WO2017039897A1

WO2017039897A1 - Drive system for actuating robotic joints

Info

Publication number: WO2017039897A1
Application number: PCT/US2016/044482
Authority: WO
Inventors: Elias BRASSITOS
Original assignee: Northeastern University
Priority date: 2015-08-31
Filing date: 2016-07-28
Publication date: 2017-03-09

Abstract

A robotic drive system (110) is operable to actuate robotic joints and other robotic applications. The drive system includes an arm carrier (140) having a central shaft and two or more arms (145a, b). The arms (145a, b) extend radially from the arm carrier (140), where each of the arms (145a, b) includes an arm shaft (142a, b) at a distal end of the arm. A ground gear train (150, 155a, 155b) is located at a ground side of the arms, and an output gear train (165a, 165b, 160) is located at an output side of the arms. The arm carrier may be driven by a motor input (120), driving the gear trains to generate a torque at an output coupling at the output gear train. The robotic drive system provides compact, high-gear, high-stiffness features for use in, for example, prosthetics, rehabilitation, and mobile robots.

Description

DRIVE SYSTEM FOR ACTUATING ROBOTIC JOINTS RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No.

62/211,939, filed on August 31, 2015. The entire teachings of the above application are incorporated herein by reference.

GOVERNMENT SUPPORT

[0002] This invention was made with government support under a NASA Space Technology Fellowship, Grant #NNX13AL80H. The government has certain rights in the invention.

BACKGROUND

[0003] Robotic drive assemblies operate to drive and control motion of robotic systems. A robotic drive assembly includes a motor, such as a stepper motor, connected to a transmission providing a modulated output. The transmission can, for example, provide a speed reduction by transmitting the motor torque through a mechanical assembly, thereby providing an output having a reduced speed but higher torque.

[0004] A common transmission for robotic drive systems includes a harmonic drive mechanism. Harmonic drives are compact, high torque transmissions that typically provide speed reductions up to 1 : 160 using relatively few components, allowing for lightweight and small transmission structures. Because of these characteristics, harmonic drives are popular in robotics and motion control, particularly where the product size and weight are decisive to its functionality, such as in applications of prosthetics and rehabilitation systems.

SUMMARY OF THE INVENTION

[0005] Example embodiments of the invention include a robotic drive system. The robotic drive system may include an arm carrier having a central shaft and at least two arms. The arms may extend radially from the arm carrier, where each of the arms includes an arm shaft at a distal end of the arm, the arm shafts rotating freely within the respective arm. A ground gear train is located at a ground side (also referred to as an "input side") of the arms, and may include a ground sun gear and a plurality of ground planet gears. The ground sun gear may be positioned around the central shaft at the ground side, while the ground planet gears are meshed with the ground sun gear and are fixed to a respective arm shaft of the arms. Further, an output gear train is located at an output side of the arms, and may include an output sun gear and a plurality of output planet gears. The output sun gear may be positioned at an output side of the arm carrier, and the output planet gears may be meshed with the output sun gear, where each of the output planet gears are fixed to a respective arm shaft at the output side of the arm carrier.

[0006] In further embodiments, the system may further include an output link coupled to the output sun gear, where the sun gear may be configured to drive a robotic arm. An enclosure may house the arm carrier and gear trains while enabling access to the central shaft and the output link, and may be configured to be positioned within a joint of a robotic arm. The ground sun gear may be fixed relative to the enclosure, or may be fixed directly to the enclosure. The output sun gear may rotate freely around the central shaft.

[0007] In still further embodiments, a motor, such as a stepper motor, may be configured to drive the central shaft. The robotic drive system may further include first and second links of a robotic arm, where the first link is coupled to the motor, and the second link is coupled to the output sun gear.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

[0009] FIG. 1 is a perspective view of a robotic joint implementing a robotic drive assembly in an embodiment of the invention.

[0010] FIG. 2 is a diagram of a cross-sectional view of the robotic drive assembly of FIG. 1.

[0011] FIGs. 3A-B are diagrams illustrating a perspective cross-sectional view of the robotic drive assembly of FIG. 1.

[0012] FIG. 4A-B are diagrams illustrating a perspective cross-sectional view of the robotic drive assembly of FIG. 1 including an enclosure. [0013] FIGs. 5A-C are free-body diagrams illustrating the input and corresponding output forces applied to a robotic drive assembly in an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] A description of example embodiments of the invention follows.

[0015] Despite its advantages, harmonic drives suffer from at least three disadvantages. First, harmonic drives generate a high degree of friction, including the radial teeth friction between the flexspline and circular spline components. Friction reduces the efficiency of the transmission and increases the breakaway torque, which necessitate the use of larger and more powerful motors. Further, the efficiency of harmonic drives depends on ambient temperature and can be as low as 30% in cold temperatures such as in outer space.

[0016] Second, harmonic drives exhibit non-linear torque amplification. In an ideal transmission, the output torque is exactly proportional to the input torque. In a harmonic drive transmission, the output torque is contaminated by noise fluctuation with respect to the input torque. These fluctuations are mainly due to the low-frequency excitations of the flexspline, which is a flexible component with low stiffness. Although the non-linear relationship between the input and output torque of a harmonic drive can be linearized through torque sensing and sophisticated control schemes, such solutions add to the complexity and cost of the application. Also, the low-stiffness of the harmonic drive transmission results in deformations under load, creating an effect similar to backlash, which reduces the accuracy of the transmission.

[0017] Third, harmonic drives typically have a high cost. Harmonic drives are intricate mechanisms that require very precise manufacturing processes. The high cost of these transmissions, combined with their non-linear performances, have restricted their use to high- end applications, and limited their expansion into becoming a platform technology.

[0018] Example embodiments of the invention provide a compact, high gear-ratio, high- stiffness, and low-cost robotic drive system. Due to these advantages, such embodiments may be particularly useful in mobile applications, such as prosthetics, rehabilitation, and mobile robots. Further, by avoiding use of a harmonic drive, embodiments of the invention do not exhibit the aforementioned shortcomings typical of harmonic drive systems.

[0019] Fig. 1 is a perspective view of a robotic joint 100 implementing a robotic drive assembly 110 in an example embodiment. The robotic joint 100 is coupled to a first robot arm 190a and a second robot arm 190b, and actuates the turning of one or both of the robotic arms 190a-b around the robotic joint 100. The roboticjoint 100 further includes a motor 120, such as a stepper motor. The motor 120 is fixed to a mounting bracket 125, which, in turn, may be coupled to the second robot arm 190b. The mounting bracket 125 includes a shaft enclosure 126, which encloses a drive shaft (not shown) of the motor 120 and connects the robotic drive assembly 110 to the motor 120.

[0020] The robotic drive assembly 110 includes an arm carrier 140 having a central shaft (not shown) that is coupled to the drive shaft of the motor 120 to be driven by the motor 120, and further includes carrier arms 145a-b. Although the illustrated embodiment includes two carrier arms, alternative embodiments may include additional arms. The arms 145a-b extend radially from the center of the arm carrier 141, where each of the arms includes an arm shaft 142a-b at a distal end of the arm 145a-b, the arm shafts 142a-b rotating freely within the respective arm 145a-b. The arms 145a-b may be radially pre-loaded to remove the backlash between the gears.

[0021] Each of the arms 145a-b supports segments of gear trains on each side of the arm carrier 141. In particular, a ground gear train is located at a ground side (also referred to an "input side") of the arm carrier 140, including a ground sun gear 150 and a plurality of ground planet gears 155a-b. The ground sun gear 150 may be positioned at the center of the arm carrier 140 (i.e., around the central shaft) at the ground side, and may be fixed relative to the motor 120 by coupling the ground sun gear 150 to the shaft enclosure 126 or to the assembly enclosure (shown in Figs. 2-4B), for example. The ground planet gears 155a-b may be meshed with the ground sun gear 150 and fixed to a respective arm shaft 142a-b of the arms. Further, an output gear train is located at an output side of the arm carrier 140 (i.e., opposite of the ground side), and may include an output sun gear 160 and a plurality of output planet gears 165a-b. The output sun gear 160 may be positioned at the center of the output side of the arm carrier 140, and configured to rotate freely relative to the arm carrier 140. Output planet gears 165a-b may be meshed with the output sun gear 160, where each of the output planet gears 165a-b may be fixed to a respective arm shaft 142a-b at the output side of the arm carrier 140.

[0022] The output sun gear 160 may be fixed to an output coupling 182. The output coupling 182 can be removably attached to a link 185 for driving a robot arm 190a, or may be coupled directly to the robot arm 190a. In alternative applications, the output coupling 182 may be coupled to other mechanical assemblies to be actuated, such as a wheel, a belt drive, or an additional gear train (not shown). [0023] In operation, the motor 120 drives the arm carrier 140, causing the arms 145a-b to rotate relative to the motor 120. The ground planet gears 155a-b move with the arms 145a-b and roll on the ground sun gear 140. The roll of the ground planet gear 155a-b, in turn, drives the turning of the output planet gears 165a-b. The output planet gears 165a-b drive the output sun gear 160, which drives the robot arm 190a (via the output coupling 182 and link 185), causing the robot arm 190a to turn relative to the associated robot arm 190b. The speed reduction between the motor 120 input and the output of the output coupling 182 can be defined by selecting given ratios between the gears 150, 155a-b, 160, 165a-b as described in further detail below with reference to Figs. 5A-B.

[0024] The robotic drive assembly 110 provides several advantages over prior drive assemblies, particularly harmonic drives, in applications such as the robotic joint 100. For example, the assembly 110 provides a relatively high torque due to the high gear-ratio of the transmission, and a high efficiency due to the low gear mesh (i.e., low number of gears meshed in the gear train). The assembly 110 may also exhibit relatively high linearity and high stiffness that supports precise and accurate performance of the robotic joint 100 in torque-controlled applications. Further, the assembly 110 can be produced at a cost that is lower than a typical harmonic drive.

[0025] Fig. 2 is a diagram of a side cross-sectional view of the robotic drive assembly 110 of Fig. 1. In contrast to Fig. 1, the assembly 110 is shown in Fig. 2 to include an enclosure 195, which houses the components of the assembly 110, including the arm carrier 140 and the gear trains. The arm carrier 140 includes a central shaft 141 extending out of the enclosure 195 for coupling to a motor drive (e.g., the motor 120 of Fig. 1), and is supported inside the enclosure 195 by a set of duplex bearings 144, allowing the arm carrier 140 to rotate freely relative to the enclosure 195. An output coupling 182 is provided to enable a connection to, for example, the first robot arm 190a via its link 185 of Fig. 1.

[0026] The arms 145a-b extend radially from the central shaft 141, and terminate with respective gear carriers 146a-b. The gear carriers 146a-b hold the respective arm shafts 142a- b via duplex bearings 143, allowing the shafts 142a-b to rotate freely within the gear carriers 146a-b. The planet gears, including the ground planet gears 155a-b and the output planet gears 165a-b, may be fixed to the respective shaft 142a-b via a set screw (not shown) or comparable means. As a result, torque generated by the roll of the ground planet gears 155a- b on the ground sun gear 150 is transferred to the output planet gears 165a-b, which, in turn, drives rotation of the output sun gear 160. Further, the ground sun gear 150 may be fixed relative to the enclosure 195, which can be configured by coupling the sun gear 150 directly to the enclosure 195 or directly via an intermediary component, or may be made integral to the enclosure 195.

[0027] Figs. 3 A-B are diagrams illustrating a perspective cross-sectional view of the robotic drive assembly 110 of Fig. 1, where Fig. 3 A shows the assembly 110 turned toward an output side, and Fig. 3B shows the assembly 110 turned toward an input side. Further, as shown in Fig. 3 A, the output-side wall of the enclosure 195 is transparent for illustrative purposes, permitting a view of the output gear train including the output sun gear 160 and output planet gears 142 A-B.

[0028] Fig. 4A-B are diagrams illustrating a perspective cross-sectional view of the robotic drive system of Fig. 1 including the enclosure 195. As shown in Fig. 4 A, the output side of the enclosure 195 allows external access to the output coupling 182. As shown in Fig. 4B, the input side of the enclosure 195 allows external access to the central shaft 141 of the arm carrier 140.

[0029] Figs. 5 A-C are free-body diagrams illustrating the input and corresponding output forces applied to the robotic drive assembly 110. For illustrative purposes, only the components of the assembly 110 that are relevant for determining the output of the assembly 110 are shown. In particular, because the ground planet gears 155a-b and the output planet gears 165a-b each function uniformly, only a single ground planet gear 155, a single output planet gear 165, and a single arm shaft 142, is shown. Thus, the ground planet gear 155 represents the gears 155a-b as described above, and the output planet gear 165 represents the gears 165a-b as described above. The referenced dimensions a-d and forces Fi, F₂ and F₄ are as follows

[0030] Dimension a: radius of the ground sun gear 150.

[0031] Dimension b: radius of the ground planet gear 155.

[0032] Dimension c: radius of the output planet gear 165.

[0033] Dimension d: radius of the output sun gear 160.

[0034] Force Fi: Input force applied at the circumference of the ground planet gear 155.

[0035] Force F₂: Output force applied by the output planet gear 165.

[0036] Force F₃ is intentionally omitted.

[0037] Force F₄: Force applied to the shaft.

[0038] As shown in Fig. 5 A, each planet subassembly (comprising the ground planet 155 and the output planet 165) behaves as one rigid body that rotates with respect to the instantaneous center of rotation between the ground planet 155 and ground sun gear (point O). This rotation causes the output force to act on a much smaller moment arm (equivalent to (b - c)) than the input force moment arm, b. Therefore, the smaller the difference between pitch diameter of the planets 155, 165, the higher the gear ratio. This relationship can be expressed by the following series equations, which are solved to express the output torque T_out as a factor of the i

[0039] The robotic drive assembly 110 differs from conventional planetary transmissions in several ways. In particular, the assembly decouples the gear-ratio from the number of parts inside the transmission. Conventional planetary transmissions require additional stages to amplify the gear-ratio. This requirement often adds complexity, inefficiencies and weight to the drive system. In contrast, the assembly 110 can alter the gear-ratio simply by varying the difference between two gears (e.g., the ground planet gear 155 and the output planet gear 165). By selecting dimensions of the gears 150, 155, 160, 165 in accordance with the relations described above, a given output torque T_out can be achieved. Further, pre-loading the arm carrier 140 radially inward can remove the backlash from the gear mesh and improve the overall accuracy of the transmission. The assembly 110 may also be constructed using standard spur gears, resulting in a transmission that may be lower in cost relative to a typical high gear-ratio transmission.

[0040] While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. For example, in alternative embodiments, at least one of the ground sun gear, ground planet gears, output sun gear, output planet gears may be implemented to include one or more cylindrical rollers having diameters equal to the corresponding gears pitch diameters.

Claims

CLAIMS What is claimed is:

1. A robotic drive system comprising:

an arm carrier having a central shaft, the arm carrier including at least two arms extending radially from the arm carrier, each of the arms including an arm shaft at a distal end of the arm, the arm shafts being configured to rotate freely within the respective arm;

a ground sun gear positioned around the central shaft at a ground side of the arm carrier;

a plurality of ground planet gears meshed with the ground sun gear, each of the plurality of ground planet gears being fixed to a respective arm shaft at the ground side of the arm carrier;

an output sun gear positioned at an output side of the arm carrier; and a plurality of output planet gears meshed with the output sun gear, each of the plurality of output planet gears being fixed to a respective arm shaft at the output side of the arm carrier.

2. The robotic drive system of claim 1, further comprising and output link coupled to the output sun gear.

3. The robotic drive system of claim 2, wherein the output link is configured to drive a robotic arm.

4. The robotic drive system of claim 2, further comprising an enclosure housing the arm carrier, ground sun gear, ground planet gears, output sun gear, and output planet gears, the enclosure enabling external access to the central shaft and the output link.

5. The robotic drive system of claim 4, wherein the enclosure is configured to be

positioned within a joint of a robotic arm.

6. The robotic drive system of claim 4, wherein the ground sun gear is fixed relative to the enclosure.

7. The robotic drive system of claim 1, wherein the output sun gear is configured to rotate freely around the central shaft.

8. The robotic drive system of claim 1, further comprising a motor configured to drive the central shaft.

9. The robotic drive system of claim 8, wherein the motor is a stepper motor.

10. The robotic drive system of claim 8, further comprising:

a first link of a robotic arm, the first link being coupled to the motor; and a second link of a robotic arm, the second link being coupled to the output sun gear.

11. The robotic drive system of claim 1, wherein at least one of the ground sun gear, plurality of ground planet gears, output sun gear, and plurality of output planet gears includes at least one cylindrical roller.

12. The robotic drive system of claim 1, wherein the at least two arms are configured to be radially pre-loaded.