CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of German patent application 10009131.8 filed Feb. 26, 2000, herein incorporated by reference  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to a method and a device for the optical detection of impurities, in particular foreign fibers, in a longitudinally traveling yarn, and relates more particularly to such a method wherein light is emitted in the direction of the yarn, the intensity of the transmitted light is measured and converted into a first electrical signal whose value is a function of the instantaneous diameter of the yarn, the light reflected by the yarn is also measured, and a second electrical signal is produced.  
         BACKGROUND OF THE INVENTION  
         [0003]    The detection of impurities is of great importance in the production of yarns. Impurities, particularly foreign fibers, can have disadvantageous effects on the final product.  
           [0004]    It is known from European Patent Publications EP 0 197 763 and EP 0 553 545 to monitor a yarn in front of a background with the same reflecting ability as the yarn. Thus, it is intended that the amount of reflected light be essentially independent of the diameter of the yarn and a change in the reflected light does not indicate a change of the yarn diameter, but an impurity, for example as a result of foreign fibers. In this case, it is necessary to match the reflections from the background and the yarn very exactly to each other, because otherwise thick or thin places in the yarn cause changes in the reflected light and distort the results of the foreign fiber detection. The matching is elaborate, can only be automated to a limited extent or not at all, and moreover must be performed with every change in the fiber material, such as after a batch change, for example.  
           [0005]    European Patent Publications EP 0 553 446 and EP 0 572 592 disclose methods and devices for detecting foreign fibers, wherein light from an illumination device is cast on the moving yarn, where it is reflected and transmitted. The reflected light and the transmitted light are each converted into electrical signals. These two electrical signals are linked to each other and a further electrical signal is thereby obtained, in which the foreign fibers are indicated and the other yarn defects, such as thick or thin places, are suppressed. With such signal linkages, the signals are customarily processed in the form of digital signals. Errors may occur in the value of a digital signal if rounding of the signal value is necessary in the course of converting analog signals into discrete digital signals which, in connection with the detection of foreign fibers, can lead to distortions of the measured results. In case of changes of the reflection signal, the signal portions which are dependent on the diameter can be ten, or even a hundred times, greater than the foreign fiber-dependent signal portions. Small errors during the linkage of the signals and the elimination of the diameter-dependent signal portions can lead to a considerable distortion of the smaller amount of the foreign fiber signal, and therefore of the results of the foreign fiber detection.  
           [0006]    Another device is known from European Patent Publication EP 0 572 592, by means of which changes of the light from the light source, which are caused by dirt, are compensated by keeping the intensity of the light source constant In this manner, it is intended to avoid changes in the light which develop over a long term or over a long time period, such as can be caused by dirt.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the present invention to reduce the disadvantages in connection with the known methods and devices for detection of foreign fibers, and to improve the detection of impurities in longitudinally traveling yarn.  
           [0008]    The method of the present invention seeks to achieve this object by optically detecting foreign fibers and other impurities in a longitudinally traveling yarn, through the steps of emitting a light in the direction of the yarn; measuring the intensity of the emitted light; producing a first electrical signal based on the measuring of the emitted light intensity, the first signal being of a value which is a function of the instantaneous diameter of the yarn; following the formation of the first electrical signal, adjusting the intensity of the emitted light as a function of the yarn diameter based on the first electrical signal to compensate for the effect of the yarn diameter on the light reflected by the yarn; then measuring light reflected by the yarn; producing a second electrical signal based on the measuring of the reflected light; and directly using the second electrical signal for detecting impurities in the yarn. A matching of the reflecting capability of the measuring background with the reflecting capability of the yarn is no longer required for this method. Since in accordance with the invention signal portions which depend on the diameter are not even entered into the second signal, and changes in the second signal directly indicate impurities, it is possible to omit computing steps for linking the second signal with another signal, or with a signal which to a large degree is dependent on the diameter, thus eliminating the dependency on the diameter for detecting the foreign fiber proportion. Any rounding or computational errors caused by such computing steps, as well as the summation of such errors, is prevented.  
           [0009]    The present invention further prevents rounding errors which occur in converting analog to digital signals in that the first electrical signal is generated in the form of an analog signal. A possible, but undesired, influencing of the measured values is prevented in a simple way by registering the first and second electrical signals at a predetermined clock frequency, or by pulsing the light source at a predetermined clock frequency.  
           [0010]    Through the present manner of controlling the intensity of the emitted light in obtaining the second electrical signal, a completely different operating mechanism exists then in comparison to the device for regulating the output of the light source known from European Patent Publication EP 0 572 592. While the known device is intended to compensate for changes and to always maintain the light intensity constant, the control of the intensity of the emitted light in the present invention for forming the second electrical signal is purposely not kept constant, in contrast to the known device, but is set as a function of the first electrical signal, and therefore as a function of the yarn diameter. Thus, in accordance with the present invention, the light intensity of the light source for forming the second electrical signal is changed when a change of the first electrical signal occurs whereby, with an increased yarn diameter, the light intensity is set correspondingly lower, and with a decreased yarn diameter, the light intensity is set correspondingly higher.  
           [0011]    The outlay for setting the intensity of the light source as a function of the first electrical signal can be reduced by setting a constant value of the light intensity of the light source in connection with the first measurement for forming the first signal. As a result, the diameter-dependent first measurement for forming the first signal always takes place under the same conditions, and the first signals can be directly used as a measurement of the instantaneous diameter of the yarn. The first electrical signal can be employed not only for controlling the intensity of a light source, but can additionally be evaluated for monitoring thick and thin places in the yarn.  
           [0012]    In a preferred embodiment of the method, the second electrical signal is obtained at a time delay in respect to the first electrical signal at a measurement point which is located downstream in the direction of the yarn travel, with the time delay being controlled as a function of the yarn velocity such that both signals are obtained at the same location or section along the yarn. An extremely great measurement accuracy and measurement dependability is thereby assured.  
           [0013]    The present invention also contemplates a novel device for performing the afore-described method and, particularly, is advantageous in compensating for changes in light intensity caused by aging or soiling of the light sources.  
           [0014]    The second electrical signal obtained in accordance with the invention leads to increased measuring sensitivity in the course of the optical detection of impurities. Critical areas, such as the differentiation between dark brown and black, or the differentiation between off-white and pure white, can be easily controlled without problems through the present invention. The invention permits a clear improvement in the detection of impurities, in particular foreign fibers, in linearly moved yarn.  
           [0015]    Further advantageous embodiments of the invention will be understood from the following disclosure of exemplary embodiments of the invention represented in the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic representation of a bobbin winding station in accordance with the present invention,  
         [0017]    [0017]FIG. 2 is a schematic representation of a detection device at the bobbin winding station of FIG. 1, in accordance with the present invention, and  
         [0018]    [0018]FIGS. 3 and 4 are schematic representations of further exemplary embodiments of detection devices in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    In the bobbin winding station represented in FIG. 1, a yarn  1  is drawn out of a spinning box  3  through a small draw-off tube  2  and is wound on a cheese  4 . Between the small draw-off tube  2  and the cheese  4 , the yarn  1  passes through a cleaning device  5  and a guide eye  6 . A drive drum  7  drives the cheese  4  by means of surface frictional contact during the winding process. Rotary movement is imparted to the drive drum  7  by a motor  8 . The cleaning device  5  is used for monitoring the quality of the running yarn  1 . The cleaning device  5  includes an integrated measuring station. The cleaning device  5  is connected with further devices for the control, data storage or data evaluation, and the triggering of further elements of the bobbin winding station, or of the spinning machine.  
         [0020]    A detection device is represented in simplified form in FIG. 2, and includes the cleaning device  5  which has a light source  9  and a control device  10 . The light source  9  emits light in the direction toward the yarn  1  and, in a known manner, provides an image of the yarn  1  on a sensor  11 . In a first measuring phase, the sensor  11  performs a first measurement and forms a signal which is representative of the instantaneous yarn diameter and is converted into an analog electrical signal. This signal is supplied to the memory  12  of a memory device and is stored therein. In this first phase, a control device  10  receives a signal of constant value coming from a data memory  14  via a change-over switch  13 . This switching position is represented in FIG. 2. The signal from the data memory  14  is used as a control signal for setting the intensity of the light emitted by the light source  9  in this first measuring phase. Light emitted by the light source  9  is detected by a sensor  15  positioned such that this detected light is not affected by the yarn  1 . The sensor  15  is a part of a control device, known per se, by means of which the effects of soiling or aging of the light source are eliminated and which causes the intensity of the light emitted by the light source  9  in the first measuring phase to be maintained constant.  
         [0021]    The length of this first measuring phase is controlled by a control device  16 . The control device  16  is connected with the memory  12 , with the control device  10 , and with the changeover switch  13  and further elements of the winding station. The control device  16  terminates the first measuring phase by actuating a switching of the change-over switch  13 . By means of the actuation of the change-over switch  13 , the control device  10  also receives the control signal coming from the memory  12 , whereby a second measuring phase starts. This analog electrical signal from the memory  12  represents the instantaneous diameter of the yarn  1  in the first measuring phase. As a function of this signal, the control device  10  now sets the intensity of the light source  9  to compensate for the effect of the yarn diameter on the light reflected by the yarn. Light reflected by the yarn  1  is detected by a sensor  17  and a second electrical signal is formed, which is supplied via an amplifier  18  to a memory  19 . The second electrical signal is forwarded from the memory  19  to an evaluation device  20 , and is directly evaluated for detecting impurities. If a change occurs in the reflected light, and therefore correspondingly changes the second signal, the change represents a yarn impurity.  
         [0022]    The first electrical signal is also forwarded from the memory  12  to the evaluation unit  20  and is evaluated there for monitoring the quality of the longitudinally traveling yarn  1  in a manner known per se.  
         [0023]    At the presence of the second electrical signal, the control device terminates the second measuring phase by actuating a reverse switching of the change-over switch  13 , so as to again provide the control device  10  with the signal from the data memory  14 . In this manner, the two-phase measuring cycle starts again at the beginning. The clock frequency of the measuring cycles is controlled by the measuring device  16  and preferably lies in the kilocycle range. The measurements follow each other so rapidly as to be equivalent to a continuous measurement. The light source  9  is switched off when the change-over switch  13  is actuated, and is switched on again immediately following the change-over process.  
         [0024]    The elements arranged inside an area indicated by dashed lines in the exemplary embodiment of FIG. 2 are located in a common housing at the measuring station of the cleaning device  5 .  
         [0025]    An alternative exemplary embodiment of the detection device is represented in FIG. 3. A light source  22  is controlled by a control device  21 . The control device  21  is connected with a data memory  23  and is provided therefrom with a control signal for setting the intensity of the light emitted by the light source  22 . This control signal is maintained constant. A control circuit can be provided in a further alternative embodiment, which compensates for aging and soiling of the light source  22  in a known manner.  
         [0026]    The light emitted by the light source  22  provides an image of the yarn  1  on the sensor  24 . The sensor  24  forms a signal which represents the instantaneous diameter of the yarn  1  and which is supplied as a first analog electrical signal to a memory  25 . The control device  21  is provided with this signal from the memory  25  for controlling a second light source  26 . The intensity of the light source  26  is set by means of this signal from the memory  25  to compensate for the effect of the yarn diameter on the light reflected by the yarn. Measurement of the reflected light takes place by means of the sensor  27 .  
         [0027]    The sensor  27  forms a second electrical signal from this second measurement, which is supplied via an amplifier  29  to an evaluation unit  28  wherein the measuring of the second electrical signal is performed with a time delay compared with the measurement of the first electrical signal. The time delay is controlled such that both measurements take place at two spaced apart measurement points, but at the same point, or the same section, along the yarn  1 . To be able to match the time delay to the yarn velocity, the control device is connected with devices for detecting the yarn velocity and receives signals from these devices, not shown for reasons of simplicity, which represent the instantaneous yarn velocity.  
         [0028]    The second electrical signal is directly used in an evaluation unit  28  for detecting yarn impurities. The measuring and control processes run continuously. It is alternatively possible to perform the clocking of measuring phases within the measuring cycles. Besides the detection of yarn impurities, additional quality monitoring takes place by means of evaluating the first electrical signal respectively transmitted from the memory  25  to the evaluation device  28 .  
         [0029]    In a further alternative exemplary embodiment of the device represented in FIG. 4, the light emitted by two light sources  30 , 31  is measured by a single sensor  32 . In a first measuring phase, a control device  33  is provided with a control signal, which is maintained constant, from a data memory  35  via a change-over device  34 . This switching position of the change-over device  34  is represented in FIG. 4. In the first measuring phase, the light source  30  is switched on by the control device  33 , and its intensity is set to a value which is a function of the control signal from the data memory  35  such that an equal intensity occurs in every first measuring phase. The light emitted by the light source  30  provides an image of the yarn  1  on the sensor  32 . The sensor  32  forms a first signal from the detected light, which represents the diameter of the yarn  1 . This signal is provided as an analog electrical signal to a memory  36  and is stored therein. The control device terminates the first measuring phase by switching off the light source  30 . The clock frequency of switching on and off the light source  30  is predetermined in the control device  33  by the control device  37 , which is connected with the latter.  
         [0030]    Then, the control device  37  actuates the change-over device  34  after which the control device  33  is provided with the first electrical signal from the memory  36 . The light source  31  is switched on by the control device  33 , and its intensity in this second measuring phase is set as a function of the first electrical signal to compensate for the effect of the yarn diameter on the light reflected by the yarn.  
         [0031]    Light reflected by the yarn  1  in the second measuring phase is measured by the sensor  32  and a second signal is formed, which is provided to the memory  36  in the form of an electrical signal. The first electrical signal, as well as the second electrical signal, are transmitted from the memory  36  via an amplifier  38  to the evaluation device  39 . Subsequently the light source  31  is switched off again by the control device  33 . The control device  37  actuates the change-over device  34  and therewith terminates the second measuring phase, and thus the entire first measuring cycle.  
         [0032]    Following the actuation of the change-over device  34 , the control device  33  is again provided with the signal from the data memory  35 , and the second measuring cycle starts. The second measurement is performed with a time delay in respect to the first measurement such that the second measurement takes place at the same location along the yarn as the first measurement. The time delay is adapted in a manner known per se to the instantaneous yarn velocity.  
         [0033]    Alternatively to these exemplary embodiments, the control and memory elements can be arranged in a single microprocessor, or in any other arrangement inside or outside of the measuring station.  
         [0034]    It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof.
Yarn impurities are detectable by a method and device wherein a first diameter-dependent signal is obtained in a first measurement of a linearly traveling yarn, the intensity of the light for a second measurement is set as a function of the first signal to compensate for the effect of the yarn diameter on the light reflected by the yarn and then the second electrical signal can be directly evaluated for detecting yarn impurities. The invention improves the detection of impurities, for example in connection with spinning and bobbin winding machines.