High-Frequency Fiber Bragg Grating Accelerometer Based on Symmetric Flexible Hinges (Invited)

Aiming at the difficulty of existing optical sensors to meet the requirements of high- frequency vibration of micro- seismic monitoring in oil and gas production field, a high frequency FBG accelerometer based on symmetrical flexible hinges is proposed. The accelerometer is based on a compact structure consisting of a base, double hole hinge, a fiber Bragg grating and a mass block. There are threaded holes in the base to install the geophone on the vibration test table. Two small semi- circular rings are cut out on a cylindrical stainless steel material along the transverse symmetry using a line cutting technology. The upper and lower parts of the base and the mass block are engraved with 0.5 mm grooves along the axis. FBG is placed in the the upper part of the base and the mass block of the fiber trench. Both ends of FBG are glued to the mass block and base by epoxy adhesive. The FBG certain is applied to some prestress during packaging. When there is an external vibration signal, the base of the detector vibrates with the measured object. The mass block vibration around the center of the hinge relative to the base under the action of inertial force, driving FBG to stretch and compress, leading to a wavelength drift of FBG. The principle of vibration sensing is analyzed. The sensitivity and the resonant frequency formula of the accelerometer are given theoretically and the influence of structural parameter on the sensitivity and resonant frequency of the accelerometer is discussed. The modal analysis of the geophone is carried out using simulation software. The first order characteristic frequency of the structure is 1 191 Hz, the vibration direction is x direction, and the second order characteristic frequency is 7 039.4 Hz. The vibration direction of the second order characteristic frequency is y direction. As the two characteristic frequencies are very different, the geophone has good transverse anti-interference performance. To obtain the sensing performance of the detector, the amplitude- frequency response, sensitivity and lateral anti-interference of the detector are tested. The packaged fiber grating geophone and standard acceleration sensor are fixed on the vibration table, PC control software controls the output signal of the vibration table, fiber grating demodulator and fiber grating geophone is connected, completing the signal demodulation. The demodulation signal is transmitted to the computer to complete the signal acquisition. The analysis of the experimental results shows:the resonance frequency of accelerometer based on the symmetrical hinge structure is 1 200 Hz, basically consistent with the resonance frequency results using the simulation software. The reason of the difference may be caused by the processing error of the sensor. The operating frequency band of the detector is 20 similar to 800 Hz. The sensitivity of the sensor is about 10.2 pm/g, and the linear sensitivity is 0.999 8. The cross axis sensitivity of the detector is about 5% of the main axis. The geophone has good application prospects in oil and gas exploitation field.