  The notion of safety in multi-agent systems assumes great significance in
many emerging collaborative multi-robot applications. In this paper, we present
a multi-UAV collaborative target-tracking application by defining bounded
inter-UAV distances in the formation in order to ensure safe operation. In
doing so, we address the problem of prioritizing specific objectives over
others in a multi-objective control framework. We propose a barrier Lyapunov
function-based distributed control law to enforce the bounds on the distances
and assess its Lyapunov stability using a kinematic model. The theoretical
analysis is supported by numerical results, which account for measurement noise
and moving targets. Straight-line and circular motion of the target are
considered, and results for quadratic Lyapunov function-based control, often
used in multi-agent multi-objective problems, are also presented. A comparison
of the two control approaches elucidates the advantages of our proposed
safe-control in bounding the inter-agent distances in a formation. A concluding
evaluation using ROS simulations illustrates the practical applicability of the
proposed control to a pair of multi-rotors visually estimating and maintaining
their mutual separation within specified bounds, as they track a moving target.
