In the production of textiles, the warp and weft thread intersect at precisely right angles. However, during subsequent working procedures, the textile can become drawn. The distortion can be removed by straightening machines which use the draft angle of the weft thread as a control value. Accordingly, the draft angle must be measured.
Optical devices are known for measuring the draft angle. In an optical device, a source of light irradiates the moving textile sheet. The transmitted or reflected light is converted to electric signals and the angular course of the weft thread is determined from the signals.
However, there are many types of materials in which the position of the weft thread can not be controlled by optical scanning means. Longitudinally ribbed corduroy, thick rough materials, and the like, are not suitable for handling by optical means.
As it will be explained in more detail hereinbelow with reference to FIGS. 1 and 2, mechanical methods which make use of a special property of the fabrics can be used for detecting the course of the weft thread in those materials that cannot be optically scanned. If forces K are now introduced in the textile sheet 1 by tension-introducing devices 20, 20', substantially no changes results in the length of the weft thread 7 or in the width of the textile sheet 1 when the weft threads extend precisely between the tension-introducing points 20 (FIG. 1). But if the weft threads lie diagonally to the connecting line between the tension-introducing devices 20, 20', then there results, as shown in FIG. 2, a displacement in relation to each other of the different weft threads 7, 7', since each weft thread 7 or 7' has tension applied to only one end. Due to the displacement of the weft threads 7, 7' relative to each other this causes the warp thread 6 to assume a diagonal position in the area between the "drawn" weft threads. This phenomenon is made use of in the apparatus according to U.S. Pat. No. 2,795,029 wherein a mechanical detector F in the form of a caster wheel follows upon the textile sheet 1. This caster wheel F follows the "trace" of the warp threads 6 so that in the case shown in FIG. 2, the angular position of said caster wheel F (which can be scanned, for instance, by a potentiometer) makes possible an indication about the draft angle .alpha., that is, the diagonal position of the weft threads 7 relative to the warp threads 6.
One disadvantage of this method is that the sensitivity of the equipment in respect to the draft angle is relatively poor and the method is subject to interferences.
There is also known a process for "automatic" alignment of weft threads in which use is made of the forces acting on the textile due to applied tension. An example of this appears in EP-A-O 136,155. In this method, the continuous textile sheet is guided by the edges over needle-wheels having axles angularly offset in respect to each other in a manner such that the textile sheet is stretched when running over a certain peripheral angle of the wheels. The wheels are free-wheelingly disposed on their axles. As long as the weft threads extend exactly at right angles with the warp threads, identical moments act upon the needle-wheels. But when a diagonal distortion is present in the textile sheet 1, as for example illustrated in FIG. 2 on the fabric 1, there are applied to the needle-wheels, due to the stretched weft threads 7, torques which make one wheel run forward or backward in relation to the other wheel. In this manner, an "automatic" rectification of the distortion is obtained. But this rectification is not satisfactory since as the draft angle decreases, the moments acting upon the wheels likewise decrease. The method is a "classical" case of a proportional adjustment in which error in the draft angle inevitably remains.
Departing from the above stated prior art, the problem to which this invention is addressed is to improve a process and an apparatus of the above-mentioned kind in the sense of making possible by simple means, an improved mechanical scanning of the course of the weft thread or draft angle and suitable method of alignment.