  This published paper investigates the distributed tracking control problem
for a class of Euler-Lagrange multi-agent systems when the agents can only
measure the positions. In this case, the lack of the separation principle and
the strong nonlinearity in unmeasurable states pose severe technical challenges
to global output-feedback control design. To overcome these difficulties, a
global nonsingular coordinate transformation matrix in the upper triangular
form is firstly proposed such that the nonlinear dynamic model can be partially
linearized with respect to the unmeasurable states. And, a new type of velocity
observers is designed to estimate the unmeasurable velocities for each system.
Then, based on the outputs of the velocity observers, we propose distributed
control laws that enable the coordinated tracking control system to achieve
uniform global exponential stability (UGES). Both theoretical analysis and
numerical simulations are presented to validate the effectiveness of the
proposed control scheme. Followed by the original paper, a typo and a mistake
is corrected.
