Based on the comparison of common gas sensing components, OPTOSENSE's latest infrared absorption methane gas sensor MIPEX was selected, and the wireless gas sensor node hardware circuit was designed. Based on the ZigBee protocol stack, the node software program is designed. The node is in a working/dormant alternating state with a period of about 10 min. In the case of three ordinary battery-powered situations, it is theoretically estimated that its working time can be more than 10 years, which is far from the traditional low-power gas sensing element. Far from being reached.
introductionGas accidents have always been a major threat to the safe production of coal mines. Although gas monitoring technology has continued to develop in recent years, gas explosions have occurred frequently. The existing coal mine safety monitoring systems at home and abroad are wired connections, which have great limitations. Since the sensor uses a wired connection, this is mainly limited to applications in the main mine. At the coal mining face with high gas concentration, due to the continuous mining of coal mines, various large-scale equipment on the working surface need to be continuously promoted, and the mutual position between the equipments is constantly changing. The wired monitoring network cannot follow the mines in time. Changes that result in monitoring blind spots. The application of wireless sensor networks in gas safety monitoring systems, combined with existing wired monitoring networks, to build a more comprehensive downhole gas monitoring system will help to improve the current problems in the field of gas monitoring.
In such systems, the sensor network nodes are battery powered and have very limited energy. However, the power consumption of commonly used low-power gas sensing components is as high as several hundred mW. How to reduce node energy consumption is a key problem to be solved by wireless gas monitoring networks.
1 hardware circuit designTable 1 lists the commonly used low-power gas sensing components and their main indicators. As can be seen from the table, the power consumption of commonly used low-power gas sensors is above 100mW, which is very disadvantageous for battery-powered wireless sensor nodes. Moreover, the sensing elements listed in the table have a certain response time, that is, after the sensing element is powered, it needs to wait for a response time to correctly reflect the gas concentration information. Longer response times limit the amount of time that a wireless gas sensor node can't work each time it collects data. For example, the response time of the TP-1.1A non-heated methane gas sensor is close to 20 s. If the gas sensor node uses the sensing element, when it collects data once, it is meaningless to collect data from the first 20 s from the power supply to the sensor. Because the sensing element is in the response phase at this time, its voltage value cannot accurately reflect the actual gas concentration information. Therefore, each time data is collected, the sensing element is powered for at least 20 s. For such high-power sensing elements, the energy consumed to acquire the data once is very large. This makes the designed wireless gas sensor node work time is too short, so that it can not meet practical requirements.
In the design of wireless sensor nodes, there is also a problem that the operating voltage of the sensing element is inconsistent with the operating voltage of the microprocessor and the wireless transceiver circuit in the node circuit. If the supply voltages of different modules in the node are different, the circuit needs to perform voltage conversion. The conversion of different voltages will increase the complexity of the circuit design, resulting in increased node energy consumption.
The infrared absorption methane gas sensor MIPEX produced by Russian OPTOSENSE company is designed by the principle of non-dispersive infrared technology (NDIR), and its light source adopts non-traditional energy-saving LED light source. The light source system uses an advanced algorithm to generate an optimized radiation spectrum, and the light is passed through a methane-filled optical system to a photodiode containing lead selenide and cadmium selenide to monitor the methane concentration. The sensor has a built-in temperature sensor and an integrated signal processing and temperature compensation system that outputs digital signals on its own. Digital signals can effectively avoid the effects of the external environment on their output signals. The digital signal output by the sensor follows the UART format.
The wireless transceiver chip selected in this paper is CC2430, and the power supply is powered by battery pack. As the battery energy is consumed, the voltage output of the battery pack changes greatly, and it is easy to exceed the operating voltage range required by the sensing component. Therefore, it is necessary to select a suitable voltage regulator device to provide a stable operating voltage for the sensor component and the wireless transceiver circuit. . The main considerations are as follows: 1 node is intended to be powered by 3 to 4 AA batteries, ie for voltage regulators, the input voltage range is 4.5 to 6 V; 2MIPEX sensor operating voltage range is 3 to 4.5 V, while CC2430 wireless transceiver The operating voltage range of the chip is 2 ~ 3.6 V, here the voltage is uniformly selected as 3.3 V, which requires the regulator output voltage to be 3.3 V; 3 wireless transceiver module maximum operating current is 27mA, MIPEX sensor average operating current It is 1 mA, so the selected regulator device is required to provide an output current of not less than 28 mA; the selected regulator device must be as small as possible to save energy when it is static.
Considering that the above four requirements are met at the same time, this paper selects the low-dropout linear regulator MH5333 produced by Shenzhen Minghe Technology Co., Ltd. Its input voltage is up to 10V, the output voltage is 3.3V, the maximum output current is 500mA, and the quiescent current is 1μA. It can be seen that the MH5333 voltage regulator device can better meet the above requirements.
The 3-5th battery is connected in series as the input of the regulator MH5333, and its output (3.3 V) supplies power to the wireless transceiver circuit and sensor components. The wireless transceiver circuit and the sensor component transmit data through the serial port. A pin LED is connected to pins P0.0 and P0.1 of CC2430 to facilitate the debugging of the program and the execution of the program. In order to reduce energy consumption, a pin of the CC2430 is used to control the power of the MIPEX sensor. The power supply requirements of the MIPEX sensor are that the power supply voltage is in the range of 3 to 4.5 V, and the output power is in the range of 0.02 to 0.25 W. The P1.0 and P1.1 pins of the CC2430 can provide a driving current of 20 mA, which can be seen from the CC2430. The output power of the P1.0 and P1.1 pins can meet this requirement. Here, the P1.0 pin of the CC2430 is selected to control the power of the MIPEX sensor. The TXD and RXD pins of the methane sensor MIPEX are connected to the P0.2 and P0.3 pins of the CC2430, respectively, to the RXD and TXD terminals of the asynchronous serial interface 0 of the CC2430.
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