American engineers from the University of Wisconsin-Madison have discovered a flaw in the method of testing NASA’s rovers on Earth.
For decades, developers have been adjusting the weight of the rovers to simulate the low-gravity conditions on the Moon or Mars, but they have not taken into account, how Earth’s gravity affects the soil itself. In fact, lunar soil is much looser and less elastic than the Earth’s. This will cause bikes to get stuck in it.
When a spacecraft worth millions of dollars if a rover gets stuck in soft alien soil, as happened with NASA’s Spirit rover in 2009, engineers on Earth have to pull it out. They act as a remote the towing team sends, carefully planned orders to the bike to move the wheels or change the path. This is a slow and very delicate process.
In particular, engineers failed to rescue Spirit in 2009, but perhaps in the future bikes will be able to avoid such a fate. Mechanical engineers from the University of Wisconsin, based on the results of modeling, found lack of modern ground testing methods alien all-terrain vehicles.
The drawback, as the researchers note, often leads to overly optimistic predictions about the behavior of rovers in real conditions on the Moon or Mars. The key to mission planning is to understand how the rover will move on extraterrestrial surfaces, in low gravity, and avoid obstacles, including soft soil and rocky areas.
Lunar gravity is only one-sixth of Earth’s. For decades, engineers have been taking this into account by creating test models that weigh 6 times less than the weight of a real rover. These vehicles were then tested in desert sands on Earth. However, this did not take into account how the Earth’s gravity affects the sand.
Using advanced modeling techniques, professor Dan Negrut and his colleagues found that Earth’s gravity compresses sand more strongly compared to the weaker lunar or Martian gravity. This makes Earth’s sand or soil harder and more stable, reducing the likelihood of it displacement under the wheels of the rover. However, on the Moon, the soil is more loose and prone to displacement, which means, that the rovers less grip and higher risk of getting stuck.
“Looking back, the idea is simple: to better understand how the rover will behave on the Moon, we need to take into account not only the gravitational pull acting on the rover, but also the effect of gravity on the sand. Our results highlight the value of using physical modeling to analyze rover mobility on loose soil”, — explains Dan Negrut.
The results of the study by the University of Wisconsin researchers are part of a NASA-funded project to model the VIPER rover for the lunar mission. The researchers used Project Chrono, an open-source physics simulation engine developed by researchers from the University of Wisconsin in collaboration with colleagues from Italy. This software allows researchers to quickly and accurately model complex mechanical systems, such as full-size rovers operating on soft sand or soil surfaces.
During the VIPER simulation, the researchers noticed a discrepancy between the results of ground tests and the results of the rover’s mobility simulation on the Moon. A more detailed study of the simulation with Chrono helped to identify the shortcoming. Chrono is used by hundreds of organizations around the world to better understand the operation of complex mechanical systems — from precision mechanical watches to off-road trucks and tanks of the US Army.
Chrono is free and freely available for use worldwide. The UW-Madison team continually puts significant effort into developing and maintaining the software and providing support to users.
The results of the study are published in the journal Journal of Field Robotics
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