GB2501233A

GB2501233A - Surface measurement apparatus and method

Info

Publication number: GB2501233A
Application number: GB1203322.1A
Authority: GB
Inventors: Daniel Ian Mansfield
Original assignee: Taylor Hobson Ltd
Current assignee: Taylor Hobson Ltd
Priority date: 2012-02-27
Filing date: 2012-02-27
Publication date: 2013-10-23
Anticipated expiration: 2032-02-27
Also published as: GB2501233B; GB201203322D0

Abstract

A metrological apparatus has a mover (9) to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface (16) and a stylus (11) such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a workpiece surface. A data processor is configured to define a representation of the stylus using stylus characteristics data, to receive nominal form data, to simulate relative movement of the stylus representation and the nominal form along a measurement path, to identify any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable, and to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.

Description

SURFACE MEASUREMENT APPARAThS AND METHOD This invention relates to a surface measurement apparatus and method for facilitating measurement of one or more surface characteristics, in particular surface form.

Surface form or profile measurements may be made by effecting relative movement between a pivotally mounted stylus arm and a workpiece along a traverse path (measurement path) and detecting, using a transducer, the deflection of the stylus arm as a tip of a stylus carried by the stylus arm follows variation in the form of the surface transverse to the traverse path. Accurate measurement requires care in the setting up of the apparatus which can be time consuming.

Measurement of surfaces having significant form, such as aspheric lenses as may be used in optical storage devices such as digital versatile discs (DYD) recorders and players, and moulds for such lenses, present particular challenges because the steepness of the local slope of the surface being measured may result in a higher than desired contact angle between stylus tip and the surface being measured increasing the likelihood of the stylus tip slipping or dragging on the surface which could render the measurement inaccurate and may also damage the stylus. Also the height (depth) to width aspect ratio of the form of the component may make access to the surface difficult, increasing the likelihood of collisions between the stylus arm and the workpiece surface which may, again, detrimentally affect the measurement and damage the stylus.

In order to address the above problems, Taylor Hobson Ltd of Leicester England have produced metrological apparatus sold under the trade name TalysurfPGl Blu" which enables precision 3-D for measurement of shallow and steep-sided aspheric lenses and moulds and offers 100 nm measurement capability.

This apparatus addresses problems discussed above by enabling the orientation (traverse angle) of a traverse unit carrying the stylus to be adjusted so that the stylus arm and the measurement path direction are inclined to the plane of a support surface, such as a turntable, on which the workpiece to be measured is mounted. Allowing the angle of the stylus arm to be adjusted reduces the possibility of the contact angle exceeding a desired limit and also should facilitate access to the surface to be measured. Nevertheless there is still a possibility that an operator may incorrectly set up the instrument at the start of a measurement procedure, for example may select an incorrect traverse angle or an incorrect stylus, which may result in a higher than desired contact angle or the possibility of a collision between the stylus arm and the Embodiments of the present invention aim to ameliorate the above issues.

Aspects and preferred examples of the present invention are set out in the appended claims.

Embodiments of the present invention facilitate setting up of a metrological instrument for a measurement procedure allowing more rapid and accurate measurements of the surface characteristic to be made.

In one aspect, there is provided a metrological apparatus for measuring a surface characteristic of a workpiccc, the apparatus comprising: a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surface of a workpiece supported on the workpiece support surface; a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path; and a data processor configured to: to receive stylus characteristics data; to define a representation of the stylus using the stylus characteristics data; to receive nominal form data representing the expected form of a surface of the to simulate relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; to identify any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable; to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.

In another aspect, there is provided a method of facilitating measurement of a surface characteristic of a workpiece using an apparatus comprising: a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiccc support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surface of a workpiece supported on the workpiece support surface; and a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path, the method comprising: receiving stylus characteristics data; defining a representation of the stylus using the stylus characteristics data; receiving nominal form data representing the expected form of a surface of the workpiccc; simulating relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; identifying any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form arc undesirable; outputting to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.

In another aspect, there is provided a data processor for a metrological apparatus comprising: a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surfacc of a workpiecc supported on the workpiece support surface; and a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path, the data processor being configured: to receive stylus characteristics data; to define a. representation of the stylus using the stylus characteristics data; to receive nominal form data representing the expected form of a surface of the to simulate relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; to identify any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable; to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.

Embodiments of the present invention facilitate setting up of a metrological instrument for a measurement procedure, enabling an operator to determine, before carrying out a measurement, whether his or her setup procedure is likely to result in any undesirable occurrences such as out of range contact angles and/or potential collisions, allowing more rapid and accurate measurements of the surface characteristic to be made.

The relative locations of the stylus representation and the nominal form may be determined to be undesirable in the event that a contact angle between the stylus tip of the stylus representation and the nominal form is outside a desired contact angle range.

The contact angle may be determined to be the angle between the normal to the local nominal form gradient or tangent and the orientation of the part of the stylus at the stylus tip. As another possibility or additionally, the relative locations of the stylus representation and the nominal form may be determined to be undesirable in the event the representation of the stylus arm intersects or contacts the nominal form indicating a potential collision point.

In the event the relative locations of the stylus representation and the nominal form are undesirable, parameters of the simulation may be adjusted and proposed alternative parameters output to the resource, to assist an operator in improving a measurement procedure. At least one of a measurement direction and a stylus characteristic may be adjusted, for example an angle of a shank of the stylus may be adjusted. As another possibility, or additionally, stylus characteristics for different available styli may be stored and parameters adjusted by selecting stylus characteristics for a different stylus, assisting an operator in selection of a correct stylus.

In an embodiment a metrological apparatus has a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a workpiece surface. A data processor is configured to define a representation of the stylus using stylus characteristics data, to receive nominal form data, to simulate relative movement of the stylus representation and the nominal form along a measurement path, to identify any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable, and to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a very schematic representation of a metrological instrument of apparatus embodying the present invention looking in a direction, y, and perpendicular to a measurement direction; Figure 2 shows a functional block diagram of data processing and control apparatus of apparatus embodying the present invention; Figure 3 shows a functional block diagram of functionality provided by programming of the control apparatus shown in Figure 2 for assisting an operator in correctly setting up the metrological instrument; Figure 4 shows a flow chart illustrating processes for assisting an operator in correctly setting up the metrological instrument; Figure 4a shows a flow chart illustrating further processes that may be carried out for assisting an operator in setting up the metrological instrument; Figures 5 to 8 show diagrams for helping to explain the setup functionality shown in Figures 3 and 4; and Figures 9 to 13 show examples of resuhing simulations.

With reference to the drawings in general, it will be appreciated that the Figures are not to scale and that for example relative dimensions may have been altered in the interest of clarity in the drawings. Also any functional block diagrams are intended simply to show the functionality that exists within the device and should not be taken to imply that each block shown in the functional block diagram is necessarily a discrete or separate entity. The functionality provided by a block may be discrete or may be dispersed throughout the device or throughout a part of the device. In addition, the functionality may incorporate, where appropriate, hard-wired elements, software elements or firmware elements or any combination of these.

Referring now to the drawings, an example metrological apparatus will be described which comprises a metrological instrument and a control apparatus.

Figure 1 shows a very diagrammatic representation of the metrological instrument 2 of the metrological apparatus 1.

The metrological apparatus 2 has a base S that is designed to be supported by a workbench 6. The base S carries a column 7 that defines a vertical or z axis reference datum. A column carriage 8 is mounted to the column 7 so as to be movable in the z direction with respect to the column 7. The movement of the column carriage 8 is effected by a motorised drive arrangement (not shown), such as for example a.

leadscrew, pulley or other suitable drive arrangement. The base 5 also carries turntable 16 to support a workpiece 14. The turntable 16 has a centring and levelling mechanism (not shown) such as that shown in Figures 2 and 3 of GB2, 1 89,604A, the whole contents of which are hereby incorporated by reference.

The column carriage 8 carries a traverse unit 9, which is arranged at an angle f3 (the transverse angle) to the x-axis (which in the example is represented by the plane of the turntable surface and is generally the horizontal). The transverse unit 9 is movable relative to the column carriage 8 by means of a motorised drive arrangement (not shown) along a straight reference datum (not shown) provided by the traverse unit 9.

The direction of this straight reference datum is determined by the orientation of the transverse unit so that the traverse unit 9 is movable in an X direction which extends at the angle f3 to the x-axis.

The traverse unit 9 carries a measurement probe (or gauge unit) 10 which consists of a pivotally mounted stylus arm (shown very diagrammatically in Figure 1 in dotted lines within the traverse unit 9) carrying at its free end a stylus arm 11 having a stylus tip 12 which in operation comes into contact with the surface of the workpiece or component under test during a measurement operation so that, as the traverse unit 9 is moved in the measurement direction, the stylus arm 11 pivots to enable the stylus tip 12 to follow surface variations along a measurement path on the surface. Deflection of the stylus arm is detected by a measurement transducer (or displacement provider) 39 shown in dotted lines in Figure 1. The measurement probe 10 may be mounted to the traverse unit 9 by a y-position adjuster (not shown) so as to be movable in the y-direction with respect to the traverse unit 9. The movement of the measurement probe in the y-dircction may be effected by a manual or motoriscd lcadscrcw, pulley or other drive arrangement (not shown).

In an example, the traverse unit 9 may be mounted to the column carriage 8 by means of a pivot pin to enable the angle of the traverse unit 9 with respect to the x-axis to be adjusted. In this particular example, the angle of the traverse unit 9 is manually adjustable and the traverse unit 9 is held in place at the manually adjusted angle by means of an air brake (not visible in the Figure). As another possibility, the adjustment of the angle may be automated. As another possbility, the angle f3 may for some applications be fixed.

Figure 2 shows a block diagram illustrating functional components of the metrological instrument 2 and the control apparatus 3 of the metrological instrument 1.

Referring now to Figure 2, the control apparatus 3 is generally a personal computer and has a processing unit 13 coupled via a bus 13a to associated data and program instruction/software storage 14 in the form generally of RAM 15, ROM 16, a mass storage device 17 such as a hard disc drive and at least one removable medium drive 18 for receiving a removable medium (RM) 19, such as a CD-ROM, solid state memory card, DVD, or floppy disc. As another possibility, the removable medium drive may itself be removable, for example it may be an external hard disc drive.

The control apparatus is also coupled via the same or a different bus to input/output devices 20 comprising in this example a display 21, a keyboard 22, a pointing device 23 such as a mouse, a printer 24 and, optionally, a communications device 25 such as at least one of a MODEM and a network card for enabling the control apparatus 3 to communicate signals S via a wired or wireless connection with other control apparatus or computers via a network such as the Internet, an intranet, a WAN or a LAN.

The processing unit 13 is programmed by program instructions and data provided by being at least one of: downloaded as a signal S via the communications device 25; pre-stored in any one or more of ROM 16, RAM 15 and mass storage device 17; read from a removable storage medium 19 received by the removable medium drive 18; and input by the user using the keyboard 22.

The metrological instrument 2 has a data acquisition and processing unit (DAPU) 30 that communicates with the processing unit 13 of the control apparatus 3 via an appropriate link, for example a serial link, 30a to enable data regarding a measurement operation to be communicated to the control apparatus 3.

The control components of the metrological apparatus 2 comprise a column drive controller 31 for driving the carriage 8 up and down the colunin in the z direction, a measurement direction position controller 32 for driving the measurement probe or gauge unit along the reference datum provided by the traversc unit 9 in the measurement direction X at an angle f3 to the x-a.xis and a.n interferometric z displacement provider 35 for providing a measure of the z displacement of the stylus tip 12 as the stylus arm 11 follows the surface being measured during movement of the traverse unit 9 along a measurement path in a direction at an angle f3 to the x-axis.

If rotation of the turntable is automated, then the metrological apparatus will also comprisc a y (where y rcpresents the angle of rotation of the turntable 16 about its spindle axis) position controller 38 for controlling rotation of the turntable 16.

Similarly, if the attitude of the traverse unit 9 is adjustable and this adjustment is automated, then a f3 position controller 36 will be provided for changing the attitude f3 of the traverse unit 9. y and 1 position providers 39, 37 (which may for example be shaft encoders, for example optical shaft encoders, or a linear grating type position provider) are provides to supply signals respectively indicating the angles y and 13 to the DAPU 30. Generally the interferometric z displacement provider 35 will be provided within the traverse unit 9.

The measurement direction position controller 32 is associated with a position provider 34 that may be, for example, a shaft encoder associated with a motor providing the position controller 32 or may be a linear grating type of transducer. The column drive 31 may also be associated with a column z position provider 33 (shown in phantom lines in Figure 4a), for example a shaft encoder associated with a motor providing the column drive 31, or the column z position may be determined in an open loop manner directly from the column motor drive signal. As show in Figure 2, the column drive 31 and position controller 32 (and other controllers if present) are coupled to the control apparatus 3 (via a link I 3b and appropriate interfaces, not shown) for control by instructions from the control apparatus 3. At least some of these instructions may be supplied by the user.

The measurement probe or gauge unit is in this example the measurement probe used in the instruments supplied by Taylor Hobson as the Form Talysurf PGT series and is described in detail in IJS-A-5,517,307 (the whole contents of which are hereby incorporated by reference) to which reference should be made for further information.

In partieula.r the measurement probe or gauge unit may be based on Taylor Hobson's Form Talysurf PGI 1240 metrological instrument, described in the brochure produced by Taylor Hobson entitled "Form Talysurf POT 1240, Aspherics Measurement system".

This Form Talysurf PGI series of metrological instruments is particularly suited to TO measuring the surface form of surfaces having significant form because, as described in US-A-5,517,307, the interferometric z displacement provider 35 uses a curved diffraction grating that has a radius of curvature which is coincident with the axis about which the stylus arm pivots to provide more accurate z displacement measurements over a longer range.

The processing unit is programmed by program instructions to enable carrying out of measurements further details of examples of such programming may be found in W02010/943906, the whole contents of which are hereby incorporated by reference.

Tn the following (see Figures 5 to 8): o is the origin, that is the location at which x0, z0 A is the nominal base diameter of the workpiece or component whose surface form is to be measured, for example an aspheric lens mould 100 as shown in solid lines in Figure 5 or an aspheric lens mounted on the attached to a base, the lens being illustrated by the dot-dash line TOT in Figure 5; a is the stylus deflection angle between the line passing through the pivot axis A and the centre of the stylus tip 12 and the x axis and represents the degree of deflection of the stylus arm; G is the gauge reading which is related to the stylus deflection angle a; J3 is the angle of the traverse unit to the x axis; X is the traverse or measurement direction which extends at the angle l to the x axis; Xi is the distance the traverse unit has moved in the traverse or measurement direction X from a zcro position Xo; z(x) is the distance in the z direction of a point on the surface being measured from a top surface of the flat part (the body of the mould or the base upon which the aspheric lens is mounted); Ax is the distance in the x direction of the centre of the stylus tip 12 from x0 where x0 corresponds to the turntable spindle axis on which the component to be measured will be centred and aligned, for examp'e as discussed in W02100/043906, so that a rotational axis of the component (the optical axis in the case of an aspheric lens) is coincident with and aligned to the spindle axis; AZ or AZ1 is the distance in the z direction when the stylus tip is at a measurement point on the surface being measured from the corresponding z position at which 0=0 (see Figure 5); AZIIa1 is the distance in the z direction from z0 to the top surface of any flat component part, part 100 in Figure 5; L0 is the length of the stylus arm 11; A is the location of the pivot axis of the stylus arm; Uo is the pivot offset angle which as shown in Figure 7 is an angle between a line parallel to the x axis passing through the pivot axis A and a line passing through the pivot axis A and the centre of the stylus tip 12 with the stylus arm parallel to the traverse axis and is determined, as illustrated in Figure 7, by the offset P of the pivot axis A from the stylus arm, the length of the stylus arm L and the length S of the stylus shank 1 Ia from the stylus arm to the centre of the stylus tip 12; L is the distance between the centre of the stylus tip 12 and the pivot axis A, which distance is determined by the length of the stylus arm L, the pivot offset P and the length S of the stylus shank 1 Ia from the stylus arm to the centre of the stylus tip 12.

Figure 3 shows a functional block diagram illustrating functionality provided by programming of the processor unit to assist an operator in setting up the metrological instrument so as to avoid or at least reduce the possibility of an out-of-range contact angle or a collision between the stylus arm and component or workpiece under test.

As shown in Figure 3, the set up assistance frmnctionality includes a data receiver 41 (which may bc providcd by the input/output devices shown in Figure 2) to receive data and store the data in a data store 40 which may be provided by, for example, any one or more of the RAM 15, ROMI6 and/or mass storage 17 shown in Figure 2. As will be explained below, data storcd in the data store 40 includes: traverse data including the traverse angle f3 and the measurement step X in the traverse direction; a nominal form of the surface of the workpiece to be measured, that is the form that the surface was designed or intended to have, and the height Az which as set out above is the distance in the z direction from z0 to the top surface of the flat part 100; stylus characteristics including, for example, the length L of the stylus arm 11, a pivot offset angle ao, the length S of a stylus shank projecting from the stylus arm 11 and carrying at its free end the stylus tip 12 and the dimensions and geometry of the stylus tip. The data store 41 also provides storage for simulation results.

The functionality shown in Figure 3 includes: a stylus geometry determiner 42 that uses the stylus characteristics to define a geometrical representation of the stylus a stylus motion determiner 43 that uses the geometrical representation of the stylus, the nominal form and the traverse data to determine what the position and orientation of that stylus arm would be at each measurement point X1 as the stylus tip follows a measurement path on a surface of the nominal form; a contact angle determiner 44 to determine the contact angle of the stylus tip at each measurement point using the determined position and orientation of that stylus arm at each measurement point X and the known the dimensions and geometry of the stylus tip; a collision determiner 45 to determine the possibility of a collision between the stylus arm and the workpiece or component at each measurement point using the determined position and orientation of the stylus arm at each measurement point and the nominal form; and a simulated measurement output provider for outputting the simulation results to a resource such as a display, printer, network connection or another computer with any measurement points with out-of-range contact angles or collisions between the stylus arm and the workpieee or component highlighted, for example displayed or printed in another colour such as red.

Processes will now to be described with reference to Figures 4 and 4a that may assist an operator in setting up the metrological instrument to avoid or rcduce the possibility of out-of-range contact angles or collisions. These processes may be carried out using the firnctionality described with reference to Figure 3 or any other appropriate flrnctionality.

In order to explain the processes shown in Figures 4 and 4a, reference is made to Figures 5 to 8 which iflustrate aspects of thc gcometry of the metrological instrumcnt.

Referring to Figures 5 to 8, the vector from origin 0 to pivot location A in Figure 6 is given by: A =(L +Xi)(icos(a0 +$)+ksin 1) where I and k are the unit vectors in the x and z directions.

(In the example illustrated in Figure 5 the traverse unit has been driven in the negative X direction from Xo and so Xi has a negative value.) The vector B from origin 0 to the stylus tip centre in Figure 6 is given by: A-L(icosa +z(Ax)) IA; +kz3 2) The gauge reading G and its relationship with the stylus deflection angle a are given by: G=L(a0+$-a) a=a0+$-(G/L) 3) Extracting the orthogonal components (x,z) from equations 1 and 2 allows a pair of relationships to bc defined that rclate thc stylus tip ccntre values (x,z) in terms of the stylus and instrument parameters as follows: Lcos(fl+a)+xcos$-Lcosa=x 4) Lsin(/3 +aj+XsinJi +AZ01 -Lsina =z Figures 7 and 8 in particular show the geometry and dimensions of the stylus. This data is either pre-stored or input by the operator. Where a number of different styli are available, the operator may select the stylus characteristics data from a number of pre-stored sets of stylus characteristics data. As another possibility, the stylus itself may carry the data in a local non-volatile memory or may carry identification data idcnti'ing the stylus so that thc control apparatus can select the correct set of stylus data from its data store.

Tn this example, the stylus data includcs thc lcngth L0 of the stylus arm 11, thc pivot offset angle uo which as shown in Figure 7 is an angle between a line parallel to the x axis passing through the pivot axis A and a line passing through the pivot axis A and the centre of the stylus tip 12 with the stylus arm parallel to the traverse axis and is determined, as illustrated in Figure 7, by the offset P of the pivot axis A from the stylus arm, the length of the stylus arm L0 and the length S of the stylus shank 1 la from the stylus arm to the centre of the stylus tip 12, and the length S of the stylus shank ha from the stylus arm to the centre of the stylus tip 12. The stylus characteristics data also includes the geometry and dimensions of the stylus tip. In this example, the stylus tip is in the form of a sphcre of givell radius r.

Thc traversc angle f3 will gencrally bc input by the opcrator but could be dctermined by detecting the degree of rotation using an appropriate transducer as dismissed above.

The measurement step X may be pre-defrned but could be operator-selectable.

Data representing the nominal form of the component to be measured may be input by the operator but may be pre-stored. Again, the data store may store data representing various different nominal surface forms for selection by the user.

If the nominal form of the component to be measured is represented as z(x) then it has a gradient of-4-= tan 1J5 For a stylus tip of radius r traversing this surface, the tip dx centre is then defined by z =z+reost S 5) x. =x-rsinP where the point of contact between the stylus tip and the surface is (x, z). These stylus tip centre values (x3, 4) are used throughout the following.

In order to determine a (and so G), Zc and X, the equations 4) above may be inverted to yield: L(sin(a-f3)-sina0)-x5sinp+zcosf3 Z0 -Z -eosf3 6) = L(eosct-cosQ3+cx0))+x5 eos f3 At Si in Figure 4, a stylus representation is defined using the stylus characteristics and the traverse data. This stylus representation represents the stylus geometry and orientation in relation to the nominal form. The stylus representation and the nominal surface are for convenience represented in the same coordinate space, either that of the nominal surface, that is (x, z) or the measurement coordinate space (G (or a), X), in accordance with the relationships set out in equation 6.

At S2, relative movement between the stylus representation and the nominal form along the measurement direction X with the stylus tip following the nominal form is simulated to provide a simulated measurement and at S3 the relative positions of the stylus and thc nominal form arc determined at each measurement point.

At S4 the contact angle between the stylus and the nominal form is determined for each measurement point. In this example, the contact angle is detcrmincd to be the angle between the norma.1 to the local nominal form gradient or tangent and the stylus shank direction or axis at the measurement point so that when the stylus shank is perpendicular to the local gradient at a measurement point the contact angle for that measurement point is zero. Other ways of defining the contact angle are possible.

At S5 any measurement points for which the contact angle is out of a desired range, for example exceeds a threshold, is identified. At S6, a determination is made at each measurement point as to whether any part of the stylus representation other than the stylus tip (that is the stylus arm or shank, for example) contacts or intersects the nominal form and if so that measurement point is identified as a potential collision point. It will of curse be understood that S6 could be carried out before S4 and/or S5.

At S7 the resulting data is output to a resource such as a display or printer of the input/output devices of Figure 2 or to the communication device for supply to a remote device, such as a computer or display or printer, directly or via a network. The output data may simply alert the operator to the possibility of an out-of-range contact angle and/or potential collision but more usefully may show the position of the stylus at each measurement point, either as a static image or as an animation, with any out-of-range contact angles and/or potential collision points highlighted, for example shown in a different colour such as red.

If the output data shows any out-of-range contact angles and/or potential collision points, the operator can then select a different traverse angle and re-run the simulation. If a traverse angle cannot be found that does not result in the simulation indicating an out-of-range contact angle and/or a potential collision, the operator may select a different stylus, for example a stylus with a different shank angle B and re-run the simulation.

Figure 4a shows a flow chart illustrating further processes that may be carried out for assisting an operator in correctly setting up the metrological instrument, if the processes shown in Figure 4 identii an out-of-range contact angle and/or a potential collision.

Thus if the processes shown in Figure 4 identify an out-of-range contact angle and/or a potential collision, the simulated traverse angle 3 and/or stylus characteristics such as the shank angle B maybe adjusted at SlO in Figure 4a and Si to 57 of Figure 4 then repeated at Si 1. If an out-of-range contact angle and/or a potential collision is detected at Si2 then the simulated traverse angle f3 and/or stylus characteristics such as the shank angle 0 may be re-adjusted and SI I to SI 2 repeated until no out-of-range contact angle and/or a potential collision is detected, at which point adjustments to the traverse angle and/or stylus characteristics may be proposed to the operator by outputting to the resource as discussed above. The process shown in Figure 4a may try different traverse angles first and only try different stylus characteristics if a traverse angle cannot be found that does not cause an out-of-range contact angle and/or a potential collision, or vice versa. Different stylus characteristics may be selected from stylus characteristics stored in the data store of the metrological apparatus or stylus characteristics for available alternative styli requested from and input by the operator.

The stylus characteristic that is adjusted could be the shank angle B or another characteristic such as the stylus arm or shank length.

Figures 9 to 13 show results of simulations (in x, z coordinate space) carried out in accordance with Figure 4 with Figure 9 showing the representation of a stylus 11 at various measurement points along a measurement path of a nominal form N with an area of out-of-range contact angles C highlighted (in the actual simulation C is shown in red, the stylus arm in green and the nominal form in blue; it will be appreciated that other colours could be selected). Figure 10 shows part of Figure 9 enlarged to illustrate the area of out-of-range contact angles C more clearly whilst Figure II shows an enlargement of one stylus position to show the simulation of the stylus tip 12 and the relative location of the stylus tip centre 12a and the path of the stylus tip centre Nc.

Figures 12 and 13 are views similar to Figures 9 and 10 to show the effect of changing the shank angle. In this example, the stylus shank has been rotated forwards 25 degrees and the measurement path no longer shows any out-of-range contact angles.

Modifications and Variations A person skilled in the art will appreciate that a number of different methods of centring and levelling could be employed with the above-described techniques. For example, as one possibility, mechanical centring is used. It may be possible to use software centring andior levelling, for example as described in US-A-5926781, the whole contents of which are hereby incorporated by reference, which may enable omission ofat least some of the centring and levelling mechanisms discussed herein.

Other forms of centring and levelling mechanism may be used. For example, it may be possible to use wedge assemblies of the type described in the Applicant's International Application Publication No. WO2007/091087, the whole contents of which are hereby incorporated by reference. Other levelling mechanism that do not use wedge assemblies may be used, for example, as discussed in US-A-473 1934, the whole contents of which are hereby incorporated by reference.

It will be appreciated that the traverse angle 1 could be zero. Also, the stylus need not necessarily be a contact stylus but could be any form of stylus that follows the frame of a surface, ahhough this may require modification of the definition of the stylus tip centre.

In the above example, the stylus tip is in the form of a sphere of given radius r but it could have another form, for example a frusto-conieal form with a part-spherical contact surface.

Also, other gauge transducers units than the ones described above may be used, for example it may be possible to use an LVDT gauge or a different form of optical interferometrie gauge.

A person skilled in the art will appreciate that the methods and apparatus described herein need not be limited in their application to instruments for the measurement of aspheric, concave or convex surfaces, and may equally be applied to instruments for the measurement of other surfaces.

As one possibility, there is provided a computer program, computer program product, or computer readable medium, comprising computer program instructions to cause a programmable computer to carry out any one or more of the methods described herein.

Various features described above may have advantages with or without other features described above.

The above embodiments are to be understood as illustrative examples of the invention.

Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS1. A metrological apparatus for measuring a surface characteristic of a workpiece, the apparatus comprising: a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surface of a workpiece supported on the workpiece support surface; a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path; and a data processor configured to: to receive stylus characteristics data; to define a representation of the stylus using the stylus characteristics data; to receive nominal form data representing the expected form of a surface of the to simulate relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; to identify any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable; to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.
2. A metrological apparatus according to Claim 1, wherein the data processor is configured to determine that the relative locations of the stylus representation and the nominal form are undesirable in the event that a contact angle between the stylus tip of the stylus representation and the nominal form is outside a desired contact angle range.
3. A metrological apparatus according to Claim 2, wherein the contact angle is determined to be the angle between the normal to the local nominal form gradient or tangent and the orientation of the part of the stylus at the stylus tip.
4. A metrological apparatus according to any preceding claim, wherein the data processor is configured to determine that the relative locations of the stylus representation and the nominal form are undesirable in the event the representation of the stylus arm intersects or contacts the nominal form indicating a potential collision point.
5. A metrological apparatus according to any of the preceding claims, wherein the data processor is configured to output to a resource such as a display, printer, network connection or another computer.
6. A metrological apparatus according to any of the preceding claims, wherein the data processor is configured to output data highlighting undesirable relative locations, for example in another colour such as red.
7. A metrological apparatus according to any of the preceding claims, wherein the data processor is configured to output data representing the position of the stylus representation relative to the nominal form at different measurement points.
8. A metrological apparatus according to Claim 7, wherein the data processor is configured to output data to show the position of the stylus at each measurement point, either as a static image or as an animation, with any out-of-range contact angles and/or potential collision points highlighted, for example shown in a different colour such as red.
9. A metrological apparatus according to Claim 7, wherein the data processor is configured to output data to provide an animation representing the position of the stylus relative to the nominal form as relative movement is effected between the stylus representation and the nominal form.
10. A metrological apparatus according to any of the preceding claims, wherein a pivotal support is provided for the stylus so that the stylus pivots about a pivot axis as the stylus tip follows surface variations.
11. A metrological apparatus according to claim 10, whcrcin the workpiecc support surface defines a workpiece coordinate system having an axis x parallel to the workpiece support surface and an axis z normal to the workpiece support surface whilst the measurement direction and the stylus define a measurement coordinate system having a. measurement direction X and a. measurement value 0 related to a.stylus deflection angle a.
12. A metrological apparatus according to claim 11, wherein the stylus representation and the nominal surface are represented in the same coordinate system, either the workpiece coordinate system or the measurement coordinate system.
13. A metrological apparatus according to any of the preceding claims, wherein the measurement direction is at an angle to the workpiccc support surface.
14. A metrological apparatus according to any of the preceding claims, wherein the stylus characteristics include at least one of a length L of the stylus arm, a pivot offset angle ao, a length S of a stylus shank projecting from the stylus arm and carrying at its free end the stylus tip, and the dimensions and geometry of the stylus tip.
15. A metrological apparatus according to any ofthe preceding claims, wherein in the event the relative locations of the stylus representation and the nominal form are undesirable, the data processor is configured to adjust parameters of the simulation and to output proposed alternative parameters to the resource.
16. A metrological apparatus according to Claim 15, wherein the data processor is configured to adjust at least one of a measurement direction and a stylus characteristic.
17. A metrological apparatus according to Claim 15, wherein the data processor is configured to adjust an angle of a shank of the stylus.
18. A metrological apparatus according to Claim 15, wherein a data store stores stylus characteristics for different available styli and thc data processor is configured to adjust parameters of the simulation by selecting stylus characteristics for a different stylus.
19. A method of facilitating mca.suremdnt of a. surfacc cha.ractcristic of a workpiecc using an apparatus comprising: a movcr to carry out a mcasurcmcnt by cffccting relative movcmcnt in a mcasurcment direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surface of a workpiece supported on the workpiece support surface; and a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path, the method comprising: receiving stylus characteristics data; defining a representation of the stylus using the stylus characteristics data; receiving nominal form data representing the expected form of a surface of the simulating relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; idcntifring any mcasurcment points along thc mcasurcmcnt path for which thc rclativc locations of the stylus representation and the nominal form are undesirable; outputting to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.
20. A method according to Claim 19, comprising determining that the relative locations of the stylus representation and the nominal form are undesirable in the eyent that a contact anglc between the stylus tip of thc stylus rcprescntation and thc nominal form is outside a desired contact angle range.
21. A method according to Claim 20, wherein the contact angle is determined to be the angle between the normal to the local nominal form gradient or tangent and the orientation of the part of the stylus at the stylus tip.
22. A method according to any of claims 19 to 21, comprising determining that the relative locations of the stylus representation and the nominal form arc undesirable in the event the representation of the stylus arm intersects or contacts the nominal form indicating a potcntia collision point.
23. A method according to any of claims 19 to 22, comprising outputting to a resource such as a display, printer, network connection or another computer.
24. A method according to any of claims 19 to 23, comprising outputting data highlighting undesirable relative locations, for example in another colour such as red.
25. A method according to any of claims 19 to 24, comprising outputting data representing the position of the stylus representation relative to the nominal form at different measurement points.
26. A method according to Claim 25, comprising outputting data to show the position of the stylus at each measurement point, either as a static image or as an animation, with any out-of-range contact angles and/or potential collision points highlighted, for example shown in a different colour such as red.
27. A method according to Claim 25, , comprising outputting data to provide an animation representing the position of the stylus relative to the nominal form as relative movement is effected between the stylus representation and the nominal form.
28. A method according to any of claims 19 to 27, wherein a pivotal support is provided for the stylus so that the stylus pivots about a pivot axis as the stylus tip follows surface variations.
29. A method according to claim 28, wherein the workpiece support surface defines a workpiccc coordinate system having an axis x parallel to the workpicce support surface and an axis z normal to the workpiece support surface whilst the measurement direction and the stylus define a measurement coordinate system having a measurement direction X and a measurement value G related to a stylus deflection angle a.
30. A method according to claim 29, comprising representing the stylus representation and the nominal surface in the same coordinate system, either the workpiece coordinate system or the measurement coordinate system.
31. A method according to any of claims 19 to 230, wherein the measurement direction is at an angle to the workpiece support surface.
32. A method according to any of claims 19 to 31, wherein the stylus characteristics include at least one of a length L of the stylus arm, a pivot offset angle uo, alengths ofa stylus shankprojecting fivmthe stylus arm and carrying at its free end the stylus tip, and the dimensions and geometry oft stylus tip.
33. A method according to any of claims 19 to 32, further comprising, in the event the relative locations of the stylus representation and the nominal fbrm are undesirable, adjusting parameters of the simulation and outputting proposed alternative parameters to the resource.
34. A method according to Claim 33, comprising adjusting at least one of a measurement direction and a stylus characteristic.
35. A method according to Claim 33, comprising adjusting an angle of a shank of the stylus.
36. A method according to Claim 33, comprising storing stylus characteristics for different available styli and adjusting parameters of the simulation by selecting stylus characteristics for a different stylus.
37. A data proccssor for a metrological apparatus comprising: a mover to carry out a measurement by effecting relative movement in a measurement direction between a workpiece support surface and a stylus such that the stylus is deflected as a stylus tip of the stylus follows surface variations along a measurement path on a surface of a workpiece supported on the workpiece support surface; and a transducer to provide measurement data representing the deflection of the stylus at measurement points along the measurement path, the data processor being configured: to receive stylus characteristics data; to define a representation of the stylus using the stylus characteristics data; to receive nominal form data representing the expected form of a surface of the workpiece; to simulate relative movement of the stylus representation and the nominal form along a measurement path to simulate a measurement; to identi' any measurement points along the measurement path for which the relative locations of the stylus representation and the nominal form are undesirable; to output to a resource data alerting an operator in the event of determination of a measurement point for which the relative locations of the stylus representation and the nominal form are undesirable.
38. A data processor according to Claim 37, wherein the data processor is configured to determine that the relative locations of the stylus representation and the nominal form are undesirable in the event that a contact angle between the stylus tip of the stylus representation and the nominal form is outside a desired contact angle range.
39. A data processor according to Claim 38, wherein the contact angle is determined to be the angle between the normal to the local nominal form gradient or tangent and the orientation of the part of the stylus at the stylus tip.
40. A data proccssor according to any of claims 37 to 39, whcrcin thc data processor is configured to determine that the relative locations of the stylus representation and the nominal form are undesirable in the event the representation of thc stylus arm intcrsccts or contacts thc nominal form indicating a potcntial collision point.
41. A data proccssor according to any of claims 37 to 40, whcrcin thc data processor is configured to output to a resource such as a display, printer, network connection or another computer.
42. A data proccssor according to any of claims 37 to 41, whcrcin thc data processor is configured to output data highlighting undesirable relative locations, for cxamplc in anothcr colour such as rcd.
43. A data processor according to any of claims 37 to 42, wherein the data processor is configured to output data representing the position of the stylus representation relative to the nominal form at different measurement points.
44. A data proccssor according to Claim 43, whcrcin thc data proccssor is configured to output data to show the position of the stylus at each measurement point, either as a static image or as an animation, with any out-of-range contact angles and/or potential collision points highlighted, for example shown in a different colour such as rcd.
45. A data processor according to Claim 43, wherein the data processor is configured to output data to provide an animation representing the position of the stylus rclativc to thc nominal form as rclativc movcmcnt is cffcctcd bctwccn thc stylus representation and the nominal form.
46. A data processor v according to any of claims 37 to 45, wherein a pivotal support is provided for the stylus so that the stylus pivots about a pivot axis as the stylus tip follows surface variations.
47. A data processor v according to claim 46, wherein the workpiece support surface defines a workpiece coordinate system having a.n axis x parallel to the workpiece support surface and an axis z normal to the workpiece support surface whilst the measurement direction and the stylus define a measurement coordinate system having a measurement direction X and a measurement value G related to a stylus deflection angle a.
48. A data processor v according to claim 47, wherein the stylus representation and the nominal surface are represented in the same coordinate system, either the workpicce coordinate system or the measurement coordinate system.
49. A v according to any ofclaims 37 to 48, wherein the measurement direction is at an angle to the workpiece support surface.
50. A data processor v according to any of claims 37 to 49, wherein the stylus characteristics include at least one of a length L of the stylus arm, a pivot offset angle ao, a length S of a stylus shank projecting from the stylus arm and canying at its free end the stylus tip, and the dimensions and geometty of the stylus tip.
51. A data processor v according to any of claims 37 to 50, wherein in the event the relative locations of the stylus representation and the nominal form are undesirable, the data processor is configured to adjust parameters of the simulation and to output proposed alternative parameters to the resource.
52. A data processor according to Claim 51, wherein the data processor is configured to adjust at least one of a measurement direction and a stylus characteristic.
53. A data processor according to Claim 51, wherein the data processor is configured to adjust an angle of a shank of thc stylus.
54. A data processor according to Claim 51, wherein a data store stores stylus S characteristics for diffcrcnt available styli and thc data proccssor is configured to adjust parameters of the simulation by selecting stylus characteristics for a different stylus.
55. A metrological apparatus substantially as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.
56. A data processor substantially as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.
57. A mcthod substantially as hcreinbcfore dcscribcd with rcfcrcncc to and//or as illustrated in Figure 4 and/or 4a of the accompanying drawings.
58. A computer program product comprising program instructions to program a processor to carry out data processing of a method according to any of claims 19 to 36 and 57 or to program a processor to provide the data processor of any of claims I to 18 and 37 to 56