Research And Design Of Coagulant Dosing Control System Based On Improved Fuzzy PID Predictive Algorithm

With the continuous acceleration of industrialization and urbanization in China,the demand and quality requirements for tap water are increasing.Therefore,reducing the production cost of water treatment plants while ensuring good effluent quality has become a goal pursued by the water treatment industry.In the water treatment process,coagulation and sedimentation are crucial steps,which have characteristics such as long-time delay,multiple interferences,non-linearity,and time varying.Traditional coagulation dosing methods have problems such as inaccurate dosing,difficult operation,unstable water quality,and high investment costs.Therefore,this paper proposes an improved fuzzy PID prediction compound control scheme based on the actual situation of a water treatment plant,studies and designs an automatic dosing system,aiming to achieve automatic operation and precise control of coagulation dosing.Firstly,based on a full understanding of the process flow of water treatment in the water plant and effluent quality standards,this paper analyzes the coagulation dosing process in detail,and elaborates the control requirements and difficulties of coagulation dosing.System process models and raw water turbidity disturbance models were established according to the water plant’s experimental data,and the model was verified through step response method and least squares method.Finally,the transfer function model of the coagulation dosing system was established.Then,based on the establishment of the transfer function control model,the control algorithm was designed.Firstly,conventional PID control algorithm was used for closed-loop control of the system.Through simulation,it was found that when the parameters of the controlled object were perturbed,the system exhibited severe oscillations.Considering that conventional PID control parameters cannot adaptively adjust,the fuzzy control algorithm was introduced to adjust the PID parameters online.The results show that the fuzzy PID is better than the traditional PID control,but due to the influence of large time delay,the system still has large overshoot and fluctuations.Finally,aiming at the large time delay part of the system,the Smith predictor was introduced based on the fuzzy PID to eliminate the influence of lagging part.However,Smith has a high requirement for the accuracy of the system model,so the Smith predictor was improved: a first-order filtering part was added in the prediction error feedback loop,and the fuzzy control algorithm was used to adjust the filtering time constant online to reduce the impact of the prediction error fluctuation caused by the perturbation of the model parameters.The simulation results show that the improved fuzzy PID prediction control algorithm has better nominal and robust performance than the fuzzy PID control algorithm and the fuzzy PID prediction control algorithm.Considering that the sudden change of raw water turbidity causes a significant disturbance to the system,a static feedforward controller was designed for the raw water turbidity disturbance based on feedback control,forming a feedforward-feedback compound dosing control system.The simulation results show that when the raw water turbidity is disturbed,the feedforward controller can effectively suppress the external disturbance.Finally,the structure and composition of the coagulation dosing system are described,and the improved fuzzy PID prediction compound control algorithm designed above is transplanted into the PLC.An upper computer monitoring interface is designed,and the control effect of the algorithm is analyzed through the historical curve analysis algorithm of the upper computer.The algorithm is written in structured text(SCL)and encapsulated into an FB function block for repeated calling by the system.It is also easy to transplant and use in other brands of PLC,DCS,and other systems.Therefore,the designed algorithm has certain reference and application value.