FIELD OF THE INVENTION 
   The invention relates to a method for controlling a weft yarn-feeding device and a yarn processing system. The yarn processing system includes a weft yarn feeding device, a power loom which consumes weft yarn and a control device. In operation, a drive motor of the yarn feeding device is switched on and off and is accelerated or decelerated, respectively, in response to control signals, in order to maintain in the yarn feeding device a yarn store size related to the yarn consumption. 
   BACKGROUND OF THE INVENTION 
   Weft yarn feeding devices used in modern power looms (jet looms, gripper looms, projectile looms, or other types) frequently are autonomic units controlling the speed of the drive motor of the winding element essentially independent from the weaving operation in the power loom and exclusively in dependence from the permanently detected size of the yarn store in the feeding device. The yarn store is permanently detected in order to generate control signals for the control device of the feeding device which control device switches on or switches off the drive motor or accelerates or decelerates the drive motor, in order to maintain a size of the yarn store sufficient to cover the consumption. In case that a yarn consumption results in a decrease of the size of the yarn store in relation to a predetermined reference size, then the drive motor either is switched on and accelerated or is only accelerated until the reference size at least partially is reached again. In case that the size of the yarn store increases in relation to the reference size, then the drive motor is decelerated or is switched off. The yarn store in the feeding device is monitored by sensors. The drive motor operates with a predetermined acceleration behaviour. Depending on the case of application of the feeding device a predetermined maximum speed may be set for the drive motor. 
   According EP 0 114 339 B, a common control device is provided for several weft yarn measuring feeding devices in a jet power loom. The common control device, depending on the weaving pattern, selects and controls only one feeding device. As all measuring feeding devices include yarn stop devices, a control routine is implemented using a preparation switch by which the yarn store is brought in each measuring feeding device to a maximum size prior to the start-up of the power loom. For this function, the drive motor is driven for a sufficiently long time period and then is stopped again. The normal control routine depending from the size of the yarn store is set out of function for the preparation phase. Furthermore, a start-up switch is provided in the power loom and upon actuation starts the weaving operation. The actuation of the start-up switch signals the control device of the measuring feeding devices so that each one will again operate with a control routine depending on the yarn store size detection. The stopping devices are brought into their respective release position in timed fashion and one by one by respective trig or trigger signals transmitted from the power loom. As soon as an under-sized yarn consumption is detected, the yarn store size monitoring device of the respective measuring feeding device responds and generates control signals to start the drive motor in order to replenish the yarn store. There is an unavoidable time delay between the start-up of the weaving operation in the power loom and the acceleration of the drive motor as controlled by the control device. Since the power loom rapidly reaches its full load operation and causes high start-up yarn consumption, the yarn store in the actuated measuring feeding device may be emptied, resulting in an operation disturbance. 
   A rapidly operating power loom equipped with one feeding device only, e.g. a water jet power loom, for processing a single weft yarn quality only causes upon start-up of the weaving operation extremely rapid high start-up yarn consumption possibly causing a quickly emptied yarn store due to the time delay between the start of the weaving operation or the occurrence of the run-signal, respectively, and the response of the drive motor of the feeding device depending on the initial yarn store size. This is not only true for power looms equipped with several measuring feeding devices or with one measuring feeding device only, but also for power looms being equipped with another type of a feeding device and/or with several feeding devices, in the case that the power loom produces a rapidly starting and strong start-up yarn consumption. This drawback can be avoided by extremely powerful and strongly accelerating drive motors of the feeding devices, i.e. by expensive special feeding devices. Such special feeding devices, however, generate undesirably high load for the respective yarn. 
   It is known in practice for measuring feeding devices used in fast jet weaving machines to switch on and to accelerate the drive motor after or in synchronism with the occurrence of the first trig signal output for the stop device and as transmitted after startup of the weaving operation of the power loom. However, since then the drive motor only starts at the same moment as the trig signal is transmitted or even later, in some cases there will not be sufficient yarn in the yarn store in order to cover the high start-up yarn consumption. 
   It is an object of the invention to provide a method of the kind as disclosed and a yarn processing system allowing to avoid an emptying of the yarn store in the feeding device despite a strong and rapidly increasing start-up yarn consumption by the weaving machine, and to achieve that function by commercial available feeding devices and in a structural simple way. 
   SUMMARY OF THE INVENTION 
   In accordance with the method the drive motor is driven at a predetermined speed already when the start-up of the weaving operation of the power loom takes place. For that reason the feeding device is apt to cover even a high start-up yarn consumption of the power loom without the danger that the yarn store will be emptied. Between the yarn windings wound on during the run-up phase of the drive motor substantially in synchronism with the start-up phase of the weaving operation and the initially starting high start-up consumption of the power loom a dynamic balance is achieved between yarn windings wound on and yarn windings wound off. By that floating balance condition an abrupt decrease of the yarn store by high start-up yarn consumption either is levelled out or is compensated for such that the yarn feeding device does not run into an emergency condition by desperately trying to not only cover the start-up yarn consumption but to reach a “safe” yarn store size. As soon as the yarn feeding device has mastered the high start-up yarn consumption the control routine depending on the yarn store size will take over and will nullify the control routine for the drive motor with the predetermined speed. In this way it is possible to reliably avoid the above-mentioned operation disturbances even with commercially available yarn feeding devices. In the yarn processing system it only has to be assured that the start-signal derived from the run-signal upon start-up of the weaving operation is transmitted to the control device and is considered by the control device such that the drive motor already will run at the predetermined speed when in an overlapping fashion the size of the yarn store starts to decrease rapidly. In order to achieve this function only slight modifications of reliable design principles of the yarn feeding device are needed, i.e., only preparations at the control side, which preparations do not influence the mechanical operation and reliability of the yarn feeding device. 
   At which point in time the usual yarn store size depending control routine for the drive motor will take over to then control the drive motor independent from any start-signals, is decided by the co-action between the feeding device and the power loom. For example control signals depending on the yarn store size will take over the control of the drive motor upon their first occurrence or even after a predetermined and selectable time period after the emittance of the run-signal. For method reasons it is possible to set any influence of the yarn store size depending out of function for a predetermined period of time control signals for the control routine of the drive motor upon start-up of the feeding device. This is independent whether the control signals are generated by sensors either sensing the size or counting the wound-on and the withdrawn windings and calculating the size of the yarn store. 
   According to the method when the start-signal is transmitted, expediently, the drive motor of the feeding device in the special control mode is driven at the maximum allowable speed or at a speed close to the maximum allowable speed, e.g. at 55%–75% of Vmax, or at a speed already stored prior to a drive motor stop. The maximum allowable speed Vmax conventionally is pre-set at the yarn feeding device, particularly in view to the design and operation behaviour of the yarn feeding device and the conditions in the power loom, e.g. the weaving width, the yarn quality, the weaving cycle frequency, and the like. Setting of the predetermined speed for the motor is expediently made such that a floating balance condition between the windings wound on into the yarn store by the drive motor and the abruptly starting start-up yarn consumption is achieved in the dynamic phase caused by the start-up yarn consumption in the power loom. By the balance condition an overfilling of the yarn store or a too strong decrease of the size of the yarn store reliably will be avoided. Basically and according to the invention it is considered in common how the start-up of the power loom up to full load operation will take place and how the drive motor of the feeding device can accelerate. 
   The start-signal by which the drive motor is brought to the predetermined speed does not need to be transmitted when the run-signal for the weaving operation is emitted but it may be generated or may be considered by the drive motor with a predetermined advance or delay. This means that the start-signal timewise may be generated earlier or later than the run-signal, however, in any case will be derived from the run-signal. Overfilling or emptying of the yarn store can be avoided reliably by a precise or adaptive timing of the start-signal. 
   A delay of the start-signal relative to the run-signal particularly is expedient for a measuring feeding device having a stopping device, because the stopping device is actuated by a trig signal correlated to a predetermined rotational angle value in the power loom, and since the respective trig signal occurs timewise later than the run-signal. Depending from the condition of the mechanical components, e.g. clutches, in the power loom the time distance between the run-signal and the first trig signal may be of different magnitude or may increase after longer operation time of the power loom. A response of the drive motor to the start-up signal at the same time as the run-signal occurs was unable to consider these circumstances reliably enough, because then the drive motor may accelerate too early and for too long before the trig signal will release the stop device and before the start-up yarn consumption will become effective for the yarn store in the feeding device. In this case, the yarn store would overfill. In order to reliably avoid this disadvantage the time distance between the start-signal or the response to the start-signal, respectively, and the trig signal should be adapted to the actual conditions in the power loom. This is considered by a delay time between the run-signal and the start-signal or the point in time, respectively, at which the start-signal activates the drive motor. The delay time may be adjusted manually, e.g. by operator and after monitoring the run-up property of the measuring feeding device. Expediently, the adaptation is carried out adaptively by a self-learning program of the control device (of the yarn feeding device or of the power loom), during which program the time distance between the run-signal and the first trig-signal is measured and a delay time between the run-signal and the start-signal or the response to the start-signal is adjusted in dependence from the result of the measurement. The delay time can be adjusted either while deriving the start-signal from the run-signal, or by delaying the transfer of the start-signal to the drive motor, respectively. In this way, e.g. stepwise increasing time distances can be used which are called up from a table in order to adjust the delay time such that emptying and overfilling of the yarn store will be omitted, i.e. that an optimum floating transition will be reached from the run-up phase into the phase of the normal operation of the yarn processing system. 
   A standard equipment of a power loom may, e.g. in the control panel, contain a first switch by which the drive system is switched on. In this case the components of the power loom which are responsive to carry out the weaving operation do not move yet. Furthermore, a second switch, in most cases a green push button, is provided, when pressed generates the run-signal for the components of the power loom which have to carry out the weaving operation such that those components will rapidly start to move, e.g. by actuating respective clutches and/or gear transmissions. The second switch e.g. actuates an electric contact switch which in turn generates the run-signal. A signal transmitting connection transmitting the external start-signal to the feeding device expediently is connected with the electric contact switch. By this it can be achieved that the run-signal initiating the start-up of the weaving operation also is transmitted as the start-signal to the yarn feeding device, such that with the help of the control device in the yarn feeding device the drive motor substantially will run up in synchronism with the startup of the weaving operation. 
   In case that, upon occurrence of the start-signal, the drive motor is driven with maximum allowable speed, the speed adjustment device for the maximum allowable speed conventionally provided in the yarn feeding device may be employed to set the speed for this control routine. If, to the contrary, a lower speed is selected than the maximum allowable speed, for this reason expediently a separate speed adjusting device may be provided. 
   Expediently, the control device interferes at the control current side of a transistorised switching device of the power supply of the drive motor. In this case low control current values or control voltages, respectively, will suffice to switch on the drive motor. In a standardised fashion the control device is equipped with at least one microprocessor which takes care of the required control functions. The microprocessor is capable enough to also carry out the additional control routine for driving the drive motor upon emission of the start-signal as soon as the start-signal is transmitted to the microprocessor. 
   In a structurally simple way the start-signal is transmitted via a separate cable to the control device. 
   Alternatively, a wireless signal transmission from the power loom to the control device of the yarn feeding device or to the yarn feeding device may be possible. 
   A selectable advance or delay of the start-signal in relation to the run-signal can be achieved in a structurally simple way by a parallel switch which is actuated together with the contact switch but responds earlier or later than the contact switch. An advance may be expedient in order to match the run-up property of the yarn feeding device to the run-up property of the components in the power loom carrying out the weaving operation in order to substantially avoid in the dynamic run-up phase a drastic decrease of the yarn store size by the assisting interference of the drive motor. A delay may be expedient to avoid an overfilling. The advance or the delay expediently can be adjusted, e.g. in steps or steplessly. 
   In case that a computerised control system with a serial date communication is provided between the power loom and the feeding device, the run-signal may be given as the start-signal to the drive motor via the already present data transmission path. 
   The feeding device implemented at the power loom may be a measuring feeding device having a stopping device, or may be a feeding device operating with a yarn brake. The respectively implemented yarn feeding device type depends on the structure and the function of the power loom. Measuring feeding devices e.g. are implemented in case of jet power looms (air jet power looms or water jet power looms). To the contrary, feeding devices having an integrated yarn brake are implemented in gripper power looms, projectile power looms or other power loom types which do not need to measure the respectively inserted weft yarn length already by the feeding device, because the insertion arrangement of the power loom automatically will measure the correct length of the inserted weft yarn. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first embodiment of a yarn processing system, 
       FIG. 2  shows another embodiment of a yarn processing system, 
       FIG. 3  shows a speed/time diagram and a yarn store size/time diagram, 
       FIG. 4  shows a yarn store size/time diagram, 
       FIG. 5  shows another yarn store size/time diagram, and 
       FIG. 6  shows a switch arrangement. 
   


   DETAILED DESCRIPTION 
   A yarn processing system S in  FIG. 1  comprises a power loom L, e.g. a water jet power or an air jet power loom, into which a weft yarn Y is inserted originating from a storage bobbin  1 . The weft yarn is inserted into a weaving shed  2  and is woven into the fabric by components  3  carrying out a weaving operation (e.g. a shed forming mechanism, a weaving reed, a warp yarn mechanism, and the like). 
   The power loom L includes a drive system  4  driving a main shaft  6 , and a drive sub-unit  5  for driving the components carrying out the weaving operation upon generation of a run-signal. The power loom L, furthermore, comprises an insertion arrangement E, e.g. a main nozzle  7  (and not shown, relay nozzles along the weft path through the weaving shed  2 ) which insertion arrangement pulls off the weft yarn Y from a weft yarn feeding device F. The control device C of the power loom L is associated to a control panel of the power loom L and includes a first switch  8  by which the drive system  4  can be switched on, and a second switch  9 , by which the run-signal can be generated. An electric contact switch  10  is unified with the second switch  9  which contact switch  10  upon actuation of the switch  9  generates the run-signal which e.g. by means of the sub-unit  5  will initiate the weaving operation. 
   At least one yarn feeding device F is functionally associated to the power loom L. The feeding device F shown in  FIG. 1  is a measuring feeding device which is designed to measure the respectively inserted length of the weft yarn. An electric drive motor M for a winding element  12  is provided in a housing  11  of the feeding device. The winding element  12  winds yarn withdrawn from the storage bobbin  1  into windings on a storage body  12 . Those windings form a yarn store  13  from which yarn store the insertion arrangement E intermittently will pull off the weft yarn. The yarn feeding device F comprises an on-board or an associated control device C 1  for the drive motor M. A speed adjusting device  14  may be provided at the control device C 1 . A power line  15  supplies electric power. A monitoring device  16  for the size of the yarn store  13  is provided in the yarn feeding device F. The monitoring device  16  includes at least one or expediently several sensors which transmit control signals to the control device C 1  depending from the detected size of the yarn store  13 . Furthermore, a stopping device  17  having an engageable and disengageable control element  18  is provided in the feeding device F for measuring the respective weft yarn length. The monitoring device  16  may comprise sensors, which either measure the number of wound on and pulled off windings and/or which detect an operation disturbance, e.g. a yarn breakage, respectively. 
   A signal transmitting connection  19  is provided between the electric contact switch  10  and the control device C 1  of the yarn feeding device F for transmitting a start signal X to the control device C 1 . The start signal X is derived from the run-signal of the power loom L. Furthermore, a signal transmitting connection  20  may be provided from the power loom L to the control device C 1  or the stopping device  17  to transmit so-called trig signals T to the control device C 1 . The trig-signals T are generated in dependence from the rotation of the main shaft  6  of the power loom L at a predetermined rotational angle position (e.g. by means of an encoder) to initiate the adjustment of the stop element  18  from the shown stop position into a retracted release position. The stop or control element  18  is adjusted by the control device C 1  from the release position into the stop position shortly before a number of windings withdrawn from the yarn store  13  is reached which corresponds to the desired weft yarn length. A computerised control system having a serial data communication may be provided which also can be used for the transmission of the start-signal X. 
   In the yarn processing system S in  FIG. 2  the power loom L e.g. is a gripper power loom comprising a bringer gripper  21  and a taker gripper  22 , both constituting the insertion arrangement of the power loom. Since the grippers  21 ,  22  automatically measure the withdrawn weft yarn length, the feeding device F does not need to have a stopping device for the weft yarn Y. A yarn brake  25 , instead, co-operates with the storage body  12  for the yarn store  13 . The withdrawn yarn runs through a withdrawal eyelet  26  downstream of the yarn brake  25  and towards the power loom L. In this case the connection  19 ′ transmitting the start-signal X from the electric contact switch  10  of the switch  9  is shown as a wireless connection. The start-signal X is transmitted by means of an emitter  23  in a wireless fashion, e.g. in the form of a radio signal, to a receiver  24  of the control device C 1  of the feeding device F. Moreover, the structure of the system in  FIG. 2  broadly corresponds to the structure of the system shown in  FIG. 1 . 
   In the yarn processing system S in  FIG. 1  or  FIG. 2 , respectively, the yarn feeding device F is switched on prior to a start of operation. Also the switch  8  in the power loom is actuated. A control routine may be stored in the control device C 1  of the feeding device by which the drive motor M first will adjust a predetermined basic size of the yarn store  13 . Then the drive motor is stopped. Upon actuation of the switch  8  the drive system of the power loom is activated. Components of the power loom responsive for weaving operations do not yet move. Subsequently the switch  9  is actuated which then generates the run-signal. The components of the power loom rapidly run to full action. A high start-up yarn consumption occurs rapidly. First when the main shaft  6  has rotated over a predetermined rotational angle a trig signal T for the stopping device  17  is emitted for the first time. The stop element  18  is retracted into the release position. The yarn consumption starts now. With the actuation of the switch  9 , however, the start-signal X was transmitted to the control device C 1 . In response to the start-signal X the control device C 1  switches on the drive motor substantially in synchronism with the beginning weaving operation and accelerates the drive motor to the predetermined speed, e.g. as adjusted at  14 . New yarn Y already is wound on prior to the first actuation of the stopping device  17 . Either with the later occurrence of first control signals from the monitoring device  16  and/or after expiration of a pre-set time duration, the drive motor control routine depending on the yarn store size will then overtake the regulation of the yarn store size for the subsequent course of the weaving operation. 
   Similarly in the yarn processing system S in  FIG. 2  the drive motor M is switched on by the start-signal X and is initially brought to the predetermined speed. 
   This will be explained with the help of  FIG. 3 . In the upper part of the diagram in  FIG. 3  the vertical axis represents the speed V or the number of revolutions, respectively, of the drive motor M and the drive systems  4 ,  5  of the power loom L. Both horizontal axes show the time or the rotational angle of the main shaft  6 , respectively. In the lower part of the diagram the yarn store size (number W of windings) is shown on the vertical axis. At point in time t 1  the switch  8  is actuated. The curve  27  represents the now running drive system  4  in the power loom. At point in time t 2  the switch  9  is actuated, and the run-signal and the start-signal X are generated. The curve  28  represents the running of the components of the power loom carrying out the weaving operation. The curve  29  represents the acceleration phase of the drive motor M of the feeding device. The first trig-signal is emitted at point in time t 3 . First at point in time t 4  the start-up yarn consumption occurring until then would have decreased the yarn store  13  so far that the monitoring device  16  would normally respond and cause a control signal to start the drive motor. If the drive motor would be switched on first at point in time t 4  and depending on the yarn store size and would then be accelerated to maximum speed following the dotted curve  31 , the yarn store could not be sufficiently replenished to cover the high start-up yarn consumption of the power loom. According to the invention, the drive motor already is started at point in time t 2  by the start-signal X and is accelerated to a predetermined speed Vd which may be lower than the maximum limited speed Vmax. First at point in time t 5  the yarn store size depending control routine takes over to then regulate the speed of the drive motor M in conformity with the curve  30 . 
   In  FIG. 3  alternatives are indicated, namely that the start-signal X for the drive motor M is generated at point in time t 2 ′ in advance of or at point in time t 2 ″ delayed with respect to the run-signal in order to drive the drive motor in accordance with the dotted curve  29 ′ or  29 ″, respectively. 
   The lower half of the diagram of  FIG. 3  shows that the size of the yarn store  13  allowably varies between a maximum value Wmax and a minimum value Wmin first, and according to the curve  32  e.g. remains close to the maximum value. Shortly after the point in time t 2 , i.e. after switching on the drive motor M due to the transmitted start signal X, the size of the yarn store  13  increases to then again decrease due to the high run-up yarn consumption, before the yarn store size will oscillate and finally will remain close to the maximum size. If the drive motor M would not have been switched on at time t 2  (or with an advance or a delay at t 2 ′ or t 2 ″, respectively), then curve  32  would continue along the dotted curve  33  and the yarn store would run empty due to the high start-up yarn consumption. 
   Since the drive motor M of the feeding device F is switched on with the start-signal upon start up of the weaving operation and is accelerated to the predetermined speed (to maximum allowable speed or to a speed close to the maximum allowable speed) winding on of new yarn material will start early such that in the dynamic run-up phase a floating balance condition will result between the high start-up consumption of the power loom and the already present windings plus newly wound on windings in the yarn store  13 . By this balance condition it is avoided that the yarn store size will decrease drastically and/or that the yarn store even will be emptied. Herewith it is to be considered that the starting behaviour of the components carrying out the weaving operation in the power loom and the acceleration behaviour of the drive motor M do not allow an abrupt start of the full weaving capacity or abrupt acceleration to maximum speed, but that between both run-up procedures an intended dynamic co-operation occurs which reliably avoids drastic or critical decreases of the yarn store size. 
     FIGS. 4 and 5  exemplarily indicate yarn store size depending control routines as conventional in yarn feeding devices, for clarity reasons, however, without the measures of  FIG. 3 . 
   In  FIG. 4  the yarn store size (number W of windings) is shown on the vertical axis, while the horizontal axis is the time axis. Maximum and minimum yarn store sizes are predetermined which should not be exceeded (for too long a time) . A predetermined reference size  34  of the yarn store is determined and provided for a microprocessor of the control device Cl. Counting or registering sensors (not shown) count the number of windings contained in the yarn store  13  so that the control device C 1  can control the drive motor such that the yarn store size e.g. follows a curve  35  which may oscillate about the reference size or may be raised or lowered somewhat upon demand, respectively. The dotted curve  37  indicates that the yarn store is emptied and will become emptied at a point in time t 6 , meaning that then the yarn feeding device would have to be stopped. The dotted line  36  indicates that the yarn store is overfilled and would become overfilled at a point in time t 7 , meaning that the yarn feeding device would have to be stopped. It is even possible to control the size of the yarn store without the reference sensor only by counting and calculating the wound on and the wound off windings and to control the drive motor accordingly. The yarn store size depending control routine as explained is replaced or overruled during run-up of the weaving operation by the earlier acceleration of the drive motor M, as shown in  FIG. 3 , in order to reliably cover the high run-up consumption of the power loom and to avoid operation disturbances (curves  36  or  37 , respectively). 
   In  FIG. 5  the yarn feeding device F e.g. is operating with a maximum size sensor  38  and a minimum size sensor  39  which generate control signals for the control device C 1  in order to e.g. guide the development of the yarn store size along the curve  40 . In this case the control device C 1  includes an intelligent logic registering the excess of the maximum and minimum store sizes, which optionally considers the time durations of such excesses and which controls the drive motor such that the yarn store size remains below the maximum size and follows the curve  40 ′. The dotted curve  41  represents a not allowed overfilling which results in a stop of the yarn feeding device at point in time t 9 . The dotted curve  32  indicates an emptying of the yarn store resulting in a stop of the feeding device in point in time t 8 . The sensor  38 ,  39  could be combined with the reference sensor  34  of  FIG. 4 . Also the control routine of  FIG. 5  will be replaced or overruled during run-up of the weaving operation, as shown in  FIG. 3 , by an advanced start of the drive motor M with the start-signal X in order to cope with the high run-up yarn consumption of the power loom. 
     FIG. 6  shows the electric contact switch  10  which is actuated by the switch  9 , e.g. a push button, in order to generate the run-signal and to start the components of the power loom carrying out the weaving operation, analogously to  FIGS. 1 and 2 . The contact switch  10  e.g. operates with a closure stroke h 1  until the run-signal is emitted. Furthermore, a parallel switch  10 ′ is provided which is actuated by the switch  9 , as soon as the contact switch  10 , e.g. by means of a relay, is closed. The parallel switch  10 ′ operates by a closure stroke h 2  which is smaller than the closure stroke h 1  of the contact switch  10 , or the contact switch  10 ′ reaches its closing position earlier. Upon actuation of the switch  9  the parallel switch  10 ′ is closed in advance to the contact switch  10  such that the start-signal X is produced with a timewise advance in relation to the run-signal for the power loom E, e.g. at point in time t 2 ′ in  FIG. 3 . By matching of both closure strokes h 1  and h 2  the magnitude of the advance may be set or varied or respectively. 
   At least one closure stroke h 1  and/or h 2  may be adjusted (arrows  43 ,  44 ) e.g. by means of a manual actuator  45 . In this way the timing or the advance or the delay of the start-signal X can be adjusted or varied respectively. 
   Alternatively the advance or the delay of the start signal X could be adjusted at the feeding device F. For this purpose  FIG. 1  shows an arrangement  46  at the control device C 1 , by which arrangement  46 , e.g. with the emission of the start-signal X at the same time as the run-signal, the start-signal X for the drive motor M is generated with advance or with delay or is output further with an advance or a delay, respectively. At a yarn feeding device not having a stopping device or at a yarn feeding device having a stopping device such adjustments can be carried out by an operator after observation of the control behaviour of the feeding device in the run-up phase upon demand. For this case, e.g. several time steps may be predetermined. 
   As a further alternative, the suitable timing by which the start signal X will switch on the drive motor M in the run-up phase could be adjusted by a self-learning program routine even automatically. The control device C 1  measures (in  FIG. 1  in the measuring feeding device F) the time duration between the occurrences of the run-signal and of the first trig-signal T which time duration depends from the condition of certain mechanical components in the power loom L. On the basis of the measured time duration, e.g. a delay time is automatically set with the help of several stepwise increasing time gaps, namely a delay time between the point in time of the run signal and the point time at which the start-signal X has to activate the drive motor M (or at which the start-signal X is considered by the control device C 1 ). The same delay time automatically will be actualised for each new run-up phase. A practical value for the delay time e.g. may lie between 50 ms and 100 ms. It always is intended to switch on the drive motor M by the start-signal X just early enough prior to the first trig-signal T in order to avoid emptying of the yarn store due to the run-up yarn consumption, but also to exclude that the time duration between the consideration of the start-signal X and the first trig-signal T becomes so large that an overfilling of the yarn store cannot be excluded. Basically the point in time at which the drive motor M is activated by the start-signal X, is adjusted such that both critical conditions “emptying or overfilling” of the yarn store are avoided and that the mentioned floating transition from the run-up phase into the normal operation can be achieved in an optimum fashion.
A yarn processing system includes a weft yarn feeding device with a control unit and a power loom which consumes weft yarn. In operation, a run-signal is generated by the power loom that initializes the start-up of the weaving operation. A start-signal that is derived from the run-signal is transmitted to the feeding device. The start-signal is generated externally of the feeding device. The drive motor of the feeding device is driven at a predetermined speed, after receiving the external start-signal in order to prevent an undesired reduction of the size of a yarn store by the initial consumption demand of the start-up of the weaving operation of the power loom. A signal transmitting connection is provided in the yarn processing system between the power loom and a control unit of the feeding device for transmitting the start-signal. On start-up of the power loom, the drive motor of the feeding device is operated at a predetermined speed by the control device independent from the size of the yarn store in the feeding device.