Diagonal grousers improve turning performance in skid-steer rovers

Skid steering is a popular choice for planetary rover missions due to its simplicity, low cost, and low mass compared to explicit steering systems. Skid steer rovers’ wheels do not turn explicitly; vehicle turning is achieved by driving the left and right wheels at different velocities. To achieve a turn, the wheels must inherently experience a high amount of slip, which can lead to high sinkage. Furthermore, previous work has shown that sinkage can increase in reduced gravity environments. This work focuses on wheel design changes to reduce sinkage for a potential future lunar rover. The rover in question employs skid-steering, and prototype testing revealed significant sinkage during point turns (i.e. turn-in-place maneuvers). This was identified as a major issue since high sinkage leads to increased entrapment risk, decreased ground clearance, and increased power consumption. To investigate risk mitigation opportunities, seven additively manufactured wheels were tested in a single-wheel testbed capable of mimicking skid-steer maneuvers by setting a representative slip angle. To represent lunar gravity performance, the tests followed Granular Scaling Laws (a scaling method employing dimensionless numbers, recently validated against parabolic flight experiments and shown to predict wheel performance in lunar gravity with < 10% error). The seven wheel designs can be summarized as (1) current baseline: rigid 150 mm diameter, 30 mm wide wheel with 27x 8mm grousers, (2) wheel 1 with closed sides, (3) wheel 1 with closed sides and ‘side grousers’, (4) wheel 1 but 200 mm in diameter, (5) wheel 1 but 60 mm wide, (6) wheel 1 with 45 degree angled grousers, and (7) wheel 1 with a rounded profile. The metrics for turning performance evaluation were sinkage and tangent turning force coefficient. It was found that increasing the wheel size (width and/or diameter) had a positive effect on turning performance, but these changes would require mass/volume increases in an already mass/volume constrained application. Diagonal grousers also enhanced turning performance, and do not require mass or volume increases. While some of the other designs decreased sinkage, they had detrimental effects on the tangent turning force coefficient, and required an increase in mass and/or volume. Additional drawbar pull tests revealed that straight-driving performance was not significantly affected by the angled grousers. Next, the diagonal grouser wheel design will be tested on a full four-wheeled rover prototype to confirm these findings and determine if slope-climbing ability and obstacle-climbing ability are negatively impacted by this design.