Human-machine Coupling Design And Performance Evaluation Of Lower Limb Exoskeleton For Load-carrying Assistance

Load-carrying is a common activity for daily life.However,the ability of human musculoskeletal system to resist external force is limited.Long-term load-carrying causes muscle fatigue and shoulder discomfort,and increases the internal forces on lower limb joints.As a wearable assistive device,the lower limb exoskeleton for load-carrying assistance enhances the ability of load-carrying by transferring the back load to the ground and assisting joint movements.Active exoskeletons have the disadvantages of complex control systems,high cost and short endurance time,which is difficult to meet the requirements of long-time use.Therefore,it is of great significance to design a humanmachine coupled passive exoskeleton for load-carrying and research the performance evaluation scientifically.In order to analyze the anatomical characteristics and range of motion of the lower limb joints,the human anatomy and kinematics were studied.The human gait and joint movement during walking with no load and 30 kg load were analyzed based on Open Sim and Vicon motion capture systems.Based on the principle of force transmission and pressure dispersion,a passive exoskeleton with 14 degrees of freedom was proposed.Modular design method was used to design the back structure,the knee structure,and the ankle structure.In order to analyze the assistant efficiency of the passive exoskeleton,the dynamics model of human-machine system was established,and the force transmission efficiency of exoskeleton during quiet standing(64%～100%),and the gravity ratio of shoulder pressure to load weight(20%～47%)were calculated.To evaluate the performance of the passive exoskeleton,the contact pressure on human shoulder,the axial force on ankle joint of the exoskeleton and the energy consumption were conducted.Contact pressure on human shoulder showed that the exoskeleton reduced the pressure on shoulder by,on average,46.56% and 34.57% during standing and walking,respectively.Axial force on ankle joint of the exoskeleton showed that the exoskeleton transmitted,on average,56.49% and 15.89% of the load gravity to the ground during standing and walking,respectively.Energy consumption derived that the exoskeleton reduced the metabolism by,on average,8.92% during standing,while slightly increased the metabolism by,on average,9.14% during dynamic walking.The experiment results show that the exoskeleton has good load-bearing support effect in static conditions,while during dynamic walking,the exoskeleton has low force transmission efficiency and slightly increased the human energy consumption.This paper designed a modular passive exoskeleton based on the principle of force transmission and force redistribution.The theory result of force transmission efficiency and the assist efficiency of human shoulder were calculated through the dynamic model of human-machine system.The human energy consumption,the contact pressure on human shoulder and the axial force on exoskeleton ankle joint were measured to evaluate the assist efficiency of passive exoskeleton.The theoretical results and the experimental results show that,when carrying load with passive exoskeleton,part of the load is transferred to the ground,and part of the interaction force of human shoulder and waist to balance the torque caused by backpack load is distributed to human legs.Results verify the working principle of force transmission and force redistribution of passive exoskeleton,and provide a new reference of the design and evaluation of passive exoskeleton.