阅读说明：本技术 一种3d打印磁性零件的双喷头装置及其工作方法 (Double-nozzle device for 3D printing of magnetic parts and working method thereof ) 是由 徐志强 陈�峰 唐志发 王晓东 陈柯文 薄新谦 姜胜强 张高峰 于 2021-05-20 设计创作，主要内容包括：本发明提供了一种3D打印磁性零件的双喷头装置及其工作方法。包括升降机构、升降平台、充磁线圈和两个挤出头。其中,一个挤出头固定在升降机构上,另一个挤出头固定在升降平台上,升降机构可实现升降平台的Z向移动。本发明采用边打印边充磁的方式,打印过程中可以快速改变充磁方向,实现对充磁区域和充磁方向的控制。同时,本发明采用两个挤出头,分别采用适合打印的聚合物和复合材料(聚合物和磁性颗粒)的打印丝,可以实现只有复合材料打印的部分被充磁,而用聚合物打印的部分没有磁性。因此,本发明可实现对磁性材料形状、充磁方向、充磁区域的自定义,从而获得复杂形状、特殊磁感线排布、分区充磁的磁性零件。(The invention provides a double-nozzle device for 3D printing of magnetic parts and a working method thereof. Comprises a lifting mechanism, a lifting platform, a magnetizing coil and two extrusion heads. One of the extrusion heads is fixed on the lifting mechanism, the other extrusion head is fixed on the lifting platform, and the lifting mechanism can realize the Z-direction movement of the lifting platform. The invention adopts a mode of printing and magnetizing, can quickly change the magnetizing direction in the printing process, and realizes the control of the magnetizing area and the magnetizing direction. Meanwhile, the invention adopts two extrusion heads and respectively adopts printing wires of polymer and composite material (polymer and magnetic particles) which are suitable for printing, so that only the part printed by the composite material is magnetized, and the part printed by the polymer has no magnetism. Therefore, the invention can realize the self-definition of the shape, the magnetizing direction and the magnetizing area of the magnetic material, thereby obtaining the magnetic part with a complex shape, special magnetic induction line arrangement and partitioned magnetizing.)

1. The utility model provides a 3D prints magnetism part's dual spray device which characterized in that: comprises a lifting mechanism (1), a lifting platform (2), an extrusion head A (3), an extrusion head B (4) and a magnetizing coil (5); the extrusion head A (3) is fixed below the lifting platform through a screw, and the extrusion head B (4) is fixed on the lifting mechanism through a screw; the lifting platform (2) is provided with 1 threaded hole and 4 through holes and is arranged on the lifting mechanism (1); the lifting mechanism (1) drives the lifting platform (2) to move in the Z-axis direction; the magnetizing coil (5) is fixed on the lower panel of the lifting mechanism (1) through a screw and covers a heating area of an extrusion head B (3) fixed on the lower panel of the lifting mechanism.

2. The dual nozzle device for 3D printing of magnetic parts according to claim 1, wherein: the lifting mechanism (1) comprises a lifting motor (1-1), a lifting frame (1-2), a ball screw (1-3) and a linear guide rod (1-4); each panel of the lifting frame (1-2) is provided with a plurality of threaded holes; the lifting motor (1-1) is fixed on the upper panel of the lifting frame (1-2) through screws; the ball screw (1-3) is fixed inside the lifting frame (1-2) along the Z-axis direction, penetrates through the upper panel of the lifting frame (1-2), and is connected with an output shaft of the lifting motor (1-1) through a coupler; the linear guide rods (1-4) are provided with 4 pieces along the Z axis and fixed in the lifting frame (1-2).

3. The dual nozzle device for 3D printing of magnetic parts according to claim 1, wherein: the extrusion head A (3) and the extrusion head B (4) have the same internal structure and comprise throats (3-1), fixing sleeves (3-2), heat dissipation assemblies (3-3), connecting pipes (3-4), heating assemblies (3-5) and nozzles (3-6); the heat dissipation assembly (3-3) is fixed on the fixed sleeve (3-2); threads are arranged at two ends of the outer surface of the connecting pipe (3-4); the heating component (3-5) is connected with the heat dissipation component through a connecting pipe (3-4); the throat pipe (3-1) penetrates through the fixed sleeve (3-2) and the heat dissipation assembly (3-3), and the outlet end of the throat pipe is fixed inside the connecting pipe (3-4); the nozzles (3-6) are fixed on the heating components (3-5) through threads.

4. The dual nozzle device for 3D printing of magnetic parts according to claim 3, wherein: the heat dissipation assembly (3-3) comprises a heat dissipation cylinder (3-3-1) and a liquid cooling pipeline (3-3-2); the inside of the heat dissipation cylinder (3-3-1) is provided with threads; the liquid cooling pipeline (3-3-2) is spirally wound outside the heat dissipation cylinder (3-3-1) and embedded in a groove outside the heat dissipation cylinder (3-3-1).

5. The dual nozzle device for 3D printing of magnetic parts according to claim 3, wherein: the heating assembly (3-5) comprises a heating cavity (3-5-1), a heating wire (3-5-2) and a temperature sensor (3-5-3); the heating cavity (3-5-1) is internally provided with threads; the heating wire (3-5-2) is spirally fixed in the heating cavity (3-5-1); the temperature sensor (3-5-3) is fixed on one side of the heating cavity (3-5-1) through threads and penetrates through the shell of the heating cavity (3-5-1).

6. The working method of the dual nozzle device for 3D printing of the magnetic parts according to claim 1 is implemented by the following steps:

step 1: establishing a three-dimensional model of the magnetic part by using modeling software such as 3DS MAX and the like, and finishing magnetic field arrangement;

step 2: slicing the magnetic part by using slicing software, and importing the magnetic part into a 3D printer control system;

and step 3: mounting a printing filament made of a polymer suitable for 3D printing to extrusion head a, mounting a printing filament of a composite material (polymer and magnetic particles) to extrusion head B;

and 4, step 4: and starting the 3D printer, introducing cooling liquid into the liquid cooling pipeline (3-3-2), and radiating the extrusion head and the used printing material. The heating component (3-3) starts to be electrified to preheat the printing wire. After the temperature sensor (3-5-3) displays that the temperature of the printing wire reaches the temperature required by the molten material, the extrusion head A (3) moves to a set initial printing position to start printing;

and 5: when the area to be printed is a magnetic area, the computer controls the lifting motor (1-1) to rotate reversely, the position of the extrusion head A (3) is raised to be higher than that of the extrusion head B (4) in a horizontal line, the extrusion head B (4) is called to print, and meanwhile, the magnetizing coil (5) is electrified according to the set magnetic field direction to perform magnetizing and printing on the composite material;

step 6: when the area to be printed is a non-magnetic area, the computer controls the lifting motor (1-1) to rotate positively, the extrusion head A (3) descends to a position where the horizontal line is lower than the extrusion head B (4), and the extrusion head A (3) is called to print;

and 7: and repeating the step 5 and the step 6 for each layer of printing until the magnetic part is printed.

Technical Field

The invention relates to the technical field of 3D printing, in particular to a double-nozzle device for 3D printing of magnetic parts and a working method thereof.

Background

3D printing is a rapid prototyping technique, also known as additive manufacturing, which is a technique that builds objects by using bondable materials such as powdered metal or plastic and the like and printing layer by layer on the basis of a digital model file. The printing head is a core component of the 3D printer, the existing printing head device for printing the magnetic material in the 3D mode can only print a single material, the material needs to be magnetized after printing is completed, and the magnetic part with special magnetic induction line arrangement and partition magnetization is difficult to produce. In the prior art, when a certain area of a magnetic material is locally magnetized, the distribution of magnetic induction lines and the direction of a magnetic field of other magnetized areas are influenced, and the magnetism of a non-magnetized area is also influenced.

At present, related devices and methods for producing magnetic parts with complex shapes, special magnetic induction line arrangement and partitioned magnetization in a 3D printing mode are few, and how to produce the magnetic parts is a technical problem which is difficult to solve.

Disclosure of Invention

The invention provides a dual-nozzle device for 3D printing of magnetic parts. The two extrusion heads are respectively used for printing wires made of polyamide and composite materials (polyamide and rubidium, iron and boron), so that only the composite materials are magnetized, and the part printed by the polymer is not magnetic, and the part in any area of the part can be magnetized according to the requirement.

The second purpose of the invention is to provide a working method of the double-nozzle device for 3D printing of the magnetic parts. The mode of 3D printing and magnetizing is adopted, the current direction of a magnetizing coil can be changed in the printing process, the magnetizing direction can be changed quickly in the printing process, and the magnetizing in the small area direction which is difficult to realize in the traditional production mode is realized, so that the self-definition of the shape, the magnetizing direction and the magnetizing area of a magnetic material is realized, and the magnetic part with a complicated shape, special magnetic induction line arrangement and partitioned magnetizing is obtained.

The technical scheme adopted by the invention for realizing the first invention is as follows: a double-nozzle device for 3D printing of magnetic parts comprises a lifting mechanism, a lifting platform, an extrusion head A, an extrusion head B and a magnetizing coil; the extrusion head A is fixed below the lifting platform through a screw, and the extrusion head B is fixed on the lifting mechanism through a screw; the lifting platform is provided with 1 threaded hole and 4 through holes and is arranged on the lifting mechanism; the lifting mechanism drives the lifting platform to move in the Z-axis direction; the magnetizing coil is fixed on the lower panel of the lifting mechanism through a screw and covers the extrusion head B fixed on the lower panel of the lifting mechanism.

Further characterized in that:

the lifting mechanism comprises a lifting motor, a lifting frame, a ball screw and a linear guide rod; each panel of the lifting frame is provided with a plurality of threaded holes; the lifting motor is fixed on the upper panel of the lifting frame through screws; the ball screw is fixed inside the lifting frame along the Z-axis direction, penetrates through the upper panel of the lifting frame and is connected with an output shaft of the lifting motor through a coupler; the linear guide rod is provided with 4 pieces along the Z axis and is fixed inside the lifting frame.

Furthermore, the internal structures of the extrusion head A and the extrusion head B are the same, and the extrusion head A and the extrusion head B comprise a throat pipe, a fixed sleeve, a heat dissipation assembly, a connecting pipe, a heating assembly and a nozzle; the heat dissipation assembly is fixed on the fixing sleeve; threads are formed at two ends of the outer surface of the connecting pipe; the heating assembly is connected with the radiating assembly through a connecting pipe; the throat pipe penetrates through the fixing sleeve and the heat dissipation assembly, and the outlet end of the throat pipe is fixed in the connecting pipe; the nozzle is fixed on the heating component through threads.

Further, the heat dissipation assembly comprises a heat dissipation cylinder and a liquid cooling pipeline; threads are formed in the heat dissipation cylinder; the liquid cooling pipeline is spirally wound outside the heat dissipation cylinder and embedded in the groove outside the heat dissipation cylinder, and is used for reducing the temperature of the printing wire and the device.

Further, the heating assembly comprises a heating cavity, a heating wire and a temperature sensor; the heating cavity is internally provided with threads; the heating wire is spirally fixed in the heating cavity and used for heating the printing wire to a molten state; the temperature sensor is fixed on one side of the heating cavity and penetrates through the heating cavity shell to control the heating temperature.

The invention adopts the technical scheme that the second invention is realized by the following steps: the working method of the double-nozzle device for 3D printing of the magnetic part is characterized by comprising the following steps:

step 1: establishing a three-dimensional model of the magnetic part by using modeling software such as 3DS MAX and the like, and finishing magnetic field arrangement;

step 2: slicing the magnetic part by using slicing software, and importing the magnetic part into a 3D printer control system;

and step 3: mounting a printing wire made of polymer to an extrusion head A, and mounting a printing wire made of composite material (magnetic particles and polymer) to an extrusion head B;

and 4, step 4: the 3D printer is started, cooling liquid is introduced into the liquid cooling pipeline, and the extrusion head and the used printing material are cooled. The heating assembly starts to be electrified to preheat the printing wire. After the temperature sensor displays that the temperature of the printing wire reaches the temperature required by the molten material, the extrusion head A moves to a set initial printing position to start printing;

and 5: when the area to be printed is a magnetic area, the computer controls the lifting motor to rotate reversely, the position of the extrusion head A is raised to be higher than that of the extrusion head B in a horizontal line, the extrusion head B is called to print, and meanwhile, the magnetizing coil is electrified according to the set magnetic field direction, so that the composite material is magnetized and printed;

step 6: when the area to be printed is a non-magnetic area, the computer controls the lifting motor to rotate positively, the extrusion head A is lowered to a position lower than the extrusion head B in a horizontal line, and the extrusion head A is called to print;

and 7: and repeating the step 5 and the step 6 for each layer of printing until the magnetic part is printed.

The invention has the beneficial effects that:

according to the invention, two extrusion heads are adopted, and printing wires made of polymer and composite material (polymer and rubidium, iron and boron) are respectively adopted, so that only the composite material is magnetized, and the part printed by the polymer has no magnetism, thus any area of a part can be precisely magnetized according to the requirement;

the invention adopts a mode of 3D printing and magnetizing, changes the current direction of the magnetizing coil, can change the direction of a magnetic field, realizes the rapid change of the magnetizing direction in the printing process, and compared with the traditional technology for manufacturing magnetic parts, realizes the magnetizing in different directions in a small area, thereby realizing the self-definition of the shape, the magnetizing direction and the magnetizing area of a magnetic material, and obtaining the magnetic parts with complicated shape, special magnetic induction line arrangement and subarea magnetizing.

Drawings

FIG. 1 is an overall three-dimensional schematic of the present invention;

FIG. 2 is a three-dimensional schematic view of the lifting mechanism of the present invention;

FIG. 3 is a three-dimensional schematic view of an extrusion head of the present invention;

FIG. 4 is a three-dimensional schematic view of a heat dissipation assembly of the present invention;

FIG. 5 is a three-dimensional schematic view of a heating assembly of the present invention;

FIG. 6 is a schematic view of a transition of the extrusion head of the present invention;

FIG. 7 is a schematic view of the printed product of the present invention;

in the drawings: (1) the device comprises a lifting mechanism, a lifting platform, an extrusion head A, an extrusion head B, a magnetizing coil, a lifting motor, a ball screw, a linear guide rod, a throat pipe, a fixed sleeve, a heat dissipation component, a connecting pipe, a heating component, a nozzle, a heat dissipation component, a liquid cooling pipeline, a heating cavity, a heating wire and a temperature sensor, wherein the lifting mechanism is used, the lifting platform is used, the extrusion head A is used, the extrusion head B is used, the magnetizing coil is used, the lifting motor is used, the ball screw is used, the linear guide rod is used, the throat pipe is used, the fixing sleeve is used, the heat dissipation component is used, the heating component is used, the connecting pipe is used, the heating component is used, the nozzle is used, the heating component is used, the nozzle is used, the heating pipe is used, the cooling is used, the heating pipe is used, the throat pipe is used, the heating pipe is used, the 3-3-2) is used for cooling the liquid cooling pipe, the heating cavity is used for 3-5-1, the heating wire, and the temperature sensor is used for the heating wire, the heating wire is used for heating wire, and the temperature sensor.

Detailed Description

The invention is further described below with reference to the accompanying drawings.

As shown in fig. 1, the dual-nozzle device for 3D printing of magnetic parts comprises a lifting mechanism (1), a lifting platform (2), an extrusion head a (3), an extrusion head B (4), and a magnetizing coil (5). Specifically, the extrusion head a (3) is fixed on the lifting platform through screws and penetrates through the lower panel of the lifting frame. The extrusion head B (4) is fixed on the lifting mechanism through screws and penetrates through the lower panel of the lifting frame. The lifting platform (2) is provided with 1 threaded hole and 4 through holes and is arranged on a ball screw and 4 linear guide rods of the lifting mechanism (1). The lifting mechanism (1) drives the lifting platform (2) to move in the Z-axis direction so as to realize the switching of the printing head in the printing process. The magnetizing coil (5) is fixed on the lower panel of the lifting mechanism (1) through 4 screws, covers the heating area of the extrusion head B (3) and is used for magnetizing the printing wire in a molten state.

The lifting mechanism (1) is of an integrated structure, the structure of which is shown in figure 2, and concretely comprises a lifting motor (1-1), a lifting frame (1-2), a ball screw (1-3) and a linear guide rod (1-4). The upper, middle and lower panels of the lifting frame (1-2) are respectively provided with 4 threaded holes, and the lifting motor (1-1) adopts a stepping motor and is fixed on the upper panel of the lifting frame (1-2) through 4 screws. The ball screw (1-3) is fixed inside the lifting frame (1-2) along the Z-axis direction, penetrates through the lifting platform (2) and the upper panel of the lifting frame (1-2), and is connected with the output shaft of the lifting motor (1-1) through a coupler. The linear guide rods (1-4) are provided with 4 pieces along the Z axis and fixed inside the lifting frame (1-2) for ensuring the stability of the lifting platform (2).

The extrusion head A (3) and the extrusion head B (4) have the same structure, and the structure is shown in figure 3, and specifically comprises a throat pipe (3-1), a fixed sleeve (3-2), a heat dissipation assembly (3-3), a connecting pipe (3-4), a heating assembly (3-5) and a nozzle (3-6). The heat dissipation assembly (3-3) is fixed on the fixing sleeve (3-2) and used for cooling the printing wire and the device and preventing the pipeline from being blocked due to backflow of materials in a molten state. Threads are arranged at two ends of the outer surface of the connecting pipe (3-4), and the heating component (3-5) is connected with the heat dissipation component through the connecting pipe (3-4) and used for heating the printing wire to a molten state. The throat pipe (3-1) is made of polytetrafluoroethylene, penetrates through the fixed sleeve (3-2) and the heat dissipation assembly (3-3), and the outlet end of the throat pipe is fixed inside the connecting pipe (3-4). The nozzles (3-6) are fixed on the heating components (3-5) through threads.

The structure of the heat dissipation assembly (3-3) is shown in fig. 4, and specifically comprises a heat dissipation cylinder (3-3-1) and a liquid cooling pipeline (3-3-2). The liquid cooling pipeline (3-3-2) is spirally wound outside the heat dissipation cylinder (3-3-1) and embedded in a groove outside the heat dissipation cylinder (3-3-1), and cooling liquid is introduced into the pipeline during printing.

The structure of the heating component (3-5) is shown in fig. 5, and specifically comprises a heating cavity (3-5-1), a heating wire (3-5-2) and a temperature sensor (3-5-3). The heating wire (3-5-2) is spirally fixed in the heating cavity (3-5-1). The temperature sensor (3-5-3) is fixed on one side of the heating cavity (3-5-1) and penetrates through the shell of the heating cavity (3-5-1) for controlling the heating temperature.

As shown in fig. 6, which is a schematic diagram of the conversion of extrusion head a (3) to extrusion head B (4), the white areas of the printed magnetic part are non-magnetic areas (6-1) and are printed by extrusion head a (3) using a polymer material; the black area of the printed magnetic part is a magnetic area (6-2) and is printed by an extrusion head B (4) using a composite material. When the region to be printed is the magnetic region (6-2), the lifting motor (1-1) drives the ball screw (1-3) to rotate reversely, the position of the extrusion head A (3) is lifted to be higher than that of the extrusion head B (4) in a horizontal line, the lifting frame (1) moves horizontally and rightwards and then moves vertically downwards, a nozzle of the extrusion head B (4) is aligned to the position to be printed, and meanwhile, the magnetizing coil (5) is electrified according to the set magnetic field direction to start printing the magnetic region (6-2). The extrusion head B (4) is switched to extrusion head A (3) as the reverse of this process.

The working method of the double-nozzle device for 3D printing of the magnetic part is characterized by comprising the following steps:

step 1: establishing a three-dimensional model of the magnetic part by using modeling software such as 3DS MAX and the like, and finishing magnetic field arrangement;

step 2: slicing the magnetic part by using slicing software, and importing the magnetic part into a 3D printer control system;

and step 3: mounting a printing filament of polymer material to an extrusion head a, and a printing filament of composite material (polymer and magnetic particles) to an extrusion head B;

and 4, step 4: and starting the 3D printer, introducing cooling liquid into the liquid cooling pipeline (3-3-2), and radiating the extrusion head and the used printing material. The heating component (3-3) starts to be electrified to preheat the printing wire. After the temperature sensor (3-5-3) displays that the temperature of the printing wire reaches the temperature required by the molten material, the extrusion head A (3) moves to a set initial printing position to start printing;

and 5: when the area to be printed is a magnetic area, the computer controls the lifting motor (1-1) to rotate reversely, the position of the extrusion head A (3) is raised to be higher than that of the extrusion head B (4) in a horizontal line, the extrusion head B (4) is called to print, and meanwhile, the magnetizing coil (5) is electrified according to the set magnetic field direction to perform magnetizing and printing on the composite material;

step 6: when the area to be printed is a non-magnetic area, the computer controls the lifting motor (1-1) to rotate positively, the extrusion head A (3) descends to a position where the horizontal line is lower than the extrusion head B (4), and the extrusion head A (3) is called to print;

and 7: and repeating the step 5 and the step 6 for each layer of printing until the magnetic part is printed.

Examples

The double-nozzle device for 3D printing of magnetic parts according to the invention shown in FIG. 1 is used for printing, and taking the finished product shown in FIG. 7 as an example, the working method is as follows:

firstly, a three-dimensional model of the magnetic part shown in fig. 6 is created by using modeling software such as 3DS MAX, selected areas are set as magnetic materials, and the magnetic field direction of each area is set to complete magnetic field arrangement;

secondly, slicing the magnetic part by using slicing software MPrint, converting the file in the STL format exported from the modeling software into a file in an X3G format which can be directly identified by a 3D printer, and copying the file to a control system of the 3D printer by using a U disk;

mounting a printing wire made of a polymer material to an extrusion head A, and mounting a printing wire made of a composite material (the material volume fraction ratio in the embodiment is neodymium iron boron: polyamide is 4: 6, and the printing wire is prepared by heating and mixing neodymium iron boron powder and a polyamide raw material through a wire making machine) to an extrusion head B;

and fourthly, starting the 3D printer, introducing cooling liquid into the liquid cooling pipeline, and radiating the extrusion head and the used printing material. The heating assembly starts to be electrified to preheat the printing wire. After the temperature sensor displays that the temperature of the printing wire reaches the temperature required by the molten material, the extrusion head A moves to a set initial printing position to start printing;

fifthly, when the area to be printed is a magnetic area, the computer controls the lifting motor to rotate reversely, the position of the extrusion head A is raised to be higher than that of the extrusion head A in the horizontal line, the extrusion head B is called and moved to the printing position without influencing the normal work of the extrusion head A, and meanwhile, the magnetizing coil is electrified according to the set magnetic field direction to perform magnetizing and printing on the composite material;

when the area to be printed is a non-magnetic area, the computer controls the lifting motor to rotate forwards, the extrusion head A is lowered to a position lower than the extrusion head B (note that the printing head is lifted at the same time when the extrusion head B is lowered so as to prevent the nozzle from colliding with the printed magnetic part), the working position of the extrusion head A is not influenced by the extrusion head A, the extrusion head A is called, and the extrusion head A is moved to the printing position for printing;

seventhly, the method comprises the following steps: repeating the fifth step and the sixth step for each layer of printing until the magnetic part shown in FIG. 7 is printed.

Through the steps, the double-nozzle device for 3D printing of the magnetic part prints the magnetic part shown in FIG. 7. S denotes the S pole of the region, and N denotes the N pole of the region. In fig. 7, the S pole and the N pole of each cylindrical magnetic region are indicated, and the magnetic regions with different region sizes and magnetic field directions are arranged on the magnetic component according to a certain rule to obtain a special magnetic induction line arrangement. The areas outside the magnetic area are printed by polyamide and have no magnetism, so the magnetic induction line arrangement is not influenced.

In the above embodiment, two extrusion heads are used, and printing wires of polyamide and composite material (with a volume fraction ratio of rubidium, iron and boron and polyamide being 4: 6) are used respectively, so that only the composite material is magnetized, and the part printed by the polyamide has no magnetism, and any specific area of the part is precisely magnetized as required. The 3D printing and magnetizing mode is adopted, the current direction of the magnetizing coil is changed, the direction of a magnetic field can be changed, and the quick change of the magnetizing direction in the printing process is realized. Finally, the magnetic part with the magnetizing direction and the magnetizing area defined by user is obtained as shown in fig. 7, and special magnetic induction line arrangement is obtained.

In addition, the embodiment of the invention is not limited to the embodiment, for example, the polymer material suitable for printing can be PLA, ABS, nylon, and the like, and the magnetic particles can be permanent magnetic ferrite, rare earth permanent magnetic material, composite permanent magnetic material, and the like, for example, by customizing the modeling shape, magnetic field distribution, and magnetic field direction, magnetic parts with various complex shapes, special magnetic induction line arrangement, and partitioned magnetization can be obtained according to the requirements, and the invention has a wide application prospect in the aspect of customized production of magnetic parts.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications can be made without departing from the technical principle of the present invention, and these modifications should also be regarded as the protection scope of the present invention.