Research On Drag Teaching And Compliant Control For Force/Torque Sensorless Collaborative Robots

Under the circumstances of “Industry 4.0” and “Made in China 2025”,that robots assist workers to complete production tasks gradually becomes a reality,and human-robot cooperation has become a new research branch in the field of robotics.Collaborative robots with human-robot cooperation functions are very different from traditional robots,collaborative robots do not need the operators to master the programming skillfully,the operators can drag the robot to complete the teaching and assembly tasks.However,when the operator drags the collaborative robot to complete the assembly tasks,it is extremely difficult to ensure that the workpieces are fully matched,and there are often certain deviations between them.Therefore,the flexibility of the collaborative robot is crucial.First,the forcefree control based on torques compensation was used to realize the drag teaching of collaborative robots.Establishing an accurate dynamics model was the basis of the force control.To facilitate programming and identification,its mathematical expressions were rewritten in the form of a minimum parameter set.Then the real dynamic parameters of the collaborative robot were obtained by two-step identification and the relationship between joint torque and joint position,velocity and acceleration can be attained.In this paper,the real-time compensation of the inertial force,Coriolis force and gravity of the robot and the compensation of the force/torque generated by the load were achieved.To improve the accuracy of robot's force control,a secondary friction compensation was proposed.In addition,this paper proposed two ways for robot trajectory reappearance.The first one was the “position point mode”,and the problem of avoiding singularity was discussed in this part.The other one was the“continuous trajectory mode”,and the recorded trajectory was smoothed by the Savitzky-Golay smoother in order to reduce the vibration of the robot.Then,in order to achieve the compliance of collaborative robot,active compliance control method was adopted in this paper.In this paper,a unified impedance model of robot and environment was established,and two methods of observing the external torque of robot were proposed.One was based on the generalized momentum,the other was based on the dynamic identification,and Kalman filter was used to improve the accuracy of external torque observation.Based on this,an active compliance control method which can realize the robot's compliance to the external environment without installing external force / torque sensor was proposed.Finally,the compliant control method was applied to the peg-in-hole assembly,the geometric analysis and force analysis of the peg-in-hole assembly were also carried out and the assembly optimization strategy was proposed.The experiment results showed that during assembly the contact force between the peg and hole was small and the position adjustment was also smooth,the experiments can fully prove the validity of active compliant control and assembly strategy of this paper.