Stability robustness properties of sampled data repetitive control systems are examined. Due to the infinite loop gain at periodic frequencies originating from the included internal model (Internal Model Principle), repetitive systems are, if properly designed, not very sensitive towards possibly time-varying gains. Uncertainty in plant delay is, however, a problem. Controller action timing becomes more or less out of order and may result in severe performance degradation, depending on model type and the number of frequencies included in the design. With a Linear Time-Invariant (LTI) controller comprising the commonly used time delay internal model, the closed loop system is stable for nominal time delay plus/minus at most one sampling interval. A controller based on a reduced order model, perhaps not modelling all harmonics, is utilized lo enhance robustness properties. Simulation runs with different controllers show how different models work in the closed loop, and also that synchronization in time is of utmost importance in order to utilize the delay internal model.
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