Design Of Anhydrous Hydrazine Detection Signal Processing System Based On TDLAS

As a widely used rocket propellant,hydrazine anhydrous itself is extremely toxic and easily endangers the environment and personnel safety of the rocket range.Electrochemical sensor equipment is usually used in the range environment to detect hydrazine anhydrous gas.However,electrochemical sensors are susceptible to external environmental contamination,and long working hours can lead to degradation of detection accuracy.We need to replace the sensor probe frequently.Therefore,it is urgent to design an anhydrous hydrazine gas detection device that can continuously measure with high accuracy for a long time,without changing the probe,non-contact and online detection.In this paper,a hydrazine gas detection system using spectral absorption technology is studied to address the above problems.By analyzing the difference in absorption spectra between ambient gas and anhydrous hydrazine gas,the absorption wavelength for anhydrous hydrazine gas detection is determined.By studying the TDLAS gas detection technique,the correspondence between the gas concentration values and the second harmonic amplitude is obtained in this study.Combining the TDLAS technique with the lock-in amplification technique,this paper presents a general design scheme for the detection of hydrazine gas using a digital lock-in amplifier.In order to enable the modulated laser wavelength output from the laser to scan near the peak of the absorption spectrum of anhydrous hydrazine gas,a highly stable laser drive source is designed and implemented in this paper.I designed the temperature control circuit to regulate the thermoelectric cooler encapsulated inside the laser for regulating the laser operating temperature.The DDS signal generation module is designed inside the FPGA to control the sawtooth waveform output from the AD5546.In this paper,the AD9832 is used to generate a high-frequency sine waveform,and the two waveforms are superimposed to generate a modulated waveform,so that the laser output has a sweep function.I designed a modulation filter circuit and a constant current source conversion circuit to provide a highly stable current drive source for the laser.To solve the problems of large size of traditional lock-in amplifier for TDLAS gas detection,easy coupling noise and the need to constantly debug the phase during detection,this paper designs and implements a two-phase quadrature digital lock-in amplifier inside the FPGA,and conducts simulations to verify each functional module in the digital lock-in amplifier.Meanwhile,this study designs the signal conditioning circuit,AD conversion circuit,and communication circuit for signal acquisition and transmission.Finally,after testing the performance of the laser driver and the digital lock-in amplifier,it’s found that the current output range of the current driver can be50～350m A.with good linearity in this current range,the stability can reach 0.0225%under long-term working conditions,which meets the laser drive requirements.By verifying the lock-in amplifier function,it’s clear that the average error of extracting harmonic amplitude is less than 1%,realizing the function of extracting harmonic amplitude information.