Abstract:
A fuser for thermally fixing a toner image on a recording medium includes: an endless fusing belt configured to transfer heat to the toner image; a heat source configured to heat the fusing belt; a detection unit configured to detect peripheral inward deformation of the fusing belt; and a power cut-off unit configured to cut off power supplied to the heat source upon detection of the peripheral inward deformation of the fusing belt by the detection unit.

Description:
The entire disclosure of Japanese Patent Application No. 2014-098801 filed on May 12, 2014 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuser and an image forming apparatus, and more particularly to a fuser having a fusing belt and an image forming apparatus including the fuser. 
     2. Description of the Related Art 
     Generally, an electrophotographic image forming apparatuses includes a fuser for thermally fixing a toner image transferred to a printed medium to the printed medium. A fuser of this kind includes an endless fusing belt configured to transfer heat to a toner image, a pressure roller configured to hold the printed medium having the toner image transferred thereon, between the pressure roller and the fusing belt, and a heat source configured to heat the fusing belt. In addition, in order to prevent ignition caused by abnormal overheating of the fusing belt, the fuser also includes a detection unit provided on the outer peripheral side of the fusing belt, and detect abnormal overheating of the fusing belt by monitoring the temperature of the fusing belt, and a mechanism for cutting off power supplied to the heat source based on the temperature of the fusing belt detected by the detection unit. As one of such fusers, a fuser described in JP 2009-276570 A (hereinafter, referred to as conventional fuser) is known. 
     When the temperature of the fusing belt is raised by abnormal heating of the heat source, the fusing belt tends to expand at an initial stage, and finally ignite through contraction. Upon contraction of the fusing belt, the fusing belt and the detection unit provided on the outer peripheral side are separated in distance, so that measurement of the temperature by the detection unit is made difficult, and detection of the abnormal overheating of the fusing belt is delayed. In order to prevent such a situation, the conventional fuser is provided with a guide member near the detection unit and on the inner peripheral side of the fusing belt to prevent separation in distance between the fusing belt and the detection unit. However, since the conventional fuser is provided with the guide member, heat from the heat source reaches the fusing belt through the guide member, and rise in temperature of the fusing belt near the guide member is delayed relative to temperature rise at the other part of the fusing belt. Accordingly, the conventional fuser has a problem that the delayed detection of the abnormal overheating of the fusing belt by the detection unit cannot prevent the ignition of the fusing belt. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fuser configured to accurately detect abnormal overheating of a fusing belt, and prevent ignition of the fusing belt, and an image forming apparatus including the fuser. 
     To achieve the abovementioned object, according to an aspect, a fuser for thermally fixing a toner image on a recording medium reflecting one aspect of the present invention comprises: an endless fusing belt configured to transfer heat to the toner image; a heat source configured to heat the fusing belt; a detection unit configured to detect peripheral inward deformation of the fusing belt; and a power cut-off unit configured to cut off power supplied to the heat source upon detection of the peripheral inward deformation of the fusing belt by the detection unit. 
     According to another aspect of the present invention, an image forming apparatus preferably includes the fuser. 
     The fuser according to an embodiment of the present invention includes a detection unit configured to detect peripheral inward deformation of the fusing belt, and a power cut-off unit configured to cut off power supply to a heat source upon detection of the peripheral inward deformation of the fusing belt. Accordingly, the fuser according to an embodiment of the present invention detects the peripheral inward deformation of the fusing belt, and overcomes the problem with the above-mentioned fuser having a mechanism for cutting off power to the heat source based on monitoring the temperature of the fusing belt. That is, the fuser according to an embodiment of the present invention can accurately detect the abnormal overheating of the fusing belt, and prevent the ignition of the fusing belt. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  is a schematic view illustrating an inner structure of an image forming apparatus; 
         FIG. 2  is a cross-sectional view of a fuser according to a first embodiment; 
         FIG. 3  is a schematic view illustrating an inner structure of the fuser according to the first embodiment, and a connection relationship with a power cut-off unit; 
         FIG. 4  is a schematic view illustrating the inner structure of the fuser according to the first embodiment, and a connection relationship with the power cut-off unit; 
         FIG. 5  is a schematic view illustrating an inner structure of the fuser according to a second embodiment, and a connection relationship with the power cut-off unit; 
         FIG. 6  is a schematic view illustrating the inner structure of the fuser according to the second embodiment, and a connection relationship with the power cut-off unit; 
         FIG. 7  is a schematic view illustrating an inner structure of the fuser according to a third embodiment; 
         FIG. 8  is a schematic view illustrating the inner structure of the fuser according to the third embodiment; 
         FIG. 9  is a schematic view illustrating a cross-section of the fuser according to a fourth embodiment, and a connection relationship with the power cut-off unit; 
         FIG. 10  is a schematic view illustrating the cross-section of the fuser according to the fourth embodiment, and a connection relationship with the power cut-off unit; and 
         FIG. 11  is a schematic view illustrating a cross-section of the fuser according to a fifth embodiment, and a connection relationship with the power cut-off unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples. 
     First Embodiment 
     Schematic Configuration of Image Forming Apparatus 
     See FIG.  1   
     An image forming apparatus  1 A according to a first embodiment will be described below with reference to the drawings. It is noted that in the drawings, the same components or portions are denoted by the same reference signs, and description thereof will be omitted. 
     As illustrated in  FIG. 1 , the image forming apparatus  1 A is an electrophotographic color printer. The image forming apparatus  1 A includes a control unit  4  configured to control each portion and each unit of the image forming apparatus  1 A, an image forming section  10 , two paper feed cassettes  21 , a conveyance unit  40 , a sensing unit  50  and a fuser  60 . 
     The image forming section  10  includes image forming units  11 Y,  11 M,  11 C, and  11 K disposed respectively with charging devices, developing devices, or the like around photoreceptor drums  12  configured to form images of yellow (Y), magenta (M), cyan (C), and black (K) colors, an intermediate transfer belt  15  configured to primarily transfer toner images formed at the image forming units  11 Y,  11 M,  11 C, and  11 K and combine the transferred toner images, a secondary transfer roller  18  configured to secondarily transfer the combined toner image from the intermediate transfer belt to a paper sheet P (recording medium), and an exposure unit  16  using a laser beam. It is noted that the configuration and function of an image forming section  10  of this kind is conventionally well known, and detailed description thereof will be omitted. 
     The paper feed cassette  21  is a box-shaped case in which paper sheets P are stacked, and is provided to be drawn from a front side (front side of  FIG. 1 ) of the image forming apparatus  1 . 
     The conveyance unit  40  is configured to convey the paper sheets P in the image forming apparatus, and includes a pickup roller  41 , a paper feed roller  42 , a separation roller  43 , a conveyance roller pair  44 , a timing roller pair  45 , and an exit roller pair  46 . In the conveyance unit  40 , one paper sheet P on the uppermost layer of the paper sheets P stacked in the paper feed cassette  21  is picked up by the pickup roller  41 , and fed by the paper feed roller  42  and the separation roller  43 . Further, the fed paper sheet P is conveyed downstream of a conveyance direction by the conveyance roller pair  44 , and fed from the timing roller pair  45  to the secondary transfer roller  18 . The secondary transfer roller  18  applies an electric field, and the toner image on the intermediate transfer belt  15  is transferred to the paper sheet P. Then, the paper sheet P is fed to the fuser  60  for thermal fusing of toner, and output by the exit roller pair  46  into a paper output tray  2  provided on an upper surface of the image forming apparatus  1 . 
     The sensing unit  50  includes a paper feed sensor  52  positioned between the paper feed roller  42  and the conveyance roller pair  44 , and a registration sensor  54  positioned immediately in front of the timing roller pair  45 . Both of the paper feed sensor  52  and the registration sensor  54  detect passage or arrival of the paper sheet P being conveyed. 
     (Detailed Description of Fuser, See  FIGS. 2 to 4 ) 
     As illustrated in  FIG. 2 , the fuser  60  is configured such that the paper sheet P having the toner image transferred thereon is held between a fusing belt  62  and a pressure roller  74 . Then the toner image is fixed on the paper sheet P by heat from the fusing belt  62 . Further, the fuser  60  includes a belt guide  64 , a pad  66 , a holding plate  68 , a halogen heater  70 , a reflection plate  72 , a detection unit  76 , and a power cut-off unit  78 , in addition to the fusing belt  62  and the pressure roller  74 . 
     The fusing belt  62  includes a release layer having a base layer including a heat resistant resin such as polyimide, an elastic layer including for example silicone rubber on the base layer, and a surface layer including for example PFA. Additionally, the fusing belt  62  is held in substantially a circular shape by the belt guides  64  disposed either end of the fusing belt  62  in a width direction as illustrated in  FIG. 3 , and the pad  66  provided on the inner peripheral side of the fusing belt  62  illustrated in  FIG. 2 , and extending in the width direction. 
     The holding plate  68  is provided on the inner peripheral side of the fusing belt  62  along the pad  66  for compensation of the rigidity of the pad  66 , holds the pad  66 , and has both ends fixed to the belt guides  64  connected to a frame of an apparatus body. 
     The halogen heater (heat source)  70  and the reflection plate  72  are provided on the inner peripheral side of the fusing belt  62  to irradiate the fusing belt  62  with infrared light. Radiant heat of the infrared light heats the fusing belt  62 . It is noted that, in the present embodiment, the halogen heater  70  is positioned on one side of an inner periphery of the fusing belt  62 , and the pad  66  and the holding plate  68  are provided on the other side of the inner periphery across the reflection plate  72 . 
     As illustrated in  FIG. 2 , the pressure roller  74  forms, together with the fusing belt  62 , a nip portion N configured to hold the paper sheet P therein. Additionally, the pressure roller  74  is rotationally driven in a direction indicated by an arrow A, and this rotational driven force drives the fusing belt  62  to be rotated in a direction indicated by an arrow B, while maintaining the substantially circular shape. It is noted that the paper sheet P is conveyed to the nip portion N, while holding a surface having the toner transferred thereon, toward the fusing belt  62 . 
     The detection unit  76  is configured to detect contraction of the fusing belt  62  (peripheral inward deformation of the belt) immediately before ignition of the belt. Additionally, the detection unit  76  is a conductive rod-shaped member, and provided at a place (on one side) between the inner peripheral surface of the fusing belt  62  and the halogen heater  70 . As illustrated in  FIG. 3 , the detection unit  76  extends in the width direction of the fusing belt  62 . Additionally, both ends of the detection unit  76  are connected to terminals T of the power cut-off unit  78  described below. Specifically, the terminals T of the power cut-off unit  78  are disposed in rod insertion holes provided in the belt guides  64 . Both ends of the detection unit  76  as the rod-shaped member are inserted into the rod insertion holes of the belt guides  64 , respectively, and the ends of the detection unit  76  are put on the terminals T of the power cut-off unit  78 , and connected with the terminal T by their own weight. 
     The power cut-off unit  78  is a so-called relay (drive circuit) for controlling power supply, received from an AC voltage source (power supply)  200 , to the halogen heater  70 . Normally, the detection unit  76  is in contact with the terminals T of the power cut-off unit  78  by its own weight. In this condition, the detection unit  76  and the power cut-off unit  78  form a closed circuit, and the power supply to the halogen heater  70  is maintained. 
     However, when the fusing belt  62  is abnormally overheated and contracted, the inner peripheral surface of the fusing belt  62  makes contact with the detection unit  76 , and the detection unit  76  is pressed toward the inner periphery of the fusing belt  62 . Therefore, the detection unit  76  having a rod shape is bent, and finally separated from and comes off the terminals T of the power cut-off unit  78 . As a result, as illustrated in  FIG. 4 , the circuit formed by the detection unit  76  and the power cut-off unit  78  is changed from the closed circuit to an open circuit, and power supply to the halogen heater  70  is stopped by the power cut-off unit  78 . That is, the detection unit  76  as the rod-shaped member is disposed to be operated as described above at a position where the detection unit  76  makes contact with part of the fusing belt  62  deformed and moved peripherally inward. 
     (Effect) 
     In the fuser  60  and the image forming apparatus  1 A including the fuser  60 , when the fusing belt  62  is deformed peripherally inward, the detection unit  76  having the rod shape is pressed peripherally inward, bent, and finally comes off. As a result, the circuit including the detection unit  76  and the power cut-off unit  78  is changed to the open circuit, and the power supply to the halogen heater  70  is stopped. That is, in the fuser  60  and the image forming apparatus  1 A including the fuser  60 , the peripheral inward deformation of the fusing belt  62  is directly detected, and the power supply to the halogen heater  70  is stopped based on the detection of the deformation. Accordingly, the fuser  60  and the image forming apparatus  1 A including the fuser  60  overcomes the problem with the conventional fuser having the mechanism for cutting off power supplied to the heat source based on monitoring of temperature of the fusing belt, for example, difficulty in measurement of the temperature by the detection unit due to separation in distance between the fusing belt and the detection unit provided on the outer peripheral side of the fusing belt. That is, the fuser  60  and the image forming apparatus  1 A including the fuser  60  can further accurately detect the abnormal overheating of the fusing belt  62 , and prevent the ignition of the fusing belt  62 . 
     Further, the detection unit  76  is provided on one side of the inner periphery of the fusing belt  62  where the halogen heater  70  is positioned. The one side where the halogen heater  70  is positioned has a temperature higher than that of the other side where the pad  66  and the holding plate  68  are positioned. Accordingly, in the fuser  60  and the image forming apparatus  1 A including the fuser  60 , the detection unit  76  is provided at a part of the fusing belt  62  where the temperature tends to rise, so that the detection unit  76  can detect the contraction caused by the abnormal overheating of the fusing belt  62  earlier, and can prevent the ignition of the fusing belt  62 , compared with the detection unit provided at the other part of the fusing belt  62 . 
     Second Embodiment 
     See FIGS.  5  and  6   
     An image forming apparatus  1 B according to a second embodiment and the image forming apparatus  1 A according to the first embodiment are different in configuration of the detection unit  76  of the fuser  60 . The detection unit  76  according to the second embodiment includes a lever  80 , and an optical sensor  82  having a set of a light emitting element and a light receiving element. Detailed description will be made below. 
     As illustrated in  FIG. 5 , the lever  80  of the detection unit  76  according to a second embodiment has one side positioned on the inner peripheral side of the fusing belt  62 . Additionally, the lever  80  has the other side mounted with a plate-shaped member  80   a  configured to block light from the light emitting element of the optical sensor  82 . Normally, the light from the light emitting element is blocked by the plate-shaped member  80   a , and power is supplied to the halogen heater  70 . 
     However, as illustrated in  FIG. 6 , when the fusing belt  62  is abnormally overheated and contracted, the inner peripheral surface of the fusing belt  62  makes contact with the one side of the lever  80  of the detection unit  76 . In this condition, the one side of the lever  80  is pressed toward the inner periphery of the fusing belt  62 . When the one side of the lever  80  is pressed, the other side of the lever  80  is moved around the fulcrum, accompanied by movement of the plate-shaped member  80   a  blocking light from the light emitting element of the optical sensor  82 . Accordingly, blocking of the light from the light emitting element by the plate-shaped member  80   a  is released, and the light from the light emitting element is received at the light receiving element. When the light is received by the light receiving element, a signal is generated. The power cut-off unit  78  receives the signal to be operated, and power supply to the halogen heater  70  is stopped. 
     Even in the image forming apparatus  1 B configured as described above, the abnormal overheating of the fusing belt  62  can be detected further accurately, and the ignition of the fusing belt  62  can be prevented. The other configurations of the present second embodiment are similar to those of the first embodiment. Accordingly, description of the present second embodiment is identical to that of the first embodiment except for the detection unit  76 . 
     Third Embodiment 
     See FIGS.  7  and  8   
     An image forming apparatus  1 C according to a third embodiment and the image forming apparatus  1 A according to the first embodiment are different in configuration of the detection unit  76  of the fuser  60 . The detection unit  76  according to the third embodiment includes an optical sensor having a set of a light emitting element  84  and a light receiving element  86 . Detailed description will be made below. 
     As illustrated in  FIG. 7 , in the detection unit  76  according to the third embodiment, the light emitting element  84  is provided at the belt guide  64  on one end side of the fusing belt  62  in a width direction, and the light receiving element  86  is provided at the belt guide  64  on the other end side. Normally, light B 1  from the light emitting element  84  is received at the light receiving element  86 . In this condition, the power supply to the halogen heater  70  is maintained. 
     However, as illustrated in  FIG. 8 , when the fusing belt  62  is abnormally overheated and contracted, the inner peripheral surface of the fusing belt  62  intrudes an optical path of the light B 1 . Therefore, the light from the light emitting element  84  is blocked. Accordingly, a signal is generated from the light receiving element  86 , the power cut-off unit  78  receiving the signal is operated, and the power supply to the halogen heater  70  is stopped. 
     Even in the image forming apparatus  1 C configured as described above, the abnormal overheating of the fusing belt  62  can be detected further accurately, and the ignition of the fusing belt  62  can be prevented. The other configurations of the present third embodiment are similar to those of the first embodiment. Accordingly, description of the present third embodiment is identical to that of the first embodiment except for the detection unit  76 . 
     Fourth Embodiment 
     See FIGS.  9  and  10   
     An image forming apparatus  1 D according to a fourth embodiment and the image forming apparatus  1 A according to the first embodiment are different in configurations of the detection unit  76  and the power cut-off unit  78 . Detailed description will be made below. 
     As illustrated in  FIG. 9 , a detection unit  76  according to the fourth embodiment includes an optical sensor having a set of a light emitting element  88  and a light receiving element  90  provided outside the fusing belt  62 . The power cut-off unit  78  according to the fourth embodiment represents a triac  92  configured to control the power supply to the halogen heater  70 . Here, the triac  92  is operated according to an ignition prevention control program of a sub CPU  100  provided at a control unit  4 . It is noted that, in the image forming apparatus  1 D, temperature control of the halogen heater  70  is performed by a triac  94  connected to a main CPU  102  provided at the control unit  4 . 
     In the detection unit  76  and the power cut-off unit  78  according to the fourth embodiment, normally, an optical axis B 2  of the light emitting element  88  crosses the outer peripheral surface of the fusing belt  62 . That is, the fusing belt  62  blocks light from the light emitting element  88 . Therefore, the light from the light emitting element  88  cannot be received by the light receiving element  90 . In this condition, the triac  92  is on, and power is supplied to the halogen heater  70 . 
     However, as illustrated in  FIG. 10 , when the fusing belt  62  is abnormally overheated and contracted, the optical axis B 2  of the light emitting element  88  and the outer peripheral surface of the fusing belt  62  are uncrossed, the light from the light emitting element  88  is received by the light receiving element  90 , and the light receiving element  90  transmits a signal to the sub CPU  100 . In the sub CPU  100  receiving the signal, the ignition prevention control program is activated, and the triac  92  is turned off. Accordingly, power supply to the halogen heater  70  is stopped. 
     Even in the image forming apparatus  1 D configured as described above, the abnormal overheating of the fusing belt  62  can be detected further accurately, and the ignition of the fusing belt  62  can be prevented. 
     Further, in the image forming apparatus  1 D, the triac  92  configured to control the power supply to the halogen heater  70  is connected to the sub CPU  100 , and the triac  94  configured to control the temperature is connected to the main CPU  102 . Accordingly, in the image forming apparatus  1 D, even if the main CPU  102  is out of control, an ignition prevention control program of the sub CPU  100  independent of the main CPU  102  is activated, and thus, the image forming apparatus  1 D has higher safety than an image forming apparatus having one CPU to control the triacs  92  and  94 . The other configurations of the present fourth embodiment are similar to those of the first embodiment. Accordingly, description of the present fourth embodiment is identical to that of the first embodiment except for the detection unit  76  and the power cut-off unit  78 . 
     Fifth Embodiment 
     See FIG.  11   
     An image forming apparatus  1 E according to a fifth embodiment is different from the image forming apparatus  1 D according to the fourth embodiment in that the detection unit  76  of the fuser  60  is a distance sensor. Detailed description will be made below. 
     As illustrated in  FIG. 11 , in the detection unit  76  according to the fifth embodiment, the detection unit  76  provided outside the fusing belt  62  is a laser displacement meter, and a distance between the detection unit  76  and the outer peripheral surface of the fusing belt  62  is measured. Normally, the distance d between the detection unit  76  and the outer peripheral surface of the fusing belt  62  is maintained constant. In this condition, the triac  92  is on, and power is supplied to the halogen heater  70 . 
     However, when the fusing belt  62  is abnormally overheated and contracted, the distance d between the detection unit  76  and the outer peripheral surface of the fusing belt  62  is increased. Then, when the distance d between the detection unit  76  and the outer peripheral surface of the fusing belt  62  exceeds a threshold, the detection unit  76  transmits a signal to the sub CPU  100 . In the sub CPU  100  receiving the signal, the ignition prevention control program is activated, and the triac  92  is turned off. Accordingly, power supply to the halogen heater  70  is stopped. 
     Even in the image forming apparatus  1 E configured as described above, the abnormal overheating of the fusing belt  62  can be detected further accurately, and the ignition of the fusing belt  62  can be prevented. 
     Further, in the image forming apparatus  1 E, even if the main CPU  102  is out of control, the ignition prevention control program of the sub CPU  100  independent of the main CPU  102  is activated, and thus, the image forming apparatus  1 E has higher safety than an image forming apparatus having one CPU to control the triacs  92  and  94 . Another method of measuring the distance D by the detection unit  76  includes a method of measuring a distance from an exciting coil to the outer peripheral surface of the fusing belt  62 , as the change of a capacitor component of an induction circuit, when the heat source of the fuser  60  uses IH. The other configurations of the present fifth embodiment are similar to those of the fourth embodiment. Accordingly, description of the present fifth embodiment is identical to that of the fourth embodiment except for the detection unit  76 . 
     Other Embodiments 
     It should be understood that the fuser and the image forming apparatus according to the present invention are not limited to the above-mentioned embodiments, and various alterations or modification may be made without departing from the scope of the invention. For example, as the power cut-off unit, a transistor may be used in place of the relay. Further, the heat source is not limited to the halogen heater, and the fusing belt may include an arbitrary material. Still further, the detection unit or the power cut-off unit according to each embodiment may be combined. 
     As described above, the present invention is useful for a fuser and an image forming apparatus, and particularly, the present invention is superior in further accurately detecting abnormal overheating of a fusing belt, and preventing ignition of the fusing belt. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustrated and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by terms of the appended claims.