Generalized Framework for Cable-Driven Lower Limb Rehabilitation Exoskeletons Modelling

What is it about?

Inspired by the benefits of cable-driven exoskeletons over link-based exoskeletons such as minimal exertion of unnecessary weight, inertia, and inertial vibration on the user limb, facilitating remote location of the actuator, lightweight design, our current work focuses on the modeling of the lower limb for the cable-driven exoskeleton. Since the majority of the distortion in the gait cycle is identified in the sagittal plane, the current model of the C-LREX (Cable driven Lower limb Rehabilitation Exoskeleton) intends to provide support to the user in sagittal plane motion only. In literature, the majority of the models were designed for a specific configuration attached with specific frames, which offered less flexibility to adapt to different routing and configurations of exoskeletons. Intrigued by frame/configuration-specific models, we have proposed a generalized framework that can accommodate a variety of routing and configurations of the cable-driven device with slight modification. The lower limb has been modeled as two link pendulums (thigh represents the first link and shank and foot combined represent the second link). C-LREX is expected mainly to support the user during the swing phase of the motion and thus we assumed that the foot is attached to the shank at a certain angle (the rotation of the foot during the swing is ignored). PD controller is employed to track the healthy reference trajectory and result in desired joint moments. The tension(s) in the cables are distributed via a quadratic programming (QP) based hybrid cost function optimization. The user's voluntary contribution is currently assumed to be passive elastic joint moments.

Featured Image

Photo by Natalia Dziubek on Unsplash

Photo by Natalia Dziubek on Unsplash

Why is it important?

In literature, few cable-driven lower limb exoskeletons have been developed, mostly for a fixed configuration. Nevertheless, the various possible configurations and their analysis have not been studied in past. Despite the benefits of cable-driven devices, the number of cables and the routing to achieve the desired performance are not straightforward. Similar performances can be obtained through a variety of cable combinations. To analyze the performances and requirements of various cable routings and configuration-based cable-driven exoskeleton models, we purpose a generalized framework in our recent work. The framework can be exploited to identify suitable configuration and routing parameters based on prescribed constraints. The configuration and routing parameters then can be used to design the actual system. The model can provide cable tension(s), tracked trajectory, motor power/speed required, component forces acting on the joints due to cables, etc as output which will serve as the foundation for the prototype design. Instead of prototyping various possible configurations of C-LREX, the framework can be exploited to identify the best among the candidates for actual prototyping. Furthermore, the generalized framework can also be exploited to obtain the optimal routing parameters for given cable configurations via optimization.

Perspectives