Pose Estimation and Traction Control for a Planetary Exploration Rover

Kurzfassung

Planetary rovers are one of the key elements for extraterrestrial on-surface exploration in our solar system, increasingly relying on autonomous robots. The Lightweight Rover Unit (LRU) developed by the Robotics and Mechatronics Center (RMC) of the German Aerospace Center (DLR) is a terrestrial prototype of an autonomous planetary exploration rover. It features autonomy both in exploration and manipulation of the environment and is designed specifically for rough terrain locomotion. Successful autonomous navigation requires correct knowledge of the current rover pose for which the LRU fuses the information of several sets of sensors using an extended Kalman filter. This includes the information obtained from the wheels: the wheel odometry. This master’s thesis focuses on implementing a slip-aware three-dimensional wheel odometry that allows to improve the overall pose estimate. The computation of the wheel odometry requires a kinematic model of the rover, which is derived and calibrated. The calibration process includes correction of the geometrical measures, selection of sensor signals, and modeling of the elasticity of system components. In rough terrain locomotion, the major error source for wheel odometry is wheel slip, degrading the accuracy of the pose estimate. The thesis provides a method to account for wheel slip and shows its successful application in experiments. At first, methods for direct slip measurement are briefly investigated. As these turn out to be not applicable, a method for slip estimation utilizing a velocity-error-model is used instead. The velocity-error-model is terrain-dependent, a dependency which is modeled by parameters and is determined experimentally. The obtained information on slip is additionally used to improve the driving performance of the LRU. Therefore, a concept for traction control is presented that reduces acceleration and velocity of the rover according to the detected slip. This concept is validated by showing the improved driving performance in experiments. The LRU was part of a test campaign on the volcano Mt. Etna in Italy that emulated lunar on-surface operation. There, the volcanic rough terrain environment allowed the rover to fully demonstrate its off-road capabilities. The master’s thesis was written in the context of this moon analogue mission and most experiments were performed on-site. The slip model is parameterized for the volcanic soil and is validated during a long-range driving test on Mt. Etna, featuring a trajectory close to one kilometer in length.