Patent Publication Number: US-7584014-B2

Title: Control unit for yarn-braking devices in weft feeders for looms, and tuning method therefor

Description:
The present invention relates to a control unit for yarn-braking devices in weft feeders for looms, in particular rapier looms, projectile looms and air-jet looms, and to a tuning method therefor. 
   BACKGROUND OF THE INVENTION 
   As it is known, weft feeders for textile machines comprise a stationary drum on which a plurality of yarn loops forming a weft reserve are wound. Upon request from the loom, the loops are unwound from the drum, then pass through a braking device which controls the tension of the yarn, and finally feed the loom. 
   In the weft feeders of the above kind, which are known from prior art documents in the name of the present Applicant, such as EP 1 059 375, the braking device typically comprises a frustoconical hollow member which is supported at the centre of an annular support on a spider assembly of springs, and is biased with its inner surface against the end of the drum from which the loops are unwound. A pair of linear actuators operatively connected to the annular support are driven by a control unit having a position control loop and a current control loop, which is capable of generating a modulated current as a function of the fluctuations of the yarn tension, in order to modulate the pressure applied upon the drum by the cone. This assembly is supported on a slide that is longitudinally movable under control of a worm screw mechanism that is manually operable in order to adjust the static pressure, or preload, applied upon the drum by the cone at rest. Therefore, the unwinding yarn runs between the drum and the frustoconical member, which modulately applies the desired braking action upon the yarn. 
   Although the above control unit allows the braking action to be modulated smoothly and dynamically, however it has the drawback that its accuracy considerably decreases when certain parameters are changed, such as the stiffness of the springs which support the frustoconical member, or the static pressure applied upon the drum by the cone, which parameters are chosen, e.g., on the basis of the type of yarn under processing, the loom speed, the loom height, and the like. In fact, as well known to the person skilled in the art, the position control loop is designed to operate accurately with a specific set of springs and with a predetermined value of preload. On the contrary, changing these parameters results in an error of compensation. The more said parameters differ from the design parameters, the more relevant said error. 
   SUMMARY OF THE INVENTION 
   Therefore, it is a main object of the present invention to provide a control unit for yarn-braking devices in weft feeders for looms, which can be tuned in an automatized way on the basis of variable parameters concerning the yarn-braking device, in particular, the stiffness of the springs and the static pressure, as well as to provide a setting or tuning method for the control unit, which can be easily automatized and requires a short execution time. 
   The above object and other advantages, which will better appear below, are achieved by a control unit having the features recited in claim  1 , while the other claims state other advantageous, though secondary, features of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be now described in more detail with reference to a few preferred, non-exclusive embodiments, shown by way of non limiting example in the attached drawings, wherein: 
       FIG. 1  is a view in side elevation of a general weft feeder provided with a yarn-braking device; 
       FIG. 2  is a perspective view which separately shows the yarn-braking device of  FIG. 1 ; 
       FIG. 3  is a block diagram of a position control loop according to this invention, which is suited to control the braking device of  FIG. 2 ; 
       FIG. 4  shows the block diagram of  FIG. 3  during the execution of a tuning method according to the invention; 
       FIG. 5  is a force-position diagram concerning the control unit according to the invention; 
       FIG. 6  shows the block diagram of  FIG. 3  during the execution of a tuning method according to an alternative embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the above Figures, a weft feeder  10  for textile machines comprises a stationary drum  12  provided with a beveled delivery edge  12   a , on which a swivel arm  14  driven by a motor  15  winds a plurality of yarn loops forming a weft reserve RT. 
   A stationary arm  17  parallel to the axis of the drum projects from the motor housing and supports a yarn-braking device  18  having the task of controlling the tension of the yarn unwinding from the drum. 
   The yarn-braking device  18  comprises a frame  20  supported on a slide  22  that is movable along the stationary arm  17  under control of a worm screw mechanism (not shown) that is operable by a knob  24 . In a known way, frame  20  supports a pair of electromechanical, linear actuators  26 ,  28  ( FIG. 2 ) arranged with their respective driving rods  26   a ,  28   a  parallel to the axis of the drum at respective, diametrically opposed positions. An annular support  30  coaxial with the drum is supported at the free ends of the driving rods. A hollow, frustoconical braking member  32  is supported at the centre of annular support  30  by elastically yielding support means, which consist of a spider assembly of springs  34  each having one end anchored to the annular support and an opposite end anchored to a ring  36  integral with the smaller base of frustoconical member  32 . The latter is arranged with its larger base coaxially facing the drum and is biased with its inner surface against beveled edge  12   a . Actuators  26 ,  28  are equipped with respective position sensors  38 , each of which comprises a magnet  40  attached to the driving rod of the respective actuator, as well as a Hall sensor  42  supported at a fixed position near the magnets and connected for sending position signals X to a control unit  44  ( FIG. 3 ). One control unit is provided per each actuator. The control unit is capable of generating a modulated current as a function of the fluctuations of the yarn tension in order to modulate the pressure applied upon the drum by the cone. 
   Having now reference to  FIG. 3 , control unit  44  comprises a position control loop  45  which receives the position signal X from position sensor  38  and compares it with a reference variable X_ref in a first subtracter block  46 , thereby obtaining a position error Xerr. A position compensator  48  processes the value Xerr and outputs a corresponding reference current Iref. An inner, current control loop  50  receives current signal Iref in a second subtracter block  52  and compares it with the current I across the actuator, thereby obtaining a current error Ierr. In a known way, current error Ierr is sent to a current compensator  54  that processes this signal to obtain a voltage value V. The latter supplies a wave form generator WFG which generates four low-level pilot signals GL 1 -GL 4  which drive respective MOS field effect transistors Q 1 -Q 4 , usually called MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which are arranged to form an H-bridge  58  which pilots a respective one of said actuators  26 ,  28 . A gate driver GD is arranged between wave form generator WFG and bridge  58 , in order to shift signals GL 1 -GL 4  to voltage levels G 1 -G 4  compatible with the gate of MOSFETs Q 1 -Q 4  of bridge  58 . 
   The braking assembly according to this invention is representable by means of an equivalent mass-spring system, with an equivalent mass corresponding to the mass of the parts in motion, i.e., rods  26   a ,  28   a , magnets  40 , annular support  30  and springs  34 , and an elastic constant k which takes into account both the stiffness of the springs forming the spider assembly, and the elastic yielding of the frustoconical member. Position compensator  48  also includes a transfer function which changes as a function of elastic constant k, and is connected for receiving variable values of said elastic constant k which are calculated by executing a preliminary tuning procedure in control unit  44 . 
   As shown in  FIG. 4 , which illustrates the block diagram of  FIG. 3  during the execution of a tuning procedure according to this invention, the driving force Fm exerted by the actuator is calculated by multiplying the current i across the actuator by a force constant Kf, which usually is assigned to the actuator, but can also be calculated, as will be better described below. The preload force F 1  of the springs, which is measured with the rod in its innermost stop position (resting position), is subtracted from the driving force Fm, thereby obtaining resulting force Fr that is applied to a transfer function of the type: 
             1         s   2     ·   m     +     s   ·   h     +   k       ,         
where m is the mass of the parts in motion, h is the viscous friction coefficient of the system, k is the elastic constant, and s is the complex pulsation, in order to obtain a corresponding displacement X.
 
   In a first embodiment of the invention, the tuning method comprises the steps of: a) positioning the rod of the actuator at a first measuring position between the opposed stop positions X 1 -X 2 , preferably a measuring position X 3  corresponding to a half of the rod stroke, with the actuator controlled by means of an accessory, slow position control loop with a narrow passband, e.g., a passband of 1 Hz, 
   b) overlaying a broad-band, variable current signal, preferably a periodical, symmetrical current signal (e.g., a rectangular signal), to the current i 3  required for maintaining the rod at the measuring position X 3 , whereby the load is excited above its mechanical resonance frequency, 
   c) calculating the coefficients a 0 , a 1 , a 2  of the numerical transfer function 
           1       a   ⁢           ⁢   0     +     a   ⁢           ⁢     1   /   z       +     a   ⁢           ⁢     2   /     z   2                 
which connects the current across the actuator to the position of the rod, by means of calculation methods well known to the person skilled in the art, such as batch identification methods (e.g., minimum squares), or recursive methods (e.g., recursive minimum squares),
 
   d) calculating the static gain, i.e., with z=1, of the numerical transfer function that connects the current across the actuator to the position of the rod, i.e., 
   
     
       
         
           
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   e) calculating the resonance frequency f ris  of the numerical transfer function by means of numerical methods well known to the person skilled in the art, such as Fourier transform methods, for example by discrete values, 
   f) calculating the value of the elastic constant k of the equivalent system by inserting the value of the measured resonance frequency into the numerical transfer function, according to the formula s=j2□f, whereby, under conditions of low viscous friction (h=0),
 
 k=m *(2π f   ris ) 2 ,
 
   g) calculating the value of the force constant kf by multiplying the transfer function in condition of direct current f.d.t.(z=1) by the calculated elastic constant k, i.e.:
 
 kf=k /( a 0+ a 1+ a 2)
 
   h) compensating the position control loop of each actuator with the calculated parameters which relate thereto. 
   The above method allows both the equivalent elastic constant k and the force constant kf of the actuators to be determined. 
   In an alternative embodiment of the invention, in which the force constant kf of the actuators is assumed to be known, the tuning method comprises the steps of: 
   a) positioning the rod of the actuator at a first measuring position X 1  very close to the innermost stop position in which the brake is at rest and the braking member applies the lowermost pressure upon the drum, 
   b) measuring the current i 1  required for maintaining the rod at the first measuring position X 1 , across each actuator, 
   c) positioning the rod of the actuator at a second measuring position X 2  very close to the outermost stop position in which the braking member applies the highermost pressure upon the drum, 
   d) measuring the current i 2  required for maintaining the rod at the second measuring position X 2 , across each actuator, 
   e) calculating the forces F 1 , F 2  exerted by the linear actuator at the measuring positions X 1 , X 2  respectively, by multiplying the force constant kf of the actuators by the measured current values i 1 , i 2  respectively, 
   f) calculating the elastic constant of the equivalent system k by dividing the difference between the forces exerted by the linear actuator at the measuring positions X 1 , X 2  by the difference between the measuring positions X 1 , X 2 , i.e.: 
   
     
       
         
           
             
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   g) similarly to the previous embodiment, compensating the position control loop of each actuator with the calculated parameters which relate thereto. 
   Therefore, the above procedure allows both the equivalent elastic constant k and the preload force F 1  to be calculated. The equivalent elastic constant is the angular coefficient of the line of  FIG. 5 , which line represents the force F as a function of the displacement X in the equivalent system (X 0  is the position with the springs at rest). 
   Advantageously, as shown in  FIG. 6 , the reference variable X_ref is calculated by using the braking force of the springs Fref as main reference value, according to the relation: 
           X_Ref   =         Fref   -     F   ⁢           ⁢   1       k     +     X   ⁢           ⁢   1             
which relation derives from simple algebrical calculations deriving from the line of FIG.  5 . This allows the differences between different actuators to be automatically compensated, so that the same desired braking action will be virtually obtained.
 
   Of course, the above-described tuning methods are particularly suited to be automatized by means of computer-assisted processing techniques, which are intended to be known to the person skilled in the art, e.g., by incorporating their procedures in the feeder-starting routine so that, when the feeder is started, the control unit is automatically set to the parameters of stiffness and preload of the system. The measured values of elastic constant k, force constant kf, and preload F 1 , may also be visualized, e.g., on a monitor accessible to the operator, in a conventional way, in order to supply the operator with informations useful for manually tuning the system. 
   A few preferred embodiments of the invention have been described herein, but of course many changes may be made by the person skilled in the art within the scope of the appended claims. 
   The disclosures in Italian Patent Application No. T02005A000484 from which this application claims priority are incorporated herein by reference.