Surface inspection device and method

Abstract

The surface inspection device comprises: a) a camera (7); b) an actuator device (3, 4) for displacing the camera (7) relative to the surface (1) to be inspected; c) a first light source (10) mounted on the actuator device (3, 4) for joint displacement with the camera (7); and d) a second light source (9) positioned such that the optical axes (17, 16) of the first and second light sources (10, 9) intersect at a non-zero angle.

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for surface inspection, in particular for detecting possible defects in recently applied paint layers. [Background technology]

[0002] Quality inspection of painted or otherwise finished surfaces of workpieces, such as automobile bumpers, is traditionally performed by workers who pick up each recently finished workpiece and examine it for irregularities that may appear when viewing the workpiece from different angles. Depending on the worker's attention, defects may remain unnoticed. With the naked eye, workers cannot accurately measure the size of defects and cannot reliably determine whether the workpiece under inspection meets the customer's quality standards. Therefore, to avoid potential customer complaints, workers may have all workpieces in which they find defects corrected, regardless of their size, thus incurring unnecessary costs. Yet, all of this effort cannot eliminate the possibility of customer dissatisfaction due to overlooked defects.

[0003] DE 10 2015 106 777 A1 discloses an apparatus for inspecting the surface of a workpiece that has been subjected to industrial cleaning, which comprises a robot arm, a camera mounted on the robot arm so that it can be moved relative to the surface to be inspected, and an image processing unit for evaluating images taken by the camera.

[0004] When the device relies on ambient light, it is clear that the reliability of the inspection results depends crucially on the quality of the lighting. However, even when reproducible lighting conditions are provided, it is difficult for the image processing unit to distinguish between finish defects and shading due to irregularities in the surface under inspection. Prior art also considers associating a light source with the camera. By moving the robot arm, the surface can be illuminated from different directions in this way, but because the camera's viewpoint also changes, differences in shading resulting from changing orientation are still difficult to assess. Furthermore, if the surface must be inspected from different perspectives, the time spent inspecting increases significantly. Summary of the Invention

[0005] SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface inspection apparatus and method that can quickly and reliably detect surface finish defects.

[0006] This object is achieved according to one aspect of the invention by a surface inspection device, the device comprising: a) a camera; b) an actuator device for displacing said camera relative to the surface to be inspected; c) a first light source mounted on an actuator device for joint displacement with the camera; and the device further comprises: d) The optical axis of the first light source and the optical axis of the second light source are arranged so that the angle at which they intersect is not zero.

[0007] The surface to be inspected is positioned at the intersection of the optical axes of the first and second light sources, so that it can be illuminated by either of them. By alternating between the light sources, the angle of incidence of the light on the surface being inspected can be varied, thus altering the brightness distribution on the surface. To clearly detect changes, the angle between the optical axes must not be less than 30°. However, the camera's viewpoint can remain the same, so that images obtained with one light source or the other can be easily compared, ideally pixel-by-pixel, without having to consider changes in viewpoint. Because there is no need to move the device to collect images of the same surface area under different lighting conditions, such images can be obtained quickly.

[0008] To facilitate the detection of defects in the form of particles adhering to a finished surface, the angle between the optical axis of the camera and at least one optical axis of the light source should not be too small, preferably 30° or greater, so that when the camera's optical axis is aligned with the surface normal, the particle casts a detectable shadow on the surface when illuminated by this light source. On the other hand, if the angle is too large, the light source may collide with the surface being inspected, and surface irregularities may cause variations in surface brightness and hide true defects. Therefore, the angle should not exceed 60°.

[0009] The emission ranges of the first and second light sources should overlap with the field of view of the camera in the same plane, so that when the surface to be inspected is placed in that plane, the field of view of the camera can be illuminated by either light source.

[0010] One of the light sources may have an optical axis that coincides with the optical axis of the camera, so that in that light, particles on the surface do not cast any shadows at all, while shadows visible only in the light of the other light sources are a clear indication of the presence of particles.

[0011] On the other hand, it is undesirable for a strong reflection of one of the light sources to reach the camera, so each light source should be offset far enough from the camera's optical axis so that the specular image of the light source lies outside the camera's field of view when the camera's optical axis is perpendicular to the surface being inspected.

[0012] To maximize the possible change in surface brightness when switching between light sources, the first plane defined by the optical axis of the camera and the first light source is preferably substantially orthogonal to the plane defined by the optical axis of the camera and the second light source.

[0013] Surface defects can also be localized irregularities on an otherwise flat, reflective surface. In manual inspection, such defects flash as the operator rotates the object being inspected in front of them, thereby changing both the direction of incidence of ambient light on the surface and the angle at which the surface is viewed. According to the present invention, a similar effect is achieved by having at least a first light source elongated in a direction perpendicular to its optical axis and the optical axis of the camera, thus allowing illumination of a given surface point from a variety of angles. The angle at which a surface point is viewed can be varied by the camera being scanned along the surface.

[0014] If the scanning direction is parallel to the longitudinal direction of the light source, the illumination conditions at a given surface point do not change substantially during the passage of at least the central part of the first light source.

[0015] To ensure substantially invariant lighting conditions for a given surface point while the given surface point is within the field of view of the camera, the length of the light source in a direction perpendicular to the optical axis of the light source must be greater than the width of the field of view of the camera in the plane where the optical axis of the camera and the optical axis of the first light source intersect.

[0016] To ensure that reflection defects are detected regardless of their orientation, the second light source may also be elongated in a direction perpendicular to its optical axis and the optical axis of the camera.

[0017] Preferably, the light source is an LED, which can be rapidly toggled between on and off states and can be used to obtain alternating images illuminated by either the first or second light source in a single scanning motion.

[0018] The actuator device preferably comprises a robot arm carrying a camera and a light source and a controller for the robot arm, which allows the camera and the light source to be scanned along the surface of the object to be inspected at a fixed distance and / or at a fixed viewing angle, even if the surface is not flat, and which can optionally change the viewing angle if necessary.

[0019] According to another aspect, the object of the present invention is achieved by a method for inspecting a surface using the surface inspection device described above, the method comprising the steps of: a) positioning a portion of the surface to be inspected at the intersection of the optical axis of the camera and the optical axis of the light source; b) then acquiring a first image of the surface portion illuminated by the first light source; c) turning off the first light source and turning on the second light source; d) then acquiring a second image of the surface portion illuminated by the second light source; e) thereafter, determining whether a feature observed in at least one of the images is indicative of a defect based on a comparison of the first and second images.

[0020] At the very least, if a feature observed in this way is found to indicate a defect, its size should be determined in order to determine, or to allow an operator to determine, whether the feature should be modified.

[0021] While the first and second light sources are alternately operated, the camera and the first and second light sources can be continuously displaced. When comparing images, the displacement can be easily compensated for, and by moving continuously, the time required for inspection can be significantly reduced.

[0022] When the direction of displacement is the direction in which the first light source is elongated, the illumination conditions from the first light source do not change substantially for a given surface point while the given surface point passes through the camera's field of view, so any change in luminosity at a surface point observed during the camera's passage is likely due to reflections from defects.

[0023] A preferred application of the present invention is the inspection of newly painted surfaces.

[0024] Another object of the invention is a computer program product comprising instructions which, when executed by a processor, cause the processor to operate as a controller in a surface inspection device as described above.

[0025] Further features and advantages of the present invention will become apparent from the following description of embodiments, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 shows a robotic system embodying the present invention. [Figure 2] FIG. 2 is an enlarged view of the end effector of the robot system of FIG. [Figure 3] FIG. 3 illustrates the process of scanning for defects using the apparatus of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] 1 is a schematic diagram of a surface inspection apparatus according to the present invention and a workpiece 2 having a surface 1 to be inspected, for example the exterior of a freshly painted automobile bumper, shown here in cross section. The surface inspection apparatus comprises a robotic arm 3, an associated controller 4, and an inspection assembly 5 mounted on a distal flange of the robotic arm 3.

[0028] The controller 4 comprises a processor 18 and a memory device 19 in which a three-dimensional model of the workpiece 2, e.g. CAD data used in its manufacture, is stored, based on which the processor 18 can control the robot arm 3 to move the camera 7 along the surface 1 while maintaining a predetermined distance so as to scan the entire surface 1.

[0029] The inspection assembly 5, shown in more detail in Figure 2, comprises a backplane 6 facing the distal flange, an electronic camera 7 mounted on the backplane 6 and having an optical axis 8, and two light sources 9, 10, each mounted on an arm 11, 12 extending from the backplane 6. The light sources 9, 10 are elongated in directions perpendicular to each other and to the optical axis 8. As shown in Figure 1, the direction of elongation y of the light source 9 is parallel to the plane of the paper, and the direction of elongation y of the light source 10 is perpendicular to the plane of the paper.

[0030] Each light source 9, 10 comprises a number of LEDs distributed along its elongation direction and illuminates an elongated strip 13, 14 respectively on the surface 1 when the inspection assembly 5 faces the surface 1.

[0031] The field of view 15 of the camera 7 is located at the intersection of the two strips 13,14 and each point within the field of view 15 can be illuminated by either of the two light sources 9,10.

[0032] Each light source 9, 10 can be assigned an optical axis 16, 17. This optical axis may, for example, be the axis of symmetry of the light emission from the light source, or, if the light is not distributed symmetrically, the direction may correspond to the center of gravity of the light distribution. The optical axis 17 of the light source 10 is close to the optical axis 8 of the camera 7, i.e., the optical axes 17, 18 intersect at a small acute angle α. As a result, when the light source 10 illuminates a strip 14 on the surface 1 and a feature capable of casting a shadow, i.e., a protrusion or depression, is present in the field of view 15, the shadow is hidden by the feature itself for the camera 7. In contrast, with the optical axes 8, 16 forming an angle β significantly larger than α, typically about 45°, the shadow cast in the light from the light source 9 is visible to the camera 7. Thus, if the camera 7 captures images of a surface portion illuminated by the light source 10 and another surface portion illuminated by the light source 9, and the image processor of the controller 4 detects a dark area in the former image but not in the latter image, it can be assumed that this dark area is associated with a three-dimensional structure on the surface 1. If no such structure is present in the model of the workpiece 2 in the storage device 19, the controller 4 concludes that the dark area is due to a surface structure defect and determines the location and size of the defect on the surface 1 based on the known pose of the arm 3, the size of the defect in the image, and the known distance between the inspection assembly 5 and the surface 1, and stores this data for later use.

[0033] A structural defect in surface 1 may also appear brighter than its surroundings if it is oriented so as to reflect light from one of the light sources 9, 10 back to camera 7. The likelihood of this happening increases the larger the angle at which the defect can receive light from one of the light sources 9, 10. This angle can be increased in one direction, say the y direction, by elongating the light source 9, and in a second direction x, by scanning the light source 9 in the second direction x, so that at some point while the defect is moving in the x direction through the field of view of camera 7, it becomes more likely that the defect will be oriented so as to reflect light from the light source 9 back to camera 7 and therefore be detected.

[0034] The controller 4 is programmed to move the inspection device in a scanning motion over the surface 1 so that all parts of the surface where defects must be controlled pass through the field of view of the camera 7. Figure 3 illustrates such a scanning process, with a reflective defect 20 present on the surface 1. The inspection assembly 5 is viewed along the y-direction, with the light source 10 performing an elongated scanning movement in the x-direction. The defect 20 appears as a bright spot in the image captured by the camera 7, indicated by a solid line at the moment of the scanning movement when the light beam 21 from the light source 9 is reflected by the defect 20 toward the camera 7. At earlier and later moments of the scanning movement, the beams 22 and 23 from the light source 9 are reflected by the defect 20, indicated by the dotted line, but the reflected beams 22' and 23' do not reach the lens of the camera 7, so the defect 20 is not visible in the image.

[0035] Light from light source 10 arrives at defect 20 at a different angle than beam 21 and is not reflected back to camera 7. Because light source 10 extends in the scanning direction, displacement of inspection assembly 5 by a small fraction of the length of light source 10 can be expected to have a negligible effect on the illumination conditions at defect 11. Thus, based on images from camera 7, controller 4 can determine that a given point on surface 1 is a structural defect when the brightness of that point changes significantly under the light from light source 10, or when the ratio of the brightness under the light from light source 10 to the brightness under the light from light source 9 changes while the point is moving through the field of view of camera 7.

[0036] Another type of defect that can occur on the workpiece surface 1 is a stain 24 (see FIG. 4), i.e., a local variation in color that can be caused by a foreign particle adhering to or embedded in the surface 1. The visibility of the stain 24 to the camera 7 must not change substantially while the inspection assembly 5 moves over the stain 24, and the visibility must be similar in the light of both light sources 9, 10, unless the camera 7 is obscured by light from the light sources 9 or 10 specularly reflected back to the camera 7, or unless a reflection of a foreign particle on the surface 1 is seen.

[0037] Camera 7 can prevent seeing reflections from foreign objects by orienting its optical axis 8 perpendicular to surface 1, so that if there is a specular reflection in the image seen by camera 7, it is a reflection from the front lens of the camera itself. Because the front lens is dark, its reflection does not hide dirt.

[0038] Although light source 10 is oriented so that its optical axis 17 intersects optical axis 8 on surface 1, and most of its light is reflected away from camera 7, there is still a light beam 25 from light source 10 that is specularly reflected into camera 7. Obstruction by this light 25 can be minimized if the light does not reach the photodetector of camera 7, i.e., if the point 26 on surface 1 from which light beam 25 is reflected is outside the field of view 15 of camera 7.

[0039] The controller 4 may determine that a given point on the surface 1 is contaminated if the brightness of that point differs from that of nearby points in a way that is not apparent from the model of the workpiece 2 in the storage device 19 or from surface brightness data previously collected from other workpieces of the same type, and if this difference remains substantially constant as the point moves through the camera's field of view.

[0040] While scanning the surface 1, the controller 4 collects data on the location, size, and type of defects encountered and compares these with predetermined quality requirements. If these quality requirements are not met, the controller 4 issues a warning message, so that the workpiece 2 can be sent back to the paint shop for correction, accompanied by a record of the defects found by the controller 4. This ensures that only workpieces that would otherwise be rejected by the customer are corrected. Furthermore, the record output by the controller 4 does not need to include all defects detected, but can be limited to those that must be repaired to meet the requirements, so that time is not lost correcting defects that are not actually critical.

[0041] Reference number 1 surface 2 workpieces 3. Robotic Arm 4 Controller 5. Inspection Assembly 6 Backplane 7. Camera 8 Optical axis 9 light source 10 light source 11 Arm 12 Arm 13 Strip 14 Strip 15 field of view 16 Optical axis 17 Optical axis 18 processors 19 Storage device 20 Defects 21 Light Beam 22 Light Beam 23 Light Beam 24 Dirt 25 Light Beam 26 points

Claims

1. A surface inspection device, a) a camera (7); b) an actuator device for displacing said camera relative to the surface to be inspected; c) a first light source (10) mounted on said actuator device for displacement with said camera (7); The surface inspection device further comprises: d) A surface inspection device characterized in that it comprises a second light source (9) arranged so that the optical axis (17) of the first light source (10) and the optical axis (16) of the second light source (9) intersect at an angle that is not zero.

2. 2. The surface inspection device according to claim 1, wherein the angle (β) of intersection between the at least one optical axis (16) of the second light source (9) and the optical axis (8) of the camera (7) is greater than 30° and / or less than 60°.

3. 3. The surface inspection device according to claim 1, wherein the light emitting range (14) of the first light source (10) and the light emitting range (13) of the second light source (9) overlap with the field of view (15) of the camera (7) in the same plane.

4. 4. The surface inspection apparatus of claim 1, wherein each of the first light source (10) and the second light source (9) is offset from the optical axis (8) of the camera (7) such that when the optical axis (8) of the camera (7) is perpendicular to the surface (1) to be inspected, the specular reflection of each light source (9, 10) on the surface (1) is located outside the field of view (15) of the camera (7).

5. 5. The surface inspection device according to claim 1, wherein a first plane (yz) defined by the optical axis (8) of the camera (7) and the optical axis (17) of the first light source (10) is substantially perpendicular to a plane (xz) defined by the optical axis (8) of the camera (7) and the optical axis (16) of the second light source (9).

6. 6. The surface inspection device according to any one of claims 1 to 5, wherein at least the first light source (10) is elongated in a direction (x) perpendicular to its optical axis (17) and to the optical axis (8) of the camera (7), and optionally the length of the first light source (10) in the direction (x) perpendicular to the optical axes (17, 8) is greater than the distance between the first light source (10) and the intersection of the optical axis (8) of the camera (7) and the optical axis (17) of the first light source.

7. 7. A surface inspection apparatus according to any one of the preceding claims, wherein at least one of the first and second light sources (10, 9) comprises an LED.

8. 8. Surface inspection device according to any one of the preceding claims, wherein the actuator device is adapted to alternately operate the first and second light sources (10, 9).

9. 9. The surface inspection apparatus according to any one of claims 1 to 8, wherein the actuator device comprises a robot arm (3) carrying the camera (7) and the first and second light sources (10, 9), and a controller (4) for the robot arm (3).

10. A surface inspection method using the surface inspection device according to any one of claims 1 to 9, comprising: a) positioning a portion of the surface (1) to be inspected at the intersection of the optical axis (8) of the camera (7), the optical axis (17) of the first light source (10) and the optical axis (16) of the second light source (19); b) then acquiring a first image of the surface portion (15) illuminated by said first light source (10); c) turning off the first light source (10) and turning on the second light source (9); d) then acquiring a second image of said surface portion (15) illuminated by said second light source (9); e) thereafter, based on a comparison of the first image and the second image, determining whether a feature observed in at least one of the first and second images indicates a defect (20, 24).

11. 11. The method of claim 10, further comprising determining the size of the feature at least when the feature is found to be indicative of a defect (20, 24).

12. 12. A surface inspection method according to claim 10 or 11, wherein the camera (7) and the first and second light sources (10, 9) are displaced continuously while the first and second light sources (10, 9) are operating alternately.

13. 13. The surface inspection method according to any one of claims 10 to 12, wherein at least the first light source (10) is elongated in a direction (x) perpendicular to its optical axis (17), and wherein the camera (7) and the first and second light sources (10, 9) are displaced in said direction (x) between the moment the first image is taken and the moment the second image is taken.

14. 14. A method for inspecting a surface according to any one of claims 10 to 13, wherein the surface (1) to be inspected is a freshly painted surface.

15. A computer program product comprising instructions that, when executed by a processor (18), cause the processor to: A computer program product that operates as a controller in the surface inspection apparatus of claim 8.

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