WO1991006396A2

WO1991006396A2 - Determination of spacial relationships

Info

Publication number: WO1991006396A2
Application number: PCT/GB1990/001617
Authority: WO
Inventors: James Bury
Original assignee: Lk Limited
Priority date: 1989-10-24
Filing date: 1990-10-22
Publication date: 1991-05-16
Also published as: GB8923947D0; JPH05501005A; EP0496776A1; WO1991006396A3

Abstract

A coordinate measuring machine has a base (10) on which is provided a linear guideway (12) supporting a vertical column (14). A movable structure (16) on the column (14) mounts a movable quill (18) having a probe (20). A rotary table (22) has a lower platen fixed relative to the base (10) and an upper platen arranged to be rotatable relative to the lower platen. A plurality of carbide spheres are provided around the periphery of an upper face of the lower platen, each sphere being set in a conical recess, with adjacent spheres providing an effective vee configuration. A pair of stalks are each mounted at one end by way of a concave bearing surface on a respective sphere, the other ends of the stalks being connected by way of bi-directional hinges to the upper platen. The arrangement provides a six point kinematic location at each angular position of the upper platen to enable accurate determination of the spacial relationship between a workpiece (24) and the probe (20).

Description

Determination of Spacial Relationships

This invention relates to apparatus for determining the spacial relationship between two relatively movable components, and is particularly concerned with co-ordinate measuring or manufacturing machines and the relationship of workpieces therewith. Such a machine may use any system of co-ordinates with any number of axes and the system may be orthogonal, cylindrical polar, spherical polar or other.

In carrying out work on a workpiece, either manufact¬ uring the workpiece or measuring the workpiece, or assessing its quality or fitness for purpose, it is sometimes necessary to present the workpiece to the machine or present the machine to the workpiece in a variety of different linear or angular positions. Usually the work carried out on the work- piece at the different positions must be accurately related so that all the dimensions produced or measured relate to a common co-ordinate system for the workpiece as a whole.

For example when measuring features on a workpiece in three dimensions it is often necessary to have access to many different orthogonal faces of the workpiece or access at various angles to the axes of the machine. A conventional way of achieving this is to use a rotary table with a verti¬ cal axis to carry the workpiece. To achieve accuracy it is necessary to use an accurate and therefore expensive rotary table in order to align this to the machine axes very accur¬ ately or alternatively to calibrate the mis-alignment at all those angles of rotation on the rotary table which are to be used. Because of coning errors of most rotary tables, multiple calibration is a preferred method. This removes the need for extreme accuracy on the rotary table but still leaves a requirement of very good repeatability which means that the rotary table will still be expensive.

With very large workpieces, it is particularly diffic¬ ult to achieve the required degree of angular accuracy and/or repeatability of the rotary table, since for a given degree of linear accuracy, the angular accuracy must be better in proportion to the increase in the radius. Also for very large workpieces the cost of a rotary table is very high indeed.

According to the present invention there is provided apparatus for determining the spacial relationship of two relatively movable components, the apparatus comprising means for moving at least one of the components relative to the other into a selected one of a plurality of positions, and means providing effectively for a six point kinematic location of said one component at each of the plurality of positions . Preferably a first of the components may be provided with a plurality of locating means providing the said plurality of positions, and the second of the components may have at least three spaced vee arrangements for selective engagement with respective ones of the locating means whereby to provide the six point kinematic location at the selected position.

The locating means on the first component may be prov¬ ided by a plurality of spherical members. The vee arrange¬ ments on the second components may each comprise a pair of stalks each connected at one end to the second component and having a bearing surface at the other end for engagement with the respective spherical member, each of the stalks further having means enabling the stalk to function as a single point kine atically. Such means may comprise a bi-directional hinge on the stalk.

Each spherical member may be formed of carbide and may be secured in effectively trihedral holes in the first comp¬ onent for example by means of expansion bolts. The spherical members may be hydraulically pressed into the recesses.

Alternatively the first of the components may provide a plurality of locating means defining the respective positions and a second of the components may have six spaced positioning means which are selectively engageable with six respective ones of the locating means whereby to provide the six point kinematic location at each position.

Each of the positioning means may comprise a stalk connected at one end to the second component and having a spherical part at the other end, each of the locating means on the first component presenting three contact surfaces for engagement by the spherical part, means being provided on each stalk to ensure functioning as a single point kine ati- cally. Such means may be in the form of a bi-directional hinge. The three surface contact may be provided by a nest of three spheres or by flat surfaces arranged to be tangen¬ tial to the spherical part of the engaging stalk.

The present invention may also provide an assembly comprising a pair of apparatus as described in any of the six preceding paragraphs, one of the components being common to each apparatus whereby there is effectively provided a six point kinematic location of a first of the components relat¬ ive to a second of the components at each of the plurality of positions, and a six point kinematic location of the second of the components relative to the third of the components at each of the respective plurality of positions. Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Fig. 1 is a schematic view in perspective showing a co¬ ordinate measuring machine with rotary table and workpiece;

Fig. 2 is an enlarged view in perspective of one embod¬ iment of a rotary table for use in the machine of Fig. 1;

Fig. 3 is a schematic view showing part of a further embodiment of a rotary table;

Fig. 4 is a schematic view showing part of yet another embodiment of a rotary table;

Fig. 5 is a schematic view of part of another embodi¬ ment of a rotary table;

Fig. 6 is an illustration of a differential rotary table;

Fig. 7 is a schematic plan of part of a rotary table showing a mechanism for lifting same; Fig. 8 is a sectional elevation of the apparatus of Fig. 7; and

Figs. 9a to 9d are schematic illustrations of various kinematic locations.

Referring to the drawings, Fig. 1 shows a co-ordinate measuring machine having a base 10 on which is provided a linear guideway 12 supporting a vertical column 14. The latter mounts a supporting structure 16 for vertical movement thereon and the structure 16 mounts a horizontally movable quill 18 mounting at one end thereof a measuring probe 20. On the base 10 there is further provided a rotary table 22 on which can be supported a workpiece 24 to be measured. Such a machine is of conventional design.

To enable accurate determination of the spacial rela¬ tionship between the workpiece 24 and the probe 20 it is appropriate in this example to move the workpiece 24 to an accurately known repeatable position each time it is rotated. The principle of six point kinematic location at each posi¬ tion or station is used to obtain the best repeatablility.

In one embodiment as shown in Fig. 2, the rotary table 22 has a lower platen 26 fixed relative to the base 10 and an upper platen 28 arranged to be rotatable relative to the lower platen 26. Around an upper face of the lower platen 26 adjacent the peripheral edge thereof, there are provided a plurality of spheres 30 adjacent one another. Each sphere 30 is preferably formed of carbide and is set in a conical recess in the platen 26, the recess being machined or hydrau- lically pressed to give effectively a trihedral hole, the sphere 30 being formed with a spark eroded hole for fitting thereto an expansion bolt for securing the sphere 30. The latter can be pressed by hydraulic or other means into its seating to provide a finite contact area able to carry a sub¬ stantial load and reduce the Hertzian stresses. Each pair of adjacent spheres 30 provides an effective vee configuration. The upper platen 28 is formed on its lower surface with three cylindrical members 32, the axes of which extend radially, the members 32 being spaced apart at 120° intervals.

When the upper platen 28 is seated on the lower platen 26, the cylindrical members 32 engage in respective vees defined by the spheres 30. In this way a change of angular position of the upper platen 28 of the rotary table 22 is exactly repeatable with the cylindrical members 32 seating in respective vees defined by the spheres 30. This arrangement provides the required six point kinematic location of the upper platen 28 in each angular position.

The kinematic locations must be disengaged and re-engaged at each new position or station and there are two fundamental choices in the direction of this movement, these being horizontal and vertical. If the mass to be moved is large then there is advantage in using horizontal movement on air bearings since little power is required. Certain problems associated with this design can be overcome by using vertical movement which is not difficult for relatively small masses.

Lifting the upper platen is most easily powered by compressed air. The problem is to ensure that the platen remains horizontal, even with the significantly out of balance workpiece.

This can be achieved by using a central air cylinder 34 (Figs. 7 and 8) and three pivotted arms 36 spaced apart by 120°, the arms 36 transferring movement of a piston through a hollow triangular plate 38 to the lower peripheral area of the platen 28. Each arm 36 has the form of an over-centre lever mechanism having a lever arm 42 pivotally mounted on the lower platen 26 of the rotary table with one end in operative engagement with a rod 44 of the piston and the other end being connected with one arm of a cranked lever 46 which is pivotally mounted on the lower platen 26 and has its other arm engaging below the hollow triangular plate 38, the apices of which engage the lower surface of the platen 28. The over-centre lever mechanism ensures that the platen 28 remains in the raised position even in the event of pressure failure during an indexing operation. A sequence of lift, index and lower is sequentially controlled by means of micro- switches or proximity detectors. The central air cylinder 34 is double acting to reverse the over-centre levers for lowering.

For powering the table rotation a friction roller drive may be provided near the periphery of the platen 28. This drive can use a roller on the end of one of the sides of the plate 38 as a drive roller with a motor 50 being carried on the plate 38.

Six point kinematic location can present certain prob¬ lems because of the indentation and Hertzian stress problems with large forces and the difficulty of ensuring full engagement of the six points due to large frictional forces. A further problem can be wear on the location surfaces due to the large stresses and friction forces which prevents repeatability over long periods of operation. A further embodiment of the rotary table 22 as shown in Fig. 3 can obviate or mitigate such problems.

In this embodiment the cylindrical members of Fig. 2 are replaced by respective pairs of elongate members or stalks 52. Each of the latter is secured by way of an elastic hinge arrangement 54 at one end to the platen 28, each pair of stalks 52 having their longitudinal axes angled towards one another and each stalk 52 at its other end having a spherical concave bearing surface 56, whereby each pair of stalks 52 engages by way of the surfaces 56 on a respective one of the spheres 30. The hinges 54 are preferably bi-directional hinges which allow the surfaces 56 to act as a kinematic vee and this provides the required six point kinematic location at each angular position. It should be appreciated that the bi-directional hinge can be replaced by two unidirectional hinges arranged orthogonally or can be replaced by a spherical bearing, or two cylindrical bearings arranged orthogonally.

In a further embodiment (Fig. 4), each angular position on the lower platen 26 is defined by a nest of three spheres 58 while six individual stalks 52 are connected to the upper platen 28 at spaced locations, each stalk 52 having on its free end a sphere 60 which can locate centrally of the spaced spheres 58 to give three points of contact. While the stalk 52 remains stiff axially, the hinge 54 of each stalk 52 enables small tranverse movement which allows all six kinema¬ tic locations to seat simultaneously and thus the three point connection of each location to function as a single point. It should be appreciated that the three spheres 58 at each location may be replaced by flat surfaces which are tangent¬ ial to the surface of the sphere 60. The arrangement may be reversed so that the nest of three spheres is on the stalk and the single sphere is on the lower platen.

In a further embodiment (Fig. 5) each of the stalks 52 is formed with a bore 62 opening on to the free end of the stalk 52, thereby defining an annulus at the free end which can locate on a sphere 30. The bore 62 may be connected to a conduit 64 for supplying compressed air into the bore 62. An air pressure gauge or pressure switch 66 is connected in the compressed air line as is a stop valve 68. When the stalk 52 is seated on the sphere 30 with compressed air being emitted into the bore 62, the air pressure registers on the gauge 66. The stop valve 68, after admitting the compressed air, can then isolate the supply so that any drop in air pressure due to leakage through incorrect seating of the stalk 52 on the sphere 30 can be monitored. This provides an arrangement for knowing whether any or all of the kinematic positions are correctly located. It will be appreciated however that other forms of monitoring may be provided such as a microswitch, proxmity sensor, or a capacitive transducer.

The six point kinematic location of one component on another may be obtained in other ways. For example one component may be pfovided with a plurality of spheres with the other component having V shaped members for locating between respective pairs of the spheres with the V members having flat sides or conformal curves. Vee recesses may be provided in one component with the other component having a plurality of spheres whereby respective ones of the spheres locate in the vee recesses in different angular positions. In a further arrangement a plurality of cylindrical members may be provided on one component with the other component having spheres each of which is adapted to locate between a pair of adjacent cylinders to determine the respective angular position.

In order to ensure that all the location points may contact and are not held apart by friction forces at those points which initially make contact, the friction forces can be removed by utilizing the stalks 52 but alternatively all six points can be held microscopically apart on gas bearings or hydrostatic bearings with the gas or fluid pressure being reduced simultaneously to allow the six points to seat. This method may be combined with the use of stalks with some locating points using gas or hydrostatic bearings and other location points using the stalks.

It may be preferred to provide locations at 5° inter¬ vals on a rotary table. Alternatively two rotary table arrangements 70,72 can be provided one above the other or one inside the other as two annular rings (Fig. 6). In the latter case the outer ring may have locations at 10° intervals with the inner ring having locations at 9° intervals. The rela¬ tive rotation of the tables 70,72 can provide a differential arrangement useful in obtaining finer divisions. For example by moving forward 10° and in reverse 9°, the net forward movement is 1° and this can be repeated 360 times to give 1° intervals. It is possible to design a rotary table with irregular intervals to suit a specific application.

Cleanliness of the location surfaces is important and in the indexing cycle the upper platen 28 acts as a piston causing air flow into and out of the enclosed centre volume, the incoming air bringing in dust particles. It is difficult to seal the joint and the easiest method is to inject clean air into the centre volume as the upper platen is lifted.

Other features included in the apparatus may include the use of spring loaded or otherwise movable surfaces to hold the upper platen of the kinematic locations until clamping forces are applied. This prevents damage to the location surfaces as the platen is moved from one station to another. Further there may be provided means for covering the location surfaces with a removable cover which may be automatically removed or retracted so as to keep the location surfaces clean. As an alternative to the injection of clean air in order to keep the location surfaces clean there may be provided a rotary or oscillating brush or a degreasing spray or any combination thereof. As a further alternative there may be provided means of pressurising an enclosed mechanism with clean compressed air at a low pressure so as to ensure there is an outward flow of the air to prevent ingress of dirt. Also there can be provided means for clamping the upper platen in its respective stations, for example in the form of pneumatic or hydraulic cylinders or actuators, springs, magnetic forces, screws, gravitational forces, or indeed any other means of applying a force.

The upper platen should be constrained to rotate about an approximate centre when disengaged from the kinematic locations and this requires to be achieved without affecting the kinematic locations on re-engagement. For example external rollers may be used with adequate clearance and with such an arrangement that the upper platen drops clear of the rollers on re-engagement. Further, during the change from one station to another, a correct indexing angle should be sensed approximately before lowering of the upper platen. This can be achieved by means of a proximity switch fixed to the base and engaging with projections on the platen. The switch should sense rotation in the raised position of the upper platen with freedom of the platen to rotate and lower. In addition, to identify which station is in use, it is possible to use a series of proximity switches arranged to sense when the kinematic locations are engaged, such proxim¬ ity switches preferably being fixed to the base to simplify wiring.

As a further improvement the apparatus may include means for calibrating the positions of the upper platen at each station relative to the other stations. Calibration can be effected by the apparatus as described in our co-pending U.K. Patent Application.

It is to be understood that the invention is not limited to use with rotary tables but may be applied to any combination of linear and angular displacements. For example a complete air frame could be moved around a static co-ordinate measuring machine, or a mobile machine could be moved around the air frame from station to station so that the working volume of the machine has access to the total surface area of the air frame.

For kinematic location there are in principle at least four layouts for the six points of contact as illustrated schematically in Figs. 9a to 9d, i.e. an arrangement of three points, two points and one point on respective faces (Fig. 9a); two points on each of three faces (Fig. 9b); three vee recesses (Fig. 9c); and a trihedral hole, slot and single point (Fig. 9d).

Various other modifications may be made without departing from the invention.

Claims

Claims : -

1. Apparatus for determining the spacial relationship of two relatively movable components, the apparatus comprising means for moving at least one of the components relative to the other into a selected one of a plurality of positions, and means providing effectively for a six point kinematic location of said one component at each of the plurality of positions.

2. Apparatus according to Claim 1, wherein a first of the components is provided with a plurality of locating means providing the said plurality of positions, and the second of the components has at least three spaced vee arrangements for selective engagement with respective ones of the locating means whereby to provide the six point kinematic location at the selected position*.

3. Apparatus according to Claim 2, wherein the locating means on the first component is provided by a plurality of spherical members.

4. Apparatus according to Claim 2 or 3, wherein the vee arrangements on the second components each comprise a pair of stalks each connected at one end to the second component and having a bearing surface at the other end for engagement with the respective spherical member, each of the stalks further having means enabling the stalk to function as a single point kine atically.

5. Apparatus according to Claim 4, wherein the enabling means comprises a bi-directional hinge on the stalk.

6. Apparatus according to Claim 4 or 5, wherein each spherical member is formed of carbide and is secured in effectively trihedral holes in the first component.

7. Apparatus according to Claim 6, wherein each spherical member is secured by means of expansion bolts.

8. Apparatus according to Claim 6, wherein the spherical members are hydraulically pressed into the recesses.

9. Apparatus according to Claim 1, wherein a first of the components provides a plurality of locating means defining the respective positions and a second of the components has six spaced positioning means which are selectively engageable with six respective ones of the locating means whereby to provide the six point kinematic location at each position.

10. Apparatus according to Claim 9, wherein each of the positioning means comprises a stalk connected at one end to the second component and having a spherical part at the other end, each of the locating means on the first component pres¬ enting three contact surfaces for engagement by the spherical part, means being provided on each stalk to ensure function¬ ing as a single point kinematically.

11. Apparatus according to Claim 10, wherein said means on the stalk is in the form of a bi-directional hinge.

12. Apparatus according to Claim 10 or 11, wherein the three surface contact is provided by a nest of three spheres.

13. Apparatus according to Claim 10 or 11, wherein the three surface contact is provided by flat surfaces arranged to be tangential to the spherical part of the engaging stalk.

14. An assembly comprising a pair of apparatus as described in any of the preceding Claims, one of the components being common to each apparatus whereby there is effectively provided a six point kinematic location of a first of the components relative to a second of the components at each of the plurality of positions, and a six point kinematic location of the second of the components relative to the third of the components at each of the respective plurality of positions.

15. Apparatus for determining the spacial relationship of two relatively movable components, substantially as herein¬ before described with reference to the accompanying drawings