CA2297879A1

CA2297879A1 - Apparatus and method for determining the shape of a piece in movement

Info

Publication number: CA2297879A1
Application number: CA002297879A
Authority: CA
Inventors: Benoit Carpentier
Original assignee: Comact Optimisation Inc
Current assignee: Comact Optimisation Inc
Priority date: 2000-02-03
Filing date: 2000-02-03
Publication date: 2001-08-03

Abstract

A method and an apparatus for obtaining the shape of a piece circulating on a conveyor is provided. Profiles of the piece are taken are intervals of time and distance with respect to the speed of the traveling piece. The profiles can be taken using laser diodes. A camera captures an image of the profiles and stores the image. The shape of the piece is then built, on a screen, using the data collected during the acquisition of the profiles. The last profiles of the first acquisition set and juxtaposed to the first profiles of the next acquisition set in order to position the subsequent profiles. The shape of the whole piece is therefore built and can be transmitted. The image can be used to discard pieces with defects or to calculate a relative quality of the piece.

Description

APPARATUS AND METHOD FOR DETERMINING
THE SHAPE OF A PIECE IN MOVEMENT
Field of the Invention The invention relates to the field of determination of the shape of pieces in linear movement. The shape comprises the bow, the crook, the twist, etc. The piece moves freely and randomly on a conveyor belt or on any other mechanical apparatus Background of the Invention Determining the exact shape of a product to be sold can be extremely important for vendors. For quality assurance purposes, the product's required shape and the produced item's shape must be the same within minimal variations. In the case of the wood industry, three main defects can appear in the processed boards:
bow, crook and twist. Each of these characteristics will affect the reliability and the performance of the board once in its final use.
To examine a workpiece for surface defects, many systems project laser beams on the workpiece. The defects found are holes, knots and the like. Most system analyze the profile line created by the laser beam with respect to a normal profile line and determine if a defect is present on the workpiece. The problem with these systems 2o is that the readings and decisions are on a line-by-line principle. While surface defects are easily spotted, the workpiece is assumed to be straight and the deformation of the piece is not analyzed.
When the workpieces are dropped on the conveyor belt, the transition is not smooth. The workpieces start wobbling as they advance on the belt and, because of the great velocity of the belt, keep wobbling with respect to the belt throughout the distance to travel on the belt. This gives rise to two problems: 1. If trying to acquire the shape of the workpiece, the shape acquisition equipment will not capture clear images of the workpieces since they are in constant movement with respect to the frame of reference, i.e. the shape acquisition equipment; 2. If the workpiece were traveling slowly on the belt, it would be possible for the shape acquisition equipment to freeze for a very short period of time a position of the workpiece and to acquire data concerning its shape, however, since the velocity of the belt is great, the equipment is not able to capture the data efficiently and precisely.
Summary of the Invention Accordingly, an object of the present invention is to provide a method of ~o determining the shape of a piece by compensating for its movement on the transport apparatus.
Another object of the present invention is to provide an apparatus for obtaining the shape of a piece using readings from one surface only of a piece circulating on a transport apparatus in a given direction.
Another object of the present invention is to provide an apparatus for obtaining the shape of a piece using readings from two adjacent surfaces of the piece circulating on the transport apparatus.
According to a first aspect of the present invention, an apparatus for obtaining data representing a shape of a wobbly elongated workpiece rapidly moving on a 2o conveyor surface in a direction of travel substantially lengthwise to the workpiece is provided. The apparatus comprises: a profile line projector for projecting at least two profile line beams on the workpiece, an image acquirer for acquiring an image signal of the at least two profile line beams on the workpiece and an analyzer for correlating consecutive overlapping image signals acquired at different times, transforming the image signals to compensate for wobble movement of the workpiece and producing data representing a shape of at least a portion of the workpiece.

-2-According to another aspect of the present invention, the apparatus further comprises a conveyor position sensor for measuring a position of the workpiece on the conveyor surface and the analyzer uses the position to correlate the image signals.
According to still another aspect of the present invention, the analyzer correlates a leading portion of a present image signal with a trailing portion of a past image signal in order to reconstruct the shape of the complete piece.
According to a preferred embodiment of the present invention, an apparatus for obtaining the shape of a piece using readings from a piece circulating on a conveyor in a given direction is provided. This apparatus takes three readings of the profile of the piece for one surface at predetermined successive intervals. Each profile is stored and recorded to reconstitute the piece. By aligning, in space, each group of profiles, the apparatus is able to recreate the original shape of the piece. The profile is determined, using a laser beam, by triangulation using a camera.
In accordance with still another aspect of the present invention, there is provided a method for obtaining data representing a shape of a wobbly elongated workpiece moving on a conveyor surface. The method comprises: acquiring a series of at least two overlapping image signals of the workpiece as it is moving on the conveyor surface and compensating for movement of the workpiece with respect to the conveyor surface by correlating the series of at least two overlapping image 2o signals in order to acquire data representing a shape of at least a portion of the workpiece.
In accordance with a preferred embodiment of the present invention, there is provided a method comprising the steps of:
a) capturing a first image signal of the workpiece;
b) measuring a displacement or position of the conveyor surface;
c) advancing the workpiece of a first distance;
d) capturing a second image of the workpiece;

-3-e) correlating the first and second image signals in order to align an overlapping portion of the image signals f) repeating steps b) through e).
Brief Description of the Drawings These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings in which:
Figure 1 illustrates typical shapes of a wood board in perspective. Figure lA
is the bow. Figure 1 B is the crook. Figure 1 C is the twist;
Figure 2 illustrates the distance between two laser profiles on a piece of wood;
Figure 3 illustrates a perspective view of the first three laser profiles of the piece, each separated by the same distance;
Figure 4 is a perspective view of a new group of three laser profiles, after the piece of ~ s wood has traveled a distance D;
Figure 5 illustrates a partial view, in perspective, of the shape of the piece, as the profiles are slowly aligned. Profile P2(1) and P3(1) are aligned with P2(2) and P3(2);
Figure 6 illustrates the resulting error when superposing profiles using only one surface;
2o Figure 7 is a perspective view of another group of laser profiles P3(3), P4(3) and PS(3) after the piece of wood has traveled another distance D separating two laser profiles;
Figure 8 is a partial view, in perspective, of the shape being reconstituted, according to the alignment principle;
25 Figure 9 is a perspective view of the shape being reconstituted at each subsequent iteration according to the alignment principle;

-4-Figure 10 is a perspective view of the apparatus according to the present invention.
The visual inspection system comprises one camera acquiring an image of the wood piece circulating on a conveyor belt according to a linear displacement, the piece being illuminated by three laser beams;
Figure 11 is a block diagram of the apparatus according to the preferred embodiment of the present invention;
Figure 12 is a perspective view of the apparatus according to a second preferred method of the present invention. It illustrates a visual inspection system composed of three cameras acquiring an image of the piece of wood which is circulating on a ~ o conveyor belt according to a linear displacement, the piece of wood being illuminated by three laser beams. Each camera records a profile of the laser beam which is associated with it;
Figure 13 illustrates a perspective view of the apparatus according to a third preferred method of this invention. The visual inspection system comprises a higher number of laser profiles operating at the same time. It necessitates four cameras and four laser beams to perform the same operation; and Figure 14 illustrates a perspective view of the apparatus according to a fourth preferred method of the invention. In this case, the visual inspection system is reduced to two laser profiles. It requires two cameras associated with their respective laser 2o beams, to sweep the piece of wood circulating on the conveyor belt according to a linear displacement.
Detailed Description of the Preferred Embodiment The difficulty in determining shapes of pieces travelling on a conveyor belt, or 25 any other mechanism that carries pieces of materials, is that since the piece can freely move on the conveyor belt, it is moving around at all times. This phenomenon gets worse as the linear velocity of the piece increases on the conveyor belt. When the

-5-linear velocity of the piece increases, the readings are also harder to take for the determination of the shape of the piece. It will be understood that this piece can be a wood board, a steel piece, an aluminum rod or anything else. In the examples that will be described, a wood board will be used for clarity.
Referring now to Figure 1, a wood board is shown with different shapes. As shown in Figure 1 A, the bow is defined as a piece bent on itself. As shown in Figure 1B, the crook is defined when the longitudinal dimension of the board has a radius.
As shown in Figure 1 C, the twist is defined when the board is twisted along its axis, in one way or the other. All of these shapes are susceptible of being present ~ o simultaneously, on the piece for which the shape is required.
The acquisition process for the shape of the piece requires that each profile be equidistant one with respect to the other. Figure 2 shows this distance called D. The method shown in Figure 2 uses both adjacent surfaces 11 and 12 of piece 10 to take the readings using the laser beam. In another preferred embodiment of the present 15 invention, only surface 11 of the piece is used to obtain its shape.
The acquisition of the shape of a piece 20 requires, firstly, the acquisition of three profiles at known places on the piece, called P1(1), P2(1) and P3(1) as shown in Figure 3. The more precise the characterization of the topology of the surface scanned is, the more precise the resulting shape of the piece will be. One method for obtaining 2o the profile of the shape consists of using a laser beam and, by triangulation with a camera, of determining the profile of the laser beam and therefore, of the shape of the piece, using the analysis of the image coming from the camera which recorded the laser beam. These three profiles are recorded and stored in order to subsequently reconstitute the shape of the piece, and are called P 1, P2 and P3 as shown in Figure 5.
25 The processing is done using a computer. When the piece 30 has traveled a distance D, as shown in Figure 4, a new series of three laser profiles is started.
Profiles P2(2), P3(2) and P4(2) are recorded as shown in Figure 4.

Then, the shape of the piece 40 can be reconstituted. By superposing, in space, readings P2 and P3 coming from P2(1) and P3(1) with P2(2) and P3(2), as shown in Figure 5, we are automatically positioning P4, given by P4(2), with respect to the first three profiles already measured. It is necessary that the three profiles coming from the same reading are considered to be a group. To be able to superpose readings P2( 1 ) and P3(1) with P2(2) and P3(2) and, at the same time, keep the relationship between P2(2), P3(2) and P4(2), a transformation of the coordinates of the group P2(2), P3(2) and P4(2) is required. This transformation must take into account the three degrees of freedom of the translation and the three degrees of freedom of the rotation.
Depending ~o on the mechanical environment, it might not be necessary to use all degrees of freedom. The parameters of the transformation are determined using P2(1), P3(1), P2(2) and P3(2). After applying the transformation on the group P2(2), P3(2) and P4(2), we obtain the profiles P1, P2, P3 and P4 of the piece, each being positioned relative to one another as shown in Figure 5. These profiles are recorded and stored because they represent the beginning of the series of data constituting the shape of the piece. It is noted that the alignment of readings P2 and P3 is easier because the profile is composed of two adjacent surfaces of the piece as shown in Figure 6C. As shown in Figure 6A, using only one surface is also sufficient to generate the profile.
In that case, it is the juxtaposition of the readings obtained which is more difficult. Figure 6B
2o illustrates the resulting error when the profiles are not aligned properly as in Figure 6A.
As shown in Figure 7, when the piece 50 has traveled another distance D, a new series of three laser profiles is begun, yielding P3(3), P4(3) and PS(3).
These profiles will be used for another iteration of the steps in the method. Using a similar alignment principle, Figure 8 shows the alignment done using a transformation of the coordinates (six degrees of freedom, if necessary) between readings P3 and P4 coming from P3(2), P4(2), P3(3) and P4(3). When this transformation is done on the group P3(3), P4(3) and PS(3), it positions the profile PS(3) with respect to the others, yielding the profiles P1, P2, P3, P4 and PS of the piece.
Repeating this process until the last profile of the piece 70, as shown in Figure 9, which is the profile P(N), we obtain profiles P 1, P2, P3,...P(N-1 ), P(N).
Using these data, we can obtain any information concerning the shape of the piece, be it the bow, the crook, the twist or any other shape resulting from the data of the piece recorded.
The process builds a 3-dimensional image of the piece which can be analyzed as wished and which is captured independently of the movement of the piece while the readings are taken.
~ o Example 1 The present invention will be more readily understood with particular reference to the following example which is given to illustrate the invention rather than to limit its scope.
Referring now to Figure 10, an apparatus according to the preferred i 5 embodiment of the present invention is described. The apparatus comprises a mechanical transportation system including a belt mounted on a conveyor 81 which is drawn by an electrical motor (not shown), in a direction indicated by arrows 82. The piece to analyze is, in this case, a wood board 80. The inspection of the surface is done by a camera 83 and three conventional monochromatic light sources 84, such as 20 lasers, illuminating their respective rays on adjacent surfaces of the piece to be analyzed, forming profiles 85, 86 and 87. Each laser is placed at a distance D
from the next laser. The preferred lasers and camera are laser diodes generating a ray from the company Lasiris Inc., together with a camera of the company Pulnix America Inc. of which the spectral response is adjusted to the frequency of the laser diodes of the 25 apparatus.
It can be seen from this figure that the conveyor belt gives a linear displacement between the piece to scan and the camera. No synchronization is _g_ required in this case since there is only one camera. The three profiles are coming from the same image. The principal goal of this operation is to be able to freeze, in a space-time, the three profiles.
A rotational encoder 88 mechanically mounted on the conveyor 81, generates impulsions towards an encoder interface of a computer, in order to synchronize the moments at which the camera has to take new images of the three laser profiles. This encoder 88 ensures that all profiles are equidistant as described earlier (see Figure 2).
When all profiles are equidistant, the readings obtained using the image from the camera can correspond with the subsequent readings of the laser profiles. The camera ~o will be positioned in such a way that its field of view will include the region illuminated by the three laser beams. In this example, the information comes from two adjacent surfaces but information from only one surface of the piece could have been used to obtain the same result.
Referring now to Figure 11, a block diagram of the apparatus is illustrated.
Camera 91 transmits using transmission line 92, the information relative to the image taken with a "FRAME GRABBER" card 93 installed in the computer 90. The synchronization of the camera 91 is done using a rotational encoder 94 mechanically coupled to the conveyor transporting the wood board. It transmits on transmission line 95, a series of impulsions towards an "ENCODER" card 96 installed in one of the 2o ports of the computer 90. The displacement of the board of a distance D
activates the capture of a new image by camera 91. Each image is processed according to the method described previously.
A screen 99 connected to the computer 90 displays a graphical interface for the operator using a graphical card 97. The computer communicates with other users using a network link 101 towards a programmable logic control unit 102 or towards another computer 102. The network card 100 has to be located in one of the ports of the control computer, in order for the computer to communicate on a network.

While the invention has been described with particular reference to the illustrated embodiment in Example 1, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. The following examples illustrate other preferred embodiments of the present invention but do not limit the scope of the invention.
Example 2 ~o Figure 12 illustrates the second preferred embodiment of the present invention.
The physical setup of the invention is very similar, although the number of cameras has changed. Three cameras 113 are now used, each recording the signal of their respective laser beam 114. This method requires the synchronization of the cameras because the three profiles 115, 116 and 117 must be taken as simultaneously as possible. The longer the time between the acquisition of each profile for the group of three, the less precise are the positions of the profiles relative to one another. This error is explained by the fact that the piece can move in between acquisitions if there is a waiting period, making the relative positions of the profiles erroneous.
However, whether the cameras are well synchronized or not, the general principle of the method 2o is the same. Only the precision is altered. The synchronization of the cameras can be done using standard control signals HD/VD (horizontal drive/vertical drive) which most cameras accept. These signals can be provided by the "FRAME GRABBER"
card, by the camera itself or by an electronic circuitry which uses the video signal of the master camera. These signals are then sent to the cameras to synchronize.
Example 3 Figure 13 illustrates a third embodiment of the present invention. The physical setting of the invention is the same as for the first example. However, more than three profiles are used to acquire the shape of the piece, that is, profiles 125, 126, 127 and 128. The process of reconstituting the shape of the piece is not changed. If four cameras 123 are associated with their respective laser beams 124, it means that we can use the first three profiles to correctly align the fourth profile giving it slightly better precision. However, one could use the first two profiles to align the profiles (as shown in Example 1 ) and, automatically, two new profiles would be positioned in space. This method would lower the number of iterations needed to complete the reconstitution of the shape of the piece.
It is understood that four laser beams could be used with only one camera, as long as it captures all of the profiles in one image.
We could increase the number of profiles in order for the piece to be scanned in less acquisitions. The maximum number of profiles would scan the whole piece in one acquisition. If using the maximum number of profiles, we would obtain an apparatus of the type "SNAP SCAN" which does not use the step-by-step process described by this invention. There is an advantage in keeping the number of profiles to three laser profiles because the physical space needed to install the apparatus and the cost of the apparatus is minimized. In addition, three profiles are sufficient to 20 obtain all the information desired.
Example 4 Figure 14 illustrates a fourth embodiment of the present invention. The physical setting of the invention is the same. However, only two profiles 145 and 146 25 are used to acquire the shape of the piece 140. Certain restrictions apply, the apparatus shown is still precise as long as the piece does not move on the conveyor belt 141. If this condition is not respected, the apparatus will not acquire all the information desired with enough precision.
The process of positioning the profiles, one with respect to the other, step-by-step, will still work. However, the only information about the piece which will stay precise and valid will be the twist. The validity of the other aspects will depend on the movement of the piece during the profile acquisition.
The method has been explained with respect to a laser profile system. Any other system of which the output yields a profile of a piece is also valid.
The acquisition of the shape principle will stay the same. The laser profile was used as an example of a preferred embodiment for clarity and ease of description. It is understood that microwave systems, x-ray systems, ultrasound systems, etc.
would increase the complexity of the system, but could be used with the same acquisition system and reconstitution of the shape system.
While the invention has been described in connection with specific embodiments and examples thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be 2o applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

We claim:

1. An apparatus for obtaining data representing a shape of a wobbly elongated workpiece rapidly moving on a conveyor surface in a direction of travel substantially lengthwise to said workpiece, the apparatus comprising:
a profile line projector for projecting at least two profile line beams on said workpiece;
an image acquirer for acquiring an image signal of said at least two profile line beams on said workpiece;
an analyzer for correlating consecutive overlapping image signals acquired at different times, transforming said image signals to compensate for wobble movement of said workpiece and producing data representing a shape of at least a portion of said workpiece.

2. An apparatus as claimed in claim 1, further comprising a conveyor position sensor for measuring a position of said workpiece on said conveyor surface and wherein said analyzer uses said position to correlate said image signals.

3. An apparatus as claimed in claim 1, wherein said analyzer correlates a leading portion of a present image signal with a trailing portion of a past image signal in order to correlate said image signals.

4. An apparatus as claimed in claim 2, wherein said analyzer correlates a leading portion of a present image signal with a trailing portion of a past image signal in order to correlate said image signals.

5. An apparatus as claimed in claim 3, wherein said profile line projector projects three profile line beams on said workpiece, wherein said image acquirer captures an image signal of said three profile line beams on said workpiece and wherein said analyzer correlates two trailing profile line beams of a past image signal with two leading profile line beams of a present image signal.

6. An apparatus as claimed in claim 1, wherein said profile line projector is a monochromatic light source.

7. An apparatus as claimed in claim 1, wherein said image acquirer is a camera.

8. An apparatus as claimed in claim 1, wherein said profile line beams are projected by said profile line projector at a discrete number of positions.

9. An apparatus as claimed in claim 1, wherein said profile line beams are projected by said profile line projector at equidistant positions on said workpiece.

10. A method for obtaining data representing a shape of a wobbly elongated workpiece moving on a conveyor surface, the method comprising:
acquiring a series of at least two overlapping image signals of said workpiece as it is moving on said conveyor surface;
compensating for movement of said workpiece with respect to said conveyor surface by correlating said series of at least two overlapping image signals in order to acquire data representing a shape of at least a portion of said workpiece.

11. A method as claimed in claim 10, wherein a position of said conveyor surface is taken into consideration during said correlating of said image signals.

12. A method as claimed in claim 10, further comprising a step of projecting profile line beams on said workpiece at equidistant intervals.

13. A method as claimed in claim 10, further comprising a step of calculating a velocity of said conveyor surface.

14. A method as claimed in claim 10, wherein said step of compensating comprises correlating a leading portion of a present image signal with a trailing portion of a past image signal in order to correlate information concerning a shape of said workpiece.

15. A method as claimed in claim 10, wherein the method comprises the steps of:
a) capturing a first image signal of said workpiece;
b) measuring a displacement or position of said conveyor surface;
c) advancing said workpiece of a first distance smaller than a length of said first image signal;
d) capturing a second image of said workpiece;
e) correlating said first and second image signals in order to align an overlapping portion of said image signals f) repeating steps b) through e).

16. A method as claimed in claim 12, wherein the method comprises the steps of:
a) projecting at least two profile line beams separated by a first distance on said workpiece;
b) capturing a first image signal of said at least two profile line beams on said workpiece;
c) measuring a displacement or position of said conveyor surface;
d) advancing said workpiece of said first distance;

e) capturing a second image signal of said at least two profile line beams;
f) correlating at least one leading profile line of said second image signal with at least one trailing profile line of said first image signal; and g) repeating steps c) through f).

17. A method as claimed in claim 10, further comprising a step of categorizing said workpiece using said data representing a shape of said workpiece.

18. A method as claimed in claim 13, wherein the method comprises the steps of:
a) projecting three profile line beams separated by a first distance on said workpiece;
b) capturing a first image signal of said three profile line beams on said workpiece;
c) measuring a displacement or position of said conveyor surface;
d) advancing said workpiece of a second distance smaller than said first distance;
e) capturing a second image signal of said at three profile line beams;
f) correlating two leading profile lines of said second image signal with two trailing profile lines of said first image signal.
g) repeating steps c) through f), wherein a third profile line of said second image signal is positioned with respect to the first profile lines.