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DAQ Dos And Don'ts

“Technologies such as digital isolators and USB 2.0 make DAQ systems where users don’t have to worry about troubleshooting problems.”

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Today’s manufacturing companies need fast, reliable data in real time in order to achieve operational excellence. The automated systems that collect much of the data are known as data acquisition, or DAQ, systems, The biggest issue in the DAQ environment is to know your entire system, according to Cal Swanson, senior principal engineer with Fenton, Mo.-based vendor Single Iteration (www.singleiteration.com), a division of Watlow Electric Manufacturing Co. (www.watlow.com), in St. Louis.

Signal level is important, Swanson says. “It has to be compatible with the levels the DAQ device can handle. Because most DAQ these days is done digitally, there is no longer the forgiveness of saturating your system.” By that, he means the digital system does not manage over-ranged signals very well—that is, those that exceed the instrument’s range. For

example, the signal may be 110 percent of the maximum-rated signal strength. Rather than recording that value, the digital systems tend to clip or trim off the top of such signals and then fold that excess back into the original signal, he explains. That causes an over-signal, which is corruption that gives incorrect readings.

Other potential corruption arises from including improper grounding and shielding, as well as improper device isolation of the DAQ measurement system, Swanson says. “Grounding is an issue where you may be injecting (unwanted) signals into your system, due to the way you have the intermediate electronics connected to one another.” Shielding means protecting wires against becoming antennae and allowing unwanted signals to enter the system, he says.

Clean up power

Yet another issue is the power supply cleanliness. “Most DAQs are not battery-driven. And most are not really designed to measure ultra-small signals,” Swanson observes. For example, if an end-user tries to measure a signal in the microvolt range and has a power supply with an alternating-current signal in the 10s or 100s of millivolts, then low-frequency signals could be overwhelmed and not seen, he explains. The solution is simple, though. “If you don’t have clean power, then you should have a dedicated power supply,” Swanson says. He also notes that special power conditioners can be applied to regular power supplies.

Agreeing with Swanson’s rundown of DAQ environment issues and solutions is Brian Betts, group manager of data acquisition for test and automation systems vendor National Instruments Corp. (www.ni.com), Austin, Texas. Betts adds that one giant step currently being made for ease-of-use of DAQ systems involves the use of Universal Serial Bus (USB) 2.0. “With USB 2.0, there’s forty times the bandwidth. That’s important, because we have a big enough data pipe to handle most of DAQ requirements we see.”

The fast USB adoption trend also plays into the bigger industrial trend of portable computing for DAQ, Betts believes. One example involves machine monitoring. While it may be cost-prohibitive to have a DAQ device on each machine, says Betts, USB 2.0-equipped laptops and portable DAQ devices can be used on the factory floor to collect data from every machine, providing a completely portable, and cost-effective DAQ system.

USB 2.0 is gaining popularity with end-users, he adds, noting an online survey NI did earlier this year, in which 30 percent of respondents indicated that the next bus they’d chose for their DAQ systems would be USB 2.0. The next two closest choices were wireless and Ethernet, respectively, he says.

“Technologies such as digital isolators and USB 2.0 make DAQ systems where users don’t have to worry about troubleshooting problems,” declares Betts.

C. Kenna Amos, ckamosjr@earthlink.net, is an Automation World Contributing Editor.

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