Patent Publication Number: US-7896282-B2

Title: Double control loop method and device for ensuring constant tension yarn feed to a textile machine

Description:
The present invention relates to a method and device for feeding a yarn to a textile machine under constant tension. 
     A yarn used by a textile machine to produce an article must preferably be fed to the machine under constant tension or constant velocity to allow defect-free production. For this purpose, devices are already known for achieving at least constant tension feed: these devices operate using a tension sensor cooperating with the yarn and connected to an electronic circuit usually comprising a microprocessor control unit, in order to provide yarn tension data in real time. The control unit receives the tension data measured by the sensor and compares them with a programmable predetermined tension value (or set point value); on the basis of this comparison, which takes place in accordance with predetermined algorithms (for example a usual PID or proportional-integral-derivative algorithm), the control unit acts on a controlled member cooperating with the yarn, to modify the yarn tension if necessary. For example, the controlled member may be a yarn brake or a rotor driven by its own electric motor controlled by said control unit. 
     As stated, these devices are commonly used to guarantee the quality of a textile production process by virtue of their capacity to ensure yarn feed to a textile machine under constant tension in real time. 
     However devices of the aforesaid type are of limited use when the distance between the spool from which the yarn unwinds and the working point or region on the textile machine is such (for example greater than one meter and up to ten meters) that the yarn has to cooperate with a plurality of deviation or thread guide members before reaching the machine. This cooperation means that the yarn undergoes direction and angle changes and is subject to friction which negatively affects its tension (regulated and maintained constant) after its unwinding from the spool and in proximity to this latter. Consequently the yarn entering the textile machine does not present the required tension, but generally presents a tension higher than or otherwise different from that required, with production quality consequences. 
     The aforegoing certainly applies to the case of textile machines for diaper production in which the spool of each yarn used may be at several meters from the machine (or rather from its productive part) and said yarn cooperates with several guide members before reaching the machine. 
     In these diaper production machines there is also the problem that the tension of the yarns (usually elastomeric fibres) fed to the machine production region cannot be guaranteed constant and homogeneous because of the manner in which the yarn is fed to this region. In this respect, these machines use yarn feed systems of “rolling takeoff” type, i.e. the type in which the yarn is unwound from its spool by rotating this latter about its axis, such systems ensuring constant velocity feed but not constant tension feed because of their nature. 
     In addition, these systems cannot accommodate yarn feed by the “head-tail” method because of the spool rotation about its axis. This affects the efficiency and production of the machine, which lacks operational continuity due to the need to halt it periodically to replace empty spools. 
     U.S. Pat. No. 6,676,054 describes a method for unwinding a yarn from a spool by the “over end takeoff” method using the head-tail method. This is achieved by special positioning of the spool from which each yarn unwinds, relative to a corresponding thread guide member. Said patent however does not describe any problem relative to the tension change which said yarn undergoes because of the distance between the spool and the region or point of its insertion into the textile machine process to which this yarn is fed, nor suggests any yarn control solution to maintain the tension of the yarn entering the machine production region constant, in particular a diaper production machine. 
     As an alternative to the “rolling takeoff” method for unwinding the yarn from the relative spool, the “over end takeoff” method is also known, by which the yarn is withdrawn from its spool without this latter moving about its axis. This known manner allows both head-tail feed and enables a constant tension to be maintained for the yarn unwound from the spool and fed to the textile machine. 
     However, this known method for feeding yarn to a textile machine, as in the case of that previously stated, enables the yarn tension to be monitored and controlled only after being unwound from the relative spool and in proximity thereto, because for obvious space reasons or for technical/physical reasons regarding the operation of the components used for their implementation, said devices are compelled to operate in strict contact with the spool (in the “rolling takeoff” case) or in proximity to it (in the “over end takeoff” case). 
     Hence neither of the methods ensures constant homogeneous tension of the yarn entering the textile machine in that region thereof in which each yarn is taken up by machine members to enable its use in producing an article. 
     An object of the present invention is therefore to provide a method and relative device by which the aforesaid limits of known methods for feeding yarn to a textile machine are overcome. 
     Another object is to provide a method and device of the aforesaid type by which a yarn can be reliably fed under constant tension to a textile machine even when the distance between this latter and the spool from which the yarn is unwound is large, for example equal to or greater than several meters. even tens of meters. 
     Another object is to provide a method and device of the aforesaid type by which any friction problems arising between the spool and the textile machine are eliminated. 
     A further object is to provide a method and corresponding device which enable the head-tail yarn feed method to be used and still maintain homogeneous constant tension of the yarn entering the textile machine. 
     A further object is to provide a method and corresponding device by which any yarn running problems over guide and/or deviation members such as thread guides, pulleys and the like can be identified and controlled. 
     Another object is to provide a method and device of the aforesaid type allowing rapid and repetitive setting of the yarn working tension, i.e. the tension with which each yarn is used by the textile machine, in particular at that point in which the yarn enters the machine, and enabling articles to be produced under working parameters which can be recalled in the case of repetition of the same production process even after considerable time. 
     These and further objects which will be apparent to the expert of the art are attained by a method and device in accordance with the accompanying claims. 
    
    
     
       The present invention will be more apparent from the accompanying drawings, which are provided by way of non-limiting example and in which: 
         FIG. 1  is a schematic view of apparatus in accordance with the invention; and 
         FIG. 2  is a flow diagram showing a method in accordance with the invention. 
     
    
    
     With reference to  FIG. 1  a device of the invention is used to control the feed of a yarn F to a textile machine T, for example a diaper production machine, i.e. a textile machine in which each spool from which a corresponding yarn worked by said machine unwinds is positioned at a distance which can vary from one meter to several tens of meters. 
     After leaving the spool  2 , the yarn F passes through a usual thread guide  3  preferably positioned in front of the two spools  1  and  2  such that both the axes of said spool coincide with the centre of the thread guide to allow regular switch-over and unwinding of the two spools when the first is empty. After cooperating with the thread guide  3 , the yarn F cooperates with the device  10  for measuring and regulating its tension. This device  10 , of known type, comprises in the example shown in the figure a tension regulator member  4  comprising a rotary element  4 A on which the yarn F sides driven by its own electric motor  4 B, for example of brushless type, and a usual tension sensor  5 . These components  4  and  5  of the device  10  are connected to a control circuit or unit preferably of microprocessor type  6  which, on the basis of tension data measured by the sensor  5  and in accordance with the predetermined control algorithm, compares each measured item of data with predetermined homogeneous data (defining a set point  1 ) and in case of difference between said data acts on the member  4  (in particular on said electric motor  4 B) such that this latter brakes or slows down its action on the yarn F to make the tension measured by the sensor  5  equal to the memorized tension values. 
     The member  4 , the sensor  5  and the unit  6  define a first yarn tension control loop  11  with very rapid intervention times, of the order of tens of nanoseconds or microseconds. This is because the first control loop has to intervene immediately as soon as the sensor  5  senses an undesired variation in the tension of the yarns F, this variation occurring naturally during unwinding of the yarn from the spool  2  because of tugging of the yarn on this latter or deriving from the progressive emptying of the spool. 
     In textile machines of the aforesaid type operating on a plurality of yarns, each being preferably guided from the corresponding spool to the machine by the use of a plurality of thread guide members  8  which by interacting with the yarn modify its trajectory of movement and spatial angulation. This interaction creates friction which can modify in a more or less considerable manner the tension of the yarn between the spool and the machine, and which can negatively influence the article produced by the machine. 
     To overcome this problem, the device of the invention is proposed, in which a second yarn tension control loop is present comprising a second tension sensor  7  positioned spaced from the first sensor  5  and in particular positioned in proximity to that region in which the yarn F is withdrawn by usual textile machine withdrawal members (not shown) for its processing. 
     This second sensor  7  is also connected to the control circuit or unit  6  (acting on the rotary member  4  or rather on its motor  4 B) and defines therewith the second yarn tension control loop. However, according to this loop, the unit  6  operates on the basis of predetermined tension values defining the set point  2 . On starting the textile machine, the set point  1  must be made to coincide with the set point  2 ; however the set point of the first control loop can change during machine use as indicated hereinafter, whereas the set point  2  remains constant. 
     In this respect, during textile machine operation, the first control loop  11  controls and regulates the tension of the yarn unwinding from the spool  2 , in known manner. During yarn feed to the textile machine T, the second sensor feeds tension data obtained by it to the unit  6  which, on the basis of these and of a predetermined operative algorithm, compares them with the set point  2 . In the case of discrepancy between the measured data (showing a tension greater than the predetermined tension) and the programmed data (set point  2 ), the unit  6  acts on the first control loop  11  to modify the set point  1  by decreasing it to essentially modify the intervention of the member  4  on the yarn F. This intervention is made to give a suitable tension to the yarn such that its tension is correct when it reaches the second sensor  7 . 
     It should be noted that the intervention times of the second control loop are different from those of the first loop and very much longer, of the order of milliseconds. This is to prevent the system defined by the two control loops from becoming unstable because of a quick intervention time for the second loop. In other words, the intervention time of the second control loop is sufficiently long to enable the tension adjustment made by the first loop on the basis of the data obtained from the second tension sensor  7  to be effectively sensed by this latter. 
     The intervention speed of each control loop is evidently the speed of execution of the corresponding control algorithm by the unit  6  and hence the speed of intervention of the tension regulator member  4 . 
     The device of the invention is of self-adapting type in that, having fixed the set point  1  for the initial tension regulation of the yarn F, and the value of the set point  2 , which instead remains fixed for the entire time for which the textile machine is in use, the device enables the tension of the yarn F entering the textile machine to be corrected totally automatically, independently of the spool from which the yarn originates (even after change-over between spool  2  and spool  1 ). 
     Referring to  FIG. 2  which is a flowchart showing a method in accordance with the invention, at step  20 , at the start of machine operation, a first actual tension in yarn F is measured (by tension sensor  5 ) at a first location proximate to the region at which the yarn is unwound from the spool  2  and the tension is regulated (by tension regulator member  4 ) to substantially equal a predetermined variable first set point. As the machine operates, at step  22 , a second actual tension in yarn F is measured (by tension sensor  7 ) at a second location proximate to the region at which the yarn enters the textile machine for processing. At step  24 , the second actual yarn tension is compared (by control unit  6 ) to a predetermined fixed second set point. At step  26 , if the fixed second set point is greater than the second actual yarn tension, the difference is calculated at step  28 A and a new first variable set point is calculated at step  30 A. Similarly, at step  26 , if the fixed second set point is less than the second actual yarn tension, the difference is calculated at step  28 B and a new first variable set point is calculated at step  30 B. At step  32 , the variable first set point is modified based on the difference between the second actual yarn tension and the fixed second set point and, at step  34 , the tension at the first location proximate to the region at which the yarn is unwound from the spool is modified based on the modified variable first set point. The method repeats after the tension at the first location is modified by returning to step  22 . 
     The device of the invention, which operates in accordance with the aforedescribed modalities and hence the aforesaid manner, also enables various degrees of alarm and pre-alarm to be obtained: if the second sensor  7  senses too high a yarn tension exceeding the value of set point  2 , the unit  6  activates a pre-alarm warning of known type (for example a visual alarm) after a predetermined number of failed control attempts. If the tension measured by the sensor  7  nearly reaches a prechosen very high value (beyond which experience or strength tests suggest that yarn breakage or article quality problems are possible), the unit  6  activates an alarm signal and halts the textile machine. This prevents production of defective articles, whereas in the first case (pre-alarm) it allows intervention to solve the problem causing said drop in yarn tension: generally this can also be due to dust or dirt accumulating on the members  8 , this dirt being easily removable by a machine operator. 
     By virtue of the said tension control on the yarn entering the textile machine, the invention enables faults in the thread guides or crossings of different yarns to be determined, and which can cause undesirable modifications (increases) in the tension of the yarn, which would essentially undergo breakage in its movement towards the textile machine. 
     It is therefore possible by virtue of the invention to maintain the tension of the yarn F entering the machine T at a constant value equal to a desired predetermined value independently of the distance between the first control loop for the tension of said yarn (close to the spool from which the yarn unwinds) form the machine. This ensures constant quality of the article produced by said machine. 
     The invention also enables the head-tail method for feeding the yarn to the textile machine to be effectively used, so preventing undesirable costly stoppages (in particular in the case of a diaper production machine) and increasing machine productivity. 
     In addition, by storing in the unit  6  the tension data measured on the yarn used in a particular production, this production can be easily and rapidly repeated. The unit  6  controlling the member  4  can measure the velocity of the yarn fed to the textile machine, the current absorbed by the motor of the member  4  and the torque used, and enables minimum and maximum thresholds for these parameters to be fixed, outside which a risk of obtaining products of quality not corresponding to that required can be identified or predicted. 
     One embodiment of the invention has been described, but others can be provided in the light of the present description, such as one in which thread guide members are not provided between the sensor  5  and the sensor  7  (even though the distance between these is considerable and exceeds one meter).