Prestressed concrete structure is getting more and more extensive application in architecture, hydraulic engineering and traffic engineering because of its significant advantages of crack later or not cracks completely. It is an internal stress concrete structure that a certain force relies on prestressing tendons. The effectivity of the prestressing tendon in concrete structure is directly related to the reliability, applicability and viability of the whole concrete structure. So it is a key program to apply accurate prestress to the prestressing tendon. According to the pressure sensing principle of the fiber Bragg grating (FBG), a circular plate diaphragm-based FBG sensor for high pressure electric oil pumps that is the pressure source device of the prestressed concrete structure was presented. To overcome the cross sensitivity of temperature and pressure, two FBGs were integrated in the sensor, one of the FBGs isolated from the pressure is used as temperature compensation grating, it is called temperature-FBG comparing to another FBG called pressure-FBG. The elastic diaphragm was chosen as the pressure sensing element whose distortion displace is proportional to the difference of the two sides’ pressure of the diaphragm. A certain stress is applied to the pressure-FBG which is stuck to the center of the diaphragm, and then the reflection wavelength of the pressure-FBG is inverse proportional to load of the diaphragm. The results indicated that the linearity is up to 99.99%, and the pressure sensitivity coefficient is 0.024nm/MPa within the measurement scope of 0-70MPa.
A wavelength demodulation system of fiber Bragg grating (FBG) sensors based on wavelength scanning mode by the
combination of the piezoelectric transducer(PZT) stack and the fiber Bragg grating has been presented. Two U-shaped
metal brackets are set to the ends of the PZT stack. Two fibers with Bragg gratings as the reference gratings are stuck on the two sides of the U-shaped metal brackets respectively, and the parts of the gratings on the fibers are just located in the gap between two metal brackets. The equal prestressing force was imposed on the fibers and held constantly before sticking the fibers. This structure will expand the scan scope of reference grating, balance the tension of the PZT stack, strengthen the operation reliability, and improve the scan linearity. The experiment results show a good linear relationship between voltage and wavelength shift in the voltage scope of 0~200V, and the wavelength scope of linear tuning is about 3.8nm with the sensitivity of 0.0097nm/V.
KEYWORDS: Signal to noise ratio, Sensors, Signal processing, Fiber optics sensors, Signal detection, Interferometry, Interference (communication), Silica, Light, Signal attenuation
We have designed an intensity-demodulated sensing system based on Fabry-Perot interferometric sensor for pressure
measurement. The structure of the sensing probe has been presented. The sensing system is interrogated by broadband
source. For compensating drift of the source power and fluctuation in fiber attenuation, the light beam is separated into
two channels by a fiber Bragg Grating, the transmitted light used as reference signal and the reflected light used as
sensing signal. In order to improve the signal-to-noise ratio(SNR) of the detection system, the input light is modulated by
pulse signal, and the low noise preamplifier is given. The more important factor to improve the SNR is that a
synchronization integrator is employed to construct a narrow band filter to restrain noises and disturbances. It has better
performance with a narrow band noise filter rather than the general RC active bandpass filter. The sensing signal and the
reference signal are transformed into DC voltage signal from AC voltage signal after they passed the synchronization
integrator circuit. Subsequently the division operation of the sensing signal and the reference signal is implemented. At
last a linear output model is established. The system has advantages of fast response, strong ability and low cost. The
dynamic range of the sensor is from 0 to 400KPa, and the resolution reaches to 200Pa.
We have designed an intensity-demodulated sensing system based on Fabry-Perot interferometer and fiber Bragg grating
(FBG) for pressure measurement. The structure of the sensor probe is presented. It is an optical fiber extrinsic
Fabry-Perot interferometer(EFPI) composed of silica tube and optical fibers. The variable quantity of cavity length of the
EFPI is equal to that of silica tube, which has direct proportion to external pressure. The sensing system is interrogated
by broadband light. In order to compensate intensity fluctuation, we use an FBG as a narrow band filter to obtain the
sensing signal and reference signal from white-light interference fringe returned from EFPI. The sensing signal is in the
reflected beam from FBG, and the reference signal is contained in reflected beam and transmitted beam of FBG
simultaneously. Then two output light signals are transformed into current signals by two photodiodes. Subsequently two
current signals are processed at the same time. After the signals being treated with circuit, the inherent disadvantage of
intensity-demodulated sensor is removed. It is also to say the sensing system has immunity from the drift of light source
power and fluctuation in fiber attenuation by using only a FBG. Meanwhile, the system has advantages of fast response,
simple circuitry, strong ability of intensity compensation and low cost. Experimental results show that the experimental
data are consistent with theoretical analysis. The dynamic range of sensor is from 0 to 500KPa, and the resolution
reaches to 0.5KPa. The sensing system can be used to measure pressure in inflammable and explosive circumstance such
as oil height of super oil storage tanks.
We designed a optical fiber acoustic and ultrasonic sensor probe based on Fabry-Perot interferometry, and gave the
principle structure of the sensor: The two mirrors of Fabry-Perot interferometer are composed of the fiber's end face and
the aluminum thin diaphragm, outside sound wave will force the thin diaphragm vibration, it is also to say the
Fabry-Perot cavity length varies with the sound wave, thus the output intensity of the interferemeter is modulated by the
wave, at last the photodetector(PD) transforms the light intensity signal to electric current signal. The thickness of the
aluminum thin diaphragm is 10μm only, and its radius is 1mm, as a result the sensitivity of the sensor is very high. But if
placing the sensor in liquid at a deep point, the static pressure would make the thin diaphragm crushed because the static
pressure of the liquid is very strong compared with sound wave. For this reason, we design a kind of small scaled air bag
linked with the Fabry-Perot cavity which spread the sensor an ability of being able to stand of the static pressure. The
maximum of static pressure the sensor could stand has improved from 0.3 MPa to 10 MPa above, so the sensor may
work normally at point of 1000m under water surface. The result of experiment in water show that the sensor sensitivity
reaches -162dB(0dB=1rad/μPa), the frequency response range is from 1KHz to 5 MHz. The sensor is fit for detecting
acoustic and ultrasonic signals in liquid.
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