  This paper studies decentralized formation control of multiple vehicles when
each vehicle can only measure the local bearings of their neighbors by using
bearing-only sensors. Since the inter-vehicle distance cannot be measured, the
target formation involves no distance constraints. More specifically, the
target formation considered in this paper is an angle-constrained circular
formation, where each vehicle has exactly two neighbors and the angle at each
vehicle subtended by its two neighbors is pre-specified. To stabilize the
target formation, we propose a discontinuous control law that only requires the
sign information of the angle errors. Due to the discontinuity of the proposed
control law, the stability of the closed-loop system is analyzed by employing a
locally Lipschitz Lyapunov function and nonsmooth analysis tools. We prove that
the target formation is locally finite-time stable with collision avoidance
guaranteed. The evolution of the vehicle positions in the plane is also
characterized.
