Unitary light tube for mounting optical sensor components on an inkjet printer carriage

A carriage-mounted optical sensor for an inkjet printer/plotter includes a unitary light tube member which holds the optical components in fixed positions relative to each other as well as relative to an outer protective casing which attaches to the carriage. The light tube member serves as a cap to capture two LEDs between itself and a protective casing, to capture an optical lens between itself and a photocell holder, and to directly engage the casing.

BACKGROUND OF THE INVENTION 
this invention relates generally to inkjet printers/plotters, and more 
specifically to carriage-mounted optical sensors in an inkjet 
printer/plotter. 
Many print quality benefits are achieved by mounting an optical sensor on a 
carriage which also carries printing elements, since the optical sensor 
can then pass over the media upon which the printing elements are applying 
alphanumeric indicia, graphics or images. For example, see commonly 
assigned U.S. Pat. No. 5,170,047 entitled OPTICAL SENSOR FOR PLOTTER PEN 
VERIFICATION, and U.S. Pat. No. 5,448,269 entitled MULTIPLE INKJET 
CARTRIDGE ALIGNMENT FOR BIDIRECTIONAL PRINTING BY SCANNING A REFERENCE 
PATTERN, both of which are incorporated herein by reference. 
The full color inkjet printer/plotters which have been developed comprise a 
plurality of inkjet pens of diverse colors. A typical color inkjet 
printer/plotter has four inkjet pens, one that stores black ink, and three 
that store colored inks, e.g., magenta, cyan and yellow. The colors from 
the three color pens are mixed to obtain any particular color. 
The pens are typically mounted in stalls within an assembly which is 
mounted on the carriage of the printer/plotter. The carriage assembly 
positions the inkjet pens and typically holds the circuitry required for 
interface to the heater circuits in the inkjet pens. 
Full color printing and plotting requires that the colors from the 
individual pens be precisely applied to the media. This requires precise 
alignment of the carriage assembly. Unfortunately, mechanical misalignment 
of the pens in conventional inkjet printer/plotters results in offsets in 
the X direction (in the media or paper axis) and in the Y direction (in 
the scan or carriage axis). This misalignment of the carriage assembly 
manifests as a misregistration of the print images applied by the 
individual pens. In addition, other misalignments may arise due to the 
speed of the carriage, the curvature of the platen and/or spray from the 
nozzles. 
However, the integration of the optical and electronic components in the 
optical sensor, as well as positioning the optical sensor on the carriage 
have been complicated, expensive and to some extent imprecise in prior 
printers/plotters. The need for reliability and precision is even greater 
in recent inkjet printers/plotters which print high resolution color 
graphics and images, often on very large poster-size printouts. 
BRIEF SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the invention to provide a modular 
structure which integrates the optical and electronic components in a 
simplified but reliable way on an optical sensor unit. 
More specifically, the invention contemplates a carriage mounted optical 
sensor for an inkjet printer/plotter which includes a unitary light tube 
member which acts as a cap for holding the optical components in fixed 
positions relative to an outer protective casing. The light tube captures 
two LEDs between itself and the casing, captures an optical lens between 
itself and a photocell holder, and directly engages the casing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 is a perspective view of an inkjet large format printer/plotter 
incorporating the teachings of the present invention. The printer 210 
includes a housing 212 mounted on a stand 214. The housing has left and 
right drive mechanism enclosures 216 and 218. A control panel 220 is 
mounted on the right enclosure 218. A carriage assembly 300, illustrated 
in phantom under a cover 222, is adapted for reciprocal motion along a 
carriage bar 224, also shown in phantom. The position of the carriage 
assembly 300 in a horizontal or carriage scan axis is determined by a 
carriage positioning mechanism 310 with respect to an encoder strip 320 
(see FIG. 2). A print medium 330 such as paper is positioned along a 
vertical or media axis by a media axis drive mechanism (not shown). As 
used herein, the media axis is called the X axis denoted as 201, and the 
scan axis is called the Y axis denoted as 301. 
FIG. 2 is a perspective view of the carriage assembly 300, the carriage 
positioning mechanism 310 and the encoder strip 320. The carriage 
positioning mechanism 310 includes a carriage position motor 312 which has 
a shaft 314 which drives a belt 324 which is secured by idler 326 and 
which is attached to the carriage 300. 
The position of the carriage assembly in the scan axis is determined 
precisely by the encoder strip 320. The encoder strip 320 is secured by a 
first stanchion 328 on one end and a second stanchion 329 on the other 
end. An optical reader (not shown) is disposed on the carriage assembly 
and provides carriage position signals which are utilized by the invention 
to achieve optimal image registration in the manner described below. 
FIG. 3 is perspective view of a simplified representation of a media 
positioning system 350 which can be utilized in the inventive printer. The 
media positioning system 350 includes a motor 352 having an axle gear 359, 
which is normal to and drives a media roller 354. The position of the 
media roller 354 is determined by a media position encoder 356 on the 
motor. An optical reader 360 senses the position of the encoder 356 and 
provides a plurality of output pulses which indirectly determines the 
position of the roller 354 and, therefore, the position of the media 230 
in the X axis. 
The media and carriage position information is provided to a processor on a 
circuit board 370 disposed on the carriage assembly 100 for use in 
connection with printhead alignment techniques of the present invention. 
The printer 210 has four inkjet print cartridges 302, 304, 306, and 308 
that store ink of different colors, e.g., black, magenta, cyan and yellow 
ink, respectively. As the carriage assembly 300 translates relative to the 
medium 230 along the X and Y axes, selected nozzles in the inkjet print 
cartridges 302, 304, 306, and 308 are activated and ink is applied to the 
medium 230. The colors from the three color cartridges are mixed to obtain 
any other particular color. Sample lines 240 are typically printed on the 
media 230 prior to doing an actual printout in order to allow the optical 
sensor 400 to pass over and scan across the lines as part of the initial 
calibration. 
The carriage assembly 300 positions the inkjet print cartridges and holds 
the circuitry required for interface to the ink firing circuits in the 
print cartridges. The carriage assembly 300 includes a carriage 301 
adapted for reciprocal motion on front and rear slider rods 303, 305. 
As mentioned above, full color printing and plotting requires that the 
colors from the individual print cartridges be precisely applied to the 
media. This requires precise alignment of the carriage assembly as well as 
precise alignment of the print cartridges in the carriage. Unfortunately, 
paper slippage, paper skew, and mechanical misalignment of the print 
cartridges results in offsets in the X direction (in the media advance 
axis) and in the Y direction (in the carriage or axis) as well as angular 
theta offsets. This misalignment causes misregistration of the print 
images/graphics formed by the individual ink drops on the media. This is 
generally unacceptable as multi-color printing requires image registration 
accuracy from each of the printheads to within 1/1000 inch (1 mil). 
FIG. 4 shows a presently preferred embodiment of printheads each having two 
groups of nozzles with a column offset 410. By comparing the relative 
positions of corresponding nozzles in different printheads along the Y 
axis, it is possible to determinine an actual horizontal offset 412 
between two printheads, and by comparison with a nominal default offset 
414 determine an actual horizontal misalignment offset 416 in the carriage 
scan axis. This is repeated for all of the different printheads while they 
remain on the carriage. 
Similarly, by comparing the relative positions of corresponding nozzles in 
different printheads along the X axis, it is possible to determine an 
actual vertical misalignment offset 418 in the media advance axis. This is 
also repeated for all of the different printheads while they remain on the 
carriage. 
In order to accurately scan across a test pattern line, the optical sensor 
400 is designed for precise positioning of all of its optical components. 
Referring to FIGS. 5A, 5B, and 6, the sensor unit includes a photocell 
420, holder 422, cover 424, lens 426, and light source such as two LEDs 
428, 430. A unitary light tube or cap 432 has a pair of notched slots 434 
which engage matching tabs 436 on a lower end of the holder 422 upon 
insertion and relative rotation between the cap and the holder. The two 
LEDs are held in opposite apertures of two shoulders 438 which have a size 
slightly less than the outside diameter of the LEDs, to prevent the LEDs 
from protruding into a central passageway which passes through the holder 
to the photocell. 
A protective casing 440 which also acts as an ESD shield for the sensor 
components is provided for attachment to the carriage as well as for 
direct engagement with the shoulders of the light tube. In that regard, 
the top of the shoulders are sized and shaped to snugly fit inside 
downwardly tapered side walls 442 of the casing, with the top of the LEDs 
abutting against an upstanding flange 444 and with a lower portion of the 
shoulders held tightly by arms 446 which flex outwardly to an open 
position while the light tube is being pushed into a position of 
engagement with the casing. Upon completion of the engagement, the arms 
return to a closed latched position with a lip 448 on the end of each arm 
446 preventing disengagement of the light tube (and its LEDs) during 
normal use. 
FIGS. 7A-7E show a preferred sequence of steps for assembling the optical 
sensor. Firstly, a modular flex-circuit assembly is created with an 
elongated TAB circuit 450 having a junction portion 452 with soldered 
through-holes which (a) connect and support a first pair of wire leads 454 
to one LED, (b) connect and support a second pair of wire leads 456 to 
another LED, and (c) connect and support a set of three wire leads 458 
coming from the photocell (FIG. 7A). Secondly a U-shaped cover 424 holds 
the photocell in nested position at the upper end of the holder, while the 
LEDs and holder are positioned by the light tube (FIGS. 7B-7C). Finally, 
the subassembly is inserted into the casing, with a free end 462 of the 
TAB circuit extending out through an access slot in the casing (FIGS. 7D 
and 7E). 
It will be appreciated by those skilled in the art from the foregoing 
description that the invention provides a self-fixturing modular assembly 
whereby the light tube acts as a cap for holding both the two LEDs as well 
as the lens/holder/photocell/cover composite in fixed relative positions. 
Accordingly, if desirable the soldering of the interconnections at the 
co-planar junction portion of the flex-circuit can be done after assembly 
of the various component parts held by the cap. 
The fully assembled optical sensor unit can then be placed inside of 
vertical rib 464 and against back plate 466 for self-attachment by rear 
tab 468, front notch 470, and lower front hook 472 to matching X/Y/Z 
datum-like surfaces on the carriage (see FIGS. 8-10). 
The benefits and details of the co-planar junction feature of the 
flex-circuit are more fully described in the previously identified 
co-pending application entitled COMT FLEX-CIRCUIT FOR MODULAR ASSEMBLY 
OF OPTICAL SENSOR COMPONENTS IN AN INKJET PRINTER. The benefits and 
details of the optical features of the unitary light tube are more fully 
described in the previously identified co-pending application entitled 
OPTICAL PATH OPTIMIZATION FOR LIGHT TRANSMISSION AND REFLECTION IN A 
CARRIAGE-MOUNTED INKJET PRINTER SENSOR. 
It should be understood that various changes and modifications can be made 
to the illustrated embodiments of the invention described herein, all 
without departing from the spirit and scope of the invention as set forth 
in the following claims.