JP2011251399A - Grasp device and grasp device-equipped robot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a grasp device capable of stably grasping a work.SOLUTION: The grasp device 1 grasping the work W includes: a plurality of base members 2 where a horizontal interval can be adjusted; grasp members 4 vertically movably installed at the base members 2, respectively, by a link mechanism 5; and stopper members 7 capable of restricting the movement of the link mechanism 5. The link mechanism 5 has: the base members 2; the grasp members 4; a pair of links 5a, 5b provided side by side; first ends 5a1, 5b1 where the respective links 5a, 5b are rotatably connected to the base members 2; and second ends 5a2. 5b2 where the respective links 5a, 5b are rotatably connected to the grasp members 4. The stopper member 7 has a first stopper 7a capable of restricting the movement of the grasp members 4 downward in a position where the second ends 5a2, 5b2 of the links 5a, 5b are located at the same heights as the first ends 5a1, 5a2 or higher than the first ends 5a1, 5b1.

Description

  The present invention relates to a gripping device that grips a workpiece.

  As described in Patent Document 1, the conventional gripping device includes a pair of tongs extending in the vertical direction, a holding member to which an intermediate portion of the pair of tongs is rotatably connected, and a lower end portion of each tongue. And a stopper that restricts the tip of the cam from being below the rotation center of the cam. Accordingly, when the lower ends of the pair of tongues are brought close to each other and the pair of cams are brought into contact with the workpiece, the tip ends of the cams tilt upward with respect to the tongue. When the tongue is lifted, the gravity of the workpiece is converted into the rotational force of the cam by the friction force generated at the tip of the workpiece and the cam, the cam pushes the workpiece by the rotational force of the cam, and the workpiece is gripped by the pair of cams.

JP-A-10-95590

  However, since the cam tilts with respect to the workpiece, the position where the cam and the worm strike can change as the cam tilts. Therefore, there are cases where the workpiece cannot be gripped stably. In addition, since the cam is in linear contact with the workpiece, stress is concentrated on a part of the workpiece. Therefore, it is not easy to grip a flexible workpiece with a cam. Therefore, a gripping device that can grip a workpiece stably has been conventionally required.

  In order to solve the above-mentioned problems, the present invention is a gripping device having a configuration as described in each claim. According to one aspect, the gripping device of the present invention includes a plurality of base members whose horizontal spacing can be adjusted, a gripping member that is operably attached to each base member by a link mechanism, and a movement of the link mechanism. And a stopper member capable of regulating the pressure. The link mechanism includes a base member, a gripping member, a pair of links arranged side by side, a first end where each link is rotatably connected to the base member, and a second where each link is rotatably connected to the gripping member. And an end portion. The stopper member is a first stopper that can regulate the downward movement of the gripping member at a position where each second end of each link is at the same height as each first end or above each first end. Have.

  Therefore, the holding member is pushed by the work by adjusting the horizontal interval between the plurality of base members and bringing the holding member into contact with the work. The gripping member moves upward substantially parallel to the base member by the link mechanism. Therefore, the gripping member can move while facing the workpiece. Thus, the position where the gripping member and the workpiece come into contact with each other does not move greatly, for example, compared with a conventional cam that tilts with respect to the base member. Therefore, the work can be stably gripped by the gripping member. Further, since the gripping member can be moved substantially in parallel by the link mechanism, the gripping member can easily be configured to come into contact with the workpiece in a relatively wide area. As a result, the concentration of stress that can be applied to the workpiece is reduced, and a flexible workpiece can be gripped by the gripping member.

It is a perspective view of a holding device and a work. It is a front view of a holding device and a work. It is a front view of the gripping device when the gripping device grips a workpiece. It is a partial front view of the gripping device when the gripping device grips a workpiece. It is a front schematic diagram of the holding | gripping apparatus in another form. It is a front schematic diagram of the gripping device in another form when the gripping device grips the workpiece.

  One embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the gripping device 1 includes a pair of base members 2 and a pair of gripping members 4. The base member 2 has a main body plate portion 2a and a flange portion 2b that stands on the front and rear ends of the main body plate portion 2a. The pair of base members 2 are opposed to each other and can be moved in the horizontal direction by the actuator 3. As a result, the horizontal distance between the pair of base members 2 can be adjusted. The horizontal direction is a direction substantially orthogonal to the direction of gravity and is the same direction as the substantially horizontal line.

The actuator 3 is preferably, for example, a cylinder type or a ball screw type. The cylinder-type actuator 3 has a cylinder and a piston that move relatively by hydraulic pressure or atmospheric pressure. One base member 2 is mounted on the cylinder, and the other base member 2 is mounted on the piston. The ball screw type actuator 3 has a male screw member that is rotated by a motor and a female screw member that is screwed into the male screw member. One base member 2 is attached to the male screw member, and another one base member 2 is attached to the female screw member side.

  As shown in FIG. 1, the holding member 4 includes a main body plate portion 4a, a flange portion 4c formed on the upper and lower edges of the main body plate portion 4a, and a plurality of ribs 4b formed on the back surface of the main body plate portion 4a. . The plurality of ribs 4 b are formed at positions corresponding to the flange portion 2 b of the base member 2. Each rib 4b protrudes from the back surface of the main body plate portion 4a toward the base member 2 and extends between the upper and lower flange portions 4c.

  A pair of link mechanisms between the base member 2 and the gripping member 4 that operatively connect the gripping member 4 to the base member 2 in the vertical direction (direction substantially along the direction of gravity) as shown in FIGS. 5 is provided. The pair of link mechanisms 5 are provided on the front side and the back side of the base member 2. Each link mechanism 5 has a first link 5a and a second link 5b arranged in parallel in the vertical direction.

  As shown in FIGS. 1 and 2, the first and second links 5 a and 5 b include first end portions 5 a 1 and 5 b 1 that are rotatably attached to the base member 2, and a second end that is rotatably attached to the holding member 4. It has parts 5a2 and 5b2. The first end 5a1 of the first link 5a is attached to the upper part of the flange part 2b so as to be rotatable in the vertical direction, and the second end part 5a2 is attached to the upper part of the rib 4b so as to be rotatable in the vertical direction. The first end portion 5b1 of the second link 5b is attached to the lower portion of the flange portion 2b so as to be rotatable in the vertical direction, and the second end portion 5b2 is attached to the lower portion of the rib 4b so as to be rotatable in the vertical direction. In this embodiment, the first and second links 5a and 5b are attached two by two.

  As shown in FIGS. 2 and 3, a stopper member 7 is provided between the base member 2 and the gripping member 4. The stopper member 7 is attached to the base member 2 and has a first stopper 7a and a second stopper 7b. The 1st stopper 7a is formed in the upper part of the stopper member 7, and protrudes below the 1st link 5a. When the first link 5a comes into contact with the first stopper 7a, the downward tilt of the first link 5a with respect to the base member 2 is restricted.

  Therefore, the first stopper 5a restricts the first link 5a from moving downward at the same height as the first end 5a1 or above the first ends 5a1 and 5b1. The Preferably, when the angle 11a between the first link 5a and the horizontal line is 30 ± 5 °, the downward tilt of the first link 5a with respect to the base member 2 is restricted.

  By restricting the downward tilt of the first link 5a, the downward tilt of the second link 5b with respect to the base member 2 is also restricted as shown in FIG. That is, the second link 5b is indirectly restricted from moving by the first stopper 7a so that the second end portion 5b2 moves downward from the first end portion 5b1. Preferably, when the angle between the second link 5b and the horizontal line is 30 ± 5 °, the downward tilt of the second link 5b is restricted.

  As shown in FIG. 3, the second stopper 7 b is formed on the side of the stopper member 7 and faces the gripping member 4. The second stopper 7 b limits the distance that the gripping member 4 is close to the base member 2. When the gripping member 4 comes into contact with the second stopper 7b, the upward tilting of the links 5a and 5b with respect to the base member 2 is restricted. Preferably, when the angle 11b between the links 5a and 5b and the horizontal line is 45 ± 5 °, the upward tilting of the links 5a and 5b is restricted.

  As shown in FIGS. 2 and 3, a biasing member 6 that biases the gripping member 4 downward with respect to the base member 2 is provided between the base member 2 and the gripping member 4. The biasing member 6 is a tension spring such as a coil spring. The biasing member 6 has a first end portion 6a attached to the base member 2 and a second end portion 6b attached to the grip member 4, and the second end portion 6b is always more than the first end portion 6a. Located above. The biasing member 6 presses the first link 5a against the first stopper 7a by biasing the gripping member 4. Therefore, the first link 5a is pressed against the first stopper 7a by the urging force of the urging member 6 and the weight of the gripping member 4 and the like.

  As shown in FIGS. 2 and 3, the gripping member 4 is provided inside the base member 2, that is, between the pair of base members 2. The pair of gripping members 4 face each other and can hold a work (luggage) W located between them. The main body plate portion 4a of the grip member 4 is formed with a grip surface 4a1 that can push the workpiece W with the surface.

  The gripping surface 4a1 is substantially flat as shown in FIGS. 2 and 3, and the anti-slip member 8 is attached to the gripping surface 4a1. The anti-slip member 8 has a sheet shape and covers substantially the entire gripping surface 4a1. The anti-slip member 8 is made of rubber or the like and has a friction coefficient of 2 or more, preferably 2.03, more preferably 2.1 or more.

  Between the base member 2 and the gripping member 4, a gripping detector 9 that can detect that the gripping device 1 grips the workpiece W is provided as shown in FIGS. The grip detector 9 is a proximity sensor and is provided in one of both the base member 2 and the grip member 4 and transmits a detection signal corresponding to the distance between the two members to a control device (not shown). Based on the detection signal from the grip detector 9, the control device determines that the distance between the two members is equal to or smaller than a predetermined value and the grip device 1 grips the workpiece W.

  When the workpiece W is gripped by the gripping device 1, first, the gripping device 1 is brought close to the workpiece W as shown in FIGS. For example, when the gripping device 1 is mounted on a robot arm, the gripping device 1 is brought close to the workpiece W by the arm. The direction in which the gripping device 1 is brought close to the work W is the horizontal direction when the work W is installed on a shelf or the like, and when the upper side of the work W is open, the direction is from the top to the bottom or the horizontal direction.

  Next, as shown in FIGS. 2 and 3, the gripping device 1 is positioned with respect to the workpiece W so that the workpiece W is positioned between a pair of gripping members 4 arranged in parallel in the horizontal direction. Subsequently, the pair of base members 2 are brought close to each other in the substantially horizontal direction by the actuator 3 that is a power source of the arm. As a result, the pair of gripping members 4 come into contact with the side surfaces of the workpiece W, and the gripping members 4 are pushed by the workpiece W. The first and second links 5a, 5b tilt upward against the urging force and gravity of the urging member 6. As a result, the gripping member 4 moves upward and parallel to the base member 2.

  As the distance between the pair of base members 2 decreases, the force with which the gripping member 4 pushes the workpiece W increases. As a result, as shown in FIGS. 2 and 3, the frictional force between the anti-slip member 8 and the workpiece W increases, and the workpiece W is gripped by the frictional force. The gripped work W can be lifted from the floor surface 10. Next, the gripping member 4 hits the second stopper 7b. At this time, based on the detection signal transmitted from the grip detector 9, the control device determines that the grip device 1 has gripped the workpiece W.

  Next, the control device controls the actuator 3 to hold the pair of base members 2 in position. As a result, the workpiece W is gripped by the gripping device 1 with a constant gripping force. When the gripping device 1 is lifted by the robot, the gripping device 1 lifts the workpiece W. At this time, the dead weight of the workpiece W is converted into the operation of the link mechanism 5 by the frictional force between the workpiece W and the gripping member 4. As a result, the gripping member 4 moves toward the workpiece W, and the force for gripping the workpiece W of the gripping member 4 is improved. Next, the robot conveys the workpiece W together with the gripping device 1 to a predetermined position.

  When the workpiece W is gripped by the gripping device 1, the frictional force N is converted into a gripping force F by the link mechanism 5 and the gripping member 4 as shown in FIG. 4. At this time, the relationship between the frictional force N and the gripping force F is expressed by the following equations (1) to (5). m: workpiece mass, μ: coefficient of static friction of the anti-slip member 8, Θ: angle between the first link 5a and the horizontal line.

N = 1/2 × mg (Formula 1)
N is decomposed into the extending direction Y1 of the first link 5a and the X1 direction perpendicular to Y1,
Fx = 1/2 × mg × cos Θ (Formula 2)
F = 1/2 × mg × cos Θ × sin Θ (Formula 3)
The conditions under which the workpiece W does not fall are
μF = 1/2 × μmg × cos Θ × sin Θ> N = 1/2 × mg (Formula 4)
From (Equation 4) μ> 1 / (cosΘ × sinΘ) (Equation 5)

From (Equation 5), the static friction coefficient μ and the angle Θ between the first link 5a and the horizontal line are shown in Tables 1 and 2.

  Therefore, from Tables 1 and 2, the static friction coefficient μ is 2 which is the minimum at Θ = 45 °. Accordingly, it can be seen that the workpiece W is most difficult to drop from the gripping device 1 at Θ = 45 °. It can also be seen that by setting μ to 2 or more at Θ = 45 °, the gripping device 1 can grip the workpiece W regardless of the weight or inertia acting on the workpiece W. At Θ = 45 ± 5 °, by setting μ to 2.03 or more, preferably 2.1 or more, the gripping device 1 can grip the workpiece W regardless of the weight or inertia acting on the workpiece W. Recognize.

  As described above, the gripping device 1 includes a plurality of base members 2 whose horizontal intervals can be adjusted as shown in FIG. 2, and grips that are attached to the base members 2 so as to be vertically movable by the link mechanism 5. It has the member 4 and the stopper member 7 which can regulate the motion of the link mechanism 5. The link mechanism 5 includes a base member 2, a gripping member 4, a pair of links 5a and 5b arranged in the vertical direction, and first ends 5a1 and 5b1 in which the links 5a and 5b are rotatably connected to the base member 2. , And second links 5a2 and 5b2 that are rotatably connected to the gripping member 4. The stopper member 7 is configured such that the second end portions 5a2 and 5b2 of the links 5a and 5b are at the same height as the first end portions 5a1 and 5b1 or at positions above the first end portions 5a1 and 5b1. Has a first stopper 7a that can restrict the downward movement of the first stopper 7a.

  Accordingly, the gripping member 4 is pushed by the workpiece W by adjusting the horizontal interval between the plurality of base members 2 and bringing the gripping member 4 into contact with the workpiece W. The gripping member 4 is moved upward substantially parallel to the base member 2 by the link mechanism 5. Therefore, the gripping member 4 can move while facing the workpiece W. Thus, the position where the gripping member 4 and the workpiece W come into contact with each other does not move greatly, for example, compared with a conventional cam that tilts with respect to the base member 2. Therefore, the workpiece W can be stably gripped by the gripping member 4. Further, since the gripping member 4 can be moved substantially in parallel by the link mechanism 5, it is easy to adopt a configuration in which the gripping member 4 can come into contact with the workpiece W with a relatively large area. Thereby, the stress concentration that can be applied to the workpiece W is reduced, and the flexible workpiece W can be gripped by the gripping member 4.

  As shown in FIG. 4, the pair of links 5a and 5b have the same distance (length) between the first end portions 5a1 and 5b1 and the second end portions 5a2 and 5b2, and the first end portions 5a1 and 5b1. The lines connecting the second ends 5a2 and 5b2 are parallel. Therefore, the link mechanism 5 having a pair of links 5 a and 5 b is a so-called parallel link, and the gripping member 4 moves in parallel to the base member 2. Therefore, the holding member 4 can move while facing a rectangular parallelepiped work W such as a cardboard box. Thus, the gripping member 4 can easily hit the workpiece W in parallel and can grip the workpiece W stably.

  As shown in FIG. 2, a pair of gripping members 4 attached to each base member 2 by each link mechanism 5 are disposed between the pair of base members 2. Each of the pair of gripping members 4 is formed with a gripping surface 4a1 that faces each other and can grip the workpiece W. Therefore, the workpiece W can be gripped by the pair of gripping members 4 by bringing the pair of base members 2 close to each other. Since the gripping surfaces 4a1 formed on each gripping member 4 face each other, the workpiece W can be stably gripped by the pair of gripping surfaces 4a1.

  As shown in FIG. 3, the gripping device 1 has a second stopper 7 b that restricts the upward movement of the gripping member 4 relative to the base member 2 at a predetermined height. Therefore, the grip member 4 moves upward with respect to the base member 2, and the links 5 a and 5 b are suppressed from being at a predetermined angle with respect to the base member 2. For this reason, the second stopper 7b can suppress the gripping force of the workpiece W by the gripping member 4 from being reduced by the links 5a and 5b having a predetermined angle or more with respect to the base member 2.

  The stopper member 7 further has a second stopper 7b as shown in FIGS. Therefore, the stopper member 7 has not only the first stopper 7a but also the second stopper 7b. Therefore, the number of parts of the gripping device 1 can be reduced.

  As shown in FIG. 3, the second stopper 7b is configured to be able to restrict the upward tilting of the pair of links 5a, 5b with respect to the base member 2 at 45 ° (or 45 ± 5 °) with respect to the horizontal line. Yes. Therefore, by pressing the gripping member 4 against the workpiece W, the links 5a and 5b are tilted to 45 ° with respect to the horizontal line by the second stopper 7b. Therefore, as shown in Tables 1 and 2, the frictional force N can be effectively converted into the gripping force F. Thus, the workpiece W can be effectively lifted by the gripping member 4.

  As shown in FIG. 3, the gripping member 4 is provided with an anti-slip member 8 having a static friction coefficient of 2 (preferably 2.03) or more and capable of coming into contact with the workpiece W. Therefore, when the gripping member 4 grips the workpiece W, the first and second links 5a and 5b are set to 45 ° from the horizontal line, and the static friction coefficient between the workpiece W and the gripping member 4 is set to 2 or more. Thereby, as shown in Tables 1 and 2, the workpiece W can be lifted by the gripping force F that can be converted from the frictional force N regardless of the weight of the workpiece W or the like.

  A proximity sensor (gripping detector 9) that can detect that the gripping member 4 and the base member 2 are close to each other is provided between the gripping member 4 and the base member 2. Therefore, it can be detected that the gripping member 4 grips the work W by the proximity sensor detecting that the distance between the gripping member 4 and the base member 2 is closer than the set distance, not the work W. . Therefore, it is possible to accurately detect that the gripping member 4 grips the workpiece W regardless of the material of the workpiece W or the like.

  The robot has a plurality of arms whose intervals are adjusted by the actuator 3. Each base member 2 is attached to each arm. Therefore, the robot can stably lift the workpiece W by the gripping device 1 provided on the arm.

(Other embodiments)
The present invention is not limited to the above-described embodiment, and may be the following form. For example, as shown in FIGS. 5 and 6, in another embodiment, the base member 2 and the gripping member 4 are inclined with respect to the vertical line, and one of the gripping members 4 has a gripping surface 4a2 facing upward. The other has a gripping surface 4a3 facing downward. When the pair of base members 2 are brought close to each other by the actuator 3, the pair of gripping members 4 press the workpiece W, and the workpiece W is inclined by the gripping surfaces 4 a 2 and 4 a 3 of the gripping member 4.

  As shown in FIGS. 5 and 6, the gripping surface 4 a 2 formed on one of the gripping members 4 is configured to face obliquely upward. Therefore, the workpiece W is supported from below by the gripping surface 4a2 that faces obliquely upward. Accordingly, the workpiece W is unlikely to fall from the gripping surface 4a2, and can be gripped by the gripping member 4 stably.

  In another embodiment, the gripping device 1 may include a pair of base members 2 and a pair of gripping members 4, and the pair of base members 2 may be disposed between the pair of gripping members 4. . Therefore, the gripping device 1 may be configured to be inserted into the workpiece W, the pair of base members 2 are separated by the actuator 3, and the gripping member 4 is pressed against the inner peripheral surface of the workpiece W.

  In another embodiment, the gripping device 1 may be used in a predetermined direction, for example, in a direction in which the pair of base members 2 are in a vertical positional relationship.

  In another form, the urging member 6 may be an air spring, a torsion spring, a compression spring, an actuator or the like that generates an urging force instead of the tension spring.

  In another embodiment, the gripping device 1 may have a first stopper member having the first stopper 7a and a second stopper member having the second stopper 7b separately.

  In another embodiment, the grip detector 9 is an encoder, and detects the angle of the links 5a, 5b with respect to the base member 2 or the angle of the links 5a, 5b with respect to the grip member 4, and transmits a detection signal to the control device. The control device may determine that the workpiece W has been gripped by the gripping device 1 based on the detection signal.

  In another embodiment, the side surface of the workpiece W may be a curved surface, for example, a curved surface, and the gripping member 4 may have a surface corresponding to the side surface of the workpiece W, for example, a curved gripping surface 4a1.

  In another embodiment, the gripping device 1 may be mounted on an industrial warehouse such as a cart of an automatic warehouse, a forklift, or a suspension device.

  In another embodiment, the gripping device 1 may rotate after gripping the workpiece W so that the gripping surface 4a2 faces upward as shown in FIG.

  In another form, the gripping device 1 has tongs extending from each base member 2, a middle portion of the tongs is rotatably connected to the support member, and a pair of tongs approaches and is separated from each other. The base member 2 may approach or separate in the horizontal direction while drawing an arc.

  In another embodiment, the urging member 6 may be configured such that the urging force is adjusted depending on the material of the workpiece W, and may be adjusted to reduce the spring coefficient in the case of the flexible workpiece W, for example. .

  In another form, a plurality of gripping members may be attached to one base member by each link mechanism. Alternatively, one holding member may be attached to a plurality of holding members by each link mechanism.

  In another form, the gripping member may be attached to the base member so as to be movable up and down by a link mechanism having another configuration having one degree of freedom like the link mechanism 5.

  In another embodiment, the pair of links 5a and 5b may have different distances (lengths) between the first end portions 5a1 and 5b1 and the second end portions 5a2 and 5b2. Alternatively, in the pair of links 5a and 5b, the lines connecting the first end portions 5a1 and 5b1 and the second end portions 5a2 and 5b2 may not be parallel. In addition, as long as the gripping member can move up and down with respect to each base member, the pair of links 5a and 5b may not be arranged in parallel in the vertical direction. Similarly, the number of the pair of links 5a and 5b is not limited to two.

DESCRIPTION OF SYMBOLS 1 ... Gripping device 2 ... Base member 3 ... Actuator 4 ... Gripping member 4a ... Main body board part 4a1, 4a2, 4a3 ... Gripping surface 5 ... Link mechanism 5a ... First link 5a1, 5b1, 6a ... First end part 5a2, 5b2, 6b ... second end 5b ... second link 6 ... biasing member 7 ... stopper member 7a ... first stopper 7b ... second stopper 8 ... anti-slip member 9 ... gripping detector

Claims (10)

A gripping device for gripping a workpiece,
A plurality of base members capable of adjusting a horizontal interval; a gripping member that is operatively attached to each of the base members by a link mechanism; and a stopper member that can restrict the movement of the link mechanism. ,
The link mechanism includes the base member, the gripping member, a pair of links arranged side by side, a first end portion where the links are rotatably connected to the base member, and the links rotating to the gripping member. Formed with a second end operably coupled,
The stopper member restricts the gripping member from moving downward when the second end portions of the links are at the same height as the first end portions or above the first end portions. A gripping device having a first stopper that can be used.   The pair of links are gripping devices in which the distances between the first end and the second end are the same, and the lines connecting the first end and the second end are parallel. The gripping device according to claim 1 or 2,
A pair of the gripping members attached to the base members by the link mechanisms are disposed between the pair of base members, and each of the pair of gripping members faces each other and holds the workpiece. A gripping device in which a gripping surface that can be gripped is formed. The gripping device according to any one of claims 1 to 3,
A gripping device provided with a second stopper for restricting upward movement of the gripping member relative to the base member at a predetermined height. The gripping device according to claim 4,
The stopper device further includes the second stopper. The gripping device according to claim 4 or 5,
The second stopper is a gripping device configured to be able to regulate upward tilting of the pair of links with respect to the base member at 45 ° with respect to a horizontal line. The gripping device according to claim 6,
The gripping device is provided with a non-slip member having a static friction coefficient of 2 or more and capable of coming into contact with the workpiece. The gripping device according to any one of claims 1 to 7,
A gripping device provided with a proximity sensor capable of detecting the proximity of the gripping member and the base member between the gripping member and the base member. The gripping device according to any one of claims 1 to 8,
A gripping device in which one of the gripping members is formed with a gripping surface that is in contact with the workpiece and faces obliquely upward. A robot comprising the gripping device according to any one of claims 1 to 9,
Having a plurality of arms whose spacing is adjusted by an actuator;
Each base member is a robot with a gripping device attached to each arm.

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