Abstract
A Nyquist-like technique using boundary image maps for the assessment of stability of two-dimensional (2-D) digital control system models was first reported by De Carlo et al. in 1977. However, this method, although attractive in principle, is time-consuming and complicated. In a recent paper by Whalley and Zeng in 1992, a more efficient technique using minimum boundary image maps was reported. The technique, if correct, turns out to be even simpler than the tabular algorithms, which are probably the most convenient of all stability algorithms to program. However, with the use of two counter-examples it is shown that this method is not consistent with either De Carlo's theory or the modified Jury table (MJT) method. It is also shown that the new method does not guarantee stability even under conditions which are stronger than those proposed by Whalley and Zeng.
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