  This paper studies output feedback consensus protocol design problems for
linear multi-agent systems with directed graphs. We consider both leaderless
and leader-follower consensus with a leader whose control input is nonzero and
bounded. We propose a novel sequential observer design approach, which makes it
possible to design fully distributed adaptive output feedback protocols that
the existing methods fail to accomplish. With the sequential observer
architecture, we show that leaderless consensus can be achieved for any
strongly connected directed graph in a fully distributed manner, whenever the
agents are stabilizable and detectable. For the case with a leader of bounded
control input, we further present novel distributed adaptive output feedback
protocols, which include nonlinear functions to deal with the effect of the
leaders's nonzero control input and are able to achieve leader-follower
consensus for any directed graph containing a directed spanning tree with the
leader as the root.
