A kiln temperature control device for firing bone china tableware

Abstract

This invention discloses a kiln temperature control device for firing bone china tableware, relating to the field of kiln temperature control technology. The device includes a kiln with ceramic plates installed inside. The invention utilizes a U-shaped mounting bracket that slides on a connecting plate based on the size of the controller body. This allows the controller body to be fixed to both sides via two U-shaped mounting brackets. Fasteners are then used to secure the U-shaped mounting brackets to their positions on the connecting plate. The distance between the side plates on the rear plate is adjusted based on the distance between the fixing blocks on the two U-shaped mounting brackets, allowing the connecting rod to pass through the fixing blocks. Once the distance between the side plates is adjusted, screws are used to install the side plates in designated positions. The fixing blocks on the U-shaped mounting brackets are then fitted onto the connecting rod, and bolts are used to secure the fixing blocks to their positions on the connecting rod. This completes the installation of the controller body and facilitates disassembly, improving maintenance convenience.

Description

Technical Field

[0001] This invention relates to the field of kiln temperature control technology, specifically to a kiln temperature control device for firing bone china tableware. Background Technology

[0002] In the production of bone china tableware, kiln temperature control is a crucial step. Traditional kiln temperature control devices often employ relatively simple temperature regulation methods, such as manually adjusting the power of the heating wire to achieve rough control of the kiln's internal temperature. However, this method has many shortcomings.

[0003] First, manual adjustment has limited precision, making it difficult to meet the precise temperature control requirements of bone china tableware firing. Even slight temperature fluctuations can affect product quality and yield.

[0004] Secondly, the structure of traditional temperature control devices is relatively fixed, making them inconvenient to install and disassemble, which brings inconvenience to equipment maintenance and repair. Once the equipment malfunctions, it often requires a lot of time and effort to troubleshoot and repair, seriously affecting production efficiency.

[0005] In addition, traditional temperature control devices often lack flexibility in adjusting the temperature and cannot quickly adjust the temperature according to actual needs, which limits their application range in different firing processes.

[0006] To address these issues, existing technologies have begun to employ more advanced temperature control techniques, such as using automated control systems to achieve precise temperature control of the kiln. However, these technologies still have some shortcomings in practical applications, such as high system complexity, high cost, and difficult maintenance.

[0007] Therefore, this invention proposes a new kiln temperature control device for firing bone china tableware, aiming to overcome the shortcomings of the prior art and provide a temperature control device that is simple in structure, easy to install and disassemble, and provides precise and flexible temperature control to meet the high requirements for temperature control during the firing process of bone china tableware. Summary of the Invention

[0008] The purpose of this invention is to provide a kiln temperature control device for firing bone china tableware, so as to solve the problems mentioned in the background art.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0010] A temperature control device for firing bone china tableware in a kiln includes a kiln, a ceramic plate installed inside the kiln, an electric heating wire installed inside the ceramic plate, a temperature adjustment control unit installed on the top of the kiln, a temperature regulating mechanism installed at the rear air outlet of the kiln, and a temperature controller installed on one side of the kiln.

[0011] The temperature controller includes a connecting plate and a rear plate. Two U-shaped mounting brackets are fitted onto both sides of the connecting plate. Fasteners are threaded onto the front and back of each U-shaped mounting bracket. Fixing blocks are fixedly installed on the side walls of each U-shaped mounting bracket, and bolts are threaded onto the side walls of the fixing blocks. The controller body is fitted between the two U-shaped mounting brackets. Several wiring grooves are formed on the front of the rear plate. Side plates are fitted onto both sides of the rear plate. A connecting rod is fixedly installed on the front of each side plate, with one end of the connecting rod passing through and slidably connected to the fixing block.

[0012] A further improvement of the technical solution of the present invention is that: the top of the controller body is provided with several sets of heat dissipation strips, one end of the connecting plate extends into the U-shaped mounting bracket and is slidably connected with the U-shaped mounting bracket, one end of the fastener extends into the U-shaped mounting bracket and is pressed against the outer wall of the connecting plate. In use, the controller body can be sleeved between two U-shaped mounting brackets, and then the U-shaped mounting bracket can slide on the connecting plate according to the size of the controller body.

[0013] A further improvement of the technical solution of the present invention is that: one end of the bolt extends into the fixing block and is pressed against the outer wall of the connecting rod; the side wall of the rear plate extends into the side plate and is slidably connected with the side plate; the two sides of the controller body are fixed by two U-shaped card holders; and then the position of the U-shaped card holders on the connecting plate is fixed by fasteners.

[0014] A further improvement of the technical solution of the present invention is that: the temperature regulating mechanism includes a fan, the fan is fixedly connected to the rear side of the kiln, one end of the fan is fixedly connected to an air outlet pipe, one end of the air outlet pipe is fixedly connected to an installation frame, a limit plate is fixedly connected to the surface of the installation frame, and a baffle is slidably connected inside the limit plate. When the kiln is burning, the fan blows air to start the motor.

[0015] A further improvement of the technical solution of the present invention is that: a connecting frame is fixedly connected to one side of the mounting frame, a motor is fixedly connected to the surface of the connecting frame, a threaded column is fixedly connected to the output end of the motor, a fixing plate is threadedly connected to the surface of the threaded column, and a baffle is fixedly connected to one end of the fixing plate. The motor drives the threaded column to rotate, the threaded column drives the fixing plate to rise and fall, and the fixing plate drives the baffle to move vertically up and down inside the limiting plate.

[0016] A further improvement of the technical solution of the present invention is that: the temperature control device includes a corrugated telescopic tube, the corrugated telescopic tube is fixedly connected to the top of the corrugated telescopic tube, an inert gas sealing box is fixedly connected to the top of the corrugated telescopic tube, a sliding rheostat is fixedly connected to one end of the inert gas sealing box, the inert gas sealing box is embedded in the top of the furnace, the corrugated telescopic tube is built into the box body, one end is fixed to the top of the box body, and the other end is connected to the carbon film contact piece of the sliding rheostat.

[0017] A further improvement of the technical solution of the present invention is that: the sliding rheostat includes a bracket and a protective sleeve connected to the bracket. A sliding rod and a ceramic tube are provided inside the protective sleeve. The ceramic tube is fixed to the center of the bracket by screws. A resistance wire is sleeved around the ceramic tube. The sliding rod is fixed above the ceramic tube by a fixing piece with a terminal post. A carbon film contact piece that can move left and right is sleeved on the sliding rod. A strip-shaped sliding groove corresponding to the carbon film contact piece is provided above the protective sleeve. The temperature measuring tube at one end of the temperature controller adopts an embedded armored thermocouple, which extends along the furnace wall to the core area of ​​the combustion chamber, collects temperature signals in real time, and transmits them to the gas box pressure regulator on the inert gas sealing box.

[0018] A further improvement of the technical solution of the present invention is that: the carbon film contact piece extends out of the protective sleeve from the sliding groove, the conductive brush connected below the carbon film contact piece contacts the resistance wire, the conductive brush can slide on the surface of the resistance wire, and a conductive piece is connected to each of the left and right ends of the resistance wire. Each conductive piece is connected to a terminal fixed on the bracket. When the furnace temperature exceeds the standard, the inert gas expands due to heat, pushing the corrugated telescopic tube to extend, which drives the carbon film contact piece to move on the sliding rheostat track. The power of the heating wire is adjusted by the change of resistance value. After the deviation is eliminated, the corrugated telescopic tube contracts back to its original position.

[0019] Due to the adoption of the above technical solution, the technical progress achieved by this invention compared to the prior art is as follows:

[0020] 1. This invention provides a kiln temperature control device for firing bone china tableware. In use, the controller body can be fitted between two U-shaped mounting brackets. The U-shaped mounting brackets slide on the connecting plate according to the size of the controller body, thereby fixing the two sides of the controller body through the two U-shaped mounting brackets. Then, the position of the U-shaped mounting brackets on the connecting plate is fixed by fasteners. Then, the distance between the two side plates of the rear plate is adjusted according to the distance between the fixing blocks on the two U-shaped mounting brackets, so that the connecting rod can pass through the fixing blocks. After the distance between the two side plates of the rear plate is adjusted, the side plates can be installed in the designated position by screws. Then, the fixing blocks on the U-shaped mounting brackets are fitted onto the connecting rod, and the position of the fixing blocks on the connecting rod is fixed by bolts, thereby completing the installation of the controller body and facilitating its disassembly, thus improving maintenance convenience.

[0021] 2. This invention provides a kiln temperature control device for firing bone china tableware. An inert gas sealing box is embedded in the top of the kiln. A corrugated expansion tube is built into the box, with one end fixed to the top of the box and the other end connected to the carbon film contact plate of a sliding rheostat. The temperature measuring tube at one end of the temperature controller is an embedded armored thermocouple that extends along the furnace wall to the core area of ​​the combustion chamber. It collects temperature signals in real time and transmits them to the gas box pressure regulator on the inert gas sealing box. When the furnace temperature exceeds the limit, the inert gas expands due to heat, pushing the corrugated expansion tube to extend and causing the carbon film contact plate to move on the track of the sliding rheostat. The power of the heating wire is adjusted by the change in resistance value. After the deviation is eliminated, the corrugated expansion tube contracts back. When the displacement of the corrugated expansion tube causes the resistance value of the sliding rheostat to change, the controller adjusts the current of the heating wire in real time to achieve a furnace temperature control accuracy of ±5℃. Thus, the principle of thermal expansion and contraction of gas is used to drive the carbon film contact plate to adjust the resistance value, thereby realizing closed-loop temperature control. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0023] Figure 2 This is a schematic diagram of the temperature controller of the present invention;

[0024] Figure 3 This is a schematic diagram of the heat dissipation strip of the present invention;

[0025] Figure 4 This is a schematic diagram of the temperature control device of the present invention;

[0026] Figure 5 This is a schematic diagram of the temperature regulating mechanism of the present invention;

[0027] Figure 6 This is a schematic diagram of the structure of the sliding rheostat of the present invention.

[0028] In the diagram: 1. Kiln; 2. Temperature controller; 21. Controller body; 22. Connecting plate; 23. U-shaped mounting bracket; 24. Fastener; 25. Fixing block; 26. Bolt; 27. Rear plate; 28. Side plate; 29. ​​Connecting rod; 291. Heat dissipation strip; 292. Wiring trough; 3. Temperature adjustment mechanism; 30. Fan; 31. Air outlet duct; 32. Mounting frame; 33. Limiting plate; 34. Baffle; 35. Connecting frame; 36. Motor; 37. Mounting plate; 38. Threaded post; 4. Ceramic plate; 5. Heating wire; 6. Temperature control switch; 60. Corrugated expansion tube; 61. Inert gas sealed box; 62. Sliding rheostat; 620. Bracket; 621. Protective sleeve; 622. Ceramic tube; 623. Resistance wire; 624. Conductive sheet; 625. Slide rod; 626. Carbon film contact piece; 627. Fixture; 628. Screw; 629. Terminal block. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments:

[0030] Example 1

[0031] like Figure 1-6 As shown, the present invention provides a kiln temperature control device for firing bone china tableware, including a kiln 1, a ceramic plate 4 installed inside the kiln 1, an electric heating wire 5 installed inside the ceramic plate 4, a temperature adjustment control 6 installed on the top of the kiln 1, a temperature adjustment mechanism 3 installed at the rear air outlet of the kiln 1, and a temperature controller 2 installed on one side of the kiln 1.

[0032] The temperature controller 2 includes a connecting plate 22 and a rear plate 27. Two U-shaped mounting brackets 23 are fitted onto both sides of the connecting plate 22. Fasteners 24 are threaded onto the front and back of each U-shaped mounting bracket 23. Fixing blocks 25 are fixedly installed on the side walls of the two U-shaped mounting brackets 23, and bolts 26 are threaded onto the side walls of the fixing blocks 25. The controller body 21 is fitted between the two U-shaped mounting brackets 23. Several sets of wiring grooves 292 are opened on the front of the rear plate 27. Side plates 28 are fitted onto both sides of the rear plate 27. The front of the side plates 28... A connecting rod 29 is fixedly installed, and one end of the connecting rod 29 passes through the fixing block 25 and is slidably connected to the fixing block 25. Several sets of heat dissipation strips 291 are opened on the top of the controller body 21. One end of the connecting plate 22 extends into the U-shaped card holder 23 and is slidably connected to the U-shaped card holder 23. One end of the fastener 24 extends into the U-shaped card holder 23 and is pressed against the outer wall of the connecting plate 22. One end of the bolt 26 extends into the fixing block 25 and is pressed against the outer wall of the connecting rod 29. The side wall of the rear plate 27 extends into the side plate 28 and is slidably connected to the side plate 28.

[0033] Specifically, in use, the controller body 21 can be fitted between two U-shaped mounting brackets 23. The U-shaped mounting brackets 23 are then slid on the connecting plate 22 according to the size of the controller body 21, thereby fixing both sides of the controller body 21 via the two U-shaped mounting brackets 23. The position of the U-shaped mounting brackets 23 on the connecting plate 22 is then fixed using fasteners 24. The distance between the side plates 28 on both sides of the rear plate 27 is adjusted according to the distance between the fixing blocks 25 on the two U-shaped mounting brackets 23, allowing the connecting rod 29 to pass through the fixing blocks 25. After adjusting the distance between the side plates 28 on both sides of the board 27, the side plates 28 can be installed in the designated position using screws. Then, the fixing block 25 on the U-shaped bracket 23 is fitted onto the connecting rod 29, and the fixing block 25 is fixed on the connecting rod 29 using bolts 26. This completes the installation of the controller body 21 and facilitates its disassembly. After the controller body 21 is installed, the wiring on the controller body 21 can be inserted into the wiring groove 292 on the front of the rear board 27 to arrange the wiring, effectively preventing it from becoming disordered and making it easier for users to use.

[0034] Example 2

[0035] like Figure 1-6 As shown, based on Embodiment 1, the present invention provides a technical solution: Preferably, the temperature regulating mechanism 3 includes a fan 30, the fan 30 is fixedly connected to the rear side of the kiln 1, one end of the fan 30 is fixedly connected to an air outlet pipe 31, one end of the air outlet pipe 31 is fixedly connected to an installation frame 32, the surface of the installation frame 32 is fixedly connected to a limiting plate 33, the inside of the limiting plate 33 is slidably connected to a baffle 34, one side of the installation frame 32 is fixedly connected to a connecting frame 35, the surface of the connecting frame 35 is fixedly connected to a motor 36, the output end of the motor 36 is fixedly connected to a threaded post 38, the surface of the threaded post 38 is threadedly connected to a fixing plate 37, one end of the fixing plate 37 is fixedly connected to a baffle 34;

[0036] Specifically, when the kiln 1 is burning, the blower 30 blows air and the motor 36 is started. The motor 36 drives the threaded column 38 to rotate. The threaded column 38 drives the fixed plate 37 to rise and fall. The fixed plate 37 drives the baffle 34 to move vertically up and down inside the limiting plate 33, so as to flexibly block the air outlet pipe 31 and thus flexibly adjust the air volume of the air outlet pipe 31.

[0037] Example 3

[0038] like Figure 1-6As shown, based on Embodiment 1, the present invention provides a technical solution: Preferably, the temperature control unit 6 includes a corrugated telescopic tube 60, the top of which is fixedly connected to the corrugated telescopic tube 60. An inert gas sealed box 61 is fixedly connected to the top of the corrugated telescopic tube 60. A sliding rheostat 62 is fixedly connected to one end of the inert gas sealed box 61. The sliding rheostat 62 includes a bracket 620 and a protective sleeve 621 connected to the bracket 620. A sliding rod 625 and a ceramic tube 622 are provided inside the protective sleeve 621. The ceramic tube 622 is fixed to the center of the bracket 620 by a screw 628. A resistor is sleeved around the ceramic tube 622. The resistance wire 623 and the slide rod 625 are fixed above the ceramic tube 622 by a fixing member 627 with a terminal 629. A carbon film contact piece 626 that can move left and right is sleeved on the slide rod 625. A strip-shaped groove corresponding to the carbon film contact piece 626 is provided above the protective sleeve 621. The carbon film contact piece 626 extends out of the protective sleeve 621 from the groove. A conductive brush connected below the carbon film contact piece 626 contacts the resistance wire 623. The conductive brush can slide on the surface of the resistance wire 623. A conductive piece 624 is connected to each of the left and right ends of the resistance wire 623. Each conductive piece 624 is connected to a terminal 629 fixed on the bracket 620.

[0039] The working principle of the kiln temperature control device for firing bone china tableware will be explained in detail below.

[0040] like Figure 1-6 As shown, an inert gas sealing box 61 (argon or nitrogen) is embedded in the top of the kiln 1. The box contains a corrugated expansion tube 60, one end of which is fixed to the top of the box, and the other end is connected to the carbon film contact plate 626 of the sliding rheostat 62. The temperature measuring tube at one end of the temperature controller 2 is an embedded armored thermocouple that extends along the furnace wall to the core area of ​​the combustion chamber. It collects temperature signals in real time and transmits them to the gas box pressure regulator on the inert gas sealing box 61. When the furnace temperature exceeds the limit, the inert gas expands due to heat, pushing the corrugated expansion tube 60 to extend, which causes the carbon film contact plate 626 to move on the track of the sliding rheostat. The power of the heating wire 5 is adjusted by the change in resistance value. After the deviation is eliminated, the corrugated expansion tube 60 contracts back. When the displacement of the corrugated expansion tube 60 causes the resistance value of the sliding rheostat 62 to change, the PID controller adjusts the current of the heating wire in real time (adjustment accuracy ±0.5A) to achieve a furnace temperature control accuracy of ±5℃. Thus, the principle of gas thermal expansion and contraction is used to drive the carbon film contact plate 626 to adjust the resistance value, thereby realizing closed-loop temperature control.

[0041] During use, the protective sleeve 621 surrounds the resistance wire 623 and the conductive brush, effectively providing waterproof and dustproof protection, protecting the resistance wire 623, ensuring sensitive response, and extending service life.

[0042] The working principle of the kiln temperature control device for firing bone china tableware will be explained in detail below.

[0043] like Figure 1-4 As shown, in use, the controller body 21 can be fitted between two U-shaped mounting brackets 23. Then, according to the size of the controller body 21, the U-shaped mounting brackets 23 slide on the connecting plate 22, thereby fixing both sides of the controller body 21 through the two U-shaped mounting brackets 23. Then, the position of the U-shaped mounting brackets 23 on the connecting plate 22 is fixed by fasteners 24. Then, according to the distance between the fixing blocks 25 on the two U-shaped mounting brackets 23, the distance between the two side plates 28 of the rear plate 27 is adjusted so that the connecting rod 29 can pass through the fixing blocks 25. When the rear plate... After adjusting the distance between the two side plates 28 on both sides of the 27, the side plates 28 can be installed in the designated position using screws. Then, the fixing block 25 on the U-shaped bracket 23 is fitted onto the connecting rod 29, and the fixing block 25 on the connecting rod 29 is fixed with bolts 26, thus completing the installation of the controller body 21 and facilitating its disassembly. After the controller body 21 is installed, the wiring on the controller body 21 can be inserted into the wiring groove 292 on the front of the rear plate 27 to arrange the wiring, effectively preventing it from becoming messy and making it easier for users to use.

[0044] An inert gas sealing box 61 (argon or nitrogen) is embedded in the top of the kiln 1. The box contains a corrugated expansion tube 60, one end of which is fixed to the top of the box, and the other end is connected to the carbon film contact plate 626 of the sliding rheostat 62. The temperature measuring tube at one end of the temperature controller 2 is an embedded armored thermocouple that extends along the furnace wall to the core area of ​​the combustion chamber. It collects temperature signals in real time and transmits them to the gas box pressure regulator on the inert gas sealing box 61. When the furnace temperature exceeds the limit, the inert gas expands due to heat, pushing the corrugated expansion tube 60 to extend, which causes the carbon film contact plate 626 to move on the track of the sliding rheostat. The power of the heating wire 5 is adjusted by the change in resistance value. After the deviation is eliminated, the corrugated expansion tube 60 contracts back. When the displacement of the corrugated expansion tube 60 causes the resistance value of the sliding rheostat 62 to change, the PID controller adjusts the current of the heating wire in real time (adjustment accuracy ±0.5A) to achieve a furnace temperature control accuracy of ±5℃. Thus, the principle of gas thermal expansion and contraction is used to drive the carbon film contact plate 626 to adjust the resistance value, thereby realizing closed-loop temperature control.

[0045] The present invention has been described in detail above. However, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, any modifications or improvements that do not depart from the spirit of the present invention are within the scope of protection of the present invention.

Claims

1. A kiln temperature control device for firing bone china tableware, comprising a kiln (1), characterized in that: The interior of the kiln (1) is equipped with a ceramic plate (4), the interior of the ceramic plate (4) is equipped with an electric heating wire (5), the top of the kiln (1) is equipped with a temperature control device (6), the rear air outlet of the kiln (1) is equipped with a temperature regulating mechanism (3), and the side of the kiln (1) is equipped with a temperature controller (2). The temperature controller (2) includes a connecting plate (22) and a rear plate (27). The connecting plate (22) is characterized in that: two U-shaped brackets (23) are sleeved on both sides of the connecting plate (22), and the number of U-shaped brackets (23) is two. Fasteners (24) are threaded on the front and back of the two U-shaped brackets (23). Fixing blocks (25) are fixedly installed on the side walls of the two U-shaped brackets (23), and bolts (26) are threaded on the side walls of the fixing blocks (25). The controller body (21) is sleeved between the two U-shaped brackets (23). The rear plate (27) has several sets of wiring grooves (292) on its front side. Side plates (28) are sleeved on both sides of the rear plate (27). A connecting rod (29) is fixedly installed on the front of the side plate (28), and one end of the connecting rod (29) passes through the fixing block (25) and is slidably connected to the fixing block (25).

2. The kiln temperature control device for firing bone china tableware according to claim 1, characterized in that: The top of the controller body (21) is provided with several heat dissipation strips (291). One end of the connecting plate (22) extends into the U-shaped card holder (23) and is slidably connected with the U-shaped card holder (23). One end of the fastener (24) extends into the U-shaped card holder (23) and is pressed against the outer wall of the connecting plate (22).

3. The kiln temperature control device for firing bone china tableware according to claim 2, characterized in that: One end of the bolt (26) extends into the fixing block (25) and is pressed against the outer wall of the connecting rod (29). The side wall of the rear plate (27) extends into the side plate (28) and is slidably connected to the side plate (28).

4. The kiln temperature control device for firing bone china tableware according to claim 1, characterized in that: The temperature regulating mechanism (3) includes a fan (30), which is fixedly connected to the rear side of the kiln (1). One end of the fan (30) is fixedly connected to an air outlet pipe (31), and one end of the air outlet pipe (31) is fixedly connected to an installation frame (32). A limit plate (33) is fixedly connected to the surface of the installation frame (32), and a baffle (34) is slidably connected inside the limit plate (33).

5. The kiln temperature control device for firing bone china tableware according to claim 4, characterized in that: A connecting frame (35) is fixedly connected to one side of the mounting frame (32), a motor (36) is fixedly connected to the surface of the connecting frame (35), a threaded column (38) is fixedly connected to the output end of the motor (36), a fixing plate (37) is threadedly connected to the surface of the threaded column (38), and a baffle (34) is fixedly connected to one end of the fixing plate (37).

6. The kiln temperature control device for firing bone china tableware according to claim 1, characterized in that: The temperature control unit (6) includes a corrugated telescopic tube (60), the corrugated telescopic tube (60) is fixedly connected to the top of the corrugated telescopic tube (60), an inert gas sealing box (61) is fixedly connected to the top of the corrugated telescopic tube (60), and a sliding rheostat (62) is fixedly connected to one end of the inert gas sealing box (61).

7. The kiln temperature control device for firing bone china tableware according to claim 6, characterized in that: The sliding rheostat (62) includes a bracket (620) and a protective sleeve (621) connected to the bracket (620). The protective sleeve (621) contains a slide rod (625) and a ceramic tube (622). The ceramic tube (622) is fixed to the center of the bracket (620) by a screw (628). A resistance wire (623) is sleeved around the ceramic tube (622). The slide rod (625) is fixed above the ceramic tube (622) by a fixing piece (627) with a terminal (629). A carbon film contact piece (626) that can move left and right is sleeved on the slide rod (625). A strip groove corresponding to the carbon film contact piece (626) is provided above the protective sleeve (621).

8. The kiln temperature control device for firing bone china tableware according to claim 7, characterized in that: The carbon film contact piece (626) extends out of the groove through the protective sleeve (621). The conductive brush connected below the carbon film contact piece (626) contacts the resistance wire (623). The conductive brush can slide on the surface of the resistance wire (623). A conductive piece (624) is connected to each of the left and right ends of the resistance wire (623). Each conductive piece (624) is connected to a terminal (629) fixed on the bracket (620).

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