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Multi-Axis Selective Decoupling Framework for Free-Floating Space Manipulators

Daegyun Choi, Donghoon Kim

arXiv:2609.26267v1cs.ROphysics.space-ph

TL;DR

Free-floating manipulators experience reactive base motion, while full-axis reactionless constraints can restrict workspace and achievable end-effector motion. This work selectively decouples momentum along mission-critical directions using a directionally constrained sub-coupling matrix. Simulations show constrained-direction disturbances are mathematically nullified across representative scenarios while remaining degrees of freedom support task-space missions.

Abstract

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Free-floating space manipulator systems face operational challenges due to dynamic coupling. While traditional reactionless manipulation eliminates base disturbances, enforcing the null-space projections across all rotational axes imposes excessive constraints on the system and drastically reduces the available workspace. To address this limitation, this work proposes a multi-axis selective decoupling framework that nullifies momentum transfer exclusively along mission-critical directions. By leveraging a directionally constrained sub-coupling matrix, the framework relaxes the null-space constraints. Numerical simulations confirm that this approach mathematically eliminates constrained directional disturbances along the targeted axes while preserving the remaining degrees of freedom, enabling the manipulator to execute task-space operations successfully.

INTRODUCTION

Free-floating SMSs can conserve fuel and avoid thruster-plume contamination, but arm motion induces reactive base motion that may hinder mission objectives. This study proposes decoupling momentum only along mission-critical directions to relax full-axis constraints and preserve workspace and redundant degrees of freedom.

INTRODUCTION

  • Free-floating operation is often preferred for long-duration missions because it is fuel-efficient and avoids thruster-plume contamination of sensitive sensors or target optical surfaces.Space manipulator systems support servicing, refueling, assembly, and debris-removal missions.Source: INTRODUCTION · p. 1
  • In free-floating operation, manipulator motion induces reactive base motion that can perturb the spacecraft’s six-degree-of-freedom motion and hinder end-effector mission objectives.Excessive base drift may saturate momentum exchange devices or require reaction-thruster desaturation maneuvers, reducing mission life.Source: INTRODUCTION · pp. 1–2
  • Full-axis reactionless manipulation can severely reduce operational workspace, limit achievable end-effector motion, and drive the manipulator into singularities.Scalar disturbance-magnitude objectives also overlook directionality, although small residuals on sensitive axes can trigger resonances or impair instrument pointing.Source: INTRODUCTION · p. 2
  • Selective axis decoupling is motivated by spacecraft vulnerability to disturbances along specific directions, while existing coupling metrics primarily assess overall magnitude rather than direction.The stated research gap is directional insight and control strategies that isolate specific base-reaction directions.Source: INTRODUCTION · p. 2
  • The framework targets and nullifies momentum transfer along a user-defined task-critical subspace, relaxing null-space constraints while preserving workspace and redundant DOF.It also aims to avoid kinematic and dynamic singularities.Source: INTRODUCTION · p. 2

DIRECTIONAL DYNAMIC COUPLING AND SELECTIVE DECOUPLING FRAMEWORK

The section presents a framework for analyzing and managing dynamic coupling in free-floating SMSs by deriving rotational coupling and using its directional characteristics to selectively mitigate base disturbances.

DIRECTIONAL DYNAMIC COUPLING AND SELECTIVE DECOUPLING FRAMEWORK

Conservation of Momentum and Rotational Coupling

With no external forces or torques, momentum is conserved; translational drift is determined by joint configuration, whereas rotational coupling depends on joint-motion history. The section focuses on rotational coupling and maps its operator into the base-fixed frame.

Conservation of Momentum and Rotational Coupling

  • Translational drift is holonomic, with base position uniquely determined by joint configuration, while rotational coupling is non-holonomic and base attitude depends on joint-motion history.The history dependence makes attitude stability a critical and challenging factor for mission success.Source: Conservation of Momentum and… · p. 3
  • The work focuses on rotational coupling because base attitude stability is history-dependent.Source: Conservation of Momentum and… · p. 3
  • Angular momentum about the system center of mass is expressed using the system inertia matrix, the joint-to-base coupling matrix, base angular velocity, and joint-rate vector.The frame superscript N indicates components expressed in the inertial frame; each link’s joint rate is measured relative to its preceding rigid body.Source: Conservation of Momentum and… · p. 3
  • Assuming zero initial momentum, the momentum relation yields a mapping from joint-space velocity to base rotational velocity.The inertia-weighted rotational coupling operator is mapped into the base-fixed frame to separate intrinsic coupling characteristics from time-varying spacecraft orientation.Source: Conservation of Momentum and… · p. 3

Multi-Axis Selective Decoupling Framework

The framework constrains momentum transfer only along user-defined, mission-critical directions rather than suppressing base motion across every rotational axis. It uses the remaining operational redundancy to execute the end-effector task while preserving degrees of freedom not needed for decoupling.

Multi-Axis Selective Decoupling Framework

  • The number and orientation of orthogonal vulnerable directions define the constrained subspace and its directionally constrained sub-coupling matrix.The framework projects base angular velocity onto the selected directions and requires the resulting constrained disturbance to vanish.Source: Multi-Axis Selective Decoupling… · pp. 3–4
  • Selective decoupling is imposed as a hard primary kinematic constraint, with end-effector regulation handled through the remaining operational redundancy.Valid joint rates lie in the decoupling matrix's null space.Source: Multi-Axis Selective Decoupling… · p. 4
  • The joint-rate optimization minimizes deviation from the task demand and joint-velocity kinetic norm, using regularization and assuming the combined task matrix has full row rank.The full-row-rank assumption means the secondary end-effector task does not conflict with the axis-decoupling constraint.Source: Multi-Axis Selective Decoupling… · pp. 4–5
  • The framework nullifies momentum transfer along selected mission-critical directions while preserving the remaining degrees of freedom for manipulation.Its closed-form law guarantees exact tracking of the primary task command while enforcing zero transfer along the axes defined by Uc.Source: Multi-Axis Selective Decoupling… · pp. 3, 5
  • For single-axis decoupling, the manipulator retains n − 1 operational degrees of freedom for the task-space mission.The stated example protects against out-of-plane bending resonance in large, flexible solar arrays.Source: Multi-Axis Selective Decoupling… · p. 6
  • For two-axis decoupling, the framework protects a critical plane while leaving n − 2 operational degrees of freedom for task execution.The antenna line-of-sight example requires nullifying disturbances along pitch and yaw.Source: Multi-Axis Selective Decoupling… · p. 6

SIMULATION STUDY

The simulations test selective decoupling on an ETS-VII reference model across principal-axis, tilted-direction, and two-axis constraints. They show that targeted disturbances can be suppressed while other axes remain available for task execution.

SIMULATION STUDY

  • The ETS-VII reference model uses actual specifications for a 6-DOF manipulator and includes solar arrays and a high-gain antenna.The manipulator is mounted at [−0.79, −0.29, 1]T m from the base center.Source: SIMULATION STUDY · p. 6, Table 1 · p. 6, Figure 1 · p. 6
  • In Case 1, yaw remains zero while roll and pitch absorb reactive momentum; the constrained angular-velocity error is bounded by 1.5×10^-19 rad/s.Yaw is selected to protect against solar-array bending-mode excitation.Source: SIMULATION STUDY · p. 7, Figure 3 · p. 8
  • In Case 2, the tilted constraint leaves all three axes moving, yet the constrained angular velocity stays within 7×10^-19 rad/s.The decoupling direction is intentionally misaligned with the base’s principal axes.Source: SIMULATION STUDY · pp. 7–8, Figure 5 · p. 9, Figure 4 · p. 9
  • For antenna line-of-sight maintenance, the two-axis constraint suppresses roll and yaw disturbances while pitch remains unconstrained.The resulting joint angle and rate profiles are smooth while satisfying end-effector tracking and the decoupling constraints.Source: SIMULATION STUDY · pp. 7–9
  • The simulations confirm that selective decoupling isolates momentum along specified mission-critical directions and redistributes it to non-critical axes, preserving feasible workspace.The study evaluates this framework in three representative scenarios on a free-floating SMS modeled on ETS-VII.Source: SIMULATION STUDY · p. 9

CONCLUSION

The framework selectively decouples mission-critical directions, avoiding the operational restrictions of full-axis reactionless manipulation. Simulations show constrained disturbances are mathematically nullified across three representative scenarios while the manipulator completes its task-space mission.

CONCLUSION

  • Selective decoupling avoids the severe workspace reduction and kinematic singularities associated with conventional full-axis reactionless manipulation.The framework projects base motion into a directional constraint space and isolates user-defined mission-critical directions.Source: CONCLUSION · p. 9
  • Case 3 time histories track joint states, base attitude, base angular velocity, and constrained disturbance angular velocity.Source: Figure 6 · p. 10, Figure 7 · p. 10
  • Across three scenarios, the framework nullifies momentum transfer along mission-critical directions while redistributing reaction into non-critical axes and completing task-space missions.The scenarios target one principal body axis, an arbitrary spatial direction, and multiple axes for pointing requirements.Source: CONCLUSION · p. 10
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