Workpiece Pose Optimization for Milling with Flexible-Joint Robots to Improve Quasi-Static Performance

Abstract

Although industrial robots are widely used in production automation, their applications in machining have been limited because of the structural vibrations induced by periodic cutting forces. Since the dynamic characteristics of an industrial robot depends on its configuration, the responses of the robot structure to the cutting forces are affected by how the workpiece is placed within the workspace of the robot. This paper presents a method for workpiece pose optimization for a robotic milling system to improve the quasi-static performance during machining. Since the milling forces are time-varying due to the characteristics of the multi-tooth and discontinuity of milling, these forces can excite vibrations inside the robot structure. To address this issue, a structural dynamics model is established for industrial robots, considering their joint flexibility, and a milling force formulation is incorporated into the robot dynamics model to investigate the forced vibrations of the flexible joints. Then, the quasi-static performance of the robotic machining system is evaluated by the vibration-induced offset of the cutter tool that is mounted on the end-effector. Finally, an optimization approach is given for the workpiece pose to minimize the cutter tool offset under the periodic milling force. A numerical simulation demonstrates that the optimal workpiece pose can significantly reduce the overall tool offset during machining and can lower the variation of the tool offset along the milling path.

Nonlinear Control of Multibody Flexible Mechanisms: A Model-Free Approach

“…Then, the obtained tuning can be refined by lowering the proportional gain k p , increasing the derivative gain k d , and introducing a small negative value for the k n gain. Before testing a new set of gains, the closed-loop stability should be checked using (16). This procedure has shown to produce good results, at least for second-order systems with lightly damped elastic modes, like the example presented in this work.…”

“…The reason can be imputed to the fact that the dynamic modeling of flexible multibody mechanisms is a very complex and challenging task. Moreover, if linearized models around a specified operating point are taken into consideration, as done in [15], the dynamics of multi-link flexible mechanisms is described with limited accuracy [16]. As proved experimentally by Milford and Asokanthan in [17], the eigenfrequencies of a two-link flexible robot can vary up to 30% as a function of the manipulator configuration.…”

A New Approach to the Consideration and Analysis of Critical Factors in Robotic Machining

“…In robotic machining operations, under certain defined machining and path conditions the deviation and tool chatter will depend strongly on the robot's axes' configuration [24,25]. In other words, the deviation of the real path with respect to the programmed path, and the chatter of the tool during the machining operations, depend essentially on the interrelation between the stiffness of the robot and the magnitude and direction of the machining forces [26,27].…”

A “Double Accuracy Theory” and Experimental Research on Precision Grinding

“…For example, Zhong Gaoyan et al [14] have optimized and improved the structure of the machine tool, thus improving the cutting speed, machining accuracy, and machining efficiency. Qin, Haojie et al [15] proposed a workpiece pose optimization method for a robot milling system to improve quasi-static performance and machining accuracy in the machining process. The above authors' research has its merits.…”