Virtual sensing in an onshore wind turbine tower using a Gaussian process latent force model

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

With the rapid development of transportation infrastructure in China, continuous arch tunnels passing under existing operational railways are increasingly encountered. This paper primarily investigates the mechanical response patterns of the lining of continuous arch tunnels under the vibrational loads generated by trains traveling on railways. To this end, a three-dimensional finite element model of the dynamic response of continuous arch tunnels under train vibrational loads was constructed using Midas GTS/NX software. This model explores the patterns of acceleration, dynamic displacement, and dynamic strain responses of continuous arch tunnels under the action of moving train loads. The study reveals that under train vibrational loads, the vertical acceleration, vertical displacement, and strain time history curves at various monitoring points in the continuous arch tunnel exhibit vibrational characteristics. The acceleration, vertical displacement, and dynamic strain increase as the train enters, fluctuate within a certain range, and then decrease to zero as the train exits. Under high-speed train loads, the acceleration peak values at the arch feet and outer sidewalls of the left and right tunnels are larger, with a greater peak value difference. The vertical displacement peak values at the arch shoulders and crown are larger, whereas those at the middle partition wall and base slab are smaller. During the process from the train's entry to its exit, the dynamic strain exhibits a trend of initially increasing, then stabilizing, and finally decreasing to zero, all while displaying vibrational characteristics. The strain response on the tunnel side where the train enters is greater than that on the side where it exits.

To study the real dynamic response regularity of asphalt pavement structure under dynamic load, a road test was conducted for three types of typical asphalt pavement. Dynamic and static deflection basins were established. The characteristics of the dynamic deflection basin were studied. The set-in dynamic strain sensors at the bottom of the surface layer collected the dynamic strain response under falling weight deflectometer (FWD) load. Results show that the static deflection basin is deeper than the dynamic deflection basin, but the range of the latter is larger than that of the former. The characteristics of the two deflection basins differ. The change trends of dynamic response with an increase in FWD load reflect nonlinear characteristics. The dynamic strain of three typical asphalt pavements at the bottom of the surface layer does not induce fatigue failure in the pavement structure. The longitudinal strain is larger than the lateral strain. Different prediction models on dynamic strain on the basis of dynamic deflection basin parameters are established.

This paper introduces a novel fiber-based sensor for dynamic strain and temperature measurements in the stator of rotating electrical machines. Fiber Bragg grating sensors were integrated into carbon fiber reinforced polymer. The encapsulation has appropriate mechanical energy transfer to measure dynamic strain and temperature. The proposed sensor is installed in the stator of the induction motor allowing to analyze the dynamic strain generated by the machine. The main failure in induction motors is attributed to bearing failures. In this work, the tests were performed operating at full load with new and defective bearings. The results show the main peaks of frequency caused by strain observed coincide with the electrical supply frequency (60 Hz) and mechanical bearing forces (30 Hz), with harmonic components at 120, 180, and 240 Hz. However, for the damaged bearings frequencies of 89.8, 116.5, 146.4, and 149.7 Hz were identified. These frequency components were generated by defects in the outer race, rolling element and inner race. The fundamental components of 30 and 60 Hz between the new and damaged bearings presents an increase of more than 75 %. Results show the potential of the proposed sensor in the monitoring of temperature and dynamic strain in electrical machines.

Theoretical and experimental kineto elastodynamic analysis of high speed linkage

An instrument for simultaneous measurement of dynamic strain and temperature in a thermally unstable ambience has been proposed, based on fiber Bragg grating technology. The instrument can function as a compact and stand-alone broadband thermometer and a dynamic strain gauge. It employs a source wavelength tracking procedure for linear dependence of the output on the measurand, offering high dynamic range. Two schemes have been demonstrated with their relative merits. As a thermometer, the present instrumental configuration can offer a linear response in excess of 500 °C that can be easily extended by adding a suitable grating and source without any alteration in the procedure. Temperature sensitivity is about 0.06 °C for a bandwidth of 1 Hz. For the current grating, the upper limit of strain measurement is about 150 µε with a sensitivity of about 80 nε Hz−1/2. The major source of uncertainty associated with dynamic strain measurement is the laser source intensity noise, which is of broad spectral band. A low noise source device or the use of optical power regulators can offer improved performance. The total harmonic distortion is less than 0.5% up to about 50 µε,1.2% at 100 µε and about 2.3% at 150 µε. Calibrated results of temperature and strain measurement with the instrument have been presented. Traces of ultrasound signals recorded by the system at 200 kHz, in an ambience of 100–200 °C temperature fluctuation, have been included. Also, the vibration spectrum and engine temperature of a running internal combustion engine has been recorded as a realistic application of the system.

Fatigue assessment in offshore wind turbine support structures requires the monitoring of strains below the mudline, where the highest bending moments occur. However, direct measurement of these strains is generally impractical. This paper presents the validation of a virtual sensing technique based on the Gaussian process latent force model for dynamic strain monitoring. The dataset, taken from an operating near-shore turbine in the Westermeerwind Park in the Netherlands, provides a unique opportunity for validation of strain estimates at locations below the mudline using strain gauges embedded within the monopile foundation. KeywordsOffshore wind turbinesVirtual sensingBayesian inferenceGaussian process

Offshore wind turbines (OWTs) are subjected to both quasi‐static loads originating from variations in the thrust force and dynamic loads linked to turbulence, waves and turbine dynamics. Both types of loads contribute to fatigue life progression and thus define the turbine's age. As a structural health monitoring solution, one could thus directly measure the stress history at fatigue critical locations. However, for OWTs on monopile foundations some fatigue critical locations are located below the seabed. Installing strain sensors at these hotspots is therefore impossible for existing wind turbines. This measurement restriction is overcome by reconstructing the full‐field response of the structure based on the limited number of accelerometers and strain sensors (installed at a few easily accessible locations) and a calibrated finite element model of the system. The system model uses a multi‐band modal expansion approach constituted of the quasi‐static and dynamic contributions. These contributions are superimposed to reconstruct the stress history at all degrees of freedom of the finite element model, and the subsequent assess fatigue life consumption at all fatigue hot spots of the OWT. In this paper, the proposed virtual sensing technique is validated by predicting the stresses in the transition piece with 12 days of consecutive measurements from an operational OWT. The data set contains both variations in environmental and operating conditions as well as extreme events. Finally, a full‐field strain assessment in the tower and foundation system of the OWT is demonstrated. Copyright © 2017 John Wiley & Sons, Ltd.

The development of a fiber laser sensor system which permits efficient interrogation of Bragg grating sensors is reported. A tunable erbium doped fiber laser which utilizes a broadband mirror and an intracore Bragg grating reflector in side-pump configuration is described. The wavelength of the laser oscillation is determined by the Bragg grating, which is remotely located and used as a strain sensor. This arrangement is used in conjunction with a passive wavelength demodulation system (WDS) to form a self-contained fiber laser strain sensor system, allowing efficient interrogation of the Bragg sensor. This device provides interrupt-immune sensing of static and dynamic strains with a bandwidth of 13.0 kHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A fiber Bragg grating can be worked as reflector, it is constructed in a short section of optical fiber that reflects light of specific wavelength and allow to pass remaining wavelengths. This is possible by creating a periodic change in refractive index within the fiber core. The change in the modal index of fiber caused due to strain results in a Bragg wavelength. This work describes performance of Fiber Bragg Gratings as a strain sensor at 1550nm wavelength is considered for simulation, corresponding dynamic strain and wavelength shifts are analysed. This performance analysis is useful especially for intrusion detection system where environments are harsh. This sensor is compatible with data communication system and can be used for remote sensing.

The study of tensoresistive characteristics of p-type conductivity GaP whiskers with [111] crystallographic orientation that is coincide with the direction of the maximal piezoresistive effect for this material, was carried out. The technology of creating the ohmic contacts to GaP whiskers that allows to use these crystals at high temperatures (400 – 6000C) was developed. Tensoresistive characteristics of p-type conductivity GaP whiskers were studied in the strain rang ± 1.2 × 10-3 rel. un. These studies show that the gauge factor for the whiskers with resistivity 0.025 – 0.03 Ohm×cm 90÷95 at 200C. The study of tensoresistive properties show that in the temperature range +20 ÷ +3000C for p-type conductivity GaP whiskers with resistivity ρ=0.01 – 0.03 Ohm×cm the gauge factor decreased as the temperature rise, but the temperature range 300÷ 5500C for this crystals very slight temperature dependence of the gauge factor was observed. In this temperature range the temperature coefficient of gage factor is no more than –0.03 %×grad-1. In the temperature range 300 ÷ 5000C the value of gage factor for this whiskers is high and it equals 40÷50. It could be noticed that in the all investigate temperature range the strain sensors on the basis of p-type conductivity GaP whiskers have the linear dependence resistance vs strain in the strain range ± 5.0× 10-4 rel. un. Developed strain sensors on the basis of p-type conductivity GaP whiskers have high mechanical strength at the static and dynamic strain (mire then 108 cycles), that show their possibility to operate in dynamic mode.

We have fabricated ultra-thin piezoelectric strain sensors 5×5 array on polyethylene naphthalate (PEN) flexible printed circuit (FPC) with Cu wiring. Ultra-thin PZT(2 μm)/Si(3 μm) plates (5 mm-long and 1 mm-wide) were fabricated by MEMS microfabrication technique. The fabricated ultra-thin PZT/Si plates were transferred onto the FPCs by a chip mounter with a specially designed rubber collet, where an adhesive layer is screen-printed on the FPCs. Twenty five of ultra-thin PZT/Si plates were arrayed by the transferring process. After that, the electrode pads on the PZT and the Cu wiring on FPCs were connected with Ag paste by screen printing. The fabricated piezoelectric dynamic strain sensors shows 0.16 mV/μe of sensitivity, which is similar to that of conventional strain gauges.

Strain sensors based on conductive elastomers face challenges like baseline drift and noise due to inherent viscoelasticity and weak electrode interfaces under dynamic strains. Herein, a synergistic structure with biphasic hierarchical networks and stable electrode interfaces is proposed to address these issues. The sensor employs a multilayer structure with polydimethylsiloxane (PDMS) substrate, carbon nanotube-doped PDMS (CNT-PDMS), and Ag film. Electrodes are fixed using a rigid island reinforced mortise and tenon joint formed with PDMS and CNT-PDMS. The Ag film dominates resistance during release, significantly reducing baseline drift. Strain-insensitive electrode interfaces further reduce baseline drift and noise. This optimized design ensures 99.999% resistance recovery without delay, even at high-speed (800 mm/min) and large (80%) strains. The sensor exhibits a high gauge factor of 55442, low detection limit (0.02%), and excellent stability (5000 cycles). With the designed algorithms, the single-channel sensor achieves 98.2% decoding accuracy for various gestures, demonstrating great potential for wearable electronics.

Mechanical and electrical forces of a three-phase induction motor (TIM), in operation, can cause strain in the stator, which may lead to frustrating vibration problems. These problems may greatly reduce reliability and ultimately lead to increased maintenance cost. This paper shows the method to overcome this problem by carrying out the dynamic strain measurement analysis in a TIM running at no-load condition, using an optical fiber sensor. The dynamic strain induced by mechanical and electrical forces was measured by using fiber Bragg gratings (FBGs), installed inside the motor, in between two stator teeth. The electromagnetic immunity and reduced size of the FBGs render them suitable for this application. The frequency components of the strain in TIM were predicted using a model of the forces present in the air gap, considering static and dynamic rotor eccentricities. Two TIM motors, one of 2-poles and the other of 4-poles, and each equipped with two FBGs strain sensors, were used. The sensors were positioned between two stator teeth, separated by 120° from each other in the 2-pole motor and 90° in the 4-pole motor. The experimental analysis shows that the frequency components of the dynamic force spectrum presented by the two motors are the same as those predicted theoretically. Rotor eccentricity was achieved by attaching an unbalanced load to the rotor shaft. The amplitude of vibration at rotor rotational frequency increased when the unbalanced load on the rotor shaft increased. The measurement presented in this paper can be used to determine some TIM parameters, such as the effect of the motor power supply on its vibration signal. It can also be used as a potential maintenance tool for the continuous monitoring of the TIM with high sensitivity.

We have proposed and constructed a system to multiplex fiber Bragg grating (FBG) strain sensors with same reflection wavelength, adopting the synthesis of optical coherence function technique (SOCF), which has realized dynamic strain measurement. The ability to control the coherence peak position of SOCF allows random access in the spatial domain. In this paper, the function of random access strain measurement has been realized by switching the coherence peak position at every FBG, synchronizing with FBG spectrum measurement, to use the sampling rate to the full. In addition, we have proposed and confirmed the novel method for improving FBG interval by simply modulating the LD intensity.