Method and apparatus for dynamically aligning a printer printhead

In order to dynamically align one or more printheads in a printer, a referencing mechanism is placed on the printer and a detector is placed on the printhead. The printhead is moved at a known speed past two spaced apart reference indicia of the referencing mechanism. The passing of a first of the spaced apart reference indicia is detected and the passing of a second of the spaced apart reference indicia is detected. The time between the detection of the first reference indicia passage and the detection of the second reference indicia passage is measured and a delay time, related to the measured period of time, is created. Energization of an ink drop ejection is delayed for the duration of the delay time.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a method and apparatus for 
aligning printing mechanisms and more particularly a method and apparatus 
for aligning multiple printheads or print cartridges in an ink droplet 
ejection printer such as a thermal inkjet printer. 
One conventional type of printer is one which forms characters and images 
on a medium, such as paper, by expelling droplets of ink in a controlled 
fashion so that the droplets land on the medium. Such a printer can be 
conceptualized as a mechanism for moving and placing the medium in a 
position such that the ink droplets can be placed on the medium, a 
printing cartridge which controls the flow of ink and expels droplets of 
ink to the medium, and appropriate control hardware and software. A 
conventional print cartridge for an inkjet type printer comprises an ink 
containment device and a fingernail-sized apparatus, commonly known as a 
printhead, which heats and expels ink droplets in a controlled fashion. 
Typically, the printhead is a laminate structure including a semiconductor 
base, a barrier material structure which is honeycombed with ink flow 
channels, and an orifice plate which is perforated with holes or orifices 
with diameters smaller than a human hair and arranged in a pattern which 
allows ink droplets to be expelled in a controlled pattern. In an inkjet 
printer the heating and expulsion mechanism consists of a plurality of 
heater resistors formed in the semiconductor substrate and associated with 
an ink chamber formed in the barrier layer and one of the orifices in the 
orifice plate. Each of the heater resistors is connected to the 
controlling mechanism of the printer such that each of the resistors may 
be independently energized to quickly vaporize to expel a droplet of ink. 
In some applications, more than one inkjet print cartridge will be designed 
into a printer. Usually this multiple print cartridge assembly is created 
to accommodate multiple colors of ink. Properly controlling the 
arrangement of various droplets of ink of different colors will result in 
a wide spectrum of perceivable colors. The clarity and quality of the 
resultant image is affected by the accuracy of the placement of the ink 
droplets on the medium. Printers which use multiple print cartridges to 
cooperatively form a single image usually require mechanical or electronic 
adjustment so that ink droplets printed by one cartridge alight at precise 
locations on the receiving medium relative to those printed by another 
cartridge in the printer. 
Cartridge-to-cartridge alignment has been eliminated in some printers with 
the use of a single multi-color ink cartridge having a printhead employing 
three sets of orifices arranged in a group and receiving one color of ink 
for each group on the printhead. Such a single multi-color print cartridge 
is inherently self-aligning due to the precise positioning of one set of 
orifices relative to another on the single orifice plate on the 
multi-color print cartridge. Even for this cartridge, however, unless 
other compensation is made, the orifice plate of the printhead should be 
oriented precisely perpendicular to the direction of travel for accurately 
printed results. 
Mechanical alignment of print cartridges is simple but expensive, requiring 
precision features created in the orifice plate of the printhead, 
precision alignment of the cartridges during manufacture to alignment 
structures or secondary milling of alignment structures or adjustment 
within the printers cartridge carriage. In each of these foregoing 
implementations, there are stringent requirements on the printer and the 
cartridge carriage for either precision during manufacture and long term 
stability, or complex adjustability and human intervention. Electronic 
alignment typically requires printing ink droplet dots on a separate 
region of the medium, scanning the medium with a detector for these dots, 
then establishing time delays within the printer to compensate for the 
measured offsets. Again, printer complexity or human intervention and 
judgment are required to optimize this form of alignment. 
Each of the foregoing techniques do not dynamically compensate for movement 
of the print cartridge within the carriage between alignment cycles due to 
thermal expansion or wear or loosening within the mechanism. Each of these 
methods add mechanical or electronic complexity to the printer. Thus, a 
need exists for a method and apparatus which readily adjusts for 
horizontal cartridge-to-cartridge alignment errors in a multiple cartridge 
printer. Furthermore, vertical and rotational offsets also need 
compensation to precisely align the ink droplets on the media. 
SUMMARY OF THE INVENTION 
A printer employs a method and apparatus for dynamically compensating 
misalignment of a printhead employing an ink drop ejection apparatus to 
expel ink in a controlled manner to effect printing in a printer having a 
referencing mechanism. Upon determining a need for a first alignment 
cycle, the printer moves the printhead at a known speed past at least two 
reference indicia having a predetermined spacing. The passing of a first 
of the at least two reference indicia and the passing of a second of the 
at least two reference indicia is detected. The period of time between the 
detection of the first reference indicia passage and the detection of the 
second reference indicia passage is measured and a delay time, related to 
the measured period of time, is created. The energization of at least a 
portion of an ink drop ejection apparatus is consequently delayed for the 
duration of the delay time until a second alignment cycle is needed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention encompasses a method and apparatus for self-aligning 
one or more print cartridges in a dynamic fashion in a printer. An 
alignment pattern having a large light-to-dark ratio is placed at a 
functional location within the printer but away from the medium. 
Photosensitive devices, created as part of the semiconductor substrate and 
associated with the orifice plate, are arranged to read the high contrast 
features of the alignment plate. Timing is derived from a clocking pulse 
and an alignment pulse provided by the printer to time the printheads 
location relative to the alignment feature on the printer. Firing of the 
heater resistors are then delayed or advanced as determined by the timing 
and is implemented by the shift registers constructed, in the preferred 
embodiment, on the semiconductor substrate of the printhead. 
A simplified diagram of a printer is shown in FIG. 1. Medium 101 is moved 
past print cartridges 103 and 105 in a direction arbitrarily designated 
the "Y" direction (into the plane of paper of FIG. 1) by a platen motor 
109. The print cartridges 103 and 105 are mounted in a cartridge carrier 
107 and are scanned back and forth across the medium in an orthogonal 
("X") direction by a carriage motor 111. The platen motor 109 and the 
carriage motor 111 are conventionally under the control of a media and 
cartridge position controller 113 such positioning and control apparatus 
are known and are further described in U.S. Pat. No. 5,070,410. Thus the 
medium is positioned in a location so that the print cartridges 103 and 
105 may eject droplets of ink as required by the data which is input to 
the droplet firing controller 115 of the printer in a band parallel to the 
"X" direction as the print cartridges 103 and 105 are scanned across the 
medium by the carriage motor 111. When the print cartridge 103 and 105 
reach the end of their travel at an edge of the medium 101, the medium 101 
typically is incrementally advanced by the media position control 113 and 
platen motor 109, and the print cartridge 103 and 105 are returned along 
the "X" axis while printing another band of ink droplet dots on the medium 
101 until the opposite end of the medium is reached. From time to time, 
the print cartridges 103 and 105 may be moved away from the medium 101 and 
aligned with a service station 117 so that the printheads of the 
cartridges 103, 105 may be wiped clean of debris and the orifices purged 
of any material which may obstruct the ejection of ink droplets. A service 
station which may be employed in the preferred embodiment of the present 
invention is further described in U.S. Pat. No. 5,103,244. In the present 
invention, an alignment reference indicia is located on the service 
station 117 to provide a location reference for each of the print 
cartridges 103 and 105. 
A print cartridge which may be used in the present invention is shown in 
FIG. 2. Generally, a majority of the volume of the print cartridge is 
dedicated to the containment of ink. At one end of the cartridge a 
printhead 203 is affixed to the print cartridge and internally coupled to 
the ink supply within the ink cartridge. Electrical connections are made 
to the heater resistors within the printhead 203 by a flexible circuit 
205. The flexible circuit 205 also mates with associated electrical 
connectors of the print carriage of the printer. When a plurality of print 
cartridges are mounted in a printer, they are arranged in a side-by-side 
carriage configuration for the preferred embodiment as shown in FIG. 3. 
Electrical connection to the print cartridges are made via mating 
connectors (not shown) disposed on the print carriage 107. The printhead 
of each cartridge is typically oriented down relative to the direction of 
gravity and positioned over the media upon which ink is to be printed. The 
service station 117 is also shown in greater detail with the print 
cartridges 107, 105 positioned over a portion 301 of the service station 
117 which caps the printheads and prevents ink from drying in the orifices 
when positioned in contact with the printheads. A wiper mechanism 303 is 
arranged in a direction from the capping mechanism such that when the 
print carriage returns from the medium being printed, the printheads are 
first wiped by the wiper mechanism 303 and then capped by the capping 
portion 301. It is a feature of the present invention that an alignment 
plate 305 be disposed within the fixed portion of the printer and that, in 
the preferred embodiment, the alignment plate 305 is placed on the service 
station. The service station also provides power for the light source (not 
shown) beneath the alignment plate 305. 
A magnified planar view of the printhead is shown in FIG. 4. A plurality of 
orifices in two columns 403, 405 are depicted in the orifice plate of the 
printhead 203. Although shown in two collinear columns, the orifices may 
be staggered in the +X or -X direction from the general line of each 
column. Such stagger amount is known and electronically compensated for 
within the control of the printer. In the preferred embodiment, a total of 
54 orifices are employed in equal division in the two columns of the 
orifice plate. At separate points of the orifice plate but created by the 
same process which creates each orifice, two alignment apertures 407 and 
409 are found in the orifice plate of the printhead 203. In the preferred 
embodiment alignment aperture 407 and alignment aperture 409 are placed 
diagonally across the surface of the orifice plate as shown. 
Alternatively, an alignment aperture 411 may be placed on an imaginary 
line with aperture 407 which is parallel to the "Y" direction. 
A cross section of the printhead 203 along the section line AA is shown in 
FIG. 5. Features relating to structure which supports the filling of ink 
have been deleted from FIG. 5 for clarity. A semiconductor substrate 501 
is conventionally processed to include heater resistors 503 and 505 which 
are appropriately connected by electrical connectors 507, 509, 511, and 
513. Ink firing chambers, which in operation are filled with ink, are 
defined by the substrate, the barrier layer material 515, and orifice 
plate 517. The orifice plate 517 overlays the barrier material 515 such 
that the orifices 403 and 405 are arranged in association with the heater 
resistors 503 and 505 and the formed ink firing chambers. As part of the 
processing steps of the semiconductor material 501 for the preferred 
embodiment, a photosensitive area 519 is created using conventional 
photolithographically defined semiconductor processes. This photosensitive 
area 519 is then connected by way of conductors 521 and 523 to appropriate 
parts in the electronic circuit to be described later. The alignment 
aperture 407 is positioned relative to photosensitive area 519 such that 
light falling perpendicularly to the surface of the orifice plate will 
fall on the photosensitive area 519. It is a feature of the present 
invention that the alignment aperture 407 is produced in the same process 
as the ink firing orifices 403 and 405 thereby providing nearly perfect 
registration between the orifices and the alignment orifice. Furthermore, 
since firing resistors 503, 505, and photosensitive area 519 are all 
produced by precision semiconductor photolithographic techniques, they 
too, are precisely aligned. 
Alignment and operation of the alignment plate 305 and the printhead can be 
apprehended from the drawing of FIG. 6. The service station 117 of FIG. 3 
has been omitted for clarity, leaving only the alignment plate 305 to 
illustrate the alignment technique employed in the preferred embodiment. A 
light source 601 is disposed within the service station 117 and arranged 
in a fashion such that the light falls perpendicular to the plane of the 
alignment plate 305. In the preferred embodiment, this is accomplished by 
using a conventional lensed light emitting diode but any source of 
essentially parallel light rays may be employed without departing from the 
spirit of the present invention. 
Two opaque stripes 603 and 605 are formed in the light transmitting 
(translucent or transparent) alignment plate 305 and cause respective 
shadows 607, 609 to fall upon the orifice plate 517 of the printhead. In 
the preferred embodiment, the alignment plate 305 is made of a 
conventional, optically translucent plastic having stripes 603 and 605 
conventionally etched and printed into the surface of the alignment plate 
plastic in such a maimer that the width of the stripes is at least as wide 
as the diameter of an alignment aperture. As the orifice plate 517 passes 
in front of the alignment plate 305 during a trip to the service station 
or as otherwise required, the shadows 607, 609 pass over the alignment 
orifices 407, 409. As illustrated in FIG. 5, the shadow 607 occludes the 
light falling upon photosensitive area 519 thereby generating an 
electrical signal which will be described later. In the preferred 
embodiment, the spacing between the alignment plate 305 of the service 
station 117 and the orifice plate 517 is 2 mm but this spacing is not 
critical as long as the light emanating through the alignment plate is 
parallel. 
In an alternative embodiment, the alignment plate 305 may be made opaque 
with transparent slits to emit light. In either embodiment, the presence 
and/or absence of light passing through apertures in the orifice plate and 
falling upon photo receptors in the printhead is used to indicate location 
of the printhead. 
After the photoreceptor signal is processed and shaped, electrical signals 
generated by the photosensitive areas are shown in the timing diagram of 
FIG. 7. The electrical output signal from the photosensitive area beneath 
the aligmnent orifice 407 is illustrated as output 701 and the electrical 
output signal from the photosensitive area beneath the alignment orifice 
409 is illustrated as output 703. In the preferred embodiment, the printer 
provides a reference indicia synchronizing signal 705 which is used to 
indicate a previously established index position of the cartridge carrier 
107 relative to the printer. This signal may be generated in a number of 
conventional ways; in the preferred embodiment, as the cartridge carrier 
passes a preselected position on the printer, an electrical reference 
pulse 707 is generated by the position controller 113. As illustrated in 
FIG. 7, the pulse 707 occurs at time t.sub.1. As the alignment orifice 407 
of the first cartridge is moved past opaque strip 605, the light falling 
upon the photosensitive area 519 is interrupted as the opaque strip 605 is 
passed and an electrical signal, represented by pulse 709 in FIG. 7, is 
generated. Pulse 709 is designated as occurring at time t.sub.2. As the 
first print cartridge continues to move, opaque strip 603 throws a shadow 
across alignment orifice 407 and the photosensitive area 519 and an 
electrical signal, represented by pulse 711 in FIG. 7, is generated and 
this pulse is designated as occurring at time t.sub.3. Evaluation of the 
difference in time between t.sub.1 and t.sub.2 yields an offset or 
position error ("X") indication for the cartridge relative to the index 
position. Since the opaque strip 603 is oriented at an angle relative to 
opaque strip 605 evaluation of the difference in time between t.sub.3 and 
t.sub.2 from an expected time (related to the translation speed of the 
print cartridge, in the preferred embodiment 32 cm/sec, and the distance 
between the opaque strips 603 and 605 at the correct "Y" elevation of the 
print cartridge) yields an indication of a position error in the "Y" 
direction. In the preferred embodiment, t.sub.3 -t.sub.2 =10 msec, but the 
absolute time is not a critical parameter in practicing the invention so 
long as it is a consistent time difference. 
Referring now to the output signal 703, it can be seen that pulses 713 and 
715, similar to pulses 709 and 711, are generated by the photosensitive 
area associated with alignment aperture 409. Since the opaque strips 603 
and 605 are further apart at the lower end of the printhead than they are 
at the upper end of the printhead, the leading edges of pulses 713 and 715 
(at times t.sub.4 and t.sub.5, respectively) are further apart in time 
(t.sub.5 -t.sub.4) than the pulses 709 and 711. If the print cartridge has 
a rotational (.theta.) error in its orientation relative to the printer 
there is at least a difference in the detected time of pulses 709 and 713. 
For example, if the expected leading edge time for pulses 709 and 713 to 
occur were t.sub.2 and t.sub.4' but the actual time detected for pulse 713 
occurred at a later time, t.sub.4, the print cartridge has a rotational 
position error in the -.theta. direction. Likewise if the actual pulse 713 
time detection preceded the expected time, the print cartridge has a 
rotational position in the +.theta. direction. In a two-or multi-print 
cartridge printer, each of the print cartridges would undergo the 
preceding measurement of position error relative to a fixed reference 
position on the printer. 
In the preferred embodiment, correction of horizontal, vertical, and 
rotational position errors of the cartridge is made by sequentially adding 
an appropriate delay in a heater resistor firing pulse output from the 
droplet firing controller 115. This process can be perceived from the 
block diagram of FIG. 8. A pulse 800 of electric energy is output from 
droplet firing controller 115 and applied to a horizontal position 
correction circuit 801 for delay, if necessary. The delayed (if needed) 
firing pulse 802 is coupled to a vertical position correction circuit 803 
for delay if necessary for correction of a vertical position error. 
The delayed (if needed) firing pulse 804 is coupled to a rotational error 
correction circuit 805 for appropriate delay to correct for rotational 
errors. A final heater resistor firing pulse 806 is then output to the 
heater resistor 809 to energize the resistor, heat and vaporize the ink, 
and expel a droplet of ink for printing on the medium. For each of the 
firing resistors there exists a similar serial correction circuit for each 
positional error, that is, for heater resistor 811, a horizontal, 
vertical, and rotational position correction circuit is available to 
modify the timing of the firing pulse output from droplet firing 
controller 115. Likewise for heater resistor 813, similar circuits exist. 
While the preferred embodiment utilizes the correction circuits as shown, 
it is obvious that the circuits could be repositioned in their sequence of 
modifying the firing pulse or that a multiple-purpose circuit could 
undertake dual or triplicate functions. In some instances, especially when 
a mechanical alignment negates the need for one of the electronic 
corrections described herein, one or more of the correction circuits may 
be deleted. 
A detailed schematic of the horizontal correction circuit 801 is shown in 
FIG. 9. A delay in the pulse 913, which is eventually coupled to a firing 
resistor, is introduced by establishing a pick-off point (an incremental 
delay) along a shift register 901. A firing pulse 800 generated by the 
droplet firing controller 115 is coupled to the shift register 901 and is 
conventionally clocked to each register of the shift register in turn. The 
print cartridge receives a reference pulse 707 from the position 
controller 113 as described relative to FIG. 7. This pulse is coupled to 
the "start up/stop" input port of the up/down counter -1 of 14 line 
selector 905 to commence the count and shift a bit in the register of the 
1 of 14 line selector. Pulse 713 generated when the shadow of opaque 
stripe 605 occludes the aperture 409, is coupled to the "start down/stop" 
input port of the up/down counter of 905 and places a counter stop after a 
number of clock pulses have been applied to the up/down counter -1 of 14 
line selector 905. This results in a bit being set in the 1 of 14 line 
selector corresponding to the time delay between time t.sub.1 and t.sub.4. 
When the firing pulse input into the shift register corresponds with the 
selected line, an "and" gate, for example "and" gate 911, has both inputs 
active and couples a pulse to gate 909 for coupling of a delayed firing 
pulse 913 out of the horizontal correction circuit. It is expected that 
the pulse 713 will lag the pulse 707 by a predetermined number of clock 
pulses. If pulse 713 occurs too soon or too late, the delay is changed to 
accommodate the error and compensate for the horizontal misalignment. Each 
cartridge in the cooperatively printing set thus has its delay offset so 
that all produce printed ink droplets at precisely the correct time 
relative to its true position in the print cartridge carriage. In this way 
horizontal ("X") alignment between the individual cartridges is 
established with a high degree of precision. 
Similarly vertical offset is established by detecting the timing of 
vertically differentiable reference indicia. The objective is to select 
the best contiguous set of orifices to be used to print a character or 
image. In the preferred embodiment where fifty orifices are used to print, 
fifty-four orifices are actually available. With a perfect vertical 
alignment, the top and bottom two orifices will remain unused while the 
centered fifty are selected for printing. If the print cartridge is 
positioned low relative to nominal, the top one orifice and bottom three 
orifices will remain unused while the fifty orifices between will become 
the selected set. 
Referring now to FIG. 10, a more detailed schematic of the vertical 
correction circuit 803 is shown. Pulses 713 and 715, generated as the 
shadows of opaque stripes 605 and 603 sequentially occlude aperture 409, 
are coupled to an up/down counter -1 of 8 line selector 1001. Pulse 713 is 
first delayed by a predetermined time corresponding to the expected time 
delay between t.sub.1 and t.sub.4 and then applied to the "start up/stop" 
port of the up/down counter and pulse 715 is applied to the "start 
down/stop" port of the up down counter. If pulses 713 and 715 are not 
essentially coincident after 713 is delayed by delay 1002, a line other 
than the line corresponding to a zero firing pulse delay is selected in 
the 1 of 8 line selector. A 1 of 7 line selection may be made by selector 
1003. When the line select is made, for example a selection of a line 
corresponding to a delay of -1 clock cycle, register -1 is the line 
selected. The state of register -1 is coupled to one input of an "and" 
gate 1005. The other input is coupled to the firing pulse designated for 
the heater resistor corresponding to orifice number (for example) 50. The 
register corresponding to no delay is coupled to "and" gate 1007 as is the 
firing pulse designated for the heater resistor corresponding to orifice 
number 49; the register corresponding to a delay of +1 clock cycle is 
coupled to "and" gate 1009 as is the firing pulse designated for the 
heater resistor corresponding to orifice number 48. Thus when a firing 
pulse 1011, destined for the heater resistor of orifice number 50, is 
input, it is converted to a firing pulse 1019 directed to the heater 
resistor of orifice number 49. The result is that the pulses for each 
resistor are electronically redirected to the firing resistor physically 
located one orifice beneath the originally selected orifice. An error in 
the vertical direction is thus compensated. A plurality of "and" gates are 
similarly connected as shown so that each resistor may have electronically 
redirected firing pulses as required. 
Rotational miss-alignment, that is, a miss-alignment in the .theta. 
direction, requires that there be two detection orifices and 
photosensitive features on the printhead of the cartridge. Assuming the 
leading detection reference indicia (relative to cartridge movement during 
alignment) on the printhead face is the feature used to establish 
horizontal alignment, a lagging detection feature is then used to 
establish degree of cartridge orifice rotation. The rotational correction 
circuit 805 of the preferred embodiment is based upon knowing the intended 
horizontal separation of the two detection features. A 1 of 8 
lineselector-up/down pulse counter 1101, as shown in FIG. 11, is started 
when the first alignment pulse 709 (from the photosensitive area 
associated with alignment orifice 407) is input to the "start up/stop" 
input to start the pulse counter after being delayed by delay 1103 for the 
expected delay time t.sub.4 -t.sub.2. The counter is stopped when the 
second alignment pulse 713 (from the photosensitive area associated with 
alignment orifice 409 produced by the same vertically extended reference 
indicia) is coupled to the "start down/stop" input of the counter. Since 
the datum in the preferred embodiment is established at the alignment 
aperture 409, rotational errors are defined as rotation about this datum. 
Orifices which are disposed furthest from the datum experience the 
greatest amount of deviation from the desired position; orifices disposed 
closest to the datum experience the least amount of deviation. Also, the 
most troublesome deviation occurs in the vertical direction rather than in 
the horizontal. Accordingly, the implementation in the preferred 
embodiment selectively corrects the vertical deviation. Those heater 
resistors associated with orifices furthest from the datum orifice are 
caused to experience a correction in the vertical direction while those 
closest to the datum are not. The line select is determined by the 
difference between the delayed pulse 709 and the pulse 713. As shown for 
the preferred embodiment in FIG. 11, the line select is set by the time 
difference in pulses 709 and 713. The state of the line select is anded 
with the input pulse 804 which is clocked through the shift register 1106 
so that, for the heater resistors corresponding to the orifices furthest 
away from the datum, when the pulse 804 reaches the register which is 
coupled to the "and" gate (for example, "and" gate 1105) which is 
connected to the line select with the active state, a heater resistor 
firing pulse 1121 is applied to the heater resistor. 
For those heater resistors which are associated with orifices closer to the 
datum, the amount of time delay allowed is compressed by coupling two or 
more of the line select registers together with an "or" gate. This is 
shown in the schematic of FIG. 12. In the preferred embodiment, line 
select registers corresponding to time increments of -3 and -2 clock 
pulses are coupled to an "or" gate 1203 and then to an "and" gate 1205. 
For line select registers corresponding to -1, 0, and +1 clock pulse 
delays, the line select register outputs are coupled to an "or" gate 1207 
and then to "and" gate 1209. For line select registers corresponding to +1 
and +2 clock pulse delays, the line select register outputs are coupled to 
an "or" gate 1211 and then to "and" gate 1213. In this way, a pulse 804 
input to the heater resistors of orifices of intermediate distance from 
the datum is output as a time shifted pulse 1123 with a compressed amount 
of time shifting. Heater resistors closest to the datum are not shifted in 
time to correct for rotational errors in the positioning of the print 
cartridge. 
In an alternative embodiment a more sophisticated mapping scheme can 
compensate vertical as well as rotational errors. Also, if the cartridge 
is capable of printing gray scale, a gray scale level adjustment could be 
made at this time. Furthermore, a more sophisticated rotation correction 
scheme would compensate for uniform change in orifice plate size due to 
manufacturing tolerance or change in temperature. In this alternative, the 
distance between the two detectors on the printhead face is essentially 
measured by using both the horizontal and vertical crossing timing 
information. This information is ratioed with the horizontal offset to 
produce a better estimate of .theta. error and therefore a more robust 
rotation correction, independent of uniform orifice size changes. 
Alignment of the cartridge in the preferred embodiment is keyed to the 
conventional servicing cycle of the print cartridges in a printer. A 
alignment cycle would also be run at printer turn on. Alternative 
alignment algorithms for the cartridges could take place as often as once 
per printing pass just prior to beginning the print swath. Also a 
detection of a change printhead temperature, a new page, or simply the 
passage of time or number of print swaths completed could also be used to 
determine when to perform an alignment cycle.