A variety of methods have traditionally been used to remove skew by mechanical means. Most document feeders incorporate some of these methods. These mechanical methods have a long history, many of them having originated in the printing equipment industry.
The length of a document along the direction of travel has an impact on its tendency to skew and on the effects of any skew that does occur.
Longer documents are typically less inclined to skew.
In the feeder, this is because a longer document has a wider base for the alignment operation, whether this is performed by a person at load time or by drive rollers at feed time. A longer distance better assures the parallelism of the document edge with the feeder guides.
In the transport, a longer document is typically held by more rollers or belts simultaneously, making it harder for it to rotate.
Unfortunately, despite the lower likelihood of skew occurring, if a given skew angle is introduced into a longer document, more translation of the document edge across the direction of travel will occur than for a shorter document. In the extreme case, a trailing corner of the document may actually leave the camera's field of view if a generous overscan is not provided.
Most scanners incorporate one or two vertical edges adjacent to the paper path of the feeder to constrain the rotation of a document as it is fed. A single alignment guide makes it possible to accommodate a variety of paper widths in the same stack. A set of two alignment guides does not permit this, but better constrains a stack of identical sheets.
Some scanner feeders use an arrangement where there is a single pick roller to draw a document out of the feeder. The document then collides with either a pair of (paused) drive rollers or a set of retractable fingers at the entry into the transport. The single pick roller does not completely constrain the document, permitting it to rotate to a proper orientation as it hits the pair of de-skewing entry rollers or fingers.
Some scanners use a tapered roller to assist in removing skew or to help prevent the introduction of further skew into the document.
The Bureau of the Census Technical Services Division has developed a prototype feeder incorporating some mechanical deskew features. While these features are not described specifically on their web site, the full set of blueprints may be downloaded from there.
This de-skewing feeder apparently uses a set of marbles or ball bearings which roll on the document while captive in a track. This track is oriented at some angle to the direction of travel of the document. The net effect of the ball bearings is to drive the document against an alignment guide. A similar method was used in an early Burroughs paper transport.
A method long used in the check and remittance processing world by such companies as BancTec and Unisys uses a large diameter drum in the transport path which has a flange on one end. The document is driven down against the flange by rollers which roll against the document in a slightly downward direction. The drum represents a folding of what would otherwise be a relatively long path. The drawback is that transitions from belts onto and off of drums are prone to jams.
While a flanged drum is reasonably effective, analysis of jam reports has shown that a reasonably high percentage of jams occur in this mechanism relative to the rest of the machine. While the mechanical de-skew function may lower the frequency of some types of jams downstream in the transport, the mechanical complication of the de-skew device itself lowers reliability of the entire device. Any place where moving paper is partially unconstrained, the possibility of jams occurring there is increased.
BancTec's latest check transport relies on electronic grayscale de-skew technology.
A technique using a pair of non-parallel rubber belts is in use in the Image Trak vacuum belt transport from Image Business Machines, LLC (IBML). It is highly effective at the modest speed at which this transport operates, but to suit long documents it requires a relatively long distance along the direction of travel to complete its work. The belt parallel to the direction of travel and nearest the alignment guide supports the paper without vacuum pressure, allowing the document to rotate as biased by the alignment guide. The non-parallel belt, farther from the alignment guide, has the document held to it by vacuum pressure and it drives this farther edge of the document toward the alignment guide, rotating it.
A case can certainly be made that mechanical de-skewing lowers the probability of jams downstream in a transport. This is especially true of complex transports with many stations, many direction changes and many hand-offs between roller or belt systems.
A stronger case can be made, however, that the simplest, shortest, straightest transport arrangement will have the highest reliability and the lowest jam rate. As noted earlier, mechanical de-skewing components themselves add to this complexity and thus account for a not insignificant portion of all jams. The trend in the design of modern production page scanners, check and remittance scanners and mail handling equipment is toward straighter, simpler, and shorter transports.
Simpler mechanical designs, while reducing jam rates, also have fewer components, leading to lower manufacturing and maintenance costs.