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1、數(shù)據(jù)采集系統(tǒng)數(shù)據(jù)采集系統(tǒng)， 正如名字所暗示的， 是一種用來采集信息成文件或分析一些 現(xiàn)象的產(chǎn)品或過程。 在最簡單的形式中， 技術(shù)人員將烤箱的溫度記錄在一張紙上 就是數(shù)據(jù)采集。 隨著技術(shù)的發(fā)展， 通過電子設(shè)備， 這個過程已經(jīng)得到簡化和變得 比較精確、多用途和可靠。 設(shè)備從簡單的存儲器發(fā)展到復(fù)雜的電腦系統(tǒng)。 數(shù)據(jù)采 集產(chǎn)品像聚焦點(diǎn)一樣為系統(tǒng)服務(wù)， 和一系列產(chǎn)品一起， 諸如傳感器顯示溫度、 水 流、程度或者過程。數(shù)據(jù)采集技術(shù)在過去 30 到 40 年以來已經(jīng)取得了很大的飛躍。舉例來說 ,在 40 年以前 ,在一個著名的學(xué)院實驗室中 ,為追蹤用青銅做的坩堝中的溫度上升情 況的裝置是由熱電偶、繼電器、查

2、詢臺、一捆紙和一支鉛筆。今天的大學(xué)學(xué)生很可能在 PC 機(jī)上自動處理和分析數(shù)據(jù)， 有很多種可供你選 擇的方法去采集數(shù)據(jù)。 至于選擇哪一種方法取決于多種因素， 包括任務(wù)的復(fù)雜度、 你所需要的速度和精度、 你想要的證據(jù)資料等等。 無論是簡單的還是復(fù)雜的， 數(shù) 據(jù)采集系統(tǒng)都能夠運(yùn)行并發(fā)揮它的作用。用鉛筆和紙的舊方式對于一些情形仍然是可行的， 而且它便宜、 易獲得、快 速和容易開始。而你所需要的就是捕捉到多路數(shù)字信息（ DMM ），然后開始用 手記錄數(shù)據(jù)。不幸的是這種方法容易發(fā)生錯誤、 采集數(shù)據(jù)變慢和需要太多的人工分析。 此 外，它只能單通道采集數(shù)據(jù)；但是當(dāng)你使用多通道 DMM 時，系統(tǒng)將很快變得 非常

3、龐大和呆笨拙。 精度取決于謄寫器的水平， 并且你可能需要自己動手依比例 輸入。舉例來說 , 如果 DMM 沒有配備處理溫度的傳感器 ,舊需要動手找比例。 考慮到這些限制，只有當(dāng)你需要實行一個快速實驗時， 它才是一個可接受的方法?，F(xiàn)代多種版本的長條圖表記錄儀允許你從多個輸入取得數(shù)據(jù)。 他們提供數(shù)據(jù) 的長備紙記錄 ,因為數(shù)據(jù)是圖解的格式 ,他們易于現(xiàn)場采集數(shù)據(jù)。一旦建立了長條 圖表記錄儀， 在沒有操作員或計算機(jī)的情況下， 大多數(shù)記錄儀具有足夠的內(nèi)部智 能運(yùn)行。缺點(diǎn)是缺乏靈活性和相對的精度低， 時常限制在百分點(diǎn)。 你能很清楚地 感覺到與筆只有小的改變。 在多通道內(nèi)較長時間的監(jiān)控， 記錄儀能發(fā)揮很好的

4、作 用，除此之外，它們的價值得到限制。舉例來說，他們不能夠與另外的裝置輪流 作用。其他的顧慮就是筆和紙的維護(hù) ,紙的供給和數(shù)據(jù)的存儲 ,最重要的是紙的濫 用和浪費(fèi)。 然而，記錄儀相當(dāng)容易建立和操作， 為數(shù)據(jù)快速而簡單的分析提供永 久的記錄。一些 benchtop DMMs 提供可選擇的掃描能力。 儀器的背面有一個槽孔接 收一張在較多輸入時能多重發(fā)訊的掃描儀卡片， 通常是 8 到 10 通道的 mux 。固 有的在儀器的前面嵌板中的受到限制。 它的柔韌性也受到限制， 因為它不能超過 可用通道數(shù)。外部的 PC 機(jī)通常處理數(shù)據(jù)采集和分析。PC機(jī)插件卡片是單板測量系統(tǒng)，它利用ISA或PCI總線在PC機(jī)

5、內(nèi)擴(kuò)大插 槽。它們時常具有高達(dá)每秒 1000 的閱讀速率。 8 到 16 通道是普遍的，采集的 數(shù)據(jù)直接存儲在電腦里，然后進(jìn)行分析。因為卡片本質(zhì)上是計算機(jī)的一部分,建立測試是容易的。PC機(jī)卡也相對的便宜，一部分地，因為他們以來主機(jī) PC去提 供能源、機(jī)械附件和使用界面。數(shù)據(jù)采集的選擇在缺點(diǎn)上，PC機(jī)插件卡片時常只有12字的容量，因此你不能察覺輸入信號 的小變化。此外 ,PC 機(jī)內(nèi)的電子環(huán)境經(jīng)常很容易發(fā)出噪聲、產(chǎn)生高速率的時鐘和 總線噪聲，電子接觸面限制 PC 機(jī)插件卡片的精度。這些插件卡片也測量一定范 圍的電壓。為了測量其他輸入信號，如電壓、溫度和阻力，你也許需要一些外部 信號監(jiān)測的器件。其它

6、關(guān)心包括復(fù)雜的校正和全部的系統(tǒng)成本,尤其如果你需要購買額外信號監(jiān)測器件或用 PC 機(jī)適應(yīng)插件卡片。把這些考慮進(jìn)去，如果你的需 要在卡片的能力和限制范圍內(nèi)變動， PC 機(jī)插件卡片給數(shù)據(jù)采集提供吸引人的方 法。數(shù)據(jù)電子自動記錄儀是典型的單機(jī)儀器，一旦配備它們,就能測量、記錄和顯示數(shù)據(jù)而不需要操作員或計算機(jī)參與。它們能夠處理多信號輸入，有時可達(dá) 120 通道。 精度可與無與倫比的臺式 DMMs 匹敵,由于它在 22 字、 0.004 個 百分率的精度范圍內(nèi)運(yùn)轉(zhuǎn)。一些數(shù)據(jù)電子自動記錄儀有能力按比例測量,檢查結(jié)果不受使用者定義的限制 ,而且輸出為控制作信號。使用數(shù)據(jù)電子自動記錄儀的一個好處就是他們的內(nèi)部

7、監(jiān)測信號。 大部分能夠 直接地測量若干不同的輸入信號， 而不需要額外的信號監(jiān)測器件。 一個通道能夠 監(jiān)測熱電偶、溫阻器（RTD ）和電壓。熱電偶為準(zhǔn)確的溫度測量提供具有參考價值的補(bǔ)償， 是很典型的配備了多路 插件卡片。內(nèi)設(shè)智能數(shù)據(jù)電子自動記錄儀幫助你設(shè)定測量周期和具體指定每個通 道的參數(shù)。一旦你全部設(shè)定好， 數(shù)據(jù)電子自動記錄儀就如同無與倫比的裝置運(yùn)行。 它們存儲的數(shù)據(jù)分布在內(nèi)存中 ,能夠容納 500000 或更多的閱讀量。與 PC 機(jī)連接容易將數(shù)據(jù)傳送到電腦進(jìn)行進(jìn)一步的分析。 大多數(shù)數(shù)據(jù)電子自 動記錄儀可設(shè)計為柔性和簡單的組態(tài)和操作 , 而且經(jīng)由電池包裹或其它方法， 多 數(shù)提供遠(yuǎn)程位置的操作選

8、項?？?A/ D 轉(zhuǎn)換技術(shù) ,一定的數(shù)據(jù)電子自動記錄儀閱 讀的速率比較低，尤其是跟PC機(jī)插件卡片比較。然而，每秒250的閱讀速率比 較少見。要牢記正在測量的許多現(xiàn)象本質(zhì)上是物理的，如溫度、壓力和流量，而且一般有較少的變動。此外，因為數(shù)據(jù)電子自動記錄儀的監(jiān)測精度,多量且平均閱讀沒有必要，就像它們經(jīng)常在 PC 記插件卡片一樣。前端數(shù)據(jù)采集經(jīng)常做成模塊而且是典型地與PC機(jī)或控制器連接。他們被用于自動化的測試中， 為其它測試裝備采集數(shù)據(jù)、 控制和循環(huán)檢測信號。 發(fā)送信號 測試裝備的零配件。前端運(yùn)轉(zhuǎn)的效率是非常高的,能與速度和精度與最好的單機(jī)儀器匹敵。前端數(shù)據(jù)采集在很多模型里都能運(yùn)行，包括 VXI 版本

9、 ,如 Agilent E1419A 多功能測量和 VXI 控制模型，還有專有的卡片升降室。雖然前端器成 本已經(jīng)降低 ,但是這些系統(tǒng)可能會非常貴 ,除非你需要提供高的運(yùn)轉(zhuǎn) ,而查找它們 的價格是禁止的。另一方面 ,它們的確能夠提供相當(dāng)多的可撓性和測量能力。好的、成本低的數(shù)據(jù)電子自動記錄儀有合適的通道數(shù) （ 20-60 通道）和掃描 速率相對低但對于多數(shù)工程師的普遍應(yīng)用已足夠。一些關(guān)鍵的應(yīng)用包括 :?產(chǎn)品特征?電子產(chǎn)品的熱靠模切削?環(huán)境的測試環(huán)境的監(jiān)測?組成物特征?電池測試建筑物和計算機(jī)容量監(jiān)測DATA ACQUISITION SYSTEMSData acquisition systems, a

10、s the name implies, are products and/or processes used to collect information to document or analyze some phenomenon. In the simplest form, a technician logging the temperature of an oven on a piece of paper is performing data acquisition. As technology has progressed, this type of process has been

11、simplified and made more accurate, versatile, and reliable through electronic equipment. Equipment ranges from simple recorders to sophisticated computer systems. Data acquisition products serve as a focal point in a system, tying together a wide variety of products, such as sensors that indicate te

12、mperature, flow, level, or pressure. Some common data acquisition terms are shown below.Data acquisition technology has taken giant leaps forward over the last 30 to 40 years. For example, 40 years ago, in a typical college lab, apparatus for tracking the temperature rise in a crucible of sodium tun

13、gsten- bronze consisted of a thermocouple, a bridge, a lookup table, a pad of paper and a pencil.Today' s coellge students are much more likely to use an automated process and analyze the data on a PC Today, numerous options are available for gathering data. The optimal choice depends on several

14、 factors, including the complexity of the task, the speed and accuracy you require, and the documentation you want. Data acquisition systems range from the simple to the complex, with a range of performance and functionality.The old pencil and paper approach is still viable for some situations, and

15、it is inexpensive, readily available, quick and easy to get started. All you need to do is hook up a digital multiple meters (DMM) and begin recording data by hand.Unfortunately, this method is error-prone, tends to be slow and requires extensive manual analysis. In addition, it works only for a sin

16、gle channel of data; while you can use multiple DMMs, the system will quickly becomes bulky and awkward. Accuracy is dependent on the transcribers level of fastidiousness and you may need to scale input manually. For example, if the DMM is not set up to handle temperature sensors, manual scaling wil

17、l be required. Taking these limitations into account, this is often an acceptable method when you need to perform a quick experiment.Modern versions of the venerable strip chart recorder allow you to capture data from several inputs. They provide a permanent paper record of the data, and because thi

18、s data is in graphical format, they allow you to easily spot trends. Once set up, most recorders have sufficient internal intelligence to run unattended without the aid of either an operator or a computer. Drawbacks include a lack of flexibility and relatively low accuracy, which is often constraine

19、d to a few percentage points. You can typically perceive only small changes in the pen plots. While recorders perform well when monitoring a few channels over a long period of time, their value can be limited. For example, they are unable to turn another device on or off. Other concerns include pen

20、and paper maintenance, paper supply and data storage, all of which translate into paper overuse and waste. Still, recorders are fairly easy to set up and operate, and offer a permanent record of the data for quick and simple analysis.Some bench top DMMs offer an optional scanning capability. A slot

21、in the rear of the instrument accepts a scanner card that can multiplex between multiple inputs, with 8 to 10 channels of mux being fairly common. DMM accuracy and the functionality inherent in the instruments front panel are retained. Flexibility is limited in that it is not possible to expand beyo

22、nd the number of channels available in the expansion slot. An external PC usually handles data acquisition and analysis.PC plug-in cards are single-board measurement systems that take advantage of the ISA or PCI-bus expansion slots in a PC. They often have reading rates as high as 100,000 readings p

23、er second. Counts of 8 to 16 channels are common, and acquired data is stored directly into the computer, where it can then be analyzed. Because the card is essentially part of the computer, it is easy to set up tests. PC cards also are relatively inexpensive, in part, because they rely on the host

24、PC to provide power, the mechanical enclosure and the user interface.In the downside, PC plug-in cards often have only 12 bits of resolution, so you can'pterceive small variations with the input signal. Furthermore, the electrical environment inside a PC tends to be noisy, with high-speed clocks

25、 and bus noise radiated throughout. Often, this electrical interference limits the accuracy of the PC plug-in card to that of a handheld DMM .These cards also measure a fairly limited range of dc voltage. To measure other input signals, such as ac voltage, temperature or resistance, you may need som

26、e sort of external signal conditioning. Additional concerns include problematic calibration and overall system cost, especially if you need to purchase additional signal conditioning accessories or a PC to accommodate the cards. Taking that into consideration, PC plug-in cards offer an attractive ap

27、proach to data acquisition if your requirements fall within the capabilities and limitations of the card.Data loggers are typically stand-alone instruments that, once they are setup, can measure, record and display data without operator or computer intervention. They can handle multiple inputs, in s

28、ome instances up to 120 channels. Accuracy rivals that found in standalone bench DMMs, with performance in the 22-bit, 0.004-percent accuracy range. Some data loggers have the ability to scale measurements,check results against user-defined limits, and output signals for control.One advantage of usi

29、ng data loggers is their built-in signal conditioning. Most are able to directly measure a number of different inputs without the need for additional signal conditioning accessories. One channel could be monitoring a thermocouple, another a resistive temperature device (RTD) and still another could

30、be looking at voltage.Thermocouple reference compensation for accurate temperature measurement is typically built into the multiplexer cards. A data logger built-in intelligence helps you set up the test routine and specify the parametersof each channel. Once you have completed the setup, data logge

31、rs can run as standalone devices, much like a recorder. They store data locally in internal memory, which can accommodate 50,000 readings or more.PC connectivity makes it easy to transfer data to your computer for in-depth analysis. Most data loggers are designed for flexibility and simple configura

32、tion and operation, and many provide the option of remote site operation via battery packs or other methods. Depending on the A/D converter technique used, certain data loggers take readings at a relatively slow rate, especially compared to many PC plug-in cards. Still, reading speedsof 250 readings

33、/secondare not uncommon. Keep in mind that many of the phenomena being monitored are physical in nature such as temperature, pressure and flow and change at a fairly slow rate. Additionally, because of a data logger superior measurement accuracy, multiple readings and averaging are not necessary, as they often are in PC plug-in solutions.Data acquisition front ends are often modular and are typically connected to a PC or controller. They are us