Position control of machine tools

 

Motivation:

The control of the feed axes of machine tools has a decisive influence on the working result of a machine tool. In conventional position control loops, the KV factor is a key parameter for evaluating axis dynamics in relation to workpiece quality. However, an increase in the KV factor results in a reduction in damping, so that undesirable vibrations can occur in the position control loop. The real machine behavior is also characterized by non-linearities due, for example, to position-dependent stiffness of the ball screws and by time-variant influences due to a decreasing workpiece mass during machining. The WZL Laboratory for Machine Tools has therefore developed robust controllers with which it is possible to dampen non-linear machine dynamics.

Results and procedure:

The aim is to develop position controllers which are robust against position-dependent or time-variant machine behaviour. The following robust controllers were developed for damping the non-linear machine behavior:

H∞-Control
Loop synthesis control
Robust model predictive control

Future position setpoints are used in model predictive control to increase dynamics. The disturbance behavior is estimated and controlled. The controllers were validated on a demonstration machine using specially developed control modules (compile cycles for the Sinumerik 840D sl control).