Method and apparatus of monitoring the quality of a false twist textured yarn

Abstract: A method and apparatus for monitoring the quality of the yarn processed on a false twist texturing machine is disclosed, and which includes sensing the tension of the advancing yarn at a location immediately downstream of the false twist unit, and determining the coefficient of variation of the sensed tension. An error signal is generated whenever the coefficient of variation exceeds a predetermined upper limit and whenever the coefficient of variation drops below a predetermined lower limit. Also, an alarm signal is generated whenever the error signal exceeds the predetermined upper limit for a predetermined time, and whenever the error signal drops below the predetermined lower limit for a predetermined time, and the alarm signals are counted so as to provide an indication of the quality of the yarn.

Description: The present invention relates to a method and apparatus for monitoring the quality of an advancing yarn comprising the steps of continuously sensing the tension of the advancing yarn and a method of false twisting a yarn according to the preamble of the independent claims and known, for instance, from EP-A-0 406 736. A method of monitoring the tension of a yarn advancing between the false twisting unit and the delivery system in the texturing zone of a multiposition false twist crimping machine, in which a mean value signal is continuously formed, is known from U.S. Patent 4,720,702 to Martens. In that known method, use is made of the fact that the yarn tension between the false twisting unit and the delivery system of the texturing zone allows essential conclusions to be drawn as to the control of the process during the texturing and the quality of the yarn produced. More particularly, the yarn tension is monitored such that, on the one hand, the minimum and maximum values of the tension do not leave a certain range on either side of the continuously determined mean value, and such that, on the other hand, the continuously determined mean value of the yarn tension does not leave a certain predetermined range. From the EP-A-0 406 736 are further known a method and an apparatus for monitoring the yarn tension at each of a plurality of yarn processing stations, wherein the mean value of the monitored tension is continuously determined at each station. The differential value between the monitored value and the mean value is continuously determined too. A first alarm signal is generated whenever the mean value leaves a predetermined tolerance range, and a second alarm signal is generated whenever the differential value leaves a second predetermined tolerance range. The alarms are logged in both number and duration to provide an indication of the quality of the processed yarn. Regarding the EP-A-0 406 736 as being the closest prior art to the present invention, it is an object of the invention to monitor the intensity and uniformity of the twisting process in the texturing zone and also to monitor and record the uniformity of the setting of the twist attained in the advancing yarn by a thermal setting process, these parameters directly reflecting the results and indicating the quality of the resulting yarn. This object is solved basically by the solution given in the characterizing part of the independent claims. The above and other objects and advantages of the present invention are achieved in the embodiment illustrated herein by the provision of a method and apparatus which includes the steps of continuously sensing the tension of the advancing yarn, while continuously determining the coefficient of variation of the sensed yarn tension, and generating an error signal whenever at least one of the following conditions is present: (1) the coefficient of variation exceeds a predetermined upper limit, and (2) the coefficient of variation drops below a predetermined lower limit. In the preferred embodiment, the step of generating an error signal includes generating the signal whenever the coefficient of variation exceeds the upper limit and whenever it falls below the lower limit. Also, an alarm signal may be generated whenever the error signal falls outside the range between the upper and lower limits for a predetermined time, and these alarm signals may be counted to provide an indication of the quality of the yarn. The present invention makes it possible to obtain a direct indication as to certain criteria of the yarn being processed irrespective of the denier. For example, the falling below the lower limit value of the coefficient of variation is not necessarily an indication of a particularly smooth and, hence, advantageous yarn path, but rather an indication of the fact that inadequate twist is imparted. From a technological viewpoint, it is advantageous to emit the error signals as short-term alarm signals. To avoid the influence of irrelevant disturbances, these alarm signals are counted, and only the exceeding of predetermined sums is used for intervening in the process or emitting quality classification signals. It has been found that falling below the lower limit value on the one hand and the exceeding of the upper limit value on the other hand give a warning of different errors. The exceeding of the upper limit value indicates primarily that the setting of the twist by the heat setting process is out of order. This may be based upon contamination of the heating plate, unsatisfactory guidance of the yarn in the heating plate, yarn defects, or irregular moistening of the yarn advancing from its supply package. A drop below the lower limit value typically indicates an inadequate impartation of twist. The cause may be that the false twister is out of operation, that the yarn is not properly guided in the false twister, and that the false twister has defective friction elements or is contaminated. For the above reason, the alarm signals are preferably counted separately, and a warning thereof is given after exceeding the upper limit value or falling below the lower limit value. Some of the objects and advantages of the present invention having been stated, others will appear as the description proceeds when considered in conjunction with the accompanying schematic drawings, in which Figure 1 is a schematic representation of a portion of the signal processing circuit of the present invention; Figures 2 and 3 are diagrams illustrating the values of the sensed tension and the coefficient of variation versus time, in accordance with the present invention; and Figure 4 is a schematic diagram of a false twisting apparatus and portion of the control circuitry in accordance with the present invention. Referring more particularly to the drawing, Figure 4 schematically illustrates a yarn false twisting apparatus and a portion of the control circuitry of the present invention. Further details of the apparatus illustrated in Figure 4 may be obtained from the U.S. Patent to Martens, No. 4,720,702, the disclosure of which is expressly incorporated herein by reference. As illustrated in Figure 4, each working position of a false twist crimping machine is provided with a supply package 39 from which a yarn 40 to be processed is withdrawn by means of a first delivery system 41. In the illustrated embodiment, a yarn cutter 42 is arranged before the delivery roll 41. After the first delivery roll 41, the yarn 40 travels along a heating plate 43, which is straight in the illustrated embodiment but which may be curved, and which is heated to, for example, 200°C. After the heating plate 43, the yarn 40 advances through a cooling zone (not illustrated) and then through a false twister 44 of conventional design. The false twister 44 is followed by a yarn tension measuring instrument 1 which permits the yarn tension to be continuously sensed. Subsequently, the yarn 40 is withdrawn by a second or delivery roll 45, and advanced either to a further heating system or, as shown, directly to a takeup where it is wound into a package 46. The yarn sensor 1 generates an output signal which in its magnitude and shape or slope is representative of the yarn tension and its fluctuations between the false twister 44 and the delivery roll 45. This signal is supplied to a circuit 47 which is within the dash-dotted lines in Figure 4, and which receives predetermined limit values as comparative values from a limit value memory 48 which will be described below. Whereas each yarn sensor 1 of each position of a texturing machine is associated with a circuit 47, one limit value memory 48 is associated with a group of texturing positions of a multiposition texturing machine. The output signal associated with the yarn tension between the false twister 44 and the delivery roll 45 is amplified by an amplifier 54 to produce an electrical voltage signal indicated at X (the corresponding signal is indicated at U in Patent 4,720,702). From the voltage X, a filter 55 forms a mean value MX. The measured signal X and its continuously formed mean value MX are supplied to a differential amplifier 58 in which the difference between measured value and mean value is formed. Reference may be made to the above cited U.S. Patent No. 4,720,702 for a more detailed description of the other elements of circuit illustrated in Figure 4. Figure 1 shows the further processing of the measured value X and its mean value MX, in accordance with the invention. More particularly, the output signal a of the differential amplifier 58 is supplied to a squaring element 3. A further filtering in filter 4 of the squared signal a ², and a determination of the square root from the filtered signal at 5 permit the standard deviation S to be determined. From the standard deviation S the quotient with the mean value MX is then determined in the computing element 6 and output as the coefficient of variation or respectively as the relative standard deviation CV in the form of a percentage figure. In comparator elements 7 and 8, this continuously formed coefficient of variation CV is continuously compared with a predetermined maximum value and a predetermined minimum value. The output signals are added in counters 9 and 10 respectively, the output sums of which indicate the aforementioned errors. As will be apparent from Figure 1, the standard deviation S may be defined as the square root of the mean square of the deviations of the signal X from the mean signal MX. Also, the coefficient of variation CV may be defined as the ratio of the standard deviation S to the mean signal MX. Figures 2 and 3 represent a measuring record of the continuous yarn tension signal X, as well as a graphic record of the continuous coefficient of variation CV derived therefrom. Figure 2 illustrates a phase with a very unsteady curve of the yarn tension or yarn tension signal X respectively. In this graph the upper limit of the CV value is exceeded and consequently output as an error signal. Figure 3 shows a phase with a periodically flawless yarn tension signal X. In this graph, the curve falls below the lower limit value of the coefficient of variation. The nature of defects which are present in this instance have been described above. It should be pointed out that the summation of the error signals includes not only the occurrence of the error signal itself, but also the duration of the occurring error signal. This is achieved in that the respective error signal is repeated in certain, fixedly input time intervals as long as the error is present, i.e., an exceeding or falling below the limit value. Stated in other words, an alarm signal is generated whenever the error signal exceeds the predetermined upper limit for a predetermined time, and whenever the error signal drops below the predetermined lower limit for a predetermined time, and these alarm signals are counted so as to provide an indication of the quality of the yarn. U.S. Patent No. 5,018,390 to Muller contains a further description of a similar counting operation, and the disclosure of such patent is expressly incorporated by reference.

Claims: A method of monitoring the quality of an advancing yarn comprising the steps of continuously sensing the tension of the advancing yarn, characterized by the steps of: deriving a signal (X) representative of the tension of the advancing yarn, filtering said signal (X) to derive a mean value (MX) thereof; deriving a signal representative of the difference (a) between said mean value and said tension signal; squaring said difference signal (a); filtering said square signal (a²); deriving the square root (S) of said filtered squared signal; and obtaining the coefficient of variation CV by determining the quotient of said mean value (MX) and said square root (S), generating an error signal whenever at least one of the following conditions is present, namely: the coefficient of variation CV exceeds a predetermined upper limit or the coefficient of variation CV drops below a predetermined lower limit; generating an alarm signal, whenever an error signal occurs for a predetermined time; and counting the alarm signals separately for the coefficient of variation exceeding the predetermined upper limit or the coefficient of variation dropping below the predetermined lower limit, so as to provide a separate indication of the quality of the yarn. A method of false twisting a yarn (40) comprising the steps of advancing the yarn along a path of travel while serially heating the yarn, cooling the yarn, false twisting the yarn, and then winding the yarn into a package (46), continuously sensing the tension of the advancing yarn (40) at a location (1) along the path of travel between the location (44) at which the false twist is applied and the location (46) at which the yarn is wound into a package, deriving a signal X representative of the tension of the advancing yarn, filtering said signal to derive a mean value MX thereof, deriving a signal representative of the difference a between said mean value MX and said tension signal X , characterized by continuously determining the coefficient of variation CV of the sensed yarn tension by a) squaring said difference signal a to get the square a ² , e) filtering said square signal a², f) deriving the square root S of said filtered squared signal a², g) deriving the quotient CV of said square root S and said mean value MX and obtaining the coefficient of variation CV by determining the quotient of said mean value (MX) and said square root (S), generating an error signal whenever at least one of the following conditions is present, namely: the coefficient of variation CV exceeds the predetermined upper limit or the coefficient of variation CV drops below the predetermined lower limit; generating an alarm signal, whenever an error signal occurs for a predetermined time; and counting the alarm signals separately for the coefficient of variation exceeding the predetermined upper limit or the coefficient of variation dropping below the predetermined lower limit, so as to provide a separate indication of the quality of the yarn. Apparatus for monitoring the tension of an advancing yarn comprising sensor means (1) for continuously sensing the tension of the advancing yarn (40) and producing a continuous output signal ( X ) representative of the sensed tension, first circuit means (3 - 5, 47, 55, 58) operatively connected to said sensor means (1) and comprising: a) means (54) for deriving a signal representative of the tension of the advancing yarn (40), b) means (55) for filtering said signal to derive a mean value thereof, c) means (58) for deriving a signal representative of the difference between said mean value and said tension signal, and being characterized by said first means furthermore comprising d) means (3) for squaring said difference signal, e) means (4) for filtering said squared signal, f) means (5) for deriving the square root of said filtered square signal, and g) means (6) for deriving the quotient of said mean value and said square root and for delivering the coefficient of variation as an output, and second circuit means (7 - 10) for generating an error signal whenever at least one of the following conditions is present: (1) the coefficient of variation CV exceeds a predetermined upper limit or (2) the coefficient of variation CV drops below a predetermined lower limit, said second circuit (7 - 10) including means for generating an alarm signal whenever the error signal exceeds the predetermined upper limit for a predetermined time, or whenever the error signal drops below the predetermined lower limit for a predetermined time, and means for counting the alarm signals so as to provide an indication of the quality of the yarn (40). Apparatus as defined in claim 3, characterized in that the means for counting the alarm signals includes means (9, 10) for separately counting the alarm signals generated when the error signal exceeds the predetermined upper limit or drops below the predetermined lower limit. Apparatus as defined in claim 9, characterized by means (41, 45) for advancing the yarn (40) along a path of travel while serially heating (43) the yarn (40), cooling the yarn (40), false twisting (44) the yarn (40), and then winding the yarn into a package (46), and wherein said sensor means (1) is positioned between the location (44) at which the yarn (40) is false twisted and the location at which the yarn is wound into a package (46). Apparatus as defined in claim 3, characterized in that said means (58) for deriving a signal representative of the difference between said mean value and said tension signal comprises a differential amplifier.

Inventors: NEUMANN BERND, NEUMANN, BERND
Applicants: BARMAG BARMER MASCHF, BARMAG AG
Application year: 1992
Main classification: D02G1
IPC codes: D02G1, G01N33