paper - Motion planning for a six-legged lunar robot Kris...

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Unformatted text preview: Motion planning for a six-legged lunar robot Kris Hauser 1 , Timothy Bretl 1 , Jean-Claude Latombe 1 , and Brian Wilcox 2 1 Computer Science Department, Stanford University { khauser,tbretl } @stanford.edu latombe@cs.stanford.edu 2 Jet Propulsion Laboratory, California Institute of Technology Brian.H.Wilcox@jpl.nasa.gov Abstract: This paper studies the motion of a large and highly mobile six-legged lu- nar vehicle called athlete , developed by the Jet Propulsion Laboratory. This vehicle rolls on wheels when possible, but can use the wheels as feet to walk when neces- sary. While gaited walking may suffice for most situations, rough and steep terrain requires novel sequences of footsteps and postural adjustments that are specifically adapted to local geometric and physical properties. This paper presents a planner to compute these motions that combines graph searching techniques to generate a sequence of candidate footfalls with probabilistic sample-based planning to generate continuous motions to reach them. The viability of this approach is demonstrated in simulation on several example terrains, even one that requires rappelling. 1 Introduction In this paper we describe the design and implementation of a motion planner for a six-legged lunar vehicle called athlete (All-Terrain Hex-Limbed Extra- Terrestrial Explorer), shown in Fig 1. This large and highly mobile vehicle was developed by the Jet Propulsion Laboratory ( jpl ). 3 It can roll rapidly on rotating wheels over flat smooth terrain and walk carefully on fixed wheels over irregular and steep terrain. In particular, athlete is designed to scramble across terrain so rough that a fixed gait (for example, an alternating tripod gait) may prove insufficient. Such terrain is abundant on the Moon, most of which is rough, mountainous, and heavily cratered particularly in the polar regions, a likely target for future surface operations. These craters can be of enormous size, filled with scattered rocks and boulders of a few centimeters to several meters in diameter (Fig. 2). Crater walls are sloped at angles of between 10-45 , and sometimes have sharp rims [19]. On this type of terrain, athlete s walking motion is governed largely by two interdependent constraints: contact (keep wheels, or feet , at a carefully 3 The views presented in this paper do not reflect those of nasa or jpl . 2 Kris Hauser, Timothy Bretl, Jean-Claude Latombe, and Brian Wilcox wheel pitch ankle roll ankle pitch knee roll knee pitch hip yaw hip pitch chassis Fig. 1. The athlete lunar vehicle (developed by jpl ). Fig. 2. Pictures of lunar terrain from Apollo missions [19]. chosen set of footfalls) and equilibrium (apply forces at these footfalls that exactly compensate for gravity without causing slip). The range of forces that may be applied at the footfalls without causing slip depends on their geom- etry (for example, average slope) and their physical properties (for example, coefficient of friction), both of which vary across the terrain. So every timecoefficient of friction), both of which vary across the terrain....
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This note was uploaded on 08/03/2010 for the course MECHANIC 65921 taught by Professor Jons during the Spring '10 term at Tampa.

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paper - Motion planning for a six-legged lunar robot Kris...

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