Design of portable mine gas detection system

Abstract: The concentration detection of combustible and dangerous gases such as gas in mines has always been an important prerequisite for ensuring safe production in coal mines. In view of this, a portable mine gas detection system is designed by using single-chip intelligent control technology and integrating functional circuits such as gas sensors. The system is small and lightweight, and can automatically detect the gas concentration in the mine and alarm. The hardware design and software design of the system are introduced in detail. After multiple tests, the system performance is stable and the effect is good.
Keywords: portable; gas; single-chip microcomputer; gas sensor

0 Introduction
In recent years, although the country has taken many favorable measures to ensure the safe production of coal mines, coal mine production accidents continue to occur, especially coal mine gas explosions, which have caused huge losses to the country and the people. Although modern gas comprehensive detection devices have been applied to mines, due to the complex structure of mines, there are often blind spots for gas detection, leaving safety hazards. In view of this, this paper uses AT89S52 single-chip microcomputers and integrates MJC4/3.0L gas sensors and other functional devices to design a portable mine gas detection system. The system is small and lightweight, and can automatically detect the gas concentration in the mine. If the concentration reaches the dangerous set value, it will sound an alarm through the buzzer to prompt the production personnel to leave safely.

1 System overall structure and working principle
1.1 System overall structure
The portable mine gas detection system is based on the main controller microcontroller as the main core, and is equipped with four major functional modules including gas sensor circuit, A/D conversion circuit, alarm circuit and key circuit. The overall structure of the system is shown in Figure 1:

1.2 System working principle
The gas sensor converts the gas concentration into an analog signal of corresponding size. The signal is amplified and converted by the signal amplifier circuit and the A/D conversion circuit, and then sent to the main controller microcontroller for data processing. Once the gas concentration exceeds the corresponding set value, the main controller immediately starts the buzzer alarm.

2 System hardware circuit design
2.1 Main control circuit design
The main control circuit is mainly used to integrate the various functional circuits of the system, complete data collection and processing, and issue alarm instructions. The amount of information processed by this design is not too large and the complexity is not too large. The 8-bit microcontroller AT89S52 is sufficient to meet the requirements of this design. It is a low-power, high-performance CMOS 8-bit microcontroller with 8 kB system programmable Flash memory, 256 bytes of RAM, 6 interrupt sources, 3 16-bit programmable timers/counters, 32 IO ports, watchdog timer and other resources.

2.2 Gas sensor and signal amplification circuit design
The system uses MJC4/3.0L as a gas sensor. The MJC4/3.0L catalytic element works according to the principle of catalytic combustion effect. The detection element and the compensation element are paired to form the arm of the bridge. When encountering combustible gas, the resistance of the detection element increases, and the output voltage of the bridge changes. The voltage variable increases in direct proportion with the increase of gas concentration. The compensation element plays a reference and temperature compensation role. It has the advantages of linear bridge output voltage, fast response speed, good repeatability, selectivity, stable and reliable component operation, and resistance to H2S poisoning.
Because the output voltage of MJC4/3.0L is too small to meet the requirements of AT89S52. Therefore, it is necessary to amplify the output signal of MJC4/3.0L. Signal amplification is achieved by adjusting the gain control voltage of the amplifier AD 602. AD602 is specially developed by AD Company in the United States for program-controlled amplification. It has two channels, and the gain range of each channel is -10 to 30 dB. Therefore, the gain control range that can be achieved by connecting two channels in series is: -20 to 60 dB. Figure 2 shows the gas sensor and signal amplification circuit.

2.3 A/D conversion circuit design
The digital-to-analog converter LTC1865 used in the system is a 16-bit SAR ADC launched by Linear Technology. It operates with a single 5 V power supply and can be guaranteed to operate within a temperature range of -40℃ to +12.5℃. The maximum current of each device is 8.50 uA, and the maximum sampling rate is 250 kS/s. The power supply current decreases as the sampling rate decreases. The MSOP-10 packaged LTC1865 provides 2 software-programmable channels, and the reference voltage can be adjusted according to requirements. The A/D conversion circuit design is shown in Figure 3.

2.4 Alarm module circuit design
The alarm module of this design is completed with an ordinary buzzer. One end of the buzzer is grounded, and the other end is connected to the emitter of the PNP transistor used to drive it to work. The base of the transistor is connected to the P3.3 port of AT89S52.
2.5 Keyboard module circuit design
The buttons in this system are mainly used to set the alarm value of gas concentration. An independent key-type keyboard is used, with a total of 3 buttons, which are connected to the P2.0~P2.2 ports of AT89S52 respectively. Usually these three pins output high level. When the key is pressed, the pin becomes low level. Therefore, as long as the level of these pins is queried in the software, it can be determined whether a key is pressed, so as to enter the corresponding subroutine.

3 System software design
The system software mainly includes two parts: the system main program and the data sampling and processing subroutine. The main program flow is shown in Figure 4, and the data sampling and processing subroutine is shown in Figure 5.

After the system is powered on, it is initialized first, and then enters the main loop to scan whether there is a key pressed. If a key is detected, the gas concentration alarm upper limit of the system is set, otherwise the data acquisition processing subroutine is directly called for data acquisition processing.
After the main program calls the data sampling processing subroutine, it enters the subroutine to run. First, the A/D conversion is started for data sampling. The obtained data signal is input to the AT89S52 for filtering, zero point correction and calculation of the gas concentration value. If the concentration exceeds the limit, the speaker sound alarm is started, otherwise the buzzer is turned off and returns.

4 Experimental results and analysis
The main component of gas is methane. There is a certain concentration range for gas explosion. The concentration range that can cause explosion after gas encounters fire in the air is usually called the gas explosion limit. The gas explosion limit is 5% to 16%. When the oxygen concentration in the air reaches 10%, the gas concentration is between 5% and 16%, and an explosion will occur.
According to the technical indicators of MJC4/3.0L (when the methane concentration is 1%, its sensitivity is 20-40 Mv), the upper limit of gas explosion is set to 0.05. When the gas concentration in the mine exceeds this upper limit, the buzzer will sound an alarm.
This design uses household biogas for simulation experiments. Since the main component of biogas is also methane (50%-80%), it should be able to achieve the expected experimental results. The biogas stove is placed in a pre-prepared 21-inch TV carton. There is an operation port at the bottom of the carton. The biogas is turned on for 5 s and then turned off. Then the experimental device is close to the operation port. The buzzer alarms at the carton port. In the process of approaching the biogas stove, the buzzer keeps ringing. Slowly move the experimental device out of the operation port. When it is about 0.1 m away from the operation port, the buzzer stops. After repeated reciprocating operations, the experimental effect is obvious, which shows that the design has achieved the expected experimental results.

5 Conclusion
This paper uses single-chip intelligent control technology and integrates gas sensors and other functional circuit modules to complete the design of a portable mine gas alarm system. The system is compact and easy to carry. After many experiments, it has achieved good results and can be used as a reference for the development of gas detection equipment to ensure safe production in coal mines.