Study On Impedance Control Methods For Flexible Robots With Series Elastic Actuators

When the robot is performing tasks such as grinding,handling,rehabilitation,and assembly,the interaction between the robot and the environment(people or objects to be operated)must have certain flexibility to avoid damage to the robot or the environment.The series elastic actuator(SEA)has the advantages of low output impedance,good post-drive,good impact resistance,stable force transmission,high energy efficiency,etc.It is an important passive compliance control device;and impedance control is an important active compliance control method,force feedback can be used to actively control the force through a certain control strategy.Therefore,it is of great value to study the impedance control method of flexible robot based on SEA,which can use active and passive compliant control coordination to improve the safety of interaction.At present,a large number of results have been achieved in the impedance control of rigid robots,but there are few results in the impedance control of SEA-driven flexible robots.Based on the singular perturbation method and dynamic surface method,this paper designs constant impedance and variable impedance control strategies for flexible robots.The main research work and contributions of this paper are summarized as follows:1)Based on singular perturbation method,nonlinear impedance control is designed for SEA-driven flexible robot.On the basis of constructing the impedance reference trajectory,the impedance control design problem is transformed into a special type of trajectory tracking problem.Then,the singular perturbation method is used to decompose the flexible robot control system into fast-changing and slow-changing subsystems.The control laws of the two subsystems are designed to ensure the exponential stability of the subsystems.Finally,using Lyapunov theory,the convergence of the impedance error is obtained,thus ensuring the realization of the expected constant impedance dynamics.The stability and effectiveness of the impedance control are verified through simulation analysis.2)An adaptive first-order variable impedance control based on dynamic surface is proposed.First,the constraints of variable stiffness and variable damping are proposed to ensure the exponential stability of first-order variable impedance dynamics.Then,on the basis of constructing the impedance reference trajectory,the impedance control design problem is transformed into a special type of trajectory tracking problem.Based on the dynamic surface method,the adaptive variable impedance control of the SEA-driven flexible robot is designed to achieve the desired first-order variable impedance dynamics.Finally,using the Lyapunov theory,the consistent and ultimately bounded impedance errors are obtained,thus ensuring the realization of the expected first-order variable impedance dynamics.The stability and effectiveness of the impedance control are verified through simulation analysis.3)A second order variable impedance control of flexible robot based on disturbance observer is proposed.First,the constraints of the second-order variable impedance parameters are given to ensure the exponential stability of the expected second-order variable impedance dynamics.Then,a disturbance observer is designed to achieve an effective estimation of low-frequency external disturbances.Then,on the basis of constructing the impedance reference trajectory,the impedance control design problem is transformed into a special type of trajectory tracking problem.The singular perturbation method is used to decompose the flexible robot control system into fast-changing and slow-changing subsystems.The control laws of the two subsystems are designed to ensure the exponential stability of the subsystems,and the subsystem control laws are combined to construct nonlinear impedance control.Finally,using Lyapunov theory,the convergence of the impedance error and the observation error is obtained,thus ensuring the realization of the expected first-order variable impedance dynamics.The stability and effectiveness of the impedance control are verified through simulation analysis.