GB2291507A

GB2291507A - Method and apparatus for measurement of tension in a moving strand

Info

Publication number: GB2291507A
Application number: GB9413944A
Authority: GB
Inventors: Boris Tchostkovski
Original assignee: HES OPTICAL FIBRES
Current assignee: HES OPTICAL FIBRES
Priority date: 1994-07-11
Filing date: 1994-07-11
Publication date: 1996-01-24
Also published as: GB9413944D0

Abstract

For undertaking continuous measurement of tension of a strand 11 during a drawing process at least one gas flow nozzle 13 is directed towards the working path X of the strand and a gas flow directed through the or each nozzle displaces the strand. An optical sensor detects the transverse displacement of the strand from the working path to the displaced path from the transverse displacement of the strand the tension in the strand is determined. There may be four nozzles to cause the displaced path to be in the form of a circle. The strand may be optical fibre. <IMAGE>

Description

METHOD AND APPARATUS FOR TENSION MEASUREMENT This invention relates to a method and to apparatus for tension measurement. It is particularly, but not exclusively, concerned with non-contact measurement of tension in a strand in a process where the strand is being drawn in a direction along its length. The term 'strand' is used as a generic term to cover items in the form of a filament, fibre, thread or suchlike and whether in the form of a mono filament or several filaments twisted, woven, plaited or otherwise drawn together to provide the required strand.

In the production of strand in the form of an optical fibre a preformed glass mass is placed in a furnace where it starts to melt. A glass fibre is drawn from a tip of the melting mass, passed through a region where it solidifies and is then passed into a coating bath where the fibre receives a protective coat of synthetic plastics material.

In drawing the fibre through the region it is necessary to measure the tension and when necessary make operating adjustments say to the furnace temperature so as to keep the tension within given tolerances during the drawing process. In practice the fibre moves at very high speed through the region where it is extremely thin and fragile prior to coating. Measurement of tension in such circumstances presents considerable difficulties. Typically any method of tension measurement involving contact with the fibre is impractical.

Known systems make use of noncontact measurement. In a known system the value of fibre tension is calculated by using an electrodynamic sound generator to generate a sound wave in the thread and measuring its velocity relative to the speed of sound in air. There are disadvantages in this system. Typically the measuring systems is of large size (typically 900 mm high) which is not conveniently mounted on the drawing system. The excitation of the fibre by the sound wave tends to disturb the drawing process and to affect fibre quality.

A method and device are disclosed in GB Patent 2 127 644 B 'Tension measurement' which are based on the laws of vibrating strings. In the described method a fibre excitation system is used and the value of fibre tension is calculated from the value of fundamental frequency of fibre vibration. There are disadvantages in this. The fibre is necessarily vibrated during the measurement process with a consequent disturbance to the drawing process and to the fibre quality. There is a need to seek the fundamental frequency by means of a servo system which tends to reduce the speed of tension measurement and also decreases the efficiency of the feedback system providing for temperature adjustment for the furnace.

An object of the present invention is to provide a continuous non-contact measurement of fibre tension during a drawing process which avoids shock excitation or vibration of the drawn fibre while providing for rapid and accurate tension measurement and to ensure effective feedback for stable control of fibre tension.

According to a first aspect of the present invention there is provided a method of measuring tension in a moving strand comprising the steps of: 1 causing the strand to pass along a working path between first and second locations; 2 directing across the strand at a predetermined position at least one jet at a rate sufficient to displace the strand from the working path to a displaced path; 3 measuring the lateral displacement of the strand from the working path to the displaced path at the predetermined position; 4 using the value of lateral displacement and the distances of the predetermined points along the strand to the first and to the second locations to establish the tension in the strand.

According to a first preferred form of the first aspect of the present invention the step of directing a jet of gas is carried out by providing a flow of gas flow whose velocity is periodically varied.

According to a second preferred form of the present invention or of the first preferred version thereof the step of directing the flow of gas across the strand occurs at a predetermined position wherein the direction of gas flow is perpendicular to the working path.

According to a third preferred form of the first aspect of the present invention or any preceding preferred version thereof the gas flow rate is varied at a rate in the range of 0.5 to 5 seconds.

According to a fourth preferred version of the first aspect of the present invention or any preceding preferred version thereof wherein the step of directing across the strand at a predetermined position is undertaken by way of a plurality of jets at a rate sufficient to displace the strand from the working path to the displaced path.

According to a fifth preferred version of the first aspect of the present invention the method according to the fourth preferred version is further characterised in that the displaced path is in the form a circle about, and perpendicular to, the working path.

According to a second aspect of the present invention there is provided apparatus for undertaking continuous measurement of tension of a strand during a drawing process comprising: 1 means defining a region through which the strand is caused to path along a working path; 2 at least one gas flow nozzle directed towards the working path; 3 means for directing a gas flow through the or each nozzle so as to displace a strand initially passing along the working path from the working to a displaced path; 4 a sensor for detecting the transverse displacement of the strand from the working path to the displaced path at the predetermined region; and 5 means for determining from the transverse displacement of the strand the tension in the strand in the displaced path.

According to a first preferred version of the second aspect of the present invention the gas flow nozzle has an outlet for gas with a cross sectional dimension greater than the greatest transverse displacement to which thew strand can be subjected at the predetermined region.

According to a second preferred version of the second aspect of the present invention or of the first preferred version thereof a pair of gas nozzles are used in horizontally opposed alignment on opposite side of, and perpendicular to, the working path and means providing for each nozzle of the pair to displace the strand in turn.

According to a third preferred version of the second aspect of the present invention or the first preferred version thereof two pairs of nozzles are used each pair of nozzles being in horizontally opposed alignment and one pair being at right angles to the other nozzle pair.

According to a fourth preferred version of the second aspect of the present invention or any preceding preferred version thereof the sensor is located perpendicular to the working path and in juxtaposition with the gas nozzle or nozzles.

The present invention is based on the discovery that fibre tension in a region between the furnace and the coating apparatus can be quantified from the transverse displacement of the fibre caused by a displacing gas flow directed across the drawn fibre. The gas flow displaces the fibre from its normal path in a given period. The gas flow can be directed across the fibre in a various ways: typically one nozzle can be used or a pair of horizontally opposed nozzles or two pairs of horizontally opposed nozzles.

The required gas flow rates are achieved using conventional flow regulators but at rates according to a pre-determined law. The displacement of the drawn fibre from a displaced path by the gas flow is detected by an optical sensor which generates a signal which is passed to a processor which calculates the amount of displacement and thereafter establishes the fibre tension.

Exemplary embodiments of the invention will now be described with reference to the accompany drawing of optical fibre drawing installations in which: Figure 1 is a diagram relating to tension measurement in a fibre; Figure 2 shows a vertical section of a first fibre drawing system using a pair of horizontally opposed gas nozzles; Figure 3 shows a vertical section of a second fibre drawing system utilising two pairs of horizontally opposed gas nozzles; and Figure 4 shows a perspective diagrammatic view of a drawing system utilising the nozzle arrangement of Figure 3.

Figure 1 A strand of glass fibre 11 is drawn from a melting preformed glass mass in a furnace (shown diagrammatically as a block GF) in the general direction of arrow W through a work region 12 into a coating bath shown as a block CB.

The fibre 11 follows a working path X which leaves the melted preform at point A and passes into the coating bath at point B. Gas nozzle 13 is off set from, and is directed perpendicularly across, the working path X at a predetermined region P. The nozzle 13 is supplied with a regulated supply of compressed air by inlet pipe 14.

Point C on working path X corresponds to an initial position of fibre 11 in the predetermined region P when there is no gas flow through the nozzle 13.

On the provision of a gas flow Q through nozzle 13 in the direction J the fibre 11 is displaced by a force G. For as long as the gas flow Q is maintained the fibre 11 thereafter no longer follows working path A C B but rather a displaced path A D B. The distance along fibre 11 from point A to point C is I1. The distance along fibre 11 from point D to point B is 12. Given a resultant tension F in the fibre the displacement H is calculated from the identity: H = C * d * 1 * I:2)/(I1 + I2) * Q2 * 1/F (1) where C is a coefficient dependent on functional parameters of the nozzle 13; d is the diameter of the fibre 11; I1, I2 are the distances from A to D and from D to B respectively; Q is the gas flow rate through the nozzle 13.Q is related to the gas flow speed and so the value of the force G displacing the fibre 11.

Relationship (1) gives that the measurement of displacement H, for example by means of a non-contact optical device located in the predetermined region P, it is possible to calculate tension F relatively easily.

In a simple embodiment it is possible to use a constant gas flow Q so providing for fibre displacement along the displaced path A D B throughout the drawing process. However this presupposes a stable geometry throughout the fibre system and in particular stability of the points A, B. In practice the tip preform point A moves slowly transverse the path X during the drawing process due to the drawing off point on the melting glass mass not remaining laterally fixed. In addition the position of point B can also move laterally due to movement of the coating bath under the control of a regulator which acts to ensure that the coating applied to the fibre is applied concentrically about the fibre in passing through the coating bath.

Both these additional factors of displacement at A and B cause additional displacement of the fibre 11 when subject to a single constant direct gas flow and this additional displacement could be interpreted as a fibre tension change. To overcome this possible source of error a further embodiment of the invention uses a varying gas flow rate instead of a constant one.

The use of variation in flow rate has the additional benefit of avoiding any shock effect of gas flow on the fibre 11 so providing for smooth fibre displacement in a direction transverse the fibre path. In this arrangement the fibre is periodically displaced in the transverse direction to its displaced path by means of the gas flow and then returns to its working path when the gas flow is periodically terminated.

The displacement amplitude H is determined during each period as the distance between the working path and displaced path positions.

The fibre position is detected with an optical sensor and a processing unit then establishes the displacement H and calculates tension F. The displacement H in this arrangement is substantially independent of any slow drift arising in the points A, B as discussed earlier since the relatively short time needed to make the repeated measurements of periodic displacement is negligible compared with the drifts associated with points A and B. Suitable periods of gas flow change in relation to glass fibre have been found to lie in a range between 0.5 and 5 seconds.

The displacement of the fibre from its working path ACB to its displaced path ADB effectively increases the length of fibre and this has an effect on the drawn fibre diameter. With periodic displacement of the fibre the fibre diameter can be modulated along the length of the fibre.

Figure 2 This shows the use of a pair of nozzles 2, 3 located in a horizontally opposed configuration on opposite sides of fibre 21. The fibre 21 is shown passing through working volume 22 of an optical sensor 4 which by way of sensor heads 4A and 4B senses the displacement of the fibre 21 from its working path X. Position D of fibre 21 corresponds to the displacement caused by the action of gas from nozzle 2 with no gas flow from nozzle 3. Position D30f fibre 21 corresponds to the displacement caused by the action of gas frolrinozzle 3 with no gas flow from nozzle 2.

Optical sensor 4 continuously detectsihe [the current]position of fibre 21 and passes the information to processor 23 which [from the limiting]position D, D' establishes the value of displacement H and so [the tension F]of the fibre 21.

In practical terms it is difficult toWocate the sensor and the gas nozzles 2, 3 in one and the same plane. NeverthelessF minimuml separation is to be preferred.

Figure 3 The removal of the modulation [effect on the fibre]diameter due to periodic displacement discussed earlier can be[achieved with]the use of additional nozzles.

The nozzles 2 and 3 are horizontallyCopposed create a first nozzle pair. A second nozzle pair made up of nozzles [5, 6 are located]in a horizontally opposed relationship at right angle to the first [pair. Both]pairs of nozzle are equidistant form working path X of fibre 31.

The nozzles 2/3 and 5/6 are supplied [with air at] a selected rate. Nozzles 2, 3 provide gas flows, respectively QI (t)FQ2(t) in accordance3 with a harmonic sine law. The phase shift between Q1 andEQ2 is IS. Nozzles 5, 6 provide gas flows, respectively Q3(t), Q4(t) in accordance [with a harmonic]sine law. The phase shift between Q3 and Q4 is 180 As a result gas flows from the nozzles, 5, 3, 4 on the fibre 31 is cause the fibre in the predetermined zone to rotate bout the workinipath X on a displaced path D in the form of a circle centred on the working path The diameter of the circle D is 2 * H where H is related to thektension in the fibre 31 in accordance with relationship (1) above.

Figure 4 This shows a practical embodiment of the arrangement discussed in relation to Figure 3. The gas nozzles are broadly indicated as a block 8 (broken outline) and include the four nozzles 2, 3, 5 and 6 which are connected through pipelines 9 with a gas supply and regulating unit 10. The nozzles 2, 3, 5, 6 are operated as described in relation to Figure 3 to cause rotation of the fibre 31. The motion of the fibre 31 is detected by the optical sensor 4 which feeds the detected displacement H to a processor 11.

Any fibre diameter sensing device which also serves to detect the fibre displacement can be used in relation to this embodiment such as BETA or ANRITSU sensors.

The processor unit 11 uses the signal relating to fibre displacement from the sensor 4 to determine the diameter of the displacement circle D as a difference between values of displacement signal during one period of the fibre circling.

Displacement H is determined by theFiameter of thifibre circle D from which the tension can be determined.

The processor 11 can be incorporated on aj circuit board along with a microprocessor for handling the various signals and to calculate the fibre tension in accordance with a suitable algorithm.

Ideally the four nozzles would be located the samgplane perpendicular to the fibre path. However it is virtually impossible in view of interaction between the various the gas flows. In addition Fthe location of theloptical sensor and the gas nozzles in one and the same plan not feasible3 Consequently for practical purposes the device shown in Figurer4 has certainlstructural features: 1 The nozzles 2, 3, 5, 6 are positioned at differenJtlevels along the fibre 31 to avoid mutual interaction betweenias flowiQ1(t), Q2(t), Q3(t) and Q4(t).

2 The nozzle 3 is located aboveir beneath optical sensor 4 relative to the fibre but with minimum vertical separation between them.

3 The nozzles 2, 3, 5, 6 have outlets of rectangular cross section. The width is more than the maximum possible diameter of the circle D that can be established by fibre 31.

The method embodied in the apparatus described in relation to figures 3 and 4 includes: forming gas flows with rates which are changed through the nozzles according to a periodic law; causing the fibre to be displaced on a circular path by gas flows applied perpendicular to the axis of the fibre; observing the current fibre position with optical sensing devices; determining the transverse displacement of the fibre; and calculating the tension of the fibre in accordance with the relationship: F=k*1/H (2) where k = C IF d * I2)/(11 + I2) * Q2 The relationship (2) is obtained from the relationship (1).

Once the tension F is established the processor 11 can be used to provide a control signal such as to the furnace to adjust the furnace temperature so as to maintain the tension value within a given working range during the drawing process.

The described embodiments disclose a method of measuring tension without causing a drawn fibre to subjected to effects likely to affect the physical characteristics of the drawn fibre. The embodiment is particularly useful where as in the present case a relatively delicate product is travelling at high speed from a source of molten material to a coating bath for the solidified material. The process enables the tension of the material to be determined rapidly and for any necessary corrective action to be applied swiftly.

The invention is also applicable to other types of strand given that the strand can be displaced from a working path to a displaced path by an amount of displacement which can be related to the tension in the moving product.

Claims

I A method of measuring tension in a moving strand comprising the steps of: I causing the strand to pass along a working path between first and second locations;

2 directing across the strand at a predetermined position at least one jet at a rate sufficient to displace the strand from the working path to a displaced path;

3 measuring the lateral displacement of the strand from the working path to the displaced path at the predetermined position;

4 using the value of lateral displacement and the distances of the predetermined points along the strand to the first and to the second locations to establish the tension in the strand.
2 A method as claimed in Claim I wherein the step of directing a jet of gas is carried out by providing a flow of gas flow whose velocity is periodically varied.
3 A method as claimed in Claim 1 or Claim 2 in which the step of directing the flow of gas across the strand at a predetermined position wherein the direction of gas flow is perpendicular to the working path.
4 A method as claimed in any preceding claim wherein the gas flow rate is varied at a rate in the range of 0.5 to 5 seconds.
5 A method as claimed in any preceding claim wherein the step of directing across the strand at a predetermined position is undertaken by way of a plurality of jets at a rate sufficient to displace the strand from the working path to the displaced path.
6 A method as claimed in Claim 6 wherein the displaced path is in the form a circle about, and perpendicular to, the working path.
7 Apparatus for undertaking continuous measurement of tension of a strand during a drawing process comprising: means defining a region through which the strand is caused to path along a working path;

2 at least one gas flow nozzle directed towards the working path;

3 means for directing a gas flow through the or each nozzle so as to displace a strand initially passing along the working path from the working to a displaced path;

4 a sensor for detecting the transverse displacement of the strand from the working path to the displaced path at the predetermined region; and

5 means for determining from the transverse displacement of the strand the tension in the strand in the displaced path.
8 Apparatus as claimed in Claim 7 wherein the gas flow nozzle has an outlet for gas with a cross sectional dimension greater than the greatest transverse displacement to which thew strand can be subjected at the predetermined region.
9 Apparatus as claimed in Claim 7 or Claim 8 wherein a pair of gas nozzles are used in horizontally opposed alignment on opposite side of, and perpendicular to, the working path and means providing for each nozzle of the pair to displace the strand in turn.
10 Apparatus as claimed in Claim 7 or Claim 8 wherein two pairs of nozzles are used each pair of nozzles being in horizontally opposed alignment and one pair being at right angles to the other nozzle pair.
11 Apparatus as claimed in Claim 7 or Claim 8 or Claim 9 or Claim 10 wherein the sensor is located perpendicular to the working path and in juxtaposition with the gas nozzle or nozzles.
12 A method of continuously measuring the tension of a strand as herein before described with reference to the accompanying drawings.
13 Apparatus for undertaking continuous measurement of tension of a strand during a drawing process as hereinbefore described with reference to and as illustrated in the accompanying drawings.
14 Apparatus for carrying out the method of any of preceding claims I to 6.