Research On Digitial Lock-in Amplifier For TDLAS

The "13th Five-Year Plan" for National Science and Technology Innovation highlights the importance of developing advanced atmospheric environment pollution monitoring technology and online monitoring equipment that are tailored to China’s specific national conditions.Among various gas detection methods,tunable diode laser absorption spectroscopy(TDLAS)has been widely applied due to its ability of high sensitivity,fast response time,and non-contact measurement.The core component of TDLAS sensing system is a lock-in amplifier that mainly used to extract harmonic signals from the absorption signal of the measured gas,hence,the gas concentration can be derived from the harmonic signals.Currently,most commercial lock-in amplifiers are expensive and bulky,which make them difficult to integrate into specific detection systems,especially some portable detection systems.Therefore,this thesis designs a dedicated lock-in amplifier from the perspective of miniaturization and integration,furthermore,the thesis studies the detection of water vapor based on TDLAS using this lock-in amplifier.The research contents of this thesis are as follows:1.An overview of infrared absorption spectrum and basic work principles of lock-in amplifier is overviewed.Beer-Lambert law is described in detail.Firstly,the basic theory of correlation detection is introduced,then the basic principle of lock-in amplifier is also introduced,and the corresponding formulas are derived.2.The hardware circuit structure of the dedicated lock-in amplifier is elaborated in detail.This includes the selections of the performance parameters for the STM32F429 chip,the pre-match impedance circuit,the serial communication circuit,the LCD drive circuit,the temperature controller and the current deriver for detected laser,and the photoelectric detector module.3.The lock-in amplifier detection program is designed based on the basic principle of lock-in amplifier.The software design of the system is explained from two parts: signal acquisition module and data processing module.The generation and debugging of scanning signal and modulation signal in the laser driver module are described in detail.The signal overlapping and phase matching are simulated by MATLAB software.This thesis also explains the implementation process of a mixer and discusses the principles of signal acquisition,as well as the selection and application of window functions in data processing.4.Gas concentration experiments were conducted.A TDLAS monitoring system was built for water vapor.The peak-to-peak value of the second harmonic signal is extracted using the lock-in amplifier.The experimental results show a strong linear relationship between the peak-to-peak value of the second harmonic signal and the measured concentration of the target gas,with a linear fitting of R2 ～ 0.999.This thesis focuses on the application of a lock-in amplifier in TDLAS gas sensing system.A lock-in amplifier with a simple hardware structure,flexible software settings,and second harmonic demodulation function is studied.In addition,based on serial communication,a real-time display function of a LCD screen is added,which is convenient for real-time observation of the harmonic signal.