  This paper addresses the problem of end-effector formation control for
manipulators that are subjected to external disturbances: input disturbance
torques and disturbance forces at each end-effector. The disturbances are
assumed to be non-vanishing and are superposition of finite number of
sinusoidal and step signals. The formation control objective is achieved by
assigning virtual springs between end-effectors, by adding damping terms at
joints, and by incorporating internal model-based dynamic compensators to
counteract the effect of the disturbances; all of which presents a clear
physical interpretation of the proposed approach. Simulation results are
presented to illustrate the effectiveness of the proposed approach.
