Abstract-This paper considers the motion planning problem that arises when a tethered robot descends and ascends steep obstacle-strewn terrain. This work is motivated by the Axel tethered robotic rover designed to provide access to extreme extra-planetary terrains. Motion planning for this type of rover is very different from traditional planning problems because the tether geometry under high loading must be considered during the planning process. Furthermore, only round-trip paths that avoid tether entanglement are viable solutions to the problem. We present an algorithm for tethered robot motion planning on steep terrain that reduces the likelihood that the tether will become entangled during descent and ascent of steep slopes. The algorithm builds upon the notion of the shortest homotopic tether path and its associated sleeve. We provide a simple example for purposes of illustration. OME of the richest potential science targets for future Mars exploration missions lie in terrains that are inaccessible to state-of-the-art Martian rovers, limiting our ability to carry out in situ analysis of these rich opportunities. Example targets include geologic flows on crater walls, ice deposits in the depths of dark craters, bedrock layers in vertical promontories, and extreme terrain underlying the source of methane plumes (see [1, 2] for details). Titan, Europa, Enceladus, and the Earth's moon also offer challenging terrains with associated scientific targets. A new generation of exploration robots is needed to access these extreme terrains in order to probe, sample, and measure. New inquiries of this sort could yield significant scientific rewards. In order to provide access and in situ sampling in extreme terrains, the Jet Propulsion Laboratory (JPL) and the California Institute of Technology have collaborated to develop the Axel rover [1,2]. The minimalistic Axel robot consists of two wheels connected by a central cylindrical body, a caster arm, and an actively controlled tether passing through the caster arm (Fig. 1) . The caster arm, in addition to controlling the tether, provides a reaction force against the terrain needed to generate forward motion when travelling on flat ground. Axel is a rappelling robot which can operate in extreme terrain via the use of a host anchor platform. Since Axel's body acts as a winch (the tether is wound and unwound around the central body as it rotates relative to the wheels), the host platform requirements are reduced to a simple mount. Fig. 2 portrays a hypothetical scenario in which Axel is deployed from the Mars Science Laboratory [3]. Axel has demonstrated high mobility in a simulated Martian environment using both its actively managed tether and its novel grouser (paddle) wheels. The rover has the capability to ascend/descend vertical cliff walls and climb over obstacles whose heights are ~70% of its wheel diameter [1, 2] . At times, however, Axel relies on large tether tensions in order to climb and maneuver [1] . An open question is how to autonomously plan Axel's ascending and descending motions
Motion planning for a tethered axel rover on steep terrain was studied in REF .
Motion planning on steep terrain for the tethered axel rover
2011 IEEE International Conference on Robotics and Automation
2011 IEEE International Conference on Robotics and Automation
Engineering Computer Science