JP2020064231A - Image forming apparatus - Google Patents

Abstract

To certainly correct, even when the revolution speed of a rotary polygon mirror is changed, deviation in a main scanning direction of a laser beam reflected on reflecting surfaces.SOLUTION: An image forming apparatus comprises: an output unit that receives laser beams reflected on reflecting surfaces of a rotary polygon mirror that is rotated at a first revolution speed or a second revolution speed larger than the first revolution speed, and outputs a signal; a surface specification unit that specifies a specific reflecting surface of the plurality of reflecting surfaces of the rotary polygon mirror based on the signal output from the output unit; a storage unit that stores in advance first correction data for correcting deviation of a laser beam associated with the specific reflecting surface and reflected on the specific reflecting surface in a main scanning direction of an image carrier, when the rotary polygon mirror is rotated at the first revolution speed, and second correction data for correcting deviation of the laser beam associated with the specific reflecting surface and reflected on the specific reflecting surface in the main scanning direction of the image carrier, when the rotary polygon mirror is rotated at the second number of rotations; and a modulation unit that generates a signal for modulating a laser beam emitted from a light source based on the first correction data or the second correction data.SELECTED DRAWING: Figure 9

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser printer, a copier, or a facsimile equipped with an optical scanning device that scans a surface to be scanned with laser light emitted from a light source and deflected by a deflector.

  An optical scanning device used in a conventional image forming apparatus such as a laser printer optically modulates a laser beam emitted from a light source according to an image signal and deflects the optically modulated laser beam with a deflector including, for example, a rotating polygon mirror. Scanning. The deflected and scanned laser light is imaged on the surface of the photosensitive drum as the surface to be scanned by a scanning lens such as an fθ lens to form an electrostatic latent image. Next, the electrostatic latent image on the photosensitive drum is visualized as a toner image by a developing device, transferred to a recording material such as recording paper and sent to a fixing device, and the toner on the recording material is heated and fixed to print. (Print) is performed.

  The reflecting surface of the rotary polygon mirror is processed with high accuracy in order to deflect and scan the laser light with high accuracy. However, due to recent miniaturization of the optical scanning device, the accuracy variation of each reflecting surface of the rotary polygon mirror has come to affect the quality of a printed image. Therefore, a technique has been proposed in which the reflecting surface of the rotary polygon mirror is detected and the uneven density of the image is electrically corrected for each reflecting surface (Patent Document 1).

  Further, a deviation occurs in the deflection scanning direction (main scanning direction) of the rotary polygon mirror due to an attachment error of the optical scanning device with respect to the image forming apparatus, a refractive index variation in the fθ lens, and the like. Since the shift is not preferable because it deteriorates the image quality, a technique for correcting the shift in the main scanning direction by changing the image clock has been proposed (Patent Document 2).

  Aluminum is generally used as the material for the rotating polygon mirror, but the use of plastic allows the surface shape of the reflecting surface to be set freely, and proposals have been made to improve the degree of freedom in design. (Patent Document 3).

Patent No. 5733897 Patent No. 5041583 Japanese Patent Laid-Open No. 2004-102006

However, when the material of the rotating polygon mirror is plastic, the reflecting surface is deformed by centrifugal force when the rotating polygon mirror rotates at high speed. FIG. 11B is a numerical simulation of the reflection surface deformation when the rotary polygon mirror 51 including the four reflection surfaces 51A to 51B shown in FIG. 11A is rotated at a high speed. The material of the rotating polygon mirror is aluminum (AL), which is a conventional metal, and polycarbonate (PC), which is a resin material. The rotation number of each rotating polygon mirror is 45,000 min ^-1 . A simulation was performed.

  In FIG. 11A, the size l × h of each reflecting surface of the rotary polygon mirror 51 is about 14 mm × 2 mm. The size 1 of the reflecting surface is the length from one end to the other end in the longitudinal direction of the reflecting surface. The size h of the reflecting surface is the length from one end to the other end in the lateral direction orthogonal to the longitudinal direction, and is the length in the axial direction passing through the rotation center of the rotary polygon mirror.

  The vertical axis of the graph shown in FIG. 11B represents the amount of deformation of the reflecting surface shown in FIG. 11A in the normal direction. The horizontal axis of the graph shown in FIG. 11B is the distance from the center of the reflecting surface to the end portion in the main scanning direction (longitudinal direction) of the reflecting surface shown in FIG. 11A (half the length of the reflecting surface in the longitudinal direction). ) Is shown. Therefore, in FIG. 11B, the amount of deformation in the normal direction of the reflecting surface from the center of the reflecting surface to the end is shown. Here, the normal direction of the reflecting surface is the outside of the reflecting surface, which is orthogonal to the tangent line of the reflecting surface center of the reflecting surface (the line orthogonal to the line connecting the rotation center of the rotating polygon mirror and the center of the reflecting surface). Refers to the direction toward. The above-mentioned amount of deformation of the reflecting surface refers to the amount of deformation of the reflecting surface in the normal direction. In FIG. 11A, since the size 1 of the reflecting surface of the rotary polygon mirror is about 14 mm, the length l / 2 from the center of the reflecting surface to the end is about 7 mm.

  From the graph shown in FIG. 11B, when the material of the rotary polygon mirror is aluminum, there is almost no deformation from the center of the reflecting surface to the end. However, when the material of the rotary polygon mirror is polycarbonate, a deformation of about 160 nm occurs from the center of the reflecting surface to the end in the main scanning direction (longitudinal direction) of the reflecting surface. Generally, the flatness of the reflecting surface is required to be λ / 5 (λ is a wavelength: λ = 632.8 nm), and the deformation amount of about 160 nm generated only by the dynamic deformation due to the rotation of the rotary polygon mirror is The amount of deformation is large in terms of the optical system.

  FIG. 12 shows the scanning time jitter within a certain section of the laser beam reflected and deflected by each reflecting surface when the rotary polygon mirror is actually rotated by using the plastic rotary polygon mirror. The vertical axis shown in FIG. 12 represents the scanning time jitter, which mainly represents the amount of jitter due to the flatness of each reflecting surface. Here, the amount of jitter (scan time jitter) is expressed as a percentage by dividing the value obtained by subtracting the minimum value from the maximum value of the scanning time of each reflecting surface (4 surfaces) of the rotary polygon mirror by the average scanning time. The horizontal axis shown in FIG. 12 is the rotation speed of the rotary polygon mirror. From FIG. 12, it can be seen that the amount of jitter due to the flatness of each reflecting surface changes depending on the rotation speed. When the amount of jitter changes depending on the number of rotations of the rotary polygon mirror, the image forming position on the photosensitive drum in the main scanning direction shifts. Normally, when the material of the rotating polygon mirror is aluminum, the amount of jitter due to the flatness of each reflecting surface hardly changes depending on the number of rotations. On the other hand, when the material of the rotary polygon mirror is plastic such as polycarbonate, as described with reference to FIGS. 11A and 11B, the rotation of the rotary polygon mirror causes a large deformation of the reflecting surface. The numerical simulation shown in FIG. 11B shows the deformation of a specific reflecting surface of the rotary polygon mirror made of plastic. Actually, as shown in FIG. 12, there is a possibility that the amount of deformation of each reflecting surface may be different between the reflecting surfaces due to the influence of manufacturing variations of the rotary polygon mirror itself, assembling variations to the deflector, and the like.

The image forming apparatus has various print modes, for example, the print speed is changed depending on the type of paper to be printed. In this case, in some cases, the number of rotations of the rotary polygon mirror in the deflector may be changed. For example, in one deflector, which may be rotated in the case and 24000Min ^-1 for rotating the rotary polygon mirror in 40000min ^-1. At this time, in FIG. 12, the jitter amount at 40,000 min ⁻¹ hour is about 0.032%, and the jitter amount at 24,000 min ⁻¹ hour is about 0.026%. The difference in the amount of jitter at each rotation speed is about 0.006%, and when the difference is converted into a distance, for example, when the short side length of A4 is 210 mm, it becomes about 12 μm. That is, when the rotary polygon mirror has four surfaces, the four scanning lines show a deviation of at least about 12 μm in the main scanning direction (deviation of the scanning lines in the main scanning direction). If a periodic misalignment of about 12 μm in the main scanning direction occurs in the four scanning lines, moire may occur in the image.

  An object of the present invention is to realize an image forming apparatus in which moire does not occur by correcting the deviation of each surface in the main scanning direction at each rotation speed even if the rotation speed of the rotary polygon mirror changes.

  In order to achieve the above object, an image forming apparatus according to the present invention scans a laser beam emitted from a light source with respect to an image carrier in a main scanning direction which is an axial direction of the image carrier by rotating a rotary polygon mirror. And an optical scanning device capable of rotating the rotary polygon mirror at a first rotation speed or a second rotation speed faster than the first rotation speed. An output unit that is provided and receives the laser light reflected by the reflecting surface of the rotary polygon mirror that rotates at the first rotation speed or the second rotation speed and outputs a signal, and outputs from the output unit Based on the signal generated, a surface identification unit that identifies a specific reflection surface of the plurality of reflection surfaces of the rotary polygon mirror, and a laser reflected by the specific reflection surface associated with the specific reflection surface. Main scanning direction of the image carrier of light Correction data for correcting the deviation of the rotary polygon mirror, the first correction data when the rotary polygon mirror rotates at the first rotation speed, and the rotation polygon mirror when the rotary polygon mirror rotates at the second rotation speed. And a modulation unit that generates a signal for modulating the laser light from the light source based on the first correction data or the second correction data. Is characterized by.

  According to the present invention, it is possible to reliably correct the positional deviation in the main scanning direction of the laser light reflected by each reflecting surface that occurs for each rotation speed of the rotary polygon mirror. Therefore, even if the number of rotations of the rotary polygon mirror of the image forming apparatus is changed, the image quality is not deteriorated and a high quality image can be held.

It is a perspective view of the optical scanning device which concerns on embodiment of this invention. It is a partial cross section figure of the deflector concerning the embodiment of the present invention. FIG. 3 is a block diagram showing a mechanism for correcting a magnification shift amount in the main scanning direction according to the embodiment of the present invention. (A) (b) is a figure which shows an example of matching with surface ID and BD period (beta) measured with the scanning period measurement part in a time series. (A) is a figure which shows the specific example of rotation speed, BD cycle, and correction data stored in the correction data memory | storage part which concerns on embodiment of this invention. (B) is a schematic diagram showing a configuration for measuring the BD period of the rotary polygon mirror and the correction data corresponding thereto. (A) is a figure which shows an example of BD cycle (alpha) stored in the correction data memory | storage part. FIG. 6B is a diagram showing an example of the BD cycle β measured and stored in the scanning cycle storage unit. (A) (b) is a conceptual diagram which shows an example of the correspondence of surface ID and BD period (beta) in a specific part, and BD period (alpha) of a correction | amendment data storage part in case pattern matching succeeds. (A) (b) is a figure which shows in time series an example of correspondence with surface ID, BD cycle (beta) measured with the scanning cycle measurement part, and the correction data memorize | stored in the correction data memory | storage part. 6 is a flowchart of a process of specifying a reflection surface and correcting a main scanning magnification deviation according to the embodiment of the present invention. 1 is a schematic cross-sectional view showing an image forming apparatus according to an embodiment of the present invention. (A) is a perspective view of a rotary polygonal mirror, (b) is a figure which shows the numerical simulation result of the deformation amount of the reflective surface at the time of rotation of a rotary polygonal mirror. It is a figure which shows an example of the scanning time jitter for every rotation speed of the rotary polygon mirror of a plastic material.

  An image forming apparatus including an optical scanning device according to an exemplary embodiment of the present invention will be described below with reference to the drawings. In the following description, an image forming apparatus including the optical scanning device according to the embodiment of the present invention will be described as an example, and then the optical scanning device in the image forming apparatus will be described. Next, the deflector assembled to the optical scanning device will be described.

[Image forming apparatus]
First, the image forming apparatus 110 will be described with reference to FIG. FIG. 10 is a schematic cross-sectional view of the image forming apparatus 110 including the optical scanning device 101 according to this embodiment.

  The image forming apparatus 110 includes an optical scanning device 101. The optical scanning device 101 scans a photosensitive drum as an image carrier, and forms an image on a recording material P such as recording paper based on the scanned image. Here, a laser beam printer will be described as an example of the image forming apparatus.

  As shown in FIG. 10, an image forming apparatus (printer) 110 emits a laser beam based on image information from an optical scanning device 101 as an exposure unit, and a photosensitive drum which is an image carrier incorporated in a process cartridge 102. Irradiate onto 103. A latent image is formed on the photosensitive drum 103 by irradiating the photosensitive drum 103 with laser light and exposing the same. The latent image formed on the photosensitive drum 103 is visualized as a toner image with toner as a developer. The process cartridge 102 integrally includes a photosensitive drum 103 and a charging unit, a developing unit, and the like as a process unit that acts on the photosensitive drum 103, and is attachable to and detachable from the image forming apparatus.

  On the other hand, the recording material P stored in the recording material stacking plate 104 is fed while being separated one by one by the feeding roller 105, and is further transported to the downstream side by the transport roller 106. The toner image formed on the photosensitive drum 103 is transferred onto the conveyed recording material P by the transfer roller 107. The recording material P on which the unfixed toner image is formed is further transported to the downstream side, and the toner image is fixed on the recording material P by the fixing device 108 having a heating body (heater) inside. After that, the recording material P is ejected out of the apparatus by the ejection roller 109.

  In this embodiment, the charging unit and the developing unit as the process unit acting on the photosensitive drum 103 are integrally provided in the process cartridge 102 with the photosensitive drum 103. However, the present invention is not limited to this, and each process means may be configured separately from the photosensitive drum 103.

[Optical scanning device]
Next, the optical scanning device in the image forming apparatus will be described with reference to FIG. FIG. 1 is a perspective view of an optical scanning device according to this embodiment. The arrow Z shown in FIG. 1 is the axial direction (axial direction) of the rotary shaft 10 shown in FIG. Arrow X is a direction orthogonal to arrow Z, and arrow Y is a direction orthogonal to arrows Z and X.

  As shown in FIG. 1, the optical scanning device 101 has the following optical members. The optical scanning device 101 has a semiconductor laser unit 1 and a compound anamorphic collimator lens 2. The semiconductor laser unit 1 is a light source that emits laser light L. The composite anamorphic collimator lens 2 is a lens integrally formed with an anamorphic collimator lens having the functions of a collimator lens and a cylindrical lens, and a sync signal detection lens (BD lens). Further, the optical scanning device 101 includes an aperture stop 3, a deflector 5 that rotationally drives the rotary polygon mirror 4, a synchronization signal detection sensor (BD sensor) 6 as an output unit, and an fθ lens (scanning lens) 7. The optical scanning device 101 accommodates the above optical members in the optical box 8.

  In the above configuration, the laser light L emitted from the semiconductor laser unit 1 is made into substantially parallel light or convergent light in the main scanning direction by the compound anamorphic collimator lens 2 and converged light in the sub scanning direction. Next, the laser beam L passes through the aperture stop 3 and has a limited laser beam width, and is focused on the reflecting surface of the rotary polygon mirror 4 to form a focal line extending in the main scanning direction. Then, the laser light L is deflected and scanned by rotating the rotary polygon mirror 4, and enters the BD lens of the composite anamorphic collimator lens 2. The laser light L that has passed through the BD lens enters the sync signal detection sensor 6. That is, the synchronization signal detection sensor 6 receives the laser light L reflected by each reflecting surface of the rotary polygon mirror 4. At this time, the synchronization signal detection sensor 6 detects a synchronization signal (BD signal), and this timing is set as the synchronization detection timing of the writing position in the main scanning direction. The synchronization signal (BD signal) is a signal for synchronizing the image writing start position in the main scanning direction on each surface of the rotary polygon mirror 4. Then, the synchronization signal detection sensor 6 outputs the signal (BD signal) to the surface identification signal generation unit 300 (see FIG. 3) described later. Next, the laser light L enters the fθ lens 7. The fθ lens 7 is designed to focus the laser light so as to form a spot on the photosensitive drum and to keep the scanning speed of the spot constant. In order to obtain such characteristics of the fθ lens 7, the fθ lens 7 is formed of an aspherical lens. The laser beam L that has passed through the fθ lens 7 is emitted from the emission port of the optical box 8 and is image-scanned on the photosensitive drum.

  Laser light is deflected and scanned by the rotation of the rotary polygon mirror 4, and main scanning is performed on the photosensitive drum in the axial direction of the photosensitive drum by the laser light. Further, the photosensitive drum is rotationally driven around the axial line of the cylinder to perform sub-scanning. Is done. The direction of scanning in the axial direction of the photosensitive drum is the main scanning direction, and the direction of scanning by rotating the photosensitive drum around the axis is the sub-scanning direction orthogonal to the main scanning direction. In this way, an electrostatic latent image is formed on the surface of the photosensitive drum.

[Deflector]
Next, the deflector in the optical scanning device will be described with reference to FIG. FIG. 2 is a partial cross-sectional view of the deflector according to this embodiment.

  As shown in FIG. 2, the deflector 5 includes a rotor 20 including the rotary polygon mirror 4, a bearing 15, a stator core 16, a stator coil 17, a circuit board 18, and a Hall element (magnetic sensor) 19 soldered onto the circuit board. And so on. The rotor 20 includes a rotary shaft 10, a rotor boss 11, a rotor frame 12, a rotor magnet 13, and a fixture 14 for the rotary polygon mirror 4, in addition to the rotary polygon mirror 4. The material of the rotary polygon mirror 4 is plastic as a resin material such as polycarbonate resin or cycloolefin resin.

  In the above configuration, when current is supplied to the stator coil 17, an electromagnetic force is generated between the stator coil 17 and the rotor magnet 13, and the rotor 20 rotates about the rotating shaft 10 pivotally supported by the bearing 15. The hall element 19 is a magnetic sensor for determining the timing (rectification timing) of the current flowing through the stator coil 17, and is arranged below the rotor magnet 13, and detects the magnetic poles (N, S) of the rotor magnet 13. There is.

[Correction of misalignment in the main scanning direction of each reflecting surface for each rotation speed of the rotary polygon mirror 4]
Next, a method of correcting the amount of jitter in the main scanning direction of each reflecting surface (positional deviation of the scanning line in the main scanning direction of each reflecting surface) for each rotation speed of the rotary polygon mirror 4 will be described with reference to the drawings. FIG. 3 is a block diagram showing a mechanism for correcting the positional deviation amount of the scanning line in the main scanning direction according to the present embodiment. FIG. 5A is a diagram showing a specific example of the rotation speed, the BD cycle of each reflecting surface of the rotary polygon mirror, and the correction data stored in the correction data storage unit according to the present embodiment. FIG. 5B is a schematic diagram showing a configuration for measuring the BD cycle of the reflecting surface of the rotary polygon mirror and the correction data corresponding thereto. FIG. 4A and FIG. 4B are time-series diagrams showing an example of the correspondence between the surface ID and the BD cycle β measured by the scanning cycle measurement unit. FIG. 6A is a diagram showing an example of the BD cycle α, the reflecting surface corresponding to the BD cycle α, and the correction data stored in the correction data storage unit. FIG. 6B is a diagram showing an example of the BD cycle β measured and stored in the scanning cycle storage unit and the surface ID of the reflecting surface. 7A and 7B are concepts showing an example of a correspondence relationship between the surface ID and the BD cycle β in the surface identification signal generation unit and the BD cycle α in the correction data storage unit when the pattern matching is successful. It is a figure. FIG. 8A and FIG. 8B are diagrams showing, in a time series, an example of associating the surface ID, the BD cycle β measured by the scanning cycle measuring unit, and the correction data stored in the correction data storage unit. is there.

  As shown in FIG. 3, this mechanism includes a surface identification signal generation unit 300, a main scanning position deviation correction unit 301, and an image signal generation unit 305. The main scanning magnification deviation correction unit 301 has a correction data control unit related to control and a surface identification unit (correction data control unit + surface identification unit 303). The surface identification unit 303a is a surface identification unit that receives information from the surface identification signal generation unit 300 and identifies a specific surface of the rotary polygon mirror. The correction data control unit 303b receives information from the surface identification signal generation unit 300, and based on the correction data of the specified specific surface, the correction data control unit 303b passes the laser light modulation unit (image clock generation unit) 304 to the laser drive unit 306. Control the drive. In this embodiment, the correction data control unit and the surface identification unit are carried by the CPU, which is a control unit that controls the operation of the apparatus. The image signal generator 305 generates an image signal and supplies it to the laser driver 306. The laser driving unit 306 outputs a laser beam from the semiconductor laser unit 1 according to the supplied image signal and correction data (an image clock generated by the main scanning position shift correcting unit 301) described later. The laser light emitted from the semiconductor laser unit 1 is reflected by the reflecting surface of the rotating rotary polygon mirror 4, and the reflected laser light is detected by the BD sensor 6 and then scans on the photosensitive drum 103. Here, when the laser light is detected by the BD sensor 6, a BD signal is generated and output.

  The surface identification signal generation unit 300 includes a scanning cycle measurement unit, a scanning cycle storage unit, and a surface identification signal unit, which are not shown. The rotary polygon mirror 4 stably rotates at a constant speed, and the process of assigning the surface ID is started. Then, the surface identification signal unit assigns a surface ID, which is a surface identification signal, to the current reflection surface with respect to the BD cycle, and updates the surface ID corresponding to the BD cycle each time the BD signal is input thereafter. Assign to the next reflective surface.

  The “current reflecting surface” is a reflecting surface to which the reflected light that is the source of the BD signal output immediately before is supplied. Each time the rotary polygon mirror 4 makes one rotation, that is, every time the BD signal is output by the same number as the number of reflection surfaces (four), the same reflection surface serves as a source of reflected light. In this example, each BD signal output once every four times corresponds to a certain reflecting surface. Therefore, the surface ID is information for identifying each reflecting surface, and also identifies each BD signal in one rotation of the rotary polygon mirror 4.

  In the scanning cycle measuring unit, the internal counter detects the “BD cycle”, which is the output interval of the BD signal, as the output interval of each reflecting surface. Therefore, the BD cycle is measured by the number of surfaces of the rotary polygon mirror 4. Then, the BD cycle of each reflecting surface is stored in the scanning cycle storage unit in the order of measurement. The BD cycle of each reflecting surface stored in the scanning cycle storage unit is measurement data measured by the scanning cycle measuring unit as an output interval for each reflecting surface. The reflection surface corresponding to the BD signal on the start side of the BD cycle that is measured first is not fixed and may be different each time.

  For example, as shown in FIG. 4A, in the case where the rotation number of the rotary polygon mirror 4 is r1 and the rotary polygon mirror 4 has four reflecting surfaces, the first BD after the step of assigning the surface ID is started. The first reflecting surface is the reflecting surface located at the position where the laser light is reflected immediately after the signal is output. In this case, the surface identification signal section assigns the surface ID “ID11” to the first surface. When the next (second) BD signal is input, the interval with the first BD signal is measured by the scan cycle measurement unit, and the measured interval is stored in the scan cycle storage unit as the BD cycle of the first surface (eg β11). Store.

  When the next (third) BD signal is further input, the interval from the immediately preceding (second) BD signal is measured as the BD cycle of the next second surface, and the BD cycle (eg β12) is scanned. The data is stored in the storage unit, and “ID12” is assigned to the second surface as the surface ID. Such a process is performed for each of two rotational speeds, that is, the first rotational speed r1 and the second rotational speed r2 that is higher than the first rotational speed r1, for the number of surfaces of the rotary polygon mirror 4. Then, for each rotation speed, the BD cycle β (β11 to β14, β21 to β24) of each reflecting surface is stored in the scanning cycle storage unit, and at the same time, the surface ID (ID11 to ID14, ID21 to ID24) is assigned. For example, in the case of the second rotation speed r2, as shown in FIG. 4B, the surface ID is assigned to the first reflection surface as “ID21”, and the IDs are assigned in the following order in order. FIG. 6B shows a correspondence relationship between the BD cycle β stored in the scanning cycle storage unit and the surface ID generated by the surface identification signal unit for each of the rotation speeds r1 and r2. The BD cycle β is measurement data measured by the scanning cycle measurement unit of the surface identification signal generation unit 300 as an output interval for each reflective surface, and is cycle data associated with the output order assigned to the surface ID.

  The main scanning misalignment correction unit 301 includes a correction data storage unit 302 that is a storage unit, a surface identification unit 303a for identifying a specific reflection surface, a correction data control unit 303b that reads out data and performs correction control, and a laser beam. A modulation unit (image clock generation unit) 304. In the present embodiment, the CPU is responsible for the correction data control unit and the surface identification unit, but the configuration of each unit is not limited to this. Each unit constituting the main scanning position deviation correction unit 301 may be realized by a dedicated circuit such as an ASIC, or may be realized by a CPU, a ROM, a RAM and a computer program. Here, as described above, the CPU that functions as the surface identification unit 303a and the correction data control unit 303b receives information from the surface identification signal generation unit 300, identifies the surface of the rotary polygon mirror, and the laser light modulation unit ( The driving of the laser driving unit 306 is controlled via the image clock generating unit) 304. As shown in FIG. 6A, the correction data storage unit 302 stores the BD cycle of each reflecting surface of the rotary polygonal mirror 4 measured in advance for each of a plurality of rotations in the assembly process and the like and the correction data corresponding thereto. Stored in association with each other. In addition, here, as a rotary polygonal mirror having a plurality of reflecting surfaces, a rotary polygonal mirror 4 having four reflecting surfaces A to D as shown in FIG. 3 is illustrated.

  As shown in FIG. 6A, the correction data storage unit 302 stores the BD cycle α corresponding to each of the reflecting surfaces A to D and the correction data data corresponding to each of the reflecting surfaces A to D. It is stored in advance. Here, the BD cycle α associated with each of the reflecting surfaces A to D is identification data.

  Further, the identification data (BD cycle α) and the correction data data corresponding to each of the reflecting surfaces A to D are the two of the first rotation speed r1 and the second rotation speed r2 that is higher than the first rotation speed r1. It is stored in advance in the correction data storage unit 302 for each rotation speed.

  That is, the correction data storage unit 302 corresponds to the BD periods α1 (α11 to α14) of the reflection surfaces A to D of the rotary polygon mirror 4 at the first rotation speed r1 and the main reflection surfaces A to D. The scanning position deviation correction data (dataL1 to dataL4) is stored in advance. The correction data (dataL1 to dataL4) is the first correction for correcting the deviation of the laser light reflected by the reflecting surface in the main scanning direction on the photosensitive drum when the rotary polygon mirror 4 rotates at the first rotation speed. The data. Further, the correction data storage unit 302 corresponds to the BD periods α2 (α21 to α24) of the reflecting surfaces A to D of the rotary polygon mirror 4 at the second rotation speed r2 and the main reflection surfaces A to D. Scanning position deviation correction data (dataH1 to dataH4) is stored in advance. The correction data (dataH1 to dataH4) is the second correction for correcting the deviation in the main scanning direction on the photosensitive drum of the laser light reflected by the reflecting surface when the rotary polygon mirror 4 rotates at the second rotation speed. The data.

The BD cycle α of each reflecting surface for each rotation speed shown in FIG. 6A and the correction data data of the main scanning position shift corresponding thereto are measured in advance in the assembly process using a jig. As a jig or the like for measuring the positional deviation of each reflecting surface of the rotary polygon mirror in the main scanning direction, Patent Literature 2 exemplifies a jig or the like for measuring a partial magnification of an optical scanning device. Patent Document 2 shows an example in which there are six scanning position detection sensors excluding the BD sensor (synchronization signal detection sensor). On the other hand, in the present embodiment, in addition to the BD sensor, three scanning position detection sensors are used to shift the position of the photosensitive drum 103 at three positions in the main scanning direction (the image writing portion, the image center portion, and the image writing end portion). The correction data to be corrected is acquired. A specific example of the measured BD cycle and the corresponding correction data is shown in FIG. In FIG. 5A, the correction data for correcting the deviation of the laser light reflected by each reflecting surface in the main scanning direction is the second correction data that is greater than the correction data for the first rotation speed r1 (25000 min ⁻¹ ). It becomes larger when the rotation speed is r2 (39625 min ⁻¹ ).

  In FIG. 6A and FIG. 5A, the BD cycle means BD signal 1 to BD when BD signal 1 is acquired on one surface of the rotating polygon mirror and BD signal 2 is acquired on the next surface. This is the time interval until signal 2. For example, the BD signal of the reflecting surface A of the rotary polygon mirror 4 shown in FIG. 3 is acquired as the BD signal 1 by the BD sensor 6. Next, the BD sensor 6 acquires the BD signal of the reflective surface B, which is the next surface in the rotation direction adjacent to the reflective surface A, as the BD signal 2. The time interval from the acquisition of the BD signal 1 to the acquisition of the BD signal 2 is the BD cycle. That is, the output interval of the signal output by the BD sensor 6 is the BD cycle. The BD cycle (time interval) between the other reflecting surfaces is also acquired in the same manner. Further, the correspondence between each reflecting surface and each BD cycle is as follows. In one BD cycle, the surface when the previous signal BD signal 1 is acquired is the time interval from BD signal 1 to BD signal 2 ( (BD cycle). For example, the time interval (BD cycle) from the BD signal 1 of the reflection surface A to the BD signal 2 of the reflection surface B acquired by the BD sensor 6 is associated with the reflection surface A of the BD signal 1 which is the previous signal. ing. The same applies to associating other reflective surfaces with the BD cycle.

  On the other hand, the correction data means the positional deviation in the main scanning direction of the laser light L reflected by each reflecting surface caused by the deformation of the surface within one surface of the rotary polygonal mirror (from the ideal reference position to the main scanning direction). This is data for correcting the (positional deviation amount). The positional deviation in the main scanning direction of the laser light L reflected by each of the reflecting surfaces means the position from the scanning start position to the image region (the region from the position of the image writing portion to the position of the image writing end portion) in the main scanning direction. It is the deviation of the distance. Here, time may be used as the distance conversion. In the present embodiment, as shown in FIG. 5B, three scanning position detection sensors S1, S2, S3 other than the BD sensor 6 are used, and three locations in the main scanning direction of the photosensitive drum 103 (the image writing unit and the image center). The correction data for correcting the positional deviation between the copy part and the image writing end part) is acquired. As shown in FIG. 5B, the BD sensor 6 is arranged at the scanning start position. At the above-mentioned three positions in the main scanning direction of the photosensitive drum, a sensor S1 is arranged at the position of the image writing portion, a sensor S2 is arranged at the position of the image center portion, and a sensor S3 is arranged at the position of the image writing end portion. In the main scanning direction, reference sign a is the distance from the scanning start position to the position of the image writing part, reference sign b is the distance from the scanning start position to the position of the center of the image, and reference sign c is the position from the scanning start position to the end part of the image writing. Is the distance to. As the correction data, the amount of deviation in the main scanning direction from the ideal positions at the three positions in the main scanning direction is stored. That is, the distances a, b, and c are measured, and the amount of deviation in the main scanning direction from the ideal distance serving as the reference of the measured distances a, b, and c is the correction data storage unit 302 illustrated in FIG. Stored in advance. In the measurement with the jig and tool, the arrangement and the number of scanning position detection sensors other than the BD sensor are not limited to this, and should be appropriately set as necessary.

  Here, the BD cycle α is a parameter that is measured in advance by specifying each of the reflecting surfaces A to D, and is a parameter corresponding to the BD cycle β. However, which reflecting surface each BD cycle β corresponds to can change in each surface identification, so it will be described later which BD cycle β each BD cycle α specifying the reflecting surface corresponds to. It is not decided unless the specific processing of the reflecting surface is performed.

  The BD cycle α can be measured by the same method as the BD cycle β, but the measuring method is not limited. That is, since it is assumed that the measurement is performed before the shipment of the image forming apparatus, it is not essential to rotate the rotary polygon mirror 4 to actually perform the scanning operation. The main-scanning position deviation correction data data (hereinafter, also simply referred to as “correction data data”) is data for correcting a previously measured deviation amount of the scanning line in the main-scanning direction during image formation. It is envisioned that this data will also be measured before the device is shipped.

  The correction data control unit 303b outputs the read address adrs to the correction data storage unit 302 according to the surface ID generated by the surface identification signal generation unit 300. Then, the correction data data stored at the read address adrs is received from the correction data storage unit 302, and the correction data is output to the laser drive unit 306.

  Next, a method of associating the surface ID generated by the surface identification signal generation unit 300 with the read address adrs stored in the correction data storage unit 401 will be described.

  The CPU reads the BD cycle β (β11 to β14, β21 to β24) from the scanning cycle storage unit of the surface identification signal generation unit 300 according to the rotation speed (r1, r2), and also reads the BD cycle from the correction data storage unit 302. α (α11 to α14, α21 to α24) is read. Then, the BD cycle β and the BD cycle α read respectively are compared with each other, and the read address adrs of the correction data data in the correction data storage unit 302 is set for the surface ID of each reflecting surface of the rotary polygon mirror 4.

  For example, when the rotation speed of the rotary polygon mirror 4 is the first rotation speed r1 and the rotary polygon mirror 4 has a four-sided structure, the BD cycle (β11 to β14) is calculated for each of the four types of combination patterns as follows. And the BD cycle (α11 to α14). First, when the BD signal is first output in the surface identification processing, the reflection surface at the position for reflecting the laser light is arbitrarily determined. The reflecting surfaces that are arbitrarily determined are set as the first surface (the surface ID = ID1) in the order in which they appear in the rotation direction of the rotary polygon mirror 4, and the reflecting surfaces that are sequentially ordered in order from the A surface. And are combined in order to form a first combination pattern.

  Each set in the first combination pattern (here, a set of the first surface and the A surface, a set of the second surface and the B surface, a set of the third surface and the C surface, and a set of the fourth surface and the D surface) For, the process of calculating the square of the difference between the BD cycle β and the BD cycle α is executed. Then, when the combination patterns are changed by shifting the reflection surfaces arranged in advance one by one, there are four kinds of combination patterns (corresponding to the number of reflection surfaces of the rotary polygon mirror). The process of calculating the square of the difference is performed for each set in all four combination patterns. For example, in the second combination pattern, it is calculated for the first surface and B surface group, the second surface and C surface group, the third surface and D surface group, and the fourth surface and A surface group. . In the third combination pattern, it is calculated for the set of the first surface and the C surface, the set of the second surface and the D surface, the set of the third surface and the A surface, and the set of the fourth surface and the B surface. The fourth combination pattern is calculated for the first surface and D surface group, the second surface and A surface group, the third surface and B surface group, and the fourth surface and C surface group.

  Then, for all the combination patterns, the sum of squared differences (that is, the sum of squared differences) of each set is obtained and used as the difference value. The difference values 1 to 4 of the combination patterns 1 to 4 are specifically calculated by the following calculation formula.

Pattern 1: (β11-α11) ² + (β12-α12) ² + (β13-α13) ² + (β14-α14) ² = difference value 1
Pattern 2: (β11-α12) ² + (β12-α13) ² + (β13-α14) ² + (β14-α11) ² = difference value 2
Pattern 3: (β11-α13) ² + (β12-α14) ² + (β13-α11) ² + (β14-α12) ² = difference value 3
Pattern 4: (β11-α14) ² + (β12-α11) ² + (β13-α12) ² + (β14-α13) ² = difference value 4

  When changing the combination pattern, the order of changing the combination of the BD cycle α and the BD cycle β does not matter. For example, in the above calculation formula, the BD cycle α is shifted one by one with respect to the BD cycle β, but the invention is not limited to this, and the BD cycle β is shifted by one with respect to the BD cycle α. You may.

  FIG. 6A is a diagram showing an example of BD cycles (α11 to α14, α21 to α24) stored in advance in the correction data storage unit 302 for each rotation speed. FIG. 6B is a diagram showing an example of BD cycles (β11 to β14, β21 to β24) for each rotation speed that are measured and stored in the scanning cycle storage unit of the surface identification signal generation unit 300.

  Here, of the four combination patterns, the combination pattern having the smallest difference value corresponds to each BD cycle β of the correction data storage unit 302 with respect to each BD cycle β of the scanning cycle storage unit of the surface identification signal generation unit 300. Is determined. At that time, a certain threshold value is set, and it is determined whether or not the matching condition "the minimum difference value is less than or equal to the threshold value and all other difference values are greater than the threshold value" is satisfied. Only when this matching condition is satisfied, it is determined that the association (pattern matching) of each BD cycle α of the correction data storage unit 302 with each BD cycle β of the scanning cycle storage unit of the surface identification signal generation unit 300 has succeeded. To do.

  Here, the surface identification signal generation unit 300 associates the surface ID with the BD cycle β (see FIG. 6B). In the correction data storage unit 302, the read address adrs in which the correction data data is stored is associated with the BD cycle α, and the BD cycle α and the correction data data are also associated with each other through the read address adrs. (See FIG. 6A). 7A and 7B exemplify the correspondence relationship in the case of the combination pattern in which the difference value 4 is the smallest among the above-mentioned four combination patterns for each rotation speed.

  If the pattern matching is successful, it corresponds to the surface ID of each reflecting surface in the correction data data of the correction data storage unit 302 based on the one-to-one correspondence between the BD cycle β and the BD cycle α in the combination pattern. This is used for the main scanning position shift correction. That is, the read address adrs corresponding to the surface ID is acquired in the order of surface ID → BD cycle β → BD cycle α → read address adrs (see FIG. 7). Then, the correction data data stored in the read address adrs is read as the correction data used for the main scanning position deviation correction.

  Since the correspondence relationship between the BD cycle β and the BD cycle α is known, it is also possible to determine which reflective surface is actually the reflective surface that is currently reflecting the laser light. That is, at the stage before the completion of surface identification, the surface ID is assigned to each reflecting surface of the rotary polygon mirror 4, and the absolute position of each reflecting surface is not actually known. However, after the surface identification is completed, each of the plurality of reflecting surfaces of the rotary polygon mirror 4 is identified. Therefore, from the correspondence with the BD signal, it is possible to determine whether each reflecting surface is a reflecting surface that reflects the laser beam or at which relative position the reflecting surface is relative to the reflecting surface. can get. In the present embodiment, this matching processing is performed by the surface specifying unit, and it is possible to specify the four reflecting surfaces of the rotary polygon mirror by the processing by the surface specifying unit. Not limited to If only one surface of the reflecting surface can be specified, it is possible to correct the deviation in the main scanning direction with the correction data for the specific surface.

  If the pattern matching fails, the correction data in the correction data storage unit 302 is not set. Here, the process of not setting the correction data when the pattern matching has failed is illustrated, but the process is not limited to this. For example, even if the pattern matching fails, the average value of the correction data data in the correction data storage unit 302 may be set as common correction data for all reflecting surfaces. Here, the same correction data may be predetermined data instead of the average value.

  When the pattern matching is successful, the minimum difference value approaches 0 infinitely. Therefore, as a method of determining the threshold value T in the matching condition, it is desirable to set an error value calculated from the rotation jitter of the rotary polygon mirror 4 when the pattern matching is successful.

  For example, if the difference value 4 which is the minimum difference value is less than or equal to the threshold value T, and the difference value 1, difference value 2 and difference value 3 other than the minimum difference value 4 are all greater than the threshold value T, pattern matching succeeds. It is considered as done.

  On the other hand, if all the difference values are larger than the threshold value T, it is considered that the pattern matching has failed. This may occur, for example, when the BD cycle of a certain reflecting surface cannot be accurately measured in the scanning cycle storage section of the surface identification signal generating section 300 due to noise in the BD signal.

  For example, when the number of rotations r1 and the difference value 4 are the minimum values and the pattern matching is successful, as shown in FIG. 7A, it corresponds to the BD cycle β11 of the scanning cycle storage unit of the surface identification signal generation unit 300. The BD cycle of the correction data storage unit 302 is the BD cycle α14. A transition pattern of the BD cycle β starting from the reflection surface corresponding to the BD cycle β11 in FIG. 6B (the surface whose surface ID is ID11) and the reflection surface corresponding to the BD cycle α14 in FIG. 6A ( It can be seen that the transition pattern of the BD cycle α starting from the (D surface) is most similar. After all, the pattern matching is also to specify the start points of the BD cycles such that the transition pattern of the BD cycle β and the transition pattern of the BD cycle α match.

  In this case, as shown in FIG. 7A, “adrs14” is set as the read address adrs in the correction data storage unit 302 of the main scanning misregistration correction unit 301 for the surface ID ID11 corresponding to the BD cycle β11. Set. In the misregistration correction, the correction data dataL4 stored in the read address adrs14 is read and used as the correction data. Note that FIG. 7B illustrates a case where the pattern matching is successful when the rotation speed r2 and the difference value 4 have the minimum values.

  In this way, if the correspondence between the surface ID and the read address adrs in the correction data storage unit 302 is determined, the correction data data corresponding to the current reflecting surface of the rotary polygon mirror 4 is determined for each rotation number according to each surface ID. Can be read and used. That is, first correction data stored in the correction data storage unit 302 in advance when the rotary polygon mirror 4 rotates at the first rotation speed r1, or when the rotary polygon mirror 4 rotates at the second rotation speed r2. It is possible to read and use the second correction data of. 8A shows the correspondence relationship between the BD signal, the surface ID, and the correction data when the rotation speed is r1 and the difference value is 4, and FIG. 8B is when the rotation speed is r2 and the difference value is 4. In the misregistration correction, the laser drive unit 306 controls the emission of laser light according to the read correction data data. That is, the CPU controls emission of laser light from the semiconductor laser unit 1 which is a light source via the laser driving unit 306 according to the read correction data data (first correction data or second correction data). As a result, it is possible to reliably correct the positional deviation in the main scanning direction of the laser light reflected by each reflecting surface that occurs for each number of rotations of the rotary polygon mirror, and suppress deterioration of image quality due to the positional deviation. it can.

  Next, description will be given of the process of specifying the reflecting surface by the CPU, which plays a role of the surface specifying unit 303a and the correction data control unit 303b, and the control process of correcting the positional deviation of the specified reflecting surface in the main scanning direction. FIG. 9 is a flowchart of the process of identifying the reflection surface and correcting the main scanning position shift.

  First, in step S101, the CPU determines whether or not the image formation is started, and when the image formation is started, the process proceeds to step S102 to determine the number of rotations of the rotary polygon mirror 4 according to various print modes. The image forming apparatus has various print modes, and the print speed is changed depending on the type of paper to be printed, for example. In this case, the number of rotations of the rotary polygon mirror in the deflector is changed to cope with this. For example, the paper type is detected by a signal from a sensor that is a detection unit that detects the type of recording material, and the printing speed (rotation speed of the rotary polygon mirror) is changed accordingly. Specifically, when the recording material is thick paper, the printing speed is made slower than in the case of plain paper which is thinner than thick paper, so the rotation speed of the rotary polygon mirror is made slower. Therefore, when the recording material is thick paper, the first rotation speed r1 is used, and when the recording paper is plain paper, the second rotation speed is faster than the first rotation speed. Here, as the plurality of rotation speeds of the rotary polygon mirror, the case of the second speed of the first rotation speed r1 and the second rotation speed r2 faster than the first rotation speed is exemplified, but the present invention is not limited to this. is not. The plurality of rotation speeds of the rotary polygon mirror should be appropriately set according to the printing speed and the paper type.

  Then, the CPU advances the process to step S103 if the rotation speed of the rotary polygon mirror 4 is the first rotation speed r1. In step S103, the CPU measures the BD cycle β1 (β11 to β14) of each reflecting surface of the rotary polygon mirror 4 according to the first rotation speed r1 by the surface identification signal generation unit 300, and also measures the measured BD cycle β1. Are stored in the scanning cycle storage unit. Then, when the measurement of the BD cycle β1 is completed for all the reflecting surfaces of the rotary polygon mirror 4, the CPU advances the process to step S104. In step S104, the CPU reads the BD cycle β1 of each reflecting surface of the rotary polygon mirror 4 according to the first rotation speed r1 from the scanning cycle storage unit of the surface identification signal generating unit 300.

  Next, in step S105, the CPU reads the BD cycle α1 (α11 to α14) of each reflecting surface of the rotary polygon mirror 4 corresponding to the first rotation speed r1 from the correction data storage unit 302. Next, in step S106, the CPU subtracts the difference between the BD cycle β1 and the BD cycle α1 of each reflecting surface corresponding to the read first rotation speed r1 in each combination pattern for the number of surfaces of the rotary polygon mirror 4. Calculate the value. For example, when the rotary polygon mirror 4 has four surfaces, there are four patterns of combinations of BD periods, and therefore the difference value 1 to the difference value 4 are calculated.

  Next, in step S107, the CPU performs pattern matching between the BD cycle α1 and the BD cycle β1 according to the first rotation speed r1 according to the above-described matching condition, and determines whether or not the pattern matching has succeeded. To do. As a result, if the pattern matching is successful, the CPU advances the process to step S108.

  In step S108, when the rotation speed of the rotary polygon mirror 4 is the first rotation speed r1, the CPU specifies the combination pattern having the smallest difference value, and determines the BD cycle β1 and the BD cycle α1 in the combination pattern. Understand the one-to-one correspondence of. That is, the pattern matching is successful, and the combination pattern having the minimum difference value is specified, whereby the correspondence relationship between the BD cycle β associated with the surface ID and the BD cycle α associated with each reflective surface in advance is specified. Then, each reflecting surface of the rotary polygon mirror 4 is specified. Then, as described above, the CPU traces the correspondence and sets the read address adrs in the correction data storage unit 302 for each surface ID (see FIG. 7).

  Then, in step S109, the CPU reads the correction data data from the correction data storage unit 302 stored at the read address adrs set for each surface ID. Here, when the rotation speed of the rotary polygon mirror 4 is the first rotation speed r1, the CPU reads the correction data data (first correction data) from the correction data storage unit 302.

  Then, in the subsequent step S110, the CPU controls the emission of the laser light by the semiconductor laser unit 1 which is the light source via the laser driving unit 306 according to the read correction data data (first correction data) to form an image. I do. More specifically, the CPU controls the read correction data data (first correction data) to be output to the laser light modulator (image clock generator) 304.

  The laser light modulation unit (image clock generation unit) 304 performs image clock modulation for each scanning line on each reflecting surface of the rotary polygon mirror 4 based on the main scanning position shift correction data data to perform main scanning position shift correction. . The details of the means for performing image clock modulation from the main scanning position deviation correction data can be obtained by using the technique of Patent Document 2 and the like, and description thereof will be omitted.

  The laser light modulator (image clock generator) 304 supplies an image clock modulated based on the correction data according to the number of rotations and the identified reflection surface to the laser driver 306. The image signal generator 305 generates an image signal and supplies it to the laser driver 306. The laser driving unit 306 outputs laser light from the semiconductor laser unit 1 according to the supplied image signal and the image clock generated by the main scanning position shift correcting unit 301, and forms an image. Then, in step S111, the CPU determines whether the image formation is completed, and when the image formation is completed, the control process is completed.

  On the other hand, when the pattern matching fails in step S107, the CPU advances the process to step S110. In step S110, the CPU does not set the correction data. Although the control process in which the correction data is not set when the pattern matching has failed is illustrated here, the present invention is not limited to this. For example, even if the pattern matching fails, the control process may be performed as follows. That is, the average value of the correction data data in the correction data storage unit 302 is set as common correction data for all reflecting surfaces. Then, the same correction is performed on the laser light for each reflecting surface of the rotary polygon mirror 4. Control processing may be performed in this way.

  Further, in step S102, when the CPU determines the rotation speed of the rotary polygon mirror 4 according to various printing modes, if the rotation speed of the rotary polygon mirror 4 is the second rotation speed r2, the processing proceeds to step S113. . The processing flow of steps S113 to S120 is the same as the above-described processing of steps S103 to S110, except that the rotation speed and the data such as the BD cycle for each rotation speed are different. Is omitted.

  Then, when the correction data (second correction data) of each reflection surface of the rotary polygon mirror is set according to the rotation speed (second rotation speed r2) and the specified reflection surface, in the subsequent step S111, the CPU Performs image formation as described above. In step S112, the CPU determines whether the image formation is completed, and when the image formation is completed, the control process is completed.

  By performing the main-scan positional deviation correction described above, it is possible to suppress the deterioration of the image quality due to the positional deviation in the main-scanning direction. Further, by performing the positional deviation correction, the maximum amount of deviation from the ideal position in the image writing unit, the image center portion, and the image writing end portion is a predetermined value or less at each rotation number (here, about 5 μm or less). ) Can be Further, the relative displacement amount of each reflecting surface of the rotary polygon mirror 4 can be about 2 μm or less. In the present embodiment, the case where the rotational speed of the rotary polygon mirror 4 is the second speed is shown, but even when the rotational speed is higher than that, the main scanning direction positional deviation correction data corresponding to each rotational speed is corrected in advance. If the data storage unit 302 has it, the same operation can be obtained.

  Further, although the case where the material of the rotary polygon mirror 4 is a resin material such as a polycarbonate resin or a cycloolefin resin is shown, the material is not limited to this. Even when the material of the rotary polygon mirror 4 is a metal material such as aluminum, the amount of deformation of the reflecting surface due to the rotation of the rotary polygon mirror 4 is not completely zero. Therefore, as described above, even when the material of the rotary polygon mirror 4 is a metal material, the same correction data may be acquired for each rotation speed, and the main scanning position deviation correction may be performed for each rotation speed.

  As described above, by providing the main scanning direction positional deviation correction data for each rotation speed of the rotary polygon mirror 4 used in the image forming apparatus, the data is reflected by each reflection surface generated for each rotation speed of the rotary polygon mirror 4. The positional deviation of the laser light in the main scanning direction can be reliably corrected. Therefore, even if the number of rotations of the rotary polygon mirror 4 of the image forming apparatus changes, the image quality does not deteriorate and a high quality image can be held.

  In the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine that combines these functions. Similar effects can be obtained by applying the present invention to these image forming apparatuses.

L ... Laser light 1 ... Semiconductor laser unit 2 ... Compound anamorphic collimator lens 3 ... Aperture stop 4 ... Rotating polygon mirror 5 ... Deflector 6 ... Sync signal detection sensor (BD sensor)
7 ... f.theta. Lens 101 ... Optical scanning device 103 ... Photosensitive drum 110 ... Image forming device 300 ... Surface identification signal generation unit 301 ... Main scanning position deviation correction unit 302 ... Correction data storage unit 303 ... Correction data control unit + specification unit 303a. Surface identification unit 303b ... Correction data control unit 304 ... Laser light modulation unit (image clock generation unit)
305 ... Image signal generating unit 306 ... Laser driving unit

Claims (11)

An optical scanning device is provided for scanning the laser beam emitted from the light source with respect to the image carrier in the main scanning direction which is the axial direction of the image carrier by rotation of the rotary polygon mirror, and the rotary polygon mirror is rotated for the first rotation. An image forming apparatus capable of rotating at a second rotation speed that is faster than the first rotation speed,
An output unit that is provided in the optical scanning device and receives a laser beam reflected by a reflecting surface of the rotary polygon mirror that is rotated at the first rotation speed or the second rotation speed and outputs a signal,
Based on the signal output from the output unit, a surface identification unit that identifies a specific reflective surface of the plurality of reflective surfaces of the rotary polygon mirror,
Correction data for correcting a deviation of the laser beam reflected by the specific reflection surface, which is associated with the specific reflection surface, in the main scanning direction of the image carrier, at the first rotation speed. A storage unit that stores in advance first correction data when the rotary polygon mirror rotates and second correction data when the rotary polygon mirror rotates at the second rotation speed;
A modulator that generates a signal for modulating the laser light from the light source based on the first correction data or the second correction data;
An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein the rotating polygon mirror is made of plastic.   The correction data for correcting the deviation of the laser light reflected by the reflecting surface in the main scanning direction is larger in the case of the second rotation speed than in the case of the first rotation speed. The image forming apparatus according to claim 1 or 2.   The said output part outputs the signal for synchronizing the image writing position in the main scanning direction in each reflective surface of the said rotary polygonal mirror, The signal of any one of Claim 1 thru | or 3 characterized by the above-mentioned. Image forming apparatus.   The correction data stored in the storage unit is measured in the main scanning direction from a scanning start position where the output unit is arranged to a predetermined position in an image area of the image carrier, and the measured distance is measured. The image forming apparatus according to any one of claims 1 to 4, wherein the image forming apparatus is a deviation amount in the main scanning direction from an ideal reference distance. The image area of the image carrier is an area from the position of the image writing unit to the position of the image writing end portion,
6. The image according to claim 5, wherein the predetermined position of the image area of the image carrier is the position of the image writing unit, the position of the image center, and the position of the image writing end in the main scanning direction. Forming equipment. The surface specifying unit stores, in advance in the storage unit, an output interval of the signals, which is measured based on two signals sequentially output from the output unit, for the number of reflecting surfaces of the rotary polygon mirror. By using the output interval of the signal is calculated, the difference value of the combination pattern for the number of reflecting surfaces of the rotating polygon mirror,
Among the combination patterns for the number of surfaces, the minimum difference value among the calculated difference values for the number of surfaces is to specify the reflection surface by the combination pattern that satisfies the matching condition that the difference value is smaller than a preset threshold value. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   If the minimum difference value among the calculated difference values for the number of faces in the combination pattern for the number of faces is smaller than a preset threshold, the face identifying unit reflects each reflection. The image forming apparatus according to claim 7, wherein the correction data is set for each surface, and the correction data is not set when the matching condition is not satisfied. Has a detector that detects the type of recording material,
The surface specifying unit changes the rotation speed of the rotary polygon mirror to the first rotation speed or the second rotation speed according to the type of the recording material detected by the detection unit. 9. The image forming apparatus according to any one of 1 to 8.   When the recording material detected by the detection unit is thick paper, the surface identification unit sets the rotation speed of the rotary polygon mirror to the first rotation speed, and when the recording material is plain paper having a thickness smaller than that of the thick paper, The image forming apparatus according to claim 9, wherein the second rotation speed is set.   In the case where the surface identification unit sets the first correction data for each reflection surface corresponding to the first rotation speed, the laser light from the light source for the reflection surface identified using the first correction data When the emission is controlled and the second correction data different from the first correction data for each reflection surface corresponding to the second rotation speed is set, the reflection surface specified using the second correction data is set. The image forming apparatus according to any one of claims 1 to 10, wherein the emission of laser light by the light source is controlled.

Images