1、PDF外文：http:/ water flowmeter using dual fiber Bragg grating sensors and cross-correlation technique Abstract In 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 no mechan
2、ical 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 the
3、 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 density of the FBG sensor with the interferometric detection was 4104 pm/(Hz)1/2 correspondi
4、ng to 0.33 n/(Hz)1/2. A water flow experiment showed 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. Introduction Fiber Bragg grating (FBG) sensors have various advantages such as small size, simplicity in sensing
5、 principle, 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 band
6、width (typically 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 m
7、aterials and used as 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 s
8、ensor applications, the choice of the wavelength-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
9、. This method is the standard technique 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 a
10、re some reports about the noise estimation 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
11、. Measurands of flowmeters are ranging over various 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 mea
12、surement. The advantage of the cross-correlation 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 the
13、ir non-intrusiveness to the flow 13. The flowmeter 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
14、and Williams 14 uses 16 light rays (eight pairs) 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 veloc
15、ities. This flowmeter is attractive because of 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 l
16、ight ray or laser beam, suited for water flow 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 prin
17、ciple and the schematic diagram of the flowmeter. 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.
18、 2. A cross-correlation flowmeter using FBG sensors Fig. 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
19、whose shape is a rectangular column generates stable 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: fGfGfGFRyyxxxyR S C OT1t &nb
20、sp; (1) where Gxx(f) and Gyy(f) are the power spectra of the upstream and downstream sensor signals, Gxy(f) is 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
21、and can detect the time delay more precisely and robustly than the simple cross-correlation function. The maximum of RSCOT() 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: t smeas dv (2)
