FIELD OF THE INVENTION

The invention relates to apparatus for and methods of performing optical metrology in the field of manufacturing engineering, e.g. in the inspection of machined components or of small sub-features formed on such components using optical testing methods. In particular, the invention is concerned with inspection processes that may use optical scanning or imaging techniques, e.g. performed using a laser triangulation sensor .

BACKGROUND TO THE INVENTION

Optical techniques for performing non-contact metrology measurements are known. The advantages of such techniques over conventional methods, e.g. using a touch probe on a coordinate measuring machine or performed measurements manually, include increased measurement speed and efficiency and an ability to work with fragile and/or flexible materials.

US 6,031,602 discusses various types of coherent optical metrology techniques, which generally operate to form an image of a sample that is overlaid with an interference pattern that provides information useful for inspection or testing

purposes .

Another successful technique is three-dimension laser triangulation sensing. In this technique, one or more laser lines (i.e. planar laser beams) may be emitted from a source towards a sample, on which they are incident as one or more lines. Light reflected from the incident lines is collected by an imaging device, which may be a camera (e.g. having a charge coupled device (CCD) or an active pixel sensor (CMOS) device) . The images captured by the imaging device can be processed to determine a data representation of the physical geometry of the sample as three-dimensional numerical data (sometimes referred to as a "point cloud") . Such processing is known.

Examples of sensors which operate using the principle of optical triangulation include the GapGun sensor, manufactured by Third Dimension Software Limited, and the LC60D digital laser scanner, manufactured by Metris. Optical triangulation sensors may be used to measure precise properties of sub- features on a component, e.g. the diameter of holes, the separation of spaced edges, the planar misalignment of adjacent surfaces, the radii of curved surfaces or chamfered edges. In other uses, the laser lines may be scanned along a surface to be measured to capture information capable of rendering a three-dimension image (e.g. by building up a model from a plurality of two-dimensional cross-sections) .

Laser triangulation scanning techniques may however be limited in cases where a feature of interest is difficult to access. The incident and reflected light beams must be able to pass unimpeded from the light source to the sample and back to the imaging device for the scanning to work properly.

Where features of interest are enclosed (e.g. internal) or covered by other parts of a component, this limitation can be a problem. For example, it may be difficult to align the laser line to take an appropriate measurement for checking precise properties of the feature of interest, or the other parts of the component may impede the scanning path to prevent measurement from being performed.

SUMMARY OF THE INVENTION

At its most general, the present invention proposes taking a physical impression of a sub-feature to be measured (i.e. effectively a three-dimensional "negative") using a tool that is arranged to locate one or more reference points or planes of the sub-feature within a notional coordinate system associated with the measurement field of view of an optical sensor. The reference points or planes of the sub-features may thus be transferred to a predetermined position or orientation within the coordinate system of the optical sensor, e.g. defined by the geometry of a substantially planar light beam or scanned laser rays of an optical sensor, e.g. an optical triangulation sensor. For example, one or more reference points relating to the sub-feature may be accurately located at a predetermined location on the reference plane (e.g. planar light beam) of an optical triangulation sensor, or a reference plane relating to the sub-feature may be accurately aligned or orientated with respect to the reference plane of the optical triangulation sensor.

By taking the physical impression, measurements can be made on the sub-feature in an indirect manner, e.g. at a location remote from the component on which the sub-feature is located. The invention can enable this to be done in an efficient and repeatable manner by providing a proxy reference frame for the sub-feature, which is related in a known way with the component.

According to one aspect of the invention, there may therefore be provided a locator jig for use with an optical sensor, the locator jig comprising: a frame defining a measurement volume for holding a deformable material (e.g. a volume of plastically deformable material and/or a mechanical probe); a first attachment portion for mounting the jig in a predetermined orientation relative to sensing apparatus comprising the optical triangulation sensor, in which

orientation a planar light beam emitted by the sensor

intersects the measurement volume; and a second attachment portion for mounting the jig on a component having a feature to be measured, the second attachment portion comprising one or more mating elements arranged to cooperate with the component to locate the measurement volume in a unique orientation with respect to the feature to be measured.

The locator jig may thus perform three functions.

Firstly, it may provide a measurement volume in which the physical impression of the feature to be measured may be transported, e.g. transferred from the component to a separate measurement location. Secondly it may fix the relative position of the measurement volume with respect to the component through the second attachment portion. Thirdly, it may fix the relative position of the measurement volume with respect to the optical sensor through the first attachment portion .

The invention may be used with other types of optical sensors. In this aspect, the invention may provide a locator jig for use with an optical sensor, the locator jig

comprising: a frame defining a measurement volume for holding a deformable element; a first attachment portion for mounting the jig relative to the optical sensor in a predetermined orientation with respect to a notional coordinate system of a measurement field of view of the optical sensor; and a second attachment portion for mounting the jig on a component having a feature to be measured, the second attachment portion comprising one or more mating elements arranged to cooperate with the component to locate the measurement volume in a unique orientation with respect to the feature to be measured.

The frame defining the measurement volume may be coupled to the first and second attachment portions in a manner to fix the position of the measurement volume relative to both the first and second attachment portions. Fixing the frame relative to the first and second attachment portions enables the locator jig to act as a proxy reference frame for the component .

The frame may act as a support for the deformable element. For example, the frame may comprise a cavity or recess for holding a block of deformable material or for housing a mechanical probe having one or more fingers arranged to locate on predetermined parts of the sub-feature. The cavity may be arranged to fit over the feature to be measured when the jig is mounted on the components and the mating elements of the second attachment portion cooperate with the component. The deformable element may be present in the measurement volume during mounting of the jig on the

component, or may be delivered (e.g. injected, extruded or the like) into the measurement volume after mounting.

The deformable element may comprise a deformable material capable of conforming its shape to the contours of a feature to be measured and retaining that shape upon removal from the feature. For example, solid materials that are softer than the material of the component and which exhibit plastic behaviour may be used. For example, modelling clay or a thin sheet of soft metal (e.g. aluminium) may be used. The deformable material may be heated before contact with the feature to improve its forming properties. In another example, the deformable material may be a transformable material, e.g. having a first formable phase and a second set phase. The change in phase may be achieved by heating. For example, the material may be a thermoplastic that is heated (e.g. to about 50°C) into a formable phase and which cools to the set phase. Alternatively, the material may be a curable polymer, which may be heat treated or otherwise cured to transform it from a formable phase to a set phase. In one embodiment, a two-part fluid or thixotropic setting mixtures (such as Microset 101 series thixotropic compounds), or materials similar to those used to create dental impressions, may be used as the deformable material. For example, the deformable material may be putty such as Microset 121 Forming Putty manufactured by Microset Products Limited

The material may be flowable (e.g. liquid, foam or powder) in the formable phase and solid in the set phase. The material may be supplied to the measurement volume by spray or injection after the jig is mounted on the component.

The type of support needed from the frame may vary depending on the type of deformable material used. For example, the frame may comprise a container for holding the deformable material. Alternatively, the deformable material after conforming (and setting where applicable) may be a self- supporting structure. The frame may comprise one or more support members, e.g. struts, ledges or the like, arranged to hold the deformable material in place in the measurement volume. The support members may penetrate into the deformable material .

Alternatively or additionally, the deformable element may comprise a physical tool, e.g. a mechanical probe or the like, adapted to contact the feature in a manner to fix the position of parts of the feature relative to the measurement volume. For example, the tool may include one or more movable fingers (e.g. spring pins or the like) that are movable to locate on known parts of the feature to be measured. When the jig is mounted on the component using the second attachment portion, the fingers may be manipulated to bring them into content with the feature to be measured. The position of the fingers may be fixed, e.g. locked, into the contact position. The spatial position of the fixed contact position of a finger within the measurement volume and/or the spatial relationship between the fixed contact positions of a plurality of fingers may be measured by the optical sensor to extract information about the feature to be measured.

The first and/or second attachment portions may be integrally formed with the frame. For example, the mating elements on the second attachment portion, which may mount the jig on the component in a unique orientation, may comprise one or more contact edges that are shaped to fit against

corresponding surfaces on the component. The contact edges may be arranged adjacent the measurement volume, e.g. on two opposite sides thereof. To aid the unique orientation, the contact sides may be positioned to overlie an irregular surface on the component, whereby the contact edges only fit flush against the component in one orientation. The irregular surface may comprise a lip, flange, shelf, web, protrusion or the like, formed on the component. The feature to be measured may be formed in or on the irregular surface.

The second attachment portion may be adapted to mount the jig on a plurality of different components. For example, the second attachment portion may have a plurality of sets of mating elements for each component. Alternatively, the mating elements may be adjustable between a plurality of positions, e.g. preset configurations, each position corresponding to a different component. In this manner, the jig may be adaptable for use on new components, i.e. may avoid the need to

manufacture a new jig for each new component or sub-feature.

In one embodiment, the contact edges may surround the measurement volume, wherein there is a closed line of contact between the jig and component when the jig is mounted on the component via the mating elements. This arrangement may be of benefit if deformable material is injected into the

measurement volume in a fluid (especially liquid) phase, as it may confine the area over which the deformable material contacts the component.

Where the feature to be measured is formed on a lip, flange or other longitudinally extending protrusion, the mating elements may be arranged to grip the protrusion. This may be achieved by forming the second attachment portion from a resilient material. A benefit of this arrangement is additional stability of the jig on the component.

Alternatively or additionally, the mating elements may comprise a plurality of support structures (e.g. arms, feet, fingers or other protrusions) arranged around the measurement volume. The support structures may provide contact points on the component to locate in a stable manner the measurement volume relative to the feature to be measured. The component may include seats for receiving the support structures. The seats may be part of existing shapes on the surface of the component, or may be specially formed for the purpose of metrology .

The first attachment portion may include one or more locating formations arranged to permit the measurement volume to be mounted in a predetermined orientation relative to a notional coordinate system of a measurement field of view of the optical sensor. The notional coordinate system may correspond to a known spatial region in which the optical sensor is capable of performing measurements. For example, it may be a region into which a detecting optical signal is transmitted. The notional coordinate system may be one- dimensional, two-dimensional or three-dimensional. For example, an optical triangulation sensor is arranged to emit a substantially planar light beam (referred to herein as a planar light beam) , which may define a notional two- dimensional coordinate system in the measurement field of view of the sensor. Additionally, the first attachment portion may be arranged to locate the measurement volume at a

predetermined position with respect to the notional coordinate system, i.e. at a known position within the measurement field of view of the optical sensor.

The locating formation of the first attachment portion may be arranged to engage with the optical sensor itself, e.g. to locate the jig relative to an aperture where an optical signal is detected. In another embodiment, however, the optical sensor may be mounted relative to a platform such that the notional coordinate system has a fixed orientation relative to the platform. In the optical triangulation sensor example, the plane of the planar light beam may have a known orientation relative to the platform, e.g. it may pass over or intersect the platform. In this embodiment, the locating formation may cooperate with a corresponding formation on the platform in a manner to cause the planar light beam to intersect the measurement volume at a predetermined

orientation. An advantage of this arrangement may be that the design of the optical triangulation sensor is not limited by requiring direct engagement with the first attachment portion.

The first attachment portion may be arranged to permit the measurement volume to be positioned in a plurality of predetermined orientations relative to the optical sensor, e.g. relative to the plane of the planar light beam emitted by the optical triangulation sensor. For example, the first attachment portion may have a plurality of locating

formations, each formation arranged to permit the locator jig to be mounted in a unique orientation relative to the optical triangulation sensor in which the plane of the emitted planar light beam intersects the measurement volume on a

predetermined plane through the measurement volume.

Alternatively or additionally, where a separate platform is used, the platform may comprise a plurality of corresponding formations for cooperating with the one or more locating formations of the first attachment portion to position the measurement volume in a plurality of orientations. Thus, in another aspect, the invention may provide optical metrology apparatus comprising: an optical triangulation sensor mounted in a fixed position relative to a platform; and a locator jig as described above mounted on the platform such that a planar light beam emitted from the sensor intersects its measurement volume; wherein the platform and first attachment portion of the locator jig comprise cooperating engagement formations for positioning the measurement volume of the locator jig in one or more (preferably a plurality of) predetermined orientations with respect to the planar light beam.

However, in a preferred embodiment the first attachment portion may be used to fixedly mount the jig on a static platform and move the optical sensor relative to the platform between a plurality of predetermined measurement positions. A robotic movement mechanism may be pre-programmed to perform the movements in an accurate and repeatable manner.

The locating formations may comprise interlocking elements, e.g. exhibiting asymmetry so that alignment is possible in only one orientation.

The invention may be used for measuring small features where manual alignment of the planar light beam may be difficult. For small features and for difficult to access locations, the jig may comprise an assembly of two or more detachable parts. The detachable parts may comprise a component mounting part having the measurement volume and the second attachment portion for mounting on the component, and a sensor mounting part having the locating formations of the first attachment portion for mounting relative to the optical triangulation sensor. In this embodiment, the component mounting part may be arranged to cooperate with the sensor mounting part, e.g. in an interlocking manner, to ensure that the locating formations of the first attachment portion are aligned uniquely with the measurement volume. The detachable nature of this embodiment means that the size of the component mounting part is not limited by the number of or size of the locating formations of the first attachment portion.

In another aspect, the invention may provide a method of measuring a feature on a component using an optical sensor using the locator jig described above. For example, where the optical sensor is an optical triangulation sensor, the method may comprise: mounting the locator jig on the component to locate the feature to be measured inside the measurement volume; forming an inverse image of the feature using

deformable material in the measurement volume; removing the locator jig from the component; and mounting the locator jig in a predetermined orientation with respect to the optical triangulation sensor, wherein a planar light beam emitted by the sensor intersects the measurement volume on the

predetermined plane. Mounting the locator jig in a

predetermined orientation with respect to the optical

triangulation sensor may comprise: mounting the locator jig on a static platform in a predetermined orientation with respect to the platform; positioning the optical triangulation sensor relative to the static platform in a first measurement configuration, in which the optical triangulation sensor emits a planar light beam that intersects the measurement volume in a first predetermined plane; and moving the optical

triangulation sensor relative to the static platform to a second measurement configuration, in which the optical triangulation sensor emits a planar light beam that intersects the measurement volume in a second predetermined plane.

Alternatively, the jig itself may be mounted on a three- dimensional movement mechanism, for example a suitably programmed robotic arm, which may be arranged to move the jig to the component and mount it thereon. If the deformable material is to be injected into the measurement volume after mounting, the robotic arm may also carry the injection mechanism. The robotic arm may be arranged to adopt one or more predetermined positions with respect to a fixed optical sensor, whereby the arm may act as the platform discussed above, to orientate the jig in a predetermined manner with respect to the optical sensor.

In a further aspect, the invention may provide optical metrology apparatus comprising: an optical triangulation sensor mounted on a positioning mechanism that is arranged to position the optical triangulation sensor relative to a static platform in a plurality of predetermined measurement

configurations; and a locator jig according to any preceding claim mounted on the static platform; wherein the static platform and first attachment portion of the locator jig comprise cooperating engagement formations for positioning the measurement volume of the locator jig at a predetermined orientation with respect to the static platform, and in each predetermined measurement configuration, the optical

triangulation sensor emits a planar light beam that intersects the measurement volume in a predetermined plane relative to the platform.

The apparatus and method disclosed above may permit accurate measurement of safety-critical or difficult to access features on manufactured (e.g. machine-made) components.

Examples of components on which features may be measured include turbine components, where features of interest include locking slots, loading slots and broach slots. The properties of these features that may be measured using the impression of the feature include chamfers, radii, gap between adjacent edges, burrs, step and angle between adjacent surface, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention is described below with reference to the accompanying drawings, in which:

Fig. 1 shows a left side perspective view of part of a dovetailed rim slot on a turbine shaft;

Fig. 2 shows a locator jig that is an embodiment of the invention mounted on the rim slot shown in Fig. 1;

Fig. 3 shows a right side perspective view of the locater jig mounted on the rim slot of Fig. 2 during injection of deformable material;

Fig. 4A shows a perspective view of the locator jig shown in Fig. 2 after dismounted from the component; Fig. 4B shows a perspective view of the locator jig shown in Fig. 4A without the deformable material;

Fig. 5 shows a perspective view of the locator jig mounted in a sensor mounting part;

Fig. 6 shows a perspective view of the locator jig mounted on a platform adjacent an optical triangulation sensor;

Fig. 7A shows a perspective view of the locator jig fixedly mounted on a platform with an optical triangulation sensor in an overhead measurement position; and

Fig. 7B shows a perspective view of the locator jig fixedly mounted on a platform with an optical triangulation sensor in a side measurement position DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

The invention is discussed in detail below with reference to a particular aerospace component. The invention need not be limited to this specific use; the general principles may be equally applicable in other manufacturing fields.

Fig. 1 shows a cut away portion of the rim 10 of a turbine shaft. The rim 10 has a groove 12 formed around its outwardly facing circumferential edge for receiving the locking key of a turbine blade. The groove 12 has inwardly tapering side surfaces 14 to form a dovetailed slot having two overhanging lips 16 at its entrance. One of the lips 16 has a recess 18 machined into it. This provides a widened portion of the groove 12 suitable for loading the locking key of a turbine blade in a loading orientation wherein it is movable along the dovetailed slot. Once in position the blade is rotated, which turns the locking key in the slot to a locked orientation. This structure is well known.

The junction 22 between the each side 20 of the recess 18 (also referred to as a loading slot) and the lip 16 needs to be rounded (e.g. chamfered) to prevent undesirable stresses from concentrating at that location during operation of the turbine. Optical metrology may be used to check that the manufactured chamfer falls within a prescribed safe range. Other properties of the recess 18 may also need to be

measured, such as the radius of curvature of its inner wall, its width, i.e. gap between the side walls 20 or the like. Performing metrology using a optical triangulation sensor may be hampered by the opposite side wall of the groove 12. This may make it difficult to align precisely a planar light beam with the recess 18.

Fig. 2 shows a locator jig 24 mounted on the rim 10. The jig 24 comprises a body 26 that acts as a frame defining a measurement volume. The body 26 effectively presents a opening to the measurement volume that is adapted to fit flush against one side of the rim 10 to enclose the recess 18. In particular, the body includes two opposing side walls 28 that have outwardly facing contact edges 30 shaped to abut the protruding lip 16. The side walls 28 and lip 16 cooperate such that the jig 24 adopts a unique stable orientation with respect to the lip 16. A top edge of the opening may abut the top surface of the lip 16 and a bottom edge may abut the interior surface of the groove 12 to completely enclose the measurement volume.

The jig 24 may be made from a plastic material that exhibits some resilient flexibility, whereby it can be snapped in place over the lip 16. The back surface of the body has a port 32 formed therein which leads to the measurement volume. As discussed below, deformable material may be injected into the port 32 to fill the measurement volume and conform to the feature to be measured (in this case, the recess 18) to create an impression thereof. The jig 24 also includes a locating formation used for aligning the jig with the planar light beam emitted from an optical triangulation sensor. In this embodiment the locating formation is an angled fin 34 that includes a sideways protruding projection 36 and a tubular end 38 to give it an asymmetrical shape. The alignment with the planar light beam may be indirect, as explained below with reference to Figs. 5 and 6, because the fin 34 is used to locate the jig 24 in a separate sensor mounting part which in turn is mountable in a plurality of predetermined orientations with respect to an optical triangulation sensor.

Alternatively, as explained below with reference to Figs . 7A and 7B, the sensor mounting part may be mounted in a fixed orientation on a stand that has a known position relative to an optical triangulation sensor that can be moved between a plurality of predetermined measurement positions, e.g. using a robotic control or the like. Fig. 3 shows a view from the other side of the rim 10 during the next stage of the metrology process. A nozzle 40 from a deformable material applicator (not shown) is coupled to the port to deliver deformable material into the interior of the jig 24. The deformable material may be any of the examples discussed above. Injection may be achieved using a syringe- or plunger-type device, such as a simple dispensing gun. In other embodiment, the port 32 may be omitted, and the measurement volume may already contain deformable material, such as modelling clay or the like, during mounting on the rim.

Fig. 3 also shows an applicator 42 for the jig 24. The applicator 42 includes a tongue portion 44 that is received in the groove 12 and a recessed portion 46 for engaging with a rib 48 (see Fig. 2) on the back of the body 26 of the jig. The applicator 42 ensures that the jig 24 is properly located against the lip 16 and also has a projecting handle 50 to facilitate insertion and removal of the jig 24 after the impression is obtained (e.g. after the deformable material has set) .

Fig. 4A shows the jig 24 after removal from the rim 10. The view in Fig. 4A looks into the measurement volume defined by the body 26. Here the measurement volume is full of deformable material exhibiting a physical impression 52 (e.g. inverse three-dimensional image) of the recess 18 and

surrounding part of the rim 10. It may be seen that

properties of the recess 18 for measurement are provided in reverse on the physical impression 52. The deformable material may be chosen to ensure that the level of detail required for measurement, i.e. effectively the resolution of the physical impression, can be achieved.

Fig. 4B shows the jig 24 without the deformable material in the measurement volume. Features in common with Fig. 4A are given the same reference number. In Fig. 4B, the

measurement volume is defined by an internal wall having an outlet 33 that communicates with the port 32 to deliver the injected deformable material. In this embodiment, the internal wall includes a hump 31 shaped roughly to conform to the recess 18 in order to reduce the amount of deformable material needed to take the physical impression. On either side of the hump 31 is a projecting foot 35. The feet 35 are arranged to engage with the side walls 20 of the recess to centre the hump 31 in the recess and hence fix a transverse position of the recess 18 in the measurement volume. In this way the jig ensures that the feature to be measured has both a unique orientation and unique position relative to the measurement volume.

Fig. 5 shows the jig 24 mounted in a separate sensor mounting part 54. The sensor mounting part 54 is a rigid block having a recessed portion 56 shaped to receive the body 26 of the jig 24 and a narrow slot 58 shaped to receive fin

34. The slot 58 includes features that correspond to the tubular end 38 and sideways protruding projection 36 in a manner that fixes the orientation in which the jig can be received in the sensor mounting part 54. Together the jig 24 and sensor mounting part 54 form an assembly 60 that is mountable in one or more predetermined orientations with respect to an optical triangulation sensor, as shown in Fig. 6. When the locating feet 35 are also used, the position of the assembly 60 relative to the sensor fixes both the position and orientation of the physical impression of the feature to be measured relative to the sensor's field of view.

In Fig. 6, an optical triangulation sensor 62 is mounted on a static work surface 64. In this embodiment, the optical triangulation sensor 62 is the GapGun model made by Third Dimension Software Limited. The sensor 62 is arranged to emit a planar light beam (not shown) from light source 72. Light reflected off a sample into the field of view of the sensor, i.e. into detector aperture 70, is detected and processed to determined spatial information about the sample. In this embodiment, a platform 66 is mounted on the work surface 66.

The platform 66 is mounted such that the planar light beam intersects with it at an angle. The platform 66 includes a plurality of mounting formations 74 (which in this embodiment are L-shaped holes) for receiving corresponding formations 76 (e.g. L-shaped protrusions) on the assembly 60. The

formations 74, 76 on the assembly 60 and platform 66 are arranged to cooperate to orientate the measurement volume in the jig 24 relative to the optical triangulation sensor 62 so that the planar light beam intersects the measurement volume in a predetermined position. Each predetermined position may be set to correspond to the measurement of a desired property of the impression in the measurement volume. As shown in Fig. 6 , the formations 74 may be provided on more than one side of the senor mounting part 54. This may permit the optical sensor to measure parts of the impression that correspond to inaccessible parts of the original feature.

Figs. 7A and 7B show an alternative arrangement in which the assembly 60 is mounted in a unique orientation on a static stand 82 . In this arrangement the sensor 62 is movable between predetermined measurement positions relative to the stand 82 , preferably under the action of a robotic movement mechanism 80 . Using a robot has the advantage of permitting the measurement positions to be pre-programmed. This allows the impression (measurement volume) to be orientated relative to the robot in a single position, e.g. using only one mounting formation, rather than the multiple mounting

formations shown in Fig. 6 .

In Fig. 7A, the robotic movement mechanism 8 0 is in a first configuration whereby the sensor 62 is located directly above the measurement volume. In Fig. 7B, the robotic movement mechanism 8 0 is in a second configuration whereby the sensor 62 is located to the side of the measurement volume. Configurations corresponding to other positions may also be used .

The robotic movement mechanism 8 0 may comprise any suitable positioning device, e.g. coordinate measuring machines or measuring arms such as a FaroArm® available from Faro Technologies UK Ltd.