GB2083795A - Manipulator mechanisms - Google Patents

Abstract

A manipulator mechanism comprises a support member shown as a tool plate (10) connected, by first universal joints (11) at at least three positions triangularly disposed thereon, to at least six rods (12) longitudinally moveably supported in bearings carried by second universal joints (13,32) hexagonally disposed in a mounting member (14), such that the position and attitude of the tool plate (10) in space relative to the bearings is fully defined by the lengths of the rods (12) between the respective joints (11) and bearings, and these lengths may be altered by driving the rods (12) up and down through the bearings by motors (44) and drive means (21,22,31,41,45) mounted thereon. <IMAGE>

Description

SPECIFICATION Manipulator mechanisms This invention relates to manipulator mechanisms, and concerns in particular such mechanisms useful as the, or part of the, manipulative structure of a robot or other automatic machinery.

On many occasions it is desirable that a physical action be performed by automatic equipment. One such occasion is during the assembly of some object borne upon a carrier from a number of individual component parts, when the machinery handling the parts and performing the assembly should, if possible, be able to pick up each part, orientate it correctly, and then fit it onto the object as so far assembled. That portion of the machinery involved in orientating the component part -the portion that "manipulates" the part - is referred to herein as the manipulator mechanism.

The design of the manipulator mechanisms of present automatic handling equipment of this type tends to be based upon that of the human arm. The mechanism has a grasping device (a "hand") attached by a first joint (a "wrist") to a first extender member (a "fore-arm") itself attached, either directly or via a second joint (an "elbow") to a second extender member (an "upper arm"), by a third joint (a "shoulder") to the body of the machine which itself may be mounted by a fourth joint (a "waist") on the floor. However, while the human arm is a superb piece of manipulative machinery, its robot imitations are not; indeed, it has become evident that "humanoid" robot structures of this general type are far from ideal for the task, particularly when they are to be used in equipment for the rapid assembly of small components.Such structures are deficient in speed, stiffness (both structural and servomechanical), and dynamic control, and thus an alternative approach is required to produce a suitable manipulator mechanism.

In a complex assembly process it may be possible - and, indeed, desirable for the manipulative capability of the equipment to be shared between the assembly carrier and the main component manipulator. It is then possible to restrict the latter's range of movement to less than a conventional "robot arm" - an arm-angle controlled five- or six-axis robot. This eases the task of designing the alternative mechanism, and opens up the way for such mechanisms which do not copy the structure of the human arm.

The present invention seeks to provide just such arl alternative design of manipulator, which design embodies the desirable features of a light-weight structure triangulated as much as possible to obtain stiffness, and (in a preferred aspect) driving motors which do not themselves have to be moved significant distances when adjusting the manipulator position.

In one aspect, therefore, this invention provides a manipulator mechanism comprising a support member connected, by first universal joints at at least three positions triangularly disposed thereon, to at least six rods longitudinally moveably supported in hexagonally-disposed bearings carried by second universal joints, such that the position and attitude of the support member in space relative to the bearings is fully defined by the lengths of the rods between the respective joints and bearings.

The support member is that part of the mechanism which supports whatever tool it is intended the mechanism should operate and/or position - for example, a power-driven spanner, or a grasping device similar to pincers orto a hand. Moreover, while the support member may actually be a part of the tool, so that the rods are then connected directly to the tool itself, most preferably it is separate therefrom, constituting a platform - a tool plate - to which the tool is then attached.

The first universal joints, by which the rods are connected to the support member, may be any such joints that enable each rod to pivot and twist in any direction relative to the support member. Such joints are quite conventional, and need no further comment here (though a particular example is shown in the accompanying drawings).

The mechanism of the invention uses at least six rods supported in hexagonally-disposed bearings and attached to at least three joints triangularly disposed on the support member. The reasons for these numbers of rods, bearings and joints, and for their dispositions, may briefly be explained as follows: A mechanical manipulator capable, say, of the assembly of small parts must move fast and accurately if it is to be competitive vis-a-vis a human, and if it is to move fast then it must have the lowest possible mass consistent with adequate strength and stiffness. To achieve this, a triangulated structure, each member of which is under purely tensile or compressive stress, is the best. The simplest triangulated structure of this type is a pair of rigid rods, connected together at one end, the other ends being joined to two separate points on a rigid datum plate.If all the joints are allowed free articulation, then for a fixed pair of rod lengths the free joint will described a circle in space; if one rod length is kept fixed, the other being varied, any point on a section of a sphere is accessible; and if both rods are variable in length any point in a limited volume is accessible. However, with only two rods no definite point can be addressed. Adding a third rod acting on the same common point (forming a tripod) will allow any point to be addressed by merely altering the three rod lengths, but unfortunately such a three-rod structure does not lead to a usable manipulator, as any tool plate pivotally secured at the tripod point will of course still rotate freely around its pivot axis.

Separating the rod attachments to the tool plate may solve that problem, but then the triangulation ability of the device is completely destroyed.

The next stage is to use a tripod structure with a common end pivotal connection to the tool plate as before, with a further rod added to another - a second - position on the tool plate. The tripod end is a fully defined point in space, and the end of the fourth rod will define an arc, the tool plate also being free to rotate about a line joining the ends of the fourth rod and tripod. To define the second point on the tool plate it is necessary to add a fifth rod to triangulate the fourth rod, and to prevent free rotation of the tool plate it is necessary to add a sixth rod, connected to another - a third - point on the tool plate.So, at least six rods are needed fully to define the position and attitude of the tool plate, and these six rods must be connected to the tool plate at at least three positions forming a triangle and to the datum plate at at least six positions forming a hexagon.

Naturally, there can be more than six rods, and the rods can be connected to the tool plate (the support member) at more than three positions and to the datum plate at more than six positions, but the necessary minimum number of rods and connection points is perfectly adequate. It should here be explained that with six only rods connected between three only positions on the support member and six only bearings on the datum plate the position and orientation of the support member can be varied (to a limited but useful extent) merely by altering the effective length of one only rod. On the other hand, if there are seven or more separate rods (however connected), at least two must be moved to move the support member. Accordingly, while more than six rods, three positions and six bearings is possible, the control problems are very much simplified if there are not.

The manner in which the rods are connected to the support member- how many rods are connected to each joint - may (as suggested above) be such that three rods are connected to one joint, two rods are connected to a second, and one rod is connected to a third. However, it is in fact neater, and more convenient for manufacture and use, to "move" one of the tripod rods to join the single rod, thus forming three idential pairs of rods.

The first universal joints on the support member are at least three in number, and the three are triangularly disposed. While for any three any irregu lartriangular disposition is operable, it is again more convenient, both in manufacture and in use, if the triangle be an equilateral triangle. Moreover, while the size of the triangle may be any that is convenient, very small triangles will lead to inaccuracies in support member positioning (especially against tilt), while very large triangles will make the mechanism unnecessarily large and bulky. Furthermore, where the triangle is similar in size (in circumcircle size) to the figure defined by the hexagonally-disposed bearings the stiffness of the resulting structure is poor. In practice an equilateral triangle side size 1 to 3 inches (2.5 to 7.5 cms) seems perfectly satisfactory.

Similarly, while the bearings in which the rods move longitudinally are at least six in number - the six can be disposed in any hexagonal shape (including non-planar hexagons, and hexagons with reentrants) - it is again very much more convenient, both in manufacture and in use, if for six the hexagon be a planar, regular hexagon. Furthermore, while again the size of the hexagon may be any that is convenient, both very small and very large hexagons are to be avoided (as are hexagons of a circumcircle size similar to that of the support member joints). A satisfactory size for a regular hexagon is from 6 to 24 inches (roughly 20 to 85 cms) across.

The rods used in the mechanism of the invention are not required to resist twisting and bending forces, but are required to resist tension and compression forces. They must therefore be rigid against the latter two, for only in this way can the position and attitude of the support member they carry be accurately defined. Naturally, the rods do not need to be actual rods, but can instead be rod-like - that is, longitudinally extending - structures of, say, a framework nature. In practice, however, actual rods, possibly in tubular form, are generally most convenient.

The rods are longitudinally moveable in bearings which are carried by second universal joints, so that no matter which way the rods are directed they can slide "up and down" through the bearings so changing their effective length (their length from the bearing to the support member joint) and thus causing the support member to take up any desired attitude. These bearings, which in one embodiment are conveniently carried by second universal joints taking the form of ball joints, carry the rods/support member combination, and are themselves borne upon any required mounting member, so effectively forming a datum plate with reference to which the position and attitude of the support member is defined.

As stated above, the second universal joints carrying the bearings in which the rods move may be ball joints, so being truly universal in that they can pivot and/or twist around three mutuallyperpendicular axes. However, because of this threeaxis freedom ball joints are not entirely satisfactory, for the additional degree of freedom (allowing the ball to twist in its socket) may result in some difficulty in "fixing" the position of the rod-borne support member against twisting forces. Preferably, therefore, each second universal joint is a two-axis only joint, and does not permit independent rotation about an axis defined by its rod's axis.Moreover, if each second universal joint has only two axes of movement, then it is advantageous if the rods are, grouped in pairs, two rods to each of the three universal joints on the support member, and the second, two-axis, universal joints are grouped in corresponding pairs, each joint allowing movement of its associated rod both in and normal to the rod pair plane. Conveniently each pair of second, twoaxis, universal joints then has one axis in common, and most preferably this is accomplished by arranging for each pair of second, two-axis, universal joints to comprise a common axle mounted for rotation about its longitudinal axis, and supporting, ohe at each end, two axially-parallel one-axis joints to which the rod bearings are attached.

The purpose of the manipulator mechanism of the invention isto allowthe support member to be positioned and orientated in a manner which is fully defined with reference to the bearings constituting the datum plate. Moreover, in practice the mechanism is to be used automatically to position a tool carried by the support member under the driving control of, say, some suitably programmed microp rocessor. Accordingly, very preferably the manipulator mechanism has associated with it drive means for each of the rods, whereby each rod may independently be moved longitudinally through its bearing, so causing the support member connected thereto to take up any desired position and attitude within the possible ranges thereof.

The drive means may be any suitable "motor" - for example, a hydraulic motor or an electric motor appropriately operatively connected to the rod, and it is a particularly useful feature of the invention that the drive means need not be carried by the main moving parts of the mechanism (the rods and the support member) but can instead effectively be borne by whatever mounting carries the bearings.

Indeed, in one embodiment it is envisaged that the drive means be contained within and supported by the ball joints carrying the bearings within which the rods move, while in another the drive means may be external to but supported by the second universal joints. In either such case the rod could be, for example, the rack of a rack-and-pinion drive powered by the drive means, or the rod/drive means combination could be a screw jack system or a linear electric motor. One particular variety of screw jack system uses a threaded rod supported bytwo corresponding nuts spaced apart within a rigid tubular cover itself mounted within bearings which also support the body of a driving motor operatively engaging the nut/tube combination. Such a system is described in more detail hereinafter with reference to the accompanying drawings.

For the mechanism of the invention correctly to position the support member upon the basis only of the lengths (between joint and bearing) of the rods it is clearly desirable to monitor these lengths. A number of methods are suitable; optical discs or other encoders on or coupled to the drive motor shafts and outputting to detecting and counting means are an obvious suggestion, but a more direct measurement may be preferable, and an attractive possibility is to contain a linear measuring device in a thin tube extending from the bearing to the moving free end of each rod.

In its more preferred aspects the manipulator mechanism of the invention has many advantages over present-day devices. Thus: a) It can be light in weight, giving good speed and dynamic performance with minimum energy usage.

sb) All the moving structural elements are in "pure" compression or tension (ignoring euler effects), allowing simple design analysis.

& It has high structural stiffness in the mid working range, due to the triangulated structure.

d) Given adequate kinematic programs, it has good accuracy and repeatability, arising from the parallel nature of the driving units as opposed to the serial nature of the more common angular joint structure.

e) It is a simple device with common drive elements; the six rod/joint/bearing units are identical, as are the three connections to the tool plate.

f) The driving power connections are simple, due to the static driving motors and the self-contained nature of the rod length measuring system (the only long flexible leads required are the electrical and/or fluid supplies to the tool itself).

Two embodiments of the invention are now described, though only by way of illustration, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of a first manipulator mechanism of the invention; Figure 2 is a part see-through part sectional side view of a portion of the mechanism of Figure 1; Figure 3 is a top plan view of a second mechanism of the invention; and Figure 4 is a side elevation, partly in section, of part of the mechanism of Figure 3 (and is similar to a view on the line IV-IV of Figure 3).

The mechanism of Figures 1 and 2 comprises a tool plate (10) having upon its upper face three universal joints (as 11 - a different type is shown in Figure 2) disposed in an equilateral triangle and to each of which are attached the lower ends of two rods (as 12) the upper ends of which are "slideably" mounted in ball joints (as 13) disposed at the points of a regular hexagon in a planar mainframe upper platform (14) supported by corner pillars (as 15) above a work surface (16).

As shown in more detail in Figure 2, each ball joint 13, through which the rods 12 move longitudnally, carries upon its upper surface the working parts of a screw jack system (20 - not shown in detail) by which an electric motor (not shown) drives a gear train (21 not shown in full) which engages a screw thread (22) cut into the surface of the upper end of the rod 12.

The motors can thus drive their rods 12 up or down by any chosen, measured, amount, so driving the tool plate 10 into any chosen position and attitude relative to the platform 14 carrying the ball joints 13.

In this embodiment, the ball joints 13 are regularly placed on a circle in the same overhead plane (14) to give the maximum degree of triangulation in the nominal centre of the work volume. Within a sphere of diameter equal to the height of the platform 14 the lengths of the rods determine the position, tilt (up to 45 in any direction) and twist about a vertical axis of the tool plate 10. The tilt is progressively restricted, as the working volume is increased, from a sphere to an oblate spheroid about twice the height across.

The mechanism of Figures 3 and 4 is a variant of that shown in Figures 1 and 2. As can be seen, it too comprises a tool plate 10 having three universal joints 11 triangularly disposed upon its upper face (one of these is shown in Figure 4) to each of which are attached the lower ends of two rods 12 the upper ends of which are threaded (as at 31) and are "slideably" mounted within a joint/bearing arrangement effectively disposed at the points of a regular hexagon in a mainframe upper platform 14 supported by pillars 15 above a work surface 16.

However, in this case each rod 12 is driveably mounted within a drive mechanism (generally as 32, and described in more detail hereinafter) which is itself pivotally held within a simple forked joint (as 33) mounted upon a shaft (as 34) supported within a sleeve bearing (as 35). Just as the rods 12 are grouped in pairs (two rods 12 joined to the same universal joint 11), so the forked joint 33 and shaft 34 combinations are grouped in corresponding pairs; their forked joint pivot axes are parallel, and the shafts 34 of each pair are common - that is, the two forked joints 33 of each pair are at opposing ends of the same shaft 34, which is coplanar with its associated rods.Thus, the triangular structure defined by each pair of rods 12 and its two forked joints 33 and common shaft 34 allows the rods - and thus the tool plate 10 - to swing and pivot both in the plane of the rod pair (about the axis of the shaft 34) and in a plane normal thereto (about the parallel pivot axes of the forked joints 33). The grouping of the rods into pairs each ultimately supported at their upper ends by a single sleeve bearing system 35 enables the mainframe upper platform 14 to take a three-lobed shape similar to that of the letter "Y", each lobe carrying one such single sleeve bearing system 35, and there being a pillar 15 at the free end of each lobe.

The rod drive mechanism 32 is best explained with reference to Figure 4, as follows. The mechanism is in essence a type of screw jack. Each rod is held by two nuts (as 41) mating with the rod's thread (31) and fixedly mounted at either end of an innertube (42) itself mounted via bearings within an outer tube (43). The outer tube 43 is fixedly secured both to the casing of a motor (44) and to the pivot fork 33, while the inner tu be 42 is drivingly engaged with the motor 44 via a gear train one element (45) of which is fixedly secured to the tube's lower (as viewed) end.

Because each rod 12 is prevented from twisting about its long axis by its physical connection at joint 11 with the other rod of the pair, rotation of the inner tube 42 (driven via geartrain element 45 bythe motor 44) relative to the outer tube 43 (held stationary by its rigid connection to the motor casing) drives the rod up and down.

Claims (15)

1. A manipulator mechanism comprising a support member connected, by first universal joints at at least three positions triangularly disposed thereon, to at least six rods longitudinally moveably supported in hexagonally-disposed bearings each carried by second universal joints such that the position and attitude of the support member in space relative to the bearings is fully defined by the lengths of the rods between the respective joints and bearings.

2. A mechanism as claimed in claim 1, wherein the support member constitutes a platform - a tool plate - to which is attached whatever tool it is intended the mechanism should operate and/or position.

3. A mechanism as claimed in either of the preceding claims, wherein there are six only rods supported in hexagonally-disposed bearings and attached to three only universal joints triangularly disposed on the support member.

4. A mechanism as claimed in claim 3, wherein the three universal joints on the support member are triangularly disposed, the triangle being an equilateral triangle.

5. A mechanism as claimed in either of claims 3 and 4, wherein the six bearings in which the rods move longitudinally are hexagonally-disposed, the hexagon being a planar, regular hexagon.

6. A mechanism as claimed in any of claims, 3 to 5, wherein the hexagon is large relative to the triangle.

7. A mechanism as claimed in any of the preceding claims, wherein each second universal joint is a ball joint within the ball of which the associated rod is longitudinally moveable.

8. A mechanism as claimed in any of claims 1 to 6, wherein each second universal joint is a two-axis joint.

9. A mechanism as claimed in claim 8, wherein the rods are grouped in pairs, two rods to each of the three universal joints on the support member, and the second, two-axis, universal joints are grouped in corresponding pairs, each joint allowing movement of its associated rod both in and normal to the rod pair plane.

10. A mechanism as claimed in claim 9, wherein each pair of second, two-axis, universal joints has one axis in common.

11. A mechanism as claimed in claim 10, wherein each pair of second, two-axis, universal joints comprises a common axle mounted for rotation about its longitudinal axis, and supporting, one at each end, two axially-parallel one-axis joints.

12. A mechanism as claimed in any of the preceding claims, wherein the second universal joints are themselves borne upon a mounting member, forming a datum plate with reference to which the position and attitude of the support member is defined.

13. A mechanism as claimed in any of the preceding claims which has associated with it drive means for each of the rods, whereby each rod may independently be moved longitudinally through its bearing, so causing the support member connected thereto to take up any desired position and attitude within the possible ranges thereof.

14. A mechanism as claimed in claim 13, wherein each drive means is borne by the combination of the respective bearings and second universal joints.

15. A manipulator mechanism as claimed in any of the preceding claims and substantially as described hereinbefore.