JP3542720B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides an image forming unit that forms an image with a developer of each color based on color image information for each color component, and a transfer that superimposes and transfers images of each color formed by the plurality of image forming units. Image forming apparatus such as a digital color copying machine, comprising: a pattern image forming unit for forming a predetermined pattern image by each of the image forming units; and a pattern image density measuring unit for measuring the density of the pattern image. In particular, the present invention faithfully reproduces a color image in which the images of the respective colors have no deviation when superimposed.
[0002]
[Prior art]
In a conventional image forming apparatus, for example, a digital color copying machine, after performing predetermined image processing on a color separation image of a document input from a scanner, a color image is output from a printer unit (image forming unit). Is going.
[0003]
In this digital color copying machine, a color document image that has been color-separated is recorded and reproduced for each color, and is superimposed on a recording medium to reproduce a color image.
At this time, when the color separation images of the respective colors are superimposed, if the color separation images of the respective colors are not accurately superimposed, a color shift of the image occurs, and the characteristics (image quality) of the image originally included in the color original image Will not be faithfully reproduced.
[0004]
Therefore, recently, color shift correction is periodically performed when the digital color copying machine is in a predetermined state so that images of respective colors are accurately overlapped on a recording medium, and an image expression close to a document image is output. There is a digital color copier.
[0005]
This color misregistration correction has been disclosed in Japanese Patent Publication No. Hei 7-19084 and Japanese Patent Laid-Open No. 6-238964.
[0006]
First, in Japanese Patent Publication No. Hei 7-19084, a measurement pattern image Bk, Y, M, and C for each color is formed on a transfer conveyance belt, and a measurement pattern image Bk for each color is formed. , Y, M, and C are measured by a reflection sensor. At this time, the image forming (image writing) timing in the image forming unit for each color is controlled by calculating the deviation from the set value of the measurement pattern images Bk, Y, M, and C for each color.
[0007]
As a method of calculating the deviation from the set value, the reflection type sensor measures the measurement pattern image Bk formed by the black image forming unit among the measurement pattern images Bk, Y, M, and C for each color. Then, by counting the timing signals until each of the other measurement pattern images Y, M, and C is measured, the deviation of the image forming (image writing) position in the image forming unit for each color is measured. I have.
[0008]
Japanese Unexamined Patent Application Publication No. 6-238954 discloses that one color or several dots of an arbitrary color in the vertical and horizontal directions (main scanning and sub scanning) are provided in a color image forming apparatus. A plurality of positioning pattern images sequentially shifted by dots are stored, and the pattern images are output from the image forming unit.
[0009]
If a group number of a timing at which a pattern image (line image) of another color matches a pattern image (line image) of an arbitrary color in the output pattern image is input, the input is performed. The image forming (image writing) timing of each color image forming unit is adjusted based on the set number.
[0010]
[Problems to be solved by the invention]
In the device disclosed in Japanese Patent Publication No. Hei 7-19084, a timing signal from when a reflective sensor measures a reference measurement pattern Bk to when each of the other measurement patterns Y, M, and C is measured is counted. Accordingly, the method of measuring the position (color) shift of image formation (image writing) of each color is difficult to accurately correct color shift due to the influence of drive unevenness in each drive unit (image formation unit and image transfer unit). There was a problem that is.
[0011]
Further, the structure disclosed in Japanese Patent Application Laid-Open No. 6-238954 has a configuration in which a pattern image formed by an image is confirmed by human eyes and the result is input to a person who has confirmed the result. Adjustment was not possible, and there were problems such as input errors. In addition, there is a problem that a sheet for determining the adjustment timing is required for each adjustment.
[0012]
The present invention has been made in view of the above-described problems, and it has been possible to accurately correct a color shift with a simple configuration by measuring the density of a superimposed pattern image and judging the superimposed state. It is intended for that purpose.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes an image forming unit that forms an image with a developer of each color based on color image information of each color component, and a plurality of image forming units. Transfer means for superimposing and transferring the images of the respective colors, a pattern image forming means for forming a predetermined pattern image by each of the image forming units, and a pattern image density measuring means for measuring the density of the pattern image. In the image forming apparatus, the pattern image is larger than the measurement range of the pattern image density measurement unit, and the line width at which the measured density value in the measurement range is substantially the same in any part of the pattern image. And a pattern image formed by a plurality of lines having a line interval and formed by an image forming unit serving as an internal reference of the image forming unit, and an image to be adjusted. Superposing a pattern image formed by the forming unit, of the overlapping pattern image density was measured by the pattern image density measuring means, by the measured density to determine the overlapping state of the pattern image, the measured density of a predetermined Control means is provided for controlling the image forming timing of the image forming section so as to obtain the density.
[0014]
In the pattern image according to the second aspect of the present invention, the line width and the line interval are set so that five or more lines exist within the measurement range of the pattern image density measuring means.
[0015]
According to the third aspect of the present invention, the width of the pattern image, which is an integral multiple of the line pitch composed of a plurality of line widths and line intervals, is set equal to the measurement range width of the pattern image density measuring means.
[0016]
The pattern image according to the fourth aspect of the present invention is formed by a plurality of lines, and the adjacent lines have different line widths.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of a digital color copying machine which is an image forming apparatus of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic sectional view showing the configuration of a digital color copying machine.
[0018]
A document table 111 and an operation panel (not shown) are provided on the upper surface of the copying machine main body 1 of the digital color copying machine, and an image reading section 110 and an image forming section 210 are provided inside the copying machine main body 1. Configuration.
[0019]
On the upper surface of the platen 111, a double-sided automatic document feeder (RADF: Recirculating Automatic Document Feeder) 112 is instructed so as to be openable and closable with respect to the platen 111, and has a predetermined positional relationship with the surface of the platen 111. It is installed.
[0020]
Further, the RADF 112 first conveys the document so that one side of the document faces the image reading unit 110 at a predetermined position on the document table 111, and after the image reading on one side is completed, the other side The document is turned over so as to face the image reading unit 110 at a predetermined position on the document table 111 and is conveyed toward the document table 111.
Then, the RADF 112 discharges this document after reading the image on both sides of one document, and executes the duplex conveyance operation for the next document.
The above-described document conveyance and reverse operation are controlled in relation to the operation of the entire copying machine.
[0021]
The image reading unit 110 is arranged below the document table 111 in order to read an image of a document conveyed onto the document table 111 by the RADF 112. The image reading unit 110 includes document scanning bodies 113 and 114 that reciprocate in parallel along the lower surface of the document table 111, an optical lens 115, and a CCD line sensor 116 that is a photoelectric conversion element.
[0022]
The document scanning bodies 113 and 114 are composed of a first scanning unit 113 and a second scanning unit 114. The first scanning unit 113 has an exposure lamp for exposing the surface of the original image, and a first mirror for deflecting a reflected light image from the original in a predetermined direction. , While reciprocating in parallel at a predetermined scanning speed while maintaining the above. The second scanning unit 114 has second and third mirrors for further deflecting the reflected light image from the document deflected by the first mirror of the first scanning unit 113 in a predetermined direction, and the first scanning unit It reciprocates in parallel with 113 while maintaining a constant speed relationship.
[0023]
The optical lens 115 reduces the reflected light image from the document deflected by the third mirror of the second scanning unit 114 and forms the reduced light image at a predetermined position on the CCD line sensor 116.
[0024]
The CCD line sensor 116 photoelectrically converts the formed light image in order and outputs it as an electric signal. The CCD line sensor 116 is a three-line color CCD that can read a black-and-white image or a color image, and output line data separated into R (red), G (green), and B (blue) color components. . The document image information converted into an electric signal by the CCD line sensor 116 is further transferred to an image processing unit (shown in FIG. 2) to be described later and subjected to predetermined image data processing.
[0025]
Next, the configuration of the image forming unit 210 and the configuration of each unit related to the image forming unit 210 will be described.
[0026]
Below the image forming unit 210, there is provided a paper feeding mechanism 211 that separates sheets (recording media) P stacked and stored in a paper feeding cassette one by one and supplies the separated paper (recording medium) P toward the image forming unit 210. . Then, the sheets P separated and supplied one by one are conveyed to the image forming unit 210 at a controlled timing by a pair of registration rollers 212 disposed in front of the image forming unit 210. Further, the sheet P on which an image is formed on one side is re-supplied and conveyed to the image forming unit 210 at the same time as the image forming by the image forming unit 210.
Below the image forming unit 210, a transfer and transport belt mechanism 213 is disposed. The transfer transport belt mechanism 213 transports the paper P by electrostatically attracting the paper P to a transfer transport belt 216 stretched so as to extend substantially parallel between the drive roller 214 and the driven roller 215.
Further, a pattern image density measurement unit is provided near the lower side of the transfer conveyance belt 216.
[0027]
Further, a fixing device 217 for fixing the toner image transferred and formed on the sheet P to the sheet P is disposed downstream of the transfer and conveyance belt mechanism 213 in the sheet conveyance path. The sheet P passing through the nip between the pair of fixing rollers of the fixing device 217 passes through a conveyance direction switching gate 218 and is discharged by a discharge roller 219 onto a discharge tray 220 attached to the outer wall of the copying machine main body 1. You.
[0028]
The switching gate 218 selectively sets a conveyance path of the sheet P after fixing between a path for discharging the sheet P to the copying machine main body 1 and a path for re-supplying the sheet P to the image forming unit 210. Switching.
The sheet P whose transport direction has been switched again toward the image forming unit 210 by the switching gate 218 is turned upside down via the switchback transport path 221, and is again supplied to the image forming unit 210.
[0029]
In addition, above the transfer conveyance belt 216 in the image forming section 210, the first image formation station Pa, the second image formation station Pb, the third image formation station Pc, and the fourth image are arranged close to the transfer conveyance belt 216. Forming stations Pd are arranged in order from the upstream side of the sheet transport path.
[0030]
The transfer conveyance belt 216 is frictionally driven by the drive roller 214 in the direction indicated by the arrow Z in FIG. 1 and carries the sheet P fed through the sheet feeding mechanism 211 as described above, and transfers the sheet P to the image forming stations Pa to. Conveyed sequentially to Pd.
[0031]
Each of the image forming stations Pa to Pd has substantially the same configuration. Each of the image forming stations Pa, Pb, Pc, and Pd includes a photosensitive drum 222a, 222b, 222c, and 222d that is driven to rotate in the direction of arrow F shown in FIG.
[0032]
Chargers 223a, 223b, 223c and 223d for uniformly charging the photosensitive drums 222a to 222d, respectively, and electrostatic latents formed on the photosensitive drums 222a to 222d are provided around the photosensitive drums 222a to 222d. Developing devices 224a, 224b, 224c, 224d for developing the respective images; transfer dischargers 225a, 225b, 225c, 225d for transferring the developed toner images on the photosensitive drums 222a to 222d to the paper P; Cleaning devices 226a, 226b, 226c, and 226d for removing toner remaining on the drums 222a to 222d are sequentially arranged along the rotation direction of the photosensitive drums 222a to 222d.
[0033]
Above the photosensitive drums 222a to 222d, laser beam scanner units 227a, 227b, 227c, and 227d are provided, respectively. The laser beam scanner units 227a to 227d include a semiconductor laser device (not shown) that emits dot light modulated according to image data, and a polygon mirror (not shown) for deflecting a laser beam from the semiconductor laser device in the main scanning direction. Deflecting device) 240, an fθ lens 241 for imaging the laser beam deflected by the polygon mirror 240 on the surfaces of the photosensitive drums 222a to 222d, and mirrors 242 and 243.
[0034]
The laser beam scanner unit 227a receives a pixel signal corresponding to the black component image of the color original image, the laser beam scanner unit 227b receives a pixel signal corresponding to the cyan component image of the color original image, and the laser beam scanner unit 227c. A pixel signal is input to the magenta color component image of the color original image, and a pixel signal corresponding to the yellow color component image of the color original image is input to the laser beam scanner unit 227d.
[0035]
As a result, an electrostatic latent image corresponding to the color-converted document image information is formed on each of the photosensitive drums 222a to 222d. The developing device 224a contains black toner, the developing device 224b contains cyan toner, the developing device 224c contains magenta toner, and the developing device 224d contains yellow toner. The electrostatic latent images on the photoconductive drums 222a to 222d are developed with the toners of these colors. As a result, the document image information color-converted by the image forming unit 210 is reproduced as a toner image of each color.
[0036]
Further, a paper suction charger 228 is provided between the first image forming station Pa and the paper feeding mechanism 211, and the paper suction charger 228 charges the surface of the transfer conveyance belt 216 to feed the paper. The sheet P supplied from the mechanism 211 is conveyed without shifting between the first image forming station Pa and the fourth image forming station Pd in a state where the sheet P is securely attracted onto the transfer conveying belt 216.
[0037]
On the other hand, a static eliminator 229 is provided almost directly above the drive roller 214 between the fourth image forming station Pd and the fixing device 217. An alternating current for separating the paper P electrostatically attracted to the transfer / transport belt 216 from the transfer / transport belt 216 is applied to the neutralizer 229.
[0038]
In the digital color copier having the above configuration, cut sheet-shaped paper is used as the paper P. When the sheet P is fed out of the sheet cassette and supplied into the guide of the sheet feeding path of the sheet feeding mechanism 211, the leading end of the sheet P is detected by a sensor (not shown). Are temporarily stopped by the pair of registration rollers 212 based on the detection signal output from the pair.
[0039]
Then, the paper P is sent onto the transfer conveyance belt 216 rotating in the direction of arrow Z in FIG. At this time, since the transfer / conveyance belt 216 is charged in a predetermined manner by the paper suction charger 228 as described above, the paper P is stably conveyed while passing through the image forming stations Pa to Pd. Supplied.
[0040]
In each of the image forming stations Pa to Pd, a toner image of each color is formed, and is superposed on the support surface of the paper P conveyed by being electrostatically attracted by the transfer conveyance belt 216. When the transfer of the image by the fourth image forming station Pd is completed, the sheet P is sequentially peeled off the transfer conveyance belt 216 from the leading end portion by the neutralizer 229, and is guided to the fixing device 217.
Finally, the paper P on which the toner image has been fixed is discharged onto a paper discharge tray 220 from a paper discharge port (not shown).
[0041]
(Explanation of transfer conveyance unit)
FIG. 2 is a cross-sectional view of a transfer and conveyance unit 216 including a transfer and conveyance belt 216 as a belt-shaped image forming medium having the configuration of the present invention and a pattern image density measurement unit.
[0042]
On the side opposite to the image forming unit side of the transfer / conveying belt 216 supported by the driving roller 214 and the driven roller 215, the back contact member 231 (231a, 231b), the pattern image density measurement sensor 232, and these components A pattern image density measurement unit 230 including a support frame 233 for supporting the positional relationship in a fixed state is provided.
The pattern image density measurement sensor 232 includes an LED as a light emitting element and a photodiode as a light receiving element.
[0043]
The pattern image density measurement unit 230 is provided with a spring 233 (233a, 233b) so as to apply a predetermined level of tension downward.
Further, the support frame 232 is provided with a long hole 234 (234a, 234b) so as to protrude from the frame 213a supporting the driving roller 214, the driven roller 215, and the like, which are components of the transfer / conveyance unit 213. And the pattern image density measuring unit 230 is displaceable downward.
[0044]
Further, springs 233 (233a, 233b) for applying a predetermined appropriate tension to the pattern image density measuring unit 230 are provided on both sides of the unit 230 under the same conditions, and are parallel to the elongated holes 234a, 234b. The transfer conveyance belt 216 is pressed down by the two rear pressing members 231a and 231b while maintaining the above state.
[0045]
As a result, a predetermined appropriate tension is always applied to the transfer / transport belt 216 with which the back contact member 231 is in contact, and the transfer / transport belt 216 of the linear portion facing the image forming unit side on which an image of each color is formed is formed. In addition, stable traveling without bending is enabled, and a flat (flat) stable region for measuring a pattern image formed on the transfer / conveying belt 216 is reliably formed.
[0046]
Then, a pattern image density measurement sensor 232 arranged in a predetermined positional relationship opposite to the flat stable area measures the pattern image density formed on the transfer conveyance belt 216. .
[0047]
Next, the measurement position of the pattern image formed on the transfer conveyance belt 216 by the pattern image density measurement sensor 232 will be described.
An eccentric motion due to rotation also occurs in the drive roller 214 for moving the transfer / transport belt 216 and appears as a predetermined periodic drive unevenness of the transfer / transport belt 216.
[0048]
In order to measure the pattern image at a fixed point (range) in the predetermined periodic drive unevenness, the transfer conveyance is started from the point where the pattern image is formed on the transfer conveyance belt 216. The distance L to the point at which the pattern image formed on the belt 216 is measured is set to be an integral multiple of the circumference l of the drive roller 214 as shown in FIG.
[0049]
(Description of formation and measurement of resist correction pattern image)
A registration correction pattern image for each color formed on the transfer conveyance belt 216 having the above-described configuration, a method of measuring the formed registration correction pattern image, and a registration correction method based on the measurement result will be described.
[0050]
(First Embodiment / First Pattern Image)
As shown in FIG. 3, the registration correction pattern image formed on the transfer conveyance belt 216 is composed of a plurality of lines parallel to the paper conveyance direction, that is, the traveling direction of the transfer conveyance belt 216. It has a configuration in which intervals of 10 dots are repeated. The operation of forming the resist correction pattern image will be described.
[0051]
The laser beam scanned by the deflecting device (porin rubber) 240 enters a beam detector sensor, and upon receiving light, outputs a beam detector signal. The beam detector sensor is provided in the laser beam scanner unit and is used for synchronizing the scanning position of the laser beam (generating a horizontal synchronization signal), and converts light into an electric signal. A counter counts a reference clock synchronized with the beam detector signal, and when the counter reaches a preset value (reference clock number N), an image formation start signal is output. When the image formation start signal is output, a pattern image signal is output in synchronization with the reference clock, and a latent image is formed on the photosensitive drum 222.
[0052]
First, a pattern image composed of the plurality of lines is formed on the photosensitive drum 222 by an image forming unit of a reference color component, and then transferred onto the transfer / conveying belt 216. A pattern image P1 having the same shape is formed by an image forming unit of a color component to be adjusted at a timing at which the pattern image P0 of the reference color component and the pattern image P0 of the reference are exactly overlapped on the transfer conveyance belt 216 (details will be described later).
[0053]
In the overlapping portion of the pattern image P0 of the reference color component and the pattern image P1 to be adjusted, the measurement area of the pattern image density measurement sensor 232 is, of course, when the two completely match and when the two are out of alignment. The concentration values measured in H are different.
[0054]
For example, when the transfer conveyance belt 216 is transparent, the smaller the displacement, the greater the amount of transmitted light and the lower the density value. In the case of the black transfer / transport belt 216, the smaller the displacement, the greater the amount of light absorbed and the higher the density value.
Further, when a pattern image is formed on a white transfer belt or paper, the smaller the shift, the more the reflection increases, and conversely, the lower the density value.
[0055]
In any case, the density value differs depending on the material, color, surface properties, and the like of the transfer / conveying belt, but in an ideal state where the pattern image P0 of the reference color component and the pattern image P1 of the color component to be adjusted exactly overlap. There is a density value. The density value is set to a predetermined value D0, and the recording start signal of the laser beam scanner 227 of the color component to be adjusted is delayed so that the measurement value of the pattern image density measurement sensor 232 falls within the allowable range of the predetermined value, or The color shift is corrected by advancing it earlier.
[0056]
As shown in the flowchart of FIG. 4, if the density value measured by the pattern image density measurement sensor 232 falls within the allowable range of the predetermined value D0, no color misregistration correction is necessary, and it must be within the allowable range of the predetermined value D0. For example, it is determined that a color shift has occurred, and the color shift is corrected.
[0057]
In this case, it is not possible to determine whether the recording start signal should be advanced or delayed, but the correction clock number Δn can be obtained from a preset table from the difference between the predetermined value D0 and the measured density value. After a delay clock number of the corrected clock number [Delta] n addition to the N + [Delta] n, and the pattern image by the image formation start signal is output, after the number of the delayed clock of the corrected clock number [Delta] n N-[Delta] n that subtract, imaging Two types of pattern images of which a start signal is output are formed. The color misregistration in the main scanning direction is corrected by measuring the density again by the pattern image density measuring sensor 232 and selecting the number of delay clocks that fall within the allowable range of the predetermined value D0 (FIG. 5).
[0058]
(1st Embodiment, 2nd pattern image)
As shown in FIG. 6, the registration correction pattern image formed on the transfer conveyance belt 216 includes a plurality of lines perpendicular to the paper conveyance direction, that is, the traveling direction of the transfer conveyance belt 216. The configuration is such that the line interval is repeated 10 dots.
[0059]
A description will be given below with Bk as a reference color component and C as a color component to be adjusted. Assuming that the length from the exposure position to the transfer position on the photosensitive drum 222 and the peripheral speed of the photosensitive drum are all the same value, the time required from exposure to transfer is the same.
[0060]
In order to exactly overlap the pattern image on the transfer / conveyance belt 216, it is necessary to delay the exposure of C of the color component to be adjusted by the moving time of the distance between the photosensitive drums 222. Assuming that the distance between the photosensitive drums 222 is L (mm) and the speed of the transfer / transport belt 216 is V (mm / sec), the pattern image signal generation timing is based on Bk.
T = L / V (sec)
It only needs to be delayed.
[0061]
In the overlapping part of the pattern image P0 of the reference color component and the pattern image P1 to be adjusted, the density measured by the pattern image density measurement sensor 232 is a matter of course when the two completely match or when they are shifted. The values are different. If the density value measured by the pattern image density measurement sensor 232 falls within the allowable range of the predetermined value D0, no color shift correction is necessary, and if it does not fall within the allowable range of the predetermined value D0, a color shift has occurred. Is determined, and color shift correction is performed.
[0062]
In this case, it is not possible to determine whether the image formation start signal should be advanced or delayed, but the correction time Δt can be obtained from a preset table from the difference between the predetermined value D0 and the measured density value. Two types of pattern images, a pattern image that outputs an image formation start signal with a delay time T + Δt obtained by adding the correction time Δt and a pattern image that outputs an image formation start signal with a delay time T−Δt obtained by subtracting the correction time Δt, are used. Form. The density is measured again by the pattern image density measuring sensor 232, and the color shift in the sub-scanning direction can be corrected by selecting the delay time that falls within the allowable range of the predetermined value D0.
[0063]
(Relation between the measurement area of the pattern image density measurement sensor 232 and the pattern image)
Here, the relationship between the measurement area H of the pattern image density measurement sensor 232 and the pattern image will be described in detail. Although the measurement area H of the pattern image density measurement sensor 232 is described as a rectangle in order to simplify the calculation, the same effect as described later can be obtained even if the measurement area H is a circle or another shape.
[0064]
As shown in FIG. 7A, if the pattern image is smaller than the measurement area H of the pattern image density measurement sensor 232, the change in the density measurement value due to the color shift is small and the color shift cannot be detected. FIG. 7B shows the relationship between the measurement area H and the pattern image size of the pattern image density measurement sensor 232 according to the present invention. It can be captured as a change in the measured concentration value. FIGS. 7C and 7D show the case where the measurement area H of the pattern image density measurement sensor 232 and the size of the pattern image are substantially equal. When the position of the measurement area H of the pattern image density measurement sensor 232 and the position of the pattern image slightly deviate due to some influence as shown in FIG. 7C, the density measurement value in FIG. Also, as shown in FIG. 7C, a reversal phenomenon may occur in which the density measured value is reduced despite the occurrence of color misregistration.
[0065]
Therefore, in order to avoid such a problem, it is necessary to make the size of the pattern image sufficiently larger than the measurement area H of the pattern image density measurement sensor 232.
[0066]
Next, regarding the line width and the line interval of the pattern image, it is necessary that the measured values have substantially the same density value even if the measurement position is slightly shifted due to some influence.
As shown in FIGS. 8A and 8B, if the number of lines of the pattern image in the measurement area H of the pattern image density measurement sensor 232 is small, the measured value greatly changes depending on the measurement position, and the accuracy is high. (A measured density value in FIG. 8A is low, and a measured density value in FIG. 8B is high).
Thus, FIG. 9 shows a result obtained by simulating the relationship between the number of lines in the measurement area H of the pattern image density measurement sensor 232 and the density variation, assuming that the line width and the line interval are the same.
The horizontal axis is the number of lines in the measurement area H, and the vertical axis is the normalized density value. The upper line connects the density MAX values, and the lower line connects the density MIN values. As the number of lines in the measurement area H of the pattern image density measurement sensor 232 increases, the difference between the density MIN value and the density MIN value decreases. When the number of lines in the measurement area H exceeds 5, the difference is 8% or less. , Which is within the allowable range in the color misregistration correction. further. If the number exceeds 8, the difference becomes 5% or less, and highly accurate color shift correction can be performed.
[0067]
Further, as can be seen from FIG. 9, there are some points where the difference between the density MAX value and the density MIN value is reduced even if the number of lines in the measurement area H is small. This is because when the width W of the measurement area H is equal to the integral multiple of the line pitch P (line width + line interval) as shown in FIG. 10, which part of the pattern image as shown in FIGS. Is measured, the concentrations are equal.
[0068]
Accordingly, since the size of the measurement area H of the pattern image density measurement sensor 232 is known in advance, the line pitch of the pattern image is set to an integral number of the width of the measurement area H, and the number of lines in the measurement area is set to 5 or more. This enables more accurate color misregistration correction.
[0069]
In the method described above, when the pattern image P1 to be adjusted with respect to the reference pattern image P0 is shifted by exactly one line pitch as shown in FIG. The value thus obtained becomes the minimum, and it is erroneously determined that no color shift has occurred.
[0070]
As shown in FIGS. 11B and 11C, by making the widths of adjacent lines different from each other in a plurality of lines constituting the pattern image, it is possible to prevent erroneous determination that no color shift has occurred. be able to.
Even in the case where there is a shift by one line pitch as shown in FIG. 11B, the measurement value by the pattern image density measurement sensor 232 does not become the minimum, and only when there is no shift as shown in FIG. The value is minimal.
[0071]
In FIG. 11, a pattern image composed of two types of line widths has been described. However, a similar effect can be obtained with a pattern image composed of various types of line widths.
[0072]
【The invention's effect】
According to the first aspect of the present invention, a plurality of lines having a line width and a line interval which are larger than the measurement range of the pattern image density measuring means and whose measured density values in the measurement range are substantially the same in any part in the pattern image. And a pattern image formed by the image forming unit serving as an internal reference of the image forming unit and a pattern image formed by the image forming unit to be adjusted. The density of the image is measured by the pattern image density measuring means, the overlapping state of the pattern images is determined based on the measured density, and the image forming timing of the image forming unit is controlled so that the measured density becomes a predetermined density. With a simple configuration using only a pattern image density measuring means for measuring density, regardless of aging or environmental changes, Since the pattern image located within the measurement range of is under the same conditions at any position, there is no measurement error due to the measurement position, so that accurate density measurement can be performed and color misregistration is reliably corrected. be able to.
[0073]
According to the second aspect of the present invention, the pattern image is formed by setting the line width and the line interval so that five or more lines exist within the measurement range of the pattern image density measuring means. The difference from the minimum measured density value is reduced, so that accurate density measurement can be performed, and more accurate correction of color misregistration can be performed.
[0074]
According to the third aspect of the present invention, the width of the pattern image, which is an integral multiple of the line pitch consisting of a plurality of line widths and line intervals, is set equal to the measurement range width of the pattern image density measuring means. The pattern image located within the measurement range of the measuring means has the same number of lines at any position, so that there is no measurement error due to the measurement position, accurate density measurement can be performed, and color misregistration can be ensured. Correction can be performed.
[0075]
According to the fourth aspect of the present invention, since a pattern is formed by forming a plurality of lines and changing the line width of adjacent lines, when the pattern images are overlapped with a shift of exactly one line pitch, the same line is formed. In the case of a pattern image formed with a width, the lines overlap at a portion deviating from the pattern image, and a color shift cannot be detected. According to the present invention, even if there is a one line pitch shift, the color shift can be accurately detected, and the color shift can be corrected without fail.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing an embodiment of a digital color copying machine according to the present invention.
FIG. 2 is a sectional view of the transfer / conveyance unit of FIG. 1;
FIG. 3 is an explanatory diagram showing a first pattern image for performing color shift correction in the main scanning direction in the digital color copying machine according to the first embodiment of the present invention.
FIG. 4 is a flowchart of color misregistration correction in the embodiment of the digital color copying machine of the present invention.
FIG. 5 is an explanatory diagram showing control signals of each unit during the operation of FIG. 4;
FIG. 6 is an explanatory diagram showing a second pattern image for performing color shift correction in the sub-scanning direction in the first embodiment of the digital color copying machine of the present invention.
FIG. 7 is an explanatory diagram showing a relationship between a measurement area of a pattern image density measurement sensor and a size of a pattern image.
FIG. 8 is an explanatory diagram showing a relationship between a measurement position by a pattern image density measurement sensor and a pattern image.
FIG. 9 is an explanatory diagram showing a relationship between the number of lines of the pattern image in the measurement area of the pattern image density measurement sensor and the density variation.
FIG. 10 is an explanatory diagram showing a relationship between a measurement area width W of a pattern image density measurement sensor and a line pitch P (line width + line interval) of a pattern image.
FIG. 11 is an explanatory diagram showing a pattern image in which adjacent line widths are different.
[Explanation of symbols]
210 Image Forming Unit 216 Transfer Conveying Belt 222 Photoconductor Drum 224 Developing Device 225 Transfer Discharger 232 Pattern Image Density Measurement Sensor

Claims (4)

An image forming unit that forms an image with a developer of each color based on color image information for each color component, a transfer unit that overlaps and transfers images of each color formed by the plurality of image forming units, In an image forming apparatus including a pattern image forming unit for forming a predetermined pattern image by each image forming unit, and a pattern image density measuring unit for measuring the density of the pattern image, the pattern image is greater than the measurement range of the density measuring device, formed from a plurality of lines measured density value in the measurement range under the line width and line spacing substantially the same density value at any part of the pattern image, the image forming The pattern image formed by the image forming unit serving as the reference of the unit is superimposed on the pattern image formed by the image forming unit to be adjusted The concentration of the overlapping pattern image measured by the pattern image density measuring means, to determine the status overlapping pattern images by the measured concentration, the image forming timing of the image forming unit so that the measured concentration of a predetermined concentration An image forming apparatus comprising a control unit for controlling. 2. The image forming apparatus according to claim 1, wherein a line width and a line interval of the pattern image are set so that five or more lines exist within a measurement range of the pattern image density measurement unit. 2. The image forming apparatus according to claim 1, wherein a width obtained by multiplying a line pitch consisting of a plurality of line widths and line intervals of the pattern image by an integer is set equal to a measurement range width of the pattern image density measuring means. apparatus. 2. The image forming apparatus according to claim 1, wherein the pattern image is formed by a plurality of lines, and adjacent lines have different line widths.

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