Research On Mechanical Design And Control Methods Of Variable Stiffness Actuator

The research on the performance of the human muscle under motion have shown the ability of stiffness variation is the key attribute of the human motion.Thus,Inspired by human joint,VSA(Variable Stiffness Actuator)which is the new type of actuation is introduced to achieve the human-like motion and human-like performance.In contrast to the rigid actuator and SEA(Series Elastic Actuator),VSA can perform better robustness and adaptivity,what's more,it can also achieve more natural human-like motion and closer to the human performance.Thus,VSA is promising in the area of application for bio-manipulator,biped robot and rehabilitation equipment.Therefore,considering the research of the key technologies of VSA,this project will make contributes on the following subjects: The mechanical design and the dynamic modeling of VSA based on the cam-roller-spring mechanism;The trajectory planning method of VSA based on the optimal control method;The anti-windup trajectory tracking method of VSA with mismatched disturbances;The building of general control platform of VSA based on Ether CAT+RTOS+Simulink and the verification by experiments.The research on mechanical design and modeling method for VSA based on camroller-spring mechanism is conducted.Oriented to extendibility and modularization as the main design goals and joint dimensions,output power,stiffness adjusting range and transmission efficiency as design indexes of VSA,the stiffness adjusting mechanism based on symmetric compression spring and cam-roller-spring mechanism is proposed.And based on combination of disc springs,the design method for the effective compression springs with desired stiffness is proposed to achieve the ability of modularization and larger output torque with small size of stiffness adjusting mechanism.Through the force analysis of cam-roller mechanism,the output-torque model and output-stiffness model of VSA are derived.Oriented to the controller design for VSA,the high order polynomial fitting method is used to derive the equivalent model of VSA to improve the computation efficiency.To verify the effectiveness of the derived model,the dynamic of the core part of VSA is simulated with ADAMS.The results show the correctness of the derived model.The research on the trajectory planning method of VSA based on the optimal control is conducted.Considering the double actuation structure of VSA and the difficulty of position and stiffness trajectory planning since lack of prior knowledge of the directmapping relationship between the output stiffness and the desired task of VSA,the optimal control method is employed.Combined with the system dynamics and system constraints of VSA,two types of optimal control problem are proposed which are time-energy optimal point-to-point motion task and payload-time-energy optimal point-to-point motion task.To solve the inefficiency of numeric optimization caused by nonlinear constraints,the normalized method of nonlinear constraints is proposed.It embedded the constraint cannot be violated into the system dynamics and transformed the other constraints as the objective function through penalty function method.The composite method combined the bisection method and i LQR framework is proposed to deal with the deficiency for approximation of one-dimensional objective function.To explore the influence of stiffness variation on the explosive movement and deal with the large deviation of the precision of optimal solutions based on i LQR framework,the payload-time-energy optimal point-topoint motion task is solved with Gauss pseudo spectral method.The simulation of these two methods is conducted to verify the effectiveness of the proposed method.The research on the anti-windup tracking controller of VSA with mismatched disturbances is conducted.Considering the strongly-nonlinear and strongly-coupled system dynamic caused by the introduction of stiffness variation mechanism and the output torque saturation phenomena have negative influence on the tracking performance,the mismatched disturbances are introduced to decouple the dynamic system of link positionside and the position of stiffness motor-side.Then the original design problem of control method for high-order dynamic system is converted to the anti-windup tracking controller design problem with mismatched disturbances.Based controller design process with the command filtered backstepping,the controller with filter tracking error compensation is proposed.The composite controller combined the momentum based disturbance observer to deal with the mismatched disturbances and matched disturbances is proposed to avoid decreasing the estimation accuracy led by introduction of high-order noise when obtaining the acceleration.On the other hand,considering the negative influence of torque windup on the tracking performance,the anti-windup auxiliary compensation system is proposed to compensate the windup torque and to improve the tracking accuracy.To verify the effectiveness of the proposed controller,the stability of the closed control system is analyzed and the simulation on the two DOF variable stiffness actuated manipulator is conducted.The simulation results show the proposed anti-disturbance and anti-windup trajectory tracking controller is effective to reject the disturbance and torque windup phenomenon.By using the proposed controller,higher tracking accuracy can be achieved.Based on the composite technology of Ether CAT,RTOS and SIMULINK,the control platform of VSA is proposed and the experiment for function verification of VSA is conducted.The hardware and operation system relied on the Ether CAT fieldbus protocol and real time operation system(RTOS)can achieve the compatibility of the non-Ether CAT slaves and improve the extendibility and generality of the control platform.On the other hand,for realization of rapid algorithm development and improving the program efficiency under different system and hardware,the software framework is developed based on Matlab/Simulink Real-Time.The application including the state machine and motion algorithm is all built under this framework.Furthermore,the experiments of stiffness variation of VSA and trajectory tracking controller are conducted on this control platform.The experiment results show the effectiveness of the performance of the proposed mechanical design of VSA and tracking controller.