On-Line Terrain Parameter Estimation for Planetary Rovers

Future planetary exploration missions will require rovers to traverse very rough terrain with limited human supervision. Wheel-terrain interaction plays a critical role in rough-terrain mobility. In this paper an on-line estimation method that identifies key terrain parameters using on-board rover sensors is presented. These parameters can be used for accurate traversability prediction or in a traction control algorithm. These parameters are also valuable indicators of planetary surface soil composition. Simulation and experimental results show that the terrain estimation algorithm can accurately and efficiently identify key terrain parameters for loose sand. 1.0 Introduction and Related Work Future planetary exploration missions will require rovers to perform challenging mobility tasks in rough terrain [S, 14, 161. Proposed future mission objectives include traversal of several kilometers with a high degree of autonomy. To accomplish these objectives, future control and planning methods must consider the physical characteristics of the rover and its environment, to fully utilize the rover’s capabilities. Wheel-terrain interaction has been shown to play a critical role in rough-terrain mobility [l, 21. For example, a vehicle traversing loose sand has very different mobility characteristics than one traversing firm clay. For planetary rovers, it is desirable to estimate terrain parameters on-line, since on-line estimation would allow a rover to accurately predict the traversability of neighboring terrain regions. Terrain parameter estimation would also allow a rover to adapt its control and planning strategy to a given terrain [7]. Off-line (i.e. Earth-based) estimation could be performed, but the associated communication time delays are extensive. Finally, on-line terrain estimation is desirable for enhancing our understanding of planetary surface composition. * Department of Mechanical Engineering, BirZeit University Palestinian Territories Many researchers have studied methods for identifylng key wheel-terrain interaction model parameters [12, 151. Generally, these methods involve off-line estimation using costly, dedicated testing equipment. Terrain parameter estimation for a legged system has been documented in [3]. This approach uses an embedded three-axis force sensor, which most planetary rovers are not equipped with. Terrain parameter estimation for tracked vehicles has been proposed in [3]. This approach assumes a highly simplified “force coefficient” model of track-terrain interaction, which is not valid in deformable rough terrain. Parameter estimation of Martian soil has been performed by the Viking landers and the Sojourner rover [lo, 111. The Viking landers used manipulator arms to conduct trenching experiments. The Sojourner rover used the rover wheel as a bevameter-type device to identify soil cohesion and internal friction angle. Both missions used visual cues and off-line analysis techniques to compute soil parameters. In this paper, a method for on-line estimation of terrain cohesion and internal friction angle using on-board rover sensors is presented. The algorithm estimates parameters of the terrain the rover is currently traversing. The algorithm relies on a simplified form of classical terramechanics equations, and uses a linear-least squares estimator to compute terrain parameters in real time. The method is computationally efficient, and is thus suitable for implementation on a rover with limited on-board computational resources. Simulation and experimental results show that the terrain estimation algorithm can accurately and efficiently identify key terrain parameters for loose sand. 2.0 Terrain Parameter Identification The purpose of terrain parameter identification is to estimate key terrain parameters on-line, using on-board rover sensors. Two key terrain parameters are the cohesion, cy and internal friction angle, @. These parameters can be used to compute the terrain shear 0-7803-7272-7/02/$17.00