Advances in assistive devices that enable individuals with leg amputations to walk, e.g., robotic prosthesis, orthoses and exoskeletons, have been considerable in recent years. However, existing systems have trouble operating in dynamic, real-world environments that include unexpected disturbances. Thus, the goal of this CAREER project is to address the need for assistive robots that can naturally respond to disturbances by exploiting the underlying dynamics of human locomotion, i.e., by providing the device, in real time, with the same abilities to change joint stiffness and damping properties that humans unknowingly regulate through muscle activations. The key idea is to understand how the nervous system regulates leg joint stiffness and resistance throughout walking, and use this information for a new, bio-inspired blueprint for design and control of assistive robotics. This project will also provide insight into how the human body responds to disturbances. The results from this project will significantly improve quality of life and productivity for over a million lower-limb amputees in the U.S. alone. Throughout the project, the integration of research and education will have broad impact through the 1) promotion of STEM and disability awareness to K-12 students through targeted events that showcase the formal and "maker" sides of engineering, 2) implementation of interdisciplinary mechanisms for knowledge transfer between engineering students and clinicians, and 3) sponsorship and mentoring of student senior design projects that directly support this research through co-oversight with clinicians.
The principal investigator's long-term research goal is to advance a transformative approach to the design and control of assistive robots that embrace and exploit the underlying dynamics of human locomotion. Towards this goal, this project will measure the mechanical impedance of the knee joint during walking and use the information obtained to develop a new generation of bio-inspired control systems that will enable users of assistive robots to move more freely in dynamic environments that include unexpected disturbances. Studies will make use of assistive devices already developed or under development in the principal investigator's lab: a torque-controllable exoskeleton for the knee and an Open-Source Robotic Leg prosthesis (OSL, developed under an NSF award) that has powered knee-ankle joints. The Research Plan is organized under three aims. The FIRST AIM is to investigate techniques for measuring knee mechanical impedance using the torque-controllable exoskeleton, which will be quantified in able-bodied subjects during walking. Methodologies will be validated in a passive mechanical system of springs and masses for which impedances are known before involving human subjects. Data that can be used to determine mechanical impedance of the knee during locomotion will be recorded from human subjects wearing the knee exoskeleton while walking on a split-belt treadmill in which small perturbations (e.g. increased ramp angle) can be applied and response torques measured by the exoskeleton. Perturbations will be applied such that a snapshot of impedance can be obtained at each point during the gait cycle. Point impedances will be quantified by fitting inertia, stiffness and damping parameters to a second-order differential equation. The SECOND AIM is to derive governing equations for bio-inspired impedance control of the leg that will be implemented in an open-source library for control of the OSL. The results obtained under Aim 1 will be synthesized with preliminary work estimating the impedance of the ankle to derive a unified control law that enables simultaneous regulation of knee-ankle kinetics, kinematics and mechanical impedance. The governing equations will be translated into a software library that is able to command the OSL and enables real-time control of knee-ankle impedance, i.e., stiffness, damping and equilibrium position parameters will be altered as a function of gait phase and, thus, not predisposed to a single speed. The ability of the OSL to achieve the desired stiffness and dampening coefficients will be validated in a benchtop testing setup. The THIRD AIM is to understand the effect of the impedance-based control system in above-knee amputees using the OSL, tested during ambulation with/without disturbances. Subjects will be studied while ground walking at a self-selected pace with both the impedance-controlled system and each person's prescribed prosthesis. A subsequent study will investigate the effect of disturbances while subjects are walking on an instrumented treadmill. Kinetics, kinematics, metabolics and stability measures across conditions will be compared to better understand how the bio-inspired control approach affects amputee mobility and how these measures compare to able-body mechanics. Though several disturbances will be applied, this project will focus on understanding the effect of walking on inclines and declines.
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.
