Abstract:
A color image forming apparatus is provided, and includes a plurality of image forming units including a photosensitive drum, a charging device and a developing device; a carriage for retaining and rotating the plurality of image forming units so as to move the image forming units between an image forming position and a waiting position; an exposing device for exposing a surface of the photosensitive drum of the image forming unit located at the image forming position; an intermediate transfer belt for successive transfer and superposition of toner images of various colors from the photosensitive drum of the image forming units located at the image forming position so as to form a color toner image; a drive member for driving the photosensitive drum and the intermediate transfer belt; a position detector for detecting a reference position of the intermediate transfer belt when the intermediate transfer belt is driven, and outputting a reference position detection signal; a secondary transfer roller for transferring the color toner image on the intermediate transfer belt onto recording paper; and a controller for controlling the operation of the above structural elements. The controller determines an operation start time of the charging device and the developing device using a rotation command signal to the driving means as a reference, and an operation start time of the exposing device, the intermediate transfer belt and the secondary transfer roller using the reference position detection signal as a reference.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a color image forming apparatus used in color printers, color copying machines or color facsimiles. More specifically, the present invention relates to a color image forming apparatus that forms a color toner image by overlapping several toner images of various colors on an intermediate transfer device in a primary transfer from several photosensitive drums, and transcribing the color toner image in a secondary transfer to a transfer material (recording paper). 
     BACKGROUND OF THE INVENTION 
     FIG. 11 shows the internal structure of a prior art example of a color image forming apparatus, as disclosed in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246. 
     The printer comprises an intermediate transfer belt unit  101  including an intermediate transfer belt  102 , a primary transfer roller  103 , a secondary transfer roller  104 , a cleaner roller  105 , and a waste toner reservoir  106 . Composition or superposition of color toner images is performed on the transfer belt  102 . A group of image forming units  108  is made up of four image forming units  107 Bk,  107 Y,  107 M and  107 C, each unit being of sector shape in cross section. As can be seen in FIG. 11, the image forming units are arranged circularly in the middle of the printer. 
     When an image forming unit  107 Bk,  107 Y,  107 M or  107 C is set properly in the printer, mechanical and electrical connection systems are established between one of the image forming units  107 Bk,  107 Y,  107 M and  107 C and the machine body side via mutual coupling members. The image forming units  107 Bk,  107 Y,  107 M and  107 C are supported by a supporter, which is rotationally driven by a motor via a cylindrical shaft  109 . Each image forming unit  107 Bk,  107 Y,  107 M, and  107 C is successively moved by rotation to an image forming position  110 . The image forming position  110  is the position where a photosensitive drum  118  of the image forming unit faces the intermediate transfer belt  102  on the primary transfer roller  103 , and is also the exposure position for exposure by a laser beam  111 . 
     A laser exposing device  112  is provided in the lower part of the printer. The laser signal beam  111  from the laser exposing device  112  passes through an opening  113  between the image forming units  107 M and  107 C, and through an opening provided in the cylindrical shaft  109 , and enters a mirror  114 . This mirror  114  is positioned inside the shaft  109  and fixed directly to the machine body. The reflected laser beam  111  enters the image forming unit  107 Bk located at the image forming position  110  through an opening  115 , and passes through the space between a developing device  116  and a cleaner  117  of the image forming unit  107 Bk, and enters an exposure portion of the photosensitive drum  118 . The laser signal beam  111  is scanned by the exposing device in the direction of the axis of the photosensitive drum  118 . The toner image, which is formed on the photosensitive drum  118  by exposure with the laser signal beam  111  and subsequent development with the developing device  116 , is transferred to the intermediate transfer belt  102 . 
     Then, the group of image forming units  108  rotates by 90 degrees, so that the yellow image forming unit  107 Y moves to the image forming position  110  to replace the black image forming unit  107 Bk. An operation similar to the operation explained above for the black image is performed to form a yellow image overlaying the black image formed on the intermediate transfer belt  102 . Subsequently, the magenta and cyan image forming units  107 M,  107 C are moved to the image forming position  110 , and similar operations as explained above are repeated to compose a full color image on the intermediate transfer belt  102 . This full color image is further transferred from the intermediate transfer belt  102  onto a recording paper using a secondary transfer roller  119 , and the image on the paper is fixed by a fixing device  120 . 
     In the color image forming apparatus as explained above, precise registration of the toner images of all four colors is very important for obtaining a high quality full color image. However, an image forming unit system of the prior art as explained above, which forms a color image by overlaying successively four toner images of four photosensitive drums at one image forming position onto an intermediate transfer device (belt) and forms a color image on the recording paper by a secondary transfer from the intermediate transfer device, has the following disadvantage: After the start-up (power on), variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. Therefore, the rotational position of the intermediate transfer device at a predetermined time after the start-up is not necessarily a predetermined position. Consequently, it is necessary to ensure that sufficient time has passed so that the rotation of the intermediate transfer device is in a stable condition in order for a high quality image to be provided. 
     On the other hand, there is the strong desire to accelerate the recording operation. The time spent until the four image forming units comprising a photosensitive drum have been switched, and the rotation of the intermediate transfer device has been stabilized, is reflected in the time that the color image forming device needs for forming an image. 
     Moreover, the circumference of the intermediate transfer belt has an influence on the size of the entire device. Therefore, to make the circumference of the intermediate transfer belt as small as possible is desirable in order to miniaturize the device. 
     A main object of the present invention is to solve the problems mentioned above by providing a color image forming apparatus combining improvement of the image quality and the image forming speed with miniaturization of the entire device. 
     SUMMARY OF THE INVENTION 
     A color image forming apparatus according to the present invention comprises: a plurality of image forming units corresponding to various colors, the image forming units including a photosensitive drum, a charging device and a developing device; a unit retaining member for retaining the image forming units and moving the image forming units between an image forming position and a waiting position; an exposing device for exposing the photosensitive drum of the image forming units when located at the image forming position; an intermediate transfer belt for successive transfer and superposition of toner images of various colors from the photosensitive drums of the image forming units located at the image forming position so as to form a color toner image; means for driving the photosensitive drums and the intermediate transfer belt; a detector for detecting a reference position of the intermediate transfer belt when the intermediate transfer belt is driven, and outputting a reference position detection signal; a secondary transfer device for transferring the color toner image on the intermediate transfer belt onto paper; and a controller for controlling the operation of the above structural elements. The controller determines an operation start time of the charging device and the developing device using a rotation command signal to the driving means as a reference, and an operation start time of the exposing device, the intermediate transfer belt and the secondary transfer device using the reference position detection signal as a reference. 
     Usually, the start and the stop of the photosensitive drum and the intermediate transfer belt are performed for each color. In that case however, variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. In the configuration according to the present invention however, the detector detects a reference position of the intermediate transfer belt after the driving of the intermediate transfer belt is started and outputs this reference position detection signal. The positioning of the overlayed toner image, which is transferred onto the intermediate transfer belt, becomes easier and more precise, because the operation start time for the exposing device, the intermediate transfer belt and the secondary transfer device are determined using the reference position detection signal. Positioning for the secondary transfer from the intermediate transfer belt onto paper becomes more precise as well. 
     On the other hand, the charging of the photosensitive drum before the photosensitive drum reaches a constant rotational velocity can be performed without problems. It is preferable that the charging of the photosensitive drum starts as early as possible after the start of the photosensitive drum, so that a larger charging portion on the surface of the photosensitive drum can be ensured. Especially in the case of the contact development method, wherein the developing roller is always contacting the photosensitive drum, it is preferable that a developing bias is impressed as early as possible after the start of the photosensitive drum for development. If this is not done, an unwanted use of toner can occur, because toner covers an unexposed area of the photosensitive drum as well. According to the configuration of the present invention, the charging and the developing of the photosensitive drum can be started as early as possible after the start of the photosensitive drum, because the operation start time of the charging device and the developing device are determined using a rotation command signal to the driving means as a reference. 
     It is preferable that a selection between a first control mode and a second control mode is possible, the first control mode being characterized in that the exposure by the exposing device begins after the photosensitive drum charged by the charging device has been rotated for at least one rotation, and the second control mode being characterized in that the exposure by the exposing device begins before the photosensitive drum charged by the charging device has been rotated for one rotation. The first control mode is a high image quality mode, wherein the exposure is started when the electric potential of the photosensitive drum is in a sufficiently stable condition, and the second control mode is a high speed mode, wherein a high recording speed is preferred. It is furthermore preferable that, when the first control mode is selected, the controller starts the rotation of the intermediate transfer belt after the photosensitive drum has been rotated for at least one rotation. By doing so, the length of the intermediate transfer belt can be shortened, and thus the miniaturization of the entire device can be enhanced. 
     It is preferable that (i) the secondary transfer device comprises a secondary transfer roller that can be switched between a state in contact to the intermediate transfer belt and a state in separation from the intermediate transfer belt, (ii) the color toner image is transferred from the intermediate transfer belt onto paper while the paper passes by when the intermediate transfer belt and the secondary transfer roller are in contact, and (iii) in the case that the first control mode has been selected, the controller maintains a separation between the secondary transfer roller and the intermediate transfer belt while the photosensitive drum is being exposed by the exposing device and while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt. By doing so, the running of the intermediate transfer belt can be stabilized and an image with a high image quality can be formed. 
     It is preferable that the color image forming apparatus further comprises a paper feed device for feeding paper synchronized by the reference position detection signal. The paper feed device is controlled by the controller so that, in the case that the first control mode has been selected, the paper feed device does not feed paper while the photosensitive drum is being exposed by the exposing device and while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt. By doing so, disturbances due to the paper feed operation can be prevented and an image with a high image quality can be formed. 
     It is preferable that the color image forming apparatus further comprises a cleaning means, which can be switched between a state in contact to the intermediate transfer belt and a state in separation from the intermediate transfer belt, and cleans remaining toner from the surface of the intermediate transfer belt in the contact state. In the case that the first control mode has been selected, the controller maintains a separation between the cleaning means and the intermediate transfer belt while the photosensitive drum is being exposed by the exposing device, while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt and while the toner image is being transferred by the secondary transfer device from the intermediate transfer belt onto paper. By doing so, the running of the intermediate transfer belt can be stabilized and an image with a high image quality can be formed. 
     It is preferable that the controller stops the driving means while the plurality of image forming units retained by the unit retaining member is moved, and the image forming unit corresponding to a color of the plurality of image forming units that is not being used for image formation is skipped and only the image forming units corresponding to colors that are used for image formation are moved successively to the image forming position. By doing so, the different colors of the color toner image can be overlayed on the surface of the intermediate transfer belt without position variation while the color image formation can be accelerated. 
     It is preferable that the driving means has a single driving source driving the photosensitive drum and the intermediate transfer belt, which driving source, after being stopped by the controller, can be driven in reverse to run back the intermediate transfer belt for a predetermined length when the image forming units are switched. With such a driving method, a miniaturization of the entire device can be enhanced, because the length of the intermediate transfer belt can be shortened. It is even more preferable that the color image forming apparatus further comprises a driving force interrupting means for interrupting the transmission of a driving force from the driving means to the photosensitive drum, wherein the controller interrupts the transmission of the driving force with the driving force interrupting means when the driving source is driven in reverse to run back the intermediate transfer belt for a predetermined length. 
     It is preferable that the controller can drive the driving source in reverse to run back the intermediate transfer belt for a predetermined length while the image forming units are moving. It is even more preferable that the running direction of the intermediate transfer belt in a portion facing the photosensitive drum is the same direction as the moving direction of the image forming unit when the driving source has been driven in reverse. In this case, the length of the intermediate transfer belt can be shortened and the life expectancy of the intermediate transfer belt can be prolonged, because friction between the photosensitive drum and the intermediate transfer belt can be kept low when the image forming unit is moved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a first embodiment of the color image forming apparatus according to the present invention, showing the inner structure in a side view; 
     FIG. 2 is a perspective view of a positioning and driving mechanism of a carriage and a photosensitive drum of the color image forming apparatus shown in FIG. 1; 
     FIG. 3 is a cross section of the carriage of the color image forming apparatus shown in FIG. 1, taken on a plane including the image forming position; 
     FIG. 4 is a perspective view of a driving mechanism that drives the photosensitive drum of the color image forming apparatus shown in FIG. 1; 
     FIG. 5 is a side view of a mechanism for positioning the axis of the photosensitive drum of the color image forming apparatus shown in FIG. 1; 
     FIG. 6 is a cross section of the carriage showing the positional relationship between an image forming unit and the carriage of the color image forming apparatus shown in FIG. 1; 
     FIG. 7 shows the power transmission of the driving mechanism, taken from the side of the machine body, that drives the photosensitive drum and the intermediate transfer belt of the color image forming apparatus shown in FIG. 1; 
     FIG. 8 is a cross section showing the positional relationship between the photosensitive drum and the intermediate belt of the color image forming apparatus shown in FIG. 1; 
     FIG. 9 is a cross section of a second embodiment of the color image forming apparatus according to the present invention, showing the inner structure in a side view; 
     FIG. 10 is a cross section of a third embodiment of the color image forming apparatus according to the present invention, showing the inner structure in a side view; and 
     FIG. 11 is a cross section of a color image forming apparatus of the prior art showing the inner structure in a side view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a color image forming apparatus according to a first embodiment of the present invention is explained with reference to the drawings. 
     First Embodiment 
     FIG. 1 illustrates the structure and operation of a color image forming apparatus according to the first embodiment of the present invention. FIG. 1 is a side view of the internal structure of the color image forming apparatus. 
     First, the image forming units are explained. In FIG. 1, image forming units  3  are provided for the four colors yellow, magenta, cyan and black. The image forming units are integrated devices comprising a photosensitive drum  30  and peripheral process elements. Each image forming unit includes a corona charger  34  that charges the photosensitive drum  30  evenly with a negative voltage, a developing device  35  having a developing roller  31 , and a toner hopper  39 . 
     The toner hopper  39  contains a toner  32  that can be negatively charged and is made of polyester resin and pigment dispersed in the resin. The toner  32  is carried by the surface of the developing roller of the developing device  35  to develop the photosensitive drum  30 . There is a cleaner  38  provided for cleaning remaining toner on the surface of the photosensitive drum  30  after image transfer is completed. The cleaner  38  comprises a cleaning blade  36  made of rubber and a waste toner reservoir  37  that collects waste toner. There is an opening  33  for a laser beam signal  8  to enter the image forming unit  3 . The photosensitive drum  30  has an outer diameter of 30 millimeters. The developing roller of the developing device  35  has an outer diameter of about 16 millimeters. The photosensitive drum  30  and the developing roller are rotatably mounted on side walls of the image forming unit  3 . 
     Next, the transfer belt unit is explained. A transfer belt unit  5  is provided for receiving a toner image formed on the photosensitive drum  30  at an image forming position  10  and reforming the toner image on a recording paper sheet. The transfer belt unit  5  is attached to the machine body  1  removably and comprises integrated members such as an intermediate transfer belt  50 , a group of guide pulleys  55 A- 55 D for supporting the belt  50 , a cleaner  51 , and a waste toner container  57  for collecting waste toner after cleaning. 
     The intermediate transfer belt  50  is an endless belt with a total thickness of 100-300 micron, comprising a urethane base that has a semiconducting property and thickness of approximately 100 micron, and a surface layer made of a fluororesin such as polytetrafluoroethylene (PTFE) or a copolymer of tetrafluoroethylene and perfluoroalkylvinylether (PFA). For example, a perimeter of the intermediate transfer belt is 377 millimeters, which corresponds to a length of A4 paper size (297 millimeters) plus half the perimeter of the photosensitive drum (diameter is 30 millimeters) plus some addition so that A4 size and letter size paper sheets can be used for printing. 
     The cleaner  51  is provided for cleaning or wiping the toner that remained on the intermediate transfer belt  50 . The cleaner  51  comprises a cleaning blade  53  made of rubber and a screw  52  for carrying the wiped toner into the waste toner container  57 . This cleaner  51  moves away from the intermediate transfer belt  50  by pivoting on a bearing  58  during the formation of a color image on the intermediate transfer belt  50 , so that it does not erase the toner image formed on the intermediate transfer belt  50 . 
     The guide pulley  55 A serves as a driving pulley for the intermediate transfer belt as well as a backup roller of the cleaning blade  53 . The guide pulley  55 B serves as a backup roller for the secondary transfer roller  9  for transferring a toner image from the intermediate transfer belt onto a paper sheet. The guide pulley  55 C applies a primary transfer bias for transferring a toner image from the photosensitive drum  30  to the intermediate transfer belt  50 . The guide pulley  55 D serves as a tension pulley for applying a tension to the intermediate transfer belt  50 . The intermediate transfer belt  50  is put over these guide pulleys and rotates in accordance with rotation of the driving pulley  55 A. The intermediate transfer belt  50  is protected by a cover  56 . 
     Next, the carriage is explained. As shown in FIG. 1, wherein the front side of the apparatus is at the right side of FIG. 1, there is a carriage  2  in the center portion of the machine body  1 . In the front side of the machine body  1 , there is a front alligator opening  1 A, and there is a top door  17  on the top of the machine body. The carriage  2  carries four color image forming units  3 Y,  3 M,  3 C, and  3 Bk. The carriage  2  is rotatably mounted on the machine body  1  so as to rotate around the axis of a cylindrical shaft  21 . Thus, each photosensitive drum  30  can move between the image forming position  10  and waiting positions. 
     By opening the top door  17 , the image forming unit  3  can be taken by its handle (not shown in the figure) and easily removed from the carriage  2  or inserted in the carriage  2 . Therefore, if one of the image forming units  3  needs to be replaced, it can be replaced by rotating the carriage  2  so that the image forming unit  3  is located under the top door  17 , and opening the door  17 . Each color image forming unit  3  operates only when it is located at the image forming position  10 , where the photosensitive drum  30  of the image forming unit  3  at the image forming position  10  is scanned by the laser beam  8  and in contact with the transfer belt unit  5 . Therefore, in the image forming position  10 , the image forming unit  3  is connected mechanically to a drive mechanism which drives the photosensitive drum  30  and the intermediate transfer belt  50  and electrically to a power source or other device of the machine body  1  that might be required for a particular image forming unit. In the waiting positions, the image forming unit  3  does not operate. 
     Next, the front alligator opening is explained. The front alligator  1 A is pivoted on the machine body  1  by a hinge  1 B so as to open to the front. A fixing device  15 , a secondary transfer roller  9 , a discharging needle  7 , and front side portions of paper guides  13   a - 13   d  are attached on the inner surface of the front alligator  1 A. These members accompany the front alligator  1 A when it opens, so that a large opening appears in the front side of the machine body when the front alligator  1 A is opened. Thus, setting or removing of the transfer belt unit  5  become easier, and removing of jammed paper becomes easier, too. 
     The transfer belt unit  5 , when placed properly in the machine body  1 , is positioned precisely and a portion of the intermediate transfer belt facing the photosensitive drum  30  is located at the image forming position  10 . Each portion of the transfer belt unit  5  is connected to the machine body electrically and the driving pulley  55 A is connected to the driving mechanism of the machine body  1  so that the intermediate transfer belt  50  can rotate. The discharging needle  7  is provided to prevent a toner image on the paper from deteriorating when the paper is separated from the intermediate transfer belt  50 . 
     Moreover, the cleaning blade  53  is pressing on the intermediate transfer belt  50  when the image formation in the machine body  1  is stopped. This is to prevent spilling of the toner from the cleaner  51  when the transfer belt unit  5  is removed from the machine body  1  or inserted in the machine body  1 . 
     Next, the exposing device is explained. A laser exposing device  6  is provided under the transfer belt unit  5 . The laser exposing device  6  comprises a semiconductor laser (not shown in the drawing), a polygon mirror  6 A, a lens system  6 B, a first mirror  6 C and other members. As shown in FIG. 1, a laser signal beam  8 , which corresponds to a sequential pixel signal of an image information, passes through an opening  22  between the waste toner reservoir  37  of the image forming unit  3 Y and the toner hopper  39  of the image forming unit  3 Bk, and passes through an opening (not shown in the drawing) provided in the cylindrical shaft  21 , and enters the mirror  19  that is located in the cylindrical shaft  21  and directly fixed to the machine body  1 . The laser beam  8 , after reflecting on the mirror  19 , enters the image forming unit  3 Y through an opening  33  of the image forming unit  3 Y that is located at the image forming position. Then, the laser beam enters a photosensitive portion of the photosensitive drum  30 . The laser beam is scanned in the direction of the axis of the photosensitive drum  30  to expose the photosensitive drum  30 . 
     Next, the paper feed system is explained. The paper feed system comprises a paper feed unit  12 , a paper feed roller  14 , a resist roller  16 , a paper ejection roller  18 , and paper guides  13   a ,  13   b ,  13   c ,  13   d  provided among these rollers, a contact portion of the intermediate transfer belt  50  and the secondary transfer roller  9 , and the fixing device  15 . 
     Next, a full color image forming process in the operation of the machine is explained. When electric power is supplied to the machine body  1 , the initializing mode is started. The presence of the transfer belt unit  5  and all image forming units  3  is confirmed and an error check of all process members is performed. 
     To be specific, the presence of the transfer belt unit  5  is confirmed with a transfer belt unit presence sensor (not shown in the drawing), and in the case that the transfer belt unit  5  has not yet been inserted, a message asking for insertion of the transfer belt unit is displayed on a display (not shown in the drawing), until the transfer belt unit  5  has been inserted. Next, the carriage  2  carrying the image forming units  3  is rotated once, and an image forming unit presence sensor (not shown in the drawing) detects whether all image forming units  3  have been inserted. 
     If not all image forming units  3  are inserted, the carriage  2  is moved to an exchange position (a position at the opening of the top door  17 ), and a message asking for insertion of the missing image forming unit  3  is displayed on a display (not shown in the drawing), until the missing image forming unit  3  has been inserted. If there is still an image forming unit missing after the designated image forming unit  3  has been inserted, the same procedure is repeated. After it has been confirmed, that all image forming units  3  have been inserted, the yellow image forming unit  3 Y is moved to the image forming position  10  and retained there. 
     Then, a process confirmation mode for all process members starts. First of all, the fixing device  15  is heated up, and the polygon mirror  6 A of the laser exposing device  6  begins to rotate. After the polygon mirror  6 A reaches a certain rotational speed, confirmation of the process members is performed using the yellow image forming unit  3 Y. The photosensitive drum  30 , the intermediate transfer belt  50  and the developing roller  31  are rotated, and an electrification voltage is impressed on the corona charger  34 . A developing bias is impressed on the developing roller  31  and a transfer bias voltage is impressed on the intermediate transfer belt  50 . After the intermediate transfer belt  50  as been rotated for about one rotation, all operations are stopped, and the magenta image forming unit  3 M is moved to the image forming position  10 . Next, using the magenta image forming unit  3 M, the confirmation of process parts is performed similar to that with the yellow image forming unit  3 Y. Then, after the confirmation of the process parts has also been performed for the cyan and black image forming units  3 C and  3 Bk in a similar manner, the initialization operation is finished, and the preparation for image formation is complete. 
     When the preparation is finished, the image formation of the yellow image forming unit  3 Y in the image forming position  10  is started. When the photosensitive drum  30 , which is connected to the driving mechanism in the machine body  1 , starts to rotate in the image forming position  10 , the developing device  35 , the charger  34  and the intermediate transfer belt  50  start to move simultaneously. When the driving pulley  55 A is driven by the driving mechanism of the machine body, the intermediate transfer belt  50  is driven in the direction of the arrow due to friction forces. The peripheral velocity of the photosensitive drum  30  and the peripheral velocity of the intermediate transfer belt  50  are set to substantially the same velocity. Moreover, the secondary transfer roller  9  and the cleaner  51  are away from the intermediate transfer belt  50 . 
     0.1 sec after the driving source has started the rotation, the charger  34  impresses an electrification voltage, and a charging operation begins. Then, the surface of the photosensitive drum  30  is charged by the charger  34 , and when an evenly charged portion comes into an exposure position, a position sensor  54  detects a home position of the intermediate transfer belt  50 . The laser signal beam  8 , which is output from the laser exposing device  6  according to an image signal, is synchronized with this detection signal. The evenly charged photosensitive drum  30  is irradiated by the laser signal beam  8 , and a static latent image is formed according to the image signal. 
     This static latent image is subsequently made manifest by the developing device  35 , and a toner image is formed. Then, the toner image formed on the photosensitive drum  30  is moved to a primary transfer position contacting the intermediate transfer belt  50 , and is subsequently copied onto the intermediate transfer belt  50 . The above operation is continued for a A4-sized image, and after the end of the image has been transferred to the intermediate transfer belt  50 , the yellow image formation process is finished. After this, the photosensitive drum  30  and the intermediate transfer belt  50  are moved to an initialization position. 
     Then, the charger  34  charges the photosensitive drum  30  at −450 volts. The exposing voltage of the photosensitive drum is −50 volts. DC potential of +100 volts is applied to the developing roller  31  when a portion of the photosensitive drum  30 , which is not charged yet, passes the developing roller. Then, 0.3 sec after the driving mechanism has begun the rotation, the evenly charged surface of the photosensitive drum  30  passes the developing roller  31 , and DC potential of −250 volts is applied to the developing roller  31 . Synchronized with the detection signal output from a position sensor  54  of the intermediate transfer belt  50 , a DC voltage of +1.0 kilovolts is applied to the guide pulley  55 C and the tension pulley  55 D of the intermediate transfer belt  50 . 
     The driving mechanism of the machine body  1  releases the coupling with the photosensitive drum  30  when the photosensitive drum  30  and the intermediate transfer belt  50  stop after the yellow image formation is completed. Then the carriage  2  rotates 90 degrees in the direction of the arrow, so that the yellow image forming unit  3 Y moves away from the image forming position  10  and the magenta image forming unit  3 M moves into the image forming position  10 . When the magenta image forming unit  3 M stops at the image forming position  10 , the driving mechanism of the machine body  1  engages the magenta photosensitive drum  30 . Then the magenta image forming unit  3 M and the transfer belt unit  5  start to operate for magenta image formation. A similar operation is performed as for yellow image formation, so that the magenta toner image is formed overlaying the yellow toner image on the intermediate transfer belt  50 . The above operation is repeated in order for cyan and black, so that a four-colored toner image is formed on the intermediate transfer belt  50 . 
     After the black toner is being formed, 1.4 sec after the generation of the detection signal from the position sensor  54 , the top of the image comes to the position of the secondary transfer roller  9 . Therefore, the secondary transfer roller  9  approaches the intermediate transfer belt  50  at 0.2 sec before the top of the toner image reaches the secondary transfer roller  9 . Simultaneously, a paper sheet is fed from the paper feed unit  12  and then is held between the secondary transfer roller  9  and the intermediate transfer belt  50 , while the timing is checked with the resist roller  16 . Thus, the four-colored toner image as a whole is transferred onto the paper sheet. At this time, a DC voltage of +300 volts is applied to the secondary transfer roller  9 . The paper on which the toner image is transferred passes through the fixing device  15 , which fixes the toner image. Then, the paper sheet is ejected by the ejecting roller  18 . 
     The remaining toner on the intermediate transfer belt  50  is wiped off after the second transfer by the cleaning blade  53 , which contacts with the intermediate transfer belt  50 . The wiped toner is collected into the waste toner container  57  with the screw  52 . Because the cleaning blade  53  is away from the intermediate transfer belt  50  while the color image is formed, the cleaning blade  53  is put into contact with the intermediate transfer belt  50  to clean the surface of the intermediate transfer belt  50 . This contacting time is determined based on the detection signal output by the position sensor  54 . 
     After finishing the second transferring and the cleaning to the intermediate transfer belt, the intermediate transfer belt  50  and the image forming unit  3  stop again. Then the carriage  2  rotates 90 degrees so that the yellow image forming unit  3 Y moves to the image forming position  10  again. Thus, the color image formation is completed. The second transferring and the cleaning of the intermediate transfer belt  50  can be performed simultaneously with the (final) recording of black (i.e. the intermediate transfer belt  50  can be cleaned as the image is being transferred to the paper), or after the recording of black by rotating the intermediate transfer belt  50  again. 
     Next, an image formation without using all four colors of toner is explained. Such an image formation also includes the cases of image formation without using the black toner, that is using only the yellow toner, the magenta toner and the cyan toner, monocolored image formation with only a single toner and multicolor image formation using an arbitrary plurality of toners. As an example, a multicolored image formation using yellow toner and cyan toner is explained below. 
     When the preparation for image forming is finished, first, the yellow image forming unit  3 Y is moved to the image forming position  10 , and image forming is performed, similar to the full color image forming process. 
     After the yellow image forming has been finished, the photosensitive drum  30  and the intermediate transfer belt  50  are stopped, and the driving mechanism of the machine body  1 , which has been connected to the yellow photosensitive drum  30 , is disconnected from the photosensitive drum  30 . The carriage  2  moves 180 degrees in the arrow direction, and the yellow image forming unit  3 Y is moved away from the image forming position  10 . This time, the magenta image forming unit  3 M is left out, and the cyan image forming unit  3 C is moved into the image forming position  10 . 
     When the cyan image forming unit  3 C is moved into the image forming position  10 , the driving mechanism of the machine body  1  is connected to the cyan photosensitive drum  30 , the image forming unit  3 C and the transfer belt unit  5  start operation, and image forming is performed similar to that of the case of yellow. As a result, a yellow toner image and a cyan toner image are formed overlapping on the intermediate transfer belt  50 , thereby forming a green toner image. 
     After this, when the secondary transfer process has been finished, the recording paper onto which the green toner image has been transferred passes the fixing device  15 , the image is fixed, and is ejected by the paper ejection roller  18 . The toner that remained on the intermediate transfer belt  50  after the secondary transfer is cleaned similarly as in the full color image formation process. The intermediate transfer belt  50  and the image forming unit  3  are stopped again, and the carriage  2  turns 180 degrees. Therefore, the yellow image forming unit  3 Y moves again into the image forming position  10 , and the forming operation of the multicolored image is finished. 
     Next, an image formation using only the black toner is explained. First of all, the driving mechanism of the machine body  1 , which has been connected to the yellow photosensitive drum  30 , is disconnected from the photosensitive drum  30 . The carriage  2  is rotated 270 degrees in the arrow direction. Consequently, the yellow image forming unit  3 Y is moved away from the image forming position  10 , and the black image forming unit  3 Bk is moved into the image forming position  10 . When the black image forming unit  3 Bk stops, the driving mechanism of the machine body  1  is connected to the black photosensitive drum  30 , and the image formation process using the image forming unit  3 Bk begins. 
     Next, the black photosensitive drum  30 , which is connected to the driving mechanism of the machine body  1 , starts to revolve in the image forming position  10 . Simultaneously, the developing device  35  and the intermediate transfer belt  50  start to move, and the paper feed unit  12  starts to feed recording paper. In addition, the secondary transfer roller  9  is pressed against the intermediate transfer belt  50 , and the cleaning blade  53  stays pressed against the intermediate transfer belt  50 . 
     0.1 sec after the driving mechanism of the machine body has started the rotation, the charger  34  impresses an electrification voltage, and a charging operation begins. The surface of the photosensitive drum  30  is charged by the charger  34 , and when an evenly charged portion comes into an exposure position, the position sensor  54  detects the home position of the intermediate transfer belt  50 . The laser signal beam  8 , which is output from the laser exposing device  6  according to an image signal, is synchronized with this detection signal. 
     When the evenly charged photosensitive drum  30  is irradiated by the laser signal beam  8 , a static latent image is formed according to the image signal. This static latent image is subsequently made manifest by the developing device  35 , and a toner image is formed. Then, the toner image formed on the photosensitive drum  30  is moved to a primary transfer position contacting the intermediate transfer belt  50 , and is subsequently copied onto the intermediate transfer belt  50 . After the toner image has been transferred, the remaining toner is taken from the surface of the photosensitive drum  30  by the cleaning blade  36  for preparation of the next image formation process, that is recharging, exposure and developing. 
     After the black toner image is formed, 1.4 sec after the generation of the detection signal from the position sensor  54 , the top of the image comes to the position of the secondary transfer roller  9 . Therefore, a paper sheet is fed from the paper feed unit  12  and further fed being held between the secondary transfer roller  9  and the intermediate transfer belt  50 , while the timing is checked with the resist roller  16 . Thus, the black toner image is transferred onto the paper sheet. The paper sheet on which the toner image is transferred passes through the fixing device  15  that fixes the toner image. Then, the paper sheet is ejected by the ejecting roller  18 . 
     Any toner remaining on the intermediate transfer belt  50  is wiped off after the second transfer by the cleaning blade  53 , which contacts the intermediate transfer belt  50 . After the secondary transfer and the cleaning is finished, the intermediate transfer belt  50  is ready to perform the next transfer process. Until successive image formation is finished, the same operations as explained above are repeated. Then, the carriage  2  rotates by 90 degrees, so that the yellow image forming unit  3 Y reaches the image forming position  10  again, and thus the image formation operation of forming a single-colored image is finished. 
     When a single-colored image formation as described above is performed successively, the successive image formation operation is interrupted regularly, and a toner supplying operation is performed by rotating the carriage  2  at least once, in order to avoid a shortage of toner supply. When a single-colored image formation is continued for a long time with the image forming unit  3  fixed in the image forming position  10 , toner  32  is accumulated at the bottom of the toner hopper  39 , because the relative position of the toner hopper  39  of the developing device  35  and the developing roller  31  are fixed, and no toner  32  is supplied to the developing roller  31 . It is preferable that the frequency of the toner supplying operation is adapted to the consumed amount of the toner. For example, it is possible to use a method wherein the time that the laser signal beam  8  is excited by the laser exposing device  6  is measured, or a method wherein the changes in the weight of the developing device  35  are detected, or a method wherein the number of printed papers is counted. 
     Usually, an imageless area can be set on a surface of the intermediate transfer belt  50  for a single-colored image formation process, same as for a multi-colored image formation process, because for a single-colored image formation process, same as for a multi-colored image formation process, the exposing operation onto the photosensitive drum  30  is synchronized with the home position of the intermediate transfer belt  50 . Consequently, even when the imageless area of the intermediate transfer belt  50  suffers some damage by abrasion through the photosensitive drum  30 , the image quality does not deteriorate. 
     Next, a positioning mechanism and a driving mechanism of the machine body for the photosensitive drum  30  in the image forming position  10  for performing precise registration for each color is explained with reference to FIGS. 2-8. As can be seen in FIG. 2, the carriage  2  has a right wall  20 R and a left wall  20 L, which are fixed at both ends of the cylindrical shaft  21 . There are partition plates  23  for partitioning the image forming unit  3  fixed between these walls  20 R and  20 L. The partition plates  23  are fixed in four places arranged at equal angular distances around the cylindrical shaft  21 . Between each two partition plates  23 , an opening  24  is formed, through which the laser beam  8  passes. The cylindrical shaft  21  has eight openings  22 . Four of them are openings through which the laser beam  8  enters from the opening  24 , and the other four openings are formed such that the laser beam  8 , which is reflected by the mirror  19 , can leave through the opening. 
     A coupling plate  42  is fixed to the photosensitive drum  30  of the image forming unit  3 , and right cutouts  26  are provided on a portion of the right wall  20 R for accepting the coupling plate  42 . The right cutouts  26  are provided with recesses, so that the coupling plate  42  and the right wall  20 R do not have contact at a regular position. On the outer periphery of the left wall  20 L, left cutouts  29  are formed. Each left cutout  29  receives a collar  43  that is provided at the left end of a shaft  40  of the photosensitive drum. The left cutouts  29  are bigger than the outer diameter of the collars  43 , so that the collars  43  and the left wall  20 L do not have contact at a regular position. 
     Guide grooves  25  are formed on the inner side of the right and left walls  20 R and  20 L. These guide grooves  25  guide a guide pin  45 R or  45 L provided on one of the two sides of the image forming unit  3 , which is thus positioned roughly in the carriage  2 . The image forming unit  3  is positioned in the carriage  2  such that the image forming unit  3  can pivot on the guide pins  45 R,  45 L by a clearance between the coupling plate  42  and the right cutouts  26  or between the collar  43  and the left cutouts  29 , as is shown in FIG.  6 . In the present example, each clearance mentioned above is set at about 1 millimeter. 
     When the photosensitive drum  30  is positioned in the image forming position  10 , the photosensitive drum  30  is supported by the carriage  2  with a clearance in every direction. To be specific, there are clearances between the guide pins  45 R,  45 L of the image forming unit  3  and the guide groove  25  of the carriage (especially in the radial direction), and between the outer surface of the image forming unit  3  and the carriage portions. 
     A mechanism for preventing the image forming unit  3  from dropping out of the carriage  2  is not shown in the figure. This mechanism is provided by using protrusions (not shown in the figure) which protrude inward from the outer periphery of the right and left walls  20 R,  20 L and which can be easily taken in and out. The image forming unit  3  may be positioned so as to be retained floating in a central position in the carriage  2  (illustrated with a chain line in FIG. 6) by using a spring or other means. 
     A carriage gear  28  is fixed on the left wall  20 L and can be connected to a carriage drive mechanism  86  of the machine body  1 . This carriage drive mechanism  86  comprises a worm gear  89  connected to a power source (not shown in the figure), a worm wheel  88  that engages the worm gear  89 , and a gear  87  that is integrated with the worm wheel  88  and engages the carriage gear  28 . The carriage  2  is rotatably mounted on the right and left main wall  1 R,  1 L via bearings  46  so that the axis of the carriage  2  is parallel to the laser exposing device  6  and the mirror  19 . The mirror  19  is fixed to the right and left main walls  1 R,  1 L directly by supporting members (not shown in the figure). 
     The photosensitive drum  30  of the image forming unit  3  has a structure shown in FIG.  3 . It comprises a pair of flanges  41  fitted in each end of the photosensitive drum, and the shaft  40  that penetrates the flanges  41 . This shaft  40  of the photosensitive drum  30  is rotatably mounted on both side walls of the image forming unit  3 . A conical concave surface  48  is formed on the right edge of the photosensitive drum shaft  40 . The coupling plate  42  is fixed on the right edge of the shaft  40  and has eight tongues  47  that are disposed in a circle around the shaft and protrude axially. When the coupling plate  42  is rotated, the photosensitive drum shaft  40  and the flanges  41  rotate together, so that the photosensitive drum  30  rotates. The collar  43 , which serves as a radial bearing, is attached rotatably on the left edge of the photosensitive drum shaft  40 . 
     Next, the driving mechanism and a detent mechanism for positioning the photosensitive drum precisely at the image forming position, which are employed at the side walls of the machine body  1 , are explained. 
     The driving mechanism  60  of the photosensitive drum  30 , which is attached on the right main wall  1 R, includes an output shaft  70 , a coupling plate  61  that rotates together with the output shaft  70 , a driving gear  71  of the output shaft  70 , and a power source (not shown). The output shaft  70  is supported rotatably and displacably in the axial (thrust) direction by bearings  77  that are fixed to the right main wall  1 R and to a base plate  67  disposed in parallel therewith. 
     The distal end of the output shaft  70  has a convex tapered tip  75 . The proximate end of the output shaft  70  has a spherical surface so as to abut on a thrust bearing  69  with little area. The driving gear  71 , which is fixed to the output shaft  70  for driving the shaft  70 , is a helical gear having left helical teeth of the same direction with the rotation of the shaft  70 . This helical gear engages a gear  72  of the power source side. 
     A compression spring  74  is inserted between the bearing  77  and the driving gear  71 . This spring  74  always applies a force to the output shaft  70  and the coupling plate  61  in the position when the coupling plate  61  and the output shaft  70  are separated from the coupling plate  42  of the photosensitive drum  30  (position indicated in FIG.  4 ). The output shaft  70  can be moved axially against the force of the compression spring  74  by a drive means (not shown) that moves the thrust bearing  69 , from the separated position (FIG. 4) where the coupling plate  61  of the output shaft  70  is away from the coupling plate  42  of the photosensitive drum  30 , to the engaging position (FIG.  3 ) where the tapered tip  75  of the output shaft  70  engages the conical concave surface  48  of the photosensitive drum shaft  40 . The gear  72  of the power source side has a sufficient length in the axial direction so that the output shaft gear  71  engages the gear  72  of the power source side at the separated position as well as the engaging position. When the output shaft  70  is moved along the axial direction, the output shaft drive gear  71  and the power source gear  72  slide against each other on the tooth faces. 
     The coupling plate  61  engages the coupling plate  42  of the photosensitive drum  30  for transmission of power. This coupling plate  61  has eight coupling tongues  65  that are disposed in a circle around the shaft and protrude axially in the same way as the tongues of the coupling plate  42  of the photosensitive drum  30 . The coupling plate  61  is fixed to the rotational output of the output shaft  70  by a pin  64 . Furthermore, the coupling plate  61  is movable axially within a predetermined distance. Thus, the coupling plate  61  goes back temporarily when the tips of the coupling tongues  65  abut the tips of the coupling tongues  47 . The coupling plate  61  is forced to the distal end of the output shaft  70  by the compression spring  62  and stopped by abutting a stopper  63 . 
     Next, the detent mechanism  80 , which is attached to the left main wall  1 L, is explained. The detent mechanism  80  comprises a guide plate  81 , a detent lever  82  and a solenoid  85  for driving the detent lever  82 . The guide plate  81 , which is fixed to the left main wall  1 L, guides the collar  43  placed at the left end of the photosensitive drum shaft  40  to position the collar at a predetermined radial distance from the center of the carriage  2  when the photosensitive drum is located substantially at the image forming position  10 . 
     The detent lever  82  is pivoted on the left main wall  1 L by a pivot pin  83  and pushes the collar  43  to the guide plate  81  with a V-shaped cutout so as to position the collar  43  correctly for the image forming position. The detent lever  82  is connected to the solenoid  85  via a lever  84 . The solenoid actuates the detent lever  82  by magnetic force. Consequently, the V-shaped cutout of the detent lever  82  forces the collar  43  to abut the guide plate  81 . 
     The axis that passes the center of the output shaft  70  of the photosensitive drum driving mechanism  60  and the center of the V-shaped cutout of the detent mechanism  80  is parallel to the plane of the mirror  19  as well as the laser exposing device  6  precisely. Clearances of the bearings are minimized. Thus, the image forming unit  30  is usually located precisely at the image forming position  10  when the photosensitive drum driving mechanism  60  and the detent mechanism  80  are actuated. 
     Next, a driving mechanism driving the photosensitive drum  30  and the intermediate transfer belt  50  is explained. As shown in FIG. 7, a driving mechanism  90  for driving the photosensitive drum  30  and the intermediate transfer belt  50  includes a motor  96  as a power source and slowdown gears  92 ,  93  that are connected to the motor  96 . The slowdown gear  92  is identical to the power source gear  72  shown in FIG.  4 . 
     A motor gear  91  engages the slowdown gears  92  and  93 . When the slowdown gear  93  is installed on the transfer belt unit  5 , the slowdown gear  93  engages a gear  94 , which engages a pulley gear  95  fixed to the drive pulley  55 A. The slowdown gear  92  engages the output shaft drive gear  71  to drive the photosensitive drum  30 . The rotation ratios among these gears are all integers. 
     An outer diameter of the drive pulley  55 A is 30 millimeters and a perimeter of the intermediate transfer belt is 377 millimeters. Four turns of the drive pulley  55 A corresponds to just one turn of the intermediate transfer belt  50 . The rotation ratio of the pulley gear  95 , which is connected to the drive pulley  55 A, to the slowdown gear  93  is 1:2, and that of the slowdown gear  93  to the motor gear  91  is 1:3. An outer diameter of the photosensitive drum  30  is also 30 millimeters. Four turns of the photosensitive drum  30  correspond to just one turn of the intermediate transfer belt  50 , so that the photosensitive drum  30  is synchronized with the drive pulley  55 A. The rotation ratio of the output shaft drive gear  71  to the slowdown gear  92  is 1:2, and that of the slowdown gear  92  to the motor gear  91  is 1:3. 
     In the present example, the outer diameter of the guide pulley  55 C of the transfer belt unit  5  is 20 mm, and the rotation ratio of the guide pulley  55 C and the intermediate transfer belt  50  is an integer. It is preferable that the rotation ratios of the backup roller  55 B and the tension roller  55 D also are integers. 
     Next, the relationship between the photosensitive drum  30  in the image forming position  10  and the intermediate transfer belt  50  is explained. FIG. 8 shows an arrangement of the photosensitive drum located at the image forming position  10  and the intermediate transfer belt  50 . When the transfer belt unit  5  is placed correctly between the right and left main walls  1 L,  1 R, the perimeter of the photosensitive drum  30  located at the image forming position  10  crosses the tangent line of the guide roller  55 C and the tension roller  55 D by about one millimeter, as shown in FIG.  8 . Therefore, the tension of the intermediate transfer belt  50  generates a constant pressure of the belt  50  against the peripheral surface of the photosensitive drum  30 . Thus, uniform contact between the intermediate transfer belt  50  and the photosensitive drum  30  is obtained. In an example, a satisfactory performance for the image transfer was obtained by applying a spring force of 2-3 kilograms onto the tension roller  55 D in the direction indicated by the arrow in FIG.  8 . In this example, the width of the intermediate transfer belt  50  was 250 millimeters. 
     When the carriage  2  rotates for changing the image forming unit  3  located at the image forming position  10 , the image forming unit  3  may move into and out of the image forming position  10  while rubbing the surface of the intermediate transfer belt  50 . In this embodiment, however, the intermediate transfer belt  50  rotates one turn per every image transfer for each color and usually stops in a predetermined position. Therefore, there is an imageless area between the beginning and the end of the image, where there is no image formed on the intermediate transfer belt  50 . Therefore, no image distortion occurs due to the color change. If the surface of the intermediate transfer belt  50  is lightly damaged due to abrasion at the imageless area, the transferred image is not affected. 
     When the photosensitive drum driving mechanism  60  actuates the photosensitive drum  30 , the image forming unit  3  in the carriage  2  can protrude about 0.5-1.0 mm in direction of the intermediate transfer belt  50 . In this case, when the photosensitive drum driving mechanism  60  is disconnected from the image forming unit  3 , the photosensitive drum  30  and the intermediate transfer belt  50  can be separated. Consequently, when the carriage  2  is moved in this condition, the photosensitive drum  30  does not abrade the surface of the intermediate transfer belt  50 , and damage of the intermediate transfer belt can be reduced. 
     Next, the operation of an apparatus with a driving mechanism as pointed out above is explained. Details concerning the installation of the image forming units  3  into the carriage  2 , and the initialization procedure of the photosensitive drum  30  and the intermediate transfer belt  50  are not included in this explanation. 
     When all image forming units  3  are installed in the carriage  2 , a motor (not shown in the drawings) for driving the carriage  2  rotates the worm gear  89 . Then the carriage  2  turns in the direction of the arrow in FIG. 1, so that the yellow image forming unit  3 Y is moved to the image forming position  10 . The output shaft  70  of the photosensitive drum driving mechanism  60  is forced to move backwards by the spring  74 . The tapered tip  75  of the shaft  70  and the coupling plate  61  are away from the coupling plate  42  of the photosensitive drum  30 . 
     The solenoid  85  of the detent mechanism  80  is not activated, and the detent lever  82  is in a waiting position, as is illustrated with a broken line in FIG.  5 . The motor  96 , which drives the photosensitive drum  30  and the intermediate transfer belt  50 , is stopped. The yellow photosensitive drum  30  is moved near the image forming position  10  while rubbing the intermediate transfer belt  50 , when the motor (not shown) for driving the carriage stops. Consequently, the worm gear  89  stops rotating, so that the carriage  2  is locked at this position. 
     When the carriage  2  stops, the solenoid  85  is actuated at once, so that the detent lever  82  forces the collar  43  of the photosensitive drum shaft  40  towards the guide plate  81 . Consequently, the V-shaped cutout of the detent lever  82  grips the collar  43  at the predetermined position. Simultaneously, the thrust bearing  69  pushes the output shaft  70  leftward in FIG. 3 against the spring force. The tapered tip  75  of the output shaft  70 , while being pushed leftward, starts to engage the conical concave surface  48  of the photosensitive drum shaft  40 . Thus, the tapered tip  75  of the output shaft  70  is moved to align the two axes of the photosensitive drum shaft  40  and the output shaft  70 . The alignment of two axes of the photosensitive drum shaft  40  and the output shaft  70  is completed and the photosensitive drum  30  is positioned precisely at the image forming position  10  when the tapered tip  75  has engaged the conical concave surface  48 , and the thrust bearing  69  pushes the output shaft  70 . 
     At this time, the thrust force on the output shaft  70  is received by the edge surface of the flange  41  pushing a side bearing of the image forming unit  3 , with this side bearing abutting the left wall  20 L of the carriage  2 . When the tapered tip  75  engages the conical concave surface  48 , the two coupling plates  42  and  61  engage each other, so that a rotation force can be transmitted to the photosensitive drum  30 . 
     As mentioned above, the yellow photosensitive drum  30  is positioned correctly by the detent mechanism  80  and the drive mechanism  60 . Moreover, the whole body of the image forming unit  3 Y, which includes the photosensitive drum  30 , is moved in the carriage  2  for positioning. However, since the image forming unit  3  is retained in the carriage  2  with some clearance, the movement of the image forming unit  3  is not disturbed during the positioning of the photosensitive drum  30 . 
     Although the carriage  2  has some clearance in the rotation direction such as a backlash between the carriage gear  28  and the gear  87 , the clearance of the carriage  2  does not effect the positioning of the photosensitive drum  30 , since the photosensitive drum  30  is positioned directly by the mechanism attached to the machine body  1 , so that the photosensitive drum  30  can be precisely positioned. 
     After the positioning of the photosensitive drum  30  is completed, the motor  96  for driving the intermediate transfer belt  50  starts to turn. When the photosensitive drum  30  and the intermediate transfer belt  50  start to turn, all the process devices begin their operation and the yellow toner image subsequently is formed on the photosensitive drum  30 . Then, the yellow toner image is transferred onto the intermediate transfer belt  50 . During this operation, the output shaft  70  is forced leftward in FIG. 2 by the thrust bearing  69 , and the solenoid  85  maintains an actuated state so that the detent lever  82  continues to retain the collar  43 . 
     After the intermediate transfer belt  50  has rotated one turn (at this time, the photosensitive drum  30  and the drive pulley  55 A have rotated four turns, and the guide pulley  55 C has rotated six turns) the yellow image forming is completed. The motor  96  stops and the intermediate transfer belt  50  stops at the initializing position. After the intermediate transfer belt  50  and the photosensitive drum  30  stop, the solenoid  85  is turned off to release the detent. At the same time, the thrust bearing  69  retreats rightward in FIG. 2, and the output shaft  70  also retreats due to the spring force. Consequently, the coupling plate  61  and the tapered tip  75  separate from the coupling plate  42  and the photosensitive drum shaft  40 , so that the carriage becomes ready to rotate. 
     After the coupling is released, the worm gear  89  starts rotating again, the carriage  2  is rotated in the direction of the arrow in FIG. 2, and the magenta image forming unit  3 M moves near the image forming position  10 . The detent mechanism  80  and the drive mechanism  60  for the photosensitive drum  30  operate again to position the magenta photosensitive drum  30  and to perform coupling. Thus, the image forming for the second color toner image starts. 
     A four-colored image can be formed on the intermediate transfer belt  50 , repeating the image forming of each color by changing the image forming unit of each color as explained above. The four-colored image formed on the intermediate transfer belt  50  is finally transferred onto a recording paper sheet. In an example, the time period for rotating the carriage  2  by 90 degrees is 0.6 seconds, the time period for engagement or release of the coupling plates  42  and  61  is 0.2 seconds, and the process velocity is 100 millimeters per second. 
     Next, the positioning for overlaying the plural color toner image is explained. It is important that both of the photosensitive drum  30  and the intermediate transfer belt  50  rotate accurately at a constant speed in order to ensure precise positioning of the plural color toner images. To realize this precise registration, a FG servo motor is used as the motor  96  for driving the photosensitive drum  30  and the intermediate transfer belt  50  in this embodiment, and to suppress load variations, the motor  96  is used exclusively for this purpose. In addition, to match the home position of the image formed on the intermediate transfer belt  50 , recording of each color is performed after the motor  96  has been started and reached a certain constant speed. Then, the home position of the intermediate transfer belt  50  is detected, and a synchronized latent image recording by the laser signal beam  8  onto the photosensitive drum  30  is started. 
     It is also necessary that the four photosensitive drums  30  are located and retained accurately at the image forming position  10  to ensure precise positioning. As mentioned before, positioning of the photosensitive drum  30  in this embodiment is performed by the output shaft  70  and the detent lever  82 , which are attached to the right and left walls  1 R,  1 L and support the photosensitive drum shaft directly. The photosensitive drum  30  is movable within a predetermined clearance in the carriage  2 , so that the carriage  2  only has to be positioned roughly, and the photosensitive drum  30  can be positioned precisely and independently from the positioning accuracy of the carriage  2 . 
     It is also necessary to rotate the precisely positioned photosensitive drum  30  at a precise speed. In order to change the photosensitive drum  30 , a clutch mechanism between the photosensitive drum  30  and the driving mechanism on the machine body  1  is necessary. When the clutch includes gears such as are usually used, variations in the transmission of a clutch (coupling) portion can occur, and the photosensitive drum  30  cannot be precisely rotated. Especially, when the four photosensitive drums are not used equally, and the toner in one image forming unit  3  is used up, it becomes necessary to exchange this image forming unit  3 , and variations in the precision of the coupling portion of the photosensitive drum  30  are likely to occur. Consequently, a coupling mechanism that might influence the precision of the positioning of the photosensitive drum  30  cannot be used. 
     In the present embodiment however, the configuration explained above is used, wherein the photosensitive drum  30  is rotated while being held by the output shaft  70 . Consequently, a variation in the angular velocity transmitted between the output shaft  70  and the photosensitive drum  30  cannot occur, and the angular velocity is transmitted precisely from the output shaft  70  to the photosensitive drum  30 . Therefore, it is not required to use coupling members for the photosensitive drum  30  that have precise dimensions. 
     Errors of rotation speeds or angular speeds, which appear in the transmission system of the machine body side between the motor  96  and the output shaft  70  or the intermediate transfer belt  50 , are eliminated by selecting integer ratios for each rotation ratio of each gear  91 - 95  and  71 , the drive pulley  55 A or the guide pulley  55 C vs. one turn of the intermediate transfer belt  50 . According to the above mentioned configuration, these elements return to the initializing position after every color image transferring, and repeat their operation under the same conditions. Thus, a displacement from the ideal recording position in the case of driving with an ideal constant speed happens always in the same amount and phase for all colors, so that the recording positions of all colors are perfectly matched, and color misregistration on the intermediate transfer belt  50  is eliminated. 
     When the photosensitive drum  30  has portions that are eccentric with respect to the center of the conical concave surface  48 , which is the rotational center of the photosensitive drum  30 , this leads to a variation of the circumferential speed of the photosensitive drum  30 . Consequently, the recording pitch changes, and if the amount and phase of eccentricity for the various photosensitive drums  30  is different, position displacement of the colors occurs. Therefore, in the present embodiment, the intermediate transfer belt  50  is pressed lightly against the photosensitive drum  30  by its own tensile force, as has been explained above, and is driven at a constant speed, regardless of the outer peripheral velocity of the photosensitive drum  30 . Consequently, due to slippage between the photosensitive drum  30  and the intermediate transfer belt  50 , when the outer peripheral velocity of the photosensitive drum  30  is higher than the velocity of the intermediate transfer belt  50 , the portion that has been recorded with an elongated recording pitch is transferred onto the intermediate transfer belt  50  with compression, and in the reverse case with elongation. As a result, the toner image for each color can be transferred precisely with a recording pitch corresponding to the angular speed, regardless of the outer peripheral speed of the photosensitive drum  30 . 
     Second Embodiment 
     Next, a color image forming apparatus according to a second embodiment of the present invention is explained. This embodiment differs from the first embodiment, in that the rotation direction of the carriage is set in an opposite direction, and that the intermediate transfer belt also can be driven in an opposite direction. 
     The operation of a color image forming apparatus according to the second embodiment of the present invention is explained with reference to FIG.  9 . After the yellow image formation using a yellow image forming unit  3 AY has been completed, the yellow image forming unit  3 AY is exchanged with a magenta image forming unit  3 AM. Synchronized with the separation of the photosensitive drum driving mechanism  60  from the photosensitive drum  30 , the driving source of the intermediate transfer belt  50  of the intermediate transfer belt unit  5 A is driven in reverse rotation. 
     The length of the intermediate transfer belt  50  is 378 mm and thus a little longer than A4 paper length (297 millimeters) plus half the perimeter of the photosensitive drum plus some addition, as has been described above. Consequently, in the difference between the length of the intermediate transfer belt  50  and an A4 paper length, a shifting distance (measuring about half the perimeter of the photosensitive drum  30 ) for the photosensitive drum  30  from the photosensitive drum  30  to the transfer position is included. 
     Considering the start-up time from starting the motor  96  until the motor runs at a perfectly constant velocity, and the shut-down time from stopping the motor  96  until the motor has come to a complete stop, the imageless area on the intermediate transfer belt  50  should be as long as possible. However, when the length of the intermediate transfer belt  50  is too long, the time required to perform one revolution of the intermediate transfer belt  50  becomes too long, so that this time has to be added to the time required to perform an image formation. 
     In order to make the length of the intermediate transfer belt  50  small, the imageless area can be made small by rotating the intermediate transfer belt  50  in the reverse direction while the photosensitive drum driving mechanism  60  is disconnected. Consequently, the time necessary to perform image formation is shortened, and a speedier print-out becomes possible, because the length of the intermediate transfer belt  50  has been shortened. 
     However, the operation of rotating the intermediate transfer belt  50  in reverse is performed after the photosensitive drum driving mechanism  60  has been separated from the photosensitive drum  30 , so that the photosensitive drum  30  is not rotated in reverse by error. That means, that the motor  96  may not be operated when the photosensitive drum driving mechanism  60  is connected to the photosensitive drum  30 . 
     If the intermediate transfer belt  50  rotates reversely in the arrow direction “a” while the image forming unit  3 A is shifted by the rotation of the carriage  2 , then the sliding of the photosensitive drum  30  and the intermediate transfer belt  50  during the shifting time of the image forming unit  3 A is small, so that the damage inflicted on the intermediate transfer belt  50  can be kept small. 
     Even if some minor damage occurs, this does not pose a problem for the image formation, because the position where the photosensitive drum  30  and the intermediate transfer belt  50  rub onto each other is in the imageless area of the intermediate transfer belt  50 . However, in order to prolong the life expectancy of the intermediate transfer belt  50 , it is preferable also to avoid rubbing between the photosensitive drum  30  and the imageless area as much as possible. 
     A configuration has been adopted, wherein the toner  32  in the toner hopper  39  accumulates in a toner gatherer  27  in the developing device  35  when carriage  2  is rotated in order to provide enough toner  32  to the developing roller  31  of the image forming unit  3 A in the image forming position  10 . To be specific, one side of the toner gatherer  27  is made bigger, so that the capacity of the toner gatherer  27  becomes bigger. Therefore, even when a one-colored image formation process is performed successively, the frequency of the toner supplying operation, which involves stopping the image formation and rotation of the carriage  2  to provide the developing roller  31  with toner  32 , can be reduced. 
     Third Embodiment 
     Next, a color image forming apparatus according to a third embodiment of the present invention is explained. As can be seen in FIG. 10, the intermediate transfer belt  50 B of this embodiment is longer than in the abovementioned embodiments. Furthermore, the gear ratio of the driving gears is set so that the ratio of the rotation periods of the photosensitive drum  30  and the intermediate transfer belt  50 B is an integer. 
     The intermediate transfer belt unit  5 B of a color image forming apparatus according to this embodiment uses a 472 mm long endless belt as an intermediate transfer belt  50 B. There are two kinds of image forming modes, namely a high speed mode and a high image quality mode. When the initializing operation is completed, and after the preparation for image formation has been finished, a judgement is performed as to which of the two modes has been selected. The high speed mode is performed with basically the same operations as described in the first embodiment. The high quality mode is explained in the following. 
     In the full color image formation process using the high quality mode, first, the yellow image forming unit  3 Y is moved into the image forming position  10 , and an image formation process using the yellow image forming unit  3 Y is performed. When the yellow photosensitive drum  30 , which is connected to the driving power source of the machine body  1 , starts to rotate in the image forming position  10 , the developing roller  31  and the intermediate transfer belt  50 B start to move simultaneously. Then, recording paper is fed by the paper feed roller  14  from the paper feed unit  12 . The cleaning blade  53 , which has been pressing onto the intermediate transfer belt  50 B so far, is separated from the intermediate transfer belt  50 B. Then, the secondary transfer roller  9  is separated from the intermediate transfer belt  50 B. 
     0.1 sec after the driving source has started the rotation, a charging voltage is applied to the charger  34 , which begins the charging operation. The surface of the photosensitive drum  30  is charged by the charger  34 , and when an even charge is starting to be applied, the photosensitive drum  30  makes at least one full rotation. Then, matched with the time when the starting position for charging the surface of the photosensitive drum  30  comes into the exposing position again, the position sensor  54  detects the home position of the intermediate transfer belt  50 B. The laser signal beam  8 , which is output from the laser exposing device  6  according to an image signal, is synchronized with this detection signal. The evenly charged photosensitive drum  30  is irradiated by the laser signal beam  8 , and a static latent image is formed according to the image signal. This static latent image is subsequently made manifest by developing device  35 , and a toner image is formed. 
     Then, the toner image formed on the photosensitive drum  30  is moved to a primary transfer position contacting the intermediate transfer belt  50 B, and is subsequently copied onto the intermediate transfer belt  50 B. The above operation is continued for a A4-sized image, and after the end of the image has been transferred to the intermediate transfer belt  50 B, the yellow image formation process is finished. After this, the photosensitive drum  30  and the intermediate transfer belt  50 B are moved to an initialization position. 
     Then, same as in the high speed mode, the charger  34  charges the photosensitive drum  30  at −450 volts. The exposing voltage of the photosensitive drum is −50 volts. DC potential of +100 volts is applied to the developing roller  31  when a portion of the photosensitive drum  30 , which is not charged yet, passes the developing roller. Then, 0.3 sec after the driving source has begun the rotation, the evenly charged surface of the photosensitive drum  30  passes the developing roller  31 , and DC potential of −250 volts is applied to the developing roller  31 . Synchronized with the detection signal output from the position sensor  54  of the intermediate transfer belt  50 B, a DC voltage of +1.0 kilovolts is applied to the guide pulley  55 C and the tension pulley  55 D of the intermediate transfer belt  50 . 
     The driving mechanism of the machine body  1  releases the coupling plates  42  and  61  with the photosensitive drum  30  when the photosensitive drum  30  and the intermediate transfer belt  50 B stop after the yellow image formation is completed. Then, the carriage  2  rotates 90 degrees in the arrow direction, so that the yellow image forming unit  3 Y moves away from the image forming position  10  and the magenta image forming unit  3 M moves to the image forming position  10 . When the magenta image forming unit  3 M stops at the image forming position  10 , the driving mechanism of the machine body  1  engages the magenta photosensitive drum  30 . Then the magenta image forming unit  3 M and the transfer belt unit  5 B start to operate for magenta image formation. Similar operations are performed as for yellow image formation, so that the magenta toner image is formed overlaying the yellow toner image on the intermediate transfer belt  50 B. 
     The above operation is repeated in order for cyan and black, so that a four-colored toner image is formed on the intermediate transfer belt  50 B. When the black toner has been transferred, the toner image comes to the position of the secondary transfer roller  9 , and about 1.4 sec after the generation of the next detection signal from the position sensor  54 , the top of the image again comes to the position of the secondary transfer roller  9 , and 0.2 sec before that, the secondary transfer roller  9  approaches the intermediate transfer belt  50 B. Simultaneously, a recording paper sheet is fed from the paper feed unit  12  and further fed while being held between the secondary transfer roller  9  and the intermediate transfer belt  50 B, while the timing is checked with the resist roller  16 . Thus, the four-colored toner image as a whole is transferred onto the recording paper sheet. At this time, a DC voltage of +300 volts is applied to the secondary transfer roller  9 . The recording paper onto which the toner image has been transferred passes through the fixing device  15  that fixes the toner image. Then, the paper sheet is ejected by the ejecting roller  18 . 
     The remaining toner on the intermediate transfer belt  50 B is wiped off after the second transfer by the cleaning blade  53  that contacts with the intermediate transfer belt  50 B. The wiped toner is collected into the waste toner container  57  with the screw  52 . Because the cleaning blade  53  is spaced away from the intermediate transfer belt  50 B while the color image is formed, the cleaning blade  53  is put into contact with the intermediate transfer belt  50 B to clean the surface of the intermediate transfer belt  50 B. This contacting time is after the secondary transfer onto the recording paper has been completed. 
     After finishing the second transferring and the cleaning of the intermediate transfer belt  50 B, the intermediate transfer belt  50 B and the image forming unit  3  are stopped again. Then the carriage  2  rotates 90 degrees so that the yellow image forming unit  3 Y moves to the image forming position  10  again. Thus, the color image formation is completed. 
     Thus, the first point in which the high image quality mode differs from the high speed mode is that the exposure for image formation begins after the photosensitive drum  30  has been rotated for at least one revolution following the start of the charging of the photosensitive drum  30 . Usually, at the time when the charging is started, the electric potential of the photosensitive drum  30  is unstable, and this instability can be a factor for deterioration of the quality of the formed image. In the present embodiment, the electric potential of the photosensitive drum  30  is stabilized by rotating the photosensitive drum  30  for at least one revolution after the start of the charging, so that the instability of the electric potential of the photosensitive drum  30  is eliminated, and a higher picture quality can be obtained for the formed image. 
     The second point in which the high image quality mode differs from the high speed mode is that the secondary transfer roller  9  and the intermediate transfer belt  50 B are kept apart from each other during the latent image formation by exposure of the photosensitive drum  30  and the transfer of the toner image onto the intermediate transfer belt  50 B. When the secondary transfer roller  9  is disconnected from the intermediate transfer belt  50 B, the rotational load of the intermediate transfer belt  50 B changes, and the conveyance velocity of the intermediate transfer belt  50 B may easily vary. In the position in which the toner image is transferred from the photosensitive drum  30  to the intermediate transfer belt  50 B, the photosensitive drum  30  and the intermediate transfer belt  50 B are in contact. Therefore, there is the possibility that a change in the velocity of the intermediate transfer belt  50 B causes a change in the velocity of the photosensitive drum  30 . In the high quality image mode of the present embodiment however, the secondary transfer roller  9  and the intermediate transfer belt  50 B are kept apart from each other during the image formation and the transfer of the toner image onto the intermediate transfer belt  50 B. Thus, a stable conveyance of the intermediate transfer belt  50 B is realized, and a higher picture quality can be obtained for the formed image. 
     The third point in which the high image quality mode differs from the high speed mode is that the paper feed of recording paper from the paper feed unit  12  and the conveyance operation of the paper by the resist roller  16  is not performed during the latent image formation by exposure of the photosensitive drum  30  and the transfer of the toner image onto the intermediate transfer belt  50 B. Especially at the start of the paper feed operation a big torque is necessary, which may cause the machine body  1  to vibrate. There are cases where the adoption of a structure for the machine body  1  that is sufficiently resistant to resonance is limited due to cost and weight considerations. In the high image quality mode of the present embodiment however, the paper feed operation is not performed during the image formation and the transfer of the toner image onto the intermediate transfer belt  50 B. Thus, a higher picture quality can be obtained for the formed image. 
     The fourth point in which the high image quality mode differs from the high speed mode is that the cleaning blade  53  and the intermediate transfer belt  50 B are kept apart from each other during the latent image formation by exposure of the photosensitive drum  30 , the transfer of the toner image onto the intermediate transfer belt  50 B and the second transfer onto the recording paper. When the cleaning blade  53  is disconnected from the intermediate transfer belt  50 B, the rotational load of the intermediate transfer belt  50 B changes, and the conveyance velocity of the intermediate transfer belt  50 B may easily vary. In the high quality image mode of the present embodiment however, the cleaning blade  53  and the intermediate transfer belt  50 B are kept apart from each other during the image formation, the transfer of the toner image onto the intermediate transfer belt  50 B, and the second transfer onto the recording paper. Thus, the stability of the conveyance of the intermediate transfer belt  50 B can be increased, and a higher picture quality can be obtained for the formed image. 
     Thus, the requirements of high image quality and high speed both can be satisfied by adopting the configuration explained above, which can be switched between a high image quality mode and a high speed mode. In the present embodiment a long perimeter is used for the intermediate transfer belt  50 B, as has been explained above, because, compared to the intermediate transfer belt  50  of the first embodiment, an additional imageless area corresponding to one revolution of the photosensitive drum  30  is necessary on the intermediate transfer belt  50 B. Therefore, instead of prolonging the perimeter of the intermediate transfer belt  50 B, it is also possible to stop the rotation of the intermediate transfer belt  50  for a period corresponding to one revolution of the photosensitive drum  30  when the image formation is started. In this case, an even smaller apparatus can be provided with a high image quality mode and a high speed mode. The high image quality mode is not limited to full color image formation, but similarly can be used for multicolor image formation with two or three colors, or for monocolor image formation using only a single color. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.