Patent Abstract:
A fixing apparatus includes a heater; a heater holding member for holding the heater; a temperature sensor, provided in contact with the heater through a hole provided in the heater holding member, for detecting a temperature of the heater; a holder for holding the sensor; and an urging member, provided between the sensor and the holder, for urging the sensor toward the heater, wherein the heater holding member includes a positioning portion for determining a position of the holder with respect to an urging direction of the urging member, and wherein the positioning portion is disposed at a position outside of a position of the sensor with respect to a direction perpendicular to a longitudinal direction of the heater.

Full Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a fixing apparatus (or device) for fixing a toner image to a sheet of a recording medium, after the transfer of the toner image onto the sheet. 
     An electrophotographic image forming apparatus such as an electrophotographic printer has a fixing device, which applies heat and pressure to a toner image on a sheet of a recording medium after the toner image is transferred onto the sheet. One of various devices is a fixing device of the so-called film type. 
       FIGS. 5(   a )- 5 ( c ) are drawings for showing the structure of a part of a fixing apparatus of the film type. More specifically,  FIG. 5(   a ) is a schematic sectional view of the film unit of the fixing apparatus, and in particular, the temperature sensor of the device and its adjacencies, at a vertical plane parallel to the lengthwise direction (indicated by arrow mark X) of the fixing device. It shows the internal structure of the film unit.  FIG. 5(   b ) is a schematic cross-sectional view of the film unit, at a plane b-b in  FIG. 5(   a ).  FIG. 5(   c ) is a perspective view of the temperature sensor, and its adjacencies, of the film unit. 
     Referring to  FIGS. 5  ( a )- 5 ( c ), the fixing apparatus of the so-called film type employs a heater  200 , which is made up of a ceramic substrate, and a heat generating member formed on the ceramic substrate by printing. The heater  200  is held by a heater holding member  201 . A cylindrical film  233  is fitted around the heater holding member  201 . A pressure roller (unshown) is kept pressed against the heater  200 , with the placement of the film  233  between the pressure roller and heater  200 . While a sheet of a recording medium on which an unfixed toner image is present is conveyed through the nip between the pressure roller and film  233 , the toner image is melted, and becomes fixed to the sheet. 
     A fixing apparatus such as the one described above is provided with a temperature sensor  202  for detecting the temperature of the heater  200 . 
     In order to ensure that the temperature sensor  202  accurately detects the temperature of the heater  200 , the fixing device is provided with a pressuring member  203 , which keeps the temperature sensor  202  in contact with the heater  200 . The pressuring member  203  is held by a sensor holding member  204  fixed to the heater holding member  201 . It keeps the temperature sensor  202  pressed toward the heater  200 . 
     The heater holding member  201  is provided with a pair of bosses  230 , which are put through a pair of through holes  231 , one for one, with which the sensor holding member  204  is provided. After the bosses  230  are put through the through holes  231 , one for one, the bosses  230  are fitted with a pair of fasteners  232 , such as locking rings, to fix the sensor holding member  204  to the heater holding member  201  to keep the sensor holding member  204  precisely positioned relative to the heater holding member  201 . A numerical referential code  206  depicts a metallic stay which plays the role of reinforcing the heater holding member  201 . 
     If the dimension W 2  of the sensor unit, in terms of the direction parallel to the lengthwise direction (indicated by arrow mark X) of the heater  100 , is large, it is possible that the temperature sensor  202  will not be desirably positioned. For example, in a case where additional sensor units and/or protective elements such as a thermo-switch have to be positioned next to the sensor unit, the temperature sensor  202  cannot be desirably positioned. Therefore, a fixing device such as the one described above needs a substantial amount of space for allowing the sensor holding member  204  to be fastened to the heater holding member  201 . Thus, the dimension W 2  has to be substantial, in order to provide the spaces for fastening the sensor holding member  204  to the heater holding member  201 . In some cases, therefore, it is possible that the positioning of the sensor unit will be restricted. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a fixing apparatus comprising a heater; a heater holding member for holding the heater; a temperature sensor, provided in contact with the heater through a hole provided in the heater holding member, for detecting a temperature of the heater; a sensor holding member for holding the temperature sensor; and an urging member, provided between the temperature sensor and the sensor holding member, for urging the temperature sensor toward the heater, wherein the heater holding member includes a positioning portion for determining a position of the sensor holding member with respect to an urging direction of the urging member, and wherein the positioning portion is disposed at a position outside of a position of the temperature sensor with respect to a direction perpendicular to a longitudinal direction of the heater. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of a typical image forming apparatus to which the present invention is applicable. 
         FIG. 2A  is a sectional view of the fixing device in accordance with the present invention. 
         FIG. 2B  is a perspective view of the fixing device. 
         FIGS. 3(   a ) and  3 ( b ) are perspective views of the temperature sensor. 
         FIGS. 4(   a )- 4 ( c ) are sectional and perspective views of the temperature sensor, and its adjacencies, of the film unit in accordance with the present invention. 
         FIGS. 5(   a )- 5 ( c ) are sectional and perspective views of the temperature sensor, and its adjacencies, of a comparative (conventional) film unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention is described in detail with reference to the appended drawings. 
     (General Structure of Image Forming Apparatus) 
       FIG. 1  is a drawing for showing the structure of the image forming apparatus. 
     Referring to  FIG. 1 , the image forming apparatus  1  is provided with four image formation units  2   a ,  2   b ,  2   c , and  2   d  which form four toner images, different in color, one for one. It is also provided with an intermediary transfer belt unit  7  having an intermediary transfer belt  8 , and a fixing device  20 . 
     The image formation units  2   a ,  2   b ,  2   c  and  2   d  have photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. They are provided with charge rollers  5   a ,  3   b ,  5   c  and  5   d , development rollers  4   a ,  4   b ,  4   c  and  4   d , and cleaning blade  6   a ,  6   b ,  6   c  and  6   d , which are arranged in the listed order, in the adjacencies of the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. Further, the image formation units  2   a ,  2   b ,  2   c  and  2   d  are aligned in parallel in the listed order, along the intermediary transfer belt  8 . 
     The charge rollers  5   a ,  5   b ,  5   c  and  5   d  uniformly charge the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. 
     Electrostatic latent images, which correspond one for one to multiple monochromatic image, different in color, into which an image to be formed has been separated, are formed on the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  by a laser-based exposing devices  26   a ,  26   b ,  26   c  and  26   d , respectively. The development rollers  4   a ,  4   b ,  4   c  and  4   d  develop the electrostatic latent images on the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , with the use of toners, one for one, which are different in color. 
     The cleaning blade  6   a ,  6   b ,  6   c  and  6   d  remove the transfer residual toner, that is, the toner remaining adhered to the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  after the transfer of the toner images from the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. 
     The intermediary transfer belt unit  7  has: the intermediary transfer belt  8 ; and a combination of a driver roller  9 , a tension roller  10 , primary transfer rollers  11   a ,  11   b ,  11   c  and  11   d , and a belt backing roller  12 , by which the intermediary transfer belt  8  is suspended and kept stretched. As the driver roller  9  is rotationally driven by a belt driving motor (unshown), the intermediary transfer belt  8  is circularly moved. 
     The image forming apparatus  1  is structured so that the primary transfer rollers  11   a ,  11   b ,  11   c  and  11   d  oppose the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively, with the intermediary transfer belt  8  being sandwiched between the primary transfer rollers  11  and photosensitive drums  3 , forming thereby primary transfer stations, one for one. 
     Further, the image forming apparatus  1  is provided with a secondary transfer roller  14 , which opposes the belt backing roller  12 , with the intermediary transfer belt  8  being sandwiched between the two rollers  14  and  12 , forming thereby the secondary transfer station  28 . 
     There is a sheet feeding device  15  in the bottom portion of the image forming apparatus  1 . The sheet feeding device  15  conveys a sheet S of a recording medium to the secondary transfer station  28 . It has a feed roller  17  which feeds a sheet S of the recording medium into the main assembly of the image forming apparatus  1  from a cassette  16  which stores multiple sheets S of the recording medium in layers. It has also a separation roller  18 . 
     As the feed roller  17  is rotated, the sheets S of the recording medium stored in layers in the cassette  16  are fed one by one into the main assembly of the image forming apparatus  1 . Then, each sheet S of the recording medium is conveyed to a pair of registration rollers  19  by way of a recording medium conveyance passage  27 . Then, it is conveyed by the pair of registration rollers  19  to the secondary transfer station  28 . 
     In the second transfer station  28 , the multiple monochromatic toner images, different in color, on the intermediary transfer belt  8 , are transferred (secondary transfer) onto the sheet S. 
     The fixing device  20  has a film unit  21  and a pressure roller  22 , which are pressed upon each other, forming thereby a fixation nip, as a heating nip, between them. 
     The sheet S on which the unfixed toner images, different in color, are present, is conveyed to the fixation nip, and then, is conveyed through the fixation nip, remaining pinched by the film unit  21  and pressure roller  22 , and being subjected to heat. Consequently, the unfixed toner images become fixed to the sheet S. After the sheet S is conveyed through the fixation nip, it is discharged by a pair of discharge rollers  24 , with which a discharge unit  23  is provided, into a delivery tray  25 . 
     (Structure of Fixing Device) 
       FIGS. 2A and 2B  are sectional and perspective views, respectively, of the fixing device  20 . They show the structure of the fixing device  20 . 
     Referring to  FIGS. 2A and 2B , the fixing device  20  has: the film unit  21 ; a pressure roller  22 ; a pair of sheet conveyance rollers  30 ; a pair of sheet conveyance guide  31 ; an external casing  32  formed of resin; and a metallic frame  41  placed in the external casing  32 . 
     The film unit  21  has a heater  100 , a heater holding member  101 , a temperature sensor  102 , a pressing member (first pressing member)  103 , a sensor holding member  104 , a second pressing member  105 , and a stay  106 . The heater  100  is a ceramic heater, which is made up of a ceramic substrate, and a heat generating member formed on the ceramic substrate by printing. The material for the heater holding member  101  and the sensor holding member  104  is heat resistant resin, such as LCD (Liquid Crystal Polymer). 
     The fixing device  20  is provided with a cylindrical film, which is fitted around the film unit  21 . The pressure roller  22  is driven by an unshown motor, through a pressure roller gear  42 . The film  33  is rotated by the friction which occurs between the film  33  and the pressure roller  22  as the pressure roller  22  is driven; the film  33  is rotated by the rotation of the pressure roller  22 . The material for the substrate layer of the film  33  is a resinous substance such as polyimide, or a metallic substance such as stainless steel. 
     The heater  100  is in contact with the inward surface of the film  33 . The film unit  21  is kept pressed against the pressure roller  22 , with the placement of the film  33  between the film unit  21  and the pressure roller  22 , forming thereby a fixation nip N, through which a sheet S of the recording medium is conveyed. As electric power is supplied to the heater  100  through a connector  43 , the heater  100  generates heat, which is transferred to the film  33  to be used for the fixation of the toner images on the sheet S. 
     Reference numeral  40  denotes a regulating member for regulating the movement of the film  33  in the direction parallel to the generatrix of the film  33 . Referring to  FIGS. 2A and 2B , the pressure roller  22  is rotatably supported by the frame  41 . The film unit  21  is attached to the frame  41  from the top side of the pressure roller  22 ; it is attached to the frame  41  after the pressure roller  22  is rotatable supported by the frame  41 . The film unit  21  is provided with a pair of regulating members  40 , which are positioned at the lengthwise ends of the film unit  21 . The regulating members  40  are kept under a preset amount of load applied in the direction indicated by a pair of arrow marks P, one for one. These loads P are the forces for forming the fixation nip N. As a sheet S of the recording medium, which is bearing an unfixed toner image (or unfixed toner images, different in color), is conveyed through the area of contact (fixation nip N) between the film unit  21  and the pressure roller  22 , the sheet S comes into contact with the film  33 . Thus, the heat from the heater  100  is transferred to the sheet S, and also, the sheet S is subjected to the pressure generated by the load P. Consequently, the unfixed toner image on the sheet P becomes fixed to the sheet S. 
       FIGS. 3(   a ) and  3 ( b ) are drawings showing the structure of the temperature sensor  102 . More specifically,  FIG. 3(   a ) is a perspective view of the temperature sensor  102 . It shows the surface of the temperature sensor  102 , on which a thermistor or thermistor chip (temperature detection element)  120  is present.  FIG. 3(   b ) is a perspective view of the temperature sensor  102 . However, it shows the opposite surface of the thermistor  120  from the surface shown in  FIG. 3(   a ), that is, the surface of the temperature sensor  102 , on which the thermistor chip  120  is not present. 
     The temperature sensor  102  has the thermistor chip  120  and a thermistor holder  121 . The thermistor holder  121 , which holds the thermistor chip  120 , is required not to be deformed by the heat generated by the heat  100 . Therefore, it is formed of heat resistant thermoplastic resin (LCP). 
     The thermistor holder  121  is provided with an electrically conductive metallic plate  122 , which is integrated into the thermistor holder  121  while the thermistor holder  121  was formed by molding. The metallic plate  122  is connected to a pair of cables  123 , which are connected to a controller (unshown). Further, the metallic plate  122  is provided with a pair of electric wires  124 , through which the metallic plate  122  is connected with the thermistor chip  120 . There is provided an elastic chip holding member (piece of sponge)  125  between the thermistor chip  120  and thermistor holder  121 . 
     The temperature sensor  102  is covered with a sheet of heat resistant and electrically insulating substance (piece of polyimide tape, unshown, for example), which is wrapped around the temperature sensor  102  in such a manner that it surrounds the thermistor chip  120 , the chip holding member  125 , and the thermistor holder  121 . 
     Further, the thermistor chip  120  is kept pressed against the heater  100 , with the presence of the abovementioned piece of polyimide tape. The thermistor chip  120  changes in electrical resistance by an amount proportional to the temperature of the heater  100 . Thus, it is possible for the controller (unshown) to detect the temperature of the heater  100  by reading the amount of the electrical resistance of the thermistor chip  120 . The electrical power to be supplied to the heater  100  is controlled according to the detected temperature of the heater  100 . 
     (Structure of Film Unit) 
       FIG. 4(   a ) is a drawing which shows the structure of the temperature sensor  102 , and its adjacencies, of the film unit  21 .  FIG. 4(   b ) is a schematic sectional view of the film unit  21 , at a plane b-b in  FIG. 4(   a ).  FIG. 4(   c ) is a schematic perspective view of the portions of the film unit  21 , shown in  FIG. 4(   a ), minus the film  33  and the stay  106 . 
     Referring to  FIGS. 4(   a ) and  4 ( b ), the stay  106  is in the form of a trough, which is roughly U-shaped in cross-section. It supports the heater holding member  101 , across the entirety of the heater holding member  101  in terms of the direction parallel to the lengthwise direction of the heater  100 , reinforcing thereby the heater holding member  101 . The material for the stay  106  is metal. In this embodiment, it is zinc alloy coated steel. A reference letter E denotes the inward space of the stay  106 . The stay  106  is under the pressure applied thereto by the load P through the regulating member  40  shown in  FIG. 2B . Therefore, the stay  106  remains roughly fixed in position relative to the frame  41 . Similarly, the heater holding member  101  and the heater  100  are under the pressure applied thereto by the load P through the stay  106 , remaining thereby roughly fixed in position relative to the frame  41 . 
     The thermistor holder  121  is provided with a through hole  121   c , whereas the heater holding member  101  is provided with a boss  101   c . Further, the boss  101   c  is fitted into the through hole  121   c , whereby the temperature sensor  102  is roughly fixed in position in terms of the direction indicated by the arrow mark X. The sensor holding member  104  is placed in the space E. The pressing member  105  (second pressing member) is placed between the stay  106  and sensor holding member  104 . Thus, it keeps the sensor holding member  104  pressed upon the heater holding member  101 . 
     Further, a pressing member  103  (first pressing member) is placed between the sensor holding member  104  and temperature sensor  102 . Thus, it keeps the temperature sensor  102  pressed toward the heater  100 . The temperature sensor  102  is in contact with the heater  100  through a through hole  101   g , with which the heater holding member  101  is provided. 
     Referring to  FIGS. 4(   b ) and  4 ( c ), the heater holding member  101  has a pair of walls  101   a  and  101   b , which have positioning portions (recesses)  101   a P and  101   b P, respectively. This pair of walls  101   a  and  101   b  play the role of a conduit through which the pair of cables  123 , which are connected to the temperature sensor  102 , are guided outward of the film  33 . Referring to  FIGS. 2B and 4(   c ), the cables  123  extend outward of the film  33  between the walls  101   a  and  101   b . The sensor holding member  104  has a pair of engaging portions  104   a  and  104   b  which engage with the positioning portions (recesses)  101   a P and  101   b P of the walls  101   a  and  101   b , respectively. Referring to  FIGS. 4(   a ) and  4 ( c ), the height of the wall  101   a  and  101   b  is h. Further, referring to  FIG. 4(   b ) and  FIG. 4(   c ), in terms of the direction (indicated by arrow mark Y) which is perpendicular to the lengthwise direction (indicated by arrow mark X) of the heater  100 , the positioning portions (recesses)  101   a P and  101   b P are on the outward side of where the temperature sensor  102  is positioned. 
     The film unit  21  is structured so that when the temperature sensor  102  is in contact with the heater  100 , the amount of pressure generated by the pressing member  105  is greater than that by the pressing member  103 . Therefore, the engaging portions  104   a  and  104   b  of the sensor holding member  104  remain in contact with the positioning portions (recesses)  101   a P and  101   b P of the walls  101   a  and  101   b , respectively, whereby the sensor holding member  104  is kept accurately positioned in terms of the direction indicated by the arrow mark Z. Because the sensor holding member  104  is kept accurately positioned in terms of the direction indicated by the arrow mark Z, it is ensured that the temperature sensor  102  is kept in contact with the heater  100  by the pressure generated by the pressing member  103 . The amount of pressure generated by the pressing member  103  is determined by the spring constant of the pressing member  103 . 
     Referring to  FIGS. 4(   a )- 4 ( c ), because the film unit  21  is structured so that the sensor holding member  104  is positioned as described above, the sensor holding member  104  can be held to the heater holding member  101  without requiring a space which is necessary in a case where fastening members such as the ones shown  FIGS. 5(   a )- 5 ( c ) are used to keep the sensor holding member  104  accurately positioned relative to the heater holding member  101 . 
     In order to fix the sensor holding member  104  to the heater holding member  101  with the use of fasteners, a certain amount of space is necessary. However, this embodiment makes it possible to eliminate the space which is required of the heater holding member  101  to hold the temperature sensor holding member  104 . Therefore, this embodiment can reduce the heater holding member  101  in size. That is, this embodiment can makes it possible for the width W 1  of the sensor unit, which is shown in  FIGS. 4(   a )- 4 ( c ), to be less than the width W 2  of the sensor unit, which is shown in  FIGS. 5(   a )- 5 ( c ). 
     Further, there is disposed the sensor holding member  104  formed of heat resistant and thermoplastic (electrically insulating) resin, between the pressing members  103  and  105 . The material for the sensor holding member  104  is LCP (Liquid Crystal Polymer), for example. Therefore, this embodiment can provide an ample distance between the temperature sensor  102  and the stay  104  formed of metallic plate, ensuring thereby that the temperature sensor  102  remains electrically insulated from the stay  106 . 
     The pressing members  103  and  105  are in the form of a compression spring. Further, the film unit  21  is structured so that the pressing members  103  and  105  are coaxially positioned, and also, that the axial lines of the pressing members  103  and  105  coincide with the portion of the heater  100 , whose temperature the temperature sensor  102  detects. That is, the pressing members  103  and  105  are positioned so that the thermistor chip  120  of the temperature sensor  102 , the center line (axial line) of the pressing member  103 , and the center line (axial line) of the pressing member  105 , coincide. 
     Referring to  FIGS. 5(   a )- 5 ( c ), in the case of the comparative fixing device, the heater holding member  201  is fixed to the sensor holding member  204  with the use of the pair of fasteners  232 . Therefore, if the sensor holding member  204  is imperfectly fixed to the heater holding member  201  with the use of the fasteners  232 , it is possible that the sensor holding member  204  will be separated from the heater holding member  201 , by the pressure generated by the pressing member  203 , and therefore, the contact pressure between the temperature sensor  202  and heater  200  will become unstable. 
     In the case of the fixing device  20  in this embodiment, the pressing member  105  is greater in resiliency than the pressing member  103 . Therefore, it is ensured that the engaging portions  104   a  and  104   b  are not separated from the positioning portions (recesses)  101   a P and  101   b P, respectively, by the resiliency of the pressing member  103 . Therefore, it is possible to reduce the contact pressure between the temperature sensor  102  and the heater  100  in the film unit  21 . Therefore, it is ensured that the heater temperature is accurately detected. 
     Further, in the case of the fixing device shown in  FIGS. 5(   a )- 5 ( c ), it is possible that the sensor holding member  204  will be deformed by the pressure from the pressing member  203 , and therefore, the pressing member  203  will be reduced in the effective amount of pressure it can applied to the sensor holding member  204 . 
     In comparison, the fixing device  20  in this embodiment is structured so that the thermistor chip  120  of the temperature sensor  102 , the axial line of the pressing member  103 , and the axial line of the pressing member  105  align (coincide). 
     Since the axial line of the pressing member  103  and the axial line of the pressing member  105  coincide (align), the amount of the pressure applied to the engaging portions  104   a  and  104   b  of the sensor holding member  104  is equal to the difference between the resiliency of the pressing member  103  and that of the pressing member  105 . Thus, this embodiment can reduce the extent to which the sensor holding member  104  is deformed. 
     Further, referring to  FIGS. 4(   b ) and  4 ( c ), the amounts of pressure generated by the pressing members  103  and  105  are borne by the engaging portions  104   a  and  014   b  of the sensor holding member  104 , which are aligned in the widthwise direction (indicated by arrow mark Y). Therefore, the sensor holding member  104  is less likely to be deformed than the counterpart of the comparative fixing device. 
     As will be evident from the foregoing description of one of the embodiments of the present invention, the present invention can substantially reduce the amount by which the sensor holding member  104  is deformed. Therefore, it can reduce the effective amount of pressure applied to the sensor holding member  104  by the pressing member  103 . Therefore, it can keep stable the state of contact between the temperature sensor  102  and heater  100 . Therefore, it can make it possible for the temperature of the heater  100  to be accurately detected. 
     Further, the present invention can provide a fixing device which is substantially smaller in the dimension of its sensor unit in terms of the lengthwise direction of its heater than any fixing device in accordance with the prior art. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims priority from Japanese Patent Applications Nos. 029721/2013 and 010990/2014 filed Feb. 19, 2013 and Jan. 24, 2014, respectively, which are hereby incorporated by reference.

Technology Classification (CPC): 6