Laser Tracking Control Implementation for SFF Applications

From a three-dimensional computer graphic model, Solid Freeform Fabrication produces solid objects directly without special tooling and human handling. In order to increas process productivity and accuracy, a time-efficient laser tracking control technique is needed. Based on the minimum time optimal control solution, the desired laser scanning control system is designed and implemented. To obtain uniform solidification during timeefficient tracking which has variable speed, laser power intensity is also controlled in real time by an acousto-optic modulator. Introduction A time-efficient tracking control for a laser scanner with on-line laser power adjustment is designed and implemented in order to increase productivity and to improve the geometric accuracy or the isotropic property of parts when one needs to trace the boundary of a part in SFF. It is not appropriate to use conventional raster scanning. Due to repetitive starts and stops, straight-line vector scanning mode can be slow when curves exist in the contour path. Several articles have presented various schemes for this problem. The preview scheme (Tomizuka, Dornfeld, Bian, and Cal, 1984) and the adaptive algorithm ( Tsao, and Tomizuka, 1987 ) need on-line computation effort which is too large for SFF application. The cross coupled compensator ( Kulkarni and Srinivasan, 1985 ) is designed to reduce the tracking error ( minimizing the contour error) at a sharp corner, however contouring analysis and optimal speed trajectories are not developed for more general paths. The control trajectory scheme (Doraiswami, and Gulliver, 1984) is directly obtained from a specified path with three simple functions regardless of the capability of the actuator. The trajectory generation scheme ( Butler, Haack and Tomizuka, 1988 ) focuses on constant tracking speed which does not give a minimum time solution. The control technology used in this paper is based upon the results ( Wu and Beaman, 1990 ) of Pontryagin's minimum principle and phase plane techniques ( Bobrow, Dubowsky and Gibson, 1985 )[6], ( Shin and McKay, 1985 ). The control system implemented in this paper uses the feedback control design model identified by Wu and Beaman, 1991.