Control of coating mass on a continuous hot dip galvanizing line

On a Continuous Hot-Dip-Galvanizing Line,steel strip is coated with a thin film of zinc to inhibit rusting. The zinc coating has to meet quality standards relating to thickness (coating mass) and uniformity (mass distribution). This thesis details the implementation of a computer based control system, to control the coating mass and distribution of zinc on steel strip, on the No. 3 Continuous Galvanizing Line at John Lysaght (Australia) Limited's Port Kembla plant. Computer control is used to overcome transport lag problems which occur because it is necessary to place the coating mass gauge some sixty metres downstream.from the coating mass control equipment. A static mathematical model of the process, which has been developed at Lysaghts, is adapted continuously on-line. Model prediction is used for feedforward control, which corrects for process and reference changes, while steady state accuracy of coating mass is maintained by feedback control. The latter is a pulse-andwait system,based on a linear incremental model, which is adapted to maintain optimum response. A ten percent saving in zinc usage resulted from the installation of the coating mass gauge and the mechanisation of the mass control equipment. Computer control added a further three percent in zinc savings and a considerable improvement in product quality (uniformity) This thirteen percent reduction in zinc usage represents a monetary saving in excess of $0.5 million per annum, giving a pay back period of less than twelve months.