Overview Structure Detection Technology Based on Optical Fiber Sensing Technology

Basic principle of optical fiber sensor

With the improvement of the level of technology, fiber optic technology is currently relatively mature. An optical fiber sensor is a broadband induction system that uses the basic principle of optical fiber transmission. The typical optical fiber sensing system consists of a light source, an optical fiber, a light sensing element, a light modulating element, and a signal processing section [3]. The working principle is as shown in the following figure: The light emitted by the light source enters the light sensor element through the optical fiber, and the light that changes in the light sensor element due to the influence of the ambient environment enters the light modulation mechanism, and the sensor will sense the light. The parameters measured by the component are modulated into information such as amplitude, phase, and polarization. This process is called the photoelectric conversion process. Finally, the microprocessor is used for signal analysis.

As mentioned before, it can be seen that the sensing mechanism of the optical fiber sensor is similar to the sensing mechanism of the electromagnetic sensor. However, since the carrier of the optical fiber sensor is a laser, the optical fiber sensor propagates inside the optical fiber and is difficult to be interfered by external electromagnetic fields. Therefore, it is suitable for detection under complex conditions, and the operation is convenient and flexible, and the signal output is automated.

Classification of fiber optic sensors have different ways according to the role of fiber in the optical fiber sensor can be divided into light transmitting and sensing type of two types.

The optical fiber of the sensor type optical fiber sensor not only transmits light but also is an optical sensor. Due to the influence of the external environment on the optical fiber itself, the physical quantity to be measured acts on the sensor through the optical fiber, and the properties of the optical waveguide (light intensity, phase, polarization, wavelength, etc.) are modulated. Sensor-type optical fiber sensors are further classified into optical emphasis, phase modulation, vibration modulation, and wavelength modulation.

The optical fiber type optical fiber sensor measures the optical signal modulated by the measured object into the optical fiber, and performs optical signal processing on the output end. This type of sensor is sensitive to the physical quantity to be measured by another photosensitive element. The optical fiber is only used as For the light-transmitting element, a sensing element capable of modulating the light transmitted by the optical fiber must be added to constitute the sensing element. Optical fiber sensors can be classified into point fiber sensors, integral fiber sensors, and distributed fiber sensors according to their measurement ranges. Among them, the distributed optical fiber sensor is used to detect the strain distribution of the large-scale structure, and it can quickly and non-destructively measure the structural displacement, internal or surface stress and other important parameters. The types of optical fiber sensors currently used in civil engineering include Math-Zender interferometric fiber sensors, Fabry-pero cavity fiber sensors, and fiber Bragg grating sensors.

Research and engineering applications show that fiber optic sensors have the following characteristics:

(1) high sensitivity, anti-electromagnetic interference. Because the fiber optic sensor detection system is difficult to be interfered by the external field, and the optical signal does not interact with the electromagnetic wave during transmission, nor is it affected by any electrical noise. Due to this feature, the optical fiber sensor is obtained in the power system detection. widely used.

(2) Optical fiber has good flexibility and toughness, so the sensor can be made into different shapes according to the needs of on-site inspection.

(3) The measured frequency bandwidth and dynamic response range are large.

(4) High portability, can be made into different physical quantity sensors, including sound field, magnetic field, pressure, temperature, acceleration, displacement, liquid level, flow, current, radiation and so on.

(5) It has strong embeddability and is easy to connect with computers and optical fiber systems. It is easy to realize remote measurement and control of the system.

Example of Application of Fiber Optic Sensors in Civil Engineering With the development of fiber optic sensing technology, fiber optic sensors in the field of civil engineering have been widely used to measure the deformation and internal stress of concrete structures, and to detect large-scale structures and bridge health conditions. The main point is to use optical fiber sensors as a new strain sensor.

The optical fiber sensor can be attached to the surface of the structure for measurement, and can also be embedded to realize the measurement of the physical quantity inside the structure. By using a pre-embedded fiber-optic sensor, the internal strain of a concrete structure can be measured during internal damage. Based on the slope of the load-strain curve, the formation and propagation of damage within the structure can be determined. Concrete experiments show that the load-strain curve of the fiber test is higher than the linearity of the strain gauge test.

In practical engineering applications, it is necessary to solve the problem of layout of optical fiber sensors in order to achieve all-round and long-term detection of structural buildings. In order not to cause damage or damage to the sensor, the following methods are generally used: 1 Put a metal tube on the fiber sensor and put it together in a concrete structure. After the concrete is poured, the metal conduit is taken out before the concrete is consolidated, so that the fiber grating sensor is well integrated with the concrete and the sensor is not damaged due to the pouring process. 2 Because the stress-strain of the steel bar is enough to reflect the concrete stress state of the steel bar attachment, the fiber grating sensor can be directly affixed to the steel bar or a small groove is formed on the surface of the steel bar so that the bare core portion of the grating is embedded in the groove. Protected or embedded in composite ribs when building bridges. 3 The FBG is embedded directly in a small prefabricated component or enclosed in a metal conduit. The small prefabricated component is then buried as part of a large component. The external load is transferred to the FBG sensor via a preform or metal conduit.

Conclusion Fiber-optic sensors have great significance in the field of structural monitoring. The light weight, durability and long-term stability of fiber optic sensors make them easy to apply to internal stress and strain detection of various building materials such as building steel structures and concrete. Achieve the healthy detection of building structures.