Higher productivity trayless duplex printer with variable path velocity

In a printing system capable of printing and outputting collated sets of plural duplex copy sheets from input job sets of plural page images using a trayless duplexing buffer loop path of a known normal plural copy sheet length for recirculating the copy sheets imaged on one side back to be imaged on their opposite sides; operating in a continuous loop burst-interleave mode, using a variable speed duplex drive, for driving at least a major portion of the duplexing path sheet feeders at at least two different sheet feeding speeds so as to initially feed a limited number of the sheets to be printed on one side, less than the duplexing path length, at full rate without skips, to the duplexing path to be selectively partially fed therein at substantially higher than normal velocities, and to variably selectively reduce the velocity of the duplexing path with the initial sheets therein, to generate sheet interleaving spaces between these duplexing path sheets, so that they may be interleaved for their second side printing with subsequent first side prints until the final sheets of the job, which are also fed through the duplexing path at higher than normal speeds for different distances so as to close up interleaving spaces. This increases the duplex copying rate of the printing system by reducing or eliminating skipped pitches without requiring an increase in the given or normal printing rate.

This disclosed system relates generally to the printing of duplex (printed 
on both sides) copy sheets with a buffer loop (endless loop) duplexing 
path. It is especially suitable for high productivity on-demand page 
printers. The disclosed system provides more efficient duplexing of 
multipage collated jobs with reduced skipped printer pitches, for more 
closely spaced or continuous production of duplex copy sheets, for higher 
overall printing productivity, without requiring increased electronic page 
buffer memory storage requirements, and with relatively simple and 
inexpensive machine paper path modifications, and while remaining fully 
compatible with normal printing of simplex (one side printed) sheets in 
the same printing apparatus. 
The term "printers" is used broadly herein, as it will be appreciated that 
the disclosed system may apply to xerographic or other 
electrostatographic, ink jet, ionographic or almost any printing system in 
which the images are reorderable electronically, including electronic or 
digital "copiers". It may even be usable in optical copiers especially 
with a recirculating document feeder capable of reordering and 
automatically feeding reordered originals, as disclosed in Xerox 
Corporation U.S. Pat. No. 4,941,023. The terminology "output", "sheets", 
"prints", or "copies" is used alternatively or interchangeably herein for 
the hardcopy sheets being printed by the printer. 
Electronic input of electronic page images in electronic page ordering is 
discussed in the examples herein, rather than a sequence of physical 
document pages for optical input, as in a conventional copier. Thus, a 
(document) "page" herein refers to the inputted information to be printed 
on one side of a copy sheet, and its page number refers to the job set 
position or copying order of that page, irrespective of any actual or 
physical page numbers, if any. Each duplex copy sheet is thus 
conventionally regarded herein as having two consecutive page numbers 
corresponding to the two respective page images printed on its opposite 
sides. 
Some prior art noted in this technology area of improved duplex systems in 
general, or endless buffer loop printing systems in particular, includes 
the following Xerox Corporation U.S. Pat. Nos. 4,459,013 by Hamlin, et 
al., issued Jul. 10, 1984, entitled "Duplex/Simplex Precollation Copying 
System"; 4,660,963 by Denis Stemmle, issued Apr. 28, 1987; 4,918,490 by 
Denis Stemmle, issued Apr. 17, 1990, entitled "Batch Mode Duplex 
Printing"; 5,159,395 to Michael Farrell, et al., issued Oct. 27, 1992, 
entitled "Method of Scheduling Copy Sheets in a Dual Mode Duplex Printing 
System"; 5,095,342, to Michael Farrell, et al. entitled "Methods for Sheet 
Scheduling in an imaging System Having an Endless Duplex Paper Path Loop"; 
and 5,184,185, issued Feb. 2, 1993, to Michael Farrell, et al., entitled 
"Duplex Printing Scheduling System Combining Finisher Interset Skipped 
Pitches with Duplex Sheet Scheduling"; and other art cited therein. [Said 
other cited art includes U.S. Pat. No. 4,453,841 to Mead Corp. and Canon 
Corp. EP 0295 612.] Also "Xerox Disclosure Journal" publication Vol. 18 
No. 4, July/August 1993, pp. 431-433. The latter-cited Xerox Corp. 
patents, such as U.S. Pat. No. 5,095,342 are of particular interest as 
showing examples of duplex buffer loops, specifically, the Xerox 
Corporation "DocuTech" printer, also shown herein in FIG. 3 by way of one 
example of a potential application of the subject system. Also, in said 
above U.S. Pat. No. 4,918,490, etc., are brief descriptions of the 
duplexing in the prior art Xerox Corporation "9700" and "5700" duplex 
printers. 
Also particularly noted re a duplexing printer is Siemens U.S. Pat. No. 
5,060,025 issued Oct. 22, 1991 to Klaus Kummel, et al. That system 
self-evidently has much more complex paper paths. Col. 11 lines 50-54 
indicates the illustrated embodiment requires 2 front-side stores for 
simplex, plus 3 reverse side stores for duplex. Col. 3, for example, 
describes paper transport elements of variable transport speed and a 
higher speed retard channel, and Col. 15 notes acceleration to a higher 
speed of paper rollers R6 for duplexing. The front side (F) and rear side 
(R) printing sequence for 8 serial duplex A4 sheets described in Col. 14, 
lines 32-40, is: 1F, 2F, 3F, 4F, 1R, 5F, 2R, 6F, 3R, 7F, 4R, 8F, 5R, 6R, 
7R, 8R. In describing the duplexing operation in Co. 15, lines 7-10 in 
particular, paper is accelerated to higher than process speed by rollers 
P6 while rollers P5 remain at process speed, and the W1 turning station 
(inverter) operates at reduced process speed. That is, only the specified 
feed rollers are sped up, not the entire duplex loop path. 
Xerox Corporation U.S. Pat. No. 4,231,567 to R. T. Ziehm discloses duplex 
path buffering with sheet shingling, as does the Canon NP-4835 copier 
duplex tray. Shingling (partially overlapping) sheets in a trayless buffer 
loop is one alternative to duplex buffer tray, if one wants to increase 
(rather than desirably decrease) the number of sheets in the duplex path, 
but it requires special paper handling and at least two different 
transport velocities in the duplex path at the same time to insure proper 
overlapping and then subsequent separation and jam clearance may be more 
complicated. 
Fuji Xerox U.S. Pat. No. 5,197,726 issued Mar. 30, 1993, is one example of 
a sheet feeding velocity control system in general. Various others are 
known in the sheet feeding art. 
There is disclosed herein a simple, low cost duplexing system for 
efficiently utilizing a printer with a simple integrated copy sheet and 
duplexing return sheet feeding path, desirably comprising an otherwise 
conventional trayless, endless loop, duplexing path, but with different 
sheet feeding velocities and timings to print duplex documents in a 
different, more efficient order or spacing. 
Further by way of background, in copier/printers where input page imaging 
and printing on both sides of the copy sheets (duplex printing) is 
provided, it is important to try to fill every available imaging panel or 
pitch space on the photoreceptor in order to maintain a constant copy 
output and the highest possible machine productivity. Most printers (vs. 
copiers) use a trayless (or quasi-trayless) duplex return path, and use 1 
to N page order printing to reduce the amount of data storage required for 
duplex copy sheets, and for other reasons further explained below. There 
are at least four types of printer duplexing modes, which are briefly 
generally described below: 
1. Interleave Mode: Sheets of paper are fed into the system at half-rate 
(every other pitch), and images are transferred to one side. These sheets 
circulate through the duplex path and fill the empty pitches left by the 
sheet feeder operating at half-rate. Images are transferred to the other 
side of the sheets, and they exit the system. Blank photoreceptor panels 
(skipped pitches) are inserted at the beginning of the job while filling 
the duplex path, and at the end while the duplex path empties. The path 
length (N) must be an odd integer for the second side images to be on 
blank photoreceptor panels between first side images. This mode would 
probably only be used with a trayless duplex path, but could also work 
with a duplex tray system. 
2. "Burst" Mode: The first N sheets of paper are fed into the system at 
full rate (N is the duplex path length). No paper is fed for the next N 
sheets. This pattern repeats. Images are transferred to one side of the 
groups of sheets. They circulate through the duplex path and then fill the 
N consecutive empty pitches skipped by the sheet feeder. Images are 
transferred to the other side of these sheets, and they exit the system. 
Any blank panels are inserted after the last first side images are 
printed. I.e., not inserted until the last group of second side images are 
printed. However, no blank photoreceptor panels are required if the number 
of sheets in the job is a multiple of N. This mode would probably be used 
only with a trayless duplex path, but would also work with a duplex tray. 
3. Immediate Duplex: Immediate duplex can be considered to be a special 
case of the three modes listed above where N=1. The second side image is 
printed with the panel immediately following the first side image. No 
blank panels would be required. This mode does not have a conventional 
duplex path and would require a special subsystem to somehow very rapidly 
invert the sheet between image transfers and/or require two separate 
transfer stations, or require image transfer to both sides of the sheet 
simultaneously, any of which is known to be difficult and to require 
unique hardware and control systems. 
4. Burst/Interleave Mode: The first N sheets of paper are fed into the 
system at full rate. The remaining sheets are fed at half rate. Images are 
transferred to one side. The sheets enter the duplex path and are stored 
temporarily as required and reintroduced into the duplex path at half rate 
to fill the empty pitches left by the feeder. Images are transferred to 
the other side of the sheets and they exit the system. The last N sheets 
are reintroduced into the duplex path at full rate. Blank panels are not 
required if the number of sheets is greater than N. For smaller jobs, this 
mode works the same as the burst mode. However, a duplex tray (or an 
alternative multiple sheet duplex buffer, such as something like a sheet 
shingler), could serve to enable a burst/interleave duplex mode. 
In view of the above, it was determined through studies by the inventors 
here that the most productive mode, with the exception of said theoretical 
immediate duplex mode, is the above burst/interleave mode, which, however, 
as noted, presently requires a duplex tray for sheet buffering. 
The system disclosed herein can achieve full duplex productivity in what 
may be considered a type of burst/interleave mode, but without a duplex 
tray, in a continuous loop duplex path system. 
Further by way of general background, it is generally known that 
electronically inputted printers can desirably provide more flexibility in 
page sequencing (page copying presentation order) than copiers with 
physical document sheet input. The printer input is electronically 
manipulatable electronic page information, rather than physical sheets of 
paper which are much more difficult to reorder or manipulate into a 
desired sequence. As also shown in the art cited herein, it is known that 
certain such reordered or hybrid document page copying orders or sequences 
may be copied onto a corresponding sequential train of copy sheets in an 
appropriate copier or printer to provide higher copying machine 
productivity, yet correct page order copy output, especially for duplex 
copies made with a copier with trayless duplexing, i.e., providing a 
limited length endless buffer loop duplexing path for the copy sheets 
being duplexed. The system disclosed herein provides for improvements 
therein. 
Further by way of background, it is preferred that the output tray or 
finisher output stacker of the printer system stacks the copy sheets face 
down. That way a simplex job can also stack face down, so that the simplex 
pages will be properly collated after being printed in a desired 1 to N 
(forward or ascending) serial order. Thus, for such a preferred facedown 
output stacking paper path configuration, in a printing system 
transferring images to the top of the sheet, with a non-inverting output, 
preferably the first sides printed within each job batch for a duplex job 
will be the odd sides, and the second sides printed will be the even 
sides. This also provides proper collation of duplex jobs in the output 
tray without requiring a (known) output path inverter, [although an 
inverting path can be used if desired]. If the paper path configuration is 
such that simplex prints are desirably outputted faceup instead, as in a N 
to 1 page order printer, or with transfer to the bottom of the sheet, or 
inversion in the output path, then the first sides printed within each 
batch for a duplex job will desirably be the even sides, and the second 
sides printed will be the odd sides. 
Many duplex printers also have a long delay or wait before the first duplex 
copy emerges from the printer (a long "first copy out time"), because many 
first side copies are being made and internally retained initially. That 
is undesirable for customer perception. Many printers are also very 
inefficient for small duplex jobs of only a few pages, which is 
particularly disadvantageous if a large number of copy sets are being made 
from such a job. 
As noted, it is desirable to provide duplexing systems using trayless 
duplex buffer loop technology, even for smaller and less expensive 
printers. Eliminating a conventional intermediate sheet stacking duplexing 
buffer tray, and its re-separating feeder, eliminates sheet jams and jam 
clearances associated therewith. It eliminates the sheet feeder/separator 
hardware and the space it requires as well as associated hardware such as 
sheet stackers, edge joggers, set separators, bail bars, and tray edge 
guide resetting means for different sheet sizes. Duplex systems that 
require a duplex buffer tray require such hardware for reliable 
intermediate copy sheet stacking in that tray after side one printing, 
sheet re-separation, and sheet re-feeding. Such duplex tray systems have 
much less positive and more error-prone sheet feeding, more complex jam 
clearance job recovery and reduce efficiency for short (small) collated 
print jobs. (Jobs with a small number of document pages and corresponding 
copy pages per set.) Yet, short jobs predominate in many user's needs. 
Irrespective of the job size, in other trayless or tray type duplexing 
systems, printers with long (multisheet length) duplex paths typically 
require a number of skipped pitches (non-print machine cycles). That is, 
in general, many current duplex printer/copier systems suffer substantial 
productivity losses due in part to skipped pitches between the imaging of 
the respective sides or pages of the duplex documents and/or between the 
copying of the first and second sides of their copy sheets. That includes 
time wasted waiting for the feeding and turning over (inversion) of copy 
sheets being duplexed and for feeding these copy sheets along duplex 
feeding paths to and from the image transfer station for receiving their 
first and second side images, and/or delays for maintaining proper 
interleaved sheet collation of the copy sheets. 
With this disclosed system, the printer does not normally have to wait 
(skip one or more copying pitches) for the time required to turn over and 
return to the transfer station a copy sheet for copying its other side in 
the desired sequence, yet collation of the copy sheets is provided at 
their output. There is high efficiency precollation copying providing 
collated copy set output with minimal skipped pitches (skipping of copying 
cycles). Copier productivity loss may be reduced or eliminated. 
Productivity can therefore more closely approach 100%. 
The document page presentation order is fully coordinated with the path 
length and velocity of the copy sheet duplex buffer loop within the 
printer for improved efficiency duplex copying. That is, coordinated with 
the sheet velocity and separation within a trayless, endless loop, 
recirculating copy sheet path, of a type known per se, which is looping 
the copy sheets to be duplexed from and back to the same imaging station 
(with inversion). This eliminates any need for intermediate copy sheet 
stacking or refeeding in a duplex tray or the like between first and 
second side printing of the duplex sheets. 
Another specific feature of the embodiments disclosed herein is to provide 
the apparatus of claim 1 herein. 
Further specific features disclosed herein, individually or in combination, 
include those wherein the features of claims 2-16 are disclosed. 
All references cited in this specification, and their references, are 
incorporated by reference herein where appropriate for appropriate 
teachings of additional or alternative details, features, and/or technical 
background.

The disclosed system provides for more efficient collated duplex copy sets 
production from printers. It may be seen that the exemplary system 
embodiments described below are intended to eliminate all, or almost all, 
"skipped" pitches on a copier/printer photoreceptor for most duplex 
copy/print jobs thereby yielding full productivity in the duplex mode, 
especially for longer jobs. This system also allows improved productivity 
even for shorter jobs in some cases. Skipped pitches can be eliminated for 
job sizes greater than or equal to two duplexed sheets for short paper 
paths with high speed duplex path servo rolls, and/or job sizes greater 
than the number of sheet pitches provided in the length "N" of a 
"trayless" duplex loop [where "N" is the number of sheet pitches in that 
particular duplex loop]. That is, skipped pitches can be eliminated for 
job sizes greater than or equal to 2.times.N.sub.f pages (N.sub.f sheets), 
where N.sub.f is the pitch time required to return a sheet to the image 
transfer station (for second side printing) at the fastest speed available 
in a variable speed duplex loop path. [Note, however, that a skipped pitch 
may be required even for long jobs if there are an odd number of pages, 
and even pages are printed first, but this can be the case for any 
duplexing mode.] 
For purposes of the examples herein, "N" is the time for a sheet to travel 
through the duplex path from transfer back to transfer. The units for "N" 
here are pitch times. A pitch time here is the time from one image 
arriving at transfer until the next image arrives at transfer. The pitch 
time is normally constant for a given machine with a fixed speed duplex 
path running a given paper size. However, for a variable speed duplex 
path, as here, "N" can vary with the path speed. The pitch time can also 
be different for different paper sizes running in the same machine. 
The avoidance of skipped pitches can be achieved, as explained in the 
examples below (including the Tables) by: (A) feeding at full rate (with 
no skipped pitches) from the designated clean paper supply sheet feeder 
the first N.sub.f sheets which are being printed on their first sides, and 
feeding these first sheets through the duplex loop at high enough 
(initial) velocities, with a controlled distributed drive (servo or 
stepper motor), to create a simplex-to-duplex sheet interleaving space 
between these first sheets, then reducing the duplex path speed 
incrementally (creating single sheet (one pitch) interleaving spaces 
between the remainder of these first N.sub.f sheets in the duplex loop) 
until a steady state process speed is reached; (B) then feeding new sheets 
at one half the regular feed rate (with skipped pitches in between feeds) 
from the paper supply while the duplexed (second) sides which are now 
printing are merging (interleaving) with first side sheets being printed; 
and finally (C) again driving the duplex path transport return nips at 
high enough velocities (higher than process speed), by increasing again 
the duplex loop speed incrementally, at the end of the job run, to 
maintain a continuous sheet output by closing up the spaces between 
duplexed sheets. 
Describing first in further detail the exemplary printer embodiments with 
reference to the Figures, there is shown a duplex laser printer 10 (FIG. 
1) or 11 (FIGS. 2 and 3) by way of examples of automatic 
electrostatographic reproducing machines of a type suitable to utilize the 
duplexing system of the present invention. Although the disclosed method 
and apparatus is particularly well adapted for use in such digital 
printers, it will be evident from the following description that is not 
limited in application to any particular printer embodiment. While the 
machines 10 or 11 exemplified here are xerographic laser printers, a wide 
variety of other printing systems with other types of reproducing machines 
may utilize the disclosed duplexing system, as noted in the second 
paragraph of this specification. 
FIG. 2 is a schematic plan view illustrating the duplex and simplex paper 
paths through which sheets are conveyed in a modification of an exemplary 
existing printing system 11 like that of the existing commercial Xerox 
Corporation "DocuTech" printer shown in FIG. 3 and described in U.S. Pat. 
No. 5,095,342 and other patents cited above. Hence, FIG. 2 will be 
discussed first. In this FIG. 2 embodiment the endless loop duplex (second 
side) paper path 12 through which a sheet travels during duplex imaging is 
illustrated by the arrowed solid lines, whereas the simplex path 14 
through which a sheet to be simplexed is imaged is illustrated by the 
arrowed broken lines. Note, however, that the output path 16 and certain 
other parts of the duplex path 12 are shared by both duplex sheets and 
simplex sheets, as will be described. These paths are also shown with 
dashed-line arrows, as are the common input or "clean" sheet paths from 
the paper trays 20 or 22. 
After a "clean" sheet is supplied from one of the regular paper feed trays 
20 or 22 in FIG. 2, the sheet is conveyed by vertical transport 24 and 
registration transport 25 past image transfer station 26 to receive an 
image from photoreceptor 28. The sheet then passes through fuser 30 where 
the image is permanently fixed or fused to the sheet. After passing 
through rollers 32, a gate 34 either allows the sheet to move directly via 
output 16 to a finisher or stacker, or deflects the sheet into the duplex 
path 12, specifically, first into single sheet inverter 36 here. That is, 
if the sheet is either a simplex sheet, or a completed duplex sheet having 
both side one and side two images formed thereon, the sheet will be 
conveyed via gate 34 directly to output 16. However, if the sheet is being 
duplexed and is then only printed with a side one image, the gate 34 will 
be positioned to deflect that sheet into the inverter 36 of the duplex 
loop path 12, where that sheet will be inverted and then fed to sheet 
transports 24 and 25 for recirculation back through transfer station 26 
and fuser 30 for receiving and permanently fixing the side two image to 
the backside of that duplex sheet, before it exits via exit path 16. 
As shown in FIG. 2 (example 11), (unlike the prior system of FIG. 3) the 
distributed sheet feeders in duplex path 12 are controlled by a variable 
speed drive 80. The variable speed drive 80 is preferably controlled by 
the existing programmable machine controller 100, which also controls the 
feed/nonfeed timing of paper trays 20 or 22. FIG. 2 shows controlled 
variable speed drives for all the feed rolls of the duplex path inverter 
36, the vertical transport 24 (the long belt transport), and the 
registration transport 25 (along the bottom here). 
Depending on which sheet feeder is supplying clean sheets, part of the 
initial (here vertical) sheet transport 24 may not need variable speed 
drives. If paper is being fed from the top feeder (tray 20) then the 
vertical transport 24 from that point down is shared here. However, if 
paper is fed in from a high capacity feeder 23 downstream of transport 24 
(see, e.g., FIG. 3) then none of the vertical transport 24 need be shared 
by the duplex path, even in this system 11. Optionally, (unlike the FIG. 3 
system) this vertical transport 24 may be divided into two separate 
transports, divided just above the input to path 24 from the top tray 20 
feeder, as shown. The upper vertical transport portion 24a could have 
variable speed drives. The lower vertical transport portion 24b could also 
have variable speed drives to be used for duplexing when paper is being 
fed from the high capacity feeder 23. 
However, in contrast, in the FIG. 1 system 10 type of duplex printing 
system, variable speed drives and controls are not required for any part 
of the duplex path which is also shared with simplex printing sheets. This 
means that (desirably) the sheet registration transport need not have 
variable speed drives, even though all sheets (both simplex and duplex) 
pass through it. As shown in FIG. 1, the trayless duplex loop path 112 
there has servo driven variable speed controlled drives N.sub.1 to 
N.sub.n, and such drives are only in the dedicated duplex path loop 112, 
not in any shared paths. 
Turning now more specifically to this FIG. 1 system 10, the respective 
comparative elements are numbered similarly, but in a 100s number series. 
That is, in FIG. 1, the photoreceptor is 128, the clean sheet or paper 
trays are 120 and 122 (with an optional high capacity input path 123), the 
vertical sheet input transport is 124, transfer is at 126, fusing at 130, 
inverting at 136 selected by gate 134, etc.. However, in this embodiment 
10 there is an overhead duplex loop path 112 with plural variable speed 
feeders N.sub.1 -N.sub.n providing the majority of the duplex path 112 
length and providing the duplex path sheet feeding nips; all driven by a 
variable speed drive 180 controlled by the controller 101. This is a top 
transfer (face down) system. An additional gate 137 selects between output 
116 and dedicated duplex return loop 112 here. 
There is disclosed in both said printing systems 10 and 11 here a system 
and method of filling a trayless duplex loop and providing time to 
interleave the first and second sides without losing a pitch on the 
photoreceptor. 
As disclosed, the duplex path roller nips and/or belt drive conveyors are 
independent or distributed drives to allow these speed differentials. This 
allows additional new design options for increased productivity. 
The system disclosed herein achieves full duplex productivity in what may 
be considered a type of burst/interleave mode, but without a duplex tray, 
in a continuous loop duplex path system. 
This can be achieved by feeding at full rate from the designated clean 
sheet feeder for the first N.sub.f sheets which is less than the normal 
number of duplex path sheets (duplex loop total pitches N), which N.sub.f 
sheets are imaged on one side and fed into the duplex loop, then feeding 
more clean sheets at one-half that feed rate while the duplexing sheets 
being second-side printed are merging (interleaving) with the half-rate 
fed sheets being first-side-copied; while, meanwhile, the duplex transport 
drives are accelerated to high enough feeding velocities to create sheet 
interleaving space between the sheets in the duplex loop, then reducing 
that speed incrementally to accommodate subsequent sheets in the loop 
until steady state speed is reached, and finally, increasing the speed 
incrementally at the end of the job run to maintain a continuous output 
flow. 
To express this another way, this duplexing sequence is made more efficient 
by eliminating skip cycles while loading up the duplex path at the 
beginning of each job, and by then depleting the duplex path without skip 
cycles at the end of that job. It does this by changing the sheet velocity 
or speed through the duplex path while loading and unloading the duplex 
path as compared to that speed during the intervening stages of the job 
when odds and even pages are being interleaved. 
This duplexing system and algorithm disclosed herein especially favors 
longer jobs. For job lengths of less than the number of sheets required to 
fill the duplex path, some skip cycles would still be required. But the 
increased speed of sheets through the duplex path would result in fewer 
skip cycles than for equivalent systems with a fixed paper path speed, 
such as in the "9700" duplex printer. 
Turning now to the Tables, Tables 1-16 below provide some operating 
examples and comparisons. In the first two (comparative) Tables are two 
examples of sequences or algorithms for duplex printing with a printer 
with a trayless duplex paper path which is 5 sheets long. Table 1 is a 
prior art system for comparison with one example of the present system in 
Table 2. Prior system tables here are 1, 3, 4, 6, 7, 9, 10, 12, and 13; 
and new system tables are 2, 5, 8, 11, 14, 15, and 16. A "page #" here is 
defined as the image on or for a single side of a single sheet. "P/R Pitch 
#" is the sequence of usable photoreceptor frames or document image areas. 
The job length or number of document pages in the particular job (collated 
document set) will obviously vary, and is 16 pages in this Tables 1 and 2 
example. Thus, this is a job of 8 duplex sheets (i.e., 1/2 the number of 
pages, for an even page duplex job). This is for a forward serial page 
order or "1 to N" printing system example, although "N to 1" systems are 
also known. "Page # SIMP" is the printing of a first side page, and "Page 
# DUP" is the printing of a second page on the opposite side of a sheet, 
in the respective vertical columns indicated for that "P/R Pitch #". "X" 
here represents each skipped (non-print) pitch. 
"No. Pitches" in the last line of Table 2 shows the number of pitches 
between the 1st and 2nd side printings (image transfers) of the respective 
(above) completed duplex sheet. This also illustrates the difference in 
their respective overall duplex path transit speeds, measured in pitches. 
E.g., here, 3, 4, 5, 5, 5, 5, 4 and 3 pitches, respectively. 
As can be seen by comparison, in the second Table 2 versus Table 1, below, 
a savings of four non-skipped (utilized) photoreceptor (P/R) panels or 
pitches can be realized in this example. No pitches are skipped. 
TABLES 
__________________________________________________________________________ 
1. Prior Trayless Duplex Interleaving Mode 
__________________________________________________________________________ 
P/R Pitch # 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
PAGE # SIMP 
1 X 3 X 5 7 9 11 13 15 X X 
PAGE # DUP 
X X 2 4 6 8 10 12 
X 14 X 
16 
__________________________________________________________________________ 
__________________________________________________________________________ 
2. New Trayless Duplex Operating Mode 
__________________________________________________________________________ 
P/R Pitch # 
1 2 3 4 5 6 7 8 9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
PAGE # SIMP 
1 3 5 7 9 11 13 15 
PAGE # DUP 2 4 6 8 10 12 
14 
16 
No. Pitches 3 4 5 5 5 5 
4 
3 
__________________________________________________________________________ 
__________________________________________________________________________ 
3. Sheet Sequence, Interleave Mode (5 pitch duplex path, 22 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 X 3 X 5 7 9 11 13 15 17 19 
21 
X X 
pg # 
Dup X X 2 4 6 8 10 12 14 16 
18 
X 20 
X 22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
4. Sheet Sequence, Burst Mode (5 pitch duplex path, 22 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 5 7 9 11 
13 
15 
17 
19 21 
X X X X 
pg # 
Dup 2 4 6 8 10 12 
14 
16 18 
20 
X X X X 22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
5. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (5 pitch 
steady state duplex path, 22 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 7 8 9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 5 7 9 11 13 15 17 19 21 
pg # 
Dup 2 4 6 8 10 12 14 16 18 
20 
22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
6. Sheet Sequence, Interleave Mode (5 pitch duplex path, 4 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 X 3 X X X 
pg # 
Dup X X X 2 X 4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
7. Sheet Sequence, Burst Mode (5 pitch duplex path, 4 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 X X X 
pg # 
Dup X X X 2 4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
8. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (5 pitch 
steady state duplex path, 4 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 X 
pg # 
Dup X 2 4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
9. Sheet Sequence, Interleave Mode (9 pitch duplex path, 22 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 X 3 X 5 X 7 X 9 11 13 15 17 19 
21 
X X X 
X 
pg # 
Dup X X X X 2 4 6 8 10 12 
14 
X 16 
X 18 X 
20 X 
22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
10. Sheet Sequence, Burst Mode (9 pitch duplex path, 22 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 5 7 9 11 
13 
15 
17 19 
21 
X 
X X X X X X 
pg # 
Dup 2 
4 
6 
8 
10 
12 
14 
16 
18 X 
X X X X X X 
20 22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
11. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (9 pitch 
steady state duplex path, 22 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 7 
8 9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 5 7 9 11 13 15 17 19 21 
pg # 
Dup 2 4 6 8 10 12 14 16 
18 
20 
22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
12. Sheet Sequence, Interleave Mode (9 pitch duplex path, 4 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 X 3 X X X X X X X 
pg # 
Dup X X X X X X X 2 
X 4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
13. Sheet Sequence, Burst Mode (9 pitch duplex path, 4 page 
__________________________________________________________________________ 
job) 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 X X X X X X X 
pg # 
Dup X X X X X X X 2 
4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
14. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (9 pitch 
steady state duplex path, 4 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 X X X 
pg # 
Dup X X X 2 4 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
15. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (9 pitch 
steady state duplex path, 22 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 
7 
8 9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 5 7 9 11 
13 15 17 19 21 
pg # 
Dup 2 4 6 8 10 
12 
14 16 
18 
20 
22 
pg # 
__________________________________________________________________________ 
__________________________________________________________________________ 
16. Sheet Sequence, Variable Speed Burst/Interleave Mode (NEW) (9 pitch 
steady state duplex path, 4 page job) 
__________________________________________________________________________ 
P/R 
1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
Pitch 
Simp 
1 3 X X X X X 
pg # 
Dup X X X X X 2 4 
pg # 
__________________________________________________________________________ 
In the printer of this Table 1-2 example, and Tables 3-8, this duplex 
buffer loop path length "N" is 5 copy sheets long when running at the 
normal process speed, determined by the photoreceptor velocity. Thus a 5 
sheet batch system may be provided. It may be seen from the second Table 
that even though the duplex loop path is normally 5 sheets in length, the 
first sheet was actually back at the transfer station for its second side 
[page #2 dup] image here by the 4th pitch, and likewise sheet 3/4, but 
that all intermediate copies 2, 7, 4, 9 are made and fed with 5 pitches 
between front and back sides as before. It may be seen that the first 3 
sheets (1, 3, 5) to be duplexed are fed by the sheet feeders of tray 20 or 
22 without any skipped pitches, and the others are fed with a skipped 
pitch in between feeds. Here, the first two sheets and the last two sheets 
run at different speeds in the duplex path than the other sheets. 
To express this in different words, in the second Table example of a 5 
pitch duplex loop, the first 3 front sides (1, 3, 5) are imaged directly 
sequentially. By accelerating the first of these sheets (page 1) to a 
higher velocity in the duplex loop, it reaches the transfer station ready 
to accept its back side image immediately after the last of the first 
three front side images has been transferred. This leaves room for another 
front side sheet (page 7) to pass through transfer before the second sheet 
(page 3) reaches the transfer station ready to accept its back side image 
(page 4). The second sheet had also been accelerated in the duplex path, 
but to a lower velocity. This, again, leaves room for another front side 
sheet to pass through transfer before the third sheet reaches the transfer 
station ready to accept its back side image. The duplex loop may then run 
at a constant velocity until the last front side is completed. Then, the 
sheets in the duplex loop are again accelerated to run at the higher 
velocity to close the gaps so that again no pitches are lost on the 
photoreceptor. This allows the elimination of 4 skipped pitches of 
photoreceptor for all jobs with more than 3 duplex sheets in this case. 
Skipped pitches are, however, provided after the first and second front 
side pages, and after the last and next-to-last back side pages, for 
machines with a 5 pitch duplex path running in an interleaved mode. Those 
"holes" are left to allow back side sheets to interleave with front side 
sheets. 
In the example of Tables 3-5, the duplex path drives can be controlled to 
cause the time for sheets to be returned to transfer to be one of three 
values: 3 pitch times (fastest), 4 pitch times (intermediate), and 5 pitch 
times (steady state). The sheets which receive pages 1 and 21 (first and 
last) pass through the path in 3 pitch times. The sheets which receive 
pages 3 and 19 (second and second from last) pass through the path in 4 
pitch times. All other sheets (at steady state) pass through the path in 5 
pitch times. 
In the example of Tables 6-8, the duplex path drives can be controlled to 
cause the time for sheets to be returned to transfer to be one of three 
values: 3 pitch times (fastest), 4 pitch times (intermediate), or 5 pitch 
times (steady state). Since there are only two sheets, both (first and 
last) pass through the path in 3 pitch times. 
In the example of Tables 9-11, the duplex path drives can be controlled to 
cause the time for sheets to be returned to transfer to be one of five 
values: 5 pitch times (fastest), 6 pitch times (intermediate), 7 pitch 
times (intermediate), 8 pitch times (intermediate), or 9 pitch times 
steady state). The sheets which receive pages 1 and 21 (first and last) 
pass through the path in 5 pitch times. The sheets which receive pages 3 
and 19 (second and second from last) pass through the path in 6 pitch 
times. The sheets which receive pages 5 and 17 (third and third from last) 
pass through the path in 7 pitch times. The sheets which receive pages 7 
and 15 (fourth and fourth from last) pass through the path in 8 pitch 
times. All other sheets (at steady state) pass through the path in 9 pitch 
times. 
In the example of Tables 12-14, the duplex path drives can be controlled to 
cause the time for sheets to be returned to transfer to be one of five 
values: 5 pitch times (fastest), 6 pitch times (intermediate), 7 pitch 
times (intermediate), 8 pitch times (intermediate), or 9 pitch times 
(steady state). Since there are only two sheets, both (first and last) 
pass through the path in 5 pitch times. 
In the examples of FIGS. 15 and 16, the range of speeds which the duplex 
path produces is somewhat arbitrary. The low speed is limited by timing. 
The speed must be fast enough to keep sheets from overlapping. The high 
speed will probably be limited by other functional problems such as noise, 
power, and reliability for a particular design. Here are some alternate 
examples for the 9 pitch steady state path. The range of speeds has been 
reduced. It can still be compared with the other trayless 9 pitch trayless 
duplex modes. The trade off is more skipped pitches for small jobs. 
In the example of Tables 15-16, the duplex path drives can be controlled to 
cause the time for sheets to be returned to transfer to be one of three 
values: 7 pitch times (fastest), 8 pitch times (intermediate), or 9 pitch 
times (steady state). The first and last sheets pass through the path in 7 
pitch times. The second and second from last sheets pass through the path 
in 8 pitch times. All other sheets (at steady state) pass through the path 
in 9 pitch times. 
These examples make comparisons between variable speed duplex paths and 
fixed speed duplex paths running at the low end of the variable speeds 
range. If the fixed speed path was running at the upper end of the speed 
range the variable speed duplex path would still be advantaged but the 
advantage would be less for smaller length jobs. For example, compare the 
variable speed case with a fastest speed of 5 pitch times to the other 
five pitch trayless duplex cases. 
It may be seen that a duplex path with at least two different sheet feeding 
speeds is required. Also, the timing (on/off) of the clean sheet feeders 
changes. The first few sheet to be duplexed are fed at the normal paper 
feeding sequential rate and velocity to the transfer station for their 
first side image, but then in the duplex path loop they are accelerated 
sufficiently to create sufficient space between those sheets such that 
when they are returned to the transfer station for their second side 
images, clean sheets (for first side images) may be interleaved between 
each second side sheet, i.e., interleaved with the first side copies. 
However, at least three different versions of implementations of the 
variable speed duplex path can still be provided even for a system with 
only a dual or two speed selection for the duplex transport. First, for 
the 5 pitch example, the first two sheets can pass through the duplex path 
at the faster speed: the first sheet in 3 pitches, the second in 4 
pitches, as shown in Table 2. At this faster speed, the duplex transport 
is run at twice the normal (process) speed for 2 pitches after the lead 
edge of the first sheet enters the transport. In this way, the first sheet 
is moved two pitch times farther ahead of subsequent sheets which pass 
through the duplex transport at the other, normal, speed. The second sheet 
enters the transport one pitch time after the first, so it travels at the 
high speed for only one pitch time. This results in the second sheet 
moving one pitch time farther ahead of the subsequent sheets which pass 
through at the normal speed. The third sheet does not enter the transport 
until two pitch times after the first. By this time the transport has 
returned to the second or normal speed. Near the end of the job, the 
duplex transport is returned to the high speed (twice the normal speed) 
immediately after the third from last sheet leaves the transport. In this 
way, the second from last sheet will be transported at the high speed for 
one pitch time and the last sheet will be transported at the high speed 
for two pitch times, moving them ahead one and two pitch times 
respectively. 
For a longer duplex loop path, this above approach can be modified to 
handle a larger range of sheet times by running the transport at the high 
speed for longer times. If the duplex transport were run at the high speed 
for 3 pitch times, then three sheets would be moved ahead: by three, two, 
and one pitches respectively. The duplex transport path length would need 
to accommodate the distance a sheet would travel for the specified time at 
the higher speed. 
If the high speed were more than twice the normal speed, then the transport 
would need to switch between normal and high speed for the appropriate 
length of time to move ahead one pitch time as each sheet to be 
accelerated entered the transport. Higher speeds may allow the length of 
the variable speed transport to be shorter. 
For a system with segmented duplex transport drives, where different 
sections of the duplex transport can change speeds independently, the 
available speeds within each section could still be limited to two. This 
segmentation would allow more flexibility to affect the speed of one sheet 
without affecting the speed of another. The sheet timing objective would 
be the same as above. Drives control to accomplish that objective should 
be straightforward. 
The most flexible system involves infinitely variable drives for a 
segmented duplex transport, rather than only two different speeds. This 
approach allows different sheets to be driven at different "higher" 
speeds, as well as controlling which sheets are affected. It may also 
allow some optimization of transport length, power, noise and related 
parameters. 
In summary, there are disclosed several embodiments of an improved copying 
sequence for duplex printing for a printer with a trayless duplex paper 
path. While the embodiment disclosed herein are preferred, it will be 
appreciated from this teaching that various alternatives, modifications, 
variations or improvements therein may be made by those skilled in the 
art, which are intended to be encompassed by the following claims: