WO1986001590A1

WO1986001590A1 - Method and plant for machine-working pieces of timber

Info

Publication number: WO1986001590A1
Application number: PCT/SE1985/000314
Authority: WO
Inventors: Bo Sjödin; Lars Olsson
Original assignee: Saab Wood Ab
Priority date: 1984-08-27
Filing date: 1985-08-22
Publication date: 1986-03-13
Also published as: EP0221903A1; FI863847A; SE8404253D0; SE447303B; FI863847A0; SE8404253L

Abstract

In a method which is carried out in a sawmill plant, blocks (B, b) are produced which are sawn from timber logs and have two plane-parallel sides (3, 4) intersecting green surfaces (1, 2) of the piece of timber. The blocks are irradiated in a measuring zone, such that the green surfaces will appear in contrast to said planar sides, and the intersections (5-8) between green surfaces and planar sides can be detected by photoelectric means. By determining the positions of said planar sides relative to said detecting means, the coordinates of the intersections are obtainable in a plurality of points in the longitudinal direction of each block. The method and the plant according to the invention make it possible to supervise how the blocks are sawn and to release an alarm signal in the event of an unacceptable defect.

Description

METHOD AND PLANT FOR MACHINE-WORKING PIECES

OF TIMBER

The present invention relates to a method for machine- working elongate pieces of timber, by which method blocks are produced which have a cross-section comprising two arcuate green surfaces facing in opposite directions and two worked planar sides which are substantially parallel and form, with said green surfaces, intersections ex¬ tending irregularly along the block.

According to this method, the blocks are moved longitudinally one by one along a reference plane extending through the block and being substantially parallel to the planar sides, the block passing through a measuring zone which extends transversely of said reference plane and in which cross-sectional dimensions of the block can be repeatedly measured, as the block is moving through said measuring zone, by means of a photoelectrically operating measuring system scanning said measuring zone.

The invention also relates to a plant making use of the above-mentioned method and comprising means for longitudinally moving the blocks one by one along a reference plane which extends through the block and is substantially parallel to said planar sides, a measuring zone extending transversely of said re¬ ference plane, such that the moving block passes there- through, said plant comprising a photoelectrically operating measuring system adapted to scan said measur¬ ing zone and to repeatedly measure cross-sectional dimensions of the block, while this is being moved through said measuring zone. During the initial sawing operation to which timber logs are subjected in a sawmill, two side boards are usually sawn off, one on either side of the log, such that the piece of timber which is discharged from the edger and which is now termed block, is defined by two parallel planar sides which form intersections with the remaining arcuate green surfaces and from which one proceeds for further working of the block. The manner in which the timber is handled by the edger, and in which positions the planar sides are cut, there¬ fore is of the utmost importance to the timber yield. For these reasons, many sawmills have introduced techniques and devices aiming at orienting each indi- vidual log at the moment of entry in such a manner relative to the saw blades of the edger that the saw cuts produced by these blades will lie in positions that are as advantageous as possible. To this end, regard must be paid on the one hand to the geometrical shape and dimensions of the log and, on the other hand, to the cutting-up alternative (the setting-up of the blades) with which the sawmill is to operate at the time in question and which determines the mutual setting of the blades on the edger and the resaw. A rule of thumb for log infeed is to seek an orientation which results in a symmetrical position of the planar sides relative to the log centre plane, in which case - if the log has an ideal cylindrical shape - the planar sides will be congruent. To minimise loss and wane formation when the block is cut up, it is important that the planar sides will have equal width and similar configuration. Laying the cuts for the planar sides symmetrically along the log also usually means that the volume of the material which is separated in the form of the two side boards and which largely goes to waste, will then be minimal.

Even if use is made of the very best techniques and expedients available for the log infeed, it is impossible in actual practice to avoid that the saw cuts provided by the edger will be slightly displaced in the lateral sense or lie obliquely relative to the contemplated position of maximum advantage in 3 the log. Such deviations may result from, for example', mechanical wear of or damage to the sawing machine, incorrect setting of the saw blades, an electric fault in a system controlling the infeed, or some other fault in the system. Another reason may be sought in the presence of nails or the like which make a saw blade blunt and may cause the log to be drawn askew, i.e. the log does not maintain the position in which is has been introduced and is displaced la- terally to an ever increasing extent.

Another fault encountered in edging work may be due to the fact that the block height, i.e. the distance between the planar sides, deviates from the dimension that must be maintained for cutting up the log in a correct manner.

If such faults occur while automatic feeding and edging of a timber lot is in progress, the waste may be considerable, unless the operator supervising the feeding and edging discovers the fault upon inspec- tion of the blocks and stops the machine, and this is something which he may be reluctant to do because a standstill will cause a decrease in production capa¬ city. In view hereof, a fault of the above-mentioned type occurring while work is in progress in an edger presumably will result in a long succession of logs being sawn with unsymmetrically or otherwise unfavour¬ ably oriented saw cuts, and this again will result in a lower yield from the blocks, before the fault can be eliminated. Even though the deviation from the contemplated ideal positions of the planar sides may seem insignificant, measured in millimetres on the individual blocks, the financial loss may be con¬ siderable.

The present invention which is based upon the insight that one should be able to discover and cor¬ rect as soon as possible a fault which occurs in a plant for edging timber to blocks, aims at providing a method and a device for determining the geometry of pieces of timber which, in a sawmill machine_? are formed into blocks, and to use the geometrical block data thus obtained for supervising that the feeding of the pieces of timber into the machine and the sub¬ sequent machining will be effected in a correct manner.

A special object of the invention is to check, on the basis of the measuring data of a block which is being discharged, or has been discharged, from an edger, if the two planar sides of the block de¬ viate in nonpermissible manner from predetermined positions and, if this is the case, to provide an indication to the sawmill workers that the edging operation in progress should be interrupted. Another aspect of the invention issues from the fact that, in order to optimise the cutting-up of pieces of timber, it is essential to know the configu¬ ration of the two planar sides of timber blocks. It therefore is a further object of the invention to pro- vide, by marking pieces of timber machined into blocks in a primary operation in a cutting-up program, data which show the mutual form and position of the planar sides and which, while being utilised during the sub¬ sequent cutting-up of the block, may give an increased yield in the form of finished timber.

The above-mentioned objects and aims are achieved according to the present invention in that the method and the plant are given the characteristic features stated in the claims. The invention will now be explained in detail, re¬ ference being had to the accompanying drawings in which Fig. 1 is a diagram showing the underlying principle of a plant according to the invention; Figs. 2-3 are respec¬ tively a side view and an end view of parts of the plant and a block conveyed edgewise therethrough; and Figs. 4-5 illustrate, in analogous manner, the corresponding parts of an embodiment in which the block is conveyed lying on its flat side. The plant illustrated in Fig. 1 is assumed to be a station in a sawmill in which timber is sawn into boards and other wood products. A primary operation in this machine work is the feeding of the timber into an edger in which the logs are conducted one by one past two parallel saw blades or cutting disks removing the outer log parts. During this operation, the pieces of timber obtain the form of blocks which, depending upon the starting material, may have highly varying thickness but are essentially uniform in cross-section. In the drawings, B denotes a block cut from a thick log, while b may be assumed to represent the smallest block cross- section occurring in the plant. Regardless of the thick¬ ness, the cross-section of the blocks has two arcuate green surfaces 1 and 2 which face in opposite directions and are rough and irregular, especially if the saw timber is delivered unbarked. 3 and 4 represent the cuts which are obtained upon edging and which are called the planar sides in the following. If edging is carried out correctly, the planar sides are mutually parallel and approximately equally wide. The lines 5-8 along which green surfaces and planar sides intersect each other, extend more or less irregularly, while at the same time they usually are somewhat divergent as seen in the longitudinal direction of each planar side since, as is well known, timber logs are not completely straight and cylindrical but frequently have a slight curvature or other deviation from the generally slightly conical basic form. Determining the geometrical shape of the intersec¬ tion lines 5-8 and, thus, the form of the two planar sides is an important feature in realising the present invention.

The sawmill station which the plant according to the invention is intended to constitute, may be located between the edger and a resaw (none of which is shown in the drawings), and in such cases the blocks B, b are longitudinally fed by a conveyor which, in Figs. 1-3, is generally designated 9 and on which the blocks arrive one by one. Alternatively, the station may be located within the working range of the edger, in which case the pieces of timber are supplied by the drive means of the sawing machine itself. In those instances where side boards are separated from the piece of timber, it is possible in this alternative, that such boards still adhere to the piece of timber on either side of the block when this is passing through the station, but this does not obstruct the operation in the station. In such a case, the term "block" should be interpreted to comprise both the block part B, b proper and the two side boards on either side thereof. In the drawings, a reference plane 10 is parallel to the planar sides 3, 4 of the blocks as these are being longitudinally fed on the conveyor 9. The direc¬ tion of this reference plane coincides with the con¬ veyor. In a position along the conveyor, a measuring zone 11 is provided which generally is a narrow area located at an angle to the conveying direction and the reference plane 10. The blocks are conducted through this area which contains two measuring planes 12, 13 directed such that their intersections with the reference plane 10 and thus with the sides 3, 4 will be perpendicular to the conveying direction. In Fig. 1, both measuring planes coincide with the plane of the drawing.

For measuring each block advanced through the measuring zone, the plant is equipped with a photo- electric measuring system comprising two cameras 14 and 15, each of which operates in one of the measuring planes 12, 13 and is directed toward the respective planar side of the block. The cameras which may be of the well-known type having an array of photodiodes and whose construction and mode of operation therefore need not be described in detail, each provide, in a rapid and repeated measuring sequence, an electro- nically readable linear image of the object which the camera at the moment is "seeing" within the area it is scanning (16 in Fig. 2). Thus, the measuring system images, at mutually close intervals, transversely locat- ed segments of the block as this is being advanced through said measuring zone. The measuring signals from the cameras are fed to an electronic unit 17 comprising a microcomputer and processing the measuring data repre¬ senting the said segments. Cooperating with the measuring system is a system of radiation sources adapted to irradiate, in a manner advantageous to the measuring, that part of the block which is in the measuring zone 11. According to a primary characteristic feature of the invention, the plant comprises two oppositely directed radiation sources 18 and 19 disposed on opposite sides of the conveyor 9 and so arranged that their radiation beams 20 and 21 fall along the reference plane 10, i.e. substantially parallel to the planar sides 3, 4 of the block. In this manner, the two green surfaces

1, 2 of the block in the measuring zone will be intense¬ ly irradiated and reflect light which is received by the cameras 14 and 15. The two planar sides, however, will not be irradiated or but slightly irradiated by the radiation sources, for which reason the planar sides will reflect substantially only the weak light from the surroundings. The result is that the two green surfaces will appear in each camera in contrast to the planar side which is facing the camera, and this in turn will result in two positions within the scanned area 16 where the received radiation shows an intense decrease or increase, respectively, which is readily indicated by the electronic unit. Correspond¬ ing points naturally occur in the scanned area of the other camera, although in positions which here may be different, depending upon the irregular form of the edged log and any faults that may occur during the edging operation. The said four positions represent points on the intersecting lines 5-8 of green surfaces and planar sides.

According to the special features of the invention, the plant also comprises means for indicating the positions of the two planar sides 3, 4 relative to the cameras 14, 15. In the embodiments illustrated, these means are laser transmitters 22, 23, one for each camera and mounted such that their lobe directions fall from outside obliquely into the measuring zone and along the measuring planes 12, 13 of the respective camera. The beam lobes 24 from the laser transmitters are of narrow cross-section, for which reason each lobe projects a small light spot 25 when, as is best seen in Fig. 1, the radiation impinges upon a block that has entered the measuring zone. The light spots will impinge upon the essentially nonirradiated planar sides 3, 4 and therefore are easily detected by the cameras and electronics of the measuring system. it will be appreciated that, for the large block cross-section B, the light spots 25„ in Fig. 1 will impinge relatively high up on the planar sides, where¬ as with decreasing block thickness the spots will impinge upon the block in an ever lower position in the measuring zone. However, by selecting a suitable direction for the laser transmitters, it is possible to cause the lobes also at the smallest block cross- section b to impinge upon the worked block surfaces fac¬ ing the cameras, now at the points 25, . As a consequence, the position of the light spots within the area 16 scanned by each camera will be in a linear relation to the posi¬ tion of the surface, i.e. the perpendicular distance between the reference plane 10 and the respective planar side 3 or 4. The positions thus are an exact measure of the distance between the planar side in question and the focal plane F of the camera (see Figs. 3 and 5) where the photodiode array is located. For each measuring sequence, the measuring system will thus have access to a combination of six positions in the camera arrays, a combination unique to the block segment which is being scanned, i.e. the four above-mentioned positions corresponding to the detected points on the intersections 5-8 and the two positions corresponding to the detected laser light spots 25. By conventional calculation technique, implemented in the electronic unit 17, it will thus be possible to determine the coordinates in the measuring planes

12, 13 for the four intersections. A measuring accuracy of 1/10 mm is thus possible.

The electronic unit also is supplied with data indicating the position of the block in the measuring zone 11 the moment a measuring sequence is carried out. These data can be obtained on the basis of a pulse signal from a pulse transducer 26 coupled with the conveyor 9, and a photo cell signal generated when the leading end of the block intersects the mea- suring planes 12, 13. When the entire block has passed through the measuring zone and thus has been measured in its entirety, the measuring system has obtained all information required for making, in a triaxial coordi¬ nate system, a complete and accurate determination of the geometry of the two planar sides 3, 4 of the piece of timber. In this manner, also the neutral position of the planar sides has been determined, i.e. the measure of the block height can be readily established at different points along the block, and such measurements can be displayed block by block on a display unit 27 connected to the microcomputer or recorded centrally in a main computer 28.

In accordance with the primary object of the invention, which is to supervise how well the form of the edged blocks corresponds to the requirements applicable in this respect, the geometrical determina¬ tion of the planar sides may be used as a basis for a check-up calculation showing the position in which the planar sides have been cut, and whether or not this is acceptable. The check-up calculation can be carried out in many different ways and preferably follows an algorithm which also may be implemented in the electronic unit.

Preferably, the values of the average width of the planar sides are used as a basis for determining the skewness that may have occurred during the edging operation, i.e. the possible lack of symmetry that can be established between the planar sides of a block and the original geometrical centre plane of the log, it being preferred first to compile values of the average width, calculated on different predetermined sections along the block length, whereupon the existing asymmetry is calculated section by section by means of experience values of the curvature of the log cross- sections and the compiled width values. The skewness values obtained are then related to the requirements valid which may be expressed in the form of limit values. According to an alternative check-up tech¬ nique, the quotient between the average widths of the two planar sides is calculated, whereupon this quotient which may be regarded as a measure of the skewness, is compared block by block with a limit value.

With normal saw timber qualities, one must tolerate that the average width of the planar sides and thus the calculated skewness values vary, and that deviations from the ideal block form occur. In each specific case, the said limit values therefore are selected such that they distinguish those deviations in the block form which affect the yield during the subsequent cutting-up. Furthermore, statistic analysis of the skewness values may show whether the equipment suffers from systematic faults, and whether its quality and status are changed. The coordinates obtained during measuring of the blocks may also be utilised for checking the block height, i.e. the distance between the planar sides, a_. dimension which is of fundamental importance to the cutting-up of the block.

In the plant according to the invention, it is possible, preferably by means of a push-button keyset 29 on the display unit, to control the automatic super¬ vision of the plant by reading into a memory in the electronic unit the tolerance values which are to apply to the varying setting-up alternatives that may occur, and of which the above-mentioned limit value is one, while another tolerance value may show permissible dimensional differences in the block height. On the basis of these read-in tolerance values, the electronic unit, in synchronism with the block transport through the measuring zone, calculates data showing how the requirements of accuracy that apply to the edging operation, are satisfied. These data can be indicated to an operator on the display unit 27.

If the check shows that nonpermissible deviations begin to appear, the operator will thus be forewarned and can quickly intervene by stopping the work in the edger. As a further security measure to make the operator notice the fault and enable him to prevent incorrect feeding and working of an unnecessarily large number of logs in the machine, the electronic unit 17, according to a special characteristic feature of the invention, can be made to give a warning signal when a predetermined tolerance value is exceeded. The warning signal is transmitted via a line 30 to a mains-supplied unit 31 which controls the current supply to the radiation sources and laser transmitters of the plant and which also releases an optical or acoustic alarm device 32 in response to the warning signal. It is pointed out that the alarm function should not be released until the electronic system 12 indicates that the nonpermissible deviation was not accidental but occurs similarly for a succession of blocks, since it will be appreciated that the occur¬ rence of an occasional defect block may not be due 5 to malfunction of the equipment or skewing but, for example, to an abnormal geometrical log form.

In Figs. 2-5, use is made of the same reference numerals as in Fig. 1 for identical parts that have been described above. When the plant is designed as

10 shown in Figs. 2-3, the conveyor 9 may comprise a driving chain 33 which is located in the reference plane 10 and is narrow in relation to the minimum block cross-section b in order not to prevent irra¬ diation of the outer parts of the lower green surface

15 2 and detection of the intersections 7 and 8. The conveyor also has a number of pressure rollers 34 which are held in engagement with the upper green surface 1 of the blocks and has a concave profile, as shown in Fig. 3, whereby said pressure rollers

20 guide the blocks during measuring so that the planar sides 3, 4 will be at least substantially vertical.

It will be appreciated that a sloping planar side causes the coordinate determination of its inter¬ sections 5, 7 or 6, 8 to be erroneous, and that these

25 errors must be compensated for before the measuring result is utilised. In the embodiment of the invention as shown in Figs. 2-3, two pairs of beam lobes 35' and 35" are projected from the laser units 22, 23, the beam lobes of each pair being essentially paral-

30 lei and mutually slightly offset in the direction of the measuring planes 12, 13, care being taken during alignment of the laser units that there are produced, on either planar side 3, 4, regardless of the block size, two light spots 36' and 36", one directly above

35 the other. In analogy with what has been said above concerning the plant according to Fig. 1 and its mea¬ suring technique, the four positions in the array cameras which are represented by the light spots, will be recorded by the measuring system, and because the system has exact information about the slope and the mutual distance of the laser lobes of each pair, it will be possible, regardless of the distance be¬ tween the camera and the planar side, to calculate, by a comparative analysis of the distance values corre¬ sponding to the said four positions, if, and in that case by how much, the planar _*sides 3, 4 are sloping relative to the reference plane 10 when the block passes through the measuring zone.

As in the variant shown in Figs. 4-5, the radiation sources 18, 19 may consist of lamps arranged in pairs and provided each with one reflector to provide for adequate parallelism.

In the embodiment shown in Figs. 4-5, the blocks are conveyed by means of belts or similar broad driv¬ ing means on which the blocks are advanced lying on one of their flat sides 3 so that the reference plane 10 will be horizontal. To enable measuring from below, the conveyor is constructed in two units 37 and 38 mu¬ tually spaced apart in the conveying direction, such that the camera 39 and the laser transmitter 41 acting in the measuring plane 40 of the said camera may be ar- ranged underneath the conveyor, while the corresponding components 42, 43 for measuring the blocks from above may be arranged in a vertical measuring plane 44 which should intersect the reference plane 10 at the point where this is intersected by the lower measuring plane 40. It is, however, acceptable to make the last-mentioned plane sloping, as shown in Fig. 4, in which case a screen 45 should be provided to protect the two components 39 and 41 which are both sloping upwardly, against dust and similar soiling. Similarly, a protective screen 46 should be provided in the embodiment according to

Figs. 2-3 so that the blocks which are conveyed edgewise will be effectively irradiated from below. It will be appreciated from the above that the- ' method and the plant according to the invention open the possibility of effectively supervising the work in an edger and, in the event of an unacceptable fault, such as skewness or lateral displacement of the saw cuts, of quickly intervening and eliminating the fault. The invention may also be utilised for the cutting- up of the blocks following upon the edging operation. After there has been obtained, in the manner described above, an electrically readable determination in the plant of the block geometry, it is possible, by con¬ tinued data processing in the main computer, to make a detailed analysis on which the control of the resaw may be based. For example, it is possible to calculate the extent by which the planar sides are mutually overlapping or offset in the plane, or to calculate if the planar sides have a curvature sufficient to justify curved sawing. Thus, the coordinate determina¬ tion of the blocks according to the present invention may also contribute to higher timber yields.

Claims

1. A method for machine-working elongate pieces of timber, by which method blocks are produced which have a cross-section (B,b_) comprising two arcuate green surfaces (1,2) facing in opposite directions and two worked planar sides (3,4) which are substantially pa¬ rallel and form, with said green surfaces, intersec¬ tions (5-8) extending irregularly along the block, and in which method the blocks are moved longitudinally one by one along a reference plane (10) extending through the block and being substantially parallel to the pla¬ nar sides, the block passing through a measuring zone (11) which extends transversely of said reference plane and in which cross-sectional dimensions of the block can be repeatedly measured, as the block is moving through said measuring zone, by means of a photoelect¬ rically operating measuring system scanning said measu¬ ring zone, c h a r a c t e r i z e d in that the block is irradiated in said measuring zone (11) by means of two oppositely directed radiation beams (20,21) falling along said reference plane (10), each radiation beam being projected against one of said green surfaces (1 , 2), such that the two green surfaces will be intensely irradiated, whereas the two planar sides (3,4) will not be irradiated or but slightly irradiated by said radia- tion beams, whereby the green surfaces will appear in contrast to the planar sides; that radiation deriving from said radiation beams and reflected by said green surfaces (1,2) is received by two oppositely directed cameras (14,15; 39,42) positioned each on one side of said reference plane and pertaining to said measuring systems, such that each camera can detect, by means of the radiation received, the intersections (5,7; 6,8) between one of said planar sides (3,4) and said green surfaces (1,2) that the two distances between the respective camera and that part of the planar side which is scanned by said camera, is determined; and that the coordinates of the two planar sides (3,4) are determined by means of data of the detected intersec¬ tions and the said distances.

2. A method as claimed in claim 1, c h a r a c ¬ t e r i z e d in that irradiation is effected by means of light which is parallel or almost parallel within each radiation beam (20, 21) and to said reference plane (10) and,thus, to, said planar sides (3,4).

3. A method as claimed in claim 1 or 2, c h a ¬ r a c t e r i z e d in that each camera (14,15; 39,42) scans linearly and within a measuring plane (12,13; 40, 44) whose intersection with said reference plane (10) and, thus, with said planar sides (3,4) is perpendicu¬ lar or substantially perpendicular to the longitudinal direction of the block.

4. A method as claimed in claim 1, 2 or 3, c h a r a c t e r i z e d in that, on the basis of the coordinates of said planar sides (3,4), a check-up of their mutual size is effected in a calculation unit (17), preferably by establishing the quotient between the average width of siad planar sides; and that, when the checkup calculation shows that the planar sides deviate from each other in the measured block or a succession of blocks so that the said quotient by a certain amount deviates from the integer 1 , a signal is provided for releasing an alarm so that continued work- ing to blocks having such deviations can be prevented.

5. A method as claimed in claim 1, 2 or 3, c h a¬ r a c t e r i z e d in that on the basis of said coor- dinates, an optimisation calculation is carried out in a computer (28) to provide data showing the most advan¬ tageous position and form of saw cuts cutting up the block, whereupon signals based on such data are gene- rated to control a resaw in which these saw cuts are carried out.

6. A method as climed in claim 3, c h a r a c¬ t e r i z e d in that narrow radiation lobes (24; 35), preferably laser lobes, extending along the measuring planes (12,13; 40,44) at an oblique angle to the re¬ ference plane (10), are projected against the planar sides (3,4); and that the said distances are determined in that the cameras (14,15;39,42) , receive radiation reflected from the spots (25; 36) on the planar sides (3,4) where the beam lobes (24; 35) impinge, and gene¬ rate position signals applicable to said spots.

7. A method as claimed in claim 6, c h a r a c¬ t e r i z e d by projecting against each planar side (3,4) two substantially parallel beam lobes (35* ,35") mutually laterally offset in the measuring plane, such that two spots (36' ,36") are generated on each planar side; and that the distance of each spot to the camera is determined, thereby to obtain data showing if and by how much the said planar sides slope relative to the reference plane.

8. A plant for machine-working elongate pieces of timber, in which plant blocks are produced having a cross-section (•B/.b) comprising two arcuate green sur¬ faces (1,2) facing in opposite directions and two worked planar sides (3,4) which are substantially pa¬ rallel and form, with said green surfaces, intersec¬ tions (5-8) extending irregularly along the block, said plant comprising means (9) for longitudinally moving the blocks one by one along a reference plane (10) which extends through the block and is substantially parallel to said planar sides (3,4), a measuring zone (11) extending transversely of said reference plane, such that the moving block passes therethrough, said plant further comprising a photoelectrically operating measuring system adapted to scan said measuring zone and to repeatedly measure cross-sectional dimensions of the block while this is being moved through said measuring zone, c h a r a c t e r i z e d in that there are provided, at said measuring zone (11), two radiation sources (18,19) which are oppositely directed and adapted to irradiate green block surfaces (1,2) lo- cated within said measuring zone so that these green surfaces will be intensely irradiated, whereas the two planar block sides (3,4) will not be irradiated or but slightly irradiated, whereby said green surfaces will appear in contrast to said planar sides, and two oppo- sitely directed cameras (14,15; 39,42) located each on one side of said reference plane and pertaining to said measuring system and adapted to detect, by means of the radiation received, the intersections (5,7; 6,8) between one of said planar sides (3,4) and said green surfaces (1,2), and a device (22,23; 41,43) by which the two distances between the respective camera and the portion of the planar side which is scanned by said camera, can be determined, said plant also comprises calculating means (17) for determining, by means of data obtained from said cameras and relating to the said intersections and distances, the coordinates of the two planar sides (3,4).

9. A plant as claimed in claim 8, c h a r a c t e¬ r i z e d in that the radiation sources (18,19) are adapted to transmit light falling parallel or almost ^'parallel to said reference plane (10) and thus to said planar sides (3,4).

10. A plant as claimed in claim 8 or 9, c a¬ r a c t e r i z e d in that each camera (14,15) has a linear array of photo detectors by which the camera scans a planar measuring area whose intersection with said reference plane (10) and thus with the planar sides (3,4) of a block within said measuring zone is perpendicular or substantially perpendicular to the direction of movement and thus to the longitudinal direction of the block.

11. A plant as claimed in claim 8, 9 or 10, c h a¬ r a c t e r i z e d in that said calculating means (17) is adapted to effect a check-up calculation, by means of the coordinates of said planar sides (3,4), concerning the mutual size of said planar sides, pre¬ ferably by establishing the quotient between the ave¬ rage width of said planar sides, and to generate a check-up signal showing the mutual size and any devia- tion from a predetermined tolerance value, and that said calculating means (17) is operatively connected to^" display and/or alarm means (27,32) receiving the check¬ up signal so that continued working to blocks showing such deviations can be prevented.

12. A plant as claimed in claim 8, 9 or 10, c h a¬ r a c t e r i z e d in that said calculating means (17) is connected to a main computer (28) adapted to calculate, on the basis of the coordinates of said planar sides (3,4), data showing the most advantageous position and form of saw cuts cutting up the block, and to generate control signals based on such data and coupled to a resaw for effecting the said saw cuts.

13. A plant as claimed in claim 10, c h a r a c¬ t e r i z e d in that the device effecting said dis¬ tance determination comprises transmitters for provid- ing narrow beam lobes (24; 35), preferably laser trans¬ mitters (22,23; 41,43), said beam lobes extending along the measuring plane (12,13; 40,44) of each of said cameras at an oblique angle to said reference plane (10) and being directed such that they impinge upon said planar sides (3,4) and the positions of the spots thereon, on which said beam lobes (24; 35) impinge, can be determined by detection in said cameras (14,15; 39,42).

14. A plant as claimed in claim 13, c h a r a c¬ t e r i s e d in that there are provided, on each side of said reference plane (10), two transmitters directed substantially parallel and laterally offset in said measuring plane (12,13), such that two light spots (36', 36") are formed on each planar side (3,4); and that said camera (14,15) is adapted for determining the positions of these two spots, whereby data relating to a possible slope of said planar sides are obtainable.