46th Lunar and Planetary Science Conference (2015) 2352.pdf A SCREW PROPULSION DESIGN FOR MOBILITY IN HIGH-SINKAGE MEDIA. N. Stein1, M. Zanetti1, B. Settle2, D. Battel2, A. Carsello2, A. Cooperberg2, D. Gebreselasse2, J. Gross2, M. Kelley2, P. Kharel2, S. Lazechko2, H. McConnell2. 1Washington University in St. Louis, Dept. of Earth and Planetary Science, St. Louis, MO ([email protected]); 2Washington University in St. Louis, School of Engineering And Applied Science, St. Louis, MO. Introduction: The Screw-Propelled Automated Regolith Collecting (SPARC) robot is Washington University in St. Louis’ (WUSTL) entry into NASA’s annual Robotic Mining Competition, in which robots must navigate and mine in a field of BP-1 lunar regolith simulant. The simulant, which is characterized by relatively low cohesion and a high shear deformation modulus, demands novel mobility approaches to minimize sinkage and slip. SPARC’s mobility platform is based on an Archimedes screw pontoon system that minimizes sinkage and provides additional mechanical advantage in low cohesion materials relative to more traditional wheel designs. Screw-propelled vehicles have long been used terrestrially to navigate through snow and swamps and, more recently, in tailings management. This abstract describes the design and testing of the SPARC screw-propelled mobility system in BP1 regolith simulant. Screw Propulsion: Screw-propelled vehicles use one or more cylinders fitted with helical flanges to navigate terrains typically characterized by high sinkage. Two-cylinder screw-propelled vehicles such as SPARC are capable of forward and backward (longitudinal) motion during which the augers are rotated inward and outward, respectively, and side-to-side (lateral) motion during which the augers are simultaneously rotated in the drive direction. During lateral drives, the screws can be treated as long, non-deformable wheels with motion dominated by shear-stresses between the cylinder surface and the soil. Ignoring flight-soil interactions, lateral motion is controlled by motion resistance (R) and traction (T), which are related to normal and shear stresses as T = τcosθ and R = σsinθ where τ and σ are shear and normal stress, respectively, and θ is the angle between the side of the screw and the terrain at the exit point (Fig. 1). In deformable surfaces such as lunar regolith, τ and σ are a function of soil properties including cohesion, internal friction angle, and shear modulus . Traction and motion resistance are functions of wheel sinkage. Due to the large area of the screw-terrain interface, which reduces surface pressure, screw-propelled vehicles are characterized by relatively low sinkage that minimizes motion resistance and maximizes traction. Fig. 1: Cutaway view of pontoon. Lateral motion is dominated by shear and normal stresses between the pontoon and surface. During longitudinal drives, the vehicle is propelled by interactions between the flanges and the surrounding medium with an efficiency proportional to the helix angle (Fig. 2). Variations in helix angle affect multiple parameters including ground deformation, drawbar pull, and slip, although no single angle optimizes every parameter . Fig. 2: Side view of SPARC pontoon. SPARC Mobility Design: SPARC consists of two 91.45 cm long, 15.25 cm wide carbon-steel tubes with oppositely threaded, 3.8 cm high continuous helix grousers made from 12-gaugue carbon steel. Each isdriven by an 88:1 geared 12 V 6.35 cm brushed DC motor. A hollow axle carries power from an external power source to the internal motors, which are shielded from dust by PVC endcaps that serve the additional purpose of bulldozing material during longitudinal drives. A helix angle of ~25⁰ was selected to balance motor efficiency with slip and drawbar pull (Fig. 3). The screws propel the 1.2 m long, 0.75 m wide, 0.75 m high, ~175 kg robot (Fig. 3). 46th Lunar and Planetary Science Conference (2015) 2352.pdf with the surrounding media as the primary form of propulsion, outperforms lateral driving in materials with high shear deformation modulus (Fig. 6). Future Work: Several changes to the screwpropulsion design will be pursued in order to further examine drive performance in BP-1 and similar media including weight reduction through the use of carbon fiber tubing, increasing drive actuator torque, and steepening flight angle to maximize drawbar pull. Fig. 3: View of fully assembled SPARC robot. Performance: Assuming limited skid, the maximum longitudinal travel distance of the robot per rotation d of the screws is πdtanϕ for flight angle ϕ, allowing lateral propulsion to drive the vehicle d/(πh) times farther per auger rotation than longitudinal driving. Consequently, lateral locomotion is preferred in terrains characterized by relatively low cohesion and shear deformation modulus (Fig. 4). In media with a high (>30 mm) shear deformation modulus, longitudinal drives approach and exceed lateral drives in displacement per auger rotation. Fig. 5: Measured wheel slip as a function of drive distance for lateral and longitudinal drives in beach sand and BP-1. Fig. 4: Displacement as a function of pontoon rotations for longitudinal and lateral drive modes. In order to compare pontoon slip as a function of drie distance, SPARC mobility was tested in two media: well-sorted beach sand with small, rounded grains, and BP-1. Slip was estimated by comparing the distance between flight marks with commanded displacement. During low sinkage (< 4cm) traverses, SPARC incurred lateral slip of less than 10% in both BP-1 and sand, compared to longitudinal slip of ~15-20% in both materials (Fig. 5). Comparing slip as a function of lateral and longitudinal shear deformation modulus indicates that longitudinal driving, which engage flights Fig. 6: Slip as a function of shear deformation modulus during lateral and longitudinal drives. References:  Cooling D. J. (2009). Austalian Centre for Geomechanics. ISBN 0-9756756-7-2.  Wong J. (2001). Theory of Ground Vehicles, 3rd ed. 9902-4.  Cole N. (1961). Proc. Inst. Mech. Eng. 175(919).
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