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Research On The Near-infrared Spectrum Absorptive Gas Detection System

Posted on:2017-01-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:B LiFull Text:PDF
GTID:1108330482491878Subject:Circuits and Systems
Abstract/Summary:PDF Full Text Request
In recent years, there are a series of domestic critical issues that cannot be neglected such as industrial safety and environmental pollution. On one hand, accidents including gas explosion in coal mines and hazard gases leakage has been frequently occurred. This causes tremendous economic losses and serious society impacts. On the other hand, the concentration of greenhouse gases has been increased every year and brings many environmental problems. Therefore, the research work of advanced gas detection sensors is very important.Infrared absorption spectroscopy technique is an advanced way for detecting gases in domestic and overseas. Contrast with traditional chemical gas sensors, it has many advantages including high sensitivity, fast response time, good selectivity, wide detection range and good stability. Infrared gas sensors also have the non-intrusive and long service life feature. In this way, infrared gas sensors are very suitable for application in coal mines, industrial control process and atmosphere monitoring fields.Tunable Diode Laser Absorption Spectroscopy(TDLAS) is a widely applied technique in the infrared gas detection field. It utilizes diode lasers, whose central wavelength can be tuned by modulation of laser temperature and injection current, to be light sources. The central wavelength is turned frequently to scan the target gas’ s absorption line to obtain the high-resolution spectroscopy for detecting the gas’ s type and concentration. Along with TDLAS, Wavelength Modulation Spectroscopy(WMS) modulates lasers with high-frequency signals and extracts harmonics signals to identify gas concentration. This method can effectively depress the influence of system noise and further increase the detection sensitivity. In this paper, based on TDLAS and WMS techniques, a near-infrared gas detection system has been developed using distributed-feedback lasers as light sources.Firstly, based on the infrared absorption spectroscopy theory, theory applied in this detection system is discussed. The data processing methods in the detection processes are presented step by step. The relative parameters of laser driving & modulation, optical to electrical conversion, harmonic extraction and gas concentrations are connected with formulas in order to deduce the relation of second order harmonic signal and gas concentration. Meanwhile, system simulation is implemented by using MATLAB for proving detection theory and guiding the experiments.Secondly, laser temperature control circuits and laser current driving circuits are self-developed for the lasers applied in the system. Under the control of those circuits, the output wavelength can be modulated to meet the demand of the WMS system. The above content is one of the core works in this paper. Based on analog PID algorithm, the parameters in the temperature compensation network are modulated and determined. The central wavelength is highly stable without shifts during a period of long time under the control of the temperature circuit. In the room temperature condition, the temperature fluctuation of the laser is within the range of ±0.02 ℃. Meanwhile, the laser current driving & modulation circuit is developed. The saw wave driving current varies the central wavelength periodically to scan across the absorption line. In addition, a high-frequency sine wave signal is superposed on the laser’s output for the extraction of harmonic signals. Moreover, a low-ripple linear power supply is self-developed to guarantee the stability of laser temperature and to decrease the system noise. The developed power module’s ripple noise is much less than normal switching power supplies.Thirdly, the detection system is developed on the basis of above work. The detection system is another core work in this paper. It consists of a few important parts including the differential optical paths system, fiber-coupled gas cell, optical to electrical conversion module and digital orthogonal lock-in amplifier(DLIA). The differential optical paths system divides the laser’s output into two power-equaled beams. The differential signal of the two beams can effectively avoid the light source’s noise. A fiber-coupled gas cell is developed by using fiber collimators that are the core optical part of the gas cell. The gas cell has advantages of stable structure, good sealing, cost-effective. The optical-to-electrical module is developed by using two high performance In Ga As photodiodes. The gas absorption signal and reference signal are converted into electrical signals via this module and the differential signal contents the information of gas concentration. A digital orthogonal lock-in amplifier is developed to extract harmonic signals. The phase sensitive detection is completed in the software programs rather than hardware circuits. Based on the result of simulation, gas detection experiments are carried out to test the performance of the DLIA. Experiment results show that the harmonics signals can be extracted successfully and stability is good. The experiment also proves the relation of harmonic signals and gas concentration.Finally, methane and carbon monoxide detection experiments are carried out to evaluate the performance of the whole detection system. By using dynamic gas distributing method, numerous characters of the gas detection system including detection limit, repeatability, stability, response time, indication errors and drifting are tested. The methane and carbon monoxide detection limit of the system are determined as 29 ppm and 148 ppm respectively by using Allen deviation. The experiment result indicates that the detection system has good sensitivity and stability. Besides, the system also have advantages such as high-integration and cost-effective. Other gas can be detected by replacing laser diodes with other central wavelength in the system. This gas detection system has the reference value for theory and application in the near-infrared gas detection research field.
Keywords/Search Tags:Infrared gas detection, wavelength modulation spectroscopy, distributed-feedback laser, laser temperature control, laser current driving, digital orthogonal lock-in amplifier
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