Modeling,Analysis,and Control Of Networked Cascade Control Systems

Cascade control systems, wherein the control loops are closed via real-time networks, are called Networked Cascade Control Systems (NCCSs for short). The NCCS has been widely employed in practical industrial process control, however, due to the complexity of its configurations and the variation of network characteristics, the theories of traditional cascade control system and networked control system are no longer valid. And therefore, it's of great theoretical and practical meanings to investigate the analysis and synthesis of NCCS systematically and thoroughly.Based on practical industrial process control, the concept and typical configurations of NCCS are proposed. To develop an intact and systematic theory system for the analysis and synthesis of NCCS, the issues of modeling, stability analysis, controller designing, performance evaluation and optimization, as well as the design and implement of a pilot plant for an NCCS have been resolved. The main contents of this thesis are as follows:First of all, four typical configurations of an NCCS are proposed. The concepts of node-device connection matrix and network transmission matrix are proposed. With the proposed matrices, system configuration diagrams, and block diagrams, the four different configurations of NCCSs are described. The fundamental issues in an NCCS are pointed out, including network-induced delays, data packet dropout, clock synchronization, quantization error, and so on.Secondly, the closed-loop system models for a class of NCCSs with different characteristics of network-induced delays, in which the controllers are both positional PID or incremental PID, are developed in discrete time domain by state space method. The concepts of general process and general controller are proposed, and they are used to develop a uniform closed-loop system model for the NCCS with uniform configurations in discrete time domain by state space method. Both the network-induced delays and data packet dropout are considered.Thirdly, based on process models and desired closed-loop system responses, the analytical expressions of the PID tuning parameters in both of the primary and secondary controllers are proposed by approximating pure delay with first-order Pade expression for two typical configurations of NCCSs with constant network-induced delays. Based on Lyapunov stability theory and LMI approach, the robust H∞state feedback and output feedback control laws for a typical kind of NCCSs with and without disturbances are designed, in which the network-induced delays are uncertain but bounded. Based onμ-synthesis theory, the uncertain but bounded delay is modeled as a multiplied perturbation. Theμ-synthesis frameworks for two typical classes of NCCSs with uncertain but bounded network-induced delays are developed, and then the D-K iteration method is used to designμ-synthesis primary controllers for the system while the secondary controller is a PID controller and has been tuned beforehand.Then, based on the performance indices of stable and dynamical states of an NCCS without networks in the control loops, a comprehensive control performance index for an NCCS is proposed in the form of weighted summation. The performances for two kinds of NCCSs are evaluated and optimized, in which the fuzzy auto-tuning PID control method is employed in the primary controller while the secondary controller still adopts traditional PID control method.Finally, an NCCS pilot plant based on FF(Foundation Fieldbus) is designed and implemented. A fuzzy PI online automatic optimization program is designed, by which the PI tuning parameters of the primary controller can be automatically adjusted based on fuzzy auto-tuning inference rules. An NCCS for the tank water temperature is implemented and commissioned. The feasibility and effectiveness of fuzzy auto-tuning PID control method in an NCCS are validated by the practical commission results.