Simple Improvement of Control of the Electromechanical Screwdown System

The fast electrohydraulic roll positioning system has become standard for new or revamped rolling mills. Nevertheless, many machines are still equipped with a classical electromechanical screwdown. Its potential can be increased by an adequate positional control in many cases. Significant backlash inherent in mechanical systems with a gear makes the system to be controlled highly nonlinear which, together with the characteristic time delays, makes the precise positional control rather difficult. The approach presented in the paper avoids modelling of the nonlinearity in the time domain. It relies on a simple approximation of the static characteristics of the system instead. The controller is than able to precalculate duration of a control pulse. Sequential alternation of the open-loop and feedback control schemes results in improved precision of positioning and decreased switching rate of the actuators.