Admittance Control Design And System Testing Of Robot Polishing Operation

Industrial robots are widely used in welding,polishing and other fields for its flexibility and high repetition efficiency.Due to the highly precision required in the polishing and grinding field,the traditional single position control of the robot cannot meet the operational requirements.Therefore,the compliance control of contact force plays a great significant role in robot polishing operations.In this paper,the control system of the inner loop of position control and the outer loop of admittance control is designed to deal with the problem of contact force in the faucet polishing operation.The feasibility of the control system is tested by simulation and experiment.Firstly,analyze the requirements of the polishing operation and elaborate the overall framework of the experiment.Establish the kinematics model of the robot through the DH parameters and the homogeneous coordinate transformation,and realize the mutual transformation between the Cartesian space and the joint space.Establish the dynamic model of two-bar linkage robot based on the Lagrange principle.Secondly,the admittance control algorithm is designed to establish the relationship between the contact force and velocity,and the excepted position is corrected by the position obtained from the velocity integration.We adjust the input torque of the two joints of the robot by PID position control to adjust the contact force between the manipulator end effector and the environment.The first-order parameter sensitivity method is used to analyze the influence of the admittance control algorithm parameters on the system output.A gravity compensation algorithm is designed to reduce the force/torque sensor error from workpiece gravity.Next,the virtual prototype of two-link manipulator is established in ADAMS,and a controller is built in the MATLIB/SIMULINK,including inner-loop position control and outer-loop admittance control.The influence of admittance parameters on system output is analyzed by calculating parameter sensitivity in MATLAB.Through the co-simulation of ADAMS and MATLAB,position control and admittance control tests were conducted to analyze the changes in the contact force between the manipulator end effector and the external environment.The result of simulation shows that the control strategy based on the inner-loop position control and the outer-loop admittance control can improve the accuracy of the position control by 2.98% compared with the single position control,which can ensure that the contact force between the robot and the outside is stable at the expected force value.Finally,we build a test system with ABB manipulator,ATI force/torque sensor and PC.We implement admittance control algorithm,design host computer user interface and solve the force sensor gravity compensation problem by Qt.We analyze the external influence factors of the polishing machine and the connectors on the polishing operation,and conduct manual traction and robot polishing experiments.The experiment shows that admittance control can improve the flexibility of the manipulator and can achieve a stable control of the contact force in the faucet polishing operation.