Research On Design And Application Of ARM-based Photoacoustic Signal Detection System

Photoacoustic spectroscopy detection technology has become the best online detection method for gas in transformer oil due to its high sensitivity and high precision.However,the current detection methods using electrical sensors still have disadvantages such as electromagnetic interference,and the detection signal is greatly affected by temperature and humidity.Therefore,a stable optical detection method with a large dynamic range is developed,which is of great significance to the stability and accuracy of the system.This article focuses on the detection method of Q-point demodulation.At present,the Q-point demodulation method has problems such as operating point drift and small dynamic range,and the main method is to use the operating point self-stabilizing system for operating point compensation.The current compensation methods can be mainly divided into two types:adjusting the laser wavelength and adjusting the mechanical structure.Among them,the method of changing the laser drive current has a smaller adjustment range,and the method of adjusting the mechanical structure and the method of adjusting the laser temperature have poor stability.In this paper,the method of adjusting current and adjusting temperature is used,and the improved control method is used to achieve stable operating point in a wide temperature range,and an improved scheme of Q-point demodulation-dual FP demodulation is proposed to eliminate the problem of limited dynamic range.The research content is as follows:(1)According to the beam interference formula,the reflection spectrum of the FP sensor is analyzed,and the spectral characteristics of the reflection spectrum are analyzed.The influence of the different reflectivity combinations of the two reflective surfaces of the FP sensor on the preparation of the sensor was discussed,and the preparation scheme of the weakly reflective film on the fiber end face was determined.The influence of operating point drift and acoustic signal size on the demodulation result is analyzed from the principle.(2)Aiming at the problem of the drift of the working point of the Q-point based intensity demodulation scheme,a feedback control method combining temperature regulation and current regulation is designed,and its structure is simple,no special laser is required,and the cost is lower.In the proposed scheme,the laser driver is used to adjust the output wavelength of the laser to eliminate the drift of the operating point caused by external environmental factors.Regarding the FP sensor,this paper uses the preparation scheme of the weak anti-thin film on the end face of the optical fiber with high anti-reflection,and designs and produces the FP sensor suitable for this scheme.Regarding the laser wavelength adjustment method,this article uses a combination of temperature adjustment and current adjustment,so that the process of stabilizing the operating point is not affected by the range of temperature changes.Compared with the pure PID control method,the response speed of this solution is faster and is not limited by the laser wavelength adjustment range.The experimental results show that this solution reduces the fluctuation of the operating point drift by a factor of 3,and at the same time can achieve a stable operating point over the entire temperature range.(3)Aiming at the problem of small dynamic range of Q-point demodulation,this paper proposes a Q-point demodulation-dual FP demodulation scheme,which combines the characteristics of Q-point demodulation method and dual-wavelength demodulation method.We designed an overall scheme based on the characteristics of Q-point demodulation and dual FP demodulation,fabricated dual FP sensors and built an experimental optical path.The Q-point demodulation scheme is used when the acoustic signal is small,and the dual FP demodulation method is used when the acoustic signal is greater than a certain threshold.This solution eliminates the influence of the acoustic signal size on Q-point demodulation and dual FP demodulation,and increases the accuracy of the system.The final experiment shows that this method can effectively expand the dynamic range of the demodulation system.