Fiber Bragg grating strain sensors in prototype testing

FBG strain sensors in prototype testingStrain measurement is imperative during prototype design and testing. Strain measurements ensure that materials perform as they should and that the equipment is safe and durable. Measuring strain is crucial for testing complex structures, like aircraft, turbines, etc. There are multiple ways in which stress can be measured; however, it is widely accepted that FBG sensors are the most efficient way of strain measurement. FBG sensors provide multiple advantages over other methods: 1) high sensitivity; 2) small size; 3) ability to be mounted on any surface; 4) immunity to electromagnetic interference; 5) reliability even in harsh environments; 6) low sensitivity to vibration and heat.

FBG strain sensors are used in wing load testing to determine the structure’s performance and possible limitations under the lifting forces during flight. There are several benefits that FBG strain sensors offer to the aerospace field for this type of application. For instance, FBG technology provides strain measurement using a big number of continuous sensors which ensures maximum coverage from a single optical fiber. The immunity to electromagnetic interference, radio frequency interference, and other electrical influences makes FBG strain sensors ideal for use in hazardous environments, such as flight. Moreover, FBG sensors are less cumbersome to install. In addition, the FBG strain sensor prices are expected to decrease by 20% which will open up new opportunities for various markets.

The repeated loading and unloading of material causes fatigue. It is estimated that around 90% of structural failures are a result of fatigue. To determine the breaking point of a structure, fatigue tests are performed. The tests indicate the number of loading cycles until failure. FBG sensors are able to provide real-time data on strain fields and load distributions. The immunity to both low and high temperatures enables the sensors to be monitored during the high-temperature cure phase of composite fabrication. The fatigue life of an FBG strain sensor far surpasses that of other methods of strain measurement.

The tendency to build longer wind turbines with longer blades complicates the maintenance. The design of stronger, lighter materials is necessary, however small imperfections during the manufacturing process can cause failures. FBG strain sensors may provide real-time knowledge of load distributions and turbine blade shape. This data will provide valuable information on the needed adjustments to the blades manufacturing.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. Our main goal is to deliver the best quality fiber optic products to our clients. We produce a wide range of fiber optic devices, including our cutting edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems. If you have any question about FBG strain sensors, please contact as info@optromix.com

Fiber Bragg grating sensors in aerospace applications

FBG sensors in aerospace applicationsAll engineering infrastructures undergo aging. The damage that appears over time is a consequence of the loads applied to them, therefore regular maintenance actions are required in order to predict the lifetime of these structures and lengthen it, which, in turn, will aid in avoiding catastrophic failures. Aviation-related infrastructures demand the highest levels of damage detection as these structures are overwhelmingly built according to a damage-tolerant principle. The structures for aerospace are designed to withstand damages with certain characteristics. Damage-tolerant design involves considerable effort for structure inspection. The inspections and maintenance tasks need to be periodic and scheduled; these are essential for safe and efficient operations.

Automation of the inspection processes is a point of capital importance to reduce inspection efforts, as maintenance is time-consuming and costly. Fiber Bragg grating sensor systems can perform real-time inspection which leads to a reduction of maintenance costs and improved the reliability and performance of the structures. Therefore, there is great interest in developing FBG sensors from the industry and academia. The most promising sensors are FBG strain sensors, FBG temperature sensors, FBG accelerometers, etc.

The intrinsic capabilities of FBG sensors, such as insensitivity to electromagnetic radiation, lightweight, small size, high sensitivity and resolution, and, most importantly, their suitability to be embedded into structures, make them suitable for most aerospace applications.

Among different approaches to a maintenance system based on FBG sensors deployment are: 1) single-point sensors; 2) distributed sensing, including distributed temperature systems. Distributed temperature sensing (DTS) systems are optoelectronic devices that measure temperatures by means of optical fibers functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. High accuracy of temperature determination is accomplished over great distances.

The successful application of FBG sensors to aircraft requires the sensors to provide reliable and accurate information about the condition of the structure and to reduce economic losses caused by unproductive downtimes. At this moment, FBG sensors and FBG based systems, such as DTS, are the most appropriate solution to most aerospace needs.

Optromix will write a tailored fiber Bragg grating-based on your specific requirements, which can be used with any type of optical fiber sensor technology. We will make sure to analyze your fiber optic applications and deliver the most suitable solution.

If you would like to purchase FBGs, please contact us: info@optromix.com or +1 617 558 9858

 

Fiber Bragg grating sensors in chemical sensing

Chemical sensing is a developing field of fiber Bragg grating sensors. It is involved in the control of chemical processes, oil recovery, environment quality improvement. There are several factors that play a role in the implementation of chemical sensors – suitability for remote sensing, sensitivity, low costs, selectivity, ability to work consistently in harsh environments. The oil and gas applications require sensors that can withstand high temperatures and pressures, and environmental sensors must have a wide coverage area and limited signal fluctuations.

Fiber Bragg grating sensors have been used in numerous applications, including aircraft monitoring, structural health monitoring, strain, and temperature measurements. In the last decade, FBG sensors have been implemented as chemical sensors due to their ability to withstand high temperatures, immunity to electromagnetic interference, and the absence of electronics.

The main principle of FBG sensors for applications in chemical sensing is based on the axial strain of the fiber. The chemicals that surround the fiber causes it to shift and deform. The strain caused by the presence of a particular chemical causes the spectral pattern of reflected light to change; the shift in reflected wavelength is observed. To monitor the shift in wavelength FBG interrogators are used. The data gathered by the interrogator can then be converted to environmental concentrations.

The key component of fiber Bragg grating sensors for chemical sensing is a highly sensitive coating. The coating should be stiff and adhere easily and firmly to the glass fiber. Most often the coating is based on polymers. However, the coating needs to have a high modulus of elasticity that remains high at high temperatures and during analytes absorption. Most polymers are restricted by temperature due to softening and material relaxation.

Some ways of FBG sensors polymer coating include:

  1. improved coating processing;

The improvement of the coating process, like improved adhesion, may aid in the reduction of material relaxation and shifting. The glass fiber may be treated before the application of polymer coating for improved adhesion.

  1. improved data processing;

The use of two different coatings that respond in equal, but the opposite manner to environmental changes. The combination of the two responses limits the temperature drift.

  1. improved material properties.

Alternative coatings, like ones based on ceramic materials, may expand the temperature application range.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line: fiber Bragg grating sensors, FBG interrogators, and multiplexers, Distributed Temperature Sensing (DTS) systems. We create and supply a broad variety of top-notch fiber optic solutions for the monitoring of various facilities all over the world. If you would like to purchase FBG sensors, please contact us at info@optromix.com

Fiber optic aircraft monitoring

Fiber optic sensors are used in aircraft structural health monitoring as they possess numerous advantages for real-time monitoring like immunity to electromagnetic interferences, low weight, small size, and high bandwidth. The latter allows multiple sensors to be used simultaneously within the same system.

It is anticipated that air traffic will grow significantly in the forthcoming years. Most modern airplanes make use of composite materials instead of conventional aluminum alloys, therefore durability and safety issues arise due to the complexity of composite materials. The different materials that compose them react differently to different environmental changes, which results in unpredictable behavior. The systems that enable the opportunity to monitor the aircraft in real-time are essential for safety and reliability improvement and reduction of maintenance costs. Fiber optic aircraft monitoring is a promising way of improving efficiency and operation costs.

FBG sensors have already found a wide range of applications. Fiber Bragg grating sensors are used in the monitoring of composite structures during on-ground aircraft testing; some airplanes are already equipped with FBG sensors that provide real-time data during a flight. The data provided by the sensors is valuable for local damage detection, and fiber optic aircraft monitoring has proven to be beneficial to the field. The integration of FBG sensors into the composite materials would enable the monitoring of the material during its whole life cycle.

Among multiple types of sensors optical sensors, namely FBG sensors, are of particular interest for aircraft monitoring due to their high sensitivity and durability. The multiplexing capability of FBG sensors reduces the weight of the wiring. Moreover, fiber Bragg grating sensors are significantly less expensive than other sensors.

However, the technology is not fully developed yet and more effort is still needed to bring it to a mature level. Among the challenges of fiber optic aircraft monitoring is the development of methods to monitor structural parameters over large structures.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. If you would like to buy FBG sensors, please contact us at info@optromix.com

Fiber Optic Sensors for Vibration Monitoring

Vibration is a common phenomenon in nature and vibration monitoring technology is of significant importance in scientific measurements and engineering applications. Accurate measurement and monitoring of vibration are crucial for the detection of the abnormal events and pre-warning of infrastructure damage. Traditional vibration sensors suffer from electromagnetic (EM) interference, which presents the difficulty for applications in harsh environments. In addition, the short monitoring distance and high maintenance cost mean they do not meet the actual needs of modern engineering measurements.

Optical fibers have attracted a significant amount of research attention in a wide range of applications during the last several decades due to the outstanding advantages of lightweight, flexible length, high accuracy, signal transmission security, easy installation, corrosion resistance, and immunity to EM interference. These characteristics render them attractive for use in harsh environments where the application of traditional sensors is severely limited. The high sensitivity to changes in external physical quantities, such as temperature, strain, and vibration, makes optical fibers suitable for sensing purposes. Up to now, fiber-optic vibration sensors mainly consist of the point, quasi-distributed, and distributed sensors. Several schemes of point sensors including fiber Bragg grating (FBG), Fabry–Perot, self-mixing, and Doppler vibrometry are deployed for vibration measurement. Among them, FBG vibration sensors have become a fast-developing scientific research field owing to intrinsic advantages such as low noise, good embeddability, and the ability to be easily multiplexed to construct a distributed sensor array. Based on the FBG sensing principle, many investigations are applied to the measurement of vibration. Distributed fiber optic vibration sensing technology is able to provide fully distributed vibration information along with the entire fiber link, and thus external vibration signals from an arbitrary point can be detected and located. Compared with point and quasi-distributed vibration sensors, which can only be used individually on a small scale and often have poor concealment, distributed fiber-optic vibration sensors inherit the advantages of general fiber sensors and offer clear advantages such as lightweight, large-scale monitoring, good concealment, excellent flexibility, geometric versatility of optical fibers, quick response, system simplicity, immunity to EM interference, high sensitivity, accurate location, etc. Distributed fiber-optic vibration sensors mainly include interferometric sensors and backscattering-based sensors. Various interferometric sensors have attracted a significant amount of research attention and are widely investigated.

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Types of gas flow measurement

Measurement of gas flow is a key point in the field of gas application. In recent decades, thermal mass flow (TMF) meters are widely used in measuring the mass of gas. The TMF meters have many advantages such as wide applicable fields it can be applied to many kinds of pipelines and different types of gasses), wide measurement range, and high measurement accuracy and repeatability.

Thermal mass flow meters generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with a knowledge of the fluid’s specific heat and density. The fluid temperature is also measured and compensated for. If the density and specific heat characteristics of the fluid are constant, the meter can provide a direct mass flow readout, and does not need any additional pressure temperature compensation over their specified range.

Today, thermal mass flowmeters are used to measure the flow of gasses in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow metering technologies. This is because thermal mass flow meters monitor variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or specific heat) of gaseous media to define the mass flow rate.

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Aerospace sensing solutions

In every infrastructure, it is important to make sure that the cracks can be detected and monitored earlier in order to avoid any unwanted incident or any deformation of structures. Recently, Fibers Bragg gratings (FBGs) are growing interest in sensing applications such as aerospace, military, structural monitoring, and many others. FBGs are very high accuracy and also high sensitivity.

Over the last two decades, the growth of air traffic has been impressive and will strongly increase in the forthcoming years. Already by 2020, it is expected that aircraft will be significantly more affordable, safer, cleaner, and quieter than at the turn of the century.

In this context, the use of composite materials is essential for the design of high-strength, lightweight aircraft structures, which may contribute significantly to the reduction of fuel consumption and pollutants without compromising flight worthiness.

Nowadays, fiber optic sensors (FOS), particularly those based on fiber Bragg gratings (FBGs), have been emerging as an increasingly interesting technology due to their distinctive advantages which include higher sensitivity, immunity to electromagnetic interference, and durability. Furthermore, their multiplexing capability offers the possibility to reduce dramatically the cumbersome wiring required by electrical strain gauges and accelerometers, traditionally employed for load monitoring.

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Structural Deformations measurement using FBG sensors

Fiber Bragg Grating (FBG) sensors are widely used to measure various physical parameters; such as liquid level, weight, temperature, vibration, etc. They are also used in structural health monitoring systems and environmental conditions.

Structural deformations are one of the most significant factors that affect machine tool (MT) positioning accuracy. These induced errors are complex to be represented by a model, nevertheless, they need to be evaluated and predicted in order to increase the machining performance. The solution is based on the use of a multiplexed optical fiber sensor with a sufficient number of Bragg gratings for strains measuring embedded in the structure. The high sensitivity of the sensors ( 0.2 με ) suggests to employ them in the MT with high stiff structures. FBG sensors are suitable to measure both strains and temperature with very high accuracy and resolution. FBG sensors may offer many advantages since they ensure a dynamic performance up to 260 Hz, permitting the exposure and measurement of distortions acting during the working operations. The determination of the tooltip displacement is a complex process if it is directly extrapolated from the strains. For this reason, it is more convenient to employ FBG as a displacement sensor and to measure the overall integral effect of some critical point-to-point dimensions of the MT geometry.

The conventional approaches are based on models able to predict the MT deformations, studying a relationship between machine accuracy and undesired loads. Nevertheless, it is difficult to identify a general robust relation and these models need to be calibrated for every machine variant limiting their success, increasing costs, and the implementation time. In the first part of the experimental tests, the model was validated by applying a set of static loads. This analysis showed a good match between the real and the predicted position of the MT tool tip point. In the same way, the tests were replicated varying the environmental temperature over time.  Scientists highlight the significant advantages of applying FBG sensors in MT calibration such as their high sensitivity, geometrical versatility, lightweight, immunity to electromagnetic interference or chemical agents, high durability. Nevertheless, it also underlines the limits of this method. In particular, the accuracy of the model depends on the number and the position configuration of the sensors implemented in the structure. The choice of specific statistical methods may improve the accuracy of the model. In light of these results, the next steps of the research will be based on the fabrication of new prototypes changing the position configuration of the FBG sensors.

Fiber optic sensor (FOS) technologies based on situ strain and temperature monitoring for tunnel structures

FBG sensorss for tunnel structuresNowadays, numerous civil infrastructures have been built in metropolitan areas all over the world. The performance of these infrastructures during construction, operation, maintenance, and upgrading is a major concern for society. The use of smart sensing technologies for structural health monitoring has attracted much attention due to their exceptional benefits. These technologies have developed rapidly and some have found widespread applications in civil and geotechnical engineering practices, such as fiber optic sensing (FOS), time-domain reflectometry (TDR) and etc.

Based on fiber optic sensor technology, a quasi-distributed fiber optic sensing array can be established to perform accurate strain and temperature measurements. Besides FBG, another popular FOS technology is the fully distributed Brillouin optical time-domain reflectometry (BOTDR), which enables the measurement of strain and temperature profiles along with single-mode optical fiber.

Compared with electrical strain gauges, fiber Bragg grating (FBG) sensing technology is a relatively novel method for tunnel structural health monitoring, which has a number of advantages including high accuracy, multiplexing, electromagnetic interference resistance, and good repeatability. In order to study the internal force of the tunnel liner and detect the potential safety hazards, series of strain monitoring tests of a tunnel, taking into account the complex stress and strain variation during tunneling, were performed by employing the tandem linear FBG sensor arrays controlled by the wavelength division multiplexing (WDM) technology.

The length of the fiber, as well as the installation position of FBG sensors, depends on the size of the tunnel cross-section. For each test section, two layout methods could be employed: only using an independent fiber for the connection and signal transmission, or using two independent fibers according to the left and right sides of the tunnel cross-section.

 

BG sensors-based in situ monitoring on the internal force of the tunnel structure is of great environmental adaptability and performance stability. However, considering the cross effects of uncertain factors, such as special engineering properties, hydration heat of cement, and shrinkage and creep of concrete, the strain, and temperature collected through FBG sensors is influenced by composite factors. The stress and strain of liner concrete under the independent action of various factors cannot be analyzed accurately by using current ways. Therefore, it is necessary to conduct relevant studies to solve cross effect problems.

Optromix provides industry sensing solutions for Structural Health Monitoring (SHM) for different types of facilities. Optromix Company also offers a wide range of fiber optic temperature sensors for monitoring tunnel structures.  To learn more about SHM please contact us: info@optromix.com or +1 617 558 98 58

 

Interrogation techniques for FBG Sensor Arrays

FBG sensors are very suitable for sensing and data acquisition, where sensor arrays can be multiplexed using similar techniques that have been applied to fiber-optic sensors like wavelength-division multiplexing (WDM), spatial-division-multiplexing (SDM), and time-division-multiplexing (TDM) as they can be directly implemented in the fiber without changing the diameter of the fiber. This feature makes FBG sensors suitable for a wide range of applications.

The main problem with the TDM system is that the sensors must be placed sufficiently far apart because the pulse returning from the adjacent sensors must be able to reach and get detected separately. In WDM systems, different sensors have the nominal central wavelength, and other sensors are separated by a few nanometers. WDM interrogation is available in two topologies i.e., series and parallel. The parallel approach is easier to implement but the series topology allows the optical power from the sensing FBG array to be used much more efficiently than parallel topology.

The number of sensors that you can incorporate within a single fiber depends on the wavelength range of operation of each sensor and the total available wavelength range of the interrogator. Because typical interrogators provide a measurement range of 60 to 80 nm, each fiber Bragg grating array of sensors can usually incorporate anywhere from one to more than 80 sensors – as long as the reflected wavelengths do not overlap in the optical spectrum. Be careful when selecting the nominal wavelengths and ranges for the FBG sensors in an array to ensure that each sensor operates within a unique spectral range.

Major limitations in interrogating FBG sensor arrays are the cross-talk, spectral shadowing, and interference. For all the interrogation approaches, some crosstalk between adjacent sensors seems to be unavoidable. The use of a serial array of FBG sensors with the same central wavelength results in the crosstalk between sensors. The amount of light reflected by the FBG sensors located nearest to the source will affect the amount of the optical power reaching and be returned from gratings further from the source. The lower the peak reflectivity of the FBGs is, the smaller the effect is. Another source of the crosstalk in a TDM serial array of identical FBG sensors arises from multiple reflections between FBGs. This can lead to pulses arriving simultaneously at the detector having undergone a direct reflection from a sensor element and also having experienced a number of multiple reflection paths between FBGs.