An Insight into Stability Conditions of Discrete-Time Systems With Time-Varying Delay

This note studies an interesting phenomenon for stability conditions of discrete-time systems with time-varying delay. The underlying reason behind this phenomenon is revealed, and thereafter some conclusions are drawn: (i) Stability conditions of discrete-time systems with time-varying delay are generally divided into two types: those obtained by summation inequalities with free-matrix variables and those obtained by the combination of summation inequalities without free-matrix variables and the reciprocally convex lemma; (ii) The conservatism between the two types of stability conditions can not be theoretically compared. To clearly demonstrate this interesting phenomenon and meanwhile, to further verify these conclusions, several bounded real lemmas are obtained via different bounding-inequality methods and applied to a numerical example.