JP3448137B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning type image forming apparatus which forms an electrostatic latent image on a photosensitive member by using multi-beams.

[0002]

2. Description of the Related Art A laser beam modulated in accordance with an image signal output from a laser diode is imaged as a spot on a photosensitive member which moves in the sub-scanning direction, and is scanned in the main scanning direction to scan an electrostatic latent image. 2. Description of the Related Art A laser beam printer (hereinafter also simply referred to as “printer”) that forms an image, converts it into a toner image by development, and transfers and fixes it on a sheet to form an image is known.

Such a printer has been widely used because of its high image quality due to its high resolution and high speed of printout, which is also supported by the cost reduction due to mass production. Since then, there is a strong demand for higher resolution and higher speed.

Generally, since high resolution and high speed are in a reciprocal relationship, if the resolution is increased, the time for printout tends to be long, and if the high speed is pursued, the resolution tends to be lowered. Therefore, in any case of increasing the level, it is necessary to increase the number of rotations of the beam deflecting means, for example, the polygon mirror, or increase the number of surfaces thereof.

However, the polygon mirror of the current printer has reached the limit near which the life of the polygon mirror rapidly deteriorates when the number of rotations is further increased, and increasing the number of surfaces causes poor yield and cost. Not only goes up,
Increasing the size of the device is unavoidable in order to obtain a predetermined image width. Further, since it is necessary to increase the modulation frequency of the laser light according to the image signal, there is a problem that circuit troubles are likely to occur due to the increase in frequency.

Therefore, by scanning N lines at the same time by using a plurality of, for example, N beams that are close to each other, N times higher speed can be achieved without increasing the number of revolutions, the number of faces and the modulation frequency of the polygon mirror. That is, a multi-beam printer that can reduce the printout time to 1 / N has come to the fore. It goes without saying that if the high-speed property is suppressed to some extent, the higher resolution can be obtained.

[0007]

However, when the multi-beam is used, the pitch P of each scan line (in the sub-scanning direction) is 42.33 μm when the pixel density is 600 dpi (dots / inch), 400 dpi
In this case, it is necessary to correctly set the pitch to 63.5 μm, but it becomes extremely difficult to adjust and maintain the pitch, which was not necessary in the case of the single beam method using a single beam.

If the pitch adjustment is insufficient,
When the pitch is changed due to temperature change or aging, the scan lines of the obtained image become periodically unequal intervals, and as a result, banding, that is, an image in which band-like density unevenness is conspicuous because the pitch is fine. Then, there is a problem that the image quality, that is, the image quality is remarkably deteriorated.

The present invention has been made in view of the above points, and an object thereof is to make it possible to easily and accurately adjust the pitch of scan lines with a simple structure. Also,
Another object is to be able to keep the adjusted pitch always accurate.

[0010]

In order to achieve the above object, the present invention provides a plurality of beam generating means for outputting a beam of laser light modulated according to an image signal, and a plurality of beam generating means. A coupling lens for converting a plurality of output diverging beams into a parallel beam, and at least a power component in the sub-scanning direction arranged on the optical axis for changing the shapes of the plurality of parallel beams passing through the coupling lens. A plurality of optical elements, a beam deflecting means for deflecting the plurality of beams passing through the plurality of optical elements in the main scanning direction, and a plurality of beams deflected by the beam deflecting means on the photoconductor, respectively. Each of the scans formed by scanning a plurality of spots on the photoconductor in the main scanning direction, the optical writing device including an image forming unit that forms an image as In order to change the pitch of a plurality of scan lines on a photoconductor in an image forming apparatus in which a plurality of beam generating means are arranged so that the pitch in the sub-scanning direction becomes a preset value, Optical element driving means for moving or rotating at least one of the plurality of optical elements arranged on the axis is provided, and the optical element driving is performed.
The means is centered on an axis substantially along the main scanning direction described above.
It is a means for rotating the optical element .

Alternatively, the optical element driving means is a means for moving the optical element in and out of the optical paths of a plurality of beams .
It is a thing .

In each of the above-mentioned image forming apparatuses, the following may be performed. That is, a pitch detecting means for detecting the pitch of a plurality of scan lines on the photoconductor is provided,
The optical element driving means may include means for moving or rotating the optical element according to a difference between the pitch of the scan line detected by the pitch detecting means and a preset value.

Further, temperature detecting means for detecting the temperature around the optical writing device is provided, and the optical element driving means has means for moving or rotating the optical element according to the temperature detected by the temperature detecting means. Good.

Alternatively, it is preferable to provide beam power changing means for changing the powers of the beams output by the plurality of beam generating means in accordance with the movement or rotation of the optical element by the optical element driving means.

Further, an aperture arranged on the optical paths of a plurality of beams and aperture diameter changing means for changing the diameter of the aperture according to the movement or rotation of the optical element by the optical element driving means may be provided.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. 2 is a perspective view showing an example of an optical writing device according to an embodiment of the present invention, and FIG. 3 is a plan view showing an arrangement of optical systems of the optical writing device shown in FIG.

The optical writing device 10 shown in FIG. 2 includes an LD (laser diode) unit 12 provided on a side surface of a housing 11, and a beam compressor 3, which is arranged on the bottom surface of the housing 11 along the optical axis. It is composed of a cylinder lens 4, a polygon mirror 5, an fθ lens 6, a long mirror 14, a synchronous mirror 15 and a PSD (position sensitive diode) 16, and a beam exit window 17 provided below the long mirror 14. Below the beam emission window 17, a photoconductor drum 8 that supports the photoconductor surface 8a on the surface is provided.

The LD unit 12 comprises an LD array 1 which is a plurality of beam generating means shown in FIG. 3, and a drive circuit which includes a coupling lens 2 and a plurality of LD drivers (not shown), and in this embodiment. 2 driven by two LD drivers according to image signals
Two divergent laser beams emitted from the LD array 1 in which one LD (laser diode) is fixedly housed in one case have a slight angle of intersection with each other via the coupling lens 2. It is output as a parallel beam of a book. A thermistor 13, which is a temperature detecting means, is provided in close contact with the back surface of the LD unit 12, and the temperature of the LD unit 12 is constantly monitored.

The two parallel beams output from the LD unit 12 have no power in the main scanning (horizontal) direction and positive and negative cylinder lenses 3a, 3b having power only in the sub-scanning (vertical) direction. Is converted into a parallel beam in which only the beam diameter in the sub-scanning direction is compressed. Similarly, a positive cylinder lens 4 having a power only in the sub-scanning direction causes only the component in the sub-scanning direction of the parallel beam to be converted. The image is converged and imaged on the mirror surface 5a of the polygon mirror 5.

These positive or negative cylinder lenses 3a, 3
b and 4 are a plurality of optical elements arranged on the optical axis for changing the shapes of the plurality of beams and having at least a power component in the sub-scanning direction (hereinafter, these are collectively referred to as "optical element group").
That)).

An arrow A is drawn by a polygon motor (not shown).
The polygon mirror 5, which is a beam deflecting means that is rotationally driven in the (counterclockwise) direction at a constant speed, reflects the laser beam incident on its mirror surface 5a, and counterclockwise (from left to right in FIG. 2). To deflect. The laser beam deflected by the polygon mirror 5 then enters the fθ lens 6.

The f.theta. Lens 6, which is an image forming means composed of the two negative toric lenses 6a and the positive toric lens 6b, has different combined focal lengths in the main scanning direction and the sub scanning direction. , A main component in the main scanning direction of the deflected laser beam which remains incident in parallel, and the polygon mirror 5
And the component in the sub-scanning direction which is imaged on the mirror surface 5a and enters as a divergent beam, respectively, are imaged on the photoconductor surface 8a.

The deflected laser beam that has passed through the fθ lens 6 is reflected by the synchronous mirror 15 provided on the front side of the long mirror 14 at a timing immediately before entering the next long mirror 14, and is reflected by the fθ lens 6. And enters the PSD 16 provided at a position common to the photoconductor surface 8a. PSD16
As will be described later, acts as a sensor that constitutes a pitch detection unit that is pitch detection means, and also acts as a sensor that outputs a synchronization signal that serves as a reference for the position of the image in the main scanning direction.

After the deflected laser beam is incident on the synchronous mirror 15, it is reflected downward by the elongated mirror 14, passes through the beam emission window 17 on the bottom surface of the housing 11, and is spotted on the photosensitive surface 8a of the photosensitive drum 8. Becomes an image, and the arrow B
The photoconductor surface 8a that moves in the sub-scanning direction by the photoconductor drum 8 that rotates in the (clockwise) direction is the arrow C that is the main scanning direction.
The scan line SL is formed by scanning in the direction.

In FIG. 4, the optical system described and shown in FIGS. 2 and 3 is developed along the optical axis, and the component in the sub-scanning direction of two laser beams whose optical axes have a slight intersecting angle is 1 It is an optical-path figure shown on behalf of the beam of a book. The diverging beam of the laser light output from one of the LDs of the LD array 1 including two LDs is converted into a parallel beam by the coupling lens 2 and the positive and negative cylinder lenses 3a and 3b.
The beam diameter of the beam is compressed by the beam compressor 3 and the parallel beam is formed, and an image is formed on the mirror surface 5a of the polygon mirror 5 by the positive cylinder lens 4.

When the beam diameter is compressed by the beam compressor 3, the intersection angle of the optical axes of the two beams is expanded by the compression ratio. If the beam diameter is expanded by the beam expander, only the expansion ratio is obtained. The intersection angle of each optical axis is reduced. That is, the beam diameter and the crossing angle of the optical axis of each beam are in inverse proportion.

If the crossing angles of the optical axes of a plurality of beams are expanded, the pitch of the plurality of scan lines SL is expanded. Therefore, the beam compressor or the beam expander is replaced to compress or expand the beam diameter to perform scanning. The pitch of the line SL can be expanded or reduced.

The sub-scanning direction component of the laser beam deflected by the fθ lens 6 is mirror surface 5a and photoconductor surface 8a.
Since there is a common relationship between and, there is an error of surface tilt (inclination in the sub-scanning direction) on each mirror surface 5a of the polygon mirror 5, and the beam reflected by the mirror surface 5a is slightly different in FIG. Even if vertically deflected, the scan lines formed by the spots on the photoconductor surface 8a do not move vertically (sub-scanning direction) at all.

In FIG. 4, since the coupling lens 2 is a normal positive lens which acts as a collimator,
Has the same power in both the main scanning direction and the sub-scanning direction, f
The focus position of the component of the θ lens 6 in the main scanning direction is the photoconductor surface 8a.
The power component in the sub-scanning direction, which is above, acts so as to form an image of the mirror surface 5a on the photoconductor surface 8a. The positive and negative cylinder lenses 3a and 3b and the cylinder lens 4 of the beam compressor 3 which form the optical element group each have only a power component in the sub-scanning direction and are zero in the main scanning direction.

Therefore, the main scanning direction component of the beam shown by the chain double-dashed line is from the LD array 1 to the coupling lens 2.
Is a diverging beam, the coupling lens 2 to the fθ lens 6 are parallel beams, and the fθ lens 6 and subsequent beams are convergent beams, which form an image on the photoconductor surface 8a as points. Therefore, the absolute value of the lateral magnification β of the LD in the main scanning direction by this optical system is a value obtained by dividing the focal length of the fθ lens 6 in the main scanning direction by the focal length of the coupling lens 2.

Unless otherwise specified, only the sub-scanning direction component of the optical system relating to the present invention will be described below.
Symbols f, d, and β indicating the focal length, the interval, and the image magnification (horizontally, respectively) have absolute values, and the magnification of the image on the mirror surface 5a with respect to the LD array 1 by the first half optical system is taken. Is β1, and the magnification of the image on the photoconductor surface 8a with respect to the mirror surface 5a by the fθ lens 6 in the latter half is β2.
And Therefore, the photoconductor surface 8 for the LD array 1
The magnification of the image on a is the product of β1 and β2.

FIG. 5 is a diagram showing an example of scan lines of an image formed when the pitch is normal and abnormal, respectively. FIG. 5A shows a normal pitch, and FIG. 5B shows an abnormal pitch. The respective states of the scan lines in the case of occurrence of are shown.

When the pixel density is 600 dpi, the reference pitch Ps of adjacent scan lines is 42.33 μm,
When the LD array 1 including two LDs, that is, LDa and LDb, is used, the moving speed of the photoconductor surface 8a in the sub-scanning direction is
Since it is doubled per scan as compared with the case of a single beam, the pitch between the scan line SL1a and the next scan line SL2a by the spot of the beam output by the same LD, for example, LDa, that is, the feed pitch Pn is 2 times the reference pitch Ps. Doubled to 84.66 μm.

As shown in FIG. 5A, LDa and L
When the pitch Pp of the scan lines SL1a and SL1b on the photoconductor surface 8a by Db is 42.33 μm which is the reference, the pitch between the scan line SL1b by LDb and the scan line SL2a by LDa in the next scan, that is, the remaining pitch Pr. Is Pr = Pn-Pp =
42.33 μm, the standard pitch Ps and the pitch P
Since it is the same as p, banding due to the synchronous variation of pitch does not occur.

However, as shown in FIG. 5B, when an error ε occurs in the pitch Pp and becomes Ps + ε, the remaining pitch Pr becomes Ps−ε, so the difference between the two is 2ε, that is, twice the error. Appears.

For example, when a slight error of about ε≈8 μm occurs and the pitch Pp reaches 50 μm, the remaining pitch is P.
Since r = 34.66 μm≅35 μm, the pitch of the scan lines of the image alternately becomes approximately 10: 7, and banding can be recognized.

FIG. 6 shows the LD imaged on the photoconductor surface 8a.
FIG. 4 is a diagram showing an example of a relationship between images (spots) LDa ′, LDb ′ of two LDs LDa, LDb of the array 1 and scan lines SL1a, SL1b resulting therefrom.
FIG. 6 is an enlarged view of a part of the normal scan line shown in FIG.

LDa and LDb of the LD array 1 are (β1
× β2) images LDa ′, LD enlarged (or reduced) by a factor of 2
However, since the distance between LDa and LDb on the LD array 1 is already determined when the LD array 1 is manufactured, when it becomes (β1 × β2) times, the reference pitch Ps (4
It is not always 2.33 μm), but in general, it is often larger than the reference pitch.

Therefore, as shown in FIG. 6, the straight line connecting LDa and LDb of the LD array 1 is set in advance with respect to the scan line in the main scanning direction, and the image LD thereof is set.
The sub-scanning direction component of the distance between a'and LDb 'is the reference pitch P.
By setting the value to be s, the degree of freedom of the magnifications β1 and β2, that is, the degree of freedom in designing the optical system and the mechanical system is secured.

Therefore, it is necessary to shift the phases of the image signals respectively input to LDa and LDb by the component in the main scanning direction of the interval between the images LDa 'and LDb', but the respective synchronization signals for each scan line SLa, SLb. It can be solved easily by synchronizing with.

In such a case, the images LDa ', LDb'
Since there is a component in the main scanning direction of the interval of, the line synchronization for each scan line can be detected at each timing by one synchronization sensor, and as will be described later, the scan line is provided by one PSD. SLa,
There is an advantage that the pitch Pp of SLb can be detected.

However, LDa and L of the LD array 1 are
Although the distance between Db is relatively accurate, the image LD on the photoconductor surface 8a is set due to the setting of the inclination angle between the straight line connecting the two and the main scanning direction and the manufacturing error of each element of the optical system and its setting position.
The sub-scanning direction component of the distance between a'and LDb '(scan line pitch Pp) does not always exactly match the reference pitch Ps. Therefore, it becomes necessary to adjust the pitch Pp so as to match the reference pitch Ps.

FIG. 1 is an optical path diagram for explaining the operation of the optical element driving means according to the first embodiment for adjusting the scan line pitch of the present invention. The optical path diagram shown in FIG. The first half of the system, that is, the LD array 1 including the optical element group to the mirror surface 5a of the polygon mirror 5 is shown enlarged.

In FIG. 1, positive and negative cylinder lenses 3a and 3 of the coupling lens 2 and the beam compressor 3 are shown.
The focal lengths of b and the cylinder lens 4 in the sub-scanning direction are f1, f2, -f3, and f4, respectively, and the cylinder lens 3
a, 3b, 4 and the mirror surface 5a are separated by d
Let 2, d3, d4. It should be noted that the distance between the coupling lens 2 and the cylinder lens 3a does not matter because the beam between them is always a parallel beam.

If the pitch of LDa and LDb of the LD array 1 is Pd and the pitch of the images of LDa and LDb on the mirror surface 5a is Pm, considering the components in the sub-scanning direction, respectively, on the photoconductor surface 8a. Since the pitches of the images LDa 'and LDb' are Pp, the relationship between the pitches Pd, Pm, and Pp is as shown in Formula 1.

[0046]

[Formula 1] Pm = β1 × Pd Pp = β2 × Pm ∴Pp = β1 × β2 × Pd

In the standard state, the positive cylinder lens 3a
Since the parallel beam incident on is again emitted as a parallel beam from the negative cylinder lens 3b, the distance d2 = f2
-F3 and the spacing d3 becomes irrelevant. The parallel beam emitted from the cylinder lens 3b is transferred to the cylinder lens 4
Since the image is formed on the mirror surface 5a by the distance d4 = f4. Assuming that the combined focal length of the optical element group in this standard state is f0, f0 and β1 are given by equation (2).

[0048]

[Formula 2] f0 = f2 × f4 / f3 β1 = f0 / f1 = F2 x f4 / (f1 x f3)

From the standard state shown in FIG. 1, as shown by the broken lines, first, the cylinder lens 3b is moved by a small amount Δ.
When parallel translation is performed to the left by d, the emitted parallel beam changes to a weak divergent beam, and the image of the LD array 1 is slightly shifted to the right from the mirror surface 5a. Next, when the cylinder lens 4 is moved in parallel to the left by a slight amount Δd 'and the image of the LD array 1 is returned to the position of the mirror surface 5a, the combined focal length of the optical element group becomes larger than f0. β1 is slightly higher than in the standard state.

Although not shown, similarly, if the cylinder lenses 3b and 4 are moved in parallel to the right by a small amount from the standard state, and the image of the LD array 1 is formed on the mirror surface 5a, the combined focal length is obtained. On the contrary, the value becomes smaller than f0, and the magnification β1 is slightly decreased from the standard state.

That is, the cylinder lens 3 of the optical element group
b and 4 constitute a zoom lens for the sub-scanning direction component, which act as a variator and a compensator, respectively.
The magnification β1 increases or decreases around the standard state. Therefore, the magnification β 2 shown in Equation 1 and the pitch Pd of the LD array 1
Is constant, the pitch Pp on the photosensitive surface 8a changes in proportion to the magnification β1.

The variator and compensator forming the zoom lens are not limited to the cylinder lenses 3b and 4, but any one of the three cylinder lenses 3a, 3b and 4 can be used as a variator and the other one. One may be the compensator. Also, the zoom lens can be constructed by moving at least two lenses, but three or more lenses may be moved.

FIG. 7 is an optical path diagram showing an example of the optical element driving means according to the second embodiment of the present invention, and FIG. 8 is a diagram for explaining the principle thereof. The parallel light incident along the optical axis of the positive lens 7 having the focal length f shown in FIG. 8 forms an image at the focal point F of the lens 7. On the other hand, the parallel light that enters at an inclination angle θ with the optical axis shown by the broken line forms an image at an image forming point Fe on the focal plane that includes the focus F and is perpendicular to the optical axis.

Needless to say, the distance between the principal point H of the lens 7 and the focal point F is the focal length f. On the other hand, if the distance between the principal point H and the image formation point Fe is fe, then fe = f / cos θ. Therefore, when the lens 7 is tilted by θ with respect to the parallel light incident along the optical axis, an image is formed at the image forming point Fe ′ on the right side of the focus F. That is, fe is an effective focal length when the lens is tilted by θ, and always fe> f regardless of whether the tilt angle θ is positive or negative.

Similarly, even when the lens 7 is a negative lens, the absolute value of fe is always larger than the absolute value of f, and this relationship does not change even if the lens 7 is a cylinder lens. Therefore, if the lens 7 is set to be tilted with respect to the optical axis by the tilt angle θ in advance, the effective focal length fe can be increased or decreased by increasing or decreasing the tilt angle θ.

FIG. 7 shows that in the first embodiment shown in FIG. 1, the cylinder lenses 3a and 3b are preliminarily tilted by tilt angles θ and θ ', respectively, and their effective focal lengths fe are f2 and -f3. The beam divergence, parallelism, and convergence in the standard state are exactly the same.
Further, the same parts are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 7, first, when the cylinder lens 3a is rotated by a small angle in the clockwise direction where the tilt angle θ increases, the effective focal length increases by Δfe, so that the image forming point moves to the right and is emitted from the cylinder lens 3b. The collimated beam becomes a slightly divergent beam. Next, the cylinder lens 3b is similarly rotated clockwise so that the tilt angle θ'increases, so that the increment of the absolute value of the effective focal length becomes the same Δfe as the increment of the effective focal length of the cylinder lens 3a. For example, the outgoing beam returns to a parallel beam.

Therefore, the LD formed by the cylinder lens 4
The image of the array 1 does not move from the mirror surface 5a, but f4 does not change in the formula of the combined focal length f0 shown in Formula 2, and f
2 and f3 both increase by Δfe. However, since the beam compressor 3 has a relationship of f2> f3, even if the increment is the same, the combined focal length becomes smaller than the standard combined focal length f0, and the magnification β1 decreases.

On the contrary, if both the cylinder lenses 3a and 3b are rotated by a slight angle in the counterclockwise direction in which the tilt angles θ and θ'decrease, the effective focal lengths f2 and f3 are reduced by the same value.
The magnification β1 increases. In other words, the tilt angles θ, θ are set so that the two cylinder lenses 3a, 3b have the same change in absolute value of the effective focal length regardless of whether the tilt angles θ, θ ′ are positive or negative.
The pitch Pp on the photoconductor surface 8a can be changed by increasing or decreasing the absolute value of θ '.

The first embodiment shown in FIG. 1, that is, the zoom lens system is an orthodox for adjusting the pitch Pp, and there is no problem in terms of aberration, but as an actual mechanical problem, optical It is not easy to translate the element along the optical axis without causing lateral blur.

On the other hand, the second embodiment shown in FIG. 7, that is, the method of changing the effective focal length of each optical element by rotating the optical element about an axis substantially in the main scanning direction is used. The mechanical accuracy is easy to obtain and the manufacturing is easy. Theoretically, it is slightly inferior in terms of aberration, but since the rotation range is not so wide, there is no practical problem.

In the first and second embodiments described above, at least two or more optical elements among a plurality of optical elements are driven to change the pitch Pp of the optical element surface 8a while forming spots on the surface 8a. Although the adjustment is performed, the case where only one of the plurality of optical elements is driven will be described in order to further simplify the mechanism, facilitate manufacturing, and suppress the cost.

In this case, the spots are imaged before and after the photoconductor surface 8a rather than on the photoconductor surface 8a, so that the positions of the principal rays of the beams forming the two spots are incident on the photoconductor surface 8a relatively. It takes advantage of changing. Therefore, it is considered that spot blurring occurs on the photoconductor surface 8a.

However, since the divergence angle of the beam when it actually enters the photoconductor surface 8a is extremely small, even if the optical system has no aberration, the spot is a geometrical optical point and the spot before and after that point. The spot diameter does not increase in proportion to the deviation of the beam, but the theoretical spot diameter obtained from physical optics is set to the minimum value, and the beams before and after that are drum-shaped, so the image formation point is Even if it moves a little, the spot diameter hardly changes.

The third embodiment of the present invention is not particularly shown in the drawing, but any one of the optical element groups in FIG.
Only the individual is moved in the optical axis direction.

For example, when the cylinder lens 4 is moved in parallel from the standard state to the left or right along the optical axis, the image of the LD array 1 moves from the mirror surface 5a to the left or right by the amount of movement Δd. Since the vertical magnification α of the image is the square of the lateral magnification β, the spot on the photoconductor surface 8a is β2 of Δd.
To the left or to the right (FIG. 4) by the squared power of, and the pitch Pp expands or contracts in accordance with the movement.

Even if only one of the other cylinder lenses 3a or 3b of the optical element group is translated along the optical axis, the ratio of the amount of movement of the spot on the photosensitive surface 8a to the amount of movement Δd thereof is The movement direction is the same as that of the parallel-moved cylinder lens, and its effect is also the same, though it changes depending on the influence of the optical element.

In the fourth embodiment of the present invention, only one of the optical element groups in FIG. 7 is rotated about an axis substantially along the main scanning direction. An optical element to be rotated in FIG. 7, for example, a cylinder lens 3a
Is set in advance with an inclination angle θ with respect to the optical axis, and the other cylinder lenses 3b and 4 are not tilted (perpendicular to the optical axis) as a standard state.

When the cylinder lens 3a is rotated by a slight angle in the counterclockwise direction to decrease the tilt angle θ or in the clockwise direction to increase the tilt angle θ, the imaging point of the parallel beam incident from the coupling lens 2 by the cylinder lens 3a becomes a standard value. Since each of them moves to the left or right with respect to the state, the same effect as in the case where the cylinder lens 3a is translated to the left or the right along the optical axis in the third embodiment is obtained. This is the same even when only the other cylinder lens 3b or 4 of the optical element group is tilted in advance and the cylinder lens is rotated counterclockwise or clockwise.

FIG. 9 is a perspective view showing an example of the optical element driving means according to the fifth embodiment of the present invention, in which the optical elements are put into and taken out from the optical paths of a plurality of beams.
Needless to say, the cylinder lenses 3a and 3 shown in FIG.
If any one of b and 4 is put in and out of the optical path as it is, the power is too strong to adjust a minute amount, etc. Therefore, one having at least some power component in the sub-scanning direction. Or a plurality of weak positive or negative lenses,
By moving the weak lens in and out of the optical path, the image forming point of the spot is moved to the front and back of the photoconductor surface 8a.

The optical element driving means shown in FIG. 9 has a weak lens 20 and two openings 21a and 21b.
A movable plate 22 having a weak lens 20 fixed to b, and a spring 2 for constantly urging the movable plate 22 to the right (in the figure).
3 and a solenoid 24 that pulls the movable plate 22 leftward against the spring 23 when energized.

Since the solenoid 24 is normally off,
The movable plate 22 is displaced to the right by the spring 23,
There is a powerless aperture 21a on the optical path, and the optical element group is in the standard state. When energized, the solenoid 24 acts to shift the movable plate 22 to the left, and the weak lens 20 fixedly installed in the opening 21b is inserted into the optical path.

The power of the weak lens 20 varies depending on the position on the optical path to be inserted, but generally, in order to obtain the same amount of movement of the spot, the closer the position is to any of the image forming points, the stronger the power is, and the farther the position is. The weaker the power. If the power is positive, the spot moves to the front (left side in FIG. 4) of the photoconductor surface 8a, and if the power is negative, the spot moves to the rear.

If the optical element driving means (FIG. 9), that is, one weak lens, is used, the standard state and the non-standard state
Although only two states can be switched, a wide range can be finely adjusted by providing a plurality of weak lenses having different positive and negative spot movement amounts and changing the combination thereof. In addition, if multiple weak lenses are provided and the combination is changed to change the magnification,
While the spot is imaged on the photoconductor surface 8a, the pitch P
It is also possible to change p.

FIG. 10 is a block diagram showing an example of the configuration of the pitch detecting section which is the pitch detecting means. The pitch detection unit 30 shown in FIG. 10 includes the PSD 16 shown in FIG.
The pitch signal, which is composed of the position signal circuit 31 and the pitch signal circuit 32 and is output from the pitch detection unit 30, is output to the error amplifier 33 in the next stage.

As shown in FIG. 2, the PSD 16 is an elongated semiconductor element whose length direction is set to coincide with the sub-scanning direction. The PSD 16 is a P-type semiconductor on the front side and an N-type semiconductor on the back side. Has a PN junction surface. The N-type semiconductor is positively biased through the conductive layer on the back surface, and two electrodes 16a and 16b are provided at both ends in the length direction of the P-type semiconductor.

The distance from the center point between the electrodes 16a and 16b to each electrode is normalized to 1, the upper side (electrode 16a side) is positively set with the center point as the origin, and when the spot is incident on the position x, the electrodes are The currents output from 16a and 16b are Ia and Ib, respectively. When no light is incident, no current flows because the PN junction surface is reverse biased.

When the spot is incident on the position x, a current proportional to the light quantity of the spot flows into the P-type semiconductor at the incident point of the PN junction surface, and the currents Ia and I from the electrodes 16a and 16b.
The currents Ia and Ib are respectively output as b, but are distributed in inverse proportion to the resistance value of the P-type semiconductor that is proportional to the distance between the incident point and each electrode, so the ratio is (1 + x):
(1-x).

Therefore, the position of the incident point is x = (Ia-
Ib) / (Ia + Ib). Since the denominator (Ia + Ib) is a value proportional to the light quantity of the spot, the position x can be obtained regardless of the magnitude of the light quantity. Also,
Even if the spot is out of focus and has a certain size or the light amount is uneven, the position x is obtained as the position of the center of gravity of the light amount of the spot.

As shown in FIG. 2, when the beam deflected by the polygon mirror 5 is reflected by the synchronous mirror 15 and enters the PSD 16, the LDa of the LD array 1 first outputs it, as shown in FIG. Beam spot LD
a'is incident on the PSD 16, and as shown in FIG.
Current I output from electrodes 16a and 16b of SD16
The a and Ib are input to the position signal circuit 31.

The position signal circuit 31 receives the input current Ia,
The difference of Ib is divided by the sum, the position signal of the spot LDa 'corresponding to the position x within the range of ± 1 with the center point as the origin is output to the pitch signal circuit 32, and the current Ia,
The sum signal of Ib is shaped and output as a synchronizing signal of the spot LDa '. Next, when the spot LDb ′ of the beam output from LDb is incident on the PSD 16, the position signal circuit 3
Similarly, 1 outputs the position signal of the spot LDb 'and its synchronizing signal.

The pitch signal circuit 32 stores and previously stores the position signal of the spot LDa 'inputted first, calculates the difference from the position signal of the spot LDb' inputted next, and respectively calculates the difference by the spots LDa 'and LDb'. The pitch signal corresponding to the pitch Pp of the formed scan lines SLa and SLb is output to the error amplifier 33.

In the optical element driving means according to the sixth embodiment of the present invention, the level of the pitch signal input from the pitch detecting section 30 is adjusted by the error amplifier 33 to the reference pitch Ps.
To a reference voltage Vref for the scan line SLa and SLb formed by LDa ′ and LDb ′, and moving or rotating the optical element according to the difference signal.
Adjust so that p becomes the reference pitch Ps.

Therefore, for example, at the time of manufacturing, the reference voltage Vref should be adjusted so that the optical signal corresponding to the scan line of the reference pitch Ps is incident on the PSD 16 by a standard jig or the like and the output of the error amplifier 33 becomes zero. For example, the pitch Pp of the scan lines SL is always maintained at the reference pitch Ps even if the environmental conditions change or the intervals between the LDs of the LD array 1 increase due to a temperature rise due to heat generation during use.

Further, as shown in FIG. 6, the spot LD
Since the straight line connecting a ′ and LDb ′ is inclined with respect to the scan line, the spots LDa ′ and LDb ′ are incident on the PSD 16 at different timings, and therefore the scan lines SLa that are difficult to separate because they are extremely close to each other. ，
The pitch Pp of SLb can be detected by one sensor (PSD16). Further, the synchronization signal of the spots LDa 'and LDb' can be obtained with one sensor.

However, although the means for detecting and adjusting the pitch of the scan lines works effectively during operation,
There is a problem that it does not work at the time of starting scanning at the beginning of operation. Even if it works after the scan is started, banding may occur at the leading edge of the image.

The optical element driving means according to the seventh embodiment of the present invention uses the thermistor 13, which is a temperature detecting means in close contact with the back surface of the LD unit 12 shown in FIG. The LDa 'and LDb' are detected by moving or rotating the optical element according to the detected temperature.
The pitch of the scan lines SLa and SLb formed by is adjusted to the reference pitch Ps.

Therefore, since the optical element is adjusted even in the initial stage of the operation, banding does not occur even at the leading edge of the image. Further, even if the ambient temperature or the temperature of the LD array 1 changes during operation, the optical element is adjusted according to the temperature change, so that banding can be prevented. further. If the adjustment according to the detected temperature according to the seventh embodiment and the adjustment according to the detected pitch according to the sixth embodiment are used in combination, a better result can be obtained by covering each other. .

When the optical element is moved or rotated by the optical element driving means according to the first to seventh embodiments, the light of the optical system on the optical path from the LD array 1 to the photoconductor surface 8a. Since the transmission efficiency changes or the spot diameter changes slightly, even if the output of the LD is constant, the light amount (exposure amount) on the photoconductor surface 8a changes and the image density changes. There is a fear.

If the fluctuation of the image density is gentle, it is inconspicuous, but if it fluctuates suddenly, it may be noticed as density unevenness. Therefore, a well-known control device (not shown), such as a CPU, gives an instruction to the LD driver provided for each LD according to the movement or rotation of the optical element, so that each L
By controlling the light emission output of D, the amount of light on the photoconductor surface 8a can be kept constant.

Each of the components in the sub-scanning direction is a surface of the photosensitive member 8.
The spot diameter on a is proportional to the combined focal length f0 of the optical element group and inversely proportional to the diameter of the parallel beam emitted from the coupling lens 2. Therefore, by moving or rotating the optical element, the combined focal length f0 of the optical element group is
, The spot diameter can be kept constant or within an allowable range by changing the diameter of the aperture provided in the optical path of the parallel beam.

FIG. 11 is a perspective view showing an example of aperture diameter changing means for that purpose. The aperture diameter changing means shown in FIG. 11 is almost the same as the optical element driving means shown in FIG. 9 for moving the weak lens in and out, except that it is provided in the optical path of the parallel beam between the coupling lens 2 and the beam compressor 3. Therefore, the same portions are denoted by the same reference numerals and the description thereof will be omitted.

The movable plate 26 corresponding to the movable plate 22 has two
Instead of the openings 21a and 21b, two apertures 2 that are long in the main scanning direction and have different widths in the sub scanning direction are provided.
7a and 27b are provided. Therefore, the diameter of the parallel beam passing through the aperture in the sub-scanning direction changes between the normal state and the time when the solenoid 24 is energized.

Therefore, in accordance with the change of the combined focal length f0 of the optical element group due to the movement or rotation of the optical element, f0 is changed.
When the value of becomes large, the wide aperture becomes f0.
When the value of becomes small, the solenoid 24 is turned on so that the narrow apertures are in the optical path.
By turning it off, the diameter of the spot can be kept unchanged.

Such switching of the aperture diameter is different from the insertion / removal of a weak lens, and the spot diameter has a certain allowable width. Therefore, if the pitch Pp is adjusted, two aperture diameters can be set. Switching is sufficient, and it is not necessary to switch three or more apertures.

[0096]

As described above, the image forming apparatus according to the present invention can easily and accurately adjust the pitch of a plurality of scan lines by multi-beams with a simple structure. Also, the adjusted pitch can always be held accurately.

[Brief description of drawings]

FIG. 1 is an optical path diagram showing an example of an optical element drive unit according to a first embodiment for adjusting the scan line pitch of the present invention.

FIG. 2 is a perspective view showing an example of an optical writing device according to an embodiment of the present invention.

FIG. 3 is a plan view showing an arrangement of optical systems of the optical writing device shown in FIG.

FIG. 4 is an optical path diagram in which the optical system shown in FIGS. 2 and 3 is developed along the optical axis.

FIG. 5 is a diagram showing an example of scan lines when the pitch of the scan lines is normal and abnormal.

FIG. 6 is a diagram showing an example of a relationship between two spots on a surface of a photoconductor and scan lines formed by the spots.

FIG. 7 is an optical path diagram showing an example of an optical element driving means that is a second embodiment of the present invention.

FIG. 8 is a diagram for explaining the principle of the optical element driving means shown in FIG.

FIG. 9 is a perspective view showing an example of an optical element driving means according to a fifth embodiment of the present invention.

FIG. 10 is a block diagram showing an example of a configuration of a pitch detection unit that is pitch detection means.

FIG. 11 is a perspective view showing an example of aperture diameter changing means.

[Explanation of symbols]

1: LD array (plural beam generating means) 2: Coupling lens 3a, 3b, 4: cylinder lens (optical element) 5: Polygon mirror (beam deflection means) 5a: Mirror surface 6: fθ lens (imaging means) 8a: photoconductor surface 10: optical writing device 13: Thermistor (temperature detection means) 16: PSD (position sensitive diode) 20: Weak lens (optical element) 30: Pitch detector (pitch detector)

─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hidetoshi Kanai 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd. (72) Kenichi Ono 1-3-6 Nakamagome, Ota-ku, Tokyo Within Ricoh Co., Ltd. (56) Reference JP-A-6-208068 (JP, A) JP-A-1-239520 (JP, A) JP-A-7-120690 (JP, A) (58) Fields investigated ( Int.Cl. ⁷ , DB name) G02B 26/10

Claims (6)

(57) [Claims] 1. A signal modulated according to an image signal.
A plurality of beam generating means for outputting a beam of laser light;
The multiple divergent beams output by the multiple beam generation means are flattened.
A coupling lens for converting into a row beam and the coupling lens
The shape of multiple parallel beams passing through
Are placed on the optical axis for at least the sub-scanning direction.
A plurality of optical elements having a power component and the plurality of optical elements
Deflecting the plurality of beams passing through the child in the main scanning direction
Beam deflection means, and the beam deflected by the beam deflection means
Imaging multiple beams as spots on the photoconductor
Image forming means for Equipped with an optical writing device consisting of The plurality of spots scan the photoconductor in the main scanning direction.
Of each scan line formed by scanning
Switch to the preset value.
In the image forming apparatus in which the image generating means is arranged, Change the pitch of multiple scan lines on the photoreceptor
Of the plurality of optical elements arranged on the optical axis to
Optical element drive to move or rotate at least one of them
Set up means, The optical element driving means has an axis substantially along the main scanning direction.
A means for rotating the optical element as a center That
A characteristic image forming apparatus. 2.Each is modulated according to the image signal.
A plurality of beam generating means for outputting a beam of laser light;
The multiple divergent beams output by the multiple beam generation means are flattened.
A coupling lens for converting into a row beam and the coupling lens
The shape of multiple parallel beams passing through
At least in the sub-scanning direction, which are arranged on the optical axis for
A plurality of optical elements having a power component of
Deflect the plurality of beams passing through the element in the main scanning direction
Beam deflecting means and the beam deflecting means
Multiple beams as spots on the photoconductor.
An image forming means for forming an image, Equipped with an optical writing device consisting of The plurality of spots scan the photoconductor in the main scanning direction.
Of each scan line formed by scanning
Switch to the preset value. Number of bees
In the image forming apparatus in which the image generating means is arranged, Change the pitch of multiple scan lines on the photoreceptor
Of the plurality of optical elements arranged on the optical axis to
Optical element drive to move or rotate at least one of them
Means The An optical element driving means connects the optical element to the plurality of beams.
It is a means for moving in and out of the optical path of
Image forming apparatus. 3. The image forming apparatus according to claim 1 or 2, wherein the provided pitch detecting means for detecting the pitch of the plurality of scan lines on the photosensitive member, the optical element driving means, by the pitch detecting means An image forming apparatus comprising means for moving or rotating the optical element according to a difference between the detected scan line pitch and a preset value. 4. The image forming apparatus according to claim 1 or 2, a temperature detection means for detecting the temperature around the optical writing device is provided, the optical element driving means, detected by said temperature detecting means temperature An image forming apparatus comprising means for moving or rotating the optical element according to the above. 5. The image forming apparatus according to claim 1 or 2, changing the beam power of said plurality of beam generating means outputs respectively in response to movement or rotation of the optical element according to the optical element driving means An image forming apparatus comprising a beam power changing unit. 6. The image forming apparatus according to claim 1 or 2, and an aperture disposed in the optical path of the plurality of beams, the diameter of the aperture to move or rotate the optical element according to the optical element driving means An image forming apparatus, comprising: an aperture diameter changing unit for changing the aperture diameter according to the change.