  Robotic exploration or monitoring missions require mobile robots to
autonomously and safely navigate between multiple target locations in
potentially challenging environments. Currently, this type of multi-goal
mission often relies on humans designing a set of actions for the robot to
follow in the form of a path or waypoints. In this work, we consider the
multi-goal problem of visiting a set of pre-defined targets, each of which
could be visited from multiple potential locations. To increase autonomy in
these missions, we propose a safe multi-goal (SMUG) planner that generates an
optimal motion path to visit those targets. To increase safety and efficiency,
we propose a hierarchical state validity checking scheme, which leverages
robot-specific traversability learned in simulation. We use LazyPRM* with an
informed sampler to accelerate collision-free path generation. Our iterative
dynamic programming algorithm enables the planner to generate a path visiting
more than ten targets within seconds. Moreover, the proposed hierarchical state
validity checking scheme reduces the planning time by 30% compared to pure
volumetric collision checking and increases safety by avoiding high-risk
regions. We deploy the SMUG planner on the quadruped robot ANYmal and show its
capability to guide the robot in multi-goal missions fully autonomously on
rough terrain.
