Electronic thread monitor for textile machines
United States Patent 3921877
Electronic thread monitor means for textile machines with a sensing element that is influenced directly or indirectly by the thread motion, for converting the mechanical input into electrical quantities which serve to actuate the shut-off device of the textile machine when these electrical quantities fall below a predetermined value, response value, for a predetermined time interval, shut-off threshold. The electrical quantities generated by the sensing element change the shut-off threshold inversely proportionally to the thread velocity.
US Patent References:
Yarn break detector and control circuit
King - July 1968 - 3391840
DEVICE FOR SENSING THREAD PASSAGE TO CONTROL MACHINE OPERATION
Rydborn - September 1972 - 3688958
Yarn break detector and control circuit
King - July 1968 - 3391840
DEVICE FOR SENSING THREAD PASSAGE TO CONTROL MACHINE OPERATION
Rydborn - September 1972 - 3688958
Application Number:
05/421782
Publication Date:
11/25/1975
Filing Date:
12/05/1973
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Export Citation:
Assignee:
W. Schlafhorst & Co. (Monchen-Gladbach, DT)
Primary Class:
Other Classes:
226/45
International Classes:
B65H63/06; B65H63/00; B65H25/32
Field of Search:
226/11,37,38,39,43,45
Primary Examiner:
Schacher, Richard A.
Attorney, Agent or Firm:
Lerner, Herbert L.
Claims:
I claim
1. In electronic thread monitor means for textile machines of the type having a sensing element influenced by the thread motion, means for converting the mechanical quantities into electrical quantities which serve to actuate the shut-off device of the textile machine if these electrical quantities exceed or fall below a predetermined response value during a predetermined time shut-off threshold interval; means responsive to the electrical quantities generated by the sensing element to change the shut-off threshold time interval inversely proportionally to the thread velocity.
2. Electronic thread monitor means according to claim 1, wherein the time constant, controlled by the generated electrical quantities, of an electric energy storage device, whose charge is a measure for the response value, serves as the shut-off threshold.
3. Electronic thread monitor means according to claim 2, wherein said generated electrical quantities cause a predetermined charging of an electric energy storage device, and also vary the time constant of the dischrage circuit connected to said energy storage device, inversely to the magnitude of the generated electrical quantity.
4. Electronic thread monitor means according to claim 3, wherein said energy storage device is a capacitor which is charged to a predetermined voltage by means of a voltage limiter, and whose discharge circuit comprises a controlled resistance, whose resistance can be controlled.
5. Electronic thread monitor means according to claim 4, wherein said controlled resistance is controlled by a capacitor, whose charging voltage is proportional to the input voltage.
1. In electronic thread monitor means for textile machines of the type having a sensing element influenced by the thread motion, means for converting the mechanical quantities into electrical quantities which serve to actuate the shut-off device of the textile machine if these electrical quantities exceed or fall below a predetermined response value during a predetermined time shut-off threshold interval; means responsive to the electrical quantities generated by the sensing element to change the shut-off threshold time interval inversely proportionally to the thread velocity.
2. Electronic thread monitor means according to claim 1, wherein the time constant, controlled by the generated electrical quantities, of an electric energy storage device, whose charge is a measure for the response value, serves as the shut-off threshold.
3. Electronic thread monitor means according to claim 2, wherein said generated electrical quantities cause a predetermined charging of an electric energy storage device, and also vary the time constant of the dischrage circuit connected to said energy storage device, inversely to the magnitude of the generated electrical quantity.
4. Electronic thread monitor means according to claim 3, wherein said energy storage device is a capacitor which is charged to a predetermined voltage by means of a voltage limiter, and whose discharge circuit comprises a controlled resistance, whose resistance can be controlled.
5. Electronic thread monitor means according to claim 4, wherein said controlled resistance is controlled by a capacitor, whose charging voltage is proportional to the input voltage.
Description:
The invention relates to an electronic thread monitor for textile machines with a sensing element which is influenced directly or indirectly by the thread motion, for converting the mechanical quantities into electrical quantities which serve for operating the shut-off device of the textile machine if these electrical quantities exceed or fall below a predetermined value, the so-called response value, for a predetermined time interval, the so-called shut-off threshold.
Electronic thread monitors for textile machines, i.e., winding machines, warping machines or cutting machines have sensing elements of the most varied kinds such as, for instance, optical, capacitive or piezoelectric types, which convert the mechanical quantities (diameter, roughness, velocity, tension, etc.) into electrical quantities (voltage, current, frequency). The response of an electronic thread monitor occurs, for instance, as a function of the presence of the thread, of the magnitude of the thread tension, or of the intensity or velocity of the thread motion. It is known that a response of the electronic thread monitor as a function of the thread motion results in the shortest possible shut-off time, as the thread motion changes its state immediately if the thread is interrupted. In the case of a break of the thread, for instance, the thread velocity drops very considerably, generally to zero. In some cases, a negative thread velocity can even occur due to the tension acting on the thread.
In those electronic thread monitors which are equipped with a sensing element that can be influenced by the thread motion, the shut-off device of the textile machine is operated if the measured quantity exceeds or falls below the response magnitude, i.e. a predetermined measurement value.
Particularly advantageous with respect to complexity and cost are electronic thread monitors which derive the control from the change of the condition of the thread during the motion. As the thread thickness, cross section, roughness or the like are not always constant, electrical quantities of different magnitude are generated when the thread moves through the sensing element. These electrical quantities of varying magnitude are the so-called "noise signal" of the running thread, the absence of which automatically indicates a broken thread, as the broken thread no longer runs and therefore also cannot generate a noise signal.
On the other hand, it happens that the mechanical property of the thread which is used for generating the noise signal, for instance, the diameter, is constant over short lengths of the thread. This means, however, that the noise signal may become zero then also, and this so-called "noise hole" simulates a broken thread.
Since, however, this noise hole is present only for a short time, one uses in electronic thread monitors of the kind described above, as an additional criterion for shutting off the textile machine, namely that time interval, during which the magnitude exceeded or fell below the response value. As soon as this time interval reaches a predetermined value, the so-called shut-off threshold, a thread break actually exists. However, most textile machines are run at different speeds for different threads, and the noise holes are longer, the lower the thread velocity. This leads to the need to choose the shut-off threshold relatively long, so that a stopping of the textile machine due to a noise hole alone is reliably avoided. This, however, has the result in high-speed textile machines that during the longer shut-off time, the ongoing part of the broken thread is removed that much farther from the stopped part.
On the other hand, it may happen in textile machines that a broken thread is taken along again by machine parts or adjacent threads immediately after the break and the thread motion is interrupted or reduced only briefly, so that too high a shut-off threshold does not lead to a shutdown of the machine in this case.
Particularly critical is the starting phase of the thread, i.e., the time until the thread has reached its normal velocity. During this phase the noise holes also become larger and can lead to a shut-down of the machine, if the shut-off threshold is set for normal operation, without the occurrence of the thread break. In order to avoid this low speed error, one may disconnect the electronic thread monitor during a certain starting phase, which, however, is accompanied by the disadvantage that a possible thread break during this starting phase can be detected only after the end of the disconnect time, which in turn leads to the disadvantages, already mentioned before, of a late machine shut-off.
It is a principal object of the invention to avoid the above disadvantages of known electronic thread monitors. According to the invention, the solution to this problem is that the electrical quantities generated by the sensing element serve to change the shut-off threshold interval inversely proportionally to the thread velocity. The proportionality need not be a linear function here, but may also follow an exponential function. It is essential that the shut-off threshold interval and therefore, the time during which the magnitude exceeds or falls below the response value, becomes smaller, the greater the thread velocity. At high thread velocities even a very brief interruption or reduction of the velocity leads to a shut-off of the textile machine, while at the low thread velocities the above-mentioned noise holes do not yet lead to crossing the shut-off threshold. The change of the shut-off threshold interval which is inversely proportional to the thread velocity, can be achieved on the one hand by providing that the electrical quantities generated by the sensing element are always inversely proportional to the thread velocity, but on the other hand also by bringing the electrical quantities which are generated proportional to the thread velocity, into a relationship of inverse proportionality to the shut-off threshold interval.
The time constant of an electric energy storage device can serve as shut-off threshold. For instance, of a capacitor which is controlled by the generated electrical quantities and whose adjustable charge, e.g., the charging voltage, is a measure for the response value. In this connection, the charging time constant as well as that for the discharge of the energy storage device can be used as the shut-off threshold. Similarly, a maximum charge as well as a minimum charge can be a measure for the response value.
In a particularly advantageous embodiment of the invention, the generated electrical quantities serve on the one hand for the predetermined charging of the electric energy storage device, and on the other hand, for varying the time constant of the discharge circuit connected to the energy storage device in such a manner that its time constant becomes smaller, the larger the value of the generated electrical quantity. If the electrical quantities generated by the sensing element are available in the form of electric voltages, it is advantageous to feed these voltages first to a highpass amplifier and to rectify them subsequently preferably in a full-wave rectifier. In this case a capacitor charged to a predetermined voltage by means of a voltage limiter, e.g., a Zener diode can serve as the energy storage device. In the discharge circuit of the capacitor is a controlled resistance, for instance, a transistor, whose resistance can be controlled in relation to the output voltage of the rectifier. In this connection, it is furthermore possible to arrange in the control circuit of the controlled resistance a capacitor, whose charging voltage, determining the control voltage, is proportional to the output voltage of the rectifier via a voltage divider, so that up to reaching the shut-off threshold, the control voltage is held at a value which prevailed shortly before the thread broke.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a particular embodiment, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The method of the invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawings, in which:
FIG. 1, is a diagram of the electric voltage corresponding to the thread velocity during a starting operation after one second and after five seconds.
FIG. 2, is a block diagram of an embodiment of the invention.
FIG. 3, is a schematic diagram of the circuit of FIG. 2.
In FIG. 1, the electric voltage U corresponding to the thread velocity is plotted at definite times as the curve A. Let this voltage U correspond to the r.m.s. value of the noise voltage. At the points a, b, c and e will be seen voltage dips caused by the so-called noise holes, while at the point d a voltage drop due to a broken thread is illustrated, where the thread was taken along by other machine parts or adjacent threads immediately after the break and the voltage is thereby generated again.
The line B indicates the response value, i.e., that electric voltage corresponding to a predetermined thread velocity which, if not reached, will cause the textile machine to be shut off. As the noise holes a, b, c and e can also drop down to the voltage zero under certain circumstances, the curve B cannot be put so low that it is reliably influenced only by thread brakes. On the other hand, it can clearly be seen that for approximately the same thread velocity the noise holes c, e are considerably shorter than the voltage dip d used by broken threads. If one compares, for instance, the time interval e', during which the voltage falls below the response value due to the noise hole e, which the time interval d', during which the voltage falls below the response value due to the thread break d, the difference in the intervals can clearly be seen. To obtain a unequivocal criterion between a noise hole and a broken thread, it is therefore only necessary to make the shut-off threshold interval correspondingly large.
On the other hand, the left part of FIG. 1 shows also how at lower velocities and therefore, lower voltages, the time interval a' for the noise hole a is of the same order of magnitude as the time interval d' for the case of a thread break d. This means that in this velocity range the machine is shut off by a noise hole corresponding to the hole a. In order to avoid this shortcoming, known electronic thread monitors are disconnected during this velocity phase.
The present invention is based on the discovery that the disadvantages described of known electronic thread monitors can be avoided if it is possible to change the time interval determining the shutting-off of the textile machine with the velocity. According to the invention, this can be achieved by the provision that the electrical quantity generated by the sensing element, i.e., the voltage according to curve A, serves to change the shut-off threshold in inverse proportion to the thread velocity. The time interval, during which the voltage must fall below the response value according to curve B to effect the shut-down of the textile machine, should therefore be longer, the lower the thread velocity, and it is to become increasingly shorter with increasing thread velocity. While in the case of the right-hand region of the time scale in FIG. 1 the shut-off threshold leading to the shutdown of the textile machine is about 20 msec, the shut-off threshold should be considerably longer at the lower velocities corresponding to the left-hand region of the time scale, as the thread velocity is considerably lower. If one has, for instance, at the thread velocity according to that of the noise hole a, which is lower by about one-third, a shut-off threshold of 60 msec instead of 20 msec, it will be seen clearly that the noise hole a cannot lead to a shutdown of the textile machine.
A particularly advantageous example of an embodiment of the invention will be explained with reference to the block diagram shown in FIG. 2.
The thread 1 to be monitored runs through the sensing element 2 as in all present electronic thread monitors. The noise signal present in the form of an AC voltage is fed to a highpass amplifier 3, so that the signal becomes larger and larger with increasing thread velocity. After the full wave rectifier 4 and the amplifier 5 following it, the rectified voltage is fed on the one hand to a voltage limiter 6, and on the other hand, to an analog storage device 7. The limited voltage from 6 is connected to a shut-off delay storage device 8, whose shut-off delay corresponds to the shut-off threshold and is controlled by the voltage of the storage device 7 in an inversely proportional manner. When the shut-off delay time has run out and the shut-off threshold has thus been reached, the buffer and threshold-value amplifier 9 triggers via the bus 10 the thyristor switch 11, which excites the stop magnet 12 of the textile machine drive. After the thread break is repaired, the circuit 11, 12, 13, 14 is re-energized by the key 13 when the textile machine is started up.
FIG. 3 shows an example of an embodiment of the circuit parts 6, 7 and 8 according to FIG. 2. The limiter 6 consists of the resistor 6a, the Zener diode 6b and the diode 6c. The analog storage device 7 consists of the resistors 7a, 7b, 7e, the diode 7c and the capacitor 7d. The shut-off delay memory 8 is composed of the capacitor 8c, which can be discharged with constant current via the transistor 8a and the resistor 8b.
The amplifier 5 supplies a positive output voltage, which corresponds to the thread velocity and is divided down to a desired magnitude by means of the resistors 7a, 7b and 7e. At the same time, the voltage behind the diode 6c is limited by the Zener diode 6b to a constant magnitude, which is stored in the capacitor 8c. The transistor 8a is, together with the resistor 8b, on the one hand an impedance transformer for the memory 7 and, on the other hand, a linear discharge resistor for the capacitor 8c.
In the event of a break in the thread, the DC output voltage of the amplifier 5 breaks down. Thereby, the capacitor 8c can discharge via the transistor 8a and the resistor 8b, as long as the voltage present at the base of the transistor 8a permits. So that the transistor 8a with the resistor 8b can constitute a constant-current load for the capacitor 8c, the base voltage must therefore be made available long enough. To this end, the time constant of the capacitor 7d, discharging via the transistor 8a and the resistor 8b, must be made about 40 times longer than that of the capacitor 8c. This means that the magnitude of the discharge current for the capacitor 8c, constant during the discharge time, depends on the magnitude of the voltage stored in the capacitor 7d. Thereby, the last thread velocity prior to the voltage dip due to a break of the thread or a noise hole determined the discharge time of the capacitor 8c in such a manner that a high thread velocity results in a short discharge time and therefore, a low shut-off threshold or a short response time of the stopping magnetic relay 12. Conversely, a low voltage at the capacitor 7d prior to the breakdown of the voltage supplied by the amplifier 5 causes a slow discharge of the capacitor 8c and therefore, a high or longer shut-off threshold.
It is not absolutely necessary that the capacitor 8c is completely discharged. Instead, the switching threshold of the amplifier 9 can be set so that at an accurately fixed partial discharge of the capacitor 8c the amplifier responds, for instance, to a reduction of the charging voltage to 30 to 35%. The operating point of the DC voltage amplifier 5 is chosen in the example of the embodiment shown as high as the voltage at the point 7f. The resistance of the resistors 7e and 7b is chosen so that the conduction threshold of the diode 7c and the base threshold of the transistor 8a are balanced. At room temperature, the voltage at the point 7f and therefore, also the operating point of the DC voltage amplifier 5 is about 1 V. Any further increase of the voltage at the amplifier 5 becomes therefore effective proportionally at the emitter resistor 8b. If necessary, the point 7f voltage can be made to track the temperature curve of the diode 7c and the transistor 8a by a thermistor.
The example of the embodiment described above represents a possible solution with a minimum of expenditure, which is completely sufficient for many cases of electronic thread monitoring in textile machines. Beyond this, the use of the noise signal as the control quantity for the shut-off threshold, among other things, has further advantage that the noise voltage is composed of the thread velocity and the thread thickness. For the same thread velocity one obtains a higher noise signal with thick threads than with thin threads. In the rectified noise voltage coming from the amplifier 5 are therefore contained two components, which act on the shut-off delay time in the correct sense.
Electronic thread monitors for textile machines, i.e., winding machines, warping machines or cutting machines have sensing elements of the most varied kinds such as, for instance, optical, capacitive or piezoelectric types, which convert the mechanical quantities (diameter, roughness, velocity, tension, etc.) into electrical quantities (voltage, current, frequency). The response of an electronic thread monitor occurs, for instance, as a function of the presence of the thread, of the magnitude of the thread tension, or of the intensity or velocity of the thread motion. It is known that a response of the electronic thread monitor as a function of the thread motion results in the shortest possible shut-off time, as the thread motion changes its state immediately if the thread is interrupted. In the case of a break of the thread, for instance, the thread velocity drops very considerably, generally to zero. In some cases, a negative thread velocity can even occur due to the tension acting on the thread.
In those electronic thread monitors which are equipped with a sensing element that can be influenced by the thread motion, the shut-off device of the textile machine is operated if the measured quantity exceeds or falls below the response magnitude, i.e. a predetermined measurement value.
Particularly advantageous with respect to complexity and cost are electronic thread monitors which derive the control from the change of the condition of the thread during the motion. As the thread thickness, cross section, roughness or the like are not always constant, electrical quantities of different magnitude are generated when the thread moves through the sensing element. These electrical quantities of varying magnitude are the so-called "noise signal" of the running thread, the absence of which automatically indicates a broken thread, as the broken thread no longer runs and therefore also cannot generate a noise signal.
On the other hand, it happens that the mechanical property of the thread which is used for generating the noise signal, for instance, the diameter, is constant over short lengths of the thread. This means, however, that the noise signal may become zero then also, and this so-called "noise hole" simulates a broken thread.
Since, however, this noise hole is present only for a short time, one uses in electronic thread monitors of the kind described above, as an additional criterion for shutting off the textile machine, namely that time interval, during which the magnitude exceeded or fell below the response value. As soon as this time interval reaches a predetermined value, the so-called shut-off threshold, a thread break actually exists. However, most textile machines are run at different speeds for different threads, and the noise holes are longer, the lower the thread velocity. This leads to the need to choose the shut-off threshold relatively long, so that a stopping of the textile machine due to a noise hole alone is reliably avoided. This, however, has the result in high-speed textile machines that during the longer shut-off time, the ongoing part of the broken thread is removed that much farther from the stopped part.
On the other hand, it may happen in textile machines that a broken thread is taken along again by machine parts or adjacent threads immediately after the break and the thread motion is interrupted or reduced only briefly, so that too high a shut-off threshold does not lead to a shutdown of the machine in this case.
Particularly critical is the starting phase of the thread, i.e., the time until the thread has reached its normal velocity. During this phase the noise holes also become larger and can lead to a shut-down of the machine, if the shut-off threshold is set for normal operation, without the occurrence of the thread break. In order to avoid this low speed error, one may disconnect the electronic thread monitor during a certain starting phase, which, however, is accompanied by the disadvantage that a possible thread break during this starting phase can be detected only after the end of the disconnect time, which in turn leads to the disadvantages, already mentioned before, of a late machine shut-off.
It is a principal object of the invention to avoid the above disadvantages of known electronic thread monitors. According to the invention, the solution to this problem is that the electrical quantities generated by the sensing element serve to change the shut-off threshold interval inversely proportionally to the thread velocity. The proportionality need not be a linear function here, but may also follow an exponential function. It is essential that the shut-off threshold interval and therefore, the time during which the magnitude exceeds or falls below the response value, becomes smaller, the greater the thread velocity. At high thread velocities even a very brief interruption or reduction of the velocity leads to a shut-off of the textile machine, while at the low thread velocities the above-mentioned noise holes do not yet lead to crossing the shut-off threshold. The change of the shut-off threshold interval which is inversely proportional to the thread velocity, can be achieved on the one hand by providing that the electrical quantities generated by the sensing element are always inversely proportional to the thread velocity, but on the other hand also by bringing the electrical quantities which are generated proportional to the thread velocity, into a relationship of inverse proportionality to the shut-off threshold interval.
The time constant of an electric energy storage device can serve as shut-off threshold. For instance, of a capacitor which is controlled by the generated electrical quantities and whose adjustable charge, e.g., the charging voltage, is a measure for the response value. In this connection, the charging time constant as well as that for the discharge of the energy storage device can be used as the shut-off threshold. Similarly, a maximum charge as well as a minimum charge can be a measure for the response value.
In a particularly advantageous embodiment of the invention, the generated electrical quantities serve on the one hand for the predetermined charging of the electric energy storage device, and on the other hand, for varying the time constant of the discharge circuit connected to the energy storage device in such a manner that its time constant becomes smaller, the larger the value of the generated electrical quantity. If the electrical quantities generated by the sensing element are available in the form of electric voltages, it is advantageous to feed these voltages first to a highpass amplifier and to rectify them subsequently preferably in a full-wave rectifier. In this case a capacitor charged to a predetermined voltage by means of a voltage limiter, e.g., a Zener diode can serve as the energy storage device. In the discharge circuit of the capacitor is a controlled resistance, for instance, a transistor, whose resistance can be controlled in relation to the output voltage of the rectifier. In this connection, it is furthermore possible to arrange in the control circuit of the controlled resistance a capacitor, whose charging voltage, determining the control voltage, is proportional to the output voltage of the rectifier via a voltage divider, so that up to reaching the shut-off threshold, the control voltage is held at a value which prevailed shortly before the thread broke.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a particular embodiment, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The method of the invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawings, in which:
FIG. 1, is a diagram of the electric voltage corresponding to the thread velocity during a starting operation after one second and after five seconds.
FIG. 2, is a block diagram of an embodiment of the invention.
FIG. 3, is a schematic diagram of the circuit of FIG. 2.
In FIG. 1, the electric voltage U corresponding to the thread velocity is plotted at definite times as the curve A. Let this voltage U correspond to the r.m.s. value of the noise voltage. At the points a, b, c and e will be seen voltage dips caused by the so-called noise holes, while at the point d a voltage drop due to a broken thread is illustrated, where the thread was taken along by other machine parts or adjacent threads immediately after the break and the voltage is thereby generated again.
The line B indicates the response value, i.e., that electric voltage corresponding to a predetermined thread velocity which, if not reached, will cause the textile machine to be shut off. As the noise holes a, b, c and e can also drop down to the voltage zero under certain circumstances, the curve B cannot be put so low that it is reliably influenced only by thread brakes. On the other hand, it can clearly be seen that for approximately the same thread velocity the noise holes c, e are considerably shorter than the voltage dip d used by broken threads. If one compares, for instance, the time interval e', during which the voltage falls below the response value due to the noise hole e, which the time interval d', during which the voltage falls below the response value due to the thread break d, the difference in the intervals can clearly be seen. To obtain a unequivocal criterion between a noise hole and a broken thread, it is therefore only necessary to make the shut-off threshold interval correspondingly large.
On the other hand, the left part of FIG. 1 shows also how at lower velocities and therefore, lower voltages, the time interval a' for the noise hole a is of the same order of magnitude as the time interval d' for the case of a thread break d. This means that in this velocity range the machine is shut off by a noise hole corresponding to the hole a. In order to avoid this shortcoming, known electronic thread monitors are disconnected during this velocity phase.
The present invention is based on the discovery that the disadvantages described of known electronic thread monitors can be avoided if it is possible to change the time interval determining the shutting-off of the textile machine with the velocity. According to the invention, this can be achieved by the provision that the electrical quantity generated by the sensing element, i.e., the voltage according to curve A, serves to change the shut-off threshold in inverse proportion to the thread velocity. The time interval, during which the voltage must fall below the response value according to curve B to effect the shut-down of the textile machine, should therefore be longer, the lower the thread velocity, and it is to become increasingly shorter with increasing thread velocity. While in the case of the right-hand region of the time scale in FIG. 1 the shut-off threshold leading to the shutdown of the textile machine is about 20 msec, the shut-off threshold should be considerably longer at the lower velocities corresponding to the left-hand region of the time scale, as the thread velocity is considerably lower. If one has, for instance, at the thread velocity according to that of the noise hole a, which is lower by about one-third, a shut-off threshold of 60 msec instead of 20 msec, it will be seen clearly that the noise hole a cannot lead to a shutdown of the textile machine.
A particularly advantageous example of an embodiment of the invention will be explained with reference to the block diagram shown in FIG. 2.
The thread 1 to be monitored runs through the sensing element 2 as in all present electronic thread monitors. The noise signal present in the form of an AC voltage is fed to a highpass amplifier 3, so that the signal becomes larger and larger with increasing thread velocity. After the full wave rectifier 4 and the amplifier 5 following it, the rectified voltage is fed on the one hand to a voltage limiter 6, and on the other hand, to an analog storage device 7. The limited voltage from 6 is connected to a shut-off delay storage device 8, whose shut-off delay corresponds to the shut-off threshold and is controlled by the voltage of the storage device 7 in an inversely proportional manner. When the shut-off delay time has run out and the shut-off threshold has thus been reached, the buffer and threshold-value amplifier 9 triggers via the bus 10 the thyristor switch 11, which excites the stop magnet 12 of the textile machine drive. After the thread break is repaired, the circuit 11, 12, 13, 14 is re-energized by the key 13 when the textile machine is started up.
FIG. 3 shows an example of an embodiment of the circuit parts 6, 7 and 8 according to FIG. 2. The limiter 6 consists of the resistor 6a, the Zener diode 6b and the diode 6c. The analog storage device 7 consists of the resistors 7a, 7b, 7e, the diode 7c and the capacitor 7d. The shut-off delay memory 8 is composed of the capacitor 8c, which can be discharged with constant current via the transistor 8a and the resistor 8b.
The amplifier 5 supplies a positive output voltage, which corresponds to the thread velocity and is divided down to a desired magnitude by means of the resistors 7a, 7b and 7e. At the same time, the voltage behind the diode 6c is limited by the Zener diode 6b to a constant magnitude, which is stored in the capacitor 8c. The transistor 8a is, together with the resistor 8b, on the one hand an impedance transformer for the memory 7 and, on the other hand, a linear discharge resistor for the capacitor 8c.
In the event of a break in the thread, the DC output voltage of the amplifier 5 breaks down. Thereby, the capacitor 8c can discharge via the transistor 8a and the resistor 8b, as long as the voltage present at the base of the transistor 8a permits. So that the transistor 8a with the resistor 8b can constitute a constant-current load for the capacitor 8c, the base voltage must therefore be made available long enough. To this end, the time constant of the capacitor 7d, discharging via the transistor 8a and the resistor 8b, must be made about 40 times longer than that of the capacitor 8c. This means that the magnitude of the discharge current for the capacitor 8c, constant during the discharge time, depends on the magnitude of the voltage stored in the capacitor 7d. Thereby, the last thread velocity prior to the voltage dip due to a break of the thread or a noise hole determined the discharge time of the capacitor 8c in such a manner that a high thread velocity results in a short discharge time and therefore, a low shut-off threshold or a short response time of the stopping magnetic relay 12. Conversely, a low voltage at the capacitor 7d prior to the breakdown of the voltage supplied by the amplifier 5 causes a slow discharge of the capacitor 8c and therefore, a high or longer shut-off threshold.
It is not absolutely necessary that the capacitor 8c is completely discharged. Instead, the switching threshold of the amplifier 9 can be set so that at an accurately fixed partial discharge of the capacitor 8c the amplifier responds, for instance, to a reduction of the charging voltage to 30 to 35%. The operating point of the DC voltage amplifier 5 is chosen in the example of the embodiment shown as high as the voltage at the point 7f. The resistance of the resistors 7e and 7b is chosen so that the conduction threshold of the diode 7c and the base threshold of the transistor 8a are balanced. At room temperature, the voltage at the point 7f and therefore, also the operating point of the DC voltage amplifier 5 is about 1 V. Any further increase of the voltage at the amplifier 5 becomes therefore effective proportionally at the emitter resistor 8b. If necessary, the point 7f voltage can be made to track the temperature curve of the diode 7c and the transistor 8a by a thermistor.
The example of the embodiment described above represents a possible solution with a minimum of expenditure, which is completely sufficient for many cases of electronic thread monitoring in textile machines. Beyond this, the use of the noise signal as the control quantity for the shut-off threshold, among other things, has further advantage that the noise voltage is composed of the thread velocity and the thread thickness. For the same thread velocity one obtains a higher noise signal with thick threads than with thin threads. In the rectified noise voltage coming from the amplifier 5 are therefore contained two components, which act on the shut-off delay time in the correct sense.