  We present three dynamic error feedback controllers for robust output
regulation of regular linear systems. These controllers are (i) a minimal order
robust controller for exponentially stable systems (ii) an observer-based
robust controller and (iii) a new internal model based robust controller
structure. In addition, we present two controllers that are by construction
robust with respect to predefined classes of perturbations. The results are
illustrated with an example where we study robust output tracking of a
sinusoidal reference signal for a two-dimensional heat equation with boundary
control and observation.
