Web steering for an image recorder

Abstract

A photosensitive medium, normally a diazo printing plate or photopolymer-based proof, is exposed in a series of exposures each of a small rectangular area. Typically several linearly-arrayed Deformable Mirror Devices (DMD's) each spatially encode a light beam in order to expose corresponding pixels within corresponding regions of the medium. Between successive exposures, the several DMD's are moved to a new position relative to the medium along a first, x, axis so that new exposures of new regions can be made. Normally the arrayed DMD's are moved relative to the imager's frame by a screw drive. There is an additional, separate relative movement between the DMD's and the medium along another, perpendicular, y, axis. Normally the medium is moved relative to the imager's frame by nip rollers. In order to provide a required precision in registering successively imaged regions of the medium along either or both axis, reference marks are exposed directly on the medium. After each movement of the medium along either or both axis, these reference marks are sensed, typically by four-quadrant photodetectors, and errors in the repositioning of the medium are determined. To compensate for these errors, the nip rollers are adjusted by a servomechanism. The medium is held by a cyclic pressure pad while it is imaged. So proceeding, the entirety of a large photosensitive medium is imaged in successive regions along two perpendicular axis while errors in image alignment and registration are substantially reduced.