The Development Of NDIR Instrument For Vehicle Emission Detection Using The Galvanometer Modulation

With the rapid development of social economy,the car ownership has increased year by year.The car brings convenience to people’s daily life.At the same time,the vehicle emission becomes one of the main sources of air pollution.To improve the living environment,the vehicle emission must be strictly detected and controlled.Nowadays,a series of means for vehicle emission detection have been developed.Compared with other methods,the non-dispersive infrared（NDIR）technique is considered as the simplest approach.The NDIR instruments are simple,easy to maintain and operate,and long service life.They have been widely used in various fields,such as environment monitoring,mining industry,fire detection and industrial processes control.In the NDIR instrument,the detector always is only sensitive to the variation of light intensity.The IR light source always needs to be modulated.There are two main methods,namely electrical pulse modulation and mechanical chopper modulation.The electrical pulse modulation is not practical for the high-powered IR thermal radiation source,which has the long run-up time.The high-frequency electrical pulse modulation often greatly shortens the service life of light source.The chopper modulation is widely used in NDIR instruments.Multiple detecting channels are needed to measure different gases respectively,which consist of multiple detectors and condenser lenses with the small aperture.This makes the structure complicated and the throughput decreased.The non-common optical path will affect the measurement accuracy of multicomponent gas which is spatially inhomogeneous.In this paper,we design a NDIR instrument with the common optical path configuration to detect CO,CO₂,HC and NO in vehicle emission.The galvanometer modulation is proposed,which makes the IR light spot uniformly scan back and forth in the line detector array.The scanning IR light is modulated for each detector element.The design of detecting system structure is proposed in this paper.The IR source,the gas cell,the condenser lens,the filter,the line detector array and the galvanometer are chosen.Compared with the chopper modulation,the galvanometer modulation produces higher detection efficiency and stability.The calibration of the system and the performance test are carried out after the system are built.The zero calibration and gas calibration are by measuring high purity nitrogen and different concentrations gases,respectively.The limits of detection and the limits of quantitation of the four gases are calculated.The repeatability and stability of the system are also tested.Finally,the results of the open path detection for CO₂ and CO validate the feasibility of the NDIR instrument.