Thermal control mechanism for multiple print bar system

The temperature of LED print bars utilized in a printing system are maintained within a specified differential range, with respect to each other. Each print bar has an associated heating and cooling element. The temperature of each print bar is monitored and compared during operation. When a temperature differential greater than a preset tolerance range is detected, either the hotter operating print bar is cooled or the lower operating print bar is heated, or a combination of cooling and heating is applied to the out-of-tolerance print bars. The cooling and/or heating of the print bars is continued until the temperature differential is reduced so as to be within the preselected or predefined range.

BACKGROUND AND MATERIAL DISCLOSURE STATEMENT 
The present invention is related to printing systems incorporating light 
emitting print bars as the imager, and, more particularly, to a print 
system using LED print bars whose operational temperatures are maintained 
within a given tolerance range. 
Image print bars used in xerographic recording systems are well known in 
the art. The print bar generally includes a linear array of a plurality of 
discrete light emitting sources on a substrate. The print bar is optically 
coupled to a linear lens array. Light emitting diode (LED) print bars are 
preferred for many recording applications. In order to achieve high 
resolution, a large number of light emitting diodes, or pixels, are 
arranged in a print bar and means are included for providing a relative 
movement between the print bar and the photoreceptor so as to produce a 
scanning movement of the print bar over the surface of the photoreceptor. 
Thus, the photoreceptor may be exposed to provide a desired image one line 
at a time as the LED print bar and associated lens array is advanced 
relative to the photoreceptor either continuously or in stepping motion. 
Each LED pixel in the print bar is used to expose a corresponding area on 
the photoreceptor to a value determined by image defining video data 
information. 
In a color xerographic system, a plurality of LED print bars may be 
positioned adjacent the photoreceptor surface and selectively energized to 
create successive image exposures, one for each of the three basic colors. 
A fourth print bar may be added if black images are to be created as well. 
FIG. 1 shows a prior art single pass color configuration having three print 
bars, 10, 12, 14. The print bars, each comprising an LED array and a 
coupling gradient index lens array (10A, 10B, 12A, 12B, 14A, 14B, 
respectively), are addressed by video image signals whose application is 
controlled by control circuit 15. Each print bar is optically coupled to 
focus the emitter outputs to form three spaced latent images l.sub.1, 
l.sub.2, l.sub.3 on the surface of photoreceptor belt 16. The optical 
coupling is accomplished by the gradient index lens arrays 10B, 12B, 14B, 
the lens array sold under the name SELFOC.sub..TM. a trademark of Nippon 
Sheet Glass Co., Ltd. Upstream of each exposure station, a charge device 
18, 20, 22 places a predetermined charge on the surface of belt 16. 
Downstream from each exposure station, a development system 26, 28, 30, 
develops a latent image of the last exposure without disturbing previously 
developed images. Further details of xerographic stations in a multiple 
exposure single pass system are disclosed in U.S. Pat. No. 4,660,059 whose 
contents are hereby incorporated by reference. 
With such a system as that disclosed in FIG. 1, each colored image must be 
precisely aligned such that all corresponding pixels in the image areas 
are registered. The print bar alignment requirements are that pixels of 
each bar be aligned in the scan or Y-direction of FIG. 1 so that each 
active write length is equal. The print bar must also be aligned in the 
skew or X-direction. This alignment must be maintained through continous 
revolutions (passes) of the photoreceptor. 
To maintain exact color registration of each image, typically to a 
tolerance of .+-.0.1.mu., the overall length of the write area, the pixel 
to pixel placement, and the straightness of the image line must all be 
within the required exacting tolerance. One of the most difficult 
manufacturing tolerances to achieve is the overall or active write length 
of an image print bar. For example, for a 14.33" LED print bar with 300 
spi resolution, 4299 pixels are aligned in the active write area and a 
.+-.15.mu. tolerance in write length is typical. 
A specific problem in correcting exact image-to-image registration, and one 
which is addressed by the present invention, is the change in length an 
LED print bar undergoes when subjected to temperature increases (thermal 
expansion), which are caused either by heat generated internally to the 
print bar, or by heat absorbed by the print bar from the surrounding 
machine environment. 
Typically, accurate LED print bars are formed on a single ceramic substrate 
with a CET (coefficient of thermal expansion) on the order of 
7.0.times.10.sup.-6 linear units /.degree.C. To achieve proper 
registration (for a .+-.10.mu. tolerance due to thermal effects) of all 
pixels over a 364 mm write zone (B4 paper size), the temperature of all 
multiple print bars would have to be held to .+-.3.9.degree. C. An 
additional factor which must be considered is the need to compensate for 
the decrease in conversion efficiency of electrical to optical energy. For 
example, GaAsP LED material illumination efficiency decreases 
approximately 0.8% per .degree.C. 
According to the principles of the present invention, a thermal controller 
system is provided which maintains multiple print bar temperatures within 
specific required limits. The temperature of each print bar is sensed and 
representative signals sent to a machine thermal controller to provde 
individual heating or cooling to maintain the print bar temperatures 
within a given tolerance as required for a dot-to-dot placement accuracy. 
More particularly, the invention is directed towards a thermal control 
system for maintaining the relative temperature of multiple print bars 
within a specified temperature differential range comprising: 
a heater connected to each print bar and adapted, when energized, to 
increase the print bar temperature, 
a temperature sensor associated with each print bar adapted to continually 
sense the operating temperature of the associated print bar and to 
generate an output signal representative thereof, 
a cooling mechanism operatively coupled to each print bar to provide a 
cooling medium to one end of the print bar, and adapted, when energized, 
to decrease the temperature of the associated print bar, 
system control means for selectively controlling the temperatures of each 
of the print bars, said temperatures represented by signals from the 
associated temperatures sensor, and for detecting a predetermined 
temperature differential between two or more print bars, said control 
means further adapted to control the operation of said heaters and cooling 
mechanisms to restore the sensed temperature differential to within the 
specified differential range. 
The following references have been identified in a prior art search: 
U.S. Pat. No. 4,865,123 to Kawashima et al. discloses an apparatus for 
circulating a cooling fluid through a plurality of cooling modules for 
cooling electronic components. The apparatus includes a plurality of 
supply lines arranged independently and in parallel to each other. Each of 
the supply lines supplies coolant to an individual cooling module. At one 
end, the supply lines draw coolant from a mixing tank having a relatively 
large volume, and at the opposite end, the supply lines return the coolant 
to the mixing tank, wherein the coolant is circulated so that its 
temperature is kept uniform throughout. Each supply line includes a pair 
of pumps 3, check valves 4, and a heat exchanger 5. 
U.S. Pat. No. 4,601,328 to Tasaka et al. discloses a method for temperature 
balancing control of a plurality of heat exchangers used in parallel. The 
temperatures of a medium flowing through the parallel heat exchangers are 
sensed at the same position in each of the plurality of heat exchangers, 
and the sensed temperature values are respectively compared with a 
temperature setting value, so as to calculate control signals for 
balancing the temperatures of the medium flowing out of the heat 
exchangers. Regulation means for each of the respective heat exchangers 
are responsive to the control signals to effect temperature balance of the 
medium. 
In addition, co-pending application Ser. No. 07/773,793, filed on Oct. 9, 
1991, and assigned to the same assignee as the present invention, 
discloses a method and apparatus for maintaining print bars at the same 
temperature by circulating a cooling medium through each print bar 
assembly.

DESCRIPTION OF THE INVENTION 
Referring again to FIG. 1, it is assumed that LED print bars 10, 12, 14 
have a resolution of 300 spots per inch (300 spi), and a pixel size of 50 
.times.50 microns on 84.67 micron centers. In an application, where an 8.5 
inch wide informational line (active write length) is to be exposed, a 
linear LED print bar of approximately 2550 pixels, arrayed in a single 
row, would be required. 
It is assumed that the print bars will be operated in an environment where 
temperature increase will be experienced that would change (increase) the 
active write length of one or more of the LED bars 10, 12, 14. For 
example, the print bars may be located within a xerographic machine frame 
which, because of other thermal loads, will experience an internal 
temperature rise (.DELTA.T). The actual rise will depend on the system and 
its specific operating parameters. Based on observations of present 
systems, an internal temperature rise of approximately 20.degree. C. can 
be expected. In addition, room ambient temperature difference adds another 
14.degree. C. of .DELTA.T to the temperature tolerance stackup. Print bar 
to print bar average operating power differences also add an additional 
complexity to the system. 
According to the present invention, and referring to FIG. 2, LED print bars 
40, 50, and 60 are shown. These print bars are shown as representative. A 
fewer or greater number of print bars may be required, depending on the 
particular configuration. For example, for a full color system, four print 
bars, one for each of the primary colors and one for black, may be 
required. The charge and development system shown in FIG. 1 would be 
modified accordingly. These print bars would be mounted, for example, in a 
printing system of the type shown in FIG. 1. For ease of description, the 
xerographic stations and the coupling lens array shown in FIG. 1 are 
omitted from this discussion, but it is understood that the print bars 
would be addressed and would expose the photoreceptor as is known in the 
art and as is shown in FIG. 1. Of particular interest for the present 
invention is how the print bars are modified so that their temperature can 
be controlled, in response to deviations in print bar to print bar 
temperatures. Each print bar has a resistive heating element (heater) 40A, 
50A, 60A, connected respectively to the substrates 40S, 50S, 60S, of print 
bars 40, 50, 60, respectively. The heaters may extend along the length of 
the bar, as shown, or may assume other configurations consistent with 
usage. 
The heater requirement is to raise an unused "cool" print bar close to the 
temperature of an active "hot" print bar. A heater power near the 
operating power of the "hot" print bar will generally be required. Each 
heater has an associated temperature sensor, 40B, 50B, 60B, which monitors 
the substrate temperature for each print bar and generates an appropriate 
signal, which represents the sensed temperature. The signal is sent to 
heater control circuit 70 in thermal controller 72. The sensors may be, 
for example, thermistors or junction devices. 
Referring still to FIG. 2, cooling mechanisms 40C, 50C, 60C are positioned 
so as to direct a cooling medium against the edge of each print bar 40, 
50, 60, respectively. The cooling medium is directed through cooling ducts 
(not shown). With the forced air system shown, it is preferable that air 
be taken from outside the machine environments, so that the temperature 
rise of the air, due to internal heating, is minimized. Alternate cooling 
systems that may be used are liquid cooling systems or Peltier cooling 
devices. 
In operation, the print heads, with usage, experience individual 
temperature changes. A predetermined temperature differential between the 
print bars has been stored in controller 72 memory 73. As an example, it 
is assumed that the print bars must operate with no more than a 
3.9.degree. C. temperature gradient differential between the bars. Machine 
thermal control circuit 72 receives signals from sensors 40B, 50B, 60B, 
and continually monitors the input from the print bars during operation 
and compares the several temperatures with each other. Upon noting a 
deviation of more than 3.9.degree. C. between at least two of the bars, 
either one or more cooling mechanisms or heaters are selectively 
activated. In a preferred embodiment, the cooling control system 74 is 
enabled to first attempt to cool the print bar which is identified as 
having the hotter relative temperature. For example, if print bar 40 
temperature is sensed along line S1 at 36.degree. C., and print bar 50 
along line S2 at 34.degree. C., and print bar 60 along line S3 at 
30.degree. C., comprarison circuitry in heater control circuit 70 
recognizes that print bars 40 and 60 have a temperature differential 
greater than 3.9.degree. C. A signal is sent to cooling control circuit 74 
to generate a signal to energize cooling mechanism 40C along line C1. Upon 
detecting a drop in print bar 40 temperature to 35.degree. C. (in heater 
control circuit 70), cooling mechanism 40C will be de-energized. If the 
print bar 40 does not respond quickly enough to the cooling, as detected 
by a timing mechanism initiated in controller 72 at the time of cooler 
mechanism 40C activation, the cooling mechanism 60C will be disabled, the 
control circuit 70 enabled, and heater 60A will be energized, raising the 
temperature of print bar 60. The heater power of heater 60A will cause the 
temperature of bar 60 to elevate, reducing the temperature differential 
between bar 40 and bar 60 to less than 3.9.degree. C. The invention may be 
practiced with a combination of heating and cooling e.g. for the example 
given, print bar 40 may be cooled and print bar 60 may be heated to 
rapidly reduce the temperature differential between the two. The control 
circuit will be set to maintain as low an average absolute print bar 
temperature as possible, to optimize LED efficiency, while maintaining the 
temperature of each of the print bars within the necessary maximum 
differential temperature. 
While the above description includes a specific example of a preselected 
temperature differential, it is understood that other differential ranges 
may be provided and other possibilities exist for maintaining 
differentials between two other print bars, or between all three print 
bars requiring cooling or heating combinations to provide both heating and 
cooling. 
While the invention has been described with reference to the structures 
disclosed, it is not confined to the details set forth but is intended to 
cover such modifications or changes as they come within the scope of the 
following claims. 
As one example, the heater shown in FIG. 2 may be modified so that areas 
adjacent the input cooling duct have a higher power density to compensate 
for the cooler air directed against that end and to compensate for 
conductive heat losses through the print bar mounts.