WO2014114663A1

WO2014114663A1 - Optical device and method for determining spatial coordinates of surfaces of macroscopic objects by triangulating two line-scan cameras

Info

Publication number: WO2014114663A1
Application number: PCT/EP2014/051210
Authority: WO
Inventors: Gunther Notni; Peter KÜHMSTEDT
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2013-01-23
Filing date: 2014-01-22
Publication date: 2014-07-31
Also published as: DE102013201061A1

Abstract

The invention relates to a device and a method for determining spatial coordinates of surfaces of macroscopic objects (1). The proposed device comprises a line projector (6), two line-scan cameras (7), and a control and evaluation unit (8), wherein the control and evaluation unit (8) is designed to perform the following steps: - controlling the line projector (6) such that the line projector projects a series of different intensity distributions (12) within a plane that remains the same for said series, - controlling the line-scan cameras (7) such that each of the line-scan cameras (7) captures a one-dimensional image for each of said intensity distributions (12), such that a series of measured values is determined for each pixel of an image line (10) of the particular line-scan camera (7), - determining pixels of the two image lines (10) of the two line-scan cameras (7) that correspond to each other by correlating the series of measured values determined by means of the two line-scan cameras (7), and - calculating spatial coordinates for a plurality of points of a line of intersection of the projected intensity distributions (12) with an object surface by triangulation.

Description

OPTICAL DEVICE AND METHOD FOR DETERMINING SPATIAL

COORDINATES OF SURFACES OF MACROSCOPIC OBJECTS BY TRIANGULATION OF TWO LINES CAMERAS

The invention relates to a device for determining spatial coordinates of surfaces of macroscopic objects and to a method that can be carried out for determining spatial coordinates of a surface of a macroscopic object.

The three-dimensional measurement of macroscopic objects is i.a. The advantage of active object lighting over approaches that work exclusively with ambient light lies in the pattern-dependent one- or one-to-one assignment of camera and / or camera

Projector points that serve as the basis of triangulation algorithms. The accuracy and reliability of these mappings form the basis of the accuracy of future 3D results, Equally important for the precision of three-dimensional coordinate determination is a given system calibration or simultaneous measurement. This calibration is valid as long as no kind of displacements and / or deformations within the 3D measuring sensor, which often consists of a projection unit and at least one area camera, has taken place. For measuring environments in which high speeds of the measuring sensor must be achieved, the necessary recording speed of the area cameras quickly becomes the limiting technical factor limiting the frame rate.

One area of three-dimensional measurement is the determination of coordinates in the case of moving objects or moving sensors, in which a very high speed of data acquisition has to be realized in order to avoid motion blur during the measurement. Also of importance is the very fast measurement of static measurement situations with frame rates that are not achieved today with conventional area cameras.

In order to keep the blurs or motion blurs within this recording as low as possible in this area, adapted to this requirement projection and camera systems are necessary.

For active object illumination in 3D surveying or SD reconstruction, either surface-measuring systems with projectors and area cameras or line scanners have been used which work with the light-section method. Such line scanners are conventionally equipped with a line projector projecting a line on the object to be measured and an area camera observing the object at a triangulation angle and taking the image of the projected line on the object. Such a system is shown in FIG. For projecting the line onto the object 1, it has a laser 2 and a cylindrical lens 3, while the area camera - that is a camera which takes two-dimensional images - is formed by a light-diffusing optics 4 and a CCD matrix 5. In addition, such known line scanners have a computer system which controls the projection and receives and processes the data from the camera. The laser line is projected onto the object in the form of a projection plane. This plane strikes the measurement object, and the intersection line between object projection plane forms the laser line on the object. When viewing this section line at a triangulation angle with a surface camera, the height information of the object is coded in the form of a lateral deflection of the line in the camera image. Geometrically, the 3D coordinates are recorded along a line on the object. To record such a line, the camera information of the area camera must be read in each case. The height information of a line on the object is thus displayed as area information in the camera image.

The arrangement of such a conventional line scanner system can be described as follows without loss of generality. A measurement object is located substantially in the x-y plane. With the projector, as shown in Fig. 1, a line from the direction of the z-axis - ie in the direction opposite to the z-axis - is projected into the x-y plane, directly to the y-axis or parallel to the y-axis. Thus, a projection beam or a fan of light is created, which spans a projection plane in the y-z plane. With an area camera, the object is observed from a direction in the x-z plane, with an angle of the optical axis to the z axis of several degrees to about 80 °. By way of example, 20 ° are chosen. The optical axis of the camera is in the x-z plane.

The camera observes the x-y plane and the measurement object located there. Thus, the height information on the measurement object in the intersection of

Object and the y-z plane are captured. This is coded in the camera for each measuring point in an offset of the observed lines. An intensity distribution of the type shown in FIG. 1 at the top right results in the camera plane. The triangulation base is spanned by the camera and the projector. It lies in the x-z plane and outside the y-z plane.

As mentioned above, systems of this type have the disadvantage that the time required to acquire a contour can not be reduced as much as would be desirable. The invention has for its object to provide a compact device for determining spatial coordinates of surfaces of macroscopic objects, with which the measurement of such surfaces in a relatively short time is possible. Also proposed is a corresponding method for determining spatial coordinates of a surface of a macroscopic object, which manages with extremely short measurement times.

This object is achieved by a device according to the main claim and by a method according to the independent claim. Advantageous embodiments and further developments emerge with the features of the subclaims.

The proposed device for determining spatial coordinates of surfaces of macroscopic objects comprises a line projector, two line scan cameras and a control and evaluation unit, wherein the control and evaluation unit is set up to carry out the following steps:

Driving the line projector so that it projects a series of different intensity distributions within a plane that is the same for that series and only one line on the object is illuminated by the line projector,

Driving the line scan cameras so that each of the line scan cameras receives a one-dimensional image for each of these intensity distributions so that a series of measured values is determined for each pixel of a line of the respective line scan camera,

Determining mutually corresponding pixels of the two image lines of the two line scan cameras by correlating the series of one-dimensional images taken with the two line scan cameras or the series of measured values determined by the two line scan cameras

Calculating spatial coordinates for a plurality of points of a line of intersection of the projected intensity distributions with an object surface By triangulation on the basis of the thus determined corresponding pixels of the two image lines of the two line scan cameras.

This device allows a measurement of three-dimensional contours and the determination of height and depth information of surfaces in a very short time. This results already from the fact that line cameras compared to surface cameras much larger rates bzg. In addition, the use of line scan cameras, due to their small dimensions, in particular perpendicular to the respective image line, results in a very compact construction is also very efficient, because at least almost all recorded image information is also used in contrast to the conventional light-section method in which ultimately only the image information of a narrow strip of the respective image sensor corresponding to the narrow illuminated line are used.

Accordingly, the proposed method for determining spatial coordinates of a surface of a macroscopic object, which comprises the following steps, is also advantageous.

Projecting a series of different intensity distributions within a plane which is constant for this series by means of a line projector directed onto the surface of the object so that only one line of inhomogeneous intensity is projected onto the surface along a line of intersection of said plane with the surface of the object,

Each taking a one-dimensional image of a strip illuminated by the line projector of the surface for each of these intensity distributions by each of two line scan cameras, so that each of the two line scan cameras each takes a one-dimensional image for each of the projected intensity distributions and for each pixel of an image line of the respective one Line camera is determined in each case a series of measured values, Determining mutually corresponding pixels of the two image lines of the two line scan cameras by correlating the series of one-dimensional images taken with the two line scan cameras or the series of measured values determined by the two line scan cameras

- Calculating spatial coordinates for a plurality of points of the intersection of the projected intensity distributions with the surface of the object by triangulation based on the thus determined corresponding pixels of the two image lines of the two line scan cameras.

In this case, it can be provided that the surface of the object is scanned by carrying out said steps for a sequence of different projection directions, such that said plane is tilted about an axis lying in this plane and oriented perpendicular to a main projection direction of the line projector becomes. Thus, a contour of the object can be detected not only along a line, but at least within certain limits over the entire surface of the object. The proposed device can accordingly be designed as a line scanner, which is designed to execute the said steps for a sequence of different projection directions, so that said plane is tilted about said axis.

The intensity distributions may e.g. in the form of (l + cos) -shaped intensity distributions or of statistical patterns or of bacon patterns. In particular, patterns of this type are suitable for enabling the necessary identification of corresponding pixels-meaning homologous points in the two image lines of the two line scan cameras-with a not too large number of projected intensity distributions. For this purpose, the control and evaluation unit can be set up to use the line projector for the projection of the intensity distributions in the form of

{l + cos) -shaped intensity distributions or of statistical patterns or bacon patterns.

It can be provided that the line projector has a laser and a diffuser for generating the intensity distributions or a digital one

Projection system or for generating the intensity distributions means discrete single-point pattern or by means of a sliding slide and a fixed projection optics is executed. The intensity distributions can thus be generated in particular by means of a laser and a diffusing disk for generating / shaping speckle or by means of a digital projection system or by means of discrete single point patterns or by means of a displaceable slide and a fixed projection optics.

It is particularly advantageous if the image lines of the line scan cameras and the line projector-preferably both a projection center of the line projector and the line direction-lie in a common plane or are arranged lying in a common plane. The device or the line scanner thus forms a substantially two-dimensional arrangement. As a result, not only can a particularly compact design be realized, but also the evaluation of the measurement results becomes particularly easy in this case due to the special geometric conditions.

An embodiment will be explained below with reference to FIGS. It shows

1 shows the already mentioned conventional line scanner,

2 shows a schematically illustrated line scanner of the proposed type with a line projector and two line scan cameras in a perspective view,

Fig. 3 shows the line scanner of Fig. 2 in a plan view of a surface to be measured and

Fig. 4 shows the same line scanner in a viewing direction perpendicular to a

Plane in which both the line projector and one image line of the two line scan cameras are located.

In each of the figures, a coordinate system is reproduced with an x-axis, y-axis and z-axis respectively identified as such, which makes orientation of the line scanner easier in the various views allowed to follow. Identical features are each provided with the same reference numerals in the figures.

The line scanner shown in FIGS. 2 to 4 forms a device for determining spatial coordinates of a surface of an object 1 shown here in transparent form. This surface lies approximately in the x-y plane. The line projector 2, whose main projection direction is parallel to the z-axis of the coordinate system and opposite to the z-axis of the coordinate system, has a line projector 6 and two line scan cameras 7 and a control and evaluation unit 8. The control and evaluation unit 8 is shown only in FIG. Each of the line scan cameras 7 has, in addition to a simple imaging optical unit 9, an image line 10 with a multiplicity of pixels not shown individually and arranged in a row. These image lines and a projection center 11 of the line projector lie in a common plane, which in the present case is parallel to the y-z plane. The line projector 6 projects one-dimensional intensity distributions 12, which respectively illuminate only one line on the object 1. In this case, the light generated by the line projector 6 in each case in the form of a light fan shown in dashed lines in Figures 2 and 4 from the line projector 6, this light fan intersects a plane parallel to the x-y plane parallel to the y-axis. The surface of the object 1 can be scanned with the line scanner, for example, by this light fan is pivoted about an axis lying parallel to the y-axis in the x direction, as shown in Fig. 2 by a double arrow. One of the various successively projected intensity distributions 12 is shown in FIG. 4 along a line projector illuminated

Strip indicated by a curve, wherein a location-dependent intensity is illustrated by a survey of this curve over the illuminated strip in the z direction. To determine spatial coordinates of the surface of the object 1, first along a line, the procedure is as follows.

The correspondingly programmed control and evaluation unit 8 controls the line projector 6 in such a way that it projects a series of different intensity distributions 12 onto the surface of the object 1 within a plane which is constant for this series. So is each along a cutting line of the last-mentioned plane and the surface of the object 1, a line of inhomogeneous intensity is projected onto the surface.

The line scan cameras 7 are simultaneously controlled by the control and evaluation unit 8, which is also correspondingly set up, so that each of the line scan cameras 7 receives a one-dimensional image of the strip of the surface illuminated by the line projector 6 for each of the aforementioned intensity distributions 12 For each pixel of the BÜdzeile 10 of the respective line scan camera 7 is determined in each case a series of measured values.

By correlating the series of one-dimensional images recorded with the two line scan cameras 7 or the series of measured values determined by the two line scan cameras 7, the control and evaluation unit 8 determines corresponding pixels of the two image lines 10 of the two line scan cameras 7 and evaluation unit 8 calculates the spatial coordinates for a plurality of points of the intersection line of the projected intensity distributions 12 with the surface of the object 1 by triangulation on the basis of the thus determined mutually corresponding pixels of the two image lines 10 of the two line scan cameras 7.

In order to determine the spatial coordinates of the surface of the object not only along a single cutting line but flatly, the surface is scanned with the line scanner, for example by carrying out said steps for a sequence of different projection directions. For this purpose, said plane, within which the intensity distributions 12 are projected, is tilted about the already mentioned axis lying in this plane and oriented perpendicular to the main projection direction of the line projector 6. The intensity distributions 12 are projected, for example, in the form of {1 + cos) -shaped intensity gradients or in the form of statistical patterns or in the form of speckle patterns or in the form of discrete single-point patterns. For this purpose, the line projector 6 can generate the intensity distributions 12 by means of a laser and a diffusing screen or by means of a digital projection system or by means of a slide displaceable for scanning the surface and a fixed projection optics. In the case of the device described here, two line scan cameras 7 are used instead of an area camera for recording the object shape along a line.

In particular, the differences described below result from the conventional structure from FIG. 1. The line scan cameras 7 are arranged as follows:

The optical axis lies in each case in a common plane, for example in the y-z plane.

- The camera viewing direction is in each case rotated against the z-axis by a few degrees up to possibly about 80 °.

The triangulation base between the two line scan cameras lies in the y-z plane.

The projector instead of the approximately constant intensity along its line, as provided in the classical line projector, a sequence of different, for example statistical intensity distributions 12. Also possible are other forms of intensity distribution 12 such as (l + cos) -shaped distributions, dot patterns or Intensity distributions 12 formed by the speckle pattern. The intensity variation takes place transversely to the propagation direction of the laser line or the projected line. The two line scan cameras 7 each record the series of intensity distributions 12. From the correlation of the images of the series in both line scan cameras 7, one pixel each of a line scan camera 7 can then be assigned to an interpolated pixel of the second line scan camera 7. The thus associated pixels, which have elsewhere been designated as corresponding to each other, may be regarded as homologous points. Then, a 3D coordinate calculation for each pixel of the cameras can be done directly. The position of the projector is not necessarily required for the coordinate calculation, since the Triangulationsbasis between the two line scan cameras 7 is spanned. This results in the following advantage over the previous solution:

The take-up speed is given instead of the maximum frame rate of a surface camera by the maximum frame rate of two comparatively fast line scan cameras 7. In a typical number of 6 to 20 shots for a series results in a speed advantage of about a factor of 10 to 100.

The geometric extent of the sensor or line scanner will essentially only be noticeable in two directions since the cameras no longer lie outside the plane of the projected light plane.

The possible application examples of this invention are in the area of three-dimensional surface measurement or coordinate determination of macroscopic objects by using active pattern projection, for example by projection of gray scale distributions on the basis of statistical patterns.

In comparison to previously known systems for active object illumination in the three-dimensional measurement or determination of spatial coordinates, the devices and methods described here have in particular the following advantages:

1. Rapid data acquisition: The use of line scan cameras 7 instead of area cameras allows much higher frequencies during image acquisition with comparable technical and financial expense.

2. The entire intensity information taken by the camera is used for the coordinate determination instead of the narrow illuminated line in the area camera.

3. Compact design: by the described arrangement, the sensor according to the proposed new approach has only a significant expansion in two dimensions. The sensor is in fact located with all essential components in the yz plane. In contrast, classic li a considerable extent in three dimensions because the camera is located outside the projection plane of the projector.

Claims

claims

1. Apparatus for determining spatial coordinates of surfaces of macroscopic objects (1), comprising a line projector (6), two line scan cameras (7) and a control and evaluation unit (8), wherein the control and evaluation unit (8) is set up, the following To perform steps:

- driving the line projector (6) so that it projects a series of different intensity distributions (12) within a plane that is the same for this series to illuminate only one line on the object,

- Controlling the line scan cameras (7), so that each of the line scan cameras (7) for each of these intensity distributions (12) each receives a one-dimensional image, so that for each pixel of a picture line (10) of the respective line scan camera (7) each have a series of esswerten is determined

Determining mutually corresponding pixels of the two image lines (10) of the two line scan cameras (7) by correlating the series of one-dimensional images taken with the two line scan cameras (7) or the series of measured values determined by the two line scan cameras (7)

- Calculating spatial coordinates for a plurality of points of a line of intersection of the projected intensity distributions (12) with an object surface by triangulation based on the thus determined corresponding pixels of the two image lines (10) of the two line scan cameras (7).

2. Apparatus according to claim 1, characterized in that the image lines (10) of the line scan cameras (7) and the line projector (6) lie in a common plane.

3. Device according to one of claims 1 or 2, characterized in that it is designed as a line scanner, which is adapted to perform the said steps for a sequence of different projection directions, so that said plane about a plane lying in this plane and perpendicular to a main projection direction the line projector (6) oriented axis is tilted.

4. Device according to one of claims 1 to 3, characterized in that the control and evaluation unit (8) is arranged, the line projector (6) for the projection of the intensity distributions (12) in the form of (l + cos) -shaped intensity distributions ( 12) or statistical patterns or bacon patterns.

5. Device according to one of claims 1 to 4, characterized in that the line projector (6) has a laser and a diffusing screen for generating the I tensitätsverteilungen (12) or a digital projection system or for generating the Intensttätsverteilungen (12) by means of discrete single-point pattern or by means of a displaceable slide and a fixed projection optics.

6. A method for determining spatial coordinates of a surface of a macroscopic object (1), comprising the following steps:

Projecting a series of different intensity distributions (12) within a plane which is constant for this series by means of a line projector (6) directed onto the surface of the object (1) so that in each case along a section line of said plane and the surface of the object (1) only a line of inhomogeneous intensity is projected onto the surface,

Each taking a one-dimensional image of a strip of the surface illuminated by the line projector (6) for each of these Intensttätsverteilungen (12) by each of two line scan cameras (7), so that for each pixel of a picture line (10) of the respective line scan camera (7) is determined in each case a series of measured values,

- Calculating spatial coordinates for a plurality of points of the intersection of the projected intensity distributions (12) with the surface of the object (1) by triangulation based on the thus determined corresponding pixels of the two image lines (10) of the two line scan cameras (7).

7. The method according to claim 6, characterized in that the image lines (10) of the line scan cameras (7) and the line projector (6) are arranged lying in a common plane.

A method according to any one of claims 6 or 7, characterized in that the surface of the object (1) is scanned by carrying out said steps for a series of different projection directions, such that said plane is perpendicular to a plane lying in said plane tilted to a main projection direction of the thin projector (6) oriented axis.

9. The method according to any one of claims 6 to 8, characterized in that the intensity distributions (12) in the form of (l + cos) -shaped intensity distributions (12) or of statistical patterns or of speckle patterns are projected.

10. The method according to any one of claims 6 to 9, characterized in that the line projector (6) the intensity distributions (12) by means of a laser and a lens or by means of a digital project tion system or by means of discrete dot dot patterns or by means of a displaceable slide and a fixed projection optics generated.