Trimming Helicoidal: A Soft Structure with High Precision, Large Workspace, and Compliant Interaction

Background Introduction

In recent years, inspired by biological systems, many researchers have gradually shifted from traditional rigid robot paradigms towards designs that incorporate compliant materials and structures. An elephant trunk is a typical example of this vision for soft robotics, offering unparalleled controllability and workspace. Its compliance provides the elephant with a versatile tool. However, even the most advanced soft robots to date have not fully matched these natural performances, especially in systems approaching or exceeding the 1-meter scale.

To address this limitation, this study proposes a solution through the use of architectured materials. Unlike metamaterials, which customize properties through internal microstructures or composites, architectured materials adjust their physical properties through geometry rather than material attributes. They utilize the spatial heterogeneity of homogeneous materials to achieve simple, low-complexity single-material manufacturing, while adjusting multiple physical properties. These structures have been used in a range of robotic applications, including actuators, deformable, and adaptive structures.

Source of the Paper

This study was collaboratively completed by Qinghua Guan, Francesco Stella, Cosimo Della Santina, Jinsong Leng, and Josie Hughes and was published in npj Robotics (2023). The research institutions include EPFL, Harbin Institute of Technology, Delft University of Technology, and the German Aerospace Center (DLR).

Research Introduction

Research Process

1. Design and Analysis of Trimming Helicoidal Structures

The study first designed an architectured material based on trimmed helicoids, which allows independent adjustment of bending and axial stiffness. Finite element analysis (FEA) and computational analysis were used to select a geometry that achieves the optimal balance between controllability, error sensitivity, and compliance.

Specific steps include:

• Setting the structure surface gradient based on the helicoidal angle, with the inner material contributing mainly to axial stiffness while the outer material primarily prevents bending.
• Selectively removing inner material through a radial trimming process, designing a structure that achieves a specific ratio between axial and bending stiffness.
• Controlling the absolute stiffness value by adjusting the thickness of the helicoidal structure.

2. Modular Assembly and Actuation Design

• Combining these trimmed helicoidal structures with actuation methods, a meter-scale soft manipulator arm was demonstrated and its performance verified. Experimental samples included a meter-scale soft manipulator arm and multi-section units.
• Designing a force transmission system to ensure that the manipulator can achieve compression, bending, and grasping functions in different directions.

3. End Effector Design

An end effector was designed so that the overall structure not only has a large workspace but can also interact with the environment and humans. For example, by selecting corresponding performance indicators, stiffness values, and geometries, the soft robot can perform well in complex task environments such as feeding and collaborative object manipulation.

Research Results

Stiffness Characteristics of Trimmed Helicoidal Structures

1. Relationship Between Geometric Parameters and Stiffness:

   • The study shows that geometric parameters (helicoidal thickness, width, number of helicoids, and helicoidal angle) significantly affect axial and bending stiffness.
   • Using the non-dimensional stiffness ratio λstiff = kaxial r²/kbend, the ratio of width to radius w/r can be adjusted to obtain different absolute stiffness values.

2. Optimization of Geometric Properties and Performance:

   • Adjusting the relative stiffness λstiff between 1.3 to 14 significantly improved the compliance and performance of the assembled structure.
   • Simulations and actual manufacturing confirmed that compliance with λstiff ≈ 2 was significantly superior to other values.

Design and Testing of the Soft Manipulator Arm

1. Workspace and Control Precision:

   • Actual modeling and simulation verified the workspace and precision of the robot end-effector on a meter-scale manipulator arm.
   • Various circular, square, and null-space trajectories under different height and offset environments demonstrated performance under different configurations.

2. Human-Robot Interaction Tasks Demonstration:

   • The experiment demonstrated two tasks, tomato sorting and soup feeding, where the robot achieved these complex tasks through high-precision control and human-robot interaction with the environment.
   • In these tasks, the soft manipulator arm exhibited excellent safety and compliance during interaction.

Research Conclusion

1. Scientific and Application Values:

   • This study demonstrates a novel application of architectured materials, providing highly efficient control performance and workspace for large-scale soft robots by adjustable axial and bending stiffness.
   • Proposes a new design method and brings important insights for the future development of soft robots.

2. Research Highlights:

   • Achieved high compliance and large workspace for soft robots through trimmed helicoidal structures.
   • Showcased the potential of architectured materials in optimizing robot structures, providing new paths for the safety of human-robot interaction.
   • Combined finite element analysis and actual manufacturing to verify the significant impact of optimized geometric parameters on physical performance.

Summary and Outlook

The study proposed a new type of soft robotic structure suitable for a variety of complex task environments. Future research can further optimize the mechanical properties of the material, explore low-viscosity materials or model-driven methods, and introduce sensory feedback to achieve closed-loop control, bringing more intelligent soft robotic systems.

The research fills the gap in high-performance, large-scale soft robots and provides robust theoretical and methodological guidance for more complex application scenarios in the future.