On Robust Output Regulation of Four Strings*

Based on our recent work [1], this paper focuses on the potential relationship between two ways to design observer based control, namely based on either three exosystem observers or two exosystem observers. Both can introduce a 3-copy internal model structure. We further explain the potential relationship between the above observer systems. The tracking errors can be converged to zero and the system states of the closed-loop system are bounded. There are two simulation examples to show the effectiveness of the observer based robust controls.