裝配圖薄膜諧振式液位高度測(cè)量用頻率計(jì)的設(shè)計(jì)與制作(論文+DWG圖紙+外文翻譯+文獻(xiàn)綜述+開題報(bào)告)
裝配圖薄膜諧振式液位高度測(cè)量用頻率計(jì)的設(shè)計(jì)與制作(論文+DWG圖紙+外文翻譯+文獻(xiàn)綜述+開題報(bào)告),裝配,薄膜,諧振,式液位,高度,測(cè)量,丈量,頻率計(jì),設(shè)計(jì),制作,論文,dwg,圖紙,外文,翻譯,文獻(xiàn),綜述,開題,報(bào)告,講演,呈文
Optical Water-Level Sensors using Fiber Bragg GratingTechnology
ABSTRACT: We developed an optical high-precision water-level sensors based on fiber Bragg grating (FBG) technology. The sensors can be applied to measure the waterlevels of rivers, lakes, and sewage systems. The sensor head consists of a diaphragm, a customized Bourdon tube and two FBGs, one for tensile measurement and the other for temperature compensation. The FBG attached to the Bourdon tube is strained as the water level increases and causes a shift of the center wavelength of the reflected light from the FBG. The center wavelength is in turn detected by the wavelength interrogation equipment composed of a tunable Fabry-Perot filter. We achieved the sensor accuracy of +/? 0.1% F.S., i.e., +/? 1 cm for the full water-level measurement range of 10 m. Several sensor heads can be connected in series through one optical fiber, and the water level at different places can be measured simultaneously by using one piece of wavelength interrogation equipment.
(1)INTRODUCTION
River administrators have the very important responsibilities to maintain their river facilities and monitor river basin conditions. Since river facilities are widely dispersed along a river basin, it is a great expense of time andlabor for the staff of river administrators to go around periodically to check these facilities. Moreover, it is difficult to perform spot checks on these facilities to ascertain their structural integrity in the event of natural disasters such as floods, washouts, and seismic disturbances. From this point of view, projects to build up the optical fiber networks around river basins have been advanced by the Japanese government. To utilize such an optical fiber network effectively, optical fiber sensors and sensing systems have been researched and developed recently, which are applicable to the safety management of river basins and their flood control facilities.
Optical fiber sensors and their systems have the following advantages:
1) The low transmission loss of optical fiber enables remote sensing over tens of kilometers.
2) Real-time monitoring is achieved by connecting sensor heads directly to the measuring equipment using optical fiber.
3) The sensor head consists of all-optical passive components and there is no need for an electric power supply where the sensor heads are installed.
4) Due to immunity from electromagnatic interference of an optical sensor, it does not suffer from the deleterious effects of electrical shock, such as the occurrence of lightning strikes. The water level sensing of a river is a most critical matter for both the river administrators and the residents living around a river basin from the standpoint of the facility maintenance and the prevention of natural disasters.
We have developed optical water-level sensors using fiber Bragg gratings (FBGs) that possess the advantages described above and a few more. In this paper, we describe details of the FBG as a strain sensor, water level sensing applications and the performance results obtained in a practical field.
(2)FIBER BRAGG GRATING TECHNOLOGY FOR SENSING APPLICATION
2.1 Principle of FBG
The FBG is a permanent periodic modulation of the refractive index along a given length of optical fiber. Figure 1 shows the schematic structure of the FBG. Due to the coupling between the forward and backward propagating modes, the specific wavelength light depending on the modulation period of the refractive index is reflected at the location of the FBG. The incoming light is reflected when its wavelength is equal to the Bragg wavelength λB of the grating, defined as(3)
λB= 2 NeffΛ, ………………………………………(1)
where Neff is the effective core refractive index and Λ is the period of the refractive index modulation. The other wavelength lights are transmitted through the FBG and irradiate the next FBGs. Thus, it is possible to achieve multi- point sensing at one optical fiber by connecting FBGs in series,each of which has a different reflected wavelength from the others.
Fig. 1 ?Schematic structure of FBG. The specific wavelength light,depending on the modulation period of the refractive index, isreflected from the FBG. The other wavelength lights are transmittedthrough the FBG and irradiate the next FBGs.
The spectrum of the reflected light from the FBG is shifted according to the applied strain and temperature. The Bragg wavelength change ?λBin response to strain and temperature is given by
λ= (1 ? Pε) ?ε＋ ξ? ?T, …………………………(2)
where Pε is the effective photo-elastic coefficient, ε is the applied strain, ξ is the thermo-optic coefficient, and ?T is the change in temperature. Using ordinary single mode fiber for telecommunication, the effective photo-elastic coefficient Pεis0.22, (3), (4)so the sensitivity to an applied axial strain and the temperature dependence are 1.2 pm/micro-strain and 10 pm/℃, respectively in 1.55 μm wavelength range.
2.2 Fabrication of FBG
Figure 2 shows the process for fabricating an FBG. We use a polyimide-coated optical fiber for the FBGs as the strain sensor, which causes no slippage on the border of the coating layer, so that externally applied strain can be precisely transferred to the FBG. The FBG is fabricated by the UV
irradiation of the fiber. In advance of writing the Bragg grating, the polyimide-coated optical fiber is subjected to a hydrogen loading treatment to enhance the effect of photo-induced refractive index change. The polyimide layer is then partially etched away by using a chemical solvent. As shown in Fig. 3, the Bragg grating is inscribed in the core region of the optical fiber by irradiating a KrF Excimer laser emitting at 248 nm through a phase mask, with the scanning in the longitudinal direction. Photo-induced refractive index modulation along the fiber, i.e., apodization profile can be controlled by the programmable scanning speed of the laser beam. We adopted a Gaussian-shape apodization to suppress side-lobe losses of the reflection spectrum. After inscribing the grating, the polyimide layer is re-coated to protect the stripped part. Finally, the optical fiber with a FBG is subjected to a high-temperature(under 300℃) heat treatment for 30 min. to ensure long-term stability.
Figure 2 Figure 3
Figure 4 shows the typical transmission and reflectionspectrum of an FBG. Transmission rejection was about ?10dB, that is, the reflection ratio was almost 90%. The FWHM(Full width at half maximum) of the reflection was less than 0.2 nm
(3)OPTICAL WATER-LEVEL SENSOR USING FBG
3.1 Structure of optical water-level sensor
Figures 5 and 6 show the schematic structure of the optical water-level sensor and the principle diagram of the pressure conversion element of the sensor, respectively. The sensor head consists of a diaphragm, a customized Bourdon tube and two FBGs, where one FBG is used for tensile measurement and the other FBG is used for temperature compensation. This water level sensor is the type used for measuring water pressure in proportion to water level. The water pressure is transmitted to the inner silicone oil in the Bourdon tube. Then the Bourdon tube converts the oil pressure to the tensile force at the tip of the tube within its elastic limit and stretches the FBG, which is strung between the tip and the base. Thus, the FBG is strained in proportion to the water level increase. The FBG is set with slight pre-tension initially, to enable water level measurement linearly from the lowest level. The Bragg wavelength varies with temperature due to the temperature dependence of the refractive coefficient of silica glass.To compensate the temperature dependency, another FBG is connected in series with the FBG for tensile measurement at the sensor head. The temperature compensation FBG is mounted in the sensor case free from any strain based on the water pressure. The intrinsic wavelength shift ?λW.L. in proportion to the water level change is calculated by
?λwl=(3)
where ?λtens. is the wavelength shift of the FBG attached to the Bourdon tube, which includes the temperature turbulence,and ?λtemp.is the wavelength shift as measured by the temperaturecompensation FBG. K is the coefficient of the temperature FBG, and we set this value to 1 according to adjust the position of FBG mounting.
The air pressure in the sensor case is balanced with the outer atmospheric pressure through the air induction pipe that contains the optical fiber approach cable inside.
3.2 Measurement result for optical water-level sensor
Figure 7 shows the schematic diagram of the measurement system for detecting the wavelength of the reflected light from the FBGs in each sensor. The broad-band light from an ASE light source is launched into the optical fiber via a 3-port circulator, and the reflected light from FBG comes back
through another port of the circulator toward the Fabry-Perot tunable filter as a spectroscopic device.(3)～ (5) Other wavelength lights pass through the FBG and transmit to the next FBGs,
where a different wavelength light will be reflected. Thus, it is possible to achieve the multi-point measurement of water level with one optical fiber when sensors are connected in series,
which are comprised of FBGs reflecting different wavelengths from each other. Switching the several optical fibers by using an optical switch, the system can connect and obtain the measurement results of multiple sensors by using a single piece of wavelength interrogation equipment.
The finesse of the Fabry-Perot tunable filter is 700, the transmission bandwidth at half maximum is almost 0.1 nm, and the used wavelength range is 1530 to 1570 nm. The tunable filter is being driven by piezoelectric transducer, and 50 Hz triangular wave voltage has been applied to it to scan the distance of the Fabry-Perot etalon.
Fig. 7?Schematic diagram of the measurement system for detecting the wavelengths of the reflected light from FBGs in each sensor
To improve the wavelength accuracy, we used a temperature-controlled FBG as a reference, which indicated a constant wavelength having less than +/? 1 pm fluctuation. And the data was averaged over 10 times to reduce the random noise. The result is shown in Fig.8. When the averaging exceeds 10 times, the wavelength fluctuation can be suppressed less than 5 pm.
We designed the maximum strain applicable to the tensile measuring FBG to be about 0.3% at 10-m water pressure, which corresponded to a 3.6-nm center wavelength shift of the FBG. To verify the accuracy of the water level measurement,we measured the wavelength shift 10 times, applying pseudo- water pressure to the sensor diaphragm using the air pressurizer. Figure 9 shows the correlation between the applied pressure measured by a pressure gauge and the wavelength measured by the sensor FBG. The result showed that there was good linearity over the full range of the water pressure, and the error at each point was less than 3 pm/cmH2O. We measured this characteristic at 0, 20 and 40℃, which was the specified temperature range of this sensor. The results are shown in Fig.10. The ordinate of this graph is the water level measurement error obtained from the linear fitting line that was calculated from the result of the dependency measurement between the pressure and the wavelength shift. There were good agreements at every temperature condition, and the error at each point was less than+/-1cm.
We confirmed the repeatability and the durability of the sensor by applying pressure from 0 to 3 mH2O repeatedly. The result is shown in Fig. 11. Even after 10,000 times pressurization, the measured water level indicated the true value at the error within +/?1cm. No drift or creep conceivable caused by temperature fluctuation or metal fatigue of elements of the sensor was observed.
3.3 Practical performance of optical water-level sensor in river
We performed a river field test of this sensor system at the Kakehashi River, Kanazawa Work Office, Hokuriku Regional Development Bureau from August 1999 to March 2001.Through this field test, we confirmed the robustness of the sensor and the measuring equipment: they operated over one and half years measuring the water level of the river with an accuracy of +/? 1 cm with no fatal system error. Next, we installed the system equipped with three sensors at the Uono and the Aburuma River, Shinanogawa Work Office, Hokuriku Regional Development Bureau in November 2001. The system diagram and the water level measurement results are shown in Figs. 12 and 13, respectively. During this measurement, water level was measured simultaneously by a conventional electric water-level gauge that was already installed at the same measuring point. The water level data measured by two types of sensor corresponded quite well and the difference between them was less than +/? 1 cm.
Through the field measurements, we demonstrated that our optical water-level sensor had superior accuracy in water level measurement, and it enabled remote and real-time monitoring without the need for an electric supply source at the sensing point.
(4)CONCLUSION
We developed an all-optical water-level sensors having accuracy corresponding to +/? 0.1% F.S., i.e., +/? 1 cm for the full water-level measurement range of 10 m.
These sensors and systems are currently set up at several sites such as river basins, lakes and sewage systems where they continue to work as designed to measure the practical water level.
REFERENCES
(1) E. Udd, “The Emergence of Fiber Optic Sensor Technology,” FIBER OPTIC SENSORS (ed. E. Udd) John Wiley & Sons, Inc., New York (1991) pp. 1-8.
(2) A. D. Kersey, “A Review of Recent Developments in Fiber Optic Sensor Technology,” Optical Fiber Technol. 2, 291-317 (1996).
(3) A. D. Kersey et al., “Fiber Grating Sensors,” J. of Lightwave Technol. 15, No. 8, 1442-1463 (1997).
(4) K. O. Hill et al., “Photosensitivity in Optical Fibers,” Annu.Rev. Mater. Sci. 23, 125-157 (1993).
(5) A. D. Kersey, T. A. Berkoff and W. W. Morey,“Multiplexed Fiber Bragg Grating Strain-Sensor Systemwith a Fiber Fabry-Perot Wavelength Filter,” Opt. Lett. 18,No. 16, 1370-1372 (1993).
Sensor Technology
So far, we have considered mainly the nature and characteristics of EM radiation in terms of sources and behavior when interacting with materials and objects. It was stated that the bulk of the radiation sensed is either reflected or emitted from the target, generally through air until it is monitored by a sensor. The subject of what sensors consist of and how they perform (operate) is important and wide ranging. It is also far too involved to merit an extended treatment in this Tutorial. However, a synopsis of some of the basics is warranted on this page. A comprehensive overall review of Sensor Technology, developed by the Japanese Association of Remote Sensing, is found on the Internet at this mirror site.Some useful links to sensors and their applications is included in this NASA site. We point out here that many readers of this Tutorial are now using a sophisticated sensor that uses some of the technology described below: the Digital Camera; more is said about this everyday sensor near the bottom of the page.
Most remote sensing instruments (sensors) are designed to measure photons. The fundamental principle underlying sensor operation centers on what happens in a critical component - the detector. This is the concept of the photoelectric effect (for which Albert Einstein, who first explained it in detail, won his Nobel Prize [not for Relativity which was a much greater achievement]; his discovery was, however, a key step in the development of quantum physics). This, simply stated, says that there will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow from the plate, collected, and counted as a signal. A key point: The magnitude of the electric current produced (number of photoelectrons per unit time) is directly proportional to the light intensity. Thus, changes in the electric current can be used to measure changes in the photons (numbers; intensity) that strike the plate (detector) during a given time interval. The kinetic energy of the released photoelectrons varies with frequency (or wavelength) of the impinging radiation. But, different materials undergo photoelectric effect release of electrons over different wavelength intervals; each has a threshold wavelength at which the phenomenon begins and a longer wavelength at which it ceases.
Now, with this principle established as the basis for the operation of most remote sensors, let us summarize several main ideas as to sensor types (classification) in these two diagrams:The first is a functional treatment of several classes of sensors, plotted as a triangle diagram, in which the corner members are determined by the principal parameter measured: Spectral; Spatial; Intensity.From this imposing list, we shall concentrate the discussion on optical-mechanical-electronic radiometers and scanners, leaving the subjects of camera-film systems and active radar for consideration elsewhere in the Tutorial and holding the description of thermal systems to a minimum (see Section 9 for further treatment). The top group comprises mainly the geophysical sensors we considered earlier in this Section.The two broadest classes of sensors are Passive (energy leading to radiation received comes from an external source, e.g., the Sun) and Active (energy generated from within the sensor system, beamed outward, and the fraction returned is measured). Sensors can be non-imaging (measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance) or imaging (the electrons released are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors (see further down this page for a discussion of detector types); since the radiation is related to specific points in the target, the end result is an image [picture] or a raster display [as in:the parallel lines {horizontal} on a TV screen).
Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum. When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to detectors, it is called a spectrometer. One type of spectrometer (used in the laboratory for chemical analysis) passes multiwavelength radiation through a slit onto a dispersing medium which reproduces the slit as lines at various spacings on a film plate. The term spectroradiometer tends to imply that the dispersed radiation is in bands rather than discrete wavelengths. Most air/space sensors are spectroradiometers.
Sensors that instantaneously measure radiation coming from the entire scene at once are called framing systems. The eye, a photo camera, and a TV vidicon belong to this group. The size of the scene that is framed is determined by the apertures and optics in the system that define the field of view, or FOV. If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system. Most non-camera sensors operating from mo
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