Optical Strain Sensor Research Articles

力を可視化できるキラル液晶エラストマーの動的挙動制御

Strain sensors are a key for the development of next-generation soft robots. For such applications, the electronic sensing using flexible electronic materials are continuing to be at the forefront of the sensing technology. Recently, optical-sensing technology have attracted much attention because of high spatiotemporal resolution, high sensitivity and noninvasively. Especially, chiral-nematic liquid-crystalline elastomers have much attention to be applied to optical strain sensors because of the change of selective reflection wavelength in response to the strain. Here, we introduce our recent research on optical strain sensors using chiral-nematic liquid-crystalline elastomers. There is remarkable maturity in the development of numerous stimuli-responsive liquid-crystalline elastomers. However, the control of the recovery process after the removal of the stimuli has remained difficult. Interestingly, our simple materials design concept of “layering the elastomers with other materials” allows us to arbitrary control the recovery process in both macroscopic deformation and microscopic molecular orientation. Based on the concept, we have succeeded to fabricate a mechano-optical sensor with high sensitivity and high spatiotemporal resolution. This concept would open a pathway to control the recovery process in various stimuli-responsive liquid-crystalline elastomers.

Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring

Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring

Stretchable reflective coating for soft optical waveguides and sensors

Soft robots must embody mechanosensing capabilities to merge with and act in the environment. Stretchable waveguides are making a mark in soft mechanical sensing since they are built from pristine elastomers. Therefore, they are insensitive to electromagnetic fields and weakly affect the deformations of the robot. However, issues in light-shielding, signal decoupling, and core-cladding interfaces are still open challenges. In this work, titanium oxide particles (TiO2) are dispersed in silicone elastomers to develop a soft optical shield coating. Results show that the added particles do not harden the matrix and reduce light transmission. Almost full NIR shielding is achieved by adding 1.0 vol% of TiO2 in 150 μm thick films. These properties make the proposed shielding coating an excellent candidate for soft mechanosensing. An open-access tool is developed to design soft optical devices by programming light transmittance at desired wavelengths by tuning, both, TiO2 concentration and film thickness. Finally, two proof-of-concepts are demonstrated, a soft waveguide and a soft strain sensor, by integrating the developed material to shield a transparent PDMS resin and a semi-transparent Ecoflex00-10 matrix, respectively. The soft waveguide can stretch up to 40% with very low optical loss, while the optical strain sensor can detect strain up to 90%. In both cases, bending, folding, and indentation of the devices have a significantly low impact on light transmission. These results can pave the way to design new optical transmission devices and sensors that exploit light reflection and that allow for discriminating different types of mechanical stimuli in soft robots.

The Effect of Fault Architecture on Slip Behavior in Shale Revealed by Distributed Fiber Optic Strain Sensing

AbstractWe use Distributed Strain Sensing (DSS) through Brillouin scattering measurements to characterize the reactivation of a fault zone in shale (Opalinus clay), caused by the excavation of a gallery at ∼400 m depth in the Mont Terri Underground Laboratory (Switzerland). DSS fibers are cemented behind casing in six boreholes cross‐cutting the fault zone. We compare the DSS data with co‐located measurements of displacement from a chain potentiometer and a three‐dimensional displacement sensor (SIMFIP). DSS proves to be able to detect in‐ and off‐fault strain variations induced by the gallery excavated 30–50 m away. The total permanent displacement of the fault is ∼200 microns at rates up to 1.5 nm/s. DSS is sensitive to longitudinal and shear strain with measurements showing that fault shear is concentrated at the top and bottom interfaces of the fault zone with little deformation within the fault zone itself. Such a localized pattern of strain relates to the architecture of the fault that is characterized by a thick “layer” including an anastomosing network of polished surfaces where clay‐rich rock splits into progressively smaller flakes conferring the entire zone very little cohesion and friction (weak zone), and favoring slipping at the edges of the “layer”. Overall, DSS shows that slow slip may activate everywhere there is a weak fault within a shale series. Thus, our work demonstrates the importance of shear strain on faults caused by remote loading, highlighting the utility of DSS systems to detect and quantify these effects at large reservoir scales.

Highly sensitive and fast response strain sensor based on evanescently coupled micro/nanofibers

Flexible strain sensors play an important role in electronic skins, wearable medical devices, and advanced robots. Herein, a highly sensitive and fast response optical strain sensor with two evanescently coupled optical micro/nanofibers (MNFs) embedded in a polydimethylsiloxane (PDMS) film is proposed. The strain sensor exhibits a gauge factor as high as 64.5 for strain ≤ 0.5% and a strain resolution of 0.0012% which corresponds to elongation of 120 nm on a 1 cm long device. As a proof-of-concept, highly sensitive fingertip pulse measurement is realized. The properties of fast temporal frequency response up to 30 kHz and a pressure sensitivity of 102 kPa⁻¹ enable the sensor for sound detection. Such versatile sensor could be of great use in physiological signal monitoring, voice recognition and micro-displacement detection.

Fault diagnosis of gear transmissions via optic Fiber Bragg Grating strain sensors

Most of the diagnostics approaches for health monitoring of gear transmissions, to date, are based on vibration analysis. A comprehensive research referring to applications of optic Fiber Bragg Grating (FBG) strain sensors for gear diagnostics is presented. The paper opens with a detailed literature review about fault diagnosis of gear transmission based on strain measurements. It is shown that diagnosis of spur and helical gears is feasible and yields similar capabilities compared to acceleration analysis. A novel diagnostic methodology based on FBG strain sensors is developed, justified, and experimentally validated for both gear types at different operating conditions, e.g., speed and load, and for local faults of different severities. The proposed methodology covers all the diagnostics stages, starting with a unique flow of signal processing, examination of condition indicators and finally, a definition of a Health Indicator (HI).

Abstract 13734: Biomechanical Comparison of Atrioventricular Valve Repair Strategies in Hypoplastic Left Heart Syndrome

Purpose: Atrioventricular valve (AV) regurgitation in hypoplastic left heart syndrome (HLHS) remains a challenging problem with no clear consensus on optimal repair strategies. We sought to evaluate the effect of AV repairs on leaflet biomechanics. Methods: Explanted fetal bovine tricuspid valves and right ventricles were sutured onto annular and ventricular mounts derived from imaging data of HLHS patients. The annulus underwent an average areal dilation of 21% with preferential dilation of the free wall (Fig 1A). The specimens were then placed in a previously validated univentricular heart simulator and subjected to the typical flows and pressures seen at Norwood, Glenn, early Fontan, and late Fontan stages. The valves underwent repair via De Vega suture annuloplasty (AP) or anteroseptal commissuroplasty (CP). Leaflet forces pre- and post-repair were measured using optical strain sensors (fiber Bragg grating) sutured to each leaflet edge. Statistical significance was determined using paired t-test. Results: A total of 8 AP and 7 CP specimens underwent testing. Regurgitation decreased from 2+ to <1+ following both repairs. With AP, forces in the tricuspid valve anterior (TVAL), posterior (TVPL), and septal leaflets (TVSL) as a percentage change from pre-repair were [TVAL:-12.6%, p<0.001 Conclusions: Force reduction was seen only in the anterior and posterior leaflets with CP in contrast to the global reduction seen in AP. This difference may compound with cardiac cycling and impact the longevity of the repair. Future studies with more complex pathologies and repair strategies can help develop more tailored techniques.

Long-Term Monitoring of a Tunnel in a Landslide Prone Area by Brillouin-Based Distributed Optical Fiber Sensors.

This paper shows the results of the monitoring of the deformations of a tunnel, carried out using a distributed optical fiber strain sensor based on stimulated Brillouin scattering. The artificial tunnel of the national railway crosses the accumulation zone of an active landslide, the Varco d’Izzo earthflow, in the southern Italian Apennines. Severely damaged by the landslide movements, the tunnel was demolished and rebuilt in 1992 as a reinforced concrete box flanked by two deep sheet pile walls. In order to detect the onset of potentially dangerous strains of the tunnel structure and follow their time trend, the internal deformations of the tunnel are also monitored by a distributed fiber-optic strain sensor since 2016. The results of the monitoring activity show that the deformation profiles are characterized by strain peaks in correspondence of the structural joints. Furthermore, the elongation of the fiber strands crossing the joints is consistent with the data derived by other measurement systems. Experiments revealed an increase in the time rate of the fiber deformation in the first and last part of the monitoring period when the inclinometers of the area also recorded an acceleration in the landslide movements.

Moving Load Identification with Long Gauge Fiber Optic Strain Sensing

Moving load identification has been researched with regard to the analysis of structural responses, taking into consideration that the structural responses would be affected by the axle parameters, which in its turn would complicate obtaining the values of moving vehicle loads. In this research, a method that identifies the loads of moving vehicles using the modified maximum strain value considering the long-gauge fiber optic strain responses is proposed. The method is based on the assumption that the modified maximum strain value caused only by the axle loads may be easily used to identify the load of moving vehicles by eliminating the influence of these axle parameters from the peak value, which is not limited to a specific type of bridges and can be applied in conditions, where there are multiple moving vehicles on the bridge. Numerical simulations demonstrate that the gross vehicle weights (GVWs) and axle weights are estimated with high accuracy under complex vehicle loads. The effectiveness of the proposed method was verified through field testing of a continuous girder bridge. The identified axle weights and gross vehicle weights are comparable with the static measurements obtained by the static weighing.

Evaluating in-service structural behaviour of an operational railway bridge using fibre optic sensing and structural model updating

Multi-girder prestressed concrete bridges are a very common structural form for bridges around the world. Their continued use is expected in the future given the advantages these structures have with respect to speed of construction, reduced maintenance and improved material efficiency. However, the in-service behaviour of these types of structures, such as their load distribution characteristics, support boundary conditions and the contribution of secondary elements, is still not well understood, and few studies have investigated these areas in depth. Gaining better insight into these aspects of bridge behaviour will enable more realistic structural assessment and thus facilitate more targeted maintenance and refurbishment of these bridges. This paper evaluates the in-service structural behaviour of an operational prestressed concrete railway bridge, which has been monitored using a dense network of fibre optic strain and temperature sensors since it was constructed in 2015. In particular, the normalised curvature profiles and the neutral axis positions across the composite solid slab bridge deck may be used to extract information about the bridge’s transverse load distribution behaviour, support boundary conditions and the stiffness contribution of secondary elements. Using this information, structural model validation and updating were performed with the goal of providing a more realistic assessment of structural behaviour and structural capacity. The proposed methodology and workflow in this study provide new insights into the real performance of multi-girder prestressed concrete bridges and thus can be used to improve their operation, maintenance, assessment and refurbishment.

A highly stretchable optical strain sensor monitoring dynamically large strain for deformation-controllable soft actuator

In this paper, we report a highly stretchable optical strain sensor that can be compatible with artificial muscle and wearable human-assistive devices. The optical strain sensor is fabricated by embedding a polymeric optical fiber thermoformed for bi-tortuous structure in silicone elastomer. The strain sensor can detect uniaxial strain using amplitude of output light intensity that increases as curvature of the curved regions becomes enlarged with uniaxial strain. Due to the benefits from curvilinear design of the optical fiber and sensor structure with the elastomeric matrix securing geometric recovery from being elongated, the strain sensor is capable of reliable detection of arbitrary tensile strain in a wide range from 0% to 120%. It possesses fast, reversible, and durable strain-sensing performance with a small hysteresis even under continuative cyclic loadings of large strain. As integrated into a soft rolled-actuator based on acrylic elastomer, we finally demonstrate the strain-sensor performance is enough to monitor dynamic deformation response of the soft actuator.

RETRACTED ARTICLE: Optical fiber bi-directional strain sensor based on coreless fiber

RETRACTED ARTICLE: Optical fiber bi-directional strain sensor based on coreless fiber

Experimental field study of floater motion effects on a main bearing in a full-scale spar floating wind turbine

In this paper we present a full-scale experimental field study of the effects of floater motion on a main bearing in a 6 MW turbine on a spar-type floating substructure. Floating wind turbines are necessary to access the full offshore wind power potential, but the characteristics of their operation leave a gap with respect to the rapidly developing empirical knowledge on operation of bottom-fixed turbines. Larger wind turbines are one of the most important contributions to reducing cost of energy, but challenge established drivetrain layouts, component size envelopes and analysis methods. We have used fibre optic strain sensor arrays to measure circumferential strain in the stationary ring in a main bearing. Strain data have been analysed in the time domain and the frequency domain and compared with data on environmental loads, floating turbine motion and turbine operation. The results show that the contribution to fluctuating strain from in-plane bending strain is two orders of magnitude larger than that from membrane strain. The fluctuating in-plane bending strain is the result of cyclic differences between blade bending moments, both in and out of the rotor plane, and is driven by wind loads and turbine rotation. The fluctuating membrane strain appears to be the result of both axial load from thrust, because of the bearing and roller geometry, and radial loads on the rotating bearing ring from total out-of-plane bending moments in the three blades. The membrane strain shows a contribution from slow-varying wind forces and floating turbine pitch motion. However, as the total fluctuating strain is dominated by the intrinsic effects of blade bending moments in these turbines, the relative effect of floater motion is very small. Mostly relevant for the intrinsic membrane strain, sum and difference frequencies appear in the measured responses as the result of nonlinear system behaviour. This is an important result with respect to turbine modelling and simulation, where global structural analyses and local drivetrain analyses are frequently decoupled.

Input Torque Measurements for Wind Turbine Gearboxes Using Fiber Optical Strain Sensors

Abstract. Accurate knowledge of the input torque in wind turbine gearboxes is key to improving their reliability. Traditionally, rotor torque is measured using strain gauges bonded to the shaft. Transferring the resulting signal from the rotating shaft to a stationary data acquisition system while powering the sensing devices is complex and costly. The magnitude of the torques involved in wind turbine gearboxes and the high stiffness of the input shaft pose additional difficulties. This paper presents a new alternative method to measure the input torque in wind turbine gearboxes based on deformation measurements of the static first stage ring gear. We have measured deformation using fiber optic strain sensors based on fiber Bragg gratings because of their advantages compared to conventional electrical strain gauges. The present study was conducted on a Siemens Gamesa Renewable Energy gearbox with a rated power of 6MW, in which a total of 54 fiber optic strain sensors were installed on the outer surface of the first stage ring gear. The gear mesh forces between the planets and the ring gear cause measurable deformations on the outer surface of the stationary ring gear. The measured strains exhibit a dynamic behavior. The strain values change depending on the relative position of the strain sensors to the planet gears, the instantaneous variations of the input torque, and the way load is shared between planets. A satisfactory correlation has been found between the strain signals measured on the static ring gear and torque. Two signal processing strategies are presented in this paper. The first procedure is based on the peak-to-peak strain values computed for the gear mesh events, and therefore, torque can only be estimated when a gear mesh event is detected. The second signal processing procedure combines the strain signals from different sensors using a Coleman coordinate transformation and tracks the magnitude of the fifth harmonic component. With this second procedure, it is possible to estimate torque whenever strain data of all sensors is available, leading to an improved frequency resolution up to the sampling frequency used to acquire strain data. The method presented in this paper could make measuring gearbox torque more cost-effective, which would facilitate its adoption in serial wind turbines and enable novel data-driven control strategies, as well as a more accurate assessment of the consumed fatigue life of the gearboxes throughout their operation.

Distributed Fiber Optic Strain Sensing for Geomechanical Monitoring: Insights from Field Measurements of Ground Surface Deformation

In recent years, distributed fiber optic strain sensing (DFOSS) technology has demonstrated a solution for continuous deformation monitoring from subsurface to surface along the wellbore. In this study, we installed a single-mode optical fiber cable in a shallow trench to establish an effective technique for ground surface deformation mapping. We conducted three experimental field tests (iron plate load, water tank filling up load, and airbag inflation) in order to confirm the strain sensitivity of DFOSS for static loads, dynamic overload, excavation, subsidence, and uplift. This paper also presents two installation methods to couple the fiber cable with the ground under various environmental conditions; here, the fiber cable was installed in a shallow trench with one part buried in the soil and another part covered with cement. Our results suggest that covering the cable with cement is a practical approach for installing a fiber cable for ground surface deformation monitoring. By combining this approach with wellbore DFOSS, accurate surface–subsurface deformation measurements can be obtained for three-dimensional geomechanical monitoring of CO2 storage and oil and gas fields in the future.

Self‐Powered Stretchable Mechanoluminescent Optical Fiber Strain Sensor

Strain sensors that can work sustainably and continuously without any external power supply are highly desirable for future wearable and implantable devices. Herein, a self‐powered stretchable strain sensor based on the integration of mechanoluminescent phosphors with an elastomer optical fiber is proposed and developed. This mechanoluminescent optical fiber is capable of emitting light just driven by external strain, without the need of an external light source or electric power. The strain‐induced emitted light can be collected and guided along the mechanoluminescent optical fiber. The sensor exhibits linear strain response up to 50% and high‐accuracy strain measurement (±1%). Moreover, this optical fiber strain sensor displays consistent signals over 10 000 stretch–release motion cycles, which demonstrates the good durability of the sensor. Due to the excellent light confinement of the elastomer optical fiber, this strain sensor is demonstrated in both bright‐ and dark‐field measurements, wearable gloves, and an implantable sensing device, thereby demonstrating potential as a promising technology for future self‐powered optical sensor systems.

Effects of bonding on the performance of optical fiber strain sensors

Effects of bonding on the performance of optical fiber strain sensors

Laboratory Investigation of the Structural Performance of a Corrugated Steel Culvert under Increasing Cover Depth

The performance of large diameter corrugated steel culverts/pipes under varying cover depth up to deep burial has previously not been investigated in a laboratory setting due to limitations associated with the application of overburden pressures. Using the new deep burial simulator at Queen’s University, it is now possible to test flexible pipes up to 3 m in diameter up to a simulated burial depth of approximately 17 m. In this study, a 2-m diameter corrugated steel pipe (CSP) culvert was instrumented with displacement transducers, strain gauges, and distributed fiber optic strain sensors and was tested under increasing overburden pressure up to a total of 358 kPa (equivalent to a depth of 17.9 m of soil with a unit weight of 20 kN/m3). The diameter change values along the length of the culvert were within 10% of each other, suggesting that the simulator produces approximately uniform loading. The Canadian bridge design code produced thrust estimates within 10% of the measured response but did not predict the development of localized plastic hinges seen in the experiment.

An extension of the strain transfer principle for fiber reinforced materials

Fiber optical strain sensors are used to measure the strain at a particular sensor position inside the fiber. In order to deduce the strain in the surrounding matrix material, one can employ the strain transfer principle. Its application is based on the assumption that the presence of the fiber does not impede the deformation of the matrix material in fiber direction. In fact, the strain transfer principle implies that the strain in fiber direction inside the fiber carries over verbatim to the strain inside the matrix material. For a comparatively soft matrix material, however, this underlying assumption may not be valid. To overcome this drawback, we propose to superimpose the matrix material with a one-dimensional model of the fiber, which takes into account its elastic properties. The finite element solution of this model yields a more accurate prediction of the strain inside the fiber in fiber direction at low computational costs.

Recent developments in polymer optical fiber strain sensors: A short review

Polymer optical fiber (POF) strain sensors have attracted increasing attention owing to the unique features of polymer over silica such as lower Young's modulus, larger elastic strain limit, higher fracture toughness, biocompatibility, and lower production cost. Several POF strain sensors have been developed in recent years for applications in various technological fields, including structural health monitoring in civil construction and aerospace industries, biomedicine, and robotics. The principles of operation, prospects, and challenges of the POF strain sensors are discussed in this article under five broad categories: POF fiber Bragg grating sensors, intensity-based POF sensors, multimodal interference-based sensors, Brillouin-frequency-based sensors, and Fabry–Perot cavity sensor. This review aims to highlight areas where further research is required for improving the performance and operating range of POF strain sensors.