Patent Publication Number: US-9897957-B2

Title: Image forming apparatus and color tone density controlling method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Korean Patent Application No. 10-2012-0061739, filed on Jun. 8, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Apparatuses and methods consistent with exemplary embodiments relate to an image forming apparatus and a color-tone density (CTD) controlling method thereof, and more particularly, to a multi-path type image forming apparatus and a CTD controlling method thereof. 
     2. Description of the Related Art 
     Image forming apparatuses are apparatuses which print printing data generated in a terminal apparatus, such as a computer, on a recording paper. As an example of the image forming apparatuses that perform such functions, there are copiers, scanners, facsimiles, or multiple function peripherals (MFPs) which multiply implement functions thereof through one apparatus. 
     In recent years, laser image forming apparatuses with remarkable effects in terms of printing quality, printing speed, noise in printing, and the like as compared with dot image forming apparatuses or inkjet image forming apparatuses which have mainly been used in the related art have been used increasingly. The laser image forming apparatuses are apparatuses using the principle which coats a toner to an organic photo conductor (OPC) using laser light ray modulated into a picture signal, transfers the toner coated on the OPC to a printing paper, and fixes the toner on the printing paper with high heat and pressure. 
     In particular, color laser image forming apparatuses which also implement color using a laser system have been increasingly used in recent years. In general, the color laser image forming apparatuses represent a color image using four color toners of cyan (C), magenta (M), yellow (Y), and black (K). 
     In the color laser image forming apparatuses, there are a single-path system including four laser scanning units and four OPCs and a multi-path system including one laser scanning unit and one OPC. 
     The time required in color printing and the time required in black and white printing are the same in the single-path system. Therefore, the single-path system is mainly used in high-speed color laser image forming apparatuses. However, since the high-speed color image forming apparatuses include the four laser scanning units and the four OPCs, the production cost becomes expensive. Thus, color laser image forming apparatuses which operate in a relatively low-speed range employ the multi-path system which includes one OPC and one laser scanning unit and repeatedly performs a writing operation, a developing operation, and a transferring operation for each color to form a color toner image on an intermediate transfer belt, and transfers and fixes the color toner image to a paper. 
     The CTD of an image formed by the color laser image forming apparatuses is changed due to various factors such as change in an environment such as a temperature or a humidity, temporal change in consumables including a developer, or change in voltages related to the development. To uniformly maintain the CTD of the image, it is necessary to measure the CTD of the image periodically or at a specific point of time and appropriately control development variables according to the measured result. 
     The method of controlling a CTD of an image in the prior multi-path type color laser image forming apparatus will be described. The CTD of a test patch formed on an OPC or an intermediate transfer belt is measured using a CTD sensor. The measuring operation for each developer is repeatedly performed to repeatedly measure the CTD and then final development variables are determined. 
     However, when the test patch is developed on the OPC in the related art, the test patches for CMYK are developed in order of the Y, M, C and K test patches. That is, the test patches are developed as shown in  FIG. 1 . This is because the development operation which is performed in order of the Y, M, C, and K developers in a color printing job is applied to the test patch development operation. 
     When the test patches are developed in order of Y, M, C, and K developers as in the related art, an unnecessary operation is caused when the test patch is developed in association with the configuration of the image forming apparatus, and it takes a long time to measure the CTD of the test patch. 
     SUMMARY OF THE INVENTION 
     One or more exemplary embodiments may overcome the above disadvantages and other disadvantages not described above. However, it is understood that one or more exemplary embodiment are not limited to overcoming the disadvantages described above, and may be directed to other features and utilities of the general inventive concept. 
     One or more exemplary embodiments provide an image forming apparatus which develops a test patch on an OPC sequentially from a developer to be developable preferentially and a CTD controlling method thereof. 
     Additional features 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. 
     Exemplary embodiments of the present inventive concept provide a method of controlling a color-tone density of an image forming apparatus including a plurality of developers configured to circularly perform a developing operation. The method may include: developing test patches on an OPC sequentially from a developer to be developable preferentially; measuring CTDs of the developed test patches; and controlling a development variable using the measured CTDs. 
     The developer to be developable preferentially may be a black (K) developer. 
     The plurality of developers may perform the developing operation in order of a yellow (Y) developer, a magenta (M) developer, a cyan (C) developer, and a black (K) developer when a color printing job is performed and the plurality of developers may develop the test patches in order of test patterns of K, Y, M, and C developers when the test patches are developed. 
     The developing may include developing the test patches in all the plurality of developers on the OPC during one cycle in which the OPC is rotated once. 
     The measuring may include measuring the CTDs of the test patches formed on an intermediate transfer belt or the OPC using a CTD sensor. 
     The image forming apparatus may have a cam type or a rotary type. 
     The developing may include developing the test patches so that a distance between a K test patch and a Y test patch among distances between two test patches from among all the developed test patches is to be shortest when the image forming apparatus has the cam type. 
     The method may further include performing a printing job using the controlled-development variable. 
     The method may further include developing in order of color developers of the plurality of developers except a black (K) developer, and the K developer when a color printing job is performed. 
     According to another aspect of an exemplary embodiment, there is provided an image forming apparatus. The image forming apparatus may include: an organic photo conductor (OPC) configured to form an electrostatic latent image; a plurality of developers configured to develop test patches on the OPC; a color-tone density (CTD) measuring unit configured to measure a CTD of each of the developed test patches; and a control unit configured to control the plurality of developers so that the plurality of developers develop the test patches on the OPC sequentially from a developer to be developable preferentially and to control a development variable using the measured CTDs. 
     The developer to be developable preferentially may be a black (K) developer. 
     The plurality of developers may perform a developing operation in order of a yellow (Y) developer, a magenta (M) developer, a cyan (C) developer, and a black (K) developer when a color printing job is performed and the plurality of developers may develop the test patches in order of test patterns of K, Y, M, and C developers when the test patches are developed. 
     The control unit may control the plurality of developers so that the test patches in all the plurality of developers are developed on the OPC during one cycle in which the OPC is rotated once. 
     The CTD measuring unit may measure the CTDs of the test patches formed on an intermediate transfer belt or the OPC using a CTD sensor. 
     The image forming apparatus may have a cam type or a rotary type. 
     The control unit may control the plurality of developers to develop the test patches so that a distance between a K test patch and a Y test patch among distances between two test patches from among all the developed test patches is shortest when the image forming apparatus has the cam type. 
     The control unit may control the plurality of developers to develop in order of color developers of the plurality of developers except a black (K) developer, and the K developer when a color printing job is performed. 
     As described above, according to the various exemplary embodiments, developers develop test patches sequentially from a test patch in a developer to be developable preferentially on an OPC so that the time required to measure the CTD of the test patch can be reduced. 
     Additional aspects and advantages of the exemplary embodiments will be set forth in the detailed description, will be obvious from the detailed description, or may be learned by practicing the exemplary embodiments. 
     According to another aspect of an exemplary embodiment, there is provided a method of controlling a color-tone density (CTD) of an image forming apparatus including a plurality of developers configured sequentially to perform a developing operation, the method comprising: developing test patches on an organic photo conductor (OPC) sequentially based on a positioning of a developer position indicating member; measuring CTDs of the developed test patches; and controlling a development variable using the measure CTDs. 
     In an exemplary embodiment, the positioning of the developer position indicating member is a home position. 
     In another exemplary embodiment, the developer position indicating member is includes an indicator for each color developer and a cam system to operate each of the color developers individually based on the position of the cam system indicated by the indicators. 
     In still another exemplary embodiment, the sequential developing of the test patches is different from a sequential developing operation of a color image. 
     In yet another exemplary embodiment, the sequential developing of the test patches begins with a black (K) developer. 
     According to another aspect of an exemplary embodiment, there is provided an image forming apparatus, comprising: an organic photo conductor (OPC) configured to form an electrostatic latent image; a plurality of developers configured to separately develop a color image and test patches on the OPC; a color-tone density (CTD) measuring unit to measure a CTD of each of the developed test patches; and a control unit configured to control the plurality of developers so that the plurality of developers develop the test patches on the OPC sequentially in a different order than the developers develop a color image. 
     In an exemplary embodiment, the control unit controls the developers to develop the test patches sequentially from a developer to be developable preferentially and controls a development variable using the measured CTDs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other features and utilities of the present general inventive concept will be more apparent by describing in detail exemplary embodiments, with reference to the accompanying drawings, in which: 
       These and/or other features 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 view illustrating a test patch developed according to the related art; 
         FIG. 2  is a block diagram illustrating an image forming apparatus according to an exemplary embodiment; 
         FIG. 3  is a detailed block diagram illustrating the image forming apparatus of  FIG. 2 ; 
         FIGS. 4A and 4B  are a cross-sectional view and a perspective view illustrating an apparatus which circularly drives a plurality of developers in a cam type image forming apparatus; 
         FIG. 5  is a graph showing an output of a sensor unit provided in an apparatus which circularly drives a plurality of developers in a cam type image forming apparatus; 
         FIG. 6  is a view illustrating a cam type image forming apparatus according to an exemplary embodiment; 
         FIG. 7  is a view illustrating a rotary type image forming apparatus according to an exemplary embodiment; 
         FIG. 8  is a view illustrating a test patch developed according to an exemplary embodiment; and 
         FIG. 9  is a flowchart illustrating a method of measuring a CTD according to exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described in more detail with reference to the accompanying drawings. 
     In the following description, same reference numerals are used for the same elements when they are depicted in different drawings. The matters defined in the description, such as detailed construction and elements, are provided to assist in a comprehensive understanding of the exemplary embodiments. Thus, it is apparent that the exemplary embodiments can be carried out without those specifically defined matters. Also, functions or elements known in the related art are not described in detail since they would obscure the exemplary embodiments with unnecessary detail. 
       FIG. 2  is a block diagram illustrating an image forming apparatus according to an exemplary embodiment. Referring to  FIG. 2 , an image forming apparatus  100  partially or wholly includes an OPC  110 , a plurality of developers  120 , a CTD measuring unit  130 , and a control unit  140 . Here, the image forming apparatus may be a color laser image forming apparatus. Further, the image forming apparatus may have a multi-path system. 
     The multi-path system may include a cam type or a rotary type. The cam type or the rotary type will be described later in detail with reference to  FIGS. 6 and 7 . 
     An operation of the color laser image forming apparatus typically includes a processing procedure of charging, writing, developing, transferring, fusing, and the like, and the color laser image forming apparatus prints an image through the processing procedure. The charging process is a process of applying a high voltage (about 7000 V) to a charger, and causing negative (−) charges to be formed on a surface of the OPC by corona discharge. The writing process is a process of scanning a laser beam on the surface of the OPC, in which the negative (−) charges are formed, to dissipate the negative (−) charges in the form of letters so that a latent image is formed. The developing process is a process of causing toner particles having a negative (−) component to be attached on a portion of a surface of the OPC in which the latent image is formed. The transferring process is a process of applying a predetermined transfer voltage to the transfer when a paper passes between the OPC and the transfer to form positive (+) charges on a rear surface of the paper and pulling the negative (−) toner particles formed on the surface of a drum in a direction of a paper side. Next, the fusing process is a process of applying appropriate heat and pressure on the toner formed on the paper to be completely fused. An image is formed and output on the paper through all the processes. Hereinafter, the image forming apparatus according to an exemplary embodiment will be described in detail with reference to the above-described operation. 
     The OPC  110  is an area in which a printing image corresponding to printing data is formed by a laser beam before the printing data is printed on a printing paper P. A charging unit applies a charging current to the OPC to cause negative (−) charges to be charged on a surface of the OPC. A laser scanning unit (LSU) modulates the laser beam according to printing data to be printed and scans the modulated laser beam on the charged surface of the OPC  110 . Therefore, an electrostatic latent image is formed on a written area of the surface of the OPC  110 . In particular, an electrostatic latent image corresponding to a test patch may be formed on the OPC  110 . 
     The plurality of developers  120  develop an image by providing toner particles to be attached onto the electronic latent image formed on the surface of the OPC  110 . Here, the plurality of developers  120  may be implemented with four developers including Y, M, C, and K developers. Therefore, the plurality of developers  120  may develop an image with respective Y, M, C, and K toners. 
     The plurality of developers  120  may develop the test patches on the OPC  110  in which the electrostatic latent images corresponding to the test patches are formed. 
     The CTD measuring unit  130  may measure a CTD of the developed test patch. Here, the CTD measuring unit  130  may be implemented with a CTD sensor. Specifically, the CTD sensor may include a light-emitting unit configured to scan light on the test patch and a receiving unit configured to receive the light reflected from the test patch. In this case, the CTD sensor may convert an intensity of light input to the receiving unit according to the CTD of the test patch into an electrical signal to measure the CTD. Further, the CTD measuring unit  130  may measure the CTD of the test patch formed on an intermediate transfer belt or the OPC. 
     The control unit  140  controls an overall operation of the image forming apparatus  100 . Specifically, the control unit  140  may partially or wholly control the OPC  110 , the plurality of developers, and the CTD measuring unit  130 . 
     In particular, the control unit  140  may control the plurality of developers so that the plurality of developers  121  perform a developing process in order of the Y, M, C, and K developers when a color printing job is performed. This is because when the developing order of the plurality of developers is changed, a color may be changed in a portion in multiple colors overlapping each other when the color printing job is performed. 
     Further, the control unit  140  may determine a point of time to measure the CTD. That is, the CTD of an image formed by the color laser image forming apparatuses is changed due to various factors such as a change in an environment such as a temperature or a humidity, temporal change in consumables including the developer, or a change in voltages related to the developing operation. To uniformly maintain the CTD of the image, it is necessary to measure the CTD of the image periodically or at a specific point of time and appropriately control a development variable according to the measured result. Thereby, the control unit  140  may determine a periodic point of time (for example, when 100 sheets of papers are printed) or a specific point of time (for example, when power is ON) as the point of time to measure the CTD. 
     Further, when the control unit  140  determines the point of time to measure the CTD, first, the control unit  140  may control the plurality of developers  120  so that the plurality of developers develop test patches on the OPC  110  sequentially from a developer to be developable preferentially. The developer to be developable preferentially may be a K developer. Therefore, the control unit  140  may control the plurality of developers so that the test patches are developed in order of the K, Y, M, and C developers when the developing process on the test patch is performed. For clarity, the operation will be described in detail with reference to  FIGS. 4 and 5 . 
       FIGS. 4A and 4B  are a cross-sectional view and a perspective view of an apparatus which circularly drives the plurality of developers in an image forming apparatus.  FIG. 5  is a graph showing an output of a sensor unit provided in an apparatus which circularly drives the plurality of developers in the image forming apparatus. Referring to  FIGS. 4A and 4B , the apparatus, which circularly drives the plurality of developers, include a cam shaft  210 , a position indicating member  220 , a sensor unit  230 , and a plurality of cams  231 K,  231 Y,  231 M, and  231 K. 
     The position indicating member  220  may be provided to detect a home position of the cam shaft  210  and perform the developing operation. The position indicating member  220  may include a plurality of indicators  221 K,  221 Y,  221 M, and  221 C. 
     The plurality of indicators  221 K,  221 Y,  221 M, and  221 C may be disposed on an outer circumference of the position indicating member  120  to be spaced from each other at predetermined intervals. Here, the plurality of indicators  221 K,  221 Y,  221 M, and  221 C may correspond to respective developers to be driven. That is, a K indicator  221 K is detected by the sensor unit  230 , the K developer may be driven according to an operation of the cam  321 K under control of the control unit  140 . That is, when the cam shaft  210  is rotated, the plurality of cams  231 Y,  231 M,  231 C, and  231 K may sequentially drive the developers corresponding to respective four sliding hubs (not shown). 
     The sensor unit  230  may sense the plurality of indicators  221 K,  221 Y,  221 M, and  221 C to output sensed signals. In this case, the control unit  140  may detect the home position using the output sensed signals. Further, the control unit  140  may control operations of the plurality of developers using the output sensed signals. 
     Here, the sensor unit  230  may be an optical sensor. Specifically, as shown in  FIG. 5 , the position indicating member  220  is rotated by a clutch, the sensor unit  230  determines whether or not the plurality of indicators  221 K,  221 Y,  221 M, and  221 C are sensed after a constant period of time from a point of time when the plurality of indicators  221 K,  221 Y,  221 M, and  221 C are sense by rotation, and output sensed signals when the plurality of indicators  221 K,  221 Y,  221 M, and  221 C are sensed. When the plurality of indicators  221 K,  221 Y,  221 M, and  221 C are not sensed after the constant period of time, the sensed signals are not sensed. When the plurality of indicators  221 K,  221 Y,  221 M, and  221 C are not sensed after the constant period of time so that the sensed signals are not sensed, the control unit  140  may determine that a corresponding indicator is the C indicator. Thus, the control unit  140  may recognize the respective indicators. In this case, the control unit  140  may drive the clutch and rotate the position indicator member  220  to a home position. Here, the home position may be disposed between the C indicator and a black (K) indicator. This home position is illustrated to be at this location because this will provide the K developer to operate quickly in a black developing operation and while printing a job. 
     However, the method of sensing the home position is not limited to the above-described method. Various methods of sensing the home position may be used according to a shape of the position indicator member  220 . 
     When a color printing job execution command is input in a state in which the position indicating member  220  is positioned at the home position as described above, the control unit  140  may control the plurality of developers to perform a developing operation in the order of the Y, M, C, and K developers. That is, the control unit  140  drives the clutch to rotate the position indicating member  220 . In this case, the sensor unit  230  first senses the K indicator  231 K positioned next to the home position and the control unit  140  passes the K indicator  221 K and does not drive the K developer corresponding to the K indicator. Next, the sensor unit  230  senses the Y indicator positioned next to the K indicator  221 K and the control unit  140  drives the Y developer corresponding to the Y indicator  221 Y. In this case, the developed Y toner image may be first transferred on the intermediate transfer belt. Next, the sensor  230  senses the M indicator  221 M, and the control unit  140  drives the M developer corresponding to the M indicator  221 M. In this case, the developed M toner image may be first transferred on the intermediate transfer belt. Next, the sensor  230  senses the C indicator  221 C and the control unit  140  devices the C developer corresponding to the C indicator  221 C. In this case, the developed C toner image may be first transferred on the intermediate transfer belt. Next, the sensor  230  senses the K indicator  221 K and the control unit  140  drives the black developer corresponding to the K indicator  221 K. In this case, the developed K toner image may be first transferred on the intermediate transfer belt. Then, to cause the position indicating member  220  to be positioned at the home position, the control unit  140  may drive the clutch so that the position indicating member  220  passes through the Y, M, C, and K indicators  221 Y,  221 M,  221 C, and  221 K and is positioned at the home position. 
     However, the test patches used for CTD measurement are not in a color in which multi colors overlap each other, but are monochrome, and therefore it is not necessary to control the developers to perform a developing operation in order of the Y, M, C, and K developers. 
     Therefore, when the control unit  140  determines the point of time to measure the CTD, the control unit  140  may control the plurality of developers  120  to perform a developing operation in order of the K, Y, M and C developers. That is, the control unit  140  drives the clutch to rotate the position indicating member  220 . In this case, the sensor  230  senses the K indicator  221 K positioned next to the home position and the control unit  150  device the K developer corresponding to the K indicator  221 K. Next, the sensor senses the Y indicator  221 Y and the control unit  140  drives the Y developer corresponding to the Y indicator  221 Y. Next, the sense  230  senses the M indicator  221 M and the control unit  140  drives the M developer corresponding to the indicator  221 M. Next, the sensor  230  senses the C indictor  221 C and the control unit  140  drives the C developer corresponding to the C indicator  221 C. When, the test patches are developed by the driving of the developers, the position indicating member  220  may be directly positioned at the home position. 
     That is, in the related art, the test patch developing operation is performed in the same order of the Y, M, C, and K developers as in the color printing job. Therefore, the rotation of the position indicating member  220  is further increased and the time required to measure the CTD is increased. Specifically, in a period in which the operation of the developer is unnecessary, since the unnecessary time, such as the time required for the K indicator  221 K to pass the home position and the time required to pass for the Y, M, and C indicators to pass the home position after developing the test patch, is taken, the time required to measure the CTD is further increased. 
     However, according to the image forming apparatus of the exemplary embodiment, the test patches are developed on the OPC sequentially from the developer to be developable preferentially so that the time required to measure the OTD can be reduced. 
     Further, the control unit  140  may control the plurality of developers  120  so that the test patches of all the plurality of developers are developed on the OPC for 1 cycle in which the OPC is rotated once. That is, in the general multi-path type color image forming apparatus, one developer performs a developing operation on one color toner for 1 cycle in which the OPC is rotated once and the developed toner image is transferred on the intermediate transfer belt. Thus, the operation for the developers is repeatedly performed to form a color image. However, since the test patch is used not to print an image but to measure the CTD, the control unit  140  may control the plurality of developers  120  so that the test patches of the all the plurality of developers are developed on the OPC  110  for 1 cycle in which the OPC  110  is rotated once. In this case, the control unit  140  may control the charging unit, the laser scanning unit, and the plurality of developers  120  so that the test patches of the plurality of developers may be developed on the OPC  110  for 1 cycle in which the OPC is rotated once. Therefore, the image forming apparatus according to the exemplary embodiment enables the CTDs of four colors through only the developing operation for 1 cycle so that the time required to measure the CTD can be reduced. 
     In addition, since the image forming apparatus according to the exemplary embodiment has the cam type, the control unit  140  may control the plurality of developers so that a distance between the K test patch and the Y test patch among distances between all of the developed test patches is to be shortest. That is, referring to the cam type image forming apparatus illustrated in  FIG. 1 , the plurality of developers  120  are mounted so that the position of the developers is different according to colors. Thus, the time to write an image to the OPC  110  by the laser scanning unit and to develop a toner image on the OPC  110  using the developer  120  is different according to the colors. That is, the time required to write an image to the OPC  110  by the laser scanning unit and then to develop a toner image on the OPC  110  using the developing unit is different according to the toner color. That is, the time required to write an image related to the Y toner to the OPC by the laser scanning unit and to develop a Y toner image using the Y developer is longest, and the time required to write an image related to the K toner to the OPC  110  by the laser scanning unit and then to develop a K toner image using the K developer is shortest. Due to the mechanical structure, the time to perform the developing operation with a developer farthest from the laser scanning unit and then to perform the developing operation with a developer nearest to the laser scanning unit is shorter than the time to perform the developing operation with the developer nearest to the laser scanning unit and then to perform the developing operation with the developer farthest from the laser scanning unit, so that the image is formed faster in the former situation than in the latter situation. This is because since the K developer is disposed to be distant from the Y developer (see, for example,  FIG. 6 ), the laser scanning unit forms an electrostatic latent image corresponding to the Y test patch on the OPC  110  immediately after forming an electrostatic latent image corresponding to the K test patch on the OPC  110 . 
     Thereby, the control unit  140  may control the plurality of developers so that a distance between the K test patch and the Y test patch is shortest among distances between the Y test patch and the M test patch, the M test patch and the C test patch, and the C test patch and the K test patch. 
     As described above, according to the image forming apparatus of the exemplary embodiment, the test patches are formed in order of from the K test patch to the Y test patch so that the distance between the Y test patch and the K test patch can be minimized, and thus the time for CTD measurement can be reduced. 
     The control unit  140  may control the development variable using the measured CTD. The control unit  140  may perform the CTD measurement through the test patch development until a termination condition is satisfied. Here, the termination condition may include the number of the CTD measurement or a deviation between the measured CTD and a reference CTD which is smaller than a preset reference value. Therefore, when the termination condition is satisfied, the control unit  140  may control the development variable using the measured CTD. Here, the development variable may be a variable to perform the developing operation in the developer when the printing job is performed, for example, a CTD of a toner. 
     When the printing job is performed, the control unit  140  may control the plurality of developers  120  to perform the developing operation using the controlled-development variable. 
       FIG. 3  is a detailed block diagram illustrating the image forming apparatus of  FIG. 2 . Referring to  FIG. 3 , an image forming apparatus  300  includes an interface unit  310 , a user interface unit  320 , a power supply unit  330 , a control unit  340 , a storage unit  350 , a printer unit  360 , and a scanner unit  370 . A description of each of components of the image forming apparatus  300  of  FIG. 3  which are the same as those in the image forming apparatus  100  of  FIG. 2  will be omitted. An MFP which performs at least two functions among those of a printer, a scanner, a copier, and a facsimile as illustrated in the configuration of  FIG. 3 . However, when the image forming apparatus  300  of  FIG. 3  may have only a printer function, some components including the scanner unit  370  may be omitted. Although not shown, the image forming apparatus  300  may further include a bus configured to exchange data between the components and a buffer configured to temporarily store data, and the like. 
     The interface unit  310  may be connected to external devices locally or through a network so that the interface unit  310  receives data and commands from the external devices. That is, the interface unit  310  may be connected to a host personal computer (PC) through a local interface or connected to a network in a wired or wireless manner so that the interface unit  310  is connected to the plurality of external devices. As to the wireless communication standards, Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, hyper local area network (LAN) standards in Europe, MMAC-PC standards in Japan and the like may be used. 
     The user interface unit  320  receives various types of selection commands from the user. The user interface unit  320  may include a display panel and at least one button. In this case, the display panel may be implemented with a touch screen. The user interface unit  320  may provide various types of user interface (UI) screens and the user may input the selection command by directly touching the UI screen or operating the button included in the user interface unit  320 . The selection command is a command to set various functions included in the image forming apparatus or to set a mode change, operation stop or operation restart. 
     The power supply unit  330  serves to supply power to respective components in the image forming apparatus. Specifically, the power supply unit  330  may receive commercial alternating current (AC) power (AC_IN) from an external source, convert the commercial AC power into direct current (DC) power having a potential level suitable for the respective components using a transformer, an inverter, a rectifier, and the like, and output the converted DC power (DC_OUT). 
     The control unit  340  controls the overall operation of the image forming apparatus according to data and commands of external devices connected through the interface unit  310  or the user&#39;s selection command input through the user interface unit  320 . Further, the control unit  340  may perform the functions described in  FIG. 2 . 
     Specifically, when a printing command is executed in a printer driver installed in the host PC or an application, the printer driver of the host PC generates printing data in which a corresponding document is converted into a predetermined printing language. The control unit  340  receives the printing data through the interface unit  310 , and may convert the printing data into a bitmap image configured of a plurality of “0s” and “1s” using a halftone table, and then provide the converted bitmap image to the printer unit  360  so that the corresponding document is printed on a paper. 
     The printer unit  360  may include a print engine controller  361  and a plurality of unit  362 - 1  to  362 - n . Here, the OPC  110 , the plurality of developers  120 , and the CTD measuring unit  130  illustrated in  FIG. 2  may be included in each of the plurality of units  362 - 1  to  362 - n  and the control unit  140  of  FIG. 2  may perform a function of the print engine controller  361 . When the printer unit  360  has a laser print type, each of the plurality of units  362 - 1  to  362 - n  may include a paper feeding unit, a charging unit, an OPC, a plurality of developers, a transferring unit, a fusing unit, a paper discharging unit, a CTD measuring unit, and the like. The print engine controller  361  controls each of the plurality of units  362 - 1  to  362 - n  and performs the printing job based on the bitmap image provided from the control unit  340 . 
     When a scan command is input through the user interface unit  320 , the control unit  340  may control the scanner unit  370  to perform the scanning job. 
     The scanner unit  370  may include a scanner engine controller  371 , a scanning unit  373 , a scan motor unit  372 , and an image processing unit  374 . 
     The scanner engine controller  371  communicates with the control unit  340  and controls the respective components of the scanner unit  370  to perform the scanning job. 
     The scanning unit  373  serves to scan an object. The scanning unit  373  may be configured of an image scanning sensor, a lens, and a light source and as the image scanning sensor, a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensor (CIS) is mainly used. The image scanning sensor may include a photoelectric conversion unit configured to absorb reflection light of light generated from a light source and radiated to the object and to generate charges, a signal detection unit (not shown) configured to detect the charges generated from the photoelectric conversion unit and convert the charges into an electric signal, and the like. The electric signal converted in the signal detection unit is provided to the image processing unit  374 . 
     The image processing unit  374  performs shading and gamma correction, dot per inch (DPI) conversion, edge emphasis, error diffusion, scaling, and the like on the image data input from the scanning unit  373  to generate scanning data. In this case, the image processing unit  374  appropriately performs the above-described processes by considering the preset resolution, a scan mode, a scan area, a reduction rate, and the like. 
     The scan motor unit  372  may move the scanning unit  373  or the paper to allow the whole object to be scanned. That is, the media moved by the scan motor unit  372  is different according to an operation type of the scanner, for example, a sheet feed type or a flat bed type. For example, the scan motor unit  372  moves the paper when the scanner is a sheet feed type scanner, while the scan motor unit  372  moves the scanning unit  373  wherein the scanner is a flat bed type scanner. The scan motor unit  372  may be implemented with a carriage return motor, and the like. 
     When the scan command is transmitted from the control unit  340 , the scanner engine controller  371  drives the scanning unit  373  and the scan motor unit  372  to scan the object and controls the image processing unit  374  to cause the scan data to be generated. 
     The storage unit  350  is configured to store various information such as a specification of the image forming apparatus, a using state, printing data, scanning data, the processed data, and printing history information and various application programs and operating system (O/S) used in the image forming apparatus. The storage unit  350  may include a volatile memory unit  351  and a nonvolatile memory unit  352 . 
     The volatile memory unit  351  may be used as a temporary storage space required to operate the image forming apparatus. That is, the volatile memory unit  351  may be implemented so that printing data transmitted from the host PC, free scanning data, data scanned for copying, and the like are temporarily stored in the volatile memory unit  351  and removed from the volatile memory unit  351  when the corresponding job is completed. Various types of data or programs may be permanently stored in the nonvolatile memory unit  352 . It has been illustrated in  FIG. 3  that one volatile memory and one nonvolatile memory are provided as the volatile memory unit  351  and the nonvolatile memory unit  352 , but the number and sizes of the volatile memory and the non-volatile memory may be variously designed to be suitable for characteristics of the image forming apparatus. 
       FIG. 6  is a view illustrating a cam type image forming apparatus according to an exemplary embodiment. Referring to  FIG. 6 , a cam type image forming apparatus  600  may partially or wholly include a charging roller  610 , a laser scanning unit  620 , four developers  640 Y,  640 M,  640 C, and  640 K, an intermediated transfer belt  650 , a cleaning unit  660 , and a discharging roller  670 , which are disposed on an outer circumference of a rotating OPC  630  in a clockwise direction in  FIG. 6 , that is, a rotation direction of the OPC  630 , a cassette  680  configured to feed a paper S, a transfer roller  690  configured to feed the paper S while allowing the paper P to be brought into contact with the intermediate transfer belt  650 , and a fusing unit  695  configured to fix a toner image transferred on the paper S to the paper S. 
     The operation of performing a color printing job of the image forming apparatus having the above configuration will be described. Light corresponding to Y image information is scanned on the OPC  630  by the laser scanning unit  620  to form an electrostatic latent image. Then, a Y toner of the Y developer  640 Y is attached to the electrostatic latent image so that a Y toner image is formed on the OPC  630 , and the Y toner image is transferred to the intermediated transfer belt  650 . When the formation of the Y toner image on the intermediate transfer belt  650  is completed, the laser scanning unit  620  scans light corresponding to M image information on the OPC  630  to form an electrostatic latent image. Then, an M toner contained in the developer  640 M is attached to the electrostatic latent image to form an M toner image on the OPC  630 , and the M toner image is transferred to the intermediate transfer belt  650 . At this time, a scanning time of the light corresponding to the M image information scanned from the laser scanning unit  620  is controlled by considering a feeding speed of the intermediate transfer belt  650  so that a front end of the Y toner image which has been already formed on the intermediate transfer belt  650  is identical with a front end of the M toner image which starts to be transferred on the intermediate transfer belt  650  from the OPC  630 . The above-described process is repeatedly performed on the C and K colors so that the Y, M, C, and K toner images are formed on the intermediate transfer belt  650  to overlap each other, and thus the overlapping toner images are transferred and fixed to the paper S to obtain a color image. 
     When the time to measure the CTD is determined, the control unit  140  controls the four developers  640 Y,  640 M,  640 C, and  640 K so that the test patches are developed in order of K, Y, M, and C test patches. 
       FIG. 7  is a view illustrating a rotary type image forming apparatus according to an exemplary embodiment. Referring to  FIG. 7 , a rotary type image forming apparatus  700  includes an OPC  730 , a laser scanning unit  720  configured to scan light to the OPC  730 , an intermediate transfer belt  750  disposed to be adjacent to the OPC  730 , and a rotating turret  740 . Four developers  740 Y,  740 M,  740 C, and  740 K are disposed at an angle of 90° on the turret  740  so that the four developers  740 Y,  740 M,  740 C, and  740 K sequentially face the OPC  730  according to the rotation of the turret  740  by 90°. 
     The operation of performing a color printing job of the rotary type image forming apparatus having the above-described configuration will now be described. When the turret  740  is rotated so that the Y developer  740 Y faces the OPC  730 , the light corresponding to the Y image information is scanned to the OPC by the laser scanning unit  720  to form an electrostatic latent image on the OPC  730 . Then, a Y toner contained in the Y developer  740 Y is attached to the electrostatic latent image to form a Y toner image on the OPC  730 , and the Y toner image is transferred to the intermediate transfer belt  750 . When the formation of the Y toner image on the intermediate transfer belt  750  is completed, the turret  740  is rotated by 90° so that the M developer  740 M faces the OPC  730 , and the laser scanning unit  720  scans light corresponding to M image information to the OPC  730  to form an electrostatic latent image. Then, an M toner contained in the M developer  740 M is attached to the electrostatic latent image to form an M toner image on the OPC  730 , and the M toner image is transferred to the intermediated transfer belt  750 . At this time, the scanning time of the light corresponding to the M image information scanned from the scanning unit  720  is controlled by considering a feeding speed of the intermediate transfer belt  750  so that a front end of the Y toner image which has been already formed on the intermediate transfer belt  750  is accurately identical with a front end of the M toner image which starts to be transferred on the intermediate transfer belt  750  from the OPC  730 . When the above-described process is repeatedly performed on the C and K colors so that the Y, M, C, and K toner images are formed on the intermediate transfer belt  750  to overlap each other, the overlapping toner images are transferred and fixed to the paper S so that a color image can be obtained. 
     In the rotary type image forming apparatus described above, the home position may be disposed between the K developer  740 K and the C developer  740 C as shown in  FIG. 7 . By disposing the home position between the K and C developers allows for the K developer to operate in a short time period in a black and while printing job. 
     In the related art, the test patches for measuring the CTDs are developed in the same order of the Y, M, C, and K developers as in performing the color printing job and the K developer  740 K, next to the home position, is passed by the turret  740 . Then, the Y, M, C, and K developers  740 Y,  740 M,  740 C, and  740 K perform the developing operations, and then the turret  740  is rotated in an reverse direction to return the K developer  740 K to the home position. Therefore, in the related image forming apparatus, even in the period in which the developer is not operated, unnecessary time, for example, the time required for the K developer to pass the home position and the time required for the K developer to rotate to return to the home position are required so that the time for measuring the CTD is increased. 
     However, the image forming apparatus of the exemplary embodiment controls the rotation of the turret  740  to develop the test patches so that the developers perform the developing operation sequentially from the K developer next to the home position so that the time required to measure the CTD can be reduced. 
       FIG. 8  is a view illustrating a test patch development sequence according to an exemplary embodiment. Referring to  FIG. 8 , it can be seen that the image forming apparatus according to the exemplary embodiment develops the test patches in order of the K, Y, M, and C developers. That is, when the test patches are developed in order of the K, Y, M, and C developers as shown in  FIG. 8 , the unnecessary operation is excluded as described above so that the time required to measure the CTD can be reduced. 
     Further, as compared with  FIGS. 1 and 8 , a distance from a first test patch of the Y test patch group to the last test patch of the K test patch group in  FIG. 1  is longer than a distance from a first test patch of the K test patch group to the last test patch of the C test patch group of  FIG. 8 . This is because the test patches are developed in order of the K, Y, M, and C developers so that the distance between the K test patch and the Y test patch can be minimized. Therefore, the image forming apparatus of the exemplary embodiment can further reduce the time required to measure the CTD. 
       FIG. 9  is a flowchart illustrating a method of measuring a CTD according to an exemplary embodiment. Referring to  FIG. 9 , first, test patches are developed on an OPC sequentially from a developer to be developable preferentially (operation S 901 ). Then, CTDs of the developed test patches are measured (operation S 902 ). A development variable is controlled using the measured CTDs (operation S 903 ). 
     Here, the developer to be developed preferentially may be a K developer. 
     That is, the plurality of developers may perform the developing operation in order of the Y, M, C, and K developers in the color printing job and the plurality of developers may perform the developing operation in order of the K, Y, M, and C developers. 
     In operation S 901 , all of the plurality of test patches in the plurality of developers may be developed on the OPC for 1 cycle in which the OPC is rotated once. 
     In operation S 902 , CTDs of the test patches formed on an intermediate transfer belt or the OPC may be measured using the CTD sensor. 
     Here, the image forming apparatus may have a cam type or a rotary type. 
     Further, in operation S 901 , when the image forming apparatus has the cam type, the plurality of developers may perform the development so that a distance between the K test patch and the Y test patch is shortest among distances of any two test patches included in all the test patches. 
     The method of measuring a CTD of the exemplary embodiment may further include performing a printing job using the controlled-development variable. 
     The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting the present inventive concept. The exemplary embodiments can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.