JP2020046473A - Image formation apparatus - Google Patents

Abstract

To prevent the output of a light reception part for receiving irregular reflection light from being saturated while maintaining a light reception amount of regular reflection light at a target light amount.SOLUTION: Control means acquires the correlation between a drive current and an output level of a first detection signal while changing the drive current of drive means when starting the adjustment of the light emission amount of irradiation means. The control means decides the drive current so that the light amount of the regular reflection light becomes the target light amount on the basis of the correlation. When the drive current that is decided so that the light amount of the regular reflection light becomes the target light amount is supplied to the irradiation means, the output level of a second detection signal may be saturated with respect to the density of the toner image. In this case, the control means reduces the drive current while increasing the gain of amplification means until the output level of the second detection signal is not saturated.SELECTED DRAWING: Figure 7

Description

    The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an image forming apparatus having a color misregistration correction function.

  The image forming apparatus adjusts various control parameters by detecting a test image with a density sensor. Therefore, the detection result of the density sensor must be highly accurate. According to Patent Literature 1, it is possible to provide a light receiving unit that receives regular reflection light and a light receiving unit that receives irregular reflection light, and generate a sensor output by correcting the light reception amount of regular reflection light with the light reception amount of irregular reflection light. Are listed. Thereby, it is said that the influence of the fluctuation of the reflectance of the intermediate transfer member on which the test image is formed is reduced.

JP-A-10-221902

    In a density sensor having both a light receiving unit for receiving specularly reflected light and a light receiving unit for receiving irregularly reflected light, the light emission amount of the light emitting unit is adjusted so that the light reception amount of the regular reflected light becomes the target light amount. Generally, as the number of images formed by the intermediate transfer member increases, the reflectance of the intermediate transfer member decreases. In this case, if the light emission amount of the light emitting unit is increased so that the light reception amount of the regular reflection light becomes the target light amount, the output of the light reception unit that receives the irregular reflection light may be saturated. When such saturation occurs, the accuracy of the detection result of the density sensor decreases. On the other hand, if the drive current of the light-emitting unit is reduced so that the output of the light-receiving unit that receives the irregularly-reflected light does not saturate, the received light amount of the specularly-reflected light becomes smaller than the target light amount. SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus that maintains the light reception amount of specularly reflected light at a target light amount and hardly saturates the output of a light receiving unit that receives irregularly reflected light.

The present invention provides, for example,
An image carrier;
Image forming means for forming a toner image on the image carrier,
Detecting means for detecting the density of the toner image carried on the image carrier;
Control means for controlling the detection means,
The detection means,
Irradiation means for irradiating light toward the toner image carried on the image carrier,
Driving means for supplying a driving current to the irradiation means;
A first light receiving unit that receives regular reflection light from the toner image and outputs a first detection signal according to the amount of the regular reflection light,
Amplifying means for amplifying the first detection signal,
A second light receiving unit that receives irregularly reflected light from the toner image and outputs a second detection signal according to the amount of the irregularly reflected light,
When the control unit starts adjusting the light emission amount of the irradiation unit, the control unit obtains a correlation between the drive current and the output level of the first detection signal while changing the drive current of the drive unit, and obtains the correlation. It is configured to determine the drive current such that the light amount of the regular reflection light becomes a target light amount based on
Further, the control unit supplies the drive current determined so that the light amount of the regular reflection light becomes the target light amount to the irradiation unit, and outputs the second detection signal with respect to the density of the toner image. When the level becomes saturated, the drive current is decreased while increasing the gain of the amplifying means until the output level of the second detection signal is not saturated.

    According to the present invention, there is provided an image forming apparatus that maintains the light reception amount of specularly reflected light at a target light amount and hardly saturates the output of a light receiving unit that receives irregularly reflected light.

Sectional view showing an image forming apparatus Diagram for explaining the density sensor Diagram for explaining the density sensor Diagram explaining controller Diagram explaining light quantity adjustment Diagram explaining the saturation of diffuse reflection output Flow chart showing light intensity adjustment and density deviation correction FIG. 4 illustrates functions of a CPU.

<Image forming apparatus>
FIG. 1 is a schematic sectional view of the image forming apparatus 10 viewed from the front side of the image forming apparatus 10. According to FIG. 1, the image forming apparatus 10 includes a reading unit 16 for reading a document and an image forming engine 15. The image forming engine 15 includes four image forming stations (image forming units) that form a multicolor image by superimposing four color toners such as yellow (Y), magenta (M), cyan (C), and black (K). Have. The character ymck added to the end of the reference numeral indicates the color of the toner, but the character ymck is omitted when items common to the four colors are described. The photoconductor 1 is a drum-shaped image carrier that carries an electrostatic latent image and a toner image. The charging roller 2 of the charging device applies a charging voltage to the photoconductor 1 to uniformly charge the surface of the photoconductor 1. The charging voltage is generated by superimposing an AC voltage on a DC voltage. The exposure device 3 is a scanning optical device having a laser light source and a rotary polygon mirror. The exposure device 3 modulates and outputs the laser light according to the image data, and deflects the laser light with a rotary polygon mirror. As a result, the laser beam scans the surface of the photoconductor 1, and an electrostatic latent image corresponding to the image data is formed. As described above, the exposure device 3 functions as an exposure unit that exposes the surface of the photoconductor uniformly charged to form an electrostatic latent image. The developing device 4 contains a toner, and forms a toner image by attaching the toner to the electrostatic latent image via a developing sleeve. The primary transfer roller 6 sandwiches the intermediate transfer belt 5 in cooperation with the photoconductor 1, and transfers the toner image carried on the photoconductor 1 to the intermediate transfer belt 5. A multicolor image is formed by sequentially transferring the toner images of the four colors to the intermediate transfer belt 5. The intermediate transfer belt 5 conveys the toner image to a secondary transfer unit. In the secondary transfer section, the secondary transfer roller 7 transports the intermediate transfer belt 5 and the sheet P fed from the cassette 9 while nipping the same. Thus, the multicolor toner image carried on the intermediate transfer belt 5 is transferred to the sheet P. The fixing device 8 fixes the toner image on the sheet P by applying heat and pressure to the sheet P and the toner image.

  The density sensor 20 is an optical sensor that detects a toner pattern formed on the intermediate transfer belt 5. The density sensor 20 is provided between the image forming station K for forming a black toner image and the secondary transfer roller 7.

<Details of concentration sensor>
FIG. 2 is a cross-sectional view of the density sensor when viewing the downstream side from the upstream side in the transport direction of the intermediate transfer belt 5. The LEDs 21 are arranged so that light enters the surface of the intermediate transfer belt 5 at a predetermined incident angle (eg, 15 °). The LED 21 may be, for example, a light emitting element that emits infrared light. LED is an abbreviation for light emitting diode. The PD 22a is a light receiving element arranged to receive light (specular reflection light) output from the LED 21 and reflected on the surface of the intermediate transfer belt 5 or the toner image 29 formed on the surface. PD is an abbreviation for photodiode. The angle between the optical axis of the PD 22a and the surface of the intermediate transfer belt 5 is basically the same as the angle between the optical axis of the LED 21 and the surface of the intermediate transfer belt 5. The PD 22b is a light receiving element arranged to receive light (irregular reflection light) output from the LED 21 and reflected on the toner image 29 formed on the surface of the intermediate transfer belt 5. The shutter 23 is disposed between the density sensor 20 and the intermediate transfer belt 5 to reduce contamination of the lens 24. When detecting the intermediate transfer belt 5 and the toner image, the shutter 23 moves to the position indicated by the solid line. When the intermediate transfer belt 5 and the toner image are not detected, the shutter 23 moves to the position shown by the broken line.

    As shown in FIG. 3, a drive circuit 30 for supplying a drive current to the LED 21 and the like are provided on the electric board 27. The IV conversion circuit 31a is a circuit that converts the photocurrent output from the PD 22a into a voltage. The amplification circuit 32a is a circuit that amplifies the voltage output from the IV conversion circuit 31a. The amplifier circuit 32a is configured to amplify the output voltage by a factor of 1, 1.2, 2.2, or 3.5. In order to switch such an amplification factor, the amplification circuit 32a may have a plurality of resistors. The amplification factor changes depending on which resistor is selected by the CPU. The IV conversion circuit 31b is a circuit that converts the photocurrent output from the PD 22b into a voltage. The amplification circuit 32b is a circuit that amplifies the voltage output from the IV conversion circuit 31b. The amplifier circuit 32b has the same circuit configuration as the amplifier circuit 32a. However, the amplification circuit 32b may be an attenuation circuit that attenuates the voltage output from the IV conversion circuit 31b.

    As shown in FIG. 2, the lens 24 is an optical component that forms a path of light emitted from the LED 21 and a path of light incident on the PDs 22a and 22b. The lens 24 is formed of, for example, an epoxy resin. The shielding member 25a is a member that reduces the light output from the LED 21 from directly entering the PD 22a. The shielding member 25b is a member that reduces the light output from the LED 21 from directly entering the PD 22b. The shielding members 25a and 25b are, for example, black resin members.

    The density sensor 20 can measure both specularly reflected light and irregularly reflected light. The PD 22a receives specularly reflected light. When a toner image is formed on the intermediate transfer belt 5, the amount of light received by the PD 22a decreases according to the optical density of the toner image (the amount of applied toner). The PD 22b receives the irregularly reflected light. The diffusely reflected light scattered from the intermediate transfer belt 5 and the black toner image formed on the intermediate transfer belt 5 and incident on the PD 22b is extremely small. On the other hand, diffusely reflected light scattered from the yellow, magenta, and cyan toner images and incident on the PD 22b is relatively large. Since the scattered light increases as the area density of the color toners such as Y, M, and C increases, the amount of light received by the PD 22b increases. The image forming apparatus 10 measures the color toner density using the specular reflection light and the irregular reflection light, adjusts the intensity of the laser light of the exposure device 3 based on the measurement result, and executes the toner density correction.

<Controller>
FIG. 4 is a block diagram of a controller that controls the image forming apparatus 10. The controller has a CPU 41 and a memory 42. The control program is stored in the ROM of the memory 42. The RAM of the memory 42 stores image data and control parameters. The motor 33 is a motor for opening and closing the shutter 23. The CPU 41 executes light amount adjustment and toner image density correction (gradation correction). The CPU 41 controls the image forming stations Y, M, C, and K, and forms a toner pattern for light amount adjustment on the intermediate transfer belt 5 and a toner pattern for density correction on the intermediate transfer belt 5.

<Output fluctuation of density sensor>
As the operation time of the image forming apparatus 10 increases, paper dust and the like adhering to the intermediate transfer belt 5 increase. Therefore, the reflectance of the surface of the intermediate transfer belt 5 gradually decreases. That is, the amount of regular reflection light received from the surface of the intermediate transfer belt 5 detected by the density sensor 20 gradually decreases. As a result, the dynamic range of the density sensor 20 decreases. Therefore, the image forming apparatus 10 executes light amount adjustment. The light amount adjustment is a process of adjusting the driving current flowing through the LED 21 so that the amount of received regular reflection light becomes a predetermined value (target light amount).

    FIG. 5A shows the output of the density sensor 20 in the light amount adjustment. The vertical axis indicates the output of the density sensor 20. Here, the output of the density sensor 20 refers to the amount of specularly reflected light received, and more specifically, the output voltage of the amplifier circuit 32a. The horizontal axis indicates time. FIG. 5B shows the target value of the output of the density sensor 20 with respect to the set value (drive current) of the light emission amount. The vertical axis indicates the output of the density sensor 20. The horizontal axis indicates the light emission amount (drive current). As shown in FIG. 5B, the CPU 41 sends three different drive currents I1, I2, and I3 to the LED 21 and measures the corresponding light reception amounts P11 to P18, P21 to P28, and P31 to P38. . The CPU 41 calculates average values P1ave, P2ave, and P3ave for the light reception amounts P11 to P18, P21 to P28, and P31 to P38. Accordingly, the CPU 41 determines a linear equation L1 indicating the relationship between the drive current and the output of the density sensor 20. In this example, since P2ave <PT <P3ave, a straight line equation L1 is determined using I2, P2ave, I3, and P3ave. Further, the CPU 41 obtains a set value It of the drive current corresponding to the target value Pt from the linear equation L1 and the target value Pt. Note that even if the drive current is set to the maximum value that can be set, the output of the density sensor 20 may not reach the target value. In this case, the CPU 41 increases the gain G of the amplifier circuit 32a by one step and performs the light amount adjustment again. Finally, the drive current It and the gain G of the amplifier circuit 32a for achieving the target value Pt are determined. In this manner, the light emission amount and the regular reflection light reception amount of the density sensor 20 are adjusted according to the change in the surface state of the intermediate transfer belt 5.

    Incidentally, irregularly reflected light is irregularly reflected light from a toner image. Therefore, the amount of the irregularly-reflected light received is less affected by the change in the surface state of the intermediate transfer belt 5. Therefore, when the light emission amount of the LED 21 increases, the light reception amount of the irregular reflection light also increases.

    FIG. 6A shows three toner densities when the intermediate transfer belt 5 is hardly deteriorated, and the corresponding regular reflection output and irregular reflection output. The specular reflection output is the output of the amplifier circuit 32a. The diffuse reflection output is the output of the amplifier circuit 32b. Patches i to iii are toner patterns formed on the intermediate transfer belt 5. The toner density of patch i is relatively low. The patch ii has a medium toner density. The toner density of patch iii is relatively high. As the toner density increases, the specular reflection output decreases, but the irregular reflection output increases. Ps1 indicates the initial value of the output of the density sensor 20 for the specularly reflected light from the patch i. Pd1 indicates the initial value of the output of the density sensor 20 for the irregularly reflected light from the patch i.

    FIG. 6B shows three toner densities when the intermediate transfer belt 5 is deteriorated to a moderate level, and the corresponding regular reflection output and irregular reflection output. As the intermediate transfer belt 5 deteriorates, the regular reflection output decreases from the initial value Ps1 shown in FIG. On the other hand, even if the intermediate transfer belt 5 is deteriorated, the diffuse reflection output does not decrease.

    FIG. 6C shows the result of the light amount adjustment executed to restore the regular reflection output when the intermediate transfer belt 5 has deteriorated to a moderate degree. The output of the density sensor 20 with respect to the regular reflection light from the patch i has been restored to the initial value Ps1 by increasing the light emission amount of the LED 21. On the other hand, due to the increase in the light emission amount of the LED 21, the diffuse reflection output also increases. However, the diffuse reflection output Pd3 ′ for the patch iii is saturated, and is lower than the original output Pd3. The original output Pd3 is a diffuse reflection output when the output is not saturated.

    As can be seen from FIGS. 6A to 6C, the regular reflection output is restored to the initial value Ps1 by executing the light amount adjustment, but the diffuse reflection output is increased from the initial value. In particular, the diffuse reflection output of the patch iii is saturated, and the detection accuracy of the density sensor 20 is reduced.

<Flow chart>
FIG. 7 is a flowchart showing the light amount adjustment and the density correction executed by the CPU 41. FIG. 8 shows functions realized by the CPU 41 executing the control program. Some or all of these functions may be realized by a hardware circuit such as an ASIC or an FPGA. The ASIC is an abbreviation for the specific application accumulation deviation. FPGA is an abbreviation for field programmable gate array. The CPU 41 is configured to execute the following processing when the start condition is satisfied. The start condition is that the power of the image forming apparatus 10 is turned on, that the number of image formations reaches a predetermined number, that execution is instructed by the user, and the like.

    In S701, the CPU 41 (light amount adjusting unit 44) forms and detects a toner pattern. As shown in FIGS. 5A and 5B, the pattern forming unit 46 of the current amount adjusting unit 45 controls the black image forming station K to form a black toner pattern on the intermediate transfer belt 5. I do. The pattern detection unit 47 of the current amount adjustment unit 45 controls the drive circuit 30 of the density sensor 20 to sample the regular reflection output Ps output from the amplification circuit 32a while switching the drive current flowing through the LED 21 from I1 to I3 in order. Then, the data is written to the memory 42. Note that the initial value of the gain Ga (amplification factor) is 1.

In S702, the CPU 41 (current determining unit 48) determines the drive current It corresponding to the target value Pt of the regular reflection output Ps. As described above, the current determination part 48, an average value P1ave~P3ave from the sampling value of the regular reflection output Ps, to determine the driving current It _n corresponding to the target value Pt. As described above, the current determination unit 48 determines the equation L1 indicating the correlation between the drive current and the regular reflection output from the data groups (I1, P1ave), (I2, P2ave), and (I3, P3ave), determining a driving current It _n corresponding from equation L1 to the target value Pt. n is an index.

S703 in CPU 41 (anticipator 51), to predict the diffuse reflection output Pd _n corresponding to the determined drive current It _n. For example, the expected unit 51 may calculate the diffuse reflection output Pd _n using the following equation. Diffuse output Pd _n is an irregular reflection output for patch iii with a maximum density among the patch i to iii.

_{Pd n = It n × c n} -1 ··· (2)
The coefficient cn _-1 may be obtained from the following equation.

cn _-1 = Pdn _-1 / Itn _-1 (3)
Here, _{Pd n-1} is the value of the diffuse reflection output Pd _n were determined by a previous light amount adjustment. It _n-1 is a drive current corresponding to the target value Pt determined by the previous light amount adjustment. These are stored in the memory 42.

S704 in CPU 41 (judging unit 52), the expected diffuse output Pd _n determines whether below the saturation threshold value Pdm. If the expected diffuse output Pd _n is below the saturation threshold value Pdm, CPU 41 advances the processing to S707. S707 in CPU 41 (update unit 55) updates the like current drive current It _n and diffuse output Pd _n were determined by the light quantity adjustment. That is, the update unit 55 overwrites the driving current _{It n-1} and diffuse output _{Pd n-1} stored in the memory 42 by the driving current It _x and diffuse output Pd _n. On the other hand, if the expected diffuse output Pd _n exceeds the saturation threshold Pdm, CPU 41 advances the processing to S705.

In S705, the CPU 41 (the gain adjustment unit 53 and the current calculation unit 54) increases the gain G an _-1 set in the amplifier circuit 32a at that time, and the drive current It _n according to the increased gain G an _-1. Decrease. For example, the gain adjusting unit 53 to increase the gain _{Ga n-1} by a predetermined value, to determine a new gain Ga _n. Further, the current calculation unit 54 calculates a new drive current It _'n corresponding to the new gain Ga _n.

_{It 'n = It n / Ga} n ··· (4)
S706 in CPU 41 (expected 51) predicts the diffuse reflection output Pd _n corresponding to the new drive current It _'n with reduced. The prediction unit 51 may calculate a new diffuse reflection output Pdn using the following equation.

_{Pd n = It 'n × c} n-1 ··· (5)
After that, the CPU 41 advances the processing to S704, and executes the determination processing of S704 again. CPU41 is irregularly reflected to the output Pd _n is equal to or less than saturation threshold Pdm, S704 to repeatedly execute the S706. Finally, the specular reflection output Ps is maintained at the target value Pt, and diffused reflection output Pd _n that is equal to or less than saturation threshold Pdm, driving current It _n, specular reflection gain Ga _n determined.

S707 in CPU 41 (update unit 55), the driving current It _n was determined by this light amount adjustment, to update the value held in the memory 42 by using the regular reflection gain Ga _n and diffuse output Pd _n.

  In S708, the CPU 41 (density deviation correction unit 43) executes the density deviation correction using the density sensor 20 for which the light amount adjustment has been completed. For example, the CPU 41 causes the image forming engine 15 to form a patch image on the intermediate transfer belt 5, and the density sensor 20 measures reflected light from the patch image. Then, the CPU 41 detects the density (adhesion amount) of the patch image from the output voltage of the PD 22a and the output voltage of the PD 22b, and controls the image forming condition based on the detection result. The image forming conditions include, for example, a charging bias, a developing bias, a transfer bias, a laser power, a gradation correction table, and the like.

<Technical ideas derived from the examples>
The intermediate transfer belt 5 is an example of an image carrier that carries and transports a toner image. The image forming stations Y, M, C, and K are examples of an image forming unit that forms a toner image on an image carrier. The density sensor 20 is an example of a detecting unit that detects the density of the toner image carried on the image carrier. The CPU 41 is an example of a control unit that controls the detection unit. The LED 21 is an example of an irradiation unit that irradiates light onto a toner image carried on the image carrier. An optical component such as a mirror may be arranged from the LED 21 to the toner image or the intermediate transfer belt 5. The drive circuit 30 is an example of a drive unit that supplies a drive current to the irradiation unit. The PD 22a and the IV conversion circuit 31a are examples of a first light receiving unit that receives regular reflection light from a toner image and outputs a first detection signal according to the amount of the regular reflection light. The regular reflection output Ps is an example of a first detection signal. The amplifier circuit 32a is an example of an amplifier that amplifies the first detection signal. The PD 22b is an example of a second light receiving unit that receives irregularly reflected light from the toner image and outputs a second detection signal according to the amount of the irregularly reflected light. The diffuse reflection output Pd is an example of a second detection signal. As described with respect to S701 and S702, when the adjustment of the light emission amount of the irradiation unit is started, the CPU 41 acquires the correlation between the driving current and the output level of the first detection signal while changing the driving current of the driving unit. Further, the CPU 41 determines a drive current such that the light amount of the regular reflection light becomes the target light amount based on the correlation. When the CPU 41 supplies a drive current determined so that the amount of specularly reflected light becomes the target amount of light to the irradiation unit, the output level of the second detection signal may be saturated with respect to the density of the toner image. In this case, the CPU 41 decreases the drive current while increasing the gain of the amplifying means until the output level of the second detection signal is no longer saturated. This provides the image forming apparatus 10 that maintains the light reception amount of the specularly reflected light at the target light amount and hardly saturates the output of the light receiving unit that receives the irregularly reflected light.

  The determining unit 52 is an example of a determining unit that determines whether the output level of the second detection signal is equal to or less than a saturation threshold when the driving current determined so that the light amount of the specular reflected light becomes the target light amount is supplied to the irradiation unit. is there. When the output level of the second detection signal does not fall below the saturation threshold, the CPU 41 increases the gain of the amplifying unit and decreases the drive current. Thus, the output of the light receiving unit that receives the irregularly reflected light does not saturate while maintaining the light reception amount of the specularly reflected light at the target light amount.

  The current calculation unit 54 is an example of a determining unit that determines a drive current corresponding to the increased gain so that the light amount of the specularly reflected light is maintained at the target light amount. The current calculation unit 54 determines the drive current corresponding to the increased gain by dividing the value of the drive current determined so that the light amount of the specularly reflected light becomes the target light amount by the increased gain. Is also good.

  The prediction unit 51 uses a mathematical expression or a table indicating the relationship between the drive current supplied to the irradiation unit and the output level of the second detection signal, and determines the drive current determined so that the amount of specularly reflected light becomes the target light amount. May be predicted for the output level of the second detection signal corresponding to. As such a mathematical expression, there are, for example, expressions (2) and (5). Equations (2) and (5) may be tabulated or programmed.

  The determination unit 52 is an example of a determination unit that determines whether the predicted value is equal to or less than the saturation threshold. When the expected value does not fall below the saturation threshold, the CPU 41 increases the gain of the amplifying unit and decreases the drive current. In particular, the CPU 41 increases the gain of the amplifying means and decreases the drive current until the predicted value becomes equal to or less than the saturation threshold. Thereby, the light emission amount of the LED 21 is reduced, so that the irregular reflection output Pd is controlled to be equal to or less than the saturation threshold Pdm.

The coefficient cn _-1 is an example of a coefficient indicating a ratio between the output level of the second detection signal smaller than the saturation threshold and the drive current corresponding to the output level. The estimating unit 51 may obtain the expected value by multiplying the coefficient cn _-1 by the driving current determined so that the amount of specularly reflected light becomes the target amount of light. Since the diffuse reflection output Pd, which is the output level of the second detection signal, is correlated with the reflected light from the toner pattern, the diffuse reflection output Pd increases as the amount of light emission (drive current) increases. Therefore, the relationship between the drive current and the diffuse reflection output Pd can be expressed by a simple coefficient cn _-1 .

  As indicated by the expression (4), the current calculation unit 54 divides the value of the drive current determined so that the amount of the specularly reflected light becomes the target amount of light by the increased gain, thereby obtaining the increased gain. A corresponding drive current may be determined.

  The memory 42 is an example of a storage unit that stores the drive current determined so that the amount of specularly reflected light becomes the target amount of light and the output level of the second detection signal. When the drive current that can achieve the target light amount and does not saturate the output level of the second detection signal and the gain of the amplification unit are determined, the CPU 41 determines the drive current and the output level of the second detection signal stored in the storage unit. And update.

  The pattern forming section 46 controls the image forming means and forms a first test image and a second test image having different densities on the image carrier by varying the drive current. Patch i or patch iii is an example of a first test image or a second test image. The density sensor 20 and the pattern detection unit 47 detect the output level of the first detection signal for the first test image and the output level of the first detection signal for the second test image. The current determining unit 48 determines the driving current and the output level of the first detection signal based on the output level of the first detection signal for the first test image and the output level of the first detection signal for the second test image. May be obtained.

  When the drive current that does not saturate the output level of the second detection signal and the gain of the amplifier are determined, the density deviation correction unit 43 corrects the gradation characteristics of the toner image to the target gradation characteristics. The tone correction process is performed. As a result, the density deviation is accurately corrected. In the gradation correction process, a so-called gradation correction table is created. The created tone correction table converts the image data of the input image so that the tone characteristics of the input image match the tone characteristics of the output image, and outputs the data to the exposure device 3. As a result, the amount of applied toner on the intermediate transfer body is controlled to the target applied amount.

  5 ... Intermediate transfer belt, YMCK ... Image forming station, 20 ... Density sensor, 21 ... LED, 30 ... Drive circuit, 22a, 22b ... PD, 32a ... Amplification circuit , 41 ... CPU

Claims (11)

An image carrier;
Image forming means for forming a toner image on the image carrier,
Detecting means for detecting the density of the toner image carried on the image carrier;
Control means for controlling the detection means,
The detection means,
Irradiation means for irradiating light toward the toner image carried on the image carrier,
Driving means for supplying a driving current to the irradiation means;
A first light receiving unit that receives regular reflection light from the toner image and outputs a first detection signal according to the amount of the regular reflection light,
Amplifying means for amplifying the first detection signal,
A second light receiving unit that receives irregularly reflected light from the toner image and outputs a second detection signal according to the amount of the irregularly reflected light,
When the control unit starts adjusting the light emission amount of the irradiation unit, the control unit obtains a correlation between the drive current and the output level of the first detection signal while changing the drive current of the drive unit, and obtains the correlation. It is configured to determine the drive current such that the light amount of the regular reflection light becomes a target light amount based on
Further, the control unit supplies the drive current determined so that the light amount of the regular reflection light becomes the target light amount to the irradiation unit, and outputs the second detection signal with respect to the density of the toner image. When the level is saturated, the drive current is decreased while increasing the gain of the amplifying means until the output level of the second detection signal is not saturated. When the drive current determined so that the light amount of the specular reflection light becomes the target light amount is supplied to the irradiation unit, a determination unit that determines whether an output level of the second detection signal is equal to or less than a saturation threshold is further provided. Have
2. The image according to claim 1, wherein when the output level of the second detection signal does not fall below the saturation threshold, the control unit increases the gain of the amplification unit and decreases the drive current. 3. Forming equipment.   3. The control unit according to claim 1, further comprising a determining unit configured to determine a drive current corresponding to the increased gain so that the light amount of the regular reflection light is maintained at the target light amount. An image forming apparatus according to claim 1.   The determining means divides the value of the drive current determined such that the light amount of the specularly reflected light becomes the target light amount by the increased gain, thereby obtaining a drive current corresponding to the increased gain. The image forming apparatus according to claim 3, wherein: Using a mathematical expression or a table indicating a relationship between the drive current supplied to the irradiation unit and the output level of the second detection signal, the drive current determined so that the light amount of the regular reflection light becomes the target light amount Prediction means for predicting an expected value of the output level of the second detection signal corresponding to
Determining means for determining whether the expected value is equal to or less than a saturation threshold,
2. The image forming apparatus according to claim 1, wherein the control unit increases the gain of the amplifying unit and decreases the drive current when the expected value does not fall below the saturation threshold. 3.   The image forming apparatus according to claim 5, wherein the control unit increases the gain of the amplifying unit and decreases the drive current until the expected value becomes equal to or less than the saturation threshold.   The equation is determined such that the light amount of the specularly reflected light becomes the target light amount in a coefficient indicating a ratio between an output level of the second detection signal smaller than the saturation threshold value and a drive current corresponding to the output level. The image forming apparatus according to claim 5, wherein the image forming apparatus obtains the expected value by multiplying the driving current.   The control means divides the value of the drive current determined so that the light amount of the specularly reflected light becomes the target light amount by the increased gain, thereby obtaining a drive current corresponding to the increased gain. The image forming apparatus according to claim 5, wherein the determination is made. Further comprising a storage means for storing the drive current determined so that the light amount of the regular reflection light is the target light amount, and the output level of the second detection signal,
When the drive current and the gain of the amplifying unit that can achieve the target light amount and do not saturate the output level of the second detection signal are determined, the drive current stored in the storage unit is determined. The image forming apparatus according to claim 1, wherein an output level of the second detection signal is updated. The control unit controls the image forming unit, and forms a first test image and a second test image having different densities on the image carrier by respectively varying the drive current,
The detection means detects an output level of the first detection signal for the first test image and an output level of the first detection signal for the second test image,
The control unit is configured to control the driving current and the first detection signal based on an output level of the first detection signal for the first test image and an output level of the first detection signal for the second test image. The image forming apparatus according to claim 1, wherein the image forming apparatus is configured to acquire a correlation with a signal output level.   When the drive current that does not saturate the output level of the second detection signal and the gain of the amplifying unit are determined, the control unit changes the gradation characteristic of the toner image to the target gradation characteristic. The image forming apparatus according to any one of claims 1 to 10, wherein the image forming apparatus performs a tone correction process for correcting the image.

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