Sep 23, 2009

Rover Wheel Traction Testing

White Label Space team member Farnoud Kazemzadeh recently completed a research project with our Partner, the Tohoku University Space Robotics Laboratory, which has advanced facilities for testing the performance of wheel designs. The topic of his project was the wheel-soil interactions of the rover we are designing for our Google Lunar X PRIZE (GLXP) mission.

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.

Sep 17, 2009

Wikipedia Page for WLS

Today I finally got around to building the wikipedia page for White Label Space.

Check it out at


Sep 11, 2009

The GLXP Panorama in One Shot

White Label Space team member Nathan Britton recently undertook a research project at our Partner the Tohoku University Space Robotics Lab in Japan, on the design of a panoramic camera system for our Google Lunar X PRIZE (GLXP) rover. Panoramic cameras use a curved mirror to capture a full 360 degree image. An example of such a mirror and the image it produces is shown below.

This approach is a more mechanically simple configuration than a camera system that incorporates pan-tilt mechanisms. Mechanical simplicity is highly desirable on space missions since it reduces the number of failure modes and the effort during design and test of the hardware, ultimately reducing the cost of the GLXP mission.

The objective of Nathan’s project was the preliminary design of a panoramic camera system that can fulfill the GLXP mission requirements for a full 360 degree panoramic still shot at peak resolution and at least 60 degrees vertical with minimum peak spatial resolution of 0.3 milli radians/pixel.

In order to satisfy the spatial resolution requirement with a mirrored panoramic camera an imager with a resolution of over 75 Megapixels would be required. A commercial supplier was identified for a space certified, ultra-HD CCD with 9216x9216 resolution.

A commercial supplier was also identified for a curved mirror to take full 360 degree panoramic shots with a 75 degree vertical window.

Using these two hardware options, the (average) spatial resolution can be calculated as;

150deg * (_/180) = 2.618 rad
2.618 rad / 9216 px = 0.000284 rad/px
0.000284 rad/px = 0.284 mrad/px

This combination of hardware therefore exceeds the GLXP requirement for spatial resolution (0.3mrad/px).

Nathan, pictured in the image at the right, has a double bachelors degree in Information and Computer Sciences and Japanese from the University of Hawaii, and has now completed his Master of Space Studies degree at the International Space University in Strasbourg, France. His research interests are Artificial intelligence developer for robotic exploration vehicles, Human-Computer Interaction and Game Programming. He is fluent in the Japanese language and is currently studying French and Russian.

Sep 3, 2009

Mond 2.0 auf Deutsch

Wir haben das Google Lünar X PRIZE Video Mond 2.0 auf Deutsch übersetzt. Man kann die Untertitel nun über das Menü rechts unten im Player aktivieren.