JP2005329636A - Line head and image forming apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a line head configured so that image quality does not deteriorate even when a driving time exceeds a certain time, and an image forming apparatus using the line head.
In the example of FIG. 1B, the light emitting element is initially controlled at a constant current with a constant current Fa. When the driving time of the light emitting element exceeds 250 hours, the driving current is increased from Fa to Fb to control the light emitting element at a constant current. For this reason, the amount of emitted light is reduced from Ia to Ix, but is improved to Ib. The driving voltage of the light emitting element has the characteristics shown in FIG. The measured drive voltage is compared with a preset threshold value Vth, and when the threshold value is exceeded, the drive current becomes Fb and the drive voltage at this time is boosted to Vb.
[Selection] Figure 1

Description

  The present invention relates to a line head and an image forming apparatus using the same, which are configured so that image quality is not deteriorated even when a driving time exceeds a certain time by performing constant current control of an organic EL element.

  An image forming apparatus using a line head provided with a large number of light emitting elements in one line as an exposure means has been developed. Patent Document 1 describes an image forming apparatus in which a light emitting element array composed of a plurality of light emitting elements is integrated on a single chip to form an exposure means. In this example, the single-chip light-emitting element array for each color is formed on a single substrate and then separated and placed in the developing device for each color, thereby eliminating variations in light emission characteristics.

  In addition to LEDs, organic EL elements have been proposed as light-emitting elements applied to line heads. The organic EL element has an advantage that the control system can be simplified because static control is possible. In a line head using a plurality of light emitting elements made of organic EL elements, the light emitting elements may be driven by constant voltage control in order to keep the light amount at a constant value.

  FIG. 11 is a characteristic diagram showing the relationship between the driving time (h) and the amount of light emitted from a light emitting element made of an organic EL element. As shown in FIG. 11A, it is assumed that a constant voltage Va is applied to each light emitting element. In this case, as shown in FIG. 11B, the amount of emitted light is constant at Ia until the driving time is constant time, and in this example, up to 200 hours. When the driving time exceeds 200 hours, the amount of emitted light starts to decrease from Ia to Ix, and when the driving time exceeds 250 hours, the amount of emitted light further decreases to become a characteristic like Iy.

JP 11-138899 A

  The technique described in Patent Document 1 also refers to the use of an organic EL element in addition to an LED as a light emitting element. However, when the organic EL element is controlled at a constant voltage, as shown in FIG. 11, no countermeasure is disclosed for reducing the amount of emitted light when the driving time exceeds a certain value. For this reason, when the organic EL element is attached to the line head for driving, there is a problem that it is not possible to cope with a situation where the amount of emitted light decreases and the image quality deteriorates when the driving time exceeds a certain value.

  The present invention has been made in view of such problems of the prior art, and an object of the present invention is to control the organic EL element at a constant current so that the image quality does not deteriorate even when the driving time exceeds a certain time. And a line head and an image forming apparatus.

  The line head of the present invention that achieves the above object includes a light-emitting element composed of a plurality of organic EL elements arranged in one line, means for controlling the light-emitting element at a constant current, and means for measuring the light emission state of the light-emitting element. And a means for comparing the measured value of the measuring means with a reference value, and performing constant current control of the light emitting element based on the output of the comparing means. By performing constant current control of a light emitting element composed of an organic EL element with such a configuration, it is possible to compensate for a decrease in the amount of light emitted due to aging deterioration of the light emitting element and to prevent deterioration in image quality.

  The line head according to the present invention is characterized in that the measuring means measures a driving voltage of the light emitting element. As described above, by measuring the amount of electricity, it is possible to compensate for the characteristic characteristic inherent in the organic EL element, in which the amount of emitted light decreases when the driving time exceeds a certain time, thereby preventing deterioration of the image quality. it can.

  In the line head according to the aspect of the invention, the reference value may be a drive voltage threshold value measured by the light emitting element. As described above, constant current control is performed by always comparing a preset threshold value with the driving voltage of the light emitting element, so that it is possible to reliably prevent a decrease in the amount of emitted light when the driving time exceeds a certain time. I can do it.

  The line head of the present invention is characterized in that constant current control of the light emitting element is performed by feedback control based on a measurement value of the measuring means. Thus, since the control system which always feeds back a measured value to a constant current control means is comprised, the constant current control of a light emitting element can be performed accurately.

In the line head according to the invention, the measuring means measures the amount of light emitted from the light emitting element. By measuring the optical characteristics of the light emitting element in this way, it is possible to compensate for a decrease in the amount of emitted light when the driving time exceeds a certain time, and to prevent image quality deterioration.

  In the line head of the present invention, the constant current control of the light emitting element is performed for each individual light emitting element. As described above, constant current control is performed for each individual light emitting element, so that high-quality image formation can be performed even when the driving time of each light emitting element exceeds a certain value.

  In the line head of the present invention, the constant current control of the light emitting elements is performed in units of blocks by dividing a plurality of light emitting elements arranged in one line into a plurality of blocks. As described above, constant current control is performed in units of blocks, so that deterioration of image quality can be prevented when various image patterns are formed.

The line head of the present invention is characterized in that the constant current control of the light emitting elements is simultaneously performed on a plurality of light emitting elements arranged in one line.
With such a configuration, constant current control for a plurality of light emitting elements is simplified.

  In the line head according to the present invention, the constant current control of the light emitting element is a driving current for recovering the light emission amount to a predetermined value when the drive amount of the light emitting element exceeds a certain time and the light emission amount decreases. It is characterized in that it is performed in a stepwise manner. As described above, since the control for the light emitting element is constant current control in which the current is changed stepwise according to the driving time of the light emitting element, the image quality can be prevented from being deteriorated by simple means.

  The line head according to the present invention is characterized in that a plurality of lines in which the plurality of light emitting elements are arranged are formed in the sub-scanning direction. As described above, since a plurality of light emitting element lines are provided in the sub-scanning direction of the line head, it is possible to cope with multiple exposures, installation of a spare row, and the like, and an image forming apparatus using the line head is applied to various uses. it can.

  The image forming apparatus according to the present invention includes at least two image forming stations in which image forming units including a charging unit, the line head described above, a developing unit, and a transfer unit are arranged around an image carrier. Two or more are provided, and the image is formed in a tandem manner by passing the transfer medium through each station. For this reason, in the tandem image forming apparatus, by performing constant current control of a light emitting element made of an organic EL element, it is possible to prevent image quality deterioration when the driving time of the light emitting element exceeds a certain time.

  The image forming apparatus of the present invention includes an image carrier configured to carry an electrostatic latent image, a rotary development unit, and any one of the line heads described above, and the rotary development unit includes a plurality of rotary development units. The toner contained in the toner cartridge is carried on the surface thereof, and the toners of different colors are sequentially conveyed to a position facing the image carrier by rotating in a predetermined rotation direction. A developing bias is applied between the developing unit and the toner is moved from the rotary developing unit to the image carrier, whereby the electrostatic latent image is visualized to form a toner image. To do. For this reason, in a rotary image forming apparatus, by performing constant current control of a light emitting element made of an organic EL element, it is possible to prevent image quality deterioration when the driving time of the light emitting element exceeds a certain time.

  In addition, the image forming apparatus of the present invention includes an intermediate transfer member. For this reason, in an image forming apparatus provided with an intermediate transfer member, by performing constant current control of a light emitting element made of an organic EL element, it is possible to prevent image quality deterioration when the driving time of the light emitting element exceeds a certain time. Can do.

  According to the present invention, it is possible to obtain a line head and an image forming apparatus having a configuration in which image quality is not deteriorated even when the driving time exceeds a certain time by performing constant current control of the organic EL element.

The present invention will be described below with reference to the drawings. FIG. 1 shows a driving time-detection voltage (a) when an organic EL element is used as a light emitting element in a line head.
It is a characteristic view showing the relationship of drive time-drive current (b), drive time-light emission quantity (c). In FIG. 1, the constant current Fa is used for control from the start of driving the light emitting element to 200 hours (b). The amount of light emitted during this period is constant at Ia (c). Further, the detection voltage of the light emitting element is Va, which is lower than the threshold value Vth (a). After 200 hours from the start of driving the light emitting element, the amount of emitted light tends to be lower than Ia (c). This is because, due to the material of the organic EL element, etc., it is due to an increase in resistance as the temperature rises, and the light emission efficiency is reduced for the same current.

  As described above, after 200 hours have elapsed from the start of driving of the light emitting element, the resistance of the light emitting element increases, and therefore, when a constant current is passed through the light emitting element, the detection voltage increases (a). In the present invention, when the current of the light emitting element is controlled at a constant current, the voltage of the light emitting element is detected and compared with a preset threshold voltage Vth (reference value), when the detected voltage exceeds the threshold voltage Vth The constant current control is performed by raising the drive current to Fb (b).

  As described above, the timing for performing the constant current control by raising the drive current from Fa to Fb is the time when the drive time of the light emitting element exceeds 250 hours in the example of FIG. At this time, the amount of emitted light recovers from Ix to Ib, and becomes substantially equal to the amount of emitted light Ia at the beginning of driving (c). Further, the detection voltage of the light emitting element rises from Vx to Vb.

  As can be seen from FIG. 1C, when the light emitting element is under constant current control and the driving time of the light emitting element approaches a certain time, for example, more than 200 hours and approaches 250 hours, the amount of emitted light decreases. A desired image cannot be formed. At this time, the detection voltage of the light emitting element approaches the level of the reference value (threshold voltage Vth) as shown in FIG.

  Accordingly, when constant current control is performed, as shown in FIG. 1C, the amount of light emitted from the light emitting element is detected by the sensor, so that a change in the amount of light emitted with time is judged and image quality deterioration is prevented. Control to increase the current step by step can be performed so as to prevent it. In addition, as shown in FIG. 1A, by detecting the driving voltage of the light emitting element, the change in the amount of emitted light is similarly judged in a stepwise manner so as to prevent image quality deterioration in advance. Control for increasing the current can be performed.

  In the example of FIGS. 1A to 1C, the driving current is changed according to the driving time of the light emitting element to control the light emitting element at a constant current. As described above, the current control for the light emitting element according to the embodiment of the present invention is a constant current control in which the current value to be applied is changed stepwise according to the driving time of the light emitting element. Can be prevented.

  FIG. 2 is a block diagram illustrating a schematic configuration of the image forming apparatus. In FIG. 2, 2 is a control unit of the line head, 3 is a control circuit, 4 is a drive circuit comprising TFTs, 5 is a voltage detector, 6 is a memory, and 7 is a plurality of light emitting elements Ea in one line (main scanning direction). A light emitting element line 8 arranged in a plurality of rows in the sub-scanning direction, 8 is a main body controller. The driving voltage of each light emitting element detected by the voltage detector 5 is fed back to the control circuit 3.

  The main body controller 8 forms print data and transmits it to the control unit 2 of the line head. The memory 6 stores the characteristics of each light emitting element Ea, for example, the threshold voltage Vth as shown in FIG. 1A and the driving time-light emission quantity characteristics as shown in FIG. The main body controller 8 creates a drive time-applied current characteristic as shown in FIG. 1B and transmits it to the control circuit 3. The control circuit 3 stores the characteristic in the memory 6.

  The control circuit 3 reads the threshold voltage Vth from the memory 6 and compares it with the detected voltage of the light emitting element fed back. Based on the comparison result, a control signal for each light emitting element is formed, and the drive circuit 4 is operated to perform constant current control of each light emitting element. As described above, constant current control is performed for each individual light emitting element by the drive circuit 4, so that even when the driving time of each light emitting element exceeds a certain value, high-quality image formation can be performed.

  The driving circuit 4 performs constant current control by applying a predetermined current to each individual light emitting element Ea. However, all the light emitting elements in one line in the main scanning direction may be driven with the same current. Is possible. Although the light emitting characteristics of the individual light emitting elements are different, the degree of image quality deterioration does not become a significant form even when driven by the same current. With such a configuration, constant current control for a plurality of light emitting elements is simplified.

  In the example of FIG. 2, a plurality of light emitting element lines are provided in the sub-scanning direction of the line head. For this reason, the line head can be applied to multiple exposure. Further, image formation can be performed with one line in the main scanning direction, and the other lines can be used for backup when the image forming line fails. As described above, in the example of FIG. 2, since the light emitting element lines of a plurality of columns are provided in the sub-scanning direction of the line head, the image forming apparatus can be applied to various uses.

  FIG. 3 is a block diagram showing another embodiment of the present invention. In FIG. 3, instead of the voltage detector 5 shown in FIG. 2, a light amount sensor 9 is provided, and a measurement signal of the light amount sensor 9 is input to the control circuit 3. The main body controller 8 transmits to the control circuit 3 the characteristics of the driving time of the light emitting element shown in FIG. The control circuit 3 stores the characteristic in the memory 6.

  When driving the light emitting element, the control circuit 3 reads the characteristic from the memory 6 and compares it with the measurement signal of the light quantity sensor 9. The measured value of the light quantity sensor 9 decreases along the characteristic Ix in FIG. 1C when the driving time of the light emitting element exceeds 200 hours. Further, when the driving time of the light emitting element is accumulated and the driving time reaches 250 hours, the value of the amount of emitted light decreases to Iy. At this time, the control circuit 3 increases the drive current from Ia to Ib, and performs constant current control of each light emitting element as in FIG. That is, the light emission amount Iy when the drive time reaches 250 hours is a reference value for determining whether to increase the drive current from Fa to Fb.

  The line head of the present invention is not limited to an example in which a plurality of light emitting element lines are provided in the sub-scanning direction as shown in FIGS. FIG. 4 is an explanatory diagram of a line head according to another embodiment of the present invention. In the example of FIG. 4, the line head 10 is provided with one light emitting element line 1. In the light emitting element line 1, a plurality of light emitting elements Ea made of organic EL elements are arranged.

  The light emitting element line 1 is divided into a plurality of blocks A, B, C. In the embodiment of the present invention, the constant current control as shown in FIG. 1A is performed in units of light emitting elements, that is, in units of dots as described with reference to FIGS. It can also be performed in units of blocks as shown in FIG. As described above, when voltage control is performed in units of blocks, it is possible to prevent deterioration of image quality when forming various image patterns. Next, a specific example of voltage control in units of blocks will be described with reference to the circuit diagram of FIG.

  In FIG. 5, the light emitting element line 1 is provided in the line head 10a. In the light emitting element line 1, light emitting elements D00 to D23 using organic EL elements are arranged. 14 is a positive power supply line, and 15 is a negative power supply line. The positive power supply line 14 is commonly connected to the anode of each light emitting element in the light emitting element line 1. The negative power supply line 15 is connected to the cathode of each light emitting element in the light emitting element line 1. The light emitting element line 1 is connected between the power supply lines 14 and 15.

  Reference numerals 11, 12, and 13 in FIG. 5 denote shift register circuits for controlling the light emitting elements D00 to D23 in units of blocks, and the output signal C0 of the shift register circuit 11 controls the block A including the light emitting elements D00 to D03. The output signal C1 of the shift register circuit 12 controls the block B including the light emitting elements D10 to D13, and the output signal C2 of the shift register circuit 13 controls the block C including the light emitting elements D20 to D23.

  SP is a start pulse input to the data terminal D of the shift register 11 from the signal line 17, and CK is a clock signal input to the shift registers 11 to 13 from the signal line 18. Reference numeral 16 denotes a signal line for supplying data signals Dat0 to Dat3 to each light emitting element, Tr2 denotes a driver transistor connected to the anode side of each light emitting element, and Tr1 denotes a control transistor whose source is connected to the gate of the driver transistor Tr2. The control transistor Tr1 and the driver transistor Tr2 are formed by, for example, an FET (Field Effect Transistor).

  The output signal C0 output from the output terminal Q of the shift register circuit 11 is applied to the gate of each control transistor Tr1 connected to the light emitting elements D00 to D03 via the signal line C0a. C1 is an output signal of the shift register circuit 12, and is applied to the gate of each control transistor Tr1 connected to the light emitting elements D10 to D13 via the signal line C1a. C2 is an output signal of the shift register circuit 13, and is applied to the gate of each control transistor Tr1 connected to the light emitting elements D20 to D23 via the signal line C2a.

  Thus, the shift register circuit 11 selects the light emitting elements D00 to D03 of the block A from the light emitting elements of the light emitting element line 1. The shift register circuit 12 selects the light emitting elements D10 to D13 of the block B, and the shift register circuit 13 selects the light emitting elements D20 to D23 of the block C. That is, the shift register circuits 11 to 13 function as block selection means for the light emitting elements.

  When the output signals C0 to C2 of the respective shift register circuits are at the H level, a signal is applied to the gate of each control transistor Tr1 that controls the light emitting elements of the block. Each light emitting element is connected in parallel between a power supply line 14 to which a positive voltage VDD is applied and a negative power supply line 15. Since the shift register is used in this way, block selection can be performed with a simple configuration of pulse driving.

  Next, the data signals Dat0 to Dat3 of the data line 16 will be described. This data signal is supplied to the drain of each control transistor Tr1. Accordingly, when the data signals Dat0 to Dat3 are supplied to the control transistor Tr1 of the light emitting element selected by the block selection signal, the driver transistor Tr2 connected to the control transistor Tr1 becomes conductive and the corresponding light emitting element operates. . The same operation is possible even when the block selection signal is connected to the drain of the control transistor Tr1 and the data line is connected to the gate of the control transistor Tr1.

  For example, for the block A, the data signals Dat0 to Dat3 are supplied to the control transistor Tr1 that controls the light emitting elements D00 to D03, respectively. That is, the data signals Dat0 to Dat3 act as selection signals for selecting individual light emitting elements in the same block. As described above, in the line head of the present invention, individual light emitting elements can be selected and lit. The data signals Dat0 to Dat3 are supplied to each light emitting element after the grayscale data is converted into time data.

  In FIG. 5, as described above, the shift register circuits 11 to 13 function as block selection means for the light emitting elements. By supplying a positive voltage VDD from the power supply line 14 to each of the light emitting elements of the blocks A, B, and C selected by the shift register circuits 11 to 13, constant voltage control using the initial Va is performed. Also, constant voltage control can be performed using a voltage boosted from Va to Vb as shown in FIG.

  FIG. 6 is a circuit diagram showing an example of constant current control of a light emitting element, and FIG. 7 is a characteristic diagram showing an operation principle of a light emitting element Ea composed of a driver transistor Tr2 and an organic EL element. The driving method shown in FIG. 6 is called current program gradation.

  This current program gradation has been developed for light-emitting polymer displays (Industrial Research Committee "Electronic Materials" July issue separate volume "Liquid Crystal Display Technology 2001" (issued July 2001) "Low-temperature poly-Si thin film transistor Driven light emitting polymer display "). The current program gradation is for suppressing variations in light emission luminance due to all characteristic variations.

  In FIG. 6, MN1 is a transistor to which a selection signal of a light emitting element (OLED) made of an organic EL element is input, MN2 is a driver transistor, MN3 is a transistor for supplying a constant current source (IDdata) current, and MN4 is a power supply voltage. This is a switching transistor that turns on and off (VDD). C1 is a capacitor connected between the gate electrode and the drain electrode of the driver transistor MN2. Each of the transistors MN1 to MN4 is composed of a TFT.

  FIG. 7 shows characteristics of the relationship between the drain-source current (Ids) of the driver transistor MN2 and the drain-source voltage (Vds). In addition, light-emitting element (OLED) current-voltage characteristics are shown. The program current corresponds to a target value for constant current control.

  In the current program period (constant current control period), the gate electrode and drain electrode of the TFT are short-circuited to Vgs = Vds, and the gradation signal is supplied to the TFT as an analog current, and Vgs at this time is stored in the capacitor C1. . In the holding period, the analog current is switched to the constant voltage VDD, and the operating point is an intersection of the IV characteristics of the TFT and the light emitting element OLED. Since the TFT operates in the saturation region in both the current program period and the holding period, the same amount of current as the program current is supplied to the holding period. That is, the light emitting element is controlled with constant current.

  A feature of this current program gradation is that since the saturation region of the TFT is used, it is possible to suppress variations in light emission luminance caused by variations in characteristics of all TFTs and light emitting elements. In addition, although the constant current control is used, the number of light emitting units is not required and constant current sources are not required, and it is only necessary to connect to one constant voltage source VDD.

  FIG. 8 is a block diagram showing the configuration of the control unit of the image forming apparatus of the present invention. In the image forming apparatus of FIG. 8, an image signal is given to the main controller 11 from an external device such as a host computer in response to an image formation request from the user. At this time, a command signal is transmitted from the main controller 20 to the engine controller 30. In response to this command signal, the engine controller 30 controls each part of the engine unit EG to form an image corresponding to the image signal on the recording medium.

  In the engine unit EG, the charging unit 62 is applied with a charging bias from the charging control unit 103, and uniformly charges the outer peripheral surface of the photosensitive member to a predetermined surface potential. Then, a light beam is irradiated from the exposure unit 61 toward the outer peripheral surface of the photosensitive member charged by the charging unit 62. The exposure unit 61 exposes a light beam on the photoreceptor in accordance with a control command given from the exposure control unit 102 to form an electrostatic latent image corresponding to the image signal. The exposure unit 61 is provided with appropriate optical elements such as a lens and a mirror.

  When an image signal is given from an external device such as a host computer to the CPU 111 of the main controller 20 via the interface 112, the CPU 101 of the engine controller 30 sends a control signal corresponding to the image signal to the exposure control unit 102 at a predetermined timing. Output. In response to this control signal, the exposure unit 61 irradiates the photosensitive member with a light beam, and an electrostatic latent image corresponding to the image signal is formed on the photosensitive member.

  The developing unit 40 is controlled by the developing device controller 104. Here, a developing bias in which a DC voltage and an AC voltage are superimposed is applied from the developing device controller 104 to the developing roller. Due to such a developing bias, the toner carried on the developing roller partially adheres to each part of the surface of the photoconductor according to the surface potential, and thus the electrostatic latent image on the photoconductor becomes the toner of the toner color. Visualized as an image.

  The vertical synchronization sensor 64 is a sensor for detecting the reference position of the intermediate transfer belt, and is a vertical synchronization sensor for obtaining a synchronization signal output in association with the rotational drive of the intermediate transfer belt, that is, a vertical synchronization signal Vsync. Function. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and accurately superimpose the toner images formed in the respective colors.

  Further, the density sensor 63 is provided to face the surface of the intermediate transfer belt, and measures the optical density of the patch image formed on the outer peripheral surface of the intermediate transfer belt in the density control process. The light quantity sensor 65 corresponds to the light quantity sensor 9 of FIG. Although not shown, the voltage detector described with reference to FIG. 2 may be provided so that the detection voltage of the light emitting element is input to the CPU 101.

  As shown in FIG. 8, each developing device (toner cartridge) 48Y, 48C, 48M, 48K is a “storage element” that stores data relating to the manufacturing lot and usage history of the developing device, the remaining amount of built-in toner, and the like. Certain memories 91 to 94 are provided. Further, connectors 49Y, 49C, 49M, and 49K are provided in the developing devices 48Y, 48C, 48M, and 48K, respectively.

  If necessary, these connectors 49Y, 49C, 49M, 49K are selectively connected to the connector 108 provided on the main body side. For this reason, data is transmitted and received between the CPU 101 of the engine controller 30 and each of the memories 91 to 94 via the interface 105 to manage various information such as consumables management for the developing device (toner cartridge). ing. In this embodiment, the main body side connector 108 and each developing device side connector 49K and the like are mechanically fitted to each other to exchange data, but for example, electromagnetic means such as wireless communication is used. Thus, data transmission / reception may be performed without contact.

  The memories 91 to 94 for storing data unique to each of the developing devices 48Y, 48C, 48M, and 48K are nonvolatile memories that can store the data even when the power is off or the developing device is removed from the main body. It is desirable. As such a non-volatile memory, for example, a flash memory, a ferroelectric memory (FRAM: Ferroelectric Random Access Memory), an EEPROM, or the like can be used.

  Further, in this image forming apparatus, a display unit 21 is provided as shown in FIG. And a required message is alert | reported with respect to a user by displaying a predetermined | prescribed message according to the control command given from CPU111 as needed. For example, when an abnormality such as a device failure or a paper jam occurs, a message is displayed informing the user to that effect. In addition, when the remaining amount of toner in any of the developing devices is reduced to a predetermined value or less, for example, to a near-end value described later, a message prompting replacement of the developing device is displayed.

  As the display unit 21, for example, a display device such as a liquid crystal display can be used, but in addition to this, a warning lamp that is lit or blinks as necessary may be used. Further, in addition to visually informing the user by displaying a message, a voice alarm such as a pre-recorded voice message or a buzzer may be used, or a combination thereof may be used as appropriate.

  An image memory 113 is provided for storing an image given from an external device such as a host computer via the interface 112. Reference numeral 106 denotes a ROM for storing a calculation program executed by the CPU 101, control data for controlling the engine unit EG, and the like, and reference numeral 107 denotes a RAM for temporarily storing calculation results in the CPU 101 and other data. The RAM 107 may be an FRAM.

  In the present invention, the organic EL array head configured as described above can be used as, for example, an exposure head of an image forming apparatus that forms an electrophotographic color image. FIG. 9 is a front view showing an example of an image forming apparatus using an organic EL array head. This image forming apparatus includes four organic EL array exposure heads 101K, 101C, 101M, and 101Y having the same configuration and corresponding four photosensitive drums (image carriers) 41K, 41C, and 41M having the same configuration. , 41Y, respectively, and is configured as a tandem image forming apparatus.

  As shown in FIG. 9, this image forming apparatus is provided with a drive roller 51, a driven roller 52, and a tension roller 53. The tension roller 53 applies tension to the image forming apparatus and stretches it in the direction indicated by the arrow (counterclockwise). ) Is circulated and driven. Photosensitive members 41K, 41C, 41M, and 41Y having photosensitive layers are arranged on the outer peripheral surface as four image carriers arranged at predetermined intervals with respect to the intermediate transfer belt 50.

  K, C, M, and Y added after the reference sign mean black, cyan, magenta, and yellow, respectively, and indicate that the photoconductors are black, cyan, magenta, and yellow, respectively. The same applies to other members. The photoreceptors 41K, 41C, 41M, and 41Y are rotationally driven in the direction indicated by the arrow (clockwise) in synchronization with the driving of the intermediate transfer belt 50.

  Around each photoconductor 41 (K, C, M, Y), charging means (corona charger) 42 (K) for uniformly charging the outer peripheral surface of the photoconductor 41 (K, C, M, Y), respectively. , C, M, Y) and the outer peripheral surface uniformly charged by the charging means 42 (K, C, M, Y) are synchronized with the rotation of the photoconductor 41 (K, C, M, Y). Thus, the organic EL array exposure head 1 (K, C, M, Y) as described above of the present invention for sequentially scanning the lines is provided.

  Further, a developing device 44 (K) that applies toner as a developer to the electrostatic latent image formed by the organic EL array exposure head 101 (K, C, M, Y) to form a visible image (toner image). , C, M, Y) and a primary transfer roller 45 as transfer means for sequentially transferring the toner image developed by the developing device 44 (K, C, M, Y) to the intermediate transfer belt 50 as a primary transfer target. (K, C, M, Y) and a cleaning device 46 (K, C, Y) as a cleaning unit for removing the toner remaining on the surface of the photoreceptor 41 (K, C, M, Y) after being transferred. M, Y).

  Here, in each organic EL array exposure head 101 (K, C, M, Y), the array direction of the organic EL array exposure head 101 (K, C, M, Y) is the photosensitive drum 41 (K, C, M). , Y) along the bus. The light emission energy peak wavelength of each organic EL array exposure head 101 (K, C, M, Y) and the sensitivity peak wavelength of the photoconductor 41 (K, C, M, Y) are set to substantially coincide with each other. Yes.

  The developing device 44 (K, C, M, Y) uses, for example, a non-magnetic one-component toner as a developer, and the one-component developer is conveyed to the developing roller by a supply roller, for example, and adheres to the surface of the developing roller. The film thickness of the developed developer is regulated by a regulating blade, and the developing roller is brought into contact with or increased in thickness by the photosensitive body 41 (K, C, M, Y), whereby the photosensitive body 41 (K, C, M, Y). The toner is developed as a toner image by attaching a developer according to the potential level.

  The black, cyan, magenta, and yellow toner images formed by the four-color single-color toner image forming station are intermediated by the primary transfer bias applied to the primary transfer roller 45 (K, C, M, Y). The toner image, which is sequentially primary transferred onto the transfer belt 50 and sequentially superposed on the intermediate transfer belt 50 to become a full color, is secondarily transferred to a recording medium P such as paper by a secondary transfer roller 66, and serves as a fixing unit. The toner is fixed on the recording medium P by passing through the fixing roller pair 61, and is discharged onto a paper discharge tray 68 formed in the upper part of the apparatus by a paper discharge roller pair 62.

  In FIG. 9, reference numeral 63 denotes a paper feed cassette in which a large number of recording media P are stacked and held, 64 denotes a pickup roller for feeding the recording media P one by one from the paper feed cassette 63, and 65 denotes a secondary transfer roller. A pair of gate rollers for defining the supply timing of the recording medium P to the secondary transfer portion 66, a secondary transfer roller 66 as a secondary transfer means for forming a secondary transfer portion with the intermediate transfer belt 50, 67 Is a cleaning blade as a cleaning means for removing the toner remaining on the surface of the intermediate transfer belt 50 after the secondary transfer. A counter for counting the number of recording sheets to be printed is provided at an appropriate position on the conveyance path of the recording medium P, for example, at an appropriate position between the paper feed cassette 63 and the gate roller pair 65.

  As described above, since the image forming apparatus of FIG. 9 uses the organic EL array as the writing means, the apparatus can be made smaller than when the laser scanning optical system is used. In the present invention, in the tandem image forming apparatus as shown in FIG. 9, when performing constant current control of the light emitting element, deterioration of image quality when the driving time of the light emitting element exceeds a certain time is prevented. be able to.

  Next, another embodiment of the image forming apparatus according to the present invention will be described. FIG. 10 is a vertical side view of the image forming apparatus. In FIG. 10, the image forming apparatus 160 includes, as main constituent members, a rotary developing device 161, a photosensitive drum 165 functioning as an image carrier, and an image writing means (line head) 167 provided with an organic EL array. In addition, an intermediate transfer belt 169, a paper conveyance path 174, a fixing roller heating roller 172, and a paper feed tray 178 are provided.

  In the developing device 161, the developing rotary 161a rotates in the arrow A direction about the shaft 161b. The inside of the development rotary 161a is divided into four, and image forming units for four colors of yellow (Y), cyan (C), magenta (M), and black (K) are provided. Reference numerals 162a to 162d are arranged in the image forming units for the four colors. The developing rollers rotate in the arrow B direction, and the toner supply rollers 163a to 163d rotate in the arrow C direction. Reference numerals 164a to 164d are regulating blades that regulate the toner to a predetermined thickness.

  As described above, reference numeral 165 denotes a photosensitive drum that functions as an image carrier, 166 denotes a primary transfer member, 168 denotes a charger, and 167 denotes an image writing unit, which is provided with an organic EL array. The photosensitive drum 165 is driven in the direction of arrow D opposite to the developing roller 162a by a drive motor (not shown), for example, a step motor.

  The intermediate transfer belt 169 is stretched between the driven roller 170b and the drive roller 170a, and the drive roller 170a is connected to the drive motor of the photosensitive drum 165 to transmit power to the intermediate transfer belt. By driving the drive motor, the drive roller 170 a of the intermediate transfer belt 169 is rotated in the arrow E direction opposite to the photosensitive drum 165.

  The paper conveyance path 174 is provided with a plurality of conveyance rollers, a pair of paper discharge rollers 176, and the like, and conveys the paper. An image (toner image) on one side carried on the intermediate transfer belt 169 is transferred to one side of the paper at the position of the secondary transfer roller 171. The secondary transfer roller 171 is separated from and brought into contact with the intermediate transfer belt 169 by a clutch, and is brought into contact with the intermediate transfer belt 169 when the clutch is turned on, so that an image is transferred onto the sheet.

  The sheet on which the image has been transferred as described above is then subjected to a fixing process by a fixing device having a fixing heater. The fixing device is provided with a heating roller 172 and a pressure roller 173. The sheet after the fixing process is drawn into the discharge roller pair 176 and proceeds in the arrow F direction. When the paper discharge roller pair 176 rotates in the opposite direction from this state, the paper reverses its direction and advances in the double-sided printing conveyance path 175 in the arrow G direction. 177 is an electrical component box, 178 is a paper feed tray for storing paper, and 179 is a pickup roller provided at the outlet of the paper feed tray 178. The number of recording sheets to be printed is counted by a sensor provided at an appropriate position in the paper feed path such as in the vicinity of the paper feed tray.

  For example, a low-speed brushless motor is used as a drive motor for driving the transport roller in the paper transport path. The intermediate transfer belt 169 uses a step motor because it requires color misregistration correction. Each of these motors is controlled by a signal from a control means (not shown).

  In the state shown in the drawing, a yellow (Y) electrostatic latent image is formed on the photosensitive drum 165, and a high voltage is applied to the developing roller 62a, whereby a yellow image is formed on the photosensitive drum 165. When all of the yellow back side and front side images are carried on the intermediate transfer belt 169, the development rotary 161a rotates 90 degrees in the direction of arrow A.

  The intermediate transfer belt 169 rotates once and returns to the position of the photosensitive drum 165. Next, two images of cyan (C) are formed on the photosensitive drum 165, and this image is carried on the yellow image carried on the intermediate transfer belt 169. Thereafter, the 90-degree rotation of the development rotary 161 and the one-rotation process after the image is carried on the intermediate transfer belt 169 are repeated in the same manner.

  For carrying four color images, the intermediate transfer belt 169 rotates four times, and then the rotation position is further controlled to transfer the image onto the sheet at the position of the secondary transfer roller 171. The paper fed from the paper feed tray 178 is transported by the transport path 174, and the color image is transferred to one side of the paper at the position of the secondary transfer roller 171. The sheet on which the image is transferred on one side is reversed by the discharge roller pair 176 as described above, and stands by on the conveyance path. Thereafter, the sheet is conveyed to the position of the secondary transfer roller 171 at an appropriate timing, and the color image is transferred to the other side. The housing 180 is provided with an exhaust fan 181.

  In the present invention, in the rotary type image forming apparatus as shown in FIG. 10, when performing constant current control of the light emitting element, deterioration of image quality is prevented when the driving time of the light emitting element exceeds a certain time. be able to. Further, in a tandem type and rotary type image forming apparatus provided with an intermediate transfer member, when performing constant current control of the light emitting element, it is possible to prevent deterioration of image quality when the driving time of the light emitting element exceeds a certain time. Can do.

  The line head and the image forming apparatus of the present invention have been described based on the embodiments. The line head and the image forming apparatus of the present invention are not limited to these embodiments, and various modifications are possible.

It is a characteristic view which shows the characteristic of the light emitting element concerning this invention. It is a block diagram which shows the example of the control part of a line head. It is a block diagram which shows the example of the control part of a line head. It is explanatory drawing which shows the example of a line head. It is a circuit diagram which shows the example of control of the light emitting element in FIG. It is a circuit diagram which shows the example of constant current control. It is an IV characteristic view of a light emitting element and a driver transistor. 3 is a block diagram illustrating an example of a control unit of the image forming apparatus. FIG. 1 is a side view showing a tandem image forming apparatus. 1 is a side view showing a rotary type image forming apparatus. It is a characteristic view showing the characteristic of a light emitting element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light emitting element line, 2 ... Control part, 3 ... Control circuit, 4 ... Drive circuit, 5 ... Counter, 6 ... Memory, 7 ... Light emitting element line, 8 ... Main body controller, 10 ... Line head, 11-13 ... Shift register, 41 (K, C, M, Y) ... Photosensitive drum (image carrier), 44 (K, C, M, Y) ... developing device, 50 ... intermediate transfer belt, 66 ... secondary transfer roller, 101K, 101C, 101M, 101Y ... organic EL array exposure head (line head), 161. Development device, 165... Photosensitive drum, 167... Exposure head (line head), 169... Intermediate transfer belt, 171... Secondary transfer roller, P. EL element.

Claims (13)

A light emitting element comprising a plurality of organic EL elements arranged in one line; means for controlling the light emitting element at a constant current; means for measuring the light emission state of the light emitting element; and a measurement value and a reference value of the measurement means. A line head comprising: a comparison unit, wherein constant current control of the light emitting element is performed based on an output of the comparison unit. The line head according to claim 1, wherein the measuring unit measures a driving voltage of the light emitting element. The line head according to claim 1, wherein the reference value is a threshold value of a driving voltage measured by the light emitting element. The line head according to any one of claims 1 to 3, wherein constant current control of the light emitting element is performed by feedback control based on a measurement value of the measuring means. The line head according to claim 1, wherein the measuring unit measures a light emission amount of the light emitting element. 6. The line head according to claim 1, wherein the constant current control of the light emitting element is performed for each individual light emitting element. 6. The constant current control of the light emitting device is performed in units of blocks by dividing a plurality of light emitting devices arranged in one line into a plurality of blocks. 6. Line head. 6. The line head according to claim 1, wherein the constant current control of the light emitting elements is performed simultaneously on a plurality of light emitting elements arranged in one line. The constant current control of the light emitting element is performed by gradually increasing the drive current in order to recover the light emission amount to a predetermined value when the light emission amount decreases after the driving time of the light emitting element exceeds a certain time. The line head according to any one of claims 1 to 8, wherein The line head according to any one of claims 1 to 9, wherein a plurality of lines in which the plurality of light emitting elements are arranged are formed in a sub-scanning direction. At least two image forming stations in which image forming units including a charging unit, a line head according to any one of claims 1 to 10, a developing unit, and a transfer unit are arranged around an image carrier. An image forming apparatus provided as described above, wherein a transfer medium passes through each station and forms an image by a tandem method. An image carrier configured to carry an electrostatic latent image, a rotary developing unit, and the line head according to any one of claims 1 to 10, wherein the rotary developing unit includes a plurality of toner cartridges. The toner stored in the toner is carried on the surface, and toners of different colors are sequentially conveyed to a position facing the image carrier by rotating in a predetermined rotation direction, and the image carrier, the rotary developing unit, An image forming method characterized in that a developing bias is applied between the toner and the toner is moved from the rotary developing unit to the image carrier to visualize the electrostatic latent image to form a toner image. apparatus. The image forming apparatus according to claim 11, further comprising an intermediate transfer member.

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