Human Mimetic Forearm Design with Radioulnar Joint using Miniature Bone-Muscle Modules and Its Applications

Abstract: The human forearm is composed of two long, thin bones called the radius and the ulna, and rotates using two axle joints. We aimed to develop a forearm based on the body proportion, weight ratio, muscle arrangement, and joint performance of the human body in order to bring out its benefits. For this, we need to miniaturize the muscle modules. To approach this task, we arranged two muscle motors inside one muscle module, and used the space effectively by utilizing common parts. In addition, we enabled the muscle module to also be used as the bone structure. Moreover, we used miniature motors and developed a way to dissipate the motor heat to the bone structure. Through these approaches, we succeeded in developing a forearm with a radioulnar joint based on the body proportion, weight ratio, muscle arrangement, and joint performance of the human body, while keeping maintainability and reliability. Also, we performed some motions such as soldering, opening a book, turning a screw, and badminton swinging using the benefits of the radioulnar structure, which have not been discussed before, and verified that Kengoro can realize skillful motions using the radioulnar joint like a human.

• The paper presents a novel forearm design that mimics human radioulnar mechanics using integrated miniature bone-muscle modules, doubling actuator density with only a 21% volume increase.
• The paper introduces dual actuator integration and embedded heat dissipators to optimize size, torque, and backdrivability in tendon-driven robotic systems.
• The paper validates its design through applications in tasks like soldering, screw turning, and badminton swings, demonstrating enhanced dexterity and stability.

Human Mimetic Forearm Design with Radioulnar Joint using Miniature Bone-Muscle Modules and Its Applications

The paper by Kento Kawaharazuka et al. discusses the development of a human-like forearm design incorporating a radioulnar joint using novel miniature bone-muscle modules. This research is anchored in the quest to emulate human forearm mechanics to enhance the performance and maneuverability of tendon-driven musculoskeletal humanoids, such as the humanoid robot Kengoro. The authors present innovative strategies and technical details to overcome the challenges inherent in creating a compact, efficient, and reliable robotic forearm that mirrors human anatomy.

Development of Miniature Bone-Muscle Module

Approach and Design

The human forearm's sophisticated mechanics, specifically its dual-bone structure (radius and ulna) and radioulnar joint, pose significant design and engineering challenges. Traditional muscle modules used in tendon-driven robots have limitations in size and spatial efficiency that hinder the replication of human-like forearm movements. The authors propose a dual strategy to overcome these hurdles:

1. Integration of Muscle and Bone Structure: By incorporating two miniature actuators within a single muscle module and utilizing shared structural components, the researchers maximize space efficiency. This integration allows the modules to function as both muscle actuators and a structural framework, significantly reducing the forearm's overall bulk while maintaining functionality.
2. Miniature Motors with Heat Dissipation: The adoption of smaller motors presents a challenge in maintaining high torque and managing heat. The authors address this by embedding heat transfer sheets between the motors and the structure, facilitating continuous high-tension operation without overheating, thus preserving backdrivability and efficiency.

Performance Evaluation

The newly developed bone-muscle modules are evaluated against conventional modules, demonstrating a substantial increase in muscle actuator density (doubling) with only a 21% increase in volume. The integration allows for versatile muscle arrangements and various connection possibilities, extending the design's adaptability.

Realization of Human Mimetic Radioulnar Structure

The constructed robotic forearm integrates four such bone-muscle modules, achieving an arrangement that mirrors human forearm proportions and muscle configuration. This setup provides six degrees of freedom (DOF), with the modules enabling a compact and efficient replication of human joint and muscle dynamics. The authors validate the structural fidelity of their forearm design by comparing it against human anatomical benchmarks in terms of link lengths, weight distribution, and joint performance metrics. The results indicate a close approximation of human forearm functionality.

Implications and Experimental Validation

To substantiate the practical benefits of the radioulnar joint design, the paper explores various application scenarios. These include:

1. Soldering: Demonstrates the ability to perform precise hand movements with the ulna fixed, leveraging the radioulnar joint's unique mechanics to stabilize the hand.
2. Opening a Book: Exploits the slanting radioulnar axis, enhancing the reach and stability of hand movements across a broader range.
3. Turning a Screw: Aligns the radioulnar joint axis with a screwdriver’s axis, optimizing torque transfer and minimizing tool-tip displacement.
4. Badminton Swing: Utilizes the increased radius of rotation afforded by the slanting radioulnar axis to boost the racket's speed, emphasizing the joint's role in high-speed, skillful movements.

Each experiment not only underscores the mechanical advantages provided by the radioulnar structure but also showcases the module's robustness in replicating human-like dexterity and movement.

Future Directions

The findings of this research pave the way for the development of more compact, efficient, and adaptable tendon-driven musculoskeletal robots. Future work aims to expand the application of these modules beyond the forearm, enhancing the biological fidelity of various robotic components. Furthermore, deepening the understanding of radioulnar joint mechanics could yield even more sophisticated and nuanced robotic movements, contributing significantly to advancements in humanoid robotics and their applications.

Conclusion

The paper presents a detailed, methodical approach to developing a human mimetic forearm with a radioulnar joint using innovative miniature bone-muscle modules. Through rigorous design and experimental validation, this work contributes valuable insights and practical advancements in the field of robotic limb design, emphasizing the potential for enhanced human-robot interaction and dexterity in future humanoid robots.