Roller-type ADF's are very well understood and can be manufactured reliably in very large quantities, eminently suitable for many applications. However, they have an upper limit in speed, complexity, and reliability; the rollers do erode and begin to slip; they may mark newly printed documents; and once the metal parts of the workings wear out replacement becomes the only option.
Alternative systems employ vacuum principles to pick and transport the paper. The mechanisms come in at least two varieties: vacuum picking and vacuum belts; these can be combined in high-end machines.
Both systems need an air pump to create the necessary vacuum, of course. In many systems, the pump is also used to separate the top sheet from the stack and to nudge it over sufficiently that the mechanism can start to operate. (Note that sheets can be fed from both the top and the bottom of the stack, depending on the design).
Vacuum picking systems literally suck the paper towards suckers, or vacuum slots, or any small mechanism that can "apply" the vacuum. (One company trumpets its patented "Paper Seeking Suckers" technology!) Whatever the application device is, it is attached to a roller of some sort so that the paper is attracted to it and moved along the paper path simultaneously. It will be appreciated that there are two significant design issues: one is to suck the air out of a rotating roller without too significant a sealing problem; the other is timing - the roller has to let go of the paper at the correct instant or else it will get rolled up and jam the mechanism. If there's more than one roller the problem is much more complex. Still, by the use of photocells and timing gears this is usually accomplished.
Vacuum belt machines use the air flow to separate and start the paper as in vacuum picking machines, but the application of the vacuum to the moving sheet is much simpler. The vacuum chamber is located below the belt, having one or more slots depending on the length of the belt; the belt itself has regularly spaced slots along its length. The operation is obvious: when one of the belt slots goes above a vacuum chamber slot, the paper is sucked down so that the friction of the belt moves it along the paper path.
It will also be obvious that static electricity can seriously bollix up these systems, so that humidity must be maintained at the proper levels. Nevertheless, these machines can operate at very great speeds: 36,000 sheets per minute is quite typical for vacuum-belt types. Apart from high-speed printing, these also find their niche in envelope-stuffing; all sorts of documents can be merged together, regardless of their relative sizes, since the speed of the belts transporting the various pieces can be adjusted easily to make sure that they all arrive at the correct place at the proper time.
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