A nonminimum phase index and its application to interacting multivariable control systems

A nonminimum phase index is defined for a linear time invariant multivariable system. It is then used to give a measure of the degree of difficulty of stabilizing the system when two or more control systems, each of which controls one output variable of the system, are applied simultaneously to the system. The index is simple to compute and so should be useful in predicting when interaction will occur in large multivariable control systems such as occur in process control. A numerical example of a distillation column with pressure control and temperature control is included.