GB2218227A - Controlling surface sensing apparatus - Google Patents

Abstract

A metrological instrument has a turntable for supporting a workpiece, a stylus 30 which is mounted on an arm 10 for movement relative thereto towards and away from the workpiece as the turntable is rotated with the workpiece thereon, and a motor for driving the arm towards and away from the workpiece. A transducer 58 coupled to the stylus produces a signal dependent upon movement of the stylus 30, relative to the arm 10, from a null position. A microprocessor controls the power applied to the motor in accordance with the magnitude of the signal from the transducer to tend to maintain the stylus in its null position. The microprocessor utilises a cubit transfer function so that the power applied to the motor is almost zero when the signal from the transducer is within a predetermined range on either side of the null value in order to minimise hunting. The transfer function is also such that the power applied to the motor reaches its maximum value before the stylus reaches its maximum deflection to ensure that the stylus reliably follows the surface of the workpiece. <IMAGE>

Description

SURFACE SENSING APPARATUS This invention relates to surface sensing apparatus and is particularly applicable to such apparatus for use in a metrological instrument in which workpiece characteristics such as form are to be sensed.

In many conventional metrological instruments, the range of operation of the surface sensor is very limited as a result of which the sensor may have to be repositioned a number of times, under operator control, during a measuring operation. In order to avoid this, our European Patent Application 0240151 discloses a system in which a stylus for engaging the workpiece surface is mounted on an arm which is radially movable relative to a turntable on which the workpiece is mounted. In operation, the workpiece is rotated with the stylus in contact with the surface thereof and the radially movable arm is driven towards or away from the workpiece in response to deflection of the stylus so as to maintain the stylus in contact with the workpiece surface and to maintain the deflection of the stylus within the range of operation of an associated transducer.Thus, as the workpiece rotates, the stylus is caused to "follow" the workpiece surface automatically. In effect, the radially movable arm upon which the stylus is mounted is driven in a direction to tend to maintain the stylus in a null position.

In one aspect of the present invention, a surface sensing apparatus comprises a surface sensor, support means carrying said surface sensor, drive means for driving the support means towards and away from a workpiece surface in operation of the apparatus in response to signals derived from the surface sensor in order to maintain the surface sensor in operative relationship with the workpiece surface, and means for at least reducing hunting of the surface sensor around a null position.

In an alternative aspect, the present invention provides a surface sensing apparatus comprising a surface sensor, support means for said surface sensor, drive means for moving said support means towards and away from the workpiece surface in response to signals from the surface sensor to maintain said surface sensor in operative relationship with said surface and means for causing said drive means to operate at increased speed with increasing signal from the surface sensor.

Either or both of said aspects may be realised by providing a non-linear transfer function between the signal from the sensor and the drive means.

In yet a further alternative aspect, the invention provides a surface sensing apparatus in which signals from a surface sensor are utilised for driving a support on which the surface sensor is mounted in order to maintain the surface sensor in operative relationship with the workpiece, such drive being accomplished utilising a non-linear transfer function.

Although, as explained above, such non-linear function may be arranged to avoid hunting and/or to provide speed control, the function may, within the scope of the invention, provide for additional or alternative effects.

In a preferred embodiment, the non-linear transfer function is a third order function.

The invention is described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic perspective view of a metrological instrument embodying a surface sensing apparatus according to a preferred embodiment of the invention; Fig. 2 is a diagrammatic sectional view through the surface sensing apparatus of the instrument of Fig.

1; Fig. 3 is a diagrammatic sectional view showing in more detail a stylus arrangement included in the apparatus of Fig. 2; Fig. 4 is a block diagram illustrating electronic circuitry included in the apparatus of Figs. 1 to 3; and Fig. 5 is a graph illustrating a transfer function in accordance with which the apparatus of Figs. 2 to 4 is arranged to operate.

The metrological instrument shown in Fig. 1 comprises a workbench 2 upon which are mounted a turntable 4 for supporting a workpiece and a column 6 carrying a vertically movable carriage 8. An arm 10 is supported by the carriage 8 for horizontal movement relative thereto, such movement being radial with respect to the turntable 4. A stylus arrangement 12 is mounted on the arm 10 at the free end thereof for engagement with the surface of a workpiece (not shown) mounted on the turntable 4 when the instrument is in operation.

As seen in Fig. 2, the arm 10 is secured to a slide 14 which is horizontally movable along a datum bar 16 which is secured, and extends horizontally, within the carriage 8. The securing means for the datum bar 16 is conventional and is not shown in Fig. 2. A screw rod 18 extending parallel to the datum bar 16 has its ends rotatably mounted in supports 20 and 22, which is secured to the carriage 8, and is rotatably drivable by a servo motor 24. The rod 18 is threadedly engaged in a ball nut 26 which is carried by the slide 14 so that the slide 14 may be driven in either direction along the datum bar 16 by actuation of the motor 24 in one direction or the other. In this way, the arm 10 may be caused to move radially towards and away from a workpiece mounted on the turntable 4.

As best seen in Fig. 3, the stylus assembly 12 comprises a housing 28 which is fixed to the arm 10 and carries a stylus 30 which protrudes through. an opening 32 in the housing 28 and has a convex end surface 34 for engagement with the surface of a workpiece. Partitions 36 and 38 are provided with central openings 40,42 which contain high precision bearings 44,46 respectively, in which an intermediate portion 48 of the stylus 30 is slidably supported so that the stylus 30 may move stably inwardly and outwardly relative to the housing 28 as shown by arrow 50, such movement being horizontal and radial with respect to the turntable 4.A compression spring 52 acting between the partition 38 and a collar 54 secured to the intermediate portion 48 of the stylus 30 urges the stylus 30 to the left as seen in Fig. 3 for maintaining the tip 34 in engagement with the workpiece surface during operation of the instrument.

The partition 36 cooperates with the collar 54 to act as a stop limiting movement of the stylus 30 to the left. In Fig. 3, the stylus 30 is shown in an intermediate position.

A linear variable displacement transducer (LVDT) 56 for detecting movement of the stylus 30 relative to the housing 28 comprises a magnetic armature 58 which is secured to a portion 60 of the stylus 30 and coils 60,62 positioned adjacent armature 58. The LVDT 56 is positioned between the partition 38 and a further partition 64 which carries terminals 66 and 68 connected to the coils 60 and 62 by leads 70 and 72.

The LVDT 56 may operate in a conventional manner to produce a null output when the armature 58 is positioned (as seen in Fig. 3) in the centre of the coils 60,62. As the stylus 30 moves away from the null position, the LVDT 56 produces a signal of increasing magnitude whose phase or polarity depends upon the direction in which the stylus 30 has moved away from the null position i.e. left or right.

A flexible membrane 74 which is sealed around its edge 76 to the interior of the housing 28 and is also sealed at 78 to the outer surface of the stylus 30, prevents dust or other contaminants from reaching the interior of the housing 28 via the aperture 32, other than the small portion of the housing 28 to the left of the membrane 74. This portion does not contain any elements sensitive to contamination.

The position of the stylus 30 relative to the housing 8 is optically detected by a fringe detector 80 mounted in the housing 8 (by means not shown) at a fixed position. The fringes are created utilising light from a laser 82 which is directed via fixed mirrors 84 and 86 to a beam splitter 88. The light transmitted by the beam splitter 88 is reflected by a retroreflector 90, secured to the right-hand end of the stylus 30, to a mirror 92 which is fixed relative to the carriage 8 and is carried by a bracket 94 projecting through a slot 96 into the interior of the arm 10. Light reflected by the mirror 92 is returned via the retroreflector 90 to the beam splitter 88 and reflected to the fringe detector 80.Light from the mirror 86 which is reflected by the beam splitter 88 is directed via a mirror 98 and a retroreflector 100 to a further mirror 102, from which it is returned via retroreflector 100 and mirror 98 to beam splitter 88 through which it passes to detector 80. Mirrors 98 and 102 and retroreflector 100 are mounted (by means not shown) at fixed positions in the carriage 8. The optical path length from beam splitter 88 to mirror 102 is approximately equal to the optical path length from beam splitter 88 to mirror 92 when the slide 14 is at about a centre position.Whenever the stylus 30 moves, whether by movement of the arm 10 or by movement of the stylus 30 within its housing 28, the latter optical path length changes thus causing interference fringes to move relative to the detector 80 which acccordingly produces a series of signals which may be counted to provide a precise indication of the position of the stylus 30 at all times.

A bellows 97 shown in broken lines has its opposite ends sealed respectively to the arm 10 and the carriage 8 to prevent dust or other foreign matter from entering the arm 10 through the slot 96 whilst permitting the arm 10 to move relative to the carriage 8.

With reference to Fig. 4, the instrument comprises a computer 110 programmed to perform metrological operations, a keyboard 112 for inputting instructions and a display 114 for outputting data. The computer 110 controls motors 116 and 118 via microprocessors 120 and 122 which respectively drive the turntable 4 and the vertically movable carriage 8. Computer 110 may also control servo motor 24 via microprocessor 124 for positioning the arm 10 as required in relation to a workpiece. When a metrological operation is performed on a workpiece mounted on the turntable 4, the turntable 4 is rotated with the stylus 30 in contact with the workpiece surface. As the surface rotates, the stylus 30 will be caused to move relative to the housing 28, which movement will be sensed by the microprocessor 124 in response to signals from the LVDT 56.In response to such signals, microprocessor 124 drives servo motor 24 so that the arm 10 is moved towards or away from the axis of the turntable 4 to maintain the stylus 30 in engagement with the surface.

The movement of the stylus is detected by the optical detector 80 which supplies signals to a counter/interpolator 126 via an amplifier 128. The counter/interpolator provides a count indicative of the position of the stylus, which count is supplied to computer 120 to enable it to obtain the data required in the operation being performed.

Microprocessor 124 is arranged to drive motor 24 in accordance with a transfer function which is illustrated in Fig. 5 in which the horizontal axis represents the signal received from LVDT 56 and the vertical axis represents the power applied to motor 24. As can be seen in Fig. 5, the transfer function, represented by curve 132 is of the form: Y = X3 In order to implement this algorithm, a look-up table 125 is provided connected to the microprocessor 124.

The microprocessor 124 obtains, from this table, the value X3 for each value of X and outputs it as value Y, which value represents the power to be applied to the motor 24.

Thus, the power applied to the motor 24 is zero at point 134 on the curve corresponding to a null input from the LVDT 56. Further, the power applied to the motor 24 remains almost at zero within a band 136 extending a short distance to each side of the null point 134, such distance extending to a point somewhere between 10% and 20% of the maximum output from the LVDT 24 remains substantially at zero or at a low level while the input from the LVDT 56 is at a value within a band including the null position, hunting may be avoided.

As can be seen in Fig. 5, values of the LVDT signal up to about 50% of its maximum result in relatively low power levels being applied to the motor 24 but, outside this 50% band, the power applied to the motor 24 increases much more rapidly. Preferably, the power applied to the motor 24 reaches its maximum value when the value of the signal obtained from the LVDT is at some point less than 100% of its maximum, preferably about 80%. Arranging that the power applied to the motor 24 is maximum at times when the LVDT signal is still well within its maximum range, ensures a rapid response whereby the tip 34 of the stylus 30 may be maintained in contact with the workpiece surface, even during relatively rapid rotation of the turntable 4, thus enabling metrological operations to be carried out more quickly.

Various modifications are possible within the scope of the invention. For example, although the invention has been described in an embodiment in which the stylus 30 is slidable, it would also be possible to apply the invention to arrangements in which the stylus is pivotable. Further, the invention may apply to surface sensing arrangements in which sensors other than contact sensors are employed.

Although in the embodiment illustrated in the drawings, a look-up table is provided for implementing the above-mentioned transfer function, such implementation may take place in other ways. For example, the microprocessor could be programed to compute the function Y = X3 for each value of X.

Although the preferred transfer function between the servo motor driving the arm 10 and the LVDT is a cubic function as described with reference to Fig. 5, this is not essential. For example, non-continuous functions could be utilised. For example, the output could be completely zero for a band on either side of the null point and thereafter could be linear.

Claims (13)

CLAIMS:

1. Surface sensing apparatus comprising: a surface sensor operable for traversing the surface of an object and producing signals dependent upon features of said surface; a support supporting said surface sensor; drive means for driving said support towards and away from the surface; and control means for actuating said drive means in response to said signals from said surface sensor, said control means being operable to supply power to said drive means as a non-linear function of said signals for maintaining said sensor in operative relationship with said surface whilst traversing said surface.

2. Apparatus according to claim 1, wherein said non-linear function is such that the power applied to the motor remains at or near to zero when said signal has a value within a predetermined range.

3. Apparatus according to claim 2, wherein said predetermined range includes a null value of Said signal.

4. Apparatus according to claim 3, wherein said null value is substantially at the centre of said predetermined range.

5. Apparatus according to any preceding claim, wherein said function is a cubic function.

6. Apparatus according to any preceding claim, wherein the value of said signal may vary within predetermined limits and said function is such that the power applied to the motor reaches a maximum value before said signal reaches its limit.

7. Apparatus according to any preceding claim, wherein said surface sensor is adapted to contact said surface and is mounted on said support for movement relative thereto, and including means for producing said signals in response to movement of the surface sensor relative to said support.

8. Apparatus according to claim 7, wherein said signal producing means comprises a linear variable displacement transducer.

9. Apparatus according to any preceding claim, including a member carrying said support for movement relative thereto, and including further signal producing means operative for sensing movement of the surface sensor relative to said member without reference to the position of the support relative to said member.

10. Apparatus according to Claim 9, wherein said further signal producing means comprises means for producing optical fringes which move in accordance with the movement of the sensor relative to said member, and means for sensing movement of the fringes.

11. Apparatus according to Claim 9 or 10, including means for producing metrological data in response to signals provided by said further signal producing means.

12. Surface sensing apparatus comprising: a surface sensor operable for traversing the surface of an object and producing signals dependent upon features of said surface; a support supporting said surface sensor; drive means for driving said support towards and away from the surface; and control means for applying power to said drive means as a function of the value of said signal, said function being such that full power is applied to the drive means before said signal reaches a limiting value for maintaining said sensor in operative relationship with said surface whilst traversing said surface.

13. Surface sensing apparatus substantially as herein described with reference to the accompanying drawings.