fiber Bragg grating; air flow sensor; strain; flow velocity; angle

A fiber optic airflow sensor based on fiber grating array is proposed. The length of the gate region of the single-mode fiber grating is 10 mm, and the three fiber gratings are evenly distributed around the silica gel substrate. The changes of the ambient temperature and flow velocity can cause changes in the reflectance spectrum of the sensor. By monitoring the reflectance spectrum, the physical quantity of the outside can be measured. Experimental analysis shows that the silica gel matrix has a good coupling effect with the fiber grating. By changing the length of the silica matrix, the sensing characteristics of the sensor are effectively improved. The experimental results show that under normal room temperature conditions, the sensor’s central wavelength drift is quadratic with the flow velocity and the sensor has directional sensing characteristics. The fiber array air flow sensor proposed in this paper is compact in structure, simple in manufacturing process and with high reliability.

Gas flow measurement plays an important role in various industrial sectors. It provides vital information for many applications such as process control, fossil fuel and nuclear power generation, transportation and environmental monitoring. For flow measurement, a large number of flow sensors based on various mechanical, electronic, and microelectromechanical systems (MEMS) structures have been developed. The first and widely used mechanical rotor-type flow sensor is gradually mature with the development of technology, but limited by the mechanical structure, the measurement error is large. In order to further improve the measurement accuracy, electromagnetic flowmeters, ultrasonic flowmeters and acoustic Doppler flowmeters have been developed, which are easy to use, but are susceptible to electromagnetic interference and have high use costs.

The fiber optic flowmeter has the advantages of strong anti-electromagnetic interference ability and low cost, but the design concept of the fiber optic flowmeter is based on the change of the flow rate to the optical signal intensity and the phase modulation. Difficult to achieve accurate detection. Fiber Bragg Grating (FBG) flow sensors monitor the flow by changing the wavelength of the reflected light, and are sensitive to changes in the external environment. Therefore, FBG flow sensors have become a research hotspot in the industry in recent years.

Compared with traditional sensing technology, fiber Bragg grating (FBG) sensing technology has the advantages of small size, high sensitivity, and anti-electromagnetic interference. FBG technology has been used in a wide variety of scenarios, most commonly for strain and temperature measurements. In addition, in recent years, FBG sensing technology has also been applied to rainfall and wind speed. However, FBG sensing technology has little application in airflow monitoring.

The measurement of gas or liquid flow rates has important practical implications in a variety of industries, such as food inspection, pharmaceuticals, oil/gas exploration, the environment, high voltage power systems, chemical plants, and marine research. Due to their many unique advantages, such as small size, light weight, immunity to electromagnetic interference, strong remote sensing capabilities, poor environmental tolerance, and the ability to distribute or quasi-distributed measurement fiber optic sensors such as temperature sensors, flow meters, or wind speed meter, has proven to be an attractive alternative to its traditional mechanical or electromagnetic counterparts.

In this paper, a real-time airflow monitoring method is proposed. The fiber grating is fixed at both ends of the simply supported beam, and the strain monitoring can be realized through the fiber grating array. The airflow sensor proposed in this paper can perform temperature compensation and tensile stress compensation, which reduces the cross-influence of temperature on the measurement results. Both simply supported beams and fiber gratings are passive materials that do not require power support. The system can be better used in pipeline airflow monitoring.

Based on the flexible and recoverable design concept, the matrix of the multi-element optical fiber airflow sensor is made of cylindrical silica gel with a radius of 2 mm, and three fiber gratings are evenly arranged on the outside of the matrix, forming a 120° angle between them.

Combined with the characteristics of the sensor, Ecoflex 00-50 silica gel is selected for the sensor package (as shown in Figure 1(a)).

Fig. 1 Basic design of airflow sensing infrastructure

The adhesive has good insulation properties, and at the same time, the shrinkage deformation after curing is small, and the coupling with the fiber grating It can also suppress the zero drift and hysteresis effect of the sensor, and it is also of great help to increase the service life of the fiber optic airflow sensor. In order to increase the service life of the fiber optic airflow sensor, a heat shrinkable tube, etc., will be added to the external part of the sensor for protection (as shown in Figure 1(c)). The advantage of the sensor infrastructure design is that the multi-directional FBG measurement points can be matched and compensated for each other, thereby reducing the effect of temperature on the sensor accuracy.

The selected sensor is Fiber Bragg Grating (FBG), and its sensing principle is shown in Figure 2.

Fig. 2 Fiber Bragg grating sensing principle

When broadband light is transmitted in the FBG, mode coupling will occur, and the center wavelength λ_B of the fiber grating satisfies the following formula:

In the formula, the center wavelength of the FBG is represented by λ_B, which is the wavelength of the reflected wave; the effective refractive index of the fiber is recorded as n_eff; Λ is the grating period. It can be seen from the above formula that the size of the center wavelength λ_B of the fiber grating is affected by n_eff and Λ, that is, the size of λ_B will also change as n_eff and Λ change, and n_eff and Λ are mainly affected by temperature and strain. The correlation functions of n_eff and Λ with temperature and strain are rewritten as follows:

Therefore, under the experimental conditions of constant temperature, the FBG center wavelength shift is only affected by the strain. Under the action of tension or compression, that is, only subjected to the axial strain ε, the period Λ of the fiber grating changes accordingly, and the center wavelength of the fiber grating shifts, and the offset is denoted as Δλ_B. The relationship corresponds to the following:

The unit area pressure W₀ provided by the wind speed can be expressed as:

where ρ is the air density; ν is the wind speed. The shear force diagram of the simply supported beam is shown in Figure 3.

Fig. 3 Shear force diagram of simply supported beam with uniformly distributed load

When the external force acts only in the vertical direction, the horizontal restraint force on the support A is zero. In addition, since the simply supported beam structure and the force are symmetrical, the vertical restraint force at the support A and the support B is the same.

From the equilibrium condition we get:

In the formula, q represents the airflow uniform load, which can be solved by the following formula:

Among them,
s_q represents the windward area;
u_q represents the windward coefficient.

The windward area of the sensor refers to the projected area perpendicular to the wind speed, which can be expressed as:

In the formula,
D represents the diameter of the sensor;
h represents the length of the sensor.

The shear force equation and bending moment equation of a simply supported beam are:

Under the condition of constant temperature, the expression of FBG subject to axial strain ε is:

In summary, the relationship between the FBG wavelength drift ΔλB and the sensor axial strain is:

The principle of airflow sensing of fiber array is shown in Figure 4.

Fig. 4 Principle of air flow characteristic measurement

In the figure, l₀ represents the length of the gate region; l₁ represents the length of the sensor silica gel substrate. Under the action of a uniform airflow field, the sensor bends in the direction shown in Figure 4

In order to verify the airflow sensing characteristics of the multi-element optical fiber airflow sensor, the experimental system shown in Figure 5 was built. In the experiment, a variable frequency duct fan is used to provide an external airflow field.

Fig. 5 Airflow characteristics test system

The maximum air volume of the fan is 710 cfm (1205 m³ / h) and the maximum speed is 3800 rpm. By adjusting the speed of the fan, the air flow rate in the system varies is from 0 to 13 m/s. The airflow velocity is incremented by 1 m/s, and the center wavelength shift of the fiber grating of the sensor is measured. Each adjustment of the airflow velocity is calibrated by a pitot tube. The sensor is placed above the angular displacement platform to ensure that the sensor surface is subjected to uniform airflow pressure, and the accuracy of the angular displacement platform can reach 1°. Adjust the angular displacement platform so that the angle between the airflow direction and the central axis of the sensor varies from 0° to 360°.

The sensor structure is placed on the fixed end of the probe, and the broadband light source and the spectrometer are connected to form an air flow monitoring experimental system, and the whole structure is placed in the built air flow detection platform.
The broad-spectrum light reaches the inside of the sensor through the single-mode fiber, where the sensor is located in the center of the fixed bracket. By changing the length of the probe base and the flow rate of the airflow field, the information of the center wavelength shift of the sensing array is further obtained, and the verification of the airflow sensing characteristics of the sensor is completed.

In order to analyze the sensing characteristics of the sensor, the following studies are now conducted.
When an airflow field with the same flow rate is applied, the sensing effect of the substrate is different with different lengths. The corresponding sensor center wavelength drift obtained in the experiment is shown in Figure 6.

Fig. 6 The center wavelength drift of the grating array with different substrate lengths at the same velocity

It can be seen from the analysis in Fig. 6 that when the length of the substrate is 9.5 cm, the center wavelength shift of the sensor is relatively obvious and is suitable for further research in this experimental system.
Under the same flow rate, the center wavelength drift of the sensor corresponding to different matrix lengths is shown in Table 1.

Table.1 Different matrix lengths correspond to the central wavelength drift of the sensor array

In a constant temperature state, the center wavelength of the fiber grating will also drift when the external stress and other parameters of the fiber optic airflow sensor change. As shown in Figure 6, the different layout positions of the fiber grating array lead to different conditions of being stretched or compressed under the air impact state. The reduced square drift is represented by the minus sign in Table 1. The fiber gratings a and c are in a stretched state, and their central wavelengths shift in the direction of increasing wavelengths.

Under the condition that the length of the sensor substrate remains the same, a uniform airflow field with different flow rates is applied to the sensing array, and the center wavelength shift of the grating array is shown in Figure 7. It can be seen that the sensor can be applied to the monitoring of a stable airflow field.

Fig. 7 The center wavelength drift of the grating array at different flow rates with the same matrix length

After determining the size of the sensor suitable for this experimental system, the relationship between the center wavelength shift of the fiber grating and the airflow velocity under the impact of different flow rates of the fiber optic airflow sensor is shown in Figure 8.

Fig. 8 Center wavelength drift fitted curves at different velocities

It can be seen from the analysis in Fig. 8 that there is a good quadratic function relationship between the center wavelength drift of the fiber grating and the flow velocity, which further verifies the derivation of formula ΔλB.

To further verify the direction sensor characteristics of the fiber optic airflow sensor, the following experiments are designed. On the premise of keeping the airflow velocity unchanged, the purpose of direction sensing monitoring is achieved by changing the direction of the impinging airflow. The airflow impact direction can be achieved by adjusting the angular displacement platform, and the angle can be gradually changed from 0° to 360°. Extract the maximum value of the center wavelength shift of the fiber grating under each air impact as the ordinate, and use the impact angle as the abscissa to complete the drawing of the relationship between the two in the coordinate system. The result is shown in Figure 9. In polar coordinates, the center wavelength shift and angle of the fiber grating are in an obvious “8” shape, indicating that the sensor has directional sensing characteristics.

Fig. 9 The relationship between the wavelength drift of FBG center and the angle in polar coordinates

In this paper, an optical fiber airflow sensor based on fiber grating array is proposed. The experimental platform built can realize the airflow monitoring of the sensor with a uniform airflow field. Through the comparative analysis of the substrates with different lengths, the appropriate sensor substrate length is selected for this experimental system. Through the airflow impact experiment, it can be seen that the drift of the center wavelength of the sensor grating array has a quadratic function relationship with the flow velocity of the airflow field.

The variance of the curve is 0.9742. The experimental study shows that the optical fiber airflow sensor can realize the monitoring of airflow sensing. Compared with other traditional airflow sensors, the structure has simple production process, compact structure and high reliability.