Deflection-limited trajectory planning in robotic milling

Abstract

The use of industrial robots in precision machining operations, such as milling, is of growing interest in aerospace manufacturing and assembly due to its potential flexible workflow and efficiency benefits. However, serial link robot manipulators are inherently less stiff than CNC machine tools, and their Cartesian stiffness is pose-dependent, which contributes to machining inaccuracies. While research exists to compensate for these deficiencies, such as offline and online position error compensation techniques, a compensation approach that combines process force-induced deflection errors during robot trajectory planning with real-time position error feedback control using an external sensor is lacking. This paper presents a process force-induced deflection-limited trajectory planning approach to improve the machining accuracy in robotic milling through variable feed rate trajectory generation and real-time position error feedback control for curvilinear trajectories. The proposed methodology generates a process-aware trajectory by adjusting the feed rate to adhere to a user-defined static deflection limit while ensuring compatibility with an external real-time closed-loop position feedback control system interfaced with the robot controller. Experimental investigations are carried out on a six degrees-of-freedom industrial robotic milling system with real-time closed-loop laser tracker-based feedback. The experimental results demonstrate that the deflection-limited variable feed rate trajectory yields improved part accuracy compared to a constant feed rate approach. Moreover, the closed-loop system executing the variable feed rate trajectory is shown to yield superior performance compared to the native robot controller utilizing native motion types. These findings highlight the significance of a process-dependent trajectory planner, and a generalization of its application is proposed.