A small series elastic stiffness variable drive based on differential gear train

Abstract

The application discloses a small-sized series elastic driver with variable stiffness based on a differential gear train and belongs to the technical field of drivers. The small-sized series elastic driver with variable stiffness comprises an input end cover, an output end cover and a shell, the two sides of the shell are connected with the input end cover and the output end cover respectively, a driving structure, a differential gear train structure, an elastic force transmission structure and a variable stiffness structure are arranged in the shell, a double power source structure is arranged on one side of the input end cover, the double power source structure is connected with the driving structure through the input end cover, the differential gear train structure is connected with the driving structure, the elastic force transmission structure is connected with the differential gear train structure, the variable stiffness structure is connected with the elastic force transmission structure, and the variable stiffness structure is connected with the output end cover. The stiffness adjustment of the variable stiffness adjustment structure, the decoupling and the accurate control of the output displacement are realized through the differential speed control of the double power source structure on the planetary carrier and the sun gear in the differential gear train.

Description

Technical Field

[0001] This invention relates to the field of drive technology, and in particular to a small series variable stiffness elastic drive based on a differential gear train. Background Technology

[0002] The rapid development of the robotics field has led to an increasing number of robot applications and increasingly complex environments. Joint actuators, as core components for robot motion execution, directly impact a robot's dynamic response, energy efficiency, environmental adaptability, and interactive safety. While traditional rigid actuators offer high precision and load capacity, their rigidity can easily lead to energy buildup during dynamic interactive tasks, resulting in decreased system stability and safety hazards. Series elastic actuators, by introducing a fixed-stiffness elastic element between the rigid drive motor and the load, enable the actuator to buffer impacts from the working environment to protect the motor, store potential energy for the robot's use during cyclic motion, and release the stored potential energy in a very short time during explosive movements to achieve significant acceleration. This significantly improves the system's compliance and safety, and to some extent, enhances the robot's driving performance. However, series elastic actuators cannot dynamically adjust stiffness to balance precise motion and compliant interaction.

[0003] The invention of variable stiffness elastic actuators aims to solve the problem of unadjustable stiffness, but existing variable stiffness elastic actuators have the following problems:

[0004] (1) The number of elastic elements is mostly arranged in a single set of antagonistic springs, which leads to low space utilization and a small adjustable stiffness range of the elastic actuator.

[0005] (2) The main methods of stiffness adjustment are cam mechanisms with Archimedean spiral grooves, ball screw mechanisms, and planetary gear trains with a diameter ratio of 2:1. Cam mechanisms with Archimedean spiral grooves are complex to manufacture, and the number of grooves on the cam disk is limited. When the number increases, it will reduce the strength of the cam disk, which will easily lead to plastic deformation under heavy loads and affect the stiffness adjustment accuracy. Ball screw mechanisms occupy a lot of space, which is not conducive to the compact design of the drive, and the response speed is slow, which is not conducive to the rapid adjustment of stiffness. Planetary gear trains with a diameter ratio of 2:1 require high strength of the pivot fixed on the planetary gears, which may also have the risk of insufficient strength under heavy loads, thus affecting the stiffness adjustment accuracy. Summary of the Invention

[0006] The purpose of this invention is to provide a small series variable stiffness elastic actuator based on a differential gear train to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides a small series variable stiffness elastic actuator based on a differential gear train, comprising an input end cover, an output end cover, and a housing. The housing is connected to the input end cover and the output end cover on both sides, respectively. The housing contains a drive structure, a differential gear train structure, an elastic force transmission structure, and a variable stiffness structure. A dual power source structure is provided on one side of the input end cover. The dual power source structure is connected to the drive structure via the input end cover. The differential gear train structure is connected to the drive structure. The elastic force transmission structure is connected to the differential gear train structure. The variable stiffness structure is connected to the elastic force transmission structure and to the output end cover.

[0008] Preferably, the drive structure includes a guide shaft, spur gear I, spur gear II, spur gear III, and a first internal gear. The guide shaft is rotatably connected to the input end cover via bearing I, and the guide shaft is located at the center of the input end cover. The spur gear III is mounted on the guide shaft. Both spur gear I and spur gear II are axially positioned via the input end cover. The spur gear III meshes with spur gear II. The first internal gear meshes with the spur gear I. The spur gear I is disposed inside the first internal gear. The first internal gear is axially positioned via the input end cover. A planetary carrier assembly is fixedly connected to one side of the first internal gear. The guide shaft passes through the planetary carrier assembly and is connected to the planetary carrier assembly via bearing assembly I.

[0009] Preferably, a bearing II is provided on the outer side of the planetary carrier assembly, and the outer side of the bearing II is connected to the housing to ensure coaxiality. The planetary carrier assembly includes a planetary carrier I and a planetary carrier II fixedly connected to the planetary carrier I. The planetary carrier I is fixedly connected to the first internal gear. The bearing assembly I includes a bushing I and bearings III and IV disposed on both sides of the bushing I.

[0010] Preferably, the differential gear train includes a second internal gear, inside which are meshing planet gears I, II, III, and IV. A sun gear is disposed between and meshes with planet gears I, II, III, and IV. The sun gear is sleeved on the guide shaft. Planet gears I, II, III, and IV are connected to the planet carrier II via elastic retaining rings.

[0011] Preferably, the elastic torque conversion mechanism includes an output shaft arm, on which elastic torque conversion rod I and elastic torque conversion rod II are symmetrically and fixedly connected, and an output shaft is fixedly connected at the center of the output shaft arm. The output shaft wall is disposed on one side of the second internal gear.

[0012] Preferably, the elastic force transmission structure includes a spring seat platform. The spring seat platform passes through the elastic torque conversion rod I, the elastic torque conversion rod II, and the output shaft and is disposed on one side of the output shaft arm. A first linear guide rail and a second linear guide rail are symmetrically arranged on the spring seat platform. A first spring-acting slider and a second spring-acting slider are respectively connected to the first linear guide rail and the second linear guide rail. A first spring stop and a second spring stop are arranged on both sides of the first spring-acting slider. A first spring is arranged between the first spring-acting slider and the first spring stop. A second spring is arranged between the first spring-acting slider and the second spring stop. A third spring stop and a fourth spring stop are arranged on both sides of the second spring-acting slider. A third spring is arranged between the second spring-acting slider and the third spring stop. A fourth spring is arranged between the second spring-acting slider and the fourth spring stop.

[0013] Preferably, the first spring stop, the second spring stop, the third spring stop, and the fourth spring stop are all fixedly connected to the spring seat platform, the output shaft is connected to the spring seat platform via bearing V, and the spring seat platform is connected to the planetary carrier II via bolts.

[0014] Preferably, the variable stiffness structure includes a lever assembly and a crank-slider assembly. The lever assembly includes a first lever and a second lever. One side of the first lever and the second lever are respectively connected to the elastic torque conversion rod I and the elastic torque conversion rod II via snap rings, and the other side is respectively floatingly connected to the first spring-acting slider and the second spring-acting slider.

[0015] The crank-slider assembly includes a second internal gear and a common crank connecting block, a common crank, a first connecting rod, a second connecting rod, and a common crank connecting block. The common crank is fixedly connected to the common crank connecting block, and the common crank and the common crank connecting block are rotatably connected to the output shaft through a bearing assembly II. A first connecting plate and a second connecting plate are symmetrically arranged on the common crank connecting block. The first connecting plate and the second connecting plate are fixedly connected to the second internal gear and the common crank connecting block. The first connecting rod and the second connecting rod are connected to the common crank connecting block through cotter pins. The pivots of the first connecting rod and the second connecting rod are respectively placed in the first lever and the second lever.

[0016] Preferably, the second internal gear and the common crank connecting block are fixedly connected to the second internal gear. The bearing assembly II includes a bushing II and bearings VI and VII disposed on both sides of the bushing II. The bearing VI is connected to the common crank, and the bearing VII is connected to the common crank connecting block. A bearing VIII is disposed on the outside of the common crank connecting block and is placed in the bearing seat of the output end cover.

[0017] Preferably, the dual power source structure includes a first power source and a second power source. The first power source includes a first motor and a first reducer connected to the first motor. The second power source includes a second motor and a second reducer connected to the second motor. The output shafts of the first reducer and the second reducer pass through the input end cover and are respectively connected to the spur gear I and the spur gear II.

[0018] Therefore, the present invention employs the above-mentioned small series variable stiffness elastic actuator based on a differential gear train, which has the following beneficial effects:

[0019] (1) By using a dual-power source structure to control the differential speed of the sun gear in the planetary carrier and the differential gear train, the stiffness adjustment of the variable stiffness adjustment structure and the decoupling and precise control of the output displacement are achieved.

[0020] (2) Using a common crank to drive two connecting rods to control the position of the fulcrum in the two levers, under the constraint of the small size of the driver, the crank-slider mechanism can make the mechanism composition simpler and the processing cost lower.

[0021] (3) By symmetrically placing two sets of antagonistic springs in the actuator, the space utilization inside the actuator can be improved, and the actuator can have a larger stiffness adjustment range in a smaller volume.

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0023] Figure 1 This is an exploded view of the power transmission section of the variable stiffness elastic actuator according to an embodiment of the present invention.

[0024] Figure 2 This is an exploded view of the elastic force conversion part of the variable stiffness elastic actuator according to an embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram of the overall structure of the small series variable stiffness elastic actuator according to an embodiment of the present invention;

[0026] Figure Labels

[0027] 1. First motor; 2. First reducer; 3. Second motor; 4. Second reducer; 5. Input end cover; 6. Bearing I; 7. Spur gear I; 8. Spur gear III; 9. Spur gear II; 10. First internal gear; 11. Planetary carrier I; 12. Bearing II; 13. Guide shaft; 14. Bearing III; 15. Bushing I; 16. Bearing IV; 17. Planetary carrier II; 18. Second internal gear; 19. Planetary gear I; 20. Planetary gear II; 21. Planetary gear III; 22. Planetary gear IV; 23. Sun gear; 24. Second internal gear and common crank connecting block; 25. Output shaft arm; 26. Elastic torque converter I; 27. Elastic torque converter II; 28. Spring seat platform; 29. 30. Bearing V; 31. First linear guide rail; 32. Second linear guide rail; 33. First spring; 34. Second spring; 35. Third spring; 36. Fourth spring; 37. First spring stop; 38. Second spring stop; 39. First spring-acting slider; 40. Third spring stop; 41. Fourth spring stop; 42. Second spring-acting slider; 43. First lever; 44. Second lever; 45. First connecting rod; 46. Second connecting rod; 47. Common crank; 48. Bearing VI; 49. Bushing II; 50. Bearing VII; 51. Output shaft; 52. Common crank connecting block; 53. First connecting plate; 54. Second connecting plate; 55. Bearing VIII; 56. Output end cap; 57. Housing. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," etc., indicating orientation or positional relationships are based on the orientation or positional relationships shown in the accompanying drawings, or the orientation or positional relationships commonly used when the product of the invention is in use. They are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.

[0029] Example

[0030] Reference Figures 1-3This invention discloses a small series variable stiffness elastic actuator based on a differential gear train, comprising an input end cover 5, an output end cover 55, and a housing 56. The housing 56 is connected to the input end cover 5 and the output end cover 55 on both sides, respectively. Inside the housing 56 are a drive structure, a differential gear train structure, an elastic force transmission structure, and a variable stiffness structure. A dual power source structure is located on one side of the input end cover 5. The dual power source structure is connected to the drive structure via the input end cover 5. The differential gear train structure is connected to the drive structure, the elastic force transmission structure is connected to the differential gear train structure, the variable stiffness structure is connected to the elastic force transmission structure, and the variable stiffness structure is connected to the output end cover 55. The housing 56 ensures that all structures except the dual power source structure are in a sealed environment, preventing dust and impurities from entering and affecting the actual operation of the actuator.

[0031] The drive structure includes a guide shaft 13, spur gears I7, II9, III8, and a first internal gear 10. The guide shaft 13 is rotatably connected to the input end cover 5 via bearing I6, and the guide shaft 13 is located at the center of the input end cover 5. Spur gear III8 is sleeved on the guide shaft 13 and is circumferentially positioned by a flat key. Spur gears I7 and II9 are both axially positioned via the input end cover 5. Spur gear III8 meshes with spur gear II9. The first internal gear 10 meshes with spur gear I7, and spur gear I7 is located inside the first internal gear 10. The first internal gear 10 is axially positioned via the input end cover 5. A planetary carrier assembly is fixedly connected to one side of the first internal gear 10. The guide shaft 13 passes through the planetary carrier assembly and is connected to the planetary carrier assembly via bearing assembly I.

[0032] A bearing II12 is mounted on the outer side of the planetary carrier assembly. The outer side of bearing II12 is connected to the outer casing 56 to ensure coaxiality. The planetary carrier assembly includes a planetary carrier I11 and a planetary carrier II17 that is bolted to the planetary carrier I11. The planetary carrier I11 is bolted to the first internal gear 10 to ensure torque transmission. Bearing assembly I includes a bushing I15 and bearings III14 and IV16 disposed on both sides of the bushing I15. Bearing assembly I is axially positioned on the guide shaft 13 by a shoulder and an elastic retaining ring. The dual bearing configuration ensures minimal deflection of the guide shaft 13, while also ensuring circumferential movement of the planetary carrier I11 and planetary carrier II17.

[0033] The differential gear train includes a second internal gear 18, inside which are meshing planetary gears I19, II20, III21, and IV22. A sun gear 23 is disposed between and meshes with planetary gears I19, II20, III21, and IV22. The sun gear 23 is connected to the guide shaft 13 via a key, bolt, and elastic retaining ring. Planetary gears I19, II20, III21, and IV22 are connected to four short shafts on the planet carrier II17 via elastic retaining rings.

[0034] The elastic torque conversion mechanism includes an output shaft arm 25. Elastic torque conversion rods I26 and II27 are symmetrically fixedly connected to both sides of the output shaft arm 25. An output shaft 50 is fixedly connected to the center of the output shaft arm 25, and the output shaft arm 25 is located on one side of the second internal gear 18. Elastic torque conversion rods I26, II27, and the output shaft 50 are all connected to the output shaft arm 25 by bolts.

[0035] The elastic force transmission structure includes a spring seat platform 28, which passes through the elastic torque conversion rod I26, the elastic torque conversion rod II27, and the output shaft 50, and is located on one side of the output shaft arm 25. The output shaft 50 is connected to the spring seat platform 28 via a bearing V29. The spring seat platform 28 is connected to four long shafts on the planetary carrier II17 via bolts. A first linear guide rail 30 and a second linear guide rail 31 are fixedly connected to the spring seat platform 28 via bolts, and the first linear guide rail 30 and the second linear guide rail 31 are symmetrically arranged. A first spring-operated slider 38 and a second spring-operated slider 41 are respectively connected to the first linear guide rail 30 and the second linear guide rail 31. Both the first linear guide rail 30 and the second linear guide rail 31 are composed of slide rails and sliders. The first spring-acting slider 38 and the second spring-acting slider 41 are both fixedly connected to the sliders of the first linear guide rail 30 and the second linear guide rail 31 by bolts. The first spring-acting slider 38 has a first spring stop 36 and a second spring stop 37 on both sides. A first spring 32 is positioned between the first spring-acting slider 38 and the first spring stop 36, and a second spring 33 is positioned between the first spring-acting slider 38 and the second spring stop 37. The second spring-acting slider 41 has a third spring stop 39 and a fourth spring stop 40 on both sides. A third spring 34 is positioned between the second spring-acting slider 41 and the third spring stop 39, and a fourth spring 35 is positioned between the second spring-acting slider 41 and the fourth spring stop 40. The first spring 32 and the second spring 33 form one set of antagonistic springs, and the third spring 34 and the fourth spring 35 form another set of antagonistic springs. The maximum deformation of each spring is determined by the gap between the end faces of the spring-acting slider and the spring stop. The first spring stop 36, the second spring stop 37, the third spring stop 39, and the fourth spring stop 40 are all fixedly connected to the spring seat platform 28 by bolts. To be suitable for heavy loads, the working length of each set of springs is within the range of 0.3-0.7 of the maximum deformation length to ensure constant stiffness and reduce errors caused by possible stiffness changes in the compression spring.

[0036] The variable stiffness structure includes a lever assembly and a crank-slider assembly. The lever assembly includes a first lever 42 and a second lever 43. One side of the first lever 42 and the second lever 53 are connected to the elastic torque conversion rod I26 and the elastic torque conversion rod II27 respectively via snap rings, and the other side is floatingly connected to the first spring-acting slider 38 and the second spring-acting slider 41 respectively, which is responsible for converting the elastic torque generated by the spring into the torque of the output shaft.

[0037] The crank-slider assembly includes a second internal gear and common crank connecting block 24, a common crank 46, a first connecting rod 44, a second connecting rod 45, and a common crank connecting block 51. The second internal gear and common crank connecting block 24 is fixedly connected to the second internal gear 18, and the common crank 46 is fixedly connected to the common crank connecting block 51. The common crank 46 and common crank connecting block 51 are rotatably connected to the output shaft 50 through bearing assembly II. A first connecting plate 52 and a second connecting plate 53 are symmetrically arranged on the common crank connecting block 51. The first connecting plate 52 and the second connecting plate 53 are respectively fixedly connected to the second internal gear and common crank connecting block 24 to ensure accurate torque transmission of the second internal gear 18. The first connecting rod 52 and the second connecting rod 53 are connected to the common crank connecting block 51 through cotter pins, and the pivots in the first connecting rod 52 and the second connecting rod 53 are respectively placed in the first lever 42 and the second lever 43, acting as fulcrums.

[0038] It should be noted that the lever assembly of the present invention can be expanded from two to four, depending on the requirements. When a larger deflection angle is required, a two-lever scheme is adopted; when a smaller deflection angle is required, a four-lever scheme is adopted.

[0039] The bearing assembly II includes a bushing II48 and bearings VI47 and VII49 disposed on both sides of the bushing II48. Bearing VI47 is connected to the common crank 46, and bearing VII49 is connected to the common crank connecting block 51. Bearing VIII54 is disposed on the outside of the common crank connecting block 51 and is placed in the bearing seat of the output end cover 55 to ensure coaxiality and axial positioning.

[0040] The dual power source structure includes a first power source and a second power source. The first power source includes a first motor 1 and a first reducer 2 connected to the first motor 1. The second power source includes a second motor 3 and a second reducer 4 connected to the second motor 3. The first reducer 2 and the second reducer 4 are fixedly connected to the input end cover 5 by bolts. The output shafts of the first reducer 2 and the second reducer 4 pass through the input end cover 5 and are respectively connected to spur gear I7 and spur gear II9.

[0041] Working principle: The variable stiffness elastic actuator of this invention has two power flow transmission paths, and based on the two power flow transmission paths, it has a displacement control mode and a stiffness control mode.

[0042] The two types of dynamic flow transmission paths include:

[0043] The first power flow transmission path is as follows: The first power source drives spur gear I7 via a key, and spur gear I7 drives the first internal gear 10 through internal meshing, thereby driving the planetary carrier assembly to rotate. The planetary carrier II17 is connected to the spring seat platform 28, realizing the synchronous rotation of the spring seat platform 28. The second power flow transmission path includes: The second power source drives spur gear II9 via a key, and then drives spur gear III8 through external meshing. It drives the sun gear 23 through the guide shaft 13, and then drives the second internal gear 18. The second internal gear drives the common crank connecting block 24, the first connecting plate 52, the second connecting plate 53, and the common crank connecting block 51 to drive the common crank, thereby driving the first connecting rod 44 and the second connecting rod 45 to rotate, so as to control the position of the pivot on the first connecting rod 44 and the second connecting rod 45 in the first lever 42 and the second lever 43.

[0044] Displacement Control Mode: In this mode, the output shaft of the second power source must remain stationary to keep the crank-slider mechanism stationary, meaning the pivots on the first connecting rod 44 and the second connecting rod 45 remain in their positions within the levers. The first power flow transmission path transfers the power from the first power source to the spring seat platform 28. During rotation, the spring seat platform 28 drives two sets of antagonistically arranged springs to drive the first lever 42 and the second lever 43 respectively. The two levers deflect and rotate around the pivots on the first connecting rod 44 and the second connecting rod 45, thereby driving the elastic torque conversion rods I26 and II27 on the other side of the first lever 42 and the second lever 43, which in turn drives the output shaft arm 25 to rotate, i.e., the output shaft 50 rotates. Because the springs are not fully compressed, both sets of antagonistically arranged springs undergo a certain degree of compression during rotation, achieving flexible drive.

[0045] Stiffness Control Mode: In this mode, the output shaft of the first power source must remain stationary to keep the planetary carrier stationary, i.e., the spring seat platform must remain stationary. The second power flow transmission path transmits the power of the second power source to the common crank 46 through the variable stiffness mechanism. The common crank 46 drives the first connecting rod 44 and the second connecting rod 45 to rotate, causing the pivots on the first connecting rod 44 and the second connecting rod 45 to change their positions in the first lever 42 and the second lever 43. This, in turn, adjusts the lever arm ratio of the antagonistic spring group and the elastic torque conversion rods I26 and II27 on the first lever 42 and the second lever 43, thereby changing the elastic torque generated by the springs in different proportions, i.e., achieving stiffness control.

[0046] Furthermore, the driver achieves passive protection through two sets of antagonistically arranged springs. Specifically, the first power source, consisting of the first motor 1 and the first reducer 2, and the second power source, consisting of the second motor 3 and the second reducer 4, both remain stationary (i.e., the driver output shaft has moved to a designated position). If an external impact load is applied to the output shaft 50, the output shaft 50 drives the output shaft arm 25, the elastic torque conversion rod I26, and the elastic torque conversion rod II27, which in turn act on the first lever 42 and the second lever 43, respectively. The two levers deflect, respectively driving the first spring-acting slider 38 and the second spring-acting slider 41, which in turn act on the first spring 32 and the second spring 33, and the third spring 34 and the fourth spring 35, respectively, causing the springs to deform and absorb the impact energy. Within the maximum deformation range of the springs, the two power sources are ensured not to be subjected to reverse loads, thus achieving the function of protecting the power sources under impact loads.

[0047] Therefore, the present invention adopts the above-mentioned small series variable stiffness elastic actuator based on differential gear train, which uses two levers as elastic force transmission mechanisms and is connected to the load at the same time. Two sets of antagonistically arranged springs act on the spring load points of the two levers respectively, thereby increasing the adjustable range of stiffness and improving the space utilization of the actuator.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A small series variable stiffness elastic actuator based on a differential gear train, characterized in that: The device includes an input end cap, an output end cap, and a housing. The housing is connected to the input end cap and the output end cap on both sides, respectively. Inside the housing, there is a drive structure, a differential gear train structure, an elastic force transmission structure, and a variable stiffness structure. A dual power source structure is provided on one side of the input end cap. The dual power source structure is connected to the drive structure through the input end cap. The differential gear train structure is connected to the drive structure. The elastic force transmission structure is connected to the differential gear train structure. The variable stiffness structure is connected to the elastic force transmission structure. The variable stiffness structure is connected to the output end cap. The drive structure includes a guide shaft, which is rotatably connected to the input end cover via a bearing I, and the guide shaft is located at the center of the input end cover. The differential gear train includes a second internal gear, inside which are meshing planet gears I, II, III and IV. A sun gear is disposed between and meshes with planet gears I, II, III and IV, and the sun gear is sleeved on the guide shaft. The elastic force transmission structure includes an output shaft arm, on both sides of which elastic torque conversion rod I and elastic torque conversion rod II are symmetrically and fixedly connected. An output shaft is fixedly connected to the center of the output shaft arm, and the output shaft arm is located on one side of the second internal gear. The elastic force transmission structure further includes a spring seat platform. The spring seat platform passes through the elastic torque conversion rod I, the elastic torque conversion rod II, and the output shaft and is disposed on one side of the output shaft arm. A first linear guide rail and a second linear guide rail are symmetrically arranged on the spring seat platform. A first spring-acting slider and a second spring-acting slider are respectively connected to the first linear guide rail and the second linear guide rail. A first spring stop and a second spring stop are arranged on both sides of the first spring-acting slider. A first spring is arranged between the first spring-acting slider and the first spring stop. A second spring is arranged between the first spring-acting slider and the second spring stop. A third spring stop and a fourth spring stop are arranged on both sides of the second spring-acting slider. A third spring is arranged between the second spring-acting slider and the third spring stop. A fourth spring is arranged between the second spring-acting slider and the fourth spring stop. The variable stiffness structure includes a lever assembly and a crank-slider assembly. The lever assembly includes a first lever and a second lever. One side of the first lever and the second lever are respectively connected to the elastic torque conversion rod I and the elastic torque conversion rod II via snap rings, and the other side is respectively floatingly connected to the first spring-acting slider and the second spring-acting slider. The crank-slider assembly includes a second internal gear and a common crank connecting block, a common crank, a first connecting rod, a second connecting rod, and a common crank connecting block. The common crank is fixedly connected to the common crank connecting block, and the common crank and the common crank connecting block are rotatably connected to the output shaft through a bearing assembly II. A first connecting plate and a second connecting plate are symmetrically arranged on the common crank connecting block. The first connecting plate and the second connecting plate are fixedly connected to the second internal gear and the common crank connecting block. The first connecting rod and the second connecting rod are connected to the common crank connecting block through cotter pins. The pivots of the first connecting rod and the second connecting rod are respectively placed in the first lever and the second lever.

2. The small series variable stiffness elastic actuator based on a differential gear train according to claim 1, characterized in that: The drive structure further includes spur gear I, spur gear II, spur gear III, and a first internal gear. Spur gear III is mounted on the guide shaft. Spur gear I and spur gear II are both axially positioned by the input end cover. Spur gear III meshes with spur gear II. The first internal gear meshes with spur gear I. Spur gear I is located inside the first internal gear. The first internal gear is axially positioned by the input end cover. A planetary carrier assembly is fixedly connected to one side of the first internal gear. The guide shaft passes through the planetary carrier assembly and is connected to the planetary carrier assembly through bearing assembly I.

3. A small series variable stiffness elastic actuator based on a differential gear train according to claim 2, characterized in that: The planetary carrier assembly has a bearing II on its outer side. The outer side of the bearing II is connected to the housing to ensure coaxiality. The planetary carrier assembly includes a planetary carrier I and a planetary carrier II fixedly connected to the planetary carrier I. The planetary carrier I is fixedly connected to the first internal gear. The bearing assembly I includes a bushing I and bearings III and IV disposed on both sides of the bushing I. The planetary gears I, II, III, and IV are connected to the planetary carrier II through elastic retaining rings.

4. A small series variable stiffness elastic actuator based on a differential gear train according to claim 3, characterized in that: The first spring stop, the second spring stop, the third spring stop, and the fourth spring stop are all fixedly connected to the spring seat platform. The output shaft is connected to the spring seat platform via bearing V, and the spring seat platform is connected to the planetary carrier II via bolts.

5. A small series variable stiffness elastic actuator based on a differential gear train according to claim 1, characterized in that: The second internal gear and the common crank connecting block are fixedly connected to the second internal gear. The bearing assembly II includes a bushing II and bearings VI and VII disposed on both sides of the bushing II. The bearing VI is connected to the common crank, and the bearing VII is connected to the common crank connecting block. A bearing VIII is disposed on the outside of the common crank connecting block and is placed in the bearing seat of the output end cover.

6. A small series variable stiffness elastic actuator based on a differential gear train according to claim 2, characterized in that: The dual power source structure includes a first power source and a second power source. The first power source includes a first motor and a first reducer connected to the first motor. The second power source includes a second motor and a second reducer connected to the second motor. The output shafts of the first reducer and the second reducer pass through the input end cover and are respectively connected to the spur gear I and the spur gear II.

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