EP1101195A1

EP1101195A1 - Measurement system

Info

Publication number: EP1101195A1
Application number: EP98901891A
Authority: EP
Inventors: Alex Van Den Bossche
Original assignee: Nv Krypton Electronic Engineering; Krypton Electronic Engineering NV
Current assignee: 3D Scanners Ltd
Priority date: 1997-02-17
Filing date: 1998-02-09
Publication date: 2001-05-23
Also published as: WO1998036381A1; AU5849198A; BE1010929A3

Abstract

The invention concerns a method and a device for determining the position of a point, involving the use of at least two separate cameras (1, 2) with a number of pixels to detect that point and identifiying the pixels where the image (4, 4') of the said point is detected and then calculating the position of that point on the basis of the position of the pixels so identified, assigning an address to each of the pixels of at least one camera (1, 2) and, from the pixels of that camera (1, 2), selecting a group of pixels that contains at least some of the pixels where the said image (4, 4') is detected, and which, when the said point is moved, is modified so that it continues to contain the pixels where the said image (4, 4') is detected.

Description

MEASUREMENT SYSTEM

The invention concerns a method for determining the position of a point, whereby this point is perceived by at least two separate cameras with a number of pixels, where the image of the said point is detected and then the position of that point is calculated on the basis of the position of the pixels so identified.

According to the state of the art, if the position of a point is to be established, each pixel of a camera must be examined in turn to see whether the image of this point falls onto it. Since the cameras used must be of a high resolution, for example 2000x2000 pixels, the search for the image of the said point requires highly complex and expensive equipment, and is also extremely time-consuming.

The invention is designed to overcome these disadvantages by putting forward a method that enables the position of the point to be determined rapidly and economically and with a high degree of accuracy.

To that end, an address is assigned to each of the pixels of at least one camera from which a group of pixels is selected containing at least some of the pixels where the said image is detected, and in such a way that when the point is moved, the group of pixels is modified so that it continues to contain the pixels where the said image is detected.

In practice, the centre of the image of the said point is calculated by constituting the said group of a fixed number of pixels and then modifying it so that its centre lies at the centre of the image of the said point as calculated. In a advantageous way, an imaginary window is selected containing the said group and the path of the image of the said point is followed with this window.

The invention also includes a device for establishing the position of a point, using at least two separate cameras, and means for determining the position of the said point on the basis of the image formed by each of the cameras.

A feature of this device is that at least one of the said cameras is capable of enabling each of the pixels to be identified separately.

In a particular embodiment of the device developed by the invention, means are provided to select a group of pixels from among the pixels of a camera, in such a way as to ensure that the group contains the pixels where the image of the said point is detected, and whereby means are provided to modify the said group when the said point is moved so that it continues to contain the pixels where the said image is detected.

In a preferred embodiment of the device pertaining to the invention, at least one of the said cameras is a so-called "addressable" camera.

Other features and advantages of the invention will become apparent from the description below of a particular embodiment of the method and the device covered by the invention; this description is offered only as an example and in no way limits the scope of the protection provided; the reference numbers used below refer to the illustrations annexed hereto.

Figure 1 is a schematic representation of a device developed by the invention.

Figure 2 is a schematic representation of the screen of a camera.

In the different figures, the same reference numbers are used for the same or similar parts.

The method and the device covered by the invention constitute a measuring system, and in particular a so-called "optical system for measuring coordinates", in which the co-ordinates of a point are measured with respect to a reference. Ideally, the point to be measured should consist of a point of light or an element that emits an optical signal such, for example, as a lamp. In this way it is possible, for instance, to determine the distance between two points or to record the shape of an object. This data can be stored in the memory of a computer for subsequent processing.

Figure 1 shows two cameras (cameras 1 and 2) that are aimed at a point identified by a point of light 3. These cameras are positioned at an angle to each other and their position relative to each other is known. The resolution of these cameras is determined, among other things by the number of pixels on which an image of an object can be detected. It is thus possible, when the position of the image of light-point 3 on cameras 1 and 2 is known precisely, to calculate using conventional trigonometry the correct position or the correct co-ordinates of the said light-point 3 in relation to these cameras or in relation to a reference point fixed in relation to cameras 1 and 2.

In the method put forward by the invention, an address is assigned to each pixel of cameras 1 and 2 with the result that each pixel can be separately read out with the aid of a computer 8. In this way it is possible to identify each pixel separately and thus detect, in respect of each separate pixel, whether an image is formed on it. Figure 2 shows all the pixels of a camera (1 or 2) represented schematically by a rectangle 7.

A group of pixels, in particular a so-called window 4, is then put together, as shown in Figure 2. Window 4 is chosen so that it contains the pixels of camera 1 or 2 on which an image 5 of the light-point 3 is formed. Ideally the centre of window 4 is made to lie at the centre of the pixels holding image 5 of the light-point 3. Once such a window 4 has been chosen, only the camera 1 or camera 2 pixels, that are located within this window 4 are taken into consideration. Thus, the processing time of such a measuring system is significantly reduced, since only a limited number out of the total number of pixels has to be used to determine the position of the said light-point 3. In other words, the data to be processed is drastically reduced.

Any movement of the said light-point 3 in relation to cameras 1 and 2 is detected by the said measuring system, since the position of image 5 of light-point 3 is changed to a new position inside window 4. In order to follow this movement, window 4 shifts in such a way as to ensure that image 5 continues to stay within it. In other words, the group of pixels that constitute window 4 is modified in such way that the pixels on which the image 5 is formed, continue to remain a part of that group.

An imaginary window 4 is thus generated that follows the path of the movement of image 5 in line with the movement of light-point 3. This is schematically illustrated in Figure 2 where, when light-point 3 moves, image 5 moves in the direction of arrow 6 to an image 5'. In the same way, the position of window 4 shifts to window 4'.

In order to shift window 4 or modify the corresponding group of pixels, a calculation will first be made, for instance, of the centre of the pixels on which an image 5 of the said light-point 3 is formed. Window 4 is then moved or the group of pixels modified so that their centre coincides with the centre of the said image 5.

When image 5 moves, a calculation is also made of the co-ordinates of the said point, or more particularly of light-point 3. This calculation is executed extremely rapidly since only a limited number of pixels have to be taken into account, namely those with an address within the said group or within the said window 4.

If the image of light-point 3 is temporarily not detected by cameras 1 or

2 because, say, an object is placed between light-point 3 and cameras 1 and 2, the said window 4 is, for instance, enlarged until it once again covers image 5. Thus, as soon as image 5 is detected, window 4 returns to its original size, doing so in such a way that the image 5 formed remains totally within its confines.

In view of the small amount of data that has to be processed in order to calculate the position of a point, it is possible with the method developed by the invention, to carry out various measurements in respect of one and the same position of a point or of light-point 3, and to calculate the co-ordinates for each of these in turn.

As a result, it is possible to generate an average of the calculated values, so that the coordinates that are finally calculated in respect of a point contain an extremely small amount of error and a very limited spread.

Using present-day methods of measuring, the enormous amount of data that has to be processed makes it impossible to carry out several measurements of one and the same position and to average those measurements out.

The method developed by the invention thus combines a high image resolution (of, say, 2,000 x 2,000 pixels) with a high pixel read-out speed that enables extremely accurate measurements to be made. Read-out speed means the speed with which data from a specific pixel is available for processing by, say, a computer 8. This high speed is attained because each pixel is assigned its own address.

In another embodiment of the method developed by the invention, the pixels are grouped into specific zones, the same address being assigned to all the pixels within the same zone. Such a zone thus consists, for example, of a column of pixels. The method described above is adapted to this approach, processing the said zone as a single pixel.

The device forming part of the invention and enabling the position of one point - or more specifically the co-ordinates of that point - to be measured with respect to a reference, is illustrated schematically in Figure 1. This shows two cameras (1 and 2) aimed at a point of light 3, and the means 8 for determining the position of that light-point 3, on the basis of the image captured by each of the cameras 1 and 2. The said unit should ideally include a computer running specially adapted software.

At least one of cameras 1 and 2 contains means enabling each pixel of camera 1 or 2 to be identified separately. It is thus possible, with the aid of computer 8, to receive a signal from a predefined pixel indicating whether an image of light-point 3 is formed on that pixel. The device forming part of the invention also enables a zone of pixels to be identified, such, for example, as a column of pixels.

Thus, in a preferred embodiment of the device pertaining to the invention, at least one of the two cameras is an addressable camera. However, it is preferable for each of the two cameras to be an addressable camera.

Provision is also made for the possibility of selecting from the pixels of a camera a group of pixels that contains the pixels where the image of the said point is detected. In the embodiment of the device, as illustrated in Figure 1 , the means to do that consists of a computer 8 with computer equipment. Computer 8 also ensures that, when the said point is ed. the said group - or, in other words, window 4 - is modified in such a way that it continues to contain the pixels where image 5 is detected.

To that end, use is made, for example, of computer 8 to calculate the centre of the image of the said point and to modify the said group of pixels in such a way that their centre lies at the centre of the image of the said point as calculated.

The device developed by the invention is also equipped with a point of light 3. This light-point 3 takes the form for example of a lamp mounted on a holder with a pointed end that is held against an object at a spot whose position has yet to be determined. Said cameras 1 and 2 and computer 8 are used to determine the positions of one or, preferably, two or more lamps that are fixed to the holder and are thus at a known distance from that pointed end. Moreover, on the basis of the detected position of the said lamps, it is possible to calculate the correct co-ordinates of the spot at which the pointed end of the said holder is located.

In a particular embodiment of the device developed by the invention the said light-point 3 takes the form of a light-emitting diode (i.e. a so-called LED).

The invention is of course not limited to the method described above or to the device illustrated in the annexed figures for determining the position of a point.

Thus, several points of light can be used to determine the position of one and the same point. Furthermore, provision can also be made for more than two cameras, thereby increasing the precision with which a position can be measured.

Claims

1. Method for determining the position of a point, involving the use of at least two separate cameras (1,2) with a number of pixels to detect that point and identifying the pixels where the image (4,4') of the said point is detected and then calculating the position of that point on the basis of the position of the pixels so identified, characterised in that an address is assigned to each of the pixels of at least one camera (1,2) and that, from the pixels of that camera (1,2), a group of pixels is selected that contains at least some of the pixels where the said image (4,4') is detected, and which, when the said point is moved, is modified so that it continues to contain the pixels where the said image (4,4') is detected.

2. Method described in claim 1, characterised in that a calculation is made of the centre of the image (4,4') of the said point and that the said group is composed of a fixed number of pixels and is modified in such a way that its centre lies at the centre of the image (4,4') of the said point as calculated.

3. Method described in claim 1 or 2, characterised in that the position of the said point is determined by means of data derived from pixels of the said group.

4. Method described in one of claims 1 to 3, characterised by the fact that an imaginary window (5,5') is selected containing the said group, and that this window (5,5') is used to follow the path (6) of the image (4,4') of the said point.

5. Method described in one of claims 1 to 4, characterised in that the pixels for the said group are selected in such a way as to obtain an entity of adjacent pixels.

6. Method described in one of the aforementioned claims, characterised in that the point to be measured is identified by means of a light source

(3) positioned at the location of that point.

7. Device for determining the position of a point, with at least two separate cameras (1,2), and means by which to determine the position of the said point on the basis of the image (4,4') formed by each of the cameras characterised in that at least one of the said cameras (1,2) is equipped with means to enable each of the pixels to be identified separately.

8. Device according to claim 7, characterised in that at least one of the cameras (1,2) is a so-called addressable camera.

9. Device according to claim 7 or 8, characterised in that means are provided to enable a group of pixels to be selected from the pixels of a camera (1,2), in such a way that that group contains the pixels where the image (4,4') of the said point is detected, whereby means are provided to modify said group, when the said point is moved, so that it continues to contain the pixels at the location where the said image (4,4') is detected.

10. Device according to one of claims 7 to 9, characterised in that it is equipped with means to enable the centre of the image (4,4') of the said point to be calculated and the said group to be modified so that its centre lies at the centre of the image (4,4') of the said point thus calculated.

11. Device according to one of claims 7 to 10, characterised in that the said point is formed by a point of light (3).