阅读说明：本技术 一种电池系统的热扩散检测方法 (Thermal diffusion detection method of battery system ) 是由 陈明生 明志茂 谢贵婷 于 2019-12-04 设计创作，主要内容包括：本发明公开了一种电池系统的热扩散检测方法,包括：在待检测电池系统内选取若干单体电池作为待替换单体电池,并将其替换为发热单元；在每一发热单元处设置一放气阀,并通过气管将放气阀与气泵连接；启动发热单元并实时监测预设数据采集点的温度数据；其中,预设温度采集点设置在待检测电池系统内；将预设数据采集点的温度与预设温度上限进行实时比对,继而在超过预设温度上限时,开启放气阀及气泵并采集待检测电池系统的监测数据；其中,气泵开启时将待检测电池系统外部的气体抽入待检测电池系统内。通过实施本发明的实施例能够模拟电池系统内部单体热失控后的产气过程,获取相应的检测数据,为电池系统的热扩散研究提供数据基础。(The invention discloses a thermal diffusion detection method of a battery system, which comprises the following steps: selecting a plurality of single batteries as single batteries to be replaced in a battery system to be detected, and replacing the single batteries with heating units; an air release valve is arranged at each heating unit and is connected with the air pump through an air pipe; starting a heating unit and monitoring the temperature data of a preset data acquisition point in real time; the system comprises a battery system to be detected, a preset temperature acquisition point, a temperature sensor and a temperature sensor, wherein the preset temperature acquisition point is arranged in the battery system to be detected; comparing the temperature of a preset data acquisition point with a preset upper temperature limit in real time, and then starting a gas release valve and an air pump and acquiring monitoring data of the battery system to be detected when the temperature exceeds the preset upper temperature limit; wherein, when the air pump is opened, the air outside the battery system to be tested is pumped into the battery system to be tested. By implementing the embodiment of the invention, the gas production process after thermal runaway of the monomer in the battery system can be simulated, corresponding detection data can be obtained, and a data basis is provided for thermal diffusion research of the battery system.)

1. A thermal diffusion detection method for a battery system, comprising:

selecting a plurality of single batteries as single batteries to be replaced in a battery system to be detected, and replacing all the single batteries to be replaced with heating units;

arranging a deflation valve at each heating unit, and connecting the deflation valve with an air pump arranged outside the battery system to be detected through an air pipe;

starting a heating unit and monitoring the temperature of a preset data acquisition point in real time; the preset temperature acquisition point is arranged in the battery system to be detected;

comparing the temperature of the preset data acquisition point with a preset upper temperature limit in real time, and then starting the air release valve and the air pump and acquiring monitoring data of the battery system to be detected when the temperature of the preset data acquisition point exceeds the preset upper temperature limit; and when the air pump is started, the air outside the battery system to be detected is pumped into the battery system to be detected.

2. The thermal diffusion detection method of the battery system according to claim 1, wherein the replacing all the single batteries to be replaced with the heat generating unit specifically includes:

selecting a filling mold with the same shape as the single battery to be replaced according to the shape of the single battery to be replaced;

and placing the heating unit in the filling mold, and then replacing the single battery to be replaced with the filling mold containing the heating unit.

3. The thermal diffusion detection method of a battery system according to claim 1, wherein the preset data acquisition points are arranged according to an arrangement structure of each battery cell in the battery system, and specifically comprises:

when all the single batteries in the battery system are arranged in order, a three-dimensional coordinate system is established by taking the heating unit as an origin of coordinates, a plurality of single batteries are respectively selected as the single batteries to be arranged along the directions of all coordinate axes, and then data acquisition points are arranged on the outer surfaces of all the single batteries to be arranged;

when the single batteries in the battery system are arranged in a staggered mode, a three-dimensional coordinate system is established by taking the heating unit as a coordinate origin, and an additional axis is established between a Y axis and an X axis; and respectively selecting a plurality of single batteries as the single batteries to be distributed along the directions of a Z axis, an X axis and an additional axis, and then arranging a data acquisition point on the outer surface of each single battery to be distributed.

4. The method for detecting thermal diffusion of a battery system according to claim 1, further comprising, before the replacing all the unit batteries to be replaced with heat generating units, performing a reference test on the battery system to be tested by:

s1, placing the battery system to be detected for a preset time at normal temperature;

s2, discharging the battery system to be detected according to a preset discharge current condition after a preset duration is reached until the voltage of the battery system reaches a cut-off voltage, and starting to acquire temperature data of the battery system to be detected;

s3, laying the discharged battery system to be detected for a second preset time, then charging the battery system to be detected according to a preset charging current condition until the voltage of the battery system reaches a second cut-off voltage, laying the discharged battery system to be detected for a third preset time, and finishing the acquisition of the temperature data of the battery system to be detected to obtain reference temperature data;

and S4, repeating the steps S2 and S3 until the repetition times reach a preset value.

5. The method for detecting thermal diffusion in a battery system according to claim 1, wherein the monitoring data of the battery system to be detected includes: and voltage, temperature and air pressure data of the battery system to be detected.

6. The thermal diffusion detection method of a battery system according to claim 5, wherein the temperature of the battery system to be detected is collected by a thermocouple temperature sensor provided at the preset data collection point; the air pressure of the battery system to be detected is collected through an air pressure sensor integrated on the air release valve.

Technical Field

The invention relates to the technical field of batteries, in particular to a thermal diffusion detection method of a battery system.

Background

At present, the thermal runaway and danger analysis and safety evaluation research of single batteries have a certain foundation. However, the heat diffusion method and mechanism for a battery system having a complicated structure (a battery system including a plurality of unit batteries) are not yet known, and the influence of the gas generation after the thermal runaway of the unit batteries in the entire battery system on the safety of the battery system is not studied.

Disclosure of Invention

The embodiment of the invention provides a thermal diffusion detection method of a battery system, which can simulate a gas production process after thermal runaway of a monomer in the battery system, obtain corresponding detection data and provide a data basis for thermal diffusion research of the battery system.

An embodiment of the present invention provides a thermal diffusion detection method for a battery system, including: selecting a plurality of single batteries as single batteries to be replaced in a battery system to be detected, and replacing all the single batteries to be replaced with heating units;

arranging a deflation valve at each heating unit, and connecting the deflation valve with an air pump arranged outside the battery system to be detected through an air pipe;

starting a heating unit and monitoring the temperature of a preset data acquisition point in real time; the preset temperature acquisition point is arranged in the battery system to be detected;

comparing the temperature of the preset data acquisition point with a preset upper temperature limit in real time, and then starting the air release valve and the air pump and acquiring monitoring data of the battery system to be detected when the temperature of the preset data acquisition point exceeds the preset upper temperature limit; and when the air pump is started, the air outside the battery system to be detected is pumped into the battery system to be detected.

Further, the replacing all the to-be-replaced single batteries with heating units specifically includes:

selecting a filling mold with the same shape as the single battery to be replaced according to the shape of the single battery to be replaced;

and placing the heating unit in the filling mold, and then replacing the single battery to be replaced with the filling mold containing the heating unit.

Further, the preset data acquisition points are arranged according to the arrangement structure of each single battery in the battery system, and specifically include:

when all the single batteries in the battery system are arranged in order, a three-dimensional coordinate system is established by taking the heating unit as an origin of coordinates, a plurality of single batteries are respectively selected as the single batteries to be arranged along the directions of all coordinate axes, and then data acquisition points are arranged on the outer surfaces of all the single batteries to be arranged;

when the single batteries in the battery system are arranged in a staggered mode, a three-dimensional coordinate system is established by taking the heating unit as a coordinate origin, and an additional axis is established between a Y axis and an X axis; and respectively selecting a plurality of single batteries as the single batteries to be distributed along the directions of a Z axis, an X axis and an additional axis, and then arranging a data acquisition point on the outer surface of each single battery to be distributed.

Further, before replacing all the single batteries to be replaced with the heating units, the benchmark test of the battery system to be tested is further carried out in the following mode:

s1, placing the battery system to be detected for a preset time at normal temperature;

s2, discharging the battery system to be detected according to a preset discharge current condition after a preset duration is reached until the voltage of the battery system reaches a cut-off voltage, and starting to acquire temperature data of the battery system to be detected;

and S3, placing the discharged battery system to be detected for a second preset time, then charging the battery system to be detected according to a preset charging current condition until the voltage of the battery system reaches a second cut-off voltage, placing the discharged battery system to be detected for a third preset time, and finishing acquiring the temperature rise and temperature difference data of the battery system to be detected to obtain reference temperature data.

And S4, repeating the steps S2 and S3 until the repetition times reach a preset value.

The embodiment of the invention has the following beneficial effects:

the embodiment of the invention provides a thermal diffusion detection method of a battery system, which comprises the steps of firstly selecting a plurality of single batteries to be replaced in a battery system to be detected, then replacing the single batteries by a heating unit, wherein the heating unit is used for simulating a thermal runaway single battery, and then arranging a vent valve at the heating unit and connecting the vent valve with an air pump arranged outside through an air pipe; after the steps are completed, the heating unit is started, the heating unit continuously heats, meanwhile, the temperature of a preset data acquisition point is monitored in real time and is compared with a preset upper temperature limit, if the preset upper temperature limit is exceeded, the temperature in the battery system at the moment is indicated, the same condition when the thermal runaway of the internal battery monomer is reached, the air release valve and the air pump are opened at the moment, the air pump sucks gas outside the battery system to be detected into the battery system to be detected, and the monomer battery can generate gas when the thermal runaway occurs, so that the gas is sucked into the battery system to be detected from the outside of the battery system through the air pump, the gas generation process is started, and the monitoring data of the battery system to be detected are acquired in real time in the process. The embodiment of the invention can simulate the gas production process when the battery monomer in the battery system is out of control due to heat, reduce the whole thermal diffusion process and acquire corresponding data, thereby providing a data basis for the subsequent thermal diffusion research of the whole battery system.

Drawings

Fig. 1 is a schematic flow chart illustrating a thermal diffusion detection method of a battery system according to an embodiment of the present invention.

Fig. 2 is a schematic structural diagram of a thermal diffusion detection system of a battery system according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a layout structure of the deflation valve and the air pump according to an embodiment of the present invention.

Fig. 4 is a schematic diagram of a layout structure of preset data acquisition points according to an embodiment of the present invention.

FIG. 5 is a schematic structural diagram of a heating unit and a filling mold according to an embodiment of the present invention;

fig. 6 is a schematic general structural diagram of a layout structure of preset data acquisition points according to an embodiment of the present invention.

Fig. 7 is another general structural diagram of the layout structure of the preset data acquisition points according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, a method for detecting thermal diffusion of a battery system according to an embodiment of the present invention includes:

s101, selecting a plurality of single batteries as single batteries to be replaced in a battery system to be detected, and replacing all the single batteries to be replaced with heating units.

And S102, arranging a deflation valve at each heating unit, and connecting the deflation valve with an air pump arranged outside the battery system to be detected through an air pipe.

S103, starting a heating unit and monitoring the temperature of a preset data acquisition point in real time; the preset temperature acquisition point is arranged in the battery system to be detected.

S104, comparing the temperature of the preset data acquisition point with a preset upper temperature limit in real time, and then starting the air release valve and the air pump and acquiring monitoring data of the battery system to be detected when the temperature of the preset data acquisition point exceeds the preset upper temperature limit; and when the air pump is started, the air outside the battery system to be detected is pumped into the battery system to be detected.

For step S101, in a preferred embodiment, the setting of the thermal diffusion detection system of the battery system according to the manner shown in fig. 2 specifically includes: the system comprises a battery system to be detected, a data acquisition instrument, an upper computer, a negative pressure distribution system, an infrared thermal imaging monitoring system, a rapid cooling system and a floor drain, wherein the data acquisition instrument is connected with the battery system to be detected and the upper computer, and is used for uploading data acquired by each sensor in the battery system to be detected to the upper computer for data analysis and infrared thermal imaging monitoring, and is used for detecting the temperature of the battery system to be detected, and when the temperature exceeds a safety threshold, the upper computer controls the negative pressure ventilation system and the rapid cooling system to cool the battery system to be detected; and the floor drain can drain water. After the arrangement is completed in the above manner, the detection of the thermal diffusion of the battery system to be detected is started.

Preferably, the negative pressure ventilation system mainly comprises an explosion-proof air draft device, a flow guide cover, an explosion-proof fan, a ventilation pipeline, a tail gas treatment device and the like;

the quick cooling system mainly comprises a liquid nitrogen or neutral low-temperature water as a medium, a spray head, a cooling medium, a flow guide pipe, a liquid storage tank and the like.

In step S101, the number of the single batteries to be replaced in the battery system is selected according to the requirement of the actual test situation, and may be one or more (in the following description, explanation of other steps is performed in the case of replacing one single battery).

In a preferred embodiment, the replacing all the unit batteries to be replaced with the heat generating unit specifically includes: selecting a filling mold with the same shape as the single battery to be replaced according to the shape of the single battery to be replaced; and placing the heating unit in the filling mold, and then replacing the single battery to be replaced with the filling mold containing the heating unit.

In a preferred embodiment, as shown in fig. 5, since the types of the unit cells in the battery system generally include a cylindrical unit cell and a block unit cell in practical cases, in the embodiment of the present invention, two types of filling molds are provided, one is a block filling mold and the other is a cylindrical filling mold; the corresponding heating units are also of two types, one is a block-shaped heating unit, and the other is a columnar heating unit; the block filling mould is subdivided into two types, one is provided with a block groove (used for embedding the block heating unit), the other is provided with a column groove (used for the column block heating unit), the column filling mould is also subdivided into two types, one is provided with a block groove (used for embedding the block heating unit), and the other is provided with a column groove (used for the column block heating unit); preferably, in practical situations, for a blocky monomer cell in the battery system, a blocky heating unit is embedded, and a blocky filling mold with a blocky filling groove is generally selected to replace the monomer cell to be replaced (in the case of no blocky heating unit, a columnar heating unit is embedded, and a blocky filling mold with a columnar filling groove is adopted);

for the single battery which is columnar in the battery system, a mode that the columnar heating unit is embedded into the columnar filling mold provided with the columnar filling groove is generally selected (under the condition that the columnar heating unit is not arranged, a mode that the blocky heating unit is embedded into the blocky filling mold provided with the blocky filling groove can be adopted) to replace the single battery to be replaced. According to the selection of different heating units and filling molds, the originally designed monomer space of the battery system can be well conformed, the fitting degree is improved, and the accuracy of the collected simulation data (namely, the temperature rise, the temperature difference and the like collected in the simulation scene) is ensured.

For step S102, the specific arrangement is shown in fig. 3: a is a heating unit which is arranged in the battery system, and an air discharge valve is arranged at the outlet of A and is connected with an air pump outside the battery system through an air pipe; in a preferred embodiment, the purge valve is integrated with a pressure sensor capable of collecting pressure data within the battery system.

Step S103, a layout structure of preset data acquisition is explained first; in a preferred embodiment, two cases are distinguished;

when all the single batteries in the battery system are arranged in order, a three-dimensional coordinate system is established by taking the heating unit as an origin of coordinates, a plurality of single batteries are respectively selected as the single batteries to be arranged along the directions of all coordinate axes, and then data acquisition points are arranged on the outer surfaces of all the single batteries to be arranged;

when the single batteries in the battery system are arranged in a staggered mode, a three-dimensional coordinate system is established by taking the heating unit as a coordinate origin, and an additional axis is established between a Y axis and an X axis; and respectively selecting a plurality of single batteries as the single batteries to be distributed along the directions of a Z axis, an X axis and an additional axis, and then arranging a data acquisition point on the outer surface of each single battery to be distributed.

Specifically, as shown in fig. 4 (fig. 4 is a cross-sectional view of a battery system), a battery system to be tested is formed by arranging single batteries in order (the arrangement refers to that the single batteries are in an aligned state in the transverse direction and the longitudinal direction, and no dislocation occurs; as shown in fig. 4, 4a and 4b), a three-dimensional coordinate system is firstly established along the direction of the external normal of a box body formed by the battery system by taking the center of a heating element as an origin, three coordinate axes, namely X, Y, Z axes, are obtained (the Z axis is not drawn in fig. 4 because fig. 4 is a cross section along the Z axis direction), then a plurality of single batteries are respectively selected from X, Y, Z axes as the single batteries to be arranged (the single battery closest to the heating unit is preferably selected), the number of the single batteries to be arranged is at least 3, namely, one single battery closest to the heating unit is selected from each coordinate axis (of course, the number of the single batteries closest to the heating unit can also be selected from each coordinate axis Selecting two, or three, or more battery monomers closest to the heating unit), and then arranging two preset data acquisition points (represented by acquisition points in the figure) on the outer surface of each single battery to be arranged; specifically, see fig. 4a (layout of preset data acquisition points when the structure of the block-shaped battery cell is adopted, and the Z-axis direction is not shown) and 4b (layout of preset data acquisition points when the structure of the columnar battery cell is adopted, and the Z-axis direction is not shown) in the drawings. The layout structure of the overall preset data acquisition points is shown in fig. 6.

For the battery system to be tested in which the single batteries are arranged in a staggered manner (the staggered arrangement mentioned here refers to the situation that all the single batteries are not aligned in the transverse direction and the longitudinal direction and have dislocation; as shown in 4c in fig. 4), a three-dimensional coordinate system is constructed along the direction of the outer normal of a box body formed by the battery system by taking the center of the heating element as an origin, three coordinate axes, namely X, Y, Z axes, are obtained (the Z axis is not drawn in fig. 4 because fig. 4 is a cross section along the Z axis direction), then an additional axis is constructed between the X axis and the Y axis, it is required to say that the additional axis is constructed by taking the battery single in which the heating element and the heating element are inclined (namely, the battery single in which the heating element is dislocated) as two points, namely, the finally constructed additional axis needs to pass through the battery single in which the heating element is disl.

Then, selecting a plurality of single batteries as the single batteries to be distributed (preferentially selecting the single battery closest to the heating unit) along the additional axis, the X-axis direction, the Y-axis direction and the Z-axis direction, wherein the preferred number of the single batteries to be distributed is at least 3, namely selecting one single battery closest to the heating unit in each coordinate axis direction (of course, two or three or more single batteries closest to the heating unit can be selected in each coordinate axis direction), and then setting two preset data acquisition points on the outer surface of each single battery to be distributed; see fig. 4c (the structure of the single cylindrical battery, and when the single batteries are arranged in a staggered manner, the arrangement of the preset data acquisition points is not shown in the Z-axis direction). The layout structure of the overall preset data acquisition points is shown in fig. 7.

According to the arrangement structure of each battery monomer in the battery system, the corresponding multi-axial acquisition method is selected, so that accidental errors of single-point detection can be avoided, and the accuracy of data is ensured.

And after the preset data acquisition points are arranged, starting the heating unit, continuously heating the heating unit, and monitoring the temperature of the preset acquisition points in real time by the thermocouple temperature sensors at all the data acquisition points.

For step S104, first, a preset upper temperature limit is explained; in a preferred embodiment of the present invention,

before the heat generating unit is used for replacing the battery cell inside the battery system to be detected, the battery system to be detected is subjected to benchmark test under the condition that no battery cell is replaced, and the method comprises the following steps of:

s1, placing the battery system to be detected for a preset time at normal temperature;

and S2, discharging the battery system to be detected according to a preset discharge current condition after a preset time is reached until the voltage of the battery system reaches a cut-off voltage, and starting to acquire temperature data of the battery system to be detected.

And S3, placing the discharged battery system to be detected for a second preset time, then charging the battery system to be detected according to a preset charging current condition until the voltage of the battery system reaches a second cut-off voltage, placing the discharged battery system to be detected for a third preset time, and finishing the acquisition of the temperature data of the battery system to be detected to obtain reference temperature data.

And S4, repeating the steps S2 and S3 until the repetition times reach a preset value.

It should be noted that, preferably, the preset time duration, the second preset time duration and the third preset time duration are all 30 min; the above-described preset charging current condition and discharging current condition are both 1C. The number of repetitions is preferably 3. Preferably, data acquisition is also performed through the thermocouple temperature sensor, and in the process of the benchmark test, the arrangement mode of the data acquisition points is consistent with the arrangement mode of the preset data acquisition points in the step S103, so that the temperature of each data acquisition point can be monitored, and the benchmark temperature data of each data acquisition point can be obtained. And extracting temperature data with the highest temperature value from the reference temperature data of each data acquisition point as a preset temperature upper limit value (namely the preset temperature upper limit) of each data acquisition point. Then, in step S104, the real-time temperature of each preset data acquisition point may be compared with the upper temperature limit value of each data acquisition point, and once the real-time temperature of any preset data acquisition point exceeds the upper temperature limit value, the deflation valve and the air pump are opened, and the deflation valve and the air pump are opened to pump the gas outside the battery system to be tested into the battery system to be tested. Therefore, the gas production process when the single battery in the battery system is out of control is simulated, and then various monitoring data are collected. In a preferred embodiment, the pumping rate of the air pump can be calculated by combining different stages of the chemical reaction process.

In addition, the capacity data of the battery system to be tested is also measured and used for determining that the battery system to be tested meets the rated capacity requirement before the thermal diffusion test. The purpose of the multiple repetitions is to activate the battery system to its optimal state of technology.

In a preferred embodiment, the monitoring data includes voltage, temperature and pressure data, gas flow rate, etc. of the battery system to be tested.

For better illustration of the present invention, the above technical solution is illustrated below by way of practical examples:

1. benchmark test

(1) The battery system charging and discharging cable, the upper computer communication cable and the data acquisition system are connected in the arrangement mode according to the attached drawing 2, and the peripheral linkage infrared monitoring and cooling system is ensured to be ready.

(2) Test procedure

① standing the object (battery system to be tested) at room temperature for at least 30 min;

② setting the discharge current condition to 1C, starting discharging the battery system to the specified cut-off voltage;

③ standing for 30 min;

④ setting the charging current condition to 1C, starting charging the battery system to the specified cut-off voltage;

⑤ standing for 30 min;

⑥ repeat steps ② - ⑤ for a total of 3 times.

(3) Recording and calculating reference data

Reference data includes, but is not limited to, capacity, internal monomer temperature, etc. The capacity data is used for determining that the tested battery system before the thermal diffusion test meets the rated capacity requirement; the internal monomer temperature is used for judging whether the temperature in the next round of test meets the upper limit temperature value judgment standard of thermal runaway.

2. Simulation test (replacing the heating unit for the battery cell in the battery system)

(1) Axial collection and gas production simulation point distribution

① disassembling the upper cover of the battery system.

② are respectively arranged in 3 axial directions with more than 6 thermocouple temperature sensors.

③ the air release valve is arranged at the single body simulation failure point (namely the heating unit) in the battery system, and is connected with an external air pump through an air pipe.

(2) Acquisition point distribution for different monomer structures and system layouts

According to fig. 5, distribution of acquisition points for different monomer structures and system layouts. In this embodiment, the test object is a square single body, and therefore, the outer surfaces of the adjacent single bodies are taken as temperature collection points to perform temperature point sensor layout with reference to the outer normal directions (the Z-axis direction is drawn) of the two sides of the cross section. The temperature sensor is connected with an external data acquisition instrument to acquire real-time temperature data.

(3) Monomer structure filling mold selection

According to fig. 5, the heating unit and the filling mold are selected for different single battery structures. In general, the monomer failure simulation uses a smaller heat generating unit than the monomer and needs to be filled to meet the installation space. In this embodiment, a block-shaped heating unit is used, and therefore, the block-shaped filling mold shown in fig. 5 is selected.

(4) Starting a heat-generating unit to perform a monomer thermal runaway simulation

And (4) restoring the upper cover of the battery system after the steps (1) to (3) are ready. And starting the heating unit to implement monomer thermal runaway simulation. And when the temperature of the set data acquisition point exceeds the preset upper temperature limit, the air release valve is opened to simulate the process of generating gas by thermal runaway of the monomer. And recording data such as voltage, temperature and air pressure acquired by the acquisition point in the whole process.

Compared with the prior art, the embodiment of the invention considers the environment and safety protection of the whole test in the method design. The problem of acquisition point arrangement caused by distribution of different battery systems in a system is solved; a gas production process replacement method of monomer failure is adopted, and a real scene of thermal runaway of a battery system is more effectively simulated by combining temperature control; according to the selection of different heating units and filling molds, the originally designed monomer space of the battery system can be well matched. The multi-axial acquisition method can avoid accidental errors of single-point detection and ensure the accuracy of the thermal diffusion test. And the whole layout principle is simple, and the operability and the flexibility are strong.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention.