Image forming apparatus having two-beam optical scanning unit with movable laser beam emitters and separate dynamic and precision adjusting of laser beams

An image forming apparatus, such as a copying machine or a printer having a two-beam optical scanning unit, includes a pair of semiconductor laser beam emitters, each emitter generating a respective laser beam. A beam composition prism composes the two laser beams and a photoreceptor is provided for holding an image written with the two laser beams. A defector deflects the two laser beams onto the photoreceptor in a primary scanning direction so that the two laser beams are emitted on a primary scanning plane. Two supporting members each respectively supports one of the pair of semiconductor laser beam emitters. A moving member is provided for moving at least one of the two supporting members so that at least one of the two laser beams is movably adjusted so as to have a predetermined emitting direction.

BACKGROUND OF THE INVENTION 
In a conventional image forming apparatus, a recording operation is 
performed by writing to a photoreceptor with a laser beam; therefore, the 
image forming apparatus consists of a semiconductor laser emitting body to 
generate a laser beam and a collimator lens or the like, which are formed 
in a single unit for an optical scanning system in an exposure unit. When 
a recording operation by writing with two beams is performed, two sets of 
the units, in each of which the semiconductor laser emitting body and the 
collimator lens or the like are unitedly formed, are provided. 
However, in this case, the optical scanning paths of the two beams need to 
be arranged precisely. Conventionally, a precise adjustment of the optical 
scanning paths in the subsidiary scanning direction is, for example, 
disclosed as the way that the pitch adjustment in the subsidiary scanning 
direction is performed with one prism (Japanese Patent Publication Open to 
the Public Inspection Nos. 58-68016/1983, and 63-50809/1988) and as the 
way that the adjustment is performed by moving the single unit, consisting 
of the semiconductor laser unit, in the subsidiary scanning direction 
(Japanese Patent Publication Open to the Public Inspection No. 
62-86324/1987). 
As explained above, in the conventional technologies, the adjustment of the 
optical scanning paths in the subsidiary scanning direction is performed. 
Further, there is a conventional way that a discrepancy in the primary 
scanning direction is adjusted by detecting the discrepancy of the two 
beams with an index sensor and delaying the signals electrically. However, 
when the discrepancy of the two beams is large, it is impossible to 
compensate the discrepancy completely. In other words, if the incident 
position of one of the two beams is discrepant in the primary scanning 
direction in relation to the another one of the two beams, the scanning 
focal positions of the two beams are discrepant from each other. 
When the two beams, generated from two semiconductor laser emitting bodies, 
are composed by a beam composition prism, if the location error of the 
beam composition prism occurs, there tends to be a problem that the beam 
at the reflection side of the beam composition prism has the discrepancy 
of its axis. For example, when the beam composition prism is positioned 
discrepant in a plane parallel to the axis of the beams, one beam, which 
is transmitted through the beam composition prism, is not affected but the 
another beam, which is reflected at the beam composition prism, is 
affected so that the irradiating direction of the beam becomes discrepant 
in the primary scanning direction. Especially, when the beam composition 
prism is fixed to a part of the optical scanning system with an adhesive, 
the precision of the beam arrangements is required to be very strict; 
therefore, when an adhesion mistake occurs or a shape precision at the 
adhesion surface is not ensured, the whole unit of the optical scanning 
system can be defective due to discrepancy of the beam axis. Further, the 
adhesion technique is not suitable for easy assembly. It requires a 
complicated inspection for the entire exposure unit after the adhesion. 
SUMMARY OF THE INVENTION 
Accordingly, the objective of the present invention is to solve the above 
explained problems and to prevent the recording apparatus, which writes 
with two beams, from the positioning discrepancy of two scanning beams, 
especially in the primary scanning direction. 
In order to accomplish the above-described objects, the present invention 
provides the following apparatus and methods. 
In a two-beam optical scanning unit for simultaneously scanning two lines 
and writing image data onto the surface of a photoreceptor by two beams 
generated from two sets of semiconductor laser beam emitting bodies, 
through a beam composition prism for composing the two beams, a deflector, 
and an image forming optical system, the beam position is adjusted by a 
moving unit for moving the two sets of semiconductor laser beam emitting 
bodies in parallel in a primary scanning direction, and an angle changing 
unit for changing the angles of the two sets of semiconductor laser beam 
emitting bodies in the primary scanning surface. The moving unit for 
moving the two sets of semiconductor laser beam emitting bodies in 
parallel in the primary scanning direction, is a unit which is moved 
relative to the base body of the beam optical scanning unit by the 
rotation of an eccentric cam. An angle changing unit for changing angles 
of the semiconductor laser beam emitting bodies, changes the angle by 
rotation of an eccentric cam rotated by a worm gear, from the position, to 
which the laser beam emitting body is moved with respect to the base body 
of the beam optical scanning unit by the moving unit. The beam position 
adjustment is carried out under the condition that the semiconductor laser 
emitting body is attached to the base body, and a beam position detection 
unit is provided on a portion of the base body. An opening is formed in 
one portion of the base body or the image forming apparatus between the 
semiconductor laser beam emitting body and the beam position detection 
unit, and the laser beam is detected by the beam position detection unit 
through the opening. 
In a two-beam optical scanning unit for simultaneously scanning two lines 
and writing image data onto the surface of a photoreceptor by two beams 
generated from two sets of semiconductor laser beam emitting bodies, a 
beam composition prism for composing the two beams, a deflector, and an 
image forming optical system, the beam composition prism and a cylindrical 
lens are integrally fixed onto a stationary member, and the stationary 
member is provided on a portion of the two-beam optical scanning unit for 
writing. 
In a two-beam optical scanning unit for simultaneously scanning two lines 
and writing image data onto the surface of a photoreceptor by two beams 
generated from two sets of semiconductor laser beam emitting bodies, a 
beam shaping optical system for shaping the two beams, a beam composition 
prism for composing the two beams, a deflector, and an image forming 
optical system, and a pair of prisms for compressing the two beams in the 
subsidiary scanning direction, a pair of prisms for adjusting beam pitches 
of the beams in the subsidiary scanning direction, a beam position 
adjusting unit for adjusting the beam position in the primary scanning 
direction by moving at least one of the two sets of semiconductor laser 
emitting bodies in parallel to the primary scanning direction, and a beam 
angle adjusting unit for adjusting the beam angle in the surface of the 
primary scanning direction, are provided in the apparatus. The beam pitch 
adjustment in the subsidiary scanning direction by the pair of prisms is 
carried out by the rotation adjustment of a screw. The adjustment of the 
beam position and the beam angle in the primary scanning direction is 
carried out by the eccentric cam and a pair of gears.

DETAILED DESCRIPTION OF THE INVENTION 
Examples will be explained below with reference to the attached drawings of 
a two-beam optical scanning system unit of the present invention. 
FIG. 1 is a view of a comprehensive structure showing an example of a 
two-beam optical scanning unit. 
In FIG. 1, numerals 1A and 1B represent semiconductor laser beam emitting 
bodies. Numerals 2A and 2B are collimator lenses (an optical system for 
beam shaping). Numerals 14 and 15 are prisms for the primary and 
subsidiary scanning adjustment. Numeral 3 is a beam composition prism. 
Numeral 5 is the first cylindrical lens. Numeral 6 is a polygonal mirror, 
and numeral 7 is an f.theta. lens. Numeral 8 is the second cylindrical 
lens, and numeral 9 is a mirror. Numeral 10 is a photoreceptor drum. 
Numeral 11 is a timing detection mirror, and numeral 12 is a synchronism 
detector. Numeral 13 is a driving motor for the polygonal mirror 6. A beam 
L.sub.1 emitted from the semiconductor laser beam emitting body 1A is made 
parallel by the collimator lens 2A, and then enters into the beam 
composition prism 3. A beam L.sub.2 emitted from the semiconductor laser 
beam emitting body 1B, arranged such that it is perpendicular to the 
semiconductor laser beam emitting body 1A, is also made parallel in the 
same way as in the semiconductor laser beam emitting body 1A by the 
collimator lens 2B, and then, enters into the beam composition prism 3. 
The pitch of this beam emitted from the semiconductor laser beam emitting 
body 1B is shifted by a predetermined value from the beam, and emitted 
from the semiconductor laser beam emitting body 1A in the subsidiary 
direction. Both beams enter into the polygonal mirror 6 through the first 
cylindrical lens 5 of the first image forming optical system. The 
reflected light passes through the second image forming optical system 
comprising of the f.theta. lens 7 and the second cylindrical lens 8, and 
simultaneously scans two lines with a predetermined spot diameter on the 
photoreceptor drum surface 10 under the condition that the pitch of one 
beam is shifted by a predetermined value from that of the other beam in 
the subsidiary scanning direction. In this connection, fine adjustment in 
the primary scanning direction is performed previously by an adjustment 
mechanism, which is not shown in the drawing. 
In order to detect the synchronism of each line, a light beam enters into 
the synchronism detector 12 before the start of scanning through the 
mirror 11. 
FIG. 2 is a plan view of the two-beam optical scanning system unit 1. 
Casings 201 and 201A, in which semiconductor laser emitting bodies 1A and 
1B, collimator lenses 2A and 2B are respectively provided, are arranged on 
a base member 111 as shown in the drawing, and beams L.sub.1 and L.sub.2 
are emitted at an angle of 90.degree. with respect to each other. The 
casings 201 and 201A are respectively arranged on angle changing members 
125 and 125A. The angle changing members 125 and 125A are respectively 
located on parallel moving members 124 and 124A, which move in parallel in 
the primary scanning direction on the base member 111. Further, the beam 
composition prism 3 and the first cylindrical lens 5 are fixed by a 
supporting member 123. The beams L.sub.1 and L.sub.2 are composed by the 
beam composition prism 3. The supporting member 123 is fixed on the base 
member 111 so that the composed beam can enter into the polygonal mirror 
6. In the optical scanning system unit 1, as shown in FIG. 2, both ends of 
the base member 111 are respectively located on the supporting members 114 
and 115 provided in the image forming apparatus 113. The optical scanning 
system unit 1 is guided in the direction perpendicular to the beam 
scanning direction by guide members 116 and 117 respectively provided at 
both end positions of the base member 111, and located at a predetermined 
position. Further, in the front position toward which the optical scanning 
system unit 1 is guided, an engagement stay 118, which is used as a 
reference position, is provided in the image forming apparatus 113 in the 
same direction as the light beam scanning direction, and engaging claw 
members 119 and 120 are respectively provided on both end positions of the 
base member 111. These claw members are respectively engaged with groove 
portions 121 and 122 formed on the engagement stay 118. In the groove 
portions 121 and 122, the width of one groove portion 121 is formed the 
same as that of the engagement claw member 119, and the width of the other 
groove portion is formed larger than that of the engagement claw member 
120, so that the engagement operation can be smoothly carried out, and the 
claw members can be accurately positioned. Further, positioning pins 128 
and 128A are fixed so that the rear end of the base member 111 can be 
positioned in a predetermined position, and positioning members 129 and 
129A for engaging with the positioning pins 128 and 128A, are respectively 
provided on the rear end of the base member 111. 
FIGS. 3 and 4 show the structure of the parallel moving member 124 and the 
angle changing member 125 provided on the base member 111. As shown in 
FIG. 3, the first guiding recesses 124B and 124C, formed on the parallel 
moving member 124 which moves in parallel in the primary scanning 
direction, are provided such that these recesses are engaged with guide 
members 132 and 133 provided on the base member 111, and the parallel 
moving member 124 is fixed onto the base member by fixing screws 134 and 
135. The second cam groove 124A, which is engaged with the eccentric cam 
130 provided on an axis 131, is formed on the base member 111. Further, 
the angle changing member 125 is located on the parallel moving member 
124, and one end of the angle changing member 125 is rotatably provided 
around a shaft 138. The third cam groove 125A, which is engaged with the 
eccentric cam 136 provided on the axis 137, is formed on the other end of 
the angle changing member 125. A fixing screw 139 is provided which fixes 
the angle changing member 125 onto the parallel moving member 124 at the 
position at which the angle is changed. A casing 201, in which the 
semiconductor laser beam emitting body 1A and the collimator lens 2A are 
provided, is fixed on the angle changing member 125 in the direction of a 
beam L.sub.1. Numerals 219 and 220 are screw rods for adjusting a prism 
200 provided in the casing 201 (refer to FIG. 8). 
Due to the above structure, the following operations are carried out when 
the parallel moving member 124 is moved parallely: initially, the hold by 
fixing screws 134 and 135 is released; the axis 131 is rotated and the 
eccentric cam 130 is rotated; and the parallel moving member 124 is moved 
in parallel in the right and left directions, shown by arrows, by the 
first guiding recesses 124B, 124C, and the guide members 132 and 133 
provided on the base member 111, through the second cam groove 124A. Due 
to this movement, the casing 201 provided on the angle changing member 125 
can be adjusted to move in parallel to the beam L.sub.1. That is, the beam 
L.sub.1 from the semiconductor laser beam emitting body 1A can be adjusted 
in the primary scanning direction. After adjustment has been completed, 
the parallel moving member 124 is fixed onto the the base member 111 by 
fixing screws 134 and 135. Next, when the angle of the angle changing 
member 125 is changed, initially, the hold by the fixing screw 139 is 
released; the eccentric cam 136, provided on the axis 137, is rotated so 
that the parallel moving member 124 is moved; and the angle changing 
member 125 is adjusted to rotate around the shaft 138 in the direction 
shown by the arrow, through the third cam groove 125A, by the rotation of 
the eccentric cam 136. Due to this adjustment, the angle of the casing 201 
provided on the angle changing member 125 is adjusted with respect to the 
beam L.sub.1. That is, the angle of the beam L.sub.1 from the 
semiconductor laser beam emitting body 1A is adjusted. 
In FIG. 4, as a rotation means of the axes 131 and 137 shown in FIG. 3, a 
worm gear G.sub.1 and a worm G.sub.2 are provided on the axis 131, and a 
worm gear G.sub.3 and a worm G.sub.4 are provided on the axis 137. When 
the worm G.sub.2 or worm G.sub.4 are rotated, and the worm gear G.sub.1 or 
worm gear G.sub.3 is rotated, fine adjustment can be performed through 
eccentric cams 130 and 136. 
FIG. 5 shows a beam position detection means for adjusting beam L.sub.1. 
Initially, as shown in the drawing, the optical member located between the 
polygonal mirror 6 and the photoreceptor drum 10 is removed. The beam 
position detection member S is arranged at a position at which the beam 
L.sub.1 reflected from the polygonal mirror 6 is directly received, and a 
supporting body S.sub.1, on which the beam position detection member S is 
provided, is arranged at the measuring position outside the apparatus. The 
beam L.sub.1 is emitted from the semiconductor laser beam emitting body 1A 
under the above conditions, and the beam pitch is adjusted so that it is 
within a predetermined specification, using the above adjustment method. 
This adjustment is simultaneously carried out on the beam L.sub.2 emitted 
from the laser beam emitting body 1B, and the beam adjustment in the 
primary and the secondary scanning directions can be carried out. Numeral 
112 is a cover, and an opening 112A for measuring is formed in a portion 
of the cover 112. Numeral 113A is an outside board of the image forming 
apparatus 113 in which the opening 112A is formed. 
FIG. 6 shows a supporting member 123 on which the beam composition prism 3 
and the first cylindrical lens 5 shown in FIG. 2 are fixed. The beam 
composition prism 3 and the first cylindrical lens 5 are integrally fixed 
on the supporting member 123. As a fixing method, an adhesive agent may be 
applied. Alternatively, the beam composition prism 3 and the first 
cylindrical lens 5 may be engaged and fixed on a holding portion, as shown 
in the drawing, which is integrally formed with the supporting member 123. 
The supporting member 123 is fixed on the base member 111 by fixing screws 
126 and 127. 
FIG. 7 shows the beam adjusting method shown in FIG. 3, and a means in 
which fine adjustment is carried out in the primary scanning direction and 
subsidiary scanning direction by a light beam compression prism 200 shown 
in FIG. 8. Initially, in FIG. 7, as also shown in FIG. 3, the first 
guiding recesses 124B, 124C formed on the parallel moving member 124, 
which is parallely moved in the primary scanning direction, are engaged 
with the guide members 132, 133 provided on the base member 111, and the 
parallel moving member 124 is fixed to the base member 111 by the fixing 
screws 134 and 135. The eccentric cam 130 is provided on the axis 131 
rotated by a gear G.sub.7 and a reduction gear G.sub.6. The second cam 
groove 124A, with which the eccentric cam 130 is engaged, is formed on the 
parallel moving member 124. The angle changing member 125 is located on 
the parallel moving member 124. One end of the angle changing member 125 
is rotatably provided on the shaft 138. An axis 137 is rotated by a gear 
G.sub.9 and a reduction gear G.sub.8. An eccentric cam 136 is provided on 
the axis 137. The third cam groove 125A with which the eccentric cam 136 
is engaged, is formed on the other end of the angle changing member 125. A 
fixing screw 139 for fixing the angle changing member 125 onto the 
parallel moving member 124 at the position at which the angle is changed, 
is provided on the angle changing member 125. Further, the casing 201, in 
which the semiconductor laser beam emitting body 1A and the collimator 
lens 2A are provided, is fixed on the angle changing member 125 along the 
direction of the beam L.sub.1. Numerals 219 and 220 are screw rods for 
adjusting a light beam compression prism 200 (refer to FIG. 8) provided in 
the casing 201. 
By the structure described above, when the parallel moving member 124 is 
moved in parallel, initially, the hold by the fixing screws 134 and 135 is 
released; the axis 131 is rotated by the gear G.sub.7 and the reduction 
gear G.sub.6 ; the eccentric cam 130 is rotated; and thereby, the parallel 
moving member 124 is moved laterally in parallel as shown by the arrow 
while the first guiding recesses 124B and 124C are engaged with guide 
members 132 and 133, provided on the base member 111. Due to this 
movement, the casing 201 provided on the angle changing member 125 can be 
adjusted so that it moves in parallel to the beam L.sub.1. That is, the 
beam L.sub.1 from the semiconductor laser beam emitting body 1A can be 
adjusted to be emitted in the primary scanning direction. After adjustment 
has been completed, the parallel moving member 124 is fixed onto the base 
member 111 by fixing screws 134 and 135. Next, when the angle of the angle 
changing member 125 is changed, the hold by the fixing screw 139 is 
initially released; the eccentric cam 136 provided on the axis 137 is 
rotated by a gear G.sub.9 and a reduction gear G.sub.8 ; and thereby, the 
angle changing member 125 is rotated around the shaft 138 in the arrowed 
direction through the third cam groove so that its angle is adjusted. By 
this rotation and adjustment, the angle of the casing 201 provided on the 
angle changing member 125 is adjusted with respect to the beam L.sub.1. 
That is, the angle of the beam L.sub.1 from the semiconductor laser beam 
emitting body 1A is adjusted. 
FIG. 8 shows the casing 201 in which the semiconductor laser beam emitting 
body 1A, the collimator lens 2A and the beam compression prism 200 are 
accommodated. Inside the casing 201, a beam transmission hole 203 is 
formed along the beam L.sub.1. A long hole 204 is formed along the beam 
L.sub.1 so that an inner barrel 202, in which the collimator lens 2A is 
fixed, is mounted in the casing 201. A female screw thread 205 is formed 
in the long hole 204 so that the the inner barrel 202 can be screwed into 
the long hole 204. On the other hand, a male screw thread 206 is formed on 
the outer surface of the inner barrel 202 so that it can be screwed into 
the female screw thread 205, and the inner barrel 202 is fixed by screws 
in the long hole 204 as shown in FIG. 8. A tapered surface 207, (at 
approximately 30.degree. with respect to the horizontal surface), is 
formed on the surface of the long hole 204 so that the tapered surface of 
the long hole 204 is extended around the beam L.sub.1 in the direction 
from the portion of the female screw thread 205 to the left in the 
drawing. A tapered surface 208 is formed on the outer surface of the inner 
barrel 202 with the same angle as that of the tapered surface 207. A 
plurality of slits 209, which penetrate the tapered surface 208 to the 
transmission hole 203 of the beam L.sub.1, are formed on the portion on 
which the tapered surface 208 is formed. The angle .theta. of the slits is 
formed at approximately 60.degree.. Numeral 210 is a rotation assembling 
hole formed at a plurality of portions formed between slits 209. The 
rotation assembling hole 210 is formed such that it can coincide with an 
assembling operation long hole 211 formed on the casing 201 at the final 
assembling position. Numeral 215 is a hole for an adhesive agent 214 and 
is formed in the casing 201. 
The beam compression prism 200 is attached to a beam compression prism 
attaching member 216 at a predetermined angle. The beam compression prism 
attaching member 216 is fixed to a cylindrical frame 217. The cylindrical 
frame 217 is rotatably attached to the beam compression prism attaching 
portion 218, formed along the long hole 204 in the casing, in the 
direction crossing the light beam L.sub.1. Screw rods 219 and 220, which 
are screwed into the casing 201, are arranged at a portion of the 
cylindrical frame 217 symmetrically to each other with respect to a 
vertical center line of the cylindrical frame in the drawing. A tip of the 
screw rod 219 directly touches a step portion 221 formed on the 
cylindrical frame 217. A tip of the screw rod 220 touches a step portion 
formed on the cylindrical frame 217 through a spring member 222. The 
cylindrical frame 217 is fixed to the casing 201 by a screw rod 226 
through a side plate 224. 
In the beam compression prism 200 structured as described above, initially, 
the screw rod 226 for fixing is loosened, and then, the screw rod 219 is 
rotated for adjusting. At this time, the step portion 221 formed on a 
portion of the cylindrical frame 217 is always contacted by the tip of the 
screw rod 219 through the force of spring member 222. When the screw rod 
219 is rotated for adjusting, the light beam compression prism 200 is 
rotated for adjusting the transmitting direction through the cylindrical 
frame 217 and the beam compression prism attaching member 216, while the 
width of the beam L.sub.1 is reduced to a predetermined value. After the 
adjustment is completed, the cylindrical frame 217 is fixed by the screw 
rod 226 in the casing 201. In this case, even when the screw rod 226 for 
fixing is rotated clockwise, the tip of the screw rod 219 is always 
blocked by the step portion 221 formed on the cylindrical frame 217, and 
the light beam compression prism 200 is not moved from the adjusted 
position. 
The same beam compression prism as the above-described prism 200 is also 
provided in the casing 201, and the primary scanning direction and the 
subsidiary scanning direction of luminous flux of the beam L.sub.1 emitted 
from the semiconductor laser beam emitting body 1A, and the beam L.sub.2 
emitted from the semiconductor laser beam emitting body 1B, are finely 
adjusted. 
As described above, according to the two-beam optical scanning unit of the 
present invention, the adjusting system for precisely adjusting each beam 
position of the primary scanning direction and the subsidiary scanning 
direction, and further, the fine adjusting system for precisely adjusting 
the rotation, are provided in the unit. Accordingly, the beam position 
adjustment of the primary scanning direction and the subsidiary scanning 
direction can be separately and accurately carried out by easy 
adjustments, which is advantageous.