Tandem cold rolling process is a nonlinear complex system with external and internal uncertainties and significant disturbances. The improvement in the quality of the final output depends on the control strategy of centerline thickness and interstand tension. This paper focuses on interstand tension control problem in 5-stand tandem cold rolling mills. Tension dynamics can be described by a nominal model perturbed by parametric uncertainties. In order to overcome the model uncertainties and external disturbances, suboptimal and controllers are proposed and the Hankel-norm approximation is used to reduce the order of controller. The performance of the proposed controllers is demonstrated by some simulations. 1. Introduction The production of cold rolled coils typically consists of separate processes such as annealing, pickling, cold rolling, and skin pass rolling. The cold rolling of strip is one of the most important processes in the areas of manufacturing and processing of metals. Tandem cold mill (TCM) is one type of cold rolling processes that strip thickness reduction is achieved with a number of nonreversing stands. The production of this process can be applied for various industries such as food packaging, automobile manufacturing, and home appliance [1, 2]. Thus, the improvement and development of new technologies of rolling have become important for researchers, engineers, and manufacturers. Tension control is one of the most important process parameters in tandem cold mill (TCM). Changing roll velocity stabilizes the interstand tension [3]. Interstand tension should be maintained at acceptable levels. Large transient variations in tension may result in a dramatic and physically damaging failure of the mill through looping, where the strip tension falls to zero, or tearing of the strip [4, 5]. Various advanced methods have been developed to analyze and control rolling mills [6–11]. Among the previous investigations, some notes focus on tension control problem in cold rolling mills [12, 13]. There are numerous uncertainties and disturbances in the model of tandem cold rolling process [14]. Thus, the control strategy for cold rolling should be robust with respect to the uncertainties and disturbances. Linear quadratic control strategy () guarantees the optimality but it has weakness in the presence of uncertainties. The suboptimal method and synthesis/analysis method have been adopted as the robust techniques in many approaches and provide systematic design procedures of robust controllers for linear system [3, 15, 16]. To promote tension control
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