Robot joint structure

Abstract

The joint structure of the present invention connects the first element and the second element of the robot's manipulator, and has a locking mechanism that switches between a free state in which the second element can move freely independently relative to the first element and a locked state in which the second element is fixed to the first element. The locking mechanism comprises: a first component that is engaged with the first element; a second component that is engaged with the second element; and a flexible linear component, one end of which is mounted on the second component, and the other end is led out to the outside of the joint structure through a through hole provided in the first component. The second component is brought into contact with the first component by pulling the linear component, thereby becoming a locked state. The second component is separated from the first component by stretching the linear component, thereby becoming a free state.

Description

Technical Field

[0001] The present invention relates to a joint structure of a robot, and in particular to a joint structure of a manipulator suitable for a soft robot (soft robot). Background Art

[0002] Industrial robots generally have a manipulator composed of a high-rigidity mechanism, which controls the three-dimensional position of the end effector while measuring its state through sensors. However, robots composed only of high-rigidity mechanisms have difficulty performing actions that involve contact with an object, actions that require higher precision than the measurement error of the sensor, etc. As a method to solve such problems, the so-called soft robot (soft robot) method has been proposed. For example, a soft mechanism that is displaced or deformed by an external force is set on a part of the robot to automatically follow the shape of the object. The compliant mechanism described in Patent Documents 1 and 2 is also an example.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 5-192892

[0006] Patent Document 2: Japanese Patent Application Laid-Open No. 8-118281 Summary of the Invention

[0007] Technical problem to be solved by the invention

[0008] Mechanisms with high rigidity and soft mechanisms each have advantages and disadvantages. Therefore, the present inventors have conducted research to realize a hybrid mechanism that can switch freely between a mechanism with high rigidity and a mechanism with softness. In addition, the compliance mechanisms proposed in Patent Documents 1 and 2 are also equipped with a function of switching between a free state in which compliance action can be performed and a locked state in which compliance action cannot be performed. However, the movable range of the compliance mechanisms of Patent Documents 1 and 2 is very narrow (to the extent that a small displacement in the horizontal plane can be performed), and the applicable scenarios or uses are limited. In addition, since an actuator (cylinder, etc.) for driving the locking mechanism is provided inside the joint or connecting rod, there is also a disadvantage that the structure of the joint or connecting rod becomes complicated and large-scale.

[0009] The present invention has been made in view of the above-mentioned actual situation, and an object of the present invention is to provide a joint structure of a robot that can switch between a locked state and a free state having a wide movable range with a simple and compact structure.

[0010] Technical solutions to solve problems

[0011] The joint structure included in the present disclosure connects the first element and the second element of the robot's manipulator, and is characterized in that the joint structure has a locking mechanism, which switches between a free state in which the second element is free to move independently relative to the first element and a locked state in which the second element is fixed to the first element. The locking mechanism has: a first component, which is engaged with the first element; a second component, which is engaged with the second element; and a flexible linear component, one end of the linear component is installed on the second component, and the other end is led out to the outside of the joint structure through a through hole provided in the first component, and the second component is brought into contact with the first component by pulling the linear component, thereby becoming the locked state, and the second component is separated from the first component by stretching the linear component, thereby becoming the free state.

[0012] According to this structure, the switching between the locked state and the free state can be achieved by pulling the linear component to bring the second component into contact with the first component / stretching the linear component to separate the second component from the first component, which is an extremely simple structure. In addition, by leading the other end of the linear component to the outside of the joint structure, the driving source of the linear component can be arranged outside the joint structure. Therefore, the joint structure itself can be constructed compactly and lightweight. In addition, since the separation distance between the first component and the second component can be adjusted by the extension amount (stroke) of the linear component, it can also be easily achieved to increase the movable range between the first element and the second element as needed.

[0013] For example, in the free state, the second component may be separated from the first component to the extent that the second element is separated from the first element to form a movable range in which the second element can move freely with six degrees of freedom independently of the first element. The six degrees of freedom of movement are translation in the x-direction, y-direction, and z-direction and rotation around the x-axis, the y-axis, and the z-axis. "The extent to which the second element can move freely with six degrees of freedom independently of the first element" means, for example, "the extent to which the second element will not be constrained by the first element or will not physically interfere with the first element when six degrees of freedom of movement are applied to the second element in a fixed state of the first element." According to the locking mechanism disclosed in the present invention, the formation of such a large movable range can be easily achieved by appropriately designing the extension amount (stroke) of the linear component, or the shapes of the first component and the second component.

[0014] Alternatively, the joint structure may further include an elastic component that connects the first component to the second component. The restoring force of the elastic component stabilizes the relative position of the first and second elements even in a free state. On the other hand, when an external force acts on the second element, the elastic component deforms, thereby preventing the movement of the second element. This allows for flexible movement, such as easily moving the tip of the second element along a concave and convex surface.

[0015] Alternatively, the elastic member may include a nonlinear spring element, and the distance between the first member and the second member may be changed according to the pulling amount of the linear member, thereby making the rigidity of the elastic member variable.

[0016] Alternatively, one of the first and second components may be provided with a protrusion and the other with a recess, and in the locked state, the protrusion engages with the recess, thereby securing the first and second components. This type of engagement is suitable for achieving a strong securing force.

[0017] Alternatively, the protrusion may have a conical or frustum-shaped shape. That is, the side surface may be inclined so that it becomes narrower as it approaches the front end of the protrusion. In addition, the recess may have a shape corresponding to the shape of the protrusion. According to the interlocking structure composed of such a protrusion and a recess, when the linear component is gradually pulled to bring the first component and the second component closer, the protrusion is guided by the inner surface of the recess and is automatically guided in a manner that the axes of the protrusion and the recess are consistent. Therefore, interlocking failure (failure to switch to the locked state) can be suppressed.

[0018] Alternatively, the convex portion may have a conical shape or a truncated cone shape. By using a convex portion with a rotationally symmetrical shape, an angular offset (rotation of the convex portion around the axis) between the first component (first element) and the second component (second element) is allowed, and thus, the failure of the fitting can be further suppressed. In addition, in this case, a protrusion may be provided on the side of the convex portion, and a guide groove for guiding the protrusion may be provided on the concave portion. According to this structure, when the convex portion is inserted into the concave portion, the protrusion is guided by the guide groove, and the angular offset between the convex portion and the concave portion is automatically corrected. Therefore, the suppression of the fitting failure and the prevention of the angular offset can be achieved at the same time.

[0019] The convex portion and the concave portion may be provided in only one set, or may be provided in a plurality of sets.

[0020] Alternatively, the first and second components may be provided with mutually parallel surfaces, and in the locked state, friction is generated by the surface contact between the first and second components, thereby securing the first and second components. This friction method is suitable for making the joint structure more compact.

[0021] The present invention can be understood as a joint structure of a robot having at least a portion of the above-mentioned structure, and can also be understood as a locking mechanism for fixing the elements of the robot's manipulator, or a state switching device for switching the elements of the robot's manipulator between a free state and a locked state. In addition, it can also be understood as a manipulator of a robot or a robot having such a joint structure, locking mechanism, or state switching device. In addition, the present invention can also be understood as a locking method for fixing the elements of the robot's manipulator using the joint structure of the above-mentioned structure, or a state switching method for switching the elements of the robot's manipulator between a free state and a locked state. In addition, the above-mentioned structures can be combined with each other as much as possible to constitute the present invention.

[0022] Effects of the Invention

[0023] According to the present invention, a joint structure of a robot can be provided that is capable of switching between a locked state and a free state having a wide movable range with a simple and compact structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of a joint structure with a locking mechanism.

[0025] Figure 2 This is a schematic diagram showing the overall structure of the robot.

[0026] Figure 3 A is a top view of the joint structure. Figure 3 B is Figure 3 AA section of A (locked state), Figure 3 C is Figure 3 AA section of A (free state), Figure 3 D is a diagram showing the behavior when an external force acts on the end effector in a free state.

[0027] Figure 4 This is a diagram showing the characteristics of the nonlinear spring used in Modification 1.

[0028] Figure 5 This is a diagram showing Modification Example 2.

[0029] Figure 6 This is a diagram showing Modification Example 3.

[0030] Figure 7 This is a diagram showing Modification Example 4. DETAILED DESCRIPTION

[0031] <Application Examples>

[0032] Reference Figure 1 , one of the application examples of the present invention is described. Figure 1 Schematic diagram of the joint structure of a robot with a locking mechanism.

[0033] The joint structure 1 connects a first element 11 and a second element 12 constituting the robot manipulator RM. The manipulator RM is a multi-jointed manipulator. The first element 11 is located at the base end of the manipulator RM, and the second element 12 is located at the tip end of the first element 11.

[0034] The joint structure 1 can take a free state in which the second element 12 is freely movable independently of the first element 11 and a locked state in which the second element 12 is fixed to the first element 11. The switching between the free state and the locked state is achieved by the locking mechanism 10.

[0035] The locking mechanism 10 generally includes a first member 110 engaged with the first element 11 , a second member 120 engaged with the second element 12 , a flexible linear member 130 , and a plurality of elastic members 140 connecting the first member 110 and the second member 120 .

[0036] The second component 120 has a protrusion 121 at its center, and the first component 110 has a recess 111 with a shape corresponding to that of the protrusion 121 at its center. A through-hole 113 and an exit hole 115 are formed on the bottom of the recess 111 and the sidewalls of the first component 110, respectively, for the linear component 130 to pass through. One end of the linear component 130 is fixed to the apex (center of the upper base) of the protrusion 121 of the second component 120, and the other end passes through the through-hole 113 and exit hole 115, leading to the outside of the joint structure 1 and connecting to the drive source M.

[0037] When the linear member 130 is pulled by the driving source M, the protrusion 121 engages with the recess 111, and the second member 120 is fixed to the first member 110, thereby entering a locked state. In this locked state, the first element 11 and the second element 12 act as a single rigid body.

[0038] When the drive source M is turned off (or the drive source M is used to feed the linear member 130), causing the linear member 130 to extend, the second member 120 separates from the first member 110 and becomes free. Extending the linear member 130 means that when the tension applied to the linear member 130 is released, the length of the linear member 130 extended from the through-hole 113 toward the second member 120 increases due to at least one of the weight of the second element 12 and the restoring force of the elastic member 140, which is compressed in the locked state. In the free state, the second element 12 is free to move independently of the first element 11. This allows the realization of a so-called "soft robot," also known as a soft robot or a compliant mechanism.

[0039] The driving source M may be a linear motor or a rotary motor. The driving source M may be driven electrically or by fluid pressure. Instead of applying a pulling force to the linear member 130 , the driving source M may drive the linear member 130 in a manner that determines the length of the linear member 130 to be drawn.

[0040] According to this structure, the switching between the locked state and the free state can be achieved by pulling the linear member 130 to bring the second member 120 into contact with the first member 110 / stretching the linear member 130 to separate the second member 120 from the first member 110, which is an extremely simple structure. In addition, by leading the other end of the linear member 130 to the outside of the joint structure 1, the driving source M of the linear member 130 can be arranged outside the joint structure 1. Therefore, the joint structure 1 itself can be constructed compactly and lightweight. In addition, since the separation distance between the first member 110 and the second member 120 can be adjusted by the extension amount (stroke) of the linear member 130, it is also possible to easily increase the movable range between the first element 11 and the second element 12 as needed.

[0041] <Implementation Method>

[0042] Reference Figure 2 , a robot and its joint structure involved in an embodiment of the present invention are described. Figure 2 This is a schematic diagram showing the overall structure of the robot.

[0043] In this embodiment, an example is shown in which the end effector E2 is attached to the manipulator RM of the vertical multi-joint robot R via the joint structure 1 having the locking mechanism. However, this is only an example, and the configuration and type of the robot R are not limited to Figure 2Example. Specifically, the robot R can be any robot as long as it has at least one manipulator, and can be applicable to various robots such as industrial robots, humanoid robots, nursing robots, transport robots, household robots, surgical support robots, etc. Among them, industrial robots are one of the robots that can be well applied to the joint structure 1 because there are many useful scenarios for switching between rigid robots and soft robots. In addition, in addition to vertical multi-joint robots, industrial robots also include horizontal multi-joint robots (SCARA robots), parallel linkage robots, orthogonal robots, etc. In addition, the applicable position of the joint structure 1 is not limited to the connection part of the end effector E2, but can also be applied to the connection part between the connecting rods.

[0044] The robot R of this embodiment mainly comprises a manipulator RM, a controller RC, and a drive source M. The manipulator RM is a multi-joint manipulator having multiple links and joints connecting the links, and is driven by a servo motor. The controller RC is a control device that controls the servo motor and drive source M of the manipulator RM.

[0045] The end effector E2 is connected to the link E1 at the front end of the robot RM via the joint structure 1. In this example, the link E1 corresponds to Figure 1 The first element 11, the end effector E2 is equivalent to Figure 1 The second element 12 is a device for driving the locking mechanism of the joint structure 1. The drive source M can be of any type and structure as long as it can pull and deliver the linear member 130. However, it is necessary to have a pulling force sufficient to ensure the rigidity of the joint structure 1 (the fixing force between the link E1 and the end effector E2) when the joint structure 1 is in the locked state, and a stroke sufficient to ensure the movable range of the end effector E2 when the joint structure 1 is in the free state. For example, a motor, hydraulic actuator, pneumatic actuator, etc. can also be used as the drive source M.

[0046] Figure 3 A to Figure 3 D shows a detail of the joint structure 1 . Figure 3 A is a top view of the joint structure 1 viewed from the link E1 side. Figure 3 B is Figure 3 AA section of A (locked state), Figure 3 C is Figure 3 AA section of A (free state), Figure 3 D is a diagram showing the operation when an external force acts on the end effector E2 in a free state.

[0047] The joint structure 1 comprises a generally cylindrical first component 110 and a generally disc-shaped second component 120 connected via a plurality of elastic components 140. In this embodiment, the first and second components 110, 120 are concentrically arranged and connected by three coil springs spaced evenly apart in the circumferential direction. In the locked state, the elastic components 140 are compressed, generating a restoring force that tends to separate the first and second components 110, 120. Even when the second component 120 is positioned below the first component 110, the weight of the second component 120 generates a force that tends to separate the first and second components 110, 120.

[0048] A truncated cone-shaped protrusion 121 is provided in the center of the second component 120, and a protrusion (locking pin) 122 is provided on the side of the protrusion 121. A through-hole 123 is formed in the upper base of the protrusion 121, and one end of the linear component 130 is inserted into and fixed to the through-hole 123. In this embodiment, a metal wire is used as the linear component 130. However, a cable (rope) made of chemical fiber or natural fiber can also be used as the linear component 130.

[0049] A recess 111 having a shape corresponding to the projection 121 is provided in the central portion of the first component 110, and a guide groove 112 for guiding and positioning the projection 122 is provided on the side surface of the recess 111. A through hole 113 is formed at the bottom of the recess 111. The linear component 130 passes through the through hole 113 and is wound around a pulley 114 provided in the hollow interior of the first component 110, and is then led out to the outside of the joint structure 1 through an outlet hole 115 formed in the side wall of the first component 110. In addition, the linear component 130 passes through the interior of a flexible tube (housing) 116 mounted on the outer wall of the first component 110 and is connected to the drive source M. In the path of the linear component 130 between the joint structure 1 and the driving source M, at least the curved portion of the linear component 130 passes through a non-elastic flexible tube 116 with fixed positions at both ends. When tension is applied to the linear component 130, the linear component 130 in the joint structure 1 displaces according to the displacement of the linear component 130 in the driving source M.

[0050] When the robot R is in operation, the driving source M pulls the linear member 130 and pulls the second member 120 toward the first member 110 until the second member 120 contacts (collides with) the first member 110. Then, the convex portion 121 engages with the concave portion 111, and the second member 120 is positioned and fixed to the first member 110, becoming a locked state ( Figure 3 B) In this locked state, the link E1 and the end effector E2 behave as a rigid body.

[0051] On the other hand, when the driving source M is turned off (or the linear member 130 is fed out by the driving source M) and the linear member 130 is extended, the second member 120 is separated from the first member 110 by the self-weight of the second member 120 and the end effector E2 or the restoring force of the elastic member 140 and becomes a free state ( Figure 3 C). In the free state, the end effector E2 is suspended or supported by three coil springs and moves independently with six degrees of freedom relative to the connecting rod E1. The six degrees of freedom are translation in the x-, y-, and z-directions and rotation around the x-, y-, and z-axes (the xyz coordinate system of the joint structure 1 can be determined, for example, by taking the axial direction of the first component 110 (i.e., the connecting rod E1) as the z-axis). At this time, when an external force acts on the end effector E2, such as Figure 3 As shown in D, only the position or posture of the end effector E2 can be changed. This makes it possible to realize a so-called "soft robot" called a soft robot or a compliant mechanism.

[0052] According to the joint structure 1 of this embodiment described above, switching between the locked state and the free state can be achieved with an extremely simple structure. Furthermore, since the drive source M for the linear member 130 is located outside the joint structure 1, the joint structure 1 itself can be constructed compactly and lightweight. Furthermore, the range of motion of the end effector E2 in the free state can be easily increased.

[0053] Furthermore, the restoring force of the elastic member 140 stabilizes the relative position of the link E1 and the end effector E2 even in the free state. Furthermore, when an external force acts on the end effector E2, the elastic member 140 deforms, thereby preventing the movement of the end effector E2. Consequently, for example, flexible motions such as moving the tip of the end effector E2 along an uneven surface can be easily achieved.

[0054] Furthermore, due to the interlocking structure formed by the truncated cone-shaped protrusion 121 and the recess 111, when the linear member 130 is gradually pulled to bring the first member 110 and the second member 120 closer together, the protrusion 121 is guided by the inner surface of the recess 111, automatically aligning the axes of the protrusion 121 and the recess 111. This prevents interlocking failures (failure to switch to the locked state). Furthermore, when the protrusion 121 and the recess 111 engage, a strong fixing force can be achieved.

[0055] Furthermore, by providing protrusions 122 and guide grooves 112 on convex portion 121 and concave portion 111, protrusions 122 are guided by guide grooves 112 when convex portion 121 is inserted into concave portion 111, and angular deviation between convex portion 121 and concave portion 111 is automatically corrected. Therefore, both suppressing mating failure and preventing angular deviation can be achieved.

[0056] Modifications

[0057] The above-mentioned embodiment is merely an example of a configuration example of the present invention. The present invention is not limited to the above-mentioned specific embodiment, and various modifications can be made within the scope of the technical concept.

[0058] In Modification 1, a nonlinear spring element is used as the elastic member 140 . Figure 4 This graph shows the difference in characteristics between linear and nonlinear springs. The horizontal axis represents displacement (deflection), and the vertical axis represents load. While linear springs have a load-displacement relationship that is directly proportional to displacement, maintaining a constant stiffness (spring constant) regardless of displacement, nonlinear springs exhibit a characteristic in which stiffness (spring constant) increases with increasing displacement. These nonlinear characteristics can be achieved by, for example, varying the coil diameter of the coil spring, varying the pitch of the coil spring, or varying the wire diameter of the coil spring.

[0059] For example, the extension of the linear member 130 in the free state can be switched in multiple stages, such as s1 [mm], s2 [mm], and s3 [mm] (s1 < s2 < s3), by controlling the drive source M, thereby changing the separation distance between the first member 110 and the second member 120 (i.e., the displacement of the spring). This allows the hardness (flexibility) of the joint structure 1 in the free state to be controlled. For example, by adjusting the hardness (flexibility) to an appropriate level to suit the robot's movements or scenarios, it is expected that the robot's application range can be expanded or its movements can be more efficient.

[0060] Figure 5 This shows Modification 2. In Modification 2, multiple interlocking structures consisting of convex portions 121 and concave portions 111 are provided. By meshing the first component 110 and the second component 120 at multiple locations, it is expected that the relative positioning accuracy of the first component 110 and the second component 120 will be improved, or the fixing force between the first component 110 and the second component 120 will be increased. It should be noted that the shape and size of the concave and convex portions can be the same or different in all interlocking structures.

[0061] Figure 6 This shows Modification 3. In Modification 3, the concave and convex portions of the interlocking structure are reversed from those in the embodiment. That is, the first component 110 is provided with a convex portion 121, and the second component 120 is provided with a concave portion 111. This way, even with the reversed concave and convex portions, the same functional effects as in the embodiment can be achieved. Furthermore, when multiple interlocking structures are provided as in Modification 2, interlocking structures with different concave and convex portions facing each other can be mixed.

[0062] Figure 7Indicates modification example 4. In modification example 4, a locking mechanism of a friction type is adopted instead of a fitting type. Specifically, mutually parallel and opposing surfaces 117 and 127 are provided on the first component 110 and the second component 120, respectively. In the locked state, the surface 117 of the first component 110 and the surface 127 of the second component 120 are in surface contact and friction is exerted, thereby fixing the first component 110 and the second component 120. According to such a friction method, there is no need to provide concave and convex parts for engaging the first component 110 with the second component 120, so that the joint structure 1 can be made more compact. It should be noted that in order to obtain sufficient friction, a material with a high static friction coefficient can be selected for the parts of the surfaces 117 and 127, or a surface treatment for increasing the static friction coefficient can be applied to the surfaces 117 and 127.

[0063] In the above embodiment, a coil spring is used as the elastic member 140, but other types of elastic elements (such as a damper) may also be used. Alternatively, a configuration may be adopted in which no elastic member 140 is provided between the first member 110 and the second member 120. In this configuration, a linear member 130 having a certain degree of hardness can be used to stabilize the posture of the second member 120 in the free state.

[0064] (Appendix 1)

[0065] (1) A joint structure (1) connects a first element (11) and a second element (12) of a manipulator (RM) of a robot (R), characterized in that the joint structure (1) has a locking mechanism (10), which switches between a free state in which the second element (12) can move independently and freely relative to the first element (11) and a locked state in which the second element (12) is fixed to the first element (11), and the locking mechanism (10) has: a first component (110) engaged with the first element (11); a second component (120) engaged with the first element (11); Two elements (12) are joined; and a flexible linear component (130), one end of the linear component (130) is mounted on the second component (120), and the other end is led out to the outside of the joint structure (1) through a through hole (113) provided in the first component (110), and the second component (120) is brought into contact with the first component (110) by pulling the linear component (130), thereby becoming the locked state, and the second component (120) is separated from the first component (110) by stretching the linear component (130), thereby becoming the free state.

[0066] Description of Reference Numerals

[0067] 1: Joint structure; 10: Locking mechanism; 11: First element; 12: Second element; 110: First component; 111: Recess; 112: Guide groove; 113: Through hole; 114: Pulley; 115: Lead-out hole; 117: Surface; 120: Second component; 121: Protrusion; 122: Protrusion; 123: Through hole; 127: Surface; 130: Linear component; 140: Elastic component; E1: Connecting rod; E2: End effector; R: Robot; RM: Manipulator; RC: Controller; M: Drive source.

Claims

1. A joint structure connecting a first element and a second element of a robot manipulator, characterized in that: The joint structure includes a locking mechanism that switches between a free state in which the second element is free to move independently of the first element and a locked state in which the second element is fixed to the first element. The locking mechanism has: a first component engaged with the first element; a second component engaged with the second element; a flexible linear member, one end of which is mounted on the second member, and the other end of which passes through a through hole provided in the first member and is led out to the outside of the joint structure; and an elastic component connecting the first component and the second component, The second member is brought into contact with the first member by pulling the linear member, thereby achieving the locked state. The second member is separated from the first member by stretching the linear member, thereby achieving the free state. In the free state, the second member is suspended and supported by the elastic member. The elastic component includes a nonlinear spring element, The rigidity of the elastic member is made variable by changing the distance between the first member and the second member according to the pulling amount of the linear member.

2. The joint structure according to claim 1, characterized in that In the free state, the second component is separated from the first component to such an extent that a movable range is formed in which the second element can freely move with six degrees of freedom independently of the first element.

3. The joint structure according to claim 1, wherein: A convex portion is provided on one of the first component and the second component, and a concave portion is provided on the other. In the locked state, the convex portion is fitted into the concave portion, thereby fixing the first member and the second member.

4. The joint structure according to claim 3, characterized in that: The convex portion has a cone shape or a frustum shape.

5. The joint structure according to claim 4, characterized in that: The convex portion has a conical shape or a truncated cone shape.

6. The joint structure according to claim 5, characterized in that A protrusion is provided on a side surface of the convex portion, and a guide groove for guiding the protrusion is provided on the concave portion.

7. The joint structure according to any one of claims 3 to 6, characterized in that: A plurality of sets of the convex portions and the concave portions are provided in the joint structure.

8. The joint structure according to claim 1 or 2, characterized in that: The first component and the second component are respectively provided with surfaces parallel to each other, In the locked state, the first member and the second member are in surface contact with each other, thereby causing friction to act and thereby fixing the first member and the second member.

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