Patent Publication Number: US-7590361-B2

Title: Image forming apparatus and control method therefor

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus and control method therefor. 
   2. Description of the Related Art 
   There is proposed an apparatus which forms an image by an electrophotographic process of irradiating a photosensitive drum serving as an image carrier with a laser beam or light from a light-emitting element (e.g., LED: Light Emitting Diode) that is modulated in accordance with recording information. 
   For example, there is proposed a color image forming apparatus having a plurality of image forming units which develop electrostatic latent images formed on photosensitive drums and transfer toner images of respective colors onto a transfer sheet or intermediate transfer belt. 
   A monochrome image forming apparatus is also proposed which develops an electrostatic latent image formed on one photosensitive drum and transfers a black toner image onto a transfer sheet. 
   For these image forming apparatuses, there is proposed an arrangement example of a copying machine which connects to a document scanning apparatus and sends document image information to an image forming apparatus to copy the document image. 
   For example, Japanese Patent Laid-Open No. 11-292335 and Japanese Utility Model Laid-Open No. 2-29063 each disclose an example of a combination of a paper feed unit and image forming apparatus. These references propose image forming apparatuses in each of which a plurality of paper feed units serving as the base of the image forming apparatus stack up replaceably and the bottom of the main body of the image forming apparatus has an opening for receiving sheets conveyed from the paper feed units. 
   An image forming apparatus is also proposed which can connect to a post-processing apparatus called a finisher for sorting transfer sheets printed by the image forming apparatus into respective copies or stapling respective copies. An image forming apparatus of this type and various post-processing apparatuses connectable to the image forming apparatus can operate to perform a series of printing and post-processing operations in cooperation with each other. 
   Some document scanning apparatuses have various scanning resolutions such as 400 dpi (dots per inch) and 600 dpi. A full-color image forming apparatus generally has a full-color CCD sensor which converts a scanned document image into a full-color image signal. A monochrome image forming apparatus often has a monochrome scanning CCD which converts a scanned document image into a monochrome image signal. Even for a monochrome image forming apparatus, a document scanning apparatus is proposed which has a full-color CCD sensor and converts a document image into a full-color image signal. Recently, a product is proposed which provides a scanner function of transmitting image information scanned by a document scanning apparatus to a desired destination via a network. 
   As described above, an apparatus arrangement is proposed in which an image forming apparatus cooperates with another apparatus to provide a function unimplementable by the single image forming apparatus. 
   Various proposals are also made for an apparatus arrangement which permits exchanging part of an image forming apparatus. For example, there is proposed an apparatus form which permits newly assembling a double-sided paper conveyance unit into an image forming apparatus of standard specifications. An image forming apparatus is also proposed in which some units in the apparatus are made detachable so that the functions of the image forming apparatus can change into an apparatus arrangement conforming to product specifications desired by a user. 
   There is also proposed an image forming apparatus of an arrangement which permits connecting the image forming apparatus to a controller arranged outside the image forming apparatus or assembling a controller into the image forming apparatus. 
   Conventionally, the user selects, from various image forming apparatuses, an image forming apparatus which implements desired functions, performance, user friendliness, and the like. To obtain a function, performance, or the like which cannot be attained by the selected image forming apparatus, the user selects an apparatus arrangement so as to utilize the desired function, performance, or the like by combining the image forming apparatus with various exchangeable apparatuses, various units, various controllers, and the like. 
   A conventional image forming apparatus can perform various operations by executing a system operation in cooperation with various apparatuses, various units, various controllers, a host computer, and the like, and provides a user with various conveniences. 
   In general, a post-processing apparatus such as a finisher is controlled to determine its operation in accordance with the printout operation mode of an image forming apparatus. There is no image forming apparatus which controls the operations of at least two subsystems to execute a series of image output operations and a series of information processing operations for image information almost simultaneously or independently. 
   A conventional image forming apparatus poses various problems owing to the above arrangement. 
   Since the conventional image forming apparatus executes a system operation in cooperation with various apparatuses, various units, various controllers, a host computer, and the like, it only operates depending on its operation mode, function, and performance. 
   For example, when the image forming apparatus connects to a paper feed apparatus or finisher, the apparatus combination may limit their functions and performance associated with apparatus control under restrictions on functions and performance. 
   For example, the image forming apparatus and finisher exchange communication information, and the finisher determines its operation mode, performance, and functions and operates in accordance with the operation of the image forming apparatus. The arrangement of the image forming unit, paper feed unit, and paper conveyance unit in the image forming apparatus also determines the whole operation performance of the image forming apparatus. 
   Among various apparatus arrangements, there is no apparatus arrangement which can flexibly meet the user&#39;s need in terms of the operation of the entire system such as a print operation or a cooperative operation (e.g., a print operation or scan operation) with a host computer. 
   That is, when weighting operation specifications in order to cope with customization to apparatus operation specifications desired by a user, apparatus specifications implementable by exchanging a subunit cannot be fully enhanced unless operation specifications can be determined on the basis of association with control information. 
   It is necessary and important to maintain the image formation quality with a combination of subsystems different in performance in an image forming apparatus made up of a plurality of subsystems. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to overcome the conventional drawbacks, and has as its object to provide an image forming technique which implements operation specifications desired by a user by a combination of subsystems. 
   It is another object of the present invention to provide an image forming technique capable of, even when a registration error occurs between subsystems upon exchanging or detaching a subsystem, correcting the registration error and maintaining the image formation quality. 
   It is still another object of the present invention to provide an image forming technique capable of, even when exchanging or detaching a subsystem, correcting a delay of the output time of the first recording material or a delay of the output time of recording materials in a continuous operation, thereby preventing a decrease in the throughput of an image forming apparatus. 
   According to the present invention, the foregoing object is attained by providing An image forming apparatus having a plurality of detachable units, the image forming apparatus permitting mounting image forming units which form an image on a recording medium and are different in performance, and recording medium conveyance units which convey the recording medium and are different in specification, the image forming apparatus comprising:
         a detection unit which detects a positional relationship between the mounted image forming unit and the mounted recording medium conveyance unit; and   a control unit which controls an operation of the image forming apparatus,   wherein said control unit controls operation timings of the image forming unit and the recording medium conveyance unit on the basis of the positional relationship detected by said detection unit.       

   According to another aspect of the present invention, the foregoing object is attained by providing an image forming apparatus which detachably mounts and supports an exchangeable image forming subsystem having an image carrier, an exposure unit, a charging unit, and a developing unit, and an exchangeable recording medium conveyance subsystem which conveys a recording medium in the image forming apparatus, comprising:
         a detection unit which detects a positional relationship between the mounted image forming subsystem and the mounted recording medium conveyance subsystem; and   a control unit which controls an operation of the image forming apparatus,   wherein the image forming apparatus permits mounting image forming subsystems different in performance and recording medium conveyance subsystems different in specification, and   said control unit controls operation timings of the image forming subsystem and the recording medium conveyance subsystem on the basis of the positional relationship detected by said detection unit.       

   According to another aspect of the present invention, the foregoing object is attained by providing an image forming apparatus which includes, as a plurality of detachable units, at least an image forming unit which forms an image on a recording medium and a recording medium conveyance unit which conveys the recording medium, and executes image formation corresponding to a combination of the units, comprising:
         a position detection unit which detects a registration error amount between the recording medium conveyance unit and the image forming unit positioned by a positioning unit, on the basis of reading of a reference pattern arranged on one of the recording medium conveyance unit and the image forming unit; and   a control unit which calculates a correction amount on the basis of the registration error amount detected by said position detection unit and controls operation timings of the image forming unit and the recording medium conveyance unit in accordance with the correction amount.       

   According to another aspect of the present invention, the foregoing object is attained by providing an image forming apparatus which includes, as a plurality of detachable units having different functions, at least an image forming unit which forms an image on a recording medium and a recording medium conveyance unit which conveys the recording medium, and executes image formation corresponding to a combination of the units, comprising:
         a calculation unit which calculates a registration error amount between the recording medium conveyance unit and the image forming unit positioned by a positioning unit, on the basis of a result of image formation on the recording medium; and   a control unit which calculates a correction amount on the basis of the registration error amount calculated by said calculation unit and controls operation timings of the image forming unit and the recording medium conveyance unit in accordance with the correction amount.       

   According to another aspect of the present invention, the foregoing object is attained by providing a method of controlling an image forming apparatus which includes, as a plurality of detachable units having different functions, at least an image forming unit which forms an image on a recording medium and a recording medium conveyance unit which conveys the recording medium, and executes image formation corresponding to a combination of the units, comprising the steps of:
         detecting a registration error amount between the recording medium conveyance unit and the image forming unit positioned by a positioning unit, on the basis of reading of a reference pattern arranged on one of the recording medium conveyance unit and the image forming unit; and   calculating a correction amount on the basis of the registration error amount detected in the step of a detecting registration error amount, and controlling operation timings of the image forming unit and the recording medium conveyance unit in accordance with the correction amount.       

   According to another aspect of the present invention, the foregoing object is attained by providing a method of controlling an image forming apparatus which includes, as a plurality of detachable units having different functions, at least an image forming unit which forms an image on a recording medium and a recording medium feed conveyance unit which conveys the recording medium, and executes image formation corresponding to a combination of the units, comprising the steps of:
         calculating a registration error amount between the recording medium feed conveyance unit and the image forming unit positioned by a positioning unit, on the basis of a result of image formation on the recording medium; and   calculating a correction amount on the basis of the registration error amount calculated in the step of calculating a registration error amount, and controlling operation timings of the image forming unit and the recording medium feed conveyance unit in accordance with the correction amount.       

   The present invention can provide an image forming technique which implements operation specifications desired by a user by a combination of subsystems. 
   The present invention can provide an image forming technique capable of, even when a registration error occurs between subsystems upon exchanging or detaching a subsystem, correcting the registration error and maintaining the image formation quality. 
   The present invention can provide an image forming technique capable of, even when exchanging or detaching a subsystem, correcting a delay of the output time of the first recording material or a delay of the output time of recording materials in a continuous operation, thereby preventing a decrease in the throughput of an image forming apparatus. 
   Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing the overall arrangement of an image forming apparatus according to an embodiment of the present invention; 
       FIGS. 2A and 2B  are views for explaining the positioning mechanism of a subsystem assembled into the image forming apparatus; 
       FIG. 3A  is a sectional view showing an example of the arrangement of a color image forming apparatus when assembling a 4-drum type color image forming subsystem having four photosensitive drums as a forming subsystem; 
       FIG. 3B  is a sectional view showing an example of the arrangement of a color image forming apparatus when assembling a 1-drum type color image forming subsystem having one photosensitive drum as an image forming subsystem; 
       FIG. 3C  is a sectional view showing an example of the arrangement of a monochrome image forming apparatus when assembling a 1-drum type monochrome image forming subsystem having one photosensitive drum as an image forming subsystem; 
       FIGS. 4A and 4B  are views showing examples of the arrangements of two types of paper conveyance subsystems; 
       FIG. 5  is a block diagram of a full-color image forming subsystem; 
       FIG. 6  is a timing chart showing the image formation timing of the full-color image forming subsystem; 
       FIG. 7  is a block diagram of another full-color image forming subsystem; 
       FIG. 8  is a timing chart showing the image formation timing of the full-color image forming subsystem; 
       FIG. 9  is a block diagram of a monochrome image forming subsystem; 
       FIG. 10  is a timing chart showing the image formation timing of the monochrome image forming subsystem; 
       FIGS. 11A and 11B  are sectional views showing the schematic structure of a paper feed unit; 
       FIGS. 12A and 12B  are sectional views showing the schematic structure of a paper conveyance unit; 
       FIGS. 13A and 13B  are sectional views showing a structure of assembling the paper feed unit and paper conveyance unit into a paper conveyance platform; 
       FIG. 14  is a sectional view of an image forming subsystem for a full-color printer; 
       FIG. 15  is a sectional view of another image forming subsystem for a full-color printer; 
       FIG. 16  is a sectional view of a monochrome image forming subsystem; 
       FIGS. 17A and 17B  are views showing parameters in configuration communication upon power-on; 
       FIGS. 18A and 18B  are ladder charts for explaining a command sequence upon power-on; 
       FIG. 19  is a view showing parameters in communication between units; 
       FIGS. 20A and 20B  are ladder charts for explaining a command sequence in communication between units when the image forming apparatus forms an image; 
       FIG. 21  is a perspective view showing a state of pulling out the image forming subsystem from the paper conveyance platform; 
       FIG. 22  is a view showing the arrangement of a position detector arranged near the fitting portion between the paper conveyance platform and the image forming subsystem; 
       FIG. 23  is a view showing the relationship between the detection position and the reference position; 
       FIG. 24  is a view for explaining position correction in the main scanning direction; 
       FIG. 25  is a view showing a concrete positional relationship between paper feed and transfer; 
       FIG. 26  is a timing chart for correcting a registration error in the sub-scanning direction (paper conveyance correction); 
       FIG. 27  is a view showing a concrete positional relationship between paper feed and transfer; 
       FIG. 28  is a timing chart for correcting a registration error (paper conveyance correction); 
       FIG. 29  is a view showing a registration error correction sheet for determining whether a registration error occurs upon exchanging a subsystem; 
       FIG. 30  is a flowchart for explaining the sequence of an image forming apparatus control method according to the first embodiment; and 
       FIG. 31  is a flowchart for explaining the sequence of an image forming apparatus control method according to the third embodiment. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments may not be construed to limit the scope of the present invention. Not all combinations of features described in the embodiments are indispensable for solving the present invention. 
   First Embodiment 
   &lt;Overall Arrangement&gt; 
     FIG. 1  is a view showing the overall arrangement of an image forming apparatus according to the first embodiment of the present invention. 
   The first embodiment will exemplify, as the image forming apparatus, a multi-functional peripheral (MFP) which comprises an electrophotographic image forming apparatus  100  and functions as a scanner, a facsimile, a copying machine, and a printer for receiving data from a PC and printing it. The image forming apparatus has a color printing function which adopts a photosensitive body and intermediate transfer method. 
   The user uses an operation unit  210  to designate a print mode, print count, or print condition, or the serviceman uses the operation unit  210  to make various operation settings in maintenance work. When the user presses a print start key (not shown) on the operation unit  210 , a document scanning apparatus  220  and document paper feed apparatus  230  start scanning a document image, and the image forming apparatus starts a desired apparatus operation such as a print operation or document image transmission. 
   The image forming apparatus  100  or an image forming apparatus  101  or  102  converts a document image into image information and prints it out. 
   The image forming apparatus  100  incorporates a paper conveyance subsystem (to be referred to as a paper conveyance platform)  60  and an image forming subsystem  150 . The paper conveyance platform incorporates a paper feed unit  70  and paper conveyance unit  80 . The image forming apparatus  100  also incorporates a power unit  90 . 
   The document paper feed apparatus  230  feeds a set document to the scan position of the document scanning apparatus  220 . The document scanning apparatus  220  converts image data of the document fed to the scan position of the document scanning apparatus  220  into image information, and sends the image information to a controller  200 . 
   The controller  200  performs a desired image process, and sends the image information of the document image scanned by the document scanning apparatus  220  to the image forming apparatus  100 . The image forming apparatus  100  prints, implementing a function of copying a document image. 
   The document scanning apparatus  220  converts a document image into image information and sends the image information to the controller  200 . Then, the controller  200  stores the image information in the storage device. (storage unit) of a server  30 - 1  via a network  10 . The server  30 - 1  transmits the image information to a client PC  20 - 1  and stores the desired image information in the storage device (storage unit) of the client PC  20 - 1 . The user can utilize the received image information. 
   By designating the destination address of email or the like as a transmission destination, the server  30 - 1  can transmit image information to a server  30 - 2  at the desired transmission destination via Internet  40 . The partner server  30 - 2  transmits the stored image information to a partner client PC  20 - 2 . The storage unit of the client PC  20 - 2  stores the image information to allow the user to utilize the image information from the partner client PC  20 - 2 . 
   The client PCs  20 - 1  and  20 - 2  respectively connected to the network  10  and a network  10 - 2  can also transmit image information to the image forming apparatus  100  via the controller  200 , and cause the image forming apparatus  100  to process the output image. 
   The first embodiment gives an exchangeable arrangement to the image forming subsystem  150  for mainly forming an image, and thereby provides various advantages to a user, serviceman, and the like. 
   &lt;Example of Exchangeable Arrangement of Image Forming Subsystem&gt; 
   The image forming apparatus according to the first embodiment of the present invention provides various advantages to a user, serviceman, and the like by exchangeably configuring the image forming subsystem  150  for mainly forming an image. The first embodiment will exemplify the arrangements of color and monochrome image forming apparatuses in which image forming subsystems are replaced. 
     FIGS. 3A to 3C  are sectional views showing examples of the arrangements of three types of image forming apparatuses.  FIG. 3A  is a sectional view showing an example of the arrangement of the color image forming apparatus  100  when assembling, as an image forming subsystem, a color image forming subsystem  150 A of a 4-drum type (to be referred to as a  4 D type hereinafter) containing an image creating unit  170 A ( FIG. 14 ) having four photosensitive drums. The color image forming subsystem  150 A comprises four photosensitive drums serving as image carriers, an exposure unit, a charging unit, and a development unit. This arrangement is particularly suitable for high-productivity color image formation, and may target an office or near-print. As the image forming subsystem  150 A, the image forming apparatus permits assembling various image forming subsystems in accordance with the user&#39;s request, such as an image forming subsystem having a productivity of 20 A4-size sheets/min in color printing, and an image forming subsystem having a productivity of 70 sheets/min in color printing. 
     FIG. 3B  shows an example of the arrangement of the color image forming apparatus  101  when assembling, as an image forming subsystem, a 1D type color image forming subsystem  150 B containing an image creating unit  170 B having one photosensitive drum. The color image forming subsystem  150 B comprises one photosensitive drum serving as an image carrier, an exposure unit, a charging unit, and a development unit. 
   The color image forming subsystem  150 B is applicable to, e.g., an image forming subsystem having different print resolutions such as 400 dpi, 600 dpi, and 1,200 dpi. The image forming apparatus permits assembling various image forming subsystems compatible with many types of toners used to print and many types of printable transfer materials, in accordance with the user&#39;s request. 
     FIG. 3C  is a sectional view showing an example of the arrangement of the monochrome image forming apparatus  102  when assembling, as an image forming subsystem, a 1D type monochrome image forming subsystem  150 C containing an image creating unit  170 C ( FIG. 16 ) having one photosensitive drum. The monochrome image forming subsystem  150 C comprises one photosensitive drum serving as an image carrier, an exposure unit, a charging unit, and a development unit. The image forming apparatus permits assembling image forming subsystems  150 C of various performances and specifications in accordance with the user&#39;s request, such as an image forming subsystem having a productivity of 20 A4-size sheets/min in monochrome printing, and an image forming subsystem having a productivity of 100 sheets/min in monochrome printing. 
   An arrangement which makes it possible to exchange the paper conveyance unit  80  having a paper conveyance function with various paper conveyance units can further provide many product lineups. 
   &lt;Example of Exchangeable Arrangement of Paper Conveyance Subsystem&gt; 
     FIGS. 4A and 4B  are views showing examples of the arrangements of two types of paper conveyance subsystems.  FIG. 4A  shows an example of a low-speed paper conveyance subsystem which incorporates a paper feed unit  70 A and paper conveyance unit  80 A. 
     FIG. 4B  shows an example of a high-speed paper conveyance subsystem which incorporates a paper feed unit  70 B and paper conveyance unit  80 B. Either of the paper feed units  70 A and  70 B, either of the paper conveyance units  80 A and  80 B, and one of the image forming subsystems  150 A,  150 B, and  150 C can be combined. 
   The paper feed unit  70 A and paper conveyance unit  80 A build a low-speed paper conveyance platform  60 A (also called a paper conveyance subsystem  60 A). The paper feed unit  70 B and paper conveyance unit  80 B build a high-speed paper conveyance platform  60 B (also called a paper conveyance subsystem  60 B). In selecting either the paper conveyance platform  60 A or  60 B, the user can select a paper conveyance platform in accordance with how the user uses the image forming apparatus, other than image formation factors such as paper conveyability, productivity, and durability. While comparing the image formation features of image forming subsystems, the user can combine image forming subsystems  150  appropriate for image quality desired by the user to selectively configure an image forming apparatus. 
     FIG. 21  is a perspective view showing a state of opening a cover  810  and pulling out the image forming subsystem  150  from the paper conveyance platform  60 . 
   Two, right and left slide rails (slide mechanisms)  811  couple the image forming subsystem  150  to the paper conveyance platform  60 , and permit pulling out and replaceable the image forming subsystem  150 . When pulling out the image forming subsystem  150 , an image creating unit  170  and fixing unit  180  mounted in the image forming subsystem  150  are pulled out together. 
   The paper feed unit  70  and paper conveyance unit  80  in the paper conveyance platform  60  will be explained. Similar to the image forming subsystem  150 , two, right and left slide rails  812  couple the paper feed unit  70  to the paper conveyance platform and permit pulling out and replaceable the paper feed unit  70 . Also similar to the paper feed unit  70 , two, right and left slide rails  813  couple the paper conveyance unit  80  to the paper conveyance platform  60  and permit to pulling out and replaceable the paper conveyance unit  80 . 
   (Positioning of Subsystem) 
     FIGS. 2A and 2B  are views for explaining the positioning mechanism (positioning unit) of a subsystem assembled into the image forming apparatus.  FIG. 2A  shows a state before fitting the image forming subsystem  150  to the paper conveyance platform  60 .  FIG. 2B  shows a state after fitting. Reference numeral  150  denoting an image forming subsystem will typify the above-described image forming subsystems  150 A,  150 B, and  150 C. It is important to meet precision and cost requirements and achieve user-friendly attachment/detachment on the assumption of insertion and removal of the image forming subsystem  150 . For this purpose, the arrangement of the attaching/detaching mechanism, the method and arrangement of the positioning mechanism, and the like are important. 
   An example of an arrangement will be explained which satisfies the positioning precision requirement while improving user operability by a positioning pin  115 , the hole shape of a positioning hole  119 , an attaching/detaching knob, and the like. 
   The shapes of the positioning pin  115  and hole  119  are optimally designed in accordance with the dimensional relationship (dimensional tolerance of a fitting system or the like) between the shaft and the hole for the purpose of smooth positioning. 
   (Design of Shapes of Positioning Pin and Positioning Hole) 
   The positioning pin  115  is used for an application purpose requiring positioning precision, and the positioning pin shape is determined in consideration of the precision requirement, reliability improvement, user operability, and the like. The positioning precision requirement and the precision levels of components (high-precision components, components greatly varying in precision, or the like) which form the positioning pin  115  and positioning hole  119  determine the shape precision and component attachment precision of components for use. 
   For example, the length of the contact surface between the positioning pin  115  of the image forming subsystem  150  (e.g., image forming subsystem  150 A,  150 B, or  150 C) and the positioning hole  119  in the paper conveyance platform (paper conveyance subsystem)  60  is determined in consideration of operability, workability, and the like. 
   The hole diameter and hole position of the positioning hole  119  are determined with sufficient precision in consideration of the tolerance of the positioning precision requirement with the image forming subsystem  150 . If necessary, it is also effective to improve the squareness precision of the positioning hole  119  with respect to the positioning pin  115 . The reference plane of the outer shape of the positioning pin  115  inserted into the positioning hole is so determined as to relatively position the hole and pin surface at high precision. In this manner, fitting of the positioning pin  115  in the positioning hole  119  is designed under proper conditions. The relative positions of the paper conveyance platform (paper conveyance subsystem)  60  and image forming subsystem  150  can fall within the precision requirement. 
   Considering operability, it is desirable to chamfer the fitting inlet port large for smooth insertion or shape it for easy removal. The shaft diameter of the positioning pin  115 , the shape of its distal end, and the like are determined in consideration of the length of the tapered portion of the positioning pin  115  and the degree of a shift between the centers of the positioning pin  115  and positioning hole  119  in positioning insertion. 
   It is also preferable to determine the length of the positioning guide and the like in terms of improvement of operability and apparatus reliability. In  FIGS. 2A and 2B , the shape of the distal end of the positioning pin  115  is tapered slightly thin, easily guiding the positioning pin  115  into the positioning hole  119  in insertion. 
   Especially, the image forming subsystem  150  is assumed to have a relatively heavy structure which incorporates various components necessary to implement an image forming function. For example, the image forming subsystems  150 A and  150 B which form a color image desirably achieve operability considering various users. 
   As for the image forming subsystem  150 C which forms a monochrome image, a high-speed monochrome image arrangement with high-productivity is assumed to be equal to or heavier than a color one. A middle-speed arrangement is assumed to be equal to or lighter than a color one. 
   As described above, it is desirable to design an arrangement which provides good user operability while achieving desired safety, durability, reliability, and high precision regardless of which of various image forming subsystems  150  is connected. 
   When the image forming subsystem  150  connectable to the paper conveyance platform (paper conveyance subsystem)  60  is lightweight or the positioning precision requirement can relax, an attaching/detaching mechanism  110  and positioning arrangement  120  can change to low-cost arrangements, expecting cost reduction. 
   (Positioning Detector) 
   As shown in  FIG. 21 , the image forming apparatus  100 ,  101 , or  102  incorporates the slide mechanism (built by the slide rails  811 ,  812 , and  813 ) so as to pull out the image forming subsystem  150 . The arrangement which permits attaching/detaching the image forming subsystem  150  requires registration between a transfer material and a toner image to be transferred onto the transfer material. 
   For this purpose, the image forming apparatus  100  has a position detector  112  which detects the relative positions of the image forming subsystem  150  and paper conveyance platform (paper conveyance subsystem)  60  (see  FIGS. 2A and 2B ) when the image forming apparatus  100  incorporates the image forming subsystem  150 . 
   As a position detection sensor used for the position detector  112 , a compact, low-cost optical displacement sensor and the like are put into practice use. Examples of the sensor suitable for the application purpose of the present invention are a micro displacement sensor available from OMRON and an area image sensor. Note that the use of these sensors does not limit the gist of the present invention, and the present invention can also adopt a sensor other than an optical one. 
   When exemplifying the micro displacement sensor available from Omron, a micro displacement sensor Z4D-B02 has a detectable distance of 9.5 mm±3 mm and a detection resolution of ±50 μm. 
   Since one dot (one pixel) is 25.4 mm/400 dots=63.5 μm in a 400-dpi image forming subsystem, the detection resolution of the micro displacement sensor is smaller than one dot (one pixel). One dot is 25.4 mm/600 dots=42.3 μm at a resolution of 600 dpi, and the detection resolution corresponds to 1.18 dots. One dot is 25.4 mm/1,200 dots=21.2 μm at a resolution of 1,200 dpi, and the detection resolution corresponds to 2.36 dots. 
   The relative positions of the image forming subsystem  150  and paper conveyance subsystem  60  are detected on the basis of the relative positions of an image to be printed and a transfer material (transfer sheet) to be printed. In other words, the resolution of about 50 μm suffices. 
   Assuming that the margin size is 2.5 mm, the resolution “±50 μm” of the micro displacement sensor of the position detector corresponds to 1/50 of the margin, and the micro displacement sensor has a detection precision enough for a normal print operation. To further increase the position detection resolution of the position detector  112 , the use of a micro displacement sensor Z4D-B01 also available from Omron can increase the detection resolution from the previous resolution “±50 μm” to ±10 μm. That is, the detector resolution increases five times. 
   In the use of the micro displacement sensor for the position detector  112 , a result detected by the micro displacement sensor is an analog output which linearly decreases a voltage output from the micro displacement sensor as the distance between the detection target and the micro displacement sensor becomes longer. Position information from the sensor of the position detector  112  is used to control an image formation position so as to print an image at a proper position on a transfer material. 
   The user operates the attaching/detaching knob to push and horizontally slide the image forming subsystem  150  into the image forming apparatus  100 . A subsystem reference surface  113  which serves as the reference position of the image forming subsystem  150  and is formed on an abutment member  117  contacts an abutment member  118  of the paper conveyance platform  60  that opposes the reference surface  113 , positioning the image forming subsystem  150 . The abutment member  118  has the position detector  112 . The positioning pin  115  of the image forming subsystem  150  is inserted into the positioning hole  119  of the image forming apparatus, and the image forming apparatus  100  accommodates the image forming subsystem  150  at a desired positioning precision. At this time, the position detector  112  measures the mechanical positions of the paper conveyance platform  60  and image forming subsystem  150 . Position detection sensor light emitted from the position detector  112  irradiates the subsystem reference surface  113 , and the position detector  112  receives light reflected by the subsystem reference surface  113 . The position detector  112  detects the position Ls of the image forming subsystem  150  on the basis of reception of light reflected by the reference surface  113 . The position detector  112  sends position information (position detection information) detected as the position Ls to a platform controller  65  (see  FIG. 1 ) of the paper conveyance platform  60 . 
   The contents of concrete measurement by the position detector  112  will be explained later with reference to  FIG. 22 , and a detailed description thereof will be omitted. 
   The platform controller  65  sends position control information to an image forming controller  160  so as to control the image formation position to an optimal position on the basis of position detection information. 
   Note that the paper conveyance platform  60  may have a reference surface, and the image forming subsystem  150  may have the position detector  112  and send position detection information to the image forming controller  160  (see  FIG. 1 ). 
   The reference surface  113  has been exemplified as the reference surface of the abutment member of the image forming subsystem  150 , but the method or detection portion may be added or changed. In other words, the arrangement suffices to be able to reflect light emitted from the position detector  112 . 
   For example, it is also possible to increase the number of position detectors  112  or change the layout position of the position detector  112  so as to detect reference surfaces  113 - 2  and  113 - 3  as other reference surfaces of the abutment member  117 . For example, it is possible to detect the registration errors of the reference surfaces  113 ,  113 - 2 , and  113 - 3  in three directions, detect the 3-dimensional registration error of the image forming subsystem  150  at higher precision, and apply the 3-dimensional registration error to image position correction control. 
   The positioning arrangement is effectively arranged near a mechanism of transferring a toner image onto a transfer material. This layout can more effectively increase the positional precisions of the transfer roller and a fed transfer material. 
   The paper conveyance platform  60 , paper feed unit  70 , and paper conveyance unit  80  will be explained. 
   (Paper Feed Unit) 
     FIGS. 11A and 11B  are sectional views showing the schematic structure of the paper feed unit  70 . A plurality of paper feed units  70 A and  70 B different in performance exchangeably connect to the paper conveyance platform  60 . 
   As paper feed units different in performance, the first embodiment will explain the paper feed unit  70 A for low-speed paper feed and the paper feed unit  70 B for high-speed paper feed. Each paper feed unit exchangeably connects to the paper conveyance platform  60 . 
   In the paper feed unit  70 A for low-speed paper feed, reference symbol P denotes a transfer material. Reference numeral  501  denotes a DC brushless motor;  502 , a pickup roller driven to rotate by the DC brushless motor  501 ;  503 , paper conveyance rollers driven to rotate by the DC brushless motor  501 ;  511 , a paper feed path; and  512 , a paper refeed path. 
   The platform controller  65  or a paper feed unit controller (not shown) in the paper feed unit controls the paper feed unit  70 A. The DC brushless motor  501  rotates at a predetermined speed. In a paper feed operation, a solenoid (not shown) or the like controls abutment/separation of the pickup roller  502  against/from the transfer material P at a predetermined timing. The pickup roller  502  driven by the DC brushless motor  501  abuts and picks up the transfer material P, and feeds it to the paper feed path  511 . The paper conveyance rollers  503  on the paper feed path  511  convey the transfer material P to the image forming subsystem  150  at a predetermined speed. The transfer material P fed again from the paper conveyance unit (to be described later) passes through the paper refeed path  512 , and is conveyed to the image forming subsystem  150  by the paper conveyance rollers  503  on the paper feed path  511 . 
   In the paper feed unit  70 B for high-speed paper feed, reference numeral  504  denotes a stepping motor which drives the pickup roller  502  and paper conveyance rollers  503 . The platform controller  65  or a paper feed unit controller (not shown) in the paper feed unit controls the paper feed unit  70 B. The stepping motor  504  at a variably controlled speed. In a paper feed operation, a solenoid (not shown) or the like controls abutment/separation of the pickup roller  502  against/from the transfer material P at a predetermined timing. The pickup roller  502  driven by the stepping motor  50 . 4  abuts and picks up the transfer material P, and feeds it to the paper feed path  511 . The paper conveyance rollers  503  on the paper feed path  511  convey the transfer material P to the image forming subsystem  150  at a predetermined speed. The transfer material P fed again from the paper conveyance unit (to be described later) passes through the paper refeed path  512 , and is conveyed to the image forming subsystem  150  by the paper conveyance rollers  503  on the paper feed path  511 . At this time, the paper conveyance speed of the transfer material P changes in accordance with the variably controlled rotational speed of the stepping motor  504 . This makes it possible to control the conveyance speed of a transfer material and the interval between successively fed transfer materials at multiple levels in a wide range. 
   The paper feed unit  70  has been described by exemplifying an arrangement having one paper feed stage. However, the paper feed unit  70  is not limited to this arrangement, and includes, e.g., a conventionally well-known arrangement which couples or connects a plurality of paper feed stages to feed transfer materials of a plurality of types and a plurality of sizes. 
   (Paper Conveyance Unit) 
     FIGS. 12A and 12B  show the schematic structure of the paper conveyance unit  80 . A plurality of paper conveyance units different in performance exchangeably connect to the paper conveyance platform  60 . As paper conveyance units different in performance, the first embodiment will explain the paper conveyance unit  80 A for low-speed paper conveyance and the paper conveyance unit  80 B for high-speed paper conveyance. 
   In the paper conveyance unit  80 A for low-speed paper conveyance, reference numeral  520  denotes a stepping motor;  521 , a DC brushless motor;  522 , paper discharge rollers driven to rotate forward and backward by the stepping motor  520 ;  523  and  524 , paper conveyance rollers driven by the DC brushless motor  521 ;  525 , a paper discharge path; and  526 , a paper conveyance path. The platform controller  65  or a paper conveyance unit controller (not shown) in the paper conveyance unit controls the paper conveyance unit  80 A. The stepping motor  520  is driven to rotate forward and backward in accordance with the operation mode. The DC brushless motor  521  rotates at a predetermined speed. In a paper conveyance operation, the transfer material P conveyed from the fixing unit  180  of the image forming subsystem  150  is fed to the paper discharge path  525 . 
   In paper discharge, the paper discharge rollers  522  rotate in a direction in which the transfer material is discharged outside the apparatus, and thereby discharge the transfer material P outside the apparatus. In reversal for double-sided formation, the paper discharge rollers  522  rotate in a direction in which the transfer material P is discharged. While the paper discharge rollers  522  pinch the trailing edge of the transfer material P, the stepping motor  520  stops and rotates backward. The paper discharge rollers  522  stop and rotate backward to convey the transfer material P to the paper conveyance path  526 . The paper conveyance rollers  523  and  524  driven to rotate by the DC brushless motor  521  rotating at a predetermined speed convey the transfer material P to the paper conveyance path  526 , feeding the transfer material P to the paper refeed path  512  of the paper feed unit  70 . 
   In the paper conveyance unit  80 B for high-speed paper conveyance, reference numerals  531  and  532  denote stepping motors. The stepping motor  531  drives and rotates the paper conveyance rollers  523 , whereas the stepping motor  532  drives and rotates the paper conveyance rollers  524 . The platform controller  65  or a paper conveyance unit controller (not shown) in the paper conveyance unit controls the paper conveyance unit  80 B. The stepping motors  520 ,  531 , and  532  rotate at a variably controlled speed. In a paper conveyance operation, the transfer material P conveyed from the fixing unit  180  of the image forming subsystem  150  is fed to the paper discharge path  525 . In paper discharge, the paper discharge rollers  522  rotate in a direction in which the transfer material is discharged outside the apparatus, and thereby discharge the transfer material P outside the apparatus. In reversal for double-sided formation, the paper discharge rollers  522  rotate in a direction in which the transfer material P is discharged. While the paper discharge rollers  522  pinch the trailing edge of the transfer material P, the stepping motor  520  stops and rotates backward. The paper discharge rollers  522  stop and rotate backward to convey the transfer material P to the paper conveyance path  526 . The transfer material P is conveyed to the paper conveyance path  526  by the paper conveyance rollers  523  driven to rotate by the stepping motor  531  whose speed is variably controlled, and by the paper conveyance rollers  524  driven to rotate by the stepping motor  532 . The transfer material P is fed to the paper refeed path  512  of the paper feed unit  70 . At this time, the paper conveyance speed of the transfer material P changes in accordance with the variably controlled rotational speeds of the stepping motors  531  and  532 . This makes it possible to control the paper conveyance speed of a transfer material and the interval between successively conveyed transfer materials at multiple levels in a wide range. 
   (Description of Arrangement of Assembling Paper Feed Unit and Paper Conveyance Unit into Paper Conveyance Platform) 
     FIGS. 13A and 13B  are sectional views showing structures of assembling the paper feed units  70 A and  70 B and the paper conveyance units  80 A and  80 B into the paper conveyance platform  60 .  FIGS. 13A and 13B  show examples of combinations with the paper conveyance platforms  60 A and  60 B, but a combination of units is not limited to them. For example, the paper feed units  70 A and  70 B and the paper conveyance units  80 A and  80 B are properly combined and assembled into the paper conveyance platforms  60 A and  60 B in accordance with the application purpose and specifications. The platform controller  65  ( FIG. 1 ) identifies or communicates with each assembled unit to collect control information corresponding to the assembled unit. The platform controller  65  communicates control information corresponding to the assembled unit with a printer engine controller  105 . The platform controller  65  comprehensively controls the paper conveyance platform  60  on the basis of control specifications determined by the printer engine controller  105 . 
   (Description of Image Forming Subsystem  150 )
         The image forming subsystem  150  will be explained.       

     FIG. 14  is a sectional view of the image forming subsystem  150 A for a full-color printer (4-drum type). The image creating unit  170 A has four photosensitive drums. A fixing unit  180 A is exchangeable with another unit of the same function, and is physically separable from the image creating unit  170 A. 
   Details of the image creating unit  170 A will be explained. 
   The image creating unit  170 A comprises an image forming portion  601 Y which forms an yellow image, an image forming portion  601 M which forms a magenta image, an image forming portion  601 C which forms a cyan image, and an image forming portion  601 Bk which forms a black image. The four image forming portions  601 Y,  601 M,  601 C, and  601 Bk align in a line at predetermined intervals. 
   The image forming portions  601 Y,  601 M,  601 C, and  601 Bk comprise drum type electrophotographic photosensitive bodies (to be referred to as photosensitive drums hereinafter)  602 A,  602 B,  602 C, and  602 D serving as image carriers, respectively. Primary chargers  603 A to  603 D, developing devices  604 A to  604 D, transfer rollers  605 A to  605 D serving as transfer units, and drum cleaners  606 A to  606 D surround the photosensitive drums  602 A to  602 D, respectively. A laser exposure device  607  is arranged below the primary chargers  603 A,  603 B,  603 C, and  603 D and the developing devices  604 A,  604 B,  604 C, and  604 D. 
   The developing devices  604 A,  604 B,  604 C, and  604 D store yellow, cyan, magenta, and black toners, respectively. 
   The photosensitive drums  602 A,  602 B,  602 C, and  602 D are negatively charged OPC photosensitive bodies having photoconductive layers on aluminum drum bases, respectively. Driving devices (not shown) drive and rotate the photosensitive drums  602 A,  602 B,  602 C, and  602 D clockwise at predetermined process speeds in  FIG. 14 . 
   The primary chargers  603 A,  603 B,  603 C, and  603 D serving as primary charging units uniformly charge the surfaces of the photosensitive drums  602 A,  602 B,  602 C, and  602 D to predetermined negative potentials by charging biases applied from a charging bias supply (not shown). 
   The developing devices  604 A,  604 B,  604 C, and  604 D store toners, and apply toners of respective colors to electrostatic latent images respectively formed on the photosensitive drums  602 A,  602 B,  602 C, and  602 D, developing (visualizing) the electrostatic latent images as toner images. 
   The transfer rollers  605 A,  605 B,  605 C, and  605 D serving as primary transfer units can abut the photosensitive drums  602 A,  602 B,  602 C, and  602 D via an intermediate transfer belt  608  at primary transfer portions  615 A,  615 B,  615 C, and  615 D. 
   The drum cleaners  606 A,  606 B,  606 C, and  606 D have, e.g., cleaning blades for removing, from the photosensitive drums  602 A,  602 B,  602 C, and  602 D, transfer toner remaining on the photosensitive drums  602 A,  602 B,  602 C, and  602 D after primary transfer. 
   The intermediate transfer belt  608  is arranged above the photosensitive drums  602 A to  602 D, and looped between a secondary transfer counter roller  609  and a tension roller  610 . At a secondary transfer portion  616 , the secondary transfer counter roller  609  can abut a secondary transfer roller  611  via the intermediate transfer belt  608 . The intermediate transfer belt  608  is formed from a dielectric resin such as a polycarbonate resin film, polyethylene terephthalate resin film, or polyvinylidene fluoride resin film. 
   A primary transfer surface  608 B of the intermediate transfer belt  608  on a side opposing the photosensitive drums  602 A,  602 B,  602 C, and  602 D inclines to the secondary transfer roller  611 . 
   The laser exposure device  607  comprises, e.g., a polygon mirror  618 , a scanner motor  617 , a reflection mirror, and a laser emitting unit (not shown) for emitting light corresponding to time series electrical digital pixel signals of supplied image information. The laser exposure device  607  exposes the photosensitive drums  602 A,  602 B,  602 C, and  602 D to form electrostatic latent images of respective colors corresponding to image information on the surfaces of the photosensitive drums  602 A,  602 B,  602 C, and  602 D charged by the primary chargers  603 A,  603 B,  603 C, and  603 D. At the same time, a beam detection signal (BD) generation circuit (not shown) in the laser exposure device  607  detects a laser beam in the main scanning direction polarized by the polygon mirror. 
   The image creating unit  170 A further comprises an image creating unit controller (not shown) for controlling the operations of these elements. The image creating unit controller controls the process speed of the image creating unit and tint/density adjustment. 
   The fixing unit  180 A will be explained. 
   The fixing unit  150 A is arranged downstream of the secondary transfer portion  616  of the image creating unit  170 A in the recording paper conveyance direction. A fixing device  612  having a fixing roller  612 A which incorporates a heat source such as a halogen heater and a press roller  612 B is installed along a vertical path. A driving device (not shown) drives and rotates the fixing roller  612 A and press roller  612 B. The surface temperature of the fixing roller is controlled by controlling power of the halogen heater in the fixing roller  612 A. Further, the fixing unit  180 A comprises a fixing unit controller (not shown) for controlling these elements. The fixing unit controller controls the rotational speed of each roller, the temperature of the fixing roller, and a process upon occurrence of an abnormality. 
   The image forming subsystem  150 A for a full-color printer comprises the image forming controller  160  ( FIG. 1 ). The image forming controller  160  communicates with the image creating unit controller and fixing unit controller, receives pieces of unit information from the respective controllers, and transmits unit control information to the respective controllers. Also, the image forming controller  160  exchanges image signals with the controller  200 , and exchanges pieces of control information with the printer engine controller  105  and platform controller  65 . 
   In the above description, the image creating unit and fixing unit have controllers, respectively. However, the image creating unit and fixing unit can operate without any controller. In this case, the image forming controller  160  ( FIG. 1 ) controls elements in the image creating unit and fixing unit. 
     FIG. 15  is a sectional view of the image forming subsystem  150 B for a full-color printer (1-drum type). Similar to the above-described color image forming subsystem  150 A, the image creating unit  170 B has one photosensitive drum. A fixing unit  180 B is exchangeable with another unit of the same function, and is physically separable from the image creating unit  170 B. 
   Details of the image creating unit  170 B will be explained. 
   A scanner unit  631  comprises a laser unit  634 , polygon mirror  635 , scanner motor  636 , and beam detection signal (BD signal) generation circuit  643 . 
   The image creating unit  170 B comprises the scanner unit  631 , a photosensitive drum  632 , an intermediate transfer belt  633 , a developing rotary unit  637 , a primary transfer roller, a secondary transfer roller  638 , and a cleaning blade  639 . The developing rotary unit  637  incorporates developing substance units  637 A to  637 D for respective colors. 
   The photosensitive drum  632  is an OPC photosensitive body having a photoconductive layer on an aluminum drum base. A driving device (not shown) drives and rotates the photosensitive drum  632  clockwise in  FIG. 15  at a predetermined process speed. 
   A primary charger  642  serving as a primary charging unit uniformly charges the surface of the photosensitive drum  632  to a predetermined potential by a charging bias applied from a charging bias supply (not shown). 
   In the scanner unit  631 , the laser unit  634  emits a laser beam modulated on the basis of time series electrical digital pixel signals of supplied image information. The polygon mirror  635  is a rotary polygon mirror which deflects a laser beam emitted by the laser unit  634  to scan the surface of the photosensitive drum  632  and form an electrostatic latent image on the photosensitive drum  632 . The scanner motor  636  drives and rotates the polygon-mirror  635 . The beam detection signal (BD signal) generation circuit  643  detects a laser beam in the main scanning direction deflected by the polygon mirror  635 . 
   The developing rotary unit  637  uses the developing substance units  637 A,  637 B,  637 C, and  637 D for yellow (Y), magenta (M), cyan (C), and black (B) to develop an electrostatic latent image formed on the photosensitive drum  632 . Similar to the above-mentioned vertical-path 4D color image creating unit, the photosensitive drum  632  applies a primary transfer bias to the primary transfer roller, and primarily transfers, to the intermediate transfer belt  633 , a developing substance supplied from the developing rotary unit  637  onto the photosensitive drum  632 . The secondary transfer roller  638  abuts the intermediate transfer belt  633 , and secondarily transfers the developing substance on the intermediate transfer belt  633  to a recording medium such as a recording sheet. 
   The cleaning blade  639  is always in contact with the photosensitive drum  632 , and cleans it by scraping toner remaining on the surface of the photosensitive drum  632 . 
   Similar to the above-mentioned color image creating unit ( FIG. 14 ), the image creating unit  170 B comprises an image creating unit controller (not shown) for controlling the operations of these elements. The image creating unit controller controls the process speed of the image creating unit and tint/density adjustment. 
   The fixing unit  180 B will be explained. 
   The fixing unit  180 B is arranged downstream of the secondary transfer roller  638  of the image creating unit  170 B in the recording paper conveyance direction. Similar to the above-described color image creating unit ( FIG. 14 ), a fixing device  640  performs a fixing operation of heating, pressing, and thereby fixing a toner image transferred on a recording medium. A driving device (not shown) drives and rotates the roller of the fixing unit. The surface temperature of the fixing roller is controlled by controlling power of the halogen heater in the fixing device  640 . 
   In addition, the fixing unit  180 B comprises a fixing unit controller (not shown) for controlling the above elements. The fixing unit controller controls the rotational speed of each roller, the temperature of the fixing roller, and a process upon occurrence of an abnormality. 
   The image forming subsystem  150 B comprises the image forming controller  160  ( FIG. 1 ). The image forming controller  160  communicates with the image creating unit controller and fixing unit controller, receives pieces of unit information from the respective controllers, and transmits unit control information to the respective controllers. Also, the image forming controller  160  communicates image signals with the controller  200 , and communicates pieces of control information with the printer engine controller  105  and platform controller  65 . 
   In the above description, the image creating unit and fixing unit have controllers, respectively. However, the image creating unit and fixing unit can operate without any controller. In this case, the image forming controller  160  ( FIG. 1 ) controls elements in the image creating unit and fixing unit. 
     FIG. 16  is a sectional view of the monochrome image forming subsystem  150 C. Each of the image creating unit  170 C and a fixing unit  180 C is exchangeable with another unit of the same function, and is physically separable. 
   Details of the image creating unit  170 C will be explained. 
   The image creating unit  170 C comprises a scanner unit  661 , photosensitive drum  662 , developing unit  666 , and transfer roller  667 . The scanner unit  661  comprises a laser unit  663 , polygon mirror  664 , scanner motor  665 , and beam detection signal (BD signal) generation circuit  672 . 
   The photosensitive drum  662  is an OPC photosensitive body having a photoconductive layer on an aluminum drum base. A driving device (not shown) drives and rotates the photosensitive drum  662  counterclockwise in  FIG. 16  at a predetermined process speed. 
   A primary charger  670  serving as a primary charging unit uniformly charges the surface of the photosensitive drum  662  to a predetermined potential by a charging bias applied from a charging bias supply (not shown). 
   In the scanner unit  661 , the laser unit  663  emits a laser beam modulated on the basis of time series electrical digital pixel signals of supplied image information. The polygon mirror  664  is a rotary polygon mirror which deflects a laser beam emitted by the laser unit  663  to scan the surface of the photosensitive drum  662  and form an electrostatic latent image on the photosensitive drum  662 . The scanner motor  665  drives and rotates the polygon mirror  664 . The beam detection signal (BD signal) generation circuit  672  detects a laser beam in the main scanning direction deflected by the polygon mirror  664 . 
   The developing unit  666  develops an electrostatic latent image formed on the photosensitive drum  662  with a black (B) developing substance. The transfer roller  667  abuts the photosensitive drum  662  and transfers the developing substance on the photosensitive drum  662  to a recording medium such as a recording sheet. 
   A cleaning blade  669  is always in contact with the photosensitive drum  662 , and cleans it by scraping toner remaining on the surface of the photosensitive drum  662 . 
   The image creating unit  170 C further comprises an image creating unit controller (not shown) for controlling the operations of these elements. The image creating unit controller controls the process speed of the image creating unit and tint/density adjustment. 
   The fixing unit  180 C will be explained. 
   The fixing unit  180 C is arranged downstream of the transfer roller  667  of the image creating unit  170 C in the transfer material conveyance direction. A fixing device  668  performs a fixing operation of heating, pressing, and thereby fixing a toner image transferred on a recording sheet. A driving device (not shown) drives and rotates the roller of the fixing unit. The surface temperature of the fixing roller is controlled by controlling power of the halogen heater in the fixing device  668 . Further, the fixing unit  180 C comprises a fixing unit controller (not shown) for controlling these elements. The fixing unit controller controls the rotational speed of each roller, the temperature of the fixing roller, and a process upon occurrence of an abnormality. 
   The image forming subsystem  150 C comprises the image forming controller  160  ( FIG. 1 ). The image forming controller  160  communicates with the image creating unit controller and fixing unit controller, receives pieces of unit information from the respective controllers, and transmits unit control information to the respective controllers. Further, the image forming controller  160  exchanges image signals with the controller  200 , and exchanges pieces of control information with the printer engine controller  105  and platform controller  65 . 
   In the above description, the image creating unit and fixing unit have controllers, respectively. However, the image creating unit and fixing unit can operate without any controller. In this case, the image forming controller  160  ( FIG. 1 ) controls elements in the image creating unit and fixing unit. 
   The image forming subsystem in the image forming apparatus, and its image forming controller will be explained. 
     FIG. 5  is a block diagram of the full-color image forming subsystem  150 A. The full-color image forming subsystem  150 A comprises an image forming controller  160 A including an image processor, the image creating unit  170 A, and the controller  200 . The image forming controller  160 A receives an image signal of the RGB color format from the controller  200 , and executes the following process. 
   First, a LOG conversion circuit  310  converts the density of the image signal, and an output masking circuit  311  converts the image signal into YMCK data. The output masking circuit  311  converts an image signal so as to minimize the average color difference in the Lab space, and the coefficients of conversion depend on the hardware characteristic of the image creating unit  170 A. 
   Then, a tone correction circuit  312  receives the YMCK data, and corrects the tone on the basis of a lookup table (to be referred to as a LUT hereinafter). The LUT is a synthesis of a table for correcting a hardware characteristic such as the individual difference of the image creating unit  170 A or a change over time, a density adjustment table changed by user settings, and an image mode table for a text mode/photographic printing paper mode. 
   The LUT also changes depending on the following halftone process. Since a halftone processing circuit  313  parallel-executes a plurality of halftone processes, the tone correction circuit  312  has LUTs by the number of processes of the halftone processing circuit  313 , and simultaneously processes and outputs all the LUTs. 
   The halftone processing circuit  313  receives the tone-corrected signal, and generates print data. The halftone processing circuit  313  simultaneously performs error diffusion and a plurality of screen processes, and outputs print data selected by a Z signal (to be described later). 
   An inter-drum delay memory  314  delays print data in accordance with the drum layout, and outputs the print data to the image creating unit  170 A. 
   The controller  200  inputs a Z signal representing the feature of an image, simultaneously with an image signal. The Z signal synchronizes with an RGB signal, and is input to the LOG conversion circuit  310 , output masking circuit  311 , tone correction circuit  312 , and halftone processing circuit  313 . 
   The Z signal contains data representing the feature of each page and data representing the feature of each pixel. More specifically, the former data represents a copy image/PDL image, and the latter data represents a text/photo, BMP/object, or the like. 
   An ITOP image sync signal and PBD signal output from a timing generator  315  control the image output timing of the controller  200 . The ITOP signal is a sync signal in the sub-scanning direction, and the PBD signal is a sync signal in the main scanning direction. 
   The controller  200  also receives an image clock PCLK, and outputs image data synchronized with PCLK. 
   The PBD signal is generated on the basis of a BD signal output from the image creating unit  170 A. The timing generator  315  also generates an REGI signal for controlling the driving timing of a registration roller, and supplies the REGI signal to the image creating unit  170 A containing the registration roller. The REGI signal is generated on the basis of the ITOP signal. The timing of the REGI signal is determined from the relationship between the image creation position, the transfer position, and the registration roller, and has a value unique to the image forming subsystem. 
   The REGI signal is also supplied to the platform controller in order to synchronize with the registration roller. 
     FIG. 6  is a timing chart showing the image formation timing of the full-color image forming subsystem  150 A.  FIG. 6  shows a case of successively creating two images. The controller  200  outputs RGB images in accordance with the ITOP timing, and the image forming controller  160 A sequentially outputs YMCK data to the image creating unit  170 A after an image processing delay t 1 . 
   A phase difference of an inter-drum delay t 2  exists between YMCK data, and the inter-drum delay memory  314  ( FIG. 5 ) performs the delay process. 
   An REGI signal is generated a registration delay t 3  after ITOP generation. At this timing, the registration roller is driven to convey a sheet to the secondary transfer portion. Secondary transfer starts at a timing delayed by a transfer delay t 4  after the REGI signal. A process for the second page starts during transfer of the first page. To process a larger number of pages, the same process is repeated. 
     FIG. 7  is a block diagram of the full-color image forming subsystem  150 B. The full-color image forming subsystem  150 B comprises an image forming controller  160 B including an image processor, the image creating unit  170 B, and the controller  200 . The image forming controller  160 B receives an image signal of the RGB color format from the controller  200 , and executes the following process. 
   An image process by the color image forming subsystem  150 B is different from the process by the image forming controller of the color image forming subsystem  150 A in that a page memory  320  replaces the inter-drum delay memory  314 . 
   The remaining blocks are the same as those of the color image forming subsystem  150 A, and a description thereof will be omitted. 
     FIG. 8  is a timing chart showing the image formation timing of the full-color image forming subsystem  150 B.  FIG. 8  shows a case of successively creating two images. The controller  200  outputs RGB signals in accordance with the ITOP timing. YMCK print data are saved in the page memory  320  after an image processing delay t 10 , and sequentially supplied to the image creating unit  170 B. Since the image creating unit  170 B creates images color by color due to its structure, the next print data is supplied upon completion of an image of each color. 
   The timing generator  315  generates an REGI signal a registration delay t 13  after ITOP generation. At this timing, the registration roller is driven to convey a sheet to the secondary transfer portion. Secondary transfer starts at a timing delayed by a transfer delay t 14  after the REGI signal. A process for the second page starts at such a timing as to prevent overlapping between an image creation process for the first page in the fourth color and an image creation process for the second page in the first color. To process a larger number of pages, the same process is repeated. 
     FIG. 9  is a block diagram of the monochrome image forming subsystem  150 C. The monochrome image forming subsystem  150 C comprises an image forming controller  160 C including an image processor, the image creating unit  170 C, and the controller  200 . 
   An image signal supplied from the controller  200  has an RGB format, and the image forming controller  160 C generates a Bk signal. First, a Bk signal generation circuit  330  converts an RGB signal into a Bk signal. Then, a LOG conversion circuit  331  converts the density of the Bk signal, a tone correction circuit  332  corrects the tone, and a halftone processing circuit  333  generates print data. 
   The LOG conversion circuit  331 , tone correction circuit  332 , and halftone processing circuit  333  have the same functions as those of the full-color system except that the number of channels is only one for Bk. 
     FIG. 10  is a timing chart showing the image formation timing of the monochrome image forming subsystem  150 C.  FIG. 10  shows a case of successively creating two images. The controller  200  outputs RGB signals in accordance with the ITOP timing, and the image forming controller  160 C outputs Bk data (Bkd) to the image creating unit  170 C after an image processing delay t 20 . An REGI signal is generated a registration delay t 23  after ITOP generation. At this timing, the registration roller is driven to convey a sheet to the transfer portion. Transfer starts at a timing delayed by a transfer delay t 24  after the REGI signal. A process for the second page starts during transfer of the first page. To process a larger number of pages, the same process is repeated. 
   (Description of Image Forming Operation) (Image Formation when Assembling Image Forming Subsystem  150 A) 
   An image forming operation by the image forming apparatus  100  when mounting the image forming subsystem  150 A corresponding to a high-speed color throughput on the paper conveyance platform  60  will be explained. 
   When receiving an image forming job start instruction from a user via the operation unit  210  ( FIG. 1 ) of the image forming apparatus, the printer engine controller  105  transmits a paper feed request command to the platform controller  65 . 
   In accordance with the paper feed request command, the paper conveyance unit  80  and paper feed unit  70  start operating. Similarly, when the printer engine controller  105  transmits an image formation request command to the image forming controller  160 , the image creating unit  170 A and fixing unit  150 A start an image forming operation ( FIG. 14 ). In the image creating unit  170 A, the primary chargers  603 A to  603 D uniformly negatively charge the photosensitive drums  602 A to  602 D of the image forming portions  601 Y to  601 Bk driven to rotate at arbitrary process speeds by driving mechanisms (not shown). 
   In the laser exposure device  607 , the laser beam emitting element emits a laser beam based on an externally input color-separated image signal to irradiate the polygon mirror  618  driven to rotate by the scanner motor  617 . The laser exposure device  607  forms electrostatic latent images of the respective colors on the photosensitive drums  602 A,  602 B,  602 C, and  602 D via a reflecting mirror or the like. 
   The developing device  604 A, which receives a developing bias of the same polarity as the charging polarity (negative polarity) of the photosensitive drum  602 A, applies yellow toner to the electrostatic latent image formed on the photosensitive drum  602 A, visualizing the latent image as a toner image. The transfer roller  605 A, which receives a primary transfer bias (of a polarity (positive polarity) opposite to that of toner), primarily transfers the yellow toner image onto the driven intermediate transfer belt  608  at the primary transfer portion  615 A between the photosensitive drum  602 A and the transfer roller  605 A. 
   The intermediate transfer belt  608  bearing the yellow toner image moves to the image forming portion  601 M. Similarly at the primary transfer portion  615 B, the image forming portion  601 M transfers the magenta toner image formed on the photosensitive drum  602 B over the yellow toner image on the intermediate transfer belt  608 . 
   At this time, the cleaner blade of the drum cleaner device or the like scrapes and recovers toner remaining on the photosensitive drums  602 A,  602 B,  602 C, and  602 D after transfer. 
   At the primary transfer portions  615 A to  615 D, cyan and black toner images are sequentially superposed on the yellow and magenta toner images superposed and transferred on the intermediate transfer belt  608 , forming a full-color toner image on the intermediate transfer belt  608 . 
   A paper feed cassette in the paper feed unit  70 A is selected at the timing when the leading edge of the full-color toner image on the intermediate transfer belt  608  moves to the secondary transfer portion  616  between the secondary transfer counter roller  609  and the secondary transfer roller  611 . The pickup roller  502  is driven to pick up a top sheet among transfer materials (sheets) P stored in the paper feed cassette. Then, the picked-up transfer material (sheet) P is conveyed to the paper feed path  511 . 
   The paper conveyance rollers  503  convey the conveyed transfer material P to registration rollers  613  of the image creating unit  170 A. The registration rollers  613  of the image creating unit  170 A convey the transfer material P to the secondary transfer portion  616 . The secondary transfer roller  611 , which receives a secondary transfer bias (of a polarity (positive polarity) opposite to that of toner), secondarily transfers the full-color toner image at once onto the transfer material P conveyed to the secondary transfer portion  616 . 
   The transfer material P bearing the full-color toner image is conveyed to the fixing unit  180 A. The full-color toner image is heated and pressed at a fixing nip portion  614  between the fixing roller  612 A and the press roller  612 B, and thermally fixed onto the surface of the transfer material P. After that, the transfer material P is conveyed to the paper conveyance unit  80 A ( FIG. 12A ). The transfer material P passes through the paper discharge path  525  of the paper conveyance unit  80 A, and is discharged by the paper discharge rollers  522  onto a paper discharge tray on the upper surface of the main body, ending a series of image forming operations. 
   The image forming operation in single-sided image formation has been described. 
   (Double-Sided Image Forming Operation) 
   A double-sided image forming operation by the image forming apparatus according to the present invention will be explained. This operation is the same as the single-sided image forming operation up to conveyance of a transfer material P to the fixing unit  180 A. After the paper discharge rollers  522  discharge most of the transfer material P having passed through the paper discharge path  525  of the paper conveyance unit  80 A ( FIG. 12A ) onto the paper discharge tray on the upper surface of the main body, they stop rotation. At this time, the discharge rollers  522  stop so that the trailing edge position of the transfer material P reaches a reversible position, i.e., the downstream side from the branch point between the paper discharge path  525  and the paper conveyance path  526 . 
   Subsequently, the paper discharge rollers  522  rotate in a direction opposite to rotation in the single-sided image forming operation, in order to feed, to the paper conveyance path  526  having the paper conveyance rollers  523  and  524 , the transfer material P which stops by stopping rotation of the paper discharge rollers  522 . By reversely rotating the paper discharge rollers  522 , the trailing edge of the transfer material P at the reversible position changes to the leading edge and reaches the paper conveyance rollers  523 . 
   The paper conveyance rollers  523  convey the transfer material P to the paper conveyance rollers  524 . The transfer material P is conveyed to the paper feed path  511  of the paper feed unit  70 A ( FIG. 11A ). 
   The paper conveyance rollers  503  convey the conveyed transfer material P to the registration rollers  613  of the image creating unit  170 A ( FIG. 14 ). During this paper conveyance, the printer engine controller  105  transmits an image formation request command to the image forming controller  160 . The registration rollers  613  move the transfer material P to the secondary transfer portion  616  at the timing when the leading edge of the full-color toner image on the intermediate transfer belt  608  moves to the secondary transfer portion  616  between the secondary transfer counter roller  609  and the secondary transfer roller  611 . 
   At the secondary transfer portion  616 , the leading edge of the toner image and that of the transfer material P match each other. After transferring the toner image, the fixing unit  180 A fixes the image on the transfer material P, similar to the single-sided image forming operation. The transfer material P is conveyed again by the paper discharge rollers  522  of the paper conveyance unit  80 A and finally discharged onto the paper discharge tray, ending a series of image forming operations. 
   (Image Formation when Assembling Image Forming Subsystem  150 B) 
   An image forming operation by the image forming apparatus  101  when mounting the image forming subsystem  150 B corresponding to a middle-speed color throughput on the paper conveyance platform  60  will be explained. 
   When receiving an image forming job start instruction from a user via the operation unit  210  ( FIG. 1 ) of the image forming apparatus, the printer engine controller  105  transmits a paper feed request command to the platform controller  65 . 
   In accordance with the paper feed request command, the paper conveyance unit  80  and paper feed unit  70  start operating. Similarly, when the printer engine controller  105  transmits an image formation request command to the image forming controller  160 , the driving mechanism (not shown) of the image creating unit  170 B drives and rotates the photosensitive drum  632  at an arbitrary process speed. The primary charger  642  uniformly negatively charges the photosensitive drum  632 . 
   In the exposure device  631 , the laser beam emitting element emits a laser beam based on an externally input color-separated image signal to irradiate the polygon mirror  635  driven to rotate by the scanner motor  636 . The exposure device  631  forms an yellow (Y) electrostatic latent image on the photosensitive drum  632  via a reflecting mirror or the like. The latent image on the photosensitive drum  632  is visualized with an yellow (Y) developing substance at the position where the photosensitive drum  632  contacts the yellow (Y) developing substance unit  637 A in the developing rotary unit  637 . 
   The transfer roller, which receives a primary transfer bias (of a polarity (positive polarity) opposite to that of toner), primarily transfers the yellow (Y) developing substance on the photosensitive drum  632  onto the driven intermediate transfer belt  633  at the position where the photosensitive drum  632  contacts the intermediate transfer belt  633 . At this time, the cleaning blade  639  of the drum cleaner device or the like scrapes toner remaining on the photosensitive drum  632  after transfer, and recovers the toner in a recovery vessel. 
   A driving unit (not shown) rotates the developing rotary unit  637  through about 90°, and the developing rotary unit  637  prepares for the next magenta (M) development. In image creation based on magenta (M) data, similar to image creation based on yellow (Y) data, a latent image of magenta (M) data is transferred onto the photosensitive drum  632 . 
   Then, the developing mechanism rotates the photosensitive drum  632 . The primary charger  642  uniformly negatively charges the photosensitive drum  632 . In the exposure device, the laser beam emitting element emits a laser beam based on an externally input color-separated image signal to irradiate the polygon mirror  635  driven to rotate by the scanner motor  636 . The exposure device forms a magenta (M) electrostatic latent image on the photosensitive drum  632  via the reflecting mirror or the like. The latent image on the photosensitive drum  632  is visualized with a magenta (M) developing substance at the same rotation position of the intermediate transfer belt  633  as the position for the yellow (Y) developing substance. 
   The transfer roller, which receives a primary transfer bias (of a polarity (positive polarity) opposite to that of toner), primarily transfers the magenta (M) developing substance on the photosensitive drum  632  onto the intermediate transfer belt  633  at the position where the rotating photosensitive drum  632  contacts the intermediate transfer belt  633 . 
   Subsequently, cyan (C) and black (Bk) are also controlled by the same image forming process. After four, yellow (Y), magenta (M), cyan (C), and black (Bk) developing substances are superposed on the intermediate transfer belt  633 , a paper feed cassette in the paper feed unit  70 B is selected at a predetermined position. The pickup roller  502  is driven to pick up the top sheet among transfer materials (sheets) P stored in the paper feed cassette, and conveys the picked-up transfer material (sheet) P to the paper feed path  511 . The paper conveyance rollers  503  ( FIGS. 11A and 11B ) convey the conveyed transfer material P to registration rollers  641  of the image creating unit  170 B ( FIG. 15 ). 
   The registration rollers  641  of the image creating unit  170 B convey the transfer material P to the secondary transfer portion. The secondary transfer roller  638 , which receives a secondary transfer bias (of a polarity (positive polarity) opposite to that of toner), secondarily transfers the full-color toner image at once onto the transfer material P conveyed to the secondary transfer portion. 
   The transfer material P bearing the full-color toner image is conveyed to the fixing unit  180 B. The fixing device  640  heats and presses the full-color toner image, and thereby thermally fixes it onto the surface of the transfer material P. Then, the transfer material P is conveyed to the paper conveyance unit  80 B. The transfer material P passes through the paper discharge path  525  of the paper conveyance unit  80 B, and is discharged by the paper discharge rollers  522  onto the paper discharge tray on the upper surface of the main body, ending a series of image forming operations. 
   Similar to the image forming subsystem  150 A, a double-sided image forming operation by the color image forming subsystem  150 B can be executed by controlling conveyance of the transfer material P in accordance with a combination of the paper conveyance unit and paper feed unit. 
   (Image Formation when Assembling Image Forming Subsystem  150 C) 
   An image forming operation by the image forming apparatus  102  when mounting the image forming subsystem  150 C on the paper conveyance platform  60  will be explained. When receiving an image forming job start instruction from a user via the operation unit  210  of the image forming apparatus, the printer engine controller  105  transmits a paper feed request command to the platform controller  65 . 
   On the basis of the transmitted paper feed request command, the paper conveyance unit  80  and paper feed unit  70  start operating. Similarly, when the printer engine controller  105  transmits an image formation request command to the image forming controller  160 , the driving mechanism (not shown) of the image creating unit  170 C drives and rotates the photosensitive drum  662  at an arbitrary process speed. The primary charger  670  uniformly negatively charges the photosensitive drum  662 . In the exposure device  661 , the laser beam emitting element emits a laser beam based on an externally input image signal to irradiate the polygon mirror  664  driven to rotate by the scanner motor  665 . The exposure device  661  forms an electrostatic latent image on the photosensitive drum  662  via a reflecting mirror or the like. 
   The latent image on the photosensitive drum  662  is visualized with a developing substance at the position where the photosensitive drum  662  contacts the developing substance unit  666 . A paper feed cassette in the paper feed unit  70 A is selected. The pickup roller  502  is driven to pick up the top sheet among transfer materials (sheets) P stored in the paper feed cassette, and conveys the picked-up transfer material (sheet) P to the paper feed path  511  ( FIGS. 11A and 11B ). The paper conveyance rollers  503  convey the conveyed transfer material P to registration rollers  671  of the image creating unit  170 C ( FIG. 16 ). The transfer roller  667 , which receives a transfer bias (of a polarity (positive polarity) opposite to that of toner), transfers the toner image onto the transfer material P conveyed to the transfer portion. The transfer material P bearing the toner image is conveyed to the fixing unit  180 C. The fixing device  668  heats and presses the toner image, and thereby thermally fixes it onto the surface of the transfer material P. Then, the transfer material P is conveyed to the paper conveyance unit  80 A. The transfer material P passes through the paper discharge path  525  of the paper conveyance unit  80 A, and is discharged by the paper discharge rollers  522  onto the paper discharge tray on the upper surface of the main body, ending a series of image forming operations. The cleaning blade  669  of the drum cleaner device or the like scrapes and recovers toner remaining on the photosensitive drum  662  after transfer. 
   Similar to the image forming subsystem  150 A, a double-sided image forming operation by the image forming subsystem  150 C can be executed by controlling conveyance of the transfer material P in accordance with a combination of the paper conveyance unit and paper feed unit. 
   (Command Sequence in Image Forming Operation) 
   Communication data of the printer engine controller  105 , the image forming controller  160  in the image forming subsystem  150 , the platform controller  65  in the paper conveyance platform  60 , and the power unit  90 , and the timings of the communication data will be explained. 
     FIGS. 17A and 17B  are views showing parameters in configuration communication upon power-on immediately after the image forming apparatus  100 ,  101 , or  102  receives power from the power unit  90  ( FIG. 1 ).  FIGS. 18A and 18B  are ladder charts for explaining a command sequence upon power-on. 
   (A) Parameters in Configuration Communication upon Power-ON 
   In  FIG. 17A , a data structure  1701  is configuration information which is shared between units and transmitted to the printer engine controller  105  upon power-on. The printer engine controller  105 , platform controller  65 , and image forming controller  160  start processing in response to power supply from the power unit  90 . At this time, the platform controller  65  transmits data to the printer engine controller  105 , and the image forming controller  160  similarly transmits data to the printer engine controller  105 . The transmitted data contents notify the printer engine controller  105  of abilities of the platform controller  65  and image forming controller  160  serving as a subsystem and platform, respectively. 
   For example, the contents include a unit ID for determining a unit which transmits information. The contents may include information such as the process speed at which the unit can operate. 
   The process speed at which fixing is possible may change between the full-color mode and the monochrome mode even with the same transfer material depending on fixing conditions, transfer conditions, and the like when the image forming subsystem  150  can print in color. In order to accurately notify the printer engine controller  105  of the ability of the image forming subsystem, the printer engine controller  105  must be notified of a set of the value of a process speed in the full-color mode, that of a process speed in the monochrome mode, and the current color mode. 
   To the contrary, the paper conveyance platform hardly changes in transfer material conveyance ability between the full-color mode and the monochrome mode. In this case, the printer engine controller  105  is notified together with the process speed value that conditions are shared between the full-color and monochrome modes. 
   When the type of transfer material changes, fixing conditions and transfer conditions often change between, e.g., thick paper and plain paper. The printer engine controller  105  must be notified of a set of material conditions and a process speed for each type of transfer material. The fixing heater temperature for ensuring the fixing property also changes depending on the difference in color mode, material conditions, or the like. Thus, the printer engine controller  105  must be notified of data on an electric energy consumed by the unit under conditions, together with data on the color mode, material conditions, and the like. 
   Considering these requirements, configuration data has the data structure  1701  which notifies the printer engine controller  105  of information containing a set of a process speed, a prerequisite color mode, an electric energy consumption amount, and material conditions. For example, the data structure  1701  notifies the printer engine controller  105  of three process speeds. When a unit has one process speed, the data structure  1701  notifies the printer engine controller  105  of only this process speed. 
   The interval between transfer materials, i.e., the distance between sheets may also change between units depending on the sensor response time serving as a paper conveyance condition, the fixing condition, or the like. Thus, the data structure  1701  contains the distance between sheets as data to be notified. 
   In  FIG. 17A , reference numeral  1702  denotes a data structure representing suppliable power data of which the power unit  90  notifies the printer engine controller  105 . The image forming apparatus according to the present invention adopts the configuration of the paper conveyance platform  60  and the image forming subsystem  150  having an arbitrary ability. Configuration data on the total electric energy suppliable from the power unit  90  and the power system is important in determining whether to enable operationg the apparatus. The printer engine controller  105  is notified of the data structure  1702  upon power-on, similar to the data  1701 . 
   In  FIG. 17A , a data structure  1703  describes data to be notified as ability data of the image forming subsystem  150  by the image forming controller  160 , other than data notified by the configuration data structure  1701 . More specifically, the data structure  1703  describes configuration information, i.e., “4D” of the image forming subsystem  150 A. For the color image forming subsystem  150 A or  150 B, ITOP signals for four colors must be generated at proper time intervals in order to develop and transfer images of the four colors. For this purpose, the data structure  1703  describes data “ITOP interval”. 
   In registration with a transfer material, the color image forming subsystem sometimes requires the time until it develops and transfers images of the four colors and the leading edge of image data reaches the secondary transfer portion after generating an ITOP signal for controlling color image data. If necessary, the data structure  1703  must describe data on the required time as one of its data. 
   In  FIG. 17B , reference numeral  1704  denotes operation condition information determined by the printer engine controller  105  for the image forming apparatus. For example, the printer engine controller  105  can derive, from the data structures  1701  to  1703 , operation conditions under which all units can operate normally and the image forming apparatus  100 ,  101 , or  102  can obtain stable performance. 
   The printer engine controller  105  can also hold several operation conditions as specified values in advance, and select operation conditions which match data collected from respective units. The operation condition information  1704  describes three process speeds and three PPMs (Print Per Minutes) in respective color modes under respective material conditions. If necessary, it is also possible to notify the printer engine controller  105  of a combination of an incompatible color mode and material. 
   In  FIG. 17B , reference numeral  1705  denotes a data structure used when the printer engine controller  105  notifies the image forming controller  160  and platform controller  65  of operation conditions, and then the image forming controller  160  and platform controller  65  determine an electric energy consumption amount again under the notified conditions and notify the printer engine controller  105  of the electric energy consumption amount. The printer engine controller  105  can use this data when comparing a suppliable electric energy received from the power unit  90  by the data structure  1702  with the total electric energy consumed by respective units under determined conditions, and determining whether to permit or inhibit the operation or correcting conditions. 
   Parameters in configuration communication upon power-on have been described. 
   In the above description, control of units accessory to the paper conveyance platform  60  and image forming subsystem  150  assumes units having no their own control units, like CPUs. When accessory units have their own control units, they may notify the platform controller  65  and image forming controller  160  of the configuration data structure  1701 , and the platform controller  65  and image forming controller  160  may communicate with the printer engine controller on the basis of the notification. 
   (B) Command Sequence of Configuration Information upon Power-ON 
     FIGS. 18A and 18B  are ladder charts showing details of the command sequence of configuration information upon power-on. Reference numerals  1701  to  1705  shown in FIGS.  1 SA and  18 B correspond to pieces of information described with reference to  FIGS. 17A and 17B .  FIG. 18A  shows a sequence when the paper conveyance platform  60  and image forming subsystem  150  form a system for storing and controlling ability information of units accessory to them. 
   After a power SW (not shown) is turned on and the power unit supplies power to each unit, the platform controller  65  and image forming controller  160  transmit ability information of the data structure  1701  to the printer engine controller  105 . At this time, the image forming controller  160  also adds the data  1703  to the data  1701 . Almost simultaneously with data communication, the power unit  90  transmits suppliable electric energy data based on the data structure  1702  to the printer engine controller  105 . 
   The printer engine controller  105  determines operation conditions (e.g., process speeds and PPMs in respective color modes for respective materials) of the image forming apparatus on the basis of the received configuration data. 
   Then, the printer engine controller  105  transmits the determined operation conditions  1704  of the data structure shown in  FIG. 17B  to the platform controller  65  and image forming controller  160 . 
   The platform controller  65  and image forming controller  160  recognize that units operate with the operation condition data  1704 . The platform controller  65  and image forming controller  160  prepare for an image forming operation such as generation of operation parameters, and at the same time calculate again electric energy consumption amounts under the received operation conditions. The platform controller  65  and image forming controller  160  transmit the results to the printer engine controller on the basis of the data structure  1705 . 
   By the above command sequence, a series of configuration communication processes upon power-on end. 
     FIG. 18B  shows a sequence when units accessory to the paper conveyance platform  60  and image forming subsystem  150  have their own control units. 
   After the power SW (not shown) is turned on and the power unit supplies power to each unit, the paper feed unit  70  and paper conveyance unit  80  accessory to the platform controller  65  transmit ability information based on the data structure  1701  to the platform controller  65 . Similarly, the fixing unit  180  accessory to the image forming controller  160  transmits ability information based on the data structure  1701  to the image forming controller  160 . The image creating unit  170  also transmits the data  1701  to the image forming controller  160 . 
   The platform controller  65  determines its ability information on the basis of the ability information transmitted from the paper feed unit  70  and paper conveyance unit  80 . 
   The image forming controller  160  performs the same operation. Thereafter, the platform controller  65  transmits ability information based on the data structure  1701  to the printer engine controller  105 . The image forming controller  160  transmits ability information based on the data structure  1703  in addition to the data structure  1701  to the printer engine controller  105 . Almost simultaneously with data communication, the power unit  90  transmits suppliable electric energy data based on the data structure  1702  to the printer engine controller  105 . 
   The printer engine controller  105  determines operation conditions (e.g., process speeds and PPMs in respective color modes for respective materials) of the image forming apparatus on the basis of the received configuration data. 
   Then, the printer engine controller  105  transmits the determined operation conditions  1704  of the data structure shown in  FIG. 17B  to the platform controller  65  and image forming controller  160 . 
   The platform controller  65  and image forming controller  160  recognize that units operate with the operation condition data  1704 . The platform controller  65  and image forming controller  160  transmit the information to the paper feed unit  70  and paper conveyance unit  80  accessory to the platform controller  65  and the image creating unit  170  and fixing unit  180  accessory to the image forming controller  160 , respectively. 
   The paper feed unit  70 , paper conveyance unit  80 , image creating unit  170 , and fixing unit  180  recognize that they operate under the received operation conditions. They prepare for an image forming operation such as generation of operation parameters, and calculate again electric energy consumption amounts under the received operation conditions. The paper feed unit  70 , paper conveyance unit  80 , image creating unit  170 , and fixing unit  180  transmit the results to the platform controller  65  and image forming controller  160  on the basis of the data structure  1705 , respectively. 
   The platform controller  65  and image forming controller  160  calculate total electric energy consumption amounts on the basis of the power consumption data transmitted from their accessory units. 
   The platform controller  65  and image forming controller  160  transmit the results to the printer engine controller on the basis of the data structure  1705 . By the above command sequence, a series of configuration communication processes upon power-on end. 
     FIG. 19  is a view showing parameters in communication between units.  FIGS. 20A and 20B  are ladder charts for explaining a command sequence in communication between units when the image forming apparatus  100 ,  101 , or  102  forms an image. Reference numerals  1911  to  1916  shown in  FIGS. 20A and 20B  correspond to pieces of information to be described with reference to  FIG. 19 . 
   In  FIG. 19 , the data structure  1911  is common information of paper feed request commands and parameters transmitted from the printer engine controller  105  to the platform controller  65  and image forming controller  160  in order to start conveying a transfer material in an image forming operation. 
   The data  1911  is a paper feed request, and can be transmitted to only the platform controller  65  or to the image forming controller  160 , too, in order to reserve an image forming operation. In the first embodiment, the data  1911  is transmitted to the image forming controller  160 , too, in order to reserve an image forming job. 
   As an example of data necessary for a paper feed start request, the data  1911  describes data such as a command ID representing a paper feed start request command, a page ID corresponding to image data to be requested, a color mode, a paper size, material information, and a print side (e.g., single-sided, double-sided upper surface, or double-sided lower surface). 
   The command data  1912  need not be notified as image forming operation reservation information to the image forming controller  160 , but is necessary to control actual conveyance of a transfer material by the platform controller  65 , and is not described in the command data  1911 . More specifically, the command data  1912  describes paper feed stage information for starting paper feed, and a paper discharge direction necessary for paper conveyance by the paper conveyance unit. 
   The paper feed request ACK command data  1913  notifies the printer engine controller  105  of the result of determining the start of a paper feed operation by the platform controller  65  on the basis of the command data  1911  and  1912 . Examples of parameters of the paper feed request ACK command data  1913  are a page ID, paper feed stage information, paper feed status information representing whether paper feed starts normally or is to start, and NG factor information when paper status information is NG representing that no paper feed starts. Examples of the NG factor are the absence of paper, an error, and a jam. In the first embodiment, the timing when the platform controller  65  transmits the paper feed request ACK command  1913  means the timing when an image forming operation can start. 
   When the platform controller  65  notifies the printer engine controller  105  of the start of paper feed by the paper feed request ACK command data  1913 , the printer engine controller  105  transmits the image formation request command data  1914  to the image forming controller  160 . The printer engine controller  105  issues this command when it becomes ready for image formation. Examples of parameters of the image formation request command data  1914  are a page ID and color mode. 
   After receiving the image formation request  1914 , the image forming controller  160  notifies the printer engine controller by the image forming operation start notification command data  1915  that the image forming operation actually starts. The image forming controller  160  generates an ITOP signal serving as a trigger to start an image forming operation, and at the same time issues the command  1915  in accordance with the arrangement of the image forming controller  160 . Upon reception of the command  1915 , the printer engine controller  105  also transmits it to the platform controller  65  in order to control conveyance of a transfer material. An example of parameters of the command  1915  is a page ID. 
   The image formation/paper conveyance end notification command data  1916  notifies the printer engine controller  105  of the result of detecting that all the image forming operation and paper conveyance operation end. From this command, the printer engine controller  105  recognizes whether the image forming operation of an image (page) normally ends. Examples of parameters of the image formation/paper conveyance end notification command data  1916  are an end status notifying the printer engine controller  105  whether the operation normally ends, and an NG factor representing a factor in failing to end normally. Examples of the NG factor are an error and jam. 
   Details of parameters of command data communicated between the printer engine controller  105 , the platform controller  65 , and the image forming controller  160  along with an image forming operation have been described. 
   In the above description, control of units accessory to the paper conveyance platform  60  and image forming subsystem  150  assumes units having no their own control units, like CPUs. 
   When accessory units have their own control units, the platform controller  65  and image forming controller  160  can also transmit received command data to their accessory units at appropriate timings. Accessory units can also control part of an image forming operation, and if necessary, the platform controller  65  and image forming controller  160  can cause their accessory units to transmit the control results, collect them, and communicate with the printer engine controller. 
     FIGS. 20A and 20B  are ladder charts showing details of a command sequence in an image forming operation. 
   The first embodiment will explain a case where a typical 1-page image forming operation starts and ends normally. 
     FIG. 20A  is a ladder chart showing a sequence when the paper conveyance platform  60  and image forming subsystem  150  control their accessory units. 
   At the start of an image forming operation, the printer engine controller  105  transmits a paper feed request command to the platform controller  65  and image forming controller  160 . The printer engine controller  105  transmits the data  1912  to the platform controller  65  in addition to the data  1911 . The printer engine controller  105  transmits the data  1911  to the image forming controller  160 . 
   Upon reception of the paper feed request command, the platform controller  65  determines whether paper feed can start, and transmits the determination result to the printer engine controller  105  by the data structure of the paper feed request ACK command  1913 . The condition to determine that paper feed can start is, e.g., a condition that a transfer material is present or a condition that no fed transfer material jams. 
   The printer engine controller  105  receives the paper feed request ACK command  1913 , and if it recognizes that the platform controller  65  determines that paper feed can start, transmits the image formation start request  1914  of the data structure shown in  FIG. 19  to the image forming controller  160 . 
   Upon reception of the image formation start request  1914 , the image forming controller  160  determines whether the time of an image forming interval obtained from a PPM set value has elapsed after image formation. If the image forming controller  160  determines that image formation is possible, it generates an ITOP signal, starts an image forming operation, and transmits the image forming operation start notification  1915  of the data structure shown in  FIG. 19  to the printer engine controller  105 . 
   The printer engine controller  105  receives the image forming operation start notification  1915 , recognizes that image formation starts normally, and also transmits the data  1915  to the platform controller  65  in order to control conveyance of a transfer material. 
   Upon reception of the data  1915 , the platform controller  65  recognizes that the registration rollers control conveyance of a transfer material so as to control transfer at the secondary transfer portion. 
   The image forming controller  160  operates the registration rollers a predetermined time after the generated ITOP signal by controlling the timing to make the position of a developed image coincide with that of a transfer material. At the same time, the image forming controller  160  transmits a registration signal to the platform controller  65 , and notifies it that conveyance of the transfer material actually starts. Upon reception of this notification, the platform controller  65  starts driving the load on the upstream side of the registration rollers for the transfer material. 
   After the platform controller  65  and image forming controller  160  control image formation and paper conveyance, the transfer material moves from the image forming subsystem  150  to the paper conveyance platform  60 . When the platform controller  65  recognizes that the paper conveyance platform  60  discharges the transfer material outside the apparatus, it issues the image formation/paper conveyance end notification command  1916  of the data structure shown in  FIG. 19  to the printer engine controller  105 . 
   Upon reception of the image formation/paper conveyance end notification command  1916 , the printer engine controller  105  recognizes the end of a series of image forming operations for the target image on the transfer material. 
   Details of the command sequence from the start to end of a 1-page image forming operation when the paper conveyance platform  60  and image forming subsystem  150  control their accessory units have been described. 
     FIG. 20B  shows a sequence when units accessory to the paper conveyance platform  60  and image forming subsystem  150  have their own control units. 
   At the start of an image forming operation, the printer engine controller  105  transmits a paper feed request command to the platform controller  65  and image forming controller  160 . The printer engine controller  105  transmits the data  1912  to the platform controller  65  in addition to the data  1911 . The printer engine controller  105  transmits the data  1911  to the image forming controller  160 . 
   Upon reception of the paper feed request command, the platform controller  65  directly transmits the data  1912  to the paper feed unit  70  in addition to the received paper feed request command  1911 . 
   The image forming controller  160  directly transmits the received paper feed request command  1911  to the image creating unit  170  and fixing unit  180 . 
   Upon reception of the paper feed request command, the paper feed unit  70  determines whether paper feed can start, and transmits the determination result to the platform controller  65  by the data structure of the paper feed request ACK command  1913 . The condition to determine that paper feed can start is, e.g., a condition that a transfer material is present or a condition that no fed transfer material jams. 
   In accordance with the paper feed request ACK command received from the paper feed unit  70 , the platform controller  65  similarly transmits, to the printer engine controller  105 , the paper feed request ACK command  1913  of the data structure shown in  FIG. 19  representing the same the determination result. 
   The printer engine controller  105  receives the paper feed request ACK command  1913 , and if it recognizes that the platform controller  65  determines that paper feed can start, transmits the image formation start request  1914  of the data structure shown in  FIG. 19  to the image forming controller  160 . 
   The image forming controller  160  directly transmits the received image formation start request command to the image creating unit  170  and fixing unit  180 . 
   Upon reception of the image formation start request  1914 , the image creating unit  170  determines whether the time of an image forming interval obtained from a PPM set value has elapsed after image formation. If the image creating unit  170  determines that image formation is possible, it generates an ITOP signal, starts an image forming operation, and transmits the image forming operation start notification  1915  of the data structure shown in  FIG. 19  to the image forming controller  160 . 
   The image forming controller  160  transmits, to the printer engine controller  105 , the same contents as the image forming operation start notification  1915  transmitted from the image creating unit  170 . Since the image creating unit  170  starts forming an image, the image forming controller  160  similarly transmits the image forming operation start notification  1915  to the fixing unit  180  so as to notify it that a transfer material is to be conveyed. The printer engine controller  105  receives the image forming operation start notification  1915 , recognizes that image formation starts normally, and also transmits the data  1915  to the platform controller  65  in order to control conveyance of a transfer material. 
   Upon reception of the data  1915 , the platform controller  65  directly transmits the same data as the received image forming operation start notification  1915  to the paper feed unit  70 . 
   Upon reception of the data  1915 , the platform controller  65  and paper feed unit  70  recognize that the registration rollers control conveyance of a transfer material so as to control transfer at the secondary transfer portion. The image creating unit  170  operates the registration rollers a predetermined time after the ITOP signal by controlling the timing so as to make the position of a developed image coincide with that of a transfer material. At the same time, the image creating unit  170  transmits a registration signal to the platform controller  65  via the image forming controller  160 , and notifies the platform controller  65  that conveyance of the transfer material actually starts. Upon reception of this notification, the platform controller  65  also transfers the notification to the paper feed unit  70  without any delay. The paper feed unit  70  starts driving the load on the upstream side of the registration rollers for the transfer material. 
   The platform controller  65  transmits a paper feed start request command to the paper conveyance unit  80  a predetermined time after the timing when the platform controller  65  receives the image forming operation start notification  1915 . By this command, the platform controller  65  causes the paper conveyance unit to prepare for reception of the transfer material. 
   The paper conveyance unit  80  receives and conveys the transfer material, and when recognizing that the transfer material is finally discharged outside the apparatus, issues the image formation/paper conveyance end notification command  1916  of the data structure shown in  FIG. 19  to the platform controller  65 . 
   Upon reception of the image formation/paper conveyance end notification command  1916  from the paper conveyance unit  80 , the platform controller  65  transmits the notification  1916  of the same contents to the printer engine controller  105 . 
   Upon reception of the image formation/paper conveyance end notification command  1916 , the printer engine controller  105  recognizes the end of a series of image forming operations on the transfer material. 
   Details of the command sequence from the start to end of a 1-page image forming operation when units accessory to the paper conveyance platform  60  and image forming subsystem  150  have their own control units have been described. 
   (Image Registration Operation when Exchanging Image Forming Subsystem) 
   An image registration operation when exchanging the image forming subsystem  150  in the above-described arrangement will be described. 
   A flowchart shown in  FIG. 30  is executed as an image forming apparatus control method according to the first embodiment. 
   A registration error amount between the positioned image forming unit (image forming subsystem  150 ) and the paper feed conveyance unit (paper conveyance platform  60 ) is detected on the basis of reading of a reference pattern  1000  (S 3010 ). 
   The correction amount is calculated on the basis of the registration error amount detected in the process of step S 3010 . The operation timings of the image forming unit and paper feed conveyance unit are controlled in accordance with the correction amount (S 3020 ). 
   Concrete contents of the above control method will be explained. 
     FIG. 22  is a view showing the arrangement of the position detector  112  arranged near the fitting portion between the paper conveyance platform  60  and the image forming subsystem  150  (see  FIGS. 4A and 4B  for the fitting state). The position detector  112  comprises an LED  1002 , area sensor  1001 , and position detection circuit  2203 . 
   The reference surface  113  of the image forming subsystem  150  has the reference pattern  1000  of a 2×2 matrix. The LED  1002  of the position detector  112  arranged on the paper conveyance platform  60  illuminates the reference pattern  1000 . The area sensor  1001  reads light reflected by the reference pattern  1000 . From the read signal from the area sensor  1001 , the position detection circuit  2203  detects registration errors of the reference pattern from a reference position in the main scanning and sub-scanning directions. 
   A vertical center line  2610  of the reference pattern  1000  represents the image center position (image reference position) of the image forming subsystem  150 , whereas a horizontal center line  2620  represents the paper conveyance reference position of the image forming subsystem  150 . The position detection circuit  2203  can detect registration errors of the reference pattern from reference positions (image reference position and paper conveyance reference position) on the basis of the differences between the image reference position and paper conveyance reference position and the measurement results of the area sensor  1001 . 
   The area sensor  1001  is an image sensor of 1,024×1,024 pixels, and its optical magnification is so adjusted as to set one pixel to 42.3 μm. The area sensor  1001  makes its center pixel ( 512 , 512 ) coincide with the image center (center in the main scanning direction) of the image forming subsystem  150  and the paper conveyance reference (center in the paper conveyance direction) of the paper conveyance platform  60 . 
   Pixel numbers are assigned such that ( 0 , 0 ) represents an upper right pixel, ( 0 , 1023 ) represents an upper left pixel, ( 1023 , 0 ) represents a lower right pixel, and ( 1023 , 1023 ) represents a lower left pixel. When the paper conveyance platform  60  and image forming subsystem  150  are ideally coupled, the center intersection P of the reference pattern  1000  is imaged at the coordinates ( 512 , 512 ) of the area sensor  1001 . 
   The position detection circuit  2203  receives an output from the area sensor  1001 , converts it into digital data, and detects the projection position of the reference pattern  1000 . The position detection circuit  2203  extracts the edge of the data output from the area sensor  1001 , detects the two edges of the center line segment of the reference pattern  1000  in the main scanning and sub-scanning directions, and determines the center position of the reference pattern  1000 . 
   Determination of the center position is based on the average value of a plurality of points in the main scanning and sub-scanning directions in order to prevent a decrease in determination precision due to contamination of the reference pattern  1000  and area sensor  1001 . 
     FIG. 23  is a view showing the relationship between a detection position  2710  and a reference position  2720 .  FIG. 23  shows that the center coordinates in the area sensor  1001  and the projection pattern at the detection position  2710  shift from each other by 2 mm in the main scanning direction and 4 mm in the sub-scanning direction. The position detection circuit  2203  detects the registration errors (2 mm in the main scanning direction and 4 mm in the sub-scanning direction), and notifies the platform controller  65  of them. The platform controller  65  manages the registration error amounts as Py (mm) in the main scanning direction and Px (mm) in the sub-scanning direction. 
   The registration error amount is detected when turning on the image forming apparatus. The platform controller  65  notifies the printer engine controller  105  of the registration error amount as the above-mentioned configuration information upon power-on. 
   The printer engine controller  105  creates image formation position correction data and paper conveyance correction data on the basis of the registration error amount. The printer engine controller  105  notifies the image forming controller  160  of the image forming subsystem  150  of the image formation position correction data, and the platform controller  65  of the paper conveyance platform  60  of the paper conveyance correction data. 
   (Correction in Main Scanning Direction) 
   Image formation position correction data created by the printer engine controller  105  aims to correct a registration error amount in the main scanning direction by changing the start position of image formation. The registration error amount can be corrected by delaying generation from a BD signal input from the image creating unit to a PBD signal to be output to the controller  200  by the timing generator  315  of the image forming controller  160  (which typifies the image forming controllers  160 A,  160 B, and  160 C). 
     FIG. 24  is a view for explaining position correction in the main scanning direction. 
   Reference numeral  1010  denotes a center of the image forming subsystem  150  in the main scanning direction; and  1011 , a center of the paper conveyance platform  60  in the main scanning direction. In the example of  FIG. 23 , the difference between the centers  1010  and  1011  is the main scanning position error amount Py=(2 mm). 
   An exposable area  1012  represents an effective area which can be irradiated with a laser beam. The exposable area  1012  is 320 mm wide, i.e., 160 mm wide on either side of the center  1010  of the image forming subsystem in the main scanning direction. 
   An effective developing area  1013  represents an image formable area, and is 310 mm wide, i.e., 155 mm on either side of the center  1010  of the image forming subsystem in the main scanning direction. The effective developing area  1013  is generally determined by the effective area of the photosensitive drum. 
   Reference numeral  1014  denotes a position of a beam detection sensor (BD sensor) for detecting a laser beam in the main scanning direction. The BD sensor functions as a beam detection signal generation circuit, and can generate a beam detection signal upon detecting a laser beam. The BD sensor falls within the exposable area but outside the effective developing area, and is spaced apart by 158 mm from the center  1010  of the image forming subsystem  150  in the main scanning direction. 
   Reference numeral  1015  denotes an output position of a sync signal (PBD signal) output from the timing generator  315  in the main scanning direction before registration error correction. The position of the reference numeral  1015  represents a PBD signal position. 
   The timing generator  315  can control the PBD signal position denoted by the reference numeral  1015  before correction by the size of the transfer material P. When the transfer material P is an A4 material (210×297 mm), the timing generator  315  receives a BD signal, performs a delay process by 9.5 mm, and generates a PBD signal. As a result, the timing generator  315  outputs the PBD signal at a position of 148.5 mm (158 mm−9.5 mm) apart from the center  1010  of the image forming subsystem  150  in the main scanning direction. 
   In  FIG. 24 , the registration error in the main scanning direction is Py=2 mm. In order to correct this registration error amount, the timing generator  315  controls the timing to delay the output position of the PBD signal. 
   Reference numeral  1016  denotes a corrected PBD signal position. In order to correct the PBD signal position, the platform controller  65  notifies the printer engine controller  105  of the registration error amount Py in the main scanning direction=2 mm as configuration information upon power-on. The printer engine controller  105  creates image formation position correction data on the basis of the registration error amount in the main scanning direction, and notifies the image forming controller  160  of it. The timing generator  315  of the image forming controller  160  changes the PED signal delay amount from 9.5 mm to 11.5 mm on the basis of the notified image formation position correction data, and sets the corrected PBD signal position  1016 . 
   The corrected PBD signal position  1016  is set to a position of 146.5 mm apart from the center  1010  of the image forming subsystem  150  in the main scanning direction. The end of the transfer material P matches the corrected PBD signal position  1016 , as shown in  FIG. 24 . This procedure can correct a registration error generated in the main scanning direction. 
   (Correction in Sub-Scanning Direction) 
   Paper conveyance correction data created by the printer engine controller  105  aims to correct a registration error amount in the sub-scanning direction at the paper feed interval. The printer engine controller  105  notifies the platform controller  65  of data on a calculated process speed (recording material conveyance speed) and a paper conveyance path length as the paper conveyance correction data. Based on these data, the platform controller  65  adjusts the paper feed timing and paper feed interval, and thereby corrects a registration error amount in the sub-scanning direction. 
     FIG. 25  is a view showing a concrete positional relationship between paper feed and transfer. The distance L(P 1 ) from the paper feed position to the reference position of the paper conveyance platform  60  is 380 mm. The registration error amount Px in the sub-scanning direction is 4 mm. The distance L(G 1 ) from the reference position of the image forming subsystem  150  to the registration roller in the image forming subsystem  150  is 20 mm. The distance L(G 2 ) from the registration roller to the transfer position is 100 mm. 
   The total distance L(PG) from the paper feed position to the registration roller is 404 mm. The ideal value of the distance L(PG) is 404 mm—the registration error amount: 4 mm=400 mm, and is defined as L(PG 0 ). 
   Letting L(ALL) be the total distance from the paper feed position to the transfer position, the respective distances meet the following relations (1) to (3): 
   
     
       
         
           
             
               
                 
                   L 
                   ⁡ 
                   
                     ( 
                     PG 
                     ) 
                   
                 
                 = 
                 
                   
                     L 
                     ⁡ 
                     
                       ( 
                       
                         P 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       ) 
                     
                   
                   + 
                   Px 
                   + 
                   
                     L 
                     ⁡ 
                     
                       ( 
                       
                         G 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         L 
                         ⁡ 
                         
                           ( 
                           
                             PG 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                           ) 
                         
                       
                       = 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             PG 
                             ) 
                           
                         
                         - 
                         Px 
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             ) 
                           
                         
                         + 
                         
                           F 
                           ⁡ 
                           
                             ( 
                             
                               G 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         L 
                         ⁡ 
                         
                           ( 
                           ALL 
                           ) 
                         
                       
                       = 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             ) 
                           
                         
                         + 
                         Px 
                         + 
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               G 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             ) 
                           
                         
                         + 
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               G 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             PG 
                             ) 
                           
                         
                         + 
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               G 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   From the relations (1) to (3), L(P 1 ) is a value (constant value) guaranteed in the paper conveyance platform  60 , and L(G 1 ) and L(G 2 ) are values (constant values) guaranteed in the image forming subsystem  150 . Since the variable factor in coupling between the image forming subsystem and the platform is only the registration error amount Px in the sub-scanning direction, the paper conveyance distance L(PG) from the paper feed position to the registration roller is the variable factor. 
   When the paper conveyance speed Ps (mm/s) is 100 mm/s, the paper conveyance time from the paper feed position to the registration roller is given by the following equations (4) and (5). 
   The paper conveyance time (T(PG)) from paper feed to the registration roller is given by
 
 T ( PG )= L ( PG )/ Ps= 404/100=4.04 s  (4)
 
   The ideal value (T(PG 0 )) of the paper conveyance time from paper feed to the registration roller is given by
 
 T ( PG 0)= L ( PG 0)/ Ps= 40/100=4 s  (5)
 
   From equations (4) and (5), an extra paper conveyance time of 40 ms is necessary owing to the registration error in the sub-scanning direction. 
   The platform controller  65  calculates a time difference from the ideal value by the above equations on the basis of paper conveyance path length data containing a registration error amount and the recording material conveyance speed that are notified as configuration information from the printer engine controller  105 . The platform controller  65  can adjust the paper feed timing and the paper time interval in order to maintain the image formation position, productivity in a continuous operation, and the output time of the first recording material. 
     FIG. 26  is a timing chart for correcting a registration error in the sub-scanning direction (paper conveyance correction). This timing chart is obtained by adding paper feed/paper conveyance timings  3020  and  3030  to an image formation timing chart  3010  (corresponding to the timing chart in  FIG. 6 ) of the full-color image forming subsystem  150 A. 
   Note that the timing chart for explaining paper conveyance correction is described by exemplifying a combination of the paper conveyance platform  60  and full-color image forming subsystem  150 A. However, the gist of the present invention is not limited to this combination. That is, paper conveyance correction can also apply to a combination of the paper conveyance platform  60  and the image forming subsystem  150 B or  150 C. 
   As described above, the REGI signal is a timing signal for starting driving the registration roller. The timing of the paper conveyance time T(PG) before the REGI signal of the first page is a paper feed timing adjusted by the platform controller  65 . 
   In  FIG. 26 , reference numeral  3020  denotes a paper feed/paper conveyance timing in the absence of any registration error (ideal state); and  3030 , a paper feed/paper conveyance timing when correcting a registration error amount of 4 mm. Since an extra, paper conveyance time of 40 ms is necessary owing to the registration error in the sub-scanning direction, the platform controller  65  starts feeding a recording material 0.04 (s) early from the ideal state. 
   The REGI signal interval—the paper conveyance time L(PG)=the paper time interval, which is 1 (s) ideally. In order to correct the registration error amount even at the paper feed time interval, the platform controller  65  shortens the paper time interval by 0.04 (s) to 0.96 (s), and controls the paper feed start timing of the next recording material. In order to correct the paper conveyance time which becomes longer due to the registration error in the sub-scanning direction, the platform controller  65  starts feeding a recording material earlier then the paper feed timing in the ideal state. 
   The paper feed interval must be maintained in order to maintain productivity without limiting processes by the image forming apparatus. The platform controller  65  hastens the paper feed start timing by the prolongation (40 ms) of the paper conveyance time owing to a registration error, shortens the paper feed time interval, and thereby corrects a registration error in the sub-scanning direction. 
   This process is done for the REGI signal generated by the timing generator  315  of the image forming controller  160 . The same control method also applies to the color image forming subsystem  150 B and monochrome image forming subsystem  150 C. 
   The first embodiment can provide an image forming technique which implements operation specifications desired by a user by a combination of subsystems. 
   Even when a registration error occurs between subsystems upon exchanging or detaching a subsystem, the registration error can be corrected to form an image and maintain the image formation quality. 
   Even when exchanging or detaching a subsystem, a delay of the output time of the first recording material or a delay of the output time of recording materials in a continuous operation can be corrected, preventing a decrease in the throughput of the image forming apparatus. 
   Second Embodiment 
   The second embodiment of the present invention will describe registration error correction when a paper conveyance platform  60  comprises registration rollers. 
     FIG. 27  is a view showing a concrete positional relationship between paper feed and transfer. This positional relationship is different from that shown in  FIG. 25  in the first embodiment in that the registration error amount Px exists between the registration roller and the transfer position. 
   As a concrete distance, the distance L(P 1 ) from the paper feed position to the reference position of the paper conveyance platform  60  is 400 mm. The distance L(P 2 ) from the registration roller to the reference position of the paper conveyance platform  60  is 20 mm. The registration error amount Px is 4 mm. The distance L(G 2 ) from the reference position of an image forming subsystem  150  to the transfer position is 100 mm. 
   The distance L(P 1 ) from the paper feed position to the reference position of the paper conveyance platform is a value (constant value) guaranteed in the paper conveyance platform  60 . 
   The total distance L(PG 2 ) from the registration roller to the transfer position is 124 mm (=L(P 2 )+Px+L(G 2 )). The ideal value of the total distance from the registration roller to the transfer position is the total distance L(PG 2 )—the registration error amount Px in the sub-scanning direction=120 mm. 
   Hence, the total distance L(PG 2 ) containing the registration error amount Px is a variable factor in coupling between the image forming subsystem and the platform. 
   When the paper conveyance speed is  100  mm/s, the paper conveyance time (T(PG 2 )) from the registration roller to the transfer position is given by
 
 T ( PG 2)= L ( PG 2)/100=1.24 ms  (6)
 
   When the paper conveyance speed is 100 mm/s, the paper conveyance time (T(PG 0 )) in the ideal state free from any registration error amount Px in the sub-scanning direction is given by
 
 T ( PG 0)=120/100=1.2 ms  (7)
 
   From equations (6) and (7), the paper conveyance time (T(PG 2 )) in the presence of the registration error amount is longer by an extra paper conveyance time of 40 ms than the paper conveyance time (T(PG 0 )) in the ideal state. 
   An image forming controller  160  calculates the time difference between the paper conveyance times (T(PG 0 ) and T(PG 2 )) by the above equations on the basis of the paper conveyance path length containing a registration error amount and the paper conveyance speed that are notified as configuration information from a printer engine controller  105 . A timing generator  315  adjusts an REGI signal generation timing on the basis of the time difference. 
   A platform controller  65  adjusts the paper feed timing in order to maintain the output time of the first recording material. 
     FIG. 28  is a timing chart for correcting a registration error (paper conveyance correction). This timing chart is obtained by adding REGI signal/secondary transfer timings  3210  to  3240  to an image formation timing chart  3250  (corresponding to ITOP to Ed in the timing chart in  FIG. 6 ) of a full-color image forming subsystem  150 A. 
   Note that the timing chart for explaining paper conveyance correction will be described by exemplifying a combination of the paper conveyance platform  60  and full-color image forming subsystem  150 A. However, the gist of the present invention is not limited to this combination. That is, paper conveyance correction can also apply to a combination of the paper conveyance platform  60  and the image forming subsystem  150 B or  150 C. 
   The transfer timing is fixed on the basis of the image formation timing. In the ideal state free from any registration error, paper feed starts 4 (s) before the REGS signal at the time t 3  after the ITOP signal, a transfer delay time of 1.2 (s) from the REGI signal is set, and transfer of the first page is executed. 
   For a registration error amount of 4 mm, an extra paper conveyance time of  40  ms is necessary in addition to the paper conveyance time in the ideal state. 
   The extra time of 40 ms from the registration roller to the transfer position due to the registration error is absorbed by adjusting the timing of the REGI signal. The REGI signal is a timing signal for starting driving the registration roller. To absorb the prolongation of the paper conveyance time caused by the registration error, the image forming controller  160  controls the timing to generate the REGI signal (t 3 −40 ms) after the ITOP signal. 
   To maintain the time (output time) until an image is formed on a transfer material and output, the platform controller  65  controls the paper feed timing so as to feed a recording material 40 ms earlier in synchronism with timing adjustment of the REGI signal. This process is performed for the REGI signal generated by the timing generator  315  of the image forming controller  160 . The same control method also applies to a color image forming subsystem  150 B and a monochrome image forming subsystem  150 C. 
   According to the second embodiment, even when a registration error occurs between subsystems upon exchanging or detaching a subsystem, the registration error can be corrected to form an image and maintain the image formation quality. 
   Even when exchanging or detaching a subsystem, a delay of the output time of the first recording material or a delay of the output time of recording materials in a continuous operation can be corrected, preventing a decrease in the throughput of the image forming apparatus. 
   Third Embodiment 
   The third embodiment of the present invention will describe a registration error correction method using a selection unit. The basic arrangement of an image forming apparatus according to the third embodiment is the same as that described in the first embodiment except that an arrangement for calculating a registration error amount does not use the position detector  112 . 
   A flowchart shown in  FIG. 31  is executed as an image forming apparatus control method according to the third embodiment. 
   A registration error amount between a positioned image forming unit (image forming subsystem  150 ) and a paper feed conveyance unit (paper conveyance platform  60 ) is calculated on the basis of the result of an image formed on a recording medium (print result on a registration error correction sheet  1025 ) (S 3110 ). 
   The correction amount is calculated on the basis of the registration error amount calculated in the process of step S 3110 . The operation timings of the image forming unit and paper feed conveyance unit are controlled in accordance with the correction amount (S 3120 ). 
   Concrete contents of the above control method will be explained. 
     FIG. 29  is a view showing a registration error correction sheet  1020  for determining whether a registration error occurs upon exchanging a subsystem, and calculating a registration error amount. The registration error correction sheet  1020  has correction patterns  1021  and  1022  printed on a transfer material P. 
   The registration error correction sheet  1020  can be printed in accordance with a user&#39;s request, but may be printed out at a predetermined timing in synchronism with exchange of a subsystem. 
   The correction pattern  1021  is a main scanning correction pattern for correcting a registration error in the main scanning direction, while the correction pattern  1022  is a sub-scanning correction pattern for correcting a registration error in the sub-scanning direction. 
   The main scanning correction pattern  1021  has a plurality of lines laid out in a direction almost perpendicular to the paper conveyance direction. Five lines are drawn in steps of 0.5 mm from a position of 9 mm measured from the image formation reference position. The respective lines are numbered. 
   The sub-scanning correction pattern  1022  has a plurality of lines laid out almost parallel to the paper conveyance direction. Five lines are drawn in steps of 0.5 mm from a position of 9 mm measured from the transfer reference position. The respective lines are numbered, similar to the main scanning correction pattern. 
   The user reads the numbers of correction pattern lines corresponding to predetermined positions from a lower end  1025  and left end  1026  in order to determine whether registration errors occur in the main scanning and sub-scanning directions. For example, when the predetermined position is 10 mm, the user reads lines (the fifth line in the main scanning direction and the first line in the sub-scanning direction) at positions of 10 mm from the lower and left ends of the registration error correction sheet  1020 , and inputs the read results from an operation unit  210 . 
   A printer engine controller  105  receives the line numbers in the respective directions input by the user from the operation unit  210 . The printer engine controller  105  calculates registration error amounts, and notifies a platform controller  65  and image forming controller  160  of them. 
   Although the original position of the line at 10 mm in the main scanning direction is, e.g., line number  3 , the printer engine controller  105  receives line number  5 , and thus calculates a registration error: 0.5×(5−3)=1 mm as a registration error amount in the main scanning direction. 
   The printer engine controller  105  notifies the image forming controller  160  of the registration error “1 mm” in the main scanning direction. The image forming controller  160  performs correction control for the timing generator to add 1 mm to the generation delay amount of the PBD signal serving as a sync signal in the main scanning direction. 
   This correction control shifts the main scanning image formation timing by 1 mm upward in  FIG. 29 , completing registration error correction in the main scanning direction. 
   Although the original position of the line at 10 mm in the sub-scanning direction is, e.g., line number  3 , the printer engine controller  105  receives line number  1 , and thus calculates a registration error: 0.5×(1−3)=−1 mm as a registration error amount in the sub-scanning direction. This means that the distance from paper feed to the transfer position is shorter by 1 mm than the ideal value, in other words, that the transfer material P reaches the transfer position earlier by 1 mm. 
   The registration error in the sub-scanning direction can be corrected by either the method of correcting the paper feed timing or paper feed interval or the method of correcting the REGI signal generation timing, which has been described in the first and second embodiments. 
   In accordance with the arrangement of an assembled subsystem, the printer engine controller  105  can determine whether to correct the paper feed timing or paper feed interval or whether to correct the REGI signal generation timing. By the correction method selected by determination of the printer engine controller  105 , the registration error “−1 mm” is corrected, completing registration error correction in the sub-scanning direction. 
   The third embodiment makes it possible to quantify registration error amounts in the main scanning and sub-scanning directions by read and operation input by a user using the registration error correction sheet. 
   Since registration error amounts in the main scanning and sub-scanning directions can be quantified, the third embodiment can maintain the quality in a combination of a subsystem and platform at a lower cost without using any detection unit for measuring a registration error amount. 
   Other Embodiment 
   The objects of the present invention are also achieved by supplying a storage medium which records program codes of software that implements the functions of the above-described embodiments to the system or apparatus. The objects of the present invention are also achieved by reading out and executing the program codes stored in the storage medium by the computer (CPU or MPU) of the system or apparatus. 
   In this case, the program code reads out from the storage medium implement the functions of the above-described embodiments, and the storage medium which stores the program codes constitutes the present invention. 
   The storage medium for supplying the program codes includes a flexible disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, nonvolatile memory card, and ROM. 
   The functions of the above-described embodiments are implemented by executing the readout program codes by the computer. Also, the present invention includes a case where an OS (Operating System) or the like running on the computer performs some or all of actual processes on the basis of the instructions of the program codes and thereby implements the functions of the above-described embodiments. 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
   This application claims the benefit of Japanese Patent Application No. 2005-284413, filed on Sep. 29, 2005, which is hereby incorporated by reference herein in its entirety.