Application of TMS320F2812 in the detection of weak gas signals in underground mines

introduction

At present, the accumulation of gas in mines has become a major problem that troubles coal mine safety production. Accurate and effective monitoring of gas is of great significance to coal mine safety production. Due to the existence of existing noise and a large amount of noise generated in production in mines, the weak gas signal submerged by the noise is extremely weak compared to the noise. For example, the signal-to-noise ratio of the input signal is 10-1, 10-2, and even 10-5. The gas signal is "deeply buried" in the noise. In addition, the detection and transmission are affected by the noise existing in the signal end, transmission devices, and conversion devices. The overall effect is that the useful weak gas signal is submerged by a large amount of noise and interference. Since the noise is random and the signal is periodic and correlated, this paper adopts the cross-correlation operation in the phase-sensitive detection technology in the phase-locked amplification principle to weaken the influence of noise, and then passes through a low-pass filter. The bandwidth of this filter is very narrow. After the phase-sensitive detection output, only the components falling within the equivalent noise bandwidth of the low-pass filter are output, and other high-frequency components are filtered out. This method has a strong noise suppression ability. It is convenient to extract low-concentration gas signals, achieves high efficiency and high sensitivity in weak gas detection, and effectively reduces the harm caused by weak gas accumulation.

Principle and method of weak gas signal detection

The basic law for measuring gas absorption is Lambert-Beer's law, and its mathematical expression is as follows:

Formula 1

In formula 1: A is the absorbance, T is the transmittance, which is the ratio of the intensity of the projected light to the intensity of the incident light. c is the concentration of the absorbing substance, and b is the thickness of the absorbing layer.

Its physical meaning is: when a beam of parallel monochromatic light passes vertically through a uniform non-scattering light-absorbing substance, its absorbance A is proportional to the concentration c of the light-absorbing substance and the thickness b of the absorption layer. In other words, the absorption of bright light is proportional to the concentration of the light-absorbing substance.

Considering the relationship between gas absorbance and gas concentration change, when there is weak gas in the coal mine, the values ​​of b and c in the formula are small, and the proportional coefficient is generally small, so the value of A, especially for weak signals, is very small, so when detecting gas concentration, it is easy to be covered by noise signals. In order to better detect weak gas concentration signals, the method used is: complete the correlation operation through the phase-sensitive detector (multiplier) and the low-pass filter (integrator), and use correlation detection to maximize bandwidth compression and suppress noise.

Phase-sensitive detector to realize correlation operation

Phase-sensitive detection PSD detects the phase between the measured signal and the reference signal. The measured signal and the reference signal are sent to the PSD and multiplied in the mixer. The working principle of PSD is shown in Figure 1:

Figure 1 PSD working principle diagram

When designing a phase-sensitive detector, assume that the measured signal is , where , , , are the amplitude, time, initial phase and frequency of the measured signal respectively. The reference signal is , where , , , are the amplitude, time, initial phase and frequency of the measured signal respectively. The output voltage of the phase-sensitive detector is:

Where n is the harmonic order. The formula shows that the output of the phase-sensitive detector consists of two parts. The former is the difference frequency component between the signal to be measured and the reference. When the useful signal to be detected is synchronized with the reference signal, that is, (n=0), the difference frequency is zero. At this time, the difference frequency component becomes a phase-sensitive DC voltage component, and the latter becomes a frequency doubling. That is, the voltage output after phase-sensitive detection is only related to the phase difference. If the initial phase of the reference signal is controlled by DSP to make it in phase with the input signal, the final DC component value is. It can be seen that as long as the useful signal in the signal to be measured is in the same frequency and phase as the reference signal, the DC output signal of the detector before filtering can be easily obtained.

Filter selection

The output signal of the phase-sensitive detector can only obtain a DC signal related to the concentration after a low-pass filter, so low-pass filtering is a very important link, and its performance directly affects the subsequent data processing. Considering the special requirements of the underground communication environment in the mine, in order to ensure the extraction of useful signals, a FIR filter is used. The response of this filter to the pulse input signal eventually tends to zero, so this filter must be stable and achievable. In addition, it is also better to use a FIR filter in places with high linear phase requirements, which also meets the requirements of the reference signal and the useful signal being of the same frequency and phase.

The FIR low-pass filter in this design adopts Butterworth low-pass filtering with a cut-off frequency of 10kHz, a sampling frequency of 40kHz, and a stop-band attenuation of 80dB.

DSP realizes low-pass filtering

The output signal of the phase-sensitive detector is directly sampled into the DSP, and the DSP optimization algorithm is used to achieve high-speed data processing capability to realize low-pass digital filtering. Based on this, this paper uses DSP to realize low-pass filtering and improves the algorithm in the filtering method. The TMS320F2812 DSP is selected as the core main control chip. It has high-speed computing capability, powerful real-time processing capability and highly integrated design structure. The instruction cycle is 6.67ns, and the pipeline technology is used to significantly improve the signal processing speed. The use of dedicated hardware multipliers and special instructions DMAC in the DSP makes it possible to obtain the result of multiplying two operands in one processor clock cycle. This structure just meets some special requirements in digital signal processing such as FIR, IIR, FFT and other operations. In addition, the TMS320F2812 chip has a FLASH program memory of up to 128K×16 bits, which can meet the storage of filter coefficients, sampling data and multiplication and accumulation of operation results in the program. [page]

At the beginning of the program, the system, registers, and working variables must be initialized, the global interrupt is turned on, the sampling clock frequency is set, the timer of the EVA module is initialized and configured, and the data sampling is completed in the interrupt subroutine. The collected data is first saved in the FIFO. After the data collection is completed, the DSP reads the data from the FIFO and starts a series of processing such as correlation and filtering, and finally displays the results. The flowchart of the program is shown in Figure 2:

Figure 2 Program flow chart

The simulation graphs are shown in Figures 3, 4 and 5. The upper part of Figure 3 is the waveform before filtering, and the lower part is the waveform after filtering.

Analysis of simulation results: From the time domain diagram, it can be seen that the irregular burrs on the original waveform have been well smoothed. It can be seen from the spectrum diagram that the low-frequency part of the input waveform passes through the filter, and most of the high-frequency signal is filtered out. The noise interference is well suppressed, and the weak gas signal is effectively extracted, which meets the requirements of phase-locked amplification for the output waveform.

Figure 3 Waveform comparison before and after filtering

Figure 4 Spectrum of input signal before filtering

Figure 5 Spectrum of the output signal after filtering

Conclusion

This paper introduces the principle and method of using phase-locked amplifier and DSP-implemented filter to jointly realize weak signal detection, and gives the flowchart and simulation results of the algorithmic implementation of low-pass filter by TMS320F2812 DSP. The results show that the method of combining DSP-implemented low-pass filtering with phase-sensitive detection principle to detect weak gas concentration signals has strong noise suppression ability and can make the entire system more stable and sensitive.