Many producers for example kuka fig1 offer

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Many producers, for example Kuka (Fig.1) offer application-specific components and tools for deployment of a robot as a machine tool for milling tasks [1]. Vibration of arm robot structure is the major limitation of robotic machining capacities. The presence of the low frequency modes will shake the entire robot body and cause instability of the dynamic system during machining. The stiffness of the CNC machine is usually hundreds of times larger than process stiffness and mode coupling chatter rarely happen. For robot, the difference is only 5 10 times. This mode coupling effect is the dominant reason for structure vibration in robotic machining process [2]. The relative orientation of the force vector and the principle stiffness axes are the dominant factors that affect the stability of machining process using robots. Methods such as changing the feed direction, using different robot configuration or changing another type of tool are all worth trying. Based on the practical investigations, this research leads to a deeper understanding of the unstable phenomenon in robotic machining process and provides a guideline as well as practical solutions to avoid such problems. 2. CHARACTERIZATION OF THE STIFFNESS OF KUKA KR-240-2 ROBOT ARM Milling process is specially designed for machining tasks using an electrically-driven spindle. It is used particularly with lightweight materials such as plastic, composite or rigid foamed material. From the HSC spindle and its controller to the special milling software, is possible to quick and easy setup the robot as a powerful milling unit [3]. Experimental research involves the application of three unidirectional accelerometers C2, C3 and C4, on the robot arm (see the figure 1) and a three-axial sensor C5 (Y), C6 (Z) and C7 (X) on the mechanical interface of the robot. Responses of the robot arm were made considering the transfer function using hammer impact method. The signal acquired was the apparent mass in dependence with frequency. Were used 10 measurement points P1 ... P10 distance between these points being equal and having a value of 200 mm. Details of the signals acquired with C4 sensor in the range 0-200 Hz are detailed in the figure 2. Robot arm was in X1 position (P1) and two situations were experimented: arm with brake or without brake in function. Significant frequencies were 23 Hz, 80 Hz and 95 Hz and it was found that there are no differences between the two operating situations (brake has no influence on the robot arm rigidity). Fig.2. Signals acquired with C4 sensor in the range 0-200 Hz; brake / without brake in function Stiffness after X axis is the least important in the milling process (the deformation is greatest on the plan ZOY). Figure 3 represents the X axis sensitivity to the impact by Y, considering measurements along the X direction, in the points P1 ... 10. Largest mechanical coupling between X and Y axes is found in section P1. Fig.3. The coupling between the X and Y axes in positions P1, … , P10 Fig.4. Comparison between the signal of C2 and C4, in Z direction
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J-Y . K’Nevez et al. / Proceedings in Manufacturing Systems, Vol. 5 (2010) , No. 1 / 1 10 3 Fig. 5. Details on the frequency shift (stiffness variation) on the Z direction (comparison of the impact signals acquired in positions 5, 7 and 10) Fig. 6.
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  • Fall '19
  • University of Bordeaux

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