FIG. 8 shows a state in which sheets S are stacked in a sheet cassette serving as a sheet holding section of an image forming apparatus. FIG. 8 shows the interior of the sheet cassette from a side where the sheets S are fed. First, in FIG. 8A, an intermediate plate 401 on which the sheets S are stacked is provided in the sheet cassette, and the sheets S stacked on the intermediate plate 401 are regulated and held by side regulation plates 204 and 205. The side regulation plate 205 is movable in the direction of an illustrated arrow A, and regulates and aligns the sheets S in accordance with the size of the sheets S in a direction orthogonal to a conveying direction of the sheets S (also referred to as a widthwise direction of the sheets S). In addition, a leaf spring 206 for applying a predetermined pressure to the sheets S is provided at the side regulation plate 205 to regulate the sheets S. Further, at a side opposite to a side towards which the sheets are conveyed, the sheets S are also regulated and held by a rear-edge regulation plate, which is movable in accordance with the size of the sheets S.
The stacked sheets S are picked up by a pickup roller 103 and are fed into the image forming apparatus. If a predetermined number of sheets S is fed, and a sheet S is no longer detected by a sheet-surface sensor 104 that detects a topmost surface of the sheets S, the intermediate plate 401 is lifted up in the direction of arrow B until a sheet is detected by the sheet-surface sensor. Accordingly, while repeatedly picking up the sheets S and lifting up the intermediate plate 401, the sheets S are supplied. According to Patent Document 1, a stepping motor for lifting up stacked sheets S is used to repeat a lifting-up operation in accordance with the number of picked up sheets.
Further, in such a sheet cassette, the sheets S may not be properly fed even if the sheets S are picked up by the pickup roller 103. In such a case, the picking-up operation by the pickup roller 103 is repeatedly executed again to also perform control for reducing the occurrence of improper sheet-feeding (also called “sheet-feed retry control”). Patent Document 2 discusses a technology that reduces the occurrence of unsuccessful sheet-feeding by executing a picking-up operation again when a sheet S is unsuccessfully fed by a pickup roller.
In such a sheet cassette, when the environment in which the sheet cassette is used is a high-temperature and high-humidity environment, the sheets S absorb moisture, thereby reducing the toughness of the sheets. The sheets may become flexed or wavy by pressure of the side regulation plate 205. An example in which the sheets are wavy is shown in FIG. 8B. Accordingly, if the sheets absorb moisture and become wavy, a height of the sheets S opposing the pickup roller 103 (height indicated by an auxiliary line 1) is less than a height of the sheets at a position where the sheet-surface sensor 104 performs detection (height indicated by an auxiliary line 2), thereby producing a height difference (h). As mentioned above, on the basis of a detection result of the sheet-surface sensor 104, the intermediate plate 401 is lifted up to control the height of the sheets S. Therefore, when the sheets are wavy, pressure generated by contacting the pickup roller 103 with the sheets S is reduced, thereby causing an improper pickup operation.
To reduce the waviness of the sheets S, for example, the pressure of the side regulation plate 205 may be reduced. However, if the pressure of the side regulation plate 205 is reduced, the degree of alignment of the sheets S in the cassette is reduced. If the degree of alignment is reduced, the sheets S may be obliquely fed. If the sheets S are obliquely fed, improper sheet-feeding results. Therefore, this method of reducing the waviness of the sheets does not satisfactorily solve the problem.
The present invention is achieved for solving the aforementioned problems, and its object is to make it possible to reduce the occurrence of improper sheet-feeding when sheets are fed.