WO2004017020A1

WO2004017020A1 - Device for measuring the relief variations of an object

Info

Publication number: WO2004017020A1
Application number: PCT/FR2003/002070
Authority: WO
Inventors: Jean Brillaud; Fabienne Lagattu
Original assignee: Ecole Nationale Superieure De Mecanique Et D'aerotechnique; Centre National De La Recherche Scientifique Cnrs
Priority date: 2002-07-16
Filing date: 2003-07-03
Publication date: 2004-02-26
Also published as: AU2003260653A8; AU2003260653A1; FR2842591A1; FR2842591B1

Abstract

The invention relates to measuring the relief variations of an object and/or to the deformations of said object in the direction of the relief thereof (z). The inventive measuring device comprises an inclined optical projection means (10) for displaying the reference image of a rough plastering whose grains are pseudo-randomly distributed, and visualisation means (20) for obtaining the test image of the thus illuminated object. The position variations of the grains of the rough plastering between the test image and the reference image which represent the variations in the relief of the object are measured by correlation with the aid of comparison means.

Description

Device for measuring variations in the relief of an object

The invention relates to the measurement of the relief of an object and / or of the deformations of an object in a relief direction of this object, by optical techniques.

Knowledge of the laws of behavior of materials is necessary today, especially in the field of precise mechanics and the design of parts in such materials. In general, it is preferable to measure the relief on the entire surface of an object, or even complete fields of deformations or stresses on the surface of this object, by non-destructive techniques, typically optical techniques.

Among these optical techniques, the following are known in particular from the publication:

"Industrial applications of shadow moiré and projection at the limit of technical possibilities", A.AUBOIN, V.VALLE, F.BRE AND (Photomechanical Congress 2001, Futuroscope (France), April 2001); a measure of the relief from a shadow moiré. This technique consists in projecting optically onto the surface of an object to be analyzed the image of a network of parallel lines spaced at a constant pitch. In an observation direction forming an angle with the normal of the surface analyzed, there are level lines characterizing the relief of the object.

However, this technique, even if it makes it possible to quickly obtain satisfactory measurements in macromechanics, is not sufficiently precise to measure variations in relief on the order of a few microns, one of the best performances achieved being an accuracy on the order of 20 microns.

Another interesting approach, but which however does not make it possible to determine a variation in the relief of an object, consists in the physical application of a speckle on the surface to be analyzed of the object.

The speckle pattern typically includes colored speckles contrasting with the background of the speckle _(for. Example black specks on white background). It is removed, after analysis of the object, by application of a solvent.

The proposed technique, details of which are described in:

"White light speckle correlation applied to strain measurement in notched composites", F. LAGATTU, TQLAM, J.BRILLAUD, M. C. LAFARIE-FRENOT (ECC 10, Brugge (Belgium), 2002); allows in particular to measure deformations of the object in the mean plane of its surface. For this purpose, the respective images are digitized, before deformation and after deformation, of the surface of the object coated with the speckle. A correlation calculation is then carried out on a plurality of groups of pixels each comprising a hundred speckle spots, which makes it possible to identify the speckle grains and to discriminate those which have been displaced by the effect of the deformation. Local movements of the speckled grains are then measured, these movements characterizing localized deformations of the object in the plane of its surface. However, as indicated above, this technique makes it possible to measure deformations in the mean plane

(x, y) from the surface of the object and not in a relief direction (z), perpendicular to the surface of the object.

In addition, the application of a speckled layer, before analysis, then of a solvent to remove this layer, after analysis, increases the number of steps in the process which is carried out to obtain the measurements. The present invention improves the situation.

To this end, it proposes a device for measuring the relief of an object and / or deformations of an object in a direction of relief of this object, comprising: - means of optical projection onto the object of an image of reference representing a pattern of substantially pseudo-random, predetermined shape;

- shooting means for obtaining at least one analysis image representing the object illuminated by said optical projection means, the shooting means and the optical projection means advantageously forming an angle relative to the object ; and

- Comparison means arranged to measure, by correlation, variations in the shape of the pattern between the analysis image and the reference image, these variations in shape being representative of variations in the relief of the object.

In a preferred embodiment, the reference image represents a speckle whose grains are distributed substantially pseudo-randomly. The comparison means are then arranged to measure deviations from distance between the respective positions of the grains in the reference image and in the analysis image.

Advantageously, the reference and analysis images are digital and the comparison means are arranged to evaluate the above-mentioned correlation on a plurality of respective groups comprising a chosen number of pixels, of the reference image and of the image. analysis.

In the embodiment where the aforementioned pattern is a speckle, the speckle grains being in this embodiment diameters of the order of a few tens of microns, the aforementioned groups of pixels comprise at least 1000 pixels while the size of a pixel is close to ten microns aside. Advantageously, a precision of measurement of the relief or of displacements in the relief is then obtained which is less than the size of a pixel and of the order of a micron.

In addition, by optically projecting only an image on the object, representing a pseudo-random pattern such as a speckle, one avoids deteriorating the object, on the one hand, and physically applying a layer of speckle on the surface of the object, on the other hand.

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings in which: - Figure 1 illustrates a device for measuring the relief of an object or local deformations of this object in a direction of relief, within the meaning of the present invention; FIG. 2 schematically represents means 10 that the measuring device comprises, capable of projecting optically the image of a pseudo-random pattern, in the form of an MT speckle in a preferred embodiment of the invention, on the surface of the object;

FIGS. 3A and 3B respectively represent an illuminated flat surface and the surface of the object illuminated by the aforementioned projection means, said surface of the object having a relief to be measured; - Figure 4 schematically illustrates .the relationship between a deformation dx of the projected pseudo-random pattern and a variation dz in the relief of the object;

- Figure 5 schematically shows the shape of the pattern in a reference image area comprising N pixels; - Figure 6A schematically shows a plurality of image areas of the object illuminated by the projection means;

- Figure 6B schematically represents the object according to section B-B of Figure 6A; - Figure 6C schematically shows the object according to section C-C of Figure 6A; and

- Figure 6D schematically shows the object according to section D-D of Figure 6A.

First of all, reference is made to FIG. 1, on which an ECH object rests on a PE sample holder, advantageously arranged to move in an xy plane perpendicular to a relief direction z of the ECH object. In a preferred embodiment, the relief measurement device within the meaning of the present invention comprises a conventional computer equipped with a central processing unit UC, comprising a microprocessor, a random access memory, a read-only memory and at least one graphics card (not shown). The central unit is connected to an input device, such as a CLA keyboard, and to an SCR display monitor, respectively to order the start and end of measurement and to visualize the reliefs of the object which are finally shown on the SCR screen.

Furthermore, shooting means 20, which are in the example described a digital camera, are connected to the graphics card of the central unit to acquire images of the ECH object.

Preferably, the digital camera 20, the sample holder PE and the object ECH which it supports are arranged in a substantially opaque enclosure 30, preferably a dark room so as to limit the stray optical reflections liable to degrade the quality of the relief measurements performed.

The opaque enclosure 30 nevertheless includes a transparent FE window for receiving lighting from projection means 10 that the device further comprises within the meaning of the present invention. By way of nonlimiting example, these lighting means 10 comprise:

- a white light lamp;

- a celluloid film representing a pseudo-random pattern, placed in front of the transparent lighting lamp, in the manner of a conventional slide;

- where appropriate, means for focusing the lighting on the object whose relief is measured; and

- A tripod for positioning the projection means 10 so that the incidence α of the lighting delivered is substantially oblique. Referring to Figure 1, the optical rays from the projection means 10 form a non-zero angle α relative to the axis z representing the relief direction of the object ECH. Referring now to FIG. 2, the projection means 10 are arranged to project a reference image MT, on the surface of the object ECH. This reference image MT preferably represents a speckle comprising, black grains on a white background. The grains are distributed randomly in this speckle. Nevertheless, the reference image MT is previously recorded, in digitized form, in a memory of the central processing unit UC. This memory also stores the instructions of a shape recognition software so that the implementation of the software makes it possible to identify the statistical position of the speckle grains. Thus, the respective positions of the speckled GR grains, in the reference image MT, are well known. For this reason, the pattern that the reference image MT represents is "pseudo-random".

The grain distribution is, in principle, never identical from one area of the speck to another, but the position of the grains in the speckle is always known.

When the projected image MT encounters a relief (hollow or bump) on the surface of the ECH object, it is understood that the speckle grains change position in the xy plane. The shooting means 20, arranged above the xy plane, capture an analysis image of the object thus lit by the projection means 10. This analysis image is then stored (in random access memory or in memory dead) in the CPU. Furthermore, the reference image MT (FIG. 2) has been projected beforehand on a flat, regular surface. It is also picked up by said shooting means 20 and stored, as a reference image, preferably in the read-only memory of the central processing unit UC. • Thus, by comparison between the reference image and the analysis image, we note differences in the position of the speckled grains in the xy plane, in the presence of a relief on the surface of the ECH object.

The optical rays from the projection means 10 are arranged around a plane xz, perpendicular to the direction y (Figure 1). The grain position variations are then measured along a direction of the analysis image (here the x axis), corresponding substantially to a parallel to the trace of the incidence plane (here the xz plane) on the surface of the object illuminated by the projection means 10.

In addition, the variations in the position of the grains GR are measured in algebraic values in a direction substantially opposite to the direction of illumination of the projection means 10 on the surface of the object ECH.

Thus, a difference in algebraic value in the position of the speckled grains, along the x axis, will be representative of a relief (hollow or bump) on the surface of the ECH object, or even of a local deformation on this surface.

FIG. 3B, by comparison with FIG. 3A, shows the action of a relief REL on the surface of the object ECH, or of out-of-plane displacements on the surface of the object, on the image of the speckle MT, in a plurality of measurement areas, on the surface of the object. In a measurement zone A, which presents a variation in height dz, outside the xy plane, the image of the speckle MT will have moved (or distorted) by a distance dx, along the x axis, such that shown in Figure 3A. Thus, measuring vectors out of plane displacement of the ECH object in all zones of its surface amounts to measuring vectors of displacement of the speckled grains in the xy plane in each of these zones. Referring in particular to FIG. 4, by measuring the displacement of one or more grains of the speckle MT, in the zone A, - we deduce therefrom, knowing the angle α of optical projection of the speckle on the surface of the ECH object, the displacement out of plane in this zone A (by variation of the relief REL of the object or even by local deformation on the surface of the object in this direction of relief). This out-of-plane displacement is obtained by the following formula: dz = -dx / tg (α) (1) where:

- dz corresponds to a variation in height of the relief,

- dx corresponds to a variation in the position of the grains along the x axis, in the analysis image relative to the reference image, - α corresponds to the angle of incidence (not zero) that form substantially the rays from the projection means 10 with the z axis perpendicular to the illuminated surface of the object, and

- tg corresponds to the "tangent" mathematical function of an angle. To obtain a mapping of out-of-plane deformations on the surface of the object, in case. this deformation is homogeneous in the thickness e ₀ of the object

(Figure 3B and Figure 4), the following equation is used more precisely: ε _z = -2dx / (e ₀ tg (α)) (2) where:

- ε _z corresponds to a local deformation, out of plane and supposed to be homogeneous in the thickness of the object, in a zone A of the object,

- e ₀ corresponds to the initial thickness of the object, without deformation, in this zone A,

- dx corresponds to a variation in the position of the grains along the x axis, in a second photograph (during the deformation) compared to a first photograph (before deformation), measured in algebraic value in the area A,

- α corresponds to the angle of incidence (not zero) that the rays from the projection means 10 substantially form with the axis z perpendicular to the illuminated surface of the object, and

- tg corresponds to the "tangent" mathematical function of an angle.

Thus, by comparing the respective positions of the grains of the speckle MT, in the reference image (optical projection of the speckle on a flat surface), on the one hand, and in the analysis image (optical projection of the speckle on the surface of the object), on the other hand, it is possible to obtain the out-of-plane displacement in all zones A on the surface of the object and, from there, a complete cartography of the relief of the object. Similarly, by comparing the respective positions of the grains of the MT speckle, in a first photograph of the object before deformation, on the one hand, and in a second photograph of the object during deformation, on the other hand, it it is possible to obtain measurements of the deformations of the object outside the xy plane.

Thus, to obtain the field of out-of-plane deformations on the surface of the ECH object, two shots are provided in digital photography, taken by the shooting means 20: a first shot before the deformation and a second shot during the deformation. One then applies the formula (2) to obtain the values of the deformations out of plane in all zones of the object ECH.

Advantageously, the comparison of the grains in the reference image and in the analysis image is obtained by carrying out a statistical analysis of the grains in these images, which will be described below.

Referring now to FIG. 5, the analysis image captured by the photographing means 20 and digitized is cut out into a plurality of pixel zones, each pixel zone corresponding to an area A of the aforementioned type, in which is measured an out-of-plane deformation and / or a variation in the relief of the analyzed ECH object. In the example described, the black GR grains of the speckle have on average a diameter of between 20 and 30 μm. Each pixel has, in the example described, dimensions of 9 μm long, 9 μm wide. Thus, there is on average a grain GR of speckles distributed over nine pixels PI (three pixels in length by three pixels in width). Preferably, each zone of pixels ZP is 40 pixels long, 40 pixels wide (N = 40), which corresponds to 1600 pixels per ZP area. Thus, each ZP zone comprises on average more than one hundred GR grains of speckles, here 178 GR grains. This number of grains GR per pixel area ZP is sufficient to identify, by correlation, the grains of the reference image in the analysis image. In principle, it is in fact preferable to have at least one hundred grains GR per "measurement point" (therefore per pixel zone ZP) to correctly perform the statistical correlation.

It is thus possible to measure the displacement of the grains ^' in the x direction, from the reference image to the analysis image.

Under such conditions and with an appropriate angle of incidence α (for example of the order of 40 °), an accuracy of the relief and / or out-of-plane displacement measurements of the order of a micron is obtained.

In a preferred embodiment, the dimensions of the analysis area of the surface of an ECH object whose relief is to be measured are approximately 20 mm long by 20 mm wide, which corresponds to more than 3000 "points in the example described (each corresponding to a zone of pixels ZP mentioned above).

Referring to FIG. 6A, on which six pixel zones ZP1 to ZP6 have been represented, the speckled grains GR whose positions are identical are identified by calculation of correlation between an analysis image and a reference image. between the two images and the grains GR whose positions are modified between the two images (here in the zones of pixels ZP3 and ZP4). In this figure 6A, there is shown the grains GR of the speckle of the analysis image in solid line, while the grains of the reference image are represented by dotted lines. There is in particular an average displacement, measured in algebraic value dxl, of the grains GR in the pixel area ZP3 (to the left, as represented in FIG. 6A>, and an average displacement, measured in algebraic value dx2, of the grains GR in the pixel area ZP4 (to the right, as shown in FIG. 6A). From these mean displacements dxl and dx2, we estimate variations in average heights in the relief of the object dzl and dz2, in zones of the surface of the object corresponding to these zones ZP3 and ZP4. The variations in height dzl and dz2 are thus representative of a bump followed, in the direction x, by a hollow, in the relief of the object Thus, in section BB of FIG. 6B, a bump of height dzl is observed in the zone ZP3, while the adjacent zones ZP5 and ZP1 have a flat relief. Furthermore, in section DD of FIG. 6D, there is a trough of depth dz2 in the zone ZP4, while the adjacent zones your ZP2 and ZP6 have a flat relief. On the other hand, in section CC of FIG. 6C, there is a slope which extends from zone ZP3 to zone ZP4, from + dzl to -dz2. The flat relief of the adjacent zones ZP1, ZP2, ZP5 and ZP6 is represented by a horizontal line PL (broken long and short).

Each zone of pixels ZP thus represents a "measurement point" in the surface of the object.

Preferably, the displacements of the GR speckles grains between the reference image and the analysis image (or even between the image before deformation and the image during an out of plane deformation of the surface of the object) are measured by parabolic interpolation of a correlation function, which allows to have a sub-pixel sensitivity. Advantageously, a software for correlation calculation and relief and / or deformation estimation ^' out of plane, provides an interface with the user (in particular for starting the start of the measurements and the end of the measurements). Preferably, this software estimates the relief and / or the out-of-plane deformations from equations (1) and (2) defined above.

Thus, according to one of the benefits of the ^'present invention, relief of the measures or deformation out of plane are made in a non destructive and contactless, which avoids degrading the analyzed object.

In addition, according to another advantage of the present invention, the accuracy of the measurements of the relief or the deformations, out of plane, along the direction z, offers great sensitivity. In particular, an accuracy of the order of a micron could be obtained under the conditions described above (178 grains of speckles on average per 40 × 40 pixel area). In addition, with the current means of digital cameras, the size of a pixel is close to 9 μm long by 9 μm wide. Advantageously, the spatial resolution (currently 360x360 μm ² ) of the device described above can be further improved with still more efficient means of shooting, in particular if the pixel size is still less than 9 μm x 9 μm. Furthermore, the dimensions of the speckle grains, here of the order of a few tens of microns in diameter, are likely to evolve with speckle printing techniques, in particular to decrease. Thus, with finer GR grains, the number of pixels per zone ZP can decrease and it is possible to obtain an even better spatial resolution.

Of course, the present invention is not limited to the embodiment described above by way of example; it extends to other variants.

Thus, it will be understood that it can also be provided to measure “simultaneously” deformations out of plane (along the z axis) and in the mean plane of the surface of the object (xy plane). By way of nonlimiting example, it may be provided to physically apply a layer of speckle to the surface of the ECH object to measure the displacements in the xy plane, while the image of another speckle is projected optically on the object to measure the out-of-plane deformations along the z axis as described above. Simultaneous measurements are obtained by playing on the respective colors of the speckles and therefore in particular on the wavelength of the lighting by the projection means 20 (colored grains on a black background). In such a case, a double correlation can be carried out, on the one hand, on the position of the grains and, on the other hand, on the color of the grains, identified by a pixel value associated with this color.

Thus, there has been described above projection means 10 capable of emitting a speckle from white light. Of course, as a variant, a monochrome emission can be provided, in particular from a laser source. Furthermore, an ECH object has been described above carried by a PE sample holder. Advantageously, if it is desired to analyze a plurality of zones of the surface of the object (each zone being 20 mm long by 20 mm wide), provision may be made for motorization of the PE sample holder. , in the xy plane, to analyze surfaces larger than 400 mm ² .

The configuration described above and according to which the shooting means 20 are arranged in front of the surface of the object to be analyzed, while the optical projection of the speckle is effected at ^an oblique angle α, although advantageous , can also be the subject of a variant according to _. which the incidence is normal while the shooting is oblique. However, it is preferable, in any configuration, that the shooting means and the optical projection means form an angle α relative to the object.

A speckle pattern has been described above as a pseudo-random pattern. Here, the deformation of the projected pattern, created by the relief of the object (roughness, hollow or bump on the surface of the object), manifests itself by a virtual displacement of the speckled grains. Of course, other variants of pseudo-random pattern can be envisaged. However, it is preferable that this pattern does not reproduce identically from one "measurement point" (or from a pixel area in the example described) to the other.

Claims

claims

1. Device for measuring the relief of an object and / or the deformations of an object in a relief direction of this object, characterized in that it comprises:

- optical projection means (10) on the object of a reference image (MT) representing a pattern of substantially pseudo-random, predetermined shape;

- shooting means (20) for obtaining at least one analysis image representing the object illuminated by said optical projection means, the shooting means (20) and the optical projection means (10) forming an angle (α) relative to the object; and

- comparison means (UC) arranged to measure, by correlation, variations in shape of the pattern between the analysis image and the reference image, said variations in shape being representative of variations (dz) in the relief of the object.

2. Device according to claim 1, characterized in that said reference image represents a speckle whose grains (GR) are distributed substantially pseudorandomly, and in that the comparison means are arranged to measure differences in distance (dx ) between the respective positions of the grains in the reference image and in the analysis image.

3. Device according to claim 2, characterized in that the comparison means are arranged to evaluate a correlation between the grains of the reference image and the grains of the analysis image, in order to identify the grains of the reference image in the analysis image.

4. Device according to one of the preceding claims, characterized in that said reference and analysis images are digital, and in that the comparison means are arranged to evaluate said correlation on a plurality of respective groups (ZP) comprising a chosen number of pixels (N), of the reference image and of the analysis image.

5. Device according to claim 4, characterized in that the comparison means are arranged to measure a variation (dx) of average shape in each group of pixels (ZP), representative of an average variation in height (dz) of an area of the object represented by this group of pixels.

6. Device according to one of claims 4 and 5, taken in combination with one of claims 2 and

3, characterized in that the speckle grains have diameters of the order of a few tens of microns, and in that said groups of pixels comprise at least 1000 pixels, while the size of a pixel is of the order about ten microns, which makes it possible to obtain a measurement accuracy of the relief or of displacements in the relief of the order of a micron.

7. Device according to one of the preceding claims, characterized in that the shooting means (20) are arranged substantially opposite a surface of the object to analyze, while the optical projection means (10) are arranged to project the reference image (MT) in substantially oblique incidence (α) on said surface of one object.

8. Device according to claim 7, characterized in that the variations in shape of the pattern are measured along a direction (x) of the analysis image corresponding substantially to a parallel to the trace of the plane of incidence on said surface of the object illuminated by the projection means.

9. Device according to claim 8, characterized in that the variations in shape of the pattern are measured in algebraic values in a direction substantially opposite to the direction of illumination of the projection means on the surface of the object to be analyzed.

10. Device according to claim 9, characterized in that the comparison means are arranged to estimate variations in relief on the surface of the object, according to the formula:

dz = - dx / tgα (1)

or :

- dz corresponds to a variation in height of the relief,

- dx corresponds to a variation in shape of the pattern in the analysis image relative to the reference image, α corresponds to the angle formed by said oblique incidence with a perpendicular to the illuminated surface of the object, and

- tg corresponds to the "tangent" mathematical function of an angle.

11. Device according to one of claims 9 and 10, characterized in that the comparison means are arranged to estimate deformations of the object in a direction perpendicular to an analysis surface of the object, according to the formula:

ε _z = - 2dx / eotgα (2)

or :

- ε _z corresponds to a local deformation, out of plane and supposed to be homogeneous in the thickness of the object,

- dx corresponds to a variation in shape of the pattern in the analysis image compared to the reference image, - e ₀ corresponds to the initial thickness of the object, without deformation, α corresponds to the angle that forms said oblique incidence with a perpendicular to the illuminated surface of the object, and - tg corresponds to the "tangent" mathematical function of an angle.