Automatic document feeder for an image forming apparatus

An automatic document feeder for an image forming apparatus, such as a scanner, is provided. The automatic document feeder comprises a document-loading means, a document-receiving means, a sheet path, a sheet-transporting mechanism, a scan window, a first sensor means and a second sensor means. The first sensor means is disposed at the entrance of the sheet path and the second sensor means is disposed at a predetermined position along the sheet path. The second sensor means includes a sensor device and a rotatable shaft attached thereto. When the sheet-transporting mechanism transports at least one sheet along the sheet path through the second sensor means, the rotatable shaft is rotated at a predetermined angle.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and more particularly to FIG. 2 and FIG. 3A to FIG. 3 C, there is shown an automatic document feeder which is constructed in accordance with the preferred embodiment of the present invention and which is adapted for use with a flatbed scanner. Although in the preferred embodiment of the present invention, reference is made to a flatbed scanner, those skilled in the art will understand any type of flatbed input device can be utilized, such as a flatbed copier. FIG. 2 is a schematic side view of an automatic document feeder 20 according to one preferred embodiment of the present invention. Referring to FIG. 2 , the automatic document feeder 20 is mounted on an image scanner 22 , e.g. a flatbed scanner. The automatic document feeder 20 includes a document-feeding means 201 , a document-receiving means 202 , a sheet-transporting mechanism including a feed-in roller 203 , a first transport roller pair 204 , a second transport roller pair 205 and an exit roller pair 206 , a first sensor means 207 , a second sensor means 208 including a sensor device 208 a and a rotatable shaft 209 attached thereto, sheet paths 210 A, 210 B and 210 C, and a scan window 211 . The sheet paths 210 A, 210 B and 210 C include at least a sheet-guiding plate. The document-feeding means 201 loads a stack of documents P 2 . The document-receiving means 202 is placed under the document-feeding means 201 for receiving the documents one by one, which have been sequentially fed out from the document-feeding means 201 . The first sensor means 207 is disposed at the entrance of the sheet path 210 A. The second sensor means 208 is disposed at a predetermined position at the sheet path 210 B. The sheet path 210 C includes a scan window 211 that is provided such that the image scanner 22 reads information carried on the document, which is being transported through the scan window 211 . The feed-in roller 203 , the first transport roller pair 204 , the second transport roller pair 205 , and the exit roller pair 206 are arranged along the sheet paths 210 A, 210 B and 210 C. The feed-in roller 203 pulls a sheet out from the stack of the documents P 2 on the document-feeding means 201 , and feeds the sheet to the first transport roller pair 204 through the sheet path 210 A. The first transport roller pair 204 , the second transport roller pair 205 , and the exit roller pair 206 sequentially transports the sheet through the sheet paths 210 B and 210 C into the document-receiving means 202 . The image scanner 22 includes a scan head 221 with a line-image-sensor such as, for example, a charge-coupled-device (CCD), and a light source (both which are not shown). The scan head 221 is under the scan window 211 . The line-image-sensor scans a widthwise line of the document to read the information carried on the line. The widthwise direction or scanning direction of the line-image-sensor is perpendicular to the surface of the sheet. The line-image-sensor reads information from the leading edge to the trailing edge of the document, line by line, while the document is being transported through the scan window 211 . Referring to FIG. 3 A, when the feed-in roller 203 pulls a sheet P 21 out from the stack of the documents P 2 on the document-feeding means 201 , and feeds the sheet P 21 to the first transport roller pair 204 through the sheet path 210 A, the first sensor means 207 will detect feed-in of the sheet P 21 to the sheet path 210 A. Referring to FIG. 3 B, when the sheet P 21 is transported along the sheet path 210 B through the second sensor means 208 , the sensor device 208 a will detect the transportation of the sheet P 21 along the sheet path 210 B. At the same time, the rotatable shaft 209 is rotated counterclockwise to a predetermined angle to block the light emitted from the light source within the scan head 221 . The line-image-sensor will sense the reflective light from the rotatable shaft 209 and detect the sheet P 21 accessing to the scanning start position. The image scanner 22 is then started. Referring to FIG. 3 C, when the trailing edge of the sheet P 21 leaves the scan window 211 , the exit transport roller pair 206 transports the sheet P 21 into the document-receiving means 202 . By means of the improved structure of the automatic document feeder 20 of this invention, the sensor means used to detect if a sheet accesses to the scanning start position is saved and replaced by the line-image-sensor of the image scanner 12 . Therefore, the present invention provides cost reduction. The preferred embodiment is only used to illustrate the present invention; it is not intended to limit the scope thereof. Many modifications of the preferred embodiment can be made without departing from the spirit of the present invention.