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为了有效抑制可调谐二极管激光吸收光谱法(TDLAS)进行红外湿度检测中的噪声,采用多阈值小波去噪方法对测量通道和参考通道间的差分信号做降噪处理,然后基于数字锁相放大技术从去噪后的差分信号中提取二次谐波信号,最终根据谐波信号的幅值大小确定湿度数值。给出了多阈值小波去噪算法的流程,并做了实验验证。分析显示,本文方法可有效抑制噪声的影响,在极低湿度条件下仍可较好地提取出二次谐波信号。利用制作的湿度检测系统,结合配备的密闭湿度环境,开展了检测实验。结果表明,采用小波去噪后,检测下限可由0.2%降至0.08%,极低湿度条件下(0.2%)的测量误差可由48%降至7.9%,其它浓度下的测量误差小于5%。同时,由于噪声被很好抑制,检测系统的测量标准差小了1个量级,从而改善了系统稳定性。本文的小波去噪方法也可用于具有类似机理的红外气体检测系统中以改善性能指标。
In order to effectively suppress the noise in tunable diode laser absorption spectroscopy (TDLAS) for detecting infrared humidity, a multi-threshold wavelet denoising method is used to denoise the differential signal between the measurement channel and the reference channel, and then based on the digital phase-locked amplification technology The second harmonic signal is extracted from the denoised differential signal, and finally the humidity value is determined according to the magnitude of the harmonic signal. The flow of multi-threshold wavelet denoising algorithm is given and verified experimentally. The analysis shows that this method can effectively suppress the influence of noise and can still extract the second harmonic signal under very low humidity conditions. Using the production of humidity detection system, combined with the enclosed humidity environment, to carry out the test. The results show that the detection limit can be reduced from 0.2% to 0.08% with wavelet denoising, and from 48% to 7.9% with very low humidity (0.2%). The measurement error under other concentrations is less than 5%. At the same time, as the noise is well suppressed, the measurement standard deviation of the detection system is one order of magnitude smaller, which improves the system stability. The wavelet denoising method in this paper can also be used in infrared gas detection system with similar mechanism to improve the performance index.