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Selection of the Best Values of Robot-Body Connection Design Parameters in Lower Limb Wearable Robots, Taking into Account the Weight of the Walking User

Mirzaei, Ali | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57451 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Behzadipour, Saeed
  7. Abstract:
  8. One of the appropriate and efficient methods to restore walking ability due to spinal cord injury is the use of exoskeletons with active joints (actuated joints), also known as wearable robots. These tools, along with their many advantages, present challenges that can affect their performance. One of the existing challenges is how to connect these robots to the body. This connection determines the amount and manner of applying force and torque from the robot to the body parts, and plays a significant role in the performance of these robots, user compliance, and user comfort. Therefore, examining the interfaces between the robot and the body and identifying the parameters that affect their performance are of critical importance. In this regard, efforts have been made, the most relevant of which are the research studies conducted to determine the mentioned parameters. Some of the considered limitations in these studies have led to the simplification of the problem and sometimes renders their results unreliable. Among these limitations are the assumption of static equilibrium, partial gait cycle analysis, and ignoring the gravity. However, given the usage of these robots, it appears necessary to investigate the stated problem under more realistic conditions. In this study, firstly, one of the mathematical models designed to assess the user's comfort and compliance with the robot was further developed, and its shortcomings were identified. Subsequently, appropriate models consisting of the user's body, the wearable robot, and their interfaces were developed in six sequential steps. Each step addressed one of the shortcomings of the preceding model and examined the robot and user interactions in more realistic conditions. These steps included creating a comprehensive model consists of all components of the robot and user, incorporating dynamics into the problem, rectifying the gait of the user and robot, and considering the effects of gravity. The variations in stiffness of the robot-body interfaces in three linear and three rotational displacement directions were analyzed across all models, and the results were evaluated quantitatively. Subsequently, with statistical tools, the most effective directions of these stiffness components were identified. Optimization methods were then utilized to determine the optimal stiffness levels of the interfaces in these directions, aiming to enhance the robot's performance (maximizing tracking accuracy and minimizing discomfort resulting from the applied forces on the body). The results showed that among all assumptions that made the model more realistic, the gravity of the earth, or in other words, the weight of the robot and the user, is the most effective on the performance indices of the robot. The changes caused by adding the Earth's gravity increased the amount of energy stored in the interfaces in the normal state of connection stiffness (stiffness coefficient α = 1) from 0.06 J (in the initial model) to 1.8 J, and the error tracking of the robot from the 1.1 degrees (in the initial model) to 24 degrees. In addition, it was noted that the stiffness of the thigh interface of the robot is much more important than its shank interface, which is contrary to the claim of the previous simple model. The results of the optimization of the problem also stated that in order to reduce the tracking error and reduce the energy stored in the interfaces, the stiffness of the shank interface in the first and third directions (in the direction of posterior/anterior displacement and medial/lateral displacement) and the stiffness of the thigh interface In the third direction (medial/lateral displacement) should be less and the stiffness of the thigh interface in the first and sixth directions (posterior/anterior displacement and rotation in the sagittal plane) should be higher. The optimization results announced a 66% reduction in the amount of tracking error and a 73% reduction for energy stored in the interfaces in exchange for the stated changes in the stiffness of the connections. Therefore, according to the stated results, the best type of connection between the user and the robot was identified and both performance indices improved compared to the normal state.
  9. Keywords:
  10. Exoskeleton ; Biomechanics ; Rehabilitation ; Connection Stiffness ; Robot-Body Interface ; Lower Limb Exoskeleton

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