Double Membrane Type Optical Fiber Bragg Grating Soil Pressure Sensor

Earth pressure is the lateral pressure of back wall which is given or loaded by the soil behind the retaining structures, and the size of the earth pressure directly decides stability and security of retaining structures and the blocked soil. In civil engineering, physical and mechanical properties of the soil and structure, deformation and displacement of retaining structures, the surface load of filled soil, groundwater situation and the add-back process all affects the size and distribution of the earth pressure. Soil retaining structures is more common in the road and the slope of water facilities, the size of the earth pressure directly relates to their security.Traditional measures of earth pressure sensor have a lot of kinds, such as string vibration sensor, resistance strain sensor, the piezoelectric sensors and etc. In signal acquisition all these traditional sensors there is one thing in common, namely make the earth pressure changes into the electromagnetic signal, and then through the test to measure the size of the signal pressure. Because the sensor element, there is poor resistance to electromagnetic influence in the traditional signal sensors, and under specified conditions they are vulnerable to the disadvantages of interference. In addition, the scattered, short, flexible monitoring of traditional sensor determined the sensor has some bad effects in the distributed, on-line monitoring, and they also have a lot of defects of retaining structure safety assessment in real-time caused great influence. Therefore, at present developed the earth pressure cell which has real-time on-line monitoring capacity and good soil adaptation is very important. In recent years, sensing technology of fiber Bragg grating is developing rapidly. It can measure the drift small changes in the quantities of the outside world by reflection wavelength of grid, and with repeatability, high linear degree effects. So the technology can realize the high precision and distributed online measurement of stress and strain; And fiber Bragg grating sensor also has a lot of merits, such as the electromagnetic interference ability, good insulation, small volume, simple operation, long-distance transmission, easy to serial reuse and collect sensing and transmission one, etc. Therefore, in geotechnical stress measurements FBG technology will have wide application prospects. This paper deals with two-diaphragm fiber Bragg grating earth pressure sensor, and makes the theory analysis and experimental research of soil pressure. This new sensor is used the two film, oil chamber and new type of sensor element. According to the pressure size and "GB150-98steel pressure vessel" regulation, pressure test devices meet the safety requirements is designed. Through the SB-4.0manual hydraulic test pump, this sensor gets the adding and discharging pressure test. The main research contents are as follows:1. Based on the theory of flat diaphragm elastic and triangle cantilever beam strain characteristics established a sensor model, designed to measure the soil pressure of double film optical fiber Bragg grating pressure sensor;2. According to the sensor, we choose316L stainless steel which has comprehensive performance as the flexibility of the diaphragm materials of a cantilever beam and the double strength such, and choose45#steel as the material of metal shell, transmitting pressure stem and cover plate. According to the selected materials and theoretical calculation, the size of various parts is identified, and after the sensor is finished,250mm in diameter and thickness of25mm.3. According to the sensor test requires, the pressure sensor test platform is developed. According to the national standard of pressure vessels "GB150-98steel pressure vessel" regulation and actual experiment pressure, the carbon steel Q235-B steel is chosen as material in1.6MPa below,25mm thickness. After Combination of pressure tank, the size of the parameters is:273.1mm in diameter,81mm,25mm of thickness, connect the diameter of30.2mm holes, the sealing ring of upper part219mm diameter, bottom sealing ring238.7mm.4. According to the selected materials, the size of the theoretical sensor sensitivity is calculated. The sensitivity in theory is474.68pm/MPa; Theoretical resolution is:0.002MPa.5. By the pressure and discharge, the testing experiment of sensor is got. The experimental results show that nonlinear error of the sensor is3.84%FS, sensitivity is399.17pm/MPa.