Patent Application
20200230810
Not applicable
Not Applicable
The present invention pertains to the field of robotics. More specifically, the invention comprises a joint design configured to amplify the motion of an actuator.
One of the goals of robotics is to approximate the performance of human and animal joints. Human and animal joints typically have a large number of degrees of freedom and a large range of motion. Creating robotic joints having a similar range of motion is difficult. Nature achieves a large range of motion for a particular limb through the use of a connecting joint having a large range of angular travel. The human hip and shoulder ball-and-socket joints are good examples. These joints are connected by multiple ligaments and muscle groups that stabilize the geometry and apply suitable forces throughout the large range of travel. Due to the large number of muscle fibers attached across a joint, no matter what position the joint is in, there is a muscle with a favorable orientation to actuate the joint. Therefore, biological joints do not suffer from singularities or range of motion constraints.
In contrast, typical robot joints have restricting range of motion constraints. For example, a one degree-of-freedom pin joint actuated by a piston has a limited range of motion. This range of motion can be increased by using a four bar mechanism, which is commonly used on backhoes and various robot limbs. Two degree-of-freedom universal joints can be actuated with two pistons utilizing a set of four bar mechanisms. However, universal joints suffer from limited range of motion due to structural limitations when components collide and due to kinematic singularities when one axis of rotation becomes aligned with another axis of rotation.
Rosheim developed several robot actuators, such as the “Compact Robot Wrist Actuator” (U.S. Pat. No. 4,686,866) and several versions of the “OmniWrist”, which improved on the range of motion of previous actuators. The “OmniWrist” utilized two coupled universal joints in order to achieve a full hemisphere, singularity free range of motion. Rosheim developed several OmniWrist variants, some utilizing gears and others utilizing linkages to couple the two universal joints and drive the motion. However, these designs can have low stiffness in some configurations, exhibit backlash, and have a high part count.
The present invention overcomes some of the drawbacks of the OmniWrist designs while still achieving singularity-free hemispherical motion (for the three-dimensional embodiments). In its simplest form the present invention uses a drive cable across two pin joints. More complex versions employ cables across two universal joints to drive two output degrees of freedom. The design achieves a high degree of stiffness, low backlash, and reduced part count as compared to previous robot joint designs.
The present invention comprises a robotic joint wherein a second link is driven relative to a first link. An intermediate link connects the second link to the first link. The first link includes a first arcuate surface centered on a first axis. The second link includes a second arcuate surface centered on a second axis. The radius of the first arcuate surface is equal to the radius of the second arcuate surface. The intermediate link is pivotally connected to the first axis and pivotally connected to the second axis.
First and second cables are provided. The first cable runs around the first arcuate surface in a first direction and runs around the second arcuate surface in a second direction that is opposite to the first direction. The second cable runs around the second arcuate surface in the first direction and around the first arcuate surface in the second direction. Tension is maintained on both cables.
An actuator is connected between the first link and the intermediate link, in order to drive the position of the intermediate link with respect to the first link. The angular motion of the intermediate link is doubled in the motion of the second link. The first and second cables produce a high degree of stiffness and minimize backlash. More complex embodiments incorporate a comparable arrangement of cables across two universal joints to drive two output degrees of freedom.
The inventive joint described in this patent application can be embodied in many different ways, including two-dimensional embodiments and three-dimensional embodiments. It is easiest to understand the two dimensional embodiments and so this description focuses on those embodiments. The term “two-dimensional” means that the motion of the links lies in a single plane.
Second link 14 is connected to first link 12 by an intermediate link. In this example the intermediate link is created by an assembly of first plate 34 and second plate 36. The first and second links are arranged in a common plane as shown. First plate 34 is placed on the top of the first and second links (“top” and other directional terms referring solely to the orientation shown in the view). Second plate 36 is moved upward against the bottom of the first and second links.
First axis 28 passes perpendicularly through first link 12 as shown. Second axis 30 passes perpendicularly through second link 14. A pivotal connection is created between plates 34, 36 and first link 12 by passing first pin 46 through corresponding holes in the components—the holes being aligned with first axis 28. A corresponding pivotal connection is created between plates 34, 36 and second link 14 by passing second pin 48 through the holes aligned with second axis 30.
The pivotal joints are depicted very simply. In other embodiments the pivotal joints will include rotary bearings and retaining features such as snap rings.
Actuator 52 applies force between mounting pin 54 (connecting one end of the actuator to first link 12) and actuator pin 50 (connecting the opposite end of the actuator to the intermediate link). Returning to
The use of tension cables across the joint is an important aspect of the present invention. These cables are shown in
A first end of second cable 18 is attached to second link 14 via anchor 24. A second end of second cable 18 is attached to first link 12 via anchor 26. The cables wrap around arcuate surfaces on each of the links. First link 12 has arcuate surface 38—which is centered on first axis 28. Second link 14 has arcuate surface 40—which is centered on second axis 30. Cable grooves are preferably provided in these arcuate surfaces. The cable grooves receive and positively locate the cables. In the view of
First cable 16 starts at anchor 22. It travels through a transverse passage through first link 12 and then wraps in a counterclockwise direction around arcuate surface 38 of first link 12. It then transfers to arcuate surface 40 of second link 14 until finally terminating in anchor 20 (which is attached to the second link). First cable 16 wraps in a clockwise direction around arcuate surface 40. The reader should note that the terms “clockwise” and “counterclockwise” are arbitrary depending on which end of a cable is considered a first end or a second end. In the convention of this disclosure, anchor 22 is the first end of first cable 16 and anchor 20 is its second end. Using that convention, as one proceeds from the first end of first cable 16 toward the second end of first cable 16, first cable 16 wraps in a counterclockwise direction around arcuate surface 38 and in a clockwise direction around arcuate surface 40.
A first end of second cable 18 is connected to second link 14 at anchor 24. Second cable 18 travels through a transverse passage through second link 14 and then wraps in a clockwise direction around arcuate surface 40. Second cable 18 then wraps in a counterclockwise direction around arcuate surface 38. The second end of second cable 18 is then connected to first link 12 at anchor 26.
As stated previously, grooves are preferably provided to receive and positively locate the cables as they travel about the arcuate surfaces.
The cables themselves are preferably made of filaments having a high longitudinal stiffness. A construction of thin steel strands can be used. Even more advantageously a construction of high-strength synthetic filaments can be used. Many different materials can be used for these filaments. These include DYNEEMA (ultra-high-molecular-weight polyethylene), SPECTRA (ultra-high-molecular-weight polyethylene), TECHNORA (aramid), TWARON (p-phenylene terephthalamide), KEVLAR (para-aramid synthetic fiber), VECTRAN (a fiber spun from liquid-crystal polymer), PBO (poly(p-phenylene-2,6-benzobisoxazole)), carbon fiber, and glass fiber (among many others). In general the individual filaments have a thickness that is less than that of human hair. The filaments are very strong in tension and have a very high modulus of elasticity, but they are not very tough. They also tend to have low surface friction. Surface friction is not necessary in the present invention, but it is preferable to provide the synthetic filaments with a jacket or coating to minimize wear and retain the cross-sectional organization of each cable.
In this example actuator 52 is a cylinder with an extendable rod (such as an air cylinder or a hydraulic cylinder commonly includes). The arrow shown on actuator 52 indicates that the rod has just started traveling outward from the cylinder in the direction indicated.
The extension of actuator 52 puts first cable 16 in tension. First cable 16 is shown as a thick black line in the view, with second cable 18 being shown as a dashed line. First cable 16 travels through passage 56 before coming in contact with—and wrapping around a part of—arcuate surface 38. The tension in first cable 16 is transmitted to anchor 20 in second link 14. The transmitted tension becomes a moment pivoting second link 14 counterclockwise about second axis 30. Of course, the intermediate link is being pivoted counterclockwise about first axis 28 via the extension of actuator 52.
The result is shown in
Throughout this motion first cable 16 has remained in tension. The geometry of the joint is important to the maintenance of tension, as can be seen by comparing
In
In the position shown in
The travel around arcuate surface 38 is 135 degrees. Thus, the total travel around an arcuate surface having a constant radius is again 270 degrees (180+90). The use of the arcuate surfaces having an equal radius means that the length of the first cable's travel remains constant and tension is maintained on the first cable throughout. During the motion between the states shown in
Returning to
The second cable does not produce the forces for moving second link 14 during the extension of actuator 52. However, the second cable does restrict backlash. It keeps the moving joint tight, and is therefore an important feature.
In the embodiment of
The anchoring of the free ends of the cables is a significant feature of the invention. In the embodiments shown, an anchor attached to each end of each cable bears against a corresponding shoulder on one of the two links. It is preferable to maintain tension on the cables at all times—as slack permits unwanted motion. Shims can be used between the anchors and the links to create the desired tension. A threaded connection can also be used, where each anchor includes a thrust bearing and a threaded portion that can be turned to adjust the anchor's overall length. It is also possible to include turnbuckle devices to adjust the cable tension.
The use of anchors to positively hold each end of the cable eliminates the need for any frictional engagement between the cables and the links. Forces are transmitted through the anchors rather than cable-to-link friction. The attachment of each cable to its corresponding anchor can be made by potting the cable filaments into an expanding cavity within the anchor. The connection can also be made using spike-and-cone hardware. Numerous other features and variations are possible, including:
1. The use of a linear actuator should not be viewed as limiting. A gear drive between the first link and the intermediate link could be used. Many other types of actuators could be used as well.
2. The cables can include parallel filaments, helically-wound filaments, braided filaments, or some other construction.
3. Cable grooves need not be used where there is adequate clearance between the two arcuate surfaces,
4. A sophisticated motion controller can be used to precisely control the motion of the joint via the motion of the actuator.
5. Braking devices can be applied across one or more of the pivoting joints.
6. The same cable construction can be used to create three-dimensional joints. As an example, the cables can be applied across a U-joint, where the axis of a first joint is orthogonal to an axis of a second joint. The routing of the cables themselves need not lie in a single plane for these more sophisticated embodiments.
Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. Those skilled in the art will be able to devise many other embodiments that carry out the present invention. Thus, the language used in the claims shall define the invention rather than the specific embodiments provided.
This non-provisional patent application claims the benefit—pursuant to 37 C.F.R. § 1.53(c)—of a provisional patent having Ser. No. 62/794,182. The provisional patent application listed the same inventors.
| Number | Date | Country | |
|---|---|---|---|
| 62794182 | Jan 2019 | US |
