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Title:
PIVOT JOINT ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2009/027660
Kind Code:
A3
Abstract:
A pivot joint assembly comprising a first structure (16) having a convex bearing member (32) and a second structure (31) having a bearing surface (30) for cooperation with the convex, bearing member (32) so as to facilitate relative rotation between the first (16) and second (31) structures about at least a first axis. A bias member (24) is provided for biasing the convex bearing member (32) into the bearing surface (30). The bias member (22) acts on the first structure (16) such that the direction of the bias force relative to the bearing surface (30) is substantially the same in all relative po.sitions of the first (16) and second (31) structures.

Inventors:
JONAS KEVYN BARRY (GB)
CURLE JASON RALPH GORDON (GB)
Application Number:
PCT/GB2008/002884
Publication Date:
April 16, 2009
Filing Date:
August 22, 2008
Export Citation:
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Assignee:
RENISHAW PLC (GB)
JONAS KEVYN BARRY (GB)
CURLE JASON RALPH GORDON (GB)
International Classes:
B25J17/02
Domestic Patent References:
WO2002006017A12002-01-24
WO2000027597A12000-05-18
Foreign References:
EP1072366A12001-01-31
US5333514A1994-08-02
GB305639A1929-10-24
US20040197132A12004-10-07
Attorney, Agent or Firm:
ROLFE, Edward, William et al. (Patent DepartmentNew Mills, Wotton-under-edge Gloucestershire GL12 8JR, GB)
Download PDF:
Claims:

CLAIMS:

1. A pivot joint assembly comprising: a first structure having a convex bearing member; a second structure having a bearing surface for cooperation with the convex bearing member so as to facilitate relative rotation between the first and second structures about at least a first axis; and a bias member for biasing the convex bearing member into the bearing surface, in which the bias member acts on the first structure such that the direction of the bias force relative to the bearing surface is substantially the same in all relative positions of the first and second structures.

2. A pivot joint assembly as claimed in claim 1 , in which the bias member is configured such that the magnitude of the bias force relative to the bearing surface is the same in all relative positions of the first and second structures.

3. A pivot joint assembly as claimed in claim 1 or 2, in which the bias member acts between the first structure and a third structure.

4. A pivot joint assembly as claimed in claim 3, in which the third structure is coupled to the second structure via a second pivot joint.

5. A pivot joint assembly as claimed in claim 4, in which the second structure is held between the first and third structures.

6. A pivot joint assembly as claimed in claim 4 or 5, in which the third structure is configured to match the movement of the first structure.

7. A pivot joint assembly as claimed in any of claims 4 to 6, in which the second pivot joint comprises a second convex bearing member on the third structure and a second bearing surface on the second structure for cooperation

with the second convex bearing member.

8. A pivot joint assembly as claimed in claim 7, in which the bias member biases the second convex bearing member into the second bearing surface such that the direction of the bias force relative to the second bearing surface is the same in all positions of the first and third structures relative to the second structure.

9. A pivot joint assembly as claimed in claim 8, in which the bias member is configured such that the magnitude of the bias force relative to the second bearing surface is the same in all positions of the first and third structures relative to the second structure.

10. A pivot joint assembly as claimed in any preceding claim, in which the convex bearing member comprises a ball fixed to the first structure.

11. A pivot joint assembly as claimed in claim 10, in which the first structure comprises at least one formation onto which the surface of the ball is mounted.

12. A pivot joint assembly as claimed in any of claim 3 to 11 , further comprising a fourth structure coupled to the first and third structures at their ends distal to the second structure via third and fourth pivot joints.

13. A pivot joint assembly as claimed in claim 12, in which the first and third structures are first and second substantially elongate struts and in which first and second bodies comprise the second and fourth structures

14. A pivot joint assembly as claimed in claim 13, in which the first and second substantially elongate struts are configured to prevent relative rotation between the first and second bodies about at least one axis.

15. A pivot joint assembly as claimed in any preceding claim, in which the bias member is a resiliently deformable member.

16. A pivot joint assembly as claimed in claim 15, in which the bias member is a spring.

17. A pivot joint assembly as claimed in any preceding claim further comprising at least one stop member arranged to face the first structure so as to restrict relative lateral movement of the first and second structure.

18. A position apparatus comprising first and second bodies held in a spaced apart relationship via at least one strut, in which the joint between the at least one strut and one of the first and second bodies comprises a pivot joint assembly as claimed in any of claims 1 to 17.

19. A pivot j oint assembly, comprising: a first structure having a first bearing surface; a second structure having a front bearing surface for cooperation with the first bearing surface to facilitate relative rotation between the first and second structures about at least a first axis, and a back surface; and a stop member arranged to face the back surface so as to restrict relative lateral movement of the first and second structure.

Description:

PIVOT JOINT ASSEMBLY

The present invention relates to a pivot joint assembly and in particular to a high precision pivot joint assembly.

In high precision movement apparatus, such as those used in metrology, instrumentation and manufacturing, one part may be joined to another via a pivot joint in order to facilitate movement relative between them. This could be, for instance, to facilitate repositioning of a manipulator, workpiece inspection device or other tool. For example, WO 03/006837 discloses a pivot joint for use in such a high precision movement apparatus in which the pivot joint comprises a ball mounted and held in location in a platform, and a bearing surface on an arm moveable relative to the platform which slides over the ball in order to facilitate pivoting between the platform and the arm.

It can be important in high precision movement apparatus that pivot joints provide repeatable and predictable movement. It has been found in known pivot joints, such as that disclosed in WO 03/006837 that their accuracy can be less than desired. It can also be important that pivot joints are not prone to dislocation. Dislocation can increase the downtime of the apparatus in which the pivot joint is used. Dislocation can also cause damage to the apparatus in which the pivot joint is used.

Accordingly, in one aspect, the invention provides a pivot joint assembly comprising: a first structure having a convex bearing member; a second structure having a bearing surface for cooperation with the convex bearing member so as to facilitate relative rotation between the first and second structures about at least a first axis; and a bias member for biasing the convex bearing member into the bearing surface, in which the bias member acts on the first structure such that the direction of the bias force relative to the bearing surface is substantially the same in all relative positions of the first and second structures.

It is an advantage of the present invention that the direction of the bias force relative to the bearing surface remains substantially constant as the first and second structures rotate relative to each other. The spread of the force between the convex bearing member and the corresponding bearing surface will therefore remain substantially the same in all relative positions of the first and second structures. This helps to avoid changes in the frictional forces between the convex bearing member and the corresponding bearing surface and therefore improves the accuracy and repeatability of the pivot joint. This also helps to avoid uneven wearing of the bearing surface and/or of the convex bearing member which could lead to uneven pivoting movement between the first and second structures and adversely affect their positional accuracy.

There are many types of bearing surface suitable for cooperation with the convex bearing member. The bearing surface can be planar. The bearing surface could comprise a single region for contact with the convex bearing member. The region could be a point contact. The region could be a contact line. The contact line could be straight. This can be preferred when then convex bearing member is cylindrical in shape.

The bearing surface could comprise a circular contact line against which a part of the convex bearing member is biased. The circular contact line could be provided by a ring shaped formation. Accordingly, the convex bearing member could be biased against a ring shaped formation's inner circular edge. In use, a part of the convex bearing member will extend through the ring's opening. The ring's inner edge could be chamfered in order to increase the contact area between the bearing surface and the convex bearing member. The circular contact line need not be a closed circle. The circular contact line could be part circular. Preferably, the circular contact line extends through more than 180 degrees, more preferably through more than 270 degrees.

The region for contact with the convex bearing member could be a contact area shaped and sized to match inversely the convex bearing member so that the convex bearing member is a close fit against the bearing surface. The contact region can be concave. For instance, if the convex bearing member is at least part spherical in shape, then the shape of the bearing surface could be like that of the inside of an at least part sphere. Optionally, if the convex bearing member is at least part cylindrical in shape, then the shape of the bearing surface could be like that of the inside of an at least part cylinder.

Optionally, the bearing surface could comprise a plurality of discrete contact regions. For example, the bearing surface could comprise at least two discrete contact regions, for instance at least three discrete contact regions. The plurality of discrete contact regions could be arranged to define a seat for the convex bearing member. For instance, the plurality of discrete contact regions could be arranged to define a concave seat for the convex bearing member.

Preferably, the bearing surface is a concave bearing surface. Preferably, the bearing surface and convex bearing member are configured such that the convex bearing member can be held against the bearing surface in one relative lateral location only. Preferably, the bearing surface provides a kinematic mount for the convex bearing member. Accordingly, even though the first and second structures can pivot relative to each other, preferably they cannot move laterally relative to each other. This is advantageous as it provides for repeatable lateral mounting of the first and second structures. The bearing surface could be a pyramidal recess, for example a trihedral recess. Optionally, the bearing surface could be a conical recess. The bearing surface could comprise a plurality of discrete contact points. The bearing surface could comprise at least three discrete contact points. The at least three discrete contact points could be arranged such that they define the corners of an equilateral triangle.

The shape of bearing surface could be closed so as to hold the convex bearing

member.

Preferably, the bearing surface is open so that the convex bearing member can be pulled away from the bearing surface against the bias force. This is advantageous as it enables easy assembly and disassembly of the pivot joint assembly. In this case, it is also advantageous that the convex bearing member is provided on the structure on which the bias member acts. This is because the amount of relative lateral movement of the convex bearing member and concave bearing surface in the dimension parallel to the bias force required to dislocate the joint remains the same in all relative rotational positions of the first and second structures.

Accordingly, the likelihood of the joint dislocating does not vary depending on the relative rotational position of the first and second structures.

As will be understood, the convex bearing member need not be provided a separate piece to the first structure, e.g. it can be formed as an integral part of the first structure. For instance, the convex bearing member and first structure can be formed from a single piece of material. For instance, the first structure and convex bearing member can be formed by a moulding and/or machining process. Optionally, the convex bearing member can be formed as a separate piece and subsequently attached to the first structure.

Many types of convex bearing members are suitable for use with the present invention. For example, the convex bearing member could comprise a plurality of discrete bearing portions arranged to define a convex protrusion for cooperation with the concave bearing surface. Preferably, the convex bearing member comprises a single bearing portion for cooperation with the bearing surface. Preferably, the convex bearing member is rounded. For example, the convex bearing member can be at least part cylindrical. Accordingly, the first and second structures can be restricted to relative rotation about one axis. Preferably, the convex bearing member is at least part spherical. Accordingly, the convex bearing member can facilitate relative rotation of the first and second structures about

three orthogonal axes.

Optionally, the convex bearing member comprises a ball fixed to the first structure. This can be advantageous as it is possible to make accurate spheres, for example by a lapping process.

The first structure can comprise at least one formation onto which the surface of a ball is mounted. The ball can be mounted to the first structure via a spigot and socket arrangement. For instance, the first structure could have a spigot which is received in a socket formed in the ball. The first structure can comprise at least one mounting surface on which the surface of the ball is mounted. The ball can be mounted on the mounting surface by, for example, an adhesive. The first structure could comprise a pyramidal recess. Preferably, the pyramidal recess provides three points of contact. Such a shaped recess provides a highly precise and repeatable mounting.

It can be preferred that the at least one bearing surface extends over not more than substantially one third of the surface area of the ball. This can be advantageous as it can facilitate a large degree of rotational movement between the first and second structure.

Optionally, the direction of the bias force extends relative to the bearing surface so as to maximise the distribution of the force across the bearing surface. Optionally, the direction of the bias force extends substantially parallel to a line extending normal to a plane containing the face of the bearing surface. As will be understood, when the bearing surface is concave, provided either by a single concave contact area or by a plurality of discrete regions to define a concave seat, then the face is the perimetral edge of the concave shape defined by the bearing surface. Accordingly, in this case, preferably the bias force extends parallel to a line that is normal to the apex of the concave bearing surface. This is advantageous as it can ensure the even spread of the bias force over the bearing

surface.

Optionally, the bias member is configured such that the magnitude of the bias force relative to the concave bearing member is the same in all relative positions of the first and second structures. This can be advantageous as it ensures that friction between the convex bearing member and the bearing surface is the same in all relative positions of the first and second structures. Therefore, this facilitates even wear and predictable movement of the pivot joint assembly.

The pivot assembly could further comprise a stop member arranged to face the first structure so as to restrict relative lateral movement of the first and second structures. The stop member can therefore help prevent dislocation of the pivot joint assembly. Preferably, the stop member is arranged to restrict relative lateral movement of the first and second structure in a dimension parallel to the direction of the bias force. The stop member could face a back surface of the first structure. The stop member could be configured to prevent any lateral movement of the first and second structure. Accordingly, the stop member could be configured to be in contact with the back surface in all relative positions of the first and second structure. The stop member could be spaced apart from the back surface. Optionally, the stop member is configured such that the space between the back surface and the stop member is the same in all relative positions of the first and second structures. The back surface could be rounded. Optionally, the back surface has a substantially constant radius from the point about which the first and second structures pivot. The back surface could be provided by the convex member itself. Preferably, the distance between the back surface and the stop member is less than the distance by which the first convex bearing member extends into the concave shape defined by the bearing surface.

Optionally, the bias member acts between the first structure and a third structure. The point on which the bias member acts on the first and/or the third structure can change in order to compensate for a change in relative rotational position of the

first and second structures. This can help ensure that the direction of the bias force relative to the bearing surface remains the same. For instance, the point at which the bias member acts on the third structure can be configured to move along the first and/or third structure. For example, the bias member could be slidingly mounted to the first and/or third structure. The third structure could also be shaped so that as the point at which the bias member acts on the third structure moves, the distance between the first and third structures remains the same, thereby ensuring that the magnitude of the force remains the same.

Optionally, the third structure is coupled to the second structure via a second pivot joint.

Optionally, the second structure is constrained between the first and third structures.

Preferably, the third structure extends substantially parallel to the first structure. Preferably, the third structure is configured to match the movement of the first structure. In other words, preferably the first and third structures are synchronously pivotable relative to the second structure. Accordingly, preferably the first and third structures are constrained to move together such that they match the movement of each other. In other words, preferably the first and third structures are configured such that their relative orientation remains substantially the same. Such an arrangement is particularly useful when the pivot joint assembly is to be used as part of a linkage between first and second bodies, as described in more detail below. This can also be advantageous as it can ensure a constant bias force direction and magnitude whilst enabling the bias member to be attached to the structures at a fixed point.

Optionally, the second pivot joint comprises a second convex bearing member on the third structure and a second bearing surface on the second structure for cooperation with the second convex bearing member.

Optionally, the bias member biases the second convex bearing member into the second bearing surface such that the direction of the bias force relative to the second concave bearing member is the same in all relative positions of the first and third structures. This can be advantageous as it can ensure even distribution and constant magnitude of force between bearing members.

Optionally, the pivot joint assembly further comprises a fourth structure coupled to the first and third structures at their ends distal to the second structure via third and fourth pivot joints.

As will be understood, the first structure and second structures can be first and second monolithic members. That is, the first structure can be formed as a single piece, and the second structure can be formed as a single piece. Optionally, the first and second structures can comprise a plurality of pieces assembled together.

The first structure can be a first substantially elongate strut. The third structure can be a second substantially elongate strut. The first and second substantially elongate struts can be substantially identical. Optionally, the first structure can be a first arm. The third structure can be a second arm. The first and second arms can be substantially identical.

The first and second structures can form part of a linkage that facilitates controlled relative movement between first and second bodies, hi particular, the first and second substantially elongate struts can form part of a linkage that facilitates controlled relative movement between first and second bodies. One of the first and second bodies can comprise a mount for a tool. For instance, the mount could be for a tool for interacting with a workpiece, for instance for machining a workpiece, for example for inspecting a workpiece, and/or for example for manipulating an object, for instance moving an object. Accordingly, the assembly can be a positioning assembly, for instance a coordinate positioning apparatus. Optionally,

the assembly can be a measurement assembly, for instance a coordinate measurement machine.

One of the first and second bodies can comprise the second structure. The other of the first and second bodies can comprise the fourth structure. Optionally, there is provided a flap attached to the other of the first and second bodies such that it can rotate relative to the body to which it is attached about an axis. Preferably the flap can rotate about a single axis only. Preferably the flap is a rigid member. The flap can comprise the fourth structure. Accordingly, the linkage can comprise the first and second elongate struts and a flap. Preferably, the flap is connected to the first or second body, and the first and second elongate struts are connected to the other of the first and second bodies, so as to prevent relative rotation of the first and second bodies about at least one rotational axis, especially preferably about at least two orthogonal rotational axes.

The first body structure can be a first platform. The second body can be a second platform. The first and second platforms can be substantially planar.

The fourth structure can be a flap, connected to a third platform so that the flap can pivot relative to the third platform about an axis. Preferably, the flap can pivot relative to the third platform about a single axis only. Preferably the flap is substantially rigid. The third platform can be substantially planar. Preferably, the flap is connected to the third platform, and the first and second substantially elongate struts are connected to the first platform, so as to prevent relative rotation of the first and third platforms about at least one rotational axis, especially preferably about at least two orthogonal rotational axes.

Accordingly, the first and second substantially elongate struts and the flap can be configured to provide a rotation restrictor. A plurality of rotation restrictors can be provided so as to prevent any relative rotation between the first and second bodies. Accordingly, the first and second bodies could be held in a parallel spaced

apart relationship by a plurality of rotation restrictors. In particular, the first and second bodies could be held in a parallel spaced apart relationship by at least three rotation restrictors. Accordingly, the first and second bodies can be held by the rotation restrictors such that the first and second bodies cannot rotate relative to each other but so that they can move laterally in three orthogonal dimensions relative to each other.

The first and second substantially elongate struts can be configured to prevent relative rotation between the first and second bodies about at least two axes.

Suitable bias members include magnetic bias mechanisms. The bias member can be a resiliency deformable member. For example, the bias member can be a spring. Optionally, the bias member is an elastically deformable material, such as an elastic band. The bias member can be a leaf spring. Preferably, the bias member is a coil spring. In one embodiment of the invention, the bias member can be a tension coil spring. The bias member can comprise more than one discrete device for providing the bias. For instance, the bias mechanism could comprise at least two springs.

According to a second aspect of the invention there is provided a movement assembly comprising first and second arms spaced apart and synchronously pivotable about respective first and second pivot joints relative to a first spacer member that extends between the first and second members, in which the first and second pivot joints comprise respective first and second convex parts provided by the first and second arms which are biased into respective first and second bearing surfaces provided by the first spacer member such that the direction of the bias force relative to the first and second bearing surfaces is substantially the same in all positions of the first and second arms relative to the first spacer member.

The first and second arms can be biased into the first and second bearing surfaces by at least one bias member. Preferably at least one of the at least one bias

member acts on the first arm. The at least one bias member can act between the first arm and the spacer member. Preferably, the same at least one bias member also acts on the second arm. Suitable bias members include magnetic bias mechanisms. For instance, at least one of the convex bearing member and corresponding bearing surface can be magnetic so as to attract the other. The bias member can be a resiliently deformable member. For example, the bias member can be a spring. Optionally, the bias member is an elastically deformable material, such as an elastic band. The bias member can be a leaf spring. Preferably, the bias member is a coil spring. In one embodiment of the invention, the bias member can be a tension coil spring. There can be at least two bias members for providing the bias force. For instance, there can be provided at least two springs. Preferably, there is provided at least one bias member that acts on both the first and second arms. Preferably the at least one bias member extends between the first and second arms. For instance, the bias member can comprise a resiliently deformable member in tension between the first and second arms.

According to another aspect, the invention provides a pivot joint assembly, comprising: a first structure having a first bearing surface; a second structure having a front bearing surface for cooperation with the first bearing surface to facilitate relative rotation between the first and second structures about at least a first axis, and a back surface; and a stop member arranged to face the back surface so as to restrict relative lateral movement of the first and second structure.

It is an advantage that the back surface and the stop member restrict lateral movement of the first and second structures and thereby help prevent the dislocation of the pivot joint assembly.

Optionally, the back surface and stop member are configured such that the distance between them is the same in all relative rotational positions of the first and second structures. The back surface and stop member can be configured such that there is no distance between them. The back surface and stop member can be

configured such that there is a small distance between them. Preferably, the stop member is spaced from the back surface by not more than the distance required for dislocation of the pivot joint. For instance, when one of the bearing surfaces is concave and the other is convex, preferably the stop member is spaced from the back surface by not more than the distance by which the convex bearing surface extends into the concave bearing surface. If the extent by which the convex bearing surface extends into the concave bearing surface varies with the rotational position of the first and second structure, then preferably the stop member is spaced from the back surface by not more than the smallest distance by which the convex bearing surface extends into the concave bearing surface.

The back surface can be smooth. Optionally, the back surface is curved. The back surface can be a constant radial distance from the at least one axis. Accordingly, when the first and second members are configured to rotate relative to each other about one axis only, preferably, the back surface is at least part cylindrical. When can pivot about at least two orthogonal axes, preferably the back surface is at least part spherical.

The first and second bearing surfaces could be configured such that the first and second structures can pivot about a single axis only. Optionally, the first and second bearing surfaces can be configured such that the first and second structures can pivot about at least two orthogonal axes. Further, the first and second bearing surfaces could be configured such that the first and second structures can pivot about at least three orthogonal axes.

The pivot joint assembly could be configured in accordance with any of the embodiments described above in connection with the first aspect of the invention. As will be understood, for the purposes of this second aspect of the invention, the structure on which the bias member acts could provide the concave or the convex bearing surface.

An embodiment of the invention will now be provided, by way of example only, with reference to the following drawings, in which:

Figure 1 shows a schematic perspective view of a rig comprising a pivot joint according to the present invention;

Figure 2a shows a schematic side view of the region B highlighted in Figure 1;

Figure 2b shows a cross-sectional side view of Figure 2a;

Figure 2c shows a cross-sectional top view of Figure 2a;

Figures 2d and 2e show cross-sectional side views of Figure 2a with the first and second arms in different rotational positions; and

Referring to Figure 1, there is shown a rig 2, comprising first 4 and second 6 stages. In use, the rig 2 will be part of a positioning apparatus, such as a coordinate positioning apparatus which is not shown for the sake of clarity. One of the first 4 and second stages 6 will be fixed relative to the positioning apparatus. The other stage will be moveable relative to the positioning apparatus by at least one, and preferably at least three independently controllable telescopic drive shafts which extend between the first 4 and second 6 stages. Again, the drive shafts are not shown for the sake of clarity. Depending on the application in which the rig 2 is to be used, the moveable stage can comprise at least one mount (not shown) for receiving a tool for machining (e.g. shaping) or manipulating (e.g. handling/moving) a workpiece, and/or at least one mount for receiving a measurement device, for instance a measurement probe. For instance, the at least one mount could be provided on the smaller of the stages, e.g. second stage. As will be understood, power could be supplied to, and signals sent to and/or received from a tool mounted on the rig 2 in the normal manner. For instance, the tool could be powered via an internal battery, or power could be supplied to the tool

via power lines connected to the mount on the rig 2. Furthermore, the tool could communicate with an external controller (not shown) wirelessly, or via signal wires connected to the mount on the rig 2.

Three rotation restrictors 8 are provided which extend between the first and second stages. The rotation restrictors 8 are configured to prevent relative rotation of the first 4 and second 6 stages, but to permit translational movement of the first 4 and second 6 stages in the orthogonal x, y and z dimensions under the control of the telescopic drive shafts (not shown).

Each rotation restrictor 8 comprises a flap 10 connected to the first stage 4 by first 12 and second 14 pivot joints. The pivot joints are configured such that the flap 10 can rotate relative to the first stage about a single axis only, illustrated by axis A. Each rotation restrictor 8 further comprises first 16 and second 18 arms which extend between the second platform 6 and the flap 10. The ends of the first 16 and second 18 arms are connected to the second platform 6 and the flap 10 by a pivot joint 20 according to the present invention, which will be described in more detail below. Each rotation restrictor 8 further comprises a first coil spring 22 which pulls the respective ends of the first 16 and second 18 arms together so as to hold a first bar 28 mounted in the flap 10 between them. A second coil spring 24 is also provided which pulls the opposite ends of the first 16 and second 18 arms together so as to hold a second bar 31 mounted in the second platform 6 between them.

The pivot joint 20 will now be described with reference to Figures 2a to 2c which show in more detail the encircled region B in Figure 1.

The first arm 16 comprises towards its end, and on its front face, a concave recess 30 which provides a seat for a ball 32. The ball 32 is mounted within the concave recess 30 through the use of a suitable adhesive such as, for example a structured hard curing adhesive. Accordingly, the ball 32 is fixed relative to the first arm 16.

The bar 31 has at its end proximal the first arm 16 a trihedral recess 34 which provides three points of contact for receiving the ball 32. As will be understood, the trihedral recess 34 provides a kinematic mount for the ball 32 against the bar 31, such that the relative lateral mounting location of the bar 31 and the first arm 16 is highly repeatable. The ball 32 is made from tungsten carbide and the material of the trihedral recess 34 is stainless steel. These materials provide for a low coefficient of friction such that the arm 16 can rotate freely relative to the elongate bar 31 about the centre of the ball 32. The second coil spring 24 is fixed to the first arm 16 via a hook 36 which loops through an opening 38 close to the end of the first arm 16.

As can be seen in Figure 2a to 2c, the pivot joint 20 for the second arml 8 is the same as that for the first arm 16. The distance between the openings 38 which receive the second coil spring's 24 hook and the pyramidal recess 30 is the same for each of the first 16 and second 18 arms. Accordingly, as shown in the Figures, the coil spring 24 extends parallel to the bar 31 which is clamped between the first 16 and second 18 arms.

Furthermore, as shown in Figures 2d and 2e, the coil spring 24 remains parallel to the bar 31 regardless of the position of the first 16 and second 18 arms relative to the bar 31. Accordingly, the force (illustrated by arrows F) on the first 16 and second 18 arms caused by the coil spring 24 always acts in the same direction relative to the trihedral recess 34. This ensures that the spread of the load on the trihedral recess 34 remains the same regardless of the position of the first 16 and second 18 arms relative to the bar 31. In the example described, the force F acts perpendicular to the face illustrated by broken line 40 (or also in this embodiment the apex) of the concave recess regardless of the position of the first 16 and second 18 arms relative to the bar 31. Accordingly, the load is spread as evenly as possible between each of the points of contact provided by the faces of the trihedral recess 34. Furthermore, the distance between the openings 38 on the first

16 and second 18 arms remains constant throughout all rotational movement.

Accordingly, the coil spring 24 is not extended or retracted during movement of the first 16 and second 18 arms and therefore the magnitude of the force F is the same in all relative rotational positions. There is therefore a constant frictional force between the ball 32 and the trihedral recess 34.

Under normal operating conditions, the bias force F of the coil spring 24 keeps the ball 32 seated within the trihedral recess 34. However, abnormal external forces, caused for example by the rig 2 being knocked, could cause the first 16 or second 18 arm to pull the ball 32 out of its trihedral recess 34. This could cause the pivot joint 20 to dislocate.

Accordingly, a stop member 42 is provided for each pivot joint 42. With reference to Figures 2a to 2c, a stop member 42 sits behind and faces the back face 44 of the end of the first elongate arm 16. The back face 44 is spherical in shape such that the smallest distance X between the back face 44 and the stop member 42 is constant for all rotational positions of the first arm 16. The distance X is smaller than the extent to which the ball 32 extends into the trihedral recess 34. Accordingly, any lateral displacement of the first arm 16 out of the bar 31 is restricted by the stop member 42 and dislocation of the pivot joint 20 is thereby prevented. As shown in Figure 1, the stop members 42 for the ends of the first 16 and second 18 arms proximal the second platform 6 are provided by the second platform 6.

As the ball 32 is provided on the first arm 16 and the trihedral recess 34 is provided on the bar 31 , the relative movement between the ball 32 and the trihedral recess 34 in a direction parallel to the direction of the bias force F which would be required to cause the pivot joint 20 to dislocate is the same in all relative rotational positions of the first arm 16 and elongate bar 31. As will be understood,- this would not be the case if the pivot joint consisted of a concave recess on the first arm and a ball on the bar. Rather, as the first arm rotates relative to the bar, the concave recess would slide over the ball and as this happens the distance in the

direction parallel to the bias force F that the first arm would be required to move relative to the bar in order to cause dislocation would vary. In particular, the distance would be much smaller at extreme relative rotational positions, than at positions in which the first arm 16 it extends substantially perpendicular to the length of the bar 31.

As will be understood, the pivot joints 20 are the same at either end of the first 16 and second 18 arms. Accordingly, the pivot joint between the first 16 and second 18 arms and the flap 10 is the same, except for that the stop member 42 is provided by the flap 10 rather than by the second platform 6.

Although in the described embodiment there is provided a coil spring 24 toward each end of the first 16 and second 18 arms, it will be understood that this need not be the case. More or less coil springs or other biasing members could be used. For instance, a rotation restrictor 8 could comprise a single coil spring 24 placed toward the mid-points of the first 16 and second 18 arms.

As also will be understood, the pivot joint 20 of the present invention need not necessarily be used in a positioning apparatus. It can be used in any apparatus that requires high-precision and highly repeatable pivot joints.