JP2008107399A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more accurately detect a toner pattern on a transfer belt than before. <P>SOLUTION: A pattern formation control section causes image forming units by color to form toner patterns PT so that the densities of the leading and trailing ends are higher than that of the middle parts in the running direction of the transfer belt. This decreases the amounts of toner on the leading and trailing ends being edges of the toner patterns PT in the running direction of the transfer belt 136, thereby decreases the thicknesses of the edges of the toner patterns PT and hence reduces the thicknesses of side faces of the toner patterns PT. Consequently, even when the edges are irradiated with light from the light emitting part of a detection sensor 245, a quantity of light returned to a light receiving part after irregularly reflected by the side faces and the transfer belt 136 decreases. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for detecting a toner pattern formed on a transfer belt.

In a so-called tandem type image forming apparatus in which image forming units for the respective colors are arranged in the running direction of the transfer belt, there may be a deviation in the superposition timing of the toner images formed by the image forming units of the respective colors. For this reason, for example, in the image forming apparatus disclosed in Patent Document 1, a toner pattern predetermined for each color image forming unit is formed on the transfer belt in order to eliminate the deviation in the overlay timing of the toner images of the respective colors. The toner image formation timing of each color image forming unit is adjusted based on the positional relationship between the toner patterns detected by the toner pattern detection sensor. In the image forming apparatus described in Patent Document 1, the toner pattern detection sensor includes a light emitting unit and a light receiving unit, and the light is emitted from the light emitting unit to the toner pattern on the transfer belt. The amount of reflected light that changes according to the presence or absence of the toner pattern is received, and based on the change in the amount of reflected light, it is detected whether or not the toner pattern exists on the transfer belt.
JP-A-8-44134

  However, in the image forming apparatus described in Patent Document 1, when the light emitting portion of the toner pattern detection sensor irradiates the toner pattern on the transfer belt with light, as shown in FIG. At the edge portion of PT ′ (the tip portion in the running direction of the transfer belt 136 in the example shown in FIG. 4A), the light emitted from the light emitting portion 2451 is reflected by the edge portion (particularly the side surface of the toner pattern PT ′). The light is further reflected by the transfer belt, and the amount of received light increases due to the irregularly reflected light. As a result, in the output waveform indicating the amount of light received by the toner pattern detection sensor, a ripple appears in a portion indicating the edge portion (tip portion) of the toner pattern PT ′, and the output waveform may be distorted. The edge portion (tip portion) of PT ′ cannot be accurately detected, and a phenomenon in which the position of each toner pattern cannot be accurately detected by the toner pattern detection sensor may occur. For example, even if the output waveform is binarized using a predetermined threshold and the distance between the toner patterns PT ′ is calculated based on the binarized output waveform, the toner pattern PT ′ Since the edge portion (tip portion) cannot be detected accurately, the distance between the toner patterns of the color toner patterns PT ′ may not be detected accurately.

  The present invention has been made to solve the above-described problem, and an object of the present invention is to make it possible to detect a toner pattern on a transfer belt more accurately than in the past.

The invention according to claim 1 of the present invention includes a transfer belt in which a plurality of color toner images are sequentially transferred and transferred to form a color toner image on the surface;
An image forming unit arranged along the running direction of the transfer belt and provided for each color for forming a toner image on the transfer belt;
Pattern formation control means for forming a predetermined toner pattern on the image forming unit for each color;
A toner on the transfer belt having a light emitting portion for irradiating light on the toner pattern on the transfer belt and a light receiving portion for receiving reflected light of light emitted from the light emitting portion, and based on the amount of light received by the light receiving portion A toner pattern detecting means for detecting a pattern,
The pattern formation control unit is configured to apply the toner pattern to the image forming unit for each color, and the density of both or at least one of the leading end and the trailing end in the running direction of the transfer belt from the density in the central portion in the same direction. Is an image forming apparatus formed at a low density.

  According to this configuration, the pattern formation control unit applies the toner pattern to the image forming unit for each color, and the density of both or at least one of the front end part and the rear end part in the transfer belt running direction is lower than that of the central part. Therefore, the toner amount (toner particle amount) of both or at least one of the leading end portion and the trailing end portion serving as the edge portion of the toner pattern in the running direction of the transfer belt is reduced. For this reason, the edge portion of the toner pattern is formed with toner on the transfer belt, and the thickness of the side surface formed by the toner pattern edge portion is reduced. , The amount of light that is diffusely reflected by the side surface and the transfer belt and returned to the light receiving portion is reduced.

  The invention according to claim 2 is the image forming apparatus according to claim 1, wherein the pattern formation control unit causes the image forming unit for each color to shift the gradation in stages. The front and rear end portions of the toner pattern are formed with the low density.

  According to this configuration, the front and rear end portions of the toner pattern formed with the low density are formed by gradually changing the density, so that the front end portion that is the edge portion of the toner pattern in the transfer belt running direction In addition, the toner amount at the rear end portion decreases stepwise, and the thickness of the edge portion formed of toner on the transfer belt also decreases stepwise. For this reason, the edge portion is formed in a stepped shape when viewed from the side. Therefore, when light is emitted from the light emitting portion of the toner pattern detecting means, the amount of light returning to the light receiving portion after being irregularly reflected by the side surface of the toner pattern and the transfer belt is further increased. Less.

  Further, the invention according to claim 3 is the image forming apparatus according to claim 1 or 2, wherein the pattern formation control unit includes the toner pattern detection unit in the image forming unit for each color. The front and rear end portions of the toner pattern are formed with the same density as or larger than the detection spot diameter and the thin density.

  According to this configuration, the formation of the leading end portion and the trailing end portion of the toner pattern with the low density is set to be equal to or larger than the detection spot diameter of the toner pattern detection unit. The amount of light that is diffusely reflected by the side surface of the toner pattern and the transfer belt and returned to the light receiving portion can be further reduced.

  According to a fourth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to third aspects, wherein the pattern formation control means applies the light density to the image forming unit other than black. The toner pattern leading edge and trailing edge are formed.

  Since the black toner pattern absorbs light, the light emitted from the light emitting portion of the toner pattern detecting means does not diffusely reflect at the edge portion. Therefore, the pattern formation control means applies the above light to the image forming unit other than black. By forming the front and rear end portions of the toner pattern depending on the density, useless toner pattern formation processing can be avoided, and the conventional toner pattern can be used as the black toner pattern.

According to a fifth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to fourth aspects, wherein each of the colors is based on a detection result of each of the toner patterns by the toner pattern detection unit. Color misregistration detection means for detecting color misregistration in toner image formation by the image forming unit
The pattern formation control means is a gradation patch in which the density is gradually changed at the leading end portion and the trailing end portion of the toner pattern having a predetermined density in the transfer belt running direction in the image forming units for each color. Is added to form the toner pattern.

  According to this configuration, the pattern formation control means includes a gradation patch in which the gradation is gradually changed at the leading end portion and the trailing end portion of the toner pattern having a predetermined density in the image forming unit for each color. Since the toner pattern is formed by adding the light, the amount of light irradiated from the light emitting portion of the toner pattern detecting means is reflected by the side surface of the toner pattern and the transfer belt and returned to the light receiving portion is further reduced. Therefore, the edge portion of each toner pattern can be accurately detected, and the positional relationship between the toner patterns is accurately detected by the color misregistration detection means based on the position of the edge portion of the toner pattern indicated by the toner pattern detection means. Therefore, it is possible to accurately detect a color shift in toner image formation by the image forming unit.

  According to the first aspect of the present invention, the output waveform indicating the amount of light received by the toner pattern detection unit can be prevented from being distorted due to the influence of the irregular reflection. Therefore, the toner pattern detection unit can detect the position of each toner pattern. Can be detected with high accuracy.

  According to the second aspect of the present invention, it is possible to more reliably avoid a decrease in the output waveform indicating the amount of light received by the toner pattern detection unit from being distorted due to the influence of the irregular reflection, and each toner pattern is detected by the toner pattern detection unit. The position can be detected with higher accuracy.

  According to the third aspect of the present invention, it is possible to further reduce the amount of light that is irradiated from the light emitting portion of the toner pattern detection means and is returned to the light receiving portion after being irregularly reflected by the side surface of the toner pattern and the transfer belt.

  According to the fourth aspect of the present invention, it is possible to avoid a useless toner pattern forming process and use a conventional toner pattern as a black toner pattern.

  According to the fifth aspect of the present invention, the positional relationship between the toner patterns can be accurately detected by the color misregistration detection means based on the position of the edge portion of the toner pattern indicated by the toner pattern detection means. It is possible to accurately detect a color shift in toner image formation by the image unit.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram showing a printer 1 according to an embodiment of an image forming apparatus according to the present invention. The printer 1 is fed out from the paper feed cassette 120 by driving the paper feed unit 12 having a paper feed cassette 120 that can be inserted into and removed from the apparatus main body 11 in order to store the recording paper P. A secondary transfer roller 137 that transfers the toner image to the recording paper P while transporting the recording paper P, and a fixing process for the toner image is performed on the recording paper P to which the toner image has been transferred by the secondary transfer roller 137. Further, a paper discharge unit 15 is provided on the upper surface of the apparatus main body 11 to discharge the recording paper P that has been subjected to fixing processing by the fixing device 14.

  The paper feeding unit 12 is provided with a pickup roller 121 at the upper left position of the paper feeding cassette 120 shown in FIG. 1, and the recording paper P stored in the paper feeding cassette 120 is driven one by one by driving the pickup roller 121. The pickup is picked up and sent out to the secondary transfer roller 137.

  The transfer unit 13 includes an image forming unit 131, a transfer belt 136 onto which a toner image is transferred by the image forming unit 131, and a recording sheet on which the toner image on the transfer belt 136 is fed from the paper feed cassette 120. And a secondary transfer roller 137 for further transferring to P.

  The image forming unit 131 includes a magenta image forming unit 131M, a cyan image forming unit 131C, and a yellow image forming unit 131Y which are sequentially arranged from the upstream side (the right side of the sheet of FIG. 1) to the downstream side. And a black image forming unit 131K. Each of the image forming units 131M, 131C, 131Y, and 131K is positioned and attached to each device in the device main body 11 with a predetermined relative positional relationship.

  Each of the image forming units 131M, 131C, 131Y, and 131K is provided with a photosensitive drum 132 that is an image carrier, and a charger 134 is provided to face each of the photosensitive drums 132. An exposure device 135 is provided downstream of the charger 134 in the rotation direction of the photosensitive drum 132 (the arrow direction in FIG. 1). The exposure device 135 irradiates the peripheral surface of the photosensitive drum 132 uniformly charged by the charger 134 with laser light based on image data input from an image reading device (not shown). As a result, an electrostatic latent image is formed on the peripheral surface of each photosensitive drum 132. A developing device 133 is provided further downstream of the exposure device 135 in the rotation direction of the photosensitive drum 132. By supplying toner from the toner container of the developing device 133 to the electrostatic latent image, a toner image is formed on the peripheral surface of the photosensitive drum 132.

  Further, each photosensitive drum 132 is provided with a cleaning device 20 that removes residual toner on the circumferential surface of the photosensitive drum 132 and cleans it. The peripheral surface of the photosensitive drum 132 cleaned by the cleaning device 20 is directed to the charger 134 for a new charging process.

  The transfer belt 136 is stretched between the driving roller 136a and the driven roller 136b so that the surface thereof is in contact with the peripheral surface of the photosensitive drum 132 at a position immediately above the image forming units 131M, 131C, 131Y, and 131K. . A primary transfer roller 136 c is provided directly above each photosensitive drum 132 via a transfer belt 136.

  The respective toner images formed on the photosensitive drums 132 of the image forming units 131M, 131C, 131Y, and 131K according to the traveling speed of the transfer belt 136 driven by the drive roller 136a are transferred to the primary transfer roller 136c. As a result, they are superimposed on each other and transferred onto the transfer belt 136, and a color toner image is formed on the surface of the transfer belt 136.

  A registration roller (an example of a conveyance roller) 145 that conveys the recording paper P to the nip portion is provided in the recording paper conveyance path from the paper feeding cassette 120 toward the nip portion of the secondary transfer roller 137 and the driving roller 136a. Yes. The registration roller 145 sets the recording paper conveyance timing to the nip portion of the secondary transfer roller 137 and the driving roller 136a, and then conveys the recording paper to the nip portion. In this nip portion, the color toner image on the transfer belt 136 is transferred to the recording paper P by the secondary transfer roller 137, and a color toner image is formed on the recording paper P.

  A transfer belt cleaning device 138 is provided on the right side of the driven roller 136b in FIG. 1, and toner remaining on the surface of the transfer belt 136 after the transfer of the toner image onto the recording paper P is transferred to the transfer belt 136b. The transfer belt 136 removed by the cleaning device 138 and cleaned thereby is supplied to the photosensitive drum 132.

  The fixing device 14 performs a fixing process on the color toner image transferred to the recording paper P by the transfer unit 13. The fixing device 14 includes a heat roller 141 that is heated by an energized heating element, and a pressure roller 142 that is disposed so as to face the heat roller 141 and whose peripheral surface is pressed against the peripheral surface of the heat roller 141. . The color toner image transferred to the recording paper P by the secondary transfer roller 137 in the transfer unit 13 is fixed by heating when the recording paper P passes between the heat roller 141 and the pressure roller 142. After fixing, the paper is discharged to the paper discharge unit 15.

  Further, the transfer belt 136 is located at a position downstream of the image forming unit 131K at the most downstream position in the running direction and upstream of the nip portion between the drive roller 136a and the secondary transfer roller 137. A detection sensor (toner pattern detection means) 245 is provided to face the surface of the belt 136. The detection sensor 245 includes a reflective photosensor and the like, and includes a light emitting unit 2451 (FIG. 2) that irradiates light on the toner pattern on the transfer belt 136 and a light receiving unit that receives reflected light of the light emitted from the light emitting unit 2451. 2452 (FIG. 2), and the presence or absence of a toner pattern (details will be described later) on the transfer belt 136 is indicated by the amount of light received by the light receiving portion 2452. The detection sensor 245 detects a toner pattern formed on the transfer belt 136 by a predetermined image forming unit among the image forming units 131M, 131C, 131Y, and 131K. In this embodiment, the transfer belt 136 is configured to have a higher reflectance than the toner pattern.

  2 is a block diagram illustrating an example of a schematic configuration of the printer 1. FIG. 3 is a schematic side view illustrating a state in which the detection sensor 245 detects an edge portion of the toner pattern. FIG. 4A is a schematic side view illustrating conventional toner pattern detection, and FIG. 4B is a diagram illustrating an output waveform of the detection sensor 245 at the time of conventional toner pattern detection.

  The printer 1 includes a control unit (control means) 110 that controls the entire printer 1. The control unit 110 is connected to a ROM 112 that stores an operation program and the like of the entire apparatus, and is connected to a RAM 113 that temporarily stores image data and the like and functions as a work area. Further, the image forming units 131M, 131C, 131Y, and 131K for the respective colors are connected to the control unit 110, and the control unit 110 includes chargers provided in the image forming units 131M, 131C, 131Y, and 131K, respectively. 134, an exposure device 135, a developing device 133, a transfer bias portion 132 that applies a transfer bias to the primary transfer roller 136c in order to transfer the toner image on the photosensitive drum 132 to a recording sheet, and a photosensitive drum 132. The drum motor 115 which is a drive source is controlled.

  The control unit 110 also functions as a deviation amount calculation unit 111, a correction value calculation unit 112, a correction unit 113, and a pattern formation control unit 114. The shift amount calculation unit (color shift detection unit) 111 is based on the detection signal output from the detection sensor 245, and the color shift amount generated when the color toner images are superimposed by the image forming units 131M, 131C, 131Y, and 131K. Is detected. The deviation amount calculation unit 111 includes a timer 111a that performs various time measurements.

  Based on the deviation amount calculated by the deviation amount calculation unit 111, the correction value calculation unit 112 changes the recording sheet conveyance speed by the registration roller 145 to a speed suitable for the secondary transfer speed by the transfer belt 136 and the secondary transfer roller 137. A correction value for calculating The correction value calculation unit 112 has a storage unit 112a that stores respective correction values (values used for accelerating or decelerating the rotation speed of the registration motor 133) corresponding to the respective deviation amounts calculated by the deviation amount calculation unit 111. Prepare. The correction unit 113 corrects the conveyance speed of the recording paper P by the registration roller 145 using the correction value calculated by the correction value calculation unit 112.

  Further, the above-described detection sensor 245 (light emitting unit 2451 and light receiving unit 2452) is also connected to the control unit 110. The detection sensor 245 sends a detection signal indicating that the toner pattern has been detected to the control unit 110.

  The pattern formation control unit (pattern formation control means) 114 performs processing and control necessary for forming predetermined toner patterns in the image forming units 131M, 131C, 131Y, and 131K for each color. The predetermined toner pattern is, for example, a long toner image as shown in FIG. 3 and FIG. 6 to be described later, and the density of the front end portion and the rear end portion in the running direction of the transfer belt 136 is It is formed at a density lower than the density at the center in the direction.

  The leading and trailing ends of the toner pattern PT are controlled by the pattern formation control unit 114, and the gradation is gradually changed by the image forming units 131M, 131C, 131Y, and 131K for each color as shown in FIG. As a result, the thin concentration is formed. The amount of toner at the leading edge and the trailing edge, which are the edge portions of the toner pattern PT formed in this way, gradually decreases (since the density is expressed by the increase / decrease in the toner amount), transfer is performed. The thickness of the edge portion formed of toner on the belt 136 varies stepwise. That is, the thickness of the front end portion of the toner pattern becomes thinner toward the upstream side in the running direction of the transfer belt 136, and the thickness of the rear end portion of the toner pattern becomes thinner toward the downstream side in the running direction of the transfer belt 136. Further, the front and rear end portions of the toner pattern composed of the thin density portion are set to be equal to or larger than the detection spot diameter by the light receiving portion 2452 of the detection sensor 245.

  The detection sensor 245 is ideally disposed in the printer 1 in a posture in which the light emitted from the light emitting unit 2451 is orthogonal to the transfer belt 136 and the toner pattern PT. Often, it is inclined to the upstream or downstream side of the traveling direction of 136. In this situation, for example, as shown in FIG. 4A, in the case of the conventional toner pattern PT ′ in which the shape such as the front end portion and the rear end portion is not formed, the light emitting portion 2451 of the detection sensor 245 is the toner. When light is applied to the tip (edge) of the pattern PT ′, the light emitted from the light emitting unit 2451 is once irregularly reflected by the side surface of the toner pattern PT ′ and the transfer belt 136, and the amount of light incident on the light receiving unit 2452 increases. . In this case, as shown in FIG. 4B, a ripple occurs in the output waveform of the detection sensor 245 (light receiving unit 2452). This ripple is a process in which the output waveform of the detection sensor 245 shifts from the portion a indicating the amount of reflected light on the transfer belt 136 to the portion b indicating the amount of reflected light on the toner pattern PT ′ (particularly the edge portion of the toner pattern PT ′). ) Is likely to be an obstacle for determining the presence of the edge portion of the toner pattern.

  On the other hand, as shown in FIG. 3, in the case of the toner pattern PT formed in this embodiment, the front end portion and the rear end portion (edge portion) are stepped in a side view with a reduced toner amount. As a result, the light irradiated from the light emitting portion 2451 of the detection sensor 245 to the leading or trailing edge of the toner pattern can be reduced from being reflected on the transfer belt 136 from the side surface of the toner pattern PT toward the light receiving portion 2452. Thus, it is possible to avoid a situation in which the amount of light returning to the light receiving unit 2452 increases due to the irregular reflection as described above. 3 shows the case where the upper part of the detection sensor 245 is inclined in the traveling direction of the transfer belt 136, but the case where the upper part of the detection sensor 245 is inclined in the opposite direction to the traveling direction of the transfer belt 136. Even in this case, it is possible to avoid an increase in the amount of light received by the light receiving portion 2452 due to the irregular reflection due to the stepped shape formed as the rear end portion of the toner pattern PT.

For example, as shown in FIG. 3, the pattern formation control unit 114 has the above-described light density obtained by shifting the density stepwise to the toner patch PT _{1 for} which the deviation amount calculation unit 111 wants to accurately determine the position of the edge portion. by adding a gradation patch PT _2, the toner pattern PT of each color image forming units 131M, 131C, 131Y, is formed on the 131K. Note that the formation of the toner pattern PT according to the present invention is not limited to the formation. For example, the edge portion of the toner patch itself for which the position of the edge portion is to be accurately determined by the deviation amount calculation unit 111 may be a gradation patch having the above-mentioned light density in which the pattern formation control unit 114 has gradually changed the density. .

  Note that the pattern formation control unit 114 does not have to form the front end portion and the rear end portion having the low density in the image forming unit 131K. This is because black does not reflect light, so that the above-described irregular reflection does not occur, and the amount of light returning to the light receiving unit 2452 does not easily increase.

  The fixing motor 140 rotationally drives the heat roller 141 and the pressure roller 142, and is controlled by the control unit 110 via the driver 120a. A clock signal is sent from the control unit 110 to the driver 120a, and the fixing motor 140 rotates at a rotation speed corresponding to the frequency of the clock signal. Therefore, the rotation speed of the fixing motor 140 can be controlled by changing the frequency of the clock signal sent from the control unit 110 to the driver 120a. The fixing heater 141a is provided in the heat roller 141, and is turned on / off by the control unit 110. In FIG. 2, each image forming unit for magenta, cyan, yellow, and black is shown as one image forming unit. However, in actuality, each image forming unit is connected to the control unit 110 for each color. It is controlled.

  The transfer belt drive motor 125 is a drive source of a drive roller 136a that travels the transfer belt 136, and is controlled by the control unit 110 via the driver 125a. A clock signal is sent from the controller 110 to the driver 125a, and the transfer belt drive motor 125 rotates at a rotational speed corresponding to the frequency of the clock signal.

  The operation unit 127 includes an operation panel to which a user operation instruction from a user is input and a display unit that displays various messages. The control unit 110 is connected to a PC (personal computer) 130 via an interface 129. The printer 1 forms an image based on the image data input from the PC 130.

  The registration motor 133 rotates the registration roller 145 (FIG. 1), and is controlled by the control unit 110 via the driver 133a. A clock signal is sent from the control unit 110 to the driver 133a, and the registration motor 133 rotates at a rotational speed corresponding to the frequency of the clock signal. Therefore, the rotation speed of the registration motor 133 can be controlled by changing the frequency of the clock signal sent from the control unit 110 to the driver 125a.

  The secondary transfer motor 129 rotates the secondary transfer roller 137 (FIG. 1), and is controlled by the control unit 110 via the driver 129a. A clock signal is sent from the control unit 110 to the driver 129a, and the secondary transfer motor 129 rotates at a rotation speed corresponding to the frequency of the clock signal. Therefore, the rotational speed of the secondary transfer motor 129 can be controlled by changing the frequency of the clock signal sent from the control unit 110 to the driver 129a.

  Further, a secondary transfer bias unit 138 that applies a transfer bias to the secondary transfer roller 137 is connected to the control unit 110.

  Next, color misregistration correction processing when the color toner images are superimposed by the printer 1 having the above-described configuration will be described. FIG. 5 is a flowchart showing color misregistration correction processing when the color toner images are superimposed by the printer 1. 6 is a view of the transfer belt 136 and each of the image forming units 131M, 131C, 131Y, and 131K when viewed from below when the toner pattern is formed by the image forming unit, and FIG. 7 is a view showing the recording paper P to which the toner pattern is transferred. It is.

  In the printer 1, the toner images of the respective colors are overlaid due to the difference in speed between the secondary transfer speed (travel speed of the transfer belt 136) by the secondary transfer roller 137 and the drive roller 136 a and the conveyance speed of the recording paper P by the registration roller 145. Processing for correcting color misregistration at the time of alignment is performed.

  When an instruction to execute color misregistration correction is input from the operator to the operation unit 127 (YES in S1), the control unit 110 and the pattern formation control unit 114 transfer the image forming units 131M, 131C, 131Y, and 131K. An image of a toner pattern for color misregistration detection is formed on the transfer belt 136 in the image forming unit 131M arranged on the most upstream side in the running direction of the belt 136 and the image forming unit 131K arranged on the most downstream side. (S2). At this time, the pattern formation control unit 114 causes the image forming unit 131M and the image forming unit 131K to form the predetermined toner pattern, and the density of the front end portion and the rear end portion in the running direction of the transfer belt 136 is determined. It is formed at a density lower than the density at the center in the direction.

  As shown in FIG. 6, the control unit 110 and the pattern formation control unit 114 transfer the toner pattern formation to the image forming unit 131M and the image forming unit 131K by the first and second two on the transfer belt 136. Let each point do it. As a result, a toner pattern P1 composed of the respective toner patterns formed by the image forming unit 131M and the image forming unit 131K is formed at the first point, and the image forming unit 131M and the image forming unit 131K are also formed at the second point. A toner pattern P2 made up of the respective toner patterns formed by the above is formed. At this time, the control unit 110 and the pattern formation control unit 114 cause the toner patterns of each color to be formed on the transfer belt 136 at predetermined intervals in the image forming unit 131M and the image forming unit 131K. The toner pattern formation timing of each image forming unit is controlled.

  As shown in FIG. 7, the first point is a position on the transfer belt 136 that is in the vicinity of the leading end of the recording paper P when the toner pattern P1 is transferred to the recording paper P. The second point is a position on the transfer belt 136 that is in the vicinity of the rear end portion of the recording paper P when the toner pattern P2 is transferred to the recording paper P. Therefore, the correction process is preferably performed for each recording paper size used for printing.

  Subsequently, the control unit 110 causes the pickup roller 121 and the registration roller 145 to convey the recording paper P from the paper feed cassette 120 toward the nip portion of the secondary transfer roller 137 and the driving roller 136a (S3).

  After the toner pattern is formed, the detection sensor 245 detects each toner pattern on the traveling transfer belt 136, and sends a detection signal indicating that the toner pattern has been detected to the control unit 110 (S4). At this time, as described above, even if the light emitting portion 2451 of the detection sensor 245 irradiates the edge portion of each toner pattern, irregular reflection does not occur. Therefore, no ripple appears in the output waveform of the light receiving portion 2452, and the toner pattern accurately. The position of the toner pattern can be detected by detecting the edge portion.

  When the recording paper P conveyed by the pickup roller 121 and the registration roller 145 reaches the nip portion between the secondary transfer roller 137 and the drive roller 136a, the secondary transfer roller 137 controls the transfer belt 136 under the control of the control unit 110. The toner image is transferred onto the recording paper P (S5). Note that, at the timing when each toner pattern is formed at the first point on the transfer belt 136, the recording paper P has not arrived at the nip portion between the secondary transfer roller 137 and the drive roller 136a, but at the second point. At the timing when each of the toner patterns is formed, the recording paper P arrives at the nip portion between the secondary transfer roller 137 and the drive roller 136a, and secondary transfer is performed.

  Further, the deviation amount calculation unit 111 of the control unit 110 detects the position detected by the detection sensor 245 for each toner pattern at the first point based on the detection signal obtained from the detection sensor 245 by the built-in timer 111a. The time from when the detection sensor 245 detects the toner pattern (for example, magenta) reaching the position until the detection pattern 245 detects the toner pattern (for example, black) that has reached the detection position by the detection sensor 245 later is measured. . The deviation amount calculation unit 111 performs the time measurement by the timer 111a in the same manner for each toner pattern at the second point. In other words, the deviation amount calculation unit 111 determines the distance between the first and second toner patterns at the first and second points, and the detection sensor 245 detects the first toner pattern and then detects the first toner pattern. Acquired by time information until a toner pattern is detected. The deviation amount calculation unit 111 subtracts the time measurement result at the second point (indicating the distance between the two toner patterns) from the time measurement result at the first point (indicating the distance between the two toner patterns). The amount of increase in the distance between the toner patterns at the first and second points in the running direction of the transfer belt 136 between the actual toner pattern forming position and the ideal position that does not cause color misregistration in the superposition of the respective color toner images. The amount of deviation is calculated (S6).

  Subsequently, the correction value calculation unit 112 reads a correction value of the rotation speed of the registration roller 145 corresponding to the time difference (deviation amount) calculated by the deviation amount calculation unit 111 from the table of the storage unit 112a (S7). For example, when the time difference is 0, the correction value is 0. When the time difference is a negative value, the traveling speed of the transfer belt 136 is greater when the second point reaches the position than when the first point reaches the detection position of the detection sensor 245. In order to indicate that the speed is slow, the correction value is a value that increases the rotational speed of the registration roller 145 in order to increase the conveyance speed of the recording paper P by the registration roller 145 and match the traveling speed of the transfer belt 136. On the other hand, when the time difference is a positive value, the time when the second point reaches the position of the transfer belt 136 is more than the time when the first point reaches the detection position of the detection sensor 245. In order to indicate that the traveling speed is high, the correction value decreases the rotational speed of the registration roller 145 in order to reduce the conveyance speed of the recording paper P by the registration roller 145 and match the traveling speed of the transfer belt 136. Value. The correction value is provided separately according to the degree of the time difference.

  Then, the correction unit 113 of the control unit 110 changes the frequency of the clock signal sent to the driver 133a based on the correction value calculated by the correction value calculation unit 112, and changes the rotational speed of the registration motor 133 to the transfer belt. Correction is made in accordance with the traveling speed of 136 (S8).

  According to such color misregistration correction processing, irregular reflection occurs at the edge portion of the toner pattern even when the light emitting unit 2451 emits light to the toner patterns P1 and P2 when the detection sensor 245 detects the toner patterns P1 and P2. Therefore, since the amount of light received by the light receiving unit 2452 does not increase, no ripple appears in the output waveform of the light receiving unit 2452, and the position of the toner pattern can be detected accurately. Therefore, it is possible to accurately detect the amount of color misregistration at the time of superimposing the respective color toner images, and to improve the image formation quality.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. For example, in the above-described embodiment, the pattern formation control unit 114 is configured such that the front end portion and the rear end portion of the toner pattern PT change stepwise in a stepwise manner in a side view. The shape of the leading end portion and the trailing end portion of the toner pattern PT is not limited to this, and may be a density lower than the central portion in the running direction of the transfer belt 136 (the toner amount is small and the thickness of the toner image is thin). For example, the shape may not be stepped when viewed from the side, in which the gradation changes gradually.

  Further, in the above embodiment, the pattern formation control unit 114 forms the portion having the low density at both the front end portion and the rear end portion of the toner pattern PT. However, the present invention is not limited to this. Depending on the posture of the detection sensor 245, the pattern forming control unit 114 may form the thin portion only at the end necessary for avoiding the irregular reflection described above. For example, when the upper part of the detection sensor 245 is inclined in the running direction of the transfer belt 136, the pattern formation control unit 114 forms a portion having the low density at the tip of the toner pattern PT, and the detection sensor 245. Is inclined to the opposite side of the transfer belt 136 in the traveling direction, the pattern formation control unit 114 forms the portion having the low density at the rear end portion of the toner pattern PT. However, as long as the portion having the low density is formed on both the front end portion and the rear end portion of the toner pattern PT, the toner can be used regardless of the direction in which the upper portion of the detection sensor 245 is inclined. The irregular reflection at the edge of the pattern PT can be avoided.

  Further, in the above-described embodiment, when the color misregistration amount is detected at the time of superimposing the respective color toner images, the pattern formation control unit 114 forms the portions having the low density at the leading end portion and the trailing end portion of the toner pattern PT. However, the present invention is not limited to this, and when the density of the toner image is detected by the detection sensor 245, the pattern formation control unit 114 may form the toner pattern PT.

1 is an explanatory diagram illustrating a printer according to an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram illustrating an example of a schematic configuration of a printer. FIG. 6 is a schematic side view illustrating a state in which a detection sensor detects an edge portion of a toner pattern. (A) is a schematic side view showing conventional toner pattern detection, and (b) is a diagram showing an output waveform of a detection sensor at the time of conventional toner pattern detection. 6 is a flowchart illustrating a color misregistration correction process when each color toner image is superimposed by a printer. FIG. 4 is a diagram of a transfer belt and each image forming unit viewed from below when a toner pattern is formed by the image forming unit. FIG. 6 is a diagram illustrating a recording sheet on which a toner pattern is transferred.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer 110 Control part 111 Deviation amount calculation part 114 Pattern formation control part 131Y Yellow image formation unit 131C Cyan image formation unit 131K Black image formation unit 131M Magenta image formation unit 136 Transfer belt 245 Detection sensor 2451 Light emission part 2452 Light reception Part
PT toner pattern
PT ₁ toner patch
PT ₂ gradation patch

Claims (5)

A transfer belt in which a color toner image is formed on the surface by sequentially transferring a plurality of color toner images;
An image forming unit arranged along the running direction of the transfer belt and provided for each color for forming a toner image on the transfer belt;
Pattern formation control means for forming a predetermined toner pattern on the image forming unit for each color;
A toner on the transfer belt having a light emitting portion for irradiating light on the toner pattern on the transfer belt and a light receiving portion for receiving reflected light of light emitted from the light emitting portion, and based on the amount of light received by the light receiving portion A toner pattern detecting means for detecting a pattern,
The pattern formation control unit is configured to apply the toner pattern to the image forming unit for each color, the density of both or at least one of the front end and the rear end in the running direction of the transfer belt, based on the density at the center in the same direction. Is an image forming apparatus that can be formed with a low density.   2. The pattern forming control unit according to claim 1, wherein the image forming unit for each color causes the toner pattern leading edge and trailing edge to be formed with the thin density by shifting the density in steps. Image forming apparatus.   The pattern formation control unit forms the front and rear end portions of the toner pattern at the thin density and in the image forming unit for each color with a size equal to or greater than the detection spot diameter of the toner pattern detection unit. The image forming apparatus according to claim 1 or 2, wherein:   4. The image forming apparatus according to claim 1, wherein the pattern formation control unit causes the image forming unit other than black to form the front end portion and the rear end portion of the toner pattern with the low density. 5. . Based on the detection result of each toner pattern by the toner pattern detection means, color misregistration detection means for detecting a color misregistration of toner image formation by the image forming unit of each color,
The pattern formation control means is a gradation patch in which the density is gradually changed at the leading end portion and the trailing end portion of the toner pattern having a predetermined density in the transfer belt running direction in the image forming units for each color. The image forming apparatus according to claim 1, wherein the toner pattern is formed by adding a toner.

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