Conceptually, document feeders are quite simple: a paper supply, a sheet-picking mechanism, a device to move the sheet through the machine, and an output hopper. But within these parameters, there is a great field of possibilities for the various mechanisms. A description of a few relevant patents will illustrate the point. (U.S. patent #'s are supplied).
Patent #4564792: January, 1986. Conventional copiers (etc.) place the input document on the processing surface using a belt which operates for a predetermined period of time. Variations in the paper (thickness, quality, etc.), in the driving motors and in the belt, especially when it's worn and loses its tension, and in the environment (humidity and heat), all combine to lessen the precision of the placement of the paper. This invention uses Hall effect sensors to determine very precisely the actual position of the paper and to stop the positioning motor at the correct time.
(The Hall effect, named for its discoverer, Edwin Hall, in 1879, occurs when a flow of electrically charged particles is subjected to a magnetic field which isn't parallel to its direction. In such a case, the path of the particles is changed and causes a buildup of charges of one electrical sign on one side of the original path. The maximum possible buildup is generated when the magnetic field is at right angles to the original path. The effect also differentiates between positive charges moving in one direction and negative charges moving in the opposite direction, and was among the first physical proofs that electricity is carried by elections and not protons. The effect is exceedingly important in semiconductors).
Patent #5377966: January, 1995. This patent describes a combined feeder and output hopper, in one unit, with the mechanisms necessary to move the paper on to the receiving machine and to take it back. This is accomplished by moving the paper in a u-shaped path. There is provision for preventing curling on the leading edge of the returning document. The general objects are to provide a compact, reliable document feeder and receiver, to guarantee that the input and output documents are separated, and to make it easy for the operator to load and unload the documents.
Patent #5946540: August, 1999. A device to handle documents that are fed via a curved u-shaped path onto the processing surface, in such a manner as to make sure that they are kept within the correct radius of the curve, and that they will arrive in the proper position on the processing surface. The rationale is that as the devices become more compact, there is less slack in the paper path, and that therefore there is a need to enforce the positioning of the paper in its path.
These examples will give an idea of how complex and subtle the design of a machine can be. There are innumerable other patents for ADF's. These can be located through the U.S. Patent Office or through Internet services such as patents.com or freepatentsonline.com.
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