The robots of Mars, from the Sojourner rover, which arrived on the Red Planet in 1997, to Perseverance, which landed in February, have one thing in common: wheels. Rolling is significantly more steady and energy-efficient than walking, which is still a challenge for even the most advanced robots on Earth. After all, NASA doesn’t want its highly expensive Martian explorer to fall over and flop around like a turtle on its back.
Wheels, on the other hand, have the drawback of limiting where rovers can go: You’ll need the kinds of legs that evolution gave animals on Earth to navigate difficult Martian terrain like steep hills. So a group of scientists from the Max Planck Institute for Solar System Research in Germany and ETH Zurich in Switzerland has been tinkering with SpaceBok, a small quadrupedal robot meant to look like a springbok antelope.
Spaceboks are also reported to “pronk” or jump into the air, though the actual reason is unknown. SpaceBok was originally created to leap and bound on the surface of the Moon in the same way humans did during the Apollo landings.
A real-life springbok bounces through the African deserts, possibly to deceive predators, as its name suggests. The robot’s original plan, which was shown in 2018, was for it to hop on the moon’s surface, as astronauts have done to move around in the weak lunar gravity. That might work on our satellite, where the ground is relatively level, but given the diverse terrain on Mars—which is full of sand, rocks, and steep slopes— it’s probably too risky. As a result, researchers are changing its limbs and gaits to determine if it can manage a more rough landscape.
The team programmed SpaceBok with more typical, less bouncy gaits in these latest studies. The researchers aimed to examine two types of gaits: a “dynamic” gait, in which more than one leg might leave the ground at the same time, and a “static” gait, in which at least three limbs make contact with the ground at any one time. Although the latter is more meticulous, the former is more efficient since it allows the robot to move more quickly.
The researchers also equipped SpaceBok with two different types of feet: point and planar. The point feet have a limited surface area, similar to that of a springbok’s hoof. The planar feet, on the other hand, are flat swiveling circles that bend at an angle when they hit the ground. Think of these as snowshoes rather than hooves. Because they’re perforated with projections that assist the foot grip the ground, they’re more like snowshoes with cleats. SpaceBok was proven to be able to maintain a steady footing on a 25-degree inclination using both the smaller feet and the round snowshoe-like feet.
Now, the team behind SpaceBok has changed the robot’s stride to make it more stable in order to adapt it for Mars, where the terrain is more dangerous and gravity is heavier. Complete research on this topic was published in IEEEE transactions on Robotics on the 14th of June, 2021.
Different gaits, and also small hoof-like feet, and flat, round feet with cleats for added stability, were tried. The researchers behind SpaceBok trained their robot on a vast slanted sandbox full of rocks to replicate Mars, as much of the research on Mars is around craters — Mars Perseverance arrived on the Jezero crater due to the perception that it may have previously been a livable river valley.
Though wheeled robots are more stable and hence preferred for off-world research, legged robots have the ability to reach locations that a rolling rover cannot.
Although NASA’s Ingenuity helicopter, the first machine to accomplish a controlled flight on Mars, has enormous potential for scanning the land, a quadrupedal robot would, in theory, allow for a comprehensive study of rocky terrain and martian cave systems.
The SpaceBok team demonstrated in their article that its machine can climb a simulated Martian hill without toppling down, which would be terrible for a mission worth billions of dollars. The findings open up new possibilities for “safe and energy-efficient global path-planning systems for reaching difficult topography on Mars,” according to the researchers.
More tricks will surely be added to SpaceBok’s repertoire in the future years, as new robotics technologies such as proprioceptive feedback for walking down the stairs develop. Though SpaceBok is primarily used as a study platform for improving space exploration robotics, a version of this model could be launched to Mars in the near future.
For complete details, visit: https://spacebok.ethz.ch/
Main paper: N. Rudin, H. Kolvenbach, V. Tsounis and M. Hutter, “Cat-Like Jumping and Landing of Legged Robots in Low Gravity Using Deep Reinforcement Learning,” in IEEE Transactions on Robotics, doi: 10.1109/TRO.2021.3084374.