Variable Hydraulic Stiffness In The Gauge Control For A Tandem Cold Mill

The control of a strip rolling mill is very complex, for it has heavy load and many disturbances. The connection of these disturbances is multiply, but high control accuracy and fast response is required. So the process of rolling is a nonlinear action, which has complex mechanism and multi-variables coupling strongly with each other. Those variables affect each other strongly. The rollers are wearing every time when rolling. As long as the value of waste becomes large enough to make the stroke of hydraulic cylinder long, the stiffness of hydraulic cylinder diminishes to a low grade. That would make the whole stiffness of rolling mill drop sharply, so as to slow down the response rapid and make worse the accuracy of automatic gauge control.This paper discusses the reasons, influence and compensation of the variability of the stiffness of hydraulic system of the first stand in WSPC's 1700mm cold tandem mill. It can be reduced as follows:1) According to the physical model of the real system, the mathematic model of both Hydraulic Automatic Position Control (APC) system and Automatic Gauge Control (AGC) system with variable stiffness are established. Then the mathematic model of APC is simulated with the SIMULINK Blockset in MATLAB, and the variables of it are modified. The simulated result of mathematic model of AGC is compared with the measured result to adjust the variables.2) The integrate stiffness characteristics of the whole rolling mill and the hydraulic system are discussed on the basis of the simulate model established. The variability of stiffness of the hydraulic system is discussed. It can be concluded that the variables which make the variability of hydraulic system are depend on both the stroke of hydraulic cylinder and the length of the hose between electro-hydraulic servo valve and hydraulic cylinder.3) How the variability of hydraulic system affects the accuracy of gauge control system is discussed in detail.4) The compensation treasures are provided separately according to the characteristic of each affecting variables that make the variability of the hydraulic system. A technique of PID control is taken to compensate the length of a hose and a technique of separate variable is employed for the stroke of the hydraulic cylinder. The simulate results show that the response of the hydraulic system becomes better after those techniques introduced.As a whole, how the variability of hydraulic system's stiffness affects the accuracy of gauge controlling system is discussed in this paper and it is try to elevate the accuracy of the gauge control in a strip rolling mill. Secondly, the rule to whom does the stiffness of hydraulic system obey is discussed. The characteristic of the stiffness of both hydraulic system and the whole rolling mill are found out, so as to offer a simple but effective way to real manufacture.