Slope traversal controls for planetary exploration rover on sandy terrain

In this paper, two control approaches are presented for exploration rovers traversing sandy-sloped terrain. One of the proposed controls is a model-based feed-forward control using a characteristic diagram, called a thrust-cornering characteristic diagram. It consists of various characteristic curves of wheel forces for varied wheel slip conditions. An appropriate steering maneuver for slope traversal can be found using the diagram with slope traversal criteria. The other control is a sensor-based feedback control. A key approach to this feedback control is to compensate for three types of slip, namely, the vehicle sideslip and longitudinal/lateral slips of a wheel. The feedback control calculates both steering and driving maneuvers that can compensate for these slips and also allow the rover to successfully traverse a sandy slope. The performances of these two control approaches are confirmed in slope traversal experiments using a four-wheeled rover test bed. The proposed controls are verified by quantitative evaluations of distance and orientation errors. Through the experiment, it was found that the two controls have advantages and disadvantages and the possibility of merging the model-based control and the sensor-based control is discussed.