The Realisation of Virtual Fluid Rendering on a Treadmill-Based Lower Limb Exoskeleton with Lateral Degrees of Freedom

Abstract

Conventional post-stroke gait rehabilitation is labour-intensive, often requiring multiple therapists for physical assistance. Both robotic and aquatic therapies have been proposed to address this: robots can reduce therapist workload and provide adaptable feedback, while aquatic environments offer apparent weight reduction, improved balance, and muscle strengthening. Virtual aquatic therapy combines both approaches by haptically rendering the fluid dynamics of virtual water. In addition to the sagittal degrees of freedom offered by existing implementations, the treadmill-based exoskeleton used in this study enables hip abduction and lateral pelvis movements. A physics-based fluid rendering model is developed to compute drag and buoyancy forces acting on virtual legs moving through fluids. To match the real-world setup, the model simulates treadmill walking rather than the conventional free walking and includes the additional axes of motion. Simulations confirm that the model's outputs are consistent with the literature and scale properly with movement speed, submersion depth and the type of fluid being rendered. Through a proof-of-concept experiment, the successful integration into the exoskeleton’s control framework is validated, demonstrating the feasibility of rendering diverse fluid environments with this system.