Optical measuring methods and optical measuring devices

Abstract

Optical measuring method for measuring the three-dimensional topography of a surface of a component, in which the surface to be measured is imaged onto a target of a sensor arrangement by means of a lens, where the distance between two object points on the surface is determined based on the distance of their image on the target, using an entocentric lens, wherein a 3D-measurable sensor arrangement is used and the distance between the respective object point and the lens is measured in the direction of the optical axis of the lens, whereby, in order to determine the distance between the object points, the distance-dependent image scale applicable to the respective distance of the object point to the lens is correctively taken into account in an evaluation device and wherein the sensor arrangement comprises at least two line scan cameras inclined relative to one another.

Description

[0001] The invention relates to optical measuring methods and optical measuring devices for measuring the three-dimensional topography of a surface of a component, in which the surface to be measured is imaged onto a target of a sensor arrangement by means of an objective lens, wherein the distance between two object points on the surface is determined based on the distance between their image on the target.

[0002] In production metrology, it is often necessary to measure lengths precisely, preferably quickly and non-destructively. Optical measurement methods are ideal for this purpose. Length measurements on components or workpieces can include, for example, width and length, or distance measurements at the edge or across the surface of the workpiece. Furthermore, it is desirable to inspect the surface of the workpiece for the absence of surface defects.

[0003] A corresponding optical measuring method is used, for example, in length measuring devices from the "Opticline" product line of Jenoptik Industrial Metrology Germany GmbH, Drachenloch 5, 78052 Villingen-Schwenningen. In this known method, the surface to be measured is imaged onto a target of a sensor array using a lens, whereby the distance between two object points on the surface of the component is determined based on the distance between their image on the target.

[0004] DE 20 2011 001 808 U1 discloses an optical measuring method for measuring the 3-dimensional topography of a component's surface. In this method, the surface to be measured is imaged onto a target of a sensor array using a lens. The distance between two object points on the surface is determined based on the distance between their image on the target, and an entocentric lens is used. In this known method, the distance-dependent image scale applicable to the respective distance of the object point from the lens is corrected for the object points in an evaluation device to determine the distance between the object points.

[0005] A similar measuring method is also known from DE 100 14 627 A1.

[0006] The invention is based on the object of providing an optical measuring method which offers high measuring accuracy with relatively low equipment expenditure.

[0007] This object is achieved by the invention defined in claim 1.

[0008] State-of-the-art measurement methods often use a telecentric lens. This ensures that the image scale does not change with the distance between the lens and the surface to be measured. If the optical axis of the lens is defined as the z-axis, the use of a telecentric lens ensures that the acquired lateral xy points do not change with their z-position, i.e., they are independent of the distance between the object point on the component to be measured and the lens.

[0009] Telecentric imaging requires an objective lens with a diameter equal to the largest component dimensions. If only a single lens is to be used with a single image sensor, either the measuring range is limited or a very large, heavy, and correspondingly expensive lens must be used. The measuring range can also be split into individual lens / image sensor combinations, whose partial images are then combined. In any case, the optical imaging device used with a telecentric lens is complex and therefore expensive.

[0010] Based on this, the invention is based on the idea of ​​using an entocentric lens to image the surface to be measured onto the target, for example, an image sensor of the sensor array, and of designing the signal processing of the image sensor's output signals in such a way that the same measurement results are obtained as when using a telecentric lens. According to the invention, perspective distortions that occur with an entocentric or approximately entocentric lens due to the distance dependence of the image scale are corrected for this purpose.

[0011] In this sense, the invention provides for the use of an entocentric lens and a 3D-measurable sensor array, and the distance between the respective object point and the lens is measured in the direction of the optical axis of the lens. To determine the distance between the object points, the image scale applicable to the respective distance to the lens is corrected for the object points in an evaluation device.

[0012] The invention according to claim 1 further provides that the sensor arrangement has at least two line scan cameras inclined relative to one another. By inclining the line scan cameras relative to one another, a sensor arrangement capable of 3D measurement is realized using simple and cost-effective means. Depending on the respective requirements and circumstances, the 3D measurement capability of the sensor arrangement can be realized in any suitable manner, for example, using a stereo triangulation method, as known from DE 10 2015 010 225 A1. The aforementioned document describes a stereo triangulation method in connection with the inspection of rotationally symmetric cavities. However, the basic principle of the method is also applicable to the measurement or imaging of non-rotationally symmetric, flat, or planar components.DE 10 2014 118 844 A1 describes the measurement of the three-dimensional topography of a surface using the shape-form shading method, also in connection with the inspection of rotationally symmetric cavities. This method is also applicable to flat or planar components.

[0013] In the optical measuring method according to the invention, in order to determine the distance between two object points on the surface of the component, the distance between the respective object point and the lens is measured in the direction of the optical axis of the lens by means of the 3D-measurable sensor arrangement.

[0014] From the measured distance and the distance-dependent image scale of the lens, the corresponding image scale can then be determined for each object point and can be taken into account individually for each object point when determining the distance between two object points.

[0015] The determination of the distance between the object points on the component's surface is then based on the distance between the image of the object points on the target. With a telecentric lens, this distance corresponds to the distance between the object points on the surface of the workpiece. In the measuring method according to the invention, the distance between the images of the object points on the target is corrected using the image scale determined for each object point. The result of the measurement is then the absolute distance (within the limits of the measurement accuracy) between the object points on the surface of the workpiece.

[0016] The invention thus enables the measurement of distances with high accuracy using a relatively simple and cost-effective setup with a 3D-measurement-capable sensor and an entocentric lens. Since each length measurement measures a distance between two object points, the measuring method according to the invention enables the measurement of any length dimensions on components or workpieces.

[0017] A particular advantage is that the correction implemented according to the invention in the signal processing of the sensor array's output signals can be performed by the software of an evaluation device. Accordingly, no additional hardware is required to implement the invention, so existing measuring devices can be retrofitted with appropriate software and equipped according to the invention.

[0018] It is advisable to measure the distance between the object points in a plane passing through the object points, inclined or perpendicular to the optical axis.

[0019] A further solution to the problem underlying the invention is specified in claim 2.

[0020] In order to be able to examine rotationally symmetric cavities using the method according to the invention, the invention, according to claim 2, provides that a lens with a panoramic view is used to measure the surface of a rotationally symmetric or approximately rotationally symmetric cavity in the component. A stereo image can thus be captured, for example, using one of the measuring devices disclosed in the aforementioned publications. DE 10 2017 111 819 A1 shows and describes a further possibility for capturing a stereo image using a lens with a panoramic view. Details of the processing of the image data can be found in the aforementioned publications and, in particular, also in DE 10 2007 031 358 A1.

[0021] An advantageous development of the invention provides that the sensor arrangement is moved relative to the component in order to scan the surface of the component.

[0022] In particular, the sensor arrangement can be formed by a scanning stereo camera.

[0023] According to the invention, an entocentric lens also includes approximately entocentric lenses.

[0024] An optical measuring device according to the invention is specified in claim 4. Another optical measuring device according to the invention is specified in claim 5. An advantageous development of the measuring devices according to the invention is specified in claim 6. The same properties and advantages result as with the measuring method according to the invention and its developments.

[0025] The invention is explained in more detail below with reference to the attached, highly schematic drawing using an exemplary embodiment.

[0026] It shows: Fig. 1 a schematic diagram to illustrate the basic principle of imaging with an entocentric lens, Fig. 2 a schematic diagram to illustrate the basic principle of a 3D measurement-capable sensor arrangement and Fig. 3 a block diagram of an embodiment of an optical measuring device according to the invention for carrying out an embodiment of the method according to the invention.

[0027] First, based on Fig. 1 The basic principle of an entocentric mapping is explained below.

[0028] In Fig. 1 schematically and simplifyingly, an entocentric lens 1 is symbolized by a pinhole, the opening of which is shown exaggeratedly large for reasons of explanation.

[0029] A target 2 of a camera is also shown purely schematically.

[0030] The measurement task to be solved is to measure the width of a component 3 in the z-direction.

[0031] Out of Fig. 1 shows how an object point 4 of the component 3 is imaged via the lens 1 (pinhole) onto an image point 5 of the target 2 and how an object point 6 is imaged via the lens 1 onto an image point 7 of the target 2.

[0032] In a corresponding manner, an object point 8 is imaged via the lens 1 onto an image point 9 of the target 2 and an object point 10 is imaged via the lens 1 onto an image point 11 of the target 2. As can be seen from Fig. As can be seen in Figure 1, the object points 4, 6 lie on top of each other in the z-direction, but due to the perspective distortion introduced by the entocentric lens 1, they are imaged at different locations 5 and 7 or onto different image points of the target 2. The same applies to the object points 8, 10.

[0033] Out of Fig. 1 it is evident that the distances of the object points 4, 8 and correspondingly the object points 6, 10 on the component are equal due to the perspective distortion introduced by the entocentric lens 1, but are imaged at different distances between the image points 5 and 9 on the one hand and the image points 7 and 10 on the other hand. A length measurement by imaging the object 3 onto the target 2 is thus possible in the Fig. 1 shown configuration is not possible.

[0034] The invention takes advantage of the fact that the Fig. 1 explained perspective distortion is defined by the degree to which the image scale varies with varying distance between lens and object, is defined by the optical design of the lens and can therefore either be calculated from the design data of the lens or determined by calibration using a calibration object.

[0035] When measuring a distance or a length using the measuring method according to the invention, it is essential that the distance between an object point and the lens 1 is known.

[0036] To determine the distance, the invention uses a 3D measuring sensor arrangement.

[0037] Fig. Figure 2 shows a highly schematic representation of an embodiment of a 3D measurement-capable sensor arrangement 12. This sensor arrangement comprises two line-scan cameras 14, 16 arranged at an angle to each other, each associated with a lens array 18, 20. The structure and operation of this embodiment are explained in more detail in DE 10 2017 126 042 A1, to which reference is made here.

[0038] A corresponding sensor arrangement 12 enables a height measurement in the z-direction and thus the determination or measurement of the distance between an object point and the lens 1. A corresponding height measurement in the z-direction can also be performed using any other stereoscopic or 3D-capable measurement method, depending on the respective requirements and circumstances. In this context, reference is made to the above-mentioned publications, which describe corresponding methods in connection with the inspection of rotationally symmetric cavities.

[0039] Fig. Figure 3 shows a highly schematic representation of an embodiment of an optical measuring device 22 according to the invention for carrying out an embodiment of a method according to the invention. The measuring device 22 comprises a camera 24, which in the illustrated embodiment is designed as a scanning stereo camera 24, with a sensor arrangement 12 corresponding to Fig.2 and an entocentric lens 26. The sensor arrangement 12 is connected to an evaluation device 28 for image data transmission.

[0040] During operation of the measuring device 22, the lens 26 images the surface of the component 3 onto a target of the stereo camera 24. The resulting image data of the surface of the component 3 are transmitted to the evaluation device, which is designed and programmed such that the distance between two object points on the surface is determined based on the distance between their images on the target 2.

[0041] According to the invention, the perspective distortion introduced by the entocentric lens 26 is corrected during the evaluation of the image data. For this purpose, the evaluation device 28 is designed and programmed such that, in order to determine the distance between the object points, the image scale applicable to the measured distance to the lens is correctively taken into account for the object points based on the distance between the object point and the lens measured by the stereo camera 24.

[0042] The optical measuring method and the optical measuring device according to the invention thus enable an exact length measurement of components or workpieces using a relatively simply constructed and cost-effective entocentric lens.

Claims

[1] Optical measuring method for measuring the three-dimensional topography of a surface of a component, in which the surface to be measured is imaged onto a target of a sensor arrangement by means of a lens, where the distance between two object points on the surface is determined based on the distance of their image on the target, using an entocentric lens, wherein a 3D-measurable sensor arrangement is used and the distance between the respective object point and the lens is measured in the direction of the optical axis of the lens, whereby, in order to determine the distance between the object points, the distance-dependent image scale applicable to the respective distance of the object point to the lens is correctively taken into account in an evaluation device and wherein the sensor arrangement comprises at least two line scan cameras inclined relative to one another. [2] Optical measuring method for measuring the three-dimensional topography of a surface of a component, in which the surface to be measured is imaged onto a target of a sensor arrangement by means of a lens, where the distance between two object points on the surface is determined based on the distance of their image on the target, using an entocentric lens, wherein a 3D-measurable sensor arrangement is used and the distance between the respective object point and the lens is measured in the direction of the optical axis of the lens, whereby, in order to determine the distance between the object points, the distance-dependent image scale applicable to the respective distance of the object point to the lens is correctively taken into account in an evaluation device and wherein a lens with an all-round view is used to measure the surface of a rotationally symmetrical or approximately rotationally symmetrical cavity in the component. [3] Measuring method according to claim 1 or 2, wherein the sensor arrangement is moved relative to the component in order to scan the surface of the component. [4] Optical measuring device (22) for measuring the three-dimensional topography of a surface of a component (3), with a sensor arrangement (12) with a target (2), with a lens (26) for imaging the surface to be measured onto the target (2) of the sensor arrangement (12) and with an evaluation device (28) which is in image data transmission connection with the sensor arrangement (12) and which is designed and programmed such that the distance between two object points (4, 8) on the surface is determined based on the distance between their images (5, 9) on the target (2), wherein the objective is an entocentric objective (26), wherein the sensor arrangement (12) is a 3D-measurable sensor arrangement for measuring the distance between the respective object point (4, 8) and the lens (26) in the direction of the optical axis of the lens (26), wherein the evaluation device (28) is designed and programmed in such a way that, in order to determine the distance between the object points (4, 8), the distance-dependent image scale applicable to the respective distance to the lens (26) is correctively taken into account for the object points (4, 8) and wherein the sensor arrangement comprises at least two line cameras (11, 14) inclined relative to one another. [5] Optical measuring device (22) for measuring the three-dimensional topography of a surface of a component (3), with a sensor arrangement (12) with a target (2), with a lens (26) for imaging the surface to be measured onto the target (2) of the sensor arrangement (12) and with an evaluation device (28) which is in image data transmission connection with the sensor arrangement (12) and which is designed and programmed such that the distance between two object points (4, 8) on the surface is determined based on the distance between their images (5, 9) on the target (2), wherein the objective is an entocentric objective (26), wherein the sensor arrangement (12) is a 3D-measurable sensor arrangement for measuring the distance between the respective object point (4, 8) and the lens (26) in the direction of the optical axis of the lens (26), wherein the evaluation device (28) is designed and programmed in such a way that, in order to determine the distance between the object points (4, 8), the distance-dependent image scale applicable to the respective distance to the lens (26) is correctively taken into account for the object points (4, 8) and wherein the lens for measuring the surface of a rotationally symmetrical or approximately rotationally symmetrical cavity in the component is a lens with an all-round view. [6] Measuring device according to claim 4 or 5, wherein a feed device for the relative movement of the sensor arrangement (12) to the component (3) is provided for scanning the surface of the component (3).

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