A Novel Method on Disturbance Analysis and Feed-Forward Compensation in Permanent Magnet Linear Motor System

Linear motor system has been developed to achieve high speed and high precision motion control in linear motion system, since rotary type motor has backlash and hysteresis due to mechanical parts like chain, belt or screw. However, paradoxically, linear motor system becomes more sensitive to the external disturbance, because it doesn´t have such transmission mechanism. Many studies have been focused on detent force which have dominant and position dependant non-linear properties among the external disturbances. But the solutions suggested by them are mostly complicated and dificult to apply due to the non-linear property of detent force. In this paper, a novel method to analyze the disturbance magnitude related to the harmonics order is introduced. It is based on a disturbance observer that is famous for its simple but robust and powerful scheme to eliminate disturbance. Nevertheless, from the implementation perspective, there exists a limitation with direct feedback of the disturbance observer output. It is because the bandwidth of disturbance observer is limited by measurement noise, while the disturbance gets higher frequency with the speed due to the position dependant property of detent force. To resolve this problem, feed-forward controller with a function of position is suggested. Through experimental results, the suggested method was verified to have much better performance compared to direct feedback of disturbance observer.