Nonlinear Hydraulic Gap Control: A Practical Approach

Abstract

This paper presents measurements and evaluates the results of a new nonlinear hydraulic gap controller (HGC) implemented in a reversing hot strip mill. Modern rolling mills contain several feedback loops and the roll gap is usually adjusted by a hydraulic system. Since the HGC is part of the most inner control loop in this cascaded structure, the use of a controller with high dynamics for the hydraulic positioning system is essential due to performance considerations. In order to fulfill the constantly rising demands on the quality as well as on the minimal strip thickness tolerances of the rolled products, it is unavoidable to improve existing control algorithms. Up today, linear design techniques are used for the HGC in most cases, although the nonlinear nature of the hydraulic system can be taken into account by means of modern nonlinear control concepts. Furthermore, the controller must be robust against friction forces, leakage flows, transducer and quantization noise in all possible operating conditions. This paper compares measured step responses of a recently proposed nonlinear controller with conventional linear ones.