Optical scanner

An optical scanner for a laser printer or the like, capable of forming dots on scanning lines which are apparently the same in length. When the respective scanning speeds of spots of laser beams reflected respectively by a plurality of reflecting surfaces of a polygonal rotating mirror are different from each other, correction clock pulses are inserted in a print control clock signal for each scanning line.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to an optical scanner of a dot matrix system 
for printing. 
A conventional optical scanner will be described with reference to FIGS. 6, 
7 and 8. An optical scanner 1 for laser printer and the like has an 
illuminating optical system 5 comprising a laser diode 2, a cylindrical 
lens 3.sub.1, a focusing lens 3.sub.2 and a reflecting mirror 4. A v 
polygonal rotating mirror 6 having reflecting surfaces 7.sub.1 to 7.sub.6 
is disposed with its center on the optical axis of the illuminating 
optical system 5 and is attached directly to the output shaft of a 
scanning motor, not shown. The laser diode 2 is connected to a print data 
circuit, not shown, which operates in synchronism with a print clock 
signal B, which will be described afterward. A correcting lens 8 is 
disposed with its optical axis in alignment with the optical axis of the 
illuminating optical system 5 so as to cover an angular range swept by 
laser beams reflected by the reflecting surfaces 7.sub.1 to 7.sub.6 of the 
polygonal rotating mirror 6. A photoconductive drum 9 is disposed behind 
the correcting lens 8 with its axis perpendicular to the optical axis of 
the illuminating optical system 5. A scanning start detecting unit 12 
consisting of a reflecting mirror 10 and a start sensor 11 is disposed on 
a scanning start line from which a laser beam reflected by the polygonal 
rotating mirror 6 starts scanning the surface of the photoconductive drum 
9. The optical scanner 1 has a reference clock, not shown, such as a 
quartz crystal oscillator, which generates a reference clock signal A 
(FIG. 8), and a print clock, not shown, which generates a print clock 
signal B (FIG. 8) obtained by dividing the frequency of the reference 
clock signal for the timing of printing dots. 
The print clock signal B has, for example, one pulse for every eight pulses 
of the reference clock signal A. Therefore, one dot 14 of an image 13 is 
formed every eight pulses of the reference clock signal A. The 
illuminating optical system 5 emits a laser beam representing image 
information in synchronism with the print clock signal B. The laser beam 
is reflected by the polygonal rotating mirror 6 and the reflected laser 
beam is focused in a spot on the photoconductive drum 9 by the focusing 
lens 3.sub.2 and the correcting lens 8. Since the photoconductive drum 9 
is rotating, the spot of the scanning laser beam moves in the direction of 
feed relative to the photoconductive drum 9. Thus, the latent image of the 
image 13 is formed in a dot matrix on the photoconductive drum 9, and then 
the the latent image is developed and transferred to a recording sheet by 
an electrophotographic process or the like to print the image 13. This 
conventional optical scanner 1 does not perform any control operation to 
synchronize the scanning operation for the image 13 and the rotation of 
the polygonal rotating mirror 6. Accordingly, the print clock signal B for 
each scanning cycle is started on the basis of synchronization of the 
reference clock signal A and start signals S.sub.1, S.sub.2, . . . 
generated by the start sensor 11 upon the detection of a scanning laser 
beam by the scanning start detecting unit 12 to arrange the dots 14 of the 
image 13 accurately. Therefore, as shown in FIG. 8, the dot 14 
corresponding to the scanning start position is dislocated by a distance 
corresponding to one pulse of the reference clock signal A due to a small 
lag in the start signals S.sub.1 and S.sub.2. The dislocation of the dot 
14 from the scanning start position can be diminished by using a print 
clock signal B obtained by dividing the frequency of the reference clock 
signal A by a further greater number. However, such diminution of the 
dislocation of the dot 14 is limited because the synchronization of the 
reference clock signal A and the start signal S must be detected. However, 
the distance of dislocation of the dot is on the order of one-eighth the 
size of the dot, and hence such dislocation of the dot is not a 
significant problem in the ordinary image forming operation. The frequency 
dividing ratio is determined so that the dislocation of the dot is within 
an allowable range; for example, when a dislocation of about a quarter of 
the size of the dot is allowed, the frequency dividing ratio may be 1/4. 
This optical scanner 1 is able to operate silently at a high speed for 
printing. The polygonal rotating mirror 6 for deflecting the laser beam 
inevitably has errors in the position of the axis of rotation and in 
angular disposition of the reflecting surfaces 7.sub.1 to 7.sub.6. 
Accordingly, although dots 14 of the image 13 on a column 13.sub.1 
corresponding to the scanning start position are aligned substantially 
exactly as shown in FIG. 7, the arrangement of dots 14 on a column 
13.sub.2 corresponding to a scanning end position varies periodically. The 
deviation of the dot 14 from a correct position is proportional to the 
length of the scanning line and, in some cases, the deviation is as large 
as a value corresponding to two to six pulses of the reference clock 
signal A. Such a deviation is attributable to dimensional errors in the 
polygonal rotating mirror 6 and the scanning motor. When the polygonal 
rotating mirror 6 has six reflecting surfaces 7.sub.1 to 7.sub.6, the 
position of the dots 14 varies periodically within six scanning lines. 
Consequently, the image is printed in a low print quality. 
To solve such a problem, a scanning beam control clock correcting apparatus 
is proposed in Japanese Patent Laid-open No. 55-133009. A conventional 
optical scanner incorporating this scanning beam control clock correcting 
device will be described hereinafter with reference to FIGS. 9, 10 and 11, 
in which parts like or corresponding to those previously described with 
reference to FIG. 6 are denoted by the same reference characters and the 
description thereof will be omitted. 
A conventional optical scanner includes a scanning end detecting unit 16 
disposed at a position corresponding to a scanning end position, a 
scanning start detecting unit 12 and a scanning beam control clock 
correcting unit 17 associated with the illuminating optical system. 
Referring to FIG. 9, the scanning start detecting unit 12, the scanning end 
detecting unit 16 and a reference clock generator 18 are connected through 
a clock gate 19 and a first counter 20 to a memory 21. The scanning start 
detecting unit 12 is connected also through a second counter 22 for 
counting the number of scanning cycles to the memory 21. The memory 21 is 
connected through a ROM 23 to a variable frequency divider 24 and an 
up-down counter 25. The up-down counter 25 is connected through a data 
selector 27 to a delay circuit 26 which is connected to the clock gate 19. 
The first counter 20 counts scanning time on the basis of the outputs of 
the scanning start detecting unit 12 and the scanning end detecting unit 
16. The memory 21 stores values of scanning time for the reflecting 
surfaces 7.sub.1 to 7.sub.6 determined by the first counter 20 and 
specified by the output signal of the second counter 22. In the subsequent 
scanning cycle, the ROM 23 gives actuation pulses to the variable 
frequency divider 24 by using the values of scanning time as addresses, 
and scanning time correction signals are given to the up-down counter 25. 
The delay circuit 26 produces pulse streams as shown in FIG. 10 having 
clock phases .phi..sub.1 to .phi..sub.7 sequentially differing from each 
other by a time .DELTA.t (sec) corresponding to the pulse width of the 
reference clock signal generated by the reference clock generator 18, and 
the data selector 27 selects a desired pulse signal among the pulse 
streams respectively of phases .phi..sub.0 to .phi..sub.7 as a corrected 
print control clock signal C according to the contents of the up-down 
counter 25. The variable frequency divider 24 counts the corrected print 
control clock signal C. Then, the up-down counter 25 is counted according 
to the count of the corrected print control clock signal C counted by the 
variable frequency divider 24. As the count increases, the up-down counter 
25 sequentially changes the pulse streams .phi..sub.0 to .phi..sub.7 
provided by the data selector 27. The pulse streams of phases .phi..sub.0 
to .phi..sub.7 are the same in frequency as the reference clock signal A 
and the phases thereof are shifted relative to each other. 
Gaps 29 are formed periodically in a predetermined scanning line as shown 
in FIG. 11 in an image 28 printed by the conventional optical scanner 
having the scanning beam control clock correcting unit 17, and dots on a 
column 30.sub.2 corresponding to the scanning end position are aligned 
satisfactorily as well as those on a column 30.sub.1 corresponding to the 
scanning start position. The size of the gap 29 corresponds, for example, 
to one pulse of the reference clock signal A, which is one-eighth the size 
of the dot in this example. 
The the image 28 printed by the conventional optical scanner employing the 
scanning beam control clock correcting unit 17 is satisfactory with the 
dots 14 in exact alignment on the column 30.sub.2 corresponding to the 
scanning end position. However, the control unit of the conventional 
optical scanner comprises many circuit elements, such as the data selector 
27, the counters 20 and 22 and the memory 21, to detect the mode of 
scanning and to repeat the correcting operation during the operation of 
the conventional optical scanner. Consequently, the conventional optical 
scanner is expensive and is not suitable for mass production. 
The foregoing disadvantages of the conventional optical scanner can be 
overcome by using a polygonal rotating mirror and a scanning motor which 
are manufactured at an extremely high accuracy so that the respective 
scanning speeds of scanning beams reflected respectively by the reflecting 
surfaces 7.sub.1 to 7.sub.6 of the polygonal rotating mirror coincide 
exactly with each other. However, manufacturing the components of the 
optical scanner including the polygonal rotating mirror and the scanning 
motor at a higher accuracy is very difficult, increases the manufacturing 
cost of the optical scanner and hence is unpractical. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a first object of the present invention to enable printing in a high 
print quality without requiring particularly high accuracy in the 
polygonal rotating mirror and the scanning motor. 
It is a second object of the present invention to enable scanning spots to 
scan scanning lines of the same length without requiring any complicated 
control means for varying the print clock signal during scanning 
operation. 
It is a third object of the present invention to print an image in an 
apparently uniform dot density with respect to the scanning direction. 
It is a fourth object of the present invention to print an image without 
causing moire to occur in the printed image. 
Other objects of the present invention will become apparent from the 
following description. 
The present invention provides an optical scanner, which prints an image on 
the basis of image information given thereto in synchronism with a print 
control clock signal of a frequency equal to the submultiple of the 
frequency of a reference clock signal, comprising: scanning time measuring 
means for measuring the scanning time of a laser beam reflected by each 
reflecting surface of a polygonal rotating mirror by a scanning start 
detecting unit disposed at the scanning start position of a scanning range 
and a scanning end detecting unit disposed at the scanning end position of 
the scanning range; correction pulse calculating means for calculating the 
number of pulses of the reference clock signal necessary for scanning a 
scanning line of a fixed length on the basis of the measured scanning time 
of the laser beam reflected by each reflecting surface of the polygonal 
rotating mirror, and calculating the difference between the number of 
pulses of the reference clock signal corresponding to the scanning time of 
the laser beam reflected by one of the reflecting surface of the polygonal 
rotating mirror and the number of pulses of the reference clock signal 
corresponding to the scanning time of the laser beam reflected by another 
reflecting surface of the polygonal rotating mirror; and scanning line 
matching means for uniformly distributing correction pulses calculated by 
the correction pulse calculating means in print control clock signals for 
the reflecting surfaces of the polygonal rotating mirror. 
Thus, the optical scanner of the present invention scans scanning lines of 
the same length without using any complicated control means for varying 
the print control clock signal during the scanning operation, so that an 
image can be clearly printed in a high print quality, the scanning motor 
and the polygonal rotating mirror need not be fabricated at an extremely 
high accuracy, and the optical scanner can be manufactured at a reduced 
cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment (FIGS. 1, 2) 
An optical scanner 31 in a first embodiment according to the present 
invention is similar in composition to the foregoing conventional optical 
scanner including the scanning end detecting unit 16. First, an image 
information output unit 32 included in the optical scanner 31 will be 
described with reference to FIG. 1. A scanning start detecting unit 12, a 
scanning end detecting unit 16 and a reference clock generator 18 are 
connected to a main counter 33 serving as a frequency divider as well as a 
counter, and four digital counters 34.sub.1 to 34.sub.4 capable of 
counting four place numbers and serving as scanning time measuring means. 
The counters 34.sub.1 to 34.sub.4 and a reflecting surface identifying 
unit 35 for identifying the reflecting surfaces of a polygonal rotating 
mirror 6 are connected to a count enable signal generator 36 serving as 
correction clock calculating means and scanning line matching means. The 
count enable signal generator 36 registers a calculated count enable 
signal, i.e., a correction clock signal for increasing or decreasing the 
number of pulses of a print control clock signal B. The count enable 
signal generator 36 is connected through the main counter 33 to a print 
data circuit 37, which in turn is connected through a laser driver 38 to a 
laser diode 2 included in an illuminating optical system 5. 
The operation of the optical scanner 31 for setting a corrected print 
control clock D. 
When the size of a recording sheet is A4 (JIS: Japanese Industrial 
Standards) and dot density is 300 dots/in., the optical scanner 31 forms a 
scanning line by about 2600 dots. Suppose that the optical scanner 31 
prints one dot every eight pulses of a reference clock signal A. Then, the 
number of pulses included in the reference clock signal A for one scanning 
line is 20,800. Since the digital counters 34.sub.1 to 34.sub.4 are 
capable of counting four place number, 65,536 pulses (16.sup.4 pulses) can 
be counted. The reflecting surfaces 7.sub.1 to 7.sub.6 of the polygonal 
rotating mirror 6 are identified by detection signals provided by the 
reflecting surface identifying unit 35. The counters 34.sub.1 to 34.sub.4 
count the number of pulses of the reference clock signal A corresponding 
to the time difference between the detection signals provided respectively 
by the scanning start detecting unit 12 and the scanning end detecting 
unit 16 to measure the values of scanning time for the reflecting surfaces 
7.sub.1 to 7.sub.6 are measured. It may be considered that the values of 
scanning time for the reflecting surfaces 7.sub.1 to 7.sub.6 are 
proportional respectively to the lengths of scanning lines swept by the 
spots of laser beams reflected respectively by the reflecting surfaces 
7.sub.1 to 7.sub.6. Therefore, the operator selects one of the measured 
values of scanning time as a reference scanning time. For example, in 
adjusting a shorter scanning line to the longest scanning line, the 
difference between the number of pulses of the reference clock signal A 
for the longest scanning line and that for the shorter scanning line is 
calculated, and then a predetermined number of count enable signals are 
inserted in the print control clock signal B for the reflecting surface 7, 
which reflected the laser beam for the shorter scanning line, to produce a 
corrected print control clock signal D. The printing operation is started 
after the corrected print control clock signals D respectively for the 
reflecting surfaces 7.sub.1 to 7.sub.6 have been set. 
The printing operation of the optical scanner will be described 
hereinafter. Upon the incidence of a laser beam emitted by the 
illuminating optical system 5 and reflected by the polygonal rotating 
mirror 6 on the scanning start detecting unit 12, the scanning start 
detecting unit 12 provides a scanning start signal S to start an image 
scanning operation. The reflecting surface identifying unit 35 provides 
identification signals for identifying the reflecting surfaces 7.sub.1 to 
7.sub.6. Upon the start of the image scanning operation, the counters 
34.sub.1 to 34.sub.4 start counting the pulses of the reference clock 
signal A, and then count enable signals respectively for the reflecting 
surfaces 7.sub.1 to 7.sub.6 are generated. Each count enable signal 
corresponds one pulse of the reference clock signal A and a dot of a width 
greater than that of a predetermined dot 14 by one-eighth the width of the 
latter. That is, the size of the dot in the feed direction remains 
unchanged while the size of the dot in the scanning direction corresponds 
to nine pulses of the reference clock signal A, so that an elliptic dot 
having a major axis extending in the scanning direction is formed. To 
reduce the length of the scanning line, a narrower dot having a width 
corresponding to seven pulses of the reference clock signal A is formed. 
At the end of one scanning cycle, the laser beam falls on the scanning end 
detecting unit 16. Then, the scanning end detecting unit 16 provides a 
signal to clear the counters 34.sub.1 to 34.sub.4 to prepare for the next 
scanning cycle by the next reflecting surface 7. Thus, the latent image of 
the image 13 is formed in a dot matrix on the photoconductive drum 9, and 
then the latent image is developed and transferred to a recording sheet by 
an electrophotographic process to print the image 13. The count enable 
signal is registered once in assembling the optical scanner 31 and need 
not be changed during the operation of the optical scanner 31. 
Although the optical scanner 31 has been described as an optical scanner 
which provides a corrected print control clock signal D for a shorter 
scanning line to adjust the shorter scanning line to the longest scanning 
line, a longer scanning line may be adjusted to the shortest scanning 
line. In the latter case, a signal to reduce the pulses of the print 
control clock signal B by a predetermined number of pulses is used. 
Second Embodiment (FIG. 3) 
In FIG. 3, parts like or corresponding to those previously described with 
reference to FIG. 1 are denoted by the same reference characters and the 
description thereof will be omitted. 
An optical scanner 39 in a second embodiment according to the present 
invention has a scanning end detecting unit 16 mounted on a jig 40 for 
assembling the optical scanner 39. The clear signal input terminals 41 of 
counters 34.sub.1 to 34.sub.4 connected to the scanning end detecting unit 
16 can be connected through a delay circuit 42 to a scanning start 
detecting unit 12. 
In assembling the optical scanner 39, a corrected print control clock 
signal D is determined and registered. Thus, the optical scanner 39 
functions in the same operating mode as that of the optical scanner 31. 
After registering the corrected print control clock signal D, the scanning 
end detecting unit 16 is removed together with the jig 40 from the optical 
scanner 39 for use in assembling and adjusting another optical scanner 39, 
and the clear signal input terminals 41 of the counters 34.sub.1 to 
34.sub.4 are connected through the delay circuit 42 to the scanning start 
detecting unit 12. In operation, the optical scanner 39 starts the image 
scanning operation upon the detection of a laser beam by the scanning 
start detecting unit 12. At a moment where the scanning is completed, the 
output signal of the scanning start detecting unit 12 is applied through 
the delay circuit 42 to the clear signal input terminals of the counters 
34.sub.1 to 34.sub.4 to clear the counters 34.sub.1 to 34.sub.4. 
Since the scanning end detecting unit 16 mounted on the jig 40 is used for 
adjusting a plurality of optical scanners 39, the manufacturing cost of 
the optical scanners 39 is reduced, the appropriateness of the optical 
scanner 39 to mass production is enhanced, and the optical scanner 39 can 
be formed in a reduced size. 
Third Embodiment (FIG. 4) 
In FIG. 4, parts like or corresponding to those previously described with 
reference to FIG. 1 are denoted by the same reference characters and the 
description thereof will be omitted. 
An optical scanner 31 in a third embodiment according to the present 
invention has a scanning start detecting unit 12, a scanning end detecting 
unit 16 and a reference clock signal generator 18 are connected to a main 
counter 33 serving also as a frequency divider, and four digital counters 
34.sub.1 to 34.sub.4 capable of counting four place numbers and serving as 
scanning time measuring means included in an image information output unit 
32. The counters 34.sub.1 to 34.sub.4 and a reflecting surface identifying 
unit 35 which provides signals for identifying the reflecting surfaces of 
a polygonal rotating mirror 6 are connected to an count enable signal 
generator 36, which calculates a count enable signal, i.e., a corrected 
clock signal for increasing or decreasing the number of pulses of a print 
control clock signal B, and registers the calculated count enable signal. 
The count enable signal generator 36 is connected through a shift circuit 
44 connected to a random number generating circuit 43 to the main counter 
33. The count enable signal generator 36, the random number generating 
circuit 43 and the shift circuit 44 comprise a scanning length matching 
unit 45. The main counter 33 is connected through a print data circuit 46 
to a laser driver 47 for controlling an illuminating optical system 5. 
The operation of the optical scanner 31 for setting a corrected print 
control clock signal D will be described hereinafter. Suppose that the 
optical scanner 31 prints an image on a recording sheet of a size A4 in a 
dot density of 300 dots/in., and the optical scanner 31, similarly to the 
optical scanner 1, prints one dot every eight pulses of the reference 
clock signal A. Then, one scanning line corresponds to about 2600 dots, 
which corresponds to 20,800 pulses of the reference clock signal A. The 
counters 34.sub.1 to 34.sub.4 capable of counting four place numbers is 
able to count 65,536 pulses (16.sup.4 pulses). The reflecting surfaces 
7.sub.1 to 7.sub.6 are identified by detection signals provided by the 
reflecting surface identifying unit 35. The counters 34.sub.1 to 34.sub.4 
count the pulses of the reference clock signal A corresponding to values 
of scanning time proportional respectively to the lengths of scanning 
lines swept by laser beams reflected respectively by the reflecting 
surfaces 7.sub.1 to 7.sub.6. The optical scanner adjusts shorter scanning 
lines to the longest scanning line. For example, at the start of the the 
scanning operation, the optical scanner 31 calculates the difference 
between the number of pulses of the reference clock signal A for the 
longest scanning line and that of pulses of the reference clock signal A 
for each shorter scanning line, and the count enable signal generator 36 
inserts a predetermined number of count enable signals corresponding to 
the calculated result in a print control clock signal B for the reflecting 
surface 7 reflected a laser beam for the shorter scanning line. The random 
number circuit 43 and the shift circuit 44 regulate the timing of 
inserting the count enable signals so that the count enable signals are 
inserted at random to provide corrected print control clock signals 
D.sub.1 to D.sub.6 respectively for the reflecting surfaces 7.sub.1 to 
7.sub.6. The corrected print control clock signals D.sub.1 to D.sub.6 are 
registered, and then the optical scanner 31 is ready to perform the 
printing operation. 
The printing operation of the optical scanner will be described 
hereinafter. A laser beam emitted by the illuminating optical system 5 is 
reflected by the polygonal rotating mirror 6 and the reflected laser beam 
falls on the scanning start detecting unit 12. Then, the scanning start 
detecting unit 12 provides a scanning start signal S to start the image 
scanning operation. The scanning length matching unit 45 identifies the 
reflecting surface 7 which reflected the laser beam by the output signal 
of the reflecting surface identifying unit 35, and then the scanning 
length matching unit 45 gives a corrected print control clock signal D 
through the main counter 33 to the print data unit 46 in synchronism with 
the counting operation of the counters 34.sub.1 to 34.sub.4. A print 
timing shift included in the corrected print control clock signal D 
corresponds to one pulse of the reference clock signal A and the increment 
or decrement of the length of a dot 14 is on the order of one-eighth the 
length of the dot 14. At the end of the scanning line, the laser beam 
falls on the scanning end detecting unit 16 and, at the same time, the 
scanning end detecting unit 16 provides an output signal to clear the 
counters 34.sub.1 to 34.sub.4 for the subsequent scanning operation of the 
next reflecting surface of the polygonal rotating mirror 6. A latent image 
thus formed in a dot matrix on the photoconductive drum 9 is developed and 
a developed image is transferred to a recording sheet by an 
electrophotographic process. 
Similarly to the image 28 shown in FIG. 11, the image printed by using the 
corrected print control clock signals D has dots exactly aligned on a 
column corresponding to the scanning end positions of the laser beams and 
gaps are formed at random between the dots, so that no moire pattern is 
formed in the printed image. 
Although the optical scanner 31 has been described as an optical scanner 
which provides a corrected print control signal D for a shorter scanning 
line to adjust the shorter scanning line to the longest scanning line, a 
longer scanning line may be adjusted to the shortest scanning line. In the 
latter case, a signal to reduce the pulses of the print control clock 
signal B by a predetermined number of pulses is used. 
Fourth Embodiment (FIG. 5) 
An optical scanner 48 in a fourth embodiment according to the present 
invention is substantially the same in constitution as the optical scanner 
31 in the third embodiment, except that a ROM 49 is connected to a count 
enable signal generator 36, and the ROM 49, the count enable signal 
generator 36, a random number circuit 43 and a shift circuit 44 comprise a 
scanning length matching unit 50 serving also as scanning speed correcting 
means. The ROM 49 stores a scanning speed correcting clock E for inserting 
dots of different lengths to make intervals between dots on the 
photoconductive drum 9 apparently uniform on the basis of the scanning 
speed of a spot formed by a laser beam on the photoconductive drum 9 
measured beforehand. 
Similarly to the optical scanner 31, the optical scanner 48 forms each 
scanning line by dots including those of different lengths distributed at 
random, so that an image is formed clearly. Furthermore, since dots are 
arranged in each scanning line at apparently uniform intervals, the dot 
density with respect to the scanning direction is uniform.