Structural health monitoring (SHM) is considered to be one of the most effective modern fiber optic technologies. Its aim is to provide robust information about engineering structures’ integrity and safety. Thanks to it, specialists can indicate the need for repair or retirement in advance and eliminate problems. Such a system of structural condition monitoring is based on fiber Bragg grating sensors.
Fiber Bragg Grating (FBG) sensors as a part of structural health monitoring systems
FBG sensors have proved to be reliable monitoring instruments. Thanks to the variety of FBG sensors, they have a great advantage in comparison with other technologies.
The main types of Fiber Bragg Grating sensors:
Temperature sensors;
Strain sensors;
Displacement sensors;
Pressure sensors, etc.
Newly developed FBG sensors are widely applied to measure and register static values in various industries. Scientists have discovered their abilities in ultrasonic detection and capacity for ultrasonic structural health monitoring. To provide the accuracy of the results, ultrasonic detection needs a broad bandwidth and high sensitivity. That’s why Fiber Bragg Grating (FBG) sensors need to be properly designed, taking into account data process and installation methods.
Main steps for structural health monitoring implementation
SHM allows the predictive maintenance of engineering structures and machines. There is no need in turning the whole system off because structural health monitoring can detect damages while operating. Mechanisms can keep working over a long period of time.
There are three main steps for SHM implementation:
Finding the most suitable locations for FBG sensors. It means the modeling of the most probable simulation. Engineers identify critical areas where it is better to install sensors.
Data gathering. At this step, FBG sensors monitor the health of the structures in normal and maximum loading conditions. They need to know the usual numbers for loading conditions to see any differences in the future. The received data also allows conducting a statistical study to determine the normal functionality of the engineering structures.
Comparison of the received data and standard behavior rates. Special algorithm(s) allows comparison even without extra manpower. If the values obtained don’t match the predetermined level, the software defines whether there is a possibility of structural failure. If ‘yes’, then the program informs about problems that have arisen with warnings and alarms.
The areas in which structural health monitoring systems are most commonly used, for example, are the gas and oil industry. In addition, recently this technology has been applied on a fish farm. Specialists have designed a customized structural health monitoring system that allows them to see the whole picture of fish farming’s biological and technological aspects. Fiber optic technology still has a wide range of industries to develop.
For the last decade, fiber Bragg grating (FBG) sensors have been attracting more attention from scientists all over the globe thanks to their numeral benefits. Nowadays, they are widely applied in health monitoring, aerospace, civil engineering, oil & gas, smart structures, etc. In this article, we will consider FBG sensors for one of the most used transportation systems, for railways.
FBG sensors in railway transport systems
Railway transport systems have faced a range of difficulties such as longer service availability, providing high safety levels, reduced energy consumption, and optimal operation. Monitoring systems should meet all of these tasks without interruption.
According to many successful pieces of research, fiber optic technology can provide all these aspects at once, it can be applied to ensure railway monitoring. It is important to understand the structural and operating conditions of rails and freight and passenger service cars to provide safe and reliable work.
The point is that FBG sensors have many crucial characteristics that perfectly suit railways’ operational conditions. For instance, it is a very long-range interrogation, that is better, especially in comparison with usual electrical sensors. Fiber Bragg Grating sensors are the only sensors that measure all the data and offer it in real-time. FBGs also are low susceptible to fluctuations. Moreover, they can be interrogated from any end.
All these features are especially significant for the railways in providing safe operations. In addition to that, FBGs are universal and can be applied for measuring many other parameters like inclination and acceleration over a large area.
Recent programs with fiber Bragg grating sensors
The most recent project where Fiber Bragg Grating sensors are involved is taken place in India. The main goal is to make the railways there safer with the help of fiber optic technology. Specialists have used sensing technology to reduce travel time and increase the trains’ speed between two states. The railway construction will be conducted by a developed structural health monitoring system.
Thanks to monitoring systems based on fiber optic solutions, rail corporations could easily get real-time data about rail structures’ conditions within 24 hours. Therefore, this feature will fast the warning alerts for possible damage detection. The fiber optic technology can be configured for any type of location and structure, even in vulnerable areas. It is specially referred to as earth slip locations, heavy rainfall areas, etc. However, recently only routine manual inspection is carried out in such cases. Constant monitoring with the FBG sensors will provide real rail safety in such areas.
The government considers that the potential of fiber Bragg grating (FBG) sensors is huge and can be applied in other projects on rail safety. It allows finding the structural defects out as early as possible, not only when an accident occurs. The preventative work gives an opportunity to avoid many troubles and mass casualties that happen. All in all, thanks to constant real-time monitoring, timely warning alerts, and the possibility of applying in severe environmental circumstances, specialists can use fiber optic technology in many spheres, as well as railways.
Over the last few decades, scientists could achieve considerable progress in creating wearable sensors. Wearable sensors are newly developed inventions that can detect physiological and biochemical markers for health monitoring. Fiber optic solutions such as fiber optic sensors were no exception and made a great contribution to this technology.
What are wearable sensors?
Wearable sensors have an enormous potential for the early detection of many diseases including Covid-19 by measuring numerous vital signs such as blood oxygen level, respiratory rate, etc. According to the studies on Covid-19, the most common symptoms are shortness of breath, fever, and fatigue. Moreover, the blood oxygen level of an infected patient is lower because it is a respiratory disease. That’s why it was important to create a fiber optic technology capable of providing accurate data that would be useful for specialists and non-specialists alike people in quarantine or self-isolated.
Fiber optic sensors for medicine
Fiber optic sensors have attracted attention by development as flexible sensors. They usually consist of optical gratings and optical fibers embedded into polymeric matrices. For example, wearable systems based on fiber Bragg grating (FBG) were developed for biomedical usage. Thanks to their small size, lightweight, and other advantages, they can be easily integrated into polymers or textiles.
FBGs were suggested as wearable sensors for respiratory measurement for the upper body like abdomen and chest movements. FBG output changes can show the respiratory rate and breathing pattern. Moreover, with the help of fiber optic sensors, specialists are able to receive information about cardiac activities. The suggested fiber optic system has high sensitivity and can respond to environmental factors, for instance, relative humidity, temperature, and water immersion. Researchers noticed that the temperature had more influence on them in comparison with water immersion and humidity. That means that this fiber optic system needs further investigation.
The new humidity sensors were also developed based on optical fibers. The humidity also influences the light intensity of the emitted light of optical fibers. This function was used for developing a fiber optic system to measure the level of the nasal exhaled breath for respiratory rate tracking. In this system, scientists have placed a Fiber Bragg grating in an agar substrate. This device with fiber optic sensors demonstrated reliable performance and nice sensitivity to relative humidity change.
Thanks to the newly designed wearable fiber optic sensors, it became possible to integrate the health data of each person and find the best option for him.
A collaboration of scientists from different countries (China, Pakistan, and Hong Kong) have developed new 3D printed FBG sensors that can help in creating the ‘smart beds’. Scientists have worked to determine the main advantages of the innovative 3D printed FBG sensors and their applications in different spheres.
3D printing has demonstrated enormous results in different scientific fields. It helps to reduce the costs and makes the production process much easier for the developers. The university scientists have already applied it in their different devices with potential healthcare applications.
The main goal of this development based on the fiber Bragg grating sensors was to track sleeping patterns with high precision. First, the research team hopes that this fiber optic solution can be helpful for hospitals to monitor the well being of patients. Moreover, these fiber Bragg grating sensors are temperature-insensitive, lightweight, and high-accurate. That increases their chances to be installed in more hospitals and improves the quality of care because the staff could respond more quickly when the patients’ condition deteriorates.
Usually, FBGs are a microstructure that length is a few millimeters. It is implemented into a short optical fiber that can transform the light in response to temperature, strain, or vibration. FBG sensors are highly applied in mechanical engineering, textile, and medical spheres, thanks to their high thermal sensitivity.
Nowadays, the production of FBG sensors for healthcare still remains a time-consuming and equipment-intensive process that is hard to replicate. While 3D printing is an advanced technology that allows the creation of complex FBG sensing devices. Moreover, fiber Bragg grating sensors have never been applied to sleep-monitoring.
During the first experiments, the 3D printed FBG sensors were tested while putting under pressure loads. Each device demonstrated a number of consistent wavelengths which leads to the possibility of providing reliable readings. The final tests on fiber Bragg grating sensors were held by placing them under the mattress of a bed. A person demonstrated several sleeping positions while the FBG sensors were tracking his changes in posture.
According to the results, the FBG sensors’ readings were almost precise and had an error rate of less than 1%. Nevertheless, the research team considers that this fiber optic technology still has potential in the future. They could, for example, track a patient’s breathing and identify when the heart rate has begun to fall. And that is not the first time that fiber optic technology can prove beneficial to medicine.
A fiber Bragg grating is an optical interferometer embedded in an optical fiber. At the same time, fiber optics combined with certain substances (usually germanium) can change its refractive factor when the fiber is exposed to ultraviolet light. If such a fiber is illuminated with ultraviolet light with a specific spatial periodic structure, the optical fiber becomes a kind of diffraction grating. In other words, this optical fiber will almost completely reflect the light of a certain, predetermined range of wavelengths, and transmit light of all other wavelengths.
For decades, electrical sensors (tensor-resistive, string, potentiometric, etc.) have been the main method of measuring physical and mechanical phenomena. Despite their widespread use, electrical sensors have several disadvantages, such as loss during signal transmission, sensibility to electromagnetic interference, the need to organize a spark-resistant electrical circuit (if there is a danger of explosion). These mentioned above limitations make electrical sensors unsuitable or difficult to use for a number of applications.
The use of fiber optic sensors is an excellent solution to these problems. In fiber optic sensors, the signal is light in the optical fiber instead of electricity in the copper wire at traditional electrical sensors.
Over the past twenty years, a huge number of innovations in optoelectronics and in the field of fiber optic telecommunications has led to a significant reduction in prices for optical components and to a significant improvement in their quality. This factor allows fiber optic sensors to move from the category of experimental laboratory tools to the category of widely used devices in various areas.
Operating principle of Bragg gratings
A fiber Bragg grating or FBG acts as a sensitive element of pointfiber optic sensors, which is capable to reflect certain wavelengths of light and transmit all others. This effect is achieved by periodically changing the refractive index in the core of the fiber optics.
When the laser light passes through an optical fiber, a part of it is reflected from the fiber grating at a certain wavelength. This peak of reflected light is registered by measuring equipment. As a result of the numerous parameters influence, the interval between the FBG bundles and the refractive index of thefiber optics change.
Consequently, the wavelength of the light reflected from the fiber Bragg gratingchanges. In addition, it is possible to determine the exact characteristics of the changes by changing the wavelength. In fiber optic sensors based on Bragg gratings, the measured value is converted to a Bragg wavelength offset. The recording system converts the wavelength offset into an electrical signal.
The sensing element of suchFBG sensordoes not contain electronic components and therefore it is completely passive, which means it can be used in the area of increased explosiveness, aggressiveness, strong electromagnetic interference. Numerous fiber Bragg gratings can be installed on a single fiber, each of which gives a response at its own wavelength. In this case, instead of a point sensor, we get a distributed sensing system with multiplexing along the wavelength.
The use of the light wavelength as an information parameter makes the FBG sensor insensitive to the long-term changes of the parameters of the source and radiation detector, as well as random attenuation of power in the optical fiber.
The principle of FGB sensoroperation is based on the modulation of one or several properties of a propagating light wave (intensity, phase, polarization, frequency), which change occurs with a change in the measured physical quantity.
The basis of fiber-optic sensing technology is an optical fiber– a thin glass thread that transmits light through its core. The optical fiber consists of three main components: core, shell, and coating. The shell reflects the scattered light back into the core, allowing light to pass through the core with minimal loss.
It can be achieved by a higher refractive index in the core relative to the shell, resulting in a complete internal reflection of light. The outer coating protects the fiber optics from external influences and physical damage. It can consist of several layers depending on the required protection.
Insensitiveness to electromagnetic and radio frequency influences;
No need for recalibration (stable over time under constant external conditions).
At the moment, most of the sensors used in the world are electrical sensors. As it was mentioned above, in optical sensorsbased onfiber Bragg gratings, the signal is light passing through an optical fiber (instead of an electric current passing through a copper wire). This fundamental difference allows FBG sensors to overcome many problems typical for electrical sensors.
Features of fiber optic sensors
Optical fibers and sensors are non-conductive, electrically passive, and immune to electromagnetic interference. Monitoring with a tunable high-power laser system allows sensing over long distances with virtually no signal loss. In addition, each optical channel is able to monitor a variety of FBG sensors unlike the electrical channel, which significantly reduces the size and complexity of such a sensing system.
Optical sensing systems are ideal for use in conditions where conventional electrical sensors (strain gauge, string, thermistor, etc.) can be difficult to use (long distances, EM fields, explosion safety, etc.). It is easy to switch to fiber optic solutions since the installation and operation of optical sensors are similar to traditional electrical sensors.
Understanding the principles of FBG operation and the benefits of Bragg grating sensor applications can greatly facilitate the solution of various problems in the field of sensing measurement (for example, monitoring of structures).
Nowadays FBG sensors are applied in various fields that require precise and fast measurements. Fiber Bragg sensing systems can be used in aeronautic, automotive, civil engineering structure monitoring, undersea oil exploration, in the mining industry, geotechnical engineering, structural engineering, tunnel construction engineering, etc.
Bragg sensors in medicine
The most promising application of FBG sensors is medicine. NowFBG technology is highly used for fiber-based biomedical sensing including biosensing, safety or security, and structural health monitoring. FBG sensors offer a new and effective way of real-time measurements. They can be applied in laser systems, medical tiny intra-aortic probes, and body sensors for biochemical analysis making.
For example, today fiber Bragg gratingsapply optical-fiber sensing probes that are able to dissolve due to such ability as controlled solubility in a physiological environment. Thus, FBG technology enables safer diagnostic of sensitive human organs and there is no need for a surgical extraction. The development of FBG continues, and it is possible that very soon new FBG sensors with improved characteristics appear.
A collaboration of scientists from different countries (China, Pakistan, and Hong Kong) have developed new 3D printed FBG sensors that can help in creating the ‘smart beds’. Scientists have worked to determine the main advantages of the innovative 3D printed FBG sensors and their applications in different spheres.
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At the moment, most of the sensors used in the world are electrical sensors. As it was mentioned above, in