INVESTIGATION OF AN ACTIVELY CONTROLLED ROBOT ARM FOR VIBRATION SUPPRESSION IN MILLING

Abstract

In recent years, the use of robotic systems to enhance the productivity of machining operations has received significant attention from the research and manufacturing communities. Robots have the potential to further improve productivity, for example by providing automated workpiece fixturing, or by providing a flexible and reconfigurable platform from which a variety of subtractive or additive manufacturing operations could be performed. One possible approach is the use of a robotic arm to provide additional fixturing or support of the workpiece during the machining operation. This can increase the stiffness of the workpiece system during machining, which can improve productivity by limiting the onset of undesirable vibrations such as chatter. Chatter is a form of self-excited vibration which leads to low surface quality of the workpiece, shortens the cutting tool life and increases the cutting forces. In this paper, an actively controlled robot arm is simulated in order to suppress the chatter, in an effort to further improve the chatter stability. During the milling operation, preload can be applied through the robot to support the flexible structure, however, the robot cannot suppress high-frequency forces. Since the stiffness and damping ratio of the large flexible structure vary during the operation due to material removal, active vibration control is performed. A proof-mass actuator is proposed that can provide 45 N force up to 2000 Hz with 2 mm stroke. The dynamic properties of the device are identified experimentally as part of a model of a robot fixture prototype. The robotic arm is modelled as a three degree of freedom system; this is combined with a simplified representation of the workpiece dynamics, and the proof-mass actuator, within a Matlab environment. The effect of active control on the chatter stability is evaluated, focussing initially on the use of direct velocity feedback as a control strategy. Estimated chatter stability predictions, along with time, frequency domain simulation results, show that the application of active control method in robotic-assisted machining can suppress the chatter vibrations during machining and hence increase productivity.