear Actuators

“…The pneumatic force is the result of the difference in the forces at the two sides of the piston, as indicated in (3). In this equation, P , A , and atm P are the absolute chamber pressure, effective piston area of the chamber, and the atmospheric pressure, respectively.…”

“…The controller structure is the same as that of Figure 3, except the MPC2 block is replaced with a linear position controller. This controller structure is similar to the two-loop structure used in [3]. Two independent controllers are used with the pneumatic and the electric actuators, with the former using feedforward + PD controller and the latter using a PD controller.…”

“…These approaches include active methods that monitor the environment and robot states using machine vision and/or sensor fusion and try to avoid risky situations; and passive methods that try to develop inherently safe robots that are not able to cause high damage in cases of system failure or unpredicted situations. Using hybrid pneumatic-electric actuators (HPEA) to drive the robot's joints has been proposed as a solution to increase inherent safety without sacrificing precision [2,3].…”

“…Although there have been some papers focused on position controller design for HPEAs [3,6,10], very few have used a controller that incorporates each actuator's characteristics and studied the allocation problem alongside the position control problem. In [7], an optimization method is proposed for the torque allocation of a HPEA consisting of a PMA and an electric motor.…”

Modeling and Networked Control of Two-rigid link Robot Arm

“…The problems inherent to the design of a good controller for a robot or part of it have beenand still are-the subject of active research (see [1][2][3][4][5][6][7][8][9][10][11][12] and references therein). A considerable volume of the research in the literature that relates to the control of robots or their parts is focused on the control of robot arms(see [13][14][15][16][17][18][19][20] and references therein).…”