Development Of The High-Performance Combustible Gas Alarm

In the present industrial production process, the safety problems are considered more and more important. In coal, chemical and petroleum industry, there are many kinds of combustible gas in the workplace. If the combustible gas gathers in the air, it will create great potential safety hazard. Therefore combustible gas alarm will then have more and more application. But the domestic combustible gas alarms have low intellectual level and low interference-elimination capacity. In view of these problems, a new kind of high-performance combustible gas alarm is designed in this thesis.This thesis describes the development of the High-Performance combustible gas alarm in four parts.The first apart is about the over-all-scheme. The system uses control bus to control every combustible gas alarm even in a long distance. In order to achieve the field regulation function, it also includes infrared remote control function in each alarm apparatus. In this part, the RS485 bus and RS232 bus are introduced. In contrast of the two bus mode, RS485 has much longer transform distance and much stronger interference elimination capacity, so I choose RS485 bus as the communication bus. Regarding the communication protocol, I choose Modbus protocol which is an industrial standard protocol as the communication protocol. It behaves well in data transmission capacity, misdate detection and real-time characteristic. According to combustible gas's type, this article uses catalytic-combustion combustible gas sensor NAP-55A. The overall diagram of the circuit and the system performance index are introduced at the end of this part.The second part is about the hardware structure of combustible gas alarm. In the alarm, the single-chip microcomputer ATmega8 is the controlling core and circuit design mainly includes the following sections: signal measurement and amplification circuit, infrared remote control circuit,RS485 communications circuit,E2PROM circuit, display circuit, alarm circuit, power circuit and so on.The measurement and amplification circuit is composed of the sensor measuring bridge and two level of operational amplifier circuit. Digital potentiometer is used to adjust the balance of the electrical bridge and to adjust the Magnification of the operational amplifier. Through this, the automatic zero setting and the automatic calibration work can be realized. The output of the amplification circuit is changed to digital signal by the A/D cell in the ATmega8. In the measurement circuit, there is a small resistance which is connected to the ground of the bridge power. The voltage of the resistance shows whether the sensor has electrical fault of short or open circuit.The parameter storage circuit is placed on the sensor probe. It uses E2PROM memory AT24C01 to record zero and the linearity information of the sensor. When electrified, the microcomputer reads memory's related information and adjusts the digital potentiometer according the information. Thus, when we change the probe, automatic adjustment is realized.The infrared remote control circuit is mainly used to give control signals to realize zero setting and calibration when the sensor has zero-drifting and linearity-drop problems. It can also give control signals to the microcomputer to adjust the high alarm point and the low alarm point.In the system, the digital circuit and the communication circuit need to use the isolates power to avoid the disturbance from each other, so this thesis uses isolated double output DCDC module D2405 to realize the main line voltage transformation to the internal isolated power supply voltage. Regarding sensor's power supply, the sensor supply current is big. In order to reduce DCDC module burden, this thesis uses LM2575 to transform the main line voltage to +2.5V output which is used for sensor power source. There is another advantage to use LM2575.It can disable the power supply when the sensor is circuit-shorted. The third part is about the software design of the combustible gas alarm. It mainly includes: Infrared receiver program, digital filtering program, zero-setting program, modbus communication program, display program and so on.The fourth part is about the EMC design of the combustible gas alarm. It makes the system much more reliable. It can also resist interphone interference. This part has first analyzed the noise source which the alarm apparatus possibly appear, and proposes the corresponding solution according to the noise source.