Study on Stability Problem of Closed-Loop Digital Servo System in NC Machine Tools

Abstract

Stability analysis of a feed drive servo system of numerically controlled machine tools is important for improving its performance but very difficult since an actual system has a lot of nonlinearities. In this paper, we propose two methods. One is a simple identification method of the nonlinearities and the dynamics of hydraulic closed-loop digital servo system used in an actual NC machine tool, which utilizes a measured hydraulic motor input pressure. The other is a semi-graphic solution method using a describing function on the stability problem of a system which has more than two nonlinearities such as DA conversion nonlinearity, nonlinear friction, and nonlinearities between spool displacement in servo valve hydraulic motor input pressure and flow rate. Using these methods, it has been shown, the nature of stability and the mechanism of self-excited oscillations observed in an actual system can be clearly explained.