Method and device for automatic registration of 3D - laser scans in real-time

Field of the invention

The suggested method and devices for automatic registration of 3D-laser scans in real-time, using photo-targets finds application in the field of 3D laser scanning used for creation of realistic 3D computer models of different kind of objects.

Background of the invention

There is a well known method for registration of laser scans using targets (reference points), that allows to obtain the Cartesian coordinates of the scanned objects.

Scanning is possible by, rotating the laser beam around the center of the scanner, covering the full surrounding space in vicinity of the scanner. During the scanning process are measured the spherical coordinates of the points reached by the laser beam. In order to calculate the Cartesian coordinates of the points in absolute coordinate system, first of all it is necessary to determine the position in Cartesian coordinates of the scans according to the absolute coordinate system. This process is well known as

registration of 3D laser scans. In order to be possible registration, preliminary in the whole scanning area have to be chosen appropriate reference points with well known Cartesian coordinates. Usually these coordinates of the reference points are determined by geodesic methods.

Then, in these points are mounted the targets which are scanned together with all other object as a part of the scan. Then with appropriate operation are calculated the Cartesian coordinates of the center of the scan and subsequently the Cartesian coordinates of the objects. By using special software is possible exporting the coordinates of the scanned objects as point clouds in CAD systems. Furthermore this can be used for creating of 3D realistic models of the objects.

The main disadvantages of the old method are, so a lot of handwork is needed to be done after the scanning which takes time and human resources, and there is no way to find out if the scans are correct during the scanning process, a waste of time for recharging again the scans in the memory after the scanning process, a possibility of mistakes caused by bad visibility, handwork and artifacts. The targets not interact with the scanner, so that the process of registration is not automatic.

There is not method and device for automatic registration of 3D laser scans in real time, using photo targets.

Summary of the invention

The invention represents method for automatic registration of 3D laser scans in real time by using photo-targets, so that the laser scanner interacts and exchange data with a new kind of targets- photo-spheres /fig1/, replacing currently used targets. The photo-sphere has the ability to respond to the laser beam when it is over the photosphere surface and to send signals to the scanner by radio channel. To the photosphere there is build-in bar code reader, keyboard or another device for entering of information. The entered information represents the name of the photosphere i.e. the name (it means the coordinates) of the reference point where the photosphere is placed.

(The method includes printing of the names of the reference points as a barcode and as a sequence of digits and letters on paper sheets, which are embedded nearby to the reference points, so that the barcode can be scanned or the name of the photo-sphere can be entered by using keyboard or another device for entering information.) The names of the photo-spheres have to be entered before the scanning process.

To be able sending of the signals to the scanner, to the scanner have to be mounted a device for receiving and recording in a convenient way these signals.

When the scanning laser beam reaches whichever point of the external sphere of the photo-sphere, the electronic is triggering the communicating radio channel. That is to say the radio channel has two positions: switched on or switched off - transmitting or not

transmitting. Always when the laser beam spot is over the external sphere of the photo-sphere (It doesn't matter in which one point) the radio channel is switched on and transmitting and contrariwise, when the laser beam spot is not over the external sphere of the photosphere the radio channel is switched off. This is equivalent to automatic finding and selecting of the external sphere of the photosphere. In order to avoid some inaccuracy when applying this method, it's possible to use and another methods of selection simultaneously. Even it's possible, if only one point of the sphere is found the furthermore selection of the sphere to be done by using shape recognition or to be used geometrical calculations if appropriate multitude of points belonging to the sphere is known found by operating the photo-sphere.

The inertia of triggering of the photo-sphere should small, so that the linear speed of movement of the laser beam spot multiplied by this inertia of triggering to determine small shift in order to have proper selection. If it's necessary, the software can make some corrections later.

For the scanning process should be used a composition of a several photo-spheres, so that at least 4-5 of them to be visible from a chosen scanning position. During the installation of the photosphere in some of the reference points should be entered the name of the reference point where the photo-sphere is placed at this moment, by using bar code reader or keyboard or another device for entering information. The name of the reference point becomes the name of the photo-sphere. In this way during irradiation of the photosphere from the laser beam, the radio channel transmits to the laser

scanner a code corresponding to the name of the photo-sphere, i.e. the coordinates of the reference point where the photo-sphere is placed at this moment. In this way, finding, selecting and the names of the photo-spheres are known during the scanning process and furthermore the registration of the scans can be done automatically in real time.

The method of automatic registration of 3D laser scans in real time is performed by using device-photo-sphere, which includes external thin wall, diffuse, semitransparent sphere connected by fitting to the internal sphere with photo-sensitive elements placed on its surface, so that in the internal volume of the internal sphere is placed the electronics. On the photo-sphere there is an aperture with a thread for embedding to the reference points and also built-in bar code reader or output for connecting a keyboard or another device for entering information nearby the aperture of the thread. There are also other versions to make the device. It's possible to use photo sensitive layer or photo-matrix on the surface of the internal sphere instead to use photo-sensitive elements. Another solution is place photosensitive layer or matrix directly on the internal surface of the external sphere, so the internal sphere is redundant. In another version, instead of using built-in bar code reader or output for keyboard or another device for entering information, to be used wireless communication between the photo-sphere and external barcode reader, keyboard or another device for entering information.

The advantages of the invention are that: during the registration are ovoid some hand work and operations, drop out the operation of

finding, selecting and naming of the targets by hand work, which is taking time and human resources, it's possible to avoid some errors and artifacts. Registration can be done during the scanning process, so eventual problems can be solved right on time and on place, there is no need to reload the scans in the memory again. The photo- targets are named with the names of the reference point preliminary, so there is no need the names of the reference points to be visible from the scan, i.e. drop out the possibility for allowing some errors and to spend time for additional references caused by poor visibility.

Furthermore, there is an opportunity to use a method for determining the quality of the scans in real time, because the registration is done in real time. In this way can be avoid the possibility to create wrong 3D models, additional scanning or corrections subsequently.

Brief description of the drawings

Figure 1 , represents the structure of the photo-sphere.

Figure 2, represents 3D model of the photo-sphere.

Figure 3, represents the interaction between the scanner and the photo-sphere.

Detailed description of the preferred embodiment of the invention

The invention is illustrated as an example as following: The method automatic registration of 3D laser scans in real time by using photo-targets includes: Printing of the name of the reference point as a barcode and as sequence of digits and letters on paper sheets /17/, embedding of these sheets close to the corresponding reference points (On the paper sheets are visible bar codes and the name of the reference points as a sequence of digits and letters), embedding of several photo-spheres /13/ in the chosen reference points, the scanner /15/ is placed on a desired position, so that at least 4-5 of the photo-spheres /13/ to be in the scanning range. Before the scanning, during the installation of the photo-sphere /13/ in some of the reference points, the name of the reference point has to be entered using bar code reader /10/, keyboard or another kind of device for entering information, so the name of the reference point where the sphere is placed becomes the name of the photo-sphere /12/. (Before installation of the photo-sphere in some of the reference points, the bar code on the paper sheet has to be read using the built- in bar code reader. Or the name of the reference point has to be entered by keyboard or another kind of device entering information). In this way, when the photo-sphere /13/ is irradiated by the laser beam, the radio channel /14/ transmitting to the scanner /15/ code corresponding to the name of the photo-sphere /12/, i.e. the

coordinates of the reference point where at the moment is mounted the photo-sphere /13/.

In this way finding, selecting and using the names of the photospheres /12/ is performed during the scanning process, so automatic registration is possible in a real time. Then the composition of photospheres has to be moved in another reference points, a new scanning position have to be chosen and so on until the whole area for scanning is covered.

The device (photo-sphere) according to the invention includes: External thin wall diffuse semitransparent sphere /5/, connected by fitting/6/ to the internal sphere /9/, where on its surface are mounted the photo-sensitive elements /8/ in the way that, in the internal volume of the internal sphere /9/ is placed the electronics. On the photo-sphere /13/, there is aperture with a thread /7/ for embedding to the reference point and built-in bar code reader /10/, output for keyboard or another device for entering information placed nearby the aperture with a thread /7/. The radius of the internal sphere /9/ should be chosen with optimal dimension, taking in to account the behavior of the diffuse passed light/3/ through the external sphere /5/. It's recommendable that the color of the internal sphere /9/ to be black in order to not reflect any more already penetrated between two spheres light.

The external sphere /5/ and the internal sphere /9/ are fixed to each other so that their centers to concur.

There are other versions, how to make the suggested device: One possible version is to apply photo sensitive layer or matrix on the surface of the internal sphere /9/ instead to use photo sensitive

elements /8/. Another possible solution is to not use internal sphere, but to apply photo sensitive layer or matrix directly on the internal surface of the external sphere /5/. Also instead of built-in bar code reader, output for keyboard or device entering information is possible to use wireless connection between the photo-sphere and external bar code reader, keyboard or another kind of device entering information.

The device (photo-sphere) according to the invention acts as follow:

When the scanning laser beam /1/ hits whichever point /4 belonging to the external sphere /5/, the diffuse passed laser light /3/ hits the photo sensitive element /8/ placed on the internal sphere /9/, then the electronics /11/ trigger the transmitting radio channel /14/ which have two positions: switched on or switched off. When the scanning laser beam /1/ is over whichever point/4 belonging to the external sphere/5/, the transmitting radio channel /14/ is switched on and contrariwise, when the beam /1/ of the scanner /15/ is not over the external sphere /5/ in this moment the radio channel /14/ is switched off.

The electronics /11/ which accepting and amplifying the signal from the photo sensitive elements/8/ eliminates the ambient light background, performs connection with the devices for entering the name of the photo-sphere /12/ and transmits via radio channel /14/ the encoded name to the receiver /16/ in the scanner /15/. The internal sphere /9/ with photo sensitive elements/8/ reacts to the diffuse passed light /3/ through the external sphere /5/ to the inside. The diffuse reflected light /2/ from the external sphere /5/ to the

outside is accepted from the scanner /15/ and is used for determining the center of the external sphere /5/ (which is equivalent to the center of the photo-sphere).

Transmitting of signal by radio channel /14/ to the scanner /15/ requires belonging to the scanner /15/ to be mounted receiver /16/ which is able to accept and record these data.

This is equivalent to hardware finding and selection of the photospheres. The inertness of switching on and switching off should be small as a time, so that if multiply the angle speed of beam rotation by the distance (between the center of the scanner and measured point) and the time inertness, it's necessary to obtain small linear shift, in order to have proper selection. But if it's necessary, subsequently some corrections from the software can be done. If before reaching the photo-sphere the laser beam is already reflected from a mirror surface, the photo-sphere can be triggered as well. To avoid that, should be used software filter which is measuring the coefficient of the diffuse reflected light from a measured point (usually there data are available in the scanner). In the case of mirror surface the coefficient of diffuse reflection of the mirror is small, so it can be eliminated easy from the software filter. In a case when, before reaching the photo-sphere the laser beam is already reflected from a diffuse reflecting surface, and then reaches the photo-sphere, undesirable triggering can be avoid by automatic adjustment of triggering level.