Plane registration for monochrome and color printing systems

A printing system for printing on a sheet of print media, the system including an electrophotographic printer operable to print on the sheet at a first print speed; and a color printer operable to print on the sheet of paper at a second print speed. The inkjet color printer includes a bidirectional sheet advancer operable to controllably move the sheet along an inkjet printer print path in either of first or second opposite directions, an inkjet pen carriage configured to support at least one inkjet pen and movable in a scan direction transverse to the inkjet printer print path for printing on the sheet, and a registration system operable to register a color image to be printed by the inkjet color printer relative to the black image printed by the electrophotographic printer. The registration system includes an optical sensor supported by the inkjet pen carriage for movement with the pen carriage along the scan direction.

FIELD OF THE INVENTION 
The invention relates generally to printing systems. More particularly, the 
invention relates to monochrome and graphics printing systems. The 
invention also relates to printing registration. 
BACKGROUND OF THE INVENTION 
Monochrome and color image printing systems are known in the art. See, for 
example, commonly assigned U.S. Pat. No. 5,140,674 issued to Anderson et 
al. (incorporated herein by reference), and commonly assigned U.S. Pat. 
No. 5,081,596 issued to Vincent et al. (incorporated herein by reference). 
These patents disclose the combination of a color printer for printing 
graphics, with a monochrome (e.g., black) electrophotographic printer. The 
color printer prints color images incrementally, and the 
electrophotographic printer prints continuously. The color printer can be 
of the inkjet type, and the electrophotographic printer can be of the 
laser type. 
An advantage of combining a color printer with an electrophotographic 
printer is that the resulting product has the combined advantages of low 
cost and high quality text printing. Color laser printers are expensive, 
and other types of color printers, such as inkjet printers, are often 
satisfactory for printing color graphics images, for certain business 
applications. On the other hand, inkjet printers sometimes do not produce 
sufficient text quality for business applications. Inkjet printers are 
less complex, and therefore less expensive than laser or 
electrophotographic printers, but do not always provide the line acuity 
and speed necessary for producing document quality text for a business 
application. Color printers of the laser or electrophotographic type 
require complex and expensive mechanisms to form and align color frames. 
Therefore, the combination of a color printer of the inkjet type to print 
color images incrementally, with a printer of the laser or 
electrophotographic type to print text images continuously, results in a 
good compromise low cost, high speed printer that can both print high 
quality text as well as high quality color graphics. 
One of the biggest problems in commercially producing a hybrid printer of 
this type relates to obtaining registration between a plane defined by the 
monochrome (typically black) image printed by the laser or 
electrophotographic printer, and planes produced by a different print 
mechanism; namely, the color printer. A further challenge is obtaining 
registration in a cost effective manner in a device for which it is an 
object to reduce cost. 
One way to deal with the problem of registration in a hybrid printing 
system is to avoid the issue by causing the color printer to print using 
process black (a combination of cyan, magenta, and yellow), where black is 
requested, in any areas of a page where black and color are to be mixed. 
Process black does not have the same quality as black printed by an 
electrophotographic or laser or printer, and, further, does not have the 
same darkness as regular black ink printed by a laser or 
electrophotographic printer, or an inkjet printer. 
Therefore, there exists a need for a low cost system for registering images 
printed by an inkjet printer with images, such as text or graphics, 
printed by an electrophotographic printer in a low cost, high speed 
printer that prints high quality color graphics as well as high quality 
black text and graphics. 
SUMMARY OF THE INVENTION 
The invention provides a printing system for printing on a sheet of print 
media. The system includes an electrophotographic printer configured to 
print monochrome images on the sheet, and an inkjet color printer 
configured to print color images on the sheet. The inkjet color printer 
further includes a bidirectional sheet advancement system configured to 
controllably move the sheet along an inkjet printer print path in either 
of first and second directions. The inkjet color printer further includes 
an inkjet pen carriage configured to support at least one inkjet pen and 
movable along a scan direction for printing on the sheet, and a 
registration system configured to register graphics printed by the inkjet 
color printer relative to image printed by the electrophotographic 
printer. The registration system includes an optical sensor supported 
relative to the inkjet pen carriage for movement with the pen carriage 
along the scan direction. 
In one aspect of the invention, a common housing supports both the 
electrophotographic printer and the inkjet printer. 
In another aspect of the invention, the electrophotographic printer is a 
laser printer. 
In another aspect of the invention, the optical sensor is a linear array 
optical sensor. 
In another aspect of the invention, the electrophotographic printer prints 
a registration mark on the sheet, and the registration system uses the 
optical scanner to locate the registration mark and effect registration. 
The registration system causes the inkjet pen carriage to move in the scan 
direction and causes the sheet advancement system to move the sheet in the 
inkjet paper path direction until the registration mark is located using 
the optical sensor, prior to printing by the inkjet printer. 
In one aspect of the invention, the color printer prints with incremental 
motion and the black printer prints with continuous motion. 
Another aspect of the invention provides a method of printing on a sheet of 
print media. The method comprises printing an image and a registration 
mark on the sheet with a laser printer, and directing the sheet to a 
staging tray after the printing with the laser printer. The image on the 
sheet is registered in an inkjet color printer that uses a bidirectional 
sheet feeder to transport the sheet from the staging tray after the sheet 
has been printed on by the laser printer. The registration includes using 
an optical sensor movable with an inkjet pen carriage, and causing the 
carriage to move and causing the bidirectional sheet feeder to move until 
the registration mark is located. A color image is printed on the sheet by 
the inkjet printer after registering the image printed by the laser 
printer.

DETAILED DESCRIPTION OF THE INVENTION 
This disclosure of the invention is submitted in furtherance of the 
constitutional purposes of the U.S. Patent Laws "to promote the progress 
of science and useful arts". U.S. Constitution, Article 1, Section 8. 
FIG. 1 shows a printing system 10, embodying the invention, for printing on 
a sheet or page 12 (FIG. 3) of print media. The printing system 10 is 
connected for control by a microprocessor based computer 11. The system 10 
includes an electrophotographic printer 14 configured to print a 
monochrome image on the sheet (e.g., including text and/or graphics), and 
an inkjet printer 16 configured to print a color image on the sheet (e.g., 
including color graphics). The term image means text, graphics, or both 
text and graphics. The term monochrome means a single color, which will 
usually be black. In the illustrated embodiment, the printer 16 prints 
with incremental motion and the printer 14 prints with continuous motion. 
More particularly, in the illustrated embodiment, the printer 14 is a 
laser printer, and the printer 16 is a multi-color inkjet printer 16. As 
such, laser printer 14 is configured to print at up to a first maximum 
speed with respect to pages per minute and the inkjet printer 16 is 
configured to print at up to a second maximum speed with respect to pages 
per minute. Although typically these maximum speeds are different, they 
may be the same. In one embodiment, the printer 14 includes internal 
components similar to those found in a LaserJet 6p printer sold by 
Hewlett-Packard Company of Palo Alto, Calif., and the DeskJet printer 16 
includes internal components similar to those found in a DeskJet 720 
printer sold by Hewlett-Packard Company of Palo Alto, Calif., except 
modified as described below. The system 10 includes a housing 18 
supporting the printer 14 above the printer 16 in the illustrated 
embodiment. In the illustrated embodiment, the printer 14 includes a laser 
scanner 76 supported in the housing 18, a toner supply (not shown), a 
photoconductor roller 78 acted on by the laser scanner 76, a charge roller 
80 imparting charges to the photoconductor roller 78 upstream of where the 
laser scanner 76 acts on the photoconductor roller 78, a developer roller 
82 acting on the photoconductor roller 78 downstream from where the laser 
scanner 76 acts on the photoconductor roller 78, a transfer roller 84 
facing the photoconductor roller 78 downstream from the developer roller 
82 and cooperating with the photoconductor roller 78 to impart an image to 
the sheet, a cleaning blade cleaning the photoconductor roller 78 after 
the image has been imparted to the sheet, and a fuser 86 spaced apart from 
and downstream from the photoconductor roller 78. 
The printing system 10 further includes a staging tray 20 which functions 
both as an output tray to receive the sheet after the sheet has been 
printed on by the printer 14, as well as an input tray to supply the sheet 
that has been printed on by the printer 14, or a fresh unprinted sheet, to 
the printer 16 before the printer 16 prints on the sheet. The staging tray 
20 decouples the variable process speed of the laser printer from the 
variable process speed of the inkjet printer. The printer 14 can 
selectively receive sheets of print media from a tray 22 (FIG. 3), or from 
an individual sheet feed area, as is known in the art. 
In one embodiment, the printer 14 further includes another output tray 23, 
and can selectively send printed media to either the staging tray or to 
the output tray 23 (e.g.; the printer 14 includes a solenoid actuated, 
pivotally mounted, paper redirection guide 24 directing the printed media 
to either the output tray 23 or the staging tray 20). For example, if the 
interpreter 28 determines that a particular sheet or set of sheets will 
not have any color image, that sheet or set of sheets can be directed to 
the output tray 23 after having a black image printed by the printer 14, 
while other sheets or sets of sheets can be directed to the staging tray. 
The printing system 10 further includes a control system 26 (FIG. 2). The 
control system 26 includes a data storage buffer 30 for the printer 16, 
and a data storage buffer 32 for the printer 14, in the housing. The 
control system 26 further includes an interpreter or formatter 28 which 
receives data from the computer. The computer 11 is connected to the 
buffer 30 for the printer 16 and to the buffer 32 for the printer 14. The 
computer 11 segregates the data into black and color components. Normally, 
such segregation is accomplished by reading printer command language (PCL) 
or postscript commands generated in the computer 11 when printing data 
blocks of black and color. Typically, each block of data is normally 
preceded by a command header that designates the size and data type of the 
block. After data is segregated, the computer 11 directs color components 
to the buffer 30 for the color printer 16 and directs the monochrome 
components to the buffer 32 for the printer 14. In an alternative 
embodiment, the segregation is performed by the interpreter 28. In the 
illustrated embodiment, the buffer 30 is in the printer 16, and the buffer 
32 is in the printer 14; however, placement anywhere in the common housing 
is possible. After segregation, the monochrome and color data are used by 
the printer 14 and the printer 16, respectively. 
In the illustrated embodiment, the computer 11 can be connected to a single 
parallel port connector for the entire printer system 10. 
The printer 16 includes a feed mechanism, having a feed roller 33, for 
separating a single sheet from a stack of sheets in the staging tray and 
feeding the single sheet into the printer 16 for printing. The printer 16 
further includes a bidirectional sheet advancement system or mechanism 34, 
including an advancement roller 38 (FIG. 3) controlled by the interpreter 
28. The bidirectional sheet advancement system 34 is substantially similar 
to the bidirectional capable sheet advancement system included in the 
Hewlett Packard DeskJet 720 printer. Though the DeskJet 720 does not 
perform bidirectional sheet advancement, it has an encoder on its paper 
shaft so is capable of bidirectional sheet advancement. 
In the illustrated embodiment, the bidirectional sheet advancement system 
34 controllably moves the sheet along an inkjet printer print path 40 in 
either of a first direction away from the staging tray 20 (into the 
printer 16), or a second direction toward the staging tray (back out of 
the printer 16). Sheet movement along the paper path is monitored by an 
encoder (not shown), so the interpreter 28 knows the position of the sheet 
along the paper path in the printer 16. 
The printer 16 further includes an inkjet pen carriage 42 (FIGS. 2 and 4) 
including a chamber 46 for removably closely receiving at least one color 
inkjet pen 48. The carriage 42 is slidably mounted on a rod 44 for 
movement therealong in a scan direction perpendicular to the print path 
40, for printing on the sheet. The inkjet pen 48 is a conventional color 
print cartridge, such as a C1823A Print Cartridge sold by Hewlett-Packard 
Company of Palo Alto, Calif. The printer 16 includes a flexible ribbon 
cable 50 electrically coupling the inkjet pen 48 to the buffer 30 and the 
interpreter 28. The interpreter 28 converts the PCL commands to a bit map 
such that the color image in the buffer 30 is converted to color dots 
which are printed by the inkjet pen 48. Movement of the inkjet pen 
carriage 42, and thus movement of the inkjet pen 48, is controlled by the 
interpreter 28. 
The printer 16 further includes a registration system 52 which registers an 
image printed by the printer 16 relative to image printed by the printer 
14. The registration system 52 includes the interpreter 28. The 
registration system 52 further includes an optical sensor assembly 54 
supported relative to the inkjet pen carriage 42 for movement with the pen 
carriage 42 along the scan direction. In the illustrated embodiment, the 
sensor assembly 54 comprises an elongated sensor housing 56, a pair of 
illumination light emitting diodes 58 on one end of the sensor assembly 54 
for facing the print media, and a charge coupled device (CCD) linear array 
60 mounted on the other end of the sensor housing 56. The linear array 60 
also faces the paper via an aperture 62 between the light emitting diodes 
58. The aperture 62 extends to the array 60. A lens 64 is mounted in the 
aperture approximately mid-way between the two ends of the sensor housing 
56. In the illustrated embodiment, the lens has a focal length of 20 mm, 
and yields 80 mm between the sheet and the array 60. In the illustrated 
embodiment, the sensor assembly 54 is mounted in an empty ink cartridge 66 
(FIG. 5) for placement in a chamber 68 (FIG. 4) of the inkjet pen carriage 
42 where a black ink cartridge would normally be placed. However, because 
black ink is printed by the printer 14, the black ink cartridge is omitted 
in the illustrated embodiment. In one embodiment, the linear array 60 is 
not mounted in a cartridge, but instead is supported by the carriage 42 
(e.g., adjacent the color ink cartridge) for movement with the carriage 
42. Thus, the chamber 68 can either be omitted, or a black ink cartridge 
can be provided. The linear array 60 is coupled to the interpreter 28 by 
the flexible ribbon connector 50. 
The printer 14 prints a registration mark 70 on the sheet, and the 
registration system 52 uses the linear array 60 to locate the registration 
mark and effect registration. The registration system 52 causes the inkjet 
pen carriage 42 to move in the scan direction and causes the sheet 
advancement system 32 to move the sheet in the inkjet paper path direction 
until the registration mark 70 is located using the linear array 60, prior 
to printing by the printer 16. 
In one embodiment of the invention, the registration system 52 registers on 
text characters intermingled with other characters, such as the letter "v" 
72. In another embodiment, the registration mark 70 comprises a series of 
dots 74 that define a pattern which is recognizable by the registration 
system 52, but which cannot be easily seen by the human eye. 
By taking advantage of the scan motion of the carriage, and the 
bi-directional sheet advancement mechanism 34, the single linear array 60 
can scan any area on the black plane pre-printed by the printer 14. This 
produces a cost savings compared with an alternative embodiment in which 
multiple arrays are employed across the width of the sheet. Scan 
information is sent to the interpreter 28 for on-the-fly registration 
correction. In an alternative embodiment, the scan information is sent to 
the computer 11 through a bidirectional parallel port, and the computer 11 
performs the registration correction. After the correction is made, the 
sheet is positioned back to top-of-form, and color printing begins. 
FIG. 7 is a flow chart illustrating logic employed by the registration 
system 52. 
In a step 100, the registration system scans a sheet printed by the printer 
14 for registration marks 72. After performing step 100, the registration 
system proceeds to step 102. 
In step 102, the registration system registers an image to be printed by 
the printer 16 with the monochrome plane printed by the printer 14. After 
performing step 102, the registration system proceeds to step 104. 
In step 104, a determination is made as to whether the image printed by the 
printer 14 is stretched or compressed in the vertical direction (along the 
inkjet print path) with respect to the image intended to be printed by the 
printer 14. If so, the registration system proceeds to step 106; if not, 
the registration system proceeds to step 108. 
In step 106, the registration system corrects for image stretch or 
compression by the printer 14 in the vertical direction. In other words, 
the bitmaps for color images to be printed are vertically stretched or 
compressed to line up with the actual image printed by the printer 14. 
After performing step 106, the registration system proceeds to step 108. 
In step 108, a determination is made as to whether the image printed by the 
printer 14 is stretched or compressed in the horizontal direction (normal 
to the inkjet print path) with respect to the intended image to be printed 
by the printer 14. If so, the registration system proceeds to step 110; if 
not, the registration system proceeds to step 112. 
In step 110, the registration system corrects for image stretch or 
compression by the printer 14 in the horizontal direction. In other words, 
the bitmaps for the color images to be printed are horizontally stretched 
or compressed to line up with the actual image printed by the printer 14. 
After performing step 110, the registration system proceeds to step 112. 
In step 112, a determination is made as to the angle (if any) at which the 
image printed by the printer 14 is rotated clockwise or counterclockwise 
with respect to the image intended to be printed by the printer 14. If so, 
the registration system proceeds to step 114; if not, the registration 
system proceeds to step 116. 
In step 114, the registration system corrects for image rotation. In other 
words, the bitmaps for the color images to be printed are rotated 
clockwise or counterclockwise to line up with the actual image printed by 
the printer 14. After performing step 114, the registration system 
proceeds to step 116. 
In step 116, the registered color image is printed on the sheet over the 
image printed by the printer 14. 
FIG. 8 shows a printing system 120 in accordance with an alternative 
embodiment of the invention. The printing system 120 is connected for 
control by a microprocessor based computer. The system 120 includes a 
laser printer 122 configured to print a monochrome (typically black) image 
on a sheet of paper, and an inkjet color printer 124 configured to print a 
color image on the sheet. In the illustrated embodiment, the printer 124 
prints with incremental motion and the printer 122 prints with continuous 
motion. More particularly, in the illustrated embodiment, the printer 124 
is a multi-color inkjet printer. In the illustrated embodiment, the 
printer 122 includes an input paper tray 126 for supporting a stack of 
print media (e.g., paper) 129, and a pick assembly 127 for separating and 
feeding a sheet from the stack. The printer 122 further includes a housing 
131 supporting a laser scanner 128, a toner supply 130, a photoconductor 
roller 134 acted on by the laser scanner 128, a transfer roller 136 facing 
the photoconductor roller 134 and cooperating with the photoconductor 
roller 134 to impart an image to the sheet, a fuser 138 spaced apart from 
and downstream from the photoconductor roller 134, and an output tray 141 
for sheets 142 that only require printing of a black image. 
The printer 124 has a pivotally movable, controlled, paper re-direction 
guide 140 which directs the sheet printed upon by the laser printer 122 
into a paper motion area 142. The sheet enters the paper motion area 142 
at the process speed of the laser printer 122. In the embodiment of FIG. 
8, the staging tray is omitted. The inkjet color printer 124 further 
includes a bidirectional sheet advancement system or mechanism 143, 
including a paper drive roller 144 controlled by an interpreter in a 
manner similar to that for the embodiment of FIG. 3. In the illustrated 
embodiment, the bidirectional sheet advancement system controllably moves 
the sheet along an inkjet printer print path in the paper motion area 142 
in either of a first direction toward the laser printer or a second 
direction away from the laser printer. 
The printer 124 further includes an inkjet pen carriage 146 including a 
chamber for removably receiving at least one color inkjet pen 148. The 
carriage 146 is slidably mounted for movement in a scan direction 
perpendicular to the print path, for printing on the sheet. The inkjet pen 
148 is a conventional color print cartridge. Movement of the inkjet pen 
carriage 146, and thus movement of the inkjet pen 148, is controlled by an 
interpreter. 
The inkjet color printer 124 further includes a registration system 150 
which registers graphics printed by the inkjet color printer 124 relative 
to text printed by the text printer 122. The registration system 150 
further includes an optical sensor assembly 152 supported relative to the 
inkjet pen carriage 146 for movement with the pen carriage 146 along the 
scan direction. In the illustrated embodiment, the sensor assembly 152 is 
similar to the sensor assembly 54 described above and includes a charge 
coupled device (CCD) linear array. 
A monochrome image is first printed on a sheet by the printer 122 and, if 
color is to be printed as well, the sheet enters a paper motion area of 
the printer 124 at the process speed for the printer 122. Scanning for 
registration marks by the sensor assembly 152 can take place while the 
paper enters at process speed for the printer 122, or can take place after 
the printer 122 completes delivery of a sheet into the printer 124. Color 
planes are then adjusted based on the scan by the sensor assembly 152, and 
printing takes place. The printer 124 further includes a color output tray 
154 which receives the sheet after color printing. More particularly, the 
printed sheet is re-directed to the color output tray 154 by the 
redirection guide 140. 
In compliance with the statute, the invention has been described in 
language more or less specific as to structural and methodical features. 
It is to be understood, however, that the invention is not limited to the 
specific features shown and described, since the means herein disclosed 
comprise preferred forms of putting the invention into effect. The 
invention is, therefore, claimed in any of its forms or modifications 
within the proper scope of the appended claims appropriately interpreted 
in accordance with the doctrine of equivalents.