Characterization of milling robot mode shape and analysis of the weak parts causing end vibration

The existing studies on milling robot dynamic characteristics pay little attention to robot structures, while structure mode shape analysis is significant to accurate vibration control. This paper applies the 3D scanning laser Doppler vibrometer to the milling robot, and proposes the R²-based compliance identification criterion to identify the robot deformation parts as the joints, greatly reducing the number of degrees-of-freedom (DOF) required for the mode shape characterization. Then, the multi-joint and full-DOF mode shape (MFMS) is proposed to succinctly characterize the robot mode shape by mapping the vibrations of numerous measurement points to the joints. MFMS shows that, different from the conventional studies, it is necessary to consider the robot joint compliance in multiple DOFs. For example, in this paper, all the 6 joints are deformed in the first two mode shapes, while the last 3 joints are deformed in the rotational DOFs in the 3 ∼ 6 mode shapes. Based on MFMS, the weak excitation direction at tool center point (TCP) is analyzed and the TCP amplitude is successfully predicted to be almost halved after changing the milling force direction. The weak joint-DOF dominating TCP vibration is analyzed, and it is found that there are only a few weak joint-DOFs in each mode and they vary with the mode, which is instructive for accurate vibration suppression and joint enhancement.