Research On FPGA Control And Signal Processing Techniques In TDLAS Gas Detecting System

Rapid economic development and industrial improvement raises the quality of our social life but brings a series of problems at the same time including the fog and haze, greenhouse effect and so on. They have been a threat to our health and ecological environment. In these circumstances, a reliable technology of gas detection is crucial to monitor gas emission and change the industry pattern. Compared with the traditional detection technology, Tunable Diode Laser Absorption Spectroscopy(TDLAS) is promising in gas detection field because of its high sensitivity and high resolution.In this paper, we firstly describe the basic principles of the TDLAS gas detection system including laser spectroscopy, and several gas detection methods. After selecting CH4 as the target and 1653.72 nm as gas absorption line, specific guidelines are introduced.Secondly, DFB laser is chosen as the optical source of the TDLAS system.The laser drivers, including the current driver and temperature-controller, are designed following the operation principles. In the current driver, the injection current changes by adjusting MOSFET through the output voltage of DAC, which is controlled by the micro control unit(MCU). The negative feedback algorithm and PID algorithm are both used in our design. Protection circuits and soft-start circuits are both used to protect the laser device. As for the temperature controller, peripheral circuits are designed, in which the temperature control chip LTC1923 acts as the core.Thirdly, a digital lock-in amplifier is designed to extract the second harmonic signal. The correlation detection, correlation demodulation and the correlation function are all explained as the theoretical basis of lock-in amplifier. Based on the above principles, basic functional blocks of lock-in amplifier, including the DDS module, the FIR filter, and root module are modeled, simulated and verified by using System Generator. Single-channel lock-in amplifier is then built and cascaded to two-channel one to further eliminate the phase effect on the output of lock-in amplifier. All the modules are finally realized in a SPARTAN 6 FPGA board from Xilinx using Verilog—one of the hardware description and modeling language.Based on all the above aspects, the demonstrated experiments on different modules and the whole TDLAS system are carried on to analysis charactericstics of our current driver and temperature-controller of laser driver modules. Several concentrations of CH4 are utilized in our experiment and different harmonic signals are extracted by using our digital lock-in amplifier. The experimental data is fitted by LabVIEW to calculate the related gas concentration and the accuracy of TDLAS gas detection system is also evaluated.