Research On High Sensitivity Ethylene Gas Concentration Detection System Based On NDIR And Dual Channel Lock-in Amplifier

Ethylene(C₂H₄),a gas hormone,is released by mature plants and can serve as a fruit ripe catalyst or preservative at appropriate concentration.Usually,the gas concentration detection in this application requires its sensitivity of several ppm or even sub-ppm level.In order to meet this application requirement,a dual-channel phase-locked amplifier circuit was carried out in this paper,combined with a highly efficient infrared reflection optical path structure,and a dual temperature compensation algorithm was proposed to reduce the temperature drift of the sensor.Based on the principle of non-dispersive infrared spectroscopy(NDIR),a highly sensitive ethylene infrared gas sensor was Successfully realizedThe main research work of this thesis is as follows:(1)The related theoretical basis of ethylene gas sensor was summarized and analyzed.From the perspective of atomic and molecular vibration,we discussed the infrared absorption spectrum of ethylene molecules.Then,the principles of eliminating the fluctuation error of the light source and the detector were analyzed theoretically in detail by the dual-wavelength difference method,and the design method of the infrared reflection optical path was discussed.Then,the principle and implementation method of the noise suppression circuit of the lock-in amplifier was introduced,as well as the modified Lambert-Beer law.(2)The hardware structure design and production of ethylene gas sensor were carried out.The hardware system could be divided into three parts:air chamber and mirror module,dual channel lock-in amplifier module,as well as signal control and data processing module.We first analyzed the driving principle of the infrared light source and the detector,and then adopted a software of Zemax to design a highly efficient infrared reflected light path structure to improve the response rate of the detector.Simulation showed that the designed optical path transmission efficiency reached 92%.And the laser 3D printing technology was implemented to make an optical absorption cell.In addition,combined with theoretical calculations and numerical simulations,a phase-locked amplifier circuit was designed.Then,the analog-to-digital converter circuit,STM32 microcontroller circuit,temperature sensor circuit,and system power supply were designed.(3)Completed the software design of ethylene gas sensor.There were four modules which was designed on the STM32,a SPI that drives the ADC,I²C that communicates with the temperature and humidity sensor,the PWM that modulates the light source,and the DMA+UART for data transmission.In the host computer program,a serial data analysis program and a GUI program for displaying the ethylene concentration curve were designed.Besides,a dual temperature compensation algorithm to reduce the temperature drift of the sensor was proposed.(4)A test device for ethylene gas sensors was built and the performance of related components and sensors was tested.The input-output relationship and the signal-to-noise ratio of the lock-in amplifier circuit were tested.The experiment result showed that the output signal-to-noise ratio was increased to 52d B.Under the condition of using dual temperature compensation algorithm,the detectable concentration range and sensitivity of the sensor were tested.The detection stability is around 1ppm,and the detection limit is close to 1ppm,the relative error of the sensor in the concentration range of 1?95.3ppm would not exceed 10%,which can be applied to the fields such as fruit maturity detection.