Research On Optimization And Control Method Of Magnetic Flux Adjusted Variable Stiffness Joint

The high load-bearing capacity and high rigidity of traditional industrial robots have become a challenge for robots to further penetrate into more complex human-machine interaction scenarios.Robots need to have functional attributes that can change the stiffness of their joints according to the application environment.This thesis introduces variable stiffness flexible joints that can change their own stiffness into robots.This paper presents a new type of variable stiffness robot joint-"flux adjusted variable stiffness joint",which uses the superposition principle of magnetic flux.It can achieve stiffness adjustment by overlaying the magnetic flux between the electromagnetic coil and the permanent magnet to change the total magnetic flux in the joint magnetic circuit.This current controlled stiffness adjustment strategy can greatly improve the response speed of joints,effectively enhancing the human-machine interaction safety and environmental adaptability of joints;The non-contact hybrid variable stiffness mechanism can make the variable stiffness joint structure compact,and has advantages such as no mechanical friction and no hysteresis.At present,the permanent magnets and electromagnetic coils in flux adjustable variable stiffness joints often have leakage and heating phenomena,leading to significant discrepancies between the theoretical and experimental values of the joint.At the same time,the strong nonlinearity of the variable stiffness joint itself also brings great difficulty to the design of the controller.Therefore,this article conducts the following research work on several issues such as the characteristics and control methods of magnetic flux adjustable variable stiffness joints.(1)A new analytical mathematical model for joint stiffness is proposed,which divides the forces inside the variable stiffness joint into mutual repulsion between permanent magnets and mutual force between electromagnetic coils and permanent magnets.The equivalent magnetic charge method and molecular current hypothesis are used for calculation and solution.This thesis establishes a more accurate analytical mathematical model.This section uses Ansys Workbench simulation software to perform lightweight design on components and perform strength verification analysis on the lightweight components.(2)Expounded the stiffness variation law of the magnetic flux adjustable variable stiffness joint.The hybrid variable stiffness module is introduced into the electromagnetic simulation software,and the Control variates is used to obtain the relationship curve between the coil current and the joint stiffness,as well as the relationship curve between the joint rotation angle and the joint stiffness,respectively,with the coil current and joint rotation angle as the simulation variables,and analyze their stiffness characteristics.(3)Considering the influence of elastic effects,system damping,and gravity term on the joint during operation,a dynamic model of a variable stiffness joint is established using Lagrangian equations.It is based on the operation mode of joints and the principle of variable stiffness,a position and stiffness coordinated control strategy is designed.PID and fuzzy adaptive PID are used to build control block diagrams in Matlab/Simulink,respectively.Controling simulations are conducted on the variable stiffness joints,and the control effects are compared and analyzed to confirm a better control strategy.In order to further improve the position accuracy of variable stiffness joints,the position error is fed back to the motion control link at the joint input end,and simulation verification is conducted to verify the position accuracy of the joint after adding the motion control link.(4)Establish a testing experimental platform for magnetic flux adjustable variable stiffness joints,and design the following experiments to investigate the impact of joint parameters on joint stiffness performance: Position step experiment,Trajectory tracking experiment,Torque control experiment,and Joint compliance experiment.Through experiments,the control effects of different control methods on variable stiffness joints and the characteristics of the joints themselves were verified.