Design of a Passive Ankle Prosthesis with Energy Return That Increases with Increasing Walking Velocity
Date of Award
Master of Science (MS)
Patients who undergo a transtibial (below the knee) amputation are often met with a difficult decision: selection of a prosthesis. Limitations of currently available prostheses motivate work on a new solution, the EaSY Walk, a passive device that mimics two key aspects of the natural ankle: non-linear rotational stiffness through implementation of a stiffening flexure mechanism and rotational work output that varies as a function of walking velocity to propel the user forward. To achieve the latter, a strategy to convert the maximum available translational energy acquired from deflection along the leg into rotational energy about the ankle joint through coupling of these two degrees of freedom is used. This strategy utilizes maxima/minima of known ankle profiles to control timing of critical device functions as well as the quantity of energy input from leg deflection. In doing so, both consistent operation of the device and maximal energy output at a given walking velocity are theoretically obtained. Optimizing for both aforementioned ankle criteria, 25.1% of the work of the average natural ankle was achieved for 15 mm of leg deflection, less deflection than is exhibited by many shock absorbing pylon prostheses. After fabricating and testing the optimized design using a repeatable robot trajectory, the device was found to convert 26.6% of input translational work as rotational work, accounting for 63.1% of modeled rotational work. Through human subject testing, the device was found to function inconsistently due to the large impact loadings associated with human gait. In order to achieve proper functionality with human gait, design modifications to the energy storage and release devices are recommended.