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This paper introduces a control strategy for a bipedal hopping robot designed for low-gravity environments, using a reaction wheel for mid-air attitude stabilization. The robot is modeled as a gyrostat to analyze the dynamic coupling between torso rotation and reaction wheel momentum, and a reduced-order model is used for control design. Simulation results in MuJoCo under lunar gravity conditions show that the reaction wheel controller reduces peak mid-air angular deviation by over 65% and limits landing attitude error to within 3.5 degrees.
Reaction wheels can dramatically stabilize bipedal hopping robots in low-gravity environments, enabling more consistent upright landings on irregular extraterrestrial terrains.
Planetary bodies characterized by low gravitational acceleration, such as the Moon and near-Earth asteroids, impose unique locomotion constraints due to diminished contact forces and extended airborne intervals. Among traversal strategies, hopping locomotion offers high energy efficiency but is prone to mid-flight attitude instability caused by asymmetric thrust generation and uneven terrain interactions. This paper presents an underactuated bipedal hopping robot that employs an internal reaction wheel to regulate body posture during the ballistic flight phase. The system is modeled as a gyrostat, enabling analysis of the dynamic coupling between torso rotation and reaction wheel momentum. The locomotion cycle comprises three phases: a leg-driven propulsive jump, mid-air attitude stabilization via an active momentum exchange controller, and a shock-absorbing landing. A reduced-order model is developed to capture the critical coupling between torso rotation and reaction wheel dynamics. The proposed framework is evaluated in MuJoCo-based simulations under lunar gravity conditions (g = 1.625 m/s^2). Results demonstrate that activation of the reaction wheel controller reduces peak mid-air angular deviation by more than 65% and constrains landing attitude error to within 3.5 degrees at touchdown. Additionally, actuator saturation per hop cycle is reduced, ensuring sufficient control authority. Overall, the approach significantly mitigates in-flight attitude excursions and enables consistent upright landings, providing a practical and control-efficient solution for locomotion on irregular extraterrestrial terrains.
