Position control of hybrid pneumatic-electric actuators

The design, modeling and control of a novel hybrid pneumatic-electric actuator for applications in robotics and automation is presented. The design incorporates a pneumatic cylinder and DC motor connected in parallel. By avoiding the need for a high ratio transmission, the design greatly reduces the joint friction torque that contributes to the danger associated with robot arms. A novel discrete-valued model-predictive control (DVMPC) algorithm is proposed for controlling the position of the pneumatic cylinder with on/off valves, rather than costly proportional or servo valves. A variant of inverse dynamics control is proposed for the DC motor. A prototype was built for validating the actuator design and control algorithms. It is used to rotate a single-link robot arm. Experimental results are presented for vertical cycloidal and sinusoidal position trajectories. Even with the poor quantization caused by the on/off valves, the pneumatic cylinder controlled by the proposed DVMPC algorithm achieved a 2.5% maximum absolute error (MAE) for the vertical cycloidal trajectory. The DVMPC algorithm also switches the valves less often than the PWM method, reducing valve wear. With the addition of the DC motor to form the hybrid actuator, the performance improvement was significant. For the vertical cycloidal trajectory, the MAE was reduced to 0.37%. With the vertical sinusoidal trajectory, the MAE was 1.1%. These results compare favorably to the 5% MAE achieved by previous researchers for a horizontal sinusoidal trajectory using a hybrid pneumatic-electric actuator controlled by servo valves.