Fiber bragg grating sensors for structural health monitoring in harsh conditions

Piezoelectric transducers have been traditionally used in structural health monitoring. These devices convert pressure to an electric voltage. The transducers are typically reliable in normal working conditions, however, they become less accurate in harsh environments, like high temperatures – over 300 degrees Celsius – or highly corrosive environments.

With the development of laser technologies the popularity of fiber Bragg grating sensors have been increasing due to their numerous advantages over traditional means of measuring strain, pressure, and temperature: 1) FBG sensors work reliably under very high temperatures because of the optical fiber material – fused silica; 2) fiber Bragg grating sensors are immune to electromagnetic interference; 3) the sensors are not susceptible to corrosion; 4) FBG sensors are very small and can be easily mounted on any surface. The development of fiber-optics technology will enhance structural health monitoring in harsh conditions, such as nuclear power plants and modern superheaters.

The following types of FBG sensors are used in structural health monitoring:

  1. FBG temperature sensors;

The main advantages of fiber Bragg grating temperature sensors are their reliability and accuracy under high temperatures, absolute temperature measurements, fast response, immunity to electric sparks. The fast response rate is particularly useful in cases where temperature shifts are extreme.

2. FBG strain sensors;

FBG sensors are the best strain sensors at the moment due to multiple advantages that are inherent to all FBG products. These sensors can determine structure parts that are stressed.  

3. FBG displacement sensors.

Fiber Bragg grating displacement sensors can detect shifts in structures and the appearance of cracks.

There are many applications of fiber Bragg grating sensors for structural health monitoring. For example, one of the possible uses of FBG sensors is pipeline monitoring. P19 pipelines, that are used in the power industry, are usually used to transmit corrosive high-temperature, high-pressure steam. The integrity of the pipes may deteriorate over time leading to eventual failure of the steam transmitting system.

If you would like to purchase FBG (Fiber Bragg Gratings) sensors, please contact us: info@optromix.com or +1 617 558 98 58

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

FBG strain sensors in structural health monitoring

Fiber Bragg grating sensors are used over conventional sensors in structural health monitoring based on a number of advantages they provide:

  • FBG sensors are more stable;
  • fiber Bragg grating sensors are more durable, namely, they don’t rust;
  • FBG sensors can be highly multiplexed;
  • the sensors can be used in highly explosive atmospheres, like natural gas or oil.

These features are desirable for the health monitoring of complex structures. FBG structural health monitoring systems are essential in the design of smart buildings. The FBG sensor systems help to ensure building safety and performance. The systems are of particular use for buildings situated in earthquake-prone zones.

The mechanism behind FBG sensors allows to monitor complex structures based on the measurements of the wavelength of the reflected light; its wavelength changes under the influence of pressure, building incline, etc. Due to the fact that a single FBG sensor reflects a narrow region of the light, it is possible to multiplex them, which provides higher data accuracy. However, the number of FBG elements should be carefully designed and spaced in accordance with the range and bandwidth of the input light source.

The FBG strain sensors are fragile on their own, therefore they need to be placed on a base of some kind to protect them from outside physical influence. Usually, the sensors have a metal protective cover that prevents damage to the sensors from environmental factors. The sensors are then welded or glued to a structural member. FBG strain sensors, or FBG deformation strain sensors, can be used in both steel and concrete structures; they offer high accuracy and resolution. The measurements received from the sensors are often used to detect the behavior of structural materials under different environmental influences. The detection of segments that are stressed is essential for proper maintenance of the building and its structural parts and prevents premature aging and failure of these structures.

FBG strain sensors may also be used in the monitoring of tunnel ribs, pipelines, ship hulls, etc. Their high accuracy, easy installation, and compact nature provide a wide area of applications.

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 purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com or +1 617 558 98 58

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 Bragg Grating Sensors in biomedical science

Biomechanical engineering experiences rapid development as a result of FBG sensor application to strain and deformation measurements. The use of fiber Bragg grating sensors in biomedicine is a promising new method of enhancing biomechanical studies.

Fiber Bragg gratings were proposed for use in medical applications at the end of the 20th century. Some of the applications were monitoring ultrasound fields, monitoring the temperature inside nMRI devices, foot pressure monitoring in diabetic patients, etc.

One of the first uses of FBG sensors in biomedicine was an electrically assisted ventilation device triggered by an FBG sensor. A deformable strap was placed on the patient’s chest; the strap had FBG sensors embedded into it that were measuring chest deformations that were caused by air inspiration. A threshold level was set to produce a trigger signal to stimulate the phrenic nerve. Nowadays FBG sensors are used in medical-grade textiles for healthcare monitoring.

Fiber Bragg grating sensors provide several advantages over traditional methods of measuring ligament and tendon deformation and strain, namely an opportunity to record the deformations under several postures. For example, a foot pressure sensing system with embedded FBG was presented; it contains several carefully calibrated FBG sensors in an optical fiber strand. The distribution of transversal pressure and its analysis help to indicate abnormal standing gait in diabetic patients.

The research suggests that FBG sensors are superior to traditionally used strain gauges in soft tissue strain measurement. FBGS easily adhere to a bone or a curved surface; their dimensions are more compatible with bone size than those of the strain gauges; they are easily implantable, highly accurate, and less invasive.

The use of fiber Bragg grating sensors in intervertebral disc pressure measurements is very promising as it is significantly more sensitive than other measuring methods. Moreover, the FBG sensors are more compact which allows them to be inserted through a needle, and to be used for small discs, e.g. cervical or biodegenerated.

Fiber Bragg sensors proved to be useful in a femoral prosthesis. The multiplexing ability of FB sensors allows us to place several sensors on a prosthesis surface and connect them using a single optical link to interrogate all of them. The sensors aid in locating potential failure areas in the prosthesis under normal strain conditions.

Optromix is a fiber Bragg grating sensor vendor; we manufacture innovative fiber optic products for the global market. We are dedicated to delivering the best products and supports to all our customers, our engineers have extensive experience and strong technical expertise in creating fiber Bragg grating products. If you would like to buy FBG strain sensors, please contact us at info@optromix.com.

Fiber Optic Strain Sensors Technology and Applications

Nowadays, different types of fiber optic strain sensors have attracted attention from all over the world. Fiber Bragg Grating (FBG) has become the most widespread technique, directly applicable to bridges, concrete, and dams for strain measurement.

To create the actual strain sensor, the optical fiber is inscribed during production with a Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.

The material interferences are placed at certain intervals. When the fiber is stretched or compressed and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa. Not only does the reflected light take a little longer or shorter to travel back when the Fiber Bragg Grating is under strain, but the wavelength that is reflected also changes. In scientific terms, the Fiber Bragg Grating has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.

For measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum.  In order to ensure the safety of personal and public property, the precise and real-time monitoring of strain becomes more and more important in all kinds of engineering applications, such as chemical plants, gas stations, power stations, bridges, tunnels, oil pipelines, etc.. In general, these application environments full of poisonous gas, intense radiation, and elevated temperature are dangerous to human health, so safe and efficient remote monitoring of strain is of great significance. Compared with conventional electrical sensing methods, an optical fiber strain sensor is more suitable for present applications because of its compact size, high sensitivity, multiplexing capability, immunity to electromagnetic interference, high-temperature tolerance, and resistance to harsh environments.

Fiber optic strain sensors are welded directly to the surface of the metal structure (pipes, beams, etc.), and it has a protective silicone cover. Fiber optic strain sensors are durable and stable, widely used for civil engineering constructions, particularly they reinforce concrete structures exceptionally well.

If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com  or +1 617 558 98 58

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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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