Phase-conditionally stable systems

In this paper, the phase-conditionally stable feature of a closed-loop system is introduced. In any control loop, time delays appear and can produce unexpected changes in the controlled-system performances. Usually, these effects are negative and performance is degraded. The possible positive effect of the delays in avoiding oscillatory plant response is here considered. For that purpose, the effect of additional measurement delays, filters or dead time in resonant systems, is analyzed. Some examples illustrate the results. Other than the possible practical relevance of the results, the concepts involved are very important for the control engineers at educational level.