Design and Implications of a Robotic Prosthetic Leg with Low-Impedance Actuation
Recent developments in the field of powered prostheses have produced several devices that implement a wide variety of actuation schemes, each presenting specific benefits and limitations to prosthetic design and acceptance of robotic prostheses. The work of this dissertation encompasses research focused on the design and implications of an actuation scheme new to robotic prosthetic leg design; low-impedance actuation. Although this style of actuation has shown promise in legged robots, it has potential benefits specifically relating to powered prosthetic legs as well. Such benefits include free-swinging knee motion, compliance with the ground, negligible unmodeled actuator dynamics, less acoustic noise, and power regeneration. To investigate these potential benefits a custom transfemoral (knee-ankle) robotic prosthetic leg with high-torque, low-impedance actuators was created. Preliminary benchtop testing established that both joints can be backdriven by small torques (~1-3 Nm), confirming the small reflected inertia and low impedance. The reduced joint-level impedance was achieved while maintaining the ability to produce very large torque (~180 Nm). Impedance control tests prove that the intrinsic impedance and unmodeled dynamics of the actuator are sufficiently small to control joint impedance without torque feedback or lengthy tuning trials. The negligible effect of the actuator’s unmodeled dynamics is further demonstrated through the direct implementation of biological impedances in amputee walking experiments. The regenerative abilities, low friction, and small reflected inertia of the presented actuators also offer practical benefits through reduced power consumption and acoustic noise compared to state-of-art powered legs. Although these benefits are mainly related to the physical device, this dissertation also extends the investigation into potential benefits to the wearer. Additional walking experiments were conducted with three amputee subjects to study how the powered prosthetic leg with low-impedance actuators affected gait compensations, specifically at the residual hip. A walking controller was implemented on the powered prosthesis to exploit the low-impedance actuators’ power density during push-off, impedance control abilities in stance, and trajectory tracking abilities to ensure toe-clearance during swing. Results show that when large push-off power is provided, less work is demanded from the residual hip to progress the limb forward. Moreover, all subjects displayed increased step length and propulsive impulses for the prosthetic side, compared to their passive prostheses. These results reduce demand on the hip to accelerate the body forward and display the ability to improve gait symmetries. Hip circumduction improved for subjects who had previously exhibited this compensation on their passive prosthesis. The improvements made to these compensations lead to reduced residual hip power and work, which can reduce fatigue and overuse injuries.