Literal Nyquist stability criterion for MIMO control systems

Nyquist stability criteria for multiple-input multiple output (MIMO) feedback systems are derived using a literal approach. In contrast to existing generalized Nyquist stability criteria, which involve the transfer function of the MIMO plant, the proposed literal approach allows the results to be expressed in terms of the poles and zeros of each of the elemental plants. Thus, explicit necessary and sufficient stability conditions for two-input and two-output and for three-input and three-output control systems are given. The results are illustrated in a flight control problem.