JP2011033756A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device wherein power consumption is low, temporal stability of images is ensured by reducing the temperature rise of a scan lens and the temperature deviation or vibration of an optical scanner, while reducing the magnification errors of printed images. <P>SOLUTION: A phase difference between a drive signal and PDa is measured by a phase comparator and a counter to calculate a difference from a referential resonance frequency. If the difference falls outside an allowable value, adjustment is made by changing a drive frequency so as to make it coincide with the resonance frequency. The allowable value is set from a limit value allowing the correction of an amplitude decrease caused by fluctuation in the resonance frequency. In order to correct (constant amplitude control) the amplitude decrease, the allowable value of the difference is determined to be the lower one of the two: the withstand voltage of a drive circuit for increasing a drive voltage and the allowable withstand voltage of a vibration mirror. The detection of a difference between the resonance frequency and the drive frequency is performed with the amplitude of the vibration mirror controlled so as to be constant. As the scan characteristics of a deflection scan will change, frequency adjustment of the drive voltage is performed in course of non-image formation, that is, while a scanned surface area is not scanned by a laser beam. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus having a laser raster writing optical system.

  Conventionally, a polygon mirror has been widely used as an optical deflector used in an optical scanning device of an image forming apparatus. In particular, in order to realize high-speed printing and high image quality of a color image forming apparatus, it is necessary to rotate the polygon scanner at a high speed of 25000 rpm or more and with high accuracy. On the other hand, in order to improve the image quality by reducing the diameter of the laser beam, the inscribed circle radius of the polygon mirror used in the polygon scanner and the length in the main scanning direction are relatively large, and the trend of increasing the load of the polygon scanner is increasing. is there.

  As the load increases, the power consumption of the polygon scanner increases, and the heat generation adversely affects the optical elements such as the scanning lens. Specifically, the temperature rise of the scanning lens closest to the polygon scanner. The heat generated from the polygon scanner is transferred to the optical housing, or the temperature of the scanning lens rises due to radiation. Actually, the temperature of the scanning lens is not raised uniformly, but the distance from the heat source (polygon scanner), the difference in the thermal expansion coefficient of each material, and the influence of the airflow, particularly in the main scanning direction that is the longitudinal direction. It will have a temperature distribution.

  If there is a temperature distribution in the main scanning direction, particularly the shape accuracy and refractive index of the scanning lens change, the spot position of the laser beam fluctuates, and the image quality deteriorates. This problem is particularly noticeable when a plastic material having a large coefficient of thermal expansion is used.

  In the color image forming apparatus, the laser beams corresponding to the respective colors (yellow, magenta, cyan, black) are scanned, so that temperature deviation between the optical scanning apparatuses corresponding to the respective colors becomes a problem in addition to the problems described above. . This temperature deviation causes a shift in the relative positional relationship between the beam spots corresponding to the respective colors, resulting in an image color shift.

  Further, the temperature rise due to heat generation of the high-load polygon mirror induces a fine movement of the rotating body component (particularly, the polygon mirror having a large mass ratio), changes the rotating body balance, and generates vibration. Even if the parts of the rotating body (polygon mirror, flange to which the rotor magnet is fixed, shaft, etc.) have different or coincide with each other, they do not strictly control or inspect the part tolerance or fixing method. As a result, a slight movement (change in balance of the rotating body) occurred during high-temperature and high-speed rotation, and as a result, vibration was increased. The vibration is transmitted and amplified to an optical element (for example, a folding mirror) in the optical scanning device to generate banding, which causes image deterioration and noise.

  In order to solve these problems, a vibrating mirror using a resonance phenomenon has been studied as a polygon mirror deflector. For example, in an image forming apparatus that uses a resonance-type oscillating mirror as a deflector, when changing the rotational movement speed of the photoreceptor or changing the resolution of the image, the drive frequency of the deflector depends on the operating conditions. Although the drive frequency Fd is changed and set, the resonance frequency Fr and the drive frequency Fd do not coincide with each other as it is, and the amplitude value of the deflecting mirror surface decreases. Therefore, the resonance characteristic of the deflector is shifted to the drive frequency side following the change setting of the drive frequency Fd. Thus, an apparatus that controls the resonance frequency Fr of the deflector having the maximum amplitude value and the drive frequency Fd so as to substantially coincide with each other is described in Patent Document 1. This method has the advantages of low power consumption, reducing temperature rise of the scanning lens used in the optical scanning device, and reducing temperature deviation and vibration of the optical scanning device of the color image forming apparatus.

  However, since the oscillating mirror uses a resonance phenomenon, a dimensional error in the shape of the reflecting surface of the oscillating mirror and the shape of the beam around the oscillating mirror makes the resonance frequency different. In addition, a dimensional error requires a processing accuracy of μm or less for a reflecting surface shape of several millimeters, but is not achieved at present. As a result, there is a problem in that variations in resonance frequency occur during mass production, and an image magnification error occurs during image formation.

  In addition, the resonance frequency fluctuates due to changes in the rigidity (Young's modulus) of the beam with respect to changes in the environmental temperature, and there is a problem that the magnification error fluctuates with time.

  Also, Patent Document 1 is a method of changing the resonance frequency of the oscillating mirror in order to make the resonance frequency and the drive frequency coincide with each other, and the resonance frequency is changed by the temperature change of the torsion spring with the electric resistance element as disclosed. Since this method requires time, there is a risk that inconveniences and restrictions that image formation cannot be performed during that time.

  Furthermore, even if the resolution of the image is changed to match the drive frequency and the resonance frequency, the resonance frequency of the resonance type oscillating mirror changes depending on the environmental temperature. There is a risk that the amplitude of the above cannot be obtained.

  The present invention solves the above-described problems of the prior art, consumes less power, stabilizes the image over time by reducing the temperature rise of the scanning lens used in the optical scanning device, and the temperature deviation and vibration of the optical scanning device. It is an object of the present invention to realize an optical scanning device and an image forming apparatus that reduce the magnification error of a printed image while having the original effect of a vibrating mirror to ensure the performance.

  In order to achieve the above object, an optical scanning device according to claim 1 of the present invention deflects and scans laser beams emitted from a plurality of light source devices in a main scanning direction by a single oscillating mirror deflector. In an optical scanning device having scanning imaging means for condensing toward a plurality of scanned surfaces, a difference between the resonance frequency of the oscillating mirror deflector and the drive frequency is detected, and the resonance is detected when the difference is outside a desired range. Control means for adjusting the drive frequency so as to match the frequency is provided.

  With this configuration, an optical scanning device that does not deteriorate the image quality can be realized.

  Further, the invention described in claims 2 and 3 is the optical scanning device according to claims 1 and 2, wherein deflection scanning is performed with reference to the voltage waveform of the driving frequency in a state where the amplitude of the oscillating mirror deflector is controlled to be constant. Measuring a phase difference with an output signal from a light receiving element (PDa, PDb) arranged in a scanning region of a laser beam to be detected, and detecting a difference between a resonance frequency and a driving frequency from a variation in the phase difference; In the deflection scanning by the oscillating mirror deflector, the difference between the resonance frequency and the driving frequency is always detected.

  With this configuration, an optical scanning apparatus that can be adjusted without stopping the apparatus can be realized.

  According to the fourth and fifth aspects of the present invention, in the optical scanning device according to any one of the first to third aspects, the adjustment of the driving frequency by the control means determines whether or not the laser beam is scanned over the surface area to be scanned. The adjustment of the drive frequency by the control means is performed at any time when the difference between the resonance frequency and the drive frequency is outside the desired range, and the adjustment is performed step by step. It is characterized in that it is performed while changing.

  With this configuration, an optical scanning apparatus that can be adjusted without stopping the apparatus can be realized.

  According to a sixth aspect of the present invention, in the optical scanning device according to any of the first to fifth aspects, the laser beams emitted from the plurality of light source devices are angled in the sub-scanning direction with respect to the reflecting surface of the vibrating mirror deflector. The plurality of laser beams to be incident and deflected are all scanned in the same direction.

  With this configuration, the optical scanning device and the image forming apparatus using the same can be reduced in size.

  According to a seventh aspect of the present invention, there is provided an image forming apparatus according to any one of the first to sixth aspects, wherein a latent image is formed on a latent image carrier by optical scanning, and the latent image is visualized to obtain a desired recorded image. The optical scanning device described in any one of the above items is used.

  With this configuration, an image forming apparatus that does not deteriorate the image quality can be realized.

  According to the present invention, the magnification of a print image is suppressed while having the effect of a vibrating mirror that suppresses power consumption, reduces the temperature rise of the scanning lens, reduces temperature deviation and vibration of the optical scanning device, and ensures temporal stability of the image. There is an effect that an optical scanning apparatus and an image forming apparatus that reduce errors can be realized.

The perspective view which shows the outline of the optical scanning device in embodiment of this invention Schematic sectional view of an image forming apparatus in the present embodiment In the vibrating mirror according to the present embodiment, (a) is a detailed view of a first substrate and (b) is a detailed view of a second substrate. Exploded perspective view of the vibrating mirror in the present embodiment The figure which shows the vibration mirror unit mounted in the optical housing in this Embodiment The figure which shows schematically the principal part of the photosensitive drum of the optical scanning device in this Embodiment. The figure which shows the waveform of the vibration mirror amplitude with respect to time in this Embodiment The figure explaining the amplitude fluctuation | variation of the rocking | fluctuation variation of the vibration mirror in this Embodiment The figure explaining the amplitude position fluctuation | variation of the rocking | fluctuation variation of the vibration mirror in this Embodiment The figure explaining the phase fluctuation | variation of the rocking | fluctuation variation of the vibration mirror in this Embodiment The figure which shows the relationship between the drive frequency fd of the vibration mirror in this Embodiment, and the resonant frequency fr. The block diagram which shows the control means in this Embodiment The figure which shows the fall displacement of the resonant frequency by the resonant frequency characteristic and phase characteristic of the vibration mirror in this Embodiment The figure which shows the resonance frequency ascending displacement by the resonance frequency characteristic and phase characteristic of the vibration mirror in this embodiment The figure which shows the relationship with the laser beam which scans the light receiving element in this Embodiment

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a perspective view showing an outline of an optical scanning device according to an embodiment of the present invention, and FIG. 2 is a schematic sectional view of the image forming apparatus. The optical scanning device 5 includes four photosensitive drums 3Y, 3M, 3C, and 3K (hereinafter, subscripts Y, M, C, and K are appropriately added to the reference numerals in the image forming apparatus 1 shown in FIG. (M: magenta, C: cyan, and K: black) are arranged above the image forming units arranged in parallel.

  The optical scanning device 5 includes four light sources 10 corresponding to each color, light deflecting means (vibrating mirror 11) for deflecting and scanning a laser beam from each light source, and four photosensitive drums 3Y, 3M, 3C, and 3K. A scanning imaging optical system for guiding the scanning surface is provided, and these components are housed in an optical housing (not shown).

  The light source 10 shown in FIG. 1 has four “light source devices” each composed of a semiconductor laser and a coupling lens so as to correspond to each color (10Y, 10M, 10C, 10K). Each of the four semiconductor lasers in the light source emits a light beam for writing each color component image of yellow, magenta, cyan, and black. The light beam emitted from each semiconductor laser is converted into a light beam form (parallel light beam or weak divergent or converging light beam) suitable for the subsequent optical system by the coupling lens, and the sub-circular lens 12 passes through the folding mirror 13 to obtain a secondary light beam. The beam is focused in the scanning direction and formed as a line image in the vicinity of the deflection reflection surface of the oscillating mirror 11 serving as a deflection scanning means in the main scanning direction.

  A laser transmitting member (not shown) is arranged on the incident side with respect to the vibrating mirror 11. Each light beam from the light source 10 side enters the vibrating mirror 11 via a laser transmitting member. The deflected light beams for the four colors deflected in the same direction by the oscillation of the oscillating mirror 11 are transmitted through the first lens 14 constituting the scanning lens group of the scanning imaging optical system. The luminous flux (for example, the position of the upper end of the lens) for writing the black component image is reflected by the mirror 16K, passes through the second lens 17K constituting the scanning lens group, and forms a drum-shaped photoconductive material that forms the actual state of the scanned surface. Is condensed as a light spot on the photosensitive drum 3K, and the surface of the photosensitive drum 3K is optically scanned in the arrow direction A. The material of the first and second lenses 14 and 17K of the scanning lens group is an aspherical shape made of an easy and low-cost plastic material. Specifically, it is a polycarbonate having low water absorption, high transmittance, and excellent moldability. And a synthetic resin mainly composed of polycarbonate is preferred.

  Similarly to the above, the light beams for writing the respective color component images of yellow, magenta, and cyan are reflected by the mirror, transmitted through the lens, formed as light spots on the photosensitive member, and scanned in the same arrow direction for each color. . By this optical scanning, an electrostatic latent image of a color component image corresponding to each photoconductor is formed (the optical elements corresponding to the respective colors other than black are not numbered, but an abbreviation for black. The parts to which “K” is attached after the number are arranged at the same optical position for yellow, magenta, and cyan).

  These electrostatic latent images are visualized with a corresponding color toner by a developing device and transferred onto the intermediate transfer belt 2. At the time of transfer, the color toner images are superimposed on each other to form a color image. This color image is transferred onto a sheet-like recording medium and fixed. The intermediate transfer belt 2 after the color image transfer is cleaned by a cleaning device (not shown).

  As described above, FIG. 1 shows that each light beam emitted from a plurality of light sources 10 corresponding to two or more color component images constituting a color image is deflected and scanned in the same direction by the vibrating mirror 11 of the deflection scanning means. A surface to be scanned corresponding to each color component image by a first lens 14 that transmits each deflected light beam in common to each color in the scanning imaging optical system and, for example, a lens 17K (black) provided in each scanning imaging unit. The optical scanning device 5 includes four scanning imaging units corresponding to the respective color components.

  Each color laser beam incident on the reflecting surface of the vibrating mirror 11 has a desired angle with respect to the sub-scanning direction (so-called oblique incidence optical system). Specifically, the maximum color laser beam is set to be 5 ° or less. When the incident angle is 5 ° or more, the scanning line is greatly bent on the surface to be scanned, and the diameter of the laser beam is increased, resulting in image deterioration. On the other hand, when the laser beams of each color are not incident obliquely and are incident horizontally (oblique incident angle is 0 °), a large width in the sub-scanning direction of the reflecting surface is required. There is a problem that the frequency cannot be increased.

  3 is a detailed view of the substrate of the vibration mirror ((a) is the first substrate, (b) is the second substrate), and FIG. 4 is an exploded perspective view of the vibration mirror. The oscillating mirror 11 includes a movable part that forms a mirror surface and forms a vibrator, a torsion beam that supports the rotating part, and a frame that forms a support shaft, and a frame that forms a support part. The oscillating mirror 11 is formed by cutting an Si substrate by etching.

  In this embodiment mode, the wafer is manufactured using a wafer in which two substrates of 60 μm and 140 μm called SOI substrates are bonded in advance with an oxide film interposed therebetween. First, a torsion beam 42, a vibration plate 43 on which a planar coil is formed, a reinforcing beam 44 forming a skeleton of a movable part, a frame 46, and the like by a dry process by plasma etching from the surface side of a 140 μm substrate (second substrate) 41 The remaining part of the film is penetrated to the oxide film, and then the movable mirror part 52 and the frame 53 are left by anisotropic etching such as KOH from the surface side of the 60 μm substrate (first substrate) 51. The other part is penetrated to the oxide film, and finally, the oxide film around the movable part is removed and separated to form the structure of the vibrating mirror.

  Here, the width of the torsion beam 42 and the reinforcing beam 44 was 40 to 60 μm. As described above, the moment of inertia I of the vibrator is preferably small in order to increase the deflection angle. On the other hand, since the mirror surface is deformed by the inertial force, the embodiment has a structure in which the movable portion is thinned. . Further, an aluminum thin film is vapor-deposited on the surface side of a 60 μm substrate (first substrate) 51 to form a reflecting surface, and a coil pattern 47 and a torsion beam 42 are formed of a copper thin film on the surface side of a 140 μm substrate (second substrate) 41. A terminal 48 wired through the terminal and a trimming patch 49 are formed. Naturally, it is also possible to adopt a configuration in which a thin film-like permanent magnet is provided on the diaphragm 43 side and a planar coil is formed on the frame 53 side.

  On the mounting substrate 54, a frame-like base 55 for mounting the movable mirror portion 52 and a yoke 56 formed so as to surround the movable mirror portion 52 are provided. The yoke 56 is opposed to the end of the movable mirror portion 52. Then, a pair of permanent magnets 57 that generate a magnetic field in a direction orthogonal to the rotation axis are joined together with the S and N poles facing each other (see FIG. 4).

  The movable mirror unit 52 is mounted on the pedestal 55 with the mirror surface facing up, and a Lorentz force is generated on each side parallel to the rotation axis of the coil pattern 47 by passing an electric current between the terminals 48, thereby twisting the torsion beam 42. Then, a rotational torque T for rotating the movable mirror portion 52 is generated, and when the current is turned off, the movable mirror portion 52 returns to the horizontal due to the return force of the torsion beam 42.

  Therefore, by alternately switching the direction of the current flowing through the coil pattern 47 (AC signal), the movable mirror 52 can be reciprocally oscillated. When the current switching period is brought close to the natural frequency of the primary vibration mode with the torsion beam 42 as the rotation axis of the structure constituting the oscillating mirror 11, the so-called resonance frequency f0, the amplitude is excited and a large fluctuation occurs. You can get a corner.

  On the other hand, by passing a direct current component current (applying a voltage) to the coil pattern 47, the movable mirror 52 can be statically changed (the center of amplitude is changed). However, since the resonance phenomenon is used, the change according to the current is within ± 1 ° in angle. By superimposing the direct current component on the alternating current signal, the amplitude center can be changed while the vibration mirror is amplitude (deflected).

  FIG. 5 shows a vibrating mirror unit 60 mounted on an optical housing. The oscillating mirror unit 60 shown in FIG. 5 fixes the oscillating mirror 11, the bracket 61 for fixing the attitude of the oscillating mirror 11 and making electrical connection (electrode part 64), the bracket 61, and an optical housing (illustrated). It is comprised with the board | substrate 62 (electrical connector 63) mounted in (not).

  The oscillating mirror 11 is much smaller in mass and inertia of the movable part than the conventional polygon mirror, so the drive part is also miniaturized, and the power consumption can be kept low due to the high efficiency of the magnetic circuit. 1/10 or less). As a result, heat generation is reduced, and it is possible to substantially eliminate the temperature rise of the optical element and housing of the writing optical system, so that the resin scanning lens group has no local temperature distribution, The occurrence of color misregistration can be suppressed without changing the scanning position of the laser beam when forming a color image.

  Furthermore, since the mass and inertia of the movable part are small, there is little vibration (vibration due to mass imbalance) transmitted to the outside even when swinging (1/100 or less of the polygon mirror), and transmitted to the optical element of the writing optical system. Therefore, the occurrence of banding (roughness variation in the sub-scanning direction) during image formation due to the vibration of the folding mirror can be eliminated.

  The image forming apparatus 1 using the optical scanning device 5 shown in FIG. 2 is a tandem type color image forming apparatus in which a plurality of photosensitive drums 3Y, 3M, 3C, and 3K are arranged in parallel. An optical scanning device 5, a developing device 6, a photosensitive drum 3, an intermediate transfer belt 2, a fixing device 7, and a paper feed cassette 4 are arranged in order from the top of the device.

  On the intermediate transfer belt 2, photosensitive drums 3Y, 3M, 3C, and 3K corresponding to the respective colors are arranged at equal intervals in the parallel order. The photosensitive drums 3Y, 3M, 3C, and 3K are formed to have the same diameter, and members are sequentially disposed around the photosensitive drums according to an electrophotographic process. The photosensitive drum 3Y will be described as an example. A charging charger (not shown), a laser beam L1 based on an image signal emitted from the optical scanning device 5, a developing device 6Y, a transfer charger (not shown), and a cleaning device (not shown). Etc. are arranged in order. The same applies to the other photosensitive drums 3M, 3C, and 3K. That is, in the present embodiment, the photosensitive drums 3Y, 3M, 3C, and 3K are to be scanned surfaces set for each color, and the laser beams L1, L2, and L3 are emitted from the optical scanning device 5 to the respective surfaces. , L4 are provided to correspond to each.

  The photosensitive drum 3Y uniformly charged by the charging charger rotates in the direction of the arrow AA, thereby sub-scanning the laser beam L1, and an electrostatic latent image is formed on the photosensitive drum 3Y. Further, a developing device 6Y that supplies toner to the photosensitive drum 3Y is disposed downstream of the irradiation position of the laser beam L1 by the optical scanning device 5 in the rotation direction of the photosensitive drum 3, and yellow toner is supplied. The The toner supplied from the developing device 6Y adheres to the portion where the electrostatic latent image is formed, and a toner image is formed. Similarly, M, Y, and K single color toner images are formed on the photosensitive drums 3M, 3C, and 3K, respectively.

  The intermediate transfer belt 2 is disposed further downstream in the rotational direction than the position where the developing unit 6 is disposed on each photosensitive drum 3. The intermediate transfer belt 2 is wound around a plurality of rollers 2a, 2b, and 2c, and is moved and conveyed in the direction of arrow BB by driving a motor (not shown). By this conveyance, the intermediate transfer belt 2 is sequentially moved to the photosensitive drums 3Y, 3M, 3C, and 3K. The intermediate transfer belt 2 sequentially superimposes and transfers the single color images developed on the photosensitive drums 3Y, 3M, 3C, and 3K to form a color image on the intermediate transfer belt 2.

  Thereafter, the transfer paper is conveyed from the paper feed tray 1 in the direction of the arrow CC, and the color image is transferred. The transfer paper on which the color image is formed is discharged as a color image after being fixed by the fixing device 7.

  The following control is indispensable for taking advantage of the vibration mirror of the optical scanning device described above, and a preferable example will be shown. FIG. 6 schematically shows a main part corresponding to one photosensitive drum in the optical scanning device for color image formation shown in FIG. The laser beam 20 deflected and scanned by the oscillating mirror 11 is scanned at a timing at which an output signal is generated by entering the laser beam 20a at the scanning position with the maximum deflection angle into the light receiving element disposed within the maximum deflection angle. The laser beam at the position to be scanned is 20b, and the laser beam at the position to scan the edge of the image area to the photosensitive drum 3 is 20c.

  FIG. 7 shows the vibration mirror amplitude with respect to time. Since the oscillating mirror 11 generates a large amplitude using a resonance phenomenon, the amplitude of the oscillating mirror 11 draws a sinusoidal locus with respect to time, and the scanning speed of the laser beam deflected and scanned is not constant, but depends on the scanning position. It will be different (when there is no scanning lens). The scanning lens group (the first lens 14 and the second lens 17) has f · arcsin characteristics so as to be constant even at such a scanning speed.

  Even if the scanning lens group having the above characteristics is used, the fluctuation variation of the oscillation mirror 11 as shown in FIGS. 8 to 10 occurs, so that control is performed to suppress the fluctuation. Even if the resonant frequency of the oscillating mirror 11 is constant, the phenomenon shown in FIGS. 8 to 10 occurs, and the ideal amplitude waveform (sinusoidal waveform shown by a broken line) changes in the scanning position of the laser beam, thereby causing image degradation. Become.

FIG. 8 shows amplitude fluctuation. When the amplitude is larger than the target (the same applies when the amplitude is smaller), the PDa output time interval A as shown in FIG. And control so that the calculated value of the time interval B of the PDb output is constant. Specifically, averaging is performed a plurality of times at (A ₁ + B ₁ ) / 2, (A ₂ + B ₂ ) / 2,..., And control is performed so that the control target value is uniquely determined from the resonance frequency.

Here, the drive frequency fd of the vibrating mirror will be described in detail with reference to FIG. The driving frequency is the frequency for realizing the number of prints, it is preferable because it can make the amplitude amount Y ₁ required for optical scanning is possible to match the resonant frequency fr (solid line in FIG. 11) is actually vibrating It may not match due to resonance frequency fluctuation (including initial fluctuation) of the mirror. At that time, as shown in FIG. 11, in order to obtain the amplitude amount Y ₁ necessary for the optical scanning, the input energy (voltage, current) to the vibrating mirror is increased, and only the amplitude amount Y ₂ at the drive frequency fd. increasing the amplitude of the oscillating mirror which is not amplitude, it is performed amplitude control of the oscillating mirror that is amplitude (broken line in FIG. 11) amplitude of Y ₁ to.

  FIG. 9 shows the positional relationship between the amplitude center of the oscillating mirror 11 and the scanning center. The state in which the amplitude waveform has an offset (difference between the amplitude center of the oscillating mirror and the optical scanning center) with respect to the scanning center. This is an example. 9A and 9B show the difference depending on the offset amount, and FIG. 9A shows an example in which the offset amount is an allowable level. The offset amount affects the linearity and beam diameter, which are optical scanning characteristics, but the offset amount does not necessarily need to be “0” in consideration of the influence on the image.

  Specifically, the linearity error is corrected by controlling the lighting timing of the laser beam based on the image information to suppress the influence and suppress the influence on the beam diameter. By controlling the light quantity (integrated light quantity) so that the latent images are equal, the influence of the offset is reduced.

  Note that the control may not be performed within the allowable offset. By setting the allowable offset amount so that the deviation from the design value of linearity is 1% or less and the deviation from the design value by beam diameter is 10% or less, the effect on the image is suppressed as much as possible and the above control is not required. It is possible not to become.

  FIG. 9B shows a state where the allowable offset is exceeded. In this case, it is necessary to adjust (correct) within the allowable range as described above. The adjustment method is to superimpose a DC component corresponding to the offset amount on the drive voltage (AC component) of the oscillating mirror, thereby changing the posture in the main scanning direction and adjusting the amplitude center to be within the allowable offset. Yes.

  As another example, it is possible to adjust by a drive mechanism that changes the posture of the vibrating mirror 11. As the drive mechanism, for example, a stepping motor is arranged (not shown) on the lower surface of the substrate 62 in FIG. 5, and the rotation axis of the stepping motor and the vibration axis of the vibration mirror 11 are arranged to coincide. The vibration mirror unit 60 is adjusted within the allowable offset by changing the posture (rotation) about the vibration axis. The rotation step resolution of the stepping motor needs to be at least equivalent to 1/2 or less of the allowable offset amount.

  Although the offset adjustment itself has been performed before, the offset amount is ideally set to “0”, not the allowable offset as described above. In this case, the DC component superimposing circuit required for offset adjustment must have a performance having an output voltage capable of adjusting all the offset amounts shown in FIG. 9B. The temperature in the optical scanning device greatly increases, and the optical scanning characteristics are deteriorated. In addition, this increases the cost associated with circuit enlargement.

  Also, the current (voltage) that is driven during offset adjustment cannot be driven indefinitely according to the offset amount from the upper limit of the current rating of the oscillating mirror (because the current rating is exceeded, the element is destroyed). In particular, in the electromagnetic drive type shown in the present embodiment, the power consumption of the DC component becomes the power consumption of the coil (copper loss), so there is a concern that the heat generation (temperature) rises as the offset adjustment amount increases.

In FIG. 9B, in order to adjust the offset within the allowable offset (as indicated by the arrow direction), the calculated values of the time interval A of the PDa output and the time interval B of the PDb output as shown in FIG. Control is performed to be constant. Specifically, A ₁ -B ₁ , A ₂ -B ₂ ,... Is averaged several times, and it is determined whether or not it is within the allowable offset by the comparison means 71 of FIG. If the allowable offset is exceeded without performing control, offset control is performed.

FIG. 10 shows the phase variation of the amplitude waveform of the oscillating mirror 11, and even if the phase variation with the reference phase clock signal (the waveform is not shown) as shown in FIG. Therefore, control is performed so that the phase of the time interval C between the reference phase clock for generating the signal for driving the oscillating mirror 11 as shown in FIG. 7 and the PDa output is constant. Specifically, averaging is performed a plurality of times with C ₁ , C ₂ ,..., And control is performed so that the target value becomes “0”. The PDa output for counting the time interval is preferably an output (the rear end side of A) that is the timing immediately before the image forming area. The phase is matched with the accuracy just before the image writing start side (the rear end side of A) (in the case of the front end side of A, the phase accuracy at the time of image formation decreases due to phase fluctuations within the time of A). .

  When the amplitude or allowable offset in FIG. 8 and FIG. 9B is exceeded (actual amplitude state: expressed by a solid line), this is a phenomenon different from the ideal scanning speed, and thus the scanning position shift in the main scanning direction. For example, image degradation such as jitter in the main scanning direction (vertical line fluctuation) and main scanning magnification error is caused, which is a common problem not only for color images but also for monochrome images.

  On the other hand, the phase fluctuation from the reference phase clock in FIG. 10 becomes a particular problem when forming a color image. As shown in FIG. 1, a single oscillating mirror 11 scans each color photosensitive drum with a laser beam emitted from each color light source in accordance with an image signal. Since the deflection scanning position of the laser beam changes, the sub-scanning position changes on the image (on the intermediate transfer belt), and color misregistration and color unevenness occur.

  FIG. 12 shows a block diagram of the vibration mirror control means, and the vibration mirror control system that realizes amplitude, offset, and phase control will be described in detail. The signals output by the laser beams deflected and scanned to scan PDa and PDb are measured with time counters A and B, respectively, and the average of (A + B) / 2 is compared with the target amplitude. The average of the differences is compared with whether it is within the allowable offset, and if it is within the allowable range, it is output to the controller without correction, and if it is out of the allowable range, the difference amount within the allowable range is adjusted. Therefore, after the adjustment, the adjustment residual is suppressed within an allowable range even if the offset amount is not “0” at the maximum.

  The reason for taking the average is that averaging processing is performed in order to prevent control by incorrect information such as when sudden electrical noise is mixed. In addition, the frequency | count of averaging is performed in the range of 2-10 times. This is because if it is 10 times or more, the correction timing is delayed and the control deviation is increased.

  The controller calculates the correction amount of the amplitude and the offset according to the comparison result, amplifies the corrected sine wave drive signal by the drive circuit (amplifier) of the vibration mirror, and drives and controls the vibration mirror. The aforementioned control loop is an amplitude and offset control loop. The offset control is performed in a state where the amplitude control is performed.

  The phase control loop controls the phase so that the reference phase clock signal and the swing angle of the oscillating mirror have a constant phase in the control state in which the amplitude and offset control described above work normally and each is within the desired range with respect to the target value. Perform a loop.

  Since phase control is highly accurate control over amplitude and offset control, if all controls are executed simultaneously, the amount of fluctuation of the drive signal will increase and all will converge within the control target value range when they interfere with each other. Cost. Therefore, it is possible to shorten the time until convergence within the control range by performing amplitude control with the first priority, then performing offset control, and then performing phase control as fine adjustment.

  The phase deviation between the output signal from PDa and the reference phase clock is detected by a phase comparator (C in FIG. 7) and measured by a counter. This is so-called PLL (Phase Locked Loop) control in which the measurement result is converted into a DC voltage corresponding to the phase deviation by an LPF (Low Pass Filter) and an integrator, and the phase is changed according to the voltage amount. The generation of the sine wave signal with respect to the phase change generates a sine wave signal having an optimum phase according to the increment of the phase change amount (resolution) prepared in advance. As a result, control is performed so that the drive signal of the vibrating mirror and the swing angle of the vibrating mirror are in a constant phase.

  Note that the generation resolution of the sine wave signal corresponding to the phase control needs to have a high accuracy exceeding the allowable range of control, but the higher the accuracy, the higher the cost because a memory is required. Therefore, the generation resolution of the sine wave signal is set to be 50 μm or less which is visually recognized as a color shift in the sub-scanning direction.

  Next, detection of the difference between the resonance frequency of the vibrating mirror and the drive frequency will be described in detail. 13 and 14 show the resonance frequency characteristic (amplitude gain) and phase characteristic (phase difference between the drive voltage waveform for driving the vibration mirror) of the vibration mirror 11. A thick solid line (thick broken line) is an initial resonance frequency characteristic (phase characteristic). The amplitude gain Gain and the phase characteristic Phase can be expressed by the following (Equation 1) and (Equation 2).

Here, Q is a Q value representing a resonance state, which is 200 in the embodiment.
ω ₀ (= 2πf ₀ ) [rad / s] is a resonance frequency, and in the embodiment, f ₀ = 3000 [Hz].
Gain and phase when driving at a frequency of ω (= 2πf) are obtained.

  The resonance frequency changes in the direction of the arrow (thick blue arrow) because the rigidity of the beam of the oscillating mirror 11 changes as the environmental temperature changes. FIG. 13 shows the fluctuation direction of the resonance frequency when the environmental temperature increases, and FIG. 14 shows the fluctuation direction of the resonance frequency when the environmental temperature decreases (here, the environmental temperature is the optical scanning device ( The environmental temperature to which the vibration mirror is exposed, including the heat generation of the unit in the image forming apparatus and the environmental temperature in which the image forming apparatus is placed).

  An example of FIG. 13 will be described with reference to FIGS. When the frequency of the drive voltage is 3000 Hz with respect to the vibration mirror having a resonance frequency of 3000 Hz, the phase difference between the drive voltage signal and the vibration mirror is −90 °. When the frequency of the drive voltage shown in FIG. 13 is constant and the resonance frequency is lowered and fluctuated in the direction of the black arrow as the environmental temperature rises, the phase characteristic also changes accordingly and the phase difference changes in the direction of the white arrow. By grasping the amount of change in the phase difference, it is possible to grasp the fluctuation of the resonance frequency.

As in this embodiment, when the resonance frequency decreases by 10 Hz (moves from the thick solid line to the thin solid line in FIG. 13), the phase difference becomes −90 ° to −144 ° (the phase difference is obtained from (Equation 2)). ). In order to detect -54 ° corresponding to the phase difference fluctuation, the time interval P between the drive voltage signal and the output of PDa in FIG. 7 (preferably averaging multiple times of P ₁ , P ₂ ,...) Is measured. Can be achieved. Since the position where PDa is arranged is not at the center of scanning, it is impossible to directly capture −90 ° or −144 °, but P = (− 90 ° −δ °) [δ: phase shift from the center of scanning] Therefore, if the fluctuation amount of P is measured, the fluctuation of the phase difference can be detected.

  In the block diagram of FIG. 12, it is indicated by a portion surrounded by a broken line (drive frequency adjusting means). The phase difference between the drive signal and PDa is measured by a phase comparator and counter, and the difference from the reference resonance frequency is calculated. If the difference is outside the allowable range, the drive frequency is changed to the resonance frequency. The drive frequency is adjusted to match. The allowable value is set from a limit value that can correct a decrease in amplitude caused by a change in resonance frequency. In order to correct the amplitude drop (control with a constant amplitude), the drive voltage is increased. Therefore, the allowable value of the difference is determined so that either the drive circuit withstand voltage or the oscillating mirror allowable withstand voltage is lower. ing. In the present embodiment, the allowable breakdown voltage of the vibrating mirror is given priority and is set within ± 10 Hz.

  The difference between the resonance frequency of the oscillating mirror and the drive frequency is detected in a state where the amplitude of the oscillating mirror is controlled to be constant. This is because if the amplitude is not constant, the output signal timing of the PDa changes and affects the time interval P.

  The drive voltage frequency adjustment timing needs to be suitable because the scanning characteristic of the deflected scanning beam changes. As a preferred example, a judgment means for judging whether image information is inputted to the light source of the optical scanning device, that is, whether a laser beam is scanned on the surface to be scanned, is not judged. This is performed at a timing during non-image formation without image information (even at this time, the scanning beams to PDa and PDb are deflected). “Non-image forming” means the interval between print papers when printing an image or the interval between print jobs.

  On the other hand, in another suitable example, in order to enable adjustment at any time (at any time when it is out of the desired value) regardless of whether or not the laser beam is scanned on the surface to be scanned as described above, This is to change the frequency fluctuation at the time of adjustment step by step. For example, when the difference exceeds 10 Hz and becomes 11 Hz, the driving frequency is not immediately changed by 11 Hz, but 110 steps are changed step by step in units of 0.1 Hz. The change time is preferably the time from the start of adjustment timing to the completion of the image formation of one sheet of print paper (the side effect of image deterioration is reduced by slowing the frequency change).

  In the conventional measurement of the resonance frequency, in order to measure the gain characteristics, the image forming of the optical scanning device is temporarily stopped and the amplitude when the drive frequency is swept is measured to resonate at the place where the amplitude becomes the largest. Since the frequency is set, there is a problem that the image forming apparatus (optical scanning apparatus) cannot be used. In the present invention, resonance is always performed at the time of deflection scanning by the vibrating mirror (at least when the laser beam is scanned in the PDa and PDb when the output signal is generated from the PDa and PDb, the laser beam is not deflected and scanned in the image area). Since the difference between the frequency and the driving frequency can be detected, there is no need to pause the apparatus even during image formation.

  If the difference between the resonance frequency of the vibrating mirror and the drive frequency is detected and the drive frequency is adjusted each time (even if the difference is within a desired range), the sub-scan magnification at the time of image formation changes. It is necessary to make corrections. Sub-scanning position magnification correction requires magnification correction by image processing and speed adjustment in the sub-scanning direction (adjustment of photosensitive unit, intermediate transfer belt, paper feed and fixing units), which is complicated and time lag, etc. Therefore, there is a time during which image formation must be stopped. For this reason, it is preferable as the image quality of the image forming apparatus that the drive frequency is not changed as much as possible.

  So far, the embodiment has shown the case where the drive voltage waveform is a sine wave. However, the present invention is not limited to the sine wave, and the effect of the present invention can also be obtained for a rectangular wave and a triangular wave.

  FIG. 15 shows a relationship with a laser beam scanned using the light receiving element PDa as an example. As shown in FIG. 1, the light receiving element PDa is disposed at a position that is optically equivalent to the laser beam scanned on the surface of the photosensitive drum (beam diameter and scanning speed). Although it is preferable to extend the scanning of the photosensitive drum surface, for the sake of layout, a configuration in which the laser beam scans inside the light receiving element via a folding mirror may be adopted. The light receiving element PDa is composed of an amplifier circuit that amplifies an output signal from a light receiving portion made of a PIN photodiode and a comparator circuit that shapes the waveform, and is packaged as one IC with a laser beam transmitting member made of resin as an IC (22a: Light receiving part, 22b: circuit part, 22c: IC lead). When the scanning beam passes through the light receiving unit 22a, a comparator output signal shown in FIG. 15 is generated.

  The broken line area of the scanning beam in FIG. 15 depicts that the light source is extinguished (or the flare light is dimmed to such an extent that it does not become a light quantity at a level that forms a latent image on the surface of the photoreceptor in the light receiving element). . When the light source emits light in the region between the maximum deflection angle of the vibrating mirror and the vicinity of the light receiving element PDa, ghost light is generated due to irregular reflection of the optical components arranged in the optical scanning device, and the light receiving elements PDa and PDb. 7 becomes noise, the time intervals of A, B, and C in FIG. 7 are disturbed, resulting in control malfunction and instability. In order not to cause this problem, it is set in advance so as to be extinguished at the timing (or dimmed to such an extent that the ghost light does not become a light quantity at a level for forming a latent image on the surface of the photoreceptor in the light receiving element). Turning off or dimming can also have the effect of extending the life of a light source made of a semiconductor laser and reducing the temperature rise of the light source. The term “in the vicinity of the light receiving element” as used herein refers to a scanning position that is a light emission timing at which each of the time intervals A, B, and C in FIG.

  Note that if the amount of light is reduced when the reflectance or transmittance of the optical element is reduced (deterioration with time), the rise time to the threshold voltage that determines the comparator output becomes longer (the inclination becomes slower). will have to go. Therefore, the above problem is solved by controlling the light source so that the light quantity is always constant when scanning the light receiving element.

  As described above, by having a vibrating mirror, power consumption is reduced, temperature rise of the scanning lens used in the optical scanning device, temperature deviation and vibration of the optical scanning device are reduced, and stability over time of the image is secured, driving By adjusting the frequency to match the resonance frequency, the magnification error of the print image can be reduced.

  The optical scanning device and the image forming apparatus according to the present invention have the effect of a vibrating mirror that suppresses power consumption, reduces the temperature rise of the scanning lens, and reduces temperature deviations and vibrations of the optical scanning device to ensure image stability over time. However, an optical scanning apparatus and an image forming apparatus that reduce a magnification error of a print image can be realized, and the apparatus is useful as an apparatus having a laser raster writing optical system.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Intermediate transfer belt 3 Photosensitive drum 4 Paper feed cassette 5 Optical scanning device 6 Developing device 7 Fixing device 10 Light source 11 Vibration mirror 12 Cylindrical lens 13 Folding mirror 14 First lens 15 Converging lens 16 Mirror 17 Second Lens 20 Laser beam 22a Light receiving portion 22b Circuit portion 22c IC lead 41 Second substrate 42 Torsion beam 43 Diaphragm 44 Reinforcement beam 46 Frame 47 Coil pattern 48 Terminal 49 Patch 51 First substrate 52 Movable mirror portion 53 Frame 54 Mounting Substrate 55 Pedestal 56 Yoke 57 Permanent magnet 58 Electrode terminal 60 Vibration mirror unit 61 Bracket 62 Substrate 63 Electrical connector 64 Electrode portion 71 Comparison means

JP 2005-305711 A

Claims (7)

In an optical scanning device having scanning imaging means for deflecting and scanning laser beams emitted from a plurality of light source devices in a main scanning direction by a single oscillating mirror deflector and collecting the laser beams toward a plurality of scanned surfaces,
And a control means for detecting a difference between the resonance frequency and the drive frequency of the vibrating mirror deflector and adjusting the drive frequency so as to match the resonance frequency when the difference is outside a desired range. Optical scanning device.   With the amplitude of the oscillating mirror deflector controlled to be constant, the voltage difference of the driving frequency is measured as a reference and the phase difference with the output signal from the light receiving element arranged in the scanning region of the laser beam to be deflected scanned is measured. 2. The optical scanning device according to claim 1, wherein a difference between a resonance frequency and a driving frequency is detected from the fluctuation of the phase difference.   3. The optical scanning device according to claim 1, wherein a difference between a resonance frequency and a driving frequency is always detected during deflection scanning by the vibrating mirror deflector.   4. The adjustment of the driving frequency by the control means is performed at a timing at the time of non-image formation by determining whether or not a scanning surface area is scanned with a laser beam. The optical scanning device according to Item.   The adjustment of the drive frequency by the control means is performed at any time when the difference between the resonance frequency and the drive frequency is outside a desired range, and the adjustment is performed by changing the drive frequency stepwise. Item 4. The optical scanning device according to any one of Items 1 to 3.   Laser beams emitted from the plurality of light source devices are incident on the reflecting surface of the oscillating mirror deflector at an angle in the sub-scanning direction, and all of the deflected laser beams scan in the same direction. The optical scanning device according to claim 1, wherein:   7. The image forming apparatus according to claim 1, wherein a latent image is formed on a latent image carrier by optical scanning, and the latent image is visualized to obtain a desired recorded image. An image forming apparatus characterized by being used.

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