Virtual Model Control Of A Lower Limb Exoskeleton To Assist Sit-to-stand Movements

Sit-to-stand(STS)movement can be commonly seen in our daily lives.For example,workers often need to stand up while bearing different loads in real-world production activities.Investigating the motion characteristics involved in human STS can not only benefit the controller design regarding to STS,but also promote the design of STS-assisted exoskeletons.However,the majority of the currently-existing studies focus on movement assistance for the disabled.Few of them have paid attention to assist load-bearing STS transfer for the healthy.Traditionally,the STS control is realized via position control.Despite can provide enough assist torque for the wearer,this method may suffer from several potential flaws,such as strict requirements on the dynamic modeling,difficulties in adapting to the interference of external environment.To achieve the position and attitude control with respect to human STS movements,an exoskeleton auxiliary control strategy based on virtual model control is first developed in this thesis.Subsequently,the efficacy and robustness of the developed method is verified both through model simulation and experimental tests.The main research works of this thesis are summarized as follows:(1)Analysis of human STS motion.A simplified human model is first established based on the motion characteristic analysis of different lower-limb joints related to human STS movement.Afterward,the dynamics in human STS motion is modeled and analyzed according to the Lagrange equation.Lastly,the musculoskeletal model and the STS simulation platform are established in Open Sim in order to lay the foundation for the design of STS control strategy.(2)Research on exoskeleton assistance strategy based on virtual model control.By analyzing the theory of virtual model control,a controller based on centroid pose of human trunk is constructed.Then,the experimental platform is built to solve the trajectory of trunk centroid,and the parameters of controller are optimized by combining Open Sim forward dynamics and simplex method.Based on the optimization results,the effectiveness and robustness of the control method are evaluated in the simulation environment.(3)STS-assisted exoskeleton design.After completing the designs of the mechanical structure,hardware platform and control system,a hip active lower-limb support exoskeleton is designed to provide assistance to the wearer.During the establishment of the control system regarding to the device,a load-bearing stand up assistance strategy is proposed based on the virtual model control and gravity compensation.Finally,the effectiveness of the proposed control strategy is verified via the established experimental platform.The experimental results show that the muscle activation levels of gluteus maximus and semimembranous are reduced in the case where the exoskeleton provides assistance during load-bearing STS transfer.Moreover,the virtual model control algorithm based on trunk centroid can not only realize the stable transformation from sitting to standing with promising controlling performance over the exoskeleton,but also adapt to different task requirements and resist external interference.Meanwhile,this exoskeleton provides supporting torque to human hip joint,such that the load-bearing STS transfer can be assisted and the trajectory of the load-bearing STS transfer can be corrected.Therefore,the proposed control strategy and developed device could provide valuable clues on the controller and mechanical structure designs of human load-bearing and STS-assisted exoskeletons.