Abstract:
A test body is illuminated by at least one light source obliquely to its direction of travel, and the reflected light is imaged onto a diaphragm and fed to a photoelectric receiver. Its signal is investigated, in an evaluating unit, for periodicties which are caused by irregularities included in the test body as a result of the twist and the wavelength of which represents a measure of the twist. A rapid and precise measurement of the twist of yarns is made possible thereby.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for measuring the twist of a running, elongate test body, such as, for example, of a yarn or of a wire rope, by optical scanning of its surface and analysis of the scanning signal obtained in this case. 
     In a process of this type which is known from DE-A-3,628,654, a thread is grazingly acted upon by light, and specifically in such a manner that a part of the light beam is shaded off by thread and the part permitted to pass by the thread impinges on a light receiver. As a result of this, it is possible to detect certain structural alterations which are a measure of the thread period. Since this process, in which the profile is scanned to a certain extent, can be employed only when a detectable profile is present at all, this process is not suitable for the measurement of the twist of yarns. 
     A process of the initially mentioned type for the measurement of the twist of yarns is to be indicated by the invention. 
     SUMMARY OF THE INVENTION 
     According to the invention, this object is achieved in that the test body is illuminated by at least one light source and the light reflected by the test body is imaged onto a diaphragm and is measured by at least one photoelectric receiver, the output signal of which is investigated for periodicities caused by irregularities included in the test body, and in that the twist is derived from the wavelength or from the frequency of these periodicities. 
     Thus, the invention proceeds from the novel finding that the twist of yarns and the like leads to the bindingin of irregularities, which occur periodically on account of the twist. If the surface of a continuous test body is investigated for such periodicities, then the twist can be determined from these; this would not be possible by a simple scanning of the yarn profile. 
     The invention further relates to a device for carrying out the process according to the invention. This device is characterized by at least one light source for illuminating the test body, a diaphragm, an optical system for imaging the light reflected by the test body onto this diaphragm, at least one photoelectric receiver disposed in the beam path downstream of the diaphragm and an evaluation unit associated with this photoelectric receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in greater detail herein below with reference to an illustrative embodiment in the drawings; in the drawings: 
     FIG. 1A shows a diagrammatic representation of a device according to the invention, 
     FIG 1B shows yet another preferred embodiment of the present invention, and 
     FIGS. 2,3 show diagrams for explaining the operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A shows a piece of a yarn G, which is conveyed by guide and transport means (not shown) in the direction of the arrow P through a device for measuring the yarn twist. As represented, this measuring device includes two light sources 1, 1&#39; for illuminating the yarn G with a respective beam L, L&#39;, a diaphragm 2, onto which the beam R reflected by the yarn is imaged, a photoelectric receiver 3 disposed in the beam path downstream of the diaphragm and an evaluation unit 4 associated with this photoelectric receiver. Respective appropriate optical systems 5, 5&#39; and 6 are provided in the beam path of the two beams L, L&#39; and of the reflected beam R. A further lens 7 is situated between diaphragm 2 and photoelectric receiver 3. The light sources 1, &#39; are preferably formed by light-emitting diodes, so-called LEDs. 
     Before the measuring device is described in greater detail, the measurement process is to be explained now with reference to FIGS. 2 and 3: any yarn G exhibiting a twist or also any wire rope or any rope and the like exhibits, on account of the twist, certain irregularities with respect to its cross-section, and specifically, in particular, deviations from the cross-sectional shape. These characteristic deviations occur periodically, in which case the spacing between two successive such irregularities or, in other words, the length of the period thereof, represents a direct measure of the twist. This is so because the yarn twisted exactly once through 360° over such a period. 
     If the yarn G is illuminated at a small angle to its longitudinal axis, then the said irregularities become clearly visible as bright or dark positions, as is indicated in FIG. 2 by the hatched regions B. The length of the period, or, in other words, the wavelength of the irregularities, is designated by d. If T designates the twist of the yarn as the number of turns per unit length, then the following is applicable for the period d:d=1/T. Customary values for T are, for example, between 300 and 1,500 turns per meter. In the case of n-fold threads, the principal period occurs at d&#39;=d/n, where here d designates the period of the thread twist; thus, for example in the case of 2-fold threads, the principal period is to be expected at d&#39;=d/2. 
     The brightness of the impinging light beam R on the photoelectric receiver 3 (FIG. 1A) then increases periodically, and if the output signal of the photoelectric receiver 3 is evaluated in the evaluation unit 4 by Fourier transformation (FFT) or autocorrelation, a clearly detectable maximum is obtained at the period of the irregularities. In the case of very well defined structures, such as, for example, in the case of wire threads or filamentary threads, this analysis can, in certain circumstances, even take place by simple determination of a trigger threshold and subsequent counting. 
     The analysis by means of Fourier transformation is diagrammatically represented in FIG. 3; in this case, the frequency f is plotted on the abscissa and the amplitude A on the ordinate. A clearly detectable maximum is obtained at a specified frequency f1; in this case, the following then applies for the twist; T=f1/v, if v designates the draw-off speed of the yarn G. In the case of analysis by means of autocorrelation, the length d of the period is obtained directly. 
     In the case of yarns with twist, cross-sectional fluctuations also virtually always occur, which, however, do not necessarily lead to irregularities which are periodic, i.e. evaluable for the determination of the twist. Accordingly, it is advantageous to compensate the cross-sectional fluctuations; this takes place, as represented in FIG. 1A, by the use of two light sources 1 and 1&#39;, which illuminate the yarn G at differing angles of incidence. The angle of incidence a of the beam L is relatively small and is between 5° and 40°, preferably 5°, and the angle of incidence a&#39; of the beam L&#39; is steeper and is between 60° and 85°, preferably 85°. 
     If the two light sources L and L&#39; are differently modulated, then, with corresponding demodulation of the signal, a single common photoelectric receiver 3 can be employed; in this case, the signal component originating from the light source 1 is divided, in this, by that originating from the light source 1&#39;. However, it is also possible to use two light sources 1, 1&#39;, which emit light of differing wavelengths. In this case, the reflected beam R must be divided on the receiver side, and the individual components originating from the two light sources 1, 1&#39; must be distributed by appropriate filters to two different photoelectric receivers. 
     A further illumination variant consists in using two light sources 1 with a small and a light source with a very steep angle of incidence of up to about 90°; in this case, in relation to FIG. 1B the two light sources 1 are disposed symmetrically on both sides of the reflected beam R and the third light source is situated between the other two. This arrangement leads, on the one hand, to an even better emphasising of periodic structures and, on the other hand, to the elimination of disturbing influences originating, for example, from neps and the like. 
     The following conditions are applicable to the diaphragm 2: if the yarn exhibits T twists per unit length, and is imaged onto the diaphragm 2 at the magnification K:1, then the diaphragm 2 must be narrower than K/T in the direction of the length of the yarn, in order that it should still be possible to achieve good detection of periodic components of the twist. In the case of yarns, in the transverse dimension the diaphragm 2 is advantageously restricted to the approximate detection of the yarn body, so that the hairiness does not have an excessively disturbing effect. In order to achieve further reduction of the disturbing influences of the hairiness, the yarn G can be singed. If the yarn G is very strongly singed, then it is possible to obtain insights into the twist in the yarn body; this can, on the one hand, be very much desired in the case of rotor yarns, but is, on the other hand, destructive and should therefore be restricted to random samples. 
     Finally, it should furthermore be mentioned that the test body G should be guided in a vibration-free manner as far as possible, since with light incident at a small angle vibrations have a disturbing effect. For this reason, it is advantageous to guide the test body G directly at the measurement position via a deflecting component. 
     The described measurement device can be constructed in a very compact manner, and is therefore outstandingly suitable for use as measurement module in a device for the automatic determination of characteristic quantities of textile test material, as is described, for example, in Swiss Patent Application No. 02,823/86-2, and is known under the designation USTER TESTER (USTER--registered trade mark of Zellweger Uster AG).