JP2012155134A - Optical scanner and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily prevent ununiformity in image quality derived from variation in FFP, variation in optical system assembling accuracy, variation in photoreceptor sensitivity, durability deterioration and the like.SOLUTION: An optical scanner forms an electrostatic latent image on a photoreceptor by exposing the photoreceptor 15 to a plurality of light beams in accordance with image data. The optical scanner comprises an LD 100 for generating the plurality of light beams and scans the photoreceptor with the plurality of light beams by using a polygon mirror 1002. A controller 1027 controls the LD by a laser driver 1029 so as to control the light quantity of the light beams in accordance with a prescribed target light quantity. When the light quantity of the light beams is out of a light quantity threshold range precedently defined, the controller controls the LD by the laser driver so as to change the number of light beams and change a rotation number of the polygon mirror. At this point, when the number of light beams is reduced, the rotation number of the polygon mirror is increased.

Description

  The present invention relates to an optical scanning device used in an image forming apparatus using an electrophotographic process.

  In recent years, in an image forming apparatus using a laser beam scanning device as an exposure device, a multi-beam laser diode is used as a light source of the laser beam scanning device in order to improve productivity or resolution.

  Now, if PS is the laser scan and the vertical process speed orthogonal to the scan, DPI is the resolution, RMS is the number of polygon mirror rotations, N is the number of polygon mirror reflecting surfaces, and M is the number of beams. Is represented by Formula (1).

PS = RMS / 60 (25.4 / DPI) (N × M) Formula (1)
From equation (1), it can be seen that increasing at least one of the number of rotations of the polygon mirror, the number of reflecting surfaces of the polygon mirror, and the number of beams can improve productivity or resolution.

  However, the rotational speed of the polygon mirror has recently become 40000 RPM to 50000 RPM, and the rotational speed is approaching its limit. Further, when the polygon mirror is rotated at a high speed, adverse effects such as temperature rise and noise of the polygon mirror itself become remarkable. For this reason, it is difficult to further increase the number of rotations of the polygon mirror.

  On the other hand, if the outer diameter of the polygon mirror is constant and the reflection surface is increased, the optical characteristics of the polygon mirror are inevitably deteriorated, and the spot diameter is enlarged. This causes deterioration in image quality, and increasing the number of reflecting surfaces is incompatible with improving the resolution.

  Further, if the reflecting surface of the polygon mirror is increased with the spot diameter being constant, the outer diameter of the polygon mirror becomes larger due to the necessity of making the length of one side of the reflecting surface constant, which causes the drive current to be reduced. Not only does it increase and the temperature rises, but also harmful effects such as noise occur.

  On the other hand, a surface emitting laser diode (VCSEL) has recently been studied as a light source for an edge emitter laser diode (edge emitter LD). Since VCSEL emits laser light from the wafer surface, it is possible to two-dimensionally arrange light emitting points with respect to the edge emitter LD that emits laser light from the wafer end face. For this reason, if VCSEL is used as a light source, there exists an advantage that it can respond easily to the increase in the number of beams.

  By the way, it is necessary to widen the range of light quantity control for the light source used in the optical scanning device. The light quantity of the laser light source is proportional to the process speed, and inversely proportional to the sensitivity of the photoreceptor, the optical efficiency, and the number of beams.

  The sensitivity of the photosensitive member inevitably varies in the manufacturing process, and the photosensitive member has a sensitivity change characteristic due to durability deterioration. When the sensitivity of the photoreceptor changes, the image density may differ even when charging, exposure, and development are performed with the same process amount. In order to suppress such density fluctuations, the exposure amount is controlled according to the change in the photoreceptor sensitivity.

  For example, in the density adjustment sequence, the photoconductor is exposed in accordance with predetermined image data for inspection, and the surface potential of the photoconductor and the density of the toner image are measured by a sensor or the like. Then, the amount of laser light is controlled so that the density of the toner image becomes the target density. At this time, since the light source is required to have a light quantity that satisfies fluctuations in the sensitivity of the photoreceptor, it is necessary to broaden the control of the light quantity range for the laser.

  Further, in the optical system, the light amount of the laser chip surface is controlled so that the light amount of the spot imaged on the photoreceptor is satisfied. There are variations in the optical system due to various factors such as variations due to assembly accuracy, variations in the transmittance and reflectance of lenses and mirrors, and variations in the laser emission angle (FFP). The relationship between the amount of light on the photoconductor and the amount of light on the laser chip surface also varies. Considering these variations, the laser is required to control a wide light amount range.

  For example, when setting the amount of laser light, the potential of the reference patch formed on the photoconductor is measured at the time of setup such as when the power is turned on, and the average potential and potential variation of the reference patch over one or more turns on the photoconductor are obtained. In some cases, the exposure amount (that is, the laser light amount) of the photosensitive member is corrected (see Patent Document 1).

JP 2000-56522 A

  As described above, in order to prevent unevenness in image quality due to variations in FFP, variations in optical system assembly accuracy, variations in photoreceptor sensitivity, durability deterioration, etc., laser devices are required to have a wide light quantity range. . However, for example, the rise time of the laser beam has a characteristic that it is fast when the light quantity is high and is slow when the light quantity is low. Then, the profile of the exposure potential is affected by the difference in the rising characteristics according to the amount of light, and it becomes difficult to make the image quality uniform. Further, when the amount of light is low, the margin (SMSR) with respect to the second peak of the oscillation wavelength is lowered, and the spot on the photoconductor is enlarged. This also has a problem that the image quality deteriorates.

  Accordingly, an object of the present invention is to provide an optical scanning apparatus and an image forming apparatus that can easily prevent non-uniformity in image quality due to variations in FFP, variations in optical system assembly accuracy, variations in photoreceptor sensitivity, durability deterioration, and the like. Is to provide.

  In order to achieve the above object, an optical scanning device according to the present invention exposes a photoconductor with a plurality of light beams emitted from a plurality of light sources according to image data, and forms a latent image on the photoconductor. In the apparatus, the plurality of light sources are arranged so as to simultaneously form a plurality of scanning lines in the sub-scanning direction of the photoconductor by the plurality of light beams, and cause the photoconductor to scan the plurality of light beams. The light quantity control means for controlling the light quantity of the light beam according to a predetermined target light quantity by controlling the scanning means, the plurality of light sources, and the light quantity of the light beam controlled by the light quantity control means are defined in advance. And changing means for changing the number of light sources used for exposure when outside the light amount threshold range, and changing the number of rotations of the rotary polygon mirror according to the number of the changed light sources. Light source Upon decrease is characterized by increasing the rotational speed of the rotary polygon mirror according to the number of light sources is the reduction.

  According to the present invention, the number of beams is determined according to the amount of light beam, and the number of rotations of the rotary polygon mirror is determined according to the number of light beams, so that the light amount range of the light beam is narrowed. Thus, exposure can be performed with constant light beam characteristics. Furthermore, since the rotational speed of the rotary polygon mirror is changed according to the number of beams, the influence on productivity can be prevented. As a result, it is possible to easily prevent non-uniformity in image quality due to variations in FFP, variations in optical system assembly accuracy, variations in photoreceptor sensitivity, durability deterioration, and the like.

It is sectional drawing which shows an example of the image forming apparatus in which an example of the optical scanning device by embodiment of this invention is used. It is a figure which shows the structure of an example of the optical scanning device shown in FIG. 1 in detail. It is a figure for demonstrating an example of a structure of the laser diode (LD) shown in FIG. 2, and a laser driver. It is a figure which shows the relationship between the sensitivity of the photosensitive drum by the number (number of beams) of a laser beam, and the light quantity of each laser beam, (A) is a figure which shows the relationship in case optical efficiency is low, (B) is optical efficiency. It is a figure which shows the relationship in the case of being high. 3 is a flowchart for explaining determination of the number of beams in the optical scanning device shown in FIG. 2. It is a figure which shows the relationship between the number of beams and the rotation speed of a polygon motor. FIGS. 3A and 3B are diagrams illustrating a flow of image data when the photosensitive drum is exposed by the optical scanning device illustrated in FIG. 2. FIG. 3A is a diagram illustrating a flow of image data when exposure is performed with 16 beams, and FIG. It is a figure which shows the flow of the image data at the time of performing exposure with a beam. FIG. 3 is a diagram illustrating a relationship between an FFP of an LD for obtaining a necessary light amount in the photosensitive drum illustrated in FIG. 2 and a light amount of each LD element.

  An image forming apparatus using an example of an optical scanning device according to an embodiment of the present invention will be described below.

  FIG. 1 is a sectional view showing an example of an image forming apparatus in which an example of an optical scanning device according to an embodiment of the present invention is used. The image forming apparatus illustrated in FIG. 1 is an image forming apparatus that forms a color image by superimposing cyan (C), magenta (M), yellow (Y), and black (K) colors.

  The illustrated image forming apparatus 1 </ b> A has four photosensitive drums (photosensitive members) 14, 15, 16, and 17, and faces these photosensitive drums 14, 15, 16, and 17 and is an intermediate transfer member. An intermediate transfer belt (endless belt) 13 is disposed.

  The intermediate transfer belt 13 is stretched around a driving roller 13a, a secondary transfer counter roller 13b, and a tension roller (driven roller) 13c, and is defined in a substantially triangular shape in a sectional view. The intermediate transfer belt 13 rotates clockwise in the drawing (rotates in the direction indicated by the solid line arrow).

  The photosensitive drums 14, 15, 16, and 17 are arranged along the rotation direction of the intermediate transfer belt 13. In the illustrated example, the photosensitive drums 14, 15 are sequentially arranged from the most upstream side in the rotation direction of the intermediate transfer belt 13. , 16 and 17 are arranged.

  Around the photosensitive drum 14, a charger 27, a developing device 23, and a cleaner 31 are arranged. Similarly, around the photosensitive drums 15, 16, and 17, chargers 28, 29, and 30, developing units 23, 24, 25, and 26, and cleaners 31, 32, 33, and 34 are arranged, respectively. Has been.

  The chargers 27, 28, 29, and 30 uniformly charge the surfaces of the photosensitive drums 14, 15, 16, and 17, respectively.

  Above the photosensitive drums 14, 15, 16, and 17, an optical scanning device (also referred to as an exposure control unit) 22 is disposed, and the optical scanning device 22 responds to image data as described later. , 15, 16 and 17 are scanned with a laser beam (light beam).

  In the illustrated example, the photosensitive drums 14, 15, 16, and 17 correspond to magenta (M), cyan (C), yellow (Y), and black (K) toners, respectively. .

  Here, an image forming (printing) operation by the image forming apparatus 1A shown in FIG. 1 will be described.

  The illustrated image forming apparatus 1 </ b> A includes two cassette sheet feeding units 1 and 2 and one manual sheet feeding unit 3. Recording paper (transfer paper) S is selectively fed from the cassette paper feeding units 1 and 2 and the manual paper feeding unit 3.

  The cassette paper feeding units 1 and 2 have cassettes 4 and 5, respectively, and the manual paper feeding unit 3 has a tray 6. The transfer sheets S are stacked on the cassette cassettes 4 and 5 or the tray 6 and are sequentially picked up by the pickup roller 7 from the transfer sheet S positioned at the uppermost position. The picked up transfer sheet S is separated only by the transfer roller S positioned at the uppermost position by the separating roller pair 8 including the feed roller 8A and the retard roller 8B.

  The transfer paper S sent out from the cassette paper supply unit 1 or 2 is sent to the registration roller pair 12 by the transport roller pairs 9, 10, and 11. On the other hand, the transfer paper S sent from the manual paper feed unit 3 is immediately sent to the registration roller pair 12. Then, the transfer sheet S is temporarily stopped by the registration roller pair 12 and the skew state is corrected.

  By the way, the image forming apparatus 1 </ b> A is provided with the document feeding device 1, and the document feeding device 1 transports the stacked documents one by one on the document table glass 19 in order. When the original is conveyed to a predetermined position on the original platen glass 19, the original surface is irradiated by the scanner unit 4A, and the reflected light from the original is guided to the lens via a mirror or the like. The reflected light is formed as an optical image on an image sensor unit (not shown).

  The image sensor unit converts the formed optical image into an electrical signal by photoelectric conversion. This electrical signal is input to an image processing unit (not shown). The image processing unit converts the electrical signal into a digital signal, and then performs necessary image processing on the digital signal to obtain image data.

  This image data is directly or once stored in an image memory (not shown) and then input to the optical scanning device (exposure control unit) 22. The optical scanning device 22 drives a semiconductor laser (not shown) according to the image data. Thereby, a laser beam (light beam) is emitted from the semiconductor laser.

  The laser beam irradiates the surfaces of the photosensitive drums 14, 15, 16, and 17 through a scanning system including a polygon mirror (rotating polygon mirror). The laser beam is scanned on the photosensitive drums 14, 15, 16, and 17 along the main scanning direction (the axial direction of the photosensitive drums 14, 15, 16, and 17).

  The photosensitive drums 14, 15, 16, and 17 are rotated in the direction indicated by the solid line arrow (sub-scanning direction) in the drawing, whereby the photosensitive drums 14, 15, 16, and 17 are sub-scanned by the laser beam. The direction is also scanned. The electrostatic latent image corresponding to the image data is formed on the photosensitive drums 14, 15, 16, and 17 by scanning with the laser beam.

  In the illustrated example, first, the photosensitive drum 14 positioned on the most upstream side is exposed by the laser beam LM based on the image data of the magenta component. Thereby, an electrostatic latent image is formed on the photosensitive drum 14. The electrostatic latent image on the photosensitive drum 14 is developed by the developing unit 23 to be a magenta (M) toner image.

  Next, when a predetermined time has elapsed from the start of exposure of the photosensitive drum 14, the photosensitive drum 15 is exposed by the laser beam LC based on the cyan component image data. Thereby, an electrostatic latent image is formed on the photosensitive drum 15. The electrostatic latent image on the photosensitive drum 15 is developed by the developing device 24 to be a cyan (C) toner image.

  Further, when a predetermined time has elapsed from the start of exposure of the photosensitive drum 15, the photosensitive drum 16 is exposed by the laser beam LY based on the image data of the yellow component. Thereby, an electrostatic latent image is formed on the photosensitive drum 16. The electrostatic latent image on the photosensitive drum 16 is developed by the developing unit 25 to be a yellow (Y) toner image.

  When a predetermined time elapses from the start of exposure of the photosensitive drum 16, the photosensitive drum 17 is exposed by the laser beam LB based on the image data of the black component. Thereby, an electrostatic latent image is formed on the photosensitive drum 17. Then, the electrostatic latent image on the photosensitive drum 17 is developed by the developing device 25 to be a black (K) toner image.

  The M toner image on the photosensitive drum 14 is transferred onto the intermediate transfer belt 13 by the transfer charger 90. Similarly, a C toner image, a Y toner image, and a K toner image are transferred from the photosensitive drums 15, 16, and 17 onto the intermediate transfer belt 13 by the transfer chargers 91, 92, and 93, respectively.

  As a result, the M toner image, the C toner image, the Y toner image, and the K toner image are sequentially superimposed and transferred onto the intermediate transfer belt 13, and the primary transfer image is transferred onto the intermediate transfer belt 13. As a result, a color toner image is formed.

  Note that the toner remaining on the photosensitive drums 14, 15, 16, and 17 after the transfer is removed by the cleaners 31, 32, 33, and 34, respectively.

  The transfer sheet S temporarily stopped by the registration roller pair 12 is conveyed to the secondary transfer position T2 by driving the registration roller pair 12. Here, at the timing when the position of the color toner image on the intermediate transfer belt 13 and the leading edge of the transfer paper S are aligned, the registration roller pair 12 is rotationally driven, and the transfer paper S is conveyed to the secondary transfer position T2.

  A secondary transfer roller 40 and a secondary transfer counter roller 13b are arranged at the secondary transfer position T2, and the color toner image on the intermediate transfer belt 13 is transferred as a secondary transfer image at the secondary transfer position T2. Transferred onto the paper S.

  The transfer sheet S that has passed the secondary transfer position T2 is sent to the fixing device 35. The fixing device 35 includes a fixing roller 35A and a pressure roller 35B. When the transfer paper S passes through the nip formed by the fixing roller 35A and the pressure roller 35B, the transfer paper S is heated by the fixing roller 35A and is pressed by the pressure roller 35B. As a result, the secondary transfer image is fixed on the transfer paper S.

  The fixing-processed transfer sheet S is sent to the discharge roller pair 37 by the transport roller pair 36 and discharged onto the discharge tray 38 by the discharge roller pair 37.

  In the illustrated image forming apparatus 1A, image formation can be performed in a so-called double-sided printing mode.

  In the duplex printing mode, the transfer-processed transfer sheet S that has passed through the fixing device 35 is sent to the reverse path 59 through the vertical path 58. At this time, the flapper 60 is in a state in which the vertical path 58 is opened, and the transfer sheet S having undergone the fixing process is conveyed to the reversing path 59 by the conveying roller pairs 36, 61, and 62 and the reverse roller pair 63.

  When the trailing edge of the fixed transfer sheet S conveyed in the direction of the arrow a passes the point P, the reverse roller pair 63 is driven in reverse. As a result, the transfer sheet S that has been subjected to the fixing process is conveyed in the direction of the arrow b with the trailing edge as the head. As a result, the surface on which the secondary transfer image is formed is on the upper side of the transfer sheet S that has been subjected to the fixing process.

  At the point P, a flapper 64 with flexible transfer paper is disposed. The flapper 64 allows the transfer paper S to enter from the vertical path 58 to the reverse path 59 and disables the transfer paper S from entering the vertical path 58 from the reverse path 59.

  Further, a detection lever 65 is disposed at the point P. The detection lever 65 detects that the trailing edge of the transfer paper S has passed the point P.

  As described above, the fixed transfer sheet S conveyed in the arrow b direction by the reverse rotation of the reverse rotation roller pair 63 is sent into the refeed path 67. Then, the fixed transfer sheet S is relayed by the plurality of re-feed path conveying roller pairs 68 and the conveying roller pair 11 and is sent to the registration roller pair 12 again.

  The transfer paper S that has been subjected to the fixing process is fed to the secondary transfer position T2 after its skew state is corrected by the registration roller pair 12. Then, the second image formation is performed based on the image data that has been subjected to the magnification correction in the main scanning direction and the sub-scanning direction, and the transfer paper S is discharged to the discharge tray 38 through the same process as the one-sided image formation described above. To be discharged.

  By the way, in order to cope with higher speed and higher image quality of the image forming apparatus, a plurality of scanning lines (scanning lines) are exposed by one scanning with a polygon mirror by using a plurality of beams in a semiconductor laser used for a laser light source. Things have been done. In particular, edge-emitting lasers to surface-emitting lasers have been put into practical use, and such multi-beam formation is easy.

  Here, an example in which a multi-beam semiconductor laser is used in an image forming apparatus will be described.

  FIG. 2 is a diagram showing in detail the configuration of an example of the optical scanning device 22 shown in FIG. Referring to FIG. 2, the illustrated optical scanning device 22 uses a semiconductor laser (here, a laser light source (LD100)) as a light emitting element, and a plurality (m, m is an integer of 2 or more) of laser beams in the LD100. Irradiation.

  In order to increase the speed and image quality of the image forming apparatus 1A, the number of laser beams (also referred to as light beams) in the LD 100 is made plural, and a plurality of scanning lines (scanning lines) are exposed by a single scan with a polygon mirror. I am doing so.

  The optical scanning device (exposure control unit) 22 has a FIFO memory (first-in first-out memory) 1028. The above-described image data is written in the FIFO memory 1028, and the controller 1027 reads out the image data Data 1 to Data 16 from the FIFO memory 1028 and gives it to the laser driver 1029. The laser driver 1029 drives the LD 100 under the control of the controller 1027. As a result, a laser beam is emitted (emitted) from the semiconductor laser 1029a. As will be described later, the controller 1027 controls the laser driver 1029 by providing the laser driver 1029 with reference voltages Vcont1 to Vcont16 and control signals cont1 to cont16.

  A laser beam emitted (emitted) from the LD 100 is converted into substantially parallel light by a collimator lens and a diaphragm 1013 and then enters a polygon mirror (rotating polygonal mirror) 1002 with a predetermined beam diameter. The polygon mirror 1002 is rotationally driven at a constant angular velocity by a motor (also referred to as a polygon motor) 1003 controlled by a control signal from the controller 1027. As the polygon mirror 1002 rotates, the laser beam L1 is converted into a deflected beam that continuously changes its angle.

  The deflected beam is subjected to distortion correction and the like by a lens group 1014 such as an f-θ lens, and then scanned in the main scanning direction of the photosensitive drum 15 (the axial direction of the photosensitive drum 15). An image 1016 is formed. In the illustrated example, one surface of the polygon mirror 1002 corresponds to one scanning, and when the number of laser beams is 16 (that is, m = 16), the laser beam is rotated by the rotation of the polygon mirror 1002. Are scanned in the main scanning direction of the photosensitive drum 15 by 16 scanning lines.

  In the illustrated example, a BD (beam detection) sensor 1017 is disposed on the side of the photosensitive drum 1010 in the vicinity of the scanning start position in the main scanning direction or a corresponding position. The laser beam reflected by each reflecting surface of the polygon mirror 1002 is detected by the BD sensor 1017 prior to each scanning. A detection signal (BD signal) from the BD sensor 1019 is given to the controller 1027 as a scanning start reference signal indicating a scanning start reference in the main scanning direction.

  The controller 1027 controls the memory 1028 and the laser driver 1029 by synchronizing the writing start position in the main scanning direction of each scanning line based on the BD signal. That is, the controller 1027 generates a write sequence according to the BD signal, provides the sequence signal to the FIFO memory 1028, and provides the control signals cont1 to cont16 to the laser driver 1029.

  Further, as shown in the figure, a density sensor 1010 is disposed in the vicinity of the photosensitive drum 15. The density sensor 1010 detects the image density (toner density) on the photoconductor 15 after development. Then, the density sensor 1010 sends a density detection signal to the controller 1027. The controller 1027 obtains the sensitivity of the photosensitive drum 15 according to the density detection signal, and applies the reference voltages Vcont1 to Vcont16 to the laser driver 1029 so that the optimum exposure amount is obtained.

  FIG. 3 is a diagram for explaining an example of the configuration of the laser diode (LD) 100 and the laser driver 1029 shown in FIG.

  Referring to FIG. 3, in the illustrated example, the LD 100 includes a plurality of laser diodes LD1 to LD16, and these laser diodes LD1 to LD16 are accommodated in a package. The laser driver 1029 also includes a plurality of driver circuits driver1 to driver16. The laser diodes LD1 to LD16 are arranged so that a plurality of scanning lines are simultaneously formed in the sub-scanning direction of the photosensitive drum by the laser beams emitted from these laser diodes.

  The cathode terminals of the laser diodes LD1 to LD16 are grounded as a common terminal, and the anode terminals of the laser diodes LD1 to LD16 are connected to the driver circuits driver1 to driver16, respectively. As will be described later, the laser diodes LD1 to LD16 are supplied with lighting currents by driver circuits driver1 to driver16, respectively.

  Since the driver circuits driver1 to driver16 have the same configuration, the operation of the driver circuits driver1 will be described here with a focus on the driver circuit driver1.

  In FIG. 3, the photodiode PD1 is for monitoring the light quantity of the laser diodes LD1 to LD16. The photodiode PD1 is installed at a position where the emitted light of the laser diodes LD1 to LD16 or a part thereof is irradiated.

  As shown in the figure, the anode terminal of the photodiode PD1 is grounded, and the cathode terminal is connected to the power supply Vcc via the resistor R1. As a result, a bias is applied to the photodiode PD1, and the output of the cathode terminal becomes the monitor output.

  The cathode terminal of the photodiode PD1 is connected to the plus (+) input terminal of the error amplifier OP1. A reference voltage Vcont1 is applied to the minus (−) input terminal of the error amplifier OP1. The output terminal of the error amplifier OP1 is connected to the analog switch SW1.

  A control signal cont1 is given to a control terminal that controls the operation of the analog switch SW1. The control signal cont1 signal is a sequence signal generated by the controller 1027 described with reference to FIG. The output of the analog switch SW1 is connected to one end of the capacitor C1 and serves as a control signal for the constant current source CC1. The other end of the capacitor C1 is grounded.

  The constant current source CC1 outputs a current corresponding to an applied voltage (that is, a control signal). The emitter terminals of the PNP transistors Q10 and Q11 are connected to this current output. The collector terminal of the PNP transistor Q10 becomes the output of the driver circuit driver1, and is connected to the anode terminal of the laser diode LD1.

  One end of the resistor RD1 is connected to the collector terminal of the PNP transistor Q11, and the other end of the resistor RD1 is grounded. A data signal (image data) data1 is input to the base terminal of the PNP transistor Q10 via the inverter Q12. Similarly, the data signal data1 is input to the base terminal of the PNP transistor Q11 via the buffer Q13. The data signal data1 is supplied from the memory 1026 shown in FIG.

  Note that the configuration of the driver circuits driver2 to driver16 is the same as that of the driver circuit driver1, but the constituent elements of the driver circuit driver2 are denoted by “2”, and the other driver circuits driver3 to driver16 are also denoted by the same symbols. “3” to “16” are attached.

  First, the controller 1027 sets both the control signal cont1 and the data signal data1 to the high level (Hi level) so that the laser diode LD1 enters the auto power control (APC) mode. At this time, the controller 1027 sets the control signals cont2 to cont16 and the data signals data2 to data16 to be supplied to the driver circuits driver2 to driver16 to the low level (Lo level).

  In the driver circuit driver1, since the data signal data1 is at the Hi level, the output of the inverter Q12 is at the Lo level. As a result, the PNP transistor Q10 is turned on. On the other hand, the PNP transistor Q11 is turned off.

  When the PNP transistor Q10 is turned on, the laser diode LD1 is turned on by the current supplied from the constant current source CC1. As the light quantity of the laser diode LD1 increases, the output current of the photodiode PD1 also increases. As a result, the voltage (output of the photodiode PD1) input to the error amplifier OP1 decreases.

  The output of the photodiode PD1 is compared with the reference voltage Vcont1 by the error amplifier OP1, and the output voltage of the error amplifier OP1 decreases. When the output voltage of the error amplifier OP1 decreases, the output current of the constant current source CC1 also decreases. When the output current of the constant current source CC1 decreases, the light amount of the laser diode LD1 decreases. Thus, the driver circuit driver1 forms a negative feedback circuit, and the laser diode LD1 is lit with a constant light amount so that the output of the photodiode PD1 and the reference voltage Vcont1 are the same voltage. The other laser diodes LD2 to LD16 are similarly controlled in the APC mode.

  Subsequently, in the print mode, the controller 1027 sets the control signals cont1 to cont16 to Lo level. On the other hand, the controller 1027 controls the FIFO memory 1028 and outputs data signals data1 to data16. Since the control signal cont1 is at the Lo level, the analog switch SW1 is turned off. As a result, the voltage in the APC mode is held in the capacitor C1. Since the voltage of the capacitor C1 is applied to the control terminal of the constant current source CC1, the output of the constant current source CC1 has the same current value as in the APC mode.

  When the data signal data1 is at Hi level, the PNP transistor Q10 is turned on, so that the laser diode LD1 is lit. On the other hand, when the data signal data1 is at the Lo level, the PNP transistor Q10 is turned off, so that the laser diode LD1 is turned off. As a result, the laser diode LD1 is driven to blink according to the image data, and the photosensitive drum 15 is exposed.

  Note that the LD 100 is known to light up with a constant light amount at the same current value in the same environment, so that when the light is turned on, the laser diodes LD1 to LD16 have a constant light amount equivalent to the APC mode. Will light up.

  On the other hand, when the data signal data1 is at the Hi level, the PNP transistor Q11 is turned off. When the data signal data1 is at the Lo level, the PNP transistor Q11 is turned on. For this reason, the current from the constant current source CC1 is applied to the resistor RD1. As a result, the current supplied from the constant current source CC1 becomes constant without being affected by the situation of the data signal data1.

  Generally, it is difficult to operate the constant current source CC1 at a speed of several tens of MHz when the constant current source CC1 is driven at high speed, particularly when an image is formed. However, in the example shown in FIG. 3, the PNP transistors Q10 and Q11 require high-speed operation, but the constant current source CC1 does not require high-speed operation. For this reason, the use of the laser driver 1029 shown in FIG. 3 facilitates image formation. The other laser diodes LD2 to LD16 operate in the same manner as the laser diode LD1 in the print mode.

  FIG. 4 is a diagram showing the relationship between the sensitivity of the photosensitive drum according to the number of laser beams (the number of beams) and the light quantity of each laser beam. FIG. 4A is a diagram illustrating a relationship when the optical efficiency is low, and FIG. 4B is a diagram illustrating a relationship when the optical efficiency is high.

As shown in FIG. 4A, when the optical efficiency is low, that is, when the FFP (far field pattern) of the LD 100 is wide and the variation in transmittance and reflectance of the optical components is low, the amount of light per beam As (mW) increases (about 0.4 mW to about 1.0 mW), the sensitivity (μJ / cm ² ) of the photosensitive drum 15 also increases (about 0.23 to about 0.357). That is, the sensitivity of the photosensitive drum 15 varies from about 0.23 to about 0.35 in accordance with the change in the amount of light per beam.

  Further, assuming that the maximum light quantity rating of each laser beam is 1 mW and the minimum light quantity that can be used without image deterioration is 0.3 mW, in the case of 16 beams, the sensitivity variation (range) of the photosensitive drum 15 is As shown in FIG. 4 (A), the light quantity of each beam is within 0.4 mW to 1 mW. For this reason, it is understood that there is no problem if image formation is performed with 16 beams.

  As shown in FIG. 4B, when the optical efficiency is high, that is, when the FFP of the LD 100 is narrow and the variation in transmittance and reflectance of optical components is high, when 16 beams are used, If the sensitivity of the photosensitive drum is high, the light amount of each beam needs to be 0.3 mW or less. In this case, if exposure / scanning is performed with 10 beams in the same manner as in the case of 16 beams as a whole, the amount of light per beam increases, regardless of the sensitivity of the photosensitive drum 15. The amount of light per beam is in the range of 0.3 mW to 0.7 mW. For this reason, it can be seen that there is no problem in image formation even when the amount of light is low.

  Next, determination of the number of beams to be used will be described. FIG. 5 is a flowchart for explaining the determination of the number of beams in the optical scanning device shown in FIG.

  2, 4, and 5, the photosensitive drum 15 is subjected to multiple exposure with 16 beams to form a latent image on the photosensitive drum 15. The latent image is developed to form a density detection image patch on the photosensitive drum 15. Then, the density of the image patch is detected by the density sensor (density detection means) 1010 (step S501).

  Subsequently, the controller 1027 determines whether or not the detected density indicated by the density detection signal is within a predetermined appropriate density range (step S502). If the detected density is within the appropriate density range (YES in step S502), controller 1027 ends the process.

  On the other hand, if the detected density is not in the proper density range (NO in step S502), the controller 1027 calculates the laser light quantity (predetermined target light quantity) that falls within the proper density range (step S503). Then, the controller 1027 determines whether or not the laser light amount is within an appropriate light amount range (a range from 0.3 mW to 1 mW) (step S504).

  When the calculated laser light quantity is out of the appropriate light quantity range (predetermined light quantity threshold range) (NO in step S504), the controller 1027 switches the number of beams (in this case, the number of beams is set to 10, for example). : Step S505), the process is returned to Step S503.

  If the calculated laser light amount is within the appropriate light amount range (YES in step S504), the controller 1027 returns the process to step S501 and exposes the photosensitive drum 15 with the laser light amount to form an image patch. Then, the density of the image patch is detected.

  In the example shown in FIG. 4A, when 16 beams are used, the light amount of the beam does not deviate from the appropriate light amount range. Therefore, the controller 1027 uses 16 beams to calculate the laser light amount that falls within the appropriate light amount range. become. As described above, the density inspection image patch is formed again on the photosensitive drum 15 based on the calculated laser light quantity, and the density is detected. If the detected density is within the appropriate density range, the subsequent image formation is performed with the number of beams and the amount of light selected in the processing.

  On the other hand, in the example shown in FIG. 4B, when 16 beams are used, the calculated laser light quantity may not be within the appropriate light quantity range. In this case, the controller 1027 calculates the laser light amount again by switching the number of beams from 16 beams to 10 beams.

  Further, it is necessary to keep productivity constant even when the number of beams is switched. For this reason, as shown in the above-described equation (1), the controller 1027 switches the rotational speed RMS of the polygon motor in order to keep the productivity PS constant when the number of beams N is switched.

  FIG. 6 is a diagram showing the relationship between the number of beams N and the rotational speed RMS of the polygon motor.

  As shown in FIG. 6, when performing exposure using 16 beams, the rotational speed of the polygon motor is about 23000 RPM. On the other hand, when exposure is performed using 10 beams, the rotational speed of the polygon motor is about 36000 RPM. That is, when the number of laser beams (number of beams) is changed, the number of rotations of the polygon mirror is changed. At this time, when the number of beams is decreased, the number of rotations of the polygon mirror is increased.

  The controller 1027 forms an image patch on the photosensitive drum 15 again at the calculated laser light quantity, beam number, and polygon motor rotation number. The density sensor 1010 detects the density of the image patch on the photosensitive drum 15. If the detected density is within the appropriate density range, the controller 1027 uses the calculated beam light quantity, the number of beams, and the polygon motor rotation speed when performing image formation thereafter.

  Here, the flow of image data during image formation will be described.

  FIG. 7 is a diagram showing the flow of image data when the photosensitive drum is exposed by the optical scanning device shown in FIG. FIG. 7A is a diagram showing a flow of image data when exposure is performed with 16 beams, and FIG. 7B is a diagram showing a flow of image data when exposure is performed with 10 beams.

  In FIG. 7A, the numbers in the image data indicate the number of lines of the image data in the sub-scanning direction (photosensitive drum rotation direction). The image data is sequentially input from the first line to the FIFO memory 1028 shown in FIG. 2 and stored in the FIFO memory 1028. When the BD signal, which is a write signal, is input to the controller 1027, the controller 1027 outputs the image data from the first row to the 16th row as image data data1 to data16 from the FIFO memory 1028, respectively. As a result, 16 rows are exposed simultaneously.

  When the image data from the 17th line to the 32nd line is stored in the FIFO memory 1028, the next BD signal is input to the controller 1027. In the next scanning, the controller 1027 outputs the image data from the 17th line to the 32nd line as image data data1 to data16, respectively, and similarly exposes 16 lines simultaneously.

  In FIG. 7B, the numbers in the image data indicate the number of lines of image data in the sub-scanning direction. The image data is sequentially input from the first line to the FIFO memory 1028 shown in FIG. 2 and stored in the FIFO memory 1028.

  Here, since the rotation speed of the polygon motor is larger than that at the time of 16 beams, the BD signal is given to the controller 1027 when the image data of the 10th row is input to the FIFO memory 1028. When the BD signal, which is a write signal, is input to the controller 1027, the controller 1027 outputs the image data from the first row to the tenth row from the FIFO memory 1028 as image data data1 to data10, respectively. In this case, since the image data data11 to data16 are not output, 10 lines are exposed simultaneously.

  When the image data from the 11th line to the 20th line is input to the FIFO memory 1028, the BD signal is input to the controller 1027. In the next scan, the controller 1027 outputs the image data from the 11th to 20th lines from the FIFO memory 1028 as the image data data1 to data10, respectively, and similarly exposes 10 lines simultaneously. .

  In this way, exposure is performed with the light amount of each beam in the range of 0.3 mW to 0.7 mW, so it is possible to prevent adverse effects such as a decrease in responsiveness and an increase in spot diameter when the light amount is low.

  In the above-described embodiment, the rotational speed 36000 RPM is close to the rotational speed limit of the polygon motor. For this reason, in order to keep productivity constant, it is necessary to perform exposure with 10 to 16 beams.

  FIG. 8 is a diagram showing the relationship between the FFP of the LD 100 and the light amount of each LD element for obtaining the light amount required for the photosensitive drum shown in FIG.

  In FIG. 8, when the FFP of the LD 100 is large, that is, when the spread angle of the laser beam is large, the coupling efficiency of the optical system decreases. For this reason, the required light quantity in the chip | tip surface of each laser diode LD1-LD16 becomes large. Further, when the light spread angle is small, the coupling efficiency of the optical system is improved, so that the required light amount on the chip surface of each of the laser diodes LD1 to LD16 is small.

  As shown in FIG. 8, in the case of 16 beams, when the FFP is about 6.6 °, the amount of light on the chip surface of each of the laser diodes LD1 to LD16 is about 0.3 mW.

  Therefore, when assembling and adjusting the optical scanning device, the FFP of the LD 100 is measured in advance and stored in a memory (storage means: not shown) mounted on the optical scanning device. When the optical scanning device is mounted on the image forming apparatus and image formation is performed, the controller 1027 reads the FFP stored in the memory and predicts how much the chip surface light amount will be.

  For example, when FFP is about 6.6 ° or more during image formation, the controller 1027 sets the number of beams to 16 and sets the polygon motor rotation speed corresponding to the 16 beams. Thus, image formation can be performed with the light amount of each beam being 0.3 mW or more.

  If the FFP is smaller than about 6.6 °, the controller 1027 sets the number of beams to 10 and the polygon motor rotation speed corresponding to the 10 beams. Thus, image formation can be performed with the light amount of each beam being 0.3 mW or more.

  In the above-described example, the example in which the density of the toner image used for determining the laser light quantity is detected by the density sensor has been described. However, if the parameter for determining the laser light quantity is detected in addition to the toner image density, the same applies. Can be applied.

  For example, the potential (latent image potential) after exposure of the surface of the photosensitive drum (photosensitive member) is detected by a potential sensor (potential detection means), and the amount of laser light, the number of beams, and the number of rotations of the polygon motor according to the latent image potential May be determined.

  As described above, according to the present embodiment, the number of beams is determined according to the light amount of the laser beam, and the number of rotations of the polygon mirror is determined according to the number of laser beams. The light amount range can be narrowed, and exposure can be performed with constant laser beam characteristics. Furthermore, since the rotation speed of the polygon mirror is changed according to the number of beams, the influence on productivity can be prevented.

  If the above-described exposure control is not used, the light amount of the laser beam needs to be low. In this case, it is necessary to supply power to the laser element in an extremely short time in an accelerated manner when starting up the optical scanning device. This complicates the laser drive circuit and increases the cost.

  Furthermore, even if the above driving problems can be solved, the laser characteristics in the low light quantity region are unstable, and in any case, the image quality is degraded.

  As is apparent from the above description, in FIG. 2, the LD 100 and the laser driver 1029 function as a multi-beam generating unit. Further, the controller 1027, the polygon mirror 1002, and the polygon motor 1003 function as scanning means. The controller 1027 and the laser driver 1029 function as a light amount control unit, and the controller 1027 functions as a changing unit.

  As mentioned above, although this invention was demonstrated based on embodiment, this invention is not limited to these embodiment, Various forms of the range which does not deviate from the summary of this invention are also contained in this invention. .

  For example, the function of the above embodiment may be used as a control method, and this control method may be executed by a computer included in the image forming apparatus. In addition, a control program having the functions of the above-described embodiments may be executed by a computer provided in the image forming apparatus. The control program is recorded on a computer-readable recording medium, for example.

  The present invention can also be realized by executing the following processing. That is, software (program) for realizing the functions of the above-described embodiments is supplied to a system or apparatus via a network or various recording media, and a computer (or CPU, MPU, etc.) of the system or apparatus reads the program. It is a process to be executed.

15 Photosensitive drum 100 Laser diode (LD)
1002 Polygon mirror (Rotating polygon mirror)
1003 Polygon motor 1010 Concentration sensor 1017 BD sensor 1027 Controller 1028 FIFO memory 1029 Laser driver

Claims (5)

In an optical scanning device that exposes a photoreceptor with a plurality of light beams emitted from a plurality of light sources according to image data to form a latent image on the photoreceptor,
The plurality of light sources are arranged so as to form a plurality of scanning lines simultaneously in the sub-scanning direction of the photosensitive member by the plurality of light beams,
Scanning means for scanning the photosensitive member with the plurality of light beams;
A light amount control means for controlling the plurality of light sources and controlling the light amount of the light beam according to a predetermined target light amount;
When the light amount of the light beam controlled by the light amount control unit is out of a predetermined light amount threshold range, the number of light sources used for exposure is changed, and the rotating polygon mirror is changed according to the changed number of light sources. Changing means for changing the rotational speed,
When the number of the light sources is decreased, the changing unit increases the number of rotations of the rotary polygon mirror according to the number of the decreased light sources. A potential detecting means for detecting a potential of a latent image formed on the photoconductor as a latent image potential in response to the scanning of the light beam;
The optical scanning device according to claim 1, wherein the target light amount is determined according to the latent image potential. Density detecting means for detecting, as a toner density, a density of a toner image obtained as a result of developing a latent image formed on the photoconductor in accordance with the scanning of the light beam;
The optical scanning device according to claim 1, wherein the target light amount is determined according to the toner density. Each of the light sources is a semiconductor laser that generates the light beam;
Comprising storage means for storing a far field pattern of the semiconductor laser;
The optical scanning device according to claim 1, wherein the changing unit changes the number of the light beams in accordance with a spread angle of the light beam indicated by the far field pattern. . 5. The optical scanning device according to claim 1, a photosensitive member exposed by the optical scanning device, and a developing device that develops a latent image formed on the photosensitive member into a toner image. Have
An image forming apparatus, wherein the toner image is transferred to a recording medium to form an image.

Images