On the performance assessment of scalar nonminimum-phase plants

The problem of minimum variance performance assessment of scalar nonminimum-phase plants requires almost full knowledge of the plant. Tyler and Morari (1995) claimed to have found a solution to this problem, which is still being cited in the literature (Huang and Shah 1999, Harris and Seppala 2001, Horch and Isaksson 2001, Eikens 2001, Ko and Edgar 2000), even though no one has ever reported using that result. This paper shows that that work is incomplete since the delay and the location of the "unstable" zeros are not enough to assess the minimum variance performance of a scalar nonminimum-phase plant.