A six-link variable stiffness bipedal wheel-legged robot

Abstract

A six-link variable stiffness bipedal wheeled robot relates to the field of robot research and development technology. Two wheeled legs are mounted on either side of the robot's head. The thigh links and drive links of the wheeled legs are connected to joint drive motors via a variable stiffness converter. An input flange connects an absolute angle sensor to the joint drive motors. An output flange is rotatably mounted on the outside of the closed end of the outer shell. The closed end of the outer shell has multiple radial through slots, and the output flange has multiple arc-shaped through slots. The outer end of a slider is inserted into the corresponding radial and arc-shaped through slots, anchored to the inner end of the absolute angle sensor, and multiple leaf springs are fixed to its edges. The inner end of the slider clamps and connects the corresponding leaf springs. The output flange meshes with the adjusting gear of the servo motor. Introducing a variable stiffness converter between the wheeled legs and the joint drive motors achieves the required variable stiffness, helping to ensure a fast and accurate response between the wheeled legs and the joint drive motors, and providing buffering characteristics against impact torque.

Description

Technical Field

[0001] This invention relates to the field of robot research and development technology, specifically a six-link variable stiffness bipedal wheel-legged robot. Background Technology

[0002] Purely wheeled robots excel on flat surfaces, moving quickly and completing tasks efficiently. However, they are prone to losing balance or becoming unable to traverse rough terrain. While purely legged bipedal robots possess excellent terrain-crossing capabilities and stability, their mobility is relatively low. To address these shortcomings, bipedal wheel-legged robots cleverly combine the advantages of both while mitigating their respective weaknesses. In wheeled mode, they can achieve rapid movement and efficient work on flat or relatively wide surfaces using the high-speed rotation of their wheels. When encountering rough terrain or obstacles, they can utilize the flexibility of their leg joints to overcome them. This combination allows bipedal wheel-legged robots to maintain high efficiency across various terrains. Compared to other robots of similar size, they exhibit superior flexibility and adaptability, helping to reduce energy consumption during movement and improving their work efficiency and accuracy.

[0003] To meet diverse task requirements, bipedal wheeled robots must possess precise and flexible joint control capabilities to ensure stable and efficient movement during task execution. However, traditional bipedal wheeled robots typically control the up-and-down squatting movements of their legs directly through joint motors, lacking variable stiffness capabilities. During robot movement, especially in rugged terrain, the legs struggle to buffer torque when encountering external impacts, easily reducing the lifespan of the joint motors. Furthermore, their slow response to torque changes in rugged terrain reduces their adaptability to different terrains. Therefore, it is crucial to optimize and improve the joint drive structure of bipedal wheeled robots to enhance the torque response performance of the joint motors, while simultaneously buffering external impacts, reducing damage to the drive motors, and minimizing collision damage to the robot. Summary of the Invention

[0004] To address the shortcomings of the prior art, this invention provides a six-link variable stiffness bipedal wheel-legged robot. It introduces a variable stiffness converter between the wheel-legs and the joint drive motor. By using a servo motor, the connection positions of multiple sliders and corresponding leaf springs can be adjusted to achieve the required variable stiffness. This helps to ensure a fast and accurate response between the wheel-legs and the joint drive motor, and also has a buffering characteristic for impact torque.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a six-link variable stiffness bipedal wheeled robot, comprising a robot head and two wheeled legs mirror-symmetrically mounted on both sides thereof. Hub motors are respectively installed at the bottom of the two wheeled legs. A control board and joint drive motors for controlling the squatting and standing movements of the wheeled legs are disposed inside the robot head. The control board sends control signals to the hub motors and joint drive motors. Four joint drive motors are arranged in a rectangular pattern on both sides of the robot head, with their output ends facing outwards. The wheeled legs include lower leg links, upper leg links, and adjustable... The system comprises a connecting rod, a support connecting rod, and a drive connecting rod. The thigh connecting rod and the drive connecting rod are arranged in an X-shape. The rear end of the lower leg connecting rod is hinged to the lower end of the thigh connecting rod. A hub motor is mounted on the front end of the lower leg connecting rod. The lower end of the adjusting connecting rod is hinged to the middle position of the lower leg connecting rod. The upper end of the support connecting rod is hinged to the middle position of the thigh connecting rod. Both the upper end of the adjusting connecting rod and the lower end of the support connecting rod are hinged to the lower end of the drive connecting rod. The upper ends of the thigh connecting rod and the drive connecting rod of each foot wheel are respectively connected to two corresponding joint drive motors on the same side via a variable stiffness converter. The variable stiffness converter includes an output flange, a housing, and an anchor. The system comprises an absolute angle sensor, an input flange, and multiple sliders. The housing is a cylindrical shell with one open end. The absolute angle sensor is coaxially rotatably mounted on the open end of the housing. The input flange connects the outer end of the absolute angle sensor to the output end of the corresponding joint drive motor. The output flange is coaxially rotatably mounted on the outer side of the closed end of the housing with a gap. Pre-fabricated mounting rings at the upper ends of the thigh connecting rod and the drive connecting rod are connected and fixed to the housing of the strain stiffness converter. Multiple radial through slots are formed at equal angles along the circumference at the middle position of the closed end of the housing, and multiple arc-shaped through slots are formed at equal angles along the circumference at the middle position of the output flange. The plurality of arc-shaped through slots are configured to mate with the plurality of radial through slots. The outer ends of the plurality of sliders are configured as guide rods and are respectively inserted into the corresponding radial through slots and arc-shaped through slots. The anchor is coaxially fixed to the inner end of the absolute angle sensor. The edge of the anchor is fixed with a plurality of radially arranged leaf springs at equal angles along the circumference. The inner ends of the plurality of sliders are configured as rectangular frames with two rollers arranged side by side inside to clamp and connect the corresponding leaf springs. The output flange of each variable stiffness converter has teeth on its circumferential surface and meshes with the adjusting gear. The adjusting gear is connected to the servo motor fixed at the corresponding position inside the robot head.

[0006] Furthermore, the input flange is connected and fixed to the corresponding joint drive motor output end through a central connecting plate. The central connecting plate is a ring-shaped component with two limiting protrusions integrally provided on both sides of its edge. A limiting plate is fixedly provided at the corresponding position of the top of the robot head and the central connecting plate. The two ends of the limiting plate cooperate with the two limiting protrusions to limit the rotation range of the joint drive motor output end.

[0007] Furthermore, multiple limiting bolts are evenly fixed circumferentially along the edge of the closed end of the outer shell, and multiple arc-shaped limiting grooves are formed on the edge of the output flange in cooperation with the multiple limiting bolts to limit the rotation range of the output flange.

[0008] Furthermore, the output flange is rotatably connected to the closed end of the housing via a thrust ball bearing, and the guide rod ends of the plurality of sliders are integrally provided with bolt heads on the outer side of the corresponding arc-shaped through groove to restrict the axial displacement of the output flange.

[0009] Furthermore, the two rollers on the rectangular frame at the inner end of the slider have variable relative positions, which can adjust the clamping degree of the leaf spring.

[0010] Furthermore, the guide rod at the outer end of the slider is divided into two sections: an inner rectangular section and an outer circular section. The rectangular section is fitted with a radial through groove, and the circular section is fitted with an arc-shaped through groove.

[0011] Compared with the prior art, the beneficial effects of this invention are as follows: The caster leg of this invention contains five links, and the two joint drive motors fixed at the upper end of the caster leg are considered as one link, forming a six-link design. The structure is stable and efficient. Furthermore, a variable stiffness converter is innovatively introduced between the caster leg and the joint drive motor. The input flange at the input end of the variable stiffness converter and the outer shell at the output end are connected and transitioned by multiple leaf springs and multiple sliders arranged circumferentially on the anchor. By driving the output flange to rotate through a servo motor, the multiple sliders can be expanded and contracted, thereby changing the clamping position between the slider and the leaf spring. As a cantilever beam, the leaf springs achieve variable stiffness due to different connection points, which not only has a certain buffering characteristic for impact torque, but also... It can reduce collision damage to joint drive motors and robots, extend the service life of robots, and adjust the output torque through the cooperation of servo motors and variable stiffness converters. This standardizes and refines the output value of joint drive motors, and makes subtle changes to torque within a certain range. This prevents the slow response caused by the excessively long torque output chain of traditional foot-wheeled legs relying solely on joint drive motors in rugged terrain. It can achieve a faster response through servo motor adjustment. In addition, the variable stiffness converter has a built-in high-resolution absolute angle sensor, which can monitor the torque output value of joint drive motors in real time during movement. It provides feedback value for robot balance control or squatting and standing adjustments, ensuring precise control while responding quickly. Attached Figure Description

[0012] Figure 1 This is an isometric view of the overall structure of the robot of this invention;

[0013] Figure 2 This is an isometric view of the variable stiffness converter of the robot of the present invention;

[0014] Figure 3 This is an exploded view of the variable stiffness converter of the robot of the present invention;

[0015] Figure 4 This is an isometric view of the wheeled legs of the robot of this invention.

[0016] In the diagram: 1-Hub motor, 2-Lower leg link, 3-Thigh link, 4-Adjusting link, 5-Support link, 6-Drive link, 7-Control board, 8-Variable stiffness converter, 9-Adjusting gear, 10-Servo motor, 11-Limit plate, 12-Joint drive motor, 13-Robot head, 14-Central connection plate, 15-Limit bolt, 16-Output flange, 17-Thrust ball bearing, 18-Outer shell, 19-Leaf spring, 20-Slider, 21-Anchor, 22-Absolute angle sensor, 23-Cross roller bearing, 24-Bearing retaining ring, 25-Input flange. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0018] like Figures 1-4 As shown, a six-link variable stiffness bipedal wheel-legged robot includes a hub motor 1, a lower leg link 2, a thigh link 3, an adjusting link 4, a support link 5, a drive link 6, a control board 7, a variable stiffness converter 8, an adjusting gear 9, a servo motor 10, a limit plate 11, a joint drive motor 12, a robot head 13, a central connecting plate 14, a limit bolt 15, an output flange 16, a thrust ball bearing 17, a housing 18, a leaf spring 19, a slider 20, an anchor 21, an absolute angle sensor 22, a cross roller bearing 23, a bearing retaining ring 24, and an input flange 25.

[0019] Combination Figure 1As shown, the robot head 13 is framed with aluminum profiles, and its interior houses the core control system, including a control board 7, a power module, an IMU (Inertial Measurement Unit) module, a camera acquisition module, and joint drive motors 12. Two wheeled legs are mirror-symmetrically mounted on both sides of the robot head 13, with hub motors 1 mounted at the bottom of each leg. The joint drive motors 12 enable the wheeled legs to squat and stand up by rotating in both directions. The power module provides power to the robot's electrical components. During autonomous walking, the camera acquisition module first collects terrain information. The chip on the control board 7 processes the collected terrain information using algorithms and selects the optimal path. Once the path is determined, the IMU module sends real-time data on the robot's overall position and speed to the control board 7 for processing. When complex terrain causes the robot to become unbalanced, the data collected by the IMU module is calculated by the chip on the control board 7, and control signals are sent to the corresponding motors (hub motors 1 and joint drive motors 12) to make corresponding rotations, thereby adjusting the robot's overall balance. Taking a scenario where the terrain on the left is higher than the terrain on the right as an example, after the IMU inertial navigation module sends the posture data to the control board 7, the chip calculates the data using an algorithm and sends control signals to the joint drive motor 12 on the left side to lift the left wheeled leg. Simultaneously, control signals are sent to the two hub motors 1 to maintain rotation. Overall, the bipedal wheeled robot's left leg lifts to adapt to the terrain on the left, while the rotation of the two hub motors 1 enables overall movement. When the robot leans forward or backward, the rotation of the two hub motors 1 can adjust the robot's overall balance. The above description illustrates the core principle of self-balancing movement in traditional bipedal wheeled robots, which is existing technology. Its drawback is the lack of variable stiffness capability between the joint drive motor 12 and the wheeled leg. In rugged terrain, the wheeled leg relies solely on the joint drive motor 12 for control, resulting in an excessively long torque output chain and slow response. Furthermore, the lack of torque buffering makes the robot, especially the joint drive motor 12, susceptible to collision damage when subjected to a certain degree of impact.

[0020] To address the aforementioned problems, this invention designs a six-link variable stiffness bipedal wheel-leg robot. The structure of the robot's two wheel-legs has been optimized, and a variable stiffness converter 8 is added between the joint drive motor 12 and the wheel-legs. This allows for flexible stiffness adjustment based on actual conditions, resulting in a faster response from the joint drive motor 12 and providing some shock absorption, thus better protecting the robot's core control system. The specific design is as follows:

[0021] Combination Figure 1 , Figure 4As shown, four joint drive motors 12 are fixed in a rectangular arrangement on both sides of the inner side of the robot head 13, with the output ends facing outwards, for controlling the two wheeled legs. Each wheeled leg includes a lower leg link 2, a upper leg link 3, an adjusting link 4, a supporting link 5, and a driving link 6. The upper leg link 3 and the driving link 6 are arranged in an X-shape. The rear end of the lower leg link 2 is hinged to the lower end of the upper leg link 3, and a hub motor 1 is mounted on the front end of the lower leg link 2. The lower end of the adjusting link 4 is hinged to the middle position of the lower leg link 2, and the upper end of the supporting link 5 is hinged to the middle position of the upper leg link 3. Both the upper end of the adjusting link 4 and the lower end of the supporting link 5 are hinged to the lower end of the driving link 6. During the assembly of the wheeled legs with the robot head 13, the upper ends of the upper leg link 3 and the driving link 6 of each wheeled leg are connected to the two corresponding joint drive motors 12 on the same side via a variable stiffness converter 8. Each caster leg has a total of five links, and the two fixed joint drive motors 12 mounted on the upper end of each caster leg are regarded as one link, forming a six-link design.

[0022] Combination Figure 2 , Figure 3As shown, the variable stiffness converter 8 includes an output flange 16, a housing 18, an anchor 21, an absolute angle sensor 22, an input flange 25, and multiple sliders 20. The housing 18 is a cylindrical shell with one open end, used to connect and fix to the upper pre-fabricated mounting rings of the thigh link 3 and the drive link 6. The absolute angle sensor 22 is coaxially rotatably mounted on the open end of the housing 18. An annular groove can be machined at the open end of the housing 18 and a cross roller bearing 23 can be embedded therein. The absolute angle sensor 22 is rotatably connected to the open end of the housing 18 via the cross roller bearing 23. A bearing retainer ring 24 is fitted on the outside of the annular groove to axially position the cross roller bearing 23. The input flange 25 connects the outer end of the absolute angle sensor 22 to the output end of the corresponding joint drive motor 12 for transmission. Multiple radial through slots are formed at equal angles along the circumference at the middle position of the closed end of the housing 18, and multiple arc-shaped through slots are formed at equal angles along the circumference at the middle position of the output flange 16. The multiple arc-shaped through slots are matched with the multiple radial through slots. Multiple sliders 20 have guide rods at their outer ends, which are respectively inserted into corresponding radial and arc-shaped through slots. Preferably, the guide rods at the outer ends of the sliders 20 have two sections: an inner rectangular section and an outer circular section. The rectangular section fits into the radial through slot to prevent twisting when the guide rods of the sliders 20 slide relative to the radial through slots, while the circular section fits into the arc-shaped through slots to make the sliding of the guide rods of the sliders 20 relative to the arc-shaped through slots smoother. The output flange 16 is coaxially rotatably mounted on the outer side of the closed end of the housing 18 with a gap. Optionally, the output flange 16 and the closed end of the housing 18 can be rotatably connected via a thrust ball bearing 17, and bolt heads are integrally set at the ends of the guide rods of the multiple sliders 20 on the outer side of the corresponding arc-shaped through slots to limit the axial displacement of the output flange 16. Anchors 21 are coaxially fixed to the inner end of the absolute angle sensor 22, and multiple radially arranged leaf springs 19 are fixed at equal angles along the circumferential direction on the edge of the anchor 21. Multiple sliders 20 have rectangular frames on their inner ends, with two rollers arranged side by side inside each frame to clamp and connect the corresponding leaf springs 19. Preferably, the relative positions of the two rollers in the rectangular frames on the inner ends of the sliders 20 are variable, so that the clamping degree of the leaf springs 19 can be adjusted.

[0023] To limit the rotation angle, a central connecting plate 14 can be added between the input flange 25 and the output end of the corresponding joint drive motor 12. The two are connected and fixed by the central connecting plate 14. The central connecting plate 14 is a ring-shaped component with two limiting protrusions integrally provided on both sides of its edge. A limiting plate 11 is fixedly installed at the corresponding position on the top of the robot head 13 and the central connecting plate 14. The two ends of the limiting plate 11 cooperate with the two limiting protrusions to limit the rotation angle range of the output end of the joint drive motor 12, thereby limiting the maximum swing position of the joint drive motor 12 and limiting the swing amplitude of the thigh link 3 and drive link 6 of the foot wheel leg. In addition, multiple limiting bolts 15 can be evenly fixed circumferentially along the edge of the closed end of the outer shell 18. Multiple arc-shaped limiting grooves are opened on the edge of the output flange 16 in cooperation with the multiple limiting bolts 15 to limit the rotation angle range of the output flange 16, preventing the output flange 16 from rotating too much and causing the slider 20 to detach from the leaf spring 19 on the anchor 21.

[0024] Combination Figure 1 As shown, the output flange 16 of each variable stiffness converter 8 has teeth on its circumferential surface and meshes with the adjusting gear 9. The adjusting gear 9 is connected to the servo motor 10 fixed at a corresponding position inside the robot head 13. The joint drive motor 12 located at the rear rotates clockwise, which drives the drive link 6 to swing clockwise through the variable stiffness converter 8, thereby driving the adjusting link 4 to move upward and pulling the lower leg link 2 to lift. The joint drive motor 12 located at the front rotates counterclockwise, which drives the thigh link 3 to swing counterclockwise through the variable stiffness converter 8, while simultaneously driving the lower leg link 2 to lift, realizing the overall retraction of the foot wheel leg. When the input flange 25 of the variable stiffness converter 8 rotates with the joint drive motor 12, the absolute angle sensor 22 and the anchor 21 rotate accordingly. The leaf spring 19 on the anchor 21 is connected to the slider 20 as a cantilever beam. During the rotation of the anchor 21, the outer shell 18 is driven to rotate through the leaf spring 19 and the slider 20, thereby realizing the swinging action of the thigh link 3 or the drive link 6 connected to it. During this period, the servo motor 10 does not work, and the output flange 16 drives the adjusting gear 9 to idle. Due to the physical characteristics of the leaf spring 19, the variable stiffness converter 8 has a certain buffering effect on the impact, which provides better protection for the joint drive motor 12 and even other core control systems of the robot. When stiffness adjustment is required, the servo motor 10 is activated to control the output flange 16 to rotate through the adjusting gear 9. Under the constraint of the arc-shaped through groove on the output flange 16 and the radial through groove on the housing 18, the multiple sliders 20 are spread out or contracted as a whole, thereby changing the connection position between the sliders 20 and the leaf spring 19. The closer the slider 20 is to the fixed end of the leaf spring 19, which is a cantilever beam, the greater the stiffness, and vice versa.

[0025] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0026] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A six-link variable stiffness bipedal wheel-legged robot, comprising a robot head (13) and two wheel-legs mirror-symmetrically mounted on both sides thereof, wherein hub motors (1) are respectively mounted on the bottom of the two wheel-legs, and a control board (7) and joint drive motors (12) for controlling the squatting and standing movements of the wheel-legs are disposed inside the robot head (13), wherein the control board (7) sends control signals to the hub motors (1) and the joint drive motors (12), characterized in that: The robot head (13) has four joint drive motors (12) arranged in a rectangular pattern on both sides of the inner side, with the output ends facing outwards. The wheeled legs include a lower leg link (2), a thigh link (3), an adjusting link (4), a supporting link (5), and a driving link (6). The thigh link (3) and the driving link (6) are arranged in an X-shape. The rear end of the lower leg link (2) is hinged to the lower end of the thigh link (3). A hub motor (1) is installed on the front end of the lower leg link (2). The lower end of the adjusting link (4) is hinged to the middle position of the lower leg link (2). The upper end of the supporting link (5) is hinged to the middle position of the thigh link (3). The upper end of the adjusting link (4) and the lower end of the supporting link (5) are hinged to the lower end of the driving link (6). The upper ends of the thigh link (3) and the driving link (6) of each foot wheel leg are respectively connected to the two corresponding joint drive motors (12) on the same side through the variable stiffness converter (8). The variable stiffness converter (8) includes an output flange (16), a housing (18), an anchor (21), an absolute angle sensor (22), an input flange (25), and multiple sliders (20). The housing (18) is a cylindrical shell with one open end. The absolute angle sensor (22) is coaxially rotatably mounted on the open end of the housing (18). The flange (25) connects the outer end of the absolute angle sensor (22) to the output end of the corresponding joint drive motor (12) for transmission. The output flange (16) is coaxially rotatably mounted on the outer side of the closed end of the housing (18) with a gap. The upper end of the thigh connecting rod (3) and the drive connecting rod (6) is prefabricated and fixed to the housing (18) of the strain stiffness converter (8). Multiple radial through slots are opened at equal angles along the circumference at the middle position of the closed end of the housing (18). Multiple arc-shaped through slots are opened at equal angles along the circumference at the middle position of the output flange (16). The multiple arc-shaped through slots are matched with the multiple radial through slots one by one. The multiple sliders ( 20) The outer end is set as a guide rod and is inserted into the corresponding radial through groove and bow-shaped through groove respectively. The anchor (21) is coaxially fixed to the inner end of the absolute angle sensor (22). The edge of the anchor (21) is fixed with multiple radially arranged leaf springs (19) at equal angles along the circumference. The inner end of multiple sliders (20) is set as a rectangular frame and two rollers are arranged side by side inside to clamp and connect the corresponding leaf springs (19). The output flange (16) of each variable stiffness converter (8) has teeth on its circumferential surface and is meshed with the adjusting gear (9). The adjusting gear (9) is connected to the servo motor (10) fixed at the corresponding position inside the robot head (13).

2. The six-link variable stiffness bipedal wheel-legged robot according to claim 1, characterized in that: The input flange (25) is connected and fixed to the output end of the corresponding joint drive motor (12) through a central connecting plate (14). The central connecting plate (14) is a ring-shaped component with two limiting protrusions integrally provided on both sides of its edge. The robot head (13) is fixedly provided with a limiting plate (11) at the corresponding position on the top of the central connecting plate (14). The two ends of the limiting plate (11) cooperate with the two limiting protrusions to limit the rotation range of the output end of the joint drive motor (12).

3. The six-link variable stiffness bipedal wheel-legged robot according to claim 1, characterized in that: Multiple limiting bolts (15) are uniformly fixed along the circumferential direction at the closed edge of the outer shell (18). Multiple arc-shaped limiting grooves are formed on the edge of the output flange (16) in cooperation with the multiple limiting bolts (15) to limit the angular range of the output flange (16).

4. The six-link variable stiffness bipedal wheel-legged robot according to claim 1, characterized in that: The output flange (16) and the closed end of the housing (18) are rotatably connected by a thrust ball bearing (17). The guide rod ends of the plurality of sliders (20) are integrally provided with bolt heads on the outer side of the corresponding arc-shaped through groove to limit the axial displacement of the output flange (16).

5. A six-link variable stiffness bipedal wheel-legged robot according to claim 1, characterized in that: The two rollers on the rectangular frame at the inner end of the slider (20) are in a variable relative position, which can adjust the clamping degree of the leaf spring (19).

6. A six-link variable stiffness bipedal wheel-legged robot according to claim 1, characterized in that: The guide rod at the outer end of the slider (20) is divided into two sections: an inner rectangular section and an outer circular section. The rectangular section is fitted with a radial through groove, and the circular section is fitted with an arc-shaped through groove.

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