Research On Instability Mechanism And Nonlinear Dynamics Behavior Of Hydraulic AGC System

Manufacturing equipment is gradually developed to the direction of intelligentization and greenization.The metallurgical industry as an important pillar for the national economic development,in addition to the higher requirements to the intelligent level of rolling equipment,the requirements to the quality of cold-rolled strip are also higher and higher.However,mass production practices prove that the frequent vibration phenomenon often occurs in the rolling process,which hinders the development process of production to high precision and intelligentization.Hydraulic automatic gauge control(AGC)system is the core control system to ensure the accuracy of the strip thickness,and the reliability of its work is the key to ensure the rolling with high-precision,high-speed,continuity and stability.Therefore,in order to improve the stability of rolling process,reduce the incidence of equipment fault and improve the product quality,hydraulic AGC system was employed as the research object.To lay a theoretical foundation for its vibration source and inhibition,the basic theory of system instability mechanism and nonlinear dynamics behavior was investigated,moreover,the causes of nonlinear vibration and the effect rule of structural parameters and nonlinear factors on the dynamic behavior were revealed.Firstly,in this paper,the composition and function of hydraulic AGC system were introduced in detail,and the mathematic model of each component was established.Based on the mathematic model and the information transfer relationship of each component,the comprehensive block diagram of the system was established,which lays the foundation for the derivation of absolute stability condition of the fundamental closed-loop.In addition,according to the derived transfer relation,we respectively established the transfer block diagram of disturbance quantity of the pivotal position closed-loop system and pressure closed-loop system.Moreover,the absolute stability conditions of aforementioned closed-loop systems were deduced by the Popov frequency criterion method,which provided theoretical basis for the subsequent instability mechanism research.Secondly,a key parameter identification method based on free vibration response signal was studied for the identification of pivotal unknown parameters in hydraulic AGC system.The vertical vibration dynamic model of the hydraulic AGC system was established and the model was solved by the complexification averaging(CA)method.Then,the shortcoming of CA method was improved by fast empirical mode decomposition method.Through the modification,the solution accuracy was improved.So,the improved CA(ICA)method can be used to identify the key structural parameters of the hydraulic AGC system and validated by experiment.Furthermore,a novel parameter modification method based on chaos wolf optimization algorithm with self-adaptive variable step-size(CWOA)was proposed by imitating wolves swarm intelligence,and the parameter identification result based on ICA method was further revised by CWOA.Thirdly,according to the derived absolute stability conditions and identified key parameters and known parameters,the instability mechanism of the position closed loop and pressure closed loop of the hydraulic AGC system was explored.The influence mechanism of many different factors on the system stability were mainly discussed,including the control cavity volume,stiffness coefficient,damping coefficient,gain coefficient and so on.The vertical vibration model of mill roll system under the nonlinear effect of the hydraulic screwdown system was established,the amplitude-frequency characteristic equation of the system was obtained by solving the model with the multi-scale method,and the bifurcation characteristic of the system was further analyzed by singularity theory.Besides,the effect coefficient of nonlinear stiffness,the rodless cavity initial displacement,the effect coefficient of nonlinear damping and the external excitation force were used as the objects to investigate the influence rules of these parameters on the amplitude-frequency response of the vertical vibration system.Furthermore,the bifurcation behavior and typical nonlinear dynamics behavior of the system under different parameters were studied to reveal the mechanism and cause of the nonlinear dynamics behavior of the system.At last,the vibration test system of hydraulic AGC system was set up and the nonlinear vibration experiments were performed.The vibration data acquisition system was independently developed by the virtual instrument development platform LabVIEW.Besides,the data preprocessing methods were also explored in this paper.In order to solve the problem that the measured signal contains noise and invalid component,a novel method was proposed to extract effective components by extreme-point symmetric mode decomposition combined with K-L divergence,which can be used to remove the influences from noise,trend items and other interference factors.Aiming at the disadvantages of low-frequency sensitivity and cumulative error amplification in the traditional frequency-domain integration method,a frequency-domain integration method based on low-frequency filter correction by vibration intensity was proposed,which can be used for the accurate conversion among the vibration acceleration signal and the vibration velocity signal and the vibration displacement signal.Finally,the validity of the theoretical research was verified by the analysis of measured data.This research can not only lay the theoretical foundation for the nonlinear dynamics performance evaluation and optimization design of hydraulic AGC system,but also provide a theoretical reference for improving system stability and preventing system from generating nonlinear vibration.Research results will contribute to ensure the safe and stable operation of steel rolling production,improve the finished product rate and product quality.Furthermore,it has great theoretical guidance and engineering practical significance to promote scientific and technological progress of national cold-rolled steel industry and accelerate the transformation from big to strong.