Online motion planning for tethered robots in extreme terrain

Several potentially important science targets have been observed in extreme terrains (steep or vertical slopes, possibly covered in loose soil or granular media) on other planets. Robots which can access these extreme terrains will likely use tethers to provide climbing and stabilizing force. To prevent tether entanglement during descent and subsequent ascent through such terrain, a motion planning procedure is needed. Abad-Manterola, Nesnas, and Burdick [1] previously presented such a motion planner for the case in which the geometry of the terrain is known a priori with high precision. Their algorithm finds ascent/descent paths of fixed homotopy, which minimizes the likelihood of tether entanglement. This paper presents an extension of the algorithm to the case where the terrain is poorly known prior to the start of the descent. In particular, we develop new results for how the discovery of previously unknown obstacles modifies the homotopy classes underlying the motion planning problem. We also present a planning algorithm which takes the modified homotopy into account. An example illustrates the methodology.