Direct Fibre Optic Shape Sensing for Large Scale Engineering Structures

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Shape conveys information about a structure that is easily visualized and interpreted, and its dynamic, absolute measurement has significance in applications that span medicine (tracking the movement of minimally invasive instruments during surgery), wind turbines (monitoring turbine blade shape for active load alleviation control), aerospace (monitoring morphing wings, hydraulic hoses and electric cable looms), civil engineering (health monitoring of onshore and offshore structures), rail (monitoring tracks and rolling stock) and the sports and gaming industries (kinematic motion measurements). Direct fibre optic shape sensing (DFOSS) is a disruptive technology that has the potential to have a transformative impact. DFOSS allows the fibre path, as well as the structure to which the fibre is attached, to be followed through space in three dimensions. A key advantage of DFOSS is that the shape is determined directly within the sensing fibre, removing the dependence on strain transfer from the structure and thus the requirement for a model of the structure. Simple surface mounting of the sensing fibre, for example using adhesive tape, is sufficient. The DFOSS approach proposed here is based on Fibre Segment Interferometry, an approach pioneered by Cranfield University, which employs a simple, cost-effective and robust interrogation system exploiting well-proven telecoms laser diodes, detectors and optical fibre components to offer highly sensitive high-speed dynamic curvature measurements. Our initial implementation of the approach is suitable for relative measurement of the shape of small structures (of length upto 5 m), with a sensing gauge length in the range 1cm to 1m and has been successfully trialled on a 5m helicopter rotor rotating at ~400rpm in a ground test on an Airbus H135 helicopter. This proposal aims to solve the scientific challenges involved in extending the capabilities of this DFOSS approach to undertake absolute measurements of shape on larger scale objects, wind turbine blades and aircraft wings (measurement lengths up to 100m, with spatial resolution of the order 1m and data rates of up to 1 kHz). The challenges introduced by the scale of the objects and the anticipated rates of change of shape will require significant innovation, driving a radical evolution of the measurement configuration while maintaining the low cost and robust nature of the approach. Innovation is also required in the processing of the long lengths of multicore optical fibre at the heart of the approach and in the means for its deployment with a known alignment. The design and development of the approach will be informed by real measurement challenges in wind energy and aerospace, with the aim to demonstrate its use on the wing of Boeing 737, for example measuring the response to the jacking of the wing, and on a wind turbine, measuring blade shape changes and blade tip displacement, undertaking vibration characterisation of the blade, and damage location identification on a faulty blade.