Abstract:
An optical cable includes a core member and a plurality of strands wound around the core member in an SZ configuration, the SZ configuration having at least two reversal sections and a helical section extending along a longitudinal length between the at least two reversal sections. A helical lay length of the wound strands is variable along the longitudinal length of the helical section. A method of forming an optical cable includes providing a core member and surrounding the core member with a plurality of strands by winding the strands in an SZ configuration that includes a helical section extending longitudinally between at least two reversal sections.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/US2015/15615, filed on Feb. 12, 2015, which claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application No. 61/940,569 filed Feb. 17, 2014, the content of which is relied upon and incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     One goal of the loose tube cable SZ stranding process is to impart as much helical length into the cable at the fastest possible speed. Reducing tube diameters may require smaller central members, which experience higher strains also cause a reduction in the helical “window” of the cable for a given lay length. Conventional loose tube cables have 6-8 turns between reversals, with a constant lay length between reversals. The reversal distances may vary somewhat based on machine technology, binder design, and processing speeds; however, the reversals naturally have a longer lay length. An average lay is typically calculated by the number of turns between reversals and the distance between reversals. This average lay is a function of the constant lay length in the helical sections, the number of turns, and the reversal distance. 
     SUMMARY 
     According to one aspect, additional helical length in the stranding process is input in the stranding process, facilitating the use smaller buffer tubes. In one embodiment, the strander rotates faster during selected sections of the RPM profile. For example, faster rotation could be used during typically constant rotational speed sections. 
     The speed limitations for SZ stranding is dominated by the time required to achieve the switch back. According to one aspect, the stranding speed can be kept at a first speed during stranding the switch back, and the stranding speed can be increased to a second speed during traditionally constant RPM portions of the lay. According to one aspect, it is possible to increase the helical window without reducing production speeds. 
     According to another aspect, tensile window is increased to enable smaller loose tube cables. It may thus be possible to, for example, to avoid the need to add yarns to a cable to reduce strain. 
     These and other advantages of the disclosure will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an example SZ cable-stranding apparatus. 
         FIG. 2  is a perspective view of an example hollow-shaft motor showing an exploded view of a guide member attached to the hollow shaft via set screws. 
         FIG. 3  is a front-on view and  FIG. 4  is a cross-sectional view of an example guide member of  FIG. 2  in the form of a layplate having a central hole sized to pass the at least one core member, surrounding strand guide holes, and peripheral set-screw holes. 
         FIG. 5  is a schematic diagram of an electronic configuration of the SZ cable-stranding apparatus. 
         FIG. 6  is a schematic overall view of a SZ cable-forming system that includes the SZ cable-stranding apparatus. 
         FIG. 7  illustrates moderate increasing &amp; decreasing of RPM. 
         FIG. 8  is an illustrative example in which lay length starts at a longer value at the reversal and continues to gradually tighten moving towards the mid-point of the helical section between reversals. 
         FIG. 9  illustrates the motor speed profile in a cable in which binders and water swellable tape may be omitted using a thin film extrusion. 
         FIG. 10  illustrates rotational angle. 
         FIG. 11  illustrates unwrapped SZ path. 
         FIG. 12  illustrates stranding angle. 
         FIG. 13  illustrates the shortest path along the inside of an SZ stranded buffer tube. 
         FIG. 14  illustrates path length comparisons inside an SZ stranded tube. 
         FIG. 15  illustrates SZ strain margin considering a single turn SZ reversal pattern and compared with the equivalent helical pattern. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to embodiments of the disclosure, exemplary embodiments of which are illustrated in the accompanying drawings. In the description below, like elements and components are assigned like reference numbers or symbols. Also, the terms “upstream” and “downstream” are relative to the direction in which the SZ-stranded cable is formed, starting upstream with the various unstranded strand elements and optional at least one core member, and ending downstream with the formed SZ-stranded assembly and SZ-stranded cable. 
       FIG. 1  is a perspective view of an example SZ cable-stranding apparatus (“apparatus”)  10 . Apparatus  10  has an upstream input end  11  and a downstream output end  13 . Apparatus  10  includes along an axis A 1  in order from an upstream to a downstream direction as indicated by arrow  12 , a stationary guide member  20 S and at least one hollow-shaft motor  100  that includes a rotatable guide member  20 R operably disposed therein. Here, the term “rotatable” refers to the fact that motor  100  causes the guide member to rotate, as described in greater detail below.  FIG. 1  shows a configuration of apparatus  10  having a plurality of axially aligned motors  100 . An example type of motor  100  is a high-precision motor such as a servo motor. Adjacent motors  100  are spaced apart by respective distances S, which in many cases is governed by space constraints and the fact that larger guide-member separations result in lower tension variation in the strands. A typical spacing S between motors  100  is between 0.1 m and 2 m, and in an example embodiment the spacing is adjustable, as described below. The spacing S may be equal between all motors  100 , or equal between some motors, while in other embodiments the spacing S is not equal between any of the motors. Providing a variable spacing S between motors  100  may be used to adjust the stranding process. A large spacing downstream helps minimize tension variation while a short spacing upstream shortens the overall length of apparatus  10  with little impact on tension variation. 
       FIG. 2  is a perspective view of a motor  100 . Motor  100  includes a guide member driver in the form of a hollow shaft  102  defined by an axial shaft hole  104  formed therein. An example size of shaft hole  104  is between 1 and 3 inches in diameter, with 2 inches being a commonly available size suitable for use in forming many types of SZ cables. The term “hollow shaft” as used herein in connection with motor  100  is intended to include a motor that contains a through passage concentric with and contained within the rotating structure of the motor. For example, certain types of servo-motors suitable for use herein and discussed in greater detail below include inductively driven rotors that surround and drive a hollow shaft. Each motor  100  includes the aforementioned rotatable guide member  20 R operably disposed within shaft hole  104  (see  FIG. 1 ) so that the guide member rotates with the rotation of the hollow shaft. A rotatable guide member  20 R is disposed in shaft hole  104  and is fixed to hollow shaft  102  by, for example, by set screws (as described below), an adhesive, a flexible or rigid mounting member or fixture, or other known fixing means. 
     Each motor  100  includes a position feedback device  106 , such as an optical encoder (see  FIG. 5 , introduced and discussed below). Positional feedback device  106  provides information (in the form of an electrical signal S 3 ) about the rotational position and speed of hollow shaft  102  and thus rotatable guide member  20 R. An exemplary motor  100  for use in apparatus  10  is one of the model nos. CM-4000 hollow-shaft inductively driven servo motors made by Computer Optical Products, Inc., Chatsworth, Calif. Another exemplary motor  100  for use in apparatus  10  is a hollow-shaft gear-based motor, such as those available from Bodine Electric Company, Chicago, Ill. 
       FIG. 3  is a face-on view and  FIG. 4  is a cross-sectional view of a guide member  20  that can be used as stationary guide member  20 S and/or as rotatable guide member  20 R. The guide member  20  is in the form of a round plate (“layplate”) having a central hole  24  with peripherally arranged smaller guide holes (e.g., eyelets)  28  (six guide holes are shown by way of example). Central hole  24  is sized to pass at least one core member  30  while guide holes  28  are sized to pass individual strand elements (“strands”)  40 . Core member  30  includes a strength element and/or a cable core member. One strength element is glass-reinforced plastic (GRP), steel or like strength elements presently used in SZ cables. Cable core members  30  include buffer tubes, optical fibers, optical fiber cables, conducting wires, insulating wires, and like core members presently used in SZ cables. Example strands  40  include optical fibers, buffer tubes, wires, thread, copper twisted pairs, etc. 
     Guide member  20  are arranged in apparatus  10  so that central hole  24  is centered on axis A 1 , and peripheral guide holes  28  are arranged symmetrically about the central hole. Guide member  20  is configured to maintain the at least one core member  30  and individual strands  40  in a locally spaced apart configuration as the core member and individual strands pass through their respective holes. Guide member  20  optionally includes hole liners  44  that line central hole  24  and/or guide holes  28  in a manner that facilitates the passing of core member  30  and/or strands  40  through the guide member. Hole liners  44  preferably have rounded edges that reduce the possibility of core member  30  and/or strands  40  from being snagged, abraded, nicked or cut as they pass through their respective holes. 
     With reference to  FIG. 2  through  FIG. 4 , rotatable guide member  20 R includes peripheral set-screw holes  25 , and hollow shaft  102  includes matching screw holes  25 ′ configured so that the rotatable guide member is attached to the hollow shaft via corresponding set screws  27 . Rotatable guide member  20 R is the same as or is similar to stationary guide member  20 S, and are both in the form of layplates such as shown in  FIG. 3  and  FIG. 4 . Motors  100  are axially aligned so that shaft hole  104  and the rotatable guide member  20 R operably disposed therein are centered on axis Al. 
     With reference again to  FIG. 1 , the stationary guide member  20 S and each motor  100  are mounted to respective base fixtures  120 , which in turn are mounted to a common platform  130 , such as a base plate or tabletop. Base fixtures  120  are configured to be fixed in place to platform  130 , or positionally adjustable relative to platform  130 . The positional adjustability is achieved by slidably mounting base fixtures  120  to rails  140 , which allows for axial adjustability of each motor  100 . Movable motors  100  can be axially moved along rails  140  and placed together for “thread up,” i.e., threading the at least one core member  30  and strands  40  through their respective holes  24  and  28  in the various rotatable guide members  20 R, and then axially moved again along the rails to be spaced apart and fixed at select positions during the SZ stranding operation, as discussed below. The positional adjustability of motors  100  allows for the spacings S to be changed so that apparatus  10  can be reconfigured for forming different types of SZ cables or to tune the cable-forming process. Base fixtures  120  and platform  130  (and optional rails  140 ) are configured so that motors  100  can be added or removed from apparatus  10 . 
     With continuing reference to  FIG. 1  and also to the schematic diagram of  FIG. 5 , at least one servo driver  150  is electrically connected to the corresponding at least one motor  100 . Each servo driver  150  is in turn operably connected to a controller  160 . The controller  160  may include a processor  164  and a memory unit  166 , which constitutes a computer-readable medium for storing instructions, such as a rotation relationship embodied as an electronic gearing profile, to be carried out by the processor in controlling the operation of apparatus  10 . Apparatus  10  also includes a linespeed monitoring device  172  operably arranged to measure the speed at which the SZ-stranded assembly  226  or core member  30  travels through the apparatus. Example locations for linespeed monitoring device  172  include downstream of the most downstream motor  100  and adjacent SZ-stranded assembly  226  as shown, or upstream of stationary guide member  20 S and adjacent core member  30 . Intermediate locations can also be used. Linespeed monitoring device  172  is electrically connected to controller  160  and provides a linespeed signal SL thereto. 
     The controller  160  includes instructions (i.e., is programmed with instructions stored in memory unit  166 ) that control the rotational speed and the reversal of rotation of each motor  100  according to a rotation relationship. This rotation relationship between motors  100  is accomplished via motor control signals S 1  provided by controller  160  to the corresponding servo drivers  150 . The rotation relationship is embodied as electronic gearing. In response thereto, each servo driver  150  provides its corresponding motor  100  with a power signal S 2  that powers the motor and drives it at a select speed and rotation direction according to the rotation relationship. Position feedback device  106  provides a position signal S 3  that in an example embodiment includes incremental positional information, speed information, and an absolute (reference) position. The reference position is typically a start position of hollow shaft  102 , while the incremental position tracks its rotational position on a regular basis (e.g., 36,000 counts per rotation). The rotational speed of hollow shaft  102  is the change in rotational position with time and is obtained from the position information contained in signal S 3 . Linespeed signal SL provides linespeed information, which is useful for comparing to the rotational speeds of motors  100  to ensure that the rotational speed and linespeed are consistent with the operational parameters of apparatus  10  and the particular SZ-cable being fabricated. 
     For apparatus  10  having a plurality of motors  100 , each motor has a different rotational speed, with less rotational speed the farther upstream the motor resides. For an SZ stranded cable, the number n of “turns between reversals”&#39; can vary, with a typical number being n=8. For this example number of turns between reversals, apparatus  10  starts at a neutral point (n=0) where all of the strands  30  and the rotational and stationary guide members  20 R and  20 S are aligned. Controller  160 , through the operation of servo drivers  150 , then causes motors  100  to execute four turns clockwise, and then reverse and execute eight turns counterclockwise. Note that after the first four counterclockwise turns, apparatus  10  returns to and then passes through the neutral point. After the eight counterclockwise turns, apparatus  10  reverses and performes eight clockwise turns. In this way, n=8 turns between reversals is obtained, with rotatable guide members  20 R turning four turns around the neutral point in each direction. 
     Rotation relationships for motors  100  are carried out in a similar manner for different numbers n of turns between reversals, a different total number m of motors, and a different maximum angular deviation θMAX between adjacent guide members. The number m of motors  100  needed in apparatus  10  generally depends on the type of SZ cable being formed and related factors, such as the maximum number n of turns between reversals, and θMAX, which in turn depends on the guide member diameter, the size of the core member  30  and the size of strands  40 . A typical number m of motors  100  ranges from 1 to 20, with between 5 and 12 being a common number for a wide range of SZ cable applications. 
     Apparatus  10  can be configured and operated in a number of ways. For example, rather than controller  160  controlling each individual servo driver  150 , in one embodiment the servo drivers are linked together via a communication line  178  and receive information about the rotation of the most downstream motor  100  via an electrical signal S 4 . The upstream servo drivers  150  then calculate the required motor signals S 2  needed to provide the appropriate rotation relationship (e.g., via electronic gearing) to their respective motors  100 . Thus, controller  160  transmits information via signal S 1  about the stranding profile (n turns between reversals, the laylength, etc.) to the first (i.e., most downstream) servo driver  150 . Each upstream servo driver  150  receives a master/slave profile (e.g. a gear ratio =R) for the motor  100  immediately in front of it via respective signals S 4 . Thus, the upstream servo drivers  150  are slaved to the most downstream servo driver. In this embodiment, controller  160  is mainly for initiating and then monitoring the operation of apparatus  10 . Linespeed information is provided to the most downstream servo driver  150  through controller  160  (i.e., from linespeed monitoring device  178  to controller  160  and then to the most downstream servo driver). 
     In a related embodiment, controller  160  transmits the aforementioned stranding profile information via signal S 1  to first servo driver  150 , while each upstream servo driver receives a master/slave profile (e.g. a gear ratio=R) that synchronizes them to the downstream servo driver. Since each upstream servo driver  150  is slaved to the most downstream servo driver, each servo driver requires the position feedback data from the first motor  100 . Linespeed information is provided to the first servo driver  150  through controller  160 . 
     In another related embodiment, controller  160  transmits the aforementioned stranding profile information to the first servo driver  150 . Controller  160  also calculates an individualized stranding profile for each upstream motor  100  based on the complete stranding profile that will result in a desired operation for apparatus  10 . In this case, there are no rotational master/slave relationships between motors  100 . Since each motor  100  operates independently of the others, each requires linespeed feedback from linespeed monitoring device  178  and only its own position information. 
     Thus, in one embodiment, each motor  100  is programmed to rotate with a select speed that is not necessarily slaved of off the “base” rotation ratio R. The rotation relationship between the motors has a non-linear form selected to optimize the SZ stranding process. The rotation relationship between two adjacent rotatable guide members  20 R can best be visualized as a function of the angular position θ M  of a “master” guide member  20 R and the angular position θ s  of a corresponding “slave” guide members. Thus, for a prior art mechanical system where the rotation ratio R is fixed, the angular position θ s  of the slave guide member is determined by the function θ s =R*θ M , which is a linear function in θ. In contrast, the rotation relationship programmed into controller  160  can allow for a much more complex functional relationships between the angular positions and rotation speeds of guide members  20 . A non-linear rotation relationship is useful, for example, to minimize tension spikes that can occur during the SZ stranding operation. 
       FIG. 6  is a schematic diagram of an SZ cable-forming system (“system”)  200  that includes apparatus  10  of the present disclosure. System  200  includes strand storage containers  210 , typically in the form of spools or “packages” that respectively hold and pay off individual strands  40  and optionally one or more individual core members  30 . System  200  include a strand-guide device  220  arranged immediately downstream of strand storage containers  210 . Strand-guide device  220  may include a series of pulleys (not shown) that collect and distribute the strands  40  and the at least one core member  30 . SZ cable-stranding apparatus  10  is arranged immediately downstream of strand-guide device  220  and receives at its input end  11  the strands  40  and the at least one core member  30  outputted from the strand-guide device. Apparatus  10  then performs SZ-stranding of the strands about the at least one core member  30 , as described above. Strands  40  and the optional core member  30  exit apparatus  10  at output end  13  as an SZ-stranded assembly  226 , as shown in the close-up view of inset A of  FIG. 6  (see also  FIG. 1 ). SZ-stranded assembly  226  consists of strands  40  wound around the at least one core member  30  in an SZ configuration. 
     System  200  includes a coating unit  228  arranged immediately downstream of apparatus  10 . Coating unit includes an extrusion station  230  configured to receive the SZ-stranded assembly  226  and form a protective coating  229  thereon, as shown in the close-up view of inset B in  FIG. 6 , thereby forming the final SZ cable  232 . In an example embodiment, extrusion station  230  includes a cross-head die (not shown) configured to combine the protective coating extrusion material with the SZ-stranded assembly. Coating unit  228  also includes a cooling and drying station  240  is arranged immediately downstream of extrusion station and cools and dries coating  228 . The final SZ cable  232  emerges from coating unit  228  and is received by a take-up unit  250  that tensions the SZ cable and winds it around a take-up spool  260 . 
     Apparatus  10  of the present disclosure eliminates the mechanical coupling between rotatable guide members  20 R and in this sense is a gearless and shaftless apparatus. Note that the strands  40  passing through the rotatable guide members  20 R do not establish a mechanical coupling between the guide members because the strands are not used to drive the rotation of the guide members. Without the added rotational inertia and bearing friction associated with mechanical components, faster reversal times and thus higher line speeds are possible for a given lay length. Gear-based SZ cable-stranding apparatus are also subject to extremely high dynamic loads during the reversals. This puts a great deal of stress on the power transmission gears, resulting in frequent maintenance issues. The gearless/shaftless SZ cable-stranding apparatus  10  eliminate these types of maintenance and reliability issues. Because the motion of rotatable guide members  20 R is electronically controlled, their rotational velocities in relation to other plates is programmable according to a rotation relationship to carry out rotation profiles (including complex rotation profiles) that result in smoother operation and lower tension variations on strands  40  and the at least on core member  30 . 
       FIG. 7  illustrates moderate increasing &amp; decreasing of RPM during the traditional “constant speed” section of the RPM profile. This will create a variable helical length between reversals. The lay length of one turn is minimized mid-way between the reversals, and then gradually lengthens going towards the reversal. In the illustrated embodiment, L 1 &lt;L 2 .  FIG. 8  is an illustrative example in which lay length starts at a longer value at the reversal and continues to gradually tighten moving towards the mid-point of the helical section between reversals. If the sample in the figure were longer, the lay lengths would begin to increase approaching the next reversal off of the page. 
     According to one aspect of the present embodiments, there are benefits of the reversal which help to offset the elongation of the helical pitch at the reversal. The optimum could be in the range of 2-3 turns between reversals as compared to the standard of 8 today. The advantages may be optimal in cable designs using 8 turns; however, there are advantages even in the case of 2-3 turns between reversals. 
     RPM profile is limited by machine capability at the “reversal” portion of the RPM profile. According to one aspect, the strander can effect a gradual speed increase and then decrease during the traditional “flat” portions of the RPM profile. The strander may effect the gradual speed increase &amp; decrease in the traditional “flat” portions of the RPM profile without any extra wear &amp; tear on the equipment. The above aspect can be effected by the hollow shaft motor as discussed above with reference to  FIGS. 1-6 . The capabilities of the above-described strander improve the ability to generate more helical window at a given line speed for any strander which is operating with 2+ turns between reversals. 
     According to another aspect, binders and water swellable tape may be omitted using a thin film extrusion. In one example, the following machine parameters are set for the rotation of the stranded: Maximum rotational speed of 3,000 rpm; Maximum rotational acceleration of 24,000 rad/s/s; and Number of turns between reversals of 4.  FIG. 9  illustrates the motor speed profile.  FIG. 10  illustrates rotational angle.  FIG. 11  illustrates unwrapped SZ path.  FIG. 12  illustrates stranding angle. 
     Referring to  FIG. 13 , conventional design rules for strain window are derived for helically stranded tubes sometimes with an SZ adjustment factor determined empirically. It is possible to calculate the shortest path along the inside of an SZ stranded buffer tube by assuming the bundle is always in contact with the inside of the tube wall and the fiber bundle is able to move to the shortest path. The blue line is adjusted until it has the shortest length. 
     Referring to  FIG. 14 , using numerical techniques it is possible to determine the shortest possible path inside an SZ stranded tube. This has been done for a range of different turn counts between reversals and an interesting conclusion can be drawn as shown in  FIG. 14 . 
     Referring to  FIG. 15 , considering a single turn SZ reversal pattern and comparing this with the equivalent helical pattern, there is a 52% increase in strain window from the profile that would typically be expected from the new direct drive strander. If the number of turns is now increased towards what we do currently, then the benefit reduces as shown in  FIG. 15 . 
     It will be apparent to those skilled in the art that various modifications to the present embodiment of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.