Fundamental analysis of the modal control applied to over-actuated machine tools

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Highly dynamic movements and process forces excite both the moving assembly (e.g. slide) and the underlying machine structure (e.g. frame) to oscillate. High vibration amplitudes result especially for lightweight structures. Vibrations in machine tools lead to a deterioration of the machining quality and must therefore be reduced to a minimum. In order to enable an increase in dynamics while maintaining or improving the quality of motion, measures to reduce or avoid vibrations are necessary. In general, the dynamic compliance at the Tool Centre Point should be reduced in order to increase the process stability and the chatter limit. This can be achieved by increasing the system’s damping, for example by using active damping systems, so-called active damping devices. In most cases, the drives required to generate motions and the auxiliary actuators are controlled independently of each other and without taking mutual interference into account. This makes the selection and parameterisation of mechanically coupled drives or additional actuators particularly difficult.The aim of the research project is to use the modal control to improve the dynamic properties of motion systems, such as machine tools, which have more drives or actuators than degrees of freedom of movement. Modal control makes it possible to control eigenmodes individually and to damp the excited oscillations in a targeted manner with all available actuators. Modal decoupling is intended to simplify the parameterisation of the controllers of all drives and actuators involved and to reduce the feedback effect of the active auxiliary systems on the position control of the feed drives. An increased control bandwidth of the feed axes and a reduction of the dynamic compliance at the TCP is aimed in order to achieve a high dynamic path accuracy and thus a high accuracy at the workpiece as well as an increased process stability.