EP0291729B1 - Method and apparatus for measuring the position of the weft threads or stitch courses in textiles - Google Patents

Description

The invention relates to a method and a device according to the preamble of claim 1 or 6 or 11.

In the manufacture of fabrics, warp and weft threads cross exactly at right angles. During various subsequent work steps in the equipment, however, the fabrics can be warped again. In the manufacture of knitted fabrics on circular knitting machines, the resulting tube is cut open, so that the knitted fabric is generally skewed after being cut open. In both cases, the warping must be eliminated by appropriate straightening machines, these straightening machines needing the warping angle as a control variable. It is therefore important to measure the draft angle.

From DE-PS 16 35 266 a device is known for measuring the draft angle, in which a single gap with a photosensor arranged behind it is rotated back and forth by an electrodynamic drive system, the rotary movement being close to the mechanical resonance frequency of the system around a central angle. The speed of the rotary movement is therefore predetermined by the system. The output signal of the photosensor is summed up via an amplifier, the sign of the amplification always being reversed when the central angle is exceeded. The signal summed up over a period therefore becomes zero when the measured values are distributed symmetrically around the central angle. This is the case when the weft has the same direction as the center angle. Furthermore, a follow-up control is provided in the known system, which adjusts the overall system or the center angle in accordance with the current measured value such that the center angle always runs parallel to the weft thread. By measuring the center angle, a direct measurement of the weft course or the draft angle is possible.

This known system is disadvantageous in that it requires mechanically moving parts that are inevitably subject to a certain amount of wear. In addition, the fact that the speed of movement (resonance frequency) has to be adapted to the conveying speed of the tissue passing by means that the conveying speed is limited by the inertial masses of the moving parts.

Furthermore, a device according to the preamble of claim 6 and a method according to the preamble of claim 1 or 11 are known from DE-A 1 109 636, two photocells with slit diaphragms lying in front of them being arranged opposite one light source, the central axes of which are angular to one another stand. A value for the angular profile of the weft thread is derived from the difference signal of the photocells without the arrangement having to be moved mechanically. This arrangement depends on the correct measurement of the luminous flux passing through the textile web, which presents certain difficulties with a single photo sensor, since the luminous flux depends not only on the spacing of the weft threads from one another and their thickness, but also on the color of the textile web . This poses difficulties for printed fabrics and irregular fabrics. However, since two optical-electrical systems have to be matched to one another in the known device, these difficulties are increased considerably. Another problem is the small "catch area" of the arrangement, which is determined by the angle between the two slit diaphragms.

Starting from the above-mentioned prior art, it is an object of the present invention to further develop methods and apparatus of the type mentioned in the introduction such that a correct measurement of the weft thread course can be achieved with simple means with less susceptibility to faults.

This object is achieved by a method according to claim 1 or 11 or an apparatus according to claim 6.

The main idea of the present invention is therefore that it is not the total luminous flux within the gap-shaped section that is considered, but rather its "pattern" within the area or the movement of the pattern within the area. This means that the brightness values can be divided into just two levels (light / dark), which significantly reduces the sensitivity to interference. Knowing this idea, the mathematical Derive rule for determining the draft angle from geometric considerations.

Fig. 1

eine perspektivische Prinzip-Darstellung der Anordnung von Beleuchtungsquelle, Textilbahn und Sensor;

Fig. 2

eine Prinzipdarstellung der Anordnung von zwei Sensoren relativ zu den Schußfäden einer Textilbahn;

Fig. 3

eine Prinzipdarstellung des Ausgangssignal-Verlaufes der Sensoranordnung nach Fig. 2;

Fig. 4 und 5

Prinzipdarstellungen zur Erläuterung der in der Beschreibung verwendeten Kurzbezeichnungen;

Fig. 6

eine besondere Anordnung von Sensorelementen gemäß einer weiteren bevorzugten Ausführungsform der Erfindung;

Fig. 7

prinzipielle Darstellungen der Ausgangssignal-Verläufe einer Anordnung nach Fig. 6; und

Fig. 8

ein Blockdiagramm einer Auswerteinrichtung für zwei CCD-Zeilen.

Preferred embodiments of the invention and further ideas essential to the invention result from the subclaims and the following description of exemplary embodiments, which are explained in more detail with reference to figures. Here show:

Fig. 1

a perspective schematic representation of the arrangement of the lighting source, textile web and sensor;

Fig. 2

a schematic diagram of the arrangement of two sensors relative to the weft threads of a textile web;

Fig. 3

a schematic representation of the output signal curve of the sensor arrangement of FIG. 2;

4 and 5

Schematic diagrams to explain the short names used in the description;

Fig. 6

a special arrangement of sensor elements according to a further preferred embodiment of the invention;

Fig. 7

basic representations of the output signal curves of an arrangement according to FIG. 6; and

Fig. 8

a block diagram of an evaluation device for two CCD lines.

As shown in Fig. 1, the arrangement is such that a light source 11 is arranged behind Reflector 12 radiates onto a textile web 10 which is conveyed past the arrangement in the direction of arrow P. A CCD line 14 or 15 with a lens 13 in front of it is arranged opposite the light source 11 with reflector 12. A number of such arrangements are provided across the entire width of the fabric web, so that, for example, even a garland warping can be detected by determining the warping angle in sections.

As shown in Fig. 2, the wefts 1 and 2 appear as dark fields, while the gaps between them appear as light fields. When conveying the fabric web in the direction of arrow P, the weft threads 1 and 2 shift, as is indicated in Fig. 2 with the weft threads 1 'and 2' shown. While the weft threads pass the CCD lines 14 and 15, the individual sensor elements 14-1, 14-2 ....; 14-n or 15-1 ...., 15-m of the CCD lines 14 and 15 progressively exposed or darkened.

The individual elements of the CCD lines 14 and 15 are (as is known) read out serially, which is to be illustrated in FIG. 3. Assuming the static case, in which the readout speed is very high in relation to the conveying speed of the fabric web, FIG. 3 also shows in the step-like course of the output signals the “blurring” which inevitably occurs in the edge areas between light and dark zones results. The sensor output signals are compared with a threshold SW in order to obtain signals that can be processed and are fault-free. All values above the threshold SW are classified as "light", all values below as "dark".

If one assumes an "ideal" black-and-white pattern, which is formed by the weft threads 1, 2, the result is not very high with respect to the conveying speed Readout speed again a signal pattern, as shown in Fig. 3. In this case, the finer graded course stems from the temporal integration of the luminous flux in the individual sensor elements 14-n, 15-m. Here too, the division SW into two groups increases the interference immunity.

If the division into light or dark was made on the basis of the threshold SW, the value of interest can be calculated. To explain the calculation, the abbreviations used are first explained in more detail with reference to FIGS. 4 and 5. 5, the distance between two dark zones (weft threads 1 to 5) is denoted by a, the thickness of the weft threads, that is to say the "dark field", by d. The length of the gap-shaped section, which corresponds to the line length of the sensor, is denoted by S. 1 denotes the maximum length of a "dark group", ie the number of successively darkened sensor elements (multiplied by their length). L denotes the "period" corresponding to the aforementioned variable 1, that is to say the distance on the CCD line within which the pattern is repeated. The drafting angle is designated by α, that is to say the angle between a weft thread 1 to 9 and the axis perpendicular to the transport direction P (transport direction normal). With β₀ or β _n the angle between the CCD lines 14, 15 and the transport direction normal is designated, while γ denotes the angle between a CCD line 14 or 15 and a weft thread.

Various options for calculating the draft angle α are described below.

If one assumes the sizes a and d given by the tissue under consideration as known, then the calculation is made the angle γ between the CCD line and weft threads according to the equation

At a predetermined angle ß of the CCD line to the normal to the conveying direction, the draft angle α then results

$\text{α = β - γ; (2).}$

As can easily be seen from the equations above, positive and negative draft angles cannot be distinguished. The distinction can be made via the speed of movement of the pattern (with known transport speed). In the upper CCD line 14 shown in FIG. 5, the movement speed would be greater with a positive draft angle α (in the definition given in FIG. 5) than with a negative draft angle α. Furthermore, it is possible to determine the "number of threads" (weft threads per unit length) by means of a further CCD line arranged in the conveying direction and to use the calculation described above as a basis.

Wobei zn bzw. z0 in Gleichung 4 definiert sind:

$$\text{z = 2 ·}\frac{\text{S}}{\text{L}}\text{ (4)}$$

Also die "Periodenzahl" über die CCD-Zeile darstellen (der Index n bzw. 0 ist in Fig. 5 definiert).A further, simpler option for calculating the draft angle α is obtained if the arrangement of two CCD lines 14 and 15 oriented at an angle to one another is selected in FIG. 5. In this case, the angle α results in

Where zn and z0 are defined in equation 4:

$$\text{z = 2}\frac{\text{S}}{\text{L}}\text{(4)}$$

So represent the "number of periods" on the CCD line (index n or 0 is defined in Fig. 5).

wobei die obigen Definitionen gelten. Besonders einfach ist die Herleitung des Verzugswinkels α auch deshalb, weil die CCD-Zeilen 14 und 15 digital arbeiten und die Werte für z ohnehin als zählbare Einzelwerte vorliegen.In this embodiment of the method according to the invention, the calculation is particularly simple if the two angles β0 and βn are chosen to be of the same magnitude. Equation 3 is then simplified

where the above definitions apply. The derivation of the draft angle α is also particularly simple because the CCD lines 14 and 15 work digitally and the values for z are available as countable individual values anyway.

zu 15° gewählt.Preferably the angle ${\text{β₀ = β}}_{\text{n}}$

selected at 15 °.

To get the greatest possible accuracy, it is advantageous if the CCD lines are as long as possible (in relation to the number of threads). This can also be achieved by appropriate optics, in which a reduced image of the tissue is projected onto the CCD lines.

Wobei selbstverständlich auch anstelle der "Dunkelstrecken" 1 auch die "Hell-Strecken" (L - 1) zur Berechnung herangezogen werden können.Instead of the above-mentioned possibilities of determining the number of weft threads which lie over a CCD line, there is also the possibility (as indicated) of using the sum of the brightly (or darkly) illuminated sections on the CCD lines for the calculation. The draft angle α then results in

Of course, instead of the "dark lines" 1, the "light lines" (L - 1) can also be used for the calculation.

In a further preferred embodiment of the invention, the numbers z or the lengths 1 are obtained over several scanning cycles of the CCD lines. This enables a significant increase in the signal-to-noise ratio.

A further preferred embodiment of the invention is described in more detail below with reference to FIGS. 6 and 7. In this (alternative) calculation method, the speed of movement of the pattern over CCD lines 14; 15 used as the basis for calculation.

If one assumes that a scanning cycle of the CCD line represents the quasi-static position of the weft threads 1 to 8 above the CCD line, the result shown in FIG. 7 results. If the first scan cycle at time t0 after division into light or dark results in the image shown in FIG. 7, the subsequent scan cycle at time t1 is shifted to the right of this image, the same applies to all subsequent scan cycles. The amount of displacement is designated τ 14 in FIG. 7.

Wobei der Verzugswinkel α nach Gleichung 2 errechnet werden kann.After the second CCD line 14 has an obtuse angle to the weft threads, the period to be observed there is shorter than that of the CCD line 15. This is shown in FIG. 7 below. The time interval τ15 according to FIG. 7 is therefore larger than the interval τ14 observed with the CCD line 14. The angle γ between the CCD line 14 and the weft threads 1 to 8 then results in

The draft angle α can be calculated according to equation 2.

This embodiment of the invention also has the advantage that the time intervals τ14 and τ15 used in equation 7 can be averaged over a particularly large number of individual values, with of course not only the time intervals between the rising edges of corresponding bright areas, but also between the falling ones Flanks of the averaging can be used.

8 shows an example of a circuit arrangement (in principle) which is suitable for carrying out the method described above.

The CCD lines 14 and 15 are, as shown in Fig. 8, driven by a common sensor driver 20 and give their, the applied amount of light output signals via buffer amplifiers 16, 16 'and blocking circuits 17 to sample and hold circuits 18 to others Buffer amplifier 19, 19 'further. The blocking circuits 17, 17 'and the sample and hold circuits 18, 18' are - like the sensor driver 20 - synchronized via a timing circuit 22.

From the buffer amplifiers 19, 19 'the output signals reach the inputs of controllable output amplifiers 23, 23', the outputs of which are routed to inputs of threshold circuits 24 which carry out the black / white discrimination. The output lines 28, 28 thus represent binary outputs which are led into an I / O interface.

The I / O interface 33 is connected to a CPU 34, which has access to a RAM 35 via data lines. Furthermore, an output interface 36 is provided, which is in a controlling connection via data line with the subsequent organs for compensating for the angle of distortion.

In order to also be able to monitor the light source or make a fault message, the output signals of the buffer amplifiers 19, 19 'are passed on to the I / O interface 33 via threshold switches 25, 25'. By appropriately setting the threshold levels, it is possible to determine whether the CCD lines 14, 15 are receiving too much light, that is to say they are operated in saturation. This saturation signal is further passed on via a latch 26, 26 'to the I / O interface 33, each latch 26, 26' being controlled via a start signal line 30, which is also routed into the I / O interface 33.

In addition to the sensor driver 20, the timing control circuit 22 also controls an exposure time control 21 to which the CPU 34 has direct access via the I / O interface 33 and an exposure control line 32. A clock line 31 connects the timing control circuit 22 to the CPU 34 for synchronization (via the interface 33).

The threshold values SW (see FIG. 3) can be set by the CPU 34 via lines 29, 29 '.

The evaluation device 37 designed in this way can be programmed so that the method described above for calculating the draft angle α is carried out.

The CCD lines 14, 15 do not necessarily have to be designed as separate line arrangements, but rather can be arranged in a single matrix arrangement. The angles β₀ and β _n are then defined by appropriate selection of the matrix elements.

die Schußfäden genau senkrecht zur Förderrichtung liegen, so sind die Änderungsgeschwindigkeiten der beiden Ausgangssignale gleich bzw. ist die Phasenlage der Signale zueinander 0°. Sobald ein Verzugswinkel α auftritt, ergibt sich auch eine Phasenverschiebung der beiden Signale zueinander, sowie eine Differenz in der Änderungsgeschwindigkeit. Aus diesen Differenzen ist nun der Verzugswinkel herleitbar.In a further preferred embodiment of the invention, two line-shaped transducers of an arrangement similar to that according to FIG. 6 are also provided. However, these are not CCD lines, but position-sensitive, line-shaped photodiodes whose output signals correspond to the brightness distribution of the light-sensitive surface. Such converters are, for example, lateral effect photodiodes or photodiodes with a gray wedge in front of them. If a line pattern, as drawn in FIG. 6, moves over such a converter, an output signal results with an essentially sawtooth-shaped AC component. These alternating current components are now compared with one another in the evaluation device, which can be constructed in a manner known per se, either with regard to the rate of change or with regard to the phase relationship of the signals to one another. The frequency of the two sawtooth signals is the same for both converters. If now in the arrangement shown in Fig. 6, the transducer ${\text{(β₀ = β}}_{\text{n}}\text{+ 90 °)}$

If the weft threads are exactly perpendicular to the conveying direction, the rates of change of the two output signals are the same or the phase relationship of the signals to each other is 0 °. As soon as a draft angle α occurs, there is also a phase shift between the two signals and a difference in the rate of change. The draft angle can now be derived from these differences.

In principle, this method of evaluation is also possible with CCD lines.

The individual features set out above are seen on their own and in combination are claimed as essential to the invention.

Reference symbol list

1 - 9

Weft

10th

Textile web

11

Lighting source

12

mirror

13

lens

14

first CCD line

14-1 to 14-n

Sensor elements

15

second CCD line

15-1 to 15-m

Sensor elements

16

Buffer amplifier

17th

Blocking circuit

18th

Sample and hold circuit

19th

Buffer amplifier

20th

Sensor driver

21

Exposure time control

22

Timing circuit

23

Output amplifier

24th

Black and white threshold circuit

25th

Saturation detector

26

Latch

27

Gain control line

28

Binary output

29

Black and white threshold

30th

Start signal line

31

Clock line

32

Exposure control line

33

I / O interface

34

CPU

35

R.A.M.

36

Output interface

37

Evaluation device

SW

threshold

a

Bright distance

b

Dark distance

S

Line length

P

Direction arrow

α

Draft angle

γ

Angle between weft and line

β

Angle between line and transport direction normal

τ

Movement time interval

t0 to tn

Time of the line passes

Claims (11)

1. A method of measuring the position of the weft threads or courses (offset angle α) in continuously advanced webs (10) of fabric, wherein at least one slit-like section of the fabric web (10) is observed under incident light or transmitted light by means of photoelectric transducers providing measured values, the width of said slit being small while its length is large in comparison with the thickness of the weft threads, the longitudinal axis of said slit extending at a defined constant angle to the direction of advance, characterized by
   setting the brightness values in said section within two stages or ranges (bright, dark) by means of at least one threshold switch (24, 24') and determining those portions within said section in which the brightness values are continuously related to one of said stages, and by determining either the number of the (overall) length of said portions of a stage by discrete evaluation of the measured values
   or by determining the velocity of movement of the portions of a stage within said section,
   whence the offset angle (α) of the weft thread (1-9) is derived and digitally analyzed.
2. The method as claimed in claim 1, characterized in that the sign of the offset angle (α) is derived from the direction of movement of the portions within said section.
3. The method as claimed in any one of the preceding claims, characterized in that two slit-like sections are observed which include a defined angle relative to one another, and that the number of portions within said sections or, respectively, the velocities within said sections are compared with one another.
4. The method as claimed in any one of the preceding claims, characterized in that prior to deriving the offset angle, averaging is performed through a plurality of chronologically spaced determinations of said numbers or velocities, respectively.
5. The method as claimed in any one of the preceding claims, characterized in that the incident or transmitted light is either applied in the form of flashes or the observation is performed for the duration of very short moments which are equidistant in time.
6. An apparatus for measuring the position of the weft threads or courses (offset angle α) in a continuously advanced web (10) of fabric, comprising at least one source of illumination (11), at least one photoelectric transducer (14, 15) and analyzing means (37) for analyzing the output signals from the transducers and for providing a value which is substantially proportional to said offset angle (α),
   characterized in that
   at least one of said transducers (14, 15) is configured as a line array (14, 15) of a plurality of sensor elements (14-1 to 14-n; 15-1 to 15-m) adapted to be scanned separately,
   that
   the analyzing means (37) comprises threshold switches (24) for grading the transducer output signals in two stages or ranges (bright, dark) and being configured such
   that
   either the number of sensor elements providing outputs of the same stage (bright, dark) is determined
   or the velocity at which the outputs of the same stage move across the line array (14, 15) is determined, and that
   said analyzing means (37) comprises a digital computing unit (34, 35) which is designed to derive and analyze said offset angle (α) from the number or the velocity, respectively.
7. The apparatus as claimed in claim 6, characterized in that in the case of a single transducer configured as a CCD line array (15) the analyzing means (37) comprises means for inputting the number of threads or, respectively, means for inputting the advancing speed.
8. The apparatus as claimed in claim 6, characterized in that two CCD line arrays (14, 15) preferentially of equal lengths are provided and are disposed at a defined angle (β₀; β_n) to the line normal to the direction of advance.
9. The apparatus as claimed in any one of the claims 6 to 8, characterized in that the source of illumination (11) is configured as a flash tube.
10. The apparatus as claimed in any one of the claims 6 to 9, characterized in that an optical system including a (cylindrical) lens (13) is provided upstream of each CCD line array (14, 15).
11. A method of measuring the position of the weft threads or courses (offset angle α) in continuously advanced webs (10) of fabric, wherein at least two slit-like sections of the fabric web (10) are observed under incident light or transmitted light by means of photoelectric transducers for providing measured values, the width of said slits being small while their lengths are large in comparison with the thickness of the weft threads, the longitudinal axes of said slits having defined constant angles to the direction of advance, characterized by
    determining the brightness distributions of the transducers (14, 15) or the position of the concentration point of illumination density on said transducers (14, 15) to produce corresponding transducer outputs,
    and by deriving and digitally analyzing the offset angle (α) from the changing rates and/or phase positions of the transducer outputs and the comparison therebetween.

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