Pneumatic actuators are low cost, clean and safe. They also have a high power to weight ratio. However, due to the compressibility of the air and the nonlinearities of their dynamics, they are very difficult to precisely model and control. Electrically powered actuators are easier to model and control, and are the most commonly used actuator in applications requiring fast and precise position control, such as robot arms. However, they require a high ratio transmission to produce sufficient torque or force, the cost of their components is greater, and they have a lower power to weight ratio compared to pneumatic actuators. This thesis presents the development of a novel hybrid pneumatic-electric actuator which combines the advantages of both actuator types.
The design and prototyping of the hybrid actuator is presented first. A pneumatic cylinder and a DC motor are connected in parallel using gears. The components are sized to provide the torque required to rotate a single-link robot arm vertically upwards. On/off solenoid valves are used rather than servo valves to keep the hardware cost low.
Next, a mathematical model of the nonlinear actuator dynamics is derived using a combination of physical laws and empirical curve fitting. The dynamics of the mechanical, electrical and pneumatic elements are included. Then a novel discrete-valued model-predictive control plus integral compensator algorithm is created for controlling the position of the pneumatic cylinder using the on/off valves. The control algorithm for the hybrid actuator is completed by using a conventional PD algorithm to control the electric motor.
The performances of the hybrid actuator and the pneumatic cylinder acting alone are investigated and compared using computer simulations and hardware experiments. Multiple experiments are done for vertical cycloidal, vertical sine wave and horizontal sine wave trajectories, and different payloads. The steady state performances of the hybrid actuator and pneumatic cylinder are found to be similar. Conversely, the DC motor added a faster acting and finer quantized force to the pneumatic cylinder force, which greatly improved the dynamic position control performance of the hybrid actuator. In experiments, the mean root-mean-square error and the maximum absolute error improved by 84% and 77%, respectively.