Design And Implementation Of A Phase-Generated Carrier Demodulation System For Fiber Grating Vibration Sensors

Compared with the traditional inductive vibration sensor,Fiber Bragg grating(FBG)vibration sensor has many advantages and is widely used in various fields.Its measurement accuracy depends on the demodulation accuracy of the sensor signal.The phase-generated carrier(PGC)technology is often used to demodulate the signals of FBG vibration sensors due to its good dynamic performance.However,due to hardware limitations and environmental disturbances,the modulation depth of the PGC algorithm is prone to drift,resulting in large errors in the solving of the FBG stretch amplitude.To avoid interference of the modulation depth with the PGC demodulation algorithm,a PGC-arctangent-peak seeking(PGC-Arctan-FP)combined demodulation algorithm is proposed in this paper to accurately calculate the frequency and amplitude of the FBG stretch caused by vibration.A phase-generating carrier demodulation system is constructed to decode the signals from the FBG vibration sensors and to verify the feasibility of the proposed algorithm.The main contents of this paper is focused on the following points:Firstly,the current state of research on FBG vibration sensors and signal demodulation algorithms is introduced,the signal generation scheme and applicability of the phase generation carrier are subjected to detailed analysis.The optical path based on unbalanced MZ interferometer is determined,thus,the movement of the central wavelength of the FBG is converted into the interferometer phase change the relationship between wavelength change and phase change is deduced.Secondly,the PGC demodulation algorithm is researched.The principle derivation and simulation verification of the differential cross multiplication and the arctangent approaches of the traditional PGC algorithms are carried out to analyze the influence of different factors on the demodulation results.To address their limitations and practical needs,a PGC-Arctan-FP demodulation algorithm is proposed.The algorithm multiplies the output signal of the interferometer with the fundamental frequency and the two-fold frequency of the carrier signal respectively and passes it through a low-pass filter to obtain quadrature signals of different amplitude containing the information to be measured.Then,finding the peaks one by one,calculating the average amplitude of the two signals separately,and multiplying the average amplitude with the cross of the two signals to make the quadrature signals equal in amplitude.At last,the frequency and amplitude of the FBG stretch are accurately solved after an inverse tangent operation and high-pass filtering.The feasibility of the algorithm is verified through simulation initially,which can effectively avoid the impact of modulation depth variation on the accuracy of the solution results.After that,the phase-generating carrier demodulation system was designed.The key parameters of the system optical path is theoretically calculated and verified,the initial arm length difference of the interferometer and the operating voltage of the piezoelectric ceramic are determined,and a suitable carrier signal is generated.The system circuit diagram is designed and soldered for debugging.The optical signal output from the interferometer is converted to a voltage signal and then to a digital signal,which is transmitted to the host computer via serial communication.Lastly,the PGC-Arctan-FP algorithm is written in the upper computer to solve and display the vibration signal.Finally,the PGC demodulation test system is built.The vibration signals of 100～1000Hz are calculated,and the vibration frequency calculation results are consistent with the measured signals,and the maximum error of the vibration amplitude calculation results is less than 0.81%.This system can be used for accurate measurement of vibration signals.