Dynamic Path Correction of an Industrial Robot Using a Distance Sensor and an ADRC Controller

Commercially available six-axis industrial robots, though highly repeatable, have relatively low accuracy. While robot calibration can improve pose accuracy, the only way for a user to improve path accuracy is by "guiding" the robot with the help of an external sensor and a control algorithm running on a separate computer. For this purpose, industrial robots, which are normally controlled with preprogrammed position-mode instructions, sometimes offer the possibility to modify the pose of the robot end-effector on the fly. In the case of Mecademic's Meca500 robot,(1) users can indirectly modify the end-effector pose by controlling the robot joint or Cartesian velocity. In this article, a practical application of an active disturbance rejection control scheme is presented to improve the path accuracy of the Meca500. The dynamic path correction is achieved by first measuring the distance between a fixed point and the robot tooltip with a linear transducer (Renishaw's QC20-W ballbar), and then feeding the tooltip velocity vector to the robot (via Ethernet TCP/IP). The (circular) path accuracy of the robot is significantly improved for different robot TCP velocities. For example, at 50 mm/s, the maximum radial error is less than 0.100 mm, and the mean error is 0.015 mm.