FR3116333A1 - Device and method for metrological monitoring of a measuring device by photogrammetry - Google Patents

Abstract

The present invention relates to a device for metrological monitoring of a photogrammetry measuring device comprising: - a three-dimensional structure; and- at least eight standard bars fixed on the three-dimensional structure so that each standard bar extends in a plane and/or in a different direction from the other standard bars, each standard bar comprising at least two standard targets. Figure for the abstract: Fig. 1

Description

Device and method for metrological monitoring of a measuring device by photogrammetry

FIELD OF THE INVENTION

The invention relates to photogrammetric measuring devices, more particularly to the calibration of such measuring devices.

STATE OF THE ART

Optical measurement by photogrammetry is based on the production of several photographic images of a scene from several different angles of view, then the three-dimensional reconstruction of the scene from the analysis of the photographic images in order to find the spatial position of points of the scene in space and thus measure distances between these points.

Over time, photogrammetric optical measuring devices can exhibit different types of errors. Periodic metrological monitoring of measuring devices by photogrammetry is therefore necessary to ensure that these measuring devices do not drift over time and continue to produce reliable measurements. The metrological monitoring of this type of measuring device has in particular been the subject of an international standard (VDI-VDE 2634 Part 1). However, there is currently no standard suitable for carrying out metrological monitoring that also meets the requirements defined in this standard.

This monitoring is usually carried out using a flat standard structure on which coded targets are placed. Photographic images of the standard structure are taken from several points of view and then their deformation is analyzed in order to determine the drift of the measuring device. However, these measurements are generally carried out by the manufacturers of the photogrammetric measuring devices. However, the manufacturers of such devices tend to apply test procedures giving the most favorable results for their own devices. In addition, since the tests must be carried out periodically, the systematic return of the photogrammetric measuring devices to the manufacturers is costly financially and in terms of time.

An object of the invention is to provide a device for metrological monitoring of a measuring device by photogrammetry which overcomes the drawbacks set out above.

More precisely, an objective of the invention is to propose a device making it possible to ensure metrological monitoring of a measuring device by photogrammetry in a simple, robust and efficient manner, for a moderate cost.

To this end, according to a first aspect of the invention, a device for metrological monitoring of a photogrammetry measuring device comprising:
- a three-dimensional structure; And
- at least eight standard bars fixed on the three-dimensional structure so that each standard bar extends in a plane and/or in a different direction from the other standard bars, each standard bar comprising at least two standard targets.

Certain preferred but non-limiting characteristics of the metrological monitoring device according to the first aspect are the following, taken individually or in combination:
- the device further comprises at least one scale ratio bar comprising at least two scale ratio targets;
- the device comprises two scale ratio bars;
- each standard bar comprises five standard targets;
- the standard targets are separated two by two by different predetermined distances;
- the standard targets are separated two by two by the following distances: 250 mm, 500 mm, 750 mm, 1000 mm;
- the standard targets and the standard bars are of different colors, for example white and black;
- the three-dimensional structure is polygonal and in which, when two standard bars are fixed on two mutually parallel faces, said bars extend at different angles; and or
- the standard bars are fixed on the faces of the three-dimensional structure so as to extend along an edge and/or a diagonal.

According to a second aspect, the invention proposes a method for metrological monitoring of a measuring device by photogrammetry comprising the following steps:
S1: making a plurality of images of a device according to the first aspect;
S2: determining distances between the standard targets of each standard bar;
S3: compare the distances measured in step S2 with reference distances; And
S4: deduce a measurement error of the measuring device by photogrammetry.

DESCRIPTION OF FIGURES

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings in which:

There schematically illustrates a device according to a first embodiment of the invention.

There schematically illustrates a device according to a first embodiment of the invention.

There schematically illustrates a device according to a first embodiment of the invention.

There is a flowchart of steps of a method according to one embodiment of the invention.

There illustrates the possible position for shots of the device of the with a measuring device to be checked according to one embodiment of the invention.

In all the figures, similar elements bear identical references.

DETAILED DESCRIPTION OF THE INVENTION

There illustrates a device for metrological monitoring of a photogrammetry measuring device according to the invention.

In order to carry out metrological monitoring of a measuring device by photogrammetry, it is proposed to produce a plurality of photographic images of a metrological monitoring device 1 comprising a three-dimensional structure 2 and at least eight standard bars 3 fixed to the structure 2 three-dimensional so that each standard bar 3 extends in a plane and / or in a direction different from the other standard bars 3. In addition, each standard bar 3 comprises at least two standard targets 4.

The distance between the two standard targets 4 of each standard bar 3 being known, it is then possible, by determining the position of the standard targets 4 by image processing in the photographic images thus taken by the device to be checked, to compare the distances measured between said standard targets 4 with reference distances and to deduce therefrom any deviation, corresponding to a difference between the distances measured between the standard targets 4 and the reference distances.

The three-dimensional structure 2 can comprise a set of uprights, crosspieces and optionally diagonals fixed to each other, so as to delimit together through openings making it possible to see through the faces of the structure 2 and therefore to make visible a greater number of standard targets 4 in each photographic image. For example, the structure 2 can be formed by assembling metal sections assembled to each other at their respective ends.

In one embodiment, the three-dimensional structure 2 can be polygonal. Such a structure is advantageous because it is stable, which is necessary in particular when performing metrology measurements. For example, the three-dimensional structure can be a rectangular parallelepiped, for example a cube.

In what follows, for the sake of simplifying the description, the invention will be more particularly described in the case of a three-dimensional structure 2 of cubic shape. However, this is not limiting, the three-dimensional structure 2 possibly having a different number of faces and/or any parallelepipedic or rectangular shape.

The three-dimensional structure 2 thus comprises a bottom face configured to be placed on a support, four side faces extending perpendicularly from the bottom wall and an upper face.

In one embodiment, the structure 2 is cubic with a side c of between 800 mm and 1500 mm, for example 1000 mm. The size and the manufacturing cost of the structure 2 are thus moderate.

The standard bars 3 can be fixed along the edges and/or the diagonals of the faces of the three-dimensional structure 2. Preferably, each standard bar 3 is fixed at the level of at least one of the ends of the uprights, of the crosspiece or of the diagonal on which it is fixed, or at the level of a corner of the three-dimensional structure 2. In one embodiment, the standard bars 3 are fixed partly on the external surface of the three-dimensional structure 2 and partly on its internal surface, in order to allow the visualization of a large number of standard targets 4 on the same photographic image . Thus, in the example shown in the , standard bars are placed on the outer surface of the four side faces of the device 1 and on the inner surface of the bottom face.

Advantageously, the standard bars 3 are distributed in the volume of the three-dimensional structure 2. In one embodiment, each side face and the bottom face of the three-dimensional structure 2 may comprise at least one standard bar 3. However, it will be noted that the eight standard bars 3 extend in a plane and/or in a direction different from the other standard bars 3. Thus, when two standard bars 3 are fixed on two mutually parallel faces, the standard bars 3 extend at different angles.

In the exemplary embodiment illustrated in FIGS. 1 and 2 , the device 1 comprises for example:

- two "vertical" standard bars 3 fixed respectively on the external surface of two lateral faces which extend perpendicularly with respect to the bottom face; it will be noted that the two side faces bearing these vertical standard bars 3 are adjacent;

- three "inclined" standard bars 3 fixed respectively on the external surface of three lateral faces which extend along a diagonal of these external faces; it will be noted for this purpose that the inclined standard bars 3 which are fixed on side faces parallel to each other extend along opposite diagonals and are therefore not parallel to each other;

- a standard bar 3 fixed to the inner surface of the bottom face which extends along a diagonal of the bottom face; And

- two "horizontal" standard bars 3 fixed respectively on the outer surface of two side faces, one of the horizontal standard bars 3 being fixed on the edge of the side face which is adjacent to the bottom of the structure 2 and the other standard bars 3 being fixed on the edge of the side face which is adjacent to its upper face.

Of course, the device 1 can comprise a greater number of standard bars 3, the additional standard bars 3 then being able to be placed in a plane and/or in the same direction or in a different direction from the other standard bars 3.

The standard bars 3 can be added and fixed on the three-dimensional structure 2, typically on the uprights, crosspieces and diagonals of the three-dimensional structure 2. This makes it possible in particular to produce the standard bars 3 and the three-dimensional structure 2 separately and in distinct materials, thus reducing its manufacturing cost. In addition, such an embodiment makes it possible, if necessary, to replace and/or repair a standard bar 3 and/or the three-dimensional structure 2 without impacting the rest of the device 1. Alternatively, the standard bars 3 can be monolithic with 2-dimensional structure. In this case, the standard bars 3 can form all or part of the uprights, diagonals and crosspieces of the device 1.

The standard bars 3 are made of a material having a very low coefficient of expansion at the temperature at which the metrological monitoring is carried out, that is to say which does not expand or expands very slightly at said temperature. Furthermore, the constituent material of the standard bars 3 is chosen so as not to deform over time. For example, the standard bars 3 can be made of a composite material comprising carbon fibers embedded in a resin, such as epoxy.

In one embodiment, a length of the standard bars 3 is substantially equal to a length of an edge of the three-dimensional structure 2, or to the length of the side c in the case of a cubic structure 2. This configuration in fact makes it possible to limit the size of the device 1 while optimizing the number of standard targets 4 and the distance separating them. Indeed, the further apart the targets, the greater the measurement error can be and therefore the better the metrological monitoring.

The standard targets 4 of each standard bar 3 are separated two by two by a predefined reference distance. In one embodiment, the reference distances are predetermined with a measurement uncertainty of less than one-fifth of an allowable limit error set for the photogrammetry measuring device to be controlled. If necessary, a calibration certificate can be used for each of the standard distances in order to check the traceability of the standard bars 3 and their connection to national standards.

In one embodiment, each standard bar 3 comprises five standard targets 4, which thus makes it possible to define, for each standard bar 3, ten reference distances (if all the pairs of standard targets 4 of the same standard bar 3). For example, with reference to the , the distance d separating two adjacent standard targets 4 is equal to a quarter of the total distance of the standard bar 3, i.e. 250 mm in the case of a standard bar 3 1000 mm in length, which makes it possible to define four distances reference distances equal to 250 mm (d), three reference distances equal to 500 mm (2d), two reference distances equal to 750 mm (3d) and one reference distance equal to 1000 mm (4d).

As a variant, the adjacent standard targets 4 of the same standard bar 3 can be separated two by two by a different distance d .

The standard targets 4 can for example be circular with a diameter between 2 mm and 8 mm, for example with a diameter equal to 5 mm.

The color of the standard targets 4 is chosen so that the standard targets 4 contrast with the standard bars 3 in order to improve the detection of the center of the standard targets 4 and to obtain precise distance measurements between the standard targets 4. For example , the standard targets 4 can be white while the standard bars 3 are black or at least dark (or vice versa) in order to ensure a sharp color transition and ensure optimum measurement accuracy. For example, standard targets 4 can be made of a white ceramic material.

As a variant, the standard targets 4 of the same standard bar 3 can be separated two by two by a predetermined distance d .

In one embodiment, the device 1 further comprises at least one scale ratio bar 5 in order to allow scaling of the distance measurements carried out during the metrological monitoring process and to allow the the scale of measurement by photogrammetry. In particular, in the absence of a 5 scale ratio bar, the 4 inter-target distances would be fair relatively to each other but might not correspond to reality. There would therefore be a deviation from the reference values. For this purpose, the scale ratio bar 5 also comprises at least two targets of scale ratio 6. If necessary, the targets of scale ratio 6 can be identical to the standard targets 4 described above. Similarly, the scale ratio bar 5 can be made of the same constituent material as the standard bars 3.

In one embodiment, the scale ratio bar 5 comprises exactly two targets of scale ratio 6. The targets of scale ratio 6 can for example be separated by a distance equal to 1000 mm.

The scale ratio bar 5 is configured to be placed in the environment of the 2 three-dimensional structure in order to be in the field of vision of the device to be controlled when taking the photographic images. For example, the scale ratio bar can be arranged on the support close to the 2-dimensional structure.

In one embodiment, the device 1 can comprise a second scale ratio bar 5 in order to provide redundancy in the event of a fault or drift over time of the other scale ratio bar 5. The presence of a second scaling bar 5 thus makes it possible to increase the reliability of the scale ratio, the probability that the two scale ratio bars 5 are defective simultaneously is very low.

Metrological monitoring process

The device 1 described above can be used during the course for the metrological monitoring of a measuring device by photogrammetry. Such a process makes it possible to verify that the photogrammetry measuring device is reliable. Preferably, during metrological monitoring, the temperature is stable so as not to taint the measurement with a bias due to any expansion of the material. Monitoring can, for example, be carried out in an air-conditioned room. Monitoring can in particular be carried out at a temperature equivalent to that which was used to characterize the scaling bars, i.e. to establish the reference values.

In a preliminary step, device 1 is placed on a support with at least one scale ratio bar 5.

During a step S1, a plurality of photographic images of the device 1 and of the at least one scale ratio bar 5 are produced so that each standard target 4 is visible at least once, preferably at least least five times at a different angle of incidence. For example, about sixty photographic images can be produced so as to be distributed along four horizontal planes (at 0°, 20°, 40° and 60°, by producing 15 to 16 images per plane) around the device 1 and at form a half-sphere (see ). If necessary, four additional photographic images can be produced in order to calibrate the lens of the camera, typically according to an angle of view forming an angle of approximately 90° with the support on which the device 1 is placed and the bars of scale ratio 5 (see also ).

During a step S2, the photographic images thus produced are processed by a processing means (which may in particular comprise a computer of the processor, microprocessor, microcontroller, etc. type, configured to execute instructions) so as to determine the distance separating the standard targets 4 of each standard bar 3 visible in the photographic images. For this, the processing means identifies the standard targets 4 and, from the position of the targets of scale ratio 6 in the photographic images and the knowledge of the different reference distances between the targets of scale ratio 6 d on the one hand and standard targets 4 on the other hand (10 distances if the standard bar 3 comprises five standard targets 4), the processing means calculates the distances between the standard targets 4 of each standard bar 3 by triangulation and determines the position spatial and orientation of each of the photographic images.

During a step S3, the distances determined during step S2 are compared with the reference distances, which are fixed and predetermined. The processing means can in particular determine a difference between the distances measured during step S2 and the corresponding reference distances and deduce therefrom, for each distance measured, a measurement deviation and deduce a measurement error therefrom.

In one embodiment, the measurement error corresponds to the maximum deviation among the measurement deviations determined in step S3. During a step S4, the measurement error is then compared with a predetermined limit value. When the measurement error is less than or equal to the limit value, then the validation test is successful and it is concluded that the photogrammetry measuring device meets the precision requirements. On the other hand, when the measurement error is greater than the limit value, the validation test is not successful and it is deduced that the photogrammetry measuring device does not meet the precision requirements.

Claims (10)

Device (1) for metrological monitoring of a measuring device by photogrammetry comprising:
- a three-dimensional structure (2); And
- at least eight standard bars (3) fixed to the three-dimensional structure (2) so that each standard bar (3) extends in a plane and/or in a different direction from the other standard bars (3), each standard bar (3) comprising at least two standard targets (4). Device (1) according to claim 1, further comprising at least one scale ratio bar (5) comprising at least two scale ratio targets (6). Device (1) according to claim 2, comprising two scale ratio bars (5). Device (1) according to one of Claims 1 to 3, in which each standard bar (3) comprises five standard targets (4). Device (1) according to claim 4, in which the standard targets (4) are separated two by two by different predetermined distances. Device (1) according to claim 5, in which the standard targets (4) are separated two by two by the following distances: 250 mm, 500 mm, 750 mm, 1000 mm. Device (1) according to one of Claims 1 to 6, in which the standard targets (4) and the standard bars (3) are of different colours, for example white and black. Device (1) according to one of Claims 1 to 7, in which the three-dimensional structure (2) is polygonal and in which, when two standard bars (3) are fixed on two mutually parallel faces, the said bars (3) s extend at different angles. Device (1) according to one of Claims 1 to 8, in which the standard bars (3) are fixed to the faces of the three-dimensional structure (2) so as to extend along an edge and/or a diagonal. Method for metrological monitoring of a measuring device by photogrammetry comprising the following steps:
S1: producing a plurality of images of a device (1) according to one of claims 1 to 9;
S2: determining distances between the standard targets (4) of each standard bar (3);
S3: compare the distances measured in step S2 with reference distances; And
S4: deduce a measurement error of the measuring device by photogrammetry.