  This paper studies distributed formation control of multiple agents in the
plane using bearing-only measurements. It is assumed that each agent only
measures the local bearings of their neighbor agents. The target formation
considered in this paper is a circular formation, where each agent has exactly
two neighbors. In the target formation, the angle subtended at each agent by
their two neighbors is specified. We propose a distributed control law that
stabilizes angle-constrained target formations merely using local bearing
measurements. The stability of the target formation is analyzed based on
Lyapunov approaches. We present a unified proof to show that our control law
not only can ensure local exponential stability but also can give local
finite-time stability. The exponential or finite-time stability can be easily
switched by tuning a parameter in the control law.
