Abstract
The CYBERLEGs Beta-Prosthesis is an active transfemoral prosthesis that can provide the full torque required for reproducing average level ground walking at both the knee and ankle in the sagittal plane. The prosthesis attempts to produce a natural level ground walking gait that approximates the joint torques and kinematics of a non-amputee while maintaining passively compliant joints, the stiffnesses of which were derived from biological quasi-stiffness measurements. The ankle of the prosthesis consists of a series elastic actuator with a parallel spring and the knee is composed of three different systems that must compliment each other to generate the correct joint behavior: a series elastic actuator, a lockable parallel spring and an energy transfer mechanism. Bench testing of this new prosthesis was completed and demonstrated that the device was able to create the expected torque-angle characteristics for a normal walker under ideal conditions. The experimental trials with four amputees walking on a treadmill to validate the behavior of the prosthesis proved that although the prosthesis could be controlled in a way that allowed all subjects to walk, the accurate timing and kinematic requirements of the output of the device limited the efficacy of using springs with quasi-static stiffnesses. Modification of the control and stiffness of the series springs could provide better performance in future work.
Highlights
Current transfemoral prostheses are most often passive, modular systems that cannot generate joint work
The design began as a passive knee/active ankle system in the AlphaProsthesis (Flynn et al, 2015) and had many new concepts added, an entirely new knee system that allowed for net positive work actuation at torques higher than normal walking as well as keeping the passive elements that were demonstrated to work in the Alpha-Prosthesis
We have created a new, active, combined ankle-knee prosthetic system which achieves as much as possible with a passive approach, using springs that were chosen to match the biological quasi-stiffness of normal gait. These springs are locked and unlocked during the gait cycle and combined with an energy harvesting system to passively provide the majority of the required torque angle characteristics during normal walking, while maintaining versatility by providing active actuation
Summary
Current transfemoral prostheses are most often passive, modular systems that cannot generate joint work. Providing positive work is an important aspect of the biological joint and there are new robotic designs that are capable of delivering it. Devices such as the OttoBock emPOWER (previously iWalk, BionX, BiOM Au and Herr, 2008), or the Össur/Springactive Odyssey (Hitt et al, 2008) ankles or the Össur Power Knee are available, or will soon be available, as commercial devices. There are many reasons for believing that active ankle and knee propulsion provides benefit such as tests which have shown a reduction in loading of the unaffected leg using a powered ankle (Grabowski and D’Andrea, 2013), reduction of the metabolic energy consumption of a transtibial amputee to the level of a non-amputee while using robotic ankles (Herr and Grabowski, 2012; Caputo and Collins, 2014), and simulations showing reductions in metabolic cost below normal human walking (Handford and Srinivasan, 2016)
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