Increasing milling precision for macro-micro-manipulators with disturbance rejection control via visual feedback

Industrial robotic manipulators can be augmented by a micro-positioning unit in order to increase their precision resulting in a so called macro-micro-manipulator. The micro-positioning unit is typically driven by piezoelectric actuators due to their beneficial properties. However, contact forces during interaction tasks induce deviations from the nominal path that can not be observed due to compliance, lack of sensors in the micro-positioning unit, or unknown interaction dynamics in constrained environments. In this paper, a model-free and decoupled disturbance rejection controller via visual feedback for macro-micro-manipulators is presented. An external stereoscopic vision system is employed to detect deviations from the nominal trajectory. We outline an image segmentation algorithm and the utilized camera calibration technique is based on two-view geometry. Afterwards, the disturbance rejection controller including visual feedback for the macro-micro-manipulator is described. In order to demonstrate the 3D capability of the proposed approach, a microscopic staircase is milled. For comparison, the milling experiment is executed without and with active disturbance rejection by the micro-positioning unit in order to show the increase in precision during the milling task. Results show that the arithmetic mean roughness falls below 2 μm for the step profiles and the maximum surface height deviation is less than ±10 μm for each steps.