  This paper addresses a formation tracking problem for nonlinear multi-agent
systems with time-varying actuator faults, in which only a subset of agents has
access to the leader's information over the directed leader-follower network
with a spanning tree. Both the amplitudes and signs of control coefficients
induced by actuator faults are unknown and time-varying. The aforementioned
setting improves the practical relevance of the problem to be investigated, and
meanwhile, it poses technical challenges to distributed controller design and
asymptotic stability analysis. By introducing a distributed estimation and
control framework, a novel distributed control law based on a Nussbaum gain
technique is developed to achieve robust fault-tolerant formation tracking for
heterogeneous nonlinear multi-agent systems with time-varying actuator faults.
It can be proved that the asymptotic convergence is guaranteed. In addition,
the proposed approach is applied to task-space cooperative tracking of
networked manipulators irrespective of the uncertain kinematics, dynamics, and
actuator faults. Numerical simulation results are presented to verify the
effectiveness of the proposed designs.
