A suite of optical fibre sensors for structural condition monitoring

This paper is to review the research activities at City University London in the development of a range of fibre Bragg grating (FBG)-based sensors, including strain, temperature, relative humidity, vibration and acoustic sensors, with an aim to meet the increasing demands from industry for structural condition monitoring. As a result, arrays of optical fibre sensors have been instrumented into various types of structures, including concrete, limestone, marine propellers, pantograph and electrical motors, allowing for both static and dynamic monitoring and thus enhanced structural reliability and integrity. Keyword List: Fibre Bragg grating, structural condition monitoring, optical fibre sensors INTRODUCTION Fibre Bragg Gratings (FBGs) have been explored widely for various structural condition monitoring [1][2]. This is due to their key characteristics of producing wavelength encoded signals that are not susceptible to instrumentation drift or environmental variations. In addition, they offer the capability of multiplexing multiple sensors on to a single length of fibre and allow for the share of the same light source and detector. Coupled with the intrinsic advantages offered by the fibres, into which the FBGs are written, including small size, light weight, immunity to electromagnetic interference and inert to harsh working conditions, FBGs have demonstrated to be suitable for monitoring various types of structures, in particular in those where their electrical counterparts have shown limitations. This paper covers both the operational principles of a suite of FBG-based sensors that has been developed at City University London and a number of case studies, highlighting their potential for wide industrial applications. FBG-BASED TEMPERATURE AND STRAIN SENSORS Optical fibre Bragg gratings (FBGs) are used as a basis for simultaneous temperature and strain measurement [12]. An FBG is a structure with the fibre core being periodically modulated and which reflects the light at a wavelength termed the Bragg wavelength ( B λ ) that satisfies the Bragg condition, given in equation (1). Λ = eff B n 2 λ (1) where eff n is the effective refractive index of the fibre core and Λ is the grating period, where both are affected by strain/vibration and temperature variations, a feature that is reflected in the sensor design. When the FBG is used for measurement of strain and/or temperature, equation (1) can be replaced by equation (2) ( ) ( ) [ ] T P P e e B B Δ + − + − = Δ ξ α ε λ λ 1 1 (2) where Pe is the photoelastic constant of the fibre, ε is the strain induced on the fibre, α is the fibre thermal expansion coefficient and ζ is the fibre thermal-optic coefficient. The first term of equation (2) represents the longitudinal strain effect on the FBG and the second term represents the thermal effect, which comprises a convolution of thermal expansion of the material and the thermal-optic effect. One of the key features that FBG-based sensors have demonstrated is their multiplexing capabilities. Figure 1 shows a typical FBG-based sensor layout based on the wavelength-division-multiplexing (WDM) with each grating (sensor point) being encoded with a specific wavelength. This characteristic is of particular importance for large-scale structural condition monitoring, allowing for simultaneous multi-point multi-parameter measurement over a long distance yet with limited number of fibres (‘wires’). As indicated in equation (2), each Bragg wavelength shift, induced either by the strain or temperature variation, is associated with the specific Bragg wavelength that is location dependent as illustrated in Figure 1. Compared to conventional strain gauge-based techniques, the optical fibre Bragg grating-based quasidistributed sensing approach has shown significant advantages in terms of ease of handling/installation and the interrogation of a large number of sensing points, i.e. FBG strain/temperature sensors, and coupled to a single source and interrogated by a single detector. In addition, there is no need to post-process the FBG raw data obtained due to their high signal-to-noise ratio compared to those from strain gauges. As illustrated in Figure 1, a Wavelength Division Multiplexing (WDM) scheme [4] can be used very effectively to address the gratings and yield both the strain/vibration/temperature values of multiple sensors and, through prior calibration, their physical locations on the target structure. As indicated clearly in equation (2) that a FBG has a cross-sensitivity, i.e. it is sensitive to both strain and temperature; therefore when strain measurement is required using a FBG, its temperature effect is required to be compensated. The latter has triggered a large number of publications in relation to the diversity of compensation mechanisms proposed. Figure 2 shows a typical example of using FBGs for monitoring a structure where its electromagnetic (EM) interference would cause a problem to electrical temperature sensors, e.g. thermocouples. Figure 2(a) shows an electric motor stator instrumented with 17 FBGs mapped on the inner circumference and Figure 2(b) the zoom-in photo, showing both the FBG sensors and their fibre connection to an interrogation system, as illustrated in Figure 1. For cross-comparison, several thermocouples are also included in the test for temperature measurement. Figure 1. Quasi-distributed FBG-based sensor system with each sensor being encoded with a specific wavelength ASE source F-P tunable filter FBG1 FBG2 FBGn