Exposure device and image forming apparatus

Abstract

To provide an image forming apparatus that comprises an exposure head, and that, even when it forms images in a mode where the rotation speed of photoreceptor drums is slow, can prevent the photoreceptor drums from carrying an excessive amount of toner.SOLUTION: In an image forming apparatus, a CPU 73 can execute a normal mode, and a low-speed mode for rotating photoreceptor drums 1 at a speed that is slower than the rotation speed in the normal mode and that is m / n times the rotation speed in the normal mode, and in the low-speed mode, controls a synchronization signal generation part 74 to generate a line synchronization signal at a period that is 1 / m times the period of a line synchronization signal generated in the normal mode, and controls a chip data conversion unit 72 to transmit the same image data signals m times to an exposure head 6 in synchronization respectively with n line synchronization signals and transmit n-m times image data signals that do not cause light emitting units 50 to emit light.SELECTED DRAWING: Figure 16

Description

Technical Field

[0001] The present invention relates to an image forming apparatus including an exposure head.

Background Art

[0002] When forming an image with an electrophotographic image forming apparatus, first, an electrostatic latent image is formed on the surface of a photosensitive drum by irradiating light corresponding to image data onto the surface of the photosensitive drum. Then, toner is attached to the electrostatic latent image on the surface of the photosensitive drum by a developing unit to form a toner image, the toner image is transferred to a sheet, and the toner image transferred to the sheet is heated by a fixing unit and fixed to the sheet to form an image.

[0003] When using thick paper as the sheet on which the image is formed, since the heat capacity of the sheet is larger than that of a sheet with a relatively small basis weight such as plain paper, the amount of heat required to fix the toner image to the sheet increases. Therefore, the conveyance speed of the sheet is set lower than normal, and the time for heating the toner image carried on the sheet by the fixing unit is lengthened, so that more heat is applied to the toner image and the sheet, and a configuration for stably fixing the toner image even to thick paper is known. That is, the image forming apparatus of this configuration has a normal mode in which the sheet is conveyed at a normal speed and a low-speed mode in which the sheet is conveyed at a speed lower than the normal mode. Since the rotation speed of the photosensitive drum is related to the conveyance speed of the sheet, when the low-speed mode is executed, the rotation speed of the photosensitive drum becomes slower than when the normal mode is executed.

[0004] Also, in Patent Document 1, as an apparatus for irradiating light on a photosensitive drum to form an electrostatic latent image, an image forming apparatus including an exposure head having a light emitting unit using an LED, an organic EL, etc., and a lens for forming the light irradiated from this light emitting unit on the surface of the photosensitive drum is described. By using such an exposure head in this way, it is possible to reduce the number of parts compared to a configuration of a laser scanning method in which laser light is deflected and scanned by a rotating polygon mirror to form an electrostatic latent image, and it is possible to reduce the size and manufacturing cost of the image forming apparatus.

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In an image forming apparatus equipped with an exposure head, when forming an image by executing the above-described low-speed mode, the amount of light irradiated to the area corresponding to one pixel on the photosensitive drum is larger compared to when forming an image by executing the normal mode. This is because when the low-speed mode is executed, the rotation speed of the photosensitive drum is slower compared to when the normal mode is executed, so the time during which the area corresponding to one pixel on the photosensitive drum is irradiated with light becomes longer. In this case, when developing the electrostatic latent image in the developing unit, too much toner is loaded and the toner consumption increases.

[0007] Therefore, an object of the present invention is to provide an image forming apparatus that can suppress excessive loading of toner on the photosensitive drum even when forming an image in a mode where the rotation speed of the photosensitive drum is slow in an image forming apparatus equipped with an exposure head.

Means for Solving the Problems

[0008] A typical configuration of the image forming apparatus according to the present invention for achieving the above object includes a photosensitive drum, an exposure head having a plurality of light emitting portions arranged along the rotation axis direction of the photosensitive drum and emitting light based on an image data signal to expose the photosensitive drum, a control signal generation unit that generates a control signal for controlling the timing of light emission of the plurality of light emitting portions at a predetermined cycle, an image data signal transmission unit that transmits the image data signal to the exposure head in synchronization with the control signal in order to cause the plurality of light emitting portions to emit light according to the image data signal, and a control unit that controls the control signal generation unit, the image data signal transmission unit, and the rotation speed of the photosensitive drum. The control unit can execute a first mode in which the photosensitive drum is rotated at a first rotation speed and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is m / n times the first rotation speed. In the second mode, in order to perform exposure for forming an image of one line on a sheet, the control signal generation unit is controlled to generate the control signal at a cycle that is 1 / m times the cycle of the control signal generated in the first mode, and the image data signal transmission unit is controlled to transmit the same image data signal m times to the exposure head in synchronization with each of the n control signals, and to transmit the image data signal that does not cause the plurality of light emitting portions to emit light n - m times.

Advantages of the Invention

[0009] According to the present invention, even when forming an image in a mode where the rotation speed of the photosensitive drum is slow in an image forming apparatus including an exposure head, it is possible to suppress excessive toner from adhering to the photosensitive drum.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Embodiments for Carrying Out the Invention

[0011] <Image Forming Apparatus> Hereinafter, the overall configuration of the image forming apparatus A according to the present invention will be described with reference to the drawings together with the operation during image formation. Note that the dimensions, materials, shapes, relative arrangements, etc. of the component parts described below are not intended to limit the scope of the present invention only to those, unless otherwise specifically described.

[0012] The image forming apparatus A is a full-color image forming apparatus that forms an image by transferring toner of four colors, yellow (Y), magenta (M), cyan (C), and black (K), onto a sheet. In the following description, although suffixes Y, M, C, and K are attached to the members that use the toner of each color, the configurations and operations of each member are substantially the same except for the different colors of the toner used. Therefore, the suffixes are omitted as appropriate unless distinction is required.

[0013] FIG. 1 is a schematic cross-sectional view of the image forming apparatus A. As shown in FIG. 1, the image forming apparatus A has an image forming unit that forms an image. The image forming unit includes a photosensitive drum 1 (1Y, 1M, 1C, 10K), a charging device 2 (2Y, 2M, 2C, 2K), and an exposure head 6 (6Y, 6M, 6C, 6K). The image forming unit also includes a developing device 4 (4Y, 4M, 4C, 4K) as a developing unit, a transfer device 5 (5Y, 5M, 5C, 5K) as a transfer unit, and a conveyance belt 11.

[0014] The image forming apparatus A also includes a touch panel type operation unit 300 (detection unit) that allows a user to perform various settings related to image formation by operating it. The user can specify the number of copies to be formed, the size of the sheet on which the image is to be formed, etc. by operating the operation unit 300. The user can also input the basis weight of the sheet S (e.g., cardboard, plain paper, brand of the sheet, etc.) stored in the sheet cassettes 99a and 99b by operating the operation unit 300. Note that the basis weight of the sheet S may be measured by a sensor 38 (detection unit) arranged in the conveyance path instead of being input from the operation unit 300.

[0015] The sensor 38 for measuring the basis weight can use, for example, a transmissive optical sensor. The transmissive optical sensor is used in combination with a light emitting element such as an LED and a light receiving element such as a photodiode. When the sheet S blocks the light emitted from the light emitting element and traveling toward the light receiving element, the amount of light received by the light receiving element decreases. The light receiving element converts the amount of received light into a voltage value and functions as a switch by setting a predetermined threshold value for the voltage amount.

[0016] Generally, for the sheet S on which an image is formed, as the basis weight increases, the paper thickness also increases. In the case of thin paper with a small basis weight, the light emitted from the LED passes through the sheet S. In the case of thick paper with a large basis weight, there is almost no transmitted light of the light emitted from the LED. Therefore, paying attention to the relationship between the sensor 38 and the basis weight of the sheet S, by measuring the amount of light received by the light receiving element at the leading edge or the trailing edge portion of the sheet S passing between the sensor 38 with the light emission amount of the light emitting element kept constant, the basis weight of the sheet S can be predicted. Since there is a tendency that an image is less likely to be formed at the leading edge or the trailing edge of the sheet S compared to the vicinity of the center, the measurement accuracy can be improved by using this portion.

[0017] Also, the operation unit 300 and the sensor 38 are electrically connected to the CPU 73 shown in FIG. 8. The CPU 73 controls the rotation speed of various rollers that convey the sheet S according to the basis weight of the sheet S detected from the operation unit 300 or the sensor 38, and sets the conveyance speed of the sheet S during image formation. The conveyance speed referred to here is defined as the speed of the sheet S when the sheet S passes through the fixing device 94 (fixing unit).

[0018] For example, when the CPU 73 executes the low-speed mode for forming an image on thick paper with a basis weight of the sheet S being equal to or more than a predetermined value, it forms an image by setting the conveyance speed of the sheet S to be lower than when it executes the normal mode for forming an image on plain paper with a basis weight of the sheet S being less than the predetermined value. Also, since the rotation speed of the photosensitive drum 1 is related to the conveyance speed of the sheet S, when the CPU 73 executes the low-speed mode (second mode), it sets the rotation speed of the photosensitive drum 1 to be lower than when it executes the normal mode (first mode).

[0019] As a result, the time for heating the toner image carried on the sheet S with a large basis weight by the fixing device 94 is lengthened to apply more heat to the toner image and the sheet S, and the toner image can be stably fixed even to the sheet S with a large heat capacity. Note that the timing at which the CPU 73 switches between the low-speed mode and the normal mode is not limited to the above timing. For example, the CPU 73 may be configured to set the low-speed mode when forming an image with higher image quality than normal and increase the resolution in the sub-scanning direction, which is the rotation direction of the photosensitive drum 1, more than in the normal mode.

[0020] Next, the image forming operation by the image forming apparatus A will be described. When forming an image, first, the sheet S stored in the sheet cassette 99a or the sheet cassette 99b is sent to the registration roller 96 by the pickup rollers 91a, 91b, the feed rollers 92a, 92b, and the conveyance rollers 93a to 93c. Thereafter, the sheet S is fed onto the conveyance belt 11 by the registration roller 96 at a predetermined timing.

[0021] On the other hand, in the image forming unit, first, the surface of the photosensitive drum 1Y is charged by the charging device 2Y. Next, the exposure head 6Y irradiates light on the surface of the photosensitive drum 10Y according to the image data read by the image reading unit 90 or the image data transmitted from an external device (not shown), and an electrostatic latent image is formed on the surface of the photosensitive drum 10Y. Thereafter, yellow toner is attached to the electrostatic latent image formed on the surface of the photosensitive drum 1Y by the developing device 4Y, and a yellow toner image is formed on the surface of the photosensitive drum 1Y. The toner image formed on the surface of the photosensitive drum 1Y is transferred to the sheet S being conveyed by the conveyance belt 11 when a transfer bias is applied to the transfer device 5Y.

[0022] By the same process, the photosensitive drums 1M, 1C, and 1K are also irradiated with light from the exposure heads 6M, 6C, and 6K to form electrostatic latent images, and magenta, cyan, and black toner images are formed by the developing devices 4M, 4C, and 4K. Then, by applying a transfer bias to the transfer devices 5M, 5C, and 5K, these toner images are superposed and transferred onto the yellow toner image on the sheet S. As a result, a full-color toner image corresponding to the image data is formed on the surface of the sheet S.

[0023] After that, the sheet S carrying the toner image is conveyed to the fixing device 94 by the conveying belt 97, and heating and pressing processes are performed in the fixing device 94. As a result, the toner image on the sheet S is fixed to the sheet S. Then, the sheet S with the fixed toner image is discharged to the discharge tray 95 by the discharge roller 98.

[0024] <Exposure head> Next, the configuration of the exposure head 6 will be described.

[0025] Fig. 2(a) is a perspective view of the photosensitive drum 1 and the exposure head 6. Fig. 2(b) is a cross-sectional view of the photosensitive drum 1 and the exposure head 6. Figs. 3(a) and 3(b) are views showing the mounting surfaces on one side and the other side of the printed circuit board 22 provided in the exposure head 6. Fig. 3(c) is an enlarged view of the region V shown in Fig. 3(b).

[0026] As shown in Fig. 2, the exposure head 6 is fixed at a position facing the surface of the photosensitive drum 1 by a fixing member (not shown). The exposure head 6 has a light-emitting element array chip 40 that emits light, and a printed circuit board 22 on which the light-emitting element array chip 40 is mounted. It also has a rod lens array 23 that forms an image (condenses light) of the light emitted from the light-emitting element array chip 40 onto the photosensitive drum 1, and a housing 24 to which the rod lens array 23 and the printed circuit board 22 are fixed.

[0027] Also, a connector 21 is mounted on the surface of the printed circuit board 22 opposite to the mounting surface of the light-emitting element array chip 40. The connector 21 is provided for transmitting the control signal of the light-emitting element array chip 40 transmitted from the image controller unit 70 (FIG. 8) and connecting the power line. The light-emitting element array chip 40 is driven via the connector 21.

[0028] As shown in FIG. 3, on the printed circuit board 22, 20 light-emitting element array chips 40 are arranged and mounted in two rows in a staggered pattern. Further, in each light-emitting element array chip 40, 748 light-emitting units 50 are arranged at a predetermined resolution pitch in the longitudinal direction (arrow X direction). Also, in each light-emitting element array chip 40, the light-emitting units 50 are arranged at a predetermined pitch in the short-side direction (arrow Y direction). That is, in each light-emitting element array chip 40, the light-emitting units 50 are two-dimensionally arranged in the arrow X direction and the arrow Y direction which is orthogonal (intersects) to the arrow X direction.

[0029] In the present embodiment, the resolution pitch of the light-emitting element array chip 40 is 1200 dpi (about 21.16 μm). Also, the distance from one end to the other end in the longitudinal direction of the light-emitting unit 50 included in each light-emitting element array chip 40 is about 15.8 mm. That is, the exposure head 6 includes a total of 14960 light-emitting units 50 in the arrow X direction, and thus exposure processing corresponding to an image width in the longitudinal direction of about 316 mm (≈ about 15.8 mm × 20 chips) is possible.

[0030] In the longitudinal direction of the light-emitting element array chip 40, the interval L1 between the light-emitting units 50 of adjacent light-emitting element array chips 40 is about 21.16 μm. That is, at the boundary portion of each light-emitting element array chip 40, the pitch in the longitudinal direction of the light-emitting units 50 is the pitch of a resolution of 1200 dpi. Also, in the short-side direction (arrow Y direction) of the light-emitting element array chip 40, the interval L2 between the light-emitting units 50 of the light-emitting element array chips 40 arranged in two rows is about 105 μm (5 pixels at 1200 dpi).

[0031] In this embodiment, the arrow X direction, which is the longitudinal direction of the light-emitting element array chip 40, is the rotation axis direction of the photosensitive drum 1, and the arrow Y direction, which is the short side direction of the light-emitting element array chip 40, is the rotation direction of the photosensitive drum 1. Also, the arrow Z direction is the stacking direction in which the layers of the light-emitting part 50 of the layer structure described later overlap, and is also the light emission direction of the light-emitting part 50. Note that the longitudinal direction of the light-emitting element array chip 40 may be inclined by about ±1° with respect to the rotation axis direction of the photosensitive drum 1. Also, the short side direction of the light-emitting element array chip 40 may be inclined by about ±1° with respect to the rotation direction of the photosensitive drum 1.

[0032] <Light-emitting element array chip> Next, the configuration of the light-emitting element array chip 40 will be described.

[0033] FIG. 4 is a schematic diagram of the light-emitting element array chip 40. FIG. 5 is a cross-sectional view taken along the M-M cross-section shown in FIG. 4. FIG. 6 is a schematic diagram for explaining the arrangement of the light-emitting part 50.

[0034] As shown in FIG. 4, the light-emitting element array chip 40 includes a light-emitting substrate 42 having a circuit part 46 for controlling the light-emitting part 50, a light-emitting region 44 in which a plurality of light-emitting parts 50 are regularly arranged on the light-emitting substrate 42, and wire bonding pads 48. The input and output of signals between the outside of the light-emitting element array chip 40 and the circuit part 46 and the power supply to the circuit part 46 are performed through the wire bonding pads 48. Note that the circuit part 46 can use a circuit including an analog drive circuit, a digital control circuit, or both.

[0035] As shown in FIG. 5, the light-emitting part 50 includes a light-emitting substrate 42, a plurality of lower electrodes 54 two-dimensionally arranged on the light-emitting substrate 42 at regular intervals (intervals d1 and d2 shown in FIG. 6) in the arrow X direction and the arrow Y direction, a light-emitting layer 56, and an upper electrode 58.

[0036] The lower electrode 54 (the first electrode layer having a plurality of electrodes) is a plurality of electrodes formed in a layered and separated manner on the light-emitting substrate 42, and is an electrode provided corresponding to each pixel. That is, each lower electrode 54 is provided to form one pixel respectively.

[0037] The upper electrode 58 (the second electrode layer) is laminated on the light-emitting layer 56 at a position opposite to the side where the lower electrode 54 with respect to the light-emitting layer 56 is disposed. The upper electrode 58 is an electrode capable of transmitting (transmittable) light having the emission wavelength of the light-emitting layer 56.

[0038] The circuit unit 46 controls the potential of the selected lower electrode 54 based on a control signal generated according to the image data, and generates a potential difference between the selected lower electrode 54 and the upper electrode 58. When a potential difference occurs between the upper electrode 58 which is the anode and the lower electrode 54 which is the cathode, electrons flow from the cathode into the light-emitting layer 56, and holes flow from the anode into the light-emitting layer 56. The light-emitting layer 56 emits light when electrons and holes recombine in the light-emitting layer 56.

[0039] The light directed toward the upper electrode 58 when the light-emitting layer 56 emits light is transmitted through the upper electrode 58 and emitted. Also, the light directed from the light-emitting layer 56 toward the lower electrode 54 is reflected from the lower electrode 54 toward the upper electrode 58, and the reflected light is also transmitted through the upper electrode 58 and emitted. In this way, the light-emitting unit 50 emits light. Although there is a time difference in the emission timing between the light directly emitted from the light-emitting layer 56 toward the upper electrode 58 and the light reflected from the lower electrode 54 and emitted from the upper electrode 58, since the thickness of the layer of the light-emitting unit 50 is extremely thin, it can be regarded as almost simultaneous.

[0040] In this embodiment, the light-emitting substrate 42 is a silicon substrate. The upper electrode 58 is preferably transparent to the light-emitting wavelength of the light-emitting layer 56. For example, by using a transparent electrode such as indium tin oxide (ITO), the aperture ratio becomes substantially 100%, and the light emitted by the light-emitting layer 56 is directly emitted through the upper electrode 58. Also, in this embodiment, the upper electrode 58 is an anode provided in common for each of the lower electrodes 54, but it may also be configured to be provided individually for each of the lower electrodes 54, or one upper electrode 58 may be provided for each of the plurality of lower electrodes 54.

[0041] Also, an organic EL film, an inorganic EL layer, or the like is used for the light-emitting layer 56. When an organic EL film is used as the light-emitting layer 56, the light-emitting layer 56 may be a laminated structure including functional layers such as an electron transport layer, a hole transport layer, an electron injection layer, a hole injection layer, an electron blocking layer, and a hole blocking layer as needed. Also, the light-emitting layer 56 may be continuously formed in the arrow X direction or may be divided into the same size as the lower electrode 54. Also, each of the lower electrodes 54 may be divided into a plurality of groups, and one light-emitting layer 56 may be laminated on the upper part of the lower electrodes 54 belonging to each divided group.

[0042] When a light-emitting material that is vulnerable to moisture, such as an organic EL layer or an inorganic EL layer, is used for the light-emitting layer 56, it is desirable to seal it in order to prevent moisture from entering the light-emitting region 44. As a sealing method, for example, a single film or a laminated sealing film of thin films such as silicon oxide, silicon nitride, and aluminum oxide is formed. As a method for forming the sealing film, a method having excellent covering performance for structures such as steps is preferable, and for example, an atomic layer deposition method (ALD method) can be used. Note that the material, configuration, formation method, etc. of the sealing film are only examples and are not limited to the examples described above, and appropriate ones may be selected as appropriate.

[0043] Also, the lower electrode 54 is preferably made of a metal with a high reflectance with respect to the emission wavelength of the light-emitting layer 56. For example, Ag, Al, or an alloy of Ag and Al is used. Further, the lower electrode 54 is formed using a Si process together with the formation of the circuit portion 46 and is directly connected to the driving portion of the circuit portion 46. By forming the lower electrode 54 by the Si process in this way, since the process rule is about 0.2 μm and the accuracy is high, the lower electrode 54 can be arranged with high density and high accuracy. Further, since the lower electrode 54 can be arranged with high density, almost all of the light-emitting region 44 can be caused to emit light, and the utilization efficiency of the light-emitting region 44 can be increased. Note that the organic material of the light-emitting layer 56 is filled between the respective lower electrodes 54, and the respective lower electrodes 54 are partitioned by the organic material.

[0044] Here, the current flowing through the light-emitting portion 50 and the light emission amount are in a substantially proportional relationship. Therefore, by controlling the current flowing through the light-emitting portion 50, the light amount of the light-emitting portion 50 can be controlled. Further, the light-emitting portion 50 has a threshold voltage, and when the voltage applied across the light-emitting portion 50 becomes equal to or higher than the threshold voltage, a current starts to flow through the light-emitting portion 50, and thereafter the current flows almost linearly. Since there is variation in the threshold voltage of each light-emitting portion 50, the voltage at which the current starts to flow is slightly different for each light-emitting portion 50. Therefore, at the stage before the product is shipped from the factory, the light-emitting portions 50 of the light-emitting element array chip 40 are caused to emit light individually and sequentially, and the current flowing through the light-emitting portion 50 is adjusted so that the light collected through the rod lens array 23 has a predetermined light amount. Note that the exposure head 6 performs focus adjustment for adjusting the distance between the light-emitting element array chip 40 and the rod lens array 23, in addition to the above-described light amount adjustment, at the stage before the product is shipped from the factory.

[0045] Also, as shown in FIG. 6, in the light-emitting region 44, the light-emitting portions 50 are two-dimensionally arranged at a predetermined interval in the arrow X direction and the arrow Y direction. That is, in the light-emitting region 44 of one light-emitting element array chip 40, a plurality of light-emitting portions 50 are arranged along the arrow X direction, and a light-emitting row is formed by the plurality of light-emitting portions 50 arranged along the arrow X direction. This light-emitting row is arranged in a plurality of rows in the arrow Y direction, and the number of rows of the light-emitting row is six rows in the present embodiment.

[0046] In this embodiment, the width W1 of the light-emitting unit 50 in the arrow X direction is 20.90 μm, and the distance d1 between adjacent light-emitting units 50 in the arrow X direction is 0.26 μm. That is, the light-emitting units 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the arrow X direction. Also, the width W2 of the light-emitting unit 50 in the arrow Y direction is also 20.90 μm, similar to the width W1, and the distance d2 is also 0.26 μm, similar to the distance d1. The light-emitting units 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the arrow Y direction as well. That is, the light-emitting unit 50 of this embodiment has a square shape with a side length of 20.90 μm, and its area is 436.81 μm2. This occupies approximately 97.6% of the area of one pixel, which is 447.7456 μm2. The organic light-emitting material has less light output compared to an LED. In contrast, by making the light-emitting unit 50 square and reducing the distance between adjacent light-emitting units 50 as described above, it becomes possible to secure a light-emitting area for obtaining a light amount sufficient to change the potential of the photosensitive drum 1.

[0047] Note that it is desirable to secure an area of the light-emitting unit 50 that is 90% or more of the occupied area of one pixel. Therefore, for the image forming apparatus A with an output resolution of 1200 dpi, it is desirable to form the width of one side of the light-emitting unit 50 to be about 20.07 μm or more. Also, for the image forming apparatus A with an output resolution of 2400 dpi, it is desirable to form the width of one side of the light-emitting unit 50 to be about 10.04 μm or more. Also, the shape of the light-emitting unit 50 is not limited to a square, and as long as it emits light with a size of the exposure area corresponding to the output resolution of the image forming apparatus A and the image quality of the output image satisfies the design specifications of the image forming apparatus A, it may be a polygon with four or more sides, a circle, an ellipse, or the like. Also, the distance d2 between adjacent light-emitting units 50 in the arrow Y direction and the number of columns of the light-emitting units 50 in the arrow Y direction are determined based on the scanning speed of the exposure head 6, the light amount required for the exposure process, the resolution, and the like.

[0048] FIG. 7 is a schematic diagram for explaining the irradiation positions on the photosensitive drum 1 of the light emitted from two light emitting units 50 located at overlapping positions in the direction of arrow Y. As shown in FIG. 7(a), when two light emitting units 50 located at overlapping positions in the Y direction are simultaneously lit, the irradiation positions on the photosensitive drum 1 of the light H emitted from the two light emitting units 50 are shifted in the rotation direction of the photosensitive drum 1 (the direction of arrow Y, the sub-scanning direction) in the same manner as the positional relationship between the two light emitting units 50. In contrast, as shown in FIG. 7(b), when the lighting timings of the two light emitting units 50 are changed according to the rotation speed of the photosensitive drum 1, the irradiation positions on the photosensitive drum 1 of the light H emitted from the two light emitting units 50 can be made substantially the same. Illuminating the photosensitive drum 1 with light from a plurality of light emitting units 50 arranged in the direction of arrow Y at substantially the same position in this way is called multi-exposure. The larger the number of light emitting units 50 arranged in the direction of arrow Y used for multi-exposure, the larger the amount of light received by a part of the photosensitive drum 1 when multi-exposure is performed.

[0049] In order to make the irradiation positions on the photosensitive drum 1 of the light emitted from two light emitting units 50 located at overlapping positions in the direction of arrow Y coincide, it is necessary to delay the lighting timing of the light emitting unit 50 on the downstream side in the rotation direction of the photosensitive drum 1 by a time of delay amount T with respect to the lighting timing of the light emitting unit 50 on the upstream side. Here, the delay amount T (μs) is calculated by the following formula 1 when the rotation speed of the photosensitive drum 1 is Vdr (mm / s), the width W2 (μm), and the interval d2 (μm).

[0050] (Formula 1) T = ((W2 + d2) ÷ 1000) ÷ Vdr

[0051] Also, in this embodiment, the maximum value Tw (s) of the light emission time of each light emitting unit 50 is generated such that the light emission signal is equal to the one-line time in the sub-scanning direction, and is represented by the following formula 2 from the resolution of 1200 dpi and the rotation speed Vdr (mm / s) of the photosensitive drum 1. (Formula 2) Tw = (25.4 ÷ 1200) ÷ Vdr

[0052] <System Configuration of Exposure Head> Next, the image controller unit 70 provided on the main body side of the image forming apparatus A and the system configuration of the exposure head 6 will be described. In the following, although the processing of a single color among the four colors of yellow, magenta, cyan, and black will be described, when performing an image forming operation, the same processing for the above four colors is performed in parallel.

[0053] FIG. 8 is a block diagram showing the system configuration of the image controller unit 70 and the exposure head 6. As shown in FIG. 8, the image controller unit 70 includes an image data signal generation unit 71 (image data signal generation unit), a chip data conversion unit 72 (image data signal transmission unit), a CPU 73 (control unit), and a synchronization signal generation unit 74 (control signal generation unit).

[0054] The image controller unit 70 performs processing of image data and processing of image forming timing by the respective units described above, and transmits a control signal for controlling the exposure head 6 to the printed circuit board 22 of the exposure head 6. Specifically, the control signal is an image data signal, a chip select signal, a clock signal, a line synchronization signal, and a communication signal of the CPU 73, and these signals are transmitted from the image controller unit 70 to the exposure head 6 via respective signal lines described later.

[0055] The image data signal generation unit 71 receives the image data of the document read by the image reading unit 90 or the image data transferred from an external device via the network. The image data signal generation unit 71 performs dithering processing on the input image data at the resolution instructed by the CPU 73, and generates an image data signal for outputting the image. In the present embodiment, the image data signal generation unit 71 performs dithering processing at a resolution of 1200 dpi. The image data signal represents 8 gradations with a density value of 0 to 7 in a 3-bit width. The density value 0 is the minimum density, and the density value 7 is the maximum density. Note that the CPU 73 transmits a communication signal via the communication signal line 79 to the image data signal generation unit 71 to give various instructions.

[0056] The line synchronization signal generation unit 74 periodically generates a line synchronization signal representing the delimiter for each line in the main scanning direction of the image data signal. In other words, the synchronization signal generation unit 74 periodically generates a line synchronization signal, which is a control signal for controlling the timing at which the light-emitting unit 50 selected according to the image data signal starts emitting light when forming an electrostatic latent image for one line in the main scanning direction. When forming an image in the normal mode, the CPU 73 sets the period during which the surface of the photosensitive drum 1 moves by a pixel size corresponding to the resolution in the sub-scanning direction in the rotation direction with respect to the preset rotation speed of the photosensitive drum 1 as one line period, and instructs the synchronization signal generation unit 74 of the period of the line synchronization signal. Specifically, when the normal mode is executed, the CPU 73 instructs the synchronization signal generation unit 74 to generate a line synchronization signal every time the photosensitive drum 1 rotates at a pitch (about 21.16 μm) with a resolution of 1200 dpi. For example, when the photosensitive drum 1 rotates at 200 mm / s in the normal mode, the synchronization signal generation unit 74 generates a line synchronization signal with a period of 105.8 μs.

[0057] As will be described later, the chip data conversion unit 72 has a line buffer 15, receives and stores the image data signal from the image data signal generation unit 71. Then, in synchronization with the line synchronization signal generated by the synchronization signal generation unit 74 and input via the line synchronization signal line 78, the chip data conversion unit 72 divides the image data signal for one line according to the instruction of the CPU 73 and transmits it to each light-emitting element array chip 40 via the image data signal line 77. The chip data conversion unit 72 also transmits a clock signal and a chip select signal representing the valid range of the image data signal to the light-emitting element array chip 40 via the chip select signal line 75 and the clock signal line 76.

[0058] The head information storage unit 171 included in the exposure head 6 is connected to the CPU 73 via the communication signal line 79. The head information storage unit 171 stores, as head information, the light emission amount and mounting position information of each light emitting element array chip 40. Also, the chip select signal line 75, the clock signal line 76, the image data signal line 77, the line synchronization signal line 78, and the communication signal line 79 are all connected to the light emitting element array chip 40. The light emitting element array chip 40 causes the light emitting unit 50 to emit light based on the set values of the respective signals input via the above-described signal lines from the image controller unit 70. Also, each light emitting element array chip 40 is cascade-connected to other light emitting element array chips 40 via the chip select signal line 75. Each light emitting element array chip 40 generates a chip select signal used by other light emitting element array chips 40 and transmits it via the chip select signal line 75.

[0059] <Chip Data Conversion Unit> Next, the configuration of the chip data conversion unit 72 will be described.

[0060] FIG. 9 is a block diagram showing the configuration of the chip data conversion unit 72. As shown in FIG. 9, the chip data conversion unit 72 includes a line buffer 15, a buffer write control unit 16, and a buffer read control unit 17. The CPU 73 controls the line buffer 15, the buffer write control unit 16, and the buffer read control unit 17 by transmitting a control signal to the chip data conversion unit 72.

[0061] The buffer write control unit 16 transmits a reception possible notification signal to the image data signal generation unit 71. As a result, an image data signal is output from the image data signal generation unit 71. The buffer write control unit 16 stores the image data signal output from the image data signal generation unit 71 in the line buffer 15 line by line. The line buffer 15 has four data storage areas (Buf000 to Buf003), and the image data signal for one line is stored in the order of Buf000 → Buf001 → Buf002 → Buf003 → Buf000 → Buf001 ···. Each time the buffer write control unit 16 stores one line of the image data signal in the line buffer 15, it transmits a write completion notification signal to the buffer read control unit 17. Also, when the data storage area of the line buffer 15 is in a full state, the buffer write control unit 16 stops transmitting the reception possible notification signal to the image data signal generation unit 71 and stops the output of the image data signal.

[0062] When the buffer read control unit 17 receives the write completion notification signal, it synchronizes with the line synchronization signal and divides the image data signal for one line stored in the line buffer 15 so as to be assigned to each light emitting element array chip 40, and transmits the image data signal to the light emitting element array chip 40 together with the clock signal and the chip select signal. Also, when the buffer read control unit 17 finishes transmitting the image data signal for one line to the light emitting element array chip 40, it transmits a read completion notification signal to the buffer write control unit 16 to notify that one data storage area of the line buffer 15 has become empty. Further, the buffer read control unit 17 can also transmit an image data signal for turning off all the light emitting parts 50 of the exposure head 6, instead of the image data signal stored in the line buffer 15, to the light emitting element array chip 40 according to the instruction of the CPU 73. The image data signals for turning off all the light emitting parts 50 of the exposure head 6 are all image data signals of "0" or "1".

[0063] <System Configuration of Light Emitting Element Array Chip> Next, the system configuration of the light emitting element array chip 40 will be described.

[0064] Figure 10 is a block diagram showing the system configuration of the light-emitting element array chip 40. As shown in Figure 10, the circuit section 46 of the light-emitting element array chip 40 is composed of a digital section 80 and an analog section 86. The analog section 86 generates a signal for driving the light-emitting section 50 based on the pulse signal generated by the digital section 80, as will be described later.

[0065] The digital section 80 includes a communication IF section 81, a register section 82, a chip select signal generation section 83, an image data storage section 84, and a pulse signal generation section 85. The digital section 80 generates a pulse signal for causing the light-emitting section 50 to emit light based on a set value, a chip select signal, an image data signal, and a line synchronization signal preset by a communication signal in synchronization with a clock signal by these parts, and transmits it to the analog section 86.

[0066] The communication IF section 81 controls the writing and reading of set values to and from the register section 82 based on the communication signal input from the CPU 73. The register section 82 stores set values necessary for operation. These set values include width information of the pulse signal generated by the pulse signal generation section 85, period information of the line synchronization signal, setting information of the drive current set by the analog section 86, and information on the rotation speeds of the photosensitive drum 1 in the normal mode and the low-speed mode.

[0067] The chip select signal generation section 83 delays the input chip select signal and generates a chip select signal used by other light-emitting element array chips 40 connected via the chip select signal line 75. The image data storage section 84 holds the image data signal while the input chip select signal is valid, and outputs the image data signal to the lighting control section 88 in synchronization with the line synchronization signal.

[0068] The pulse signal generation unit 85 generates a pulse signal for controlling the light emission timing of the light emitting unit 50 based on the width information of the pulse signal stored in the register unit 82 and the period of the line synchronization signal, and outputs it to the lighting control unit 88. The lighting control unit 88 selects whether to output the pulse signal generated by the pulse signal generation unit 85 to the analog unit 86 for each light emitting unit 50 based on the image data signal output from the image data storage unit 84, and outputs the pulse signal to the analog unit 86.

[0069] <Image data storage unit> Next, the configuration of the image data storage unit 84 will be described.

[0070] FIG. 11 is a circuit configuration diagram of the image data storage unit 84. In FIG. 11, the image data signal is also denoted as "data". Although the chip select signal and the line synchronization signal are negative logic signals, they may be positive logic signals.

[0071] As shown in FIG. 11, the image data storage unit 84 includes a clock gate circuit 30 and flip-flop circuits 31 to 37. The flip-flop circuit 31 (31-000 to 31-747) uses the image data signal data input to the image data storage unit 84 as the main input, and 748 pieces, which is the same number as the number of light emitting units 50 in the arrow X direction of the light emitting element array chip 40, are connected in series.

[0072] Similarly, the flip-flop circuits 32 to 37 are provided in the same number as the number of light emitting units 50 in the arrow X direction of the light emitting element array chip 40 (32-000 to 32-747, 33-000 to 33-747, 34-000 to 34-747, 35-000 to 35-747, 36-000 to 36-747, 37-000 to 37-747).

[0073] The clock gate circuit 30 outputs the logical product of the inverted signal of the chip select signal and the clock signal, and outputs the clock signal to the flip-flop circuit 31 only when the chip select signal is valid. The flip-flop circuit 31 operates with the clock signal sent from the clock gate circuit 30 and outputs the image data signal (dly_data_000).

[0074] The flip-flop circuit 32-000 takes the output of the flip-flop circuit 31-000 as input and operates with the line synchronization signal. The output (buf_data_0_000) of the flip-flop circuit 32-000 is input to the flip-flop circuit 33-000 and the lighting control unit 88.

[0075] The flip-flop circuit 33-000 takes the output of the flip-flop circuit 32-000 as input and operates with the line synchronization signal. The output (buf_data_1_000) of the flip-flop circuit 33-000 is input to the flip-flop circuit 34-000 and the lighting control unit 88.

[0076] The flip-flop circuit 34-000 takes the output of the flip-flop circuit 33-000 as input and operates with the line synchronization signal. The output (buf_data_2_000) of the flip-flop circuit 34-000 is input to the flip-flop circuit 35-000 and the lighting control unit 88.

[0077] The flip-flop circuit 35-000 takes the output of the flip-flop circuit 34-000 as input and operates with the line synchronization signal. The output (buf_data_3_000) of the flip-flop circuit 35-000 is input to the flip-flop circuit 36-000 and the lighting control unit 88.

[0078] The flip-flop circuit 36-000 takes the output of the flip-flop circuit 35-000 as input and operates with the line synchronization signal. The output (buf_data_4_000) of the flip-flop circuit 36-000 is input to the flip-flop circuit 37-000 and the lighting control unit 88.

[0079] The flip-flop circuit 37-000 takes the output of the flip-flop circuit 36-000 as input and operates with a line synchronization signal. The output (buf_data_5_000) of the flip-flop circuit 37-000 is input to the lighting control unit 88.

[0080] Note that each of the flip-flop circuits 32-001 to 32-747, 33-001 to 33-747, 34-001 to 34-747, 35-001 to 35-747, 36-001 to 36-747, and 37-001 to 37-747 also operates in the same manner as the above-described flip-flop circuits 32-000 to 37-000.

[0081] Figure 12 is a timing chart showing the operation in the main scanning direction in the image data storage unit 84. The meanings of the symbols shown in Figure 12 are the same as those of the symbols shown in Figure 11. As shown in Figure 12, between the time T0 and T1 when the low output of the chip select signal is captured at the rising edge of the clock signal, the image data signal is shifted in order as data → dly_data_000 → dly_data_001. The low output of the chip select signal is input 748 times, which is the same number as the number of light emitting units 50 in the main scanning direction of the clock signal. As a result, the image data signal for one line is held in dly_data_000 to dly_data_747.

[0082] After the time T1, since the chip select signal is high, the shift operation is not performed and it is held. When the low output of the line synchronization signal is captured at the rising edge of the clock signal at the time T2, the image data signal for one line is shifted all at once as dly_data_000 → buf_data_0_000 → dly_data_001 → buf_data_0_001 and output to the lighting control unit 88 as buf_data_0_000 to buf_data_0_747.

[0083] Figure 13 is a timing chart showing the operation in the sub-scanning direction in the normal mode in the image data storage unit 84. The meanings of the symbols shown in Figure 13 are the same as those of the symbols shown in Figure 11. Hereinafter, the outputs buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, buf_data_5_000 of the flip-flop circuits 32-000, 33-000, 34-000, 35-000, 36-000, 37-000 shown in Figure 11 will be described. Although the description will be omitted hereinafter, the same applies to buf_data_0_001~buf_data_0_747, buf_data_1_001~buf_data_1_747, buf_data_2_001~buf_data_2_747, buf_data_3_001~buf_data_3_747, buf_data_4_001~buf_data_4_747, buf_data_5_001~buf_data_5_747.

[0084] As shown in Figure 13, every time the line synchronization signal rises from low to high, it shifts in the order of dly_data_000→buf_data_0_000, buf_data_0_000→buf_data_1_000. Therefore, the value of B000 of dly_data_000 at time S0 is output to the lighting control unit 88 at time S1 for buf_data_0_000, at time S2 for buf_data_1_000, and at time S3 for buf_data_2_000.

[0085] In this way, multiplexed exposure is achieved by connecting buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, buf_data_5_000 in order from the light-emitting unit 50 that is first exposed on the photosensitive drum 1.

[0086] <Analog section> Next, the configuration of the analog section 86 will be described. In the following description, although two drive sections 61 for driving two light emitting sections 50 will be described, all the light emitting sections 50 are driven in the same manner by the same drive sections 61.

[0087] FIG. 14 is a block diagram showing the configuration of the analog section 86. As shown in FIG. 14, the analog section 86 includes a drive section 61 for driving the light emitting section 50, a DAC 62 which is a digital-to-analog converter, and a drive section selection section 67.

[0088] The DAC 62 supplies an analog voltage that determines the drive current to the drive section 61 via the signal line 63 based on the data set in the register section 82. The pulse signal output from the lighting control section 88 is input to the drive section 61 via the signal line 66. In this way, an analog voltage that determines the drive current and a pulse signal are input to the drive section 61. Then, based on these signals, the drive section 61 controls the drive current and the emission time of the light emitting section 50 by a drive circuit described later.

[0089] The drive section selection section 67 supplies a drive section select signal for selecting the drive section 61 to the two drive sections 61 via the signal lines 64 and 65 based on the data set in the register section 82. Here, the drive section select signal is generated such that only the signal connected to the selected drive section 61 becomes high. For example, when the upper drive section 61 shown in FIG. 14 is selected, only the signal line 64 is supplied with high, and the signal line 65 is supplied with low. The two drive sections 61 have an analog voltage that determines the drive current set from the DAC 62 at the timing when the drive section select signal becomes high. In this way, the CPU 73 sequentially selects the drive section 61 via the register section 82 and sets the analog voltage of the selected drive section 61, thereby setting the analog voltages of all the drive sections 61 using one DAC 62.

[0090] <Drive section> Next, the configuration of the drive section 61 will be described.

[0091] FIG. 15 is a circuit diagram of the driving unit 61. As shown in FIG. 15, the driving unit 61 includes MOSFETs 112 to 115, a capacitor 116, and an inverter 117.

[0092] MOSFET 112 supplies a driving current to the light emitting unit 50 according to the value of the gate voltage, and controls the current so that the driving current turns off (lights out) when the gate voltage is at a low level. A signal line 63 is connected to the gate of MOSFET 114. When the pulse signal input via the signal line 66 is high, MOSFET 114 transfers the voltage charged in the capacitor 116 to MOSFET 112.

[0093] The gate of MOSFET 115 is connected to the driving unit selection signal transmitted from the driving unit selection unit 67 via the signal line 64. MOSFET 115 turns on when the input driving unit selection signal is high, and charges the capacitor 116 with the analog voltage output from the DAC 62 and transmitted via the signal line 63. In the present embodiment, the DAC 62 sets an analog voltage in the capacitor 116 at the timing before image formation, and keeps the voltage level by turning off MOSFET 115 during the image formation operation.

[0094] By the above operation, MOSFET 112 supplies a driving current to the light emitting unit 50 according to the set analog voltage and the pulse signal. Further, when the input capacitance of the light emitting unit 50 is large and the response speed at the time of turning off is slow, the response speed at the time of turning off can be increased by MOSFET 113. A signal obtained by logically inverting the pulse signal by the inverter 117 is input to the gate of MOSFET 113. When the pulse signal is low, the gate of MOSFET 113 becomes high, and the charge charged in the input capacitance of the light emitting unit 50 is forcibly discharged.

[0095] <Control of the light emitting unit in the low speed mode> As described above, the CPU 73 can execute the normal mode and the low-speed mode during image formation, and changes the rotation speed of the photosensitive drum 1 when switching between these modes. Here, when the CPU 73 controls the synchronization signal generation unit 74 with the cycle of the line synchronization signal as the time for the photosensitive drum 1 to rotate by the pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode in the same manner as in the normal mode, the following problems occur.

[0096] That is, when the rotation speed of the photosensitive drum 1 decreases, the cycle of the line synchronization signal becomes longer, so the amount of light irradiated on the area corresponding to one pixel on the photosensitive drum 1 increases. For example, when the rotation speed of the photosensitive drum 1 in the low-speed mode is half that in the normal mode, the cycle of the line synchronization signal becomes twice that in the normal mode, and the amount of light irradiated on the area corresponding to one pixel on the photosensitive drum 1 becomes twice. When the amount of light irradiated on the area corresponding to one pixel on the photosensitive drum 1 increases, too much toner accumulates on the photosensitive drum 1 when developing the electrostatic latent image.

[0097] On the other hand, in the low-speed mode, a configuration can be considered in which the value of the current for driving the light-emitting unit 50 is lowered to reduce the amount of light compared to the normal mode, thereby suppressing the overloading of toner on the photosensitive drum 1. However, a configuration for making the amount of light of the light-emitting unit 50 variable complicates the configuration of the circuit for driving the light-emitting unit 50. Therefore, the CPU 73 performs the control described below in the low-speed mode to reduce the amount of light irradiated on the area corresponding to one pixel on the photosensitive drum 1 and suppress the overloading of toner on the photosensitive drum 1.

[0098] FIG. 16 is a timing chart showing the operation in the sub-scanning direction in the low-speed mode in the image data storage unit 84. Here, it is assumed that the rotation speed of the photosensitive drum 1 in the low-speed mode is 2 / 3 times the rotation speed of the photosensitive drum 1 in the normal mode. Note that the meanings of the symbols shown in FIG. 16 are the same as the symbols shown in FIG. 11.

[0099] As shown in FIG. 16, first, the CPU 73 instructs the synchronization signal generation unit 74 to set the period of the line synchronization signal in the low-speed mode to 1 / 2 times the period of the line synchronization signal in the normal mode. The numerical value of "1 / 2" is determined as follows. That is, it is assumed that the rotation speed of the photosensitive drum 1 in the low-speed mode is m / n times the rotation speed of the photosensitive drum 1 in the normal mode. Since the rotation speed of the photosensitive drum 1 is slower in the low-speed mode, m < n. In this case, the CPU 73 first obtains a value that is n / m (the reciprocal of m / n) times the period of the line synchronization signal in the normal mode. The value obtained here is the time for the photosensitive drum 1 to move by a pixel size corresponding to the resolution in the sub-scanning direction in the rotation direction in the low-speed mode.

[0100] Next, the CPU 73 obtains a value that is further 1 / n times the value of n / m times the period of the line synchronization signal in the normal mode that was obtained. The CPU 73 instructs the synchronization signal generation unit 74 to use the value thus obtained as the period of the line synchronization signal. It can also be said that the period of the line synchronization signal in the low-speed mode thus obtained is the time obtained by dividing the time for the photosensitive drum 1 to rotate by a pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode into n parts.

[0101] That is, when the CPU 73 executes the low-speed mode in which the photosensitive drum 1 rotates at a speed slower than the rotation speed (the first rotation speed) in the normal mode and m / n times the rotation speed in the normal mode (the second rotation speed), the synchronization signal generation unit 74 is controlled as follows. That is, the CPU 73 controls the synchronization signal generation unit 74 to generate a line synchronization signal at a period that is 1 / m times the period of the line synchronization signal generated in the normal mode in the low-speed mode.

[0102] Next, the CPU 73 controls the chip data conversion unit 72 as follows. That is, the CPU 73 causes the buffer read control unit 17 to read out one-line worth of image data signals from Buf000 of the line buffer 15 and transmit them to the light-emitting element array chip 40 in synchronization with the first line synchronization signal and the second line synchronization signal among the three line synchronization signals generated while the photosensitive drum 1 rotates by a pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. That is, the buffer read control unit 17 transmits the same one-line worth of image data signals to the light-emitting element array chip 40 in synchronization with the first line synchronization signal generated at time S0A and the second line synchronization signal generated at time S0B.

[0103] Next, the CPU 73 causes the buffer read control unit 17 to transmit an image data signal that does not cause the light-emitting unit 50 to emit light (non-lighting) to the light-emitting element array chip 40 in synchronization with the third line synchronization signal generated at time S0C. As a result, the buffer read control unit 17 transmits the same one-line worth of image data signal among the three image data signals transmitted to the exposure head 6 in synchronization with the three line synchronization signals twice, and transmits an image data signal for not causing the light-emitting unit 50 to emit light once. Generalizing the number of transmissions of these image data signals by the buffer read control unit 17 using the above n and m is as follows. That is, the CPU 73 controls the buffer read control unit 17 so that the same one-line worth of image data signal is output m times and the image data signal for not causing the light-emitting unit 50 to emit light is output n - m times in synchronization with each of the n line synchronization signals generated while the photosensitive drum 1 rotates by a pixel size corresponding to the resolution in the sub-scanning direction in the low-speed mode. In other words, the CPU 73 controls the chip data conversion unit 72 as described above in order to perform exposure for forming an image of one line extending in the main scanning direction (the rotation axis direction of the photosensitive drum 1, the width direction of the sheet S) on the sheet S.

[0104] Here, the time during which the light emitting unit 50 emits light when the chip data conversion unit 72 transmits the same image data signal for one line to the light emitting element array chip 40 twice is equal to the time during which the light emitting unit 50 emits light while the photosensitive drum 1 rotates by a pixel size corresponding to the resolution in the sub-scanning direction in the normal mode. That is, the time from time S0A to time S0B shown in FIG. 16 is the same as the time from time S0 to time S1 shown in FIG. 13. Also, the time from time S0A to time S0B shown in FIG. 16 is the time during which the photosensitive drum 1 rotates by a pixel size corresponding to the resolution in the sub-scanning direction in the low speed mode. The same applies to other times. Therefore, in the low speed mode, by the CPU 73 controlling the chip data conversion unit 72 and the synchronization signal generation unit 74 as described above, it is possible to make the amount of light irradiated to the area corresponding to one pixel on the photosensitive drum 1 equal in the normal mode and the low speed mode.

[0105] Note that after the CPU 73 transmits the image data signal to the exposure head 6 in synchronization with the second line synchronization signal to the buffer read control unit 17, the CPU 73 instructs the buffer write control unit 16 to transmit a read completion notification signal. When the buffer write control unit 16 receives the read completion notification signal, the buffer write control unit 16 generates a reception possible notification signal and outputs it to the image data signal generation unit 71. When the image data signal generation unit 71 receives the reception possible notification signal, the image data signal generation unit 71 transmits the image data signal for the next one line to the buffer write control unit 16. The buffer write control unit 16 stores the image data signal for the next one line received from the image data signal generation unit 71 in the empty line buffer Buf000. Also, the CPU 73 instructs the buffer read control unit 17 to read the image data signal from Buf001 and transmit it to the printed circuit board 22 in synchronization with the line synchronization signal generated at time S1A. The CPU 73 repeatedly executes the above control after time S1B to perform the exposure process.

[0106] Summarizing the above, when the CPU 73 executes the low-speed mode in which the photosensitive drum 1 is rotated at a speed slower than the rotation speed in the normal mode and m / n times the rotation speed in the normal mode, the following control is performed. That is, in the low-speed mode, the CPU 73 controls the synchronization signal generation unit 74 to generate a line synchronization signal at a period of 1 / m times the period of the line synchronization signal generated in the normal mode. In addition, in order to perform exposure for forming an image of one line extending in the main scanning direction on the sheet S, the same image data signal is transmitted m times in synchronization with each of the n line synchronization signals generated by the synchronization signal generation unit 74, and the buffer read control unit 17 is controlled to transmit the image data signal for preventing the light emitting unit 50 from emitting light n - m times. Thereby, in the normal mode and the low-speed mode, the amount of light irradiated to the area corresponding to one pixel on the photosensitive drum 1 can be made equal. Therefore, it is possible to suppress the toner from being overloaded on the photosensitive drum 1 in the low-speed mode.

[0107] In the above description, although the configuration in which the rotation speed of the photosensitive drum 1 in the low-speed mode is 2 / 3 times that in the normal mode is exemplified, the present invention is not limited to this. That is, the speed difference between the low-speed mode and the normal mode is not limited to the above-described one, and the same effect as described above can be obtained even with other speed differences.

[0108] Also, in the present embodiment, although the configuration in which a plurality of light emitting units 50 are arranged in the arrow Y direction to perform multi-exposure in order to compensate for the insufficient amount of light per one light emitting unit 50 is described, the present invention is not limited to this. That is, when the amount of light per one light emitting unit 50 is high and multi-exposure by the light emitting units 50 arranged in the arrow Y direction is not necessary, even if only one row in which a plurality of light emitting units 50 are arranged along the arrow X direction is provided and the above control is performed, the same effect as described above can be obtained.

Explanation of Reference Numerals

[0109] 1... Photosensitive drum 6... Exposure head 42... Light emitting substrate (substrate) 50... Light emitting unit 54…Lower electrode (first electrode layer including a plurality of electrodes) 56…Light-emitting layer 58…Upper electrode (second electrode layer) 72…Chip data conversion unit (image data signal transmission unit) 73…CPU (control unit) 74…Synchronization signal generation unit (control signal generation unit) A…Image forming apparatus

Claims

1. A photosensitive drum; an exposure head having a plurality of light emitting units arranged along a rotation axis direction of the photosensitive drum and emitting light based on image data signals to expose the photosensitive drum; a control signal generating unit that generates a control signal for controlling timing at which the plurality of light emitting units emit light at a predetermined cycle; an image data signal transmitting unit that transmits the image data signal to the exposure head in synchronization with the control signal so as to cause the plurality of light emitting units to emit light in response to the image data signal; a control unit that controls the control signal generating unit, the image data signal transmitting unit, and a rotation speed of the photosensitive drum; Equipped with The control unit is A first mode in which the photosensitive drum is rotated at a first rotation speed and a second mode in which the photosensitive drum is rotated at a second rotation speed that is slower than the first rotation speed and is m / n times the first rotation speed can be executed, In the second mode, in order to perform exposure to form an image for one line on a sheet, the control signal generating unit is controlled to generate the control signal at a period that is 1 / m times the period of the control signal generated in the first mode, and the image data signal transmitting unit is controlled to transmit the same image data signal m times to the exposure head in synchronization with each of the n control signals, and to transmit the image data signal n-m times that does not cause the multiple light-emitting elements to emit light.

2. The image forming apparatus of claim 1, characterized in that in the second mode, the control unit controls the image data signal transmitting unit to transmit the same image data signal m times to the exposure head in synchronization with each of the n control signals in order to perform exposure to form an image of one line on a sheet, and then to transmit the image data signal n-m times that does not cause the multiple light-emitting elements to emit light.

3. a developing unit that develops the electrostatic latent image formed on the surface of the photosensitive drum by the exposure head into a toner image; a transfer section that transfers the toner image formed on the surface of the photosensitive drum onto a sheet; a fixing section that fixes the toner image transferred onto the sheet onto the sheet; Equipped with The control unit is controlling a conveying speed of the sheet passing through the fixing unit based on information regarding a basis weight of the sheet on which an image is formed; When a conveying speed of the sheet passing through the fixing unit is a first conveying speed, the first mode is executed; 3. The image forming apparatus according to claim 1, wherein the second mode is executed when a conveying speed of the sheet passing through the fixing unit is a second conveying speed that is slower than the first conveying speed.

4. the plurality of light-emitting units are provided on the exposure head such that light-emitting rows each composed of the plurality of light-emitting units arranged in the direction of the rotation axis are arranged in a plurality of rows in a direction intersecting the direction of the rotation axis, 4. The image forming apparatus according to claim 1, wherein the exposure head performs multiple exposure of an area corresponding to one pixel on the surface of the photosensitive drum by using the light emitting portions of a plurality of light emitting rows, each of which has a light emitting portion.

5. 5. The image forming apparatus according to claim 4, wherein the plurality of light-emitting sections include a first electrode layer including a plurality of electrodes arranged two-dimensionally in the direction of the rotation axis and the intersecting direction and separated and disposed on a substrate, a light-emitting layer that is stacked on the first electrode layer and emits light when a voltage is applied thereto, and a second electrode layer that is arranged on the opposite side of the light-emitting layer from the side on which the first electrode layer is disposed and that is capable of transmitting light.

Images