Inherently Compliant Biomimetic Upper Limb Mechanism Inspired by Ligamentous Structure

Abstract

Different from robotic joints, the joints of the human body are not shaped as that bones are firmly fixed to each other, but shaped as that one bone rests on the other bone. The joints of this unstable structure are stabilized by multiple ligaments around the joints, so they become rotating joints having a specific axis of rotation. Different from the traditional robotic joints, these ligamentous structured joints have inherent compliance against external forces, caused by the elastic nature of the ligaments. These compliant joints are characterized by protecting bones and the environment from damage by contact or shock with the external environment as well as reducing the impact to transfer to the chest. Also, the joint capsule covering the human joints is filled with the synovial fluid, which lubricates the joints to prevent bone damage caused by friction. This paper proposes an upper limb mechanism with a new type of intrinsic compliance that could not be seen in the existing mechanism by mimicking these joint characteristics of the human body. In particular, the hinge joints of the human body are characterized by the same structure having different movements. This is caused by the position of the origins of the lateral ligaments that restricts the movement of the outer and inner forces, and the movement of the hinge joints depends on this position of the origins of the lateral ligaments. Applying these features, this paper proposes three types of hinge joints with different movements of the isomorphic structure. The proposed joint mechanism consists of rigid bodies and elastic strings that substitute bones and ligaments, similar to human ligament structures. The joint mechanisms are inherently compliant, such as the characteristics of human joints, and do not cause friction between rigid bodies. The dissertation also proposes a finger mechanism implemented this proposed joint to each joint and based on this, suggests a hand mechanism with a self-adaptive grasp. Unlike human other joints, the wrist joint is not rotated by a single joint but rather rotated by two joints connected systematically as if a single joint. In other words, commonly referred to as the wrist is operated by two joints consisting of three layers. This wrist structure has a wider range of motion than any other joint and has a better capacity of shock-absorb for external forces. In this dissertation, a wrist joint mechanism mimicking this bony structure of the human wrist which consists of three-layer structures is proposed. The forearm of human has four substructured joints composed of one bone in the upper arm and two bones in the lower arm, which are systematically joined by multiple ligaments, representing the movement of two degrees of freedom as a whole system. The noteworthy characteristic of forearm structure is that the bones that received mainly load on the wrist and the bones that support the forearm on the elbow are different. And this enables efficient rotation of the forearm to the longitudianl axis. Therefore, there is an important ligament between the two bones in the lower arm, called the interosseous membrane, which distributes and transmits the force received from one bone to the other. This dissertation proposes a forearm mechanism that mimics the bony and ligamentous structure of humans including the interosseous membrane. The proposed forearm mechanism is also inherently compliant and has the advantage of being relatively light compared to the counterpart of traditional mechanisms, as it is based on joints stemming from the aforementioned ligament structure. In conclusion, this dissertation aims to solve the conditions required in the field of service or wearable robots, even medical robots, rather than industrial robots such as light-weight mechanisms, compliant mechanisms for external forces, and safety mechanisms in contact, by using the biomimetics. To achieve this, this dissertation analyzes the characteristics of the human joint and constructs a new type of mechanism structure by applying the ligamentous structure of the human body in an engineering way. Through this process, the structures, which has compliant characteristic against external forces, light-weight, and capacity to reduce wear of mechanism due to no contact between rigid bodies, are proposed. It also suggests hand, wrist, and arm mechanisms designed based on this, showing that actual human movement can be implemented.