Collaborative Research: Integrating Optimal Function and Compliant Mechanisms for Ubiquitous Lower-Limb Powered Prostheses

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The majority of lower-limb prostheses are passive. They can dissipate and store mechanical power but cannot generate positive net power. The lack of power generation limits movements that require the user to move against gravity, such as going upstairs or transitioning from sitting to standing. This lack of power may induce uneven loads in the body, which can increase the likelihood of chronic back pain and increase the effort to walk. Active prostheses have the potential to overcome these fundamental challenges. However, commercially available powered prostheses are heavier, noisier, more expensive, and generally less accessible than unpowered versions. The overall goal of this project is to reimagine existing rigid prosthetic components as compliant mechanisms that reduce mass, energy consumption, audible noise, and part count of powered prostheses. Muscles take advantage of the elasticity of tendons (in series with the muscle) and ligaments (in parallel with the muscle) to efficiently transfer power from the muscle to the joints. This project will provide a new understanding of how to engineer elastic components in parallel with electric motors as engineered ligaments to make powered prostheses more attractive and accessible.The performance and behavior of compliant mechanisms depend on three fundamental factors: 1) material properties, 2) geometry, and 3) load-deflection response. This project will develop new knowledge to design the load-deflection response and geometry of compliant mechanisms that connect in parallel with electric motors to reduce motor torque. This reduction implies lighter, more energy-efficient, and quieter prostheses, as it requires lower reduction ratios, fewer gears meshing, lighter motors, and less heat dissipation. This collaborative project between the University of Notre Dame and Brigham Young University will establish two scientific contributions: 1) a robust convex optimization framework to design the load-deflection response of a parallel spring that guarantees motor torque reduction in multiple locomotion activities despite parametric uncertainty (e.g., user mass, walking speed); and 2) a design framework for compliant mechanisms with optimal load-deflection profiles to reimagine existing rigid components and implement the benefits of parallel compliance without a tradeoff in terms of mechanical complexity or extra components. The application of these innovations will result in an Open-Source Compliant Ankle, with open-source designs available online that complement the existing NSF-funded Open-Source Leg. The research team will collaborate with non-profit organization 2ft Prosthetics and local Amputee Support Groups to incorporate the feedback from prosthetic users, manufacturers, and clinicians into new designs. The outcomes of this research will include the organization of a conference workshop, new content in a graduate-level class on Wearable Robotics, and a 3-week summer program for local middle schoolers interested in STEM education.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.