Design a passive mechanism acting on the frontal DoF of an existing ankle-foot prosthesis

The aim of this work is to improve the quality of life of lower limb amputees, their main frustration being their lack of mobility. Many types of ankle-foot prostheses are already on the market, but most of them are limited by the fact that they have only one degree of freedom (DoF) that acts in the sagittal plane. Indeed, it is the most useful rotation for flat ground walking. In the last two decades, there have been many attempts to add a second DoF enabling a wider range of motion. It allows for rotation in the frontal plane, which comes into play when walking on uneven ground, avoiding obstacles and bending to the side, among others. In 2019, an active prosthesis with one DoF, called ELSA, was presented in the context of a PhD at UCLouvain. Last year, a thesis proposed a module that can be fitted to ELSA in order to add a second DoF. It is entirely passive and takes advantage of the user weight. Following on from those works, a new solution is proposed, also passive and weight activated, but more suitable and universal, i.e. able to be attached to any existing prosthesis. To achieve this, a new iteration of each design step was carried out. The outcome is a mechanism that swaps between two stiffnesses during the gait cycle thanks to the user's weight. The switching is done by a locking mechanism, i.e. a pawl that locks a wheel with several hooks. An epicyclic gear allows the hook wheel to rotate faster than the ankle. Ropes are used to mimic the mechanical impedance of the ankle. This was then implemented on a computer-aided design (CAD) software, taking into account possible fatigue effects. Finally, numerical simulations using finite element methods were performed on some parts considered as critical, in order to quantify their strength as well as their deformation.