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We present a demodulation method that partially compensates for the nonlinear and random transfer function of fiber-optic direct detection coherent Rayleigh optical time-domain reflectometry (C-OTDR). Specifically, the proposed method is shown to improve the detection of small amplitude, high frequency dynamic optical fiber strain as it occurs in acoustic emission sensing. The method is applicable provided that the dynamic fiber strain to be sensed is known to affect a longer section of sensing fiber in a spatially homogeneous way. It is shown that this knowledge can be used to extract more quantitative information from the measured C-OTDR signal by averaging signal components from the affected fiber section in a suitable and efficient way. The theoretical basis of the method is developed and supporting experimental results are presented.
Es werden aktuelle Ergebnisse der quasi-verteilten Faseroptische Sensorik der BAM bzgl. der Diagnostik von Hochspannungskomponenten vorgestellt. Hierbei werden Ergebnisse der akustischen Teilentladungs-Überwachung von Kabelmuffen präsentiert.
Ferner werden Resultate zur Verbesserung der Fasersensoreigenschaften einer Faser durch induzierte Streuer dargestellt.
We propose the application of Distributed Acoustic Sensing (DAS) based on Rayleigh Coherent Optical Time-Domain Reflectometry (C-OTDR) to unconventional sensing tasks in industrial condition monitoring. As examples we present results on the way to fibre-optic remote sensing of dielectric damage processes in high voltage cable joints as well as to condition monitoring of passive rollers in large industrial belt conveyor systems.
We report on the successful application of distributed acoustic sensing (DAS) to the detection of partial discharge (PD). A detection limit of about 1 nC discharge magnitude was achieved for PD in a real-scale model of a high voltage termination. Dedicated ultrasonic fibre-optic transducers were interrogated using coherent optical time-domain Rayleigh backscatter reflectometry (C-OTDR). Random quadrature demodulation was employed for retrieving relevant acoustic information from the raw C-OTDR backscatter traces. To our knowledge, our results are a first-time demonstration that quasi-distributed fibre-optic acoustic sensing is a candidate technology for the acoustic partial discharge monitoring of power cable joints and terminations.
Due to their high strength-to-weight ratio and excellent fatigue resistance, glass fiber reinforced polymers (GFRP) are used as a construction material in a variety of applications including composite high-pressure gas storage vessels. Thus, an early damage detection of the composite material is of great importance. Material degradation can be determined via measuring the distributed strain profile of the GFRP structures. In this article, swept wavelength interferometry based distributed strain sensing (DSS) was applied for structural health monitoring of internal pressure loaded GFRP tube specimens. Measured strain profiles were compared to theoretical calculation considering Classical Lamination Theory. Reliable strain measurements with millimeter resolution were executed even at elongations of up to 3% in the radial direction caused by high internal pressure load. Material fatigue was localized by damaged-induced strain changes during operation, and detected already at 40% of burst pressure.
We present the results of distributed fiber optic strain sensing for condition monitoring of a hybrid type IV composite fully wrapped pressure vessel using multilayer integrated optical fibers. Distributed strain sensing was performed for a total number of 252,000 load cycles until burst of the vessel. During this ageing test material fatigue could be monitored and spatially localized. Critical material changes were detected 17,000 cycles before material failure. Results have been validated by acoustic emission analysis.
We present results demonstrating several beneficial effects on distributed fiber optic vibration sensing (DVS) functionality and performance resulting from utilizing standard single mode optical fiber (SMF) with femtosecond laser-inscribed equally-spaced simple scattering dots. This modification is particularly useful when using traditional single-wavelength amplitude-based coherent optical time domain reflectometry (C-OTDR) as sensing method. Local sensitivity is increased in quasi-distributed interferometric sensing zones which are formed by the fiber segments between subsequent pairs of the scattering dots. The otherwise nonlinear transfer function is overwritten with that of an ordinary two-beam interferometer. This linearizes the phase response to monotonous temperature variations. Furthermore, sensitivity fading is mitigated and the demodulation of low-frequency signals is enabled. The modification also allows for the quantitative determination of local temperature gradients directly from the C-OTDR intensity traces. The dots’ reflectivities and thus the induced attenuation can be tuned via the inscription process parameters. Our approach is a simple, robust and cost-effective way to gain these sensing improvements without the need for more sophisticated interrogator technology or more complex fiber structuring, e.g., based on ultra-weak FBG arrays. Our claims are substantiated by experimental evidence.
Condition monitoring of industrial infrastructures using distributed fibre optic acoustic sensors
(2017)
Distributed fibre optic acoustic sensing (DAS) can serve as an excellent tool for real-time condition Monitoring of a variety of industrial and civil infrastructures. In this paper, we portray a subset of our current Research activities investigating the usability of DAS based on coherent optical time-domain reflectometry (C-OTDR) for innovative and demanding condition Monitoring applications. Specifically, our application-oriented Research presented here aims at acoustic and vibrational condition monitoring of pipelines and piping systems, of rollers in industrial heavy-duty conveyor belt systems and of extensive submarine power cable installations, respectively.
Distributed Fibre Optic Acoustic and Vibration Sensors for Industrial Monitoring Applications
(2017)
We investigate the usability of distributed fibre optic acoustic sensing (DAS) for innovative and advanced monitoring applications in industrial and civil infrastructure installations. In this paper, we report on our ongoing application-oriented research activities regarding the utilization of DAS based on coherent optical time-domain reflectometry (C-OTDR) for condition monitoring of a variety of infrastructures. Specifically, our research presented here aims at acoustic condition monitoring of and fault detection in pipelines and industrial piping systems, at acoustic condition monitoring of rollers in industrial conveyor belt installations, and at acoustic condition monitoring of and threat detection in extensive submarine power cables, respectively. Furthermore, we show a method to mitigate the effect of sensitivity fading of C-OTDR based DAS due to unstable environmental conditions via the modification of the sensor fibre. This can help to provide a continuous adequate sensor functionality for a number of different industrial monitoring applications.
We have shown that for C-ODTR based DAS two UV-inscribed scattering segments within an optical fiber have a significant influence on the local sensitivity Fading characteristics. This method was experimental proven to be suitable for distributed dynamic strain measurement in a thermally unstable environment. Signal-to-noise ratio analyses for detecting a signal peak within the time-dependent spectra were performed. Results show an increase in SNR and additionally that the occurrence of total loss of sensitivity (SNR <1) could be decreased by a factor of six when compared to an untreated standard fiber. We have shown that for C-ODTR based DAS two UV-inscribed scattering segments within an optical fiber have a significant influence on the local sensitivity Fading characteristics. This method was experimental proven to be suitable for distributed dynamic strain measurement in a thermally unstable environment. Signal-to-noise ratio analyses for detecting a signal peak within the time-dependent spectra were performed. Results show an increase in SNR and additionally that the occurrence of total loss of sensitivity (SNR <1) could be decreased by a factor of six when compared to an untreated standard fiber.
We have shown that for C-ODTR based DAS two UV-inscribed scattering segments within an optical fiber have a significant influence on the local sensitivity Fading characteristics. This method was experimental proven to be suitable for distributed dynamic strain measurement in a thermally unstable environment. Signal-to-noise ratio analyses for detecting a signal peak within the time-dependent spectra were performed. Results show an increase in SNR and additionally that the occurrence of total loss of sensitivity (SNR <1) could be decreased by a factor of six when compared to an untreated standard fiber.
Glass-fibre-reinforced plastics (GFRP) are commonly used for the construction of pressure vessels and tanks for example in automotive and aerospace industries. Especially for pressure vessels used for the storage of natural gas or hydrogen with operation pressures up to 700 bar, an early material fatigue detection is of great interest. Measuring the distributed strain profile of GFRP structures helps to understand and detect material fatigue. In this article, we demonstrate the great potential of swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of pressure vessels made from GFRP. A resin transfer molding (RTM) process was used to manufacture GFRP-tube specimens. Commercial optical fibres with polyimid coating were glued to the surface externally in circumferential and axial direction. A cyclic load of up to 150 bar was applied to the samples using a servo-hydraulic test bench. Comparing the loaded and unloaded test conditions, we determined up to 2 % elongation in circumferential direction. We demonstrate reliable distributed strain measurements with sub-centimetre spatial resolution. By monitoring these high-resolution strain profiles, we were able to detect local material degradation that manifested itself as localized strain changes. Crucially, the material Degradation could be detected already after 75 % of dat the fatigue life before a crack appeared that led to leakage.
Glass-fibre-reinforced plastics (GFRP) are commonly used for the construction of pressure vessels and tanks for example in automotive and aerospace industries. Especially for pressure vessels used for the storage of natural gas or hydrogen with operation pressures up to 700 bar, an early material fatigue detection is of great interest. Measuring the distributed strain profile of GFRP structures helps to understand and detect material fatigue. In this article, we demonstrate the great potential of swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of pressure vessels made from GFRP. A resin transfer molding (RTM) process was used to manufacture GFRP-tube specimens. Commercial optical fibres with polyimid coating were glued to the surface externally in circumferential and axial direction. A cyclic load of up to 150 bar was applied to the samples using a servo-hydraulic test bench. Comparing the loaded and unloaded test conditions, we determined up to 2 % elongation in circumferential direction. We demonstrate reliable distributed strain measurements with sub-centimetre spatial resolution. By monitoring these high-resolution strain profiles, we were able to detect local material degradation that manifested itself as localized strain changes. Crucially, the material Degradation could be detected already after 75 % of dat the fatigue life before a crack appeared that led to leakage.
Fibre-reinforced plastics (FRP) especially carbon-fibre-reinforced polymer (CFRP) and glass-fibre-reinforced polymer (GFRP) are commonly used materials in high pressure vessels and storage units for automotive and aerospace purposes. Optical fibres are suitable to be integrated or directly applied to the surface of FRP components. Using optical fibres it is possible to monitor the distributed strain profiles and changes within the fatigue life of a pressure vessel to ensure the operational safety. Within artificial ageing experiments we used swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of commercial high-pressure composite cylinder. This artificial ageing was performed using test conditions of 503bar pressure load (service pressure 300 bar) and 89 °C for 100 h. The polyimide coated optical fibres were glued to the surface externally in circumferential and axial direction. Using distributed strain sensing (DSS) material expansion of over 0.5% were monitored with sub-centimetre spatial resolution. Within the circumferential direction we observed up to 10 % local fluctuation compared to the median strain caused by inhomogeneous material expansion, which could cause local material fatigue. In addition, we determined material degradation manifested itself as localized remaining material expansion and/or contraction. Results have been validated by other non-destructive methods like digital strip projection.
Fibre-reinforced plastics (FRP) especially carbon-fibre-reinforced polymer (CFRP) and glass-fibre-reinforced polymer (GFRP) are commonly used materials in high pressure vessels and storage units for automotive and aerospace purposes. Optical fibres are suitable to be integrated or directly applied to the surface of FRP components. Using optical fibres it is possible to monitor the distributed strain profiles and changes within the fatigue life of a pressure vessel to ensure the operational safety. Within artificial ageing experiments we used swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of commercial high-pressure composite cylinder. This artificial ageing was performed using test conditions of 503bar pressure load (service pressure 300 bar) and 89 °C for 100 h. The polyimide coated optical fibres were glued to the surface externally in circumferential and axial direction. Using distributed strain sensing (DSS) material expansion of over 0.5% were monitored with sub-centimetre spatial resolution. Within the circumferential direction we observed up to 10 % local fluctuation compared to the median strain caused by inhomogeneous material expansion, which could cause local material fatigue. In addition, we determined material degradation manifested itself as localized remaining material expansion and/or contraction. Results have been validated by other non-destructive methods like digital strip projection.
Distributed vibration sensing (DVS) based on Rayleigh single pulse optical time-domain reflectometry (C-OTDR) is an attractive and robust method for a variety of sensing applications including geosensing. A key issue for the usability of DVS systems is the signal-to-noise ratio (SNR) and thus the feasibility to demodulate a measured signal, especially in the case of small dynamic strain amplitudes. We present a simple method to locally boost the sensitivity of DVS using pairs of fs-laser-written strongly scattering fiber segments.
Distributed vibration sensing (DVS) based on Rayleigh single pulse optical time-domain reflectometry (C-OTDR) is an attractive and robust method for a variety of sensing applications including geosensing. A key issue for the usability of DVS systems is the signal-to-noise ratio (SNR) and thus the feasibility to demodulate a measured signal, especially in the case of small dynamic strain amplitudes. We present a simple method to locally boost the sensitivity of DVS using pairs of fs-laser-written strongly scattering fiber segments.
Due to high specific stiffness, fibre reinforced plastics are the dominant material group for the design of mobile pressure vessels. At the Federal Institute of Materials Research and Testing (BAM) aging process of composite pressure vessels is studied to be able to give more accurate lifetime predictions in future. Investigations are based on type III breathing air cylinders consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. Goal is to detect changes of residual stresses over life time which directly affect fatigue strength. Within this paper an approach is presented to monitor residual stresses via an experimental modal analysis (EMA). First, the influence of changed stress conditions on modal parameters is analysed via a numerical study. Secondly, a test bench for an EMA is set up. To be able to analyse cylinders of different prestress condition, several specimens are prestress modified via high-temperature and high-pressure treatment. During modification processes, specimens are monitored via optical fibres to control prestress modifications. Through experimental measurements of the modified specimens via EMA changes in prestressing can be detected. Finally, validity and accuracy of the EMA is evaluated critically by comparing all numerically and experimentally obtained data.