GB2240623A

GB2240623A - An imaging process for detecting the shape of three dimensional objects

Info

Publication number: GB2240623A
Application number: GB9100845A
Authority: GB
Inventors: Neil Starsmore
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-01-31
Filing date: 1991-01-15
Publication date: 1991-08-07
Also published as: GB9100845D0; GB9002193D0

Abstract

In a method and apparatus for detecting the shape of a three dimensional object 5, an assembly 8 comprises at least one electronic camera 4 mounted on a framework 6 which is rotatable by a motor 7 about a fixed axis 9 which is approximately coaxial with the object 5 and line sources of light 1 also mounted on the framework 6 and to illuminate the object 5. As the assembly is rotated with respect to the object 5, the camera 4 detects a reflection of the line source of light 1 from the object 5 and supplies a signal to computing means which processes the signal into stored information. The stored information can be used to program a machine tool to make a three dimensional replica of the object 5. <IMAGE>

Description

AN IMAGING PROCESS FOR DETECTING THE SHAPE OF THREE DIMENSIONAL OBJECTS This invention relates to an imaging process for detecting the shape of three dimensional objects.

Traditionally the creation of a facsimile of a three dimensional object, such as the sculpture of a person's head, has been a time consuming and skilled art, so much so that only relatively few people have been able to obtain busts of themselves.

Photogrammetric technology, namely the process of extracting geometrical and other data from two dimensional images of an object has evolved over many decades, and the basic mathematics is well defined. In particular, techniques for extraction of three dimensional geometry using stereo images have become widely used.

Given an object though, it is still a substantial task, using known techniques, to extract sufficient geometrical data to obtain a reasonable three dimensional facsimile of that object.

According to one aspect of the invention there is provided apparatus for detecting the shape of a three dimensional object comprising: an assembly of a line source of light and an electronic camera to detect a reflection of the line source of light from the object; means for moving the assembly relative to the object while the electronic camera detects light from the line source reflected from successive strips on the surface of the object; and computing means for receiving a signal from the electronic camera and processing the signal into stored information.

Advantageously the apparatus includes reprocessing means for converting the stored information into a signal to control an automatic tool, so as to work a material and produce a three dimensional facsimile of the object.

Preferably the assembly is rigidly mounted onto a frame and the relative motion between the assembly and the object is brought about by a motor drivably connected to the frame so as to rotate it about the object.

Also preferably the line source of light is produced by a laser and the assembly may include a plurality of lasers which can emit electromagnetic waves of a different frequency so that reflections from more than one line source of light can be received simultaneously by the computing means, and the processor can distinguish between the different reflections.

The line source of light produced by the or each laser is preferably a straight line source of light.

The object is preferably placed approximately co-axial with the rotation axis of the laser and electronic camera assembly, and the line sources of light extend approximately parallel to the axis of rotation.

Parallel light beams from the straight line sources advantageously either converge or diverge so that each beam of light will be incident on a strip of the surface of the object at a different angle.

The apparatus preferably includes means for modulation of the laser output by computer control in order to provide variation of the intensity, or brightness, of the observed image. This can provide active feedback to improve resolution and maximize coverage of the object being digitised. Modulation can be achieved by using laser diodes and pulsing them at high frequency with respect to the frame rate of the cameras.

Each line of light can be associated with two electronic cameras, with the electronic cameras positioned adjacent to the laser and on each side of the beam of light formed by the line source of light emitting from the laser, so that should a 'highspot' on the object obscure the reflected laser light from the view of one of the electronic cameras, it may still be detected by the other of the electronic cameras.

The straight line of light produced by the laser can be augmented by a second, coplanar line of light from a second laser so as to create a longer line source of light.

Preferably the beam of light produced by the laser passes approximately through the axis of rotation of the assembly.

The information from the object can be obtained by the computer during one complete revolution of the assembly.

The relative movement between the object and the assembly may be other than a rotational movement, for example the assembly could overlie a conveyor belt on which objects were conveyed, the stored information obtained from the assembly being used selectively to treat the objects in accordance with their determined shape. For detecting the shape of a particularly large object, such as the hull of a boat, the laser and camera assembly could be moved linearly with respect to the object, along one or more axes.

The electronic cameras are advantageously of the charged coupled device or Video Cassette Recorder type.

The computer used to process the data acquired by the electronic cameras is preferably of the digital electronic type.

According to a second aspect of the invention there is provided a method for detecting the shape of a three dimensional object comprising the steps of: shining a line source of light onto the object; using an electronic camera to detect the reflection of the line source of light from the surface of the object; moving the object relative to the line light source of light so that light from the line source of light is reflected from successive strips on the surface of the object; and processing the detected signal in a computer and then storing the information.

Thus with a configuration of line light source and electronic camera which are integrated into a fixed geometry relative to one another, the line light source can provide controlled illumination of the object by creating a pattern of narrow parallel lines and the electronic camera is able to observe reflections of these lines from the object. To measure the geometry of any object, the light pattern illuminates the surfaces of the object for which the geometry is required while the object is moved relative to the camera or vice versa. As a traverse takes place the electronic camera acquires a series of images which can be digitised and processed by the computer to obtain geometrical information. This information relates to the geometry observed at the instant that a single camera image was taken. Images are acquired several times a second.By combining the data from several images together with data on the movement of the camera and line source assembly, the three dimensional geometry can be calculated. The fidelity of the system is governed primarily by the field of view of the camera or a plurality of cameras, the orientation of the cameras with respect to the light source, the resolution of the digitised image, the image acquisition rate, and the speed of traversing.

The invention is diagrammatically illustrated by way of example in the accompanying drawings in which: Figure 1 illustrates a basic principle by which a system can operate to detect the shape of a three dimensional object; Figure 2 shows a second embodiment of the apparatus which can be used to generate data in order to detect the shape of an object; and Figure 3 shows a further embodiment of apparatus to detect the shape of an object.

Referring to Figure 1, a laser 1 creates a straight line light source which can be reflected from the surface of an object 2. The line source can be considered positioned in an x-Xrz rectangular cartesian co-ordinate system local to the laser with the straight line light source orientated substantially along the z axis and in the yrz plane with x zero. The straight line light source illuminates a single strip 3 of the object, this strip therefore also lies approximately in the z plane at x zero.

A camera 4 is placed at a known orientation within the same co-ordinate system so as to observe the reflections of the straight line light source from the surface of the object 2. The camera 4 is offset from the z plane, and any variation in the shape of the reflected line observed by the camera 4 from other than a straight line is therefore a function only of the z displacement of the surface of the object surface at any given value of . The arrangement is first calibrated by using an object surface of known dimensions (not shown) and correlating these dimensions with the reflections observed by the camera 4.Thereafter, providing the orientation of the camera 4 is kept fixed with respect to the straight line light source, then the position of the surface of the object producing the reflections of the single strip 3 observed by the camera Li can be determined with respect to the previously described co-ordinate system.

If a global rectangular cartesian co-ordinate system is defined that is fixed in a space X-Y-Z as shown in Figure 1, providing the relationship between the x--z co-ordinate system and the X-Y-Z co-ordinate system is known, then the co-ordinates of the reflected single strip 3 with respect to this global cartesian coordinate system can also be obtained. The process therefore defines a geometry of a thin strip of the object 2. If the object 2 is traversed, with respect to the camera 4 and the laser 1 producing the straight line light source in a controlled way the relationship between the local and global co-ordinate systems is known at all times, the surface of the object 2 is progressively illuminated by the straight line light source and the geometric shape of the surface of the object 2 can be defined.

In Figure 2 a practical implementation of the technique is illustrated. Here, two lasers 1 and 1' are used to create an even straight line light source over an extended distance. Also, two pairs of cameras 4 and Ii' are positioned on opposite sides of the plane of the light beam. This increases the accuracy in defining the geometry of the surface of an object 5 as both cameras may see the reflections 3 from the object 5 and so two estimates of position are obtained. There is however compensation for significant variations in surface geometry, since reflections may not be visible to one of the cameras if there is an obstruction ie. a high spot, on the object 5.

In the embodiment of Figure 2, the light sources 1 and 1' and cameras 4 and 4' are mounted on a rigid frame 6 that is in turn attached to a motor drive mechanism 7 to form an assembly 8. With power applied to the motor 7, the cameras 4 and 4' and the light sources 1 and 1' will be rotated about a fixed axis 9. If an object is placed approximately along the fixed axis, a single rotation of the assembly 8 can define a substantial part of the geometry of the object 5 ie. that area illuminated by the straight line light source and visible to the cameras 4 and 4'. The rate of rotation of the assembly 8 should be matched to the frequency of acquisition of images so that the positions of a surface strip as each image is acquired matches the required accuracy in surface definition.Using standard charged coupled device (CCD) or video cassette recorder (VCR) style cameras, images can be acquired at a rate of 25 per second.

In the apparatus as described above, the line source will only illuminate the surface of the object 5 which is not obstructed by high spots. This problem can be partially overcome by using a series of line sources. As illustrated in Figure 3 several straight line light sources 11, 12 and 13 emanate approximately from the same point 14 but along different lines in the x, z plane so that a greater part of the surface of the object 15 is illuminated than is possible with a single straight line light source. These additional line sources 11, 12 and 13 can be at different frequencies (ie.

colours) so that when observed by cameras, they can be correlated correctly for restitution of the position information. By using additional lasers and cameras, the surface of the object can be resolved in even greater detail.

Claims

1. Apparatus for detecting the shape of a three dimensional object comprising: an assembly of a line source of light and an electronic camera to detect a reflection of the line source of light from the object; means for moving the assembly relative to the object while the electronic camera detects light from the line source reflected from successive strips on the surface of the object; and computing means for receiving a signal from the electronic camera and processing the signal into stored information.

2. Apparatus according to claim 1, including reprocessing means to convert the stored information into a signal to control an automatic tool, so as to work a material and produce a three dimensional facsimile of the object.

3. Apparatus according to claim 1 or claim 2, in which the assembly is rigidly mounted onto a frame and the relative motion between the assembly and the object is brought about by a motor drivably connected to the frame so as to rotate it about the object.

4. Apparatus according to claim 3, in which the line source of light is produced by a laser.

5. Apparatus according to claim Li, in which the assembly includes a plurality of lasers which can emit electromagnetic waves of a different frequency so that reflections from more than one line source of light can be received simultaneously by the computing means which can distinguish between the different reflections.

6. Apparatus according to claim 5, in which the line source of light produced by the or each laser is a straight line source of light.

7. Apparatus according to claim 5, in which the object is placed approximately co-axial with the rotation axis of the laser and electronic camera assembly, and the line sources of light extend approximately parallel to the axis of rotation.

8. Apparatus according to claim 7, in which the parallel light beams from the straight line sources either converge or diverge so that each beam of light will be incident on a strip of the surface of the object at a different angle.

9. Apparatus according to claim 4, including means for modulation of the laser output by computer control to provide variation of the intensity, or brightness, of the observed image.

10. Apparatus according to claim 9, in which modulation is achieved by using laser diodes and pulsing them at high frequency with respect to the frame rate of the cameras.

11. Apparatus according to claim 5, in which each line of light is associated with two electronic cameras, with the electronic cameras positioned adjacent to the laser and on each side of the beam of light formed by the line source of light emitting from the laser, so that should a 'high-spot' on the object obscure the reflected laser light from the view of one of the electronic cameras, it may still be detected by the other of the electronic cameras.

12. Apparatus according to claim 4, in which the line of light produced by the laser can be augmented by a second, coplanar line of light from a second laser so as to create a longer line source of light.

13. Apparatus according to claim 4, in which the beam of light produced by the laser passes approximately through the axis of rotation of the assembly.

14. Apparatus according to claim 13, in which information from the object can be obtained by the computer during one complete revolution of the assembly.

15. Apparatus according to claim 1 or claim 2, in which the relative movement between the object and the assembly is other than a rotational movement.

16. Apparatus according to claim 15, in which the assembly overlies a conveyor belt on which objects are conveyed, the stored information obtained from the assembly being used selectively to treat the objects in accordance with their determined shape.

17. Apparatus according to claim 15, in which the laser and camera assembly are movable linearly with respect to the object, along one or more axes.

18. Apparatus according to any one of claims 1 to 19, in which the electronic camera is of the charged coupled device.

19. Apparatus according to any one of claims 1 to 19, in which the electronic camera is of the Video Cassette Recorder type.

20. Apparatus according to any one of claims 1 to 19, in which the computing means is of the digital electronic type.

21. A method for detecting the shape of a three dimensional object comprising the steps of: shining a line source of light onto the object; using an electronic camera to detect the reflection of the line source of light from the surface of the object; moving the object relative to the line light source of light so that light from the line source of light is reflected from successive strips on the surface of the object; and processing the detected signal in a computer and then storing the information.

22. A method according to claim 21, in which with a configuration of line light source and electronic camera which are integrated into a fixed geometry relative to one another, the line light source can provide controlled illumination of the object by creating a pattern of narrow parallel lines and the electronic camera is able to observe reflections of these lines from the object.

23. A method according to claim 22, in which to measure the geometry of any object, the light pattern illuminates the surfaces of the object for which the geometry is required while the object is moved relative to the camera or vice versa.

24. A method according to claim 23, in which as a traverse takes place the electronic camera acquires a series of images which can be digitised and processed by the computer to obtain geometrical information.

25. A method according tD claim 24, in which the geometrical information relates to the geometry observed at the instant that a single camera image was taken.

26. A method according to claim 25, in which images are acquired several times a second.

27. A method according to claim 26, in which by combining the data from several images together with data on the movement of the camera and line source assembly, the three dimensional geometry can be calculated.

28. Apparatus for detecting the shape of a three dimensional object substantially as hereinbefore described and illustrated with reference to the accompanying drawings.

29. A method for detecting the shape of a three dimensional object as claimed in claim 21 and substantially as hereinbefore described.