Light beam scanning apparatus and image forming apparatus

A beam position detector output processor converts an output from a beam position detector into beam position information. A main control unit detects a total offset value of a plurality of operational amplifiers constituting the processor. The detected offset value is used to compensate a determination reference value used to control the sub-scanning position of a beam, or the beam position information obtained from the beam position detector output processor. As a result, the offsets of the operational amplifiers constituting the beam position detector output processor are compensated.

BACKGROUND OF THE INVENTION 
The present invention relates to a light beam scanning apparatus for 
simultaneously scanning and exposing a single photosensitive drum with a 
plurality of laser beams, thereby to form a single electrostatic latent 
image on the photosensitive drum, and an image forming apparatus such as a 
digital copying machine or a laser printer using the light beam scanning 
apparatus. 
In recent years, various digital copying machines have been developed in 
which image formation is performed by scanning and exposing with a laser 
beam and electronic photographing processing. 
More recently, in order to obtain higher image forming speed, developments 
have been made to a digital copying machine adopting a multi-beam method 
in which a plurality of laser beams are generated and scanning is 
simultaneously carried out for a plurality of scanning lines with use of a 
plurality of beams. 
This kind of digital copying machine which adopts such a multi-beam method 
comprises a plurality of laser oscillators for generating laser beams, a 
multi-face rotation mirror such as a polygon mirror for reflecting the 
laser beams generated by the plurality of laser oscillators toward a 
photosensitive drum to scan the photosensitive drum with the laser beams, 
and an optical unit serving as a light beam scanning device consisting 
mainly of a collimator lens and an f-.theta. lens. 
However, in the structure of a conventional optical unit, it is very 
difficult to obtain an ideal positional relationship between a plurality 
of light beams on a photosensitive drum (or a surface to be scanned). In 
order to obtain an ideal positional relationship, respective components as 
well as assembling thereof require high accuracy, and hence the cost of 
the device is increased. 
Even if an ideal positional relationship is obtained, the shape of a lens 
may vary slightly or the positional relationship between respective 
components may vary slightly due to circumferential changes, such as 
changes in temperature and humidity or time-based changes. Consequently, 
the positional relationship between light beams varies, and as a result a 
high quality image cannot be formed. Therefore, to construct this kind of 
optical system, it is necessary to adopt a structure and components which 
are strong against changes as described above. 
In the following, defects in the multi-beam method, which are caused when 
an image is formed with light beams whose passing positions are 
erroneously dislocated, will be explained with reference to FIGS. 48A and 
48B and FIGS. 49A and 49B. 
For example, in a case where character "T" shown in FIG. 48A is formed, an 
image as shown in FIG. 48B is formed when a passing position of a light 
beam is erroneously dislocated from a predetermined position. In the 
example of this figure, the passing position of a light beam b is shifted 
from its predetermined position so that the distance between light beams a 
and b is reduced while the distance between light beams b and c is 
increased, among four light beams a to d used. 
FIG. 49A shows an example of an image in which emission timings of 
respective light beams are not controlled correctly. As is apparent from 
this figure, if the emission timings of respective light beams are not 
controlled correctly, the image forming position in the main scanning 
direction is dislocated so that a longitudinal line cannot be formed 
straight. 
FIG. 49B shows an image in which neither the passing positions of light 
beams nor the emission timings of light beams are controlled correctly, 
defects in an image appear both in the main scanning direction and in the 
direction perpendicular to the main scanning direction, i.e., the 
sub-scanning direction. 
Thus, when an image is formed in a multi-beam method, a beam position 
detector for detecting the passing positions of a plurality of light beams 
must be provided at high accuracy to control light beam passing positions 
in the sub-scanning direction so as to be arranged at predetermined 
intervals, and to control the emission timings of respective light beams 
so as to align the image forming position in the main scanning direction. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a light beam scanning 
apparatus and an image forming apparatus in which the position of a light 
beam on a surface to be scanned can be properly controlled with high 
precision at all times, thereby always maintaining high image quality. 
It is another object of the present invention to provide a light beam 
scanning apparatus and an image forming apparatus in which the positional 
relationship between a plurality of light beams on a surface to be scanned 
can always be controlled to be ideal with high precision, thereby 
maintaining high image quality at all times. 
To achieve the above objects, according to the present invention, there is 
provided a light scanning apparatus comprising a scanning unit for 
deflecting a light beam generated by a laser oscillator to scan a surface 
to be scanned, a beam position detection unit for detecting the light beam 
with a plurality of light detection elements laid out in a sub-scanning 
direction of the light beam, and supplying a position signal corresponding 
to a passing position of the light beam in the sub-scanning direction, 
first and second beam passage detection units which are arranged in an 
area including areas on two sides of the beam position detection unit in a 
main scanning direction, and detect passage of the light beam made to scan 
by the scanning unit, and supply a beam passage detection signal, a signal 
processing unit which includes an integrator that starts/ends integration 
of the position signal from the beam position detection unit in response 
to the beam passage detection signal output from the first and second beam 
passage detection units, supplies an integrated value of the position 
signal, and has an offset value, a first control unit for controlling the 
scanning unit so as to make the light beam scan an undetectable region of 
the beam position detection unit during an offset compensation period of 
the signal processing unit, an offset determination unit for determining 
the integrated value supplied from the signal processing unit as an offset 
value corresponding to an offset voltage, and storing the offset value 
after the scanning unit controlled by the first control unit makes the 
light beam scan over the beam passage detection unit during the offset 
compensation period, and a second control unit which has a compensation 
unit for compensating the offset voltage of the signal processing unit by 
using the offset value stored in the offset determination unit, compares 
the integrated value from the signal processing unit with a preset 
reference value during position control in the sub-scanning direction by 
the scanning unit, and controls the scanning unit on the basis of a 
comparison result so as to set the passing position of the light beam in 
the sub-scanning direction to a proper position. 
The compensation unit compensates the offset voltage of the signal 
processing unit by changing the set reference value in accordance with the 
offset value. 
The compensation unit compensates the offset voltage of the signal 
processing unit by changing the integrated value from the signal 
processing unit in accordance with the stored offset value. 
The integrator of the signal processing unit comprises an operational 
amplifier, the apparatus comprises a D/A converter for applying a 
reference voltage to one input of the operational amplifier, and the 
compensation unit has a unit for transferring the stored offset value to 
the D/A converter, thereby compensating the offset voltage of the signal 
processing unit. 
According to the present invention, there is provided a light beam scanning 
apparatus comprising a scanning unit for deflecting a light beam generated 
by a laser oscillator to scan a surface to be scanned, a beam position 
detection unit for detecting the light beam made to scan by the scanning 
unit with a plurality of light detection elements laid out in a 
sub-scanning direction of the light beam, and supplying a position signal 
corresponding to a passing position of the light beam in the sub-scanning 
direction, first and second beam passage detection units which are 
arranged in an area including areas on two sides of the beam position 
detection unit in a main scanning direction, detect passage of the light 
beam made to scan by the scanning unit, and supply a first beam passage 
detection signal, third and fourth beam passage detection units which are 
arranged on one side near the beam position detection unit in the main 
scanning direction, detect passage of the light beam made to scan by the 
scanning unit, and supply a second beam passage signal, a selection unit 
for selecting one of the first and third beam passage detection units and 
one of the second and fourth detection units, and supplying beam passage 
signals output from the selected beam passage detection units, a first 
control unit for controlling the selection unit so as to select the first 
and second beam passage detection units during sub-scanning position 
control by the scanning unit and to select the third and fourth beam 
passage detection units during an offset compensation period of a signal 
processing unit, the signal processing unit which includes an integrator 
(42) that starts/ends integration of the position signal from the beam 
position detection unit in response to the beam passage detection signal 
output from the selection unit, supplies an integrated value of the 
position signal, and has an offset value, a second control unit for 
controlling the light generation unit so as to generate the light beam 
only when the scanning unit scans over the third and fourth beam passage 
detection units during the offset compensation period, an offset 
determination unit for determining the integrated value supplied from the 
signal processing unit as an offset value corresponding to an offset 
voltage, and storing the offset value after the light beam from the light 
generation unit controlled by the second control unit is made to scan over 
the third and fourth beam passage detection units during the offset 
compensation period, and a third control unit which has a compensation 
unit for compensating the offset voltage of the signal processing unit by 
using the offset value stored in the offset determination unit, compares 
the integrated value from the signal processing unit with a preset 
reference value during sub-scanning position control by the scanning unit, 
and controls the scanning unit on the basis of a comparison result so as 
to set the passing position of the light beam in the sub-scanning 
direction to a proper position. 
According to the present invention, there is provided a light beam scanning 
apparatus comprising a scanning unit for deflecting a light beam generated 
by a beam oscillator to scan a surface to be scanned, a beam position 
detection unit for detecting the light beam made to scan by the scanning 
unit with a plurality of light detection elements laid out in a 
sub-scanning direction of the light beam, and supplying a position signal 
corresponding to a passing position of the light beam in the sub-scanning 
direction, first and second beam passage detection units which are 
arranged in areas on two sides of the beam position detection unit in a 
main scanning direction, detect passage of the light beam made to scan by 
the scanning unit, and supply a beam passage signal, a signal processing 
unit which includes an integrator that starts/ends integration of the 
position signal from the beam position detection unit in response to the 
beam passage signal output from the first and second beam passage 
detection units, supplies an integrated value of the position signal, and 
has an offset value, a first control unit for controlling the light 
generation unit so as to stop generating the light beam in response to the 
beam passage signal from the first beam passage detection unit during an 
offset compensation period of the signal processing unit, a timer unit for 
measuring a predetermined time interval in response to the beam passage 
signal from the first beam passage detection unit (S1) during the offset 
compensation period, and supplying a measurement completion signal after 
lapse of the predetermined time interval, the signal processing unit 
stopping an integration operation in response to the measurement 
completion signal, and supplying the integrated value of the position 
signal, an offset determination unit for determining the integrated value 
supplied from the signal processing unit as an offset value corresponding 
to an offset voltage, and storing the offset value during the offset 
compensation period, and a second control unit which has a compensation 
unit for compensating the offset voltage of the signal processing unit by 
using the offset value stored in the offset determination unit, compares 
the integrated value from the signal processing unit with a preset 
reference value during sub-scanning position control by the scanning unit, 
and controls the scanning unit on the basis of a comparison result so as 
to set the passing position of the light beam in the sub-scanning 
direction to a proper position. 
According to the present invention, there is provided a light beam scanning 
apparatus comprising a light generation unit for generating a light beam, 
a scanning unit for deflecting the light beam generated by the light 
generation unit to make the light beam to scan a surface to be scanned, a 
beam position detection unit for detecting the light beam made to scan by 
the scanning unit with a plurality of light detection elements laid out in 
a sub-scanning direction of the light beam, and supplying a position 
signal corresponding to a passing position of the light beam in the 
sub-scanning direction, first and second beam passage detection units 
which are arranged in areas on two sides of the beam position detection 
unit in a main scanning direction, detect passage of the light beam made 
to scan by the scanning unit, and supply a beam passage detection signal, 
a signal processing unit which includes an integrator that starts/ends 
integration of the position signal from the beam position detection unit 
in response to the beam passage detection signal output from the first and 
second beam passage detection units, supplies an integrated value of the 
position signal, and has an offset value, a shielding unit for shielding 
the plurality of light detection elements of the beam position detection 
unit from external light with a shielding plate, a first control unit for 
controlling the shielding unit so as to cause the shielding plate to cover 
the plurality of light detection elements during an offset compensation 
period of the signal processing unit, an offset determination unit for 
determining the integrated value supplied from the signal processing unit 
as an offset value corresponding to an offset voltage, and storing the 
offset value after the light beam is made to scan over the first and 
second beam passage detection units during the offset compensation period, 
and a second control unit which has a compensation unit for compensating 
the offset voltage of the signal processing unit by using the offset value 
stored in the offset determination unit, compares the integrated value 
from the signal processing unit with a preset reference value during 
position control in the sub-scanning direction by the scanning unit, and 
controls the scanning unit on the basis of a comparison result so as to 
set the passing position of the light beam in the sub-scanning direction 
to a proper position. 
According to the present invention, there is provided a light beam scanning 
apparatus comprising a scanning unit for deflecting a light beam generated 
by a laser oscillator to scan a surface to be scanned, a beam position 
detection unit for detecting the light beam made to scan by the scanning 
unit with a plurality of light detection elements laid out in a 
sub-scanning direction of the light beam, and supplying a position signal 
corresponding to a passing position of the light beam in the sub-scanning 
direction, first and second beam passage detection units which are 
arranged in areas on two sides of the beam position detection unit in a 
main scanning direction, detect passage of the light beam made to scan by 
the scanning unit, and supply a beam passage signal, a signal processing 
unit which includes an integrator that starts/ends integration of the 
position signal from the beam position detection unit in response to the 
beam passage signal output from the first and second beam passage 
detection units, supplies an integrated value of the position signal, and 
has an offset value, a timer unit for measuring a predetermined time 
interval, and supplying a measurement completion signal after lapse of the 
predetermined time interval, a first control unit for controlling the 
timer unit and the integrator so as to perform integration by the 
integrator for only the predetermined time interval during an offset 
compensation period of the signal processing unit, the signal processing 
unit supplying an integrated value of the position signal after lapse of 
the period, an offset determination unit for determining the integrated 
value supplied from the signal processing unit as an offset value 
corresponding to an offset voltage, and storing the offset value during 
the offset compensation period, and a second control unit which has a 
compensation unit for compensating the offset voltage of the signal 
processing unit by using the offset value stored in the offset 
determination unit, compares the integrated value from the signal 
processing unit with a preset reference value during sub-scanning position 
control by the scanning unit, and controls the scanning unit on the basis 
of a comparison result so as to set the passing position of the light beam 
in the sub-scanning direction to a proper position. 
In addition, according to the present invention, there is provided an image 
forming apparatus comprising: a light scanning apparatus including: (a) a 
plurality of light generation means for simultaneously generating light 
beams; (b) scanning means for deflecting the light beams generated by the 
light generation means to make the light beams scan a surface to be 
scanned; (c) beam position detection means for detecting the light beams 
made to scan by the scanning means with a plurality of light detection 
elements laid out in a sub-scanning direction of the light beams, and 
supplying position signals corresponding to passing positions in the 
sub-scanning direction of the light beams; (d) first and second beam 
passage detection means, arranged in an area including areas on two sides 
of the beam position detection means in a main scanning direction, for 
detecting passage of the light beams made to scan by the scanning means, 
and supplying beam passage detection signals; (e) signal processing means 
which includes an integrator that starts/ends integration of the position 
signals from the beam position detection means in response to the beam 
passage detection signals output from the first and second beam passage 
detection means, supplies an integrated value of the position signals, and 
has an offset value; (f) first control means for controlling the scanning 
means so as to make the light beams to scan undetectable regions of the 
beam position detection means during an offset compensation period of the 
signal processing means; (g) offset determination means for determining 
the integrated value supplied from the signal processing means as an 
offset value corresponding to an offset voltage, and storing the offset 
value after the scanning means controlled by the first control means makes 
the light beams to scan over the beam passage detection means during the 
offset compensation period; and (h) second control means, having 
compensation means for compensating the offset voltage of the signal 
processing means by using the offset value stored in the offset 
determination means, for comparing the integrated value from the signal 
processing means with a preset reference value during position control in 
the sub-scanning direction by the scanning means, and controlling the 
scanning means on the basis of a comparison result so as to set the 
passing position of the light beam in the sub-scanning direction to a 
proper position; developing means for developing an electrostatic latent 
image formed on an image carrier; and transfer means for transferring the 
image developed by the developing means onto a paper sheet. 
Additional object and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The object 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the present invention will now be described with reference 
to the accompanying drawings. 
FIG. 1 shows the structure of a digital copying machine as an image forming 
apparatus which adopts a light beam scanning device according to an 
embodiment of the present invention. Specifically, this digital copying 
machine comprises a scanner unit 1 or image reading means and a printer 
unit 2 or image forming means. The scanner unit 1 comprises a first 
carriage 3 and a second carriage 4 which are movable in the arrow 
direction in the figure, an imaging lens 5, and a photoelectric conversion 
element 6. 
In FIG. 1, an original O is placed on an original mount 7 made of 
transparent glass such that the original faces downwards. The original O 
aligned with a mount index which is the center of the shorter side of the 
original mount 7 in the right-hand side in the figure is pressed against 
the original mount 7 by an openable fixing cover 8. 
The original O is illuminated by a light source 9 and the reflection light 
therefrom is converged onto a light receiving surface of the photoelectric 
conversion element 6, by mirrors 10, 11 and 12 and the imaging lens 5. The 
first carriage 3 equipped with the light source 9 and the mirror 10 and 
the second carriage 4 equipped with the mirrors 11 and 12 are moved at a 
relative speed of 2:1 such that the length of the light path is maintained 
to be constant. The first and second carriages 3 and 4 are moved by a 
carriage drive motor (not shown) from the right-hand side to the left-hand 
side in synchronization with a read timing signal. 
As has been described above, an image of the original O placed on the 
original mount 7 is sequentially read in units of a line by the scanner 
unit 1. An output obtained by thus reading the image is converted to 8-bit 
digital image signals representing gradation of the image by an image 
processing unit (not shown). 
The printer unit 2 comprises an optical unit 13 and an image forming unit 
14 adopting an electronic photographing method in which an image can be 
formed on a paper sheet P as a medium on which an image is formed. 
Specifically, image signals read out from the original O by the scanner 
unit 1 are processed by the image processing unit (not shown), and 
thereafter are converted into laser beams (hereinafter referred to as 
simply "light beams") from semiconductor laser oscillators. This 
embodiment employs a multi-beam optical system using a plurality of (two 
or more) semiconductor laser oscillators. 
Although the structure of the optical unit 13 will e specifically described 
later, a plurality of semiconductor laser oscillators provided in the unit 
carry out emission operation in accordance with laser modulation signals 
output from the image processing unit (not shown). The light beams output 
from the oscillators are reflected by a polygon mirror to form scanning 
light beams which are output to the outside of the unit. 
A plurality of light beams output from the optical unit 13 are imaged as 
spotted scanning light beams having a resolution necessary for an exposure 
position X on a photosensitive drum 15 as an image carrying member, and 
thus scanning and exposing are performed. As a result of this, an 
electrostatic latent image is formed on the photosensitive drum 15 in 
accordance with image signals. 
In the periphery of the photosensitive drum 15, there are provided an 
electric charger 16 for electrically charging the surface of the drum, a 
developer device 17, a transfer charger 18, a separation charger 19, a 
cleaner 20, and the like. The photosensitive drum 15 is rotated at a 
predetermined circumferential speed by a drive motor (not shown), and is 
electrically charged by the electric charger 16 provided so as to face the 
surface of the drum. A plurality of light beams (or scanning light beams) 
are spotted on an exposure position X on the charged photosensitive drum 
15, thereby forming an image. 
An electrostatic latent image formed on the photosensitive drum 15 is 
developed with toner (or developer) supplied from the developer device 17. 
A toner image formed on the photosensitive drum 15 by developing is 
transferred by the transfer charger 18 at a transfer position onto a paper 
sheet P fed from a sheet supply system at a certain timing. 
The sheet supply system sequentially supplies paper sheets P in a sheet 
supply cassette 21 provided at a bottom position, while the sheets P are 
being separated from one another by a sheet supply roller 22 and a 
separation roller 23. The sheet P is sent to a resist roller 24, and fed 
to a transfer position at a predetermined timing. In the downstream side 
of the transfer charger 18, there are provided a sheet conveyer mechanism 
25, a fixing device 26, and delivery rollers 27 for discharging the sheets 
P with the formed images. Therefore, a paper sheet P on which a toner 
image has been transferred is fed out onto an external sheet supply tray 
28 through the delivery rollers 27, after the toner image is fixed by the 
fixing device 26. 
A cleaner 20 removes toner remaining on the surface of the photosensitive 
drum 15 from which a toner image has been transferred onto a paper sheet 
P, and the drum thereby recovers an initial condition in a standby 
condition. 
Image forming operation is continuously performed by repeating the 
processing operation as described above. 
As has been described above, data is read out from the original O placed on 
the original mount 7 by the scanner unit 1, and the data thus read is 
subjected to a series of processing at the printer unit 2. Thereafter, the 
data is recorded as a toner image on the paper sheet P. 
The optical unit 13 will now be described. 
FIG. 2 shows the structure of the optical unit 13 and the positional 
relationship between the unit 13 and the photosensitive drum 15. The 
optical unit 13 includes, for example, four semiconductor laser 
oscillators 31a, 31b, 31c, and 31d or light beam generator means, and each 
of the oscillators 31a to 31d performs image formation for one scanning 
line at the same time, so that high speed image formation is realized 
without extremely increasing the rotation speed of the polygon mirror. 
The laser oscillator 31a is driven by a laser driver 32a. A light beam 
output therefrom passes through a collimator lens (not shown) and 
thereafter strikes on a galvanomirror 33a or light beam path changing 
means. The light beam reflected by the galvanomirror 33a passes through 
half-mirrors 34a and 34b and falls on a polygon mirror 35 serving as a 
rotational polygonal mirror. 
The polygon mirror 35 is rotated at a constant speed by a polygon motor 36 
driven by a polygon motor driver 37. In this manner, the light beam 
reflected by the polygon mirror 35 swings such that scanning is performed 
in a constant direction at an angle speed depending on the rotation speed 
of the polygon motor 36. 
The scanning light beam swung by the polygon mirror 35 passes through an 
f-.theta. lens (not shown), thereby scanning the light receiving surface 
of a beam position detector 38, which serves as beam position detector 
means, and the photosensitive drum 15, owing to the f-.theta. 
characteristics of the f-.theta. lens. 
The laser oscillator 31b is driven by a laser driver 32b. A light beam 
output therefrom passes through a collimator lens (not shown) and is 
thereafter reflected by a galvanomirror 33b and further by the half-mirror 
34a. The light beam reflected by the half-mirror 34a passes through the 
half-mirror 34b and strikes on the polygon mirror 35. The route of the 
light beam after the polygon mirror 35 is the same as that of the beam 
from the laser oscillator 31a, i.e., the light beam passes through an 
f-.theta. lens (not shown), thereby scanning the light receiving surface 
of the beam position detector 38 and the photosensitive drum 15. 
The laser oscillator 31c is driven by a laser driver 32c. A light beam 
output therefrom passes through a collimator lens (not shown) and is 
thereafter reflected by a galvanomirror 33c. The light beam further passes 
through a half-mirror 34c, is reflected by the half-mirror 34b, and then 
falls on the polygon mirror 35. The route of the light beam after the 
polygon mirror 35 is the same as that of the beam of the laser oscillators 
31a and 31b, i.e., the light beam passes through an f-.theta. (not shown), 
thereby scanning the light receiving surface of the beam position detector 
38 and the photosensitive drum 15. 
The laser oscillator 31d is driven by a laser driver 32d. A light beam 
output therefrom passes through a collimator lens (not shown) and is 
thereafter reflected by a galvanomirror 33d. The light beam is further 
reflected by the half-mirrors 34c and 34b, and falls on the polygon mirror 
35. The route of the light beam after the polygon mirror 35 is the same as 
that of the beams of the laser oscillators 31a, 31b, and 31c, i.e., the 
light beam passes through an f-.theta. lens (not shown), thereby scanning 
the light receiving surface of the beam position detector 38 and the 
photosensitive drum 15. 
Thus, light beams output from the individual laser oscillators 31a, 31b, 
31c, and 31d are synthesized by the half-mirrors 34a, 34b, and 34c so that 
four light beams travel in the direction toward the polygon mirror 35. 
Therefore, the photosensitive drum 15 can be simultaneously scanned with 
four light beams so that an image can be recorded at a speed four times 
higher than in a conventional single light beam on condition that the 
polygon mirror 35 is rotated at an equal rotation speed. 
The galvanomirrors 33a, 33b, 33c, and 33d are used to adjust (or control) 
the positional relationship between light beams in the sub-scanning 
direction, and are respectively connected to galvanomirror drive circuits 
39a, 39b, 39c, and 39d for driving corresponding galvanomirrors. 
The beam position detector 38 serves to detect passing positions and 
passing timings of four light beams, and is provided near an end portion 
of the photosensitive drum 15 such that the light receiving surface of the 
detector 38 is situated at a level equal to the surface of the 
photosensitive drum 15. On the basis of a detection signal from the beam 
position detector 38, control of the galvanomirrors 33a, 33b, 33c, and 33d 
(i.e., control of image forming positions in the sub-scanning direction), 
control of emission power (i.e., intensity) of the laser oscillators 31a, 
31b, 31c, and 31d, as well as control of emission timings (i.e., control 
of image forming positions in the main scanning direction) are 
respectively performed in accordance with the light beams, although the 
details of the control will be described later. The beam position detector 
38 is connected to a beam detector output processor 40, in order to 
generate signals for performing the control as described above. 
The beam position detector 38 will now be described. 
FIG.3 shows the relationship between the structure of the beam position 
detector 38 and the scanning direction of the light beams. Light beams a 
to d from the four laser oscillators 31a to 31d are made to scan from the 
left to the right by the rotation of the polygon mirror 35, thus crossing 
the beam sensor 38. 
The beam position detector 38 has two sensor patterns S1 and S2 serving as 
elongated light beam detectors, as well as seven sensor (light reception) 
patterns SA, SB, SC, SD, SE, SF and SG serving as light beam detectors 
which are interposed between the patterns S1 and S2. The sensor patterns 
S1, S2, and SA to SG are constituted by, e.g., photodiodes. 
The sensor pattern S1 is a pattern for detecting passage of a light beam to 
generate a reset signal (integration start signal) for an integrator (to 
be described later), and the sensor pattern S2 is a pattern for similarly 
detecting passage a light beam to generate a conversion start signal for 
an A/D converter (to be described later). The sensor patterns SA to SG are 
patterns for detecting the passing positions of light beams. 
As shown in FIG. 3, the sensor patterns S1 and S2 are formed to be longer 
in a direction perpendicular to the scanning direction of a light beam 
such that the light beams a to d made to scan by the polygon mirror 35 
always cross the sensor patterns S1 and S2. For example, in this 
embodiment, widths W1 and W3 of the sensor patterns S1 and S2 in the 
scanning direction of a light beam are 200 .mu.m, while their length L1 in 
the direction perpendicular to the scanning direction of a light beam is 
2,000 .mu.m. 
The sensor patterns SA to SG, as shown in FIG. 3, are arranged between the 
sensor patterns S1 and S2 in a stacking fashion in the sub-scanning 
direction of a light beam. The arrangement lengths of the sensor patterns 
SA to SG are equal to the length L1 of the sensor patterns S1 and S2. A 
width W2 of the sensor patterns SA to SG in the scanning direction of a 
light beam is, e.g., 600 .mu.m. 
FIG. 4 is an enlarged view showing the pattern shapes of the sensor 
patterns SA to SG of the beam position detector 38. 
The sensor patterns SB to SF have rectangular pattern shapes of, e.g., 32.3 
.mu.m.times.600 .mu.m, and small gaps G of about 10 .mu.m are provided in 
the sub-scanning direction of a light beam. Accordingly, the alignment 
pitch between the gaps is 42.3 .mu.m. In addition, the sensor patterns SA 
and SB and the sensor patterns SF and SG are also arranged to have gaps of 
about 10 .mu.m therebetween. The widths of the sensor patterns SA and SG 
in the sub-scanning direction of a light beam are set larger than those of 
the sensor patterns SB to SF. 
A control operation using the output of the beam position detector 38 will 
be described later in detail. It should be noted, however, that the gaps 
provided at the pitch of 42.3 .mu.m serve as targets for setting the 
passing positions of beams a, b, c, and d at a predetermined pitch (42.3 
.mu.m in this embodiment). Specifically, the gap G (B-C) defined by the 
sensor patterns SB and SC serves as the target for the passing position of 
the beam a; the gap G (C-D) defined by the sensor patterns SC and SD 
serves as the target for the passing position of the beam b; the gap G 
(D-E) defined by the sensor patterns SD and SC serves as the target for 
the passing position of the beam c; and the gap G (E-F) defined by the 
sensor patterns SE and SF serves as the target for the passing position of 
the beam d. 
The features of the beam position detector 38 having the above-described 
sensor patterns will now be described with reference to FIGS. 5A and 5B. 
As has been described above, the beam position detector 38 is situated near 
the end portion of the photosensitive drum 15 or at the position where the 
length of an obtained light path is equal to the distance from the polygon 
mirror 35 to the photosensitive drum 15, such that the light reception 
surface of the detector 38 is situated on an axis extended from the 
surface of the photosensitive drum 15. In order to exactly detect the 
passing positions of beams by means of the beam position detector 38 
situated in this manner, it is necessary that the above-described sensor 
patterns are aligned perpendicular or parallel to the beam passing 
direction. However, the beam position detector 38 is actually mounted at 
some inclination to the ideal position. 
To solve this problem, in the beam position detector 38 of the present 
invention, the sensor a patterns are arranged such that detection points 
for detecting the beam passing positions are aligned in the beam passing 
direction. Thereby, even if the beam position detector 38 is mounted at 
some inclination, the error in detection pitch can be limited to a 
minimum. 
Furthermore, an integrator is added to a circuit for processing the output 
of the beam position detector 38, though this will be described later in 
detail. Thus, even if the beam position detector 38 is inclined, the 
influence on the detection result of the beam passing positions can be 
limited to a minimum. 
The length of each of the sensor patterns SA and SG in the beam 
sub-scanning direction is much greater than that of each of the sensor 
patterns SB to SF. In this embodiment, the length of each of the sensor 
patterns SA and SG in the sub-scanning direction is more than 20 times 
greater than the pitch (42.3 .mu.m) of the sensor patterns SB to SF. The 
reason is that the sensor patterns SB to SF are used to set the beam 
passing position within 1 .mu.m from the target position while the sensor 
patterns SA and SG are used to generally determine the beam passing 
position. 
FIG. 5A shows the relationship between the sensor patterns SA to SG and the 
scan positions of beams a to d in the case where the beam position 
detector 38 of the present invention is situated with an inclination to 
the beam scanning direction. In FIG. 5A, however, this relationship is 
shown as if the scanning direction of beams a to d is inclined with 
respect to the beam position detector 38. The scanning lines of beams a to 
d in FIG. 5A are controlled at ideal intervals (pitch=42.3 .mu.m). 
Control target points (indicated by circles in white) are set among the 
sensor patterns SA to SG. The target points, as will be described later in 
detail, remain at the centers among the patterns even when the beams a to 
d are made incident at an angle, by virtue of the advantages of 
integrators. 
As is clear from FIG. 5A, the loci of scanning lines controlled at ideal 
intervals (pitch=42.3 .mu.m) pass through substantially the central points 
of control targets on the sensor patterns SA to SG. In other words, even 
when the beam position detector 38 is mounted at a small angle, the 
influence on the detection precision is very small. 
For example, when the beam position detector 38 is mounted at an angle of 
5.degree. to the beam scanning lines, the beam scan position pitch, which 
is normally to be set at 42.3 .mu.m, is set at 42.14 .mu.m in 
consideration of a detection error of the beam position detector 38 due to 
the inclination. In this case, the error is about 0.16 .mu.m (0.03%). If 
the pitch is controlled in this manner, the influence on the image quality 
is very small. Although not described in detail, the value of the pitch 
can be easily calculated by using a trigonometric function. 
If the sensor patterns SA to SG of the beam position detector 38 of the 
present invention is used, the beam scan positions can be exactly detected 
even if the beam position detector 38 is mounted with a slight 
inclination. 
FIG. 5B shows an example of the conventional sensor patterns of a beam 
position detector 80 for achieving the same function as the beam position 
detector 38 of the present invention. 
In the case where such conventional sensor patterns are adopted, the beam 
passing positions cannot exactly be detected if the sensor patterns are 
slightly inclined with respect to the scanning directions of beams a to d. 
The reason is that the sensor patterns (S3*, S4*, S5*, S6*: *=a and b) for 
detecting the passing positions of beams a to d are arranged at a distance 
in the beam scanning direction. The greater the distance between the 
sensor patterns in the beam scanning direction, the greater the detection 
error for a sensor pattern. 
FIG. 5B, like FIG. 5A, shows loci of scanning lines controlled at ideal 
intervals (pitch=42.3 .mu.m), on the assumption that the beam position 
detector 80 is mounted with an inclination. As is obvious from FIG. 5B, 
the conventional beam position detector 80 requires much higher precision 
in mounting position than the beam position detector 38 of the present 
invention as shown in FIG. 5A. 
Suppose that the beam position detector 80 shown in FIG. 5B, like the beam 
position detector 38 shown in FIG. SA, is mounted with an inclination of 
5.degree. and the distance between the sensor patterns S3a and S3b, on the 
one hand, and the sensor patterns S6a and S6b, on the other, is 900 .mu.m. 
In this case, the control target point of the beam d is dislocated from 
the ideal position by 78.34 .mu.m This value is much greater than the 
target control pitch of 42.3 .mu.m and adversely affects the image quality 
very seriously. In the case where the beam position detector 80 is used, 
very high sensor mounting precision is required at least with respect to 
the inclination to the beam scanning direction. 
In order to solve this problem in the prior art, it is necessary to reduce 
the sensor pattern width W in the beam scanning direction as much as 
possible and locate the beam passing position detection points as close as 
possible in the beam scanning direction, even though the sensitivity of 
the sensor is sacrificed to some extent. Furthermore, in order to 
compensate the deficiency in sensitivity of the sensor, it is necessary to 
increase the power of the laser oscillator or decrease the rotation speed 
of the polygon motor at the time of detecting the beam passing positions. 
The control system will now be described. 
FIG. 6 shows the structure of the control system mainly for control of the 
multi-beam optical system. A main control unit 51 performs the general 
control of the optical unit and comprises, for example, a CPU. The main 
control unit 51 is connected to a memory 52, a control panel 53, an 
external communication interface (I/F) 54, the laser drivers 32a, 32b, 
32c, and 32d, the polygon mirror motor driver 37, the galvanomirror drive 
circuits 39a, 39b, 39c, and 39d, the beam detector output processor 40, a 
sync circuit 55 and an image data interface (I/F) 56. 
The sync circuit 55 is connected to the image data I/F 56. The image data 
I/F 56 is connected to an image processor 57 and a page memory 58. The 
image processor 57 is connected to the scanner unit 1. The page memory 58 
is connected to an external interface (I/F) 59. 
The flow of image data at the image formation will now be described in 
brief. 
In the copying operation mode, as described above, the image on the 
original O set on the original mount 7 is read by the scanner unit 1 and 
sent to the image processor 57. In the image processor 57, the image 
signal from the scanner unit 1 is subjected to, for example, well-known 
shading correction, filtering processes, gray scale processing, or gamma 
correction. 
The image data from the image processor 57 is delivered to the image data 
I/F 56. The image data I/F 56 distributes the image data to the four laser 
drivers 32a, 32b, 32c, and 32d. The sync circuit 55 generates clocks in 
synchronism with the timing of the light beams passing over the beam 
position detector 38. In synchronism with the clocks, the image data is 
supplied from the image data I/F 56 as laser modulation signals to the 
laser drivers 32a, 32b, 32c, and 32d. Since the image data is transferred 
in synchronism with the scanning of the beams, the image formation can be 
performed in correct synchronism (at correct positions) in the main 
scanning direction. 
The sync circuit 55 includes a sample timer and a logic circuit. The sample 
timer controls the power of each beam by forcibly activating the laser 
oscillators 31a, 31b, 31c, and 31d in non-image areas. The logic circuit 
activates the laser oscillators 31a, 31b, 31c, and 31d on the beam 
position detector 38 according to the order of the beams, thereby to 
establish the image formation timing of the beams. 
The control panel 53 is a man-machine interface for starting the copying 
operation, setting the number of copies, and the like. 
This digital copying machine can perform not only the copying operation but 
also the image formation of image data input from the outside via the 
external I/F 59 connected to the page memory 58. The image data input from 
the external I/F 59 is temporarily stored in the page memory 58 and then 
delivered to the sync circuit 55 via the image data I/F 56. 
When the digital copying machine is externally controlled by means of a 
network or the like, the external communication I/F 54 serves as the 
control panel 53. 
The galvanomirror drive circuits 39a, 39b, 39c, and 39d drive the 
galvanomirrors 33a, 33b, 33c, and 33d in accordance with instruction 
values generated from the main control unit 51. Thus, the main control 
unit 51 can freely control the angles of the galvanomirrors 33a, 33b, 33c, 
and 33d by using the galvanomirror drive circuits 39a, 39b, 39c, and 39d. 
The polygon motor driver 37 drives the polygon motor 36 for rotating the 
polygon mirror 35 for causing the above-mentioned four light beams to 
scan. The main control unit 51 can instruct the polygon motor driver 37 to 
start/stop and switch the rotation speed of the polygon motor driver 37. 
The rotation speed of the polygon motor 36 is made lower than a 
predetermined value, where necessary, in order to confirm the passing 
positions of the beams by means of the beam position detector 38. 
The laser drivers 32a, 32b, 32c, and 32d, as mentioned above, produce laser 
beams in accordance with the laser modulation signals synchronized with 
the beam scanning generated from the sync circuit 55. In addition, the 
laser drivers 32a, 32b, 32c, and 32d, upon receiving forcible emission 
signals from the main control unit 51, activate the laser oscillators 31a, 
31b, 31c, and 31d, irrespective of the image data. 
The main control unit 51 sets, by means of the laser drivers 32a, 32b, 32c, 
and 32d, the light emission power of the laser oscillators 31a, 31b, 31c, 
and 31d. The light emission power is varied in accordance with a change in 
process conditions or detection results of the beam passing positions. 
The memory 52 stores information necessary for the control operations. For 
example, the memory 52 stores data on the control of the galvanomirrors 
33a, 33b, 33c, and 33d and on the order of incoming light beams. Thereby, 
the optical unit 13 can be immediately set in the image formation state, 
after the power is turned on. 
The beam passing (scanning) position control will now be described in 
detail. 
FIG. 7 is a block diagram for explaining beam passing position control and 
offset detection/compensation processing (to be described later). FIG. 7 
shows in detail a portion associated with beam control which is extracted 
from a block diagram in FIG. 6. 
As described above, pulse signals indicating passage of light beams are 
output from the sensor patterns S1 and S2 of the beam position detector 
38. Independent signals are output from the sensor patterns SA to SG in 
accordance with the beam passing positions. 
Of the sensor patterns SA to SG, the sensor patterns SA and SG respectively 
supply output signals to amplifiers 61 and 62 (to be also referred to as 
amplifiers A and G hereinafter). The amplifier factors of the amplifiers 
61 and 62 are set by the main control unit 51 constituted by the CPU. 
Of the sensor patterns SA to SG, the sensor patterns SB to SF respectively 
supply output signals to differential amplifiers 63 to 66 (to be also 
referred to as differential amplifiers B-C, C-D, D-E, and E-F hereinafter) 
each for amplifying a difference between output signals from adjacent ones 
of the sensor patterns SB to SF. The differential amplifier 63 amplifies a 
difference between output signals from the sensor patterns SB and SC; the 
differential amplifier 64, a difference between output signals from the 
sensor patterns SC and SD; the differential amplifier 65, a difference 
between output signals from the sensor patterns SD and SE; and the 
differential amplifier 66, a difference between output signals from the 
sensor patterns SE and SF. 
The output signals from the amplifiers 61 to 66 are input to a selection 
circuit (analog switch) 41. The selection circuit 41 selects a signal to 
be input to an integrator 42 in response to a sensor selection signal from 
the main control unit (CPU) 51. The output signal from the amplifier which 
is selected by the selection circuit 41 is input to the integrator 42 to 
be integrated. 
On the other hand, a pulse signal output from the sensor pattern S1 is also 
input to the integrator 42. The pulse signal from the sensor pattern S1 
resets the integrator 42. At the same time, this signal is used to permit 
the integrator 42 to start a new integration operation. Although described 
later in detail, the integrator 42 has functions of removing noise and 
eliminating an influence of inclination in mounting position of the beam 
position detector 38. 
An output from the integrator 42 is input to an A/D converter 43. A pulse 
signal output from the sensor pattern S2 is also input to the A/D 
converter 43. The A/D conversion operation of the A/D converter 43 is 
started upon reception of the pulse signal from the sensor pattern S2 as a 
conversion start signal. Specifically, the A/D conversion operation is 
started at the timing at which the beam passes through the sensor pattern 
S2. 
As has been described above, the integrator 42 is reset by the pulse signal 
from the sensor pattern S1 immediately before the light beams have passed 
over the sensor patterns SA to SG, and at the same time the integration 
operation is started. While the beams are passing over the sensor patterns 
SA to SG, the integrator 42 integrates signals indicating the beam passing 
positions. Immediately after the beams have passed over the sensor 
patterns SA to SG, the integration result of the integrator 42 is 
A/D-converted by the A/D converter 43, with the pulse signal from the 
sensor pattern S2 supplied as a trigger signal. Thus, the noise-free 
sensor signal, from which the influence of the inclination in mounting 
position of the beam position detector 38 has been eliminated, can be 
converted to a digital signal. 
The A/D converter 43, which has completed the A/D conversion, outputs an 
interrupt signal INT indicating the completion of the A/D conversion 
process to the main control unit 51. 
The amplifiers 61 to 66, the selection circuit 41, the integrator 42, and 
the A/D converter 43 constitute the beam detector output processor 40. 
Thus, the digitally converted beam position detection signal from the beam 
position detector 38 is input as beam position information to the main 
control unit 51, thereby determining the passing position of the beam. 
On the basis of the beam position detection signal thus obtained, the main 
control unit 51 calculates control amounts for galvanomirrors 33a to 33d, 
and the calculated results are stored in the memory 52, where necessary. 
The main control unit 51 delivers the calculated results to the 
galvanomirror drive circuits 39a to 39d. 
As shown in FIG. 7, the galvanomirror drive circuits 39a to 39d are 
provided with latches 44a to 44d for holding the data of the calculated 
results. Once the data is written in the latches 44a to 44d by the main 
control unit 51, the latches 44a to 44d maintain the data until the data 
is updated. 
The data held in the latches 44a to 44d is converted to analog signals 
(voltage) by D/A converters 45a to 45d and input to drivers 46a to 46d for 
driving the galvanomirrors 33a to 33d. The drivers 46a to 46d drive the 
galvanomirrors 33a to 33d in accordance with the analog signals (voltage) 
input from the D/A converters 45a to 45d. 
In this embodiment, only one of the amplified output signals from the 
sensor patterns SA to SG is selected by the selection circuit 41 and 
integrated, and the integrated output is A/D-converted. Thus, all output 
signals from the sensor patterns SA to SG cannot be input to the main 
control unit 51 at the same time. 
Consequently, in a state in which the passing positions of the beams are 
not specified, it is necessary to successively operate the selection 
circuit 41 and supply the output signals from all the sensor patterns SA 
to SG to the main control unit 51, thereby to determine the passing 
positions of the beams. 
However, once the passing positions of the beams have been recognized, the 
passing positions of the beams can be estimated unless the galvanomirrors 
33a to 33d are excessively moved. Therefore, there is no need to input the 
output signals from all sensor patterns to the main control unit 51. This 
process will be described later in detail. 
FIGS. 8A to 8C illustrate the relationship among the beam passing 
positions, the outputs from the beam position detector 38, the outputs 
from the differential amplifiers 63 to 66, and the output from the 
integrator 42 in the operation of the circuit shown in FIG. 7. 
FIG. 8A illustrates a case where the light beam passes a just middle point 
between the sensor patterns SB and SC. FIG. 8B illustrates a case where 
the light beam passes a point closer to the sensor pattern SB, as compared 
to the case illustrated in FIG. 8A. FIG. 8C illustrates a case where the 
beam position detector 38 is mounted with an inclination to the beam 
passing direction. 
The output from the beam position detector 38, the output from the 
differential amplifier 63, and the output from the integrator 42 will now 
be described with respect to each case. 
Circuit Operation in FIG. 8A 
The light beam crosses the sensor pattern S1, and a pulse signal is output 
from the sensor pattern S1. As shown in FIG. 8A, the pulse signal resets 
the output of the integrator 42 to "0". Accordingly, when the beam crosses 
the sensor pattern S1, the previous detection result is reset, and a new 
detection result is integrated. 
When the light beam passes through the middle point between the sensor 
patterns SB and SC, the outputs from the sensor patterns SB and SC are 
equal to each other, as shown in FIG. 8A. However, since the outputs from 
sensor patterns are very small, a small amount of noise component may be 
superimposed, as shown in FIG. 8A. 
Such output signals are input to the differential amplifier 63 and a 
difference therebetween is amplified. When the outputs from the sensor 
patterns SB and SC are substantially equal, the output from the 
differential amplifier 63 is substantially zero, as shown in FIG. 8A, but 
a small amount of noise component may be superimposed. The differential 
amplification result thus obtained is input to the integrator 42 through 
the selection circuit 41. 
The integrator 42 integrates the output from the differential amplifier 63 
and outputs the integration result to the A/D converter 43. As shown in 
FIG. 8A, noise components have been removed from the output from the 
integrator 42. The reason is that a high-frequency noise component 
superimposed on the differential amplification result is removed by the 
integration. As described above, at the same time as the light beam has 
passed over the sensor patterns SB and SC, the difference between outputs 
from the sensor patterns SB and SC is amplified and integrated, and then 
input to the A/D converter 43. 
On the other hand, the output from the sensor pattern S2 is input to the 
A/D converter 43. Specifically, a pulse signal, as shown in FIG. 8A, is 
output from the sensor pattern S2 to the A/D converter 43, when the light 
beam has passed over the sensor patterns SB and SC. With the pulse signal 
received as a trigger, the A/D converter 43 starts to A/D-convert the 
output of the integrator 42. Accordingly, the A/D converter 43 can timely 
convert the high-S/N analog information on the beam passing position, from 
which the noise component has been removed, to a digital signal. 
Circuit Operation in FIG. 8B 
The operation of the circuit in FIG. 8B is basically the same as that of 
the circuit in FIG. 8A. However, since the beam passing position deviates 
towards the sensor pattern SB, the output from the sensor pattern SB is 
greater by the degree of deviation and the output from the sensor pattern 
SC is smaller. Thus, the output from the differential amplifier 63 
deviates to the positive side accordingly. Like the case of FIG. 8A, the 
integrator 42 is reset when the beam passes over the sensor pattern S1. 
Then, the differential amplification result is input to the integrator 42. 
The integrator 42 gradually increases the output thereof, as long as the 
input (i.e., the output from the differential amplifier 63) to the 
integrator 42 is on the positive side. When the input has restored to 
zero, the integrator 42 keeps this value. Thus, the output from the 
integrator 42 represents the deviation of the beam passing position. 
In the same manner as in the case of FIG. 8A, the A/D converter 43 
A/D-converts the integration result at the timing the light beam passes 
over the sensor pattern S2. Thus, the correct beam passing position is 
timely converted to digital information. 
Circuit Operation in FIG. 8C 
The operation of the circuit in FIG. 8C is basically the same as those in 
FIGS. 8A and 8B. However, since the light beam passes over the beam 
position detector 38 in a slanting direction, the outputs from the sensor 
patterns SB and SC, the output from the differential amplifier 63, and the 
output from the integrator 42 are characterized accordingly. 
As shown in FIG. 8C, the light beam, after passing over the sensor pattern 
S1, enters the region of the sensor patterns SB and SC at an angle from 
the pattern SC side. Then, the beam passes over substantially the middle 
point between the sensor patterns SB and SC and goes out of the region of 
the sensor patterns SB and SC at an angle toward the sensor pattern SB. In 
this case, the output from the sensor pattern SB is low immediately after 
the incidence of the beam and then gradually increases as the beam is 
passing, as shown in FIG. 8C. On the other hand, the output from the 
sensor pattern SC is high immediately after the incidence of the beam and 
then gradually decreases as the beam is passing. 
As shown in FIG. 8C, the output from the differential amplifier 63, to 
which such outputs from the sensor patterns SB and SC have been input, is 
high on the negative side just after the incidence of the beam, and then 
gradually decreases. When the beam has passed over the middle point 
between the sensor patterns SB and SC, the output from the differential 
amplifier 63 becomes substantially zero. Then, the output gradually 
increases on the positive side and takes a maximum positive value just 
before the beam has passed the sensor patterns SB and SC. 
The output from the integrator 42, to which the output of the differential 
amplifier 63 has been input, increases on the negative side just after the 
incidence of the beam. The value on the negative side continues to 
increase until the output from the differential amplifier 63 becomes zero. 
When the output from the differential amplifier 63 turns to the positive 
side, the negative value of the output of the integrator 42 decreases. 
Then, the output of the integrator 42 becomes substantially zero when the 
beam has passed over the sensor patterns SB and SC. 
Although the light beam crosses the beam position detector 38 in a slanting 
direction, it is considered that the beam passes through the middle point 
between the sensor patterns SB and SC on average. When the light beam has 
passed over the sensor pattern S2, the A/D converter 43 starts the A/D 
conversion operation. In this case, the integrated value is zero and also 
the digital data indicating the beam passing position is zero. In other 
words, it is considered that the beam has passed through the middle point 
between the sensor patterns SB and SC. 
The relationship among the beam passing position, the outputs from the 
sensor patterns S1, S2, SB, and SC, the output from the differential 
amplifier 63, the output from the integrator 42, and the operation of the 
A/D converter 43 has been described. The operations of the sensor patterns 
SC, SD, SE, and SF and differential amplifiers 64, 65, and 66 are 
basically the same as those of the sensor patterns SB and SC and 
differential amplifier 63 and thus a description of the respective 
operations will be omitted. 
The relationship between the beam passing position and the output from the 
A/D converter 43 will now be described with reference to FIG. 9. 
The ordinate of the graph of FIG. 9 indicates the magnitude of the output 
of the A/D converter (12 bits) 43 as shown in FIG. 7, and the abscissa 
indicates the passing positions of the beams. As regards the beam passing 
positions on the abscissa, the beam passing position closer to the 
left-hand end indicates that the beam passes through a position closer to 
the sensor pattern SG, and the beam passing position closer to the 
right-hand end indicates that the beam passes through a position closer to 
the sensor pattern SA. 
The output from the differential amplifier (63, 64, 65, or 66; to be 
referred to as B-C, C-D, D-E, or E-F hereinafter) may have either a 
positive value or a negative value. In this case, the output from the A/D 
converter 43 may vary as follows. When the output of the differential 
amplifier (B-C, C-D, D-E, or E-F) has a positive value, the A/D converter 
43 outputs values (A/D coverted values) from 000H (minimum value) to 7FFH 
(maximum value) as the output from the differential amplifier increases. 
On the other hand, when the output of the differential amplifier (B-C, C-D, 
D-E, or E-F) has a negative value, the A/D converter 43 outputs values 
(A/D coverted values) from 800H (minimum value) to FFFH (maximum value). 
In the case of the values on the negative side, the A/D coverted value 
with a large absolute value (close to FFFH) indicates that the output of 
the integrator 42 is close to zero, i.e., the target value. On the other 
hand, the A/D coverted value with a small absolute value (close to 800H) 
indicates that the output of the integrator 42 has a large negative value. 
A detailed description will now be given of the case where the output of 
the differential amplifier B-C associated with the sensor patterns SB and 
SC is A/D-converted by the A/D converter 43. 
The output of the sensor pattern SB is connected to a positive terminal of 
the differential amplifier B-C, and the output of the sensor pattern SC is 
connected to a negative terminal of the differential amplifier B-C. As 
shown in FIG. 9, the output from the differential amplifier B-C takes a 
maximum value when the light beam passes near the center of the sensor 
pattern SB, and the A/D coverted value from the A/D converter 43 is 7FFH. 
The reason is that the output from the sensor pattern SB becomes maximum 
near the center. 
When the light beam deviates from this position either toward the sensor 
pattern SA side or toward the sensor pattern SC side, the A/D coverted 
value (output of the differential amplifier B-C) decreases. 
In addition, when the beam passing position has deviated toward the sensor 
pattern SA, neither the sensor pattern SB nor SC is able to detect the 
passage of the beam, and the A/D coverted value (output of the 
differential amplifier B-C) becomes substantially zero. 
By contrast, when the beam passing position has deviated toward the sensor 
pattern SC, the A/D coverted value (output from the differential amplifier 
B-C) gradually decreases. The A/D coverted value becomes zero when the 
beam passes over the very middle point between the sensor patterns SB and 
SC. The reason is that the outputs from the sensor patterns SB and SC are 
equalized. In this embodiment, this point is the target passing position 
of the light beam a. 
When the beam passing point deviates toward the sensor pattern SC, the 
output of the differential amplifier B-C has a negative value, and the A/D 
coverted value varies from 000H to FFFH. Then, the A/D coverted value 
gradually decreases. Besides, if the beam passing position becomes close 
to the center of the sensor pattern SC, the output of the differential 
amplifier B-C takes a maximum negative value, and the A/D coverted value 
becomes 800H. 
When the beam passing position deviates toward the sensor pattern SD, the 
output of the differential amplifier B-C has a smaller negative value, and 
the A/D coverted value increases from 800H. At last, the A/D coverted 
value varies from FFFH to 000H. The reason is that the beam passing 
position deviates toward the sensor pattern SD (SE) side excessively and 
neither the sensor pattern SB nor SC is able to detect the passing of the 
beam, so that both outputs from the sensor patterns SB and SC become zero 
and no difference arises therebetween. 
The control characteristics of the galvanomirror 33 will now be described. 
FIGS. 10 and 11 show the relationship between the data delivered to the 
galvanomirror drive circuits 39a to 39d and the beam passing positions on 
the beam position detector 38 (i.e., on the photosensitive drum 15). As 
shown in FIG. 7, the input signals to the D/A converters 45a to 45d of the 
galvanomirror drive circuits 39a to 39d have 16-bit construction. 
FIG. 10 shows the state of variation of the beam passing position, which is 
associated with the upper 8 bits of the input 16-bit signal. As shown in 
FIG. 10, the beam passing position varies in the range of 2,000 .mu.m (2 
mm) in relation to data 00H to FFH. The inputs near 00H and FFH are out of 
the range of responsivity of the galvanomirror, and the beam passing 
position is unchanged. 
However, in the range of inputs between 18H and E8H, the beam passing 
position varies substantially linearly in relation to these inputs. As to 
the ratio of variation, 1LSB corresponds to a distance of about 10 .mu.m. 
FIG. 11 shows the state of variation of the beam passing position, which is 
associated with the lower 8 bit of the input 16-bit signal to the D/A 
converters 45a to 45d of the galvanomirror drive circuits 39a to 39d. It 
should be noted that FIG. 11 shows the state of variation of the beam 
passing position, which is associated with the lower 8 bit, in the case 
where the value of the range, within which the beam passing position 
linearly varies, is input as the upper 8 bit. As is clear from the figure, 
the beam passing position varies within the range of about 10 .mu.m in 
relation to data between 00H and FFH in association with the lower 8 bit. 
1LSB corresponds to 0.04 .mu.m. 
In this manner, the main control unit 51 delivers 16-bit data to the 
galvanomirror drive circuits 39a to 39d. Thereby, the beam passing 
position on the beam position detector 38 or the photosensitive drum 15 
can be moved with the resolution of about 0.04 .mu.m within the range of 
about 2,000 .mu.m (2 mm). 
The operation of the printer unit 2 at the time of power on will now be 
generally described with reference to a flow chart in FIG. 12. A 
description of the scanner unit 1 will be omitted. 
When the copying machine is switched on, the main control unit 51 rotates 
the fixing rollers in the fixing device 26 and starts the heating control 
for the fixing device 26 (S1 and S2). The beam passing position control 
routine in the sub-scanning direction is executed, and the beam passing 
position is controlled at a predetermined position (S3). 
If the beam passing position is controlled correctly, the offset 
compensation routine is executed to detect the offset value of the beam 
detector output processor 40 and perform compensation (S4). Then, the beam 
passing position control routine is executed again (S5). 
Pull-in in the main scanning direction is carried out to perform APC 
(Auto-Power Control) in a hardware structure so as to simultaneously emit 
each light beam with a desired power (S6). The photosensitive drum 15 is 
rotated, and process-related initialization is effected so as to regulate 
the conditions of, e.g., the surface of the photosensitive drum 15 (S7). 
After the series of initialization steps have been completed, the fixing 
rollers are kept rotating until the temperature of the fixing device 26 
rises up to a predetermined level, and the copying machine is set in the 
standby state (S8). When the temperature of the fixing device 26 has 
reached the predetermined level, the rotation of the fixing rollers are 
stopped (S9), and a copying instruction is awaited (S10). 
When no copying (printing) instruction is sent through the control panel 53 
in the copying instruction wait state (S10), after the beam passing 
position control routine is executed, e.g., after 30 minutes have passed 
(S11), the offset compensation routine is automatically executed (S12), 
and the beam passing position control routine is executed again (S13). 
Thereafter, the flow returns to step S10, and the copying machine waits 
for a copying instruction again. 
When a copying instruction is sent through the control panel 53 in the 
copying instruction standby state (S10), a copying operation is carried 
out (S14). The beam passing position control routine is executed again 
(S15), and the flow stands by until the copying is complete (S16). Upon 
completion of the copying, the flow shifts to step S12 to repeatedly 
perform the above operation. 
The beam passing position control routine in steps S3, S5, S13, and S15 in 
FIG. 12 will now be generally described with reference to a flow chart 
shown in FIG. 13. 
At first, the main control unit 51 activates the polygon motor 36 and 
rotates the polygon mirror 35 at a predetermined rotational speed (S20). 
The main control unit 51 reads out from the memory 52 the latest drive 
values of the galvanomirrors 33a to 33d, and drives the galvanomirrors 33a 
to 33d on the basis of these values (S21). 
Then, the main control unit 51 controls the passing position of the beam a 
(S22). In this control, the passing position of the light beam a is 
detected, and it is determined whether the passing position is within a 
prescribed value range. If the detected passing position is not within the 
prescribed value range, the angle of the galvanomirror 33a is altered. If 
the detected passing position is within the prescribed value range, a flag 
indicating that the passing position of the beam a is within the 
prescribed value range is set. 
Subsequently, the main control unit 51 detects the passing positions of the 
light beams b, c, and d, like the beam a, and determines whether each of 
the passing positions of these beams is within the prescribed value range. 
If the passing positions are not within the prescribed value range, the 
associated galvanomirrors 33b to 33d are altered. If the detected passing 
positions are within the prescribed value range, flags indicating that the 
passing positions of the beams are within the prescribed value range are 
set (S23, S24, and S25). 
After the passing positions of the beams a, b, C, and d are controlled, the 
main control unit 51 examines the flags and determines whether the beam 
passing position control should be finished (S26). Specifically, if all 
flags are set, the beam position control is finished. If any one of the 
flags is not set, the control returns to step S22 and the passing position 
control of each light beam is performed. 
The operations of galvanomirrors 33a to 33d in the above control flow will 
now be described in brief. 
As has been described above, the angles of the galvanomirrors 33a to 33d 
are altered on the basis of the control values supplied from the main 
control unit 51, thereby varying the passing positions of the scanning 
beams. However, the angles of the galvanomirrors are not necessarily 
controlled immediately in response to the instructions from the main 
control unit 51. Specifically, a time period on the order of "ns" or 
".mu.s" is needed from the time the control data is output from the main 
control unit 51, latched in the latches 44a to 44d, and D/A-converted by 
the D/A converters 45a to 45d, to the time drive signals proportional to 
the magnitude of the D/A-converted data are output from the drivers 46a to 
46d. On the other hand, the response time of the galvanomirrors 33a to 33d 
used in, e.g., this embodiment is on the order of 4-5 ms. 
In this context, the response time refers to a time period from the time 
the angles of the galvanomirrors 33a to 33d begin to be varied by a new 
drive signals and moved (oscillated), to the time the movement 
(oscillation) is stopped and the galvanomirrors rest at the new angles. 
Thus, in order for the main control unit 51 to confirm the control result 
after new control data is delivered to the galvanomirrors 33a to 33d, the 
main control unit 51 needs to confirm the beam passing position after at 
least the response time. 
As is apparent from FIG. 13, in this embodiment, the control effects of a 
given galvanomirror are confirmed after another beam position is detected 
or another galvanomirror is controlled. The effects are confirmed after 
the lapse of a sufficient time required for the galvanomirror to respond. 
When the time needed for a single scan operation is 330 .mu.m, a time of 
2.64 ms is required to acquire outputs from at least one amplifier or 
differential amplifier with respect to all surfaces (e.g., eight surfaces) 
of the polygon mirror 35 in steps S21, S22, S23, and S24. Accordingly, 
after a certain galvanomirror is controlled, and the passing positions of 
the three beams detected, at least a time interval of 7.92 ms is required 
to confirm the effects. The beam passing position can be confirmed after 
the movement (oscillation) of the galvanomirror is already stopped. 
Outputs of the amplifier or differential amplifier are acquired for all 
the surfaces of the polygon mirror 35 in order to prevent inclination of 
the surfaces of the polygon mirror 35. 
FIGS. 14 and 15 are flow charts for explaining in detail the operations in 
step S22 in FIG. 13. That is, FIGS. 14 and 15 are flow charts for 
explaining the passing position control of the light beam a. As has been 
described above, FIG. 9 shows the relationship between the beam passing 
position and the output from the A/D converter 43. Thus, FIG. 9 will also 
be referred to. 
At first, the main control unit 51 forcibly activates the laser oscillator 
31a to emit light (S31). Thereby, the light beam a is made to cyclically 
scan over the beam position detector 38 by the rotation of the polygon 
mirror 35. 
The main control unit 51 then reads A/D coverted values of the outputs from 
each amplifier and differential amplifier in accordance with interrupt 
signals INT output from the A/D converter 43. In general, the scan 
positions of light beams slightly vary, depending on the respective 
surfaces of the polygon mirror 35, because of inclinations of the 
surfaces. In order to eliminate the influence of the inclinations of the 
surfaces of the polygon mirror 35, it is desirable to read A/D coverted 
values a number of times corresponding to the number of surfaces of 
polygon mirror 35 or an integer number of times of the number of surfaces 
of polygon mirror 35. The main control unit 51 averages output values from 
the A/D converter 43 corresponding to the respective amplifiers and 
differential amplifiers, and recognizes the averaged result as the outputs 
of the amplifiers and differential amplifiers (S32). 
Accordingly, if the values from the A/D converter 43 are read a number of 
times corresponding to the number (8) of surfaces of the polygon mirror 35 
with respect to the amplifiers 61 and 62 (A and G) and the differential 
amplifiers 63 to 66 (B-C, C-D, D-E, and E-F), it is necessary to make the 
light beams scan 48 times. 
The main control unit 51 compares the obtained output (A/D coverted value) 
from the amplifier 61 (A) with the determination reference value 100H 
stored in the memory 52 in advance to determine whether the output from 
the amplifier 61 is greater than the determination reference value 100H 
(S33). 
If it is determined that the output from the amplifier 61 is greater than 
100H, it is found that the passing position of the beam a is on the sensor 
pattern SA or closest to the sensor pattern SA. In other words, it is 
found that the light beam a passes over the area A in FIG. 9. Since the 
target passing position of the beam a is at the middle between the sensor 
patterns SB and SC, the galvanomirror 33a is controlled so that the light 
beam a may pass on the sensor pattern SG side (S34). 
The control amount (the amount of movement of the beam) in this case is set 
at about 120 .mu.m. The reason is that the sensor patterns SA and SG are 
large patterns located on both sides of the area of control target points, 
as described with reference to FIGS. 3 and 4, and the beam passing 
position needs to be altered to a relatively great degree while the beam 
is passing over these sensor patterns, in order to approach the beam 
passing position to the target point as quickly as possible. Even when the 
output of the amplifier 61 is greater than 100H, if the light beam a is 
passing over a point near the sensor pattern SB, the beam passing position 
may be altered excessively. However, in consideration of the total 
efficiency, the amount of movement of such a degree is necessary. 
If it is determined in step S33 that the output from the amplifier 61 is 
not greater than 100H, the output (A/D coverted value) from the amplifier 
62 (G) is compared with the determination reference value 100H stored in 
the memory 52 in advance to determine whether the output from the 
amplifier 62 is greater than the determination reference value 100H (S35). 
If it is determined that the output from the amplifier 62 is greater than 
100H, it is found that the passing position of the beam a is on the sensor 
pattern SG or closest to the sensor pattern SG. In other words, it is 
found that the light beam a passes over the area G in FIG. 9. In this 
case, the galvanomirror 33ais controlled so that the light beam a may pass 
on the sensor pattern SA side and may approach the middle point between 
the sensor patterns SB and SC, which is the target passing position of the 
beam a (S36). The control amount (amount of movement) in this case needs 
to be about 120 .mu.m, like the control in step S34. 
If it is determined in step S3S that the output from the amplifier 62 is 
not greater than 100H, the output (A/D coverted value) from the 
differential amplifier 66 (E-F) is compared with the determination 
reference value 800H stored in the memory 52 in advance to determine 
whether the output from the differential amplifier 66 is greater than the 
determination reference value 800H (S37). 
If it is determined that the output from the differential amplifier 66 is 
800H or more, it is found that the passing position of the beam a is near 
the sensor pattern SF. In other words, it is found that the light beam a 
passes over the area F in FIG. 9. In this case, the galvanomirror 33a is 
controlled so that the light beam a may pass on the sensor pattern SA side 
and may approach the middle point between the sensor patterns SB and SC, 
which is the target passing position of the beam a (S38). The control 
amount (amount of movement) in this case needs to be about 120 .mu.m, in 
consideration of the distance between the target point and the area F. 
If it is determined in step S37 that the output from the differential 
amplifier 66 is not greater than 800H, the output (A/D coverted value) 
from the differential amplifier 65 (D-E) is compared with the 
determination reference value 800H stored in the memory 52 in advance to 
determine whether the output from the differential amplifier 65 is greater 
than the determination reference value 800H (S39). 
If it is determined that the output from the differential amplifier 65 is 
800H or more, it is found that the passing position of the beam a is near 
the sensor pattern SE. In other words, it is found that the light beam a 
passes over the area E in FIG. 9. In this case, the galvanomirror 33a is 
controlled so that the light beam a may pass on the sensor pattern SA side 
and may approach the middle point between the sensor patterns SB and SC, 
which is the target passing position of the beam a (S40). The control 
amount (amount of movement) in this case needs to be about 80.mu.m, in 
consideration of the distance between the target point and the area E. 
If it is determined in step S39 that the output from the differential 
amplifier 65 is not greater than 800H, the output (A/D coverted value) 
from the differential amplifier 64 (C-D) is compared with the 
determination reference value 800H stored in the memory 52 in advance to 
determine whether the output from the differential amplifier 64 is greater 
than the determination reference value 800H (S41). 
If it is determined that the output from the differential amplifier 64 is 
800H or more, it is found that the passing position of the beam a is near 
the sensor pattern SD. In other words, it is found that the light beam a 
passes over the area D in FIG. 9. In this case, the galvanomirror 33a is 
controlled so that the light beam a may pass on the sensor pattern SA side 
and may approach the middle point between the sensor patterns SB and SC, 
which is the target passing position of the beam a (S42). The control 
amount (amount of movement) in this case needs to be about 40.mu.m, in 
consideration of the distance between the target point and the area D. 
If it is determined in step S41 that the output from the differential 
amplifier 64 is not greater than 800H, the output (A/D coverted value) 
from the differential amplifier 63 (B-C) is compared with the 
determination reference values 400H and 7FFH stored in the memory 52 in 
advance to determine whether the output from the differential amplifier 63 
is greater than the determination reference value 400H and is 7FFH or less 
(S43). 
If it is determined that the output from the differential amplifier 63 is 
greater than 400H and is 7FFH or less, it is found that the passing 
position of the beam a is near the middle point between the sensor 
patterns SB and SC, which is the target passing position, but slightly 
deviates towards the sensor pattern SB. In other words, it is found that 
the light beam a passes over the area BA of the area B in FIG. 9. In this 
case, the galvanomirror 33a is controlled so that the light beam a may 
pass on the sensor pattern SG side and may approach the middle point 
between the sensor patterns SB and SC, which is the target passing 
position of the beam a (S44). The control amount (amount of movement) in 
this case needs to be about 10 .mu.m, in consideration of the distance 
between the target point and the area D. 
If it is determined in step S43 that the output from the differential 
amplifier 63 is not greater than 400H and is not 7FFH or less, the output 
from the differential amplifier 63 is compared with the determination 
reference values 60H and 400H stored in the memory 52 in advance to 
determine whether the output from the differential amplifier 63 is greater 
than the determination reference value 60H and is 400H or less (S45). 
If it is determined that the output from the differential amplifier 63 is 
greater than 60H and is 400H or less, it is found that the passing 
position of the beam a is near the middle point between the sensor 
patterns SB and SC, which is the target passing position, but slightly 
deviates towards the sensor pattern SB. In other words, it is found that 
the light beam a passes over the area BC of the area B in FIG. 9. In this 
case, the galvanomirror 33a is controlled so that the light beam a may 
pass on the sensor pattern SG side and may approach the middle point 
between the sensor patterns SB and SC, which is the target passing 
position of the beam a (S46). The control amount (amount of movement) in 
this case needs to be about 0.5 .mu.m, in consideration of the distance 
between the target point and the area D. 
If it is determined in step S45 that the output from the differential 
amplifier 63 is not greater than 60H and is not 400H or less, the output 
from the differential amplifier 63 is compared with the determination 
reference values 800H and A00H stored in the memory 52 in advance to 
determine whether the output from the differential amplifier 63 is the 
determination reference value 800H or more and is smaller than A00H (S47). 
If it is determined that the output from the differential amplifier 63 is 
800H or more and is smaller than A00H, it is found that the passing 
position of the beam a is near the middle point between the sensor 
patterns SB and SC, which is the target passing position, but slightly 
deviates towards the sensor pattern SC. In other words, it is found that 
the light beam a passes over the area CD of the area C in FIG. 9. In this 
case, the galvanomirror 33a is controlled so that the light beam a may 
pass on the sensor pattern SA side and may approach the middle point 
between the sensor patterns SB and SC, which is the target passing 
position of the beam a (S48). The control amount (amount of movement) in 
this case needs to be about 10 .mu.m, in consideration of the distance 
between the target point and the area CD. 
If it is determined in step S47 that the output from the differential 
amplifier 63 is not 800H or more and is not smaller than A00H, the output 
from the differential amplifier 63 is compared with the determination 
reference values A00H and FA0H stored in the memory 52 in advance to 
determine whether the output from the differential amplifier 63 is the 
determination reference value A00H or more and is smaller than FA0H (S49). 
If it is determined that the output from the differential amplifier 63 is 
A00H or more and is smaller than FA0H, it is found that the passing 
position of the beam a is near the middle point between the sensor 
patterns SB and SC, which is the target passing position, but slightly 
deviates towards the sensor pattern SC. In other words, it is found that 
the light beam a passes over the area CB of the area C in FIG. 9. In this 
case, the galvanomirror 33a is controlled so that the light beam a may 
pass on the sensor pattern SA side and may approach the middle point 
between the sensor patterns SB and SC, which is the target passing 
position of the beam a (S50). The control amount (amount of movement) in 
this case needs to be about 0.5 .mu.m, in consideration of the distance 
between the target point and the area CB. 
When it is determined in step S49 that the output from the differential 
amplifier 63 is neither A00H or more nor smaller than FA0H, it is found 
that the passing position of the beam a is within the prescribed range 
(.+-.1 .mu.m from the target point). Thus, the main control unit 51 sets 
the control completion flag A for the galvanomirror 33a (S51). 
When the light beam a does not pass within the range of .+-.1 .mu.m from 
the ideal passing point (steps S34, S36, S38, S40, S42, S44, S46, S48, and 
S50), the main control unit 51 controls the galvanomirror 33a by a 
predetermined amount and writes the control amount in the memory 52 (S52). 
As has been described above, the main control unit 51 sets the control 
completion flag A for the galvanomirror 33a when the beam a passes within 
the range of .+-.1 .mu.m from the ideal passing point. When the beam a 
does not pass within the range of .+-.1 .mu.m from the ideal passing 
point, the main control unit 51 sets the galvanomirror control amount in 
accordance with the beam passing position (area) and writes the control 
amount in the memory 52. 
Then, the main control unit 51 stops the forcible activation of the laser 
oscillator 31a and completes the series of the control operations for the 
passing position of the beam a (S53). 
As has already been described with reference to FIG. 13, when the control 
completion flag A for the galvanomirror 33a is not set, the passing 
position control routine for the beam a is executed once again. In other 
words, this routine is repeated until the beam a passes within the range 
of .+-.1 .mu.m from the ideal passing point. 
The above description is related to control of the light beam a. Control of 
the light beams b, c, and d is basically the same as that of the light 
beam a. After the main control unit 51 forcibly activates the 
corresponding laser oscillators 31b to 31d to emit light beams, outputs 
from the amplifiers 61, 62 and the differential amplifiers 63 to 66 are 
determined. When each beam passes within the range of .+-.1 .mu.m from the 
ideal control point, control completion flags B to D for the 
galvanomirrors 33b to 33d are set. When each beam does not pass within 
this range, the main control unit 51 determines passing areas of the light 
beams b to d, controls the galvanomirrors 33b to 33d in accordance with 
the passing areas, and writes the control values in the memory 52. 
According to the above-described embodiment, the beam position detector 
having the above-described sensor patterns is used. Thereby, the scan 
positions of the light beams can be exactly detected even if the precision 
in mounting angle of the beam position detector is not high. 
In the digital copying machine using the multi-beam optical system, the 
passing positions of light beams are detected by the beam position 
detector situated on a level with the surface of the photosensitive drum. 
Based on the detected result, control amounts for optimally controlling 
the relative positions of the beams on the surface of the photosensitive 
drum are calculated. Based on the calculated control amounts, the 
galvanomirrors are controlled to alter the relative positions of the beams 
on the surface of the photosensitive drum. Thereby, the positional 
relationship among the light beams on the surface of the photosensitive 
drum can be optimally controlled with no particular precision or 
adjustment for the assembly of the optical system, even if some change 
occurs in the structure of the optical system due to a variation in 
ambience or a variation with the passing of time. Therefore, high image 
quality can be maintained at all times. 
Detection and compensation of the offset value in the beam detector output 
processor 40, which are the most important parts of the present invention, 
will be described below. 
FIG. 16 shows a detailed example of the arrangement from the sensor 
patterns SB and SC to the integrator 42 in the beam detector output 
processor 40. In FIG. 16, currents flowing through the sensor patterns 
(photodiodes) SB and SC are respectively converted to voltages by 
resistors RP1, RL1, RP2, and RL2, then amplified by operational amplifiers 
A1 and A2 serving as voltage follower circuits, and sent to the 
differential amplifier 63. The differential amplifier 63 is constituted by 
resistors R1 to R4 and an operational amplifier A3. 
An output from the differential amplifier 63 is sent to the integrator 42 
via an analog switch SW1 included in the selection circuit 41. The 
integrator 42 is constituted by an operational amplifier A4, an 
integrating resistor R5, an integrating capacitor C, an integrator reset 
analog switch SW7, and a protective resistor R6. An output from the 
integrator 42 is sent to the A/D converter 43 to be converted from an 
analog signal to a digital signal. Upon completion of the A/D conversion, 
the A/D convertor 43 transmits a conversion completion signal to the main 
control unit 51. Upon reception of the conversion completion signal, the 
main control unit 51 reads the beam position information converted into 
the digital value. 
An example of the arrangement from the sensor patterns SD, SE, and SF to 
the integrator 42 is also basically the same as the example of the 
arrangement from the sensor patterns SB and SC to the integrator 42, and a 
description thereof will be omitted. 
The offset voltage (offset value) of the operational amplifier will be 
described with reference to FIGS. 17A and 17B. 
In FIG. 17A, an ideal operational amplifier does not send any output when a 
non-inverting input and an inverting input have no difference in voltage. 
In practice, however, even if the non-inverting input and the inverting 
input are connected to the ground potential (GND), and the voltage 
difference between these inputs is eliminated, an output voltage Vout is 
generated at the output terminal. 
In FIG. 17B, a given voltage Vos is applied across the input terminals so 
as to set the output voltage Vout to 0 V. This voltage value is an input 
offset voltage Vos. The offset voltage Vout is mainly generated by 
variations in transistor characteristics of a differential input circuit 
in the operational a amplifier. The input offset voltage of a general 
operational amplifier is several mV at room temperature. The input offset 
voltage also varies depending on the temperature. 
The influence and problem of the offset voltage of the operational 
amplifier with respect to beam passing position detection will be 
explained with reference to FIG. 18. 
For example, when the passing position of the light beam a is the middle 
position between the sensor patterns SB and SC, outputs from the sensor 
patterns SB and SC are equal to each other (V1=V2). 
Assume that the operational amplifiers A1 to A4 constituting the circuit of 
the beam detector output processor 40 shown in FIG. 18 have the following 
offset voltages: 
Offset Voltage of Operational amplifier A1:-Vo1 V! 
Offset Voltage of Operational amplifier A2:+Vo2 V! 
Offset Voltage of Operational amplifier A3:+Vo3 V! 
Offset Voltage of Operational amplifier A4:+Vo4 V! 
Considering these offset voltages, outputs from the operational amplifiers 
are 
Output from Operational amplifier A1: V1+Vo1 V! 
Output from Operational amplifier A2: V2+Vo2 V! 
Output from Operational amplifier A3: (Vo1-Vo2).times.R3/R1 
Output from Operational amplifier A4: 
-(Vo1-Vo2).times.(R3/R1)+Vo4!t/R5.times.C.times.! 
where V1=V2 
R1=R2, R3=R4 
R5: integrating resistor 
C: integrating capacitor 
t: integrating time 
When the outputs from the sensor patterns SB and SC are equal to each other 
(V1=V2), an output from the operational amplifier A4 (integrator) is 
ideally 0 V. However, the output from the operational amplifier A4 is not 
"0" owing to the influence of the offset voltage of each operational 
amplifier. That is, even when the beam passing position is ideal, the beam 
detector output processor 40 outputs erroneous information indicating that 
the beam position deviates. 
For example, when the constants are 
Vo1=Vo2=Vo3=Vo4=5 mV 
R2/R1=R4/R5=3 
R5=200.OMEGA. 
C=150 pF 
t=406 ns (time required for the light beam to 
pass between the sensor patterns S1 and S2) an integrated output is about 
0.615V, which can be converted to beam position information of about 1.23 
.mu.m. 
Offset detection/compensation according to the first embodiment will be 
described below. 
In summary, as described above, in passing position detection control of 
detecting the beam passing position on the beam position detector 38, a 
difference between outputs from sensor patterns after the beam passes over 
the beam position detector 38 is calculated, and the result is integrated 
and A/D-converted to detect the beam passing position. 
As has already been described above, the integration start and end timings 
of the integrator 42 are the timings of outputting signals from the sensor 
patterns S1 and S2. That is, when the beam is made by the polygon mirror 
35 to scan, and passes over the sensor pattern S1, the integrator 42 is 
reset, and integration starts immediately after the reset. In addition, 
integration is complete when the beam passes over the sensor pattern S2, 
and at the same time A/D conversion starts. 
The offset value of the beam detector output processor 40 is stationarily 
generated as far as the circuit is powered on. During the integration 
period from the start to end of integration, the offset value becomes an 
error factor of beam position information upon beam passing position 
detection processing. Therefore, if the offset value during the 
integration period can be measured, beam passing position control can be 
performed while considering the offset value (compensating the offset). 
In this embodiment, the integration start and end timings of the integrator 
42 are obtained using output signals from the sensor patterns S1 and S2. 
In this case, a correct offset value cannot be detected if the light beam 
is detected by the sensor patterns SB, SC, SD, SE, and SF. 
By shifting the light beam to a region where the beam is not detected by 
the sensor patterns SB, SC, SD, SE, and SF, the offset value can be 
detected (measured) without changing the integration start/end timing. 
FIG. 19 is a view showing a state wherein the beam position is shifted to 
the sensor pattern SA, and the offset value is detected. In this manner, 
the offset value can be detected by integration while the light beam is 
not irradiated on the sensor patterns SB to SF. 
The offset compensation routine will be explained with reference to flow 
charts in FIGS. 20 and 21. 
The main control unit 51 activates the polygon motor 36 and rotates the 
polygon mirror 35 at a predetermined rotational speed (S61). The main 
control unit 51 reads out the previous drive values of the galvanomirrors 
33a to 33d from the memory 52, and drives the galvanomirrors 33a to 33d on 
the basis of these values (S62). 
Then, the main control unit 51 emits a light beam, and controls the 
galvanomirror 33a so as to shift the light beam a to an undetectable 
region of the beam position detection pattern (S63). In this control, the 
main control unit 51 grasps the current beam passing position, and 
controls the galvanomirror 33a so as to shift the light beam a to a region 
where the light beam a is not detected by the sensor patterns SB, SC, SD, 
SE, and SF. 
Subsequently, the main control unit 51 detects the offset value of a beam 
passing position detector for the light beam a in the beam detector output 
processor 40 (offset detection; S64 and S65). Based on the detected offset 
value, the main control unit 51 executes offset compensation (offset 
compensation; S66). 
The main control unit 51 sequentially carries out steps S67 to S78 to 
perform the same control: (shift a light beam to an undetectable 
region).fwdarw.(offset detection).fwdarw.(offset compensation) for the 
light beams b, c, and d. 
A routine operation of shifting the light beam a to the undetectable region 
of the beam position detection pattern in step S63 of FIG. 20 will now be 
described with reference to flow charts shown in FIGS. 22 and 23. 
The main control unit 51 forcibly activates only the laser oscillator 31a 
to emit a light beam (S81). Thereby, the light beam a is made to 
cyclically scan over the beam position detector 38 by the rotation of the 
polygon mirror 35. 
The main control unit 51 then reads A/D-converted values of the outputs 
from the amplifiers 61 and 62 and the differential amplifiers 63 to 66 in 
accordance with interrupt signals INT output from the A/D converter 43. In 
general, the scan positions of light beams slightly vary depending on the 
respective surfaces of the polygon mirror 35, because of inclinations of 
the surfaces. In order to eliminate the influence of the inclinations of 
the surfaces of the polygon mirror 35, it is desirable to read 
A/D-converted values a number of times corresponding to the number of 
surfaces of polygon mirror 35 or a plurality of times of the number of 
surfaces of polygon mirror 35. The main control unit 51 averages output 
values from the A/D converter 43 corresponding to the respective 
amplifiers and differential amplifiers, and recognizes the averaged result 
as the outputs of the amplifiers and differential amplifiers (S82). 
The main control unit 51 compares the obtained outputs from the amplifiers 
and the differential amplifiers with determination reference values, and 
controls the galvanomirrors on the basis of the results so as to shift the 
light beam to the undetectable region. 
Assume that the undetectable regions are set near the centers of the sensor 
patterns SA and SG. As described above, the sensor patterns SA and SG have 
a length of 800 .mu.m. Even a light beam having a shape of about 
100.times.100 .mu.m (or more) is not detected by the sensor patterns SB, 
SC, SD, SE, and SF as long as the light beam passes through a point 
falling within range from the center to 400 .mu.m. 
The main control unit 51 compares the output (A/D-converted value) from the 
amplifier 61 (A) with the determination reference value 100H stored in the 
memory 52 in advance to determine whether the output from the amplifier 61 
is the determination reference value 100H or more (S83). 
If the output from the amplifier 61 is determined to be 100H or more, the 
passing position of the light beam a is found to be closer to the sensor 
pattern SA side than the center of the sensor pattern SB or on the sensor 
pattern SA. In this case, the main control unit 51 controls the 
galvanomirror 33a so as to shift the light beam a to the sensor pattern SA 
side by about 450 .mu.m (S84). 
After shifting the light beam a, the main control unit 51 reads the output 
from the sensor pattern SA again, and compares outputs before and after 
the shift (S85). If the output after the shift is equal to or larger than 
the output before the shift (output after the shift.gtoreq.output before 
the shift), the light beam a is determined to be at least on the sensor 
pattern SA on an upper side with respect to the sheet surface from the 
center of the sensor pattern SA. The shift to the undetectable region is 
complete. 
If the output after the shift is smaller than the output before the shift, 
the passing position of the light beam a partially falls within the upper 
side of the sensor pattern SA with respect to the sheet surface or 
completely falls outside the sensor pattern SA. This means that the 
passing position of the light beam a has been in the undetectable region 
before the shift. In this case, the main control unit 51 controls the 
galvanomirror 33a so as to shift the position of the light beam a to the 
sensor pattern SG side by about 450 .mu.m (S86). 
If NO in step S83, the main control unit 51 compares the output 
(A/D-converted value) from the amplifier 62 (G) with the determination 
reference value 100H stored in the memory 52 in advance to determine 
whether the output from the amplifier 62 is the determination reference 
value 100H or more (S87). 
If the output from the amplifier 62 is determined to be 100H or more, the 
passing position of the light beam a is found to be closer to the sensor 
pattern SG side than the center of the sensor pattern SF or on the sensor 
pattern SG. In this case, the main control unit 51 controls the 
galvanomirror 33a so as to shift the light beam a to the sensor pattern SG 
side (lower side with respect to the sheet surface) by about 450 .mu.m. 
After shifting the light beam a, the main control unit 51 reads the output 
from the sensor pattern SA again, and compares outputs before and after 
the shift (S89). If the output after the shift is equal to or larger than 
the output before the shift (output after the shift.gtoreq.output before 
the shift), the light beam a is determined to be at least on the sensor 
pattern SG on a lower side with respect to the sheet surface from the 
center of the sensor pattern SA. The shift to the undetectable region is 
complete. 
If the output after the shift is smaller than the output before the shift, 
the passing position of the light beam a partially falls within the lower 
side of the sensor pattern SG with respect to the sheet surface or 
completely falls outside the sensor pattern SG. This means that the 
passing position of the light beam a has been in the undetectable region 
before the shift. In this case, the main control unit 51 controls the 
galvanomirror 33a so as to shift the position of the light beam a to the 
sensor pattern SA side by 450 .mu.m (S90). 
If NO in step S87, the main control unit 51 compares the output 
(A/D-converted value) from the differential amplifier 66 (E-F) with the 
determination reference values 800H and A00H stored in the memory 52 in 
advance to determine whether the output from the differential amplifier 66 
falls within the range of the determination reference values 800H to A00H 
(S91). 
If the output from the differential amplifier 66 is determined to fall 
within the range of 800H to A00H, the passing position of the light beam a 
is found to be closer to the sensor pattern SF side than the target 
passing position of the light beam d. In this case, the main control unit 
51 controls the galvanomirror 33a so as to shift the light beam a to the 
sensor pattern SG side by about 450 .mu.m (S92). 
If NO in step S91, the main control unit 51 compares the output 
(A/D-converted value) from the differential amplifier 65 (D-E) with the 
determination reference values 800H and A00H stored in the memory 52 in 
advance to determine whether the output from the differential amplifier 65 
falls within the range of the determination reference values 800H to A00H 
(S93). 
If the output from the differential amplifier 65 is determined to fall 
within the range of 800H to A00H, the passing position of the light beam a 
is found to be closer to the sensor pattern SE side than the target 
passing position of the light beam c. In this case, the main control unit 
51 controls the galvanomirror 33a so as to shift the light beam a to the 
sensor pattern SG side by about 500 .mu.m (S94). 
If NO in step S93, the main control unit 51 compares the output 
(A/D-converted value) from the differential amplifier 64 (C-D) with the 
determination reference values 800H and A00H stored in the memory 52 in 
advance to determine whether the output from the differential amplifier 64 
falls within the range of the determination reference values 800H to A00H 
(S95). 
If the output from the differential amplifier 64 is determined to fall 
within the range of 800H to A00H, the passing position of the light beam a 
is found to be closer to the sensor pattern SD side than the target 
passing position of the light beam b. In this case, the main control unit 
51 controls the galvanomirror 33a so as to shift the light beam a to the 
sensor pattern SG side by about 540 .mu.m (S96). 
If NO in step S95, the main control unit 51 compares the output 
(A/D-converted value) from the differential amplifier 63 (B-C) with the 
determination reference values 800H and A00H stored in the memory 52 in 
advance to determine whether the output from the differential amplifier 63 
falls within the range of the determination reference values 800H to A00H 
(S97). 
If the output from the differential amplifier 63 is determined to fall 
within the range of 800H to A00H, the passing position of the light beam a 
is found to be closer to the sensor pattern SC side than the target 
passing position of the light beam a. In this case, the main control unit 
51 controls the galvanomirror 33a so as to shift the light beam a to the 
sensor pattern SA side by about 520 .mu.m (S98). 
Since the purpose of this control is to shift the light beam a to the 
undetectable region, beam position control suffices to be rough without 
requiring high precision (e.g., 1 .mu.m or less). 
The light beams b, c, and d are similarly shifted to the undetectable 
region in steps S67, S71, and S75 in FIGS. 20 and 21. The shift of the 
light beams to the undetectable region (steps S63, S67, S71, and S75) can 
be successively performed. 
A routine operation of detecting (measuring) the offset value of the beam 
passing position detector for the light beam a in step S64 of FIG. 20 will 
be explained with reference to a flow chart shown in FIG. 24. 
The main control unit 51 selects a beam passing position detector for the 
light beam a in the beam detector output processor 40 (S101). In step 
S101, the analog switch SW1 is turned on to connect the output terminal of 
the differential amplifier 63 for calculating a difference between outputs 
from the sensor patterns SB and SC which detect the passing position of 
the light beam a, to the input terminal of the integrator 42. 
The main control unit 51 forcibly activates the laser oscillator 31a to 
emit a light beam (S102). Thereby, the light beam a is made to cyclically 
scan over the beam passing position detector 38. While the ight beam a 
passes over the beam passing position detector 38, a signal is output from 
the sensor pattern S1. The output signal from the sensor pattern S1 resets 
the integrator 42, and at the same time integration starts (S103). 
At this time, since the light beam a is shifted to the undetectable region, 
the light beam a is not detected by the sensor patterns SB and SC which 
control the passing position of the light beam a, and only the offset 
component is integrated. In other words, if the operational amplifier is 
ideal, the light beam a does not strike the sensor patterns SB and SC, so 
that the outputs from the operational amplifiers A1 and A2 are 0 V, and 
the output from the operational amplifier A3 is also 0 V. Since the output 
from the operational amplifier A4 is also 0 V, a value A/D-converted and 
read by the main control unit 51 is also 000H. 
However, since the operational amplifier constituting the beam passing 
position detector has an offset voltage, the value read by the main 
control unit 51 is not 000H, and the main control unit 51 reads a given 
value, which is an offset value. 
The integration operation ends in response to an output from the sensor 
pattern S2, and at the same time A/D conversion is performed (S104). Upon 
completion of the A/D conversion, the A/D converter 43 outputs an A/D 
conversion completion signal (S1O5). Upon reception of the A/D conversion 
completion signal, the main control unit 51 cancels the forcible emission 
of the laser oscillator 31a(S106), and reads the A/D-converted value 
(S107). 
The main control unit 51 stores the read offset value in the memory 52 
(S108). Finally, the main control unit 51 cancels the selection of the 
beam passing position detector for the light beam a (S109). That is, the 
main control unit 51 turns off the analog switch SW1. 
The same control is executed in offset detection of the light beams b, c, 
and d in steps S68, S72, and S76 in FIGS. 20 and 21. As a result, the 
offset values of the beam passing position detectors for all the light 
beams a to d, i.e., the offset values of the beam detector output 
processor 40 are detected. 
A routine operation of compensating the offset in steps S66, S70, S74, and 
S78 in FIG. 20 will be described below with reference to a flow chart 
shown in FIG. 25. 
The main control unit 51 determines the polarity of the detected offset 
value (S111). The polarity of the offset value varies depending on an 
operational amplifier. The beam detector output processor 40 is 
constituted by a plurality of operational amplifiers, and its polarity 
varies depending on an image forming apparatus which uses the beam 
detector output processor 40. For this reason, the polarity must be 
determined. 
The main control unit 51 determines the polarity of the read offset value. 
If the polarity is positive, the main control unit 51 subtracts the 
absolute value (.vertline.+Vos.vertline.) of the offset value from the 
determination reference value (100H in steps S33 and S35, 800H in step 
S37, or the like) in the above-mentioned beam passing position control 
routine in FIGS. 14 and 15 (S112). If the polarity of the read offset 
value is negative, the main control unit 51 adds the absolute value 
(.vertline.+Vos.vertline.) of the offset value to the determination 
reference value (S113). 
If normal beam passing position control is executed upon completion of the 
offset detection/compensation processing, no control error of the beam 
passing position is generated by the offset value because the offset value 
is considered in the determination reference value for beam passing 
position control. 
Offset detection/compensation according to the second embodiment will be 
described below. Offset compensation is performed by compensating the 
determination reference value using the offset value in the first 
embodiment, whereas offset compensation is performed by compensating 
detected beam position information using the offset value in the second 
embodiment. 
In the second embodiment, offset compensation is performed in the beam 
passing position control routine. Only the offset compensation method will 
be explained below. 
FIG. 26 is a flow chart showing a routine in which a main control unit 51 
compensates passing position information on a light beam a in the passing 
position control routine for the light beam a. The main control unit 51 
selects a beam passing position detector for the light beam a in a beam 
detector output processor 40 (S121). In step S121, an analog switch SW1 is 
turned on to connect the output terminal of a differential amplifier 63 
for calculating a difference between outputs from sensor patterns SB and 
SC which detect the passing position of the light beam a, to the input 
terminal of an integrator 42. 
The main control unit 51 forcibly activates a laser oscillator 31a to emit 
a light beam (S122). Thereby, the light beam a is made to cyclically scan 
over a beam passing position detector 38. While the light beam a passes 
over the beam passing position detector 38, a signal is output from a 
sensor pattern S1. The output signal from the sensor pattern S1 resets the 
integrator 42, and at the same time integration starts. 
The integration ends in response to an output from a sensor pattern S2, and 
at the same time A/D conversion is performed. Upon completion of the A/D 
conversion, an A/D converter 43 outputs an A/D conversion completion 
signal. Upon reception of the A/D conversion completion signal, the main 
control unit 51 reads the A/D-converted value (position information on the 
light beam a) (S124). 
Since the main control unit 51 has already grasped the offset value in the 
offset detection routine described in the first embodiment, it determines 
the polarity of the offset value (S125), and performs the following 
processing in accordance with the determination results. More 
specifically, if the polarity of the detected offset value is positive, 
the main control unit 51 subtracts the absolute value of the offset value 
from the read beam position information (beam position information-offset 
value; S126). If the polarity of the offset value is negative, the main 
control unit 51 adds the absolute value of the offset value to the read 
beam position information (beam position information+offset value; S127). 
In this manner, the offset value can be removed from the read beam position 
information by performing calculation in accordance with the polarity of 
the offset value so as to eliminate the offset. The main control unit 51 
determines the calculation results as final beam position information 
(S128), and controls a galvanomirror 33a. A method of controlling the 
galvanomirror 33a is the same as in the first embodiment. 
Offset detection/compensation according to the third embodiment will be 
described below. 
FIG. 27 is a block diagram for explaining offset detection/compensation 
processing according to the third embodiment. The basic arrangement is the 
same as in the first embodiment of FIG. 7 described above except that a 
beam detector output processor 40 includes a D/A converter 67. 
FIG. 28 shows in detail the relationship between an integrator and the D/A 
converter 67 in the beam detector output processor 40. In FIG. 28, the 
digital input terminal of the D/A converter 67 is connected to a main 
control unit 51, and receives a digital offset compensation value from the 
main control unit 51. The analog output terminal of the D/A converter 67 
is connected to the non-inverting input terminal of an operational 
amplifier A4 constituting the integrator 42. 
A method of detecting an offset value in the third embodiment is the same 
as in the first embodiment. By shifting a light beam to the undetectable 
region of a beam position detection pattern, sensor patterns S1 and S2 set 
timings to detect an offset value. 
In the third embodiment, the detected offset value is fed back as the 
reference voltage of the integrator 42, and offset detection is performed 
again. This operation is repeatedly performed until the offset value falls 
within a prescribed range. This operation will be explained in detail. 
FIGS. 29 and 30 are flow charts for explaining the operation of the offset 
compensation routine. The main control unit 51 activates a polygon motor 
36 and rotates a polygon mirror 35 at a predetermined rotational speed 
(S131). The main control unit 51 reads out the previous drive values of 
galvanomirrors 33a to 33d from a memory 52, and drives the galvanomirrors 
33a to 33d on the basis of these values (S132). 
The main control unit 51 emits only a light beam a, and controls the 
galvanomirror 33a so as to shift the light beam a to the undetectable 
region of a beam position detection pattern (S133). In this control, the 
main control unit 51 grasps the current beam passing positions and 
controls the galvanomirror 33a so as to shift the light beam a to a region 
where the light beam a is not detected by sensor patterns SB, SC, SD, SE, 
and SF. 
Subsequently, the main control unit 51 detects the offset value of a beam 
passing position detector for the light beam a in the beam detector output 
processor 40 (offset detection; S134). 
The main control unit 51 determines whether the detected offset value falls 
within the prescribed range (S135). Ideally, the defined value is 000H 
when no offset value exists. However, the defined value is generally set 
with a margin to a certain degree. When the offset value falls outside the 
prescribed range, the main control unit 51 sets the read offset value as 
an offset compensation value in the D/A converter 67 (S136). The D/A 
converter 67 D/A-converts the set offset compensation value, and inputs 
the obtained value as the reference voltage of the integrator 42 to the 
non-inverting input terminal of the operational amplifier A4. 
Thereafter, the main control unit 51 detects the offset value again (S134), 
and determines whether the offset value falls within the prescribed range 
(S135). 
An integrated output from the integrator 42 in FIG. 28 is negative when the 
voltage at the input terminal (inverting input terminal:-) is higher than 
the voltage at the reference voltage terminal (non-inverting input 
terminal:+), and positive when the voltage at the input terminal is 
smaller. When the reference voltage is equal to the input voltage, the 
integrated output is "0". That is, when the offset value is set at the 
reference voltage terminal, the offset value becomes "0" by performing 
offset detection again as far as the offset voltage of the circuit is 
equal to the previous voltage. When the offset voltage falls outside the 
prescribed range, offset detection is performed again. 
When the offset value falls within the prescribed range, the main control 
unit 51 stores the offset value in the memory 52, and sequentially carries 
out steps S137 to S148 to perform the same offset value compensation for 
light beams b, c, and d. 
In performing beam passing position control, the offset voltage (addition 
value of the detected offset voltage) settled by the above control is set 
as the reference voltage of the integrator 42. 
The offset voltage of the operational amplifier varies upon a change in 
temperature. For example, in powering on the image forming apparatus, an 
error may occur in one offset detection operation. According to the third 
embodiment, however, high-precision offset compensation can be attained 
because (offset detection).fwdarw.(offset compensation) is repeatedly 
executed until the offset value falls within the prescribed range. 
Offset detection/compensation according to the fourth embodiment will be 
described below. 
According to the fourth embodiment, in performing offset detection, the 
reset and integration start/end timings of an integrator 42, or the A/D 
conversion start timing of an A/D converter 43 is generated not by sensor 
patterns S1 and S2 for detecting a normal beam position but by a timing 
sensor for detecting an offset. That is, a beam passing position detector 
38 includes an offset detection sensor pattern, which will be explained 
below. 
FIG. 31 shows the arrangement of the beam passing position detector 38 
according to the fourth embodiment. As shown in FIG. 31, offset detection 
sensor patterns S3 and S4 having the same shape as that of the sensor 
pattern S1 are arranged parallel to the sensor pattern S1 on the left side 
of the beam position detection sensor pattern S1 with respect to the sheet 
surface. 
The sensor pattern S3 is a pattern for generating the reset signal and 
integration start timing of the integrator 42 in offset detection. The 
sensor pattern S4 is a pattern for generating integration end and A/D 
conversion start signals in offset detection. The positional relationship 
between the sensor patterns S3 and S4 is the same as the positional 
relationship between the sensor patterns S1 and S2. In other words, the 
distance between the sensor patterns S3 and S4 is W4. 
FIG. 32 is a block diagram for explaining offset detection/compensation 
processing according to the fourth embodiment. The basic arrangement is 
the same as in the first embodiment of FIG. 7 described above except that 
the beam passing position detector 38 shown in FIG. 31 is used, and 
selection circuits 68 and 69 (A and B) for switching between outputs from 
the sensor patterns S1 to S4 serving as timing sensors in normal beam 
passing position control and offset detection are arranged. 
The selection circuit 68 (A) receives output signals from the sensor 
patterns S1 and S3, and selects either signal in accordance with a 
selection signal from a main control unit 51. The selection circuit 69 (B) 
receives output signals from the sensor patterns S2 and S4, and selects 
either signal in accordance with a selection signal from the main control 
unit 51. That is, the output signals from the sensor patterns S1 and S2 
are selected in beam passing position control, and the output signals from 
the sensor patterns S3 and S4 are selected in offset detection. 
FIG. 33 is a flow chart for explaining the operation of the offset 
compensation routine. The main control unit 51 activates a polygon motor 
36 and rotates a polygon mirror 35 at a predetermined rotational speed 
(S151). The main control unit 51 reads out the previous drive values of 
galvanomirrors 33a to 33d from a memory 52, and drives the galvanomirrors 
33a to 33d on the basis of these values (S152). 
The main control unit 51 detects the offset value of a beam passing 
position detector for a light beam a in a beam detector output processor 
40 (offset detection; S153). The fourth embodiment is different from the 
first embodiment in that the light beam a is not shifted to the 
undetectable region of a passing position detection sensor pattern at this 
time. 
Subsequently, the main control unit 51 determines whether the detected 
offset value falls within the prescribed range (S154). When the offset 
value falls outside the prescribed range, the main control unit 51 
executes offset compensation on the basis of the detected offset value 
(offset compensation; S155). The offset compensation method is the same as 
in the first embodiment described above. 
The main control unit 51 sequentially carries out steps S156 to S164 to 
perform the same control: (offset detection).fwdarw.(offset compensation) 
for light beams b, c, and d. 
Upon completion of offset compensation for all the light beams a to d, the 
main control unit 51 performs beam passing position control. 
A routine operation of detecting (measuring) the offset value of the beam 
passing position detector for the light beam a in step S153 of FIG. 33 
will be explained with reference to a flow chart shown in FIG. 34. 
The main control unit 51 outputs sensor selection signals to the selection 
circuits 68 and 69 to select the sensor patterns S3 and S4 (S171). The 
selection circuit 68 inputs an output from the sensor pattern S3 as a 
reset signal (also serving as an integration start signal) for the 
integrator 42 to a reset switch (analog switch) SW7 of the integrator 42. 
The selection circuit 69 inputs an output from the sensor pattern S4 as an 
A/D conversion start signal (also serving as an integration end signal) 
for the A/D converter 43 to the A/D converter 43. 
The main control unit 51 selects a beam passing position detector for the 
light beam a in the beam detector output processor 40 (S172). In step 
S172, an analog switch SW1 is turned on to connect the output terminal of 
a differential amplifier 63 for calculating a difference between outputs 
from sensor patterns SB and SC which detect the passing position of the 
light beam a, to the input terminal of the integrator 42. 
The main control unit 51 forcibly activates a laser oscillator 31a to emit 
a light beam (S173). Thereby, the light beam a is made to cyclically scan 
over a beam passing position detector 38. While the light beam a passes 
over the beam passing position detector 38, a signal is output from the 
sensor pattern S3. The output signal from the sensor pattern S3 resets the 
integrator 42, and at the same time integration starts (S174). 
The integration operation ends in response to an output from the sensor 
pattern S4, and at the same time A/D conversion is performed (S175). 
An offset value is detected by performing integration while no light beam 
is irradiated on the beam passing position detection sensor patterns SB to 
SF. According to this method, offset detection is performed before the 
light beam a reaches the beam passing position detection sensor patterns 
SB to SF. The timing is formed using the sensor patterns S3 and S4 
identical to the sensor patterns S1 and S2 for generating integration 
start and end timings used in beam passing position detection control, and 
an error upon offset detection is minimized. 
Upon completion of the A/D conversion, the A/D converter 43 outputs an A/D 
conversion completion signal (S176). Upon reception of the A/D conversion 
completion signal, the main control unit 51 cancels the forcible emission 
of the laser oscillator 31a (S177), and reads the A/D-converted value 
(offset value) (S178). 
The main control unit 51 stores the read offset value in the memory 52 
(S179). Finally, the main control unit 51 cancels the selection of the 
beam passing position detector for the light beam a (S180). That is, the 
main control unit 51 turns off the analog switch SW1. 
The same control is executed in offset detection of the light beams b, c, 
and d in steps S156, S159, and S162 in FIG. 33. As a result, the offset 
values of the beam passing position detectors for all the light beams a to 
d, i.e., the offset values of the beam detector output processor 40 are 
detected. 
Offset detection/compensation according to the fifth embodiment will be 
described below. 
In the fifth embodiment, an offset value is detected by the same method as 
in the fourth embodiment (using the sensor patterns S3 and S4), and the 
offset value is compensated by the same method as in the second embodiment 
(compensation of beam position information). Therefore, a description of 
the fifth embodiment will be omitted. 
Offset detection/compensation according to the sixth embodiment will be 
described below. 
In the sixth embodiment, an offset value is detected by the same method as 
in the fourth embodiment (using the sensor patterns S3 and S4), and the 
offset value is compensated by the same method as in the third embodiment 
(offset compensation of the operational amplifier via the D/A converter). 
FIG. 35 is a block diagram for explaining offset detection/compensation 
processing according to the sixth embodiment. The basic arrangement is the 
same as in the fourth embodiment of FIG. 32 described above except that a 
beam detector output processor 40 includes a D/A converter 67, similar to 
the third embodiment. Therefore, a description of the sixth embodiment 
will be omitted. 
Offset detection/compensation according to the seventh embodiment will be 
described below. 
In the seventh embodiment, the integration start timing is managed by a 
timing sensor for beam passing position control, and the integration end 
timing is managed by a timer serving as a timepiece means. The offset 
compensation method is the same as in the first embodiment (compensation 
using a determination reference value), and a description thereof will be 
omitted. 
FIG. 36 is a block diagram for explaining offset detection/compensation 
processing according to the seventh embodiment. The basic arrangement is 
the same as in the first embodiment of FIG. 7 described above except that 
a timer 70 serving as a timepiece means, and a selection circuit 71 are 
arranged. 
The selection circuit 71 receives an output signal from a sensor pattern S2 
and a measurement completion signal from the timer 70, either of which is 
selected by a selection signal from a main control unit 51. That is, the 
output signal from the sensor pattern S2 is selected in beam passing 
position control, and the measurement completion signal from the timer 70 
is selected in detecting an offset value. The selected signal is input as 
an A/D conversion start signal (integration end) to an A/D converter 43. 
The timer 70 starts measurement upon reception of an output signal from a 
sensor pattern S1 as a measurement start signal. After the lapse of a time 
set by the main control unit 51, the timer 70 outputs a measurement 
completion signal. The measurement completion signal is sent as an A/D 
conversion start signal to the A/D converter 43 via the selection circuit 
71. At the same time, the measurement completion signal is also output to 
the main control unit 51. 
The measurement time of the timer 70 can be arbitrarily set by the main 
control unit 51. In the seventh embodiment, the measurement time is a time 
interval between output of a signal from the sensor pattern S1 and output 
of a signal from the sensor pattern S2 (i.e., a time interval between 
integration start and integration end). 
FIG. 37 is a flow chart for explaining the operation of the offset 
detection routine in the seventh embodiment. A method of detecting an 
offset value will be explained with reference to FIG. 37. Although FIG. 37 
exemplifies the case of a light beam a, the same operation is also 
performed for light beams b, c, and d. 
The main control unit 51 outputs a selection signal to the selection 
circuit 71 to select the measurement completion signal from the timer 70 
as an A/D conversion start signal (S181). The measurement completion 
signal from the timer 70 is input to the A/D converter 43 as the A/D 
conversion start signal for the A/D converter 43 (also serving as an 
integration end signal). 
The main control unit 51 selects a beam passing position detector for the 
light beam a in a beam detector output processor 40 (S182). In S182, an 
analog switch SW1 is turned on to connect the output terminal of a 
differential amplifier 63 for calculating a difference between outputs 
from sensor patterns SB and SC which detect the passing position of the 
light beam a, to the input terminal of an integrator 42. 
The main control unit 51 forcibly activates a laser oscillator 31a to emit 
a light beam (S183). Thereby, the light beam a is made to cyclically scan 
over a beam passing position detector 38. While the light beam a passes 
over the beam passing position detector 38, a signal is output from the 
sensor pattern S1. The output signal from the sensor pattern S1 resets the 
integrator 42, and at the same time integration starts (S184). 
At this time, the main control unit 51 cancels the forcible emission of the 
laser oscillator 31a. Since the timer 70 receives the output from the 
sensor pattern S1 as a measurement start signal for the timer 70, the 
timer 70 starts a measurement operation (S185). 
The forcible emission of the laser oscillator 31ais canceled because, in 
this embodiment, since the light beam is not shifted to the undetectable 
region of the beam passing position detection sensor pattern, if the light 
beam is continuously emitted, the light beam may scan over the beam 
passing position detection sensor pattern, and the beam position 
information may be superposed on a measured offset value, failing to 
measure a correct offset value. 
Subsequently, the timer 70 starts measuring the time set by the main 
control unit 51. After measuring the predetermined time (S186), the timer 
70 outputs a measurement completion signal. The measurement completion 
signal is input as a conversion start signal to the A/D converter 43 via 
the selection circuit 71. 
Upon reception of the conversion start signal, the A/D converter 43 starts 
A/D conversion (S187). Upon completion of the A/D conversion, the A/D 
converter 43 outputs an A/D conversion completion signal to the main 
control unit 51 (S188). Upon reception of the A/D conversion signal, the 
main control unit 51 reads an A/D-converted value (offset value) (S189). 
The main control unit 51 stores the read offset value in a memory 52 
(S190). Finally, the main control unit 51 cancels the selection of the 
beam passing position detector for the light beam a (S119). That is, the 
main control unit 51 turns off the analog switch SW1. 
The same control is executed in offset detection of the light beams b, c, 
and d to detect the offset values of the beam passing position detectors 
for all the light beams a to d, i.e., the offset values of the beam 
detector output processor 40. 
Offset detection/compensation according to the eighth embodiment will be 
described below. 
In the eighth embodiment, an offset value is detected by the same method as 
in the seventh embodiment (using the sensor pattern S1 and the timer), and 
the offset value is compensated by the same method as in the second 
embodiment (compensation of position information). Therefore, a 
description of the eighth embodiment will be omitted. 
Offset detection/compensation according to the ninth embodiment will be 
described below. 
In the ninth embodiment, an offset value is detected by the same method as 
in the seventh embodiment (using the sensor pattern S1 and the timer), and 
the offset value is compensated by the same method as in the third 
embodiment (offset compensation of the operational amplifier via the D/A 
converter). 
FIG. 38 is a block diagram for explaining offset detection/compensation 
processing according to the ninth embodiment. The basic arrangement is the 
same as in the seventh embodiment (using the sensor pattern S1 and the 
timer) of FIG. 36 described above except that a beam detector output 
processor 40 includes a D/A converter 67, similar to the third embodiment. 
Therefore, a description of the ninth embodiment will be omitted. 
Offset detection/compensation according to the 10th embodiment will be 
described. 
In the 10th embodiment, output signals from sensor patterns S1 and S2 are 
used as integration start and end timings in offset compensation. To 
prevent a light beam from being irradiated on beam passing position 
detection sensor patterns SB to SF to superpose beam position information 
on an offset value, the beam passing position detection sensor patterns SB 
to SF are shielded from the light beam by a beam-shielding plate in an 
offset detection mode. The offset compensation method is the same as in 
the above-mentioned first embodiment (compensation using a determination 
reference value), and a description thereof will be omitted. 
FIG. 39 shows the relationship between a beam position detector 38 and a 
beam-shielding plate 72 used in the 10th embodiment. The layout and 
function of the sensor patterns are the same as in the above-described 
first embodiment, and a description thereof will be omitted. The 
beam-shielding plate 72 serving as a beam-shielding means is formed of a 
beam-shielding member which does not transmit a light beam. In offset 
detection, as shown in FIG. 39, the beam-shielding plate 72 is set at a 
position where it completely covers the sensor patterns SA to SG to 
prevent a light beam from being irradiated on the beam passing position 
detection sensor patterns SB to SF. 
In beam passing position control, the beam-shielding plate 72 is moved by 
an actuator (not shown) to a position where the plate 72 does not 
influence the control (i.e., a position where any of all the sensor 
patterns is not shielded from a light beam). 
FIG. 40 is a block diagram for explaining offset detection/compensation 
processing according to the 10th embodiment. The basic arrangement is the 
same as in the first embodiment of FIG. 7 described above except that the 
beam-shielding plate 72 is arranged in the beam passing position detector 
38, as shown in FIG. 39, and a pulse motor 73 serving as an actuator for 
moving the beam-shielding plate 72, and a driver 74 for driving the pulse 
motor 73 are arranged. 
The beam-shielding plate 72 has normally been moved by the pulse motor 73 
to a position where the plate 72 does not influence beam passing position 
control (i.e., a position where any of all the sensor patterns is not 
shielded from a light beam). Only in offset detection, the beam-shielding 
plate 72 is moved by the pulse motor 73 to a position where the plate 72 
completely covers the sensor patterns SA to SG as shown in FIG. 40. 
The pulse motor 73 for moving the beam-shielding plate 72 is driven by the 
driver 74. The driver 74 is connected to a main control unit 51, and 
controlled by an ON/OFF instruction and a forward/backward rotation 
instruction from the main control unit 51. 
FIGS. 41 and 42 are flow charts for explaining the operation of the offset 
compensation routine. The main control unit 51 activates a polygon motor 
36 and rotates a polygon mirror 35 at a predetermined rotational speed 
(S201). The main control unit 51 reads out the previous drive values of 
galvanomirrors 33a to 33d from a memory 52, and drives the galvanomirrors 
33a to 33d on the basis of these values (S202). 
The main control unit 51 sends an ON signal and a forward or backward 
rotation signal to the driver 74 of the pulse motor 73 to operate the 
pulse motor 73 and move the beam-shielding plate 72, thereby shielding the 
sensor patterns SA to SG from light beams (S203). 
The main control unit 51 detects the offset value of a beam passing 
position detector for a light beam a in a beam detector output processor 
40 (offset detection; S204). Subsequently, the main control unit 51 
determines whether the detected offset value falls within a prescribed 
range (S205). When the offset value falls outside the prescribed range, 
the main control unit 51 executes offset compensation on the basis of the 
detected offset value (offset compensation; S206). The offset compensation 
method is the same as in the first embodiment described above. 
The main control unit 51 sequentially carries out steps S207 to S216 to 
perform the same control: (offset detection).fwdarw.(offset compensation) 
for light beams b, c, and d. 
Upon completion of offset compensation for all the light beams a to d, the 
main control unit 51 performs beam passing position control. 
FIG. 43 is a view for explaining an actuator for moving the beam-shielding 
plate 72 in the 10th embodiment. This embodiment exemplifies a case using 
the pulse motor 73 as the actuator for moving the beam-shielding plate 72. 
A rotating shaft 75 of the pulse motor 73 is ball-screwed, and a stage 76 
mounting the beam-shielding plate 72 is attached to the rotating shaft 75. 
The two sides of the stage 76 are supported by a guide (not shown). As the 
pulse motor 73 rotates, the beam-shielding plate 72 fixed to the stage 76 
reciprocally moves in the arrow direction in FIG. 43. 
The main control unit 51 instructs the rotation direction (forward/backward 
rotation) to the pulse motor 73 via the driver 74. When a drive pulse is 
output in response to an ON signal, the pulse motor 73 starts rotating. 
For example, when an offset value is to be detected, the main control unit 
51 sets the rotation direction to a forward rotation direction, and 
outputs an ON signal. In response to the output of the ON signal, the 
pulse motor 73 starts rotating to move the beam-shielding plate 72. The 
main control unit 51 manages the number of drive pulses. After outputting 
a predetermined number of pulses, the main control unit 51 outputs an OFF 
signal to stop the rotation of the pulse motor 73. The predetermined 
number of pulses is the number of pulses necessary to move the 
beam-shielding plate 72 from a standby position until the beam passing 
position detection sensor patterns SA to SG are shielded from a light 
beam. 
Note that, if the OFF signal is output to stop the rotation of the pulse 
motor 73, the beam-shielding plate 72 does not move because the pulse 
motor 73 has a holding force. 
In performing beam passing position detection control, the beam-shielding 
plate 72 is moved to a standby position (in the state of FIG. 39). That 
is, the main control unit 51 sets the rotation direction to a backward 
rotation direction, and outputs an ON signal. In response to the output of 
the ON signal, the pulse motor 73 starts rotating to move the 
beam-shielding plate 72 to the standby position. After outputting a 
predetermined number of pulses, the main control unit 51 outputs an OFF 
signal to stop the rotation of the pulse motor 73. 
Offset detection/compensation according to the 11th embodiment will be 
described below. 
In the 11th embodiment, an offset value is detected by the same method as 
in the 10th embodiment (using the shielding plate), and the offset value 
is compensated by the same method as in the second embodiment 
(compensation of position information). Therefore, a description of the 
11th embodiment will be omitted. 
Offset detection/compensation according to the 12th embodiment will be 
described below. 
In the 12th embodiment, an offset value is detected by the same method as 
in the 10th embodiment (using the shielding plate), and the offset value 
is compensated by the same method as in the third embodiment (compensation 
of the operational amplifier via the D/A converter). 
FIG. 44 is a block diagram for explaining offset detection/compensation 
processing according to the 12th embodiment. The basic arrangement is the 
same as in the 10th embodiment (using the shielding plate) of FIG. 40 
described above except that a beam detector output processor 40 includes a 
D/A converter 67, similar to the third embodiment. Therefore, a 
description of the 12th embodiment will be omitted. 
Offset detection/compensation according to the 13th embodiment will be 
described below. 
In the 13th embodiment, the integration end timing is managed by a timer 
serving as a timepiece means. The offset compensation method is the same 
as in the first embodiment, and a description thereof will be omitted. 
FIG. 45 is a block diagram for explaining offset detection/compensation 
processing according to the 13th embodiment. The basic arrangement is the 
same as in the first embodiment of FIG. 7 described above except that 
selection circuits 68 and 69, and a timer 70 serving as a timepiece means 
are arranged. 
The selection circuit 68 receives an output signal from a sensor pattern S1 
and a measurement start signal for the timer 70, either of which is 
selected by a selection signal from a main control unit 51. That is, the 
output from the sensor pattern S1 is selected in beam passing position 
control, and the measurement start signal for the timer 70 is selected in 
offset detection. The selected signal is input to an integrator 42 as 
integration reset and integration start signals. 
The selection circuit 69 receives an output signal from a sensor pattern S2 
and a measurement completion signal from the timer 70, either of which is 
selected by a selection signal from the main control unit 51. That is, the 
output signal from the sensor pattern S2 is selected in beam passing 
position control, and the measurement completion signal from the timer 70 
is selected in offset detection. The selected signal is input as an A/D 
conversion start signal (integration end) to an A/D converter 43. 
The timer 70 starts measurement upon reception of the measurement start 
signal output signal from the main control unit 51. After the lapse of a 
time set by the main control unit 51, the timer 70 outputs a measurement 
completion signal. The measurement start signal is sent as integration 
reset and integration start signals to an analog switch SW1 of the 
integrator 42 via the selection circuit 68. The measurement completion 
signal is sent as an A/D conversion start signal to the A/D converter 43 
via the selection circuit 69. At the same time, the measurement completion 
signal is also output to the main control unit 51. 
The measurement time of the timer 70 can be arbitrarily set by the main 
control unit 51. In this embodiment, the measurement time is a time 
interval between output of a signal from the sensor pattern S1 and output 
of a signal from the sensor pattern S2 (i.e., a time interval between 
integration start and integration end). 
FIG. 46 is a flow chart for explaining the operation of the offset 
detection routine in the 13th embodiment. A method of detecting an offset 
value will be explained with reference to FIG. 46. Although FIG. 46 
exemplifies the case of a light beam a, the same operation is also 
performed for light beams b, c, and d. 
The main control unit 51 outputs a selection signal to the selection 
circuit 68 to select the measurement start signal for the timer 70 as 
integration reset and integration start signals. The measurement start 
signal for the timer 70 is input to an analog switch SW7 of the integrator 
42. The main control unit 51 outputs a selection signal to the selection 
circuit 69 to select the measurement completion signal from the timer 70 
as an A/D conversion start signal. The measurement completion signal from 
the timer 70 is input to the A/D converter 43 as the A/D conversion start 
signal (also serving as an integration end signal) (offset detection mode; 
S221). 
The main control unit 51 selects a beam passing position detector for the 
light beam a in a beam detector output processor 40 (S222). In S222, the 
analog switch SW1 is turned on to connect the output terminal of a 
differential amplifier 63 for calculating a difference between outputs 
from sensor patterns SB and SC which detect the passing position of the 
light beam a, to the input terminal of the integrator 42. 
The main control unit 51 outputs the measurement start signal to the timer 
70 and the analog switch SW7 of the integrator 42. After this measurement 
start signal resets the integrator 42, integration starts. At the same 
time, the timer 70 starts measurement (S223). 
Subsequently, the timer 70 starts measuring the time set by the main 
control unit 51. After measuring the predetermined time (S224), the timer 
70 outputs a measurement completion signal. The measurement completion 
signal is input as a conversion start signal to the A/D converter 43 via 
the selection circuit 69. 
Upon reception of the conversion start signal, the A/D converter 43 starts 
A/D conversion (S225). That is, the A/D converter 43 converts the 
integrated offset value as an analog signal into a digital signal. Upon 
completion of the A/D conversion, the A/D converter 43 outputs an A/D 
conversion completion signal to the main control unit 51 (S226). 
Upon reception of the A/D conversion signal, the main control unit 51 reads 
the A/D-converted value (offset value) (S227). Finally, the main control 
unit 51 stores the offset value in a memory 52 (S228), and cancels the 
selection of the beam passing position detector for the light beam a 
(S229). That is, the main control unit 51 turns off the analog switch SW1. 
The same control is executed in offset detection of the light beams b, c, 
and d to detect the offset values of the beam passing position detectors 
for all the light beams a to d, i.e., the offset values of the beam 
detector output processor 40. 
According to the 13th embodiment, the offset value of the beam detector 
output processor 40 can be detected without making a light beam scan by a 
polygon mirror 35. That is, since no light beam is emitted, no light beam 
is irradiated on the beam passing position detector 38, no beam position 
information is superposed on an offset component, and an offset value can 
be detected with high precision. 
Offset detection/compensation according to the 14th embodiment will be 
described below. 
In the 14th embodiment, an offset value is detected by the same method as 
in the 13th embodiment (using no sensor but the timer), and the offset 
value is compensated by the same method as in the second embodiment 
(compensation of position information). Therefore, a description of the 
14th embodiment will be omitted. 
Offset detection/compensation according to the 15th embodiment will be 
described below. 
In the 15th embodiment, an offset value is detected by the same method as 
in the 13th embodiment (using no sensor but the timer), and the offset 
value is compensated by the same method as in the third embodiment 
described above. 
FIG. 47 is a block diagram for explaining offset detection/compensation 
processing according to the 15th embodiment. The basic arrangement is the 
same as in the 13th embodiment of FIG. 45 described above except that a 
beam detector output processor 40 includes a D/A converter 67, similar to 
the third embodiment. Therefore, a description of the 15th embodiment will 
be omitted. 
As has been described above, according to the above embodiments, beam 
position control almost free from a control error can be realized by 
detecting the offset value of a beam position detector output processor 
for converting an output from a beam position detector into beam position 
information, and performing compensation processing in accordance with the 
detected offset value. Accordingly, the position of a light beam can 
always be controlled at a proper position on a photosensitive drum with 
high precision, thereby always maintaining high image quality. 
The beam position detector having the above-described sensor patterns is 
used. Thereby, the scan positions of the light beams can be exactly 
detected even if the precision in mounting angle of the beam position 
detector is not high. 
In the digital copying machine using the multi-beam optical system, the 
passing positions of light beams are detected by the beam position 
detector situated on a level with the surface of the photo-sensitive drum. 
Based on the detected result, control amounts for optimally controlling 
the relative positions of the beams on the surface of the photo-sensitive 
drum are calculated. Based on the calculated control amounts, the 
galvanomirrors are controlled to alter the relative positions of the beams 
on the surface of the photo-sensitive drum. Thereby, the positional 
relationship among the light beams on the surface of the photo-sensitive 
drum can be optimally controlled with no particular precision or 
adjustment for the assembly of the optical system, even if some change 
occurs in the structure of the optical system due to a variation in 
ambience or a variation with the passing of time. Therefore, high image 
quality can be maintained at all times. 
In the above-described embodiments, the present invention is applied to the 
digital copying machine using the multi-beam optical system. However, the 
present invention is not limited to this, and is applicable to image 
forming apparatuses such as a high-speed printer, other than the digital 
copying machine. 
As has been described above, according to the present invention, a light 
beam scanning apparatus and an image forming apparatus in which the 
position of a light beam on a surface to be scanned can always be 
controlled to a proper position with high precision, thereby maintaining 
high image quality at all times can be provided. 
In addition, according to the present invention, a light beam scanning 
apparatus and an image forming apparatus in which the positional 
relationship between a plurality of light beams on a surface to be scanned 
can always be controlled to be ideal with high precision, thereby 
maintaining high image quality at all times can be provided. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details and representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalent.