Abstract:
An image forming apparatus comprises a main body having a mounting portion and a sensing portion disposed on one sidewall of the mounting portion. A developing unit assembly has a plurality of developing units and a contacting portion is disposed on one of the developing units. The contacting portion selectively contacts the sensing portion. The bottom surface of the mounting portion is declined to block the contacting portion from contacting the sensing portion prior to the installation of all of the developing units on the mounting portion. The angle of the bottom surface becomes larger when going towards the interior of the mounting portion. The contacting portion is installed on only one developing unit, but whether the other developing units are mounted can be sensed by using the contacting portion, thereby avoiding the use of multiple contacting portions. An image forming apparatus fabricated with this structure is smaller, simpler, and has reduced manufacturing costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application Serial No. 2004-55139, filed on Jul. 15, 2004, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image forming apparatus. More particularly, the present invention relates to an apparatus for detecting the presence of a developing unit assembly formed by a plurality of developing units and an image forming apparatus employing the same.  
         [0004]     2. Description of the Related Art  
         [0005]     An electrophotographic image forming apparatus, such as a laser printer, a light emitting diode (LED) printer, a facsimile, a digital photocopier, or a multifunction device, generates visible images on a printing medium in response to digital signals input from a computer or a scanner by performing a series of image forming processes.  
         [0006]     In a typical image forming apparatus, a developing unit assembly is an integral structure so that the entire developing unit assembly can be exchanged as necessary. There are various reasons for exchanging a developing unit, including removing and replacing the developing unit when inner parts of a main body of the image forming apparatus must be cleaned or repaired; replacing the unit when the photosensitive medium reaches the limit of its useful lifespan, and replacing the unit when the developer in the unit is exhausted. Integral developing unit assemblies are generally one of two types. In the first type, a photosensitive medium, a developing roller and a developer container are all integrated into one, single developing unit. In the second type, the developer container and the developing roller are formed into one unit and the photosensitive medium is formed as another unit. These two units together make up the developing unit assembly.  
         [0007]      FIG. 1  is a cross-sectional view of a conventional image forming apparatus with the second type of developing unit assembly, that is, a developing unit assembly formed by two units. The developing unit assembly  150  of the image forming apparatus is mounted inside a mounting portion  120  of a main body  110 . The developing unit assembly  150  includes a first developing unit  160  and a second developing unit  170 . The first developing unit  160  includes a developer container  164  and a developing roller  162 , and the second developing unit  170  includes a photosensitive medium  172 . In a large capacity image forming apparatus, the second developing unit  170  typically has a lifespan three to four times longer than that of the first developing unit  160 . Hence, the first developing unit  160  is exchanged more frequently than the second developing unit  170 .  
         [0008]     The first and second developing units  160  and  170  are mounted on the mounting portion  120  so that they function integrally during operation of the image forming apparatus. In other words, although not illustrated, a charging roller charges a surface of the photosensitive medium  172  uniformly, and an electrostatic image is formed by a scanned light generated by a laser scanning unit (not shown). A developer supplied from the developer container  164  is applied to the electrostatic image to form a visible image. The visible image is then transferred onto a printing medium (which is provided from a printing paper feed unit  180 ) by a transfer roller (not shown). The transferred image is fixed onto the printing medium by a fixing unit and is delivered to a stacker.  
         [0009]     A first contacting portion  166  and a second contacting portion  174  are formed on the rear sides of the first developing unit  160  and the second developing unit  170 , respectively. The first and second contacting portions  166  and  174  indicate whether or not the first and second developing units  160  and  170  are installed in the mounting portion  120 . A first sensing portion  116  and a second sensing portion  114  are mounted inside the mounting portion  120  of the main body  110 . The first and second sensing portions  116  and  114  are located at regions corresponding to the first and second contacting portions  166  and  174  so that they contact the first and second contacting portions  166  and  174  when the developing units are installed. Preferably, the first and second contacting portions  166  and  174  include a CRUM (Customer Replaceable Unit Monitor) in the form of semiconductor chip. The CRUM stores information regarding the characteristics and lifespan of elements such as the photosensitive medium, the developing roller, and the developer.  
         [0010]     When the first developing unit  160  and the second developing unit  170  are mounted in the mounting portion  120  of the main body  110 , the first and second contacting portions  166  and  174  contact the first and second sensing portions  116  and  114 , respectively. Therefore, the image forming apparatus detects the presence of the developing unit assembly  150 , as well as any information stored by the CRUM which is required for printing operations.  
         [0011]     In the conventional image forming apparatus described thus far, the first and second contacting portions  166  and  174  are formed in the first and second developing units  160  and  170 , respectively. Thus, the first and second sensing portions  116  and  114  must be installed at corresponding regions of the main body  110 . However, a driving unit  117  for driving the developing roller  162  and the photosensitive medium  172 , and a power transfer unit  118  for transferring power from the driving unit  117  to the developing roller  162  and the photosensitive medium  172 , must also be installed in the main body. Since all of these parts must be installed in the same general area, it is difficult and complicated to find suitable mounting locations for the multiple contacting portions  166  and  174  and sensing portions  116  and  114 . Thus, the use of a plurality of contacting and sensing parts increases the required size of the image forming apparatus as well as its manufacturing cost, and hinders the development of a smaller, more cost-effective image forming apparatus.  
         [0012]     Accordingly, there is a need for an image-forming apparatus with an improved apparatus for detecting the presence of a developer unit formed of a plurality of parts.  
       SUMMARY OF THE INVENTION  
       [0013]     An aspect of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an object of the present invention to provide a smaller, more cost-effectively manufactured, image forming apparatus.  
         [0014]     In accordance with an aspect of the present invention, an image forming apparatus includes a main body provided with a mounting portion and a sensing portion disposed on one sidewall of the mounting portion. A developing unit assembly having a plurality of developing units is attachable to and detachable from the mounting portion. A contacting portion is disposed on one of the plurality of developing units to make contact with the sensing portion. The bottom surface of the mounting portion is sloped to prevent the contacting portion from making contact with the sensing portion before the plurality of developing units are mounted on the mounting portion. Preferably, the angle of the sloped bottom surface increases when moving towards the interior of the mounting portion from the exterior of the mounting portion.  
         [0015]     With this structure, when only one developing unit is mounted on the mounting portion of the main body, the contacting portion is prevented from making contact with the sensing portion. Thus, the apparatus senses when other developing units are not mounted on the mounting portion, thereby preventing erroneous operation of the image forming apparatus.  
         [0016]     The image forming apparatus preferably further comprises at least one position adjustment guide member for causing the developing unit on which the contacting portion is disposed to contact the sensing portion.  
         [0017]     The developing unit assembly can comprise a first developing unit with a developing roller and a second developing unit with a photosensitive medium. The contacting portion is preferably installed on the first developing unit.  
         [0018]     The at least one position adjustment guide members can include a first position adjustment guide member installed on the first developing unit and a second position adjustment guide member installed on the second developing unit. It is preferred that the second position adjustment guide member interferes with the first position adjustment guide member to cause the first developing unit to contact the sensing portion. Hence, because of this interference, the mounting of the first developing unit and the second developing unit can be sensed only when both of the first developing unit and the second developing unit are mounted on the mounting portion.  
         [0019]     Additionally, it is preferred that the mounting portion includes a guide rail and the second developing unit further includes a sliding guide member guided by the guide rail.  
         [0020]     In accordance with another aspect of the present invention, an image forming apparatus includes a main body with a mounting portion, and a sensing portion disposed on one sidewall of the mounting portion. A developing unit assembly including a plurality of developing units is attachable to and detachable from the mounting portion. A contacting portion is disposed on one of the plurality of developing units and contacts the sensing portion. An anti-contact member blocks the contacting portion from contacting the sensing portion before the plurality of developing units is mounted on the mounting portion.  
         [0021]     Preferably, the anti-contact member is an elastic member installed on either the mounting portion or one developing unit selected from the plurality of developing units. When only one of the developing units on which the contacting portion is mounted is installed on the mounting portion, the elastic member forces the developing unit away from the sensing portion. Accordingly, the contacting portion and the sensing portion do not contact each other. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above and other objects, features, and advantages of certain embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
         [0023]      FIG. 1  is a schematic cross-sectional view of a conventional image forming apparatus;  
         [0024]      FIG. 2  is a schematic perspective view of an image forming apparatus in accordance with a first embodiment of the present invention;  
         [0025]      FIG. 3  is a perspective view of the developing unit assembly of the image forming apparatus shown in  FIG. 2 ;  
         [0026]      FIGS. 4 and 5  are perspective views showing the first developing unit of the developing unit assembly shown in  FIG. 3 ;  
         [0027]      FIG. 6  is a perspective view showing the second developing unit of the developing unit assembly shown in  FIG. 3 ;  
         [0028]      FIGS. 7A  to  7 C are diagrams illustrating the operation of a first position adjustment guide member and a second position adjustment guide member in accordance with the first embodiment of the present invention;  
         [0029]      FIG. 8  is a top view of the image forming apparatus of  FIG. 2  with only the first developing unit mounted on the mounting portion;  
         [0030]      FIGS. 9A and 9B  are diagrams showing the bottom side of the mounting portion viewed in directions I and II shown in  FIG. 8 ;  
         [0031]      FIG. 10  is a top view of the image forming apparatus of  FIG. 2  with both the first developing unit and the second developing unit mounted on the mounting portion; and  
         [0032]      FIG. 11  is a top view of a second embodiment of the present invention with only a first developing unit mounted on a mounting portion. 
     
    
       [0033]     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
         [0035]     In accordance with a first embodiment of the present invention, an image forming apparatus is constructed with a developing unit assembly including one unit with a developing roller and a developer container and another unit with a photosensitive medium. For convenience, the unit with the developing unit and the developer container will be referred to as the first developing unit and the unit with the photosensitive medium will be referred to as the second developing unit.  
         [0036]     Referring to  FIG. 2 , the image forming apparatus includes a main body  100 . A front door  90  is disposed in the front of the main body  100 . The front door  90  provides access to a mounting portion  80 . A side door  62  is disposed on the side of the main body  100 . A developing unit assembly  25  is disposed in, and is attachable to and detachable from, the mounting portion  80  of the main body  100 . The developing unit assembly  25  includes a first developing unit  20  and a second developing unit  40 . The image forming apparatus further includes a printing paper feed unit  10  for storing printing paper and supplying sheets of printing paper. Preferably, the printing paper feed unit  10  is disposed beneath the mounting portion  80 .  
         [0037]     A bottom portion of the side door  62  is attached to the main body  100  in a hinged manner to allow the lateral side of the main body  100  to be opened and closed. A transfer roller  60  (shown in  FIG. 3 ) and a printing paper transfer roller (not shown) are disposed inside the side door  62 . When the developing unit assembly  25  is mounted on the mounting portion  80 , the transfer roller  60  is disposed so that it corresponds to a photosensitive medium  50  (shown in  FIG. 3 ) included in the second developing unit  40 . During the printing process, a paper being printed on passes between the photosensitive medium  50  and the transfer roller  60 .  
         [0038]     Referring to  FIG. 3 , the developing unit assembly  25  is mounted on the mounting portion  80  so that the first developing unit  20  and the second developing unit  40  are combined together. Although not illustrated, a laser scanning unit creates an electrostatic image on the photosensitive medium  50  of the second developing unit  40 . When the side door  62  is closed and the second developing unit  40  is mounted on the mounting portion  80 , the photosensitive medium  50  contacts the transfer roller  60 . A first position adjustment guide member  29  and a second position adjustment guide member  49  are included in the first developing unit  20  and the second developing unit  40 , respectively. A detailed description of the first and the second position adjustment guide members  29  and  49  is provided later.  
         [0039]      FIGS. 4 and 5  are perspective views of the first developing unit of the first embodiment of the present invention. The first developing unit  20  includes a developing roller  30 , a developer container  24 , a waste developer storage unit  22 , a contacting portion  26 , and the above mentioned first position adjustment guide member  29 .  
         [0040]     The developing roller  30  is disposed in a lengthwise direction of the first developing unit  20 ; that is, the developing roller  30  is disposed generally parallel to the mounting portion  80 . When the first developing unit  20  and the second developing unit  40  are mounted on the mounting portion  80  and combined together, the photosensitive medium  50  of the second developing unit  40  shown in  FIG. 3  is supplied with developer.  
         [0041]     The developer container  24  is disposed lengthwise along the developing roller  30  and faces the developing roller  30  so that the developer container  24  contacts the developing roller  30 . The developer container  24  contains developer and provides the developer to the developing roller  30 .  
         [0042]     The waste developer storage unit  22  is disposed at one side of the developing roller  30  and stores waste developer transferred by a waste developer transfer unit  48  of the second developing unit  40  (refer to  FIG. 6 ). The waste developer storage unit  22  preferably covers the front sides of the developing roller  30  and the photosensitive medium  50  when the first developing unit  20  and the second developing unit  40  are mounted on the mounting portion  80  and combined together. The reason for this configuration is because the lifetime of the photosensitive medium  50  is shortened when it is exposed to direct light, such as when the front door  90  shown in  FIG. 2  is opened.  
         [0043]     The contacting portion  26  is placed on one lateral surface of the developing unit  20  and contacts a sensing portion  86  mounted on the mounting portion  80 , thereby providing the ability to detect whether or not the first developing unit  20  is mounted, and to detect any characteristic history stored in a CRUM. The contact between the contacting portion  26  and the sensing portion  86  also makes it possible to sense whether or not the second developing unit  40  is mounted on the mounting portion  80 .  
         [0044]     The sensing portion  86  is disposed on one sidewall  82  of the mounting portion  80 . More particularly, the sensing portion  86  is disposed at a region corresponding to the contacting portion  26  when the first developing unit  20  is mounted on the mounting portion. As illustrated in  FIGS. 4 and 5 , a contact part  87  is made of a springy metal and is encased by a case  89 . The contact part  87  protrudes outwardly so that the sensing portion  86  contacts the contacting portion  26 . A printed circuit board  88  is disposed at the rear side of the sensing portion  86 . Preferably, the printed circuit board  88  is connected with a main circuit board (not shown) of the main body  100  by a flex-print connector (FPC) to allow the first developing unit  20  to communicate signals with the image forming apparatus.  
         [0045]     The first position adjustment guide member  29  is disposed in an upper part of the first developing unit  20  and interferes with the second position adjustment guide member  49  of the second developing unit  40 , thereby moving the first developing unit  20  toward the sidewall  82  on which the sensing portion  86  of the mounting portion  80  is disposed. The operation of the position adjustment guide members will be described in further detail below.  
         [0046]      FIG. 6  is a perspective view showing the second developing unit shown in  FIG. 2 . The second developing unit  40  includes a front frame  41 , a rear frame  42 , the photosensitive medium  50 , the waste developer transfer unit  48 , a sliding guide member  44 , and the second position adjustment guide member  49 .  
         [0047]     The sliding guide member  44  is disposed on the upper portion of the second developing unit  40 , and the waste developer transfer unit  48  and the photosensitive medium  50  are disposed on the bottom portion of the sliding guide member  44 . They are disposed between the front frame  41  and the rear frame  42 .  
         [0048]     The photosensitive medium  50  is disposed in a lengthwise direction of the second developing unit  40 , that is, the photosensitive medium  50  is disposed generally parallel to the mounting portion  80 . The ends of the photosensitive medium  50  are rotatably mounted in supports in the front frame  41  and the rear frame  42 . Furthermore, the photosensitive medium  50  is disposed in such a manner to receive power from a driving unit  83  (refer to  FIG. 8 ) placed on the mounting portion  80 . Additionally, when the photosensitive medium  50  is mounted on the mounting portion  80 , the photosensitive medium  50  makes contact with the transfer roller  30  disposed in the main body  100 .  
         [0049]     The waste developer transfer unit  48  is formed at an upper part of the photosensitive medium  50 , and the ends of the waste developer transfer unit  48  are attached to the front frame  41  and the rear frame  42 . The waste developer transfer unit  48  separates any developer remaining on the photosensitive medium  50  after the completion of printing, and collects and transfers the collected waste developer into the waste developer storage unit  22  of the first developing unit  20 .  
         [0050]     One end of the sliding guide member  44  is connected with an upper part of the front frame  41  and helps stabilize the second developing unit  40  in the mounting portion  80  of the main body  100 . The sliding guide member  44  is formed along the length of the second developing unit  40  so that the sliding guide member  44  is able to slide along a guide rail  74  (refer to  FIG. 10 ) located along the length of the mounting portion  80 . As a result of this sliding action, the second developing unit  40  is accommodated into the mounting portion  80 .  
         [0051]     The second position adjustment guide member  49  contacts the first position adjustment guide member  29  of the first developing unit  20  and, subsequently interferes with the first position adjustment guide member  29 , thereby forcing the first developing unit  20  to the sidewall  82  of the mounting portion  80  on which the sensing portion  86  is disposed. As a result, the contacting portion  26  of the first developing unit  20  contacts the sensing portion  86 .  FIGS. 7A  to  7 C are diagrams showing the operation of the first position adjustment guide member and the second position adjustment guide member. As illustrated in  FIG. 7A , when the first developing unit  20  is mounted on the mounting portion  80 , and the second developing unit  40  is installed on the mounting portion  80  by moving it in the direction indicated by the arrow, the second position adjustment guide member  49  contacts the first position adjustment guide member  29 . This contact is illustrated in  FIG. 7B . As shown in  FIG. 7C , as the second developing unit  40  is pushed in farther, the second position adjustment guide member  49  interferes with the first position adjustment guide member  29  and presses the first position adjustment guide member  29 . The first position adjustment guide member  29 , in turn, presses the first developing unit  20  towards the sidewall  82  of the mounting portion  80 . Therefore, the sensing portion  86  and the contacting portion  26  of the first developing unit  20  are forced into contact with each other.  FIG. 10  illustrates the contact between the sensing portion  86  and the contacting portion  26 .  
         [0052]     In a similar fashion, when the first developing unit  20  is mounted on the mounting portion  80  after the second developing unit  40  is mounted on the mounting portion  80 , the first position adjustment guide member  29  and the second position adjustment guide member  49  interfere with each other, thereby resulting in contact between the sensing portion  86  and the contacting portion  26 .  
         [0053]     In the first embodiment of the present invention, the first position adjustment guide member  29  and the second position adjustment guide member  49  are disposed respectively on the first developing unit  20  and the second developing unit  40  to force the contacting portion  26  and the sensing portion  86  into contact with one another. It is also possible to install a further position adjustment guide member that forces the contacting portion  26  of the first developing unit  20  into contact with the sensing portion  86  of the mounting portion  80 .  FIG. 8  is a top view showing the first embodiment of the present invention with just the first developing unit mounted.  FIGS. 9A and 9B  are diagrams showing the mounting portion viewed in the direction of arrows I and II shown in  FIG. 8 .  FIG. 10  is a top view showing the first embodiment of the present invention with both the first developing unit and the second developing unit mounted.  
         [0054]     Referring to  FIG. 9A , the bottom surface  81  of the mounting portion  80  declines as it progresses away from the sensing portion  86 . As shown in  FIG. 9B , the bottom surface  81  of the mounting portion  80  declines when moving from the outside to the inside of the mounting portion  80 . Hence, when only the first developing unit  20  is mounted on the mounting portion  80  of the main body  100 , the first developing unit  20  is spaced apart from the sensing portion  86  installed on the mounting portion  80  because of the weight of the first developing unit  20  and the slope of the bottom surface  81  of the mounting portion  80 . That is, the first developing unit  20  moves away from a position A that enables printing because the contacting portion  26  contacts the contact part  87  of the sensing portion  86  to a position B where printing is disabled because the contacting portion  26  installed on the first developing unit  20  is separated from the contact part  87  of the sensing portion  86 .  
         [0055]     To explain in more detail, as a result of the separation between the contacting portion  26  and the sensing portion  86 , the image forming apparatus cannot detect whether or not the first developing unit  20  is mounted on the mounting portion  80 . Thus, the printing operation is deactivated, and, the image forming apparatus is not susceptible to erroneous operation. In contrast, in a conventional image forming apparatus such as that illustrated in  FIG. 1 , the first developing unit  160  and the second developing unit  170  are provided with a first contacting portion  166  and a second contacting portion  174 , respectively. The first sensing portion  116  and the second sensing portion  114  are installed at regions corresponding to the first and the second contact point units  166  and  174 , respectively. The use of multiple sensing portions and contacting portions produces a high risk of erroneous operation when only one of the developing units is installed on the mounting portion  80 . In the first embodiment of the present invention, however, the contacting portion  26  is installed only on the developing unit  20 , and the sensing portion  86  is installed at a corresponding region of the main body  100 . Therefore, it is possible to sense whether or not the first developing unit  20  and the second developing unit  40  are mounted using a single detector, and there is little likelihood of erroneously detecting that a developing unit is installed. Furthermore, the structure of the present invention therefore allows a smaller apparatus that can be more cost-effectively manufactured.  
         [0056]     Similarly, when the second developing unit  40  is removed to exchange it for a new one, the first developing unit  20  is pushed back to the original mounting position B and thus, the contacting portion  26  and the contact part  87  of the sensing portion  86  are disconnected. As a result of this disconnection, it is possible to sense that the second developing unit  40  is not mounted.  
         [0057]     Referring to  FIG. 10 , when the first developing unit is installed and the second developing unit  40  is installed by sliding it toward the interior of the mounting portion  80 , the second developing unit  40  combines with the first developing unit  20 . Specifically, when the second developing unit  40  is installed on the mounting portion  80 , the guide rail  74  guides the sliding guide member  44  to press the first developing unit  20  in a direction toward the sensing portion  86 . As the second developing unit  40  is mounted on the mounting portion  80 , the second position adjustment guide member  49  of the second developing unit  40  and the first position adjustment guide member  29  of the first developing unit  20  contact each other. As the second developing unit  40  slides further into the interior of the mounting portion  80 , the second position adjustment  49  presses the first position adjustment guide member  29 . Therefore, the first developing unit  20  is pressed towards the sidewall  82  of the mounting portion  80 , in other words, towards the sensing portion  86  installed on the mounting portion  80 . As a result, the first developing unit  20  moves from the initial position B when only the first developing unit  20  is mounted to position A where the contacting portion  26  can make a contact with the sensing portion  86  of the main body  100 , thereby activating the printing operation.  
         [0058]     Hence, the image forming apparatus is capable of sensing whether or not both the first developing unit  20  and the second developing unit  40  are mounted on the mounting portion  80 . Further, the second developing unit  40  is connected with the driving unit  83  when it is installed in the printer.  
         [0059]      FIG. 11  is a diagram showing a second embodiment of the present invention with only a first developing unit is mounted on a mounting portion. In this embodiment, an anti-contact member is disposed on the surface where the contacting portion  26  of the first developing unit  20  is installed. In the illustrated exemplary embodiment, the anti-contact member is an elastic member  84 .  
         [0060]     In this embodiment of the invention, instead of using the declined bottom surface  81  shown in  FIGS. 9A and 9B , the elastic member  84  is disposed on the surface where the contacting portion  26  of the first developing unit  20  is installed. The elastic member  84  forcefully pushes the first developing unit  20  away from the surface where the sensing portion  86  is disposed. Thus, when only first developing unit  20  is installed, as illustrated, the sensing portion  86  and the contact part  87  of the first developing unit  20  are separated from each other. That is, the elastic member  84  pushes the first developing unit  20  from a position A at which a printing operation can be carried out because the contacting portion  26  and the contact part  87  contact each other to a position B where the contacting portion  26  and the contact part  87  are separated. As a result, printing operation is disabled. In the same manner as the first embodiment, a second developing unit  40  is mounted on the mounting portion  80  by sliding it along a guide rail  74 , which is the same as the one shown in  FIG. 10 . This mounting of the second developing unit  40  causes the first developing unit  20  to be pressed toward the side wall  82 , causing the sensing portion  86  to contact the contacting portion  26 . The configuration and operation of the first and second embodiments of the image forming apparatus are similar in other respects, and, therefore, a detailed description is not repeated.  
         [0061]     Furthermore, as another embodiment of the present invention, the elastic member  84  can be installed on the sidewall  82  on which the sensing portion  86  of the mounting portion  80  is disposed. The elastic member  84  performs the same function as described with respect to the second embodiment.  
         [0062]     In accordance with the exemplary embodiments of the present invention described above, an image forming apparatus has a contacting portion that is mounted on only one of the developing units included in the developing unit assembly. Using this contacting portion, it is possible to detect whether or not the other developing unit is mounted. Accordingly, the present invention allows a simplified structure along with reduced manufacturing costs by using only one contacting portion.  
         [0063]     Furthermore, in the exemplary embodiments of the invention described above, the contacting portion is disposed on a surface generally parallel to the direction in which the developing unit is mounted on the mounting portion of the main body. The contacting portion is located at a region where the contacting portion does not overlap with the driving unit of the other developing unit of the main body. With this configuration, it is possible to eliminate the layout limitations with respect to the driving unit of the developing units and the image forming apparatus, thereby allowing a smaller apparatus.  
         [0064]     While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.