Nonminimum-phase difficulties in muitivariable-control-system design

Much attention has been paid to the design of the numerator functions in a multivariable control problem. A paper by Mayne presents an essentially practical approach to the problem of multivariable design, and removes some of the restrictions of Rosenbrock´s earlier inverse Nyquist method. The letter points out that there are still some major unsolved problems relating to the design of the numerator functions.