Principle and design scheme based on a new type of high reliability methane sensor-EEWORLD

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0 Introduction

In order to adapt to the development of coal industry towards mechanization and automation, ensure the production safety of mines, and prevent gas explosion accidents, the research and design of mine methane sensors have never stopped. Existing methane sensors generally have the disadvantages of high power consumption, single function, and low accuracy. In addition, the use of analog circuit technology results in low anti-interference ability and intelligence of the system. Therefore, the development of a high-reliability methane detector that is easy to carry, multifunctional, high-precision, and has strong anti-interference ability has great application value.

1 New features of the instrument

Methane sensor is a mining instrument that must first meet the regulations for underground safety production, but compared with other common sensors, it has the following main features:

(1) High reliability processing

Low power consumption uses AT89LV51 single chip microcomputer for centralized processing, which greatly reduces the complicated analog circuits and components and saves circuit power consumption.

(2) High precision requirements

The settings of the black and white component circuits within the chip.

Coal mine safety regulations stipulate that when the methane concentration exceeds 1%, the sensor should alarm; when it exceeds 1.5%, the underground equipment should be powered off; when it exceeds 2%, all personnel should be evacuated immediately. The detection range of the methane sensor studied in this paper is 0% to 4%, which fully meets the requirements of coal mine safety regulations.

The measurement error within the effective detection range of 0% to 4% methane concentration is:

When the methane concentration is 0% to 1%, the error range is ≤±0.1%;

When the methane concentration is 1% to 2%, the error range is ≤±0.2%;

When the methane concentration is 2% to 4%, the error range is ≤±0.3%.

(3) High intelligence

It adopts infrared remote control and onboard buttons. When working normally, the instrument is equipped with time display.

(4) The powerful digital processing method breaks the shortcoming of the single function of the previous methane sensor and adds auxiliary functions that are convenient for underground workers to use, such as mine ambient temperature display, year, month and day time display, instrument power detection, and historical concentration data storage.

2. Instrument working principle

The sensor system is centered on the AT89LV51 microcontroller. The sensor detects the methane concentration in the well. The methane sensor linearly outputs a voltage signal proportional to the concentration, which is amplified by the amplifier and input into the A/D converter. After conversion, the digital signal is sent to the microcontroller. The microcontroller performs corresponding processing and then sends it to the LCD to display the corresponding concentration value. When the methane concentration exceeds the safe value, an audible and visual alarm is issued. The working principle is shown in Figure 1.

Figure 1. Working principle of methane sensor

Temperature measurement: The temperature sensor detects the ambient temperature and outputs the corresponding voltage signal which is directly sent to the analog input channel of the A/D converter. After being converted into a digital signal, it is sent to the microcontroller for data processing and finally the temperature is displayed on the LCD.

Power detection part: A corresponding detection circuit is set at the power supply end, and the remaining power is displayed on the LCD screen in a graphical way after being processed by the microcontroller.

Infrared remote control part: TOSH IBA's TC9148P infrared modulation transmitter chip, with three buttons for selection, adjustment up and adjustment down.

Watchdog part: It uses XICOR's watchdog chip X5045 with serial interface EEPROM. It is set at the beginning of the sensor and its setting data is saved in the EEPROM.

Time display part: Add a dedicated clock chip to generate the calendar and clock, and connect to the microcontroller for time adjustment and display control.

3 Working Principle of Hardware Circuit

3.1 Detection Circuit

3.1.1 Working principle of methane sensor

There are many methods for methane detection, such as thermal conductivity method, infrared spectrum coefficient method, ultrasonic measurement method, gas-sensitive semiconductor method, hot carrier catalytic element detection method, etc. The instrument adopts the hot carrier catalytic element detection method. This element has a platinum wire as the core inside and an aluminum oxide as the carrier outside. The carrier is coated with a catalyst. When a certain current passes through the platinum wire and the element is in a gas containing methane, flameless combustion will occur on the surface, causing the platinum wire to increase in resistance due to temperature increase, thereby realizing the detection of methane. At present, methane sensors all use this method. The principle of using a hot carrier catalytic element to detect methane concentration is shown in Figure 2. It is a simple bridge, with the catalytic element r1 (black element) as the working element and r2 (white element) as the comparison element. R2 is used to compensate for the inconsistency of r1 and r2. When there is no methane, adjust RP to make the bridge in a balanced state, the same constant current flows through r1 and r2, and the temperature of the two elements rises to about 500 ℃. When methane is present, methane reacts with oxygen on the surface of the working element. The heat released causes the temperature of the working element to rise by ΔT, thereby causing the resistance of the working element to increase by Δr, causing the bridge to lose balance and generating a voltage signal output that is proportional to the methane concentration. By measuring the size of this signal, the methane concentration can be determined, and the signal will be output to the A/D conversion circuit.

Figure 2 Principle of carrier catalytic element detecting methane concentration

3.1.2 Methane detection bridge

Figure 2 is simplified as shown in Figure 3. When the output end of the bridge is connected to a high input impedance device (such as an operational amplifier or a digital voltmeter), the bridge is equivalent to working in an output open circuit state, and its output voltage is:

Figure 3 Simplified circuit

3.2 A/D conversion circuit

According to the multi-function and high-precision requirements of the sensor, the A/D conversion circuit uses TLC2543C to convert the measured analog signal into a digital signal. TLC2543C is a 10-bit switched capacitor successive approximation analog-to-digital converter. It is connected to the main processor or other peripheral devices through a serial 3-state output terminal, reducing the hardware routing. In addition to high-speed conversion and general control functions, the device has 11 analog input terminals, which can fully meet multi-channel sampling and function upgrades. The device's converter combines the external input differential high-impedance reference voltage, which has the characteristics of simplified proportional conversion and isolation of analog circuits from logic circuits and power supply noise. The design of the switched capacitor can also reduce conversion errors over the entire temperature range and improve the accuracy of the system.

3.3 Display circuit, RS232 serial port communication circuit and hardware watchdog circuit

In order to adapt to the mine environment, the sensor uses a wide LCD display with backlight to enhance the visual effect, and has calendar and time display functions.

Communicate with PC and store data, so as to establish a database for the mine environment and analyze the environment. The serial port of the microcontroller is added with MAX232 level converter, and the data is transmitted with PC by 3-wire system, and the baud rate is 9600 bps. The T1 counter of the microcontroller is used as the baud rate generator for sending and receiving data of the serial port. The watchdog chip X5045 with serial interface (SPI) EEPROM of XI2COR company is used, and the setting data is saved in EEPROM when the sensor is initialized. The reliability of the whole system is greatly improved, and the system resources are saved to the greatest extent.

3.4 External remote control circuit and amplifier circuit

The system also uses infrared communication mode. TOSH IBA's TC9148P infrared modulation transmitter chip has single-key transmission function and continuous transmission function. The three buttons of the remote control transmitter, such as selection, adjustment up and adjustment down, all adopt the continuous transmission mode. The receiving part only uses the infrared receiving head, and the software decoding function is realized inside the single-chip microcomputer. The use of infrared remote control makes the operation convenient and reliable.

The digital potentiometer X9313 is used to replace the analog potentiometer. The precision and sensitivity of the sensor are adjusted to improve the precision and accuracy.

4 Software Design and Algorithm Implementation

After the system is powered on, the microcontroller is initialized first, then the A/D is initialized, and then the system starts working. First, the methane concentration is sampled and displayed, and an alarm is triggered when it exceeds the safety value; then the ambient temperature is sampled and displayed; finally, the time is displayed, and the sampling is cyclic and displayed in real time.

On the other hand, the external interrupt INT0 is used to wait for keyboard input. The INT0 interrupt enters the keyboard setting program to adjust the calendar time and set the safety alarm value. The main program and the PC and infrared and keyboard interrupt program flow chart are shown in Figure 4. After the initial state setting of the system is completed, the methane sensor can be used for gas measurement.