CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of German Application 19938433.9 filed Aug. 3, 1999, herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for centrifugal spinning of textile yarn within a rotatable spinning centrifuge, typically an open-ended cylinder, commonly referred to as pot spinning. More particularly, the present invention relates to a method of pot spinning on a spinning machine with a plurality of spinning stations whereat a fibrous rope or other suitable continuous supply of spinnable fiber, typically and preferably in the form of a sliver or a roving, is drawn by means of a drafting device, is guided via a tubular reciprocating yarn guide into a rotating spinning centrifuge, is spun into a yarn and, while forming a rotating yarn leg as the yarn exits the guide, is deposited in the form of an annular spinning cake on the inner wall of the spinning centrifuge. The present invention further relates to a pot spinning machine, having a plurality of work stations, each of which has a centrifuge rotatable about a shaft and a reciprocable yarn guide for introducing into the centrifuge a drawn fibrous sliver received from a drafting device. 
     BACKGROUND OF THE INVENTION 
     In pot spinning, a sliver is delivered from a sliver can and drawn in a drafting device, or a roving is delivered in the form of a speed frame bobbin, referred to collectively herein as a slubbing, is introduced into a rotating spinning centrifuge by means of a yarn guide reciprocable downwardly and upwardly within the centrifuge, is spun into a yarn, and is deposited as a so-called spinning cake on the inner wall of the spinning centrifuge. 
     More specifically, the slubbing delivered from the mouth of the yarn guide is transported by the action of an air flow rotating along with the spinning centrifuge to the inner wall of the spinning centrifuge and becomes fixed thereon centrifugally such that a moving yarn leg is created extending from the guide to the inner centrifuge wall and rotates around the centrifuge axis. The required twisting of the fibers of the sliver or roving necessary to form them into a yarn is imparted by means of the rotation of this yarn leg before it is deposited on the rotating inner surface of the centrifuge in the form of the spinning cake. At the end of a spinning cycle, e.g., after a predetermined amount of yarn has accumulated in the centrifuge, the yarn of the spinning cake which has been spun theretofore is rewound onto a rewinding tube introduced into the centrifuge. This rewinding process is initiated, for example, by pushing the rewinding tube forwardly into the centrifuge and into the path of the yarn leg. In the course of doing so, the rewinding tube grasps the rotating yarn leg coming out of the yarn guide by means of the leading edge of the tube, whereby the spinning cake deposited on the inner wall of the spinning centrifuge is then rewound onto the rewinding tube. 
     A yarn break can occur in the course of this rewinding process, resulting in a yarn body remnant remaining in the centrifuge after the rewinding of the yarn body onto the rewinding tube. A method and a device for restarting the rewinding process in pot spinning following such a yarn break are known from the later published patent application German Patent Publication DE 198 02 656. This document discloses the introduction of a special yarn loosening element into the centrifuge for grasping a yarn end of the yarn body remnant left in the centrifuge. Although yarn body remnants can be quite dependably removed from the spinning centrifuges by such a device, the occurrence and the presence of such yarn body remnants is often not noticed or noticed only very late. 
     Since such yarn body remnants also further reduce the already relatively limited receiving volume of the spinning centrifuges, it is important in the interest of the highest possible efficiency of a pot spinning machine to swiftly detect a yarn body remnant which has remained in the centrifuge, and to initiate appropriate counter-measures. 
     A method for pot spinning is known from German Patent Publication DE 195 23 835 A1, wherein the length of the rotation time of the yarn legs rotating in the spinning centrifuges is measured whereby a change of the length of the rotating time of the yarn leg occurring is immediately recognized and the already spun yarn body can be saved by introducing a rewinding tube. 
     It is furthermore known to detect the presence of a roving yarn introduced into the spinning centrifuge by means of a yarn sensor arranged between the drafting device and the reciprocable yarn guide tube. In the presence of the roving yarn, the yarn sensor provides an appropriate output signal. Here, the yarn sensor detects the movement of the roving yarn and provides an output signal which is proportional to the movement, preferably a noise signal. Such monitoring of the movement of the roving yarn is described, for example, in German Patent Publications DE 42 06 030 A1 or DE 42 06 031. 
     In case of a yarn break at a spinning station, an interruption of the material feed takes place as a rule. Thus, delivery of roving yarn is interrupted by means of a stop device acting on the slubbing. Such slubbing stop devices arranged in the area of the drafting devices are known from German Patent Publication DE 38 22 930 A1, for example. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Based on the above mentioned prior art, it is an object of the present invention to provide a method and a device which assures in a simple and cost-effective manner that the individual spinning centrifuges of a pot spinning machine are in an optimal state at the start of the spinning cycle, in particular that the spinning centrifuges are empty without any spinning cake remnants. 
     This object is achieved in accordance with the present invention by a method wherein all spinning centrifuges of the pot spinning machine are checked for the presence of possible yarn body remnants at the start of a spinning cycle, which makes it possible to dependably avoid that spinning centrifuges which having a yarn remnant remaining in the spinning centrifuge, and hence would produce poor spinning cops, are placed into the normal spinning process. 
     More specifically, the present method contemplates that the rotary frequency of the yarn legs of the individual work stations of the pot spinning machine is determined at the start of a spinning cycle of the pot spinning machine, and the rotary frequencies for each work station is compared with a predetermined set value. Thus, possible yarn body remnants in one or several of the spinning centrifuges of the work stations can be immediately detected because of the appearance of differences in the rotary frequency in comparison with the set value. In the course of measuring the rotary frequency of the yarn leg, the fact is taken into account that piecing takes place on the possibly still present yarn body remnant and that the reduction in the free radius as a result of this also leads to a reduction of the rotary frequency of the yarn leg. 
     More specifically, in a centrifuge rotating at a constant rotary frequency, a yarn body remnant left in the spinning centrifuge immediately leads to a lower rotary frequency of the respective yarn leg, based on the equation: 
     
       
           f   G   =f   Z −V L /2 π   *r   
       
     
     wherein f G  represents the rotary frequency of the yarn leg, f Z  represents the rotary frequency of the centrifuge, V L  represents the delivery speed of the yarn, and r represents the free radius of the centrifuge. It is therefore possible by means of an appropriate evaluation of the rotary frequency of the yarn legs to draw a direct conclusion regarding the presence of a yarn body remnant in the respective spinning centrifuge, which reduces the free radius. 
     In an advantageous embodiment of the invention, the rotary frequency of the rotating yarn leg is measured by means of a yarn sensor detecting the movement of the yarn. That is, a yarn sensor is arranged between the drafting device and the reciprocable tubular yarn guide to detect a yarn movement occurring in the area of the yarn sensor which corresponds to the rotary frequency of the yarn leg in the spinning centrifuge. 
     According to a further aspect of the invention, this yarn movement corresponding to the rotary frequency of the yarn leg is filtered out of the noise signal, which is detected by means of the yarn sensor and represents the movement of the yarn. For this purpose, the noise signal is conducted over a filter stage, for example, which detects a frequency signal based on the rotary frequency of the yarn leg. Processing of this frequency signal, for example by means of a fast Fourier transformation or the like, leads to frequency lines located in a defined spectral range, which can be unequivocally assigned to clearly defined free radii within the centrifuge. 
     By means of comparing these spectral lines with set values which are to be expected with empty centrifuges, the presence of a yarn body remnant can be immediately determined, so that appropriate counter-measures can be initiated. Since the free radius of an empty centrifuge is known because of the known geometric dimensions, the determination of a set value to be expected is possible in a simple manner. 
     In an advantageous feature of the invention, it is further provided that the rotary frequency of the yarn leg at each work station is determined simultaneously at all work stations. An unequivocal association of this data with each respective individual spinning station takes place in a measuring and evaluation unit, whereby it is possible by means of a comparison of the respective rotary frequencies from the work stations with the preset value to immediately determine whether and in which of the spinning centrifuges a yarn body remnant could possibly still be present. Since it is assumed that a yarn body remnant occurs seldom, and even more seldom at the same time at several work stations, the detected numbers of revolution of the yarn legs can also be compared with each other, so that it is possible to omit the provision of an external set value. In effect, the spinning stations not showing a yarn body remnant provide an internal set value. 
     Upon detecting a yarn body remnant in a spinning centrifuge, an appropriate control signal is immediately issued to the slubbing stop device of the respective work station via the evaluation unit, whereby the further delivery of roving yarn is stopped at the respective work station. 
     The objective of the invention is furthermore attained by providing the pot spinning machine with a sensor arrangement which makes it possible to detect at least one of the physical values appearing during a spinning cycle. The detected value is compared with a predetermined set value, for example in a work station computer or in a central spinning machine computer, and conclusions regarding the load status of the individual spinning centrifuges are drawn from the result of this comparison. 
     In this invention, the sensor arrangement is preferably designed as a yarn sensor, located between the drafting device and the yarn guide, which detects the presence of the slubbing. Because a filter stage is assigned to this yarn sensor, a signal corresponding to the rotary frequency of the yarn leg can be filtered out of the output signal of the yarn sensor and the signal may therefore be used for comparison with a set value signal in an evaluation circuit. 
     In this manner, a detection and evaluation of the rotary frequency of the yarn legs rotating with the spinning centrifuges can take place without extensive structure or structural modifications to the spinning machine, which might possibly require additional structural space. More specifically, only a filter stage, and a comparator which compares the signals, are required for modifying an already existing device such that, through the measurement of the rotary frequency of the yarn legs, conclusions can be immediately drawn regarding the possible presence of a yarn body remnant in one of the spinning centrifuges. 
     According to a further aspect of the invention, the pot spinning machine has a central control device, in which the signals provided by the individual yarn sensors are evaluated in regard to the detected rotary frequency of the yarn legs. In this manner, it is possible advantageously to perform a comparison with the set value in a compact unit, wherein the set value need only be made available once for all spinning stations. It is furthermore possible in an alternative embodiment to perform a comparison of the rotary frequency of the yarn legs of the individual work stations in respect to each other in a simple way by means of the control device. 
     Further aspects, features and advantages of the present invention will be explained in greater detail and will be understood form the following disclosure of an exemplary embodiment represented in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic front elevational view of three work stations of a pot spinning machine, in accordance with the method and device of the present invention, 
     FIG. 1 a  is a further schematic representation of the pot spinning machine of FIG. 1, and 
     FIG. 2 is a schematic top plan view of a spinning centrifuge of the pot spinning machine of FIG. 1, in accordance with the method and device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the accompanying drawings and initially to FIG. 1, a pot spinning machine is schematically represented at  10  and basically comprises a plurality of work stations  12 . Three of these work stations,  12 ,  12 ′,  12 ″, are partially represented in FIG.  1 . 
     Here, each work station  12 ,  12 ′,  12 ″ respectively includes a spinning centrifuge  14  supported, for example, on magnetic bearings (not represented), for rotation of the spinning centrifuge  14  at high speeds. 
     A reciprocable tubular yarn guide  16  is associated with each spinning centrifuge  14  and is disposed with a longitudinal axis of the yarn guide  16  extending coaxial with the axis of rotation of the spinning centrifuge  14 , whereby the yarn guide  16  can be lowered in the direction R into its respective spinning centrifuge  14 . 
     At each spinning station, a roving  24  is fed initially through a drafting device  26 , only representative shown in FIG. 1, wherein the roving  24  is drawn, and therefrom the drawn roving  24  is fed through the yarn guide  16  from which the drawn roving  24  exits the lower end of the yarn guide  16  and extends in the form of a moving leg  20  which deposits onto an inner wall  22  of the rotating centrifuge  14  during which the roving  24  becomes twisted into a finished spun yarn  18  by the rotation of the centrifuge  14 . 
     A drive mechanism, not represented, is respectively assigned to the yarn guides  16  and provides the yarn guides  16  with a continuous axial traversing motion CH and also lowers the yarn guides  16  continuously in the direction R during the spinning process. 
     A yarn sensor  28  is also assigned to each spinning station  12 , preferably in the area of the yarn guide inlet, to detect the presence of the roving  24  drawn to create the yarn  18 . In this case, it is contemplated that the yarn sensor  28  can be combined in a manner not shown or described in detail with and injector (not represented), which assures that the roving yarn  24  is threaded into the yarn guide  16  and the spinning station  12  can start spinning. More specifically, such an injector aspirates the roving  24  leaving the drafting device  26  and blows it into the yarn guide  16 . 
     Since these pot spinning machines are generally well known within the textile industry, it is not believed to be necessary to illustrate or describe further details of the pot spinning machine for purposes of describing and understanding the present invention. 
     The pot spinning machine  10  represented in FIG. 1 functions as follows. Following an essentially simultaneous spinning start of all work stations  12 ,  12 ′,  12 ″, the roving  24  leaving the mouth of the respective yarn guide  16  at each spinning station  12 ,  12 ′,  12 ″ is placed against the inner wall  22  of the rotating spinning centrifuge  14 , so that a yarn leg  20  is created which rotates with the spinning centrifuge  14 . The yarn  18  being created in the process is deposited on the inner wall  22  of the spinning centrifuge  14  in the form of a yarn body, i.e., a spinning cake  8 , as indicated by way of example by the dashed lines in the work station  12 ″ of FIG.  1 . 
     At the end of a predetermined spinning time or when reaching a predetermined amount of yarn, rewinding tubes (not represented) are delivered to the centrifuges  14 , on which the yarn bodies  8  are rewound. In the course of this rewinding operation, a yarn body remnant  30 , indicated by way of example at the spinning station  12 ′, can occur, for example, by a yarn break occurring during the rewinding process. This yarn body remnant  30  remains in the respective spinning centrifuge  14  even after the rewinding tube has been removed. If this yarn body remnant  30  is not removed, then upon the start of subsequent spinning operation in the centrifuges  14 , the new yarn leg  20  forming in the concerned work station  12 ′ will encounter the yarn body remnant  30  left in the spinning centrifuge. 
     To facilitate an understanding of the present invention, the relevant parameters affecting the course of spinning in the centrifuges will be explained by means of FIG.  2 . To this end, a spinning centrifuge  14  has been schematically represented in FIG. 2 in a top plan view. 
     As already indicated above, the axis of rotation  32  of the spinning centrifuge  14  coincides with the longitudinal axis of the tubular yarn guide  16 . It can be furthermore seen that the roving  24 , or respectively the yarn  18 , exiting from the yarn guide  16  is deposited on the rotating inner wall  22  of the spinning centrifuge  14 , forming the yarn leg  20 , and is taken along in the direction. In the process, the yarn  18  is deposited on the inner wall  22  at a constant delivery speed (winding speed) V L . At the same time, the centrifuge  14  rotates at a constant angular velocity V Z . At the impact point  34  of the yarn leg  20  on the inner wall  22  of the spinning centrifuge  14 , a free radius r therefore results between the mouth of the yarn guide  16  and the inner wall  22 . 
     The circumferential speed of the impact point  34  correspondingly results from the angular velocity V Z  and the free radius r of the spinning centrifuge  14 . It is true in principle that the circumferential speed results from the angular velocity and the radius r as V=ω*r. 
     Because the angular velocity ω can be replaced by 2 π f, wherein f represents the frequency, the result for the circumferential speed is V Z =2π*r. 
     In what follows, f G  indicates the rotary frequency of the yarn leg  20  and f Z  the rotary frequency of the inner centrifuge wall  22 . 
     The following equation then results in respect to the circumferential speed V G  of the yarn leg end  36  in the area of the impact point  34 : 
     
       
         V G =V Z −V L . 
       
     
     From this the following can be derived: 
     
       
         2π f   G   r =2 π   f   Z   *r− V L . 
       
     
     
       
           f   G =2 π   f*r− V L   
       
     
     
       
           f   G   =f   Z −V L /2 π   r.   
       
     
     It follows from this derivation, that the rotary frequency f G  of the yarn leg  20  is a function of the free radius r, i.e., the smaller the free radius r, the lower the rotary frequency f G  of the yarn leg  20 . 
     This knowledge means that, by measuring the rotary frequency f G  of the yarn leg  20 , it is possible to draw conclusions as to the actual free radius r, and therefore as to a possible yarn body remnant  30  present in the spinning centrifuge  14 . If, as indicated in FIG. 1 by means of the example of the work station  12 ′, a yarn body remnant  30  is present, a reduction of the free radius r occurs as a result, so that in accordance with the above explained relationships, a lower rotational frequency f G  of the yarn leg  20  exists at the work station  12 ′. 
     In respect to FIG.  1  and based on the above considerations in principle, the following results. As a rule, the yarn legs  20  each rotate around the yarn guide  16  at a rotary frequency f G , which corresponds to the free radius r of the spinning centrifuge  14 . 
     The movement of the respective roving  24  or yarn  14  is detected by means of the yarn sensors  28 . This movement leads to an output signal of the yarn sensor, which indicates in the form of a noise signal that the respective yarn  18  is moving and therefore exists. Because of twisting in accordance with the rotary frequency f G  of the yarn leg  20 , a further yarn movement is superimposed on the delivery speed V L  of the yarn  18 . This additional yarn movement is directly proportional to the rotary frequency f G  of the yarn leg  20 . 
     Accordingly, the output signal of the yarn sensors  28  contains, besides the movement information corresponding to the delivery speed V L  of the yarn  18 , a signal portion which corresponds to the rotary frequency f G  of the yarn leg  20 . 
     As indicated in FIG. 1, the yarn sensors  28  are connected with a control device  40  via signal lines  38 . As made clear by the schematically enlarged representation in FIG. 1 a , the control device  40  has a number of inputs  42 ,  42 ′,  42 ″ corresponding to the number of work stations  12  of the pot spinning machine  10 , so that every output signal of the yarn sensor  28  of each winding station  12  can be individually processed. 
     The inputs  42 ,  42 ′,  42 ″ are each connected with a filter module  44 , each of which is followed by a frequency evaluation module  46 . The frequency evaluation modules  46  are connected to a comparator  48 , which is moreover connected to a memory element  50 . 
     The comparator  48  is connected with the outputs  52  of the control device  40 , wherein a number of outputs  52 ,  52 ′,  52 ″ corresponding to the number of work stations  12  of the pot spinning machine  10  is again provided. Each one of the outputs  52 ,  52 ′,  52 ″ is connected with a so-called slubbing stop device  54 , not represented, which is assigned to the work stations  12  and which, with proper activation, causes the delivery of material by the respective drafting device  26  to be stopped. 
     An evaluation in accordance with the present invention of the output signals of the yarn sensors  28  by the control device  40 , which customarily takes on the control functions for monitoring the operation of the pot spinning machine  10 , can take place as follows, but it is to be explicitly understood that the control device  40  can of course also evaluate the output signals of the yarn sensors in other ways. 
     First, the output signals of the yarn sensors  28  are conducted through the filter modules  44  which are designed as bandpass filters, for example. In this manner, signal portions in a defined frequency band, which are located in the range of the balloon frequency at the yarn sensor  28 , are filtered out of the noise signal present thereat. Thereafter, this filtered-out frequency range is conducted through the frequency evaluation modules  46 . It is possible here to perform a spectral analysis, for example by means of a fast Fourier transformation or other known frequency evaluation methods, in such a way that the spectral portions, which correspond to the additional yarn movement, and therefore to the rotary frequency f G  of the yarn leg  20 , are filtered out. These are subsequently compared in the comparator  48  with stored set values, which are made available by the memory element  50 , for example. The rotary frequency f G  of the yarn legs  20  can be pre-calculated, based on the known geometry of the spinning centrifuges  14 , and therefore the known free radius r of an empty spinning centrifuge  14 . 
     If it is found in the course of this operation that the rotary frequency f G  of the yarn leg  20  is lower than the preset value, the conclusion may be drawn that this must result from a reduced free radius r, and therefore must indicate the presence of a yarn body remnant  30  in the corresponding spinning centrifuge  14 . A signal assigned to the respective work station  12  is provided via the comparator  48  to the respective output  52 , so that the delivery of roving material to the respective work station  12 ′ is directly interrupted by means of the slubbing stop device  54 , which is known and therefore not represented or explained in greater detail. 
     In accordance with a further variation, a comparison of each respective rotary frequency for a given work station with each other of the rotary frequencies f G  of the yarn legs  20  of the work stations  12  can be performed by means of the comparator  48 . 
     It will therefore be clear from the above disclosure of the present invention that it is thereby possible directly following the start of a spinning process of the pot spinning machine  10  to detect whether, as indicated by means of the example of the work station  12 ′ in FIG. 1, a yarn body remnant  30  is still present in one of the spinning centrifuges  14 . It is then possible to remove the respective work station  12 ′ immediately from production and clean it. The spinning process is preferably continued at the other work stations  12 ,  12 ″, etc., and the affected work station  12 ′ may be later actuated after being cleaned. By means of the method of the present invention, it is achieved that all work stations  12  are fully available again at the time of the next mutual spinning process, except if in the meantime another yarn body remnant  30  occurs, for example because of another yarn break, in another spinning centrifuge  14 . 
     It will be understood that the detection of the rotary frequency of each yarn leg need not necessarily take place by means of the yarn sensor  28  explained in the present exemplary embodiment. It is easily conceivable within the scope and substance of the present invention to use a sensor arrangement for determining the rotary frequency of the yarn leg, such as has been described, for example, in German Patent Publication DE 195 23 835 A1 in connection with the initiation of an emergency rewinding process. 
     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.
A method for operating a pot spinning machine which has a plurality of identical work stations each having a drafting device with a slubbing stop device, a spinning centrifuge rotating at high speed around an axis, and a tubular reciprocable guide. A slubbing emerging from the yarn guide mouth forms a rotating yarn leg which is deposited on the inner wall of the spinning centrifuge in the form of a spinning cake. At the start of a spinning cycle of the spinning machine, the individual spinning centrifuges of the spinning machine are checked for the presence of yarn cake remnants.