| Human-machine collaboration working mode is a hot topic at present,and collaborative robots have become an indispensable tool for people to free their hands and improve productivity.In recent years,collaborative robots have been rapidly popularized,and robot teaching technology is facing increasing challenge.Free force control is one of the most important methods to implement direct teaching technology for robots,and it is an important direction in the field of robot research.The essence of free force control lies in the precise compensation of robot joint friction and the gravity of the link side.However,the joint torsion and the complex friction characteristics in the joint during the robot's operation affect the estimation accuracy of the gravity and the friction torque.Therefore,based on the joint transmission mechanism of the cooperative robot,this thesis studies the joint stiffness characteristic and friction characteristic,so as to improve the compensation accuracy of the gravity torque and friction torque under circumstance of free force control.The transmission mechanism of the robot joint system is firstly studied,the relationship between the stiffness characteristics and friction characteristics of the joint system and the system structural parameters is theoretically analysed.Then an experimental platform of robot joint system is built to conduct experimental modeling and research of two mentioned characteristics above.Aimed at the hysteretic asymmetric stiffness characteristic of the robot joint system during the stiffness characteristic experiment,a piecewise polynomial function model was used to fit it.According to the convergence of the fitting error with the degree of the polynomial,the two-segment quartic polynomial was finally adopted to model the stiffness characteristics.The LuGre friction model is established according to the friction characteristic of the joint system to estimate the friction torque in the joint system,and then the model parameters are obtained by parameter identification on the constructed robot joint system experimental platform.Based on the modeling results of the stiffness and friction characteristic,a free force control strategy based on contact torque assist is proposed in view of the existing free force control strategies.The proposed control strategy realized free force control of the robot joint system without a link-side encoder and a joint torque sensor.The reactive joint torque is estimated through observing of generalized momenta,combining with the inverse model of stiffness the torsional angle is estimated,and the angle on the link side together with gravity torque is worked out,and further the contact torque is estimated using the dynamic equation of the link side.Based on this,a desired motor driving torque is constructed,including the estimated gravity torque,friction torque and contact torque.Free force control is achieved by tracking this desired motor driving torque.Subsequently,a comparative experiment of the same teaching process was performed,and the results show that the proposed free force control strategy can effectively reduce the contact torque required during direct teaching,and is helpful to improve the efficiency of direct teaching. |
PDF Full Text Request