GB2244822A - Monitoring and control of surface curvature - Google Patents

Abstract

There is described an arrangement for monitoring and control of the surface curvature of a deformable platen, which is used in equipment for the accurate grinding of surfaces. The lapping platen 12 is mounted peripherally on turntable 14 such that a gap 20 is formed. Whilst the platen is rotating, transducers 30 and 92 measure the displacement of the platen at respectively its centre and a point corresponding to the location of a workpiece. The displacement signals are filtered and compared with a setpoint in control unit 74, the results of said comparison being used to control the application of a fluid-pressure source, preferably a vacuum source, to the air gap 20 such that the curvature of the platen surface 18 is adjusted to a degree required for obtaining a precise flat surface on the workpiece. Details of the preferred displacement transducers (30, 92) are disclosed (Figs 2, 3). This arrangement is used to overcome the problem of unevenly finished surfaces of a workpiece caused by thermal distortion effects and the workpiece rocking slightly in its holder. <IMAGE>

Description

"Monitorina and Control of Surface Curvature" This invention relates to the monitoring and control of surface curvature of a deformable platen, and more particularly but not exclusively to the monitoring and control of the degree of flatness of the working surface of a lapping platen.

In the field of forming a highly precise flat surface on a workpiece, for example, an engineering component or an optical lens, it is standard practice to lap the workpiece surface on the working surface of a lapping platen. The lapping platen is normally rotated about a vertical axis with the working surface horizontal and facing upwards. The workpiece is laid on the working surface of the lapping platen with the workpiece surface to be lapped flat facing downwards and resting on the working surface of the lapping platen at a location radially offset from the rotational axis of the lapping platen.The workpiece is loosely constrained from being dragged around the lapping platen as the platen rotates, but the workpiece is usually free to rotate around its own vertical axis, for example by the workpiece being generally circular and located within a somewhat larger yoke anchored to a suitable static point near the lapping platen. A liquid suspension of an abrasive material is spread as a thin film on the lapping platen such that as the lapping platen rotates under the workpiece, the underside of the workpiece is ground and polished.

The working surface of the lapping platen is nominally flat such that the lapped surface of the workpiece supposedly becomes truly flat. However, because there is a tendency for the workpiece to rock slightly within its yoke, and also for the workpiece to undergo thermally-induced distortion due to frictional heating, the working surface of the lapping platen may actually need to be of slightly concave or convex form to produce a workpiece surface with the requisite degree of flatness. Moreover, as the working surface of the lapping suffers wear in use, this must also be compensated.

A conventional procedure for measuring the degree of flatness of the working surface of a lapping platen involves making interferometric measurements, a time-consuming process requiring skilled personnel.

The curvature of the working surface can then be adjusted by application of a selected weight. Needless to say, the lapping platen is incapable of productive use during measurement and adjustment of the degree of flatness of its working surface.

Accordingly it is an object of the invention to obviate or mitigate these disadvantages of conventional equipment and procedures for the monitoring and control of surface curvature of a platen.

According to a first aspect of the present invention there is provided an arrangement for the monitoring and control of the surface curvature of a deformable platen, said arrangement comprising platen mounting means for mounting the deformable platen in a manner which allows controllable deformation thereof, displacement measuring means for measuring the displacement of a predetermined portion of the platen as the platen undergoes deformation and for producing a corresponding displacement signal, a controllable fluid pressure source coupled to receive said displacement signal and operable to produce a fluid pressure controlled to a magnitude dependent on the deviation of said displacement signal from a setpoint, and fluid pressure coupling means coupling said pressure source to said platen mounting means for application of said controlled fluid pressure to the deformable platen in a sense tending to diminish said deviation by fluid-pressure-induced variation of the deformation of said platen.

Said source of fluid pressure is preferably a vacuum source, such that a controlled sub-atmospheric air pressure is applied in use to one major face of the deformable platen while ambient atmospheric air pressure acts on the other major face of the deformable platen to provide a differential air pressure thereacross tending to cause controlled deformation of said platen with consequent control of the surface curvature of said other major face of the deformable platen.

Said platen mounting means is preferably formed to support the periphery of the deformable platen.

According to a second aspect of the present invention there is provided an overall displacement monitor for use as the displacement measuring means of an arrangement according to the first aspect of the present invention wherein said platen mounting means is formed to support the periphery of the deformable platen, said displacement monitor comprising a linear displacement transducer and transducer mounting means for mounting said transducer in a position such that said transducer measures the linear displacement of a substantially central portion of said one major face of the deformable platen with respect to said platen mounting means, in a direction substantially normal to said one major face as a measurement of the overall surface curvature of said deformable platen.

According to third aspect of the present invention there is provided a local displacement monitor for use as the displacement measuring means of an arrangement according to the first aspect of the present invention wherein said platen mounting means is formed to support the periphery of the deformable platen, said displacement monitor comprising a linear displacement transducer and transducer mounting means for mounting said transducer in a position such that said transducer measures the linear displacement of an non-central portion of said other major face of the deformable platen with respect to adjacent portions thereof, in a direction substantially normal to said other major face as a measurement of the local surface curvature of said deformable platen.

Said local displacement monitor preferably comprises a flexible diaphragm disposed to have one face thereof in substantially overall contact with said non-central portion of said other major face of the deformable platen, the transducer being supported on or adjacent the periphery of said flexible diaphragm and bearing on a substantially central portion of said diaphragm to measure the displacement thereof in substantially direct correspondence with the displacement of the underlying portion of the deformable platen. Said flexible diaphragm is preferably formed as a glass disc.

According to a fourth aspect of the present invention there is provided a method of adjusting the surface curvature of a peripherally-supported lapping platen, said method comprising the step of applying a sub-atmospheric air pressure to the underside of the lapping platen whereby ambient atmospheric pressure acting on the upper or working surface of the lapping platen in combination with said sub-atmospheric air pressure on the underside of the lapping platen produces a differential air pressure on the lapping platen to deform the platen centrally downwards relative to the periphery of the platen.

According to a fifth aspect of the present invention there is provided a lapping apparatus, said apparatus comprising a peripherally-supported lapping platen, and vacuum-applying means for applying a sub-atmospheric air pressure to the underside of the lapping platen to vary the curvature of the upper or working surface of the lapping platen.

The lapping platen is preferably mounted with its upper or working surface substantially horizontal and rotatably supported for rotation about a substantially vertical axis.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings wherein Fig. 1 is a schematic diagram of a lapping apparatus together with an arrangement for monitoring and control of the surface curvature of the lapping platen; Fig. 2 is a vertical section to an enlarged scale of the lapping apparatus of Fig. 1, incorporating an overall surface curvature monitor; and Fig. 3 is a vertical section to a much-enlarged scale of a local surface curvature monitor for use in the lapping apparatus of Fig. 1.

Referring first to Fig. 1, a lapping apparatus 10 comprises a circular lapping platen 12 peripherally supported on a turntable 14 which is rotatably mounted on a vertical spindle assembly 16 (detailed in Fig. 2).

The upper or working surface 18 of the lapping platen 12 is substantially horizontal, while the spindle assembly 16 rotates the turntable 14 and the lapping platen 12 supported theron about a substantially vertical central axis.

As is diagrammatically depicted in Fig. 1, the lapping platen 12 is supported only at its periphery on the turntable 14 to leave a gap 20 therebetween at all non-peripheral regions, such that the centre of the lapping platen 12 can vary its separation from the turntable 14 under the influence of variations in vertical loading on the platen 12. The lapping platen 12 is sealed at its periphery to the turntable 14 such that the volume of the gap 20 is air-tight. Vertical loading is applied to the lapping platen 12 in a downward direction by the application of a "vacuum" or sub-atmospheric air pressure to the gap 20. The ambient atmospheric air pressure simultaneously acting on the upper or working surface 18 results in a differential air pressure across the lapping platen 12 from top to bottom, causing the platen 12 to bow centrally downwards relative to its supported periphery.Even when the lapping platen 12 is formed of a nominally rigid material such as steel or marble, such material has a non-zero elasticity, and the differential air pressure induced by the vacuum applied to the underside of the platen 12 is sufficient when acting over the area of the platen 12 to cause the platen 12 to deflect in the manner of a uniformly loaded elastic beam end-supported on pin joints, the circular platen 12 being the rotational development of such a notional beam. Even a central deflection of the lapping platen 12 by a few hundred microns is sufficient to vary the overall surface curvature of the working surface 18 by an amount which is adequate for the purposes of precision lapping.

The vacuum is applied to the gap 20 through a vacuum conduit 22 incorporated into the spindle assembly 16, and arranged to connect a suitable external vacuum source (detailed subsequently) to the turntable 14.

Since the external vacuum source will be static and the turntable 14 will be rotating in use of the lapping apparatus 10, the vacuum conduit 22 and the spindle assembly 16 function, inter alia, as a vacuum 'slip-ring' transferring the sub-atmospheric pressure in a substantially leak-tight manner between static and rotating assemblies. Thus the present invention enables the curvature of the working surface 18 to be adjusted during use. of the lapping apparatus 10, without the turntable 14 having to be stopped for such adjustments. This facility is in contrast to prior art lapping apparatus wherein rotation of the lapping table had to be stopped during adjustments of working surface curvature, for example by the removal or addition of discrete weights.

The lapping platen 12 can be formed to have a predetermined degree of flatness of its working surface 18 in the absence of a differential air pressure between its top and bottom surfaces, and when peripherally supported to carry only its own weight, without external loading. For example, the platen 12 may be formed with the working surface 18 substantially flat, or convex (centre higher than periphery), or concave (centre lower than periphery). Since the method of adjusting the curvature of the working surface 18 results in the centre of the lapping platen 12 becoming increasingly lower with respect to the periphery of the platen 12 with increasing levels of curvature-adjusting vacuum, it is preferred that the platen 12 be formed to have its working surface 18 initially convex, ie with its centre marginally higher than its periphery.With a suitable choice of initial (zero pressure differential) dimensions, the working surface 18 can be adjusted from convexity through flatness to concavity with levels of vacuum in the gap 20 varying from zero to a theoretical maximum of 1 Bar sub-atmospheric. However, any requisite degree of initial flatness, convexity, or concavity can be selected according to working requirement of the lapping apparatus.

The level of vacuum applied to the gap 20 in use of the lapping apparatus 10 can be selected empirically, or by indirect measurement, for example by measuring the degree of flatness of finished lapped surfaces produced on the working surface 18 of the lapping platen 12.

However, better control of flatness may be achieved by direct measurement of the curvature of the working surface 18 during operation of the lapping apparatus 10, such that on-line control can be exercised.

Surface curvature monitoring arrangements will now be described by way of examples. An overall surface curvature monitoring arrangement will first be described, and followed by a description of an arrangement for monitoring of local surface curvature.

The overall surface curvature monitoring arrangement functions by measuring the deflection of the centre of the lapping platen 12 relative to the turntable 14 which peripherally supports the platen 12. This monitoring arrangement comprises a linear displacement transducer 30 schematically depicted in Fig. 1, and detailed in Fig. 2 to an enlarged scale. The transducer 30 has its static body 32 mounted in a static central hub 34 (Fig. 2) within the spindle assembly 16. A displacement-sensing linearly movable transducer plunger 36 is disposed to bear against the centre of the underside of the lapping platen 12, to sense the displacement of the centre of the lapping platen 12 along its vertical axis of rotation, ie normal thereto, and to produce a proportionate electrical displacement signal as the transducer output. The transducer plunger 36 may bear directly on the underside of the lapping platen 12, or a suitable thrust bearing arrangement can be interposed therebetween to compensate for the rotation of the platen 12 relative to the stationary transducer 30 and obviate wear at the contact point of the transducer plunger 36. Adjustment means (not shown) may be provided to permit initial mains calibration and setting of the transducer plunger. The transducer output signal is carried on an electrical signal lead 38 which passes through an air-tight gland 40 in one branch of a T-piece 42 attached to the lower end of the hub 34. The other branch of the T-piece 42 constitutes the external connection point of the vacuum conduit 22.

To transfer the vacuum to the gap 20 between the turntable 14 and the underside of the lapping platen 12, the hub 34 is hollow and provided with a port 44 leading to a space 46 within the hollow spindle assembly 16. In turn, the space 46 communicates through a port 48 in the turntable 14 to the gap 20.

The spindle assembly 16 is rotatably mounted on the static central hub 34 by an upper bearing 50 and a lower bearing 52 which each provide radial and axial support. A pulley 54 secured to the lower end of the spindle assembly 16 provides for a belt drive (not shown) to rotate the turntable 14.

Vacuum-tightness of the space 46 in particular and of the vacuum 'slip-ring' in general is assured by a lip seal 56 extending between the hub 34 and the spindle assembly 16 immediately above the upper bearing 50.

A further lip seal 58 is fitted between the upper end of the hub 34 and a central bore 60 in the turntable 14 which allows the transducer plunger 36 to contact the underside of the lapping platen 12. This further seal 58 inhibits the ingress of abrasive material and debris to the hub assembly 16 when the lapping platen 12 is not in place on the turntable 14.

The turntable 14 has a shallow upstanding peripheral annulus 62 upon which the lapping platen 12 rests to be peripherally supported thereby. The lapping platen 12 is elsewhere vertically spaced above the upper surface of the turntable 14 (when not depressed by differential air pressure) so as to create the gap 20. A large-diameter 0-ring 64 encircles the annulus 62 to form a vacuum-tight seal between the platen 12 and the turntable 14 around the outer edge of the gap 20. The lapping platen 12 is laterally located against radial and angular displacement relative to the turntable 14 by means of intervening locating pins 66.

The lapping platen 12 is vertically retained upon the turntable 14 by the weight of the platen 12 (augmented by whatever sub-atmospheric pressure exists in the gap 20), and specifically without the use of screws or other such fasteners. The absence of clamping fasteners eliminates a source of localised distortions in the lapping platen 12, and so obviates resultant imperfections in the flatness of the working surface 18.

Reverting to Fig. 1, the deflection measurement signal output from the transducer 30 along the lead 38 is passed through a signal path changeover switch relay 70 (whose purpose will be described below), and a signal conditioner circuit 72 for amplification and/or noise reduction of the transducer output, to a filter/control circuit 74 for reduction or elimination of unwanted components of frequency and amplitude, and exercise of a control function to be detailed below.

A display unit 76 is coupled to the filter/control circuit 74 in order.to give a display (preferably visual) of the deflection of the centre of the lapping platen 12 as measured by the linear displacement transducer 30.

The filter/control circuit 74 may incorporate a manual control (not shown) by which vacuum control is manually exercised in accordance with the deflection measurement as displayed by the display unit 76. The filter/control circuit 74 may alternatively incorporate a set-point circuit (not shown) and a differential amplifier circuit (not shown) or the like by which negative feedback control of vacuum is automatically carried out, with the display unit 74 serving as confirmation of the correct functioning of the automatic control circuit. A manual trimming control (not shown) may be incorporated in the set-point circuit for manual trimming of the set-point level.

Whether functioning manually or automatically, the control output of the filter/control circuit 74 actuates a stepper-motor-controlled valve 78 coupling a vacuum pump 80 to the vacuum conduit 22 so as to apply the requisite level of vacuum to the gap 20 to cause the appropriate level of vacuum-induced central deflection of the lapping platen 12 and hence of overall curvature of the working surface 18 of the lapping platen 12.

A further surface curvature monitoring arrangement 90, for monitoring local surface curvature of the working surface 18, functions by resting under its own weight on, and monitoring the deflection of, a portion of the working surface 18 at a location radially between the vertical rotation axis of the lapping platen 12 and periphery of the platen 12, as schematically depicted in Fig. 15 This non-central portion of the working surface 18 radially offset between central rotation axis and periphery approximates to the radial position of a workpiece (not shown) being lapped on the lapping platen 12 in use of the lapping apparatus 10, and hence the local deflection measurement from the monitoring arrangement 90 is a more accurate measurement of the degree of flatness of the portion of the working surface 18 under the workpiece as compared to overall curvature measurement produced by the central deflection monitoring transducer 30, albeit that the deflection measured by the local monitoring arrangement 90 will have a smaller magnitude than that of the deflection measured by the transducer 30 of the monitoring arrangement internal to the spindle assembly 16. The monitoring arrangement 90 is loosely restrained against being dragged around the lapping platen 12 as the platen rotates, by means of an anchored yoke (not shown) similar to that employed to restrain a workpiece.

The curvature monitoring arrangement 90 is schematically depicted in Fig. 1 and shown in vertical section to a much-enlarged scale in Fig. 3. The monitoring arrangement 90 comprises a linear displacement transducer 92 having a body 94 rigidly mounted in a hollow monitor housing 96 formed with a relatively broad base 98. A glass disc 100 covers the bottom face of the monitor housing base 98 which is formed with a peripheral shoulder 102 such that the base 98 rests on the periphery of the disc 100 but does not rest on the central area of the disc 100. The glass disc 100 is laterally restrained to remain directly under the monitor housing 96 by means of a base rim member 104. with an interposed rubber ring 106 acting as both resilient edge restraint and seal to inhibit ingress of abrasive material and debris.

A downwardly extending transducer plunger 108 projects from the transducer body 94 to rest against the centre of the glass disc 100 such that the transducer 92 measures the vertical displacement of the centre of the glass disc 100 relative to the periphery of the glass disc 100 which is rested on by the shoulder 102 of the monitor housing base 98.

The glass disc 100 functions as a flexible diaphragm to transmit the local curvature of the underlying portion of the working surface 18 to the transducer 92, the response of the glass disc 100 in this curvature-transmitting function mimicking that of a glass workpiece. The glass disc 100 also functions to prevent the tip of the transducer plunger 108 from gouging the working surface 18 as the lapping platen 12 is rotated under the curvature monitoring arrangement 90.

As previously described, the local curvature monitoring arrangement 90 is loosely restrained against rotation with the lapping platen 12 by means of an externally anchored restraint yoke similar to that employed for loosely restraining a workpiece. Such a yoke allows the monitoring arrangement 90 to rotate about its own vertical axis under the influence of differential drag across the width of the disc 100, as does a workpiece being lapped, thus further improving the accuracy of the response of the monitoring arrangement 90 to variations in local curvature of the underlying portion of the working surface 18. However, the consequent rotation of the transducer body 94 requires that its output signal be coupled to the immobile deflection monitoring and vacuum control equipment (70-80 in Fig.

1) through a signal transmitting arrangement that allows such rotation to be indefinitely sustained without loss of signal. To this end, the upper part of the hollow monitor housing 96 is fitted with an electrical slip-ring assembly 110. The output signal lead 112 from the transducer 92 is connected to the lower end of the slip-ring assembly 110. The relatively rotatable upper end of the slip-ring assembly 110 supports a sub-housing 114 which does not rotate with the monitor housing 96, and through which a continuation 116 of the output signal lead 112 passes from the upper end of the slip-ring assembly 110 to the signal path changeover switch relay 70 (Fig. 1).The output signal lead continuation 116 (and a possible flexible conduit therefor) can serve to anchor the sub-housing 114 against rotation with the remainder of the curvature monitoring arrangement 90 in a manner imposing no vertical or toppling loads impairing correct deflection measuring operation of the transducer 92.

The changeover switch relay 70 can be operated manually or automatically to change the signal received by the circuits 72 and 74 from the overall deflection signal on the signal lead 38 to the local deflection signal on the signal lead continuation 116, and vice versa. Thus it is possible in accordance with operator wishes or automatic control circuit requirements, to change over between the relatively coarse deflection signal from the overall deflection monitoring transducer 30 to the relatively fine deflection signal from the local deflection monitoring transducer 92.

In a typical practical embodiment of the lapping apparatus 10, the lapping platen 12 would have an outside diameter of about 30 centimetres, with lapping taking place in a peripheral zone of about 10-12 centimetres radius. The material of the lapping platen 12 could be stainless steel with a thickness of 8 millimetres, or an unplasticised polymer such as polyurethane or "polytron" (Trade Name) with a thickness of 2.4 millimetres. The lapping platen 12 could alternatively be a disc of marble or a similar mineral.

A suitable working range for the overall curvature-measuring linear displacement transducer 30 is plus/minus 300 microns, and a suitable working range for the local curvature-measuring linear displacement transducer 92 is plus/minus 10 microns.

Modifications and variations in the above-described arrangements can be made without departing from the scope of the invention.

Claims (13)

1. An arrangement for the monitoring and control of the surface curvature of a deformable platen, said arrangement comprising platen mounting means for mounting the deformable platen in a manner which allows controllable deformation thereof, displacement measuring means for measuring the displacement of a predetermined portion of the platen as the platen undergoes deformation and for producing a corresponding displacement signal, a controllable fluid pressure source coupled to receive said displacement signal and operable to produce a fluid pressure controlled to a magnitude dependent on the deviation of said displacement signal from a setpoint, and fluid pressure coupling means coupling said pressure source to said platen mounting means for application of said controlled fluid pressure to the deformable platen in a sense tending to diminish said deviation by fluid-pressure-induced variation of the deformation of said platen.

2. An arrangement as claimed in Claim 1 wherein said source of fluid pressure is a vacuum source, such that a controlled sub-atmospheric air pressure is applied in use to one major face of the deformable platen while ambient atmospheric air pressure acts on the other major face of the deformable platen to provide a differential air pressure thereacross tending to cause controlled deformation of said platen with consequent control of the surface curvature of said other major face of the deformable platen.

3. An arrangement as claimed in either Claim 1 or 2 wherein said platen mounting means is formed to support the periphery of the deformable platen.

4. A displacement monitor for use as a displacement measuring means as claimed in Claims 1 to 3 wherein said platen mounting means is formed to support the periphery of the deformable platen, said displacement monitor comprising a linear displacement transducer and transducer mounting means for mounting said transducer in a position such that said transducer measures the linear displacement of a substantially central portion of said one major face of the deformable platen with respect to said platen mounting means, in a direction substantially normal to said one major face as a measurement of the overall surface curvature of said deformable platen.

5. A displacement monitor for use as the displacement measuring means of an arrangement as claimed in any one of Claims 1 to 3 wherein said platen mounting means is formed to support the periphery of the deformable platen, said displacement monitor comprising a linear displacement transducer and transducer mounting means for mounting said transducer in a position such that said transducer measures the linear displacement of an non-central portion of said other major face of the deformable platen with respect to adjacent portions thereof, in a direction substantially normal to said other major face as a measurement of the local surface curvature of said deformable platen.

6. A monitor as claimed in Claim 5 and including a flexible diaphragm disposed to have one face thereof in substantially overall contact with said non-central portion of said other major face of the deformable platen, the transducer being supported on or adjacent the periphery of said flexible diaphragm and bearing on a substantially central portion of said diaphragm to measure the displacement thereof in substantially direct correspondence with the displacement of the underlying portion of the deformable platen. Said flexible diaphragm is preferably formed as a glass disc.

7. A method of adjusting the surface curvature of a peripherally-supported lapping platen, said method comprising the step of applying a sub-atmospheric air pressure to the underside of the lapping platen whereby ambient atmospheric pressure acting on the upper or working surface of the lapping platen in combination with said sub-atmospheric air pressure on the underside of the lapping platen produces a differential air pressure on the lapping platen to deform the platen centrally downwards relative to the periphery of the platen.

8. A lapping apparatus, said apparatus comprising peripherally-supported lapping platen, and vacuum-applying means for applying a sub-atmospheric air pressure to the underside of the lapping platen to vary the curvature of the upper or working surface of the lapping platen.

9. Lapping apparatus as claimed in Claim 8 wherein the lapping platen is preferably mounted with its upper or working surface substantially horizontal and rotatably supported for rotation about a substantially vertical axis.

10. An arrangement for the monitoring and control of the surface curvature of a deformable platen substantially as hereinbefore described with reference to the accompanying drawings.

11. A displacement monitor for use as a displacement measuring means substantially as hereinbefore described with reference to the accompanying drawings.

12. A method of adjusting the surface curvature of a peripherally-supported lapping platen substantially as hereinbefore described with reference to the accompanying drawings.

13. 'lapping apparatus substantially as hereinbefore described with reference to the accompanying drawings.