Decentralized control of web processing lines

The focus of the research is on the design of a decentralized controller for web processing lines that provide improved web tension regulation. First, an accurate dynamic model is developed for the unwind (rewind) roll in a web processing line by taking into account explicitly the time-varying nature of the roll inertia and its radius. The unwind (rewind) roll in a web processing line releases (accumulates) unfinished (finished) web to (from) the process section. Second, a strategy for computing the equilibrium control inputs and reference velocities for each driven roll/roller is given; this strategy is based on dividing the web processing line into tension zones and using the reference web tension of each zone and the reference velocity of the master speed roller which sets the desired web transport speed. Based on the new model developed, a decentralized controller is proposed. Variations in web tension and transport velocity in each tension zone are shown to asymptotically converge to zero. Extensive comparative experiments were conducted with the proposed decentralized controller and an often used decentralized PI controller in industry. A representative sample of the experimental results conducted on a large experimental web line, which mimics most of the features of an industrial process line, is shown and discussed