Research On Control Strategy Of Pneumatic Muscle Flexible Joint Of Lower Limb Exoskeleton Robot

In 2022,China issued a specific outline of the " 14 th Five-Year Plan," which pointed out that wearable pneumatic muscle exoskeleton robot technology is one of the "transformative manufacturing science engineering " contents of the major national needs in the future.With the goal of realizing intelligent,super-sensitive bionic and integrated design,it has a wide range of needs in many fields such as helping the elderly and the disabled,rehabilitation medicine,emergency rescue,industrial assembly and social services.Therefore,it is of great significance to conduct in-depth and systematic research on pneumatic muscle exoskeleton robots in line with national strategies.The key problem of the design of the pneumatic muscle exoskeleton robot is the design and control of the robot joint.In this paper,a new type of magnetorheological variable damping flexible joint of the antagonistic pneumatic muscle of the lower extremity exoskeleton robot is designed.The control strategy of the pneumatic muscle flexible joint is the main research content,which solves the strong nonlinear relationship,high frequency chattering and hysteresis characteristics of the pneumatic muscle flexible joint.It meets the requirements of flexibility,human-machine interaction and safety in the process of using the lower extremity exoskeleton users,and provides a new control idea for the flexible joint of the lower extremity exoskeleton robot.The main research work includes :(1)The magnetorheological variable damping flexible joint of antagonistic pneumatic muscle of lower extremity exoskeleton robot is designed.In order to solve the defects of large swing,weak flexibility,difficult safety guarantee and inaccurate existing model of pneumatic muscle flexible joint.The test experiment of air pressure and output tension is designed and the accurate mathematical model of pneumatic muscle is obtained by fitting the experimental data.The magnetorheological damper is used to adjust the joint damping,and the pneumatic muscle is used to pull the synchronous belt wheel to drive the joint movement.The design of magnetorheological variable damping flexible joint of pneumatic muscle is completed,and the joint dynamics equation and space state equation are derived,which lays a foundation for the design of control system.(2)The STC-HOSMO inner loop trajectory(position)tracking controller of the pneumatic muscle flexible joint is established.Aiming at the problems of strong nonlinear relationship,high frequency chattering and large hysteresis of pneumatic muscle flexible joint.Based on the sliding mode control algorithm,the second-order system super-twisting algorithm with a relative degree of 1 is used to reduce the sliding mode chattering phenomenon.The high-order sliding mode observer is used to observe and estimate the state variables of the sliding manifold,so that the super-twisting algorithm can be applied to the second-order pneumatic muscle flexible joint system with a relative degree of 2.The STC-HOSMO inner loop trajectory(position)tracking controller of the pneumatic muscle flexible joint is established,and the stability of the controller is proved.(3)The variable parameter admittance outer loop(force)controller of the pneumatic muscle flexible joint is constructed.Aiming at the requirements of human-machine interaction performance and safety of exoskeleton robot joints.The Sigmoid function discriminator is used to judge the user ’s motion intention and human-computer interaction state,obtain the joint damping parameters and rigid parameters,input the joint magnetorheological damper and admittance controller,adjust the joint damping and rigidity in real time,and correct the expected angle of the system,so as to construct the variable parameter admittance outer loop(force)controller of the pneumatic muscle flexible joint,and complete the stability proof of the controller.(4)The simulation,analysis and experiment of magnetorheological variable damping flexible joint of pneumatic muscle are carried out.The simulation platform of control system is built in Simulink simulation environment,and the simulation experiment of flexible joint is carried out.Based on the designed magnetorheological variable damping flexible joint structure of the antagonistic pneumatic muscle of the lower extremity exoskeleton robot,an experimental prototype was built to carry out physical experiments to analyze the rationality of the structure and the feasibility of the control system.Design trajectory tracking experiment,load robustness and human knee gait tracking experiment to obtain experimental data.The tracking effect,man-machine interaction,flexibility and safety of the control system for flexible joints are analyzed.