Method for detecting torsion in a cable, electric cable with torsion sensor and method for manufacturing said cable

Patent metadata

Field	Value
Publication number	US-9400221-B2
Application number	US-201214382919-A
Country	US
Kind code	B2
Filing date	Mar 5, 2012
Priority date	Mar 5, 2012
Publication date	Jul 26, 2016
Grant date	Jul 26, 2016

Abstract

A method for monitoring a torsional state of a cable having a central longitudinal axis, includes providing a cable including a torsion sensor longitudinally extending along the cable, the torsion sensor including a single-mode optical fiber arranged substantially along the central longitudinal axis of the cable, and at least three longitudinal structural elements, at least one of the longitudinal structural elements being an electrically conductive core, wherein the torsion sensor is mechanically coupled with at least one of the longitudinal structural elements; measuring a torsional state of the single-mode optical fiber by polarization-sensitive optical reflectometry; and associating the torsional state of the cable along the longitudinal axis with the measured torsional state of the single-mode optical fiber.

First claim

The invention claimed is:

1. A method for monitoring a torsional state of a cable having a central longitudinal axis, comprising:
providing a cable comprising a torsion sensor longitudinally extending along the cable, said torsion sensor comprising a single-mode optical fibre arranged substantially along the central longitudinal axis of the cable, and at least three longitudinal structural elements, at least one of the longitudinal structural elements being an electrically conductive core, wherein the torsion sensor is mechanically coupled with at least one of the longitudinal structural elements;
measuring a torsional state of the single-mode optical fibre by polarisation-sensitive optical reflectometry, said measuring comprising:
injecting a plurality of probe optical signals having distinct input polarisation states into an end of the single-mode optical fibre;
detecting a plurality of backscattered optical signals, each of the plurality of backscattered optical signals having an output state of polarisation;
measuring the output states of polarisation of the backscattered optical signals;
calculating a birefringence angle function as a function of longitudinal position of the single-mode optical fibre from the measured output states of polarization;
calculating a rotational angle function of the single-mode optical fibre from the birefringence angle function as a function of the longitudinal position in the single-mode optical fibre; and
associating the torsional state of the single-mode optical fibre with the calculated rotational angle function, and

wherein the method further comprises associating the torsional state of the cable along the longitudinal axis with the measured torsional state of the single-mode optical fibre.

2. The method of claim 1 , wherein the torsion sensor is mechanically coupled to the at least three longitudinal structural elements.

3. The method of claim 1 , wherein the torsion sensor is embedded in a coupling filler mechanically coupling the torsion sensor with at least one of the at least three longitudinal structural elements.

4. The method of claim 1 , further comprising comparing the measured torsional state of the single-mode optical fibre with a reference torsional state of the single-mode optical fibre before associating the torsional state of the cable with the measured torsional state of the single-mode optical fibre.

5. The method claim 1 , further comprising, before injecting a plurality of polarised probe optical signals, preparing a reference birefringence angle function relative to a reference rotational state of the cable, wherein calculating a rotational angle function comprises calculating a variation of the calculated birefringence angle function with respect to the reference birefringence angle function.

6. The method of claim 1 , wherein the plurality of polarised probe optical signals is a plurality of N probe optical signals having distinct input states of polarisation, with N≧3, so that the amount of measured output states of polarisation is equal to N, and the method further comprises:
grouping N measured output states of polarisation in G groups of measured output states of polarisations, each group comprising at least two measured output states of polarization, each group being distinct from remaining (G-I) groups by at least one measured output state of polarisation,
calculating a birefringence angle function from each group so as to obtain a plurality of group birefringence angle functions,
calculating an average value of the group birefringence angle functions, and
calculating the rotational angle function from an average value of the group birefringence angle functions.

7. The method of claim 6 , wherein calculating a birefringence angle function further comprises:
analysing the plurality of group birefringence angle functions to reveal a discontinuity in a group birefringence angle function;
correcting the discontinuity when a discontinuity in a first group birefringence angle function is revealed at a longitudinal position along a fibre length; and
calculating an average value from the first group angle function and from a remaining plurality of group angle functions comprises eliminating the discontinuity.

8. The method of claim 6 , wherein calculating a birefringence angle function further comprises:
calculating a difference function between each pair of group birefringence angle functions of two different groups;
analysing each difference function to detect a presence of a discontinuity in a group birefringence angle function in a pair at a longitudinal position in the single-mode optical fibre, the discontinuity representing an angle jump of 2 πm, where m is an integer number;
determining which group birefringence angle function of the pair comprises the discontinuity, and
marking a discontinuity length region in the group birefringence angle function of the pair containing the discontinuity, the discontinuity length region being around and comprising the longitudinal position corresponding to the discontinuity,

wherein the birefringence angle function containing the discontinuity is taken into account in calculation of the average value of the group birefringence angle functions only for longitudinal positions outside the discontinuity length region.

9. A method for manufacturing an electric cable having a central longitudinal axis, the electric cable comprising:
at least three longitudinal structural elements, at least one of the at least three longitudinal structural elements being an electrically conductive core comprising an electrical conductor; and
a torsion sensor comprising a single-mode optical fibre, said torsion sensor arranged substantially along the central longitudinal axis and being mechanically coupled with at least one of the at least three longitudinal structural elements;

the method comprising forming the torsion sensor by:
pre-twisting the single-mode optical fibre with a twisting pitch having a first value and a twisting hand;
coating the single-mode optical fibre with at least one protective layer;
embedding the fibre optic torsion sensor in a coupling filler;
mechanically coupling the coupling filler embedding the fibre optic torsion sensor to a longitudinal structural element; and
stranding the longitudinal structural elements around the coupling filler with a stranding pitch having a second value substantially equal to the first value of the twisting pitch and a stranding hand opposite to the twisting hand whereby the electric cable has the single-mode optical fibre with a twisting pitch substantially equal to zero.

10. The method of claim 9 , wherein coating the single-mode optical fibre comprises applying at least one of a tight buffer and a protective sheath.

11. The method of claim 9 , wherein pre-twisting the optical fibre and coating the optical fibre can be carried out in any order.

12. The method of claim 9 , wherein the twisting pitch and the stranding pitch have a first and second value of from 2 to 3 turns/meter.

Assignees

• Sarchi Davide
• Palmieri Luca
• Prysmian Spa

Inventors

• Sarchi Davide
• Palmieri Luca

Classifications

• G01M5/0091

  by using electromagnetic excitation or detection · CPC title

• G01L1/242 Primary

  the material being an optical fibre · CPC title

• Y10T29/49004

  including measuring or testing of device or component part · CPC title

• G01M5/0025 Primary

  of elongated objects, e.g. pipes, masts, towers or railways (G01M5/0058 takes precedence) · CPC title

• G01M11/086

  Details about the embedment of the optical fiber within the DUT · CPC title