In the early stage, a scanning apparatus is used to scan the image of a single document. For scanning both sides of the document, the document should be manually turned over after one side of the document has been scanned in order to sequentially scan the other side of the document. For scanning a stack of documents, after one document has been scanned, the document should be removed from the scanning apparatus and then a next document could be placed on the scanning apparatus in order to be further scanned. Since the process of manually turning over the document or manually replacing the document is very troublesome, the conventional scanning apparatus is not feasible. Recently, an automatic document feeder is usually integrated into the scanning apparatus. The automatic document feeder is suitable to perform a duplex scanning operation and successively scan plural documents without the need of manually turning over or replacing the documents.
Generally, the automatic document feeder has a sheet input tray for placing a stack of documents. The automatic document feeder also has a sheet pick-up module for successively feeding the stack of documents from the sheet input tray to the internal portion of the automatic document feeder in a sheet-feeding direction. For allowing only one document to be fed into the internal portion of the automatic document feeder at each feeding time, the sheet pick-up module has a sheet separation roller and a separation pad. The separation pad is disposed under the sheet separation roller. The sheet separation roller may provide a frictional force to the document that is contacted with the separation pad. The frictional forces between the sheet pick-up module, the separation pad and the documents should be elaborately controlled. Generally, the frictional force between the sheet pick-up module and the document contacted with the sheet pick-up module is greater than the frictional force between the documents. In addition, the frictional force between the separation pad and the document contacted with the separation pad is also greater than the frictional force between the documents. As a consequence, only one document is allowed to be fed into the internal portion of the automatic document feeder at each feeding time. As the automatic document feeder is used for a long time, the separation pad is usually abraded, or even losses the function of separating documents. In this situation, the separation pad needs to be replaced with a new one. Since the separation pad is usually securely fastened on the automatic document feeder, the process of replacing the separation pad is very complicated.
For solving the above drawbacks, U.S. Pat. No. 6,659,450 disclosed an automatic document feeder with an easily disassembled separation pad and retard roller. During the sheet-feeding process, the retard roller generates a damping torque in a sheet-feeding rotating direction. In response to the damping torque, the retard roller provides a frictional force to the document. Although the separation pad and the retard roller of the automatic document feeder described in U.S. Pat. No. 6,659,450 are disassembled more easily when compared with the prior art, there are still some drawbacks. For example, after the retard roller is disassembled, the spring for providing a normal force on the retard roller is still retained in the automatic document feeder. During the process of assembling the retard roller, the retard roller needs to be installed in the automatic document feeder while aligning the retard roller with the spring. In other words, the retard roller needs to be precisely combined with the spring in order to achieve a normal function of the retard roller. The process of assembling the retard roller is not user-friendly.
Moreover, in a case that the documents are jammed in the internal portion of the automatic document feeder, the installation of the retard roller or separation pad incurs some drawbacks. For example, due to the frictional force between the jammed documents and the retard roller (or separation pad), the jammed documents fail to be pulled out of the automatic document feeder in a sheet-returning direction, which is opposed to the sheet-feeding direction. For releasing the jammed documents, the user needs to open the upper cover of the automatic document feeder to uplift the sheet pick-up module. As such, the sheet pick-up module is separated from the upper sides of the jammed documents and the frictional force between the jammed documents and the retard roller (or separation pad) is eliminated. Meanwhile, the jammed documents could be effectively released.
Since the process of removing the document from the automatic document feeder is very troublesome, there is a need of providing a retard roller for moving the document in the sheet-feeding direction and the sheet-returning direction without the need of opening the upper cover.