Design, Analysis, and Validation of a Passive Parallel Continuum Ankle Exoskeleton for Support and Walking Assistance

In this paper, we propose a novel passive parallel continuum ankle exoskeleton that canprovide assistive torque during ankle plantarflexion. Due to theflexible branches arrangedin compliance with ankle motion and shape, the compact design can also offer some verticalsupport. The proposed parallel mechanism consists of two types of branches. Thefirst typeis a pre-bentflexible rod, mainly used to apply assistive force/torque during ankle plantarflexion. The second type of branch consists of a bounded sphere joint,flexible rod, andbounded sphere joint (BFB), which is mainly used for support. We formulate the kinetostaticmodel of the BFB branch as a series of parallelizable unconstrained optimization problemsto ensure efficient solvability. After that, we derive the kinetostatic model of the proposedmechanism. After calibration, the wrench error of the kinetostatic model is 9.07%. Simula-tion analysis based on the calibrated model shows that the designed mechanism has highsupporting stiffness and low rotational stiffness. The assistive torque caused by the nonlin-ear rotational stiffness in the sagittal plane is similar to that of passive clutch-like mecha-nisms. These properties can still be maintained when the joint center changes within a smallrange. Besides, a walking experiment was conducted, and the results show that the pro-posed design can reduce gastrocnemius activity.