Stiffness Modulation in an Elastic Articulated-Cable Leg-Orthosis Emulator: Theory and Experiment

Abstract

There has been an increasing interest in cable-driven robotic rehabilitation systems with adjustable stiffness to provide safe and natural interactions for physical therapy. In this paper, we investigate the effectiveness of various stiffness modulation schema and alternate attachment configurations within a scaled planar elastic articulated-cable leg-orthosis emulator for gait training. Elasticity, incorporated by series-elastic springs or adjustable stiffness modules connected to nonflexible cables, can add substantial robustness during forceful interactions with uncertain environments. Other benefits include force-sensor-free tension control, i.e., without using force sensors connected to cables and better overall tension distribution. However, elasticity also degrades the accuracy of positioning and makes the system more disposed to external disturbances. Hence, we examine the performance of actively modulating the effective stiffness to smooth out any perturbations to the scaled normative rehabilitative motion patterns and achieve natural gaits (by simulation and experiment). Stiffness modulation can now be achieved by varying system configuration, antagonistic cable tension, or joint stiffness, and we examine the benefits of each. Finally, we also comparatively evaluate the functional performance of alternate attachment configurations ankle–cable configuration, articulated-cable configuration by way of simulation and experiments.