Numerical modelling of a 6-high mill rolling stand and development of a closed-loop control system to attenuate chatter vibrations

When aiming to maximize productivity in rolling plants, the bottleneck is usually represented by a self-excited vibration of the machinery, known in the literature as chatter, which occurs at high strip velocities; The objective of this thesis is the development of a closed-loop control system which is going to damp and bound the chatter vibrations, with the final goal to increase the productivity of a tandem of five 6-high cold rolling mills property of the Danieli corporation, which also issued this project. The idea is that of using an injection system coming from vehicle application to control the pressure inside the hydraulic pistons which controls the gauge dimension, introducing a damping effect. The first part of this thesis is devoted to the realization of the numerical models that will be used to design the control loop, consisting of: a lumped parameters model of the stand and its hydraulic lines, a code to simulate the rolling process forces and a model for the injection system; At the time of the writing of this thesis the 6-high mill stand was still in production, thus the realization of the model was based entirely on a 3D CAD drawing of the latter. In the second part of the thesis the assembled models are used to design the control system with the aid of the software MATLAB Simulink; The design consisted in the tuning of the PID controller involved, and most importantly in the choice of the number of injectors to be used, or equivalently the maximum flow rate of fluid that can be injected. In conclusion, the controller was verified to be nominally capable to damp the instability in a range of oscillation proportional to the “injection power”.