Study On Characteristics And Control Methods Of Permanent Magnetic Variable Stiffness Robot Joint

In the interaction of robots,human beings and environments,the compliant motion can not only ensure stable contact and operational safety,but also enhance the performance of robot's motion effectively and reduce energy consumption,which has become the main direction of the future robot development.Rigid robots can achieve compliant motion by using torque sensor.However,with the decrease of the robot required stiffness and the sensitivity increase of the sensor,the control instability increase.The mechanical variable stiffness robot joint can ensure stable contact and operational safety due to application of elastic components with adjustable stiffness in series or parallel,is received more and more attention.However,the mechanical variable stiffness robot joint has been mainly concentrated in the variable stiffness principle and the joint structure design stage so far.There are still some problems such as insufficient stiffness change capability,high energy consumption,and insufficient structure.The wire-driven variable stiffness joint uses an antagonistic drive similar to the combination of the biceps and triceps,which is the closest to the compliant movement of the human joint.However,the wire-driven antagonistic variable stiffness joint often has the coupling problem between the joint stiffness,output force and joint position,which brings great challenges to the design of the controller.In this paper,several problems involved in the characteristics and control of the wire-driven antagonistic permanent magnet variable stiffness robot joints are studied.(1)A permanent magnet variable stiffness component is proposed for the wire-driven antagonistic variable stiffness robot joint.The nonlinear combination of the permanent magnet spring and the wire unit increases the joint stiffness variable range without increase of the wire-tension.Similarly,to realize the same joint variable stiffness range,the advantage of using permanent magnet spring replacing the conventional metal spring is decrease of the motor power,volume,etc.The wire-tension and stiffness analytic mathematical model of the permanent magnet variable stiffness component is established by the virtual displacement method.(2)The antagonistic variable stiffness robot joint based on the permanent magnet component and its stiffness model were proposed.The antagonistic variable stiffness robot joint with active and passive compliance is constructed by using trapezoidal layout.The variable stiffness joint has small structural size,weight,and strong stiffness adjustment ability,due to application of the permanent magnetic variable stiffness component.The relationship between the joint stiffness,joint position and the stiffness of the permanent magnetic component is established.Then the stiffness model of the variable stiffness robot joint is obtained based on the Jacobian matrix and the static relationship.This model is suitable for both this robot joint stiffness relationship description and the parallel multi-degree-freedom variable stiffness joints.(3)The hybrid control strategy of position and stiffness is designed for the wire-driven antagonistic variable-stiffness joint.The proposed control strategy realizes the decoupling of joint position and stiffness independently.The advantage of the variable stiffness joint such as a short stiffness adjustment time,a large stiffness adjustment range,and strong motion performance,is verified by experiments.The experiment reveals also the variable stiffness joint has energy-saving characteristics.The decoupling method is also suitable for parallel multi-degree-freedom variable stiffness joints.(4)Two controllers are designed for the variable stiffness joint to deal with the position control accuracy and stability issues.The double closed-loop decoupling controller is designed to deal with the disadvantage of variable stiffness joint that position control accuracy is easily affected by the joint stiffness changing.The controller is obtained by variable stiffness joint system dynamic and stability analysising.Then the position accuracy of the variable stiffness joint is improved.The dual-loop decoupling robust controller is designed to deal with the shortcomings of the variable stiffness joint that controller is sensitive to unknown disturbances.The uncertainty stiffness perturbation and external disturbance are substituted into the constraint conditions.The convex optimization method is used to obtain the optimal H?controller.It realizes the joint stiffness and position decoupling control with better disturbance suppression ability.The simulation and the experiments verify the accuracy of the analysis above.(5)The active compliant controller of the variable stiffness joint is designed,and external force compensation is proposed.This method realizes the joint compliant control and improves the position tracking accuracy.The wire-driven variable stiffness joint is regarded as a series-parallel hybrid structure with a constraint link due to the three-dimensional force vector closure principle.The mechanical model of the wire-tension,external force and the equivalent joint driving force is obtained through the static analysis of the wire space,joint space and operating space with consideration of the shielding effect of the restraining link between the wire tension and the external force.Furthermore,the position accuracy of the variable stiffness joint is improved based on the external force compensation method in different joint stiffness conditions.