CN101038205A - Chirp optical fiber grating sensor and intensity demodulation system thereof - Google Patents

Abstract

The invention proposes a chirped fiber grating weighing sensor and its intensity demodulation system. The system is used for weighing vehicles during driving. The system selects two chirped fiber gratings with the same reflection spectrum, one of which is used For the induced strain, it is pasted on the pressure-sensitive area of the sensor, which is called the sensing grating, and another chirped grating is pasted on the pressure-insensitive area of the sensor, called the demodulation grating. After the sensing grating and the demodulation grating are connected in series, through The optical fiber splitter 13 is respectively connected with the broadband light source 14 and the photoelectric converter 15, and the photoelectric converter 15 is connected with the computer 16 to form a pressure sensing and demodulation system. Fix the equal-strength beam 1 on the base 5 of the weighing instrument with screws through the screw hole 2 during detection, and install the pulley 3 at the free end of the equal-strength beam 1, and the pulley 3 is installed on both sides of the equal-strength beam 1 through the bolt 4. The signal provided by the sensor can be transmitted over a long distance, and is widely used in various dynamic weighing and billing systems such as roads and bridges. The sensor can work reliably for a long time in harsh outdoor environments such as humid water, electromagnetic interference, etc., with high measurement accuracy and low cost.

Description

Chirped fiber grating load cell and its intensity demodulation system

technical field

The invention relates to a chirped optical fiber grating weighing sensor and its intensity demodulation system, which is especially suitable for weighing vehicles in motion.

technical background

There are currently two types of dynamic vehicle weighing systems, one is a portable weighing instrument, and the other is a fixed weighing instrument, which is used for expressway management and toll collection. Most of these weighing instruments reported in Chinese and foreign literature use electrical sensors, including: resistance strain gauges, piezoelectric sensors, capacitor strips, linear variable differential transformers, etc. Since the fixed weighing instruments are all installed in the harsh outdoor environment, they have poor reliability, are susceptible to electromagnetic interference, are afraid of moisture, and cannot transmit signals over long distances. Most of the dynamic vehicle weighing systems are installed on highways, where the working environment is harsh, and the sensing part is always located below the road surface, which is easy to be soaked in stagnant water. Therefore, the existing weighing sensor is easy to be damaged and cannot work reliably for a long time. At the same time, the drainage problem needs to be fully considered, so the structure is complicated. In order to solve the above problems, the U.S. discloses a patent, the patent No. 4560016 "Method and Apparatus for measuring the weight of a vehicle while the vehicle is in motion". This invention uses the principle of optical fiber microbending effect to invent a weighing instrument, which avoids the shortcomings of electrical sensors, but uses optical fiber microbending as the sensing mechanism, which has low measurement accuracy, poor reliability, and complex structure.

Fiber Bragg grating sensor and demodulation system is a new type of sensing technology developed in recent years. It adopts wavelength coding and has the advantages of high measurement accuracy and long-term working reliability. However, the cost of the wavelength demodulator based on the Fabry-Perot filter is relatively high. This type of sensor system is only suitable for large-scale engineering fields of distributed multi-point measurement, and instruments and meters that measure at a single point or a small number of points Field applications are limited. In the document "Identical broadband chirped FratinF interroFationtechnique for temperature and strain sensinF", R.W.Fallon and others proposed to use the same two chirped fiber gratings to sense and demodulate strain and temperature measurement systems, avoiding the use of Fabry-Perot The wavelength demodulator of the filter reduces the cost. However, this system does not solve the cross-sensitivity problem of temperature and strain; in addition, as reported in the literature, the demodulation chirped fiber grating adopts a transmission connection method, and a fiber coupler is required to input the optical signal reflected by the sensing grating into the solution. In the dimming grating, two optical fibers are needed to connect the sensor and the system, and the structure is complex.

Contents of the invention

The purpose of the present invention is to develop a fiber grating weighing sensor and a signal demodulation system that meet the requirements of precision, can work reliably in a harsh outdoor environment for a long time, has a simple structure, and is low in cost.

In order to meet the above requirements, the present invention designs a dynamic weighing sensor and demodulation system for expressways. The system uses double-chirped fiber gratings, connects two identical chirped fiber gratings in series, and measures the reflection spectrum of the grating A dual-chirped fiber grating load cell and its demodulation system for expressway dynamic weighing are designed. This system not only solves the problem of cross-sensitivity between temperature and strain, but also has the advantages of simple system structure, low cost, long transmission distance, anti-electromagnetic interference, high measurement accuracy, and good long-term stability. It is very suitable for dynamic weighing of expressways system.

The chirped fiber grating weighing sensor and its intensity demodulation system of this invention adopt an equal-intensity cantilever beam structure. Choose two chirped fiber gratings with exactly the same reflection spectrum, one of which is used to induce strain, and is pasted on the pressure-sensitive area of the sensor, called the sensing grating, and the other chirped grating is pasted on the pressure-insensitive area of the sensor , called the demodulation grating, after the sensing grating and the demodulation grating are connected in series, they are respectively connected to the broadband light source 14 and the photoelectric converter 15 through the optical fiber splitter 13, and the photoelectric converter 15 is connected to the computer 16 to form a pressure sensor Sense and demodulation system. The sensor is composed of equal strength beam 1, screw hole 2, bearing 3, bolt 4, base 5, base screw hole 6, sensing grating 7, demodulation grating 8, and optical fiber 9. When detecting, use a screw to pass through the screw hole 2 to pass the equal intensity The beam 1 is fixed on the base 5 of the weighing instrument, and the free end of the equal-strength beam 1 is installed with a bearing 3 , and the bearing 3 is installed on both sides of the equal-strength beam 1 through bolts 4 . Sensing grating 7 and demodulation grating 8 are chirped fiber gratings with matching reflection spectrum, wherein sensing grating 7 is pasted on the pressure sensitive area, and the strain in this area is evenly distributed due to the equal intensity design. The paste area of the demodulation grating 8 is fixed to the base by screws, which is an insensitive area for pressure. However, considering the transmission of strain, we paste the chirped fiber grating horizontally to ensure that it is not affected by strain. Due to the stress-bearing end of the equal-strength beam, the external pressure acts on the equal-strength beam through the bearing, which can only accept the longitudinal pressure and avoid the influence of lateral impact on the weighing result. When there is pressure acting on the bearing, the bearing transmits the longitudinal pressure to the equal-intensity beam, and the sensing grating senses the strain of the equal-intensity beam, and converts the load pressure into the drift of the reflection spectrum of the sensing grating, while the reflection spectrum of the demodulation grating does not At this time, the area under the reflection spectrum envelope of the two gratings is equal to the reflection light intensity of the two gratings. Obviously, the change ΔP of the total reflected light intensity of the two gratings is proportional to the distance Δλ of the spectral translation of the sensing grating, that is

ΔP=K ₁ ·Δλ (1)

In the formula, _K1 is a constant of proportionality. The wavelength change of the sensing grating corresponds to the strain of the constant-strength beam, and is proportional to the pressure on the constant-strength beam, which is given by

Δλ=K ₂ ·F (2)

In the formula, _K2 is a constant of proportionality. Substituting (2) into (1), we get

                                                 

In the formula, K=K ₁ K ₂

The optical signal reflected by the fiber grating is converted into a voltage through a photoelectric cell, and then sent to a computer for analysis and processing. Then the voltage V has a linear relationship with the load F of the sensor, and its proportional constant can be determined in advance through a calibration test. Therefore, the computer can obtain the weighing result only by simple calculation.

Due to the above-mentioned technical scheme, the dynamic vehicle weighing system provided by the present invention has the following beneficial effects:

1. The pressure is transmitted through the bearing, which can effectively offset the impact of the horizontal impact force of the driving car on the weighing result, so as to improve the weighing accuracy.

2 There are no electronic components on the weighing site, and the sensor can work reliably for a long time in harsh environments such as wet water and electromagnetic interference.

3 The sensor integrates "sensing" and "demodulation" functions, directly outputs light intensity signals, and has low requirements for back-end equipment.

4 Signal measurement with high precision and good repeatability.

Description of drawings

Figure 1 is a schematic diagram of the chirped fiber grating weighing sensor system

Figure 2 is the chirped fiber grating reflection spectrum

Figure 3 is a structural schematic diagram of a chirped fiber grating load cell

Figure 4 is the assembly diagram of the chirped fiber grating load cell and weighing platform

Figure 5 is the chirped fiber grating layout scheme 1

Figure 6 is the chirped fiber grating layout scheme 2

Figure 7 is the chirped fiber grating layout scheme 3

In the figure, 1-equal strength beam, 2-screw hole, 3-bearing, 4-bolt, 5-base, 6-base screw hole, 7-sensing grating 1, 8-demodulation grating, 9-optical fiber, 10 -Chirped fiber grating load cell, 11-weighing platform, 12-top cover, 13-fiber splitter, 14-broadband light source, 15-photoelectric converter, 16-computer, 17-sensing grating 2

Detailed ways

The technical solution of the present invention will be further described below in conjunction with accompanying drawings 1, 2, 3, 4, 5, 6, and 7.

The structural schematic diagram of the chirped fiber grating weighing sensor of the present invention is shown in FIG. 3 , and the sensor adopts the structure of an equal-intensity beam 1 . The equal strength beam 1 is fixed on the sensor base 5 through the screw hole 2 by the screw. A bearing 3 is installed at the free end of the equal-strength beam, and the bearing 3 is installed on both sides of the equal-strength beam through bolts 4 . The sensing grating 7 and the demodulating grating 8 are pasted in series on the surface of the equal-intensity beam. There are three methods for the specific layout of the two chirped fiber gratings.

When in use, the chirped fiber grating weighing sensor 10 is fixed on the frame of the weighing instrument through the screw holes 6 and screws on the base, and the installation direction and position of the sensor are shown in FIG. 4 . The direction of the arrow in the figure is the road direction, that is, the driving direction of the vehicle. The upper surface of the weighing platform 11 is flush with the road surface, and only the bearing of the load cell is in contact with the lower surface. The movement through the bearing can counteract the impact force of the driving vehicle in the horizontal direction, thereby improving the influence of the vehicle speed on the weighing result.

Fig. 1 is a schematic diagram of the working principle of the chirped fiber grating weighing sensor system. After the fixed sensing grating 7 and the demodulating grating 8 are connected in series on the chirped fiber grating weighing sensor 10, they are respectively connected to the broadband through a fiber splitter 13. The light source 14 is connected to the photoelectric converter 15, and the light emitted by the broadband light source 14 is transmitted to the sensing grating 7 and the demodulation grating 8 through the optical fiber 9, and the light signal reflected by the grating is transmitted through the optical fiber 9 and enters the photoelectric tube 15 to be converted into a voltage signal and enters the computer 16 Perform process analysis and display weighing results.

In the present invention, chirped gratings with matching reflection spectra and wavelengths are selected as the sensing grating 7 and the demodulating grating 8 respectively. The sensing grating 7 is pasted on the pressure-sensitive area, and the strain in this area is evenly distributed due to the equal-intensity design. The paste area of the demodulation grating 8 is fixed to the base by screws, which is an insensitive area for pressure. However, considering the transmission of strain, we paste the chirped fiber grating horizontally to ensure that it is not affected by strain. The chirped fiber grating load cell works on the principle that the grating strain causes the wavelength drift, which causes the reflected light intensity to change. The top reflectance of the reflection spectrum of the chirped fiber grating with saturation exposure is close to 1, and it is very flat. The reflection spectrum of the sensing grating 7 and the demodulation grating 8 we choose are completely matched. The reflection spectrum of the two gratings connected in series is shown in Figure 2 As shown by the solid line, the reflected light intensity of the two gratings is equal to the area contained by the solid line and the X axis. When the force F acts on the equal-intensity beam, the reflection spectrum of the sensing grating 7 is shifted by Δλ, as shown by the dotted line in Fig. 2, while the reflection spectrum of the demodulation grating 8 remains unchanged. At this time, the reflected light intensity of the two gratings is equal to The area contained by the two grating reflectance spectrum envelopes and the X-axis. Obviously, the amount of change in the total reflected light intensity of the two gratings is proportional to the translational distance Δλ of the sensing grating 7 . Therefore, by detecting the reflected light intensity of the grating, the moving distance of the reflective spectrum of the sensing grating can be known, so as to calculate the magnitude of the pressure F applied on the sensor. There are three layout methods for chirped fiber gratings. Combined with the accompanying drawings, the way of layout and the change of reflected light are explained as follows.

Fiber Bragg Grating Layout Scheme 1: As shown in Figure 5, two chirped gratings with matching reflection spectrum wavelengths are used. Both gratings are pasted on the upper surface of the equal-intensity beam. The sensing grating 7 is pasted parallel to the symmetrical central axis of the equal-intensity beam. Located on the central axis is the best. The demodulation grating 8 is pasted perpendicular to the symmetrical central axis of the equal-intensity beam. When the free end of the equal-intensity beam is subjected to pressure, the sensing grating 7 stretches, and its reflection spectrum translates to the long-wave direction under the condition that the spectrum type remains unchanged. Obviously, the variation of the total reflected light intensity of the two gratings is proportional to the translational distance Δλ of the sensing grating 7 . In this solution, the sensing grating 7 can also be pasted on the lower surface of the equal-intensity beam. At this time, when the force F acts on the equal-intensity beam, the reflection spectrum of the sensing grating 7 will shift to the short-wave direction by Δλ, but the total reflected light of the two gratings The change in intensity is still proportional to the translational distance Δλ of the sensing grating 7 .

Fiber Bragg Grating Layout Scheme 2: As shown in Figure 6, two chirped gratings with matching reflection spectrum wavelengths are used. Both gratings are sensing gratings, that is, pasted on the pressure-sensitive area of the equal-intensity beam, parallel to the symmetrical central axis of the equal-intensity beam For pasting, it is best to be located on the central axis. The sensing grating 7 is pasted on the upper surface of the equal-intensity beam, which is a pulling grid, and the other sensing grating 17 is pasted on the lower surface of the equal-intensity beam, which is a pressure grating. When the free end of the equal-intensity beam is subjected to pressure, the sensing grating 7 is stretched, and its reflection spectrum is translated to the long-wave direction under the condition that the spectrum type remains unchanged, while the sensing grating 17 is compressed, and its reflection spectrum is ensured under the condition that the spectrum type remains unchanged. Pan down to the shortwave direction. The total reflected light intensity of the two gratings is equal to the area contained by the two grating reflection spectrum envelopes and the X axis. The variation of the total reflected light intensity is still proportional to the sum of the translational distances of the two sensing gratings 7, 17 Δλ ₁ +Δλ ₂ . By adopting this solution, the change amount of reflected light intensity can be increased, thereby increasing the weighing resolution.

Fiber Bragg grating layout scheme three: as shown in Figure 7, three chirped gratings with matched reflection spectra and wavelengths are used, and the two gratings are sensing gratings, which are pasted on the pressure-sensitive area of the equal-intensity beam and pasted parallel to the symmetrical central axis of the equal-intensity beam , preferably located on the central axis, the sensing grating 7 is pasted on the upper surface of the equal-intensity beam, which is a pulling grid, and the sensing grating 17 is pasted on the lower surface of the equal-strength beam, which is a pressure grating. The demodulation grating 8 is located in the pressure-insensitive area and pasted perpendicular to the symmetrical central axis of the equal-intensity beam. When the free end of the equal-intensity beam is subjected to pressure, the reflection spectrum of the demodulation grating 8 remains unchanged, the sensing grating 7 stretches, and its reflection spectrum shifts to the long-wave direction under the condition that the spectrum type remains unchanged, while the sensing grating 17 compresses , its reflection spectrum shifts to the short-wave direction under the condition that the spectrum type remains unchanged. At this time, the reflected light intensity of the three gratings is equal to the area contained by the reflection spectral envelopes of the three gratings and the X axis. The change of the total reflected light intensity is still proportional to the sum of the translational distances of the two sensing gratings 7, 17 Δλ ₁ +Δλ ₂ . By adopting this solution, the maximum variation of reflected light intensity can be further increased, and the total reflected light intensity can be increased, thereby increasing the weighing accuracy.

Claims (3)

1, a kind of chirp optical fiber grating sensor and intensity demodulation system thereof, it is characterized in that: this system adopts the equi intensity cantilever structure, after chirp optical fiber grating sensor (10) is gone up fixing sensing grating (7) reconciliation light modulation grid (8) serial connection, be connected with photoelectric commutator (15) with wideband light source (14) respectively by optical fiber splitter (13), the light that wideband light source (14) sends is transferred to sensing grating (7) again through optical fiber (9), the optical grating reflection light signal enters photoelectric tube (15) through optical fiber (9) transmission and is converted into voltage signal, enters computing machine (16).

2, a kind of chirp optical fiber grating sensor according to claim 1 and intensity demodulation system thereof is characterized in that sensing grating (7) sticks on the pressure-sensitive area of sensor, and demodulation grating (8) sticks on the non-sensitive district of pressure of sensor.

3, a kind of chirp optical fiber grating sensor, it is characterized in that: this sensor is made up of the beam of uniform strength (1), screw (2), pulley (3), bolt (4), base (5), base screw (6), sensing grating (7), demodulation grating (8), optical fiber (9), the beam of uniform strength (1) is fixed on the weighing instrument base (5) by screw (2) with screw during detection, the beam of uniform strength (1) free end is installed pulley (3), pulley (3) is installed on the both sides of the beam of uniform strength (1) by bolt (4), and sensing grating (7) is conciliate light modulation grid (8) and is serially connected.

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