JP2006056010A - Driving device and method for line head, line head, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving device for a line head whereby a constant luminance can be obtained from the line head by correcting irregularities of an emission luminance between EL elements and a change of an emission strength of EL elements with time, and to provide a method, a line head and an image forming apparatus. <P>SOLUTION: The line head 1 is equipped with a light emitting element array 3a which consists of an arrangement of a plurality of organic EL elements 3. A current flowing to a power supply line 8 is measured while a voltage applied to a gate electrode of a transistor 60 for driving is changed by varying a voltage supplied to each of analog voltage signal cables 62. A voltage/current characteristic of each of the organic EL elements 3 is thus measured beforehand. Analog voltages V0-V15 applied to respective analog voltage signal cables 62 are adjusted on the basis of the measured result of the voltage/current characteristic so that a current flowing to each of the organic EL elements 3 becomes nearly equal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a line head driving apparatus and method used as exposure means in an image forming apparatus, a line head, and an image forming apparatus.

  Conventionally, as a high-speed and high-quality printer (image forming apparatus), a laser scanner type generally called a laser printer is known. In this type of printer, typically, a laser beam emitted from a laser light source such as a semiconductor laser is scanned on a photosensitive drum using a rotating mechanism such as a polygon mirror, and an electrical latent image is formed on the photosensitive drum. An image is formed. For this reason, in order to respond to high speed and high image quality, high-speed and high-precision control of the rotation mechanism unit is necessary.

  However, in recent years, printers are required to have higher speed and higher image quality, and downsizing of the printer itself is also required. In order to meet such a demand, an image forming apparatus including a line head having a light emitting element array in which a plurality of organic electroluminescent elements (organic EL elements) are arranged in place of a laser light source is known. In this image forming apparatus, since a rotation mechanism such as a polygon mirror provided in the laser printer is not required, the speed can be further increased and the size can be reduced.

In this type of image forming apparatus, in order to form a uniform image, it is necessary to make the light intensity distribution of the light emitted from the line head uniform. For this reason, for example, in Patent Document 1 below, error data approximating the light intensity distribution of the light emitting element array, fluctuation data of the condensing characteristic of the lens array, and relative position data between the light emitting element array and the lens array Is disclosed in advance, and a technique for making the light intensity of the line head uniform using these data is disclosed.
Japanese Unexamined Patent Publication No. 5-57955

  By the way, EL light-emitting elements, particularly organic EL elements, have a specification that their light emission efficiency changes depending on the light emission time or the like. Therefore, even in a line head formed by arranging a plurality of EL light emitting elements manufactured simultaneously in the same manufacturing process, there is no variation in light emission luminance at the initial point if the light emission intensity of each EL light emitting element is different. In both cases, there arises a problem that the emission luminance varies over time. Further, as time elapses, there arises a problem that not only variations in light emission luminance among EL light emitting elements but also overall luminance reduction occurs. For this reason, in a conventional image forming apparatus provided with a line head having a light emitting element array, for example, if the usage time is long, there is a possibility that the formed image may be uneven or the image may be totally faded. It was.

  The present invention has been made in view of the above circumstances, and a line head capable of obtaining a constant luminance from a line head by correcting variations in light emission luminance between EL elements and changes in light emission intensity of the EL elements over time. It is an object to provide a driving apparatus and method, a line head, and an image forming apparatus.

In order to solve the above-described problems, a line head drive device according to the present invention is a line head drive device including a light emitting element array in which a plurality of EL elements are arranged, and the voltage-current characteristics of each of the EL elements are determined. And a control unit that controls a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements measured by the measurement unit.
According to the present invention, the voltage-current characteristics of each of the plurality of EL elements are measured, and the voltage applied to each of the EL elements is controlled based on the measurement result. Here, there is a strong correlation between the luminance of the EL element and the current flowing through the EL element. That is, the luminance increases when a large amount of current flows through the EL element, and the luminance decreases when the current flowing through the EL element is small. Therefore, as in the present invention, if the voltage applied to each EL element is controlled based on the measurement result of the voltage-current characteristics of each EL element, the current flowing through each EL element can be controlled. The variation in the emission luminance and the change in the emission intensity of the EL element over time can be corrected, and as a result, a constant intensity of light emission can be obtained from the line head.
In addition, the line head driving device of the present invention includes a storage unit that stores voltage-current characteristics of each of the EL elements measured by the measurement unit, and the control unit stores the EL element stored in the storage unit. It is desirable to control the voltage applied to each of the EL elements based on the voltage-current characteristics.
According to the present invention, voltage-current characteristics of each EL element provided in the line head can be measured and stored at an arbitrary timing such as a timing when the line head is not used. The formation efficiency is not deteriorated. In addition, a desired timing can be obtained regardless of whether the EL element provided in the line head has a characteristic in which the voltage-current characteristic changes in a short time or a characteristic in which the voltage-current characteristic does not change so much. The voltage-current characteristics can be measured with.
In the line head drive device of the present invention, the control unit controls a voltage applied to each of the EL elements so that a current flowing through each of the EL elements becomes equal to a reference current.
According to the present invention, since the voltage applied to each EL element is controlled so that the current flowing through each EL element becomes equal to a predetermined reference current, the luminance variation among the EL elements is made uniform. Of course, even if the EL element characteristics deteriorate so that the luminance of the EL element provided in the line head gradually decreases with time, the luminance of the line head can be reduced to the initial luminance. Can keep.
In the line head drive device of the present invention, the control unit controls the light emission amount of each of the EL elements based on gradation-processed image data.
According to the present invention, the EL device is controlled so that a current substantially equal to the reference current flows through each EL element, and the light emission amount of each EL element is controlled based on the gradation-processed image data. As a result, for example, a high-definition image can be formed.
In order to solve the above problems, a line head driving method according to the present invention is a line head driving method including a light emitting element array in which a plurality of EL elements are arranged, and the voltage-current characteristics of each of the EL elements are determined. And a control step of controlling a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements measured in the measurement step.
According to the present invention, since the current applied to each EL element is controlled by controlling the voltage applied to each EL element based on the voltage-current characteristic measurement result of each EL element, the light emission luminance between the EL elements is controlled. Variation of the EL element and the change in the emission intensity of the EL element over time can be corrected, and as a result, light emission with a constant intensity can be obtained from the line head.
The line head driving method according to the present invention includes a storage step of storing the voltage-current characteristics of each of the EL elements measured in the measurement step.
According to the present invention, voltage-current characteristics of each EL element provided in the line head can be measured and stored at an arbitrary timing such as a timing when the line head is not used. The formation efficiency is not deteriorated. In addition, a desired timing can be obtained regardless of whether the EL element provided in the line head has a characteristic in which the voltage-current characteristic changes in a short time or a characteristic in which the voltage-current characteristic does not change so much. The voltage-current characteristics can be measured with.
In the line head driving method of the present invention, the control step includes a step of controlling a voltage applied to each of the EL elements so that a current flowing through each of the EL elements becomes equal to a reference current. It is said.
According to the present invention, since the voltage applied to each EL element is controlled so that the current flowing through each EL element becomes equal to a predetermined reference current, the luminance variation among the EL elements is made uniform. Of course, even if the EL element characteristics deteriorate so that the luminance of the EL element provided in the line head gradually decreases with time, the luminance of the line head can be reduced to the initial luminance. Can keep.
The line head driving method of the present invention is characterized in that the control step includes a step of controlling a light emission amount of each of the EL elements based on gradation-processed image data.
According to the present invention, the EL device is controlled so that a current substantially equal to the reference current flows through each EL element, and the light emission amount of each EL element is controlled based on the gradation-processed image data. As a result, for example, a high-definition image can be formed.
In order to solve the above problems, a line head according to the present invention is a line head in which a light emitting element array in which a plurality of EL elements are arranged and a drive element group for driving the EL elements are integrally formed on an element substrate. A measurement unit for measuring the voltage-current characteristics of each of the EL elements on the element substrate; and a voltage-current characteristic of each of the EL elements measured by the measurement unit. And a control unit for controlling the voltage to be applied.
According to the present invention, since the voltage applied to each of the EL elements can be controlled based on the voltage-current characteristic measurement result of each EL element, the current flowing through each of the EL elements can be controlled. Variations in luminance and changes in the light emission intensity of the EL element over time can be corrected, and as a result, light emission with a constant intensity can be obtained from the line head. Moreover, since the control part which controls the voltage applied to each of the EL elements is provided on the element substrate, the size can be reduced as compared with the case where the control part and the line head are provided separately.
Moreover, the line head of the present invention includes a storage unit that stores voltage-current characteristics of each of the EL elements measured by the measurement unit on the element substrate, and the control unit is stored in the storage unit. The voltage applied to each of the EL elements is controlled based on the voltage-current characteristics of each of the EL elements.
According to the present invention, voltage-current characteristics of each EL element provided in the line head can be measured and stored at an arbitrary timing such as a timing when the line head is not used. The formation efficiency is not deteriorated. In addition, a desired timing can be obtained regardless of whether the EL element provided in the line head has a characteristic in which the voltage-current characteristic changes in a short time or a characteristic in which the voltage-current characteristic does not change so much. The voltage-current characteristics can be measured with. Furthermore, in addition to the control unit described above, a storage unit is provided on the element substrate, which is extremely suitable for downsizing.
An image forming apparatus according to the present invention includes a line head provided with a light emitting element array in which a plurality of EL elements are arranged, and a line head driving device according to any one of claims 1 to 5 as an exposure unit. It is characterized by providing as.
An image forming apparatus according to the present invention includes the above-described line head as an exposure unit.
According to the present invention, it is possible to obtain a constant intensity of light emission from the line head by correcting variations in light emission luminance between EL elements and changes in light emission intensity of the EL elements over time, so that high speed image quality can be achieved. An image forming apparatus can be obtained. Further, it is possible to obtain an image forming apparatus that is reduced in size as compared with a conventional image forming apparatus.

  Hereinafter, a line head driving apparatus and method, a line head, and an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

[Line head]
1A and 1B are diagrams schematically showing a line head according to an embodiment of the present invention, in which FIG. 1A is a bottom view and FIG. 1B is a side view. The line head 1 of the present embodiment is used as an exposure unit of an image forming apparatus described later, and has a plurality of organic EL elements on a long and thin rectangular element substrate 2 as shown in FIG. 3 is integrated, a drive element group including a drive element 4 that drives the organic EL element 3, and a control circuit group 5 that controls the drive of these drive elements 4 (drive element group). Formed.

  Moreover, as shown in FIG.1 (b), the sealing substrate 6 is affixed on the element substrate 2 by the transparent adhesive in the state which sealed the organic EL element 3 on the surface side. . Here, the line head 1 of the present embodiment is not a bottom emission type in which the light emitted from the organic EL element 3 is emitted from the element substrate 2 side, but a top emission type in which the light is emitted from the sealing substrate 6 side. ing. Therefore, as described later, the sealing substrate 6 is made of a transparent substrate such as glass. The line head 1 of the present embodiment includes a heat radiating plate 7 made of a metal such as aluminum on the back side of the element substrate 2. The heat radiating plate 7 prevents a decrease in luminous efficiency and life of the organic EL element 3 by suppressing an increase in the ambient temperature of the organic EL element 3.

  As shown in FIG. 1A, power source lines 8 and 9 are connected to the drive element 4 formed on the element substrate 2, and a power source (not shown) is connected via these power source lines 8 and 9. A voltage is applied to the drive element 4. In such a configuration, the light emitting operation of the organic EL element 3 is controlled by the drive element 4 controlled by the control circuit group 5.

  Here, the configuration of the organic EL element 3 and the driving element 4 in the line head 1 will be described. FIG. 2 is a side cross-sectional view showing the main configuration of the line head 1, and FIG. 3 is a bottom view showing the main configuration of the line head 1. As described above, the line head 1 of the present embodiment is a top emission type and has a configuration in which emitted light is extracted from the sealing substrate 6 side opposite to the element substrate 2. Any of the substrates can be used. When an opaque substrate is used as the element substrate 2, for example, a thermosetting resin, a thermoplastic resin, or the like is used in addition to a ceramic sheet such as alumina or a metal sheet such as stainless steel that has been subjected to an insulation treatment such as surface oxidation. Can do. However, in this embodiment, since a transparent glass substrate is used as the sealing substrate 6, a glass substrate is also used as the element substrate 2 in order to prevent problems such as warpage due to a difference in thermal expansion coefficient.

  On the element substrate 2, a circuit portion 35 including a driving TFT 21 (driving element 4) connected to the pixel electrode 20 is formed, and the organic EL element 3 is provided thereon. The organic EL element 3 includes a pixel electrode 20 that functions as an anode, a hole transport layer 32 that injects / transports holes from the pixel electrode 20, a light emitting layer 31 made of an organic EL material, and a cathode 30. Has been. Here, FIG. 3 shows a schematic diagram in which the organic EL element 3 and the driving TFT 21 (driving element 4) correspond to FIG. In FIG. 3, the power supply line 8 is connected to the source / drain electrode of the drive element 4, and the power supply line 9 is connected to the cathode 30 of the organic EL element 3. As shown in FIG. 2, the organic EL element 3 emits light when holes injected from the hole transport layer 32 and electrons from the cathode 30 are combined in the light emitting layer 31.

  The pixel electrode 20 functioning as an anode is usually formed of ITO (Indium Tin Oxide). In addition, particularly in the case of the top emission type, when it is desired to obtain high contrast, titanium, tungsten, titanium-tungsten alloy or the like is formed as a reflective layer, and ITO is laminated on the reflective layer as an interference layer. The pixel electrode 20 may be formed by a laminated structure including these reflective layers and interference layers.

  As a material for forming the hole transport layer 32, in particular, a dispersion of 3,4-polyethylenedithiothiophene / polystyrene sulfonic acid (PEDOT / PSS), that is, 3,4-polyethylenedioxy in polystyrene sulfonic acid as a dispersion medium. A dispersion in which thiophene is dispersed and further dispersed in water is preferably used. The material for forming the hole transport layer 32 is not limited to the above, and various materials can be used. For example, those obtained by dispersing polystyrene, polypyrrole, polyaniline, polyacetylene or a derivative thereof in an appropriate dispersion medium such as the above-described polystyrene sulfonic acid can be used.

  As a material for forming the light emitting layer 31, a known light emitting material capable of emitting fluorescence or phosphorescence is used. In the present embodiment, for example, a light emitting layer whose emission wavelength band corresponds to red is adopted. Of course, a light emitting layer whose emission wavelength band corresponds to green or blue may be adopted.

  Specific examples of the material for forming the light emitting layer 31 include (poly) fluorene derivative (PF), (poly) paraphenylene vinylene derivative (PPV), polyphenylene derivative (PP), polyparaphenylene derivative (PPP), and polyvinylcarbazole (PVK). ), Polythiophene derivatives, and polysilanes such as polymethylphenylsilane (PMPS) are preferably used. In addition, these polymer materials include polymer materials such as perylene dyes, coumarin dyes, rhodamine dyes, rubrene, perylene, 9,10-diphenylanthracene, tetraphenylbutadiene, nile red, coumarin 6, and quinacridone. It can also be used by doping a low molecular weight material such as.

  The cathode 30 is formed so as to cover the light emitting layer 31, and is formed of a transparent conductive material particularly in the case of this embodiment which is a top emission type. As the transparent conductive material, a structure in which a co-deposited film of bathocuproine and cesium is used and ITO is laminated to impart conductivity is preferably employed. Instead of the co-deposited film of bathocuproine and cesium, Ca may be formed to a thickness of about 5 nm. Further, the sealing substrate 6 (not shown in FIG. 2) shown in FIG. 1 is stuck on the cathode 30 via a transparent adhesive layer.

Further, as described above, the circuit unit 35 is provided below the organic EL element 3. The circuit portion 35 is formed on the element substrate 2. That is, a base protective layer 36 mainly composed of SiO ₂ is formed on the surface of the element substrate 2 as a base, and a silicon layer 41 is formed thereon. A gate insulating layer 37 mainly composed of SiO ₂ and / or SiN is formed on the surface of the silicon layer 41. In the silicon layer 41, a region overlapping with the gate electrode 42 with the gate insulating layer 37 interposed therebetween is a channel region 41a. The gate electrode 42 is a part of a scanning line (not shown). On the other hand, a first interlayer insulating layer 38 mainly composed of SiO ₂ is formed on the surface of the gate insulating layer 37 covering the silicon layer 41 and forming the gate electrode 42.

  Further, in the silicon layer 41, a low concentration source region 41b and a high concentration source region 41S are provided on the source side of the channel region 41a, while a low concentration drain region 41c and a high concentration drain are provided on the drain side of the channel region 41a. The region 41D is provided to form a so-called LDD (Light Doped Drain) structure. Among these, the high concentration source region 41 </ b> S is connected to the source electrode 43 through a contact hole 43 a that is opened across the gate insulating layer 37 and the first interlayer insulating layer 38. The source electrode 43 is configured as a part of a power supply line (not shown). On the other hand, the high-concentration drain region 41D is connected to the drain electrode 44 made of the same layer as the source electrode 43 through a contact hole 44a that opens between the gate insulating layer 37 and the first interlayer insulating layer 38. .

  On the upper layer of the first interlayer insulating layer 38 on which the source electrode 43 and the drain electrode 44 are formed, for example, a planarizing film 39 mainly composed of an acrylic resin component is formed. This flattening film 39 is formed of a heat-resistant insulating resin such as acrylic or polyimide, and has surface irregularities caused by the driving TFT 21 (driving element 4), the source electrode 43, the drain electrode 44, and the like. It is a well-known thing formed in order to eliminate.

  On the surface of the flattening film 39, the pixel electrode 20 made of the above-described ITO or the like is formed and connected to the drain electrode 44 through a contact hole 23a provided in the flattening film 39. That is, the pixel electrode 20 is connected to the high concentration drain region 41 </ b> D of the silicon layer 41 through the drain electrode 44.

The surface of the planarization film 39 on which the pixel electrode 20 is formed is covered with the pixel electrode 20, a lyophilic control layer 25 mainly composed of a lyophilic material such as SiO _2, and the partition wall 22. On the pixel electrode 20, the hole transport layer 32 and the light emitting layer described above are formed inside the opening 25 a provided in the lyophilic control layer 25 and the opening inside the opening provided in the partition wall 22 (that is, the pixel region). 31 are stacked in this order from the pixel electrode 20 side.

[Line head drive unit]
FIG. 4 is a block diagram showing a schematic configuration of a line head driving apparatus according to an embodiment of the present invention. As shown in FIG. 4, the line head drive device 51 of this embodiment is connected to host control means, for example, a host computer 50, and drives the line head 1 based on image data output from the host computer 50. To do. 4 shows a configuration in which the line head 1 is provided in the line head driving device 51 for convenience, the line head 1 and the driving device 51 may be provided separately. .

  The drive device 51 includes a data processing unit 52, a storage unit 53, a power supply drive unit 54, and a current detector 55. The data processing device 52 includes a CPU (central processing unit), an image processing device, and the like. The data processing device 52 performs gradation processing such as screen processing and color conversion processing on image data output from the host computer 50. Processing for converting the image data subjected to the gradation processing into data for transmission to the line head 1 is performed. Further, the line head 1 is driven using the converted transmission data, and gradation control of the organic EL element 3 provided in the line head 1 is performed. Further, the voltage / current characteristic of the organic EL element 3 provided in the line head 1 is measured from the detection result of the current detector 55, and the measurement result is stored in the storage unit 53.

  The data processing device 52 outputs a start pulse SP, a clock signal CK, and analog voltage signals V0 to V15 to the line head 1. The start pulse SP is a pulse for instructing the light emission start of the line head 1. The clock signal CK is a signal that defines the operation timing of the line head 1. The analog voltage signals V <b> 0 to V <b> 15 are used for correcting light emission luminance variations between the organic EL elements 3 and changes in the light emission intensity of the organic EL elements 3 over time to emit light having a constant luminance from the line head 1. Is.

  The storage unit 53 temporarily stores the image data output from the host computer 50 to the data processing unit 52, and stores the measurement result of the voltage / current characteristics of each organic EL element 3 measured by the data processing unit 52. . The power source drive unit 54 supplies a power source VEL for driving the organic EL element 3 provided in the line head 1. This power supply VEL is supplied to each organic EL element 3 through, for example, the power supply line 8 shown in FIG. The current detector 55 is provided, for example, in the power supply line 8, detects a current flowing through the power supply line 8, and outputs the detection result to the data processing unit 52.

  In FIG. 4, for the sake of simplicity, a driving device 51 that drives one line head 1 is illustrated. However, a plurality of line heads 1 are provided and a control signal is sent from the data processing unit 52 to each line head 1. A plurality of line heads 1 can be driven by providing signal lines to be output. When the driving device 51 is provided in an image forming apparatus such as a monochrome printer, usually only one line head 1 is provided. When the driving device 51 is provided in an image forming apparatus such as a color printer, usually a plurality of driving devices 51 are provided. Line heads 1 (yellow line head, magenta line head, cyan line head, black line head) are provided.

  Next, the electric circuit configuration of the line head 1 will be described. FIG. 5 is a block diagram showing an electrical circuit configuration of the line head 1. As shown in FIG. 5, the line head 1 is provided with a light emitting element array 3a in which a plurality of organic EL elements 3 are arranged. For driving, the line head 1 corresponds to each of the organic EL elements 3 constituting the light emitting element array 3a. A transistor 60 and a switching transistor 61 are provided. The drive transistor 60 corresponds to the drive TFT 21 (see FIG. 2) and the drive element 4 shown in FIG.

  Each of the organic EL elements 3 has a negative electrode (cathode) connected to the power supply line 9, and an anode (anode) corresponding to each of the organic EL elements 3. , Drain electrode). The other one electrode (for example, source electrode) of each of the driving transistors 60 is connected to the power supply line 8. That is, one organic EL element 3 and one driving transistor 60 are connected in series to form a series circuit, and this series circuit is connected between the power supply line 8 and the power supply line 9. Similarly, one electrode (for example, drain electrode) of the switching transistor 61 provided corresponding to each of the organic EL elements 3 is connected to the gate electrode of the driving transistor 60 provided corresponding to each of the organic EL elements 3. ) Are connected to each other.

  In the present embodiment, the organic EL elements 3 constituting the light emitting element array 3a are divided into 16 blocks. Therefore, the driving transistor 60 and the switching transistor 61 provided in correspondence with the organic EL element 3 are also divided into 16 blocks. In the example shown in FIG. 5, only two blocks B1 and B2 out of a plurality of blocks are shown.

  Further, 16 analog voltage signal lines 62 connected to the data processing unit 52 shown in FIG. 4 and supplied with the analog voltage signals V0 to V15 are provided. Each of the analog voltage signal lines 62 is connected so that the other electrodes (for example, source electrodes) of the 16 switching transistors 61 included in one block do not overlap. When the number of blocks is n (n ≧ 1), n switching transistors 61 are connected to one analog voltage signal line 62.

  Further, a signal line 63 connected to the data processing unit 52 shown in FIG. 4 and supplied with a start pulse SP and a signal line 64 supplied with a clock pulse CK are provided. The signal line 63 is connected to the input end of the shift register 65a, and the output end of the shift register 65a and the input end of the shift register 65b are connected. That is, the shift register 65a and the shift register 65b are connected in cascade. The signal line 64 is connected to the clock input terminal of each shift register 65a, 65b,. Although only the shift registers 65a and 65b are shown in FIG. 5, when the number of blocks is n, n shift registers are provided. That is, a shift register is also provided for each block.

  Each gate electrode of the switching transistor 61 included in one block is connected to an output terminal of a shift register provided corresponding to the block. That is, the gate electrode of each switching transistor 61 included in the block B1 is connected to the output terminal of the shift register 65a, and the gate electrode of each switching transistor 61 included in the block B2 is connected to the output terminal of the shift register 65b. .

[Line head drive method]
Next, a driving method of the line head 1 having the above configuration will be described. When the start pulse SP is output from the data processing unit 52 illustrated in FIG. 4, the start pulse SP is input to the shift register 65 a via the signal line 63. The start pulse SP is input to the shift register 65 in synchronization with the rising edge of the clock signal CK, and sequentially transferred to the shift registers 65b,... At the rising edge of the clock signal CK.

  Here, the time interval of the start pulse SP is set so that only one of the outputs of the shift registers 65a, 65b,... Becomes H (high) level. Therefore, when the start pulse SP is output from the data processing unit 52 and the clock signal CK rises, only the output of the shift register 65a becomes H level, and when the clock signal CK rises next, only the output of the shift register 65b becomes H level. Similarly, the output of the shift register connected to the subsequent stage sequentially becomes H level.

  Now, assuming that the output terminal of the shift register 65a becomes H level, the 16 switching transistors 61 corresponding to the 16 organic EL elements 3 included in the block B1 connected to the output terminal of the shift register 65a are turned on. It becomes a state. As described above, since only one of the outputs of the shift registers 65a, 65b,... Becomes H level at a time and the outputs of a plurality of shift registers do not become H level at the same time. The switching transistor 61 corresponding to the included organic EL element 3 remains off.

  When the switching transistor 61 included in the block B1 is turned on, the analog voltage signals V0 to V15 applied to the analog voltage signal line 62 via the switching transistor 61 are applied to the driving transistor 60 included in the block B1. Applied to the gate electrode. This is called block B1. As a result, a current corresponding to the analog voltage signals V0 to V15 applied to the gate electrode flows to the organic EL element 3 included in the block B1 via the driving transistor 60, and light having luminance corresponding to the current is organic. It is emitted from the EL element 3.

  Next, when the clock signal CK is output from the data processing unit 52 shown in FIG. 4, the output of the shift register 65a becomes L (low) level and the output of the shift register 65b becomes H level. Note that the outputs of the shift registers other than the shift registers 65a and 65b remain at the L level. As a result, the switching transistor 61 connected to the output terminal of the shift register 65a is turned off, but the gate electrode of the driving transistor 60 has a parasitic capacitance (not shown), and the gate applied thereby Since the voltage is maintained, the light emission of the organic EL element 3 is also maintained.

  On the other hand, the 16 switching transistors 61 corresponding to the 16 organic EL elements 3 included in the block B2 connected to the output terminal of the shift register 65b are turned on. When the switching transistor 61 included in the block B2 is turned on, the analog voltage signals V0 to V15 applied to the analog voltage signal line 62 via the switching transistor 61 are included in the driving transistor 60 included in the block B2. Applied to the gate electrode.

  As a result, a current corresponding to the analog voltage signals V0 to V15 applied to the gate electrode flows to the organic EL element 3 included in the block B1 via the driving transistor 60, and light having luminance corresponding to the current is organic. It is emitted from the EL element 3. Thereafter, the same operation is performed for different blocks each time the clock signal CK is output from the data processing unit 52 shown in FIG. After sequentially selecting each block and selecting all the blocks, the start pulse SP is started again. This is repeated and this is called one scan. Then, each block is periodically selected. If each of the organic EL elements 3 in the block is turned on or off during each selection period, the state is maintained until the next selection. It will be. Accordingly, the light emission and extinction states of each organic EL element are set in units of one scanning period. For example, the G (G is a positive integer) scanning period is defined as one unit, and J (J is a positive value less than G). (Integer) A pulse width modulation method for emitting light only during a scanning period (a constant light amount) or an amplitude modulation method for varying the amount of emitted light in units of a predetermined G scanning period is performed by the data processing unit 52. This is performed based on the gradation-processed image data. Therefore, the light emission amount of the organic EL element 3 is performed based on the gradation-processed image data.

  When the line head 1 described above is provided in an image forming apparatus such as a printer, unevenness is formed in the formed image if the light emission intensities of the plurality of organic EL elements 3 provided in the line head 1 are different. . Further, as the time elapses, the overall luminance of the organic EL element 3 is reduced, resulting in unevenness in the formed image or the entire image being blurred. Therefore, in the present embodiment, the voltage-current characteristics of the organic EL element 3 are measured, and the analog voltage signals V0 to V15 applied to the organic EL element 3 are individually controlled based on the measurement result, and each organic EL element 3 is controlled. By controlling the current flowing through the organic EL element 3, variations in the light emission intensity between the organic EL elements 3 are eliminated and a change in the light emission intensity over time is prevented. Next, a method for measuring voltage-current characteristics of the organic EL element 3 will be described.

  FIG. 6 is a flowchart showing a procedure for measuring voltage-current characteristics of the organic EL element. This processing is performed by the data processing unit 52 shown in FIG. In FIG. 6, for convenience, the number of clock signals after outputting the start pulse SP is i, and the variable for selecting one of the 16 analog voltage signal lines 62 is k. When the process is started, the data processing unit 52 outputs the clock signal CK after outputting the start pulse SP (step S11). As a result, the output of the first shift register (shift register 65a) is set to the H level (step S12). In addition, since it takes time to measure the voltage-current characteristic of the organic EL element 3, it is desirable to set a longer interval for outputting the clock signal CK.

  When the output of the shift register 65a becomes H level, the 16 switching transistors 61 corresponding to the organic EL elements 3 included in the block B1 are turned on. Next, the data processing unit 52 sets the value of the variable k to “0” (step S13), and outputs the analog voltage signal V0 through one of the analog voltage signal lines 62. The analog voltage signal V0 is applied to the gate electrode of the driving transistor 60 through one of the switching transistors 61 that is in the ON state, and a current corresponding to the value of the analog voltage signal V0 is applied to the block B1. The light that flows through the included organic EL element 3 and has a luminance corresponding to the current is emitted.

  Next, the data processing unit 52 takes in the detection result of the current detector 55 (see FIG. 4), and stores the value of the analog voltage signal V0 and the detection result of the current detector 55 in association with each other in the storage unit 53. Next, the data processing unit 52 captures the detection result of the current detector 55 while changing the value of the analog voltage signal V0, and stores them in the storage unit 53 by associating them with one organic EL. A voltage-current characteristic of the element 3 is obtained (step S15). Of course, the other analog voltage signals V1 to V15 are set to, for example, the GND potential so that no current flows in the organic EL elements 3 other than the organic EL element 3 to be measured.

  Next, the data processing unit 52 increments the value of the variable k (step S16), and then determines whether or not the value of the variable k has become larger than “15” (step S17). Here, since the value of the variable k is “1”, the determination result in step S16 is “NO”. The data processing unit 52 outputs one of the remaining 15 analog voltage signal lines 62 (via the analog voltage signal V1). The other analog voltage signals V0, V2 to V15 are set to the GND potential, for example.

  The analog voltage signal V1 is applied to the gate electrode of the driving transistor 60 through one of the switching transistors 61 that is in the on state, and a current corresponding to the value of the analog voltage signal V1 is applied to the block B1. It flows into one of the remaining 15 organic EL elements 3 included, and light having a luminance corresponding to the current is emitted. Similarly, the detection result of the current detector 55 is taken in while changing the value of the analog voltage signal V1 (step S14), and the voltage-current characteristic of the organic EL element 3 is obtained (step S15). Similarly, the voltage characteristics of 16 organic EL elements 3 included in the block B1 are measured. After measuring the characteristics of all the organic EL elements 3 in the block B1, all the analog voltage signals V0 to V15 are set to the GND potential, for example, and no current flows through all the organic EL elements 3 in the block B1. To.

  When the voltage characteristic measurement of the 16 organic EL elements 3 included in the block B1 is completed, the value of the variable k becomes “16”, and the determination result in step S17 becomes “YES”. Next, the data processing unit 52 determines whether or not the number of clock signals CK output after outputting the start pulse SP is equal to or greater than n (the number of blocks dividing the organic EL element 3) (step S18). Here, since the number of clock signals i is “1”, the determination result is “NO”, and the data processing unit 52 outputs the clock signal CK (step S19). As a result, the output of the second shift register (shift register 65b) is set to the H level (step S12).

  When the output of the shift register 65b becomes H level, the 16 switching transistors 61 corresponding to the organic EL elements 3 included in the block B2 are turned on. Next, the data processing unit 52 sets the value of the variable k to “0” (step S13), repeats the processing of steps S14 to S16, and sets the voltage / current characteristics of the organic EL element 3 included in the block B2. Measure in the same way. When the number of clock signals i reaches “n” by repeating the above processing, the determination result in step S18 becomes “NO”, and the series of processing ends.

  Here, if the variation in the light emission luminance of the organic EL element 3 is eliminated, the voltage applied to the electrode of the driving transistor 60 provided corresponding to each of the organic EL elements 3, and the It is necessary to measure the relationship with the emission luminance. However, in this embodiment, the luminance increases when a large amount of current flows through the organic EL element 3, and conversely, the luminance decreases when the current flowing through the organic EL element 3 is small. The voltage-current characteristic of the organic EL element 3 is measured by paying attention to a strong correlation between the current flowing through the organic EL element 3. Thereby, the measuring apparatus which measures the light emission luminance of the organic EL element 3 can be omitted, and the configuration of the line head 1 can be simplified.

  When driving the line head 1, analog voltage signals V <b> 0 to V <b> 15 based on the voltage / current characteristics of each organic EL element 3 measured by the above processing are applied to the analog voltage signal line 62. That is, the data processing unit 52 is necessary for reading out the voltage / current characteristics of each of the organic EL elements 3 included in the line head 1 from the storage unit 53 and flowing a constant current (reference current) to all the organic EL elements 3. Is obtained for each organic EL element 3. Then, in synchronization with the clock signal CK, the voltage applied to the gate electrode of the driving transistor 60 included in the block is synchronized with the timing when the 16 switching transistors 61 included in the block are turned on. The analog voltage signals V0 to V15 are output to each of the analog voltage signal lines 62.

  By performing the above control for all the blocks, substantially equal current flows to each of the organic EL elements 3 provided in the line head 1, and light having substantially equal luminance is emitted from each organic EL element 3. (In the case of pulse width modulation method). Also in the amplitude modulation method, an appropriate voltage for applying a desired current can be applied. Thereby, the dispersion | variation in the brightness between the organic EL elements 3 provided in the line head 1 can be eliminated. Further, as described above, when the data processing unit 52 reads out the voltage / current characteristics of each organic EL element 3 from the storage unit 53, the voltage required to flow a constant current (reference current) to the organic EL element 3. Since the value is obtained, there is no problem that the overall luminance of the organic EL element 3 decreases with time.

  In the above description, the EL element of the present invention is configured by the organic EL element 3 in which the light emitting layer is formed of an organic material. You may comprise by an EL element. In the line head 1 described above, a top emission type that emits light emitted from the EL element from the sealing substrate 6 side is adopted, but a bottom emission type that emits light from the element substrate 2 side may be used. Further, the line head 1 may be of an end surface emission type as shown in, for example, JP-A-2-233268, instead of the top emission type or the bottom emission type. Further, the line head 1 and the driving device 51 shown in FIG. 4 may be provided separately, or the driving device 51 may be provided integrally with the line head 1.

[Image forming apparatus]
Next, an image forming apparatus provided with the line head 1 of the present invention will be described. FIG. 7 is a diagram showing the configuration of the image forming apparatus according to the first embodiment of the present invention. As shown in FIG. 7, the image forming apparatus 80. The image forming apparatus 80 includes organic EL array line heads 81K, 81C, 81M, and 81Y, which are examples of the line head of the present invention, and four corresponding photosensitive drums (image carriers) 82K having the same configuration. These are arranged in 82C, 82M, and 82Y exposure apparatuses, respectively, and are configured as a tandem system.

  The image forming apparatus 80 includes a driving roller 83, a driven roller 84, and a tension roller 85, and an intermediate transfer belt 86 is stretched around each of the rollers so as to be circulated and driven in an arrow direction (counterclockwise direction) in FIG. Is. Photoconductors 82K, 82C, 82M, and 82Y are arranged at predetermined intervals with respect to the intermediate transfer belt 86. These photoreceptors 82K, 82C, 82M, and 82Y have a photosensitive layer as an image carrier on their outer peripheral surfaces.

  Here, K, C, M, and Y in the above symbols mean black, cyan, magenta, and yellow, respectively, and indicate that the photoconductors are for black, cyan, magenta, and yellow, respectively. The meanings of these symbols (K, C, M, Y) are the same for the other members. The photoconductors 82K, 82C, 82M, and 82Y are driven to rotate in the arrow direction (clockwise) in FIG. 7 in synchronization with the driving of the intermediate transfer belt 86.

  Around each photosensitive member 82 (K, C, M, Y), charging means (corona charger) 87 (K) for uniformly charging the outer peripheral surface of the photosensitive member 82 (K, C, M, Y), respectively. , C, M, Y) and the outer peripheral surface uniformly charged by the charging means 87 (K, C, M, Y) are synchronized with the rotation of the photosensitive member 82 (K, C, M, Y). And an organic EL array line head 81 (K, C, M, Y) of the present invention that sequentially scans the line.

  Further, a developing device 88 (K) that applies a toner as a developer to the electrostatic latent image formed by the organic EL array line head 81 (K, C, M, Y) to form a visible image (toner image). , C, M, Y) and a primary transfer roller 89 as transfer means for sequentially transferring the toner image developed by the developing device 88 (K, C, M, Y) to the intermediate transfer belt 86 as a primary transfer target. (K, C, M, Y) and a cleaning device 90 (K, C, Y) as a cleaning unit for removing toner remaining on the surface of the photosensitive member 82 (K, C, M, Y) after being transferred. M, Y).

  Each organic EL array line head 81 (K, C, M, Y) is installed such that each array direction is along the bus line of the photosensitive drum 82 (K, C, M, Y). The emission energy peak wavelength of each organic EL array line head 81 (K, C, M, Y) and the sensitivity peak wavelength of the photoconductor 82 (K, C, M, Y) are set so as to substantially match. ing. Each organic EL array line head 81 (K, C, M, Y) is supported and attached to the image forming apparatus 80 by a support member.

  The developing device 88 (K, C, M, Y) uses, for example, a non-magnetic one-component toner as a developer. The film thickness of the developed developer is regulated by a regulating blade, and the developing roller is brought into contact with or pressed against the photoreceptor 82 (K, C, M, Y), thereby the photoreceptor 82 (K, C, M, Y). In accordance with the potential level, a developer is attached and developed as a toner image.

  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 89 (K, C, M, Y). Primary transfer is sequentially performed on the transfer belt 86. The toner images, which are sequentially superimposed on the intermediate transfer belt 86 and become full color, are secondarily transferred to the recording medium P such as paper by the secondary transfer roller 91 and further pass through the fixing roller pair 92 as a fixing unit. Then, the toner image is fixed on the recording medium P and then discharged onto a paper discharge tray 94 formed on the upper part of the apparatus by a pair of paper discharge rollers 93.

  In FIG. 7, reference numeral 95 denotes a paper feed cassette in which a large number of recording media P are stacked and held, 96 denotes a pickup roller for feeding the recording media P from the paper feed cassette 95 one by one, and 97 denotes secondary transfer. A pair of gate rollers for defining the supply timing of the recording medium P to the secondary transfer portion of the roller 91; a secondary transfer roller 91 as a secondary transfer means for forming a secondary transfer portion with the intermediate transfer belt 86; A cleaning blade 98 serves as a cleaning unit that removes toner remaining on the surface of the intermediate transfer belt 86 after the secondary transfer. As described above, since the image forming apparatus of FIG. 7 uses the organic EL array line head 81 (K, C, M, Y) as the exposure means (writing means), for example, when a laser scanning optical system is used. In comparison, the size of the apparatus can be reduced.

  Next, an image forming apparatus according to a second embodiment of the present invention will be described. FIG. 8 is a longitudinal side view of the image forming apparatus according to the second embodiment of the present invention. In FIG. 8, the image forming apparatus 100 is provided with a rotary developing device 101, a photosensitive drum 105 functioning as an image carrier, and the line head (organic EL array head) as main components. Means (exposure means) 107, intermediate transfer belt 109, paper conveyance path 114, fixing unit heating roller 112, and paper feed tray 118 are provided.

  The developing device 101 is configured such that the developing rotary 101a rotates in the direction of arrow A about a shaft 101b. The interior of the development rotary 101a 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 102a to 102d are arranged in the image forming units for the four colors, and developing rollers that rotate in the direction of arrow B, and 103a to 103d are toner supply rollers that rotate in the direction of arrow C. 104a to 104d are regulating blades that regulate the toner to a predetermined thickness.

  In FIG. 8, reference numeral 105 denotes a photosensitive drum that functions as an image carrier as described above, 106 denotes a primary transfer member, and 108 denotes a charger. Reference numeral 107 denotes an image writing means as an exposure means in the present invention, which comprises the organic EL line head of the present invention. The photosensitive drum 105 is rotationally driven in the direction of arrow D, which is the opposite direction to the developing roller 102a, by a drive motor (not shown), for example, a step motor. The organic EL line head constituting the image writing means 107 is disposed in a state where the alignment (optical axis alignment) is performed between this and the imaging lens (not shown) and the photosensitive drum 105. Yes.

  The intermediate transfer belt 109 is stretched between the driving roller 110a and the driven roller 110b. The drive roller 110 a is connected to the drive motor of the photosensitive drum 105 and transmits power to the intermediate transfer belt 109. That is, by driving the drive motor, the drive roller 110a of the intermediate transfer belt 109 is rotated in the direction of arrow E, which is the opposite direction to the photosensitive drum 105.

  The paper transport path 114 is provided with a plurality of transport rollers, paper discharge roller pairs 116, and the like, so that the paper is transported. An image (toner image) on one side carried on the intermediate transfer belt 109 is transferred to one side of the paper at the position of the secondary transfer roller 111. The secondary transfer roller 111 is brought into contact with and separated from the intermediate transfer belt 109 by a clutch. When the clutch is turned on, the secondary transfer roller 111 is brought into contact with the intermediate transfer belt 109 so that an image is transferred onto a 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 H. The fixing device is provided with a heating roller 112 and a pressure roller 113. The sheet after the fixing process is drawn into the discharge roller pair 116 and proceeds in the direction of arrow F. When the paper discharge roller pair 116 rotates in the opposite direction from this state, the paper reverses its direction and advances on the duplex printing conveyance path 115 in the direction of arrow G. 117 is an electrical component box, 118 is a paper feed tray for storing paper, and 119 is a pickup roller provided at the outlet of the paper feed tray 118.

  For example, a low-speed brushless motor is used as a drive motor for driving the transport roller in the paper transport path. Further, a step motor is used for the intermediate transfer belt 109 because color misregistration correction or the like is necessary. Each of these motors is controlled by a signal from a control means (not shown).

  In the state shown in FIG. 8, a yellow (Y) electrostatic latent image is formed on the photosensitive drum 105, and a high voltage is applied to the developing roller 102a, whereby a yellow image is formed on the photosensitive drum 105. Is done. When all of the yellow back side and front side images are carried on the intermediate transfer belt 109, the development rotary 101a rotates 90 degrees in the direction of arrow A.

  The intermediate transfer belt 109 rotates once and returns to the position of the photosensitive drum 105. Next, two images of cyan (C) are formed on the photosensitive drum 105, and this image is carried on the yellow image carried on the intermediate transfer belt 109. Thereafter, the 90-degree rotation of the development rotary 101 and the one-rotation process after the image is carried on the intermediate transfer belt 109 are repeated in the same manner.

  For carrying four color images, the intermediate transfer belt 109 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 111. The paper fed from the paper feed tray 118 is transported by the transport path 114, and the above color image is transferred to one side of the paper at the position of the secondary transfer roller 111. The sheet on which the image is transferred on one side is reversed by the pair of discharge rollers 116 as described above, and waits on the conveyance path. Thereafter, the sheet is conveyed to the position of the secondary transfer roller 111 at an appropriate timing, and the above color image is transferred to the other side. The housing 120 is provided with an exhaust fan 121.

  The image forming apparatus according to the first and second embodiments of the present invention has been described above. However, the image forming apparatus provided with the line head of the present invention is not limited to the above-described embodiment, and various modifications are possible. is there.

It is a figure showing typically a line head by one embodiment of the present invention. 3 is a side cross-sectional view showing a main part configuration of the line head 1. 2 is a bottom view showing the configuration of the main part of the line head 1. FIG. 1 is a block diagram illustrating a schematic configuration of a drive apparatus for a line head according to an embodiment of the present invention. 2 is a block diagram showing an electrical circuit configuration of the line head 1. FIG. It is a flowchart which shows the measurement procedure of the voltage-current characteristic of an organic EL element. 1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. It is a vertical side view of the image forming apparatus by 2nd Embodiment of this invention.

Explanation of symbols

1 …… Line head 2 …… Element substrate 3 …… Organic EL element (EL element)
3a: Light emitting element array 4: Drive element 8: Power line 9 ... Power line 51 ... Drive device 52 ... Data processing unit (measurement unit, control unit)
53 …… Storage unit (storage unit)
55 …… Current detector (measurement unit)

Claims (12)

A line head driving device including a light emitting element array in which a plurality of EL elements are arranged,
A measurement unit for measuring voltage-current characteristics of each of the EL elements;
A line head drive device comprising: a control unit that controls a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements measured by the measurement unit. A storage unit for storing voltage-current characteristics of each of the EL elements measured by the measurement unit;
2. The line head drive according to claim 1, wherein the control unit controls a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements stored in the storage unit. apparatus.   3. The line head driving apparatus according to claim 2, wherein the control unit controls a voltage applied to each of the EL elements so that a current flowing through each of the EL elements becomes equal to a reference current.   4. The line head drive device according to claim 3, wherein the control unit controls the light emission amount of each of the EL elements based on image data subjected to gradation processing. A driving method of a line head including a light emitting element array in which a plurality of EL elements are arranged,
A measurement step of measuring voltage-current characteristics of each of the EL elements;
And a control step of controlling a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements measured in the measuring step.   6. The line head driving method according to claim 5, further comprising a storage step of storing voltage-current characteristics of each of the EL elements measured in the measurement step.   7. The line head driving method according to claim 6, wherein the control step includes a step of controlling a voltage applied to each of the EL elements so that a current flowing through each of the EL elements becomes equal to a reference current. .   8. The line head driving method according to claim 7, wherein the control step includes a step of controlling a light emission amount of each of the EL elements based on image data subjected to gradation processing. A line head in which a light emitting element array in which a plurality of EL elements are arranged on an element substrate and a drive element group for driving the EL elements are integrally formed,
On the element substrate, a measurement unit that measures voltage-current characteristics of each of the EL elements,
A line head comprising: a control unit that controls a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements measured by the measurement unit. A storage unit that stores voltage-current characteristics of each of the EL elements measured by the measurement unit on the element substrate,
The line head according to claim 9, wherein the control unit controls a voltage applied to each of the EL elements based on a voltage-current characteristic of each of the EL elements stored in the storage unit.   An image comprising: a line head including a light emitting element array formed by arranging a plurality of EL elements; and a line head driving device according to any one of claims 1 to 4 as exposure means. Forming equipment. An image forming apparatus comprising the line head according to claim 9 as exposure means.

Images