JP4269914B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a technique for detecting output image density, color, gradation, and the like in an image forming apparatus such as a copying machine or a printer, and controlling image forming conditions based on the detection result.

JP-A-6-102734 JP 2001-209292 A

  Currently, the image quality of the output image by the image forming apparatus is remarkably improved, and as a result, the user's demand for image quality fluctuations has become more severe. The requirement is close to the limit of human color discrimination ability, such as a color difference of 3 or less.

  Among image forming apparatuses, those using an electrophotographic method using an electrostatic process generally have large image quality fluctuations. Particularly problematic in this image quality variation are the positional deviation, color reproduction and gradation of each color toner image in a color image, and image density and fog in a monochrome image. These image quality fluctuations occur mainly because the image quality reproducibility of the electrostatic process is easily affected by environmental conditions such as temperature and humidity, deterioration with time, and the like.

  As a method for solving such image quality fluctuation, there is a method of measuring a reference pattern image formed on a moving body such as an intermediate transfer belt and controlling image forming conditions (Japanese Patent Laid-Open No. 6-102734). Publication). In this method, the amount of developer attached to a high density reference pattern image and the amount of developer attached to a low density reference pattern image are measured, and a deviation between the measured value and a target value is calculated. Then, the change amount of the contrast voltage and the change amount of the background voltage corresponding to the deviation between the high density and the low density are extracted from the storage means, and according to the extracted change amount of the contrast voltage and the change amount of the background voltage. The image forming conditions are controlled by calculating the grid bias value and the developing bias value, and changing the grid bias value and the developing bias value. In this apparatus, an actual image formed as a reference pattern image is measured by a sensor, and the difference between the measured value and the reference value is feedback-controlled, so that highly accurate control is possible. As described above, a method of detecting a reproduction state of image quality using a reference pattern image (hereinafter referred to as an ADC method) is widely applied.

  The ADC type sensor is provided to face a position where a reference pattern image formed on the photosensitive member or the moving body passes, and when the reference pattern image passes through a detection area of the sensor, the reference pattern image is displayed. It can be read. Usually, the irradiation light spot diameter of the LED used in such a sensor is generally about 3 to 10 mm in half width. On the other hand, the intensity of the irradiation light is strong near the center, and the intensity decreases as it goes to the periphery, and the outer portion of the full width at half maximum is weakly irradiated with light that affects the measured value. For this reason, when the reference pattern image is smaller than the irradiation light spot, if the position through which the reference pattern image changes due to misalignment or the like, it passes through a position where the irradiation light intensity is different, and accurate measurement can be performed. There is no problem.

  Therefore, as a device for reducing the adverse effects of measuring the reference pattern image due to misalignment, the density error of the minimum size is corrected by correcting the formation position of the reference pattern image for density error detection based on the misalignment detection information. There has been proposed an image forming apparatus capable of using a reference pattern image for detection and improving the accuracy of density error detection (Japanese Patent Laid-Open No. 2001-209292). In this image forming apparatus, when the misregistration is correctly detected, it can be expected to accurately control the image forming conditions. However, if the density of the reference pattern image is drastically reduced due to electrical noise or out of toner, and the misregistration detection result becomes abnormal, the formation position of the density error detection reference pattern image cannot be set correctly. As a result, the accuracy of density error detection is reduced. For this reason, there is a problem that a problem occurs in density control when toner is replenished as the toner runs out.

  The present invention has been made in view of such problems, and the object of the present invention is to adjust the size, interval, number, etc. of the reference pattern image for density error detection based on the position shift detection information. An object of the present invention is to provide an image forming apparatus capable of preventing a decrease in density error detection accuracy even when there is an abnormality in deviation detection information.

In order to achieve the above object, an image forming apparatus according to the present invention includes an image forming engine for forming a toner image according to an image data signal, a moving body on which toner images are multiplex-transferred from the image forming engine, and a moving body. A reference pattern image control unit for forming a reference pattern image for density error detection on the body; a photosensor for reading the reference pattern image; and a density error from a signal output when the reference pattern image passes through the photosensor. In a color image forming apparatus that includes a detection information extraction unit that detects image density and a control unit that corrects image density according to density error information sent from the detection information extraction unit, and executes a density detection mode at a predetermined timing. In the first density detection mode, the reference pattern image control unit generates a reference pattern image for detecting a density error having a predetermined size at a predetermined interval from the moving object. On the other hand, in the second density detection mode, the reference pattern image controller controls the density error detection reference pattern image on the moving body with a different size and a different interval from the first density detection mode. The reference pattern image control unit forms a reference pattern image for position shift detection on the moving body, and is detected from a signal output when the reference pattern image for position shift detection passes through the photosensor. A positional deviation is detected by the information extraction unit, and either the first density detection mode or the second density detection mode is selected according to the detection result of the positional deviation .

  In such an image forming apparatus of the present invention, it is preferable that an image forming engine used in an image forming apparatus using an electrophotographic system is applied as the image forming engine. That is, an image processing unit that inputs an original image signal read from an original by an image reading unit or an original image signal created by an external computer, and the input original image signal are decomposed into image information of each color. After that, it is composed of ROS (laser output unit) for raster-irradiating a modulated laser beam and each color photoconductor that is uniformly charged by a contact charger and raster-irradiated with the laser beam on its surface.

  The moving body may be any material that can transfer a toner image formed on each color photoconductor and is formed of a material that is difficult to expand and contract. And as the drive means, it is preferable to have an output sufficient to drive the movable body. In addition, it is only necessary to apply a support method in which the moving body that repeatedly moves along the moving path is less likely to be displaced with respect to the main scanning direction and the sub-scanning direction of the moving body itself.

  The reference pattern image control unit is an image data for causing the imaging engine to form a reference pattern image for position shift detection and a reference pattern image for density error detection on the moving body when position shift detection or density error detection is required. Must be set to send a signal. The formation position of each reference pattern image must be a position corresponding to the reading position of the photosensor on the moving body.

  A photosensor that detects a positional deviation or a density error from each reference pattern image formed on a moving body combines a light emitting element and a light receiving element, and each light is emitted to the light receiving element when each reference pattern image is irradiated by the light emitting element. Any device capable of receiving the reflected light from the reference pattern image may be used.

  The detection information extraction unit can perform calculation processing based on the interval and output value of the output signals detected by the photosensor, and can detect a positional deviation amount and a density error of the reference pattern image formed on the moving body. These detection results must be sent to the control unit as positional deviation information and density error information.

  The positional deviation of the reference pattern image formed on the movable body such as the intermediate transfer belt is relatively small, such as several hundred μm or less due to, for example, thermal expansion due to temperature change under normal use conditions. Furthermore, as a cause of a larger displacement, for example, when replacing consumable parts such as a toner cartridge and a photoconductor used in the image forming apparatus, the apparatus housing is opened, and the parts constituting the image forming engine are replaced with the apparatus. For example, when the image forming apparatus is once taken out and then accommodated in the apparatus again, when the image forming apparatus is moved, or when some external force is applied to the apparatus. In addition, an abnormality is observed in the misregistration detection result when the density of the reference pattern image is extremely lowered due to electrical noise or toner exhaustion. On the other hand, the image density is easily affected by changes in temperature and humidity, and the number of output images. It is necessary to detect density errors when the temperature inside the device changes by a predetermined temperature or when the number of output images reaches a predetermined number. It becomes. Then, both the position shift detection and the density error detection are performed by one detection operation.

  Here, for example, when a displacement occurs in the moving body due to some trouble, the reference pattern image for density error detection is not formed at the reading position of the photosensor. In this case, even if the positional deviation is small, each reference pattern image for density error detection passes through a position where the intensity of light emitted from the photosensor is weak, and the detection accuracy is lowered. Also, depending on the degree of positional deviation, each reference pattern image cannot be read by the photosensor, and it may be impossible to detect the density error. In this case, one detection operation is wasted. End up. Therefore, when no misregistration has occurred and the detection result is normal, a setting for applying a predetermined density error detection reference pattern image and when an abnormality has occurred in the misregistration detection result, Depending on the degree, a setting for adjusting the reference pattern image for density error detection is provided, and a first density detection mode and a second density detection mode are set, respectively.

  The first density detection mode is preferably applied in a situation in which a large positional deviation does not occur, for example, between image forming jobs. In this detection mode, it is not necessary to adjust the reference pattern image for detecting the density error accompanying the positional deviation, so it is preferable to form the reference pattern image for detecting the density error having a predetermined size on the moving body at a predetermined interval. .

  Next, the second density detection mode is preferably applied in situations where a large positional deviation may occur, for example, after replacement of the toner cartridge or after some external force is applied to the apparatus. Therefore, it is preferable that the setting is applied immediately after opening / closing of the apparatus casing or immediately after the apparatus main power is turned on. Then, the size, interval, and number of the density error detection reference pattern image must be adjusted according to the detection result of the positional deviation, and the density error must be reliably detected.

  According to the image forming apparatus of the present invention configured as described above, the size of the reference pattern image for density error detection, based on the position shift detection information or according to the situation where the occurrence of position shift is a concern, By adjusting the interval, the number, etc., it is possible to prevent a decrease in density error detection accuracy even when there is an abnormality in the positional deviation detection information.

  Hereinafter, an image forming apparatus of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic configuration diagram of a color image forming apparatus using an electrophotographic system to which the present invention is applied. In this configuration diagram, after charging the surface of the photoreceptor with a contact charger, an electrostatic latent image is formed by irradiation with a laser beam, and the xerographic engine that develops the electrostatic latent image with toner is yellow, magenta, cyan, An outline of an IOT (image output terminal: image output unit) of a tandem type color electrophotographic image forming apparatus provided for each color of black is shown. In the drawing, the image reading unit and the image processing unit of the image forming apparatus are omitted.

  The IOT of this image forming apparatus includes four photoconductors 1Y, 1M, 1C, and 1K that rotate in the direction of arrow A in the figure, and contact chargers 2Y, 2M, 2C, and 2K that charge the surface of each photoconductor. ROS (laser output units) 3Y, 3M, 3C, which expose the charged photoreceptor surfaces with exposure light modulated based on the image information of each color and form electrostatic latent images on the photoreceptors. 3K and developing units 4Y, 4M, 4C, and 4K that develop the electrostatic latent image on each photoconductor with each color developer to form a toner image on the photoconductor, and intermediate transfer each color toner image on the photoconductor The primary transfer units 5Y, 5M, 5C, and 5K that transfer to the body belt 6, the secondary transfer unit 7 that transfers the toner image on the intermediate transfer body belt 6 to the paper P, and the toner image transferred to the paper P are fixed. The fixing device 9 for carrying out, the paper tray T for containing the paper P, and the surface of each photoconductor Cleaner for cleaning (not shown), static eliminator (not shown) for removing residual charges on the surface of each photoconductor, reference pattern image for detecting displacement and density error detection transferred to the surface of the intermediate transfer belt 6 The photo sensor 10 detects the reference pattern image for use, and the belt cleaner 8 that cleans the surface of the intermediate transfer belt.

  As an image forming operation in the image forming apparatus shown in the configuration diagram, first, an original image signal read from an original by an image reading unit (not shown) or an external computer (not shown) is used. The original image signal is input to an image processing unit (not shown). The input image signal is decomposed into image information of each color and then input to ROS (laser output units) 3Y, 3M, 3C, and 3K, and the laser beam L is modulated. The modulated laser beam L is applied to the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K that are uniformly charged by the contact chargers 2Y, 2M, 2C, and 2K. When the laser beam L is raster-irradiated on the surface of each photoconductor, an electrostatic latent image corresponding to the input image signal is formed on each photoconductor. Subsequently, the electrostatic latent images on the photoconductors are developed with toner by the color developing devices 4Y, 4M, 4C, and 4K, and toner images are formed on the photoconductors. The toner image formed on each photoconductor is transferred to the intermediate transfer belt 6 by the primary transfer units 5Y, 5M, 5C, and 5K. Each photoreceptor after the transfer of the toner image to the intermediate transfer belt 6 is cleaned of adhering matter such as residual toner adhering to the surface by a cleaner, and the residual charge is removed by a static eliminator.

  Next, the toner image on the intermediate transfer belt 6 is transferred onto the paper P sent from the paper tray by the secondary transfer device 7 and then transferred onto the paper P by the fixing device 9. Is fixed and a desired image is obtained. The intermediate transfer belt 6 on which the transfer of the toner image onto the paper P has been completed is cleaned of adhering matters such as residual toner adhering to the surface by the belt cleaner 8, and one image forming operation is completed.

  In an electrophotographic color image forming apparatus, image fluctuations such as image density, positional deviation of each color toner image, color reproduction, gradation, and fogging occur due to environmental conditions such as temperature and humidity and deterioration over time. For this reason, it is necessary to correct misalignment and density error before outputting an image to the paper P or while waiting for output. As a method therefor, first, a reference pattern image for position shift detection and density error detection is formed on the intermediate transfer member belt 6. Each reference pattern image is detected by the photosensor 10 and an output signal is sent to the control unit. Further, the positional deviation and the density error are corrected as necessary from the result of the positional deviation amount and the density error obtained from the output signal.

  FIG. 2 is a block diagram showing a flow of correction of positional deviation and density error in the color image forming apparatus shown in FIG. The photosensitive member 1 is charged by the contact charger 2, and the photosensitive member 1 is exposed by the ROS 3 in accordance with the reference pattern image signal output from the reference pattern image control unit 11, thereby forming an electrostatic latent image. Then, the reference pattern image is transferred to the intermediate transfer belt 6. Then, the reference pattern image transferred onto the intermediate transfer belt 6 is read by the photosensor 10.

  The detection information extraction unit 12 detects a positional deviation and a density error from the signal output from the photosensor 10, and the control unit 100 performs ROS 3 according to the positional deviation information and the density error information sent from the detection information extraction unit 12. The image power is corrected by controlling the laser power. Further, the control unit 100 controls the writing timing of the ROS 3 according to the output signal output from the photosensor 10 to correct the image forming position.

FIG. 3 is a layout diagram of a reference pattern image formed on the intermediate transfer belt 6. In the present embodiment, the positional deviation detection reference pattern image M (FIG. 3A), the density error detection reference pattern image D ₁ in the first density detection mode (FIG. 3B), and the second density detection. An arrangement of three types of reference pattern images with the reference pattern image D ₂ for density error detection in the mode (FIG. 3C) is used. The misregistration detection reference pattern image M is a V-shaped pattern image having a size of 3.5 mm × 7 mm with halftone dot coverage: Cin = 100% for each color, cyan, yellow, cyan, magenta, cyan, An arrangement of pattern images arranged in the order of black and cyan is employed. The reference pattern image D ₁ for density error detection in the first density detection mode is a single color, a dot pattern coverage: Cin = 60% rectangular pattern image of 1.8 mm × 1.3 mm size, black, A pattern image arrangement is employed in which the pattern image intervals are 15 mm, 1.3 mm, and 1.3 mm in the order of cyan, yellow, and magenta. The reference pattern image D ₂ for density error detection in the second density detection mode is a single color, a halftone dot coverage: a square pattern of 20 mm × 1.3 mm with Cin = 60%, black, cyan, yellow, In the order of magenta, an arrangement of pattern images in which the interval between the pattern images is 20 mm, 5 mm, and 5 mm is employed. Thus, the density error detection reference pattern image D ₂ in the second density detection mode has a width in the main scanning direction and the interval between the reference pattern images of the respective colors as compared with the reference pattern image D ₁ in the first density detection mode. It is wide. The reason for this is that even if the position detection result becomes abnormal and the formation position of the density error detection reference pattern image is greatly shifted, the measurement position of the photosensor is reliably detected without overlapping the density error detection reference pattern images of the respective colors. Is to pass through.

  Each reference pattern image is created by the reference pattern image control unit 11 during the operations of position adjustment and density control in accordance with an instruction from the control unit 100. Details of the specific operation will be described later.

  FIG. 4 is a schematic configuration diagram of the photosensor 10 in the present embodiment. The photosensor 10 includes an illumination unit, a light receiving optical system, and a light receiving element. This illumination means consists of two LEDs 10a, 10b. The light receiving optical system includes a lens 10c and a mask 10e. In this figure, the left-right direction is the main scanning direction. FIG. 5 is a block diagram showing a flow in which the output signal from the photodiode 10d is processed by the detection information extraction unit 12, and includes an AMP, a peak detection circuit, an under-peak detection circuit, and two sample-and-holds. The output signal from each circuit is sent to the control unit 100 in FIG.

In order to detect misalignment and density error by the photosensor 10, the misregistration detection reference pattern image M shown in FIG. 3A and the density error detection shown in FIG. It is necessary to irradiate the reference pattern image D with illumination means. For example, in the case of detecting the density error, as described above, irradiating black, cyan, yellow, by the illumination means the density error detection reference patterns D ₁ arranged in the order of magenta. However, the reflected light from the black reference pattern image is different from the reflected light from the cyan, yellow, and magenta reference pattern images. Therefore, in order to detect these two types of reflected light with a single light receiving element (photodiode 10d), the reference pattern image must be irradiated from a position where each reflected light can enter the light receiving element. Two illumination means are used properly according to the reference pattern image to be irradiated.

  The lens 10c of the light receiving optical system is arranged so that one of the two types of reflected light can be imaged on the light receiving surface of the photodiode 10d. However, when the reflected light is incident on the photodiode 10d, it is not limited to whether the reflected light is imaged or not formed on the light receiving surface, and reflected light that is unnecessary for the detection of positional deviation and density error also enters. End up. Therefore, it is necessary to block this unnecessary reflected light and guide only the reflected light component effective for each detection onto the light receiving surface of the photodiode 10d. Therefore, a mask 10e for restricting the visual field area of the light receiving surface of the photodiode 10d is provided immediately before the photodiode 10d. The mask 10e is black to prevent stray light. The lens 10c and the mask 10e constituting the light receiving optical system can make the viewing area of the light receiving surface of the photodiode 10d substantially equal regardless of which reflected light is incident.

  When the reflected light from the reference pattern image is projected onto the light receiving surface of the photodiode 10d, the photodiode 10d outputs a current corresponding to the amount of reflected light, that is, the density of the reference pattern image. As shown in FIG. 5, the current output from the photodiode 10d is converted from current to voltage by the AMP 20 and then converted into a sensor output signal by a control unit (not shown), a peak detection circuit 21, and an under peak detection circuit 23. , And two sample and hold circuits 22 and 24.

  The peak detection circuit 21 detects the maximum position of the sensor output signal, supplies it to the sample & hold circuit 22 as a peak detection signal, and outputs it to the control unit. By using this peak detection circuit 22 to detect the maximum position of the sensor output signal, it is possible to detect the center position in the thickness direction of the reference pattern image for position shift detection. Then, the control unit calculates the positional deviation based on the peak detection signal and corrects the positional deviation.

  The sample & hold circuit 22 holds the sensor output signal output from the AMP 20 with the peak detection signal output from the peak detection circuit 21 as a trigger. As a result, the maximum value of the sensor output signal is held and output to the control unit as a hold signal. The control unit calculates the density error based on the maximum value hold signal, and corrects the image density.

  The under peak detection circuit 23 detects the minimum position of the sensor output signal and supplies it to the sample and hold circuit 24 as an under peak detection signal. The sample and hold circuit 24 holds the sensor output signal output from the AMP 20 with the under peak detection signal output from the under peak detection circuit 23 as a trigger. As a result, the minimum value of the sensor output signal is held and output to the control unit as an under-peak hold signal. In the control unit, the density error is calculated based on the hold signal having the minimum value, and the image density is corrected. Note that a general electric circuit may be applied to the AMP 20, the peak detection circuit 21, the under peak detection circuit 23, and the sample and hold circuits 22 and 24, and the description thereof is omitted.

  In order to detect the density of the reference pattern image based on the output signal from the photodiode 10d, it is necessary to compare the reference output signal with the output signal detected from the reference pattern image. Therefore, it is necessary to have means capable of switching between the case where the reference light is incident on the photodiode 10d and the case where the reflected light from the reference pattern image is incident. Therefore, a shutter 10f as shown in FIG. 6 is attached to the photosensor 10 in a slidable state on the housing of the photosensor 10 facing the intermediate transfer belt 6 (FIG. 4). . This figure is a plan view of the shutter 10f as seen from the LED side. The shutter 10f is provided with a measurement window 10g and a reference plate 10h for obtaining a reference for the output voltage of the sensor. A mechanism is provided that moves by a drive device (not shown) in the direction of the arrow in the figure in accordance with the reflected light incident on the photodiode 10d. The shutter 10f is in such a position that the reference plate 10h is disposed on the light receiving system optical axis in the normally closed state, and the shutter 10f is opened only when measuring the reference pattern image, and the measurement window 10g is disposed on the light receiving system optical axis. Move to be.

FIG. 7 shows the positional relationship between the positional deviation detection reference pattern image M formed on the intermediate transfer belt 6 and the visual field region R of the photosensor 10 on the intermediate transfer belt 6 with time. The lower graph (a) shows the waveform of the sensor output signal corresponding to the position of the visual field region R of the photosensor 10. The lowermost graph (b) shows the peak detection signal of the reference pattern image M for detecting misalignment output from the peak detection circuit in correspondence with the passage of time. Here, the control mark is formed such that the thickness t of each side M ₁ and M ₂ is slightly smaller than the diameter d (1 mm) of the visual field region R.

The misregistration detection reference pattern image M primarily transferred onto the intermediate transfer belt 6 passes through the front surface of the photosensor 10 with the rotation of the intermediate transfer belt 6 and crosses the visual field region R of the photosensor 10. It will be. When the misregistration detection reference pattern image M moves together with the intermediate transfer belt 6 and the visual field region R of the photosensor 10 reaches the point A on the intermediate transfer belt shown in FIG. Since one side M _{1 of the} misregistration detection reference pattern image M enters, the sensor output signal starts to change. When the positional deviation detection reference pattern image M further moves, the area of the positional deviation detection reference pattern M included in the visual field region R, that is, the overlapping area of the visual field region R and one side M ₁ of the positional deviation detection reference pattern image M. Therefore, the sensor output signal gradually increases, and the sensor output signal becomes maximum at the point B where the visual field region R is substantially covered with the positional deviation detection reference pattern image M.

As described above, since the thickness t of each side M ₁ , M ₂ of the reference pattern image M for detecting misregistration is formed slightly smaller than the diameter d of the visual field region R of the photosensor 10, misregistration detection is performed. When the reference pattern image M passes the point B, the area of the misregistration detection reference pattern image M included in the visual field region R, that is, the overlapping area of the visual field region R and the misregistration detection reference pattern image M is reduced. As a result, the sensor output signal gradually decreases, and the sensor output signal becomes the minimum when the reference pattern image M for detecting displacement is completely removed from the visual field region R of the photosensor 10 (point C).

In this way, in the example shown in FIG. 7, when one side M _{1 of the} misregistration detection reference pattern image M passes through the field region R of the photosensor 10 (between point A and point B), the field region R And the control mark M overlap continuously with the progress of the intermediate transfer belt 6 so that a sensor output signal having the same intensity is not continuously output from the photosensor 10. It is configured. That is, the maximum value is instantaneously generated in the sensor output signal. The waveform of the sensor output signal is such that the visual field region R of the photosensor 10 is formed in a circular shape, and the thickness of the reference pattern image M for detecting displacement is the same as the diameter of the visual field region R, or Can also be easily obtained.

  In multi-color printing presses, color copiers, color printers, etc., when the reference pattern image M for detecting misregistration is formed on a moving body such as an intermediate transfer belt, misregistration is detected depending on the environmental conditions such as temperature and humidity at that time. Since the thickness of the reference pattern image M may change, it is difficult to form the reference pattern image M for detecting misregistration having the same thickness as the diameter of the visual field region R of the photosensor 10. Therefore, as described above, even when the thickness of the reference pattern image M for detecting misalignment is smaller than the diameter of the visual field region R, it is actually that an instantaneous maximum value is generated in the waveform of the sensor output signal. This is advantageous when configuring a color printer or the like.

As shown in FIG. 7, when an instantaneous maximum value is generated in the waveform of the sensor output signal, the maximum value is the center position (center of gravity position) in the thickness direction of _one side M ₁ of the reference pattern image M for positional deviation detection. This occurs when the center position of the visual field region R of the photosensor 10 is matched. Therefore, if the peak detection circuit detects the maximum value (peak) of the sensor output signal and, as shown in FIG. 7B, a pulse-shaped peak detection signal is output in accordance with this maximum value. The rising edge portion of the peak detection signal indicates the center position (center of gravity position) of one side M ₁ of the positional deviation detection reference pattern image M, and the position of M ₁ can be accurately detected. .

The misregistration detection reference pattern image M shown in FIGS. 3A and 7 has two sides M ₁ and M ₂ that are inclined at approximately 45 degrees in different directions with respect to the moving direction of the intermediate transfer belt 6. Therefore, by detecting one of the misregistration detection reference pattern images M by the photosensor 10 of this embodiment, the positions in the main scanning direction and the sub-scanning direction are detected. The amount of deviation can be grasped at once. That is, the sensor output signal is once minimized when the visual field region R reaches the point C. However, when the visual field region R passes the point D, the side M ₂ of the reference pattern image M for misalignment detection and the visual field region again. Since R starts to overlap, it starts to rise again, and the center position in the thickness direction of the side M ₂ shows the maximum value at the point E where it overlaps with the center position of the visual field region R. As the overlapping area of M ₂ and the visual field region R decreases, the sensor output signal also decreases, and the positional deviation detection reference pattern image M returns to the minimum output at the point F where it has left the visual field region R.

For this reason, when the V-shaped misregistration detection reference pattern image M shown in FIG. 3A is read by the photosensor 10, the misregistration detection reference pattern image M is read as shown in FIG. A pair of pulsed peak detection signals are output from the peak detection circuit corresponding to points B and E where the center position (center of gravity position) in the thickness direction of each side M ₁ and M ₂ overlaps the center position of the visual field region R. Result.

  Next, the operation of misregistration correction and density error correction according to this embodiment will be described with reference to the flowchart of FIG. When there is an image output instruction from the user, first, in step S1, it is determined whether or not to perform misalignment correction. Here, for example, when the power is turned on, when the image forming apparatus is stopped for a certain period of time, or when the environmental temperature changes by a predetermined value (for example, 4 ° C.), it is determined that the positional deviation correction is performed. This determination criterion is merely an example, and a conventionally known misregistration correction execution determination criterion may be used. If it is determined that the positional deviation correction is to be performed, the process proceeds to step S2, and after the positional deviation correction is performed, the process proceeds to step S3. The specific content of the misregistration correction will be described in detail later. If it is determined not to perform misalignment correction, the process proceeds directly from step S1 to step S3.

  Prior to the description of step S3 and subsequent steps, the operation of correcting misalignment in this embodiment will be described with reference to the flowchart of FIG. In the misregistration correction, first, in step S11, the misregistration detection reference pattern image M shown in FIG. 3A is formed on the intermediate transfer belt. In step S12, the reference pattern image is measured by the photosensor 10, and then in step S13, the peak detection signal output from the peak detection circuit in the detection information extraction unit 12 based on the output signal of the photodiode 10d. Accordingly, the control unit 100 measures and calculates the absolute position displacement amount of the reference color cyan with respect to the target value in the main scanning direction and the relative displacement amounts of yellow and magenta with respect to the reference color cyan.

  In this embodiment, the positional deviation amount of the reference pattern image is obtained by calculation from the timing chart at the time of measuring the reference pattern image shown in FIG. In FIG. 10, the waveforms of the operation signal of the shutter 10f of the photosensor 10, the lighting signal of the LED 10b of the photosensor 10, the sensor output signal, and the peak detection signal are shown from the top.

  As shown in FIG. 10, the measurement of the positional deviation amount is started from a state where the shutter 10f is closed and the LED 10b is turned off. After the shutter is opened before the reference pattern image passes the measurement position of the sensor, the LED 10b is turned on. At this time, the sensor output signal is 0V. This is because the intermediate transfer belt 6 used in this embodiment has a black surface and has a mirror surface or gloss, and the LED illumination light is almost diffused in the non-image portion on the surface of the intermediate transfer belt 6. Therefore, the sensor output signal is 0V.

  When one side of the cyan reference pattern image passes with the shutter opened, the sensor output signal has a pulse-like waveform corresponding to the cyan toner amount. At this time, as shown in FIG. 5, the peak detection circuit 21 detects the maximum value of the sensor output signal and outputs the peak detection signal. Here, the time from the start of misalignment measurement until the peak detection signal is output is tA1. The time until the peak detection signal detected by the peak detection circuit 21 is output as the remaining one side of the reference pattern image passes is assumed to be tA2.

  Thereafter, similarly, the times tT1, tT2, tB1, and tB2 until the peak detection signal is output as the reference pattern images of yellow, cyan, magenta, cyan, black, and cyan pass are sequentially measured. Note that FIG. 10 shows a state of passing through cyan, yellow, and cyan reference pattern images. Thereafter, after all the reference pattern images have passed the measurement position of the sensor, the shutter 10f is closed and the LED is turned off. As a result, one reference pattern image measurement operation is completed.

Next, the calculation of the positional deviation amount is performed by obtaining the absolute value positional deviation amount with respect to the target value of the reference color cyan in the main scanning direction and the relative positional deviation amounts of yellow and magenta with respect to the reference color cyan. First, the absolute value position shift amount of the reference color cyan in the main scanning direction is
Absolute value positional deviation amount in the main scanning direction = {(tA2-tA1) -target value} / 2
The relative positional deviation of yellow relative to the reference color cyan is
Sub-scanning direction displacement = [(tT2 + tT1) / 2-((tA2 + tA1) / 2 + (tB2 + tB1) / 2) / 2] × PS
= [(tT2 + tT1) / 2- (tA2 + tA1) / 4- (tB2 + tB1) / 4] × PS
Misalignment in main scanning direction = [(((tB1 + tA1) / 2-tT1 + sub-scanning direction error
+ tT2- (tB2 + tA2) / 2-sub-scanning direction error) / 2] × PS
= [((tB1 + tA1) / 2-tT1 + tT2- (tB2 + tA2) / 2) / 2] × PS
Can be obtained. Here, tA1, tA2, tT1, tT2, tB1, and tB2 are times (μs) from the start of positional deviation measurement until the peak detection signal is output, and PS is a process speed (mm / s). The relative misregistration amounts of magenta and black with respect to the reference color cyan are similarly calculated.

  This calculation corresponds to step S13 in FIG. 9, and after the measurement and calculation of the positional deviation amount is completed by the control unit 100, in step S14, the image forming position at the time of forming the output image, that is, the main scanning direction by ROS and the sub-scanning. Sets the direction exposure timing. With this series of operations, one misalignment correction operation is completed.

  Next, returning to the flowchart of FIG. 8, the operation after step S3 will be described. In step S3, it is determined whether to execute density error correction. Here, for example, it is determined that the density error correction is performed immediately after the power is turned on or when the cumulative number of output images after the previous density error correction exceeds a predetermined value (for example, 30 sheets). As this determination criterion, a conventionally known determination criterion may be used. If it is determined that the density error correction is to be executed, the process proceeds to step S4. If it is determined that the density error correction is not to be executed, the process proceeds directly from step S3 to step S7.

  In step S4, the positional deviation detection information at the time of the previous positional deviation correction execution is compared with a preset reference value (= presumed position detection range) to determine whether the positional deviation detection information is normal or abnormal. Judging. If it is determined that the positional deviation detection information is normal, the process proceeds to step S5, and the first density detection mode is performed. On the other hand, when it is determined that the positional deviation detection information is abnormal or when there is a concern about the occurrence of positional deviation, the process proceeds to step S6 and the second density detection mode is performed. Note that a conventionally known determination criterion may be used as the determination criterion shown here. For example, whether or not a predetermined number of peak detection signals can be measured may be used as a criterion.

Next, operations in the first concentration detection mode and the second concentration detection mode of the present embodiment will be described. Here, the difference between the first density detection mode and the second density detection mode is that only the density error detection reference pattern image to be created is different and the other operations are the same. explain. In the second density detection mode, instead of the density error detection reference pattern image D ₁ (FIG. 3B) in the first density detection mode, the density error detection reference pattern image D ₂ (FIG. 3C) is used. Output and detect density error.

The density error correction operation in this embodiment is as shown in the flowchart of FIG. First, in step S21, to form a density error detection reference patterns D ₁ that shown in FIG. 3 (b) to the intermediate transfer belt on, then, in step S22, measuring a reference pattern image by the photosensor 10 . Thereafter, in step S23, the control unit 100 calculates the density error of the reference pattern image from the hold signal output from the sample and hold circuit in the detection information extraction unit 12 based on the detection signal of the photodiode 10d.

  The density error of the reference pattern image is obtained by calculation from the timing chart when measuring the density error detecting reference pattern image shown in FIG. In FIG. 12, each of the operation signal of the shutter 10f of the photo sensor 10, the lighting signal of the LEDs 10a and 10b of the photo sensor 10, the sensor output signal, the peak detection signal, the peak hold signal, the under peak detection signal, and the under peak hold signal from the top. The waveform is shown.

  As shown in FIG. 12, the density measurement is started from a state in which the shutter 10f is closed and the LEDs 10a and 10b are turned off. After the shutter is opened before the reference pattern image passes the measurement position of the sensor, the LED 10a is turned on. At this time, the sensor output signal is output at a voltage value corresponding to the reflected light from the intermediate transfer belt 6. Thereafter, as the black reference pattern image passes, the output voltage of the sensor has a pulse-like waveform that decreases in accordance with the amount of black toner. At this time, as shown in FIG. 5, the under peak detection circuit 23 detects the minimum value of the sensor output signal and outputs an under peak detection signal. The sample & hold circuit 24 holds the minimum value of the sensor output signal corresponding to the amount of black toner by using the rising pulse of the under peak detection signal output from the under peak detection circuit 23 as a trigger, thereby causing the black density voltage to be held. (Vk) is measured. Next, after passing the black reference pattern image, the sensor output signal again indicates a voltage value corresponding to the specularly reflected light from the intermediate transfer belt, and this value is measured as the belt surface voltage (Vbelt). After the belt surface voltage is measured, the LED 10a is turned off and the LED 10b is turned on, so that the sensor output signal becomes 0V.

  Thereafter, the output signal of the sensor becomes a pulse-like waveform corresponding to the amount of cyan toner due to the passage of the cyan reference pattern image. At this time, the peak detection circuit detects the maximum value of the sensor output signal and outputs a peak detection signal. The sample & hold circuit 22 holds the maximum value of the sensor output signal corresponding to the amount of cyan toner by using the rising pulse of the peak detection signal output from the peak detection circuit as a trigger, and thereby the cyan density voltage (Vc). Is measured. Thereafter, similarly, the density voltage (Vy) of yellow and the density voltage (Vm) of magenta are measured by passing through the yellow and magenta pattern.

  After all the reference pattern images have passed the measurement position of the sensor, the shutter 10f is closed. As a result, a voltage value corresponding to the reflected light from the reference plate 10h of the shutter 10f is output as the sensor output signal, and this is measured as the reference plate output voltage (Vref) of the sensor. Thereafter, the LED 10b is turned off, and one reference pattern image measurement operation is completed.

The image density is calculated differently between black and color (CYM). The image density of black is a relative value with respect to the non-image surface of the intermediate transfer belt 6. Image density: Dk = Vk / Vbelt
And calculate. On the other hand, the color (CYM) image density is a relative value to the output of the reference plate 10h.
Image density: Dn = ((Vn average value) / Vref)
Where n = toner color (c, y, m)
Define and calculate.

  As described above, the reason why the relative value for the output of the intermediate transfer belt 6 or the reference plate 10h is used as the image density is that the sensor light is fouled, changes with time, changes in temperature, changes in LED light quantity, PD sensitivity, and the like occur. This is because the density of the reference pattern image is measured with high accuracy. In this manner, the image density of the reference pattern image is calculated in step S23 of FIG. 11, and the error between the predetermined density target value and the calculated image density is calculated in step S24.

In step S25 of FIG. 11, the correction amount of the ROS laser power: ΔLP is
Laser power correction amount: ΔLP = ΔDn / An
Where n = toner color (k, c, y, m)
Is required. Here, ΔDn is the density error of the reference pattern image obtained in step S24, and An is a coefficient indicating the correspondence between the laser power and the image density of the reference pattern image. This coefficient is obtained in advance by experiments or the like.

  Next, in step S26, the laser power set value is corrected by subtracting the correction amount ΔLP of the laser power obtained in step S25 from the laser power when the reference pattern image is formed. The laser power setting value obtained at this time is supplied to the ROS 3 as the laser power at the time of output image formation. With the above operation, one density error correction operation is completed, and then the process proceeds to step S7 in FIG. 8 to form an output image. As described above, the image forming position and the output image density are kept constant by periodically repeating the operation of correcting the positional deviation and the operation of correcting the density error.

  As described above, the image forming apparatus of the present invention has a configuration in which the first density detection mode or the second density detection mode can be selected in accordance with the position shift detection result. The first concentration detection mode is applied when the position shift or the position shift detection result is normal. The second density detection mode is applied when an abnormality occurs in the position shift or the position shift detection result. In the second density detection mode, the size, interval, number, and the like of the reference pattern image for density error detection are adjusted according to the degree of the position shift and the abnormality of the position shift detection result. Thereby, it is possible to prevent a decrease in density error detection accuracy.

  In the above-described embodiment, two types of density error detection reference pattern images are used. However, the present invention is not limited to this. For example, the length of the density error detection reference pattern image in the main scanning direction and / or according to the amount of positional deviation The interval may be varied to three or more types in proportion to the positional deviation amount. Further, either the main scanning direction size or the interval may be changed, and the number of density patterns may be two or more for each color.

  In the above embodiment, the density error detection reference pattern image is one type of halftone dot coverage: 60%, but this is merely an example, and the scope of application of the present invention is not limited to this embodiment. Absent. Halftone dot coverage: The value of Cin can be arbitrarily selected according to the image forming apparatus. Two or more types of reference pattern images having different Cin values may be formed. Also, the size of the reference pattern image is not limited to this embodiment, and the size may be changed in accordance with the assumed positional deviation amount of the image forming apparatus.

  The shape of the misregistration detection reference pattern image is not limited to the V-shaped reference pattern image. For example, a linear pattern extending in the main scanning direction is used, and the image density and the sub-scanning direction position are used. You may apply the reference pattern image which detects this. By using such a reference pattern image, the length of the reference pattern image in the sub-scanning direction can be further shortened. It is also possible to use other known reference pattern images. Furthermore, two or more types of adjustment modes such as coarse adjustment and fine adjustment may be provided, and the size of the reference pattern image may be switched according to the adjustment mode.

  In the above embodiment, the IOT includes a xerographic engine including a photoconductor, a contact charger, a ROS, and a developer for each of yellow, magenta, cyan, and black, and a toner image on the photoconductor is placed on the intermediate transfer belt. An example of a color electrophotographic system in which a primary transfer is performed and then transferred and fixed on a sheet has been described. However, the scope of application of the present invention is not limited to this embodiment. For example, instead of an intermediate transfer belt, a paper transport belt is used, and a toner image on a photoconductor is directly transferred onto a paper transported by a paper transport belt, or a belt-shaped or drum-shaped photoconductor The same effect can be exhibited even in an image forming apparatus in which each color toner image is superposed on the photosensitive member by a charger and an exposure device developer provided for each color. Alternatively, the same effect can be exhibited even with a black monochrome image forming apparatus. Further, the exposure apparatus is not limited to the ROS, and may use an LED array.

  In the above embodiment, the reference pattern image is formed on the intermediate transfer body belt. However, the reference pattern image may be formed on the photoreceptor, the paper transport belt, or the paper depending on the configuration of the image forming apparatus. . The photo sensor may be configured to use, for example, a CCD or a CMOS image sensor according to the formation location of the reference pattern image.

  In the above embodiment, both the positional deviation correction and the density error correction of the output image are performed. However, the present invention is not limited to this. For example, in a black monochromatic image forming apparatus, only the positional detection is performed, and the positional deviation is performed. Correction may not be performed.

  Furthermore, in the above embodiment, both the positional deviation and the density error are detected by using a single optical sensor (in this embodiment, a photodiode). However, the present invention is not limited to this. For example, the positional deviation detection and the density error are detected. The detection may be performed using another optical sensor. For example, it is possible to detect a positional deviation at both ends of the image forming area in the main scanning direction on the intermediate transfer belt and detect a density error at the center. In this case, it is preferable to obtain in advance the relationship between the position shift detection position and the density error detection position in the main scanning direction on the intermediate transfer belt.

  Furthermore, in the photosensor of the above embodiment, the detection of the positional deviation information and the density error information of the reference pattern image is shared by the same photosensor. However, the present invention is not limited to this, and the positional deviation information and the density error information are used. May be detected by separate photosensors. In the above embodiment, an example in which one photosensor is used has been described. However, this is merely an example, and the scope of application of the present invention is not limited to this embodiment. A photosensor may be arranged to correct magnification deviation in the main scanning direction, skew deviation (the image formed on the intermediate transfer belt having a certain inclination), and the like.

  Finally, in the above-described embodiment, an example in which the laser power setting value is used as the correction amount of the density error is shown. However, this is merely an example, and the scope of application of the present invention is not limited to this embodiment. Any correction amount may be used as long as the output image quality can be adjusted. For example, a charging voltage setting value, a developing bias setting value, a toner supply amount, a coefficient of an input image signal conversion table, or the like may be used. Further, a plurality of these operation amounts may be used for control.

1 is a schematic configuration diagram of an image forming apparatus to which a photosensor device of the present invention is applied. FIG. 2 is a functional block diagram of the image forming apparatus in FIG. 1. It is a top view of a reference pattern for image density control and image formation position adjustment. It is a schematic block diagram of a photosensor. It is a block diagram which shows the flow of the output signal from the photodiode in the detection information extraction part comprised from AMP, a peak detection circuit, an under-peak detection circuit, and two sample & hold circuits. It is a block diagram of a shutter. FIG. 6 is a diagram illustrating a positional relationship between a field of view of a photosensor and an image formation position adjustment reference pattern on an intermediate transfer belt, and changes in sensor output signals and peak detection signals at that time. It is a flowchart of the whole control operation. It is a flowchart of an image position adjustment operation. It is a timing chart at the time of measurement of a reference pattern for image position adjustment. It is a flowchart of an image density control operation. It is a timing chart at the time of measurement of a reference pattern for image density control.

Explanation of symbols

1Y, 1M, 1C, 1K ... photoreceptor, 2Y, 2M, 2C, 2K ... contact charger, 3Y, 3M, 3C, 3K ... ROS (laser output unit), 4Y, 4M, 4C, 4K: Developing unit, 5Y, 5M, 5C, 5K ... Primary transfer unit, 6 ... Intermediate transfer belt, 7 ... Secondary transfer unit, 8 ... Belt cleaner, 9 ... Fixing device, 10 ... photo sensor, 10a, 10b ... LED, 10c ... lens, 10d ... photodiode, 10e ... slit, 10f ... shutter, 10g ... window for measurement 10h: reference plate, 11: reference pattern image control unit, 12: detection information extraction unit, 100: control unit, M: reference pattern image for detecting displacement, D ₁ , D ₂ ... Reference pattern image for density error detection

Claims (5)

An image forming engine that forms a toner image according to an image data signal, a moving body on which toner images are multiplex-transferred from the image forming engine, and a reference pattern image that forms a reference pattern image for density error detection on the moving body A control unit; a photosensor that reads the reference pattern image; a detection information extraction unit that detects a density error from a signal output when the reference pattern image passes through the photosensor; A color image forming apparatus configured to execute a density detection mode at a predetermined timing.
In the first density detection mode, the reference pattern image control unit forms a density error detection reference pattern image of a predetermined size on the moving body at a predetermined interval, while in the second density detection mode, the reference pattern image control unit pattern image controller, wherein the first concentration detection mode to form the density error detecting reference pattern image at a different size and different intervals on the movable body, the reference pattern image control unit, the movable body A misalignment detection reference pattern image is formed upward, and the misalignment detection reference pattern image is detected by a detection information extraction unit from a signal output when the misalignment detection reference pattern image passes through the photosensor. A color image forming apparatus, wherein either the first density detection mode or the second density detection mode is selected according to the detection result .   The reference pattern image for density error detection in the second density detection mode is larger in size and wider than the reference pattern image for density error detection in the first density detection mode. The color image forming apparatus according to claim 1.   3. The color image forming apparatus according to claim 2, wherein the second density detection mode is applied after replacement of the image forming engine.   3. The color image forming apparatus according to claim 2, wherein the second density detection mode is applied immediately after the apparatus casing is opened and closed.   3. The color image forming apparatus according to claim 2, wherein the second density detection mode is applied immediately after the apparatus main power is turned on.

Images