CA1295881C - Single-pass color plotter - Google Patents

Abstract

SINGLE-PASS COLOR PLOTTER

ABSTRACT OF THE DISCLOSURE
A printer/plotter incorporates four individ-ual microprocessor-based print stations, each for print-ing on a print media a separate color image for super-imposition with one another, forming a final full-color image. The four print stations are located along a transport path for single-pass operation, and each print station includes a transport system that allows the media to traverse a print station with controlled forces exerted on the media by that station. The invention further includes a precise registration system wherein each print station monitors registration marks to detect variations of the media (i.e., stretching or shrinkage) during the printing process and to correct for such variations on obtaining accurate registration of the individual images for a full-color result.

Description

64 . 100 SINGIE-PASS COI.OR P];OTTER

BACKGROUND OF l~IE IN~TION
S l~is inYe~tion relates generally:to electro-~tatic printing, and particularly to elec~rostatic color pri~ting/plot~ing using mul~iple color print stations to produce a full-color image in a single-pas~.
Electrostatic printing is accomplished by 10 placing electrostati~ charges in the form of the image . to be printed on an electrographic media, usually paper.
The paper is ~hen exposed to a liquid tonèr to produce a permanent ~isible image. In electrostatic color print-ing, separate images are electrostatically printed on the paper~and toner applied, each image typical}y c~r-responding to one of four colors: The three colors of yellow, cyan and magenta, and for true black, a fourth cclor of~black. ~ ;
eretofore, some electrostatic color printers use a recording head containing an elongate arrangement of styli in combination with~ a back-plane of one form or another. Electrostatic printing paper is drawn be-t~een the recording head and styli while i~dividual ones of the styli are~selected and impressed with a 25~ voltage potential that, together with th~ back-plane;;~
usually~also impressed with a~voltage~potential) creates~
dot"~ of ~hàrge~on the~paper. The image for each color may~be thoug~ht~of as comp~ising lines or~"rasters"
of~these dots~hich, when to~ed combine to form the 30 ~image o~that~color. For full col;or, ~he yellow, magen-ta,~cyan,~and (if used~ black ima~es~are~separatel~ ~
printed, ~each registered to be~relatively superimposed on one another to form~he f ll-color image. Depending upon the~f~inal color desired, any particular dot location on~the paper may have o~e or~more colors printed thereat.
Printing the fu~ color image has been performed by :

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passi~g the paper through the print statio~ (which usual-ly has a single electrostatic recorder and multiple ~oner~dryers - one for each separate image~ to print the first image rewinding the paper and passing the paper through again for each subsequen~ image.
~ mong ~he problems associa~ed with ~his tech-nique o ~l~ctrostatic color printing are the-~ime re-quired to ~omplete the total image and maintai~ing the relative position of each successive Image upon the paper relative to all the primary Lmages. As the paper is transported forward and backward through the pxint bed, .. proper registration of the individual imageæ can be guite dif~icult. In addition, these problems are exacer.
lS bated by changes in the media caused, pri~arily, by humidity which, in turn, causes the paper to change size, ~oth in the direction of travel and laterally.
Thus, the advantages of a color printer/plot-ter cap2ble of printing a full-color image in a single-.
pass are certainly evident to t~ose skilled in thisart. The problems of time and registration are reduced, if not obviated, allowing precise ~ull-color images to be printed, one after the other in a single-pass by print stations located in seguence along the ~ransport path. Unfortunately, the drag forces imposed upon the paper by each print station, unless somehow compensated for, would ultimately be too great. Attempting to pull the paper through all print stations would require addi-tional paper stre~gth to prevent tearing. ~he paper becomes still less f}exible, and more expe~sive. Al-ternatively, me~ely adding additional drives between the print stations does not necessarily solve the prob~
lem - and can create other problems. Proper tension across the print head must be mainta-ned within prede-termined limits ln order for the printing process tooccur correctly. Adding extra paper drives can cause paper tensi~on~to be unacceptable at some of the print stations.

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SUMMARY OF 1~ INVENTION
The present invention provides an electro-static, single-pass printer/plotter capable of produc-ing a full-color image from four separate, superimposed images (yellow, magenta, cyan, and black). Broadly, the invention includes a regis~ration system that con-strains one edge of the print media to a predetermined line of travel along a transport path to accurately guide the media therealong, and drive ppara~us that, in ad~ition to moving the media along the transport path, imparts a lateral ~orce (i.e., directed away from the constrained edge). Thereby, any change in media size is limited to the lateral direction. A registration line is printed proximate ~he media edge opposite the constrained edge, as are egually spaced registration marks. Sensors, incorporated in each of a plurality of ~ print stations (one for each separate color image~ lo-:
: cated along the transport path are optical sensors ~hat :~ 20 monitor the registration line and mark to, under micro-:~ : processor control, modify the printing process to com-: : pensate for media variations in size. In short, the ; registration marks (which define each print linej and the registration line es~ablish the position of eve~y :
point on the media. Should media variations in size cause any of these points to vary their relative posi-tion, particularly if:one of the images has been printed ;.
: : thereon,~e relevant print stations can detect such~
variations~and~, knowing:the direction of occurrence, : 30~ make the~necessary correction.
The~inven~ion:further includes a transport :drive:~system for each print station that, in additionto~ransportinq the:~media through that station to the : next~print~station,~maintains a correct media te~sion 35~ ~:and:operates; to~eLiminate; any~in~luence on the~mediaj : by an immediately:preceding subsequent:print station.
:The transport system includes~a media drive located at : ; : : ~he output of each print station that is separately 1;2~35~

controllable (relative to the other print stations~ in cooperation with adjacent print stations to adjust a ' pa~h along which the media ~ravels within the pri~t S station to maintain media tension.
According to the preferred embodiment of the i~vention, a support defines a transport path along which are mounted four individual print ~ta~ions. Each print station is adapted ~o receive data for printing a separate color image for superposition with the other color images printed.
A registration system includes precision, ,. free-wheeling sprocketed rollers located along the transport path to operably engage sprocket holes formed along one edge of the media, which is, in the embodiment described an electrostatic paper. The sprockets function to constrain the edge as the paper moves along the trans-port path, providing an accurate alignment of the paper.
Proximate the opposing edge of the paper are placed registration ~arks and a registration line. Each print station is provided wi~h optical sensors that monitor ~
the registration marks to synchronize the printins ope-ration with each mark, and to,monitor the alignment strip.
The transport system used to move the paper from print station to print stak~o~ uses fine separately controlled drive rollers. One~ located near an entry area of.the ~ransport path, pulls the paper from a roll : : onto the transport path and through the p~int station : 30 that marks the paper with ~he registration line and arks, and feeds t:he paper to a first of the ~our print stations. In addition, each print station has a drive ~ roller to~pull the paper through that station and supply " ~ ~ : it to the next~successive print station (or, in the case of the final print station, to the output of the transport path).
All drive rollers of the transport system are skewed a slight amount, relative to the direction of ~ 88~

tra~el of the paper, to impart to the paper a small Xorce (o~ the order of 0.1 oz.) in a direction transverse ~he direction of travel, and away from the constrained S ~dge. This ~erves two important purposes: First, it ensures ~hat any lateral variations are in one direction only - away from the constrained edge (and toward the edge carrying the r~gistration line). The amo~nt of lateral variation is detected by the optical sensor o~
each print station that monitors the registration line, and the microprocessor of that print station can modify the data sent to the print head to obtain proper corr~c-.. tion for variation. Second, the paper is pulled againstthe sprockets to accurately align the paper with, and ,~ 15 guide the paper along, the transport path.
Each of the print stations includes at the exit thereof, one of the drive rollers of the transport ., system. ~receding each (exit) drive roller, each print station also includes an electrostatic print head and back plane combination toner station, and a tension bar that assists in maintaining the force with which the ~; paper is drawn across the print head within predetermined limits. A microprocessor, also included in each print station, responds to signals indicative of paper tension wi~hin the print station to separateIy operate the print stations drive roller relative to the other drive rollers to control paper tension within the print station. In this manner the drag force imparted to the paper by the print station is~ accurately maintained and controlled 3Q as needed for paper print operation.
A number of ad~antages are obtained by the ~presen~ invention. First, by providing the capa~ility of single pass, full-color print/plot operation, the time reguired for the ~inal image is reduced. Fur~her, ;~ 35 ~he printing/plotting procedure can be varied to accom-modate data flow frQm the source (i.e., computer or the like) supplying image data and the complexity of the image being printed by any one print station. For .

~9~&~ 64157-~0 example, the data to be printed could be supplied from a main frame computer, and the data flow may vary. The printing speed can be varied (within limits of the printer itself~ to accommodate such data flow variations on a real time basis.
The registration system employed by the present invention allows each print station to calculate exactly where any particular point on the paper is located, despite media variations, allowing accurate placement and reglstration of the separate image printed on the paper. Additionally, constraining one edge of the paper to a specific line of travel, toget.her with applying a small lateral force to the print media, away from the constrained edge, accurately aligns and guides the media along the transport path and through the print stations.
Additionally, by providing each print station with a drive roller that is independently variable, and by monitoring the tension of the media (i.e., paper) within the print station, the tension of the paper across the print head can be accur-ately controlled. In addition, the overall drag imposed by the print station can be kept to a predetermined minimum, thereby allowing a plurality of print stations to be sequen-tially placed along a~transport path for producing single-pass full-color printing/plotting.
According to a broad aspect of the invention there :: ~
~ is provided transport apparatus for a printer/plotter having .~
~plural print heads arran~ed successively along a transport path~for single-pass printing of an image on a prlnt media, said transport apparatus comprising: a plurality of separately controllable drive means for movlng the print media along the transport path while maintaining media tension, a first one of said drive means being positioned at an input location of a . .

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- 6a - 64157-240 first one of said print heads, the remaining ones of said drive means being positioned at an output location of each print head, said first drive means being operable as an entry drive for the first print head, a last one of said drive means being operable as an exit drive for a last one of said print heads, the remaining ones of said drive means simultaneously being operable as an exit drive of an immediately preceding print head and as an entry drive of an immediately succeeding print head; a plurality of tension monitoring means, each engageable with the print media, for providing a tension signal indicative of the print media tension as the print media moves between said drive means; and control means, coupled to each said drive means, for providing drive signals in response to receipt of the tension signals to maintain print media tension within a predetermined range between each sald drive means.
According to another broad aspect of the invention ; there is provided transport apparatus for a single-pass printer/
plotter for printing of an image on a print media~ the trans-port apparatus comprising: means for defining a transport path along which the print media travels; means, mounted to the transport path, for constraining one periphery of the print media along a predetermined line of travel of the transport path; and drive means for moving the media along the transport path by imparting a first force to the print mediain the direc-tion of media travel while imparting a second force in a direction generally transverse the direction of media travel, parallel the media surface, and away from the constrained peri-phery.
According to~another broad aspect of the invention there is provided a single-pass color printer for imprinting D

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- 6b - 64157-240 color images on a media by superimposing a plurality of indi-vidual images one upon the other, comprising: support means defininy a transport path for movement of the media therealong;
a plurality of print stations mounted in sequential order along the transport path, each print station being operable to print a corresponding one of the individual images; and transport means for moving the media from an entry to ~he transport path to an exit of the transport path through each of the print stations, the transport means including a first drive means mounted to the transport path upstream of the print stations and, for each print station, a second drive means mounted to pull the media through the corresponding print station, means for sensing a media path length within each print station, the media path length being variable within a predetermined range, and control means for each print station for controlling the velocity with which the immediately preceding first or second drive means provides the media to the print station relative to the second drive means of the print station.
According to another broad aspect of the invention there is provided transport apparatus for a printer-plotter hav-ing plural print heads arranged successively along a transport path for a single-pass printing of an image on a print media, the transport apparatus comprising: means, mounted to the transport path, for constraining one periphery of the print media along a predetermined line of travel of the transport path; a plurality o separately controllable drive means for moving the print media along the transport path by imparting a first force to the print media in the direction of media travel while imparting a second force in a direction generally transverse the direction of media travel, parallel the media ~2~
- 6c - 64157-240 surface, and away from the constrained periphery, a first one of said drive means being positioned at an input location of a Eirst one of said print heads, the remaining ones of said drive means being positioned at an output location of each print head, said first one of said drive means being operable as an entry drive for the first print head, a last one of said drive means being operable as an exit drive for a last one of said print heads, the remaining ones of said drive means simul-taneously being operable as an exit drive of an immediately preceding print head and as an entry drive of an immediately succeeding print head; a plurality of tension monitoring means, engageable with said print media, for providing a tension signal indicative of a print media tension as the print media moves between said drive means; and control means, coupled to each said drive means, for providing drive signals in response to receipt of said tension signals to maintain said print media tension within a predetermined range between each said drive means.
These and other advantages will become evident to those skilled in this art upon a reading of the following detailed description, which should be made with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
; Figure l is a perspective view of a single-pass, full-color r electrostatic printer/plotter constructed in accordance with the teachings of the present inventio~;
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Fig. 2 is a block diagr~m representation of one of the four pri~t stations usled in connection with the present invention;
Fig. 3 is a representation of a por~ion of ~he print media, e.g., electrostatic prin~er paper used, for printing images by the present i~vention, illustrat-in~ placement of the apertures at one edge of the paper, and the registration marks and registration line placed at the opposing edge thereof; also illustrated in Fig.
3 is the skewing of a drive roller to impart a lateral force to the paper;
Fig. 3A is an amplified view o the registra-tion marks a~d registration line placed proximate one edge of the printer paper, and illustrating sensor loca-tions relative thexeto; and Fig. 4 is a representation of the shuttered optical sensor used to detect tension of the media trans-:~ ~ ported through each print station of Fig. 2.
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~DETA1LED DESCRIPTION OF T~E INVENTION
Referring to Fig. l, a printerjplotter con-structed in accordance with the teachings of the present ; invention, and designated generally with the reference numeral l0, is illustrated. The printer/plotter }O
. 25 includes a pai~ of side plates 12 and 14, mounted in generally ~lxed,~spaced and parallel relation to one ..
another by appropriate means (not shown). The side plates I2~and~14 g~nera1ly define, at ~he upper portion thereof,~a transport path l8 along which ~he media 16 (such as~,~: for~example, electrostatic paper3 tra~els f~om a paper supply roll (not shown), located proximate an entry~area 20~;of the ~rans~port~path 18, to a take-up roll 22,~ located~at an~exit area 24 of the transport path. At spaced locations along the transport path 18 ;: 35 are four print stations 30Y, 30M, 30C and 30B, each respectively structured to print;an image in the separate colors of yellow, magenta, cyan, and black.
- -Proximate the entry area 20 of the transport path 18, and mounted to the side E~late 14, is a marker station 32, which ~unctions to pxint registration indicia S along one edge of the paper 16 as it enters the transport path 18. As will be seen, ~he registration indicia, comprising a registration line and spaced markers (104 ~nd 102, respectively, Figs. 3 ~nd 3A) are used to track any variations in paper size as the paper moves along the transport path. The print stations 30Y, 30~, 30C
and 30B, as will be described more ~ully below, are each equipped to sense a~y such variations and adjust the print operation to compensate. ~ach o the print stations 30Y, 30M, 30C and 30B operate inde~endently to electrostatically print their corresponding yellow, magenta, cyan or black images on the paper 16 in regis-tered relation to one another by monitoring the regis-tration indicia.
- Fig. 1 illustrates the print station 30Y being preceded by an entry drive 36, which functions to pull paper from the paper supply roll (now shown) onto the transport path 18 and through the marker station 32, feeding the paper to the print station 30Y. The print station 30Y includes an exit dri~e 38 to pull the paper through the print station, ~nd, preceding the exit drive 38 is a back plane 40, and a tension bar 42. Tension bar 42 is coupled to a detector 44 for determining the depth of ~he tension bar 42, obtaining thereby ~n indi-cation of the tension of the paper 16 between the entry and exit drives 36 and 38.
The print stations~ 30Y, 30M, 30C and 30B are identically stru~tured, so that hereinafter only the print station 30Y will be described. It should be under-stood, therefore, that any description of the print 35 statio~ 30Y will apply equally to ~he print stations 3UM, 30C and 30B - unless otherwise noted. Further, the individual print stations 30Y-30B use an electrosta-tic print process generally of the type that emp oys a ~:

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printer head that extends between the side plates 12 and 14 (and underlies the back-plane bar 40 of Fiyure 1) and comprises an elongate array of staggered rows of styli. The print heads, and the elec~ronics to operate the print heads, are described in U.S. Patent No. 4r419,679r and the documents referred therein, entitled "Service and Maintenance Manual" r published in April r 1981 by "Benson-Varian of 2690 Orchard Park Wayr San Joser California 95152-2059r Model 9424 Printer/Plotterr Publication No. 03996-336E." Only the novel features of each prink stationr as they apply to the present invention, are descrihed herein.
Figure 2 illustrates, in greater detail, the marker station 32, the entry drive 36, and the configuratlon of the print station 30Y. As Figure 2 shows, data and command information is supplied from a source of data to the print station 30Y and received by an input controller 48, which functions as an intelligent buffer for the information to be printed. Status information respecting operation of the print station 30Y is generated by a microprocessor 50 r and is communicated to the input controller 48 via a command~control data path 52, and made available for transmission to a data source (not shown) by the input controller 48.
The data path 52 connects the microprocessor 50 to a memory unlt 54, which includes both random access memory ~RAM) and read-only memory (ROM). The memory unit 54 contains the necessary programming to operate the microprocessor 50 to e~fect control over printing operations performed by the print station 30~. An input~output (I~O) port 56 provides a bufferPd inputtoutput data path between the print stations 30Y, 30M, 30C
and 30B. Status information such as, for example, adjustment of the tension, is ~ransmitted via the I~O port 56 between the various print stations to coordinate paper flow there~hrough.

Also coupled to the com~and/control data path 52 is a motor controll~r 58 which, in turn, is connec-t~d to micro-stepper motars 60 and 62. In response *o command aata from ~he microprocessor 50, in the form of a count specifying the number of micro-steps to be taken by one or ~he other (or both) o~ the micro stepper motors 60, 62, the motor controller 58 controls operation of the micro-stepper motors 60 aad 62. By issuing pulses on one or ~he other (or both for synchronous operation) of the signal lines 59a and 59b, the motor conl:roller 58 causes the micro-stepper motors 60 and 62 to take the required numbex of micro-steps ~one for each pulse) indi~idually, together at di~erent rates, or together in synchronism.
The micro-stepper motor 60 operatively connects to drive roller 66 that, toge~her with a pinch roller 68, form.the entry drive 36 (Fig. 1) to;grip and pull the paper 16 into the print station 30Y~ The drive :~ : 20 xoller &6 is provided with a knurled sur~ace, and the :: ~ pinch roller 68 is coated with an elastomeric material together forming a combination that extends between side plates 12 and 14 (Fig. 1) to grip the paper 16 therebetween to pull the paper into ~he print station OY or hold the paper fixed, as the case may be.
In similar fashion, the micro-stepper motor :62 connects ~o and powers the exit drive ~8,~also com~
prising a knurled drive roller 70 and an elastomeric-:
coated~pinch roller 72.
:~ ~; 30 The drive:rollers 66 and 70, as well as pinch rollers 68 and~72, are alI aligned in substantially parallel relatlon to one another, but skewed slightly elative~o:the direction of paper traveI, as:il}ustrated Fig;. 3. Fig.~3 shows the pinch roller 68 mounted :
~:~ 35 between~the side~plates 12 and 14 and skewed an angle 9 relative to:the direction of travel (arrow 76) of th~
~;: paper 16 along the transport path 18. In the preferred e~ood1ment of the invention the angle 9 of skew is ~: .

approximateiy 0.072. Since the companion drive roller 66 (which would generally underlie the pinch roller 66, but is not seen in Fig. 3 because it is obscured by the paper 16), and the drive and pinch rollers 70 and 72, respectively, are parallel to the pinch roller 66, they also are skewed the same angle ~. This slight skew of the entry nd exit drives 36 and 38 i~parts a force vector 78 (Fig. 3) ~o the paper 16, when moving the paper, with armajor component 78a, but a lesser compo-nent 78b (approximateIy 0.1 oz.).
~s Fig. 3 also illu~trates, the paper 16 is . perforated proximate one edge 16a. The print station 30Y is provided with a sprocketed roller 99 (Fig. 2) that is journalled to exte~d between side plates 12 and }4. The sprockets are located proximate the end jour-; nalled to side plate 12 to opera~ively engage the aper-, tures 92 (Fig. 3) formed in the paper. I~ addition, an identical sprocketed roller 98 (Fig. 2) is located prox-imate the entry~20 of the transport path 18.
Th`e five sprocket rollers (sprocketed rol}er 98, and the sprocketed rollers 99 in each of the print stations 30Y-30B) cooperatively operate with the aper-tures 92 to constrain the edge 16a in a line along the transport path 18. This constraint operates with the skewing of the entry and exit drives 36 and 38 to any size variation of the paper 16 to two~directions only:
- One in~he direction of travel, and one in a transverse direction. Knowing which way paper size ~aries allows 3Q one to~more accurately detect, and compensate for, such variations. The preferred detection and compensation is discussed further~hereinafter.
The;~sprocket/paper aperture combinatioA also accura~ely align~and guide the paper along the transport path. The forward and lateral forces imparted to the paper 16 by the drive~rolIers pull the paper against the sprockets, accurately aligni~g the paper as it travels through each print station and, in particular, ' ~: ' :

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just prior to entering the print head/back-plane 86/40 combination. In the preferred e~odiment, the teeth of the sprockets are 0.103~0.001 inches; the paper 16 is perforated with aper~ures 0.104~Q.OO1 inches in diameter, set 0.250iO.OO1 apart (center-to-center).
Returning to Fig. 2, the microprocessor 50 also communicates via the comma~d/control data path 52 with a write control circuit 80 which, i~ response to instructions from the microprocessor 50, controls the write operation via print head drivers 82 and back-plane drivers 84. The print head drivers 82 are co~nected to .. an electrostatic print head 86. The electrosta~ic print hea~ 86 is, as indicated above, an elongate array of individual styli (not shown) held by a matrix of epo~y and positioned beneath the back-plane bar 40 ~Figs.
1 and 2). In response to signals from the write control circuit 80, and the microprocessor 50, the pri~t head drivers 82 and back-plane drivers 84 function to produce, :~ . 20 between individual selected ones of the styli (not s~own) of the:eiectrostatic print head 86 and the back-plane 40, approximately a 600-volt potential to produce elec-.
` trostatic charges on the paper 16.
Located downstream of the print head 86 -back-plane 80 combination is a toner station 88. It is : here that toner (for print station 30Y - yellow; for~; ~print stations 30M, 30C and 30B - magenta, cyan and black, respectively) is applied to the electrostatic-: ally charged paper (adhering on}y to the charged area),~
dried, and any residual charge neutralized. The toning : : : ; roller~89 of the print sta~ion 88 is driven by the step-`
: ~: per motor 62~to~apply ~he liguid toner to the charged : paper. The printing process used~by the present inven-; tion is known to those skilled in the:art, except as to : : :35 the construction and operation of the electrostatic pri~ting head 86, in which case resort should be had to the above~referenced U.S. Patents 4,419,679 for further information.

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Located upstream and proximate the electro-static print head 86 is an optical sensor array 90, comprising a pair of optical sensors ~not shown, but whose locations are indicated in :Fig. 3A at 91 a~d 92 by the dotted circles) that are u,sed to detec~ and track the registratio~ marks placed on an edge of the paper 16 by the marker station 32 (Figs. 1 and 2~. The ~arker ~tation 32 comprises a small electrostatic reyi.stration print station 96 (including toning and dryiIlg appara-tus) for printing the registration i~dicia on t:he paper 16. As Figs. 3 and 3A illustrate, the registration indicia, consisting of r~gistration marks 102 c~d line 104, is placed proximate the edge 16b - oppoæite the constrained edge 16a. The registration line and marks 104, 1~2 are printed on the downward-facing surface of ~ the paper 16.
: ~ ~ The registration marks 102 define the portion of each line o~ dots to be printed at each of:the print 20 :stations 30Y-30B. Between each registration mark 102 -~
.that is from leading e:dge to leading edge - are 50 raster lines (i.e., lines of dots) across the paper 16, and 800 micro-steps of the micro-stepper motors 60 and 62.
The printing of each registration mark 102 is keyed to rotation of the sprocketed roller 98. As the paper 16 tra~els along the transport pa~h 18, the apertures 92 captur~ ~nd rotate the sprocketed roller 98. An optical encod~r 94 is coùpled to the sprocketed roller 98 to produce timing~signals indicative of rotatio~. These timing signals; are con~eyed to the registration print tation 96, via the optics control unit 92, to cause a registration mark 102 to be printed on the paper 16.
Thus,::in addition to functioning to constraining the : edge 16a of the paper 16 at that point, the sprocketed : 35 roller 94 sees to it that registration marks are accu-~rately spaced on the paper 16 -:independent of:paper :~ :veIocity. The print stations 30~-30B can rely on the registration marks to determine ii any varia1:ion in .

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size of the paper, in the direc-tion of travel, has oc-cuxred, and act accordingly to compensate ~or any such variation.
S Similarly, the registrat:ion line 104 estab-lishes a trans~er~e dimension of the paper 16 as it enters ~he transport path. Detected variations of the - registration.line 104 at the prin~ stations 30Y-30B
allow the print stations to signal th~ print process to compensate in a manner discussed ~urther below. It is the function ~f the optical sensor array 90 to detect these variations, if any. ~s indicated above, the reg-. istration marks 102 so placed on the paper :L6 correspondto the start of a print line formed by the print head 86. The markers are read by the optical sensor array 90, and used to accurately position the lines of print data at the print station 3QY and at each print station 30M-30B relative to any preceding print station.
::: In order to achieve proper printing/plotting of the color images by each of the print stations 30Y-30B, the tension of the paper 16 while in the print station 30Y (i.e., while between the entry and exit drives 36 and 38) must be correctly maintained so that it is pulled across the drive head 86 and back-plane 40 . 25 in a proper manner. Too much te~sion will create too ~u~h wear of the head 86, too little paper tension im-pairs ~he print operation. In addition, each print : ~tation must see to it that it imposes a predetermined minimum drag for~e on the paper 16. Otherwise, the problem~of pap~r tears and poor plotting/printing become ~ ~ery real. To alleviate ~hese problems, separate control ~ ~ of the~the micro-stepper motors 60 and 62 is possible (and~, correspondingly, separate control o~ the micro-: stepper motors o print stations 30M, 30C, a~d 30B, correspo~ding to micro-stepper motor 62). Thus, for : : example, to increase paper tension within the print station 30Y, the micro-stepper motor 60 can be slowed down, relativ~ to the micro-stepper motor 62, ~o slow , ~z~

down the paper being fed into the print station 30Y.
At the same time, operation of the micro-stepper motor - 62 is conti~ued or speeded up to cause the exit drive 5 38 to continue to pull the paper 16 from the prin~ sta-tion 30Y but at a faster rate - until the desired paper tension is obtained, at which time both micro-stepper motors 60 and 62 can be synchronously operated.
If the speed of micro-stepper motor 62 is 10 increased, the transport system downstream thereo must adjust accordingly~ Thus, the microprocessor S0 places status data on the Internal Sta~us Bus via the I/0 port 56 that is communicated to the downstream prin1: stations 30M, 30C, and 30Y. They, in turn, will adjust the speed 15 of their micro-stepper motors 62 to take up the slack.
Proper tension o the paper within the print station 30Y is monitored by the tension bar 42. But s the tension bar 42 performs a variety of addition func-tions: First, it operates the maintain a paper path of 20 predetermined length between the upstream entry drive 36 and the downstream exit drive 30; second, it operates . to straighten the paper as it is fed from the upstream entr~ drive 36 i~to the print station 30Y ~the skewed : rollers tend to cause the paper to buckle or wrinkle 25 somewhat); third, it operates to ensure that the con-strained edge 16a properly engages the sprocketed roller 99 to align the paper just before it is pulled across the electrostatic print head 86; and fourth, ~he vertical ; position of the bias bar ~a~ vary ~o compensate for 30 disparities between ~he micro-stepper motors 60 and 62.
Thus, while indeed it is tension that the : : tension bar 42 monitors, it may be more accurate to say that what really is being monitored is the force exerted on the print head 86. If this paper path shortens, it 35 is an indication ~hat the paper is tightening, imposing higher~ forces:on ~he pri~t:head 86 and possibly causing a de~erioration in print/plot guality. In any;event, : : the vertical position::of the tension bar 42 is monitored .

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by a shuttered optic l ~etec~or 44 to which the tension bar 42 is connected. Two signals are produced by the optical detector 44: A ~I signal that indica~es when ~he tension bar 42 has been moved by a shortening of the paper path to its vertically upward extreme; and a LO signal to indicate that an extension of ~he paper pa~h has released the tension bar 42 to its extreme low position. Absence of both the ~I and LO signals indicate the tension bar 42 is in a median range and that the paper path is within an acceptable range.
The shuttered optical detector 44, used to monitor the position of the tension bar 42, is illus-trated in greater detail in Fig. 4. As shown, the de-tector 44 includes a planar shutter 106 mounted in the detector 44 in a vertical orientation. The shutter ,' 106, so mounted, is capable of ver~ical movement with~
the tension bar 42 which is biased downward by, ~or example, a spring 106 that exerts approximately a two-pound force thereon at the end connected to the shutter 106. An egual bias force is applied to the opposite end of the tension bar 42 in order to e~ert an egual force against the paper 16 along the length of the ten-sion bar.
The shutter 106 is apertured at 110 to, depend-ing upon the relative vertical position of the shutter la6 and the ap~rture lI0, allow light communication betw~en one or the other light sources 112a, 112b and their corresponding optical sensors 114a, 114b, respec-tively.~ Optical communication between the light source/optical sensors 112ajll4a and 112b/114b produces the HI and LO signals, respectively; no communication no signals. ~ ~
Preferably, the paper is tensioned i~ print station 30Y so;that a force o~ approximately two pounds is exerted on the paper 16 (in the plane of the paper) at the input of the head/back plane combi~ation 40~86, and five pounds at the output. It is one ~unction of : ~

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the tension bar. 42 to maintain a paper path through the print station 30Y so that the paper 16 is drawn properly, and wi~h ~he correct force, acros~; the print head 86.
S The range within which the paper path can vary is estab-lished by the limits of vertical travel o the tensio~
bar 42, as measured by the optical shuttered detector 44. The ~I and LO signals, therefore, provide indicia : of ~he lengthening or shortening of the paper path, in the form of paper tension, so that the microprocessor 50, via the motor control 58, can independently control the micro-stepper motors 60 and 62, and with them the . drive rollers 66 and 70, to maintain the desired paper path length. This is accomplished in the following manner: Assume, at the outset, that the paper 16 has ust been installed in the print station 30Y (as well ~ as the following print stations 30M, 30C and 30B). At ; the initiation of operation, the paper will have very little tension, indicating that the path of the paper 16 in print station 30~ is too long for proper print operation. The tension bar 42, therefore, will be al-lowed by the paper 16 to drop to its lower extreme, positioning the aperture 110 of the shutter 106 (Fig.
4) so that optical co~munication is established between the light source 112b a~d its corresponding optical ~:~ sensor 114b. The LO signal becomes active, indicating :
to the microprocessor 50, via the optics control u~it 92: and~commandJcontroI data path 52, that little or no : :tension is present on the paper 16 and that ~he paper : 30 path must b~ shortened. Accordingly, the microprocessor 50 will issue commands (i.e., a micro-step count) to : the motor contr~ller 58 to begin operation of the micro-stepper mo~or 62 while,:at the same time, holding the micro-stepper motor 60 fixed.
The number of micro-steps that a micro-stepper ~ motor must take to move the tension bar from its low : ~ : Iimit to its high limit is known; in the preferred em bodiment this number is approximately 6,000 micro-steps.

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Accordingly, ~he microprocessor program knows that from the point the LO signal terminates there will be approx-imately 3,000 micro-steps reguire~ by the micro-stepper motor 62 to position ~he te~sion bar 42 midway its limits.
Thus, the microprocessor S0 will issue commands ~counts) to the motor controller 58 to cause the micro-stepper motor 62 to set through approximately 3,000 micro-steps while holding the micro-stepper motor 60 inactive. With the paper held fixed between the drive rollers 66 and pinch roller 68, the drive roller 70 begins pulling the paper 16 to shorten the paper path ,. m the prlnt station 30Y a~d ~hereby increase paper tension, moving the tension bar 42 until the micro-step-per motor 62 makes its approximately 3,000 steps.
This paper-tightening operation also takes ~ place in the remaining print stations 30M, 30C and;30B
; ' so that, when~complete, the path of the paper 16 in `~ each print station is the desired length and the paper . 20 properly tensioned. However, the tightening operation is somewhat different - only because ~he print stations 30~, 30C, and 30B directly control only one micro-stepper motor - the one corresponding the micro-stepper motor 62 of the print stations exit drive 38. Print station 30Y is the only one having direct microprocessor control over two micro-stepper motors; control over what would correspond to the micro-stepper motor 60 for print sta-tions 30M, 30C,~a~d 30B is exercised by the microproce~-sor immediately preceding the print station. For exam-ple, paper tightening in the downs*ream print station ~; 30M wlll be identi~al to that described aboYe~ except that ~he microprocessor 50 of print station 30Y will control its micro-stepper 62 at the exit sta~ion 38 to hold the paper 16 fixed while the corresponding micro-; 35 processor SO of print station 30M will control its cor-responding micro-stepper motor 62 to shorten the paper path therein. (Remember, the entry drive 36 for print stations 30M, 30C, and 30B are also the exit drives 38 .

of print stations 30Y, 30M, a~d 30C, respectively.) This reguires communication between the respective microprocessors 50 of the print st.ations 30Y a~d 30M so that their efforts are synchronize!d as described. This communication is made between the microprocessors 50 ~via the I/O ports 56 (of each pri~t station 30Y and 30M) and the Internal Status Bus. This paper tightening pxocedure is substantially identical for print stations 30C and 30B, é~cept that it ~hould be evident that all upstream print stations are involved in the paper-tight-ening operation of any particular downstream print sta-. tion.
In addition to tensioning the paper a~ the outset, the ~I and L0 signals from the optical detector 44 also allow the microprocessor 50 to readjust tension at any of the pxint stations 30Y-30B, independent of the others, during print operations. For example,~as-sume that the paper is moving at a constant speed through the pxint station and ~hat, for whatever reason, the paper path at the print station 30Y (Fig. 2) has de-/ creased. This decrease, or shortening, of the paper path causes the tension bar 42 to be moved upward against ~ the bias spring 108 (Fig. 43. Ultimately, the paper path will shorten, moving tension bar 42 so that theaperture 110 in the shutter 106 of the optical detector 44 is positioned to allow light communication between the light source 112a and *he optical sensor }14a.
Thereby, the ~I~signal is active, and communicated to the microprocessor via the optics control unit 92 and 1 : ` ~
the command/control data path 52. ~This will cause the microprocessor 50 to issue commands to the motor con-troller 58 to ~ slow down o~eration of ~he~micro-s~tepper~motor 62 while;(2~ continuing operation of the micro-stepper motor 60 at a slightly higher rate. Thus, the micro-stepper motor 62 will operate at a slightly lower ratc of micro-steps per second than before; the micro-stepper motor 60 will, at the same ~ime, operate at a slightly hi~her rate of micro-steps than before;
and, since ~he paper 16 is being ied into the print station 30Y at a faster rate than it leaves, the paper 5 path lengthens in a smooth, continuous ~ashion. While the micro-stepper motors 60 and 62 are operated at these-different rates, the microprocessc)r 50 keeps a ru~ning accumulation of the excess micro-steps taken by the micro-stepper motor 60 relative to the micro-stepper motor 60. When ~his excess totals approxima~ely 3000, the microprocessor 50 will te~minate running the micro-stepper motors 60 and 6~, and begin operating them at .. the same rate, thereby terminating this paper path lengthening procedure.
The procedure is somewhat dif~erent for down-stream print stations 30M, 30C, and 30B~ For exampl~, if it is the print station 30B that experiences a short-ened paper path, all upstream print stations 30Y, 30M, and 30C must increase their respective paper transport rates while the microprocessor 50 o the print station 30~ slows its micro stepper motor 62. Coordination of this procedure is established between the respective ` microprocessors 50 of the print stations 30Y-30B by communication of status information (i.e., micro-stepper i 25 motor rates) on the Internal Status Bus.
The actual size of the paperj both laterally and in the direction of travel along the transport path 18, can change - primarily due to humidity differences to which the~paper was subjected whèn rolled and un-~
3~ rolled.~ Changes in ~he direction of travel are measuredby detecting;the timing marks 102 at each of the print stations 30Y-30B by the optical sensor group 90. The optical sensor group, as indicated in Fig. 5A comprises two independent sensors ~indicated, in Fig. 5A in dotted line~ 118 and 120. The sensor 118 monitors the regis~
tration marks~102, providing a signal every 50 raster }ines ~i~e., lines~o printed dots) at the detection of each mark to the microprocessor 50 via the optical ~ ~3~

control unit 9~ and comman~/control data path 52. The microprocessor 50 then functions i:o register the write operation with the registration mark. If, due to varia-tions in paper length along the d~irection of travel,the registration mark has not yet appeared, or alterna-ti~ely has appeared earlier than the 16 micro-steps allotted between raster lines, the microprocessor S0 coordinates the write operation accordingly, and resets the motor controller 58. In this manner, ~he lines of (yellow) image printed by the print station 30Y can be registered accurately, re~ative to the registration . marks 102, 50 that a printed yellow "dot" can be super-imposed (either in ovexlapping or side-by-side rela-tion) with a magenta dot at print station 30M, a cyandot at print station 30C, a black dot at print station 30B, or any combination thereof, to produce the desired color. The registration is checked by each print sta-tion 30Y-30B every 50 raster lines and brought back into exact registration if necessary.
If the media, i.e., paper 16, experiences a lateral change in size, this chan~e will be in one di-rection o~ly as a result of the skewing o~ the drive rollers, as explained above, with reference to Fig. 3.
Any change will be detected by the optical sensor 120 which produces an analog voltage varied by the black strip relative to the white background of the paper that is-communicated to the optics control unit 92.
There, the analog voltage is converted to digital infor-mation via a conventio~al digital-to-analog converter (not shown) that is con~eyed to the microprocessor S0 on the command/control data path 52.
Paper can stretch, in the lateral direction, as much as 3-5 dot positions leach "dot" b~ing approxi-mately 1/254 of an i~ch). Thus, for example, if a par-ticular dot is printed by the print station 30Y and is to have a mayenta dot superimposed thereo~, the prin~
station 30M for placing the magenta dot must know the ~L2~Sl3~3~

X, Y location of that yellow dot. This information is provided both by the registration marks 102 and the registration line 104. Registration marks 102, when counted by the microprocessor 50, d~termine exactly which line contains the yellow do~: of our example, and the signal produced by the optical sensor 120 provides information as to whether that dot position has shifted away from the edge 16a (Fig. 3) of the paper that has been constrai~ed. Accordingly, when that particular line is to be printed by the print station 30M, the microprocessor 50 can act on the dot information applied to the print head drivers 82 accoxding}y, adding or subtracting dots to superimpose the proper magenta dot printed by the print station 30M over the yellow dot previously printed by the print station 30Y.
In summary, there has been disclosed a single-pass full-color plotter/printer, among the significant ~eatures of which are (1) monitoring and adjusting the paper path and paper tension independentIy within each of the four color print stations, thereby keeping the tension forces imposed on the paper by each print sta-tion at a predetermined state; (2) maintaining the reg-istration of each image produced by each color station relative to those images produced by the others. This latter aspect is achieved, in part, by ensuring that lateral changes in the print media (i.e., paper) are i~
a single~ direction, and by using registration mar~s that determine, relative to the paper, the placement of each print line and each dot in that print line, there~y :
aIlowing microprocessor compensation "on the fly" for ~`; media variations; and ~3) guide and monitor the exact position of~the~media ~e.g., print paper) with sprock-~ eted rollers and apertured media together with transport`~ ~ 35 drive apparatus, including the tension bars 42, to ac-curately locate the media to the sprockets~ These fea-tures combine to provide a printer/plotter that gener-; ates, in a single pass, a ~ull color image~from a number 2~

of individual, superimposed, accurately registered mono chrome images each printed by separate pxint statio~s -indPpe~dent of velocity.
Although a preferred embodiment of the present invention has been described, those skilled in the art will recognize that other embodiments are capable of being implemented based upon the principles of this invention. For example, accordiny to the above disclo sure, each individual print station 30Y, 30M, 30C and 30B are provided with a separate microprocessor. Other embodiments may incorporate the teachings of the present invention using a single microprocessor to control all four print stations, thereby obviating much of the data communications therebetween via the internal status bus 56. Therefore, the above description should not be * taken as limiting the present invention, the scope of which is deined by the appended claims.

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Claims (19)

1. Transport apparatus for a printer/plotter having plural print heads arranged successively along a transport path for single-pass printing of an image on a print media, said transport apparatus comprising:
a plurality of separately controllable drive means for moving the print media along the transport path while maintaining media tension, a first one of said drive means being positioned at an input location of a first one of said print heads, the remaining ones of said drive means being positioned at an output location of each print head, said first drive means being operable as an entry drive for the first print head, a last one of said drive means being operable as an exit drive for a last one of said print heads, the remaining ones of said drive means simultaneously being operable as an exit drive of an immediately preceding print head and as an entry drive of an immediately succeeding print head;
a plurality of tension monitoring means, each engage-able with the print media, for providing a tension signal indicative of the print media tension as the print media moves between said drive means; and control means, coupled to each said drive means, for providing drive signals in response to receipt of the tension signals to maintain print media tension within a predetermined range between each said drive means

2. The transport apparatus of claim 1, wherein the drive means include an elongate drive roller and an elongate pinch roller positioned in parallel relation to one another with the print media therebetween in gripping relation.

3. The transport apparatus of claim 2, and wherein the drive means each includes a microstepper motor operable in response to the control means to drive the drive roller.

4. The transport apparatus of claim 1, and wherein the drive means each includes a microstepper motor operable in response to the control means.

5. The transport apparatus of claim 1, and wherein the tension monitor means includes an element and means for movably biasing the element in engagement with the print media as the print media moves through the print station, and means coupled to the element to provide said signal as a function of posi-tional movement of the element.

6. Transport apparatus for a single-pass printer/
plotter for printing of an image on a print media, the trans-port apparatus comprising:
means for defining a transport path along which the print media travels;
means, mounted to the transport path, for constrain-ing one periphery of the print media along a predetermined line of travel of the transport path; and drive means for moving the media along the transport path by imparting a first force to the print media in the direction of media travel while imparting a second force in a direction generally transverse the direction of media travel, parallel the media surface, and away from the constrained periphery.

7. A single-pass color printer for imprinting color images on a media by superimposing a plurality of individual images one upon the other, comprising:
support means defining a transport path for movement of the media therealong;
a plurality of print stations mounted in sequential order along the transport path, each print station being oper-able to print a corresponding one of the individual images; and transport means for moving the media from an entry to the transport path to an exit of the transport path through each of the print stations, the transport means including a first drive means mounted to the transport path upstream of the print stations and, for each print station, a second drive means mounted to pull the media through the corresponding print station, means for sensing a media path length within each print station, the media path length being variable within a pre-determined range, and control means for each print station for controlling the velocity with which the immediately preceding first or second drive means provides the media to the print station relative to the second drive means of the print station.

8. The single-pass color printer of claim 7, and includ-ing a marker station for imprinting sequential registration marks on the media for establishing predetermined positional relation between the registration marks and the media; and wherein each of the print station includes sensing means for detecting said registration marks and means for registering the image printed by the print station to the registration marks.

9. The single-pass color printer of claim 7, wherein the transport means includes means for sensing variations of the media path length within each print station.

10. The single-pass color printer of claim 9, wherein the transport means includes means for sensing the variations of the media path length and for adjusting the media path length to a predetermined nominal length.

11. The single-pass color printer of claim 7, wherein the media includes a substantially linear edge the transport means including means for constraining the media edge to a predetermined line of travel along the transport path.

12. The single-pass color printer of claim 11, the media having a plurality of spaced-apart apertures proximate the media edge and wherein the constraining means includes a plurality of sprocketed means mounted to the support means at spaced locations along the line of travel, the sprockets being operable to engage the apertures to constrain said apertures to said line of travel.

13. The single-pass color printer of claim 12, wherein the first and second drive means each impart a force to the media having a first component oriented generally in a direction of media travel along the transport path and a second component in a direction away from the media edge.

14. The single-pass color printer of claim 12, the maintaining means includes means for biasing the media into engagement with the sprocket means.

15. The transport apparatus of claim 6, the drive means including a pair of elongate drive rollers extending across and forming an angle less than 90 degrees with the transport path, the drive rollers being mounted substantially parallel to one another and spaced for receiving the media therebetween.

16. The transport apparatus of claim 6, the media being of the type having spaced apertures formed adjacent the constrained periphery, the constraining means including a sprocketed roller for engaging the spaced apertures.

17. The transport apparatus of claim 6, including means for monitoring the opposing periphery to detect transverse media variations along the transport path.

18. The transport apparatus of claim 17, the monitoring means including optical sensing means mounted to be located proximate the opposing periphery as the media travels the transport path.

19. Transport apparatus for a printer/plotter having plural print heads arranged successively along a transport path for a single-pass printing of an image on a print media, the transport apparatus comprising:
means, mounted to the transport path, for constrain-ing one periphery of the print media along a predetermined line of travel of the transport path;
a plurality of separately controllable drive means for moving the print media along the transport path by impart-ing a first force to the print media in the direction of media travel while imparting a second force in a direction generally transverse the direction of media travel, parallel the media surface, and away from the constrained periphery, a first one of said drive means being positioned at an input location of a first one of said print heads, the remaining ones of said drive means being positioned at an output location of each print head, said first one of said drive means being operable as an entry drive for the first print head, a last one of said drive means being operable as an exit drive for a last one of said print heads, the remaining ones of said drive means simultaneously being operable as an exit drive of an immediate-ly preceding print head and as an entry drive of an immediately succeeding print head;
a plurality of tension monitoring means, engageable with said print media, for providing a tension signal indicative of a print media tension as the print media moves between said drive means; and control means, coupled to each said drive means, for providing drive signals in response to receipt of said tension signals to maintain said print media tension within a predetermined range between each said drive means.