Sensing System

Sensing System

The fibre-optic sensing system comprises three key elements:

  • The fibre Bragg grating sensors including their method of deployment. That is the method by which they are integrated in to, or attached to the structure.

  • The interrogation unit which comprises the light source and also accurately measures the wavelength of the light reflected from each sensor.

  • The interconnecting fibre-optic cables and connectors

 

 

 

 

 

Sensor Deployment Methods

Epsilon has developed three prime methods for deploying the sensors:

1. Surface bonded sensor patch:

The optical fibre is mounted within a patch normally manufactured from low modulus glass fibre. The patch is shaped to suit the component to which it is to be attached and the FBGs are located in their required positions.

Standard strain sensor patch   Custom strain sensor patch   Standard temperature sensor patch
Standard strain sensor patch
 
Custom strain sensor patch
 
Standard temperature sensor patch
Note: When compared with an installation using conventional resistive strain gauges sensor patches can offer significant weight savings as only a single optical fibre is required to multiple measurement points compared with conventional gauges, each of which requires a minimum of 3 connecting wires.

 

 
Key Features Heading

Surface bonded sensor patch:

  • Ease of installation which significantly reduces both time and cost.
  • The patch’s low stiffness and relatively large bond area greatly reduces the shear strain on the bond when compared with conventional resistive strain gauges significantly improving the sensors life.
  • The ingress and egress of the optical fibre can be engineered into the patch adding to the robustness of the installation.
  • Sensor patches can be used on both metallic and composite structures

 

2. Embedded Sensors:

The optical fibre with its FBGs can be embedded in composite structures with minimal intrusion and no detrimental effect on the mechanical properties of the structure (see note). Although the diameter of an optical fibre is much larger than that of a glass or carbon fibre used in a composite structure their mechanical properties are very similar. In this way a properly embedded sensor can be considered an integral part of the structure and will last the lifetime of the structure, even over many millions of fatigue cycles.

Embedded Sensor Installing
Picture taken by electron microscope showing the cross-section of an optical fibre embedded in e-glass/epoxy composite
Installation of FBG sensors in
pre-preg composite

For a successful installation a number of factors need to be considered including:

  • The fibre type used, in particular the coating type needs to be selected to match the temperatures and pressures of the moulding process.
  • The fibre needs to be positioned such that the risk of micro-bending is minimised.
  • The ingress/egress point needs to be engineered into the component.

Epsilon optics has worked with a number of manufacturers of composite components to successfully embed optical sensors and has developed techniques for all common composite manufacturing processes including:

  • Hand lay-up
  • Vacuum infusion
  • Resin transfer moulding
  • Filament winding
  • Pultrusion
  • Pre-preg
  • Autoclave processing
Note: A number of trials have been conducted by several of our Aerospace customers, comparing the strength, and resistance to fatigue, of components with and without Epsilon optics embedded sensors. Provided the sensors are embedded in accordance with Epsilon Optics best practice, no measureable knock-down in performance was detected.

 

 
Key Features Heading

Embedded Sensors:

  • No impact on the component’s external surface. This is important where the component provides an aerodynamic surface such as a helicopter rotor blade.
  • It is possible to measure strains within the component
  • The embedded sensor is very well protected and is available for the life of the component.

 

3. Sensor Carriers:

The optical fibre and its FBGs are mounted within a composite carrier which is shaped such that it can be locked or clamped to the structure. The carrier follows the changes in shape of the parent structure and thus measures the loads applied to it. A number of variations have been developed including sensors for detecting cracks and for measuring displacements of civil engineering structures. They have also been used on complex metallic structures such as aircraft landing gears.

Sensor Carriers
In-axle sensor for aircraft landing gear
Long guage length displacement sensor
Micro-Tenex © pultruded rod sensor

 

 
Key Features Heading

Sensor Carriers:

  • They are easy to install
  • Can be removed for re-use
  • Very robust

 

Cables and connectors

Epsilon can supply appropriate cabling and connector solutions for a wide range of harsh environments and applications including: sub-sea, aerospace, marine and civil engineering. We have also developed a number of unique solutions for integrating connectors into composite components. These can be embedded or surface mounted, and are capable of withstanding the production process and cure cycle.

Cables and Connectors
Surface mounted
sub-sea connector
Surface-mount connector,
suitable for vacuum bagging
During lay-up
   
Design and implementaion of complete fibre-optic sensing solutions

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Fiber-optic sensing for smarter, stronger, safer structures. Call us on +44 (0) 1425 655027