Abstract:
An image heating apparatus includes first image heater, having a nip through which a sheet passes, for heating toner image on the sheet; second image heater, having a nip through which the sheet passes, capable of heating the toner image on the sheet having passed through the first image heater; speed changing means for changing a feeding speed of the sheet of the second image heater; a first detector for detecting an amount of slack of the sheet at one end of the sheet when the sheet is nipped by both of the nip of the first image heater and the nip of the second image heater; a second detector for detecting an amount of slack of the sheet at the other end of the sheet when the sheet is nipped by both of the nip of the first image heater and the nip of the second image heater; adjusting means adjusting a distance between the first image heater and the second image heater at the one end and a distance therebetween at the other end; and an executing portion for executing a correction mode when the amount of slack at the one end detected by the first detector and the amount of slack at the other end detected by the second detector are different from each other, wherein the executing portion executes the correction mode in which after the adjusting means reduces the different, the speed changing means is controlled.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating apparatus which is desirable as a fixing apparatus to be mounted in an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer, etc. 
         [0002]    In the field of an image forming apparatus such as an electrophotographic copying machine, a printer, etc., demands have been increasing for an image forming apparatus which can be adjusted in the glossiness level at which it outputs images, in accordance with the glossiness level of the sheet of recording medium. An example of such an image forming apparatus is an image forming apparatus which outputs images which are relatively high in glossiness when it is used to output images on coated paper, photographic printing paper, etc., and yet, outputs images which are relatively low in glossiness when it is used for images such as black-and-white texts and/or color texts for business, on ordinary paper or the like. 
         [0003]    As an example of the embodiment of the technologies capable of satisfying these demands, there have been known image forming apparatuses equipped with two or more fixing apparatuses (fixing devices) arranged in the so-called tandem fashion. The greater the number of fixing apparatuses which an image forming apparatus is provided, the wider the range in which the image forming apparatus can be controlled in the amount by which heat, which affects the apparatus in the glossiness with which the apparatus outputs images, can be applied to a toner image, and the amount by which pressure, which also affect the apparatus in the glossiness with which the apparatus outputs images, can be applied to a toner image. That is, increasing an image forming apparatus in the number of fixing apparatuses can widen the image forming apparatus in the glossiness range in which the image forming apparatus can output images. 
         [0004]    As the structural arrangement for positioning two or more fixing apparatuses in tandem in an image forming apparatus, the following may be listed. For example, Japanese Laid-open Patent Application H04-245275 discloses an image forming apparatus which can output glossy images by being provided with two fixing apparatuses, each of which comprises a heat roller and a pressure roller, which are kept pressed upon each other. More specifically, in the case of this image forming apparatus, after a toner image is transferred onto a sheet of recording paper, the sheet is sequentially conveyed through the first and second compressing portions so that the toner image on the sheet will be glossier than it will be if the sheet is conveyed through only a single compressing portion. Further, Japanese Laid-open Patent Application 2000-221821 discloses a technology for controlling an image forming apparatus in the glossiness level at which the apparatus outputs images. More specifically, according to this technology, an image forming apparatus is provided with two or more fixing devices which are sequentially positioned in tandem in terms of the recording medium (paper) conveyance direction, and a desired level of glossiness is achieved by changing the amount by which heat is applied to the recording medium by changing the image forming apparatus in the number of fixing apparatuses (fixing nips) to be used for fixation, and also, in the selection of the fixing apparatuses to be used for fixation (that is, changing the location of fixation). Further, Japanese Laid-open Patent Application 2000-075710 discloses a technology for preventing the image on a sheet of recording medium from being shifted in location by the tension to which the sheet is subjected between the two fixing devices (fixation nips). More specifically, according to this technology, the first fixing device is made faster in the recording medium conveyance speed than the second fixing device, so that the sheet of recording medium is slightly slackened between the two fixing devices. 
         [0005]    A fixing apparatus thermally fixes the toner image on a sheet of recording medium by conveying the sheet of recording medium (on which the toner image is present) through its fixation nip which is high in temperature and pressure. Thus, it is substantial in the amount of force it applies to a sheet of recording medium to convey the sheet. Further, it is likely to be nonuniform in the temperature and/or pressure distribution in terms of the lengthwise direction of the nip. Also in terms of the lengthwise direction of the nip, it is likely to be nonuniform in the nip width because of the nonuniformity in the preciseness of the components which form the nip. Therefore, the nip is likely to become nonuniform in the recording medium conveyance speed in terms of its lengthwise direction. Thus, when a large sheet of recording medium is in the two fixing devices of a fixing apparatus of the tandem-type at the same time while it is being conveyed through the fixing apparatus, the portion of the sheet, which is being slackened between the two fixing devices, sometimes becomes substantially nonuniform in the amount of slack in terms of the widthwise direction of the path of the sheet. Therefore, if the portion of the sheet, which is being slackened, becomes substantial in the amount of slack, it sometimes comes into contact with the components by which the nip is formed, by one side or the other side in terms of the lengthwise direction of the nip. The contact between the sheet of recording medium and these components sometimes causes the toner image on the sheet of recording medium to be shifted relative to the sheet and/or scratched, resulting in the outputting of defective copies (images) by the image forming apparatus. 
         [0006]    As the means for minimizing the amount by which a sheet of recording medium is slackened as described above, there is the structural arrangement disclosed in Japanese Laid-open Patent Application 2002-351237. This structural arrangement makes it possible to change the downstream fixation unit in the recording medium conveyance speed. 
         [0007]    However, this structural arrangement suffers from the following problem. That is, assuming that a sheet of recording medium is being conveyed through the fixing apparatus structured as described above, and there is a difference in the amount of the slack between one end of the sheet and the other in terms of the lengthwise direction of the fixing apparatus. If the downstream fixation unit is adjusted in the recording medium conveyance speed to reduce the slackened portion of the sheet in the amount of slack on one end, the sheet is reduced in the amount of slack on one end, but it may be excessively pulled by the downstream fixation on the other side, although whether or not this phenomenon occurs depends on the difference in the amount of slack between one side of the sheet and the other. 
       SUMMARY OF THE INVENTION 
       [0008]    Thus, the primary object of the present invention is to provide an image heating apparatus which is significantly smaller in the extent of nonuniformity across the direction perpendicular to the recording medium conveyance direction, in terms of the recording medium conveyance speed, being therefore significantly more stable in the recording medium conveyance, than any of image heating apparatuses in accordance with the prior arts. 
         [0009]    According to an aspect of the present invention, there is provided an image heating apparatus comprising first image heating means, having a nip through which a recording material passes, for heating toner image on the recording material; second image heating means, having a nip through which the recording material passes, capable of heating the toner image on the recording material having passed through said first image heating means; speed changing means for changing a feeding speed of the recording material of said second image heating means; a first detector for detecting an amount of slack of the recording material at one end of the recording material when the recording material is nipped by both of the nip of said first image heating means and the nip of said second image heating means; a second detector for detecting an amount of slack of the recording material at the other end of the recording material when the recording material is nipped by both of the nip of said first image heating means and the nip of said second image heating means; adjusting means adjusting a distance between said first image heating means and said second image heating means at said one end and a distance between said first image heating means and second image heating means at the other end; and an executing portion for executing a correction mode when the amount of slack at said one end detected by said first detector and the amount of slack at the other end detected by said second detector are different from each other, wherein said executing portion executes the correction mode in which after said adjusting means reduces the different, said speed changing means is controlled. 
         [0010]    These and other objects, features, and advantages of the present invention will become more apparent upon 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 
         [0011]      FIG. 1  is a schematic sectional view of an example of an image forming apparatus to which the present invention is applicable, at a plane perpendicular to the axial line of each of the photosensitive drums of the apparatus. 
           [0012]      FIG. 2  is a schematic sectional view of the first and second fixing devices of the fixing apparatus in the first preferred embodiment of the present invention, at a plane perpendicular to the axial line of each of the four rollers of the fixing apparatus. 
           [0013]      FIG. 3  is a schematic perspective view of the first and second fixing devices of the fixing apparatus in the first preferred embodiment of the present invention, and shows the positioning of the slack amount sensors which detect the amount of the slack of a large sheet of recording medium as the sheet becomes slackened between the nip of the first fixing device of the fixing apparatus in the first embodiment, and the nip of the second fixing device of the fixing apparatus while the sheet is conveyed through the fixing apparatus. 
           [0014]      FIG. 4  is a graph which shows the relationship between the output of the slack amount sensors for detecting the amount of the slack of a sheet of recording medium, on one side of the fixing apparatus in terms of the lengthwise direction of the fixing apparatus and on the other side, and the recording medium conveyance speed of the first and second fixing devices. 
           [0015]      FIG. 5  is an example of a control sequence for changing the second fixing devices in the recording medium conveyance speed. 
           [0016]      FIG. 6(   a ) is a top plan view of the second fixing device of the fixing apparatus in the first preferred embodiment, as seen from above the fixation roller.  FIG. 6(   b ) is a schematic sectional view of the fixation roller shifting mechanism, and shows its structure. 
           [0017]      FIG. 7  is a graph which shows the relationship between the output of the slack amount sensors for detecting the amount of the slack of a sheet of recording medium, on one side of the fixing apparatus in terms of the lengthwise direction of the fixing apparatus and on the other side, and the recording medium conveyance speed of the first and second fixing devices. 
           [0018]      FIG. 8  is a schematic top plan view of the fixing apparatus, in the first embodiment, the fixation roller of the second fixing device of which is tilted upstream relative to the recording medium conveyance direction. 
           [0019]      FIG. 9  is a graph which shows the relationship between the angle by which the fixation roller of the second fixing device of the fixing apparatus in the second preferred embodiment of the present invention was tilted and the angle by which the fixation roller is restored in attitude. 
           [0020]      FIG. 10  is a schematic sectional view of the first and second fixing devices of the fixing apparatus in the second preferred embodiment of the present invention, at a plane perpendicular to the axial lines of each of the four rollers of the fixing apparatus. 
           [0021]      FIG. 11  is a drawing which shows the distribution of the heat from the halogen heater in the hollow of the fixation roller of the first fixing device of the fixing apparatus in the second preferred embodiment of the present invention. 
           [0022]      FIG. 12(   a ) is a schematic drawing which show the pattern in which the sections  24   a ,  24   b , and  24   c  of the halogen heater of the first fixing device of the fixing apparatus in the second embodiment are turned on and off to keep the temperature of the fixation roller remains at the fixation temperature level. 
           [0023]      FIGS. 12(   b ) and  12 ( b ) are schematic drawings which show the patterns, respectively, in which the sections  24   a ,  24   b ,  24   c  of the halogen light are turned on and off, and which are different from  FIG. 12(   a ) in terms of the ratio with which the three sections  24   a ,  24   b , and  24   c  of the halogen heater are turned on per unit length of time. 
           [0024]      FIG. 13  is a schematic sectional view of the first and second fixing devices of the fixing apparatus in the third preferred embodiment of the present invention, at a plane perpendicular to the axial lines of each of the four rollers of the fixing apparatus. 
           [0025]      FIG. 14  is a schematic top plan view of the first and second fixing devices and slack amount sensors of the fixing apparatus in the fourth preferred embodiment of the present invention, and is for describing the operation of the sensor moving mechanism,  FIGS. 14(   a ) and  14 ( b ) being different in the location of the first and second sensor moving racks of the sensor moving mechanism. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Hereafter, the preferred embodiments of the present invention are described with reference to the appended drawings. 
       Embodiment 1 
     (A) General Structure of Image Forming Apparatus 
       [0027]      FIG. 1  is a schematic sectional view of an example of an image forming apparatus having an image heating apparatus in accordance with the present invention, at a plane perpendicular to the axial line of each of the photosensitive drums of the apparatus. This image forming apparatus is an electrophotographic color printer. 
         [0028]    The structure of the image forming apparatus in this embodiment is such that four monochromatic toner images, different in color, are formed through charging, exposing, developing, and transferring processes carried out by the first, second, third, and fourth image forming portions Py, Pm, Pc, and Pb, respectively, which are in the main assembly of the apparatus and are in alignment in parallel in the recording medium conveyance direction. 
         [0029]    The image forming apparatus in this embodiment starts a preset image forming operation sequence in response to a print command outputted from an external apparatus (unshown) such as a host computer. As the sequence is started, the image forming portions Py, Pm, Pc, and Pb sequentially begin to be driven, whereby the photosensitive drum  1  (image bearing member) of each image forming portion is rotated in the direction indicated by arrow marks at a preset peripheral velocity (process speed). The intermediary transfer belt  7  of the apparatus, which is supported, being stretched thereby, by the driver roller  6   a , follower roller  6   b , and tension roller  6   c  of the apparatus, in such a manner that it remains in contact with the photosensitive drum  1  of each of the image forming portions Py, Pm, Pc, and Pb, is circularly moved by the driver roller  6   a  in the direction indicated by another arrow mark at a velocity which matches the peripheral velocity of each photosensitive drum  1 . 
         [0030]    Referring to the image forming portion Py, that is, the image forming portion for forming a monochromatic image of yellow color (first color), the peripheral surface of the photosensitive drum  1  is uniformly charged by a charging device  2  to preset polarity and potential level. Then, the uniformly charged portion of the peripheral surface of the photosensitive drum  1  is scanned (exposed) by the beam of laser light outputted from an exposing apparatus while being modulated in accordance with information of image to be formed, whereby an electrostatic latent image, which reflects the information of the image to be formed, is effected on the peripheral surface of the photosensitive drum  1 . This latent image is developed by the developing apparatus  4  which uses yellow toner (developer). That is, a visible image of yellow color is formed on the peripheral surface of the photosensitive drum  1 . The steps similar to those carried out in the image forming portion Py are carried out also in the image forming portions Pm, Pc, and Pb, which are for forming monochromatic images of magenta color (second color), cyan color (third color), and black color (fourth color), respectively. As four toner images, different in color, are formed in the four image forming portions Py, Pm, Pc, and Pb, one for one, they are sequentially transferred in layers onto the outward surface of the intermediary transfer belt  7  by the first transfer rollers  8 , each of which is kept pressed against the peripheral surface of the corresponding photosensitive drum  1  with the presence of the intermediary transfer belt  7  between itself and peripheral surface of the photosensitive drum  1 . 
         [0031]    Meanwhile, a sheet P of recording medium is moved out of a sheet feeder cassette  10  by a sheet feeder roller  11 , and then, is conveyed to a pair of registration roller  12 . Then, the sheet P is conveyed by the pair of registration roller  12  to the second transfer nip, which is the interface between the intermediary transfer belt  7  and second transfer roller  13  (second transferring member). Then, the sheet P is conveyed through the second transfer nip while remaining pinched by the intermediary transfer belt  7  and second transfer roller  13 . While the sheet P is conveyed through the nip, the full-color toner image, that is, the combination of the four layers of monochromatic toner images, different in color, on the intermediary transfer belt  7  is transferred onto the surface of the sheet P by the second transfer roller  13 . That is, an unfixed full-color toner image is borne on the surface of the sheet P. Then, the sheet P is introduced into the first fixing device  14  which is the image heating upstream unit in terms of the recording medium conveyance direction, and then, is introduced into the second fixing device  14  which is the image heating downstream unit. While the sheet P is conveyed through the first and second fixing devices  14  and  15 , the unfixed toner image on the sheet P becomes fixed to the surface of the sheet P. Then, as the sheet P comes out of the second fixing device  15 , it is discharged into the delivery tray  16 , which is outside the main assembly of the image forming apparatus. 
         [0032]    The toner remaining on the peripheral surface of the photosensitive drum  1  after the transfer (first transfer) of the toner image from the peripheral surface of the photosensitive drum  1  onto intermediary transfer belt  7  is removed by the drum cleaner  5 , so that the peripheral surface of the photosensitive drum  1  can be used for the next image formation. The toner remaining on the intermediary transfer belt  7  after the transfer of the full-color toner image from the intermediary transfer belt  7  onto the sheet P, is removed by the belt cleaner  9  so that the portion of the intermediary transfer belt  7 , from which the residual toner has just been removed, can be used for the next image formation. 
       (2) Description of First and Second Fixing Devices 
       [0033]    In the following description of the first and second fixing devices, the “lengthwise and widthwise” directions of any of the components of which the first and second fixing devices are made mean the directions perpendicular and parallel, respectively, to the recording medium conveyance direction. The “width” of any component means the measurement of the component in terms of the widthwise direction. The “lengthwise and widthwise” directions of a sheet of recording medium which is being conveyed through the fixing apparatus mean the directions perpendicular and parallel, respectively, to the recording medium conveyance direction. The “width” of a sheet of recording medium in the fixing apparatus means the measurement of the sheet in terms of the widthwise direction. 
         [0034]      FIG. 2  is a schematic sectional view of the combination of the first and second fixing devices of the fixing apparatus (image heating apparatus) of the so-called tandem type, which is in the image forming apparatus, in this embodiment (first preferred embodiment), at a plane perpendicular to the lengthwise direction of both fixing devices. 
         [0035]    Both the first and second fixing devices  14  and  15 , respectively, in this embodiment are of the so-called heat roller type. That is, the first fixing device  14  has a fixation roller  21  (rotational heating member), a pressure roller  22  (rotational pressing member), a halogen heater  23  (heat generating member), etc. The lengthwise direction of each of the fixation roller  21 , pressure roller  22 , and halogen heater  23  (heat generating member) is parallel to the above-mentioned “lengthwise” direction. The fixation roller  21  is 30 mm in diameter. It is made up of a cylindrical metallic core  21   a  and an elastic layer  21   b . The cylindrical metallic core  21   a  is made of iron. The elastic layer  21   b  is formed of silicone rubber, and is 1.0 mm in thickness. It covers the entirety of the peripheral surface of the metallic core  21   a . The fixation roller  21  has also a parting layer  21   c , which is formed of a piece of PFA tube. It is 30 μm in thickness, and covers the entirety of the elastic layer  21   b . In the case of a fixing device to be mounted in a color image forming apparatus, the parting layer  21   c  of the fixation roller  21  may be formed of silicone rubber impregnated with silicone oil, instead of being formed of the piece of PFA tube. The pressure roller  22  is under the fixation roller  21 , and is in parallel to the fixation roller  21 . It is 30 mm in diameter. It is made up of a metallic core  22   a  and an elastic layer  22   b . The metallic core  22   a  is in the form of a piece of iron rod which is 10 mm in diameter. The elastic layer  22   b  is made of sponge, more specifically, foamed silicone rubber, and covers the entirety of the peripheral surface of the metallic core  22   a . The pressure roller  22  also has a parting layer  22   c , which is 30 μm in thickness. The parting layer  22   c  is a piece of PFA tube, and covers the entirety of the peripheral surface of the elastic layer  22   b . The fixation roller  21  and pressure roller  22  are rotatably supported by the frame (unshown) of the fixing apparatus, at their lengthwise ends of their metallic cores  21   a  and  22   a , respectively, with the placement of a pair of bearings between the lengthwise ends of the metallic cores  21   a  and  22   a , and the frame of the fixing apparatus. Further, the bearings for the pressure roller  22  are under the pressure generated by a pressure applying members (unshown) such as compression springs which keep the pressure roller  22  pressured in the direction (toward) perpendicular to the generatrix of the fixation roller  21 . The overall amount of the pressure applied to the bearings is roughly 392 N (40 kgf). Therefore, the elastic layer  22   b  of the pressure roller  22  and the elastic layer  21   b  of fixation roller  21  remain deformed across the portion by which they are in contact with each other. Thus, there is a nip N between the peripheral surface of the fixation roller  21  and the peripheral surface of the pressure roller  22 . The nip N has a preset width. The halogen heater  23  is in the hollow of the metallic core  21   a  of the fixation roller  21 . It is supported by the frame of the fixing apparatus, at its metallic contacts which are at its lengthwise ends. The second fixing device  15  is the same in structure as the first fixing device  14 . In this embodiment, the components, members, etc., of the second fixing device  15 , which are the same as the counterparts of the fixing device  14 , are given the same referential codes as those given to the counterparts, and are not going to be described to avoid the repetition of the same descriptions. The distance between the nip N of the first fixing apparatus  14  and that of the second fixing apparatus  15  is less than the length of a largest sheet Pa of recording medium (preset in size, and will be referred hereafter as large sheet Pa of recording medium, or simply as large sheet Pa, among various sheets of recording medium, different in size, usable by the image forming apparatus in this embodiment). Therefore, when the leading edge portion of a large sheet Pa of recording medium is conveyed through the nip N of the second fixing device  15 , remaining pinched between the fixation roller  21  and pressure roller  22  of the second fixing device  15 , while the sheet P is conveyed through the fixing apparatus to thermally fix the unfixed toner image t on the sheet Pa, the trailing edge portion of the sheet Pa is still in the nip N of the first fixing device  14 , remaining pinched between the fixation roller  21  and pressure roller  22  of the first fixing device  14 . 
       (3) Thermal Fixing Operation of First and Second Fixing Devices 
       [0036]    As a control portion  60  (controlling means), which is made up of a CPU, and memories such as a ROM and a RAM, receives a print command, it begins to drive a first fixation motor M 1  (driving means) and a second fixation motor M 2  (driving means) roughly at the same time in response to the print command, while carrying out a preset control for keeping the second fixation motor M 2  slower in rotational speed than the first fixation motor M 1 . More specifically, as the first fixation motor M 1  is driven, the rotation of its output shaft is transmitted to the fixation roller  21  of the first fixing device  14  through a speed reduction gear train (unshown), whereby the fixation roller  21  is rotated at a rotational speed V 1  in the direction indicated by an arrow mark. Further, as the second fixation motor M 2  is driven, the rotation of its output shaft is transmitted to the fixation roller  21  of the second fixing device  15  through a speed reduction gear train (unshown), whereby the fixation roller  21  is rotated in the direction indicated by an arrow mark at a rotational speed V 3 , which is slower than the rotational speed V 1  of the fixation roller  21  of the first fixing device  14 . The rotation of the fixation roller  21  of the first fixing device  14  is transmitted to the pressure roller  22  of the first fixing device  14  through the nip N between the two rollers  21  and  22 . The rotation of the fixation roller  21  of the second fixing device  15  is transmitted to the pressure roller  22  of the second fixing device  15  through the nip N between the two rollers  21  and  22 . Thus, the two pressure rollers  22  are rotated by the corresponding fixation rollers  21  at the same rotational speeds as the rotational speeds V 1  and V 3 , respectively, in the direction indicated by arrow marks. Hereafter, the rotational speed of the fixation roller  21  of the first fixing device  14 , and that of the pressure roller  22  of the first fixing device  14 , are referred to as the recording medium conveyance speed of the first fixing device  14 , whereas the rotational speed of the fixation roller  21  of the second fixing device  15 , and that of the pressure roller  22  of the second fixing device  15 , are referred to as the recording medium conveyance speed of the second fixing device  15 . The halogen heater  23  is supplied with a preset amount of electric power by a power supply control portion (unshown). As it is turned on, it generates heat. As heat is generated by the halogen heater  23 , the metallic core  21   a  of the fixation roller  21  is heated from within. As the fixation roller  21  is heated, its surface temperature is detected by a temperature detecting member (unshown) such as a thermistor located in the adjacencies of the peripheral surface of the fixation roller  21 . The output signals from the temperature detecting member are taken in by the control portion  60 , which controls the power supply control portion in response to the output signals from the temperature detecting member, in such a manner that the surface temperature of the fixation roller  21  remains at a preset fixation level (target level). In this embodiment, the fixation temperature level (target temperature level) is 170° C. 
         [0037]    While the first and second fixation motors M 1  and M 2 , respectively, are being driven, and the halogen heaters  23  are being supplied with electric power, a large sheet Pa of recording medium, on which an unfixed toner image t is present, is introduced into the nip N of the first fixing device  14 , and is conveyed through the nip N, remaining pinched by the peripheral surface of the fixation roller  21  and the peripheral surface of the pressure roller  22 . While the sheet Pa is conveyed through the nip N of the first fixing device  14 , the toner image t on the sheet Pa is heated by the heat from the fixation roller  21  while being pressed by the pressure in the nip N. Thus, the toner image t, which is made of the four monochromatic images, more specifically, yellow, magenta, cyan, and black monochromatic images, are melted, becoming thereby mixed. Then, as they cool down, they become fixed to the surface of the sheet Pa. As they cool down, the texture (glossiness) of the peripheral surface of the fixation roller  21  is transferred onto the surface of the cooling mixture of the toner images. Thus, as the mixture of the toner images cools down, it results in a full-color image, the surface of which reflects the glossiness (texture) of the peripheral surface of the fixation roller  21  of the first fixing device  14 . As the toner image t is being fixed to the large sheet Pa of recording medium in the nip N of the first fixing device  14 , the sheet Pa is conveyed out of the nip N from its leading edge side, that is, the side from which it was introduced into the nip N. Then, it is introduced into the nip N of the second fixing device  15 , and is conveyed through the nip N while remaining pinched by the peripheral surface of the fixation roller  21  and pressure roller  22  of the second fixing device  15 . While the sheet Pa is conveyed through the nip N of the second fixing device  15 , the fixed toner image ta on the surface of the large sheet Pa of recording medium is subjected to the heat from the fixation roller  21  and the pressure in the nip N of the second fixing device  15 . As the fixed toner image ta is subjected to the heat from the fixation roller  21 , it is softened by the heat. Then, as the softened toner images ta cools down, the texture (glossiness) of the peripheral surface of the fixation roller  22  is transferred onto the surface of the toner image ta by the pressure in the nip N. Thus, after the softened toner image ta cools down, the glossiness of the toner image ta reflects the glossiness of the peripheral surface of the fixation roller  21  of the second fixing device  15 . Then, the large sheet Pa of recording medium is discharged from the fixing device  15  while the surface of the toner images ta thereon is made glossier by the peripheral surface of the fixation roller  21  of the second fixing device  15  than it was before the sheet Pa was introduced into the second fixing device  15 . 
       (4) Structural Arrangement for Prevention of Image Deterioration on Large Sheet of Recording Medium in Nip N in Second Fixing Device 
       [0038]      FIG. 3  is a schematic perspective view of the fixing apparatus, and shows the location of the sensor for detecting the amount of the slack which occurs to a large sheet Pa of recording medium between the first and second fixing devices while the sheet Pa is conveyed through the fixing apparatus. It sometimes occurs that while a large sheet Pa of recording medium is conveyed through the fixing apparatus, in particular, when the leading edge portion of the sheet Pa is in the nip N of the second fixing device  15  and the trailing edge portion of the sheet Pa is still in the nip N of the first fixing device  14 , the nips N become different in the recording medium conveyance speed (nip N of second fixing device becomes slower in the recording medium conveyance speed than nip N of first fixing device), the large sheet Pa begins to slack toward its image bearing surface. If the two nips N become nonuniform in the recording medium conveyance speed in terms of the widthwise direction of the large sheet Pa, the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa, which correspond in position to one of the widthwise edge of the large sheet Pa and the other edge, respectively, become different in the amount of slack. Further, the greater the speed with which the sheet Pa is conveyed, the greater the difference in the amount of slack between the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa. Thus, it is possible that the image bearing surface of the larger of the slackened portion δ 1  or  52  will come into contact with the peripheral surface of the fixation roller  21  of the first fixing device  14  and/or the peripheral surface of the fixation roller  21  of the second fixing device  15 , and therefore, the toner image on the sheet Pa will be damaged, more specifically, scratched and/or dislodged. Here, a “slackened” portion means the portion of the sheet Pa, which has curved toward the image bearing surface of the sheet Pa between the nip N of the first fixing device  14  and the nip N of the second fixing device  15 . 
         [0039]    In this embodiment, therefore, in order to detect the amount of the slack of each of the portions  51  and  52  of the slackened portion of the large sheet Pa of recording medium, the fixing apparatus was provided with a pair of slack amount sensors  31  and  32 , which were placed so that they would be on the opposite side of the sheet Pa from the image bearing surface, and also, that they would be on the inward side of the recording medium sheet passage in terms of the widthwise direction of the sheet Pa when the sheet Pa is conveyed through the fixing apparatus. Thus, the amount of the slack of the portion δ 1 , which corresponds in position to one of the widthwise edges of the sheet Pa, is detected by the slack amount sensor  31 , whereas the amount of the slack of the portion δ 2 , which corresponds in position to the other widthwise edge of the sheet Pa is detected by the slack amount sensor  32 . The slack amount sensors  31  and  32  are on the sensor supporting member  33 . In terms of the recording medium conveyance direction, the sensor supporting member  33  (and slack amount sensors  31  and  32 ) is between the first and second fixing devices  14  and  15 . In terms of the direction perpendicular to the recording medium conveyance direction, the slack amount sensor supporting member  32  is located so that when the large sheet Pa of recording medium is conveyed between the first and second fixing devices  14  and  15 , it will be on the opposite side from the image bearing surface of the sheet Pa. The slack amount sensors  31  and  32  in this embodiment are such sensors that project a beam of light upon the widthwise end portions of the opposite surface of the large sheet Pa of recording medium from the image bearing surface, and outputs electric signals, the level of which corresponds to the point of the sheet Pa by which the beam of light from each of the sensors  31  and  32  is reflected. That is, the slack amount sensors  31  and  32  are such sensors that output electric signals, the level of which corresponds to the amount of the slack of the portions δ 1  and δ 2 , respectively, of the slackened portion of the large sheet Pa of recording medium. 
         [0040]    Referring to  FIG. 3 , described next is an example of an operation in which if the portion δ 1  of the slackened portion of the large sheet Pa of recording medium becomes in the amount of slack than the portion δ 2  of the slackened portion of the sheet Pa, the second fixing device  15  is changed in the recording medium conveyance speed by controlling the driving of the second fixation motor M 2 .  FIG. 4  is a graph which shows the relationship between the outputs of the two slack amount sensors  31  and  32  which correspond in position to the widthwise ends of the sheet Pa in the fixing apparatus in this embodiment, and the recording medium conveyance speeds of the first and second fixing devices, respectively. Point 0 on a vertical line (T 1 ), which stands for the elapsed time in  FIG. 4 , corresponds to the point in time at which the leading edge of the large sheet Pa of recording medium was introduced into the nip N of the second fixing device  15 , whereas point 0 on a vertical line (T 7 ) corresponds to the point in time at which the trailing edge of the sheet Pa is discharged from the nip N of the second fixing device  15 . Points 0 which correspond to the elapsed lengths (T 1 ) and (T 7 ) of time, one for one, are reset each time a large sheet Pa of recording medium is conveyed through the second fixing device  15 . 
         [0041]    Referring to  FIG. 4 , it is assumed that a large sheet Pa of recording medium begins to be conveyed through the second fixing device  15  (T 1 ) while remaining pinched in the nip N. While the large sheet Pa is conveyed through the nip N, the portions  51  and  52  of the slackened portion the large sheet Pa, which correspond in position to the widthwise end portions, one for one, increase in the amount of slack, increasing thereby the slack amount sensors  31  and  32  in output strength (T 1 -T 2 ). Then, as the smaller of the output from the slack amount sensor  31  and the output from the slackness sensor  32  reaches a preset slack amount control start level L 1  (T 2 ), the second fixing device is changed in the recording medium conveyance speed from V 3  to a preset recording medium conveyance speed V 2 , which is greater than the recoding medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). Here, the slack amount control start level L 1  is a level preset for the smaller of the outputs of the slack amount sensor  31  and  32 , one for one, above which the large sheet Pa of recording medium is controlled in the amount of slack to prevent the larger of the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa from coming into contact with the peripheral surface of the fixation roller  21 . As the second fixing device  15  is increased in the recording medium conveyance speed from V 3  to V 2 , the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa begin to reduces in the amount of slack, and therefore, the slack amount sensors  31  and  32  weaken in output (T 2 -T 3 ). Then, as the output of the weaker of the slack amount sensors  31  and  32  in terms of output strength reaches a preset slack amount control start level L 2  (T 3 ), the second fixing device  15  is reduced in the recording medium conveyance speed from V 2  to V 3 , which is the original recording medium conveyance speed. Here, the slack amount control start level L 2  means the slack amount level above which the large sheet Pa of recording medium is controlled in the amount of slack to prevent the smaller of the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa from being excessively pulled by the second fixing device  15 , whose recording medium conveyance speed of which is V 2 . As the second fixing device  15  is reduced in the recording medium conveyance speed from V 2  to V 3 , the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa begin to increase in the amount of slack, and therefore, the slack amount sensors  31  and  32  become stronger in output (T 3 -T 4 ). The above described steps are repeated until the trailing edge of the large sheet Pa of recording medium is discharged from the nip N of the second fixing device  15 . 
         [0042]    Next, referring to  FIG. 5 , the control sequence through which the second fixing device  15  is changed in the recording medium conveyance speed by the control portion  60  is described.  FIG. 5  is an example of a flowchart of the control sequence through which the second fixing device  15  is changed in the recording medium conveyance speed by the control portion  60 . As a printing operation is started, the second fixing device  15  is started at V 3  in the recording medium conveyance speed (S 1 ). In step (S 2 ), it is determined whether or not the output of the slack amount sensor  31  is greater than the output of the slack amount sensor  32 . If the output of the slack amount sensor  31  is greater than the output of the slack amount sensor  32  (Y), the control portion  60  proceeds to step (S 3 ), whereas if the former is less than the latter (N), the control  60  proceeds to (S 4 ). In step (S 3 ), it is determined whether or not the output of the slack amount sensor  31  is greater than the slack amount control start level L 1 . If the output of the slack amount sensor  31  is greater than slackness amount control start valueL 1  (Y), the control portion  60  proceeds to step (S 5 ), whereas if the output of the slack amount sensor  31  is less than the slackness amount control start value L 2  (N), the control portion  60  returns to step (S 2 ). In step (S 4 ), the control determines whether or not the output of the slack amount sensor  32  is greater than the slack amount control start level L 1 . If the output of the slack amount sensor  32  is greater than the slack amount control start level L 1  (Y), the control portion  60  proceeds to step (S 5 ), whereas if the output of the slack amount sensor  32  is less than the slack amount control start level L 1  (N), the control portion  60  returns to step (S 2 ). In step (S 5 ), a preset control is carried out to increase the second fixation motor M 2  in rotational speed so that the second fixing device  15  is increased in the recording medium conveyance speed to V 2 . In other words, the second fixing device  15  is changed in the recording medium conveyance speed from V 3  to V 2 . In step (S 6 ), the control portion  60  determines whether or not the output of the slack amount sensor  31  is larger than the output of the slackness sensor  32 . If the output of the slack amount sensor  31  is larger than the output of the slack amount sensor  32  (Y), the control portion  60  proceeds to step (S 8 ). If the output of the slack amount sensor  31  is smaller than the output of the slack amount sensor  32  (N), the control portion proceeds to a step (S 7 ). In the step (S 7 ), the control portion  60  determines whether or not the output of the slack amount sensor  31  is greater than the slack amount control start level L 2 . If the output of the slack amount sensor  31  is larger than the slack amount control start level L 2  (Y), the control portion  60  advances to a step (S 9 ). If the output of the slack amount sensor  31  is smaller than the slack amount control start level L 2  (N), the control portion  60  returns to the step (S 6 ). In a step (S 8 ), the control portion  60  determines whether or not the output of the slack amount sensor  32  is larger than the slack amount control start level L 2 . If the output of the slack amount sensor  32  is larger than the slack amount control start level L 2  (Y), the control portion  60  proceeds to the step (S 9 ). If the output of the slack amount sensor  32  is smaller than the slack amount control start level L 2  (N), the control portion  60  returns to the step (S 6 ). In the step (S 9 ), the control portion  60  carries out a preset control sequence to reduce the second fixation motor M 2  in rotational speed so that the second fixing device  15  is reduced in the recording medium conveyance speed from V 2  to V 3 . Thus, the second fixing device  15  is changed in the recording medium conveyance speed from V 2  to V 3 . 
         [0043]    Next, a shifting mechanism  35 , in this embodiment, for adjusting the fixing apparatus in the distance between one of the lengthwise ends of the first fixing device  14  and the corresponding lengthwise end of the second fixing device  15 , and also, the distance between the other lengthwise end of the first fixing device  14  and the corresponding lengthwise end of the second fixing device, before the starting of the above described sequence for changing the first and second fixing devices in the recording medium conveyance speed, is described. In this embodiment, the first fixing device is not changeable in the attitude (angle) of its fixation roller  21 , but, the second fixing device is changeable in the attitude (angle) of its fixation roller  21 . The shifting mechanism  35  is made up of a rack and a pinion gear, for example. Referring to  FIG. 6(   b ), designated by a referential code  25  is a bearing, which is at each of the lengthwise ends of the metallic core  21   a  of the fixation roller  21 . The bearings  25  are supported by the aforementioned frame in such a manner that they can be moved upstream and downstream in terms of the recording medium conveyance direction. The shifting mechanism  35  is at each of the lengthwise ends of the fixation roller  21 . It has: a rack  35   a  which is under the bearing  25 ; a pinion gear  35   b  which is in mesh with the rack  35   a ; and a shift motor M 3  (driving means) for rotating the pinion gear  35   b.    
         [0044]    Referring to  FIGS. 6-8 , the control sequence to be carried out by the control portion  60  to change the second fixing device  15  in the recording medium conveyance speed when either of the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa of recording medium, which correspond in position to the widthwise direction of the sheet Pa, is greater in the amount of slack than the other, is described along with the operation of the shifting mechanism  35 .  FIG. 7  is a graph which shows the relationships among: the output of each of the slack amount sensors  31  and  32  which correspond in location to the widthwise end portions of the large size sheet Pa of recording medium while the sheet Pa is conveyed through the fixing apparatus, the amount by which the fixation roller  21  of the second fixing device  14  is changed in attitude (angle); recording medium conveyance speed of the first fixing device  14 ; and recording medium conveyance speed of the second fixing device  15 . 
         [0045]    Next, first, referring to  FIG. 7 , the operation carried out when the fixing apparatus is in a correction mode is described. The portion of the image forming apparatus, by which the fixing apparatus is operated in the correction mode is the control portion  60 . In this embodiment, it is the second fixing device  15  that is adjusted first. More specifically, first, the second fixing device  15  is adjusted in the attitude (angle) of its fixation roller  22 , and then, in the recording medium conveyance speed. 
         [0046]    Referring to  FIG. 7 , as a large sheet Pa of recording medium begins to be conveyed through the nip N of the second fixing device  15  by being pinched in the nip N (T 1 ), the large sheet Pa begins to increase in the amount of the slackness across both the widthwise end portions, whereby the slack amount sensors  31  and  32  also are increased in output (T 1 -T 2 ). As the control portion  60  detects, based on the outputs of the slack amount sensors  31  and  32 , that either the portion δ 1  or δ 2  of the slackened portion of the large sheet Pa, which correspond in position to the widthwise end portions of the large sheet Pa, is greater in the amount of slack than the other, the control portion  60  drives forward the shift motor M 3  which corresponds to the larger of the portions δ 1  and δ 2  in terms of the amount of slack, for example, the portion δ 1 . Thus, the output shaft of the shift motor M 3  rotates, and the rotation of the output shaft is transmitted to the supporting shaft  35   b   1  of the pinion gear  35   b . Therefore, the pinion gear  35   b  rotates, whereby the rack  35   a  is moved downstream in terms of the recording medium conveyance direction. As the rack  35   a  is moved, the bearing  25  is displaced downstream (+direction in  FIG. 6(   a )). This displacement of the bearing  25  causes the fixation roller  21  to tilt so that the corresponding lengthwise end of the fixation roller  21  moves in the +direction, in the plane parallel to the large sheet Pa which is being conveyed. Thus, the corresponding side of the large sheet Pa is pulled in the +direction by the corresponding side of the fixation roller  21 . Therefore, the portion δ 1 , for example, is gradually reduced in the amount of slack until the slack amount sensors  31  and  32  become equal in output, that is, until the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa, which correspond in position to the lengthwise end portions of the large sheet Pa of recording medium, one for one, become equal in the amount of slack. As the two portions δ 1  and δ 2  become equal in the amount of slack (t 1 ), the control portion  60  stops driving the above described shifting motor M 3 , which in turn stops the corresponding bearing  25  from shifting in the +direction. Thus, the fixation roller  21  stops being tilted as described above, and remains tilted at such an angle that its lengthwise end which corresponds to the shifted bearing  25  is on the plus side as depicted with a broken line in  FIG. 6(   a ). 
         [0047]    Next, the control portion  60  adjusts the second fixing device  15  in the recording medium conveyance speed. This sequence for adjusting the second fixing device  15  in the recording medium conveyance speed is carried out based on the flowchart shown in  FIG. 5 . 
         [0048]    That is, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 1  (T 2 ), the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to V 2 , which is greater than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). As the second fixing device  15  is changed in the recording medium conveyance speed from V 3  to V 2 , the large sheet Pa of recording medium begins to reduce in the amount of the slack of the portion δ 1 , and that of the portion δ 2 , which in turn reduces the output of the slack amount sensor  32  (T 2 -T 3 ). After the changing of the recording medium conveyance speed of the second fixing device  15  from V 3  to V 2 , if the slack amount sensors  31  and  32  become different in output (t 2 ), the control portion  60  drives forward the shift motor M 3  which corresponds in position to one of the widthwise ends of the large sheet Pa of the recording medium so that the slack amount sensors  31  and  32  become equal in output as described above. As the slack amount sensors  31  and  32  become equal in output (t 3 ), the control portion  60  stops driving the shift motor M 3  to make the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa, which correspond in position to the widthwise end portions of the large sheet Pa, one for one, the same in the amount of slack. Then, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 2  (T 3 ), the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 2  to V 3 . As the second fixing device  15  is reduced in the recording medium conveyance speed from V 2  to V 3 , the large sheet Pa begins to increase in the amount of the slack of its portions δ 1  and δ 2 , causing the slack amount sensors  31  and  32  to increase in output (T 3 -T 4 ). The control portion  60  continuously repeats the above described steps until the trailing edge of the large sheet Pa is discharged from the nip N of the second fixing device  15 . 
         [0049]    As soon as the trailing edge of the large sheet Pa of recording medium comes out of the nip N of the second fixing device  15  (T 7 ), the shift motor M 3  of the second fixing device  15  is driven in reverse for a preset length of time, whereby the output shaft of the shift motor M 3  rotates. This rotation of the output shaft of the shift motor M 3  causes the pinion gear  35   b  to rotate, causing thereby the rack  35   a  to move upstream in terms of the recording medium conveyance direction, and stops. This movement of the rack  35   a  causes the bearing  25  to move upstream in terms of the recording medium conveyance direction (− direction in  FIG. 6(   a )), until the fixation roller  21  is tilted by a preset angle so that one of the lengthwise end of the fixation roller  21  moves in the − direction as depicted by a broken line in  FIG. 8(   a ). 
         [0050]    By the way, as one of the mechanical reasons why the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa of recording medium, which correspond in position to the widthwise end portion of the large sheet Pa, the following is thinkable. That is, it is possible that the fixing devices  14  and  15  may not be uniform in the recording medium conveyance speed, in terms of their lengthwise direction, or that the fixation roller  21  and pressure roller  22  of the first fixing device  14 , and the fixation roller  21  and pressure roller  22  of the second fixing device  15 , may be, or may have become tilted relative to the direction perpendicular to the recording medium conveyance direction. 
         [0051]    Thus, in order to reduce, in the amount of slack, the portion δ 1  of the slackened portion of the large sheet Pa, which corresponds in position to one of the widthwise end portions of the large sheet Pa, the fixation roller  21  is tilted in the − direction by the same angle as that by which the fixation roller  21  is tilted in the +direction (so that lengthwise end of fixation roller  21  moves in − direction), and is kept at that angle. This arrangement makes it possible to introduce the leading side of the upstream edge of the large sheet Pa of recording medium in terms of the recoding medium conveyance direction, into the nip N of the second fixing device  15 , before the trailing side of the upstream edge of the large sheet Pa. That is, by tilting the fixation roller  21  relative to the direction perpendicular to the recording medium conveyance direction, in the plane which coincides with the surface of the sheet Pa in the nip N, the distance of the fixation roller  21  of the second fixing device from the fixation roller  21  of the first fixing device  14  can be reduced, and therefore, the fixing apparatus is reduced in the amount by which the next large sheet Pa of recording medium is slackened when the sheet Pa is conveyed between the nip N of the first and the nip N of the second fixing devices. This in turn makes it possible to reduce the amount by which the bearing  25  has to be moved upstream to reduce the amount by which the portion δ 1  (which will be slackened) of one of the widthwise end portions of the large sheet Pa is slackened. Thus, the leading side of the upstream edge of the large sheet Pa is introduced into the nip N of the second fixing device  15 , and begins to be conveyed through the second fixing device  15 , before the trailing side of the upstream edge of the large sheet Pa, as shown in  FIG. 8 . Therefor, it is possible to reduce the fixing apparatus in the amount of the difference between the amount by which the portions δ 1  and δ 2  of the large sheet Pa, which correspond in position to the widthwise end portions of the large sheet Pa, one for one, are slackened between the first and second fixing devices  14  and  15 . 
         [0052]    The relationship between the attitude (angle by which it is tilted) of the fixation roller  21  of the second fixing device  15  during the conveyance of a large sheet Pa of recording medium through the second fixing device  15 , and the attitude (angle) of the fixation roller  21  of the second fixing device  15  after the passage of the large sheet Pa through the second fixing device  15 , is as shown in  FIG. 9 . The broken line represents the case in which the ratio between the amount by which the fixation roller  21  was tilted, and the amount by which the fixation roller  21  is reduced in angle was 1:1. In this embodiment, the design of the fixing apparatus is such that the greater the angle by which the fixation roller is tilted during the conveyance of the large sheet Pa through the second fixing device  15 , the greater the angle by which the fixation roller  21  is tilted back after the passage of the large sheet Pa through the second fixing device  15  as indicated by the solid line. A point “0” in  FIG. 9  corresponds to the point in time when the leading edge of the large sheet Pa was introduced into the nip N of the second fixing device  15 , and was reset each time a large sheet Pa of recording medium was introduced into the second fixing device  15 . 
         [0053]    Also in the case of the fixing apparatus in this embodiment, if one of the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa of recording medium, which correspond in position to the widthwise end portions of the large sheet Pa, one for one, in the fixing apparatus, is larger in the amount of slack than the other, for example, the portion δ 2  is larger in the amount of slack than the portion δ 1 , the fixation roller  21  is tilted by a preset angle in the plane which coincides with the surface of the large sheet Pa in the second fixing device  15  so that its lengthwise end which corresponds to the portion δ 2  is moved in the +direction. That is, the control portion  60  moves the rack  35   a  by a preset distance in the recording medium conveyance direction by rotating forward the shift motor M 3 , which corresponds in position to the portion δ 2  (opposite portion of slackened portion of large sheet Pa from portion δ 1 ) which happened to be larger in the amount of slack than the portion δ 1 . Thus, the fixation roller  21  tilts by a preset angle so that the lengthwise end of the fixation roller  21  moves in the + direction. Then, as the output of the slack amount sensor  31  reaches the slack amount control start level L 1 , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to the preset recording medium conveyance speed V 2  which is greater than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). Further, after the second fixing device  15  is changed in the recording medium conveyance speed from V 3  to V 2 , if the slack amount sensors  31  and  32  become different in output, the control portion  60  drives forward the shift motor M 3 , which corresponds in position to the other widthwise end of the large sheet Pa of recording medium. Then, as the output of the slack amount sensor  31  reaches the preset slack amount control start level L 2 , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 2  to V 3 . The above described operational sequence is continuously repeated by the control portion  60  until the trailing edge of the large sheet Pa is discharged from the nip N of the second fixing device  15 . 
         [0054]    As soon as the trailing edge of the large sheet Pa of recording medium is conveyed out of the nip N of the second fixing device  15 , the control portion  60  drives the shift motor M 3  of the second fixing device  15  in reverse for a preset length of time, whereby the pinion gear  35   b  is rotated, causing thereby the rack  35   a  to move upstream in terms of the recording medium conveyance direction. Therefore, the fixation roller  21  is tilted by roughly the same angle as the angle by which it was tilted in the +direction, so that the other lengthwise end of the fixation roller  21  moves in the − direction. Thus, the other widthwise end of the following large sheet Pa of recording medium is introduced into the nip N of the second fixing device  15  before the opposite widthwise end of the large sheet Pa, and the large sheet Pa begins to be conveyed through the second fixing device  15 . Therefore, it becomes possible to reduce the difference between the amount by which the portions  51  of the large sheet Pa is slackened between the first and second fixing devices  14  and  15 , respectively, while the large sheet Pa is conveyed through the fixing apparatus, and the amount by which the portion δ 2  of the large sheet Pa is slackened between the first and second fixing devices  14  and  15 , respectively, while the large sheet Pa is conveyed through the fixing apparatus. 
         [0055]    In this embodiment, the fixing apparatus is structured so that the fixation roller  21  of the second fixing device is tilted downstream in terms of the recording conveyance direction, in the plane which coincides with the surface of the sheet P of recording medium in the fixing apparatus, so that the two slack amount sensors  31  and  32  become equal in output. Thus, the fixing apparatus is adjusted in such a manner that when a large sheet Pa of recording medium is conveyed through the fixing apparatus, the amount by which one side of the large sheet Pa in terms of the widthwise direction of the sheet Pa is slackened between the first and second fixing devices equals the amount by which the other side of the large sheet Pa is slackened between the first and second fixing devices. Further, as the two sides of the large sheet Pa in terms of the widthwise direction of the sheet Pa become equal in the amount of slack, the recording medium conveyance speed of the second fixing device is increased or decreased relative to the recording medium conveyance speed of the first fixing device so that one or/and the other end of the slackened portion of the large sheet Pa does not come into contact with the peripheral surface of either of the fixation rollers  21 . Therefore, the fixing apparatus in this embodiment can prevent the problem that when a large sheet of recording medium is conveyed through the fixing apparatus, one or both of the end portions of the sheet in terms of the widthwise direction come into contact with the fixation roller of the first fixing device and/or the fixation roller of the second fixing device, between the first and second fixing devices. Therefore, it can prevent an image forming apparatus from outputting defective images, the defects of which are attributable to the contact between the slackened portion of the sheet of recording medium and one or both of the fixation rollers. 
       Embodiment 2 
       [0056]    First, another example of a fixing apparatus of the so-called tandem type is described.  FIG. 10  is a schematic sectional view of the first and second fixing devices of the fixing apparatus in this embodiment (second embodiment) of the present invention, at a plane perpendicular to the axial line of each of the fixation rollers and pressure rollers of the fixing apparatus.  FIG. 11  is a drawing which shows the distribution of the amount by which heat is generated by the halogen heater in the hollow of the fixation roller of the first fixing device of the fixing apparatus in the second embodiment, in terms of the lengthwise direction of the heater.  FIG. 12(   a ) is a schematic drawing which show the pattern in which the sections  24   a ,  24   b , and  24   c  of the halogen heater of the first fixing device of the fixing apparatus in the second embodiment are turned on and off to keep the temperature of the fixation roller remains at the fixation temperature level.  FIGS. 12(   b ) and  12 ( b ) are schematic drawings which show the patterns, respectively, in which the sections  24   a ,  24   b ,  24   c  of the halogen light are turned on and off, and which are different from  FIG. 12(   a ) in terms of the ratio with which the three sections  24   a ,  24   b , and  24   c  of the halogen heater are turned on per unit length of time. 
         [0057]    The fixation roller  21  and pressure roller  22  of the second fixing device  15  of the fixing apparatus in this embodiment are 50 mm in external diameter. The second fixing device has three halogen heaters  24   a ,  24   b , and  24   c  which are heating members for changing how the fixation roller  21  is heated. The heaters  24   a ,  24   b , and  24   c  are in the hollow of the fixation roller  21 . Otherwise, the fixing apparatus in this embodiment is the same in structure as the fixing apparatus in the first embodiment. 
         [0058]    The fixation roller  21  is made up of a cylindrical metallic core  21   a  and an elastic layer  21   b . The cylindrical metallic core  21   a  is made of iron. The elastic layer  21   b  is formed of silicone rubber, and is 1.0 mm in thickness. It covers the entirety of the peripheral surface of the metallic core  21   a . The fixation roller  21  has also a parting layer  21   c , which is formed of a piece of PFA tube. It is 30 μm in thickness, and covers the entirety of the elastic layer  21   b . The pressure roller  22  is made up of a metallic core  22   a  and an electric layer  22   b . The metallic core  22   a  is in the form of a piece of iron rod which is 20 mm in diameter. The elastic layer  22   b  is made of sponge, more specifically, foamed silicone rubber, and covers the entirety of the peripheral surface of the metallic core  22   a . The pressure roller  22  has also a parting layer  22   c , which is 30 μm in thickness. The parting layer  22   c  is a piece of PFA tube, and covers the entirety of the peripheral surface of the elastic layer  22   b . The fixation roller  21  and pressure roller  22  are rotatably supported by the frame (unshown) of the fixing apparatus, at their lengthwise end portions of the metallic cores  21   a  and  22   a , respectively, with the placement of a pair of bearings between the lengthwise ends of the metallic cores  21   a  and  22   a , and the frame of the fixing apparatus. Further, the bearings for the pressure roller  22  are under the pressure generated by a pressure applying members (unshown) such as compression springs which keep the pressure roller  22  pressured in the direction (toward) perpendicular to the generatrix of the fixation roller  21 . The overall amount of the pressure applied to the bearings is roughly 490 N (50 kgf). Therefore, the elastic layer  22   b  of the pressure roller  22  remains deformed across the portion which is in contact with the fixation roller  21 . Thus, there is a nip N between the peripheral surface of the fixation roller  21  and the peripheral surface of the pressure roller  22 . The nip N has a preset width. 
         [0059]    Referring to  FIG. 11 , among the three halogen heaters  24   a ,  24   b , and  24   c , the halogen heater  24   a  is such a heater that its center portion, in terms of its lengthwise direction, is greater in the amount of heat generation than its lengthwise end portions. The halogen heater  24   b  is such a heater that its lengthwise end portions are greater in the amount of heat generation than its center portion. The halogen heater  24   c  is such a heater that one of its lengthwise end portions is greater in the amount of heat generation than the other lengthwise end portion. Further, the amount by which each of the lengthwise end portions of the halogen heater  24   b  is capable of generating heat is greater than the amount by which the center portion of the halogen heater  24   c  is capable of generating heat. It has been known that normally, as the fixation roller  21  increases in temperature, the nip N widens because of the thermal expansion of the fixation roller  21 . Thus, if the halogen heater  24   b  is greater in the ratio at which it is kept turned on per unit length of time than the halogen heater  24   c  ( FIG. 12(   b )), the portion of the nip N, which corresponds to one of the lengthwise portions of the fixation roller  21  (which hereafter may be referred to as first side) becomes wider than the portion of the nip N, which corresponds to the other lengthwise end portion of the fixation roller  21  (which hereafter may be referred to as second side). Thus, the portion of the nip N, which corresponds to the first lengthwise end portions of the fixation roller  21 , becomes faster in the speed with which a large sheet Pa of recording medium is conveyed, than the portion of the nip N, which corresponds to the second lengthwise end portion of the fixation roller  21 . On the other hand, if the halogen heater  24   c  is made greater in the ratio at which it is kept turned on per unit length of time than the halogen heater  24   b  (unshown), the portion of the nip N, which corresponds to the second lengthwise end portion of the fixation roller  21  becomes wider than the portion of the nip N, which corresponds to the first lengthwise end portion of the fixation roller  21 . Thus, the portion of the nip N, which corresponds to the second lengthwise end portion of the fixation roller  21 , becomes faster in the speed with which a large sheet Pa of recording medium is conveyed, than the portion of the nip N, which corresponds to the first lengthwise end portion of the fixation roller  21 . The halogen heaters  24   a ,  24   b , and  24   c  are supplied with a preset amount of electric power by a power supply control portion (unshown). As they are turned on ( FIG. 12(   a )), they generate heat. As heat is generated by the halogen heaters  24   a ,  24   b , and  24   c , the metallic core  21   a  of the fixation roller  21  is heated from within. As the fixation roller  21  is heated, its surface temperature is detected by a temperature detecting member (unshown) such as a thermistor located in the adjacencies of the peripheral surface of the fixation roller  21 . The output signals from the temperature detecting member are taken in by the control portion  60 , which controls the power supply control portion in response to the output signals from the temperature detecting member, in such a manner that the surface temperature of the fixation roller  21  remains at a preset fixation level (target level). In this embodiment, the fixation temperature (target temperature level) is 170° C. 
         [0060]    Next, referring to  FIGS. 4 and 12 , the control sequence to be carried out by the control portion  60  to change the second fixing device  15  in the recording medium conveyance speed and also, in the ratio at which the halogen lamp heater  24   a - 24   c  are kept turned on per unit length of time, when the amount by which one side of a large sheet Pa of recording medium in terms of its widthwise direction becomes greater in the amount of slack than the amount by which the other side of the large sheet Pa, is described. 
         [0061]    As a large sheet Pa of recording medium is pinched by the nip N of the second fixing device  15 , and begins to be conveyed, remaining pinched, through the fixing device  15  (T 1  in  FIG. 4 ), the large sheet Pa begins to increase in the amount of slack. Thus, the slack amount sensors  31  and  32  increase in output (T 1 -T 2  in  FIG. 4 ). 
         [0062]    As soon as the control portion  60  determines based on the outputs of the slack amount sensors  31  and  32  that one side of the large sheet Pa of recording medium in terms of the widthwise direction of the sheet Pa is greater in the amount of slack than the other side, it changes the halogen lamps  24  in the ratio at which they are kept turned on per unit length of time. That is, in order to increase, in the recording medium conveyance speed, one side of the nip N of the second fixing device  15  in terms of the lengthwise direction of the nip N, the control portion  60  increases the halogen lamp  24   c  in the ratio at which the lamp  24   c  is kept turned on per unit length of time, while leaving the halogen lamp  24   a  as it is in the ratio at which the  24   a  is kept turned on per unit length of time ( FIG. 12(   b )). Thus, one side of the nip N of the second fixing device  15  in terms of the lengthwise direction of the nip N becomes faster in recording medium than the other side. Therefore, the corresponding side of the large sheet Pa is pulled faster in the recording medium conveyance direction by the corresponding side of the fixation roller  21 . Thus, the corresponding side (portion δ 1 ) of the large sheet Pa gradually becomes smaller in the amount of slack. Then, if it is determined based on the outputs from the slack amount sensors  31  and  32  that the portion δ 1  of slackened portion of the large sheet Pa, which corresponds in position to one side of large sheet Pa in terms of the widthwise direction of the sheet Pa is greater in the amount of slack than the portion δ 2 , which corresponds in position to the other side, the control portion  60  changes again the halogen lamp  24  in the ratio of the length of time the halogen light  24  is kept turned on. That is, the control portion  60  makes the halogen lamp  24   b  even greater in the ratio of the length of time the lamp  24   b  is kept turned on per unit length of time, while keeping the same the ratio of the length of time the lamp  24   a  is kept turned on per unit length of time ( FIG. 12(   c )). Thus, one side of the nip N of the second fixing device  15  in terms of its lengthwise direction becomes even faster in the recording medium conveyance speed than the other side. Therefore, the first widthwise end portion of the large sheet Pa is pulled even faster in the recording medium conveyance direction than the second widthwise end portion of the sheet Pa. Therefore, the portion δ 1 , which corresponds to the first widthwise end portion of the large sheet Pa becomes even smaller in the amount of slack. Then, as the slack amount sensors  31  and  32  become equal in output, that is, as the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa, which correspond in position to the first and second widthwise end portions, one for one, of the large sheet Pa, the control portion  60  changes the halogen heaters  24   b  and  23   c  in the ratio of the length of time they are kept turned on per unit length of time, to the value at which the temperature of the fixation roller  21  remains at a preset level ( FIG. 12(   a )). Thereafter, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 1  (T 2  in  FIG. 4) , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to V 2  which is faster than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). As the second fixing device  15  increases in the recording medium conveyance speed from V 3  to V 2 , the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa of recording medium begin to reduce in the amount of slack. Therefore, the slack amount sensors  31  and  32  reduce in output (Figure T 2 -T 3  in  FIG. 4 ). If the slack amount sensors  31  and  32  become different in their outputs as described above even after the second fixing device  15  is changed in the recording medium conveyance speed from V 3  to V 2 , the control portion  60  changes the halogen lamps  24   b  and  24   c  in the ratio at which they are kept turned on per unit length of time. Then, as the slack amount sensors  31  and  32  become equal in output, the control portion  60  changes the halogen heaters  24   b  and  24   c  in the ratio at which they are kept turned on per unit length of time, back to the value at which the temperature of the fixation roller  21  remains at the fixation level. Then, as the output of the slack amount sensor  32  reaches the slack amount control start level L 2  (T 3  in  FIG. 4 ), the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 2  to V 3 . As the second fixing device  15  changes in the recording medium conveyance speed from V 2  to V 3 , the large sheet Pa of recording medium gradually increases in the amount of slack. Thus, the slack amount sensors  31  and  32  increase in output (T 3 -T 4  in  FIG. 4 ). The control portion  60  repeats the above described steps until the trailing edge of the large sheet Pa is discharged from the nip N of the second fixing device  15 . 
         [0063]    In the case of the fixing apparatus in this embodiment, if the portion δ 2  of the slackened portion of the large sheet Pa of recording medium, which corresponds in position to the second widthwise end portion of the large sheet Pa of recording medium is greater in the amount of slack than the portion δ 1  of the slackened portion of the large sheet Pa, which corresponds in position to the first widthwise end portion of the sheet Pa, the control portion  60  changes the halogen heaters in the ratio at which the they are kept turned on per unit length of time, in order to increase the second fixing device  15  in its recording medium conveyance speed on the second side of its nip N. That is, the control portion  60  increases the halogen lamp  24   c  greater in the ratio at which it is kept turned on per unit length of time than the halogen lamp  24   b , while keeping the halogen lamp  24   b  the same in the ratio at which it is kept turned on per unit length of time. Thus, the second side of the nip N of the second fixing device  15  in terms of the lengthwise direction of the nip N becomes faster in the recording medium conveyance speed than the first side of the nip N. Thus, the portion of the large sheet Pa of recording medium, which corresponds in position to the second lengthwise end portion of the fixation roller  21  is pulled faster in the recording medium conveyance speed than the portion of the large sheet Pa which corresponds in position to the first lengthwise end portion of the fixation roller  21 . Therefore, the portion δ 2  of the slackened portion of the large sheet Pa, which corresponds in position to the second widthwise end portion of the large sheet Pa gradually reduces in the amount of slack. Then, as the slack amount sensors  31  and  32  become equal in output, that is, as the portions  51  and  52  of the large sheet Pa, which correspond in position to the first and second sides of the fixation roller  21  in terms of the lengthwise direction, become equal in the amount of slack, the control portion  60  changes the halogen heaters  24   b  and  24   c  in the ratio at which they are kept turned on per unit length of time, back to the value for keeping the temperature of the fixation roller  21  at the fixation level. Then, as the output of the slack amount sensor  31  reaches the slack amount control start level L 1 , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to V 2  which is faster than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). If the slack amount sensors  31  and  32  become different in output, as described above, even after the second fixing apparatus  15  was changed in the recording medium conveyance speed from V 3  to V 2 , the control portion  60  changes the halogen lamps  24   b  and  24   c  in the ratio at which they are kept turned on per unit length of time. Then, as the output of the slack amount sensor  31  reaches the preset slack amount control start level L 2 , the control portion  60  changes the second fixing apparatus  15  in the recording medium conveyance speed from V 2  to V 3 . The control portion  60  repeats the above described steps until the trailing edge of the large sheet Pa of recording medium is discharged from the nip N of the second fixing device  15 . 
         [0064]    The fixing apparatus in this embodiment is controlled in the halogen heater settings so that the slack amount sensors  31  and  32  which correspond in position to the lengthwise end portion of the fixation roller of the second fixing device become equal in output. Thus, it is adjusted in such a manner that when a large sheet of recording medium is conveyed through the fixing apparatus, two sides of a large sheet of recording medium, in terms of its widthwise direction, become equal in the amount of slack. Further, the second fixing device is increased or reduced in the recording medium conveyance speed relative to the first fixing device to prevent the slackened portion of the large sheet of recording medium from coming into contact with the peripheral surface of the fixation rollers after the large sheet of recording medium become uniform in the amount of slack in terms of its widthwise direction. That is, the fixing apparatus in this embodiment also can prevent the problem that the portion of the large sheet P of recording medium, which is being slackened between the first and second fixing devices, comes into contact with the fixation rollers by one of its widthwise end portion or the entirety of the slackened portion. Therefore, it can prevent an image forming apparatus from outputting defective images, the defects of which are attributable to the contact between the slackened portion of recording medium and the fixation rollers. 
       Embodiment 3 
       [0065]    Next, another example of a fixing apparatus of the so-called tandem type is described.  FIG. 13  is a schematic sectional view of the first and second fixing devices of the fixing apparatus in the third embodiment of the present invention, at a plane perpendicular to the axial line of the fixation rollers and pressure rollers of the fixing apparatus. 
         [0066]    The fixation roller  21  of the second fixing device  15  of the fixing apparatus in the third embodiment is a roller which is 32 mm in external diameter. Further, the second fixing device of this fixing apparatus has a pressure application mechanism  40  as the pressure applying means of the second fixing device  15 . Otherwise, the fixing apparatus in the third embodiment is the same in structure as the fixing apparatus in the first embodiment. 
         [0067]    The fixation roller  21  is made up of a cylindrical metallic core  21   a  and an elastic layer  21   b . The cylindrical metallic core  21   a  is made of iron. The elastic layer  21   b  is formed of silicone rubber, and is 1.0 mm in thickness. It covers the entirety of the peripheral surface of the metallic core  21   a . The fixation roller  21  has also a parting layer  21   c , which is formed of a piece of PFA tube. It is 30 μm in thickness, and covers the entirety of the elastic layer  21   b . In the case of a fixing device to be mounted in a color image forming apparatus, the parting layer  21   c  of the fixation roller  21  may be formed of silicone rubber impregnated with silicone oil, instead of being formed of the piece of PFA tube. Referring to  FIG. 13 , designated by a referential code  28  are a pair of bearings, which are at the lengthwise ends of the metallic core  22   a  of the pressure roller  22  of the second fixing device  15 . The bearings  28  are supported by the frame in such a manner that they can be moved upward or downward (direction perpendicular to recording medium conveyance direction). There are two pressure application mechanisms  40 , which are at the lengthwise ends of the pressure roller  22 , one for one. Each pressure application mechanism  40  has a lever  41  and an eccentric rotational cam  42 . The lever  41  supports a pressing member  29  for pressing the bearing  28 . The eccentric rotational cam  42  is roughly in the form of a disc, and is for changing the lever  41  in the angle relative to the pressure roller  22 . The rotational axis of the rotational eccentric cam  42  coincides with the rotational axis of a cam shaft  42   a . The rotational eccentric cam  42  is rotated by a cam motor M 4  (driving means) by way of the camshaft  42   a . The cam motor M 4  is driven by the control portion  60  so that it will stop after being rotated a preset number of times. Therefore, the rotational eccentric cam  42 , which is rotated by the cam motor M 4 , stops after being rotationally moved by a preset number of times. The attitude in which the rotational eccentric cam  42  stops is such that the area  42   b   1  of the peripheral surface  42   b  (cam surface) of the rotational eccentric cam  42 , which is closest to the camshaft  32   a , is in contact with the lever  41 . As the rotational eccentric cam  42  rotationally moves, the lever  41  rotationally moves (tilts) by a preset angle toward the pressure roller  22  about a lever supporting shaft  41   a  as the fulcrum of the lever  41   a , and stops. As the lever  41  is moved as described above, the pressing member  29  is made to press the bearing  28  in the direction perpendicular to the direction of the generatrix of the fixation roller  21 . Thus, the elastic layer  22   b  of the pressure roller  22  and the elastic layer  21   b  of the fixation roller  21  are compressed by a preset amount across their lengthwise direction, creating thereby a nip N having a preset dimension in terms of the recording medium conveyance direction. Generally speaking, as the pressure applied to the fixation roller  21  and pressure roller  22  in the direction to press them against each other is increased, the nip N between the two rollers widens in the direction perpendicular to the recording medium conveyance direction, which in turn increases the fixing device in the recording medium conveyance speed. Thus, if the pressure applied to the lengthwise ends of the pressure roller  22  to press the pressure roller  22  upon the fixation roller  21  is increased only at one of the lengthwise ends of the pressure roller  22 , the nip N changes in shape in such a manner that it becomes widest at its lengthwise end which corresponds to the lengthwise end of the pressure roller  22 , at which the pressure was increased. Thus, the fixing device becomes nonuniform in terms of its lengthwise direction, in the recording medium conveyance speed, in such a manner that the closer it is to the lengthwise end at which the pressure was increased, the faster the speed with which it conveys the large sheet Pa of recording medium. On contrary, if the pressure applied to the pressure roller  22  to press the roller  22  upon the fixation roller  21  is made greater at the other lengthwise end of the pressure roller  22  than the aforementioned lengthwise end, the nip N changes in shape so that the it becomes widest at the lengthwise end of the pressure roller  22 , which was increased in the amount applied to the pressure roller  22 . Thus, the fixing device becomes nonuniform in terms of the lengthwise direction, in its recording medium conveyance speed, so that the closer it is to the lengthwise end of the pressure roller  22  at which the pressure applied to the pressure roller  22  was increased, the faster the speed with which the large sheet Pa of recording medium is conveyed. 
         [0068]    Next, referring to  FIGS. 4 and 13 , the control sequence to be carried out by the control portion  60  to change the second fixing device  15  in the recording medium conveyance speed when the large sheet Pa of recording medium, which is being conveyed through the fixing apparatus, is nonuniform in terms of the widthwise direction of the sheet Pa, in the amount of slack between the first and second fixing devices, and the operation of the pressure application mechanism  40 , are described. 
         [0069]    As the large sheet Pa of recording medium is pinched by the nip N of the second fixing device  15 , and begins to be conveyed, remaining pinched in the nip N, by the second fixing device  15  (T 1  in  FIG. 4 ), the large sheet Pa begins to increase in the amount of slack across its widthwise direction, whereby the slack amount sensors  31  and  32  are increased in output (T 1 -T 2  in  FIG. 4 ). As the control portion  60  determines based on the outputs of the slack amount sensors  31  and  32  that the slackened portion δ 1  of the large sheet Pa, which corresponds in position to one of the widthwise end of the sheet Pa, is greater in the amount of slack than the slacked portion δ 2  of the sheet Pa, which corresponds in position to the other widthwise end of the sheet Pa, the control portion  60  activates the pressure application mechanism  40  to make the second fixing device  15  change in the recording medium conveyance speed in such a manner that the lengthwise end of the nip N of the second fixing device  15 , which corresponds in position to the other widthwise end of the sheet Pa, increases in the recording medium conveyance speed. That is, the control portion  60  drives forward the cam motor M 4 , which is at the lengthwise end at which the large sheet Pa was larger in the amount of slack, to rotate the rotational eccentric cam  42  by a preset angle, and then, stops the cam  42 . The attitude in which the rotational eccentric cam  42  will be when the cam motor M 4  stops being driven is such that the area  42   b   1  of the peripheral  42   b  (cam surface) of the rotational eccentric cam  42 , which is farthest from the camshaft  32   a , is in contact with the lever  41 . As the rotational eccentric cam  42  is rotated as described above, the lever  41  is rotationally moved about the lever supporting shaft  41   a  (as fulcrum) by a preset angle toward the pressure roller  22  from where it was, and then, stops. As the lever  41  is rotationally moved, the pressing member  29  presses the bearing  28  in the direction perpendicular to the direction of the generatrix of the fixation roller  21 . Thus, the nip N changes in shape so that the corresponding lengthwise end of the nip N becomes wider than the other lengthwise end. Therefore, the second fixing device  15  becomes nonuniform in the recording medium conveyance speed in terms of its lengthwise direction in such a manner that the closer it is to the corresponding lengthwise end a given point in the nip N, the faster in the recording medium conveyance speed. Therefore, the closer it is to the corresponding lengthwise end of the second fixing device  15 , the faster the large sheet Pa is pulled in the recording medium conveyance direction. Therefore, the large sheet Pa is conveyed in such a manner that its portion δ 1  of the slackened portion of the large sheet Pa, which corresponds in position to the first widthwise edge of the sheet Pa, gradually reduces in the amount of slack. Then, as the slack amount sensors  31  and  32  become equal in output, that is, as the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa, which correspond in position to the first and second widthwise edges of the large sheet Pa, become equal in the amount of slack, the control portion  60  changes the rotational eccentric cam  42  in attitude back into the fixation temperature retention attitude, by rotating the cam motor M 4  in reverse ( FIG. 13 ). Then, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 1  (T 2  in  FIG. 4 ), the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to V 2  which is greater than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). As the second fixing device  15  is increased in the recording medium conveyance speed from V 3  to V 2 , the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa begin to reduced in the amount of slack. Thus, the slack amount sensors  31  and  32  reduce in output (T 2 -T 3  in  FIG. 4 ). If the slack amount sensors  31  and  32  become different in output as described above even after the second fixing device  15  was changed in the recording medium conveyance speed from V 3  to V 2 , the control motor  60  rotationally moves the rotational eccentric cam  42  by a preset angle by rotating forward the cam motor M 4 , and then, stops the cam  42 . As the slack amount sensors  31  and  32  become equal in output because of the rotation of the rotational cam  42  by the preset angle as described, the control portion  60  rotates the rotational eccentric cam  42  back into the fixation temperature retention attitude by rotating the cam motor M 4  in reverse. Then, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 2  (T 3  in  FIG. 4 ), the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 2  to V 3 . As the second fixing device  15  is changed in the recording medium conveyance speed from V 2  to V 3 , the portions δ 1  and δ 2  of the slackened portion of the large sheet Pa begin to gradually increase in the amount of slack. Thus, the slack amount sensors  31  and  32  increase in output (T 3 -T 4  in  FIG. 4 ). The control portion  60  repeats the above described steps until the trailing edge of the large sheet Pa is discharged from the nip N of the second fixing device  15 . 
         [0070]    In this embodiment, as the portion δ 2  of the slackened portion of the large sheet Pa of recording medium, which corresponds in position to the second widthwise edge of the large sheet Pa, becomes larger in the amount of slack than the portion δ 1  of the slackened portion of the large sheet Pa, which corresponds in position to the first widthwise edge of the large sheet Pa, the control portion  60  activates the pressure application mechanism  40  to control the second fixing device  15  in such a manner that the second side of the second fixing device  15  becomes faster in the recording medium conveyance speed than the first side. That is, as the control portion  60  rotates the rotational eccentric cam  42  by a preset angle by rotating forward the corresponding cam motor M 4 , that is, the cam motor M 4  which corresponds in position to the widthwise edge of the large sheet Pa, which was determined to be larger in the amount of slack, and then, stops the rotational eccentric cam  42 . Thus, the nip N of the second fixing device  15  changes in shape so that its lengthwise end on the second side in terms of the lengthwise direction becomes wider than its opposite lengthwise end. Therefore, the recording medium conveyance speed of the nip N becomes faster on the second side than on the first side. Thus, the large sheet Pa is pulled faster in the recording medium conveyance direction on the second side of the fixing apparatus than on the first side. Thus, the portion δ 2  of the slackened portion of the large sheet Pa, which is on the second side, gradually reduce in the amount of slack. Then, as the slack amount sensors  31  and  32  become equal in output, that is, as the portions δ 1  and δ 2 , which are on the first and second side become equal in the amount of slack, the control portion  60  rotates the rotational eccentric cam  42  back into the fixation temperature retention attitude by rotating the cam motor  4 M in reverse. Then, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 1 , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 3  to V 2  which is faster than the recording medium conveyance speed V 1  of the first fixing device  14  (V 2 &gt;V 1 &gt;V 3 ). If the slack amount sensors  31  and  32  become different in output as described above even after the second fixing device was changed in the recording medium conveyance speed from V 3  to V 2 , the control portion  60  rotates forward the cam motor M 4  by a preset angle the cam motor M 4 , and stops the rotation of the cam motor M 4 . Then, as the output of the slack amount sensor  32  reaches the preset slack amount control start level L 2 , the control portion  60  changes the second fixing device  15  in the recording medium conveyance speed from V 2  to V 3 . The control portion  60  repeats the above described steps until the trailing edge of the large sheet Pa is discharged from the nip N of the second fixing device  15 . 
         [0071]    The fixing apparatus in the third preferred embodiment is changed (adjusted) in the amount by which pressure is applied to the lengthwise end portions of the pressure roller and fixation roller of the second fixing device so that the slack amount sensors become equal in output. Thus, it also is adjusted so that when a large sheet of recording medium is conveyed through the fixing apparatus, not only is the portion of the large sheet Pa between the first and second fixing devices slackened, but also, the slackened portion becomes uniform in the amount of slack in terms of the lengthwise direction of the fixing apparatus. Further, in order to prevent the slackened portion of the large sheet Pa from coming into contact with the peripheral surface of the fixation rollers, the second fixing device is increased or decreased in the recording medium conveyance speed relative to the first fixing device after the slackened portion of the large sheet becomes uniform in the amount of slack in terms of the widthwise direction of the large sheet (lengthwise direction of fixing apparatus). That is, the fixing apparatus in this embodiment also can prevent the problem that when a large sheet of recording medium is conveyed through a fixing apparatus structured so that when a large sheet of recording medium is conveyed through the fixing apparatus, it is slackened between the first and second fixing devices of the fixing apparatus, the slackened portion of the large sheet comes into contact with the fixation rollers of the first and second fixing devices at one of the lengthwise end of the fixing apparatus, and/or across the entirety of the fixing apparatus. Therefore, it can prevent an image forming apparatus from outputting defective images, the defects of which are attributable to the contact between a sheet of recording medium and the fixation rollers. 
       Embodiment 4 
       [0072]    Next, another example of a fixing apparatus of the so-called tandem type is described.  FIG. 14(   a ) is a schematic top plan view of the first and second fixing device and sensor moving mechanism of the fixing apparatus in the fourth preferred embodiment of the present invention, as seen from above the fixation roller.  FIG. 14(   b ) is a schematic top plan view of the first and second fixing devices and sensor moving mechanism of the fixing apparatus, and shows the operation of the sensor moving mechanism. 
         [0073]    The fixing apparatus in the fourth embodiment is structured so that the slack amount sensors  31  and  32  can be moved into the path of the large sheet Pa of recording medium with the use of a sensor moving mechanism  50  (sensor moving means). Otherwise, this fixing apparatus is the same in structure as the fixing apparatus in the first embodiment. 
         [0074]    The sensor moving mechanism  50  in this embodiment is a rack-and-pinion mechanism. It has a first rack  51  and a second rack  52 . The first rack  51  supports the slack amount sensor  31 , which is the sensor on the first side in terms of the widthwise direction of the large sheet Pa of recording medium, and the second rack  52  supports the slack amount sensor  32 , which is the sensor on the second side. The sensor moving mechanism  50  has also a supporting member  55 , which is between the first and second fixing device  14  and  15  of the fixing apparatus, and extends in the direction parallel to the lengthwise direction of the fixation roller  21  of the first fixing device  14  and the fixation roller  21  of the second fixing device  15 . The first and second racks  51  and  52 , respectively, are on the surface of the supporting member  55 , which faces the path of the large sheet Pa. The first and second racks  51  and  52  are perpendicular to the recording medium conveyance direction and oppose each other. They are movable in the widthwise direction of the large sheet Pa. As the pinion  53  which is supported between the first and second racks  51  and  52  by the supporting member  55  in such a manner that it is in mesh with the two racks  51  and  52  is rotated, the first and second racks  51  and  52  are moved in the widthwise direction of the large sheet Pa, while remaining symmetrical to each other about the axial line of the pinion gear  53 . Referring to  FIG. 14 , a pair of two-headed arrow marks designed by referential codes W 1  and W 2  are the width of the path of the largest sheet of recording medium and the width of the path of the smallest sheet of recording medium, respectively, which are usable with the image forming apparatus. The recording medium sheet size is inputted into the control portion  60  by a user, or the recording medium sheet width is inputted into the control portion  60  based on the information from an automatic recording medium sheet width detecting mechanism (unshown) of a recording medium sheet feeder cassette or the like. The control portion  60  controls (drives) a sensor moving motor M 5  (sensor moving means supported by the supporting member  55 ), based on the inputted information.  FIG. 14(   b ) shows the state of the second fixing device  15  after the slack amount sensors  31  and  32  have been moved immediately inward of the path of the smallest sheet of recording medium on the first and second sides, respectively, in terms of the widthwise direction of the sheet path. 
         [0075]    The fixing apparatus in this embodiment is structured so that the slack amount sensors are movable in the widthwise direction of the recording medium sheet path, to preset locations according to the width of the recording medium sheet. Therefore, regardless of the size of the sheet of recording medium, it can prevent the problem that the slackened portion of the sheet of recording medium comes into contact with the fixation rollers at one of the lengthwise end of the fixing apparatus, or across the entirety of the fixing apparatus. Therefore, it can prevent an image forming apparatus from outputting defective images, the defects of which are attributable the contact between the slackened portion of the sheet of recording medium and one or both of the fixation rollers. Although the fourth preferred embodiment of the present invention was described with reference to the image forming apparatus created by providing the image forming apparatus in the first embodiment with the sensor moving mechanism, the application of the sensor moving mechanism is not limited to image forming apparatuses such the one in the first embodiment. That is, the sensor moving mechanism may be applied to image forming apparatuses such as those in the third and fourth embodiments. 
         [0076]    According to the present invention, it is possible to provide an image heating apparatus which can reliably convey a sheet of recording medium, while slightly slackening the sheet of recording medium between its upstream and downstream image heating units in terms of the recording medium conveyance direction, even if it temporarily becomes nonuniform in the recording medium conveyance speed, in terms of the widthwise direction of the sheet of recording medium. 
         [0077]    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 purposes of the improvements or the scope of the following claims.