阅读说明：本技术 可抑制谐振的电磁干扰滤波器设计方法 (Design method of electromagnetic interference filter capable of inhibiting resonance ) 是由 陈恒林 郑晓燕 于 2020-05-06 设计创作，主要内容包括：本发明公开了一种可抑制谐振的电磁干扰滤波器设计方法,通过逆变器干扰源模型得到逆变器的差/共模干扰电压和干扰源内阻,计算得到系统所需插入损耗,然后选定滤波器拓扑,采用最优化算法提取差/共模滤波器的元件参数,在考虑滤波器件高频等效寄生参数的情况下仿真得到最优的滤波器元件值。本发明根据仿真得到的元件参数值进行滤波器制作和实验验证,分析其滤波效果,结合滤波器高频等效模型,找出滤波器自谐振频点,通过添加阻尼电阻的方法解决部分频点共模EMI超标的问题,降低了滤波器设计过程中的试错成本,实现了EMI滤波器的定量设计,通过分析系统高频等效模型,精确定位造成EMI超标的关键元器件,实现了滤波器的优化设计。(The invention discloses a design method of an electromagnetic interference filter capable of inhibiting resonance, which comprises the steps of obtaining differential/common mode interference voltage and interference source internal resistance of an inverter through an inverter interference source model, calculating to obtain insertion loss required by a system, then selecting filter topology, extracting element parameters of the differential/common mode filter by adopting an optimization algorithm, and simulating to obtain an optimal filter element value under the condition of considering high-frequency equivalent parasitic parameters of a filter device. According to the invention, filter manufacturing and experimental verification are carried out according to element parameter values obtained by simulation, the filtering effect is analyzed, the self-resonant frequency points of the filter are found out by combining a high-frequency equivalent model of the filter, the problem that common-mode EMI (electro-magnetic interference) of part of the frequency points exceeds the standard is solved by adding damping resistors, the trial-and-error cost in the design process of the filter is reduced, the quantitative design of the EMI filter is realized, the key components causing the EMI exceeding the standard are accurately positioned by analyzing the high-frequency equivalent model of the system, and the optimal design of the filter is realized.)

1. A design method of an electromagnetic interference filter capable of inhibiting resonance comprises the following steps:

(1) establishing an inverter interference source equivalent model and obtaining an equivalent difference/common mode interference voltage V of an inverter system through measurement and calculation_DM,CMInternal resistance Z of sum-difference/common-mode interference source_DM,CMAccording to the EMC standardThe voltage limit value is calculated to obtain the insertion loss IL of the difference/common mode filter required by the inverter system_DM,CM；

(2) According to the internal resistance Z of the differential/common mode interference source of the inverter system_DM,CMSum load difference/common mode equivalent internal resistance Z_loadSelecting a difference/common mode filter topology, and connecting the filter topology into an inverter system;

(3) setting an optimization target, determining parameters of components in the filter by adopting a least square method, and calculating to obtain the differential/common mode insertion loss IL of the inverter system after the filter is accessed_filterAnd further judging whether the designed filter meets the requirements: if not, executing the step (4), and if so, executing the step (5);

(4) adopting different strategies to adjust the topology and the devices of the filter according to the frequency band of the standard exceeding frequency point;

(5) manufacturing a filter according to the determined filter topology and component parameters thereof, testing the difference/common mode electromagnetic interference of the inverter system after the filter is accessed, and verifying the EMI inhibition effect of the filter;

(6) when the EMI frequency spectrum of the inverter system is measured to have the problem that partial frequency points exceed the standard after the filter is connected, a high-frequency equivalent model of the filter is established, the path and the frequency range of resonance of each component are analyzed, and damping resistors are connected in series on the corresponding resonance path to inhibit the problem that the EMI of the resonance points exceed the standard.

2. The emi filter design method of claim 1, wherein: in the step (1), the insertion loss IL of the difference/common mode filter is calculated by the following formula_DM,CM；

IL_DM,CM＝20logV_DM,CM-20logV_PK

Wherein: v_PKEMC standard voltage limit.

3. The emi filter design method of claim 1, wherein: the load equivalent internal resistance Z_loadWith LISN connected to output terminal for standard testing of electromagnetic compatibility of inverter systemImpedance value, said differential/common mode insertion loss IL_filterFrom Z_DM,CM、Z_loadAnd the capacitance and inductance values in the filter.

4. The emi filter design method of claim 1, wherein: the optimization goal in step (3) is as follows:

min(|IL_filter-IL_DM,CM|)。

5. the emi filter design method of claim 1, wherein: the criterion for judging whether the designed filter meets the requirement in the step (3) is as follows: for the frequency interval [150kHz,30MHz]At any frequency point f within the range, if the frequency point f corresponds to the differential/common mode insertion loss IL_filter(f) Sum/difference common mode filter insertion loss IL_DM,CM(f) Satisfies IL_filter(f)>IL_DM,CM(f) If yes, the filter is judged to meet the requirement; if IL is present_filter(f)<IL_DM,CM(f) Then the filter is determined to be unsatisfactory.

6. The emi filter design method of claim 1, wherein: the specific implementation method of the step (4) is as follows: for exceeding frequency point f_bI.e. the frequency point f_bCorresponding differential/common mode insertion loss IL_filter(f_b) Sum/difference common mode filter insertion loss IL_DM,CM(f_b) Presence of IL_filter(f_b)<IL_DM,CM(f_b) (ii) a If f_b∈[150kHz,1MHz]Increasing the order of the filter topology and returning to execute the step (3); if f_b∈[1MHz,30MHz]And (4) increasing the high-frequency parasitic parameters of the components in the filter and returning to execute the step (3).

7. The emi filter design method of claim 1, wherein: in the step (6), the resistance value of the damping resistor should be larger than the impedance value of the filter at each resonance frequency point to achieve a good suppression effect, and meanwhile, the resistance value is ensured not to be too large, otherwise, the suppression effect of the filter is affected.

8. The emi filter design method of claim 1, wherein: according to the method, differential/common mode interference voltage and interference source internal resistance of an inverter are obtained through an inverter interference source model, insertion loss required by a system is obtained through calculation, then filter topology is selected, element parameters of the differential/common mode filter are extracted through an optimization algorithm, and an optimal filter element value is obtained through simulation under the condition that high-frequency equivalent parasitic parameters of a filter device are considered.

Technical Field

The invention belongs to the technical field of filter design, and particularly relates to a design method of an electromagnetic interference filter capable of inhibiting resonance.

Background

An EMI (electromagnetic interference) filter is used as an important part of the electromagnetic compatibility design of a converter, and is widely applied to high-frequency power electronic products.

The topology of the filter is divided into a single-stage structure and a multi-stage structure, the single-stage filter has an L type and a pi type, and the multi-stage topology structure is formed by cascading a plurality of single-stage filters. The topology of the EMI filter is selected according to the internal resistance value and the load resistance value of an interference source, the source impedance and the load impedance are assumed to be 50 ohm resistors in the traditional design method, and the filter is designed according to the impedance mismatch principle to achieve the maximum insertion loss. Considering that components such as an inductor, a transformer, an X capacitor and the like may exist in an actual inverter, a source impedance of a system generally does not have a constant 50 ohms and presents a resistive property, but presents a capacitive property or an inductive property, that is, an internal resistance of an interference source changes with a change of a frequency and does not have a constant 50 ohms, and if the internal resistance of the interference source is assumed according to a resistance characteristic of 50 ohms and a filter design is performed, an insertion loss cannot meet a requirement.

In the early filter design, after a filter topology is selected, insertion loss values of the filter are designed and measured for multiple times, and element parameters meeting requirements are obtained through gradual improvement so as to approach insertion loss required by a system as much as possible. In actual filter design, the turning angle frequency of the filter is mostly determined by the attenuation curve of the filter and the theoretically required insertion loss value, so as to perform element selection, but this method also has a defect in practical application, and the filter is often not good in high-frequency band effect because the high-frequency characteristics of the filter elements are less considered.

In order to obtain the component values through simulation calculation more accurately at the initial stage of Filter Design, some researchers have proposed methods for determining the high frequency characteristics of filters (e.g., Ye S, Eberle W, Liu Y F.A Novel EMI Filter Design method for Switching Power Supplies [ J ]. IEEE Transactions on Power Electronics,2004,19(6) (6):1668 + 1678). these methods include establishing circuit models for analyzing the high frequency characteristics, extracting parasitic parameters using finite element simulation software, and performing electromagnetic compatibility aided Design by combining the PCB structure with the coupling coefficients. In the stage of filter pre-design, the high-frequency characteristics of components are fully considered, and filter element values meeting requirements can be calculated more accurately.

In summary, a systematic filter design scheme should fully consider the influence of the source impedance in the pre-design stage, and simultaneously analyze the influence of the high-frequency parasitic parameters of the filter device on the filtering effect. However, after the filter element obtained by calculation by the method is put into practical application, the problem that the EMI of part of frequency points exceeds the standard due to circuit resonance still occurs, so that the filtering effect is influenced. On one hand, in order to ensure that the filter can have a required filtering effect after being put into use, engineers often leave enough allowance for the filter during the presetting to meet the requirement of electromagnetic compatibility, which leads to the increase of the size of the filter, the increase of the weight of equipment and the great increase of the design cost; on the other hand, after the system is put into use, if the problem that the EMI exceeds the standard still exists after the system is connected to the filter, the filter generally needs to be redesigned again, and a large amount of manpower and material resources are consumed.

Disclosure of Invention

In view of the above, the invention provides a method for designing an electromagnetic interference filter capable of suppressing resonance, which selects a differential/common mode (differential mode or common mode) filter topology based on the equivalent electromagnetic interference source internal resistance of an inverter, and then obtains the parameter values of filter components by utilizing an optimization algorithm fitting, thereby reducing the trial-and-error cost in the filter design process; after the filter is connected, key components causing EMI exceeding are accurately positioned by analyzing a system high-frequency equivalent model, and the problem that the EMI exceeding of partial frequency points is solved by adding damping resistors into the filter.

A design method of an electromagnetic interference filter capable of inhibiting resonance comprises the following steps:

(1) establishing an inverter interference source equivalent model and obtaining an equivalent difference/common mode interference voltage V of an inverter system through measurement and calculation_DM,CMInternal resistance Z of sum-difference/common-mode interference source_DM,CMAnd calculating the insertion loss IL of the differential/common mode filter required by the inverter system according to the EMC (electromagnetic compatibility) standard voltage limit value_DM,CM；

(2) According to the internal resistance Z of the differential/common mode interference source of the inverter system_DM,CMSum load difference/common mode equivalent internal resistance Z_loadSelecting a differential/common mode filter topology and inverting the filter topologyIn a machine system;

(3) setting an optimization target, determining parameters of components in the filter by adopting a least square method, and calculating to obtain the differential/common mode insertion loss IL of the inverter system after the filter is accessed_filterAnd further judging whether the designed filter meets the requirements: if not, executing the step (4), and if so, executing the step (5);

(4) adopting different strategies to adjust the topology and the devices of the filter according to the frequency band of the standard exceeding frequency point;

(5) manufacturing a filter according to the determined filter topology and component parameters thereof, testing the difference/common mode electromagnetic interference of the inverter system after the filter is accessed, and verifying the EMI inhibition effect of the filter;

(6) when the EMI frequency spectrum of the inverter system is measured to have the problem that partial frequency points exceed the standard after the filter is connected, a high-frequency equivalent model of the filter is established, the path and the frequency range of resonance of each component are analyzed, and damping resistors are connected in series on the corresponding resonance path to inhibit the problem that the EMI of the resonance points exceed the standard.

Further, the step (1) calculates the insertion loss IL of the differential/common mode filter by the following formula_DM,CM；

IL_DM,CM＝20log V_DM,CM-20log V_PK

Wherein: v_PKEMC standard voltage limit.

Further, the load equivalent internal resistance Z_loadThe impedance value of LISN (line impedance stabilizing network) connected to the output end of the inverter system during the standard test of electromagnetic compatibility, and the differential/common mode insertion loss IL_filterFrom Z_DM,CM、Z_loadAnd the capacitance and inductance values in the filter.

Further, the optimization goal in step (3) is as follows:

min(|IL_filter-IL_DM,CM|)

further, the criterion for determining whether the designed filter meets the requirement in the step (3) is as follows: for the frequency interval [150kHz,30MHz]At any frequency point f within the range, if the frequency point f corresponds to the differential/common mode insertion loss IL_filter(f) Sum/difference common mode filter insertion loss IL_DM,CM(f) Satisfies IL_filter(f)>IL_DM,CM(f) If yes, the filter is judged to meet the requirement; if IL is present_filter(f)<IL_DM,CM(f) Then the filter is determined to be unsatisfactory.

Further, the specific implementation method of the step (4) is as follows: for exceeding frequency point f_bI.e. the frequency point f_bCorresponding differential/common mode insertion loss IL_filter(f_b) Sum/difference common mode filter insertion loss IL_DM,CM(f_b) Presence of IL_filter(f_b)<IL_DM,CM(f_b) (ii) a If f_b∈[150kHz,1MHz]Increasing the order of the filter topology and returning to execute the step (3); if f_b∈[1MHz,30MHz]And (4) increasing the high-frequency parasitic parameters of the components in the filter and returning to execute the step (3).

Further, the resistance value of the damping resistor in the step (6) should be greater than the impedance value of the filter at each resonance frequency point to achieve a better suppression effect, and meanwhile, it is ensured that the resistance value is not too large, otherwise, the suppression effect of the filter will be affected.

The method comprises the steps of designing a filter according to an interference source model of an inverter, obtaining differential/common mode interference voltage and interference source internal resistance of the inverter through the interference source model of the inverter, calculating to obtain insertion loss required by a system, selecting filter topology, extracting element parameters of the differential/common mode filter by adopting an optimization algorithm, and simulating to obtain an optimal filter element value under the condition of considering high-frequency equivalent parasitic parameters of a filter device. According to the invention, filter manufacturing and experimental verification are carried out according to element parameter values obtained by simulation, the filtering effect is analyzed, the self-resonant frequency point of the filter is found out by combining a high-frequency equivalent model of the filter, and the problem that the common-mode EMI (electro-magnetic interference) of part of the frequency points exceeds the standard is solved by adding a damping resistor.

Compared with the existing EMI filter design method, the method obtains the component parameters through the simulation of the optimization algorithm, reduces the trial and error cost in the filter design process, realizes the quantitative design of the EMI filter, accurately positions the key components causing the EMI exceeding through analyzing the high-frequency equivalent model of the system, and realizes the optimal design of the filter.

Drawings

Fig. 1 is a schematic diagram of an inverter differential/common mode interference voltage waveform.

Fig. 2 is a schematic diagram of inverter differential/common mode interferer impedance and load impedance waveforms.

Fig. 3 is a schematic diagram of the insertion loss waveform of the differential/common mode filter required by the inverter.

Fig. 4 is an equivalent circuit diagram of the inverter after the inverter is connected to the differential mode filter.

Fig. 5 is a schematic diagram of the insertion loss simulation result of the first-order pi-type differential mode filter.

Fig. 6 is a diagram illustrating the differential mode EMI results of the inverter after the differential mode filter is connected.

Fig. 7 is a diagram illustrating the insertion loss simulation result of the first-order pi-type common mode filter.

Fig. 8 is an equivalent circuit diagram of the inverter connected to the second order pi-type common mode filter.

Fig. 9 is a schematic diagram of an insertion loss simulation result of the second-order pi-type common mode filter.

Fig. 10 is a diagram illustrating the common mode EMI results of the inverter after the second order pi-type common mode filter is connected.

Fig. 11 is a high-frequency equivalent circuit diagram of a second-order pi-type common mode filter.

FIG. 12 is an EMI equivalent circuit diagram of a second order pi-type common mode filter.

Fig. 13 is a diagram illustrating the comparison result of the common-mode EMI of the inverter after the filter is connected to different damping resistors.

Fig. 14 is a graph illustrating the differential/common mode EMI results of the inverter.

Fig. 15 is a schematic diagram of EMI results on the inverter L and N lines.

Detailed Description

In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.

The invention provides a quantitative design method of an electromagnetic interference filter capable of inhibiting resonance, which comprises the steps of obtaining port interference voltage and an interference source internal resistance value according to a known three-port electromagnetic interference equivalent model of an inverter, and determining the insertion loss of the filter required by a system by combining an EMC standard limit value and the port interference source voltage of the inverter; selecting a filter topology according to an internal resistance value and a load impedance value of an inverter interference source, extracting element parameters of a difference/common mode filter by adopting an optimization algorithm, and simulating to obtain an optimal filter element model selection scheme under the condition of considering high-frequency equivalent parasitic parameters of a filter device; the method comprises the following steps of carrying out filter manufacturing and experimental verification according to element parameter values obtained by simulation, analyzing the filtering effect, finding out the self-resonant frequency points of the filter by combining a high-frequency equivalent model of the filter, and solving the problem that common-mode EMI (electro-magnetic interference) of partial frequency points exceeds the standard by adding damping resistors, and specifically comprises the following steps:

(1) obtaining equivalent-effect difference/common-mode interference voltage V according to known inverter interference source model_DM,CMSum/difference/common mode internal resistance value Z_DM,CMAccording to the EMC standard limit value V_PKObtaining the insertion loss IL of the differential/common mode filter required by the inverter_DM,CM：

IL_DM,CM＝20logV_DM,CM-20logV_PK

(2) According to the internal resistance Z of the difference/common mode interference source_DM,CMAnd internal load resistance Z_lo_adOf a value selected difference/common mode filter topology, Z_loadThe impedance value of the LISN connected with the output end when the electromagnetic compatibility standard test is carried out on the system; obtaining the insertion loss IL of the system after accessing the differential/common mode filter according to the specific filter topology_filterSpecifically, it is represented as:

IL_filter＝f(Z_DM,CM,Z_load,C₁,...C_n,L₁,...L_n)

wherein: c₁,…C_nIs the capacitance value in the filter, L₁,…L_nIs the inductance value in the filter and n is the order of the filter.

(3) Extracting parameters of filter components by adopting a least square method, and setting an optimization target of an algorithm as follows:

min(|IL_filter-IL_DM,CM|)

the insertion loss of the filter obtained by simulation can not meet the requirement if the simulation result (f ∈ [150k,30M exists)]Make IL_filter(f)<IL_DM,CM(f) Go to step (4) if the simulation result meets the requirement (for any f ∈ [150k, 30M)]，IL_filter(f)>IL_DM,CM(f) Step (5) is performed.

(4) If the insertion loss of the filter obtained by simulation does not meet the requirement in a low frequency band (f <1MHZ), increasing the order of the filter, and performing the step (3); and (3) if the requirement is met in the low frequency band and the requirement is not met in the high frequency band (f is more than or equal to 1MHZ), considering the high frequency parasitic parameters of the filter device and carrying out the step.

(5) According to the optimal selection scheme of the filter obtained by simulation, the filter is manufactured, the difference/common mode electromagnetic interference of a system after the filter is accessed is tested, and the suppression effect of the filter on EMI is verified; if the measured EMI result is not over-standard, analyzing the path and frequency range of each element in which resonance occurs by establishing a high-frequency equivalent model of the filter, specifically: the equivalent series inductance of the Y capacitor and the capacitance value thereof generate series resonance, and the series resonance frequency is about dozens of MHz; the equivalent parallel capacitance and inductance of the common mode inductor generate parallel resonance, and the resonance frequency is about several MHz; the Y capacitor is in series resonance with the common mode inductance, with a resonance frequency of about several hundred KHz. After defining the resonant path, by connecting damping resistors R in series on the respective resonant path_dampThe problem of EMI exceeding is solved.

The following describes a specific implementation method of the present invention in further detail with reference to an example of a design of an emi filter capable of suppressing resonance.

In this embodiment, the differential/common mode interference voltage of the inverter is shown in fig. 1, the differential/common mode interference source internal resistance and the load equivalent internal resistance are shown in fig. 2, and the product adopts the electromagnetic compatibility standard EN55022classA, and the insertion loss of the differential/common mode filter required by the product is shown in fig. 3.

Because the internal resistance amplitude of the inverter interference source and the impedance amplitude of the LISN are in one order of magnitude, the topology of the selected filter is in a pi-type structure. Firstly, designing a differential mode filter, enabling a differential mode equivalent circuit after the differential mode filter is connected into a first-order pi type differential mode filter to be shown in fig. 4, fitting filter element parameter values by adopting a least square method, setting simulated initial values and fitting obtained element value-taking results to be shown in table 1:

TABLE 1

The insertion loss simulation results of the differential mode filter obtained according to table 1 are shown in fig. 5, and according to the simulation insertion loss results, specific values of the differential mode filter elements are selected as follows: c₁＝5uF，C₂＝3uF，L₁＝L₂After experimental verification, 6uH is obtained, and the differential mode EMI of the system after the differential mode filter is accessed is shown in fig. 6 and meets the standard of EN55022 ClassA.

Similarly, the common mode filter designed according to the above method mainly acts in the high frequency range, so the high frequency parasitic parameters need to be considered, and the element parameter values of the simulated first-order pi-type common mode filter are shown in table 2:

TABLE 2

The insertion loss of the filter obtained through simulation is shown in fig. 7, the low-frequency insertion loss of the filter obtained from fig. 7 cannot meet the requirements, so that a second-order pi-type common mode filter is selected, an equivalent common mode circuit of a system after the second-order pi-type common mode filter is connected is shown in fig. 8, and parameter values of a filter element are extracted by using an MATLAB optimization algorithm and are shown in table 3:

TABLE 3

The insertion loss result of the filter obtained by simulation is shown in fig. 9, and can be obtained from fig. 9, the simulated insertion loss of the filter element meets the requirement, the following experiment verifies that the selection type of the second-order pi-type common mode filter device is shown in table 4:

TABLE 4

The common-mode interference under the three schemes obtained by the test is shown in fig. 10, and can be obtained from fig. 10, and the common-mode EMI of the system has a plurality of standard exceeding frequency points which are 200KHz, 500KHz and 10MHz respectively.

The following analysis system accesses a high-frequency equivalent circuit model of a second-order pi-type common mode filter, as shown in fig. 11, the loops in the circuit that may resonate are:

(1) capacitor C_Y3The equivalent series inductance of (2) and its capacitance value are in series resonance.

C_Y3The capacitance value of (A) is nF level, the pin inductance value of a general Y capacitor can be considered as 10nH with the length of 1mm, so that the equivalent series inductance is nH level, and the series resonance frequency is near 10 MHz; when the frequency is more than 10MHz, the Y capacitor is inductive, and when the frequency is less than 10MHz, the Y capacitor is capacitive.

(2) Common mode inductor Lcm₂The equivalent parallel capacitance and the inductance value of the capacitor are in parallel resonance.

The common-mode inductance value is mH level, the equivalent parallel capacitance is pF level, so the frequency of the parallel resonance is about 1 MHz; when the frequency is less than 1M, the common mode inductor is inductive, and when the frequency is more than 1MHz, the common mode inductor is capacitive.

(3) Capacitor C_Y3And common mode inductance Lcm₂Series resonance occurs.

In the frequency range less than 1MHz, C_Y3For compatibility, Lcm₂The frequency at which series resonance occurs is about several hundred KHz for inductive purposes.

Through the analysis, several frequency points with the standard exceeding EMI can be determined to be caused by self resonance and mutual resonance of common-mode capacitance and common-mode inductance. In order to suppress these resonance frequencies, a common-mode capacitor C is used_Y3A damping resistor is connected in series, as shown in fig. 12, and after the damping resistor is connected into the damping resistor, two loops can be restrainedThe resonance of the circuit is self-resonance of the Y capacitor, and the resonance of the common-mode capacitor and the common-mode inductor in series. The damping resistors are selected to be 160 Ω, 400 Ω and 1k Ω respectively, experimental comparison is carried out, common mode interference of the system is obtained as shown in fig. 13, the common mode interference can be obtained from fig. 13, when the damping resistors are 160ohm and 400ohm, a plurality of over-standard frequency points of the system can be well inhibited, common mode EMI of a full frequency band meets the limit requirement of EN55022clasSA, and when the damping resistor is 400ohm, the EMI inhibition effect of the high frequency band is more obvious; at a low frequency (f) when an impedance value of 1K ohm is added<1MHz), available:

|R_damp|＞＞|Z_Y3|≈100

at the moment, the second-order filter is equivalent to the effect of the first-order filter, and the high-frequency suppression effect is not obvious; in summary, 400ohm may be selected as the resistance of the damping resistor.

A difference/common mode filter is connected to an output port of the inverter, the measured difference/common mode interference of the system and the EMI on the L/N line are respectively shown in fig. 14 and fig. 15, and after the difference/common mode filter is added, the difference/common mode EMI of the system and the EMI on two output lines can both accord with the standard of EN55022classA, and the effectiveness of the filter design method is proved.

The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.