Automatic Document Feeders - Where?

    Where are document feeders used?

As stated before, all paper-handling machines have a feed mechanism. It's worth thinking about them a bit to clarify the subject.

The machines of which we speak can be divided into four broad categories: printers, copiers, scanners, and faxes. They have two functions in common, although not all of them do both: these being scanning (input) and printing/saving the image (output). Faxes are input only, in this reckoning; printers are output only, while copiers and scanners do both. Of course, these days many machines combine all or most of the categories - and why not? Additional functions can be added routinely since the technology for any of them is readily available.

Printer are the simplest of the categories: they create marks on paper under the command of a computer, which prepares the image (including, or not, printer functions depending on the sophistication of the printer; at the bottom of the line the printer can only advance lines or pages, whereas the most sophisticated ones have a complex programming language). The feeding requirement is that blank paper shall be fed into the machine. All printers have an ADF, even tiny portable ones. The smallest and simplest printers have a roll of paper as ADF, which may be cut by the machine or may be a tear-off. Most printers in common experience work off a stack of paper which is fed by the various mechanisms already described. Very high-speed printers with no need of graphics use continuous forms which are propelled through the machine with pin-feed tractors.

Faxes are simple, if they just transmit messages: they have only a mechanism to feed the papers in, one at a time and usually quite slowly. Of course, they may not have even this: many just consist of a scanner on which one piece of paper at a time is placed and read for transmission. Most these days produce activity reports, which usually are printed. These machines, then, use an ADF to feed in the input documents, and another to provide blank paper for the printed reports. The latter, if they include an image of what was transmitted, must be graphics printers.

Copiers may also have only a scanner for one page at a time: in that case they have just the ADF for the printing, which (again) may be a roll of paper or a stack. Otherwise, for any more complex machine, there's an input ADF and a blank-paper supply ADF. And they can get very complicated indeed, with multiple paper sizes and two-sided printing options. Multiple paper supply ADF's are needed; the input ADF must be made to handle the largest paper size but must have guides, in the input hopper and internally, to position the paper correctly for reading. For off-size documents it's easier from the design and use points of view to place them on the scanning surface directly and not attempt to feed them in automatically. Two-sided printing usually means reading the paper twice, with the output paper flipped over during the cycle, although some very expensive copiers have two reading devices and two printing devices.

Finally, scanners: everything that's been said about copiers applies to scanners, except that they don't print! The input ADF's have to be more forgiving as they typically have to handle far more varied input media, which have varying characteristics of stiffness, slipperiness, durability, etc. Two-sided scanning causes problems in collating the output images.

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