Patent Publication Number: US-9423746-B2

Title: Image forming apparatus and method in which a developer carrying member is rotated for a predetermined time during a startup or removal operation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2014-005705, filed on Jan. 16, 2014, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming apparatus such as a copy machine, a printer, a facsimile, a multifunction peripheral of the aforementioned, etc. 
     2. Description of the Related Art 
     Generally, a developer carrier is used in an image forming apparatus, such as a copy machine, a printer, a facsimile, a multifunction peripheral, etc., to develop a latent image formed on a photoconductor, which is a latent image bearer. The developer carrier is arranged at a position facing the photoconductor so as to develop the latent image on the photoconductor by carrying the developer stored in a developer accommodating container and conveying the developer to a developing position. 
     Conventionally, there are suggested various structures to remove a developer that remains on the photoconductor when an operation of a developing unit is stopped during a developing operation in such an image forming apparatus. For example, Japanese Laid-Open Patent Application No. 2001-209277 suggests a structure of collecting a developer remaining on a photoconductor by a developer carrier after rotating the photoconductor to convey the developer to the position at which the developer carrier is arranged while changing the electrostatic characteristic of the developer remaining on the photoconductor. 
     However, if the photoconductor is driven and rotated to remove the remaining developer, there may occur a rotation abnormality of the photoconductor due to locking of a photoconductor drive motor, which is caused by a load fluctuation generated by the remaining developer. 
     Thus, it is desirous to suppress the occurrence of the rotation abnormality of the photoconductor during the removing operation of removing the developer remaining on the photoconductor, which condition is caused by a stopping operation during a developing operation, when performing a subsequent startup operation. 
     SUMMARY OF THE INVENTION 
     There is provided according to an aspect of the present invention an image forming apparatus including a latent image bearer that carries a latent image and a developer carrier including a developer carrying member that carries a developer on a surface thereof. The developer carrier conveys the developer to a developing position opposite to the latent image bearer by moving the developer carrying member so as to develop the latent image carried by the latent image bearer. In a startup operation after a stop operation performed during a developing operation by the developer carrier, the developer carrying member is rotated for a predetermined time while said latent image bearer is set in a stopped state before starting a rotation. 
     There is provided according to another aspect of the present invention an image forming apparatus including a latent image bearer that carries a latent image and a developer carrier including a developer carrying member that carries a developer on a surface thereof. The developer carrying member of the developer carrier is rotated to convey the developer to a developing position opposite to the latent image bearer so as to develop the latent image carried by the latent image bearer. In a startup operation of the image forming apparatus after a stop operation, the developer carrying member is rotated for a predetermined time while the latent image bearer is set in a stopped state before starting a rotation. 
     There is provided according to a further aspect of the present invention an image forming method performed by an image forming apparatus including a latent image bearer that carries a latent image and a developer carrier including a developer carrying member that carries a developer on a surface thereof. The image forming method includes: in a startup operation of said image forming apparatus, determining whether an immediately preceding stop operation of said image forming apparatus is a normal stop or an abnormal stop; when the immediately preceding stop operation is the abnormal stop, rotating the developer carrying member for a first predetermined time while the latent image bearer is set in a stopped state before starting a rotation; after the first predetermined time has passed, rotating the latent image bearer and charging a surface of the latent image for a second predetermined time; and after the second predetermined time has passed, rotating the latent image bearer; conveying the developer carried by the developer carrying member to a developing position opposite to the latent image bearer by moving the developer carrying member; and developing the latent image carried by the latent image bearer by the developer conveyed by the developer carrying member at the developing position. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a hardware structure of an image forming apparatus according to an embodiment; 
         FIG. 2  is an illustration of a plotter hardware part; 
         FIG. 3  is an enlarged cross-sectional view of a developing device; 
         FIGS. 4A and 4B  are illustrations illustrating potential relationships between a photoconductor and a developing sleeve at a developing position; 
         FIG. 5  is an illustration of a developer for explaining an event that occurs when an abnormal stop is performed; 
         FIG. 6  is a block diagram of a startup operation controlling part; 
         FIG. 7  is a time chart illustrating an operation timing of each part during a startup operation performed by a printer; 
         FIG. 8  is a time chart illustrating an operation timing of each part during a startup operation performed by the image forming apparatus according to the embodiment; 
         FIG. 9  is an illustration of a developer for explaining an event that occurs during a startup operation after an abnormal stop; 
         FIGS. 10A and 10B  are illustrations illustrating potential relationships between the photoconductor and the developing sleeve at a developing position after a long time has passed from an abnormal stop; 
         FIG. 11  is a flowchart of a startup operation controlled by a startup operation controlling part; and 
         FIG. 12  is a flowchart of another startup operation controlled by the startup operation controlling part. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A description will now be given, with reference to the drawings, of embodiments of the present invention. In the embodiments described below, it is assumed that the “abnormal stop” designates i) a stop due to an activation of an emergency stop control during a developing operation in an image forming apparatus, ii) a stop due to a power OFF of an image forming apparatus during a developing operation, etc. The emergency stop control is performed when it is determined that the image forming apparatus is set in an abnormal state during a developing operation, which may lead to a malfunction, such as, for example, a case where an opening/closing door is open during a developing operation. As a case where a power of the image forming apparatus is turned OFF during a developing operation, there may be a power failure, an interruption of a power supply breaker, an erroneous operation of a power supply switch by a user, etc. 
     &lt;1. Hardware Structure of Image Forming Apparatus&gt; 
     First, a description is given of a hardware structure of an image forming apparatus according to an embodiment.  FIG. 1  is a block diagram of a hardware structure of an electrophotographic printer  100  (hereinafter, simply referred to as the “printer”), which is an example of the image forming apparatus according to the present embodiment. 
     As illustrated in  FIG. 1 , the printer  100  includes a CPU (computer)  111 , ROM  112 , RAM  113  and storage device  114  such as an HDD (Hard Disk Drive) or the like. The printer  100  further includes an engine part, operating part  116  and communication I/F part  117 . These elements constituting the printer  100  are mutually connected through a bus  118 . 
     The CPU  111  controls the entire printer  100  by executing various programs recorded in the ROM  112  or the storage device  114  using the RAM  113  as a work area. The CPU  111  also materializes various functions including a startup operation control function mentioned later. 
     The storage device  114 , which is a non-volatile storage medium, records programs executed by the CPU  111  and various kinds of data. The programs recorded in the storage device  114  include a program executed by the CPU  111  to materialize the startup operation controlling part  130  that provides a startup operation control function mentioned later. 
     The engine part  115  is provided with hardware (a plotter hardware part  140 ) for materializing a printing function. Details of the plotter hardware part  140  will be mentioned later with reference to  FIG. 2 . 
     The operating part  116  is used when a user performs various operations, such as inputting various settings to cause the printer  100  to perform a printing function and inputting an instruction to cause the printer  100  to perform a printing function. The communication I/F  117  is an interface for communication with an external device (not illustrated in the figure). 
     &lt;2. Outline Structure of Plotter Hardware&gt; 
     A description is given below of an outline structure of the plotter hardware part  140  that constitutes the printer  100 .  FIG. 2  is an illustration of a structure of the plotter hardware part  140 . 
     As illustrated in  FIG. 2 , the plotter hardware part  140  includes four toner image forming parts  206 Y,  206 M,  206 C and  206 K to create toner images in yellow, magenta, cyan and black (hereinafter, represented by Y, M, C and K) 
     The toner image forming parts  206 Y,  206 M,  206 C and  206 K use a Y toner, an M toner, a C toner and a K toner, respectively, but they have the same structure except for the usage of the different color toners. Thus, a description is given below of only the toner image forming part  206 Y for creating a Y toner image as a representative of the four toner image forming parts  206 Y,  206 M,  206 C and  206 K. 
     The toner image forming part  206 Y includes a drum-shaped photoconductor  201 Y, which is a latent image bearer, a drum cleaning device  202 Y, a discharging device (not illustrated), a charging device  204 Y, a developing device  205 Y, and an exposure device  207 Y. The charging device  204 Y charges the entire surface of the photoconductor  201 Y at a uniform potential while the photoconductor  201 Y is driven by a driving means (not illustrated) to rotate in a counterclockwise direction in the figure. 
     The surface potential of the photoreceptor  201 Y charged by the charging device  204 Y is hereinafter indicated as V 0 . The photoconductor  201 Y carries an electrostatic latent image for Y by being scan-exposed by a laser light emitted by the exposure device  20 . The surface potential of the electrostatic latent image portion of the photoconductor  201 Y, which is scan-exposed by the laser light, is hereinafter indicated as V 1 . The electrostatic latent image for Y is developed to be a Y toner image by the developing device  205 Y provided with the Y toner. Then, the Y toner image is transferred onto an intermediate transfer belt, which is an intermediate transfer member. 
     The drum cleaning device  202 Y performs cleaning to remove the toner remaining on the surface of the photoconductor  201 Y after being subjected to an intermediate transfer process. The discharging device discharges the residual electric charge of the photoconductor  201 Y after the cleaning by the drum cleaning device  202 Y. According to the discharging by the discharging device, the surface of the photoconductor  201 Y is initialized to be prepared for a subsequent image forming operation. 
     Similarly in other toner image forming parts  206 M,  206 C and  206 K, an M toner image, C toner image and K toner image are formed on the photoconductors  201 M,  201 C and  201 K, and are transferred onto the intermediate transfer belt  208 . 
     The exposing device  207 Y,  207 M,  207 C and  207 K, which are latent image forming units, project laser lights, which are emitted based on image information, onto the respective photoconductors  201 Y,  201 M,  201 C and  201 K in the toner image forming parts  206 Y,  206 M,  206 C and  206 K so as to expose the photoconductors  201 Y,  201 M,  201 C and  201 K with the respective laser lights. According to the exposure, the electrostatic latent images for Y, M, C and K are formed on the respective photoconductors  201 Y,  201 M,  201 C and  201 K. 
     An intermediate transfer unit  215  is arranged underneath the toner image forming parts  206 Y,  206 M,  206 C and  206 K. The intermediate transfer unit  215  includes the intermediate transfer belt  208  is an endless belt that rotates to move the toner images transferred thereon. The intermediate transfer unit  215  further includes four primary transfer bias rollers  209 Y,  209 M,  209 C and  209 K, a cleaning device  210  and a secondary transfer backup roller  212 . 
     The intermediate transfer belt  208  rotates in a clockwise direction in the figure. The primary transfer bias rollers  209 Y,  209 M,  2090  and  209 K sandwich the intermediate transfer belt  208  with the respective photoconductors  201 Y,  201 M,  201 C and  201 K so as to form primary transfer nip portions, respectively. In the primary transfer nip portions, the primary transfer bias rollers  209 Y,  209 M,  209 C and  209 K apply a transfer bias, which is a reverse polarity (for example, a plus bias voltage), to the backside of the intermediate transfer belt  208  (the inner surface of the loop). All of the rollers excluding the primary transfer bias rollers  209 Y,  209 M,  209 C and  209 K are electrically grounded. 
     In the process of sequentially passing through the primary transfer nip portions for Y, M, C and K with the rotation of the intermediate transfer belt  208 , the Y, M, C, and K toner images on the photoconductors  201 Y,  201 M,  201 C and  201 K are primarily transferred onto the intermediate transfer belt  208  in an overlapping manner. Thereby, a four color overlapping toner image (hereinafter, referred to as the “4-color toner image”) is formed on the intermediate transfer belt  208 . 
     The secondary transfer backup roller  212  forms a secondary nip portion by sandwiching the intermediate transfer belt with a secondary transfer roller  219 . The 4-color toner image formed on the intermediate transfer belt  208  is transferred onto a transfer paper P at the secondary nip portion. 
     A reflection type photosensor  240  is arranged to oppose to the intermediate transfer belt  208  in an area between the lowermost toner image forming part  206 K and the secondary transfer nip portion so that the photosensor  240  outputs a signal corresponding to a reflectance of the surface of the intermediate transfer belt  208 . Specifically, the photosensor  240  includes reflection-type photosensors for each color Y, M, C and K that are arranged in a line in a direction of depth in  FIG. 2  so as to individually detect an image density of each color in the 4-color toner image. 
     A remaining toner, which has not been transferred to the transfer paper P, adheres to the intermediate transfer belt  208  after passing through the secondary transfer nip portion. The transfer remaining toner is removed from the intermediate transfer belt  208  by the cleaning device  210 . In the secondary transfer nip portion, the transfer paper P is conveyed by being sandwiched between the intermediate transfer belt  208  and the secondary transfer roller  219  each of which rotates in a normal direction. The 4-color toner image is transferred to the surface of the transfer paper P conveyed out of the secondary transfer nip portion. The 4-color toner image is fixed by heat and pressure when the transfer paper P passes through an area between rotating rollers of the fixing device  220 . 
     &lt;3. Structure of Developing Device&gt; 
     A description is given below of the details of the structure of the developing device  205 Y.  FIG. 3  is an enlarged cross-sectional view of the developing device  205 Y. The developing device  205 Y includes a developing roller  301 Y. A part of the circumferential surface of the developing roller  301 Y is exposed outside through an opening part provided in a casing  310 Y. 
     The developing roller  301 Y, which is a developer carrying roller, includes a developing sleeve  311 Y (developer carrying member) and a magnet roller  312 Y. The developing sleeve  311 Y is made of a non-magnetic pipe, and is rotatable by a driving means (not illustrated in the figure). The magnet roller  312 Y is encircled by the developing sleeve  311 Y, and does not rotate together with the rotation of the developing sleeve  311 Y. 
     A certain amount of Y developer (not illustrated in the figure) is stored in the developing device.  205 Y. The Y developer contains a magnetic carrier and a Y toner having a minus charging property. 
     The Y developer is agitated and conveyed by two conveyance screws  304 Y and  305 Y. Thereby, the Y toner contained in the Y developer is frictionally charged. Then, the Y developer is attached to the surface of the rotating developing sleeve  311 Y by the magnetic force of the magnet roller  312 , which is a magnetic field generating means in the developing sleeve  301 Y, and is conveyed by the developing sleeve  311 Y. 
     The Y developer is conveyed with a rotation of the developing sleeve  311 Y and passes through a position opposite to a developing doctor  30 , which is a restricting member. The thickness of the Y developer is restricted by the developing doctor  30 , and, then, the Y developer is conveyed to a developing position opposite to the photoconductor  201 Y. 
     In the developing position, between the developing sleeve  311 Y to which the developing bias of a negative polarity is applied and the electrostatic latent image portion (surface potential=V 1 ) on the photoconductor  201 Y, a developing potential is applied to the Y toner to electrostatically move the Y toner having a negative polarity from the side of the developing sleeve  311 Y to the side of the electrostatic latent image. Additionally, between the developing sleeve  311 Y and the uniformly charged portion (surface portion (surface potential=V 0 )) on the photoconductor  201 Y, a non-developing potential is applied to the Y toner to electrostatically move the Y toner having a negative polarity from the side of the surface portion to the side of the developing sleeve  311 Y. It is assumed that the developing bias Vb having a negative polarity is supplied to from a power supply source (not illustrated in the figure). 
     The Y toner contained in the Y developer on the developing sleeve  311 Y is separated from the developing sleeve  311 Y due to an action of the developing potential, and is transferred to the electrostatic latent image portion of the photoconductor  201 Y. According to the transfer, the latent image on the photoconductor  201 Y is developed and changed into a Y toner image. The Y developer of which the Y toner is consumed by the development is returned to the interior of the casing  310 Y with the rotation of the development sleeve  311 Y. 
     As illustrated in  FIG. 2 , the developing device  205 Y has a toner density sensor  230 Y, which is constituted by a magnetic permeability sensor. The toner density sensor  230 Y outputs a voltage corresponding to the magnetic permeability of the Y developer stored in the developing device  205 Y. Because the magnetic permeability of the developer exhibits an excellent collation with the toner density of the developer, the toner density sensor  230 Y outputs a voltage corresponding to the toner density. The value of the output voltage is sent to a toner supply controlling part (not illustrated in the figure). 
     The toner supply controlling part includes a storage unit, such as a RAM, in which a target value (Vtref for Y) of the voltage output from the toner density sensor for Y is stored. The storage unit also stores data of Vtref for M, C and K, which represent target values of voltages output from the toner density sensors mounted on other developing devices. 
     In the developing device  205 Y, the value of the voltage output from the toner density sensor  230 Y is compared with Vtref, which is a target vale of the output voltage for Y so as to cause the Y toner density supply device (not illustrated in the figure) to drive for a time corresponding to a result of the comparison. Thereby, the replenishing Y toner can be supplied to the developing device  205 . 
     Accordingly, by the control of the drive of the Y toner density supply device, an appropriate amount of the Y toner is supplied to the Y developer of which the Y toner density is decreased with the developing operation. Thus, the density of Y toner contained in the Y developer stored in the developing device  205 Y can be maintained within a predetermined range. For example, the toner density is maintained within a range from 5 weight % to 9 weight % in a developer that is a combination of a toner having a particle diameter of 6 μm and a carrier having a particle diameter of 35 μm. 
     &lt;4. Event Occurring at Developing Position at Abnormal Stop Time&gt; 
     A description is given below of an event which occurs at the developing position when an operation of the printer  100  having the above-mentioned structure is stopped due to an abnormality (hereinafter, referred to as the “abnormal stop”). In a state where a developing operation is carried out normally, at the developing position, the surface potential V 0  of the uniformly charged portion (surface portion) of the photoconductor  201 Y, the developing bias Vb having a negative polarity and the surface potential V 1  of the electrostatic latent image portion have a relationship as illustrated in  FIG. 4A . 
     On the other hand, if an operation of the printer  100  is stopped due to an abnormality and a power supply is interrupted, the developing bias Vb drops to the ground potential because a power supply to a power board, which has applied the developing bias, is interrupted. On the other hand, the surface potential V 0  of the uniformly charged portion (surface portion) and the surface potential V 1  of the electrostatic latent image portion on the photoconductor  201 Y are almost unchanged and maintained at the values before the abnormal stop. As a result, the relationship in potential between the photoconductor  201 Y and the developing sleeve  311 Y at the developing position in the case of the abnormal stop is as illustrated in  FIG. 4B . 
     In the state illustrated in  FIG. 4B , an extremely large electric field is generated between the developing sleeve  311 Y, which has become a ground potential, and the uniformly charged portion (surface potential=V 0 ) of the photoconductor  201 Y to move the carrier having a positive polarity from the developing sleeve  311 Y to the surface side of the photoconductor  201 . 
     Here, at the time of the abnormal stop, the power supply to a drive motor, which is a driving unit to rotate the photoconductor  201 Y and the developing sleeve  311 Y, and the power supply to the power board, which has supplied the developing bias Vb, are interrupted almost simultaneously. On the other hand, after the developing bias Vb dropped to the ground potential and the condition of the potentials is set to the relationship illustrated in  FIG. 4B , the photoconductor  201 Y and the developing sleeve  311 Y continue to rotate due to an inertial force for a time of about several hundred milliseconds. 
     At this time, because the extremely large electric field is generated at the developing position to move the carrier having a positive polarity from the developing sleeve  311 Y to the surface side of the photoconductor  201 Y, an adhesion of the carrier to the photoconductor  201  occurs. As a result, as illustrated in  FIG. 5 , a developer pool  500 Y in which the developer is accumulated is formed on an upstream side of the developing position. 
     Note that an amount of the developer pool  500 Y becomes larger as the potential of the uniformly charged portion (surface portion (surface potential=V 0 )) of the photoconductor  201 Y is higher (larger) because the intensity of the electric field that electrostatically moves the carrier having a positive polarity from the developing sleeve  311 Y to the surface side of the photoconductor  201 Y is increased. 
     &lt;5. Functional Structure of Startup Operation Controlling Function&gt; 
     A description is given below of a functional structure of the startup operation controlling part  130  in the printer  100 , which suppresses an occurrence of a rotation abnormality of the photoconductor  201 Y while removing the above-mentioned developer pool  500 Y.  FIG. 6  is a block diagram illustrating a functional structure of the startup operation controlling part  130  in the printer  100 . 
     As illustrated in  FIG. 6 , the startup operation controlling part  130  includes a detecting part  600 , a photoconductor drive motor controlling part  601 , a charging device controlling part  602 , a developing roller drive motor controlling part  603  and a developing bias controlling part  604 . 
     The detecting part  600  determines whether an immediately preceding stop operation was “normal stop” or “abnormal stop”. Specifically, the detecting part  600  sets a flag during the developing operation and resets the flag after ending the developing operation. The detecting part  600  refers to the flag when performing a startup operation after a stop operation to determine whether the stop was “normal stop” or “abnormal stop”. A result of the comparison is sent to the photoconductor drive motor controlling part  601 , the charging device controlling part  602 , the developing roller drive motor controlling part  603  and the developing bias controlling part  604 . 
     The photoconductor drive motor controlling part  601  controls a photoconductor drive motor (not illustrated in the figure), which is a drive unit of rotating the photoconductor  201 Y, to drive/stop. 
     The charging device controlling part  602  controls a charging operation to the photoconductor  201 Y by the charging device  104  to start/end. Note that the photoconductor derive motor controlling part  601  and the charging device controlling part  602  are configured to operate in synchronization with each other. Additionally, each of the photoconductor drive motor controlling part  601  and developing roller drive motor controlling part  603  is configured to operate in response to an operating condition of the other. 
     &lt;6. Startup Operation After Abnormal Stop&gt; 
     A description will be given of a startup operation performed by the startup operation controlling part  130  of the printer  100  after an abnormal stop. First, as a target for comparison, a description is given of a typical startup operation performed in a popular printer. 
     &lt;6.1 Startup Operation After Abnormal Stop in General Printer&gt; 
       FIG. 7  is a time chart indicating operation timings of a photoconductor, charging device, developing sleeve and developing bias in a startup operation after an abnormal stop in a general printer. As illustrated in  FIG. 7 , in the startup operation after an abnormal stop in a popular printer, the photoconductor and the charging device start operations first, and, then, a rotation of the developing sleeve and an application of the developing bias are started after a fixed time period (section  701 ) has passed. 
     In this case, there is no change in the developer pool  500 Y on the upstream side of the developing position, which is formed due to the abnormal stop during the section  701  where the photoconductor rotates first in the state where the developing sleeve is stopped. 
     Thereafter, when the rotation of the development sleeve is started (in section  702  in  FIG. 7 ), the developer accumulated in the developer pool  500 Y is pressed into the developing position all at once. As a result, a load fluctuation is generated, which causes the photoconductor drive motor to lock, and, thereby, a rotation abnormality of the photoconductor occurs. 
     Especially, if a stepping motor is used as the photoconductor drive motor, which is a drive unit for driving the photoconductor, the locking of the photoconductor drive motor occurs more easily when the developer accumulated in the developer pool  500 Y is pressed into the developing position all at once and a load fluctuation is generated. 
     &lt;6.2 Startup Operation After Abnormal Stop in Printer  100 &gt; 
     A description is given below of a startup operation after an abnormal stop performed by the startup operation controlling part  130  of the printer  100  after according to the present embodiment. In the printer according to the present embodiment, if the detecting part  600  determines that an immediately preceding stop operation is the “abnormal stop”, a startup operation different from the above-mentioned startup operation after an abnormal stop in the popular printer is performed in order to suppress an occurrence of a rotation abnormality of the photoconductor  201 Y. 
       FIG. 8  is a time chart indicating operation timings of the photoconductor  201 Y, charging device  204 Y, developing sleeve  311 Y and developing bias in the startup operation after an abnormal stop performed by the startup operation controlling part  130  of the printer according to the present embodiment. As illustrated in  FIG. 8 , in the printer according to the present embodiment, the developing sleeve  311  is rotated for a predetermined time period (section  803 ) in a stopped state before the rotation of the photoconductor  201 Y and the charging by the charging device  204  are started. Additionally, the developing bias is applied to the developing sleeve  311 Y for the predetermined time period (section  803 ). 
     If the developing sleeve  311 Y is rotated in a state where the photoconductor  201 Y is stopped, the developer accumulated in the developer pool  500 Y flows and passes through the developing position (the closest point between the developing sleeve and the photoconductor). Thereby, the amount of the developer in the developer pool  500 Y is reduced. In this circumstance, because the photoconductor  201 Y is not rotated yet, there is no possibility that the photoconductor drive motor locks due to a load fluctuation. 
     Here, in order to reduce the developer in the developer pool  500 Y to the extent that the photoconductor drive motor does not lock when the photoconductor drive motor is driven in the section  701 , it is necessary to rotate the developing sleeve  311 Y by more than a fixed angle (for example, a half rotation) to carry the developer in the developer pool  500 Y to a downstream side. 
     Although the time period (section  803 ) for rotating the developing sleeve  311 Y by more than the fixed angle depends on the characteristic of the developer and the photoconductor drive motor, an environment, etc., the section  803  is preferably about 20 milliseconds to about 1 second. Note that an upper limit is set to the time of rotation of the developing sleeve  311 Y because there is a possibility of generating an uneven wear of the photoconductor  201 Y due to local abrasion of the photoconductor  201 Y if the developing sleeve  311 Y is rotated for a long time in a state where the photoconductor  201 Y is stopped. Additionally, it is not desirable from the viewpoint of user convenience to spent a long time in the startup operation due to the rotation of the developing sleeve  311 Y. 
     Note that if the process speed (a linear velocity of the photoconductor surface) of the printer  100  is set to 440 mm/s, the diameter of the development sleeve  311 Y is set to 30 mm, and the development linear velocity ratio is set to 1.5, a time required by the developing sleeve  311 Y to rotate a half rotation is about 71 milliseconds. If such a time period is taken, it can contribute to the prevention of locking of the photoconductor drive motor to that extent of a range almost the same as the time required by the startup operation after an abnormal stop in the above-mentioned popular printer. 
     Note that the amount of the developer in the developer pool  500 Y may be grasped previously so as to calculate a time period required for reducing the amount of developer to the extent that the photoconductor drive motor does not lock. The thus-calculated time period is stored in a memory of the printer  100 , and the section  803  may be determined based on the stored time period. 
     &lt;6.3 Reason for Applying Developing Bias in Startup Operation After Abnormal Stop in Printer  100 &gt; 
     As mentioned above, in the printer  100  according to the present embodiment, the developing sleeve  311 Y is rotated for the predetermined time (section  803 ) and a developing bias is applied during the startup operation after an abnormal stop in the state where the photoconductor  201  is stopped. However, the photoconductor drive motor can be prevented from being locked by merely rotating the developing sleeve  311 Y without applying a developing bias. The reason for applying a developing bias in synchronization with the rotation of the developing sleeve  311  is mentioned below. 
     Because the dark attenuation speed of the photoconductor  201 Y is slow, it takes a considerably long time (several minutes to several ten minutes) until the surface potentials V 0  and V 1  fall below the developing bias Vb. Thus, the surface potential V 0  of the uniformly charged portion (surface portion) and the surface potential V 1  of the electrostatic latent image portion of the photoconductor  201 Y are almost unchanged from the state at the time of abnormal stop in the printer  100 . 
     Accordingly, if the developing sleeve  311 Y is rotated without applying a developing bias in the section  803 , similar to the time of an abnormality stop, an extremely large electric field is generated, which electrostatically moves the carrier having a positive polarity from the developing sleeve  311 Y to the photoconductor  201 Y. As a result, an amount of the developer flowing through the developing position (closest position between the developing sleeve and the photoconductor) becomes small and, thereby, the amount of the developer stored in the developer pool  500 Y hardly reduces. Additionally, because the carrier adheres to the surface of the photoconductor  201 Y, if the photoconductor  201 Y is rotated in the section  701 , the carrier adhering to the surface of the photoconductor  201  is input into a drum cleaning device  202 Y and a cleaning device  210  in a subsequent stage. As a result, there may be a problem of an occurrence of an adverse reaction that the photoconductor  201 Y and the intermediate transfer belt  208  are damaged. 
     On the other hand, if a developing bias is applied in synchronization with the rotation of the developing sleeve  311 Y in the section  803 , the photoconductor drive motor can be reliably prevented from being locked, and also such an adverse reaction can be prevented from being occurred. 
     An advantage of applying a developing bias in synchronization with the rotation of the developing sleeve  311 Y in the section  803  can be acquired even in a case where a considerable time has passed after an abnormal stop of the printer  100  and until a startup operation is performed. 
     If a considerable time has passed after an abnormal stop of the printer  100  and until a startup operation is performed, the surface potential of the photoconductor  201  may be dark-attenuated (refer to an arrow  1000  indicated between  FIGS. 10A and 10B ), and the surface potential falls below the developing bias Vb. In this condition, an electric field, which causes the Y toner having a negative polarity to move from the side of the developing sleeve  311 Y to the side of the electrostatic latent image, is generated in an area between the developing sleeve  311 Y and the electrostatic latent image portion (surface potential=V 1 ) on the photoconductor  201 Y. Thus, the Y toner having a negative polarity merely adheres to the surface of the photoconductor  201 Y, and the above-mentioned adverse reaction does not occur. 
     As mentioned above, it is more desirable to apply a development bias in synchronization with the rotation of the developing sleeve  311 Y in the startup operation after an abnormal stop. 
     &lt;7. Flow of Startup Operation by Startup Operation Controlling Part&gt; 
     A description will be given of a flow of a startup operation performed by the startup operation controlling part  130 .  FIG. 11  is a flowchart illustrating a flow of a basic startup operation performed by the startup operation controlling part  130 . 
     First, in step S 1102 , the development roller drive motor controlling part  603  sends an instruction to drive the developing roller drive motor to rotate the developing sleeve  311 Y. Additionally, the developing bias controlling part  604  sends an instruction to apply a developing bias to the developing sleeve  311 Y. 
     In step S 1103 , the startup operation controlling part  130  determines whether a predetermined time (section  803 ) has passed after the instruction of driving the development roller drive motor and the instruction of applying a developing bias were sent. If it is determined in step S 1103  that the predetermined time (section  803 ) has not passed yet, the startup operation controlling part  130  waits for passage of the predetermined time (section  803 ). 
     On the other hand, if it is determined in step S 1103  that the predetermined time (section  803 ) has passed, the startup operation controlling part  130  proceeds to step S 1104 . In step S 1104 , the developing roller drive motor controlling part  603  sends an instruction to stop the developing roller drive motor to stop the rotation of the developing sleeve  311 Y. Additionally, the developing bias controlling part  604  sends an instruction to end the application of the developing bias to the developing sleeve  311 Y. 
     Then, in step S 1105 , the photoconductor drive motor controlling part  601  sends an instruction to drive the photoconductor drive motor to rotate the photoconductor  201 Y. Additionally, the charging device controlling part  602  sends an instruction to start charging by the charging device  204 Y. 
     Then, in step S 1106 , the startup operation controlling part  103  determines whether a predetermined time (section  701 ) has passed after the instruction of driving the photoconductor drive motor and the instruction of starting the charging were sent. If it is determined in step S 1106  that the predetermined time (section  701 ) has not passed yet, the startup operation controlling part  103  waits for passage of the predetermined time (section  701 ). 
     On the other hand, if it is determined in step S 1106  that the predetermined time (section  701 ) has passed, the startup operation controlling part  130  proceeds to step S 1107 . In step S 1107 , the developing roller drive motor controlling part  603  sends an instruction to drive the developing roller drive motor to rotate the developing sleeve  311 Y. Additionally, the developing bias controlling part  604  sends an instruction to start applying a developing bias to the developing sleeve  311 Y. Thereby, the startup operation after an abnormal stop is completed. 
     A description is given below, with reference to another example of the startup operation.  FIG. 12  is a flowchart of another example of the startup operation performed by the startup operation controlling part  130 . The startup operation illustrated in  FIG. 12  is the same as the startup operation illustrated in  FIG. 11  except for a determining process performed by the detecting part  600 , and duplicate descriptions of the steps will be omitted. 
     In the startup operation illustrated in  FIG. 12 , the process of step S 1101  is first performed by the detecting part  600  of the startup operation controlling part  130 . That is, in step S 1101 , the detecting part  600  determines whether the immediately preceding stop operation is a normal stop or an abnormal operation. If the detecting part  600  determines in step S 1101  that the immediately preceding step is an abnormal stop, the process proceeds to step S 1102  to perform the above-mentioned startup operation explained with reference to  FIG. 11 . Thereby, the startup operation after an abnormal stop is completed. 
     On the other hand, if the detecting part  600  determines in step S 1101  that the immediately preceding stop operation is a normal stop, the process proceeds to step S 1105  without performing the process of steps S 1102  through S 1104  to perform the above-mentioned process of steps S 1105  through S 1107 . Thereby, the startup operation after a normal stop is completed. 
     &lt;8. Summary&gt; 
     As mentioned above, in the printer  100  according to the present embodiment, the developing sleeve is rotated for the predetermined time in the stopped state before the rotation of the photoconductor is started, and a developing bias is applied in synchronization with the rotation of the developing sleeve for the predetermined time. 
     That is, the image forming apparatus according to the present embodiment rotates the developing sleeve and applies a developing bias for the predetermined time in the state where the photoconductor is stopped. Accordingly, an amount of the developer in the developer pool created due to an abnormal stop can be reduced before the rotation of the photoconductor is started. 
     As a result, it becomes possible to avoid an occurrence of a rotation abnormality of the photoconductor due to a load fluctuation caused by a remaining developer when the photoconductor is started to rotate in the startup operation after an abnormal stop. 
     In the present embodiment, a DC motor or a stepping motor can be used as the photoconductor drive motor, which is a drive unit for rotating the photoconductor  201 Y. However, as mentioned above, the stepping motor has a characteristic of being more easily locked than the DC motor when a load fluctuation occurs due to the developer accumulated in the developer pool  500 Y being pushed into the developing position (the closest position between the developing sleeve and the photoconductor) all at once. Thus, the present embodiment is especially effective to the case where a stepping motor is used as the photoconductor drive motor, which is a drive unit for rotating the photoconductor  201 Y. 
     Moreover, according to startup operation illustrated in  FIG. 12 , different startup operations are performed depending on whether the immediately preceding stop operation is a normal stop or an abnormal stop. However, the startup operation is not limited to such operations, and the startup operation after a normal stop may be the same as the startup operation after an abnormal stop. 
     However, if the developing sleeve  311 Y is rotated in the state where the photoconductor  201 Y is stopped, a specific portion of the photoconductor  201 Y is scrubbed by the developer even though it is in an extremely short time. Thus, it is desirous to perform the startup operation illustrated in  FIG. 8  only after an abnormal stop. 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventors to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.