Fibre Optic Cable Force Transformer for Temperature Insensitive Sensing

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Solution Overview

Problem

Conventional fibre optic distributed acoustic sensing (DAS) systems struggle to distinguish between mechanical strains and temperature-induced variations, leading to erroneous signals due to the indistinguishable effects of dynamic strains and thermal changes on the optical path length of the fibre optic cable.

Innovation Solution

A fibre optic cable design incorporating a force transformer mechanically coupled to the optical fibre, which transforms transverse forces from temperature variations into longitudinal forces, counteracting the longitudinal component of dimension changes and reducing the sensitivity to temperature variations, thereby minimizing changes in optical path length.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional fibre optic cable is used for DAS sensing, then the cable can detect acoustic waves through optical path length changes, but temperature variations cause erroneous signals that cannot be distinguished from mechanical strains

Engineering Contradiction:

Improvedetection accuracy of mechanical strainsVSAvoidtemperature-induced erroneous signals

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The cable is segmented into multiple layers with different thermal expansion properties: an inner tube containing the optical fibre, an outer tube, and a compensating element positioned between them. This segmentation allows each layer to respond differently to temperature changes, enabling the compensating element to counteract thermal effects on the optical fibre while maintaining mechanical strain detection capability

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A compensating element acts as an intermediary between the inner and outer tubes. This element is mechanically coupled to both tubes and is designed to expand or contract in response to temperature variations, thereby compensating for the thermal expansion/contraction of the inner tube and reducing temperature-induced optical path length changes in the optical fibre

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-affected harmful factors

If the cable structure is made more complex to compensate for temperature effects, then temperature sensitivity is reduced, but the device complexity increases

Engineering Contradiction:

Improvetemperature sensitivityVSAvoidcable structure complexity

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The compensating element serves multiple functions simultaneously: it mechanically couples the inner and outer tubes, compensates for thermal expansion/contraction differences, and maintains the structural integrity of the cable. This multi-functionality reduces the need for additional separate compensation mechanisms, thereby limiting the increase in device complexity

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The compensating element is designed with specific material properties and geometric parameters (such as length, cross-sectional area, and elastic modulus) that allow it to undergo controlled dimensional changes in response to temperature variations. By carefully selecting these parameters, the cable achieves temperature compensation without requiring overly complex structural modifications

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The fibre optic cable with a force transformer significantly reduces the variation in effective optical path length due to temperature changes, making it less sensitive to thermal effects and allowing for more accurate detection of mechanical strains without interference from temperature-induced signals.

Implementation Method 1

a force transformer mechanically coupled to the at least one optical fibre, such that transverse forces acting on the force transformer due to dimension changes of the cable induce longitudinal forces in the at least one optical fibre

Methodology Applied

Scientific EffectForce transformation: Mechanical Advantage

Implementation Method 2

the intensity of Rayleigh backscatter from a given channel in response to separate interrogations of the sensing fibre is monitored to determine any acoustic stimulus acting on the fibre

Methodology Applied

Scientific EffectRayleigh backscatter: Rayleigh Scattering

Implementation Method 3

transverse forces due to dimension changes of the cable arising from a temperature variation of the cable

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS11796353B2Cable for distributed sensing

Publication Date: 2023.10.24 OPTASENSE HOLDINGS LIMITED

AI summary

This application relates to a fibre optic cable (104, 300) suitable for use with a distributed fibre optic sensor apparatus (106). The fibre optic cable includes at least one optical fibre (301) and a force transformer (304) mechanically coupled to the at least one optical fibre. The cable may also include at least one cladding later (302) and/or a compliant material (303). The cable may be surrounded by a jacket layer (306). The force transformer (304) is configured to transform transverse forces due to dimension changes of the cable arising from a temperature variation of the cable into longitudinal forces to counteract the longitudinal component of said dimension change over a tuned temperature range. In this way optical path length changes due to a change of temperature can be reduced or eliminated providing a cable which is insensitive to temperature.