0 引言

列車在高速運行的過程中，機車與鋼軌的頻繁沖擊會造成車輛軸承的發(fā)熱，當軸承磨損和產(chǎn)生缺陷時，會造成機損從而影響車輛的正常運行，甚至出現(xiàn)熱切軸，直接導(dǎo)致火車發(fā)生故障翻車，給國家和社會在鐵路運輸造成巨大的經(jīng)濟損失。目前我國大部分采用紅外軸溫監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)" target="_blank">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" 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title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)" title="監(jiān)測系統(tǒng)">監(jiān)測系統(tǒng)，但這種設(shè)備易受外界環(huán)境影響、探測點受車身擺動影響定位困難等原因，使得軸溫過高告警兌現(xiàn)率低、誤報率極高、而且外界因素對其工作狀態(tài)容易產(chǎn)生很大的干擾，失真嚴重，極有可能給發(fā)出錯誤的溫度提示，影響鐵路運輸?shù)恼９ぷ鱗1-2].針對這種情況，設(shè)計了系統(tǒng)簡潔、布局小巧、靈敏度高、收發(fā)信息能力快速的高速列車軸溫集中監(jiān)測系統(tǒng)，可在第一時間發(fā)現(xiàn)運行中的列車軸承是否溫度過高，如軸溫超過預(yù)設(shè)定值，就發(fā)出報警信號，機車司機可及時的停車檢查，有效地降低事故發(fā)生率，避免因事故而產(chǎn)生的巨大經(jīng)濟損失，維護鐵路運輸?shù)恼＿\行。

1 系統(tǒng)總體設(shè)計

高速列車有多節(jié)車廂組成，每節(jié)列車車廂有兩個車軸架，每個車軸架上有兩個車軸，每個車軸兩端又有兩個軸箱。設(shè)計中在每個軸箱上安裝一個軸溫傳感器，每個軸架上設(shè)一個監(jiān)測節(jié)點。絕大多數(shù)列車軸溫報警裝置仍然沿用了傳統(tǒng)的單車分散報警的模式，又考慮到列車存在經(jīng)常調(diào)換機車車頭和車廂的問題，所以在傳輸方式上系統(tǒng)選用了2.4G無線通信傳輸方式。

為了防止數(shù)據(jù)阻塞，采用了輪詢問答方式進行通信，大大提高了通信質(zhì)量和可靠度。系統(tǒng)主要由多個監(jiān)測節(jié)點和一個部署在駕駛艙內(nèi)的監(jiān)測臺組成。系統(tǒng)結(jié)構(gòu)示意如圖1所示。

圖1 系統(tǒng)結(jié)構(gòu)示意圖

監(jiān)測節(jié)點通過溫度傳感器將模擬信號轉(zhuǎn)換成數(shù)字信號，實時地將列車每個車軸的軸溫情況傳送到安裝在機車頭的監(jiān)測控制臺，進行分析、顯示，供機車司機或者地面車輛檢修部門隨時監(jiān)測車輛運行時軸溫的變化情況。在列車運輸過程中，當出現(xiàn)軸溫異常時，監(jiān)測臺能發(fā)出報警信號給司機，提醒及時停車采取一定的措施排除故障，降低了機車在運行過程中因軸溫過高發(fā)生事故的幾率，從而確保列車運行的安全。

2 硬件結(jié)構(gòu)

系統(tǒng)的無線節(jié)點硬件主要由微控制器ATmega128L、溫度傳感器PT100、無線通信模塊nRF24L01、存儲器K9F5608和電源管理模塊等組成。其中監(jiān)測臺還包括報警和顯示屏單元。

硬件部件的控制器單元主要功能在于控制溫度傳感器采集列車的軸溫信息，并對信息進行處理和轉(zhuǎn)發(fā)，以及對接受到的數(shù)據(jù)進行分析、顯示。數(shù)據(jù)采集單元主要是通過溫度傳感器對軸溫信息的采集。無線通信單元用于發(fā)送采集到的軸溫數(shù)據(jù)，并與列車監(jiān)測臺進行實時通信。節(jié)點硬件構(gòu)成如圖2所示。

圖2 節(jié)點硬件構(gòu)成

2.1 微控制器ATmega128L

考慮到控制器既要滿足系統(tǒng)的需求，又要保持低功耗和小體積的特性，故選用了8位微控制器ATmega128L,相對于其他通用的8位微控制器來說，它具有非常豐富的資源，工作于16MHz時性能高達16MIPS,具有片內(nèi)128k字節(jié)的程序存儲器，4k字節(jié)的數(shù)據(jù)存儲器和4k字節(jié)的E2PROM;具有兩個16位定時器/計數(shù)器；具有53個通用I/O 口線、實時時鐘RTC、兩個USART、可工作于主機/從機模式的SPI串行接口、8路10位ADC、兩路8位PWM、與IEEE1149.1規(guī)范兼容的JTAG測試接口用于片上調(diào)試，以及6種可以通過軟件選擇的省電模式?？刂破鞯腁DC口與PT100的輸出連接，SPI接口與無線通信模塊nRF24L01連接實現(xiàn)數(shù)據(jù)的收發(fā)，PWM口用來驅(qū)動監(jiān)測臺的報警單元，數(shù)據(jù)總線和地址總線與存儲器配合使用。

2.2 無線通信模塊nRF24L01

nRF24L01是工作在2.4GHz~2.5GHz的ISM 頻段的單片射頻收發(fā)器，內(nèi)置頻率合成器、功率放大器、晶體振蕩器、調(diào)制器等功能模塊，支持Shock-Burst和Enhanced Shock-Burst兩種數(shù)據(jù)傳輸方式，其中輸出功率和通信頻道可通過程序進行配置。nRF24L01功耗低，在以-6dBm的功率發(fā)射時，工作電流也只有9mA,接收時，工作電流只有12.3mA,多種低功率工作模式（掉電模式和空閑模式）使節(jié)能設(shè)計更方便，接口最高速率可以到達8Mbps,工作電壓1.9~3.6V,采用SPI接口進行數(shù)據(jù)的收發(fā)，無線數(shù)據(jù)的傳輸速率最高達2Mbps,并有自動應(yīng)答和自動重發(fā)射功能。輸出功率、頻道分配及協(xié)議的選擇可以通過SPI接口進行設(shè)置[3].

2.3 溫度傳感器PT100

常用的接觸式測溫傳感器有熱電偶、熱電阻、半導(dǎo)體石英晶體等，鉑熱電阻在0℃時的電阻值稱R （0℃）和100℃時的電阻值稱R （100℃）以及R （100℃）/R （0℃）叫做比值W100.監(jiān)測節(jié)點溫度傳感器采用的是鉑電PT100,PT100的含義為（0℃）時的名義電阻值為100Ω，在溫度作用下，鉑熱電阻絲的電阻值隨之變化而變化，且電阻與溫度的關(guān)系即分度特性完全和IEC標準等同，因此PT100主要用來測量-200~+600℃的溫度。鉑電阻的電阻值與溫度成非線性關(guān)系，所以需要進行非線性校正[4].校正分為模擬電路校正和微處理器數(shù)字化校正，模擬校正有很多現(xiàn)成的電路，其精度不高且易受溫漂等干擾因素影響，數(shù)字化校正需要在微處理系統(tǒng)中使用，將鉑電阻的電阻值和溫度對應(yīng)起來后存入EEPROM 中，根據(jù)電路中實測值以查表方式計算相應(yīng)溫度值。常用的Pt電阻采樣電路有兩種：一為橋式測溫電路；另一為恒流源式測溫電路。

系統(tǒng)中監(jiān)測節(jié)點采用的是是第一種橋式電流測溫電路，其具有測量準確度高、測量范圍大、復(fù)現(xiàn)性和穩(wěn)定性好等優(yōu)點。

2.4 存儲器K9F5608

存儲器K9F5608 是三星公司生產(chǎn)的32MBytes容量的NAND Flash芯片，用來存儲臨時的軸溫數(shù)據(jù)，當通信中斷時可以把數(shù)據(jù)存在K9F5608中，待通信恢復(fù)正常后再將這些數(shù)據(jù)發(fā)送至監(jiān)測臺。它具有8位數(shù)據(jù)/地址復(fù)用接口以及CLE、ALE、WP、R/B、CE等控制信號，硬件接口比較簡單，但讀寫時序相對較復(fù)雜，需要依次送入命令、地址、數(shù)據(jù)等。實際需要輸入24位的地址用來尋址，由于該芯片與控制器ATmega128L的接口是8位地址數(shù)據(jù)復(fù)用線，所以需要3個周期才能完成24位地址的輸入[5].存儲器K9F5608外圍電路如圖3所示。

圖3 K9F5608外圍電路

控制器ATmega128L的PA口完成對Flash數(shù)據(jù)﹑地址與命令的傳輸；用控制器的PG口來完成對Flash傳輸狀態(tài)的選擇，如CLE、ALE、R/B、CE等；用控制器的WE和RD信號來選擇FLASH是讀還是寫，在總線上傳輸?shù)囊话沩樞驗椋?/p>

命令->地址->命令/數(shù)據(jù)。

3 軟件設(shè)計

系統(tǒng)軟件包括監(jiān)測節(jié)點軟件和監(jiān)測臺軟件兩部分。為了確保正常通信，采用問答方式，即控制臺依次與每個監(jiān)測節(jié)點建立通信，收到節(jié)點發(fā)來的數(shù)據(jù)后，再收取下一個節(jié)點的溫度數(shù)據(jù)信息，這樣就避免了阻塞的情況，確保數(shù)據(jù)的可靠性[6].

3.1 監(jiān)測節(jié)點軟件

監(jiān)測節(jié)點上電后，首先進行系統(tǒng)初始化，包括控制器各端口、寄存器和通信模塊nRF2401的配置等，之后進入接收控制臺指令狀態(tài)。當接收到指令時，立即讀取傳感器PT100的輸出值，經(jīng)過查表處理得出當前軸溫信息，與節(jié)點ID打包正數(shù)據(jù)幀后發(fā)送到監(jiān)測臺，并繼續(xù)進入到接收指令狀態(tài)。監(jiān)測節(jié)點軟件流程如圖4所示。

3.2 數(shù)據(jù)格式

系統(tǒng)了制定了簡單的數(shù)據(jù)幀格式，如表1所示。

圖4 監(jiān)測節(jié)點軟件流程

表1 監(jiān)測節(jié)點數(shù)據(jù)幀結(jié)構(gòu)

每個幀的幀頭都以‘start'’開始，占用5個字節(jié)；然后是占用3個節(jié)點的ID,可以認為是節(jié)點在網(wǎng)絡(luò)中的地址；再接下來就是占用6個字節(jié)的溫度值。當監(jiān)測臺發(fā)送指令時，每個監(jiān)測節(jié)點都收到數(shù)據(jù)，但只有指令中ID與節(jié)點ID匹配時才會將該數(shù)據(jù)返回給監(jiān)測臺，如果不匹配則不做任何處理。

3.3 監(jiān)測臺指令幀結(jié)構(gòu)

監(jiān)測臺與各監(jiān)測節(jié)點通信的時候主要是通過發(fā)送指令完成的，指令幀結(jié)構(gòu)如表2所示。

表2 監(jiān)測臺數(shù)據(jù)幀結(jié)構(gòu)

每個指令幀也是以“start”開始，占用5個字節(jié)；然后是需要提供溫度數(shù)據(jù)的節(jié)點ID,占用3個字節(jié)；再接下來是指令的有無效，占用1個字節(jié)；最后是以“end”的幀尾結(jié)束符，占用3個字節(jié)。

3.4 數(shù)據(jù)發(fā)送和接收

發(fā)送數(shù)據(jù)。首先將nRF24L01配置為工作在PTX 模式，接著把地址TX_ADDR和數(shù)據(jù)TX_PLD按照時序由SPI口寫入nRF24L01緩存區(qū)，置CE為高電平并保持至少10μs,拉低CE后發(fā)射數(shù)據(jù)，并立即進入接收模式等待應(yīng)答信號。如果收到應(yīng)答，則認為此次通信成功，TX_DS置高，同時TX_PLD從發(fā)送堆棧中清除；若未收到應(yīng)答，則自動重新發(fā)射該數(shù)據(jù)，若重發(fā)次數(shù)ARC_CNT達到上限，MAX_RT置高，TX_PLD 不會被清除；MAX_RT 或TX_DS置高時，使IRQ變低，通知處理器做相應(yīng)處理。發(fā)射成功后，進入空閑模式。

接收數(shù)據(jù)。首先將nRF24L01配置為PRX接收模式，接著延遲130μs進入接收狀態(tài)等待數(shù)據(jù)的到來。當接收方檢測到有效的地址和CRC時，就將數(shù)據(jù)包存儲在接收堆棧中，同時中斷標志位RX_DR置高，IRQ變低，通知處理器讀取數(shù)據(jù)，接收完數(shù)據(jù)后進入發(fā)射狀態(tài)并回傳應(yīng)答信號，接收成功，CE變低進入空閑模式[5].

4 試驗結(jié)果與分析

試驗采用雙層集裝箱車，分別對空載和載重的兩種不同情況車廂進行了測試?？蛰d車廂自重為25t,載重車廂重為78t.

兩種情況的測量結(jié)果如圖5所示。

圖5 測試結(jié)果

由于列車運行情況相對復(fù)雜，軸溫與速度、運行時間、制動、載重和環(huán)境溫度等情況都有密切關(guān)系。從對圖5的分析看，具體關(guān)系是：速度越快，軸溫越高；列車啟動階段，即0~30km之間，隨著運行時間的增加軸溫越高，當達到一定程度時，運行時間不再軸溫；在行駛到43~50km處，列車制動使車軸急劇升高，溫度達到71.45℃；在運行穩(wěn)定狀態(tài)下，載重78t的車廂明顯比載重25t的車廂，溫度高6℃左右；另外，環(huán)境越高，軸溫越高。不難看出，載重越大，軸承運轉(zhuǎn)溫度越高，并且上升速度越快。

5 結(jié)論

針對目前列車軸溫探測存在的不足，借助于現(xiàn)代無線傳感器網(wǎng)絡(luò)技術(shù)，設(shè)計了系統(tǒng)簡潔、布局小巧、靈敏度高、收發(fā)信息能力快速的高速列車軸溫集中監(jiān)測系統(tǒng)，擺脫了傳統(tǒng)有線的傳輸方式，可實時監(jiān)測運行中的列車軸承情況，如超過軸溫監(jiān)測預(yù)設(shè)定值，就發(fā)出報警信號，便于及時采取有效措施，有效地降低事故發(fā)生率，避免因事故而產(chǎn)生的巨大經(jīng)濟損失。經(jīng)過實驗得到兩種載重情況運行列車的軸溫數(shù)據(jù)，分析了提速列車的軸溫變化規(guī)律，給改進熱軸預(yù)報提供了數(shù)據(jù)基礎(chǔ)，對維護鐵路運輸?shù)恼＿\行和高速鐵路的發(fā)展具有重要意義。