Image forming device

Abstract

To provide an image formation apparatus which has an exposure head and can suppress excessive-mounting of toner on a photoreceptor drum even when forming an image in a mode in which the rotational speed of the photoreceptor drum is slow.SOLUTION: In an image formation apparatus, a CPU 73 can execute the normal mode and the slow mode that rotates a photoreceptor drum 1 at a speed that is slower than the rotational speed in the normal mode and is m / n times of the rotational speed in the normal mode, controls a synchronous signal generation unit 74 so as to generate, in the slow mode, a line synchronous signal in a cycle of the 1 / m time of a cycle of the line synchronous signal generated in the normal mode, and controls a chip data conversion unit 72 so as to transmit the same image data signal to an exposure head 6 m times in synchronization with each of the n line synchronous signals and transmit the image data signal that does not cause a light emitting unit 50 to emit light n-m times.SELECTED DRAWING: Figure 16

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus equipped with an exposure head. [Background technology]

[0002] When forming an image with an electrophotographic image forming apparatus, first, an electrostatic latent image is formed on the surface of the photosensitive drum by irradiating the surface of the photosensitive drum with light according to image data, then, a toner image is formed by attaching toner to the electrostatic latent image on the surface of the photosensitive drum by a developing unit, the toner image is transferred to a sheet, and the toner image transferred to the sheet is heated by a fixing unit to fix it to the sheet, thereby forming an image.

[0003] Furthermore, when thick paper is used as the sheet on which an image is formed, the heat capacity of the sheet is large compared to sheets with a relatively small basis weight such as plain paper, so that a large amount of heat is required to fix the toner image to the sheet. Therefore, a configuration is known in which the sheet conveying speed is slower than normal and the time for heating the toner image carried on the sheet by the fixing unit is extended, thereby applying more heat to the toner image and the sheet, and thus stably fixing the toner image even to thick paper. That is, an image forming apparatus with 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 slower than the normal mode. Since the rotation speed of the photosensitive drum is related to the sheet conveying speed, when the low-speed mode is executed, the rotation speed of the photosensitive drum is slower than when the normal mode is executed.

[0004] Patent Document 1 also describes an image forming apparatus that includes a light-emitting unit using an LED or an organic electroluminescence (EL) light, and an exposure head having a lens that focuses the light emitted from the light-emitting unit on the surface of the photosensitive drum, as a device that irradiates the photosensitive drum with light to form an electrostatic latent image. By using an exposure head in this manner, it is possible to reduce the number of parts compared to a laser scanning type configuration in which a laser beam is deflected and scanned by a rotating polygon mirror to form an electrostatic latent image, thereby making it possible to reduce the size of the image forming apparatus and the manufacturing costs. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-112856 A Summary of the Invention [Problem to be solved by the invention]

[0006] In an image forming apparatus equipped with an exposure head, when an image is formed by executing the above-mentioned low-speed mode, the amount of light irradiated onto an area on the photosensitive drum corresponding to one pixel is greater than when an image is formed by executing the normal mode. This is because when executing the low-speed mode, the rotation speed of the photosensitive drum is slower than when executing the normal mode, so the time during which light is irradiated onto an area on the photosensitive drum corresponding to one pixel is longer. In this case, when the electrostatic latent image is developed in the developing unit, too much toner is applied, resulting in increased toner consumption.

[0007] Therefore, the present invention aims to provide an image forming apparatus equipped with an exposure head that can prevent excessive toner from being placed on the photosensitive drum even when forming an image in a mode in which the rotation speed of the photosensitive drum is slow. [Means for solving the problem]

[0008] A representative configuration of an 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 elements 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 generation unit that generates a control signal at a predetermined cycle to control the timing at which the plurality of light-emitting elements emit light, an image data signal transmission unit that transmits the image data signal to the exposure head in synchronization with the control signal to cause the plurality of light-emitting elements to emit light in response 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, and the control unit rotates the photosensitive drum at a first rotation and a second mode in which the photosensitive drum is rotated at a second rotation speed which 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 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 which 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 nm times that does not cause the multiple light-emitting elements to emit light. Effect of the Invention

[0009] According to the present invention, in an image forming apparatus equipped with an exposure head, even when an image is formed in a mode in which the rotation speed of the photosensitive drum is slow, it is possible to prevent excessive toner from being placed on the photosensitive drum. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] 2A and 2B are a perspective view and a cross-sectional view of a photosensitive drum and an exposure head. [Diagram 3] FIG. 2 is a diagram showing the mounting surface of a printed circuit board provided in the exposure head. [Figure 4] FIG. 2 is a schematic diagram of a light-emitting element array chip. [Diagram 5] 1 is a cross-sectional view of a light-emitting element array chip; [Figure 6] FIG. 2 is a schematic diagram for explaining the arrangement of light-emitting units. [Figure 7] 5A and 5B are schematic diagrams for explaining irradiation positions on a photosensitive drum of light emitted from two light-emitting elements adjacent to each other in the sub-scanning direction. [Figure 8] 2 is a block diagram showing a system configuration of an image controller unit and an exposure head. FIG. [Figure 9] 4 is a block diagram showing a configuration of a chip data conversion unit; FIG. [Figure 10] FIG. 2 is a block diagram showing a system configuration of a light-emitting element array chip. [Figure 11] FIG. 4 is a circuit diagram of an image data storage unit. [Figure 12] 6 is a timing chart showing an operation in the main scanning direction in the image data storage unit. [Figure 13] 6 is a timing chart showing the operation in the sub-scanning direction in the image data storage unit in a normal mode. [Figure 14] FIG. 2 is a block diagram showing a configuration of an analog section. [Figure 15] FIG. 4 is a circuit diagram of a drive unit. [Figure 16] 6 is a timing chart showing the operation in the sub-scanning direction in the image data storage unit in a low-speed mode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] <Image forming device> The overall configuration of the image forming apparatus A according to the present invention will be described below with reference to the drawings, together with the operation during image formation. Note that the dimensions, materials, shapes, relative positions, etc. of the components described below are not intended to limit the scope of the present invention, unless otherwise specified.

[0012] 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. Note that in the following description, although the components that use the above-mentioned toners of each color are given the suffixes Y, M, C, and K, the configurations and operations of the components are essentially the same except for the color of the toner used, and therefore the suffixes will be omitted as appropriate unless a distinction is required.

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

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

[0015] The sensor 38 for measuring the basis weight may be, for example, a transmission type optical sensor. A transmission type 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 directed toward the light receiving element, the amount of light received by the light receiving element is reduced. The light receiving element converts the amount of light received into a voltage value and functions as a switch by setting a predetermined threshold value for the amount of voltage.

[0016] Generally, the sheet S on which an image is formed increases in thickness as the basis weight 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, almost no light emitted from the LED passes through. Therefore, by focusing on the relationship between the sensor 38 and the basis weight of the sheet S and measuring the amount of light received by the light receiving element at the leading or trailing end of the sheet S passing between the sensor 38 with the light emitting element's light emission amount kept constant, the basis weight of the sheet S can be predicted. Since there is a tendency for images to be formed less at the leading or trailing end of the sheet S compared to the center, the aim is to improve the measurement accuracy by using this portion.

[0017] The operation unit 300 and the sensor 38 are electrically connected to a CPU 73 shown in Fig. 8. The CPU 73 controls the rotation speed of various rollers that transport the sheet S in accordance with the basis weight of the sheet S detected by the operation unit 300 or the sensor 38, and sets the transport speed of the sheet S during image formation. The transport speed here is defined as the speed of the sheet S when the sheet S passes through the fixing device 94 (fixing section).

[0018] For example, when executing a low-speed mode in which an image is formed on thick paper having a basis weight of a predetermined or more, the CPU 73 forms an image by setting the conveying speed of the sheet S to a lower speed than when executing a normal mode in which an image is formed on plain paper having a basis weight of less than a predetermined value. Since the rotation speed of the photosensitive drum 1 is related to the conveying speed of the sheet S, when executing the low-speed mode (second mode), the CPU 73 sets the rotation speed of the photosensitive drum 1 to a lower speed than when executing the normal mode (first mode).

[0019] This lengthens the time for which the fixing device 94 heats the toner image carried on the sheet S with a large basis weight, thereby applying more heat to the toner image and the sheet S, and thus the toner image can be stably fixed even on the sheet S with a large heat capacity. 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 to increase the resolution in the sub-scanning direction, which is the rotation direction of the photosensitive drum 1, compared to the normal mode.

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

[0021] Meanwhile, in the image forming section, the surface of the photosensitive drum 1Y is first charged by the charging device 2Y. Next, the exposure head 6Y irradiates the surface of the photosensitive drum 10Y with light in accordance with image data read by the image reading section 90 or image data transmitted from an external device (not shown), and forms an electrostatic latent image on the surface of the photosensitive drum 10Y. Thereafter, the developing device 4Y causes yellow toner to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1Y, 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 transported by the transport belt 11 by applying a transfer bias to the transfer device 5Y.

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

[0023] Thereafter, the sheet S carrying the toner image is conveyed by a conveyor belt 97 to a fixing device 94, where it is subjected to a heating and pressure treatment. This causes the toner image on the sheet S to be fixed to the sheet S. Thereafter, the sheet S with the toner image fixed thereto is discharged to a discharge tray 95 by a 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 diagrams showing mounting surfaces on one side and the other side of a printed circuit board 22 provided in the exposure head 6. Fig. 3(c) is an enlarged view of an area V shown in Fig. 3(b).

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

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

[0028] As shown in Fig. 3, 20 light-emitting element array chips 40 are mounted on the printed circuit board 22 in a staggered arrangement in two rows. Each light-emitting element array chip 40 has 748 light-emitting sections 50 arranged at a predetermined resolution pitch in its longitudinal direction (arrow X direction). Each light-emitting element array chip 40 has light-emitting sections 50 arranged at a predetermined pitch in its lateral direction (arrow Y direction). That is, in each light-emitting element array chip 40, the light-emitting sections 50 are two-dimensionally arranged in the arrow X direction and the arrow Y direction, which is a direction perpendicular to (intersecting) the arrow X direction.

[0029] In this embodiment, the resolution pitch of the light-emitting element array chip 40 is 1200 dpi (approximately 21.16 μm). The distance from one end to the other end of the light-emitting section 50 of each light-emitting element array chip 40 in the longitudinal direction is approximately 15.8 mm. That is, the exposure head 6 has a total of 14960 light-emitting sections 50 in the direction of the arrow X, which enables exposure processing corresponding to an image width in the longitudinal direction of approximately 316 mm (≈approximately 15.8 mm × 20 chips).

[0030] In the longitudinal direction of the light-emitting element array chip 40, the interval L1 between the light-emitting sections 50 of adjacent light-emitting element array chips 40 is approximately 21.16 μm. In other words, the longitudinal pitch of the light-emitting sections 50 at the boundary between each light-emitting element array chip 40 is a pitch with a resolution of 1200 dpi. In the lateral direction of the light-emitting element array chip 40 (direction of arrow Y), the interval L2 between the light-emitting sections 50 of the light-emitting element array chips 40 arranged in two rows is approximately 105 μm (equivalent to five 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 direction of the rotation axis of the photosensitive drum 1, and the arrow Y direction, which is the transverse direction of the light-emitting element array chip 40, is the rotation direction of the photosensitive drum 1. The arrow Z direction is the stacking direction in which the layers of the light-emitting section 50 having a layered structure described below overlap, and is also the emission direction of light from the light-emitting section 50. The longitudinal direction of the light-emitting element array chip 40 may be inclined by about ±1° with respect to the rotation axis of the photosensitive drum 1. The transverse direction of the light-emitting element array chip 40 may also 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 MM cross section shown in Fig. 4. Fig. 6 is a schematic diagram for explaining the arrangement of the light-emitting sections 50.

[0034] 4, the light-emitting element array chip 40 has a light-emitting substrate 42 incorporating a circuit section 46 for controlling the light-emitting sections 50, a light-emitting region 44 in which a plurality of light-emitting sections 50 are regularly arranged on the light-emitting substrate 42, and wire-bonding pads 48. Signals are input and output between the outside of the light-emitting element array chip 40 and the circuit section 46, and power is supplied to the circuit section 46 through the wire-bonding pads 48. The circuit section 46 can be an analog driving circuit, a digital control circuit, or a circuit including both.

[0035] As shown in FIG. 5, the light-emitting section 50 is composed of 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 directions of arrows X and Y, a light-emitting layer 56, and an upper electrode 58.

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

[0037] The upper electrode 58 (second electrode layer) is laminated on the light emitting layer 56 at a position opposite to the side on which the lower electrode 54 is disposed with respect to the light emitting layer 56. The upper electrode 58 is an electrode that can transmit (is transmissive to) light of 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 image data, generating a potential difference between the selected lower electrode 54 and the upper electrode 58. When a potential difference is generated between the upper electrode 58, which is an anode, and the lower electrode 54, which is a 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 recombination of electrons and holes in the light-emitting layer 56 causes the light-emitting layer 56 to emit light.

[0039] When the light-emitting layer 56 emits light, the light travels toward the upper electrode 58 and is emitted after passing through the upper electrode 58. Light travels from the light-emitting layer 56 toward the lower electrode 54 is reflected by the lower electrode 54 toward the upper electrode 58, and the reflected light also travels through the upper electrode 58 and is emitted. In this manner, the light-emitting section 50 emits light. Note that although there is a time difference in the emission timing between the light emitted from the light-emitting layer 56 directly toward the upper electrode 58 and the light reflected by the lower electrode 54 and emitted from the upper electrode 58, the emission can be regarded as being nearly simultaneous because the layer thickness of the light-emitting section 50 is extremely thin.

[0040] In this embodiment, the light emitting substrate 42 is a silicon substrate. The upper electrode 58 is preferably transparent to the emission 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. In this embodiment, the upper electrode 58 is an anode provided in common to each of the lower electrodes 54, but it may be configured to provide an individual electrode for each of the lower electrodes 54, or a single upper electrode 58 may be provided for each of the plurality of lower electrodes 54.

[0041] The light-emitting layer 56 may be an organic EL film or an inorganic EL layer. 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 necessary. The light-emitting layer 56 may be formed continuously in the direction of the arrow X, or may be divided into portions of the same size as the lower electrodes 54. Each lower electrode 54 may be divided into a plurality of groups, and one light-emitting layer 56 may be laminated on the upper portion of the lower electrodes 54 belonging to each divided group.

[0042] When using a moisture-sensitive light-emitting material such as an organic EL layer or an inorganic EL layer as the light-emitting layer 56, it is desirable to seal the light-emitting region 44 to prevent moisture from entering the region. As a sealing method, for example, a sealing film is formed by forming a single or laminated thin film of silicon oxide, silicon nitride, aluminum oxide, etc. As a method for forming the sealing film, a method that has 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, and forming method of the sealing film are merely examples and are not limited to the above examples, and appropriate methods may be selected.

[0043] The lower electrode 54 is preferably made of a metal having 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. The lower electrode 54 is formed by a Si process together with the formation of the circuit section 46, and is directly connected to the driving section of the circuit section 46. By forming the lower electrode 54 by a Si process in this way, the process rule is about 0.2 μm, which is highly accurate, so that the lower electrodes 54 can be arranged with high accuracy and high density. Furthermore, since the lower electrodes 54 can be arranged with high density, most of the light-emitting region 44 can be made to emit light, and the utilization efficiency of the light-emitting region 44 can be improved. The organic material of the light-emitting layer 56 is filled between each of the lower electrodes 54, and each of the lower electrodes 54 is partitioned by the organic material.

[0044] Here, the current flowing through the light-emitting unit 50 and the amount of emitted light are in a substantially proportional relationship. Therefore, by controlling the current flowing through the light-emitting unit 50, the amount of light of the light-emitting unit 50 can be controlled. The light-emitting unit 50 has a threshold voltage, and when the voltage applied to both ends of the light-emitting unit 50 becomes equal to or higher than the threshold voltage, a current starts to flow through the light-emitting unit 50, and thereafter the current flows substantially linearly. Since the threshold voltage of each light-emitting unit 50 varies, the voltage at which the current starts to flow in each light-emitting unit 50 differs slightly. Therefore, before the product is shipped from the factory, the light-emitting units 50 of the light-emitting element array chip 40 are individually and sequentially made to emit light, and the current flowing through the light-emitting unit 50 is adjusted so that the light collected through the rod lens array 23 has a predetermined amount of light. In addition to the above-mentioned light amount adjustment, the exposure head 6 is also subjected to focus adjustment to adjust the distance between the light-emitting element array chip 40 and the rod lens array 23 before the product is shipped from the factory.

[0045] 6, the light emitting units 50 are two-dimensionally arranged at predetermined intervals in the directions of the arrow X and the arrow Y in the light emitting region 44. That is, in the light emitting region 44 of one light emitting element array chip 40, a plurality of light emitting units 50 are arranged along the direction of the arrow X, and a light emitting row is formed by the plurality of light emitting units 50 arranged along the direction of the arrow X. The light emitting rows are arranged in a plurality of rows in the direction of the arrow Y, and in this embodiment, the number of rows of the light emitting rows is six.

[0046] In this embodiment, the width W1 of the light-emitting section 50 in the direction of the arrow X is 20.90 μm, and the interval d1 between adjacent light-emitting sections 50 in the direction of the arrow X is 0.26 μm. That is, the light-emitting sections 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the direction of the arrow X. The width W2 of the light-emitting section 50 in the direction of the arrow Y is also 20.90 μm like the width W1, and the interval d2 is also 0.26 μm like the interval d1, so that the light-emitting sections 50 are also arranged at a pitch of 21.16 μm (1200 dpi) in the direction of the arrow Y. That is, the light-emitting section 50 in this embodiment is in the shape of a square with one side measuring 20.90 μm, and its area is 436.81 μm. 2 This is the size of one pixel, which is 447.7456 μm 2 The organic light-emitting material emits less light than an LED. In response to this, by making the light-emitting section 50 square and reducing the distance between adjacent light-emitting sections 50 as described above, it is possible to ensure a light-emitting area sufficient to obtain a light quantity sufficient to change the potential of the photosensitive drum 1.

[0047] It is desirable to secure an area of ​​the light-emitting section 50 of 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 section 50 to be about 20.07 μm or more. 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 section 50 to be about 10.04 μm or more. The shape of the light-emitting section 50 is not limited to a square, and may be a polygon having a size of four or more sides, a circle, an ellipse, or the like, as long as the light is emitted in an exposure area size 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. The interval d2 between the light-emitting sections 50 adjacent to each other in the direction of the arrow Y and the number of rows of the light-emitting sections 50 in the direction of the arrow Y are determined based on the scanning speed of the exposure head 6, the amount of light required for exposure processing, the resolution, and the like.

[0048] FIG. 7 is a schematic diagram for explaining the irradiation position on the photosensitive drum 1 of the light emitted from two light-emitting units 50 that are overlapping in the direction of the arrow Y. As shown in FIG. 7(a), when two light-emitting units 50 that are overlapping in the direction of the arrow Y are turned on simultaneously, the irradiation position on the photosensitive drum 1 of the light H emitted from the two light-emitting units 50 shifts in the rotation direction of the photosensitive drum 1 (the direction of the arrow Y, the sub-scanning direction) in the same manner as the positional relationship of the two light-emitting units 50. In contrast, as shown in FIG. 7(b), when the timing of turning on the two light-emitting units 50 is 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. Irradiating the photosensitive drum 1 with light from a plurality of light-emitting units 50 arranged in the direction of the arrow Y in this manner is called multiple exposure. The more the number of light-emitting units 50 arranged in the direction of the arrow Y used for multiple exposure is, the greater the amount of light received on a portion of the photosensitive drum 1 when multiple exposure is performed.

[0049] In order to align the irradiation positions on the photosensitive drum 1 of the light emitted from the two light-emitting units 50 that are overlapping in the direction of the arrow Y, it is necessary to delay the turn-on timing of the light-emitting unit 50 on the downstream side in the rotation direction of the photosensitive drum 1 by a delay amount T relative to the turn-on timing of the light-emitting unit 50 on the upstream side. Here, the delay amount T (μs) is calculated by the following formula 1, where the rotation speed of the photosensitive drum 1 is Vdr (mm / s), the width is W2 (μm), and the interval is d2 (μm).

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

[0051] Furthermore, in this embodiment, an emission signal is generated so that the maximum emission time Tw(s) of each light-emitting element 50 is equal to one line time in the sub-scanning direction, and is expressed by the following equation 2 using the resolution of 1200 dpi and the rotational speed Vdr(mm / s) of the photosensitive drum 1. (Formula 2) Tw = (25.4 ÷ 1200) ÷ Vdr

[0052] <Exposure head system configuration> Next, a description will be given of the system configuration of the image controller unit 70 provided on the main body side of the image forming apparatus A and the exposure head 6. Although the following describes processing for a single color out of the four colors of yellow, magenta, cyan, and black, when performing image forming operations, similar processing is performed in parallel for the above four colors.

[0053] Fig. 8 is a block diagram showing a system configuration of the image controller section 70 and the exposure head 6. As shown in Fig. 8, the image controller section 70 includes an image data signal generating section 71 (image data signal generating section), a chip data converting section 72 (image data signal transmitting section), a CPU 73 (control section), and a synchronization signal generating section 74 (control signal generating section).

[0054] The image controller unit 70 processes image data and image formation timing using the above-mentioned components, and transmits control signals for controlling the exposure head 6 to the printed circuit board 22 of the exposure head 6. Specifically, the control signals are an image data signal, a chip select signal, a clock signal, a line synchronization signal, and a communication signal for the CPU 73, and these signals are transmitted from the image controller unit 70 to the exposure head 6 via each signal line described below.

[0055] The image data signal generating unit 71 receives image data of a document read by the image reading unit 90 and image data transferred from an external device via a network. The image data signal generating unit 71 performs dithering processing on the input image data at a resolution instructed by the CPU 73, and generates an image data signal for outputting an image. In this embodiment, the image data signal generating unit 71 performs dithering processing at a resolution of 1200 dpi. The image data signal has a width of 3 bits and represents eight gradations of density values ​​0 to 7. Density value 0 is the minimum density, and density value 7 is the maximum density. The CPU 73 transmits communication signals via a communication signal line 79 to the image data signal generating unit 71 to give various instructions.

[0056] The synchronization signal generating unit 74 periodically generates a line synchronization signal that represents a division of each line in the main scanning direction of the image data signal. In other words, the synchronization signal generating unit 74 periodically generates a line synchronization signal that is a control signal that controls 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 determines that the period during which the surface of the photosensitive drum 1 moves in the rotation direction by a pixel size corresponding to the resolution in the sub-scanning direction for a preset rotation speed of the photosensitive drum 1 is one line period, and instructs the synchronization signal generating unit 74 to generate the period of the line synchronization signal. Specifically, the CPU 73 instructs the synchronization signal generating unit 74 to generate a line synchronization signal every time the photosensitive drum 1 rotates by a pitch (about 21.16 μm) of the resolution of 1200 dpi during execution of the normal mode. For example, when the photosensitive drum 1 rotates at 200 mm / s in the normal mode, the synchronization signal generating unit 74 generates a line synchronization signal at a period of 105.8 μs.

[0057] As described later, the chip data conversion unit 72 has a line buffer 15, and receives and stores image data signals from the image data signal generation unit 71. Then, in synchronization with a line synchronization signal generated by a synchronization signal generation unit 74 and input via a line synchronization signal line 78, the chip data conversion unit 72 divides one line's worth of image data signals to each light-emitting element array chip 40 in response to an instruction from the CPU 73, and transmits the divided signals via an image data signal line 77. The chip data conversion unit 72 also transmits a chip select signal, which indicates the effective range of the clock signal and the image data signal, to the light-emitting element array chip 40 via a chip select signal line 75 and a clock signal line 76.

[0058] The head information storage unit 171 provided in the exposure head 6 is connected to the CPU 73 via a communication signal line 79. The head information storage unit 171 stores information on the amount of light emitted and the mounting position of each light-emitting element array chip 40 as head information. In addition, 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 setting values ​​of each signal input from the image controller unit 70 via the above-mentioned signal lines. In addition, 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 the 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 control signals 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. This causes the image data signal to be 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 one line of image data signal is stored in the order of Buf000 → Buf001 → Buf002 → Buf003 → Buf000 → Buf001, . . . The buffer write control unit 16 transmits a write completion notification signal to the buffer read control unit 17 every time one line of image data signal is stored in the line buffer 15. Furthermore, when the data storage area of ​​the line buffer 15 is full, the buffer write control unit 16 stops transmitting the reception possible notification signal to the image data signal generation unit 71 and stops outputting the image data signal.

[0062] When the buffer read control unit 17 receives the write completion notification signal, it divides the image data signal for one line stored in the line buffer 15 in synchronization with the line synchronization signal so that it can 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. 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 free. In addition, the buffer read control unit 17 can also transmit an image data signal for turning off all the light-emitting elements 50 of the exposure head 6 to the light-emitting element array chip 40 in response to an instruction from the CPU 73, instead of the image data signal stored in the line buffer 15. The image data signal for turning off all the light-emitting elements 50 of the exposure head 6 is an image data signal of all "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] Fig. 10 is a block diagram showing the system configuration of the light-emitting element array chip 40. As shown in Fig. 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 in the digital section 80, as will be described later.

[0065] The digital unit 80 includes a communication IF unit 81, a register unit 82, a chip select signal generation unit 83, an image data storage unit 84, and a pulse signal generation unit 85. The digital unit 80 uses these components to generate a pulse signal for causing the light emitting unit 50 to emit light in synchronization with a clock signal, based on a preset setting value, a chip select signal, an image data signal, and a line synchronization signal, which are determined by a communication signal.

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

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

[0068] The pulse signal generating unit 85 generates a pulse signal that controls the light emission timing of the light emitting unit 50 based on the pulse signal width information stored in the register unit 82 and the period of the line synchronization signal, and outputs the generated pulse signal to the lighting control unit 88. The lighting control unit 88 selects, for each light emitting unit 50, whether or not to output the pulse signal generated by the pulse signal generating unit 85 to the analog unit 86 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 section> Next, the configuration of the image data storage unit 84 will be described.

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

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

[0072] Similarly, the flip-flop circuits 32 to 37 are provided in the same number as the light-emitting units 50 in the direction of the arrow X 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 a clock signal to the flip-flop circuit 31 only when the chip select signal is valid. The flip-flop circuit 31 operates on the clock signal sent from the clock gate circuit 30, and outputs an image data signal (dly_data_000).

[0074] The flip-flop circuit 32-000 receives the output of the flip-flop circuit 31-000 as an input and operates in response to a 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 receives the output of the flip-flop circuit 32-000 as an input and operates in response to a 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 receives the output of the flip-flop circuit 33-000 as an input and operates in response to a 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 receives the output of the flip-flop circuit 34-000 as an input and operates in response to a 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 receives the output of the flip-flop circuit 35-000 as an input and operates in response to a 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 receives the output of the flip-flop circuit 36-000 and operates in response to a line synchronization signal. The output of the flip-flop circuit 37-000 (buf_data_5_000) is input to the lighting control unit 88.

[0080] 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 operates in the same manner as the above-described flip-flop circuits 32-000 to 37-000.

[0081] FIG. 12 is a timing chart showing the operation of the image data storage unit 84 in the main scanning direction. The symbols in FIG. 12 have the same meaning as those in FIG. 11. As shown in FIG. 12, during the time period from T0 to T1 when the low output of the chip select signal is captured by the rising edge of the clock signal, the image data signal shifts in the order of data → dly_data_000 → dly_data_001. The low output of the chip select signal is input by 748 clock signals, which is the same number as the number of light emitting units 50 in the main scanning direction. As a result, one line's worth of image data signal is held in dly_data_000 to dly_data_747.

[0082] After time T1, the chip select signal is held high and no shift operation is performed. When the low output of the line synchronization signal is captured by the rising edge of the clock signal at time T2, the image data signal for one line is shifted simultaneously as buf_data_0_000 to buf_data_0_747 in the order of dly_data_000 → buf_data_0_000 → dly_data_001 → buf_data_0_001, and output to the lighting control unit 88.

[0083] Fig. 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 in Fig. 13 are the same as those in Fig. 11. Below, the outputs buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, and buf_data_5_000 of the flip-flop circuits 32-000, 33-000, 34-000, 35-000, 36-000, and 37-000 shown in Fig. 11 will be described. Although not explained below, the same applies to buf_data_0_001 to buf_data_0_747, buf_data_1_001 to buf_data_1_747, buf_data_2_001 to buf_data_2_747, buf_data_3_001 to buf_data_3_747, buf_data_4_001 to buf_data_4_747, and buf_data_5_001 to buf_data_5_747.

[0084] 13, each time the line synchronization signal rises from low to high, it shifts from dly_data_000 to buf_data_0_000, buf_data_0_000 to buf_data_1_000, etc. For this reason, the value of B000 of dly_data_000 at time S0 is output to the lighting control unit 88 as buf_data_0_000 at time S1, buf_data_1_000 at time S2, and buf_data_2_000 at time S3.

[0085] In this manner, multiple exposure is realized by connecting buf_data_0_000, buf_data_1_000, buf_data_2_000, buf_data_3_000, buf_data_4_000, and buf_data_5_000 in order from the light-emitting element 50 that is exposed first on the photosensitive drum 1.

[0086] <Analog section> Next, a description will be given of the configuration of the analog unit 86. In the following description, two drive units 61 that drive two light-emitting units 50 will be described, but all of the light-emitting units 50 are driven in the same manner by the same drive unit 61.

[0087] Fig. 14 is a block diagram showing the configuration of the analog unit 86. As shown in Fig. 14, the analog unit 86 includes a drive unit 61 that drives the light emitting unit 50, a DAC 62 that is a digital-to-analog converter, and a drive unit selection unit 67.

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

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

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

[0091] 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] The MOSFET 112 supplies a drive current to the light emitting unit 50 according to the value of the gate voltage, and controls the current so that the drive current is turned off (light is turned off) when the gate voltage is at a low level. A signal line 63 is connected to the gate of the MOSFET 114. When a pulse signal input via a signal line 66 is high, the MOSFET 114 passes the voltage charged in the capacitor 116 to the MOSFET 112.

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

[0094] Through the above operation, MOSFET 112 supplies a drive current to light-emitting unit 50 in accordance with the set analog voltage and pulse signal. Furthermore, if the input capacitance of light-emitting unit 50 is large and the response speed when turned off is slow, the response speed when turned off can be increased by MOSFET 113. A signal obtained by logically inverting the pulse signal by 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 stored in the input capacitance of light-emitting unit 50 is forcibly discharged.

[0095] <Light emitting unit control in low speed mode> As described above, the CPU 73 can execute the normal mode and the low-speed mode during image formation, and when switching between these modes, changes the rotation speed of the photosensitive drum 1. Here, when the CPU 73 controls the synchronization signal generating unit 74 in the low-speed mode, as in the normal mode, by setting the time it takes for the photosensitive drum 1 to rotate by an amount corresponding to the pixel size corresponding to the resolution in the sub-scanning direction as the period of the line synchronization signal, the following problem arises.

[0096] That is, when the rotation speed of the photosensitive drum 1 decreases, the period of the line synchronization signal becomes longer, and therefore, an increased amount of light is irradiated onto an area corresponding to one pixel on the photosensitive drum 1. For example, if the rotation speed of the photosensitive drum 1 in the low-speed mode is half that of the normal mode, the period of the line synchronization signal becomes twice that of the normal mode, and the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 becomes twice as much. If the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1 increases, too much toner will be placed on the photosensitive drum 1 when the electrostatic latent image is developed.

[0097] In response to this, a configuration may be considered in which, in the low-speed mode, the value of the current for driving the light-emitting unit 50 is lowered compared to the normal mode to reduce the amount of light, thereby preventing excessive toner from being deposited on the photosensitive drum 1. However, a configuration in which the amount of light of the light-emitting unit 50 is variable would complicate the configuration of the circuit for driving the light-emitting unit 50. Thus, the CPU 73 performs the control described below in the low-speed mode to reduce the amount of light irradiated onto an area corresponding to one pixel on the photosensitive drum 1, thereby preventing excessive toner from being deposited on the photosensitive drum 1.

[0098] 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 in FIG. 16 are the same as those 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 rotational speed of the photosensitive drum 1 in the low-speed mode is m / n times the rotational speed of the photosensitive drum 1 in the normal mode. Since the rotational 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 it takes for the photosensitive drum 1 to move by a pixel size corresponding to the resolution in the sub-scanning direction in the rotational 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. Note that the period of the line synchronization signal in the low-speed mode obtained in this way can also be said to be the time obtained by dividing the time it takes 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 rotational speed (the first rotational speed) in the normal mode and m / n times the rotational speed in the normal mode (the second rotational speed), the CPU 73 controls the synchronization signal generation unit 74 as follows. That is, in the low-speed mode, the CPU 73 controls the synchronization signal generation unit 74 to generate the line synchronization signal at a period that is 1 / m times the period of the line synchronization signal generated in the normal mode.

[0102] Next, the CPU 73 controls the chip data conversion unit 72 as follows. That is, the CPU 73 makes the buffer read control unit 17 read one line of image data signals from Buf000 of the line buffer 15 in synchronization with the first and second line synchronization signals 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, and transmit the image data signals to the light-emitting element array chip 40. That is, the buffer read control unit 17 transmits the same one line 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 (to be turned off) 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 image data signal for one line twice, and transmits an image data signal for not causing the light emitting unit 50 to emit light once, among the three image data signals transmitted to the exposure head 6 in synchronization with the three line synchronization signals, respectively. The number of times that the buffer read control unit 17 transmits the image data signal is generalized using the above n and m as follows. That is, the CPU 73 controls the buffer read control unit 17 to output the same image data signal for one line m times and output the image data signal for not causing the light emitting unit 50 to emit light nm times in synchronization with 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 to perform exposure to form an image of one line extending in the main scanning direction (the direction of the rotation axis 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 by the chip data conversion unit 72 transmitting the same image data signal for one line twice to the light emitting element array chip 40 is equivalent 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. 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 is true for other times. Therefore, in the low-speed mode, the CPU 73 controls the chip data conversion unit 72 and the synchronization signal generation unit 74 as described above, so that the amount of light irradiated on an area corresponding to one pixel on the photosensitive drum 1 can be made equivalent in the normal mode and the low-speed mode.

[0105] The CPU 73 instructs the buffer read control unit 17 to transmit the image data signal to the exposure head 6 in synchronization with the second line synchronization signal, and then to transmit the read completion notification signal to the buffer write control unit 16. When the buffer write control unit 16 receives the read completion notification signal, it 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, it transmits the next one line's worth of image data signals to the buffer write control unit 16. The buffer write control unit 16 stores the next one line's worth of image data signals received from the image data signal generation unit 71 in the now empty line buffer Buf000. The CPU 73 also instructs the buffer read control unit 17 to read the image data signal from Buf001 in synchronization with the line synchronization signal generated at time S1A, and transmit it to the printed circuit board 22. The CPU 73 repeatedly executes the above control even after time S1B to perform the exposure process.

[0106] To summarize 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 normal mode rotation speed and m / n times the normal mode rotation speed, the CPU 73 performs the following control. That is, in the low-speed mode, the CPU 73 controls the synchronization signal generating unit 74 to generate a line synchronization signal at a period 1 / m times the period of the line synchronization signal generated in the normal mode. In addition, the CPU 73 controls the buffer read control unit 17 to transmit the same image data signal m times in synchronization with each of the n line synchronization signals generated by the synchronization signal generating unit 74, and transmit an image data signal nm times for preventing the light emitting unit 50 from emitting light, in order to perform exposure for forming an image for one line extending in the main scanning direction on the sheet S. This makes it possible to equalize the amount of light irradiated to an area corresponding to one pixel on the photosensitive drum 1 in the normal mode and the low-speed mode. Therefore, it is possible to suppress excessive toner from being placed on the photosensitive drum 1 in the low-speed mode.

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

[0108] In addition, in this embodiment, a configuration has been described in which a plurality of light-emitting units 50 are arranged in the direction of the arrow Y to perform multiple exposure in order to compensate for a lack of light intensity per light-emitting unit 50, but the present invention is not limited to this. That is, when the light intensity per light-emitting unit 50 is high and multiple exposure by light-emitting units 50 arranged in the direction of the arrow Y is not necessary, the same effect as above can be obtained even if the above control is performed by providing only one row of multiple light-emitting units 50 arranged in the direction of the arrow X. [Explanation of symbols]

[0109] 1...Photosensitive drum 6...Exposure head 42...Light emitting substrate (substrate) 50…Light emitting part 54...Lower electrode (first electrode layer including multiple electrodes) 56...Emitting layer 58...Top electrode (second electrode layer) 72...Chip data conversion unit (image data signal transmission unit) 73...CPU (control unit) 74...Synchronization signal generating unit (control signal generating unit) A: Image forming device

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