An Agnostic End-Effector Alignment Controller for Robust Assembly of Modular Space Robots

2025-10-24

AI Summary

Modular space robots for lunar missions face significant challenges in autonomous assembly due to mechanical tolerances, sensor noise, and non-ideal dynamics—including backlash and structural flexibility—degrading alignment accuracy and stability. Method: This paper proposes a hardware-agnostic adaptive alignment controller featuring a novel dynamic hyperspherical clamping mechanism. It jointly constrains translational and rotational velocity bounds in real time based on end-effector pose measurements relative to the target, ensuring motion smoothness, asymptotic convergence, and robustness. The controller admits two implementations: discrete-step and continuous-velocity variants, balancing stability and convergence speed. Results: Evaluated on the JAXA Lunar Environment Simulator, both variants achieve sub-millimeter positioning accuracy. The continuous-velocity variant converges faster, while the discrete-step variant exhibits lower jitter. Both demonstrate strong robustness against multiple non-ideal factors, enabling plug-and-play control of modular robotic arms in lunar surface operations.

Technology Category

• Intelligent Robots: Manipulation
• Search and Optimization: Mixed Discrete/Continuous Search
• Planning, Routing, and Scheduling: Replanning and Plan Repair

Application Category

• Responsible Web: Machine-in-the-loop, human agency and autonomy
• Search and Retrieval-Augmented AI: Agentic search
• Systems and Infrastructure for Web, Mobile and WoT: Experiences and lessons learnt from Web-based algorithms and system deployments

Abstract

Modular robots offer reconfigurability and fault tolerance essential for lunar missions, but require controllers that adapt safely to real-world disturbances. We build on our previous hardware-agnostic actuator synchronization in Motion Stack to develop a new controller enforcing adaptive velocity bounds via a dynamic hypersphere clamp. Using only real-time end-effector and target pose measurements, the controller adjusts its translational and rotational speed limits to ensure smooth, stable alignment without abrupt motions. We implemented two variants, a discrete, step-based version and a continuous, velocity-based version, and tested them on two MoonBot limbs in JAXA's lunar environment simulator. Field trials demonstrate that the step-based variant produces highly predictable, low-wobble motions, while the continuous variant converges more quickly and maintains millimeter-level positional accuracy, and both remain robust across limbs with differing mechanical imperfections and sensing noise (e.g., backlash and flex). These results highlight the flexibility and robustness of our robot-agnostic framework for autonomous self-assembly and reconfiguration under harsh conditions.

Problem

• Develops adaptive controller for modular robots' safe real-world assembly
• Ensures smooth alignment using real-time pose measurements without abrupt motions
• Maintains robustness across mechanical imperfections and sensing noise conditions

Innovation

• Dynamic hypersphere clamp enforces adaptive velocity bounds
• Step-based version ensures predictable low-wobble motions
• Continuous version maintains millimeter-level positional accuracy