Key Takeaways & Executive Findings
- •• The proposed RFHJ achieves motion decoupling and independent variable stiffness in two orthogonal planes, addressing a key limitation of existing continuum robots. • The hybrid rigid-flexible design combines safe human-robot interaction with accurate control and reliable stiffness variation, offering a promising solution for practical applications. • Experimental validation shows a wide variable stiffness range of [1.2, 49.9] N·m/rad, demonstrating the joint's tunability and effectiveness. • The RFHA prototype with three RFHJs in series exhibits decoupled driving, bidirectional stiffness tunability, and self-stability, paving the way for advanced continuum robotic systems.
Abstract
Continuum robots have been a hot topic in recent years due to their intrinsic features of agility, flexibility, and safety. To successfully deploy continuum robots in practical applications, further enhancements in variable stiffness, decoupled motion, and embedded sensing are highly desirable. Since continuum robots are usually composed of multiple joints assembled in series, their mechanical properties and performance will certainly rely on the connected joints. This paper proposes a motion-decoupled variable stiffness-decoupled pneumatic rigid-flexible hybrid joint (RFHJ), which is modular designed and integrated with a rigid hinge, a stiffness-tuning module, and soft actuators. The soft pneumatic muscle actuators are pre-stretched during assembly, ensuring the stable initial state of RFHJ. A novel musculature-mounting configuration is also presented, which enables RFHJs to achieve independent motions in two orthogonal planes. Furthermore, the variable stiffness module is embedded in the RFHJ’s structure to offer real-time and independent stiffness tunability across multiple scales in two perpendicular directions. The proposed RFHJ makes most of the advantages of soft continuum robots and conventional rigid serial robots by introducing a hybrid structure to provide both safe human-robot interaction (HRI), accurate control and reliable stiffness variation, presenting promising potentials for robotic systems, which have been theoretically proved and experimentally verified on the physical prototype. The experimental results also indicate that the developed RFHJ can work with variable stiffness ranging in [1.2, 49.9] N·m/rad. A variable stiffness rigid-flexible hybrid continuum arm (RFHA) is designed with three RFHJs in series. Primary tests on the developed RFHA prototype demonstrate that it has the characteristics of decoupled driving, bidirectional stiffness tunability and self-stability.
1. Introduction
Continuum robots are a novel type of bionic robots [1] that mimic the behaviors of soft creatures such as elephant trunks and octopus tentacles. The rapidly advancing field of soft robotics provides continuum robots with flexible and variable stiffness actuators, as well as innovative mechanisms, enhancing their ability to adapt their shape in unknown environments. Continuum robots have been widely utilized in constraint environments to perform various tasks, including surgery [2], elder assistance [3, 4], exoskeleton [5], grasping and manipulation [6].
However, continuum robots are often challenged by the drawback of low joint stiffness in applications, primarily due to their long-arm structure [7]. To address this problem, researchers have proposed numerous methods to achieve variable stiffness [8], include electro active polymers (EAPs) [9], fluidic actuators [10], shape memory alloy [11], electro- and magneto-rheological materials [12, 13], as well as low melting point materials. These material-based approaches to variable stiffness have predominantly been used as actuators in soft robots. In addition, stiffness variation can be achieved through particle interference, layer interference, and fiber interference, collectively referred to as jamming [14–17]. The jamming technique enables the miniaturization of variable stiffness designs.
The implementation of variable stiffness mechanisms enables continuum robots to partially overcome the issue of low stiffness, utilizing approaches such as antagonistic principles [18, 19]. Stiffness adjustment in continuum robots can also be achieved through the use of inflatable or negative pressure extrusion structures [10, 20, 21], as well as by incorporating deformable materials within the central plane of the actuators [22, 23]. These methods constitute a range of approaches for achieving stiffness modulation in continuum robots. However, these methods for stiffness modulation couple the joints’ movements with stiffness adjustment. Furthermore, the coupling of motion and stiffness regulation also prevents the independent variation of stiffness in a specific degree of freedom within the same joint, while allowi
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Wenbiao Wang, Jiahao Shi, Ke Wu, Rui Chen, Zean Yuan, Shibo Cai, Guanjun Bao (2025). A Motion-decoupled Pneumatic Rigid-Flexible Hybrid Joint with Independently-Controlled Variable Stiffness for Continuum Robot. Chinese Journal of Mechanical Engineering. https://doi.org/10.1186/s10033-025-01266-y
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Frequently Asked Questions
What is the main innovation of the proposed rigid-flexible hybrid joint (RFHJ)?
The RFHJ achieves motion decoupling and independent variable stiffness in two orthogonal planes, overcoming the coupling issue in existing continuum robots. It integrates a rigid hinge, a stiffness-tuning module, and soft pneumatic muscle actuators in a modular design.
How does the RFHJ achieve variable stiffness?
The RFHJ embeds a variable stiffness module that provides real-time and independent stiffness tunability across multiple scales in two perpendicular directions, with a measured stiffness range of [1.2, 49.9] N·m/rad.
What are the potential applications of the RFHJ and RFHA?
The hybrid design offers safe human-robot interaction, accurate control, and reliable stiffness variation, making it suitable for applications such as surgery, elder assistance, exoskeletons, and grasping/manipulation in constrained environments.
How was the RFHJ validated?
The RFHJ was theoretically analyzed and experimentally verified on a physical prototype. The RFHA, composed of three RFHJs in series, demonstrated decoupled driving, bidirectional stiffness tunability, and self-stability in primary tests.
What are the advantages of the rigid-flexible hybrid structure?
The hybrid structure combines the flexibility and safety of soft continuum robots with the accuracy and stiffness of conventional rigid serial robots, providing a balanced solution for robotic systems.
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