  Autonomous exploration of unknown environments has been widely applied in
inspection, surveillance, and search and rescue. In exploration task, the basic
requirement for robots is to detect the unknown space as fast as possible. In
this paper, we propose an autonomous collaborative system consists of an aerial
robot and a ground vehicle to explore in unknown environments. We combine the
frontier based method and the harmonic field to generate a path. Then, For the
ground robot, a minimum jerk piecewise Bezier curve which can guarantee safety
and dynamical feasibility is generated amid obstacles. For the aerial robot, a
motion primitive method is adopted for local path planning. We implement the
proposed framework on an autonomous collaborative aerial-ground system.
Extensive field experiments as well as simulations are presented to validate
the method and demonstrate its higher efficiency against each single vehicle.
