GB2113404A - A method for detecting a thread failure in tufting machines - Google Patents

Abstract

The invention relates to a method for detecting a thread failure in tufting machines using thread-sensing elements 17,18 emitting electric signals corresponding to the thread tension. The periodic variation of the thread tension is used as a criterion for determining whether or not the thread is running correctly and the non- appearance of this periodic signal is regarded as indication of a failure, and further that the signals are compared at 33 with reference signals and an alarm 35 is released only when several defect signals are encountered in uninterrupted series. The reference signals are generated by pick-up devices during each cycle of operation. The signals corresponding to thread tension periodically change between maximum and minimum values. These may be counted to detect thread leakage. <IMAGE>

Description

SPECIFICATION A method for detecting a thread failure in tufting machines The present invention relates to a method for detecting a thread failure in textile machines, in particular tufting machines, using thread-sensing devices emitting signals corresponding to the thread tension.

Thread-sensing devices emitting an electric signal when the thread tension exceeds or drops below a pre-determined value have been previously known in spooling machines and spinning frames. But in the case of spooling machines and spinning frames, the thread tension is quite exactly defined and maintained at relatively constant values which for instance when spooling cops vary within the limits of a few grams (centinewtons) only inspite of varying package diameters and, thus, varying yarn speeds.

The conditions existing in these known machines in which thread-sensing devices have been used heretofore, are not comparable to those encoun tered in tufting machines. First of all, the last mentioned machines have very often several hun dred threads arranged side by side in an extremely confined space so that there is absolutely no room for larger thread-sensing devices. Secondly, the thread tension varies within very broad limits in tufting machines, namely from a maximum value given when the needles penetrate the backing material to a minimum value given when the needles are in their uppermost position at the point of reversal of their movement. At this point, the thread tension may be reduced to zero.

In an effort to overcome the difficulties encoun tered in the use of thread-sensing devices in tufting machines, a device has been developed in which the thread is slightly but constantly blown against a light barrier. It is hoped that in the case of breakage the broken thread will be carried by this slight air current into the area of the light barrier and that the interruption of the light barrier will provide a useful pulse. These systems are however not reliable in operation. On the one hand, the thread tension may be so low that the yarn loop will interrupt the light barrier even when no breakage has occurred; on the other hand, the broken thread may be caught and entrained by neighbouring threads so that the light barrier will not be interrupted at all.This known solution shows clearly that, to solve the problems of tufting machines, it has been felt heretofore to depart a long way from the usual technical solutions known for thread-sensing devices.

Further, there has been known a method in which each ofthethreads lifts a U-shaped electrically conductive lamina covering a strip with 2 conducting paths which are short-circuited when any one of the laminas drops due to a thread breakage. This method provides the drawback that in the presence of a relatively low average thread tension the lamina tends to lower itself each time the minimum tension is reached so that finally a thread breakage may be signalled even though no breakage has occurred.

Moreover, it is a disadvantage of both methods described before that they give a general failure signal only without identifying the broken thread.

Now, it is the object of the present invention to develop a method for detecting a thread breakage which can be successfully used even under extreme conditions, for instance in tufting machines.

According to the invention, this problem is solved by an arrangement using thread-sensing element emitting electric signals corresponding to the thread tension in which the signals emitted by the threadsensing element are compared with reference signals given during at least one machine-cycle period, and that a defect signal is released when the difference between the said signals differs from a given value. Accordingly, it is the principle of the invention to use the periodic variation of the thread tension as a criterion for determining whether or not the thread is running correctly, and tp regard the absence of such periodicity as indication of a failure.

So, when the signals emitted by the threadsensing element do not alternate between "high" and "low" level, but remain constantly at "high" or "low" level for one or more periods - a condition indicative of either a thread breakage or a blocked thread, depending on the arrangement of the error sensing device, a defect signal will be released. This measuring method provides a reliable defect indication even under extreme conditions as those existing in tufting machines, and the risk of false alarms is considerably lower than in the case of the known methods.This is to a large extent due to the fact that the periodic alternation of the thread tension and, thus, the periodic drop of the thread tension to zero or close to zero is used as a criterion for the defect indication and that the value of the amplitude affects the defect indication only when the amplitude does no longer exceed a given value which is determined by the noise level. The method of the invention offers, however, the advantage that it can be used even in cases where the values not indicative of defects vary within broad limits.

Accordingly, the method of the invention is suited for use in tufting machines and other machines where extreme conditions are to be observed and in which the thread tensions vary within extremely broad limits relative to their mean value, which is for instance also the case in sewing machines.

The method of the invention can be realized in different ways. For example, the reference signal corresponding to the machine cycle may also be a signal which after a certain number of machine cycles reads out a counter in which the number of maximum values and/or minimum values of the signals emitted by the thread-sensing device are counted. When the number of values read out after a number of machine cycles differs from a nominal value, there is no coincidence and a defect signal is emitted.

Another solution consists in that when the thread tension has reached its minimum value this minimum value is converted by a converter circuit for instance into a signal of a polarity opposite to the polarity of the signal emitted for the maximum value of the tension. Now, the minima and/or maxima obtained during a period of time defined by a number of machine cycles must be counted only to determine if there exists a thread failure. When a thread failure exists, a plurality of minimum values and/or a plurality of maximum values will be missing, a fact which is immediately discovered when the counter is read out.

In other embodiments of the method of the invention, however, the amplitude of the signals emitted by the thread-sensing elements is compared to a signal having an amplitude in excess ofthe noise level so that no differential signal will be emitted as long as the amplitude of the signal emitted by the thread-sensing element does not exceed, but remains below the said noise level.

Then, the non-appearance of the differential signals at moments when they should normally have appeared indicates the presence of a failure.

Thus, in this embodiment of the method of the invention the signals representing the thread tension are checked only to determine whether their level is "high" or "low". To this end, the signals generated are compared with reference signals, such as clock pulses characterized by the facts that the difference between their maximum and their minimum values is considerably lower than that of the signals produced at the thread-sensing element by intact threads but that the mean level of their range of variations is identical to that of the signals generated by intact threads.

In other words: the reference signals have the same frequency as the signals produced by intact threads, but a considerably smaller amplitude. A defect signal will be obtained only when the amplitude produced by the thread is below the amplitude of the reference signals, for instance equal to zero.

So, when the needles are in their lowermost position, i.e. stuck in the backing, the arrangement checks with the aid of reference pulses representing the condition "high" whether or not all threadsensing devices have emitted a pulse representing the conditions "high". When the needles are in their uppermost reversing position in which they have a certain distance to the backing, the signals emitted by the thread-sensing elements are compared with low level reference signals.

The constant amplitude reference signal may in this method be formed by the threshold value of an electronic circuit, for instance an electronic storage, or for example by the threshold value of a Schmitt trigger.

According to one embodiment of the method of the invention, an alarm is released only when several consecutive defect signals are encountered.

It is a main advantage of this embodiment of the method of the invention that due to this quasistatistical method false alarms are largely excluded.

The thread tensions vary within broad limits relative to their mean value. It is absolutely possible that when a loop is being formed and a needle is stuck in the backing the thread tension does not assume the expected high value, but remains below the same. This does not in any way affect the quality of tufting material produced. But when the expected tension is not reached say ten times in sequence, there is a relatively high probability that the thread has broken or that some other component is out of adjustment, in which case the alarm will be justified.

Although the probability of the existence of a thread failure is the greatest when several defect signals are encountered in series for the same thread-sensing element, it will in many cases suffice to determine only the defect-signal rate of the signal emitted by a thread-sensing element, because a series of successive defect signals will be obviously deteriorate the defect-signal rate as compared to that resulting from occasional variations of the maximum tension values that there is little probability that an alarm released due to the measurement of the defect rate should be a false alarm. If, for instance, a defect signal is obtained for a specified percentage of the cycles, for instance for 20 % of the operating cycles during any period, the probability that there may exist a thread failure is high enough to justify an alarm.The percentage to be pre-set for the release of a signal depends of course on the given sensitivity for emitting defect signals during a period.

Considering that a series of successive defect signals entail a steep rise of the defect rate, the probability of a false alarm will not even considerably rise when instead of checking each individual thread for the presence of a failure, i.e. instead of processing and checking the signals of each threadsensing element, all signals of the thread-sensing elements of the whole block are commonly processed.

This embodiment of the invention can be realized by very simple means. If for instance the maximum values and the minimum values are merely counted, an alarm will be released only when the difference between the number of extreme values counted for the entire block and the reference value determined by the number of machine cycles exceeds a predetermined value or a pre-determined percentage of the machine cycles.

The method of the invention may be carried out in that after having been formed, peferably via amplifiers or Schmitt triggers, the signals of several thread-sensing elements associated with threads leading to needles working at the same rhythm are supplied to the parallel data inputs of a shift register which is read out by a series of "shift" pulses, that the read-out pulse sequence is compared in a comparator circuit with reference signals and that the defect pulses of the same thread-sensing element are supplied to a storage which will release an alarm when a pre-settable number of defect pulses has been received by it in series.

This embodiment of the method of the invention requires particularly little space and cost, factors which, as has been mentioned before, are of particular importance in the case of tufting machines.

The object of the invention can be achieved also by a method characterized in that the A.C. component of the signals emitted by the thread-sensing elements is rectified, that the output voltage is compared with a constant reference voltage and that a defect signal is released when this D.C. voltage drops below the reference value. In this case again, an alarm may be released either by a single defect signal or only after a number of defect signals have been encountered in series. And again, this method may be used regardless of whether the signals associated with one thread are processed separately from those associated with other threads, or whether the signals of a whole group of threadsensing elements are connected to one rectifier.

Further, the present invention relates to devices for carrying out the method of the invention. As mentioned before, the absolute and mean thread tensions encountered in these tufting machines are extraordinarily low, and in addition the distance between the individual threads is very small so that there is only little room for the arrangement of devices intended to measure the thread tension. As a result, the deflection performed by the individual thread-sensing element in response to any variation of the thread tension must be extremely small to ensure that a thread-sensing element does not interfere with a neighbouring thread or threadsensing element.In addition, the deflection enabling the measurement must be very small so that the thread will not be required to work because with the thread tension equalling zero the thread would be deflected by a great distance and, with the thread tension rising to maximum it would have to accelerate the thread-sensing element into the opposite direction. If, however, the thread could not exert the accelerative force required, the elastic force of the thread-sensing element would increasingly retain the thread in a deflected position so that there would any way hardly remain any spring travel. This means, however, that inspite of the very small thread forces the thread-sensing element has only a very short spring travel at its disposal.

This problem is solved by a device for carrying out the method of the invention in that the end of the thread-sensing element opposite the thread-guiding eye is mounted on a base plate via a leaf spring and that the leaf spring is equipped with measuring elements for determining the degree of flexion of the leaf spring. It is the advantage of this arrangement that it solves the above-described problem in a perfect manner.

According to a further improvement of this embodiment of the invention, the whole thread-sensing advice, except for the leaf spring, is made from a material which is inflexible to the forces encountered. The advantage of this arrangement lies in the fact that the movement of the thread-sensing elements produced by the varying thread tension makes itself felt only at one point in the form of the flexion of the leaf spring. Due to this, the flexion produced even by a relatively small deflection of the thread-sensing device is still important enough to enable the usual measuring elements for determining the flexion of a leaf spring to generate pulses of an amplitude which enables them to be properly processed.

The solution to mount the thread-sensing element via a leaf spring offers the double advantage of being, on the one hand, simple and maintenancefree and, on the other hand, of requiring only little space.

The measuring elements for determining the degree of flexion of the leaf spring may take the form of piezoelectric crystals, resistance strain gauges or the like. The leaf springs may also be replaced by bar springs or other similar resilient elements permitting the thread-sensing element to move against their resilient force in the direction of the thread tension component acting upon the thread-sensing element.

In one embodiment of the device of the invention, the thread-sensing elements of the neighbouring threads are provided in staggered arrangement in two rows disposed one behind the other. This arrangement helps a little to save space because in tufting machines the threads and the needles are arranged very close together. In a further improvement of this embodiment of the invention, the foremost thread-sensing element may come to bear against a support mounted on the base when the thread tension rises to an excessively high value likely to endanger the measuring system.In this improvement of the invention, the thread-sensing elements disposed in the rows behind the said foremost thread-sensing elements are provided each with a projection which extends beyond the width of the thread-sensing element and comes to bear against the thread-sensing element arranged before it when the tension becomes excessively high. So, this row, too, is protected against excessive tension. As a result of this arrangement, the foremost thread-sensing device may perhaps no longer function properly, butthis is of no importance because it will emit an additional defect signal only when a thread-sensing element positioned behind it is any way required to signal a defect.On the other hand, however, it is an advantage of this design that a fixed support must be provided on the base only for the first row of thread-sensing elements and that for space reasons such fixed supports can be dispensed with for the other thread-sensing elements not arranged in the foremost row.

Further features and details of the invention will be apparent from the following description of certain embodiments of the invention and the claims when read with reference to the drawings in which Figure lisa diagrammatic representation of those parts of a tufting machine which are essential for the understanding of the invention; Figure 2 shows a thread-sensing element in enlarged scale; Figure 3 is a cross-section along line Ill-Ill in Figure 2, in smaller scale; Figure 4 is a block diagram; Figure 5a and 5b represent, jointly, one detailed diagram; Figure Gis a longitudinal section through a pulse generator; Figure 7is a schematic diagram of another embodiment of the invention;; Figure 8 is a modified form of the schematic diagram shown in Figure 7 Figure 9 is the schematic diagram of an arrangement in which the signals of a whole group of thread-sensing elements are commonly processed.

Figure 10 shows a circuit for replacing a piezoelectric crystal by a resistance strain gauges; Figure "shows a modified detail of the diagram of Figure 5; Figure 12 is a schematic diagram of another embodiment of the invention; and Figure 13 shows a numerical indication for connection to the circuit shown in Figure 5.

In the embodiment of the invention shown in the drawing, the tufting machine comprises a plurality of threads 1. Each of the said threads 1 is drawn off a spool 2 and guided to the needle 10 and through its eye via one or more tubes 3 in which an air current may or may not be produced, further transport rollers 4 and 5, a hole 6 in a perforated plate 7 and the hole 8 in a perforated plate 9. The needle 10 performs stiches into the backing 11 which consists for instance of a rubber or plastic-reinforced fabric.

When the needle penetrates into the backing 11 it entrains the thread and when it moves upward again the thread remains stuck in the hole formed by the needle, and forms a loop. When the needle 10 makes the next stich as the backing 11 advances, the thread forms the next loop.

Between the perforated sheets 7 and 9 there is provided a thread-sensing device 12 comprising an eye 13 which is passed by the thread and which deflects the thread a little from its normal path between the two holes 6 and 8. The eye 13 is moulded en bloc with a plastic body 14 which has a cross-section in the form of an oblong rectangle and its lower end fastened to a leaf spring 15 which is in turn fixed in the base plate 16.

Figure 2 shows two thread-sensing elements 17 and 18 of the thread-sensing device 12 which extends overthe full width of the needlebed. The base plate 16 is equipped with a fixed support 19 carrying a stop surface 20 against which the body 21 of the thread-sensing element 18 comes to bear when the thread 22 exerts an excessive tension upon the hook 24. To save space, the thread-sensing elements 17 and 18 are provided in staggered arrangement in two rows disposed one behind the other (Figue 3). The projection 25 moulded upon the body of the thread-sensing element 17 extends laterally beyond the body 14. Now, when an excessive tension is exerted upon the thread-sensing element 17, it does not bear against a support 19 fixed to the base plate 16, but its projection 25 comes to rest upon the neighbouring thread-sensing ele ment 18.This saves a lot of space which is anyway very limited at this point ofthetufting machine.

When the thread tension is zero, for instance when the needle occupies its uppermost reversing position, the leaf springs 15,23 are released and the voltage ouputofthe measuring elements 27,28 is zero.

When the needle 10 penetrates the backing 11, the thread 1 gets a little tensioned, but when the needle 10 returns to its raised position the tension is immediately relieved by the fact that the feed system supplies an additional length ofyarn. So, the thread-sensing elements 17 and 18 perform a recip rocating, if because of the extraordinary low tension of the thread only very small movement in the direction indicated by the double arrow 26. This movement is converted by piezoelectric crystals 27 and 28 bonded to the leaf springs 15 and 23 into electric pulses which are supplied via lines 29 and 30 and the Schmitt triggers 37 and 38 to the parallel data inputs of a shift register 31. The latter comprises a plurality of parallel data inputs the number of which conforms to the number of threads arranged in one row in the tufting machine.Consequently, when the needles stick in the backing, the pulses emitted by the piezoelectric crystals 27 and 28 all create in the individual storage positions of the shift register 31 a condition corresponding to a "high" pulse level. When the needles 10 are outside the backing 11, the storage positions of the shift register 31 all obtain a condition corresponding to the "low" pulse level. Instead of the piezoelectric crystals 27, 28, resistance strain gauges may be provided in other embodiments of the invention.

In the two extreme positions of the needles 10 a series of shift pulses is entered into the shift register 31 so that the conditions of the individual storage positions appear one after the other in the output line 32. When all threads are intact, the pulses sent into the output line 32 all correspond to the "high" level. However, when one of the threads 1, for instance thread 22, is broken, the line 30 applies to the corresponding storage position of the shift register the condition "low" instead of the condition "high". The corresponding pulse on line 32 which corresponds to the condition applied to input 30, then also corresponds to the condition "low", contrary to all the other pulses delivered from the shift register 31 during this cycle which all correspond to the condition "high".The pulse sequence on line 32 is then compared in a comparator circuit 33 with pulses generated in synchronism with the operating cycle of the machine. The comparator circuit emits a defect pulse when a reference pulse does not conform to the pulse supplied into the comparator circuit by line 33, i.e. when for instance the reference pulses indicate the condition "high" while a pulse on the line 32 received from the input 30 indicates the condition "low". The comparator circuit 33 is connected to a counter 34 which indicates which one of the pulses delivered in series on line 30 from the shift register 1 exhibits a deviating condition so that the thread whose condition differs from the expected condition can be identified.The counter 34 may be followed by a storage for storing the defect pulses and emitting an alarm, and possibly switching off the machine when instead of the expected condition "high" the condition "low" appears several times, for instance 10 times, in series.

In shift registers in which the individual storage positions can assume two conditions only, the storage position wiil always, regardless of the level of the pulses applied via lines 29 and 30, assume the "high" condition when the amplitude of the pulses exceeds the threshold value of the Schmitt triggers 37 and 38. By a suitable selection of this threshold value it can be ensured that the full range of thread tensions that do not yet signalize a thread breakage will generate the condition "high" which will then be compared in circuit 33 with the reference pulses.

The moment when the conditions "high" or "low" of the pulses generated by the thread-sensing elements are checked need not necessarily be the moment when the needle 10 occupies its upper or lower reversal point, but may also be shifted against the latter.

Figures 5a and 5b show, jointly, a complete circuit arrangement for carrying out the method of the invention. In this example a total of 96 piezoelectric crystals 27, 28 are provided for monitoring 96 threads. The said piezoelectric crystals are connected in groups of 8 with an integrated shift register type 4021 B. There are provided a total of 12 such shift registers of the type 4021 B, but for simplicity's sake not all of them are shown in the drawing. The input into the integrated shift register of the signals supplied by the piezoelectric crystals 27,28 is effected in parallel, the output in series.The rotary shaft of the tufting machine which performs one full revolution when the needles perform one complete upward and downward movement is coupled to a pulse generator - Figure 6 - having two pick-ups PU 1 and PU 2 (Figure 5) for generating during each complete revolution of the machine shaft two pulses shifted by 1800. The JKflip flop Q 3 serves to form from these pulses the reference signal which is compared by the integrated cuircuit 0 4 which comprises several NAND gates and is set up as an EXCLUSIVE OR circuit, with the signal delivered at any time by the connection of the uppermost register in Figure 5. While the described shaft of the machine rotates at a speed of about 10 to 20 revolutions per second, the data are delivered from the shift register 4021 B at a far higher frequency.

The shifting frequency generated by the integrated circuit 0 5 equals approx. 50 kHz. There are provided several integrated counters type 4518, namely the integrated circuits 09,010 and 011. Each of the said counters is connected via diodes to manually settable BCD code switches for presetting the maximum number to be counted. The counter comprising the integrated circuit Q 10 has been preset by means of the associated BCD switches so that an alarm signal will be generated when five defects have been counted. The counter 0 11 counts the shift pulses generated by the circuit Q 5 and has been preset by means of the associated BCD switches to the number of th reads existing in the machine.

The counter Q 9 ensures that the counter Q 10 will be reset to zero after a predetermined number of revolutions of the machine shaft. In the example shown, the counter will be reset after ten revolutions of the machine shaft. This ensures that defects which are rarely encountered and which do not affect the product produced by the tufting machine will not sum up in the counter 0 10 in the course of time and finally switch off the machine. It is thus ensured that an alarm will be generated, orthe machine will be switched off, only when defect signals are encountered in a pre-determined percentage of the working cycles.

Switching-off of the machine is effected by a positive voltage appearing at connection 3 of the integrated circuit Q 8, which positive voltage actuates the relay 50. At the same time, a voltage impulse appears at the output X1, 1 which can be used, together with the other ouputs X1,2 and X1,3 to identify by means of a digital display that thread which produced the last defect signal that led to the alarm signal.

All diodes shown in Figure 5 are of the type 1 N4148. The integrated circuits 01, 02, Q4, 06, Q7 and Q12 are of the type 74C00. The integrated circuit 03 is of the type 74C76. The integrated circuits 05, 013 and 014 are of the type SFC 2301. The voltage Vcc is 15 V. The piezoelectric crystals 27, 28 are of the ITT type MK129PPK22.127.6. All other data are apparent from the diagram shown in Figure 5.

The lines ending on the right-hand side in Figure 5a and continuing on the left-hand side of Figure 5b are identified by the same letters.

The electronic pulse generator 55 shown in Figure 6 comprises a shaft 56 coupled with the abovementioned machine shaft 57. A metal disk 58 coupled with the shaft 56 is provided with a hole 59 which in the view shown in Figure 6 is positioned exactly in front of one electric pulse pick-up 60 which is identical to pulse pick-up PU1 in Figure 5. When the disk 58 has performed a rotary movement by 1800, the hole 59 is exactly positioned in front of the pulse pick-ups 61 (PU2).

The electric signals from these pulse pick-ups 60 and 61 are supplied to the circuit shown in Figure 5.

The total height of the pulse pick-up shown in Figure 6 is 167 mm. Figure 6 is drawn in correct scale so that the other dimensions can be directly taken from the drawing. The pulse pick-ups 60 and 61 are of the type FGL 4/1.5-5, make Dr. E. Horn GmbH, D7036 Schönaich/West Germany.

While in the circuit shown in Figure 5 binary signals are formed from the signals supplied by the pulse pick-up and the piezoelectric crystals, which signals are then compared with each other, the arrangement shown in Figure 7 forms initially binary signals only from the pulses supplied by the pulse pick-ups 60 and 61. The signal generated by the piezoelectric crystal 27 is, however, applied to one input of a comparator 65 whose other input is supplied with the signal generated from the signals supplied by the pulse pick-ups 60 and 61. The comparator 65 is connected to an EXCLUSIVE OR gate 66 represented by a special symbol whose output signal is applied to an AND gate 67 which emits a defect signal when the output signal of the piezoelectric crystal 27 does not exceed or remains below certain threshold values determined by the voltage dividers 68, 69 and 70, 71.

Figure 8 shows a modification of Figure 7 in which the output signal of the piezoelectric crystal 27 is applied to the input of a Schmitt trigger 74 whose ouput is connected to the one input of the EXCLUStV OR gate 66 which has its other input connected as shown in Figure 7. Contrary to Figure 7, the arrangement shown in Figure 8 does not comprise a comparator 65, nor the associated voltage dividers.

The circuit shown in Figure 9 illustrates the common evaluation of a group of - in the example shown - 8 piezoelectric crystals of which three are shown in the drawing only for simplicity's sake.

When a single one of the piezoelectric crystals 27, 28 applies to its associated EXCLUSIVE OR gate a signal differing from the other signal applied to the said gate, a defect signal will be produced at the output 80 of the circuit.

Figure 10 shows an example for the use of resistance strain gauges instead of piezoelectric crystals. A resistance strain gauge 82 is arranged in a bridge circuit with resistors 83 to 85. Instead of a piezoelectric crystal, the resistance strain gauge is bonded to the leaf spring 15 of a thread-sensing element. Any deflection of the leaf spring 15 changes the resistance of the resistance strain gauge 82, and the voltage encountered at the bridge as a result of such change is amplified by an amplifier 86. In the arrangement shown in Figure 8, the output of the said amplifier 86 may be connected to the input of the Schmitt trigger 74. In the example shown in Figure 10, a resistance strain gauge of the type LY11-6/350, make Hottinger Baldwin, Darmstadt/ West-Germany, has been used.

Figure 11 shows a modification of Figure 5 in which the Schmitt trigger 90 of the type ELA4/1, make Dr. E. Horn GmbH, D-7036 Sch6naich/West- Germany (corresponds to hybrid module 18943918700, make Röderstein GmbH, D-8300 Landshut, West-Germany) is arranged between the output of a piezoelectric crystal and the associated input of the shift register 4021 B.

The thread-sensing elements 17 and 18 shown in Figure 2 comprise a body 14 or 21 made from the polycarbonate known under the name Makrolon S with a 20 % by weight proportion of glass fibres. The leaf springs 15 and 23 consist of brass. The total height visible in Figure 2, from the lower face of the base plate 16to the upper end of the plastic body 14 or 21 is 50 or 52 mm, respectively. The other dimensions can be taken from the drawing which is drawn in correct scale.

It appears from the diagrammatic representation in Figure 12 that the A.C. voltage content of the signals supplied by the piezoelectric crystal 27 is applied via a capacitor 90 to the input of an amplifier 91 whose output signal is rectified through a diode 92. The other connection of the diode 92 is connected to a resistor 93, a capacitor 94 and the input of a Schmitt trigger 95. In one embodiment of the invention, the time constant RxC is approximately equal to the period of one machine cycle. So, when the signal of the piezoelectric crystal 27 is not received during one single machine cycle, the threshold of the Schmitt trigger 95 is not reached and the defect signal will be generated. In another embodiment of the invention, the time constant RxC of the resistor 93 and the capacitor 94 is equal to n times, for instance 5 times, the period of the machine cycle. In this case, a defect signal will be generated only when the piezoelectric crystal 27 does not emit an output signal during five machine cycles.

Figure 13 shows a numerical indication which can be connected to the connections X1,1, X1,2 and X1,3 of the circuit shown in Figure 5 and which uses the integrated circuit 74C925. The integrated circuit 74C925 counts the shiftpulses and transfers the counter content into the indication storage when a defect is encountered.

The reference numbers in the claims do not in any way restrict the claims but serve merely the purpose to enhance understanding.

Claims (23)

1. A method of detecting a thread failure in textile machines with periodically varying thread tension, for example tufting machines, using threadsensing elements emitting signals corresponding to the thread tension, wherein the signals emitted by the thread-sensing elements are compared with reference signals given during at least one machinecycle period, and a defect signal is released when the difference between the said signals differs from a given value.

2. A method as claimed in claim 1, wherein the signals corresponding to the thread tension are checked substantially in the needle end positions for the presence of the levels "high" or "low" only.

3. A method as claimed in claim 1 or 2, wherein the reference signal consists of a signal of constant amplitude related to the machine-cycle period, whose amplitude exceeds the noise level.

4. A method as claimed in claim 3, wherein the reference signal of constant amplitude is formed by the threshold value of an electronic circuit.

5. A method as claimed in claim 4, wherein the threshold of a Schmitt trigger is used for the amplitude comparison.

6. A method as claimed in any one of the preceding claims, wherein an alarm is released only when several defect signals are received in uninterrupted series.

7. A method as claimed in any one of claims 1 to 5, wherein an alarm is released only when defect signals are encountered in a certain percentage of the working cycles.

8. A method as claimed in any one of claims 1 to 7, wherein the signals of each thread-sensing element are individually processed.

9. A method as claimed in any one of claims 1 to 7, wherein the signals of a whole group of threadsensing elements are jointly processed.

10. A method as claimed in claim 9, wherein the signals of several thread-sensing elements associated with threads leading to needles working at the same rhythm are supplied to the parallel data inputs of a shift register which is read out by a series of shift pulses, the read-out pulse sequence is compared in a comparator circuit with reference signals and the defect pulses of the same thread-sensing element are supplied to a storage which will release an alarm when a pre-settable number of defect pulses has been received by it in series.

11. A method as claimed in claim 10, wherein the signals may be formed by amplifiers or Schmitt triggers prior to being enetered in the parallel data inputs.

12. A method of detecting a thread failure in textile machines with periodically varying thread tensions, for example tufting machines, using thread-sensing devices emitting electric signals corresponding to the thread tension, wherein the A.C.

component of the signals emitted by the threadsensing elements is rectified, the output voltage is compared with a constant reference voltage and a defect signal is released when this D.C. voltage drops below the reference value.

13. A method as claimed in claim 12, wherein the differential voltage between the D.C. voltage and the reference value that will release the defect signal corresponds to the non-appearance of several A.C.

voltage periods.

14. Apparatus for detecting a thread failure in textile machines with periodically varying thread tension, wherein the ends of the thread-sensing elements opposite the eyes for guiding the threads are mounted on a base plate via leaf springs and each leaf spring is equipped with a measuring element for determining the degree of flexion of the leaf spring.

15. Apparatus as claimed in claim 14, wherein the thread-sensing element, with the exception of the leaf spring is made from a material which is inflexible to the forces encountered.

16. Apparatus as claimed in claim 14 or 15 wherein the measuring element consists of a piezoelectric crystal.

17. Apparatus as claimed in claim 14 or 15, wherein the measuring element consists of a resistance strain gauge.

18. Apparatus as claimed in any one of claims 14 to 17, wherein the thread-sensing elements are arranged in at least two rows provided one behind the other.

19. Apparatus as claimed in any one of claims 14 to 18, wherein the foremost thread-sensing element comes to bear against a fixed support when the thread tension rises to an excessively high value.

20. Apparatus as claimed in claim 19, wherein the thread-sensing element disposed in a row behind the said foremost row of thread-sensing elements is provided with a projection which comes to bear against the thread-sensing element arranged directly before it when the thread tension become excessively high.

21. Apparatus for detecting a thread failure in textile machines with periodically varying thread tension, for example tufting machines, using theadsensing elements emitting signals corresponding to the thread tension, wherein a device is provided which compares the signals emitted by the threadsensing element with reference signals given during at least one machine-cycle period, and which releases a defect signal when the difference between the said signals differs from a given value.

22. A method of detecting a thread failure in textile machines with periodically varying thread tension, substantially as herein described with reference to the accompanying drawings.

23. Apparatus for detecting a thread failure in textile machines with periodically varying thread tension, substantially as herein described, with reference to the accompanying drawings.