Abstract:
An image forming apparatus has a function for correcting a condition for an image formation in accordance with a detection result given by a detecting unit which detects a toner pattern formed on a transfer medium by an image holding component. A deviation obtaining unit obtains a deviation in the main scanning direction between a detecting position on the transfer medium and a predetermined point of the toner pattern to be formed on the transfer medium, the detecting position being a position where the detecting unit detects the toner pattern. An adjusting unit adjusts, in accordance with the deviation obtained by the deviation obtaining unit, a positional relation in the main scanning direction between the detecting position and the predetermined point to reduce the deviation.

Description:
This application is based on an application No. 10-9426 filed in Japan, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an image forming apparatus which forms toner patterns and detects them using a sensor to correct image forming positions in accordance with detection results, and particularly relates to an adjustment to a toner pattern detecting mechanism provided in the image forming apparatus. 
     (2) Description of the Related Art 
     A so-called “tandem-type” image forming apparatus, as one example of a conventional full-color image forming apparatus, has image holding components (photosensitive drums, for example) set in line corresponding to reproduction colors cyan, magenta, yellow, and black. The reproduction colors are respectively referred to as C, M, Y, and K hereinafter and components related to these colors are assigned numerals with a corresponding C, M, Y, or K. Toner images for different colors formed on the image holding components are sequentially transferred onto a transfer material (a recording sheet, for example) which is transported on a transfer belt or onto the transfer belt as a transfer intermediate component. The toner images are superimposed to form a color image. 
     In general, for the tandem-type image forming apparatus having the stated construction, a so-called “registration correction” is performed to correct forming positions of images formed on surfaces of the image holding components corresponding to the reproduction colors. By means of the registration correction, deterioration in the image quality caused by color deviations is prevented. 
     The following is a brief description of a method of the registration correction, with reference to FIGS. 1A and 1B. FIG. 1A is a diagrammatic illustration of a tandem-type image forming apparatus which has a registration correction mechanism. For the registration correction performed in this image forming apparatus, an optical sensor  925  for optically detecting a toner density is set after photosensitive drums  941 C to  941 K in the transportation direction of a transfer material, as shown in FIG.  1 A. The photosensitive drums  941 C to  941 K are set along a transfer belt  931 . Registration marks  960 C to  960 K, for example, corresponding to the photosensitive drums  941 C to  941 K are formed on the transfer belt  931  as shown in FIG.  1 B. 
     The registration marks  960 C to  960 K are formed in the same shape, and are V-shaped in FIG.  1 B. Each of the V-shaped registration marks is composed of a standard line making a right angle with a transportation direction when no deviation is detected and a sloped line forming a 45° angle with the standard line. When image writing positions on the photosensitive drums  941 C to  941 K are correct and transfer positions are also correct, this means that no color deviations occur. In this case, the registration marks  960 C to  960 K are formed on the exactly same line that is parallel to the transportation direction as shown in FIG. 1B, with the standard lines being formed with a distance D between them in the transportation direction. 
     The optical sensor  925  set after the photosensitive drums  941 C to  941 K detects the registration marks  960 C to  960 K. Due to its detection characteristics, the optical sensor  925  detects a toner density of a point located directly under the optical sensor  925 . More specifically, the optical sensor  925  sequentially detects toner densities of points on a dash line shown in FIG.  1 B. Here, a high density value is detected at each point of intersection of the dash line and the standard line or the sloped line of the corresponding registration mark  960 C to  960 K. 
     If the registration marks  960 C to  960 K are formed on the transfer belt  931  at respective correct positions, a time period taken from the detection of the standard line to the detection of the next standard line is obtained by dividing the distance D by a moving speed of the transfer belt  931 . In addition, time periods respectively taken from the detection of the standard line to the detection of the sloped line of the registration marks  960 C to  960 K are the same. 
     Meanwhile, if a timing at which a registration mark is formed is different between the photosensitive drums  941 C to  941 K, the distance D varies according to the different timings. This means that the time period taken from the detection of the standard line to the detection of the next standard line varies. In this case, an address of image data read from an image memory in the sub-scanning direction is corrected for each pixel so that deviations of the registration marks  960 C to  960 K are corrected. 
     If the image forming positions on the photosensitive drums  941 C to  941 K are deviated in the main scanning direction, the time period taken between the detections the standard line and the sloped line varies with the registration marks  960 C to  960 K. Since the standard line and the sloped line of each of the registration marks  960 C to  960 K intersect at a 45° angle, relative differences in time periods between the detections of the standard line and the sloped line correspond with deviations of the registration marks  960 C to  960 K in the main scanning direction. In this case, an address of image data read from the image memory in the main scanning direction is corrected for each pixel, with one of the registration marks  960 C to  960 K being set as a standard mark. As a result, the time periods respectively taken between the detections of the standard lines and the sloped lines of the registration marks  960 C to  960 K are the same. 
     The above operations are performed by a pattern position determining unit  916   a , a color deviation calculating unit  916   b , and an address correcting unit  916   c  shown in FIG.  1 A. The pattern position determining unit  916   a  determines timings at which the registration marks  960 C to  960 K are respectively detected. The color deviation calculating unit  916   b  calculates the color deviations. In doing so, the color deviation calculating unit  916   b  obtains the color deviations in the main scanning direction from the relative differences in the time periods respectively taken between the detections of the standard lines and the sloped lines of the registration marks  960 C to  960 K, and obtains the color deviations in the sub-scanning direction from the relative differences in the time periods respectively taken between the detections of the standard line and the corresponding next standard line. The address correcting unit  916   c  corrects an address value for each pixel in accordance with the color deviations in the main scanning and sub-scanning directions. 
     Here, the optical sensor  925  should reliably detect the registration marks  960 C to  960 K so that the registration correction is correctly performed as stated above. In general, a registration mark has a width equal to or shorter than 8 mm in the main scanning direction. To obtain a valid detection value, about 4 mm middle short part of the 8 mm-wide registration mark needs to be detected. Meanwhile, since tolerances are established for an installation position of the optical sensor  925 , a timing at which the optical sensor  925  detects the registration marks  960 C to  960 K varies according to the position of the optical sensor  925 . Also, deviations of the image forming positions of the registration marks  960 C to  960 K and magnification deviations in the main scanning and sub-scanning directions may be initially great. 
     When the registration correction is performed, the optical sensor  925  may not be able to reliably detect the registration marks  960 C to  960 K due to the deviated installation position of the optical sensor  925  and the variations in the image forming positions of the registration marks  960 C to  960 K which have been determined before the registration correction is performed. 
     To avoid this problem, the registration mark can be formed in a large size. However, it is desirable to have a distance between the registration marks  960 C to  960 K as short as possible so that fluctuations in the moving speed of the transfer belt  931  does not adversely affect the formations of the registration marks  960 C to  960 K. Moreover, to improve accuracy of the registration correction, registration marks need to be formed as many as possible in one correction cycle and a deviation needs to be measured a plurality of number of times. For this reason, the registration marks  960 C to  960 K should be formed in a small size. It is not desirable to avoid the stated problem by forming the registration marks  960 C to  960 K in a large size. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an adjustment mechanism in an image forming apparatus which forms toner patterns and includes a correction mechanism for correcting image forming positions in accordance with results obtained by the optical sensor that reliably detects the toner patterns by means of the adjustment mechanism. 
     The object of the present invention can be achieved by an image forming apparatus which corrects a condition for an image formation in accordance with a detection result of a toner pattern formed on a transfer medium transported in a first direction, the image forming apparatus made up of: a specific pattern forming unit for forming at least one specific pattern on the transfer medium; a detecting unit, which is located at a downstream side of the specific pattern forming unit in the first direction, for detecting the specific pattern formed on the transfer medium; a deviation obtaining unit for obtaining a deviation in a second direction between a detecting position on the transfer medium and a predetermined point of the toner pattern to be formed on the transfer medium in accordance with a detection result given by the detecting unit, the second direction intersecting the first direction and the detecting position being a position where the detecting unit detects the toner pattern; and an adjusting unit for adjusting, in accordance with the deviation obtained by the deviation obtaining unit, a positional relation in the second direction between the detecting position and the predetermined point to reduce the deviation. 
     The object of the present invention can be also achieved by an image forming apparatus made up of: a photosensitive component; a registration mark forming unit for forming a registration mark on the photosensitive component; a specific pattern forming unit for forming a specific pattern on the photosensitive component; a transfer medium on which the registration mark and the specific pattern formed on the photosensitive component are transferred; a sensor for detecting the registration mark and the specific pattern transferred onto the transfer medium; and an adjusting unit for adjusting, in accordance with a detection result of the specific pattern given by the sensor, a positional relation between a detecting position on the transfer medium and a predetermined point of the registration mark to be formed on the transfer medium to reduce a deviation between the detecting position and the predetermined point, the detecting position being a position where the sensor detects the registration mark. 
     The object of the present invention can be also achieved by an adjusting method of a toner pattern detecting system for an image forming apparatus which corrects a condition for an image formation in accordance with a detection result given by a detecting unit that detects a toner pattern and a specific pattern formed by an image holding component on a transfer medium transported in a first direction, the adjusting method including: a deviation obtaining step for obtaining a deviation in a second direction between a detecting position on the transfer medium and a predetermined point of the toner pattern to be formed on the transfer medium in accordance with a detection result of the specific/pattern, the second direction intersecting the first direction and the detecting position being a position where the detecting unit detects the toner pattern; and an adjusting step for adjusting, in accordance with the deviation obtained in the deviation obtaining step, a positional relation in the second direction between the detecting position and the predetermined point to reduce the deviation. 
     The object of the present invention can be also achieved by an adjusting method of a registration mark detecting system for an image forming apparatus which forms a registration mark as a toner pattern on a transfer medium, the adjusting method including: a specific pattern forming step for forming a specific pattern as a toner image on the transfer medium; a specific pattern detecting step for detecting the specific pattern formed on the transfer medium using a sensor; and an adjusting step for adjusting, in accordance with a detection result obtained in the specific pattern detecting step, a positional relation between a detecting position on the transfer medium and a predetermined point of the registration mark to be formed on the transfer medium so as to reduce a deviation between the detecting position and the predetermined point, the detecting position being a position where the sensor detects the registration mark. 
     With these constructions, when the registration correction is performed, for example, a deviation in the main scanning direction between a detecting position of the detecting unit and forming positions of toner patterns (or, registration marks) can be minimized. Consequently, the detecting unit can reliably detect each valid width of the registration marks for the density detection, so that the registration correction can be correctly performed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. In the drawings: 
     FIG. 1A is a diagrammatic illustration of a tandem-type image forming apparatus which has a registration correction mechanism; 
     FIG. 1B is a plan view showing an example of registration marks formed on a transfer belt when the registration correction is performed; 
     FIG. 2 is a schematic sectional view showing a construction of a tandem-type digital copying machine of embodiments of the present invention; 
     FIG. 3 is a block diagram showing the construction of a controlling section  100  of the first embodiment of the present invention, which performs the registration correction and the adjustment operation for the registration correction; 
     FIG. 4 is a flowchart showing the adjustment operation performed for the registration correction in the first embodiment of the present invention; 
     FIG. 5 is a plan view showing an example of specific patterns formed on the transfer belt in the first embodiment of the present invention; 
     FIG. 6 shows an example of a detection signal obtained when the optical sensor  25  detects the specific patterns shown in FIG. 5; 
     FIG. 7 shows the specific pattern of the first embodiment of the present invention; 
     FIG. 8 is a perspective view showing a driving mechanism of the optical sensor  25  used in the second embodiment of the present invention; 
     FIG. 9 is a block diagram showing the construction of the controlling section  100  of the second embodiment of the present invention, which performs the registration correction and the adjustment operation for the registration correction; 
     FIG. 10 is a flowchart showing the adjustment operation performed for the registration correction in the second embodiment of the present invention; 
     FIG. 11 is a plan view showing that a line image reaches a position which aligns with a detecting position of the optical sensor on a main scanning line in the second embodiment of the present invention; and 
     FIG. 12 is an example of a detection signal obtained when the optical sensor  25  detects the line image shown in FIG.  11 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following is a description of embodiments of the image forming apparatus of the present invention, with reference to the drawings. In these embodiments, a tandem-type digital full-color copying machine (simply referred to as the “copier” hereinafter) is used as an example of such image forming apparatus. FIG. 2 is a schematic view showing the construction of the copier of the present invention. Although the copier is used as an example in the embodiments, the present invention can be applied to various image forming apparatuses, such as a printer and a facsimile. 
     First Embodiment 
     In the copier, a paper feeding cassette  12  is set on a right-side wall  11  of an enclosure  10  and can be freely slid in and out of the copier. A paper discharging tray  14  is set on a left-side wall  13  and protrudes outward. A transfer belt  31  is horizontally set in a lower space between the paper feeding cassette  12  and the paper discharging tray  14 . Image forming units  40 C,  40 M,  40 Y, and  40 K are set above the transfer belt  31  along its length. The transfer belt  31  transports a recording sheet S, and the image forming units  40 C to  40 K successively transfer toner images for each color onto the recording sheet S. The toner images are superimposed on the recording sheet S to form a full-color image. 
     An image reading unit  15  is set at the upper part of the enclosure  10 . The image reading unit  15  optically reads an original document, so-that image data of the original document is obtained. An image processing unit of a controlling unit  16  performs the necessary processes on this obtained image data and separates the image data into each color C, M, Y, and K. Laser diodes (referred to as the “LDs” hereinafter)  18 C to  18 K of optical units  17 C to  17 K set above the image forming units  40 C to  40 K are driven to perform light modulation in accordance with the image data for each color. Light-modulated laser beams are respectively deflected by polygon mirrors  19 C to  19 K in the main scanning direction and guided into the image forming units  40 C to  40 K corresponding to the respective colors. 
     The image forming units  40 C to  40 K respectively have photosensitive drums  41 C to  41 K as main components, chargers, and developing units. The light-modulated laser beams respectively expose the corresponding photosensitive drums  41 C to  41 K which are rotated in the direction of arrows C shown in FIG.  2 . Electrostatic latent images formed by means of the exposure are developed into visible toner images by the corresponding developing units. The developing units respectively supply the photosensitive drums  41 C to  41 K with toners C, M, Y, and K as developers corresponding to the light-modulated colors of the optical units  17 C to  17 K. 
     Transfer chargers  20 C to  20 K are respectively set at positions located directly under the photosensitive drums  41 C to  41 K of the image forming units  40 C to  40 K, with the transfer belt  31  in between. The toner images formed on the surfaces of the photosensitive drums  41 C to  41 K are transferred onto the recording sheet S which is transported on the transfer belt  31 . Here, the toner images are superimposed on the recording sheet S to form a color image. After the toner image transfer, the recording sheet S is transported by the transfer belt  31  to fixing rollers which fix the transferred toner image onto the recording sheet S. Finally, the recording sheet S is discharged onto the discharging tray  14 . 
     A transmission type optical sensor  25  (referred to as the “optical sensor  25 ” hereinafter) is set after the photosensitive drums  41 C to  41 K in the transportation direction of the transfer belt  31 . The optical sensor  25  is used for the registration correction and the adjustment operation for the registration correction described later in this specification. The transfer belt  31  of the present embodiment is made of transparent synthetic resin material, such as polyethylene terephthalate (PET). As such, the optical sensor  25  can detect a toner density of an image formed on the transfer belt  31 . A reflection type optical sensor can be used as the optical sensor  25  if opaque material is used as the transfer belt  31  or if the registration marks are formed on the recording sheet S. 
     The optical sensor  25  includes a light-emitting diode (referred to as the “LED” hereinafter) and a photo diode (referred to as the “PD” hereinafter). Receiving a control signal from a CPU  101  described later and shown in FIG. 3, the optical sensor  25  has the LED emit a light which is then converged by a converging lens (not illustrated). This light exposes the surface of the transfer belt  31 . The light passing through the transport belt  31  is received by the PD and converted into an electric signal. This detection signal is amplified by an amplifier. The amplified detection signal is further converted into a multivalued digital signal by an A/D converter and outputted to the CPU  101 . 
     The following is a description of a controlling section which performs the registration correction and the adjustment operation for the registration correction. FIG. 3 is a block diagram showing the construction of a controlling section  100  which performs the registration correction and the adjustment operation for the registration correction. The controlling section  100  is part of the controlling unit  16  which controls the entire copier, and is composed of a CPU  101  for performing calculation processing, a RAM  102  for serving as a work area of the CPU  101 , a ROM  103  for storing programs, and a time counter  104  for programmatically counting a time. The CPU  101  is connected to an image processing unit  201 , an image memory  202 , an LD driving unit  203 , an image formation controlling unit  204 , and a transportation system controlling unit  205  which are controlled by another CPU included in the controlling unit  16 . 
     The image processing unit  201  converts the electric signals for red(R), green(G), and blue(B) obtained by scanning the original document into the multivalued digital signals to generate image data. After performing the well-known correction processing, such as a shading correction process, the image processing unit  201  generates C, M, Y, and K image data for each pixel and outputs the image data to the image memory  202 , where the image data is stored for each reproduction color. In doing so, the image memory  202  stores the image data for each pixel in a storing position (or, an address) corresponding to a position of the pixel. 
     The LD driving unit  203  drives the LDs  18 C to  18 K in accordance with the image data. The image formation controlling unit  204  has the image forming units  40 C to  40 K perform the stated operation, so that the electrostatic latent images formed by means of the exposure of the LDs  18 C to  18 K are developed into the toner images. The toner images are then sequentially transferred onto the recording sheet S. The transportation system controlling unit  205  controls operations, such as the transportation of the recording sheet S by transfer belt driving rollers  32  and  33 . 
     The ROM  103  stores programs which the CPU  101  reads to perform the registration correction and the adjustment operation for the registration correction, and also stores data required for printing a registration mark and a specific pattern used for the adjustment operation. 
     According to the programs stored in the ROM  103 , the CPU  101  controls the registration correction and the adjustment operation for the registration correction. When performing the registration correction, the CPU  101  has the image memory  202  store the data for printing the registration mark that is stored in the ROM  103 . Then, the CPU  101  gives instructions to the LD driving unit  203 , the image formation controlling unit  204 , and the transportation system controlling unit  205  so that the registration mark is formed on the transfer belt  31  for each color using the data stored in the image memory  202 . Here, the registration marks are formed with a certain distance between them. Each of the registration marks, which is in the same shape as shown in FIG. 1B, is about 8 mm wide in the main scanning direction and its valid width for the density detection is about 4 mm. This V-shaped registration mark is composed of a standard line making a right angle with a transportation direction when no deviation is detected and a sloped line forming a 45° angle with the standard line. 
     As the transfer belt  31  moves, the standard and sloped lines of the registration marks formed on the transfer belt  31  approach the optical sensor  25 . When one of the standard or sloped lines passes directly under the optical sensor  25 , a waveform signal having a peak value as shown in FIG. 6 is detected. The detection signal is converted to a digital signal and outputted to the CPU  101 . The time counter  104  counts each time period taken between the detections of the peak values. 
     In this way, the time period is measured between the detections of the peak values of the registration marks. If each time period measured between the detections of the standard lines differs from a predetermined time period, the CPU  101  corrects the deviation by correcting the addresses in the sub-scanning direction for each color stored in the image memory  202 , with consideration given to the moving speed of the transfer belt  31 . If the time period measured between the detections of the standard line and the sloped line varies with the registration marks, the CPU  101  sets the registration mark for K as the standard mark. Then, the CPU  101  corrects the deviation by correcting the addresses in the main scanning direction for the reproduction colors aside from black stored in the image memory  202  so that the registration marks for C, M, and Y are aligned with the registration mark for K. According to the above operations, the registration correction is achieved. 
     Next, the adjustment operation for the registration correction is explained. This adjustment operation is performed so that the registration marks used for the registration correction are formed within a valid detection range of the optical sensor  25 . FIG. 4 is a flowchart of the adjustment operation. As a general rule, this operation is performed at the factory prior to shipment or when the copier is set up. However, the operation may be performed when necessary, such as every time an image formation is performed. The same can be said of the second embodiment. 
     The CPU  101  stores the data for printing specific patterns stored in the ROM  103  into an area of the image memory  202  associated with black which is used as the standard color (step S 101  of FIG.  4 ). Each of the specific patterns is in the same shape as the registration mark and composed of a standard line and a sloped line. 
     The CPU  101  gives the instructions to the LD driving unit  203 , the image formation controlling unit  204 , and the transportation system controlling unit  205  so that the specific patterns stored in the image memory  202  are formed on the transfer belt  31  in the arrangement as shown in FIG. 5 (step S 102 ). More specifically, forty-one of the specific patterns ( . . . Pn+5, Pn+4, Pn+3, Pn+2, Pn+1, Pn, Pn−1, Pn−2, Pn−3, . . . as shown in FIG. 5) are formed on the transfer belt  31  with the middle pattern Pn being formed at a standard forming position of the registration marks (simply referred to as the “standard forming position” hereinafter). The standard forming position is determined at the factory prior to shipment. Each of the specific patterns is deviated 5 dots with respect to the adjacent specific pattern(s) in the main scanning direction, and are formed with a certain distance L between them in the sub-scanning direction so that the specific patterns are not overlaid one another. As such, the specific patterns are formed in a range of ±100 dots in the main scanning direction, with the standard forming position being located at the middle in the main scanning direction. Note that the downward direction in FIG. 5 is a forward direction of the main scanning direction. 
     The specific patterns formed on the transfer belt  31  are sequentially detected by the optical sensor  25  and inputted to the CPU  101  (step S 103 ). The CPU  101  measures the time period taken between the detections of the standard line and the sloped line for each of the specific patterns using the time counter  104 . 
     More specifically, the detection waveforms as shown in FIG. 6 are outputted from the optical sensor  25  and inputted to the CPU  101  after the A/D conversion. From these detection signals, the CPU  101  obtains the central position (or, peak position) of each detection value as a standard position using a barycenter calculating method. This standard position is determined as a correct position of the standard or sloped line of the corresponding specific pattern. The CPU  101  then measures a time period taken between the detections of the standard line and the sloped line for each of the specific patterns ( . . . tn+5, tn+4, tn+3, tn+2, tn+1, tn, tn−1, . . . as shown in FIG.  6 ). It should be noted here that the standard line or the sloped line may not be detected due to the forming position of the specific pattern in the main scanning direction. Since a distance between the photosensitive drum  41 K and the optical sensor  25 , the moving speed of the transfer belt  31 , and a time when each of the specific patterns is formed are apparent, the CPU  101  can identify the specific pattern by the time when the standard line of the specific pattern is detected by the optical sensor  25 . 
     The moving speed of the transfer belt  31  and the shape of the specific patterns are apparent. Therefore, the time period taken between the detections of the standard line and the sloped line can be obtained beforehand, with the optical sensor  25  detecting the respective middle points of the standard and sloped lines in the main scanning direction. The CPU  101  sets this time period as the standard time period and compares the time period actually taken between the detections of the standard line and the sloped line of each specific pattern detected by the optical sensor  25  with the standard time period. Then, the CPU  101  records the specific pattern whose time period is the closest to the standard time period (step S 104 ). 
     The CPU  101  obtains a deviation of the recorded specific pattern from the specific pattern formed at the standard forming position. From this deviation, the CPU  101  obtains the deviation of the detecting position of the optical sensor  25  from the standard forming position in the main scanning direction (step S 105 ). As one example, suppose that the time period of the specific pattern Pn+3 shown in FIG. 5 is the closest to the standard time period. The specific pattern Pn+3 is situated three patterns ahead of the specific pattern Pn formed at the standard forming position in the transportation direction. As described above, each of the specific patterns is deviated 5 dots with respect to the adjacent specific pattern(s) in the main scanning direction. Therefore, the deviation of the specific pattern Pn+3 from the specific pattern Pn in the forward direction is calculated at 15 (=5×3) dots. This is to say, when a specific pattern is situated k specific patterns ahead of or behind the specific pattern Pn in the transportation direction, the specific pattern is deviated by ±5×k dots from the specific pattern Pn in the forward direction. 
     After the calculation of the deviation, the CPU  101  adjusts the forming positions of the registration marks by correcting the addresses in the image memory  202  in accordance with the calculated deviation. When doing so, the CPU  101  corrects the addresses corresponding to all of the reproduction colors in accordance with the calculated deviation (step S 106 ). The deviation among the forming positions of the registration marks on the photosensitive drums  41 C to  41 K are not so great. For this reason, the forming positions of the registration marks on the photosensitive drums  41 C to  41 Y can be correctly adjusted in accordance with the deviation of the forming position of the photosensitive drum  41 K from the standard forming position. It should be obvious that each deviation of the forming positions of the photosensitive drums  41 C to  41 Y may be also calculated in the same way and the addresses in the image memory  202  may be corrected for each color in accordance with the calculated deviation. 
     The adjustment operation for the registration correction is performed as stated above. Consequently, the optical sensor  25  can reliably detect each valid width of the registration marks for the density detection when the registration correction is performed for image formation. 
     In the present embodiment, the optical sensor  25  detects the specific patterns which are deviated with respect to one another. However, only one specific pattern P 0  may be formed as shown in FIG.  7 . The specific pattern P 0  is composed of a standard line having a width of ±100 dots in the main scanning direction and a sloped line forming a 45° angle with the standard line. As shown in FIG. 7, the middle of the specific pattern P 0  in the main scanning direction is located at the standard forming position. 
     The deviation of the detecting position of the optical sensor  25  from the standard forming position in the main scanning direction is obtained using the specific pattern P 0  as follows. First, a time period taken between the detections of the standard line and the sloped line of the specific pattern P 0  is measured using the optical sensor  25  in the stated way. The shape of the specific pattern P 0  and the moving speed of the transfer belt  31  are apparent. Therefore, the time period taken between the detections of the standard line and the sloped line in a case when there is no deviation of the detecting position of the optical sensor  25  from the standard forming position in the main scanning direction can be obtained beforehand. The CPU  101  sets this time period taken when there is no deviation as the standard time period, and calculates a difference between the standard time period and an actually measured time period. 
     Suppose that this time difference is “T 0 ” and the moving speed of the transfer belt  31  is “V 0 ”. By calculating an equation V 0 ×T 0 , a distance “D” is obtained. The distance D indicates a distance in the sub-scanning direction between the sloped line actually detected by the optical sensor  25  and the sloped line detected by the optical sensor  25  in a case when there is no deviation of the detecting position of the optical sensor  25  from the standard forming position in the main scanning direction. Here, the sloped line forms a 45° angle with the standard line. Therefore, the distance D is equivalent to the deviation of the detecting position of the optical sensor  25  from the standard forming position in the main scanning direction. 
     The deviation obtained in this way is converted into the number of dots. Then, the addresses in the image memory  202  in the main scanning direction are corrected according to the number of dots. Accordingly, the adjustment operation can be performed for the registration correction. 
     Second Embodiment 
     In the second embodiment, an image forming apparatus basically has the same construction as the image forming apparatus of the first embodiment shown in FIG.  2 . However, the image forming apparatus of the second embodiment differs from the image forming apparatus of the first embodiment in that a reflection type optical sensor is used as the optical sensor  25 . As shown in FIG. 8, this optical sensor  25  is set to be freely shifted in the direction of the arrow A (i.e., in the main scanning direction) by a driving mechanism  50  which is composed of a stepping motor  51 , a ball screw  52 , and a guide  53 . The driving mechanism  50  is driven by a sensor motor driving unit (not illustrated). 
     Due to the addition of the driving mechanism  50  to the construction, the controlling section  100  controls a sensor motor driving unit  206  which drives a motor of the driving mechanism  50 . As such, the ROM  103  stores control programs that are not included in the first embodiment. 
     The following is a description of the adjustment operation for the registration correction performed by the image forming apparatus having the stated construction. FIG. 10 is a flowchart of the adjustment operation. Note that, in the initial state, the optical sensor  25  is located at its home position which is located at the end of a shift range of the optical sensor  25 . 
     The CPU  101  stores a line image having a predetermined length in the sub-scanning direction in the area of the image memory  202  associated with black (step S 201 ). The line image is formed on the transfer belt  31  (step S 202 ). Here, the middle of the line image in the main scanning direction is located at the standard forming position which is set at the factory prior to shipment. Then, the CPU  101  controls the transfer belt  31  to stop when the middle point of the line image P 1  in the sub-scanning direction reaches a position that is aligned with the detecting position of the optical sensor  25  on a main scanning line as shown in FIG. 11 (step S 203 ). 
     After this, the CPU  101  gives an instruction to the sensor motor driving unit  206  to drive the driving mechanism  50  of the optical sensor  25 . The CPU  101  then has the optical sensor  25  shift within the shift range, so that the toner density on the transfer belt  31  is detected (step S 204 ). In doing so, the CPU  101  counts driving pulses of the stepping motor  51  of the driving mechanism  50 , and as a result, a signal as shown in FIG. 12 is outputted from the optical sensor  25 . From this signal, the CPU  101  obtains the central position (or, peak position) of the detection value as a standard position using the barycenter calculating method. Then, the CPU  101  obtains the number of pulses N that were counted before the optical sensor  25  detected the toner density of the image formed at the standard position (step S 205 ). This number of pulses N is equivalent to the deviation of the home position of the optical sensor  25  from the standard forming position. 
     The CPU  101  has the optical sensor  25  return to the home position, and has the stepping motor  51  driven by the number of pulses N so that the location of the optical sensor  25  is correctly adjusted (step S 206 ). Accordingly, the optical sensor  25  is located at the correct position, with no deviation from the standard forming position in the main scanning direction, thereby reliably detecting each valid width of the registration marks. 
     In the second embodiment, the location of the optical sensor  25  is corrected to eliminate the deviation after the deviation is calculated. However, as in the case of the first embodiment, after the deviation is calculated and the optical sensor  25  returns to the home position, the addresses in the image memory  202  may be modified so that the image forming positions are corrected. 
     In the stated embodiments, the registration marks are transferred onto the transfer belt  31  and the optical sensor  25  detects these registration marks. However, patterns formed on the transfer belt  31  are not limited to the registration marks and may be different patterns as long as the image forming apparatus forms toner patterns and includes a mechanism which detects the toner patterns using an optical sensor. 
     In the stated embodiments, the present invention has been described for the copier as an example, and in particular, a tandem-type color copier has been described. However, the present invention is not limited to the tandem-type image forming apparatus, and can be applied to a monochrome image forming apparatus or a color image forming apparatus which forms multicolor images using a single photosensitive drum. 
     The present invention is not limited to the image forming apparatus in which toner images formed on the photosensitive drums are transferred directly onto a recording sheet. The present invention can be applied to image forming apparatuses which have transfer intermediate components of various types and employ the intermediate transfer method. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. 
     Therefore, unless such changes and modifications depart from the scope of the present invention, they should be constructed as being included therein.