GB2127544A - Measuring tension - Google Patents

Abstract

Method and apparatus for continuously measuring the tension of a moving product e.g. yarn or sheet, wherein a moving product 2 is subjected to transverse vibrations between two fixed supports over a predetermined length of its path between set supports, and the vibrations are detected remotely by an optical system 6-9 including a laser 6. A signal representative of the detected vibrations is transmitted to an electronic servo system (Fig. 6) which controls the exciting system which may, for example, be a loudspeaker (1) or a ceramic finger to maintain resonance. The signal also is fed to a display system (16, 17) which indicates the tension, which may be deduced from the resonant frequency of the product or one of the harmonics thereof. <IMAGE>

Description

SPECIFICATION Measuring tension This invention relates to a method and apparatus for measuring, without contact, the tension of piliform products and moving surfaces.

"Filoform products" are understood to mean any textile, plastic, metallic or optical yarn: textile yarns may be in the form of continuous, multifilament or multi-ply yarn, mono-filaments or spun yarn, employed alone or in combination and made of any natural, regenerated, synthetic or inorganic material.

"Surface" is understood to mean any woven, knitted or non-woven textile surface, surface of a plastic sheet, a sheet of yarns, tapes or strips of a plastic, paper or metal.

In the textile industry, the uniformity of tension of a yarn which is being manufactured or being converted is particularly important; in fact, during the manufacture of yarns, irregularities of tension lead, for example, to irregularities in orientation causing variations in dyeing affinity, to zones of weakness causing ply breakage, and to irregularities of appearance of the finished articles using these yarns. During the conversion of the yarns, either by weaving or by knitting, the irregularities of tension can, for example, cause breakages on the machines or defects of appearance in the finished woven or knitted material. The same applies to sheets of yarns, for example during the warping.

With regard to metallic yarns, the regularities of tension are of prime importance in drawing and standing. The same applies to the manufacture of plastic films for example when they are drawn and wound onto reels. The same also applies to the regularity of tension of filiform products in sheathing or coating.

In the description that follows, for the sake of simplicity, the "filiform products" and "surfaces" will simply be referred to as "products".

An object of the present invention is to offer a simple method and apparatus making it possible to monitor continuously the tension of the various sections of a path which is followed by a moving product.

According to the present invention we provide a method of continuously measuring the tension of a moving product, said method comprising subjecting the moving product to transverse vibrations between two fixed supports, over a predetermined length of its path, detecting the vibrations remotely by an optical system and transmitting a signal representative of the detected vibrations to an electronic servo system to control the means subjecting the product to the transverse vibrations and also to a means for reading the signal to give the tension.

The two fixed supports on the path followed by the moving product are determined by components of the means for manufacturing or for converting the said product.

Because the method involves the use of an optical detector system the method can be carried out without contact between the product and the detection system.

By "without contact" is understood to mean the total absence of contact or a slight contact through very faint rubbing having no influence on the quality of the product.

According to another aspect of the present invention we provide apparatus for continuously measuring the tension of a moving product, said apparatus comprising two spaced supports for the moving product, said supports being spaced apart by a predetermined distance, a vibration exciting system for causing a product fed between said supports to vibrate transverse to the direction of feed, an optical system for the remote detection of the vibrations produced and an electronic servo system controlling the vibration exciting system in response to signals received from said optical system.

The principle of the measurement is based on the law of vibrating strings: the fundamental frequency f0 (in Hz) of a transverse vibration of a portion of length L (in metres) of the product, for example a portion included between two guides or two rollers or guide and roller, or the like, is related to the tension t (force per unit of surface area in g/tex) by the relationship

The optical observation of this frequency therefore enables the tension to be determined in a simple manner. It is also possible to provide for the harmonics of the fundamental frequency when the latter is unsuitable for the measurement, when the fundamental frequency becomes too low; in this case the relationship between frequency and tension, for the harmonic order N, becomes fN=N.fo.

The measurement can be carried out in air or in an immersion medium.

The vibration-exciting system employed is preferably a loudspeaker which is suitable for the frequency range to be transmitted and is placed in the neighbourhood of the middle of the vibrating string and at a distance from the yarn which can vary preferably between 1 centimetre and 20 centimetres. The power may be concentrated by incorporating an acoustic cone nozzle: it is also possible to employ an electrodynamic vibrator whose vibrating part terminates in a ceramic finger which is placed against the product and which forms one of the ends of the vibrating string. The advantage of the loudspeaker is the absence of any contact with the product, the vibration being transmitted by air; some slight disadvantages can nevertheless arise in the case of a high tension because of the appearance of a dephasing between the loudspeaker vibration and the movement of the product.The electrodynamic vibrator has the advantage of never producing dephasing, but, on the other hand, it involves a slight contact with the product and can sometimes induce a triangulation motion which is superposed on the normal mode of vibration when it vibrates at a frequency which is well away from the resonance of the product, this triangulation motion being prejudicial to the operation of the electronic servo. This parasitic phenomenon can be suppressed by adjusting the electronic filters.

As a general rule it can be taken that the loudspeaker is the most convenient exciter for measuring tensions which are predominantly below an upper limit which is a function of the nature of the product and of its linear density (filiform products), and which is in the region of 1 0 g/tex for the textile yarns. When the apparatus is used, the principles to be applied relate to the optical aim and to the adjustment of the filters and the gain; the servo electronics will then lock without difficulty onto the resonance to be measured. In measurements of tensions which are greater than the abovementioned limit, a dephasing appears between the loudspeaker and the detector, becoming more pronounced the higher the tension. If this high tension shows little fluctuation, of the order of +20% around the mean, the dephasing I is approximately constant.

An opposite dephasing -1, which restores the normal operation of the apparatus is then introduced into the servo loop. The measurement of a high tension can thus also be carried out in this case by the use of a loudspeaker requiring only one additional adjustment, which is advantageous since, as already stated, the loudspeaker makes no contact with the yarn.

However, it is possible to resort to the electrodynamic vibrator when dealing with high tensions.

In order that the present invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawing, in which: Figure 1 is a schematic view of a moving yarn having its tension measured by one example of method according to the invention.

Figure 2 is a simliar view of a modified method: Figure 3 is a schematic view of a laser vibration detector system forming part of one embodiment of apparatus of the invention: Figure 4 shows the Gaussian curve of the transverse intensity of vibration plotted against distance from the centre of vibration; Figures 5a to 5d show various types of laser aiming conditions: Figure 6 is a schematic block diagram of one embodiment of apparatus according to the invention; and Figure 7 is a curve showing the relationship between the tension measured in the yarn and the peripheral speed of a pair of overdraw rollers according to example 4 below.

Figure 1 shows a loudspeaker 1, placed in the neighbourhood of the middle of a vibrating yarn forming a moving product between two points 3 and 4 along the path of the moving product.

Figure 2 shows an electrodynamic vibrator 5 in contact with the product 2, the vibrator being placed at one end 3 of the vibrating string formed by the moving product 2 between the two point 3 and 4.

The optical system for remote detection of vibrations preferably comprises a low-power laser, for example a helium/neon laser, which illuminates the product at an approximately normal incidence, and an objective sending back a part of the light which is diffused, reflected or refracted by the product to a light-detector consisting of a phototransistor and its polarising electronics, this light-detector measuring the intensity of a part of the light which is reflected by the product.The transverse movements whose amplitude exceeds a few tens of microns modify the illumination of the product by the laser, and hence the intensity received by the light-detector; the latter provides a signal with an intensity which is modulated at the frequency f0 or fN. The transverse distribution of intensity in a section of laser beam is Gaussian in character; when the product moves in a section, its illumination changes and the intensity received by the phototransistor reflects the motions of the product. It should be stressed that the frequency of the optical detector is identical to that of the exciter. The aiming of the laser should be adjusted so that the transverse movements of the yarn scan one side of the Gaussian distribution.The spread of the distribution can be increased by incorporating a cylindrical lens between the laser and the product; this accessory can be useful for keeping the product in the beam when the product floats transversely because of, for example, a poor tension control, a mechanical defect in the path followed by the product, caused, for example, by a blower under the spinneret during the manufacture of a yarn or by a poor stability of wind-up on rollers or on reels. The distance between the optical system and the product varies preferably between 5 centimetres and 50 centimetres. The attached Figure 3 shows the product 2, a laser 6, a movable lens 7, an objective 8 which generally consists of a converging lens, and a phototransistor 9.

Figure 4 shows the Gaussian distribution of the transverse intensity I in a section of the laser beam at a distance R from the centre 0. In Figure 5 there are shown various types of laser aiming conditions in which 0 refers to the centre of the beam, P to the mean position of the yarn in the beam and A is the vibration amplitude of the product; thus 5a corresponds to the optimum aim condition, 5b an acceptable condition, and Sc and 5d unacceptable conditions.

The electronic servo system which connects the two aforementioned systems consists of a number of means, the principle of the electronic servo control of the device being shown in the attached Figure 6.

The exciter, such as a loudspeaker 1, is fed by a low-frequency generator 12 through a gain stage 10 and a power amplifier 11. The signal produced by the detecting system TS is processed by a pair of adjustable high-pass and low-pass filters 1 4 whose function is to select the frequency range containing the required resonance or its harmonic. The filtered signal S1 and the signal S2 which are addressed to the exciter 1 are compared in a phase detector 1 5 which drives the generator 12 by means of a control voltage. The loop which is thus formed locks automatically onto the frequency producing the correspondence in phase between S, and S2. This frequency is the required resonance.To obtain a measurement of this frequency, the output signal of the filters 1 4 is converted into a square form 1 6 and sent to a pulse counter 1 7 which permits the frequency measurement to be read by means of a display and/or a recording, the tension being then derived by means of the aforesaid formula either manually or by an appropraite computing instrument installed in line. It is possible for example to use a frequency meter, a periodmeter or a computer which is capable of performing this function. In most cases it is sufficient to adjust the gain 10 manually to a constant value which gives a signal of a good quality. However, when the amplitude of the detection system signal varies greatly, the gain stage 10 can be driven so as to keep the level of the signal S1 constant.In order to facilitate the adjustment of the optical aiming of the laser, the adjustment of the filters 14 and the gain 10, the filter stage can be provided with two control outputs (signal before and after filtration) for display on an oscilloscope.

The device can also be employed to carry out the measurement of the linear density of a filiform product such as a textile yarn; in face when a length of the yarn is stretched by a force F in grammes, the measurement of the tension tin g/tex enables its linear density to be obtained from the relationship:

It has thus been found that it is possible to measure the in-line tension of these products by virtue of two principal advantages: a remote optical detection employing a laser which avoids a disturbance of the air flow, for example around a strand or a yarn, and a phase-sensitive servo control (phase-detector) allowing an adequate precision to be obtained in all the zones of the path followed by the product.

As already mentioned, the device can be used for measuring tension during product manufacture or conversion operations, whatever the speed of the product.

The following examples illustrate the present application without limiting it.

Example 1 It is required to measure the tension of a mono-filament yarn during its manufacture, between the spinneret and the take-off roller: Spinning conditions: -yarn of poly (hexamethylene adipamide) -ply linear density on the take-off roller 1 dtex (finished yarn 44/13) -roller speed: 1,030 m/min --spinneret-roller distance:2.00 metres Measurement conditions: -2mW helium/neon laser equipped with a cylindrical lens of focal length 80 mm, optics-yarn distance; 30 cm -a loudspeaker with a membrane of diameter 20 cm, without a cone nozzle, mounted in a protective panel, loudspeaker/adjustable yarn distance from 2 to 10 centimeters.

The measured tension is 2.4 g/tex+0.1 g/tex.

Although it has been obtained under difficult conditions, given the turbulence under the spinneret, this tension measurement nevertheless makes it possible, in view of the consistency of its value, to determine possible variations in temperature or in the rheological behaviour of the polymer.

Example 2 This example relates to the measurement of the tension between the abovementioned take-off roller and the oiling roller.

Spinning conditions: -yarn of polyethylene terephthalate with a ply linear density of 4 dtex/ply distance between take-off rollers and contact on oiling rollers: 0.50m, the oiling roller being placed between the take-off roller and the drawing rollers, and depositing a film of oil on the yarn speed of take-off roller: 600 m/min speed of drawing rollers: 2,500 m/min Measurement conditions: -2mW helium/neon laser without cylindrical lens, placed 30 cm from the wire -loudspeaker of diameter 20 cm, without a cone nozzle, placed at a distance of 1 to 5 centimetres from the measured yarn.

The yarn produced has a value of 140 dtex/30 plies, The tension measured to +2% are: 6.25 g/tex at a draw ratio of 4.6 6.75 g/tex at a draw ratio of 4.9 Example 3 This relates to the measurement of a 280 dtex/60 plies yarn of polyethylene terephthalate between the delivery rollers and an interlacing jet.

speed of delivery rollers: 3,000 m/min Measurement conditions: -2 mW helium/neon laser without cylindrical lens, placed 30 cm from the yarn -loudspeaker of 20 cm diameter, without a cone nozzle, placed at a distance from 1 to 5 cm from the yarn.

The measured tension was 0.80 g/tex+2% at an interlaced yarn wind-up speed of 2,900 m/min.

Example 4 In order to allow characterisation of the dynamo-metric behaviour of a yarn of the polyethylene terephthalate, a pair of rollers, or overdrawing pair, is inserted into the production line after the drawing rollers. The speed of the pair can vary linearly with time and thus impart a progressive additional extension to the yarn. An in-line dynamometric curve is obtained by recording the change in the tension while the overdrawing pair of rollers is accelerated. This curve, shown in Figure 7, was obtained with the use of the electrodynamic vibrator, the signal being measured by an SEMS computer employed for the surveillance of the position; the ceramic finger is placed at the delivery of the drawing rollers to locate precisely the upper node of the vibrating string.

The manufacturing conditions are as follows: spinneret throughput: 40g/minute for a yarn of 140 dtex/30 plies, speed of drawing rollers: 3,000 m/min, speed of the overdrawing pair from 3,000 m/min to 3,150 m/min, length of the vibrating string between the ceramic finger and the electrodynamic vibrator: 0.34 m, acquisition on the computer: 1 point in Figure 7 corresponds to a count over 10 periods, rate: 1 point/second.

Example 5 The tension of a 0.385 g/metre brass-coated steel wire is measured upstream of a winder which winds the wire at a speed of 90 m/min.

Measured with the device of the invention, the value is 1.8 kilogrammes. When controlled with a Rotschild tension meter, the measured tension is 1.9 kilogrammes, which gives a comparable measurement. However, with the contact tensiometer, the value is not constant owing to the friction against the means for measuring and the variations, however minimal, in the diameter of the brass-coated steel wire.

Example 6 The tension of a strand of 4 filaments of brasscoated steel is measured upstream of a winder which winds the strand at a speed of 90 m/min.

The measured tension is 7 kilogrammes using the device of the invention and 8 kilogrammes using the Rotschild contact tensiometer.

At a winding speed of 320 m/min, the tension is 12.5 kilogrammes, when measured with the device of the invention and 12 kilogrammes with the other, contact-type device having the disadvantages, alrady referred to, associated with the contact.

Example 7 The tension is measured on a twisted cable of 17 g/metre at a winding speed of 90 m/min.

When measured with the device of the invention the value is 10 kilogrammes and is stable, while, when measured with the contact tensiometer, the value is not reliabie on account of the twist and varies between 9 and 11 kilogrammes, demonstrating the advantage of the use of the contactless tensiometer of the invention.

Claims (9)

Claims

1. A method of continuously measuring the tension of a moving product, said method comprising subjecting the moving product to transverse vibrations between two fixed supports, over a predetermined length of its path, detecting the vibrations remotely by an optical system and transmitting a signal representative of the detected vibrations to an electronic servo system to control the means subjecting the product to the transverse vibrations and also to a means for reading the signal to give the tension.

2. A method according to claim 1, wherein the product is a filiform product.

3. A method according to claim 1 or 2, wherein the optical system for detecting the vibrations includes a laser.

4. A method according to claim 1,2 or 3, wherein the optical detector supplies an oscillating signal at the same frequency as the exciting signal driving the means for subjecting the product to transverse vibrations.

5. Apparatus for continuously measuring the tension of a moving product, said apparatus comprising two spaced supports for the moving product, said supports being spaced apart by a predetermined distance, a vibration exciting system for causing a product fed between said supports to vibrate transverse to the direction of feed, an optical system for the remote detection of the vibrations produced and an electronic servo system controlling the vibration exciting system in response to signals received from said optical system.

6. Apparatus according to claim 5, wherein the optical system for detecting the vibrations includes a laser.

7. Apparatus according to claim 5 or 6, wherein the optical detector supplies an oscillating signal at the same frequency as the exciting signal driving the means subjecting a product to transverse vibrations.

8. A method of continuously measuring the tension of a moving product, said method being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. Apparatus for continuously measuring the tension of a moving product, said apparatus being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.