Design and Testing of an Inerter to Examine Acceptable Reflected Inertia in Knee Prostheses

Abstract

Leg dynamics approximate that of a passive double pendulum in ballistic walking, where the leg’s trajectory is governed by inertia and gravity. Passive knee prostheses are able to replicate healthy knee ballistic walking well, but they cannot provide positive power for activities like sit-to-stand or stair ascent. Powered knees can provide positive power but have degraded passive dynamics due to the inertia of a motor, and it is wholly infeasible to generate stance-phase knee torques in a package acceptable to users without a sizeable transmission ratio to boost motor torque. While a powered prosthesis can produce correct kinematics, the poor passive dynamics cause users to prefer passive prostheses despite their limitations. Currently there is no analysis of how low reflected inertia must be to avoid disrupting ballistic gait. This thesis encompasses the design of an experimental device, called an inerter, which can apply variable amounts of reflected inertia to a user’s passive knee prosthesis. This is accomplished through a chain drive system, where inertia is added near the user’s hip, so as to minimize the effects of increased mass, and reflected to an output shaft aligned with the knee center of the user’s daily use prosthesis. This inerter is first validated in a benchtop experiment and then tested on participants’ daily use prostheses. Qualitative data regarding the participants’ perceptions of various reflected inertia conditions is recorded along with biomechanical marker data. The results of these participant experiments can inform the extent to which other design sacrifices should be made to reduce reflected inertia.