JP2010181538A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus using a transfer belt having a large coefficient of friction between the transfer belt and a photoreceptor, which prevents a significant color shift even in continuous image formation. <P>SOLUTION: The color printer 1 includes: a drive motor 33 which rotationally drives the intermediate transfer belt 11; a torque meter 34 which measures a drive torque value of the drive motor 33; and a control section 35 which corrects the rotating speed of the drive motor 33 based on the drive torque value obtained by the torque meter 34. In the case the obtained value is positive, the control section 35 reduces the rotating speed of the drive motor 33, In the case the obtained drive torque value is negative, the control section 35 increases the rotating speed of the drive motor 33. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of forming a color image, such as a printer or a copier. More specifically, the present invention relates to an image forming apparatus having a transfer belt that is moved while being in contact with a plurality of photoconductors and onto which toner images are transferred.

  2. Description of the Related Art Conventionally, for example, a so-called tandem image forming apparatus has a transfer belt that contacts a photoconductor of each color and receives a toner image of each color. For example, the transfer belt is stretched around a plurality of rollers or the like, and rotated by their rotational driving. Then, the toner images of the respective colors formed on the surfaces of the photoreceptors of the respective colors are transferred so as to be superimposed on the toner images up to the previous color moving on the transfer belt. Therefore, in order to form a color image without color misregistration, the toner image up to the previous color may face the photoconductor at a desired timing so that the toner image of the current color is arranged at the same position. Desired.

  Conventionally, in a belt member such as this transfer belt, the speed is detected by various methods in order to set the traveling speed at least at a position facing the photoconductor to a desired speed. For example, there is a method for detecting the speed of the belt by forming a mark for speed measurement on the belt or by previously forming a mark on the belt and detecting the position of the mark (for example, patents). Reference 1, Reference 2). Based on the belt speed detected by these methods, a desired speed can be obtained by controlling so as to suppress periodic rotational fluctuations due to eccentricity of the rotating body, unevenness of the belt thickness, and the like.

  As a conventional transfer belt, for example, a synthetic resin endless belt or the like is often used. In such a case, at least the outer surface in contact with the photoreceptor is made of a resin having a relatively small friction coefficient. On the other hand, in recent years, it has been studied to use a transfer belt having an elastic layer such as rubber on the outer surface. Such an elastic belt has high flexibility of the belt itself and excellent adhesion to a recording material such as paper. Therefore, there is an advantage that it is less likely to drop out during the primary transfer as compared with a conventional resin belt. Furthermore, transfer efficiency is better at the time of secondary transfer than a belt made of synthetic resin.

Japanese Patent No. 3186610 JP 2001-16883 A

  However, in a transfer belt having an elastic layer, it is not sufficient to control the running speed by various conventional speed detection methods. This is because the frictional force between the photosensitive member and the photosensitive member is larger than that of the conventional resin belt because of the elastic layer. Usually, when there is a relative speed difference between the surface of the photoreceptor and the transfer belt that are in contact with each other, slip occurs between them. In a resin belt with a small coefficient of friction, small slips frequently occur, so misalignment due to slip is not a problem. However, in the case of an elastic belt having a large coefficient of friction and high flexibility, small slips are unlikely to occur. For this reason, the bending of the belt gradually accumulates, and a relatively large slip may occur when the deflection accumulates. When such a large slip occurs, there is a risk that a relatively large color shift may occur at that point.

  Furthermore, it has been found that the speed of the transfer belt may slightly change during image formation. This is considered to be due to a slight change in the outer diameter of the drive roller that drives the transfer belt due to, for example, temperature change in the machine or double-sided printing. Even if the relative speed difference between the surface of the photosensitive member and the transfer belt is adjusted in advance so as to be as small as possible, the speed difference naturally occurs if the speed of the transfer belt changes. If the speed difference occurs, there is a problem that the belt bends and causes color shift.

  The present invention has been made to solve the problems of the conventional image forming apparatus described above. That is, an object of the present invention is to provide an image forming apparatus that does not cause a large color shift in an image forming apparatus that uses a transfer belt having a large coefficient of friction with a photoconductor.

  An image forming apparatus of the present invention, which has been made for the purpose of solving this problem, is an image forming unit that forms a toner image on a rotationally driven photoconductor, and is rotationally driven while being in contact with the photoconductor. An image forming apparatus having a transfer belt onto which a toner image is transferred, a drive unit that rotationally drives the transfer belt, a torque measurement unit that measures a drive torque value of the drive unit, and a drive acquired by the torque measurement unit A rotation speed correction unit that corrects the rotation speed of the drive unit based on the torque value, and the rotation speed correction unit, when the acquired drive torque value is a positive value, When the acquired drive torque value is a negative value, the rotational speed of the drive unit is increased.

  According to the image forming apparatus of the present invention, the drive torque value of the drive unit that rotationally drives the transfer belt is corrected so as to approach zero. The drive torque value is close to 0 when the relative speed difference between the photosensitive member and the transfer belt is the smallest overall. That is, it is the state where the bending of the transfer belt is minimized. By approaching this state, the occurrence of a large slip is prevented. Therefore, even in an image forming apparatus using a transfer belt having a large coefficient of friction with the photosensitive member, a large color shift is not generated.

Further, in the present invention, it is desirable that the torque measuring unit obtains an average value of the driving torque value as the measured value while the transfer belt rotates a plurality of times.
In this way, for example, variations in measured values due to roller eccentricity can be leveled.

Furthermore, in the present invention, the rotational speed correction unit has a second value that is less than a first value that is a predetermined positive value and that is a predetermined negative value. If it is above, no correction is made, and if the drive torque value is greater than or equal to the first value and less than a predetermined third value greater than the first value, whether or not image formation is in progress. Regardless of whether the image is being formed or not, if the driving torque value is less than the second value and greater than or equal to a predetermined fourth value that is smaller than the second value, the correction is performed. If the drive torque value is greater than or equal to the third value or less than the fourth value, the image formation is temporarily stopped, the rotational speed of the drive unit is corrected, and the image formation is resumed after the correction. Is desirable.
In this way, since a small amount of correction is performed even during image formation, productivity is high. In addition, since the large amount of correction is performed after the image formation is temporarily stopped, the image being formed is not affected.

Furthermore, in the present invention, the rotational speed correction unit has a second value that is less than a first value that is a predetermined positive value and that is a predetermined negative value. If it is above, no correction is made, and if the drive torque value is greater than or equal to the first value and less than a predetermined third value greater than the first value, whether or not image formation is in progress. Regardless, the predetermined minimum correction amount is corrected, and the image is being formed if the drive torque value is less than the second value and greater than or equal to a predetermined fourth value that is smaller than the second value. Regardless of whether or not the predetermined minimum correction amount is corrected, and the drive torque value is greater than or equal to the third value or less than the fourth value, the minimum correction amount is set without stopping image formation. It is desirable to repeat the correction a plurality of times.
In this way, the correction can be performed during the image formation without affecting the image being formed, so that the productivity is high.

Further, in the present invention, it is desirable that the rotation speed correction unit does not perform correction when the toner area ratio of the image being formed is equal to or greater than a predetermined level.
No correction is particularly required for a toner that has a high toner area ratio and whose frictional force is reduced by the toner.

Furthermore, in the present invention, it is desirable that an image forming unit is provided for each color, and a photoconductor for each color is disposed along the transfer belt.
Such a so-called tandem image forming apparatus is particularly effective.

  According to the image forming apparatus of the present invention, in the image forming apparatus using the transfer belt having a large coefficient of friction with the photosensitive member, the image forming apparatus does not cause a large color shift.

1 is a schematic configuration diagram illustrating a color printer according to an embodiment. FIG. 3 is an explanatory diagram showing an intermediate transfer belt and its drive system. 6 is a graph showing the relationship between the running speed of the intermediate transfer belt and the drive torque. It is a graph which shows the drive torque during rotation. It is a graph which shows the example of a timing of correction processing. It is a graph which shows the example of a timing of correction processing.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to a color printer having a so-called tandem intermediate transfer belt.

  As shown in FIG. 1, the color printer 1 of this embodiment is a so-called tandem image forming apparatus capable of forming a color image. In the middle of the figure, four color image forming portions are arranged along the intermediate transfer belt 11 in the order of yellow 10Y, magenta 10M, cyan 10C, and black 10K from left to right in the figure. Above the intermediate transfer belt 11, toner storage portions 12 (12Y, 12M, 12C, and 12K) that store toner of each color are provided.

  A detachable paper feed cassette 13 is mounted on the lower portion of the color printer 1 in FIG. Further, on the right side in the figure, a paper transport path 14 is provided from the bottom upward. A paper feed roller 15, a secondary transfer unit 16, a fixing unit 17, and a paper discharge roller 18 are provided in order from the bottom along the paper conveyance path 14. A discharge tray 19 is provided on the upper surface of the color printer 1.

  The image forming units 10Y, 10M, 10C, and 10K for the respective colors include a charging unit 22, an exposing unit 23, a developing unit 24, a primary transfer unit 25, and a cleaner 26 in the clockwise direction in FIG. Have. The exposure unit 23 is provided below the image forming unit for each color, and is common to each color. A belt cleaner 27 is provided in contact with the left end of the intermediate transfer belt 11 in the figure.

  As shown in FIG. 2, the intermediate transfer belt 11 is stretched between a driving roller 31 and a stretching roller 32. Furthermore, this embodiment has a drive motor 33, a torque meter 34, and a control unit 35. The drive motor 33 is for rotating the drive roller 31. The torque meter 34 is attached to the drive shaft of the drive motor 33 and is used for measuring the rotational torque of the drive motor 33. The control unit 35 receives the detection result of the torque meter 34 and controls the drive speed of the drive motor 33. 2 is a view of the intermediate transfer belt 11 and its drive system in FIG. 1 as viewed from above.

  At the time of image formation, the driving roller 31 is driven to rotate counterclockwise in FIG. 1 by the driving motor 33, and the intermediate transfer belt 11 is rotated in the direction indicated by the arrow in the drawing. The color printer 1 of this embodiment can adjust the system speed (traveling speed of the intermediate transfer belt 11) within a range of 100 to 400 mm / sec. For this purpose, a drive motor 33 whose drive torque can be adjusted by the control unit 35 is used. The torque required when only the intermediate transfer belt 11 is driven in a state where the photosensitive member 21 or the like is not in contact with the drive motor 33 is, for example, about 200 to 300 mN · m.

  Next, the operation of each part when a color image is formed by the color printer 1 will be briefly described. At the time of image formation, the intermediate transfer belt 11 and the photosensitive members 21 of the respective colors are rotated as indicated by arrows in FIG. The photoreceptor 21 is uniformly charged by the charging unit 22 and then exposed by the exposure unit 23. As a result, an electrostatic latent image based on the image data is formed on the surface of the photoreceptor 21. Next, toner is supplied to the electrostatic latent image by the developing unit 24, and a toner image is formed on the photoreceptor 21. The toner images of the respective colors are transferred onto the intermediate transfer belt 11 by the primary transfer unit 25 and superimposed. The toner remaining on the photoreceptor 21 even after passing through the primary transfer area is scraped off by the cleaner 26.

  On the other hand, the sheets stored in the sheet feeding cassette 13 are pulled out one by one by the sheet feeding roller 15. Then, the toner image superimposed on the intermediate transfer belt 11 is transferred by the secondary transfer unit 16 to the sheet conveyed through the sheet conveyance path 14. The sheet on which the toner image is transferred is further conveyed upward and reaches the fixing unit 17. In the fixing unit 17, the toner is fixed to the sheet by being heated and pressurized. As a result, the paper on which the image has been formed is discharged to the paper discharge tray 19 by the paper discharge roller 18. Note that the toner remaining on the intermediate transfer belt 11 even after passing through the secondary transfer region is scraped off by the belt cleaner 27.

  The intermediate transfer belt 11 of this embodiment is an elastic belt having a rubber layer. Therefore, it is relatively flexible and has a certain degree of elasticity. The intermediate transfer belt 11 has a coefficient of friction with the surface of the photosensitive member 21 of about μ = 0.7 to 1.0, for example. Therefore, a large frictional force acts between the intermediate transfer belt 11 and the surface of the photosensitive member 21 as compared with a resin belt (for example, μ = about 0.2 to 0.4).

  In this embodiment, as shown in FIG. 1, the four photoconductors 21 sandwich the intermediate transfer belt 11 together with the opposing primary transfer portions 25. Each of the photoconductors 21 is forcibly driven so that the surface thereof is at a constant speed, and each primary transfer unit 25 is driven to rotate. Therefore, in this embodiment, not only the rotation of the driving roller 31 but also the frictional force due to the rotation of each photosensitive member 21 is a part of the force for driving the intermediate transfer belt 11.

  Further, the intermediate transfer belt 11 of the present embodiment is an elastic belt and is flexible, so that it bends relatively easily. For this reason, if there is a speed difference between the outer surface of the intermediate transfer belt 11 and the outer surface of the photosensitive member 21, or if there is a speed difference between the photosensitive members 21, the belt is likely to bend. Furthermore, if this belt deflection accumulates between the photoreceptors 21, a large slip may occur. A large slip is not preferable because it causes a large color shift. That is, it is preferable that the speed difference between the intermediate transfer belt 11 and the photoreceptor 21 is as small as possible.

  Therefore, in this embodiment, when a new product is manufactured or when the environment changes (for example, when the temperature changes from 30 ° C. or more to 10 ° C. or less from the normal environment), when every 10,000 sheets are printed, or when the belt is replaced, etc. At the time of formation, setting processing of the rotational speed of the drive motor 33 is performed. Specifically, first, the rotational speed of the drive motor 33 is changed little by little, and the drive torque at each rotational speed is measured by the torque meter 34. For example, the measurement is performed at a plurality of different speeds shifted by about 0.1% in both positive and negative directions at a speed ratio with respect to the photoconductor 21 around the rotational speed at which the peripheral speed of the driving roller 31 is equal to the peripheral speed of each photoconductor 21. To do.

  By this measurement, the relationship between the rotational speed of the drive motor 33 and the drive torque value at that time is acquired as shown by a solid line A in FIG. There is a range where the driving torque changes sharply in the center of the graph. In the range where the rotational speed of the drive motor 33 on the left side of the drawing is slower than that range, the drive torque value is a negative and substantially constant value. In this state, the rotational speed of the intermediate transfer belt 11 is smaller than the speed when driven by each photoconductor 21 alone. That is, the brake is substantially applied by the drive motor 33.

  Further, in the range where the rotational speed of the right drive motor 33 in FIG. 3 is fast, the drive torque value is a substantially constant positive value. In this state, the rotation speed of the intermediate transfer belt 11 is higher than the speed when driven by each photoconductor 21 alone. In this case, the drive motor 33 moves the intermediate transfer belt 11 by overcoming the friction of the entire transfer system of the intermediate transfer belt 11 including the respective photoreceptors 21.

  As in this example, when the driving torque changes greatly from negative to positive, the rotational speed of the driving motor 33 (speed V in FIG. 3) at which the driving torque becomes zero can be obtained. When the rotational speed V at which the drive torque of the drive motor 33 becomes 0 is determined in this way, at the time of image formation, the rotational speed V is set as a target speed to control the drive of the drive motor 33 at the time of image formation. Thereby, the relative speed with respect to the surface of each photoconductor 21 can be made as small as possible as a whole.

  Here, in the color printer 1 of the present embodiment, as shown in FIG. 1, the drive roller 31 also serves as a counter roller of the secondary transfer unit 16. For this reason, a relatively high-temperature transfer material comes into contact with the drive roller 31 during the transfer to the second surface of double-sided printing. As a result, the temperature of the drive roller 31 may rise. Depending on the degree of temperature rise, the roller diameter of the drive roller 31 slightly changes during the image forming process. Even if the rotational speed of the drive motor 33 is kept constant at the above-mentioned rotational speed V, if the roller diameter of the drive roller 31 changes, the running speed of the intermediate transfer belt 11 will change.

  Therefore, in this embodiment, the drive torque of the drive motor 33 is acquired even during image formation, and the rotational speed is corrected based on the acquired value. First, the torque meter 34 measures the driving torque when the intermediate transfer belt 11 is rotationally driven by the driving motor 33 during image formation or the like. This measurement may be performed at an appropriate sampling interval (for example, about 30 ms). Then, this measurement result often fluctuates at a substantially constant pitch as shown in FIG. This variation is caused by, for example, a shake due to the eccentricity of the drive roller 31.

  Therefore, the control unit 35 measures the drive torque over a period of at least one rotation of the drive roller 31 and calculates the average value. In the present embodiment, the average value of the driving torque during a period corresponding to three rotations of the driving roller 31 is acquired as the driving torque value at that time. For example, the average value of the periods described as the period S in FIG. The measurement period is preferably a multiple of the time for one rotation of the drive roller 31 as a unit. The time interval for calculating the average value may be longer than the sampling interval.

  However, this period is not included in the measurement period, as described below, because the drive torque may have a unique value that differs from normal. For example, it is a period including the moment when the conveyed paper enters or exits between the drive roller 31 and the secondary transfer unit 16. Alternatively, in the case where the belt cleaner 27 has a mechanism that separates and presses against the intermediate transfer belt 11, the period includes the moment of the separation or press contact. Further, after correcting the rotational speed as described later, it is preferable to interrupt the measurement of the drive torque for a while until the drive torque is stabilized. For example, the measurement is not performed for the time required for the drive roller 31 to rotate three times. Note that FIG. 4 does not include a period showing these unique values.

  Based on the calculated average value of the drive torque values, the control unit 35 adjusts the rotational speed of the drive motor 33 so that the drive torque value approaches zero. For example, when the acquired average value is positive and exceeds a predetermined first value, the rotational speed of the drive motor 33 is reduced. As a result, the running speed of the intermediate transfer belt 11 becomes slow, the drive torque value becomes small and approaches zero. In addition, when the acquired average value is negative and is equal to or less than a predetermined second value (more than the absolute value), the rotational speed of the drive motor 33 is increased. As a result, the traveling speed of the intermediate transfer belt 11 increases, and the drive torque value increases and approaches zero. The control unit 35 stores the first value and the second value in advance.

  For example, the first value is preferably about + 1% of the driving torque value of the belt alone. The drive torque value of the belt alone is a torque value required when only the intermediate transfer belt 11 is driven by the drive motor 33 in a state where the photosensitive member 21 or the like is not in contact with the intermediate transfer belt 11. This torque value is hereinafter referred to as single torque. This single torque is a predetermined value within the range of 200 to 300 mN · m. Usually, if the amount of change in the torque value is within ± 1% of this, no particular correction is required. Therefore, in this embodiment, the torque value corresponding to + 1% of the single unit torque is set as the first value, and the torque value corresponding to -1% of the single unit torque is previously determined and stored as the second value.

  The first value and the second value are, for example, values indicated by W1 and W2 in FIG. 3, and are within a range in which the drive torque changes sharply with respect to the rotational speed of the drive motor 33. . This change in driving torque does not have a significant effect on the rotational speed of the intermediate transfer belt 11. Therefore, in the present embodiment, when the average value of the acquired drive torque values deviates outside the range of W1 to W2 shown in FIG. 3, the rotational speed of the drive motor 33 is corrected. In this embodiment, the absolute values of W1 and W2 are both 1%, but they may be different from each other.

  Furthermore, as indicated by W3 in FIG. 3, if the average value of the acquired drive torque values is a deviation of about + 3% of the single torque, the rotational speed correction amount is very small. In this case, even if correction is performed in parallel during image formation, there is almost no effect on the image. Therefore, it is preferable to perform speed correction immediately even during image formation. Also, since the necessary correction amount is small, the entire amount is corrected at once. For example, if the drive motor 33 is a stepping motor, a minimum one-step correction is performed. This is the minimum correction amount. If it is not a stepping motor, a minimum correction amount may be determined in advance.

  The control unit 35 predetermines the range of the average value of the driving torque values for which the correction of the minimum correction amount is sufficient as the third value (positive value) and the fourth value (negative value). I remember it. That is, if the acquired average value is between the first value and the third value, the rotational speed of the drive motor 33 is reduced by one step. If the acquired average value is between the second value and the fourth value, the rotational speed of the drive motor 33 is increased by one step. Although FIG. 3 does not particularly show the driving torque value corresponding to the fourth value, this diagram is merely an example, and the driving torque value is negative by appropriately setting the fourth value. Of course, there can be control on the value side.

  However, when the average value of the drive torque values is larger than the third value or smaller than the fourth value, the necessary correction amount of the rotational speed is larger than the minimum correction amount. For example, this may occur due to a sudden environmental change or continuous double-sided printing. In this case, if the entire amount of speed correction is performed during image formation, especially during primary transfer, there is a risk of color misregistration due to the correction itself. For this reason, it is desirable to perform correction of an amount larger than the minimum correction amount when the image is not being formed.

  Therefore, for example, the image formation is temporarily interrupted at a timing such as between pages, and all necessary correction amounts are corrected at once. That is, the intermediate transfer belt 11 is idly rotated while keeping the photosensitive member in contact. It is preferable to correct the speed of the drive motor 33 during the idling and restart the image formation after the correction is completed. In this way, the image that is being formed is not adversely affected by the correction, and the image to be formed next time is suppressed in color misregistration.

  FIG. 5 shows an example of changes in the drive torque value when the correction process is performed in this way. At the times indicated by black circles in the figure, the average value of the drive torque values described above was obtained. As described above, since the environmental temperature rises due to image formation, the roller diameter usually changes in an increasing direction. For this reason, if the driving is continued with the rotation speed kept constant, the peripheral speed and the driving torque often increase. In the example of this figure, the average value acquired at time t1, t2, t3 exceeds the allowable upper limit value (single torque + 1%), and the rotational speed is corrected at this point.

  Since the average value acquired at the times t1 and t2 is within the range of +1 to + 3% of the single torque, the speed correction of the minimum correction amount was performed in parallel during the image formation. Here, an example in which the drive motor 33 is a stepping motor is shown. That is, the rotational speed of the drive motor 33 is reduced by one step. As a result, the drive torque value also decreased, and the next average value measurement was within the range of + 1% of the single unit torque. In such a one-step correction, the drive torque value is not always exactly zero. Also in the example of FIG. 5, although it is not 0, it is close to 0 before the correction and is not necessary to be corrected, and the bending of the intermediate transfer belt 11 is suppressed.

  On the other hand, as shown in FIG. 5, at the time t3, the acquired average value exceeded + 3% of the unit torque. Therefore, here, the image formation is temporarily interrupted and correction processing is performed. That is, the intermediate transfer belt 11 is idled and the rotation speed is corrected during that time. The necessary correction amount is calculated based on the relationship between the drive torque value and the rotational speed of the drive motor 33 (see FIG. 3). This relationship is preferably stored in advance as a table or the like. After the correction, when the rotation of the intermediate transfer belt 11 is stabilized, the next image formation is performed.

  Even when the acquired average value exceeds + 3% of the single torque, the correction can be made by repeating the correction by the minimum correction amount after a certain interval, instead of making a correction at once. In this case, it is not necessary to interrupt image formation. For example, if the necessary correction amount at this time is about twice the minimum correction amount, correction is performed in two times at times t3 and t4 as shown in FIG. That is, first, the minimum correction amount is corrected, and the correction is performed again at a time interval (usually shorter than the average value calculation interval) at which the speed of the intermediate transfer belt 11 is stabilized. During this time, it is not necessary to measure the driving torque value. The number of repetitions is acquired from the above table.

  In this embodiment, since the intermediate transfer belt 11 is an elastic belt having a rubber layer, the frictional force acting between the surface of the photoreceptor 21 is large. This causes a large deflection and slip of the belt. This is the reason why the speed correction as described above is necessary. However, when the frictional force between the intermediate transfer belt 11 and the photosensitive member 21 is small, such processing is not necessary. Therefore, when the toner area of the image to be primarily transferred is large, the frictional force is reduced by the toner, so that the speed correction of the drive motor 33 may not be performed. For example, when the toner covering area of each photoconductor 21 exceeds a predetermined level, the correction may not be performed.

  For example, the correction may not be performed when the toner area ratio of the formed image exceeds a predetermined reference value (for example, 50%). Alternatively, the average or total of the toner covered areas of the respective photoconductors 21 may be used as a determination criterion. Alternatively, the photosensitive member 21 having the smallest toner covering area can be used as a determination criterion.

  In the present embodiment, each primary transfer unit 25 is driven to rotate. Therefore, as described above, only the frictional force between the intermediate transfer belt 11 and the photosensitive member 21 has to be considered. However, there is also an image forming apparatus that drives and rotates the primary transfer roller. In such a case, it is necessary to consider the frictional force between the primary transfer roller and the intermediate transfer belt 11. Therefore, in such a case, it is desirable to control the rotational speed of the drive motor 33 of this embodiment regardless of the toner area ratio.

  As described in detail above, according to the color printer 1 of the present embodiment, the drive torque is appropriately measured even during image formation, and the drive motor 33 is controlled so that the drive torque value is close to zero. Therefore, even during image formation, the speed difference from the entire photoreceptor of the intermediate transfer belt 11 can be further reduced. Since the deflection of the intermediate transfer belt 11 can be reduced, a large slip can be suppressed. As a result, even a transfer belt having a large coefficient of friction with the photosensitive member does not cause a large color shift.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, in this embodiment, a torque meter is provided to measure the driving torque, but the torque value can also be obtained by detecting the current value that flows through the driving motor 33. This is because a large current is required to generate a large torque value. In this case, the relationship between the current value of the drive motor 33 and the torque value may be measured in advance, and the current value corresponding to the state where the torque value becomes 0 is obtained.
Further, for example, in the present embodiment, the present invention is applied to a tandem color printer, but a so-called four-cycle image forming apparatus or monochrome image forming apparatus in which developing devices for each color are provided around one photosensitive member. It can also be applied to.

DESCRIPTION OF SYMBOLS 1 Color printer 10Y, 10M, 10C, 10K Image formation part 11 Intermediate transfer belt 21 Photoreceptor 33 Drive motor 34 Torque meter 35 Control part

Claims (6)

In an image forming apparatus, comprising: an image forming unit that forms a toner image on a rotationally driven photoconductor; and a transfer belt that is rotationally driven while being in contact with the photoconductor and onto which a toner image is transferred from the photoconductor ,
A drive unit for rotationally driving the transfer belt;
A torque measuring unit for measuring a driving torque value of the driving unit;
A rotation speed correction unit that corrects the rotation speed of the drive unit based on the drive torque value acquired by the torque measurement unit;
The rotational speed correction unit is
When the acquired drive torque value is a positive value, the rotational speed of the drive unit is reduced,
The image forming apparatus, wherein when the acquired drive torque value is a negative value, the rotational speed of the drive unit is increased. The image forming apparatus according to claim 1,
The torque measuring unit acquires an average value of driving torque values while the transfer belt rotates a plurality of times as a measured value. The image forming apparatus according to claim 1, wherein:
The rotational speed correction unit is
If the driving torque value acquired by the torque measuring unit is less than a first value that is a predetermined positive value and greater than or equal to a second value that is a predetermined negative value, correction is not performed.
If the drive torque value is greater than or equal to the first value and less than a predetermined third value that is greater than the first value, correction is performed regardless of whether or not image formation is in progress;
If the drive torque value is less than the second value and greater than or equal to a predetermined fourth value that is smaller than the second value, correction is performed regardless of whether or not image formation is in progress.
If the drive torque value is greater than or equal to the third value or less than the fourth value, the image formation is temporarily stopped, the rotational speed of the drive unit is corrected, and the image formation is resumed after the correction. An image forming apparatus. The image forming apparatus according to claim 1, wherein:
The rotational speed correction unit is
If the driving torque value acquired by the torque measuring unit is less than a first value that is a predetermined positive value and greater than or equal to a second value that is a predetermined negative value, correction is not performed.
If the drive torque value is greater than or equal to the first value and less than a predetermined third value greater than the first value, a predetermined minimum correction is performed regardless of whether or not an image is being formed. Correct the amount,
If the drive torque value is less than the second value and greater than or equal to a predetermined fourth value that is smaller than the second value, a predetermined minimum value is set regardless of whether the image is being formed or not. Correct the correction amount,
If the drive torque value is greater than or equal to the third value or less than the fourth value, the minimum correction amount is corrected a plurality of times without stopping image formation. Forming equipment. The image forming apparatus according to any one of claims 1 to 4, wherein:
The image forming apparatus according to claim 1, wherein the rotation speed correction unit does not perform correction when the toner area ratio of the image being formed is equal to or greater than a predetermined level. In the image forming apparatus according to any one of claims 1 to 5,
An image forming apparatus comprising: the image forming unit for each color, and the photoconductor for each color is disposed along the transfer belt.

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