Optical sensor calibration target

Abstract

TITLE: Calibration Target for Optical Sensor This disclosure relates to a calibration target (1) comprising a face (10) having a first zone (11) and a second zone (12), the first zone (11) being defined by a pattern configured to be detectable by a camera (2), the first zone having a reflectivity factor of at least 94% for which at least 90% of the reflection is diffuse, the second zone having an emissivity factor of at least 50% for an emission wavelength range between 2 and 12 µm. Figure 1

Description

Title of the invention: Calibration target for optical sensor technical field

[0001] The present invention relates to the field of optical calibration and more particularly to the calibration of infrared cameras.

[0002] For various applications, it is known to equip a robotic arm with one (or more) camera(s) equipped with an optical matrix sensor for the purpose of acquiring data. As with other effectors equipping the robot, it is necessary to know precisely the actual position and orientation of the camera, which serves as a reference point for the robot's computer and allows it to correctly position the camera for the needs of the operations (generally control) to be performed.

[0003] A common application example is non-destructive testing (NDT) by thermography of a part (e.g., a sheet of metal) with a complex surface. For large parts, it is then necessary to perform several optical acquisitions from different viewpoints using an infrared camera positioned on the movable arm of a robot. Alternatively, the part itself can be moved by a robotic arm, so as to provide several viewpoints for a fixed camera.

[0004] This is also the case for smaller parts, where the aim is to detect smaller defects. For example, to inspect defects on the order of ten microns, it is necessary to perform a zoom operation, which reduces the camera's field of view. For example, if we consider that the camera has a rectangular field of view, this can measure several tens of centimeters on each side if no zoom is used. The same camera, with zoom, will have its field of view reduced to, for example, 1 or 2 centimeters on each side. It is therefore sometimes necessary to combine several hundred acquisitions to cover a part.

[0005] Using the available information on the robot's position and orientation, for each measurement point, the images from the camera can be projected onto a virtual 3D model of the part. However, conventional procedures only provide a very approximate projection of the images onto the three-dimensional virtual model of the part. In fact, the different projected images do not perfectly overlap the virtual model of the part, and discontinuities are observed between them. These discontinuities result, for example, from a misalignment between two acquisitions, caused by an inaccuracy in the knowledge of the camera's position relative to the subject.

[0006] In the case of a non-destructive testing operation, these discontinuities may be taken for defects by the person or algorithm in charge of checking the image, which may slow down the diagnostic operation or even lead to erroneous conclusions about the condition of the part in question. Previous technique

[0007] To limit discrepancies between images, a theoretical solution consists of precisely determining the dimensions of the tooling used to attach the camera to the robot in order to obtain better control of its positioning. However, the assembly's low tolerance for angular positioning makes its implementation complex and drastically increases the cost of the tooling for an uncertain effect.

[0008] It is therefore preferable to use a calibration target to accurately determine the camera's position relative to the subject. For example, in the case of non-destructive testing, this allows the actual camera position to be known in real time, enabling the precise stitching of several images and thus limiting the risk of errors due to imprecise stitching.

[0009] For example, it is known from patent US2012069193 to use a calibration target comprising a pattern visible to an infrared camera. The pattern being, in one example, a checkerboard in which some squares have a different emissivity than the other squares. This difference in emissivity is sufficient, with the addition of heat, for the checkerboard pattern to appear in the infrared image and be used for calibrating the camera.

[0010] In theory, using two different emissivities is satisfactory, particularly for large target pieces. However, maintaining a satisfactory level of accuracy with this solution is difficult for small target pieces, for example, those on the order of a few centimeters on each side. Furthermore, such a calibration target is difficult to manufacture, since it involves applying a plurality of squares of each of the two materials to create the checkerboard pattern. In short, such a solution is not sufficiently precise for industrial applications.

[0011] It is also known from the publication by R. Usamentiaga et al., "Highly accurate geometry calibration for infrared cameras using inexpensive calibration targets," to print a high-emissivity pattern with ink onto a low-emissivity metallic substrate. The printing principle described is not entirely satisfactory, particularly for small targets. Indeed, the position of the parts of the printed pattern is not sufficiently precise to meet industrial requirements.

[0012] Finally, state-of-the-art test patterns are not entirely satisfactory in terms of image quality. Indeed, it is essential for test patterns, particularly at high precision, to provide a very high contrast image, that is to say, one in which the high emissivity area is very "clearly" distinguished from the lower emissivity area.

[0013] To increase image contrast, it is essential to increase the reflectivity of the area of ​​lowest emissivity so that it exhibits greater contrast in the infrared. Therefore, materials with so-called "specular" reflection are systematically chosen, as they offer the highest reflectivity and thus allow for achieving the best levels of contrast.

[0014] However, such test patterns are difficult to use because they produce very noisy images due to the brightness of the materials used. These images then require significant digital processing before they can be used, which slows down inspection operations and increases their cost.

[0015] The state of the art is therefore not fully satisfactory in that it is systematically faced with an inevitable compromise: choosing between a material with a high reflectivity factor, but generating restrictive noise, or a material with a lower reflectivity factor, generating little noise, but producing low contrast images unusable for high precision applications such as industrial non-destructive testing.

[0016] The purpose of this disclosure is therefore to mitigate at least in part the disadvantages of the prior art mentioned above.

[0017] In particular, one objective of this disclosure is to propose a solution that both provides a high-contrast image, meeting the accuracy requirements of an industrial application, and reduces image noise, which was previously unavoidable on state-of-the-art test charts. Summary

[0018] The objectives mentioned above are achieved in particular by a calibration target comprising a face having a first zone and a second zone, the first zone being defined by a pattern configured to be detectable by a camera, the first zone having a reflectivity factor of at least 94% for which at least 90% of the reflection is diffuse, the second zone having an emissivity factor of at least 50% for a range of emission wavelengths within the mid and long infrared spectrum, i.e. between 2 and 12 pm.

[0019] In other examples, the second zone has an emissivity value (or factor) of at least 75%, or at least 90%, or at least 95%.

[0020] Indeed, the inventors determined during research that the calibration target generates significantly less "noise" in the image when the reflection of the first zone is diffuse, particularly at 90% diffuse, and that it retains its highly reflective nature, in particular that it has a reflectivity factor greater than 94%.

[0021] The features described in the following paragraphs may optionally be implemented independently of each other or in combination with each other:

[0022] According to examples, the calibration target comprises a plate, one face of which includes the first zone and the second zone, the second zone being defined by a plurality of unitary patterns, preferably circular, made on said face of the plate, the first zone being defined as the part of the face which is not covered by the plurality of unitary patterns.

[0023] According to examples, the calibration target comprises a pattern made up of at least three unit patterns.

[0024] According to examples, the unitary motifs are distributed regularly in a plurality of rows and columns.

[0025] According to examples, each unit motif is a few mm, or a few cm away from the other unit motif(s).

[0026] According to examples, the first zone comprises one of the following materials: a metal, a polymer.

[0027] According to examples, the first zone contains aluminium.

[0028] This disclosure further includes a method for manufacturing the calibration target described above, comprising: - Supply of a plate comprising one face defining a first zone having a reflectivity factor of at least 94%, and for which at least 90% of the reflection is diffuse, - Printing, with ink, of one or more unitary patterns to form a pattern on said first zone of said plate, said pattern defining a second zone having an emissivity factor of at least 50%,

[0029] According to examples, the ink used for printing one or more unitary patterns defining the second zone is an ink polymerized under UV radiation.

[0030] According to examples, each unit motif has a height, when measured perpendicular to the plate, of 200 microns or less, preferably 50 microns or less. This thickness can correspond to the thickness of the ink. This avoids shadow phenomena that generate localization errors during image acquisition.

[0031] According to examples, the unit motifs have an elliptical shape, more advantageously a circular shape.

[0032] This disclosure further relates to a method of using a calibration target described above, comprising: - Supply of a robotic arm equipped with a camera, - Provision of a calibration target as described above, the pattern of said calibration target comprising a plurality of distinct unitary patterns, - Acquisition, using the camera, of a plurality of images of the target according to various orientations and spatial positions of the robot arm, - Calculation, from said image plurality and the position of said plurality of unit patterns, of the calibration parameters of said camera.

[0033] This disclosure further relates to a thermography installation capable of operating in a mid-infrared wavelength range between 2 and 12 pm comprising a calibration target as described above, a robotic arm and a camera, either the calibration target or the camera being mounted on the robotic arm.

[0034] According to examples, the camera is an infrared camera, the wavelength of which is between 2 and 5 pm or between 7 and 12 pm. Brief description of the drawings

[0035] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analysis of the accompanying drawings, on which:

[0036] [Fig.1] shows a schematic representation of a robotic arm supporting a camera, oriented towards a calibration target according to an example in the present disclosure.

[0037] [Fig.2] shows several views 2a, 2b, 2c, 2d illustrating different examples of patterns possible for a calibration target.

[0038] [Fig.3] shows a cross-section of a calibration target according to an example of the present disclosure.

[0039] [Fig.4] shows a first representation 4a of a calibration target of which the the first area does not exhibit diffuse reflection, and a second representation 4b of a calibration target according to an example in this disclosure in which the first area exhibits diffuse reflection. Description of the implementation methods

[0040] The drawings and description below contain, essentially, elements of a definite nature. They may therefore not only serve to better understand this disclosure, but also contribute to its definition, if necessary.

[0041] In the various figures, the same reference numerals designate identical or similar elements. For the sake of brevity, only the elements that are useful for understanding are shown. The embodiments described are shown in the figures and are described in detail below.

[0042] Reference is now made to [Fig. 1], which shows an example of a thermography installation. In this example, the aim is to accurately determine the position of a camera 2 on a robotic arm 3 using a calibration target 1, as shown in an example in this disclosure. As explained above, it is essential to accurately determine the position parameters of the camera 2 in order to reconstruct, particularly during a non-destructive testing operation, a faithful view of the object being inspected by assembling several acquisitions together. Inaccuracies in the camera position result in inaccuracies at the junctions between the stitched images, which can distort or slow down the inspection operation.

[0043] In the solution proposed in this document, the calibration target 1 comprises a pattern to be detected in the infrared range by the camera 2, within a mid-infrared (2 to 5 pm) or long-infrared (7 to 12 pm) wavelength range. However, it is possible to adapt the calibration target to other wavelength ranges without departing from the scope of this disclosure.

[0044] Such detection can be performed by an image processing algorithm, executed by a processing unit comprising a processor (not shown). This pattern to be detected can take several forms depending on the camera used: checkerboard, dot matrix, etc., as shown as an example in [Fig. 2]. The calibration target can be made of several materials (paper, plastic, metal, wood, etc.). In the context of an application using an infrared camera, presented here as a non-limiting example, the pattern differs from the classic checkerboard and comprises a network of circular unit patterns P.

[0045] The calibration target 1 illustrated in [Fig.1] comprises a plate 4, presenting a first zone 11 of a first emissivity value, the plate 4 also comprising a pattern defining a second zone 12 of a second emissivity value.

[0046] To better understand the concept of emissivity, it can be considered as inversely proportional to reflectivity. If a body is particularly emissive, it is relatively low in reflectivity, and vice versa.

[0047] In our case, according to examples, the first zone 11 can then be characterized as being particularly reflective, and the second zone 12 as being particularly emissive. For example, the first emissivity value may be on the order of 30%, while the second emissivity value may be on the order of 90%. Although the present disclosure focuses on configurations in which the first zone 11 is reflective and the second zone 12 is emissive, it is also It is possible to predict a reverse configuration, in which the first zone 11 is particularly emissive, and the second zone 12 is particularly reflective. It is therefore implicit in the description that the reverse configuration is possible. The essential point is that the first and second zones 11, 12 have different emissivity values. Advantageously, they are significantly different: the greater the difference, the more contrasted the calibration target 1 will produce in the infrared.

[0048] In other examples, it is possible that the first emissivity value is between 5 and 10%, and that the second emissivity value is between 70 and 80%.

[0049] In other examples, the second zone has an emissivity value of at least 75%, or at least 90%, or at least 95%.

[0050] In order to obtain high contrast for the camera, it is advantageous to use a highly reflective material for the first zone 11 of the calibration target 1. This type of material has, according to DIN 5036, Part 3, a total reflectance of at least 85%, preferably at least 90%. The first zone 11 of the calibration target 1 according to this disclosure advantageously has a reflectance, i.e., total reflectance, of at least 94%.

[0051] In some examples, the first zone 11 is complementary to the second zone 12 on the face 10 of the plate 4. In this way, regardless of its shape, the second zone 12 is always included within the contour of the first zone 11. The second zone 12 can typically comprise a plurality of distinct regions. In some examples, each region is represented by a circular unit motif P. In other examples already discussed, the regions will be squares, possibly touching at their corners, arranged in a regular checkerboard pattern. A "regular" motif is understood to be constructed from one or more repetitions of a unit motif. The motif may include identical unit motifs, that is, motifs of the same 3D conformation.

[0052] In examples shown in [Fig. 1] and in Figures 2a and 2b, the second zone 12 is defined on the plate 4 by unit motifs P, regularly spaced on the plate 4 so as to form a pattern. The background of the plate 4 on which the second zone 12 is defined then defines the first zone 11. [Fig. 2] shows various, non-limiting, examples of possible patterns. Typically, the network of unit motifs P can comprise a number M of columns and a number N of rows. It is also possible that one or more columns, or rows, have a different number of unit motifs P. Since the invention is not limited to infrared camera inspections, it is possible to use a target formed by any pattern on a background such that there is a contrast between the background and the pattern.

[0053] It is finally possible to adapt the embodiment of the calibration target 1 according to the application cases: the type of camera, the type of operation, etc. However, it is advantageous for locating the position of the camera to be calibrated that the second zone 12, which forms a pattern on the first zone 11, allows the majority of points appearing in the image to be easily and accurately located.

[0054] The plates shown by way of non-limiting example in [Fig. 1] and 2 are substantially flat and generally rectangular or square in shape. However, in other examples, the plate may be curved. Furthermore, the plate may also be of a shape other than a rectangle, for example, an ellipse or any other shape including or encompassing the motif.

[0055] In one example, the plate 4 is made of a highly reflective material as defined above, and the second, particularly emissive zone 12 is created by adding material to this plate 4, the surface of the plate 4 not covered by the second zone 12 constituting the first zone 11. Such an example is shown in particular in [Fig. 3]. The plate 4 can be fixed to a more rigid support 5, for example made of a plastic with high mechanical strength, particularly in bending, or of any other material chosen more for its mechanical properties than for its optical properties. In a preferred example, the plate 4 comprises aluminum, and the face on which the pattern is formed is thus made of aluminum. In another example, the plate 4 comprises a polymer.

[0056] According to an example of a method for manufacturing the calibration target 1, of which [Fig.3] represents an example of the product obtained, a plate 4 is provided comprising a face defining a first zone 11 having a reflectivity factor of at least 94%, and for which at least 90% of the reflection is diffuse, and on which one or more unit motifs P are printed, for example with ink, to form a pattern on said first zone 11 of said plate 4, said motif defining a second zone 12 having an emissivity factor of at least 50%,

[0057] In one example, the plate 4 is less than 1 mm thick and is glued to a thicker support 5, which stiffens the assembly. Stiffening the calibration target 1 can be advantageous to prevent it from deforming and losing its flat shape. While not essential, it is still beneficial for the plate to be substantially flat, as this can facilitate image acquisition.

[0058] The support 5 may also include a heating means 6 which makes it possible to further increase the contrast of the image by supplying heat, for example during acquisition by the camera 2.

[0059] The pattern defining the second zone 12 can be produced by printing ink, preferably black, onto the plate 4. The black color notably allows for obtaining the highest emissivity factor. Such a process using an operation The printing process is particularly well-suited to the high precision required for this type of target, especially for applications in the inspection of industrial parts. However, it is possible to produce the pattern defining the high-emissivity zone using any other method known to those skilled in the art.

[0060] The printing can be carried out using an ink that hardens upon exposure to UV rays, but can also be carried out with any ink known to those skilled in the art.

[0061] It may be advantageous for the pattern to have a height of less than 200 micrometers, advantageously less than 50 micrometers. This ensures that no shadows obscure parts of the first zone 11 and make the learning of the position of the unit patterns by the camera 2 inaccurate.

[0062] Reference is now made to [Fig. 4]. According to this disclosure, the first zone 11 allows for what is called "diffuse" reflection. Such reflection is characterized by the fact that it reflects light in a large number of directions. Ideally, a reflection that is 100% "diffuse" reflects light, regardless of its direction of incidence, uniformly in all directions. Such reflection is opposed to what is called "specular" reflection, in which light is reflected in a direction that depends on the direction of incidence of the light on the reflecting surface, obeying Snell's law. In other words, a perfectly specularly reflecting plane surface will return an incident ray in a direction symmetrical to a plane of incidence, normal to the reflecting surface.This is the case, for example, with a mirror, or with known calibration targets, an example of which is shown in part 4a on the left of [Fig. 4]. Conversely, a diffusely reflective surface will reflect an incident ray in a multitude of directions, making it impossible to determine the source of the incident ray by perceiving one of the reflected rays. In other words, this is a visual rendering similar to that obtained by a surface with a so-called "matte" appearance. For example, a sheet of paper or a brushed metal coating. However, the materials used for this type of rendering do not have a sufficient reflectivity factor. Furthermore, on materials that are already sufficiently reflective, the surface treatments used to obtain this appearance degrade their reflectivity factor and therefore do not constitute a solution to the problem either.Calibration target 1, as described in this disclosure, offers a solution to overcome this compromise. A representation of such an example of calibration target 1 is shown in Part 4b to the right of [Fig. 4].

[0063] The unit motifs P are an example of a motif embodiment that the camera 2 must detect, and are in principle highly emissive. In particular, a unit motif P comprises a width 1 on the order of a millimeter. Preferably, the width 1 can be approximated as a diameter of the unit motif P if it is circular, the height H of which pattern corresponding to an ink thickness necessary for the formation of the unit pattern (We understand that the surface of the pattern is not smooth) and is controlled so as not to generate shadow phenomena when taking images.

[0064] Figure 1 thus represents a method of using a calibration target 1 according to an example in this disclosure, the method comprising: - The provision of a robotic arm 3 carrying a camera 2, the camera possibly being an infrared camera, - The provision of a calibration target 1 according to this disclosure, the pattern of said calibration target 1 comprising a plurality of distinct unit patterns P, - The acquisition, using camera 2, of a plurality of images of the test pattern 1 according to various orientations and spatial positions of the robot arm 3, - The calculation, from said plurality of image and the position of said plurality of unitary patterns P, of the calibration parameters of said camera 2 in order to know precisely its position.

Claims

Demands

1. Calibration target (1) comprising a face (10) having a first zone (11) and a second zone (12), the first zone (11) being defined by a pattern configured to be detectable by a camera (2), the first zone having a reflectivity factor of at least 94% for which at least 90% of the reflection is diffuse, the second zone having an emissivity factor of at least 50% for an emission wavelength range between 2 and 12 pm.

2. Calibration target (1) according to claim 1, comprising a plate (4) of which one face (10) comprises the first zone (11) and the second zone (12), the second zone (12) being defined by a plurality of unitary patterns (P), preferably circular, made on said face (10) of the plate (4), the first zone (11) being defined as the part of the face (10) which is not covered by the plurality of unitary patterns (P).

3. Calibration target (1) according to any one of claims 1 to 2, wherein the first zone (11) comprises a material from: a metal, a polymer.

4. Calibration target (1) according to claim 3, wherein the first zone (11) comprises aluminum.

5. A method for manufacturing the calibration target (1) according to any one of claims 1 to 4 comprising: - Supplying a plate (4) comprising a face defining a first area having a reflectivity factor of at least 94%, and for which at least n is diffuse, - Printing, with ink, one or more unit patterns (P) to form a first area (11) of said plate (4), said pattern defining a second area having an emissivity factor of at least 50%,

6. A method according to claim 5, wherein the ink used for printing one or more unitary patterns (P) defining the second zone (12) is an ink polymerized under UV radiation.

7. A method according to any one of claims 5 to 6, wherein each unit motif (P) has a height (H), when measured perpendicular to the plate (4), which is less than or equal to 50 microns

8.

9. Method of using a calibration target (1) according to any one of claims 1 to 4, comprising: - Supply of a robotic arm (3) carrying a camera (2), - Provision of a calibration target (1) according to any one of claims 1: a calibration target (1) comprising a plurality of unit motifs P c - Acquisition, using the camera (2), of a plurality of images of the movements and various spatial positions of the robot arm (3), - Calculation, from said image plurality and the position of said plurals (P), of the calibration parameters of said camera (2). Thermography installation capable of operating in a wavelength range between 2 and 12 pm comprising a calibration target (1) according to claims 1 to 5, a robotic arm (3) and a camera (2), either the calibration target (1) or the camera (2) being mounted on the robotic arm (3).

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