This invention relates to an improved device able to determine the geometrical dimensions of a given object by a process of surface probing.

Whereas prior art shows that it is possible to construct such a measuring machine using a novel five parallel linkage kinematic architecture, such methods have relied on the accurate preparation of required parts by generally costly precision engineering.

It is clearly desirable to limit the requirement for precision parts to items that can be readily made at both low cost and high accuracy such as spherical ball bearings. Also a preferred design would avoid requiring parts that may be subject to bending moments, or at least ensure they are as small and stiff as possible.

This invention achieves these goals by employing novel solutions that enables the manufacturing cost of co-ordinate measuring machines to be reduced while maintaining and potentially even enhancing their accuracy.

Background

Coordinate Measuring Machines (CMMs) are a common tool for the metrology of 3D objects. They tend to either use a gantry type mechanism that articulates in a Cartesian fashion about 3 degrees of freedom (DOF) and gathers single point data with the aid of a touch trigger probe, or they use a polar mechanism that articulates like a 6 DOF arm and gathers data from a fixed probe point or laser scanner.

Recently new forms of kinematic architecture have been employed that can articulate about at least 5 DOF without necessitating a series of connecting linkages each facilitating a degree of freedom. They have an architecture where the end effectors multi-axis position is determined from a set of variable length struts acting in parallel. The advantage is that errors do not add up through the series chain but can to a degree average themselves out. Also stiffness does not similarly degrade and in the case of polar based mechanisms, nor does displaced leverage vary throughout the structure. These parallel architectures do though rely on a precise knowledge of the relative position of the focal point of their articulating nodes as well as their node to node displacements in order to accurately calculate the forward kinematic transforms that translate these displacements into the required Cartesian co-ordinates. Maintaining a known and consistent focal point for these linkages is therefore a key determinant of final accuracy.

The most repeatable and unambiguous positional settling is provided by three contact points held rigidly and suitably displaced around a sphere, constituting a conceptual 'three legged stool'. This device is well known in the engineering world and is characterised as a metrology tool by a 'ball bar' used for calibrating the accuracy of machine tools. In this guise one end is connected to the machine bed by such a linkage, generally magnetically preloaded, and the other to the turret of a multi axis machine tool. The connecting bar is of variable length with a suitable transducer measuring any length offset. In operation the machine tool turret is driven in a circular path about the end of the ball bar which is attached to the machine tool bed by the machines CNC controller, with any deviation from the optimum constant radius measured by the transducer.

The calibration accuracy attained is a factor of the sensitivity of the transducer but importantly also the sphericity of the balls about which the bar linkage can articulate. A very high degree of sphericity can be attained at low cost by using a simple ball bearing. Small ball bearings with sphericities of less than a tenth of a micron cost only a few pence.

However this style of linkage can only attach a single bar to a node focal point, whereas it can be seen as described in the Summary with reference to the requirements of a Pentapod linkage solution that there is a need for up to four struts to articulate with respect to each other about a single focal point. Four struts because the handle also acts like a strut. Objects of the Invention

It is an object of this invention to improve the accuracy of parallel kinematic coordinate measuring machines by better maintaining constant and known node focal points.

It is a further object of this invention to construct node linkages in such a way such as to minimise their build cost.

It is a further object of this invention to construct such linkages with covers that protect them from the ingress of matter that may effect their accuracy.

It is a further object of this invention to apply a Pentapod 5 linkage architecture in a compact style suitable for use on a desktop.

It is a further object of this invention to package the solution in a way that enables the necessary interconnecting signal cables to be concealed and to articulate with minimal mechanical stresses.

It is a further object of this invention that various styles of encoders can be applied to measure the mechanism's node to node displacements.

Summary

In accordance with a first aspect of the invention, a linkage system for a parallel kinematic co-ordinate measuring machine is provided comprising: a magnetically permeable ball; means for holding the ball; means to attach two or more struts to the ball such that each strut can track around the ball with up to three degrees of freedom; and a magnetically permeable seat connected to an end of each strut, each seat retaining three spatially offset magnetic pads, one at each of three points on the seat, each pad having a friction reducing surface for contact with the ball.

In accordance with a second aspect of the invention, a cradle support for a spherical node is provided. In longitude the cradle support is just short of a complete hemisphere. It comprises three low friction pads held rigidly at an offset with a vertex angle of at least 20 degrees from each other with such pads offering a tangential sliding surface with the node's spherical surface and where the cradle extends up to a notional horizontal equatorial axis which also passes through the node's focal point and at the ends of said axis a sliding or rotating constraint bears down against the upper surface of the node such that the node can rotate underneath it or rock underneath it about the said notional axis but cannot rotate about an axis substantially horizontally orthogonal to this.

In accordance with a third aspect of the invention a cover is provided for a two axis articulating joint. The cover comprises: a mounting part for attachment to a fixed frame; an arcuate member extending from the mounting part to a pivot remote from the mounting part; a bush coaxial with the pivot and surrounding the mounting part; a substantially hemispherical shell which is supported by the bush, between the bush and the pivot, the shell encompassing the arcuate member; and a substantially hemispherical upper shell attached to the arcuate member. The upper shell is constrained by the arcuate member and can rotate in a polar fashion about the lower shell.

This invention is preferably applied to a 'Pentapod' kinematic configuration in which a handle with attached stylus is supported by two linkages (one at each end), and in which one linkage forms the vertex of a three-strut tripod and the other another tripod vertex, but using the handle as one of the struts and the first vertex as one of its tripod bases. The five strut lengths described between the handle linkage and corresponding linkages retained on a known stationary frame unambiguously define the stylus position in space, with unnecessary rotation about the handle axis also constrained by the linkages.

Although described in the context of a Pentapod it can be seen by those reasonably skilled in the art how these mechanical solutions may be similarly applied to either a simple tripod or a more complex hexapod.

So it can been seen that the Pentapod architecture requires four different types of linkage as follows. On the handle:

At one end two struts couple to a common focal point, these two struts being able to vary their vertex angle sharing the same pivot axis but do not need to rotate about their own axes, only to articulate about their shared focal point together as a pair.

At the other end a similar situation arises, but with the addition of a third strut which shares the same focal point but which acts independently in enabling two degrees of tilt. Advantageously this two axis tilt is best facilitated by a single tilt axis retained to a rotational axis about the handle centre line axis. In this way the handle centre line (which extends at one end to the stylus tip measuring point) has a constrained and hence repeatable angle of rotation which thereby facilitates the calibration of any tilt offsets resulting from the attachment of the stylus. Also advantageously this third strut would be permitted to rotate about its own axis so that such rotation which needs to be supported for the complete articulation of the handle does not have to be enabled by a further in-line swivel or by providing for a third degree of rotation at the node that supports an encoder at the other end of the strut.

On the frame:

The frame supports five nodes acting essentially in two pairs with a fifth node supporting the other end of the third strut at the tripod end of the handle.

These nodes accommodate linear displacement encoders preferably of the non contact type such that the struts emanating from the handle pass through the nodes where their displaced length offset from the handle node is measured. This is preferable to other methods because there can be a large ratio between maximum and minimum extended length (between the base node and the handle) unlike what would be possible with telescopic solutions. The struts can then also remain uncomplicated and stiff.

The frame nodes which act in pairs have struts passing through them which share the single degree of freedom common pivot axis located at their common vertex at the handle node. As such the connecting struts do not need to be able to rotate about their own axis as they act in a common plane. However experience teaches that a further constraint to the rotation of the struts about their own axis at the frame nodes is helpful in mitigating any parasitic torque loads, avoiding the requirement of providing this constraint at their common pivot axis.

The fifth independently acting node on the frame could be allowed to rotate about its own axis to facilitate the additionally required degree of freedom, but as mentioned above when discussing the fifth strut's attachment to the handle it is better to enable this further freedom at the handle where the node is small and hence attracts the least friction. It is of course also lower cost to provide an accurate focal point in a small linkage rather than a large one.

By limiting all the nodes articulation to only two tilt degrees of freedom their position is more repeatable and as such if there are any errors inducing unwelcome offsets from the nodes focal point during articulation for which they are easier to calibrate.

Handle Linkages:

In one embodiment, the linkage nodes on either end of the handle are both based on precision balls about which the struts can articulate. At the stylus end of the handle the ball needs to fit on a shaft that passes as closely as possible through its focal point. At the other end of the handle where the tripod of struts share a common focal point the ball needs to be mounted from one side by a shaft that then ideally continues through the axis of the handle and subsequently through the other node ball before terminating at the stylus.

The balls consequently require a minimum of engineered preparation, being in one case a through hole and in the other a hole that is deep enough in order to fit it to a shaft. These holes can be made at relatively low cost by the process of spark erosion.

The requirement is then to enable the five struts to connect to these spherical nodes such that their articulation about them does not compromise their ability to maintain their focal point. The preferred method sees all these attachments based upon the aforementioned 'three legged stool' principle.

In a preferred embodiment, the 'three legged stool' is comprised of a multi finger fabricated steel part that extends on one end to provide an axial mounting for its attached strut. The fingers are terminated by magnets that hold the 'stool' (or seat) with unambiguous settling against the ball. These magnets are advantageously of a high flux Neodymium Iron Boron type. They can then have a low friction coating such as Molybdenum Disulphide which has a coefficient of friction of between 0.01 and 0.06 and an extremely low specific wear rate of 4*10-17m ^A 3/Nm on their contact face. Recent advances now suggest that near-frictionless carbon (NFC) will have friction coefficients of 0.001 or less in a clean environment and still 0.02 in air with even lower wear rates.

The magnets are arranged such that one foot of the 'stool' contacts the ball with a 'large' magnet of specific polarity and the other two feet have smaller magnets with opposite polarities in contact such that the three feet generate two magnetic circuits, each small magnet sharing the flux of the large one. As the above low friction coatings are so thin (< 1 micron) they introduce very little resistance to the free transmission of flux.

An alternative embodiment sees six magnets deployed with two on each foot. In this case each pair form a magnetic circuit with one member acting as the in contact foot and the other member being slightly offset leaving a small air gap. The existence of the air gap does reduce the flux flowing through the circuit, but this is compensated for by the larger volume of magnetic material in each circuit.

The disposition of the 'feet' is arranged such as to enable the maximum articulation as they slide over the ball and yet still avoid interfering with each other. This is enhanced by recognising that two of the struts act like scissors about a common tilt axis thereby reducing the situational permutations. Furthermore the 'feet' can be orientated with an offset with respect to their strut such that although their struts may need to achieve a small vertex angle, the 'feet' can act in the largest available spaces around the ball.

Where the third strut is attached at the tripod end of the handle, the covers constrain the articulation to the required single tilt axis combined with the rotation about the handle axis. Although this constraint is not kinematic, it is not a critical determinant of the positional accuracy. The effect of the constraint is to enable any angular offset in the stylus with respect to the main handle axis to be calibrated. As any such offset will be a product of the tilt error and its rotation about the handle axis, a small error in the latter factor will have a very small impact on the calibration value. For example if the stylus is 40 mm long and the tilt error is measured as 0.02 degrees the calibrated positional error at the tip of the stylus would be ~ 14 micron. A further error in the rotational displacement of this value of as much as 1 degree would only amount to an additional - 0.25 micron.

Frame Linkages:

The nodes that hold the encoders against the frame are all independent so can each have a simple 'three legged stool' support to ensure their focal point is accurately maintained. Once again this can be based on magnets with a low friction coating applied on their sliding contact face. In this case as the frame remains static, the nodes can sit in their 'stool' such that gravity augments their positional settling.

The accuracy of the nodes articulation is again a factor of their sphericity so it is advantageous to keep the nodes as small as possible. In one embodiment of the device these nodes are spark eroded from complete balls into semi-hemispheres with further cut outs to accommodate the encoder and the roller axles that retain the struts that pass through them.

The nodes are described as semi-hemispheres because advantageously they are a little short of a complete hemisphere, being rebated by an amount consistent with the radius of constraining pins or rollers. These act on either side against the rim 180 degrees opposed such that they hold the spherical surface against its three bearing supports preventing them becoming fully dislodged if the device is inappropriately handled, but importantly also constrain the motion of the hemisphere to a tilt axis along the shared axis of these constraints and an orthogonal rotation axis. In this way the nodes prevent the struts that pass through them from rotating about their own axis.

Packaging:

The frame based nodes have to be held in a fixed and stable position relative to each other. In this embodiment the cradles that support the encoder nodes are connected to a carbon fibre tube which is in turn held vertically by a base moulding.

In this embodiment each cradle and node assembly is protected by a three stage cover system. A fixed cover extends out from the central tube. A second cover is supported between the outer side of this cover and a pivot point attached to the node support cradle and co-axial with the above mentioned node motion constraints such that it can rotate about this axis. A further largely hemispherical cover shell then sits on top of this shell which can rotate in a polar axis about its shared equator. This upper cover includes two cut outs on either side to enable the strut to pass through it. It can be located with respect to the internal hemispherical node because it articulates with it. Advantageously the equator rotational interface and the strut cut outs are lined with a dust excluding member like a felt gasket.

The encoders are installed in the hemispherical nodes and need to connect electrically with the body of the machine where the necessary decoding electronics can be housed. In order to enable the wires to articulate without mechanical stress from the tilt and pivot node to the frame's tubular tower they are best accommodated along the axes of rotation where the articulation displacement can be limited in each case to rotation. Also by this means the wires are neatly concealed behind the covers.

In order to protect the handle linkage components a scheme of multi-stage 'eyelids' can be employed to provide for a sufficient degree of articulation while still packing away into a compact space. Multi-stage in that the strut has a fixed spherical shell segment attached to it. This is overlapped by a further segment that has a central hole such that the strut shell at its extremity of travel is still covered by the shell ring segment. This ring segment is then retained behind a cover that is common to both strut eyelid apertures and that extends from the handle shell to the stylus.

At the upper tripod end the covers are complicated by the need to accommodate the third strut. This strut only has a single tilt axis, its further articulation being enabled by its rotation with and about the handle axis. As such it can act through a gap generated centrally between the two sides - in effect an equator between the two poles. A band can then pass through a local expansion in the handle shaft covering this equatorial slot irrespective of the degree of rotation of this top strut. A collateral benefit of this arrangement is that the covers can now act as the desirable constraint on rotation about the handle shaft axis other than that provided for by the actual axial rotation of the strut with the handle shaft. In other words fixing the handle shaft rotation to the tilt plane of the strut thereby ensures that it is determinate and consequently enables any stylus offset to be calibrated for as previously described. Brief Description of the Drawings

An embodiment will now be described with reference to the accompanying drawings in which:

Fig. 1 shows an isometric view of the device.

Fig. 2 shows the device's handle with upper covers removed to reveal the kinematic strut linkages.

Fig. 3 shows the detail of the three point contact linkages.

Fig. 4 shows the three point contact cradle for the encoder nodes.

Fig. 5 shows an embodiment of an encoder fitted into a partial hemisphere.

Fig. 6 shows the partial hemisphere fitted into its cradle with side and lower covers.

Description of an embodiment with reference to the drawings.

One embodiment of the device is shown in Fig. 1 and comprises of a moulded base (16) with a fitted pillar 15 made out of carbon fibre tube. The pillar 15 supports a top node 1 having a strut 10 passing therethrough. A series of pylons 17 extend out from the pillar 15. The pylons support articulating nodes 2, 3, 4 and 5. These nodes have struts 11, 12, 13 and 14 passing therethrough . The device further comprises a handle 7 connected between an upper pivot 6 and a lower pivot 8. Each pivot houses a node within (described below). A stylus tip 9 extends downwards from lower pivot 8.

The nodes substantially act as two pairs. Struts 11 and 12 couple to the upper pivot 6 and to the top strut 10, and struts 13 and 14 couple to the lower pivot 8. Consequently the upper pivot 6 is the vertex of a tripod leading to nodes 1,2, and 3, whereas the lower pivot 8 is the vertex of a triangle with nodes 4 and 5 or notionally a tripod for kinematic transform purposes when the handle 7 is considered as a strut with its base being the vertex point (pivot 6). Each strut 10, 11, 12, 13, and 14 is of pulltruded carbon fibre with a triangular cross section. Each has a steel encoder track on one face and either steel or polyester running tracks on the other two faces.

By knowing the fixed node positions 1, 2 and 3 and the length of the struts 10, 11 and 12 between the nodes and the pivot 6, the Cartesian position of pivot 6 can be calculated. Similarly, by knowing the relative fixed positions of nodes 4 and 5, the lengths of the struts 13 and 14, the length of the handle 7, and the calculated position of the pivot 6, the position of the handle 7 can be unambiguously calculated, and then extrapolated to the final measurement point at the stylus tip 9.

Fig. 2 shows the handle 7 in more detail with certain left and right covers of upper pivot 6 removed. A set of three point contact fingers together form a seat 25. One such seat is attached to each of fittings 20, 21 and 22, with the contact fingers in each seat attaching to sphere 23 via magnets. A band 24 extends forwards from fitting 20 and back to fitting 20, and traverses the gap between the left and right covers. Fittings 18 and 19 connect to lower pivot 8, which comprises a cover, a partial spherical shell 26 and spherical shell ring 27.

When constructed, the fittings 20, 21 and 22 are epoxy bonded into the hollow ends of the struts 10, 11 and 12 respectively.

The sets of three point contact fingers track around the sphere 23. The band allows fitting 20 to articulate in a tilt axis about a horizontal axis passing through the sphere 23.

The lower triangle vertex (lower pivot 8) has a node similar to pivot 6 which supports the fittings 18 and 19 which connect to struts 13 and 14. The node is protected from contamination by the covers shown from this view where the partial spherical shell 26 is rigidly attached to the fitting 18 and then tracks within the spherical shell ring 27 in turn retained behind the cover at lower pivot 8.

Instead of three contact fingers, the seat 25 can be a triangle shape or part- spherical cup, in each case with three (or more) points at which magnets contact the sphere 23. Fig. 3 shows the internals of pivot 6 (the tripod vertex) . The internal structure comprises the sphere 23 attached to a handle axle 127, and a local thickening 28 with a slot 29 through the local thickening. It further comprises three struts which are attached to sphere 23 via end fitting core parts 124, 125 and 126, which are spark eroded out of a stiff magnetic metal material.

Further spark eroded metal parts are fitted to the distal ends of these in the manner of "T" bars. These form the seat 25 with its three point contact fingers. The T-bars and fingers that form the seat 25 are magnetically permeable. The three rigid points on each seat preferably have magnets 32, 33, and 34 attached. Each of these magnets has low friction coatings on the sliding side in contact with the node.

The slot 29 allows the band 24 to pass through the centre of the handle axle 127.

In the case of end fitting core part 126 small magnets 32 and 34 each form a magnetic circuit through the node sphere 23 with a thicker magnet 33. In the case of end fitting core part 124 there are six magnets attached in pairs where one (e.g. 30) is in contact with the node sphere 23 and another is slightly offset (e.g. 31).

Fig. 4 shows a support structure for any one of nodes 2, 3, 4 and 5. The support structure comprises a spark eroded pylon part 134 that is attached (not shown in Fig. 4) to the pillar 15. The pylon part 134 includes an arcuate portion 170 that extends from bush 40 to pivot 43. A transverse arcuate spark eroded part 35 is located on the arcuate portion 170. A cover shell 39 is attached to the bush 40 and pivot 43.

Between them, the pylon part 134 and the transverse part 35 provide three rigidly fixed locations for low friction magnetic pads 36, 37 and 38. They also have a provision for constraint pins that act against partial hemispherical encoder nodes (described below) to be attached along their pivot axis at 41 and 42.

The cover shell 39 rides on the bush 40 and pivot 43 to enable it to tilt about that described axis.

Fig. 5 shows the internals of nodes 1, 2, 3, 4, and 5. The internal structure comprises a partial hemisphere 44 housing a non-contact encoder 45, opposing rollers 46 and 47, and a further two pairs of opposing rollers 48, 49 and 50, 51. Retained block 54 is proximate rollers 49 and 50, and block 52 is proximate rollers 48 and 51. Part 53 is horizontally preloaded against retained block 54. Partial hemisphere 44 sits within shell 39 of the respective note and is supported by pads 36, 37 and 38.

In this embodiment a triangular profile strut that passes over the non-contact encoder 45 runs over rollers 46 and 47 and between opposing roller pairs 48, 49 and 50,51, thereby being fully constrained apart from movement along its own axis.

The block 52 jams and thereby retains axles of the rollers 48 and 51. The rollers 49 and 50 are held by part 53 which is horizontally preloaded against retained block 54 by a central sprung point set screw. By this means it is adjustably preloaded to apply a relatively constant clamping force to the strut against its fixed rollers. This ensures that the strut maintains a consistent vertical offset from the encoder and also a consistent axial tracking.

Fig. 6 shows the partial hemisphere 44 of Figure 5 in the support structure of Fig. 4. Cover part 55 is fixed against the fitted pillar 15 and covers the pylon part 134. Felt seals 56, 57 and 58 are present.

The cover part 55 is fixed against the tower 15 (Fig. 1) covering the pylon part 434 and supporting the bush 40. The bush is fixed to the shell and moves relative to the cover part 55. Felt seal 56 acts as a radial seal against a top spherical shell that can rotate in a polar axis above it. Seals 57 and 58 act between the axially displacing strut and against the top spherical cover shell.

Further modifications of the invention will also occur to persons skilled in the art, and all such are deemed to fall within the spirit and scope of the invention as defined by the appended claims.