1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile machine, and more particularly, to an image forming apparatus which corrects skew feeding of a sheet while conveying the sheet, and which conducts positioning of a side edge (end) of the sheet in a width direction intersecting with a sheet conveying direction.
2. Description of Related Art
Generally, image forming apparatuses of an electrophotographic system, an offset printing system and an ink-jet system are known. As image forming apparatuses using the electrophotographic system, image forming apparatuses of various systems are known, such as an image forming apparatus of a direct transfer system which transfers a toner image from a photosensitive drum directly to a sheet, and an image forming apparatus of an intermediate transfer system which once transfers a toner image to an intermediate transfer member and then transfers the toner image to a sheet. As image forming apparatuses using the electrophotographic system, an image forming apparatus of a tandem system in which a plurality of image forming portions is arranged, and an image forming apparatus of a rotary system in which a plurality of image forming portions is cylindrically arranged are known.
In recent years, in image forming apparatuses of the electrophotographic system, making full use of a merit that a plate is not formed, apparatuses targeting a printing market of small copies are provided. In order to be accepted by such a light printing market, high velocity (high productivity) and high image quality must be achieved in various kinds of materials, and a requirement for sheet conveying precision is increased. An image position precision with respect to a sheet is most required to be high, an image position deviation of front and back when images are formed on both surfaces is also included. There is a method for adjusting a position of an image with respect to a sheet, but a method for adjusting a sheet with respect to an image predominates.
The precision of an image position is determined by registration of a sheet in a sheet conveying direction, registration of a sheet in a width direction intersecting with the sheet conveying direction, magnification and skew feeding. Among them, it is difficult to correct the skew feeding of a sheet by electrical control. For example, if skew feeding of a sheet is detected and an image which inclines corresponding to the skew feeding is formed, it is possible to correct an image position with respect to the sheet. However, in the case of a color image on which three or four colors are superposed, if an image is inclined every sheet, color is changed in every sheet due to deviation of dot formation of each color. Further, since it takes time to calculate for inclination of an image, productivity is largely reduced. Thus, the skew feeding of a sheet is determined by performance of conveying precision of a sheet.
Generally, the skew feeding of a sheet and registration are independently controlled, but in recent years, there is proposed a method for correcting the skew feeding and correcting registration in a direction intersecting with the sheet conveying direction at the same time or by the same driving operation (see Japanese Patent Laid-Open No. 10-310289). More specifically, it includes two moving drive motors which independently slide two rollers arranged in the sheet conveying direction in a direction intersecting with the sheet conveying direction, and two optical sensors which detect a side edge (end) of a sheet are arranged in the sheet conveying direction corresponding to the roller. Control is carried out such that the rollers are slid in the width direction so that the side edge of the sheet follows the optical sensor.
According to the conventional method, however, since the rollers are slid in the direction intersecting with the sheet conveying direction to correct the skew feeding of the sheet or correct the position of the sheet in the width direction, stress is applied to the sheet when the rollers slide. Especially, in the case of a thin paper sheet, since a sheet is bent between the two rollers, it is difficult to precisely correct the skew feeding of a sheet or correct a position of a sheet in the width direction.
According to the conventional method, since the moving drive motor is rotated normally and reversely by an ON/OFF operation of the optical sensor, a sheet overshoots in the width direction and reciprocates, and it takes time for a side edge of the sheet to reach a target position. If an attempt is made to enhance the position precision of an image with respect to a sheet, the conveying velocity of the sheet cannot be increased, and the productivity cannot be enhanced.
Hence, the present invention provides an image forming apparatus which can handle various kinds of materials including a sheet having weak elasticity such as a thin paper sheet, and which has excellent position precision of an image with respect to a sheet, and which enhances the productivity.