Review and Assessment of Walking Assist Exoskeleton Knee Joints

Walking Assist Exoskeletons (WAEs) are wearable devices that are used to support an individual's mobility. With a wide range of potential users including the elderly and others with mobility challenges, demand for WAE devices is rapidly increasing.Whilst WAE technologies have recently progressed significantly, their mechanical designs do not adequately reflect the complexity of human joint articulations; most notably, the knee, which plays a significant role in human mobility. The human knee joint, a 3-dimensional joint, is often over-simplified during WAE design processes. Typically modelled as a single degree of freedom joint, kinematic and kinetic incompatibilities yield at the user-device interface. This, in turn, restricts the natural motion of the user, causes discomfort, and reduces the efficiency of the device.Polycentric joints are known to reduce the inherent kinematic mismatch between exoskeletons and their users, creating a more comfortable interaction. Researchers have begun to implement polycentric exoskeleton knee joint designs, as they take into account the complexity of the biological joint. Some devices follow a pre-defined path of rotation based on average biological joint geometries, whereas others decouple translations and rotations to allow for passive adjustment to the user.This paper presents a review of current WAEs that incorporate powered polycentric knee joints. Subsequently, a theoretical comparison between conventional single-axis joints, polycentric joints that follow pre-defined paths of rotation, and polycentric joints that allow passive translation is also provided.