Abstract:
A fixing unit for an image forming apparatus includes a heating element  102 , a temperature sensor  119  to detect the temperature of the hating element, and an arm to hold the temperature sensor  119 , and positioning means  108  and  110  to adjust the position of the arm  105  with respect to the heating element.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fixing unit for an image forming apparatus. 
   2. Description of the Related Art 
   An image forming apparatus such as an electronic photo printer usually forms an electrostatic latent image on a photosensitive drum using a laser beam or the like, and forms a toner image by developing the electrostatic latent image with a toner. Then, it transfers the toner image onto printing paper, fixes the toner image on the printing paper with a fixing device having a fixing roller, and then ejects the paper. Such electronic photo printer and fixing device are disclosed, for example, in Unexamined Japan Patent Application Publication 07-181833 and Unexamined Japan Patent Application Publication 2001-242741. 
     FIG. 15  schematically illustrates the constitution of the electronic photo printer disclosed in Unexamined Japan Patent Application Publication 07-181833. This electronic photo printer is comprised of a paper cassette  1  to store printing paper  20 , a paper feeding roller  2  to feed the printing paper, a paper conveyance roller  4  to convey the printing paper  20 , a photosensitive drum  5  to transfer a printing toner (not illustrated) onto the printing paper  20 , a fixing device to fix the transferred toner on the printing paper  20 , and a stacker  22  to store the paper  20  after printing. 
   This electronic photo printer further comprises a cassette sensor  25  to detect if the paper cassette  1  is inserted and in a position, and a paper feeding sensor  3   a  which is provided between the paper feeding roller  2  and the paper conveyance roller  4  to detect the presence of the printing paper  20 . In addition, a charger  6  to uniformly charge the surface of the photosensitive drum  5 , an LED head  7  to form an electrostatic latent image that corresponds to the shapes of characters and the like by irradiating a specified light beam onto the surface of the charged photosensitive drum  5 , a developing device  8  to form a toner image by developing the electrostatic latent image through adhering toner on the electrostatic latent image, a transferring device  9  to transfer the toner image onto printing paper  20 , and a cleaner  10  to remove the toner remained on the photosensitive drum  8  are provided around the photosensitive drum  5 . 
   Furthermore, the fixing device  21  has a heater  21   a  provided inside the roller, and a thermistor  21   b  to detect temperature change of the roller. By heating and pressuring with the roller the toner image transferred on the printing paper  20 , the toner image is fixed. A paper ejection sensor  3   b  to detect the presence of the printing paper  20  passing through the fixing device  21  is provided between the fixing device  21  and the stacker  22 . 
   Moreover, the electronic photo printer has a first controlling unit  23   a  to control the operation of the mechanism of each element, an operation panel  24  to notify an operator of specified alarm and to set a menu, and a second controlling unit  23   b  to control the operation panel  24 , communicate with upper device(s), edit printing data, and so on. The first controlling unit  23   a  and the second controlling unit  23   b  are connected to each other via a serial interface and a video interface. 
     FIG. 16  is a circuit diagram of the first controlling unit  23   a  which controls the operation of the mechanism of the above-described electronic photo printer. A microprocessor  13   c  of the first controlling unit  23   a  mainly outputs a motor  1  control signal (a), a motor  2  control signal (b), a motor  3  control signal (c) and a heater control signal (h) based on a comparator output signal (g), a paper feeding sensor signal (d), a paper ejecting sensor signal (e), and a cassette sensor signal (f). 
   Here, the motor  1  control signal (a) is a control signal for the motor  1 , which is outputted from an output port P 3 . 0  of a microprocessor  13   c , and drives the photosensitive drum  5  and the roller of the fixing device  21  shown in  FIG. 15  via the motor controlling circuit  14 . The motor  2  control signal (b) is a control signal for a motor  2 , which is outputted from an output port P 3 . 1  of the microprocessor  13   c , and drives the paper feeding roller  2  shown in  FIG. 15  via the motor control circuit  14 . The motor  3  control signal (c) is a control signal for a motor  3 , which is outputted from an output port P 3 . 2  of the microprocessor  13   c , and drives the conveyance roller  4  shown in  FIG. 15  via the motor control circuit  14 . The motor control circuit  14  is connected to an oscillator  15 . The rotational speeds of the motors  1 - 3  are determined by the values of frequency oscillated by the oscillator  15 . 
   The paper feeding sensor signal (d) is an output signal from the paper feeding sensor  3   a  shown in  FIG. 15 , and is connected to an input port P 2 . 1  of the microprocessor  13   c . The paper ejecting sensor signal (e) is an output signal from the paper ejecting sensor  3   b  shown in  FIG. 15 , and is connected to an input port P 2 . 2  of the microprocessor  13   c . The cassette sensor signal (f) is an output signal from the cassette sensor  25 , and connected to an input port P 2 . 3 . 
   A comparator output signal (g) is a signal outputted from a comparator  12 , and is connected to an input port P 2 . 0  of the microprocessor  13   c . The input terminal a of the comparator  12 , to which the reference voltage is applied, is connected to a pull-up resistance R 3 , a pull-down resistance R 4 , and a hysteresis resistance R 2 . The input terminal b of the comparator  12  is connected to the pull-up resistance R 5  and the terminal a of the thermistor  21   b , while the terminal b of the thermistor  21   b  is connected to ground. The output terminal c of the comparator  12  is connected to the pull-up resistance R 1 , the hysteresis resistance R 2 , and the input port P 2 . 0  of the microprocessor  13   c.    
   In addition, the heater control signal (h) is outputted from an output port P 1 . 0  of the microprocessor  13   c , and is connected to a power control circuit (not illustrated) of the heater  21   a  of the fixing device  21  shown in  FIG. 15  so as to perform ON/OFF control of the heater  21   a.    
   The thermistor  21   b  shown in  FIGS. 15 and 16  has the characteristics of having smaller resistance as the temperature becomes higher, and larger resistance as the temperature becomes lower. For this reason, the voltage Vb applied to the input terminal b of the comparator  12  becomes lower as the temperature becomes higher, and the voltage Vb becomes higher as the temperature becomes lower. 
     FIG. 17  is a time chart in the temperature control of the heater  21   a  using the above-described characteristics. Here, Vb shows the waveform of the voltage applied to the input terminal b of the comparator  12 , Va-High is the reference voltage applied as a high slice level to the input terminal a of the comparator  12 , and Va-Low is the reference voltage applied as a low slice level to the input terminal a of the comparator  12 . TL is the length of time from when Vb reaches Va-High to when Vb reaches Va-Low, and TH is the length of time from when Vb reaches Va-High to when Vb reaches Va-Low. 
   Once the voltage Vb applied to the input terminal b of the comparator  12  exceeds the high slice level voltage applied to the input terminal a of the comparator  12 , the comparator output signal (g) becomes low level. The microprocessor  13   c  shown in  FIG. 16  detects the comparator output signal (g) which is low level at the input port P 2 . 0 , and turns “ON” the heater  21   a , setting the output port P 1 . 0  as high level. 
   When the heater  21   a  is turned “ON”, the temperature of the roller of the fixing device  21  becomes higher, the resistance of the thermistor  21   b  becomes smaller, and the voltage Vb applied to the input terminal b of the comparator  12  becomes lower. Once Vb becomes lower than the low slice level Va-Low applied to the input terminal a of the comparator  12 , the comparator output signal (g) becomes High level. The microprocessor  13   c  detects the comparator output signal (g) which becomes high level at the input port P 2 . 0 , and turns “OFF” the heater  21   a , setting the output port P 1 . 0  as low level. By repeating the above-described control of the heater  21   a  from when the power of the electronic photo printer is ON till the power is OFF, the temperature of the roller of the fixing device is maintained at a substantially constant temperature. 
   Referring now to a time chart of  FIG. 18 , the method of controlling the above-described electronic photo printer is described. Here, the reference numerals not indicated in  FIG. 18  shall be referred to those in  FIGS. 15 and 16 . Once the second controlling unit receives printing data from the upper device (not illustrated), it produces an image data for printing, and then writes the data in a memory (not illustrated). Then, once editing one page of the image data is completed, a printing instruction is sent to the controlling unit  23   a.    
   Once the first controlling unit  23   a  receives a start printing instruction from the second controlling unit  23 , it rotates the photosensitive drum  5  and the roller of the fixing device  21 , setting the motor  1  control signal (a) as high level. At the same time, by driving the charger  6 , the surface of the photosensitive drum  5  is uniformly charged. After that, the motor  2  control signal is set as high level, and a sheet of the printing paper  20  in the paper cassette  1  is fed toward the conveyance roller  4 . Once the paper feeding signal (d) becomes high level (i.e. if the paper  20  is passing the paper feeding sensor  3   a ), the rotation of the paper feeding roller  2  stops, setting the motor  2  control signal (b) as low level after a specified length of time (until the front edge of the paper contacts the conveyance roller  4 ). 
   Once the rotation of the paper feeding roller  2  is stopped, the paper  20  is fed toward the fixing device  21  by rotating the roller  4 , setting the motor  3  control signal (c) as high level. At this time, the second controlling unit  23   b  sends the image data for printing to the LED head  7  via the first controlling unit  23   a , and forms an electrostatic latent image on the surface of the photosensitive drum  5 . Then, the toner image is formed from the electrostatic latent image by the developing device  8 , and then transferred onto the paper  20 . The paper having the transferred toner image is conveyed to the fixing device  21 , and the toner image is fixed on the paper  20  by heating and pressuring. Thereafter, the paper that passed the fixing device  21  reaches the paper ejecting sensor  3   b , and the paper ejecting sensor signal (e) becomes high level. Then, if the paper is further conveyed, and the paper ejecting sensor signal (e) becomes low level, the paper  20  after printing is stored in the stacker, and the printing is completed. 
   After completing the printing, the first controlling unit  23   a  notifies the completion of the printing to the second controlling unit  23   b . Once the second controlling unit  23   b  receives the notification of the completion of the printing, it clears the memory, in which a page of image is written. If there is no printing instruction from the second controlling unit  23   b  within a specified period of time after the paper  20  passes the paper ejecting sensor  3   b , the first controlling unit  23   a  stops the rotation of the photosensitive drum  5  and the fixing device  21 . On the other hand, if there is a printing instruction from the second controlling unit  23   b  within the specified period of time, the paper feeding roller  2  is rotated, setting the motor  2  control signal (b) as high level, and printing on the next sheet of the paper  20  is performed. By repeating the above-described operations, a series of printing process is performed. 
   As described above, in the image forming device such as the electronic photo printer, the temperature of the roller of the fixing device can be detected without contacting the roller surface. Therefore, the thermistor is attached to the frame being specified distance away from the surface of the fixing roller, and the timing for turning ON/OFF the heater in the fixing roller is controlled based on the temperature detected by the thermistor. By heat of the heating element and pressure applied by the fixing roller, the toner is fixed on the printing paper, and then the printing is completed. 
   However, since the distance between the fixing roller and the thermistor varies depending on the dimensional precision or precision of attachments related to a frame or other components, the temperature detected by the thermistor of conventional fixing device varies, which affects the temperature control of the roller surface of the fixing device. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to improve the precision of the fixing temperature. It is another object of this invention to stabilize the printing quality by controlling the dispersion of the temperature detected by the thermistor due to the dispersion of the distance between the fixing roller and the thermistor in the fixing device used in the image forming device such as electronic photo printer. 
   According to the invention there is provided a fixing device for an image forming device which comprises an arm to hold a temperature sensor, and a positioning means to adjust the position of the arm with respect to the heating element. 
   According to this invention, the dispersion of the temperature detected by the thermistor due to the dispersion of the distance between the fixing roller and the thermistor in the fixing device used in the image forming device such as electronic photo printer can be controlled, and therefore, the precision of the fixing temperature can be improved, and the printing quality can be stabilized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the fixing device in the first embodiment of this invention. 
       FIG. 2  is an enlarged view of the frame opening and the sensor arm of the fixing device of  FIG. 1 . 
       FIG. 3  is a perspective view of the sensor arm of the fixing device of  FIG. 1 . 
       FIG. 4  is a cross-sectional view taken along a line A-A′ of  FIG. 2 . 
       FIG. 5  is a cross-sectional view taken along a line B-B′ of  FIG. 2 . 
       FIG. 6  is a perspective view of the sensor arm of the fixing device in the second embodiment. 
       FIG. 7  is a perspective view of the sensor arm of the fixing device in the third embodiment of this invention. 
       FIGS. 8(   a ) and  8 ( b ) are a perspective view of the position adjusting spacer to use for the fixing device of the third embodiment, respectively. 
       FIG. 9  is a front view of the sensor arm of the fixing device of the fourth embodiment, which is viewed from the protrusion provided at the end of the sensor arm. 
       FIG. 10  is a partial cross-sectional view of the fixing device of the fourth embodiment. 
       FIG. 11  is a perspective view of the sensor arm of the fifth embodiment of this invention. 
       FIG. 12  is a partial cross-sectional view of the fixing device of the fifth embodiment. 
       FIG. 13  is a front view of the sensor arm of the fixing device of the fifth embodiment, when it is viewed from the protrusion side provided at the end of the sensor arm. 
       FIG. 14  is an example of a projected image of the heat roller and the protrusion of the sensor arm of the fixing device of the fifth embodiment, which is taken by a camera. 
       FIG. 15  illustrates the constitution of the electronic photo printer having a conventional fixing device. 
       FIG. 16  is a circuit diagram of the controlling unit which controls the mechanism of the electronic photo printer of  FIG. 15 . 
       FIG. 17  is a time chart in the temperature control of the heater of the fixing device of the electronic photo printer of  FIG. 15 . 
       FIG. 18  is a time chart in the method of controlling the operation of the electronic photo printer of  FIG. 15 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the fixing device  101  of the first embodiment. A heat roller bearing  117  is attached to a frame  106  of the fixing device  101 , and a heat roller  102  is supported by the heat roller bearing  117  so as to be freely rotatable. A heating element (e.g. halogen lamp)  103  is provided inside the heat roller  102 . A pressure roller  104  supported by the pressure roller bearing  118  so as to be freely rotatable is provided under the heat roller  102 . The pressure roller bearing  118  is attached to the frame  106  so as to be movable and biased toward the heat roller  102  by a coil spring (not illustrated). 
     FIG. 2  shows the sensor arm  105  of the fixing device  101 . As shown in this figure, the frame  106  has a frame opening  116  for adjusting the position of the sensor arm  105 .  FIG. 3  shows the sensor arm  105  of the fixing device  101 . As shown in  FIGS. 2 and 3 , the frame  106  has a first contact section  122  and a second contact section  123 , which contact with respective edges of a flat spring  110 . The sensor arm  105  is attached to the frame with a screw  108  via the flat spring  110 . A temperature detector  119  such as the one comprised of a thermistor is provided to the sensor arm at its base section, and the heat roller is disposed right under the temperature detector  119 . 
   As shown in  FIG. 4 , the sensor arm  105  and the flat or leaf spring  110  are attached to the frame  106  with the screw and a nut  109  fitted to the screw. The sensor arm  105  is interposed between the flat spring  110  and a positioning section  107  defined by the collar of the screw head of the screw  108 , and extends generally parallel to the tangential line of the highest point of the circumference of the heat roller  102 . 
   In  FIG. 5 , since the flat spring  110  is supported by the first contact section  122  and the second contact section  123  of the frame  106  and its generally center part is flexed being pushed by the sensor arm  105 , the sensor arm  105  is energized toward the positioning section  107  by the elastic force of the flat spring  110 . Therefore, by rotating the screw  108 , the positioning section moves upward/downward, and therefore the sensor arm  105  moves upward/downward as the positioning section  107  moves, being energized by the flat spring  110 . 
   With the above-described constitution, the positioning section  107  of the screw  108  can be moved toward/away from the heat roller  102  supported to the frame  106  by rotating the screw  108 . If the positioning section  107  moves away from the heat roller  106 , the amount of flexion of the flat screw  110  is reduced, so that the sensor arm  105  biased toward the screw head moves with the positioning section  107  away from the heat roller  102 . If the positioning section  107  is moved toward the heat roller  102 , the amount of flexion of the flat spring  110  increases, so that the sensor arm  105  biased toward the screw head moves with the positioning section  107  toward the heat roller  102 . Therefore, the position of the sensor arm  105  with respect to the heat roller  102  can be adjusted by rotating the screw  108 . 
   Since the screw  108  is energized by the flat spring  110 , it is difficult to loosen. In addition, since the sensor arm  105  is elastically supported by the flat spring  110 , the sensor arm  105  will not be damaged by undesired force from paper jam between the heat roller  102  and the sensor arm  105 . In this case, if the jammed paper is removed, the sensor arm will be back to the original position by the energizing or biasing force of the flat spring  110 . 
   As described above, according to the first embodiment, since the sensor arm  105  is attached to the frame via the flat spring  110  and is movable upward/downward by rotating the spring  108 , the distance between the heat roller  102  and the sensor arm  105  can be adjusted, and the surface temperature of the heat roller can be precisely measured. Furthermore, since the sensor arm  105  is attached so as to extend generally along the tangential line of the highest point of the circumference of the heat roller  102 , the clearance can be precisely measured using a clearance gauge or the like. 
     FIG. 6  shows the sensor arm of the fixing device of the second embodiment of this invention. In this embodiment, another flat or leaf spring  111  is used in place of the flat spring  110 . This flat spring  111  has openings  120  provided between the sensor arm  105  and the first contact section  122  of the frame  106  and between the sensor arm  105  and the second contact section  123  of the frame  106 . In addition, the flat spring  111  has a restricting member  121  to restrict the sensor arm from moving in the lateral direction (the direction perpendicular to the longitudinal directions of the screw  108  and the sensor arm  105 , i.e. the direction parallel to the axial direction of the heat roller  102 ). The fixing device of this embodiment is similar to that of the first embodiment, but differs only in having the flat spring  111 , therefore the explanation is omitted. 
   Similarly to the first embodiment, in the second embodiment, the position of the sensor arm  105  with respect to the heat roller  102  can be adjusted by rotating the screw  108  through the opening  116  of the frame  106 . 
   In this embodiment, since the movement of the sensor arm  105  in the lateral direction is restricted by the restricting member  121 , when the position of the sensor arm  105  is adjusted by rotating the screw  108 , the sensor arm  105  will not rotate about the screw  108 , but moves only upward/downward. Therefore, the position of the temperature detector  119  will not be off in the lateral direction. 
   Furthermore, since the portions of the flat spring  111 , which are between the respective edges that contact with the first and the second contact sections and the generally central part of the flat spring  111 , are made narrow, these portions are easily flexed, so that strong force does not have to be applied to adjust the screw  108 . On the other hand, the surface of the portion of the flat spring  110  to attach the sensor arm  105  is difficult to be flexed and generally flat, so that the sensor arm  105  can be stably positioned. Accordingly, since the rotational movement of the screw  108  does not cause rotation of the sensor arm around the screw, the position of the sensor arm  105  will not be off in lateral direction. 
   As described above, in the flat spring  111  of this embodiment, the section that contacts the first contact section  122  and the section that contacts the second contact section  123  are respectively connected to the generally central portion of the flat spring  111  to attach the sensor arm by easily flexible narrow sections. Therefore, there is an advantage that strong force is not required to adjust the screw  108 . In addition, since the surface of the flat spring  111  for attaching to the sensor arm  105  is difficult to flex and made generally flat, there is another advantage that the sensor arm  105  can be stably positioned. 
     FIG. 7  shows the sensor arm  105  of the fixing device of the third embodiment. This embodiment is featured by having a position adjusting spacer  115 , which is configured as shown in  FIG. 8(   a ). While this spacer  115  is interposed between the positioning section  107  of the screw  108  and the sensor arm  105 , the position of the sensor arm  105  is adjusted by rotating the screw  108  such that the sensor arm  105  contacts with the surface of the heat roller  105 . Thereafter, the position adjusting spacer  115  is removed, so that the sensor arm  105  will move upward for the thickness of the position adjusting spacer  115 . Accordingly, the sensor arm  105  is moved away from the surface of the heat roller  102  for the distance equal to the thickness of the position adjusting spacer. Therefore, according to this embodiment, the position of the sensor arm can be easily adjusted without the clearance gauge. 
   Conventionally, to adjust the clearance between the sensor arm and the heat roller, the clearance gauge was inserted between the sensor arm and the heat roller, the position of the sensor arm is adjusted such that the sensor arm contacts with the inserted clearance gauge, and the clearance gauge was removed. In this conventional technique, however, the thermistor or the surface of the heat roller can be damaged by the clearance gauge through scraping against the thermistor or the heat roller when the clearance gauge is pulled out. As described above, according to this embodiment, since the position of the sensor arm can be adjusted without using the clearance gauge, there is no concern of damaging the thermistor or the heat roller surface. 
   As shown in  FIG. 8(   b ), the position adjusting spacer  115  can have a stepwise structure consist of a first step section, which is interposed between the position adjusting section  107  and the sensor arm  105  when the image forming device is used as usual, and a second step section which is interposed between the position adjusting section  107  and the sensor arm  105  when the position of the sensor arm  105  is adjusted. In this case, by moving the position adjusting spacer  115  forward/backward at the time of adjusting the position of the sensor arm  105 , the position of the sensor arm  105  can be adjusted such that the sensor arm  105  is away from the surface of the heat roller  102  for the distance equal to the difference of the thickness between the first and the second step sections. Also in this case, the position of the sensor arm can be easily adjusted without using a clearance gauge. In addition, since this position adjusting spacer having stepwise structure can be kept attached even when the image forming device is used as usual, it does not have to be attached or removed whenever the position of the sensor arm has to be adjusted. 
     FIG. 9  shows the sensor arm of the fixing device of the fourth embodiment, which is viewed from the temperature detector side, and  FIG. 10  is a cross-sectional view taken along a line A-A′ of  FIG. 9 . As shown in those figures, this embodiment is different from the first embodiment because a protrusion  125 , which extends generally parallel to the tangential line of the highest point of the circumference of the heat roller  102  is provided at the end of the sensor arm  105 , and a restricting section  127  having a hole  126  to put the protrusion therein is provided at the frame  106 . 
   In this embodiment, even if the screw  108  is excessively tighten, since the protrusion  125  works as a stopper by contacting the lower part of the wall surface of the hole  126  of the restricting part  127 , the sensor arm  105  will not touch the heat roller  102 . In this case, since the sensor arm  105  pivots upward/downward having the contact point between the protrusion  125  and the wall surface of the hole  126  as a fulcrum as the flat spring  111  is flexed, the sensor arm  105  will not be damaged. According to this embodiment, since the protrusion  125  of the sensor arm  105  and the hole  126  of the restricting part  127  work as the stopper when the screw is excessively tighten and the interference of the sensor arm  105  with the heat roller  102  can be prevented, the heat roller  102  will not be damaged and therefore stable printing quality is assured. Moreover, even if a sheet of printing paper is wound around the heat roller and pushes up the sensor arm  105 , since the protrusion  125  of the sensor arm  105  contacts with the upper portion of the wall of the hole  126  of the restricting part  127 , the attaching position of the sensor arm  105  will not be off excessively. 
     FIG. 11  shows the sensor arm of the fixing device of the fifth embodiment of this invention, and  FIG. 12  is a cross-sectional view of the embodiment. As shown in those figures, this embodiment is different from the fourth embodiment because a second protrusion  128  is respectively provided on two edges  129  of the sensor arm  105 . The second protrusion  128  has a surface  128   f  facing the protrusion  125 . The protrusion  128  is formed such that the surface  128   f  is included in a plane defined by the center axes of the heat roller  102  and the pressure roller  104 .  FIG. 13  illustrates the sensor arm  105  viewed from the protrusion  125  side, which is formed at the end of the sensor arm. As shown in this figure, in this embodiment, the height of the upper edge of the surface  128   f  measured from the lower surface of the temperature detector  119  is made constant H. 
   Similarly to the precedent embodiments, the position of the sensor arm  105  with respect to the heat roller  102  can be adjusted by rotating the screw  108  through the opening  116  of the frame  106 . In this embodiment, the distance between the surface of the heat roller  102  and the protrusion  128  can be adjusted using a camera. More specifically, the image of the upper part of the heat roller  102  and the surface  128   f  of the protrusion  128  is first taken by a camera, and projected on a monitor.  FIG. 14  shows an example of the image projected on the monitor. Based on this image, the distance F between the upper edge of the heat roller and the upper edge of the surface  128   f  of the protrusion  128  is measured. In this case, the distance between the surface of the heat roller  102  and the bottom surface of the temperature detector  119  can be obtained as a value calculated by subtracting H from F. Once the calculated value agrees with the specified value after rotating the screw  108 , the adjustment of the position is completed. 
   As described above, according to the fifth embodiment, since the protrusion  128  for measuring the height of the sensor arm  105  is provided to the sensor arm  105 , the clearance between the temperature detector  119  of the sensor arm  105  and the heat controlling unit  102  can be measured even if the clearance cannot be observed from the front side. Therefore, the measured value will not vary with the people who conduct the measurement, and therefore the adjustment of the position can be precisely done.