During the detection process of atmospheric laser absorption spectroscopy in open space, the useful signal may be submerged in the noise due to the inherent noise of the instrument and external environmental noise. In order to effectively improve the signal-to-noise ratio, a new method of cascaded stepwise singular value decomposition has been proposed. By constructing different matrices, the low-frequency noise and high-frequency noise are removed in stages. A laser absorption spectroscopy atmospheric detection system in open space has been established to experimentally verify. The results show that, compared with the traditional method, the signal-to-noise ratio of the spectral signal processed by the method of cascaded stepwise singular value decomposition is increased from 0.211 to 1.029, the standard deviation is also reduced by 1.927 times. It is proved that the proposed method can effectively remove noise and obtain high-quality laser absorption spectrum signal to improve gas detection accuracy, which has practical value.
For the technical requirements of automobile emission CO and CO<sub>2</sub> detector’s data processor, the scheme is based on the detection principle of NDIR method and the implementation of the data processor software as well as hardware is discussed. High-speed, high-precision DSP is selected as the core of the detector’s data acquisition and processing, while four-channel thermoelectricity sensor TPS4339 as infrared detector, digital-analog data acquisition circuit of NDIR is designed and simulated. Then Fast Fourier Transform (FFT) is adopted for signal processing. Automobile emission CO and CO<sub>2</sub> concentration can be accurately obtained by appropriately adjusting sampling period and the light source modulation frequencies, the system SNR is improved and the detection limit is reduced. The experimental results show that the detector’s data processor has 3% accuracy and stability which can meet the measurement and analysis of automobile emission CO and CO<sub>2</sub> concentration.
Hydrogen cyanide gas leakage may exist in the petrochemical industry, smelting plant, and other industrial processes, causing serious harm to the environment, and even threatening the safety of personnel. So the continuous detection of HCN gas plays an important role in the prevention of risk in production process and storage environment that existing hydrogen cyanide gas. The Tunable Diode Laser Technology (TDLAS) has advantages of non-contact, high sensitivity, high selectivity, and fast response time, etc., which is one of the ideal method of gas detection technologies and can be used to measure the hydrogen cyanide concentration. This paper studies the HCN detection system based on TDLAS technology, selects the absorption lines of hydrogen cyanide in 6539.12cm-1, and utilizes the center wavelength of 1.529μm distributed feedback (DFB) laser as a light source. It is discussed in detail on technical requirements of a high frequency modulated laser signal detection circuit, including noise level, gain, and bandwidth. Based on the above theory, the high frequency modulation preamplifier circuit and main amplifier circuit are designed for InGaAs photoelectric detector. The designed circuits are calculation analyzed with corresponding formula and simulation analyzed based on the Multisim software.
With the increasing number of vehicles, the harm from NO to the environment becomes more and more prominent. So the monitoring of the NO concentration of the vehicle exhaust emissions is very important to assess the emission levels. In this paper, the NO detection system designing for vehicle exhaust emissions based on the non-dispersive ultraviolet principle (NDUV) has been researched. The technical indexes of the two-way modulation UV signal detection circuit are discussed in detail. And then a precision detection circuit is designed, which is composed of a trans-impedance amplifier and a lock-in amplifier, with which the output of the UV photoelectric detector can be amplified to a suitable voltage range, and the DC noise of the pre-stage amplifier is effectively removed by the lock-in amplifier. An experimental system was set up to test the designed circuit. To ensure the consistency of the two channels, the method of exchange calibration was adopted in the test. It’s drawn that the designed circuit is of high SNR, measuring accuracy and a large dynamic range from the test results. The NO concentration detection limit of vehicle emissions can reach 1ppm, and the detection precision is ±15ppm.