Human movement assistance using a variable stiffness exoskeleton

  • Shaikha F. Abdulmajeed

Student thesis: Master's Thesis


Assistive exoskeleton technology has made significant advances during the last decade, resulting in a considerable variety of solutions for gait assistance and rehabilitation. The mechanical design of these devices is a crucial aspect that affects the efficiency and effectiveness of their performance and interaction with the user. Recent developments have leaned towards compliant mechanisms or biomimicry, which have shown better results in terms of adaptability, safety, and efficiency. However, improvements in efficiency are still possible showing that there are challenges to overcome. In specific, assistive technologies rely either- on strong actuation, or passive structures to augment users, or support users who have lost the ability to perform lost functions. At one end of the spectrum are powered devices with strong actuators, complex control systems and heavy power sources. At the other end are passive devices which are generally unpowered, light weight and rely on their mechanical behavior to enhance users support or performance. Ideally, assistive technologies should be able to achieve both systems characteristics while remaining lightweight, efficient, and adaptable. This can be achieved by using distinct mechanical mechanisms to harness gait energy towards enhancing human mobility, performance yet still reduce the cost of muscular expenditure. In specific, a passive exoskeleton exhibiting variable system stiffness is the proposed concept of achieving such characteristics in exoskeletons. Stable and flexible walking with minimal energy consumption of the human body is achieved by the compliance of human joints. Essentially, the ankle joint plays the biggest role as the stiffness of the ankle joint varies continuously while the stiffness of the knee and hip joints remain nearly constant during the gait cycle. With inspiration from the human leg biomechanics, this thesis presents a new compliant ankle exoskeleton to assist in human walking and reduce the biological demands of the calf muscle. The ankle exoskeleton is unpowered, lightweight and adaptable. A variable stiffness mechanism was developed and integrated onto the ankle exoskeleton to harness gait energy and compliment the human ankle biomechanical abilities. To validate the proposed exoskeleton design, the prototype was fabricated and preliminary experimental tests on healthy subjects were carried out. The device uses a variable stiffness mechanism, which is based on a slider with five levels of optimum stiffness to compliment the human ankle range of motion. The slider is triggered by a passive mechanical clutch, which controls spring engagement on the slider. By engaging different levels on the slider, the system produced five different levels of stiffness ranging from 150 N/m to 200 N/m, with a system torque of 200 Nm/kg to 400 Nm/kg being enough to assist the ankle, and resulting in a 42% decrease in soleus muscle activity and 36% decrease in gastrocnemius muscle activity. To our knowledge, this is the first ankle assistance exoskeleton, which is passive, and demonstrates variable stiffness while reducing the demands of the biological calf muscles for healthy users. In summary, the ankle exoskeleton presented herein offers a promising opportunity to adjust ankle compliance, and improve the robustness of walking by giving the user further adaptability.
Date of AwardMay 2020
Original languageAmerican English


  • Passive exoskeleton
  • variable stiffness
  • ankle assistance.

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