Design And Dynamics Research Of Variable Stiffness Manipulator

Variable stiffness manipulator is an important direction in the field of robot research.Compared with rigid manipulator,it has better adaptability and human robot interaction ability.Safely interacting with human in unstructured environment is one of the core tasks of variable stiffness manipulator,which requires that the variable stiffness manipulator has good motion performance.Taking the 7-DOF hybrid manipulator with variable stiffness joints as the research object,this paper studies several key aspects,such as the inverse kinematics solution based on anthropomorphic motion characteristics,the design and dynamic stiffness control of the variable stiffness actuator,the joint stiffness configuration strategy of variable stiffness manipulator in anthropomorphic motion,and the dynamic modeling,hoping to improve the motion performance of variable stiffness manipulator.The main research work of this paper is summarized as follows:(1)Good anthropomorphic performance is conducive to the efficient and friendly communication and cooperation between the variable stiffness manipulator and human in the human robot interaction environment.In order to meet the needs of human robot interaction,the configuration of variable stiffness manipulator is designed,and an inverse kinematics algorithm based on anthropomorphic arm angle method is proposed.Based on the forward kinematics derivation of the variable stiffness manipulator,the analytical solution of the inverse kinematics of the manipulator is obtained by adaptively parameterizing the shoulder joint posture angle,which improves the adaptability of the parameterized joint posture angle to the end pose.The redundant manipulator has self-motion characteristics.By introducing the "arm angle" as an auxiliary parameter,the evaluation function of the motion discomfort of each joint of the variable stiffness manipulator is established,and the particle swarm optimization algorithm is used to optimize the "arm angle",and then the inverse kinematics solution with anthropomorphic characteristics is obtained by analytical method.The simulation results show that this method makes the arm angle of the variable stiffness manipulator coincide with the range of motion of the human arm,and the motion of the variable stiffness manipulator has anthropomorphic characteristics.(2)Variable stiffness joint has the advantages of adjustable stiffness and better cushioning,which can improve the joint performance of the manipulator.Taking the lightweight collaborative manipulator as the application object,a novel variable stiffness actuator is designed in this paper.Since the crank-slider mechanism can quickly adjust the stiffness of the joint,and the symmetrical structure is conducive to maintaining the force balance of each part of the joint,therefore,based on the method of changing the effective length of the spring bar,the design and dimensional synthesis of the variable stiffness actuator are completed by using symmetric crank-slider mechanism.On this basis,the joint stiffness of the variable stiffness actuator is calculated,and then the dynamic model is established.Using the improved PID closed-loop control method based on feedforward and feedback,the dynamic stiffness tracking simulation of random signal and sine wave signal are realized,which verifies the effectiveness of the proposed joint stiffness control method,and provides a basis for the real-time control of joint stiffness of variable stiffness manipulator in anthropomorphic motion.(3)Facing the complex and changeable working environment,reasonable joint stiffness configuration can improve the environmental adaptability of variable stiffness manipulator.In order to protect the human body from injury in the event of accidental collision in human-robot interaction,a real-time joint stiffness configuration strategy of variable stiffness manipulator in anthropomorphic motion is proposed in this paper.Based on the safety index of manipulator and human injury threshold,the motion speed and surface contact stiffness of the variable stiffness manipulator are optimized.According to the force at the end of the manipulator in all directions,the torque of each joint can be calculated through the force Jacobian matrix.In order to maintain the motion stability of variable stiffness manipulator,a hyperbolic tangent relationship model between joint torque and passive joint deflection angle is proposed,and the stiffness of each joint is reasonably configured in real time.Simulation results show that the proposed algorithm can effectively improve the safety of human body in human-robot collision.(4)Dynamic model is the basis of dynamic analysis and control of manipulator.For the variable stiffness manipulator,the dynamic modeling and related simulation analysis are carried out in this paper.Compared with the serial manipulator,the number of bars of the redundant hybrid manipulator is larger,which increases the complexity of dynamic calculation.Therefore,the bars of the variable stiffness manipulator are classified,and the bars with consistent rotation are treated as a bar,which effectively reduces the computational complexity.The kinetic energy and potential energy of different bars are calculated,and the dynamic equations of variable stiffness manipulator are established by using Lagrange method and virtual work principle.Through a numerical example,the theoretical value of each joint torque is compared with the simulation value in Adams environment,which verifies the correctness of the dynamic modeling of the variable stiffness manipulator.