Japanese Patent Application Publication No. 2007-91369 discloses a sheet processing apparatus to perform processes of sorting, stitching and punching.
The apparatus includes a punch unit, an adjustment unit, a sensor unit and a changeover switch. The punch unit punches the sheets discharged sequentially from the image forming apparatus. The adjustment unit slides the punch unit in the direction crossing the sheet conveying direction and adjusts the punching position. The sensor unit is installed in the sliding punch unit and comprises a trailing edge sensor to detect the leading edge and trailing edge of a sheet in the conveying direction and a lateral register sensor to start movement at predetermined timing after detection of the leading edge of the sheet by the trailing edge sensor and detecting the lateral edge of the sheet. The changeover switch goes over between a high productivity mode and a precision mode. In the high productivity mode, the trailing edge sensor detects the leading edge of the sheet and then the lateral register sensor starts movement at early timing and detects the lateral edge of the sheet on the leading edge side of the sheet conveyed, thus the time required for the punching process is shortened. In the precision mode, the lateral register sensor starts movement inversely at late timing and detects the trailing edge side of the sheet when the conveyance of the sheet is stopped, thus the hole position is decided accurately at the sacrifice of the processing time.
However, in the aforementioned apparatus, even in the high productivity mode or the precision mode, regardless of the sheet size and sheet conveying speed, the lateral register sensor starts movement after the trailing edge sensor detects the leading edge of the sheet. Therefore, if the conveying speed is increased to improve the processing performance, a problem arises that the driving up to the detection position is too late. Particularly, as the size of the sheet in the width direction crossing the conveying direction becomes smaller, the movement distance from the standby position outside the lateral edge of the sheet to the lateral edge on the sheet becomes longer is increased. Therefore, the time until the position for detecting the lateral edge of the sheet becomes longer, so that as the sheet size in the width direction becomes smaller, it is impossible to increase the conveying speed and improve the performance.