A Reachability-Based Planner for Sequences of Acyclic Contacts in Cluttered Environments

Multiped locomotion in cluttered environments is addressed as the problem of planning acyclic sequences of contacts, that characterize the motion. In order to overcome the inherent combinatorial difficulty of this problem, we separate it in two subproblems: first, planning a guide trajectory for the root of the robot and then, generating relevant contacts along this trajectory. This paper proposes theoretical contributions to these two subproblems. We propose a theoretical characterization of the guide trajectory, named “true feasibility”, which guarantees that a guide can be mapped into the contact manifold of the robot. As opposed to previous approaches, this property makes it possible to assert the relevance of a guide trajectory without explicitly computing contact configurations. Indeed, this property is efficiently checked using a low dimensional sampling-based planner (e.g. we implemented a visibility PRM). Since the guide trajectories that we characterize are easily mapped into a valid sequence of contacts, we then focus on how to select a particular sequence with desirable properties, such as robustness, efficiency and naturalness, only considered in cyclic locomotion so far. Based on these novel theoretical developments, we implement a complete acyclic contact planner and demonstrate its efficiency by producing a large variety of motions with three very different robots (humanoid, insectoid, dexterous hand) in five challenging scenarios. The quality of the obtained motions and the performance of the algorithm make it the first acyclic contact planner suitable for interactive applications.