Fragility and robustness: a case study on PM headbox control

It has been claimed in Keel and Bhattacharyya (1997) that standard optimal and robust control synthesis techniques tend to produce fragile controllers (that is controllers which result in high sensitivity of closed-loop stability and/or performance against small changes in controller coefficients). In this paper we study controller fragility in more detail. It is explained why fragility can be a problem if the optimization criterion and controller realization are not chosen with care. Furthermore, it is emphasized that several rather standard rules of thumb in robust control design may fail to indicate the presence of fragility and other robustness problems. Finally, a case study on a paper machine (PM) headbox control problem is presented. This case study shows that even with a good robust controller design method such as the McFarlane-Glover loopshaping method, it is advisable to avoid certain fragile controller realizations such as the companion form.