Patent Publication Number: US-7590372-B2

Title: Multi-pass type color image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Patent Application No. 2006-0097189, filed on Oct. 2, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present general inventive concept relates to a multi-pass type color image forming apparatus, and more particularly, to a multi-pass type color image forming apparatus which is provided with one photosensitive drum and one exposure unit, forms a color toner image on an intermediate transfer member by repeating exposure, development and transfer with respect to respective colors, and transfers and fixes the color toner image on a recording medium, such as, a paper. 
   2. Description of the Related Art 
   Generally, a color image forming apparatus using an electrophotographic process is configured such that a light beam is scanned to a photosensitive member charged with a certain electric potential to form an electrostatic latent image thereon. The electrostatic latent image is developed to a toner image by using a certain color toner, and the toner image is transferred and fixed to a paper to form a color image. 
   Color of the toner used in the color image forming apparatus generally includes yellow (Y), magenta (M), cyan (C) and black (K). Accordingly, in order to adhere the four-color toner to the electrostatic latent image, four developing devices are needed. 
   A color image forming process is classified into a single pass process which is equipped with four exposure units and four photosensitive members, and a multi-pass process which is equipped with a single exposure unit and a single photosensitive member. 
   A multi-pass type color image forming apparatus includes one photosensitive drum and one exposure unit, and is configured such that exposure, development and transfer are repeated with respect to respective colors to form a color toner image on an intermediate transfer member, and the color toner image is transferred and fixed to a paper. 
   In such a multi-pass type color image forming apparatus, four developing devices get power from a driving motor. All four developing devices are not necessarily driven at the same time, and are selectively driven according to the color image. 
   In such a multi-pass type color image forming apparatus, four developing devices are sequentially operated. It is preferable that only a developing roller of the selected developing device (e.g., Y) is rotated, and developing rollers of the remaining developing devices (e.g., M, C and K) are not rotated. 
   To this end, a conventional multi-pass type color image forming apparatus includes a power transmission device for selectively transmitting a driving force from a driving motor to four developing devices, and a cam device for operating the power transmission device so that the driving force is transmitted to the selected developing device as a cam provided at a camshaft is rotated. 
   However, in operating the cam device, a slip is generated between the cam device and the power transmission device so that the driving force cannot be transmitted timely to the selected developing device or the driving force is transmitted to the wrong developing device. 
   SUMMARY OF THE INVENTION 
   The present general inventive concept provides a multi-pass type color image forming apparatus which can transmit a driving force timely and reliably to respective developing devices. 
   Additional aspects and utilities of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
   The foregoing and/or other aspects and utilities of the present general inventive concept are achieved by providing a multi-pass type color image forming apparatus including a photosensitive drum, a laser scanning unit, a driving motor, a plurality of developing devices, a cam device which has a camshaft that rotates by the driving motor and a plurality of cams provided at the camshaft corresponding to the developing devices, and a power transmission device which is provided between the developing devices and the cam device and selectively transmits a rotational force from the driving motor to the developing devices according to a rotational phase of the driving motor, including an indicating member which is provided at the camshaft and has a plurality of indicating portions; a sensing part which detects the plurality of indicating portions, and a control unit which receives a signal from the sensing part when the indicating member passes by the sensing part by the rotation of the camshaft, and determines a home position by a rotational position of the camshaft when a signal pattern from a sequence of signals from the sensing part accords with a preset reference pattern. 
   The indicating member can have two indicating portions which are spaced apart from each other by a predetermined distance in a circumferential direction on an outer periphery of the indicating member and protrude in the circumferential direction. 
   The control unit can receive the signal from the sensing part while controlling the camshaft to be intermittently rotated. 
   The control unit can receive the signal from the sensing part while controlling the camshaft to be continuously rotated. 
   The control unit can determine the home position by the rotational position of the camshaft when the signal pattern from a sequence of signals from the sensing part is changed into a pattern of 0→0→1→0. 
   The foregoing and/or other aspects and utilities of the present general inventive concept are also achieved by providing a multi-pass type color image forming apparatus, including a driving motor to drive a camshaft based on a signal received, an actuator device to selectively engage the driving motor with the camshaft to operate a plurality of color developing units, a control unit to generate a signal to activate the driving motor and to activate the actuator device based on a control signal, and a sensing device to determine a rotational position of the camshaft and to generate and transmit the control signal to the control unit based on the determination. 
   The foregoing and/or other aspects and utilities of the present general inventive concept are achieved by providing a method of operating a developing unit of a multi-pass type color image forming apparatus, the method including determining a rotational position of a rotating camshaft and generating a control signal based on the determination and selectively engaging or disengaging a driving motor of the camshaft based on the generated control signal. 
   The foregoing and/or other aspects and utilities of the present general inventive concept are achieved by providing method of operating a developing device of a multi-pass type color image forming apparatus, the method including sensing a rotational position of a camshaft of the developing device to operate individual developing units thereof, generating and transmitting a control signal based on the sensing, and selectively engaging or disengaging a driving motor with the camshaft based on the control signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and utilities of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, of which: 
       FIG. 1  is a schematic constitutional view illustrating a multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept; 
       FIG. 2  is a constitutional view illustrating a power transmission device of a multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept; 
       FIG. 3  is a fragmentary sectional view of  FIG. 2 ; 
       FIG. 4  is a rear perspective view of  FIG. 2 ; 
       FIG. 5  is a sectional view of an indicating member in  FIG. 4 ; 
       FIG. 6  is a perspective view of a sliding hub and a fixing hub in  FIG. 2 ; 
       FIG. 7  is an exploded perspective view of a spring clutch of  FIG. 2 ; 
       FIG. 8  is a schematic view illustrating an operation of a spring clutch and an actuator in  FIG. 2 ; 
       FIG. 9  is a control block diagram of a multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept; 
       FIG. 10  is a flow chart of a method of determining a home position of a camshaft in a multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept; 
       FIG. 11  is a flow chart of a method of determining a home position of a camshaft in a multi-pass type color image forming apparatus in accordance with another exemplary embodiment of the present general inventive concept; and 
       FIG. 12  is a view illustrating a signal pattern from a sensing part in  FIG. 11 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to exemplary embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below to explain the present general inventive concept by referring to the figures. 
   As illustrated in  FIG. 1 , a multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept includes a photosensitive drum  1 , a charge roller  2 , an exposure unit  3 , developing devices  4 , an intermediate transfer belt  6 , a first transfer roller  7 , a second transfer roller  8 , and a fixing device  9 . 
   The photosensitive drum  1  can be a cylindrical metallic drum which is coated with a photoconductive layer on a circumference. 
   The charge roller  2  can charge the photosensitive drum  1  with a uniform electric potential. The charge roller  2  can be rotated in contact or non-contact with the outer peripheral surface of the photosensitive drum  1  to supply an electric charge, to thereby charge the outer peripheral surface of the photosensitive drum  1  with a uniform electric potential. 
   The exposure unit  3  can scan a light beam corresponding to image information to the photosensitive drum  1  that is charged with a uniform electric potential to form an electrostatic latent image. The exposure unit  3  generally includes a laser scanning unit (LSU) which uses a laser diode as a light source. 
   In order to print the color image, the multi-pass type color image forming apparatus of this embodiment uses a yellow (Y) toner, a magenta (M) toner, a cyan (C) toner and a black (K) toner. Accordingly, four developing devices  4  are provided to respectively contain the yellow (Y) toner, the magenta (M) toner, the cyan (C) toner and the black (K) toner. 
   The developing devices  4  are respectively provided with developing rollers  5 . The developing devices  4  are positioned such that the developing rollers  5  are spaced from the photosensitive drum  1  by a development gap to perform a non-contact type development. The development gap can be in the range of several tens of microns to several hundreds of microns. The developing devices  4  may further include supply rollers (not illustrated) to supply the toner to the developing rollers  5 , and agitators (not illustrated). 
   The intermediate transfer belt  6  can be supported by support rollers  61  and  62 , so as to run at the same linear velocity as the rotational velocity of the photosensitive drum  1 . A length of the intermediate transfer belt  6  can be the same as or larger than a length of a paper P (or other recording medium) of the maximum size which is used in the multi-pass type color image forming apparatus. 
   The first transfer roller  7  is disposed opposite to the photosensitive drum  1 . A first transfer bias is applied to the first transfer roller  7  to transfer the toner image developed on the photosensitive drum  1  to the intermediate transfer belt  6 . 
   The second transfer roller  8  is disposed opposite to the intermediate transfer belt  6 . While the toner image is transferred to the intermediate transfer belt  6  from the photosensitive drum  1 , the second transfer roller  8  is spaced apart from the intermediate transfer belt  6 . When the toner image is completely transferred to the intermediate transfer belt  6 , the second transfer roller  8  comes into contact with the intermediate transfer belt  6  with a specific pressure. A second transfer bias is applied to the second transfer roller  8  to transfer the toner image to the paper P. 
   Hereinafter, a color image forming process of the multi-pass type color image forming apparatus structured as above will be described. Firstly, the light beam corresponding to the image information of yellow (Y) color, for example, is scanned from the exposure unit  3  to the photosensitive drum  1  charged with a uniform electric potential by the charge roller  2 . The electrostatic latent image corresponding to the image of yellow (Y) color is formed on the photosensitive drum  1 . The development bias is applied to the developing roller  5  of the yellow developing device  4 Y. Then, the toner of yellow (Y) color is adhered to the electrostatic latent image, and the toner image of yellow (Y) color is developed on the photosensitive drum  1 . The toner image of yellow (Y) color is transferred to the intermediate transfer belt  6  by the first transfer bias applied to the first transfer roller  7 . When the toner image of yellow (Y) color for one page is completely transferred, the exposure unit  3  scans the light beam corresponding to the image information of magenta (M) color, for example, to the photosensitive drum  1  re-charged with a uniform electric potential by the charge roller  2 , to form the electrostatic latent image corresponding to the image of magenta (M) color. The magenta developing device  4 M supplies the toner of magenta (M) color to the electrostatic latent image to develop the same. The toner image of magenta (M) color formed on the photosensitive drum  1  is transferred to the intermediate transfer belt  6  so as to be overlapped on the toner image of yellow (Y) color which has already been transferred. By repeating the above process with respect to cyan (C) and black (K), the color toner image in which the toner images of yellow (Y), magenta (M), cyan (C) and black (K) colors are overlapped is formed on the intermediate transfer belt  6 . The color toner image is transferred to the paper P which passes between the intermediate transfer belt  6  and the second transfer roller  8  by the second transfer bias. The fixing device  9  fixes the color toner image to the paper by applying heat and pressure to the color toner image. 
   As described above, the multi-pass type color image forming apparatus according to this embodiment is configured such that four developing devices  4  are sequentially operated. The developing roller  5  of the selected developing device (e.g.,  4 Y) is applied by the development bias. On the other hand, the developing rollers  5  of the remaining developing devices (e.g.,  4 M,  4 C and  4 K) are not applied by the development bias, or are applied by the development-prevention bias to prevent the development of the toner. Also, it is preferable that only the developing roller  5  of the selected developing device (e.g.,  4 Y) is rotated, and the developing rollers  5  of the remaining developing devices (e.g.,  4 M,  4 C and  4 K) are not rotated. To this end, the multi-pass type color image forming apparatus according to this embodiment includes a power transmission device to selectively transmit a driving force from a driving motor to four developing devices  4 , and a cam device to operate the power transmission device. 
     FIG. 2  is a constitutional view illustrating the power transmission device of the multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept,  FIG. 3  is a fragmentary sectional view of  FIG. 2 ,  FIG. 4  is a rear perspective view of  FIG. 2 ,  FIG. 5  is a sectional view of an indicating member in  FIG. 4 , and  FIG. 6  is a perspective view of a sliding hub and a fixing hub in  FIG. 2   
   Referring to  FIGS. 2 through 6 , four shafts  101  are rotatably mounted to a bracket  100 . Each of the shafts  101  is provided with a cylinder portion  102  and a surface cutting portion  103 . The cylinder portion  102  is coupled with a sliding hub  104 . The surface cutting portion  103  is coupled with a fixing hub  106  on one end and a driving gear  109  on the other end. An elastic member  112  elastically biases the sliding hub  104  away from the fixing hub  106 . As illustrated in  FIG. 2 , the sliding hub  104 Y is connected to the driving motor  10  through gears  11  and  12 . The sliding hub  104 Y and the sliding hub  104 M are connected to each other through a gear  13 . The sliding hub  104 C is connected to the driving motor  10  through a plurality of gears (not illustrated). The sliding hub  104 C and the sliding hub  104 K are connected to each other through a gear  14 . As illustrated in  FIG. 6 , the sliding hub  104  and the fixing hub  106  are respectively provided with complementary engagement portions  105  and  107 . Accordingly, if the sliding hub  104  is engaged with the fixing hub  106 , the driving force of the driving motor  10  is transmitted to the fixing hub  106 , and the shaft  101  and the driving gear  109  are rotated. The driving gear  109  is connected to a driven gear (not illustrated) provided at the developing device  4 . The driven gear is connected to the developing roller  5  and other components mounted inside the developing device  4 . 
   By the above structure, four sliding hubs  104  are selectively slid to be engaged with four fixing hubs  106 , thereby selectively driving four developing devices  4 . 
   Referring to  FIG. 4 , the multi-pass type color image forming apparatus according to this embodiment is provided with a cam device to selectively slide four sliding hubs  104 . The cam device includes a camshaft  120  and four cams  131 . 
   Four cams  131  are fixed to the camshaft  120  at positions corresponding to four sliding hubs  104 . The four cams  131  and the camshaft  120  can be formed by a plastic injection molding in an integral manner. The four cams  131  have different phases. When the camshaft  120  is rotated, the four cams  131  sequentially push the four sliding hubs  104  to be sequentially engaged with corresponding four fixing hubs  106 . 
   The multi-pass type color image forming apparatus according to this embodiment is further provided with four push caps  110 . The cams  131  push the push caps  110  to slide the sliding hubs  104 . 
   The cams  131  can have a locus capable of smoothly engaging the sliding hubs  104  with the fixing hubs  106 , and disengaging the sliding hubs  104  from the fixing hubs  106  as promptly as possible. 
   Referring to  FIG. 4 , the cams  131 Y,  131 M and  131 C can directly push the push caps  110 Y,  110 M and  110 C, respectively. But, because the cam  131 K is disposed apart from the push cap  110 K, the cam  131 K cannot directly push the push cap  110 K. Therefore, a connecting member  170  to connect the cam  131 K and the push cap  110 K is provided. The connecting member  170  is pivotably coupled to a cover  180 , and the cover  180  is coupled to the bracket  100 . If the cam  131 K pushes a first end portion  171  of the connecting member  170 , the connecting member  170  is swiveled, and a second end portion  172  of the connecting member  170  pushes the push cap  110 K. 
   To this end, the cams  131 Y,  131 M,  131 C and  131 K are arranged as illustrated in  FIG. 4 . The cams  131 M and  131 C respectively have phase differences of 90 degrees and 180 degrees from the cam  131 Y in a reverse direction to a rotational direction A of the camshaft  120 . The cam  131 K operates the connecting member  170  to push the push cap  110 K. The first end portion  171  of the connecting member  170  is located in an opposite direction of the push cap  110 K. Accordingly, the cam  131 K has a phase difference of 270 degrees from the cam  131 C in a reverse direction of the rotational direction A of the camshaft  120 . 
   As illustrated in  FIGS. 2 and 3 , the camshaft  120  is rotated by the driving motor  10 . The rotational force of the driving motor  10  is intermittently transmitted to the camshaft  120 . The image forming apparatus includes a spring clutch  150  to intermit the rotational force transmitted to the camshaft  120  from the driving motor  10 , and an actuator  160  to selectively operate the spring clutch  150 . 
     FIG. 7  is an exploded perspective view of the spring clutch  150  depicted in  FIG. 2 , and  FIG. 8  is a schematic view illustrating an operation of the spring clutch and the actuator depicted in  FIG. 2 . 
   Referring to  FIGS. 7 and 8 , the spring clutch  150  includes a clutch gear  151 , a clutch spring  159 , a clutch hub  157  and a bushing  152 . 
   The bushing  152  is fixed to an end of the camshaft  120 , and the clutch gear  151  is rotatably coupled to the bushing  152 . The clutch spring  159  is inserted by cylinder portions  153  and  154  of the clutch gear  151  and the bushing  152 . 
   The clutch hub  157  surrounds the clutch spring  159 . The clutch hub  157  is provided with four coupling portions  158 Y,  158 M,  158 C and  158 K which correspond in phase to four cams  131 , and a single home position coupling portion  158   h . A first end  159   a  and a second end  159   b  of the clutch spring  159  are respectively inserted into insertion holes  155  and  156  which are provided at the bushing  152  and the clutch hub  157 , respectively. The clutch gear  151  is connected to a gear  15  (see  FIG. 2 ) which is rotated by the driving motor  10 . The clutch gear  151  is rotated in a direction of an arrow A shown in  FIG. 7  by the driving motor  10 . 
   The clutch spring  159  is contractingly twisted to reduce an inner diameter thereof, so as to tightly hold the cylinder portions  153  and  154  of the clutch gear  151  and the bushing  152 . Accordingly, when the clutch gear  151  is rotated in the direction of the arrow A ( FIG. 7 ), the clutch spring  159  and the bushing  152  are rotated, and the camshaft  120  is also rotated together therewith. Because the second end  159   b  of the clutch spring  159  is inserted into the insertion hole  156  of the clutch hub  157 , the clutch hub  157  is also rotated. 
   When electric current is not applied to a coil part  161 , as illustrated by a real line in  FIG. 8 , a stopper  164  of a moving rod  162  is moved forward, so that the coupling portions  158  are caught by the stopper  164  and the rotation of the clutch hub  157  is prevented. 
   As illustrated in  FIG. 7 , because the second end  159   b  of the clutch spring  159  is inserted into the insertion hole  156  of the clutch hub  157 , if the clutch hub  157  is not rotated, the clutch spring  159  is expandingly twisted to enlarge the inner diameter thereof. As the force with which the clutch spring  159  holds the cylinder portion  153  of the clutch gear  151  is weakened, the inner peripheral portion of the clutch spring  159  slips over the cylinder portion  153  of the clutch gear  151 , and the clutch spring  159  and the bushing  152  are not rotated. Accordingly, the rotation of the camshaft  120  is stopped. 
   If electric current is applied to the coil part  161 , as illustrated by a dotted line in  FIG. 8 , the moving rod  162  adheres to the coil part  161 , and the stopper  164  is moved away from the coupling portions  158 . Then, as described above, the clutch gear  151  and the camshaft  120  are rotated together. 
   Referring to  FIGS. 2 ,  3  and  5 , in order to check an initial position of the camshaft  120 , the camshaft  120  is provided with an indicating member  132 . 
   The indicating member  132  is provided with a plurality of indicating portions  133 H and  133 Y (e.g., two indicating portions) which protrude at different lengths on a circumference of the indicating member  132 . For instance, the indicating portion  133 H is a home position indicating portion, and the indicating portion  133 Y is a yellow-color indicating portion. The home position indicating portion  133 H and the yellow-color indicating portion  133 Y are spaced apart from each other by a predetermined distance in a circumferential direction on the outer periphery of the indicating member  132 . 
   The bracket  100  is provided with a sensing part  140  to detect (sense) two indicating portions  133 H and  133 Y. The sensing part  140  can be an optical sensor. Before the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs a “0.” When the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs a “1.” 
   The clutch hub  157  is provided with a plurality of coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h  (e.g., five coupling portions) corresponding to the indicating member  132 . 
   If the stopper  164  of the actuator  160  comes into contact with one of the coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h , the rotation of the camshaft  120  is stopped at the home position or at each of the development positions (yellow (Y), magenta (M), cyan (C) and black (K)). 
   At this time, the home position refers to the non-driving state of four developing devices  4 , i.e., the state in which all four sliding hubs  104  are disengaged from the corresponding fixing hubs  106 . Accordingly, the home position coupling portion  158   h  is not overlapped in phase with any of the four coupling portions  158 Y,  158 M,  158 C and  158 K. 
   If electric current applied to the actuator  160  is interrupted when the indicating portions  133 H and  133 Y are detected by the sensing part  140 , the moving rod  162  is located at a position illustrated by the real line in  FIG. 8 . 
   If the camshaft  120  is rotated and one of the coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h  is caught by the stopper  164 , the rotational force from the driving motor  10  is interrupted, and the rotation of the camshaft  120  is stopped at the home position or at one of the development positions.  FIG. 8  illustrates the state in which the home position coupling portion  158   h  is caught by the stopper  164 , and thus the camshaft  120  is stopped at the home position. 
     FIG. 9  is a control block diagram of the multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept. As illustrated in  FIG. 9 , the multi-pass type color image forming apparatus of this embodiment further includes a control unit  200  to control the overall operation of the apparatus. The control unit  200  is electrically connected to the sensing part  140  at an input part, and electrically connected to the driving motor  10  and the actuator  160  at an output part. 
   The control unit  200  supplies electric power to the actuator  160  to operate the same. If electric power is supplied to the actuator  160 , the moving rod  162  becomes adhered to the coil part  161  (the position illustrated by the dotted line in  FIG. 8 ), and the coupling portions  158  go into a state of freely moving. 
   The control unit  200  drives the driving motor  10  to rotate the camshaft  120 . According to the rotation of the camshaft  120 , the indicating member  132  is also rotated, and two indicating portions  133 H and  133 Y of the indicating member  132  pass by the sensing part  140 . 
   In response to the signal from the sensing part  140  when the indicating member  132  passes by the sensing part  140  by the rotation of the camshaft  120 , the control unit  200  determines the home position of the camshaft  120 . 
     FIG. 10  is a flow chart of a method of determining the home position of the camshaft in the multi-pass type color image forming apparatus in accordance with an exemplary embodiment of the present general inventive concept. 
   Referring to  FIG. 10 , the control unit  200  supplies electric power to the actuator  160  to turn on the same at operation S 210 . If the electric power is supplied to the actuator  160 , the moving rod  162  of the actuator  160  becomes adhered to the coil part  161  (the position illustrated by the dotted line in  FIG. 8 ), and five coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h  go into a state of freely moving. 
   The control unit  200  drives the driving motor  10  to rotate the camshaft  120  at operation S 220 . As the camshaft  120  is rotated, the indicating member  132  is also rotated. Accordingly, as the indicating member  132  is rotated, two indicating portions  133 H and  133 Y of the indicating member  132  pass by the sensing part  140 . 
   When the indicating member  132  is rotated, the control unit  200  determines whether the sensing part  140  detects front ends of the indicating portions  133 H and  133 Y at operation S 230 . At this time, before the front ends of the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs a “0.” When the front ends of the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs a “1.” 
   If it is determined that the sensing part  140  does not detect the front ends of the indicating portions  133 H and  133 Y at operation S 230 , the control unit  200  continuously determines whether the front ends of the indicating portions  133 H and  133 Y are detected. If it is determined that the sensing part  140  detects the front ends of the indicating portions  133 H and  133 Y, the control unit  200  counts the time until rear ends of the indicating portions  133 H and  133 Y are detected since the front ends of the indicating portions  133 H and  133 Y are detected at operation S 240 . 
   The control unit  200  determines whether the sensing part  140  detects the rear ends of the indicating portions  133 H and  133 Y at operation S 250 . At this time, before the rear ends of the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs the “1.” When the rear ends of the indicating portions  133 H and  133 Y pass by the sensing part  140 , the sensing part  140  outputs the “0.” 
   If it is determined that the sensing part  140  does not detect the rear ends of the indicating portions  133 H and  133 Y at operation S 250 , the control unit  200  continuously determines whether the rear ends of the indicating portions  133 H and  133 Y are detected. If it is determined that the sensing part  140  detects the rear ends of the indicating portions  133 H and  133 Y, the control unit  200  terminates the time counting at operation S 260 . The counted time T_ 0  is a time elapsed until the rear end of one of the indicating portions  133 H and  133 Y passes by the sensing part  140  after the front end of one of the indicating portions  133 H and  133 Y passes by the sensing part  140 . 
   The control unit  200  determines whether the counted time T_ 0  is equal to a time T_home which is taken for the home position indicating portion  133 H to pass by the sensing part  140  at operation S 270 . The time T_home taken for the home position indicating portion  133 H to pass by the sensing part  140  is measured in advance and stored in the control unit  200 . 
   If it is determined that the time T_ 0  is not equal to the time T_home at operation S 270 , the process goes to operation S 230 . On the other hand, If it is determined that the time T_ 0  is equal to the time T_home at operation S 270 , the control unit  200  turns off the actuator  160  at operation S 280 . If the actuator  160  is turned off, the moving rod  162  is returned to the position illustrated by the real line in  FIG. 8  by the elastic force of the spring  163 , and the home position coupling portion  158   h  is caught by the stopper  164 . Accordingly, the camshaft  120  is located at the home position. 
   From the state in which the camshaft  120  is located at the home position, the camshaft  120  is moved to the respective development positions according to the development control, and the electrostatic latent images of yellow (Y), magenta (M), cyan (C) and black (K) colors are formed on the photosensitive drum  1 . 
   The above method has features of counting the time since the front ends of the indicating portions  133 H and  133 Y are detected by the sensing part  140  until the rear ends of the indicating portions  133 H and  133 Y are detected by the sensing part  140 , comparing the counted time with the pre-stored time taken for the home position indicating portion  133 H to pass by the sensing part  140 , and determining that the position when the counted time is equal to the pre-stored time is the home position of the camshaft  120 . 
   When rotating the camshaft  120  in order to determine the home position of the camshaft  120 , the cams  131 Y,  131 M,  131 C and  131 K contact the push caps  110 Y,  110 M,  110 C and  110 K and push them by the rotation of the camshaft  120 . However, when the cams  131 Y,  131 M,  131 C and  131 K are released from the push caps  110 Y,  110 M,  110 C and  110 K, slip may be generated, and so the camshaft  120  may be rotated instantly faster than the original rotational speed. Accordingly, because the time taken for the indicating portions  133 H and  133 Y to pass by the sensing part  140  is not constant, it may be difficult to accurately determine the home position of the camshaft  120 . 
   In another embodiment of the present general inventive concept, there is a method to increase the reliability in determining the home position of the camshaft  120  in the shortest amount of time regardless of a slip between the cams  131 Y,  131 M,  131 C and  131 K and the push caps  110 Y,  110 M,  110 C and  110 K, so that a warm up time and a noise generation time can be minimized, and a power can be timely transmitted to the correct developing device. 
   According to this embodiment, in contrast with the method of determining the home position of the camshaft  120  by using the time taken for the indicating member  132  to pass by the sensing part  140 , a signal pattern from the sensing part  140  can be detected while the indicating member  132  passes by the sensing part  140 . 
   Hereinafter, the method of determining the home position of the camshaft  120  by using a signal pattern from the sensing part  140  while the indicating member  132  passes by the sensing part  140  will be described. 
   Different from the configuration of the indicating member  132  depicted in  FIG. 5 , an indicating member  132  having a yellow-color indicating portion  133 Y and a black-color indicating portion  133 K can be used. 
     FIG. 11  is a flow chart of a method of determining the home position of the camshaft in the multi-pass type color image forming apparatus in accordance with another exemplary embodiment of the present general inventive concept using a signal pattern from the sensing part  140 . 
   Referring to  FIG. 11 , the control unit  200  supplies electric power to the actuator  160  to turn on the same at operation S 310 . If the electric power is supplied to the actuator  160 , the moving rod  162  of the actuator  160  becomes adhered to the coil part  161  (the position illustrated by the dotted line in  FIG. 8 ), and five coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h  go into a state of freely moving. 
   The control unit  200  drives the driving motor  10  to rotate the camshaft  120  at operation S 320 . As the camshaft  120  is rotated, the indicating member  132  is also rotated. Thus, as the indicating member  132  is rotated, the indicating portions of the indicating member  132  pass by the sensing part  140 . 
   While the indicating member  132  is rotated, the control unit  200  receives a signal pattern from the sensing part  140  at operation S 330 . 
   The control unit  200  determines whether the signal pattern from the sensing part  140  corresponds with a preset reference pattern at operation S 340 . As illustrated in  FIG. 12 , if the indicating member  132  is rotated counterclockwise, a circular loop having an order of yellow (Y), magenta (M), cyan (C), black (K), home position (H), and again yellow (Y) is formed, and such a circular loop passes by the sensing part  140 . According to the rotational position of the indicating member  132 , the signal pattern from the sensing part  140  is diversely changed for one rotation of the above circular loop. For example, when the output signal from the sensing part  140  has the pattern of 0→0→1→0, the rotational position of the camshaft  120  when the last “0” signal is output becomes the home position H. At this time, the output of the signal from the sensing part  140  in the pattern of 0→0→1→0 is limited to one time while the indicating member  132  is rotated in the range of about two-thirds of a rotation to one and a half rotations. 
   Accordingly, the signal pattern of 0→0→1→0 is pre-stored as the reference pattern. The rotational position of the camshaft  120  when the signal pattern from the sensing part  140  accords with the reference pattern is the home position. 
   For instance, the signal pattern which can be output from the sensing part  140  according to the rotational position of the indicating member  132  may include 1→0→0→1→0, 0→1→0→0→1→0, 1→0→1→0→0→1→0, 0→1→0→1→0→0→1→0, etc. By detecting that the signal pattern from the sensing part  140  is 0→0→1→0, the home position of the camshaft  120  can be easily determined. 
   If it is determined that the signal pattern from the sensing part  140  corresponds with the preset reference pattern at operation S 340 , the control unit  200  turns off the actuator  160  at operation S 350 . If the actuator  160  is turned off, the moving rod  162  is returned to the position illustrated by the real line in  FIG. 8  by an elastic force of the spring  163 , and the home position coupling portion  158   h  is caught by the stopper  164 . When the home position coupling portion  158   h  is caught by the stopper  164 , the rotational force transmitted to the camshaft  120  from the driving motor  10  is interrupted by the spring clutch  150 , and the rotation of the camshaft  120  is stopped. Accordingly, the camshaft  120  becomes located at the home position. 
   From the state in which the camshaft  120  is located at the home position, the camshaft  120  is moved to the respective development positions according to the development control, and the electrostatic latent images of yellow (Y), magenta (M), cyan (C) and black (K) colors are formed on the photosensitive drum  1 . 
   The above described method of  FIG. 11  using the signal pattern minimizes or eliminates slipping between the cams  131 Y,  131 M,  131 C and  131 K and the push caps  110 Y,  110 M,  110 C and  110 K. 
   Another embodiment of the present general inventive concept provides a method of overcoming slipping between the cams  131 Y,  131 M,  131 C and  131 K and the push caps  110 Y,  110 M,  110 C and  110 K is to detect the signal from the sensing part  140  while one of the coupling portions  158 Y,  158 M,  158 C,  158 K and  158   h  is caught by the stopper  164  of the actuator  160  by turning on the actuator  160  for a constant time, and to repeatedly perform the above process with respect to the other coupling portions in the same manner. In other words, the home position of the camshaft  120  is determined based on the signal pattern from the sensing part  140  detected by repeatedly performing the process of detecting the signal from the sensing part  140  by intermittently rotating the camshaft  120  to rotate the indicating member  132  by partial sections. 
   As apparent from the above description, according to the multi-pass type color image forming apparatus of the present general inventive concept, the home position of the camshaft can be found out in the shortest time regardless of a slip between the cams and the push caps, so that the warm up time can be shortened and the generation time of noise caused by the contact between the cams and the push caps can be minimized. 
   Also, the reliability about the home position of the camshaft can be increased, thereby transmitting the power timely to the correct developing device. 
   Although embodiments of the present general inventive concept have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the claims and their equivalents.