FR3000265A1 - METHOD FOR EVALUATING THE METROLOGICAL PERFORMANCE OF A CINEMATIC FIELD MEASUREMENT BY CORRELATION OF DIGITAL IMAGES - Google Patents

Abstract

The invention essentially relates to a method for evaluating the metrological performance of a measurement by correlation of digital images, of a kinematic field of a structure subjected to mechanical and / or thermal stresses, characterized in that it comprises the steps of: a) recording (200) a digital reference image of the surface of the structure by means of a digital image acquisition device, b) recording (201) a digital image, comprising a determined optical modification of the surface of the structure by means of the digital image acquisition device; c) calculating (206) the field of displacement of the points of the surface of the structure, measured by the acquisition device; digital images (100), d) evaluating (207, 208) the accuracy of the measurement of the field of displacement of the points of the surface of the structure by the digital image acquisition device.

Description

TECHNICAL FIELD The invention relates to a method for evaluating the metrological performance of a measurement of a kinematic field by correlation of digital images. The invention is more particularly, but not exclusively, adapted to the field of measurement techniques of deformations and displacements of a structure subjected to mechanical and / or thermal stresses. STATE OF THE ART According to the prior art, techniques for measuring a kinematic field by correlation of digital images are used in order to know the behavior of a structure subjected to mechanical and / or thermal stresses. Kinematics is here the movement of the structure. These techniques make it possible to measure displacements and to calculate the deformations of said structure.

Said techniques implement a digital image acquisition device and a digital image correlation software. The structure has a surface defined in an object frame (X, Y, Z). Said surface is either naturally heterogeneous or heterogeneous by creating a random pattern called mouchetis. The digital image acquisition device records a reference digital image of said surface. Said digital image is divided into pixels defined in an image frame (X ', Y', Z '). The heterogeneity of the surface of the structure is then rendered at the level of the digital image by gray levels different from one pixel to another. A point P of the surface defined in the object reference therefore corresponds to a pixel P 'of the reference digital image, defined in the image reference and having a determined gray level. Mechanical or thermal stress is applied to the structure. Said bias distorts the structure and in particular the surface studied. The point P of the surface is thus displaced during the application of the mechanical forces in a final position of unknown coordinates in the object reference. The digital image acquisition device records at regular intervals a series of digital images of the structure during the application of the mechanical forces.

The digital image correlation software makes it possible to find the coordinates in the image reference (X ', Y', Z ') of the pixel corresponding to the point P for each of the digital images recorded. Said coordinates make it possible to obtain a kinematic field. This kinematic field makes it possible to deduce the behavior of the studied surface. However, these techniques do not allow evaluation of the metrological performance of the measurement where the measurement is made, without moving and / or deforming the structure. According to the prior art, a first method makes it possible to measure the accuracy of the measurement of the digital image acquisition device. This precision measurement is performed before measuring the kinematic field by digital image correlation. In said precision measurement a second structure is implemented. A reference point sensor is associated with said second structure to be studied to evaluate the accuracy of the measurement of the digital image acquisition device by comparison of measurements. However, the measurement chain for the evaluation of the accuracy is different from the measurement chain of the kinematic field. Indeed, the structure used is different, so the mouchetis is different. In addition, the optical settings and lighting conditions vary. In addition, this method does not fully validate the displacement field measurement and the digital image correlation software is not evaluated. A second method implements virtually transformed images to evaluate the digital image correlation software. However, this method does not validate the measurement of the kinematic field, the digital image acquisition device not being evaluated. There is therefore no method for the complete evaluation of the metrological performance of the measurement at the place where the measurement is made, without moving and / or deforming the structure. DISCLOSURE OF THE INVENTION The object of the invention is in particular to solve this problem. To this end, the invention relates to a method for evaluating the metrological performance of a measurement by correlation of digital images, of a kinematic field of a structure subjected to mechanical stresses, said method implementing a digital image acquisition device, characterized in that it comprises the steps of: a) recording a digital reference image of the surface of the structure by means of the digital image acquisition device, b) recording a digital image, comprising a determined optical modification, of the surface of the structure by means of the digital image acquisition device, c) calculating the displacement field of the points of the surface of the structure, measured by the device of acquisition of digital images, from the set of digital images obtained by the digital image acquisition device, d) evaluating the accuracy of the measurement of the displacement field acement of the points of the surface of the structure by the digital image acquisition device by comparing the displacement field calculated in step c) with a field corresponding to the determined optical modification of step b). Thus, the displacement in translation of the digital image acquisition device makes it possible to simulate a displacement of the structure without actually moving said structure. The method according to the invention is therefore not destructive, the structure is not requested and is then used for the measurement of the displacement field. The invention can be implemented according to the advantageous embodiments described below, which can be considered individually or in any technically operative combination. Advantageously, step b) is repeated for different optical modifications. Thus, the evaluation of the metrological performance is more complete, this evaluation making it possible to apprehend the metrological performance of the device in the presence of a non-uniform kinematic field.

Advantageously, the method which is the subject of the invention comprises, after step d), the step of: e) identifying the sources of inaccuracy of the measurement. Thus, an operator implementing the method that is the subject of the invention knows the sources of inaccuracies in the measurement. Advantageously, the sources of inaccuracies of the measurement are divided into two categories, the first category concerning the configuration of the digital image acquisition device and / or the digital image correlation software, the second category concerning data sources. global inaccuracies including the configuration of the digital image acquisition device and / or the image correlation software as well as the experimental conditions such as brightness, structure position and / or quality of a speckle of the surface of the structure. Thus, a complete evaluation of the metrological performance of the measurement is carried out Advantageously, step e) of the method which is the subject of the invention comprises the steps of: ei) calculating, for each digital image recorded in step b), a standard deviation from the displacement field calculated in step c), this standard deviation being related to the inaccuracies caused by the two categories of sources of inaccuracies. eu) establish, for each digital image recorded in step b), a variogram from the displacement field obtained in step c), eiii) calculate, for each digital image recorded in step b), the value the corresponding variogram established in step eu), for a distance between two neighboring points of said digital image tending towards zero. eiv) comparing, for each digital image recorded in step b), the standard deviation calculated in step ei) and the value of the variogram calculated in step eiii), a difference in value between said standard deviation and said variogram being related to the inaccuracies due to the experimental conditions of the second category.

Thus, the operator is able to correct the experimental conditions of the second category if necessary. In addition, the processing time of the digital images is reduced because the statistical data is calculated for the entire digital image and not for each point of the digital image. Advantageously, the method according to the invention comprises, after step eiv) the step of: ev) calculating, for each digital image recorded in step b), a percentage corresponding to the value of the variogram calculated at step eiii) divided by the standard deviation calculated in step ei), a percentage higher than a threshold indicating that the inaccuracies are due to the experimental conditions of the second category. Advantageously, the optical modification is brought about by a displacement of the optical centers of the digital image acquisition device relative to the structure. Advantageously, the optical modification is provided by the introduction of distortion by means of an optical deformation device positioned between the structure and the digital image acquisition device. Thus, the method according to the invention is not destructive, the structure is not stressed and is then used for measuring the kinematic field. In addition, deformations are taken into account in the evaluation of metrological performance. The invention also relates to a metrology bench for the evaluation of the metrological performance of a digital image correlation measurement, of a kinematic field of a structure subject to mechanical stresses, said metrology bench comprising: a digital image acquisition device, characterized in that: the digital image acquisition device is positioned on a translation device that is adapted to move in translation the digital image acquisition device according to the three axes of space, the metrology bench further comprises a precision displacement sensor being adapted to measure the translation displacement of the digital image acquisition device. Advantageously, the bench further comprises an optical deformation device. PRESENTATION OF THE FIGURES The invention will be better understood on reading the following description, given by way of non-limiting example, and with reference to the figures which represent: FIG. 1: a schematic representation of a metrology bench according to an exemplary embodiment of the invention; - Figures 2a-2b: tables of successive positions of a digital image acquisition device of a metrology bench according to an embodiment of the invention; - Figure 3: a functional diagram showing different steps of the method according to an exemplary embodiment of the invention. In these figures, identical references from one figure to another designate identical or similar elements. For the sake of clarity, the elements shown are not to scale unless otherwise stated.

DETAILED DESCRIPTION OF EMBODIMENTS The invention implements a metrology bench for evaluating the metrological performance of a measurement, by correlation of digital images, of a kinematic field of points of a structure subjected to mechanical stresses. In addition, the invention implements the structure used for measuring the kinematic field. The evaluation is performed at the place where said measurement is made, without moving and / or deforming the structure and before said measurement. In one embodiment, the metrology bench comprises a digital image acquisition device 100 positioned on a support 110 (see FIG. In one example, said digital image acquisition device 100 delivers digital images in three dimensions and comprises either two digital cameras, two fast cameras or two cameras. In another example, said digital image acquisition device 100 delivers digital images in two dimensions and comprises either a digital camera, a fast camera, or a camera. The support 110 is positioned on a translation device 120 comprising three motorized micrometer tables 130. The first micrometer table 130 can be displaced in translation along a first axis X. The second micrometer table 130 can be displaced in translation along a second axis Y perpendicular to the first axis X. The third micrometer table 130 can be displaced in translation along a third axis Z perpendicular to the first axis X and the second axis Y. In a variant, the translation device 120 comprises a micrometric table displaceable in translation along the first axis X, the second axis Y and the third axis Z. The displacement of the first micrometer table 130 is controlled by a calibrated first precision displacement sensor. The displacement of the second micrometer table 130 is controlled by a second calibrated precision displacement sensor. The displacement of the third micrometer table 130 is controlled by a third calibrated precision displacement sensor. Thus, the displacement of the digital image acquisition device 100 is metrologically controlled. In addition, the metrology bench includes an optical deformation device. In one example, this device takes the form of a magnification lens known from a metrological point of view. The metrology bench further includes digital image correlation software and statistical data processing software. As a variant, the metrology bench comprises other devices allowing specific optical modifications of the digital images recorded by the digital image acquisition device. Metrological performance evaluation is performed prior to kinetic field measurement by digital image correlation. In a step 200, the digital image acquisition device 100 records a reference digital image of the surface of the structure (see Fig. 3). In a step 201, the digital image acquisition device 100 records a digital image, comprising a determined optical modification, of the surface of the structure. This optical modification is identical for each point of the surface of the structure and therefore for each point of said digital image.

In one example, the optical modification is brought about by a displacement of the optical centers of the digital image acquisition device 100 relative to the structure. According to a particular mode of implementation, the digital image acquisition device 100 is moved in translation by means of the translation device 120 to a predefined location. The digital image acquisition device 100 thus moved records a digital image of the surface of the structure. In addition, the displacement sensors measure the displacement of the digital image acquisition device 100. Thus, the displacement in translation of the digital image acquisition device 100 makes it possible to simulate a displacement of the structure without actually moving said structure . In another example, the optical modification is brought about by the introduction of distortion by means of an optical deformation device positioned between the structure and the digital image acquisition device 100. According to a mode of implementation, In particular, the optical deformation device is positioned on the digital image acquisition device 100. The digital image acquisition device 100 records a digital image of the surface of the structure. The positioning of the optical deformation device between the structure and the digital image acquisition device 100 makes it possible to simulate a deformation of the structure without actually deforming said structure. Thus, the method according to the invention is not destructive, the structure is not stressed and is then used for measuring the displacement field. The step 201 of recording a digital image, comprising a determined optical modification, of the surface of the structure is repeated for different optical modifications. Thus, the acquisition of the digital image is performed for different displacements and different distortions, advantageously combined according to a plan of experiments. A complete orthogonal experiment plan is used preferentially to determine, through statistical processing such as variance analysis, the simple and combined effects of image modification that have a significant influence on metrological performance and, where appropriate, to quantify the extent of this effect. Thus, according to an exemplary embodiment the digital image acquisition device is moved eight times in eight different positions listed in Figure 2a. In another example, the digital image acquisition device is moved twenty-seven times in twenty-seven different positions listed in Figure 2b. Alternatively, the measurement combinations are made according to another type of experimental design. The amplitude of the displacement corresponding to a "1" in FIGS. 2a and 2b is chosen by an operator as a function of the expected displacement of the structure during the application of the desired mechanical stress. This choice is restricted by the physical limitations of the translation device 120. In a step 206, the digital image correlation software calculates the field of displacement of the points of the surface of the structure, measured by the acquisition device. digital images 100, from the set of digital images obtained by the digital image acquisition device. More precisely, the initial coordinates of each point of the structure in the image space (X ', Y', Z ') and the displacement of each point of the structure in the image space (X', Y ', Z' ) are obtained by the digital image correlation software. The parameters of the image correlation software are chosen by the operator. In a step 207, a first evaluation of the accuracy of the measurement of the displacement field of the points of the surface of the structure by the statistical data processing software is performed. This first evaluation is global and concerns the whole area of the studied surface. This first evaluation is therefore based on all the part of the digital images corresponding to the surface studied. In said step 207, the displacement field calculated in step 206 is compared with the displacement field corresponding to the optical modifications determined. In one example, the displacement measurements provided by the displacement sensors and / or the deformation induced by the optical deformation device make it possible to know the displacement field corresponding to the optical modifications determined. This comparison makes it possible to evaluate the overall accuracy of the measurement. In one example, a standard deviation is calculated for each digital image, this standard deviation allowing the development of a tolerance interval from which the accuracy of the measurement is deduced. Then, an identification of the nature of the sources of inaccuracies of the measurement is carried out. There are two categories of sources of inaccuracies. The first category concerns the configuration of the digital image acquisition device 100 and / or the digital image correlation software. The second category concerns sources of global inaccuracy including the configuration of the digital image acquisition device and / or the image correlation software as well as experimental conditions such as brightness, position of the structure and / or mouchetis quality. In a step 217, a histogram is performed for each digital image and for each displacement along one of the three axes of the image space (X ', Y', Z '). In a step 218, for each digital image, an empirical probability distribution is calculated from the three histograms associated with said digital image. In a step 219, for each digital image, from the probability distribution of said digital image, an average value and a standard deviation are calculated. The standard deviation is related to the inaccuracies caused by the two categories of sources of inaccuracies, it is an overall estimate of measurement inaccuracies. The data provided by the image correlation software further makes it possible to establish an empirical variogram in a step 220 for each digital image and for each point of the digital image. A variogram is a mathematical function describing the degree of spatial dependence of a random spatial field. Thus, a variogram makes it possible to eliminate in a statistical series the outlier data from the values of the neighboring data. In one example, for a digital image, the simulated displacement Zx of the structure on the X axis is considered. This displacement Zx being the same for all the points of said digital image, said displacement Zx is a stationary variable. The mathematical formula of the variogram associated with two points A and B of said digital image is then y (A, B) = 1/2 Var [Zx (A) -Zx (B)] = 1/2 E [IZ (A) - Z (B) I2], where Var [Zx (A) - Zx (B)] is the variance of the difference between the displacement of the point A and the displacement of the point B and E is the expectation. Point A is close to point B and these two points A and B are as close as possible. Said two points A and B are separated by a distance H. Since the displacement is stationary, the variance of the difference between the displacement of the point A and the displacement of the point B depends solely on the distance H. The variogram of the distance H is then the half-average of the square of the difference between the displacement of the point A and the displacement of the point B. The formula of the said variogram of a distance H is y (H) = 1/2 EIA _ BI = H [IZX ( A) - Zx (B) I2]. The variogram may have a discontinuity at the origin, this discontinuity being called nugget effect or nugget effect. This nugget effect, is related to inaccuracies of the measurement caused by the second category of sources of inaccuracies. Indeed, in theory, the displacement Zx being a stationary variable, a property of the variogram of the distance H is that this variogram is null for a zero distance H. The value of the variogram of the distance H is thus calculated, for a distance H tending to zero in a step 221. If said value is greater than a threshold, the nugget effect exists. The difference between the value of the standard deviation and the variogram value of the distance H for a distance H tending to zero thus makes it possible to detect the inaccuracies due to the experimental conditions of the second category. However, once detected, the inaccuracies due to the second category are easily reduced by the operator. In one example, in a step 222, a percentage corresponding to the variogram value of the distance H for a distance H tending to zero divided by the standard deviation is calculated for each digital image. If said percentage is above a threshold, measurement inaccuracies are due to the experimental conditions of the second category. In one example, the threshold is equal to ten percent. The operator then checks and modifies, in a step 223, the experimental conditions of the measurement.

In another example, the absolute value of the difference between the variogram value of the distance H for a distance H tending to zero and the standard deviation is calculated for each digital image. If said absolute value is less than a predetermined desired precision value divided by two, the operator checks and modifies the experimental conditions of the measurement. In one embodiment, if the operator wants to obtain more precise information, a second evaluation of the accuracy of the measurement of the field of displacement of the points of the surface of the structure by the statistical data processing software is carried out in a step 208. This second evaluation is local, the studied surface is divided into several zones and a measurement accuracy value is associated with each zone. The digital images are then divided into several parts.

Claims (6)

REVENDICATIONS1. Method for the evaluation of the metrological performance of a digital image correlation measurement, a kinematic field of a structure subject to mechanical and / or thermal stresses, said method implementing a data acquisition device digital image (100), characterized in that it comprises the steps of: a) storing (200) a digital reference image of the surface of the structure by means of the digital image acquisition device (100) , b) storing (201) a digital image, comprising a determined optical modification, of the surface of the structure by means of the digital image acquisition device (100), c) calculating (206) the field of movement of the points of the surface of the structure, measured by the digital image acquisition device (100), from the set of digital images obtained by the digital image acquisition device (100), d) evaluating ( 207, 208) the meadow determining the field displacement measurement of the points of the surface of the structure by the digital image acquisition device (100) by comparing the displacement field calculated in step c) with a field corresponding to the optical modification determined from step b) 2. Method according to claim 1, characterized in that step b) is repeated for different optical modifications. 3. Method according to claim 1 or 2, characterized in that it comprises after step d) the step of: e) identifying the sources of inaccuracy of the measurement. 4. Method according to claim 3, characterized in that the sources of inaccuracies of the measurement are divided into two categories, the first category concerning the configuration of the digital image acquisition device and / or the correlation software. digital images, the second category relating to global inaccuracy sources including the configuration of the digital image acquisition device and / or the image correlation software as well as the experimental conditions such as brightness, position of the structure and / or the quality of a speckled surface of the structure. 5. Method according to claim 4, characterized in that step e) comprises the steps of ei) calculating (219), for each digital image recorded in step b), a standard deviation from the field of displacement calculated in step c), this standard deviation being related to the inaccuracies caused by the two categories of sources of inaccuracies. eu) establish (220), for each digital image recorded in step b), a variogram from the displacement field obtained in step c), eili) compute (221), for each digital image recorded at the step b), the value of the corresponding variogram set in step eu) for a distance between two neighboring points of said digital image tending to zero, eiv) comparing (222), for each digital image recorded in step b), the standard deviation calculated in step ei) and the value of the variogram calculated in step eiii), a difference in value between said standard deviation and said variogram being related to the inaccuracies due to the experimental conditions of the second category. 6. Method according to claim 5, characterized in that it comprises after step eiv) the step of: ev) calculating (222), for each digital image recorded in step b), a percentage corresponding to the value of the variogram calculated in step eiii) divided by the standard deviation calculated in step ei), a percentage greater than a threshold indicating that the inaccuracies are due to the experimental conditions of the second category 7 Method according to one of claims 1 to 6, characterized in that the optical modification is provided by a displacement of the optical centers of the digital image acquisition device (100) relative to the structure, 8. A method according to one of claims 1 to 6, characterized in that the optical modification is provided by introducing distortion by means of an optical deformation device positioned between the structure and the digital image acquisition device (100).