Abstract:
A system for determining a zero position of a yarn guide ( 25 ) of a textile winding device for cross-winding yarn into cheeses via a step motor ( 34 ). The yarn guide ( 25 ), acted upon by the step motor ( 34 ), is initially displaced in the direction of its zero position (NS) and positioned at a slow speed against a defined detent ( 15 ) downstream of the zero position (NS). Then, the step motor ( 34 ) is switched off, causing the rotor ( 41 ) of the step motor ( 34 ) to drop into one of two possible stop positions (RS 1 , RS 2 ). Subsequently, the step motor ( 34 ) is switched on by an electrical current supply to its stator windings ( 20 A,  20 B) such that, when switched off, the rotor ( 41 ) of the step motor ( 34 ) is in the stop position (RS 2 ) in which the yarn guide ( 25 ) is in its zero position (NS).

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of German patent application DEP102005002409.2 filed Jan. 19, 2005, herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method and apparatus for determining a zero position of a yarn guide of a winding device of a textile machine producing cheeses, and relates more particularly to such a textile machine which is capable of cross-winding cheeses by means of a step motor. 
   For producing a textile bobbin, it is known to be necessary, on the one hand, to put the respective textile bobbin into rotation and, on the other hand, to cross-wind the yarn running up on the bobbin along the bobbin axis. By means of a relatively rapid cross-winding of the yarn, it is possible to produce a so-called cross-wound textile bobbin, which is not only distinguished by a comparatively stable bobbin body, but also by good unwinding properties. 
   In connection with the production of such cheeses, there are already different yarn placement systems in actual use, which are extensively described in numerous patent applications. 
   For example, in connection with bobbin-winding machines operated at relatively high winding speeds, the use of so-called yarn guide drums as yarn cross-winding devices is widespread. Such yarn guide drums have a groove for cross-winding the yarn and at the same time they drive the textile bobbin by frictional contact. However, such yarn guide drums, which have proven themselves per se, are relatively expensive to produce and, because of the limits of the system, can only be used for producing cheeses of the “random cross-wound” type, since for producing a so-called “precision,” or “stepped precision winding,” the drive mechanism of the cheese and the drive mechanism of the-yarn cross-winding device must be separate. 
   Different yarn cross-winding devices are known in connection with winding heads which respectively have a separate drive mechanism for rotating the take-up bobbin and a separate drive mechanism for cross-winding the arriving yarn. For example, cross-winding devices are known which have a yarn guide which can be shifted parallel with the axis of rotation of the cheese and are for example connected via traction means with a reversible individual drive mechanism. Installations are furthermore known which operate with a so-called finger yarn guide, i.e. a yarn guide which has a finger-like yarn displacement lever, which is pivotable-through a specific angular range around an axis which is arranged substantially perpendicularly in relation to the cheese axis. 
   Yarn cross-winding devices with a yarn guide which can be shifted parallel in relation to the axis of rotation of the cheese have been described, for example, in German Patent Publication DE 37 34 478 A1, in German Patent Publication DE 100 21 963 A1 or in the later published German Patent Publication DE 10 2004 003 173.8. 
   The yarn cross-winding device described in German Patent Publication DE 37 34 478 A1 has a yarn guide which is guided on a yarn guide rod in the cross-winding area and is driven via endless traction means by a microprocessor-controlled step motor. 
   A yarn cross-winding device is also described in the later published German Patent Publication DE 10 2004 003 173.8, whose yarn guide is guided on a yarn guide rod and is connected via a toothed belt with a reversible individual drive mechanism. Here, the yarn guide rod, the gear wheels for the toothed belt, as well as the associated individual drive mechanisms are arranged in a housing which is closed to the greatest possible extent. This means that the interior of this yarn cross-winding device, embodied as a modular structural component, is sealed at the front by the endless traction means. 
   A yarn cross-winding device with a cross-winding yarn guide which is fixed on an endless belt and can be operated by an individual drive mechanism is furthermore described in German Patent Publication DE 100 21 963 A1. Here, the individual drive mechanism of the yarn guide can be controlled in such a way that a cross-winding stroke can be performed which can be changed in length. To this end, the individual drive mechanism of the cross-winding yarn guide is equipped with an angle encoder which detects the rotor position of the electric motor and reports the detected rotor position to a corresponding work station control. 
   However, in connection with the above-mentioned generally advantageous yarn cross-winding devices, the comparatively large control outlay, which is performed for assuring a correct traversing of a yarn, is disadvantageous. Thus, the known yarn cross-winding devices have relatively elaborate, and therefore expensive control and monitoring devices. 
   SUMMARY OF THE INVENTION 
   In view of the above described known winding devices, it is an object of the present invention to provide a method and a device which overcomes the disadvantages mentioned. A more particular object is to make possible an assured and accurate cross-winding of a yarn being wound onto a cheese, wherein the expenditure for control means should be as low as possible. 
   In accordance with the invention, this object is addressed by a method for determining a zero position of a yarn guide device of a winding device of a textile machine which is operative for cross-winding a yarn by means of a step motor for producing yarn cheeses. The method basically comprises the steps of, first, displacing an element of the yarn guide device by the step motor in the direction of the zero position, and positioning the element of the yarn guide device at a comparatively slow speed against a defined detent arranged downstream of the zero position in the direction of the displacement. The step motor is then switched off for positioning a rotor of the step motor into one of two possible stop positions. Then, the step motor is switched on by a definite electrical current supply to stator windings of the step motor such that, when again switched off, the rotor of the step motor is in the stop position in which the yarn guide is in its zero position. 
   The method of the present invention has the particular advantage of assuring in a simple manner, i.e. without additional sensor devices or the like, that at the beginning of the cross-winding process the yarn guide can always be started from a defined position. In connection with a step motor, it is assured that the cross-winding process can be correctly performed, i.e. that in the course of cross-winding the incoming yarn, the yarn guide is always in the right position at the right time. 
   As mentioned above, no additional sensor devices are required for executing the method in accordance with the invention, only hardware or software, which is needed anyway for operating a yarn guide which can perform cross-winding by means of a step motor. Thus, the method in accordance with the invention can be realized quite cost-efficiently. 
   In an advantageous embodiment, an apparatus for executing the method in accordance with the invention has a yarn guide drive embodied as hybrid step motor. The rotor of this hybrid step motor is equipped with an axially magnetized permanent magnet and has salient poles in the form of toothed pole caps in each rotor end. Rotors designed in this way with a relatively large number of poles in the area of their pole caps lead to small angular steps during the operation of the step motor which, on the one hand, has a positive effect on the rotation of the step motor and, on the other hand, leads to a multitude of so-called stop positions of the rotor when the step motor is switched off. Thus, a step motor designed in this way is distinguished, on the one hand, by good rotation, but can also be sensitively positioned in a pre-defined stop position when needed. 
   The rotation of the step motor can be further optimized in a simple manner by arranging the poles of the first pole cap in an angularly offset manner in respect to the poles of the second pole cap. 
   In an advantageous embodiment, the angular offset between the poles of the first pole cap and the poles of the second pole cap may be half a divisional spacing between the poles, which is effective, together with continuous magnetic stator poles, for halving of the increments of the respective angular steps of the motor and therefore to a very even rotation of the step motor. 
   In a preferred embodiment, it is furthermore provided that the stator of the hybrid step motor has a multitude of stator segments, each of which is enclosed by windings which can be charged with electrical current. In this case, one half of the windings is connected to a first phase of a d.c. current source, while the other half of the windings is in connection with a second phase. Thus, the stator segments on the circumference of the stator which are supplied with electrical current via the first phase alternate with stator segments connected to the second phase. Charged appropriately by means of a control, such a step motor runs dependably in bipolar operation. 
   The stator segments preferably have pole teeth which, in accordance with the electrical current supply of the windings of the stator segments, correspond with the poles, of the pole caps of the rotor. Thus, when the windings of the stator segments are supplied with electrical current, the N-poles of the front pole cap of the rotor, as well as the S-poles of the rear pole cap of the rotor are affected. 
   In a further design of the invention, it may be furthermore provided that a toothed drive wheel is placed, fixed against relative rotation, on the motor shaft of the step motor, which drives a toothed belt with a tooth spacing of preferably 2 mm. In an advantageous embodiment, the drive wheel here has 48 teeth, for example. 
   Such a design of the mechanical drive elements assures that every movement of the step motor which relates to the cross-winding range, the cross-winding width or the cross-winding speed, is exactly converted, i.e. without slippage. Correct yarn cross-winding, as well as a sufficiently exact positioning of the yarn guide in its zero position, is possible in this way in connection with hybrid step motors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail in the following disclosure with reference to an exemplary embodiment represented in the accompanying drawings, wherein: 
       FIG. 1  is a lateral view of a work station of a cheese-producing textile machine with a yarn cross-winding device in accordance with the present invention in the area of the bobbin-winding device, 
       FIG. 2  is a top perspective view in an enlarged scale of a yarn cross-winding device in accordance with the present invention, 
       FIG. 3  depicts the yarn guide of the yarn cross-winding device in accordance with  FIG. 2  in its zero position, 
       FIG. 4  is a front view of a yarn guide drive mechanism designed as a hybrid step motor with the front motor cover removed, and 
       FIG. 5  depicts the rotor of the hybrid step motor of  FIG. 4  in a lateral view and partially in section. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the accompanying drawings, one side (i.e., one half) of a cheese-producing textile machine  1 , in the exemplary embodiment an open-end rotor spinning machine, is schematically represented in a lateral end view in  FIG. 1 . 
   As known, such textile machines have multiple identical work stations  2  arranged side-by-side between their end frames (not represented). In this case, each of the work stations  2  has a spinning unit  3 , as well as a bobbin-winding device  4 . Slivers  6  stored in sliver cans  5  are processed into yarn  7  in the spinning units  3  and are subsequently wound into cheeses  8  on the bobbin-winding devices  4 . The finished cheeses  8  are conveyed via a cheese transport installation  12  to a loading station (not represented) arranged at the machine. 
   As indicated in  FIG. 1 , the work stations  2  each have still further manipulating arrangements besides the spinning unit  3  and the bobbin-winding device  4 , for example a yarn draw-off device  10 , a suction nozzle  17 , or a waxing device  14 . The functions of these components are known and extensively explained in numerous patent publications, and therefore need not be explained herein. 
   The bobbin-winding device  4  has a creel  9 , a bobbin drive roller  11 , as well as a yarn cross-winding device  16 . The bobbin drive roller  11 , which can be driven by a drive mechanism  13  with an individual motor, drives the cheese  8 , which is seated in a freely rotatable manner in the creel  9 , by friction. 
   The yarn cross-winding device  16 , such as respectively arranged in the area of the bobbin-winding device  4  of the numerous work stations  2  of the textile machine  1 , is shown in detail in  FIG. 2 . 
     FIG. 2  here shows a top perspective view of a yarn cross-winding device  16  embodied as a modular yarn guide unit  22  and sealed to the greatest extent. That is, the interior  37  of the housing  23  of the yarn guide unit  22  is sealed off at the front by the endless traction means  30 , can be covered at the top by means of a cover element (not represented) and is then protected to the greatest extent against the penetration of textile dust and flying fibers. 
   As can be further seen, the yarn guide  25 , which is made in the customary way from a wear-resistant material, preferably an oxide ceramic material, is fastened to a guide shoe  26 , which in turn is slidingly conducted on a guide element  27 . In this case the sliding element  27  itself is fixed on the lateral walls of the yarn guide unit  22 . The guide shoe  26  is furthermore fastened on the exterior of an endless traction means, preferably a toothed belt  30  which, as indicated, is guided via direction-reversing wheels  31 ,  32  arranged laterally next to the cross-winding section B of the yarn guide  25 , as well as a drive wheel  33 . 
   The drive wheel  33  is arranged, fixed against relative rotation, on the motor shaft  24  of a reversible electric motor drive mechanism, preferably a hybrid step motor  34 , which is connected via a control line  36  to a control computer  35 , for example a central computer of the textile machine. 
   In an alternative embodiment, it is of course also possible to employ a section computer or a separate work station computer as the control computer  35 . 
     FIG. 3  shows the yarn guide  25  in its zero position NS, i.e. in a defined position at a distance a in respect to a run-up edge  15  of the housing  23  of the yarn guide unit  22 . This zero position NS respectively represents the initial position of the yarn guide  25 , and therefore of the step motor  34 ′ prior to the start of a cross-winding process, or during a fresh start of the yarn cross-winding device  16 . 
     FIG. 4  shows a front view of a yarn guide drive mechanism designed as a hybrid step motor  34 . As can be seen, the stator  18  has eight stator segments  19 A and  19 B with salient (toothed) poles  40 . Each of the stator segments  19 A and  19 B is here surrounded by windings  20 A or  20 B, which can be supplied with electrical current. In this case, the windings  20 A of the stator segments  19 A are connected to a d.c. current source  38  via a first electrical current phase  21 A, while the windings  20 B of the stator segments  19 B are connected to the d.c. current source  38  via a second electrical current phase  21 B. 
   The circuit arrangement  39  is designed such that bipolar operations are possible, i.e. that the magnetization of the teeth  40  arranged on the stator segments  19 A and  19 B can be selectively switched to the N-pole or the S-pole. 
   As represented in  FIG. 5 , the rotor  41  substantially consists of a motor shaft  42 , an axially magnetized, preferably ring-shaped bipolar permanent magnet  43 , as well as two, also toothed pole caps  44 A and  44 B. For example, the teeth  45 A of the pole cap  44 A are N-poles, while the teeth  45 B of the pole cap  44 B form S-poles. Moreover, the teeth  45 A of the pole cap  44 B are offset by half a tooth division in respect to the teeth  45 B of the pole cap  44 B. 
   The method in accordance with the present invention is carried out as follows. In connection with the embodiment selected for the exemplary embodiment of a hybrid step motor  34  with respectively twenty-five teeth per pole cap, as well as two-phase operation, the following relationship applies in regard to the number z of steps of the motor in accordance with the equation: z=k×p, 
   wherein k=2×m s , 
   and m s  represents the number of phases of the stator, 
   as well as p=the number of poles of the rotor, 
   z=2×2×50=200 single steps per rotor revolution. 
   Thus, with each step, the rotor of the motor moves by the increment 360°/200 steps=1.8°. 
   Since the rotor has a total of fifty poles, which are respectively spaced apart by 7.2°, four individual steps are required between the individual poles. 
   Moreover, in connection with the teeth at the stator element, the fifty poles of the rotor respectively form a so-called stop moment, i.e. a defined position at which the rotor stops as soon as the motor current is switched off. 
   Furthermore, in the exemplary embodiment, a displacement path s per motor step=path steps=2 mm×48/200=0.48 mm results for the yarn guide arranged on the motor shaft of the step motor from the divisional spacing of the toothed belt connecting the yarn guide with its associated drive mechanism of 2 mm, as well as from the number of teeth, forty-eight, of the drive wheel. Thus, two stop positions of the motor are respectively 4×0.48 mm=1.92 mm apart. 
   At the start of carrying out the present method, the yarn guide  25  is at any arbitrary distance from its desired zero position NS which, in the exemplary embodiment represented in  FIG. 3 , is for example located 0.2 mm in front of the run-up edge  15  of the housing  23  of the yarn guide unit  22 . Initially, the yarn guide  25  is shifted by the step motor  34  slowly in the direction of this zero position NS until it is assured that it has reached or passed the zero position NS. Thus, at any arbitrary time the guide shoe  26  of the yarn guide  25  reaches the run-up edge  15  of the housing  23  and continues to run against this run-up edge  15  until the step motor  34  is switched off Upon switch-off, the rotor  41  of the step motor  34  drops into a so-called stop position. 
   The location of this stop position results from the type of the step motor  34 , as well as from the electrical switching step in which the step motor  34  had been at the time of the switch-off, as explained above. Thus, the rotor  41  either is in the last stop position, in which the guide shoe  26  of the yarn guide  25  is placed directly against the run-up edge  15  of the housing  23 , or in the next-to-last position, in which the guide shoe  26  of the yarn guide  25  is positioned spaced apart from the run-up edge  15 , wherein this distance would be 1.92 mm in the exemplary embodiment, as explained above. 
   The step motor  34  is subsequently again charged with electrical current. Thus, the windings  20 A and  20 B of the stator elements  19 A and  19 B of the step motor  34  are controlled in such a way that the rotor  41  turns further, or attempts to turn further, by four steps of 1.8° in the direction of the last stop position, and therefore the zero position NS of the yarn guide  25 . 
   Thus, if, at the time of the switch-on of the step motor  34 , the yarn guide  25  is already in its zero position NS at the run-up edge  15  of the housing  23 , the rotor  41  cannot obey the electrical control and, at the time of the subsequent switch-off of the electrical current, drops back again into the last stop position. 
   However, if, at the time of switching-on of the electrical current, the rotor  41  is positioned in the next-to-last stop position, the rotor  41  of the step motor  34  turns forward by four switching steps so that, at the time of the subsequent switch-off of the electrical current, it is also in the last stop position, and therefore the yarn guide  25  is in its zero position NS. 
   At each start of a cross-winding process, this zero position NS respectively represents the initial value for the control device. 
   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.