Method and device for detection of untextured yarn sections in textured filament yarns

A method for detecting untextured or defectively textured yarn segments in textured filament yarns during the texturing process, in which the textured yarn is continuously measured by the measurement, conditioning and evaluation of high-frequency measurement signals as evidence of untextured or defectively textured yarn segments.

The invention concerns a method for detecting untextured or defectively
 textured yarn segments in textured filament yarns during the texturing
 process and a device for carrying out this method.
 The texturing of filament yarns, particularly multifilament yarns, is
 known. Texturing serves to a produce from a synthetic, flat and smooth
 multifilament yarn a crimped and structured yarn that has textile
 character because of its bulky and lofty structure. For this purpose, the
 multifilament yarn is generally unwound from a spool, guided through a
 first delivery device, then heated in a heater, cooled on a cooling rail,
 and guided through a twister and a second delivery device arranged
 thereafter, a so-called draw-off delivery device, ultimately to be wound
 onto a yarn spool. The twister serves to temporarily impart high torque to
 the multifilament yarn in one pass (temporary torque is also known as
 false twist) and makes it possible, by means of the residual torque that
 simultaneously develops, to heat-set the twisted state by heating and
 cooling in the region ahead of the twister. After the twister, the torque
 is removed again and the filament yarn is untwisted. Because of the
 heat-setting treatment effected in the twisted state, the yarn has the
 desired crimped structure.
 The torque is imparted primarily with a three-shaft disk friction unit or
 by means of so-called crossed belts. The use of friction to impart the
 torque permits very high rotation speeds and thus also high production
 speeds. However, if the friction ratios between the twister and the yarn
 are not constant, this leads to disruptions of the process that are also
 known as instabilities, and thus to loss of quality in the yarn. Such
 defects can result from spinning problems, uneven application or
 distribution of the spinning preparation on the surface of the yarn,
 temperature fluctuations during texturing or contamination by the heating
 and/or cooling rails. Such disruptions can cause so-called ballooning of
 the yarn, which occurs in particular at high rotation speeds. Ballooning
 of the yarn causes uncontrolled travel of the yarn, fluctuations of the
 yarn tension, and losses of quality. In unstable processes, in particular,
 the yarn can skip across the surfaces of the disks in the friction unit.
 This so-called "closing up" of the twist.sup.1 causes a torque deficit in
 the twist zone. Yarn that has been imparted high torque before the twister
 can therefore pass through the unit briefly or by segments without the
 torque being removed. This results in short untextured yarn segments
 (temporary twist slippage), so-called tight spots, and long untextured
 yarn segments ("surging," prolonged twist slippage).

FNT .sup.1 TRANSLATOR'S NOTE: German Drallschlu.beta.. Schlu.beta. can also be
 translated as "termination."
 The quality of the textured yarn is generally monitored by spot-check-type
 inspection of the finished textured yarn. Only a very small fraction of
 the total output can be inspected in this way. In recent years, so-called
 on-line production controls have been introduced in texturing, with the
 result that nearly the entire output can be monitored for quality.
 However, the yarns are not examined by the measuring equipment in an
 uninterrupted manner, since the measuring sensors and scanning rates used
 operate at low frequencies, and undesirable short tight spots thus often
 go undetected. On-line control in the texturing process is generally
 accomplished by measuring the yarn tension. Known are monitoring systems
 that are able to detect low-frequency fluctuations.sup.2 and peaks of yarn
 tension, i.e., fluctuations of yarn tension that are due to relatively
 long defects in the yarn. These systems are unable to detect short
 malfunctions and defects.

FNT .sup.2 TRANSLATOR'S NOTE: The German actually says "thread tension pivots"
 or perhaps "changes in the direction of thread tension"
 (Fadenzugkraftschwenkungen). We assume that this is a typographical error
 for "fluctuations of thread tension" (Fadenzugkraftschwenkungen), there
 being only a one-letter difference.
 A yarn tension sensor for a textile machine has been disclosed (EP 0 531
 753 A1) for the determination of short-period fluctuations of yarn
 tension. This device is intended to measure relatively high-frequency
 fluctuations of yarn tension--of up to 50 Hz, for example--at high yarn
 speeds. It can also, therefore, detect relatively high-frequency
 fluctuations of yarn tension caused by the displacement of the yarn. This
 is intended to give the yarn tension sensor a much broader field of
 application. Examples of such applications that are disclosed include the
 compensation of aperiodic fluctuations of yarn tension via yarn braking
 means, the generation of trouble signals in response to periodically
 occurring deviations from the specified yarn tension characteristic, and
 detection of the gradual increase in yarn tension that occurs when the
 last third of a cop is being wound at the spool location, with appropriate
 throttling of the winding speed to prevent breakage of the yarn.
 German Patent Application (Unexamined) 41 19 780 proposes, for quality
 monitoring in a false-twist crimping machine, measuring the yarn tension
 continuously between the false-twister and the exit delivery device from
 the texturing zone. The mean of the measurement values is constantly taken
 and the position of this mean and/or the relative position of the current
 measurement value with respect to the mean is evaluated. To permit a
 statement regarding the nature of the defects that affect quality, it is
 further proposed to determine a plurality of means differing with respect
 to the length of the evaluation time and, in order to generate a quality
 signal, to determine the relative position of the measurement value with
 respect to one or more of the means and/or the relative positions of the
 means with respect to one another. However, this method comprising taking
 the mean is unable to specifically and independently detect and evaluate
 sudden, brief irregularities in the process, such as voltage peaks, which
 occur, for example, in the event of hindrances to the winding of the yarn
 onto the spool and which subsequently disappear completely.
 EP 0 406 736 B1 also proposes monitoring the tension of a traveling yarn by
 the uninterrupted determination of the mean. A difference from the mean
 and the current yarn tension are also constantly determined. Whenever the
 mean and/or the difference pass out of prescribed tolerance ranges for set
 periods of time, corresponding alarm signals are generated. Quality data
 for the yarn are derived on the basis of the occurrence of such alarm
 signals during preset time intervals. However, time delay elements with
 time constants of about 10 milliseconds are used to filter out output
 signals caused by brief disturbances in the yarn texturing process that
 are classed as irrelevant.
 The technical problem underlying the present invention is, therefore, to
 provide an improved method, performed during texturing, for detecting
 undesired, untextured or defectively textured yarn segments, in particular
 short lengths in filament yarns manufactured in particular by the friction
 false twist process.
 The solution to the technical problem lies in the provision of a method
 according to the main claim, in particular a method for detecting
 untextured or defectively textured yarn segments in textured filament
 yarns during the texturing process, in which high-frequency yarn signals,
 in particular yarn tension signals, are measured and evaluated. The
 measured yarn signals permit statements as to the degree and evenness of
 crimping of the yarn. Tight spots in the yarn, particularly short,
 untextured tight spots, are determined, by the detection process effected
 during the texturing process, as correspondingly brief signals which
 according to the invention are measured and evaluated as evidence of such
 tight spots. In the context of the present invention, a short tight spot
 is understood to be a tight spot with a length of no more than
 approximately 50 mm, preferably 1 to 50 mm. In the context of the present
 invention, high-frequency yarn tension signals are understood to be peaks
 or fluctuations of yarn tension which at a given yarn speed have a
 frequency of more than 0.2 kHz, preferably of 1 to 6 kHz, but also higher
 frequencies.
 The method according to the invention provides in particular that
 high-frequency yarn tension signals are measured, conditioned and
 evaluated, thereby permitting statements as to the degree and evenness of
 crimping of the yarn. Since the method according to the invention detects
 high-frequency yarn tension signals, it is also able to register, in
 particular, brief disruptions caused by so-called tight spots, making it
 possible to detect at an early stage--i.e., before a tangible decline in
 yarn quality--when a texturing process is unstable and is therefore
 subject to error.
 The invention provides preferably that the yarn tension of the textured
 yarn is measured after the yarn passes through the twister, i.e., in a
 state in which the yarn is essentially untwisted again. The performance of
 the method according to the invention with untwisted yarn, i.e., after
 passage through the twister, is especially advantageous, since the
 imparting of the twist and thus the texturing process are not adversely
 affected by the diversion of the yarn to the sensor that must be done in
 order to measure the yarn tension.
 In a preferred embodiment, the detection method according to the invention
 is used during the friction false-twist process. The twister in this case
 is a disk friction unit.
 The detection according to the invention of brief disturbances that occur
 during the texturing process, in particular high-frequency yarn tension
 signals, is made possible by a measuring sensor able to pick up
 high-frequency fluctuations or peaks of the yarn tension. The measured
 high-frequency yarn tension signals are conditioned and evaluated and thus
 provide information according to the invention concerning the degree and
 evenness of crimping of the yarn. The evaluation of the measured yarn
 tension signals is performed, for example, by setting a threshold value
 and detecting the number of times this threshold is exceeded per time
 unit. The number of excursions is a measure of irregularities and
 disturbances. The magnitude of the mean is a relative measure of the
 intensity of crimping of the yarn.
 The method according to the invention therefore permits, in an advantageous
 manner, the detection in particular of short tight spots that are detected
 by the measurement of high-frequency yarn (tension) signals.
 The invention also concerns a device for detecting untextured or
 defectively textured yarn segments in textured filament yarns, in
 particular for the carrying out of the aforesaid method, the device
 comprising a measuring sensor, a device for signal conditioning and
 processing and for signal analysis, data processing and potentially data
 output, and the measuring sensor being able to detect brief yarn tension
 signals and deliver them for data evaluation. The evaluation is performed
 by a device according to the invention, in that its signal processing
 device can comprise means for selectable smoothing or filtration,
 determination of the mean, construction of scattering matrices,
 determination of differentials, selectable setting of threshold values
 and/or counting of the number of times the threshold is exceeded in the
 conditioned signal, in which case the last counting circuit detects the
 number of signal readings that exceed the threshold value within a set
 period of time. The number of readings per time unit is to be considered a
 measure of the unevenness of crimping of the yarn. The device according to
 the invention therefore permits the detection of high-frequency yarn
 signals, in particular yarn tension signals, and their subsequent
 evaluation, taking into account the relationship, recognized according to
 the invention, between the occurrence of brief tight spots and that of
 high-frequency yarn signals. The measuring sensors to be used according to
 the invention are, in particular, able to pick up yarn tension signals in
 a frequency range of up to 6 kHz or in even higher frequency ranges.
 The measured high-frequency yarn signals can then, for example, be
 evaluated further by FFT (fast Fourier transformation) analysis of the
 yarn signal, thus permitting the detection even of minuscule periodic
 fluctuations that are not visible, for example, in the normal curve of
 yarn tension over time. The use of high scanning rates, for example of 50
 kHz or 250 kHz, prevents the occurrence of aliasing effects during the FFT
 analysis.
 Since the method according to the invention includes the detection of a
 large number of measurement signals, it is also provided according to the
 invention to condition and partly evaluate the analog measurement signals,
 preferably at the site of measurement, for purposes of data compression,
 and then to transmit the digital results to a central computer for
 storage, computeraided overall evaluation and documentation. The
 evaluation can be performed by the reception or determination of amplitude
 spectra, autocorrelation functions, curves of yarn tension over time, mean
 values, standard deviations, coefficients of variation, scattering
 matrices, FFT analyses and/or amplitude histograms of the yarn signal,
 particularly the yarn tension signal.
 Alternatively or additionally, the solution to the above problem lies in a
 method for detecting untextured or defectively textured yarn segments in
 textured filament yarns during the texturing process, in which the yarn
 thickness is determined optically or laser-optically and is then
 evaluated. The examination of yarn thickness provides, in an advantageous
 manner, information regarding the texturing process and the quality of the
 yarn obtained, since comparatively thin yarn locations that can be
 recognized by optic or laser-optic means indicate untextured or
 defectively textured yarn segments that have not been untwisted. According
 to the invention, deviations of yarn thickness greater than 20%, in
 particular, indicate short, untextured yarn locations. In particular,
 short tight spots could not be detected by the methods of prior art, but
 this can now be done in an advantageous manner by means of the present
 invention. Brief instabilities that cause tight spots, for example
 slippage of the yarn on a disk of the friction unit, can therefore be
 detected in an advantageous manner.
 The invention also provides a method for detecting untextured or
 defectively textured yarn segments in textured filament yarns during the
 texturing process, in which the yarn position and/or variations of the
 yarn position, in particular fluctuations of the transverse travel of the
 yarn, are determined optically and/or laser-optically and evaluated. The
 use of optic or laser-optic devices makes it possible, in this embodiment
 of the invention as well, to detect brief instabilities that cause
 undesired brief disruptions. The present invention is therefore based,
 inter alia, on the realization that the position of the yarn or the
 variation of its position during travel over a disk surface of the twister
 preferably realized as a friction unit can be used to detect short,
 untextured or defectively textured yarn segments. In an especially
 preferable manner, it is provided according to the invention to determine
 the behavior of the yarn as it travels over the exit disk of the friction
 unit--i.e., the last friction disk, which is usually a yarn guide disk. In
 a stable texturing process, i.e., a texturing process in which no tight
 spots are formed, the yarn travels to the exit disk in a sharp curve,
 which indicates that a frictional force is acting on the yarn. The
 curvature of the path of travel of the yarn becomes smaller in a slightly
 unstable process, while in a very unstable process the yarn passes almost
 rectilinearly over the last disk, which indicates that the frictional
 force being exerted is tending toward 0. An unstable process is subject in
 particular to variations of the path of travel of the yarn on the exit
 disk that have a wave-like appearance. From this it will be appreciated
 that in an unstable process the yarn skips across the surface of the disk.
 This twist slippage causes a torque deficit in the twist zone. If this
 wave occurs on the exit disk, high-twist yarn can briefly pass through the
 unit without relaxation of the twist, and thereby form tight spots.
 The optic or laser-optic examination of the travel behavior of the yarn,
 especially on the exit disk, therefore makes it possible to detect short
 tight spots in textured yarn. The detection of short untextured yarn
 segments by determining and evaluating the position and travel behavior of
 the yarn can be used, either alone or in combination with the
 yarn-thickness detection method described hereinabove, for the
 determination of short untextured yarn segments.
 In an especially preferred manner, the present invention provides a method
 for detecting short, untextured or defectively textured yarn segments in
 textured filament yarns during the texturing process, which uses to
 determine both the thickness of the yarn and the position and/or travel
 behavior of the yarn, optic and/or laser-optic devices that are able to
 detect short tight spots in the yarn, i.e., that have high scanning rates,
 preferably &gt;10 kHz. According to the invention, the scanning rates
 selected should in particular be so high that at the chosen production
 speed the yarn thickness is scanned at 1-mm intervals and fluctuations of
 yarn position of 6 kHz or more can be detected. The higher the scanning
 rates and the resolution of the optic and/or laser-optic devices used, the
 shorter the tight spots that can be detected and/or the higher the
 production speeds at which the method can be employed. The yarn thickness
 can be determined by means of a photocell and the variation of the yarn
 position by means of a photocell strip (CCD strip).
 According to a further preferred embodiment of the invention, it is
 provided to perform both the determination of the yarn thickness and the
 determination of the position and/or travel behavior of the yarn during
 the imparting of the twist in the region of the friction disks of the disk
 friction unit, in particular to perform said determination on or at the
 last working disk or the last yarn guide disk, i.e., the exit disk.
 Depending on the instability of the texturing process, the travel
 behavior, in particular, of the yarn exhibits marked differences at or on
 the exit disk, and the optic or laser-optic measurements therefore are
 preferably to be performed there. However, the detection of the yarn
 thickness can also take place over the entire distance from the exit disk
 of the twister to the winding of the textured yarn onto the spool.
 The described method according to the invention can also, of course, be
 used to detect long untextured yarn segments (so-called surging spots).
 Further advantageous embodiments of the invention will be understood from
 the dependent claims.

FIG. 1 schematically illustrates a device 1 for texturing filament yarns by
 a friction false-twist method. The device 1 comprises a spool 3 from which
 the multifilament yarn 5 is unwound and guided through a delivery device 7
 into a heater 9 [syntax sic]. The multifilament yarn 5 is guided out of
 the heater 9 over a cooling rail 11 and through a twister 13 realized as a
 disk friction unit into a draw-off delivery device 17 and from there to
 the yarn spool 19. Disposed between twister 13 and draw-off delivery
 device 17 is a device 15 for detecting unstructured, particularly
 untextured or defectively textured, yarn segments in textured filament
 yarns. It can also be provided according to the invention that device 15
 is disposed downstream, i.e., after draw-off delivery device 17 and before
 yarn spool 19.
 FIGS. 2A and 2B illustrate the basic structure of the monitoring device 15
 and the force components K1 and K2 exerted on the yarn 5. Device 15,
 comprising measuring pin 21, wire strain gauge 16 and a measurement and
 evaluation system (not shown), measures the yarn tension F2 after twister
 13 of the yarn 5 guided across measuring pin 21 and two guide pins 23, 25.
 The deflection of measuring pin 21 can be measured by means of wire strain
 gauge 16, Hall sensors, or capacitive, inductive or optical sensors. It
 will also be understood from FIG. 2A that the yarn 5 passes over guide
 pins 23, 25 and pin strain gauge 21, so that these are disposed inside the
 radius of curvature of the path of the yarn. In FIG. 2B, the bending
 direction of measuring pin 21 (bending beam) is indicated by an arrow. The
 bracket indicates the bendable region 21 of device 15, the wire strain
 gauges 16 being disposed in the region of potentially greatest deflection.
 Other measurement systems can, of course, be used to determine the yarn
 tension, provided that they are able to receive high-frequency yarn
 tension signals in the range of at least 0.2 kHz to 6 kHz.
 FIG. 3 shows the structure of device 15 in the form of a block diagram.
 Device 15 comprises a measuring sensor 27, to which a buffer amplifier
 (not shown) can potentially be assigned. The signals S received by
 measuring sensor 27 and potentially amplified are routed through a filter
 29, preferably a low-pass filter, to an analog-to-digital converter 31.
 The digitized signals are stored in a memory 33 and can be evaluated by
 means of suitable devices and methods, for example by the determination of
 means and standard deviations or the plotting of histograms (block 35), or
 by the performance of an autocorrelation (block 37) or an FFT analysis
 (fast Fourier transformation) (block 39). The data digitized by
 analog-to-digital converter 31 can also be routed to a digital display 34
 or to a digital-to-analog converter 43. To compress the data sets, it is
 advantageous to a condition the analog measurement signal (S) or the
 filtered signal electronically by analog means, for example by analog
 differentiation, taking of the mean, determination of the scattering
 matrices (block 30) and/or the setting of selectable thresholds for
 counting the number of excursions over the threshold value (block 32). The
 digitized results of the count (block 36) already represent extreme
 compression of the data set and can be delivered to a central evaluating
 computer for storage (block 33) and for further processing and hard-copy
 printout or display (block 34).
 Said signal processing devices can comprise devices for the selectable
 setting of threshold values and counting devices that determine the number
 of high-frequency yarn tension signals exceeding the threshold value per
 time unit and supply this number as a measure of the degree of crimping of
 the yarn and of irregularities and tight spots.
 The operation of the device can be described as follows. The smooth
 multifilament yarn 5 on spool 3 is guided through delivery device 7,
 heater 9, cooling rail 11 and twister 13. Twister 13, which can be
 realized as a disk friction unit, for example, twists the filament yarn,
 causing the turns to "pile up" in the upstream direction, i.e., the
 direction of delivery device 7. The twisted filament yarns are in a state
 of tension that is heat-set by heating in heater 9, for example to
 200.degree. C., and cooling on cooling rail 11. As soon as the yarn 5
 leaves twister 13, the torque is again removed from the yarn 5, although
 at least some of the state of tension of the yarn 5 obtained by the
 heat-setting remains and causes the yarn leaving twister 13 to be crimped,
 structured, bulky and lofty. This yarn is guided through device 15 and a
 draw-off delivery device 17, and after leaving-draw-off delivery device 17
 in the relaxed state, can be wound onto yarn spool 19.
 The method according to the invention now provides for the localization in
 particular of short tight spots in the yarn 5, i.e., short, untextured
 segments of yarn, thereby providing a statement as to the degree and
 evenness of crimping of the yarn. Such short tight spots in the yarn 5 are
 manifested by brief yarn tension signals. The method according to the
 invention permits the determination and evaluation of the crimping of the
 yarn through the detection of high-frequency--i.e., brief--yarn tension
 signals. For this purpose, the twisted yarn 5 is guided over the guide pin
 23 shown in FIG. 2 to a measuring pin 21 comprising, for example, one or
 more, for example two or four, wire strain gauges. The yarn 5 is then
 routed over a further guide pin 25 to draw-off delivery device 17.
 Measuring sensor 27 realized as a measuring pin has, for example, a
 periodic resonance preferably in excess of 6 kHz, especially preferably in
 excess of 10 kHz, permitting the reception of high-frequency yarn tension
 signals in frequency ranges of 0.2 to 6 kHz or 0.2 to 10 kHz. Such yarn
 tension signals can, for example, be measured and analyzed at a scanning
 rate of 250 kHz (FIG. 5).
 FIG. 4A shows a curve of yarn tension F2 over time (compare FIG. 2) for a
 yarn 5 exiting twister 13. The high-frequency yarn tension signals 45
 shown indicate short tight spots in the yarn 5.
 The high-frequency yarn tension signals 45 received are smoothed by means
 of a low-pass filter whose resonance is, for example, approximately 60% of
 the periodic resonance, so that it filters out (FIG. 4B) the extremely
 high-frequency yarn tension signals 47, for example above 10 kHz, caused
 by vibrations of the yarn 5 on the friction disk. FIG. 4B further shows
 that a threshold value 49 is set for the evaluation of the signals
 obtained; a counting device registers the number of yarn tension signals
 45' exceeding the threshold value 49 per time unit (threshold excursions
 51, 53). The number of yarn tension signals exceeding threshold value 49
 per time unit provides information on the existence of short tight spots
 in the yarn 5.
 The described method according to the invention and the device for carrying
 out this method can also, of course, be used to detect longer untextured
 yarn segments (surging spots) on the yarn 5.
 FIGS. 5A to 5C show amplitude spectra of the yarn tension F2 with different
 D/Y ratios (D: circumferential speed of the disk; Y: speed of the yarn).
 D/Y ratios of 1.9 to 2.5 (FIG. 5B) indicate the presence of a stable
 process in which no tight spots are produced, whereas in the ranges
 D/Y&lt;1.9 and &gt;2.5, tight spots occur (FIGS. 5A and 5C). High
 amplitudes characteristic of short tight spots are marked with an arrow.
 FIGS. 5D and 5E show amplitude spectra of the yarn tension F2 at different
 heater temperatures, taking as an example a microfilament yarn PES 50 dtex
 f80. At a heater temperature of 230.degree., amplitudes of fluctuation of
 the yarn tension (FIG. 5D) occur, especially in the high-frequency range,
 that result in tight spots and filament breakage. High heater temperatures
 cause an unstable process, recognizable by the high-frequency fluctuations
 of the yarn tension, and therefore result in damage to the yarn. FIG. 5E
 illustrates the amplitude spectrum of a stable process at a heater
 temperature of 190.degree. C.
 FIGS. 5F and 5G show amplitude spectra of the yarn tension F2 with
 different degrees of stretch V. With low stretch (V=1.55), tight spots are
 formed; at a degree of stretch V=1.60 a stable process is present.sup.3.

FNT .sup.3 TRANSLATOR'S NOTE: Last clause of sentence garbled (" . . . at which
 degree of stretch of V=1.60 is a stable process are present.")
 FIG. 6 schematically depicts the structure of a twister 13 realized as a
 friction unit 248. FIG. 6 illustrates the structure of the disk friction
 unit 248 comprising three shafts 250, 252, 254. Disposed on each of the
 three shafts 250, 252, 254 are concentric friction disks 256. The
 direction of travel VF of the yarn is indicated by an arrow. The
 determination according to the invention of the thickness or travel
 behavior of the yarn is preferably performed at or on the last working
 disk 256' or the exit disk 256".
 FIG. 7 schematically illustrates the geometry of the path of travel of the
 yarn on exit disk 256" under different process conditions. The figure
 shows the path of travel of the yarn in direction VF from entry point E
 over exit disk 256" with its equator M (the center of the disk) in the
 direction of exit point A of unit 248. The axis 258 of the disk is also
 shown. The travel behavior and/or thickness of the yarn on exit disk 256"
 is determined by means of a high-speed video camera with an evaluating
 system and the ability to receive up to 6000 images/s. The optic or
 laser-optic determination of the thickness and/or travel behavior of the
 yarn can be performed on (block 260, for example) or after (block 262, for
 example) exit disk 256". The images taken by the high-speed video camera
 reveal that the yarn moves essentially perpendicular to its direction of
 travel VF on disk 256", i.e., in the direction VD, during both stable
 processes and unstable processes, that is, texturing processes that result
 in tight spots. In a stable process--i.e., a process in which no tight
 spots occur--the yarn traces a sharp curve (Position 1). The amplitude of
 the transverse movement of the yarn is small. The maximum amplitude of the
 transverse movement of the yarn at the center M of the disk is app. 0.2 mm
 (the diameter of the PES yarn 50 dtext f80 in the friction unit is 0.09
 mm, and the travel speed of the yarn is, for example, 600 m/min).
 In a slightly unstable process there is relatively large transverse
 movement of the yarn, the maximum amplitude of said transverse movement
 being 0.5 mm. The yarn oscillates periodically at a low frequency back and
 forth between Positions 1 and 3.
 In a frankly unstable process, the yarn is usually in Position 4. The yarn
 oscillates periodically at a low frequency back and forth between
 Positions 4 and 2, the maximum amplitude of the transverse movements of
 the yarn being app. 1 mm.
 Undesired short, untextured locations in the yarn (tight spots) occur when
 the process is unstable.
 Since tight spots can also be detected in textured yarn by measuring the
 yarn tension in the kHz frequency range, the optical method according to
 the invention can be used to check or replace yarn tension measurements.
 According to the invention, results concerning the travel behavior of the
 yarn or the yarn thickness can be correlated with the results of yarn
 tension measurement in order to detect tight spots. At higher yarn
 tensions, the yarn tends to follow a rectilinear, more gently curved path
 over disk 256", i.e., it tries to take the shortest route. In so doing,
 the yarn can, for example, slip from Position 1 to Position 2, thereby
 giving rise to twist slippage and undesired tight spots. The yarn
 thickness varies little in Zone M-A (see FIG. 7) during a stable process,
 whereas an unstable process is recognizable by a fluctuating yarn
 thickness with the formation of short tight spots.
 The method according to the invention can, of course, be used to detect not
 only short tight spots, but also long, untextured segments of yarn
 (surging spots).