Imperfect register correcting method to be carried out on a multicolor image forming apparatus

An imperfect register correcting method is applied to correcting the incorrect disposition and functions of the components of a multicolor image forming apparatus which forms a multicolor image by sequentially superposing color images formed by a plurality of image forming units thereof, causing imperfect register of the color images formed by the plurality of image forming units. The imperfect register correcting method comprises a first imperfect register correcting cycle in which skews of the image forming units are corrected, and a second imperfect register correcting cycle in which factors of imperfect register other than the skews are corrected. The second imperfect correcting cycle is carried out subsequent to the first imperfect register correcting cycle to obviate the introduction of errors into data for correcting the factors of imperfect register by the correction of the skew of the image forming units.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an imperfect register correcting method to 
be carried out on a multicolor image forming apparatus having a plurality 
of image forming units capable of sequentially transferring color images 
in register in a superposed arrangement on a recording medium to form a 
multicolor image on the recording medium. 
2. Description of the Prior Art 
The use of color documents in offices has rapidly become prevalent in 
recent years and color image forming apparatus for producing color 
documents, including color copying machines, color printers and color 
facsimile equipments have rapidly become widespread. The enhancement of 
the operating speed of the color image forming apparatus is the trend of 
the times. 
A so-called tandem color printer is an example of a high-speed color image 
forming apparatus. The tandem color printer is provided with four ROSs 
(raster output scanners), i.e., image forming units, respectively for 
forming, for example, an yellow (Y) image, a magenta (M) image, a cyan (C) 
image and a black (K) image. The tandem color printer superposes four 
color images formed sequentially by the ROSs to form a multicolor image 
efficiently. When the tandem color printer jams or malfunctions, some of 
the ROSs are removed or displaced, recording sheets clogging the tandem 
color printer are removed or faulty parts are repaired or replaced, and 
then the removed or displaced ROSs are restored to their original 
positions. However, the ROSs are not necessarily restored correctly to 
their original positions. If the ROSs are dislocated from the correct 
positions, the superposed color images are imperfectly registered. 
Factors that cause imperfect register are dislocation of the ROS with 
respect to the scanning direction, dislocation of the ROS with respect to 
the feed direction, i.e., the direction of advancement of the recording 
sheet, incorrect scanning width, i.e., deviation of the image in size with 
respect to the scanning direction, angular deviation of the scanning 
direction, i.e., skew of the ROS. 
An imperfect register correcting method is disclosed in Japanese Patent 
Laid-open (Kokai) No. Hei. 1-142671. According to this known imperfect 
register correcting method, a register mark is formed by each of a 
plurality of ROSs, the register mark generator of each ROS generates the 
image of the register mark according to a specific rule to form the image 
of the register mark on the conveyor belt, the image of the register mark 
is sampled at predetermined moments by CCD image sensors to obtain 
register mark data. Therefore, the positional differences of the register 
marks formed by the ROSs containing such as dislocations of the ROSs with 
respect to the scanning direction, dislocations of the ROSs with respect 
to the feed direction, incorrect scanning width of the ROSs and skews of 
the ROSs are corrected at once on the basis of the calculated positional 
differences between the register marks. 
However, when the skew is corrected, at least one of the factors of 
imperfect register, i.e., dislocation with respect to the scanning 
direction, dislocation with respect to the feed direction and incorrect 
scanning width, changes. Therefore, if the changed factor is corrected on 
the basis of corrections calculated on the basis of the register mark data 
sampled before the correction of the skew, the factor cannot be corrected 
exactly and hence perfect correction of imperfect register is impossible; 
that is, since secondary errors are introduced additionally into the 
factors of imperfect register by the correction of the skew, the secondary 
errors remain uncorrected even if the factors are corrected on the basis 
of the corrections calculated on the basis of the register mark data 
sampled before the correction of the skew. 
The secondary errors will be described concretely with reference to FIGS. 5 
and 6 showing the images of two register marks in terms of the effect of 
skew correction on the dislocation of the images with respect to the feed 
direction. Referring to FIG. 5, a first image 100a, i.e., the image of a 
first register mark, and a second image 101a, i.e., the image of a second 
register mark, are formed on the basis of the image data obtained by 
sampling. The second image 101a is skewed at a skew angle with respect 
to the first image 100a and dislocated by a dislocation 102a from the 
first image 100a. If the skew angle of the second image 101a is 
corrected, the second image is shifted to a first corrected position 
101a'. The second image 101a at the first corrected position 101a' is 
dislocated with respect to the feed direction by a dislocation 102b from 
the first image 100a. Thus, the dislocation 102b of the second image 101a 
at the first corrected position 101a' from the first image 100a with 
respect to the feed direction differs considerably from the dislocation 
102a of the second image 101a from the first image 100a calculated on the 
basis of the image data sampled before skew correction. If the dislocation 
of the second image 101a at the first corrected position 101a' is 
corrected subsequent to skew correction on the basis of a correction 
determined on the basis of the dislocation 102a calculated on the basis of 
the image data sampled before skew correction, the second image 101a is 
shifted to a second corrected position 101a" as shown in FIG. 6, in which 
the second image 101a is still dislocated with respect to the feed 
direction from the first image 100a by a dislocation equal to the 
difference between the dislocations 102a and 102b. Thus, the positional 
error of the second image 101a with respect to the feed direction cannot 
be correctly corrected if the dislocation of the second image 101a with 
respect to the feed direction is corrected after skew correction on the 
basis of the dislocation 102a calculated on the basis of the image data 
sampled before skew correction. This known imperfect register correcting 
method, which corrects the factors of imperfect register on the basis of 
the image data sampled before skew correction, is unable to precisely 
correct other factors of imperfect register as well as the dislocation 
with respect to the feed direction. 
Even if a skew correcting mechanism capable of correcting the skew of an 
image without affecting other factors of imperfect register is available, 
the skew correcting mechanism will be very complex and very expensive. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
imperfect register correcting method for correcting the imperfect register 
of images formed by a multicolor image forming apparatus, capable 
perfectly correcting the factors of imperfect register without being 
adversely effected by skew correction carried out prior to the correction 
of the factors of imperfect register other than the skew of the image 
forming units. 
In one aspect of the present invention, an imperfect register correcting 
method for correcting imperfect register of images formed by a multicolor 
image forming apparatus which forms a multicolor image by sequentially 
superposing color images formed by a plurality of image forming units 
thereof comprises steps of forming images of specific register marks by 
pattern generators included in the plurality of image forming units on a 
recording sheet transporting device included in the multicolor image 
forming apparatus, obtaining register mark data by reading the images of 
the register marks by image sensors, calculating corrections on the basis 
of the differences of the register mark data representing the positions of 
the images of the register marks formed by the image forming units 
excluding one specified image forming unit among the plurality of image 
forming unit from the register mark data representing the position of the 
image of the register mark formed by the specified image forming unit, and 
correcting the position of each image forming unit in respect of 
dislocation with respect to the scanning direction, dislocation with 
respect to the feed direction, incorrect scanning width and skew. When 
correcting imperfect register, the register mark data for the correction 
of the skew and that for the correction of the factors of imperfect 
register other than the skew are obtained respectively in different 
imperfect register correcting cycles. 
Thus, the correction of the factors of imperfect register other than the 
skew is carried out independently of the correction of the skew on the 
basis of the register mark data in an imperfect register correcting cycle 
other than that for obtaining the register mark data for the correction of 
the skew, and hence the correction of the factors of imperfect register 
other than the skew is not effected by the correction of the skew. 
Accordingly, all the factors of imperfect register can be accurately 
corrected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, in a digital multicolor copying machine, an image 
scanner provided with a color CCD image sensor 3 samples the image of a 
document 2 placed on a platen 1 and provides an analog signal R 
representing the red component of the image, an analog signal G 
representing the green component of the image, and an analog signal B 
representing the blue component of the image. An image processing unit 4 
converts the analog signals R, G and B into an yellow image signal Y, a 
magenta image signal M, a cyan image signal C and a black image signal K, 
stores the image signals Y, M, C and K temporarily in a storage included 
therein, and sends the image signals Y, M, C and K respectively to an 
yellow scanning device 5Y, a magenta scanning device 5M, a cyan scanning 
device 5C and a black scanning device 5K. Then, the scanning devices 5Y, 
5M, 5C and 5K scan corresponding photoconductive drums 6Y, 6M, 6C and 6K 
to form electrostatic latent images on the photoconductive drums 6Y, 6M, 
6C and 6K, respectively. Developing devices 7Y, 7M, 7C and 7K combined 
respectively with the photoconductive drums 6Y, 6M, 6C and 6K develop the 
electrostatic latent images in visible color images. 
The yellow scanning device 5Y, the yellow photoconductive drum 6Y and the 
yellow developing device 7Y form, in combination, an yellow image forming 
unit. The magenta scanning device 5M, the magenta photoconductive drum 6M 
and the magenta developing device 7M form, in combination, a magenta image 
forming unit. The cyan scanning device 5C, a cyan photoconductive drum 6C 
and the cyan developing device 7C form, in combination, a cyan image 
forming unit. The black scanning device 5K, the black photoconductive drum 
6K and the black developing device 7K form, in combination, a black image 
forming unit. The scanning devices may be either laser beam scanning 
devices or LED scanning devices. 
Recording sheets 11 stored in a sheet feed tray 12 are fed onto a sheet 
transporting belt 8 one by one by a feed roller 13 according to a sheet 
feed signal. The sheet transporting belt is formed of a transparent, 
dielectric material and extended at a specified tension between a driving 
roller 9 driven by a constant-speed motor, not shown, and a driven roller 
10. The sheet transporting belt 8 is turned counterclockwise, as viewed in 
FIG. 1, to transport the recording sheet 11 via the photoconductive drums 
6Y, 6M, 6C and 6K. The recording sheet 11 fed onto the sheet transporting 
belt 8 is attracted to the sheet transporting belts 8. The visible yellow, 
magenta, cyan and black images are transferred sequentially in that order 
respectively from the photoconductive drums 6Y, 6M, 6C and 6K to the 
recording sheet 11. 
The transportation of the recording sheet 11 by the sheet transporting belt 
8, and the operation for forming the electrostatic latent image on the 
yellow photoconductive drum 6Y are timed so that the leading edge of the 
recording sheet 11 and the leading edge of the electrostatic latent image 
formed on the yellow photoconductive drum 6Y coincide with each other at 
an yellow image transfer position corresponding to the line of contact 
between the sheet transporting belt 8 and the circumference of the yellow 
photoconductive drum 6Y. At the image transfer position, the yellow 
visible image is transferred from the yellow photoconductive drum 6Y to 
the recording sheet 11 by the agency of a transfer corona discharger or 
the like. At a magenta image transfer position, the magenta visible image 
is transferred in a similar manner from the magenta photoconductive drum 
6M to the recording sheet 11 so as to be superposed on the yellow visible 
image. Similarly, the cyan visible image and the black visible image are 
transferred at a cyan image transfer position and a black image transfer 
position from the cyan photoconductive drum 6C and the black 
photoconductive drum 6K, respectively, to the recording sheet 11 so as to 
be superposed on the yellow and magenta visible images formed previously 
on the recording sheet 11. Thus, the yellow, magenta, cyan and black 
visible images are transferred sequentially from the photoconductive drums 
6Y, 6M, 6C and 6K to the recording sheet 11 in a superposed relation to 
form a multicolor image on the recording sheet 11. 
After all the color visible images have been transferred to the recording 
sheet 11, the recording sheet 11 is further advanced by the sheet 
transporting belt 8, separated from the sheet transporting belt 8 at a 
position near the driven roller 10 by the agency of a corona discharger or 
a sheet stripper, and delivered to a fixing unit 14, which fixes the 
multicolor image to the recording sheet 11 and delivers the recording 
sheet 11 to a delivery tray 15. 
FIG. 2 shows an imperfect register correcting system for carrying out the 
imperfect register correcting method embodying the present invention. The 
imperfect register correcting system corrects the imperfect register of 
the color visible images attributable to the minute dislocation of the 
photoconductive drums and/or the variation of image transfer timing caused 
by an external force or temperature variation. The imperfect register 
correcting system has a detecting unit 23 comprising a pair of image 
sensors 16 disposed above the sheet transporting belt 8 opposite to 
positions near the side edges of the sheet transporting belt 8, 
respectively, and a pair of light sources 17 disposed under the upper side 
of the sheet transporting belt 8 opposite to the image sensors 16 to 
illuminate register marks formed on the sheet transporting belt 8, 
respectively. The image sensors 16 may be, for example, CCD image sensors, 
and the light sources 17 may be, for example, LEDs (light emitting diodes) 
or halogen lamps. The luminous intensity of the light sources 17 can 
optionally be adjusted to compensate the reduction of luminous intensity 
due to the deterioration of their performance, the deterioration of the 
transparency of the sheet transporting belt 8 and/or the variation of the 
environmental conditions including the environmental temperature for the 
optimum illumination of the register marks. 
The imperfect register correcting system comprises, in addition to the 
detecting unit 23, interfaces 18Y, 18M, 18C and 18K formed on printed 
wiring boards to send image signals respectively to the scanning devices 
5Y, 5M, 5C and 5K, an imperfect register correcting unit 19 formed on a 
printed wiring board, an image processing unit 20 formed on a printed 
wiring board, and a control unit 21 formed on a printed wiring board to 
control the operation of the imperfect resister correcting system. 
The imperfect register correcting system starts an imperfect register 
correcting cycle when the image forming unit or the image forming units 
are removed from the digital multicolor copying machine to remove 
recording sheets clogging the digital multicolor copying machine and put 
on the digital multicolor copying machine after removing the recording 
sheet or when the temperature within the digital multicolor copying 
machine rises beyond a predetermined temperature. 
In the imperfect register correcting cycle, first the control unit 21 gives 
command signals to the interfaces 18Y, 18M, 18C and 18K, the imperfect 
register correcting unit 19 and the image processing unit 20. Then, the 
interfaces 18Y, 18M, 18C and 18K function as pattern generators to 
generate register mark forming signals representing register marks and 
send the register mark forming signals to the scanning devices 5Y, 5M, 5C 
and 5K of the image forming units. Register marks 22Y, 22M, 22C and 22K 
formed on the photoconductive drums 6Y, 6M, 6C and 6K are transferred to 
the sheet transporting belt 8, and the imperfect register correcting unit 
19 prepares for sampling the register marks 22Y, 22M, 22C and 22K formed 
on the sheet transporting belt 8. 
When the imperfect register correcting cycle is started, first the yellow 
interface 18Y gives the register mark forming signal for forming the 
yellow register marks 22Y to the yellow scanning device 5Y, the yellow 
scanning device 5Y forms electrostatic latent images corresponding to the 
register mark forming signal on the photoconductive drum 6Y, the 
electrostatic latent images are developed by the developing device 7Y in 
visible yellow register mark images, and then the visible yellow register 
mark images are transferred to the sheet transporting belt 8 to form 
yellow register marks 22Y at positions near the opposite side edges of the 
sheet transporting belt 8. 
Then, a fixed time corresponding to a time interval in which the sheet 
transporting belt 8 advances by a distance between the yellow image 
transfer position and the magenta image transfer position after the 
register mark forming signal for forming the yellow register marks 22Y has 
been given to the yellow scanning device 5Y, the magenta interface 18M 
sends a register mark forming signal for forming the magenta register 
marks 22M to the magenta scanning device 5M. Thus, the magenta register 
marks 22M are formed, similarly to the yellow register marks 22Y, on the 
sheet transporting belt 8 at positions near the opposite side edges of the 
sheet transporting belt 8. The cyan register marks 22C and the black 
register marks 22K are formed by the same procedure on the sheet 
transporting belt 8. The register marks 22Y, 22M, 22C and 22K are thus 
superposed to form register patterns 22. 
Upon the arrival of the register patterns 22 at a position directly below 
the image sensors 16, the light sources 17 illuminate the register 
patterns 22 and the image sensors 16 forms images of the register patterns 
22. 
The imperfect register correcting unit 19 monitors at least one of the 
times when the interfaces 18Y, 18M, 18C and 18K provide the register mark 
forming signals. The imperfect register correcting unit 19 determines the 
time when the register patterns 22 arrive directly below the image sensors 
16 on the basis of the time when the interface 18Y, 18M, 18C or 18K 
provides the register mark forming signal, and determines the sampling 
start time and the sampling end time to provide a time interval sufficient 
for sampling the register patterns 22 detected by the image sensors 16. 
At the sampling start time, the imperfect register correcting unit 19 
starts reading image signals provided by the image sensors 16 and storing 
the image signals in its internal high-speed memory and terminates reading 
the image signals at the sampling end time. Subsequently, the imperfect 
register correcting unit 19 determines the positions of the register 
patterns 22 on the basis of the image signals stored in the high-speed 
memory by a method of elastic center or the like and stores the positions 
of the register patterns 22 as image position addresses in a main memory. 
This procedure is repeated to determine a plurality of definite image 
position addresses for each image forming unit. To obtain accurate 
definite image position addresses, the plurality of definite image 
position addresses for each image forming unit may be averaged. 
Then, the imperfect register correcting unit 19 processes the definite 
image position addresses for each image forming unit by a predetermined 
algorithm to calculate corrections for a plurality of imperfect register 
parameters for each image forming unit. The corrections thus obtained are 
set directly or indirectly for the scanning devices 5Y, 5M, 5C and 5K and 
the interfaces 18Y, 18M, 18C and 18K. In this register correcting cycle, 
the skew of each image forming unit is corrected with respect to the other 
image forming units. Thus, calculations for correcting the skew of one 
specified image forming unit need not be made. Corrections to the other 
image forming units may be made with respect to the one specified image 
forming unit. The imperfect register parameters are the dislocation of the 
scanning device with respect to the scanning direction, the dislocation of 
the scanning device with respect to the feed direction, a deviation of 
scanning width, i.e., a deviation of the image in size with respect to the 
scanning direction, and the skew with respect to the scanning direction. 
The skews of the image forming units are corrected by adjusting the angular 
positions of the respective reflecting mirrors 24Y, 24M, 24C and 24K of 
the scanning devices 5Y, 5M, 5C and 5K according to the calculated 
corrections by means of stepping motors. The correction of other factors 
causing imperfect register are achieved by correcting the scanning start 
times and the scanning intervals of the scanning devices 5Y, 5M, 5C and 5K 
in the software according to the calculated corrections. 
The imperfect register correcting cycle is repeated twice respectively for 
correcting the skews and other functions, or factors, of the image forming 
units due to imperfect register. After correcting the skews in the first 
imperfect register correcting cycle, the second imperfect register 
correcting cycle is performed to correct other factors of imperfect 
register. 
Referring to FIG. 3, after the first incorrect register correcting cycle 
has been started, images of the register patterns are formed and register 
mark data representing the values of the factors of imperfect register are 
determined on the basis of the register patterns data in step S1. Then, 
the skews are calculated on the basis of the register pattern data in step 
S2 and the skews are examined in step S3 to see if the skews need 
correction. If the skews need correction, corrections for correcting the 
skews are calculated in step S4. The corrections for correcting the skews 
are angular data for correcting the respective angular positions of the 
reflecting mirrors 24Y, 24M, 24C and 24K of the scanning devices 5Y, 5M, 
5C and 5K. Then, in step S5, the skews are corrected according to the 
corrections calculated in step S4. When correcting the skews, the angular 
positions of the reflecting mirrors 24Y, 24M, 24C and 24K are adjusted 
according to the corrections by means of the stepping motors. When the 
skews need correction, only the corrections for correcting the skews are 
calculated. 
In step S6, the second imperfect register correcting cycle is carried out, 
in which images of the register patterns are formed and register pattern 
data representing the values of the factors of imperfect register are 
determined on the basis of the register patterns data. The register 
pattern data obtained in the second imperfect register correcting cycle 
represents the values of factors of imperfect register after the 
correction of the skews. Then, in step S7, the values of the factors other 
than the skews, i.e., the dislocations of the image forming units with 
respect to the scanning direction, the dislocations of the image forming 
units with respect to the feed direction and the error in scanning width, 
are calculated on the basis of the register pattern data, and corrections 
for correcting the factors of imperfect register are calculated on the 
basis of the values of the factors in step S8, and then the factors of 
imperfect register are corrected according to the corrections in step S9, 
in which the scanning starting times and scanning intervals of the 
scanning devices 5Y, 5M, 5C and 5K set in the software are corrected. 
After all the factors of imperfect register have been thus corrected, the 
normal copying operation of the digital multicolor copying machine is 
started. 
Thus, the factors of imperfect register other than skew are corrected on 
the basis of the register pattern data obtained after the correction of 
the skews. Therefore, both the skews and the factors of imperfect register 
can be accurately corrected. 
If it is decided in step S3 that the skews need not be corrected, values of 
the factors of imperfect register other than the skews are calculated in 
step S7 on the basis of the register pattern data obtained in the first 
imperfect register correcting cycle, and then steps S8 and S9 are 
executed. In this case, the second imperfect register correcting cycle is 
unnecessary. 
Once the skews are corrected, the skew of the image forming units, in 
general, does not occur any more. Therefore, when the digital multicolor 
copying machine does not require the removal of the image forming units 
for removing jamming recording sheets or for repair, an imperfect register 
correcting procedure as shown in FIG. 4 is carried out to correct 
imperfect register. Referring to FIG. 4, upon the connection of the 
digital multicolor copying machine to the power source, steps S1, S7, S8 
and S9, which are the same respectively as steps S1, S7, S8 and S9 of the 
imperfect register correcting procedure shown in FIG. 3, are executed to 
correct the factors of imperfect register other than the skews and the 
correction of the skew is omitted. 
Since the register pattern data for correcting the factors of imperfect 
register other than the skews of the image forming units is obtained in an 
imperfect register correcting cycle carried out after correcting the 
skews, the register pattern data represents the actual values of the 
factors of imperfect register. Accordingly, the factors of imperfect 
register other than the skews can be accurately corrected without being 
effected by errors that may be introduced into the data by the correction 
of the skews. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, obviously many changes and variations are 
possible therein. It is therefore to be understood that the present 
invention may be practiced otherwise than as specifically described herein 
without departing the scope and spirit thereof.