GB2173472A

GB2173472A - Manipulator

Info

Publication number: GB2173472A
Application number: GB08509402A
Authority: GB
Inventors: Mark Gordon Vickers
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1985-03-30
Filing date: 1985-04-12
Publication date: 1986-10-15
Also published as: GB2173472B; GB8508387D0; GB8509402D0

Abstract

A manipulator comprises three arms 1 which can pivot at different points on a supporting structure 6 and which can be driven longitudinally to adjust their effective lengths. The longitudinal axes of the three arms intersect at a common point but their lower ends are offset from this point and are pivoted about respective axes which also intersect at the aforementioned common point. They are pivotted to an end effector mounting flange 8 which optionally comprises a fourth arm 11 which slides and pivots on the supporting structure. The fourth arm serves to ensure that the flange 8 always faces at a known orientation. It can also be used in cooperation with suitable sensors to produce signals on lines 15, 16 and 18 to a control unit which drives the arms 1 in their linear directions. <IMAGE>

Description

SPECIFICATION A manipulator This invention relates to a manipulator for controlling the position of what is referred to as an "end effector"; which term covers such things as tools, gripping devices and holding devices. The invention concerns the type of manipulator which includes at least three arms each connecting the end effector to a fixed point on a supporting structure. In such a construction the effective length of each arm is adjusted to position the end effector as required.

One known manipulator of the aforementioned type is described in our U.K. Patent Specification No: 2083795. This system is able to manipulate an end effector to any desired point in a volume of space and also to orientate it at a desired orientation at that point. For this purpose the manipulator includes six, instead of three, control arms. The problem with this arrangment is that it is relatively complex and that the degree of adjustment of orientation of the end effector, for any particular position of it in the aforementioned volume of space is limited.

Another known manipulator is described with reference to Figure -1 in U.S. Patent Specification No: 4407625. This comprises three arms connected at their bottom ends by a pivot-mechanism allowing all three to pivot relative to each other about a common point.

The end effector is connected to one of the three arms. A manipulator of this type does not allow orientation of the end effector when it has been moved to a desired point. Such adjustment of orientation is not necessarily a disadvantage since the end effector itself can be provided with a special wrist mechanism providing a freedom for adjustment of orientation through large angles (greater than the angles provided for by the arrangement of U.K. Specification No: 2083795). Also ofcourse many types of end effector, including that shown in U.S. Specification No: 4407625, do not require such adjustment of orientation. A problem however with the arrangement described in the U.S. Specification is that the pivot arrangement at the bottom of arms is relatively complex and that there is no way in which the end effector can be con- nected directly to the pivot point.This means that, to move the end effector to a given point, it is necessary to take intro consideration an offset between the end effector and the pivot point. The need for this complicates the control system.

This invention provides a manipulator having three arms of variable effective length connecting fixed points on a supporting structure to an end effector mounting characterised in that the three arms are connected to different points on the mounting for rotation about respective axes fixed with respect to the mounting. The axes preferably intersect at a common point.

By employing the invention only three simple pivots are required instead of the complex pivotting arrangement of the aforementioned U.S. Patent Specification. Also, the difficulty of mounting the end effector in the design of the system described in the U.S. Patent Specification No: 4407625 is removed. This is because an inherent part of the invention is an end effector mounting member. Furthermore, by suitably shaping each arm (e.g., by providing a double bend) the longitudinal axes of movement of the arms can be made to intersect at a common point fixed with respect to the support. This is preferably made to be the same common point as previously referred to, in order to simplify calculations required in the control process.

In a preferred form of the invention the end effector mounting includes a fourth arm which is preferably straight, and which joins the end effector to the supporting structure by connections which allow the fourth arm to pivot about two intersecting orthogonal axes perpendicular to its length and to slide iongitudinally relative to the frame. One advantage of this arrangement is that the position of the end effector can be determined by three sensors which monitor the angles of rotation and the longitudinal displacement of the fourth arm. Furthermore, the end effector is held more rigidly than would be the case without the fourth arm.

The provision of the fourth arm is considered to be a useful and novel feature in itself applicable to other systems, for example to systems with a complex pivot arrangement like that described in U.S. Patent Specification No: 4407625. Thus, according to another aspect of this invention there is provided a manipulator having three arms of variable effective length connecting fixed points on a supporting structure to an end effector mounting characterised in that the end effector mounting comprises a fourth arm mounted for sliding and pivotting movement at a point on the supporting structure.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawing of a manipulator constructed in accordance with the invention.

Referring to the drawing three screw threaded control arms 1 are driven longitudinally in the direction of axes I by drive mechanisms 2 each including a motor 3 and drive 4 co-operating with the screw thread. Axes I intersect at a common point which will be referred to later.

Each drive mechanism 2 is pivotted about an axis II to a bifurcated support 5. The later is pivotted about an axis 111 to a supporting structure 6. Each associated group of axes I, II and liy are orthogonal to each other and intersect at a common point.

Each arm 1, has a double bend which offsets its lower end from its axis I of longitudinal movement. This offset lower end is pivotted about an axis IV to respective positions spaced around a ring 7 forming part of an end effector support (formed by components 7-13). The axes IV are symmetrically arranged in a plane (which is horizontal in the position illustrated) and they intersect at a common point in that plane.

The end effector support comprises a mounting plate 8 which is attached to the ring 7. In an alternative construction (not shown) the-parts 7 and 8 are replaced by a single rigid mounting bracket to which the end effector is attached; and no further components are needed since the longitudinal position of adjustment of the three arms can be used to move the end effector to any desired position within a space between the legs of the supporting structure. However in such an alternative arrangement there may be undue twisting effects on the end effector support particularly if it is required to handle a heavy load. Also, the orientation of the end effector support may be difficult to calculate and this information will be needed in some circumstances e.g. when the end -effector includes a wrist mechanism. The preferred features now to be described help to overcome these problems.

In the illustrated construction the end effector support also includes a ring 9 (see Figure 2) which is pivotted to the ring 7 about an axis V and to an inner bush 10 about an axis VI. Axes V and VI intersect at the same point, and this point is the same as that at which axes II intersect. The bush 10 is secured rigidly to the bracket 8 at its lower end and has a central bore which receives a reduced diameter portion at the lower end in of a fourth arm 11 in such a way as to allow the bush 10 to rotate relative to the arm 11. This rotation takes place about a longitudinal central axis VII which passes through the same point as that at which the axes I, IV, V and VI intersect.The fourth arm 11 is connected to a fixed point on the supporting structure 6 by a gimbal mechanism shown in detail in Figure 3 and comprising a ring 12 pivotted about an axis IIX and to the supporting structure 6 about an axis IX. The axes VII IIX and IX are orthogonal to each other and intersect -at a common point. The fourth arm 11- is able to slide longitudinally in the direction- of axis VII through an inner ring 13. It has a slot 14 which receives an inwardly pointing projection on the ring 13 to key it against axial rotation- relative to the supporting structure 6. Operation of the motors 5 control the effective lengths of the arms 1. The effective lengths are the distances between the points where axes II and III intersect and the point where axes IV and I intersect.The choice of appropriate effective lengths of the arms 1 uniquely defines the position in space of the last mentioned point which is the effective point at which the end effector is mounted. The orientation of the end effector is constrained by the fourth arm 15 always to be the line adjoining the last mentioned point to the fixed point on the supporting frame-6 where axes VIII and IX intersect.

Suitable sensors, not shown produce signals on lines 15 and 16 representing the degree of rotation of the arm 11 about axes IIX and IX respectively whilst a linear position encoder 17 produces a signal on line 18 representing its position along the axis VII. This information completely defines the position of the end effector and is compared in a control circuit 19 with the desired position entered at 19 to produce correction signals on lines 20, 21 and 22 to operate the motors 3 in directions such as to reduce the difference between the real and required positions.

Claims

1. A manipulator having three arms of variable effective length connecting fixed points on a supporting structure to an end effector mounting characterised in that the three arms are connected to different axes fixed with respect to the mounting.

2. A manipulator according to Claim 1 in which the three axes intersect at a common point.

3. A manipulator according to Claim 2 in cludingmeans for varying the effective lengths of the three arms by sliding them through the said fixed points on the supporting structure along axes passing through the aforementioned common point.

4. A manipulator according to any preceding claim in which the mounting includes a fourth arm pivotally mounted to a point on the supporting structure.

5. A manipulator substantially as described with reference to the accompanying drawing and substantially as described therein.

5. A manipulator according to Claim 4- including means for sensing movement and/or position of the fourth arm and for adjusting the aforementioned effective lengths in response.

6. A manipulator having three arms of variable effective length connecting fixed points on a supporting structure to an end effector mounting characterised in that the end effector mounting comprises a fourth arm mounted for sliding and pivotting movement at a point on the supporting structure.

7. A manipulator according to Claim 6 including means for sensing movement and/or position of the fourth arm and for adjusting the aforementioned effective lengths in response.

8. A manipulator substantially as described with reference to the accompanying drawing -and substantially as illustrated therein.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1 to 8 above have been deleted.

New claims have been filed as follows:

1. A manipulator having: three arms connecting fixed points on a supporting structure to an end effector mounting the three-arms being cnnected to different fixed points on the mounting in a way which allows rotation of the arms relative to the mounting about respective axes which intersect at a common point; and three drive means each acting on a part of a corresponding arm so as to drive that part along a straight line passing through the said common point.

2. A manipulator according to claim 1 in which each arm is connected to the mounting by a pivot which allows rotation about only one fixed axis.

3. A manipulator according to any preceding claim in which the mounting includes a fourth arm pivotally mounted to a point on the supporting structure.

4. A manipulator according to claim 3 including means for sensing movement and/or position of the fourth arm and for controlling the drives in response.