DSP-based high-speed portable infrared gas analyser

Infrared gas Analyzer is based on the different gas molecules on a specific wavelength infrared absorption principle of selectively into a physical analysis instrument, is a typical optical, mechanical and electrical integration, intelligent sensor systems.

And compared to other gas sensor system with high sensitivity and fast response, analyze gases variety, the quantum of the wide, continuous measurement characteristics. In earthquake prediction, mine safety, oil exploration, atmospheric physics, health, pollution monitoring, high-voltage equipment fault diagnosis, chemical process control, metallurgical and other traditional industries and now all of the new technological revolution lead in disciplines such as biology, electronics, new materials, and other areas are becoming more and more widely used there.

From domestic existing infrared Analyzer, most online monitor desired position machine with complete data for post-processing and save, front-end signal detection and only complete collection of functions, this design in some limited or permanent installed without supervision (e.g. pollution monitoring) occasions would not be applicable, in response to this situation, this article on the development of an approach based on TI's DSP for non-simultaneous-infrared gas analyzers, ability to independently perform monitoring, and data can be stored in large-capacity Flash memory or via GPRS remote transmission, the instrument of the simple man-machine interface enables calibration of measuring instruments or whatever can be easily completed, at the same time, the USB interface so that the instrument has added a greater expansion of space, and the instrument has a variety of signal output mode, you can easily connect with the various systems.

1 infrared Analyzer system structure

Infrared Analyzer system architecture diagram as shown in Figure 1, send a certain frequency of TMS320F2812DSP modulation control infrared light source.

IR infrared originate from shooting, after a full test gas chamber was a specific gas absorption, filtered through selective x-ray, finally arrived at the corresponding infrared detector, detector measured absorption of light energy intensity, reflects the strength of the gas absorption infrared, also reflects the concentration of gas. Infrared detector output of weak signal, after precision of pre-amplification and II zoom filter circuit, get stable signal, the signal a/d conversion, sent to the analysis and processing DSP, passes through the filter and nonlinear correction, the final measurement data will be in the system settings and status of the current instruments, save, transmit or display refresh. Note that here the probe measurement channels at least 2 road, where 1 road to measuring road, 1 way to reference road, so you can achieve the effect of differential measurement, formed on the system noise and interference suppression, the other road sensor and A/D channels used to measure temperature, humidity and air pressure, involved in the concentration of compensation calculated.

2 system hardware design

Hardware design of the instrument in infrared sources, probes, DSP system and peripheral circuits, signal amplifier selection is the key.

The apparatus is infrared light source select IRL715 infrared light source, the source of light in the wavelength range from visible light wavelength to 5 μ m, 5 V voltage driven working life up to 40 000 h. Detector chose two road surveying pathway TPS2534G2. TI DSP has adopted the United States company TMS320F2812 digital signal processor, 32-bit fixed-point DSP, the kernel provides up to 150 MIPS computing bandwidth, greatly improving the control system of control and data handling capabilities, its peripheral circuits include: data capture, digital output, man-machine interface. Storage system and USB communication interface. Signal amplifier circuit using a two-level AD8552 amplifier circuit concatenation, there are automatic AD855X series offset adjustment features, is the low frequency of weak signal detection amplifier system preferred. Because of this instrument is high speed DSP system in infrared application of analytical instruments, the following special on DSP external circuit to be a description.

2.1 data acquisition

Data acquisition is mainly responsible for analog-digital conversion and signal acquisition and measurement of the various needs of analog by .A/D converted to the DSP.

Which measurement channels and reference channel signal generated by the infrared detector, consisting of a A/D converter, convert, convert 16 bit precision, upon request, select direct serial interface with TMS320F2812 16 bit A/D converter; other participation in gas concentration calculation of compensation, including the signal humidity, temperature and atmospheric pressure signal by TMS320F2812 with A/D conversion time conversion, 12 bit precision. These signals converted to DSP processor cache. The series of complex calculation processing the resulting concentration of the measured gases. Here the reference is used to measure the channel and channel signal acquisition of circuit design, as shown in Figure 2.

2.2 switch reset output

Switch output including detector temperature control signal, optical lens window temperature control signal, elements of the alarm trigger signal (concentration, temperature, humidity) 3 and 1-channel time trigger signal.

Using DSP processor for general purpose I/O port for digital output, achieving 74HC244 control signals of the driver output. Figure 3 circuit diagram for the digital output. Note: the power supply voltage TMS320F2812 processor 3.3 V, and the power supply voltage 74HC244 5 V, two devices exist interface circuit level translation problems that cannot be directly connected, using LVCl * 5 as level translator for TMS320F2812 and the level of match between 74HC244.

2.3 man-machine interface

Man-machine interface uses LCMl68651 LCD module as shown, the use of matrix keyboard interrupt mode, man-machine interface of the task is receiving keyboard commands, complete sets of equipment, calibration, measurement, and so on, to provide real-time concentration curve plotting, and provides historical data in the query and display functions.

2.4 USB connected

Interface circuit

USB interface for PC and PC communication.

USB data transfer rate is very high, so not only can be used to transfer command, and can transmit data in real time, including the original measured value, the current concentrations of values, or history, Figure 4 shows the USB interface circuit schematic.

2.5 data storage

Infrared Analyzer requires the storage of data including: gas measurement record number, date, time, concentration, temperature, humidity, atmospheric pressure, record the amount of data depends on the size of the saved entry, save, save frequency and save time, considering the online measurement and management measure, the use of large-capacity Flash K9F1G08UOA as instruments of data storage media, data storage volume larger than 128 MB.

3 system software design

Software handled directly determine the system handle the level of speed and accuracy of the calculation results, Figure 5 gives the instrument software's primary processes, the basic principle is to use the ring buffer for data and other information necessary to cache, without affecting the system, man-machine interactive experience to achieve higher speed measuring and data transmission.

In the software throughout the process the most time-consuming and most affect performance of the local processing of raw data, which relates to the filter, the environment and probe compensation correction, eliminate gas absorption cross interference and other algorithms, this instrument in the such algorithms are made specifically for DSP optimization, thus ensuring system running quickly.

The system first completed initialization, including DSP and its peripheral circuits initialization, create and set the original data and target data, keyboard, and other information of a ring buffer and so on.

Set the infrared light modulation frequency and start, equipment on standby, you can do this on the instrument, press the key or the parameter setting remote start command will make the instrument into the measurement State. In the measurement State apparatus in order to distinguish a non-loop processing data or command, and then make the appropriate treatment, such as remote command execution, the original data calculation, press the command is executed, the target data transfer and storage, LCD interface refresh, etc.

A variety of original data will set the sample rate is interrupted program read and stored in the original data ring buffer, the original data including measuring channel and reference channel readings, various compensation signals to measure time, etc.

The original data is used after processing. The calculated target values into the target data buffer, if the system has a transmission, display or save settings to the destination data buffer of data will be used in sequence. All the ring buffer will maintain their own read/write pointer, and the corresponding operation completes. Some priority higher command and special situations are not affected by the above process limitations, such as a stop command, like this one interruption of operations Council to deal with directly.

4 experimental results

This is not mentioned in the earlier instrument specific measurement of gas type, because the instrument in the swap different wavelengths of light filter and do the appropriate parameter modification can be used to measure the concentration of various gases such as CO2, CO and HC.

In select the gas CO2 as a test, the ambient temperature is 25 ° c, a standard atmospheric pressure, on the national metrology standards in a variety of standard concentrations of CO2 gas measurement using this instrument, the actual results show that absolute error in 0.3%, relative error in less than 2 per cent, has better measurement accuracy.

5 conclusions

Infrared gas Analyzer involve optical, mechanical, electronic, computer, communications, information fusion and other areas, on the hardware and software design and integration capabilities requires a relatively high, described by infrared gas Analyzer with SP's powerful computing capacity, not only satisfied the demand for high-speed measurements, but also because the PC makes instrument portability and installation of greatly enhanced, through the scene run and debug the benefits of the instrument are displayed, such as the structure is simple, reliable, convenient installation and maintenance, easy operation, long time to save the data, USB extension, and believe that with the online gas analysers of the renewal application, the tester and wider.