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

Shamistan Karimov, Elian Neppel, Shreya Santra, Kentaro Uno, Kazuya Yoshida

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.

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

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.
Paper Structure (17 sections, 11 equations, 5 figures, 1 table)

This paper contains 17 sections, 11 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Method
  3. Controller Version 1: Two-Stage LERP Alignment
  4. Controller Version 2: Continuous Adaptive Hypersphere Clamp
  5. Experiments and Results
  6. Self-Assembly
  7. Task Sequence and Metrics
  8. Results and Comparison
  9. Trajectory Analysis and Controller Validation
  10. Version 1 (two‐stage LERP) on both limbs
  11. Version 2 (adaptive hypersphere clamp)
  12. Summary and Trade-Offs
  13. Velocity Clamping Dynamics
  14. Discussion
  15. System Agnosticism
  16. ...and 2 more sections

Figures (5)

  • Figure 1: End-effector of the MoonBot Limb V2 (blue) approaching the grasping point on the MoonBot Wheel V2 (red).
  • Figure 2: Test stages of the MoonBot Minimal self-assembly. Blue masking highlights the end-effector (Limb V2), Red masking highlights the target (grasping point of Wheel V2).
  • Figure 3: MoonBot Limb V1 executing the self-assembly sequence toward the MoonBot Wheel V1 target to assemble the Dragon configuration. End-effector is marked in blue and target in red.
  • Figure 4: Alignment-only trajectories for both MoonBot limbs converging on a stationary wheel at $(871,\,-459,\,221)$ mm using both controller versions. All trials used the same 30 Hz high-level loop.
  • Figure 5: Time‐series of alignment dynamics for MoonBot Limb V2 under Controller Version 2.