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1、个人资料整理仅限学习使用1 / 12 Selecting the Right Data Acquisition System Engineers often must monitor a handful of signals over extended periods of time, and then graph and analyze the resulting data. The need to monitor, record and analyze data arises in a wide range of applications, including the design-ver
2、ification stage of product development, environmental chamber monitoring, component inspection, benchtop testing and process trouble-shooting. This application note describes the various methods and devices you can use to acquire, record and analyze data, from the simple pen-and-paper method to usin
3、g todays sophisticated data acquisition systems. It discusses the advantages and disadvantages of each method and provides a list of questions that will guide you in selecting the approach that best suits your needs. Introduction In geotechnical engineering, we sometime encounter some difficulties s
4、uch as monitoring instruments distributed in a large area, dangerous environment of working site that cause some difficulty for easy access. In this case, operators may adopt remote control, by which a large amount of measured data will be transmitted to a observation room where the data are to be c
5、ollected, stored and processed. The automatic data acquisition control system is able to complete the tasks as regular automatic data monitoring, acquisition and store, featuring high automation, large data store capacity and reliable performance. The system is composed of acquisition control system
6、 and display system, with the following features: 1. No. of Channels: 32 ( can be increased or decreased according to users real needs. 2. Scanning duration: decided by user, fastest 32 points/second 3. Store capacity: 20G( may be increased or decreased 4. Display: (a Table of parameter (b History t
7、endency (c Column graphics. 5. Function: real time monitoring control, warning 6. Overall dimension: 50cm 50cm 72cm Data acquisition systems, as the name implies, are products and/or processes used to 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 1 页，共 12 页个人资料整理仅限学习使用2 / 12 collect information to
8、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 simplified and made more accurate, versatile, and reliable through electronic equ
9、ipment. 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 temperature, flow, level, or pressure. Some common data acquistion terms are shown
10、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 sodiumtungsten- bronze consisted of a thermocouple, a bridge, a lookup table, a pad of pape
11、r and a pencil. Todays college 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 factors, including the complexity of the task, the speed and accuracy you require, and
12、 the documentation you want. Data acquisition systems range from the simple to the complex, with a range of performance and functionality. Pencil and paperThe old pencil and paper approach is still viable for some situations, and it is inexpensive, readily available, quick and easy to get started. A
13、ll you need to do is hook up a digital multimeter (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 single channel of data 。 while you can use multiple DMMs, the system will quic
14、kly 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 will be required. Taking these limitations into account, this is often an acc
15、eptablemethod when you need to perform a quick experiment. 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 2 页，共 12 页个人资料整理仅限学习使用3 / 12 Strip chart recorderModern versions of the venerable strip chart recorder allow you to capture data from several inputs. They provide a permanent paper record of the
16、 data, and because this 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, whi
17、ch is often constrained 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
18、 concerns include pen 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. Scanning digital multimeterSome benchtop DM
19、Ms offer an optional scanning capability. A slot 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
20、limited in that it is not possible to expand beyond the number of channels available in the expansion slot. An external PC usually handles data acquisition and analysis. PC plug-in cardsPC plug-in cards are single-board measurement systems that take advantage of the ISA or PCI-bus expansion slots in
21、 a PC. They often have reading rates as high as 100,000 readings per 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 a
22、re relatively inexpensive, in part, because they rely on the host PC to provide power, the mechanical enclosure and the user interface. Data acquisition optionsIn the downside, PC plug-in cards often have only 12 bits 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 3 页，共 12 页个人资料整理仅限学习使用4 / 12 of res
23、olution, so you cant perceive small variations with the input signal. Furthermore, the electrical environment inside a PC tends to be noisy, with high-speed clocks and bus noise radiated throughout. Often, this electrical interference limits the accuracy of the PC plug-in card to that of a handheld
24、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 some sort of external signal conditioning. Additional concerns include problematic calibration and overall system cost, especially if you ne
25、ed to purchase additional signal conditioning accessories or a PC to accommodate the cards. Taking that into consideration, PC plug-in cards offer an attractive approach to data acquisition if your requirements fall within the capabilities and limitations of the card. Data loggersData loggers are ty
26、pically 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 some instances up to 120 channels. Accuracy rivals that found in standalone bench DMMs, with performance in the 22-bit, 0.004
27、-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 using data loggers is their built-in signal conditioning. Most are able to directly measure a number of different inputs with
28、out the need for additional signal conditioning accessories. One channel could be monitoring a thermocouple, another a resistive temperature device (RTD and still another could be looking at voltage. Thermocouple reference compensation for accurate temperature measurement is typically built into the
29、 multiplexer cards. A data loggers built-in intelligence helps you set up the test routine and specify the parameters of each channel. Once you have completed the setup, data loggers can run as standalone devices, much like a recorder. They store data locally in internal memory, which can accommodat
30、e 50,000 readings or more. PC connectivity makes it easy to transfer data to your computer for in-depth 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 4 页，共 12 页个人资料整理仅限学习使用5 / 12 analysis. Most data loggers are designed for flexibility and simple configuration and operation, and many provide the op
31、tion 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 speeds of 250 readings/second are not uncommon. Keep in mind tha
32、t 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 loggers superior measurement accuracy, multiple readings and averaging are not necessary, as they often are in PC plug-in solutions.
33、 Data acquisition front endsData acquisition front ends are often modular and are typically connected to a PC or controller. They are used in automated test applications for gathering data and for controlling and routing signals in other parts of the test setup. Front end performance can be very hig
34、h, with speed and accuracy rivaling the best standalone instruments. Data acquisition front ends are implemented in a number of formats, including VXI versions, such as the Agilent E1419A multifunction measurement and control VXI module, and proprietary card cages. Although front-end cost has been d
35、ecreasing, these systems can be fairly expensive, and unless you require the high performance they provide, you may find their price to be prohibitive. On the plus side, they do offer considerable flexibility and measurement capability. Data Logger ApplicationsA good, low-cost data logger with moder
36、ate channel count (20 - 60 channels and a relatively slow scan rate is more than sufficient for many of the applications engineers commonly face. Some key applications include: ? Product characterization ? Thermal profiling of electronic products? Environmental testing。environmental monitoring ? Com
37、ponent characterization? Battery testing? Building and computer room monitoring? Process monitoring, evaluation and troubleshooting No single data acquisition 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 5 页，共 12 页个人资料整理仅限学习使用6 / 12 system works for all applications. Answering the following questi
38、ons may help you decide which will best meet your needs: 1. Does the system match my application? What is the measurement resolution, accuracy and noise performance? How fast does it scan? What transducers and measurement functions are supported? Is it upgradeable or expandable to meet future needs?
39、 How portable is it? Can it operate as a standalone instrument? 2. How much does it cost? Is software included, or is it extra? Does it require signal conditioning add-ons? What is the warranty period? How easy and inexpensive is it to calibrate? 3. How easy is it to use? Can the specifications be u
40、nderstood? What is the user interface like? How difficult is it to reconfigure for new applications? Can data be transferred easily to new applications? Which application packages are supported? ConclusionData acquisition can range from pencil, paper and a measuring device, to a highly sophisticated
41、 system of hardware instrumentation and software analysis tools. The first step for users contemplating the purchase of a data acquisition device or system is to determine the tasks at hand and the desired output, and then select the type and scope of equipment that meets their criteria. All of the
42、sophisticated equipment and analysis tools that are available are designed to help users understand the phenomena they are monitoring. The tools are merely a means to an end. 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 6 页，共 12 页个人资料整理仅限学习使用7 / 12 正确选择数据采集系统工程师经常要对很长时间内的很多信号进行监测、画图和分析产生的数据。监测、记录和
43、分析数据在大范围内的应用趋势上升，包括设计- 检定产品发展状态、环境的监测、组成物的检验、benchtop 测试和处理故障检查。从简单的纸笔记录方法到现在所使用的复杂的数据采集系统这些应用笔记描述了你能获得、记录和分析数据的很多种不同方法和装置。讨论每一种方法的缺点和优点，然后列出一张能够指引你选择适合你实际需要的问题单。前边在工程上，我们有时遇到一些困难，诸如监控设备分散在大范围内，危险的工作环境很容易引发一些困难。在这种情况下，操作者可能采用视讯遥控，通过大量检测数据传输到检测室，在那里收集、存储和处理数据。自动数据采集的控制系统能够完成很多任务，像自动定时数据监测，数据采集和存储，高度自
44、动化为特色，大量数据存储能力和可靠功能。自动数据采集的控制系统和显示系统优如下特点：1 多通 路 数 ： 32 根 据 使用 者 的 需 要增 长 或 减 少 ）2 扫 描 持 续 时 间 ： 由 使 用 者 决 定 ， 最 快 一 秒 种 扫 描32点3 存储容量：20G可能增加或减少）4显 示 ： a ）参 数 表b ） 历史趋 势 c ）专 栏 图 表5 功能：实时监测、控制和报警6 大体尺寸：50cm 50cm 72cm数据采集系统，正如名字所暗示的，是一种用来采集信息成文件或分析一些现象的产品或过程。在最简单的形式中，技术人员将烤箱的温度记录在一张纸上就是数据采集。随着技术的发展，通
45、过电子设备，这个过程已经得到简化和变得比较精确、多用途和可靠。设备从简单的存储器发展到复杂的电脑系统。数据采集产品像聚焦点一样为系统服务，和一系列产品一起，诸如传感器显示温度、水流、程度或者过程。数据采集技术在过去30 到 40 年以来已经取得了很大的飞跃。举例来说, 在 40 年以前 , 在一个著名的学院实验室中, 为追踪用青铜做的坩埚中的温度上升情况的装置是由热电偶、继电器、查询台、一捆纸和一支铅笔。今天的大学学生很可能在PC 机上自动处理和分析数精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 7 页，共 12 页个人资料整理仅限学习使用8 /
46、 12 据，有很多种可供你选择的方法去采集数据。至于选择哪一种方法取决于多种因素，包括任务的复杂度、你所需要的速度和精度、你想要的证据资料等等。无论是简单的还是复杂的，数据采集系统都能够运行并发挥它的作用。铅笔和纸用铅笔和纸的旧方式对于一些情形仍然是可行的，而且它便宜、易获得、快速和容易开始。而你所需要的就是捕捉到多路数字信息DMM），然后开始用手记录数据。不幸的是这种方法容易发生错误、采集数据变慢和需要太多的人工分析。此外，它只能单通道采集数据；但是当你使用多通道DMM时，系统将很快变得非常庞大和呆笨拙。精度取决于誊写器的水平，并且你可能需要自己动手依比例输入。举例来说, 如果 DMM 没有
47、配备处理温度的传感器,旧需要动手找比例。考虑到这些限制，只有当你需要实行一个快速实验时，它才是一个可接受的方法。长条图表记录仪现代多种版本的长条图表记录仪允许你从多个输入取得数据。他们提供数据的长备纸记录, 因为数据是图解的格式, 他们易于现场采集数据。一旦建立了长条图表记录仪，在没有操作员或计算机的情况下，大多数记录仪具有足够的内部智能运行。缺点是缺乏灵活性和相对的精度低，时常限制在百分点。你能很清楚地感觉到与笔只有小的改变。在多通道内较长时间的监控，记录仪能发挥很好的作用，除此之外，它们的价值得到限制。举例来说，他们不能够与另外的装置轮流作用。其他的顾虑就是笔和纸的维护 , 纸的供给和数据
48、的存储, 最重要的是纸的滥用和浪费。然而，记录仪相当容易建立和操作，为数据快速而简单的分析提供永久的记录。数字扫描传多用表一些 benchtop DMMs 提供可选择的扫描能力。仪器的背面有一个槽孔接收一张在较多输入时能多重发讯的扫描仪卡片，通常是8 到 10 通道的 mux 。固有的在仪器的前面嵌板中的受到限制。它的柔韧性也受到限制，因为它不能超过可用通道数。外部的PC机通常处理数据采集和分析。PC机插件卡片PC机插件卡片是单板测量系统，它利用ISA 或 PCI 总线在 PC机内扩大插槽。它们时常具有高达每秒1000 的阅读速率。 8 到 16 通道是普遍的，采集的数据直接存储在电脑里，然后
49、进行分析。因为卡片本质上是计算机的一部分, 建立测试是容易的。 PC机卡也相对的便宜 , 一部分地，因为他们以来主机PC去提精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 8 页，共 12 页个人资料整理仅限学习使用9 / 12 供能源、机械附件和使用界面。数据采集的选择在缺点上， PC机插件卡片时常只有12 字的容量 , 因此你不能察觉输入信号的小变化。此外,PC 机内的电子环境经常很容易发出噪声、产生高速率的时钟和总线噪声，电子接触面限制 PC机插件卡片的精度。这些插件卡片也测量一定范围的电压。为了测量其他输入信号，如电压、温度和阻力，你也许需
50、要一些外部信号监测的器件。其它关心包括复杂的校正和全部的系统成本, 尤其如果你需要购买额外信号监测器件或用 PC机适应插件卡片。把这些考虑进去，如果你的需要在卡片的能力和限制范围内变动， PC机插件卡片给数据采集提供吸引人的方法。数据电子自动记录仪数据电子自动记录仪是典型的单机仪器，一旦配备它们, 就能测量、记录和显示数据而不需要操作员或计算机参与。它们能够处理多信号输入，有时可达120 通道。精度可与无与伦比的台式 DMMs 匹敌 , 由于它在22 字、0.004 个百分率的精度范围内运转。一些数据电子自动记录仪有能力按比例测量, 检查结果不受使用者定义的限制, 而且输出为控制作信号。使用数