For motion systems, such as robotic pick and place units, or precise electron microscopes, friction in the motion stages is often limiting the accuracy of the motion performance. In practice, a friction compensating feed forward signal can be tuned (using the gain K in the figure) on the basis of repeated experiments, while observing the time domain error of the motion. The results depend on the conditions of the signals and disturbances. In order to improve this tuning, a new method has been invented, exploiting a frequency domain based method. It is based on our on-going research to exploit the benefits of linear control theory towards classes of nonlinear systems, and makes use of the analysis of the positioning error in the frequency domain. By summarizing all higher harmonics of the error, a measure can be determined for the correctness of the feedforward gain K. In a case study, this method is applied to optimally design the feed forward friction compensator for an industrial motion stage in a transmission electron microscope as an example. It is shown that the new frequency domain approach yields a tool to fast and easily design friction control in practice with high detection sensitivity, while providing a well defined notion of optimal performance.
In the figure the results are shown for low speed scanning of the microscoop motion system,left: no feed forward compensation and, right: optimal feed forward compensation.
More details of the method and results can be found in the full paper :
David Rijlaarsdam, Pieter Nuij, Johan Schoukens, Maarten Steinbuch, “Frequency domain based nonlinear feed forward control design for friction compensation”, Mechanical Systems and Signal Processing, Volume 27, February 2012, Pages 551-562