The inner loop rejecting and the outer loop tracking set-point controllers are coupled with each other in the cascade control, so they can't regulate respectively during the process of the design, which will result in redesigning the system in the application of engineering, since either is inappropriate. For this problem, this paper adopts a control structure of Two-Degree-Of-Freedom based on the Internal Model Control, which can deal with the decoupling between the nominal set-point response and the rejecting function. According to the H2 optimal performance index, this paper designs the set-point tracking and the inner rejecting controllers on the basis of the control trajectory structure proposed, the parameters of the both can be regulated respectively. The superheated steam temperature system is a kind of large lag and inertia control problem in the thermal power plant, which usually adopts cascade control manner in practice. The decoupling two-degree-freedom cascade control strategy is used to the superheated steam temperature control. From the simulation results, the application in the superheated steam temperature improves the ability of tracking set-point response and rejecting disturbance.
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