Development Of A Digital Orthogonal Lock-in Amplifier For Infrared Laser Gas Detection

The project is supported by the National Science and Support Technology Program “Infrared carbon monoxide detector?sensor?research and development in mine environment”.It is necessary to monitor the concentration of CH₄,CO and such other gases all the time under coal mine to ensure production safety.There are a lot of gas detection methods,among these methods TDLAS technology was widely adopted,as its high detection precision,fast response and non-contact measurement.In gas detection system based on TDLAS,the laser driving signal consists of a high-frequency sin-wave signal and a low-frequency saw-wave signal,thus LIA?Lock-in Amplifier?is necessary to extract harmonic signals from absorption signal and then to obtain gas concentration.Although there are many commercial LIAs provided by different companies,they have many disadvantages such as expensive,big-size and hard to be integrated.In order to construct a portable gas detector,a digital orthogonal LIA especially used for TDLAS detection system was developed in this paper,which has cheap price,small size,simple hardware circuit and capability to be integrated.In this paper,firstly,infrared absorption spectroscopy and TDLAS theories were introduced,including the Lambert-Beer Law,DAS technology theory,WMS technology theory,harmonic detection theory and lock-in amplifier theory.Besides,simulation based on MATLAB was carried out to verity the algorithm.Secondly,the hardware circuits of digital orthogonal LIA were designed,which consists of photoelectric convert circuit,subtract circuit,AD sampling circuit,keyboard matrix,LCD driving circuit,SCI communication circuit and the DSP minimum system.Then,the program flow chart of digital orthogonal LIA was explained,and modules of the two interrupts,low pass filter and display of harmonic signals were introduced in detail.The functional model of LIA was built by Simulink and has passed all the test cases.Finally,experiments were carried out to obtain the LIA's performance.When sinwave signal was input to the designed LIA,a good linear relationship was observed between the peak value of 1f signal and the peak to peak value of the input sin-wave signal.And when simulated absorption signal was detected by the LIA,there are also good linear relationships between the peak to peak values of harmonic signals and the gas concentration.Then gas detection experiments of CH₄ were conducted and harmonic signals were extracted.The detection system was calibrated and its stability was tested.The concentration fluctuation was-61.2457 ppm +84.4715 ppm when CH₄ was 0 ppm.The Allan Deviation – Integral Time curve of 0 ppm shows the 1 ? detection limit was 16.89212 ppm at an integral time of 0.1 s,and it will decrease to 3.05046 ppm when integral time increased to 3.8 s.The innovations of this paper are 1.Developing a digital orthogonal LIA especially used for TDLAS detection system.Its hardware circuit is simple and it only needs to store very few reference signals' magnitudes and digital filter's coefficients.2.The developed digital orthogonal LIA can be used in any TDLAS detection system as a signal processing or harmonic signal extracting module.