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86 A. Wawrzak, K. Karwowski, K. Karwowski, S. Mandra, M. Mizan

1. INTODUCTION

High precision machining is one of the most important targets in machinę construction. It is particularly difficult to get this in case of high speed and high acceleration machining. Generally the machining errors depend on machinę construction and CNC (Computer Numerical Control) system.

The most of CNC systems are based on servo-drives with DC (Direct Current brush motors) and PMSM (Permanent Magnet Synchronous Motors) motors. The servo-drive controllers are generally Cascade PID controllers. The PID controllers of servo-drives do not decrease tracking errors enough by fast changes of the motion velocity. The good method of decreasing of the tracking errors in Cascade servo-drives is to use the feedforward command and feedforward friction compensation [1, 2], The feedforward loops are the open loops. The correctly designated parameters of feedforward loops give very good results in tracking errors compensation. Most Solutions for feedforward loops parameters settings are based on theoretical models of: servo-drive, axle of power transmission and machining. The parameters of feed forward loops are stable and off-line designated. Friction and elasticity may change during machinę motion and change due to machinę exploitation. Therefore the designated models loose their accuracy. Inaccurate models of servo-drive, axle or machining can generate tracking errors. These problems appears particularly in light, inexpensive and not high precision machines. The good method to improve the machinę precision and decrease the tracking errors is the model verification during the motion and machining process and adequately on-line designation of the feed-forward parameters.

2. CNC CONTROL SYSTEM

WITH FEEDFORWARD CONTROL

There are two kinds of feedforward Controls applied in CNC control system to improve the tracking performance. The one is the command feedforward control. The other is the friction feedforward control. These are open loop Controls. The błock diagram of servo system with the command feedforward is shown in Fig. 1. The velocity reference value is the sum of position loop output

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