  This paper tackles the distributed leader-follower (L-F) control problem for
heterogeneous mobile robots in unknown environments requiring obstacle
avoidance, inter-robot collision avoidance, and reliable robot communications.
To prevent an inter-robot collision, we employ a virtual propulsive force
between robots. For obstacle avoidance, we present a novel distributed
Negative-Imaginary (NI) variant formation tracking control approach and a
dynamic network topology methodology which allows the formation to change its
shape and the robot to switch their roles. In the case of communication or
sensor loss, a UAV, controlled by a Strictly-Negative-Imaginary (SNI)
controller with good wind resistance characteristics, is utilized to track the
position of the UGV formation using its camera. Simulations and indoor
experiments have been conducted to validate the proposed methods.
