Numerical Study On The Cascade Control System Of Backwater Temperature PI~λD~μ And The Flow Of Water Supply PI~λ For Air Conditioning Chilled Water System With Variable Flow In Secondary Pump

With the increase of large buildings and intelligent buildings,the corresponding scales of air conditioning chilled water systems has increased accordingly.Air conditioning chilled water system with variable flow in secondary pump(VFSP-ACCWS)has been gradually gotten promotion and application.The backwater temperature control method for VFSP-ACCWS is widely used because of its features of ensuring the operation with small flow and large temperature difference,simplicity,easy operation and energy-saving,etc.However,because of the characteristics of large inertia,large time-delay and more interference for the controlled plant with backwater temperature in VFSP-ACCWS,the traditional PID single-loop control strategy for backwater temperature often leads to problems on larger steady-state errors and larger overshoots longer adjustment time,etc.and reduces the control quality of backwater temperature in VFSP-ACCWS,so it is difficult to get the desired control effect.In view of the above problems of controlling backwater temperature in VFSP-ACCWS,the purpose of this paper is to further improve the backwater temperature control method,in order to obtain better control effects and to meet the requirements of the technical process for VFSP-ACCWS.Firstly,combining the knowledge of fractional order calculus,fractional order PID control technology,cascade control theory and the requirements of the technical process for VFSP-ACCWS,a design scheme of backwater temperature PI~λD~μand the flow of water supply PI~λcascade control strategy is proposed.Secondly,aiming at the characteristic of time-delay for the controlled plant with backwater temperature in VFSP-ACCWS,Smith predictive compensator is added to this cascade control system to improve the stability of the system and speed up its response speed.Accordingly,for tuning parameters of a fractional order PID controller for backwater temperature(BT-FOPIDC)and a fractional order PI controller for the flow of water supply(FWS-FOPIC),an improved particle swarm optimization algorithm(IPSOA)is designed to tune parameters of these two controllers so that the optimal values of eight parameters are found.Finally,by MATLAB/Simulink tools,the simulation model of this cascade control system including BT-FOPIDC and FWS-FOPIC is configured and the corresponding simulation is carried out.The results show that this fractional order cascade control system and controller parameters tuned by IPSOA are theoretically feasible,and the control effect meets the relevant requirements of the technical process for VFSP-ACCWS.The corresponding research contents are as follows.1.An improved Oustaloup filter is used to accurately fit the PI~λD~μcontroller.By means of MATLAB/Simulink tools,the modules of PI~λD~μand PI~λcontrollers and MITAE are constructed and the corresponding functions are packaged.For the same fractional order controlled plant,it is verified that the PI~λD~μcontroller has better control performance than the PID one.2.On the basis of basic particle swarm optimization algorithm(BPSOA),by introducing a tangent trigonometric function to make its inertia weight(denoted asω)non-linearly,the model structure and calculation process of IPSOA are constructed.Through the verification of Sphere and Rastrigin function examples,the results show that IPSOA has significant improvements in diversity and convergence by comparison with BPSOA,and the ability to find optimization is improved.Furthermore,it is verified that this IPSOA is feasible to tune parameters of PID controller in terms of numerical simulation and experimental test on the PID control effect of the water tank level.3.Analyzing and comparing three frequently used control methods for the related performance parameters of VFSP-ACCWS,the ranking of energy-saving effects of these three control methods is backwater temperature control method,terminal constant pressure difference control one and main pipe constant pressure difference control one.However,the traditional PID control method for backwater temperature has problem such as control hysteresis and low control accuracy,etc.Therefore,this paper proposes a design scheme of backwater temperature PI~λD~μand flow of water supply PI~λcascade control strategy.With the help of MATLAB/Simulink tool,the simulation model of this cascade control system is configured and the corresponding simulation is simulated.The results indicate that this control system has a smaller overshoot and a short adjustment time on controlling backwater temperature,and can effectively eliminate steady-state error and has stronger anti-jam ability.Moreover,the stability and response speed of this control system are further improved by adding Smith predictive compensator.4.For the same controlled plant with backwater temperature,the backwater temperature PID single-loop control strategy and the backwater temperature PID and the flow of water supply PI cascade control strategy are simulated,respectively.By comparison of the simulation results,the backwater temperature PI~λD~μand the flow of water supply PI~λcascade control strategy is better than the above two control ones.