FR2530175A1 - METHOD FOR TRANSVERSAL CUTTING OF LONG WOOD PARTS AND MACHINE FOR CARRYING OUT THE PROCESS - Google Patents

Abstract

THE INVENTION CONCERNS A PROCESS AND A MACHINE FOR TRANSVERSAL CUTTING OF LARGE LENGTH PIECES OF WOOD. FOR CROSS-SAWING OF THE EXTREME BASE PART OF A PIECE OF WOOD 20 FOR THE PURPOSE OF LENGTHENING OR FAULT ELIMINATION, THE DISTANCE N BETWEEN THE RECESSED CORNER H OF THIS EXTREME PART AND A REFERENCE LINE L IS MEASURED AND THE RESULT OF THE MEASUREMENT IS TRANSMITTED TO AN ELECTRONIC UNIT 40 WHICH PRODUCES A CONTROL SIGNAL APPLIED TO AN ACTUATION DEVICE 16 WITH THE HELP OF WHICH A TRANSVERSE SAWING BLADE 12 IS MOVED IN RELATION TO THE WOOD PIECE, OR ON THE OTHER HAND, SO THAT THE RECESSED CORNER H COMES INTO THE CUTTING PLANE U OF THE SAW BLADE. APPLICATION TO AREAS OF SAWMILLS.

Description

2530175

  PROCESS FOR TRANSVERSAL CUTTING OF LONG WOOD PARTS

  AND MACHINE FOR IMPLEMENTING THE METHOD

  The present invention relates to a method of transverse cutting at the base end of long pieces of wood such as panels and boards having an end portion and a base end portion, the latter generally ending in an angled edge. oblique with respect to the longitudinal axis of the piece and having as end points a projecting corner and a recessed corner The invention

  also relates to a machine for carrying out the method.

  When transversely cutting, for example panels, to a desired length, generally a length adapting to an established system of longitudinal modules (when a module is for example equal to 3 dm),

  the cutting operation is started at the base end, then the end

  head is cut to an appropriate integer multiple of longitudinal modules

  tudinal so that the panel thus formed adapts to the modular system.

  The fact that a cut is also made at the base end partly results from the obligation to remove possible defects such as

  cracks or crevices existing in this area, but

  first and foremost, a clean, straight edge oriented perpendicular to the longitudinal axis of the

  wood, the last mentioned reason being often the only one existing.

  It is obvious that the cut-out portion at the base end should be as small as possible. The modular system ensures that,

  when a cut at the base end is unnecessarily too long

  such as 1 cm, it can result that almost a complete module of 3 dm (exactly one section of 29 cm) can be lost at the end.

of head.

  The base end is usually cut in what is called a wrought ironing or trimming plant from green wood (not dried) or in another roughing plant where the panel:

  are brought transversely to a stationary circular saw blade.

  An operator is placed a little upstream of this trans-

  versal and it examines whether or not it is necessary to cut

  to lift defective parts at the base end In the affirmative case

  tif, the operator pulls the panel outward so that all the defective part is removed by the cut As for the others

  panels, in most cases only a final

  This is automatically performed upstream of the trans-

  versal so that the part that is cut out has the same length for all the panels. It is obvious that in such a case, the length of the

  cut-out part must be dimensioned in correspondence with the

  bearing the most oblique end edge, that is to say the greatest distance in the longitudinal direction between the projecting corner and the

  corner, although such panels are in the minority in the majority

  As a result, on most of the panels, an

  Tiny is removed by cutting.

  The invention aims to create a method and a machine for

  to reduce to a minimum the part cut in pieces of wood or the non

  extreme edge is the only reason for a cut made in the ex-

  basic tremity The invention is characterized by the properties defined

  in the appended claims and it will now be explained

  in more detail with reference to the accompanying diagrammatic drawings which relate to exemplary embodiments and in which: Figure 1 is a plan view of a first embodiment of the invention; Fig. 2 is a plan view of a portion of a second embodiment; Figures 3 and 4 show, in plan view and on a somewhat enlarged scale, how the measuring devices of the figures

1 and 2 operate;

  Figure 5 is a sectional view of a third embodiment of a measuring device; and

  Figure 6 is a block diagram of an electronic unit.

  For parts fulfilling identical functions, references

  identical or similar numerals are used in all the figures.

  According to Figure 1, a number of panels 20, 21

  are supported by a transverse conveyor 10 which moves in the di-

  The panels were obtained by cutting logs which were

2530-175

  were in turn obtained from long tree trunks that have been

  cut into sections, that is to say that they have been cut transversely

  The panels have plane faces a and b which are parallel

  between them, and lateral or longitudinal edges c, d, also parallel

  between them (see also Figure 5) On the contrary, the transverse edge e of the base end and the transverse edge f of the end of

  head are irregular and, as a general rule, they are not perpendicular

  to any of the faces a to d.

  The panels 20, 21 are supported by load bearing elements

  on the conveyor 10, so that it is ensured, among other things, that the longitudinal axes x and the lateral edges c, d of the panels are always oriented perpendicular to the direction of advance R. Due to the obliquity of the base end edge e with respect to the side edges c, d and the longitudinal axis x which is parallel to the side edges, a projecting corner G and a recessed corner H are obtained at the base end The drawings show that we obtain a minimal part

  cut at the base end when the cutting line of the saw is di-

  along a Z plane which passes through the recessed corner H and which is

pendicular to the x axis.

  Assuming that in the panel 20, the largest usable integer multiple of a given measurement module, for example 3 dm, is located in a plane V just before a defective area K at the leading end and

  assuming that this area is to be removed by cutting, it can arrive

  ver, if the cutting at the base end is erroneously performed in a plane Z ', for example only 15 mm beyond the corner H, the cutting plane at the leading end will be shifted into a position V '

  which is already 15 mm inside the defective zone K

  sequence, the cutting done at the head end must be shifted

  down to a plane corresponding to the lowest integer multiple

  close to the measuring module m and all the zone M, containing perfect wood,

is lost.

  Transverse cutting means, constituted by a saw blade

  circular 12 having a cutting plane U, is placed in an adjacent zone

  The saw blade 12 is mounted on the output shaft 14 of a motor 13. The motor 13 can be placed on guides 15a, 15b.

  in the direction S perpendicular to the direction of advance R of the con-

  voyeur 10 and parallel to the x-axes of the transported panels 20, 21.

  The displacement is effected by a double-acting cylinder and piston mechanism 16, actuated by a pressurized fluid, and whose piston rod 16a is fixed to the motor 13 Instead of causing the displacement to occur. 13 together with the relatively heavy motor 13, the saw blade 12 could be arranged so that it alone could be moved in both directions of the boom S under the action of the assembly 16. There is provided a measure 30 upstream of the device

  transverse cutting 12 to 15, this device 30 comprising a rectifying ramp

  measurement line 31 associated with a plurality of photodiodes or photocells 32 In front of photodiodes 32 (that is to say, upstream with respect to the direction of movements R), an additional photodiode is provided 33 All Photodiodes 32 and 33 are placed below the conveyor 10. The ramp 31 may for example have a length of about 7.5 cm and may comprise 16 photodiodes 32 placed at 5 mm intervals. conveyor 10 and above

  from the center of the ramp 31, there is provided a light emitting diode 34 '.

  The diode 34 'is powered, as are all the photodiodes 32, 33, by

  an electronic unit 40 which can be constituted by a set of cir-

  simple electronic cooking or by a programmed microprocessor, a

  known way, in order to determine, on the basis of signals received in

  of the measuring device 30, the position of the recessed corner H with respect to a reference line or zero L, and for generating a control signal of a position adjusting means displacing the saw blade 12 into the direction S to the same spacing with respect to the reference line L The position adjusting device is constituted by said double-acting cylinder and piston mechanism 16, connected to a

  fluid under pressure from a source, not shown, through

  a conduit 17a and a distributor 17, the latter being itself

  even controlled by the electronic unit 40 in the manner already mentioned.

  In practice, several cylinder and piston mechanisms, stacked one above the other, can be used advantageously. For example, four cylinder and piston mechanisms having respective races of

  0.5, 1, 2 and 4 cm, allow to position the saw blade 12 per

  0.5 cm in an area between 0 and 7.5 cm.

  A right stop or a stationary stop ramp 19 is mounted

  in an area adjacent to the conveyor 10 The side of the ramp which is

  to the conveyor is oriented parallel to the direction of advance-R and is placed exactly on the reference line L At least one end position adjusting roller 18 is mounted in a known manner below the conveyor 10 and it comes into contact with a panel passing over it so as to move it in the T direction until the panel

  stopped by the extreme stop ramp 19.

  Another measuring ramp 31 "(interrupted extracts) can replace the ramp 31 and has the same basic structure as the ramp 31, although the elements 31, 32, and 34 'have not been shown to clarify the drawing The ramp 31 "is however considerably longer than the ramp 31 and its function will be described below. The stop 19 may preferably be made retractable so that the operator can, in the conventional manner which has been described above, conditioning panels in which, due to defects, a large part must be removed by cutting (the contact of the roller 18 with such a panel being interrupted) Alternatively, in the device of Figure 2 which will be described later, the operator can intervene in a gap provided for this purpose, in the direction of travel, between the adjustable stop 19 'and the saw blade 12 (stationary in this embodiment) and none

  stop needs to be retractable.

  The set described above works as follows:

  panels 20, 21, etc. are conveyed on the conveyor 10 in the di-

  R, usually in an arbitrary longitudinal position (ie

  say according to the distance n 'existing between the recessed corner H and a side edge of the conveyor 10 which may coincide with said reference line or zero L), but, taking into account the carrier elements 11, which are always oriented perpendicular to the direction of advance R Before a panel reaches the measuring device 30, it is moved in the direction T by one or more rollers 18, of known structure, and this roll, coming into contact with the panel from below, the incline so that its projecting corner G comes into contact with the right stop

  stationary (rule) 19 (which corresponds to what is called an "alignment

  "end panel"). All panels are, therefore, driven

  the measuring ramp 31 in a position where their projecting corners G make a zero spacing with respect to the reference line L The distance between the recessed corner H and the reference line L is measured in the

  measuring device 30 in a manner to be explained later.

  This distance is equal to the distance that the saw blade 12, or more precisely the sawing plane of this blade, must be spaced from the reference line L so that the cross section made at the base end passes through the recessed corner H, that is to say is situated in the plane Z A corresponding control signal of the device 16, 17 is generated in the electronic unit 40 and the necessary displacement of the motor 13 on the guides 15a 15 b, or alternatively only of the saw blade 12, is carried out. In the example shown, the displacement is equal to the distance N between the corners H and G (since it has been assumed that the position initial saw blade 12 was located on the reference line or zero L) Accordingly, in Figure 1, the blade of

  saw 12 is shown in a position which it occupies, as a rule

  only after the panel 21 has passed the measuring device 30.

  FIG. 2 shows a modified embodiment in FIG.

  wherein the motor 13 and the saw blade 12 are mounted stationary.

  The measuring device 30 'is essentially the same, but it comprises a plurality of light emitting diodes 34 and a light emitting diode

  additional 33 'on the ramp 31' and a single photodiode 32 'on

  above the ramp The measurement is carried out in the same way as for fig 1,

  after the panel 21 has been applied by its protruding corner G against the door

  stationary tee 19 under the action of the roller 18 (or similar) There is provided behind this stationary stop 19 a short movable stop 19 '

  which is attached to the piston rod 16 'has a cylinder and piston mechanism

  your 16 'double acting pressurized fluid The signal of com-

  Means from the electronic unit 40 control, through the distributor 17 'a, the assembly 16' so that the stop 19 'is placed at the distance n, determined in the measuring device 30', by relative to the stationary saw blade 12 The measuring ramp 31 'is provided with a series of light-emitting diodes 34 as well as an additional diode

  33 'placed in an advanced position, as indicated above A photo-

  diode 32 'is mounted above the conveyor 10 The dimensions and spacers

  may be the same as what has been specified with reference to the

figure 1.

  FIG. 2 shows, in addition, by lines in broken lines, another position 31 'a of the measuring ramp 31' as will be explained in more detail in the following. However, it should be noted that the embodiment comprising a displaceable saw blade according to FIG. 1 can be provided with an inclined measurement ramp and / or equipped with light emitting diodes and that the embodiment of FIG. 2 can comprise a ramp of FIG. measurement (oriented at right angles or

  oblique) equipped with photodiodes or photocells.

2 530175

  A third and a fourth embodiment correspond to FIGS. 1 and 2, with the difference that the stationary abutment-19, and by

  consequently the "final alignment" operation are eliminated.

  The panels are transported on the conveyor 10 in their transposed positions.

  The measuring ramp must in this case be considerably longer than the distance corresponding only to the marginal zone of the conveyor 10 and it must cover, for example, a device of the type shown in FIG. third of the total width (transverse dimension) of the conveyor, as indicated for the ramp 31 "on

  FIG. 1 The distance n 'between the recessed corner G and the reference line

  in this rather hypothetical case is determined in the

  30, and in the electronic unit 40, in the same manner as

  above and the saw blade 12 is moved the same distance from the reference line (in the embodiment similar to Figure 1) or preferably the short ruler 19 'is moved so that the plane Z coincides with the plane of the saw blade 12 (in the embodiment similar to FIG. 2). It should be noted that in the embodiment

  according to FIG. 2 and in the embodiment which has just been-

  described and which is analogous thereto, a driven roller 18 of "end alignment" is placed below the operational zone of the rule 19 '

  and moves the panel so that it comes up against the ruler.

  In principle, any arbitrary and suitable non-contact measuring device suitable for the intended application can be used.

  preferably, as mentioned above, either a group of

  diodes (or photocells) in combination with a diode emitting

  of light, ie a group of light-emitting diodes in combination with

  with a photodiode or a photocell, the elements

  Each unit is also determined by the electronic unit 40 In this unit, the position of the saw line (plane Z) is also determined.

  and the positioning of the saw blade 12 or the short

  An appropriate electronic unit for this purpose may be constituted, as indicated above, by several simple electronic circuits, or by a microprocessor such as, for example, that known under the

  commercial designation INTEL 8085.

  A measuring device comprising a single light-emitting diode

  34 'and several photodiodes or photocells 32, function

  according to Figure 3, as follows: The diode emits

  light channel 34 '(FIG. 1) placed above the measurement ramp 31 emits a light in a continuous manner When the additional photodiode 33,

  placed a little further upstream, is masked by a transported panel 20, the

  The photodiodes 32 can be divided into two groups: a group IA which is never lit and a group IIA which is lit for at least part of the time when the panel 20 passes in front of the delimitation between these two photodiodes 32. groups indicates the position of the recessed corner H When the position of the measuring ramp 31 with respect to the reference line L is known, the position of this corner is thus also established with respect to the reference line The analysis is stopped either when the additional photodiode 33 is again on, or so as to eliminate the influence of the small distance between the diode 33 and the row of the other diodes 32 (this distance can in practice be made considerably smaller than that which is indicated on the drawing to clarify it), when all the diodes 32, that is to say the

  two groups IA and IIA are lit simultaneously.

  It is obvious that no modification of this principle of measurement

  occurs when a measuring ramp 31 "(Figure 1) has a longer length than

  than that of the ramp 31 of Figure 3.

  A measuring device comprising a single photodiode or photocell, as shown in FIG. 2, operates in correspondence with FIG. 4, in the following manner. The light-emitting diodes 34

  on the measuring ramp 31 'are switched on and off sequentially.

  During an analysis, ie one after the other. The photodiode 32 '(FIG. 1) is analyzed each time a light emitting diode is analyzed.

  34 is lit The illumination of this photodiode 32 'indicates that at this moment

  a panel 20 does not cut the ray path between the emitting diode

  light beam 34 which has just been turned on and the photodiode 32 'When the photodiode 32' does not receive light, it means that the diode

  light emitter 34, which has just been switched on, is masked by the

20.

  The analysis begins when the light-emitting diode

  33 ', which emits light continuously, indicates that it is masked, a panel 20 is coming in and then all the light emitting diodes 34 are analyzed up to what

  the additional diode 33 'is no longer masked, which completes the analysis.

  Alternatively, a shutdown can be triggered when all the diodes emit

  light is masked, that is to say that the group IB, as long as a given panel passes in front of it, can never emit light towards

2501? 5

  the photodiode 32 'and that the group IIB can do it, at least during a

  part of the time, when said panel passes in front of it.

  An analysis movement is carried out quickly enough (appro-

  approximately 10 ms, or even in a shorter time), so that the position of the panel in the direction of the longitudinal axis x is determined approximately on each cm of its width (i.e. direction R), if the transport speed is approximately one meter

per second.

  The measuring ramp 31, 31 'is placed in such a close position.

  as far as possible below panel 20, passing in front of it, so as to minimize the parallax error

  distance transmitter-transmitter and the length of the ramp With the parameters

  very indicated above, that is to say 16 elements spaced from each other by 5 mm on the ramp and with the element 32 'or 43' placed at 50 cm above the ramp, the aforementioned error must be negligible.

  eliminate it completely if, in accordance with Figure 5, provision is made for a

  positive measurement 30 'having two parallel measuring ramps 31 "a, 31" b, one of which is placed close to the underside of the panel 20, while the other is placed at a height just above the conveyor 10 to allow the thickest measured object encountered in practice to pass in front of, for example 15 cm above the conveyor 10 on which the panels are transported Light guides 36a, 36b, for example 16 guides of light of this kind, are anchored by their end portions in each ramp 31 "a, 31" b, at intervals of 5 mm, each pair of light guides, whose end portions are placed opposite one of the the other in the two measuring ramps 31 "a, 31" b is connected to a separate photoelectronic switch, as indicated for the guides.

  36 "a, 36" b, which are connected to a switch 37 A photo-switch

  In principle, it is assumed that all the light guides 36a are connected to the light source in the respective switch and that all the light guides 36b are connected to the light source. are connected to the photocell The single upstream indicator element is

  defined by another pair of light guides ending in a po-

  The radiation from a first IC group of light guides 36a is stopped by the panel 20 passing in front of it while radiation from a second group 2C can reach the light guides unhindered. In a manner analogous to the measuring devices described above, the position of the recessed corner of the panel 20 is determined as the selvedge between the light guides which are never stressed when the panel passes and those which are subjected to less during part of the interval crossing Also, in analogy with the two other measuring devices described above, a pair of unrepresented light guides is placed a little upstream so

  starting the process of analysis Preferably, electrical switches

  neighboring tronics are designed to operate at different wavelengths

  different, so as to completely do without special logical commands

  or other means to prevent "crosstalk"

  appropriate electronic device is, for example, the switch sold in

  trade under the designation "Hemomatik" type FM-30-500-3 L SNO.

  The resolving power of the measuring devices is defined by the spacing of the indicator elements placed on the measuring ramps. If necessary, the resolving power can be increased by arranging the

  ramps of an angle between 0 and 900 in relation to the direction of

  R, as indicated by broken lines in the measuring ramp 31 "a in FIG. 2, and the electronic unit 40 is programmed for distance conversion in the direction of the longitudinal axis x the measuring ramps 31, 31 ', 31 ", 31' a, 31" b, are always turned towards one of the flat faces a, b, of the measured panel 20, 21, more exactly towards the inner flat face b in the examples represented. The individual elements 32 ' , 34 ', or the second parallel measurement ramp 31' a, are turned towards the opposite plane face of the panel, that is to say the face a in the

examples shown.

  The electronic unit 40 functions in principle in the same way in all the embodiments. Considering, by way of example, the embodiment described with reference to FIGS. 2 and 4, the unit block diagram of the unit electronic 40 may be in accordance with

  what has been indicated in Figure 6 As soon as the presence of a new

  the measuring zone has been indicated (by the light-emitting diode 33 '), the light-emitting diodes lying

  on ramp 31 'are lit and extinguished in succesive analyzes.

  If in an analysis more diodes are visible for the photodiode 32 '

  than during the previous analysis (which can only happen when

  that the extreme edge e is oblique), a function of

  which has zeroed at the beginning of the analysis is increased by

  corresponding to the number of light-emitting diodes which are newly visible When the indication indicates that the piece of wood 20 has left the measurement zone, said function is suppressed and a corresponding control signal of the distributor 17/17 'is generated, said function being reset and the unit 40 being ready to wait for a new piece of wood. Other variants are possible while remaining within the scope of the invention. An additional indicator element (photodiode, diode, light-emitting diode, pair of light guides) may for example be placed on the measurement ramps downstream of the light source. set of elements so as to produce a stop signal for measuring operations The additional element mentioned above, placed downstream, can also be used for a function check

  In addition, any other electromagnetic radiation may be used

  that, for example infrared light, instead of visible light, to avoid, in a known manner, disturbances caused by surrounding illumination The stationary stop 19 can also be placed

  at any other selected spacing point relative to the line of

fence L.

Claims (8)

REVENDI CATIONS   1. Method of transverse cutting of the base end part of   longitudinal pieces of wood, such as panels (20,21) and planes   having a leading end (t), a base end (r) and being transported in an arbitrary longitudinal position, in a direction   cross-section in front of a longitudinal measuring device (30) extending   versalement and not parallel to the direction of advance of the piece of wood, and then to a transverse saw blade   (12), said pieces of wood being movable in their longitudinal direction   tudinal (T) until their base ends rest against an abutment means (19, 19 '), characterized in that for pieces of wood having at the base end an inclined edge (e ) which is bounded by a recessed corner (H) and a projecting corner (G), the distance (n), considered in the longitudinal direction of the piece of wood between the   protruding corner and the recess corner, is measured in the measuring device   and signals are generated to be applied to an electronic unit (40) at the output of which a control signal which is applied to a displacement device (16, 17,   16 ', 17'), in that subsequently the piece of wood is moved longitudinally-   until the projecting corner comes into contact with a straight stop.   gne (19, 19 ') extending parallel to the feed direction (R) and is further conveyed to the transverse saw blade with the projecting wedge sliding along the stop, and in that said device of   displacement, controlled by said control signal, moves, in the direction   longitudinally of the piece of wood, at least a portion of upstream (19 ') of said stop, or said transverse saw blade, until the recessed corner is located dansél transverse cutting plane (U ) of the transverse saw blade so as to minimize the   fall when transverse cutting at the base end.   Method according to claim 1, characterized in that the piece of wood, before it reaches the measuring device, is moved longitudinally (T) from the head end to the base end until the protruding corner comes touching a rectilinear lying abutment and   stationary which extends parallel to the direction of transit   versal of the piece of wood, and possibly defining an upstream portion of said stopper, and in that the piece of wood is transported in front of the measuring device with the projecting wedge which slides along   of the stationary thrust mentioned last.   3. Machine for transverse cutting of elongated pieces of wood, such as panels (20, 21) and boards, having an extracted end of head (t) and a base end (r) and placed on a transverse conveyor (10) in an arbitrary longitudinal position so as to be transported in a transverse direction in front of a stationary and elongate measuring device (30, 31 ') extending transversely to said transport direction, and further to a screener device; e transverse (12 to 14), said machine further comprising a device (18) for longitudinally moving the piece of wood on the conveyor and for applying it continuously against at least one stop (19, 19 '), characterized in that for the cutting of the base end part of pieces of wood having, at their base ends, an inclined edge   (e) bounded by a recessed corner (H) and a protruding corner (G), the provision   The measuring device is arranged to measure the distance (n) in the longitudinal direction of the wood piece, between the protruding and recessed corners, and is electrically connected to an electrical unit (40) arranged to receive the signal signals. measuring and producing corresponding control signals, the stop being defined by a straight ruler extending parallel to the transport direction and positioned from   stationary in relation to the said direction, a moving device   motor-driven cement (16, 17, 16 ', 17') controllable by said control signals and providing for the displacement, in the longitudinal direction (T) of the piece of wood, of at least a portion (19 ') of the abutment, or of the transverse saw blade, until the recessed corner is situated in the transverse cutting plane (U) of the transverse saw blade.   4. Machine according to claim 3, characterized in that   elongated rectilinear abutment (19) extends parallel to the direction of a-   at least in part also upstream of the measuring device and   in that it is also stationary in the oriented direction   pendicular to the direction in advance.   5. Machine according to claim 4, characterized in that a rectilinear portion of a stop (19 ') extending parallel to the direction of advance is disposed between said stationary stop (19) and the saw blade, in it defines said movable part and in that it is displaceable by said displacement device perpendicular to the direction in advance. 253-175   6. Machine according to one of claims 4 or 5, characterized in   the measuring device comprises at least one rectilinear assembly formed of a plurality of measuring elements (32, 34) placed on one side of the measured piece of wood and at least one additional cooperating element (32 '). , 34 ') placed on the opposite side. 7. Machine according to claim 6, characterized in that the element or elements placed on one side of the piece of wood are constituted by   photodiodes and, on the other hand, light-emitting diodes.   8. Machine according to one of claims 6 or 7, characterized in that   that the measuring device comprises two measuring ramps (31 "a, 31" b)   parallel to each other and placed on opposite sides of a piece of wood passing in front, a plurality of light guides (36b) being arranged so that their end portions are placed opposite one another in two measuring ramps , and each pair of light guides, the end portions of which are placed facing one another in two measuring ramps,   being connected to a separate switch forming part of a plurality of photoelectronic mutators.