Weaving method with control or adjustment of the yarn tension in warp threads. and weaving machine for producing a fabric using said weaving method

Abstract

Disclosed is a weaving method with which the yarn tension of several part groups with at least one warp thread per part group is controlled or adjusted separately, in order to follow a respective reference yarn tension profile during weaving, where, for at least one part group, the reference yarn tension profile is changed during weaving, where the reference yarn tension profile is determined and changed separately for at least two part groups, and where each reference yarn tension profile is selected from a collection of different reference yarn tension profiles. Also disclosed is a weaving machine provided with yarn tensioning elements, a storage unit in which said collection is provided, and a control or steering unit in order, in cooperation with the yarn tensioning elements, to adjust or control the yarn tension in separate warp threads using the indicated weaving method.

Claims

1. Method for weaving a fabric on a weaving machine, wherein: in successive weft insertion cycles, at least one weft thread is inserted at a weft insertion level between warp threads, the warp threads in each weft insertion cycle are positioned relative to each weft insertion level such that the warp threads and the weft threads inserted in between together form a fabric according to a predefined weaving pattern, and the yarn tension of a group of warp threads which comprises at least some of the warp threads is controlled or adjusted by means of a yarn tensioning device, wherein the group of warp threads comprises several part groups with at least one warp thread, that the yarn tension of the warp threads is controlled or adjusted separately per part group in order to follow a respective reference yarn tension profile during weaving; that for at least one part group, the reference yarn tension profile to be followed is changed during weaving; and that for at least two part groups, the reference yarn tension profile to be followed during weaving is determined and changed separately, wherein each reference yarn tension profile is selected from a collection of at least two different reference yarn tension profiles.

2. Method for weaving a fabric according to claim 1, wherein a respective different reference yarn tension profile is provided for at least two different statuses of a yarn tension influencing property of a warp thread, and that for at least one part group, the reference yarn tension profile to be followed during weaving is determined and changed as a function of the status of each warp thread of the part group.

3. Method for weaving a fabric according to claim 2, wherein the at least two different statuses of the yarn tension influencing property of a warp thread are: at least two different phases of the weaving cycle in which a warp thread is processed into the fabric, or at least two different places on the weaving machine at which a warp thread is located during weaving process, or at least two different paths which a warp thread follows from a yarn store to the fabric, or at least two different degrees of contact which a warp thread makes with other warp threads and/or with yarn guide means on its path from a yarn store to the fabric, or at least two different sizes of forces which counter the movement of a warp thread towards the weaving machine on its path from a yarn store to the fabric, or at least two different inertias of a yarn storage bobbin from which the warp thread is unwound during the weaving process by rotation of the yarn storage bobbin, or at least two different bobbin places at which the warp thread is unwound.

4. Method for weaving a fabric according to claim 1, wherein a respective different reference yarn tension profile is provided for at least two different weave statuses of a warp thread in the fabric to be woven, and that for at least one part group, the reference yarn tension profile to be followed during weaving is determined and changed as a function of the weave structure of each warp thread of the part group, as provided according to the weaving pattern.

5. Method for weaving a fabric according to claim 1, wherein at least a number of part groups, preferably all part groups, comprise only one warp thread.

6. Method for weaving a fabric according to claim 4, wherein it is a method for weaving pile fabrics, in which at least one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into a ground fabric without forming pile, according to the weaving pattern; that a pile-forming pile-warp thread has a first weave status and a pile-warp thread which is incorporated into a ground fabric without forming pile has a second weave status; that a first and a second reference yarn tension profile are provided for the first and second weave statuses respectively; and that the reference yarn tension profile to be followed during weaving is determined and changed as a function of the presence or absence of a first or a second weave status of each pile-warp thread of the part group, according to the weaving pattern.

7. Method for weaving a fabric according to claim 4, wherein it is a method for weaving a pile fabric, in which at least one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into one of the ground fabrics without forming pile, according to the weaving pattern; that at least one pile-warp thread has a pile-forming part and a non-pile-forming part; that the transition from a pile-forming part to a non-pile-forming part of a pile-warp thread has a third weave status; that a third reference yarn tension profile is provided for the third weave status; and that the reference yarn tension profile to be followed during weaving is determined and changed as a function of the presence or absence of a third weave status of each pile-warp thread of the part group, according to the weaving pattern.

8. Method for weaving a fabric according to claim 4, wherein it is a method for weaving a pile fabric, in which at least one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into one of the ground fabrics without forming pile, according to the weaving pattern; that at least one pile-warp thread has a pile-forming part and a non-pile-forming part; that the transition from a non-pile-forming part to a pile-forming part of a pile-warp thread has a fourth weave status; that a fourth reference yarn tension profile is provided for the fourth weave status; and that the reference yarn tension profile to be followed during weaving is determined and changed as a function of the presence or absence of a fourth weave status of each pile-warp thread of the part group, according to the weaving pattern.

9. Method for weaving a fabric according to claim 4, wherein it is a face-to-face weaving method in which two ground fabrics are woven one above the other from respective warp threads and weft threads, wherein the pile-warp threads on the mutually facing sides of the ground fabrics form a pile on at least one of the ground fabrics in that pile-warp threads are interlaced alternately into the one and the other ground fabric and cut through between the two ground fabrics so as to form cut pile on both ground fabrics, and/or in that pile loops are formed on at least one of the ground fabrics, and/or in that pile-warp threads on at least one of the ground fabrics form ribs running over weft threads on the fabric surface.

10. Method for weaving a fabric according to claim 4, wherein it is a fabric with a cut pile and/or a loop pile and/or a rib-forming structure, such as amongst others a false bouclé fabric or a fabric with sisal appearance.

11. Method for weaving a fabric according to claim 1, wherein, to influence the yarn tension of the warp threads, a yarn tensioning element is provided per part group and comprises at least one roller that can be driven by an electric motor and that is in contact with each warp thread of the part group, wherein the electric motor has a cogging torque which is at least 5% and most 20% of the nominal torque of the motor.

12. Method for weaving a fabric according to claim 1, wherein, per part group, a yarn tensioning element is provided which comprises at least one roller that can be driven by an electric motor and is in contact with each warp thread of the part group, wherein the electric motor has a nominal torque of at least 0.005 Nm and at most 0.2 Nm.

13. Weaving machine comprising: weft insertion means in order, in successive weft insertion cycles, to insert at least one weft thread at a weft insertion level between warp threads, shed-forming means for positioning the warp threads in each weft insertion cycle relative to each weft insertion level such that the warp threads and the weft threads inserted in between together form a fabric according to a predefined weaving pattern, and a yarn tensioning device for controlling or adjusting the yarn tension of the group of warp threads which comprises at least part of the warp threads, wherein the yarn tensioning device comprises several yarn tensioning elements which are provided for changing the yarn tension in the warp threads of respective part groups of the group of warp threads, and a control or steering unit which is provided, in cooperation with the yarn tensioning elements, to adjust or control the yarn tension in the warp threads separately per part group in order to follow a respective reference yarn tension profile during weaving, wherein each part group comprises at least one warp thread; that the control or steering unit is provided to change the reference yarn tension profile to be followed during weaving for at least one part group; that the yarn tension device comprises a storage unit in which a collection of at least two different reference yarn tension profiles is provided; and that the control or steering unit is provided, for at least two part groups, to determine the reference yarn tension profile to be followed during weaving by selection from said collection.

14. Weaving machine according to claim 13, wherein the weaving machine is provided with a group of warp threads which comprises several part groups with at least one warp thread; that in the storage unit a respective different reference yarn tension profile is provided for at least two different statuses of a yarn tensioning influencing property of a warp thread; and that the control or steering unit is provided, for at least one part group, to determine the reference yarn tension profile to be followed during weaving and change this as a function of the status of each warp thread of the part group.

15. Weaving machine according to claim 14, wherein the at least two different statuses of a yarn tension influencing property of a warp thread are: at least two different phases of the weaving cycle in which a warp thread is processed into the fabric, or at least two different places on the weaving machine at which a warp thread is located during the weaving process, or at least two different paths which a warp thread follows from a yarn store to the fabric, or at least two different degrees of contact which a warp thread makes with other warp threads and/or with yarn guide means on its path from a yarn store to the fabric, or at least two different sizes of forces which counter the movement of a warp thread towards the weaving machine on its path from a yarn store to the fabric, or at least two different inertias of a yarn storage bobbin from which the warp thread is unwound during the weaving process by rotation of the yarn storage bobbin, or at least two different bobbin places at which the warp thread is unwound.

16. Weaving machine according to claim 13, wherein the yarn tensioning device comprises a storage unit in which a respective different reference yarn tension profile is provided for at least two different weave statuses of a warp thread in the fabric to be woven; and that the control or steering unit is provided, for at least one part group, to determine the reference yarn tension profile to be followed during weaving and change this as a function of the weave status of each warp thread of the part group, as provided according to the weaving pattern.

17. Weaving machine according to claim 13, wherein at least a number of part groups, preferably all part groups, comprise only one warp thread.

18. Weaving machine according to claim 16, wherein it is a weaving machine which is provided for weaving pile fabrics, wherein at least one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into a ground fabric without forming pile, according to the weaving pattern; that a pile-forming pile-warp thread has a first weave status and a pile-warp thread which is incorporated into a ground fabric without forming pile has a second weave status; that a first and a second reference yarn tension profile are provided for the first and second weave statuses respectively; and the control or steering unit is provided in order to determine the reference yarn tension profile to be followed during weaving and change this as a function of the presence or absence of a first or a second weave status of each pile-warp thread of the part group, according to the weaving pattern.

19. Weaving machine according to claim 16, wherein it is a weaving machine which is provided for weaving a pile fabric, wherein at one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into a ground fabric without forming pile, according to the weaving pattern; that at least one pile-warp thread has a pile-forming part and a non-pile-forming part; that the transition from a pile-forming part to a non-pile-forming part of a pile-warp thread has a third weave status; that a third reference yarn tension profile is provided for the third weave status; and that the control or steering unit is provided in order to determine the reference yarn tension profile to be followed during weaving and change this as a function of the presence or absence of a third weave status of each pile-warp thread of the part group, according to the weaving pattern.

20. Weaving machine according to claim 16, wherein it is a weaving machine which is provided for weaving a pile fabric, wherein at least one ground fabric is woven from warp threads and weft threads, and wherein pile-warp threads are provided in order to form pile and/or be incorporated into one of the ground fabrics without forming pile, according to the weaving pattern; that at least one pile-warp thread has a pile-forming part and a non-pile-forming part; that the transition from a non-pile-forming part to a pile-forming part of a pile-warp thread has a fourth weave status; that a fourth reference yarn tension profile is provided for the fourth weave status; and that the control or steering unit is provided in order to determine the reference yarn tension profile to be followed during weaving and change this as a function of the presence or absence of a fourth weave status of each pile-warp thread of the part group, according to the weaving pattern.

21. Weaving machine according to claim 16, wherein it is a face-to-face weaving machine.

22. Weaving machine according to claim 21, wherein the weaving machine is provided to weave two ground fabrics one above the other from respective warp threads and weft threads, wherein the pile-warp threads on the mutually facing sides of the ground fabrics form a pile on at least one of the ground fabrics in that pile-warp threads are interlaced alternately into the one and the other ground fabric and cut through between the two ground fabrics so as to form cut pile on both ground fabrics, and/or in that pile loops are formed on at least one of the ground fabrics, and/or in that pile-warp threads on at least one of the ground fabrics form ribs running over weft threads on the fabric surface.

23. Weaving machine according to claim 13, wherein said yarn tensioning elements comprise at least one roller that can be driven by an electric motor and is intended to be in contact with at least one warp thread, wherein the electric motor has a cogging torque which is at least 5% and most 20% of the nominal torque of the motor.

24. Weaving machine according to claim 13, wherein the yarn tensioning elements comprise at least one roller that can be driven by an electric motor and is intended to be in contact with at least one warp thread, and that the electric motor has a nominal torque which is at least 0.005 Nm and at most 0.2 Nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure is now explained further with reference to the description which follows of a possible embodiment of a yarn tensioning device according to this disclosure and a possible weaving method according to this disclosure. It is emphasised that the device and method described are merely examples of the general principle of the disclosure, and thus may in no way be regarded as a limitation of the scope of protection or of the area of application of the disclosure.

(2) In this description, reference signs are used to refer to the attached figures, in which:

(3) FIG. 1 is a diagrammatic representation of the shed geometry on a face-to-face weaving machine, indicating the movements of a heddle eye which positions a pile-forming pile-warp thread;

(4) FIG. 3 is a diagrammatic representation of the shed geometry on a face-to-face weaving machine, indicating the movements of a heddle eye which positions a non-pile-forming pile-warp thread on its incorporation into the top ground fabric;

(5) FIGS. 2, 4 and 5 show graphs which, for a warp thread in a number of successive weft insertion cycles, represent: the development of the yarn tension (in grams) in the pile-forming pile-warp thread, the development of the position of the heddle eye (in mm), and the total rotation angle (in degrees), over a complete machine cycle, of the brake roller of a yarn tensioning element; wherein

(6) FIGS. 2 and 4 respectively are related to a pile-forming pile-warp thread and a pile-warp thread incorporated into the top ground fabric, on use of a face-to-face weaving machine according to FIG. 1 and a yarn tensioning device which, according to the prior art, exerts a constant force on the warp threads, and

(7) FIG. 5 is related to a pile-forming pile-warp thread on use of a face-to-face weaving machine from FIG. 1 and a yarn tensioning device which, according to the disclosure, adjusts the yarn tension in the warp threads in order to follow a reference yarn tension profile;

(8) FIG. 6 shows a block diagram of the principle of control of the yarn tension according to a method of this disclosure; and

(9) FIG. 7 shows a block diagram of the principle of steering of the yarn tension according to a method of this disclosure.

DETAILED DESCRIPTION

(10) Firstly, with reference to FIGS. 1 to 4, it is explained how, during weaving on a face-to-face weaving machine, the yarn tension profile develops in a pile-warp thread which forms pile and in a pile-warp thread which is incorporated into one of the ground fabrics. It is shown that these yarn tensions differ greatly from each other, and it is also shown that the yarn tension in a pile-forming and in a non-pile-forming pile-warp thread varies greatly over the course of the weaving process. The yarn tension profile shows great differences between the maximum values (peaks) and the minimum values (troughs) for both the pile-forming and non-pile-forming pile-warp threads.

(11) With reference to FIG. 5, it is shown that, according to the disclosure, a yarn tension profile may be obtained with lower maximum values and higher minimum values (lower peaks and higher troughs), with less variation in the yarn tension of a warp thread, as a first advantageous effect. In addition, because the yarn tension varies within a range with higher minimum values, this range can be lowered to a level at which the minimum values are still higher than the minimum required to guarantee good shed formation, good progress of the weaving process and excellent fabric quality. A second advantageous effect is therefore that the mean yarn tension can be lowered.

(12) FIGS. 1 and 3 show the various possible positions of the warp threads during shed formation by means of a jacquard device on a face-to-face weaving machine, indicated symbolically by four straight position lines (1), (2), (3), (4) and by two straight position lines (1), (2) respectively.

(13) These position lines (1), (2), (3), (4) run from the symbolically depicted upper bridge (5) or lower bridge (6) of a face-to-face weaving machine, via a jacquard machine (7) symbolically depicted by means of a vertical dotted line, to a grid (8) shown symbolically on the right of the drawings as a row of small circles. From the grid (8), the warp threads run to a bobbin creel which is not shown on the drawings. Part of the latter path of the warp threads is presented symbolically by means of a straight line (9).

(14) The jacquard machine (7) is a known jacquard machine provided with a large number of heddles with respective heddle eyes and associated hooks, selection means and positioning means for positioning the heddles and the warp threads extending through these heddle eyes in successive weft insertion cycles, in a number of possible positions corresponding to a predefined weaving pattern.

(15) In FIG. 1, a jacquard machine is shown with four possible positions for shed formation: a “bottom (O)” position, a “middle 1 (M1)” position, a “middle 2 (M2)” position and a “top (B)” position. The top position line (1) indicates the position of the warp threads which extend from the upper bridge (5) to a heddle brought to the “top (B)” position, and on to the grid (8). The position line (2) indicates the position of the warp threads which extend from the upper bridge (5) to a heddle brought into the “middle 1 (M1)” position and on to the grid (8). The position line (3) indicates the position of the warp threads which extend from the lower bridge (6) to a heddle brought to the “middle 2 (M2)” position and on to the grid (8). The bottom position line (4) indicates the position of the warp threads which extend from the lower bridge (6) to a heddle brought to the “bottom (O)” position and on to the grid (8).

(16) A pile-warp thread which forms pile is brought successively, in successive weft insertion cycles, into the following positions: “middle 2 (M2)”, “top (B)”, “middle 1 (M1)” and “bottom (O)”. See the indication of these movements on FIG. 1. The movement of the warp threads is determined in advance by the movement of the heddle eye (by the jacquard machine) but partly also by the geometry of the weaving machine.

(17) FIG. 2 indicates how the yarn tension develops during a number of successive jacquard cycles for a pile-forming pile-warp thread with the successive heddle positions indicated above, wherein two weft insertion cycles take place during one jacquard cycle. The horizontal axis of FIG. 2 shows the degrees of rotation of the main shaft of the weaving machine. During two machine cycles or 720° on the horizontal axis, one jacquard cycle takes place. The vertical axis shows the values of the yarn tension (in gram) which are also the values of the movement of the heddle (in mm) and of the rotation of the roller of a yarn tensioning element (in degrees). FIG. 2 shows four graph lines (G1), (G2), (G3) and (G4) which are hereafter referred to as graph lines G1, G2, G3 and G4.

(18) Graph line G1 shows the development of the yarn tension in the pile-forming warp thread.

(19) Graph line G2 shows how the heddle eye which positions this pile-warp thread is moved in the meantime.

(20) Graph line G3 indicates the total rotation of the roller of the yarn tensioning element which controls the tension of the warp thread during one jacquard cycle (after each jacquard cycle, the value of this rotation is returned to zero), wherein we emphasise that this yarn tensioning element according to the prior art exerts a constant force on the warp thread in order to keep this under tension.

(21) Graph line G4 indicates the mean value of the yarn tension according to graph line G1.

(22) Since the roller of the yarn tensioning element only rotates when the pile-warp yarn in contact with this roller is moved, both in the feed direction and in the opposite direction (on recuperation), the number of degrees of rotation of this roller can be used to derive the length of the pile-warp thread used. Accordingly, graph line G3 may also be regarded as an indication of the consumption of the supplied pile-warp yarn.

(23) FIG. 2 shows the following for a number of successive jacquard cycles (2 weft insertion cycles): The heddle eye is moved from the “middle 2 (M2)” position to the “top (B)” position, as shown from the curve of graph line G2 from 0° on the horizontal axis. This movement begins slightly before 0°, which can be seen from the accumulated yarn tension at 0°. On graph line G3, we see that this is accompanied by a large rotation of the roller of the yarn tensioning element (hence a large consumption of pile-warp yarn), and on graph line G1 we see that this is accompanied by a rapid increase in yarn tension leading to a peak (P1). When the heddle eye is stationary in the “top (B)” position (the horizontal top part of graph line G2), there is still a further take-up of yarn (see graph line G3). This surplus yarn feed, also called overflow, causes a fall in tension (graph line G1) until the yarn tension in the warp thread is normalized. Then the heddle eye moves from the “top (B)” position to the “middle 1 (M1)” position (see graph line G2). This causes a large fall in yarn tension (see graph line G1) and sometimes a recuperation of warp thread occurs (see the small fall in graph line G3 just before reaching 360° machine cycle). When the heddle eye then moves from the “middle 1 (M1)” position to the “bottom (O)” position (see graph line G2), the distance to be covered is smaller than in the movement from the “middle 2 (M2)” position to the “top (B)” position. The yarn tension therefore builds up more slowly. In addition, there is now a pull-back element, e.g. a spring, which exerts force on the heddle and hence on the yarn to pull it down. This slower tension build-up with a very small peak at the position of arrow (P2) is apparent from graph line G1. This graph line G1 also shows that the tension is constant when the heddle eye is moved in the “bottom (B)” position (the horizontal bottom part of graph line G2 in the region between 360° and 720°). Furthermore, it appears from the rotation of the roller of the yarn tensioning element (graph line G3) that a quantity of warp thread has been supplied in the meantime. Then the heddle eye is again moved upward (see graph line G2), whereby the yarn tension falls (see graph line G1). This fall persists until the heddle eye has reached the “middle 2 (M2)” position. In this “middle 2 (M2)” position, the tension does not reach such a low value as in the “middle 1 (M1)” position. From there, the jacquard cycle begins again. Graph line G4 is a horizontal line which indicates the mean value of the yarn tension according to graph line G1.

(24) FIG. 3 shows a jacquard machine in which two possible positions are used: a “middle (M)” position and a “top (B)” position. The top position line (1) indicates the position of the warp threads which extend from the upper bridge (5) to a heddle brought to the “top (B)” position. The bottom position line (2) indicates the position of the warp threads which extend from the upper bridge (5) to a heddle brought to the “middle (M)” position.

(25) A pile-warp thread which is incorporated into the top ground fabric is moved successively, in successive weft insertion cycles, into the “top (B)” and “middle (M)” positions. See the indication of these movements on FIG. 3.

(26) FIG. 4 shows how the yarn tension develops during a number of successive jacquard cycles for a pile-warp thread incorporated into the top ground fabric with the successive heddle positions indicated above. Similarly to FIG. 2, the horizontal axis shows the rotation of the main shaft of the weaving machine (in degrees). During two machine cycles, or 720° on the horizontal axis, one jacquard cycle takes place. The vertical axis, just as in FIG. 2, shows the values of the yarn tension (in grams) which are also values of the movement of the heddle (in mm) and of the rotation of the roller of the yarn tensioning element (in degrees). FIG. 4 again shows four graph lines (G5), (G6), (G7, (G8) which are hereafter referred to as graph lines G5, G6, G7, and G8, and which respectively indicate the development of the yarn tension in the incorporated pile-warp thread, the movements of the heddle eye which positions this pile-warp thread, the total rotation of the roller of the yarn tensioning element which controls the tension of the pile-warp threads during one jacquard cycle (this yarn tensioning element according to the prior art exerts a constant force on the warp thread in order to keep this under tension), and the mean value of the yarn tension according to graph line G5. The indications on the horizontal and vertical axes of FIG. 4 are identical to those of FIG. 2.

(27) FIG. 4 shows the following for a number of successive jacquard cycles (2 weft insertion cycles): The heddle eye is moved from the “top (B)” position to the “middle (M)” position under the influence of the downward force exerted by a spring or other return element on the heddle, as apparent from the curve of graph line G6 from 0° on the horizontal axis. As the curve of graph line G6 shows, the yarn tension thereby falls to a minimum and remains approximately the same when the heddle is stationary in the “middle (M)” position, the warp thread is still under tension but under a much lower tension than in the “top (B)” position. The heddle eye is then moved from the position “middle (M)” to the position “top (B)” (see graph line G6), whereby the yarn tension builds up again to a maximum when the heddle eye is in the “top (B)” position. In the meantime, there is a small consumption of the pile-warp yarn (see graph line G7). From there, the jacquard cycle begins again. Graph line G8 is a horizontal line which shows the mean value of the yarn tension according to graph line G5.

(28) As can be clearly seen from comparison of the graph line G1 on FIG. 2 and graph line G5 on FIG. 4, the development of the yarn tension in a pile-forming pile-warp thread differs greatly from the yarn tension in a pile-warp thread which is incorporated. When a pile-warp thread is incorporated, there is only one peak of yarn tension per jacquard cycle, while there are two tension peaks in a pile-forming pile-warp thread. Also, the yarn is not pulled as hard, whereby for an incorporated pile-warp thread, the yarn tensions achieved are not as high. As a result, there is rarely or never any yarn overflow.

(29) When a method and a yarn tensioning device according to this disclosure are used, wherein each pile-warp thread cooperates with a respective yarn tensioning element and wherein a control unit controls the yarn tension via this yarn tensioning element in order to follow a first reference yarn tension profile when the pile-warp thread forms pile, and to follow a second reference yarn tension profile when the pile-warp thread is incorporated into the top ground fabric, a yarn tension profile may be obtained with lower maximum values and higher minimum values (lower peaks and higher troughs), whereby lower yarn tensions may be applied. These advantageous effects are illustrated in FIG. 5, which shows, for a number of successive jacquard cycles, the yarn tension profile of a pile-forming pile-warp thread, with the same successive heddle positions as in FIG. 2, while the yarn tension is controlled according to this disclosure.

(30) The horizontal axis of FIG. 5 shows the rotation of the weaving machine main shaft (in degrees). The vertical axis again shows the values of the yarn tension (in grams) which are also the values of the movement of the heddle (in mm) and of the rotation of the roller of the yarn tensioning element (in degrees). FIG. 5 shows four graph lines (G9), (G10), (G11) and (G12), which are referred to below as graph lines G9, G10, G11 and G12, and which respectively indicate the development of the same variables as the graph lines (G1)-(G4) on FIG. 2, namely yarn tension in the pile-warp thread (G9), movement of the heddle eye (G10), rotation of the roller of the yarn tensioning element (G11), and the mean yarn tension in the pile-warp thread (G12).

(31) By comparing the development of yarn tension according to graph line G1 on FIG. 2 and the development of yarn tension according to graph line G9 on FIG. 5, it is clear that the yarn tension according to graph line G9 builds up as quickly as according to graph line G1, but that the maximum value of the peak (P1) of graph line G9 is lower than the maximum value of the peak (P1) of graph line G1.

(32) Both graph lines (G1, G9) reach approximately the same minimum value in their trough (D1), which indicates that the yarn tension remains sufficiently high to be able to guarantee a good progress of the weaving process in general and of the shed formation in particular, and provides fabrics of excellent quality. The variation in yarn tension (the difference between the maximum value and minimum value) according to graph line G9 is thus also lower than according to graph line G1.

(33) By comparing graph line G4 on FIG. 2 and graph line G12 on FIG. 5, it is also shown that the mean yarn tension according to graph line G12 is significantly lower than the mean yarn tension according to graph line G4.

(34) FIG. 6 shows the principle of a control unit for a weaving machine according to this disclosure in a block diagram. The yarn tension (T.sub.M) in a warp thread is measured and compared in a comparator (10) with a specific reference value (T.sub.R) for this yarn tension. Alternatively, a variable which is a measure of this yarn tension may be measured and compared with a reference value for this variable.

(35) If a difference is found between the measured value (T.sub.M) and the reference value (T.sub.R), a regulator (11) is activated so as to intervene on the motor torque or current which controls the motor of the yarn tensioning element (12), so that this yarn tensioning element (12) changes the yarn tension such that the established difference is reduced. The yarn tension (T) in a warp thread is thus brought closer to or up to the reference value (T.sub.R).

(36) FIG. 7 shows the principle of a steering unit for a weaving machine according to this disclosure in a block diagram. A reference value (T.sub.R) for the yarn tension is input into a regulator, which as a result intervenes on the motor torque or current which controls the motor of the yarn tensioning element (12), so that this yarn tensioning element (12) brings the yarn tension (T) to a value which corresponds to the reference value (T.sub.R).

(37) Machine parameters such as the machine position or machine speed or data connected with the weaving pattern or weave structure may be made available to the regulator according to FIG. 6 and according to FIG. 7, wherein one or more of these parameters may be used for control or adjustment.