Modular Isoperimetric Soft Robotic Truss for Lunar Applications

Technical Summary: Modular Isoperimetric Soft Robotic Truss for Lunar Applications

Problem Statement
Establishing a permanent human presence on the Moon requires adaptable structures and robots capable of operating safely alongside astronauts or performing autonomous tasks. Traditional rigid truss structures and lunar robots often suffer from high mass, limited stowage volume, specialized single-task designs, and safety risks associated with high-impact collisions. While inflatable structures offer lightweight and compact alternatives, previous soft robotic truss designs (specifically isoperimetric pressurized tubes) were largely limited to single-octahedron geometries, restricting their scalability and functional versatility for complex lunar missions.

Methodology
The authors present a large-scale, modular, and reconfigurable robotic system based on isoperimetric inflatable tubes. The core methodology involves:

• System Architecture: The robot consists of triangular units formed by continuous inflated fabric tubes routed through two active robotic roller units and one passive connecting unit. The active rollers pinch the tubes to create localized joints with reduced bending stiffness. Electric motors translate these rollers along the tube, shifting the pinch point to lengthen one edge while shortening another, thereby preserving a constant perimeter (isoperimetric constraint) without requiring additional compressed air.
• Novel Spherical Joint: A newly developed modular spherical joint connects the roller units of adjacent triangles at a vertex. Unlike previous hinge joints limited to two connections, this design allows up to three triangles to connect at a single node while maintaining a virtual center of rotation, enabling the construction of stacked octahedra and more complex truss assemblies.
• Hardware Improvements: The active roller units were redesigned with larger roller shafts and gap heights compared to prior work, allowing operation at higher internal pressures (up to 76 kPa, tested up to 221 kPa) to increase structural strength. The units utilize a simplified gear train and topology-optimized components, resulting in a 31% mass reduction for active units and a 38% reduction for passive units.
• Control and Kinematics: The system is modeled as a graph where node positions, edge lengths, and roller positions are related. Inverse kinematics are solved using a quadratic program with linear constraints to enforce constant triangle perimeters and specific node velocities (e.g., fixed ground nodes or moving target nodes). This allows the robot to execute complex motions via coordinated geometric reconfiguration.
• Configurations: Two primary configurations were demonstrated:
  1. Solar Array: Two vertically stacked octahedra (6 triangles, 12 degrees of freedom) with a rigid top panel.
  2. Locomotion: Two symmetric octahedra connected by a shared central triangle (7 triangles, 14 degrees of freedom) utilizing a step-and-slide gait.

Key Contributions
The paper claims the following primary contributions:

1. Modular Scalability: Extending the isoperimetric truss concept beyond a single octahedron to create scalable, stackable structures capable of compact stowage with a high stowed-to-deployed volume ratio.
2. Novel Joint Design: The development of a spherical joint that enables the connection of three or more components at a node while preserving a virtual center of rotation, facilitating complex 3D truss geometries.
3. Demonstrated Versatility: Validating the system's ability to perform complex motions including extension, squatting, twisting, top-plane tilting, sweeping, and locomotion.
4. Performance Enhancements: Improving reliability and load capacity over previous iterations by utilizing high-pressure tubing, strengthening roller units, and implementing robust gear trains, resulting in significant weight reductions.

Results
Experimental results demonstrate the system's functional capabilities:

• Storage Efficiency: The system achieves a stowed-to-deployed volume ratio of 1:18.3. A 6-triangle solar array configuration stows in 0.30 m³, while a 7-triangle locomotion configuration stows in 0.301 m³.
• Solar Array Motion: The 12-DOF solar array configuration achieved a vertical range of motion of approximately 1.5 m (from 2.03 m to 3.52 m height) when ground-fixed. It demonstrated independent twisting of top and bottom sections, tilting of the top plane up to 65° (about a joint) or 62° (about an edge), and sweeping motions. By combining tilting and sweeping, the system achieved an effective 360° rotational coverage.
• Locomotion: The 14-DOF locomotion configuration successfully executed a step-and-slide gait. Each cycle advanced the robot 0.46 m over 26 seconds, with individual foot steps covering 0.61 m.
• Durability: Active roller units successfully operated at 221 kPa (2.9x operating pressure) for two hours without significant elastic deformation or creep. At the standard operating pressure of 76 kPa, motors draw approximately 2.94 A, providing roughly 26 minutes of continuous operation per battery charge (though practical operation involves short bursts).
• Failure Modes: The paper notes that sweeping motions beyond ±35° can induce high bending moments leading to tube buckling and truss collapse. However, the soft nature of the tubes attenuates impact, and the springback effect allows for relatively easy re-erection of the structure, a process facilitated by low lunar gravity.

Significance
The paper positions this modular isoperimetric inflatable robot as a valuable solution for sustainable lunar operations. By addressing the critical challenges of mass, volume, and adaptability, the system offers a reconfigurable structure capable of performing diverse tasks such as solar array deployment, crane-like manipulation, and terrain interaction. The ability to compactly stow the system for transport and reconfigure it on-site for various functions, combined with its inherent safety and modularity for repair, makes it a promising candidate for future space missions requiring versatile, human-robot collaborative environments.