Device for recognising the leading edge of a printing medium and a method for avoiding bending, which may occur when gripping in a sheet-processing subsystem

摘要

The method involves picking a media sheet from a stack, then moving the picked sheet along a media path. Any skewing of the media sheet existing in the media stack or occurring during the pick cycle is removed before the sheet reaches a position to receive print markings. In particular, the alignment of the skewed media sheet is altered (i.e., the sheet is moved) to square the media sheet to the media path. An electro-optic sensor detects when the top of a media sheet enters between a drive roller and pinch roller. Upon entering, the media sheet moves a mechanical flag into the light circuit of the optical sensor. After the media sheet trips the flag, the drive roller moves the top edge of the media sheet backward along the media path out of the grasp of the pinch roller and drive roller. As the sheet moves out of the grasp, the top edge of the sheet falls into squared alignment with the drive roller and pinch roller. The squared media sheet then is moved forward tripping the flag again. The drive roller then pulls the sheet along the media path into the path of the optical sensor so that the optical sensor detects the top of the page. The sensor then is shuttled to scan for a side of the page. With the top of page and side of page known, and with it known that the page is squared to the media path, markings are placed accurately on the media sheet.