Principle design of an energy-efficient transfemoral prosthesis
Ramazan Unal, promovendus, UT
In both robotics and biomechanics, the research in the design of lower limb prostheses is of great interest due to the crucial impact in the outcome to human life. However, energy efficiency and mobility of the lower limb prosthesis are still open issues that should be deeply addressed.
In this work, the main purpose is to propose the design principle for an energy efficient transfemoral prosthesis. Starting from the energetic analysis of the healthy human gait by David Winter, it is possible to highlight three main phases:
- Stance: the knee absorbs a certain amount of energy and employs it. In the meantime, the ankle joint absorbs energy due to the weight bearing.
- Pre-swing: the ankle generates the main power (80 percent of the overall generation) for push-off and, consequently, the knee absorbs about 25 percent of the energy due to push-off.
- Swing: the knee absorbs energy during the early and the late phase, while the power in the ankle joint is negligible.
These three phases are the foundation of the design of a prosthesis that can capture the idea of energetically coupling the ankle and the knee through suitable springs. In the swing phase, the energy transferred to the knee joint due to push-off and in the beginning of the swing and the potential energy of the lower leg are stored by changing the spring configuration and, in particular, by changing the spring attachment point on the foot to the front. This operation is realized ideally without using energy, since the length of the spring is not changing. During the swing period until heel strike, the spring is loaded and right after that, this load is locked and the attachment point is changed to the heel, again ideally without using energy. Therefore, the swing absorption part of the energy is ready for the push-off, ideally without any dissipation. In the stance phase, the energy due to weight bearing during roll-over is stored. During the stance period the torque is applied to the ankle in order to bear the weight of the body. Since this brake torque is provided by a spring, a considerable amount of energy is stored so to be delivered with the energy stored at the swing storage part for the push-off.