Robot transmission structure and rigidity calibration method

Abstract

The invention belongs to the technical field of six-degree-of-freedom vertical multi-joint industrial robots, and particularly relates to a robot transmission structure and a rigidity calibration method. The transmission structure comprises a forearm shell, a forearm cover, a wrist shaft, an input gear shaft, a motor and a three-stage gear transmission assembly, the wrist shaft is rotationally connected to the tail end of the forearm shell, the forearm cover is connected with the forearm shell, a transmission cavity is formed between the forearm cover and the forearm shell, and the input gear shaft, the motor and the three-stage gear transmission assembly are all arranged in the transmission cavity; the input gear shaft is arranged at the head end of the small arm shell and is parallel to the wrist shaft, one end of the input gear shaft is connected with the motor, the three-stage gear transmission assembly is arranged between the input gear shaft and the wrist shaft, and the three-stage gear transmission assembly transmits the rotating torque of the input gear shaft to the wrist shaft to drive the wrist shaft to rotate. The transmission inertia is smaller, the controllability is good, and the wrist part is small in size and light in weight.

Description

Technical Field

[0001] The present invention belongs to the technical field of six-degree-of-freedom vertical multi-joint industrial robots, and particularly relates to a robot transmission structure and a stiffness calibration method. Background Art

[0002] Six-degree-of-freedom vertical multi-joint industrial robots are widely used in the industrial manufacturing field. In existing six-degree-of-freedom vertical multi-joint industrial robots, an RV reducer is usually placed on the wrist axis, and a reducer steel casting shell is covered outside the RV reducer. This structure increases the volume and weight of the wrist part and has a large moment of inertia. In the transmission system of a robot, the transmission stiffness plays a decisive role in indicators such as meshing error, transmission stability, and transmission smoothness. Therefore, detecting the composite transmission stiffness of the robot transmission system in advance is of great significance for ensuring the stability, reliability, and smoothness of the system. Summary of the Invention

[0003] Aiming at the above problems, the purpose of the present invention is to provide a robot transmission structure and a stiffness calibration method to ensure the stability, reliability, and smoothness of the system.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] On the one hand, the present invention provides a robot transmission structure, including a small arm shell, a small arm cover, a wrist axis, an input gear shaft, a motor, and a three-stage gear transmission assembly. The wrist axis is rotatably connected to the end of the small arm shell, the small arm cover is connected to the small arm shell, a transmission cavity is formed between the small arm cover and the small arm shell, the input gear shaft, the motor, and the three-stage gear transmission assembly are all arranged in the transmission cavity, the input gear shaft is arranged at the head end of the small arm shell and is parallel to the wrist axis, one end of the input gear shaft is connected to the motor, the three-stage gear transmission assembly is arranged between the input gear shaft and the wrist axis, and the three-stage gear transmission assembly transmits the rotational torque of the input gear shaft to the wrist axis to drive the wrist axis to rotate.

[0006] The three-stage gear transmission assembly includes a first-stage gear shaft, a first-stage large gear, a second-stage large gear, a second-stage gear shaft, and an output gear shaft. The first-stage gear shaft, the second-stage gear shaft, and the output gear shaft are sequentially and rotatably installed in the transmission cavity parallel to the input gear shaft, and the second-stage gear shaft also has a degree of freedom of axial movement; the first-stage large gear is fixed on the first-stage gear shaft and meshes with the input gear shaft; the second-stage large gear is fixed on the second-stage gear shaft and meshes with the first-stage gear shaft; the output gear shaft meshes with the second-stage gear shaft through a variable tooth thickness gear to form a variable tooth thickness gear meshing kinematic pair; a gear clearance compensation module for adjusting the meshing clearance of the variable tooth thickness gear meshing kinematic pair is provided at the end of the second-stage gear shaft.

[0007] Both ends of the secondary gear shaft are connected to the small arm housing and the small arm cover respectively through cylindrical roller bearings, and the outer rings of the cylindrical roller bearings are axially limited by the ball pressing shaft threaded gland; on the end face of one end of the secondary gear shaft connected to the small arm cover, a stepped blind hole is provided along the axis, and the gear clearance compensation module is accommodated in the stepped blind hole, and the ball pressing shaft threaded gland

[0008] is provided with a central hole for the gear clearance compensation module to pass through.

[0009] The gear clearance compensation module includes steel balls, a spring, a ball pressing threaded plug and an internal hexagon plug screw. Among them, the steel balls, the spring and the ball pressing threaded plug are accommodated in the stepped blind hole provided at the end of the secondary gear shaft from the inside to the outside. The ball pressing threaded plug is threadedly connected to the central hole of the ball shaft threaded gland, and the internal hexagon plug screw is arranged on the outside of the ball pressing threaded plug and is threadedly connected to the ball pressing shaft threaded gland.

[0010] At the central holes of the ball pressing shaft threaded glands at both ends of the secondary gear shaft, an outer seal end cover and an inner seal end cover are respectively provided.

[0011] On the other hand, the present invention provides a method for calibrating the stiffness of a robot transmission structure as described above, including the following steps:

[0012] Step S1: Remove the outer seal end cover and the inner seal end cover on the ball pressing shaft threaded glands at both ends of the secondary gear shaft, then remove the internal hexagon plug screw, and rotate the ball pressing threaded plug to make the variable tooth thickness gear meshing pair between the output gear shaft and the secondary gear shaft have no meshing clearance;

[0013] Step S2: Install a calibration tooling on the small arm housing, and the calibration tooling is located at the other end of the secondary gear shaft away from the gear clearance compensation module;

[0014] Step S3: The calibration tooling applies a calibration force to the secondary gear shaft to obtain the axial displacement of the secondary gear shaft, compares the obtained axial displacement with the calibrated displacement value, and by adjusting the ball pressing threaded plug as the system input variable, changes the pre-deformation and pre-pressure of the non-linear spring to make the axial displacement of the secondary gear shaft reach the calibrated displacement value; if the optimization adjustment fails, then replace the non-linear spring and repeat the above steps S1-S3;

[0015] Step S4: Remove the calibration tooling and perform a torsional stiffness test.

[0016] The calibration tooling includes a constant-force cylinder, support columns, a support plate and a micrometer. The constant-force cylinder includes a cylinder block, a piston and a piston rod. The piston rod passes through the cylinder block, and the piston rod is slidably engaged with the inner wall of the cylinder block through the piston. The support plate is connected to the cylinder block through the support columns. The micrometer is arranged on the support plate, and the measuring end contacts with one end of the piston rod. The other end of the piston rod contacts with the end of the secondary gear shaft.

[0017] In step S3, when the axial displacement of the secondary gear shaft detected by the micrometer is greater than the calibrated displacement value, the spring is compressed by screwing the ball pressure plug inward, so that the axial displacement of the secondary gear shaft detected by the micrometer reaches the calibrated displacement value.

[0018] When the axial displacement of the secondary gear shaft detected by the micrometer is less than the calibrated displacement value, the ball pressure plug is screwed outward to loosen the spring, so that the axial displacement of the secondary gear shaft detected by the micrometer reaches the calibrated displacement value.

[0019] A calibration force is applied to the secondary gear shaft through the constant-force cylinder, and the calibration force is 61±3.05N; the calibrated displacement value is 0.07±0.02mm.

[0020] In step S4, the torsional stiffness test process is as follows: fix the input gear shaft, and then apply a torque to the output gear shaft, then a twist corresponding to the torque will be generated, and a hysteresis curve is drawn, so as to obtain the stiffness characteristics of the robot transmission structure.

[0021] The advantages and beneficial effects of the present invention are as follows: A robot transmission structure provided by the present invention places the transmission components inside the small arm shell and the small arm cover, and arranges them sequentially from the wrist axis to the elbow joint axis, with a more uniform distribution, smaller moment of inertia, good controllability, making the wrist part smaller in volume and lighter in weight. Compared with the RV reduction mechanism type, one reducer steel casting shell is saved, and the production cost is low.

[0022] A robot transmission stiffness calibration method provided by the present invention can apply a load to the robot three-stage gear reduction transmission structure in a relatively simple manner, and obtain the angular displacement changes of the main and driven shafts of the transmission structure under load in a relatively simple, accurate and reliable manner, and then the composite transmission stiffness of the shaft gear can be calculated through the transmission stiffness calculation formula. Generally speaking, it has the technical advantages of reasonable structure design, convenient measurement and use, and high result accuracy, which helps to better evaluate and understand the composite transmission stiffness of the three-stage gear reduction transmission structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of a robot transmission structure in Embodiment 1 of the present invention;

[0024] Figure 2 is Figure 1Partial enlarged view of area A in the middle;

[0025] Figure 3 Schematic diagram of a stiffness calibration method for a robot transmission structure in the second embodiment of the present invention;

[0026] Figure 4 Flow chart of the stiffness calibration of a robot transmission structure in the second embodiment of the present invention.

[0027] In the figure: 1 - small arm housing, 2 - small arm cover, 3 - wrist shaft, 4 - output gear shaft, 5 - tapered roller bearing, 6 - variable tooth thickness gear meshing pair, 7 - secondary gear shaft, 8 - cylindrical roller bearing, 9 - steel ball, 10 - spring, 11 - ball pressure thread plug, 12 - hexagon socket plug screw, 13 - ball pressing shaft thread gland, 14 - outer side sealing end cover, 15 - inner side sealing end cover, 16 - primary gear shaft, 17 - input gear shaft, 18 - motor, 19 - primary large gear, 20 - secondary large gear, 27 - piston rod, 28 - piston, 29 - cylinder block, 30 - support column, 31 - support plate, 32 - dial indicator. Specific implementation mode

[0028] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Embodiment 1

[0030] As shown in Figure 1 , Figure 2 The present invention provides a robot transmission structure, including a small arm housing 1, a small arm cover 2, a wrist shaft 3, an input gear shaft 17, a motor 18 and a three-stage gear transmission assembly. The wrist shaft 3 is rotatably connected to the end of the small arm housing 1, the small arm cover 2 is connected to the small arm housing 1, a transmission cavity is formed between the small arm cover 2 and the small arm housing 1, the input gear shaft 17, the motor 18 and the three-stage gear transmission assembly are all arranged in the transmission cavity, the input gear shaft 17 is arranged at the head end of the small arm housing 1 and is parallel to the wrist shaft 3, one end of the input gear shaft 17 is connected to the motor 18, the three-stage gear transmission assembly is arranged between the input gear shaft 17 and the wrist shaft 3, and the three-stage gear transmission assembly transmits the rotational torque of the input gear shaft 17 to the wrist shaft 3 to drive the wrist shaft 3 to rotate.

[0031] In an embodiment of the present invention, the three-stage gear transmission assembly includes a first-stage gear shaft 16, a first-stage large gear 19, a second-stage large gear 20, a second-stage gear shaft 7, and an output gear shaft 4. Among them, the first-stage gear shaft 16, the second-stage gear shaft 7, and the output gear shaft 4 are sequentially and rotatably installed in the transmission cavity parallel to the input gear shaft 17, and the second-stage gear shaft 7 also has the freedom of axial movement; the first-stage large gear 19 is fixed on the first-stage gear shaft 16 and meshes with the input gear shaft 17; the second-stage large gear 20 is fixed on the second-stage gear shaft 7 and meshes with the first-stage gear shaft 16; the output gear shaft 4 and the second-stage gear shaft 7 are meshed through a variable tooth thickness gear to form a variable tooth thickness gear meshing kinematic pair 6; a gear clearance compensation module is provided at the end of the second-stage gear shaft 7 for adjusting the meshing clearance of the variable tooth thickness gear meshing kinematic pair 6.

[0032] Specifically, the output gear shaft 4 is installed on the small arm cover 2 through a tapered roller bearing 5. Both the small arm housing 1 and the small arm cover 2 are made of cast aluminum alloy. The second-stage large gear 20 and the second-stage gear shaft 7 are combined by hot fitting. The hot fitting method is that the second-stage gear shaft 7 is cooled with liquid nitrogen at low temperature, the second-stage large gear 20 is heated, and then the second-stage gear shaft 7 is inserted into the central hole of the second-stage large gear 20 through a tooling.

[0033] In an embodiment of the present invention, both ends of the second-stage gear shaft 7 are respectively connected to the small arm housing 1 and the small arm cover 2 through cylindrical roller bearings 8. The outer rings of the cylindrical roller bearings 8 are axially limited by a ball shaft thread gland 13; a stepped blind hole is provided along the axis on the end face of the end of the second-stage gear shaft 7 connected to the small arm cover 2, and the gear clearance compensation module is accommodated in the stepped blind hole, and the ball shaft thread gland 13

[0034] is provided with a central hole for the gear clearance compensation module to pass through.

[0035] In an embodiment of the present invention, the gear clearance compensation module includes a steel ball 9, a spring 10, a ball pressure thread plug 11, and an inner hexagon plug screw 12. Among them, the steel ball 9, the spring 10, and the ball pressure thread plug 11 are accommodated in the stepped blind hole provided at the end of the second-stage gear shaft 7 from the inside to the outside. The ball pressure thread plug 11 is threadedly connected to the central hole of the ball shaft thread gland 13. The inner hexagon plug screw 12 is provided on the outside of the ball pressure thread plug 11 and is threadedly connected to the ball shaft thread gland 13.

[0036] Further, outer seal end caps 14 and inner seal end caps 15 are respectively provided at the central holes of the ball shaft thread glands 13 at both ends of the second-stage gear shaft 7.

[0037] Preferably, the bottom of the stepped blind hole of the secondary gear shaft 7 is a conical surface, which forms a ring contact with the steel ball 9. The upper end of the spring 10 abuts against the steel ball 9, and the lower end of the spring 10 abuts against the ball pressure threaded plug 11. The ball pressure threaded plug 11 is threadedly engaged with the fine threads of the ball pressure shaft threaded gland 13, and the ball pressure shaft threaded gland 13 is threadedly engaged with the fine threads of the small arm cover 2. The hexagon socket head plug 12 is used to lock the relationship between the ball pressure threaded plug 11 and the ball pressure shaft threaded gland 13, and the outer seal end cover 14 and the inner seal end cover 15 are respectively used to block the small arm cover 2 and the small arm housing 1.

[0038] A robot transmission structure provided by the present invention places the transmission components inside the small arm housing and the small arm cover, and arranges them sequentially from the wrist axis to the elbow joint axis. The distribution is more uniform, the moment of inertia is smaller, the controllability is good, and the wrist part is small in volume and light in weight. Compared with the RV reduction mechanism type, one reducer steel casting shell is saved, and the production cost is low.

[0039] Due to the tolerances of the variable tooth thickness transmission of the wrist of the six-degree-of-freedom vertical multi-joint industrial robot and the batches of related components, each batch can be different, but it is necessary to efficiently control the product quality.

[0040] Embodiment 2

[0041] Based on the above design concept, another embodiment of the present invention provides a stiffness calibration method for the robot transmission structure in any of the above embodiments, as Figures 1 to 4 shown, the method includes the following steps:

[0042] Step S1: Remove the outer seal end cover 14 and the inner seal end cover 15 on the ball pressure shaft threaded gland 13 at both ends of the secondary gear shaft 7, then remove the hexagon socket head plug 12, and rotate the ball pressure threaded plug 11 to make the variable tooth thickness gear meshing pair 6 between the output gear shaft 4 and the secondary gear shaft 7 have no meshing clearance; specifically, it is indirectly judged by the height difference between the exposed shaft ends of the output gear shaft 4 and the secondary gear shaft 7. It can be indirectly determined by tools such as inspection fixtures, coordinate measuring machines, or inspection fixtures;

[0043] Step S2: Install a calibration tooling on the small arm housing 1, and the calibration tooling is located at the other end of the secondary gear shaft 7 away from the gear clearance compensation module;

[0044] Step S3: The calibration tooling compares the axial displacement obtained after applying a constant force to the secondary gear shaft 7 with the calibrated displacement value, and by adjusting the ball pressure threaded plug 11 as the system input variable, changes the pre-deformation and pre-pressure of the non-linear spring 10 to make the axial displacement of the secondary gear shaft 7 reach the calibrated displacement value; if the optimization adjustment fails, then replace the non-linear spring 10 for re-optimization, and repeat the above steps S1-S3;

[0045] Step S4: Remove the calibration tooling and perform a torsional stiffness test.

[0046] As Figure 3 shown, in the embodiment of the present invention, the calibration tooling includes a constant force cylinder, a support column 30, a support plate 31 and a dial indicator 32. The constant force cylinder includes a cylinder block 29, a piston 28 and a piston rod 27. The piston rod 27 passes through the cylinder block 29, and the piston rod 27 is slidably engaged with the inner wall of the cylinder block 29 through the piston 28. The support plate 31 is connected to the cylinder block 29 through three support columns 30, and the outside of the support plate 31 can abut against the inner side of the casting draft imitation small arm shell 1. The dial indicator 32 is arranged on the support plate 31, and the measuring end contacts with one end of the piston rod 27. The other end of the piston rod 27 contacts with the end of the secondary gear shaft 7. The probe of the dial indicator 32 can measure the moving amount of the piston rod 27.

[0047] Further, in step S3, a calibration force is applied to the secondary gear shaft 7 through the constant force cylinder. The calibration force is 61 ± 3.05 N; the calibration displacement value is 0.07 ± 0.02 m. That is to say, when the constant force cylinder outputs a calibration force of 61 ± 3.05 N and the displacement value of the secondary gear shaft 7 measured by the dial indicator 32 is 0.07 ± 0.02 m, it is determined that the spring 10 is qualified, otherwise it is unqualified. The constant force cylinder is controlled by a servo constant force pneumatic control system.

[0048] Specifically, in step S3, when the axial displacement amount of the secondary gear shaft 7 detected by the dial indicator 32 is greater than the calibration displacement value, it indicates that the axial force of the secondary gear shaft 7 is too small, resulting in a small transmission stiffness, and the absolute accuracy of the robot causes the robot to be unable to complete, etc. The function replaces the mission of the RV reducer. By screwing the ball pressure screw plug 11 inward to compress the spring 10, the axial displacement amount of the secondary gear shaft 7 detected by the dial indicator 32 reaches the calibration displacement value, so that the transmission stiffness reaches the calibration target; when the axial displacement amount of the secondary gear shaft 7 detected by the dial indicator 32 is less than the calibration displacement value, it indicates that the axial force of the secondary gear shaft 7 is too large and the transmission will be locked. By screwing the ball pressure screw plug 11 outward to relax the spring 10, the axial displacement amount of the secondary gear shaft 7 detected by the dial indicator 32 reaches the calibration displacement value, so that the transmission stiffness reaches the calibration target.

[0049] In the embodiment of the present invention, in step S4, the torsional stiffness test process is as follows: fix the input gear shaft 17, and then apply a torque to the output gear shaft 4, then a twist corresponding to the torque will be generated, and a hysteresis curve will be drawn, so as to obtain the stiffness characteristics of the robot transmission structure.

[0050] Furthermore, a torque is applied to the output gear shaft 4 through an external stiffness test bench. The stiffness test bench obtains test parameters through a torque sensor and a code disk. The servo motor brake is braked, and the maximum torque is applied to the output end of the reducer in the forward direction and then gradually reduced to the maximum torque in the reverse direction. Due to a series of factors such as structural non-linear stiffness, gear non-linear contact stiffness, bearing non-linear stiffness, variable tooth thickness meshing, and spring stiffness at the output end, the output shaft rotation angle changes, and the torque-loading torque - output shaft torsion curve of the output shaft is plotted. The curve characteristics represent the stiffness characteristics of the reducer.

[0051] The compensation principle of the gear clearance compensation module is as follows: the spring force of the spring 10 always compensates to keep the variable tooth thickness gear meshing pair 6 between the output gear shaft 4 and the secondary gear shaft 7 in contact without clearance. Therefore, the transmission stiffness is that the motor at the input end of the reducer is locked, and the output end is loaded in both forward and reverse directions, and the forward and reverse rotation angle lag curves of the output end are obtained. Fix the input gear on the input shaft, and then apply a torque to the output shaft, then a twist corresponding to the torque will be generated, and a lag curve will be drawn. The angular changes of the driving and driven shafts of the transmission structure under load can be obtained from this curve, and then the composite transmission stiffness of the shaft-mounted gear can be calculated through the transmission stiffness calculation formula. In a mechanical transmission state with a certain variable tooth thickness meshing clearance, the gear axial force is calibrated to the design value, and then the meshing stiffness is calibrated. If the meshing stiffness is too large, the service life of the industrial robot variable tooth thickness reducer is insufficient; if the meshing stiffness is too small, the transmission accuracy of the robot cannot meet its performance indicators.

[0052] In this embodiment, the spring preload and pre-deformation of the spring 10 are non-linear variables, so the spring stiffness is also non-linear. The present invention optimizes the transmission stiffness of the robot by calibrating the axial pre-tension force and calibrating the variable tooth thickness backlash to achieve a dual-target variable optimization mathematical model.

[0053] In this embodiment, the transmission structure of the robot of the present invention is used in the variable tooth thickness transmission five-axis and small arm of the industrial robot, and can be analogized to the variable tooth thickness three-axis and large arm of the industrial robot, the variable tooth thickness transmission two-axis and waist seat of the industrial robot, and the variable tooth thickness transmission one-axis and base of the industrial robot. The calibration object structure of the present invention is complex. From the perspective of R & D, it is necessary to design and verify the overall calculation system stiffness, and from the perspective of mass production quality control, it is necessary to calibrate the stiffness performance of the components of the entire system. In a mechanical transmission state with a certain variable tooth thickness meshing clearance, the gear axial force is calibrated to the design value, and then the meshing stiffness is calibrated. If the meshing stiffness is too large, the service life of the industrial robot variable tooth thickness reducer is insufficient; if the meshing stiffness is too small, the transmission accuracy of the robot cannot meet its performance indicators.

[0054] The above is only the implementation mode of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, expansion, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A robot transmission structure, characterized in that, It includes a small arm housing (1), a small arm cover (2), a wrist shaft (3), an input gear shaft (17), a motor (18) and a three-stage gear transmission assembly. The wrist shaft (3) is rotatably connected to the end of the small arm housing (1). The small arm cover (2) is connected to the small arm housing (1), and a transmission cavity is formed between the small arm cover (2) and the small arm housing (1). The input gear shaft (17), the motor (18) and the three-stage gear transmission assembly are all arranged in the transmission cavity. The input gear shaft (17) is arranged at the head end of the small arm housing (1) and is parallel to the wrist shaft (3). One end of the input gear shaft (17) is connected to the motor (18). The three-stage gear transmission assembly is arranged between the input gear shaft (17) and the wrist shaft (3), and the three-stage gear transmission assembly transmits the rotational torque of the input gear shaft (17) to the wrist shaft (3) to drive the wrist shaft (3) to rotate.

2. The robot transmission structure according to claim 1, wherein, The three-stage gear transmission assembly includes a first-stage gear shaft (16), a first-stage large gear (19), a second-stage large gear (20), a second-stage gear shaft (7) and an output gear shaft (4). The first-stage gear shaft (16), the second-stage gear shaft (7) and the output gear shaft (4) are sequentially and rotatably installed in the transmission cavity parallel to the input gear shaft (17), and the second-stage gear shaft (7) also has the freedom of axial movement. The first-stage large gear (19) is fixed on the first-stage gear shaft (16) and meshes with the input gear shaft (17). The second-stage large gear (20) is fixed on the second-stage gear shaft (7) and meshes with the first-stage gear shaft (16). The output gear shaft (4) and the second-stage gear shaft (7) are meshed through a variable tooth thickness gear to form a variable tooth thickness gear meshing kinematic pair (6). A gear clearance compensation module is provided at the end of the second-stage gear shaft (7) for adjusting the meshing clearance of the variable tooth thickness gear meshing kinematic pair (6).

3. The robot transmission structure according to claim 2, wherein, Both ends of the second-stage gear shaft (7) are connected to the small arm housing (1) and the small arm cover (2) respectively through cylindrical roller bearings (8), and the outer rings of the cylindrical roller bearings (8) are axially limited by a ball shaft thread gland (13). A stepped blind hole is provided along the axis on the end face of the end of the second-stage gear shaft (7) connected to the small arm cover (2), and the gear clearance compensation module is accommodated in the stepped blind hole. The ball shaft thread gland (13) is provided with a central hole for the gear clearance compensation module to pass through.

4. The robot transmission structure according to claim 3, wherein, The gear clearance compensation module includes a steel ball (9), a spring (10), a ball pressure thread plug (11) and an internal hexagon plug screw (12). The steel ball (9), the spring (10) and the ball pressure thread plug (11) are accommodated in the stepped blind hole provided at the end of the second-stage gear shaft (7) from the inside to the outside. The ball pressure thread plug (11) is threadedly connected to the central hole of the ball shaft thread gland (13). The internal hexagon plug screw (12) is arranged outside the ball pressure thread plug (11) and is threadedly connected to the ball shaft thread gland (13).

5. The robot transmission structure according to claim 4, wherein Outer seal end caps (14) and inner seal end caps (15) are respectively provided at the central holes of the ball shaft thread glands (13) at both ends of the second-stage gear shaft (7).

6. A stiffness calibration method for a robot transmission structure as described in claim 5, characterized in that, It includes the following steps: Step S1: Remove the outer seal end cover (14) and the inner seal end cover (15) on the threaded gland (13) at both ends of the secondary gear shaft (7), then remove the hexagon socket plug screw (12), and rotate the ball press threaded plug (11) so that there is no meshing clearance in the variable tooth thickness gear meshing pair (6) between the output gear shaft (4) and the secondary gear shaft (7); Step S2: Install a calibration tooling on the forearm housing (1), and the calibration tooling is located at the other end of the secondary gear shaft (7) away from the gear clearance compensation module; Step S3: The calibration tooling applies a calibration force to the secondary gear shaft (7) to obtain the axial displacement of the secondary gear shaft (7), compares the obtained axial displacement with the calibrated displacement value, and adjusts the ball press threaded plug (11) as the system input variable to change the pre-deformation and pre-pressure of the non-linear spring, so that the axial displacement of the secondary gear shaft (7) reaches the calibrated displacement value; If the optimization adjustment fails, then replace the non-linear spring and repeat the above steps S1 - S3; Step S4: Remove the calibration tooling and conduct a torsional stiffness test.

7. The stiffness calibration method according to claim 6, characterized in that The calibration tooling includes a constant force cylinder, a support column (30), a support plate (31) and a dial indicator (32). The constant force cylinder includes a cylinder body (29), a piston (28) and a piston rod (27). The piston rod (27) passes through the cylinder body (29), and the piston rod (27) is slidably matched with the inner wall of the cylinder body (29) through the piston (28). The support plate (31) is connected to the cylinder body (29) through the support column (30). The dial indicator (32) is arranged on the support plate (31), and the measuring end contacts the end of one end of the piston rod (27), and the other end of the piston rod (27) contacts the end of the secondary gear shaft (7).

8. The stiffness calibration method according to claim 7, wherein In step S3, when the axial displacement of the secondary gear shaft (7) detected by the dial indicator (32) is greater than the calibrated displacement value, the ball press threaded plug (11) is screwed inwards to compress the spring (10) so that the axial displacement of the secondary gear shaft (7) detected by the dial indicator (32) reaches the calibrated displacement value; When the axial displacement of the secondary gear shaft (7) detected by the dial indicator (32) is less than the calibrated displacement value, the ball press threaded plug (11) is screwed outwards to relax the spring (10) so that the axial displacement of the secondary gear shaft (7) detected by the dial indicator (32) reaches the calibrated displacement value.

9. The stiffness calibration method according to claim 8, wherein A calibration force of 61 ± 3.05 N is applied to the secondary gear shaft (7) by the constant force cylinder; the calibrated displacement value is 0.07 ± 0.02 mm.

10. The stiffness calibration method according to claim 6, wherein In step S4, the process of the torsional stiffness test is to fix the input gear shaft (17), then apply a torque to the output gear shaft (4), which will generate a twist corresponding to the torque, and draw a hysteresis curve, so as to obtain the stiffness characteristics of the robot transmission structure.

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