Carmine Senatore

Simulations of Mars Rover Traverses

Artemis (Adams-based Rover Terramechanics and Mobility Interaction Simulator) is a software tool developed to simulate rigid-wheel planetary rover traverses across natural terrain surfaces. It is based on mechanically realistic rover models and the use of classical terramechanics expressions to model spatially variable wheel-soil and wheel-bedrock properties. Artemiss capabilities and limitations for the Mars Exploration Rovers (Spirit and Opportunity) were explored using single-wheel laboratory-based tests, rover field tests at the Jet Propulsion Laboratory Mars Yard, and tests on bedrock and dune sand surfaces in the Mojave Desert. Artemis was then used to provide physical insight into the high soil sinkage and slippage encountered by Opportunity while crossing an aeolian ripple on the Meridiani plains and high motor currents encountered while driving on a tilted bedrock surface at Cape York on the rim of Endeavour Crater. Artemis will continue to evolve and is intended to be used on a continuing basis as a tool to help evaluate mobility issues over candidate Opportunity and the Mars Science Laboratory Curiosity rover drive paths, in addition to retrieval of terrain properties by the iterative registration of model and actual drive results.

Dynamic Modeling and Soil Mechanics for Path Planning of the Mars Exploration Rovers

To help minimize risk of high sinkage and slippage during drives and to better understand soil properties and rover terramechanics from drive data, a multidisciplinary team was formed under the Mars Exploration Rover (MER) project to develop and utilize dynamic computer-based models for rover drives over realistic terrains. The resulting tool, named ARTEMIS (Adams-based Rover Terramechanics and Mobility Interaction Simulator), consists of the dynamic model, a library of terramechanics subroutines, and the high-resolution digital elevation maps of the Mars surface. A 200-element model of the rovers was developed and validated for drop tests before launch, using MSC-Adams dynamic modeling software. Newly modeled terrain-rover interactions include the rut-formation effect of deformable soils, using the classical Bekker-Wong implementation of compaction resistances and bull-dozing effects. The paper presents the details and implementation of the model with two case studies based on actual MER telemetry data. In its final form, ARTEMIS will be used in a predictive manner to assess terrain navigability and will become part of the overall effort in path planning and navigation for both Martian and lunar rovers.Copyright

General scaling relations for locomotion in granular media

Inspired by dynamic similarity in fluid systems, we have derived a general dimensionless form for locomotion in granular materials, which is validated in experiments and discrete element method (DEM) simulations. The form instructs how to scale size, mass, and driving parameters in order to relate dynamic behaviors of different locomotors in the same granular media. The scaling can be derived by assuming intrusion forces arise from resistive force theory or equivalently by assuming the granular material behaves as a continuum obeying a frictional yield criterion. The scalings are experimentally confirmed using pairs of wheels of various shapes and sizes under many driving conditions in a common sand bed. We discuss why the two models provide such a robust set of scaling laws even though they neglect a number of the complexities of granular rheology. Motivated by potential extraplanetary applications, the dimensionless form also implies a way to predict wheel performance in one ambient gravity based on tests in a different ambient gravity. We confirm this using DEM simulations, which show that scaling relations are satisfied over an array of driving modes even when gravity differs between scaled tests.

Publisher's Note: General scaling relations for locomotion in granular media [Phys. Rev. E 95 , 052901 (2017)]

This corrects the article DOI: 10.1103/PhysRevE.95.052901.

Analysis of Shear Bands in Sand Under Reduced Gravity Conditions

The strength of granular material, specifically sand is of pivotal importance for understanding physical phenomena on other celestial bodies. However, relatively few experiments have been conducted to determine the dependence of strength properties on gravity. In this work, we experimentally investigated three measures of strength (peak, confined flow, and unconfined flow friction angle) in Earth, Martian, Lunar, and near-zero gravity. The angles were captured in a passive Earth pressure experiment conducted on a reduced gravity flight. The results showed no dependence of the peak friction angle on gravity, a weak dependence of the confined flow friction angle on gravity, and no dependence of the unconfined flow friction angle on gravity. These results highlight the importance of understanding strength and deformation mechanisms of granular material at different levels of gravity.

Discrete Element Method Simulations of Mars Exploration Rover Wheel High-Slip Mobility Tests.

Mars Exploration Rovers (MERs) experienced mobility challenges when crossing wind-blown ripples and deformable sands while exploring Martian terrain. Analysis of MER wheel mobility using a 3D discrete element method (DEM) model indicate three stages of mobility: (1) low slip ( 60% slip) defined by residual soil strength and depth of wheel sinkage. MER wheel sinkage and drawbar pull increased then decreased during contact between soil and the wheel tie-down patch, compared to wheel cleats, in high slip conditions, but had little affect on overall wheel mobility. DEM simulations well describe intermediate and high slip average wheel sinkage and replicate the signature of drawbar pull and sinkage for the tie-down patch, but not its magnitude. Improved DEM simulation accuracy can be achieved by using smaller polyhedral particles to represent soil.