| Variable stiffness robots have the advantages of strong task adaptability and high safety performance by adjusting their own stiffness.The variable stiffness actuator is usually a dualdrive system.When an actuator is driven to perform a task,the introduction of elastic elements can result in a coupling of position and stiffness.Therefore,the control problem of antagonistic variable stiffness robots has become a key factor limiting their development.This thesis focuses on the characteristics of antagonistic variable stiffness joints and controller design.The main content is as follows:(1)This article analyzes the principle and compliance of the structure of the antagonistic variable stiffness joint.It explains the influence of the magnetic spring and pulley group on the stiffness of the variable stiffness structure.The motion principle and range of motion of the antagonistic variable stiffness joint are compared and analyzed with physical objects.The article introduces how compliance is achieved in antagonistic variable stiffness joints.Stiffness simulation is conducted on the magnetic spring element used to improve compliance,and the range of joint stiffness variation is simulated by combining the amplification function of the pulley group.Tension tests are conducted on the used wire rope to verify the actual stiffness range of the antagonistic variable stiffness joint.(2)In response to the problem of position-stiffness coupling in joints,this article decouples and implements compliant control for the joints.By conducting a mechanical analysis of the joints and combining it with the variable stiffness principle,a joint stiffness model is established.The mechanical equilibrium relationship and joint stiffness model are used to separately calculate the changes in the cable caused by position and stiffness variations,thus achieving decoupling of position and stiffness.Based on the lagrangian equation,a dynamic model of the joint is established,and a PID-based position-stiffness decoupling controller is designed by using the dynamic model.The controller is used to conduct active/passive compliant control simulations on the antagonistic variable stiffness joint.(3)In response to the precise positioning control issue of the antagonistic variable stiffness joint,a PID-based position closed-loop decoupling controller is proposed.By analyzing the stability of the joint,it is found that the stiffness change may affect the effectiveness of the controller.Through a series of simulations,it is verified that the controller has good position tracking ability,but as the joint stiffness changes,the controller parameters need to be re-adjusted for optimal control of the joint.In order to solve the disadvantage of control parameters needing to be adjusted with stiffness,a PID-based parameter adaptive decoupling controller is proposed based on the previous work,and position control simulations are carried out.A comparison of the simulation results of the two controllers is made,and it is found that both controllers can achieve good position control,but the PID-based parameter adaptive decoupling controller has less workload and better control effect.(4)Utilizing the existing facilities,an experimental platform was set up to test the controllers with the joints.Three controllers were tested by using the experimental platform.The performance of joint compliance was tested by using the position and stiffness decoupling controller based on PID,and the impact of joint stiffness on the accuracy of joint output torque was verified.The compliance variation under different joint stiffness was tested by applying external force to the joint.The position control and continuous motion of the joint were tested by using the PID-based position closed-loop decoupling controller and the PID-based parameter adaptive decoupling controller respectively,and the performance of the two controllers was compared.The experimental results show that the latter is superior to the former in terms of stability in position control accuracy. |
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