RoMeLa team designs tethered jumping robot for planetary exploration

Researchers at RoMeLa, UCLA’s Robotics and Mechanisms Laboratory, designed SPLITTER, a planetary exploration system of two sub-10-kilogram quadrupedal robots linked by a tether, and disclosed the work on Feb. 11, 2025.

SPLITTER stands for Space and Planetary Limbed Intelligent Tether Technology Exploration Robot. The system joins two small four-legged units into a dumbbell-like structure intended to move across planetary surfaces. It jumps across lunar or asteroid surfaces and gathers information from the areas it reaches. The researchers presented the concept as able to gather environmental information during surface movement.

The researchers intended SPLITTER for low-gravity bodies where conventional wheeled rovers cannot operate. Traditional rovers are bulky, heavy and slow, which restricts how much ground they can cover and how efficiently they explore, the team said on Feb. 11, 2025. Drones can survey wider areas than rovers but cannot work on the moon or asteroids because those bodies lack an atmosphere. This limitation motivated RoMeLa to seek a different locomotion method for planetary bodies.

RoMeLa had introduced SCALER, the Spine-enhanced Climbing Autonomous Legged Exploration Robot, designed for planetary exploration. Testing showed SCALER moved slowly both when walking and when climbing surfaces. Because of that performance, the researchers explored methods to enhance efficiency without drastically changing the robot’s structure, work that led to the SPLITTER concept. The lab’s expertise lies in legged robots for ground locomotion on Earth, and decades of research on these systems now apply to space applications.

SPLITTER consists of two small four-legged robots called Hemi-SPLITTERs, joined by a tether into a dumbbell shape. Each unit weighs less than 10 kilograms. The tether can anchor one unit while the other enters a crater or cave, allowing one robot to explore difficult terrain while the other remains fixed.

Yusuke Tanaka, the paper’s first author, said the work began because moving and controlling orientation in low gravity on the moon or asteroids is difficult. Tanaka told Tech Xplore that aerial drones are impractical on the moon or asteroids because they lack atmosphere. The paper’s main goal was to show attitude control using inertial morphing, which changes inertia through limb arrangements and tether length. This method avoids heavy reaction wheels or gas thrust by altering inertia through tether length and limb placement.

Tanaka said the approach is among the earliest, possibly the first, to use a Model Predictive Controller for inertial morphing attitude control. An MPC-based inertial morphing mechanism controls the robot’s orientation during flight and improves stability. It avoids heavy reaction wheels or gas thrust by dynamically changing inertia through tether length and limb positioning.

Tanaka explained that the mechanism uses the Tennis Racket Theorem, or Dzhanibekov effect, under which asymmetrically inert objects can flip spontaneously while spinning around their intermediate axis. The limbs can jump in reduced gravity but need stable control during flight, he said. Successful jumping stores energy in rotation, allowing the robot to keep accelerating with each jump.

The researchers said SPLITTER suits low-gravity settings because wheeled robots work poorly there and aerial robots are not always easy to deploy. Simulation results show SPLITTER can control its posture in midair using inertial morphing with MPC. The team used MPC to show that the system can control angular velocity and orientation and stay stable without external forces or momentum wheels. That result came from simulation work described in the paper.

The researchers envision deploying the robots as a swarm that can efficiently cross and explore large, unstructured terrain. Mass efficiency is a key advantage because it removes the need for dedicated attitude control gear such as gas thrusters, reaction wheels, or wings while adding agility. The MPC-based inertial morphing mechanism might also help other robots, satellites, and spacecraft maintain stability in space, extending the tool beyond planetary rovers to space vehicles generally.

Tanaka said future work will test SPLITTER in high-fidelity simulation to further validate inertial morphing MPC. The UCLA RoMeLa team is currently working on further enhancing the robot’s hardware, including new actuators and sensing mechanisms. These developments could broaden the robot’s capabilities. The paper is part of Project SPLITTER, foundational work researching technologies needed to build SPLITTER hardware.

Researchers planned to present the SPLITTER system at the IEEE Aerospace Conference in 2025 as part of an effort to share the design with the aerospace community. The work also appeared on the arXiv preprint server with the identifier 10.48550/arxiv.2501.10156. The paper is titled Tethered Variable Inertial Attitude Control Mechanisms through a Modular Jumping Limbed Robot by Yusuke Tanaka and co-authors.

Dennis Hong, the project’s principal investigator and RoMeLa director, said new challenges lead the team to create new solutions and new knowledge that make the work more meaningful and interesting. Hong leads the RoMeLa laboratory at UCLA. Space applications introduce a whole new set of difficult challenges. “With new challenges, we create new solutions and new knowledge which make this job more meaningful and interesting,” Hong said.