Inverted Decoupling Control For Multivariable Thermal Processes

Recent years in China, the rapid industrial development along with the vast energyconsumption calls for power plants to adopt power units with high-parameter andlarge-capacity. Increasingly strict demand on high-performance of thermal processeshas been put forward subsequently. Since a large part of thermal processes areessentially multivariable systems, the decoupling control is well worth studying.This thesis did a comparative study of the existing decoupling method, demonstratingthat traditional decoupling has high computational complexity, which increasesexponentially with plant dimension. The inverted decoupling, however, avoids thecomputational problem by replacing the matrix inverse in the decoupler with twofeedforward loops, bringing several practical advantages. Still, the design of inverteddecoupler requires accurate mathematical model, which is hard to obtain for thermalprocesses. The results of some pre-study shows that the dependency of inversedecoupling on mathematical model could be freed by active disturbance rejectioncontroller(ADRC) with its ability to estimate and actively compensate the unknowndisturbance, nonlinear dynamics as well as uncertainties in the controlled plant. Thus,this thesis focus on inverted decoupling control for multivariable thermal processesusing active disturbance rejection control technique(IDADRC).Firstly, this thesis formulates a general inverted decoupling structure for multivariableprocesses and introduces the basic principle of ADRC. Considering that there might beimproper, non-causal and unstable elements in the decoupler, this thesis proposes astandardized procedure to design compensator matrix. For those can’t be compensated,approximation methods are suggested for the realization issue.Secondly, simulations on thermal processes like ball mill coal-pulverizing system andultra supercritical coal-fired units are illustrated for demonstrating the validity of theproposed control schemes. Due to the fact that the diagonal elements of apparentprocess in inverted decoupling is the same as those in the original model, the switch ofcontrol or decoupling mode in one loop do not affect the other loops. Inverteddecoupling with only one feedforward path is proposed to improve the controlperformance of ultra supercritical boiler. The strong coupling from power to specific enthalpy is dynamically compensated by the decoupling network. And ADRCeffectively rejected the input disturbances at the same time.Finally, the level control experiment of quadruple-tank process based on the CoupledTanks33-040s platform developed by Feedback Instrument Ltd. is carried out. Withproposed IDADRC scheme, loop interactions can be decoupled by inverted decouplingstructure and real-time closed loop level control is achieved.The proposed decoupling control scheme has achieved desired performance formultivariable thermal plants both in simulation and experimentation, demonstrating thepromising future of application in engineering.