Research On Passive Flexible Variable Stiffness Actuator And Its Characteristics

In the scenes of human-robot or robot-environment physical interactions such as rehabilitation training,mobility assistance,mechanical prosthetics,industrial production,and special tasks,high safety and strong flexibility are two common key indicators of interactive robots.Joints are the core actuators that drive the robot body and meet these two key indicators.From the physical hardware level,by introducing elastic elements into the actuator transmission chain,the actuator will have inherent compliant properties,called passive flexible actuators,which can greatly reduce the reflected inertia and the difficulty of compliant control,and have a certain passive elastic energy storage and force perception ability,good human-robot interaction experience,and high inherent safety.Compared with the traditional rigid transmission actuators or fixed stiffness elastic actuators,variable stiffness actuator(VSA)has a wide range of stiffness adjustment,large passive elastic energy storage capacity,strong task adaptability,and flexible control bandwidth,high safety,which is suitable for physical interactive robots that require both safety and accuracy.However,the additional stiffness adjustment mechanism greatly increases the design complexity and control difficulty of the VSA.How to make the VSA obtain a large range,high speed,low energy consumption,weak coupling active stiffness adjustment ability,accurate elastic force/torque sensing ability,and higher power density through compact design is a major challenge.Moreover,the inevitable nonlinear motion coupling disturbance,system complexity and uncertainty in the controller design caused by VSA are the main obstacles to its precise motion control and expansion of applications.This research aims to break through the above-mentioned challenges and obstacles,and conducts related theories,methods,technologies and experimental research around the design and control of VSA and its characteristics.The main contents are as follows:(1)Taking the three primary design goals of wide range,fast response,and low resistance of stiffness adjustment,combined with the advantages of the existing variable stiffness principle,the variable stiffness principle based on variable elastic elements structure parameters is proposed.The corresponding elastic element and its application scheme are designed,and then the implementation is established.The stiffness model of the actuator is established,and the simulation analyses of the stiffness,elastic torque,passive elastic energy and the change characteristics of the stiffness adjustment torque of the proposed principle are conducted.Then,the multiple constraint conditions of the maximum elastic deflection of the actuator are analyzed in detail.Finally,the corresponding constraint mechanism to ensure the elastic force and torque sensing ability of the actuator is designed.(2)Starting from the application backgroun,the basic composition and the design requirements of a rotary VSA are analyzed,firstly.Then,the overall design of the VSA is propose.Combined with the proposed variable stiffness principle and the overall design of the VSA,the selection of key components of the VSA is analyzed.The development of a physical prototype of the mechanical-electrical-control system of the proposed passive flexible variable stiffness actuators(named as S3VSA)is completed.Finally,the developed S3VSA is tested for its stiffness adjustment performance,its static stiffness characteristics is identified.Its stiffness adjustment speed and actual adjustment energy consumption are investigated.Moreover,the change characteristics of its stiffness adjustment resolution are also analyzed.(3)Aiming at the suppression of S3VSA's motion-coupled disturbance,a dual-loop nonlinear controller based on disturbance observer is proposed.Firstly,the dynamic model of S3VSA is established and the system disturbance is also analyzed.According to the error dynamics,a nonlinear disturbance observer is designed to estimate the coupled disturbance,and then the inner and outer loop position tracking controllers of the actuator are designed.Its stability is proved by Lyapunov theory,and the controller parameter tuning principle is obtained.Finally,it is verified by comparative experiments under various stiffness and load changes.(4)Aiming at the difficulty of controller design caused by S3VSA's complex dynamic modeling,the learning adaptive control of S3VSA in the task space is the research focus.First,the problem is analyzed to obtain the control target,and then The composite learning control algorithm based on the local weighting theory with incremental learning is proposed.The feedbacks of S3VSA are used as the sample points,and the model parameters in the local weighted linear regression are updated by the composite learning algorithm.The estimation of each receptive field is incrementally weighted by the local weighted learning to obtain the inner and outer loop dynamics of the actuato.The stability and convergence of the controller are proved by the Lyapunov theory.Finally,the generalization ability of the proposed algorithm is verified by comparative experiments.(5)Based on the designed S3VSA and related algorithms,its safety response performance in collision simulation applications and energy-saving ability in periodic motion simulation applications are analyzed and verified,respectively.First,an online collision monitoring method based on the combination of actuator deflection rate and energy method is proposed.Then a post-incident safety response strategy based on flexible joints is designed,and the safety collision comparison experiments are carried out based on the proposed DNC controller.Then,based on the dynamic characteristics of S3VSA,the stiffness dynamics required for its lowest power consumption is explored.Comparative experiments are carried out under constant stiffness,variable stiffness and rigid transmission modes to verify the energy-saving ability of S3VSA.The comprehensive performance comparison between S3VSA and some classic VSA solutions confirms the competitiveness of S3VSA in terms of compactness,stiffness adjustment speed,energy consumption,and power density.