Analysis and design of variable structure control systems for chemical processes.

摘要

Chemical processes typically have unfavorable nonlinear dynamics that must be modified by feedback control. Method of Variable Structure Control (VSC) is well suited for just such a purpose. A generalized methodology is developed for analysis and design of VSC systems with emphasis on disturbance rejection.; The design of continuous action Sliding Mode control systems for non-linear processes is investigated for robust control of processes subject to system model uncertainties and disturbances not in the span of control input space. A state-feedback sliding mode controller is developed for non-linear systems under the influence of disturbances with constraints on the control action and its rate of change.; The controller guides the system to a subspace sliding manifold and subsequently maintains it on the manifold to achieve the desired dynamics. A new method of construction of the switching function is introduced in which the significant process output exhibits linear dynamics in the sliding mode. Dynamic compensation is added to handle disturbance rejection. A direct analogy of the switching function structure to a PID controller allows use of existing tuning rules for various PID controller specifications to determine optimum parameter values associated with the switching function.; Conditions for stability of the sliding manifold are developed for constrained control action and its rate of change. Region of sliding manifold stability and stability margins are shown to be in tradeoff with controller performance. An effective maximum feedback gain of the control system is established when controller constraints are in effect.; These developments are then applied to the design of a sliding mode controller to maintain a constant outlet product concentration or temperature in the CSTR. Both cases are investigated and are found to be equivalent in terms of system behavior.; Robustness of the example reactor control system is illustrated with numerical simulations. System performance is analyzed for set-point regulation, Damkohler number variations and disturbance rejection. Fast and safe control is observed. Control action and its rate of change are bounded--to ensure feasibility of industrial applications, which limits the range of operating conditions. Nevertheless, the range of realistic operating conditions is well within the constrained boundaries.