阅读说明：本技术 一种考虑燃料电池启停策略的复合电源系统能量分配方法 (Energy distribution method of composite power supply system considering fuel cell start-stop strategy ) 是由 丁一 周健豪 孙静 何龙强 于 2019-10-14 设计创作，主要内容包括：本发明提供了一种考虑燃料电池启停策略的复合电源系统能量分配方法,属于燃料电池领域,具有减少燃料电池系统启停次数,提高系统效率,延长电源寿命的优点。本发明包括上层控制和下层控制；所述上层控制是对燃料电池的启停控制：采集蓄电池的电量数据以及汽车的负载电流,根据燃料电池当前的开闭状态以及蓄电池的电量数据,确定燃料电池进行启动和关机操作；所述下层控制是对能量分配的控制：根据数据采集模块采集到的蓄电池的电量,超级电容的电量以及负载的电流需求,对燃料电池模块,超级电容模块进行能量的分配。(The invention provides a composite power supply system energy distribution method considering a fuel cell start-stop strategy, belongs to the field of fuel cells, and has the advantages of reducing the start-stop times of a fuel cell system, improving the system efficiency and prolonging the service life of a power supply. The invention comprises an upper layer control and a lower layer control; the upper control layer is used for controlling the starting and stopping of the fuel cell: acquiring electric quantity data of a storage battery and load current of an automobile, and determining the fuel battery to start and shut down according to the current on-off state of the fuel battery and the electric quantity data of the storage battery; the lower layer control is control of energy distribution: and according to the electric quantity of the storage battery, the electric quantity of the super capacitor and the current demand of the load, which are acquired by the data acquisition module, energy is distributed to the fuel battery module and the super capacitor module.)

1. A composite power supply system energy distribution method considering a fuel cell start-stop strategy is characterized by comprising an upper layer control and a lower layer control;

the upper control layer is used for controlling the starting and stopping of the fuel cell: acquiring electric quantity data of a storage battery and load current of an automobile, and determining the fuel battery to start and shut down according to the current on-off state of the fuel battery and the electric quantity data of the storage battery;

the lower layer control is control of energy distribution: and according to the electric quantity of the storage battery, the electric quantity of the super capacitor and the current demand of the load, which are acquired by the data acquisition module, energy is distributed to the fuel battery module and the super capacitor module.

2. The hybrid power system energy distribution method considering the fuel cell start-stop strategy according to claim 1, wherein the upper-layer control comprises the following steps:

(1) judging the current state of the automobile according to the positive and negative acquired load currents of the automobile, if the current is a negative value, the automobile is in a braking state, the reference current value of the fuel cell is 0, and if the current is a positive value, the automobile is in a driving state;

(2) according to the collected electric quantity data of the storage battery and the current on-off state of the fuel cell, the fuel cell is started or shut down; if the fuel cell is in a shutdown state, when the electric quantity of the storage battery is smaller than the lower limit threshold value, the fuel cell is immediately started without considering the constraint of the minimum shutdown time toff, and the reference current value of the fuel cell is the maximum current Imax of the fuel cell; if the fuel cell is in the starting state, when the electric quantity of the storage battery is larger than the upper limit threshold value of the storage battery, the fuel cell is immediately shut down without considering the constraint of the minimum starting time ton, and the reference current of the fuel cell is 0 at the moment; if the fuel cell is in an open state, when the electric quantity of the storage battery is between an upper limit threshold and a lower limit threshold of the storage battery, the output current of the fuel cell changes along with the change of the automobile load current, and the reference current of the fuel cell is Id; if the fuel cell is in a shutdown state, the load current is always greater than the maximum current of the fuel cell within 10 seconds, and the time of the shutdown state of the fuel cell is greater than the minimum shutdown time, starting the fuel cell; if the fuel cell is in a starting state, the load current is always less than the optimal current of the fuel cell within 10 seconds, and the starting time is greater than the minimum starting time, then the fuel cell is closed; if the load current is larger than the maximum current which can be provided by the super capacitor and the storage battery, the fuel cell is started.

3. The hybrid power system energy distribution method considering the fuel cell start-stop strategy according to claim 1, wherein the lower layer control comprises the following steps:

step a: determining the reference current of the fuel cell according to the collected automobile load current and the electric quantity data of the storage battery and combining the upper and lower limit thresholds of the fuel cell current;

step b: and b, according to the acquired automobile load current and the reference current of the fuel cell determined in the step a, the difference value of the automobile load current and the reference current of the fuel cell is the difference current required to be provided by the super capacitor and the storage battery, and the reference current of the super capacitor is determined according to the positive and negative of the difference current and the electric quantity data of the super capacitor.

4. The hybrid power system energy distribution method considering the fuel cell start-stop strategy according to claim 3, wherein the step a in the lower layer control specifically comprises the following steps:

according to the start-stop state of the fuel cell judged by the upper layer control, if the fuel cell is in the shutdown state, the reference current of the fuel cell is 0; if the fuel cell is in an open state, adding the load current and the charging current of the storage battery according to the collected load current of the automobile to obtain the required current of the fuel cell, and comparing the required current of the fuel cell with the upper and lower limit threshold currents of the current of the fuel cell to obtain the corresponding reference current of the fuel cell; if the required current of the fuel cell is smaller than the lower current limit threshold of the fuel cell, the reference current of the fuel cell is the lower current limit threshold of the fuel cell at the moment; if the required current of the fuel cell is between the upper and lower current limit thresholds of the fuel cell, the reference current of the fuel cell is equal to the required current value; if the required current of the fuel cell is larger than the upper current threshold of the fuel cell, the reference current of the fuel cell is the upper current threshold of the fuel cell.

5. The energy distribution method of the hybrid power system considering the fuel cell start-stop strategy according to claim 3 or 4, wherein the upper and lower limit thresholds of the fuel cell current are upper and lower limit thresholds of the current of the fuel cell working in the high-efficiency region, which are drawn for the efficiency test data measured by the fuel cell system.

6. The hybrid power system energy distribution method considering the fuel cell start-stop strategy according to claim 3, wherein the step b in the lower layer control specifically comprises the following steps:

b, judging whether the difference value between the acquired automobile load current and the reference current of the fuel cell determined in the step a is the difference current required to be provided by the super capacitor and the storage battery, if the difference value is a positive value, entering a step s1, and if the difference value is a negative value, entering a step s 2;

said step s1 comprises the steps of:

if the difference current required to be provided by the super capacitor and the storage battery is a positive value, whether the super capacitor can supply power is judged, the difference current is compared with the current lower limit threshold of the storage battery, if the difference current is smaller than the current lower limit threshold of the storage battery, the reference current of the super capacitor is 0, if the difference current is larger than the current lower limit threshold of the storage battery, the electric quantity of the super capacitor is compared with the electric quantity lower limit threshold of the super capacitor, and if the electric quantity of the super capacitor is larger than the electric quantity lower limit threshold of the super capacitor, the reference current of the super capacitor is output to be Is; otherwise, outputting the reference current of the super capacitor as 0;

said step s2 comprises the steps of:

and if the difference current required to be provided by the super capacitor and the storage battery is a negative value, judging whether the super capacitor can be charged, comparing the electric quantity of the super capacitor with the initial electric quantity of the super capacitor, if the electric quantity of the super capacitor is smaller than the initial electric quantity of the super capacitor, taking the charging current of the super capacitor as the current lower limit threshold of the super capacitor, otherwise, setting the charging current of the super capacitor to be 0.

Technical Field

The invention belongs to the field of fuel cells, and particularly relates to a composite power supply system energy distribution method considering a fuel cell start-stop strategy.

Background

In recent years, fuel cells have been widely used in the field of vehicles due to their advantages of high efficiency and no pollution. In consideration of the disadvantage of slow dynamic response of the fuel cell, the fuel cell and other energy storage energy sources such as a storage battery, a super capacitor and the like form a composite energy source for use. The super capacitor has the advantage of high power density, and the storage battery has the advantage of high energy density, so that the respective advantages of the power supplies are fully exerted, and the design of a fuel cell composite power supply system with high efficiency, long service life and good reliability is particularly important.

The existing fuel hybrid power system generally only comprises two power sources, particularly, the damage of frequent switching on and off of the fuel battery system is not considered in an energy management strategy, the start-stop control of the fuel battery is not considered while the energy distribution of the hybrid power system is carried out, and further, the characteristics of the fuel battery and other energy storage energy sources cannot be fully met in the design of an energy management controller, so that the defects that the efficiency of the hybrid power system is low, the service life of the battery system is damaged and the like are caused.

Disclosure of Invention

The invention provides a composite power supply system energy distribution method considering a fuel cell start-stop strategy, which can reduce the start-stop times of a fuel cell system, improve the working efficiency of the composite power supply system and prolong the service life of a battery.

In order to achieve the purpose, the invention adopts the following technical scheme:

a hybrid power system energy allocation method that takes into account a fuel cell start-stop strategy, the hybrid power system comprising: the system comprises a fuel cell module, a super capacitor module, a storage battery module, a data acquisition module and a composite power supply controller; the composite power supply controller is connected with the data acquisition module, and the data acquisition module is respectively connected with the fuel cell module, the storage battery module and the super capacitor module; the storage battery module is directly connected with the load module; the fuel cell module is connected with a unidirectional DC-DC controller, and the unidirectional DC-DC controller is respectively connected with a load and the composite power supply controller; the super capacitor module is connected with a bidirectional DC-DC controller, the bidirectional DC-DC controller is respectively connected with a load and the composite power supply controller, the composite power supply controller comprises an upper layer and a lower layer, the upper layer is used for controlling the start-stop strategy of the fuel cell, the lower layer is used for distributing energy, and the energy is distributed to the fuel cell module and the super capacitor module according to the electric quantity of the storage battery, the electric quantity of the super capacitor and the current demand of the load, which are collected by the data collection module;

the data acquisition module comprises a load current and voltage acquisition module, a fuel cell current acquisition module, a super capacitor electric quantity acquisition module and a storage battery electric quantity acquisition module; the load current acquisition module is connected with a load in the automobile in series and then connected with the composite power supply controller; the load voltage acquisition module is connected with the load in parallel and then connected with the composite power supply controller; the fuel cell electric quantity acquisition module is respectively connected with the fuel cell module and the composite power supply controller; the super capacitor electric quantity acquisition module is respectively connected with the super capacitor module and the composite power supply controller; the storage battery electric quantity acquisition module is respectively connected with the super capacitor module and the composite power supply controller;

the energy distribution method comprises upper layer control and lower layer control; the upper control layer is used for controlling the starting and stopping of the fuel cell, the control method is to collect the electric quantity data of the storage battery and the load current of the automobile, and determine the starting and stopping operations of the fuel cell according to the current opening and closing state of the fuel cell and the electric quantity data of the storage battery,

the method comprises the following specific steps:

(1) judging the current state of the automobile according to the positive and negative acquired load currents of the automobile, if the current is a negative value, the automobile is in a braking state, the reference current value of the fuel cell is 0, and if the current is a positive value, the automobile is in a driving state;

(2) according to the collected electric quantity data of the storage battery and the current on-off state of the fuel cell, the fuel cell is started or shut down; if the fuel cell is in a shutdown state, when the electric quantity of the storage battery is smaller than the lower limit threshold value of the storage battery, the fuel cell is immediately started without considering the constraint of the minimum shutdown time toff, and the reference current value of the fuel cell is the maximum output current I max of the fuel cell; if the fuel cell is in the starting state, when the electric quantity of the storage battery is larger than the upper limit threshold value of the storage battery, the fuel cell is immediately shut down, and the constraint of the minimum starting time ton is not considered, wherein the reference current of the fuel cell is 0 at the moment; if the fuel cell is in an open state, when the electric quantity of the storage battery is between an upper limit threshold and a lower limit threshold of the storage battery, the output current of the fuel cell changes along with the change of the load current, and the reference current of the fuel cell is Id; if the fuel cell is in a shutdown state, the load current is always greater than the maximum current of the fuel cell within 10 seconds, and the time of the shutdown state of the fuel cell is greater than the minimum shutdown time, starting the fuel cell; if the fuel cell is in a starting-up state, the load current is always less than the optimal current of the fuel cell within 10 seconds, the optimal current is a current value corresponding to the maximum efficiency point of the fuel cell system, and the starting time is greater than the minimum starting-up time, the fuel cell is closed; if the load current is larger than the maximum current which can be provided by the super capacitor and the storage battery, the fuel cell is started.

The lower layer control comprises the following steps:

step a: determining the reference current of the fuel cell according to the collected automobile load current and the electric quantity data of the storage battery and by combining the upper limit threshold and the lower limit threshold of the high-efficiency interval of the fuel cell current;

step b: b, according to the acquired automobile load current and the reference current of the fuel cell determined in the step a, the difference value of the automobile load current and the reference current of the fuel cell is the difference value current required to be provided by the super capacitor and the storage battery, and according to the positive and negative values of the difference value current and the electric quantity data of the super capacitor, the reference current of the super capacitor is determined;

the step a in the above lower layer control includes the following steps:

according to the start-stop state of the fuel cell judged by the upper layer control, if the fuel cell is in the shutdown state, the reference current of the fuel cell is 0; if the fuel cell is in the starting state, adding the load current and the charging current of the storage battery according to the collected load current to obtain the required current of the fuel cell, and comparing the required current of the fuel cell with the upper and lower limit threshold currents of the fuel cell to obtain the corresponding reference current of the fuel cell; the upper and lower limit threshold values of the fuel cell current are the upper and lower limit threshold values of the current of the fuel cell working in the high-efficiency area which are defined by actually measured efficiency test data of the fuel cell system; if the required current of the fuel cell is smaller than the lower current limit threshold of the fuel cell, the reference current of the fuel cell is the lower current limit threshold of the fuel cell at the moment; if the required current of the fuel cell is between the upper and lower current limit thresholds of the fuel cell, the reference current of the fuel cell is equal to the required current value; if the required current of the fuel cell is larger than the upper current threshold of the fuel cell, the reference current of the fuel cell is the upper current threshold of the fuel cell.

The step b comprises the following steps:

judging whether the difference current is positive or negative according to the acquired automobile load current and the determined reference current of the fuel cell, wherein the difference current is the difference current required to be provided by the super capacitor and the storage battery, if the difference current is positive, entering a step s1, and if the difference current is negative, entering a step s 2;

step s1 includes the following steps:

if the difference current is a positive value, judging whether the super capacitor can supply power, comparing the difference current with a current lower limit threshold of the storage battery, wherein the lower limit threshold refers to the minimum discharge current of the storage battery under the current SOC, if the difference current is smaller than the current lower limit threshold of the storage battery, the reference current of the super capacitor is 0, if the difference current is larger than the current lower limit threshold of the storage battery, comparing the electric quantity of the super capacitor with the electric quantity lower limit threshold of the super capacitor, and if the electric quantity of the super capacitor is larger than the electric quantity lower limit threshold of the super capacitor, outputting the reference current of the super capacitor to be Iscmax; otherwise, the reference current of the output super capacitor is 0.

Step s2 includes the following steps:

and if the difference current is a negative value, judging whether the super capacitor can be charged, comparing the electric quantity of the super capacitor with the initial electric quantity of the super capacitor, if the electric quantity of the super capacitor is smaller than the initial electric quantity of the super capacitor, taking the lower current limit threshold of the super capacitor as the charging current of the super capacitor, otherwise, taking the charging current of the super capacitor as 0.

Has the advantages that: the invention provides a composite power supply system energy distribution method considering a fuel cell start-stop strategy, which combines upper-layer control and lower-layer control, wherein the upper layer is the fuel cell start-stop strategy, the lower layer is energy distribution, and energy is distributed to a fuel cell module and a super capacitor module according to the electric quantity of a storage battery, the electric quantity of a super capacitor and the current demand of a load, which are collected by a data collection module; the fuel cell is used as the main energy source of the composite power supply system, and the upper limit threshold and the lower limit threshold of the current of the fuel cell working in a high-efficiency area are regulated through the actually measured efficiency test data of the selected fuel cell system, so that the fuel cell meets the high-efficiency interval during working, and the working efficiency of the whole system is improved; adding a start-stop strategy for a fuel cell system in the energy management of the hybrid power system, and introducing the limits of minimum start-up time and minimum shutdown time in consideration of the defect of slow dynamic response of the fuel cell and prolonging the service life of the fuel cell, thereby reducing the frequent start of the fuel cell as much as possible; and for the super capacitor, the normal electric quantity level of the super capacitor is ensured, and the service life level of the whole system is greatly improved by the method.

Drawings

FIG. 1 is a schematic structural diagram of a hybrid power supply system according to the present invention;

FIG. 2 is a block diagram of a hybrid power supply control method of the present invention;

FIG. 3 is a flow chart of the energy distribution method of the storage battery and the super capacitor in the invention.

Detailed Description

The invention is described in detail below with reference to the following figures and specific examples:

as shown in fig. 1, the hybrid power supply system includes: the system comprises a fuel cell module, a super capacitor module, a storage battery module, a data acquisition module and a composite power supply controller; in the figure, a dotted line is a signal connecting line, a solid line is an electrical connecting line, the fuel cell module is used for providing a main power source for an automobile, the super capacitor module is used for providing peak power for the automobile, the storage battery module is used for supplementing the power required by the rest automobile, the composite power source controller is connected with the data acquisition module, and the data acquisition module is respectively connected with the fuel cell module, the storage battery module and the super capacitor module; the storage battery module is directly connected with the load module, so that the supplement of the power requirement of the remaining automobile after the energy supply of the fuel battery and the super capacitor is realized; the fuel cell module is connected with a unidirectional DC-DC controller, and the unidirectional DC-DC controller is respectively connected with a load and the composite power supply controller to realize unidirectional energy supply of the fuel cell module to the direct current bus; the super capacitor module is connected with a bidirectional DC-DC controller, the bidirectional DC-DC controller is respectively connected with a load and the composite power supply controller to realize charging and discharging of electric energy of the super capacitor, the composite power supply controller comprises an upper layer and a lower layer, the upper layer is used for controlling a start-stop strategy of a fuel cell, the lower layer is used for distributing energy, and the energy is distributed to the fuel cell module and the super capacitor module according to the electric quantity of the storage battery, the electric quantity of the super capacitor and the current demand of the load, which are collected by the data collection module;

the data acquisition module comprises a load current and voltage acquisition module, a fuel cell current acquisition module, a super capacitor electric quantity acquisition module and a storage battery electric quantity acquisition module; the load current acquisition module is connected with a load in the automobile in series and then connected with the composite power supply controller; the load voltage acquisition module is connected with the load in parallel and then connected with the composite power supply controller; the fuel cell electric quantity acquisition module is respectively connected with the fuel cell module and the composite power supply controller; the super capacitor electric quantity acquisition module is respectively connected with the super capacitor module and the composite power supply controller; and the storage battery electric quantity acquisition module is respectively connected with the super capacitor module and the composite power supply controller.

Take a fuel cell of the german EK company as an example:

as shown in fig. 2, a hybrid power system energy allocation method considering a fuel cell start-stop strategy includes an upper layer control and a lower layer control:

the upper control is the start and stop control of the fuel cell, the control method is to collect the electric quantity data of the storage battery and the load current of the automobile, and according to the current on-off state of the fuel cell and the electric quantity data of the storage battery, the fuel cell is determined to start and shut down, and the start and stop control method of the fuel cell in the upper control comprises the following steps: and judging the current state of the automobile according to the positive and negative of the collected load current of the automobile. If the current is a negative value, the fuel cell is in a braking state, and the reference current value of the fuel cell is 0. If the current is a positive value, the automobile is in a driving state, and the fuel cell is started or shut down according to the acquired electric quantity data of the storage battery and the current on-off state of the fuel cell; if the fuel cell is in a shutdown state, when the electric quantity of the storage battery is smaller than the lower limit threshold value, the constraint of the minimum shutdown time toff (5 s is taken here and is taken in combination with a specific system value) is not considered, the fuel cell is immediately started, and the reference current value of the fuel cell is the maximum current of the fuel cell, wherein the maximum current can be obtained through the test data of the fuel cell, in the embodiment, a cell stack with the peak power of 35kW of the company EK of germany is selected, and the corresponding maximum current is about 230A; if the fuel cell is in an open state, when the electric quantity of the storage battery is larger than the upper limit threshold value of the storage battery, the fuel cell is immediately shut down, and the constraint of the minimum startup time ton is not considered (the value is 5s, and the specific number of the storage battery needs to be adjusted by combining with the specific system value), at this time, the reference current of the fuel cell is 0; if the fuel cell is in an open state, when the electric quantity of the storage battery is between an upper limit threshold and a lower limit threshold of the storage battery, the output current of the fuel cell changes along with the change of the load current; if the fuel cell is in a shutdown state, the load current is always greater than the maximum current of the fuel cell within 10 seconds, and the time of the shutdown state of the fuel cell is greater than the minimum shutdown time, starting the fuel cell; if the fuel cell is in a starting state, the load current is always less than the optimal current of the fuel cell within 10 seconds, and the starting time is greater than the minimum starting time, then the fuel cell is closed; and if the load current is greater than the maximum current which can be provided by the super capacitor and the storage battery, starting the fuel cell, wherein the optimal current is a current value corresponding to the maximum efficiency point of the fuel cell system, the optimal current of the fuel cell of Germany EK company is 82A, and the reference current of the fuel cell in the upper-layer control is the total required current Id of the whole vehicle.

The control content of the lower layer control comprises:

step a: determining the reference current Ifcref of the fuel cell according to the collected load current and the electric quantity data of the storage battery and combining an upper limit threshold and a lower limit threshold of a high-efficiency interval of the current of the fuel cell; firstly, the starting and stopping state of the fuel cell is judged by upper-layer control, and if the fuel cell is in a stopping state, the reference current of the fuel cell is 0; if the fuel cell is in a starting state, the acquired load current Iload is added with the charging current of the storage battery to obtain the required current of the fuel cell, and the required current of the fuel cell is compared with the upper and lower limit threshold currents of the fuel cell to obtain the corresponding reference current Ifcref of the fuel cell; the upper and lower limit thresholds of the fuel cell current are defined by measured efficiency test data of the fuel cell system, and the upper and lower limit thresholds of the current of the fuel cell working in the high-efficiency area are defined by the measured efficiency test data of the fuel cell system. If the required current of the fuel cell is smaller than the lower current limit threshold of the fuel cell, outputting the reference current of the fuel cell as the lower current limit threshold of the fuel cell at the moment; if the required current of the fuel cell is between the upper and lower current limit thresholds of the fuel cell, the reference current of the fuel cell is equal to the required current value; if the required current of the fuel cell is larger than the upper current threshold of the fuel cell, the reference current of the fuel cell is the upper current threshold of the fuel cell; for the fuel cell system of EK, the high efficiency section refers to the region with system efficiency above 45%, the current section is about 40A-100A, the lower limit is 40A, and the upper limit is 100A.

Step b: and determining the reference current of the super capacitor according to the acquired automobile load current Iload and the reference current Ifcref of the fuel cell determined in the previous step, wherein the difference Idi between the automobile load current Iload and the reference current Ifcref is the difference current required by the super capacitor and the storage battery, and the positive and negative of the difference current and the electric quantity data of the super capacitor.

The fuel cell reference current in the lower layer control is the fuel cell final distribution current Ifcref.

In the lower layer control, a flowchart of the energy control method in step b is shown in fig. 3, and specifically includes the following steps:

and judging the positive and negative of the difference current according to the acquired automobile load current and the determined reference current of the fuel cell, wherein the difference between the automobile load current and the determined reference current is the difference current Idi required to be provided by the super capacitor and the storage battery, if the difference current is a positive value, the step s1 is carried out, and if the difference current is not a positive value, the step s2 is carried out.

The control method of the step s1 is as follows:

if the difference current Idi is positive, then it is determined whether the super capacitor can supply power. And comparing the difference current with a current lower limit threshold Ibamin of the storage battery, wherein the lower limit threshold refers to the minimum discharge current of the storage battery under the current SOC, and if the difference current is smaller than the current lower limit threshold Ibamin of the storage battery, the reference current Isc of the output super capacitor is 0. If the difference current is larger than the lower current limit threshold of the storage battery, comparing the electric quantity of the super capacitor with the lower current limit threshold of the super capacitor, and if the electric quantity of the super capacitor SOCsc is larger than the lower current limit threshold of the super capacitor SOCmin, outputting the reference current of the super capacitor as Iscmax; otherwise, the output super capacitor reference current Isc is 0.

The control method of the step s2 is as follows:

the difference current Idi is a negative value, and whether the super capacitor can be charged or not is judged. And comparing the electric quantity SOCsc of the super capacitor with the initial electric quantity SOCint of the super capacitor, if the electric quantity SOCsc of the super capacitor is smaller than the initial electric quantity SOCint of the super capacitor, taking the charging current Isc of the super capacitor as a current lower limit threshold of the super capacitor, otherwise, setting the charging current of the super capacitor to be 0.

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