Fiber Optic Sensor Array Research Articles

Partial Discharge Online Monitoring and Localization for Critical Air Gaps Among SiC-Based Medium-Voltage Converter Prototype

As a consequence of higher blocking voltage and power density by using SiC devices, detailed analysis and careful design of insulation are important for future medium-voltage power converters. Because of the insulation or mechanical coordination, air gaps inside the converters cannot be avoided. Moreover, some gaps are more critical than others, since much oversized design or electric shielding there cannot be acceptable. Admittedly, the final design should be partial discharge (PD) free under normal operation with a safety margin. However, due to possible change of air properties and insulation degradation, massive PD can be still initiated from high electric field regions along the gaps and may lead to the damage of devices, through the highly coupled power and driving circuitry. Therefore, with online PD monitoring and localization, insulation health condition of the critical gaps can be provided, which could have great values for converter operation, maintenance, and design. In this paper, after detailed acoustic signal propagation study, a fiber-optic acoustic sensor array has been implemented into a 6-kV H-bridge power electronics building block prototype, in order to realize online PD monitoring along the critical gaps. Finally, from experimental results, the proposed sensor array has good sensitivity, localization accuracy, and high immunity to the electromagnetic interference. It unveils a novel access to insulation online monitoring for future high voltage, high power density SiC-device-based converters.

Analytical models for predicting required response of mandrel-type fiber optic acoustic sensors for ocean and seismic applications

Fiber optic acoustic sensor systems have been developed by industry, university, and government research teams which meet enhanced requirements for arrays with high sensor counts, high dynamic range, wide bandwidths, and will operate reliably for a decade or more in high-pressure-high-temperature pipeline, ocean, and seismic environments. Optical sensors do not require vulnerable electrical telemetry in the “wet” end of the system, and can reliably transmit high sensor count string outputs which are 10's of km from the light source and acoustic data processors. This paper introduces a modified version of the SONAR equation which allows a straight forward estimation of the required fiber optic acoustic sensor array response to capture the desired acoustic signal detail necessary to interpret the target or seismic event. This enables system design engineers to define and optimize sensor response requirements early on, and help minimize prototype and production system performance and budget risks.

Partial Discharge Localization Inside Transformer Windings via Fiber-Optic Acoustic Sensor Array

Partial discharge (PD) is the main sign of the insulation deterioration; therefore, the online PD localization of the power transformers can provide the equipment degradation characteristics and has great value for the power grid. Because of some unique merits, acoustic PD detection is one of the best options for such applications. However, when discharges happen inside the windings, precise PD localization by acoustic method becomes very challenging. As the acoustic PD signal can be reverberated and distorted along its propagation, the type and number of sensors, with their installation, should be carefully studied and selected. In this paper, the acoustic wave propagation inside the transformer is explored via numerical simulation. Based on the understanding of wave propagation, a novel-structure fiber-optic acoustic sensor array is properly designed and installed into one phase winding, which is from a real 35 kV transformer. Experimental verification shows that, by using this proposed design, PD localization with less than 5-cm error can be achieved. Therefore, the novel sensor array developed in this paper, together with the installation and localization method, unveils a novel access to accurate online PD localization, especially for those PDs which happen inside transformer windings.

FPGA-based interrogation controller with optimized pipeline architecture for very large-scale fiber-optic interferometric sensor arrays

Fiber-optic sensor arrays are always organized utilizing hybrid time division multiplexing (TDM)/wavelength division multiplexing (WDM) techniques to share some common optical components and reduce the cost. Modern sensor arrays can support up to several thousands of sensors by employing distributed amplification technique. They generate huge amount of data and pose other big challenge on the interrogation controller of the sensor system, such as long demodulation time and heavy computational burden. Aiming to solve these problems, we present a complete design of the interrogation controller based on a field programmable gate array (FPGA) by lending its powerful parallelism potential. The interrogation controller adopts a hardware-implementation-friendly modulation method called the rectangular-pulse binary (RPB) modulation. The functional modules in the FPGA are properly organized according to the working principle of the RPB method and carefully designed by optimizing their pipeline architecture to minimize the output latency, which is essential to increase the maximal supportable number of sensors. By analyzing the resource budget and the actual resource consumption result, it demonstrates that the controller can support up to 1000+ sensors in real time by using a single middle-end Stratix III FPGA chip. A 8-sensors array prototype is constructed to validate its functionality. This work can greatly reduce the cost of the sensor system and push it closer to the practical applications.

Wave separation and pipeline condition assessment using in-pipe fibre optic pressure sensors

Abstract The use of two pressure transducers in close proximity can enable the separation of the directional travelling pressure waves in pipelines. However, the implementation of this measurement strategy in real water pipes is difficult due to the lack of closely located access points. This paper reports the use of a customised in-pipe fibre optic pressure sensor array for hydraulic transient wave separation and pipeline condition assessment. The fibre optic pressure sensor array can be inserted into a pressurised pipeline through a single access point. The array consists of multiple fibre Bragg grating (FBG)-based pressure sensors in close proximity (∼0.5 m apart). A previously developed wave separation algorithm is adapted to analyse the transient pressure measurement from the FBG sensors. The resultant directional pressure waves are then used to detect pipe sections with a thinner wall thickness. A challenge is the influence of the in-pipe fibre optic sensing cable on the transient pressure measurement. The impact is analysed and adjustments to the pipeline condition assessment algorithm are undertaken to resolve the issue. The successful experimental application verifies the usefulness of the in-pipe fibre optic sensor array, which can facilitate transient-based pipeline condition assessment for buried water pipes with limited access points.

Localization of partial discharge in transformer oil using Fabry-Pérot optical fiber sensor array

Partial discharge is the main reason for insulation deterioration and failure of high voltage electrical asset. Thus, the online detection and localization of partial discharge can provide the information on the equipment degradation characteristics, and moreover, is of great value for improving the safe operation of the power grid and the developing of optimal maintenance. Accounting for the good sensitivity, the high immunity to electromagnetic interference, the compact size with outstanding insulation level, the Fabry-Perot optical fiber sensor is attracting increasing attention as a promising way for online acoustic partial discharge localization. In this study, the partial discharge localization in the transformer oil is realized by an ultrasonic detection system, which is consisting of 2×2 Fabry-Perot optical fiber sensor array with steered response power algorithm. In general, the Fabry-Perot optical fiber acoustic sensor array shows good direction sharpness, and achieves accurate localization. This detection and localization system is further applied to a 35 kV transformer single-phase winding and verified with partial discharges on the outer winding.

Precisely Automatic Time Window Locating for an Interferometric Fiber-Optic Sensor Array Based on a TDM Scheme.

Interferometric fiber-optic sensors are often organized in the form of large-scale arrays by lending the technique of time division multiplexing (TDM) to reduce the system cost. Discriminating the time windows for different sensor units is the prerequisite to successfully demodulate the sensing message, but it traditionally calls for a very time-consuming manual calibration process. To combat this problem, a novel automatic time window locating method is proposed in this paper. It introduces the concept of shape function and carries out the cross-correlation operation between the shape function and the sensor signal. The shape function is defined as the function whose curve profile reflects the main data characteristics of the sensor signal. The time window information is then extracted from the correlation result. This whole process is carried out automatically by the interrogation controller of the sensor system without any manual intervene. Experiments are conducted to validate this method. The proposed method can greatly reduce the complexity of locating time windows in large-scale TDM sensor arrays, and make the practical use of the TDM scheme much more convenient.

Correction of Peak Tracking Ripple in Solid State Spectrometers

Errors in peak wavelengths from fiber Bragg gratings (FBGs) when monitored using a pixel-based solid state spectrometer have been investigated. These errors show a pseudo-sinusoidal ripple superimposed upon the expected linear response. The error is analogous to a peak-locking effect common to subpixel resolution particle tracking by CCD camera in particle image velocimetry. The amplitude of the error is calibrated as a proportion of the scaled variation of the change in the peak power of the calculated peak as the spectral peak is scanned across the pixel array. An error correction algorithm has been developed and verified in order to provide an increase in the accuracy of sensing solutions when using pixel-based spectrometers. The methods presented here permit the correction of pixel induced ripple errors in real time using data commonly available from many interrogation spectrometers. Calibration prior to use can be achieved by scanning each FBG spectral peak across the width of a single pixel. The proposed method of error correction is applied to a fiber optic sensor array utilized for biomedical pressure and temperature measurement and demonstrates a clear ability to increase the accuracy of measured signals.

Diaphragm-based optical fiber sensor array for multipoint acoustic detection.

Multipoint acoustic sensing system plays an important role in industrial applications. Here, a diaphragm based optical fiber sensor array is proposed, in which each sensor tip is made of 10-layer graphene diaphragm and optical fiber pigtail, with the compact size of about 2.5 mm in diameter. In particular, coherent phase detection is adopted to improve detection signal-to-noise ratio (SNR) and eliminate the demodulation dependence on structural parameters of sensor tips, and thus to achieve the multiplexing ability. Through time division multiplex (TDM), a multiplexing capacity up to 248 in theory can be realized, which is the first time to theoretically demonstrate large-scale acoustic sensor array for diaphragm based fiber sensor by phase detection, to the best of our knowledge. A prototype of 2 *2 sensor array is built to demonstrate the acoustic sensing performance. The field test results show excellent acoustic sensitivity of higher than -136 dB re 1 rad/μPa within the frequency range of 300 Hz~15 kHz, as well as the MDP of only 75 μPa/Hz1/2. Besides, good temperature stability and wide directivity are demonstrated. The proposed sensor array is promising in sound source localization, where the positioning accuracy of 3.55 cm is successfully realized.

Study of gain factor effect of erbium doped fiber amplifier on noise floor level of fiber-optic interferometric sensor

Subject of Research. The paper presents experimental study results of the gain factor effect of an erbium doped fiber amplifier on the noise floor level of a fiber-optic interferometric acoustic sensor. Fiber-optic sensor is based on a Michelson interferometer. The optical amplifier is located behind the compensating interferometer which induces the auxiliary phase modulation to the interference signal. The homodyne demodulation algorithm is used to recover the sensor phase signal. Method. During the experiment there were no external acoustical impacts on the sensor. The power control of optical signals from the fiber amplifier was performed. Noise signals from the sensor were written into data files under the different values of the fiber amplifier gain factor. Spectral estimations of the noise floor level of the fiber-optic interferometric sensor were performed by the averaged modified periodogram method under the different values of the fiber amplifier gain factor. Obtained results were used to define the dependence of the noise floor level of the fiber-optic interferometric sensor on the fiber amplifier gain factor. Main Results. Mean noise floor levels were equal to 64 urad/Hz0.5 at 355 Hz, 68 urad/Hz0.5 at 450 Hz and 66 urad/Hz0.5 at 500 Hz. Experimental results showed insignificant increase of the noise floor level of the fiber-optic interferometric sensor with the growth of the optical amplifier gain factor from 14.6 dB to 25.8 dB. The gain factor increase was about several percent and did not exceed inaccuracies of performed measurements. Practical Relevance. The absence of significant changes in the noise floor level of the considered fiber-optic sensor with the changing of the fiber amplifier gain factor is caused by the intensity noise suppression of the used demodulation scheme. The considered fiber amplifier might be used for the amplification of optical signals from the fiber-optic multiplexed array of fiber-optic sensors without the significant deterioration of their noise performance on condition that the homodyne demodulation algorithm with the intensity noise suppression is used.

In-pipe fibre optic pressure sensor array for hydraulic transient measurement with application to leak detection

Controlled hydraulic transient pressure waves can be used to actively assess the condition of pressurised pipelines (e.g. leak detection). One practical challenge to the application of the technique to buried water pipelines is in the pressure measurement – fast-response pressure transducers are costly and the deployment is limited to one sensor per access point (e.g. at air valves or fire hydrants). The measurement from a single sensor is always the superposition of the waves travelling upstream and downstream along a pipe. It is often difficult to determine the direction of the propagating waves, unless there are suitable locations nearby where additional measurements can be made. The research reported here develops a fibre optic pressure sensor array for in-pipe transient pressure measurement. The sensor array consists of five Fibre Bragg Gratings (FBGs), each formed into a custom designed pressure transducer. The cable containing the array has a diameter of approximately 4 mm, making it easy to insert into a pipeline through a single access point for distributed transient pressure measurement. The usefulness of the fibre optic pressure sensors for transient-based pipeline leak detection is validated through laboratory experiments. The measurement from each individual fibre optic sensor is generally consistent with conventional piezo-resistive pressure transducers over a wide frequency range. The sensor array configuration enables the determination of the directional information of waves, therefore enhancing the ability of pipeline defect detection and localisation.

High-quality hybrid TDM/DWDM-based fiber optic sensor array with extremely low crosstalk based on wavelength-cross-combination method

A high-quality hybrid time-division-multiplexing (TDM)/dense wavelength-division-multiplexing (DWDM)-based fiber optic sensor array with extremely low crosstalk is proposed and experimentally demonstrated in this paper. The array is based on a novel wavelength-cross-combination (WCC) method, aiming to combat the annoying problem of crosstalk between adjacent sensors. The core idea of this method is to reassign optical pulses of different wavelengths rather than of the same wavelength to the same photodetector to eliminate the possibility of coherent optical pulse overlapping and interference. Based on the WCC method and a so-called rectangular-pulse binary phase demodulation scheme, a sensor array with 4 TDM time windows and 16 DWDM wavelengths is set up and experimentally tested. Test results show that even utilizing an optical pulse modulator with less than 28 dB extinction ratio, the obtained crosstalk between adjacent sensors can be still suppressed down to less than −60 dB. This method provides an excellent choice for a practical sensor array application where the sensing performance is of the top consideration.

Robust fibre optic sensor arrays for monitoring early-age performance of mass-produced concrete sleepers

This study investigates integrating fibre optic sensing technology into the production process of concrete railway sleepers. Robust fibre Bragg grating strain and temperature sensor arrays were developed specifically for this application and were designed for long-term monitoring of sleeper performance. The sensors were used to monitor sleeper production and to help gain a deeper understanding of their early-age behaviour which can highly influence long-term performance. In total, 12 sleepers were instrumented and strain data were collected during the entire manufacturing process including concrete casting and curing, prestressing strand detensioning and qualification testing. Following the production process, sleepers were stored temporarily and monitored for 4 months until being placed in service. The monitoring results highlight the intrinsic variability in strain development among identical sleepers, despite high levels of production quality control. Using prestress loss as a quality control indicator, the integrated sensing system demonstrated that sleepers were performing within Eurocode-based design limits prior to being placed in service. A three-dimensional nonlinear finite element model was developed to provide additional insight into the sleepers’ early-age behaviour. Based on the fibre Bragg grating–calibrated finite element model, more realistic estimates for the creep coefficient were provided and found to be 48% of the Eurocode-predicted values.

A Highly Compact and Efficient Interrogation Controller Based on FPGA for Fiber-Optic Sensor Array Using Interferometric TDM

We demonstrate a highly compact and efficient design of an interrogation controller based on FPGA for TDM sensor array. The interrogation controller precisely and adaptively synchronizes the round trip time for each sensor by introducing an extra adjustable delay and fastly retrieves the measurand induced phase signals throughout real-time phase calculation and phase unwrapping. A four-sensor array is experimentally built and interrogated by the controller to evaluate the performance as proof-of-principle. Test results validate the efficient functionality of the controller. The interrogation controller design, which consumes 8867/85200 (11%) ALUTs, 7920/85200 (9%) registers and 47/896 (6%) DSP 18-bit elements on a Stratix-III FPGA per TDM channel, greatly reduces the overall cost which is a central consideration for large-scale sensor array applications.

Analysis and mitigation of the carrier phase delay effect of the digital phase generated carrier algorithm.

We present an improved digital phase generated carrier algorithm based on the synchronous carrier restoration (SCR) method to mitigate the carrier phase delay effect. The most distinguishing feature of this method is that it directly picks up the carrier signal information (frequency and phase) from the interference signal and synchronically accomplishes the processing of carrier signal restoration. In comparison to the traditional method, which adopts the initial carrier signal, total-harmonic-distortion with SCR is only 0.091%, lower than the traditional SCR of 18.38%, and the signal-to-noise ratio increases by 29 dB. Further, we derived the analytic expression of the distortion component and verified it by experiments. This technique may be potentially applied to a long-distance large-scale distributed fiber-optic interferometric sensor array.

Evaluating the early-age behaviour of full-scale prestressed concrete beams using distributed and discrete fibre optic sensors

This paper evaluates the results of a monitoring study that captures the early age behaviour of four 11.9m prestressed concrete bridge beams utilising both distributed and discrete fibre optic sensor (FOS) arrays. The performance of the beams is evaluated before they are placed in-service as part of new concrete railway bridges in the Midlands in the UK. Two types of prestressed beams were monitored, two TY7 internal beams and two TYE7 edge beams. The beams incorporated high strength (up to 90.7MPa) self-consolidating concrete. The entire manufacturing process which included early-age curing and the detensioning process was captured in great detail using the installed FOS system. An analysis of the curing strains within the beams revealed the significant effect that ambient temperature, curing duration, and formwork restraint has on the development of prestress losses prior to detensioning. Based on the distributed FOS readings, it was observed that the strain remained uniform along the length of the beams during the various beam monitoring stages. The measured strain data was then used to calculate prestress losses in the first six months after casting (prior to casting of the in-situ concrete bridge deck). The TY7 and TYE7 beams experienced losses that were 79% and 72% of the ultimate losses predicted using Eurocode 2 equations, respectively. Distributed strain measurements were used to provide estimates of the change in beam camber with time. The pre-camber values calculated using the recorded FOS strain data at the time of detensioning closely match the theoretically calculated values. However, camber values increased by up to 1.7 times in the first six months compared with the post-detensioning values and deviated significantly from the theoretically calculated values. The future aim of this research is to establish integrated FOS systems as viable tools for monitoring strain evolution in concrete bridges in order to establish comprehensive baselines to facilitate long term data-driven bridge monitoring programmes.

Investigation of the dynamic strain responses of sandstone using multichannel fiber-optic sensor arrays

In the process of rock mechanical experiments, strain-response measurement is a most fundamental and most essential procedure for geomechanical researchers. The main objective of this paper is to point out the feasibility and the superiority of the application of a novel multichannel fiber Bragg grating (FBG) sensor arrays for dynamic strain-response measurements of cylindrical specimen subjected to uniaxial compression. The principle, design, and embedment of multichannel FBG sensors used in the experiment are briefly described. To fully monitor the strain history of the sandstone cylinder in uniaxial compression, six circumferential FBG sensors, four lateral FBG sensors, linear variable differential transformers (LVDT) built-in machine have been utilized for spatially monitoring small radial and axial strains along the height of the specimen, respectively. The experimental results indicate that the proposed FBG sensors can successfully provide a full-field view of the surface strains, as well as detect the potential crack locations within the specimen, and strains measured by multichannel FBG sensors are in good agreement with the results of LVDT, especially in the axial strains. Hence, it could be inferred that multichannel FBG sensor arrays are capable of measuring dynamic strain responses of sandstone specimen in multistage compression, which would greatly strengthen experimental basis for further application and theoretic research of in-situ field monitoring.

Low-Crosstalk and Polarization-Independent Inline Interferometric Fiber Sensor Array Based on Fiber Bragg Gratings

In this paper, a low-crosstalk and polarization-independent inline interferometric optical fiber sensor array with four time division multiplexing (TDM) channels was demonstrated. The low level of the multireflection (MR) induced crosstalk was realized by the ultra-weak ( $ 1%) fiber Bragg gratings (FBGs). The FBGs were fabricated in a single-mode fiber without any fusion points with a pulling force and a phase mask matching method. A polarization switching (PS) method based on the phase-generated carrier (PGC) scheme was proposed to eliminate the polarization-induced signal fading and the input-polarization-induced phase noise simultaneously. The PGC-based PS method was validated and the MR-induced crosstalk was tested in the experiment. The equivalent visibilities of the 4-TDM sensors separately range from 0.9 to 0.92 for sensor 1, from 0.95 to 0.96 for sensor 2, from 0.97 to 0.98 for sensor 3, and from 0.87 to 0.88 for sensor 4 for multiple measurements and the phase noise induced by the polarization fluctuation of the leading fiber was eliminated effectively. The background noise could reach $-$ 97 dB/ $\sqrt{\text{Hz}}$ at 1 kHz. The crosstalk level lower than $-$ 45 dB was achieved for the 4-TDM sensor array. The PGC demodulation scheme is first combined with the PS method using a complexing-exponent synthesis skill which can be used in the heterodyne interrogation scheme as well. Besides, though there are only four TDM channels in the study, the low crosstalk about $-$ 40 dB can be achieved if the TDM channels are expanded to five using the ultra-weak FBGs. The ultra-weak FBGs to reduce the crosstalk is especially suitable for the high-resolution applications of short-distance fiber sensing system such as the towed-array.

A Fiber-Optic System for the Quasi-Distributed Detection of Liquids

This letter presents a multiplexed fiber-optic sensor array suitable for detection of liquid intrusions, spills, and leaks. The proposed detection systems is based on active heating and observation of temperature changes in short sections of optically absorbing (vanadium-doped) optical fibers that were interconnected into a sensing array by a standard single-mode optical fiber. The heating of the vanadium-doped fiber sections was achieved by application of a common 980-nm pump laser source. The associated signal processing and integration utilizes a simple, custom-designed optical time domain reflectometry that relies on only a few low-cost standard telecom components.

Development of a novel proton dosimetry system using an array of fiber-optic Cerenkov radiation sensors

This study describes the development and evaluation of a new dosimetric system for proton therapy using an array of fiber-optic Cerenkov radiation sensors (AFCRS). The AFCRS was superior to a conventional, multi-layer ion chamber (MLIC) system in real-time data acquisition and cost effectiveness.