Quadrant glitch compensation by a modified disturbance observer for linear motor stages

Abstract

In this paper, we propose to develop a novel disturbance force compensator to effectively suppress quadrant glitches in a feed drive system using a linear motor and linear motion ball guides (LMBGs). In the circular motions, feed drive systems using LMBGs generate a following error called quadrant glitches at the motion direction changing point of each axis. Quadrant glitches are engendered by disturbance force due to friction, and disturbance force compensation is necessary to compensate quadrant glitches. A disturbance observer is a disturbance force compensation method in the motion control field. If a disturbance observer causes quadrant glitch compensation, the height of quadrant glitches decreases. However, after a quadrant glitch, a deviation with a sign opposite to that of the quadrant glitch, called “inverse response”, is generated. An inverse response degrades the contouring motion accuracy and causes an excessive cutting of the cylindrical process. The proposed compensator, called a “disturbance suppressor”, presents two advantages over an ordinary disturbance observer: 1) there is no inverse response, and 2) the compensator exhibits a superior quadrant glitch compensation capability. The disturbance suppressor has two structural differences compared to an ordinary disturbance observer: 1) a pull-off point and a summing point are interchanged, and 2) two gain constants, K and β, are added to a disturbance observer. In this study, we investigate the causes of the inverse response suppression and quadrant glitch compensation improvement when applying the disturbance suppressor. We also propose a gain tuning method for the disturbance suppressor in a quadrant glitch compensation based on our results. The effectiveness of this compensator was verified by experiments using a feed-drive system. In the case of applying the disturbance observer, the height of the quadrant glitch was 10.4 nm and the amplitude of the inverse response was 3 nm. In the case of applying the optimized disturbance suppressor, the height of the quadrant glitch was 7.7 nm and there was no inverse response.