Milling using two mechatronically coupled robots

TL;DR

This paper addresses the limited dynamic stiffness of serial robotic arms in milling by proposing a parallel, flange-coupled milling system formed by two identical robots. The redundant degrees of freedom are leveraged to shift natural frequencies through tension applied to the coupling module, a concept validated by modal analysis that shows directionally dependent frequency shifts and a roughly uniform stiffness distribution across the workspace. Milling experiments demonstrate that machining is feasible under tension, with deformation of the coupling module being largely constant along the path and compensable via a rigid transformation. The work suggests a practical route for chatter suppression in robotic milling using mechatronically coupled robots, while outlining future needs in coupling design, tension prediction, and material testing.

Abstract

Industrial robots are commonly used in various industries due to their flexibility. However, their adoption for machining tasks is minimal because of the low dynamic stiffness characteristic of serial kinematic chains. To overcome this problem, we propose coupling two industrial robots at the flanges to form a parallel kinematic machining system. Although parallel kinematic chains are inherently stiffer, one possible disadvantage of the proposed system is that it is heavily overactuated. We perform a modal analysis to show that this may be an advantage, as the redundant degrees of freedom can be used to shift the natural frequencies by applying tension to the coupling module. To demonstrate the validity of our approach, we perform a milling experiment using our coupled system. An external measurement system is used to show that tensioning the coupling module causes a deformation of the system. We further show that this deformation is static over the tool path and can be compensated for.

Figures (9)

• Figure 1: Block diagram of the proposed robotic milling system. The toolpath gets translated for both robots, additionally one robot introduces tension via an offset in position using a linear spring model.
• Figure 2: Lab setup of the proposed robotic milling system. Both robots are coupled and a spindle and a 6 DOF Lasertracker target is attached.
• Figure 3: Visualization of the modal analysis positions with relative offsets from the starting position (1). The first dimension points from the left robot to the right.
• Figure 4: Frequency response for first measure position and impact in positive x direction over tension forces. The peaks in compliance shift towards higher frequencies with an increase of tension force.
• Figure 5: Frequency response for first measure position and impact in +Y over tension forces. No shifts in frequency under 500hz were measured, and shifting starts at 700hz for increasing tension forces.