Method for chatter detection with standard PLC systems

The aim of growing productivity together with increasing quality demands in machining leads to milling processes that are near their stability limits. The remaining stability reserves become smaller and the risk of unstable processing conditions like chatter increases. Unstable processes cause unwanted vibrations with high amplitudes in bearing loads. As a result, the lifetime of the bearings of the main spindle is reduced. Besides this, the surfaces of unstable processed work-pieces have unwanted chatter marks and do not fulfill the quality demands. To basically avoid unstable processing,a continuous monitoring of the process state is necessary. In this paper, a method for monitoring cutting processes using a standard programmable logic controller which is integrated in the drive controller of the machine tool spindle unit is presented. The method is real time-capable and based on the hypothesis that unstable process conditions result in a modulation of the amplitudes of the cutting forces. To detect this, an order tracking method is implemented, which uses a recursive parameter estimation algorithm together with inherently given signals of the drive controller. It is shown that the characteristic property of the used estimation algorithm and allowed aliasing lead to a reliable chatter detection even at sub-sampling. Finally some results of the first experimental investigation of the method are given.