JP2005156877A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent color unevenness and color shift by accurately controlling an intermediate transfer belt to a target speed even if the intermediate transfer belt extends or a conveyance system for the belt expands owing to heat. <P>SOLUTION: The color-matching-time conveying speed of the intermediate transfer belt 12 after color matching operation for correcting shifts of toner images of respective colors formed on photoreceptor drums 10 for the respective colors in the respective colors at a primary transfer position on the intermediate transfer belt 12 is stored in a RAM of a control unit 50, and the conveying speed in the color matching operation is compared with the conveying speed of the intermediate transfer belt 12 detected in prescribed timing after the color matching operation; when there is a speed difference between the both, the conveying speed of the intermediate transfer belt 12 detected in the prescribed timing is corrected by the speed difference to control the conveying speed of the intermediate transfer belt 12 to the conveying speed in the color matching operation. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus that primarily transfers toner images respectively formed on a plurality of image carriers in an overlapping state on an intermediate transfer belt, and collectively transfers the synthesized image onto a recording medium. The present invention relates to an image forming apparatus that detects a mark formed thereon with a sensor and controls an intermediate transfer belt at a constant speed based on the information.

  For example, in an electrophotographic image forming apparatus such as a copying machine or a printer capable of forming a full-color image, a plurality of image bearing members (for example, a drum type) and a latent image formed on the photosensitive member are provided. A development unit that develops toner images of the respective colors, and sequentially transfers the toner images of different colors formed on the respective photoconductors in an overlapping state on the intermediate transfer belt, There is an indirect transfer type in which secondary transfer is performed collectively on a recording medium.

  In such an indirect transfer type image forming apparatus using an intermediate transfer belt, the running unevenness of each photosensitive member and intermediate transfer belt that form images of different colors, and further, the position of the outer peripheral surface of the photosensitive drum and the intermediate transfer belt If the positional relationship between the toner images of the respective colors varies at the primary transfer position due to a deviation from the movement amount of the toner, they do not match when the toner images of the respective colors are superimposed on the intermediate transfer belt, resulting in a color deviation. It will end up.

Therefore, in a conventional indirect transfer type image forming apparatus, for example, a color misregistration detection pattern is formed for each color on an intermediate transfer belt, and the pattern is detected by an optical sensor (toner image). The amount of misregistration is detected, and the toner images of the respective colors are adjusted to match on the intermediate transfer belt according to the misregistration amount.
As a measure for preventing color misregistration of the toner image on the intermediate transfer belt, for example, one method is to keep the conveyance speed of the intermediate transfer belt constant.

However, in general, an intermediate transfer belt is an endless belt that is stretched between a driving roller and a driven roller with a predetermined tension and rotated by the rotation of the driving roller. When expanded due to heat, even if the rotational speed of the driving roller is kept constant, the conveyance speed (belt speed) of the intermediate transfer belt is increased by the increase in the roller diameter due to expansion, resulting in uneven conveyance.
Further, for example, two marks are formed on the belt at intervals along the moving direction of the belt, and the conveyance speed is determined by the time from detection of the mark by one sensor to detection by the other sensor. In this case, when the temperature in the apparatus rises, the intermediate transfer belt also extends due to heat, and the extension also spreads between the two marks, so that the accurate belt conveyance speed cannot be detected. . Therefore, unevenness occurs in the conveyance speed.

If such unevenness occurs, the toner image that was primarily transferred to the intermediate transfer belt during image formation will be misaligned. Therefore, if a color image using multiple colors is formed, the color misregistration will result in a composite color image in which each color is superimposed. Occurs. Therefore, color irregularity and registration error occur in the recorded image that is secondarily transferred to the recording medium, so that the image quality is deteriorated.
Therefore, in an indirect transfer type image forming apparatus using a conventional intermediate transfer belt, two sensors are arranged at intervals in the belt moving direction, and one of these sensors detects a mark. There is one in which the time until the other sensor detects the mark is measured, the conveyance speed of the intermediate transfer belt is detected from that time, and the conveyance speed is controlled to be constant (for example, Patent Documents). 1).
Japanese Patent No. 3344614

However, even the one described in Patent Document 1 has a problem in that the mounting distance between the two sensors varies due to an assembly error, and the error causes unevenness in the speed of the intermediate transfer belt. It was.
The present invention has been made in view of the above-described problems. Even if the intermediate transfer belt is stretched by heat or its transport system is expanded, the intermediate transfer belt is accurately controlled to a target speed. An object of the present invention is to prevent color unevenness and color misregistration from occurring in a color image formed on a recording medium.

In order to achieve the above object, the present invention provides a plurality of image carriers, a plurality of developing units for developing latent images formed on the respective image carriers into toner images of different colors, and toners of different colors. A rotating intermediate transfer belt on which images are primarily transferred in a superimposed state, two sensors arranged at a fixed distance to detect marks provided on the intermediate transfer belt, and one of the sensors Measures the time from when the mark is detected until the other sensor detects the same mark, and detects the transfer speed of the intermediate transfer belt from that time, and the transfer speed of the intermediate transfer belt becomes constant. In the image forming apparatus provided with the conveyance speed control means for controlling as follows:
The transport speed control means includes means for storing a transport speed at the time of color matching of the intermediate transfer belt after performing a color matching operation for correcting a shift at a primary transfer position of a plurality of color toner images. The stored transport speed at the time of color matching is compared with the transport speed of the intermediate transfer belt detected at a predetermined timing after the color matching operation, and when there is a speed difference between them, the speed difference is equal to the predetermined timing. The intermediate transfer belt conveyance speed detected in step (1) is corrected to control the intermediate transfer belt so that the conveyance speed of the intermediate transfer belt is equal to the color matching conveyance speed.

In the image forming apparatus, the time from when one of the sensors detects the mark until the other sensor detects the same mark is measured a plurality of times, and the intermediate transfer belt is conveyed from the average value. It is good to ask for speed.
The time measurement by the sensor is preferably performed indefinitely while the intermediate transfer belt is rotating.
Also, a plurality of the marks are formed on the intermediate transfer belt along the rotation direction of the belt until one of the sensors detects the mark and the other of the sensors detects the same mark. Is measured for each of a plurality of marks, and the conveyance speed of the intermediate transfer belt is obtained from the average value.

The sensor may be a reflective optical sensor, and the time may be measured by detecting an edge portion of the mark with a beam with a reduced beam spot diameter.
The light receiving element of the optical sensor may be a photodiode.
At least one of the sensors may be disposed in the vicinity of a driving roller that rotates the intermediate transfer belt.
The distance between the two sensors may be an integral multiple of the distance between the development units for each color.

  According to the present invention, the conveyance speed at the time of color matching is compared with the conveyance speed of the intermediate transfer belt detected at a predetermined timing after the color matching operation. Since the conveyance speed of the belt is corrected so that the conveyance speed becomes the conveyance speed at the time of color matching, even if the intermediate transfer belt is extended by heat or the conveyance system that conveys the intermediate transfer belt expands, Since it is possible to accurately control the conveyance speed when the colors of the intermediate transfer belt are matched, it is possible to prevent color unevenness and color misregistration from occurring in the color image formed on the recording medium. In addition, it is possible to avoid the influence of variations in the mounting interval between the two sensors.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic diagram showing an intermediate transfer belt of an image forming apparatus according to the present invention and a control system for controlling the conveyance speed, and FIG. 2 is a schematic configuration diagram showing a full-color laser printer which is also the image forming apparatus.
The full-color laser printer that is the image forming apparatus of FIG. 2 includes four photosensitive drums 10Y, 10C, 10M, and 10K (hereinafter, simply referred to as the photosensitive drum 10 unless otherwise specified), A plurality of developing units 11Y, 11C, 11M, and 11K that respectively develop latent images formed on the photoconductive drums 10Y, 10C, 10M, and 10K into toner images of different colors (hereinafter simply referred to as the developing unit 11 if not specified). And an intermediate transfer belt 12 that rotates in the direction indicated by the arrow A, in which toner images of different colors are primarily transferred in a superimposed state.

  Further, two sensors 14A and 14B arranged at a fixed distance in the belt moving direction for detecting the mark 13 (only one is shown in FIG. 2) provided on the intermediate transfer belt 12, and the sensor One sensor 14A detects the mark 13 until the other sensor 14B detects the same mark 13, and from that time, the conveyance speed of the intermediate transfer belt 12 is detected. A control device 50 functioning as a transport speed control means for controlling the transport speed of 12 to be constant is also provided.

  The control device 50 performs a color matching operation for correcting the deviation of the toner images of the respective colors formed on the respective photosensitive drums 10Y, 10C, 10M, and 10K at the primary transfer position on the intermediate transfer belt 12 with different colors. 1 has a RAM 51 (FIG. 1) serving as means for storing the color transfer speed at which the intermediate transfer belt 12 is stored, and the color transfer speed stored in the RAM 51 and a predetermined timing after the color matching operation. The intermediate transfer belt 12 is compared with the conveyance speed of the intermediate transfer belt 12, and if there is a speed difference between them, the intermediate transfer belt 12 is corrected by correcting the conveyance speed of the intermediate transfer belt 12 detected at the predetermined timing. Is controlled so that the transport speed is equal to the transport speed during color matching.

As shown in FIG. 1, a plurality of marks 13 are provided on the back surface (inner surface) of the intermediate transfer belt 12 over the entire circumference of the belt at predetermined intervals.
The two sensors 14A and 14B are similar sensors, for example, emit measurement light to the mark 13, receive the reflected light, and output a pulsed detection signal (see FIG. 3). Use optical sensors.
The intermediate transfer belt 12 is an endless belt. In this embodiment, as shown in FIG. 2, the intermediate transfer belt 12 is disposed on the lower side of the intermediate transfer belt 12 along the rotation direction (arrow A direction). The four photosensitive drums 10Y, 10C, 10M and 10K for the respective colors of yellow, cyan, magenta and black are arranged in parallel.

Around each photosensitive drum 10, a charging device (not shown), the developing unit 11, the primary transfer roller 17 constituting the primary transfer device, a cleaning unit (not shown), and the like are arranged. .
Further, the charging surface of each photosensitive drum 10 is charged with light corresponding to each color of yellow, cyan, magenta, and black from the exposure device 16 to form a latent image there. It is like that.

A primary transfer roller 17 is disposed opposite to each of the photosensitive drums 10Y, 10C, 10M, and 10K, and the intermediate transfer belt 12 is sandwiched between each primary transfer roller 17 and the photosensitive drum 10. It is designed to rotate.
The intermediate transfer belt 12 is supported by a drive roller 21, a driven roller 22, and a steering roller 23 that serve as support rollers, and the drive roller 21 is rotated by the drive motor 5 to rotate in the direction of arrow A. A secondary transfer roller 25 is disposed at a position facing the driving roller 21 with the intermediate transfer belt 12 interposed therebetween.

In this laser printer, when the image forming operation is started, each photosensitive drum 10 starts to rotate in the clockwise direction in FIG. 2, the surface thereof is uniformly charged by the charging device, and each charged surface is exposed. The device 16 emits light corresponding to each color image of yellow, cyan, magenta, and black, and a latent image is formed there.
The latent images are developed by the developing units 11Y, 11C, 11M, and 11K, and become toner images of colors of yellow, cyan, magenta, and black.

The toner images of the respective colors are accurately transferred to the intermediate transfer belt 12 rotating in the direction indicated by the arrow A by the primary transfer rollers 17 so as to be superposed on the intermediate transfer belt 12. A composite color image (toner image) is formed.
On the other hand, a transfer sheet P, which is a recording medium, is fed at a predetermined timing from a sheet feeding unit 1 disposed below the photosensitive drum 10, and is transferred between the driving roller 21 and the secondary transfer roller 25. When fed, the composite color image carried on the intermediate transfer belt 12 is secondarily transferred collectively onto the transfer paper P by the secondary transfer roller 25. The toner image on the transfer paper P is fixed by the fixing unit 18 and discharged onto a paper discharge tray (not shown).

Thus, this laser printer forms a composite toner image on the intermediate transfer belt 12 when two or more color images are formed. Therefore, if a shift occurs in the position of each color image to be synthesized, a color shift or color unevenness occurs.
For this reason, in this laser printer, a color matching operation for correcting a shift of the toner images of the respective colors formed in different colors on the photosensitive drums 10Y, 10C, 10M, and 10K at the primary transfer position on the intermediate transfer belt 12 is performed. This is performed prior to the regular image forming operation.

In the color matching operation, for example, a misregistration detection pattern (four color toner images) is formed on the intermediate transfer belt 12, and the misregistration amounts of the toner images of the respective colors are detected so that they are accurately matched. Further, it is performed by adjusting the light emission timing of the light source corresponding to each color of the exposure device 16.
By the way, this laser printer measures the time from when the mark 13 provided on the intermediate transfer belt 12 is detected by one sensor 14A until the other sensor 14B detects the mark 13 as described above. From that time, the conveyance speed (belt speed) of the intermediate transfer belt 12 is detected, and the conveyance speed of the intermediate transfer belt 12 is controlled to be constant.

Therefore, when the conveyance speed of the intermediate transfer belt 12 fluctuates, toner images of different colors superimposed on the intermediate transfer belt 12 cause color misregistration. Since the conveyance speed can be kept constant, it is possible to prevent color misregistration.
However, the conveyance speed of the intermediate transfer belt 12 is likely to fluctuate for the following reasons.

That is, the drive roller 21 for driving the intermediate transfer belt 12 is generally used in which a rubber roller portion is formed on the outside of an iron core from the viewpoint of transferability (this is also used in this embodiment). Since the driving roller 21 is close to the fixing unit 18 as shown in FIG. 2, the roller portion expands due to heat from the fixing unit 18 and the diameter of the rubber roller increases.
In such a case, even if the rotation speed of the driving roller 21 is the same, the conveyance speed of the intermediate transfer belt 12 is increased by the increase in the roller diameter. For this reason, when the toner images on the respective photosensitive drums 10 are primarily transferred onto the intermediate transfer belt 12 in an overlapping state in order, the toner images are actually faster than the predetermined conveying speed by the intermediate transfer belt 12. Only a color shift will occur.

In the case of a full-color image in which four color toner images are overlaid, the color misregistration becomes maximum between yellow and black of the photosensitive drums 10Y and 10K that are in the most distant positions, and the amount of expansion of the roller diameter of the drive roller 21 The color shift is × π × 3 (number between drums).
Therefore, in this laser printer, the two sensors 14A and 14B are provided as described above, and from when one sensor 14A of the sensor detects the mark 13 to when the other sensor 14B detects the same mark 13. The time is measured, and the conveyance speed of the intermediate transfer belt 12 is detected from that time. By doing so, even if the roller diameter of the driving roller 21 is increased, it is possible to accurately detect the conveyance speed of the intermediate transfer belt 12 that has been accelerated by the increase. Therefore, the conveyance speed of the intermediate transfer belt 12 can be controlled to be constant.

Thereby, a plurality of color toner images can be accurately matched on the intermediate transfer belt 12 to prevent color misregistration.
The method for obtaining the conveyance speed of the intermediate transfer belt 12 will be described in detail later. The time from when the sensor 14A detects the same mark 13 to when the sensor 14B detects the mark 13 is described as follows. The distance is calculated from the distance from the sensor 14A to the sensor 14B (in this embodiment, the distance between the developing units is the same).

  In this embodiment, as described above, the transport speed of the intermediate transfer belt 12 is detected using the two sensors 14A and 14B. Here, the conveyance speed of the intermediate transfer belt 12 is determined by the time when two marks provided at intervals in the moving direction on the intermediate transfer belt 12 are detected by one sensor (optical sensor) (between marks). (Passing time).

However, when the belt conveyance speed is detected by one sensor in this way, for example, when the drive roller 21 shown in FIG. As the roller diameter of the roller 21 increases, the belt conveyance speed increases, and the intermediate transfer belt 12 extends and becomes longer.
Therefore, in this case, since the distance between two marks provided on the intermediate transfer belt 12 with an interval in the moving direction becomes longer, the actual belt conveying speed is increased, As a result, the passage time between the two marks is detected almost the same, and the increase in the conveyance speed due to the expansion of the drive roller 21 cannot be detected accurately.

Therefore, in this embodiment, even if the intermediate transfer belt 12 is extended by detecting the same mark 13 using the two sensors 14A and 14B as described above, the conveying speed of the belt is increased. Can be detected accurately.
The control device 50 described above detects the conveyance speed of the intermediate transfer belt 12 using the sensors 14A and 14B.

That is, when the OR circuit 52 inputs a pulse-like detection signal obtained by detecting the mark 13 from any of the sensors 14A and 14B shown in FIG. 1 at a predetermined timing after the color matching operation, the control device 50 receives the sensor 14A. The time from when the mark 13 is detected until the sensor 14B detects the same mark 13 is measured by the timer of the counter unit 53.
Next, the calculation unit 54 calculates the conveyance speed of the intermediate transfer belt 12 from the measured time, and the conveyance speed and the conveyance speed at the time of color matching stored in the RAM 51 (more precisely, the intermediate transfer immediately after the color matching is performed). Belt 12 conveyance speed) and a speed difference (relative speed difference) between them is calculated.

When there is a speed difference, the motor correction unit 55 corrects the drive frequency of the drive motor 5 by the speed difference, and the motor drive unit 56 controls the drive motor 5 with the corrected drive frequency. Therefore, even if the driving roller 21 expands due to the temperature rise at the predetermined timing and the intermediate transfer belt 12 extends in the transport direction and becomes longer, the correction causes the intermediate transfer belt 12 to accurately detect the color toner images. Since it is driven at the conveyance speed at the time of color matching overlapping at various positions, no color shift occurs in the formed color image.
Further, if the conveyance speed of the intermediate transfer belt 12 is controlled as described above, it is not affected by variations in the two sensors 14A and 14B themselves, or variations in the mounting interval between the two sensors. Errors can be eliminated. Thereby, color misregistration can be suppressed.

By the way, in this embodiment, the time measured using the above-described sensors 14A and 14B is measured a plurality of times, and the conveyance speed of the intermediate transfer belt 12 is obtained from the average value.
Therefore, a detection error due to meandering that moves in a direction perpendicular to the conveyance direction of the intermediate transfer belt 12, a detection error due to uneven conveyance due to fluctuations in the thickness of the belt, a detection error due to behavioral behavior such as belt deflection, etc. It is possible to minimize unsteady detection errors. As a result, color misregistration between the toner images of the respective colors caused by the detection error can be suppressed.

The time measurement by the sensors 14A and 14B is performed indefinitely while the intermediate transfer belt 12 is rotating. As a result, the conveyance speed of the intermediate transfer belt 12 can be controlled at all times, and color misregistration of the color image can be prevented over time.
Furthermore, in this embodiment, as described above, a plurality of marks 13 are formed on the intermediate transfer belt 12 along the rotational direction of the belt, and the sensor 14B detects a single mark 13 and then the sensor 14B The time until the same mark 13 is detected is measured for every plurality of marks 13, and the time is measured using all the marks 13 over the entire circumference of the intermediate transfer belt 12. The conveyance speed of the intermediate transfer belt 12 is obtained from the average value.

Accordingly, since the steady mechanical fluctuations of the developing unit 11 and the intermediate transfer belt 12 provided for each color are made constant, the detection error can be averaged, and as a result, the detection error depends on the detection error. Color shift can be suppressed.
In this embodiment, one sensor 14A is arranged in the vicinity of the driving roller 21 that rotates the intermediate transfer belt 12. As a result, the vibration (flutter) of the intermediate transfer belt 12 is reduced in the vicinity of the drive roller 21, so that an unsteady detection error due to the vibration of the belt can be reduced, and detection accuracy is improved. The sensor 14B may also be arranged in the vicinity of the drive roller 21.

  The distance between the two sensors 14A and 14B is preferably an integral multiple of the distance between the developing units 11 for each color (in this embodiment, it is set to 1). By doing so, the sensor always detects the mark 13 at the same point even with respect to periodic fluctuations between the developing units 11, so that detection errors due to steady mechanical fluctuations can be eliminated. Therefore, color misregistration caused by the detection error can be suppressed.

Next, the conveyance speed V of the intermediate transfer belt 12 obtained by calculation will be described with reference to FIG.
FIG. 3 is a bottom view showing a part of the inner surface of the intermediate transfer belt 12. When the conveyance speed V of the intermediate transfer belt 12 is detected, first, one of the plurality of marks 13 formed on the intermediate transfer belt 12 is detected by the sensor 14A. When the edge of the mark 13 is applied to the beam spot Bs ₁ of the sensor 14A during the detection, the sensor 14A outputs a pulse wave as shown in the figure. The pulse wave is output until the beam spot Bs ₁ passes through the mark 13.

Then, when the mark 13 detected by the sensor 14A moves to a position where the sensor 14B is located due to the movement of the intermediate transfer belt 12 in the direction of arrow A (the position indicated by the broken line), this time, the beam spot of the sensor 14B is moved to the mark 13. When the edge of Bs ₂ is applied, the sensor 14B outputs a pulse wave as illustrated.
Then, the detection signal output from the sensor 14A and the detection signal output from the sensor 14B (respectively pulse waves) are OR-connected, and as a result, a detection signal as shown in FIG. 3 is formed. The time t between the pulses of the two detection signals is the time for the same mark 13 to pass between the beam spots of the sensors 14A and 14B. Further, since the two sensors 14A and 14B are arranged at a predetermined interval L (in this embodiment, an integral multiple of the distance between the developing units 11), the intermediate transfer can be performed by calculating L / t. The conveyance speed V of the belt 12 can be obtained.

The beam spot diameters of the sensors 14A and 14B are preferably made as narrow as possible so that the edge portion of the mark 13 can be detected as described above.
By doing so, when the mark 13 on the intermediate transfer belt 12 must be detected by a reflective sensor because it is not a transparent material such as polyimide, the intermediate transfer belt 12 is perpendicular to the transport direction. The belt moves in the direction of the belt, or uneven speed fluctuation occurs due to the unevenness of the belt due to the fluctuation of the belt thickness, or the behavior of the belt due to the deflection of the belt. Even if an unsteady speed fluctuation occurs, the change in waveform can be reduced.

That is, if both the beam spot diameters of the sensors 14A and 14B are reduced, the rising / falling of the photocurrent that occurs when the beam spot is located at the edge of the mark 13 is accelerated, so that the rising / falling waveform changes. Can be made as small as possible. As a result, detection errors can be minimized, and color misregistration due to detection errors can be prevented.
The light receiving elements of the sensors 14A and 14B, which are optical sensors, may be photodiodes. By doing so, the reaction speed with respect to the edge change of the mark 13 is increased, and the rise / fall of the photocurrent can be further accelerated, so that the detection error can be reduced and the color shift can be prevented.

In this printer, as described above, the mark 13 provided on the intermediate transfer belt 12 is detected by the two sensors 14A and 14B, and the conveyance speed of the intermediate transfer belt 12 is detected based on the detected information. The intermediate transfer belt 12 is controlled so that the conveyance speed is constant, and the control is performed by the control device 50 shown in FIGS.
The control device 50 includes a central processing unit (CPU) having various determination and processing functions, a ROM storing each processing program and fixed data, and a RAM (RAM 51 in FIG. 1) which is a data memory storing processing data. And a microcomputer comprising an input / output circuit (I / O).

  The microcomputer included in the control device 50 starts the belt speed control process shown in FIG. 4 at a predetermined timing. That is, when the processing of FIG. 4 starts, first, the drive motor 5 is started in step 1 to rotate the intermediate transfer belt 12. In step 2, the counter unit 53 counts between pulses of the detection signals output from the sensors 14A and 14B by the method described with reference to FIG. 3, and measures the time tn (n = n + 1).

In the next step 3, using the measured time tn, the speed Vn of the intermediate transfer belt 12 at the time of measurement is calculated from the distance L between the sensors 14A and 14B as described with reference to FIG. Calculate.
In step 4, it is determined whether n = N times, that is, whether n has reached N, which is the number of marks 13 provided on the circumference of the intermediate transfer belt 12, and if n has not yet reached N times. Returning to step 2, the processing of step 2 and step 3 is repeated, and when n = N times, the process proceeds to step 5.

In step 5, Vav = V _n +... + V _N / N is calculated by the calculation unit 54. Here, N is the number of marks 13 provided on one circumference of the intermediate transfer belt 12 as described above, and therefore the average speed Vav during one round (one rotation) of the intermediate transfer belt 12 is obtained.
In the next step 6, whether or not the average speed Vav obtained in step 5 is the transport speed that matches the transport speed at the time of color matching (the transport speed at the time of color matching) is the time of color matching stored in the RAM. Judgment is made by comparing with the conveyance speed. If it is determined that the average speed Vav measured at a predetermined timing matches the conveyance speed at the time of color matching (YES determination), the process proceeds to step 7 where Vav is temporarily stored in the RAM 51 as Vav ₀ .

If it is determined in step 6 that the average speed Vav does not coincide with the conveyance speed at the time of color matching, the process proceeds to step 8, and the speed difference between the average speed Vav and the conveyance speed at the time of color matching (matches Vav ₀ ) ( Relative speed difference) ΔV is calculated by the calculation unit 54.
In the next step 9, the drive frequency of the drive motor 5 is corrected by the motor correction unit 55 according to ΔV, and the motor drive unit 56 drives the drive motor 5 at the corrected drive frequency. Therefore, the conveyance speed of the intermediate transfer belt 12 can always be kept constant at the color matching conveyance speed. Note that ΔV increases and decreases when the roller diameter of the drive roller 21 expands or contracts due to a temperature change.

  In step 10, it is determined whether or not an instruction to continue driving the drive motor 5 has been issued. If an instruction to continue driving has been issued (determination of YES), the process returns to step 2 and the subsequent steps are performed. Repeat processing and judgment. If it is determined in step 10 that an instruction to continue driving the drive motor 5 has not been issued, the drive of the drive motor 5 is stopped in step 11 and this routine is terminated.

As described above, the laser printer according to this embodiment matches the conveyance speed (Vav ₀₎ at the time of color matching of the intermediate transfer belt 12 immediately after performing the color matching operation for correcting the deviation of the four color toner images at the primary transfer position. ) Is stored in the RAM 51, and the color-conveying conveyance speed stored in the RAM 51 is compared with the conveyance speed Vav of the intermediate transfer belt 12 detected at a predetermined timing after the color-matching operation. When there is a speed difference ΔV, the transport speed Vav of the intermediate transfer belt 12 detected at the predetermined timing is corrected by the speed difference, and the transport speed of the intermediate transfer belt 12 becomes the transport speed (Vav ₀ ) during color matching. Control to be.

In this way, when the conveyance speed Vav of the intermediate transfer belt 12 is measured at a predetermined timing after the color matching operation (it can be arbitrarily set and the number of times can be set arbitrarily), the conveyance speed Vav is measured. Even if the toner changes due to expansion of the driving roller 21 that drives the intermediate transfer belt 12, the change (corresponding to the speed difference ΔV) is corrected, and the toner image of four colors is accurately transferred on the belt. Therefore, the color image does not cause color misregistration.
Since this printer controls the conveyance speed of the intermediate transfer belt 12 by performing the above-described control, for example, the distance L between the sensors 14A and 14B shown in FIG. Even so, the variation can be canceled by the correction described above, so that no color shift occurs in the color image.

  The present invention can be widely applied to an image forming apparatus that forms a composite color image by superimposing different color toner images on an intermediate transfer member such as an intermediate transfer belt.

FIG. 2 is a schematic diagram illustrating an intermediate transfer belt of the image forming apparatus according to the present invention and a control system for controlling the conveyance speed. 2 is a schematic configuration diagram showing a full-color laser printer that is also the image forming apparatus. FIG. FIG. 5 is a bottom view showing a part of the belt and a sensor in order to explain the conveyance speed of the intermediate transfer belt that is calculated by the full-color laser printer. FIG. 3 is a flowchart showing a belt speed control process routine performed by a control device included in the full-color laser printer of FIG. 2.

Explanation of symbols

10Y, 10C, 10M, 10K: photosensitive drum (image carrier), 11Y, 11C, 11M, 11K: development unit, 12: intermediate transfer belt, 13: mark, 14A, 14B: sensor, 17: primary transfer roller, 21: Drive roller, 50: Control device (conveying speed control means), 51: RAM

Claims (8)

A plurality of image carriers, a plurality of developing units for developing the latent images formed on the image carriers into different color toner images, respectively, and the different color toner images are primarily transferred in a superimposed state. A rotating intermediate transfer belt, two sensors arranged at a certain distance for detecting a mark provided on the intermediate transfer belt, and one of the sensors after detecting the mark, the sensor The conveyance speed control means for measuring the time until the other of the two detects the same mark, detecting the conveyance speed of the intermediate transfer belt from the time, and controlling the conveyance speed of the intermediate transfer belt to be constant In an image forming apparatus comprising:
The transport speed control means has means for storing a transport speed at the time of color matching of the intermediate transfer belt after performing a color matching operation for correcting a shift of the toner images of the plurality of colors at the primary transfer position; The conveyance speed at the time of color matching stored in the means is compared with the conveyance speed of the intermediate transfer belt detected at a predetermined timing after the color matching operation. If there is a speed difference between them, only the speed difference is detected. An image forming apparatus, comprising: means for correcting the conveyance speed of the intermediate transfer belt detected at the predetermined timing and controlling the conveyance speed of the intermediate transfer belt to be the conveyance speed at the time of color matching.   The time from when one of the sensors detects the mark until the other of the sensors detects the same mark is measured a plurality of times, and the conveyance speed of the intermediate transfer belt is obtained from the average value. The image forming apparatus according to claim 1, wherein:   The image forming apparatus according to claim 1, wherein the time measurement by the sensor is performed indefinitely while the intermediate transfer belt is rotating.   A plurality of the marks are formed on the intermediate transfer belt along the rotation direction of the belt, and from when one of the sensors detects the mark until the other of the sensors detects the same mark. 2. The image forming apparatus according to claim 1, wherein time is measured for each of the plurality of marks, and the conveyance speed of the intermediate transfer belt is obtained from an average value thereof.   5. The sensor according to claim 1, wherein the sensor is a reflective optical sensor, and the time is measured by detecting an edge portion of the mark with a beam with a reduced beam spot diameter. The image forming apparatus according to claim 1.   6. The image forming apparatus according to claim 5, wherein the light receiving element of the optical sensor is a photodiode.   The image forming apparatus according to claim 1, wherein at least one of the sensors is disposed in the vicinity of a driving roller that rotates the intermediate transfer belt. The image forming apparatus according to claim 1, wherein the distance between the two sensors is an integral multiple of the distance between the developing units for each color.

Images