JP2018189799A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably detect a BD signal by changing the inclination of a detection signal at a point where a detection signal of the BD sensor and a threshold intersect with each other according to the amount of laser light when the amount of light of a laser input to a BD sensor fluctuates. It may not be possible. SOLUTION: When a light irradiating means is subjected to microexposure, a control means irradiates a first light emitting element with a laser beam with a first light amount, and causes a second light emitting element with a second light amount larger than the first light amount. The laser beam is irradiated, and the detection means detects the laser beam irradiated with the first light amount from the first light emitting element and outputs a horizontal synchronization signal. [Selection diagram] FIG. 11

Description

  The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine, and a facsimile using mainly an electrophotographic process.

  In an image forming apparatus of a laser beam printer, in order to form a stable image even if a change in film thickness of the photosensitive drum or a change in environmental temperature occurs, the amount of laser light when scanning the photosensitive drum is set to the film thickness of the photosensitive drum. A method of correcting according to a change or a change in environmental temperature is widely performed.

  In recent years, the life of a photosensitive drum has been extended, and the amount of change in film thickness of the surface layer of the photosensitive drum accompanying deterioration in durability has increased. In order to correct the necessary laser light amount for exposing the photosensitive drum in accordance with the change in the film thickness of the photosensitive drum, the range of use of the laser light amount of the laser scanner unit mounted on the laser beam printer is widened. Due to the wide use range of the laser light quantity, a horizontal synchronization detection sensor (hereinafter also referred to as BD (Beam Detect) sensor) that detects the synchronization timing when scanning the photosensitive drum also detects laser light in a wide light quantity range Performance that can be done is necessary. For example, the BD sensor detects a BD signal by switching the logic of the output signal based on the timing at which the detection signal corresponding to the amount of laser light incident on the sensor exceeds a predetermined threshold as in Patent Document 1. It is something that can be done.

JP, 2006-109734, A

  When a BD signal is detected using a conventional BD sensor, if the light quantity of the laser is corrected in accordance with a change in the film thickness of the photosensitive drum or a change in the environmental temperature, it is input to the BD sensor as shown in FIG. The amount of laser light also varies. As a result, the inclination of the detection signal at the point where the detection signal of the BD sensor and the threshold intersect changes according to the amount of laser light, and there is a problem that the BD signal may not be detected stably. .

  The invention according to the present application has been made in view of the above situation, and an object thereof is to stably detect a BD signal.

  In order to achieve the above object, a photosensitive member and a plurality of light emitting elements that emit laser light, and a light irradiation unit that irradiates the photosensitive member with laser light, and a laser beam emitted from the light irradiation unit are provided. A detecting means for detecting and outputting a horizontal synchronizing signal; a laser beam is irradiated to the image area of the photoconductor by normal exposure; and a non-image area of the photoconductor is microexposure having a light amount smaller than the normal exposure. Control means for controlling the light irradiating means so as to irradiate laser light, and the control means causes the first light emitting element to emit a first light amount when the light irradiating means performs the minute exposure. Laser light is irradiated so that the second light emitting element is irradiated with laser light with a second light amount greater than the first light amount, and the detecting means is a laser light irradiated with the first light amount from the first light emitting element. Detect the horizontal And outputting a period signal.

  According to the configuration of the present invention, a BD signal can be detected stably.

Schematic configuration diagram of an image forming apparatus Diagram showing transition of potential of photosensitive drum Figure showing the output image Diagram showing transition of potential of photosensitive drum Figure showing the output image Top view of optical device Schematic diagram of BD sensor BD sensor input / output waveform Timing chart for laser light quantity control The figure which showed the characteristic of the laser light quantity according to the usage-amount of the photosensitive drum Diagram showing usage of photosensitive drum and light quantity control for normal exposure and micro exposure of multi-beam laser Signal waveforms detected by the BD sensor at the beginning and end of use of the photosensitive drum Diagram showing usage of photosensitive drum and light quantity control for normal exposure and micro exposure of multi-beam laser Signal waveforms detected by the BD sensor at the beginning and end of use of the photosensitive drum The figure which shows the input waveform of the conventional BD sensor

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the invention according to the scope of claims, and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention.

(First embodiment)
[Image forming apparatus]
FIG. 1 shows a schematic configuration diagram of an image forming apparatus according to the present embodiment. A color laser beam printer as an image forming apparatus corresponds to each of four colors in order to form a color image by superimposing images of four colors (Y: yellow, M: magenta, C: cyan, Bk: black). An image forming unit is provided.

  When the image data 203 is input from the host computer 202, the color laser beam printer 201 develops the image data by the video controller 204 in the color laser beam printer 201 and converts it into image data for image formation. Then, the video controller 204 generates a video signal 205 that is video signal format data for controlling light emission of the laser diode based on the image data. The engine controller 206 as control means has arithmetic processing means such as a CPU 209. The video signal 205 generated by the video controller 204 is transmitted from the video controller 204 to the engine controller 206.

  Then, laser diodes 211y, 211m, 211c, and 211k as a plurality of light emitting elements are driven in accordance with video signals by laser control ICs 226a and 226b disposed on a laser control substrate 255 in an optical device 210 as a light irradiation unit. To do. Laser beams 212y, 212m, 212c, and 212k emitted from the laser diodes are reflected by the reflecting surface of a polygon mirror 703 that is a rotating polygon mirror. Then, the light passes through lenses 213y, 213m, 213c, and 213k and is deflected by folding mirrors 214y, 214m, 214c, and 214k. The changed laser beams are irradiated onto photosensitive drums 215y, 215m, 215c, and 215k as photosensitive members arranged in the process cartridges 225y, 225m, 225c, and 225k, respectively.

  The photosensitive drums 215y, 215m, 215c, and 215k are charged to a desired potential by the charging units 216y, 216m, 216c, and 216k. An electrostatic latent image is formed on the surface of the photosensitive drum by irradiating the charged photosensitive drum with a laser beam to partially lower the surface potential. The formed electrostatic latent image is developed by developing means 217y, 217m, 217c, and 217k, whereby a toner image corresponding to the electrostatic latent image is formed on the photosensitive drum.

  The toner image formed on the photosensitive drum is primarily transferred onto an endless intermediate transfer belt 219 that is an intermediate transfer member by applying a primary transfer bias to the primary transfer members 218y, 218m, 218c, and 218k. . First, a yellow toner image is primarily transferred onto an intermediate transfer belt, and magenta, cyan, and black toner images are sequentially transferred thereon to form a color image. The intermediate transfer belt is driven by an intermediate transfer belt driving roller 256.

  The recording material 221, which is paper stacked in the paper feed cassette 220, is fed by the paper feed roller 222. Then, the toner image is conveyed to the secondary transfer unit 223 so as to be synchronized with the color image primarily transferred onto the intermediate transfer belt. Then, by applying a secondary transfer bias to the secondary transfer roller, a color image is secondarily transferred onto the recording material. The recording material 221 onto which the color image has been secondarily transferred is thermally fixed by heat and pressure in the fixing device 224, and the color image is fixed on the recording material. Then, the paper is discharged from a paper discharge unit to a paper discharge tray. The above is a schematic description of the image forming apparatus. Next, the development process of the image forming apparatus will be described, and then the configuration of the optical apparatus and laser emission control will be described in detail.

[Development process]
When multicolor printing is performed by the color image forming apparatus, the toner image primarily transferred onto the intermediate transfer belt at the upstream station is conveyed to a primary transfer nip portion including a photosensitive drum and an intermediate transfer belt at the downstream station. At this time, a portion where the toner image of the upstream station is formed and a portion where the toner image is not formed are generated on the intermediate transfer belt. When the toner image formed at the upstream station is transported to the primary transfer nip portion of the downstream station, the portion where the toner image is formed and the portion where the toner image is not formed in the primary transfer nip portion A difference occurs in the resistance value between the transfer belts.

  As a result, the transfer current flowing from the intermediate transfer belt to the photosensitive drum is different between the toner image forming portion and the non-toner image forming portion. Due to the difference in the transfer current amount, as shown in FIG. Unevenness ΔV1 remains in the surface potential of the drum (FIG. 2A). When charging is performed again with this surface potential unevenness, the surface potential unevenness cannot be sufficiently filled, and the drum surface potential after charging remains a portion where ΔV2 is higher than VD (FIG. 2B). When an electrostatic latent image is formed with a laser in this state, the surface potential unevenness on the photosensitive drum is not the portion where the surface potential is VL after exposure with the laser, but the portion where the potential is locally higher than VL. It remains (FIG. 2 (c)).

  If development with toner is performed with the surface potential unevenness remaining on the photosensitive drum, unevenness occurs in the density of the toner image after development (FIG. 2D). When the toner image is transferred (FIG. 2E), a locally light toner image is formed as shown in FIG. Here, 401 is an output image, and 402 is an image (for example, a yellow solid pattern) formed at an upstream station. Reference numeral 403 denotes an image formed at the downstream station (for example, a black halftone image), and reference numeral 404 denotes an image density unevenness portion generated in the image formed at the downstream station. That is, if charging is performed again with unevenness in the surface potential of the photosensitive drum at the downstream station, this surface potential unevenness cannot be sufficiently eliminated, and the amount of toner adhering to the surface of the photosensitive drum in the next development process is reduced. Unlikely, image density unevenness occurs.

[Background exposure]
Next, a method for solving the image density unevenness will be described. As shown in FIG. 4, in order to eliminate the unevenness of the surface potential after transfer (FIG. 4A), the charging bias setting value is set higher by ΔV3 than the conventional bias setting value VD, and VD ′ = -850V. Thereby, the surface potential unevenness of the photosensitive drum after the transfer can be reduced to ΔV4 (ΔV4 <ΔV2) in the charging process (FIG. 5B). However, by increasing the output value of the charging bias after transfer to VD ′ = − 850 V, the surface potential of the photosensitive drum after charging becomes higher by ΔV3 than the surface potential VD of the conventional photosensitive drum. Therefore, in order to form an image with a desired density, it is necessary to cancel the surface potential ΔV3 in the exposure process.

  That is, in order to cancel the increase ΔV3 in the surface potential of the photosensitive drum, the light emission amount of the beam that exposes the photosensitive drum is set to L1 in the image area (toner image forming area). Further, in the non-image area (non-toner image forming area), in order to cancel ΔV3, the light emitting element emits light with an amount of light emission L2 (L1> L> L2) that does not cause excessive toner adhesion (hereinafter, referred to as “light exposure”). , Also referred to as background exposure). By performing background exposure, the surface potential of the non-image area after exposure can be set to VD, and the surface potential of the image forming area can be set to VL. Thereby, the surface potential difference ΔV3 caused by increasing the surface potential of the photosensitive drum to VD ′ = − 850 V by charging can be canceled (FIG. 4C). Note that the light emission amount L2 that does not cause toner adhesion is a light emission amount in the laser emission region that is larger than the laser emission threshold. If it falls below the threshold value of laser emission, there is a possibility that light cannot be emitted stably.

  If development with toner is performed in a state where the surface potential unevenness on the photosensitive drum is reduced, unevenness in the density of the toner image after development can be suppressed (FIG. 4D). Here, the laser emission for setting the surface potential of the photosensitive drum to the surface potential VD in the non-image area is fine exposure (weak emission), and the laser emission for setting the surface potential VL in the image area is normal exposure (strong exposure). Also called light emission. In addition, the amount of laser light required at that time is a weak light emission amount L2 and a strong light emission amount L1, respectively. When the toner image subjected to background exposure is transferred (FIG. 4E), a toner image having a uniform density is formed as shown in FIG. The image formed at the downstream station has a uniform density in the areas 603 and 605, and a toner image in which density unevenness is suppressed can be formed.

  In the case of performing fine exposure, the drive current Ib for fine exposure may be supplied to the laser diode, and in the case of performing normal exposure, the drive current Idrv + Ib for normal exposure may be supplied to the laser diode. By adjusting the current amounts of the drive current Ib and the drive current Idrv + Ib, it is possible to control the light emission amount of fine exposure and the light emission amount of normal exposure.

[Optical device]
FIG. 6 shows a top view of the optical device 210. Since the optical device 210 has the same configuration for each color, the yellow station is illustrated and described for convenience of explanation.

  The laser diode 211y disposed on the laser control substrate 255 is a twin beam laser diode (hereinafter also referred to as a laser diode) having two light emitting elements that can emit light independently in one package. The laser beams 701a and 701b emitted from the laser diode 211y are shaped into a parallel beam by the collimator lens 702 and shaped into a parallel beam. Then, it is reflected by the reflecting surface of the rotating polygon mirror 703 and deflected and scanned. The laser beams 701a and 701b subjected to the deflection scanning pass through the lens 213y, and are then folded back by the folding mirror 214y to scan the photosensitive drum 215y. The laser light imaged on the surface of the photosensitive drum 215y becomes a dot-like spot and moves from the main scanning direction S indicated by the arrow in the figure to S ′ (parallel to the rotational axis direction of the photosensitive drum 215y). A scanning line is formed and the photosensitive drum 215y is exposed.

  On the other hand, a lens 704 is provided at a position where laser light is incident at a position upstream of the scanning start position S of the scanning line on the photosensitive drum 215y in the main scanning direction. The laser light incident on this lens is collected. The light is incident on a BD sensor 705 serving as a detection unit. This BD (Beam Detect) sensor 705 detects that a laser beam has entered, and outputs a BD signal that is a horizontal synchronization signal corresponding to the laser beam. Based on the BD signal from the BD sensor 705, the start timing of the laser beam scanning on the photosensitive drum 215y is determined. Further, after scanning the photosensitive drum 215y and before scanning the photosensitive drum 215y again, APC (Auto Power Control) which is automatic light amount control of the laser light amount is performed by the laser control IC. Then, the amount of light emitted from the laser diode 211y is adjusted for the next scan.

[Configuration of BD sensor]
FIG. 7 is a schematic configuration diagram of the BD sensor 705. When the light receiving unit 801 of the BD sensor 705 is irradiated with laser light, a current I flows through the photodiode 802. A current I flowing through the photodiode 802 is amplified α times by a current amplifier 803 and converted into a voltage by a threshold setting resistor 804. The voltage Vin calculated by the following equation (1) is input to the inverting input terminal of the comparator 805, and is compared with the voltage Vref input to the non-inverting input terminal.

Vin = I × α × threshold setting resistor 804 (1)
When the voltage Vin at the inverting input terminal of the comparator 805 is equal to or lower than Vref, the output at the BD signal output terminal maintains the H level, and when Vin becomes higher than Vref, the output is inverted and becomes the L level. As described above, when the laser beam is incident on the BD sensor 705, the BD signal output is inverted from the H level to the L level. That is, the timing at which the signal at the BD signal output terminal is inverted from the H level to the L level can be recognized as the timing at which the laser beam passes through the BD sensor 705. Note that the threshold when the BD sensor 705 detects the laser light is determined by the relationship between the expression (1) and Vref as described above. Therefore, when the constant of the threshold setting resistor 804 is increased, the threshold is set low, and when the constant of the threshold setting resistor 804 is decreased, the threshold is set high.

  As shown in FIG. 8, the threshold value 903, which is a threshold voltage for detecting the BD signal, may be set in the vicinity of a voltage half the peak voltage 902 of the voltage value 901 obtained by performing IV conversion on the amount of light incident on the BD sensor. desirable. By setting the threshold value 903 in the vicinity of a voltage half the peak voltage 902, the input waveform of the BD sensor 705 can be compared with the threshold value at a steep timing, depending on the result of the comparison with the threshold value. The BD signal 904 can be output. By setting the threshold value 903 in this way, it is possible to suppress the jitter of the BD signal, which is a variation in detection of the BD signal that is repeatedly detected each time the laser beam is scanned, and to suppress the deterioration of the image quality.

[Control of laser light quantity (APC)]
FIG. 9 is a timing chart regarding control of the laser light quantity. A section 1001 is a 1BD period, and is a period in which the polygon mirror rotates by a quarter in a scanner unit using a four-sided polygon mirror. Based on the timing of the BD signal 1009 output from the BD sensor 705, the rotation of the polygon motor is performed so that the polygon mirror has a desired rotational speed, that is, the scanning speed of the laser beam on the photosensitive drum becomes a desired speed. Control the speed.

  In the image area 1002, APC is performed to adjust the light amount of the microexposure and the normal exposure in order to perform the microexposure in the non-image area and the normal exposure in the image area. APC is performed at the timing when the laser beam scans the non-image area 1003. The APC of the normal exposure amount by the first laser in the multi-beam laser is performed in the section 1004, and the APC of the normal exposure amount by the second laser is performed in the section 1005. Further, the APC of the minute exposure amount by the first laser is performed in the section 1006, and the APC of the minute exposure amount by the second laser is performed in the section 1007.

  Here, as an example, after the BD signal is detected, the first laser microexposure APC, the second laser microexposure APC, the second laser normal exposure APC, and the second laser normal exposure APC are in this order. However, it is not limited to this. Ordinary exposure APC may be performed first after detecting the BD signal. A section 1008 is a section in which laser light is emitted in order to detect a BD signal. Details of the laser light amount control method for detecting the BD signal will be described later.

[Control of laser light intensity according to changes in photosensitive drum]
Even if the ambient temperature and humidity of the image forming device changes or the amount of process cartridge usage changes, such as the change in film thickness of the photosensitive drum, the laser is used to keep the toner development amount on the photosensitive drum constant. Control the amount of light. That is, control is performed to change the target light quantity for the above-described microexposure and normal exposure according to the environmental temperature and humidity of the image forming apparatus and the amount of process cartridge used. In the following, the control for changing the target light amount according to the change in the film thickness of the photosensitive drum (the amount of use of the photosensitive drum) will be described.

  FIG. 10 is a diagram showing the characteristics of the laser light amount according to the usage amount of the photosensitive drum. Reference numeral 1101 denotes the relationship between the usage amount of the photosensitive drum and the light amount of fine exposure, and 1102 denotes the relationship between the usage amount of the photosensitive drum and the light amount of normal exposure. Depending on the usage amount of the photosensitive drum, the optimum light amount changes as shown in FIG.

  For example, let A be the maximum amount of light for normal exposure and B be the maximum amount of light for minute exposure. When a new process cartridge 225 in the initial state is mounted, the target light amount for normal exposure is set to about 50% (A × 0.5) of the maximum light amount. Thereafter, as the usage amount of the photosensitive drum increases and the film thickness decreases, the drum sensitivity with respect to the laser light amount becomes dull. Therefore, the amount of light is gradually increased as the usage amount increases until the usage amount of the photosensitive drum reaches the final state. When the usage amount reaches the final state, the target light amount for normal exposure is set to the maximum light amount A.

  Similarly, when a new process cartridge 225 in the initial state is mounted, the target light amount for fine exposure is set to about 50% (B × 0.5) of the maximum light amount. Thereafter, the amount of light is gradually increased as the amount of use increases until the amount of use of the photosensitive drum reaches a terminal state. Then, when the usage amount reaches the final state, the target light amount for minute exposure is set to the maximum light amount B. As described above, the photosensitive drum 215 can be exposed to a desired potential by appropriately changing the amount of laser light in accordance with a change in the usage amount such as wear of the photosensitive drum 215. Therefore, a decrease in image density due to a change in the usage amount. Can be suppressed.

[Control of laser light quantity in this embodiment]
FIG. 11 is a diagram showing the usage amount of the photosensitive drum and the light amount control of the normal exposure and the microexposure of the multi-beam laser in the present embodiment. Reference numeral 101 denotes a laser light quantity characteristic for normal exposure of the first laser, and reference numeral 102 denotes a laser light quantity characteristic for normal exposure of the second laser. The amount of light for normal exposure is the same for both the first laser and the second laser in accordance with the amount of use of the photosensitive drum in order to form an image with an appropriate density even if the amount of use of the photosensitive drum changes. Control. In this manner, by adjusting the amount of light for normal exposure in the same manner for each of the multi-beam lasers, it is possible to perform image formation while suppressing fluctuations in image density.

  Reference numeral 103 denotes a laser light quantity characteristic for the fine exposure of the first laser, and reference numeral 104 denotes a laser light quantity characteristic for the fine exposure of the second laser. Reference numeral 105 denotes an average light amount of the fine exposure of the first laser and the second laser, which corresponds to the light amount characteristic as a result of performing the fine exposure by the first laser and the second laser on the photosensitive drum. The purpose of performing the micro exposure on the photosensitive drum is to suppress deterioration of image quality due to image fogging or the like by making the surface potential of the photosensitive drum appropriate in the non-image area as described above. Accordingly, if the average light amount 105 per unit area of the light amount of the micro exposure by the first laser and the second laser is set so as to satisfy the following conditions, the light amount of the micro exposure of the first laser and the micro exposure of the second laser Can be changed to any ratio. That is, the average light amount 105 satisfies the exposure amount 1101 for making the potential of the non-image area of the photosensitive drum appropriate, and the exposure unevenness due to the first laser and the second laser on the surface of the photosensitive drum is within an allowable range. To.

  In the present embodiment, the change amount 103 of the light exposure amount of the first laser is smaller than the change amount 104 of the light exposure amount of the second laser that changes as the usage amount of the photosensitive drum increases. To control. And the average light quantity 105 of a 1st laser and a 2nd laser is made to satisfy | fill the above-mentioned conditions. The reason for setting the light amount of the minute exposure of the first laser and the second laser in this way will be described later. Note that the amount of change in the light amount of the micro exposure may be set so that the first laser is larger. That is, it is only necessary to control the amount of change of minute exposure of any laser to be equal to or less than a predetermined amount of change and the average light quantity 105 satisfies the above-described conditions. In other words, it is possible to control a laser beam that is selected from a plurality of lasers so that the amount of change in the amount of light for microexposure is less than a predetermined amount of change, and which is incident on the BD sensor.

[Control of laser light quantity input to BD sensor]
In the configuration as described above, the amount of laser light input to the BD sensor in order to detect the BD signal is a minute exposure of the first laser as shown in a section 1008 in FIG. In the present embodiment, the amount of change in the amount of light for the micro exposure of the first laser is adjusted to be smaller than the amount of change in the amount of light for the micro exposure of the second laser. If the amount of change in the light amount of the second laser microexposure is smaller, the BD signal may be detected using the second laser microexposure.

  FIG. 12 shows signal waveforms detected by the BD sensor at the beginning and end of use of the photosensitive drum. The threshold value 903 is a fixed value for detecting the BD signal. A waveform 1201 is a waveform in the case where the photosensitive drum is in an initial state and the first laser is finely exposed and a beam is incident on the BD sensor. As the amount of use of the photosensitive drum increases, the amount of light of the first laser minute exposure is gradually increased, so that the waveform detected by the BD sensor changes from the initial waveform 1201 to the final waveform 1202. In addition, the slope of the waveform at the point where each waveform intersects the threshold for detecting the BD signal is an initial slope 1203 and a final slope 1204.

  A BD signal is detected based on the intersecting point. In the present embodiment, the amount of change in the light amount of the micro exposure of the first laser is set to be smaller than a predetermined amount of change according to the detection accuracy of the BD signal. Therefore, the difference between the initial inclination 1203 and the final inclination 1204 also decreases from the initial use to the final use of the photosensitive drum. Thereby, the jitter at the time of detecting a BD signal can be suppressed. Thereby, the stability of the rotation control of the polygon motor can be improved.

  On the other hand, a waveform 1205 is also shown for comparison. A waveform 1205 is a final waveform when the light amount of the minute exposure of the first laser incident on the BD sensor in order to detect the BD signal is changed as the average light amount 105 in FIG. In other words, in both the first laser microexposure and the second laser microexposure, the amount of light is gradually increased as the usage amount of the photosensitive drum increases. The slope of the waveform at a point that intersects the threshold for detecting the BD signal is a slope 1206. Since the amount of change in the light exposure of the first laser in the comparative example is larger than the amount of change in the light exposure of the first laser in the present embodiment described above, the variation in the tilt is larger in the comparative example. ing. Therefore, it can be seen that the present embodiment with less variation in tilt can suppress the jitter when detecting the BD signal. In addition, since the inclination 1204 of this embodiment is steeper than the inclination 1206 of the comparative example, the BD signal can be detected stably.

  In the present embodiment, a method of stably detecting a BD signal has been described in a multi-beam laser, particularly a twin beam laser having two light emitting elements. However, the configuration is not limited to the twin beam laser, and a beam laser having more light emitting elements may be used. For example, the same control method can be applied to a quad beam laser having four light emitting elements. More specifically, among the four lasers of the quad beam laser, the amount of change in the amount of minute exposure of one laser is made smaller than that of the other lasers and is incident on the BD sensor. As a result, even in the quad beam laser, it is possible to reduce the jitter in detecting the BD signal.

  As described above, in order to detect the BD signal, the first laser having a small amount of change in the light amount of the micro-exposure of the multi-beam laser is caused to perform the micro exposure so that the beam enters the BD sensor. By making the light of the first laser with a small amount of change in the amount of light for microexposure incident, for example, even if a change in the amount of laser light occurs according to the usage amount of the photosensitive drum, the BD signal can be detected stably. Can do. Further, even if the amount of change in the light amount of the micro exposure of the first laser is reduced, it is compensated by the amount of change of the light amount of the micro exposure of the second laser. Specifically, the average light amount 105 satisfies the exposure amount 1101 for making the potential of the non-image area of the photosensitive drum appropriate, and the exposure unevenness by the first laser and the second laser on the surface of the photosensitive drum is within the allowable range. To be. This makes it possible to perform background exposure appropriately while stabilizing the detection of the BD signal.

  For example, in order to suppress jitter when detecting a BD signal, it is conceivable to make the threshold variable. Making the threshold variable is a method in which the peak value of the incident laser is detected and a value of a predetermined ratio of the peak value is set as the threshold. Since the threshold value is set as an optimum value according to the incident laser, it is possible to detect a laser having a wide light amount range. However, since an arithmetic circuit that detects a peak value and determines a threshold value inside the BD sensor is required, the component cost tends to increase. In this respect, the BD signal detection method according to the present embodiment does not require an arithmetic circuit for determining the threshold value, and has an effect that the BD signal can be stably detected with an inexpensive configuration. As described above, in the present embodiment, even when an inexpensive BD sensor is used, the influence of jitter when detecting a BD signal can be reduced.

(Second Embodiment)
In the first embodiment described above, the method for reducing the amount of change in the light amount of the fine exposure incident on the BD sensor in order to detect the BD signal has been described. In the present embodiment, a method for further suppressing jitter in detecting a BD signal will be described. Note that detailed description of the configuration of the image forming apparatus, the configuration of the scanner unit, and the like that are the same as those of the first embodiment is omitted here.

[Control of laser light quantity in this embodiment]
FIG. 13 is a diagram showing the usage amount of the photosensitive drum and the light amount control of the normal exposure and the microexposure of the multi-beam laser in the present embodiment. Reference numeral 1301 denotes a laser light quantity characteristic of the first laser for normal exposure, and 1302 denotes a laser light quantity characteristic of the second laser for normal exposure. The amount of light for normal exposure is the same for both the first laser and the second laser in accordance with the amount of use of the photosensitive drum in order to form an image with an appropriate density even if the amount of use of the photosensitive drum changes. Control. In this manner, by adjusting the amount of light for normal exposure in the same manner for each of the multi-beam lasers, it is possible to perform image formation while suppressing fluctuations in image density.

  Reference numeral 1303 denotes a laser light quantity characteristic of the first laser for fine exposure, and 1304 denotes a laser light quantity characteristic of the second laser for fine exposure. Reference numeral 1305 denotes an average light amount of the first laser and the second laser for microexposure, which corresponds to the light amount characteristic as a result of performing the microexposure by the first laser and the second laser on the photosensitive drum.

  In the present embodiment, the light amount of the minute exposure of the first laser is constant regardless of the amount of use of the photosensitive drum. The amount of minute exposure light of the second laser is variable, and the average amount of light 1305 per unit area of the amount of minute exposure light by the first laser and the second laser satisfies the following condition. That is, the average light amount 1305 satisfies the exposure amount 1101 for making the potential of the non-image area of the photosensitive drum appropriate, and the exposure unevenness due to the first laser and the second laser on the surface of the photosensitive drum is within an allowable range. To.

[Control of laser light quantity input to BD sensor]
In the configuration as described above, the amount of laser light input to the BD sensor in order to detect the BD signal is a minute exposure of the first laser as shown in a section 1008 in FIG. In the present embodiment, the amount of light for the minute exposure of the first laser is constant regardless of the amount of use of the photosensitive drum. Note that the amount of fine exposure of the second laser may be constant and the amount of fine exposure of the first laser may be variable.

  FIG. 14 shows signal waveforms detected by the BD sensor at the beginning and end of use of the photosensitive drum. The threshold value 903 is a fixed value for detecting the BD signal. A waveform 1401 is a waveform in the case where the photosensitive drum is in an initial state and the first laser is finely exposed and a beam is incident on the BD sensor. Even if the usage amount of the photosensitive drum increases, the light amount of the minute exposure of the first laser is constant, so the waveform detected by the BD sensor does not change between the initial waveform 1401 indicated by a solid line and the final waveform 1402 indicated by a broken line. In addition, the slope of the waveform at the point where each waveform intersects the threshold for detecting the BD signal is the same slope 1403 at the initial slope and the final slope.

  A BD signal is detected based on the intersecting point. In the present embodiment, since the amount of light of the micro exposure of the first laser is constant, the inclination 1403 is constant from the beginning to the end of use of the photosensitive drum. Therefore, the timing for detecting the BD signal is further stabilized as compared with the first embodiment, and jitter at the time of detecting the BD signal can be suppressed. Thereby, the stability of the rotation control of the polygon motor can be improved.

  In the present embodiment, a method of stably detecting a BD signal has been described in a multi-beam laser, particularly a twin beam laser having two light emitting elements. However, the configuration is not limited to the twin beam laser, and a beam laser having more light emitting elements may be used. For example, the same control method can be applied to a quad beam laser having four light emitting elements. More specifically, among the four lasers of the quad beam laser, the amount of fine exposure of one laser is made constant and enters the BD sensor. As a result, even in the quad beam laser, it is possible to reduce the jitter in detecting the BD signal.

  As described above, in order to detect the BD signal, the beam is incident on the BD sensor by causing the first laser having a constant amount of microexposure of the multi-beam laser to perform microexposure. Even if the amount of use of the photosensitive drum is changed, the amount of light of minute exposure of the first laser is constant, so that the detection of the BD signal is stable from the beginning to the end of use of the photosensitive drum, and jitter can be suppressed. Further, since the threshold value is fixed, there is no need for an arithmetic circuit for determining the threshold value, and there is an effect that the BD signal can be stably detected with an inexpensive configuration.

206 Engine Controller 210 Optical Device 211 Laser Diode 215 Photoconductor 705 BD Sensor

Claims (10)

A photoreceptor,
A plurality of light emitting elements for emitting laser light, and a light irradiation means for irradiating the photosensitive member with laser light;
Detecting means for detecting the laser light emitted from the light irradiation means and outputting a horizontal synchronization signal;
The light irradiation unit is configured to irradiate the image area of the photoconductor with a laser beam at a normal exposure, and to irradiate the non-image area of the photoconductor with a laser beam having a light exposure smaller than the normal exposure. Control means for controlling,
The control means causes the first light emitting element to emit laser light with a first light quantity and causes the second light emitting element to emit laser light with a second light quantity greater than the first light quantity when the light irradiating means performs the minute exposure. Let it shine,
The image forming apparatus according to claim 1, wherein the detection unit detects the laser light emitted from the first light emitting element with the first light amount and outputs the horizontal synchronization signal.   The image forming apparatus according to claim 1, wherein the control unit performs control so as to increase a light amount of the microexposure in accordance with an increase in a usage amount of the photoconductor.   In the case where the light amount of the minute exposure is increased in accordance with an increase in the usage amount of the photoconductor, the control means determines the change amount of the light amount of the second light emitting element rather than the change amount of the light amount of the first light emitting element. The image forming apparatus according to claim 2, wherein the image forming apparatus is controlled so as to increase.   The control means increases the light amount of the second light emitting element while keeping the light amount of the first light emitting element constant when increasing the light amount of the minute exposure according to an increase in the usage amount of the photoconductor. The image forming apparatus according to claim 2.   5. The control unit according to claim 1, wherein the control unit controls the second light amount so that an average light amount of the first light amount and the second light amount becomes a predetermined light amount. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the control unit increases the amount of light for the normal exposure in accordance with an increase in a usage amount of the photoconductor. A photoreceptor,
A plurality of light emitting elements for emitting laser light, and a light irradiation means for irradiating the photosensitive member with laser light;
Detecting means for detecting the laser light emitted from the light irradiation means and outputting a horizontal synchronization signal;
The light irradiation unit is configured to irradiate the image area of the photoconductor with a laser beam at a normal exposure, and to irradiate the non-image area of the photoconductor with a laser beam having a light exposure smaller than the normal exposure. Control means for controlling,
The control means changes the light amount of the first light emitting element by the first change amount, changes the light amount of the second light emitting element by the second change amount,
The detection means detects the laser light that has been finely exposed from a light emitting element with a small amount of change among the first light emitting element and the second light emitting element, and outputs the horizontal synchronization signal. Forming equipment.   The image forming apparatus according to claim 7, wherein the control unit controls the light amount of the microexposure to increase in accordance with an increase in usage amount of the photoconductor.   When the light amount of the minute exposure is increased in accordance with an increase in the amount of use of the photoconductor, the control unit makes the light amount of the light emitting element with a small amount of change constant and increases the light amount of the light emitting element with a large amount of change. The image forming apparatus according to claim 8, wherein: 10. The image forming apparatus according to claim 7, wherein the control unit increases the amount of light for the normal exposure in accordance with an increase in a usage amount of the photoconductor.

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