GB2255636A

GB2255636A - High-precision positional control

Info

Publication number: GB2255636A
Application number: GB9209975A
Authority: GB
Inventors: Keith Carlisle
Original assignee: CRANFIELD PRECISION ENGINEERIN
Current assignee: CRANFIELD PRECISION ENGINEERIN
Priority date: 1991-05-09
Filing date: 1992-05-08
Publication date: 1992-11-11
Also published as: GB9110070D0; GB2256606A; GB9209975D0

Abstract

Positioning apparatus comprises a support structure 1 bearing first 2 and second 3 carriage members which are movable respectively, relative to the support structure, along first X and Z mutually-transverse axes. The first carriage member 2 carries first 213b and second 213a reflecting devices which extend parallel respectively to the said first and second axes. The second carriage member 3 carries first and second interferometers 214b, 214a which respectively direct light beams onto the first and second reflecting devices 213b, 213a for reflection back to the interferometers. Interference in the first and second interferometers 214a, 214b is monitored to derive measures of relative displacement between the said first and second carriage members parallel to the second Z and first X axes respectively. Such apparatus can be employed in a measuring machine or in a machine tool such as a grinding/turning machine to determine displacement of a tool relative to a workpiece without reference to the machine base. <IMAGE>

Description

HIGH-PRECISION POSITIONAL CONTROL The present invention relates to high-precision position measurement and to high-precision positional control, for example in measuring machines or in precision machine tools such as grinding and turning machines for use in ductile-regime grinding of glass and other brittle materials to produce complex components having highly smooth surfaces suitable for optical applications.

Figure 1 of the accompanying drawings shows a diagrammatic perspective view of a precision grinding and turning machine which comprises a bed 1, known as a Tbed, which supports a first carriage (referred to alternatively as a workslide) 2 that is movable relative to the bed 1 along a first horizontal axis (the X-axis) of the machine. The T-bed 1 also supports a second carriage (toolslide) 3 which is movable relative to the bed 1 along a second horizontal axis (the Z-axis) perpendicular to the X-axis.

The workslide 2 is supported on the bed 1, at opposite sides of the toolslide 3 in the manner of a bridge spanning the toolslide 3. The workslide 2 supports a rotatable workspindle 4 which projects through a front face of the workslide 2 and whose axis of rotation is parallel to the said Z-axis. A workhead (faceplate) 5 is mounted on the forward end of the workspindle 4 and serves to retain a workpiece to be ground/turned when the machine is in use.

The toolslide 3 may comprise a rotary table 6, sometimes known as a B-axis rotary table, supported on a rotary table spindle 7 of the machine. The central axis of the spindle 7, about which it rotates, is parallel to a third axis (Y-axis) which extends vertically.

The rotary table 6 carries on its upper main face a toolpost 8 which at its upper end supports a turning tool 9 having a cutting edge made, for example, of diamond.

The workslide 2 and toolslide 3 have respective linear drive mechanisms 10 and 11, for example friction drive mechanisms, for displacing them along their respective axes of movement. Rotary drive means 12 and 13 are also provided for rotating the workspindle 4 and rotary table spindle 7.

Displacement monitoring means, denoted schematically at 14 and 16, are provided for measuring such linear displacement of the workslide 2 and toolslide 3 along their respective axes of movement. Such monitoring means may advantageously be based upon laser interferometry.

An angular position encoder 15, associated with the rotary table 6, produces data indicative of the angular position of the tool 9.

A tool setting station 17 is mounted on the front face of the workslide 2, adjacent to the workhead 5.

Respective horizontal and vertical air bearing probes (not shown) projecting from a corner portion of the tool setting station 17 serve to facilitate presetting of the position of the tool 9 prior to a machining operation.

Computer numeric control means 18 are provided for controlling operation of the machine.

The machine as a whole is preferably enclosed in a containment 19 (shown dotted in Fig. 1) supporting an oil shower thermal control system, in order to protect the machine components from undesirable environmental effects.

In use of the grinding and turning machine of Fig.

1, the workpiece to be machined is mounted on the front face of the workhead 5, and a tool 9 is mounted on the toolpost 8. Before machining can begin, the position of the tool must be preset, such that the cutting point of the tool (i.e. the point on the tool which contacts the workpiece) lies in a working disposition along the central axis of the rotary table spindle 7 and at the same height as the central axis of the workspindle 4. To facilitate such presetting, the workslide 2 is driven along the X-axis to a position such that the air bearing probes can move into contact with the cutting edge of the tool 9. The position of the tool may then be adjusted in situ with reference to measurement values provided by the air bearing probes, until the cutting edge of the tool is in the desired position.

The computer numeric control means 18 is supplied with control data defining a required profile of the workpiece. During machining this control data is translated by the control means into the necessary drive signals for the linear drive mechanisms of the workslide 2 and toolslide 3, and for the rotary drive means of the workspindle 4 and rotary table spindle 7, so as to cause the required profile to be generated.

As mentioned above, a precision grinding and turning machine may incorporate a laser interferometry arrangement for monitoring the respective positions of the workpiece and tool during machining. Highly precise position data from such an interferometry arrangement is then used by the machine control means to enable the tool to be positioned relative to the work piece in the manner required, during the machining operation, to obtain a desired workpiece profile.

In a previously-considered interferometry arrangement the displacements of the toolslide and workslide along their respective axes of movement (Z and X axes) are measured relative to the stationary bed of the machine on which the toolslide and workslide are movably supported. Thus, the bed, which is intermediate in the Kinematic chain between the tool and the workpiece, is also included in the metrology chain therebetween. Such inclusion of the bed in the metrology chain is undesirable because any distortion of the bed, due for example to changes in the shape of the bed brought about by changes in the loading pattern of the bed as the toolslide and workslide are moved, results in measurement errors.

Furthermore, in such an arrangement the detection of inevitable imperfections in the nominally-linear motions of the toolslide and workslide involves the monitoring of two interfaces (workslide/bed and bed/toolslide) requiring two sets of metrological equipment, introducing more error sources and necessitating the carrying out of two sets of calculations to determine the relative carriage positions.

According to the present invention there is provided positioning apparatus comprising: a support structure bearing first and second carriage members which are movable respectively, relative to the support structure, along first and second mutually-transverse axes; first and second reflecting devices carried by the said first carriage member and extending parallel respectively to the said first and second axes; a first displacementmeasuring device, including a first interferometer mounted on the said second carriage member, for employing a first light beam, directed onto the said first reflecting device and reflected back therefrom to produce interference in the said first interferometer, to produce a measure of relative displacement between the said first and second carriage members parallel to the said second axis; and a second displacement-measuring device, including a second interferometer mounted on the said second carriage member, for employing a second light beam, directed onto the said second reflecting device and reflected back therefrom to produce interference in the said second interferometer, to produce a measure of relative displacement between the said first and second carriage members parallel to the said first axis.

In such apparatus the measures of relative displacement are obtained directly between the two carriage members of interest, so that the support structure is not involved in the metrological chain.

Thus, distortions in the support structure, caused by displacement of the carriage members (which, in a machine tool, for example, may be quite heavy), do not introduce measurement error.

The apparatus may further comprise a third displacement-measuring device, including a third interferometer mounted on the said second carriage member, for employing a third light beam, directed onto the said first reflecting device and reflected back therefrom to produce interference in the said third interferometer, to produce a further measure of relative displacement between the said first and second carriage members parallel to the said second axis, which further measure together with the measure produced by the said first displacement-measuring device is employed to detect relative angular displacement between the said first and second carriage members about an axis perpendicular to the said first and second axes.

Such an extra interferometer is usefully provided in situations in which it is not practical to arrange the first interferometer so that it operates in line with the point of interest at which the first and second carriage members interact with one another. The two interferometers (first and third interferometers) measuring parallel to the said second axis permit relative angular error motion (caused, for example, by yaw of the first carriage member) to be detected directly. In a previously-considered arrangement for detecting angular error, individual measurements of carriage member position were made relative to the machine base, requiring more equipment, introducing more error sources and involving more calculations to detect the angular error.

In one application of the apparatus, the said first carriage member supports a workpiece to be machined when the apparatus is in use and the said second carriage member supports a tool for operating on the said workpiece. The apparatus is particularly useful in such a machine tool application because the relative displacement that is of interest there is that of the tool relative to the workpiece, the positions of the workpiece and tool relative to the machine support structure being of less relevance. The apparatus can be employed, for example, in a precision grinding and turning machine where the carriage members (workslide and toolslide) are very heavy and accordingly cause distortion of the machine support structure during displacement thereof.In previously-considered arrangements any such distortion has led to significant measurement errors because measurements were taken relative to the nominally-fixed base of the machine.

The said interferometers are preferably arranged at the same height, measured in a direction perpendicular to the said first and second axes, as the operating point of the said tool, so that each of the said light beams, at least in the region between the interferometer and reflecting device concerned, lies in a common plane which passes through the said operating point. This can reduce the undesirable effects on measurement accuracy of pitching of the first and second carriage members during displacement thereof.

The said second interferometer is advantageously mounted so that the said second light beam, between the said first interferometer and the said first reflecting device, extends along a line that is parallel to the said first axis and passes through the operating point of the said tool. This can reduce the undesirable effects on measurement accuracy of yaw of the second carriage member.

The said first carriage member preferably carries a tool setting station having means for determining the position of the said tool relative to the said first carriage member, before commencement of machining, to establish a reference position of the tool. The said means for determining the position of the tool may comprise air bearing probes operable to contact the tool.

In a preferred machine tool application of the apparatus, the said first carriage member carries a rotatable workhead for supporting such a workpiece and the said second carriage member comprises a rotary table carrying a toolpost for supporting such a tool.

In such an application the apparatus usefully also comprises tool error storage means connected with the said means for determining the position of the tool for storing measurement values provided thereby, the apparatus being operable, in a tool setting mode thereof, to cause the said rotary table to be rotated to bring the said tool to different angular positions so that the position of the operating point of the tool at each such angular position can be stored by the said tool error storage means.

The toolpost may have tool position adjustment means, for example screw-driven slides, operable to adjust the position of the tool as necessary to ensure that the operating point of the tool lies along the axis of rotation of the said rotary table when the apparatus is in use. Alternatively, if the tool has an arcuate cutting edge, of a predetermined radius, the tool position adjustment means may be operable to adjust the position of the tool as necessary to ensure that the centre of curvature of the said arcuate cutting edge lies along the axis of rotation of the said rotary table when the apparatus is in use.

The said tool position adjustment means may be operable also to adjust the position of the tool as necessary to ensure that the operating point/arcuate cutting edge of the tool lies along the axis of rotation of the said workhead when the apparatus is in use.

Preferably the said displacement-measuring devices each include means for deriving their respective light beams from a single laser source mounted on the said support structure, which source may comprise, for example, a stabilised He:Ne laser. Such a laser source can provide a highly stable source beam common to all the displacement-measuring devices and of a wavelength suitable for precision measurement purposes.

To ensure even better accuracy, the apparatus may further comprise a wavelength compensator disposed adjacent to the said interferometers so as to receive a reference beam, directed through the apparatus so as to be subject to ambient conditions similar to those to which the said light beams are also subject when the apparatus is in use, for detecting changes in the wavelength of the light making up those light beams.

Such changes in wavelength can then be compensated for when processing the measurement data provided by the interferometers.

Preferably, the said reference beam is derived from the same source as the said light beams so that it is closely matched to those light beams.

In view of variation of the wavelength of the light beams with ambient conditions in the apparatus, as the beams pass therethrough, it can be advantageous that the path length of each of the said light beams, at least in the region between the interferometer and reflecting device concerned, is substantially the same.

In a preferred machine tool application, the said first and second carriage members have respective linear drive mechanisms for displacing them along the said first and second mutually-transverse axes respectively.

The said first and second reflecting devices may comprise respective elongate planar reflecting surfaces and the apparatus advantageously comprises error map storage means storing, for each of the said reflecting devices, an error map, representative of non-linearity in the surface shape of the reflecting device concerned along its length, the said displacement-measuring devices being connected with the said error map storage means for employing the said error maps to compensate for any such non-linearity when processing measurement signals provided by the said interferometers.

Reference will now be made by way of example to the accompanying drawings, in which: Fig. 1 shows a diagrammatic perspective view of a grinding and turning machine; Fig. 2 shows a schematic perspective view of previously-considered laser metrology apparatus suitable for use in the Fig. 1 machine; Fig. 3 shows a schematic perspective view of laser metrology apparatus embodying the present invention; Fig. 4 shows a front elevational view of parts of a grinding and turning machine incorporating the metrology apparatus of Fig. 3; Fig. 5 shows a plan view corresponding to Fig. 4; and Fig. 6 shows a block diagram of machine control means for use in a grinding and turning machine.

Fig. 2 shows a schematic perspective view of a laser metrology apparatus previously considered for use in a precision grinding/turning machine. As discussed hereinbefore, such apparatus is required to measure respective displacements of the workpiece and the tool during machining.

In Fig. 2 a T-bed 1 of a machine such as that shown in Fig. 1 supports a movable workslide 2 carrying a workhead (not shown), and also supports a movable toolslide 3 comprising a rotary table (not shown) carrying a toolpost (not shown). The workslide is guided to move along a first horizontal axis (the X-axis) and the toolslide is guided to move along a second horizontal axis (the Z-axis) perpendicular to the X axis.

The apparatus is based upon heterodyne interferometry and includes a stabilised He:Ne laser 110, beam splitters lila to lllc, beam deflectors 112a to 112f, plane mirrors 113a to 113c, interferometers 114a to 114c, optical receivers 115a to 115c and an optical wavelength compensator 116. With the exception of the plane mirrors 113a to 113c the components 110 to 116 are all fixed relative to the machine bed 1.

The plane mirror 113a (referred to as the X-axis mirror) is mounted on the workslide 2 adjacent to the workhead at spindle centre height, and the two further mirrors 113b and 113c (referred to as the Z-axis mirrors) are mounted one above the other on the toolslide 3.

In use of the metrology arrangement of Fig. 2, the laser 110 produces a highly stable optical beam B which is split into three measuring beams M1, M2 and M3 which are directed via the interferometers 114a to 114c to be incident respectively on the X-axis mirror 113a and on the two Z-axis mirrors 113b and 113c. A reference beam R for the optical wavelength compensator 116 is also split off from the measuring beam M1.

The interferometer 114a facing the X-axis mirror 113a directs the beam M1 onto that mirror and receives the resulting reflected beam from the mirror. The interferometer 114a provides an output beam to the receiver 115a, from which the receiver derives a measure x of the displacement of the X-axis mirror 113a relative to the interferometer 114a. The measure x can thus be employed to determine the position of the workpiece along the X-axis.

Similarly, the interferometers 114b and 114c facing the Z-axis mirrors 113b and 113c provide respective output beams to the receivers 115b and 115c, from which those receivers derive respective measures Zl and z2 of Z-axis displacement of the mirrors 113b and 113c relative to the interferometers 114b and 114c. These measures can be employed to determine the position of the tool along the Z-axis, as explained in more detail hereinafter.

In the vicinity of the interferometer 114a, changes in the wavelength of the reference beam R (which wavelength varies with pressure, temperature and humidity) are detected by the compensator 116 so that such changes can be compensated for by the machine control means when deriving the measures x, zl, and z2 It will be apparent from Fig. 2 that although it is desirable to determine accurately the respective positions of the workpiece and the tool, in practice the metrological components must be located at a distance from the tool and workpiece.Because the arrangement does not measure the actual positions of the tool and workpiece, but rather measures the positions of respective metrological components (the plane mirrors) connected with the tool and workpiece, the machine control means must effectively infer from those measured positions the actual positions of the tool and workpiece.

However, such inference can give rise to error if the respective carriages supporting the workslide and toolslide are mechanically imperfect so as to give rise to pitch (angular movement of the carriage about a transverse axis), yaw (angular movement of the carriage about a vertical axis, perpendicular to the main face of the carriage) and roll (angular movement to the carriage about its intended axis of movement). In addition, it is possible for the guideways constraining the carriage movement to be out of alignment, so that the carriage does not move precisely along its intended axis of movement.

In the arrangement of Fig. 2, the X-axis mirror 113a is mounted on the workslide 103 at tool height so that the effects of errors due to pitching of the X-axis carriage, during movement thereof, are minimised.

However, the single X-axis mirror 113a does not permit account to be taken of errors due to yaw and roll of the X-axis carriage supporting the workslide 2. Such errors can result in the position of the workpiece being inferred incorrectly by the machine control means, so that during machining the workpiece may be positioned inaccurately. Nor is it possible in the Fig. 2 arrangement to determine whether the X-axis carriage is out of alignment, i.e. does not move precisely along the X-axis.

In the case of the Z-axis carriage 3 (carrying the toolslide), it is not possible to detect roll (but the effects of errors due to yawing of the Z-axis carriage, during movement thereof, are minimized because both the Z-axis mirrors and their interferometers 114b and 114c are arranged such that their measuring beams M2 and M3 lie in a vertical plane, parallel to the X-axis direction, containing the central axis of the rotary table carrying the tool). The two interferometers 114b and 114c permit pitch errors of the Z-carriage to be detected (on the basis of any difference between the two Z-axis measures Zl and z2). The two interferometers are required principally because the mounting point for the Z-axis mirrors must, for practical reasons, be at a different height from the tool itself.If only one Zaxis mirror were provided, at a height other than tool height, the effects of pitch errors on the Z-axis carriage measurement would tend to be more pronounced than in the case of the X-axis carriage, whose mirror 113a can be mounted at the same height as the workpiece.

Furthermore, it is not possible in the Fig. 2 arrangement system to detect any error in the alignment of the workslide along the Z-axis.

It should also be understood that the Fig. 2 arrangement provides respective measures x, Zl and z2 representing displacements of the tool and workpiece relative to the fixed base (T-bed) of the machine, these measures being then used by the machine control means to calculate the displacement of the tool relative to the workpiece, which is the displacement that is of real interest. In use of the machine, the loading pattern of the base changes as the carriages supporting the tool and workpiece are displaced. The carriages can be up to 2 tonnes in weight, resulting in a deflection of the base of up to 1pm as they move.Thus, because in the Fig. 2 metrological arrangement the relative displacement between the tool and workpiece is determined on the basis of a metrological chain involving the base, distortions in the base result in significant measurement errors in high precision machining applications. Moreover, two interfaces are involved, namely workpiece/base and base/tool, in the metrological chain, so that two sources of error are introduced and more calculations must be performed (potentially slowing down machining and/or requiring greater processing power) to determine the position of the tool relative to the workpiece.

It is desirable to provide an improved laser metrology apparatus wherein displacement of the tool relative to the workpiece is determined more directly, without reference to the machine base.

Fig. 3 shows a schematic perspective view of such an improved laser metrology apparatus, embodying the present invention.

The apparatus comprises a laser 210 mounted on the machine base 1, beam splitters 211a to 211c, beam deflectors 212a to 212h, interferometers 214a to 214c, linear mirrors 213a to 213b, optical receivers 215a to 215c, and an optical wavelength compensator (refractometer) 216.

It will be appreciated that in the Fig. 3 arrangement the components 210 to 216 are, with the exception of the linear mirrors 213a and 213b, generally similar to the corresponding components 110 to 116 in Fig. 2. However, the components 210 to 216 of the Fig. 3 arrangement are mounted differently to the components 110 to 116 of the Fig. 2 arrangement, as will now be described.

In the Fig. 3 arrangement, a platform 220 is mounted, by way of an arm 221, on the toolslide 3 so as to be movable therewith. The platform 220 is arranged to be at the same height as the workspindle and tool.

The interferometers 214a to 214c are mounted on the platform 220, being spaced along the X-axis direction.

The linear mirrors 213a and 213b are carried at right angles to one another by the workslide 2, adjacent to the workhead 5. The mirrors 213a and 213b are perpendicular to one another, and face towards the interferometers 214a to 214c which are located in a first plane containing the intended point of contact between the tool and the workpiece (the operating point of the tool).

Of the remaining optical components, the beam deflectors 212f to 212h and the optical wavelength compensator 216 are located in a second plane, parallel to the first plane but a small height thereabove. The beam deflector 212f is carried by the base 1, and the deflectors 212g and 212h and the optical wavelength compensator 216 are carried by the workslide 2.

The remaining optical components: laser 210, beam deflectors 211a to 211c, and also the receivers 215a to 215c are carried by the machine base 1.

In use of the Fig. 3 arrangement, the laser 210 generates a highly stable optical beam B which is split to provide three measuring beams M1, M2 and M3 and a reference beam R.

The measuring beam M1 is directed by the interferometer 214a to be incident perpendicularly upon the linear mirror 213a and is reflected back by that mirror to the interferometer 214a. An output beam from the interferometer 214a is directed to the optical receiver 215a via the beam deflector 212c. It will be appreciated that the beam deflector 212c is a double beam deflector (as are the deflectors 212d and 212e) and serves not only to transmit the measuring beams M1 to the interferometer 214a but also to direct the output beam from that interferometer to the receiver 215a. The receiver 215a senses changes in the received beam and derives therefrom a measure x of the X-axis displacement of the tool relative to the workpiece.

The measuring beams M2 and M3 are directed by the interferometers 214b and 214c to be incident in parallel upon the linear mirror 213b and are reflected back in parallel by the mirror to these interferometers.

Respective output beams from the interferometers 214b and 214c are directed to the receivers 215b and 215c via the double beam deflectors 212d and 212e. The receivers 215b and 215c derive from these output beams respective measures Zl and z2 of the Z-axis displacement of the tool relative to the workpiece.

It will immediately be apparent that, by virtue of the platform 220, in the Fig. 3 arrangement the interferometers 214a to 214c are all mounted in the said first plane in which the tool contacts the workpiece.

Accordingly, errors due to pitch of the toolslide (and indeed the workslide) are minimised.

The interferometer 214a which measures parallel to the X-axis operates in line with the operating point of the tool, regardless of movements of the Z-axis. This means that the X-axis displacement measurement accords with the Abbe principle, so that the tool position in the X-axis direction is measured correctly in spite of small angular error motions of the carriages.

Practical considerations prevent the same principle from being applied to the Z-axis displacement measurement in the particular machine shown in Figs. 1 and 3.

However, substantially the same immunity to small angular error motions is obtained by measuring at the same height (in the Y-axis direction) as the tool and by using the two interferometers 214b and 214c to measure in the Zaxis direction. The two interferometers measure to the same reference straight-edge (linear mirror) 213b. The separation of the interferometers 214b and 214c in the Xaxis direction is known, as is the distance from each interferometer to the operating point. Thus, the angular error motion between the workslide 2 and toolslide 3 (relative angular displacement about an axis perpendicular to both the X and Z axes) can be determined on the basis of the ratio of the difference between the interferometer readings to the separation of the interferometers.Using this angular error ratio, the actual displacement measured by one of the interferometers 214b and 214c and the spacing, measured in the X-axis direction, of that one interferometer from the operating point of the tool, the machine control means can then calculate relative displacement between the tool and workpiece in the Z-axis direction satisfactorily despite the offset (Abbe offset) between the line of measurement and the cutting point.

Such offset (in the X-axis direction) arises because in the Fig. 3 arrangement the platform 220 is spaced laterally (in the X-axis direction) at some distance from the tool. The offset means that yaw of the toolslide 3 is potentially the most significant error motion, but such yaw can be detected in the Fig. 3 apparatus by virtue of the two Z-axis interferometers 214b and 214c, the degree of yaw being represented by the difference between the two measures Zl and z2 associated with these two interferometers.

It will also be apparent that the measures x, Zl and Z2 of the Fig. 3 arrangement are relative measures, indicating directly displacement of the tool relative to the workpiece. Thus, the Fig. 3 arrangement complies with the important metrological principle of measuring as directly as possible between the two points of interest (the tool which is on one carriage and the workpiece which is on the other carriage).

Angular (error) relative motion between the two carriages is also measured directly and can be compensated for by the machine control means with a single calculation. If this measurement were made relative to the base (as in previously-considered arrangements) there would be two interfaces to monitor (workslide to base, base to toolslide), requiring more equipment, introducing more error sources and calling for more calculations for compensation to be applied.

The arrangement of Fig. 3 can also permit straightness and orthogonality errors of the two carriages to be detected. If, for example, the workslide is maintained in a stationary position, and the toolslide is moved through its full stroke, any systematic alignment error (orthogonality error) of the toolslide will show up in the x measure (after the effects of toolslide yaw have been removed using the measures zl and z2).

In addition, any straightness error in the workslide can be detected by means of the interferometers 214b and 214c when the toolslide is held stationary.

To ensure optimum accuracy, it is possible to compensate for any straightness errors in the linear mirrors 213a and 213b themselves by deriving respective error maps for these mirrors. Such error maps can then be stored within the machine control means, to enable compensation for these errors to be carried out during machining.

Figs. 4 and 5 show a front elevation view and a plan view respectively of a grinding/turning machine incorporating the Fig. 3 laser metrology arrangement.

As shown in Fig. 4, the beam splitters 211a to 211c, the beam reflectors 212a and 212b, and the optical receivers 215a and 215c are all mounted in an enclosure 225 connected to the base 1 by buttons 226. The enclosure 225 serves to provide a controlled environment for these optical components and to exclude contaminants.

The laser beam from the laser 210 is delivered via a window in the base of the enclosure 225 to the beam splitter 212a, there being in the enclosure 225 two further beam deflectors 212i and 212j (not shown in Fig.

3) interposed between the window and the beam splitter 211a.

The beam deflectors 212c to 212e are mounted on a support 227 connected to the enclosure 225 by a bracket 228. The platform 227 supports the beam deflectors 212c to 212e in the above-mentioned first plane.

A further platform 230 above the support 227 carries the beam deflector 212f in the above-mentioned second plane.

A generally rectangular cover 231 is mounted fixedly on the front face of the workslide 2. Within the cover, a sub-frame 232 supports the mirrors 213a and 213b and the beam deflectors 212g and 212h, as well as the optical wavelength compensator (refractometer 216). The beam deflector 212h and the refractometer 216 are suspended from the sub-frame 232.

Figure 6 shows a block diagram of machine control means. Such control means may comprise, for example, the applicant's Cuproc CNC Series 3000 Advanced Computer Numeric Control System. Such control means may be used to control operation of a grinding and turning machine such as that shown schematically in Fig. 1.

As shown in Fig. 6 the measures x, Zl and z2 from the optical receivers 215a to 215c, the angular position data e from the angular position encoder 15 (see Fig. 1) are received by the machine control means 18. The control means 18 also includes a store 18a in which data is stored which defines the required workpiece profile.

In dependence upon the measures x, Zl and z2, the data stored in the store 18a, and the angular position data e the control means 18 supplies respective drive signals Dx and Dz to the drive mechanisms 10 and 11 (discussed hereinbefore with reference to Fig. 1) to control displacement of the workslide 2 and the toolslide 3 along their respective axes. The control means 18 also supplies drive signals Dw and Dg to the rotary drive means 12 and 13 of the workspindle 4 and the rotary table spindle 7.

Although the metrology arrangement of Figs. 3 to 5 is capable of high precision operation it is important that a reference position for the tool be established accurately before machining can begin.

A preferred toolpost for use in a precision grinding and turning machine such as that shown in Fig. 1 (described in the applicant's co-pending United Kingdom patent application no. 9110070.1) has only one positional adjustment mechanism, which permits the height of the tool (in the Y direction) to be controlled. For this reason, the tool must be preset to a nominal height and length relative to its tool carrier before the tool carrier is introduced to the machine. Such presetting may be preformed with the aid of an optical measuring system, and height and length adjustments are made to the tool carrier to bring the tool to a specified position.

Once preset in that position, the tool and its carrier are transferred to the toolpost on the machine. A typical tool set routine, involving the above-mentioned tool setting station 17, may then be followed, except that only tool height is adjusted while the tool coordinates are measured. This approach is sufficient for a fixed toolpost operation but additional measurements are required when the toolpost is mounted on a rotary table such as that shown in Fig. 1.

In such a case, the tool position is desirably preset such that during machining the operating (cutting) point of the tool lies along the axis of rotation of the table (i.e. along the central axis of rotary table spindle 7). Alternatively, the cutting edge of the tool, which may have a radius of for example 5mm, may be positioned such that the origin of the tool radius (i.e.

the centre of curvature of the cutting edge) lies along the axis of rotation of the table, so that the tool edge is concentric with that axis of rotation. Such adjustment is conventionally performed by adjustment of the toolpost. However, as explained hereinbefore, the screw-driven slide mechanisms used to perform such adjustments in conventional toolposts can give rise to positional errors.

In addition to such mechanical errors in the adjustments mechanisms, the rotary table spindle and/or toolpost may give rise to rotational errors at tool height, and the tool itself may have a radius error.

To overcome the above problems when employing a tool post having only a height adjustment (see UK application no 9110070.1), it is possible to measure all the abovementioned sources of errors in a single tool setting routine by rotating the rotary table supporting the tool post and taking measurements of the position of the cutting point relative to the fixed point of the tool set station (17 in Fig. 1). The information derived in this way is then stored in the memory of the machine control means to permit error compensation to be performed during machining.

According to one novel aspect of the tool error compensation system described above, there is provided a method of compensating for errors in the position of a rotatable tool, wherein the tool is moved to successive angular positions, and at each such position the location of the working point of the tool is measured by probe means brought into contact therewith, position data from the probes being stored and used subsequently to compensate for errors in the tool position.

According to another such aspect, for example, there is provided a tool setting apparatus, mounted on a movable workslide of the machine, which apparatus comprises measurement means that can be brought selectively into contact with a rotatable tool of the machine to determine the position thereof at successive angular positions of the tool, and which also comprises storage means for storing data indicative of the tool position at each such angular position.

Claims

CLAIMS:

1. Positioning apparatus comprising: a support structure bearing first and second carriage members which are movable respectively, relative to the support structure, along first and second mutually-transverse axes; first and second reflecting devices carried by the said first carriage member and extending parallel respectively to the said first and second axes; a first displacement-measuring device, including a first interferometer mounted on the said second carriage member, for employing a first light beam, directed onto the said first reflecting device and reflected back therefrom to produce interference in the said first interferometer, to produce a measure of relative displacement between the said first and second carriage members parallel to the said second axis; and a second displacement-measuring device, including a second interferometer mounted on the said second carriage member, for employing a second light beam, directed onto the said second reflecting device and reflected back therefrom to produce interference in the said second interferometer, to produce a measure of relative displacement between the said first and second carriage members parallel to the said first axis.

2. Positioning apparatus as claimed in claim 1, further comprising a third displacement-measuring device, including a third interferometer mounted on the said second carriage member, for employing a third light beam, directed onto the said first reflecting device and reflected back therefrom to produce interference in the said third interferometer, to produce a further measure of relative displacement between the said first and second carriage members parallel to the said second axis, which further measure together with the measure produced by the said first displacement-measuring device is employed to detect relative angular displacement between the said first and second carriage members about an axis perpendicular to both of the said first and second axes.

3. Positioning apparatus as claimed in claim 1 or 2, wherein the said first carriage member supports a workpiec#e to be machined when the apparatus is in use and the sssid second carriage member supports a tool for operating on the said workpiece.

4. Positioning apparatus as claimed in claim 3, wherein the said interferometers are arranged at the same height, measured in a direction perpendicular to the said first and second axes, as the operating point of the said tool, so that each of the said light beams, at least in the region between the interferometer and reflecting device concerned, lies in a common plane which passes through the said operating point.

5. Positioning apparatus as claimed in claim 3 or 4, wherein the said second interferometer is mounted so that the said second light beam, between the said first interferometer and the said first reflecting device, extends along a line that is parallel to the said first axis and passes through the operating point of the said tool.

6. Positioning apparatus as claimed in any one of claims 3 to 5, wherein the said first carriage member carries a tool setting station having means for determining the position of the said tool relative to the said first carriage member, before commencement of machining, to establish a reference position of the tool.

7. Positioning apparatus as claimed in claim 6, wherein the said means for determining the position of the tool comprises air bearing probes operable to contact the tool.

8. Positioning apparatus as claimed in any one of claims 3 to 7, wherein the said first carriage member carries a rotatable workhead for supporting such a workpiece and the said second carriage member comprises a rotary table carrying a toolpost for supporting such a tool.

9. Positioning apparatus as claimed in claim 8 read as appended to claim 6 or 7, further comprising tool error storage means connected with the said means for determining the position of the tool for storing measurement values provided thereby, the apparatus being operable, in a tool setting mode thereof, to cause the said rotary table to be rotated to bring the said tool to different angular positions so that the position of the operating point of the tool at each such angular position can be stored by the said tool error storage means.

10. Positioning apparatus as claimed in claim 8 or 9, wherein the toolpost has tool position adjustment means operable to adjust the position of the tool as necessary to ensure that the operating point of the tool lies along the axis of rotation of the said rotary table when the apparatus is in use.

11. Positioning apparatus as claimed in claim 8 or 9, wherein the said tool has an arcuate cutting edge, of a predetermined radius, and the toolpost has tool position adjustment means operable to adjust the position of the tool as necessary to ensure that the centre of curvature of the said arcuate cutting edge lies along the axis of rotation of the said rotary table when the apparatus is in use.

12. Positioning apparatus as claimed in claim 10 or 11, wherein the said tool position adjustment means are operable also to adjust the position of the tool as necessary to ensure that the operating point of the tool lies along the axis of rotation of the said workhead when the apparatus is in use.

13. Positioning apparatus as claimed in any preceding claim, wherein the said displacement-measuring devices each include means for deriving their respective light beams from a single laser source mounted on the said support structure.

14. Positioning apparatus as claimed in claim 13, wherein the said laser source comprises a stabilised He:Ne laser.

15. Positioning apparatus as claimed in any preceding claim, further comprising a wavelength compensator disposed adjacent to the said interferometers so as to receive a reference beam, directed through the apparatus so as to be subject to ambient conditions similar to those to which the said light beams are also subject when the apparatus is in use, for detecting changes in the wavelength of the light making up those light beams.

16. Positioning apparatus as claimed in claim 15, wherein the said reference beam is derived from the same source as the said light beams.

17. Positioning apparatus as claimed in any preceding claim, wherein the path length of each of the said light beams, at least in the region between the interferometer and reflecting device concerned, is substantially the same.

18. Positioning apparatus as claimed in any preceding claim, wherein the said first and second carriage members have respective linear drive mechanisms for displacing them along the said first and second mutually-transverse axes respectively.

19. Positioning apparatus as claimed in any preceding claim, wherein the said first and second reflecting devices comprise respective elongate planar reflecting surfaces.

20. Positioning apparatus as claimed in claim 19, further comprising error map storage means storing, for each of the said reflecting devices, an error map, representative of non-linearity in the surface shape of the reflecting device concerned along its length, the said displacement-measuring devices being connected with the said error map storage means for employing the error maps to compensate for any such non-linearity when processing measurement signals provided by the said interferometers.

21. Positioning apparatus substantially as hereinbefore described with reference to Figs. 1 and 3 to 6 of the accompanying drawings.

22. A method of employing apparatus as claimed in claim 9, wherein the said measurement values stored by the said tool error storage means are used by control means of the apparatus to compensate for errors in the position of the said tool, as it is moved between the said different angular positions, during machining.