CONTROL PHASE OF OPC BASED ON ACCELERATION/DECELERATION SECTION OF MOTOR BACKGROUND [0001] An image forming device may drive a yellow (Y) OPC, a magenta (M) OPC, a cyan (C) OPC, and a black (K) OPC for Y, M, C, and K color print jobs. In that case, each OPC may be driven by a motor. For example, for each OPC, a motor may be mounted on the image forming device. More specifically, on the image forming device, a motor to drive the K OPC and a motor to drive the Y OPC, the M OPC, and the C OPC may be mounted. BRIEF DESCRIPTION OF THE DRAWINGS [0002] The present disclosure may be easily understood by the combination of the following detailed descriptions and accompanying drawings, in which reference numerals refer to structural elements. [0003] FIG. 1A is a conceptual diagram illustrating an operation of controlling a plurality of motors such that a phase difference between a plurality of OPCs is constant in an image forming device, according to an example. [0004] FIG.1B is a diagram illustrating a phase difference between a plurality of OPCs by controlling a plurality of motors in an image forming device, according to an example. [0005] FIG. 2 is a block diagram illustrating a configuration of an image forming device, according to an example. [0006] FIG. 3 is a diagram illustrating a phase difference between a plurality of OPCs, according to an example. [0007] FIG. 4 is a diagram illustrating a change in an acceleration time according to a load of a motor, according to an example. [0008] FIG. 5 is a diagram illustrating a deceleration section according to a stop operation and an acceleration section according to a start operation of a plurality of motors in an image forming device, according to an example. [0009] FIG. 6 is a diagram illustrating a process of controlling a phase between a plurality of OPCs by differently controlling start times of a plurality of motors in an image forming device, according to an example. [0010] FIG. 7 is a diagram illustrating a process of controlling a phase between a plurality of OPCs by differently controlling stop times of a plurality of motors in an image forming device, according to an example. [0011] FIG. 8 is a diagram illustrating a process of controlling a phase between a plurality of OPCs when a second OPC stops and restarts while a first OPC rotates in an image forming device, according to an example. [0012] FIG.9 is a flowchart of an operation method of an image forming device, according to an example. [0013] FIG. 10 is a diagram illustrating instructions stored in a computer- readable storage medium, according to an example. DETAILED DESCRIPTION [0014] Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings, in which examples of the present disclosure are shown such that those skilled in the art may easily work the present disclosure. However, the present disclosure may be implemented in various different forms and is not limited to the examples described herein. [0015] An "image forming device" may be any kind of device capable of performing an image forming operation, such as a printer, a scanner, a fax machine, a multi-function printer (MFP) or a display device, etc. The image forming device may also be a two dimensional (2D) image forming device or a 3D image forming device. An "image forming operation performed by the image forming device" may be an operation related to printing, copying, scanning, faxing, storage, transmission, coating, etc., or a combination of two or more of the operations described above. [0016] An organic photoconductor (OPC) may be one of the components in an electrophotographic (EP) image forming device. [0017] An "OPC drum unit" may include a plurality of OPC drums and a plurality of sensors to detect an OPC reference position of each of the plurality of OPC drums. For example, a sensor may be mounted on an OPC drum. In addition, the sensor may be a photo sensor. [0018] An "OPC rotation amount" may be a numerical value indicating a degree of rotation while an OPC rotates. For example, the OPC rotation amount may be represented based on a certain parameter. For example, the certain parameter may include at least one of a movement distance, a rotation time, and a rotation angle of the OPC drum (also referred to as “OPC”). The OPC rotation amount may be represented by one of the movement distance, the rotation time, and the rotation angle of the OPC. [0019] An "OPC phase" may refer to a parameter to identify a position from a reference position to a certain position of the OPC. For example, controlling a phase between a plurality of OPCs to be constant may refer to controlling a distance or an angle of a reference position of the plurality of OPCs to be constant. [0020] FIG. 1A is a conceptual diagram illustrating an operation of controlling a plurality of motors such that a phase difference between a plurality of OPCs is constant in an image forming device, according to an example. [0021] Referring to FIG. 1A, a plurality of OPC drums (also referred to as “OPCs”) 101, 102, 103, and 104, and motors 111 and 112 to drive the plurality of OPC drums 101, 102, 103, and 104 may be mounted on an image forming device 10. An example of an image forming device 10 is shown in FIG.2. [0022] For example, a first OPC drum 101 may by driven by a first motor 111, and a second OPC drum 102, a third OPC drum 103, and a fourth OPC drum 104 may be driven by a second motor 112. [0023] Also for example, a color of the first OPC drum 101 may be a reference color such as black (K). For example, the first OPC drum 101 may be black (K), the second OPC drum 102 may be yellow (Y), the third OPC drum 103 may be magenta (M), and the fourth OPC drum 104 may be cyan (C). [0024] As the first motor 111 rotates the first OPC drum 101, and the second motor 112 rotates the second OPC drum 102, the third OPC drum 103, and the fourth OPC drum 104, positions between respective colors may become misaligned due to a difference in a rotation amount of each OPC. [0025] For example, start characteristics of the motors 111 and 112 may be different due to a difference in a control method according to a capacity or due to a difference in a vendor of the motors 111 and 112, and thus, the rotation amount of each OPC may be different in an acceleration section or in a deceleration section of the motors 111 and 112. [0026] For example, depending on a magnitude of a load applied to the motors 111 and 112, the rotation amount of each OPC may be different in the acceleration section or in the deceleration section of the motors 111 and 112. For example, a load of the first OPC drum 111 is applied to the first motor 111, whereas loads of the second OPC drum 102, the third OPC drum 103, and the fourth OPC drum 103 are applied to the second motor 112. Therefore, in the example, the loads applied to the second motor 112 may be greater than the load applied to the first motor 111. [0027] For example, when start speed profiles of the motors 111 and 112 are controlled differently according to OPC characteristics, the rotation amount of each OPC may be different. [0028] Therefore, in order to correct the misalignment of the positions between the respective colors due to the difference in the rotation amount of each OPC, the image forming device 10 may control operations of the motors 111 and 112 to maintain a constant phase difference between the plurality of OPC drums 101, 102, 103, and 104. [0029] For example, as illustrated in FIG. 1A, the image forming device 10 (shown in the example of FIG. 2) may control the operations of the motors 111 and 112 to maintain a constant angle deviation between reference positions 121, 122, 123, and 124 of each of the plurality of OPC drums 101, 102, 103, and 104. [0030] The second OPC drum 102, the third OPC drum 103, and the fourth OPC drum 104 are driven by the second motor 112 in the example shown in FIG.1A, but the second OPC drum 102, the third OPC drum 103, and the fourth OPC drum 104 may be driven by motors different from each other. [0031] FIG.1B is a diagram illustrating a phase difference between a plurality of OPCs by controlling a plurality of motors in the image forming device 10, according to an example. [0032] Due to a difference in a rotation amount of each OPC, positions between respective colors may be misaligned. An image 150 of FIG. 1B is a graph showing variations in a periodic surface speed occurring in each OPC when the positions between the respective colors are misaligned. For example, a first line 131 represents variations in a surface speed of the first OPC drum 101, a second line 132 represents variations in a surface speed of the second OPC drum 102, a third line 133 represents variations in a surface speed of the third OPC drum 103, and a fourth line 134 represents variations in a surface speed of the fourth OPC drum 104. [0033] The image forming device 10 may control operations of the motors 111 and 112 to maintain a constant phase difference between the plurality of OPC drums 101, 102, 103, and 104. An image 160 of FIG. 1B is a graph showing variations in a periodic surface speed occurring in each OPC when the constant phase difference between the plurality of OPC drums 101, 102, 103, and 104 is maintained. [0034] FIGS. 2 to 9 illustrate an operation of controlling operations of a plurality of motors to maintain a constant phase difference between a plurality of OPC drums in the image forming device 10, according to an example. [0035] FIG.2 is a block diagram illustrating a configuration of the image forming device 10, according to an example. [0036] Referring to the image forming device 10 of FIG. 2, the image forming device 10 may include an OPC drum unit 210, a driving device 220, a memory 230, and a processor 240. However, the illustrated components are not essential components. The image forming device 10 may be implemented by more components than the illustrated components, and the image forming device 10 may be implemented by fewer components than the illustrated components. Hereinafter, the illustrated components will be described in detail. [0037] For example, the OPC drum unit 210 may include a plurality of OPC drums (also referred to as “OPCs”), and a plurality of sensors to detect an OPC reference position of each of the plurality of OPC drums. For example, the plurality of OPC drums may include a K OPC drum, a Y OPC drum, a M OPC drum, and a C OPC drum such as OPC drums 101, 102, 103, and 104 shown in FIG. 1A. For example, a photo sensor may be mounted on each of the plurality of OPC drums to detect the OPC reference position such as 121, 122, 123, and 124. For example, each of the plurality of sensors may detect a signal while each of the plurality of OPC drums rotates. The plurality of sensors may transmit detected signals to the processor 240. The processor 240 may detect a reference position of each of a plurality of OPCs based on the signals obtained from the plurality of sensors. [0038] For example, the driving device 220 may rotate the plurality of OPC drums such as OPC drums 101, 102, 103, and 104 shown in FIG. 1A. For example, the driving device 220 may be a motor, and may include first motor 111 and second motor 112 shown in FIG. 1A. The number of motors may be less than or equal to the number of the plurality of OPC drums. For example, the K OPC drum may be driven by a first motor, and the Y OPC drum, the M OPC drum, and the C OPC drum may be driven by a second motor. In that case, a sensor may be mounted on the K OPC drum and the Y OPC drum to detect the OPC reference position. [0039] For example, the memory 230 may store software or a program. For example, the memory 230 may store a program including instructions for an operation method of the image forming device 10 to control a plurality of motors such that a phase difference between the plurality of OPCs is constant considering an OPC rotation amount for a deceleration section at a stop time or an acceleration section at a start time. [0040] The memory 230 may include at least one type of storage medium of a flash memory type, a hard disk type, a multimedia card micro type, card type memory (e.g., SD or XD memory, etc.), random access memory (RAM), static random access memory (SRAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), programmable read- only memory (PROM), magnetic memory, a magnetic disk, and an optical disk. [0041] The processor 240 may control the operation of the image forming device 10, and may include at least one processor such as a central processing unit (CPU). The processor 240 may include at least one processor specialized for each function, or may be an integrated processor. [0042] The processor 240 may execute a program stored in the memory 230, read data or files stored in the memory 230, or store new data or files in the memory 230. The processor 240 may execute instructions stored in the memory 230 to perform an operation to control the plurality of motors such that the phase difference between the plurality of OPCs is constant based on the OPC rotation amount for the deceleration section at the stop time or the acceleration section at the start time. [0043] The processor 240 may calculate the OPC rotation amount based on the OPC reference position of each of the plurality of OPC drums, based on the signals detected by the plurality of sensors. [0044] For example, the OPC rotation amount may be a numerical value of a degree of rotation as an OPC rotates. For example, the OPC rotation amount may be represented based on a certain parameter. The certain parameter may include at least one of a movement distance, a rotation time, and a rotation angle of the OPC. The OPC rotation amount may be represented by the movement distance, the rotation time, and the rotation angle of the OPC. For example, the movement distance of the OPC may represent a movement distance of a surface of the OPC. That is, the movement distance of the OPC may be equal to a length of an image. For example, the rotation time of the OPC may be calculated by dividing the movement distance of the OPC by a speed of an image forming operation. For example, the rotation angle of the OPC may be calculated by dividing the movement distance of the OPC by a circumferential length of an OPC drum and multiplying the movement distance of the OPC by 360 degrees. [0045] For example, the processor 240 may calculate a difference in rotation amounts between the plurality of OPCs corresponding to the deceleration section according to a stop operation of the plurality of motors or the acceleration section according to a start operation of the plurality of motors. The processor 240 may control the plurality of motors such that the phase difference between the plurality of OPCs is constant based on the difference in the rotation amounts between the plurality of OPCs. [0046] For example, the processor 240 may control start times of the plurality of motors differently to control the phase difference between the plurality of OPCs to be constant based on the difference in the rotation amounts between the plurality of OPCs. [0047] More specifically, and for example, the first motor may rotate a first OPC drum, and the second motor may rotate a second OPC drum. When a rotation amount of the second OPC corresponding to a deceleration section according to a stop operation of the second motor is greater than a rotation amount of the first OPC corresponding to a deceleration section according to a stop operation of the first motor, the processor 240 may control a start time of the second motor to be later than a start time of the first motor based on a difference between the rotation amount of the second OPC and the rotation amount of the first OPC to control a phase difference between the first OPC and the second OPC to be constant. For example, the processor 240 may control the start time of the second motor such that the start time of the second motor is later than the start time of the first motor by the difference between the second OPC rotation amount and the first OPC rotation amount. [0048] As another example, when the rotation amount of the second OPC corresponding to the acceleration section according to the start operation of the second motor is greater than the rotation amount of the first OPC corresponding to the acceleration section according to the start operation of the first motor, the processor 240 may control the start time of the second motor to be earlier than the start time of the first motor based on the rotation amount of the second OPC and the rotation amount of the first OPC to control the phase difference between the first OPC and the second OPC to be constant. For example, the processor 240 may control the start time of the second motor such that the start time of the second motor is earlier than the start time of the first motor by the difference between the second OPC rotation amount and the first OPC rotation amount. [0049] For example, the processor 240 may control stop times of the plurality of motors differently to control the phase difference between the plurality of OPCs to be constant, based on the difference in the rotation amounts between the plurality of OPCs. [0050] More specifically, for example, the first motor may rotate the first OPC drum, and the second motor may rotate the second OPC drum. When the rotation amount of the second OPC corresponding to the deceleration section according to the stop operation of the second motor to rotate the second OPC drum is greater than the rotation amount of the first OPC corresponding to the deceleration section according to the stop operation of the first motor, the processor 240 may control the stop time of the second motor to be earlier than the stop time of the first motor based on the difference between the rotation amount of the second OPC and the rotation amount of the first OPC to control the phase difference between the first OPC and the second OPC to be constant. For example, the processor 240 may control the stop time of the second motor to be earlier than the stop time of the first motor by the difference between the second OPC rotation amount and the first OPC rotation amount. [0051] As another example, when the rotation amount of the second OPC corresponding to the acceleration section according to the start operation of the second motor is greater than the rotation amount of the first OPC corresponding to the acceleration section according to the start operation of the first motor, the processor 240 may control the stop time of the second motor to be later than the stop time of the first motor based on the rotation amount of the second OPC and the rotation amount of the first OPC to control the phase difference between the first OPC and the second OPC to be constant. For example, the processor 240 may control the stop time of the second motor to be later than the stop time of the first motor by the difference between the second OPC rotation amount and the first OPC rotation amount. [0052] For example, rotation amounts of the plurality of OPCs may be represented by one of rotation angles, surface movement distances, and rotation times of the plurality of OPCs. [0053] For example, the plurality of motors may include the first motor to rotate the first OPC drum and the second motor to rotate the second OPC drum. When a load driven by the first motor is greater than a load driven by the second motor, the processor 240 may control the acceleration section according to the start operation or the deceleration section according to the stop operation of the first motor to be longer than the acceleration section according to the start operation or the deceleration section according to the stop operation of the second motor. [0054] For example, the processor 240 may control an interval between the OPC reference positions of each of the plurality of OPCs to be constant based on the difference in the rotation amounts of the plurality of OPCs. [0055] By controlling the phase difference between the plurality of OPCs to be constant in the image forming device 10, an idle time of an OPC drum may be decreased and the life of the OPC drum may be prolonged. [0056] FIG. 3 is a diagram illustrating a phase difference between a plurality of OPCs, according to an example. [0057] Referring to FIG.3, a first signal 310 represents a signal obtained from a first sensor while a first OPC drum rotates. The first signal 310 may include sections 311 and 312 indicating a reference position of a first OPC. A section 313 in the first signal 310 may represent a signal for one rotation of the first OPC. That is, the section 313 in the first signal 310 may represent a signal for a 360 degree rotation of the first OPC. [0058] A second signal 320 may represent a signal obtained from a second sensor while a second OPC rotates. The second signal 320 may include seconds 321 and 322 indicating a reference position of the second OPC. A section 323 in the second signal 320 may be a signal representing a phase difference between the first OPC and the second OPC. For example, the section 323 may represent a signal for a certain angle rotation of the second OPC. Here, the certain angle may represent the phase difference between the first OPC and the second OPC. [0059] FIG. 4 is a diagram illustrating variations in an acceleration time according to a load of a motor, according to an example. [0060] Referring to FIG.4, the acceleration time when the motor starts may vary depending on the load driven by the motor, even under the same speed condition as the same motor. Referring to FIG. 4, the acceleration time of the motor may increase as a magnitude of the load driven by the motor, i.e., torque, increases. In addition, as the revolutions per minute (rpm) and torque of the motor increase, the acceleration time of the motor may be increased. Therefore, a phase difference may be generated between OPCs by a difference between acceleration sections of each motor. [0061] FIG. 5 is a diagram illustrating a deceleration section according to a stop operation of a plurality of motors and an acceleration section according to a start operation of the plurality of motors in the image forming device 10, according to an example. [0062] For example, the acceleration section or the deceleration section of the plurality of motors may vary because start characteristics of a motor may vary due to a difference in a control method according to a capacity or a vendor of the motor to drive an OPC. In addition, even if the plurality of motors are the same, the acceleration section or the deceleration section of the plurality of motors may vary due to a difference in a magnitude of a load or inertia applied to each motor. Moreover, when start speed profiles of the plurality of motors are differently controlled, the acceleration sections or deceleration sections of the plurality of motors may vary. [0063] Referring to FIG. 5, a first motor may rotate a first OPC drum, and a second motor may rotate a second OPC drum. For example, at the end of an image forming operation, the first motor and the second motor may stop at the same time after deceleration. A graph 511 represents speed variations according to a stop operation of the first motor, and a graph 512 represents speed variations according to a stop operation of the second motor. In that case, a deceleration section 514 according to the stop operation of the second motor may be longer than a deceleration section 513 according to the stop operation of the first motor. That is, a rotation amount of a second OPC corresponding to the deceleration section according to the stop operation of the second motor may be greater than a rotation amount of a first OPC corresponding to the deceleration section according to the stop operation of the first motor. [0064] Therefore, the processor 240 may control a phase interval between the first OPC and the second OPC to be constant by using the OPC rotation amounts according to a difference between the deceleration section of the first motor and the deceleration section of the second motor. [0065] In addition, a graph 521 represents speed variations according to a start operation of the first motor, and a graph 522 represents speed variations according to a start operation of the second motor. In that case, an acceleration section 524 according to the start operation of the second motor may be longer than an acceleration section 523 according to the start operation of the first motor. In other words, a rotation amount of the second OPC corresponding to the acceleration section according to the start operation of the second motor may be greater than a rotation amount of the first OPC corresponding to the acceleration section according to the start operation of the first motor. [0066] Therefore, the processor 240 may control the phase interval between the first OPC and the second OPC to be constant by using the OPC rotation amounts according to a difference between the acceleration section of the first motor and the acceleration section of the second motor. [0067] FIG. 6 is a diagram illustrating a process of controlling a phase between a plurality of OPCs by differently controlling start times of a plurality of motors in the image forming device 10, according to an example. [0068] A first motor may rotate a first OPC drum and a second motor may rotate a second OPC drum. Referring to FIG. 6, a first signal 610 represents a signal obtained from a first sensor while the first OPC drum rotates. The first signal 610 represents a sensor value over time. For example, a sensor value for a reference position of a first OPC may be lower than a sensor value for a position other than the reference position of the first OPC. A first graph 620 represents speed variations of the first motor. At the end of an image forming operation, the processor 240 may control an operation of the first motor such that the first motor stops operating. The first motor may perform a stop operation according to a stop command. Thereafter, at the beginning of the image forming operation, the processor 240 may control the operation of the first motor such that the first motor starts operating. The first motor may perform a start operation according to a start command. [0069] For example, in the first signal 610, a position 611 indicates a time point at which the reference position of the first OPC is detected. In addition, a position 612 indicates a start point of a deceleration section according to the stop operation of the first motor. The position 612 indicates a time point at which the stop command of the first motor is received. In addition, a position 613 indicates an end point of the deceleration section according to the stop operation of the first motor. [0070] For example, a section 621 in the first graph 620 represents a speed from the time when the reference position of the first OPC is detected to the time when the stop command of the first motor is received. For example, the section 621 in the first graph 620 may be represented by α. In addition, a section 622 represents the deceleration section according to the stop operation of the first motor. The section 622 represents a section in which a speed change occurs from a time point at which the stop command of the first motor is received to a time point at which the first motor is stopped. For example, the section 622 in the first graph 620 may be represented by K Decel. In addition, a section 623 represents a stop section of the first motor calculated from the reference position of the first OPC. For example, the section 623 in the first graph 620 may be represented by K Stop, and may be expressed by Equation 1. [0071] ^Equation 1^ [0072] K Stop = α + K Decel [0073] For example, a position 614 in the first signal 610 indicates a start point of an acceleration section according to the start operation of the first motor. The position 614 may indicate a time point at which the start command of the first motor is received. In addition, a position 615 indicates an end point of the acceleration section according to the start operation of the first motor. In the first signal 610, a position 616 may indicate a time point at which the reference position of the first OPC is detected. A position 617 may indicate a time point separated from the reference position of the first OPC by a target section indicating a phase difference between the first OPC and the second OPC. [0074] For example, a section 624 in the first graph 620 represents the acceleration section according to the start operation of the first motor. For example, the section 624 in the first graph 620 may be represented by K Accel. The section 624 may represent a section in which a speed change occurs from the time when the start command of the first motor is received to the time when the first motor reaches a predetermined speed. A section 625 may represent the target section indicating the phase difference between the first OPC and the second OPC. For example, the section 625 in the first graph 620 may be represented by Target. For example, a total movement section K Total of the first motor may be expressed by Equation 2. [0075] [Equation 2] [0076] K Total = K Stop + K Accel - Target [0077] A second signal 630 represents a signal obtained from a second sensor while the second OPC drum rotates. The second signal 630 represents a sensor value over time. For example, a sensor value for a reference position of the second OPC may be lower than a sensor value for a position other than the reference position of the second OPC. A second graph 640 represents speed variations of the second motor. At the end of the image forming operation, the processor 240 may control an operation of the second motor such that the second motor stops operating. The second motor may perform a stop operation according to a stop command. Thereafter, at the beginning of the image forming operation, the processor 240 may control the operation of the second motor such that the second motor starts operating. The second motor may perform a start operation according to a start command [0078] For example, in the second signal 630, a position 631 indicates a time point at which the reference position of the second OPC is detected. A position 632 indicates a start point of a deceleration section according to the stop operation of the second motor. The position 632 indicates a time point at which the stop command of the second motor is received. A position 633 indicates an end point of the deceleration section according to the stop operation of the second motor. [0079] For example, a section 641 in the second graph 640 represents a speed from the time when the reference position of the second OPC is detected to the time when the stop command of the second motor is received. For example, the section 641 in the second graph 640 may be represented by β. A section 642 represents the deceleration section according to the stop operation of the second motor. The section 642 may indicate a section in which a speed change occurs from a time point at which the stop command of the second motor is received to a time point at which the second motor stops operating. For example, the section 642 in the second graph 640 may be represented by C Decel. A section 643 may indicate a stop section of the second motor calculated from the reference position of the second OPC. For example, the section 643 in the second graph 640 may be represented by C Stop, and may be expressed by Equation 3. [0080] [Equation 3] [0081] C Stop = β + C Decel [0082] For example, a position 634 in the second signal 630 indicates a start point of the acceleration section according to the start operation of the first motor. A position 635 may indicate a time point at which the start command of the second motor is received, and may indicate a start point of an acceleration section according to the start operation of the second motor. In addition, a position 636 may indicate an end point of the acceleration section according to the start operation of the second motor. Moreover, in the second signal 630, a position 637 may indicate a time point at which the reference position of the second OPC is detected. [0083] For example, in the second graph 640, a section 644 represents an interval between a start time of the first motor and a start time of the second motor. A section 645 indicates the acceleration section according to the start operation of the second motor. For example, the section 645 in the second graph 640 may be represented by C Accel. The section 645 may indicate a section in which a speed change occurs from a time point at which the start command of the second motor is received to a time point at which the second motor reaches a predetermined speed. For example, a total movement section C Total of the second motor may be expressed by Equation 4. [0084] [Equation 4] [0085] C Total = C Stop + C Accel [0086] The processor 240 may control the plurality of motors such that a phase difference between the plurality of POCs based on rotation amounts between the plurality of OPCs corresponding to a deceleration section according to a stop operation of the plurality of motors. [0087] Referring to the first graph 620 and the second graph 640 of FIG. 6, the deceleration section 642 according to the stop operation of the second motor is larger than the deceleration section 622 according to the stop operation of the first motor. Therefore, a rotation amount of the second OPC according to the deceleration section 642 of the second motor is greater than a rotation amount of the first OPC according to the deceleration section 622 of the first motor. [0088] Since a movement amount of the second OPC according to the deceleration section 642 of the second motor is greater than a movement amount of the first OPC according to the deceleration section 622 of the first motor while the first motor and the second motor stop operating, the processor 240 may control the operation of the second motor such that a start time of the second motor is later than a start time of the first motor by a difference between the movement amount of the second OPC corresponding to the deceleration section 642 of the second motor and the movement amount of the first OPC corresponding to the deceleration section 622 of the first motor. For example, a movement section Δ corresponding to the section 644 may be calculated by Equation 5. [0089] [Equation 5] [0090] Δ = C Total - K Total [0091] In other words, after the start operation of the first motor begins and a time corresponding to the section 644 elapses, the processor 240 may control the operation of the second motor to perform the start operation of the second motor. [0092] The movement amount corresponding to the acceleration section or deceleration section of the first motor and the second motor may be calculated by a movement amount of an actual acceleration section or deceleration section of an OPC by using a speed detection means, or may be calculated by a predetermined profile calculation formula. [0093] FIG.7 is a diagram illustrating a process of controlling phases between a plurality of OPCs by differently controlling stop times of a plurality of motors in the image forming device 10. [0094] A first motor may rotate a first OPC drum and a second motor may rotate a second OPC drum. Referring to FIG. 7, a first signal 710 represents a signal obtained from a first sensor as the first OPC drum rotates. The first signal 710 represents a sensor value over time. A first graph represents a speed change of the first motor. At the end of an image forming operation, the processor 240 may control an operation of the first motor such that the first motor stops. In that case, after the image forming operation is finished and a reference position of a first OPC is detected, the processor 240 may control the operation of the first motor to stop the first motor. The first motor may perform a stop operation according to a stop command. Thereafter, at the beginning of the image forming operation, the processor 240 may control the operation of the first motor such that the first motor starts. [0095] For example, a position 711 in the first signal 710 indicates a time point at which an end command of the image forming operation is received. In addition, a position 712 indicates a time point immediately before the reference position of the first OPC is detected. A section 711 - 5 is a section from a time point when the end command of the image forming operation is received to a time point immediately before the reference position of the first OPC is detected, and represents a section in which the first OPC additionally rotates such that the first motor performs the stop operation based on the reference position of the first OPC. In addition, a position 713 indicates a time point at which the reference position of the first OPC is detected. A position 714 indicates a time point at which the stop command of the first motor is received. That is, the position 714 indicates a start point of a deceleration section according to the stop operation of the first motor. A position 715 indicates an end point of the deceleration section according to the stop operation of the first motor. [0096] For example, a section 721 in the first graph 720 represents a speed from the time point when the reference position of the first OPC is detected to the time point when the stop command of the first motor is received. For example, the section 721 in the first graph 720 may be represented by α. In addition, a section 722 represents a section in which a speed change occurs from the time when the stop command of the first motor is received to the time when the first motor stops. For example, the section 722 in the first graph 720 may be represented by K Decel. A section 723 indicates a stop section of the first motor calculated from the reference position of the first OPC. For example, the section 723 in the first graph 720 may be represented by K Stop, and may be expressed by Equation 6. [0097] [Equation 6] [0098] K Stop = α + K Decel [0099] For example, a position 716 in the first signal 710 indicates a start point of an acceleration section according to a start operation of the first motor. The position 716 indicates a time point at which a start command of the first motor is received. A position 717 indicates an end point of the acceleration section according to the start operation of the first motor. A position 718 in the first signal 710 indicates a time point at which the reference position of the first OPC is detected. A position 719 may indicate a point from the reference position of the first OPC to a target section indicating a phase difference between the first OPC and the second OPC. [0100] For example, a section 724 in the first graph 710 represents the acceleration section according to the start operation of the first motor. For example, the section 724 in the first graph 720 may be represented by K Accel. The section 724 may indicate a section in which a speed change occurs from a time when the start command of the first motor is received to a time when the first motor reaches a predetermined speed. A section 725 may represent a target section indicating the phase difference between the first OPC and the second OPC. For example, the section 725 in the first graph 720 may be represented by Target. For example, a total movement section K Total of the first motor may be expressed by Equation 7. [0101] [Equation 7] [0102] K Total = K Stop + K Accel [0103] A second signal 730 represents a signal obtained from a second sensor as the second OPC drum rotates. The second signal 730 represents a sensor value over time. A second graph 740 represents a speed change of the second motor. At the end of the image forming operation, the processor 240 may control an operation of the second motor to stop the second motor. In that case, after the image forming operation is finished and a reference position of the second OPC is detected, the processor 240 may control the operation of the second motor to stop the second motor. The second motor may perform a stop operation according to a stop command. Thereafter, at the beginning of the image forming operation, the processor 240 may control the operation of the second motor to operate the second motor. The second motor may perform a start operation according to a start command. [0104] A position 731 in the second signal 730 indicates a time point at which an end command of the image forming operation is received. A position 732 indicates a time point immediately before a reference position of the second OPC is detected. A section 731-5 is a section from the time point when the end command of the image forming operation is received to the time point immediately before the reference position of the second OPC is detected, and represents a section in which the second OPC further rotates to perform the stop operation of the second motor based on the reference position of the second OPC. A position 733 indicates a time point at which the reference position of the second OPC is detected. A position 734 indicates a time point at which the stop command of the second motor is received. A position 735 indicates an end point of the deceleration section according to the stop operation of the second motor. [0105] For example, a section 741 in the second graph 740 indicates a speed from the time point at which the reference position of the second OPC is detected to the time point at which the stop command of the second motor is received. The section 742 in the second graph 740 may be represented by β. A section 742 represents a section in which a speed change occurs from the time when the stop command of the second motor is received to the time when the second motor stops. The section 742 in the second graph may be represented by C Decel. A section 743 represents a stop section of the second motor calculated from the reference position of the second OPC. In the second graph 740, the section 743 may be represented by C Stop, and may be expressed by Equation 8. [0106] [Equation 8] [0107] C Stop = β + C Decel [0108] A position 736 in the second signal 730 indicates a start point of the acceleration section according to the start operation of the second motor. The position 736 may indicate a time point at which the start command of the second motor is received. A position 737 indicates an end point of the acceleration section according to the start operation of the second motor. A position 738 indicates a time point at which the reference position of the second OPC is detected. [0109] A section 744 in the second graph 740 represents the acceleration section according to the start operation of the second motor. The section 744 in the second graph 740 may be represented by C Accel. The section 744 may indicate a section in which a speed change occurs from the time when the start command of the second motor is received to the time when the second motor reaches a predetermined speed. A total movement section C Total of the second motor may be expressed by Equation 9. [0110] [Equation 9] [0111] C Total = C Stop + C Accel [0112] A movement distance of the target section representing the phase difference between the first OPC and the second OPC may be expressed as in Equation 10. [0113] [Equation 10] [0114] Target = K Total - C Total [0115] The processor 240 may control the plurality of motors such that the phase difference between the plurality of OPCs is constant based on a difference in rotation amounts between the plurality of OPCs corresponding to the acceleration section according to the start operation of the plurality of motors. [0116] Referring to the graph 720 and graph 740 of FIG. 7, the acceleration section 744 according to the start operation of the second motor is larger than the acceleration section 724 according to the start operation of the first motor. Therefore, a rotation amount of the second OPC in the acceleration section 744 of the second motor is greater than a rotation amount of the first OPC in the acceleration section 724 of the first motor. [0117] Since a movement amount of the second OPC according to the acceleration section 744 of the second motor is greater than a movement amount of the first OPC according to the acceleration section 724 of the first motor as the first motor and the second motor start operating, the processor 240 may control operations of the first motor and the second motor such that a stop time of the second motor is delayed than a stop time of the first motor based a difference between the movement amount of the second OPC corresponding to the acceleration section 744 of the second motor and the movement amount of the first OPC corresponding to the acceleration section 724 of the first motor. A movement section β from the reference position of the second OPC to the start point of the deceleration section of the second motor may be calculated by Equation 11. In addition, a movement section α from the reference position of the first OPC to the start point of the deceleration section of the first motor may be calculated by Equation 12. [0118] [Equation 11] [0119] β = K Total – Target – (C Accel + C Decel) [0120] [Equation 12] [0121] α > Target + (C Accel + C Decel) - (K Accel + K Decel), (β>0) [0122] For example, after the image forming operation is finished, the reference position of the first OPC is detected, and a time corresponding to the section 721 elapses, the processor 240 may control the operation of the first motor to perform the stop operation of the first motor. After the image forming operation is finished, the reference position of the second OPC is detected, and a time corresponding to the section 741 elapses, the processor 240 may also control the operation of the second motor to perform the stop operation of the second motor. [0123] FIG. 8 is a diagram illustrating a process of controlling a phase between a plurality of OPCs when a second OPC stops and restarts operating while a first OPC rotates in the image forming device 10, according to an example. [0124] A first motor may rotate a first OPC drum and a second motor may rotate a second OPC drum. For example, it is assumed that the first OPC is K and the second OPC is Y. It is also assumed that the image forming device 10 performs a color print job while performing a mono print job. [0125] Referring to FIG.8, a first signal 810 represents a signal obtained from a first sensor while the first OPC drum rotates. A first graph 820 represents a speed of the first motor. For example, even if the image forming device 10 changes from the mono print job to the color print job, the first OPC rotates continuously, so that a reference position of the first OPC is periodically detected from the first signal 810. A position 811 in the first signal 810 indicates a time point at which the reference position of the first OPC is detected immediately before the color print job is performed. Since the first motor continuously rotates the first OPC, as shown in the first graph 810, the speed of the first motor may be constant. For example, a movement section of the first motor may be expressed by Equation 13. [0126] [Equation 13] [0127] K Total = C Stop + C Accel - Target [0128] A second signal 830 represents a signal obtained from a second sensor while the second OPC drum rotates. A second graph 840 represents speed variations of the second motor. In the second signal 830, a position 831 indicates a time point at which a reference position of the second OPC is detected. A position 832 indicates a time point at which a stop command of the second motor is received. That is, the position 832 indicates a start point of a deceleration section according to a stop operation of the second motor. A position 833 indicates an end point of the deceleration section according to the stop operation of the second motor. [0129] A section 841 in the second graph 840 is a section representing a speed from the time when the reference position of the second OPC is detected to the time when the stop command of the second motor is received. A section 842 represents a section in which a speed change occurs from the time when the stop command of the second motor is received to the time when the second motor stops operating. A section 843 represents a stop section of the second motor calculated from the reference position of the second OPC. [0130] A position 834 in the second signal 830 indicates a time point at which the reference position of the first OPC is detected immediately before the color print job is performed. A position 835 indicates a time point at which a start command of the second motor is received. That is, the position 835 indicates a start point of an acceleration section according to a start operation of the second motor. A position 836 indicates an end point of the acceleration section according to the start operation of the second motor. [0131] The processor 240 may calculate a time corresponding to a predetermined phase interval between the first OPC and the second OPC, and may control an operation of the second motor according to the calculated time. More specifically, a section 844 in the second graph 840 indicates an interval between the time when the reference position of the first OPC is detected and a start time of the second motor just before the color print job is performed. A section 845 represents the acceleration section according to the start operation of the second motor. For example, a movement section of the second motor may be expressed by Equation 14. [0132] [Equation 14] [0133] C Total = C Accel [0134] A movement time of the first motor may be calculated by Equation 15. A speed of an image forming operation may be represented by process speed. [0135] [Equation 15] [0136] K Time = K Total / process speed [0137] In addition, a movement time of the second motor may be represented by C Time. A time γ from the time when the reference position of the first OPC is detected to the start time of the second motor just before the color print job is performed may be calculated by Equation 16. [0138] [Equation 16] [0139] γ = K Time - C Time [0140] FIG.9 is a flowchart of an operation method of the image forming device 10, according to an example. [0141] Referring to FIG. 9, in operation 910 of the image forming device 10, the image forming device 10 may obtain signals to detect OPC reference positions of each of a plurality of OPC drums from a plurality of sensors while the plurality of OPC drums are rotated by a plurality of motors. [0142] In operation 920 of the image forming device 10, the image forming device 10 may calculate OPC rotation amounts based on the OPC reference positions of each of the plurality of OPC drums, based on the signals obtained from the plurality of sensors. [0143] In operation 930 of the image forming device 10, the image forming device 10 may control the plurality of motors such that a phase difference between a plurality of OPCs is constant based on a difference in rotation amounts between a plurality of OPC sections corresponding to a deceleration section according to a stop operation of the plurality of motors or an acceleration section according to a start operation of the plurality of motors. [0144] For example, the image forming device 10 may control the phase difference between the plurality of OPCs by differently controlling start times of the plurality of motors based on the difference in the rotation amounts between the plurality of OPCs. [0145] More specifically, for example, a first motor may rotate a first OPC drum and a second motor may rotate a second OPC drum. When a rotation amount of a second OPC corresponding to a deceleration section according to a stop operation of the second motor is greater than a rotation amount of a first OPC corresponding to a deceleration section according to a stop operation of the first motor, the image forming device 10 may control a phase difference between the first OPC and the second OPC to be constant by controlling a start time of the second motor to be later than a start time of the first motor based on a difference between the rotation amount of the first OPC and the rotation amount of the second OPC. For example, the image forming device 10 may control the start time of the second motor to be later than the start time of the first motor by the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. [0146] As another example, when a rotation amount of the second OPC corresponding to an acceleration section according to a start operation of the second motor is greater than a rotation amount of the first OPC corresponding to an acceleration section according to a start operation of the first motor, the image forming device 10 may control the phase difference between the first OPC and the second OPC to be constant by controlling the start time of the second motor to be earlier than the start time of the first motor based on the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. For example, the image forming device 10 may control the start time of the second motor to be earlier than the start time of the first motor by the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. [0147] For example, the image forming device 10 may control the phase difference between the plurality of OPCs by differently controlling stop times of the plurality of motors based on the difference in the rotation amounts between the plurality of OPCs. [0148] More specifically, for example, the first motor may rotate the first OPC drum and the second motor may rotate the second OPC drum. When the rotation amount of the second OPC corresponding to the deceleration section according to the stop operation of the second motor to rotate the second OPC drum is greater than the rotation amount of the first OPC corresponding to the deceleration section according to the stop operation of the first motor, the image forming device 10 may control the phase difference between the first OPC and the second OPC to be constant by controlling a stop time of the second motor to be earlier than a stop time of the first motor based on the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. For example, the image forming device 10 may control the stop time of the second motor to be earlier than the stop time of the first motor by the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. [0149] As another example, when a rotation amount of the second OPC corresponding to an acceleration section according to a start operation of the second motor is greater than a rotation amount of the first OPC corresponding to an acceleration section according to a start operation of the first motor, the image forming device 10 may control the phase difference between the first OPC and the second OPC to be constant by controlling the start time of the second motor to be later than the start time of the first motor based on the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. For example, the image forming device 10 may control the stop time of the second motor to be later than the stop time of the first motor by the difference between the rotation amount of the second OPC and the rotation amount of the first OPC. [0150] For example, the rotation amounts of the plurality of OPCs may be represented by one of rotation angles, surface movement distances, and rotation times of the plurality of OPCs. [0151] For example, the plurality of motors may include the first motor to rotate the first OPC drum and the second motor to rotate the second OPC drum. When a load driven by the first motor is greater than a load driven by the second motor, the image forming device 10 may control the acceleration section according to the start operation or the deceleration section according to the stop operation of the first motor to be longer than the acceleration section according to the start operation or the deceleration section according to the stop operation of the second motor. [0152] For example, the image forming device 10 may control an interval between the OPC reference position of each of the plurality of OPCs to be constant based on the difference in the rotation amounts between the plurality of OPCs. [0153] FIG. 10 is a diagram illustrating instructions stored in a computer- readable storage medium, according to an example. [0154] A computer-readable storage medium 1000 illustrated in FIG. 10 may store instructions for an operation method of the image forming device 10 to control a plurality of motors such that a phase difference between a plurality of OPCs is constant based on an OPC rotation amount for a deceleration section at a stop or an acceleration section at a start of the image forming device 10. The computer-readable storage medium 1000 may be a non-transitory computer-readable storage medium. [0155] For example, the computer-readable storage medium 1000 may store instructions 1010 to obtain signals for detecting OPC reference positions of each of a plurality of OPC drums from a plurality of sensors while the plurality of OPC drums are rotated by a plurality of motors, instructions 1020 to calculate OPC rotation amounts based on the OPC reference positions of each of the plurality of OPC drums based on the signals obtained from the plurality of sensors, and instructions 1030 to control the plurality of motors such that a phase difference between the plurality of OPCs is constant based on a difference in the rotation amounts between the plurality of OPCs corresponding to a deceleration section according to a stop operation or an acceleration section according to a start operation of the plurality of motors. [0156] The above-described operation method of the image forming device 10 may be implemented in the form of a computer-readable storage medium to store instructions or data executable by a computer or a processor. The above- described operation method of the image forming device 10 may be written in a program executable by a computer, and may be implemented in a general- purpose digital computer that operates such a program using a computer- readable storage medium. Examples of such a computer-readable storage medium may include read-only memory (ROM), random-access memory (RAM), flash memory, compact disc (CD)-ROMs, CD-recordables (Rs), CD+Rs, CD- rewritables (RWs), CD+RWs, and digital versatile disc (DVD)-ROMs, DVD-Rs, DVD+Rs, DVD-RWs, DVD+RWs, DVD-RAMs, blu-ray disc (BD)-ROMs, BD-Rs, BD-recordable low to highs (R LTHs), BD-REs, magnetic tapes, floppy disks, magneto-optical data storage devices, optical data storage devices, hard disks, solid-state disks (SSDs), and any device capable of storing instructions or software, associated data, data files, and data structures, and providing a processor or computer with instructions or software, associated data, data files, and data structures such that the processor or computer may execute the instructions. [0157] It should be understood that examples described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. While one or more examples have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.