Patent Publication Number: US-8532518-B2

Title: Image forming apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine or a multi-function machine having a plurality of functions of these machines. More specifically, the direction relates to the image forming apparatus, of an electrophotographic type or an electrostatic recording type, in which a patch as a control image (image for control) is formed and an image density is controlled. 
     In a conventional image forming apparatus of the electrophotographic type, in general, the surface of a drum-like photosensitive member as an image bearing member is uniformly charged by a charger and the charged photosensitive member is exposed to light by an exposure device depending on image information to form an electrostatic latent image on the photosensitive member. The electrostatic latent image formed on the photosensitive member is visualized as a toner image with a toner as a developer by using a developing device. Then, the visualized image is transferred onto the recording material. Thereafter, the toner image transferred on the recording material is melt-fixed on the recording material under heat and pressure by a fixing device. 
     In such an image forming apparatus, in order to control a density and gradation of the image, it has been conventionally practiced that the control image (patch) is formed separately from a normal image which is an image based on image data and the density of the patch is detected. The patch is formed between output images (normal images) in general. That is, as shown in  FIG. 20 , a plurality of patches β different in density are formed between an output image α Band an output image α which are formed on a photosensitive drum  1  which is the photosensitive member. Then, these patches β formed between the images are detected by a density detecting sensor and a detection result is fed back to an image forming condition for charging, exposure, development and the like, so that control of the density and gradation of the output image is effected. Further, in the case where the patch is detected on the photosensitive drum  1  and between the images, in order to alleviate a cleaning load at a downstream side, a bias (reverse bias) of an identical polarity to a toner charge polarity is applied to a transfer means. As a result, most of the patches can be sent to a cleaning means for removing the toner on the photosensitive drum  1 . In the case of a structure in which the toner image is primary-transferred from the photosensitive drum onto an intermediary transfer belt as an intermediary transfer member and then is secondary-transferred from the intermediary transfer belt onto the recording material, a structure in which the toner remaining on the intermediary transfer belt is electrostatically removed has been known. For example, a fur brush is disposed downstream of a secondary transfer portion of the intermediary transfer belt and electrostatically attracts and removes the toner remaining on the intermediary transfer belt (Japanese Laid-Open Patent Application (JP-A) 2008-129472). Incidentally, in the case of a structure described in JP-A 2008-129472, in order not to transfer the patches onto the intermediary transfer belt, a bias of an opposite polarity to that for the normal image is applied when the patches reach a primary transfer portion. 
     Further, with respect to a structure including the intermediary transfer belt, a structure in which an untransferred toner image such as the patch transferred on the intermediary transfer belt is charged and a collecting efficiency of the untransferred toner image by the fur brush for electrostatically removing the toner on the intermediary transfer belt is improved has also been known (JP-A 2008-122625). In the case of the structure described in JP-A 2008-122625, the patch transferred on the intermediary transfer belt is sent to the fur brush, without being deposited on a secondary transfer roller, by separation between the secondary transfer roller and the belt at the secondary transfer portion. 
     In the case where the patch is formed between the images as described above, as shown in  FIG. 20 , there is a need to arrange the patches different in density in a main scan direction which is an axial direction of the photosensitive drum  1 . Further, in order to detect the density of the plurality of patches arranged in the main scan direction, sensors are required correspondingly to the number of the patches. Here, the number of the sensors which can be provided is limited and therefore the number of the patches which can be formed between the images is also limited. For this reason, the control of the density and the gradation by the patch is not readily effected with high accuracy. 
     On the other hand, it would be considered that the patches are arranged and formed in sub-scan direction perpendicular to the main scan direction at a position in which the patches are adjacent to the normal image with respect to the main scan direction. When the plurality of patches are arranged in the sub-scan direction, the density of the patches can be detected by a single sensor and therefore the number of the patches can be easily increased, so that the control of the density and the gradation can be effected with high accuracy. 
     However, in the case where the patches are formed at the adjacent position to the normal image, the reverse bias application at the patch portion as described in JP-A 2008-129472 cannot be performed. For this reason, as shown in ( a ) of  FIG. 21 , the patch β is also transferred together with the normal image α onto an intermediary transfer belt (intermediary transfer member)  6 . However, at the secondary transfer portion where the image is transferred from the intermediary transfer belt  6  onto a recording material  7 , the normal image α is transferred onto the recording material  7  and on the other hand, the patch β is transferred onto a secondary transfer bias  9 . Here, at the secondary transfer portion, a bias to which a paper-shaving voltage in view of impedance of the recording material  7  is added is to be applied but the patch β located at a position apart from the recording material  7  is, as shown in ( b ) of  FIG. 21 , subjected to electric discharge at a degree more than that for the normal image by the paper-sharing voltage. As a result, e.g., there is a case where a part of the patch β carried by the negative-polarity voltage is discharged to about 0 volts or a positive polarity. 
     As the structure in which the toner deposited on the secondary transfer roller  9  is electrostatically removed, there is a structure in which an electrostatic cleaning means such as the fur brush is disposed. However, as described above, a part of the patch β which is discharged to about 0 volts or is positively charged is less liable to be removed. In the case where the toner remains on the secondary transfer roller  9  without being removed by the electrostatic cleaning means, the back surface of a subsequent recording material  7  to be subjected to the image formation after the recording material  7  passes through the secondary transfer portion is contaminated with the residual toner. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an image forming apparatus capable of suppressing contamination of a transfer portion even when cleaning of the transfer portion at which a toner image is transferred from an intermediary transfer member onto a recording material is electrostatically effected. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising: 
     an image bearing member; 
     image forming means for forming a toner image on the image bearing member, the image forming means being capable of forming an image to be formed on a recording material and forming a control image for controlling an image density at a position adjacent to the image to be formed on the recording material; 
     an intermediary transfer member rotatable while carrying the toner image transferred from the image bearing member; 
     a transfer member for forming a transfer portion where the toner image is to be transferred from the intermediary transfer member onto the recording material; 
     electrostatic cleaning means for electrostatically removing the toner deposited on the transfer member; 
     density detecting means, provided upstream of the transfer member with respect to a rotational direction of the intermediary transfer member, for detecting a density of the control image transferred on the intermediary transfer member; 
     adjusting means for adjusting an image forming condition of the image forming means depending on an output of the density detecting means; and 
     charge amount changing means for adjusting a charge amount of the control image transferred onto the intermediary transfer member. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural view of an image forming apparatus in First Embodiment according to the present invention. 
         FIG. 2  is a schematic view showing a positional relationship among a charging device, a normal image and patches. 
         FIG. 3  is a block diagram of a controller of the image forming apparatus in First Embodiment. 
         FIG. 4  is a flow chart of control in First Embodiment. 
       Parts ( a ) and ( b ) of  FIG. 5  are schematic views for illustrating a charge polarity relationship of toners at a secondary transfer position, in which ( a ) shows the case where the patch is not charged and ( b ) shows the case where the patch is charged. 
         FIG. 6  is a schematic view for illustrating a positional relationship between the normal image and the patch in Embodiment 1. 
         FIG. 7  is a schematic view for illustrating the type of a patch screen in First Embodiment. 
         FIG. 8  is a schematic view for illustrating the positional relationship between the normal image and the patch during black (monochromatic) image formation. 
         FIG. 9  is a schematic view for illustrating the type of the patch screen during the black image formation. 
         FIG. 10  is a graph showing a distribution of a toner charge amount when the toner is charged and uncharged. 
         FIG. 11  is a table showing a relationship among a basis weight of a recording material, a secondary transfer voltage and a charger current. 
         FIG. 12  is a schematic structural view of an image forming apparatus in Second Embodiment of the present invention. 
         FIG. 13  is a schematic view for illustrating positional relationship among the charging device, the normal image and the patch. 
         FIG. 14  is a flow chart in Third Embodiment. 
         FIG. 15  is a graph showing a distribution of the toner charge amount in different environments. 
         FIG. 16  is a graph showing a relationship between input signal of the patch and the charger current in different environments. 
         FIG. 17  is a graph for illustrating the effect of the present invention. 
         FIG. 18  is a schematic structural view of another example of the image forming apparatus. 
         FIG. 19  is a schematic structural view of a further example of the image forming apparatus. 
         FIG. 20  is a perspective view of a photosensitive drum in the case where the patches are formed at a sheet interval. 
       Parts ( a ) and ( b ) of  FIG. 21  are schematic views, for illustrating a problem in the case where the patches are formed at a side of the normal image, as seen from a side portion of a secondary transfer position. 
         FIG. 22  is a schematic view showing the positional relationship among the charging device, the normal image and the patch. 
       Parts ( a ) and ( b ) of  FIG. 23  are schematic views for illustrating a structure of a shutter of the charging device, in which ( a ) is a schematic view as seen from the shutter side and ( b ) is a schematic view as seen from a side portion. 
         FIG. 24  is a graph showing the distribution of the toner charge amount when the toner is charged and uncharged. 
         FIG. 25  is a schematic structural view of an image forming apparatus in Fifth Embodiment of the present invention. 
         FIG. 26  is a table showing an open/close state of the shutter of the charging device. 
         FIG. 27  is a flow chart of control with respect to a recording material of sizes up to a large size. 
         FIG. 28  is a flow chart of control with respect to a recording material of an overlarge size. 
         FIG. 29  is a graph showing the distribution of the toner charge amount indifferent environments. 
         FIG. 30  is a schematic view for illustrating the positional relationship among the charging device, the normal image and the patch. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     First Embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 11 . First, with reference to  FIG. 1 , an image forming apparatus in this embodiment will be described. The image forming apparatus in this embodiment has a constitution of a so-called tandem type in which a plurality of image forming stations for different colors are juxtaposed in a rotational direction of an intermediary transfer member. Incidentally, in the following description, to reference numerals or symbols for constituent elements for yellow, magenta, cyan and black, suffixes Y, M, C and k are added, respectively. However, with respect to the same constitution, these suffixes are omitted in some cases. 
     Image Forming Apparatus 
     In  FIG. 1 , around photosensitive drums (photosensitive members, image bearing members)  1 Y,  1 M,  1 C and  1   k , members including charging devices  2 Y,  2 M,  2 C and  2   k , exposure devices  3 Y,  3 M,  3 C and  3   k , developing devices  4 Y,  4 M,  4 C and  4   k , an intermediary transfer belt (intermediary transfer member)  6  and the like are disposed. The photosensitive drum  1  is rotationally driven in an arrow F direction at a predetermined peripheral speed (process speed). The charging device  2  electrically charges the peripheral surface of the photosensitive drum  1  to a predetermined polarity and a predetermined potential (primary charging). A laser beam scanner as the exposure device  3  outputs laser light which is ON/OFF-modulated correspondingly to image information inputted from an unshown external device such as an image scanner or a computer, so that the charged surface of the photosensitive drum  1  is subjected to scanning exposure. By this scanning exposure, an electrostatic latent image depending on objective image information is formed on the surface of the photosensitive drum  1 . 
     The developing devices  4 Y,  4 M,  4 C and  4   k  incorporates therein color component toners of yellow (Y), magenta (M), cyan (C) and black (K), respectively. Further, on the basis of the image information, the developing device  4  to be used is selected and develops the electrostatic latent image with the developer (toner) on the surface of the photosensitive drum  1 , so that the electrostatic latent image is visualized as the toner image. In this embodiment, a normal triboelectric chargeability (normal charge polarity) is negative. In this embodiment, as described above, a reverse developing method in which the toner is deposited on the exposed portion of the electrostatic latent image to develop the electrostatic latent image. The charging device, the exposure device and the develop device constitute an image forming means. 
     The intermediary transfer belt  6  is an endless belt and is disposed in contact with the surface of the photosensitive drum  1 , and is stretched around a plurality of a plurality of stretching rollers  20 ,  21  and  22 . The intermediary transfer belt  6  is rotationally moved in an arrow G direction at a speed of, e.g., 300 mm/sec. In this embodiment, the stretching roller  20  is a tension roller configured to control the tension of the intermediary transfer belt  6  at a constant level, the stretching roller  22  is a driving roller for driving the intermediary transfer belt  6 , and the stretching roller  21  is an opposite roller for secondary transfer. Further, at the primary transfer position, each of primary transfer rollers  5 Y,  5 M,  5 C and  5   k  is disposed opposed to the associated photosensitive drum  1  while sandwiching the intermediary transfer belt  6  between itself and the photosensitive drum  1 . 
     In this embodiment, as the intermediary transfer belt  6 , the endless belt prepared by containing carbon black as an antistatic agent in an appropriate amount in various resin or rubber materials such as polyimide and polycarbonate. The intermediary transfer belt  6  has a volume resistivity of, e.g., 1×10 9 -1×10 14  ohm·cm and a thickness of, e.g., 0.07-0.5 mm. 
     The respective unfixed color toner images formed on the photosensitive drums  1  are successively primary-transferred electrostatically onto the intermediary transfer belt  6  by applying a primary transfer bias of a positive polarity opposite to the (negative) toner charge polarity from a constant voltage source or a constant current source to the primary transfer rollers  5 . Thus, on the intermediary transfer belt  6 , a full-color image obtained by superposing the unfixed four color toner images is obtained. The intermediary transfer belt  6  is rotated while carrying the toner images transferred from the photosensitive drums  1 . Every one rotation of the photosensitive drum  1  after the primary transfer, the surface of the photosensitive drum  1  is subjected to cleaning of a transfer residual toner by a cleaning device  11  and then repeatedly enters a subsequent image forming process. 
     Further, at the secondary transfer position of the intermediary transfer belt  6  facing a conveying path of the recording material  7 , a secondary transfer roller  9  (transfer position) is press-contacted to the intermediary transfer belt  6  at a toner image-carrying surface side. Further, inside the intermediary transfer belt  6  at the secondary transfer position, the opposite roller  21  which constitutes an opposite electrode to the secondary transfer roller  9  and to which a bias is to be applied is disposed. When the toner image on the intermediary transfer belt  6  is transferred onto the recording material  7 , to the opposite roller  21 , the bias of an identical polarity to the toner charge polarity is applied from a transfer bias applying means  28 . For example, the bias voltage of −1000 to −3000 V is applied, so that a current of −10 to −50 μA flows. The transfer voltage at this time is detected by a high transfer voltage detecting means  29 . Further, at a downstream side of the secondary transfer position, a cleaning device (belt cleaner)  12  for removing the toner remaining on the intermediary transfer belt  6  after the secondary transfer is provided. 
     In this embodiment, each of the primary transfer roller  5  and the secondary transfer roller  9  is prepared by forming an electroconductive layer on an outer peripheral surface of a core metal of, e.g., 8-12 mm in outer diameter, so as to provide an outer diameter of 16-30 mm. The electroconductive layer is prepared by mixing an ion conductive substance into a base material using a polymeric elastomer or polymeric foam material of hydrin rubber, EPDM or the like, and is adjusted at a medium resistance level of 1 MΩ to 100 MΩ as electroconductivity. Further, as a surface layer of the secondary transfer roller  9 , a resin coating layer formed of urethane or nylon in the thickness of 2-10 μm is used. The hardness of the whole transfer roller is 25-40 degrees in terms of Asker C hardness. A load of 0.6-1.5 kgf (5.88-14.7 N) is exerted on the photosensitive drum  1 , and a load of 1.5-5 kgf (14.7 N-49.03 N) is exerted on the opposite roller  21  for the secondary transfer. 
     Further, in an unshown sheet feeding cassette, sheets of the recording material  7  are accommodated. On the basis of a sheet feeding start signal, an unshown sheet feeding roller is driven, so that the recording material  7  in the sheet feeding cassette is fed one by one and then is conveyed in an arrow H direction by a registration roller  8 . The conveyance of the recording material  7  is controlled by the registration roller  8  so as to be synchronized with timing when a leading end of the toner image on the intermediary transfer belt  6  reaches the secondary transfer position. 
     The recording material  7  introduced into the secondary transfer position is nip-conveyance at the secondary transfer position and at that time, a constant voltage bias (transfer bias) controlled in a predetermined manner is applied from a secondary transfer bias applying means  28  to the opposite roller  21  of the secondary transfer roller  9 . The transfer bias of the identical polarity to the toner charge polarity is applied to the opposite roller  21 , so that the four color-based full-color images (toner images) superposed on the intermediary transfer belt  6  are collectively transferred onto the recording material at the transfer position to form an unfixed full-color toner image on the recording material. 
     The recording material  7  on which the toner images are transferred at the transfer position is separated from the intermediary transfer belt  6  and then is conveyed and introduced into an unshown fixing device, so that the recording material  7  is subjected to a fixing step of the toner image under heat and pressure. On the other hand, the intermediary transfer belt  6  after the transfer separation is subjected to cleaning by a cleaning device  12  to remove transfer residual toner, thus repeatedly entering the image forming step. The cleaning device  12  electrostatically removes the toner on the intermediary transfer belt  6 . 
     The cleaning device  12  includes a fur brush  12   a  and a metal roller  12   d . To the metal roller  12   b , a power source  12   d  is connected. The stretching roller  22  which is grounded is disposed at a position in which the roller  22  opposes the cleaning device  12  via the intermediary transfer belt  6  in contact with the intermediary transfer belt  6 . Further, the fur brush  12   a  is rotated counterdirectionally to the intermediary transfer belt  6 . Then, to the power source  12   d , a constant voltage-controlled high voltage (cleaning bias) is applied, so that the residual toner deposited on the surface of the intermediary transfer belt  6  is attracted by the fur brush  12   a  through an electrostatic force to be collected and removed. The toner collected by the fur brush  12   a  is collected by the metal roller  12  and is removed by a blade  12   c.    
     The secondary transfer roller  9  is provided with a secondary transfer portion cleaning device (electrostatic cleaning means)  60  for electrostatically removing the toner deposited on the secondary transfer roller  9 . The secondary transfer portion cleaning device  60  includes fur brushes  60   a  and  60   b  and a metal roller  61 . To the metal roller  61 , a power source  62  is connected. The secondary transfer roller  9  is grounded. The fur brushes  60   a  and  60   b  are rotated counterdirectionally to the secondary transfer roller  9 . Then, to the power source  62 , a constant voltage-controlled high voltage (cleaning bias) is applied, so that the residual toner deposited on the surface of the secondary transfer roller  9  is attracted by the fur brushes  60   a  and  60   b  through the electrostatic force to be collected and removed. The toner collected by the fur brushes  60   a  and  60   b  is collected by the metal roller  61  and is removed by a blade  63 . In this embodiment, the two fur brushes  60   a  and  60   b  are provided, so that a contact area between the secondary transfer roller  9  and the fur brushes is increased and thus a collection efficiency is improved. It is also possible to use a single fur brush. 
     Further, in this embodiment, as shown in  FIG. 2 , the normal image α based on the image data and the patch β which is the control image are formed, and the patch βB is detected, so that the density and gradation of the output image (normal image) are controlled. The patch β is formed at a position spaced from the normal image α with respect to the direction (main scan direction) perpendicular to the rotational direction (sub-scan direction) of the intermediary transfer belt  6 . For example, as shown in  FIG. 2 , the patch β is formed at the position spaced from an area, in which the output image on an A3-sized sheet is to be outputted, with respect to the main scan direction. Further, the patch β is constituted by a plurality of images different in density and gradation arranged in the sub-scan direction. 
     Density Detecting Portion 
     In this embodiment, in order to detect such a patch β, a single patch sensor (density detecting means or portion)  17  is provided, upstream of the secondary transfer roller  9  with respect to the rotational direction of the intermediary transfer belt  6 , at a position in which the patch sensor  17  opposes the intermediary transfer belt  6 . Further, an opposite roller  19  is disposed, in contact with the intermediary transfer belt  6 , at a position in which the roller  19  opposes the patch sensor  17  via the intermediary transfer belt  6 . These positions of the patch sensor  17  and the opposite roller  19  are downstream of a downstreammost image forming station and are upstream of the secondary transfer position with respect to the rotational direction of the intermediary transfer belt  6 . In  FIG. 1 , the patch sensor  17  and the opposite roller  19  are disposed between the block photosensitive drum  1   k  and the stretching roller  20 . 
     Further, the positions of the patch sensor  17  and the opposite roller  19  with respect to the mains can direction are such that the positions opposite to the position in which the patch β is formed. That is, the patch sensor  17  is disposed at the position spaced from the area, in which the normal image α is formed, with respect to the main scan direction. In  FIG. 2 , the patch β is disposed at the rear side with respect to a widthwise direction (main scan direction) of the intermediary transfer belt  6  and therefore the patch sensor  17  is also disposed at the rear side. Incidentally, in the case where the image forming apparatus is mounted, a side where a user operates the image forming apparatus is referred to as a front side and its opposite side is referred to as the rear side. 
     Charge Amount Changing Portion 
     In this embodiment, between the secondary transfer position and the patch sensor  17 , a charging device  50  (charge amount changing means or portion) for changing the charge amount of the patch β transferred on the intermediary transfer belt  6  is disposed. That is, with respect to the rotational direction, the charging device  50  is disposed, upstream of the secondary transfer roller  9  and downstream of the patch sensor  17 , so as to oppose a portion where the patch β is formed on the outer peripheral surface (image transferred surface) of the intermediary transfer belt  6 . Further, an opposite roller  51  is disposed, in contact with the intermediary transfer belt  6 , at a position in which the roller  51  oppose the charging device  50  via the intermediary transfer belt  6 . 
     Such a charging device  50  is a corona charger and is disposed at the position in which the charging device  50  opposes the patch β but does not oppose the normal image α. In this embodiment, by the charging device  50  having such a constitution, the patch β is charged without charging the normal image α. That is, when the charging device  50  is turned on, the patch β is charged, so that only the charge amount of the patch β can be increased. Further, when the charging device  50  is turned off, the patch β can be prevented from being charged. Further, the charge amount of the patch β can be lowered by applying an opposite voltage to the charging device  50 . Thus, the charge amount of the patch β can be changed by on/off control of the charging device  50  or by changing the applied voltage. 
     Controller 
     Next, control in this embodiment will be described also with reference to  FIGS. 3 and 4 . As shown in  FIG. 3 , a controller (control means)  101  controls a printer  205  as an image forming means on the basis of data (not shown) read by a scanner  201  as an image reading means or sent from an external device. Further, the controller  101  controls the charging device  50  so that a removing efficiency of the patch β, transferred on the secondary transfer roller  9 , by the secondary transfer cleaning device  60  can be improved. For this purpose, the controller  101  includes a reading controller  202 , a CPU  203 , a pattern designating portion  204 , an output controller  206 , a gradation correcting portion  207 , a patch charge controller  208  and a paper type setting portion (sheet type setting means)  209 . The reading portion  202  controls reading of the image data by the scanner  201 . The patch charge controller  208  controls the charging device  50 . The pattern designating portion sets the output image. The output controller  206  sets the secondary transfer voltage and the like. The gradation correcting portion  207  corrects the gradation (level) of the output image. The paper type setting portion  209  sets the paper type (e.g., plain paper, thick paper, coated paper, etc.) inputted by the user by an unshown inputting means. The CPU  203  controls these portions. 
     Incidentally, setting of the paper type can also be performed by providing a sensor (paper type detecting means) for detecting the paper type at any portion of a conveying path of the recording material  7 . Such a sensor detects the paper type on the basis of information on reflection of ultrasonic wave applied to the recording material. 
     An example of the control by the controller  101  will be described along a flow chart shown in  FIG. 4 . First, an original is read by driving the scanner  201  by the reading controller  202 , so that the output image is set by the pattern designating portion  204  (S 1 ). Next, reading of the paper type setting is effected by the paper type setting portion  209  (S 2 ). Then, the secondary transfer voltage depending on the paper type is set by the output controller  206  (S 3 ), and a patch charging output depending on the secondary transfer voltage is set by the patch charge controller  208  (S 4 ). Incidentally, the secondary transfer voltage can also be set by taking a detection result of an environment sensor  30  described later into consideration. Thereafter, the pattern designating portion  204  provides an instruction so that the output image (normal image &amp;a) and the patch β located at the side of the output image are formed on the intermediary transfer belt  6 , with the result that the patch β is formed at the side of the control image (S 5 ). Then, the patch sensor  17  is turned on, so that gradation correction is made on real time by the gradation controller  207  (S 6 ). The patch which reaches the charging device  50  is charged by performing the patch charging output by the patch charge controller  208  (S 7 ). The patch charging is continued until a final patch on the last sheet designated for output passes through the charging device  50 . 
     As in this embodiment, when the patch β is formed adjacently to the normal image α, the plurality of patches can be provided in line in the sub-scan direction without increasing the number of the patch sensor  17 . For this reason, the control of the density and gradation of the output image can be effected inexpensively with high accuracy. Further, the charge amount of the patch β is changed by the charging device  50  and even when the patch β is formed adjacently to the normal image α, the charge amount of the patch β can be adjusted at a proper level. 
     Specifically, in the case where the patch β is formed at the position spaced from the recording material  7 , the patch β is transferred onto the secondary transfer roller  9  at the secondary transfer position. At this time, a voltage larger than that applied to the normal image α transferred onto the recording material  7  is applied to the patch β. For this reason, when the charge amount of the patch β is small, the charge amount can be become about zero during the voltage application, so that the electrostatic cleaning of the patch β on the secondary transfer roller  9  cannot be sufficiently performed by the secondary transfer portion cleaning device  60 . In this embodiment, as described above, the charge amount of the patch β is increased by the charging device  50 , so that a degree of the decrease in charge amount to about zero can be reduced even when the patch β is transferred onto the secondary transfer roller  9 . 
     That is, as shown in ( a ) of  FIG. 5 , in the case where the patch β is not charged, the toner with no charge amount is generated at the secondary transfer position and cannot be electrostatically removed. On the other hand, as shown in ( b ) of  FIG. 5 , when the patch β is charged to the negative polarity relative to the normal image α at the position upstream of the secondary transfer position, substantially all of the patches β are transferred onto the secondary transfer roller  9  and can be electrostatically removed. 
     On the other hand, when the charge amount of the normal image α transferred onto the recording material  7  is increased, a transfer efficiency by the secondary transfer bias determined in view of the paper-sharing voltage is lowered since the charge amount is out of a good charge amount range, so that there is a possibility that improper transfer occurs. In this embodiment, it is possible to prevent an increase in charge amount of the normal image α by disposing the charging device  50  at the position in which the charging device  50  opposes the patch β but does not oppose the normal image α. As a result, the normal image α can be properly transferred onto the recording material  7 , and the patch β transferred on the secondary transfer roller  9  can be sufficiently removed and thus contamination of the secondary transfer roller  9  can be suppressed. When the contamination of the secondary transfer roller  9  can be suppressed, a degree of an occurrence of back-side (surface) contamination on the recording material  7  to be subjected to subsequent image formation. 
     In the control described above, the case where the patch β is transferred onto the secondary transfer roller  9  is described. However, depending on the size of the recording material  7 , there is a case where the patch β is transferred onto the recording material  7 . For this reason, in this embodiment, the paper type setting portion  209  can read not only the paper type but also the size of the recording material  7 . Further, the environment sensor (environment detecting means)  30  for detecting a temperature and humidity in the image forming apparatus is provided ( FIGS. 1 and 3 ). As a result, it is also possible to effect the above-described control also by making reference to the environment detected by the environment sensor  30  and the size of the recording material read by the paper type setting portion  209 . 
     Embodiment 1 
     Next, a specific embodiment of First Embodiment will be described. First, in this embodiment, the rotational movement speed of the intermediary transfer belt  6  is 300 mm/sec and a measurement temperature and humidity environment is normal temperature and normal humidity environment (NN environment) (23° C., 50% RH).  FIG. 6  shows constitutions of the output image (normal image α) and the patch β in this embodiment. The output image size in A3 and at the position (side) adjacent to the output image with respect to the main scan direction, the patch β is formed along a longitudinal direction (sub-scan direction). At the side of the output image on a first sheet, patches for Y and M are formed, and at the side of the output image on a second sheet, patches for C and k are formed. An input signal for patch gradation levels is common to the respective colors, and patches with 10 gradation levels of C 0   h , B 0   h , A 0   h ,  90   h ,  70   h ,  50   h ,  40   h ,  30   h ,  20   h  and  10   h  from a leading end side with respect to the belt movement direction are formed. These values represent associated ones of 256 gradation levels by hexadecimal notation and represent descending gradation levels in this order. 
       FIG. 7  shows screen types of the patches. With respect to the first and second sheets, these two sheets are treated as one set for which each color is changed on the same screen. Similarly, third and later sheets are treated. In this embodiment, a screen A for the first sheet is judged as a photographic portion during copying and is a line screen with 200 lines. A screen B for the third sheet is the photographic portion during printing and is judged as the line screen with 160 lines. A screen C for a fifth sheet is judged as a character portion during the printing and is a 260-line dot screen. Further, a screen D for a seventh sheet is judged as the character portion during the copying and is error diffusion. 
       FIG. 8  shows the constitution of the output image (normal image α) and the patch β when the black (k) image is formed. In this case, the basic constitutions are similar to those in  FIG. 6  but the patches for Y, M and C are not formed. That is, the patch for k is formed at the latter half portion for the second sheet.  FIG. 9  shows the patch screen types for k (black). The basic constitution is similar to that in  FIG. 7  but the patches for Y, M and C are not formed. That is, the patch is formed at the latter half portion for even-numbered sheets. 
     These patches β formed at the side of the output images are always formed in a default state except for the case of an overlarge size (330.2×487.7 mm) be can be not formed by user selection. 
     In this embodiment, the patches having the above-described constitutions are transferred onto the secondary transfer roller  9  and are removed by the secondary transfer portion cleaning device  60 . Further, in this embodiment, the charging device  50  is the corona charger of 25 mm in width and is constituted so that ion current flowing from the charging device  50  covers the entire patches with respect to the longitudinal direction but does not cover the output image. Further, in this embodiment, the densest patch (input signal C 0 H) is set to be transferred onto the secondary transfer roller  9 . The current passing through a wire of the charging device  50  is −6 μA and a voltage applied to a shield is 0.1 kV. In this embodiment, the patch is not formed at the sheet interval and therefore the charging device  50  is turned on at the leading end of the first sheet and kept in the on state since the sheet interval during continuous sheet passing is short, and is turned off when a large sheet interval is provided, e.g., when potential control of the photosensitive drum  1  is effected. 
     Further, in the case where the overlarge-sized paper (sheet) is passed, the charging device  50  is turned off since there are two cases including the case where the patch is transferred onto the paper on the premise that the paper is to be cut and the case where the patch is formed at the sheet interval. 
       FIG. 10  shows a toner charge amount distribution (triboelectric charge distribution). Measurement was made by using a measuring apparatus (“Espart Analyzer EST-3”, mfd. by Hosokawa Micron Corp.) (cyan toner, 3000 counts). In the case of the NN environment (23° C., 50% RH), the triboelectric charge distribution on the intermediary transfer belt  6  is indicated by a dotted line in  FIG. 10  and a center value is −40 μC/g. This value is the same with respect to the output image and the patch. The triboelectric charge distribution of the patch in the case where the charging device  50  is turned off is indicated by a solid line (“UNCHARGED”) in  FIG. 10 , so that the toner with no charge amount is generated in a large amount. Therefore, in the case where the secondary transfer roller  9  is of an electrostatic cleaning type, the toner with substantially no charge amount deposited on the secondary transfer roller  9  cannot be removed, so that back-side contamination can occur in the case where a subsequent overlarge-sized paper is passed. 
     A curve indicated by a chain double-dashed line represents the triboelectric charge distribution of the patch in the case where the charging device  50  is turned on. The center value is about −60 μC/g which is larger than that of the output image by about 20 μC/g in terms of an absolute value. In this state, when the patch reaches the secondary transfer roller  9  to which the high transfer voltage is applied, the charge amount of the patch is substantially balanced with the amount of the current, passing through an area other than the paper area, applied to the secondary transfer roller  9 . Further, the toner with no charge amount does not occur and therefore all the patches can be transferred onto the secondary transfer roller  9  and can be then removed by the electrostatic cleaning. 
     Incidentally, in the image forming apparatus in this embodiment, the type of the paper can be designated by the user, and a value of the secondary transfer bias applying means  28  is changed depending on the type (basis weight) of the paper, so that the output of the charging device  50  is correspondingly changed. That is, depending on the type of the recording material, the charging device  50  is controlled. A modified example of such an output is shown in  FIG. 11 . With an increase in basis weight of the paper, the paper-sharing voltage is increased, so that the secondary transfer voltage is set at a larger value. Correspondingly, the amount of the current passing through the patch portion is increased, so that a total amount of the current for the charging device is set at a larger value. 
     Second Embodiment 
     Second Embodiment of the present invention will be described with reference to  FIGS. 12 and 13 . First, the image forming apparatus in this embodiment includes, as shown in  FIG. 12 , a cleaning device  120  of a blade for cleaning the intermediary transfer belt  6 . That is, the blade is contacted to and slid on the intermediary transfer belt  6  to remove the toner from the intermediary transfer belt  6 . Other constitutions except for a charging device  50 A described below are the same as those in First Embodiment described with reference to  FIG. 1 . 
     The charging device  50 A is, as shown in  FIG. 13 , constituted by a charging roller. That is, the charging roller is disposed at a position in which the charging roller opposes the patch β on the intermediary transfer belt  6 , and the charge amount of the patch β is changed by applying a voltage from the power source  52  to the charging roller. In this embodiment, the charging device  50 A is controlled depending on the density of the patch β detected by the patch sensor  17 , a detection result of the environment sensor  30  and the size of the recording material  7 . Other structures and functions are the same as those in First Embodiment. 
     Embodiment 2 
     A specific embodiment in Second Embodiment will be described. In this embodiment, the charging device (charging roller)  50 A is a sponge roller prepared by disposing an elastic member such as sponge on the peripheral surface of a core metal. This roller has a length of 50 mm in which the patch is covered with respect to the widthwise direction, and is 20 mm in outer diameter, 4 mm in thickness of the sponge, and 10 6  to 10 8 Ω in resistance. Further, to the charging roller, a positive bias can be applied. In this embodiment, the voltage of 2 kV is applied when the patch passes through the charging roller. An opposite stretching roller  51  is constituted by a rubber tube of 20 mm in outer diameter and 1 mm in thickness and has a resistance of 10 4 -10 5 Ω and is grounded. The bias sequence of the charging device  50 A is also the same as that in Embodiment 1. 
     Third Embodiment 
     Third Embodiment of the present invention will be described with reference to  FIG. 14  to  FIG. 17 . A basic constitution in this embodiment is the same as that in First Embodiment or Second Embodiment described above. In this embodiment, the temperature and humidity are detected by the environment sensor  30  ( FIG. 1 ) and depending on the detected environment, a value of the current applied to the charging device  50  is set. 
     Embodiment 3 
       FIG. 14  is a flow chart in this embodiment. A difference from the flow chart in  FIG. 4  is that a temperature and humidity detecting step (S 21 ) is provided and the environment is set at a level closest to any one of HH (30° C., 80% RH), NN (23° C., 50% RH) and NL (23° C., 5% RH). Subsequent steps are the same as those in  FIG. 4 . The triboelectric charge distribution in each environment is shown in  FIG. 15 . The center value in each environment is −30 μC/g in HH environment, −40 μC/g in NN environment and −50 μC/g in NL environment. 
       FIG. 16  is a table showing a relationship among patch input signals and currents of the charging device  50  in the respective environments. Each column represents the current values when the temperature and humidity environment is changed and each row represents the current values when the patch input signal (density) is changed. Depending on each environment, the charger current is set. As a basic currant setting, the current is increased when the charge amount (toner amount) per unit area is large (the case where the density is high) and is decreased when the charge amount is small (the case where the density is low). Further, the current is increased when the patch charge amount is large (the case where the water content in the air is small) and is decreased when the patch charge amount is small (the case where the water content in the air is large). Further, in the case where the effect cannot be obtained by excessively decreasing the current, the lower limit is set. In the case where the effect cannot be obtained, the upper limit is set. For example, when the patch input signal is C 0   h  or B 0   h , the change in effect is small even when the current amount is made larger than −4 μA in HH environment, −6 μA in NN environment and −8 μA in NL environment when the patch input signal is C 0   h  or B 0   h , and therefore the above current amounts are set at the upper limit. Further, in HH environment, in the case where the current amount is made smaller than −2 μA when the patch input signal is  30   h ,  30   h  or  10   h , the effect cannot be obtained and therefore the lower limit is set at −2 μA. 
     Effect 
       FIG. 17  is a table showing the effect of the present invention in each of the embodiments since the same effect is achieved in all the embodiments of the present invention. Checking of the effect was made by using the image forming apparatus shown in  FIG. 1  in the following manner. In a state in which the patch was formed at the side of the A3-sized output image with respect to the longitudinal direction, the overlarge-sized paper was passed after a predetermined number of the A3-sized sheets were passed and then a degree of the back-side (surface) contamination of the paper was checked. When the overlarge-sized paper was passed, the charging device was turned off. 
     In the case where the patch β was not charged, the back-side contamination occurred after the passing of 10 sheets. On the other hand, as in the respective embodiments, in the case where the patch β was charged, the back-side contamination did not occur even after the passing of 1,000 sheets. That is, it can be said that the constitutions in the present invention are effective in suppression of the occurrence of the back-side contamination. 
     Fourth Embodiment 
     In this embodiment, a charging device  500  as the charge amount changing means is used in place of the charging device  50 . Other constitutions are the same as those in the above embodiments. 
     Charge Amount Changing Means 
     In this embodiment, between the secondary transfer position and the patch sensor  17 , the charging device  500  (charge amount changing means) for changing the charge amount of each of the normal image α and the patch β which are transferred on the intermediary transfer belt  6  is disposed. That is, with respect to the rotational direction, the charging device  500  is disposed, upstream of the secondary transfer roller  9  and downstream of the patch sensor  17 , so as to oppose the outer peripheral surface (image transferred surface) of the intermediary transfer belt  6 . Further, an opposite roller  51  is disposed, in contact with the intermediary transfer belt  6 , at a position in which the roller  51  oppose the charging device  50  via the intermediary transfer belt  6 . 
     Such a charging device  500  includes a charger  500   a  and a shutter  500   b . In this embodiment, the charger  500   a  is a corona charger and has a width such that the charger  500   a  opposes both of the normal image α and the patch β which are transferred on the intermediary transfer belt  6 . Further, between the charger  500   a  and the intermediary transfer belt  6 , the shutter  500   b  which is slidably movable is disposed. In an example as shown in  FIG. 22 , the charger  500   a  is disposed so as to cover the entire width area of the intermediary transfer belt  6  and the shutter  500   b  is disposed so as to oppose the normal image α as shown in  FIG. 22  and ( a ) of  FIG. 23 . Further, by moving the shutter  500   b  in a direction (arrow K direction in ( a ) of  FIG. 23 ) along the rotational direction of the intermediary transfer belt  6 , the portion of the charger  500   a  opposing the normal image α can be freely opened and closed. In other words, the shutter  500   b  does not cover the portion of the charger  500   a  opposing the patch β but is opened and closed at the portion opposing the normal image α. 
     A moving mechanism  52  for slidably moving such a shutter  500   b  moves the shutter  500   b , as shown in ( b ) of  FIG. 23 , by engagement between a pinion  52   a  rotated by an unshown motor and a rack  52   b  provided on the shutter  500   b . That is, the pinion  52   a  is rotated in an arrow direction, so that the rack  52   b  and the pinion  52   a  are engaged with each other to move the shutter  500   b  in an arrow direction. Incidentally, the moving mechanism  52  may also be, in addition to such a rack-and-pinion mechanism, other mechanisms such as a hydraulic actuator, a linear drive actuator and a ball screw mechanism driven by a driving source such as the motor. 
     In this embodiment, the normal image α and the patch β can be charged by the above-constituted charging device  500 . That is, when the shutter  500   b  is opened and the charger  500   a  is turned on, both of the normal image α and the patch β are charged, so that the charge amounts of the both images can be increased. Further, when the shutter  500   b  is closed and the charger  500   a  is turned on, the normal image α is not charged but the patch β can be charged, so that only the charge amount of the patch β can be increased. Further, when the charger  500   a  is turned off, the both images can be prevented from being charged. Further, the charge amounts of the normal image α and the patch β can be lowered by applying an opposite voltage to the charging device  50 . Thus, the charge amount of each of the normal image α and the patch β can be changed by on/off control of the charger  500   a , by changing the applied voltage or by opening and closing the shutter  500   b.    
     In the case where the patch β is not charged, the toner with no charge amount is generated at the secondary transfer position and cannot be electrostatically removed. On the other hand, when the patch β is charged to the negative polarity relative to the normal image α at the position upstream of the secondary transfer position, substantially all of the patches β are transferred onto the secondary transfer roller  9  and can be electrostatically removed. 
     On the other hand, when the charge amount of the normal image α transferred onto the recording material  7  is increased, a transfer efficiency by the secondary transfer bias determined in view of the paper-sharing voltage is lowered since the charge amount is out of a good charge amount range, so that there is a possibility that improper transfer occurs. In this embodiment, it is possible to prevent an increase in charge amount of the normal image α by closing the shutter  500   b . As a result, the normal image α can be properly transferred onto the recording material  7 , and the patch β transferred on the secondary transfer roller  9  can be sufficiently removed and thus contamination of the secondary transfer roller  9  can be suppressed. When the contamination of the secondary transfer roller  9  can be suppressed, a degree of an occurrence of back-side (surface) contamination on the recording material  7  to be subjected to subsequent image formation. 
     When the operation as shown in  FIG. 4  is performed, the shutter  500   b  of the charging device  500  is closed, so that only the patch is charged. On the other hand, depending on the environment in which the image forming apparatus is placed, there is a case where the normal image α may also preferably be charged. Further, depending on the size of the recording material  7 , there is a case where the patch β is transferred onto the recording material  7 . For this reason, in this embodiment, the environment sensor (environment detecting means)  30  for detecting a temperature and humidity in the image forming apparatus is provided. Further, e.g., the paper type setting portion  209  can read not only the paper type but also the size of the recording material  7 . As a result, in addition to the above-described control, the opening and closing operation of the shutter  500   b  is performed also by making reference to the environment detected by the environment sensor  30  and the size of the recording material read by the paper type setting portion  209 . 
     For example, in the case where the size of the recording material is the overlarge size such that the area in which the patch β is formed is included and the environment detected by the environment sensor  30  is the high temperature and high humidity environment, the shutter  500   b  is opened and both of the normal image α and the patch β are charged by the charger  500   a . This is because the toner charge amount is liable to become low in the high temperature and high humidity environment and the patch B is transferred onto the recording material  7  and therefore the transfer condition of each of the patch β and the normal image α is the same (there is no difference in paper-sharing voltage component). As a result, the transfer efficiency for transferring the normal image α and the patch β onto the recording material  7  can be improved, so that the amount of the toner remaining on the intermediary transfer belt  6  can be decreased and the cleaning by the cleaning device can be satisfactorily performed. 
     Embodiment 4 
     Next, a specific embodiment in Fourth Embodiment will be described. 
     In this embodiment, the patches having the above-described constitutions are transferred onto the secondary transfer roller  9  and are removed by the secondary transfer portion cleaning device  60 . Further, in this embodiment, the shutter  500   b  shown in  FIG. 22  is constituted by an electroconductive material and is in a closed state. Further, in this embodiment, the charger  50  is the corona charger of 360 mm in width and is constituted so that ion current flowing from the charger  500   q  is partly blocked by the shutter  500   b  and ion current passing through an opening of 25 mm in which cover the entire patches with respect to the longitudinal direction but does not cover the output image. Further, the current passing through the opposite roller  51  of the charger  500   a  is −6 μA and a voltage applied to a shield of the charger  500   a  and to the shutter  500   b  is 0.1 kV. In this embodiment, the patch is not formed at the sheet interval and therefore the charger  500   a  is turned on at the leading end of the first sheet and kept in the on state since the sheet interval during continuous sheet passing is short, and is turned off when a large sheet interval is provided, e.g., when potential control of the photosensitive drum  1  is effected. 
     Further, in the case where the overlarge-sized paper (sheet) is passed, the charger  500   a  is turned off since there are two cases including the case where the patch is transferred onto the paper on the premise that the paper is to be cut and the case where the patch is formed at the sheet interval. 
       FIG. 24  shows a toner charge amount distribution (triboelectric charge distribution). Measurement was made by using a measuring apparatus (“Espart Analyzer EST-3”, mfd. by Hosokawa Micron Corp.) (cyan toner, 3000 counts). In the case of the NN environment (23° C., 50% RH), the triboelectric charge distribution on the intermediary transfer belt  6  is indicated by a dotted line in  FIG. 24  and a center value is −40 μC/g. This value is the same with respect to the output image and the patch. The triboelectric charge distribution of the patch in the case where the charger  500   a  is turned off is indicated by a solid line (“UNCHARGED”) in  FIG. 24 , so that the toner with no charge amount is generated in a large amount. Therefore, in the case where the secondary transfer roller  9  is of an electrostatic cleaning type, the toner with substantially no charge amount deposited on the secondary transfer roller  9  cannot be removed, so that back-side contamination can occur in the case where a subsequent overlarge-sized paper is passed. 
     A curve indicated by a chain double-dashed line represents the triboelectric charge distribution of the patch in the case where the charging device  50  is turned on. The center value is about −60 μC/g which is larger than that of the output image by about 20 μC/g in terms of an absolute value. In this state, when the patch reaches the secondary transfer roller  9  to which the high transfer voltage is applied, the charge amount of the patch is substantially balanced with the amount of the current, passing through an area other than the paper area, applied to the secondary transfer roller  9 . Further, the toner with no charge amount does not occur and therefore all the patches can be transferred onto the secondary transfer roller  9  and can be then removed by the electrostatic cleaning. 
     Incidentally, in this embodiment, A3-sized paper is used in NN environment and therefore the shutter  500   b  is in the closed state. In the case where, e.g., the overlarge-sized paper is used in HH environment, the shutter  500   b  is placed in an open state. Further, in the image forming apparatus in this embodiment, the type of the paper can be designated by the user, and a value of the secondary transfer bias applying means  28  is changed depending on the type (basis weight) of the paper, so that the output of the charger  500   a  is correspondingly changed. That is, depending on the type of the recording material, the charging device  50  is controlled. 
     Fifth Embodiment 
     Fifth Embodiment of the present invention will be described with reference to  FIGS. 25 and 26 . First, the image forming apparatus in this embodiment includes, as shown in  FIG. 25 , a cleaning device  120  of a blade for cleaning the intermediary transfer belt  6 . That is, the blade is contacted to and slid on the intermediary transfer belt  6  to remove the toner from the intermediary transfer belt  6 . Other constitutions except for a charging device  500 A described below are the same as those in First Embodiment described with reference to  FIG. 1 . 
     The charging device  500 A, as shown in  FIG. 26 , includes two shutters  500   c  and  500   d  for opening and closing the charger  500   a . Of these shutters, the shutter  500   c  is disposed opposed to the area in which the normal image α shown in  FIG. 22  is formed, and the shutter  500   d  is disposed opposed to the area in which the patch β shown in  FIG. 22  is formed. Each of the shutters  500   c  and  500   d  is independently movable in the opening and closing direction. In this embodiment, the charging device  500 A is controlled depending on the density of the patch β detected by the patch sensor  17 , a water content in air using a detection result of the environment sensor  30  and the size of the recording material  7 . Other structures and functions are the same as those in First Embodiment. Incidentally, the water content in the air is larger in the order of HH&gt;NN&gt;NL. 
     Embodiment 5 
     A specific embodiment in Fifth Embodiment will be described. As shown in  FIG. 26 , the shutter  500   c  of the charging device  500 A is used for the output image portion of 335 mm in width, and the shutter  500   d  is used for the patch portion of 25 mm in width. In the following, four states (1) to (4) based on combination of opening and closing of these (two) shutters  500   c  and  500   d.    
     (1) Shutter  500   c :open and shutter  500   d :open 
     In this case, as after the image formation on 500×10 3  sheets in HH environment, in the case where the toner charge amount is low (center value: about −10 μC/g) and the patch is formed on the overlarge-sized paper as the recording material, the charge amounts of the normal image and the patch are intended to be increased. Incidentally, this combination may also be applied to the case where the patch is formed at the sheet interval. By turning the charger  500   a  on and by setting the current passing through the opposite roller  51  at −60 μA and the shield voltage at 1 kV, the toner charge amount is increased up to about −30 μC/g (center value), so that good transfer can be effected. 
     (2) Shutter  500   c :closed and shutter  500   d :open 
     In this case, as in NN environment, in the case where the toner charge amount is normal (center value: about −4 μC/g) and the patch is formed at the position spaced from and at the side of the recording material up to the large size (297×420 mm), the charge amount of the patch is intended to be increased. By turning the charger  500   a  on and by setting the current passing through the opposite roller  51  at −6 μA and the shield voltage at 1 kV, the patch charge amount is increased up to about −60 μC/g (center value) as indicated by the chain double-dashed line in  FIG. 24 , so that good transfer of the patch β onto the secondary transfer roller  9  can be effected. 
     (3) Shutter  500   c :open and shutter  500   d :closed 
     In this case, as in NL environment (23° C., 5% RH), in the case where the toner charge amount is higher (center value: about −6 μC/g) than those in  FIG. 29  and the patch is formed at the position spaced from and at the side of the recording material up to the large size (297×420 mm), the charge amounts of the image and the patch are intended to be decreased. By turning the charger  500   a  on and by setting the current passing through the opposite roller  51  at 41.7 μA and the shield voltage at 1 kV, the patch charge amount is decreased down to about −50 μC/g (center value), so that good transfer can be effected. 
     (4) Shutter  500   c :closed and charge  500   d :closed 
     In this case, as in NN environment, the toner charge amount is normal (center value: about −40 μC/g) and the recording material size is not limited, and the patch is formed at the sheet interval as shown in  FIG. 24 . There is a possibility that the wire of the charger  500   a  is contaminated due to scattering of the toner. Therefore, in the case where the off state of the charger  500   a  is continued for a time, the shutters  500   c  and  500   d  are closed and thus the contamination of the wire is prevented. 
       FIG. 27  is a flow chart in the case of the recording material up to the large size, i.e., the case where the patch can be formed at the side of the normal image. First, an unshown potential sensor is disposed, upstream and downstream of the developing device  4 , on the photosensitive drum  1 . From the potentials before and after the formation of a solid patch and the toner amount per unit area (mg/cm 2 ), the toner charge amount (μC/g) is obtained (S 11 ). Here, the toner amount per unit area is estimated from a detection result of the patch sensor  17 . When the center value of the toner charge amount is larger than −50 μC/g in terms of the absolute value (S 12 ), the shutter  500   c  is opened and the shutter  500   d  is closed, so that the operation goes to the same control step as that in  FIG. 4 . Here, a bias of the opposite polarity is applied to the charger  500   a , so that the charge amount of the normal image is lowered. When the center value of the toner charge amount is not larger than −50 μC/g in terms of the absolute value (S 13 ), the shutter  500   c  is closed and the shutter  500   d  is opened, so that the operation goes to the same control step as that in  FIG. 4 . Here, a bias of the positive polarity is applied to the charger  500   a , so that the charge amount of the patch is increased. 
       FIG. 28  is a flow chart in the case of the recording material of the overlarge size, i.e., the case where the patch cannot be formed at the side of the normal image (output image). First, an unshown potential sensor is disposed, upstream and downstream of the developing device  4 , on the photosensitive drum  1 . From the potentials before and after the formation of a solid patch and the toner amount per unit area (mg/cm 2 ), the toner charge amount (μC/g) is obtained (S 21 ). Here, the toner amount per unit area is estimated from a detection result of the patch sensor  17 . When the center value of the toner charge amount is larger than −20 μC/g in terms of the absolute value (S 12 ), the shutter  500   c  is closed and the shutter  500   d  is closed, so that the operation goes to the same control step as that in  FIG. 4 . When the center value of the toner charge amount is not larger than −20 μC/g in terms of the absolute value (S 13 ), the shutter  500   c  is opened and the shutter  500   d  is also opened, so that the operation goes to the same control step as that in  FIG. 4 . Here, a bias of the positive polarity is applied to the charger  500   a , so that the charge amount of the output image is increased. 
     Sixth Embodiment 
     Sixth Embodiment of the present invention will be described with reference to  FIG. 30 . In this embodiment, a charging device  5000  does not include, different from those in the above embodiments, the shutter structure but includes two chargers  501  and  502 . These chargers  501  and  502  are, as shown in  FIG. 30 , different in widthwise dimension. The smaller charger  501  is disposed at the position in which the charger  501  opposes the patch β, and the larger charger  502  is disposed so as to oppose the normal image d and the patch β. These chargers  501  and  502  are disposed side by side with respect to the rotational direction of the intermediary transfer belt  6 . 
     For that reason, in the case where the bias is applied to both of the chargers  501  and  502 , the patch β is charged by both of the chargers  501  and  502  and the normal image α is charged by the charger  502 . Further, in the case where the bias is applied to only the charger  501 , only the patch β is applied to only the charger  501 , only the patch β is charged by the charger  501 . Further, in the case where the bias is applied to only the charger  502 , the normal image α and the patch β are charged by the charger  502 . Therefore, by effecting ON/OFF control of these chargers  501  and  502 , each of the charge amounts of the normal image α and the patch β can be changed. Incidentally, the width of the charger  502  is decreased and thus only the normal image α may be charged by the charger  502 . Other structures and functions are the same as those in Fourth and Fifth Embodiments. 
     Other Embodiments 
     The structure of the image forming apparatus to which the present invention is applicable is not limited to that of the tandem type as shown in  FIG. 1  and  FIG. 12 . For example, as shown in  FIG. 18  and  FIG. 19 , the present invention is also applicable to a one-drum type image forming apparatus. In  FIGS. 18 and 19 , a single photosensitive drum is used and a drum  40  for rotationally driving develop devices  4 Y,  4 M,  4 C and  4   k  is provided. In the case where each color is formed, the drum  50  is rotated to switch the associated develop device opening the photosensitive drum  1 . Incidentally, the electrostatic cleaning means for cleaning the secondary transfer roller  9  is omitted. Other structures and functions are the same as those of the image forming apparatus of the tandem type shown in  FIGS. 1 and 12 . 
     As described above, according to the present invention, the control image is formed adjacently to the normal image, so that it is possible to form a plurality of control images without increasing the density detecting means in number. For this reason, the control of the density and gradation of the output image can be effected with high accuracy. Further, the charge amount of the control image can be changed by the charging means, so that the charge amount of the control image can be adjusted at a proper level even when the control image is formed adjacently to the normal image. Further, the control image transferred onto the transfer portion can be sufficiently removed by the electrostatic cleaning means, so that the contamination of the transfer portion can be suppressed. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Applications Nos. 170286/2010 filed Jul. 29, 2010 and 170287/2010 filed Jul. 29, 2010, which are hereby incorporated by reference.