K. Havre, J. Morud and S. Skogestad, ``Selection of feedback variables for implementing optimizing control schemes,'' UKACC Conference Control'96, Exeter, UK, Sept. 1996, 491-496.

Abstract.
This paper considers selection of controlled variables when implementing optimizing control schemes. As a special case we treat indirect control. The selection criterion derived is to maximize the smallest singular value of the selected subsystem to be controlled using feedback control. A procedure for selecting outputs according to this criterion is outlined. The selection criterion is dependent on scaling, so we discuss appropriate scaling.