Dynamical adaptive backstepping sliding mode control for servo pneumatic positioning applications: controller design and experimental evaluation
Servo control of pneumatic actuators is difficult due to the high compressibility and non linear flow of air. Friction as well as uncertainties in the parameters and model characterizing dynamics of the pneumatic systems further contribute to control challenges. These drawbacks cause stick slip motion, larger tracking error and limit cycles, which degrades the control performances. Selection of a controller that satisfies requirements of the performing tasks is thus crucial in servo pneumatic applications. This thesis focuses on the design and experimental evaluation of a model based, nonlinear controller known as Dynamical Adaptive Backstepping Sliding Mode Control (DAB SMC) Originally designed for chemical process control and applied only in simulations, the DAB SMC is adopted in this thesis and applied to the new area of servo pneumatic control of a single rod, double acting pneumatic cylinder and antagonistic pneumatic artificial muscles (PAMs) The controller is further enhanced by augmenting it with LuGre based friction observers to compensate the adverse frictional effect presents in both actuators. Unlike other research works, the actuators are subject to a varying load that influences control operations in two different modes: motion assisting or resisting. The implementation of DAB SMC for such servo pneumatic control application is novel. The mass flow rates of compressed air into and out of the actuators are regulated using one of the following valve configurations: 5/3 way proportional directional valve, two 3/2 way of four 2/2 way Pulse Width Modulation (PWM) controlled valves. Over the entire range of experiments which involve various operating conditions, the DAB SMC is observed to track and regulate the reference input trajectories successfully and in a stable manner. Average root mean square error (RMSE) values of tracking for cylinder and PAMs when the compressed air is regulated using the 5/3 way proportional valve are 1.73mm and 0.10deg, respectively. In case of regulation, the average steady state error (SSE) values are 0.71mm and 0.04deg, respectively. The DAB SMC exhibits better control performance than the standard PID and classical SMC by at least 33% The DAB SMC also demonstrates robustness for up to 78% in uncertainty of load parameter. When the control valve is replaced by the PWM controlled valves of 3/2 way and 2/2 way configurations, performance is slightly compromised.