BACKGROUND OF THE INVENTION Printers which use individual sheets of printing media usually transport the sheets along a path, for example using conveyer belts or pneumatic means or gravity. Each sheet is brought to a stop at certain stopping points, for example to stack the sheets, or to flip them over, or to transfer them to a cylinder where printing takes place, or to align them, or to process them in some other way. In order to maximize throughput, it is generally desirable to move the sheets as quickly as possible along the path, and to have several sheets in the path at the same time, following one another as closely as possible. As used herein, the term"printer"includes both printers and copiers.

However, if a sheet hits a stopping point while moving at a high speed, it will experience a very high momentary force, which may cause the sheet to bounce if it is a heavy weight sheet. Bouncing is undesirable, since it can cause the sheet to become misaligned, or positioned incorrectly. Bouncing can be minimized if the sheet is coupled to an element of high inertia, for example by using a vacuum conveyer belt. But if the sheet is a light weight sheet, coupling it to a high inertia when it is slammed into a stopping point may cause the sheet to buckle, wrinkling and creasing it, and possibly causing the printer to jam.

If the printer is to be used for both light weight and heavy weight sheets, then both buckling and bouncing can be avoided by slowing the sheet down before it hits the stopping point. This method is described, for example, in US patent 5,967, 506, to Miki et al. But slowing the sheet down means that the sheets in the path must be spaced further apart, so that the following sheet will not catch up to the first sheet when the first sheet slows down. This decreases the throughput of the printer.

US patent 4,674, 739 to Smith describes a machine for producing cardboard boxes, in which the boxes are conveyed to various stopping points where they are cut, creased, and printed. In order to avoid having a box slam into a stopping point at high speed, grippers begin moving away from the box as the box approaches, and close onto the box when the grippers and the box are moving at nearly the same speed. The grippers continue to speed up, while holding the box, and finally bring the box to a stop at the next stopping point. This patent also mentions a prior art machine, described in US patent 3,042, 398 to A. F. Shields, in which a box

speeds up as it approaches fixed grippers at a stopping point, in order to increase the distance between that box and the next box, and then slams into the grippers at high speed.

SUMMARY OF THE INVENTION An aspect of an embodiment of the invention concerns a printer in which a sheet of printing media moving at an initial velocity is first sped up, and then slowed down below its initial velocity, before it hits a stop. The speed up is great enough, and of long enough duration, that the slow down does not allow the next sheet of printing media to catch up to that sheet before it reaches the stop. The sheet slows down enough before reaching the stop, so that it does not bounce or buckle significantly when it hits the stop.

If the sheets do not accumulate at the stop, but each sheet stops momentarily and then moves on, then the speed up of each sheet is optionally great enough, and of great enough duration, so that the next sheet does not catch up to that sheet before that sheet leaves the stopping point. For example, the excess speed integrated over the period of higher speed is equal to or greater than the deficit in speed integrated over the periods of lower speed and stopping.

Optionally, in addition to momentarily speeding up before slowing down and stopping at the stop, each sheet also momentarily speeds up after leaving the stop, to a velocity greater than the initial velocity. This prevents the following sheet from colliding with that sheet after that sheet leaves the stop, when the following sheet speeds up. If the sheets only speed up before the stop, but do not also speed up after the stop, then they may still collide, depending on the initial spacing between the sheets, and on the time profile of the velocity of the sheets.

There is thus provided, in accordance with an embodiment of the invention, a method of serially moving a plurality of sheets of a printing media along a same path through a printer to a stop, the method comprising: a) moving at least a first of the sheets and a second of the sheets along the path at a same initial speed, with the first sheet ahead of the second sheet and separated from the second sheet by an initial gap; b) after moving the first sheet at the initial speed, moving the first sheet along the path at one or more speeds, faster than the initial speed, thereby increasing the gap; c) after moving the first sheet at the one or more faster speeds, moving the first sheet along the path at one or more speeds, slower than the initial speed; d) impinging the first sheet against the stop, while the first sheet is moving at one of the slower speeds.

In an embodiment of the invention, the gap between the first sheet and the second sheet does not close completely during the moving of the first sheet at the one or more speeds slower than the initial speed.

In an embodiment of the invention, excess speed integrated over the period of higher speed is equal to or greater than a deficit in speed integrated over the periods of lower speed and stopping.

In an embodiment of the invention, a decrease in the gap while the first sheet is moving at the one or more slower speeds is greater than the initial gap.

Optionally, the speed at which the first sheet impinges against the stop gate is sufficiently low so that the first sheet does not bounce substantially, and wherein the first sheet would bounce substantially, at least sometimes at the initial speed.

Optionally, the first sheet bounces by less than 5,3 or 1 mm when the first sheet impinges on the stop, and the first sheet would bounce by more than 5,3 or 1 mm respectively, at least sometimes, if it were to impinge on the stop while moving at the initial speed.

Optionally, the first sheet bounces a short enough distance so that the first sheet returns to the stop again in less than 10-30 milliseconds after the first sheet impinges on the stop, wherein the first sheet would bounce, at least sometimes, a sufficient distance so that it would take more than 30 milliseconds to return to the stop, at the initial speed.

Optionally, the first sheet bounces a short enough distance so that the leading and the trailing edges of the first sheet are both displaced laterally by less than. 045-1 mm when the first sheet returns to the stop after impinging on the stop, but the leading or trailing edge or both would be displaced by more than 1 mm, at least sometimes, if the first sheet were to impinge on the stop at the initial speed.

In an embodiment of the invention, the speed at which the first sheet impinges against the stop is sufficiently low so that the leading edge of the first sheet is not substantially damaged, wherein the first sheet would be substantially damaged, at least sometimes, if it were to impinge on the stop at the initial speed.

In an embodiment of the invention, the speed at which the first sheet impinges against the stop is sufficiently low so that the first sheet does not buckle substantially, wherein the first sheet would buckle substantially, at least sometimes, if it were to impinge on the stop at the initial speed.

Optionally, the method includes stopping the first sheet at the stop after impinging the first sheet against the stop. Optionally, the path extends past the stop, and including moving the

first sheet along the path past the stop after stopping the first sheet at the stop. Optionally, moving the first sheet past the stop comprises moving the first sheet at a speed faster than the initial speed.

Optionally, the method includes aligning the first sheet, wherein aligning the first sheet comprises the impinging of the first sheet against the stop. Optionally, aligning the first sheet also includes moving the first sheet against a side alignment guide during the moving of the first sheet along the path.

Optionally, the method includes repeating the method one or more times, using for each repetition a different one of the plurality of sheets in the role of the first sheet, and the sheet following said one of the plurality of sheets in the role of the second sheet.

In an embodiment of the invention, said at least one or more speeds faster than said initial speed is a single faster speed.

In an embodiment of the invention, said at least one or more speeds slower than said initial speed is a single slower speed.

There is further provided, in accordance with an embodiment of the invention, a printer for printing on a printing media, including a transport system for moving the printing media, the transport system comprising: a) a first conveyer which serially moves a plurality of sheets of a printing media at a same initial speed along the path; b) a multi-speed conveyer system which sequentially receives each of the sheets of printing media from the first conveyer, and first moves each sheet at a speed faster than an initial speed and then moves the sheet at a speed slower than the initial speed, along the path; and c) a stop against which each of the sheets of printing media impinges, after the multi- speed conveyer system moves said sheet slower than the initial speed.

Optionally, the multi-speed conveyer system comprises a variable speed conveyer belt.

Optionally the multi-speed conveyer system comprises a plurality of conveyer belts arranged serially, moving at different speeds.

Optionally, the system includes a feeder which feeds the sheets of printing media at regular intervals onto the first conveyer, such that, if the sheets are all a same length less than or equal to a maximum length, then there is a same initial gap separating consecutive sheets on the first conveyer.

Optionally, the multi-speed conveyer system moves each sheet slower than the initial speed for a long enough period, at a low enough speed, so that, during said period, the gap between said sheet and the following sheet decreases by an amount greater than the initial gap, for sheets of the maximum length.

Optionally, the multi-speed conveyer moves each sheet faster than the initial speed for a long enough time, at great enough speed, so that, when the variable speed conveyer moves said sheet slower than the initial speed, a gap between said sheet and the following sheet does not close completely, for sheets of length less than or equal to a maximum length.

Optionally, the stop has a closed state in which sheets of printing medium do not move past the stop, and an open state in which sheets of printing medium can move past the stop, and wherein the stop is in the closed state when each sheet of printing media impinges against the stop. Optionally, the system includes a transport which moves each sheet of the printing media past the stop when the stop is open.

Optionally, the multi-speed conveyer system moves each sheet of the printing media past the stop faster than the initial speed for a sufficient period and at a great enough speed so that, when the multi-speed conveyer system moves the following sheet faster than the initial speed before moving the following sheet slower than the initial speed, the gap between said sheet and the following sheet does not close completely, for sheets of length less than or equal to the maximum length.

Optionally, the printer includes an output tray which comprises the stop, wherein the sheets accumulate in the output tray when the sheets impinge against the stop.

Optionally, the transport system includes a side alignment guide parallel to the path, wherein the first conveyer comprises a conveyer belt oriented at an oblique angle to the path, thereby moving each sheet of the printing media against the side alignment guide while moving the sheet along the path, and if any one of the sheets of the printing media is not already well aligned with the side alignment guide, the first conveyer brings said sheet into better alignment with the side alignment guide when the first conveyer moves said sheet along the path.

Optionally, if any one of the sheets of printing media is not already well aligned with the path, the stop brings said sheet into better alignment with the path when said sheet impinges against the stop.

Optionally, the transport system includes a vacuum system having at least one"on" state and at least one"off"state, which holds at least one of the sheets of printing media to one or both of the first conveyer and the multi-speed conveyer system when the vacuum system is

in the"on"state, and releases said sheet when the vacuum system is turned from the"on"state to the"off"state.

BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the invention are described in the following sections with reference to the drawings, which are generally not to scale.

Fig. 1 is a schematic overhead view of a paper path in a printer, according to an exemplary embodiment of the invention; Fig. 2 is a schematic plot of the speed of the paper as a function of time, according to the embodiment of the invention shown in Fig. 1; Fig. 3 is a schematic overhead view of a paper path in a printer, according to an exemplary embodiment of the invention different from that shown in Fig. 1; Fig. 4 shows an exemplary plot of the velocity of a sheet in the embodiment of Fig. 3 as a function of time; and Fig. 5 is a schematic side view of a printer, incorporating the embodiments of the invention shown in Figs. 1 and 3.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Fig. 1 shows a paper path in a printer or copier. It should be understood that any printing media may be used, not only paper, and henceforth a reference to"paper"means any sheet printing media. On a portion 102 of the paper path on the right, a series of sheets of paper 104 are traveling to the left, at a fairly high constant velocity Vo, for example 1200 mm/sec.

The sheets each have a length L in the direction of motion, and are separated from each other by a distance Do. The total length L + Do from one sheet to the next is optionally fixed, for example, by the circumference of an impression roller on which the sheets are printed. If the same impression roller is used for sheets of different length, then, particularly for the longest sheets, the distance Do between the sheets may be relatively small. For example, if L + Do is equal to the circumference of an impression roller which is 536 mm, and the longest sheets are 475 mm long, then for the longest sheets Do is only 61 mm.

Sheets 104 may become misaligned on portion 102 of the paper path, a circumstance which could lead to paper jams, or an improperly aligned printed image, or other undesired results. Consequently, it is desired to align the paper, but to do so in a manner that will not slow down the throughput of the printer, and will not make it necessary to vary the speed Vo of the paper over most of the paper path.

This goal is accomplished by means of an obliquely oriented conveyer 106. When paper 104 reaches conveyer 106, the paper is pulled toward a side alignment guide 108, at the same time as it continues to move to the left at speed Vo. If paper 104 is not oriented properly, then one corner will reach the side alignment guide first, and will be forced by the guide to travel along the guide, while optionally the rest of the paper continues to rotate toward the guide, until an entire edge of the paper is in contact with the guide, and the paper is moving parallel to the guide. Even if the entire edge of the paper does not quite come into contact with the guide, so that the paper is not oriented perfectly, conveyer 106 and side alignment guide 108 may still position the paper properly in the direction lateral to the paper path.

Conveyer 106 optionally has special belts 110 of rounded cross-section, which will not damage the surface of the paper (for example, if there is a printed image on the bottom face of the paper) if the paper slides across them, as the side alignment guide forces the paper into the proper alignment. Conveyer 106 also optionally has holes 112 attached to a vacuum system, which keeps the paper on the conveyer. For example, the vacuum system keeps the paper from bouncing or jumping off the conveyer as a result of the impact of the paper with the side alignment guide. However, the force of the vacuum is preferably not so great that the paper cannot rotate when it comes in contact with the side alignment guide. The rounded belts cause a greater fraction of the area of the paper to come into contact with the belt, the stronger the vacuum is, optionally keeping the pressure of the paper against the belts at an optimum value.

The belts are optionally close enough together so that even the lightest weight paper will not be pulled into direct contact with holes 112, and rub against them.

Alternatively or additionally, the vacuum system attached to conveyer 106 is capable of being turned on and off, and is used in order to put the motion of the paper under the control of conveyer 106 or another conveyer, as will be described later.

As the paper continues to move to the left, along side alignment guide 108, it reaches a multi-speed conveyer system 114. Optionally conveyer system 114 is a variable speed conveyer belt. Alternatively, conveyer system 114 comprises a plurality of conveyer belts arranged serially, each belt moving at a different constant speed, or a plurality of conveyer belts at least some of which move at variable speeds. Henceforth, the multi-speed conveyer system is referred to as"the variable speed conveyer, "but it should be understood that the term includes other configurations that provide the same function.

The variable speed conveyer, like conveyer 106, optionally has belts of rounded cross- section and vacuum holes. The vacuum helps to keep the paper in place, moving at the same

speed as the variable speed conveyer, when the variable speed conveyer accelerates or decelerates. The variable speed conveyer brings the paper, whose position now corresponds to outline 116 in Fig. 1, towards a stop gate 118. The front edge of the paper hits stop gate 118, which optionally brings the paper into proper orientation, with its front edge parallel to the stop gate and its side edge parallel to the side alignment guide, if it was not already fully aligned by the side alignment guide. Optionally, the stop gate is capable of being raised and lowered, or is hinged and can fold flat, so that the paper can move past it after stopping and becoming aligned. The stop gate also optionally serves to ensure that the paper is in the right position at the right time. For example, the stop gate briefly holds the paper in place and releases it at the proper time, so that the paper will later be picked up smoothly by grippers of an impression roller. Optionally, the stop gate consists of only two pins or other protuberances which can be raised and lowered. The paper continues to move to the right on portion 120 of the paper path.

Outline 122 shows the position of the paper then. Optionally, conveyer 114 causes the paper to continue moving to the right on portion 120, at least initially. Alternatively, another mover, for example a motorized feed roller (not shown in Fig. 1), is used to move the paper to the right on portion 120.

If the paper hits stop gate 118 at too great a speed, then it may have a tendency to bounce if it is of too heavy a stock, or it may have a tendency to buckle, perhaps becoming permanently creased, or suffer other damage at the leading edge, if it is of too light a stock. In order to avoid both these results when different weights of paper are used in the same printer, it may be desirable to keep the speed of the paper sufficiently low when the paper hits the stop gate. For example, in one test done by the inventors, it was found that bouncing, and damage to the front edge, could be avoided if the paper hit the stop gate traveling at less than 800 mm/sec.

If the desired speed for hitting the stop gate is less than the speed Vo used over most of the paper path, then the variable speed conveyer slows the paper down before it reaches the stop gate, so that it is traveling slowly enough not to bounce substantially or to buckle substantially, or to suffer substantial damage at its leading edge, when it hits the stop gate.

"To bounce substantially, "as used herein, means to bounce a long enough distance so that one or both of the following conditions occurs: 1) the paper returns to the stop gate in a long enough time, so that the delay causes a problem later, for example failure of the paper to be picked up by grippers of an impression roller; 2) the paper returns to the stop gate sufficiently out of alignment, so as to cause a problem later. For example, the maximum tolerable delay is 10 milliseconds, or 20 milliseconds, or 30 milliseconds, or a longer time, and

the maximum lateral displacement is 0.2 mm, or 0.45 mm, or 1 mm, or a greater distance, depending on the details of operation of the printer. These values of lateral displacement refer to one or both of the lateral displacement of the paper as a whole, and the relative lateral displacement of the leading and trailing portions of the paper. The corresponding maximum tolerable bounce distance is, for example, 1 millimeter, or 3 millimeters, or 5 millimeters, or a greater distance.

"To buckle substantially, "as used herein, means to buckle sufficiently so that the paper does not become flat again before it moves past the stop. If the paper remains buckled when it is picked up by a pinch between rollers, or when another sheet of paper falls on top it, or when something else presses down on it, or when it is sucked against a surface by a vacuum, then the paper may crease. "To suffer substantial damage"at the leading edge, as used herein, means that the leading edge is torn, creased, or otherwise irreversibly damaged, to an extent that the paper may misfeed and jam later, for example when it is taken up by a pinch between two rollers.

The collision speed at which substantial bouncing occurs, and the collision speed at which substantial buckling and damage to the leading edge occurs, depend on the weight of the paper. For example, tests have shown that at a speed of 1200 mm/s, substantial bouncing occurs for paper with weight greater than 225 grams per square meter, and substantial leading edge damage occurs for paper with weight less than 115 grams per square meter. When the speed was reduced to 600 mm/s, then substantial bouncing did not occur even for the heaviest paper tested, which weighed 350 grams per square meter, and substantial leading edge damage did not occur even for the lightest paper tested, which weighed 60 grams per square meter.

It is understood that there will be situations in which every sheet is buckled, bounced or damaged. However, for marginal situations, the damage or malfunction only occurs occasionally, with the incidence generally increasing as the paper gets lighter or heavier than a light or heavy malfunction onset rate.

If a sheet of paper must slow down to a certain velocity before hitting the stop gate and if the following sheet of paper is still moving at constant velocity Vo, and if the initial distance Do between sheets of paper is too small, then the next sheet may catch up to the first sheet and collide with it, before it has passed the stop gate and gotten back up to a speed of Vo. In order to prevent this collision, in accordance with an embodiment of the invention, the variable speed conveyer speeds up the sheet of paper before slowing it down. This period of increased speed increases the spacing between the sheets.

If the first sheet of paper simply accelerates back up to a speed of Vo after its leading edge passes the stop gate, and thereafter maintains a constant speed of Vo, then, under some circumstances, the next sheet may catch up to it and collide with it when the next sheet is in its speeding up phase prior to slowing down and stopping. In order to prevent this collision, the first sheet of paper optionally accelerates up to a speed greater than Vo, before slowing back down to a speed of Vo as it moves along portion 120 of the paper path.

Fig. 2 shows a plot 200 of an example of how the speed of a sheet of paper varies with time. The abscissa 202 shows the time t, and ordinate 204 represents the speed V (t). Initially the speed is Vo. At time tl, as the leading edge of the sheet starts to approach the stop gate, it speeds up, and then slows down again, reaching a speed Vo again at time t2, and continuing to slow down, reaching a speed of Vl at time t3, shortly before hitting the stop gate at time t4.

Speed V, is low enough so that the sheet does not bounce or buckle when hitting the stop gate.

When the sheet hits the stop gate, its speed drops to zero, and remains at zero, generally for a short time, until time tS. The sheet then begins to accelerate again, reaching a speed of Vo at time t6, and optionally continuing to speed up, then slowing back down to a speed of Vo at time t7, and remaining at that speed afterwards. Optionally, instead of continuing to speed up above Vo at time t6, the sheet remains at speed of Vo for a period of time after t6 and then speeds up above Vo at a later time, and then slows back down to a speed of Vo.

Before a sheet starts to accelerate at time tl, its trailing edge is separated from the leading of the next sheet by a distance Do. If all of the sheets are separated by the same distance Do initially, and if all of the sheets undergo the same velocity profile V (t), then the leading edge of the sheet will be separated by the same distance Do from the sheet ahead of it, at time t7.

The shaded areas 206, 208, and 210 in plot 200 indicate how the spacing between the sheets changes with time. Area 206 corresponds to a distance Dl, area 208 corresponds to a distance D2, and area 210 corresponds to a distance D3. At time t2, if the next sheet has not started to accelerate yet, the distance between the trailing edge of this sheet and the leading edge of the next sheet has increased to Do + Dl, the maximum separation distance. At the same time t2, if the sheet ahead had already returned to a constant speed of Vo before this sheet started accelerating, then the distance between the leading edge of this sheet and the sheet ahead has a local minimum in value of Do + D3-D2. At time t6, the distance between the trailing edge of this sheet and the leading edge of the next sheet (assuming it hasn't started accelerating yet) reaches a local minimum in value of Do + Dl-D2. The condition for avoiding

collisions between sheets is that both these local minimum separation distances are greater than zero, in other words: min (D1, D3) > D2-Do This condition, of course, is not necessarily valid if the assumptions are not satisfied, for example, if more than one sheet at a time has a speed that differs from Vo, or if all the sheets are not separated by the same distance Do initially, or do not have the same speed when they pass the same place. The fact that this formula depends on min (Di, D3) shows why it is useful for the sheet to move faster than Vo after the leading edge passes the stop gate, between t6 and t7, as well as before it reaches the stop gate, between tl and t2. Optionally, Dl, D2, and D3 are all equal, and the distance between sheets never is less than Do.

Optionally, when the paper moves from conveyer 106 to conveyer 114, the vacuum in conveyer 106 is turned off sometime before conveyer 114 begins to accelerate the paper. Then, when conveyer 114 and conveyer 106 are moving at different speeds, the paper will stick only to conveyer 114, and its speed will be determined only by the speed of conveyer 114, even if part of the paper is still overlapping conveyer 106. Optionally, only one of the conveyers has its vacuum on at any given time, since the two conveyers may be going at slightly different speeds even before conveyer 114 starts to speed up, and in any case, the two conveyers are moving in slightly different directions. If conveyer 114 comprises two conveyer belts in series, the first one, adjacent to conveyer 106, moving at a constant speed greater than Vo, and the second one, adjacent to stop gate 118, moving at a constant speed less than Vo, then adjacent conveyer belts will always be moving at different speeds when the paper is transferred between them.

Optionally, the vacuum in different parts of one or both of the conveyers is capable of being turned on and off independently. Then, for example, when one sheet of paper is stuck by vacuum to conveyer 114, but its trailing portion is still overlapping 106, the vacuum under that sheet of paper in conveyer 106 can be turned off, while at the same time the vacuum under the next sheet of paper, which is beginning to overlap conveyer 106, can be turned on. Similarly, when two sheets of paper overlap conveyer 114, the vacuum can be turned on under one of the sheets, whose speed in being controlled by conveyer 114, while the vacuum is turned off under the other sheet, whose speed is being controlled by a different conveyer.

Optionally, conveyer 114 causes more than one sheet at a time to have a speed different from Vo. For example, at the same time that conveyer 114 causes one sheet to go faster than Vo, as that sheet is approaching the stop gate, conveyer 114 causes the preceding sheet, which

is moving past the stop gate, to go at the same speed as the sheet which is approaching the stop gate. This automatically prevents the sheet approaching the stop gate from catching up to the sheet moving past the stop gate, without any need to program conveyer 114 to undergo a special increase in speed above Vo after each sheet passes the stop gate, as shown in area 210 of Fig. 2, for example.

The usefulness of the invention is not limited to the apparatus for aligning sheets of paper, shown in Fig. 1. It is potentially useful in any situation where sheets of a printing media hit a stop, and have to slow down first in order to avoid bouncing or buckling. For example, the invention is optionally used when sheets of paper are being stacked. In that case, of course, the paper remains at the stop, and there is no acceleration of the paper after a time tS, as there is in Fig. 2.

Fig. 3 shows a paper path 300 leading to an output tray 302. A sheet of paper 304 is first conveyed to the right by a conveyer belt 306 which runs at a constant speed Vo. There are a series of such sheets, not shown in Fig. 3, which optionally enter conveyer belt 306 at regular intervals, and are optionally separated from each other (when carried by conveyer belt 306) by a constant distance Do. The sheet, now labeled 308, then moves onto a variable speed conveyer belt 310. Although the sheets rest on the conveyer belts, the sheets of paper in Fig. 3 are shown slightly above the conveyer belts, for clarity. Variable speed conveyer belt 310 first accelerates sheet 308 to a speed greater than Vo, and then slows sheet 308 down to a speed below Vo.

When the sheet, now labeled 312, leaves variable speed conveyer belt 310, it is going at a slow enough speed Vl so that, when it impinges on stop 314, it does not bounce or buckle significantly. Sheet 312 then falls into output tray 302, stacked on top of any sheets that are already in the output tray.

Any bouncing that sheet 312 does when it impinges on stop 314 is small enough so that sheet 312 does not miss the output tray, and does not interfere with the next sheet on paper path 300, the sheet that is in the position of the sheet labeled 308 in Fig. 3. Any buckling that sheet 312 does when it impinges on stop 314 is slight enough so that sheet 312 does not crease.

Fig. 4 shows an exemplary plot 400 of the velocity V of the sheet as a function of time t. The velocity is the ordinate 404 of the plot, and the time is the abscissa 402. Initially, the velocity is Vo. The velocity starts to increase at time tl, then decreases again, passing through V = Vo at time t2, and falling to a slower speed V, at time t3. At time t4, the sheet impinges on stop 314, and its speed goes to zero. Since the sheet falls onto the output tray at that time, its speed remains at zero, in contrast to the case in Fig. 2. Area 406 is a distance D4, and area 408

is a distance Ds. At time t2, the distance between the sheet that is decelerating and the following sheet is Do + D4, assuming that the following sheet is still moving at its initial speed Vo. When the sheet impinges on the stop at time t4, the sheet is separated from the next sheet by a distance Do + D4-D5, again assuming that the next sheet is still moving at its initial speed. As long as this distance is greater than zero, one sheet will not catch up to the sheet that is ahead of it before they both reach the output tray.

Optionally, conveyer belts 306 and 310 are both equipped with vacuum systems, which can be selectively turned on and off for each conveyer. The vacuum systems are optionally used to transfer control of each sheet from one conveyer to the next, so that, when a sheet is overlapping both conveyers, the conveyers can be moving at different speeds without causing the sheets to rub against one or both of the conveyers. Optionally, the vacuum system on each conveyer can be selectively turned on and off on different parts of that conveyer, so that, for example, one conveyer can be controlling one sheet, while another sheet, that is overlapping both conveyers, can be controlled by the other conveyer at the same time.

Although conveyer 310 ends some distance before stop 314, optionally sheet 312 continues going at the final speed it is given by conveyer 310, due to its momentum, until sheet <BR> <BR> 312 impinges on stop 314. Alternatively, conveyer 310 is located above the paper path (i. e. , it is an overhead conveyer), and holds sheet 312 onto its bottom surface by suction, and conveyer 310 extends almost up to stop 314. Sheet 312 is released from the suction of conveyer 310 just before sheet 312 impinges on stop 314, and sheet 312 then falls into output tray 302.

Alternatively, conveyer 310 moves at a constant speed greater than Vo, and accelerates sheet 312 up to this higher speed. After sheet 312 leaves the control of conveyer 310, other means are used to slow sheet 312 down to speed Vi. For example, sheet 312 rubs against a surface, or an air stream slows down sheet 312, or sheet 312 is caught in a pinch between two rollers which are then slowed down by friction (without any surface rubbing against the sheet).

Various combinations of these methods of speeding up and slowing down sheet 312 will be apparent to one skilled in the art.

Fig. 5 is a partial schematic illustration of a printer 500, which incorporates one or both of the aligning apparatus shown in Fig. 1, and the output path shown in Fig. 3. An input tray 502 holds sheets of paper 504. A sheet 506 from the input tray travels on paper path 508, which is optionally identical to paper path 100 shown in Fig. 1. The sheet is then taken up on an impression roller 510, and an image is printed on it from intermediate transfer member 512, which has acquired the image from photosensitive cylinder 514. After being printed with one

or more images (for example, color separations), the sheet of paper, now labeled 516, travels along paper path 518, which is optionally identical to paper path 300 shown in Fig. 3. The sheet ends up in output tray 520.

The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawing or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention. Furthermore, variations on the method and apparatus shown are included within the scope of the invention, which is limited only by the claims. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms"have", "include"and"comprise"or their conjugates mean"including but not limited to."