Multivariable control of active vibration compensation modules of a portal milling machine

Abstract

The Z-ram of a Portal Milling machine presents a weak point in the dynamic behaviour of the machine tool which makes it prone to the occurrence of chatter vibration. Since chatter vibrations directly limit the maximum allowable cutting depth during machining, an improvement in dynamic behaviour of the machine tools by means of active vibration control of the spindle will result in an increase of maximum cutting depth. An active vibration control of the Tool Center Point of a Portal Milling machine using four hydraulic compensation modules integrated in the Z-ram structure is proposed. A test bench for the Z-ram was constructed at Machine Tools Laboratory (WZL) of RWTH Aachen University and it’s modal analysis revealed the occurring dominant vibration mode. A polyreference-LSCF modal parameter estimation was employed for identification of the measured MIMO Frequency Response Functions (FRF) of the Z-ram test bench. Using this mathematical model, the MIMO controller was synthesized with the Glover-McFarlane $\text{H}\infty$ Loop Shaping Design procedure. The implementation of the controller on the test bench resulted in significant improvement in the compliance behaviour of the Z-ram structure. The dynamic compliance at dominant mode of vibration at 75 Hz was reduced by a factor of 3.5. Furthermore, the attenuation of the maximum negative real part resulted in a direct increase in maximum stable depth of cut by a factor of 2.1.