Inspection method and system for inspecting dimensional or geometric feature of mechanical component

Abstract

An inspection method for inspecting the dimensions or geometric features of a mechanical component (4) using an optoelectronic system of the shadow projection type, said method comprising calculating spatial coordinates of points (pi) of the surface of said component, allowing a suitable 3D reconstruction of the surface of said component, and inspecting the dimensions or geometric features using such spatial coordinates. The spatial coordinates of the points are calculated starting from a two-dimensional image (5i) and an associated contour, which are obtained by the shadow projection system when the mechanical part is rotated about the axis of rotation (A). The position of a point or edge (Ejk) of the contour and a change in such position are detected at a specific height (zj) as the angle of rotation ([theta] i) changes. Based on such a change, the position of the component to be inspected, which generates the points of the contour, relative to the measurement plane is identified and the spatial coordinates of the points of the surface are calculated.

Description

Technical Field

[0001] The present invention relates to a method and a system for inspecting the dimensions or geometric features of a three-dimensional mechanical component rotating about a rotation axis by means of an optical system. Background Art

[0002] It is known to measure and inspect a three-dimensional mechanical component by means of a device in which the mechanical component can rotate about a rotation axis and, during rotation, an optoelectronic system acquires images of the mechanical component in corresponding cross-sectional planes. More specifically, in order to obtain extremely accurate results, it is known to use an optical measurement system of the shadow projection type, which system comprises a transmitter with a light source and a telecentric lens, a receiver with an optical sensor, and a processing device. It is known that extremely accurate linear or two-dimensional images of the mechanical component can be obtained using such a device.

[0003] If the component to be inspected has axial symmetry and rotates about its own axis of symmetry, appropriate information on the position of the points of the entire surface of the component can be obtained. However, if the shape of the component is different and / or it does not rotate about its own axis of symmetry, this is generally not possible, and more specifically, it is not possible to obtain a correct three-dimensional reconstruction of the surface of the component by extracting the contours from the images acquired in different cross-sectional planes during rotation. In fact, the detected contours of each two-dimensional image acquired during the rotation of the mechanical component do not necessarily correspond to the true contours of the mechanical component in the corresponding cross-sectional plane: the points constituting the detected contours do not necessarily lie in that cross-sectional plane and may have different, unknown distances from that cross-sectional plane, without information related thereto. Since it is generally not possible to perform an appropriate 3D reconstruction of the component (which is for evaluating the position of the points of the entire surface of the component), the inspection operations that can be performed are also limited. This occurs, for example, in the case of a mechanical component with an external thread.

[0004] It should be noted that systems and methods for obtaining a 3D digital object corresponding to a mechanical component to be inspected are known and, for example, include coordinate measuring machines with contact sensors or devices using optoelectronic distance sensors, such as laser scanners, which are appropriately arranged and oriented with respect to the mechanical component to be controlled.

[0005] However, such instruments with optoelectronic sensors, although generally reliable and widely used, are not able to obtain a three-dimensional digital object that precisely corresponds to the component to be controlled, and this level of precision is required in many cases and can be obtained using a projection measurement system, which is, for example, due to interference caused by unwanted reflection phenomena. Summary of the Invention

[0006] An object of the present invention is to provide a method and a device for inspecting the dimensions or geometric features of a mechanical component by means of an optoelectronic system (more specifically, an optical measurement system of the shadow projection type), capable of obtaining a three-dimensional reconstruction of the mechanical component, that is, calculating the spatial coordinates of the points on the surface of the mechanical component, so as to measure the component in a more accurate and complete manner.

[0007] The inspection method according to claim 1 and the device according to claim 7 achieve this object.

[0008] In particular, according to the method of the present invention, the mechanical component to be inspected is rotated about a rotation axis, two-dimensional images of the mechanical component are acquired at successive rotation angles by means of an optoelectronic system, points of the detected contour are extracted from each two-dimensional image, the points of each detected contour are converted into points of the measured contour in a Cartesian reference system that includes a measurement plane and in which the axis of the first coordinate coincides with the rotation axis, edge points of each measured contour corresponding to the same value of the first coordinate are identified in the Cartesian reference system, the variation of the positions of these edge points as a function of the rotation angle is evaluated, and based on this variation, the positions of the points on the surface of the mechanical component corresponding to the edge points of the measured contour relative to the measurement plane and the spatial coordinates of these points are calculated.

[0009] The previous two steps are repeated for a predetermined number of values of the first coordinate, and the dimensions or geometric features of the mechanical component are inspected based on the spatial coordinates of these points. Description of the Drawings

[0010] With reference to the accompanying drawings, the method and device according to the present invention will now be described by way of non-limiting examples, where - Figure 1 The basic components of a shadow projection system for acquiring two-dimensional images of a mechanical component are shown in a highly schematic manner,

[0011] - Figure 2A is a graphical representation of some contours that are obtained by Figure 1 the system when the mechanical component rotates about Figure 1 the axis A in

[0012] - Figure 2B shows how the positions of the corresponding points of the Figure 2A contours in

[0013] - Figure 3 is a schematic diagram of a shadow projection system for inspecting a mechanical component defining an external thread, and

[0014] - Figures 4 to 9 is a graphical representation of the output of each step during the inspection process, taking as an example the inspection of a mechanical component with an external thread. Detailed Description of the Invention

[0015] Figure 1 An inspection device including a shadow projection type optoelectronic system is shown in a minimalist schematic manner and is designated by reference numeral 1. It includes a transmitter 2 having a light source and a telecentric lens and a receiver 3 having a sensor (more specifically, a matrix sensor). The mechanical component 4 to be inspected is arranged in a reference and support device and can rotate about a rotation axis A, and the sensor 3 generates a two-dimensional image 5 i . The control and processing unit 10 is connected to the reference and support device to control the rotation of the mechanical component 4 and is connected to the optoelectronic system to acquire two-dimensional images at different rotation angles θ i . The detected values E i of the positions (or "edges") of the contours of the two-dimensional image 5 j k are represented, that is, the edge points of the contour at the same z j coordinate (or height) are grouped to obtain curves, and each curve represents how the value of the edge E j k changes, and more specifically represents how the value of the corresponding coordinate x in the measurement plane XZ changes with the rotation angle θ i ( Figure 2B ). In the illustrated example, the intersection between the z = z j line and the contours of the grouped two-dimensional image 5 i produces two edges (k = 1, k = 2), but the number of edges K produced varies and depends on the shape of the mechanical component 4 to be inspected. Information about the depth (i.e., the distance of the points belonging to the surface of the mechanical component 4, and the surface of the mechanical component 4 corresponds to the detected edge E i from the measurement plane XZ at a specific angle θ j k ) can be obtained by calculating the change of the position as a function of the rotation angle θ.

[0016] In practice, for each angle θ i and in the case where the mechanical component 4 rotates clockwise and the angle θ increases (or the mechanical component 4 rotates counterclockwise and the angle θ decreases), the position of each point p i in space at the height z j , that is, the spatial coordinates of the point p i on the surface of the mechanical component 4, are obtained by the following formula, where as described above, E j k is a value of one of the K edges generated at the z j height, that is, one of order k (k = 1,..., K):

[0017]

[0018] When the mechanical component 4 rotates clockwise and the angle θ decreases, or when the mechanical component 4 rotates counterclockwise and the angle θ increases, the formula changes as follows:

[0019]

[0020] In summary, for point p i the calculation formula of the spatial coordinates can be expressed as follows, where, depending on the rotation direction of the mechanical component 4 and whether the angle θ increases or decreases during such rotation, the two occurrences of ± can both be represented as + or -:

[0021]

[0022] It should be noted that the derivative can be calculated, for example, by appropriate, numerical, and known methods per se. This derivative provides information about the distance of the points on the contour relative to the measurement plane XZ, and thus the spatial coordinates of the points p corresponding to these points on the surface of the mechanical component 4. i of the points.

[0023] The three-dimensional model consists of a cloud or set of points p i where i = l,..., N, obtained at different predetermined heights z j where j = l,..., M.

[0024] A specific preferred embodiment of the method according to the present invention will be briefly described below. This embodiment includes other steps in addition to the above steps. In Figure 1 the shadow projection device 1 of , the controlled mechanical component 4 is arranged between the transmitter 2 and the receiver 3 and rotates around the rotation axis A, and the latter defines the axis of the first coordinate Z of the Cartesian reference system S f of.

[0025] During the rotation of the mechanical component 4, the control and processing unit 10 uses the sensor 3 to obtain N two-dimensional images 5 i at continuous rotation angles θ i , for example, these images can cover a range of 180° or 360° at a predetermined frequency. In particular, considering all K edges at the height Z j and the entire 360° rotation, there is a certain information redundancy, because each point of the mechanical component 4 obtained at the angle θ i is also obtained at the angle θ i + 180. Therefore, in order to calculate all the spatial coordinates of the point p i , all K edges in the 180° rotation can be considered, or alternatively, K / 2 edges in the entire 360° rotation can also be considered.

[0026] From each two-dimensional image 5i Extract a contour (“detected contour”) and in the reference frame S associated with sensor 3 c Identify the points C of the detected contour i at the positions (x, z), where the plane X c Z c coincides with the plane of sensor 3.

[0027] Using the parameters (transformation matrix and vector) obtained in a known manner during the calibration phase, transform the points C i (x, z) of the detected contour into the points F f of the corresponding contour (“measured contour”) in the Cartesian reference frame S i (x, z).

[0028] M values z of the Z coordinate or height j (j = 1,..., M) are selected, at which the measured contour is “sectioned”, i.e., the height z i of the points F j (x, z) of such a measured contour, for which it is necessary to evaluate the position in space of the corresponding points in the actual contour of the mechanical component 4 and calculate their spatial coordinates.

[0029] Group the edges E of the various measured contours j k to obtain at most K trajectories (K = maximum number of edges detectable when the mechanical component 4 rotates), which describe the profile of the edges (i.e., the respective coordinates x) as a function of the rotation angle θ. Typically, the edges are grouped so that the same trajectory contains edges representing the same part of the mechanical component 4 at different angles θ i The edges of two consecutive measured contours belonging to the same trajectory are typically those whose values (x coordinates) are closest to each other.

[0030] As the rotation angle θ i varies, the trend of the k-th order (k = 1,..., K) edge at height z j is defined as

[0031]

[0032] As described above, by applying the formula, the spatial coordinates of the points p i resulting from the edges defined above are obtained:[[]]

[0033]

[0034] where, as described above, depending on the rotation direction of the mechanical component 4 and whether the angle θ increases or decreases during such rotation, the two occurrences of the sign ± are equal to + or -.

[0035] For each height z j (j = l,..., M), the spatial coordinates of the calculated points constitute a point cloud (or set) representing the three-dimensional numerical model of the mechanical component 4. Using or based on these spatial coordinates, the required inspections and measurements (such as the geometric or dimensional characteristics of the mechanical component 4) are performed in a known manner. The method according to the invention can be carried out by means of a device comprising an optoelectronic system having a matrix, a two-dimensional sensor (as described above) or different sensors. For example, a linear, one-dimensional sensor can be used. In this case, during the rotation of the mechanical component, a two-dimensional image can be acquired by performing an appropriate scan in a direction parallel to the rotation axis A.

[0036] The method according to the invention can calculate the spatial coordinates of the point p on the surface of the mechanical component 4 to be inspected i , thus allowing a more comprehensive and precise inspection / measurement of the geometric and dimensional characteristics of the mechanical component 4 compared to known methods using a shadow-projection type optoelectronic system that uses a two-dimensional or linear sensor. It highly accurately identifies how the points of the profile "move" when the mechanical component 4 rotates about the axis A by analyzing the instantaneous changes in the profile position (detected from the images provided by the shadow projection technique). In this way, the missing information on the distances of the points on the surface of the mechanical component corresponding to the points of the measurement profile relative to the measurement plane can be obtained, and then the spatial coordinates of said points can be calculated, and the spatial coordinates can be used to obtain a three-dimensional model of the mechanical component 4 and to perform an inspection / measurement of the geometric and / or dimensional characteristics of the mechanical component 4.

[0037] It should be noted that the formulas used in the preferred embodiment of the method according to the invention are always applicable, regardless of the specific type of the component.

[0038] Referring to Figures 3 to 9 , an example of the device and method according to the invention will be described in relation to the inspection of the geometric and dimensional characteristics of a mechanical threaded component 4 having a helical outer profile defining an external thread T and an operating axis. Figure 3 A shadow-projection type optoelectronic system 1 is schematically shown, comprising a transmitter 2 with a light source 21 and a telecentric lens 22, and a receiver 3 with appropriate optical elements 32 and a matrix sensor 31. The mechanical component 4 with the thread T is arranged in a reference and support device (not shown in Figure 3 ) and can rotate about a rotation axis A parallel to the coordinate Z, which generally does not coincide with the operating axis, contrary to the situation shown in Figure 3 . The receiver 3 is connected to a processing and control unit 10, which acquires two-dimensional images of the thread T at different rotation angles θ i .

[0039] It should be noted that due to the helical development of the surface, the contour of the thread image obtained using the shadow projection technique is generated by points that do not belong to a single cross-sectional plane. That is, the points of the measured contour obtained from the detected contour extracted from the acquired image (shadow) correspond to points on the surface of the thread T that are not on each cross-section or measurement plane (reference Figure 3 , including the plane XZ of the axis A), and these points have different and unknown distances from the measurement plane. Therefore, in order to obtain a contour that approximates the true contour of the thread for proper inspection / measurement operations, it is necessary to apply an algorithm to correct the "errors" caused by the geometry of the part and detect or calculate the distance of each point of the measured contour from the measurement plane of the optoelectronic system.

[0040] During the rotation of the part 4 with the thread T, the rotation is not necessarily centered relative to the rotation axis A. The sensor 31 generates a sufficient number of images, which are acquired by the processing and control unit 10. For each image, the rotation angle θ i is stored, and the detected contour is extracted from each image. Curves (multiple) located in the XZ plane are obtained. Using axis calibration, the detected contour is rotationally translated so that the Z axis coincides with the rotation axis. Figure 4 The resulting measured contour is shown.

[0041] The information related to the rotation angle θ i is used as the X axis in order to "package" the various curves into the XYZ system.

[0042] The number M of different cross-sections with height z j is defined, and the above-mentioned packaged curves are dissected into planes parallel to the XY plane and located at different z j heights ( Figure 5 ). For each z j and each edge k, a function can be obtained that represents the trend of the points of the measured contour changing with the rotation angle θ ( Figure 6 ):

[0043] E j k (θ)

[0044] By calculating the derivative of this function using an appropriate method and using the formula that has been cited before and is repeated here, the spatial coordinates of the point p i on the surface can be obtained:

[0045]

[0046] The spatial coordinates thus obtained form a point cloud, which represents the workpiece in a rather realistic way ( Figure 7A ). Figure 7BA threaded part is shown that is identical to a part that can be reconstructed by a method according to the background art without applying the method of the present invention, i.e., considering the points of the measurement profile that correspond to all the points of the part surface lying in the same measurement plane. Figure 7B The model of Figure 7B has parts that do not correspond to the actual surface of the threaded part to be inspected, so the points forming the model cannot accurately inspect the geometric and dimensional features of the threaded part.

[0047] The spatial coordinates obtained by the method according to the present invention are rotated and translated in a reference system, where the Z-axis coincides with the operating axis of the thread. This can be achieved by appropriately segmenting the region of the thread T from the measurement profile to obtain the thread axis. For example, try to isolate the points belonging to the thread crest, and after applying the above formula, they can become part of a cylinder (or cone), and the axis of the cylinder (or cone) can be calculated. The region of the mechanical part 4 outside the thread region can also be used to obtain the same axis.

[0048] In this way, a curve lying in a plane containing the operating axis can be obtained, or the cylindrical coordinates of point p i ( Figure 8 ) can be calculated to obtain the points lying in the axial plane at each defined angle θ i . For the curve obtained in this way, an appropriate segmentation algorithm can be used to extract the profile of the thread from the curve, and the profile truly reflects the true profile of the thread T Figure 9 , thereby providing the required accurate information about the dimensional and geometric features of the mechanical part 4.

Claims

1. A checking method for checking the dimensions or geometric features of a mechanical component (4) by means of a device comprising a shadow projection type optoelectronic system (1), said optoelectronic system having a linear or matrix sensor and telecentric optics, said method comprising the following steps: - rotating the mechanical component (4) to be checked about a rotation axis (A), -Obtaining a two-dimensional image (5 i ) of the mechanical component (4) at continuous rotational angles (θ i ) by means of the optoelectronic system (1), - Extract the points (C i i (x, z)) of the corresponding detected contours from each two-dimensional image (5), - Convert the points (C i (x,z)) of each detected contour to the points (F f (x,z)) of the measured contour in a Cartesian reference system (S i ), the Cartesian reference system including a measurement plane (XZ) and in which the axis of the first coordinate (Z) coincides with the axis of rotation (A). - Identify, in the Cartesian reference system (S f ), each edge point (E j ) of each measurement profile corresponding to the same value (z j k ) of the first coordinate (Z), - Evaluate the change in the position of the edge point (E j k ) as a function of the rotation angle (θ i ), and calculate, based on this change, the position of the points on the surface of the mechanical component (4) corresponding to the edge point (E j k ) of the measurement profile relative to the measurement plane (XZ), and the spatial coordinates of these points (p i ) on the surface of the mechanical component (4), - Repeat the previous two steps for a predetermined number (M) of values (z) of the first coordinate (Z) j ) and - Check the dimensions or geometric features of the mechanical component (4) based on the spatial coordinates of these points (p i ).

2. The inspection method according to claim 1, wherein, The step of evaluating the change of the position of the edge point (Ej k ) as a function of the rotation angle (θ i ) and calculating, based on this change, the position of the points on the surface of the mechanical component (4) corresponding to the edge points (E j k ) of the measurement profile relative to the measurement plane (XZ) and the spatial coordinates of these points (p i ) on the surface of the mechanical component (4), includes applying the following formula wherein E j k (θ i ) represents the trend of the edge points of order k (k = 1,..., K) at the same value (z i ) of the first coordinate (Z) as the rotation angle (θ j ) changes, and said ± sign is a positive sign when the mechanical component (4) rotates clockwise and the rotation angle (θ) increases, or when the mechanical component (4) rotates counterclockwise and the rotation angle (θ) decreases, and is a negative sign when the mechanical component (4) rotates counterclockwise and the rotation angle (θ) increases, or when the mechanical component (4) rotates clockwise and the rotation angle (θ) decreases.

3. The inspection method according to claim 1 or claim 2, wherein the two-dimensional image (5 i ) of the mechanical component (4) is acquired by the two-dimensional sensor (3) of the optoelectronic system (1).

4. The inspection method according to claim 1 or claim 2, wherein the two-dimensional image (5 i ) of the mechanical component (4) is obtained by a linear sensor of the optoelectronic system (1) and scanning the mechanical component (4) in a direction parallel to the rotation axis (A).

5. The checking method according to any one of claims 1 to 4, for checking the dimensions or geometric features of a mechanical component, defining an external thread (T) and an operating axis.

6. The checking method according to claim 5, further comprising the following steps: rotating and translating the calculated spatial coordinates in a reference system in which the axis of the first coordinate (Z) coincides with the operating axis, and extracting the exact profile of the external thread (T).

7. A checking device for checking the dimensions or geometric features of a mechanical component (4), said device comprising: reference and support means adapted to rotationally support the mechanical component (4) to be checked and to define a rotation axis (A), an optoelectronic system (1) which is of the shadow projection type and has a linear or matrix sensor and telecentric optics, and a control and processing unit connected to said reference and support means and said optoelectronic system, said control and processing unit being adapted to - control the rotation of the mechanical component (4) to be checked about the rotation axis (A), - Obtaining a two-dimensional image (5 i ) of the mechanical component (4) at continuous rotational angles (θ i ) through the optoelectronic system (1), - From each two-dimensional image (5 i ) extract the points (C i (x, z)) of the corresponding detected contour, - Convert the points (C i (x, z)) of each detected contour into the points (F f (x, z)) of the measured contour in the Cartesian reference system (S i ) which includes the measurement plane (XZ) and in which the axis of the first coordinate (Z) coincides with the axis of rotation (A). - Identify, in the Cartesian reference system (S f ), each edge point (E j ) of the measurement profile corresponding to the same value (z j k ) of the first coordinate (Z), - Evaluate the position of the edge point (E j k ) as a function of the rotation angle (θ i ), and calculate, based on this variation, the position of the points on the surface of the mechanical component (4) corresponding to the edge points (Ej k ) of the measurement profile relative to the measurement plane (XZ), and the spatial coordinates of these points (p i ) on the surface of the mechanical component (4). - repeat the previous two steps for a predetermined number (M) of values (Zj) of the first coordinate (Z), and - Check the dimensions or geometric features of the mechanical component (4) based on the spatial coordinates of these points (p i ).

Images