Validation of Artemis Mobility Simulations for the Spirit, Opportunity, and Curiosity Mars Rovers

Artemis is a framework for modeling the dynamical motions of rovers over realistic planetary terrains that include topography generated from orbital or rover-based data and interactions of driven wheels with deformable soils with compaction resistance due to wheel sinkage into soils (Bekker-Wong-Reece model) or with hard-surface contacts dominated by static and dynamic coefficients of friction (contact model). Artemis is being used to simulate flight-based drives for the Mars Exploration Rovers (Spirit, Opportunity) and Mars Science Laboratory (Curiosity). Critical to realistic simulations is validation of the models by comparison and registration to single-wheel tests in the laboratory using spare flight wheels and testing of rover drives on various surfaces on Earth. In this abstract we report results from modeling test rover drives on deformable soil and hard surfaces. First, a MER test rover was driven over a tilted hard-surfaced plane covered with high friction paint, with separate runs with tilts ranging from 0 to 20 degrees. The Artemis contact model with static and dynamic coefficients of friction of 0.780 and 0.580 reproduced the runs in a manner in which the tests and models were shown to be statistically indistinguishable. Second, the MSL Test Rover was driven over bedrock plates with slopes ranging from 0 to 30 degrees, and Artemis successfully modeled these runs with static and dynamic coefficients of friction of 0.577 and 0.450, respectively. The MSL Test Rover simulations yielded low, linearly increasing slip between 0% and 15% on slopes ranging from 0 to 20 deg, increasing nonlinearly to nearly 100% on the 30 degree slope. Third, both the MER and MSL Test Rovers were deployed to the Dumont Dunes in the Mojave Desert to do side-by-side runs up dune slopes with tilts ranging from 0 to 15 degrees. Test runs and Artemis (Bekker-Wong-Reece model) results are comparable in terms of wheel sinkages and slippage values. Uphill drives led to non-linearly increasing wheel slip-sinkage and slippage with increasing slope, with the MER Test Rover reaching 100% slippages at a 12 deg slope whereas the MSL Test Rover ascended up and over the dunes, but with significant slippage at highest slopes. Downhill runs led to significant wheel skid for both rovers. Artemis was able to simulate the trends seen in all of the runs and provided physical insight as to why the MSL runs were able to ascend the slopes whereas the MER runs were not. Specifically, the models showed that larger radius wheels (21 cm for MER and 50 cm for MSL) are able to direct more torque to the longitudinal drive direction and thus provide more forward thrust for the same pressure-induced sinkage (MER and MSL have approximately the same ground pressures beneath the wheels). Artemis also showed that softer sands with more sinkage and slippage were needed near the crests of the dunes, consistent with in-situ measurements. Results thus show that rover-based tests can be simulated with Artemis. Work is continuing on validation using single-wheel tests in soil chambers at MIT, and initial work has begun on simulating rover drives for Opportunity and Curiosity on Mars.