Patent Publication Number: US-6701117-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an image forming apparatus adopting the electrophotographic process, the electrostatic recording process or the like, and particularly to an image forming apparatus using a transfer material transporting member and a transfer medium as an intermediate transfer member, and having the function of automatically correcting image misregister during image-on-image formation. 
     2. Description of Related Art 
     There has heretofore been proposed an image forming apparatus in which there are disposed a plurality of image forming means each for applying a laser beam modulated in conformity with recording information to a photosensitive drum which is an image bearing member or light emitted by a light emitting element such as an LED (light emitting diode), developing an electrostatic latent image formed on the photosensitive drum by the electrophotographic process and transferring a toner image of each color to transfer paper or an intermediate transfer belt, and the toner images of respective colors are multi-transferred on the transfer paper while the transfer paper is sequentially transported to the respective image forming means by a transfer material transporting belt or the toner images of respective colors are multi-transferred on the intermediate transfer belt, thereafter a color image can be formed by a method of collectively transferring the polychromatic toner images primary-transferred to the intermediate transfer belt to the transfer paper. 
     In the image forming apparatus of this type, there is a case where the positions (registrations) of the respective color images formed on the respective photosensitive drums are not registered with one another on the transfer material to which they are finally multi-transferred, for such reasons as the mechanical mounting errors among the photosensitive drums, the optical path length errors of the respective laser beams, the changes in the optical path and the warp of the LED by the environmental temperature. 
     Therefore, as shown in FIG. 2 of the accompanying drawings, an image misregister detection pattern  3  formed from each photo-sensitive drum onto the transfer material transporting belt or the intermediate transfer belt  31  which is a transfer medium is read by light sensors  2   a ,  2   b , and the deviation of registration on the photosensitive drum corresponding to each color is detected, and electrical correction is effected on an image signal to be recorded and a turn-back mirror provided in the optical path of the laser beam is driven to thereby effect the correction of any change in the optical path length or any change in the optical path. 
     Various patterns have been proposed as the image misregister detection pattern  3 , and for example, in Japanese Patent Application Laid-Open No. 2000-98810, there is proposed a pattern comprising a first segment disposed with a predetermined angle with respect to a process direction which is the direction of movement of a transfer belt and a second segment disposed axisymmetrically with the first segment with respect to an imaginary line orthogonal to the process direction. 
     FIG. 2 shows the manner in which the light sensors  2   a ,  2   b  detect the image misregister detection pattern  3  on the intermediate transfer belt  31  which is a belt member, and the image misregister detection pattern  3  is read by the light sensors  2   a ,  2   b  of an LED  4   a  which is a light emitting element and a phototransistor  4   b  which is a light receiving element. These light sensors  2   a ,  2   b  are disposed in two sets ( 2   a  and  2   b ) at a predetermined distance therebetween in a direction orthogonal to the process direction, and the image misregister detection pattern  3  is formed so as to pass on the light sensors  2   a ,  2   b.    
     As the material of the intermediate transfer belt  31 , use is made of a material of which the reflectance for the light (e.g. infrared light) applied from the LED  4   a  which is a light emitting element in the light sensors  2   a ,  2   b  is great as compared with the reflectance of the image misregister detection pattern  3 , and by this difference in reflectance, the pattern detection of the image misregister detection pattern  3  is made possible. 
     FIG. 3 of the accompanying drawings shows a light receiving circuit  17  for reflecting the light applied from the LED  4   a  to the image misregister detection pattern  3  or the intermediate transfer belt  31 , and converting the output signal when the reflected light thereof is received by the phototransistor  4   b  which is a light receiving element into an electrical signal. 
     In FIGS. 2 and 3, when a region of the intermediate transfer belt  31  is detected by the light sensors  2   a ,  2   b , the quantity of reflected light is great and therefore a great deal of photocurrent passes through the phototransistor  4   b  and it is current/voltage-converted by a resistor  5 , and is amplified by resistors  6 ,  7 ,  8  and an operational amplifier  9 . 
     On the other hand, when the image misregister detection pattern  3  is detected by the light sensors  2   a ,  2   b , the quantity of reflected light is small and therefore, a photocurrent small as compared with that in the region of the intermediate transfer belt  31  passes through the phototransistor  4   b , and it is likewise current/voltage-converted by the resistor  5 , and is amplified by the resistors  6 ,  7 ,  8  and the operational amplifier  9 . 
     FIG. 4 of the accompanying drawings shows a state in which the light receiving circuit  17  has detected the reflected light in the order of a region of the intermediate transfer belt  31 →the image misregister detection pattern  3 →a region of the intermediate transfer belt  31 . In FIG. 4, a threshold level Vt is set intermediately of a transfer belt detection level Va at which the intermediate transfer belt  31  has been detected by the light sensors  2   a ,  2   b  and a pattern detection level Vb at which the image misregister detection pattern  3  has been detected. 
     This threshold level Vt is set by a variable resistor  18  shown in FIG. 3, and by a voltage value outputted from the operational amplifier  9  after the photocurrent passing through the phototransistor  4   b  has been current/voltage-converted and the voltage value of the threshold level Vt set by the variable resistor  18  being compared with each other by a comparator  19 , a pattern detection output  28  shown in FIG. 4 can be created. 
     Design is made such that the pattern detection output  28  sequentially sent is read, and the deviation of registration is detected from the width, interval or the like of the image misregister detection pattern  3  and electrical correction is effected on an image signal to be recorded and further, the turn-back mirror provided in the optical path of the laser beam is driven to thereby effect the correction of any change in the optical path length or any change in the optical path. 
     However, when in the aforedescribed example of the conventional art, an unexpected stain, scar or the like occurs on the intermediate transfer belt  31 , reflectance is reduced on such portions and the reflected light may not be received in some cases by the phototransistor  4   b  in the light sensors  2   a ,  2   b.    
     In that case, the stain or scar on the intermediate transfer belt  31  is recognized as the detection pattern  3 , and the accurate width or interval of the image misregister detection pattern  3  cannot be read, and this has led to the problem that the deviation of registration cannot be corrected accurately. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above-noted problem and the object thereof is to provides, in a construction for detecting an image misregister detection pattern, and on the basis of the result thereof, effecting the registration correction of image forming means, an image forming apparatus which is free from wrong detection due to the stain, scar or the like of a transfer medium and which precisely reads only the image misregister detection pattern to thereby effect highly accurate correction and realize a high quality of image. 
     A preferred form of the present invention for achieving the above object is an image forming apparatus comprising: 
     at least two image forming means; 
     transferring means for successively superimposing and transferring images formed by the image forming means onto a transfer medium; 
     pattern forming means for forming an image misregister detection pattern for detecting the positional deviation between the images formed by the image forming means; 
     pattern detection means for detecting the image misregister detection pattern formed on the transfer medium by the pattern forming means; 
     pattern position detecting means for detecting the position of the image misregister detection pattern from a detection signal obtained from the pattern detecting means; and 
     positional deviation correcting means for correcting the positional deviation of at least one of the image forming means on the basis of the result of the detection of the pattern position detecting means; 
     wherein the pattern positional detecting means detects the time at a point of time whereat the detection signal of the pattern detecting means has been continuously outputted for a predetermined time, and the time at a point of time whereat thereafter the outputting of the detection signal has been stopped, to thereby detect the position of the image misregister detection pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional illustration showing the construction of an image forming apparatus according to the present invention. 
     FIG. 2 is a typical illustration showing the manner in which an image misregister detection pattern on a belt member is read by a light sensor. 
     FIG. 3 shows the construction of a light receiving circuit for receiving the output of the light sensor. 
     FIG. 4 shows the output of the light sensor and the pattern detection output of the light receiving circuit when the image misregister detection pattern has been read. 
     FIG. 5 shows an example of the image misregister detection pattern formed on the belt member. 
     FIG. 6 is a timing chart when the data of the image misregister detection pattern are stored. 
     FIG. 7 is a timing chart when the data of the image misregister detection pattern are stored. 
     FIG. 8 is a flowchart illustrating the registration correcting operation. 
     FIG. 9 is a block diagram showing the construction of a control system. 
     FIG. 10 shows the constructions of a pattern width and position storing portion. 
     FIG. 11 shows another form of the image forming apparatus to which the present invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As an example of an image forming apparatus according to the present invention, an embodiment in which the present invention is applied to a tandem type color image outputting apparatus having a plurality of image forming means arranged in a row will hereinafter be described specifically with reference to the drawings. 
     Referring to FIG. 1, the image forming apparatus  1  is of the electrophotographic type and is constructed as a so-called tandem type color image outputting apparatus having a plurality of image forming means arranged in a row. 
     The color image forming apparatus  1  shown in FIG. 1 comprises an image reading portion  1   a  and an image outputting portion  1   b , and the image reading portion  1   a  optically reads the image of an original placed on a platen glass plate  1   c  or transported by an auto original feeder (not shown) and converts it into an electrical signal and sends it to the image outputting portion  1   b.    
     The image outputting portion  1   b  is broadly divided into an image forming portion  10 , in which four stations a, b, c and d as image forming means are juxtaposed and are the same in construction, a feed unit  20  for feeding transfer materials P contained in feed cassettes  21   a ,  21   b  and a manual feed tray  27 , an intermediate transfer unit  30  for secondary-transferring to the transfer material P toner images primary-transferred to an intermediate transfer belt  31  which is a transfer medium and comprises an intermediate transfer member at the stations a, b, c and d, a fixing unit  40  for fixing the toner images secondary-transferred to the transfer material P, a cleaning unit  50  for removing any residual toners on the intermediate transfer belt  31 , and a control unit  60  for comprehensively controlling these units. 
     In the image forming portion  10 , photosensitive drums  11   a ,  11   b ,  11   c  and  11   d  as image bearing members are supported at the centers thereof and are rotatively driven in the directions indicated by the arrows in FIG.  1 . Primary chargers  12   a ,  12   b ,  12   c ,  12   d , optical systems  13   a ,  13   b ,  13   c ,  13   d , turn-back mirrors  16   a ,  16   b ,  16   c ,  16   d  and developing devices  14   a ,  14   b ,  14   c ,  14   d  are disposed around the photosensitive drums  11   a  to  11   d  and in opposed relationship with the outer peripheral surfaces of the photosensitive drums  11   a  to  11   d.    
     First, charges of a uniform charging amount are given to the surfaces of the photosensitive drums  11   a  to  11   d  by the primary chargers  12   a  to  12   d , whereafter by the optical systems  13   a  to  13   d , the photosensitive drums  11   a  to  11   d  are exposed to a light such as a laser beam modulated in conformity with a recording image signal to thereby form electrostatic latent images thereon. 
     Further, developers of four colors, i.e., yellow, cyan, magenta and black (hereinafter referred to the toners) are supplied to the electrostatic latent images by the developing devices  14   a  to  14   d  containing these toners therein to thereby visualize the electrostatic latent images. 
     Downstream of primary transfer areas Ta, Tb, Tc and Td for transferring the visualized visible images to the intermediate transfer belt  31  which is an intermediate transfer member, any residual toners left on the photosensitive drums  11   a  to  11   d  without being transferred to the transfer material P are scraped off by the cleaning devices  15   a ,  15   b ,  15   c  and  15   d  to thereby effect the cleaning of the surfaces of the photosensitive drums  11   a  to  11   d . By the image forming process shown above, the image formation by the toners of the respective colors is sequentially effected. 
     The feed unit  20  is comprised of the feed cassettes  21   a ,  21   b  and the manual feed tray  27  for containing the transfer materials P therein, pickup rollers  22   a ,  22   b ,  26  for feeding the transfer materials P one by one from the feed cassettes  21   a ,  21   b  or the manual feed tray  27 , a pair of feed rollers  23  and a feed guide  24  for transporting the transfer materials P fed by the pickup rollers  22   a ,  22   b ,  26  to a pair of registration rollers  25 , and the pair of registration rollers  25  for feeding the transfer material P to a secondary transfer area Te in timed relationship with the image formation by the image forming portion  10 . 
     The construction of the intermediate transfer unit  30  will now be described in detail. The intermediate transfer belt  31  which is a belt member is formed, for example, of PET (polyethylene terephthalate), PVdF (polyvinylidene fluoride) or the like, and is looped around a drive roller  32  for transmitting a rotative driving force to the intermediate transfer belt  31 , a tension roller  33  for giving moderate tension to the intermediate transfer belt  31  by the biasing of a spring or the like (not shown) and a driven roller  34  opposed to the secondary transfer area Te with the intermediate transfer belt  31  interposed therebetween. 
     The intermediate transfer belt  31  has a primary transfer flat surface A formed between the drive roller  32  and the tension roller  33 . 
     The drive roller  32  comprises a metal roller having its surface coated with rubber (e.g. urethane rubber or chloroprene rubber) having a thickness of several millimeters to thereby prevent the slip thereof relative to the intermediate transfer belt  31 . Also, the drive roller  32  is rotatively driven by a pulse motor (not shown). 
     Primary transfer chargers  35   a ,  35   b ,  35   c , and  35   d  are disposed on the backside of the intermediate transfer belt  31  at the primary transfer areas Ta, Tb, Tc, and Td in which the photosensitive drums  11   a  to  11   d  are opposed to the intermediate transfer belt  31 . 
     A secondary transfer roller  36  is disposed in opposed relationship with the driven roller  34  with the intermediate transfer belt  31  interposed therebetween, and the secondary transfer area Te is formed by the nip portion thereof with respect to the intermediate transfer belt  31 . 
     The secondary transfer roller  36  is urged against the intermediate transfer belt  31 , which is a belt member and is an intermediate transfer member under a moderate pressure. Also, the cleaning unit  50  for cleaning the image forming surface of the intermediate transfer belt  31  is provided on the intermediate transfer belt  31  and downstream of the secondary transfer area Te in the direction of rotation of the intermediate transfer belt  31 . 
     The cleaning unit  50  is provided with a cleaning blade  51  abutting against the surface of the intermediate transfer belt  31 , and a waste toner box  52  for containing therein the residual toners scraped off by the cleaning blade  51 . 
     The fixing roller  40  is comprised of a fixing roller  41   a  provided with a heat source such as a halogen heater therein, a pressure roller  41   b  urged against the fixing roller  41   a  (in some cases, the pressure roller  41   b  is also provided with a heat source therein), a transport guide  43  for guiding the transfer material P to the nip portion between the above-described pair of rollers  41   a  and  41   b , fixing adiabatic covers  46 ,  47  for confining the heat of the fixing unit  40  therein, a pair of inner delivery rollers  44  and a pair of outer delivery rollers  45  for further directing the transfer material P delivered from the above-described pair of rollers  41   a  and  41   b  to the outside of the image forming apparatus  1 , and a delivery tray  48  for stacking thereon the transfer materials P delivered to the outside of the apparatus. 
     The control unit  60 , as shown in detail in FIG. 9, is comprised of a CPU (central processing unit)  61  for controlling the operation of a mechanism in each of the above-described units, a RAM (random access memory)  62 , a ROM (read only memory)  63 , a motor driver portion  64 , etc., and further a light receiving circuit  17 , a pattern width shaping portion  29  and a pattern width and position storing portion (register)  37  which will be described later in detail. 
     The image forming operation of the image forming apparatus  1  will now be described in detail. When an image forming operation starting signal is generated from the CPU  61  shown in FIG. 9, the feeding operation is started from the feeding means selected in conformity with the selected paper size or the like of the transfer material P. 
     Describing a case where the transfer material has been fed, for example, from the upper feeding means shown in FIG. 1, the transfer materials P are first fed one by one from the feed cassette  21   a  by the pickup roller  22   a . The transfer material P is then guided between the feed guides  24  by the pair of feed rollers  23  and is transported to the pair of registration rollers  25 . 
     At that time, the pair of registration rollers  25  are at a stop and the leading edge of the transfer material P hits against the nip portion between the pair of registration rollers  25 . Thereafter, the pair of registration rollers  25  start to be rotated in timed relationship with the start of the image formation by the image forming portion  10 . 
     The rotation timing of the pair of registration rollers  25  is set so that the toner images primary-transferred onto the intermediate transfer belt  31  by the image forming portion  10  and the transfer material P may just be registered with each other in the secondary transfer area Te. 
     On the other hand, in the image forming portion  10 , when an image forming operation starting signal is generated, the toner image formed on the photosensitive drum  11   d  lying most upstream in the direction of rotation of the intermediate transfer belt  31  by the aforedescribed image forming process is primary-transferred to the intermediate transfer belt  31  in the primary transfer area Td by a primary transfer charger  35   d  to which a high voltage is applied. 
     The primary-transferred toner image is transported to the next transfer area Tc. There is being effected there image formation with a delay of the time for which the toner image is transported between the adjacent ones of the image forming portions  10 , and the next toner image is registered with and transferred onto the previous toner image. Thereafter a similar process is repeated and after all, the toner images of the four colors are successively primary-transferred on the intermediate transfer belt  31 . 
     Thereafter, the transfer material P comes into the secondary transfer area Te and comes into contact with the intermediate transfer belt  31 , whereupon a high voltage is applied to the secondary transfer roller  36  in timed relationship with the passage of the transfer material P. 
     Then, the toner images of the four colors formed on the intermediate transfer belt  31  by the aforedescribed image forming process are transferred to the surface of the transfer material P. Thereafter, the transfer material P is accurately guided to the nip portion between the fixing roller  41   a  and the pressure roller  41   b  by the transport guide  43 . 
     The toner images are fixed on the surface of the transfer material P by the heat of the pair of rollers  41   a  and  41   b  and the pressure of the nip. Thereafter, the transfer material P is transported to the outside of the apparatus by the pair of inner and outer delivery rollers  44  and  45  and is stacked on the delivery tray  48 . 
     The registration correcting operation will now be described with reference to the block diagram of the control unit shown in FIG.  9 . The control unit  60  of FIG. 9 comprises the CPU  61  for controlling the image output portion  1   b , the ROM  63  and RAM  62  for storing a control program and data therein, the motor driven portion  64  for driving various motors, the light receiving circuit  17  for receiving the output from the light sensors  2   a ,  2   b  shown in FIG. 2, and converting it into a waveform which can be processed by the pattern width shaping portion  29 , the pattern width shaping portion  29  for receiving the output from the light receiving circuit  17  and shaping the pattern width of the image misregister detection pattern  3 , and the pattern width and position storing portion (the registers D to S of FIG. 10)  37  for storing the pattern width and position of the image misregister detection pattern  3  therein. 
     The registration correcting operation is started by the instructions from the CPU  61 , and when the image misregister detection pattern  3  is detected, it is converted into an electrical signal by the light sensors  2   a ,  2   b  shown in FIG.  2  and the light receiving circuit  17  shown in FIG. 3, and is inputted to the pattern width shaping portion  29 . 
     In the pattern width shaping portion  29 , as shown in FIG. 6, only when the output continues for a predetermined time T set by the CPU  61  or longer, it is discriminated as the image misregister detection pattern  3 , and the control of storing the pattern width and the pattern position in the pattern width and position storing portion (register)  37  is effected. This pattern width shaping portion  29  and the control of the pattern width and position storing portion  37  will be described later in detail. 
     The position of the pattern is detected on the basis of the data stored in the pattern width and position storing portion  37 , and the deviation of registration on the photosensitive drums  11   a  to  11   d  corresponding to the respective colors is calculated by the use of a table or the like stored in the CPU 61  and the ROM  63 , and electrical correction is effected on an image signal to be recorded or the motor for controlling the turn-back mirrors  16   a  to  16   d  is drive-controlled by the motor driver portion  64  to thereby control the turn-back mirrors  16   a  to  16   d  provided in the optical path of the laser beam, and effect the correction of any change in the optical path length or any change in the optical path. 
     These correcting operations are suitably performed to the plurality of image forming means as required. 
     In the present embodiment, the photosensitive drums  11   a  to  11   d  which are a plurality of image forming means for forming images serve also as pattern forming means for forming the image misregister detection pattern  3  for correcting the misregister of the images formed by the photosensitive drums  11   a  to  11   d , and pattern detecting means for detecting the image misregister detection pattern  3  uses light sensors  2   a ,  2   b  similar to the aforedescribed conventional ones. 
     The light sensors  2   a ,  2   b  have an LED  4   a  which is a light emitting element, and a phototransistor  4   b  which is a light receiving element, and are designed to output a signal when light emitted from the LED  4   a  is reflected by the intermediate transfer belt  31  which is a belt member looped in proximity to the photosensitive drums  11   a  to  11   d  which are the image forming means and rotatively driven and the quantity of reflected light received by the phototransistor  4   b  is a predetermined value or greater. 
     As registration correcting means for correcting the registration among the photosensitive drums  11   a  to  11   d  which are the image forming means on the basis of the result of the detection by pattern detecting means constituted by the light sensors  2   a ,  2   b , the light receiving circuit  17 , the pattern width shaping portion  29  and the pattern width and position storing portion  37 , electrical correction is effected on the image signal to be recorded or the motor for controlling the turn-back mirrors  16   a  to  16   d  is drive-controlled by the motor driver portion  64  to thereby control the turn-back mirrors  16   a  to  16   d  provided in the optical path of the laser beam and effect the correction of any change in the optical path length or any change in the optical path. 
     A construction for accurately detecting the pattern width and interval of the image misregister detection pattern  3  even if a stain or a scar is on the intermediate transfer belt  31 , which construction is to be said to be the feature of the present embodiment, will now be described with reference to the timing charts of FIGS. 6 and 7. 
     As regards an electrical signal outputted from the light receiving circuit  17  for receiving the output from the light sensors  2   a ,  2   b  shown in FIG. 2, and converting it into a waveform which can be processed by the pattern width shaping portion  29  shown in FIG. 9, the signal is outputted as in the case of the image misregister detection pattern  3  because reflectance also lowers when a scar or a stain is on the intermediate transfer belt  31 . 
     So, a control is effected in a manner that the waveform due to the scar or the stain by the pattern is eliminated from the signal outputted from the light receiving circuit  17  by the width shaping portion  29 , which is the pattern detecting means. When there is obtained an output from the light sensors  2   a ,  2   b  as shown, for example, in FIG. 6, such an output as will not reach a preset threshold level Vt can be eliminated by the light receiving circuit  17 . 
     An output exceeding the threshold level Vt, however, cannot be eliminated by the light receiving circuit  17 . So, in the pattern width shaping portion  29 , control is effected by the use of a counter or the like so that no signal may be outputted until an inputted signal continues for the predetermined time T or longer. 
     By effecting this control, it becomes possible to eliminate a signal due to the scar or the stain or the like which is finer than the image misregister detection pattern  3 . However, when the outputting of the signal has stopped, the signal is stopped at that point of time. This is because if it is not effected, there is the possibility that when there are many fine scars or stains, the signal will continue to be outputted. Thereby, as a waveform of the pattern width of the image misregister detection pattern  3  and other width due to a scar or a stain or the like, it becomes possible to obtain a pattern width shaping portion output signal from the pattern width shaping portion  29 , as shown in FIG.  6 . 
     The data storing timing in the pattern width and position storing portion (register)  37  will now be described with reference to FIG.  7 . On the basis of the pattern width shaping portion output signal obtained by the pattern width shaping portion  29  shown in FIG. 7, the counter is operated and further, a latch timing signal is generated and data are stored. 
     When for example, the image misregister detection pattern  3  as shown in FIG. 5 is obtained by a signal as shown in FIG. 7, a counter value “0” is stored in the D register of the pattern width and position storing portion  37  shown in FIG.  10 . 
     Subsequently, the storing of counter value data into respective registers is effected in such a manner that “100” is stored in E register, “150” is stored in F register, “110” is stored in G register, and so on. By these data, it is possible to detect the pattern width and further, the pattern interval of the image misregister detection pattern  3 , and it also becomes possible to find the absolute position (the position of the pattern) from a signal detected at first. 
     However, it must be considered in calculation that in the pattern width shaping portion  29 , the actual sensor output by the light sensors  2   a ,  2   b  is delayed by a time T that is a predetermined time. 
     For example, assuming that the waveform shown in FIG. 7 is the output waveform when it has been set as being delayed by 10(T=10) in terms of the counter value, the width of the pattern ( 1 ) is 110 counts. Also, the pattern ( 2 ) has a pattern width of 120 counts (110 counts+10(=T) counts), and the absolute distance of the central value of the pattern ( 2 ) from the central value of the pattern ( 1 ) is 255 counts (pattern ( 1 ) (55 counts)+interval (150 counts)+pattern ( 3 )(50 counts)). 
     Such a count value correcting operation can be performed to the data stored in the register  37  by the CPU  61 , which is control means, for example, during the registration correcting operation of the CPU  61 . Thus, by this count value correcting operation being performed, the position of the pattern can be detected. 
     By the above-described control, it becomes possible to prevent the wrong detection of the image misregister detection pattern  3  due to a scar or a stain on the intermediate transfer belt  31 , and accurately detect the pattern width and interval and position of the image misregister detection pattern  3 . 
     That is, in the present embodiment, when in the pattern width shaping portion  29  which is pattern detecting means, the time for which the signal from the light sensors  2   a ,  2   b  is outputted is to be read by the counter, the counting of the time is started at a point of time whereat the signal from the light sensors  2   a ,  2   b  has been continuously outputted for the predetermined time T, and the count value of that time is read at a point of time whereat the signal output from the light sensors  2   a ,  2   b  has been stopped. 
     Also, when the time for which the signal from the light sensors  2   a ,  2   b  is not outputted is to be read by the counter, the counting of the time is started at the point of time whereat the signal output from the light sensors  2   a ,  2   b  has been stopped, and the count value of that time is read at the point of time whereat the signal has been continuously outputted for the predetermined time T. 
     Reference is now had to the flowchart of FIG. 8 to describe the registration correcting operation sequence by the registration correcting means for correcting the registration of the photosensitive drums  11   a  to  11   d  which are the image forming means on the basis of the result of the detection by the pattern width shaping portion  29  which is the above-described pattern detecting means. 
     The CPU  61  shown in FIG. 9 performs the registration correcting operation, for example, when the power supply switch of the image forming apparatus  1  is closed or when a predetermined time has passed after the power supply switch is closed. 
     When the registration correcting operation is started, the intermediate transfer belt  31  is rotatively driven at a step S 1  shown in FIG. 8, and at a step S 2 , the writing of the image misregister detection pattern  3  onto the intermediate transfer belt  31  is started by the photosensitive drums  11   a  to  11   d . The LED  4   a  is turned on (step S 3 ) before the image misregister detection pattern  3  written onto the intermediate transfer belt  31  passes the light sensors  2   a ,  2   b , and at a step S 4 , the detecting operation for the image misregister detection pattern  3  is started. 
     At the step S 4 , as previously described, the signal from the light sensors  2   a ,  2   b  is passed through the light receiving circuit  17  and the pattern width shaping portion  29  for shaping the pattern width of the image misregister detection pattern  3  to thereby eliminate the wrong detection signal due to a scar or a stain or the like, and the pattern widths and positions of the image misregister detection pattern  3  are sequentially stored in registers D to S shown in the pattern width and position storing portion (register)  37 . 
     At a step S 5 , the LED  4   a  is turned off and the rotative driving of the intermediate transfer belt  31  is stopped, and the pattern width and interval detecting operation is terminated and advance is made to a step S 6 , where electrical correction is effected on the image signal to be recorded on the basis of the data stored in the aforementioned registers D to S and the table or the like stored in the ROM  63 , and the turn-back mirrors  16   a  to  16   d  provided in the optical path of the laser beam are driven to thereby effect the correction of any change in the optical path length or any change in the optical path, thus terminating the registration correcting operation. 
     For example, FIG. 5 shows a state in which the image misregister detection patterns  3  are read and output thereof are stored. The position data and width data of a image misregister detection pattern  3   a  are stored in the registers D, E, F and G on the basis of an image misregister detection pattern output obtained by the light sensors  2   a ,  2   b  reading the image misregister detection pattern  3   a.    
     Likewise, the position data and width data of image misregister detection patterns  3   b  to  3   d  are stored in registers H to S, respectively, on the basis of image misregister detection pattern outputs obtained by the light sensors  2   a ,  2   b  reading the image misregister detection patterns  3   b  to  3   d.    
     Also, while in the present embodiment, description has been made of the registration correcting process in the intermediate transferring process (collectively transferring process) by the intermediate transfer belt  31  on which images are formed by the photosensitive drums  11   a  to  11   d  which are the image forming means, of course the present invention is also effectively applicable to the multi-transfer process by a transfer material transporting belt which is transfer material transporting means for transporting the transfer materials P on which images are formed by the image forming means. 
     FIG. 11 shows another form of the image forming apparatus to which the present invention is applicable, and this apparatus uses a transfer material transporting belt, which is a transfer material transporting member. In this apparatus, toner images formed by a plurality of image forming means  110 Y to  110 K are successively superimposed and transferred onto a transfer material borne on and transported by a transfer material transporting belt  108 , whereby a color image is formed. Describing the image forming means  110 Y in detail, an electrostatic latent image is formed on the surface of an image bearing member  113 Y uniformly charged by primary charging means  114 , by the exposure by exposing means  115 , and this latent image is developed as a toner image by developing means  116 . This toner image is transferred to a transfer material by transferring means  121 Y, and any untransferred toner is collected by a cleaner  117 . 
     The transfer material fed from a cassette  101  by feeding means  102 ,  103 , and  104  starts to be fed by registration rollers  107  in synchronism with the image formation timing in the image forming means, and is borne on and transported by the transfer material transporting belt  108  and at the same time, toner images formed by the image forming means  110 Y to  110 K are successively superimposed and transferred onto the transfer material. After the transfer of all toner images is terminated, the transfer material is separated from the belt  108 , and the fixing of the toner images is effected by fixing means  118 . 
     In such an image forming apparatus, for the registration of the images among the image forming means, an image misregister detection pattern is formed on the transfer material transporting belt  108  and the detection thereof by the light sensors  2   a ,  2   b  is effected. During this detection, the method shown in the previous description is used. 
     While in the aforedescribed embodiment, description has been made of a case where the time until the signal of the pattern width shaping portion  29  is outputted is defined as the predetermined time T and set as a fixed value, it becomes possible for the CPU  61  of the control unit  60  to perform the detecting operation for detecting any stain or scar on the intermediate transfer belt  31  prior to performing the operation of detecting the image misregister detection pattern  3 , detect in advance the time during which the scar or stain can be removed, and determine the time T until the signal of the pattern width shaping portion  29  is outputted, to thereby enhance accuracy. 
     That is, before the correction of the registration of the photosensitive drums  11   a  to  11   d  which are the image forming means is effected by registration correcting means, the scar or stain detecting operation for detecting any scar or stain on the intermediate transfer belt  31  which is a belt member is performed by the light sensors  2   a ,  2   b  which are pattern detecting means, and in conformity with the width of the scar or stain detected by the pattern detecting means, the predetermined time T regarding the reading of the pattern detecting means is controlled. 
     For example, the detecting operation for the surface of the belt is performed while the belt is moved in a state in which the formation of the image misregister detection pattern  3  is not effected. If at this time, there is a signal detected by the length of a time T′ exceeding the time T, the scar or stain will be wrongly detected if the value of the time T remains unchanged. So, by changing the value of this time T to a value exceeding T′, it is possible to prevent the wrong detection due to the scar or stain.