Design of wireless monitoring system for coal mine temperature and gas concentration

O Introduction

As an important energy source, coal plays a vital role in many aspects such as industrial production and is related to the lifeline of the national economy. Safety issues in coal mining have always been given great attention. If safety problems occur, it will not only cause huge economic losses, but also directly threaten the lives of coal workers. In recent years, the safety situation of coal mining in my country is not optimistic. Mining accidents have occurred from time to time in various places, especially in some small coal mines, which have serious safety hazards. Therefore, it is imperative to take modern safety monitoring measures.

With the continuous progress of science and technology, the modernization of safety monitoring in coal mining is also advancing. There are many parameters for coal mine safety monitoring, among which gas concentration is a very important parameter. If the concentration is too high, it is easy to cause explosion danger, and the consequences are disastrous. Similarly, temperature is also a very important parameter.

Many systems use wired transmission, but there are disadvantages such as difficult wiring and high price. Based on the above, this paper designs a wireless monitoring system for coal mine temperature and gas concentration.

1 System composition and working principle

The whole system consists of a PC in the main control room, a host computer and a slave computer. The communication adopts wireless communication. The overall block diagram of the system is shown in Figure 1.

The whole system includes 240 slave computers, one master computer and a PC. This system adopts wireless communication. The master computer sends synchronization signals, and the slave computers send data to the master computer in their respective time slots in turn, once every 15 minutes. The gas sensor converts the gas concentration into an electrical signal, and the output voltage amplitude is proportional to the concentration. The temperature sensor uses a digital temperature sensor. The slave computer consists of a single-chip microcomputer and a wireless transceiver module, which is used for on-site monitoring, packaging the collected data, and transmitting the data to the master computer through the wireless transceiver module. The master computer is also composed of a single-chip microcomputer and a wireless transceiver module. Its main function is to synchronize the entire system and receive data sent by the slave computer, and forward it to the PC.

This system uses the MSP430F135 microcontroller produced by TI, which has the following characteristics: low power consumption, working current is microampere level; fast speed, the fastest instruction cycle reaches 125 ns, rich peripheral components, with A/D converter, comparator, two 16-bit timers, 6 8-bit parallel ports, etc. The wireless communication uses the CCl000 wireless transceiver chip produced by CHIPCON, which works in the ISM frequency band within the range of 300 MHz to 1 000 MHz and is usually used for low-power wireless communication. The temperature sensor uses the DS18820 of DALLAS. The gas sensor uses the MJC4/3.0L produced by my country.

2 Data Collection

DS18820 adopts single bus communication mode. When connected to the single chip microcomputer, it only needs to connect an external pull-up resistor of about 5.1 kΩ and select external power supply. The peripheral circuit is simple and clear. The temperature measurement range is -55℃～+125℃. The conversion result is 9 to 12 bits. When the result is 12 bits, the minimum resolution can reach 0.0625℃. In this system, the conversion result is set to 9 bits in the initialization stage, and the corresponding resolution is 0.5℃. It only takes 93.75 ms to complete a conversion, which is much less than the 750 ms required for 12-bit conversion. Since one byte is used to represent the temperature in the data frame, a right shift is required, and the result is an integer.

Before each data transmission, the temperature data is collected 10 times and then the average value is taken to ensure that the data is more reliable and accurate.

The MJC4/3.0L catalytic element works according to the principle of catalytic combustion effect. It is composed of a detection element and a compensation element to form two arms 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 to the increase of gas concentration, and the compensation element plays a role in temperature and humidity compensation. It has the following characteristics: the output voltage of the bridge is linear, the response speed is fast, the element works stably, and it can resist H2S poisoning. It is widely used in the concentration detection of combustible gases such as natural gas, liquefied gas, coal gas, alkanes, and organic solvent vapors such as gasoline, alcohol, ketone, and benzene at industrial sites.

In this system, MJC4/3.OL is followed by an amplifier circuit. The gas concentration is a variable, and the corresponding output voltage also changes accordingly. In practice, the staff hopes to obtain a more accurate peak value, so this system uses a peak detection circuit, as shown in Figure 2.

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Before each gas concentration measurement, switch K is disconnected and the time delay is 50 ms to collect the peak value. A/D conversion is performed in 50 ms and the data is stored. When the time is reached, switch K is closed to discharge. Similarly, data is collected 10 times and averaged before each data transmission. If the result is greater than the set voltage, the lower computer alarms and the on-site LED flashes.

3 System Workflow

This system is a wireless monitoring system. The lower computer circuit is powered by lithium batteries and is required to work continuously for more than one year. Therefore, energy consumption must be considered first when designing the system process. Since the change of gas and temperature is a relatively slow process, data can be sent and received once every 15 minutes. Therefore, the sensor circuit can be in sleep state most of the time to save energy. The working current of the wireless transceiver chip CCl000 is about 10 mA, and only 1μA in sleep state; the RF power amplifier consumes about 70 mA when sending data.

The operating current of the MSP430F135 microcontroller is 300μA, and less than 2μA in sleep state.

In order to save energy, 15 minutes is divided into 90 time slots. The host computer sends a time slot code Tc every 10 seconds, Tc = 0x00 ~ 0x59, and switches to the receiving state after each transmission. The data frame sent by the host computer includes a 4-byte preamble, a 1-byte start delimiter 0xF3, a 1-byte time slot code TC, and a byte CRC checksum to improve the reliability of wireless transmission. Therefore, a data frame has a total of 7 bytes and takes about 12 ms. (The transmission rate of CCl000 is set to 4.8 kbps)

The data to be sent by the lower computer includes a 4-byte preamble, a 1-byte start delimiter 0xF2, a 1-byte address code, a 1-byte temperature value data, a 1-byte gas concentration data, and a 1-byte CRC check code. Therefore, a data frame has a total of 9 bytes. It takes 15 ms to send data.

On this basis, the wireless monitoring system process was designed, and 20 ms of sending time was allocated to each slave computer. It takes a total of 4.8 s for 240 slave computers (serial numbers from 0x01 to 0xF0) to complete a communication.

The lower machine process is as follows:

(1) After the lower computer is initialized, it switches to the receiving state; after receiving the time slot code Tc, it calculates the sleep time t and enters sleep. t=10×(90-Tc)-1.5 s.

At the same time, timer t2 is cleared and timing starts.

(2) When timer t2 ends, t2 is reset and the signal acquisition subroutine begins to execute.

(3) When t2=1 s, disconnect the signal acquisition circuit and switch to the receiving state.

(4) When t2 = 1.5 s, the synchronization signal 0x00 is received, and the time to send data is calculated according to the local address code n, t1 = (n-1) × 20 ms.

At the same time, timers t1 and t2 are cleared, timing starts, and the lower computer enters sleep mode.

(5) The nth slave computer is awakened at tl = (n-1) × 20 ms and sends data. After sending, it enters the sleep state again and goes to step (2). The process of the upper computer is relatively simple. The following is only the flowchart of the lower computer work, as shown in Figure 3.

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4 Conclusion

This system effectively collects the temperature and gas concentration of multiple measuring points in the coal mine; low power consumption extends the working life of the system; wireless transmission reduces the complexity of the system and wiring costs. The system has high reliability and cost-effectiveness.