Robust stability evaluation of sampled-data control systems with time-invariant nonlinearity in a gain-phase plane

Describes a graphical evaluation of the robust stability in the frequency domain based on the results from our previous paper (1996) in which the extension of Popov´s criterion to discrete-time systems was expressed in an explicit form. The control system described herein is a sampled-data control system with one time-invariant nonlinear element (sector nonlinearity) in the forward path. Considering the application to computer-aided control system design (CACSD), we present an evaluation method of the robust stability in connection with the size of sector nonlinearity and the gain margin on a gain-phase diagram (i.e. a modified Nichols chart). We show two results as numerical examples: one is where Aizerman´s conjecture was approved, and the other is where it was not