The interaction of wheels or tracks on loose soil has been well investigated in the field called terramechanics, and understanding it is one of the most crucial steps of designing a new wheel or traction system. Since soil characteristics greatly affect the effectiveness of the mobility system it is very important to understand the planetary surface's properties. In soft soils, loss of traction due to excessive wheel slippage can lead to wheel sinkage and ultimately vehicle entrapment. The Moon's relatively low gravity leads to soils with lower confining stresses, and hence bearing strength, than soils on Earth. To be accurate, tests conducted on Earth must accurately simulate the lunar soil's mechanical properties.
Farnoud conducted the tests in a sandbox containing simulated lunar regolith that closely matches the properties of the lunar soil. This lunar regolith simulant was created based on samples returned by the Apollo missions. Using this sandbox the performance and behavior of the wheel assembly can be tested in a condition which very closely replicates the conditions on the Moon. The sandbox is capable of being tilted to simulate the rover climbing a slope.
In this research project, Farnoud tested three candidate wheel designs shown below. One wheel has a bare outer surface, one wheel has spikes of length 9mm and the third wheel has spikes of 18mm.
For each wheel design, three main parameters were measured in the experiments; the slip ratio, the drawbar pull and the torque.
The slip ratio is given by the equation;
SlipRatio = 1 - (D_measured/D_theoretical)
Where D_measured was the actual distance travelled (measured by a ruler) and D_theoretical is the distance that the wheel should have traveled if there was no slippage (derived from the encoder counts). Each run was conducted three times and an average of the slip ratio with uncertainty was calculated.
Drawbar pull is the amount of force that is exerted by the motor axle minus the rolling resistance between the wheel and surface in the direction of travel. For any given wheel and solid system the drawbar pull depends a function of slip ratio for different traveling speeds.
When the slip ratio is increased, drawbar pull also increases. This is because the larger slip ratio results in the larger soil deformation, hence inducing larger drawbar pull.
The torque was measured to determine the power requirements for the motors that drive the wheels.
The above parameters were measured for a range of slope angles and wheel rotation speeds. Through the experiments it was observed that the wheel with the 18 mm long spikes had approximately 30% less slip ratio for the same drawbar pull than the other two wheels, verifying the expected result that spikes help to increase the wheel-soil traction.
Researchers at Tohoku University will continue to experimentally investigate other options for increasing the traction such as using a larger wheel diameter or deformable (compliant) wheels to have increased contact area. Based on their results, the White Label Space team will design and thoroughly test a mobility system that is suitable for the terrain expected at our GLXP landing site (see also: Where Shall I Land my GLXP Mission?).
Farnoud Kazemzadeh conducted this research project at the Tohoku University Space Robotics Laboratory whilst undertaking his Master of Space Sciences at the International Space University in Strasbourg, France. He previously completed a Bachelor of Science specialized in Astrophysics at the University of Waterloo in Canada and is currently a candidate for the Master of Applied Sciences in System Design Engineering at the same university.