Research in machine tool axis control
- Time：2013-12-18 13:50:11
Affecting a multi-axis machine are defined as the difference between the real and measured position and orientation of the end effector. These cause a deterioration of the machine performance, which in turn directly influences the final product specifications. To maintain high quality of performance for a machine, these have to be detected and eliminated. Machine tool control systems maintain two control objectives: to follow a pre-specified trajectory as closely as possible, and to maintain a pre-specified advancing speed. These systems break the desired trajectory into small segments, and step along these segments to form the path. When the trajectories are multi-dimensional, the systems divide the motion into simultaneous movements in different axes. The drivers of different axes have to move synchronous with one another to obtain the desired trajectory. If the controllers operate independent of each other, then any load disturbance or any difference in the performance of each axis may cause contour. Hence, two major approaches have emerged for improving the control performance in drive systems. The first approach, known as tracking control concentrates on only reducing the tracking in each axis, which indirectly results in the reduction of the contour. The second approach, known as contouring control aims at estimating the contour in real-time and using this estimate in the feedback control law.
Poo et al. studied the effect of dynamic in two-axis, contouring systems. Two specific cases of constant acceleration and constant velocity along a straight line contour were studied. It was found that when the axis gains of the two axes (Kx and Ky) were perfectly matched, the contour was zero even though there were individual axis errors with respect to time. However, when there is mismatch between the axis characteristics, there results a contour. In the case of circular contours, it was found that for perfectly matched axes, the amount of radial was very small and a perfect circle was generated with a radius larger or smaller than the desired radius depending upon the amount of damping and the angular velocity. Mismatch in system gains however has considerable effect on the maximum radial contour with the steady-state contour being elliptical in shape.