A Bola Robot Could Provide Stable Jumping Capability on Low-Gravity Bodies

New research on locomotion techniques that could be used in space exploration is constantly coming out. A lab from UCLA known as the Robotics and Mechanisms Laboratory (RoMeLa) is presenting a paper at the upcoming IEEE Aerospace Conference in March that details a unique system. The Space and Planetary Limbed Intelligent Tether Technology Exploration Robot (SPLITTER) consists of two miniaturized jumping robots tethered together.

Such a system might sound like a recipe for chaos and bring back memories of ladder ball games where no amount of control seems to make the tether go where you want it to. But, according to the paper, that system is actually quite stable, even in airless environments.

Mechanically, their system consists of two four-legged robots designed for jumping and tied together at their tops by a tether. Jumping is much more effective than “roving” on the surface of an asteroid because of all the jagged obstacles that need to be avoided. It is also more effective than flying since there is no atmosphere to push against in many space environments. Jumping robots, however, have been around for a while, but the real secret sauce is in the controls the RoMeLa team has developed.

The concept they used is called inertial morphing. In the case of SPLITTER, the robots “adjust inertia with changes in limb configurations and tether length,” according to lead author Yusuke Tanaka in an interview with TechXplore. The researchers turned to a technique called Model Predictive Control (MPC) to determine how each variable needs to be adjusted.

MPC is used in various industries and comes as advertised, with a model (i.e., a mathematical representation of the robots) and a prediction, which reflects what the software estimates will happen to the model next. With the model’s current state and expected next state, a controller can change the variables that affect the model’s state. Those changes will result in a stable flying path, allowing SPLITTER to soar through the skies, even without air. It also uses a physical phenomenon known as the Tennis Racket Theorem, which describes how an object can flip rotation around its intermediate axis while rotating around it. Most famously, this was demonstrated on the ISS with a t-handle. It looks chaotic, but the mathematics behind the motion are well-understood.

Implementing it in a tethered robotic system is another matter altogether, though. While SPLITTER is flying, it looks a lot like a bola used in ladder ball, except instead of round spheres on each end, it’s a robot body with four legs splayed out in different directions. The orientation of how those legs are spread out and the length of the tether connecting the two ends are the variables the MPC controls to stabilize its flight. SPLITTER can operate without heavy attitude control hardware, like reaction wheels or thrusters.

It also allows the system to perform other actions, like spelunking, where one robot is anchored firmly to the top of a cave system while the other rappels using the tether. Both robots only weigh about 10kg each on Earth, as well, which would make them even more agile on a world with smaller gravity like the Moon or an asteroid.

This isn’t the first robot system the RoMeLa lab designed for this purpose. They initially worked on a robot called the Spine-enhanced Climbing Autonomous Legged Exploration Robot) (SCALER), which had its limitations as they found the limbed climbing robot was too slow.

With SPLITTER, the research team thinks they have a better concept that can both traverse terrain faster and collect data that a robot tied to the ground would be unable to do. Unfortunately, for now, at least, SPLITTER is best described as a computer model, though some preliminary work has been done on the physics of MPC controlling a reaction wheel. Researchers at the lab intend to continue working on the concept, so maybe soon we’ll see a bola robot test jumping near Los Angeles.

Learn More:
TechXplore – Modular robot design uses tethered jumping for planetary exploration
Tanaka, Zhu, & Hong – Tethered Variable Inertial Attitude Control Mechanisms through a Modular Jumping Limbed Robot
UT – Miniaturized Jumping Robots Could Study An Asteroid’s Gravity
UT – A Jumping Robot Could Leap Over Enceladus’ Geysers

Lead Image:
Depiction of one SPLITTER robot descending into a crater while the other anchors on the rim.
Credit – Yusuke Tanaka, Alvin Zhu, & Dennis Hong

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