A Four-Legged Robotic System Can Move on Tough Landscapes

Massachusetts Institute of Technology, Cambridge, MA

Researchers created DribbleBot, a system for in-the-wild dribbling on diverse natural terrains including sand, gravel, mud, and snow using onboard sensing and computing. In addition to these football feats, such robots may someday aid humans in search-and-rescue missions. (Image: Mike Grimmett/MIT CSAIL)

Researchers from MIT’s Improbable Artificial Intelligence Lab, part of the Computer Science and Artificial Intelligence Laboratory (CSAIL), have developed a legged robotic system that can dribble a soccer ball under the same conditions as humans. The bot used a mixture of onboard sensing and computing to traverse different natural terrains such as sand, gravel, mud, and snow, and adapt to their varied impact on the ball’s motion. Like every committed athlete, “DribbleBot” could get up and recover the ball after falling.

Programming robots to play soccer has been an active research area for some time. However, the team wanted to automatically learn how to actuate the legs during dribbling, to enable the discovery of hard-to-script skills for responding to diverse terrains like snow, gravel, sand, grass, and pavement. Enter, simulation.

A robot, ball, and terrain are inside the simulation — a digital twin of the natural world. You can load in the bot and other assets and set physics parameters, and then it handles the forward simulation of the dynamics from there. Four thousand versions of the robot are simulated in parallel in real time, enabling data collection 4,000 times faster than using just one robot. That’s a lot of data.

The robot starts without knowing how to dribble the ball — it just receives a reward when it does, or negative reinforcement when it messes up. So, it’s essentially trying to figure out what sequence of forces it should apply with its legs. “One aspect of this reinforcement learning approach is that we must design a good reward to facilitate the robot learning a successful dribbling behavior,” said MIT PhD student Gabe Margolis, who co-led the work along with Yandong Ji, Research Assistant in the Improbable AI Lab. “Once we’ve designed that reward, then it’s practice time for the robot: In real time, it’s a couple of days, and in the simulator, hundreds of days. Over time it learns to get better and better at manipulating the soccer ball to match the desired velocity.”

The bot could also navigate unfamiliar terrains and recover from falls due to a recovery controller the team built into its system. This controller lets the robot get back up after a fall and switch back to its dribbling controller to continue pursuing the ball, helping it handle out-of-distribution disruptions and terrains.

“The whole point of studying legged robots is to go terrains outside the reach of current robotic systems,” said Pulkit Agrawal, MIT Professor, CSAIL Principal Investigator, and Director of Improbable AI Lab. “Our goal in developing algorithms for legged robots is to provide autonomy in challenging and complex terrains that are currently beyond the reach of robotic systems.”

Compared to walking alone, dribbling a soccer ball imposes more constraints on DribbleBot’s motion and what terrains it can traverse. The robot must adapt its locomotion to apply forces to the ball to dribble. The interaction between the ball and the landscape could be different than the interaction between the robot and the landscape, such as thick grass or pavement. For example, a soccer ball will experience a drag force on grass that is not present on pavement, and an incline will apply an acceleration force, changing the ball’s typical path. However, the bot’s ability to traverse different terrains is often less affected by these differences in dynamics — as long as it doesn’t slip — so the soccer test can be sensitive to variations in terrain that locomotion alone isn’t.

“Past approaches simplify the dribbling problem, making a modeling assumption of flat, hard ground. The motion is also designed to be more static; the robot isn’t trying to run and manipulate the ball simultaneously,” said Ji. “That’s where more difficult dynamics enter the control problem. We tackled this by extending recent advances that have enabled better outdoor locomotion into this compound task which combines aspects of locomotion and dexterous manipulation together.”

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