JP2006201510A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the deterioration of picture quality due to the variation in rotation speed of a optical scanning means. <P>SOLUTION: In the image forming apparatus 10 of the present invention, the frequency of an oscillator 76 is decided by preliminarily deciding a PWM clock frequency to be an intermediate value between N times (N is integer) and N+1 times a reference clock frequency for realizing a low speed rotation when a picture is formed under a low speed environment, i.e., at the low speed rotation of the polygon mirror 38, thus the variation in the rotation speed of the polygon mirror 38 is suppressed and the generation of jitter is suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus that form an image by an electrophotographic method by scanning a light beam with an optical scanning unit.

  2. Description of the Related Art Conventionally, as an image forming apparatus, an image forming apparatus that scans a photosensitive member with a laser beam by an optical scanning device and forms an image by an electrophotographic method is known. As an image forming process, an electrophotographic method is generally used in which an electrostatic latent image is formed on a photosensitive member by scanning and exposing a charged photosensitive member with a laser beam, and developed with toner. The optical scanning device scans and exposes a photosensitive member by irradiating a rotating polygon mirror with a laser beam.

  In the optical scanning device, an optical deflector for rotating the rotary polygon mirror by a brushless DC motor or the like whose PLL speed is controlled is used. In the PLL speed control, the rotation reference signal generated by a crystal oscillator or the like is the same as the rotation detection signal generated in synchronization with the rotation of the motor, such as the output signal of the Hall element that detects the magnetization pattern of the rotor. This is a method of controlling the drive current applied to the motor coil based on the rotation control voltage signal obtained by converting the phase comparison signal adjusted to have the frequency and phase by a low-pass filter. In such control of the drive current, a PWM (Pulse Width Modulation) method is often used to reduce power loss and suppress heat generation. In the PWM method, a triangular wave as a pulse width modulation clock output from a PWM oscillator that oscillates independently and a rotation control voltage signal are input to a PWM comparator, whereby the frequency of the triangular wave output from the PWM oscillator is obtained. In addition, a drive current corresponding to a pulse width modulation signal having a duty ratio proportional to the rotation control voltage signal is applied to the motor coil. In this way, the motor can be rotated at a constant speed according to the rotation reference signal. As the oscillation frequency of the PWM oscillator, a frequency that is within the drive capability of the current drive transistor and is higher than the audible frequency is usually selected, and a frequency higher than the rotation reference signal and the commutation frequency of the motor is used.

In such an image forming apparatus, when there is a need to change the image forming speed due to switching between black and white image formation or color image formation, changing the writing density, difference in paper thickness, etc., the image carrier The rotation speed of the rotary polygon mirror and the rotation speed of the rotary polygon mirror are changed (for example, see Patent Document 1).
Japanese Patent Laid-Open No. 2-16580

  However, the conventional technique has a problem that when the rotation speed of the rotating polygon mirror is changed, the rotation speed of the rotating polygon mirror varies. In an optical scanning device, a laser beam scanning start position is generally detected every time an image carrier is scanned by a rotary polygon mirror, and a laser based on image data at predetermined time intervals based on this detection signal. An image is recorded by emitting a beam, but if the rotational speed of the rotary polygon mirror fluctuates, the time from when the scan start position is detected until the laser beam actually reaches the photoconductor changes. However, there is a problem that the scanning position on the photosensitive member fluctuates and a fine distortion of an image called jitter occurs as shown in FIG.

  For this reason, in the image forming apparatus, it is necessary to have very high-precision rotational safety of the rotary polygon mirror, but stable rotation control is difficult due to the relationship between the frequency of the rotation reference signal and the frequency of the PWM oscillator. was there. Specifically, a method that does not have a phase relationship between the frequency of the rotation reference signal and the frequency of the PWM oscillator and a method that starts oscillation of the PWM oscillator in synchronization with the rotation reference signal are known. In any of the methods, it has been found that when the integer multiple of the frequency of the rotation reference signal is close to the oscillation frequency of the PWM oscillator, the rotation speed fluctuation of the rotary polygon mirror occurs.

  Specifically, in the case of a system that does not have a phase relationship between the frequency of the rotation reference signal and the frequency of the PWM oscillator, if the integer multiple of the frequency of the rotation reference signal is close to the oscillation frequency of the PWM oscillator, the phase relationship Fluctuations (beats) occur, and the ON time of the drive current varies according to the pulse width modulation signal applied to the coil of each phase, resulting in fluctuations in the rotational speed of the rotary polygon mirror.

  Also, in the case of the method of starting the oscillation of the PWM oscillator in synchronization with the rotation reference signal, the PWM oscillator starts to oscillate in synchronization with the period of the rotation reference signal, and once before the start of the next period oscillation, the PWM oscillator It is necessary to reset the pulse width modulation clock output from, but if the integer multiple of the frequency of the rotation reference signal is close to the oscillation frequency of the PWM oscillator, the preceding reset operation is delayed by one period of the pulse width modulation clock. Sometimes. In this case, since the ON time of the drive current to the one-phase coil fluctuates during one rotation cycle, the rotation speed fluctuation of the rotary polygon mirror occurs as a result.

  Furthermore, the low-pass filter suppresses a signal having a cycle shorter than the rotation cycle of the motor in order to suppress an error included in the rotation detection signal. Even if the setting is made so as to optimally remove the error, it becomes difficult to suppress the error if the rotational speed is changed. In particular, as the motor rotates at a lower speed, there is a problem in that the rotational stabilization due to the inertia of the rotor and the rotary polygon mirror is weakened, the rotational speed fluctuation increases, and as a result, the jitter tends to increase.

  The present invention has been made to solve the above problems, and provides an optical scanning apparatus and an image forming apparatus capable of suppressing image quality deterioration due to fluctuations in rotational speed of an optical scanning unit as a rotating polygon mirror. Objective.

  In order to achieve the above object, an optical scanning device of the present invention comprises a scanning unit that scans a light beam by rotating, a detection signal output unit that outputs a rotation detection signal according to the rotation of the scanning unit, and the scanning The rotation reference signal output means for outputting a rotation reference signal having a frequency corresponding to the target rotation speed of the means, the rotation reference signal input from the rotation reference signal output means, and the rotation detection signal have the same phase and frequency. Phase comparison signal output means for outputting the phase comparison signal, and conversion means for converting the phase comparison signal input from the phase comparison signal output means into a rotation control voltage signal for controlling the rotation of the scanning means, Pulse width modulation clock output means for outputting a pulse width modulation clock having a predetermined frequency; and the rotation control voltage signal as the pulse width modulation clock. And a rotation control means for controlling the rotation of the scanning means based on the pulse width modulated signal, and the frequency of the pulse width modulation clock is N of the frequency of the rotation reference signal. An optical scanning device that is intermediate between N times and N + 1 times (N is an integer).

  The frequency of the rotational speed fluctuation of the optical scanning means is obtained by the difference between the frequency of the pulse width modulation clock and the integral multiple of the frequency of the rotation reference signal. Note that the value of the integer multiple at this time is determined so that the integer multiple of the frequency of the rotation reference signal is the closest value to the frequency of the pulse width modulation clock. As this difference increases, the frequency of the rotational speed fluctuation of the optical scanning means increases and becomes higher, so the actual fluctuation does not affect image quality degradation. However, the smaller this difference is, the lower the rotational speed fluctuation frequency of the optical scanning means becomes, and the lower the frequency, the more the rotational speed fluctuation of the optical scanning means affects the image quality degradation. In the optical scanning device of the present invention, the frequency of the pulse width modulation clock output from the pulse width modulation clock output means is set to N times and N + 1 times the frequency of the rotation reference signal of the frequency corresponding to the target rotation speed (N is N Since the difference between the frequency of the pulse width modulation clock and the integer multiple of the rotation reference signal frequency can be made larger, the rotational speed fluctuation of the optical scanning means can be increased. Can be suppressed. Therefore, it is possible to suppress image quality deterioration due to fluctuations in the rotation speed of the optical scanning unit.

  Further, the frequency fpwm of the pulse width modulation clock and the frequency fpd of the rotation reference signal can be expressed by the following equations.

fpwm = fpd (N + 0.5) (N is an integer) (2)
For this reason, the difference between the frequency of the pulse width modulation clock and the integer multiple of the frequency of the rotation reference signal can be made larger, so that fluctuations in the rotational speed of the optical scanning means can be suppressed. Therefore, it is possible to suppress image quality deterioration due to fluctuations in the rotation speed of the optical scanning unit.

  The rotation reference signal may have a frequency corresponding to the target rotation speed that is lower than a predetermined speed.

  The fluctuation in the rotational speed of the optical scanning means increases as the speed decreases due to inertia. For this reason, the frequency of the pulse width modulation clock is set to be intermediate between N times and N + 1 times (N is an integer) the frequency of the rotation reference signal having a frequency corresponding to the target rotation speed lower than a predetermined speed. Thus, it is possible to suppress fluctuations in the rotational speed of the optical scanning means. Therefore, it is possible to suppress image quality deterioration due to fluctuations in the rotation speed of the optical scanning unit.

  An image forming apparatus according to the present invention includes: the optical scanning device according to claim 1 or 2; and image formation in which an image is formed by scanning a light beam according to image data on an image carrier by the optical scanning device. And an image forming apparatus capable of suppressing deterioration in image quality due to fluctuations in the rotational speed of the optical scanning unit.

  In addition, the image forming apparatus can suppress deterioration in image quality due to fluctuations in the rotational speed of the optical scanning unit even when the image forming speed is changed by making the rotational speed of the optical scanning unit changeable. .

  Furthermore, the rotation reference signal has a frequency corresponding to the target rotation speed lower than a predetermined speed, and particularly when an image is formed at a target rotation speed lower than a predetermined speed. Image quality deterioration due to fluctuations in the rotation speed of the optical scanning means can be suppressed.

  As described above, according to the optical scanning device and the image forming apparatus of the present invention, the frequency of the pulse width modulation clock output from the pulse width modulation clock output means is set to the rotation reference signal corresponding to the target rotation speed. Since the frequency is between N times and N + 1 times (N is an integer), the difference between the frequency of the pulse width modulation clock and the integer multiple of the frequency of the rotation reference signal can be increased. Therefore, the rotational speed fluctuation of the optical scanning unit can be suppressed, and the image quality deterioration due to the rotational speed fluctuation of the optical scanning unit can be suppressed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the image forming apparatus 10 according to the first embodiment includes a fixing device 52 including a heat fixing roller 52 </ b> A and a pressure roller 52 </ b> B. It is formed in a shape.

  A plurality of paper trays 37 (in the present embodiment, three paper trays 37A, 37B, and 37C are shown) are disposed at the lower portion of the image forming apparatus 10. Each of the paper trays 37A, 37B, and 37C is provided with paper 48 having different sizes such as B5 size, B4 size, A4 size, and A3 size. A half-moon roller 39 is disposed in the vicinity of the paper discharge portion in the paper tray 37, and the paper 48 is sent out one by one from the designated paper tray 37 and conveyed in a predetermined direction by a plurality of conveyance roller pairs 41. In addition, when the image forming apparatus 10 instructs to form images on both surfaces (front and back) of the paper 12, a paper reversing unit 17 is provided to reverse the front and back of the paper 48 on which image formation on the front surface has been completed. Yes.

  On one side surface of the image forming apparatus 10, a manual feed tray 25 for manually inserting the paper 48 is disposed as necessary. Similar to the paper tray 37 described above, a half-moon roller 39 is disposed in the vicinity of the paper discharge portion in the manual feed tray 25 so that the paper 48 can be fed one by one.

  On the upper part of the image forming apparatus 10, a discharge tray 27 for discharging the paper 48 on which a desired image is fixed by the fixing device 52 is provided.

  Image data obtained by reading a document with a scanner and performing various image processes is placed above the paper tray 37 in the image forming apparatus 10 (since the image forming apparatus 10 in this embodiment targets color images, image processing is performed. Is converted into image data of four colors of yellow, magenta, cyan, and black), and a fixing device that fixes a desired image on the paper 48 and a light scanning device 13 that irradiates the photosensitive member 33 with light beams. An image forming unit 12 including 52 and the like is provided.

  The optical scanning device 13 includes a light source, a rotary polygon mirror 38, an fθ lens, a cylindrical mirror, a reflection mirror 36, and the like. Laser light emitted from a light source (not shown) is deflected by the rotary polygon mirror 38 and irradiated to the photosensitive member 33 through an fθ lens, a cylindrical mirror, a reflection mirror, and the like.

  Further, around the photosensitive member 33, a charger 39, a rotary developing device 49, an intermediate transfer member 43 constituted by an IBT belt, a photosensitive member cleaner 44, and a neutralizing lamp (not shown) are disposed.

  The photoconductor 33 rotates at a constant speed in the direction of arrow A shown in FIG. The charging member 39 uniformly charges the photosensitive member 33 negatively. The rotary developer 49 is supplied with toners 46Y, 46M, 46C, and 46B of four colors of yellow, magenta, cyan, and black. The intermediate transfer member 43 is wound around a plurality of rollers 47, and moves at a constant speed in the direction of arrow B shown in FIG. The photoreceptor cleaner 44 removes toner remaining on the photoreceptor 33 without being transferred to the intermediate transfer body 43. Further, the static elimination lamp neutralizes the photosensitive member 33 after the residual toner is removed by the photosensitive member cleaner 44.

  Further, an image detection sensor 51 and an intermediate transfer body cleaner 50 are disposed around the intermediate transfer body 43. The image detection sensor 51 is provided upstream of the transfer unit 53 in the rotation direction of the intermediate transfer body 43 and detects the presence or absence of a toner image formed on the intermediate transfer body 43. The intermediate transfer body cleaner 50 is provided on the downstream side of the transfer portion 53 in the rotation direction of the intermediate transfer body 43, and after the transfer of the toner image from the intermediate transfer body 43 to the paper 48 is completed, The toner remaining on the surface is removed.

  The photosensitive member 33 that rotates in the direction of arrow A is uniformly negatively charged by the charger 39, and first a black latent image of the first color is formed on the photosensitive member 33 by the laser light emitted from the optical scanning device 13. The This latent image is developed with black toner by the black developer of the rotary developer 49. The developed black toner image is transferred to the intermediate transfer member 43. The toner remaining without being transferred onto the photosensitive member 33 is removed by a cleaner (not shown), and the photosensitive member 33 is discharged by a discharging lamp.

  Then, the photosensitive member 33 is uniformly negatively charged again by the charger 39, and the second color yellow image is formed. In this way, a total of four color toner images are sequentially transferred to the intermediate transfer body 26 from the third color magenta to the fourth color cyan. When the transfer of the four color toner images to the intermediate transfer member 43 is completed, a final toner image is formed on the surface of the intermediate transfer member 43.

  Below the intermediate transfer member 43, a transfer unit 53 for transferring the final toner image formed on the intermediate transfer member 43 to the paper 48 is provided. The paper 48 is transported from the paper tray 37 provided with the paper 48 of the designated size as described above to the transfer unit 53 by the transport roller pair 41.

  A fixing device 52 is disposed on the downstream side in the transport direction of the paper 48 from the position where the transfer unit 53 is disposed. In the fixing device 52, a predetermined image is formed on the paper 48 by subjecting the paper 38 to which the final toner image has been transferred to heat and pressure.

  As shown in FIG. 2, the optical scanning device 13 includes a semiconductor laser 16 for emitting a laser beam, a collimator lens 22, and a slit member 24 for shaping a light beam in which a rectangular restriction opening 32 is formed. The laser output unit 14 and the optical deflector 62 are included. In the optical scanning device 13, an expander lens 26, a cylindrical lens 28, and a bending mirror 30 are sequentially arranged from the multi-beam laser emission unit 14 side along the optical path of the laser beam emitted from the semiconductor laser 16. Yes. The optical deflector 62 is connected to a rotating body (rotor) 63 and a rotor 63, which will be described later in detail, through the control board 61 serving as the base, and is rotated in one direction around the axis 38C by the rotor 63. A polygon mirror 38, a motor 70 for rotating the polygon mirror 38, and a drive control circuit 64 for performing drive control of the motor 70 are configured. In the present embodiment, the polygon mirror 38 is described as having 12 surfaces, but is not limited to 12 surfaces.

  The laser beam 20 emitted from the semiconductor laser 16 spreads in a conical shape and then becomes substantially parallel light by the collimator lens 22, and then only a part of the beam around the optical axis passes through the limiting aperture 32. The laser beam that has passed through the limiting aperture 32 is converted into diffused light by the expander lens 26 that is a spherical concave lens, and then becomes focused light in the direction orthogonal to the main scanning direction by the cylindrical lens 28. Thereafter, the optical path is bent by the bending mirror 30, passes through the fθ lens 34 and the fθ lens 36, and then enters the polygon mirror 38.

  The polygon mirror 38 includes a plurality of reflecting surfaces 38A in the circumferential direction, and the laser beam 20 reflected and deflected by the rotation of the polygon mirror 38 passes through the fθ lens 34, the fθ lens 36, and the cylindrical mirror 40. Thus, the photosensitive member 33 is scanned in the main scanning direction from the scanning start position (for example, the position 57) of the photosensitive member 33. A light beam position detector 58 for synchronizing the scanning start timing of the laser light is disposed in the vicinity of the scanning start position. The light beam position detection unit 58 is provided with a reflection mirror 56 that reflects the laser beam and a sensor unit 60 that senses the laser beam in order to synchronize the timing of the main scanning start (Start Of Scan: SOS) with the laser beam. When a laser beam is not sensed, a high level signal (hereinafter referred to as H signal) is output, and when a laser beam is sensed, a low level signal (hereinafter referred to as L signal) is output. .

  The photosensitive member 33 is formed in a cylindrical shape, and its outer peripheral surface is a photosensitive surface sensitive to a laser beam. The photosensitive member 33 is supported so that its axial direction (arrow A direction) coincides with the main scanning direction of the laser beam. That is, in the image forming apparatus 10, the laser beam 20 emitted from the laser emitting unit 14 converges as a light spot on the photosensitive member 33, and the light spot moves on the photosensitive member 33 along the main scanning direction. A latent image is recorded along the main scan line. Further, sub-scanning drive means (not shown) is connected to the photoconductor 33. The sub-scanning drive means is synchronized with the completion of the main scanning by the optical scanning device 13 for the photoconductor 33 once. Is rotated by a predetermined amount. As a result, the photosensitive member 33 is scanned in the main scanning direction and the sub-scanning direction, and an electrostatic latent image is formed on the photosensitive member 33.

  As shown in FIGS. 3 and 4, the rotor 63 of the optical deflector 62 includes a cylindrical cover member 63 </ b> A having an open bottom surface and a flange at the periphery thereof, and the hollow portion of the cover member 63 </ b> A. A ring-shaped drive magnet 63B in which N poles and S poles are alternately connected is inscribed on the inner side surface facing the side. Further, the polygon mirror 38 having 12 reflecting surfaces is connected to the cover member 63A of the rotor 63 so as to share the rotating shaft 66 by the connecting member 63C. As a result, the polygon mirror 38 and the rotor 63 can be rotated integrally around the rotation shaft 66. The rotating shaft 66 is supported in the vertical direction by a bearing 68 provided in a resin housing 67 of the optical scanning device 13 on which the control board 61 is installed.

  Further, on the control board 61, a stator coil 69 is fixed at a position facing the circumferential direction of the inner peripheral portion of the drive magnet 63B, and the rotor 63 is positioned at a position facing the lower side in the vertical direction of the drive magnet 63B. Three Hall elements 56 (Hall elements 56A, 56B, and 56C) are provided as rotational position detectors. In the present embodiment, the drive magnet 63B is magnetized in 12 divisions, and the Hall elements 56A, 56B, and 56C are arranged at a phase angle of 80 °.

  That is, in the optical deflector 62, a brushless motor (hereinafter referred to as a motor) 70 is configured by the rotor 63 including the rotating shaft 66, the stator coil 69, and the Hall element 56. In this case, the stator coil 69 preferably has three phases (U phase, V phase, and W phase).

  As shown in FIG. 5, the drive control circuit 64 for controlling the drive of the motor 70 includes a PLL control unit 71, a low-pass filter (hereinafter referred to as LPF) 72, a pulse width modulator 73, an oscillator 76, a drive circuit 74, and An amplifier 75 is included. The drive control circuit 64 is connected to a control circuit board 80 for controlling the entire image forming apparatus 10.

  The control circuit board 80 includes a control unit 77 for controlling each device provided in the image forming apparatus 10, a crystal oscillator 78 for oscillating a clock having a predetermined frequency, and a set motor 70. A frequency divider 79 is included to divide the frequency of a predetermined clock output from the crystal oscillator 78 so as to have a frequency corresponding to the rotation speed. An input unit 81 for inputting image forming speed information indicating the image forming speed in the image forming apparatus 10 and various information is connected to the control unit 77 so as to be able to exchange data.

The image forming speed information needs to be changed in order to form an image on the paper 48 based on input information such as black and white image forming, color image forming, writing density, and image forming speed. Such information. In the present embodiment, it is assumed that black-and-white image formation information indicating black-and-white image formation or color image formation information indicating color image formation is input. Frequency information indicating a frequency (details will be described later) corresponding to the rotational speed of the polygon mirror 38 corresponding to each image forming speed is stored in advance in association with the forming speed information.

  In the present embodiment, the crystal oscillator 78 outputs a clock having a frequency of 9.0 MHz. In the frequency divider 79, the frequency dividing number of the frequency divider 79 is changed so as to have a frequency corresponding to a target rotation speed (hereinafter referred to as rotation speed) corresponding to the image forming speed information input from the input unit 81. A control unit 77 for controlling the entire image forming apparatus 10 is connected. The frequency divider 79 divides the clock output from the crystal oscillator 78 with the frequency division number changed under the control of the control unit 77, and outputs a rotation reference signal having a frequency corresponding to the rotation speed of the polygon mirror 38. To do.

  One input terminal of the PLL control unit 71 is connected to the output terminal of the crystal oscillator 78 via the frequency divider 79. The other input terminal of the PLL control unit 71 is connected to one of the plurality of Hall elements 56 </ b> A via an amplifier 75. An output terminal of the PLL control unit 71 is connected to one input terminal of the pulse width modulator 73 via the LPF 72. The other input terminal of the pulse width modulator 73 is connected to the output terminal of the oscillator 76, and the output terminal of the pulse width modulator 73 is connected to the drive circuit. The hall elements 56A, 56B, and 56C are each connected to a drive circuit 74, and a three-phase stator coil 69 of the motor 70 is connected to the drive circuit 74.

  The clock output from the crystal oscillator 78 is frequency-divided by the frequency divider 79 so as to have a frequency corresponding to the rotational speed input from the control unit 77, and a reference clock having a frequency corresponding to the rotational speed is generated by the PLL control unit. When the signal is input to 71, the PLL control unit 71 compares the input reference clock with a frequency corresponding to the rotation speed and the output signal from the Hall element 56A so that the phase and the frequency are the same. A comparison signal is output. The LPF 72 converts the phase comparison signal output from the PLL control unit 71 into an integrated voltage, and outputs the rotation control voltage signal to the pulse width modulator 73. The pulse width modulator 73 performs pulse width modulation based on a rotation control voltage signal input from the LPF 72 and a PWM clock input from an oscillator 76 that oscillates a PWM clock having a predetermined frequency, which will be described in detail later. The digital signal is output to the drive circuit 74 as a pulse width modulation signal (hereinafter referred to as PWM signal). In this embodiment, it is assumed that there is no phase relationship between the frequency of the PWM clock and the frequency of the PWM clock. The drive circuit 74 supplies a drive current proportional to the PWM signal voltage to each of the three-phase stator coils 69 based on the Hall signal input from each of the Hall elements 56A, 56B and 56C and the input PWM signal. To the stator coil 69.

  Specifically, for example, as shown in FIG. 6, the U-phase, V-phase of the three-phase stator coil 69 according to the Hall signals H1, H2, and H3 input from the Hall elements 56A, 56B, and 56C, respectively. When each Hall signal H1, H2, and H3 is switched from the low level to the high level for each of the phase and the W phase, current is drawn from the corresponding U phase, V phase, and W phase, respectively, When each of the signals H1, H2, and H3 is switched from the high level to the low level, a drive current proportional to the PWM signal voltage is supplied to each of the corresponding U phase, V phase, and W phase.

  Further, the drive circuit 74 detects the rotation of the motor 70 using the hall signals input from the hall elements 56A, 56B, and 56C as rotation detection signals. As shown in FIG. 6, the current flow supplied to the stator coil 69 by the Hall signals input from the Hall elements 56 </ b> A, 56 </ b> B, and 56 </ b> C is V-phase to U-phase, W-phase to U-phase, W-phase. To V phase, U phase to V phase, U phase to W phase, V phase to W phase, and V phase to U phase. In the present embodiment, the motor 70 rotates 1/6 in one cycle of the switching pattern.

  By configuring the drive control circuit 64 and the control circuit board 80 in this way, the motor 70 is controlled to rotate at a rotation speed corresponding to the image formation speed information input by the input unit 81. . That is, by controlling the rotational speed of the motor 70, the polygon mirror 38 is rotated, and the incident angle of the laser beam on each reflecting surface is continuously changed, so that the laser beam is deflected and moved to the mirror 28 side. Irradiation can be performed on the photosensitive member 33 in each developing unit along the main scanning direction.

  Here, a reference clock having a frequency corresponding to the rotational speed of the motor 70 is input to the PLL control unit 71. The Hall signal input from the Hall elements 56A, 56B, and 56C in advance is the frequency of the motor. It is determined in advance in consideration of how many pulses are generated for one rotation of 70. That is, the frequency of the reference clock is determined in advance according to the structure of the image forming apparatus 10 and the rotation speed of the motor 70.

  In the present embodiment, when the motor 70 rotates once, a 6-cycle hall signal (rotation detection signal of the motor 70) is generated. Therefore, in order to rotate the motor 70 at, for example, 2000 rotations / min, The frequency fref of the clock is fref = 20000 (rpm) × 6 (pulses) / 60 (seconds) = 2000 (Hz).

  On the other hand, the frequency of the PWM clock output from the oscillator 76 is usually preset in the range of 20 KHz to 50 KHz. As described above, the drive circuit 74 switches the input / output of the current to the motor 70 by switching control of the switching circuit (not shown) included in the drive circuit 74 according to the amplitude of the PWM signal. This is because there is a problem that noise is generated from the motor 70 when the frequency in the range is set. However, if the frequency is increased, the switching speed of the switching circuit (transistor) not shown in the figure included in the drive circuit 74 cannot catch up, and the drive circuit 74 generates heat and loss increases. As a frequency within the drive capability of the drive circuit 74 and higher than the audible frequency, a range of 20 KHz to 50 KHz is normally set in advance.

  The oscillator 76 that outputs a PWM clock is set to output a PWM clock having a specific frequency at the time of manufacture.

  In the image forming apparatus 10 of the present embodiment, it is assumed that writing is performed at an image forming speed of 203.2 mm / s during monochrome image formation and an image forming speed of 158.75 mm / s during color image formation. At this time, the writing density is assumed to be 600 dpi.

  If a black and white image is formed at a writing density of 600 dpi and the image forming speed is 203.2 mm / s, the rotation speed of the motor 70 is 24000.0 rpm, so the frequency fref of the reference clock is 2400.0 Hz. Thus, the period of the reference clock is 416.7 μs. In this case, the reference clock having a frequency of 2400.0 Hz is generated by dividing the clock having a frequency of 9.0 MHz output from the crystal oscillator 78 by the frequency divider 79 by 3750.

  In the image forming apparatus 10 according to the present embodiment, when a color image is formed at a writing density of 600 dpi, if the image forming speed is 158.75 mm / s, the motor 70 at this image forming speed is used. Since the rotation speed is 18750.0 rpm, the frequency fref of the reference clock is 1875.0 Hz, and the cycle of the reference clock is 533.3 μs. In this case, the reference clock having a frequency of 1875.0 Hz is generated by frequency-dividing the clock having a frequency of 9.0 MHz output from the crystal oscillator 78 by the frequency divider 79 by 4800.

  The memory not shown in the figure of the control unit 77 stores reference clock frequency information 2400.0 Hz in association with black and white image formation information, and in association with color image formation information, the reference clock frequency. Information 1875.0 Hz is stored. The image formation speed information and frequency information stored in the memory are not limited to such values.

  In FIG. 9, two types of frequencies 37.5 KHz and 31.0 KHz are defined as the frequency of the PWM clock, and when the frequency of the PWM clock is each of these values, the frequency of the motor 70 is changed by changing the frequency of the reference clock. The measurement result of the amount of fluctuation (hereinafter referred to as jitter) (refer to FIG. 12) in the main scanning direction of the image formed on the paper 48 when the rotation speed is changed is shown.

  As shown in FIG. 8, the rotation speed of the motor 70 is controlled when the frequency of the PWM clock is set to 37.5 KHz (FIG. 8A) and when set to 31.0 KHz (FIG. 8 ( B)) For each, a reference clock having a frequency that is an integer (N) multiple of each frequency was applied (details will be described later).

  As a result, when the frequency of the PWM clock is 37.5 KHz, the result of the solid line 90 shown in FIG. 9 is obtained, and when the frequency of the PWM clock is 31.0 KHz, the result of the dotted line 92 shown in FIG. 9 is obtained.

  As shown in FIG. 9, the amount of jitter generation decreases as the motor rotation speed increases, but the amount of jitter generation increases as the motor rotation speed decreases. In particular, it can be seen that as the rotational speed of the motor 70 becomes lower, a jitter of 10 μm or more indicating that the rotational speed fluctuation that affects the quality of the formed image occurs in the polygon mirror 38. .

  More specifically, the jitter generation amount is 10 μm or less when the motor rotation speed is 24000.0 rpm when the image formation speed is 203.2 mm / s (during black-and-white image formation). The polygon mirror 38 has no fluctuation in rotational speed that affects the image quality. However, the amount of jitter generated exceeds 10 μm when the motor rotation speed is 18750.0 rpm when the image forming speed is 158.75 mm / s (during color image formation). It can be said that the rotational speed fluctuation that affects the quality of the generated image has occurred in the polygon mirror 38.

  That is, it can be said that the lower the rotation speed, the greater the influence of image quality deterioration due to the rotation speed fluctuation of the polygon mirror 38.

  Further, as indicated by each of the solid line 90 and the dotted line 92 in FIG. 9, the amount of jitter is generated when the rotation speed is such that the PWM clock frequency and the integer (N) times the reference clock frequency are close to each other. You can see that it is getting bigger.

  The frequency of the rotational speed fluctuation (beat) of the polygon mirror 38 is obtained by the difference between the PWM clock frequency and the integer (N) times the reference clock frequency. Note that the value of N at this time is set to a value such that the value of N times the frequency of the reference clock and the frequency of the PWM clock are closest. As this difference becomes larger, the beat frequency of the polygon mirror 38 becomes higher, so that jitter that affects image quality degradation does not occur. However, when this difference becomes small, the beat frequency becomes low, and jitter that affects the image quality occurs.

  Therefore, in the present embodiment, a reference clock for realizing a rotation speed of 18750.0 rpm of the motor 70 for realizing a low image forming speed of 158.75 mm / s at the time of color image formation, particularly at a low speed rotation. When the above-mentioned 37.5 KHz is preset as the frequency of the PWM clock so as to match the integer multiple of the frequency 1875.0 Hz, and N times the frequency of the reference clock 1875.0 Hz (N is an integer) When the frequency of the PWM clock is set to 31.0 KHz in advance so that the frequency is halfway between N + 1 and N + 1 times, the amount of jitter generated when the number of revolutions of the motor 70 is changed was measured. .

  As indicated by the solid line 90 in FIG. 9, when the frequency of the PWM clock is preset to 37.5 KHz, the rotational speed 18750 of the motor 70 for realizing a low image forming speed of 158.75 mm / s during color image formation. The jitter generation amount at 0.0 rpm is 10 μm or more as shown at point 90A, indicating that the rotational speed fluctuation that affects the quality of the formed image is generated in the polygon mirror 38.

  At this time, the cycle of the reference clock when the rotation speed of the motor 70 is 24000.0 rpm for realizing a high-speed image formation speed of 203.2 mm / s during monochrome image formation is shown in FIG. As described above, when the reference clock period is adjusted to be 16 times the PWM clock period, the motor rotation speed is 23438 rpm, and the reference clock period is set to be 15 times the PWM clock period. It is an intermediate value between the rotational speed of the motor when adjusted and 25000 rpm. The amount of jitter generated at this time is 10 μm or less, as indicated by a point 90B in FIG. 9, and it can be seen that there is no rotational speed fluctuation that affects the image quality.

  On the other hand, as shown by a dotted line 92 in FIG. 9, when the frequency of the PWM clock is preset to 31.0 KHz, the rotation of the motor 70 for realizing a low image forming speed of 158.75 mm / s during color image formation. The jitter generation amount at several 18750.0 rpm is 10 μm or less as indicated by point 92A, and it can be seen that the rotational speed fluctuation that affects the quality of the formed image does not occur in the polygon mirror 38.

  At this time, the cycle of the reference clock when the rotational speed of the motor 70 for realizing a high-speed image formation speed of 203.2 mm / s at the time of monochrome image formation is 24000.0 rpm is shown in FIG. Thus, it does not coincide with a period N times the period of the PWM clock. The amount of jitter generated at this time is 10 μm or less as indicated by a point 92B in FIG. 9, and it can be seen that there is no fluctuation in rotational speed that affects the image quality.

  Therefore, in the image forming apparatus 10 of the present invention, an image is formed with the frequency of the oscillator 76, particularly in a low-speed environment, in order to suppress the occurrence of rotational speed fluctuations that affect the image quality in the polygon mirror 38. In other words, when the polygon mirror 38 is rotated at a low speed, a frequency (for example, 31.N) is intermediate between N times (N is an integer) and N + 1 times the frequency of the reference clock for realizing the low speed rotation. 0 kHz).

  As shown in FIG. 9, this low speed means an image forming speed and a rotational speed (rotational speed) when the rotational speed of the polygon mirror 38 is equal to or lower than a threshold at which a rotational speed fluctuation that affects image quality occurs. Speed), for example, a speed in a range determined in advance by measurement results so that the amount of jitter generation is 10 μm or less.

  Next, the operation of this embodiment will be described.

  When power is supplied to the image forming apparatus 10 by a power switch (not shown) of the image forming apparatus 10, the control unit 77 executes the processing routine shown in FIG. When image forming speed information indicating the image forming speed is input by the operation instruction, the process proceeds to step 102. As the image forming speed information, in order to form an image on a sheet based on information input on the image forming apparatus 10 side, such as black and white image forming, color image forming, writing density, and image forming speed, image forming is performed. This information needs to be changed. In the present embodiment, monochrome image formation information indicating monochrome image formation or color image formation information indicating color image formation is input.

  In the next step 102, the frequency information of the reference clock corresponding to the image forming speed information acquired in step 100 is read from a memory (not shown), and output from the crystal oscillator 78 so as to be the frequency of the read frequency information. Information indicating the frequency division number for dividing the frequency of the clock to be output is output to frequency divider 79. In the frequency divider 79, when the information indicating the frequency division number is input, the frequency of the clock output from the crystal oscillator 78 based on the input frequency division number is set to the image forming speed information input in the step 100. The reference clock frequency-divided so as to be in accordance with the frequency is output to the PLL control unit 71 of the drive control circuit 64.

  When the reference clock is input to the PLL control unit 71, the PLL control unit 71 compares the input reference clock with the output signal (rotation detection signal) from the Hall element 56A, and the phase and frequency are the same. The phase comparison signal is output so that The LPF 72 converts the phase comparison signal output from the PLL control unit 71 into an integrated voltage, and outputs the rotation control voltage signal to the pulse width modulator 73. In the pulse width modulator 73, the rotation control voltage signal input from the LPF 72 and N times (N is an integer) and N + 1 times the frequency of a reference clock for realizing the low speed rotation when the polygon mirror 38 is rotated at a low speed. Based on the PWM clock input from the oscillator 76 in which the frequency oscillating at an intermediate frequency (for example, 31.0 KHz) is preset, the pulse width modulated digital signal is output to the drive circuit 74 as a PWM signal. To do. The drive circuit 74 supplies a drive current proportional to the PWM signal voltage to each of the three-phase stator coils 69 based on the Hall signal input from each of the Hall elements 56A, 56B and 56C and the input PWM signal. To the stator coil 69.

  By controlling in this way, the motor 70 is controlled to rotate at a rotational speed corresponding to the image forming speed information input by the input unit 81.

  In the next step 104, rotation control of the photosensitive member 33 provided in the image forming apparatus 10 and control of various other devices are performed so that an image is formed at an image forming speed corresponding to the image forming speed information acquired in step 100. In step 100, the optical scanning device 13 is controlled to emit laser light corresponding to the image data to the polygon mirror 38 that rotates at the rotational speed corresponding to the input rotational speed. After the image is formed on the recording paper at the image forming speed, this routine is finished.

  As described above, in the image forming apparatus 10 of the present invention, the frequency of the oscillator 76 is set to a low speed environment in order to prevent the polygon mirror 38 from generating a rotational speed fluctuation that affects the image quality. When an image is formed by the above method, that is, when the polygon mirror 38 is rotated at a low speed, the PWM is set to a frequency that is intermediate between N times (N is an integer) and N + 1 times the frequency of the reference clock for realizing the low speed rotation. Since the clock frequency is determined in advance, fluctuations in the rotational speed of the polygon mirror 38 can be suppressed, and the occurrence of jitter can be suppressed.

  For example, as described in the above embodiment, when a phase relationship is not provided between the reference clock and the PWM clock (see FIG. 7A), the low-speed image formation speed 158 at the time of color image formation. When the frequency of the reference clock for realizing the rotational speed 18750.0 rpm of the motor 70 for realizing .75 mm / s is 1875.0 Hz, the frequency of the PWM clock is an integral multiple of the frequency of the reference clock (20 If the frequency close to 37.5 KHz is set to 37.8 KHz, the beat frequency is the difference between the PWM frequency and a frequency that is an integral multiple of the reference clock, and 37.8 KHz- (1875 Hz × 20) = 300 Hz. The beat cycle is 333 μs, and jitter that affects the image quality occurs.

  The frequency of the rotational speed fluctuation (beat) of the polygon mirror 38 is obtained by the difference between the PWM clock frequency and an integer (N) times the reference clock frequency. Note that the value of N at this time is set to a value such that the value of N times the frequency of the reference clock and the frequency of the PWM clock are closest. This is because the smaller the difference is, the lower the beat frequency is, and hence jitter that affects the image quality.

  On the other hand, the PWM clock when the frequency of the PWM clock is set to a value close to a frequency such that the difference between the frequency of the PWM clock and an integer multiple of the reference clock is an integer multiple of the reference clock frequency The larger the difference between the reference clock frequencies, the higher the beat, so the beat cycle becomes shorter and the image quality deterioration becomes less noticeable.

  In this embodiment, since the frequency of the PWM clock is preset to a frequency that is between N times (N is an integer) and N + 1 times the frequency of the reference clock, jitter caused by fluctuations in the rotational speed of the polygon mirror 38 Generation can be suppressed, and image quality deterioration can be suppressed.

  In addition, as described in the above embodiment, the case where the phase relationship between the reference clock and the PWM clock is not given has been described, but the case where the reference clock and the PWM clock are synchronized is similarly described. If the frequency of the PWM clock is set in advance to a frequency that is between N times (N is an integer) and N + 1 times the frequency of the reference clock, the occurrence of jitter due to the rotational speed fluctuation of the polygon mirror 38 can be suppressed. Image quality deterioration can be suppressed.

  As shown in FIG. 7B, when the reference clock and the PWM clock are synchronized, it is necessary to synchronize (reset) the output timing of the reference clock and the output timing of the PWM clock. Is difficult to synchronize completely. Specifically, it is known that this shift in the reset timing occurs in units of several ns or less. However, if the frequency of the PWM clock is set in advance to a frequency that is between N times (N is an integer) and N + 1 times the frequency of the reference clock, the occurrence of jitter due to the rotational speed fluctuation of the polygon mirror 38 is suppressed. Image quality deterioration can be suppressed.

  In the present embodiment, the case where monochrome image formation information indicating monochrome image formation or color image formation information indicating color image formation is input has been described. However, the present invention is not limited to such a form.

  For example, each image forming information such as information indicating the thickness and writing density of the paper may be input. In this case, frequency information indicating the frequency of the reference clock corresponding to the rotation speed of the polygon mirror 38 for realizing the image formation speed corresponding to each image formation information information is stored in advance in a memory not shown in the control unit 77. At the same time, the PWM clock frequency is set to a threshold value that causes a fluctuation in rotational speed to an extent that affects image quality, among image forming speeds according to information such as paper thickness and writing density. The frequency of the PWM clock may be determined in advance so that it does not coincide with an integer multiple of the frequency of each of the reference clocks corresponding to the image forming speed that affects the image quality below this threshold, obtained from the measurement result.

  In this way, even if information that requires the image forming speed to be changed is input by the input unit 81, the rotational speed fluctuation of the polygon mirror 38 can be suppressed. It is possible to suppress the image quality deterioration due to.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic configuration diagram of a laser scanning device according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of the optical deflector according to the embodiment of the present invention along the direction of arrow AA in FIG. 2. It is a schematic block diagram of the roller which concerns on embodiment of this invention. It is a schematic block diagram which shows the electrical structure of the optical deflector and image control part of this invention. It is a timing chart which shows the waveform of the drive signal by the drive current applied or drawn in to each three-phase stator coil corresponding to the Hall signal from the Hall element. (A) is a timing chart showing waveforms of the reference clock and the PWM clock when there is no phase relationship between the reference clock and the PWM clock and the period of the reference clock is not N times the period of the PWM clock; (B) is a timing chart showing waveforms of the reference clock and the PWM clock when there is a phase relationship between the reference clock and the PWM clock and the period of the reference clock is N times the period of the PWM clock. (A) shows the measurement result of the rotation speed of the motor 70 when the period of the reference clock is adjusted so that the period becomes N times the period of the PWM clock when the frequency of the PWM clock is 37.5 KHz. (B) shows the number of rotations of the motor 70 when the period of the reference clock is adjusted to be N times the period of the PWM clock when the frequency of the PWM clock is 31.0 KHz. It is the table | surface which showed the measurement result. It is a diagram showing the amount of jitter generated when the number of rotations of the motor is changed when the frequency of the PWM clock is 37.5 KHz and 31.0 KHz, respectively. 4 is a flowchart illustrating processing executed by the image forming apparatus according to the present embodiment. 6 is a timing chart showing waveforms of the reference clock and the PWM clock when there is no phase relationship between the reference clock and the PWM clock. It is a schematic diagram which shows the generation amount of jitter in the conventional technique.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Optical scanning apparatus 38 Polygon mirror 56 Hall element 62 Optical deflector 64 Drive control circuit 70 Motor 71 PLL control part 72 LPF
73 Pulse Width Modulator 74 Drive Circuit 76 Oscillator 77 Control Unit 78 Crystal Oscillator 79 Frequency Divider 80 Image Processing Unit

Claims (6)

Scanning means for scanning the light beam by rotating;
Detection signal output means for outputting a rotation detection signal according to the rotation of the scanning means;
Rotation reference signal output means for outputting a rotation reference signal having a frequency corresponding to a target rotation speed of the scanning means;
Phase comparison signal output means for outputting a phase comparison signal so that the phase and frequency of the rotation reference signal input from the rotation reference signal output means and the rotation detection signal are equal;
Conversion means for converting the phase comparison signal input from the phase comparison signal output means into a rotation control voltage signal for controlling the rotation of the scanning means;
A pulse width modulation clock output means for outputting a pulse width modulation clock having a predetermined frequency;
A rotation control means for controlling the rotation of the scanning means based on a signal obtained by pulse width modulation of the rotation control voltage signal in accordance with the pulse width modulation clock;
An optical scanning device in which the frequency of the pulse width modulation clock is intermediate between N times and N + 1 times (N is an integer) the frequency of the rotation reference signal. The optical scanning device according to claim 1, wherein the frequency fpwm of the pulse width modulation clock and the frequency fpd of the rotation reference signal are represented by the following equations.
fpwm = fpd (N + 0.5) (N is an integer) (1)   The optical scanning device according to claim 1, wherein the rotation reference signal has a frequency corresponding to the target rotation speed that is lower than a predetermined speed. An optical scanning device according to claim 1 or 2,
Image forming means for forming an image by scanning the image carrier with a light beam according to image data by the optical scanning device;
An image forming apparatus.   The image forming apparatus according to claim 4, wherein the image forming apparatus is capable of changing an image forming speed.   The image forming apparatus according to claim 4, wherein the rotation reference signal has a frequency corresponding to the target rotation speed lower than a predetermined speed.

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