Patent Publication Number: US-9842690-B2

Title: Apparatus and method for bending and winding conductors to make superconductive coils

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Italian Patent Application No. TO2013A000942, filed on Nov. 20, 2013, the disclosure of which is expressly incorporated herein by reference in its entirety. 
     The present invention relates to an apparatus and a method for bending and winding conductors to make superconductive coils, in particular superconductive coils having turns of circular shape. 
     A typical apparatus for bending and winding conductors to make superconductive coils basically comprises an unwinding and straightening unit and a bending and winding unit. The unwinding and straightening unit has the function of unwinding a coil with vertical axis, formed by a conductor that is bent with a constant radius and is wound along a cylindrical helical path, providing straightened conductor. To this end, the unwinding and straightening unit drives the coil into rotation about its vertical axis and at the same time straightens the conductor leaving the coil by means of a roller straightening device. The coil is usually unwound continuously and at a constant speed, but the speed may also be changed by the operator or by the control system for various reasons, for example may be reduced during some critical phases of the subsequent winding operation. The bending and winding unit comprises a bending device arranged to bend the straightened conductor and a rotary table on which the bent conductor leaving the bending device is laid, whereby a set of turns is formed to make the superconductive coil. Additional devices may be provided for between the unwinding and straightening unit and the bending and winding unit, which devices are arranged to treat the straightened conductor leaving the unwinding and straightening unit, such as for example one or more fine straightening devices placed downstream of the roller straightening device to further straighten the conductor, a cleaning device and a sandblasting device. The sandblasting device may, however, be placed downstream, instead of upstream, of the bending device. Further devices may be arranged between the bending device and the rotary table to treat the bent conductor leaving the bending device. 
     Typically, the superconductive coil is not obtained by winding the conductor along a cylindrical helical path with a vertical axis, and hence with the conductor being bent with a constant bending radius, but in the following mode. First, the conductor is bent with a constant radius for a wide angle (for example 330 degrees) and then a joining portion is made, usually referred to as “turn-to-turn transition”, which takes the remaining angle (for example 30 degrees) up to a round angle. Such a joining portion and is made so as to end with the conductor arranged again tangentially to the coil axis, but spaced therefrom, inwardly or outwardly, by one turn pitch (which is normally equal to the transverse size of a turn plus the space taken by the insulating layer). This mode allows to obtain a perfectly axially symmetrical flat winding for a wide angle (which is important for ensuring a proper operation of the coil), the non-axially symmetrical path being limited to a relatively narrow angle with respect to the round angle. 
     The transition from a turn to the adjacent one may be made in the shape of an S by means of a hydraulically-operated die. This operation has to be done manually and with the rotary table stopped, and thus involves an increase in the overall time required to make the coil, as well as the risk of positioning errors. Therefore, although this first solution allows to limit the angle of the transition, it is not currently the preferred one. According to an alternative solution, that is currently the preferred one, the transition from a turn to the next one is obtained by making with the bending device, at the end of the portion having a constant bending radius, a joining portion comprising a section with a smaller bending radius (with respect to the aforesaid constant bending radius) and a section with a larger bending radius (again, with respect to the aforesaid constant bending radius). Making first the section with the smaller bending radius and then the section with the larger bending radius allows to shift from the previously formed turn to a new inner turn, while making the two sections in the reversed order allows to shift from the previously formed turn to a new outer turn. Preferably, the section with the larger bending radius is a straight section, i.e. a section having an infinite bending radius, since making this section as a straight one allows to minimize, all the other conditions being the same, the overall length of the joining portion. 
     The above-mentioned second solution for making the turn to turn transition requires a wider transition angle, but is quicker and more accurate and does not involve stops of the apparatus. 
     In order to allow the apparatus to carry out the turn to turn transition using this second solution, it is known to have the bending device stationary, along with the unwinding and straightening unit and with the further devices, if any, upstream of the bending device, and to have the rotary table able to translate in the horizontal plane (in particular both in the forward direction of the straightened conductor, hereinafter referred to as longitudinal direction, or x-direction, and in a direction perpendicular to the x direction, hereinafter referred to as transverse direction, or y-direction), so as to allow the rotary table to change its position in the horizontal plane (hence, both in the x-direction and in the y-direction) when the bending radius is changed at the beginning of the transition phase, and up to the end of that phase. At the end of the transition phase, the rotary table will be in the same position along the x-direction as the initial one, whereas along the y-direction it will be shifted by a distance equal to one turn pitch. Once the transition phase has been completed and until the next transition phase, the rotary table will only be subject to rotary motion. 
     When superconductive coils of great size, with a diameter in the order of 20 meters or more, have to be produced, it can be very difficult to make the rotary table translate in the horizontal plane. An apparatus that has to produce coils of such sizes and to obtain the turn to turn transition according to the second solution described above is therefore very complicated and expensive. 
     It is an object of the present invention to provide an apparatus and a method for bending and winding conductors to make superconductive coils that allows to obtain a turn to turn transition according to the second solution discussed above and that are less complicated than the prior art. 
     This and other objects are fully achieved according to the present invention by virtue of an apparatus and a method for bending and winding conductors to make superconductive coils as defined in the enclosed independent claims  1  and  4 , respectively. 
     Further advantageous features of the invention are set forth in the dependent claims, the content of which is to be regarded as being an integral and integrating part of the following description. 
     In short, the invention is based on the idea of providing the rotary table with only a rotary movement about its axis, of providing the whole part of the apparatus upstream of the rotary table (i.e. the unwinding and straightening unit, the bending device and the further devices, if any, provided for between the unwinding and straightening unit and the bending device) with a translational movement along the transverse direction, and of providing only the bending device with also a longitudinal translational movement, whereby the turn to turn transition phase is carried out by suitably combining the rotary movement of the rotary table, the translational movement of the part of the apparatus upstream of the rotary table, bending device included, in the transverse direction and the translational movement of the bending device in the longitudinal direction. 
    
    
     
       Further features and advantages of the invention will result more clearly from the following detailed description, given purely by way of non-limiting example with reference to the appended drawings, where: 
         FIG. 1  is a schematic plan view of an apparatus for bending and winding conductors to make superconductive coils according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of the bending device of the apparatus of  FIG. 1 ; and 
         FIGS. 3 a  to 3 g    are schematic views showing in sequence how the turn to turn transition phase is carried out with an apparatus and a method according to the invention. 
     
    
    
     With reference first to  FIG. 1 , an apparatus for bending and winding conductors C to make superconductive coils B basically comprises:
         an unwinding and straightening unit  10  for unwinding a coil with vertical axis, which is formed by the conductor C that is bent with a constant radius and wound along a cylindrical helical path, and for providing straightened conductor C,   a bending and winding unit  12  comprising a bending device  14  arranged to bend the straightened conductor C leaving the unwinding and straightening unit  10  and a rotary table  16  on which the bent conductor C leaving the bending device  14  is laid, whereby a set of turns is formed to make the superconductive coil B, and   a plurality of intermediate devices that are placed between the unwinding and straightening unit  10  and the bending and winding unit  12  and are arranged to treat the conductor C upstream of the bending and winding unit  12 , such as for example one or more fine straightening devices  18  arranged to further straighten the conductor C leaving the unwinding and straightening unit  10 , a cleaning device  20  and a sandblasting device  22 .       

     The rotary table  16  is mounted so as to be rotatable about its axis z (vertical axis), as well as translatable along that axis. The rotary table  16  is not, however, movable in the horizontal plane, and therefore the position of its axis z is fixed. The bending device  14  is translatable along a x-direction (hereinafter referred to as longitudinal direction) coinciding with the direction of the longitudinal axis of the straightened conductor C that is fed by the unwinding and straightening unit  10  to the bending device  14 . All the part of the apparatus that is placed upstream of the rotary table  16 , that is to say the bending device  14 , the unwinding and straightening unit  10  and the intermediate devices  18 ,  20  and  22 , if any, interposed between the bending device  14  and the unwinding and straightening unit  10 , is translatable along a y-direction (hereinafter referred to as transverse direction) that is oriented horizontally and perpendicular to the longitudinal direction. 
       FIG. 2  shows a typical example of a bending device  14  that can be used in an apparatus for bending and winding conductors to make superconductive coils, and more specifically a so called three-roller bending device, that is to say a bending device which comprises three rollers  24 ,  26  and  28 , usually referred to as first roller, middle roller and bending roller, respectively, which are placed in such a manner that the conductor C that is being fed through the bending device  14  passes between the first roller  24  and the bending roller  28  on one side and the middle roller  26  on the opposite side. In the embodiment shown in  FIG. 2 , the bending device  14  comprises additional rollers  30  and  32 , which are placed upstream and downstream of the three above-mentioned rollers, respectively, but these additional rollers might also be omitted. Moreover, the bending device  14  might also have a configuration different from the one shown herein. 
     The way the turn to turn transition phase, and more specifically the transition from an outer turn S e  to an inner turn S i  of the coil B, is carried out in an apparatus according to the present invention will be described now with reference to  FIGS. 3 a  to 3 g   , with regard to the case where the joining portion between the two turns comprises a first curved section having a smaller bending radius and a second straight section. 
       FIG. 3 a    shows the condition at the end of the main constant-radius turn portion. During the entire process of making this turn portion, the bending device  14  is not moved along the x-direction, the part of the apparatus that is placed upstream of the rotary table  16  (bending device  14  included) is not moved along the y-direction and the rotary table  16  is set into rotation about the axis z (for example at a constant speed), with the conductor C being forwarded along the x-direction (for example at a constant speed as well), from the unwinding and straightening unit  10  to the bending device  14 . 
     During the turn to turn transition phase, the translational movement along the y-direction of the part of the apparatus that is placed upstream of the rotary table  16  and the translational movement along the x-direction of the bending device  14 , as well as the rotary movement of the rotary table  16  about its axis z, are controlled as explained below. 
     As far as the translational movement of the bending device  14  along the x-direction is concerned, the law of movement that is preferably applied is the following one:
 
Δ x ( a )= R ·sin α,
 
wherein α is the current angular position of the rotary table  16  (hence of the coil B that is being formed on the rotary table  16 ), measured from the starting point of the transition, and R is the distance between the axis of rotation z of the rotary table  16  (i.e. of the coil B) and the centre of curvature of the first section (curved section) of the transition, that is to say the difference between the radius of the turn S e  that has already been formed and the radius of the first section of the transition.
 
     As soon as the constant-radius turn portion has been completed, the bending device starts to move in the x-direction (see  FIGS. 3 b  and 3 c   ), preferably according to the above-mentioned law of movement, so as to meet the requirement of tangency of the longitudinal axis of the conductor C with the arc of the transition in the current point. During the process of making the curved portion of the transition, the position of the rollers of the bending device  14  is adjusted to define the correct radius of the curved portion of the transition. 
     Moreover, during the process of making the curved portion of the transition, the part of the apparatus that is placed upstream of the rotary table  16  is caused to move along the y-direction towards the radial position—with respect to the rotary table  16 —corresponding to the inner turn S i . 
       FIG. 3 d    shows the end point of the curved portion of the transition. In this condition, the bending device  14  has reached its maximum forward position along the x-direction, while the part of the apparatus that is placed upstream of the rotary table  16  has reached a position along the y-direction corresponding to the inner turn S i , as it has moved along this direction by one turn pitch. In the condition illustrated in  FIG. 3 d   , both the rotation of the rotary table  16  and the forward movement of the conductor C have been stopped to allow the bending device  14  to move back to the correct position along the x-direction to be able to start the bending of the main constant-radius portion, which will have a radius equal to that of the previous turn S e  minus one turn pitch (see  FIG. 3 f   ). 
     In order to allow the bending device  14  to move along the x-direction in the opposite direction to that of the previous movement, it is necessary first to adapt the position of the rollers of the bending device  14 , in particular of the bending roller  28 , to the straightened portion of the conductor C. This phase is shown in  FIG. 3   e.    
       FIG. 3 f    refers to the condition where the transition portion has been completely made. In this figure the curved section of the transition portion is indicated L 1 , while the straight section is indicated L 2 . 
       FIG. 3 g    shows a first constant-radius portion of the inner turn S i  that has already been made. The bending roller  28  has reached—as from the end of the phase shown in  FIG. 3 e   —the position suitable for forming the inner turn S i . Throughout the constant-radius portion of the inner turn S i  the same considerations apply as those already set forth with reference to  FIG. 3   a.    
     With regard to the movement of the rollers of the bending device  14  in the y-direction, i.e. the movement that produces and controls the bending of the conductor C, it is normally adjusted depending on the forward movement of the conductor C through the bending device itself, and more specifically depending on the movement of the conductor leaving the bending device. In this case, this will be a relative forward movement, i.e. a forward movement of the conductor C leaving the bending device  14  relative to the bending device itself. Indicating with Δt the current arc of the transition and with r the radius of the transition, the following equation applies:
 
 Δt=α·r.  
 
     It is to be taken into account that the above-mentioned equations refer only to “after bending” parameters, such as α and Δt, while the forward movement of the conductor relative to the bending device is to be intended as “leaving the bending device”. The reason is that in this way the equations are not affected by approximation errors due to changes in length of the conductor inside the bending device. However, it is not in practice easy to measure the forward movement of the conductor relative to the bending device after bending, particularly in case of a transition involving radius changes. Therefore, it is admissible—as far as the transition is concerned—to use in practice the forward movement before bending, as this is easy to measure with an appropriate encoder system, thereby overlooking the small error connected to the length change through a relatively short length. 
     In addition to providing a structurally less complicated solution for making the turn to turn transition, which is particularly advantageous in case of large-size coils, the present invention offers the advantage of allowing to make the position corrections required to compensate for the errors due to the elasticity of the portion of conductor comprised between the rollers of the bending device. Typically, the centre of curvature of the conductor leaving the bending device does not lie in the middle transverse plane of the bending device itself, i.e. in the plane that is perpendicular to the longitudinal of the conductor entering the bending device and passes through the axis of the middle roller of the bending device. This is due to the elastic component of the portion of conductor comprised between the rollers of the bending device. The elastic component is then released when the conductor leaves the bending device. Generally, the position of the centre of curvature of the conductor leaving the bending device is significantly spaced from said middle transverse plane, both in the longitudinal direction x and in the transverse direction y. This effect must be compensated, in that the elastic stresses in the bent portion of the conductor comprised between the bending device and the rotary table must be cancelled as much as possible, since these stresses may cause deformations of the conductor which are, of course, undesirable. The required corrections can be made, with an apparatus according to the invention, by suitably moving the bending device along the x- and y-directions and/or by suitably moving the part of the apparatus upstream of the bending device along the y-direction. 
     Naturally, the principle of the invention remaining unchanged, the embodiments and the constructional details may be greatly modified with respect to those described and illustrated purely by way of a non-limiting example, without thereby departing from the scope of protection as defined in the appended claims.