Fixed-gain and variable-gain PID control of longitudinal tension in the winding section of a simple web transport system were evaluated. An open-loop mathematical model for the web transport system was derived and used for the design of the PID controllers. The winding roll radius is a time-varying parameter in the model.

The fixed-gain PID controller designed for a particular winding roll radius did not meet the desired specifications, whereas the variable-gain PID controller compensated for the time-varying parameter and produced accurate tension control. In comparison with other controllers, the variable-gain PID controller is easy to implement and shows promise for applications where the time-varying parameters are easily measured.