A One-Degree-of-Freedom Spherical Wrist for the Modelling of Passive Motion of the Human Ankle Joint

Abstract

The passive motion of the human ankle complex joint, i.e. the motion in virtually unloaded condition, has been recently modelled by means of a type 5-5 equivalent one-degreeof- freedom (DOF) fully parallel mechanism (FPM). The 5-5 FPM can replicate the complicated joint passive motion very well while at the same time it outlines the basic role of the main anatomical structures such as bones and ligaments. However, more simple mechanisms, although less accurate to simulate the passive motion, can be of great utility and of practical interest, in particular for the design of prostheses and orthoses. In this contest this paper shows that a one-DOF fully parallel spherical wrist mechanism (one-DOF SWM), which is more simple than the 5-5 FPM, can replicate the ankle passive motion fairly well. The one-DOF SWM mechanism still retains a strong connection with some of the main anatomical structures such as the bone shapes and the principal ligaments (TiCaL and CaFiL). With respect to previous papers still proposing a one-DOF SWM as an equivalent mechanism, this paper presents a novel iterative synthesis procedure which makes it possible to define an optimal geometry of the one-DOF SWM which replicates the experimental data much better