A tension amplification constant torque constant stiffness rope-driven humanoid elbow joint

Abstract

The application discloses a tension amplification constant torque constant stiffness rope-driven humanoid elbow joint and belongs to the technical field of robots. The elbow joint comprises an elbow joint fixed end and an elbow joint moving end. The fixed end is provided with a quarter-arc upper engaging plate, and the moving end is provided with a quarter-arc lower engaging plate. The two are kept to rotate on one side through a coupling rope, and a rope driving assembly with tension amplification and constant torque is arranged therebetween. Through a movable pulley set mechanism formed by cooperation of a stepped double-pulley of the moving end and a multi-stage pulley of the fixed end, tension amplification is realized in a limited radial space. Differential rope winding arrangement of the double-pulley makes rope length change and joint angle present a linear relationship, and cooperation of an absolute value encoder realizes constant torque output and accurate control. The rope driving structure isolates motor impact, and rolling engagement movement constrained by the coupling rope guarantees safety of human-machine interaction. The application has compact structure and high flexibility, and is suitable for an upper limb elbow joint of a humanoid robot.

Description

Technical Field

[0001] This invention belongs to the field of robotic elbow joint technology, specifically relating to a tension-amplified, constant-torque, constant-stiffness rope-driven humanoid elbow joint. Background Technology

[0002] Traditional rigid direct-drive elbow joints have high torque density, but their concentrated mass and large inertia mean that the reverse impact is directly transmitted to the robot and the external environment upon collision, resulting in poor safety and interaction. While elastic drive solutions introduce flexible elements, they rely on a complex series connection of harmonic reducers and torque sensors, which leads to severe stiffness nonlinearity, sensitivity to assembly tolerances, and high costs. Furthermore, they cannot simultaneously achieve high-magnification tension amplification and constant stiffness, limiting the application of humanoid robot elbow joints in high-dynamic and high-load scenarios. Summary of the Invention

[0003] This invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a tension amplification constant torque constant stiffness rope-driven humanoid elbow joint.

[0004] The technical solution of the present invention is: a tension-amplified, constant-torque, and constant-stiffness rope-driven humanoid elbow joint, comprising an elbow joint fixed end and an elbow joint movable end. The elbow joint fixed end is integrally formed with an upper engagement plate with a quarter-circle arc, and the elbow joint movable end is integrally formed with a lower engagement plate with a quarter-circle arc. A coupling rope is provided between the upper engagement plate with a quarter-circle arc and the lower engagement plate with a quarter-circle arc to maintain unilateral rotation of the two. A tension-amplified, constant-torque, and constant-stiffness rope-driven assembly is provided between the elbow joint fixed end and the elbow joint movable end.

[0005] Furthermore, the lower end of the elbow joint fixing end is provided with two support platforms, and the support platforms are located on both sides of the meshing plate with a quarter-circle arc. Small pulleys for guiding the fixing end are provided on the inclined surfaces of both sides of the support platforms.

[0006] Furthermore, the upper end of the meshing plate with a quarter-circle arc protrudes to form an extension, and large fixed-end pulleys for guidance are provided on the inclined surfaces on both sides of the extension.

[0007] Furthermore, the meshing plate with a quarter-circle arc has coaxial fixed-end central shafts on both sides. The fixed-end central shafts are stepped, and a rotating fixed-end guide wheel is provided on the large-diameter section of the fixed-end central shaft. The small-diameter section of the fixed-end central shaft is movably connected to the double-end connector.

[0008] Furthermore, the lower meshing plate with a quarter-circle arc has coaxial moving end central shafts fixed on both sides, and the moving end central shafts are stepped shafts.

[0009] Furthermore, a rotatably fitted pulley seat is provided on the central shaft of the mobile end, and the outer side of the mobile end pulley seat is fixed to the double-end connector.

[0010] Furthermore, a symmetrical mounting slope is formed on the outer wall of the movable end pulley seat, and a stepped shaft is provided on the mounting slope. The stepped shaft shoulder and bolt work together to axially fix the movable end stepped double-layer pulley used as a secondary rope winding guide.

[0011] Furthermore, an absolute encoder rotating side is provided on the outer side of the central shaft of the mobile terminal, and an absolute encoder magnetic base is correspondingly provided on the inner side of the double-ended connector. The absolute encoder rotating side and the absolute encoder magnetic base measure the rotation angle of the elbow joint in real time.

[0012] Furthermore, a tensioning plate is provided in the lower engagement plate with a quarter-circle arc, which tensions the coupling rope.

[0013] Furthermore, the rope drive assembly includes a first set of steel wire ropes and a second set of steel wire ropes, which are arranged diagonally in a mirror image. Tension amplification is achieved by forming a movable pulley group through a fixed-end guide wheel, a fixed-end small pulley, a fixed-end large pulley, and a movable-end stepped double-layer pulley.

[0014] The present invention provides a cable-driven humanoid elbow joint with tension amplification characteristics, constant torque, and a wide range of motion, which can be used as an upper limb motion joint component of a humanoid robot. Its beneficial effects are as follows: This invention features a compact axial design and high tension amplification for the elbow joint: a dynamic pulley system integrating double-layer pulleys on a single pulley seat shortens the axial dimension while amplifying the tension, making it more suitable for confined spaces in the upper limbs and reducing motor torque requirements and joint weight.

[0015] This invention relates to a precision constant torque control for the elbow joint: a double-layer pulley differential rope winding ensures a linear relationship between the rope length and the joint angle, and with real-time feedback from an absolute encoder, a constant torque output is achieved throughout the entire range of motion of the upper limb, meeting the precision requirements for fine grasping and load holding.

[0016] This invention provides a safe elbow joint interaction system: a rope-driven flexible transmission isolates the rigid impact of the motor, the rolling engagement of the coupling rope eliminates mechanical backlash, and the elastic deformation of the rope absorbs energy in the event of accidental contact, ensuring the safety of human-machine collaboration. Attached Figure Description

[0017] Figure 1 This is an isometric view of the articulated module of the present invention; Figure 2 This is a side view of the present invention; Figure 3 This is an exploded isometric view of the present invention; Figure 4 This is a front view of the present invention; Figure 5 This is a cross-sectional view of the present invention; The components include: 1. First set of wire rope; 2. Bowden conduit; 3. Fixed end of elbow joint; 4. Double-end connector; 5. Stepped double-layer pulley at the moving end; 6. Moving end of elbow joint; 7. Pulley seat at the moving end; 8. Guide wheel at the fixed end; 9. Small pulley at the fixed end; 10. Second set of wire rope; 11. Large pulley at the fixed end; 12. Tensioning plate; 13. Central shaft at the moving end; 14. Rotating side of the absolute encoder; 15. Magnet seat of the absolute encoder; 16. Central shaft at the fixed end; 17. Coupling rope; 18. Joint module; 19. Upper meshing plate with quarter-circle arc; 20. Lower meshing plate with quarter-circle arc. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and embodiments: like Figures 1 to 5 As shown, a tension-amplified, constant-torque, and constant-stiffness rope-driven humanoid elbow joint includes an elbow joint fixed end 3 and an elbow joint moving end 6. The elbow joint fixed end 3 is integrally formed with an upper engagement plate 19 with a quarter-circle arc, and the elbow joint moving end 6 is integrally formed with a lower engagement plate 20 with a quarter-circle arc. A coupling rope 17 is provided between the upper engagement plate 19 with a quarter-circle arc and the lower engagement plate 20 with a quarter-circle arc to maintain unilateral rotation of the two. A tension-amplified, constant-torque, and constant-stiffness rope-driven assembly is provided between the elbow joint fixed end 3 and the elbow joint moving end 6.

[0019] The lower end of the elbow joint fixing end 3 is provided with two support platforms, and the support platforms are located on both sides of the meshing plate 19 with a quarter-circle arc. The fixed end small pulleys 9 are provided on the inclined surfaces of the two sides of the support platforms for guidance.

[0020] The upper end of the quarter-circle upper meshing plate 19 extends out to form an extension, and fixed-end large pulleys 11 are provided on the inclined surfaces on both sides of the extension for guidance.

[0021] The upper meshing plate 19 with a quarter-circle arc is provided with coaxial fixed end central shafts 16 on both sides. The fixed end central shaft 16 is stepped. The large diameter section of the fixed end central shaft 16 is provided with a rotating fixed end guide wheel 8. The small diameter section of the fixed end central shaft 16 is movably connected to the double-end connector 4.

[0022] The lower meshing plate 20 with a quarter-circle arc has a coaxial moving end center shaft 13 fixed on both sides, and the moving end center shaft 13 is a stepped shaft.

[0023] The central shaft 13 of the mobile end is provided with a rotatingly engaged mobile end pulley seat 7, and the outer side of the mobile end pulley seat 7 is fixed to the double-end connector 4.

[0024] The outer wall of the movable end pulley seat 7 forms a symmetrical mounting slope, and a stepped shaft is provided on the mounting slope. The movable end stepped double-layer pulley 5, which is used as a secondary rope winding guide, is axially fixed by the joint action of the shoulder of the stepped shaft and the bolt.

[0025] An absolute encoder rotating side 14 is provided on the outer side of the central shaft 13 of the mobile terminal, and an absolute encoder magnetic base 15 is provided on the inner side of the double-ended connector 4. The absolute encoder rotating side 14 and the absolute encoder magnetic base 15 measure the rotation angle of the elbow joint in real time.

[0026] A tensioning plate 12 is provided in the lower engagement plate 20 with a quarter-circle arc, and the tensioning plate 12 tensions the coupling rope 17.

[0027] The rope drive assembly includes a first set of steel wire rope 1 and a second set of steel wire rope 10, which are arranged diagonally in a mirror image. Tension amplification is achieved by forming a movable pulley group through a fixed end guide wheel 8, a fixed end small pulley 9, a fixed end large pulley 11, and a movable end stepped double-layer pulley 5.

[0028] Specifically, the four sides of the elbow joint fixing end 3 are fixedly connected to one side of the robot arm. The upper side is provided with two Bowden tubes 2 for threading and two knot fixing ends. The lower side is provided with two protruding support platforms. The support platforms are pentagonal in shape. Each support platform is provided with four stepped shafts. The shoulders of the eight stepped shafts and the open retaining ring work together to fix eight small fixed end pulleys 9 of the same diameter for guiding the rope winding.

[0029] Specifically, the upper engagement plate 19 with a quarter-circle arc is perpendicular to the four sides of the elbow joint fixing end 3. The upper end of the upper engagement plate 19 with a quarter-circle arc protrudes from the elbow joint fixing end 3 and forms a symmetrical inclined surface in the protruding section. Through the joint action of the shoulders of the two stepped shafts and the open retaining ring, two large pulleys 11 with different diameters used as rope guides are fixed respectively.

[0030] Specifically, the upper engagement plate 19 with a quarter-circle arc has a circular groove at the center of the arc and eight threaded holes are evenly arranged. The two fixed end central shafts 16 are respectively fixedly connected to the left and right end faces of the upper engagement plate 19 with a quarter-circle arc of the elbow joint fixed end 3 by bolts.

[0031] Specifically, the fixed end central shaft 16, through the joint action of the shoulder of the stepped shaft and the shaft retaining ring, respectively fixes the inner sides of two deep groove ball bearings of the same diameter and a third deep groove ball bearing of a different diameter in the axial direction. The outer sides of the two deep groove ball bearings of the same diameter are axially fixed to the fixed end guide wheel 8 through the hole retaining ring.

[0032] Specifically, the four sides of the elbow joint moving end 6 are fixedly connected to one side of the forearm. The lower engagement plate 20 with a quarter-circle arc is perpendicular to the elbow joint moving end 6 and extends out of the elbow joint moving end 6. The lower engagement plate 20 with a quarter-circle arc has a circular groove at the center of the arc and eight threaded holes are evenly arranged. The two moving end central shafts 13 are fixedly connected to the left and right end faces of the lower engagement plate 20 with a quarter-circle arc of the elbow joint moving end 6 by bolts.

[0033] Specifically, the central shaft 13 of the moving end is axially fixed to the inner side of the deep groove ball bearing through the joint action of the shoulder of the stepped shaft and the circlip of the shaft, and the outer side of the deep groove ball bearing is fixed to the pulley seat 7 of the moving end through the circlip of the hole.

[0034] Specifically, the movable end pulley seat 7 is provided with two stepped shafts. The stepped shaft shoulders and bolts work together to axially fix the movable end stepped double-layer pulley 5, which is used as a secondary rope winding guide.

[0035] Specifically, four threaded holes are evenly arranged on the movable end pulley seat 7, and are fixed to one side of the double-end connector 4 by bolts. The other side of the double-end connector 4 has a bearing hole, and a retaining ring is used to axially fix it to the outermost third deep groove ball bearing on the central shaft 16 of the fixed end through the hole, thereby connecting the fixed end 3 of the elbow joint to the movable end 6 of the elbow joint.

[0036] Specifically, the rotating side 14 of the absolute encoder is fixedly connected to the top of the central shaft 13 of the moving end via a set screw. Correspondingly, the magnetic base 15 of the absolute encoder is embedded in a circular groove on one side of the double-ended connector and fixedly connected by bolts. A gap of 0.8mm is left between the rotating side 14 of the absolute encoder and the magnetic base 15 of the absolute encoder, which are used to measure the rotation angle of the elbow joint in real time.

[0037] Specifically, the elbow joint fixing end 3 has two wire grooves and two rope knot holes on its vertical surface with a quarter-circle upper engagement plate 19. Correspondingly, the elbow joint moving end 6 also has two wire grooves and two rope knot holes of the same size and position on its vertical surface with a quarter-circle lower engagement plate 20. One end of each of the two coupling ropes 17 is fixed along the wire groove to the rope knot hole of the elbow joint fixing end 3 and the tensioning plate 12 on the side of the elbow joint moving end 6, respectively. The other end of each of the two coupling ropes 17 is fixed along the wire groove to the rope knot hole of the elbow joint moving end 6 and the tensioning plate 12 on the side of the elbow joint fixing end 3, respectively.

[0038] Specifically, the tensioning plate 12 has three holes, and the coupling rope 17 is knotted and fixed to one of the through holes on both sides of the tensioning plate 12. A long bolt is installed on the bolt hole on the vertical surface of the four sides of the elbow joint fixing end 3. The middle threaded hole of the tensioning plate 12 is connected to the long bolt by thread, and the axial displacement of the tensioning plate 12 can be achieved by turning the bolt, thereby achieving the tensioning of the coupling rope 17.

[0039] Since the radii of the two quarter-circle arcs are the same, and the length of the connecting line between the two circular grooves of the double-ended connector 4 is the same as the diameter of the quarter-circle arc, under the fixed length of the two coupling ropes 17, the elbow joint fixed end 3 and the elbow joint moving end 6 always maintain a tangential gear-like meshing motion state.

[0040] Specifically, the two sets of steel wire ropes are respectively fixed to the release end and the take-up end of the joint module 18, and then transmitted through the Bowden conduit to the Bowden conduit threading end on the elbow joint fixing end 3, and further wound around the elbow joint.

[0041] More specifically, the first set of wire rope 1, after being guided by the fixed end guide wheel 8, winds around to the small groove of the stepped double-layer pulley 5 on the opposite moving end. After being guided by the pulley, it is again guided by the fixed end guide wheel 8 on the opposite side and winds around to the two fixed end small pulleys 9. Then, it winds further through the fixed end guide wheel 8 to the large groove of the stepped double-layer pulley 5 on the opposite moving end. After being guided by the stepped double-layer pulley 5 on the moving end, it is again guided by the fixed end guide wheel 8 on the opposite side and winds around to the fixed end large pulley 11. Here, after being guided by the fixed end large pulley 11, it winds around to the other end of the elbow joint fixed end 3 with the quarter-circle upper meshing plate 19. In this case, the winding method is exactly the same as described above. That is, after the first set of wire rope 1 is guided by the fixed end guide wheel 8, it winds into the large groove of the stepped double-layer pulley 5 on the opposite side. After being guided by the pulley, it is guided again by the fixed end guide wheel 8 on the opposite side and winds into the two fixed end small pulleys 9. Then, it winds further through the fixed end guide wheel 8 into the small groove of the stepped double-layer pulley 5 on the opposite side. After being guided by the stepped double-layer pulley 5 on the moving end, it is guided again by the fixed end guide wheel 8 on the opposite side and then the knot is fixed to the knot fixing end of the elbow joint fixing end 3, or fixed to the tensioned end of the rope at the elbow joint fixing end 3.

[0042] The complete path of the first set of steel wire rope 1 after passing through the Bowden conduit end on the elbow joint fixed end 3 is as follows: Fixed end guide wheel 8 Mobile terminal stepped double-layer pulley 5 Fixed end guide wheel 8 Fixed end small pulley 9 Fixed end guide wheel 8 Mobile terminal stepped double-layer pulley 5 Fixed end guide wheel 8 Fixed end large pulley 11 Fixed end guide wheel 8 Mobile terminal stepped double-layer pulley 5 Fixed end guide wheel 8 Fixed end small pulley 9 Fixed end guide wheel 8 Mobile terminal stepped double-layer pulley 5 Fixed end guide wheel 8 The knotted end of the elbow joint fixing end 3.

[0043] Similarly, the winding method of the second set of steel wire ropes 10 is exactly the same as that of the first set of steel wire ropes 1. Since the second set of steel wire ropes 10 and the first set of steel wire ropes 1 are arranged in mirror image of each other, when the joint module 18 rotates clockwise or counterclockwise, the first set of steel wire ropes 1 and the second set of steel wire ropes 10 shorten / extend by the same length at the same time, causing the elbow joint moving end 6 to rotate relative to the center of a quarter circle arc of the elbow joint fixed end 3. Its intuitive movement is a tangential pure circle rolling relative to the outer contour of the elbow joint fixed end 3, and the change in rope length has a linear relationship with the change in the pure circle rolling angle.

[0044] More specifically, let the changes in rope length of the first set of wire rope 1 and the second set of wire rope 10 be respectively... and The change in the rolling angle of a pure circle is The diameter of the fixed end guide wheel is d The number of wire ropes passing around the movable pulley block is n Then its linear relationship can be expressed as formula (1): (1).

[0045] The tension amplification principle is achieved using the designed movable pulley system: Let the change in the tensile length of the wire rope be... The change in length of the movable pulley is The number of wire ropes passing around the movable pulley block is n The tension at the tension end of the wire rope is The output tension at the end of the movable pulley is The stiffness coefficient of the wire rope body is The combined stiffness coefficient of the movable pulley end is Formulas (2)-(5) can be derived as follows.

[0046] It is evident that this mechanism achieves speed reduction and force amplification through its constructed movable pulley system, and also achieves an exponential increase in the stiffness coefficient. Since the mechanism uses 8 steel wire ropes that pass through the movable pulley system, it can achieve an 8-fold increase in tension and a 64-fold increase in the stiffness coefficient. .

[0047] The principle by which the elbow joint can achieve constant stiffness and constant torque is as follows: Let... This represents a small change in the output torque of the elbow joint. For joint angle The minute changes These represent the tension at the taut end and the slack end of the rope, respectively. This is the preload length of the rope. This represents a tiny change in the amount of change in the rope. is the stiffness coefficient of the wire rope body. d The diameter of the fixed-end guide wheel, n The number of wire ropes that need to be bypassed by the movable pulley block, k This refers to the stiffness of the elbow joint.

[0048] Differentiating formula (1) yields: (6).

[0049] Based on the principle of virtual work, assuming the elbow joint rotates by a certain angle, Δ T Defined as the tension end of a rope and relaxation end From the tension difference, we can obtain: (7); Substituting equation (6) into equation (7) yields (8); Expand and achievable (9); (10); Then Δ T It can also be expressed as (11); Substituting equations (6) and (11) into equation (8) yields... (12); Therefore, the elbow joint stiffness can be obtained as follows: (13); As can be seen from equations (12) and (13), the torque and stiffness of the elbow joint do not change with the angle, and it has the characteristics of constant stiffness and constant torque.

[0050] The tensioning solutions at the rope end include, but are not limited to, bolt-slider tensioning solutions, tension / compression spring tensioning solutions, disc spring tensioning solutions, and temperature-controlled spring tensioning solutions.

[0051] The motion process of the tension-amplified constant torque constant stiffness rope-driven anthropomorphic elbow joint is as follows: The output end of the joint module 18 drives the upper meshing plate 19 with a quarter-circle arc on the elbow joint moving end 6 to rotate around the center of the arc of the lower meshing plate 20 with a quarter-circle arc on the elbow joint fixed end 3 by pulling the first set of steel wire rope 1 or the second set of steel wire rope 10. Simultaneously, due to the constraint of the coupling rope 17, the elbow joint fixed end 3 and the elbow joint moving end 6 always maintain a tangential gear-like meshing motion state. During this process, the angle between the elbow joint fixed end 3 and the elbow joint moving end 6 decreases, and the elbow joint moving end 6 gradually lifts up. Similarly, by pulling the steel wire rope 10 or 1 in the opposite direction, the output end of the joint module 18 can increase the angle between the elbow joint fixed end 3 and the elbow joint moving end 6 until it reaches the limit of 0°.

[0052] Thus, the present invention, a tension amplification constant torque constant stiffness rope-driven humanoid elbow joint, is complete.

Claims

1. A tension-amplified constant torque constant stiffness rope-driven humanoid elbow joint, comprising a fixed end (3) and a movable end (6) of the elbow joint, characterized in that: The elbow joint fixed end (3) is integrally formed with the upper engagement plate (19) with a quarter-circle arc, and the elbow joint moving end (6) is integrally formed with the lower engagement plate (20) with a quarter-circle arc. A coupling rope (17) is provided between the upper engagement plate (19) with a quarter-circle arc and the lower engagement plate (20) with a quarter-circle arc to maintain the rotation of the two on one side. A rope drive assembly with tension amplification and constant torque and constant stiffness is provided between the elbow joint fixed end (3) and the elbow joint moving end (6) to drive the rotation of the elbow joint moving end (6) relative to the elbow joint fixed end (3). The rope drive assembly includes a first set of steel wire rope (1) and a second set of steel wire rope (10), and the first set of steel wire rope (1) and the second set of steel wire rope (10) are arranged diagonally in a mirror image. The tension is amplified by forming a movable pulley group through a fixed end guide wheel (8), a fixed end small pulley (9), a fixed end large pulley (11) and a movable end stepped double-layer pulley (5). The first set of wire ropes (1) is wound as follows: it is led out from the Bowden tube end of the elbow joint fixed end (3), and the fixed end guide wheel (8) is used as the basic guide. It is wound around the movable end stepped double-layer pulley (5) multiple times to form the core structure of the movable pulley group. At the same time, it passes through the auxiliary guide of the fixed end small pulley (9) and the fixed end large pulley (11) in sequence, and is finally fixed to the knot fixed end of the elbow joint fixed end (3) to form a complete closed loop winding path. The winding method of the second set of wire ropes (10) is the same as that of the first set of wire ropes (1); The elbow joint fixing end (3) has two support platforms at its lower end, and the support platforms are located on both sides of the meshing plate (19) with a quarter circle arc. The two inclined surfaces of the support platforms are provided with small fixed end pulleys (9) for guidance. The upper end of the meshing plate (19) with a quarter-circle arc protrudes to form an extension, and large fixed-end pulleys (11) are provided on the inclined surfaces on both sides of the extension for guidance. The upper meshing plate (19) with a quarter-circle arc is provided with a coaxial fixed end center shaft (16) on both sides. The fixed end center shaft (16) is stepped. A rotating fixed end guide wheel (8) is provided on the large diameter section of the fixed end center shaft (16). The small diameter section of the fixed end center shaft (16) is movably connected to the double-end connector (4). The lower meshing plate (20) with a quarter-circle arc is fixed with a coaxial moving end center shaft (13) on both sides, and the moving end center shaft (13) is a stepped shaft; The central shaft (13) of the mobile end is provided with a rotatingly fitted mobile end pulley seat (7), and the outer side of the mobile end pulley seat (7) is fixed to the double-end connector (4); The outer wall of the movable end pulley seat (7) forms a symmetrical mounting slope. A stepped shaft is provided on the mounting slope. The movable end stepped double-layer pulley (5), which is used as a secondary rope winding guide, is axially fixed by the joint action of the shoulder of the stepped shaft and the bolt.

2. The tension amplification constant torque constant stiffness rope-driven human elbow joint according to claim 1, characterized in that: An absolute encoder rotating side (14) is provided on the outside of the central shaft (13) of the mobile end, and an absolute encoder magnetic base (15) is provided on the inside of the double-ended connector (4). The absolute encoder rotating side (14) and the absolute encoder magnetic base (15) measure the rotation angle of the elbow joint in real time.

3. The tension amplification constant torque constant stiffness rope-driven human elbow joint according to claim 1, characterized in that: A tensioning plate (12) is provided in the lower engagement plate (20) with a quarter-circle arc, and the tensioning plate (12) tensions the coupling rope (17).

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