What DAQ System Should We Use?

Readers help a reader solve this control problem. Next month: How can we measure load profiles?

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...Mid-priced systems offer more flexibility, input channels, accuracy and programming flexibility. Accuracy will be 12-18 bit with available faster speeds in the sub-second area. This is the first area where flexibility is a product feature. Expect to be able to configure input channels for different signal types and levels. Additional features include communication options of modem, RF, cellular and Ethernet are possible. While mid-priced units offer more means for communication they are still standalone instruments and capable of conditional and intelligent logging scenarios. Alarm functionality is greatly expanded as well.

...High-priced units will offer faster sample rates, slightly higher accuracy (18-plus bit), and higher channel count. These units may also be more ruggedized for a specific application, for example, automotive or extreme temperatures. The new breed of data loggers built into multimeters fall into this category as well. Increased flexibility comes from the addition of the data logger to a multimeter, making a good digital multimeter and a limited-functionality data logger. Memory, programmability, and flexibility are the limiting factors of this segment within the low cost range.

...Computer-hosted data acquisition instruments are used in combination with data acquisition cards or data collection nodes and specialized software. The simpler system will consist of a data acquisition card inserted into a PC. The card will be selected for the required types and number of inputs that are connected to the PC card. Application software is loaded and run on the computer, allowing the operator to create programs controlling data collection. Additional data acquisition cards may be added for increased channel count and functionality.

...Another computer-hosted system makes use of computer communication protocols. Signal conditioning nodes can be placed on an Ethernet, RS-232, USB, or RS-485 bus into which the signals are fed and conditioned. The data may remain in memory in the remote signal-conditioning node for the host computer to retrieve later at a scheduled time, or it can be sent in real time to the host PC for storage. Sample rates will determine the best method for collection.

...These systems are found in the lab or on a plant floor and are typically faster than portable instruments due to the processing power available from the PC. That also means they are more potentially more fragile, susceptible to environmental conditions and data backbone issues, and power-hungry. Systems that rely on laptop-based units will also find battery life a constraint.

...Chart recorders can be divided into two segments. The very common circular chart recorder uses pen on paper to record information. These are slowly being replaced by electronic methods although there is still a continued legacy requirement. The more current method is videographic, which uses a video display for presentation of the data, typically in a horizontal manner instead of circular. Data is now stored electronically in memory, on a disc, or in a PCMCIA card, and either reviewed/analyzed on the display or downloaded into a computer.

...These systems vary widely in capabilities but have in common a graphic display. Displays can vary from 3x1.5 in. monochromatic to 10-in. circular color displays designed to mimic the paper chart recorders they are trying to replace. Similar to portable systems, chart recorders are available in many sizes with a varying number of inputs and acquisition speeds. The more you spend the faster you can go, or the more channels you can monitor. Application for these products is typically a permanent installation where real-time or near real-time display is important at the location. These units are not battery-operated due to the demand for power from the display.

...Supervisory control and data acquisition (SCADA) systems consist of three important components: the central host, usually called a master station or master terminal unit (MTU); one or more remote devices called remote stations or remote terminal units (RTUs) that gather field data; and software, either a standard or custom program, designed to monitor and control remotely located field data elements. SCADA systems are typically custom designed for a specific facility and the functionality of that facility. Installation and programming are a major component of a SCADA system. These systems are designed to monitor and facilitate the operation of a, typically, very large system. They are discussed here as a possible alternative method for collecting data. They are not portable and generally very difficult to modify for simple tests, so once a system is up and running in a manufacturing facility there is reluctance to modify it.

...Finally, there are exotic systems: custom systems made specifically for a unique application. From the ultra-fast system to a worldwide distributed system, these applications are not solved by off-the-shelf hardware and software. Expensive and complex, they are designed to do a single job.

Mark Albert, Sales & Marketing Mgr.

Logic Beach

PC-Based Systems High on Flexibility

Programmable digital multimeters are inherently different than recorders and PC DAQ systems. DMMs usually get better resolution than the alternatives, but have limited acquisition rates and include only one input channel. Expanding your channel count requires front-end switching, which further limits your acquisition rates. For low-speed measurements in limited channel-count systems, DMMs are a good fit. To measure more than a couple of channels or to achieve sampling rates higher than 1,000 samples per second requires a recorder or PC DAQ system.

...Even though built-in processing and data storage have made recorders more powerful, it is still intrinsically less expensive to leverage the state-of-the-art processors and hard drives available in today's PCs. To accomplish this, some recorders support high-speed data transfer across a USB or Ethernet bus. The limitation of recorders now is not speed, but functionality. The fact that recorders have signal conditioning, digitizers, processors, memory chips, and displays all in a single box usually means that they are either too much or too little for any one application. For example, expanding the channel count or adding a measurement type not supported by the recorder requires adding an additional system. On the other hand, if you are using the PC to analyze and display your data, the built-in processors and displays on a recorder are overkill.

...The most versatile and cost-effective way to acquire and log data is to use a PC-based data acquisition system. Plug-in DAQ vendors such as National Instruments offer boards starting at $395, with models capable of sampling up to 800 kS/s at 16-bit resolution. The true benefit of the PC-based DAQ system is the flexibility that it provides. You can configure each piece of your system, from the speed and disk space of the PC to the signal conditioning for your specific measurements. This unique capability allows you to select the functionality you need to exactly meet your application requirements.

...Another often overlooked benefit of PC-based systems is the ease of expanding or upgrading the system. To expand a DMM or recorder requires buying an additional instrument (duplicating your system). Modular signal conditioning, such as our SCXI, allows you to increase your channel count or add a measuremen

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