ROBUSTNESS PROPERTIES OF REPETITIVE CONTROLLERS

Stability robustness properties of sampled data repetitive control sys tems are examined. Due to the infinite loop gain at periodic frequenci es originating from the included internal model (Internal Model Princi ple), repetitive systems are, if properly designed, not very sensitive towards possibly time-varying gains. Uncertainty in plant delay is, h owever, a problem. Controller action timing becomes more or less out o f 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 nomin al time delay plus/minus at most one sampling interval. A controller b ased on a reduced order model, perhaps not modelling all harmonics, is utilized to enhance robustness properties. Simulation runs with diffe rent controllers show how different models work in the closed loop, an d also that synchronization in time is of utmost importance in order t o utilize the delay internal model.