Particle simulation – important tool for simulation-based optimization of mars vehicles
Mars has always been the center of attention of astronomy research – small wonder, with it being top candidate for possible future settlement on other planets. Scientists have used telescopes, probes and, for a while now, also unmanned exploration vehicles (rovers), to explore the red planet. These mobile science platforms can perform many different scientific tasks and are impressively mobile. Based on measurement and camera data, which are continually transmitted back to earth, their course can be spontaneously altered when something of interest appears in the vicinity or when the vehicle meets with obstacles.
In order to also reach more distant places extraterrestrial exploration vehicles generally need to have the abilities of all-terrain vehicles. Similar to ralley vehicles in desert sand, mars rovers must be able to climb steep inclines without getting stuck in soft soil. If this happens rescue is impossible and elaborate missions which have been extensively prepared for are doomed to fail. Unfortunately this is what happened to the NASA rover Spirit1, which had got stuck in Mars soil.
In order to prevent this from happening in the first place the wheels of rovers are already optimized during the development process. The goal is to develop wheels which can drive safely on the largest possibl range of ground surfaces and conditions. The German Aerospace Center (DLR) has, in cooperation with various aerospace agencies and institutes (e.g. working on ExoMars BB2 for ESA, cooperation project with JPL), long been involved in the development of exploration vehicles. For this, computer based tools are very important. They not only make it possible to save costs by making the production of prototypes unncessary, butare also often the only way to reproduce the physical conditions of other worlds in virtual experiments. Just taking into account for conventional experiments the gravity of other planets, which differs a lot from that on earth, can be complicated, expensive and sometimes even impossible.
In order to gain reliable information about the driving dynamics of the rover during the development phase, coupled modeling of the vehicle and of the ground it drives on is used. Unlike the simulation of the vehicle itself, which is done via conventional multi-body modelling, the virtual reproduction of the contact between wheels and ground has not yet been really understood. Difficulties are created by the small grain size of the ground and especially by its unknown material properties. The latter ones can often only be estimated, since only little reliable information, inormation based on measurements, is available about Mars ground. Therefore, often worst case scenarios are used for simulations.
Scientists of DLR’s Institute of System Dynamics and Control have now developed a special, particle-based ground model in order to master this complex task. This model, which was particularly adapted to simulating the driving dynamics of the rover, was validated through bevameter tests and other means and, supported by Inpartik, implemented in Pasimodo® by members of DLR’s Institute of System Dynamics and Control. It has already been successfully used for analyzing and optimizing wheel geometry. This innovative simulation tool will in the future help to make sure that newly developed rovers can perform their tasks reliably and with suitable safety margins, thus providing without interruptions fascinating findings about our red neighbor. The goal is to additionally increase the traction of future Mars rovers in order to gain access to heretofore unreachable places.
1) NASA/JPL: MER Rover Homepage (Opportunity); http://mars.nasa.gov/mer
2) R. Lichtenheldt, B. Schäfer: Planetary rover locomotion on soft granular soils - efficient adaption of the rolling behaviour of nonspherical grains for discrete element simulations, Particle-based methods III : fundamentals and applications ; proceedings of the III International Conference on Particle-Based Methods, Fundamentals and Applications (Particles 2013), Stuttgart, 18 - 20 September 2013. -Barcelona : CIMNE. - 2013, S. 807-818