A six-degrees of freedom dynamic ankle simulator

María Ruiz Rincón

doi:10.17918/pz15-nk44

A clear understanding of foot locomotion and the function of each of its bones, muscles and ligaments, would allow for a fast identification of pathologies as well as the improvement of surgical procedures. So far, many different methods have been used in the hopes that they will provide thorough information on the mechanics and physiology of the foot, while also stay non-invasive and ethical for the living subjects. However, these approaches come with limitations, such as the impossibility to determine individual bone kinematics. It is due to this that researchers depend on in vitro experiments of the foot. These highly invasive studies on cadavers provide information on the kinetic interaction between bones, including particularly shielded ones like the talus. Their application in reproducible and repetitive testing is also welcome for implant evaluation or any other surgical procedure. Different ankle simulators have been used for in vitro investigations in the past aiming to replicate foot dynamic functional activities. However, an accurate reproduction of gait would require a complicated combination of applied high-speed motions and large loadings. Due to this, some simplifications and assumptions are often needed during the design of the simulator. Current simulators do provide six-degree-of-freedom controlled movement. However, these are highly coupled, which leads to control difficulties and accumulative inaccuracies. In addition, the existing systems are slow and expensive. For this study, it was then considered that a new more accurate device was required to address these deficiencies. This led to the development of an innovative structure of the linkage with decoupled degrees of freedom, which provides a significant advantage over other simulators. Therefore, its control is much simpler and quick, it requires minimum online computation and its accuracy increases since error is not propagated. This is also a much cheaper solution, capable of delivering high forces. The design process of the linkage included dynamic simulations that were performed iteratively to ensure smooth performance free of collisions. These analyses showed the components are able to rotate perfectly, having enough available space to meet the ranges of motion required. Additionally, extremely high loads are applied by the linear actuators connected to the simulator, which may lead to unwelcome deformations. Therefore, the design process also included iterative FEM analysis to evaluate stresses and deformations, as well as to modify the design in order to produce proper mechanical loading on the elements of the structure. While some parts of the device proved to have minimal stresses, other parts showed a dangerous amount of deformation. This led to a change in their design which greatly improved their ability to withstand the required loads.

A six-degrees of freedom dynamic ankle simulator

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A six-degrees of freedom dynamic ankle simulator

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