Patent Publication Number: US-7917048-B2

Title: Image forming apparatus and electromagnetic induction heating type fixing device having magnetic flux regulating section

Description:
This application is based on an application No. 2007-125748 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to fixing devices and, in particular, to an electromagnetic induction heating type fixing device. 
     The present invention relates also to an image forming apparatus having an electromagnetic induction heating type fixing device. 
     In general, the electromagnetic induction heating type fixing device of this kind has a fixing member (roller or belt) whose outer peripheral surface is brought in pressure contact with a sheet (paper) conveyed, and an induction coil for heating the fixing member by electromagnetic induction. By heating a metal layer of the fixing member by electromagnetic induction caused by the induction coil, a toner image adhering to the sheet is melted by the heat of the fixing member and fixed onto the sheet. 
     Conventionally, a system for preventing an excessive temperature rise in the end portion of the fixing member (i.e., a region through which a small-size sheet does not pass) by movably providing a magnetic flux shield member in the vicinity of the end portion of the fixing member in the widthwise direction of the sheet and moving the position of the magnetic flux shield member according to the dimension (width dimension) in the widthwise direction of the sheet to be subjected to image fixing has been proposed (refer to, for example, JP H10-74009 A and JP 2006-195408 A). 
     SUMMARY OF THE INVENTION 
     In the fixing devices of JP H10-74009 A and JP 2006-195408 A, only the portion, with which each sheet is actually brought in contact, of the outer peripheral surface of the fixing member is heated in the widthwise direction. In other words, the whole range of each sheet is heated in the widthwise direction. 
     However, the portion that needs to be actually heated in the widthwise direction of each sheet is only the image region to which the toner is adhering. Therefore, the fixing devices of JP H10-74009 A and JP 2006-195408 A have the problem that they are wastefully consuming energy. 
     Moreover, generally in the conventional fixing devices, the whole range of each sheet conveyed is heated in the conveyance direction of the sheet. For the above reasons, there is a problem that energy is wastefully consumed. 
     An object of the present invention is to provide a fixing device capable of heating only the image region of each sheet and therefore achieving energy conservation. 
     Another object of the present invention is to provide an image forming apparatus capable of heating only the image region of each sheet for image fixing and therefore achieving energy conservation. 
     In order to accomplish the object, a fixing device of the present invention comprises: 
     a fixing member that extends along a widthwise direction of a sheet conveyed and has an outer peripheral surface to be brought in pressure contact with the sheet; 
     an induction coil that is placed elongate in the widthwise direction of the sheet along the fixing member, for inductively heating a heat generating layer of the fixing member; 
     a high frequency power source section that heats the heat generating layer of the fixing member via the induction coil by supplying an alternating current to the induction coil; 
     a magnetic flux regulating section that restricts heating of a specified region of the fixing member in the widthwise direction of the sheet; 
     a first image region detecting section that detects an image region on the sheet occupied by an image to be fixed in the widthwise direction of the sheet; and 
     a first control section that specifies a region in which generation of heat should be restricted for the magnetic flux regulating section so that only a region of the fixing member corresponding to the image region is substantially heated in the widthwise direction of the sheet, sheet-by-sheet according to a result of detection of the first image region detecting section. 
     It is noted that the verbal phrase of “substantially heat” means exclusion of the case where heating takes effect as a result of the diffraction of magnetic fluxes despite that the heating is restricted by the magnetic flux regulating section. 
     In the fixing device of the present invention, the first image region detecting section detects the image region on the sheet in the widthwise direction of the sheet. The first control section specifies the region in which the generation of heat should be restricted for the magnetic flux regulating section so that only the region corresponding to the image region of the fixing member is substantially heated in the widthwise direction of the sheet, sheet-by-sheet according to the result of detection of the first image region detecting section. The high frequency power source section supplies an alternating current to the induction coil, and the magnetic flux regulating section restricts the generation of heat in the specified region of the fixing member in the widthwise direction of the sheet. By this operation, only the region corresponding to the image region of the fixing member is substantially heated in the widthwise direction of the sheet, and the generation of heat in other regions is restricted. In the above state, the sheet is brought in pressure contact with the outer peripheral surface of the fixing member. As a result, only the image region of each sheet can be heated in the widthwise direction of the sheet. Therefore, energy conservation can be achieved. 
     In an another aspect, a fixing device of the present invention comprises: 
     a fixing member that extends along a widthwise direction of a sheet conveyed and has an outer peripheral surface to be brought in pressure contact with the sheet; 
     an induction coil that is placed elongate in the widthwise direction of the sheet along the fixing member, for inductively heating a heat generating layer of the fixing member; 
     a high frequency power source section that heats the heat generating layer of the fixing member via the induction coil by supplying an alternating current to the induction coil; 
     a magnetic flux regulating section that restricts heating of a specified region of the fixing member in the widthwise direction of the sheet; 
     a second image region detecting section that detects an image region on the sheet occupied by an image to be fixed in a conveyance direction of the sheet; and 
     a second control section that specifies turning-on and -off of power supply to the induction coil for the high frequency power source section so that only the image region on the sheet is heated in the conveyance direction of the sheet, sheet-by-sheet according to a result of detection of the second image region detecting section. 
     In the fixing device of the present invention, the second image region detecting section detects the image region on the sheet occupied by the image to be fixed in the conveyance direction of the sheet. Then, the second control section instructs the high frequency power source section to turn on and off the power supply to the induction coil so that only the image region on the sheet is heated in the conveyance direction of the sheet, sheet-by-sheet according to the result of detection of the second image region detecting section. As a result, only the image region of each sheet can be heated in the conveyance direction of the sheet. Therefore, energy conservation can be achieved. 
     In the fixing device of one embodiment, 
     the magnetic flux regulating section comprises: 
     a support member made of a nonmagnetic material placed elongate in the widthwise direction of the sheet along the fixing member and the induction coil; 
     a magnetic flux shield portion that is formed with a prescribed pattern on a surface of the support member and interrupts a magnetic flux directed from the induction coil toward the fixing member; and 
     a displacement mechanism that displaces the support member having the magnetic flux shield portion relatively to the fixing member and the induction coil. 
     In the fixing device of the present one embodiment, the displacement mechanism can restrict the generation of heat in the specified region of the fixing member in the widthwise direction of the sheet by displacing the support member having the magnetic flux shield portion relative to the fixing member and the induction coil. 
     In the fixing device of one embodiment, 
     the fixing member is a metal sleeve that extends elongate in the widthwise direction of the sheet, 
     the support member is a cylindrical member placed so as to be brought in contact with an inner peripheral surface of the fixing member, 
     the induction coil is placed inside the support member, 
     the support member is divided into a first support member and a second support member in the widthwise direction of the sheet, and the magnetic flux shield portion is provided for each of the first support member and the second support member, and 
     the displacement mechanism displaces the first support member and the second support member of the support member by independently rotating the members around a central axis of each of the members. 
     In the fixing device of the present one embodiment, the variations of the regions of the fixing member in which the generation of heat can be restricted in the widthwise direction of the sheet are increased. 
     In the fixing device of one embodiment, 
     the pattern of the magnetic flux shield portion on the first support member comprises: a first pattern portion whose dimension in the direction of the central axis is gradually decreased from its total length as located farther away from a reference position on the support member toward one side in a circumferential direction; and a second pattern portion whose dimension in the direction of the central axis is gradually increased from zero to the total length as located farther away from the reference position toward the other side in the circumferential direction, 
     the pattern of the magnetic flux shield portion on the second support member is formed symmetrically to the pattern of the magnetic flux shield portion on the first support member with respect to a boundary plane located between the first support member and the second support member, and 
     the displacement mechanism is able to independently rotate the first support member and the second support member forwardly or reversely. 
     In this fixing device of the present one embodiment, the first support member and the second support member have the first pattern portion whose dimension in the direction of the central axis is gradually decreased from the total length as located farther away from the reference position on the support member to one side in the circumferential direction, and the second pattern portion whose dimension in the direction of the central axis is gradually increased from zero to the total length as located farther away from the reference position to the other side in the circumferential direction. Therefore, in order to make the displacement mechanism displace the first support member and the second support member to the respective target angular positions, it is proper to move the first support member and the second support member to the one side or the other side in the circumferential direction, preferably to the side where the required angle of rotation is smaller. Therefore, time required for the displacement-mechanism to displace the first support member and the second support member to the respective target angular positions is shortened. 
     In the fixing device of one embodiment, 
     the first image region detecting section comparts the sheet into a plurality of areas in the widthwise direction of the sheet and detects whether or not each comparted area is occupied by the image that should be fixed, and 
     each of the first and second patterns of the magnetic flux shield portion on the first support member and the second support member has the dimension in the direction of the central axis increased or decreased step-by-step in blocks defined in correspondence with the area as located farther away from the reference position in the circumferential direction. 
     In the fixing device of the present one embodiment, the first and second patterns of the magnetic flux shield portions on the first support member and the second support member have the dimensions in the direction of the central axis increased or decreased step-by-step in specified blocks in correspondence with the areas as located farther away from the respective reference positions. Therefore, it is proper for the displacement mechanism to rotate the first support member and the second support member to the one side or the other side in the circumferential direction every angle at which the dimension in the direction of the central axis of the first and second patterns of the magnetic flux shield portions is increased or decreased step-by-step. Therefore, the control of the angular positions of the first support member and the second support member is simplified. 
     In the fixing device of one embodiment, each of the first and second patterns of the magnetic flux shield portion on the first support member and the second support member has the dimension in the direction of the central axis linearly increased or decreased as located farther away from the reference position in the circumferential direction. 
     In the fixing device of the present one embodiment, the region in which the generation of heat of the fixing member can be restricted in the widthwise direction of the sheet can be continuously variably set. 
     In the fixing device of one embodiment, 
     the first image region detecting section comprises: 
     a counting section that comparts the sheet into a plurality of areas in the widthwise direction of the sheet and detects an amount of image of each comparted area; 
     a memory section that stores the amount of image of each area detected by the counting section; and 
     a determining section that determines whether or not the amount of image of each area exceeds a prescribed threshold value and determines that such an area where the amount of image exceeds the threshold value belongs to the image region. 
     In this case, the “amount of image” indicates the amount of toner in the case of, for example, a toner image. The “amount of image”, which is defined for each area, corresponds to the image density (toner density in the case of, for example, a toner image). 
     In the fixing device of the present one embodiment, it is determined area-by-area whether or not the area belongs to the image region in the widthwise direction of the sheet. Therefore, detection of the image region on the sheet can be performed comparatively easily. 
     In the fixing device of one embodiment, 
     the second image region detecting section comprises: 
     a counting section that comparts the sheet into a plurality of areas in the conveyance direction of the sheet and detects an amount of image of each comparted area; 
     a memory section that stores the amount of image of each area detected by the counting section; and 
     a determining section that determines whether or not the amount of image of each area exceeds a prescribed threshold value and determines that such an area where the amount of image exceeds the threshold value belongs to the image region. 
     In the fixing device of the present one embodiment, it is determined area-by-area whether or not the area belongs to the image region in the conveyance direction of the sheet. Therefore, detection of the image region on the sheet can be performed comparatively easily. 
     When the temperature difference is excessively large in the sheet passing region of the fixing member where the sheet should pass, a trouble of the generation of wrinkles in the sheet after fixation possibly occurs. 
     Thus, in the fixing device of the present one embodiment, 
     a temperature distribution obtaining section that obtains a temperature distribution of the fixing member in the widthwise direction of the sheet; 
     a passing region detecting section that preparatorily obtains a sheet passing region, through which the sheet should pass, of the fixing member sheet-by-sheet on a basis of a width dimension of the sheet conveyed; and 
     a third control section that obtains a difference between a maximum temperature and a minimum temperature represented by the temperature distribution within the sheet passing region and permits the magnetic flux regulating section to heat whole area of the sheet passing region on the fixing member when the difference exceeds a prescribed threshold value. 
     In the fixing device of the present one embodiment, the temperature distribution obtaining section obtains the temperature distribution of the fixing member in the widthwise direction of the sheet. The third control section obtains the difference between the maximum temperature and the minimum temperature expressed by the temperature distribution within the sheet passing region, and executes control to permit the magnetic flux regulating section to heat the whole area of the sheet passing region on the fixing member when the difference exceeds the prescribed threshold value. Therefore, wrinkles can be prevented from generating in the sheet after fixation. 
     In the fixing device of one embodiment, the temperature distribution obtaining section comprises a plurality of temperature sensors that are arranged in the widthwise direction of the sheet oppositely to the fixing member and each detect a temperature of an opposite portion of the fixing member. 
     In this case, the phrase of “oppositely to the fixing member” of the temperature sensor includes both the case where the temperature sensor is put in contact with the fixing member and the case where the sensor is spaced apart from the fixing member. That is, the temperature sensor may be a contact type or a noncontact type. 
     In the fixing device of the present one embodiment, the difference between the maximum temperature and the minimum temperature can be obtained simply and accurately. 
     In the fixing device of one embodiment, the temperature distribution obtaining section obtains by simulation a temperature distribution of the fixing member in the widthwise direction of the sheet. 
     In the fixing device of the present one embodiment, the parts cost can be reduced by reducing the number of temperature sensors. 
     An image forming apparatus of the present invention comprises: 
     an image forming section that forms a toner image by making toner adhere to a sheet; 
     a fixing member that extends along a widthwise direction of the sheet conveyed and has an outer peripheral surface brought in pressure contact with the sheet to which the toner image has adhered; 
     an induction coil that is placed elongate in the widthwise direction of the sheet along the fixing member, for inductively heating a heat generating layer of the fixing member; 
     a high frequency power source section that heats the heat generating layer of the fixing member via the induction coil by supplying an alternating current to the induction coil; 
     a magnetic flux regulating section that restricts heating of a specified region of the fixing member in the widthwise direction of the sheet; 
     a first image region detecting section that detects an image region on the sheet occupied by an image to be fixed in the widthwise direction of the sheet; and 
     a first control section that specifies a region in which generation of heat should be restricted for the magnetic flux regulating section so that only a region corresponding to the image region of the fixing member is substantially heated in the widthwise direction of the sheet, sheet-by-sheet according to a result of detection of the first image region detecting section. 
     It is noted that the verbal phrase of “substantially heat” means exclusion of the case where heating takes effect as a result of the diffraction of magnetic fluxes despite that the heating is restricted by the magnetic flux regulating section. 
     In the image forming apparatus of the present invention, the image forming section forms a toner image by making toner adhere to the sheet. Meanwhile, the first image region detecting section detects the image region on the sheet in the widthwise direction of the sheet. The first control section specifies the region in which the generation of heat should be restricted for the magnetic flux regulating section so that only the region corresponding to the image region of the fixing member is substantially heated in the widthwise direction of the sheet, sheet-by-sheet according to the result of detection of the first image region detecting section. The high frequency power source section supplies an alternating current to the induction coil, and the magnetic flux regulating section restricts the generation of heat in the specified region of the fixing member in the widthwise direction of the sheet. By this operation, only the region corresponding to the image region of the fixing member is substantially heated in the widthwise direction of the sheet, and the generation of heat in other regions is restricted. In the above state, the sheet to which the toner image has adhered is brought in pressure contact with the outer peripheral surface of the fixing member. As a result, only the image region of each sheet can be heated for fixation in the widthwise direction of the sheet. Therefore, energy conservation can be achieved. 
     In an another aspect, an image forming apparatus of the present invention comprises: 
     an image forming section that forms a toner image by making toner adhere to a sheet; 
     a fixing member that extends along a widthwise direction of the sheet conveyed and has an outer peripheral surface brought in pressure contact with the sheet to which the toner image has adhered; 
     an induction coil that is placed elongate in the widthwise direction of the sheet along the fixing member, for inductively heating a heat generating layer of the fixing member; 
     a high frequency power source section that heats the heat generating layer of the fixing member via the induction coil by supplying an alternating current to the induction coil; 
     a magnetic flux regulating section that restricts heating of a specified region of the fixing member in the widthwise direction of the sheet; 
     a second image region detecting section that detects an image region on the sheet occupied by an image to be fixed in a conveyance direction of the sheet; and 
     a second control section that specifies turning-on and -off of power supply to the induction coil for the high frequency power source section so that only the image region on the sheet is heated in the conveyance direction of the sheet, sheet-by-sheet according to a result of detection of the second image region detecting section. 
     In the image forming apparatus of the present invention, the image forming section forms a toner image by making toner adhere to the sheet. Meanwhile, the second image region detecting section detects the image region on the sheet occupied by the image to be fixed in the conveyance direction of the sheet. Then, the second control section instructs the high frequency power source section to turn on and off the power supply to the induction coil so that only the image region on the sheet is heated in the conveyance direction of the sheet, sheet-by-sheet according to the result of detection of the second image region detecting section. As a result, only the image region of each sheet can be heated in the conveyance direction of the sheet. Therefore, energy conservation can be achieved. 
     In the image forming apparatus of one embodiment, 
     the magnetic flux regulating section comprises: 
     a support member made of a nonmagnetic material placed elongate in the widthwise direction of the sheet along the fixing member and the induction coil; 
     a magnetic flux shield portion that is formed with a prescribed pattern on a surface of the support member and interrupts a magnetic flux directed from the induction coil toward the fixing member; and 
     a displacement mechanism that displaces the support member having the magnetic flux shield portion relatively to the fixing member and the induction coil. 
     In the image forming apparatus of the present one embodiment, the displacement mechanism can restrict the generation of heat in the specified region of the fixing member in the widthwise direction of the sheet by displacing the support member having the magnetic flux shield portion relative to the fixing member and the induction coil. 
     In the image forming apparatus of one embodiment, 
     the fixing member is a metal sleeve that extends elongate in the widthwise direction of the sheet, 
     the support member is a cylindrical member placed so as to be brought in contact with an inner peripheral surface of the fixing member, 
     the induction coil is placed inside the support member, 
     the support member is divided into a first support member and a second support member in the widthwise direction of the sheet, and the magnetic flux shield portion is provided for each of the first support member and the second support member, and 
     the displacement mechanism displaces the first support member and the second support member of the support member by independently rotating the members around a central axis of each of the members. 
     In the image forming apparatus of the present one embodiment, the variations of the regions of the fixing member in which the generation of heat can be restricted in the widthwise direction of the sheet are increased. 
     In the image forming apparatus of one embodiment, 
     the pattern of the magnetic flux shield portion on the first support member comprises: a first pattern portion whose dimension in the direction of the central axis is gradually decreased from its total length as located farther away from a reference position on the support member toward one side in a circumferential direction; and a second pattern portion whose dimension in the direction of the central axis is gradually increased from zero to the total length as located farther away from the reference position toward the other side in the circumferential direction, 
     the pattern of the magnetic flux shield portion on the second support member is formed symmetrically to the pattern of the magnetic flux shield portion on the first support member with respect to a boundary plane located between the first support member and the second support member, and 
     the displacement mechanism is able to independently rotate the first support member and the second support member forwardly or reversely. 
     In the image forming apparatus of the present one embodiment, the first support member and the second support member have the first pattern portion whose dimension in the direction of the central axis is gradually decreased from the total length as located farther away from the reference position on the support member to one side in the circumferential direction, and the second pattern portion whose dimension in the direction of the central axis is gradually increased from zero to the total length as located farther away from the reference position to the other side in the circumferential direction. Therefore, in order to make the displacement mechanism displace the first support member and the second support member to the respective target angular positions, it is proper to move the first support member and the second support member to the one side or the other side in the circumferential direction, preferably to the side where the required angle of rotation is smaller. Therefore, time required for the displacement mechanism to displace the first support member and the second support member to the respective target angular positions is shortened. 
     In the image forming apparatus of one embodiment, 
     the first image region detecting section comparts the sheet into a plurality of areas in the widthwise direction of the sheet and detects whether or not each comparted area is occupied by the image that should be fixed, and 
     each of the first and second patterns of the magnetic flux shield portion on the first support member and the second support member has the dimension in the direction of the central axis increased or decreased step-by-step in blocks defined in correspondence with the area as located farther away from the reference position in the circumferential direction. 
     In the image forming apparatus of the present one embodiment, the first and second patterns of the magnetic flux shield portions on the first support member and the second support member have the dimensions in the direction of the central axis increased or decreased step-by-step in specified blocks in correspondence with the areas as located farther away from the respective reference positions. Therefore, it is proper for the displacement mechanism to rotate the first support member and the second support member to the one side or the other side in the circumferential direction every angle at which the dimension in the direction of the central axis of the first and second patterns of the magnetic flux shield portions is increased or decreased step-by-step. Therefore, the control of the angular positions of the first support member and the second support member is simplified. 
     In the image forming apparatus of one embodiment, each of the first and second patterns of the magnetic flux shield portion on the first support member and the second support member has the dimension in the direction of the central axis continuously increased or decreased as located farther away from the reference position in the circumferential direction. 
     In the image forming apparatus of the present one embodiment, the region in which the generation of heat of the fixing member can be restricted in the widthwise direction of the sheet can be continuously variably set. 
     In the image forming apparatus of one embodiment, 
     the first image region detecting section comprises: 
     a counting section that comparts the sheet into a plurality of areas in the widthwise direction of the sheet and detects an amount of image of each comparted area; 
     a memory section that stores the amount of image of each area detected by the counting section; and 
     a determining section that determines whether or not the amount of image of each area exceeds a prescribed threshold value and determines that such an area where the amount of image exceeds the threshold value belongs to the image region. 
     In this case, the “amount of image” indicates the amount of toner in the case of, for example, a toner image. The “amount of image”, which is defined for each area, corresponds to the image density (toner density in the case of, for example, a toner image). 
     In the image forming apparatus of the present one embodiment, it is determined area-by-area whether or not the area belongs to the image region in the widthwise direction of the sheet. Therefore, detection of the image region on the sheet can be performed comparatively easily. 
     In the image forming apparatus of one embodiment, 
     the second image region detecting section comprises: 
     a counting section that comparts the sheet into a plurality of areas in the conveyance direction of the sheet and detects an amount of image of each comparted area; 
     a memory section that stores the amount of image of each area detected by the counting section; and 
     a determining section that determines whether or not the amount of image of each area exceeds a prescribed threshold value and determines that such an area where the amount of image exceeds the threshold value belongs to the image region. 
     In the image forming apparatus of the present one embodiment, it is determined area-by-area whether or not the area belongs to the image region in the conveyance direction of the sheet. Therefore, detection of the image region on the sheet can be performed comparatively easily. 
     When the temperature difference is excessively large in the sheet passing region of the fixing member where the sheet should pass, a trouble of the generation of wrinkles in the sheet after fixation possibly occurs. 
     Thus, in the image forming apparatus of one embodiment, 
     a temperature distribution obtaining section that obtains a temperature distribution of the fixing member in the widthwise direction of the sheet; 
     a passing region detecting section that preparatorily obtains a sheet passing region, through which the sheet should pass, of the fixing member sheet-by-sheet on a basis of a width dimension of the sheet conveyed; and 
     a third control section that obtains a difference between a maximum temperature and a minimum temperature represented by the temperature distribution within the sheet passing region and permits the magnetic flux regulating section to heat whole area of the sheet passing region on the fixing member when the difference exceeds a prescribed threshold value. 
     In the image forming apparatus of the present one embodiment, the temperature distribution obtaining section obtains the temperature distribution of the fixing member in the widthwise direction of the sheet. The third control section obtains the difference between the maximum temperature and the minimum temperature expressed by the temperature distribution within the sheet passing region, and executes control to permit the magnetic flux regulating section to heat the whole area of the sheet passing region on the fixing member when the difference exceeds the prescribed threshold value. Therefore, wrinkles can be prevented from generating in the sheet after fixation. 
     In the image forming apparatus of one embodiment, the temperature distribution obtaining section comprises a plurality of temperature sensors that are arranged in the widthwise direction of the sheet oppositely to the fixing member and each detect a temperature of an opposite portion of the fixing member. 
     In this case, the phrase of “oppositely to the fixing member” of the temperature sensor includes both the case where the temperature sensor is put in contact with the fixing member and the case where the sensor is spaced apart from the fixing member. That is, the temperature sensor may be a contact type or a noncontact type. 
     In the image forming apparatus of the present one embodiment, the difference between the maximum temperature and the minimum temperature can be obtained simply and accurately. 
     In the image forming apparatus of one embodiment, the temperature distribution obtaining section obtains by simulation a temperature distribution of the fixing member in the widthwise direction of the sheet. 
     In the image forming apparatus of the present one embodiment, the parts cost can be reduced by reducing the number of temperature sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a view showing the constituent elements of an electromagnetic induction heating type fixing device according to one embodiment of the present invention, viewed obliquely; 
         FIG. 2  is a view showing the cross section of the constituent elements of the fixing device; 
         FIG. 3  is a view showing a state in which a magnetic flux shield portion is provided on the outer peripheral surface of a holder of the fixing device; 
         FIG. 4  is a view showing the construction of the holder of the fixing device; 
         FIG. 5  is a view showing a metal sleeve and thermistors of the fixing device; 
         FIG. 6  is a view showing one example of the pattern of the magnetic flux shield portion provided on the outer peripheral surface of each holder, shown in a state in which each holder is developed in a plane; 
         FIG. 7  is a view showing another example of the pattern of the magnetic flux shield portion provided on the outer peripheral surface of each holder, shown in a state in which each holder is developed in a plane; 
         FIG. 8  is a diagram showing the block construction of a control system of an image forming apparatus of one embodiment that includes the fixing device; 
         FIG. 9A  is a view showing one example of the angular position taken by each holder having the pattern of the magnetic flux shield portion shown in  FIG. 6 ; 
         FIG. 9B  is a view showing a histogram of the amount of toner in the widthwise direction of the sheet when the angular position shown in  FIG. 9A  is taken; 
         FIG. 9C  is a view showing the state of the sheet that has an image region in which the histogram of the amount of toner shown in  FIG. 9B  is obtained; 
         FIG. 10A  is a view showing another example of the angular position taken by each holder having the pattern of the magnetic flux shield portion shown in  FIG. 6 ; 
         FIG. 10B  is a view showing a histogram of the amount of toner in the widthwise direction of the sheet when the angular position shown in  FIG. 10A  is taken; 
         FIG. 10C  is a view showing the state of the sheet that has an image region in which the histogram of the amount of toner shown in  FIG. 10B  is obtained; 
         FIG. 11A  is a view showing one example of the angular position taken by each holder having the pattern of the magnetic flux shield portion shown in  FIG. 7 ; 
         FIG. 11B  is a view showing a histogram of the amount of toner in the widthwise direction of the sheet when the angular position shown in  FIG. 11A  is taken; 
         FIG. 11C  is a view showing the state of the sheet that has an image region in which the histogram of the amount of toner shown in  FIG. 11B  is obtained; 
         FIG. 12A  is a view showing another example of the angular position taken by each holder having the pattern of the magnetic flux shield portion shown in  FIG. 6 ; 
         FIG. 12B  is a view showing a histogram of the amount of toner in the widthwise direction of the sheet when the angular position shown in  FIG. 12A  is taken; 
         FIG. 12C  is a view showing the state of the sheet that has an image region in which the histogram of the amount of toner shown in  FIG. 12B  is obtained; 
         FIG. 13A  is a graph showing a uniform temperature distribution when the metal sleeve is controlled to a target temperature T 1 ; 
         FIG. 13B  is a graph showing a temperature distribution when some of the blocks on the metal sleeve are heated and the generation of heat in the remaining blocks is restricted; 
         FIG. 14A  is a view showing another example of the angular position taken by each holder having the pattern of the magnetic flux shield portion shown in  FIG. 6 ; 
         FIG. 14B  is a view showing the state of the sheet that has an image region when the angular position shown in  FIG. 14A  is taken; 
         FIG. 14C  is a view showing a histogram of the amount of toner in the conveyance direction of the sheet shown in  FIG. 14B ; 
         FIG. 15  is a view showing an example of control by a control section when sheets of A4 size are successively fed into a nip portion in portrait orientation by a multi print job; 
         FIG. 16  is a view showing an example of control by the control section when, in particular, a sheet of A4 size and a sheet of A3 size are successively fed into the nip portion in portrait orientation by a multi print job; 
         FIG. 17  is a view showing an example of control by the control section when, in particular, three sheets of A4 size and a sheet of A3 size are successively fed into the nip portion in portrait orientation by a multi print job; 
         FIG. 18  is a schematic flow chart (main routine) of the control section for executing the control of  FIG. 17 ; 
         FIG. 19A  is a chart showing a concrete processing flow for executing an operation plan forming routine in  FIG. 18 ; 
         FIG. 19B  is a chart showing a concrete processing flow for executing an operation plan forming subroutine in  FIG. 18 ; 
         FIG. 20A  is a chart showing a concrete processing flow for executing an paper wrinkle correcting routine in  FIG. 18 ; 
         FIG. 20B  is a chart showing a concrete processing flow for executing a block search subroutine in  FIG. 20A ; 
         FIG. 21  is a chart showing a concrete processing flow for executing a holder rotating routine in  FIG. 18 ; 
         FIG. 22  is a chart showing a concrete processing flow for executing a temperature control routine in  FIG. 18 ; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail below by the embodiments shown in the drawings. 
       FIG. 1  shows constituent elements of an electromagnetic induction heating type fixing device according to one embodiment of the present invention, viewed obliquely.  FIG. 2  shows a cross section of the constituent elements of the fixing device. The fixing device constitutes part of an image forming apparatus and serves to fix the unfixed toner image  91  that has been adhered to a sheet-shaped paper  14  as a recording medium to be conveyed by melting the image by heating on the sheet  14 . 
     As shown in  FIG. 1 , the fixing device has an elongate cylindrical metal sleeve  11  as a fixing member and a cylindrical pressure roller  13  brought in pressure contact with the metal sleeve  11 . A sheet  14 , which has a variety of sizes as indicated by the solid lines and the two-dot chain lines in  FIG. 1 , is fed into a nip portion (indicated by reference numeral  23  in  FIG. 2 ) constructed of the metal sleeve  11  and the pressure roller  13  as indicated by arrow “b”. It is noted that the effective axial length L in the axial direction of the metal sleeve  11  is defined by deflection stopping members  30 ,  30  provided projecting annularly at both ends in the axial direction of the metal sleeve  11 . In the following description, the axial dimension of the metal sleeve  11  is assumed to be L. In this example, the axial dimension L of the metal sleeve  11  approximately coincide with the dimension of the short side (297 mm) of a sheet of A3 size, which is the predetermined largest size. It is noted that the sheets of sizes provided by JIS (Japanese Industrial Standards) are used in the present embodiment. 
     As shown in  FIG. 2 , the metal sleeve  11  is provided around a cylindrical holder  12  that extends along the axial direction of the metal sleeve  11  and held between the holder  12  and the pressure roller  13 . The holder  12  is generally constructed of a cylindrical support member made of a heat-resistant electrically insulative engineering plastic, which is a nonmagnetic material, and a magnetic flux shield portion  32  (constructed of a thin copper foil or the like, see  FIG. 3 ) integrally formed with its outer peripheral surface. The holder  12  is rotated around the central axis of the holder for magnetic flux regulation at need (described later). The metal sleeve  11  is brought in sliding contact with the outer peripheral surface of the holder  12  and made rotatable around the holder  12 . The pressure roller  13  is provided rotatably in the direction of arrow “a” in  FIG. 2 , and the metal sleeve  11  is driven to rotate in the direction of arrow “c” in accordance with the rotation of the pressure roller  13 . 
     Around the metal sleeve  11  are provided a separation claw  15  that is made of a heat-resistant electrically insulative engineering plastic and has a tip end portion brought in contact with the outer peripheral surface of the metal sleeve  11 , a temperature sensor  21  that detects the temperature of the metal sleeve  11 , and a thermostat  22  that serves as a safety mechanism when the temperature abnormal rises. 
     As shown in  FIG. 5  in the present example, the temperature sensor  21  is put in pressure contact with the surface of the metallic sleeve  11  so as to face an induction coil  18  with interposition of the metallic sleeve  11 . The temperature sensor  21  is constructed of a total of six thermistors Th 1 , Th 2 , . . . , Th 6  provided for blocks X 1 , X 2 , . . . , X 6 , respectively, which are obtained by equally dividing the metal sleeve  11  into six parts in the direction of the central axis  11 A of the metal sleeve  11 . While detecting the temperature of the metal sleeve  11  by the thermistors Th 1 , Th 2 , . . . , Th 6 , the power supply to the induction coil  18  is controlled so that the temperature of the metal sleeve  11  becomes an optimum temperature. The thermistors Th 1 , Th 2 , . . . , Th 6  constitute a temperature distribution obtaining section. 
     The thermostat  22  shown in  FIG. 2  is put in contact with the surface of the metal sleeve  11 . The thermostat  22  cuts off the power supply to the induction coil  18  by opening its contact when the temperature becomes a preset temperature to prevent the metal sleeve  11  from being raised to a temperature higher than the predetermined temperature. 
     A coil assembly  10  is placed in the holder  12  and retained by the holder  12 . The coil assembly  10  generates a high frequency magnetic field upon receiving an alternating current by a high frequency power source section (not shown) and generates Joule heat by inducing an induction current (eddy current) in the metal sleeve  11 . 
     The metal sleeve  11  is a thin hollow metal conductor having flexibility and has a conductive layer formed of a conductive magnetic material of, for example, nickel, iron, stainless steel SUS430 or the like. Then, a heat-resistant mold release layer is formed of fluororesin coating on the outer peripheral surface of the metal sleeve  11 . The metal layer of the metal sleeve  11  has a thickness of 20 μm to 60 μm. 
     The coil assembly  10  has a core  16  (corresponding to the core member) made of a magnetic material, a bobbin  17  formed having a through hole in which the core  16  is inserted, and the induction coil  18  that is formed by winding a copper wire around the bobbin  17  and induces an induction current in the metal sleeve  11  to heat the sleeve. The core  16  is preferably made of a material of a large magnetic permeability and a small self-loss and appropriately made of, for example, ferrite, Permalloy, sendust or the like. The bobbin  17  is made of a heat-resistant electrically insulative engineering plastic and functions as an insulating portion that provides insulation between the core  16  and the induction coil  18 . The lengthwise dimension (total length) of the coil assembly  10  substantially coincides with the axial dimension L of the metal sleeve  11 , and the coil assembly  10  is housed in the holder  12  formed separately from the bobbin  17  so as not to be exposed to the outside. In the sectional view of  FIG. 2 , the position of the coil assembly  10  is biased from the central axis of the metal sleeve  11  toward the nip portion  23  side. With this arrangement, the coil assembly  10  generates heat mainly in the vicinity of the nip portion  23  of the metal sleeve  11 . 
     The pressure roller  13  is constructed of an axial core  19  and a silicone rubber layer  20  that is a surface release type heat-resistant rubber layer formed around the axial core  19 . 
     The sheet  14  on which the unfixed toner image  91  has been transferred is conveyed from the direction indicated by arrow “b” in  FIG. 2  and fed into the nip portion  23  that holds the sheet  14 . The sheet  14  is conveyed through the nip portion  23  while receiving heat of the heated metal sleeve  11  and a pressure applied from the pressure roller  13 . As a result, the unfixed toner  91  is fixed, and a fixed toner image is formed on the sheet  14 . The sheet  14 , which has passed through the nip portion  23 , is separated from the metal sleeve  11  by the separation claw  15  and conveyed in the rightward direction in  FIG. 2 . The sheet  14  is conveyed by a sheet ejection roller (not shown) and discharged onto a copy receiving tray. 
       FIG. 4  schematically shows the holder  12  viewed obliquely with the metal sleeve  11  removed. In this example, the holder  12  is divided in the direction of the central axis  12 A into two parts of a holder  12 M that serves as a first support member and a holder  12 N that serves as a second support member via a boundary plane  12 C. As a result, each of the holder  12 M and the holder  12 N has an axial dimension of L/2. By thus dividing the holder, the variations of the regions in which the generation of heat can be restricted by the magnetic flux shield portion  32  described later are increased. 
     A first motor  111  and a second motor  112 , which serve as displacement mechanisms, are placed in the vicinity of the end portions of the holder  12 M and the holder  12 N oppositely from the boundary plane  12 C. The first motor  111  and the second motor  112  are each constructed of a stepping motor, which is rotatable forwardly and reversely. With this arrangement, the holders  12 M,  12 N can independently be rotated around the central axis  12 A (indicated by arrows “d” and “e”). By virtue of the first motor  111  and the second motor  112  that are each arranged rotatable forwardly and reversely, time required for displacing the holder  12 M and the holder  12 N to respective target angular positions can be shortened. It is noted that the displacement mechanism may be constructed of one motor, a clutch and so on. Since the arrow “e” is similar to the arrow “d”, the following description is made by using the arrow “d”. 
     Moreover, a first circumferential position sensor  113  and a second circumferential position sensor  114  for detecting displacements (angular positions) in the circumferential direction “d” of the holder  12 M and the is holder  12 N are provided in the vicinity of the end portions of the holder  12 M and the holder  12 N. 
       FIG. 6  shows one example of the pattern of the magnetic flux shield portion  32  provided on the outer peripheral surface of the holder  12 M and the holder  12 N in a state in which the holders are each developed in a plane. In this example, the pattern on the holder  12 M includes a first pattern portion G 1  whose dimension in the direction of the central axis (corresponding to the transverse direction in  FIG. 6 , and so forth) is gradually decreased from the total length L/2 (see  FIG. 4 ) and deviated toward the holder  12 N side as located farther away from a reference position P 1  on the holder  12 M toward one side f 1  in the circumferential direction, and a second pattern portion G 2  whose dimension in the direction of the central axis is gradually increased toward the holder  12 N side from zero to the total length L/2 as located farther away from the reference position P 1  toward the other side f 2  in the circumferential direction. Then, the first and second patterns G 1 , G 2  on the holder  12 M have the dimensions in the direction of the central axis increased or decreased step-by-step in units of a dimension (i.e., L/6) corresponding to the blocks X 1 , X 2 , X 3  of the metal sleeve  11  as located farther away from the reference position P 1  in the circumferential direction. 
     Patterns G 1 ′ and G 2 ′ on the holder  12 N are formed symmetrically to the patterns G 1 , G 2  (transversely symmetrical in  FIG. 6 ) on the holder  12 M with respect to the boundary plane  12 C (see  FIG. 4 ). That is, the pattern on the holder  12 N includes a first pattern portion G 1 ′ whose dimension in the direction of the central axis is gradually decreased from the total length L/2 (see  FIG. 4 ) and deviated toward the holder  12 M side as located farther away from the reference position P 2  on the holder  12 N toward the one side f 1  in the circumferential direction, and a second pattern portion G 2 ′ whose dimension in the direction of the central axis is gradually increased to the holder  12 N side from zero to the total length L/2 as located farther away from the reference position P 2  toward the other side f 2  in the circumferential direction. Then, the first and second patterns G 1 ′, G 2 ′ on the holder  12 N have the dimensions in the direction of the central axis increased or decreased step-by-step in units a dimension (i.e., L/6) corresponding to the blocks X 4 , X 5 , X 6  of the metal sleeve  11  as located farther away from the reference position P 2  in the circumferential direction. 
     As described above, the dimensions in the direction of the central axis of the first and second patterns G 1 , G 2  on the holder  12 M and the first and second patterns G 1 ′, G 2 ′ on the holder  12 N are increased or decreased step-by-step in blocks. Therefore, when the generation of heat is restricted in blocks on the metal sleeve  11  described later, it is proper to rotate the angular positions of the holder  12 M and the holder  12 N every angle of increase or decrease step-by-step. Therefore, it becomes easy to control the angular positions of the holder  12 M and the holder  12 N. 
       FIG. 8  shows the block construction of the image forming apparatus including the fixing device. The image forming apparatus has a control section  100  including a central processing unit (CPU) and an image forming unit  170  as an image forming section. The control section  100  includes an image processing circuit  180  for processing an image signal that is taken in from a network or read by an accessory scanner, and an area table  190  that serves as a memory section. The image processing circuit  180  converts the inputted image signal into a laser signal to make a laser diode section of the image forming unit  170  expose the photoconductor surface to light. 
     The image forming unit  170  includes a photoconductor, a laser diode section for forming a latent image by exposing the photoconductor surface to light, a developing section for developing the latent image on the photoconductor surface by making toner adhere, and a transfer section for transferring the toner image on the photoconductor surface onto a paper as a sheet, although every member of which is not shown. Then, the toner image is formed on the paper as the sheet by executing the well-known electrophotography process. 
     The control section  100  operates as first and second image region detecting sections for detecting the image region on the sheet occupied by the image to be fixed. In concrete, the section operates as a counting section to detect the density of toner transferred to the area, area-by-area on the sheet. In detail, the sheet fed into the nip portion  23  is divided into a plurality of areas in the widthwise direction of the sheet. In this example, a length (297 mm) corresponding the dimension L in the axial direction of the metal sleeve  11  is equally comparted into six areas (corresponding one by one to the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  already described). Then, the on- and off-states of the laser signal of each comparted area is counted. The count values (dot counts) of the areas in the widthwise direction of the sheet are stored as data D 1 , D 2 , . . . , D 6  representing the amounts of image of the areas into the area table  190 . Concomitantly, the sheet fed into the nip portion  23  is also comparted into a plurality of areas in the conveyance direction of the sheet. In this example, a length (420 mm) corresponding to the long side of the sheet of A3 size, which is the maximum size is comparted into nine areas. Then, the on- and off-states of the laser signal of each comparted area is counted. The count values (dot counts) of the areas in the conveyance direction of the sheet are stored as data D 11 , D 12 , . . . , D 19  representing the amounts of image in the areas into the area table  190 . 
     When the areas where the dot counting is performed correspond one by one to the blocks on the metal sleeve  11  as described in this example, it is easily determined block-by-block whether or not the block belongs to the image region as described later. Therefore, the control of specifying the region in which the control section  100  should restrict the generation of heat becomes easy. 
     The dot counting, which needs to express the density of the toner transferred to the sheet, may be achieved by counting the number of on- and off-states when the laser beam is applied to the photoconductor on the basis of the image data or measuring the number (amount) of on-states by discharge duration of on-state data stored in a capacitor or counting the number of on-states by software when image processing is carried out. Since these methods are already well known, no description is provided therefor. Moreover, strictly speaking, the equation of: amount of exposure of photoconductor to laser light=amount of toner development=amount of transfer does not hold. However, means for counting the amount of transfer itself is hard to achieve, and it is acceptable to perform the counting by using a value obtained through conversion from the amount of exposure in consideration of the development efficiency as the amount of development and using a value obtained through conversion from the amount of exposure in consideration of the transfer efficiency as the amount of transfer. 
     Moreover, a temperature signal representing the temperature of the metal sleeve  11  is inputted from the thermistors Th 1 , Th 2 , . . . , Th 6  to the control section  100 . On the basis of the temperature signal, for example, a temperature signal representing the temperature of the blocks corresponding to the region in which the sheet passes on the metal sleeve  11 , the control section  100  outputs a signal for controlling the on- and off-states of the power supply (by the high frequency power source section) to the induction coil  18 . 
     Moreover, an angular position signal representing the displacement (angular position) in the circumferential direction “d” of the holder  12 M and the holder  12 N is inputted from the first circumferential position sensor  113  and the second circumferential position sensor  114  to the control section  100 . The control section  100  moves and retains the holder  12 M and the holder  12 N into the respective desired angular positions by outputting a control signal to the first motor  111  and the second motor  112  while referring to the angular position signal. 
       FIGS. 9A through 9C  show a relation between the dot count of each area in the widthwise direction of the sheet  14  and the angular positions where the holder  12 M and the holder  12 N should be placed in accordance with the dot count in the case where the sheet  14  of A4 size is fed in landscape orientation into the nip portion (in the case of “A4 landscape” sheet feed).  FIG. 9C  shows that, when the sheet  14  is viewed upright, it can be understood that the region of an upper portion of the sheet (about ⅓ of the longitudinal dimension in this example) and the region like a lower footer are image regions IA onto which the toner images (indicated by dotted lines) actually adhere. In this case, the data D 1 , D 2 , . . . , D 6  in the area table  190  after detection of the amount of image are expressed by a histogram D as shown in  FIG. 9B . In this example, the data D 1 , D 2 , D 6  have comparatively large values, and the data D 3 , D 4 , D 5  have comparatively small values. 
     An example of the control of heating only the image region on the sheet by detecting the image region in the widthwise direction of the sheet is described with reference to  FIGS. 9A through 9C . 
     The control section  100  operates as a determining section to determine whether or not the data representing the amount of image of each area exceeds a prescribed threshold value K and determine that the area whose data exceeds the threshold value K belongs to the image region IA. In this example, it is determined that the areas corresponding to the blocks X 1 , X 2 , X 6  on the metal sleeve  11  belong to the image region IA and the areas corresponding to the remaining blocks X 3 , X 4 , X 5  belong to no image region. Therefore, it can be understood that the blocks X 1 , X 2 , X 6  on the metal sleeve  11  need to accurately have the target temperature by heating and the remaining blocks X 3 , X 4 , X 5  are allowed to undergo restricted heating in the fixing operation. 
     In the above case, the control section  100  operates as a first control section to heat only the blocks X 1 , X 2 , X 6  on the metal sleeve  11  and restrict the generation of heat in the remaining blocks X 3 , X 4 , X 5 . In concrete, the angular positions of the holder  12 M and the holder  12 N are controlled by outputting a control signal to the first motor  111  and the second motor  112  as shown in  FIG. 9A . In this example, the holder  12 M is retained in an angular position rotated by 120° in a reverse rotational direction d 2 , and the holder  12 N is retained in an angular position rotated by 90° in the reverse rotational direction d 2  from a home position P 3  (position located closest to the nip portion  23 ). By this operation, the magnetic fluxes directed from the induction coil  18  toward the blocks X 3 , X 4 , X 5  on the metal sleeve  11  are interrupted by the patterns that form the magnetic flux shield portions  32  of the holder  12 M and the holder  12 N. Therefore, only the blocks X 1 , X 2 , X 6  on the metal sleeve  11  are heated, and the generation of heat in the remaining blocks X 3 , X 4 , X 5  is restricted. As a result, only the image region IA can be heated in the widthwise direction of the sheet  14 . Therefore, energy conservation can be achieved. 
       FIGS. 10A through 10C  show an example in which only the position of the image region IA on the sheet  14  differs from that of the example of  FIGS. 9A through 9C . As is apparent from  FIGS. 10C and 10B , the areas corresponding to the block X 2 , X 3 , X 4  on the metal sleeve  11  belong to the image region IA, and the areas corresponding to the remaining blocks X 1 , X 5 , X 6  are not image regions. 
     In the above case, the control section  100  operates as the first control section to heat only the blocks X 2 , X 3 , X 4  on the metal sleeve  11  and restrict the generation of heat in the remaining blocks X 1 , X 5 , X 6  on the metal sleeve  11 . In concrete, the angular positions of the holder  12 M and the holder  12 N are controlled as shown in  FIG. 10A  by outputting a control signal to the first motor  111  and the second motor  112 . In this example, the holder  12 M is retained in an angular position rotated by 54° in the forward rotational direction d 1 , and the holder  12 N is retained in an angular position rotated by 90° in the forward rotational direction d 1  from the home position P 3  (position located closest to the nip portion  23 ). By this operation, the magnetic fluxes directed from the induction coil  18  toward the blocks X 1 , X 5 , X 6  on the metal sleeve  11  is interrupted by the patterns that form the magnetic flux shield portions  32  of the holder  12 M and the holder  12 N. Therefore, only the blocks X 2 , X 3 , X 4  on the metal sleeve  11  are heated, and the generation of heat in the remaining blocks X 1 , X 5 , X 6  is restricted. As a result, only the image region IA can be heated in the widthwise direction of the sheet  14 . Therefore, energy conservation can be achieved. 
     In the construction in which the magnetic flux from the induction coil  18  are interrupted by the magnetic flux shield portion  32 , the metal sleeve  11  is heated by the magnetic fluxes diffracted around the magnetic flux shield portion  32  at the boundary in the vicinity of the pattern of the magnetic flux shield portion  32 . Therefore, when the blocks on the metal sleeve  11  are fragmented into a greater number of blocks, a situation in which the blocks of the metal sleeve  11  shielded by the magnetic flux shield portion  32  are also disadvantageously heated by the magnetic fluxes diffracted around the blocks occurs. In such a case, it is desirable to perform adjustment by, for example, setting large the pattern area of the magnetic flux shield portion  32 . 
       FIG. 7  shows another example of the pattern of the magnetic flux shield portion  32  provided on the outer peripheral surfaces of the holder  12 M and the holder  12 N in a state in which the holders are each developed in a plane. In this example, the pattern on the holder  12 M includes a first pattern portion H 1  whose dimension in the direction of the central axis (corresponding to the transverse direction in  FIG. 7 , and so forth) is gradually decreased from the total length L/2 (see  FIG. 4 ) and deviated toward the holder  12 N side as located farther away from the reference position P 1  on the holder  12 M toward one side f 1  in the circumferential direction, and a second pattern portion H 2  whose dimension in the direction of the central axis is gradually increased from zero to the total length L/2 toward the holder  12 N side as located farther away from the reference position P 1  toward the other side f 2  in the circumferential direction. Then, the first and second patterns H 1 , H 2  on the holder  12 M have the dimensions in the direction of the central axis linearly increased or decreased as located farther away from the reference position P 1  in the circumferential direction. 
     Patterns H 1 ′ and H 2 ′ on the holder  12 N are formed symmetrically to the patterns H 1 , H 2  (transversely symmetrical in  FIG. 7 ) on the holder  12 M with respect to the boundary plane  12 C (see  FIG. 4 ). That is, the pattern on the holder  12 N includes a first pattern portion H 1 ′ whose dimension in the direction of the central axis is gradually decreased from the total length L/2 (see  FIG. 4 ) and deviated toward the holder  12 M side as located farther away from the reference position P 2  on the holder  12 N toward the one side f 1  in the circumferential direction, and a second pattern portion H 2 ′ whose dimension in the direction of the central axis is gradually increased from zero to the total length L/2 toward the holder  12 M side as located farther away from the reference position P 2  toward the other side f 2  in the circumferential direction. Then, the first and second patterns H 1 ′, H 2 ′ on the holder  12 N have the dimensions in the direction of the central axis linearly increased or decreased as located farther away from the reference position P 2  in the circumferential direction. 
     In short, the pattern of the magnetic flux shield portion  32  of  FIG. 7  differs from the patterns of  FIG. 6  in that the dimension in the direction of the central axis is linearly increased or decreased as located farther away from the reference position P 2  in the circumferential direction. Therefore, the region, in which the generation of heat can be restricted in the widthwise direction of the sheet, of the metal sleeve  11  can be set continuously variably. 
       FIGS. 11A through 11C  show a relation between the dot count of each area in the widthwise direction of the sheet  14  and the angular positions where the holder  12 M and the holder  12 N should be placed in accordance with the dot count in the case where the sheet  14  of A4 size is fed in landscape orientation into the nip portion (in the case of “A4 landscape” sheet feed) in correspondence with  FIGS. 9A through 9C . In this case, the way of control by the control section  100  is similar to that described with reference to  FIGS. 9A through 9C . Also, in this case, only the image region IA can be heated in the widthwise direction of the sheet  14 . Therefore, energy conservation can be achieved. 
       FIGS. 12A through 12C  show a relation between the dot count of each area in the widthwise direction of the sheet  14  and the angular positions where the holder  12 M and the holder  12 N should be placed in accordance with the dot count in the case where the sheet  14  of A4 size is fed in landscape orientation into the nip portion (in the case of “A4 landscape” sheet feed) in correspondence with  FIGS. 10A through 10C . In this case, the way of control by the control section  100  is similar to that described with reference to  FIGS. 10A through 10C . However, in this example, the block X 5  also belongs to the image region IA in addition to the blocks X 2 , X 3 , X 4  on the metal sleeve  11 . Therefore, the control section  100  heats the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  and restricts the generation of heat in the remaining blocks X 1 , X 6 . Also, in this case, only the image region IA can be heated in the widthwise direction of the sheet  14 . Therefore, energy conservation can be achieved. 
       FIG. 13A  shows a uniform temperature distribution detected by the thermistors Th 1 , Th 2 , . . . , Th 6  (see  FIG. 5 ) when the metal sleeve  11  is controlled to the target temperature T 1  (setting temperature for fixation) over all the blocks X 1 , X 2 , . . . , X 6  immediately before the start of the fixing operation. 
     However, when the control of heating only the blocks X 1 , X 2 , X 6  on the metal sleeve  11  and restricting the generation of heat in the remaining blocks X 3 , X 4 , X 5  is executed as in the example of  FIGS. 9A through 9C  continuously on, for example, a plurality of sheets, the temperatures Th 1 , Th 2 , Th 6  of the blocks X 1 , X 2 , X 6  are maintained at the target temperature T 1 , whereas the temperatures of the remaining blocks X 3 , X 4 , X 5  are lowered to the temperature T 2  lower than the target temperature T 1  as shown in  FIG. 13B . In this case, if a temperature difference (T 1 −T 2 ) is excessively large in the sheet feed region (sheet passing region), through which the sheet should pass, of the metallic sleeve  11 , then a trouble of the generation of wrinkles (paper wrinkles) in the sheet after fixation possibly occurs. 
     Therefore, the control section  100  operates as a third control section to obtain a difference (T MAX −T MIN ) between a maximum temperature T MAX  and a minimum temperature T MIN  in the sheet feed region on the metal sleeve  11  by using the temperatures detected by the thermistors Th 1 , Th 2 , . . . , Th 6 . When the difference (T MAX −T MIN ) exceeds a prescribed threshold value (50° C. in this example), control of permitting the generation of heat in the whole area of the sheet feed region of the metal sleeve  11  (paper wrinkle correction) is executed. By this operation, wrinkles can be prevented from generating in the sheet after fixation. Moreover, since the plurality of thermistors Th 1 , Th 2 , . . . , Th 6  are arranged along the axial direction of the metal sleeve  11  in this example, the difference (T MAX −T MIN ) between the maximum temperature T MAX  and the minimum temperature T MIN  can be obtained simply and accurately. 
       FIGS. 14A through 14C  show a relation between the dot count of each area in the conveyance direction of the sheet  14  (indicated by arrow “b”) and the angular positions where the holder  12 M and the holder  12 N should be placed in accordance with the dot count in the case where the sheet  14  of A4 size is fed in portrait orientation into the nip portion (in the case of “A4 portrait” sheet feed).  FIG. 14B  shows that, when the sheet  14  is viewed upright, it can be understood that the region of the upper half of the sheet is the image region IA to which the toner image (indicated by the dotted lines) actually adheres. The image region IA occupies the whole area in the widthwise direction of the sheet (transverse direction in  FIG. 14B ). In this case, data D 11 , D 12 , . . . , D 16  in the area table  190  after detection of the amount of image are expressed by a histogram D as shown in  FIG. 14C . In this example, the data D 11 , D 12 , D 13  have comparatively large values, and the data D 14 , D 15 , D 16  have comparatively small values. Moreover, with regard to the data D 1 , D 2 , . . . , D 6  in the area table  190 , the data D 2 , D 3 , D 4 , D 5  have comparatively large values, and the data D 1 , D 6  have comparatively small values although not shown. 
     An example of control of heating only the image region on the sheet by detecting the image region in not only the widthwise direction of the sheet but also the conveyance direction of the sheet is described with reference to  FIGS. 14A through 14C . 
     The control section  100  operates as a determining section to determine whether or not the data representing the amount of image of each area exceeds the prescribed threshold value K and determine that such an area in which the data exceeds the prescribed threshold value K belongs to the image region IA. In this example, it is determined that the areas corresponding to the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  belong to the image region IA, and the areas corresponding to the remaining blocks X 1 , X 6  are not the image region in the widthwise direction of the sheet. Therefore, it can be understood that the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  need to accurately have the target temperature by being heated, whereas the generation of heat in the remaining blocks X 1 , X 6  may be restricted in the fixing operation. Moreover, it can be understood that the areas corresponding to the data D 11 , D 12 , D 13  need to accurately have the target temperature by being heated, whereas the generation of heat in the remaining areas corresponding to the data D 14 , D 15 , D 16  may be restricted in the conveyance direction of the sheet. 
     In the above case, the control section  100  operates as the first control section to heat only the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  and restricts the generation of heat in the remaining blocks X 1 , X 6 . In concrete, a control signal is outputted to the first motor  111  and the second motor  112  to control the angular positions of the holder  12 M and the holder  12 N as shown in  FIG. 14A . In this example, the holder  12 M and the holder  12 N are retained in angular positions rotated by 54° in the reverse rotational direction d 2  from the home position P 3  (position located closest to the nip portion  23 ). By this operation, the magnetic fluxes directed from the induction coil  18  toward the blocks X 1 , X 6  on the metal sleeve  11  are interrupted by the patterns that form the magnetic flux shield portions  32  of the holder  12 M and the holder  12 N. Therefore, only the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  are heated, and the generation of heat in the remaining blocks X 1 , X 6  is restricted. Therefore, energy conservation can be achieved. 
     Concomitantly, the control section  100  operates as a second control section to control the on- and off-states of the power supply (by the high frequency power source section) to the induction coil  18  in synchronization with the conveyance of the sheet  14  through the nip portion  23  so that the areas corresponding to the data D 11 , D 12 , D 13  of the sheet  14  are heated and the remaining areas corresponding to the data D 14 , D 15 , D 16  are not heated. For example, the power supply to the induction coil  18  is turned on until the areas corresponding to the data D 11 , D 12 , D 13  of the sheet  14  pass through the nip portion  23 , and the power supply to the induction coil  18  is turned off while the areas corresponding to the data D 14 , D 15 , D 16  pass through the nip portion  23 . As a result, only the image region IA can be heated in the conveyance direction of the sheet  14 . Therefore, further energy conservation can be achieved. 
     Strictly speaking, the present control possibly generates paper wrinkles as a consequence of the occurrence of a temperature difference between the first half portion and the latter half portion of the sheet  14 . However, since the fall velocity of the temperature of the metal sleeve  11  is usually comparatively gradual in comparison with the conveyance speed of the sheet  14 , it is considered that no trouble occurs. 
     Although the example of the control in  FIGS. 14A through 14C  has been in the case where an image is formed on one sheet  14 , the present invention is also applied to the case of a multi print job (job to form images continuously on a plurality of sheets) as follows. 
       FIG. 15  shows an example of control by the control section  100  in a case where particularly sheets  14 - 1 ,  14 - 2  of A4 size are successively fed in portrait orientation into the nip portion by the multi print job. A flow of times t 0 , t 1 , t 2 , . . . in certain units of time (hereinafter referred to as a “frame”) is shown at the left-hand end of  FIG. 15  (and so forth in  FIGS. 16 and 17  described later). 
     As is apparent from  FIG. 15 , the sheet  14 - 1  passes through the nip portion during time t 6  to t 10 , and the sheet  14 - 2  passes through the nip portion during time t 15  to t 19 . It is noted that the conveyance speed of each sheet is constant. In this example, with regard to the conveyance direction of the sheet, only the area of the sheet  14 - 1  passing through the nip portion during time t 6  to t 7  is an image region IA 1  to which the toner image has actually adhered, and only the area of the sheet  14 - 2  passing through the nip portion during time t 15  to t 17  is an image region IA 2  to which the toner image has actually adhered. With regard to the widthwise direction of the sheet, the areas that belong to the image regions on the sheets  14 - 1  and  14 - 2  correspond to the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11 . 
     In this case, the control section  100  immediately turns on the power supply to the induction coil  18  upon receiving the job at time t 1 . The reason why the power supply to the induction coil  18  is immediately turned on is to perform control of preparatorily raising the temperature of (the sheet feed region of) the metal sleeve  11  to the target temperature by performing heating of five frames by time t 6  when the first sheet  14 - 1  reaches the nip portion. By controlling the angular positions of the holder  12 M and the holder  12 N to the angular positions shown in  FIG. 14A  by the control section  100 , a state in which only the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  are heated is provided. A timewise and spatial range of heating at this time is indicated by H 1  in the figure. When the image region IA 1  of the sheet  14 - 1  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 8 ). Subsequently, the control section  100  turns on the power supply to the induction coil  18  before the next sheet  14 - 2  reaches the nip portion (time t 13 ). The reason for the above is to control the temperature of (the sheet feed region of) the metal sleeve  11  again to the target temperature by performing heating of two frames by time t 15  when the sheet  14 - 2  reaches the nip portion. A timewise and spatial range of heating at this time is indicated by H 2  in the figure. When the image region IA 2  of the sheet  14 - 2  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 18 ). 
     By thus restricting the range of heating timewise and spatially, energy conservation can be achieved. 
       FIG. 16  shows an example of control by the control section  100  in a case where particularly a sheet  14 - 3  of A4 size and a sheet  14 - 4  of A3 size are successively fed in portrait orientation into the nip portion by a multi print job. 
     As is apparent from  16 , the sheet  14 - 3  passes through the nip portion during time t 6  to t 10 , and the sheet  14 - 4  passes through the nip portion during time t 15  to t 21 . It is noted that the conveyance speed of each sheet is constant. In this example, with regard to the conveyance direction of the sheet, only the area of the sheet  14 - 3  passing through the nip portion during time t 6  to t 7  is an image region IA 3  to which the toner image has actually adhered, and only the area of the sheet  14 - 4  passing through the nip portion during time t 15  to t 16  is an image region IA 4  to which the toner image has actually adhered. With regard to the widthwise direction of the sheet, the area that belongs to the image region on the sheet  14 - 3  corresponds to the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11 , and the area that belongs to the image region on the sheet  14 - 4  corresponds to all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11 . 
     In this case, the control section  100  immediately turns on the power supply to the induction coil  18  upon receiving the job at time t 1 . The reason why the power supply to the induction coil  18  is immediately turned on is to perform control of preparatorily raising the temperature of (the sheet feed region of) the metal sleeve  11  to the target temperature by performing heating of five frames by time t 6  when the first sheet  14 - 3  reaches the nip portion. The control section  100  controls the angular positions of the holder  12 M and the holder  12 N to the angular positions shown in  FIG. 14A . By this operation, a state in which only the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  are heated is provided. A timewise and spatial range of heating at this time is indicated by H 3  in the figure. When the image region IA 3  of the sheet  14 - 3  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 8 ). 
     Subsequently, the control section  100  turns on the power supply to the induction coil  18  before the next sheet  14 - 4  reaches the nip portion (time t 10 ). Concomitantly, the control section  100  switches the angular positions of the holder  12 M and the holder  12 N so that only the blocks X 1 , X 6  on the metal sleeve  11  are heated. In concrete, the angular position P 1  of the holder  12 M is rotated by 144° in the reverse rotational direction d 2 , and the angular position P 2  of the holder  12 N is rotated by 144° in the reverse rotational direction d 2  with respect to the angular positions shown in  FIG. 14A . The reason for the above is to perform control of preparatorily raising the temperatures of the blocks X 1 , X 6  of the metal sleeve  11  to the target temperature by performing heating of five frames of the blocks X 1 , X 6  of the metal sleeve  11  by time t 15  when the sheet  14 - 4  reaches the nip portion. A timewise and spatial range of heating at this time is indicated by H 4  in the figure. 
     At time t 13 , the control section  100  switches the angular positions of the holder  12 M and the holder  12 N to the home positions (see  FIG. 6 ) so that all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  are heated. In concrete, the angular position P 1  of the holder  12 M is rotated by 90° in the forward rotational direction d 1 , and the angular position P 2  of the holder  12 N is rotated by 90° in the forward rotational direction d 1  with respect to the immediately preceding angular positions. The reason for the above is to preparatorily control the temperatures of all the blocks X 1 , X 2 , . . . , X 6  of the metal sleeve  11  to the target temperature by performing the heating of two frames of the blocks X 2 , . . . , X 5  of the metal sleeve  11  by time t 15  when the sheet  14 - 4  reaches the nip portion. A timewise and spatial range of heating at this time is indicated by H 5  in the figure. When the image region IA 4  of the sheet  14 - 4  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 17 ). 
     By thus restricting the range of heating timewise and spatially, energy conservation can be achieved. 
       FIG. 17  shows an example of control by the control section  100  in a case where particularly sheets  14 - 11 ,  14 - 12 ,  14 - 13  of A4 size and a sheet  14 - 14  of A3 size are successively fed in portrait orientation into the nip portion by a multi print job. 
     As is apparent from  FIG. 17 , the sheet  14 - 11  passes through the nip portion during time t 8  to t 12 , the sheet  14 - 12  passes through the nip portion during time t 15  to t 19 , the sheet  14 - 13  passes through the nip portion during time t 24  to t 28 , and the sheet  14 - 14  passes through the nip portion during time t 33  to t 39 . It is noted that the conveyance speed of each sheet is constant. In this example, with regard to the conveyance direction of the sheet, the whole area of the sheets  14 - 11 ,  14 - 12  of the job  1  (two copies) are image regions IA 11 , IA 12  to which the toner images have actually adhered, and only the area of the sheet  14 - 13  passing through the nip portion during time t 24  to t 25  is an image region IA 13  to which the toner image has actually adhered. Moreover, only the area of the sheet  14 - 14  passing through the nip portion during time t 33  to t 34  is an image region IA 14  to which the toner image has actually adhered. With regard to the widthwise direction of the sheet, the areas that belong to the image regions on the sheets  14 - 11  and  14 - 12  correspond to the blocks X 2 , X 3 , X 4  on the metal sleeve  11 . The area that belongs to the image region on the sheet  14 - 13  corresponds to the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11 . Moreover, the area that belongs to the image region on the sheet  14 - 14  corresponds to all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11 . 
     In this case, the control section  100  turns on the power supply to the induction coil  18  immediately upon receiving the job  1  at time t 1  in the standby state to control the angular positions of the holder  12 M and the holder  12 N to the home positions (see  FIG. 6 ) so that all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  are heated by time t 3  when exposure of the photoconductor to laser light ends. The reason for the above is to heat all the blocks of the metal sleeve  11  for the time being, since the image region cannot be found if image region detection is not performed in the standby state. A timewise and spatial range of heating at this time is indicated by H 11  in the figure. Subsequently, at time t 4  after the exposure of the photoconductor to light ends, the image region IA 11  on the sheet  14 - 11  is identified by image region detection. At time t 4 , the control section  100  switches the angular positions of the holder  12 M and the holder  12 N from the home positions to the angular positions shown in  FIG. 10A . By this operation, a state in which only the blocks X 2 , X 3 , X 4  on the metal sleeve  11  are heated is provided. A timewise and spatial range of heating at this time is indicated by H 12  in the figure. Since the job  1  is the print job of two copies, and all the areas of the sheets  14 - 11 ,  14 - 12  are the image regions IA 11 , IA 12  in the conveyance direction of the sheet, it may be proper to maintain the heating of only the blocks X 2 , X 3 , X 4  until time t 19  when the whole area of the second sheet  14 - 12  passes through the nip portion, as fundamental control. 
     However, in this example, it is assumed that the difference (T MAX −T MIN ) between the maximum temperature T MAX  and the minimum temperature T MIN  based on the temperatures detected by the thermistors Th 1 , Th 2 , . . . , Th 6  (see  FIG. 5 ) within the sheet feed region on the metal sleeve  11  has exceeded the prescribed threshold value (50° C. in this example) partway when the second sheet  14 - 12  is passing through the nip portion (time t 16 ) (paper wrinkle detection). At this time, the control section  100  switches the angular positions of the holder  12 M and the holder  12 N to the angular positions shown in  FIG. 14A . In concrete, the angular position P 1  of the holder  12 M is maintained with respect to the angular position shown in  FIG. 10A , and the angular position P 2  of the holder  12 N is rotated by 36° in the reverse rotational direction d 2 . By this operation, a state in which the heating of the blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11  is permitted in correspondence with the whole sheet feed region on the metal sleeve  11 , corresponding to the “A4 portrait” sheet feed in this example. A timewise and spatial range of heating at this time is indicated by H 13  in the figure. 
     Even if the second sheet  14 - 12  of the job  1  has passed through the nip portion past the time t 19 , the control section  100  maintains the heating of the blocks X 2 , X 3 , X 4 , X 5  without immediately turning off the power supply to the induction coil  18 . The reason for the above is that the image region IA 13  on the sheet  14 - 13  of the job  2  corresponds to the same blocks X 2 , X 3 , X 4 , X 5  on the metal sleeve  11 . When the image region IA 13  of the sheet  14 - 13  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 25 ). 
     Subsequently, the control section  100  turns on the power supply to the induction coil  18  before the next sheet  14 - 14  reaches the nip portion (time t 28 ). Concomitantly, the control section  100  switches the angular positions of holder  12 M and the holder  12 N so that only the blocks X 1 , X 6  on the metal sleeve  11  are heated. In concrete, the angular position P 1  of the holder  12 M is rotated by 144° in the reverse rotational direction d 2 , and the angular position P 2  of the holder  12 N is rotated by 144° in the reverse rotational direction d 2  with respect to the angular positions shown in  FIG. 14A . The reason for the above is to perform control of preparatorily raising the temperatures of the blocks X 1 , X 6  of the metal sleeve  11  to the target temperature by performing heating of five frames of the blocks X 1 , X 6  of the metal sleeve  11  by time t 33  when the sheet  14 - 14  reaches the nip portion (end portion temperature correction). A timewise and spatial range of heating at this time is indicated by H 14  in the figure. 
     At time t 31 , the control section  100  switches the angular positions of the holder  12 M and the holder  12 N to the home positions (see  FIG. 6 ) so that all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  are heated. In concrete, the angular position P 1  of the holder  12 M is rotated by 90° in the forward-rotational direction d 1 , and the angular position P 2  of the holder  12 N is rotated by 90° in the forward rotational direction d 1  with respect to the immediately preceding angular positions. The reason for the above is to preparatorily control the temperatures of all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  to the target temperature by performing the heating of two frames of the blocks X 2 , . . . , X 5  of the metal sleeve  11  by time t 33  when the sheet  14 - 14  reaches the nip portion. A timewise and spatial range of heating at this time is indicated by H 15  in the figure. When the image region IA 14  of the sheet  14 - 14  has passed through the nip portion, the control section  100  immediately turns off the power supply to the induction coil  18  (time t 35 ). 
     By thus restricting the range of heating timewise and spatially, energy conservation can be achieved. 
       FIG. 18  shows a general processing flow (main routine) of the control section  100  to perform the control of  FIG. 17 . 
     When the control section (CPU)  100  is reset to start the program, initial setting of the CPU, peripheral circuits and products are executed in S 101 . Subsequently, an operation plan forming routine (S 102 ), a paper wrinkle correcting routine (S 103 ), a holder rotating routine (S 104 ) and a temperature control routine (S 105 ) are sequentially executed. Moreover, sheet conveyance, electrophotography process control, operation panel control and so on are executed as other processing (S 106 ). Then, one routine ends in S 107  after awaiting for the end of the time measured by a reference timer (not shown). The reference timer is set to, for example, 10 ms, and timers for controlling the image forming apparatus of the present embodiment are counted by multiples of the reference timer. 
       FIGS. 19A and 19B  show a concrete processing flow for executing the operation plan forming routine (S 102  in  FIG. 18 ). 
     When job start is received from a personal computer (PC) or the like (not shown) on the network in S 201  in  FIG. 19A , it is determined whether or not the image forming apparatus is in a standby state in S 202 . Since it is supposed that the temperature of the metal sleeve  11  (fixing member) is low when job start is received in the standby state, control of heating all the blocks X 1 , X 2 , . . . , X 6  on the metal sleeve  11  is executed in S 203  (during time t 1  to t 3  in  FIG. 17 ). Next, since the dot counting of the toner to be subjected to development and transfer ends when the exposure of the photoconductor to laser light ends in S 204 , an operation plan forming subroutine is executed in S 205 . It is determined whether or not all the jobs have ended in S 206 , and the program flow proceeds to a standby mode in S 207  when it is determined that the jobs have ended. It is noted that the standby mode in the operation plan forming routine means a mode in which the power supply to the induction coil  18  is turned off. 
     As shown in  FIG. 19B , in the operation plan forming subroutine, by referring to the data D 1  through D 6  and D 11  through D 19  of the area table  190  (see  FIG. 8 ) in S 208 , the heating area in the widthwise direction of the sheet is determined in S 209 , and the heating area in the conveyance direction of the sheet is determined in S 210 . Further, a schedule for managing the time of the power supply to the induction coil  18 , such as a heating method to be carried out when the sheet is passing through the nip portion  23  or before the next sheet is fed into the nip portion  23 , is determined in S 211 . 
       FIGS. 20A and 20B  show a concrete processing flow for executing the paper wrinkle correcting routine (S 103  in  FIG. 18 ). 
     In S 301  of  FIG. 20A , a block search subroutine for preparatorily obtaining the region of the metal sleeve  11  (i.e., sheet feed region) through which the sheet being currently conveyed passes is executed. In concrete, as shown in S 305  of  FIG. 20B , the blocks on the metal sleeve  11  are obtained on the basis of the width dimension of the sheet conveyed. For example, the sheets of A3 size and B4 size are conveyed consistently in portrait orientation, and all the blocks X 1 , X 2 , . . . , X 6  are the sheet feed regions. When a sheet of A4 size is conveyed in landscape orientation (“A4 landscape” sheet feed) and when a sheet of B5 size is conveyed in landscape orientation (“B5 landscape” sheet feed), all the blocks X 1 , X 2 , . . . , X 6  are the sheet feed regions. Moreover, when a sheet of A4 size is conveyed in portrait orientation (“A4 portrait” sheet feed) and when a sheet of B5 size is conveyed in portrait orientation (“B5 portrait” sheet feed), the blocks X 2 , . . . , X 5  are the sheet feed regions. Next, in S 302  of  FIG. 20A , the control section  100  inputs the temperatures of the thermistors corresponding to all the blocks within the obtained sheet feed region among the plurality of thermistors Th 1 , Th 2 , . . . , Th 6 . Next, in S 303 , the difference (T MAX −T MIN ) between the maximum temperature T MAX  and the minimum temperature T MIN  is obtained within the sheet feed region on the metal sleeve  11 , and it is determined whether or not the difference (T MAX −T MIN ) exceeds the prescribed threshold value (50° C. in this example). When the difference (T MAX −T MIN ) exceeds 50° C., the control of permitting the heating of the whole area of the sheet feed region of the metal sleeve  11  (paper wrinkle correction) is executed. By this operation, wrinkles can be prevented from generating in the sheet after fixation. 
     It is noted that the temperature difference (50° C. in this example) serving as the criterion of determination in S 303  may be set not fixed but variable. Moreover, the difference may be allowed to be set according to the kind and the thickness of the sheet to be conveyed. 
     With regard to temperature control in the axial direction of the metal sleeve  11 , it is sometimes the case where the system of providing a thermistor for each block as in the present embodiment becomes disadvantageous from the viewpoint of parts cost as a consequence of an increase in the number of necessary thermistors when more minute blocks are set for improvement in control accuracy. In such a case, it is possible to execute control equivalent to that in the case where many thermistors are employed, by preparatorily storing the quantity of heat to be absorbed by the sheet and so on in heating blocks and non-heating blocks as data and carrying out simulation in consideration of environments such as the temperature and humidity of the place where the fixing device is installed. With this arrangement, parts cost can be reduced with a reduction in the number of temperature sensors. 
       FIG. 21  shows a concrete processing flow for executing the holder rotating routine (S 104  in  FIG. 18 ). 
     First of all, it is determined in S 401  whether or not the heating pattern should be changed, i.e., it is determined whether or not the range of heating on the metal sleeve  11  should be changed on the basis of the image region detection result in the widthwise direction of the sheet conveyed. In this case, when the heating pattern should be changed, the program flow proceeds to S 402  to compare the current heating pattern with a new heating pattern to substitute for the pattern. Next, the directions of rotation and the angles of rotation of the first motor  111  and the second motor  112  are calculated in S 403  on the basis of the comparison result in order to rotate the holder  12 M and the holder  12 N. Then, the first motor  111  and the second motor  112  are controlled in S 404  to rotate the holder  12 M and the holder  12 N by the obtained angles of rotation in the obtained directions of rotation. 
       FIG. 22  shows a concrete processing flow for executing the temperature control routine (S 105  in  FIG. 18 ). 
     First of all, the blocks being currently heated on the metal sleeve  11  are confirmed in S 501 . The temperatures of the thermistors corresponding to the blocks being currently heated are inputted in S 502 . For example, when the blocks X 2 , . . . , X 5  are being currently heated, the temperatures (data) detected by the thermistors Th 2 , Th 3 , Th 4 , Th 5  are inputted. Since all the detection temperatures fundamentally express almost same temperatures, it is acceptable to input only the temperature of one representative thermistor (e.g., the thermistor Th 2  or Th 3  located almost in the center portion in the axial direction of the metal sleeve  11 ). Next, the inputted thermistor temperature Thx is compared with a preset target temperature (temperature necessary for fixation) Ttg in S 503 . In this case, when the thermistor temperature Thx is higher than the target temperature Ttg, the program flow proceeds to S 504  to turn off the induction coil. When the thermistor temperature Thx is lower than the target temperature Ttg, the program flow proceeds to S 505  to turn on the induction coil. The temperature of the metal sleeve  11  is thus controlled to the target temperature Ttg. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.