Sensor-less on-line chatter detection in turning process based on phase monitoring using power factor theory

This paper presents a sensor-less on-line chatter detection method for a turning process by introducing a mechanical energy factor (MEF) and a mechanical power factor (MPF). The MEF and MPF serve as indexes for self-excited chatter and forced chatter, respectively. The indexes are based on the power-factor theory, which generally represents the electrical-power efficiency as having a correlation with the phase difference between the current and the voltage. By applying this theory to a mechanical system, the MEF and MPF can be employed to monitor the phase difference between the cutting force and the displacement/velocity of the tool system, respectively. By monitoring the phase difference, chatter vibration can be detected in time domain with a high response and small number of computations. The MEF and MPF can be calculated without using additional external sensors employing the sensor-less cutting-force estimation technique based on the disturbance observer. The monitoring performance of the proposed method was evaluated through several outside turning tests with a prototype precision lathe. The results showed that both the self-excited and forced chatters were successfully detected with unique thresholds, which did not depend on the cutting condition or the workpiece material.