A Study On The Balance Index Of Paraplegic Human Exoskeleton System

With the development of lower exoskeleton for walking assistance,exoskeletons are able to assist paraplegia patients with daily activities.But exoskeletons are still not reliable to prevent patients from falling down which may cause secondary damage.Since maintaining balance is becoming an important part of follow-up studies,exoskeleton researchers should consider how to quantify the balance of human-exoskeleton system.This paper aims to establish a stability index of the human-exoskeleton system,mainly includes the following four parts:Based on the studies of previous stability criterions which are widely applied to biped robot or quadruped robot are only partially applicable to human-exoskeleton system.By studying the strengths and weaknesses of the above criterions applied in the paraplegic human-exoskeleton system,an index based on dynamic center of mass(CoM)is proposed.The CoM estimation of human-exoskeleton system is firstly calculated by definition.Parameters of human segments are estimated through the regression equation proposed in anthropometric table while that of exoskeleton is obtained according to the mechanical parameters of actual system.But the coordinate information of segments can merely collected in experimental setting.Hence,the formula is optimized by the coordinate transfer principle and transformed into a serial chain model.The chain model of human and exoskeleton are obtained respectively,and the coupling correction of the bind parts of human-exoskeleton system is added to get the couple serial chain model which can estimate CoM of the actual system in real time from joint angles.The stability index of human-exoskeleton based on dynamic CoM is proposed.Firstly,the human-exoskeleton system is simplified to a pyramid,with CoM as vertex and the convex hull of supporting points as the bottom.Secondly,the stability boundary is determined by the constraint parameters such as range of motion and moment of joints.Then,according to different supporting states,the gait of human-exoskeleton system is divided and corresponding stability index are calculated.Finally,the indexes are spliced to a complete walking period through states transition.Finally,an experimental scheme is put forward to verify the above theoretical methods.Through the CoM estimation experiment,the coupled serial chain model of human-exoskeleton system is realized.Compared with the kinematics method the prediction error of the coupled serial chain model is all within 2cm which means the addition of the coupling term improves the accuracy of the model to a greater extent.Through dynamic CoM stability index experiments,the pyramid model is established,and stability index corresponds to base of support are calculated.The validity of the index is verified by applying varying degrees of perturbations to the system.The results shows that dynamic CoM stability index can accurately reflect the balance degree of the humanexoskeleton system.Finally,the stability switch of the whole gait cycle obtained.