GB2157649A - Robotic wrist assembly - Google Patents

Abstract

The robotic wrist assembly is adapted by plate (27) for mounting at one end of a collapsible tube assembly, i.e. as incorporated in an overhead gantry robot system, and provides for three-axis rotation of an end effector (49) while permitting the effector to be mounted very close to the axis of the tube assembly. Compound rotational capability in a compact assembly is obtained by utilizing a gear differential (45) driven by a pair of elongate motor assemblies (35 and 37) which are mounted closely adjacent and parallel to the tube assembly, the end effector (49) being connected to the output shaft of the differential. <IMAGE>

Description

SPECIFICATION Rohotic wrist assembly The present invention relates to a three-axis robotic wrist assembly and more particularly to such an assembly which is adapted for mounting on a vertically-oriented collapsible tube assembly carried by an overhead gantry.

In particular, the present invention pertains to an improved wrist assembly for a robot manipulator in which an overhead gantry system carries a conventional three-axis wrist, each of the three axes being independently driven by motor means with the motor axes extending generally along the driven axis. While the motor for rotation around the vertical axis could be readily located within the collapsible tube assembly, the motor for swinging the effector projected laterally from the assembly, somewhat limiting its movement in certain applications, and the motor for rotating the end effector itself was included within an arm of appreciable length extending from the wrist so that the end effector itself could not be mounted close to the intersection of the three axes.

Among the objects of the present invention may be noted the provision of a three-axis robotic wrist assembly which permits an end effector to be mounted close to the axes; the provision of such a wrist assembly which is exceptionally compact allowing the collapsible tube assembly to closely approach a workpiece from any side; the provision of such a wrist assembly in which all three axes intersect at a point; the provision of such an assembly which permits compound movement of the end effector; the provision of such an assembly which permits unlimited rotation of the end effector around the final or third axis; the provis highly reliable; the provision of such an assembly which can handle heavy loads; and the provision of such an assembly which is of relatively simple and inexpensive construction.

The present invention is a robotic wrist assembly comprising an elongate tubular mounting member, a base member, aligned with said tubular member, rotary drive means for rotating said base member around the axis of said tubular member, mounted on said base member, adjacent and parallel to said tubular mounting member, a pair of elongate motor means, gear housing means, hournalled within said housing means, a differential gear means having a pair of input members rotatable about aligned, opposing axes, each operatively connected to one of said elongate motor means, and an output shaft extending perpendicular to said input members, and on said output shaft, means for mounting an effector, whereby an effector carried by said mounting means can be rotated around the output shaft axis by oppositely operating said motor means and can, with said housing means, be swung about the input member axes by operation of said motor means in similar directions and can be rotated around the axis of said tubular mounting member by operation of said rotary drive means.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a gantry robot employing a three-axis wrist in accordance with the present invention; Figure 2 is a front view of the wrist assembly; Figure 3 is a side view of the wrist assembly; Figure 4 is a partial sectional view, taken substantially on the line 4-4 of Fig. 3, showing the rotary drive for the first, i.e. vertical, axis; Figure 5 is a sectional view, taken substantially on the line 5-5 of Fig. 2, showing one of two similar motor means employed in driving the second and third axes; and Figure 6 is a partial sectional view, also taken substantially on the line 4-4 of Fig. 3, showing a differential gear assembly combining the motions of the two similar motor means to obtain rotations around the second and third axes.

Corresponding reference characters include corresponding parts throughout the several view of the drawings.

Referring now to Fig. 1, a frame assembly 11 carries a pair of rail assemblies 13. A bridge assembly 17 is movable along the rails and itself mounts a carriage 19 on which is supported a vertically-collapsible tube assembly 23. At the lower end of the collapsible tube assembly 23 is mounted a three-axis wrist of the present invention, designated generally by reference character 24. on the wrist assembly is mounted an end effector or tool 25.

As for the operation of the bridge carriage and collapsible tube assemblies for the purposes of the present description, it is appropriate to note that movement along each of the three orthogonal axes is controlled by a servo motor system. Typically, each of the servo loops employs a d.c. servo motor for driving force and an independent sensing system, e.g. a resolver or tagline, for independently determining the extent of actual motion imparted along each axis. It is an advantage of gantry robots of this type that they can operate relatively uniformly over a large volume and that the volume can be easily reconfigured and expanded. Further, since the translational axes are orthogonal, the manipulator can be programmed relatively easily.

Significant in the success of such robot manipulators is the provision, at the iower end of the collapsible tube assembly 23, of a wrist assembly which permits rotation of the end effector, preferably about three intersecting orthogonal axes, so that complete flexibility in positioning the end effector is obtainable.

Referring now to Fig. 2, the three-axis wrist assembly illustrated there incorporates a mounting plate 27 for securing the wrist assembly to the bottom of the collapsible tube assembly 23. Below the mounting plate 27 is a tubular casing 29 which essentially constitutes an extension of the last of the collapsing tubes. Within this tubular housing is a rotary drive, described in greater detail hereinafter, which operates to rotate a base member 33 around the axis (vertical) of the collapsible tube assembly 23.

Base member 33 carries a pair of elongate motor assemblies 35 and 37 which are mounted vertically so as to be parallel to and closely adjacent the tube assembly 29 and the rotary drive contained therein. Each of the motor assemblies 35 and 37 drives a respective right angle bevel gear assembly 41 and 43, respectively, which changes the direction of the motor rotation by 90 degrees. Carried between the bevel gear assemblies is a gear case 45 containing a differential gear system 47. The differential gear system 47 provides a pair of aligned but oppositely facing input members or shafts, each of which is driven by a respective one of the right angle bevel gears 41 and 43. The differential gear system also provides an output shaft which carries a mounting plate 49 on which may be mounted a robotic end effector.Each of these subassemblies is described in greater detail hereinafter with reference to a respective sectional view.

Referring now to Fig. 4, it can be seen that the tubular housing assembly 29 comprises an inner tube 51 which is rigidly connected to the mounting plate 27 and an outer tube 53 which is journalled on the inner tube by means of bearings 54 and 55.

An elongate d.c. servo motor 56 is mounted within the inner tube and the output shaft of the motor drives the input shaft of a harmonic speed reducer 57. The harmonic speed reducer 57 in turn drives the outer tube 53, i.e. by means of an output shaft 58 which is connected to the mounting plate 49 along with the outer tube 53. A servo resolver 59 is provided for directly sensing the angular position of the output shaft 58 and providing a feedback signal which can be used in controlling the energization of the servo motor 56 independently of any backlash or offset error in the mechanical mounting system.

The motor means 35 is illustrated in somewhat greater detail in Fig. 5. As is understood, the motor means 37 is essentially identical, though mounted on the opposite side of the mounting plate 33, and thus is not described separately.

Like the rotary drive for the vertical axis, each of the two motor means 35 and 37 comprises an elongate d.c. servo motor 61 driving a respective harmonic speed reducer 62. (See Fig. 5.) Associated with each motor 61 and speed reducer 62 is a right angle bevel gear, preferably of the hypoid type in which the input and output shaft axes are offset. The output shaft 63 of the speed reducer 62 drives the smaller of the bevel gears, i.e. the pinion 64. The larger or ring gear 65 drives an output shaft 66. The two similar motor assemblies are mounted on opposite sides of the mounting plate or base member 33 and are oriented so that the respective output shafts 66 are aligned but oppositely facing so as to mate with the input members or shafts of the differential gear system 47.The actual angular position of each of the output shafts 66 is sensed by a respective servo resolver 67 which provides a feedback signal which the robot controller can utilize in controlling the energization of the d.c. servo motor 61.

Referring now to Fig. 6, the differential gear system input shafts are indicated by reference characters 71 and 72. These shafts are journalled in the differential gear casing 45 and are connected to respective bevel gears 73 and 74. An output shaft 75 is also journalled in the gear case 45, though at right angles to the input shafts 71 and 72. Output shaft 75 carries a bevel gear 76 which meshes with both of the gears 73 and 74. The other end of the shaft 75 carries the robot end effector mounting plate 49 as described previously.

As may be seen, the differential gear case 45 is floating, i.e. not attached to the base member 33 but, rather, is merely held between by means of the two input shafts 71 and 72. Accordingly, it will be understood that, when the input shafts 71 and 72 are driven in opposite directions, the output shaft will rotate around its own axis, i.e. spinning the end effector around the final (gamma) axis. On the other hand, if the two input shafts 71 and 72 are rotated in the same direction, the output shaft 75, together with the gear case 45 and the end effector, will be swung around the common axis of the differential input shafts.

From the foregoing, it will be seen that neither of the two motor means 35 or 37 independently controls either one of the final two rotational axes, but rather there is a compound relationship. While this relationship somewhat complicates the task of the controller, the combining of movements merely involves a simple summing of the positional signals generated by the resolvers associated with the respective motor means. This is in contrast with the complex geometrical relationship which can occur as in other robotic systems where the rotational axes are not orthogonal or are non-intersecting.

The advantages of the differential drive scheme of the present invention is, as noted previously, that the various elongate motor means may be mounted closely adjacent and parallel to the vertical tube assembly and that the end effector can be mounted very close to the intersection of the three axes so that no significant arm length is required in providing space for the motor driving the end effector around the third (gamma) axis.

Claims (8)

1. A robotic wrist assembly comprising an elongate tubular mounting member, a base member, aligned with said tubular member, rotary drive means for rotating said base member around the axis of said tubular member, mounted on said base member, adjacent and parallel to said tubular mounting member, a pair of elongate motor means, gear housing means, journalled within said housing means, a differential gear means having a pair of input members rotatable about aligned, opposing axes, each operatively connected to one of said elongate motor means, and an output shaft extending perpendicular to said input members, and on said output shaft, means for mounting an effector, whereby an effector carried by said mounting means can be rotated around the output shaft axis by oppositely operating said motor means and can, with said housing means, be swung about the input member axes by operation of said motor means in similar directions and can be rotated around the axis of said tubular mounting member by operation of said rotary drive means.

2. A wrist assembly as claimed in claim 1, wherein the axis of said tubular mounting member and the axes of the differential input members and the axis of the output shaft all intersect at a common point.

3. A wrist assembly as claimed in claim 1, wherein each of said motor means comprises a d.c. motor, a harmonic reducer for reducing the motor speed, and a right angle bevel gear for changing the direction of the reduced motor speed.

4. A wrist assembly as claimed in claim 3, wherein each of said motor means also includes a resolver for generating a feedback signal indicative of the position of the respective input member.

5. A robotic wrist assembly comprising an axially collapsible elongate tubular mounting member, a base member, aligned with said tubular member, rotary drive means for rotating said base member around the axis of said tubular member, mounted on said base member, adjacent and parallel to the axis of said tubular mounting member, a pair of elongate motor means, bevel gear means associated with each elongate motor means for changing the axis of rotation of the movement provided by the respective motor means by 90 degrees, gear housing means located between said bevel gear means, journalled within said housing means, a differential gear means having a pair of input members rotatable about aligned, opposing axes, each operatively connected to a respective one of said bevel gear means, and an output shaft extending perpendicular to said input members, and on said output shaft, means for mounting an effector, whereby an effector carried by said mounting means can be rotated around the output shaft axis by oppositely operating said motor means and can, with said housing means, be swung about the input member axes by operation of said motor means in similar directions and can be rotated around the axis of said tubular mounting member by operation of said rotary drive means.

6. A wrist assembly as claimed in claim 4, wherein said bevel gears are of the hypoid type in which the gear axes are offset.

7. A wrist assembly as claimed in claim 4, wherein the axis of said tubular mounting member and the axes of the differential input members and the axis of the output shaft all intersect at a common point.

8. A robotic wrist assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.