【正文】 the overall schematic diagram of system is shown in Figure 1. The reason for selecting singlechip microputer as a control core is that it possesses advantages of small size, high reliability, low price which made it very suitable to be used in industries of intelligent instrument and real time control . The operating interface of system is shown in Figure 2. Number at upper right corner shows the default or userdefined gas concentration value, number at upper left corner shows detected gas concentration value. One alarm lamp is equipped. All functions are controlled by keys arranged on the control panel, including POWER key, RESET key, DATA COLLECTION key. Other keys including ten number keys, ADJUST VALUE key and ENTER key are used to change threshold values. Basic operating procedures are as follows: Firstly press POWER key, the system initialized. Press DATA COLLECTION key, LED at upper right corner displays the threshold value 。它可以用于實時甲烷氣體濃度監(jiān)測。當按下 Enter 鍵，系統(tǒng)開始數(shù)據(jù)采集的運行和 A / D轉(zhuǎn)換，然后將轉(zhuǎn)換后的數(shù)據(jù)為二進制格式調(diào)用一個子程序的雙字節(jié)乘法。 串行通信模塊，信號控制器發(fā)送到主機計算機上通過 RS485，和 MAX485是用于轉(zhuǎn)換電壓。信號放大器電路原理圖如圖 5所示。當它被暴露可燃氣體，測量元件電阻增大，輸出通 過測量電橋電壓參數(shù)，參數(shù)的電壓成正比的氣體濃度值。如圖 3所示。所有的功能通過設(shè)置控制面板上的按鍵控制，包括電源鍵，復位鍵，數(shù)據(jù)采集的關(guān)鍵。該系統(tǒng)的主要功能如下：濃度的實時監(jiān)測 CH4和顯示的濃度值；發(fā)射聲光報警信號，如果 CH4濃度值超過報警值通過鍵盤面板輸入；串行通信口發(fā)送數(shù)據(jù)地面以上主機。軟件調(diào)試和硬件仿真上述系統(tǒng)也實現(xiàn)在同一時間。其他鍵包括十個數(shù)字鍵，調(diào)整值鍵和回車鍵來改變閾值。 圖 3. 主控制單元 和信號放大電路 在氣體檢測系統(tǒng)設(shè)計的一個關(guān)鍵問題是如何選擇的氣體傳感器。補償元件的工作溫度補償和參考。 圖 5. 信號放大電路 A / D轉(zhuǎn)換器模塊 如圖 6所示，該模塊由多路 CD4051，采樣保持器 lf389， A / D轉(zhuǎn)換器 AD574A [ 12 ]和并行 I/O 芯片 8255A作為核心的一部分， AD574A是一個 12位的逐次逼近 A/D轉(zhuǎn)換器芯片三的狀態(tài)緩沖區(qū)，其轉(zhuǎn)換時間約為 25μS. AD574A能夠直接連接到單片沒有額外的接口邏輯電路，內(nèi)置高精度的參考功率電源和時鐘電路，無需外部時鐘源 AD574A能夠正常運行參考電源。 RS485是多點雙向半雙工通信基于單平衡線電路具有高噪聲抑制鏈接，傳輸速率高，傳輸距離長、寬共模范圍，其最大傳輸率達到 10Mbps，電纜的最大距離達到 1200 [ 13,14 ]。我們需要三個存儲單元存儲這些數(shù)據(jù)是因為他們 24位二進制數(shù)據(jù)格式。測量從零到百分之 2本系統(tǒng)的范圍內(nèi)，傳感器的靈敏度達到百分之 。 User can customized threshold value by press ADJUST VALUE key and ten number keys, then press ENTER to confirm the change. System starts to detect gas concentration and display these parameters on upper left LED area, at meantime transmit realtime data by RS485 to host puter above ground. 3 Hardware System Design of Gas Detection System The Hardware architecture of system mainly including main control unit, sensors and signal amplifier circuit, A/D converter module, soundlight alarming circuit, keyboard and display module, serialmunication module. Main Control Unit Featured by high integration level, small size and low prices, Single chip microputer has been widely used in a broad range of industrial applications including process controlling, data collection, electromechanical integration, intelligent instrument, household appliances and work technology, and significantly improved the degree of technology and automation. Two factors are taken into account here in chip selecting, first one is antiinterference ability, increase the interferences in SCM application systems, so the SCM must have highresistance to outside interference。 using serial munication port to transmit data to the host puter above ground. The software debugging and hardware simulating of the system above ar