Abstract:
An image-forming device includes an image bearing member, a data providing unit, an image-forming unit, a detecting unit, a first calculating unit, a determining unit, and a second calculating unit. The image bearing member has a surface moving at a surface speed. The data providing unit provides pattern data indicative of a target test pattern to be formed on a target position of the surface in response to a test instruction. The image-forming unit forms an actual test pattern on an actual position of the surface in accordance with the pattern data provided from the data providing unit, and configure to form an image in response to an image-forming instruction. The detecting unit detects the actual position. The first calculating unit calculates a deviation of the actual position from the target position. The determining unit performs a first determination of whether the surface speed when the data providing unit receives the test instruction is stable or unstable. The second calculating unit calculates a correction amount based on the deviation and the first determination. The image-forming unit forms, in response to the image-forming instruction, an image on a position of the surface. The position is corrected based on the correction amount.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2007-082476 filed Mar. 27, 2007. The entire content of each of these priority applications is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image-forming device. 
     BACKGROUND 
     An image-forming device can form an image at an image formation position on a recording medium, which deviates from the right position due to a physical shock from the outside or another cause, for example. For this reason, some conventional image-forming devices have a function for correcting the deviation of the image formation position based on a test pattern. Responsive to a test instruction, such image-forming device forms a test pattern for registration on a belt which drives rotationally to transfer recording medium. Then, the image-forming device detects the position of the pattern formed on the belt with an optical sensor or another device, and corrects the image formation position based on the result of the detection. After that, responsive to an image forming instruction, the image-forming device forms an image at the corrected image formation position. 
     Japanese unexamined patent application publication No. 2001-228670 describes that variation in the temperature of the inside of the image-forming device affects the image formation position. Especially, the fixing unit is the major cause of the variation in the inner temperature of the image-forming device. Thus, the image-forming device described in Japanese unexamined patent application publication No. 2001-228670 forms a pattern at the time when a predetermined time has elapsed since the fixing unit had been turned on. Then, the image-forming device detects the pattern and corrects the image formation position based on the result of the detection. 
     In addition, Japanese unexamined patent application publication No. HEI11-231750 discloses an image-forming device that obtains information of the position of a pattern and the temperature of the inside of the apparatus at a predetermined timing, and substitutes the obtained information and temperature into a predetermined function to calculate a position deviation amount that occurs after a predetermined time period has elapsed. 
     SUMMARY 
     In recent years, a large demand is arising to reduce time between the time when the power switch is turned on or the image forming instruction is issued to the image-forming device that is in a sleep state and the time when an image forming process is started. However, if the image formation process is started immediately after the turning on the image-forming device, the image formation process is compelled to be carried out in a state where the inner temperature of the apparatus is unstable since the temperature of the fixing unit has not reach the target temperature. If the image formation position is corrected based on a pattern detected in the state in which the inner temperature of the apparatus is stable at the target temperature, the image formation position relative to a recording medium deviates from the right position due to the difference between the inner temperature when the pattern is detected (the test instruction is issued) and the inner temperature when the image formation instruction is issued. To avoid this problem, it is necessary to take into consideration the state in which the image-forming device has detected the pattern. 
     The image-forming device disclosed in Japanese unexamined patent application publication No. 2001-228670 detects the pattern formed when a predetermined time period has passed since the fixing unit had turned on. However, if the correction result obtained based on the detection is used without being modified under a different temperature condition, the image formation position relative to a recording medium also deviates from the right position. Further, the image-forming device disclosed in Japanese unexamined patent application publication No. HEI11-231750 does not consider the state of the image-forming device when the pattern position information to be substituted into the calculation function has been detected. 
     In view of the above-described drawbacks, it is an objective of the present invention to provide an image-forming device capable of restraining a correction error of an image formation position which occurs due to the difference in a state of the image-forming device between the time of a test instruction and the time of an image forming instruction. 
     In order to attain the above and other objects, the present invention provides an image-forming device including an image bearing member, a data providing unit, an image-forming unit, a detecting unit, a first calculating unit, a determining unit, and a second calculating unit. The image bearing member has a surface moving at a surface speed. The data providing unit provides pattern data indicative of a target test pattern to be formed on a target position of the surface in response to a test instruction. The image-forming unit forms an actual test pattern on an actual position of the surface in accordance with the pattern data provided from the data providing unit, and configure to form an image in response to an image-forming instruction. The detecting unit detects the actual position. The first calculating unit calculates a deviation of the actual position from the target position. The determining unit performs a first determination of whether the surface speed when the data providing unit receives the test instruction is stable or unstable. The second calculating unit calculates a correction amount based on the deviation and the first determination. The image-forming unit forms, in response to the image-forming instruction, an image on a position of the surface. The position is corrected based on the correction amount. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a sectional side view illustrating the schematic configuration of a printer according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram schematically showing the electrical configuration of the printer of  FIG. 1 ; 
         FIG. 3  is a diagram schematically showing patterns formed on a belt while the belt makes half revolution in a first operation; 
         FIG. 4  is a diagram schematically showing patterns formed on the belt while the belt makes a revolution in the first operation; 
         FIG. 5  is a diagram schematically showing patterns formed on the belt while the belt makes half revolution in a second operation; 
         FIG. 6  is a diagram schematically showing patterns formed on the belt while the belt makes a revolution in the second operation; 
         FIG. 7  is a graph showing an experiment result of variation in mark positions of measurement colors from the time when the power switch is turned on or the like; 
         FIG. 8  is a graph showing the relationship of the variation in a mark position with execution timings of the first operation and the second operation; 
         FIG. 9  is a flow diagram showing a succession of procedural steps performed to correct a position deviation; 
         FIG. 10  is a flow diagram showing a succession of procedural steps of a test process according to a second embodiment of the present invention; 
         FIG. 11  is a flow diagram showing a succession of procedural steps of an image formation process; and 
         FIG. 12  is a graph showing the relationship of the variation in a mark position with execution timings of the first operation and the second operation. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A first embodiment of the present invention will be described with reference to  FIGS. 1-9 . 
     (The Entire Configuration of a Printer) 
       FIG. 1  is a sectional side view illustrating a schematic configuration of a printer  1  according to the first embodiment. In the following description, the right side (rightward) of  FIG. 1  is assumed to be the front side (forward) of the printer  1 . 
     As shown in  FIG. 1 , the printer  1  is a tandem-electrophotographic direct-transferring color laser printer and is provided with a casing  3 . A tray feeder  5  in which recording medium (exemplified by paper sheets)  7  are stocked is disposed at the bottom of the casing  3 . 
     The recording medium  7  is pressed against a pickup roller  11  by a pressing board  9 , and is sent to a resist roller  13  by rotation of the pickup roller  11 . The resist roller  13  corrects a skew of the recording medium  7  and then sends the recording medium  7  to a belt unit  15  at a predetermined timing. 
     An image forming unit  17  includes the belt unit  15 , a scanner unit  19 , process units  21 , a fixing unit  23  and other elements. 
     The belt unit  15  includes an endless belt  29  provided between a pair of supporting rollers  25  and  27 . The belt  29  is circularly rotated in the counter-clockwise direction in  FIG. 1  by, for example, rotation of the rear supporting roller  27 , so that a recording medium on the belt  29  is transferred to the rearward. 
     Further, a cleaning roller  31  is provided below the belt unit  15  in order to remove toner, such as a registration pattern  91  described below, paper dusts, and others adhered to the belt  29 . 
     The scanner unit  19  includes a laser light emitting section (not shown) which is on/off-controlled based on image data, and irradiates a photosensitive drum of each color with laser beam L corresponding to an image of the color and concurrently makes high-speed scan. 
     Four process units  21  corresponding to the four colors of black, cyan, magenta, and yellow respectively are same in configuration except the colors of toner. Hereinafter, reference numbers  21  with corresponding subscripts of K (black), C (cyan), M (magenta) and Y (yellow) are used when it is necessary to discriminate the process units  21  in colors from one another, but the subscripts are to be omitted when no discrimination is needed. 
     Each process unit  21  includes a photosensitive drum  33 , a charger  35 , a developer cartridge  37 , and other elements. 
     The developer cartridge  37  has a toner container  39 , a supplying roller  41 , a developing roller  43 , and a layer thickness limiting blade  45 . 
     Toner is supplied to the developing roller  43  by rotation of an agitator  47  and rotation of the supplying roller  41 . The toner supplied to the surface of developing roller  43  enters a space between the layer thickness limiting blade  45  and the developing roller  43  to thereby be formed into a thin layer having a uniform thickness carried on the developing roller  43 . 
     The surface of each photosensitive drum  33  is uniformly and positively charged by the charger  35 , and then exposed by laser beam L from the scanner unit  19 . Consequently, on the surface of the photosensitive drums  33 , electrostatic latent images corresponding one to each of the colors are formed. 
     The toners born on the developing rollers  43  are supplied to electrostatic latent images formed on the surfaces of photosensitive drums  33 , so that the electrostatic latent images become visible in the form of toner images, one in each of the corresponding colors. 
     While a recording medium  7  passes through each transferring position between the photosensitive drum  33  and a transferring roller  49  by the belt  29 , a negative transferring bias is applied to the transferring roller  49 . Thus, toner images born on the surface of the photosensitive drums  33  are transferred onto the recording medium  7 . The recording medium  7  is then transferred to the fixing unit  23 . 
     A heating roller  51  and a pressure roller  53  of the fixing unit  23  heats the recording medium  7  holding the toner image thereon while transferring the recording medium  7 , so that the toner image is thermally fixed to the surface of the recording medium  7 . Then, the recording medium  7  is discharged onto a discharging tray  57  by a discharging roller  55 . 
     As shown in  FIG. 1 , the printer  1  further includes an optical sensor  81  arranged under the rearward of the belt  29 . The optical sensor  81  is a reflective sensor including a phototransmitter and a photoreceptor. The phototransmitter diagonally irradiates the surface of the belt  29  with light. The photoreceptor receives light reflected by the surface of the belt  29  and outputs a binary signal indicating whether or not there is a mark  93  of a registration pattern  91  (described later) in the detection region. 
     (Electric Configuration of the Printer) 
       FIG. 2  is a block diagram schematically showing the electrical configuration of the printer  1 . 
     The printer  1  includes a CPU  61 , a ROM  63 , a RAM  65 , an EEPROM (a non-volatile memory)  67 , an operating unit  69 , a display unit  71 , the above-described image forming unit  17 , a network interface  73 , the optical sensor  81 , and others. 
     The ROM  63  stores various programs for controlling operations of the printer  1 . The CPU  61  controls operations of the printer  1  in accordance with programs read from the ROM  63 , while storing the process results into the RAM  65  and/or the EEPROM  67 . 
     The operating unit  69  has a plurality of buttons with which a user can perform various input operations, such as an instruction to start printing. The display unit  71  is formed by an LCD and lamps and can display various setting screen and an operation state thereon. The network interface  73  is connected to an external computer (not shown) through a communication line  75  and consequently makes mutual data communication possible. 
     (Process to Correct Position Deviation) 
     If image formation positions (transferring positions) on a recording medium are deviated from one another for each color, color images with color registration error is formed. In order to avoid such color registration error, a position deviation correcting process is performed. In the position deviation correcting process, black is regarded as the reference color and the remaining colors (yellow, magenta, and cyan) are regarded as measurement colors, and a deviation of the image formation position of each measurement color from the image formation position of the reference color is corrected.  FIGS. 3-6  are schematically show patterns formed on the belt  29  at various operation stages. Each of the drawing shows the top view, the side view and the bottom view of the belt  29  from the top of the drawing. 
     1. Registration Pattern 
       FIGS. 4-6  show first registration patterns (hereinafter, simply called first patterns  91 A). The first patterns  91 A are used to detect deviations of image formation positions on the belt  29  in the rotation direction (the machine direction of the printer  1 , hereinafter called a “sub-scanning direction”). Specifically, the first patterns  91 A are formed by a plurality of bar-shaped marks  93  which extend in the side-to-side direction, and are arranged in the movement direction of the belt  29 . In addition, the first pattern  91 A includes one or more mark sets each having a black mark  93 K, a yellow mark  93 Y, a magenta mark  93 M, and a cyan mark  93 C arranged in this order in the sub-scanning direction. 
       FIGS. 3 ,  4 , and  6  show second registration patterns (hereinafter, simply called second patterns  91 B). The second patterns are used to detect deviations of image formation positions on the belt  29  in the direction (the side-to-side direction of the printer  1 , hereinafter called the “main-scanning direction”) perpendicular to the above sub-scanning direction. Specifically, the second patterns  91 B are formed by a plurality of pairs of bar-shaped marks  95  forming respective different angles with respect to the main-scanning direction, and are arranged in the movement direction of the belt  29 . The plurality of pairs of marks  95  includes a plurality of pairs of black marks  95 K, a plurality of pairs of yellow marks  95 Y, a plurality of pairs of magenta marks  95 M, and a plurality of pairs of cyan marks  95 C. Data of the first pattern  91 A and data of the second pattern  91 B are recorded in, for example, the EEPROM  67 . 
     2. Relationship Between the Belt Speed and a Variation in the Image Formation Position 
     When a power switch is turned on, the printer  1  starts control of rotational driving of the belt  29  and raising the temperature of the fixing unit  23  to a target temperature (at which an image can be thermally fixed, e.g., 200° C.). In the present embodiment, if an image forming instruction is not issued by a user for a predetermined time period after the power switch has been turned on, the printer  1  comes to be in a sleep state. In the sleep state, the temperature of the fixing unit  23  becomes lower than the target temperature and the belt  29  halts the rotational driving. Then, when the printer  1  returns from the sleep state, the control of rotational driving of the belt  29  and raising the temperature of the fixing unit  23  to the target temperature are started again. 
       FIG. 7  is a graph showing an experimental result obtained by sequentially sampling the positions of the marks  93  of each measurement color after the power switch is turned to on or the printer  1  returns from the sleep state (hereinafter referred to as “the time when the power switch is turned on or the like”). In  FIG. 7 , zero position of the position deviation amount is the position of the mark  93 K of the reference color on the belt  29 . The distance between the zero position and each plot represents the position deviation amount of the measurement color with respect to the reference-color mark  93 K. 
     As shown in  FIG. 7 , immediately after the power switch is turned on or the like, the temperature of the fixing unit  23  is unstable since the temperature has not reach the target temperature yet. Thus, the thickness of the belt  29  is not constant. Further, the rotation speed of the belt  29  (i.e., the rotation speed of supporting rollers  25  and  27 ) is also unstable since the speed has not reach the set speed yet. Accordingly, the surface speed of the belt  29  can be estimated to be unstable (hereinafter, this state is referred to as an “unstable state”). Therefore, the position of the mark  93  of each measurement color varies with time passage. 
     After that, when the temperature of the fixing unit  23  reaches the target temperature and becomes stable at the target temperature, and the rotation speed of the belt  29  reaches the set speed and becomes stable at the set speed, the surface speed of the belt  29  is assumed to become substantially stable at a substantially constant speed and the position of the mark  93  of each measurement color is assumed to become stable at a substantially constant position (hereinafter, this state is referred to as a “stable state”). On the other hand, if the position of the marks  93  is detected in the unstable state and the image formation positions of measurement colors are corrected in the stable state based on only the marks  93  detected the unstable state, images with the registration error may be formed, since the surface speed of the belt  29  when the positions of the marks  93  are detected is different from the surface speed of the belt  29  when the image is formed. 
     Note that even if the control of either rotational driving of the belt  29  or the temperature of the fixing unit  23  to the target temperature is started at the time when the power switch is turned on or the like, the variations are estimated to show the substantially same tendency as those shown in  FIG. 7  while the position deviation amounts themselves are less than those of  FIG. 7 . 
     3. Detection of Pattern Position 
     When a test instruction is issued, the image forming unit  17  carries out the following first and second operations during the position deviation correcting process. 
     In the first operation, as shown in  FIG. 3 , the second pattern  91 B is formed on a first region  29 A of about the half of the belt  29  while a predetermined reference point P on the belt  29  reaches, for example, the side of the supporting roller  27  from the side of the supporting roller  25 , in other words, while the belt  29  makes half revolution from the start of the first operation. Next, the first pattern  91 A is formed on the second region  29 B of the remaining half of the belt  29  while the predetermined reference point P reaches the side of the supporting roller  25  from the side of the supporting roller  27 , in other words, while the belt  29  further makes half revolution after the completion of the formation of the second pattern  91 B, as shown in  FIG. 4 . 
     After completion of the first operation, the cleaning roller  31  cleans off the first pattern  91 A. After  30  seconds has elapsed since the first operation had been completed, the second operation is performed. The image forming unit  17  starts performing the second operation at the timing when the reference point P on the belt  29  reaches a position that is same as a position at which the first operation is started (the same position as that shown in  FIG. 3 ). This timing can be determined in advance based on the starting timing of the above first operation and the set speed of the belt  29 . 
     In the second operation, the first pattern  91 A is formed on the first region  29 A of the belt  29 , as shown in  FIG. 5 , while the reference point P makes half revolution from the start of the first operation, and successively the second pattern  91 B is formed on the second region  29 B as shown in  FIG. 6 , while the belt  29  further makes half revolution from the completion of formation of the first pattern  91 A. 
     In short, the second operation forms the second pattern  91 B on a region on which the first operation has formed the first pattern  91 A and forms the first pattern  91 A on a region on which the first operation has formed the second pattern  91 B. After the completion of the second operation, the cleaning roller  31  also cleans the belt  29 . 
     Further, the image forming unit  17  obtains binary signals sequentially sent from the optical sensor  81  during the first operation and the second operation.  FIG. 8  is a graph showing the variation in the position of the mark  93  having one measurement color. In  FIG. 8 , immediately after the power switch is turned on or the like, the test instruction is issued, that is, first operation is carried out and after a predetermined time period (e.g., 30 seconds) has elapsed since the first operation had been completed, the second operation is carried out. Further, in this embodiment, relative to the position of the mark  93  formed during the first operation, the position of the mark  93  formed during the second operation comes close to approximately 70-80 percent of the mark  93  formed in the stable state. 
     4. Contents of Control of the Position Deviation Correcting Process 
     When a predetermined execution condition is satisfied, the CPU  61  determines that the test instruction is issued, and carries out the position deviation correcting process shown in  FIG. 9 . For example, the CPU  61  determines that the test instruction is issued, when a predetermined time has elapsed since the previous position deviation correcting process had completed or the number of recording medium on which images has been formed has reached the threshold value. 
     First of all, the CPU  61  determines whether the surface speed of the belt  29  is currently in the stable state or the unstable state. However, it is difficult to measure the actual surface speed of the belt  29 . For this reason, in the present embodiment, the CPU  61  determines whether or not the elapsed time T 1  from the start of control of rotational driving of the belt  29  or the temperature of the fixing unit  23  to the target temperature is longer than a predetermined time period N in S 1 . For example, if the predetermined time has elapsed while the printer  1  is forming an image, it can be assumed that the surface speed is in the stable. On the other hand, if the predetermined time has elapsed while the printer  1  is in the sleep state, it can be assumed that the surface speed is in the unstable. 
     If the elapsed time T 1  is shorter than the predetermined time period N (S 1 :NO), the CPU  61  determines that the surface speed is currently in the unstable state, and carries out correction  1  in S 2 . In the correction  1 , a position deviation amount A 1  is detected in the first operation and a position deviation amount A 2  is detected in the second operation, as shown in  FIG. 8 , an average value H 1  of the position deviation amount A 1  and the position deviation amount A 2  is calculated, and a predetermined reference value (a position deviation amount of a right position of a measurement-color mark from a right position of the reference-color mark) is subtracted from the calculated average value H 1 . Then in S 4 , the value obtained as the result of the subtraction is stored in the RAM  65  or the EEPROM  67  as a correction amount of an image formation position of the measurement color. 
     On the contrary, if the elapsed time T 1  is longer than the predetermined time period N in S 1  (S 1 : YES), the CPU  61  determines that the surface speed is currently in the stable state, and carries out correction  2  in S 3 . In the correction  2 , a position deviation amount A 3  is detected in the first operation and a position deviation amount A 4  is detected in the second operation, as shown in  FIG. 8 , a value H 3  is calculated by adding a first adjustment amount B 1  to one of the position deviation amount A 3  and the position deviation amount A 4  or an average value H 2  of the position deviation amounts A 3  and A 4 , and the predetermined reference value is subtracted from the value H 3 . 
     The first adjustment amount B 1  is set to be a value so that the value of H 1  obtained in the unstable state is substantially same as the value H 3  obtained in the stable state, for example, the substantial half value of the difference between the position deviation amount A 1  detected in the unstable state and the position deviation amount A 3  (or A 4 ) detected in the stable state (i.e., the substantial half value of a distance difference between the position of the mark of the measurement color formed in the unstable state and the position of the corresponding mark formed in the stable state). Then in S 4 , the value obtained as the result of the subtraction is stored in the RAM  65  or the EEPROM  67  as the correction amount of the image formation position of the measurement color. 
     After the correction  1  or  2  has been carried out, upon receipt of the image forming instruction, the image forming unit  17  forms an image of each color at the image formation position which has been corrected using the correction amount stored in RAM  65 , on a recording medium  7 . 
     Note that if the image forming instruction is issued immediately after the power switch is turned on or the like, the temperature of the fixing unit  23  may not reach the target temperature at the time when the image forming unit  17  is forming an image on (transferring an image onto) a recording medium  7 . However, the printer  1  of the present embodiment controls the temperature of the fixing unit  23  to become stable at the target temperature by the time the recording medium reaches the fixing unit  23 . 
     Effect of the First Embodiment 
     In the first embodiment, the state of the surface speed when the test instruction is issued is taken into consideration. Therefore, even if the state of the surface speed when the test instruction is different from the state of the surface speed when the image forming instruction is issued, it is possible to prevent the image formation position from deviating excessively. 
     Further, in the first embodiment, the value of H 1  obtained in the unstable state is substantially same as the value H 3  obtained in the stable state in the stable state as shown in  FIG. 8 . In other words, the correction amount of the image formation position of the measurement color is substantially the same (i.e., approximately the average amount of the position deviation amount detected in a stable state and that detected in unstable state) irrespective of whether the first operation and the second operation are performed in the stable state or the unstable state. Accordingly, even if the surface speed of the belt  29  at the time when the test instruction is issued is different from the surface speed of the belt  29  at the time when the image forming instruction is issued, for example, the surface speed of the belt  29  at the time when the test instruction is issued is stable and the surface speed of the belt  29  at the time when the image forming instruction is issued is unstable, it is possible to prevent the image formation position from deviating excessively. 
     Further, in the first embodiment, whether or not the surface speed of the belt  29  is in the stable state is determined based on whether or not the elapsed time T 1  is longer than the predetermined time period N. Accordingly, there is no need to install a temperature sensor to measure the temperature of the fixing unit  23 , a sensor to measure the rotation speed of the belt  29  or a similar device. 
     Further, in the first embodiment, the fixing unit  23  is controlled to reach the target temperature not only when the image forming instruction is issued but also when the test instruction is issued. Since an environment in which the test instruction is issued is same as an environment in which the image forming instruction is issued, the printer  1  of the first embodiment can correct an image formation position with higher accuracy as compared with the configuration in which the fixing unit  23  keeps an off state even if the test instruction is issued. 
     The image formation position in the sub-scanning direction is more easily affected by the surface speed of the belt  29  than the image formation position in the main-scanning direction. In particular, at the beginning of formation of the above pattern, the surface speed of the belt  29  is frequently unstable. For this reason, the printer of the first embodiment forms the first pattern  91 A, which is easily affected by variation in the surface speed, after the formation of the second pattern  91 B. 
     Second Embodiment 
       FIGS. 10-12  show a second embodiment, which is different in the position deviation correction process from the first embodiment and which is the same in the remaining as the first embodiment. Accordingly, repetitious description is omitted here by giving elements and parts with the same reference numbers as those of the first embodiment, so only the difference will now be detailed below. 
     (Position Deviation Correction Process) 
     The position deviation correction process according to this embodiment consists of a test process performed at the time when the test instruction is issued and a correction process performed during an image forming process. 
     1. Test Process 
     When the execution condition described in the first embodiment is satisfied, the CPU  61  determines that the test instruction is issued, and executes the procedure of the test process shown in  FIG. 10 . In S 11 , the CPU  61  determines whether the surface speed is currently in the stable state or the unstable state, which determination is the same as that made in S 1  in  FIG. 9 . If the elapsed time T 1  from the start of control of the rotational driving of the belt  29  or the temperature of the fixing unit  23  to the target temperature is longer than the predetermined time period N (S 11 :YES), the CPU  61  determines that the surface speed is currently in the stable state, and sets a flag “R” to “1” (S 12 ). On the other hand, the elapsed time T 1  is shorter than the predetermined time period N (S 11 :NO), the CPU  61  determines that the surface speed is currently in the unstable state, and sets the flag “R” to “0” (S 13 ). 
     Then in S 14 , the CPU  61  provides data of the first pattern  91 A and the second pattern  91 B stored in the EEPROM  67  to the image forming unit  17 . The image forming unit  17  carries out the first operation and the second operation based on the data of the first pattern  91 A and the second pattern  91 B, and obtains binary signals sequentially sent from the optical sensor  81  during the first operation and the second operation. 
     In S 15 , based on the obtained binary signals, the CPU  61  stores the position deviation amounts of the marks  93 C ( 95 C),  93 M ( 95 M), and  93  Y ( 95 Y), one for each of the measurement colors, with respect to mark  93 K ( 95 K) of the reference color into RAM  65  or EEPROM  67  and terminates the test process. 
     Normally, the position deviation amounts stored in the RAM  65  or the like can vary depending on the surface speed of the belt  29 . However, for simplification of the description in the present embodiment, the position deviation amounts A 1  and A 2  in  FIG. 12  are assumed to be stored in RAM  65  or the like if the CPU  61  determines that the surface speed is currently in the stable state in S 11 , and the position deviation amounts A 3  and A 4  in  FIG. 12  are assumed to be stored if the CPU  61  determines that the surface speed is currently in the unstable state in S 11 . 
     2. Image Forming Process (Correction Process) 
     Responsive to the image forming instruction from the user, for example, the CPU  61  carries out the image forming process shown in  FIG. 11 . First of all, the flag “R” is read in S 21  to determine whether the deviation amounts of the marks  93  ( 95 ) stored in EEPROM  67  or the like have been detected in the stable state or the unstable state. 
     (1) Correction  3   
     If the CPU  61  determines that the deviation amounts of the marks  93  ( 95 ) have been detected in the unstable state (R=“0”) (S 21 : YES), in S 22  the CPU  61  determines whether a current surface speed is in the stable state or the unstable state in the same manner as performed in S 11  in  FIG. 10 . 
     If the CPU  61  determines that the current surface speed is in the unstable state (S 22 : YES), the CPU  61  carries out a correction  3  in S 23 . That is, the correction  3  is carried out when the test process (detection of the position deviation amounts of the marks  93  ( 95 ) in  FIG. 10 ) was performed in the unstable state and the image formation will be also performed in the unstable state. For this reason, in the correction  3 , the correction amount of the image formation position of each measurement color is calculated while weighing the position deviation amount A 1  of each of the marks  93  ( 95 ) formed and detected in the unstable state. 
     Specifically, the below calculation expression is used in the correction  3 .
 
 H 4= X 1· A 1+ X 2· A 2, where  X 1+ X 2=1 and  X 1&gt; X 2
 
     This expression weights the position deviation amount A 1  detected in the unstablest state among the position deviation amounts A 1  and A 2  detected in the unstable state. In other words, the expression weights the position deviation amount A 1  by multiplying a multiplier X 1  larger than a multiplier X 2  multiplied to the other position deviation amount A 2 , and then calculates an average value H 4  (see  FIG. 12 ) of the position deviation amount. For example, the multipliers can be set to be “X 1 =0.6, X 2 =0.4”, “X 1 =0.7, X 2 =0.3”, or “X 1 =1.0, X 2 =0”. 
     Then, the correction amount of the image formation position of the corresponding measurement color is calculated by subtracting the predetermined reference value described in the first embodiment from the average value H 4 . In S 24 , the CPU  61  expands image data received along with the image forming instruction, using the image formation position corrected based on the correction amount obtained in the correction  3 , and provides the expanded image data to the image forming unit  17 . Thus, an image in each measurement color is formed at the image formation position that has been corrected on a recording medium  7 , and the image forming process is terminated. 
     (2) Correction  4   
     If the CPU  61  determines that the current surface speed is in the stable state (S 22 : YES), the CPU  61  carries out the correction  4  in S 25 . That is, the correction  4  is carried out when the test process was performed in the unstable state and the image formation will be performed in the stable state. For this reason, in the correction  4 , the correction amount of the image formation position of each measurement color is calculated while weighing the position deviation amount A 2  of each of the marks  93  ( 95 ) formed and detected in the unstable state, since the position deviation amount A 2  is closer to the stable state than the position deviation amount A 1 . 
     Specifically, the below calculation expression is used in the correction  4 .
 
 H 5= Y 1· A 1+ Y 2· A 2, where,  Y 1+ Y 2=1 and  Y 1&lt; Y 2
 
     This expression weights the position deviation amount A 2  closer to the stable state than the position deviation amount A 1 . In other words, the expression weights the position deviation amount A 2  by multiplying a multiplier Y 2  larger than a multiplier Y 1  multiplied to the other position deviation amount A 1  and then calculates an average value H 5  (see  FIG. 12 ) of the position deviation amounts. For example, the multipliers can be set to be “Y 1 =0.4, Y 2 =0.6”, “Y 1 =0.3, Y 2 =0.7”, or “Y 1 =0.2, Y 2 =0.8”. 
     Then, the correction amount of the image formation position of the corresponding measurement color is calculated by subtracting the predetermined reference value described in the first embodiment from the average value H 5 . Then the procedure proceeds to S 24 . 
     (3) Correction  5   
     If the CPU  62  determines that the deviation amounts of the marks  93  ( 95 ) have been detected in the stable state (R=“1”) (S 21 : NO), in S 26  the CPU  61  determines whether the current surface speed is in the stable state or the unstable state in the same manner as performed in S 11  in  FIG. 10 . 
     If the CPU  61  determines that the current surface speed is in the stable state (S 26 : YES), the CPU  61  carries out a correction  5  in S 27 . That is, the correction  5  is carried out when the test process was performed in the stable state and the image formation will be also performed in the stable state. For this reason, in the correction  5 , the correction amount of the image formation position of each measurement color is calculated based on the position deviation amounts A 3  and A 4  of each of the marks  93  ( 95 ) formed and detected in the unstable state. 
     Specifically, the below calculation expression is used in the correction  5 .
 
 H 6= Z 1· A 3+ Z 2· A 4, where  Z 1+ Z 2=1
 
     This expression calculates the average value H 6  (see  FIG. 12 ) of the position deviation amounts A 3  and A 4  detected in the stable state. The combination of the multipliers of the position deviation amount A 3  and A 4  is exemplified by a combination in which both position deviation amount A 3  and A 4  are equally weighed (i.e., “Z1=0.5, Z2=0.5”), a combination in which the position deviation amount A 4  is weighed (i.e., “Z1=0.4, Z2=0.6” or “Z1=0.3, Z2=0.7”), and a combination in which either position deviation amount A 3  or A 4  is used (i.e., “Z1=1.0, Z2=0” or “Z1=0, Z2=1.0”). 
     Then, the correction amount of the image formation position of the corresponding measurement color is calculated by subtracting the predetermined reference value described in the first embodiment from the average value H 6 . Then the procedure proceeds to S 24 . 
     (4) Correction  6   
     If the CPU  61  determines that the current surface speed is in the unstable state (S 26 : NO), the CPU  61  carries out the above described correction  5  in S 28  and a correction  6  in S 29 . That is, the correction  6  is carried out when the test process was performed in the stable state and the image formation will be performed in the unstable state. 
     In the correction  6 , a value H 7  is calculated by adding a second adjustment amount B 1  to the above average value H 6 , and the predetermined reference value is subtracted from the value H 6 . The second adjustment amount B 1  is set to be, for example, the difference value between the position deviation amount A 1  detected in the unstable state and the position deviation amount A 3  (or A 4 ) detected in the stable state (i.e., the distance difference between the mark  93  ( 95 ) of the measurement colors formed in the unstable state and the corresponding mark  93  ( 95 ) formed in the stable state) and exemplified by the twice the above first adjustment amount. Then, the procedure proceeds to S 24 . 
     The present second embodiment corrects the image formation positions, considering whether or not the image formation has been carried out in the stable state in addition to whether or not the test process has been carried out in the stable state. Accordingly, the second embodiment can further inhibit the influence caused by the difference between the surface speed of the belt  29  at the time when the test instruction is issued and the surface speed of the belt  29  at the time when the image forming instruction is issued, so that the image formation positions can be corrected with higher accuracy than the first embodiment. 
     Other Embodiments 
     Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. 
     (1) In the foregoing embodiments, the image-forming device is a direct-transferring color laser printer. Alternatively, the present invention may be applied to an intermediate-transferring color laser printer, or a printer using two, three, or more than four coloring agents. Further alternatively, application of the present invention even to a mono-color printer can accurately form an image on a proper position of a recording medium. 
     (2) The “image bearing member” of the foregoing embodiments takes the form of the belt  29  for transferring a recording medium. If an intermediate-transferring printer is used as the image-forming device, the image bearing member may be an intermediate-transferring belt. 
     (3) Differently from the foregoing embodiments, whether the surface speed of the belt  29  is in the stable state or in the unstable state may be determined by means of a temperature sensor which measures the temperature of the fixing unit  23 , a sensor (e.g., an encoder) which measures the rotational speed of the belt  29 , or another device. Further, the judgment method performed as a test process may be different from that performed as image formation. For example, a temperature sensor may be used for the determination that is to be made as a test process and a time elapsed from turning on the fixing unit  23  may be used for the determination to be made as image formation. 
     (4) A position deviation amount of each measurement color is detected in the first operation and in the second operation during the position deviation correcting process, i.e., twice in total. Alternatively, each position deviation amount may be detected three times or more.