Design and Fabrication of Soft Locomotion Robots based on Spatial Compliant Mechanisms

TL;DR

This paper tackles the challenge of rapid, scalable soft robot design by introducing Combots—soft locomotion robots built from a net of spatially distributed thin elastic beams acting as 3D compliant mechanisms. It combines an Evolutionary Algorithm–driven synthesis framework with a Ground Structure Approach to optimize a single appendage for high end-effector displacement and, when included, favorable force transmission, validated by nonlinear FEM and real-world experiments. Key contributions include a unified framework for automated 3D compliant-structure design, demonstration of locomotion up to $3.76$ body lengths per second terrestrial, payload-bearing capability, and underwater operation using common PP material and electromagnet actuation. The work broadens the design space for soft robots, enabling faster production and broader application potential in exploration, inspection, and safe human-robot interaction scenarios.

Abstract

Soft robotics has emerged as a promising technology that holds great potential for various application areas. This is due to soft materials unique properties, including flexibility, safety, and shock absorption, among others. Despite many advancement in the field, the development of effective design methodologies and production techniques for soft robots remains a challenge. Although numerous robot prototypes have been proposed in recent years, their designs are often complex and difficult to produce. As such, there is a need for more efficient and unified design approaches that can facilitate the production of soft robots with desirable properties. In this paper, we propose a method for designing soft robots using elastic beams and spatial compliant mechanisms. The method is based on an evolutionary approach that enables the creation of designs with both high motion and force transmission ratios. Specifically, we focus on the development of locomotion mechanisms using a central linear actuator. Our approach involves the use of commonly available plastic materials and a 3D printer to manufacture the designs. We demonstrate the feasibility of our approach by presenting experimental results that show successful production and real world operation. Overall, our findings suggest that the use of elastic beams and an evolutionary approach can facilitate the creation of soft robots with desirable locomotion properties, including fast locomotion up to 3.7 body lengths per second, locomotion with a payload, and underwater locomotion. This method has the potential to enable the development of more efficient and practical soft robots for various applications.

Figures (12)

• Figure 1: Concept of soft locomotion robots based on a net of thin elastic beams (spatial compliant mechanisms): (a) illustration of one robot design, (b) working principle - active state, c) working principle – passive state, (d) examples of different robot designs and actuator placements.
• Figure 1: Locomotion principle: (a) Robot 1 design, (b) Robot 2 design. The columns containt key points in the locomotion cycle. Cases are shown for manual control and when using a controller, and without and with pads to increase friction.
• Figure 2: Synthesis framework overview for soft locomotion robots: (a) problem formulation, (b) discretization, (c) evaluation of one solution for the optimization process, (d) best found solution, (e) nonlinear Finite Element Method verification, (f) two-leg robot prototype.
• Figure 3: (a-c) Problem formulation and discretization of the design domain for the soft compliant locomotion robot synthesis. Different investigated cases, where input and output displacement direction are varied together with location of the appendage end-effector point. (d) Fully connected ground structure with overlaying elements, (e) partially connected ground structure containing overlapping intersecting elements, (f) ground structure with identified crossing - intersecting points.
• Figure 4: Results of the soft compliant appendage robot synthesis. Solutions where only motion transmission ratio is optimized (GA values) are marked with circle. Solutions where both motion and force transmission ratio are optimized (MA values) are marked with rectangle. Results are shown for the various problem setups defined in Fig. \ref{['fig:formulation']}a-c. Solutions 1-4 correspond to problem setup at Fig. \ref{['fig:formulation']}a, Solutions 5-8 correspond to problem setup at Fig. \ref{['fig:formulation']}b, Solutions 9-12 correspond to problem setup at Fig. \ref{['fig:formulation']}c. This is repeated for solutions 13-24, where both GA and MA were optimized for the same setups. Circles signify structures optimized for GA only, while squares signify optimization for both GA and MA.