Investigation Of Variable Hydraulic Parameters In The Gauge Control System For A Four-high Strip Mill

The accuracy of strip gauge and the strip flatness are the main quality targets of a strip product. Currently, there have been huge advancements in the two targets above with the development of rolling theory, control theory, and artificial intelligence theory and their application in the rolling process. However, the application of the special control technology in singular-stand rolling mill and the realization of the control process with micron-level precision is still one of the hot spots in the control of strip thickness.The control of the strip rolling mill is very complex because there exist heavy loads and all kinds of disturbances and complicated relationships between them, and meanwhile high control accuracies and instant response are required. The whole rolling process is a nonlinear one with multiple variables, strong couplings and close relationships and interactions between those variables. Owing to the change in the operating range, the corresponding change in the stroke of the hydraulic cylinder, such parameters as the system stiffness and damping ratio vary a deal, and as a result of the entire system is unable to maintain the basically consistent response time and unable to achieve a higher control precision within the whole operating range as well.The paper discusses the ranges, influences and compensations of the variable parameters in the Automatic Gauge Control (AGC) in the first stand at WSPC's 1700-millimetre cold tandem mill. The works discussed in this paper are as follows.(1) The basic theory of the AGC system and the control strategy of the AGC system with variable stiffness are summarized, and the control principles in the traditional AGC used in the rolling mill mentioned above are analyzed.(2) Based on the physical model in actual system, the mathematic model of both the Hydraulic Automatic Position Control (HAPC) system and the traditional AGC system with feedforward and monitors are established, and the main parameters of the system are provided.(3) Simulations are implemented subsequently with the SIMULINK Blockset in MATLAB based on the mathematic model already established. The natural frequencies in electro-hydraulic servo valve, the displacement of the actuators and the damping ratio are variable. The feature of the system with the variable parameters and their influences on the accuracies and stabilities in AGC are discussed. The simulation results and the measured results are compared to verify the validity of the model.(4) In order to solve the problems of variable stiffness and relatively small damping ratio, a compensation combining phase lag with positive differential feedback of displacement was advanced with the target for a constant response time in the whole operating range. The simulations demonstrated that the system compensated has higher control precision.This paper discusses the influences of the variable parameters on the accuracy of the gauge control in accordance with the characteristics in the variations of the parameter in the APC system. Aimed at the realization of a constant response time in the whole operating range, a compensation combining phase lag with positive differential feedback of displacement are utilized, which offers a new solution to actual rolling process.