# Multicriteria Optimization of Lower Limb Exoskeleton Mechanism
## 1 Introduction
The exoskeleton is based on a seven-link mechanism and is cost-effective and easy to implement in practical activities. This leg mechanism can ensure the mobility of knee and hip joints, and the ankle joint was not considered in favor of reducing the cost. Also, the paper states that its design may require additional improvements in future developments, such as the foot shape optimization. Another study [7] focuses on the experimental and numerical study of human gait. Its results have practical applications in the design and development of human-inspired robotic structures for use in medical, assistive, or rehabilitation fields. Specifically, the study aims to investigate the flexion-extension movements of lower limb joints in humans and analyze the ground reaction forces generated during walking on force platforms. [PERSON] proposed a synthesis method for six-bar mechanisms [16], which was applied to explore one million tasks, resulting in the synthesis of one hundred and twenty-two practical linkage designs. In [17], the author presents a design procedure for Stephenson I-III six-bar linkages that is demonstrated in the design of legs. These mechanisms are advantageous for their simplicity, characterized by a reduced number of links. Although suitable for applications such as walking robots, they may not meet the requirements for exoskeletons, which demand designs that closely conform to the human anatomy while emphasizing compactness. Hence, the quest for more compact solutions in the context of exoskeletons continues. Demonstration of a 1 DOF closed-loop mechanical linkage that can be designed to the shape and movement of a biped human walking apparatus is also presented in [18]. A single DOF eight-bar path generator that typifies the shape and motion of a human leg is proposed. However, the relative foot stride is notably small. This concept was developed by the authors in [19] towards minimizing the number of links, and a six-link leg mechanism for a biped robot was synthesized. Unfortunately, they used prismatic joints, and the foot stride is small as well. An eight-bar walking mechanism was designed in [20], but the foot path does not have a straight-line segment that will correspond to the support phase (when the foot contacts the ground). Design and optimization of a 1 DOF eight-bar leg mechanism for a walking machine was proposed in [21]. The leg mechanism is considered to be very energy efficient, especially when walking on rough terrains. Furthermore, the mechanism requires very simple controls since a single actuator is required to drive the leg. Dynamic analysis was performed to evaluate the joint forces and crank torques of each solution, thus taking into account inertia forces in the design. However, the total design obtained was cumbersome since the mechanism was based on [PERSON]'s straight-line generator. The synthesis of leg mechanisms inspired by [PERSON]'s solution has been a research topic in the last years [22; 23] for its advantages regarding the reduced number of DOFs, which makes it easier to control, the scalable design, the reduced impact on the ground during walking, and because of less energy consumption. The [PERSON] mechanism was developed in [22] considering an adaptive and controllable mechanism on irregular ground. This research sets a basis for further extension of the [PERSON] mechanism, considering the bending of the leg linkages while turning and providing high stiffness of design. The prototype developed was tested at various speeds and torques due to the presence of the speed control system provided in an electronic system, which enables the use of the prototype for various load-handling capacities. An eight-bar leg mechanism dimensional synthesis is presented in [23] as well. Compared to the prototype, the advantage of the proposed geometric model consists of a greater step height that helps the robotic structure overcome larger obstacles. The Klann linkage is another single DOF walking mechanism that is patented and is widely used on multi-legged robots [24; 25; 26; 27].