6ES7315-1AF03-0AB0 飛行器的工作狀態參數和環境參數主要通過無線遙測和回收遙測獲得，是評定飛行器性能和分析飛行器故障的依據[1]。隨著技術的發展，飛行器內部的工作參數越發復雜，遙測數據的信息量越來越大,遙測系統現有的存儲能力和信道帶寬已經很難滿足如此大數據量的存儲、傳輸要求。考慮到技術、成本等條件的限制，一味增加信道帶寬和存儲器的容量是不現實的。目前，數據壓縮技術被廣泛應用于遙測系統[2]。根據遙測數據的特點，采用合適的算法對大數據量的遙測數據進行編碼壓縮，不僅減輕了遙測系統數據存儲的壓力，也降低了對信道帶寬的要求，提高了通信效率。
遙測噪聲信號的頻率和幅度變化很大且無規則，信號的相關性差，為反映信號的完整特性，需要較高的采樣頻率，這樣就會產生很大的數據量。為有效地完成對噪聲信號的測量，這里采用ARC（算術編碼）算法對噪聲數據進行無損壓縮，以DSP+FPGA為硬件平臺[3]，充分利用FPGA高速、并行的特性和DSP在算法實現上的優勢，很好地實現了對遙測噪聲數據的實時、無損壓縮。
1 系統設計
系統整體設計框圖如圖1所示，由噪聲傳感器采集的噪聲信號經調理電路濾波、放大后，進行A/D轉換得到量化噪聲數據；FPGA將噪聲數據寫入內部8 KB FIFO，直到FIFO達到半滿[4]，DSP才會讀取噪聲數據進行算術編碼；編碼壓縮后的噪聲數據先是被DSP緩存至SDRAM，然后通過McBSP串口發送到FPGA，FPGA通過內建的4 KB FIFO對接收的壓縮數據進行緩沖；422通信控制模塊會接收讀數命令并在4 KB FIFO達到半滿時按照HDLC協議的要求將壓縮數據傳輸到外部設備進行存儲、傳輸和分析等操作。
2 數模轉換電路設計
噪聲信號的模數轉換采用TI公司的ADS8365芯片實現。它是16位6通道并行A/D，采樣率可達250 kS/s，完全滿足對4路噪聲信號進行27 kHz采樣的要求。
ADS8365的6個模擬輸入通道可分為3組,分別為A、B和C組[5]。每組都有一個保持信號(分別為HOLDA、HOLDB和HOLDC)，用于啟動各組的A/D轉換。6個通道可以進行同步并行采樣和轉換。當ADS8365的HOLDX保持20 ns的低電平后開始轉換。當轉換結果被存入輸出寄存器后，引腳EOC的輸出將保持半個時鐘周期的低電平，以提示FPGA進行轉換結果的接收，FPGA通過置RD和CS為低電平使數據通過并行輸出總線讀出。
ADS8365的數據的讀出模式有3種，分別是：直接地址讀取、FIFO讀取、循環讀取，是由地址/模式信號A0、A1和A2來選擇的[6]。本系統FPGA將數據讀出配置為FIFO讀取模式。A/D轉換電路如圖2所示。
3 FPGA與DSP通信設計
FPGA控制ADS8365完成對4路噪聲信號的A/D轉換，各路量化數據加入通道標志后依次寫入FPGA內部FIFO。當FIFO達到半滿后，通知DSP讀取2 048 B數據進行編碼，編碼時間最長為40 ms，平均20 ms。DSP在編碼過程中不能與FPGA進行數據通信，FPGA要對由A/D產生的量化數據進行緩存。按最長耗時40 ms計算，每路27 kHz的采樣率會產生為4.32 K個采樣點。采用16位FIFO，則FIFO的深度應大于4 320，這里為增加可靠性，設計FIFO深度為8 192。
6ES7315-1AF03-0AB0
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