JP4816006B2 - Print head and image forming apparatus - Google Patents

Description

  The present invention relates to a print head used for a print head of an image forming apparatus such as a copying machine or a printer, and more particularly to a print head using an LED as a light emitting element.

  In recent years, an LED print head (LPH: LED Print Head) composed of a self-scanning LED (SLED) array is used as a print head for an image forming apparatus such as an electrophotographic copying machine or printer. ) Has been developed. Such an SLED can employ a thyristor structure in a switching unit that selectively turns on and off an LED, which is a light emitting element, so that the switching unit and the LED can be arranged on the same chip. In addition, since the LED can be driven by two signal lines that transmit a timing signal for setting the LED to an on / off state and one signal line that transmits an image signal to the LED. It is also possible to simplify the wiring. Therefore, LPH using SLED is a very effective print head for downsizing the apparatus.

In LPH using such an SLED, the lighting unit (ON state) in each LED is sequentially transferred (scanned) by a switching unit having a thyristor structure, and a pulse width modulated lighting signal is transmitted in synchronization with the LED. By doing so, each LED is turned on. Then, with respect to the lighting pulse width in the image signal, the overall light amount control for adjusting the light amount of the entire LPH and the light amount correction control for adjusting the variation of the light amount for each LED are executed, so that the light emission amount of each LED is adjusted. Optimization is planned.
Here, the total light amount control is to uniformly control the light amounts of all the LEDs. That is, changes in the sensitivity of the photoreceptor exposed by LPH over time, potential fluctuations of the electrostatic latent image caused by environmental conditions such as temperature and humidity, and the amount of developer in the developing device that develops the electrostatic latent image As a result, the density fluctuation occurs in the formed image. Therefore, the total light amount control is executed in order to suppress density fluctuation due to such factors. The light amount correction control controls the light amount independently for each LED. That is, an error in the amount of light for each LED caused by a difference in the light emission characteristics of each LED, an error in the arrangement position, and the like causes density unevenness on the image. Therefore, the light amount correction control is executed to set the light amounts of all the LEDs within a predetermined range in order to suppress such image density unevenness.

  As a conventional technique related to the control of the lighting pulse width in such LPH, a lighting time reference value for controlling the total light amount of LPH is set, and each LED for correcting the uneven density of the image by using this lighting time reference value. There is a technique of performing correction based on a correction amount for each LED and setting a lighting time (lighting pulse width) for each LED (see, for example, Patent Document 1).

JP 2002-36628 A (Pages 5-8)

  By the way, the light quantity correction control by LPH described above corrects the reference lighting pulse width for setting the light emission quantity of each LED for each LED, and each LED to which the same reference lighting pulse width is input is substantially omitted. Adjustment is made so that the same amount of emitted light is output. In such light quantity correction control, the fluctuation range of the light quantity characteristic representing the relationship between the reference lighting pulse width and the light emission quantity of the LED is a light quantity characteristic (target light quantity characteristic) that is a design target in the entire use light quantity range. ) To be within a predetermined range (for example, 0.5% of the target light quantity characteristic).

However, a driver output unit that outputs a lighting signal to the LED is normally provided for each SLED chip on which a predetermined number (for example, 128) of LEDs are mounted (for example, 58). In each driver output unit, the linearity of the output characteristics is different, and power supply voltage fluctuation (crosstalk) also occurs according to the number of LEDs that emit light. Therefore, even if the light amount correction control is actually performed, it is difficult to set the fluctuation range of the light amount characteristic of each LED within the predetermined range in the entire use light amount range. In particular, when it is necessary to switch the resolution in the sub-scanning direction between high resolution and low resolution, it is necessary to set the light amount range to be wide. It is extremely difficult to suppress.
If the fluctuation range of the light quantity characteristic in each LED cannot be suppressed within a predetermined range with respect to the target light quantity characteristic, there arises a problem that the occurrence of the above-described image density unevenness cannot be sufficiently suppressed. In addition, since the setting accuracy of the total light quantity also decreases, it becomes difficult to sufficiently cope with changes in image density due to changes in the sensitivity of the photoconductor over time, environmental conditions, and the like. Such inconvenience tends to be particularly noticeable when a high-definition image is formed.

  Accordingly, the present invention has been made to solve the technical problems as described above, and an object of the present invention is to increase the optimization of the light emission amount of each LED in the entire use light amount range in the LED print head. It is to achieve accuracy.

  For this purpose, the print head of the present invention is a print head that exposes an image carrier in an image forming apparatus, and corrects the light amount of each of the light emitting elements arranged in a line and the light emitting elements. Each of the light emitting elements based on the reference clock and the correction data, a correction data storage section for storing correction data for generating the reference clock, a reference clock generating section for generating a reference clock used as a reference when setting the light emission amount of the light emitting element A lighting pulse width setting unit for setting a lighting pulse width for driving the light source, and a constant addition processing unit for adding a predetermined constant determined corresponding to the lighting pulse width to the lighting pulse width set by the lighting pulse width setting unit It is characterized by having.

Here, the constant addition processing unit can be characterized in that a constant having a different value is set for each predetermined region in the range in which the lighting pulse width is set by the lighting pulse width setting unit. In addition, the constant addition processing unit sets constants having different values for each predetermined area divided in accordance with the resolution of the image forming apparatus within the range in which the lighting pulse width is set by the lighting pulse width setting unit. It can also be characterized.
Furthermore, it is possible to further include a memory for storing a constant added to the lighting pulse width in the constant addition processing unit in association with the lighting pulse width. In particular, the constant addition processing unit may be characterized in that a constant corresponding to the lighting pulse width set by the lighting pulse width setting unit is downloaded from the memory.
In addition, a predetermined number of light emitting elements are arranged on a plurality of chip members, and one lighting pulse width setting unit is provided for each chip member, and a constant addition processing unit corresponds to each lighting pulse width setting unit. It can also be characterized in that a fixed constant is set.

  Further, the present invention is regarded as an image forming apparatus, and the image forming apparatus of the present invention includes a photosensitive member, a print head that exposes the photosensitive member, and a control unit that outputs an instruction signal that indicates the total light amount of the print head. The print head emits light based on a plurality of light emitting elements arranged in a line, a correction data storage unit that stores correction data for correcting the amount of light of each light emitting element, and an instruction signal from the control unit. A reference clock generation unit that generates a reference clock serving as a reference when setting the light emission amount of the element, and a lighting pulse width setting unit that sets a lighting pulse width for driving each light emitting element based on the reference clock and the correction data And a constant addition processing unit that adds a predetermined constant to the lighting pulse width set by the lighting pulse width setting unit. For each predetermined region in the range in which the scan width is set, and characterized in that the constant of different values are set.

Here, a resolution switching control unit that outputs a resolution switching signal for switching the printing resolution by the print head is further provided, and the constant addition processing unit of the print head is a range in which the lighting pulse width is set by the lighting pulse width setting unit. A constant having a different value is set for each predetermined region divided based on the resolution switching signal from the resolution switching control unit.
In addition, it may further include a memory that stores constants set in the constant addition processing unit of the print head in association with each predetermined area.
Furthermore, the constant addition processing unit has a variation width of the light amount characteristic of the light emitting element with respect to the lighting pulse width within the range of the lighting pulse width that can be set by the lighting pulse width setting unit. A constant may be set so as to be within a predetermined range. In addition, the constant addition processing unit has a variation range of the light amount characteristic of the light emitting element with respect to the lighting pulse width within the range of the lighting pulse width that can be set by the lighting pulse width setting unit. An area may be set so as to be within a predetermined range.

  According to the present invention, the light quantity characteristic of each LED in the LED print head can be accurately matched with the target light quantity characteristic in the entire use light quantity range, so that the occurrence of uneven image density can be suppressed to a very low level. In addition, since it is possible to adjust the total light amount with high accuracy, it is possible to maintain uniformity in image density in response to changes in the sensitivity of the photoconductor over time, changes in environmental conditions, and the like.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram showing an overall configuration of an image forming apparatus using an LED print head to be measured in the present embodiment. The image forming apparatus shown in FIG. 1 is a so-called tandem digital color printer 1, and includes an image forming process unit 10 that forms an image corresponding to image data of each color, a control unit 30 that controls the image forming process unit 10, For example, an image processing unit (IPS: Image Processing System) 40 that is connected to a personal computer (PC) 2 and an image reading device (IIT) 3 and performs predetermined image processing on image data received from these devices is provided. .

The image forming process unit 10 includes four image forming units 11Y, 11M, 11C, and 11K that are arranged in parallel at a predetermined interval. The image forming units 11Y, 11M, 11C, and 11K uniformly charge the surface of the photosensitive drum 12 as an image carrier that forms an electrostatic latent image and carries a toner image with a predetermined potential. The charger 13, the LED print head (LPH) 14 as an exposure device that exposes the photosensitive drum 12 charged by the charger 13, the developing device 15 that develops the electrostatic latent image obtained by the LPH 14, and the photosensitive after transfer A cleaner 16 for cleaning the surface of the body drum 12 is provided.
Further, a density detection circuit 17 that detects the toner image density of a test patch (density sample) formed on the photosensitive drum 12 is provided in the vicinity of the downstream side of the developing device 15 so as to face the photosensitive drum 12. It has been. The density detection circuit 17 is connected to the control unit 30 and outputs a toner image density detection value.
Here, the image forming units 11Y, 11M, 11C, and 11K are configured in substantially the same manner except for the toner stored in the developing unit 15. The image forming units 11Y, 11M, 11C, and 11K respectively form yellow (Y), magenta (M), cyan (C), and black (K) toner images.

  In addition, the image forming process unit 10 includes an intermediate transfer belt 21 to which the toner images of the respective colors formed on the photosensitive drums 12 of the image forming units 11Y, 11M, 11C, and 11K are transferred in a multiple manner, and the image forming units 11Y and 11Y. A primary transfer roll 22 as a primary transfer charger for sequentially transferring (primary transfer) each color toner image of 11M, 11C, and 11K to the intermediate transfer belt 21, and a superimposed toner image transferred onto the intermediate transfer belt 21 as a recording material (recording) A secondary transfer roll 23 serving as a secondary transfer charger that performs batch transfer (secondary transfer) on the paper P, which is a paper, and a fixing device 25 that fixes the secondary transferred image on the paper P.

  In the digital color printer 1 of the present embodiment, the image forming process unit 10 performs an image forming operation based on a control signal such as a synchronization signal supplied from the control unit 30. At that time, the image data input from the PC 2 or IIT 3 is subjected to image processing by the image processing unit 40 and supplied to each of the image forming units 11Y, 11M, 11C, and 11K via the interface. In the yellow image forming unit 11Y, for example, the surface of the photosensitive drum 12 uniformly charged at a predetermined potential by the charger 13 is exposed by the LPH 14 that emits light based on the image data obtained from the image processing unit 40. Thus, an electrostatic latent image is formed on the photosensitive drum 12. The formed electrostatic latent image is developed by the developing device 15, and a yellow toner image is formed on the photosensitive drum 12. Similarly, magenta, cyan, and black toner images are formed in the image forming units 11M, 11C, and 11K.

The color toner images formed by the image forming units 11Y, 11M, 11C, and 11K are sequentially electrostatically attracted by the primary transfer roll 22 onto the intermediate transfer belt 21 that rotates in the direction of arrow A in FIG. A toner image superimposed on the belt 21 is formed. The superimposed toner image is conveyed to a region (secondary transfer portion) where the secondary transfer roll 23 is disposed as the intermediate transfer belt 21 moves. When the superimposed toner image is conveyed to the secondary transfer unit, the paper P is supplied to the secondary transfer unit in accordance with the timing at which the toner image is conveyed to the secondary transfer unit. Then, the superimposed toner images are collectively electrostatically transferred onto the conveyed paper P by the transfer electric field formed by the secondary transfer roll 23 in the secondary transfer portion.
Thereafter, the sheet P on which the superimposed toner image has been electrostatically transferred is peeled off from the intermediate transfer belt 21 and conveyed to the fixing device 25 by the conveying belt 24. The unfixed toner image on the paper P conveyed to the fixing device 25 is fixed on the paper P by being subjected to a fixing process by heat and pressure by the fixing device 25. Then, the paper P on which the fixed image is formed is conveyed to a paper discharge mounting portion (not shown) provided in the discharge portion of the image forming apparatus.

  FIG. 2 is a view showing a configuration of an LED print head (LPH) 14 that is an exposure unit. 2, the LPH 14 includes a housing 61 as a support, a self-scanning LED array (SLED) 63 constituting a light emitting unit, and a drive circuit (signal generation circuit) 100 (drive signal generation means for driving the SLED 63 and SLED 63). The LED circuit board 62 and the rod lens array 64, which are optical members for forming an image of the light from the SLED 63 on the surface of the photosensitive drum 12, and the rod lens array 64 are supported from the outside. A shielding holder 65 and a leaf spring 66 for urging the housing 61 toward the rod lens array 64 are provided.

The housing 61 is formed of a block or sheet metal such as aluminum or SUS, and supports the LED circuit board 62. The holder 65 supports the housing 61 and the rod lens array 64, and is set so that the light emitting point of the SLED 63 and the focal point of the rod lens array 64 coincide. Furthermore, the holder 65 is configured to seal the SLED 63. Therefore, it is possible to prevent dust from adhering to the SLED 63 from the outside. On the other hand, the leaf spring 66 urges the LED circuit board 62 toward the rod lens array 64 via the housing 61 so as to maintain the positional relationship between the SLED 63 and the rod lens array 64.
The LPH 14 configured as described above is configured to be movable in the optical axis direction of the rod lens array 64 by an adjustment screw (not shown), and the imaging position (focal plane) of the rod lens array 64 is the surface of the photosensitive drum 12. It is adjusted so that it is located above.

As shown in FIG. 3 (plan view of the LED circuit board 62), the LED circuit board 62 includes, for example, SLEDs 63 composed of 58 SLED chips (CHIP1 to CHIP58) in parallel with the axial direction of the photosensitive drum 12. It is arranged in a line with high accuracy. In this case, the SLED chips are alternately staggered so that each LED array is continuously arranged at the connection portion between the SLED chips at the end boundary of the LED arrays arranged in each SLED chip (CHIP1 to CHIP58). Is arranged.
The LED circuit board 62 includes a signal generation circuit 100, a level shift circuit 104, a three-terminal regulator 101 that outputs a power supply voltage, an EEPROM 102 that stores light amount correction value data in the SLED 63, and the digital color printer 1 main body. A harness 103 for transmitting and receiving signals is provided.

Next, the wiring configuration on the LED circuit board 62 will be described.
FIG. 4 is a diagram showing a wiring diagram formed on the LED circuit board 62. As shown in FIG. 4, on the LED circuit board 62, a + 3.3V power supply line 105 that supplies power to each SLED chip, a grounded (GND) power supply line 106, and the signal generation circuit 100 to each SLED chip. Signal lines 107 (107_1 to 107_58) for transmitting lighting signals ΦI (ΦI1 to ΦI58), signal lines 108 (108_1 to 108_6) for transmitting transfer signals CK1 (CK1_1 to 1_6), and transfer signals CK2 (CK2_1 to 2_6) ) For transmitting signal lines 109 (109_1 to 109_6).
And the lighting signal (PHI) I with respect to CHIP1-CHIP58 is input into each SLED chip (CHIP1-CHIP58) via the signal line 107. FIG. Further, the transfer signal CK1 (CK1_1 to 1_6) is input to the CHIP1 to CHIP58 via the signal line 108, and the transfer signal CK2 (CK2_1 to 2_6) is input to the CHIP1 to CHIP58 via the signal line 109, respectively.

Next, the circuit configuration of the SLED 63 will be described.
FIG. 5 is a diagram illustrating the circuit configuration of the SLED 63. The SLED 63 of this embodiment is connected to the signal generation circuit 100 via the level shift circuit 104. The level shift circuit 104 has a configuration in which a resistor R1B and a capacitor C1, and a resistor R2B and a capacitor C2 are arranged in parallel, one end of which is connected to the input terminal of the SLED 63, and the other end of the signal generating circuit 100. Connected to the output terminal. Based on the transfer signals CK1R and CK1C and the transfer signals CK2R and CK2C output from the signal generation circuit 100, the transfer signal CK1 and the transfer signal CK2 are output to the SLED 63.
In addition, although 58 SLED chips are arranged in series in the SLED 63 of the present embodiment, only one SLED chip is shown in FIG. In the following description, the SLED chip is referred to as SLED 63 for convenience.

As shown in FIG. 5, the SLED 63 includes 128 thyristors S1 to S128 as switching elements, 128 LEDs L1 to L128 as light emitting elements, 128 diodes D1 to D128, and 128 resistors R1 to R128. In addition, the transfer current limiting resistors R1A and R2A are configured to prevent excessive current from flowing through the signal lines Φ1 and Φ2.
Here, a part mainly composed of thyristors S1 to S128 and diodes D1 to D128 for controlling supply of current to the LEDs L1 to L128 is referred to as a transfer unit.

In the SLED 63 of the present embodiment, the anode terminals (input terminals) A1 to A128 of the thyristors S1 to S128 are connected to the power supply line 105. A power supply voltage VDD (= + 3.3 V) is supplied to the power supply line 105.
A transfer signal CK1 is transmitted from the signal generation circuit 100 to the cathode terminals (output terminals) K1, K3,..., K127 of the odd-numbered thyristors S1, S3,. Is done.
.., S128 to the even-numbered thyristors S2, S4,..., S128 from the signal generation circuit 100 through the level shift circuit 104 and the transfer current limiting resistor R2A. CK2 is transmitted.

On the other hand, gate terminals (control terminals) G1 to G128 of the thyristors S1 to S128 are respectively connected to the power supply line 106 via resistors R1 to R128 provided corresponding to the thyristors S1 to S128. The power supply line 106 is grounded (GND).
The gate terminals G1 to G128 of the thyristors S1 to S128 are connected to the gate terminals of the LEDs L1 to L128 provided corresponding to the thyristors S1 to S128, respectively.
Furthermore, the cathode terminals of the diodes D1 to D128 are connected to the gate terminals G1 to G128 of the thyristors S1 to S128. The gate terminals G1 to G127 of the thyristors S1 to S127 are connected to the anode terminals of the next-stage diodes D2 to D128, respectively. That is, the diodes D1 to D128 are connected in series with the gate terminals G1 to G127 interposed therebetween.
The anode terminal of the diode D1 is connected to the signal generation circuit 100 via the transfer current limiting resistor R2A and the level shift circuit 104, and the transfer signal CK2 is transmitted. Further, the cathode terminals of the LEDs L1 to L128 are connected to the signal generation circuit 100, and the lighting signal ΦI is transmitted.

  Further, the light shielding mask 50 is disposed in the SLED 63 so as to cover the thyristors S1 to S128 and the diodes D1 to D128 in the transfer unit. This is because during the image forming operation, light emission from the thyristors S1 to S128 in the on state and current is flowing, and from the diodes D1 to D128 in the current flow state is blocked, and unnecessary light is removed from the photoconductor. It is provided to suppress exposure of the drum 12.

Next, a signal (drive signal) for driving the SLED 63 output from the signal generation circuit 100 and the level shift circuit 104 will be described.
FIG. 6 is a timing chart showing drive signals output from the signal generation circuit 100 and the level shift circuit 104. Note that the timing chart shown in FIG. 6 shows a case where all LEDs perform optical writing (light emission).
(1) First, when a reset signal (RST) is input from the image forming apparatus to the signal generation circuit 100, the signal generation circuit 100 transfers the transfer signal CK1C to a high level (hereinafter referred to as “H”) and transfers. The signal CK1R is set to “H”, the transfer signal CK1 is set to “H”, the transfer signal CK2C is set to low level (hereinafter referred to as “L”), the transfer signal CK2R is set to “L”, and the transfer signal CK2 is set. Is set to the low level “L”, and all the thyristors S1 to S128 are set to the off state (FIG. 6A).
(2) Following the reset signal (RST), the line synchronization signal Lsync output from the signal generation circuit 100 becomes “H” (FIG. 6A), and the operation of the SLED 63 is started. Then, in synchronization with the line synchronization signal Lsync, as shown in FIGS. 6E, 6F, and 6G, the transfer signal CK2C and the transfer signal CK2R are set to “H”, and the transfer signal CK2 is set to “H”. (FIG. 6B).
(3) Next, as shown in FIG. 6C, the transfer signal CK1R is set to “L” (FIG. 6C).

(4) Subsequently, as shown in FIG. 6B, the transfer signal CK1C is set to “L” (FIG. 6D).
In this state, the gate current of the thyristor S1 starts to flow. At that time, the tri-state buffer B1R of the signal generation circuit 100 is set to high impedance (Hiz) to prevent current backflow.
Thereafter, the thyristor S1 starts to be turned on by the gate current of the thyristor S1, and the gate current gradually increases. At the same time, when a current flows into the capacitor C1 of the level shift circuit 104, the potential of the transfer signal CK1 also gradually increases.

(5) After a lapse of a predetermined time (time when the transfer signal CK1 potential becomes close to GND), the tristate buffer B1R of the signal generation circuit 100 is set to “L” (FIG. 6E). As a result, the potential of the signal line Φ1 and the potential of the transfer signal CK1 rise due to the rise of the gate G1 potential, and accordingly, a current starts to flow to the resistor R1B side of the level shift circuit 104. On the other hand, the current flowing into the capacitor C1 of the level shift circuit 104 gradually decreases as the potential of the transfer signal CK1 increases.
When the thyristor S1 is completely turned on and becomes a steady state, a current for maintaining the on state of the thyristor S1 flows to the resistor R1B of the level shift circuit 104, but does not flow to the capacitor C1.
At this time, as shown in FIG. 6B, the tristate buffer B1C of the signal generation circuit 100 is set to high impedance (Hiz) (FIG. 6E).

  (6) With the thyristor S1 fully turned on, the lighting signal ID is set to “L” as shown in FIG. 6H (FIG. 6F). At this time, since the potential of the gate G1> the potential of the gate G2, the LED L1 having a thyristor structure is turned on earlier and lights up. As the LED L1 is turned on, the potential of the signal line Φ1 rises, so that the LEDs after the LED L2 are not turned on. That is, for the LEDs L1, L2, L3, L4,..., Only the LED L1 having the highest gate voltage is turned on (lighted).

(7) Next, as shown in FIG. 6 (F), when the transfer signal CK2R is set to “L” (FIG. 6 (g)), a current flows as in the case of FIG. A voltage is generated across the capacitor C2 104.
(8) As shown in FIG. 6E, when the transfer signal CK2C is set to “L” in this state (FIG. 6H), the thyristor switch S2 is turned on.
(9) Then, as shown in FIGS. 6B and 6C, when the transfer signals CK1C and CK1R are simultaneously set to “H” (FIG. 6I), the thyristor switch S1 is turned off and passes through the resistor R1. As a result, the potential of the gate G1 gradually decreases. At that time, the thyristor switch S2 is completely turned on. Therefore, by turning on / off the lighting signal ΦI corresponding to the image data from the lighting signal terminal ID, the LED L2 can be turned on / off. In this case, since the potential of the gate G1 is already lower than the potential of the gate G2, the LED L1 is not turned on.

  (10) The above-described operation is sequentially performed to sequentially turn on the LEDs L1 to L128. Then, after the “transfer operation period” in FIG. 6 in which the terminal LED L128 is extinguished, the transfer signals CK1C and CK1R are set to “H”, the transfer signal CK1 is set to “H”, and the transfer signals CK2C and CK2R are set to “H”. The transfer signal CK2 is set to “H” as “H”, and both the transfer signal CK1 and the transfer signal CK2 are kept in the “H” state for a predetermined time (“transfer thyristor is turned off” in FIG. 6). As a result, all thyristors S1 to S128 are turned off. Therefore, in this state, no current flows through all the thyristors S1 to S128, so that the thyristors S1 to S128 are held in the off state (not lit).

(11) Furthermore, after keeping both the transfer signals CK1 and CK2 at the “H” state for a predetermined time, the transfer signals CK2C and CK2R are set to “L” and the transfer signal CK2 is set to “L” (in FIG. 6, “Period during which no current flows through the transfer section” Thereby, since no current flows through the diodes D1 to D128, all the diodes D1 to D128 are also kept in the non-lighted state.
As a result, during the unsteady operation including the state where the photosensitive drum 12 (see FIG. 1) stops rotating after the lighting signal ΦI is output and the image formation is completed, the transfer unit of the SLED 63 is operated. No current is applied. Therefore, in a state where the photosensitive drum 12 stops rotating, current does not flow through the thyristors S1 to S128 and the diodes D1 to D128 arranged in the transfer unit as well as the LEDs L1 to L128, and the thyristors S1 to S128. Since no light is emitted from the diodes D1 to D128, unnecessary exposure of the photosensitive drum 12 is suppressed.

Next, the configuration of the signal generation circuit 100 will be described in detail.
FIG. 7 is a block diagram showing the configuration of the signal generation circuit 100. The signal generation circuit 100 includes an image data development unit 110, a density unevenness correction data unit 112, a timing signal generation unit 114, a reference clock generation unit 116, and lighting time control / corresponding to each SLED chip (CHIP1 to CHIP58). The driving unit 118-1 to 118-58 constitutes a main part.
Image data is serially transmitted from the image processing unit (IPS) 40 to the image data development unit 110. The image data development unit 110 divides the transmitted image data into image data for each SLED chip (CHIP1 to CHIP58) and 1st to 128th dot, 129 to 256th dot,..., 7297 to 7424th dot. The image data developing unit 110 is connected to the lighting time control / drive units 118-1 to 118-58, and outputs the divided image data to the corresponding lighting time control / drive units 118-1 to 118-58.

The density unevenness correction data section 112 stores density unevenness correction data Corr for correcting image density unevenness at the time of image formation caused by variations in the amount of light for each LED in the SLED 63. Then, in synchronization with the data read signal from the density unevenness correction data unit 112, the density unevenness correction data is output to the lighting time control / drive units 118-1 to 118-58. The density unevenness correction data Corr is data set for each LED in accordance with variations in the amount of light by each LED, and is formed as 8-bit (0 to 255) data in this embodiment.
The EEPROM 102 stores light amount correction value data for each LED. When the machine power is turned on, the light amount correction value data for each LED is downloaded from the EEPROM 102 to the density unevenness correction data unit 112. Accordingly, the light amount correction value data is stored in the density unevenness correction data unit 112 as the density unevenness correction data Corr.

Here, the light amount correction value data for each LED stored in the EEPROM 102 is obtained as follows. The light amount correction value data for each LED is generated based on the measured light amount data by measuring the light amount of each LED with an optical profile measuring device or the like.
First, the LPH 14 is installed in an optical profile measuring device (not shown), each LED is turned on, and light quantity distribution data of each LED is measured. Then, with respect to the measured light amount distribution data, an integrated value in a direction (sub-scanning direction) orthogonal to the main scanning direction at each coordinate position in the LED arrangement direction (main scanning direction) is calculated, and the light amount in the main scanning direction. Obtain the distribution (light profile).
Subsequently, the light quantity from the valley to the valley in the light profile is integrated, and the integrated value is divided by the distance from the valley to the valley to calculate the light quantity (exposure energy) density in the area between the valleys. . The exposure energy density of each region obtained in this way is used as the correction characteristic value of each LED. Further, the amount of light is increased / decreased in accordance with the difference from the target value so that the correction characteristic value matches the predetermined target value, so that the correction characteristic value in all regions becomes flat. The light amount correction value for each area subjected to such flattening processing is used as the light amount correction value for each LED. The light amount correction values for all the LEDs are stored in the EEPROM 102 as light amount correction value data.

Next, the reference clock generator 116 is connected to the main controller 30, the timing signal generator 114, and the lighting time control / drive units 118-1 to 118-58.
As shown in FIG. 8 (a block diagram illustrating the configuration of the reference clock generation unit 116), the reference clock generation unit 116 includes a crystal oscillator 140, a frequency divider 1 / M142, a frequency divider 1 / N144, and a phase comparator. 146 and a voltage controlled oscillator 148, and a lookup table (LUT) 132.
The LUT 132 stores a table for determining the frequency division ratios M and N based on the light amount adjustment data from the control unit 30. The crystal oscillator 140 is connected to the frequency divider 1 / N144, oscillates at a predetermined frequency, and outputs the oscillated signal to the frequency divider 1 / N144. The frequency divider 1 / N 144 is connected to the LUT 132 and the phase comparator 146, and divides the signal oscillated by the crystal oscillator 140 based on the frequency division ratio N determined by the light amount adjustment data from the LUT 132. The phase comparator 146 is connected to the frequency divider 1 / M142, the frequency divider 1 / N144, and the voltage controlled oscillator 148, and the output signal from the frequency divider 1 / M142 and the frequency divider 1 / N144. Is compared with the output signal. The control voltage supplied to the voltage controlled oscillator 148 is controlled according to the comparison result (phase difference) by the phase comparator 146. The voltage controlled oscillator 148 outputs a reference clock signal at a frequency based on the control voltage. In the present embodiment, a control voltage corresponding to a frequency that divides the possible lighting period into 256 is supplied, and a reference clock signal having this frequency is generated, and all lighting time control / drive units 118-1 to 118-58 are generated. Output to. The voltage controlled oscillator 148 is also connected to the frequency divider 1 / M142, and the reference clock signal output from the voltage controlled oscillator 148 is also branched and input to the frequency divider 1 / M142. The frequency divider 1 / M 142 divides the reference clock signal fed back from the voltage controlled oscillator 148 based on the frequency division ratio M determined by the light amount adjustment data from the LUT 132.

The timing signal generation unit 114 is connected to the control unit 30 and the reference clock generation unit 116, and is synchronized with the horizontal synchronization signal (Lsync) from the control unit 30 based on the reference clock signal from the reference clock generation unit 116. Thus, transfer signals CK1R and CK1C and transfer signals CK2R and CK2C are generated. The transfer signals CK1R and CK1C and the transfer signals CK2R and CK2C are transferred to the LPH 14 as the transfer signal CK1 and the transfer signal CK2 via the level shift circuit 104.
The timing signal generation unit 114 is connected to the density unevenness correction data unit 112 and the image data development unit 110 and is synchronized with the Lsync signal from the control unit 30 based on the reference clock signal from the reference clock generation unit 116. Then, a data read signal for reading image data corresponding to each pixel (each LED) from the image data development unit 110, and data for reading density unevenness correction data corresponding to each pixel from the density unevenness correction data unit 112 A read signal is output to each. Further, the timing signal generation unit 114 is also connected to the lighting time control / drive units 118-1 to 118-58, and based on the reference clock signal from the reference clock generation unit 116, the Lsync signal from the control unit 30. In synchronization, a trigger signal for starting lighting of the SLED 63 is output.

The lighting time control / drive units 118-1 to 118-58 set the lighting time (lighting pulse width) of each pixel (each LED) based on the density unevenness correction data Corr and the linearity correction value data Offset, and the SLED 63 Control signals (lighting signals) ΦI1 to ΦI58 for lighting each LED are generated. Then, lighting signals ΦI1 to ΦI58 are output to the SLED chips (CHIP1 to CHIP58), respectively.
Specifically, as shown in FIG. 9 (block diagram for explaining the configuration of the lighting time control / driving unit 118), the lighting time control / driving units 118-1 to 118-58 are connected to the lighting pulse width setting unit. It includes a presettable digital one-shot multivibrator (PDOMV) 160 that is an example, a linearity correction unit 162 that is an example of a constant addition processing unit, and an AND circuit 170.

The AND circuit 170 is connected to the image data development unit 110 and the timing signal generation unit 114. When the image data from the image data development unit 110 is 1 (ON), the trigger signal (TRG) from the timing signal generation unit 114 is displayed. ) Is output to the PDOMV 160, and when the image data is 0 (OFF), the trigger signal is set not to be output.
PDOMV 160 is connected to AND circuit 170, linearity correction unit 162, density unevenness correction data unit 112, and reference clock generation unit 116. Then, in synchronization with the trigger signal from the AND circuit 170, the reference pulse signal from the reference clock generation unit 116 is corrected (gain correction) according to the density unevenness correction data Corr from the density unevenness correction data unit 112. A signal is generated and output to the linearity correction unit 162.
Specifically, in PDOMV 160, the calculation of the following equation (1) is performed to set the lighting pulse width (gain corrected lighting pulse width) Pg that has been gain-corrected.
Pg = BASE · (1 + Corr / 128) (1)
In the expression (1), “BASE” is a reference pulse width generated by the reference clock signal. In addition, since the density unevenness correction data Corr of the present embodiment is 8-bit data (0 to 255), the light amount correction width (adjustment range) related to density unevenness correction (gain correction) is maximum corrected in the equation (1). In this example, the value / minimum correction value = 3 is set.

Next, the linearity correction unit 162 corrects and outputs the lighting pulse signal from the PDOMV 160 in order to correct the variation in the light emission start time of the LED generated for each SLED chip (CHIP1 to CHIP58). Specifically, the linearity correction unit 162 includes a plurality of delay circuits 164 (eight in this embodiment, 164-0 to 164-7), a delay selection register 166, a delay signal selection unit 165, and an AND circuit 167. , An OR circuit 168, and a lighting signal selection unit 169.
The delay circuits 164-0 to 164-7 are connected to the PDOMV 160, and different times for delaying the lighting pulse signal from the PDOMV 160 are set. The delay selection register 166 is connected to the delay signal selection unit 165 and the lighting signal selection unit 169. The delay selection register 166 stores linearity correction value data Offset corresponding to the delay time set for each of the lighting time control / drive units 118-1 to 118-58 and lighting signal selection data. Here, the linearity correction value data Offset is a predetermined constant set for each of the lighting time control / drive units 118-1 to 118-58. The linearity correction value data Offset and the lighting signal selection data for each of the lighting time control / drive units 118-1 to 118-58 are measured in advance and stored in the EEPROM 102 as an example of a memory. The linearity correction value data Offset and lighting signal selection data stored in the EEPROM 102 are downloaded to the delay selection register 166 when the machine power is turned on. Note that a flash ROM can also be used as the memory. In that case, the flash ROM itself can function as the delay selection register 166.

The delay signal selection unit 165 is connected to the AND circuit 167 and the OR circuit 168, and is based on linearity correction value data (hereinafter also referred to as “offset amount”) Offset stored in the delay selection register 166. One of the outputs from the circuits 164-0 to 164-7 is selected. The AND circuit 167 is a logical product of the lighting pulse signal from the PDOMV 160 and the delayed lighting pulse signal selected by the delay signal selection unit 165, that is, both the lighting pulse signal before the delay and the lighting pulse signal after the delay are in the lighting state. If so, a lighting pulse is output. The OR circuit 168 is a logical sum of the lighting pulse signal from the PDOMV 160 and the delayed lighting pulse signal selected by the delay signal selection unit 165, that is, at least one of the lighting pulse signal before the delay and the lighting pulse signal after the delay is in the lighting state. If so, a lighting pulse is output.
The lighting signal selection unit 169 selects one of the outputs from the AND circuit 167 or the OR circuit 168 based on the lighting selection data stored in the delay selection register 166. Then, the selected lighting pulse is output as a lighting signal ΦI to the SLED chips (CHIP1 to CHIP58) in the LPH 14 via the MOSFET 172.
That is, the delay signal selection unit 165 performs the calculation of the following equation (2) for adding a predetermined offset amount Offset to the gain-corrected lighting pulse width Pg (offset correction). Then, the lighting pulse width Pout in the final lighting signal ΦI is set.
Pout = BASE. (1 + Corr / 128) + Offset (2)

  Further, as shown in FIG. 7, a three-terminal regulator 101 is connected to the LPH 14, and a stable + 3.3V voltage is supplied from the three-terminal regulator 101 to the LPH 14.

Next, linearity correction value data (offset amount) Offset stored in the EEPROM 102 will be described.
First, FIG. 10 shows three different lighting time control / drive units 118-K, 118-L, and 118-M (K, L, and K) when the linearity correction unit 162 is not functioning, that is, when offset correction is not performed. The relationship between the lighting pulse width Pout of the lighting pulse signal output from M is an integer of 1 to 58 and the exposure energy (= light intensity × lighting pulse width) output by the LED that has received the lighting pulse signal (this relationship) Is referred to as “light quantity characteristic”). In FIG. 10, since the linearity correction unit 162 is not functioned, the lighting pulse width Pout is the gain-corrected lighting pulse width Pg.
In FIG. 10, the lighting pulse width Pout (horizontal axis) is converted into a normalized pulse width for each light quantity characteristic curve. That is, when the maximum value of the exposure energy range of the LPH 14 is set to 3 mJ / m ² and the exposure energy is set to ³ mJ / m ² , the lighting time control / drive units 118-K, 118-L, 118 Normalization is performed with the lighting pulse width Pout output from −M as a reference (Pout = 1). This corresponds to the fact that the density unevenness correction amount Corr used at the time of gain correction has a different value for each LED, and the lighting pulse width Pout and exposure energy in each lighting time control / drive unit 118-1 to 118-58. This was done to compare the relationship with A broken line is a target light quantity characteristic (target light quantity characteristic) required when the LED is actually driven by the lighting pulse signal from the lighting time control / drive units 118-1 to 118-58.

As shown in FIG. 10, the light quantity characteristics of the LED when the linearity correction unit 162 is not functioning draw different curves for the lighting time control / drive units 118 -K, 118 -L, and 118 -M. Become. Such a light quantity characteristic difference is caused by a characteristic inherent to each of the lighting time control / drive units 118-1 to 118-58. That is, first, when light is actually emitted from the LED, the influence of the inductance, capacity, etc. of the signal line connecting the lighting time control / drive units 118-1 to 118-58 and each LED In response, the output waveform of the lighting signal ΦI does not become a complete rectangle. This is the main factor, and usually a delay time of about 3 to 15 nsec occurs in the light emission start time. Second, in the lighting time control / drive units 118-1 to 118-58, the linearity of the output characteristics is different. Further, power supply voltage fluctuation (crosstalk) also occurs according to the number of LEDs that emit light. For this reason, there are variations peculiar to the linearity of the light quantity characteristics.
As described above, the different delay times of the lighting time control / drive units 118-1 to 118-58 and the variation in linearity cause a difference in light amount characteristics.

  Therefore, in the LPH 14 of the present embodiment, the lighting time control / drive unit 118-is set so that the light amount characteristics of the LEDs substantially coincide with the target light amount characteristics in the entire exposure energy region (use exposure energy range) to be used. Corresponding to each of 1 to 118-58, the offset amount Offset used for the calculation according to the above equation (2) is set. As the offset amount Offset, the used exposure energy range is divided into a plurality of regions, and different offset amounts Offset are set for the divided regions.

Here, the setting of the offset amount Offset for each divided area in the used exposure energy range will be described. First, in the LPH 14 of the present embodiment, it is assumed that the adjustment range of the used exposure energy range is set to about three times (the maximum value of used exposure energy / the minimum value of used exposure energy≈3). Specifically, the use exposure energy range in the LPH 14 of the present embodiment is assumed to be in the range of 0.9 to 3 (mJ / m ² ) as an example.
Then, in FIG. 10, the light quantity characteristics of each LED by the lighting time control / drive units 118-K, 118-L, and 118-M in the entire first area and second area when the used exposure energy range is divided into two. It is necessary to set the offset amount Offset so that the curve substantially matches the target light quantity characteristic (broken line). However, as shown in FIG. 10, the light amount characteristic curves of the lighting time control / drive units 118-K, 118-L, and 118-M have high linearity in the first region, but have a lower pulse than the first region. In the second region on the width region side, the linearity is low. Therefore, it is difficult to make the light quantity characteristic of the LED substantially match the target light quantity characteristic by setting one offset amount Offset in the entire first area and second area. That is, when an offset correction is performed using one offset amount Offset, the fluctuation range of the light amount characteristic curve by the lighting time control / drive units 118-K, 118-L, and 118-M with respect to the target light amount characteristic is as follows. It is inevitable that the allowable value (for example, 0.5% of the target light amount characteristic) is exceeded in any of the second regions.

  Therefore, in the LPH 14 of the present embodiment, first, in the first region with high linearity, the fluctuation range of the light amount characteristic curve by the lighting time control / drive units 118-K, 118-L, and 118-M with respect to the target light amount characteristic is large. A first offset amount Offset1 that falls within the allowable range is set. FIG. 11 is a diagram illustrating an example of the light quantity characteristic of each LED by the lighting time control / drive units 118-K, 118-L, and 118-M when the offset correction is performed using the first offset amount Offset1. . As shown in FIG. 11, by performing offset correction using the first offset amount Offset1, it is possible to set the fluctuation range of the light amount characteristic curve within the allowable range in the first region. The first area is a first use exposure energy range corresponding to the light amount adjustment data from the control unit 30.

Specifically, the first offset amount Offset1 is set as follows. First, if offset correction is performed by the linearity correction unit 162, the lighting time control / drive units 118-K, 118-L, and 118-M (actually, the lighting time control / drive units 118-1 to 118-58) The first region is determined such that the fluctuation range of the light amount characteristic curve of each LED with respect to the target light amount characteristic falls within an allowable range (for example, 0.5% of the target light amount characteristic). That is, as a first stage, a first area is selected which is an area in which the linearity correction unit 162 can set the fluctuation range of the light quantity characteristic curve with respect to the target light quantity characteristic within an allowable range. In the example shown in FIG. 11, such a first region has an exposure energy in the range of 1.5 to 3 (mJ / m ² ) and a corresponding normalized pulse width of m to 1 (0 < This shows a case where m <1).
Then, as the second stage, at the point (lower limit point) Q where the exposure energy is 1.5 (mJ / m ² ) and the normalized pulse width is m, which is the lower limit on the low pulse region side of the first region. The lighting time control / drive units 118-K, 118-L, and the lighting time control / drive units 118-K, 118-L, and 118-M match the target light quantity characteristic (broken line). A first offset amount Offset1 at 118-M is set.

Next, in the second region with low linearity, the fluctuation range with respect to the target light amount characteristic of the light amount characteristic curve of each LED by the lighting time control / drive units 118-K, 118-L, and 118-M is within an allowable range. 2 Set the offset amount Offset2. When setting the second offset amount Offset2, as shown in FIG. 12, the lighting time control / drive units 118-K, 118-L, and 118-M when the linearity correction unit 162 is not functioned are used. For the light quantity characteristic curve, the lighting pulse width Pout (= Pg) is normalized at the lower limit point Q of the first region used when setting the first offset amount Offset1. That is, in the example shown in FIG. 12, the light is output from the respective lighting time control / drive units 118-K, 118-L, and 118-M when the exposure energy is set to be 1.5 mJ / m ^2. Normalization is performed with the lighting pulse width Pout as a reference (Pout = 1).

Then, in the normalized region where the exposure energy is 1.5 mJ / m ² or less, the second offset amount Offset2 is set similarly to the case where the first offset amount Offset1 is set. FIG. 13 is a diagram illustrating an example of the light amount characteristic of each LED by the lighting time control / drive units 118-K, 118-L, and 118-M when the offset correction is performed using the second offset amount Offset2. . As shown in FIG. 13, by performing offset correction using the second offset amount Offset2, it is possible to set the fluctuation range of the light amount characteristic curve within the allowable range in the second region. This second region is a second used exposure energy range corresponding to the light amount adjustment data from the control unit 30.
Specifically, the second offset amount Offset2 is set as follows. First, in the normalized region where the exposure energy is 1.5 mJ / m ² or less, if the offset correction is performed by the linearity correction unit 162, the lighting time control / drive units 118-K, 118-L, and 118-M. (Actually, the variation range of the light amount characteristic curve of each LED by the lighting time control / drive units 118-1 to 118-58 with respect to the target light amount characteristic is within an allowable range (for example, 0.5% of the target light amount characteristic). The second region is determined as follows. In other words, as the first stage, the second region, which is a region in which the variation range of the light amount characteristic curve with respect to the target light amount characteristic can be set within the allowable range by the linearity correction unit 162, is selected. In the example shown in FIG. 13, such a second region has an exposure energy in the range of 0.9 to 1.5 (mJ / m ² ) and a corresponding normalized pulse width of n to 1 ( This shows a case where 0 <n <1).
Then, as a second stage, at the point (lower limit point) R where the exposure energy is 0.9 (mJ / m ² ) and the normalized pulse width is n, which is the lower limit of the second region on the low pulse region side. The lighting time control / drive units 118-K, 118-L, and the lighting time control / drive units 118-K, 118-L, and 118-M match the target light quantity characteristic (broken line). A second offset amount Offset2 at 118-M is set.

Thus, when the first offset amount Offset1 and the second offset amount Offset2 are set for each exposure energy region (first used exposure energy range and second used exposure energy range) within the used exposure energy range, In the lighting time control / drive units 118-1 to 118-58, the following equations (3) and (4) are calculated to set the lighting pulse width Pout of the lighting signal ΦI. That is, when the first use exposure energy range (first region) within the use exposure energy range is designated based on the light amount adjustment data from the control unit 30,
Pout = BASE · (1 + Corr / 128) + Offset1 (3)
Further, when the second use exposure energy range (second region) within the use exposure energy range is designated based on the light amount adjustment data from the control unit 30,
Pout = BASE · (1 + Corr / 128) + Offset2 (4)

As described above, in the LPH 14 of the present embodiment, it is assumed that the exposure energy is set to be adjustable in the range of 0.9 to 3 (mJ / m ² ). Therefore, when the exposure energy is in the range of 0.9 to 3 (mJ / m ² ), the light amount characteristic curve by the lighting time control / drive units 118-K, 118-L, and 118-M matches the target light amount characteristic (broken line). Thus, the offset amount Offset in each lighting time control / drive unit 118-K, 118-L, 118-M is set.
Accordingly, when the second region where the second offset amount Offset2 is set is a region where the exposure energy is in a range larger than 0.9 (mJ / m ² ), that is, the lighting pulse at the lower limit point Q of the first region. Normalizing the width Pout, the target light amount of the light amount characteristic curve by the lighting time control / drive units 118-K, 118-L, 118-M (actually, the lighting time control / drive units 118-1 to 118-58). When the second region in which the fluctuation range with respect to the characteristic is within the allowable range is obtained, and the exposure energy at the lower limit R of the second region is 0.9 (mJ / m ² ) or more, It is necessary to further set the third offset amount Offset3 in a region between the lower limit R of the second region and the exposure energy of 0.9 (mJ / m ² ).

  Also in this case, similarly to the method in which the first offset amount Offset1 and the second offset amount Offset2 are set, the lighting time control / drive units 118-K, 118-L, 118- when the linearity correction unit 162 does not function. For the light quantity characteristic curve by M, the lighting pulse width Pout (= Pg) is normalized at the lower limit point R of the second region used when setting the second offset amount Offset2 for each light quantity characteristic curve. If the offset correction by the linearity correction unit 162 is performed in the normalized region on the lower pulse side than the second region, the lighting time control / drive units 118-K, 118-L, 118-M (actually Determines the third region in which the fluctuation range of the light amount characteristic curve with respect to the target light amount characteristic by the lighting time control / drive units 118-1 to 118-58) is within the allowable range. Further, at the lower limit of the third region, each lighting time control / drive is performed so that the light quantity characteristic curve by the lighting time control / drive units 118-K, 118-L, and 118-M matches the target light quantity characteristic (broken line). The third offset amount Offset3 in the units 118-K, 118-L, and 118-M is set.

In this way, in the normalized area, the fluctuation range of the light quantity characteristic curve by the lighting time control / drive units 118-K, 118-L, and 118-M with respect to the target light quantity characteristic is within the allowable range. The offset amount Offset is set until the lower limit becomes 0.9 (mJ / m ² ) or less, which is the minimum value of the used exposure energy range, as the exposure energy. Accordingly, it is naturally assumed that a plurality of offset amounts Offset of 3 or more are set corresponding to the used exposure energy range.
Note that the example shown in FIG. 13 shows a case where the second region includes the minimum value 0.9 (mJ / m ² ) of the used exposure energy range. In this case, the first region in the first region is shown. By setting the offset amount Offset1 and the second offset amount Offset2 in the second region, the lighting time control / drive units 118-K and 118-L in the entire region of the exposure energy range 0.9 to 3 (mJ / m ² ). , 118-M, the fluctuation range of the light amount characteristic curve with respect to the target light amount characteristic can be suppressed within an allowable range.

The first offset amount Offset1 and the second offset amount Offset2 set for each exposure energy region (first use exposure energy range and second use exposure energy range) within the use exposure energy range are set to each lighting time control / drive unit 118. Each of −1 to 118-58 is stored in advance in the EEPROM 102 in association with the lighting pulse width. Here, when the first offset amount Offset1 and the second offset amount Offset2 are set, the lighting pulse width is set to the first use exposure energy range and the lighting time control / drive unit 118-1 to 118-58, respectively. Normalization processing is performed in the second use exposure energy range. Therefore, by converting the normalized pulse width to the lighting pulse width, each offset amount Offset is associated with the lighting pulse width and stored in the EEPROM 102.
Therefore, when the reference clock signal is generated in the reference clock generation unit 116 based on the light amount adjustment data (instruction value) from the control unit 30 and output to the lighting time control / drive units 118-1 to 118-58. The offset amount Offset corresponding to the lighting pulse width generated based on the reference clock signal is downloaded from the EEPROM 102 to each lighting time control / drive unit 118-1 to 118-58.

  Thus, in the LPH 14 of the present embodiment, the first offset amount Offset1 and the second offset amount Offset2 are set for each exposure energy region (first use exposure energy range and second use exposure energy range). In almost all areas within the exposure energy range, the light amount characteristic curve of each LED by each lighting time control / drive unit 118-1 to 118-58 is set so that the fluctuation range with respect to the target light amount characteristic is within the allowable range. Is done. Therefore, the occurrence of image density unevenness can be suppressed extremely small. At the same time, as described below, the total light amount can be adjusted with high accuracy.

That is, in the image forming units 11Y, 11M, 11C, and 11K (see FIG. 1), a test patch (density sample) is formed on the photosensitive drum 12 at a predetermined timing, and the density detection circuit 17 determines the toner image density. To detect. The toner image density data detected by the density detection circuit 17 is output to the control unit 30, and the control unit 30 calculates the light quantity at the LPH 14 from the input toner image density data to generate light quantity adjustment data. The generated light amount adjustment data is output to the reference clock generation unit 116.
The light amount adjustment data is caused by, for example, a change in sensitivity of the photosensitive drum 12, a change in latent image potential (dark portion potential _VH or bright portion potential _VL ), a change in developer amount in the developing device 15, and the like. Since the toner image density fluctuates, this data is used to adjust the total light amount in the LPH 14 (the total light amount of the LEDs in the LPH 14) in order to keep this constant. Therefore, the light amount adjustment data is output as an instruction value for uniformly adjusting the light amount of each LED in the SLED 63 of the LPH 14. Then, the reference clock generator 116 generates a reference clock signal corresponding to the light amount adjustment data.

  In the LPH 14 of the present embodiment, the first offset amount Offset1 and the second offset amount Offset2 are set for each exposure energy region (first use exposure energy range and second use exposure energy range). Thereby, in each lighting time control / drive unit 118-1 to 118-58 that has received the reference clock signal from the reference clock generation unit 116, the light quantity characteristic curve of each LED in almost all regions within the used exposure energy range. The fluctuation range with respect to the target light quantity characteristic is set to be within an allowable range. Therefore, as shown in FIG. 14 (a diagram showing the relationship between the instruction value and the exposure energy of the LPH 14), a linear relationship between the light amount adjustment data (instruction value) and the exposure energy of the LPH 14 can be formed. As a result, since it is possible to set the total light amount of the LPH 14 corresponding to the light amount adjustment data with high accuracy in almost all the used exposure energy ranges, there is a clear density difference between the images before and after the adjustment of the total light amount. It is possible to suppress the occurrence and maintain the uniformity of the image density.

The LPH 14 according to the present embodiment can switch between a mode in which the digital color printer 1 forms a normal image such as 1200 dpi and a mode that forms a high-definition image such as 4800 dpi required in the graphic arts market such as a photographic image. Even when it is set, offset correction corresponding to the resolution can be performed by setting the offset amount Offset for each resolution and storing it in the EEPROM 102.
For example, in the normal image (1200 dpi) mode, an area having a large lighting pulse width is used, and thus the first area (first used exposure energy range) is associated. Further, in the high-definition image (4800 dpi) mode, an area with a small lighting pulse width is used, so the above-described second area (second use exposure energy range) is made to correspond. When the LPH 14 receives a resolution switching signal from the control unit 30 that also functions as a resolution switching control unit, the first offset amount Offset 1 or the second offset corresponding to the resolution is read from the EEPROM 102 by the control signal from the control unit 30. The offset amount Offset2 can be read and set to be written in the delay selection register 166 of each lighting time control / drive unit 118-1 to 118-58. As a result, even in the exposure energy range used when the resolution is switched, the fluctuation range for the target light amount characteristic is allowed in the light amount characteristic curve of each LED by the lighting time control / drive units 118-1 to 118-58. It becomes possible to keep within the range. Therefore, even when a high-definition image is formed, it is possible to suppress a clear density difference between the images before and after the adjustment of the total light amount.

  As described above, in the LPH 14 of the present embodiment, the used exposure energy range is divided into a plurality of exposure energy regions. At that time, each exposure energy region to be divided has a fluctuation range with respect to the target light amount characteristic of the light amount characteristic curve by each lighting time control / drive unit 118-1 to 118-58 due to the offset correction in the linearity correction unit 162. This is an area that can be set to be within the allowable range. Then, a first offset amount Offset1 and a second offset amount Offset2 are set for each of the divided exposure energy regions (first use exposure energy range and second use exposure energy range). Accordingly, by switching between the first offset amount Offset1 and the second offset amount Offset2 according to the exposure energy region to be used, each lighting time control / drive is performed in almost all regions within the used exposure energy range. It is possible to set the fluctuation range of the light quantity characteristic curve by the units 118-1 to 118-58 with respect to the target light quantity characteristic to be within an allowable range (for example, 0.5% of the target light quantity characteristic).

Therefore, the occurrence of image density unevenness can be suppressed extremely small. In addition, since it is possible to adjust the total light amount with high accuracy, it is possible to maintain uniformity in image density in response to changes in the sensitivity of the photoconductor over time, changes in environmental conditions, and the like.
In particular, even in an image forming apparatus capable of switching between a mode for forming a normal image such as 1200 dpi and a mode for forming a high-definition image such as 4800 dpi required in the graphic arts market such as a photographic image, an offset amount for each resolution. By switching Offset, it is possible to set the total light amount of the LPH 14 corresponding to the light amount adjustment data with high accuracy in almost all use exposure energy ranges. Therefore, even when a high-definition image is formed, it is possible to suppress the occurrence of a clear density difference between the images before and after the adjustment of the total light amount, and provide a uniform image.

1 is a diagram illustrating an overall configuration of an image forming apparatus using an LED print head according to an embodiment. It is the figure which showed the structure of the LED print head (LPH). It is a top view of a LED circuit board. It is the figure which showed the wiring diagram currently formed on the LED circuit board. It is a figure explaining the circuit structure of SLED. 3 is a timing chart showing drive signals output from a signal generation circuit and a level shift circuit. It is a block diagram which shows the structure of a signal generation circuit. It is a block diagram explaining the structure of a reference clock generation part. It is a block diagram explaining the structure of lighting time control and a drive part. Relationship between the normalized pulse width of the lighting pulse signal output from the lighting time control / drive units 118-K, 118-L, and 118-M and the exposure energy output from the LED that has received the lighting pulse signal (light quantity characteristics) ) Is a diagram showing an example. It is the figure which showed an example of the light quantity characteristic of each LED by a different lighting time control and drive part at the time of implementing offset correction using the 1st offset amount. It is the figure which normalized and expressed the lighting pulse width in the lower limit point Q of the 1st field for every light quantity characteristic curve about the light quantity characteristic curve of each LED by a different lighting time control and drive part. It is the figure which showed an example of the light quantity characteristic of each LED by a different lighting time control and drive part at the time of implementing offset correction using 2nd offset amount. It is the figure which showed the relationship between the instruction | indication value and the exposure energy of LPH.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Digital color printer, 10 ... Image formation process part, 11Y, 11M, 11C, 11K ... Image formation unit, 12 ... Photosensitive drum, 14 ... LED print head (LPH), 30 ... Control part, 40 ... Image processing part (IPS), 62 ... LED circuit board, 63 ... self-scanning LED array (SLED), 64 ... rod lens array, 100 ... drive circuit (signal generation circuit), 101 ... three-terminal regulator, 102 ... EEPROM, 104 ... level Shift circuit 110 ... Image data development unit 112 ... Density unevenness correction data unit 114 ... Timing signal generation unit 116 ... Reference clock generation unit 118-1 to 118-58 ... Lighting time control / drive unit 160 ... Preset Double digital one-shot multivibrator (PDOMV), 162... Linearity correction unit, 164- ～164-7 ... delay circuit, 165 ... delay signal selecting section, 166 ... delay selected register, 167 ... the AND circuit, 168 ... OR circuit, 169 ... lighting signal selection unit

Claims (4)

A print head for exposing an image carrier in an image forming apparatus,
A plurality of light emitting elements arranged in a line;
A lighting pulse width setting unit for setting a lighting pulse width for driving the light emitting element for each of the plurality of light emitting elements ;
Each provided corresponding to each of the plurality of light emitting elements, and based on the lighting pulse width, comprising a plurality of drive units for supplying a lighting signal to each of the light emitting elements,
In the plurality of driving units, a plurality of correction values for correcting the lighting pulse width are set for each of the plurality of driving units according to the exposure energy of the light emitting element, and a plurality of correction values are used among the set correction values. A print head, characterized in that the lighting signal is supplied to the light emitting element with the lighting pulse width corrected by the correction value corresponding to the exposure energy . 2. The print head according to claim 1, wherein the plurality of correction values set for each of the plurality of driving units are set corresponding to each of a plurality of print resolutions . A photoreceptor,
A print head for exposing the photoreceptor;
A control unit that outputs an instruction signal instructing the total light amount of the print head,
The print head is
A plurality of light emitting elements arranged in a line;
A lighting pulse width setting unit for setting a lighting pulse width for driving the light emitting element for each of the plurality of light emitting elements ;
Each provided corresponding to each of the plurality of light emitting elements, and based on the lighting pulse width, comprising a plurality of drive units for supplying a lighting signal to each of the light emitting elements,
In the plurality of driving units, a plurality of correction values for correcting the lighting pulse width are set for each of the plurality of driving units according to the exposure energy of the light emitting element, and a plurality of correction values are used among the set correction values. An image forming apparatus , wherein the lighting signal is supplied to the light emitting element with the lighting pulse width corrected by the correction value corresponding to the exposure energy . A resolution switching control unit for outputting a resolution switching signal for switching the printing resolution by the print head;
4. The image forming apparatus according to claim 3 , wherein a plurality of the correction values set for each of the plurality of driving units are switched based on the resolution switching signal from the resolution switching control unit .

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