Design of perimeter fence alarm system based on FBG sensor

Abstract: Although traditional perimeter security or fence alarm systems (such as active infrared counter-radiation, microwave counter-radiation, leakage cable, vibration cable, electronic fence, power grid, etc.) have made certain contributions to security technology prevention, they are limited by some objective technical conditions and other factors, and there are also certain defects, such as false alarms and missed alarms. To solve this problem, scholars at home and abroad are competing to study fiber optic fence sensing technology. This article introduces the principles and advantages of FBG sensors, the composition and working principle of the perimeter fence alarm system based on FBG sensing, wavelength shift demodulation, multiple intrusion positioning...

Although traditional perimeter security or fence alarm systems (such as active infrared counter-radiation, microwave counter-radiation, leakage cables, vibration cables, electronic fences, power grids, etc.) have made certain contributions to security technology prevention, they are limited by some objective technical conditions and other factors, and there are also certain defects, such as false alarms and missed alarms. To solve this problem, scholars at home and abroad are competing to study fiber optic fence sensing technology. This article introduces the principles and advantages of FBG sensors, the composition and working principle of the perimeter fence alarm system based on FBG sensing, wavelength shift demodulation, multiple intrusion positioning and intrusion mode characteristics. Experiments have proved that the system is reliable and has achieved the desired purpose.

introduction

Over the years, traditional perimeter security or fence alarm systems (such as active infrared counter-radiation, microwave counter-radiation, leakage cables, vibration cables, electronic fences, etc.) have made certain contributions to security technology prevention. However, due to some objective technical conditions and other factors, there are also certain defects: For example, the active infrared counter-radiation fence alarm system has poor ability to adapt to the environment and is easily restricted by the terrain conditions such as height, twists, turns, and bends. Moreover, they are not suitable for harsh climates and are easily affected by natural climates such as high temperature, low temperature, strong light, dust, rain, snow, fog, and frost, and are prone to false alarms; for example, the fence alarm systems such as leakage cables, vibration cables, electronic fences, and power grids are all active electrical sensors, and the system power consumption is very large. In addition, electronic fences, power grids, etc. are harmful to a certain extent; they are also susceptible to electromagnetic interference, signal interference, crosstalk, etc., which reduce sensitivity, increase false alarm rates, and increase missed alarm rates (such as covering the electric fence with electrical insulation).

Compared with the above perimeter security or fence alarm system, the perimeter security or fence alarm system made of new fiber optic sensing technology has obvious technical advantages:

· Anti-electromagnetic interference, good electrical insulation, safe and reliable, corrosion-resistant, stable chemical properties, so it is completely unaffected by lightning and can work in harsh chemical environments, field environments and places with strong electromagnetic interference;

Small size, light weight, flexible geometry, low transmission loss, large transmission capacity, and very good reliability and stability;

· It can not only detect external disturbances, but also determine the location of external disturbances. The system has low cost, simple structure, easy expansion and installation;

No radiation, no flammable or explosive materials, waterproof and environmentally friendly;

Low energy dependence, which can greatly save the cost of power supply equipment and lines, and is suitable for long-distance use;

Different detection methods can be selected according to the conditions of the object being tested. In addition, it has little impact on the medium being tested, so it is very beneficial for application in fields with complex environments such as structural testing.

The fiber optic perimeter fence alarm system can be implemented using three methods: one is optical time domain reflectometry (OTDR) technology; the second is fiber optic interferometry fiber optic sensor; and the third is FBG distributed fiber optic sensor.

In recent years, fiber Bragg grating is one of the fastest growing and most widely used fiber passive devices. Because its sensitive variable parameter is the wavelength of light, it is not affected by the light source, transmission line loss and other factors that cause the change of light intensity. It is easy to connect with the system and other fiber optic devices to form a distributed sensing system, so it can achieve real-time measurement and distributed measurement. Because FBG has excellent temperature and strain response characteristics, it can be used to make stress, pressure, vibration, fire and temperature sensors, especially convenient for perimeter security and fence intrusion alarm systems, so it will have great practical and social significance in national and people's security and anti-terrorism struggle.

FBG sensor principle and advantages

FBG (Fiber Bragg Grating) is a development of the concept of diffraction grating, and its diffraction is achieved by the change of the refractive index inside the optical fiber. FBG was introduced in 1978. It uses the photosensitivity of doped optical fibers (such as germanium, phosphorus, etc.) and uses ultraviolet writing to make the external incident photons interact with the doped particles in the fiber core, resulting in periodic or non-periodic permanent changes in the refractive index of the fiber core along the fiber axis, forming a spatial phase grating in the fiber core (as shown in Figure 1). In the figure, the period Λ of FBG is generally less than 1μm.

Figure 1 Structure of uniform periodic FBG

The basic principle of FBG sensing is shown in Figure 2. When a beam of light is sent into the FBG, according to the grating theory, when the Bragg condition is met, total reflection will occur, and its reflection spectrum will peak at the Bragg wavelength. When the grating is affected by an external physical field (such as stress, strain, temperature, etc.), its grating pitch Λ changes accordingly, thereby changing the wavelength of the back-reflected light. The change in the corresponding physical quantity of the measured part can be determined based on the size of the change in ΔλB.

Figure 2 FBG sensing principle

FBG is like a narrow-band mirror, reflecting only one wavelength and transmitting the rest. The reflected wavelength is called the Bragg wavelength, which satisfies the Bragg equation of the fiber grating, that is, it satisfies the condition

λB=2neffΛ (1)

In the formula, ∧ is the Bragg grating period; neff is the effective refractive index of the reverse coupling mode. This equation provides a theoretical tool for the sensing response of the Bragg wavelength of the fiber Bragg grating under external influences, that is, any process that changes these two parameters will cause the shift of the Bragg wavelength of the grating. Therefore, the common FBG sensor detects the measured value by measuring the movement (or drift) of the Bragg wavelength.

Among all the external factors that cause the Bragg wavelength shift of the grating, the most direct ones are stress and strain parameters. Because whether the grating is stretched or squeezed, it will cause the grating period ∧ to change, and the elastic-optical effect of the optical fiber itself makes the effective refractive index change with the change of the external stress state. Based on this, the fiber Bragg grating can be used to make a sensitive optical fiber sensor. Among them, the shift of the Bragg wavelength of the grating caused by stress can be uniformly described by the following formula

ΔλB=2neffΔΛ+2ΔneffΛ (2)

In the formula, ΔΛ is the elastic deformation of the optical fiber itself under stress; Δneff is the photoelastic effect of the optical fiber. Different external stress states will lead to different changes in ΔΛ and Δneff. Therefore, as long as the shift ΔλB of the grating Bragg wavelength in the reflected signal is detected, the change of the sensor quantity to be measured can be detected.

From the perspective of the photoelastic effect, the fiber Bragg grating is more sensitive to longitudinal pressure than transverse pressure. Combining the photoelastic and waveguide effects, the fiber Bragg grating is less sensitive to uniform transverse stress than longitudinal expansion, so under complex stress conditions, the wavelength shift caused by longitudinal pressure will dominate.

If only the axial strain (i.e. longitudinal pressure) is considered, the relative change in the displacement of the central wavelength is

In the formula, is the fiber Bragg grating strain sensitivity coefficient, is the axial strain. It can be seen from formula (3) that the change of reflection wavelength is proportional to the strain stress. In other words, the corresponding strain force can be obtained from the change of reflection wavelength.

Changes in external temperature will also cause the Bragg wavelength of the fiber Bragg grating to shift. From a physical point of view, the main reasons for the wavelength shift are: fiber thermo-optic effect, fiber thermal expansion effect, and elastic-optic effect caused by thermal stress inside the fiber. Starting from the Bragg equation (1), when the external temperature changes, the equation (2) is expanded to obtain the shift of the Bragg wavelength of the fiber Bragg grating caused by the temperature change ΔT. It is confirmed through theoretical derivation that when the material is determined, the sensitivity coefficient of the fiber Bragg grating to temperature is basically a constant related to the material coefficient. Therefore, for pure fused silica optical fiber, when the influence of external factors is not considered, its temperature sensitivity coefficient basically depends on the temperature coefficient of the refractive index of the material, and the elastic-optic effect and waveguide effect will not have a significant effect on the wavelength shift of the fiber Bragg grating. Therefore, the following expression can be obtained, that is,

Where αn is the thermo-optical coefficient and αΛ is the linear thermal expansion coefficient. For fused silica optical fiber, αn=0.86×10-5/oC, and αΛ=5.5×10-7/oC.

It can be seen from formula (4) that the change in reflected wavelength is also proportional to the temperature change ΔT. That is, the corresponding temperature can be obtained from the change in reflected wavelength. For a wavelength of 1.55μm, the wavelength shift caused by a unit temperature change is 10.8pm/oC.

In addition to the advantages of general optical fiber sensors, fiber grating sensors also have the following advantages:

· Stronger anti-interference ability, high reliability and stability;

· High measurement sensitivity, high resolution, high accuracy and good repeatability;

· Large dynamic range, good linearity, self-calibration, and can be used for absolute measurement of external parameters;

· Multiple sensors can be integrated into the same optical fiber for multiplexing, making it easy to form various forms of optical fiber sensing networks;

It is convenient for monitoring bridges and other buildings from a distance, and is easy to make into smart sensors, which are widely used;

· Simple structure, long service life, easy maintenance, expansion and installation;

The grating writing process is mature, which is convenient for large-scale production.

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Design of FBG sensing perimeter fence alarm system

The FBG sensing perimeter fence alarm system is a security alarm system built using high-tech technologies such as laser, fiber optic sensing and optical communication. It is a modern defense system that monitors and alarms emergencies that threaten public safety. It is a new system based on FBG distributed fiber optic sensing technology applied to perimeter monitoring and protection.

The designed FBG sensing perimeter fence alarm system consists of a broadband light source, a coupler, a sensing optical fiber and a transmission optical fiber with a required number of FBG sensors (according to the perimeter length), a wavelength shift demodulation device, a signal processing system, etc. (as shown in Figure 3).

Figure 3 Composition and principle of FBG sensing perimeter fence alarm system

As can be seen from Figure 3, this system uses a single optical fiber (the required number of Bragg gratings are written on the photosensitive optical fiber using ultraviolet light) as a two-in-one sensing and transmission device. Various disturbances transmitted to the optical fiber (cable) through direct contact with the optical fiber or through carriers such as covering soil, barbed wire, fences, pipelines, etc., can be monitored continuously and in real time at any point in the entire process around the clock.

The working principle of the FBG sensing perimeter fence alarm system is as follows: the light emitted by the broadband light source is transmitted through the coupler to the FBG perimeter sensor optical fiber with the required number of FBGs. When the optical fiber of a certain point of the FBG sensor is disturbed by external intrusion, even if the wavelength of the FBG changes, the wavelength modulation data of the reflected light is sent to the monitoring room through the coupler through the transmission optical fiber. The input light reaches the wavelength shift demodulation device, is first received and amplified by the photoelectric detector, and then sent to the wavelength shift demodulation circuit for demodulation. Finally, it is analyzed and processed by the signal processing system and intelligently identified to determine whether there is any external harmful intrusion. Non-hazardous environmental interference such as thunder and firecrackers can also be identified, and harmless judgments can be made; when climbing barbed wire, pressing walls, running or walking in prohibited areas, and mechanical construction that may threaten perimeter buildings are identified, in order to improve reliability, the monitoring images taken by the perimeter monitoring camera must be judged and verified before the system early warning or real-time alarm can be realized to achieve the purpose of real-time monitoring of threatening behaviors that invade the perimeter of the fortified area.

For local high-risk areas, the system can also monitor and record voice. This function does not require electrical or metal sensors, but can be achieved with optical fiber, thus enriching the functions and protection level of the monitoring system using a single optical fiber.

Wavelength shift demodulation

In fact, the key issue in studying FBG sensors is how to accurately measure the shift of the FBG reflected wavelength. Traditionally, spectrometer demodulation systems are generally used. Although the miniature spectrometers that have appeared in recent years are small in size and cheap, their spectral resolution is only at the order of 0.1 nm, far from the pm-level resolution required for FBG demodulation. In order to improve the measurement accuracy of Bragg wavelength drift, scholars from various countries have studied many demodulation methods, such as edge filter method, tunable filter method, interferometer scanning method and dual-cavity interferometer scanning method. In 2007, the Key Laboratory of Optoelectronic Information Technology Science of the Ministry of Education of the School of Precision Instruments of Tianjin University developed a portable FBG wavelength demodulator for the application field of large-scale building structure monitoring. It is based on the principle of passive proportional demodulation, uses fused taper devices as linear filters, uses phase-locked amplification technology to extract weak signals, and uses a single-chip microcomputer to control the collection, display and storage of FBG wavelength information. The demodulator has a simple structure and low cost, and can achieve a wide range of wavelength measurement. Experiments show that the demodulation range of the fiber grating demodulator is 15 nm, and the wavelength measurement accuracy is 12.4 pm.

A method based on FP tunable filter and wavelength reference is adopted, and the interpolation-correlation spectrum method is used for processing. First, some points are linearly inserted between each two adjacent points in the original spectrum, and then the Bragg wavelength shift is obtained by the correlation spectrum method. This method can not only effectively suppress noise, but also accurately measure the Bragg wavelength shift, thereby realizing high-precision measurement of external parameters such as temperature and strain. Theoretical analysis and experiments show that it is feasible to measure the Bragg wavelength shift by correlation spectrum method, and it can improve the signal-to-noise ratio and thus improve the demodulation accuracy. On this basis, combined with the linear interpolation method, a certain number of points are inserted between each two adjacent points in the original spectrum, which can further improve the demodulation accuracy. This demodulation method can make the Bragg grating wavelength resolution reach 1 pm and the temperature measurement accuracy reach ±0.2℃.

Multiple intrusion locations and intrusion pattern characteristics

Generally, the Fiber Bragg Grating perimeter fence alarm system has the problem of judging multiple simultaneous interferences. Therefore, when judging that a threatening intrusion has occurred, the system needs to be able to locate the intrusion point in real time based on the analysis of the optical signal modulation, so that security personnel can take effective measures in a timely manner with clear targets to prevent subsequent intrusion events from occurring.

In order to solve the problem that when multiple FBG sensors in the optical fiber fence alarm system are disturbed at the same time, it is difficult to locate the alarm signal and cannot effectively identify and judge the alarm signal, an analysis method based on empirical mode decomposition (EMD) and wavelet packet feature entropy algorithm can be used to solve it. We first perform empirical mode decomposition on the alarm signal, and then combine it with wavelet packet decomposition to obtain the wavelet packet coefficient to extract the energy distribution of its signal; secondly, normalize it to obtain the energy distribution feature vector of the signal; finally, use correlation analysis to realize the recognition and judgment of the alarm signal.

Experiments have shown that the combination of empirical mode decomposition and wavelet packet characteristic entropy algorithm can effectively solve the problem of simultaneous interference judgment positioning alarm in the optical fiber fence alarm system of FBG sensors. Therefore, it is fully proved that the use of this method is effective in solving the cascade judgment alarm signal of FBG sensors in the optical fiber fence alarm system.

From the general intrusion and experimental data of fiber perimeter fences, we have obtained six different intrusion alarm modes, and their typical feature diagrams are shown in Figure 4. In Figure 4: (a) is the walking intrusion mode; (b) is the running intrusion mode; (c) is the vehicle intrusion mode; (d) is the climbing intrusion mode; (e) is the shearing intrusion mode; (f) is the shaking intrusion mode. Obviously, the first three are ground intrusion modes; the last three are fence intrusion modes. This system first extracts various feature data such as length, extreme value, amplitude, period, mean, wavelet coefficient mean, signal fluctuation, strain feature, peak value feature, impact feature, vibration feature, etc. from the alarm signal; secondly, it determines which features the signal contains according to the threshold unit; finally, it analyzes and distinguishes these features according to the membership function determined by experience, and uses the feature combination method to determine which intrusion mode the signal is.

Figure 4 Six intrusion modes

Experiments have proved that the design is feasible, the system is reliable, and it has fully achieved the expected purpose.

Conclusion

From the above introduction, it can be seen that the fiber optic fence based on long-distance quasi-distributed FBG sensors, as a new type of security monitoring system, is not only anti-electromagnetic interference, anti-corrosion, and easy to reuse, but also has the advantages of mature technology, low cost, accurate alarm positioning, and high reliability. It is currently a mainstream development direction of intelligent security monitoring and is bound to have broad application prospects in the security field.

It is worth pointing out that in order to improve reliability, this solution is finally verified by the video surveillance system integration linkage before driving the sound and light alarm. The characteristics of this solution are: simple and efficient, easy to install, convenient for users, simple maintenance, and the sensitivity can be adjusted according to the actual installation environment. It is stable and reliable. Therefore, after it is transformed into finalized production, it will be very suitable for large, medium and small perimeter fence deployment users, and it is believed that it will be rapidly popularized and promoted in the civilian market.