Dynamical Adaptive Backstepping-Sliding Mode Control of Pneumatic Actuator

Pneumatic actuators are popular due to their extensive use in production lines of automobile industry in 1950s. Advantages of pneumatic actuators over other power systems include high power to weight ratio, cleanness, low costs, and low maintenance. The challenges in pneumatic actuators are the high compressibility of fluid medium, the nonlinearities in the system dynamics (i.e., pressure dynamics and valve flow dynamics), and the modeling of those characteristics. Another great challenge in the pneumatic actuators is the stick-slip friction caused by pneumatic cylinder seals, which keep pressurized air inside the pneumatic cylinder chambers. As the motion of actuator commences and stops, a jerky motion caused by the stick-slip friction can create adverse effects to the performance of pneumatic actuators, e.g., instability, slow response, large tracking errors, and limit cycle. Conventionally, the friction caused adverse effects in mechanical systems are reduced by increasing the stiffness of the control mechanisms. However, due to the compliance nature of pneumatically operated system, an alternative approach via adaptive friction compensation is considered herein.;This thesis documents the development of a novel nonlinear controller for servo pneumatic actuators that give good reference tracking at low speed motion, where friction has strong effect to the system behaviors. The design of the nonlinear controller presented in this thesis is based on the formalism of Lyapunov stability theory. The controller is constructed through a dynamical adaptive backstepping-sliding mode control algorithm. The conventional Lyapunov-based control algorithm is often limited by the order of the dynamical system; however, the backstepping design concept allows the control algorithm to be extended to higher order dynamical systems. In addition, the friction is estimated on-line via the Lyapunov-based adaptive laws embedded in the controller; meanwhile, the sliding mode control provides high robustness to the system parameter uncertainties. The simulation results clearly demonstrating the improved system performance (i.e., fast response and the reduced tracking error) are presented. Finally, the integration of the controller with a Lyapunov-based pressure observer reduces the state feedback of the servo pneumatic actuator model to only the piston displacement.