Tension control of webs - A review of the problems and solutions in the present and future

Abstract

Production plants with continuous moving webs have a complex structure where mechanical and electrical problems are involved. To solve the problems of this systems, it is necessary to take into account the total system to find an optimum. In other words, we have to achieve an Integrated Design.

A global presentation of specific demands and problems referring to tension control is given. After an excursion to the modelling of such systems, we can study the steady-state and dynamic behaviour.

In industrial plants, usually the tension is controlled either in an open loop system with a speed control of the driven rollers or the tension is controlled in a closed loop cascade control with PI or PID controllers. The optimization of the control is often done without the influence of the coupling of the rollers with the web. Under special conditions, if the parameters of the system fulfill particular conditions, such a control leads to an acceptable tension control.

Nowadays the demands on a tension control increase because of higher speed in the plant. In many cases the limits of the cascade control are reached and new solutions are demanded. The goal in the future is to realize non-interacting, decentralized control loops. In the control science, the state space control is an effective tool to solve complex problems and to improve the dynamic behaviour of the control. As a state space control of the total system is complex and often unpractical in industrial plants, decentralized control methods are discussed. As the measurement of the tension causes sometimes more problems than solutions, observers which estimate the web forces are used.

Nevertheless, some problems of tension control cannot be solved by using this methods. In the reality, we do not have linear systems. Therefore some new methods as Fuzzy Control and Neural Networks are discussed which may be able to solve such non-linear problems.