阅读说明：本技术 电机定子及其极靴加工方法、永磁电机 (Motor stator, pole shoe machining method thereof and permanent magnet motor ) 是由 高东东 吴广荣 陈华杰 于 2019-10-22 设计创作，主要内容包括：本申请提供一种电机定子及其极靴加工方法、永磁电机。该电机定子包括轭部(1)、极靴(2)和定子齿(3),在垂直于轭部(1)的中心轴线的截面内,至少部分极靴(2)具有削边,具有削边的极靴(2)的径向内周壁包括圆弧段(4)和位于圆弧段(4)直线段(5),直线段(5)和圆弧段(4)的交点为削极点I,其中一端的削极点I和定子中心O的连线为OI,OI与OH所形成的夹角为φ,θ≥φ≥θ/2,θ＝α-β,其中α为相邻两个定子齿(3)的中心线之间的夹角,β＝π/P,P为转子极数。根据本申请的电机定子,能够改善电机气隙的磁场分布,大幅度地降低反电动势谐波含量,提高反电动势正弦化程度。(The application provides a motor stator, a pole shoe machining method thereof and a permanent magnet motor. The motor stator comprises a yoke portion (1), pole shoes (2) and stator teeth (3), at least part of the pole shoes (2) are provided with chamfered edges in a section perpendicular to the central axis of the yoke portion (1), the radial inner peripheral wall of the pole shoes (2) with the chamfered edges comprises an arc section (4) and a straight section (5) located on the arc section (4), the intersection point of the straight section (5) and the arc section (4) is a chamfered point I, the connecting line of the chamfered point I at one end and the center O of a stator is OI, the included angle formed by the OI and OH is phi, theta phi is larger than or equal to theta/2, theta is alpha-beta, alpha is the included angle between the central lines of two adjacent stator teeth (3), beta is pi/P, and P is the number of the rotor poles. According to the motor stator, the magnetic field distribution of the motor air gap can be improved, the counter electromotive force harmonic content is greatly reduced, and the sine degree of the counter electromotive force is improved.)

1. A motor stator is characterized by comprising a yoke (1), pole shoes (2) and stator teeth (3), wherein the stator teeth (3) are connected between the yoke (1) and the pole shoes (2), at least part of the pole shoes (2) are provided with chamfered edges in a section perpendicular to the central axis of the yoke (1), the radial inner peripheral wall of the pole shoes (2) with the chamfered edges comprises circular arc sections (4) extending along the circumferential direction and straight line sections (5) positioned at two ends of the circular arc sections (4), the two straight line sections (5) are symmetrical relative to the central line OH of the stator teeth (3) where the pole shoes (2) are positioned, the intersection point of the straight line sections (5) and the circular arc sections (4) is a chamfered point I, the connecting line of the chamfered point I at one end and the stator center O is OI, and the included angle formed by the OI and the central line OH of the stator teeth (3) is phi, theta is larger than or equal to phi and larger than or equal to theta/2, theta is alpha-beta, wherein alpha is an included angle between center lines of two adjacent stator teeth (3), beta is pi/P, and P is the number of poles of the rotor.

2. The electric machine stator of claim 1, wherein Φ is 3 θ/4.

3. The stator of the motor according to claim 1, wherein the pole shoe (2) further comprises radial side edges (6) located at two circumferential ends of the radially inner circumferential wall, an intersection point between an extension line of the radial side edge (6) and the inner circle of the stator is G, and an included angle δ is formed between a cutting line segment IG ' and a central line OH, wherein δ 2 is greater than or equal to δ > δ 1, δ 1 is a theoretical minimum value of an included angle formed between IG and the central line OH, δ 2 is a theoretical maximum value of an included angle formed between IG ' and the central line OH, δ 2 is 90 ° + θ/2, and G ' is an intersection point between the cutting line segment and the radial side edge (6) of the pole shoe (2) when the included angle δ is at the theoretical maximum value.

4. A machine stator according to claim 3, wherein δ is 90 °.

5. Stator for an electric machine according to claim 1, characterized in that each pole shoe (2) has a chamfered edge.

6. The electric machine stator according to claim 1, further comprising stator windings wound around the stator teeth (3), wherein the pole shoes (2) corresponding to one phase stator winding each have a chamfered edge; or the motor stator further comprises stator windings wound on the stator teeth (3), wherein the pole shoes (2) corresponding to the two-phase stator windings are provided with chamfered edges.

7. A permanent magnet electric machine comprising an electric machine stator and an electric machine rotor (7), said electric machine rotor (7) being arranged inside said electric machine stator, characterized in that said electric machine stator is an electric machine stator according to any of claims 1-6.

8. A method of machining pole shoes (2) of stators of dynamoelectric machines, as claimed in any one of claims 1 to 6, comprising:

determining a pole shoe (2) needing edging;

determining critical pole cutting points E 'and E' on the pole shoe (2) in a section perpendicular to the central axis of the yoke part (1), wherein E 'and E' are intersection points of two corner lines of one magnetic pole of the motor rotor (7) and the inner circle of the stator;

determining a connecting line OE ', OE' of one critical pole cutting point E 'and a stator center O, wherein an included angle between the OE' and a center line OH 'of a stator tooth (3) corresponding to the critical pole cutting point E' is theta, and theta is ∠ E 'OH' ═ alpha-beta, wherein alpha is an included angle between center lines of two adjacent stator teeth (3), beta is pi/P, and P is the number of rotor poles;

determining ∠ EOH theta alpha beta according to the space uniform distribution and symmetry;

determining a pole-cutting point I, wherein an included angle between an OI connecting line of the pole-cutting point I and the center O of the stator and a central line OH of the stator teeth (3) corresponding to the pole shoe (2) is phi, theta is not less than phi and not less than theta/2, when phi is theta/2, an intersection point between the OI and the inner circle of the stator is F, and the track of the pole-cutting point I is an arc EF.

9. Method for machining a pole piece (2) of a stator of an electric machine, according to claim 8, characterized in that the step of determining the pole-clipping point I comprises: and determining the middle point I 'of the arc EF, wherein the included angle phi between the radial line where the pole cutting point I' is located and the central line OH of the stator teeth (3) is 3 theta/4.

10. A method for machining a pole shoe (2) of a stator of an electric machine according to claim 8, characterized in that the method for machining a pole shoe (2) further comprises:

determining an intersection point G of the radial side (6) of the pole shoe (2) and the inner circle of the stator;

determining an included angle delta 1 between the line segment IG and the central line OH of the stator teeth (3), wherein the delta 1 is a theoretical minimum value of the included angle between a cutting line segment IG' passing through the pole cutting point and the central line OH of the stator teeth (3);

and cutting and edging the inner peripheral wall of the pole shoe (2) by using a cutting line segment IG ', wherein an included angle delta between the cutting line segment IG' and a central line OH of the stator teeth (3) is larger than delta 1.

11. A method for processing a pole shoe (2) of a stator of an electric machine according to claim 10, wherein the step of cutting and edging the inner circumferential wall of the pole shoe (2) by using a cutting line segment IG' further comprises the following steps:

determining a theoretical maximum value delta 2 of an angle between the cutting line segment IG' and a central line OH of the stator teeth (3), wherein delta 2 is 90 DEG + theta/2,

determining an intersection point G ' of the cutting line segment IG ' and the radial side edge (6) of the pole shoe (2) when the included angle between the cutting line segment IG ' and the central line OH of the stator teeth (3) is delta 2;

such that G 'is located between G and G'.

12. A method of manufacturing a pole piece (2) of a stator of an electric machine, as claimed in claim 11, characterized in that δ is 90 °.

Technical Field

The application relates to the technical field of motors, in particular to a motor stator, a pole shoe machining method of the motor stator and a permanent magnet motor.

Background

The permanent magnet motor is widely applied to the refrigeration compressor due to simple structure and good running performance. In the traditional motor structural design, the inner diameter and the outer diameter of the stator are designed into equal circles, so that the design is convenient to process, the size inspection is convenient, and the control on the processing size is technically convenient.

However, the requirements of the existing refrigeration compressors including piston compressors on noise and vibration are higher, low noise and low vibration are the development trend of the existing compressors, the defects of non-sinusoidal air gap flux density waveform, high back electromotive force waveform harmonic distortion rate and the like exist in the traditional motor stator inner diameter equal circle design, the existing technical requirements cannot be met, and the traditional motor back electromotive force waveform is poor in sinusoidal degree. Although the harmonic distortion rate of the motor can be reduced by adopting the stator chute, the process of adopting the stator chute is complex, the quality control of the stator is difficult, and the production cost is high. With the development of motor technology, the inner diameter of the stator is adjusted from a traditional full-circle arc to a form of a line segment and an arc, namely, a partial arc is chamfered, the form can effectively adjust the pole arc coefficient of the motor and optimize the waveform of the counter electromotive force of the motor, but the harmonic content of the counter electromotive force is increased due to the improper line segment determining mode, so that the noise and vibration performance of the motor during operation are poor.

Disclosure of Invention

Therefore, the technical problem to be solved by the present application is to provide a motor stator, a pole shoe processing method thereof, and a permanent magnet motor, which can improve the magnetic field distribution of the motor air gap, greatly reduce the content of counter electromotive force harmonic waves, and improve the sinusoidal degree of the counter electromotive force.

In order to solve the problems, the application provides a motor stator which comprises a yoke part, pole shoes and stator teeth, wherein the stator teeth are connected between the yoke part and the pole shoes, in a section perpendicular to the central axis of the yoke part, at least part of the pole shoes are provided with chamfered edges, the radial inner peripheral wall of each pole shoe with the chamfered edges comprises an arc section extending along the circumferential direction and straight line sections positioned at two ends of the arc section, the two straight line sections are symmetrical about the central line OH of the stator tooth where the pole shoe is positioned, the intersection point of the straight line sections and the arc sections is a chamfered point I, the connecting line of the chamfered point I at one end and the center O of a stator is OI, the included angle formed by the OI and the central line OH of the stator teeth is phi, the phi is not less than or equal to theta/2, the theta phi is not less than or equal to alpha-beta, the alpha is the included angle between the.

Preferably, Φ is 3 θ/4.

Preferably, the pole shoe further comprises radial side edges located at two circumferential ends of the radial inner circumferential wall, an intersection point between an extension line of the radial side edge and the inner circle of the stator is G, an included angle δ is formed between the cutting line segment IG' and the central line OH, δ 2 is greater than or equal to δ > δ 1, δ 1 is a theoretical minimum value of the included angle formed between IG and the central line OH, δ 2 is a theoretical maximum value of the included angle formed between IG ″ and the central line OH, δ 2 is 90 ° + θ/2, and G "is the intersection point between the cutting line segment and the radial side edge of the pole shoe when the included angle δ is at the theoretical maximum value.

Preferably, δ is 90 °.

Preferably, each pole shoe has a chamfered edge.

Preferably, the motor stator further comprises stator windings wound on the stator teeth, wherein the pole shoes corresponding to the stator windings of one phase are provided with chamfered edges; or the motor stator further comprises stator windings wound on the stator teeth, wherein pole shoes corresponding to the two-phase stator windings are provided with chamfered edges.

According to another aspect of the present application, there is provided a permanent magnet motor, including a motor stator and a motor rotor, the motor rotor is disposed in the motor stator and forms an air gap with the motor stator, and the motor stator is the above-mentioned motor stator.

According to another aspect of the present application, there is provided a method for processing pole shoes of a stator of an electric machine, including:

determining pole shoes needing edging;

determining critical pole cutting points E 'and E' on the pole shoe in a section perpendicular to the central axis of the yoke part, wherein the E 'and the E' are intersection points of two corner lines of one magnetic pole of the motor rotor and the inner circle of the stator;

determining a connecting line OE ' between one critical pole cutting point E ' and the stator center O, wherein an included angle between the OE ' and a stator tooth center line OH ' corresponding to the critical pole cutting point E ' is theta, and the theta is ∠ E ' OH ' ═ alpha-beta, wherein alpha is an included angle between center lines of two adjacent stator teeth, beta is pi/P, and P is the number of rotor poles;

determining ∠ EOH theta alpha beta according to the space uniform distribution and symmetry;

and determining a pole cutting point I, wherein an included angle between OI and a central line OH of the stator tooth corresponding to the pole shoe is phi, theta is not less than phi and not less than theta/2, when phi is not less than theta/2, an intersection point between OI and the inner circle of the stator is F, and the track of the pole cutting point I is an arc EF.

Preferably, the step of determining the pole-clipping point I comprises: and determining the middle point I 'of the arc EF, wherein the included angle phi between the radial line where the pole cutting point I' is located and the central line OH of the stator teeth is 3 theta/4.

Preferably, the pole shoe machining method further comprises:

determining an intersection point G of the radial side edge of the pole shoe and the inner circle of the stator;

determining an included angle delta 1 between the line segment IG and the central line OH of the stator teeth, wherein the delta 1 is a theoretical minimum value of the included angle between the cutting line segment IG' passing through the pole cutting point and the central line OH of the stator teeth;

and cutting and edging the inner peripheral wall of the pole shoe by using a cutting line segment IG ', wherein an included angle delta between the cutting line segment IG' and a central line OH of the stator teeth is larger than delta 1.

Preferably, the step of performing edge cutting on the inner peripheral wall of the pole shoe by using the cutting line segment IG' further comprises the following steps:

determining a theoretical maximum value delta 2 of the angle between the cutting line segment IG' and the center line OH of the stator teeth, where delta 2 is 90 ° + θ/2,

determining an intersection point G ' of the cutting line segment IG ' and the radial side edge of the pole shoe when the included angle between the cutting line segment IG ' and the central line OH of the stator teeth is delta 2;

such that G 'is located between G and G'.

Preferably, the first and second electrodes are formed of a metal,

δ＝90°。

the stator of the motor comprises a yoke, pole shoes and stator teeth, wherein the stator teeth are connected between the yoke and the pole shoes, at least part of the pole shoes are provided with chamfered edges in a section perpendicular to the central axis of the yoke, the radial inner peripheral wall of each pole shoe with the chamfered edges comprises an arc section extending along the circumferential direction and straight line sections located at two ends of the arc section, the two straight line sections are symmetrical about the central line OH of the stator tooth where the pole shoe is located, the intersection point of the straight line sections and the arc section is a chamfered point I, the connecting line of the chamfered point I at one end and the center O of the stator is OI, the included angle formed by the OI and the central line OH of the stator teeth is phi, theta is not less than theta/2, theta is alpha-beta, alpha is the included angle between the central lines of two adjacent stator teeth, beta is pi/P, and P is the number of. The motor stator can optimize the chamfered edge structure of the pole shoe, so that the chamfered edge of the pole shoe can have a more optimized structure, the magnetic field distribution of a motor air gap can be improved, the counter electromotive force harmonic content is greatly reduced, and the counter electromotive force sinusoidal degree is improved.

Drawings

FIG. 1 is a schematic structural diagram of a motor stator before being chamfered according to an embodiment of the present application;

fig. 2 is a first structural view of a stator of an electric machine according to an embodiment of the present application;

fig. 3 is a second structural view of a stator of the motor according to the embodiment of the present application;

FIG. 4 is a diagram illustrating pole clipping determination for a stator of an electric machine according to an embodiment of the present application;

FIG. 5 is an enlarged view of the structure at L of FIG. 4

Fig. 6 is a diagram illustrating a process of determining a cutting line segment of a stator of a motor according to an embodiment of the present application;

FIG. 7 is an enlarged schematic view of the structure at M of FIG. 6;

fig. 8 is a schematic structural view of a permanent magnet motor according to another embodiment of the present application;

fig. 9 is a comparison graph of torque ripple of the permanent magnet motor according to the embodiment of the present application and a conventional motor.

The reference numerals are represented as:

1. a yoke portion; 2. a pole shoe; 3. stator teeth; 4. a circular arc section; 5. a straight line segment; 6. a radial side; 7. a motor rotor; 8. and (4) mounting the groove.

Detailed Description

Referring to fig. 1 to 9 in combination, according to an embodiment of the present application, a stator of an electric motor includes a yoke 1, a pole shoe 2, and a stator tooth 3, the stator tooth 3 is connected between the yoke 1 and the pole shoe 2, at least a part of the pole shoe 2 has a chamfered edge, a radially inner peripheral wall of the pole shoe 2 having the chamfered edge includes a circular arc segment 4 extending in a circumferential direction and straight line segments 5 located at both ends of the circular arc segment 4, the two straight line segments 5 are symmetrical with respect to a center line OH of the stator tooth 3 where the pole shoe 2 is located, an intersection point of the straight line segment 5 and the circular arc segment 4 is a chamfered pole I, a connecting line between the chamfered pole I at one end and a stator center O is, OI forms an angle with the center line OH of the stator tooth 3, θ ≧ θ/2, θ ≧ α - β, where α is an angle between center lines of two adjacent stator teeth 3, β ═ P, p is the number of rotor poles.

The motor stator of the application can optimize the chamfered edge structure of the pole shoe 2, so that the chamfered edge of the pole shoe 2 can have a more optimized structure, the magnetic field distribution of a motor air gap can be improved, the counter electromotive force harmonic content is greatly reduced, and the counter electromotive force sinusoidal degree is improved.

Through the biggest scope of injecing the utmost point I of cutting of edging at motor stator circumference, can avoid cutting utmost point I and set up the influence range that the position surpassed a magnetic pole of electric motor rotor, lead to cutting utmost point I and can't play effectual edging effect, cause adverse effect to adjacent magnetic pole, through injecing the minimum scope of cutting utmost point I at motor stator circumference of edging, can prevent that the harmonic content increases too much after the edging, influence the motor performance.

Preferably, Φ is 3 θ/4.

In the embodiment of the application, critical pole cutting points E ' and E ' of a pole shoe 2 of a motor stator are determined according to an angle 2 β occupied by each magnetic pole of a motor rotor 7, points E ' and E ' are intersection points of two side angular lines of ∠ β and an inner circle of the stator respectively, an included angle between the critical pole cutting point E ' and a stator tooth center line OH ' is θ ∠ E ' OH ' α - β, where α is an angle between two adjacent stator tooth center lines, α is 2 pi/s, the included angle is known according to space distribution and symmetry, ∠ EOH is α - β, the included angle between the critical pole cutting point I and a stator radial center line OH is θ/2, the intersection point of the critical pole cutting point I and an inner circle hole of the stator is f, the value range of the included angle between the critical pole cutting point I and the stator radial center line OH is θ/2, the motion trajectory of the cutting point EF. is a sine curve with high harmonic content, and the optimum cut arc angle between the critical pole cutting point I and the stator radial center line OH is selected as θ EF 3 ', and the optimum cut arc line.

The pole shoe 2 further comprises radial side edges 6 located at two circumferential ends of the radial inner circumferential wall, an intersection point between an extension line of each radial side edge 6 and the inner circle of the stator is G, an included angle delta is formed between a cutting line segment IG ' and a central line OH, delta 2 is larger than or equal to delta and larger than delta 1, delta 1 is a theoretical minimum value of the included angle formed between the IG and the central line OH, delta 2 is a theoretical maximum value of the included angle formed between the IG ' and the central line OH, delta 2 is 90 degrees + theta/2, and G ' is the intersection point between the cutting line segment and the radial side edges 6 of the pole shoe 2 when the included angle delta.

The point G is the intersection point of the radial side 6 of the pole shoe 2 before the pole shoe 2 is chamfered and the inner circle of the stator. The included angle between the line segment IG and the radial central line OH of the stator is delta 1, and the delta 1 is the theoretical minimum value of the included angle between the cutting line segment IG' passing through the pole cutting point I and the radial central line OH of the stator; the cutting line segment IG' cuts and trims the bottom of the polar shoe to obtain the straight line segment 5 of the present application. In order to ensure the sine degree of the counter electromotive force, the maximum value delta 2 of an included angle between a line segment IG 'and a stator tooth central line OH is 90 degrees + theta/2, the intersection point of a cutting line segment and the side edge of the pole shoe is G', namely the value range of delta angle is more than or equal to 90 degrees + theta/2 and more than or equal to delta > delta 1.

Preferably, δ is 90 °. In order to obtain an optimal cutting line segment, an optimal pole cutting point I ' is selected for pole cutting, and when an included angle delta between the cutting line segment I ' G ' and a radial center line OH of the stator is 90 degrees, namely the cutting line segment I ' G ' is perpendicular to the OH, the optimal cutting line segment is obtained. The cutting line segment on the right side of the bottom edge of the pole shoe 2 is symmetrical to the line segment I 'G' about the centre line OH of the stator tooth, whereby the cutting of one stator pole shoe 2 is completed. According to the space uniform distribution and symmetry, the same trimming can be carried out on other stator pole shoes 2.

Preferably, each pole shoe 2 has a chamfered edge.

In another embodiment, the stator of the electrical machine further comprises stator windings wound on the stator teeth 3, wherein the pole shoes 2 corresponding to one phase of the stator windings are chamfered.

In another embodiment, the stator of the electrical machine further comprises stator windings wound on the stator teeth 3, wherein the pole shoes 2 corresponding to the two-phase stator windings each have a chamfered edge.

For a three-phase motor, the motor comprises three UVW phases, the number of stator teeth in each phase is S/3, when partial pole shoes are chamfered, the number of chamfered edges of the stator pole shoes 2 is S/3 or 2S/3, and at the moment, the chamfered pole shoes or the non-chamfered pole shoes are symmetrical in 120 degrees in space, as shown in FIGS. 2 and 3.

According to the embodiment of the application, the permanent magnet motor comprises a motor stator and a motor rotor 7, wherein the motor rotor 7 is arranged in the motor stator and forms an air gap with the motor stator, and the motor stator is the motor stator.

In the embodiment, the motor rotor 7 is provided with a mounting groove 8, wherein the mounting groove 8 may be circular arc, straight line, V-shaped, W-shaped, or the like.

According to the embodiment of the application, the method for machining the pole shoe 2 of the motor stator comprises the steps of determining the pole shoe 2 needing edge cutting, determining critical pole cutting points E ' and E ' on the pole shoe 2 in a cross section perpendicular to the central axis of the yoke 1, wherein E ' and E ' are intersection points of two corner lines of one magnetic pole of a motor rotor 7 and an inner circle of the stator, determining a connecting line OE ' of one critical pole cutting point E ' and a center O of the stator, determining an included angle theta between the OE ' and a central line OH ' of a stator tooth 3 corresponding to the critical pole cutting point E ', and determining ∠ E ' OH ', alpha-beta, wherein alpha is an included angle between central lines of two adjacent stator teeth 3, beta pi/P and P are the number of rotor poles, determining ∠ EOH-theta-alpha-beta according to space uniform distribution and symmetry, determining a pole cutting point I, wherein the connecting line of the pole cutting point I and the center O of the stator, phi between OI and the central line OH of the stator tooth 3 corresponding to the pole shoe 2 is equal to or more, and theta 2 is equal to or more than or equal to theta/beta, and theta-beta.

The motor stator can conveniently and reasonably carry out edging on the pole shoe 2 of the motor stator by adopting the processing method, and can optimize the edging structure of the pole shoe 2, so that the edging of the pole shoe 2 can have a more optimized structure, thereby improving the magnetic field distribution of a motor air gap, greatly reducing the content of counter electromotive force harmonic waves and improving the sinusoidal degree of the counter electromotive force.

The step of determining the pole clipping point I comprises: the center point I 'of the arc EF is determined, and the included angle phi between the radial line where the pole I' is cut and the center line OH of the stator teeth 3 is 3 theta/4.

The pole shoe 2 machining method further comprises the following steps: determining an intersection point G of the radial side 6 of the pole shoe 2 and the inner circle of the stator; determining an included angle delta 1 between the line segment IG and the central line OH of the stator teeth 3, wherein the delta 1 is a theoretical minimum value of the included angle between the cutting line segment IG' passing through the pole cutting point and the central line OH of the stator teeth 3; and cutting and edging the inner circumferential wall of the pole shoe 2 by using a cutting line segment IG ', wherein an included angle delta between the cutting line segment IG' and a central line OH of the stator teeth 3 is larger than delta 1.

The step of cutting and edging the inner circumferential wall of the pole shoe 2 by using the cutting line segment IG' also comprises the following steps: determining a theoretical maximum value delta 2 of an included angle between the cutting line segment IG ' and the central line OH of the stator teeth 3, wherein delta 2 is 90 ° + θ/2, and determining an intersection point G "between the cutting line segment IG ' and the radial side edge 6 of the pole shoe 2 when the included angle between the cutting line segment IG ' and the central line OH of the stator teeth 3 is delta 2; such that G 'is located between G and G'.

Preferably, δ is 90 °.

In the preferred embodiment of the present application, all pole shoes 2 of the motor stator are trimmed according to an optimal cutting line segment. The measured motor back electromotive force waveform is shown in fig. 9, the motor torque fluctuation obtained through simulation is shown in fig. 9, a curve 1 in the graph is a torque curve graph of a traditional motor, and a curve 2 is a torque curve graph of the permanent magnet motor in the embodiment of the application.

It is readily understood by a person skilled in the art that the advantageous ways described above can be freely combined, superimposed without conflict.

The present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. The foregoing is only a preferred embodiment of the present application, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present application, and these modifications and variations should also be considered as the protection scope of the present application.