On the robust tuning of controllers with recycle compensators

The control of processes with recycle normally considers a recycle compensator and a main controller. This work analyses the use of a H∞ approach to design the main controller and compares its performance with IMC and PI controllers. Stability conditions are derived for assessing the robustness of the main controller in presence of uncertainty in the forward and recycle model. These robust stability conditions along with practical issues, like time delay in the recycle model and integral action, are all considered in the controller design. Several simulations examples compare the behavior of the different controllers and highlight the positive features of the H∞ design approach.