Laser beam raster scan under variable process speed - an application of time varying model reference repetitive control system

A laser beam raster scanning problem, where the laser beam must traverse certain trajectories on a moving platform is addressed in this paper. The raster scanning motion is periodic with respect to the platform coordinate. The periodicity depends on the feed speed of the platform. This problem is addressed by transforming the time domain system dynamics to the platform´s coordinate frame. This results in a problem of designing a controller for a time varying system to track periodic trajectory. In the control scheme, a time varying model reference control is first designed to render a time invariant system. A repetitive control is then added to the compensated time invariant system to track periodic profiles. This paper presents the scheme´s real time implementation and its experimental results. Different speed variation profiles and reference signals have been tested. The experimental data show the effective of this scheme in reducing the tracking error induced by the dynamic speed variations