A Novel Precision Measuring Parallel Mechanism for the Closed-Loop Control of a Biologically Inspired Lower Limb Exoskeleton

The knee joint of the human body involves both rotation and translation, while the magnitude of anterior-posterior translation during flexion/extension movement of the knee joint is very small compared with the length of the human lower limb. It is, therefore, desirable for an exoskeleton leg to have two degrees of freedom (DOFs) to accommodate the motion of the human knee joint, and for there to be a precision measuring method to obtain its trajectory. This paper presents a novel parallel mechanism which can be used as a precise measuring device to realize closed-loop control for a biologically inspired three-DOF lower limb exoskeleton (BLLE-3) for human gait rehabilitation. In this paper, the mechanical design and kinematics of the exoskeleton are described. Errors of exoskeleton motion are modeled and analyzed. Closed-loop control law is implemented to enable accurate trajectory following the motions of the exoskeleton. Simulations and experimental results are included to show the effectiveness of the new measuring and control method.