Mechanical and Control System Analysis of a Tandem Cold Mill to Solve a Torsional Drive System Problem

Flat rolled steel strip is reduced to its final thickness in a tandem cold mill. This mill consists of a series of close coupled rolling stands. In order to maximize strip quality it is necessary to have precise and quick response mill drives. During a modernization study of an existing mill it was discovered that the mill had a low torsional resonance that would be excited by improved, fast drives. This paper summarizes the steps taken to analyze and understand this problem. These steps included measuring the torsional resonance frequencies on the mill, modeling the mechanical and electrical systems and tuning the models. The measured resonance frequencies were found to vary with operating variables. A finite element model of the mechanical system was developed and tuned. Studies with this model showed that changes in the mechanical system to increase the frequencies by increasing stiffness were not practical. A model of the control and drive system was developed and tuned. The model was used to determine the effectiveness of applying analog filters in the control system to reduce the excitation of the resonant frequency.