💻 computer science

A Dexterous and Compliant Gripper With Soft Hydraulic Actuation for Microgravity Manipulation

This paper presents the integration of DexCoHand, a dexterous and compliant soft hydraulic gripper, with the Astrobee free-flying robot to enable stable microgravity manipulation that minimizes disturbance to the base platform while outperforming existing underactuated grippers in both simulation and hardware experiments.

Original authors: William Su, Jordan Kam, Yixiao Wang, Jianshu Zhou

Published 2026-05-19

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CC BY 4.0

Original authors: William Su, Jordan Kam, Yixiao Wang, Jianshu Zhou

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the International Space Station (ISS) as a giant, floating office building. Inside, there are robots called Astrobee that fly around like friendly drones, checking equipment and taking pictures. Currently, these robots have a very simple "hand"—a single-fingered claw that acts like a parking brake. It's great for grabbing onto a handrail and holding still (perching), but it's terrible at doing anything fancy, like turning a screw or moving a tool around without letting go.

The problem in space is that everything floats. If a robot pushes against an object, the object pushes back, and the whole robot gets knocked off course. It's like trying to unscrew a jar while floating in a swimming pool; if you push too hard, you spin away instead of turning the lid.

The New Solution: "DexCoHand"
The authors of this paper built a new, smarter hand for the Astrobee robot called DexCoHand. Think of it as upgrading from a simple clothespin to a human hand with soft, squishy fingers.

Here is how it works, using some everyday comparisons:

The "Squishy" Hydraulic System: Instead of hard metal joints, this gripper uses soft hydraulic actuation. Imagine holding a stress ball. If you squeeze it, it gives way a little bit before pushing back. This "give" is crucial. When the robot touches something, the soft fingers absorb the shock, preventing the robot from bouncing off its handrail like a pinball.
The "Dexterous" Fingers: Unlike the old claw that only opens and closes, DexCoHand has two fingers, each with three different ways to move. It's like the difference between a pair of tongs and a human hand that can pinch, slide, and rotate an object all at once. This allows the robot to manipulate objects while holding them, rather than having to drop and pick them up repeatedly.

The Experiment: The "Tilt and Pan" Test
To see if this new hand was better, the researchers put it in a computer simulation (a video game world called MuJoCo) that mimics zero gravity. They made the robot grab a handrail and then try to do two specific moves:

Tilt: Leaning side-to-side.
Pan: Sweeping side-to-side.

The Results

The Old Claw: When the robot tried to tilt or pan, the old claw was too rigid. It acted like a stiff stick. Every time the robot tried to move, the rigid grip caused the whole robot to wobble and drift sideways. It was like trying to walk on a tightrope while holding a stiff pole; you end up swaying uncontrollably.
The New DexCoHand: The new hand kept the robot steady. Because the fingers were soft and could adjust their grip, the robot could tilt and pan smoothly without drifting off course. It stayed exactly where it was supposed to be, just like a tightrope walker with a flexible pole that absorbs the wind.

What This Means for Space
The paper shows that this new hand allows the Astrobee robot to do "in-hand manipulation." This means it can hold an object and turn it, align a tool, or route a cable without letting go.

The authors suggest this is a big step toward robots that can do more complex, "intelligent" tasks in space, such as:

Aligning tools precisely.
Routing cables (like untangling headphones).
Handling flexible or squishy objects.

In short, the paper proves that by giving a flying robot a soft, multi-fingered hand, we can let it do delicate work in space without knocking itself off balance. It's the difference between a robot that can just "hold on" and one that can actually "work" while floating.

Technical Summary: A Dexterous and Compliant Gripper With Soft Hydraulic Actuation for Microgravity Manipulation

Problem Statement
The Astrobee free-flying robot aboard the International Space Station (ISS) currently utilizes a one-degree-of-freedom (1-DOF) underactuated compliant claw gripper. While effective for perching on ISS handrails, this design restricts the robot to a limited "grasp-and-release" manipulation regime. More complex microgravity tasks—such as reorienting payloads for handoff, aligning tool tips, threading cables, or manipulating small objects—require continuous pose regulation within the grasp rather than repeated regrasping. In microgravity, the absence of gravity means that every contact impulse couples directly into the free-flying base, inducing undesired translation or rotation and necessitating compensatory control. Existing gripper developments (e.g., gecko-inspired adhesives, rigid-soft hybrids) have improved attachment and transport robustness but offer limited support for continuous, dexterous in-hand manipulation.

Methodology
The authors present the integration of DexCoHand, a dexterous, compliant, two-finger, six-degree-of-freedom (2F–6-DOF) gripper, with Astrobee's perching arm. The system is evaluated through two primary avenues:

Simulation (MuJoCo): The authors modeled Astrobee with both the existing perching gripper and the new DexCoHand as free-floating rigid bodies in a zero-gravity environment. They evaluated both systems using a standard handrail perching sequence comprising approach, perching, and subsequent pan and tilt motions.
- Kinematics & Compliance: DexCoHand features fingers with three controlled DOFs each (fingertip flexion, proximal flexion, and proximal adduction/abduction). The soft hydraulic actuation introduces compliance between the actuator command and contact force, modeled via an effective compliance matrix ( $C_c = J_c C_q J_c^\top$ ). This compliance is designed to reduce abrupt force changes during stick-slip transitions and distribute interaction forces, thereby limiting unintended base motion.
- Dynamics: A simplified translational model ( $m_b \dot{v}_b = F_{thr} - \sum f_{c,i}$ ) was used to analyze how contact forces ( $f_{c,i}$ ) disturb the base. The study logs base position trajectories along x, y, and z axes to measure unintended coupling during commanded maneuvers.
Hardware Experiments: Earth-based hardware demonstrations were conducted to validate DexCoHand's dexterous manipulation capabilities, specifically focusing on tasks requiring compliance and multi-DOF control.

Key Contributions

System Integration: The successful integration of a 6-DOF dexterous gripper with a free-flying robot platform, extending its capabilities from discrete grasping to continuous, contact-rich manipulation.
Compliant Actuation: The implementation of soft hydraulic transmission, which provides compliant force transmission under contact uncertainty, mapping joint-space compliance directly to contact-interface compliance.
Motion Preservation: A demonstration that a dexterous gripper can maintain the stable perching behavior required for free-flyers while enabling complex in-hand manipulation.

Results

Simulation: Both grippers successfully executed pan and tilt motions. However, DexCoHand significantly reduced unintended cross-axis base motion compared to the existing perching gripper.
- During tilt maneuvers, the existing gripper exhibited lateral deviation of approximately 80 mm along the orthogonal y-axis, whereas DexCoHand maintained deviation at approximately 20 mm.
- During pan maneuvers, DexCoHand showed near-zero deviation (0 to −2 mm), while the existing gripper varied between −2 and 4 mm.
- These results indicate that DexCoHand preserves commanded motions while minimizing coupling-induced disturbances.
Hardware: Earth-based experiments demonstrated capabilities not achievable with compliant claw-like grippers, including in-hand object reorientation, controlled pivoting, and compliant pinching on irregular geometries.

Significance and Claims
The paper claims a conceptual validation that dexterous manipulation can be introduced into free-flying robots without compromising the stability required for microgravity operations. The integration of DexCoHand preserves baseline perching capabilities while expanding the manipulation space toward more intelligent, adaptable tasks. The authors assert that this architecture provides a pathway toward more autonomous robotic operations in microgravity, applicable to tasks such as tool alignment, cable routing, and interaction with deformable payloads. The work suggests that this manipulation architecture is generalizable to other free-flyers, such as the SPHERES satellite, though the current validation is specific to the Astrobee platform. Future work is identified as extending this framework to full in-hand manipulation tasks under closed-loop control in simulation.

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