Digital Phase Locked Technology In Vehicle Exhaust Remote Sensing System Based On TDLAS Technology

Automobile exhaust has become the main source of air pollution with the rapid development of economy and the number of motor vehicles in China.And so,it is necessary to develop roadside on-line monitoring system in order to monitor vehicle exhaust emission comprehensively,truly and quickly.TDLAS（tunable diode laser absorption spectrum）is a real-time online detection technology with high sensitivity,precision and strong anti-interference among many gas detection methods.TDL（tunable diode laser）is an ideal light source for gas detection because of its narrow linewidth and fast wavelength tuning.It can greatly improve the detection accuracy and sensitivity by combining with the wavelength modulation spectroscopy technology which is high sensitivity,strong anti noise ability and short response time.This paper realizes digital phase-locked from the perspective of digital signal processing in order to run stably,has strong reliability and high detection accuracy,in view of the characteristics that TDLAS exhaust gas detection system is easily disturbed and inaccurate due to the traditional analog phase-locked circuit.In this paper,the TDLAS detection technology is deeply studied,and the relationship between the second harmonic spectrum curve of TDLAS and gas concentration is analyzed theoretically.Then,DFB（distributed feedback laser）is selected as the light source.The influence of driving current on the output center wavelength of DFB at a constant temperature is studied.Finally,a saw tooth wave scanning circuit is designed to make the emission wavelength of the laser cover the absorption band of CO₂and CO.Based on above research,the corresponding experimental system is built.A sawtooth wave scanning circuit is used to modulate the distributed feedback laser,so that its emission spectrum covers 1579.0251nm～1580.0095nm.The beam is collimated through the gas and measured,photoelectric conversion and amplification are carried out at the light receiver,then acquisition and phase-locked amplification are processed.High-precision analog-to-digital converter AD9243 is used to convert analog signal to digital signal,and ds PIC30f6014A is used to collect data and implement digital phase-locked algorithm to complete the extraction of absorption spectrum.In order to improve the detection accuracy,the background noise under continuous detection is analyzed.The signal curve and concentration inversion results are analyzed with and without the background signal.The results show that after digitizing,the system stability can be effectively improved.By eliminating the background signal,the linear correlation between the fitted second harmonic amplitude and the concentration can reach 0.9978 and 0.9934.The relative error after inversion is controlled around±8%,and the system detection accuracy is effectively improved.