| Flexible lower limb exoskeletons can provide strength assistance to the wearer during walking,running,weight-bearing,carrying,and other movements,to share the load on the hip and knee joints,thereby reducing metabolic expenditure caused by exercise and improving the body’s long-term exercise ability.For flexible lower limb assist exoskeletons with a force transmission form of flexible Bowden cable and fabric,due to friction and elastic deformation during the transmission of force between the Bowden cable and fabric,the output force/position of the driving system will be lost and delayed.In response to these issues,this article conducts research on the precise force transmission control of the flexible Bowden cablefabric force transmission system through theoretical modeling analysis and the construction of an experimental platform.Firstly,based on the biomechanical analysis of the human lower limbs,the motion data and plantar force data of the human body during normal walking were tested using a 3D motion capture system and a force testing platform system.Then,Open Sim software was used to process the lower limb motion angle and plantar force data to obtain the motion angle and joint torque of the lower limb joints of the subjects,providing a foundation for the study of joint angle and assistance curve in the following text.Secondly,Secondly,in response to the friction force in the Bowden cable system,this article adopts the "static + Coulomb" friction model to establish a static force transfer model and analyze the characteristics of force transfer.An experimental platform was built to test the influence of the friction coefficient,bending radius,bending angle,and motion speed of the Bowden cable on the force transmission characteristics.After designing friction compensation and linear active disturbance rejection control algorithms,the maximum relative error of force tracking can be reduced from 43.5% to 8.17%,verifying the effectiveness of the proposed compensation control algorithm.Then,in response to the friction phenomenon of the fabric in contact with the cylinder,the micro element analysis method was used to model the mechanics of the fabric and establish an experimental platform for force transfer characteristics.Then,the two key influencing factors,the surrounding angle and different materials,were analyzed,and the friction coefficient was obtained through experiments.Simulation and experimental results were compared,and the tensile characteristic curve of the fabric was tested.A force compensation control algorithm without sensing feedback at the end was proposed,and the relative error of force tracking was reduced from38.66% to 7.92%,verifying that the compensation control algorithm can effectively reduce force tracking error.Finally,in order to achieve more coordinated assistance and consider individual kinematics differences,ensure the validation of control effectiveness and consider the safety and physiological characteristics of human testing.Conduct research on human knee joint motion and assist curve,and validate it by building an experimental platform to simulate knee joint motion control.Set the DC brushless motor to control the knee joint to simulate walking gait,add the friction compensation of Bowden cable and fabric,adopt the impedance control strategy based on the inner loop PI position control,and combine the stiffness of the Bowden cable fabric force transmission system and the change of individual kinematics as the feedforward model,and build a Simulink simulation platform for verification.Through comparative experiments with LADRC control,it was found that this algorithm can effectively reduce the tracking error of the assist curve,verifying that this control strategy can significantly improve the tracking accuracy and compliance of the assist.Through the research on the kinematics and dynamics of human lower limbs,Bowden cable force transmission and fabric force transmission,this paper explores the friction loss,time delay and other phenomena in the process of force transmission of Bowden cable and fabric,analyzes the changes in the stiffness of the force transmission system and human kinematics,and solves the key technology of the precision assistance and compliance of the Bowden cable fabric force transmission system,The proposed impedance control algorithm based on inner loop PI position control has been verified through an experimental platform to have good assist tracking accuracy and compliance. |
