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System Structure of Infrared Gas Analyzer

Infrared gas analyzer is a physical analysis instrument based on the principle of selective absorption of specific wavelength infrared light by different gas molecules. It is a typical optical, mechanical, and electrical integrated intelligent sensor system. Compared with other gas sensor systems, it has the characteristics of high sensitivity, fast response, various types of analysis gases, wide range, and continuous measurement. In earthquake prediction, mine safety, petroleum exploration, atmospheric physics, medical and health, pollution source monitoring, high-voltage equipment fault diagnosis, chemical process control, metallurgy and other traditional industries, and even now all new technologies ** take the lead in disciplines such as biological sciences, microelectronics, New materials and other fields have more and more extensive applications.

Judging from the existing infrared analyzers in China, most online monitors need the upper computer to complete the post-processing and storage of the data, and the front end only completes the function of signal detection and acquisition. This design is limited in installation or unmanned for a long time. Supervision (such as pollution source monitoring) is not applicable. In response to this situation, this article researches and develops a non-dispersive infrared gas analyzer that can independently complete the monitoring work, and can store data in large-capacity flash memory or remotely through GPRS Transmission. The simple man-machine interface of the instrument makes it easy to complete both measurement and instrument calibration. At the same time, the addition of the USB interface makes the instrument have a larger expansion space, and the instrument has a variety of signal output methods, which can be easily communicated with various System connection.

1 System structure of infrared analyzer

The system structure diagram of the infrared analyzer is shown in Figure 1. TMS320F2812DSP sends a certain frequency modulation signal to control the infrared light source. The infrared light emitted by the infrared light source passes through the gas chamber filled with the gas to be measured and is absorbed by the specific gas. It is selectively transmitted by the filter and reaches the corresponding infrared detector. The detector measures the intensity of the absorbed light energy. Reflects the strength of the gas’s absorption of infrared light, which also reflects the concentration of the gas. The weak signal output by the infrared detector passes through a precise pre-amplification and two-stage amplifying filter circuit to obtain a stable signal. The signal is converted by A/D and sent to the DSP for analysis and processing. After filtering and nonlinear correction, the final The measured data will be transmitted, saved, or displayed refreshed according to the system settings and current instrument status. Note that there are at least 2 measurement channels of the detector here. Among them, 1 is the measurement channel and 1 is the reference channel. This can achieve the effect of differential measurement and form the suppression of system noise and interference. Road sensors and A/D channels are used to measure temperature, humidity and gas pressure, and participate in the calculation of concentration compensation.

2 System hardware design

In the hardware design of the instrument, the choice of infrared light source, detector, DSP system and peripheral circuit, signal amplification circuit is the key. The infrared light source of this instrument selects IRL715 infrared light source, the wavelength range of the light source is from visible light wavelength to 5μm, and the working life can reach 40 000 h under the driving of 5 V voltage. The detector chose TPS2534G2 with two measurement channels. The DSP adopts the TMS320F2812 digital signal processor of American TI Company, which provides a 32-bit fixed-point DSP with a computing bandwidth of up to 150 MIPS, which greatly improves the control accuracy and data processing capacity of the control system. Its peripheral circuits mainly include: data acquisition, Switch output, man-machine interface. Storage system and USB communication interface. The signal amplifier circuit uses two AD8552 operational amplifier circuits in series. The AD855X series has the function of automatic offset adjustment, which is the best choice for low-frequency weak signal detection amplifier systems. Because this instrument focuses on the application of high-speed DSP system in infrared analysis instrument, the following is a special description of the DSP peripheral circuit.

2.1 Data acquisition

Data acquisition is mainly responsible for analog-to-digital conversion and signal acquisition, passing various analog quantities that need to be measured. After A/D conversion, it is sent to DSP. The signals of the measurement channel and the reference channel are generated by the infrared detector and converted by an A/D converter with a conversion accuracy of 16 bit. According to the requirements, a serial 16 bit A/D converter can be directly interfaced with TMS320F2812; others The signals involved in the gas concentration compensation calculation, including humidity, temperature and atmospheric pressure signals, are converted by the A/D time-sharing included in the TMS320F2812, with a conversion accuracy of 12 bit. The above signals are converted and sent to the DSP processor buffer. After a series of complicated calculations*, the concentration value of the measured gas is finally obtained. The circuit design for signal acquisition of the measurement channel and reference channel is given here, as shown in Figure 2.

2.2 Switch output

The switch output mainly includes 3 channels of detector temperature control signal, optical lens window temperature control signal, element alarm trigger signal (concentration, temperature, humidity) and 1 channel time trigger signal. The universal I/O port of the DSP processor can be used to realize the output of digital quantities, and the 74HC244 can realize the drive output of each control signal. Figure 3 is a digital output circuit diagram. It should be noted that the power supply voltage of the TMS320F2812 processor is 3.3 V, while the power supply voltage of the 74HC244 is 5 V. The two devices have the level conversion problem of the interface circuit and cannot be directly connected. The LVC16245 is used as the level converter. Achieve level matching between TMS320F2812 and 74HC244.

2.3 Man-machine interface

The human-machine interface uses the LCMl68651 liquid crystal module as a display, and a matrix keyboard with interrupt mode. The task of the human-machine interface is to receive keyboard commands, complete instrument settings, calibration, measurement and other operations, and provide real-time concentration change curve drawing, and Provide query and display functions for historical data.

2.4 USB interface circuit

USB interface is used for communication between lower computer and upper computer. The data transmission rate of USB is very high, so it can not only be used to transmit commands, but also can transmit data in real time, including the original measured value, current concentration value, or historical record value. Figure 4 shows the schematic diagram of the USB interface circuit.

2.5 Data storage

The data that the infrared analyzer needs to store includes: gas measurement record number, date, time, concentration value, temperature, humidity, atmospheric pressure, etc. The size of the recorded data depends on the size of the saved item, the type of storage, the storage frequency and the length of storage , Taking into account the requirements of online measurement and unmanned measurement, the large-capacity flash memory K9F1G08UOA is used as the data storage medium of the instrument, and the total data storage capacity is more than 128 MB.

3 System software design

The quality of the software processing directly determines the processing speed of the system and the accuracy of the calculation results. Figure 5 shows the main flow of the instrument software. The basic principle is to use the ring buffer to cache data and other information as necessary without affecting the system personnel. High-speed measurement and data transmission can be achieved in the case of machine interaction. In the entire processing process of the software, the *time-consuming and* performance-affecting part is the processing of raw data, which involves filtering, environment and detector compensation correction, eliminating gas absorption cross-interference and other algorithms. These algorithms are used in this instrument. They are all optimized specifically for DSP to ensure the rapid operation of the system.

The system first completes the initialization, including the initialization of the DSP and its peripheral circuits, the creation and setting of the ring buffer for the original data, target data, keyboard and other information. Set the modulation frequency of the infrared light and start it. The meter enters the waiting state. The user can set the parameters of the meter at this time. The key or remote start command will make the meter enter the measurement state. In the measurement state, the instrument will determine whether there are data or commands to be processed in turn, and then perform corresponding processing, such as remote command execution, raw data calculation, key command execution, target data transmission and storage, LCD interface refresh, etc.

Various raw data will be read by the interrupt program according to the set sampling rate and stored in the raw data ring buffer. The raw data includes the readings of the measurement channel and reference channel, various compensation signals, measurement time, etc. The original data is used after processing. Put the calculated target value into the target data buffer. If the system has settings for transmission, display or saving, the data in the target data buffer will be used in turn. All ring buffers maintain their own read and write pointers and modify them after the corresponding operations are completed. Some commands with higher priority and special cases are not restricted by the above process, such as stop commands and similar operations will be directly processed in the interrupt.

4 Experimental results

The previous article did not mention the specific types of gases measured by this meter, because the meter can be used to measure the concentration of multiple gases, such as CO2, when changing to filters of different wavelengths and modifying the corresponding parameters. , CO and HC etc. In the experiment, CO2 was selected as the type of gas to be tested. At an ambient temperature of 25℃ and a standard atmospheric pressure, the meter was used to measure a variety of standard concentrations of CO2 gas qualified by the national metrology department. The actual results show that the ** error is 0.3%, the relative error is within 2%, with good measurement accuracy.

5 Conclusion

Infrared gas analyzers involve multiple disciplines such as optics, mechanics, electronics, computers, communications, and information fusion, and require relatively high software and hardware design and integration capabilities. The infrared gas analyzers described in this article rely on the powerful computing capabilities of SP , Not only meets the requirements of high-speed measurement, but also greatly enhances the portability and installability of the instrument because it is separated from the host computer. Through on-site operation and debugging, many advantages of the instrument are displayed, such as simple and reliable structure, convenient installation and maintenance , Convenient operation, long-term storage of data, USB expansion, etc. I believe that with the upgrading of online gas analyzers, the application prospects of this instrument will become wider and wider.

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