GB2261485A

GB2261485A - A linkage for a robot arm

Info

Publication number: GB2261485A
Application number: GB9124330A
Authority: GB
Inventors: Jonathan Charles Kieffer
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1991-11-15
Filing date: 1991-11-15
Publication date: 1993-05-19
Anticipated expiration: 2011-11-15
Also published as: GB2261485B; GB9124330D0

Abstract

A planar linkage for a robot arm has four, interconnected links (11, 12, 13, 14). The links are capable of defining the skies of a parallelogram, and the linkage incorporates a belt-and-pulley arrangement (18, 19, 20), chain-and-sprocket arrangement or gears which prevents the linkage for adopting a crossed-configuration. Separate coaxial drives 16, 17 to links 11, 12 provide the planar positioning, the linkage as a whole being mounted in a frame for movement perpendicular to the plane. <IMAGE>

Description

A LINKAGE FOR A ROBOT ARM This invention relates to a linkage for a robot arm.

According to a first aspect of the invention there is provided a planar linkage for a robot arm comprising, four, interconnected links wherein a first of the links is connected to a second and a third of the links at respective first ends thereof, and the fourth of the links is connected to the second and third links at respective second ends thereof such that the links forming respective pairs of directly-connected links are capable of relative rotational movement about mutually parallel axes, the links being so configured and arranged that they are capable of defining the sides of a parallelogram as viewed in a direction parallel to said axes of rotation, the first and the fourth links are spaced apart from each other along said direction and the second and third links cannot collide with each other when the links define the sides of a parallelogram, and further comprising means for coupling movement of one of the links to another of the links to which said one link to indirectly connected.

Preferably, said first link and either the second or the third link are adapted to be independently rotatably driven. The independently driven links may have coaxial drive shafts.

The defined arrangement enables the drive shafts to be driven entirely independently of each other without any risk of there being a collision between individual links, and said coupling means prevents the links from assuming a crossed-configuration.

In a preferred embodiment, the coupling means couples the second link to the third link.

The coupling means may comprise a pulley and belt arrangement comprising respective pulleys, having the same radius, in fixed relation to each coupled link, and an endless belt trained around the pulleys. Alternatively, the coupling means may comprise a sprocket and chain arrangement or a pair of meshing gear wheels.

According to a further aspect of the invention, there is provided a robot system, such as a SCARA-type robot system, incorporating a linkage as defined in accordance with said first aspect of the invention.

A planar linkage in accordance with the invention is now described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a side elevation view of the linkage forming part of a SCARA-type robot; Figure 2 shows a bottom plan view of the linkage; Figure 3 illustrates schematically how the linkage shown in Figures 1 and 2 is effective to position an endeffector in a plane; and Figures 4(a) to 4(d) show different configurations of the linkage.

Referring to Figure 1, the planar linkage 10 forms part of a SCARA-type robot, being mounted on a cross-member C which can be displaced in the vertical (Z-axis) direction by means of a suitable drive arrangement D.

The linkage 10 has two degrees of freedom and enables an end-effector E, mounted on one of its links, to be positioned in a horizontal (X-Y) plane.

Referring now to Figures 1 and 2, the linkage 10 comprises four, interconnected, links 11,12,13,14, and the end-effector E is mounted on one of the links, referenced 14.

The link 11 is connected to links 12 and 13 at their respective upper ends, whereas link 14 is connected to links 12 and 13 at their respective lower ends.

The links forming each interconnected pair (e.g. 11,12; 13,14) are capable of relative rotational displacement about respective vertical axes in response to rotational drive applied to links 11 and 12 via respective, coaxial drive shafts 16 and 17. With this arrangement, each point in a link can only move in a respective horizontal plane in response to applied drive.

As shown in Figure 1, the drive shafts 16,17 are positioned remotely from the four links, and this arrangement reduces the overall inertia of the linkage.

Each drive shaft 16,17 is fitted with a respective gear 16',17', or similar means, to which the rotational drive is applied.

The links 12 and 13 are both inclined relative to the vertical (Z) axis, and the linkage is able to adopt a configuration which, as viewed in plan (along the Z-axis direction), has the form of a parallelogram, as shown in Figure 2.

Figure 3 illustrates schematically how the respective angular orientations e1,e2 of the driven links 11,12 determine the angular orientations of links 13 and 14, which, in turn, determine the x,y coordinates of the end-effector E in the X-Y plane.

Provided the links define a parallelogram, the coaxial drive shafts 16,17 can be driven entirely independently of each other without any risk of there being a collision between individual links, and this offers a significant advantage over known linkages that have been used hitherto in robot arm systems.

Figures 4(a) to 4(d) show four different configurations that could, in principle, be adopted by the linkage in positioning the end-effector at the same x,y coordinates.

Two of these configurations, referenced 4(a) and 4(b), have the desired form of a parallelogram, whereas, the two other configurations, referenced 4(c) and 4(d), do not; instead, they have crossed-configurations in which one of the links overlies another link. Such crossedconfigurations are generally undesirable in that the links are difficult to control and collisions between links can occur.

In order to prevent the linkage from adopting such crossed-configurations, a belt-and-pulley arrangement is used to couple link 12 to link 13. This arrangement comprises an endless belt 18 which is trained around respective pulleys 19,20 which have the same diameter and are fixed to the respective links 12,13.

The belt and pulley arrangement does not affect the position of the end-effector, which is determined by the respective drives applied to the drive shafts 16,17, but limits relative rotational displacement of the links preventing the linkage from assuming the undesirable crossed-configurations, such as are shown in Figures 4(c) and 4(d).

It will be appreciated that alternative forms of coupling could be used to achieve the same end, such as a sprocket-and-chain arrangement or a pair of meshing gear wheels having the same diameter, each gear wheel being fixed to a respective one of the links 12,13.

The described linkage offers significant advantages compared with hithertoknown linkages used in robot arm structures: (i) Because the arms define a parallelogram, the linkage has improved structural stiffness; (ii) Actuators for applying rotational drive to the linkage can be positioned remotely, thereby reducing the inertia of the linkage; and (iii) Provided the links define the afore-mentioned parallelogram there is no risk of the links colliding, and undesirable crossed-configurations can be eliminated by means of a suitable coupling.

Claims

1. A planar linkage for a robot arm comprising, four, interconnected links wherein a first of the links is connected to a second and a third of the links at respective first ends thereof, and the fourth of the links is connected to the second and third links at respective second ends thereof such that the links forming respective pairs of directly-connected links are capable of relative rotational movement about mutually parallel axes, the links being so configured and arranged that they are capable of defining the sides of a parallelogram as viewed in a direction parallel to said axes of rotation, the first and the fourth links are spaced apart from each other along said direction and the second and third links cannot collide with each other when the links define the sides of a parallelogram, and further comprising means for coupling movement of one of the links to another of the links to which said one link is indirectly connected.

2. A planar linkage as claimed in claim 1, wherein said first link and either the second or the third link are adapted to be independently rotatably driven.

3. A planar linkage as claimed in claim 2, wherein the independently driven links have coaxial drive shafts.

4. A planar linkage as claimed in any one of claims 1 to 3, wherein the coupling means couples the second link to the third link.

5. A planar linkage as claimed in any one of claims 1 to 4, wherein the coupling means comprises a pulley and belt arrangement comprising respective pulleys, having the same radius, in fixed relation to each coupled link, and an endless belt trained around the pulleys.

6. A planar linkage as claimed in any one of claims 1 to 4, wherein the coupling means comprises a sprocket and chain arrangement or a pair of meshing gear wheels.

7. A robot system incorporating a linkage as claimed in any one of claims 1 to 6.

8. A robot system as claimed in claim 7, being of the SCARA-type.

9. A planar linkage substantially as herein described with reference to the accompanying drawings.

10. A robot system substantially as herein described with reference to the accompanying drawings.