The use of series compliance and variable transmission elements in the design of a powered knee prosthesis

Abstract

Compared to non-amputees, above knee amputees expend significantly more metabolic energy. This is a result of the passive nature of most knee prostheses, as the development of clinically successful powered knee prostheses has remained a challenge. The addition of powered elements, such as electric motors, allow prosthetic knees to more closely emulate natural knee biomechanics. However, the addition of powered elements presents a new challenge of creating energy efficient devices that do not require frequent charging or excessively large batteries. In this thesis, a general optimization routine was developed to simulate and evaluate the electrical economy of various actuator architectures. Advanced actuators utilizing variable transmissions with elastic elements were compared to direct drive actuators, series elastic actuators, and two novel mechanisms known as the continuously-variable series-elastic actuator (CV-SEA) and the clutchable series-elastic actuator (CSEA). The CV-SEA is similar to a traditional series-elastic actuator (SEA), but uses a controllable continuously-variable transmission (CVT) in between the series-elastic element and the motor. The CSEA included a low-power clutch in parallel with an electric motor within a traditional series-elastic actuator. The stiffness of the series elasticity was tuned to match the elastically conservative region of the knees torque-angle relationship during early stance phase knee flexion and extension. During this region of the gait cycle, the clutch was engaged and elastic energy was stored in the spring, thereby providing the reactionary torque at a substantially reduced electrical cost. The optimization routine showed that the electrical economy of knee prostheses can be greatly improved by implementing variable transmissions in series with elastic elements. The optimization routine also estimated that a CSEA knee prosthesis could provide an 83% reduction in electrical cost, when compared to an SEA knee prosthesis. Although the variable transmission actuators were predicted to be more electrically economical than the CSEA knee, their design complexity limits their current feasibility in a knee prosthesis. Thus, a fully autonomous knee prosthesis utilizing the CSEA was designed, developed and tested. The CSEA Knee was actuated with a brushless electric motor; ballscrew transmission and cable drive as well as commercial electrical components. The knee was lighter than the 8th percentile and shorter than the 1st percentile male shank segment. The CSEA Knee was tested in a unilateral above knee amputee walking at 1.3 m/s. During walking, the CSEA Knee provided biomechanically-accurate torque-angle behavior, agreeing within 17% of the net work and 73% of the stance flexion angle produced by the biological knee during locomotion. Additionally, the process of locomotion reduced the net electrical energy consumed of the CSEA Knee. The knees motor generated 1.8 J/stride, while the electronics consumed 5.4 J/Stride. Thus the net energy consumption was 3.6 J/stride, an order of magnitude less electrical energy consumption than previously published powered knee prostheses. Future work will focus on a custom, power-optimized embedded system and the expansion of the CSEA architecture to other biomechanically relevant joints for bionic prosthesis development.