JP7175705B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a laser printer, a copying machine, and a facsimile, which is equipped with an optical scanning device for scanning a surface to be scanned with laser light emitted from a light source and deflected by a deflector.

A conventional optical scanning device used in an 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 composed of, for example, a rotating polygonal mirror. scanning. The deflected and scanned laser light forms an image on the surface of a photosensitive drum as a 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 into a toner image by a developing device, which is transferred to a recording material such as recording paper, sent to a fixing device, and printed by heating and fixing the toner on the recording material. (printing) is performed.

The reflecting surfaces of the rotating polygon mirror are processed with high precision in order to deflect and scan the laser light with high precision. However, due to the recent miniaturization of optical scanning devices and the like, variations in the precision of each reflecting surface of the rotating polygon mirror affect the quality of the printed image. Therefore, a technique has been proposed for detecting the reflecting surfaces of a rotating polygon mirror and electrically correcting the density unevenness of an image for each reflecting surface (Patent Document 1).

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

Aluminum is generally used as a material for rotary polygon mirrors, but by using plastic, the shape of the reflective surface can be freely set, leading to a proposal to improve the degree of freedom in design. (Patent Document 3).

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

However, if the rotating polygon mirror is made of plastic, the reflective surface is deformed by centrifugal force when the rotating polygon mirror rotates at high speed. FIG. 11(b) is a numerical simulation of reflection surface deformation when the rotating polygon mirror 51 composed of the four reflection surfaces 51A to 51B shown in FIG. 11(a) is rotated at high speed. As for the material of the rotating ^polygon mirror, the case of aluminum (AL), which is a conventional metal, and the case of polycarbonate (PC), which is a resin material, are set. I did a simulation.

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

The vertical axis of the graph shown in FIG. 11(b) indicates the amount of deformation in the normal direction of the reflecting surface shown in FIG. 11(a). The horizontal axis of the graph shown in FIG. 11(b) is the distance from the center of the reflecting surface to the end in the main scanning direction (longitudinal direction) of the reflecting surface shown in FIG. ). Therefore, FIG. 11B shows the amount of deformation in the normal direction of the reflecting surface from the center of the reflecting surface to the edge. Here, the normal direction of the reflecting surface refers to the outer side of the reflecting surface that is perpendicular to the tangent to the center of the reflecting surface (the line perpendicular to the line connecting the rotation center of the rotating polygon mirror and the center of the reflecting surface). It refers to the direction towards The aforementioned amount of deformation of the reflecting surface refers to the amount of deformation of the reflecting surface in this normal direction. In FIG. 11(a), since the size l of the reflecting surface of the rotating polygon mirror is about 14 mm, the length l/2 from the center of the reflecting surface to the edge is about 7 mm.

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

FIG. 12 shows the scanning time jitter within a certain section of the laser beam reflected by each reflecting surface and deflected and scanned when the rotating polygonal mirror is actually rotated using a plastic rotating polygonal mirror. The vertical axis shown in FIG. 12 is the scanning time jitter, which mainly represents the amount of jitter caused by the flatness of each reflecting surface. Here, the amount of jitter (scanning 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 (four surfaces) of the rotating polygon mirror by the average scanning time. The horizontal axis shown in FIG. 12 is the rotational speed of the rotating polygon mirror. From FIG. 12, it can be seen that the amount of jitter caused by the flatness of each reflecting surface varies with the number of revolutions. When the amount of jitter changes according to the number of rotations of the rotating polygon mirror, the imaging position in the main scanning direction on the photosensitive drum shifts. Normally, when the material of the rotating polygon mirror is aluminum, the amount of jitter caused by the flatness of each reflecting surface hardly changes with the number of revolutions. On the other hand, if the material of the rotating polygon mirror is plastic such as polycarbonate, as described with reference to FIGS. The numerical simulation shown in FIG. 11(b) shows deformation of a specific reflecting surface of a rotating polygon mirror made of plastic. In practice, as shown in FIG. 12, there is a possibility that the amount of deformation of each reflecting surface will differ among the reflecting surfaces due to the influence of variations in manufacturing of the rotating polygon mirror itself, variations in assembly to the deflector, and the like.

An image forming apparatus has various print modes, and for example, changes the print speed depending on the type of paper used for printing. In this case, in some cases, the deflector changes the number of rotations of the rotating polygon mirror. For example, one deflector may rotate a rotating polygon mirror at 40000 min ⁻¹ or at 24000 min ⁻¹ . At this time, in FIG. 12, the jitter amount at 40000 min ⁻¹ is about 0.032%, and the jitter amount at 24000 min ⁻¹ is about 0.026%. The difference in the amount of jitter at each rotational speed is about 0.006%, and when the difference is converted into a distance, for example, considering the length of the short side of an A4 sheet of 210 mm, the difference is about 12 μm. That is, when the rotating polygon mirror has four faces, the four scanning lines show a relative displacement of at least about 12 μm in the main scanning direction (scanning line displacement in the main scanning direction). If there is a periodic misalignment of about 12 μm in the main scanning direction in the four scanning lines, moiré may occur in the image.

SUMMARY OF THE INVENTION It is an object of the present invention to realize an image forming apparatus that does not generate moire by correcting the displacement of each surface in the main scanning direction at each rotational speed even if the rotational speed of the rotating polygon mirror changes.

In order to achieve the above object, an image forming apparatus according to the present invention scans an image carrier with laser light emitted from a light source in a main scanning direction, which is the axial direction of the image carrier, by rotating a rotating polygonal mirror. and an image forming apparatus capable of rotating the rotating polygon mirror at a first rotational speed or at a second rotational speed faster than the first rotational speed, wherein the optical scanning device an output unit provided for receiving a laser beam reflected by a reflecting surface of the rotating polygon mirror rotated at the first rotation speed or the second rotation speed and outputting a signal; a surface specifying unit for specifying a specific reflecting surface among the plurality of reflecting surfaces of the rotating polygon mirror based on the received signal; and a laser beam reflected by the specific reflecting surface associated with the specific reflecting surface. correction data for correcting deviation of light in the main scanning direction of the image carrier, the first correction data when the rotating polygon mirror rotates at the first number of rotations; a storage unit pre-stored with second correction data when the rotating polygon mirror rotates at the number of revolutions; and a modulator that generates a signal.

According to the present invention, it is possible to reliably correct positional deviation in the main scanning direction of laser light reflected by each reflecting surface that occurs for each number of rotations of the rotating polygon mirror. Therefore, even if the number of rotations of the rotating polygon mirror of the image forming apparatus changes, image quality does not deteriorate, and high-quality images can be maintained.

1 is a perspective view of an optical scanning device according to an embodiment of the invention; FIG. FIG. 4 is a partial cross-sectional view of a deflector according to an embodiment of the invention; 4 is a block diagram showing a mechanism for correcting the amount of magnification deviation in the main scanning direction according to the embodiment of the present invention; FIG. (a) and (b) are diagrams showing in chronological order an example of correspondence between surface IDs and BD periods β measured by a scanning period measuring unit. (a) is a diagram showing a specific example of rotation speed, BD cycle, and correction data stored in a correction data storage unit according to the embodiment of the present invention. (b) is a schematic diagram showing a configuration for measuring the BD period of a rotating polygon mirror and correction data corresponding thereto. (a) is a diagram showing an example of a BD period α stored in a correction data storage unit. (b) is a diagram showing an example of a BD cycle β measured and stored in a scanning cycle storage unit. 8A and 8B are conceptual diagrams showing an example of the correspondence relationship between the surface ID and the BD cycle β in the specific unit and the BD cycle α in the correction data storage unit when pattern matching is successful; FIG. 10A and 10B are diagrams showing an example of correspondence between a surface ID, a BD cycle β measured by a scanning cycle measurement unit, and correction data stored in a correction data storage unit in chronological order; FIG. 8 is a flowchart of processing for identifying a reflecting surface and correcting 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 invention; FIG. (a) is a perspective view of a rotating polygon mirror, and (b) is a diagram showing a numerical simulation result of a deformation amount of a reflecting surface during rotation of the rotating polygon mirror. FIG. 4 is a diagram showing an example of scanning time jitter for each number of revolutions of a rotating polygon mirror made of plastic;

An image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention will be described below with reference to the drawings. In the following description, first, an image forming apparatus including an optical scanning device according to an 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, a deflector to be assembled in 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 an 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, which 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 exposure means, and a photosensitive drum, which is an image carrier built in a process cartridge 102 . 103 is irradiated. A latent image is formed on the photosensitive drum 103 by irradiating the photosensitive drum 103 with laser light and exposing the photosensitive drum 103 . A latent image formed on the photosensitive drum 103 is visualized as a toner image by toner as a developer. The process cartridge 102 integrally includes the photosensitive drum 103 and the charging means and developing means acting on the photosensitive drum 103, and is detachable from the image forming apparatus.

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

In the present embodiment, the charging means and the developing means acting on the photosensitive drum 103 are integrated with the photosensitive drum 103 in the process cartridge 102 . However, the present invention is not limited to this, and each process means may be constructed 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. An arrow Z shown in FIG. 1 indicates the axial direction (axial direction) of the rotating shaft 10 shown in FIG. Arrow X is a direction orthogonal to arrow Z, and arrow Y is a direction orthogonal to arrow Z and arrow 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. As shown in FIG. The compound anamorphic collimator lens 2 is a lens in which an anamorphic collimator lens having both functions of a collimator lens and a cylindrical lens and a lens for detecting a synchronization signal (BD lens) are integrally formed. Further, the optical scanning device 101 has an aperture stop 3 , a deflector 5 for rotationally driving the rotary polygon mirror 4 , a synchronization signal detection sensor (BD sensor) 6 as an output section, and an fθ lens (scanning lens) 7 . The optical scanning device 101 accommodates the above optical members in an optical box 8 .

In the above configuration, the laser light L emitted from the semiconductor laser unit 1 is converted by the compound anamorphic collimator lens 2 into substantially parallel light or convergent light in the main scanning direction, and convergent light in the sub-scanning direction. Next, the laser beam L passes through the aperture stop 3, the width of which is restricted, and forms an image on the reflecting surface of the rotary polygon mirror 4 in the form of a focal line extending in the main scanning direction. The laser beam L is deflected and scanned by rotating the rotary polygon mirror 4 and enters the BD lens of the compound anamorphic collimator lens 2 . The laser beam L that has passed through the BD lens is incident on the synchronization signal detection sensor 6 . That is, the synchronization signal detection sensor 6 receives the laser light L reflected by each reflecting surface of the rotating polygon mirror 4 . At this time, the synchronization signal (BD signal) is detected by the synchronization signal detection sensor 6, and this timing is used as the synchronization detection timing of the writing position in the main scanning direction. A synchronizing signal (BD signal) is a signal for synchronizing the image writing 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 section 300 (see FIG. 3), which will be described later. Next, the laser beam L enters the fθ lens 7 . The f.theta. lens 7 is designed to condense 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 f.theta. lens 7, the f.theta. lens 7 is formed of an aspherical lens. The laser beam L that has passed through the fθ lens 7 is emitted from the exit port of the optical box 8 and is imaged and scanned on the photosensitive drum.

The rotation of the rotary polygon mirror 4 deflects and scans the laser beam, and main scanning is performed on the photosensitive drum by the laser beam in the axial direction of the photosensitive drum. is done. The direction of scanning along the axis of the photosensitive drum is defined as a main scanning direction, and the direction of scanning by rotating the photosensitive drum about its axis is defined as a sub-scanning direction perpendicular to the main scanning direction. Thus, 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 a rotating 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 on the circuit board. etc. In addition to the rotating polygon mirror 4 , the rotor 20 includes a rotating shaft 10 , a rotor boss 11 , a rotor frame 12 , a rotor magnet 13 , and a fixture 14 for the rotating polygon mirror 4 . The rotating polygon mirror 4 is made of plastic such as polycarbonate resin or cycloolefin resin.

In the above configuration, when a current is supplied to the stator coil 17 , electromagnetic force is generated between the stator coil 17 and the rotor magnet 13 , causing the rotor 20 to rotate about the rotating shaft 10 supported by the bearing 15 . The Hall element 19 is a magnetic sensor for determining the timing (commutation timing) of current flow through the stator coil 17, 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 number of rotations of the rotating 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 number of rotations 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 amount of misregistration of scanning lines in the main scanning direction according to this embodiment. FIG. 5A is a diagram showing specific examples of the number of rotations, the BD period of each reflecting surface of the rotating 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 period of the reflecting surface of the rotating polygon mirror and the correction data corresponding thereto. FIGS. 4(a) and 4(b) are diagrams showing an example of correspondence between surface IDs and BD periods β measured by the scanning period measuring unit in chronological order. FIG. 6A is a diagram showing an example of the BD period α stored in the correction data storage unit, the reflecting surface corresponding thereto, and the correction data. FIG. 6B is a diagram showing an example of the BD period β measured and stored in the scanning period storage unit and the surface ID of the reflecting surface. FIGS. 7(a) and 7(b) are conceptual diagrams showing an example of the 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 pattern matching is successful. It is a diagram. 8(a) and 8(b) are diagrams showing an example of correspondence between the surface ID, the BD period β measured by the scanning period measuring unit, and the correction data stored in the correction data storage unit in time series. be.

As shown in FIG. 3, this mechanism includes a surface identification signal generator 300 , a main scanning position shift corrector 301 and an image signal generator 305 . The main scanning magnification shift correction unit 301 has a correction data control unit and a surface identification unit (correction data control unit + surface identification unit 303) related to control. 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 rotating 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 identified specific surface, controls the laser driving unit 306 through the laser light modulation unit (image clock generation unit) 304. control the drive. In this embodiment, the correction data control section and the surface identification section are performed by the CPU, which is a control section for controlling the operation of the apparatus. The image signal generator 305 generates an image signal and supplies it to the laser driver 306 . The laser drive unit 306 outputs laser light from the semiconductor laser unit 1 according to the supplied image signal and correction data (image clock generated by the main scanning position shift correction unit 301), which will be described later. A laser beam emitted from the semiconductor laser unit 1 is reflected by the reflecting surface of the rotating rotary polygon mirror 4 , and after the reflected laser beam is detected by the BD sensor 6 , the photosensitive drum 103 is scanned. 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 has a scanning cycle measurement unit, a scanning cycle storage unit, and a surface identification signal unit (not shown). The rotating polygon mirror 4 is stably rotated at a constant speed, and the process of assigning a surface ID is started. Then, the surface identification signal unit assigns a surface ID, which is a surface identification signal, to the current reflecting surface for the BD period, and updates the surface ID corresponding to the BD period each time a BD signal is input thereafter. Assign to the next reflective surface.

The “current reflective surface” is the reflective surface that supplied the reflected light that is the source of the BD signal that was output immediately before. Each time the rotary polygon mirror 4 rotates once, that is, each time the same number of BD signals as the number of reflecting surfaces (four) are output, the same reflecting surface becomes a source of reflected light. In this example, each BD signal output once every four times corresponds to one reflective surface. Therefore, the surface ID is information for specifying each reflecting surface and at the same time identifies each BD signal in one rotation of the rotating polygon mirror 4 .

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

For example, as shown in FIG. 4(a), when the rotation speed of the rotating polygon mirror 4 is r1 and the rotating polygon mirror 4 has four reflecting surfaces, after starting the step of allocating surface IDs, the first BD A reflecting surface located at a position where the laser beam is reflected immediately after the signal is output is defined as the first surface. 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 from the first BD signal is measured by the scanning cycle measurement unit, and stored in the scanning cycle storage unit as the BD cycle (for example, β11) of the first surface. Store.

Furthermore, when the next (third) BD signal is input, the interval from the immediately preceding (second) BD signal is measured as the BD period of the next second surface, and the BD period (for example, β12) is the scanning period. The second surface is stored in the storage unit, and "ID12" is assigned as the surface ID to the second surface. Such processing is performed for the number of faces of the rotating polygon mirror 4 for each two rotation speeds, that is, the first rotation speed r1 and the second rotation speed r2, which is greater than the first rotation speed r1. Then, for each number of rotations, the BD period β (β11 to β14, β21 to β24) of each reflecting surface is stored in the scanning period storage unit, and at the same time, surface IDs (ID11 to ID14, ID21 to ID24) are assigned. For example, in the case of the second rotational speed r2, as shown in FIG. 4(b), the surface ID "ID21" is assigned to the first reflecting surface, and then IDs corresponding to the number of surfaces are assigned in order. FIG. 6B shows the 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 numbers r1 and r2. The BD cycle β is measurement data measured as an output interval for each reflecting surface by the scanning cycle measurement unit of the surface identification signal generation unit 300, and is cycle data associated with the output order assigned to the surface ID.

The main scanning positional deviation correction unit 301 includes a correction data storage unit 302 as a storage unit, a surface specifying unit 303a for specifying a specific reflecting surface, a correction data control unit 303b for reading data and performing correction control, and a laser beam. and a modulation unit (image clock generation unit) 304 . In this embodiment, the CPU serves as the correction data control section and the surface identification section, but the configuration of each section is not limited to this. Each unit that configures the main scanning misregistration correction unit 301 may be implemented by a dedicated circuit such as an ASIC, or may be implemented by a CPU, ROM, RAM, and computer program. Here, as described above, the CPU serving 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 surfaces of the rotating polygon mirror, and uses the laser light modulation unit ( It controls driving of the laser driver 306 via an image clock generator 304 . As shown in FIG. 6(a), the correction data storage unit 302 stores the BD period of each reflecting surface of the rotating polygon mirror 4, which is measured in advance for each of a plurality of rotational speeds in the assembly process or the like, and the corresponding correction data. They are stored in association with each other. Here, as a rotating polygon mirror having a plurality of reflecting surfaces, a rotating polygon mirror 4 having four reflecting surfaces AD as shown in FIG. 3 is exemplified.

As shown in FIG. 6A, the correction data storage unit 302 stores the BD period 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. Pre-stored. Here, the BD period α associated with each of the reflecting surfaces AD is the 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 divided into two types: a first rotational speed r1 and a second rotational speed r2 greater than the first rotational speed r1. It is stored in advance in the correction data storage unit 302 for each number of revolutions.

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

The BD period α of each reflecting surface for each number of revolutions shown in FIG. 6A and the correction data data of the main scanning positional deviation corresponding thereto are measured in advance using jigs and tools in the assembly process. Japanese Laid-Open Patent Publication No. 2002-100001 discloses a tool for measuring the partial magnification of an optical scanning device as an example of a tool for measuring the positional deviation of each reflecting surface of the rotating polygon mirror in the main scanning direction. Patent Document 2 discloses an example in which six scanning position detection sensors are provided, excluding BD sensors (synchronization signal detection sensors). On the other hand, in the present embodiment, three scanning position detection sensors other than the BD sensor are used to detect positional deviations at three locations in the main scanning direction of the photosensitive drum 103 (the image writing portion, the image center portion, and the image writing end portion). Correction data to be corrected is acquired. A specific example of the measured BD period and the correction data corresponding thereto is shown in FIG. In FIG. 5A, the correction data for correcting the deviation in the main scanning direction of the laser beam reflected by each reflecting surface is the second rotation speed r1 (25000 min ⁻¹ ). It is larger in the case of the rotational speed r2 (39625 min ⁻¹ ).

6(a) and 5(a), the BD period means that the BD signal 1 is obtained when the BD signal 1 is obtained on one surface of the rotating polygon mirror, and the BD signal 2 is obtained on the next surface. is the time interval to signal 2; For example, the BD signal of the reflecting surface A of the rotating polygon mirror 4 shown in FIG. Next, the BD signal of the reflecting surface B, which is the next surface in the rotational direction adjacent to the reflecting surface A, is acquired as the BD signal 2 by the BD sensor 6 . The time interval from acquisition of the BD signal 1 to acquisition of the BD signal 2 is the BD period. That is, the output interval of the signal output by the BD sensor 6 is the BD period. BD periods (time intervals) between other reflecting surfaces are similarly obtained. In addition, the correspondence relationship between each reflecting surface and each BD period is such that the surface when BD signal 1, which is the previous signal, is acquired in one BD period is the time interval from BD signal 1 to BD signal 2 ( BD cycle). For example, the time interval (BD period) from the BD signal 1 on the reflecting surface A acquired by the BD sensor 6 to the BD signal 2 on the reflecting surface B is associated with the reflecting surface A of the BD signal 1, which is the previous signal. ing. The same applies to the correspondence between other reflecting surfaces and the BD period.

On the other hand, the correction data refers to positional deviation in the main scanning direction of the laser light L reflected by each reflecting surface caused by deformation of one surface of the rotating polygon mirror (displacement in the main scanning direction from the ideal reference position). This is data for correcting the amount of positional deviation). The positional deviation in the main scanning direction of the laser light L reflected by each reflecting surface means the distance from the scanning start position to the image area (the area 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 difference in distance. Here, time may be used as conversion of distance. In this embodiment, as shown in FIG. 5B, in addition to the BD sensor 6, three scanning position detection sensors S1, S2, and S3 are used to scan the photosensitive drum 103 at three points in the main scanning direction (the image writing portion and the image center). Correction data for correcting the positional deviation between the image writing end portion and the image writing end portion) is acquired. As shown in FIG. 5B, the BD sensor 6 is arranged at the scanning start position. At the three positions in the main scanning direction of the photosensitive drum, the sensor S1 is arranged at the position of the image writing portion, the sensor S2 is arranged at the position of the central portion of the image, and the sensor S3 is arranged at the position of the end portion of the image writing. In the main scanning direction, symbol a is the distance from the scanning start position to the image writing position, symbol b is the distance from the scanning start position to the image center position, and symbol c is the position from the scanning start position to the image writing end part. is the distance to The amount of deviation in the main scanning direction from the three ideal positions in the main scanning direction is stored as the correction data. That is, the respective distances a, b, and c are measured, and the amounts of deviation in the main scanning direction from the ideal distances serving as the reference for each of the measured distances a, b, and c are stored as correction data in the correction data storage unit 302 shown in FIG. pre-stored in In addition, in the measurement using the jigs and tools, the arrangement and number of the scanning position detection sensors other than the BD sensor are not limited to this, and should be appropriately set as necessary.

Here, the BD period α is measured in advance by specifying each of the reflecting surfaces A to D, and is a parameter corresponding to the BD period β. However, which reflecting surface each BD period β corresponds to can change in each surface identification. It is not determined unless processing for specifying the reflecting surface to be performed is performed.

The BD period α can be measured by the same method as for the BD period β, but the measurement method does not matter. That is, since it is assumed that the image forming apparatus will be measured before shipment, it is not essential to rotate the rotary polygon mirror 4 to actually perform scanning. The main scanning positional deviation correction data data (hereinafter also simply referred to as “correction data data”) is data for correcting the pre-measured deviation amount of scanning lines in the main scanning direction during image formation. It is assumed that this data is also measured before shipment of the device.

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. FIG. Then, it receives the correction data data stored in the read address adrs from the correction data storage unit 302 and outputs the correction data to the laser driving 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 number of rotations (r1, r2), and reads the BD cycle from the correction data storage unit 302. Read α (α11 to α14, α21 to α24). Then, the BD period β and the BD period α read out are compared, 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 rotating polygon mirror 4 .

For example, when the rotation speed of the rotating polygon mirror 4 is the first rotation speed r1 and the rotating polygon mirror 4 has a four-face configuration, the BD period (β11 to β14) is obtained for each of the four types of combination patterns as follows. and the sum of the squares of the differences between the BD periods (α11 to α14). First, when the BD signal is first output in the surface specifying process, the reflecting surface at the position where the laser light is reflected is arbitrarily determined. This arbitrarily determined reflecting surface is defined as the first surface (surface ID=ID1), and each reflecting surface is ordered in order of appearance along the rotation direction of the rotary polygon mirror 4, and each reflecting surface is ordered in advance from surface A. are combined in order to form a first combination pattern.

Each set in this first combination pattern (here, the set of the first surface and the A surface, the set of the second surface and the B surface, the set of the third surface and the C surface, and the set of the fourth surface and the D surface) , the process of calculating the square of the difference between the BD period β and the BD period α is executed. If the combination pattern is changed by shifting the pre-ordered reflecting surfaces one by one, there are four types of combination patterns (the number of reflecting surfaces of the rotating polygon mirror). For each set in all four combination patterns, the process of calculating the square of the difference is performed. For example, in the second combination pattern, calculations are made for the first surface and B surface pair, the second surface and C surface pair, the third surface and D surface pair, and the fourth surface and A surface pair. . In the third combination pattern, calculations are made for the first surface and C surface pair, the second surface and D surface pair, the third surface and A surface pair, and the fourth surface and B surface pair. In the fourth combination pattern, calculations are made for the set of the first surface and the D surface, the set of the second surface and the A surface, the set of the third surface and the B surface, and the set of the fourth surface and the C surface.

Then, for all the combination patterns, the sum of the squared differences of each pair (that is, the sum of the squared differences) is obtained and used as the difference value. Difference values 1 to 4 of combination patterns 1 to 4 are specifically calculated by the following 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 period α and the BD period 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 present invention is not limited to this, and the BD cycle β is shifted one by one with respect to the BD cycle α. You can

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

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

Here, the surface ID and the BD period β are associated with each other in the surface identification signal generator 300 (see FIG. 6B). In the correction data storage unit 302, the readout 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 also have a corresponding relationship through the readout address adrs. (See FIG. 6(a)). 7(a) and 7(b) exemplify the correspondence relationship in the case of the combination pattern with the minimum difference value of 4 among the above-described four combination patterns for each number of revolutions.

When the pattern matching is successful, based on the one-to-one correspondence between the BD period β and the BD period α in the combination pattern, the correction data data in the correction data storage unit 302 corresponds to the surface ID of each reflective surface. The resulting image is used for main scanning positional deviation correction. That is, the read address adrs corresponding to the face ID is obtained in the order of face ID→BD cycle β→BD cycle α→read address adrs (see FIG. 7). Then, the correction data data stored at the read address adrs is read out as correction data to be used for main scanning positional deviation correction.

Since the corresponding relationship between the BD period β and the BD period α is found, it is also possible to find out which reflecting surface is actually reflecting the laser beam at present. That is, at the stage before the completion of surface identification, only the surface ID is assigned to each reflecting surface of the rotating 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 multiple reflecting surfaces of the rotary polygon mirror 4 is identified. Therefore, from the correspondence relationship with the BD signal, it is possible to identify whether each reflecting surface is a reflecting surface that reflects the laser beam, or the relative position of the reflecting surface with respect to the reflecting surface. can get. In this embodiment, this matching processing is performed by the surface identification unit, and it is possible to identify the four reflecting surfaces of the rotating polygon mirror by the processing in the surface identification unit. is not limited to If only one of the reflecting surfaces can be specified, it is possible to correct the shift in the main scanning direction with correction data for that specified 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 correction data when pattern matching fails has been exemplified, but the present invention is not limited to this. For example, even if pattern matching fails, the average value of the correction data stored 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 instead of the average value.

If pattern matching succeeds, the minimum difference value approaches 0 infinitely. Therefore, as a method of determining the threshold value T in the above matching condition, it is desirable to set the value of the error calculated from the rotational jitter of the rotating polygon mirror 4 when the pattern matching is successful.

For example, if the difference value 4, which is the minimum difference value, is equal to or less than the threshold T, and the difference values 1, 2, and 3 other than the minimum difference value 4 are all greater than the threshold T, pattern matching is successful. deemed to have been

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

For example, when the rotation speed r1 and the difference value 4 take the minimum value and pattern matching is successful, as shown in FIG. The BD cycle of the correction data storage unit 302 is the BD cycle α14. The pattern of transition of the BD period β starting from the reflective surface (the surface whose surface ID is ID11) corresponding to the BD period β11 in FIG. 6B, and the reflective surface corresponding to the BD period α14 in FIG. It can be seen that the transition pattern of the BD period α starting from the D plane) is most similar. After all, the pattern matching is also to specify the starting points of the BD cycles such that the pattern of transition of the BD cycle β and the pattern of transition of the BD cycle α match each other.

In this case, as shown in FIG. 7A, "adrs14" is set as the readout address adrs in the correction data storage unit 302 of the main scanning position shift correction unit 301 for ID11, which is the surface ID corresponding to the BD period β11. set. In the positional deviation correction, the correction data dataL4 stored at the read address adrs14 is read and used as correction data. FIG. 7B exemplifies a case where the minimum value is taken with the number of revolutions r2 and the difference value 4, and the pattern matching is successful.

If the correspondence between the surface ID and the readout address adrs in the correction data storage unit 302 is determined in this way, the correction data data corresponding to the current reflecting surface of the rotating polygon mirror 4 can be obtained according to each surface ID for each number of rotations. can be read and used. That is, the first correction data stored in advance in the correction data storage unit 302 when the rotary polygon mirror 4 rotates at the first rotation speed r1, or the case where the rotary polygon mirror 4 rotates at the second rotation speed r2. can be read out and used. FIG. 8A shows the correspondence relationship between the BD signal, the surface ID, and the correction data for the rotation speed r1 and the difference value 4, and FIG. 8B shows the rotation speed r2 and the difference value 4. In the positional deviation correction, the laser drive unit 306 controls the emission of the laser light according to the read correction data data. That is, the CPU controls the emission of laser light from the semiconductor laser unit 1, which is the light source, via the laser driving section 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 displacement in the main scanning direction of the laser beam reflected by each reflecting surface that occurs for each number of revolutions of the rotating polygon mirror, and to suppress the deterioration of the image quality due to the displacement. can.

Next, control processing for specifying a reflective surface and correcting positional deviation of the specified reflective surface in the main scanning direction by the CPU that serves as the surface specifying unit 303a and the correction data control unit 303b will be described. FIG. 9 is a flowchart of processing for specifying a reflecting surface and correcting main scanning positional deviation.

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

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

Next, in step S105, the CPU reads the BD period α1 (α11 to α14) of each reflecting surface of the rotating polygon mirror 4 according to the first rotational speed r1 from the correction data storage unit 302. FIG. Next, in step S106, the CPU calculates the difference in each combination pattern corresponding to the number of faces of the rotating polygon mirror 4 from the BD period β1 and the BD period α1 of each reflecting surface corresponding to the read first rotation speed r1. Calculate the value. For example, when the rotating polygon mirror 4 has four faces, there are four combinations of BD periods, so difference values 1 to 4 are calculated.

Next, in step S107, the CPU performs pattern matching between the BD period α1 and the BD period β1 according to the first rotation speed r1 according to the matching conditions described above, and determines whether the pattern matching is successful. do. As a result, if pattern matching is successful, the CPU advances the process to step S108.

In step S108, the CPU identifies a combination pattern that has the smallest difference value when the number of rotations of the rotating polygon mirror 4 is the first number of rotations r1, and determines the BD period β1 and the BD period α1 in the combination pattern. grasp the one-to-one correspondence of That is, when the pattern matching is successful and the combination pattern with the minimum difference value is identified, the correspondence relationship between the BD period β associated with the surface ID and the BD period α previously associated with each reflecting surface is identified. , and each reflecting surface of the rotating polygon mirror 4 is specified. Then, as described above, the CPU traces the correspondence relationship 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 number of rotations of the rotary polygon mirror 4 is the first number of rotations r1, the CPU reads out the correction data data (first correction data) from the correction data storage section 302 .

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

A laser light modulation unit (image clock generation unit) 304 performs image clock modulation for each scanning line of each reflecting surface of the rotary polygon mirror 4 based on the main scanning positional deviation correction data data to correct the main scanning positional deviation. . The details of the means for performing image clock modulation from main scanning positional deviation correction data can be obtained by using the technique disclosed in Japanese Patent Laid-Open No. 2002-200030, and the description thereof is omitted.

A laser light modulating unit (image clock generating unit) 304 supplies an image clock modulated based on the number of rotations and correction data corresponding to the specified reflecting surface to the laser driving unit 306 . The image signal generator 305 generates an image signal and supplies it to the laser driver 306 . The laser drive unit 306 outputs a laser beam from the semiconductor laser unit 1 according to the supplied image signal and the image clock generated by the main scanning position shift correction unit 301 to form an image. Then, in step S111, the CPU determines whether or not the image formation is completed, and ends this control process when the image formation is completed.

On the other hand, if pattern matching fails in step S107, the CPU advances the process to step S110. In step S110, the CPU does not set correction data. Here, the control process of not setting correction data when pattern matching fails has been exemplified, but the present invention is not limited to this. For example, even if pattern matching fails, the following control processing may be performed. That is, the average value of the correction data stored 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 rotating polygon mirror 4 . Control processing may be performed in this way.

In step S102, when the CPU determines the number of rotations of the rotating polygon mirror 4 associated with various printing modes, if the number of rotations of the rotating polygon mirror 4 is the second number of rotations r2, the process proceeds to step S113. . The processes from steps S113 to S120 are the same as the processes from steps S103 to S110 described above, except that the number of rotations and data such as the BD cycle for each number of rotations are different. are omitted.

Then, when correction data (second correction data) for each reflecting surface of the rotating polygon mirror is set according to the number of rotations (second number of rotations r2) and the specified reflecting surface, in subsequent step S111, the CPU performs image formation as described above. In step S112, the CPU determines whether or not the image formation is completed, and terminates this control process when the image formation is completed.

By performing the main scanning positional deviation correction described above, it is possible to suppress deterioration of image quality caused by the positional deviation in the main scanning direction. In addition, by performing the positional deviation correction, the maximum amount of deviation from the ideal position in the image writing portion, the image center portion, and the image writing end portion is a predetermined value or less (here, about 5 μm or less) at each rotation speed. ). Also, the amount of relative displacement between the reflecting surfaces of the rotary polygon mirror 4 can be set to about 2 μm or less. In this embodiment, the rotational speed of the rotating polygon mirror 4 is 2nd speed. If the data storage unit 302 is provided with the same effect, the same effect can be obtained.

Also, although the rotating polygon mirror 4 is made of a resin material such as a polycarbonate resin or a cycloolefin resin, the present invention is not limited to this. Even if the rotating polygon mirror 4 is made of a metal material such as aluminum, the amount of deformation of the reflecting surface due to the rotation of the rotating polygon mirror 4 is not completely zero. Therefore, as described above, even when the material of the rotating polygon mirror 4 is a metal material, similar correction data may be obtained for each number of rotations, and main scanning positional deviation correction may be performed for each number of rotations.

In this way, by providing main scanning direction positional deviation correction data for each number of rotations of the rotary polygon mirror 4 used in the image forming apparatus, the light reflected by each reflecting surface generated for each number of rotations of the rotary polygon mirror 4 is reflected. It is possible to reliably correct the positional deviation of the laser light in the main scanning direction. Therefore, even if the number of rotations of the rotating polygon mirror 4 of the image forming apparatus changes, the image quality does not deteriorate, and high-quality images can be maintained.

In the above-described embodiments, a printer is used as an example of the image forming apparatus, but the present invention is not limited to this. For example, other image forming apparatuses such as a copying machine and a facsimile machine, or other image forming apparatuses such as a multi-function machine combining these functions may be used. Similar effects can be obtained by applying the present invention to these image forming apparatuses.

L: laser light 1: semiconductor laser unit 2: composite anamorphic collimator lens 3: aperture diaphragm 4: rotary polygon mirror 5: deflector 6: synchronization signal detection sensor (BD sensor)
7 ... fθ lens 101 ... optical scanning device 103 ... photosensitive drum 110 ... image forming apparatus 300 ... surface identification signal generation unit 301 ... main scanning position deviation correction unit 302 ... correction data storage unit 303 ... correction data control unit + specifying unit 303a ... Surface identification unit 303b ... correction data control unit 304 ... laser light modulation unit (image clock generation unit)
305 ... Image signal generation unit 306 ... Laser driving unit

Claims (11)

an optical scanning device for scanning an image carrier with a laser beam emitted from a light source by rotating a rotating polygon mirror in a main scanning direction, which is an axial direction of the image carrier; or a second rotation speed faster than the first rotation speed,
an output unit provided in the optical scanning device for receiving a laser beam reflected by a reflecting surface of the rotating polygon mirror rotated at the first rotation speed or the second rotation speed and outputting a signal;
a surface identifying unit that identifies a specific reflecting surface among a plurality of reflecting surfaces of the rotating polygon mirror based on the signal output from the output unit;
Correction data for correcting deviation in the main scanning direction of the image carrier of the laser beam reflected by the specific reflecting surface associated with the specific reflecting surface, the correction data comprising: a storage unit pre-stored with first correction data when the rotating polygon mirror rotates and second correction data when the rotating polygon mirror rotates at the second rotation speed;
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;
An image forming apparatus comprising: 2. An image forming apparatus according to claim 1, wherein said rotating polygon mirror is made of plastic. Correction data for correcting deviation in the main scanning direction of the laser beam reflected by the reflecting surface is larger at the second rotation speed than at the first rotation speed. 3. The image forming apparatus according to claim 1 or 2. 4. The apparatus according to any one of claims 1 to 3, wherein the output unit outputs a signal for synchronizing image writing positions in the main scanning direction on each reflecting surface of the rotating polygon mirror. Image forming device. The correction data stored in the storage section is obtained by measuring the distance from the scanning start position where the output section is arranged to a predetermined position in the image area of the image carrier in the main scanning direction, and calculating the measured distance. 5. The image forming apparatus according to claim 1, wherein the amount of deviation in the main scanning direction from an ideal distance serving as a reference is . The image area of the image carrier is an area from the position of the image writing part to the position of the image writing end part,
6. The image according to claim 5, wherein the predetermined positions of the image area of the image carrier are the position of the image writing portion, the position of the image central portion, and the position of the image writing end portion in the main scanning direction. forming device. The surface specifying unit stores in advance an output interval of the signals, which is measured based on the two signals output in order from the output unit, for the number of reflecting surfaces of the rotating polygon mirror, and an output interval of the signals. calculating a difference value of a combination pattern corresponding to the number of reflecting surfaces of the rotating polygon mirror using the output intervals of the signals,
The reflective surface is specified by a combination pattern that satisfies a matching condition that the minimum difference value among the calculated difference values for the number of surfaces, among the combination patterns for the number of surfaces, is smaller than a preset threshold value. 2. The image forming apparatus according to claim 1. If a matching condition is satisfied that the minimum difference value among the calculated difference values for the number of surfaces among the combination patterns for the number of surfaces is smaller than a preset threshold value, correction data for each reflecting surface is changed. 8. The image forming apparatus according to claim 7, wherein the correction data is not set if the matching condition is not satisfied. having a detection unit for detecting the type of recording material,
9. The number of rotations of the rotating polygon mirror is changed to the first number of rotations or the second number of rotations according to the type of recording material detected by the detection unit. 1. The image forming apparatus according to item 1. When the recording material detected by the detection unit is thick paper, the rotational speed of the rotary polygon mirror is set to the first rotational speed, and when the recording material is plain paper thinner than the thick paper, the second rotational speed. 10. The image forming apparatus according to claim 9, wherein: When the first correction data for each reflecting surface corresponding to the first rotation speed is set, the first correction data is used to control the emission of laser light from the light source to the specified reflecting surface, When second correction data different from the first correction data for each reflecting surface corresponding to the second rotation speed is set, laser light emitted from the light source to the specified reflecting surface using the second correction data is set. 11. The image forming apparatus according to any one of claims 1 to 10, wherein the emission of the light is controlled.

Images