Research On Constant Disturbance Rejection For Multivariable Processes With Multiple Time Delays

Tracking reference and rejecting disturbance have always been the two main issues in designing control systms.Among the two issues,disturbance rejection is especially important to process industry.The reason is that in process industry,the set-point cannot be changed frequenctly,which means the control system will run at the steady-state for a long time.When the control system runs at the steady-state,disturbance rejection is the main task of the controllers.Considering that most controlled processes are multivariable processes with multiple time delays in process industry,we will study the control strategies for rejecting constant disturbance in cases of multivariable processes with multiple time delays.The main research contents are as follows..1.The design of decentralized controllers for disturbance rejection.The decentralized controllers have advantages of simple structures and easy implement.In order to design effcctive decentralized controllers,we propose the equivalent disturbance transfer function based on the idea of equivalent open-loop transfer function.Whereas the equivalent disturbance transfer function usually has a complicated form,model simplification is necessary before utilizing.Then,according to the simplified equivalent disturbance transfer function,the optimal controllers can be obtained by optimizing the infinite norm of each closed-loop disturbance transfer function.The controller parameters are determined by considering robust index.The final controllers can be realized in the form of PID controller by Maclaurin series.2.The design of centralized controllers for disturbance rejection.Although the structure of decentralized controllers is simple,they have limitations in control performance.Therefore,we propose novel centralized IMC-PID controllers based on internal model control strategy.By structure transformation and rational design,the centralized IMC-PID controller matrix which consists of a full-element filter matrix and a diagonal PID controller matrix can be obained.The elements of the filter matrix are designed using the Nyquist set of the compensated inverse model and the complex curve fitting technique.The method for obtaining stable filter elements is provided.The parameters of the diagonal PID controllers are determined by considering the robustness and disturbance rejection performance.The propsed centralized IMC-PID controllers can not only be used for square processes,but also for non-square processes.3.The research of disturbance rejection methods based on inverted decoupling.The decoupling control strategies are necessary when the controlled process has strong couplings.The inverted decoupling is selected as the main decoupling method because of its good decoupling performance.In this paper,the disturbace rejection strategies based on inverted decoupling can be divided into two parts.The one is designing 2 degree of freedom controllers based on full inverted decoupling.In this part,inverted decoupling is extended to non-square processes and the method for designing stable and realizable decoupler is provided.Based on the decoupled process,the method for designing 2 degree of freedom controller is proposed,which can improve the performance for set-point tracking and disturbance rejection simultaneously.The other is proposing partical inverted decoupling control strategy.Considering that the coupling may be helpful for rejecting disturbance,inverted decoupling is applied into partical decoupling control strategy.After judging the couplings,the helpful couplings remain while the harmful couplings are removed by the inverted decoupler.Compared with full decoupling,the partical decoupling has simpler structure but better performance for disturbance rejection.4.The research of disturbance rejection methods based on disturbance observer.The disturbance observer-based structure can esitimate and rejecte external disturbance effectively in SISO systems.But,the disturbance observer-based structure is rarely studied in MIMO systems.The reason is that the external disturbance cannot be esitimated exactly because of the couplings in MIMO system.In this paper,a modified disturbance observer which can esitimate the external disturbance effectively is proposed.After the disturbance estimation is obtained,a disturbance rejection controller is designed by optimizing H2 performance.The proposed controller can improve the performance for disturbance rejection under the desired robustness condition.