Patent Publication Number: US-2023152736-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus that forms a toner image on a recording medium. 
     Description of the Related Art 
     The image forming apparatus includes a fixing unit that fixes an unfixed toner image carried on a recording medium to the recording medium. 
     The fixing unit includes a pair of rotating members consisting of a heating rotary member and a pressing rotary member. The heating rotary member is rotated and applies heat to the unfixed toner, while the pressing rotary member is rotated and presses the heating rotary member. The heating rotary member and the pressing rotary member form a fixing nip portion therebetween. When a recording medium with unfixed toner being carried thereon is conveyed to the fixing nip portion, the heating rotary member heats the recording medium and the pressing rotary member presses the recording medium, thereby fixing the unfixed toner to the recording medium. 
     A phenomenon called a “hot offset” may occur in the fixing unit. In the hot offset, too much heat is introduced from the heating rotary member to unfixed toner on a recording medium, which causes the unfixed toner to adhere to the surface of the heating rotary member without being fixed onto the recording medium. Due to the hot offset, the toner adhering to the surface of the heating rotary member (otherwise called “hot-offset toner”) is fixed to subsequent recording media, leading to faulty image forming. 
     To avoid this, a known image forming apparatus is equipped with a web unit for removing the hot-offset toner (Japanese Patent Laid-Open No. 2001-282029). The web unit uses a web made of a nonwoven fabric or the like to remove the hot-offset toner from the heating rotary member. 
     The web used to clean the surface of the heating rotary member is wound up by a winding roller that rotates. The more the amount of wound web, the greater the outside diameter of the wound web. To wind up the web accurately, the amount of rotation of the winding roller in winding up the web is adjusted in accordance with the outside diameter of the wound web. 
     A variable resistor is used in the web unit for accurate winding up of the web. In the web unit using the variable resistor, the winding roller is rotated by a motor to wind up the web. 
     An abnormality in which the winding roller does not rotate and the web is not wound may occur. Even in this case, the fixing operation may continue without noticing the abnormality of the web being not wound up. As a result, the toner is not collected appropriately, the hot-offset toner may adhere to subsequent recording media and thereby deteriorate image quality. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image forming apparatus that can reduce the likelihood of deterioration of image quality when an abnormality in which the web is not wound up occurs in the web mechanism that uses a variable resistor. 
     According to an aspect of the present invention, an image forming apparatus includes a heating rotary member and a pressing rotary member. The pressing rotary member and the heating rotary member form a fixing nip portion and are configured to apply heat and pressure to a toner image carried on a recording medium at the fixing nip portion, thereby fix the toner image onto the recording medium. The image forming apparatus further includes a winding roller configured to wind up a web used to collect toner that is not fixed to the recording medium and is adhered to a surface of the heating rotary member, a motor configured to rotate the winding roller to wind up the web that has been used for the collection of the toner, a contact member configured to contact an outside surface of the web wound around the winding roller and to be movable so as to follow the outside surface of the web, a variable resistor disposed so as to be able to change a resistance in accordance with a position of the contact member, and a control unit configured to control an amount of rotation of the motor in response to information related to the resistance. The control unit is configured to report an abnormality when an amount of variation of the information in a predetermined time period is equal to or less than a predetermined amount. 
     Further features of the present invention will become apparent from the following description of embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view illustrating a structure of an image forming apparatus. 
         FIG.  2    is a schematic cross-sectional view illustrating a fixing unit and a web unit. 
         FIGS.  3 A and  3 B  are schematic views illustrating a structure of an outside-diameter detection device. 
         FIG.  4    is a diagram illustrating a detection circuit with a variable resistor. 
         FIG.  5    is a view illustrating a relationship between a rotation angle of the variable resistor and an outside diameter of a wound web. 
         FIG.  6    is a view illustrating a relationship between the rotation angle of the variable resistor and a detected voltage. 
         FIG.  7    is a schematic cross-sectional view illustrating a fixing unit and a web unit according to a modification example. 
         FIGS.  8 A and  8 B  are schematic views illustrating a structure of an outside-diameter detection device according to the modification example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Image Forming Apparatus 
       FIG.  1    is a schematic view illustrating a structure of an image forming apparatus  100 . As illustrated in  FIG.  1   , the image forming apparatus  100  includes four image formation units, in other words, a yellow image formation unit  120   a , a magenta image formation unit  120   b , a cyan image formation unit  120   c , and a black image formation unit  120   d , which are disposed along an intermediate transfer belt  115  in the rotation direction thereof. The following describes a process of forming a toner image onto the intermediate transfer belt  115  with the yellow image formation unit  120   a  being taken as an example. 
     First, a charging device  112  uniformly charges the surface of a rotating photosensitive drum  111  with electricity (charging). Subsequently, an exposure device  113  emits laser light to the surface of the photosensitive drum  111  in accordance with an input image data and thereby forms an electrostatic latent image on the photosensitive drum  111  (exposure). Subsequently, a developing device  114  forms a yellow toner image on the photosensitive drum  111  (development). A primary transfer roller  117  applies a voltage, of which the polarity is opposite to the charged polarity of the yellow toner image, to the intermediate transfer belt  115 . The yellow toner on the photosensitive drum  111  is thereby transferred to the intermediate transfer belt  115  (primary transfer). Residual yellow toner remaining on the surface of the photosensitive drum after the primary transfer is scraped away from the surface of the photosensitive drum  111  by a toner cleaner. This sequence is performed also in the magenta image formation unit  120   b , the cyan image formation unit  120   c , and the black image formation unit  120   d . As a result, a full-color toner image is formed on the intermediate transfer belt  115 . 
     The toner image on the intermediate transfer belt  115  is conveyed to a secondary transfer portion N 2  formed by a secondary transfer roller pair  116 . In synchronization with the toner image being conveyed, a recording medium S is picked up one by one from a recording media cassette  103  and fed to the secondary transfer portion N 2 . Subsequently, the toner image on the intermediate transfer belt  115  is transferred onto the recording medium S (secondary transfer). 
     The recording medium S on which the toner image has been transferred is conveyed to a fixing unit  200 , and the fixing unit  200  fixes the toner image to the recording medium S by applying heat and pressure. The recording medium S with the fixed toner image is discharged to a discharge tray. 
     The image forming apparatus  100  can also form monochrome images. In the case of monochrome image forming, only the black image formation unit  120   d  is activated. 
     The following describes a double-sided printing that forms images on both sides of a recording medium S. A recording medium S of which an image has been formed on one side is discharged from the fixing unit  200  and guided by a flapper  132  to a sheet conveyance path  134 . The recording medium S is further conveyed from the sheet conveyance path  134  to a reversing path  136  and is switched back along the reversing path  136 . The recording medium S subsequently passes a double-sided printing path  137 . In this process, the recording medium S is reversed with the top side down. The recording medium S is subsequently conveyed again to the secondary transfer portion N 2 , another toner image is transferred thereon, and the fixing unit  200  fixes the toner image. The recording medium S with both sides having printed images is discharged to the discharge tray. 
     The process starting from the charging until the recording medium S to which a toner image is fixed is discharged onto the discharge tray is called an “image forming process” or otherwise called a “print job”. In other words, image forming is being performed during the image forming process or during the print job. 
     Fixing Unit 
     Next, the fixing unit  200  of the present embodiment is described with reference to  FIG.  2   . 
     In  FIG.  2   , a recording medium is conveyed in the direction of arrow a. The fixing unit  200  includes a heating rotary member  201  and a pressing rotary member  202 . The heating rotary member  201  has a heat source for heating an unfixed toner image carried on a recording medium S, and the pressing rotary member  202  comes into contact with the outside surface of the heating rotary member  201 . The pressing rotary member  202 , which is urged toward the heating rotary member  201 , applies pressure to the heating rotary member  201 , thereby forming a fixing nip portion N between the pressing rotary member  202  and the heating rotary member  201 . The toner image is fixed at the fixing nip portion N where the recording medium S is heated and pressed. 
     The heating rotary member  201  is made of a metal, such as aluminum or stainless steel and is shaped like a hollow cylinder. The heating rotary member  201  is excellent in heat conduction and heat resistance. The heating rotary member  201  of the present embodiment has a rubber layer of a predetermined thickness on the outside surface of the metal core shaped like the hollow cylinder. The rubber layer has a three-tier structure that includes a base layer, an elastic layer formed over the base layer, and a releasing layer formed over the elastic layer. The material of the base layer is polyimide resin (PI). The elastic layer is made of silicone rubber, and the releasing layer is made of a fluoro-resin, such as perfluoroalkoxy alkane or PFA. The releasing layer formed on the outside surface facilitates separation of toner therefrom. The heating rotary member  201  has a heater  205  therein. The heater  205  is the heat source for heating the recording medium. The heater  205  is a halogen heater that produces heat, and the heat is conducted to the surface of the heating rotary member  201 . A thermistor  206  is provided to detect the surface temperature of the heating rotary member  201 . Fixation is performed when the surface temperature of the heating rotary member  201  reaches a predetermined target temperature required for fixing. The heating rotary member  201  can have multiple heaters  205  that are oriented and distributed differently. Due to the heating rotary member  201  having multiple heaters  205  that are oriented and distributed differently, the heating region can be changed in accordance with the sizes of recording media. The heat source of the heating rotary member  201  is not limited to the halogen heater but may be an induction heating device. The heating rotary member  201  is rotated in the direction of arrow R 1  by receiving the driving force of a motor (not illustrated). 
     The pressing rotary member  202  has an aluminum core shaped like a hollow cylinder. The pressing rotary member  202  also has a one-millimeter-thick elastic layer formed over the core and a releasing layer formed over the elastic layer for facilitating separation of the toner. 
     The pressing rotary member  202  is rotated in the direction of arrow R 2 . When a recording medium S is conveyed to the fixing nip portion N formed between the heating rotary member  201  and the pressing rotary member  202 , the recording medium S is heated and pressed to fix the toner image thereto. 
     A connection-separation mechanism is provided to bring the pressing rotary member  202  into contact with the heating rotary member  201  or to detach the pressing rotary member  202  from the heating rotary member  201 . The connection-separation mechanism includes a frame  385  and a drive motor (not illustrated). The frame  385  is supported by the image forming apparatus  100 . The frame  385  supports the pressing rotary member  202 . The frame  385  is rotated about a pivot  332  by receiving the driving force of the drive motor (not illustrated). When the frame  385  is rotated about the pivot  332  by the drive motor in the clockwise direction as viewed in  FIG.  2   , the pressing rotary member  202  is moved in the direction of arrow P. The pressing rotary member  202  is thereby brought into contact with the heating rotary member  201  in a direction perpendicular to the conveyance direction a of recording media (connection state). The fixing nip portion N is thereby formed. In the present embodiment, the pressing rotary member  202  is pressed against the heating rotary member  201  with a total pressing force of 2000 N. The width of the fixing nip portion N is 24 mm. When the frame  385  is rotated about the pivot  332  in the counterclockwise direction as viewed in  FIG.  2   , the pressing rotary member  202  is separated from the heating rotary member  201  (separation state). 
     As described above, the recording medium S carrying the unfixed toner image thereon is nipped and conveyed at the fixing nip portion N by the heating rotary member  201  and the pressing rotary member  202 , and the recording medium S is heated and pressed to fix the toner image thereto. 
     In the fixing unit  200  illustrated in  FIG.  2   , the fixing nip portion formed by a pair of rollers performs fixation. The method of fixation is not limited to this. For example, the fixing unit  200  may have a belt member, such as a fixing belt, and multiple suspension members, such as a fixing pad and a heating roller, and the suspension members may suspend the belt member. 
     Web Unit 
     Next, a web unit  210  and a collection roller  204  are described with reference to  FIG.  2   . 
     The fixing unit fixes the toner to the recording medium by heat and pressure. If too much heat is introduced to the toner on the recording medium, the toner on the recording medium melts excessively and may adhere to the heating rotary member  201  without being fixed to the recording medium. This phenomenon is called “hot offset”. If the toner adhering to the heating rotary member  201  due to the hot offset is not collected, the hot-offset toner on the heating rotary member  201 , which is rotating, is fixed to subsequent recording media. A faulty image may be formed in the region to which the hot-offset toner is fixed. 
     To prevent this, the web unit  210  for collecting the hot-offset toner is known. The web unit  210  collects the toner adhering to the heating rotary member  201 . This reduces the occurrence of faulty image forming due to the hot offset. 
     The web unit  210  includes a web  212 , a supply roller  211 , a winding roller  214 , and a pressing roller  213 . 
     The collection roller  204  is in contact with the surface of the heating rotary member  201  and is rotated passively. The hot-offset toner melted by the heat at the fixing nip portion N remains on the surface of the heating rotary member  201  in the melted state. The collection roller  204  of the present embodiment has an outside diameter of 20 mm and is made of a stainless steel (SUS303) that has a higher affinity for the melted toner compared with the releasing layer of the heating rotary member  201 . Accordingly, the melted toner is transferred to the surface of the collection roller  204 . 
     The toner on the surface of the collection roller  204  is collected therefrom using the web  212  that is made of a nonwoven fabric. The web unit  210  includes the pressing roller  213  that presses the web  212  against the collection roller  204 . The web  212  is pressed against the collection roller  204  by the pressing roller  213 , thereby forming a predetermined nip width between the web  212  and the collection roller  204 . The toner transferred onto the collection roller  204  is collected by the web  212 . 
     The web  212  having been used for collecting the toner is wound up by the winding roller  214 . In the present embodiment, the winding roller  214  winds up the web  212 , for example, by 0.2 cm at every four A4-size sheets of paper. One end portion of the web  212  is wound around the winding roller  214 , and the other end portion of the web  212  is wound around the supply roller  211 . The portion of the web  212  wound around the supply roller  211  is an unused portion. When the winding roller  214  winds up the web  212 , the unused portion of the web  212  is supplied from the supply roller  211 . The unused portion of the web  212  is thereby supplied to the contact portion between the web and the collection roller  204  to collect the toner adhering to the surface of the heating rotary member  201 . 
     The web  212  is made, for example, of a nonwoven fabric having a total length of about 50 m. The web  212  is consumed to collect the toner during the fixation for recording media, and finally the web  212  needs to be replaced with a new one. When the web  212  for collecting the toner runs out, a user of the image forming apparatus  100  calls a serviceperson to replace the web unit  210  with a new one. The longer the lifespan of the web unit  210 , the better it is, because the number of calling the serviceperson depends on the lifespan of the web unit  210 . The “new web” as used above is the one that is not yet used for toner collection. 
     The following is the reason why the toner is collected by the web  212  indirectly via the collection roller  204 . The web  212  is made of a nonwoven fabric or the like. If the web  212  comes into direct contact with the heating rotary member  201  without using the collection roller  204 , the web  212  accelerates the surface deterioration of the heating rotary member  201 . The acceleration of the surface deterioration increases the number of replacements of the heating rotary member  201 . To prevent this, the metallic collection roller  204  is provided to collect the toner from the heating rotary member  201 . Accordingly, the web  212  does not come into direct contact with the heating rotary member  201 , which can prolong the lifespan of the heating rotary member  201 . In addition, the surface roughness of the heating rotary member  201  affects the glossiness of images formed on recording media. If the web  212  comes into direct contact with the surface of the heating rotary member  201 , the web  212  produces irregularities on the surface of the heating rotary member  201 . This may cause irregularity in glossiness of images formed on recording media. Accordingly, the web  212  collects the toner via the collection roller  204 , and the web  212  does not come into direct contact with the heating rotary member  201 , which reduces the occurrence of irregularity in glossiness. 
     Web Unit Connection Mechanism 
     The web unit  210  has a mechanism (not illustrated) for bringing the web  212  into contact with the collection roller  204  and for separating the web  212  from the collection roller  204 . The collection roller  204  has a mechanism (not illustrated) for bringing the collection roller  204  into contact with the heating rotary member  201  and for separating the collection roller  204  from the heating rotary member  201 . When a print job is not received by a control unit  151 , the collection roller  204  is separated from the heating rotary member  201 . When a print job is received by the control unit  151 , the collection roller  204  is brought into contact with the heating rotary member  201 , which increases the surface temperature of the collection roller  204 . This causes the toner on the surface of the heating rotary member  201  to move easily to the collection roller  204 . After the surface temperature of the collection roller  204  rises sufficiently, the web  212  is brought into contact with the collection roller  204  approximately one second before a recording medium is conveyed to the fixing nip portion. The web  212  remains in contact with the collection roller  204  and the collection roller  204  also remains in contact with the heating rotary member  201  until the print job is completed. 
     When the last recording medium S finishes passing the fixing nip portion N at the time of completion of the print job, the web  212  is separated from the collection roller  204 . 
     Control of Amount of Rotation of Winding Roller in Accordance with Outside Diameter of Wound Web 
     The winding roller  214  is activated to wind up a used portion of the web  212 . As the winding roller  214  winds up the used portion of the web  212  and the wound portion of the web  212  increases, the outside diameter of the web  212  wound around the winding roller  214  increases. If the amount of rotation of the winding roller  214  were set to be constant, the winding rate of the web  212  would increase as the outside diameter of the wound web  212  increases. Accordingly, it would be difficult to wind a constant amount of the web  212  unless the amount of rotation of the winding roller  214  is controlled in accordance with the outside diameter of the wound web  212 . If the amount of rotation of the winding roller  214  were set to be constant without taking into account of the outside diameter of the wound web  212 , the amount of web consumption would increase uselessly as the winding amount of the web  212  increases, which would shorten the service life of the web  212 . 
     It is a known practice, however, to control the amount of rotation of the winding roller  214  while taking into account of the outside diameter of the wound web  212 . 
     Winding Mechanism Using Variable Resistor and Motor 
     The following describes a method of controlling the winding amount of the web  212  in accordance with the outside diameter of the wound web  212 . According to the present embodiment, as illustrated in  FIGS.  3 A and  3 B , a variable resistor  225  converts the outside diameter of the wound web  212  into a resistance in collaboration with a contact member  221 . This mechanism is described first as below. 
     The fixing unit  200  includes an outside-diameter detection device  220  that detects the outside diameter of the wound web.  FIGS.  3 A and  3 B  are schematic views illustrating an example structure of the outside-diameter detection device  220  according to the present embodiment.  FIG.  3 A  illustrates a state of the outside-diameter detection device  220  when the web unit  210  is new and the web  212  is not wound yet by the winding roller  214 .  FIG.  3 B  illustrates a state of the outside-diameter detection device  220  after the web  212  is wound up by the winding roller  214  in the direction of arrow A and the outside diameter of the web  212  wound around the winding roller  214  increases. 
     As illustrated in  FIGS.  3 A and  3 B , the contact member  221  of the outside-diameter detection device  220  includes a lever  221   a  that is in contact with, and urged by, the outside surface of the wound web  212 . The contact member  221  also includes a portion  221   b  being in contact with a link gear  222 . The lever  221   a  of the contact member  221  is rotated in the direction of arrow B in  FIG.  3 B  by following the outside diameter of the wound web  212 . The portion  221   b  of the contact member  221  is thereby rotated in the direction of arrow C, and the link gear  222  is thereby rotated in the direction of arrow D. The link gear engages a double-gear  223 . Accordingly, when the link gear  222  is rotated in the direction of arrow D, the double-gear  223  is thereby rotated in the direction of arrow E. The double-gear  223  engages a gear portion  224  of the variable resistor  225 . The gear shaft of the gear portion  224  of the variable resistor  225  has a cross section shaped like the letter D, and the gear shaft engages a rotating member  225   a  of the variable resistor  225 . Accordingly, when the double-gear  223  is rotated in the direction of arrow E, the rotating member  225   a  of the variable resistor  225  is thereby rotated in the direction of arrow G. In short, as the winding roller  214  winds up the web  212  and the outside diameter of the wound web  212  increases, the rotating member  225   a  of the variable resistor  225  is rotated in the direction of arrow G. The resistance of the variable resistor  225  changes in accordance with the amount of rotation of the rotating member  225   a . As described above, the variable resistor  225  used in the present embodiment is a rotary-type variable resistor or a so-called “rotary potentiometer”. 
     Detected Voltage Vsns Obtained From Resistance of Variable Resistor 
     The following describes the relationship between the rotating member  225   a  of the variable resistor  225  and the resistance with reference to  FIG.  4   . 
       FIG.  4    is a diagram illustrating a detection circuit with the variable resistor  225  of the present embodiment. The variable resistor  225  has a terminal  1 , a terminal  2 , and a terminal  3 . The terminals  1  to  3  of the variable resistor  225  of  FIG.  4    correspond to the terminals  1  to  3  of the variable resistor  225  of  FIGS.  3 A and  3 B . The terminal  2  is connected to the rotating member  225   a . A resistance R 12  between the terminals  1  and  2  and a resistance R 23  between the terminals  2  and  3  change in accordance with the angle (amount of rotation) of the rotating member  225   a.    
     The terminals  1  to  3  of the variable resistor  225  are connected to a control circuit board  150 , in which the terminal  1  is connected to GND, the terminal  2  is connected to Vsns for a detected voltage and further connected to a terminal of the control unit  151 , and the terminal  3  is connected to a 3.3 V power source. 
     In the present embodiment, the total resistance of the variable resistor between R 1  and R 3  is 10 kΩ. When the angle (amount of rotation) of the rotating member  225   a  of the variable resistor  225  changes and accordingly, the resistance R 12  between the terminals  1  and  2  and the resistance R 23  between the terminals  2  and  3  change, the following equation hold true. 
         R 13= R 12+ R 23=10 KΩ  Equation 1
 
     The following describes an example method of calculating information related to the resistance of the variable resistor  225 . In the present embodiment, a voltage is calculated from the resistance of the variable resistor  225 . The information calculated from the resistance is not limited to this. A current may be calculated from the resistance. The resistance of the variable resistor  225  may be stored in a storage unit  152 . The resistance, however, changes in accordance with the ambient temperature. Accordingly, the voltage or the current can be stored in the storage unit  152 . 
     The terminal  3  of the variable resistor  225  is connected to the 3.3 V power source, and the resistance is set by the variable resistor  225 . Accordingly, a detected voltage Vsns is input into the control unit  151  that is connected to the terminal  2 . The detected voltage Vsns is a voltage obtained by dividing 3.3V by the resistance R 12  and the resistance R 23 , which can be obtained from the following equation. 
       Vsns=3.3V×( R 12/( R 12+ R 23))   Equation 2
 
     Accordingly, the outside diameter of the web  212  wound around the winding roller  214  can be obtained as an electrically detected signal using the detected voltage Vsns. 
     Relationship Between Detected Voltage and Outside Diameter of Web 
     The relationship between the rotation angle of the variable resistor  225  of the present embodiment and the output voltage is described below with reference to  FIG.  5   . Assume that the outside diameter of the winding roller  214  of the present embodiment is 12 mm and the outside diameter of the wound web  212  becomes 50 mm when the winding roller  214  fully winds up the web  212  (when all the web  212  is consumed). In  FIG.  5   , the time at which a virgin web  212  is started to use is indicated by point (1), and the rotation angle of the rotating member  225   a  of the variable resistor  225  is, for example, 45 degrees at point (1). The detected voltage at point (1) is denoted by Va. As the outside diameter of the wound web  212  increases while the web  212  is consumed, the resistance of the variable resistor  225  (the resistance between R 1  and R 2 ) increases. This increases the detected voltage Vsns until the entire web  212  is consumed at point (2) or at the end of life of the web  212 . At point (2), the rotation angle of the rotating member  225   a  of the variable resistor  225  is, for example, 315 degrees. The detected voltage at this point is denoted by Vb. A characteristic line connected between point (1) and point (2) is denoted by TYP. The control unit  151  controls the amount of rotation of a winding motor  240  (to be described later) in accordance with TYP. 
     The storage unit  152  stores information related to the web  212 . The information include the voltage Va detected before shipment and the voltage Vb to be detected at the end of life of the web  212  (end-of-life voltage). 
     Note that in the present embodiment, the voltage Vb indicates the voltage when the entire web  212  is consumed, and the voltage Vb is calculated from the voltage Va detected before shipment. Accordingly, when the entire web  212  is consumed, the detected voltage Vsns does not always become equal to, or more than, the voltage Vb because the amount of toner collected by the web  212  varies. In other words, the outside diameter of the wound web  212  depends also upon the amount of collected toner. The more the amount of collected toner, the greater the outside diameter of the wound web  212 . 
     In the present embodiment, the voltage Vb is a voltage that indicates the point of time when the entire web  212  is consumed, but the voltage Vb is not limited to this. The voltage Vb may be a voltage for urging a user to replace the web  212  when the remaining amount of the web  212  reaches a predetermined value or less. 
     Web Winding by Winding Motor 
     The following describes how the control unit  151  controls a winding motor  240  in accordance with obtained voltages with reference to  FIG.  2   . The control unit  151 , which is electrically connected to the terminal of the variable resistor  225 , can obtain the resistance of the variable resistor  225 . The control unit  151  can obtain the detected voltage Vsns using Equation 2. The control unit  151  is also connected to the winding motor  240 . The “winding motor  240 ” here is a motor provided to rotate the winding roller  214 . The control unit  151  controls the amount of rotation of the winding motor  240  in accordance with the detected voltage Vsns obtained. More specifically, in the present embodiment, the winding roller  214  winds up the web  212  by 0.2 cm at one operation. According to the present embodiment, with this amount of winding, the toner on the surface of the heating rotary member  201  can be collected, and the likelihood of faulty image forming can be reduced for subsequent recording media. Accordingly, 0.2 cm is a desirable amount of the web  212  to be wound up by one operation of the winding roller  214 . Accordingly, the amount of rotation of the winding motor  240  is reduced as the outside diameter of the wound web  212  increases so that the amount of the web  212  wound up by the winding roller  214  can stay 0.2 cm. The control unit  151  rotates the winding motor  240  and controls the amount of rotation in accordance with the voltage Vsns obtained. 
     In the present embodiment, the winding motor  240  is a stepping motor. The control unit  151  controls the amount of rotation of the stepping motor and thereby controls the winding amount of the web  212  by controlling the number of pulses to be input to the stepping motor. 
     The winding motor  240  winds up the web  212 , and the unused portion of the web  212  is consumed gradually. The web  212  comes to the point of replacement. The storage unit  152  stores the voltage Vb as a voltage for urging the user to replace the web  212 , which enables the control unit  151  to determine if the detected voltage Vsns reaches the voltage Vb. This enables the user to determine when to replace the web  212 . 
     Note that the web  212  is a consumable item. When the web  212  to be wound runs out, the web  212  needs to be replaced with a new one. To replace the web  212 , it is normally necessary to replace the web unit  210 . When the web unit  210  needs to be replaced, the user calls a serviceperson for the replacement of the web  212 . When the web  212  is replaced with the new one, the new web  212  has no wound portion, and the outside diameter for the wound portion of the web  212  becomes zero. Accordingly, the voltage Vsns returns to the vicinity of the voltage Va because the contact member  221  is urged to be in contact with the outside surface of the wound web  212 . 
     In summary, the variable resistor  225  converts the position of the contact member  221  into the resistance. The control unit  151  detects the voltage from the converted resistance and controls the winding motor  240  and the winding roller  214  in accordance with the detected voltage. Compared with a known structure using a solenoid or the like, the structure using the variable resistor  225  and the winding motor  240  of the present embodiment is advantageous because the number of contacts between levers is small. This reduces the occurrence of error caused by the contacts between levers and reduces the amount of the error even if it occurs. As a result, the web  212  can be wound up accurately, which can prolong the life of the web  212 . 
     The following describes advantageous effects of the structure in which the control unit  151  controls the winding motor  240  using the detected voltage Vsns. The control unit  151  controls the amount of rotation of the winding motor  240 . Here, the control unit  151  uses the detected voltage Vsns instead of the resistance R 12  of the variable resistor  225 . The variable resistor  225  is made of a metal. The metal has a general tendency in which the resistance increases as the temperature increases. As a result, the resistance R 12  of the variable resistor  225  varies depending on the ambient temperature. If, for example, the control unit  151  controlled the amount of rotation of the winding motor  240  on the basis of the resistance R 12 , the amount of rotation would vary depending on the ambient temperature. As a result, the winding amount of the web  212  would fluctuate largely. The detected voltage Vsns obtained from Equation  2 , however, is less dependent on the ambient temperature compared with the resistance R 12 . Accordingly, the control unit  151  controls the winding motor  240  on the basis of the detected voltage Vsns. Thus, the winding motor  240  winds up the web  212  accurately. 
     Abnormality Detection Using Variable Resistor 
     In the web unit  210  using the variable resistor  225 , the winding motor  240  rotates the winding roller  214  to wind up the web  212 . 
     Assume that an abnormality in which the winding roller  214  does not rotate occurs (due to the failure of the motor  240 , for example). As a result, the web  212  ceases to be wound up. If there was no method available to detect the abnormality of the web being not wound up, it would be difficult for a user to be aware of this. The image forming continues in the state of the abnormality, and the fixing unit  200  continues to perform fixing. The fixation is performed while the same portion of the web  212  continues to be in contact with the collection roller  204 , which leads to insufficient collection of the toner. As a result, the residual toner that has not been collected may adhere to subsequent recording media, which leads to deterioration of image quality. In the present embodiment, however, the abnormality in which the winding roller  214  does not rotate can be detected and reported to the user, thereby reducing the likelihood of deterioration of the image quality. The following describes the detection method in detail. 
     The variable resistor  225  converts the position of the lever  221   a  into the resistance using multiple gears in the outside-diameter detection device  220 . The resistance of the variable resistor  225  changes in accordance with the position of the lever  221   a . In the present embodiment, as the outside diameter of the wound web increases, the resistance of the variable resistor  225  increases. 
     When the abnormality of the winding roller  214  not rotating occurs, the outside diameter of the wound web  212  stops increasing. As a result, the resistance of the variable resistor  225  does not change because the outside diameter of the wound web  212  does not increase. The detected voltage Vsns does not change, either. In the present embodiment, when the resistance does not change beyond predetermined limits in a predefined period of time, the control unit  151  determines that the abnormality has occurred and reports this. 
     The “predefined period of time” as used herein is defined as follows. The predefined period of time here corresponds to a period of time of one rotation of the winding roller  214  to wind up the web  212 . The method of calculating the period of time of one rotation is as follows. The control unit  151  detects the detected voltage Vsns. The control unit  151  obtains the outside diameter of the wound web  212  from the graphs in  FIGS.  5  and  6    and also obtains the winding amount of the web  212  required for one rotation of the winding roller  214 . The control unit  151  predicts the period of time of one rotation on the basis of the winding amount of the web  212  obtained as above. The period of time predicted this way is referred to as the “predefined period of time” in the present embodiment. More specifically, the winding roller  214  winds up the web  212  by 0.2 cm per each operation in the present embodiment. The diameter of the winding roller  214  itself is 12 mm. Accordingly, a minimum predefined period of time corresponds to a period of time of one rotation of the winding roller  214  when the diameter of the web  212  wound around the winding roller  214  is 12 mm. 
     A threshold value is used in the present embodiment. Even in the state of the abnormality in which the winding roller  214  does not rotate, the detected voltage Vsns may change after the predefined period of time due to an error and resulting variation of information. Accordingly, a threshold can be determined in advance. The control unit  151  can detect the abnormality by determining whether the variation of the detected voltage Vsns before and after the predefined period of time exceeds the threshold. The threshold can be greater than an expected amount of error. 
     If the detected voltage Vsns exceeds the threshold in the predefined period of time, which is a predicted period of time, the control unit  151  determines that the winding roller  214  rotates properly. If the detected voltage Vsns is below the threshold in the predefined period of time, which is a predicted period of time, the control unit  151  determines that the winding roller  214  does not rotate. The control unit  151  reports that an abnormality has occurred. 
     In the present embodiment, the predefined period of time is defined as the period of time of one rotation of the winding roller  214  but is not limited to this. The web  212  is thin. Accordingly, even in the case of using the variable resistor  225  or the like, it may be difficult to detect an increase of one-turn thickness in the outside diameter of the wound web  212 . In order to detect a change of the outside diameter of the wound web  212  easily, the control unit  151  may predict the period of time of one or more rotations, for example, five rotations, of the winding roller  214  and the control unit  151  may detect the abnormality using this as the predefined period of time. 
     The control unit  151  obtains the predefined period of time and determines whether the variation of the detected voltage Vsns before and after the predefined period of time exceeds the threshold. Accordingly, the control unit  151  can determine whether the winding roller  214  rotates properly. As a result, the control unit  151  can detect whether an abnormality occurs to the outside-diameter detection device  220  including the winding motor  240 . 
     When the control unit  151  detects the abnormality of the winding roller  214  not rotating, the control unit  151  reports this. As a method of reporting, a display unit  180  can display the nature of the abnormality. This enables the user to know that an abnormality occurs using the display unit  180 . The user is aware of the abnormality and can take a measure of suspending the fixing operation in the state of toner not being collected appropriately. For example, the user can call for a serviceperson. In addition to displaying the nature of the abnormality in the display unit  180 , a sound may be generated to give notice to the user. Any method of giving notice can be used insofar as the user can be aware of an abnormality occurring. 
     Image forming is suspended while the control unit  151  detects an abnormality and reports this using an adopted method of giving notice. Suspending the image forming means suspending the image forming process or the print job, in other words, suspending the process starting from the charging until the recording medium S to which the toner image is fixed is discharged onto the discharge tray. The control unit  151  detects the abnormality and suspends the image forming, which enables the image forming to stop in the state of the web  212  not being wound up. This reduces the likelihood of the image quality being deteriorated. 
     In the state of the winding roller  214  not rotating, the same portion of the web  212  continues to be in contact with the collection roller  204 . If the same portion of the web  212  is in contact with the collection roller  204 , the web  212  may be broken since the collection roller  204  continues to be rotated passively by the heating rotary member  201 . To avoid this, when the control unit  151  detects an abnormality, the web  212  can be separated from the collection roller  204 . 
     Second Embodiment 
     Next, a second embodiment will be described with reference to  FIGS.  7 ,  8 A, and  8 B . Description of the same elements as those of the first embodiment will be omitted. 
       FIG.  7    is a schematic cross-sectional view illustrating a fixing unit according to the second embodiment.  FIGS.  8 A and  8 B  are schematic views illustrating an outside-diameter detection device for a supply roller according to the second embodiment. 
     In the second embodiment, as illustrated in  FIG.  7   , an outside-diameter detection device  250  is provided to detect the outside diameter of the web  212  wound around the supply roller. The outside-diameter detection device  250  converts the outside diameter into an electric signal using a second variable resistor  225  and outputs the electric signal to the control circuit board  150 .  FIG.  8 A  illustrates a state of the outside-diameter detection device  250  when the web unit  210  is new and the web  212  is not sent out yet by the supply roller  211 .  FIG.  8 B  illustrates a state of the outside-diameter detection device  250  after the web  212  is sent out by the supply roller  211  in the direction of arrow A′ and the outside diameter of the web  212  wound around the supply roller  211  decreases. 
     As illustrated in  FIGS.  8 A and  8 B , a contact member  251  of the outside-diameter detection device  250  for the supply roller includes a lever  251   a  that is in contact with, and urged by, the outside surface of the web  212  wound around the supply roller  211 . The contact member  251  also includes a portion  251   b  being in contact with a link gear  222 . The lever  251   a  of the contact member  251  is rotated in the direction of arrow B′ in  FIG.  8 B  by following the outside diameter of the wound web  212 . The portion  251   b  of the contact member  251  is thereby rotated in the direction of arrow C′, and the link gear  222  is thereby rotated in the direction of arrow D′. The link gear engages a double-gear  223 . Accordingly, when the link gear  222  is rotated in the direction of arrow D′, the double-gear  223  is thereby rotated in the direction of arrow E′. The double-gear  223  engages a gear portion  224  of the variable resistor  225 . The gear shaft of the gear portion  224  of a second variable resistor  225  has a cross section shaped like the letter D, and the gear shaft engages a rotating member  225   a  of the variable resistor  225 . Accordingly, when the double-gear  223  is rotated in the direction of arrow E′, the rotating member  225   a  of the variable resistor  225  is thereby rotated in the direction of arrow G′. In short, as the winding roller  214  winds up the web  212  and the outside diameter of the wound web  212  decreases, the rotating member  225   a  of the variable resistor  225  is rotated in the direction of arrow G′. The resistance of the variable resistor  225  changes in accordance with the amount of rotation of the rotating member  225   a . As described above, the second variable resistor  225  used in the second embodiment is a rotary-type variable resistor or a so-called “rotary potentiometer”. The outside diameter of the web  212  wound around the supply roller  211  is detected using the rotary-type variable resistor and the contact member. 
     As described above, as the web  212  is consumed, the outside diameter of the supply roller  211  decreases, and the outside diameter of the web  212  wound around the winding roller  214  increases. The detected voltages Vsns detected by respective variable resistors  225  increase. The outside-diameter detection device  250  for the supply roller detects a change in the detected voltage Vsns when one turn of the web  212  is sent out from the circumference of the supply roller  211 . The outside-diameter detection device  220  for the winding roller detects a change in the detected voltage Vsns when one turn of the web  212  is wound around the circumference of the winding roller  214 . 
     When an abnormality of the winding roller  214  not rotating occurs, the outside diameter of the wound web  212  stops increasing. As a result, the resistance of the variable resistor  225  does not change because the outside diameter of the wound web  212  does not increase. The detected voltage Vsns does not change, either. In the second embodiment, the fixing unit  200  includes the outside-diameter detection device  250  that detects the outside diameter of the web  212  wound around the supply roller  211 . When the resistance does not change beyond predetermined limits in a predefined period of time, the control unit  151  determines that the abnormality has occurred and reports this. 
     Regarding the variation of the detected voltage Vsns, a smaller diameter one of the supply roller  211  and the winding roller  214  responds more quickly. In the first embodiment in which the supply roller  211  is not equipped with the outside-diameter detection device  250 , if the outside diameter of the winding roller  214  becomes larger than the outside diameter of the supply roller  211 , it takes a longer period of time to detect the abnormality. In the second embodiment, however, the supply roller  211  is equipped with the outside-diameter detection device  250 . Accordingly, the control unit  151  can detect the abnormality using the detected voltage Vsns of the smaller diameter one of the supply roller  211  and the winding roller  214 . 
     With the above-described embodiment, if an abnormality occurs in the contact member, in the winding motor  240 , or in the outside-diameter detection device  220  (the outside-diameter detection device  250 ), the abnormality can be detected and reported quickly even in a later stage of the service life of the web  212 , which can reduce the likelihood of deterioration of the image quality due to the web not being supplied. 
     While the present invention has been described with reference to embodiments, it is to be understood that the invention is not limited to the disclosed embodiments but is defined by the scope of the following claims. 
     This application claims the benefit of Japanese Patent Application No. 2021-186663, filed Nov. 16, 2021, which is hereby incorporated by reference herein in its entirety.