This paper addresses analysis and synthesis of robust stability and robust
performance repetitive control systems. The repetitive control design probl
em is formulated as a standard feedback form in the lin ear fractional tran
sformation form such that the standard numerical optimization software can
be used to obtain the solution. The main idea of the robust repetitive cont
rol system design lies in introducing a fictitious complex uncertainty to r
eplace the long delay chain in the internal model of the repetitive control
system. This drastically reduces the order of the augmented plant for cont
roller synthesis and hence generates a low order compensator, which in conj
unction with the pure delay renders a repetitive controller that can be imp
lemented efficiently in real time. The proposed approach can be applied to
both the continuous and discrete-time domain repetitive control design for
unstable open-loop plant. Sufficient conditions for the robust stability an
d robust performance repetitive control systems are presented. Conservatism
analysis shows that the sufficient conditions become necessary when the pu
re delay approaches infinity. The robust repetitive control is applied to a
n electrohydraulic actuator for tracking periodic trajectories. Experimenta
l results are presented to illustrate the design procedure and control syst
em performance.