1、外文翻译使用双光纤布拉格光栅传感器和互相关技术的水流量计A water flowmeter using dual fiber Bragg grating sensors and cross-correlation techniqueAbstractIn this paper, a principle and experimental results of a cross-correlation flowmeter using fiber Bragg grating (FBG) sensors are presented.The flowmeter has no electronics and
2、no mechanical parts in its sensing part and the structure is thus simple and immune to electromagnetic interference (EMI). For water flow measurement, the flowmeter uses the time delay of the vortex signal generated by a bluff body. Karman vortex shedding frequency is also detected and utilized for
3、the flow velocity estimation in the system. In order to realize a low noise and wide bandwidth system, we employed interferometric detection as a FBG wavelength-shift detection method. The noise spectral densityof the FBG sensor with the interferometric detection was 4104 pm/(Hz)1/2 corresponding to
4、 0.33 n/(Hz)1/2. A water flow experimentshowed that the flowmeter had a linear characteristic at velocity range from 0 to 1.0 m/s and the minimum detectable velocity of 0.05 m/s.1. IntroductionFiber Bragg grating (FBG) sensors have various advantages such as small size, simplicity in sensing princip
5、le, electromagnetic interference (EMI) immunity and capability of multiplexing. Because of these advantages, a number of basic researches and applications on FBG sensors have been made 13. In telecommunication systems, FBGs are used as add-drop multiplexers because of their narrow bandwidth (typical
6、ly 0.1 nm). The FBG application to optical tunable filters is also useful for discrimination of the signals in FBG sensor systems 4. The applications to smart structures and health monitoring are attractive and have been investigated actively 5,6. FBGs are embedded in composite materialsand used as
7、strain and temperature sensors in the application. In the field of civil engineering, strain measurements for bridges and buildings are made using FBG sensor arrays with wavelength division multiplexing (WDM) and time division multiplexing (TDM) 7.In the FBG sensor applications, the choice of the wa
8、velength-shift detection method is very important because the noise level and the measurement bandwidth of the system are mainly determined by the detection method. The most commonly used detection method is the tunable filter detection using FabryPerot filter. This method is the standard technique
9、and provides static or quasi-static measurement with a strain resolution of 1 _. Another promising method is the interferometric detection 8,9. This method has the capability of dynamic measurement with high strain resolution in the order of n/(Hz)1/2. There are some reports about the noise estimati
10、on of the FBG sensor with interferometric detection 1012. Our subject of research is the FBG application to a water flowmeter. There are various kinds of flowmeters including turbine flowmeters, vortex flowmeters and differential pressure type flowmeters. Measurands of flowmeters are ranging over va
11、rious flow including water flow, gas flow and multiphase flow. Cross-correlation flowmeter, which utilizes a time delay of signals by coherent structures including vortices and naturally existing unsteady pressure field, is usually used for pipe flow measurement. The advantage of the cross-correlati
12、on flowmeter is its simplicity in sensing principle. The only parameter required to the flowmeter is the distance between two sensors. In the cross-correlation flowmeter, two pair of a ultrasonic transmitter and a receiver are usually used because of their non-intrusiveness to the flow 13. The flowm
13、eter using the ultrasonic transducers has a good linearity at wide velocity range. The problem with the flowmeter is complexity of the sensing part because the system needs at least four transducers. The cross-correlation flowmeter reported by Dyakowski and Williams 14 uses 16 light rays (eight pair
14、s) to detect flow signals in gassolid mixture. The velocities are obtained from cross-correlation of the intensity modulated light signals, and the average velocity and the velocity distribution in the pipe are then obtained by combining calculated velocities. This flowmeter is attractive because of
15、 EMI immunity and the passive nature. However, the system needs particles, which reflect or scatter the light rays, in the fluid and the application is limited. There are few reports concerning the cross-correlation flowmeter using optical sensors, not light ray or laser beam, suited for water flow
16、measurement.In this paper, we present a water flowmeter using dual FBG sensors and cross-correlation technique. The flowmeter has no electronics and no mechanical parts in its sensing part, and thus the structure is simple. At first, we explain the principle and the schematic diagram of the flowmete
17、r. Next, we present the noise estimation of the FBG sensor with the interferometric detection using a MachZehnder interferometer comprised of a 2 2 and a 3 3 couplers 9. Finally, we describe experimental performances of the FBG sensor and the flowmeter.2. A cross-correlation flowmeter using FBG sens
18、orsFig. 1 shows the principle of the flowmeter. The cross-correlation flowmeter presented here uses FBG strain sensors comprised of FBGs and metal cantilevers. In the flow measurement section, the FBG sensors and a bluff body are used. The bluff body whose shape is a rectangular column generates sta
19、ble vortices. The time delay between the vortex signals detected by the FBG sensors are estimated using the smoothed coherence transform RSCOT() 15. The function RSCOT() is expressed as follows: (1)where Gxx(f) and Gyy(f) are the power spectra of the upstream and downstream sensor signals, Gxy(f) is
20、 the cross-spectrum of two signals and F1 denotes the inverse Fourier transform. The function RSCOT() is a cross-correlation function weighted with the coherence of the signals and can detect the time delay more precisely and robustly than the simple cross-correlation function. The maximum of RSCOT(
21、) is the best estimate _t of the time delay between two FBG sensors. The measured velocity v meas is then calculated from the following simple equation: (2)where ds is the distance between two sensors.Fig. 2 illustrates the schematic diagram of the whole system. We used an amplified spontaneous emis
22、sion (ASE) as an optical source of the system. The ASE has output power of 22 dBm and full width at half maximum (FWHM) of 50 nm at C-band. The light from the ASE is separated by an optical 3 dB coupler and then illuminates two FBG sensors installed to the PVC pipe whose inner diameter is 20 mm. The
23、 light reflected by the FBG sensors is fed to MachZehnder interferometers, which are comprised of a 22 and a 33 couplers, with the optical path differences of 1.635 and 3.169 mm. In the 33 coupler, three fibers are arranged in a triangular array. These interferometers are used as wavelength-shift de
24、tectors for interferometric detection. The incident light is phase-modulated by the MachZehnder interferometer and then converted to voltage signals by photodetectors. Six output signals are simultaneously digitized by an A/D converter with a resolution of 16 bit and sampling frequency of 10 kHz, an
25、d the detected signals are then processed to obtain the time delay. The FBG reflects the light wave with a certain wavelength B called Bragg wavelength and the wavelength is then expressed as follows:，（3）where n is the effective refractive index of the FBG and is the modulation pitch of the refracti
26、ve index of the FBG. The Bragg wavelength B changes by longitudinal strain z applied to the FBG, and the Bragg wavelength-shift B is expressed as follows 3: (4)where p12 is the elasto-optic constant of the optical fiber and is approximately 0.22. This yields the strain sensitivity of 1.2 pm/_.To obt
27、ain the shift B, the outputs of the interferometer are used, and the outputs Vm (m = 1, 2, 3) are expressed as follows 9:Vm = mVin + Re() = mVin1 + cos(MZI + m), （5）where () is the auto-correlation function of light wave reflected by the FBG sensor, Vin is the voltage corresponding to optical power
28、reflected by the FBG sensor, and m is the coefficient compensating differences of photodetector sensitivities and obtained from preliminary experiments. If the split ratios of the 2 2 and 3 3 couplers are 1:1 and 1:1:1, respectively, one can obtain 1 = 0, 2 = 2/3 and 3 = 2/3, and the outputs V1, V2
29、and V3 are derived as follows: （6） The signal MZI can be then calculated using the following equation: (7)where L is the optical path difference of the interferometer. The relationship between the signal variation MZI and the shift B is expressed as follows: (8)where the term (B + B) was assumed to
30、be nearly equal to B because the shift B is significantly smaller than B. An accidental loss of optical power while measurement, which causes problems in optical intensity modulation type sensors, is admissible in some measure because the wavelength-to-phase sensitivity (=2L/2B) depends on only the
31、path difference L.3. Noise estimation of the FBG sensor with interferometric detectionThere are some reports about the noise estimation of FBG sensors with interferometric detection. However the noise estimation reports about the interferometric detection using a 2 2 and a 3 3 couplers have not been
32、 presented.3.1. Noise of the photodetectorFig. 3 shows the circuit diagram of the photodetector. The noise of the photodetector is defined by the noise of a photodiode and a transimpedance amplifier. The noise of the photodetector is determined by thermal noises due to the feedback resistance Rf of the transimpedance amplifier Rf and shunt resistance Rsh of the photodiode and by shot noise due to the output current im (=Vm/Rf ) of the photodiode. The equivalent input noise voltage and current of
