US3532138A

US3532138A - Arrangement for the optical monitoring of weft threads

Info

Publication number: US3532138A
Application number: US756847A
Authority: US
Inventors: Rudolf Schlappi
Original assignee: Maschinenfabrik Rueti AG
Current assignee: Ruti Machinery Works Ltd; Maschinenfabrik Rueti AG
Priority date: 1967-09-22
Filing date: 1968-09-03
Publication date: 1970-10-06
Anticipated expiration: 1987-10-06
Also published as: DE1785393B1; GB1231886A; ES358381A1; CH460664A; BE720826A; AT277898B; FR1581302A

Description

United States Patent Inventor RudolfSchlappi,

Ruti, Zurich; Switzerland Appl. No. 756,847 Filed Sept. 3, 1968 Patented Oct. 6, 1970 Assignee Ruli Machinery Works Ltd., formerly Caspar Honegger Ruti, Zurich, Switzerland, a corporation of Switzerland Priority Sept. 22, 1967 Switzerland ARRANGEMENT FOR THE OPTICAL MONITORING OF WEFI THREADS 16 Claims, 5 Drawing Figs.

US. Cl 139/370,

139/394 Int. Cl 003d 51/34 Field of Search 139/125, 126, 336, 342, 370, 273.1; 340/259; 250/209, 220, 225

I 56] References Cited UNITED STATES PATENTS 3,139,911 7/1964 Breitmeier 139/370 3,373,773 3/1968 Ballentine et: al 139/336 3,440,634 4/1969 Maurmann et al 139/370 Primary Examiner- Henry S. Jaudon Altarneyx- Donald D. Benton and David T. Terry ABSTRACT: An arrangement for optical monitoring of weft threads in a loom wherein free ends of the weft threads project from the shed which comprises a light-sensitive cell, a light source (Le a light-emitting diode of semiconducting material) directing a light beam towards the light-sensitive cell, and means for causing the ends of the weft threads to execute a prescribed movement outside of the shed; the light beam being encountered by the thread ends during their prescribed movement whereby the light-sensitive cell monitors the presence of the thread ends.

Patented Oct. 6, 1970 I of 4 Sheet Sheet FIG. 5

ARRANGEMENT FOR THE OPTICAL MONITORING OF WEFI THREADS SPECIFICATION The present invention relates to an arrangement for the optical monitoringof weft threads in a loom, wherein ends of the weft threads project in a prescribed manner from the shed after they are inserted therein, and wherein a light source havinga beam directed towards a light-sensitive cell is provided to uniquely monitor the presence of the ends of the weft threads.

It is already known to monitor for the presence of weft threads by optical means. In such arrangements, the weft thread to be monitored passes between a light source and a light-sensitive cell. When the signal supplied by the cell indicates that a weft thread is not present, theloom is automatically stopped. These automatic weft thread monitors have'the advantage that they enable the looms to be run athigh speeds and ,do not involve any mechanical contact with the weft threads. Up to the present, however, the known optical weft thread monitors have had the disadvantage that the constant vibrations stemming from the loom have an adverse effect upon the light sources hitherto used. The vibrations also have the effect of causing the light beam to act with varying intensity on the light-sensitive cell, since the light source (heated coil) changes its position. Additionally, with the known arrangements, it has been necessary to provide diaphragms or lenses. If diaphragms are provided for producing the light beam, there then arises the problem of undesirable heatingup. The use of lenses results in an expensive construction and, moreover, in this case the thread to be sensed has, as far as possible, to be positioned at the focal point, i.e., if it is displaced from its nominal position, the loom may be stopped in error. If, for example, when inserting the weft thread by the single pick method it is desired to monitor the free ends of the threads, it is not possible to rely completely upon the free ends of the threads being correctly positionedv The known optical monitoring arrangements employing lenses would therefore hardly be applicable in this case.

The use of diaphragms and lenses in the known arrange ments also calls for a relatively large and therefore cumbersome construction. Additionally, the light sources employed in these cases use up a considerable amount of energy. Because of the cumbersome construction, the choice of location for carrying out the monitoring is limited with these arrangements, and the monitoring often cannot be carried out at the most advantageous place.

Advantageously, the present invention avoids these disadvantages. Thus, this invention contemplates an arrangement for optical monitoring of weft threads in a loom, characterized in that a light-emitting diode of semiconducting material is used as the light source for directing a light beam towards a light-sensitive cell, means are provided for causing the ends of the weft threads to execute a prescribed movement, and the position of the light beam is so selected that the beam is encountered by the ends of the threads when the threads execute their prescribed movements.

The special and unique advantages of this arrangement of monitoring resides in the fact that the weft thread is monitored at a particularly favorable point. If, for some reason, no thread is present, then there will also be no thread end projecting from the shed. Since the thread ends may encounter the light source for only a short period, and since (because of the occurrence of short ends), a small zone ofinfluence 0r actuation exists, the smallness of the light source is of considerable importance. Moreover, because the lightsensitive cell can also be kept small, the arrangement according to the invention can be made very small and it can be brought into the immediate vicinity of the thread ends. As regards order of size, the light source and the light-sensitive cell are approximated to the thread to be monitored, so that the relative sensitivity of the arrangement is increased.

In a particularly advantageous embodiment of the invention, the light source and the light-sensitive cell are disposed in a device or means for sucking in the free ends of the threads.

The invention will now be described in more detail by reference to certain embodiments and to the accompanying drawings in which:

FIG. 1- is a schematic plan view of an apparatus fon inserting weft threads ona loom and the monitoring arrangement of this invention;

FIG. 2 shows an optical means of the monitoringarrangernent fitted in a suction device which. results in aparticularly favorable construction of the invention;

FIG. 3 illustrates another embodiment'of an arrangement of theoptical means relative to the suctionvdevice of theinventron;

FIG. 4 shows an electrical circuit of the monitoring arrangementutilizing-the signals produced'by the optiCaLmeans; and

FIG. 5. is a perspective view further showing; the optical monitoring means of FIG. 2 supported. on the suction:device on the.loom.

In all the figures. like reference numerals indicate like elements.

FIG. I shows schematically an embodiment of aweaving apparatus.(and its method-of operation) in which the present invention is used. The reference numerals 1 1 and 12 indicate weft thread feed bobbins positioned-on each side of the loom frame. Threads 13 and 1-4, respectively, run from these bobbins through the

rings

15 and 16, respectively, fixed to the loom frame, the thread-braking devices Hand 18, respectively, and further to

thread guides

19 and 20, respectively. In FIG. 1, the thread 13 ends within the suction nozzle 21, which serves as athread-retaining means, and. thread 14 is drawn away through a corresponding suction nozzle 22, also functioning as a thread-retaining means. The suction nozzles 21 and 22' are secured to the frame of the loom. The thread l4.extends through the shed formed by the warp threads 23. For the sake of clarity, of the entire group of warp threads, only the threads 23 (at the sides) are shown. The free end of thread 14 is held by the suction device or means 25. Another suction device or means (corresponding to the latter and located on the left-hand side of the loom) is indicated by the reference numeral 26. The

suction devices

25 and 26 are disposed on the movable sley and are movable relatively thereto in a vertical direction.

On the sley 30 (of which the front or cloth side limit is not shown in the drawing) is located the shuttle 31, which serves as a pick-inserting member or means. On its bottom running face, the shuttle has a portion 35 of ferromagnetic material, e.g. iron, which is sunk into the face. The shuttle also has two

horizontal openings

43 and 44 extending through it. These narrow towards the center of the shuttle 31 for the purpose of guiding threads drawn through the

openings

43 or 44 towards an appropriately narrowed zone. Fitted in the narrowed zones are thread-clamping means which are able to retain a thread located in that zone with a predetermined force; it being possible to withdraw the thread from the shuttle 31, i.e., this predetermined force is smaller than the breaking strength of the weft threads (threads 13 and 14) and is such that the weft threads are withdrawn by the application of a breaking force.

The reed 32 is secured to the sley 30. Also provided in the sley 30 are the magnetic

brake control systems

33 and 34, the electrical characteristics of which can be altered by means of a ferromagnetic material. In the present embodiment, each of the

systems

33 and 34 comprises a permanent magnet surrounded by a wire coil (not shown in FIG. 1). Each magnet is U-shaped, ile., a horseshoe magnet, and is fitted into the sley in such a way that its poles point upwards (i.e., the magnet forms an upright U). When the pick-inserting member 31 moves over the sley 30, the iron portion 35 of the shuttle 31 thus moves over the

magnetic systems

33 and 34. This movement produces voltage variations in the wire coils.

On each side of the loom there are also provided pullin devices or means 36 and 37 incorporating

hooks

41 and 42, respectively. These devices are mounted on the shafts 38 and 39, carried by the sley 30, and can be swung about these shafts and serve to pull the

weft threads

13 and 14, respectively, into the shuttle 31. The

numerals

52 and 40 indicate two shears for cutting off the

weft threads

13 and 14, respectively. The thread-braking devices 'or means 17 and 18 incorporate the thread-clamping means 46 and 47, which are actuable by means of the

controllable magnets

48 and 49, respectively. These magnets, as hereinafter described, are controlled by the

magnetic systems

33 and 34.

In the working phase of the loom that is illustrated in FIG. 1, the shuttle 31 has just moved from the left to the right-hand side of the machine. The thread 14, inserted during its flight, is held by the closed clamp 47 of the braking device; the magnet 49 associated therewith having been appropriately actuated. Due to the thread 14 being clamped by the clamp 47, the thread-end held by the shuttle 31 has been drawn out of the shuttle 31.

As shown in FIG. 1, the extracted end of the thread is sucked up and retained by the suction device 25. On the righthand side the magnet 48 of the braking device 17 is energized to the extent that the clamp 46 is closed.

In the course of further operation of the loom, the thread 14 drawn into the shed is beaten up on the cloth beat-up and a change of the shed comprised of the warp threads 23 takes place. The

shears

52 and 40 cut the weft thread 14 at both edges of the cloth. The end thereby released at the left-hand side is sucked into the retaining nozzle 22. The short thread end hanging in the suction device 25 is sucked up by this device. Furthermore, in the foremost position of the sley 30, the pullin device 36 rotates in the anticlockwise direction (as seen in FIG. 1) until its hook 41 extends through the opening 44 running horizontally through the shuttle 31. This movement is produced by the roller 50 which is caused to move over the cammed surface Sl'by the movement of the sley. The hook 51 thereby seizes the thread 13 offered by the movable thread guide 19 and, since the clamp 46 is closed, pulls it out of the thread retaining means 21 and through the opening 44 of the shuttle 31. The movement of the pullin device 37 (which takes place at the same time) has no effect on thread 14 since the thread is not passed to it via the thread guide 20.

Due to the shuttle 31 being shot to the left immediately thereafter, the thread 13 is drawn along in the shuttle 31 and possibly a part of it is drawn through the shuttle, since the clamp 46 is still closed. On account of the thread brake or clamping means which is contained in the slot 44 and into which it is drawn, the thread is subjected to a predetermined tensile force. Shortly after the shuttle 31 has been shot, the clamp 46 is opened by the magnet 48 being appropriately actuated. During its flight, the shuttle 31 draws the weft thread 13 from the feed bobbin 11 through the shed. At the instant at which the ferromagnetic plate 35 in the shuttle 31 moves over the electrical magnetic system 34, a voltage impulse is produced in the latter. This impulse or signal actuates the magnet 48 by means of a circuit (not illustrated) so that the clamp 46 is closed and the weft thread 13 is retained thereby. The thread 13 is thus drawn out ofthe shuttle 31 and leaves it at the front side of the shuttle, Le. the side facing the edge of the cloth. The suction device 26, secured to the sley 30 and movable backwards and forwards relatively thereto, has meanwhile moved backwards relatively to the sley 30, [.e. towards the path of the shuttle. It takes over the end of the thread 13 which emerges from the shuttle flying past, so that the thread is tensioned in the shed between the clamp 46 and the suction device 26.

For monitoring the insertion of the weft threads, i.e. in order to ascertain whether a weft thread has been fully inserted with each passage of the shuttle 31, an arrangement for optical monitoring which includes a light source and a light-sensitive cell is provided in each of the

suction devices

25 and 26. FIG. 2 shows the detailed construction of such a suction device. This figure illustrates an elevational view as seen in the direction of the orifice in the suction device. The suction nozzle 66 is made of a suitable plastic material. It is held by the

metallic retaining elements

81, 82. The

sidewalls

54, 55 are attached to the retaining elements by means of screws 53. The

small plates

58 and 59 are retained by the

sidewalls

54 and 55 and

appropriate rings

56, 57. The plate 58 serves as a base or attachment for the light-sensitive cell 60 and the plate 59 acts as a base or attachment for the light source 61.

Openings

62 and 63 are provided in the suction nozzle 66 to enable the light beam 64 (shown in broken lines) from the light source 61 to reach the cell 60, which in the embodiment shown is a photoelectric cell.

A light-emitting diode of semiconducting material is used as the light source 61. Such light sources can be considered as the converse of cells in which current is produced by the effect of light. In these cells, a voltage difference occurs due to the absorption of light at a PN junction. In the case of the lightemitting diode, the PN junction shines when voltage is applied. The emission of light is achieved by the reconstruction of cavities with electrons. Its excess energy is thereby given off in the form of light radiation. The amount of excess energy is typical for each semiconductor material and thus determines the wave length of the radiation. The light radiation can, if this is desired, be concentrated by means of a small lens.

The electrical connection with the

photoactive elements

60 and 61 is carried out in such manner that the sidefaces of each

small plate

58 and 59, made of insulating material, carry the appropriate electrical conductors. These conductors can be applied to the sidefaces as a printed circuit.

The suction device or means 25 will now be specifically considered with reference to the operation of the loom. When the shuttle 31, traveling from left to right, leaves the shed at the right-hand side, the suction device 25 and its nozzle 66 are located in the immediate vicinity of the shuttle 31, flying past. At the instant at which the opening 43 in the shuttle 31 is approximately or precisely at the level of the suction device 25, the outermost end of the thread 14 is drawn out of the shuttle 31 by the clamp 47 that just previously has closed. At this moment the free end of the thread 14 is taken over by the suction device 25. This phase in the process of thread takeover is illustrated in FIG. 2. It can be seen that when the thread is taken over, the end of the thread 14 is sucked into the interior of the suction device 25. The end thereby passes across the light beam 64. The change in the intensity of the light striking the light-sensitive cell 60, caused thereby, is registered by the cell. The resulting electrical signal, supplied by the cell 60, is passed to the circuit illustrated in FIG. 4 and causes no stoppage of the loom to take place.

In the embodiment shown in FIGS. 2 and 5 the suction nozzle 66 has an opening of elongate form. The longer sides of the suction nozzle 66 lie parallel with the path of movement of the shuttle 31 that inserts the

weft threads

13 and 14 into the shed. The light-sensitive cell 60 and the light source 61 are disposed, the one opposite the other, on the long sides of the nozzle 66.

FIG. 3 shows another embodiment, which differs from that of FIG. 2, in that the light-sensitive cell 60 and the light source 61 are fitted at other points in the suction nozzle 66. FIG. 3 again illustrates a view seen in the direction of the orifice in the suction nozzle. Again assuming that it is the suction device 25 that is being considered, then in this view the end of the thread 14 (when sucked in) is applied against the right-hand side of the nozzle 66 as indicated by the thread portion 14 seen in this drawing. In this embodiment it is important that the side of the nozzle 66, which faces the middle of the sley 30, should incorporate a convex curvature directed towards this middle area of the loom, since the end of the thread 14 is always moved by the suction action on the outermost zone of this curvature (or the zone thereof nearest the middle area). When the

photoactive elements

60, 61 are then so arranged that the light beam 64 is also directed onto this outermost zone, then it can be guaranteed that the thread end is reliably registered.

The use of a diode of semiconducting material as the light source, offers the advantage that a fine, concentrated light beam can be formed and that the arrangement of the photoactive elements is very compact and is, advantageously, extremely insensitive to vibrations. By providing the fine beam the changes in light caused by the thread are large compared with the effect. of the light beam 64 on the photoelectric cell 60. These changes are such that even when the thread flicks past extremely rapidly, its movement suffices to give a clearly discernible signal. The fitting of the photoactive elements in the suction devices and 26 also offers the advantage that there is practically no danger of the equipment becoming covered with dust. The particles of dust are subjected to continuous suction and therefore have no opportunity for settling.

It will be appreciated, however, that the photoactive elements need not necessarily be fitted inside the suction device. These elements can be positioned at any other place in which the light beam directed by the light source onto the photoelectric cell is encountered by the piece of thread (to be monitored) that makes the prescribed movement. As shown in FIG. 5, the light-sensitive cell 60 and the light source 61 are mounted on the end of the nozzle 66 which acts as a suction device for drawing the thread past the cell 60. The nozzle 66 is, as heretofore described, secured to the sley 30 andis mova ble backwards and forward relative thereto.

FIG. 4 shows an embodiment of a circuit in which the signals for controlling the loom (Le. those supplied by the photoelectric cells 60 of the suction devices 25 and 26) are used. The photoelectric cells 60 of the

suction devices

25 and 26 are connected to the common input of the amplifier 70. The signals passed to the

magnets

48 and 49 for closing the

clamps

46 and 47, respectively, are also used in the circuit of FIG. 4. These signals are passed to the common input point of the timer member or means 71. When an impulse reaches the input of the timer member 71, this member supplies an impulse 79 of prescribed duration at its output. In regard to the monitoring of the lengths of the free ends of the threads (hereinafter described in greater detail), this timer member 71 can be so constricted, that the duration of the impulse 79 produced by it can be varied or accurately adjusted to prescribed values. The output of the amplifier 70 and of the timer member 71 are passed to the AND circuit 72. The output of this circuit 72 is connected to one input of each of the flip-

flop circuits

73 and 74. Starting signals are supplied by the starting

impulse transmitters

75 and 76 to the other two inputs ofthe flip-

flop circuits

73 and 74. The impulses supplied to the flip-flop 73 by the starting impulse transmitter 75 coincide roughly, as regards timing, with the start of the movement of the shuttle on the left-hand side of the loom, and the impulses supplied by the starting impulse transmitter 76 to the flip-flop 74 coincide with the start of the movement of the shuttle on the right-hand side of the loom. An impulse sent by the starting

impulse transmitters

75 or 76 to the flip-

flop circuits

73 or 74 respectively produces a positive signal at the output of the flip-flop circuit to which this impulse was sent. lmpulses supplied by the AND circuit 72 on the other hand shift the outputs of these flip-

flop circuits

73 and 74 to zero voltage. lf the voltage at the outputs of the

circuits

73 and 74 is already zero, it will not be changed by impulses from the AND circuit 72. The outputs of the

circuits

73 and 74 are each connected to an input of the AND circuit 77. if these connections are under positive voltage at the same time, a signal occurs at the output of the AND circuit 77 which indicates that there is no weft thread present and which can therefore be used for stopping the loom.

When the circuit of FIG. 4 is set in operation, the voltages at the outputs of the flip-

flop circuits

73 and 74 are zero. When the shuttle 31 is shot from the right (as viewed in FIG. 1), the starting impulse transmitter 76 supplies an impulse, whereby positive voltage is applied to the output of the flip-flop 74. After the shuttle has passed the element 34 fitted in the shed 30, element 34 supplies a signal whereby the magnet 48 is actuated. With the signal for actuating the magnet 48, there is also formed a signal 80, which is passed to the timer member 71.,The timer member 71 therefore provides an impulse 79 of predetermined duration which is passed to the input of the AND circuit 72. The weft thread 13 is thereupon drawn out of the shuttle 31 arriving at the left-hand side of the loom and is taken over by the suction device 26. At this moment, the

photoelectric cell 60, contained in device 26, supplies a signal which is amplified in the amplifier 7'0 and is then passed, likewise as an impulse 78, to the AND circuit 72. The duration of the impulse 79 from the timer member 71 is such that, when the loom is operating correctly, the impulse 78 from the amplifier 70 coincides with the impulse 79 as regards timing. Thus, the AND circuit 72 supplies a signal whereby the output of theflip-flop 74 is again brought to zero voltage. The output of the flip-flop 73 remains at zero voltage during this operatron.

1f the shuttle 31 is now shot from left to right, a cycle of events corresponding precisely to that just described takes place. The differences are that the starting signal is now supplied by the starting impulse transmitter 75 that switches the flip-flop 73 to produce a positive output signal and, simultaneously with the signal for actuating the magnet 49, a signal 81 is produced and passed to the timer member 71. The end of the inserted weft thread 14 causes a signal to occur in the photoelectric cell 60 of the suction device 25. This signal is supplied as an impulse 78 to the AND circuit 72. This cycle of events is not described in detail since it is completely analogous with that already describedwith reference to device 26.

Let it now be assumed, however, that when the shuttle is again shot from right to left, no thread end is sucked into the nozzle 26. Under these conditions the photoelectric cell 60 of the suction device 26 does not supply an impulse. No impulse therefore occurs at the output of the amplifier 70 and in turn at the output of the AND circuit 72. The flip-flop 74 in which a positive voltage has been produced at the output by the starting signal from the starting impulse transmitter 76 will no longer be switched back because no'impulse now enters from the AND circuit 72. The positive voltage is therefore maintained at its output.

When the shuttle is again shot from left to right, first of all a signal is again supplied by the starting impulse transmitter 75. The signal switches the flip-flop 73 so that a positive voltage occurs at its output. At this moment, the AND circuit 77 becomes active, however, and supplies at its output a signal which can be used for stopping the loom.

It can be seen from the above operation that the signal 78 must occur before the end of the signal 79, since otherwise it will not be allowed through the AND circuit 72. The signal 78, on the other hand, is created by the thread end encountering the light beam 64 in the

suction devices

25 and 26, respectively. The circuit illustrated thus also enables the lengths of these thread ends to be monitored. This is based on the following:

When a weft thread, inserted by the shuttle 31 is too long, Le. longer than the rest of the threads, this overlong thread,

, when sliding out of the shuttle 31, is also released from the shuttle 31 somewhat later than the other threads. For this reason it is sucked into the suction nozzle 25 or 26 a brief instant'later. Again referring to FIG. 2, it can also be seen that the end of such a thread (which is longer than thread 14 shown in FIG. 2) projects by its end over the left-hand side of the nozzle 66. The sucking in of this thread end takes place more slowly than in the case of a shorter end portion, because the longer thread end is braked somewhat on the left-hand side of the nozzle 66. If the thread end is longer than the length of the opening in the nozzle 66, then it first becomes inwardly curved against the interior of the nozzle 66, and is only sucked in after some delay.

This somewhat longer period of time required for the registering of the thread can be measured in relation to the passage of shuttle 31, at a prescribed point along the path over which the shuttle is inserted. Since actuation of the

magnets

48 and 49, respectively, always takes place when the shuttle 31 is at a very well defined point, the use of the signals for actuating the magnets 48 and 49 (which in themselves serve to control the release of the

weft threads

13 and 14 from the shuttle 31) is particularly suitable for measuring this longer period. In the embodiment illustrated, the :signals 79 are therefore produced in synchronism with the signals controlling the the case of thread ends that are too long, however, the impulse 78 will no longer coincide with the impulse 79, as regards timing, and no signal will occur at the output of the AND circuit 72, so that the output of the corresponding flip-flop circuit (e.g. 74) when the shuttle is moving from the right, is not returned to zero voltage. The loom is thus stopped at the beginning of the next picking of the shuttle.

It can be seen from FIG. 1 that the sucking in of the ends of the weft threads by the

devices

25 and 26 coincides more or less precisely, as regards timing, with the travel of the shuttle 31 past the

suction devices

25 and 26. In other words, this travel takes place in a period of time which coincides approximately with the presence of the impulse 79 shown in FIG. 4. It has now been found that the shuttle 31 can acquire an electrical charge, discharge of which occurs by way of the

suction device

25 or 26. When nozzles 66 made of insulating material were used as suction devices, these discharges produced interference signals which simulated the registering of the thread. In these circumstances, the loom was not stopped either when the weft threads were too short or when the threads were not present.

This defective operation can be remedied by electrostatically screening the nozzle by an electrically conducting surface. This screening can be achieved, for example, by surrounding the nozzle on the outside with a metallic layer made of an electrically conducting material, or, as shown in FIG. 2 by housing the nozzle 66 in

metal elements

81, 82, 54 and 55, which completely surround it.

In a further embodiment of the invention, use has been made of the fact that these interference signals are steeper than the signals produced in the photoelectric cell 60 by the end of the threads. To this end, an electrical filter is incorporated in the input of the amplifier 70. The filter suppresses signals that are steeper than the signals produced by the

weft threads

13 and 14 in the photoelectric cell 60; these signals, however, being able to pass through the filter.

I claim: ,g

I. In a loom, an arrangement for the optical monitoring of weft threads, each having at least one free end wherein upon insertion of each weft thread into the successive sheds, the free ends thereof project from the sheds, said monitoring arrangement comprising a light-sensitive cell, a light source having a light beam directed on said light-sensitive cell, means operativcly associated with said cell for indicating the intensity of the light striking said cell and thread takeover means for causing the free ends of the weft threads projecting from the sheds to execute a prescribed movement; said light source comprising a light-emitting diode of semiconducting material and being so positioned with respect to said thread takeover means that during the prescribed movement of the ends of said weft threads the light beam is encountered by the thread ends and the intensity of the beam is changed, whereby the light-sensitive cell via the means associated therewith, monitors the presence of each of the thread ends projecting from the shed by the change of intensity of the light beam or the absence of the thread ends by the constant intensity of said light beam.

2. The arrangement of claim I wherein said loom includes a weft inserting shuttle and in which the thread takeover means for causing the free ends of the weft to execute said prescribed movement are suction means for sucking the free ends therein.

3. The arrangement of claim 2 in which the light source is arranged so that the light beam passes through the interior of the suction means.

4. The arrangement of claim 3 in which the suction means comprises a suction nozzle having a suction orifice of elongated form, the longer sides of which lie parallel with the path of movement of the shuttle for inserting the weft threads into the successive sheds and the light source bein positioned so that the light beam is directed transversely of t e longitudinal sides of said orifice.

5. The arrangement of claim 3 in which the suction means comprises a suction nozzle which has a side portion with a curvature directed towards the middle area of the sley and the light beam is directed substantially towards the center of that curvature.

6. The arrangement of claim 3 in which the light source and the light-sensitive cell are secured to the suction means.

7. The arrangement of claim 6 in which the light source and the light-sensitive cell are oppositely positioned in relation to each other in the interior of said suction means.

8. The arrangement of claim 6 in which said suction means has a suction orifice and said orifice is of a length, in the direction in which the weft threads are inserted, that is approximately equal to the length of the free ends of the weft threads so that excessively long free thread ends remain suspended, at least for a short time, by being curved in against the interior of the suction'ori'fice whereby the operation of sucking said ends within the interior is delayed.

9. The arrangement of claim 6 in which the suction means incorporates an electrically conductive surface which acts as an electrical screen.

10. The arrangement of claim 9 in which the suction means comprises a suction nozzle made of plastic material, said nozzlc being held by metallic retaining means which completely surround the nozzle.

11. The arrangement of claim 10 in which carrier plates are incorporated in the retaining means, said plates serving as supports for the light source and the light-sensitive cell.

12. The arrangement of claim 11 in which said light source and said light-sensitive cell have electrical connections which are printed on said carrier plates.

13. The arrangement of claim 1 in which the light beam from the light source takes the form of an at least roughly focused, cylindrical beam.

14. The arrangement of claim 1 in which the means operatively associated with said light-sensitive cell comprises an electrical circuit means, said light-sensitive cell being con nected to the input of said electrical circuit means and emitting signals thereto indicative of the presence of ends of the weft threads projecting from the shed within said light beam, the signals emitted by the light-sensitive cell being passed through an electrical filter which suppresses signals that are steeper than the signals produced in the light-sensitive cell by the presence of the ends of the weft threads.

15. The arrangement of claim 2 in which the means operatively associated with said light-sensitive cell comprises an I electrical circuit means, said light-sensitive cell emitting signals indicative of the presence of the ends of the weft threads in the light beam due to the change of intensity of said light beam, said electrical circuit means producing second signals that are in timed relationship with the signals produced by said light-sensitive cell and that are in response to the passage of the shuttle carrying the weft thread to be monitored at a prescribed point along the path of the shuttle along the sley of the loom, said electrical circuit means also producing an indication when the period between the signal from the light-sensitive cell and the second signal denotes that the free thread ends are excessively long.

16. The arrangement of claim 15 in which control means are provided for receiving said second signals, said control means controlling the release of the weft threads from the shuttle carrying the threads through the sheds.