KR20240020017A - System for producing electrodes of battery - Google Patents

Abstract

The present invention is an electrode production system for secondary batteries that includes a transfer unit for transferring electrodes to a magazine for loading electrodes, wherein the transfer unit includes a plurality of transfer units arranged along the front-back direction and spaced apart in the left and right directions, and left and right It includes a bracket disposed along a direction and a driving unit connected to the transfer unit, wherein the transfer unit includes a suction plate, a body including a column connected to the suction plate and disposed in the vertical direction, and a body along the body. The belt and the column that are disposed and connected to the driving unit are arranged to be able to slide left and right on the bracket, thereby providing a secondary battery electrode production system in which the distance between the transfer units in the left and right directions is adjusted.

Description

Electrode production system for secondary batteries {SYSTEM FOR PRODUCING ELECTRODES OF BATTERY}

The present invention relates to an electrode production system for secondary batteries.

In general, a chemical battery refers to a battery that includes an anode, a cathode, and an electrolyte and generates electrical energy using a chemical reaction. This includes primary batteries that are disposable and secondary batteries that can be charged and discharged for repeated use. It can be divided into:

The use of secondary batteries is gradually increasing due to the advantage of being able to charge and discharge. Among such secondary batteries, lithium secondary batteries have a high energy density per unit weight, and are therefore widely used as a power source for electronic communication devices or in high-output hybrid vehicles.

The electrodes used in these secondary batteries are used as the anode and cathode of the battery and are used to electrically connect the battery to the outside of the battery.

Electrode tabs can be formed on the electrode by notching at regular intervals.

The notched electrode is sealed in the form of a pouch or square or cylindrical can in the stack process and manufactured into a secondary battery.

The electrode material wound in the form of a reel is unwound from the unwinding section and fed into the notching section. And, a plurality of electrode tabs are continuously formed on the electrode material at regular intervals. The electrode on which the tab is formed can be rewound in reel form or cut to a certain size and transported through a transfer unit to be loaded into a magazine.

The transfer unit includes a belt with a plurality of suction holes and a belt drive unit that moves the belt. The electrode on which the tab is formed is adsorbed to the belt and moved to the magazine side. At this time, the belt is located above the electrode, and the electrode does not fall due to the suction force, but is transported to the point where the magazine is located.

At this time, the electrode has a constant width in the left-right direction. If the left-right edges of the electrode are not aligned with the suction hole of the belt and both left and right ends of the electrode are located outside the suction hole, the left and right ends of the electrode are not absorbed. Because this is not possible, a problem occurs in which both left and right ends of the electrode sag due to their own weight. In this way, if both left and right ends of the electrode sag, there is a problem that the electrode is damaged when it falls from the belt and is loaded into the magazine.

In addition, because the left and right positions of the belt are fixed, when producing electrodes with different widths in the left and right directions, the position of the pusher that pushes the electrode toward the magazine is not aligned with the position of the electrode, so the electrode cannot be pushed toward the magazine. As mentioned above, a problem may arise in which the left and right edges of the electrode are not aligned with the suction hole of the belt, and a separate ㅇ transfer unit must be prepared whenever electrodes with different widths in the left and right directions are transferred. There is a problem that needs to be resolved.

Republic of Korea Patent Publication No. 10-2015-0089803 (published on August 5, 2015)

The purpose of the present invention is to provide an electrode production system for secondary batteries that can align the left and right edges of the electrode with the suction holes of the belt.

The purpose of the present invention is to provide an electrode production system for secondary batteries that prevents sagging of the electrode by adjusting the gap between the belts of the transfer unit according to the size of the width of the electrode in the left and right directions.

An embodiment is an electrode production system for secondary batteries that includes a transfer unit that transfers electrodes to a magazine for loading electrodes, wherein the transfer unit includes a plurality of transfer units arranged along the front-back direction and spaced apart in the left and right directions, It includes a bracket disposed along the left and right directions, and a driving unit connected to the transfer unit, wherein the transfer unit includes a suction plate, a body including a column connected to the suction plate and disposed in the vertical direction, and the body. The belt and the column, which are arranged along and connected to the driving unit, are arranged to be able to slide left and right on the bracket, providing a secondary battery electrode production system in which the left and right separation distance between the transfer units is adjusted. .

The body includes a guide block, one end of the column is coupled to the suction plate in the vertical direction, the other end of the column is coupled to the guide block, and the belt can be guided by being placed inside the guide block. .

The column may include a horizontal portion and a vertical portion disposed perpendicular to the horizontal portion, the horizontal portion may be coupled to the guide block, and the vertical portion may be slidably coupled to the bracket.

The bracket includes a rail disposed along the left and right directions, the column includes a sliding block disposed opposite the rail, and the sliding block can be movably coupled to the rail.

It further includes a fixing part, wherein the fixing part includes a fixing block movably disposed inside the rail part, a shaft part that penetrates the horizontal part and is rotatably fastened to the fixing block, and a handle coupled to the shaft part. , As the shaft portion rotates by the handle, the fixing block can fix or release the horizontal portion from the rail portion.

The driving unit includes a spline shaft, a spline nut coupled to the spline shaft, and a roll rotatably coupled to the spline nut, the belt is mounted on the roll, and the spline nut moves left and right along the spline shaft. You can move in any direction.

It further includes a pusher disposed on the body, and the pusher moves in a vertical direction, thereby selectively contacting the electrode and pushing the electrode from the suction plate to the magazine.

The pusher may be disposed between adjacent suction plates in the left and right directions.

The plurality of columns may be spaced apart along the front-to-back direction.

The plurality of brackets may be spaced apart along the front-to-back direction.

According to the embodiment, the transfer unit including the belt is configured to be movable in the left and right directions, so that the left and right edges of the electrodes are aligned with the suction holes of the belt, thereby preventing sagging of the electrodes.

According to the embodiment, there is an advantage in that sagging of the electrode can be prevented by aligning the left and right edges of the electrode with the suction holes of the belt in response to the various widths of the electrode in the left and right directions.

According to the embodiment, the pusher is configured to be movable in the left and right directions together with the transfer unit, so that there is an advantage in that the electrode can be easily pushed toward the magazine in response to various widths of the electrode in the left and right directions.

1 is a perspective view of the transfer unit of the electrode production system for secondary batteries according to an embodiment, viewed from below;
Figure 2 is a perspective view of the transfer unit shown in Figure 1 viewed from above;
Figure 3 is an enlarged view of the transfer unit shown in Figure 1;
Figure 4 is a side view of the transfer unit shown in Figure 1;
FIG. 5 is a plan view of the transfer unit shown in FIG. 1 viewed from above, and FIG. 6 is a plan view of the transfer unit shown from below.
Figure 7 is an enlarged view of the transfer unit;
Figure 8 is a perspective view of the column and bracket of the transfer unit shown in Figure 7 from one side;
Figure 9 is a perspective view of the column and bracket of the transfer unit shown in Figure 7 as seen from the other side;
10 is a view showing a fixing part,
11 is a diagram showing a driving unit;
Figure 12 is an enlarged view of the driving unit shown in Figure 11;
Figure 13 is a plan view showing a state in which both left and right ends of the electrode are not aligned with the belt;
Figure 14 is a front view showing a state in which both left and right ends of the electrode are not aligned with the belt;
Figure 15 is a plan view showing a state in which the transfer unit is moved outward and both left and right ends of the electrode are aligned with the belt;
Figure 16 is a front view showing a state in which the transfer unit is moved outward and both left and right ends of the electrode are aligned with the belt;
Figure 17 is a plan view showing another state in which both left and right ends of the electrode are not aligned with the belt;
Figure 18 is a front view showing another state in which both left and right ends of the electrode are not aligned with the belt;
Figure 19 is a plan view showing a state in which the transfer unit is moved inward and both left and right ends of the electrode are aligned on the belt;
Figure 20 is a front view showing a state in which the transfer unit is moved inward and both left and right ends of the electrode are aligned with the belt;
Figure 21 is a diagram showing a pusher pushing an electrode.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art, and the following embodiments may be modified in various other forms, and the scope of the present invention is limited. It is not limited to the examples below. Rather, these embodiments are provided to make the disclosure more faithful and complete and to fully convey the spirit of the invention to those skilled in the art.

The terms used herein are used to describe specific embodiments and are not intended to limit the invention. As used herein, the singular forms include the plural forms unless the context clearly indicates otherwise. Additionally, when used herein, “comprise” and/or “comprising” means specifying the presence of stated features, numbers, steps, operations, members, elements and/or groups thereof. and does not exclude the presence or addition of one or more other shapes, numbers, operations, members, elements and/or groups. As used herein, the term “and/or” includes any one and all combinations of one or more of the listed items.

Although terms such as first, second, etc. are used herein to describe various members, regions, and/or portions, it is obvious that these members, parts, regions, layers, and/or portions should not be limited by these terms. do. These terms do not imply any particular order, superiority or inferiority, or superiority or inferiority, and are used only to distinguish one member, region or portion from another member, region or portion. Accordingly, a first member, region or portion described below may refer to a second member, region or portion without departing from the teachings of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to drawings schematically showing embodiments of the present invention. In the drawings, variations of the depicted shape may be expected, for example, depending on manufacturing techniques and/or tolerances. Accordingly, embodiments of the present invention should not be construed as limited to the specific shape of the area shown herein, but should include changes in shape resulting from, for example, manufacturing.

The electrode production system for secondary batteries is a device for automatically and continuously producing electrodes used in secondary batteries.

FIG. 1 is a perspective view of the transfer unit of the electrode production system for secondary batteries according to an embodiment, viewed from the bottom, and FIG. 2 is a perspective view of the transfer unit shown in FIG. 1, viewed from the top.

below. The x-axis direction in the drawing represents the front-back direction of the secondary battery electrode production system and represents the transport direction of the electrode, and the y-axis direction in the drawing represents the left and right direction of the secondary battery electrode production system and represents the width direction of the electrode. The z-axis direction represents the vertical direction of the secondary battery electrode production system. Hereinafter, in describing the embodiments, ‘front’ and ‘back’ are based on the transfer direction of the electrode, and ‘top’ and ‘bottom’ are based on the vertical direction.

The electrode is supplied from the unwinding part to the notching part, and an electrode tab is formed in the notching part and supplied to the magazine through the transfer part.

Referring to Figures 1 and 2, the transfer unit 10 may include a plurality of transfer units 100 and a plurality of brackets 200.

The transfer unit 100 is arranged along the front-back direction (x). These plurality of transfer units 100 may be arranged at regular intervals in the left and right direction (y). For example, three transfer units (100:100A, 100B, 100C) may be arranged, and the three transfer units (100:100A, 100B, 100C) may be arranged at regular intervals in the left and right direction (y).

The bracket 200 is arranged along the left and right direction (y). Each transfer unit 100 is connected to a bracket 200. A plurality of brackets 200 may be arranged. A plurality of brackets 200 may be arranged at regular intervals in the front-back direction (x).

A plurality of brackets 200 may be fixed to the base 11. The base 11 may be coupled to the frame 12 to be movable in the vertical direction (z). The vertical driving unit 180 is connected to the base 11 and can adjust the vertical position of the transfer unit 10 by moving the base 11 in the vertical direction (z).

FIG. 3 is an enlarged view of the transfer unit 100 shown in FIG. 1, and FIG. 4 is a side view of the transfer unit 10 shown in FIG. 1.

Referring to FIGS. 1 to 4 , each transfer unit 100 may include a suction plate 110, a body 120, and a belt 130.

The suction plate 110 is disposed on the lower side of the body 120. The suction plate 110 may be arranged long along the front-to-back direction (x).

The body 120 forms the body of the transfer unit 100. The body 120 may include a column 121 and a guide block 150.

Column 121 is arranged along the vertical direction (z). The lower end of the column 121 is fixed to the suction plate 110. A guide block 150 is placed at the top of the column 121. The guide block 150 contacts the belt 130 and guides the movement of the belt 130. A plurality of such columns 121 may be arranged. The plurality of columns 121 are arranged to be spaced apart along the front-back direction (x).

As shown in FIG. 4, when the belt 130 rotates counterclockwise, the belt 130 moves along the suction plate 110 at the lower side of the transfer unit 100, and It moves along the guide block 150 from the upper side.

The electrode S is adsorbed to the belt 130 at the lower side of the transfer unit 100 and is transferred from left to right in the drawing.

FIG. 5 is a plan view of the transfer unit shown in FIG. 1 viewed from the top, and FIG. 6 is a plan view of the transfer unit shown from the bottom.

The transfer unit 10 of the electrode production system for secondary batteries according to the embodiment is a transfer unit for each transfer unit to align the electrode S and the belt 130 so that both ends of the electrode S in the left and right direction (y) are not sagging. It is configured to adjust the left and right (y) position of (100).

Referring to FIGS. 5 and 6 , for example, the transfer unit 100 may include a first transfer unit 100A, a second transfer unit 100B, and a third transfer unit 100C. The first transfer unit 100A, the second transfer unit 100B, and the third transfer unit 100C are arranged at regular intervals in the left and right directions (y).

The bracket 200 is arranged along the left and right direction (y). The first transfer unit 100A, the second transfer unit 100B, and the third transfer unit 100C are each slidably coupled to the bracket 200. A plurality of such brackets 200 may be arranged. A plurality of brackets 200 are arranged at regular intervals along the front-back direction (x).

FIG. 7 is an enlarged view of the transfer unit, FIG. 8 is a perspective view of the column 121 and bracket 200 of the transfer unit shown in FIG. 7 from one side, and FIG. 9 is a perspective view of the column 121 and bracket 200 of the transfer unit shown in FIG. 7. This is a perspective view of the bracket 200 from the other side.

7 to 9, the bracket 200 includes a rail 210 disposed along the left and right direction (y).

Column 121 may include a vertical portion 121a and a horizontal portion 121b. The horizontal portion 121b is disposed at the top of the vertical portion 121a. The horizontal portion 121b is disposed perpendicular to the vertical portion 121a. The horizontal portion 121b is coupled to the guide block 150.

The vertical portion 121a includes a sliding block 121c. The sliding block 121c is disposed opposite to the rail 210. The sliding block 121c is slidably coupled to the rail 210.

As the column 121 moves along the bracket 200, the left and right (y) positions of the first transfer unit 100A, the second transfer unit 100B, and the third transfer unit 100C can be adjusted, respectively. .

Meanwhile, on one side of the bracket 200, there is a scale mark (T) that visually confirms the left and right (y) positions of the first transfer unit (100A), the second transfer unit (100B), and the third transfer unit (100C). It can be provided. And an indicator (ID) aligned with the scale mark (T) may be mounted on the horizontal portion (121b).

The user checks the scale mark (T) and indicator (ID) and uses the first transfer unit (100A), the second transfer unit (100B), and the third transfer unit in response to the left and right (y) positions of the electrode (S). The left and right (y) position of the unit (100C) can be directly adjusted.

Figure 10 is a diagram showing the fixing part 170.

Referring to FIGS. 8 to 10 , it may include a fixing part 170 connecting the column 121 and the bracket 200. The fixing unit 170 is a device for selectively fixing or releasing the transfer unit 100 to the bracket 200.

A slot (ST) is disposed inside the rail 210. The slot (ST) is arranged long along the left and right directions (y). The fixing part 170 is arranged to be movable along the slot ST.

The fixing part 170 may include a fixing block 171, a shaft part 172, and a handle 173. The fixed block 171 is coupled to one side of the shaft portion 172. The handle 173 is coupled to the other side of the shaft portion 172. The shaft portion 172 is rotationally fastened to the fixed block 171. The fixed block 171 is placed in the slot (ST).

A portion of the horizontal portion 121b is disposed above the slot ST. The shaft portion 172 passes through the horizontal portion 121b and is fastened to the fixed block 171.

When the handle 173 is operated and the shaft portion 172 rotates, the shaft portion 172 and the fixing block 171 tighten the rail 210 and the horizontal portion 121b, so that the fixing portion 170 is connected to the transfer unit 100. ) can be fixed to the rail 210. Conversely, when the handle 173 is operated in the opposite direction and the shaft portion 172 rotates in the opposite direction, the fixing block 171 and shaft portion 172 that tightened the rail 210 and the horizontal portion 121b are released and the transfer unit 100 This can be released from the rail 210 and set to move along the slot (ST).

FIG. 11 is a diagram showing the driving unit 140, and FIG. 12 is an enlarged view of the driving unit 140 shown in FIG. 11.

Referring to FIGS. 11 and 12 , the drive unit 140 provides driving force to move the belt 130. The driving unit 140 may include a spline shaft 141, a spline nut 142, and a roll 143.

The spline shaft 141 is connected to a driving source such as a motor. The spline nut 142 is coupled to the spline shaft 141. The spline nut 142 rotates as the spline shaft 141 rotates, but is movable along the spline shaft 141. A roll 143 may be disposed between the spline nuts 142. The roll 143 rotates together with the spline shaft 141 to induce rotation of the belt 130. Belt 130 is mounted on roll 143.

The driving unit 140 includes a connecting bar 144. One side of the connecting bar 144 is connected to the spline nut 142, and the other side of the connecting bar 144 is connected to the column 121. Accordingly, as the left-right (y) position of the column 121 changes, the left-right (y) position of the spline nut 142 also changes, and in conjunction with this, the position of the roll 143 also changes.

Figure 13 is a plan view showing the left and right ends Sa of the electrode S are not aligned with the belt 130, and Figure 14 shows the left and right ends Sa of the electrode S aligned with the belt 130. ) is a front view showing the state in which it is not aligned.

13 and 14, if the left and right ends Sa of the electrode S are not aligned with the belt 130, the left and right ends Sa of the electrode S may sag due to their own weight. You can. For example, if the width (D) of the electrode (S) is too large to fit the position of the belt 130, or the left-right (y) position of the electrode (S) is misaligned with the belt 130, the electrode ( Both left and right ends (Sa) or one end of S) may not be aligned with the suction hole 131 of the belt 130.

In this way, the end Sa of the electrode S disposed outside the suction hole 131 of the belt 130 of the first transfer unit 100A and the belt 130 of the third transfer unit 100C is adsorbed. Because it does not work, sagging may occur due to its own weight.

FIG. 15 is a plan view showing a state in which the transfer unit 100 is moved outward and both left and right ends (Sa) of the electrode (S) are aligned with the belt 130, and FIG. 16 shows the transfer unit 100 being moved to the outside. This is a front view showing a state in which both left and right ends (Sa) of the electrode (S) are aligned with the belt 130 by moving to .

15 and 16, in a state where both left and right ends (Sa) of the electrode (S) are not aligned with the belt 130, the user uses the first transfer unit 100A as a reference in the left and right direction (y). The third transfer unit (100C) is moved to one side away from the second transfer unit (100B), and the third transfer unit (100C) is moved to the other side so as to be away from the second transfer unit (100B) based on the left and right direction (y), and the electrode (S ) The left and right direction (y) position of the transfer unit 100 can be adjusted so that both left and right ends (Sa) are aligned with the suction hole 131 of the belt 130.

Since the left and right ends Sa of the electrode S are aligned and absorbed into the suction hole 131 of the belt 130, it is possible to prevent the left and right ends Sa of the electrode S from sagging.

FIG. 17 is a plan view showing another state in which the left and right ends Sa of the electrode S are not aligned with the belt 130, and FIG. 18 shows the left and right ends Sa of the electrode S aligned with the belt 130. This is a front view showing another state that is not aligned in 130).

Referring to FIGS. 17 and 18 , since the width of the electrode S is small compared to the position of the belt 130, both left and right ends Sa of the electrode S may not be aligned with the belt 130. In this case, because the electrode S is adsorbed only by the belt 130 disposed in the second transfer unit 100B, significant sagging may occur at both left and right ends Sa of the electrode S.

FIG. 19 is a plan view showing a state in which the transfer unit 100 is moved inward and both left and right ends (Sa) of the electrode (S) are aligned with the belt 130, and FIG. 20 shows the transfer unit 100 being moved inward. This is a front view showing a state in which both left and right ends (Sa) of the electrode (S) are aligned with the belt 130 by moving to .

Referring to FIGS. 19 and 20, in a state where both left and right ends (Sa) of the electrode (S) are not aligned with the belt 130, the user uses the first transfer unit (100A) as a reference in the left and right direction (y). moved to the other side to be closer to the second transfer unit (100B), and moved the third transfer unit (100C) to one side to be closer to the second transfer unit (100B) based on the left and right direction (y), electrode (S) ) The left and right direction (y) position of the transfer unit 100 can be adjusted so that both left and right ends (Sa) are aligned with the suction hole 131 of the belt 130.

In this way, in response to the width of the electrode (S) or the position of the left and right (y) end portions of the electrode (S), the user adjusts the left and right (y) position of the transfer unit 100 to move the left and right sides of the electrode (S). By aligning both ends Sa and the suction hole 131 of the belt 130, it is possible to prevent the ends of the electrode S from sagging.

Figure 21 is a diagram showing a pusher 160 that pushes the electrode (S).

Referring to FIGS. 6 and 22 , the pusher 160 serves to push the electrode S adsorbed on the belt 130 and cause the electrode S to fall toward the magazine.

When the electrode (S) is transferred to the upper side of the magazine, the pusher 160 operates to push the electrode (S) so that the electrode (S) falls to be loaded in the magazine.

As shown in FIG. 6, the pusher 160 is disposed between adjacent suction plates 110 based on the left and right direction (y). Since the pusher 160 along with the transfer unit 100 also moves along the left and right direction (y), it corresponds to the electrodes (S) of different widths or the left and right (y) positions of the electrodes (S) in a balanced manner. There is an advantage in being able to push S) to the magazine side.

Above, specific embodiments of the electrode production system for secondary batteries of the present invention have been described, but it is obvious that various implementation modifications are possible without departing from the scope of the present invention.

Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined by the claims and equivalents thereof as well as the claims described later.

That is, the above-described embodiments should be understood in all respects as illustrative and not restrictive, and the scope of the present invention is indicated by the claims to be described later rather than the detailed description, and the meaning and scope of the claims and All changes or modified forms derived from the equivalent concept should be construed as falling within the scope of the present invention.

10: transfer unit
100: transfer unit
110: Suction plate
120: body
121: column
121a: vertical part
121b: horizontal part
130: belt
140: Belt driving part
150: Guide block
160: pusher
170: fixing part
200: bracket

Claims (10)

An electrode production system for secondary batteries that includes a transfer unit for transferring electrodes to a magazine for loading electrodes,
The transfer unit
It includes a plurality of transfer units arranged along the front and rear directions and spaced apart in the left and right directions, a bracket arranged along the left and right directions, and a driving unit connected to the transfer units,
The transfer unit is,
A body including a suction plate and a column connected to the suction plate and arranged in a vertical direction;
A belt disposed along the body and connected to the driving unit; And
An electrode production system for a secondary battery in which the column is arranged to slide left and right on the bracket, and the distance between the transfer units in the left and right directions is adjusted. According to claim 1,
The body includes a guide block,
In the vertical direction, one end of the column is coupled to the suction plate, and the other end of the column is coupled to a guide block,
An electrode production system for secondary batteries in which the belt is disposed and guided inside the guide block. According to clause 2,
The column includes a horizontal portion and a vertical portion disposed perpendicular to the horizontal portion,
The horizontal portion is coupled to the guide block,
An electrode production system for secondary batteries in which the vertical portion is slidably coupled to the bracket. According to clause 3,
The bracket includes rails arranged along the left and right directions,
The column includes a sliding block disposed opposite the rail,
An electrode production system for secondary batteries in which the sliding block is movably coupled to the rail. According to clause 4,
Further comprising a fixing part,
The fixing part,
A fixed block movably disposed inside the rail;
a shaft portion that penetrates the horizontal portion and is rotationally fastened to the fixed block;
Includes a handle coupled to the shaft portion,
An electrode production system for a secondary battery in which the fixing block fixes or releases the horizontal portion to the rail portion as the shaft portion rotates by the handle. According to claim 1,
The driving unit includes a spline shaft, a spline nut coupled to the spline shaft, and a roll rotatably coupled to the spline nut,
The belt is mounted on the roll,
An electrode production system for secondary batteries in which the spline nut moves left and right along the spline shaft. According to claim 1,
Further comprising a pusher disposed on the body,
The pusher moves in an upward and downward direction, selectively contacting the electrode and pushing the electrode from the suction plate to the magazine. According to clause 7,
The pusher is an electrode production system for secondary batteries in which the pusher is disposed between adjacent suction plates in the left and right directions. According to claim 1,
An electrode production system for secondary batteries in which a plurality of the columns are spaced apart along the front-back direction. According to claim 1,
An electrode production system for secondary batteries in which the plurality of brackets are spaced apart along the front-back direction.

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