Design And Research Of Fiber Bragg Grating Dual Differential Osmotic Pressure Sensors

In structural engineering, especially in the large civil engineering structure which has a long term of service. There is a main problem endangers the structure stability and safety that the effect of osmotic pressure. Osmotic pressure is also known as pore pressure, which is the pressure of pore fluid inside soil. Its value includes the size of the pore water pressure and pore air pressure. The influence factors of osmotic pressure is relatively complex, for example, the physical properties of soil mass medium, saturation of the soil, hydrostatic pressure, uplift pressure, seepage and the external load and so on.The fiber Bragg grating sensor based on fiber Bragg grating sensor technology. It not only has the basic functions of traditional electric measuring sensor, but also has some new features and advantages as high precision, absolute measuring, distributed sensing and anti-electromagnetic interference and so on. It can be achieve a real-time and on-line monitoring of civil engineering structures to well. According to physical characteristics of fiber grating, the fiber Bragg grating is more sensitive to temperature than on pressure. So we must package fiber Bragg grating in efficiently and reasonably, in order to increase the pressure sensitivity and solve the problem of temperature strain cross-sensitive issue, and make it able to meet the practical requirements.This paper based on the analysis of osmotic pressure, developed a new type of fiber Bragg grating sensor in osmotic pressure. The main completed work as follows:1、Established the sensing model based on the elastic theory of metal bellows and strain characteristic of cantilever beam. Use existing method to solve the problem of temperature strain cross-sensitive issue with differential type. It this approach overcomes the effects of temperature change on measurement, and it don’t need to be repeated calibration after the temperature change. This guarantees the osmotic pressure in the practical application of fiber Bragg grating in measurement accuracy.2、According to the structure of the sensor, the mathematical relationships between the fiber Bragg grating wavelength shifts and the osmotic pressure is established.3、According to the material choice and size design of the sensor, the characteristics of the sensor were calculated. The theory pressure sensitivity is2.34nm/MPa, theory resolution is427Pa.4、The fiber Bragg grating osmotic pressure sensor is processed and the related tests were conducted.5、The test results showed that, the test pressure sensitivity of the fiber Bragg grating osmotic pressure sensor as follows:load1.96nm/MPa, unload1.9nm/MPa; nonlinearity error:load2.3%, unload1.6%; repeatability error:load3.76%, unload2.37%; hysteresis:load5.6%, unload2.9%.