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Data Acquisition Fundamentals[/

Thảo luận trong 'Labview' bắt đầu bởi bmnhy, 19 Tháng một 2007.

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    Data Acquisition Fundamentals


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    This document discusses the elements of a PC-based data acquisition system and considerations for users who are selecting components for such a system.

    Table of Contents
    The Personal Computer
    DAQ Hardware
    Sensors and High-Voltage Signals
    Extensions of PC Technology

    Today, most scientists and engineers use personal computers (PCs) with PCI, PXI, CompactPCI, PCMCIA, USB, FireWire, parallel, or serial ports for data acquisition in laboratory research, test and measurement, and industrial automation. Many applications use plug-in devices to acquire data and transfer it directly to computer memory. Others use data acquisition (DAQ) hardware remote from the PC that is coupled via Ethernet, parallel, or serial port. Obtaining proper results from a PC-based DAQ system depends on each of the following system elements (see Figure 1):
    The PC
    DAQ Hardware
    Signal Conditioning

    This document gives an overview of each element and explains the most important criteria of each element. The document also defines much of the terminology common to each element of a PC-based DAQ system.

    Figure 1. The Typical PC-Based DAQ System
    The Personal Computer

    The computer you use for your DAQ system can drastically affect the maximum speeds at which you can continuously acquire data. As computers continuously improve, your DAQ system can take advantage of the computer’s enhanced capabilities, including improved real-time processing, the ability to usdse complex video graphics, and higher streaming-to-disk throughput. Today's technology boasts Pentium IV and PowerPC class processors coupled with the high-performance bus architectures. The PCI bus and USB port are standard equipment on most of today's desktop computers and yield up to 132 Mbytes/s theoretical data transfer capabilities. External and portable PC buses such as PCMCIA, USB, and FireWire offer a flexible alternative to desktop PC-based DAQ systems while achieving up to 40 Mbytes/s transfer rates. For remote or distributed DAQ applications, you can place measurement nodes near sensors and signal sources and use standard networking technology, such as Ethernet, serial, or wireless. When choosing a DAQ device and bus architecture, keep in mind the data transfer methods supported by your chosen device and bus and the transfer rates.
    The data transfer capabilities of your computer can significantly affect the performance of your DAQ system. Twenty years ago, PCs were capable of transferring at rates around 5 MHz, whereas today’s computers can transfer significantly faster. As PC speed continuously increases, DAQ system speed increases as a result.

    Today’s PCs are capable of programmed I/O and interrupt data transfers. Direct memory access (DMA) transfers increase the system throughput by using dedicated hardware to transfer data directly into system memory. Using this method, the processor is not burdened with moving data and is therefore free to engage in more complex processing tasks. With National Instruments driver software, NI-DAQ 7, which serves as the interface between the hardware and the computer, the DMA routines to transfer waveform data across the PC bus were optimized, thus providing the ability to transfer data as fast as possible. To reap the benefits of DMA or interrupt transfers, your DAQ device must be capable of these transfer types. For example, while PCI and FireWire devices offer both DMA and interrupt-based transfers, PCMCIA and USB devices use interrupt-based transfers. Depending on how much processing is needed during data transfer, the rate at which the data is transferred from the DAQ device to PC memory may be affected by the data transfer mechanism.

    The limiting factor for real-time storage of large amounts of data often is the hard drive. Hard drive access time and hard drive fragmentation can significantly reduce the maximum rate at which data can be acquired and streamed to disk. For systems that must acquire high-frequency signals, select a high-speed hard drive for your PC and ensure that there is enough contiguous (unfragmented) free disk space to hold the data. In addition, dedicate a hard drive to the acquisition and run the operating system (OS) on a separate disk when streaming data to disk.

    In the past, applications requiring real-time processing of high-frequency signals needed a high-speed, 32-bit processor with its accompanying coprocessor or a dedicated plug-in processor such as a digital signal processing (DSP) board. However, with today’s processors, you can perform the same real-time analysis without a specialized DSP because they are capable of rates around 2.5 GHz.

    Determine which operating system and computer platform will yield the greatest long-term return on investment while still meeting your short-term goals. Factors that influence your choice may include the experience and needs of both your developers and end users, other uses for the PC (now and in the future), cost constraints, the availability of different computers with respect to your implementation time frame, and software support on that particular operating system. Traditional platforms include Mac OS, which is known for its simple graphical user interface, and Windows 2000 or XP which include native plug and play and power management. Furthermore, real-time operating systems provide reliability and robustness that may appeal to your particular application.

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