JP4841086B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical scanning device and an image forming apparatus used in a laser beam printer, a digital plain paper copying machine, a plain paper facsimile, and the like.
[0002]
[Prior art]
Some color laser beam printers, color copiers, color facsimiles, and the like are provided with a plurality of scanning optical systems in order to speed up image writing or image formation. The invention described in Japanese Patent Laid-Open No. 10-244708 is one of them, and a plurality of scanning optical systems that perform optical scanning with a polygon scanner are provided. And the synchronous rotation control means which adjusts the rotational speed of each polygon motor is provided.
[0003]
Further, as a new method of the optical scanning device, an optical scanning device having a deflecting unit that performs optical scanning by vibration is being studied. This is an optical scanning device having deflection means using sinusoidal vibration. An optical scanning device characterized in that the mechanical resonance frequency of the deflecting means is made variable has also been studied.
[0004]
[Problems to be solved by the invention]
Conventionally, an optical scanning device that deflects and scans a light beam by a deflector is well known. As the deflector, a polygon scanner that rotates at a constant speed is widely used. However, since the polygon scanner requires a large device, there are problems such as banding due to vibration, temperature rise, noise, and power consumption increase.
[0005]
There is also known an optical scanning device in which a plurality of scanning optical systems are arranged in parallel in the main scanning direction. This method has the following advantages.
-A compact optical scanning device can handle a wide scanning area.
Since each scanning optical system and scanning optical element become small, correction of wavefront aberration becomes easy, and variations in beam spot diameter due to component variations and component mounting errors are reduced. Therefore, the beam spot can be easily reduced in diameter.
It is also known that a plurality of scanning optical systems are arranged in the sub-scanning direction and applied to a multicolor image forming apparatus.
[0006]
On the other hand, a micromirror has been proposed in which light is deflected by using a micromachine technique to generate a sine wave vibration with a resonant structure. When this is used, the apparatus is miniaturized, and problems such as banding due to vibration, temperature rise, noise, and increase in power consumption can be greatly reduced.
[0007]
However, the scanning frequency of the micromirror that performs sinusoidal vibration depends on the inherent resonance frequency, and the resonance frequency may be a different value due to manufacturing variations.
If the resonance frequency is different for each scanning optical system, the position of the scanning line is different for each scanning optical system. When a plurality of scanning optical systems are arranged in the main scanning direction, the secondary scanning line is connected at the joint of the scanning lines in the main scanning direction. A dot position shift occurs in the scanning direction. Further, when a plurality of scanning optical systems are arranged in the sub-scanning direction, color misregistration occurs in the sub-scanning direction.
[0008]
The present invention has been made in order to solve the above-described problems of the prior art. In an optical scanning apparatus provided with a plurality of scanning optical systems having deflection means for performing optical scanning by vibration, dots in the sub-scanning direction are provided. The object is to reduce misregistration and color misregistration.
The present invention is also an image forming apparatus using an optical scanning device provided with a plurality of scanning optical systems having a deflecting unit that performs optical scanning by vibration, and has high image quality with little color shift and image degradation at joints. An object is to provide an image forming apparatus.
[0009]
First aspect of the present invention, there is provided an image forming apparatus having an optical scanning device scanning optical system having a deflection means for performing optical scanning is more deployed by the reciprocal vibration, said plurality of light scanning optical system, the main scanning Arranged in the direction, and configured to record one image by connecting the scanning areas of the respective optical scanning optical systems, and performs image recording only in the forward section of the reciprocating vibration of the deflection means, and the plurality of scanning optical systems For at least one of the above , image recording is not performed every specific scanning period so that the scanning line position deviation in the sub-scanning direction at the scanning line joint between the plurality of scanning optical systems is equal to or less than the line interval of one period. A section is provided.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an optical scanning device and an image forming apparatus according to the present invention will be described below with reference to the drawings.
FIG. 1 shows an embodiment of a vibrating mirror deflector used in an optical scanning device. In FIG. 1, the mirror substrate etches the silicon substrate 102 so that the back side is cut into a square shape, and the frame portion and the top plate portion are left at a predetermined thickness. In the top plate portion, the vibrating mirror 100 and the torsion bar 101 that pivotally supports the vibrating mirror 100 are left, and the periphery thereof is formed to penetrate. In the embodiment, a 200 μm silicon substrate was used, and the thickness of the vibrating mirror 100, that is, the thickness of the top plate portion was 60 μm. The width of the oscillating mirror 100 is 4 mm × 2 mm, and a metal film such as Au (copper) is vapor-deposited on the upper surface to form a reflecting surface. In addition, comb-like irregularities are formed at both end portions, and this is the movable electrode 104.
[0014]
A fixed electrode 105 is formed on the end face of the substrate 102 facing the movable electrode 104 so as to mesh with the above-mentioned comb-tooth shape and with a clearance of 5 μm. The substrate 102 is opposed to the substrate 102 in the same direction in the thickness direction. The torsion bar 101 is sized so as to resonate in a vibration mode with the torsion bar 101 as a rotation axis in accordance with the scanning frequency of the vibration mirror 100. In the embodiment, the torsion bar 101 has a width of about 100 μm and a length of about 1 mm.
[0015]
The surface of the silicon substrate 102 is a polished surface of the wafer, and a nitride film is formed for insulation between the electrode and the substrate. Therefore, the vibrating mirror 100 penetrated by etching is slightly inclined due to the internal stress difference between the front and back surfaces, and the movable electrode 104 and the fixed electrode 105 have a step of several μm. When a voltage is applied to one of the fixed electrodes 105, the oscillating mirror 100 is rotated by twisting the torsion bar 101 to a horizontal state by electrostatic force. When AC voltage is applied, it vibrates back and forth. Although this amplitude is very small, when the frequency of the AC voltage to be applied is set to a frequency corresponding to the mechanical resonance frequency unique to the vibrating mirror 100, the vibrating mirror 100 can be excited to expand the amplitude. In the embodiment, the sine wave is oscillated at ± 5 °.
[0016]
The reason why the electrodes 104 and 105 are comb-toothed is that the outer peripheral length becomes longer and the area of the electrode can be increased. Therefore, consideration is given to obtaining a larger electrostatic torque at a low voltage.
[0017]
Incidentally, the resonance frequency fd is approximately given by the following equation.
fd = √ (K / J)
J is the moment of inertia of the vibrating mirror, K is the spring constant of the torsion bar,
K = G · I / L
Here, G: elastic coefficient, I: secondary moment of section, L: length, however, the resonance frequency of the oscillating mirror varies by about 2% due to processing errors and dimensional variations in the manufacturing process.
[0018]
FIG. 3 shows an embodiment of the optical scanning device. In FIG. 3, three oscillating mirror deflectors 201, 202, and 203 as deflection means for performing optical scanning by vibration are mounted on the electrical board 204 side by side in the main scanning direction. Each vibrating mirror deflector 201, 202, 203 is combined with a semiconductor laser 205 as a light source, a coupling lens 206, a first scanning lens 207, and a second scanning lens 208 to constitute an optical scanning device. In the embodiment, the second scanning lens 208 of each optical scanning device is linearly connected in the main scanning direction and formed integrally.
[0019]
The light beam emitted from the semiconductor laser 205 of each optical scanning device is made into a parallel light beam by the coupling lens 206 and is incident on the vibrating mirror deflector. The light beam is incident on the normal line of the oscillating mirror 100 with an inclination of about 20 ° in the sub-scanning direction, deflected by the oscillating mirror 100 and emitted from the deflector. Further, an image is formed on the image carrier 210 such as a photosensitive drum by the scanning lenses 207 and 208, and an area obtained by dividing one image in the main scanning direction is scanned and exposed to form an electrostatic latent image. The electrostatic latent image scanned and recorded by each optical scanning device and joined in the main scanning direction is visualized with toner by the developing device 211 and transferred to the transfer paper 212. The toner image on the transfer paper 212 is fixed by a fixing unit (not shown), and the image carrier 210 after the transfer is subjected to an exposure process again through processes such as cleaning, charge removal, and charging. The process is executed.
[0020]
In the exposure process, when the resonance frequency varies by about 2% as described above, a dot position shift in the sub-scanning direction occurs at the joint of the scanning lines. For example, even if the writing positions are matched, a shift of one line or more occurs when 50 lines are scanned.
[0021]
FIG. 6 shows another embodiment of the optical scanning device. In FIG. 6, four oscillating mirror deflectors 220, 221, 222, and 223 as deflecting means for performing optical scanning by vibration are mounted on the electrical board as in the above-described embodiment. However, the four oscillating mirror deflectors 220, 221, 222, and 223 are arranged in parallel in the sub-scanning direction. Each of the oscillating mirror deflectors is combined with the semiconductor laser 225, the coupling lens 226, the first scanning lens 227, and the second scanning lens 228 to constitute an optical scanning unit. Each optical scanning unit includes yellow (Y), magenta (M), cyan (C), and image carriers 230, 231, 232, and 233 that are arranged along the feeding direction of the transfer belt 229, respectively. An electrostatic latent image corresponding to black (K) is recorded, and developed with each color toner by developing units 234, 235, 236, and 237.
[0022]
The toner images formed on the image carriers 230, 231, 232, and 233 are sequentially transferred to the transfer belt 229 as the transfer belt 229 moves, and are superimposed to form a color image. The color image on the transfer belt 229 is transferred to the transfer paper 238. As is well known, after that, it is subjected to a fixing process, a cleaning process, a charge eliminating process, a charging process, etc., and is again subjected to an exposure process to form images on the respective image carriers 230, 231, 232, 233.
[0023]
In the case of the embodiment shown in FIG. 6, as in the embodiment shown in FIG. 3, if the resonance frequency varies by about 2%, even if the writing start position is adjusted, the color shift of one line or more is caused by scanning 50 lines. Will occur.
[0024]
In order to solve this problem, it is necessary to make the scanning frequency of the deflecting means substantially the same. As described above, since the resonance frequency is expressed by fd = √ (K / J) and K = G · I / L, the resonance frequency is adjusted by changing any of the parameters, and the scanning frequency is changed. Can be adjusted. FIG. 2 shows various means for adjusting the scanning frequency.
[0025]
In the example shown in FIG. 2A, the length L of the torsion bar 101 is substantially adjusted by providing the wide portion 111 at the base of the torsion bar 101 and forming the notch 112 using a carbon dioxide gas laser or the like. It is configured as follows. In the example shown in FIG. 2B, a groove 113 is provided at the base of the torsion bar 101, and the groove 113 is filled with an adhesive 114 or the like, so that the length L of the torsion bar 101 is substantially reduced. It is configured to adjust. In any case, the resonance frequency of the oscillating mirror 100 can be adjusted, and thereby the scanning frequency can be adjusted.
[0026]
FIG. 2C shows the vicinity of the torsion bar 101 by forming a heater 115 in the vicinity of the root of the torsion bar 101 by forming a thin film resistor in a comb-like pattern and energizing and heating the heater 115. The resonance frequency of the oscillating mirror 100 is adjusted by adjusting the moment of inertia J, and the scanning frequency can be adjusted.
[0027]
A sinusoidal oscillating mirror using resonance has the advantage that the gain is maximized when the scanning frequency matches the resonance frequency, and the drive power supply can be low. However, the input reference clock can be varied with respect to the resonance frequency. By doing so, the scanning frequency can be made variable. FIG. 7 is a block diagram showing an example of the control means. In FIG. 7, the reference clock is frequency-divided by the programmable frequency divider 121 and supplied to the pulse generation unit 122. The pulse generation unit 122 generates a clock by counting the number of pulses based on the divided clock pulses. When the original reference clock is adjustable, the formed clock is also adjusted. The PLL unit 123 detects the phase of the clock, and outputs an updated clock if there is a phase difference. The gain control unit 124 controls the voltage applied to the fixed electrode 105 of the oscillating mirror according to the clock cycle. The amplitude of the oscillating mirror increases as the voltage is increased. The voltages are set so that the amplitudes are uniform in each optical scanning unit.
[0028]
At this time, the amplitude is shifted because the scanning frequency deviates from the resonance frequency, but the amplitude can be kept constant by increasing the drive voltage. The target value of the scanning frequency is preferably set between the maximum value and the minimum value of the resonance frequency unique to each deflecting unit. By doing so, the dynamic range of adjustment can be reduced, and more accurate frequency adjustment is possible.
[0029]
Although the above example is an example in which the scanning frequency is adjusted, a method in which the scanning frequency is not adjusted will be described below. FIG. 8 shows the state of the scanning lines connected in the main scanning direction. In order to simplify the description, an example in which two scanning optical systems are arranged will be described. In FIG. 8, the left-right direction is main scanning and the up-down direction is sub-scanning, and image recording is performed only in the forward section of reciprocating vibration. (A) is a comparative example, (b) is a case of an Example. As described above, when each drive is performed at an individual scanning frequency, a shift in operation frequency occurs. For example, in the embodiment shown in FIG. 8B, the scanning frequency on the right side is higher than that on the left side, so that it shrinks in the vertical direction, and the deviation of the scanning position at the right and left joints gradually increases. In the nth line, the scanning line position is shifted by about one cycle, and in the 2nth line, the scanning line position is shifted by about two cycles.
[0030]
Therefore, in the right scanning optical system, the timing of image recording is changed every n cycles, and the n-th image data is recorded where n + 1 lines are to be initially recorded. Similarly, the 2n-th image data is recorded where the 2n + 2 line is to be recorded. At this time, image recording is not performed on the n line and the 2n + 1 line. As described above, for at least one of the plurality of scanning optical systems, the timing of image recording is changed for each specific scanning cycle, whereby the sub-scan at the scanning line joint between the plurality of scanning optical systems is changed. The scanning line position deviation in the scanning direction can be made equal to or shorter than the line interval for one cycle, and a high quality image can be obtained.
[0031]
For at least one of the plurality of scanning optical systems as described above, the timing of image recording is changed for each specific scanning period, and the same can be applied to the example of the optical scanning device shown in FIG. This can similarly reduce the color misregistration in the sub-scanning direction.
[0032]
In the above embodiment, the image recording timing is changed in the same manner for each specific scanning cycle. However, the image recording timing may be changed for each scanning cycle. FIG. 9 shows a state of the scanning line in which the deviation is corrected according to the embodiment. Here too, in order to simplify the description, an example in which two optical scanning means are arranged is shown. In FIG. 9, the horizontal direction indicates main scanning and the vertical direction indicates sub-scanning. In addition, image recording is performed only in the forward section of the reciprocating vibration. As described above, when driving at each individual scanning frequency, for example, the right scanning frequency is high in the example of FIG. 9, the image is shrunk in the vertical direction and the scanning position shift Δ at the joint gradually increases.
[0033]
In general, if the pitch is 1/8 line or less, it is difficult to identify the scanning line deviation. Therefore, in the embodiment, the phase of the right scanning frequency is delayed in the section of the 5th line and the 6th line and connected every 6 lines. Correction is performed so that the eye coincides with the optical scanning means on the left side. Therefore, by repeating this periodically, the seams of the scanning lines can be made to coincide with each other in a specific scanning period, and the scanning position at the joints can be prevented from being shifted beyond an allowable value.
[0034]
Similarly, any number of optical scanning means can be corrected. FIG. 5 is a block diagram showing an example of driving means of the vibrating mirror deflector for performing the control. In FIG. 5, the reference clock is frequency-divided by the programmable frequency divider 121 and supplied to the pulse generator 122. The pulse generator 122 counts the number of pulses based on the divided clock pulses and generates a clock at the resonance frequency. In this embodiment, write control is performed every six lines in this embodiment. In accordance with the phase selection signal generated from the unit, the pulse width is changed as shown in FIG.
[0035]
The PLL unit 123 detects the phase of the clock, and outputs an updated clock if there is a phase difference. The gain control unit 124 controls the voltage applied to the fixed electrode 105 of the oscillating mirror according to the clock cycle. The amplitude of the oscillating mirror increases when the voltage is increased, and the voltage is set so that the amplitude is uniform in each optical scanning unit.
[0036]
FIG. 4B shows another embodiment for changing the phase of the scanning frequency. A plurality of phase clocks a, b... Are prepared in advance and switched according to the phase selection signal. However, if the above changes are made at the time of image recording, the scanning speed will change in the middle, causing a magnification error, making it impossible to perform accurate image recording.
Therefore, if the above change is performed in a region where image recording is not performed, that is, a region other than the effective scanning period, a magnification error does not occur.
The idea of changing the timing of image recording for each specific scanning cycle and the idea of providing phase variable means for shifting the phase of the scanning frequency for each specific scanning cycle are based on the fact that the deflection means is a polygon scanner. Can be applied, and belongs to the technical scope of the present invention.
[0037]
In any of the above-described embodiments of the optical scanning apparatus having the deflecting means for performing optical scanning by vibration, at least one of the plurality of deflecting means is provided with the gain adjusting means for changing the amplitude, so that any scanning can be performed. The optical system also has the same scanning frequency, and dot position shift and color shift in the main scanning direction can be reduced.
In general, the vibration of the deflecting means is often a sine wave vibration, but even a vibration other than a sine wave does not depart from the technical scope of the present invention.
[0038]
In the above description, the light source of each scanning optical system is a single light source, but it may be a multi-beam and does not depart from the technical scope of the present invention.
[0044]
According to the first aspect of the present invention, a plurality of scanning optical systems are arranged in the main scanning direction by using the above-described optical scanning device, and one image is recorded by connecting the scanning areas of the respective optical scanning optical systems. By configuring the image forming apparatus in this way, by changing the timing of image recording for each specific scanning period, it is possible to reduce dot position deviation and color deviation in the sub-scanning direction without adjusting the number of scanning periods. In addition, it is possible to provide an image forming apparatus capable of reducing dot position deviation and color deviation in the sub-scanning direction by providing a section in which image recording is not performed .
[Brief description of the drawings]
FIG. 1 is an exploded perspective view showing an example of deflection means applicable to the present invention.
FIG. 2 is a plan view showing various examples of resonance frequency adjusting means of the deflecting means.
FIG. 3 is a perspective view showing an embodiment of an image forming apparatus using the optical scanning device according to the present invention.
FIG. 4 is a timing chart showing an operation of deflecting image recording timing for each specific scanning cycle.
FIG. 5 is a block diagram illustrating an example of a driving unit of a vibrating mirror deflector that performs scanning position deviation correction of a plurality of optical scanning units.
FIG. 6 is a perspective view showing another embodiment of the image forming apparatus using the optical scanning device according to the present invention.
FIG. 7 is a block diagram showing an example of scanning frequency control means.
FIGS. 8A and 8B show a state of scanning lines connected in the main scanning direction, where FIG. 8A is a comparative example, and FIG. 8B is a diagram showing a case according to an embodiment of the present invention.
FIG. 9 is a diagram showing the state of scanning lines when correcting the shift of scanning lines by changing the method of changing the timing of image recording for each scanning cycle.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 100 Vibration mirror 101 Torsion bar 104 Movable electrode 105 Fixed electrode 201 Vibration mirror deflector 202 Vibration mirror deflector 203 Vibration mirror deflector 207 Scan lens 208 Scan lens 201 Image carrier

Claims (1)

An image forming apparatus having an optical scanning device in which a plurality of scanning optical systems having a deflecting unit that performs optical scanning by reciprocating vibration is provided,
The plurality of optical scanning optical systems are arranged in the main scanning direction, and are configured to record one image by joining the scanning regions of the respective optical scanning optical systems,
Perform image recording only in the forward section of the reciprocating vibration of the deflection means,
For at least one of the plurality of scanning optical system, a particular scan cycle each as the sub-scanning direction of the scanning line position shift in th joint scanning lines between the plurality of scanning optical systems is equal to or less than the line spacing of one period An image forming apparatus characterized in that a section in which image recording is not performed is provided.

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