  A robust output feedback consensus problem for networked identical nonlinear
negative-imaginary (NI) systems is investigated in this paper. Output consensus
is achieved by applying identical linear output strictly negative-imaginary
(OSNI) controllers to all the nonlinear NI plants in positive feedback through
the network topology. First, we extend the definition of nonlinear NI systems
from single-input single-output (SISO) systems to multiple-input
multiple-output (MIMO) systems and also extend the definition of OSNI systems
to nonlinear scenarios. Asymptotic stability is proved for the closed-loop
interconnection of a nonlinear NI system and a nonlinear OSNI system under
reasonable assumptions. Then, an NI property and an OSNI-like property are
proved for networked identical nonlinear NI systems and networked identical
linear OSNI systems, respectively. Output feedback consensus is proved for a
network of identical nonlinear NI plants by investigating the stability of its
closed-loop interconnection with a network of linear OSNI controllers. This
closed-loop interconnection is proposed as a protocol to deal with the output
consensus problem for networked identical nonlinear NI systems and is robust
against uncertainty in the individual system's model.
