A result on output regulation of lower triangular systems with unknown high-frequency gain sign

This paper concerns global robust output regulation of a class of nonlinear lower triangular systems with an unknown high-frequency gain as well as an unknown exosystem. A novel class of internal model candidates is integrated with the output regulation framework. As a result, stabilization of the augmented system can be performed without parameter estimators. However, the new internal models bring challenges to the stabilization of the augmented system. To overcome these challenges, we propose a new recursive controller design procedure and use it to develop a Nussbaum-gain-based controller. This work extends the existing results on nonlinear output regulation of lower triangular systems to the case where both the high-frequency gain sign and the exosystem are unknown.