1、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) sensor

2、s are presented.The flowmeter has no electronics and 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 us

3、es the time delay of the vortex signal generated by a bluff body. Karman vortex shedding frequency is also detected and utilized for the flow velocity estimation in the system. In order to realize a low noise and wide

4、 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 4×10?4 pm/(Hz)1/2 corresponding

5、 to 0.33 nε/(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 (FB

6、G) sensors have various advantages such as small size, simplicity in sensing principle, electromagnetic interference (EMI) immunity and capability of multiplexing. Because of these advantages, a number of basic research

7、es and applications on FBG sensors have been made [1–3]. In telecommunication systems, FBGs are used as add-drop multiplexers because of their narrow bandwidth (typically 0.1 nm). The FBG application to optical tunable

8、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 i

9、n composite materialsand 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 divi

10、sion multiplexing (WDM) and time division multiplexing 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 m

11、easurement.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 simpl

12、e. At first, we explain the principle and the schematic diagram of the flowmeter. Next, we present the noise estimation of the FBG sensor with the interferometric detection using a Mach–Zehnder interferometer comprised o

13、f 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 sensorsFig. 1 shows the principle of the flowmet

14、er. 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 i

15、s 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(τ) [15]. The function RSCOT(τ) is express

16、ed as follows:(1) ? ? ? ?? ? ? ?????? ???? ?f G f Gf G F Ryy xxxyRSCOT1 twhere 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 F?1 de

17、notes the inverse Fourier transform. The function RSCOT(τ) 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-corre

18、lation function. The maximum of RSCOT(τ) 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) t ? ? smeasd vwhere ds is