BACKGROUND OF THE INVENTION
[0001] This invention relates generally to electrostatic printing, and particularly to electrostatic
color printing/plotting using multiple color print stations to produce a full-color
image in a single-pass.
[0002] Electrostatic printing is accomplished by placing electrostatic charges in the form
of the image to be printed on an electrographic media, usually paper. The paper is
then exposed to a liquid toner to produce a permanent visible image. In electrostatic
color printing, separate images are electrostatically printed on the paper and toner
applied, each image typically corresponding to one of four colors: The three colors
of yellow, cyan and magenta, and for true black, a fourth color of black.
[0003] Heretofore, 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 between the recording head and styli while individual
ones of the styli are selected and impressed with a voltage potential that, together
with the back-plane (usually also impressed with a voltage potential) creates a "dot"
of charge on the paper. The image for each color may be thought of as comprising lines
or "rasters" of these dots which, when toned combine to form the image of that color.
For full color, the yellow, magenta, cyan, and (if used) black images are separately
printed, each registered to be relatively superimposed on one another to form the
full-color image. Depending upon the final color desired, any particular dot location
on the paper may have one or more colors printed thereat. Printing the full-color
image has been performed by passing the paper through the print station (which usually
has a single electrostatic recorder and multiple toner/dryers -one for each separate
image) to print the first image, rewinding the paper and passing the paper through
again for each subsequent image.
[0004] Among the problems associated with this technique of electrostatic color printing
are the time required to complete the total image and maintaining the relative position
of each successive image upon the paper relative to all the primary images. As the
paper is transported forward and backward through the print bed, proper registration
of the individual images can be quite difficult. In addition, these problems are exacerbated
by changes in the media caused, primarily, by humidity which, in turn, causes the
paper to change size, both in the direction of travel and laterally.
[0005] Thus, the advantages of a color printer/plotter capable of printing a full-color
image in a single-pass are certainly evident to those skilled in this art. The problems
of time and registration are reduced, if not obviated, allowing precise full-color
images to be printed, one after the other in a single-pass by print stations located
in sequence along the transport 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
additional paper strength to prevent tearing. The paper becomes still less flexible,
and more expensive. Alternatively, merely adding additional drives between the print
stations does not necessarily solve the problem -and can create other problems. Proper
tension across the print head must be maintained within predetermined limits in order
for the printing process to occur correctly. Adding extra paper drives can cause paper
tension to be unacceptable at some of the print stations.
SUMMARY OF THE INVENTION
[0006] The present invention provides an electrostatic, single-pass printer/plotter capable
of producing a full-color image from four separate, superimposed images (yellow, magenta,
cyan, and black). Broadly, the invention includes a registration system that constrains
one edge of the print media to a predetermined line of travel along a transport path
to accurately guide the media therealong, and drive apparatus that, in addition to
moving the media along the transport path, imparts a lateral force (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 the media edge opposite
the constrained edge, as are equally spaced registration marks. Sensors, incorporated
in each of a plurality of print stations (one for each separate color image) located
along the transport path are optical sensors that monitor the registration line and
mark to, under microprocessor control, modify the printing process to compensate for
media variations in size. In short, the registration marks (which define each print
line) and the registration line establish the position of every point on the media.
Should media variations in size cause any of these points to vary their relative position,
particularly if one of the images has been printed thereon, the relevant print stations
can detect such variations and, knowing the direction of occurrence, make the necessary
correction.
[0007] The invention further includes a transport drive system for each print station that,
in addition to transporting the media through that station to the next print station,
maintains a correct media tension and operates to eliminate any influence on the media,
by an immediately preceding subsequent print station. The transport system includes
a media drive located at the output of each print station that is separately controllable
(relative to the other print stations) in cooperation with adjacent print stations
to adjust a path along which the media travels within the print station to maintain
media tension.
[0008] According to the preferred embodiment of the invention, a support defines a transport
path along which are mounted four individual print stations. Each print station is
adapted to receive data for printing a separate color image for superposition with
the other color images printed.
[0009] A registration system includes precision, freewheeling 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 transport path, providing
an accurate alignment of the paper. Proximate the opposing edge of the paper are placed
registration marks and a registration line. Each print station is provided with optical
sensors that monitor the registration marks to synchronize the printing operation
with each mark, and to monitor the alignment strip.
[0010] The transport system used to move the paper from print station to print station uses
fine separately controlled drive rollers. One, located near an entry area of the transport
path, pulls the paper from a roll onto the transport path and through the print station
that marks the paper with the registration line and marks, and feeds the paper to
a first of the four 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).
[0011] All drive rollers of the transport system are skewed a slight amount, relative to
the direction of travel of the paper, to impart to the paper a small force (on the
order of 0.1 oz.) in a direction transverse the direction of travel, and away from
the constrained edge. This serves two important purposes: First, it ensures that any
lateral variations are in one direction only -away from the constrained edge (and
toward the edge carrying the registration line). The amount of lateral variation is
detected by the optical sensor of 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 correction for variation. Second, the paper is pulled
against the sprockets to accurately align the paper with, and guide the paper along,
the transport path.
[0012] Each of the print stations includes at the exit thereof, one of the drive rollers
of the transport system. Preceding 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 within the
print station to separately 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 as needed for paper print operation.
[0013] A number of advantages are obtained by the present invention. First, by providing
the capability of single-pass, full-color print/plot operation, the time required
for the final image is reduced. Further, the printing/plotting procedure can be varied
to accommodate data flow from 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 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.
[0014] 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 registration of the separate images printed
on the paper. Additionally, constraining one edge of the paper to a specific line
of travel, together 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.
[0015] 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 accurately 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 sequentially placed
along a transport path for producing single-pass full-color printing/plotting.
[0016] 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
[0017]
Fig. 1 is a perspective view of a single-pass, full-color, electrostatic printer/plotter
constructed in accordance with the teachings of the present invention;
Fig. 2 is a block diagram representation of one of the four print stations used in
connection with the present invention;
Fig. 3 is a representation of a portion of the print media, e.g., electrostatic printer
paper used, for printing images by the present invention, illustrating 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 of the registration marks and registration line placed
proximate one edge of the printer paper, and illustrating sensor locations relative
thereto; and
Fig. 4 is a representation of the shuttered optical sensor used to detect tension
of the media transported through each print station of Fig. 2.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Referring to Fig. 1, a printer/plotter constructed in accordance with the teachings
of the present invention, and designated generally with the reference numeral 10,
is illustrated. The printer/plotter 10 includes a pair of side plates 12 and 14, mounted
in generally fixed, spaced and parallel relation to one another by appropriate means
(not shown). The side plates 12 and 14 generally define, at the upper portion thereof,
a transport path 18 along which the media 16 (such as, for example, electrostatic
paper) travels from a paper supply roll (not shown), located proximate an entry area
20 of the transport 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 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.
[0019] Proximate the entry area 20 of the transport path 18, and mounted to the side plate
14, is a marker station 32, which functions to print registration indicia along one
edge of the paper 16 as it enters the transport path 18. As will be seen, the registration
indicia, comprising a registration line and spaced markers (104 and 102, respectively,
Figs. 3 and 3A) are used to track any variations in paper size as the paper moves
along the transport path. The print stations 30Y, 30M, 30C and 30B, as will be described
more fully below, are each equipped to sense any such variations and adjust the print
operation to compensate. Each of the print stations 30Y, 30M, 30C and 30B operate
independently to electrostatically print their corresponding yellow, magenta, cyan
or black images on the paper 16 in registered relation to one another by monitoring
the registration indicia.
[0020] 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 drive 38 to pull the paper through the
print station, and, 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 the
tension bar 42, obtaining thereby an indication of the tension of the paper 16 between
the entry and exit drives 36 and 38.
[0021] The print stations 30Y, 30M, 30C and 30B are identically structured, so that hereinafter
only the print station 30Y will be described. It should be understood, therefore,
that any description of the print station 30Y will apply equally to the print stations
30M, 30C and 30B -unless otherwise noted. Further, the individual print stations 30Y-30B
use an electrostatic print process generally of the type that employs a printer head
that extends between the side plates 12 and 14 (and underlies the back-plane bar 40
of Fig. 1) and comprises an elongate array of staggered rows of styli. The print heads,
and the electronics to operate the print heads, are described in U.S. Patent No. 4,419,679,
the contents of which are hereby incorporated by reference.the documents referred
therein, entitled "Service and Maintenance Manual", published in April, 1981 by Benson-Varian
of 2690 Orchard Park Way, San Jose, California 95152-2059, Model 9424 Printer/Plotter,
Publication No. 03996-336E." Only the novel features of each print station, as they
apply to the present invention, are described herein.
[0022] Fig. 2 illustrates, in greater detail, the marker station 32, the entry drive 36,
and the configuration of the print station 30Y. As Fig. 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, 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.
[0023] The data path 52 connects the microprocessor 50 to a memory unit 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 effect control over
printing operations performed by the print station 30Y. An input/output - (I/O) port
56 provides a buffered input/output data path between the print stations 30Y, 30M,
30C and 30B. Status information such as, for example, adjustment of the tension, is
transmitted via the I/O port 56 between the various print stations to coordinate paper
flow therethrough.
[0024] Also coupled to the command/control data path 52 is a motor controller 58 which,
in turn, is connected to micro-stepper motors 60 and 62. In response to command data
from the microprocessor 50, in the form of a count specifying the number of micro-steps
to be taken by one or the other (or both) of the micro-stepper motors 60, 62, the
motor controller 58 controls operation of the micro-stepper motors 60 and 62. By issuing
pulses on one or the other (or both for synchronous operation) of the signal lines
59a and 59b, the motor controller 58 causes the micro-stepper motors 60 and 62 to
take the required number of micro-steps (one for each pulse) individually, together
at different rates, or together in synchronism.
[0025] The micro-stepper motor 60 operatively connects to drive roller 66 that, together
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 roller 66 is provided with a knurled surface,
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 the print station 30Y or hold the paper fixed,
as the case may be.
[0026] In similar fashion, the micro-stepper motor 62 connects to and powers the exit drive
38, also comprising a knurled drive roller 70 and an elastomeric-coated pinch roller
72.
[0027] The drive rollers 66 and 70, as well as pinch rollers 68 and 72, are all aligned
in substantially parallel relation to one another, but skewed slightly relative to
the direction of paper travel, as illustrated in Fig. 3. Fig. 3 shows the pinch roller
68 mounted between the side plates 12 and 14 and skewed an angle 0 relative to the
direction of travel (arrow 76) of the paper 16 along the transport path 18. In the
preferred embodiment of the invention the angle e of skew is approximately 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 0. This slight skew of the entry and exit drives 36
and 38 imparts a force vector 78 (Fig. 3) to the paper 16, when moving the paper,
with a major component 78a, but a lesser component 78b (approximately 0.1 oz.).
[0028] As Fig. 3 also illustrates, 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 extend between side plates 12 and 14. The sprockets are located proximate the end
journalled to side plate 12 to operatively engage the apertures 92 (Fig. 3) formed
in the paper. In addition, an identical sprocketed roller 98 (Fig. 2) is located proximate
the entry 20 of the transport path 18.
[0029] The five sprocket rollers (sprocketed roller 98, and the sprocketed rollers 99 in
each of the print stations 30Y-30B) cooperatively operate with the apertures 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 the direction of travel, and one in a transverse
direction. Knowing which way paper size varies allows one to more accurately detect,
and compensate for, such variations. The preferred detection and compensation is discussed-further
hereinafter.
[0030] The sprocket/paper aperture combination also accurately align and guide the paper
along the transport path. The forward and lateral forces imparted to the paper 16
by the drive rollers pull the paper against the sprockets, accurately aligning the
paper as it travels through each print station and, in particular, just prior to entering
the print head/back- plane 86/40 combination. In the preferred embodiment, the teeth
of the sprockets are 0.103±0.001 inches; the paper 16 is perforated with apertures
0.104±0.001 inches in diameter, set 0.250±0.001 apart (center-to-center).
[0031] Returning to Fig. 2, the microprocessor 50 also communicates via the command/control
data path 52 with a write control circuit 80 which, in 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 connected to an electrostatic
print head 86. The electrostatic print head 86 is, as indicated above, an elongate
array of individual styli (not shown) held by a matrix of epoxy 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 print head drivers 82 and back-plane drivers
84 function to produce, between individual selected ones of the styli (not shown)
of the electrostatic print head 86 and the back-plane 40, approximately a 600-volt
potential to produce electrostatic charges on the paper 16.
[0032] 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 electrostatically
charged paper (adhering only to the charged area), dried, and any residual charge
neutralized. The toning roller 89 of the print station 88 is driven by the stepper
motor 62 to apply the liquid toner to the charged paper. The printing process used
by the present invention is known to those skilled in the art, except as to the construction
and operation of the electrostatic printing head 86, in which case resort should be
had to the above-referenced U.S. Patents 4,419,679 for further information.
[0033] Located upstream and proximate the electrostatic 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 and 92 by the dotted circles) that are used to detect and
track the registration marks placed on an edge of the paper 16 by the marker station
32 - (Figs. 1 and 2). The marker station 32 comprises a small electrostatic registration
print station 96 (including toning and drying apparatus) for printing the registration
indicia on the paper 16. As Figs. 3 and 3A illustrate, the registration indicia, consisting
of registration marks 102 and line 104, is placed proximate the edge 16b -opposite
the constrained edge 16a. The registration line and marks 104, 102 are printed on
the downward-facing surface of the paper 16.
[0034] The registration marks 102 define the portion of each line of dots to be printed
at each of the print stations 30Y-30B. Between each registration mark 102 -that is
from leading edge 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 travels along the transport path 18, the apertures 92 capture and rotate
the sprocketed roller 98. An optical encoder 94 is coupled to the sprocketed roller
98 to produce timing signals indicative of rotation. These timing signals are conveyed
to the registration print station 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 roller 94 sees to it that
registration marks are accurately spaced on the paper 16 - independent of paper velocity.
The print stations 30Y-30B can rely on the registration marks to determine if any
variation in size of the paper, in the direction of travel, has occurred, and act
accordingly to compensate for any such variation.
[0035] Similarly, the registration line 104 establishes a transverse dimension of the paper
16 as it enters the transport path. Detected variations of the registration line 104
at the print stations 30Y-30B allow the print stations to signal the print process
to compensate in a manner discussed further below. It is the function of the optical
sensor array 90 to detect these variations, if any. As indicated above, the registration
marks 102 so placed on the paper 16 correspond to 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 30Y and at each
print station 30M-30B relative to any preceding print station.
[0036] 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 in a proper manner. Too
much tension will create too much wear of the head 86, too little paper tension impairs
the print operation. In addition, each print station must see to it that it imposes
a predetermined minimum drag force on the paper 16. Otherwise, the problem of paper
tears and poor plotting/printing become very real. To alleviate these problems, separate
control of the the micro-stepper motors 60 and 62 is possible - (and, correspondingly,
separate control of the microstepper motors of print stations 30M, 30C, and 30B, corresponding
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, relative to the
micro- stepper motor 62, to slow down the paper being fed into the print station 30Y.
At the same time, operation of the micro-stepper motor 62 is continued or speeded
up to cause the exit drive 38 to continue to pull the paper 16 from the print station
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.
[0037] If the speed of micro-stepper motor 62 is increased, the transport system downstream
thereof must adjust accordingly. Thus, the microprocessor 50 places status data on
the Internal Status Bus via the I/O port 56 that is communicated to the downstream
print stations 30M, 30C, and 30Y. They, in turn, will adjust the speed of their micro-stepper
motors 62 to take up the slack.
[0038] Proper tension of the paper within the print station 30Y is monitored by the tension
bar 42. But the tension bar 42 performs a variety of addition functions: First, it
operates the maintain a paper path of 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 entry drive 36 into the print station 30Y (the skewed
rollers tend to cause the paper to buckle or wrinkle somewhat); third, it operates
to ensure that the constrained 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, the vertical position of the bias bar can vary to compensate for disparities
between the micro-stepper motors 60 and 62.
[0039] 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 is an indication that the paper is tightening,
imposing higher forces on the print head 86 and possibly causing a deterioration in
print/plot quality. In any event, the vertical position of the tension bar 42 is monitored
by a shuttered optical detector 44 to which the tension bar 42 is connected. Two signals
are produced by the optical detector 44: A HI signal that indicates when the 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 the paper path has released the tension
bar 42 to its extreme low position. Absence of both the HI and LO signals indicate
the tension bar 42 is in a median range and that the paper path is within an acceptable
range.
[0040] The shuttered optical detector 44, used to monitor the position of the tension bar
42, is illustrated in greater detail in Fig. 4. As shown, the detector 44 includes
a planar shutter 106 mounted in the detector 44 in a vertical orientation. The shutter
106, so mounted, is capable of vertical movement with the tension bar 42 which is
biased downward by, for example, a spring 106 that exerts approximately a two-pound
force thereon at the end connected to the shutter 106. An equal bias force is applied
to the opposite end of the tension bar 42 in order to exert an equal force against
the paper 16 along the length of the tension bar.
[0041] The shutter 106 is apertured at 110 to, depending upon the relative vertical position
of the shutter 106 and the aperture 110, allow light communication between one or
the other light sources 112a, 112b and their corresponding optical sensors 114a, 114b,
respectively. Optical communication between the light source/optical sensors 112a/114a
and 112b/114b produces the HI and LO signals, respectively; no communication, no signals.
[0042] Preferably, the paper is tensioned in print station 30Y so that a force of approximately
two pounds is exerted on the paper 16 (in the plane of the paper) at the input of
the head/back plane combination 40/86, and five pounds at the output. It is one function
of the tension bar 42 to maintain a paper path through the print station 30Y so that
the paper 16 is drawn properly, and with the correct force, across the print head
86. The range within which the paper path can vary is established by the limits of
vertical travel of the tension bar 42, as measured by the optical shuttered detector
44. The HI and LO signals, therefore, provide indicia of the 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 just 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 30Y is too
long for proper print operation. The tension bar 42, therefore, will be allowed by
the paper 16 to drop to its lower extreme, positioning the aperture 110 of the shutter
106 (Fig. 4) so that optical communication is established between the light source
112b and its corresponding optical sensor 114b. The LO signal becomes active, indicating
to the microprocessor 50, via the optics control unit 92 and command/control data
path 52, that little or no tension is present on the paper 16 and that the paper path
must be shortened. Accordingly, the microprocessor 50 will issue commands (i.e., a
micro-step count) to the motor controller 58 to begin operation of the micro-stepper
motor 62 while, at the same time, holding the micro-stepper motor 60 fixed.
[0043] The number of micro-steps that a micro-stepper motor must take to move the tension
bar from its low limit to its high limit is known; in the preferred embodiment this
number is approximately 6,000 micro-steps. Accordingly, the microprocessor program
knows that from the point the LO signal terminates there will be approximately 3,000
micro-steps required by the micro-stepper motor 62 to position the tension bar 42
midway its limits.
[0044] Thus, the microprocessor 50 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 in the print station 30Y and thereby increase paper tension,
moving the tension bar 42 until the micro-stepper motor 62 makes its approximately
3,000 steps.
[0045] 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 properly tensioned. However, the tightening operation
is somewhat different -only because the print stations 30M, 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 stations 30M, 30C, and 30B is exercised by
the microprocessor immediately preceding the print station. For example, paper tightening
in the downstream print station 30M will be identical to that described above, except
that the microprocessor 50 of print station 30Y will control its micro-stepper 62
at the exit station 38 to hold the paper 16 fixed while the corresponding microprocessor
50 of print station 30M will control its corresponding 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, and 30C, respectively.)
This requires communication between the respective microprocessors 50 of the print
stations 30Y and 30M so that their efforts are synchronized as described. This communication
is made between the microprocessors 50 via the I/0 ports 56 (of each print station
30Y and 30M) and the Internal Status Bus. This paper tightening procedure is substantially
identical for print stations 30C and 30B, except that it should be evident that all
upstream print stations are involved in the paper-tightening operation of any particular
downstream print station.
[0046] In addition to tensioning the paper at the outset, the HI and LO signals from the
optical detector 44 also allow the microprocessor 50 to readjust tension at any of
the print stations 30Y-30B, independent of the others, during print operations. For
example, assume that the paper is moving at a constant speed through the print station
and that, for whatever reason, the paper path at the print station 30Y (Fig. 2) has
decreased. This decrease, or shortening, of the paper path causes the tension bar
42 to be moved upward against the bias spring 108 (Fig. 4). Ultimately, the paper
path will shorten, moving tension bar 42 so that the aperture 110 in the shutter 106
of the optical detector 44 is positioned to allow light communication between the
light source 112a and the optical sensor 114a. Thereby, the HI signal is active, and
communicated to the microprocessor via the optics control unit 92 and the command/control
data path 52. This will cause the microprocessor 50 to issue commands to the motor
controller 58 to (1) slow down operation of the micro-stepper 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 rate of micro-steps per second than before;
the micro-stepper motor 60 will, at the same time, operate at a slightly higher rate
of micro-steps than before; and, since the paper 16 is being fed into the print station
30Y at a faster rate than it leaves, the paper path lengthens in a smooth, continuous
fashion. While the micro-stepper motors 60 and 62 are operated at these different
rates, the microprocessor 50 keeps a running accumulation of the excess micro-steps
taken by the micro-stepper motor 60 relative to the micro-stepper motor 60. When this
excess totals approximately 3000, the microprocessor 50 will terminate running the
micro-stepper motors 60 and 62, and begin operating them at the same rate, thereby
terminating this paper path lengthening procedure.
[0047] The procedure is somewhat different for downstream print stations 30M, 30C, and 30B.
For example, if it is the print station 30B that experiences a shortened paper path,
all upstream print stations 30Y, 30M, and 30C must increase their respective paper
transport rates while the microprocessor 50 of the print station 30B 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
motor rates) on the Internal Status Bus.
[0048] The actual size of the paper, 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 when rolled and unrolled. Changes in the direction of travel
are measured by 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 registration marks 102, providing a signal every 50 raster
lines (i.e., lines of printed dots) at the detection of each mark to the microprocessor
50 via the optical control unit 92 and command/control data path 52. The microprocessor
50 then functions to register the write operation with the registration mark. If,
due to variations in paper length along the direction of travel, the registration
mark has not yet appeared, or alternatively has appeared earlier than the 16 micro-steps
allotted between raster lines, the microprocessor 50 coordinates the write operation
accordingly, and resets the motor controller 58. In this manner, the lines of (yellow)
image printed by the print station 30Y can be registered accurately, relative to the
registration marks 102, so that a printed yellow "dot" can be superimposed (either
in overlapping or side-by-side relation) with a magenta dot at print station 30M,
a cyan dot 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 station
30Y-30B every 50 raster lines and brought back into exact registration if necessary.
[0049] If the media, i.e., paper 16, experiences a lateral change in size, this change will
be in one direction only as a result of the skewing of 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 information via a conventional digital-to-analog
converter (not shown) that is conveyed to the microprocessor 50 on the command/control
data path 52.
[0050] Paper can stretch, in the lateral direction, as much as 3-5 dot positions (each "dot"
being approximately 1/254 of an inch). Thus, for example, if a particular dot is printed
by the print station 30Y and is to have a magenta dot superimposed thereon, the print
station 30M for placing the magenta dot must know the 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, determine
exactly which line contains the yellow dot 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 constrained. 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 accordingly,
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.
[0051] In summary, there has been disclosed a single-pass full-color plotter/printer, among
the significant features of which are (1) monitoring and adjusting the paper path
and paper tension independently within each of the four color print stations, thereby
keeping the tension forces imposed on the paper by each print station at a predetermined
state; (2) maintaining the registration 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 in a single
direction, and by using registration marks that determine, relative to the paper,
the placement of each print line and each dot in that print line, thereby allowing
microprocessor compensation "on the fly" for media variations; and (3) guide and monitor
the exact position of the media (e.g., print paper) with sprocketed rollers and apertured
media together with transport drive apparatus, including the tension bars 42, to accurately
locate the media to the sprockets. These features combine to provide a printer/plotter
that generates, in a single pass, a full color image from a number of individual,
superimposed, accurately registered monochrome images each printed by separate print
stations -independent of velocity.
[0052] 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, according to the above disclosure,
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 defined
by the appended claims.
1. In a printer/plotter apparatus of the type having plural print stations arranged
along a transport path for single-pass printing of an image on a print media, a drive
system for each print station to move the print media along the transport path, the
drive system characterized by:
separately controllable entry and exit drive means respectively positioned at input
and output locations of the print station for moving the print media;
tension monitoring means engageable with the print media for providing a signal indicative
of the print media tension as it moves between the entry and exit drive means; and
control means coupled to the entry and exit drive means for providing drive signals
in response to. receipt of the signal to maintain media tension within a predetermined
range.
2. The drive system of claim 1, characterized in that the entry and the exit 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 drive system of claim 1 or 2 characterized in that the entry and exit drive
means each includes a microstepper motor operable in response to the control means.
4. The drive system of any of claims 1 to 3 characterized in that 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 positional movement
of the element.
5. The drive system of any of claims 1 to 4 characterized in that the plural print
stations include a first print station and a number of remaining print stations arranged
to seqentially receive the print media; and wherein the exit drive means of each immediately
preceding print station forms the entry drive means for each of the number of print
stations.
6. A transport system for moving a media across through a print station and along
a transport path of predetermined length for printing indicia on the media, the transport
system characterized by:
entry and exit drive means mounted to the print station respectively for respectively
pulling the media into and out of the print station;
means defining the transport path between the entry and the exit drive means, the
defining means including sensing means mounted to the defining means for producing
a signal indicative of the tension of the media; and
control means coupled to the sensing means, and to the entry and exit drive means,
for providing drive signals to each ofthe drive means in response to the signal to
maintain the length of the transport path within predetermined limits.
7. A single-pass color plotter for providing a color image on a media, the plotter
characterized by:
means defining a transport path along which the media travels;
means mounted to the transport path for constraining one periphery of the media along
a predetermined line of travel of the transport path;
drive means for moving the media along the transport path and for imparting a first
force to the media in the direction of media travel and a second force in a direction
generally transverse the direction of media travel and away from the constrained periphery;
and
means for monitoring the opposing periphery to detect transverse media variations
along the transport path.
8. A single-pass color printer for imprinting color images on a media by superimposing
a plurality of individual images one upon the other, characterized by:
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 operable to print a corresponding to 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
means for maintaining a media path length within each print station, the media path
length being variable within a predetermined range.
9. The single-pass color printer of claim 8 characterized by a marker station for
imprinting sequential registration marks on the media for establishing predetermined
positional relation between the registration marks of the media; and wherein each
of the print stations includes sensing means for detecting said registration marks
and means for registering the image printed by the print station to the registration
marks.
10. The single-pass color printer of claim 8 or 9 characterized in that the transport
means includes means for sensing variations of the media path length within each print
station.
11. The single-pass color printer of any of claims 8 to 10 characterized in that 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.
12. The single-pass color printer of any of claims 8 to 11 characterized in that the
transport means includes a first drive means mounted to the transport path upstream
of the print stations and, for each station, a second drive means mounted to pull
the media through the corresponding print station.
13. The single-pass color printer of any of claims 8 to 13 characterized in that the
media includes a substantially linear edge, the transport means including means for
constraining the media edge to a predefermined line of travel along the transport
path.
14. The single-pass color printer of claim 13, 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.
15. The single-pass color printer of any of claims 12 to 14, characterized in that
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.
16. The single-ass color printer of claim 14, the maintaining means includes means
for biasing the media into engagement with the sprocket means.