Image forming apparatus

A tandem-type image forming apparatus in which a photoreceptor for black is controlled to be contacted to an intermediate transfer member and the photoreceptors for colors are controlled to be separated from the intermediate transfer member in a case of forming a black and white image, is disclosed, including a current control part which controls so that in the case of forming the black and white image, when the intermediate transfer member is driven, a value of a current flowing to the motor is equal to the value of the current in the case of forming the color image. The current control part controls the value of the current by controlling a rotational velocity of the photoreceptor for black.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a technology of an image forming apparatus, and more particularly to the technology for making load torque of a motor driving an intermediate transfer member or a transfer belt not be reduced when photoreceptors for colors are separated from the intermediate transfer member or the transfer belt.

2. Description of the Related Art

Recently, the number of electrophotographic devices, which includes a color printing function such as a color copier, a color printer, or a like, has increased.

Among others, the number of tandem type of the electrophotographic devices has increased in which a developing device is individually provided to each of a plurality of photoreceptors. Single toner images are formed on respective photoreceptors, and then sequentially transfer the single toner images to an intermediate transfer member (belt), and transfer a synthetic color image onto a sheet. In this case, in order to improve a life duration of the photoreceptors for colors, Japan Laid-open Patent No. 2006-139063 and a like disclose the photoreceptors other than photoreceptors for black and white are separated from the intermediate transfer member when a black and white image is formed. The intermediate transfer member is driven by a motor. The load torque for the motor driving the intermediate transfer member is different between a case of forming a color image and a case of forming the black and white image, depending on the number of the photoreceptors contacting with the intermediate transfer member.

Accordingly, in the black and white image formation, a torque for the motor is lower than a torque for the motor while conducting the color image formation, and there is a problem in that a stable motor control becomes difficult.

SUMMARY OF THE INVENTION

The present invention solves or reduces one or more of the above problems.

In an aspect of this disclosure, there is provided a tandem-type image forming apparatus including a photoreceptor for black which is rotatable, a plurality of photoreceptors for colors which is rotatable, an intermediate transfer member or a transfer belt, the intermediate transfer member which is an endless belt shape and transfers a toner image formed on the photoreceptors, the transfer belt which is an endless belt shape and carries a printing medium to transfer the toner image, and a motor which drives the intermediate transfer member or the transfer belt, in which the photoreceptor for black and the plurality of the photoreceptors for colors are controlled to be contacted to the intermediate transfer member or the transfer belt in a case of forming a color image, and in which the photoreceptor for black is controlled to be contacted to the intermediate transfer member or the transfer belt and the plurality of the photoreceptors for colors are controlled to be separated from the intermediate transfer member or the transfer belt in a case of forming a black and white image, the tandem-type image forming apparatus including: a current control part configured to control so that in the case of forming the black and white image, when the intermediate transfer member or the transfer belt is driven, a value of a current flowing to the motor is equal to the value of the current in the case of forming the color image, wherein the current control part controls the value of the current by controlling a rotational velocity of the photoreceptor for black.

In another aspect of this disclosure, there is provided a tandem-type image forming apparatus including a photoreceptor for black which is rotatable, a plurality of photoreceptors for colors which is rotatable, an intermediate transfer member which is an endless belt shape and transfers a toner image formed on the photoreceptors, a motor which drives the intermediate transfer member, a polygon mirror which is rotatable and reflects a laser light to scan the photoreceptors and to form an electrostatic latent image on the photoreceptors, and a transfer belt which caries a printing medium when the toner image transferred onto the intermediate transfer member is secondarily transferred onto the printing medium, in which the photoreceptor for black and the plurality of the photoreceptors for colors are controlled to be contacted to the intermediate transfer member in a case of forming a color image, and in which the photoreceptor for black is controlled to be contacted to the intermediate transfer member and the plurality of the photoreceptors for colors are controlled to be separated from the intermediate transfer member in a case of forming a black and white image, the tandem-type image forming apparatus including: a current control part configured to control so that in the case of forming the black and white image, when the intermediate transfer member is driven, a value of a current flowing to the motor is equal to the value of the current in the case of forming the color image, wherein the current control part controls the value of the current by controlling a transfer velocity of the transfer belt and a rotational velocity of the polygon mirror. In a further aspect of this disclosure, there is provided a tandem-type image forming apparatus including a photoreceptor for black which is rotatable, a plurality of photoreceptors for colors which is rotatable, a transfer belt which is an endless belt shape and carries a printing medium to transfer a toner image formed on the photoreceptors, a motor which drives the transfer belt, and a polygon mirror which is rotatable and reflects a laser light to scan the photoreceptors and to form an electrostatic latent image on the photoreceptors, in which the photoreceptor for black and the plurality of the photoreceptors for colors are controlled to be contacted to the transfer belt in a case of forming a color image, and in which the photoreceptor for black is controlled to be contacted to the transfer belt and the plurality of the photoreceptors for colors are controlled to be separated from the transfer belt in a case of forming a black and white image, the tandem-type image forming apparatus including: a current control part configured to control so that in the case of forming the black and white image, when the transfer belt is driven, a value of a current flowing to the motor is equal to the value of the current in the case of forming the color image, wherein the current control part controls the value of the current by controlling a driving velocity of the transfer belt by the motor and a rotational velocity of the polygon mirror.

Accordingly, it is possible to provide the image forming apparatus which can stably rotate and drive the motor driving the intermediate transfer member or the transfer belt and can form a high quality image, even in the case of forming the black and white image in which the photoreceptors for colors are separated from the intermediate transfer member or the transfer belt.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, an embodiment of the present invention to will be described with reference to the accompanying drawings.

<Brief Overview of Image Forming Apparatus>

A brief overview of image forming apparatuses according to the present invention will be described with reference toFIG. 1andFIG. 2.FIG. 1is a diagram illustrating a schematic configuration of an image forming apparatus of an intermediate transfer system according to an embodiment.

As illustrated inFIG. 1, an image forming apparatus100according to the present invention includes developing devices2Y,2C,2M, and2B (hereinafter, simply called developing devices2if not necessary to specify), and transfer devices3Y,3C,3M, and3B (hereinafter, simply called transfer devices3if not necessary to specify), respectively for a plurality of photoreceptors1Y,1C,1M, and1B (hereinafter, simply called photoreceptors1if not necessary to specify). The photoreceptors1Y,1C,1M, and1B are driven by corresponding control motors14Y,14C,14M, and14B. In the image forming apparatus100, toner images corresponding to single colors are formed on respective photoreceptors1. The toner images for the single colors are contacted to an intermediate transfer member (an intermediate transfer belt)5, and are sequentially transferred onto the intermediate transfer member5. Then, a synthetic color image is formed. Moreover, the image forming apparatus100is an electrophotographic device (a color copier or a color printer) being a tandem type in which a color image can be formed by collectively transferring a formed synthetic color image to a printing medium P of a sheet.

The photoreceptors1Y,1C,1M, and1B being a drum shape are rotatable image carriers being separately independent in which different toner images are formed on their surfaces. Thus, each of toner images formed on respective photoreceptors1is formed on the intermediate transfer member5moving rotationally at transfer positions corresponding to the photoreceptors1. Since the image forming apparatus100transfers a toner image on each of the photoreceptors1onto the intermediate transfer member5, the image forming apparatus100operates the transfer devices (transfer rollers)3Y,3C,3M, and3B upward and downward as appropriate. The transfer devices3can be positioned up and down by moving contact/separation mechanisms4YMC and4B. Thus, it is possible to make a contact or a separation between the photoreceptors1Y,1C,1M, and1B and the intermediate transfer member5.

Moreover, in addition to the developing devices2, charging devices6, cleaning devices7, charge eliminating devices8, and the like are circumferentially provided for the corresponding photoreceptors1. In each of the photoreceptors1, a laser light corresponding to an image of a respective color is scanned from a laser writing part9based on an image signal. Electrostatic latent images are formed on the photoreceptors1Y,1C,1M, and1B, respectively.

FIG. 2is a diagram illustrating a schematic configuration of an image forming apparatus100-2of a direct transfer system which directly transfers toner images on the photoreceptors1Y,1C,1M, and1B to the printing medium P. InFIG. 2, parts that are the same as those shown inFIG. 1are given the same reference numbers.

In the image forming apparatus100-2of the direct transfer system, mechanisms for generating toner images on the photoreceptors1Y,1C,1M, and1B and of transferring the generated toner images to the printing medium P are the same as the image forming apparatus100of an intermediate transfer system. Also, in the image forming apparatus100-2of the direct transfer system, the transfer devices (transfer rollers)3Y,3C,3M, and3B are positioned up and down by moving the contact/separation mechanisms4YCM, and4B. By this operation, a contact and a separation can be made between a transfer belt33for transferring the printing medium P and the photoreceptors1Y,1C,1M, and1B. Moreover, the transfer belt33is stretched between a driving roller32being rotationally driven by a control motor16and a driven roller34, and rotationally moves in a belt rotation direction illustrated inFIG. 2.

Image forming operations in the image forming apparatus100of the intermediate transfer system and the image forming apparatus100-2of the direct transfer system are the same as an image forming operation of a well-known color image forming apparatus, and the explanation thereof will be omitted.

<Operation Principle of Image Forming Apparatus>

An operation principle of the image forming apparatus100of the intermediate transfer system according to this embodiment will be described with reference toFIG. 3A.FIG. 3Ais a diagram for explaining the operation principle of the image forming apparatus100of the intermediate transfer system.

As illustrated inFIG. 3A, the image processing apparatus100includes an image forming part110. The image forming part110is a part to realize functions of forming a regular color image and/or a regular black and white image. For example, the image forming part110realizes a copier function for forming a color image or a black and white image on a sheet from image data which are acquired by scanning a manuscript by a scanner and outputting the sheet, a facsimile function for forming a color image or a black and white image on a sheet from image data which are received through a telephone line and outputting the sheet, and a printer function for forming a color image or a black and white image on a sheet from image data which are received through a LAN (Local Area Network) and outputting the sheet.

Also, the image forming part110in the image forming apparatus100of the intermediate transfer system includes a current control part120, an intermediate transfer member5, a control motor15for activating a roller21to drive the intermediate transfer member5, the photoreceptor1B for black, the control motor14B for activating the photoreceptor1B for black, a transfer belt33, a control motor17for activating a roller24to drive the transfer belt33, a driven roller22, a control motor31for activating the driven roller22, a polygon mirror42, and a control motor41for activating the polygon mirror42.

In the image forming apparatus100of the intermediate transfer system, the transfer belt33carries the printing medium P (a paper sheet or a like) to which a toner image on the intermediate transfer member5is secondarily transferred at a secondary transfer roller23(FIG. 1).

The polygon mirror42scans the corresponding photoreceptors1by reflecting a laser light, and forms electrostatic latent images on the corresponding photoreceptors1. The polygon mirror42is rotated by the control motor41.

On one hand, when a color image is formed, the image forming part110forms the color image by controlling so that the photoreceptors1Y,1C,1M, and1B are contacted to the intermediate transfer member5. On the other hand, when a black and white image is formed, the image forming part110forms the black and white image by controlling so that the photoreceptor1B for black is contacted to the intermediate transfer member5and the photoreceptors1Y,1C, and1M for colors are separated from the intermediate transfer member5.

The current control part120controls a value of a current flowing to the control motor15when the black and white image is formed, so that the value of the current flowing to the control motor15when the black and white image is formed becomes equal to a value of the current flowing to the control motor15when the color image is formed. An object of the present invention can be achieved if both of the values of the current are approximately equal to each other even though both of the values of the current do not become equal to each other.

An operation principle of the image forming apparatus100-2of the direct transfer system according to this embodiment will be described with reference toFIG. 3B.FIG. 3Bis a diagram for explaining the operation principle of the image forming apparatus100-2of the direct transfer system.

As illustrated inFIG. 3B, the image processing apparatus100-2includes an image forming part110-2. The image forming part110-2is a part to realize functions of forming a regular color image and/or a regular black and white image. For example, the image forming part110-2realizes a copier function, a facsimile function, and a printer function, similar to the image forming part110in the intermediate transfer system.

Also, the image forming part110-2in the image forming apparatus100-2of the direct transfer system includes a current control part120-2, a transfer belt33, a control motor16for activating the transfer belt33, a photoreceptor1B for black, a control motor14B for activating the photoreceptor1B for black, a polygon mirror42, and a control motor41for activating the polygon mirror42.

In the image forming apparatus100-2of the direct transfer system, the transfer belt33carries the printing medium P to which a toner image on the photoreceptor1B for black is directly transferred.

On one hand, when a color image is formed, the image forming part110-2controls so that the photoreceptors1Y,1C,1M, and1B are contacted to the transfer belt33, and forms the color image. On the other hand, when a black and white image is formed, the image forming part110-2controls so that the photoreceptor1B for black is contacted to the transfer belt33and the photoreceptors1Y,1C, and1M are separated from the transfer belt33, and forms the color image.

The current control part120-2controls a value of a current flowing to the control motor16, when the black and white image is formed, so that the value of the current flowing to the control motor16when the black and white image is formed becomes equal to a value of the current flowing to the control motor16when the color image is formed. An object of the present invention can be achieved if both of the values of the current are approximately equal to each other even though both of the values of the current do not become equal to each other.

In the following, a process of the current control part120in the image forming apparatus100of the intermediate transfer system will be described. The number of the photoreceptors1contacting to the intermediate member5is four when the color image is formed. On the other hand, the number of the photoreceptors1contacting to the intermediate member5is one when the black and white image is formed. Accordingly, frictional force acting onto the intermediate transfer member5(load torque with respect to the control motor15) by the photoreceptors1in a case of forming the black and white image becomes smaller than frictional force in a case of forming the color image. Thus, in order to control so that the value of the current flowing to the control motor15becomes the same value in both cases in that the black and white image is formed and in that the color image is formed, the current control part120conducts a velocity control which will be described in the following, and increases the frictional force with respect to the intermediate transfer member5(load torque with respect to the control motor15).

In one aspect, the current control part120increases the frictional force acting to the intermediate transfer member5by the photoreceptor1B for black, by controlling the control motor14B which rotationally drives the photoreceptor1B for black. In detail, the current control part120controls the control motor14B, and makes a rotational velocity of the photoreceptor1B for black be a predetermined velocity Vb which is slower than a rotational velocity in a case of forming the color image. The predetermined velocity Vb is determined by torque of the control motor15and a range of allowable deterioration of image quality which is caused by making the rotational velocity of the photoreceptor1B for black slower. By this control, it is possible to increase the frictional force acting to the intermediate transfer member5(load torque with respect to the control motor15) by the photoreceptor1B in the transfer device3B.

Moreover, in a sub-scan direction of the toner image formed on the photoreceptor1B for black, shrinkage caused by making the rotational velocity of the photoreceptor1B for black slower is offset (enlarged) when the toner image is transferred onto the intermediate transfer member5. Thus, the toner image transferred onto the intermediate transfer member5becomes a desired image. Alternatively, in this aspect, the current control part120may decrease the frictional force acting to the intermediate transfer member5by the photoreceptor1B for black, by controlling the control motor14B rotationally driving the photoreceptor1B for the black. In detail, the current control part120controls the control motor14B, and sets the rotational velocity of the photoreceptor1B for black to be the predetermined velocity Vb which is faster than the rotational velocity in the case of forming the color image. In this case, in the sub-scan direction of the toner image formed on the photoreceptor1B for black, elongation caused by making the rotational velocity of the photoreceptor1B for black faster is offset (shrunk) when the toner image is transferred onto the intermediate transfer member5.

Moreover, in another aspect, the current control part120increases the frictional force acting to the intermediate transfer member5by the photoreceptor1B for black, by controlling the control motor15rotationally driving the intermediate transfer member5. In detail, the current control part120controls the control motor15, and makes the rotational velocity of the intermediate transfer member5be a predetermined velocity Vm which is faster than the rotational velocity in the case of forming the color image. The predetermined velocity Vm is determined by the torque of the control motor15and a range of allowable of deterioration of image quality which is caused by the control motor15which makes the rotational velocity of the transfer member5faster. By this control, it is possible to increase the frictional force acting to the intermediate transfer member5by the photoreceptor1B for black (the load torque with respect to the control motor15) in the transfer device3B. Moreover, in the sub-scan direction of the toner image formed on the intermediate transfer member5, the elongation caused by making the rotational velocity of the intermediate transfer member5faster is offset (shrunk) when the toner image is secondary transferred. Thus, the toner image transferred onto the printing medium P becomes a desired image.

Moreover, in the another aspect, the current control part120increases the frictional force acting to the intermediate transfer member5by the transfer belt33, by controlling the control motor17rotationally driving the transfer belt33and the control motor41rotationally driving the polygon mirror42. In detail, the current control part120controls the control motor17, and makes the rotational velocity of the transfer belt33be a predetermined velocity Vs which is slower than the rotational velocity in the case of forming the color image. Simultaneously, the current control part120controls the control motor41, and makes the rotational velocity of the polygon mirror42be a predetermined velocity Vp which is faster than the rotational velocity in the case of forming the color image. The predetermined velocities Vs and Vp are determined by the torque of the control motor15and a range of allowable deterioration of image quality which is caused by making the rotational velocity of the transfer belt33slower. By this control, it is possible to increase the frictional force acting to the intermediate transfer member5by the transfer belt33(the load torque with respect to the control motor15). Also, in the sub-scan direction of the toner image which is to be transferred onto the printing medium P, the shrinkage caused by making the rotational velocity of the transfer belt33slower is offset (enlarged) by making a scan velocity of the polygon mirror42slower. Thus, the toner image transferred onto the printing medium P becomes the desired image.

Also, in a relationship between the predetermined velocities Vs and Vp, absolute values of change rates, in which each of the rotational velocities in forming the color image is set as a reference, are equal to each other. Alternatively, in this aspect, the current control part120may decrease the friction force acting to the intermediate transfer member5by the transfer belt33, by controlling the control motor17rotationally driving the transfer belt33and the control motor41rotationally driving the control motor17and the polygon mirror42. In detail, the current control part120controls the control motor17, and makes the rotational velocity of the transfer belt33be a predetermined velocity Vs which is faster than the rotational velocity in the case of forming the color image. Simultaneously, the current control part120controls the control motor41, and makes the rotational velocity of the polygon mirror42be a predetermined velocity Vp which is slower than the rotational velocity in the case of forming the color image. In this case, in the sub-scan direction of the toner image, the elongation caused by increasing the rotational velocity of the transfer belt33is offset (shrunk) due to a decrease of the scan velocity of the polygon mirror42.

In another aspect, the current control part120directly increases the frictional force acting to the intermediate transfer member5by the driven roller22, by controlling the control motor31which is additionally provided to the driven roller22rotationally driving the intermediate transfer member5. In detail, the current control part120controls the control motor31, and applies a load (brake) to a rotation of the driven roller22. The control motor31may be directly connected to the driven roller22. Alternatively, in order to be approximately the same as the load torque of the control motor15in the case of forming the color image, the control motor31may be connected to the driven roller22through a gear30. Therefore, it is possible to increase the load torque which the control motor15receives, to be the same as a level of forming the color image. By this control, the shrinkage in the sub-scan direction of the toner image formed on the intermediate transfer member5is offset (enlarged) when the toner image is secondarily transferred onto the printing medium P. Thus, the toner image transferred onto the printing medium P becomes the desired image.

Next, a process of the current control part120-2in the image forming apparatus100-2of the direct transfer system will be described. The number of the photoreceptors1contacting to the transfer belt33is four when the color image is formed. On the other hand, the number of the photoreceptors1contacting to the transfer belt33is one when the black and white image is formed. Accordingly, the frictional force acting to the transfer belt33by the photoreceptors1in the case of forming the black and white image becomes smaller than the frictional force in the case of forming the color image. Thus, in order to control so that the values of the current flowing to the control motor16become equal to each other when the black and white image is formed and when the color image is formed, the current control part120-2conducts a velocity control which will be described in the following, and increases the frictional force with respect to the transfer belt33(the load torque with respect to the control motor16).

In an aspect, the current control part120-2increases the frictional force acting to the transfer belt33of the photoreceptor1B for black, by controlling the control motor14B rotationally driving the photoreceptor1B for black. In detail, the current control part120-2controls the control motor14B, and makes the rotational velocity of the photoreceptor1B for black be the predetermined velocity Vb which is slower than the rotational velocity in the case of forming the color image. The predetermined velocity Vb is determined by the torque of the control motor16and a range of allowable deterioration of image quality which is caused by making the rotational velocity of the photoreceptor1B for black slower. By this control, it is possible to increase the frictional force acting to the transfer belt33(load torque with respect to the control motor16) by the photoreceptor1B in the transfer device3B.

Moreover, in the sub-scan direction of the toner image formed on the photoreceptor1B for black, the shrinkage caused by making the rotational velocity of the photoreceptor1B for black slower is offset (enlarged) when the toner image is transferred onto the printing medium P. Thus, the toner image transferred onto the printing medium P becomes the desired image. Alternatively, in this aspect, the current control part120-2may control the control motor14B, and may make the rotational velocity of the photoreceptor1B for black be the predetermined velocity Vb which is faster than the rotational velocity in the case of forming the color image. In this case, in the sub-scan direction of the toner image formed on the photoreceptor1B for black, the elongation caused by making the rotational velocity of the photoreceptor1B for black faster is offset (shrunk) when the toner image is transferred onto the printing medium P.

Moreover, in another aspect, the current control part120-2increases the frictional force acting to the transfer belt33by the photoreceptor1B for black, by controlling the control motor16rotationally driving the transfer belt33and the control motor41rotationally driving the polygon mirror42. In detail, the current control part120-2controls the control motor16, and makes the rotational velocity of the transfer belt33be the predetermined velocity Vm which is faster than the rotational velocity in the case of forming the color image. Simultaneously, the current control part120-2controls the control motor41, and makes the rotational velocity of the polygon mirror42be a predetermined velocity Vp′ which is slower than the rotational velocity in the case of forming the color image. The predetermined velocities Vm and Vp′ are determined by the torque of the control motor16and a range of allowable deterioration of image quality which is caused by making the rotational velocity of the transfer belt33faster. By this control, it is possible to increase the frictional force acting to the transfer belt33by the photoreceptor1B for black (the load torque with respect to the control motor16).

Moreover, in the sub-scan direction of the toner image which is to be transferred onto the printing medium P, the elongation caused by making the rotational velocity of the transfer belt33faster is offset (shrunk) by making the scan velocity of the polygon mirror42faster. Thus, the toner image transferred onto the printing medium P becomes the desired image. Also, in a relationship between the predetermined velocities Vm and Vp, absolute values of change rates, in which each of the rotational velocities in the case of forming the color image is set as a reference, are equal to each other.

Each of the image forming apparatuses100and100-2includes a CPU (Central Processing Unit), a ROM (Read-Only Memory), a RAM (Random Access Memory), and an HDD (Hard Disk Drive). The CPU is a device to execute programs. The ROM is a device to store the programs and data to be executed by the CPU. The RAM is a device to develop (load) the programs and the data when the CPU executes the programs and to temporarily retain operation data during an operation. The HDD is a device to store an OS (Operating System) being basic software, application programs, and a like according to this embodiment with related data.

Process parts included in each of the image forming apparatuses100and100-2can be realized by the CPU executing respective programs stored in the ROM or HDD. Alternatively, the process parts may be realized by hardware.

A process example (part 1) conducted by the image forming apparatus100according to the embodiment will be described with reference toFIG. 4.FIG. 4is a flowchart for explaining the process example (part 1) by the image forming apparatus100. Since a process conducted by the image forming apparatus100-2of the direct transfer system is similar to the process conducted by the image forming apparatus100of the intermediate transfer system, the process example (part 1) will be described mainly for the image forming apparatus100. InFIG. 4, in a case of the intermediate transfer system, the current control part120controls the value of the current flowing to the control motor15by using the rotational velocity of the photoreceptor1B for black as a parameter. In a case of the direct transfer system, the current control part120-2controls the value of the current flowing to the control motor16by using the rotational velocity of the photoreceptor1B for black as a parameter.

In step S10, the image forming apparatus100begins a print process. In step S20, it is determined whether or not the print process (of which an execution request is made) to be conducted by the image forming part110is a black and white print process. When the print process is a black and white print (Yes in the step S20), in step S30, the image forming part110separates the photoreceptors1Y,1C, and1M for colors from the intermediate transfer member5. In the case of the direct transfer system, the image forming apparatus110-2separates the photoreceptors1Y,1C, and1M for colors from the transfer belt33.

In step S40, the image forming part110conducts the black and white print process with respect to image data to which the execution request is made, while conducting a control as described in step S50through step S70. In the step S50, the image forming part110drives the control motor14B driving the photoreceptor1B for black, the control motor15driving the intermediate transfer member5, and the control motor17driving the transfer belt33. In the case of the direct transfer system, the image forming apparatus110-2drives the control motor14B driving the photoreceptor1B for black, and the control motor16driving the transfer belt33.

In step S60, the current control part120controls the control motor14B, and changes the rotational velocity of the photoreceptor1B for black to the predetermined velocity Vb. The predetermined velocity Vb is a velocity slower than the rotational velocity in the case of forming the color image, and is determined by the torque of the control motor15or the control motor16and the range of allowable deterioration of image quality which is caused by making the rotational velocity of the photoreceptor1B for black slower. Alternatively, the predetermined velocity Vb may be a velocity slower than the rotational velocity in the case of forming the color image.

Subsequently, in the step S70, the current control part120controls so that the value of the current flowing to the control motor15driving the intermediate transfer member5is equal to the value of the current flowing to the control motor15in the case of forming the color image. In the case of the direct transfer system, the current control part120-2controls so that the value of the current flowing to the control motor16driving the transfer belt33is equal to the value of the current flowing to the control motor16in the case of forming the color image. Thus, in step S90, the image forming part110terminates the black and white print process.

When it is determined that the print process (of which the execution request is made) to be executed by the image forming part110is a color print (No in the step S20), in the step S80, the image forming part110conducts the color print process with respect to image data of which the execution request is made, and executes the color print process. Then, in step S90, the image forming part110terminates the color print process.

As described above, by making the rotational velocity of the photoreceptor1B for black slower (faster), a contact resistance with the intermediate transfer member5can be increased (decreased). Therefore, even in the case of forming the black and white image, it is possible to stably drive the control motor15of the intermediate transfer member5. In the case of the direct transfer system, by making the rotational velocity of the photoreceptor1B for black slower (faster), it is possible to increase (decrease) a contact resistance with the transfer belt33. Therefore, even in the case of forming the black and white image, it is possible to stably drive the control motor16of the transfer belt33.

A process example (part 2) conducted by the image forming apparatus100according to the embodiment will be described with reference toFIG. 5.FIG. 5is a flowchart for explaining the process example (part 2) by the image forming apparatus100. Since a process conducted by the image forming apparatus100-2of the direct transfer system is similar to the process conducted by the image forming apparatus100of the intermediate transfer system, the process example (part 2) will be described mainly for the image forming apparatus100.

InFIG. 5, in the case of the intermediate transfer system, the current control part120controls the value of the current flowing to the control motor15by using the rotational velocity of the intermediate transfer member5as a parameter. In a case of the direct transfer system, the current control part120-2controls the value of the current flowing to the control motor16by using the rotational velocity of the transfer belt33as a parameter.

In step S110, the image forming apparatus100begins the print process. In step S120, it is determined whether or not the print process (of which the execution request is made) to be executed by the image forming part110is a black and white print process. When the print process is a black and white print (Yes in the step S120), in step S130, the image forming part110separates the photoreceptors1Y,1C, and1M for colors from the intermediate transfer member5. In the case of the direct transfer system, the image forming apparatus110-2separates the photoreceptors1Y,1C, and1M for colors from the transfer belt33.

In step S140, the image forming part110executes the black and white print process with respect to image data to which the execution request is made while conducting a control which will be described in step S150through step S170. In the step S150, the image forming part110drives the control motor14B driving the photoreceptor1B for black, the control motor15driving the intermediate transfer member5, and the control motor17driving the transfer belt33. In the case of the direct transfer system, the image forming apparatus110-2drives the control motor14B driving the photoreceptor1B for black, and the control motor16driving the transfer belt33.

In step S160, the current control part120controls the control motor15, and changes the rotational velocity of the intermediate transfer member5to the predetermined velocity Vm. In the case of the direct transfer system, the image forming apparatus110-2controls the control motor16and the changes the rotational velocity of the transfer belt33to the predetermined velocity Vm. The predetermined velocity Vm is a velocity faster than the rotational velocity in the case of forming the color image, and is determined by the torque of the control motor15or the control motor16and the range of allowable deterioration of image quality which is caused by making the rotational velocity of the intermediate transfer member5or the transfer belt33faster.

In the case of the direct transfer system, the current control part120makes the rotational velocity of the transfer belt33be the predetermined velocity Vm. Simultaneously, the current control part120controls the control motor41, and makes the rotational velocity of the polygon mirror42be the predetermined velocity Vp′ which is slower than the rotational velocity in the case of forming the color image. The predetermined velocity Vp′ is determined by the torque of the control motor16and the range of allowable deterioration of image quality which is caused by making the rotational velocity of the transfer belt33faster.

Subsequently, in step S170, the current control part120controls so that the value of the current flowing to the control motor15driving the intermediate transfer member5is equal to the value of the current flowing to the control motor15in the case of forming the color image. In the case of the direct transfer system, the current control part120-2controls so that the value of the current flowing to the control motor16driving the transfer belt33is equal to the value of the current flowing to the control motor16in the case of forming the color image. Thus, in step S190, the image forming part110terminates the black and white print process.

When it is determined that the print process (of which the execution request is made) to be executed by the image forming part110is a color print (No in the step S120), in the step S180, the image forming part110conducts the color print process with respect to image data of which the execution request is made, and executes the color print process. Then, in step S190, the image forming part110terminates the color print process.

As described above, by making the rotational velocity of the intermediate transfer member5faster, a contact resistance between the photoreceptor1B for black and the intermediate transfer member5can be increased. Therefore, even in the case of forming the black and white image, it is possible to stably drive the control motor15of the intermediate transfer member5. In the case of the direct transfer system, by making the rotational velocity of the transfer belt33faster, it is possible to increase a contact resistance between the photoreceptor1B for black and the transfer belt33. Therefore, even in the case of forming the black and white image, it is possible to stably drive the control motor16of the transfer belt33.

A process example (part 3) conducted by the image forming apparatus100according to the embodiment will be described with reference toFIG. 6.FIG. 6is a flowchart for explaining the process example (part 3) by the image forming apparatus100. InFIG. 6, the current control part120controls the value of the current flowing to the control motor15by using the rotational velocity of the transfer belt33as a parameter.

In step S210, the image forming apparatus100begins a print process. In step S220, it is determined whether or not the print process (of which an execution request is made) to be conducted by the image forming part110is a black and white print process. When the print process is a black and white print (Yes in the step S220), in step S230, the image forming part110separates the photoreceptors1Y,1C, and1M for colors from the intermediate transfer member5.

In step S240, the image forming part110executes the black and white print process with respect to image data to which the execution request is made while conducting a control which will be described in step S250through step S270. In the step S250, the image forming part110drives the control motor14B driving the photoreceptor1B for black, the control motor15driving the intermediate transfer member5, the control motor17driving the transfer belt33, and the control motor41driving the polygon mirror42.

In the step S260, the current control part120changes the rotational velocity of the polygon mirror42to the predetermined velocity Vp, by controlling the control motor17and the control motor41, and changing the rotational velocity of the transfer belt33to the predetermined velocity Vs. The predetermined velocity Vs is a velocity slower than the rotational velocity in the case of forming the color image, and the predetermined velocity Vp is a velocity faster than the rotational velocity in the case of forming the color image. The predetermined velocities Vs and Vp are determined by the torque of the control motor15and the range of allowable deterioration of image quality which is caused by making the rotational velocity of the transfer belt33slower. In a relationship between the predetermined velocities Vs and Vp, absolute values of change rates, in which each of the rotational velocities in forming the color image is set as a reference, are equal to each other. Alternatively, the predetermined velocity Vs may be faster than the rotational velocity in the case of forming the color image. Simultaneously, the predetermined velocity Vp may be slower than the rotational velocity in the case of forming the color image.

Subsequently, in the step S270, the current control part120controls so that the value of the current flowing to the control motor15driving the intermediate transfer member5is equal to the value of the current flowing to the control motor15in the case of forming the color image. Thus, in step S290, the image forming part110terminates the black and white print process.

When it is determined that the print process (of which the execution request is made) to be executed by the image forming part110is a color print (No in the step S220), in the step S280, the image forming part110conducts the color print process with respect to image data of which the execution request is made, and executes the color print process. Then, in step S290, the image forming part110terminates the color print process.

As described above, by making the rotational velocity of the transfer belt33for conducing the secondary transfer slower (faster), the contact resistance with the intermediate transfer member5can be increased (decreased) Accordingly, it is possible to stably drive the control motor15driving the intermediate transfer member5.

A process example (part 4) conducted by the image forming apparatus100according to the embodiment will be described with reference toFIG. 7.FIG. 7is a flowchart for explaining the process example (part 4) by the image forming apparatus100. InFIG. 7, the current control part120controls the value of the current flowing to the control motor15by using the rotational velocity of the driven roller22as a parameter.

In step S310, the image forming apparatus100begins a print process. In step S320, it is determined whether or not the print process (of which an execution request is made) to be conducted by the image forming part110is a black and white print process. When the print process is a black and white print (Yes in the step S320), in step S330, the image forming part110separates the photoreceptors1Y,1C, and1M for colors from the intermediate transfer member5.

In step S340, the image forming part110conducts the black and white print process with respect to image data to which the execution request is made, while conducting a control as described in step S350through step S370. In the step S350, the image forming part110drives the control motor14B driving the photoreceptor1B for black, the control motor15driving the intermediate transfer member5, and the control motor17driving the transfer belt33.

In step S360, the current control part120controls the control motor31, and applies a load (brake) to the rotation of the driven roller22. Also, the control motor31may be directly connected to the driven roller22. Alternatively, in order to be approximately the same as the load torque of the control motor15in the case of forming the color image, the control motor31may be connected to the driven roller22through the gear30. Moreover, the current control part120increases the load torque which the control motor15receives, to be the same level in the case of forming the color image.

Subsequently, in the step S370, the current control part120controls so that the value of the current flowing to the control motor15driving the intermediate transfer member5is equal to the value of the current flowing to the control motor15in the case of forming the color image. Thus, in step S390, the image forming part110terminates the black and white print process.

When it is determined that the print process (of which the execution request is made) to be executed by the image forming part110is a color print (No in the step S320), in the step S380, the image forming part110conducts the color print process with respect to image data of which the execution request is made, and executes the color print process. Then, in step S390, the image forming part110terminates the color print process.

As described above, it is possible to stably drive the control motor15driving the intermediate transfer member5in the case of forming the black and white print process, by connecting the control motor31as a motor for a brake to the driven roller22as a drive shaft of the intermediate transfer member5.

According to the present invention, even in the case of forming the black and white image in which the photoreceptors1Y,1C, and1M for colors are separated from the intermediate transfer member5or the transfer belt33, it is possible to provide the image forming apparatus100or100-2which can stably rotate and drive the motor15or16driving the intermediate transfer member5or the transfer belt33and can form a high quality image.

The present application is based on the Japanese Priority Patent Applications No. 2008-200038 filed Aug. 1, 2008 and No. 2009-168442 filed Jul. 17, 2009, the entire contents of which are hereby incorporated by reference.