Design And Optimization Of Lower Limb Exoskeleton With Variable Axis Knee Joint Mechanism

The assisted lower extremity exoskeleton is a humanoid robot that can be worn in healthy people and has the ability to assist the body's weight-bearing,which can improve the body's weight-bearing capacity and increase the duration of continuous exercise.The knee joint is the most complicated joint of the human body,and the knee joint mechanism is also an important component of the lower extremity exoskeleton.The form of movement of the human knee joint is very complicated.The instantaneous movement of the thighs and calves is not fixed.However,the existing knee joint mechanisms of the lower extremity exoskeleton are mostly reduced to a rotating pair,resulting in poor coordination between the exoskeleton and the human knee.Therefore,this paper designs a variable axis knee joint mechanism based on an undriven lower extremity exoskeleton to achieve the variable axis function of the exoskeleton knee joint and improve human-machine coordination.Firstly,by analyzing the movement characteristics of the lower limbs and knee joints of the human body,the study of the exoskeleton and knee joint mechanism is mainly based on the sagittal plane.Through the analysis of the movement characteristics of the knee joint,the configuration of the variable-axis knee joint was determined,and the overall configuration was determined by combining the original structure of the exoskeleton.The initial verification of the knee joint by the calculation of the degree of freedom of the unilateral exoskeleton and the graphical method ensure the correctness of the configuration.Secondly,the kinematics modeling of the lower extremity exoskeleton with the variable axis knee joint was carried out.The instantaneous trajectory equation of the variable axis knee joint in the absolute coordinate system and the local coordinate system was calculated,and the coordinates were transformed into the coordinate system,It laid the foundation for pole length optimization.The least-squares fitting of the theoretical trajectory coordinates of the instantaneous center of the knee of the human body is performed,and the fitted trajectory curve of the instantaneous center of the knee joint is obtained,which can provide a reference for the design of knee joint mechanism and prosthetic limb.Then,the degree of coincidence between the knee joint mechanism and the fitting trajectory of the human knee joint is the objective function.The length of each rod and thigh rod of the knee joint is taken as the design variable,and the standing stability condition and the bionic condition are used as constraints.The optimized rod length parameters were verified by MATLAB calculation,and compared with the human knee joint fitting trajectory to verify the correctness of the rod length parameters.Finally,the motion simulation was performed by ADAMS software to verify the instantaneous trajectory of the knee joint.And a new exoskeleton prototype was machined,and a motion capture experiment was performed to obtain the instantaneous trajectory of the variable-axis knee joint mechanism to verify the correctness of the rod length parameters of the knee joint mechanism.The results show that the optimized variable length knee joint length parameter can make the knee joint mechanism and the human knee joint's instantaneous trajectory trend basically match,and the relative error is less than 10%,which can effectively improve the man-machine coordination.