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Issue Date: April 2005, Posted On: 3/29/2005
Digital data recorders deliver:
This month's Technically Speaking column compares mechanical, analog and digital recorders for data acquisition and concludes that local data logging can be the easiest way to satisfy regulatory requirements.
By Dan Hebert, Senior Technical Editor
To clearly illustrate the advantages of digital data recorders, it helps to revisit problems associated with analog chart recorders. “Mechanical chart recorders need constant manual calibration and have moving parts which introduce errors due to friction and repetitive strain,” says Gricha Raether, the industrial control and distributed I/O product manager at National Instruments.
“Paper data can fade with time, making readings inaccurate. Data storage is cumbersome because it involves storing thousands of sheets of paper in folders and warehouses. Data retrieval means going through all those sheets to find the correct time segment,” concludes Raether.
Others second Raether’s opinions. “Digital recorders have reduced maintenance costs because of the elimination of mechanical parts such as chart drives and pen mechanisms,” according to Douglas Bradway, the recorder product manager for Honeywell International. “The elimination of consumables such as the charts, pens and ink wheels provides additional savings,” adds Bradway.
Analog chart recorders are inferior in other areas. “Chart recorders are limited to the process they are directly connected to, offer little in the way of higher level analysis, and cannot address ad-hoc analysis,” says Roy Kok, the HMI/SCADA manager at GE Fanuc Automation.
Problems with and limitations of analog chart recorders are clear, and digital recorders address and correct these issues.
The first step in the process of data recording is data acquisition, usually accomplished at the I/O level. Two innovations are of note at this level. The first is the ability to digitally record and store data locally. The second is the ability to transmit this recorded data in industry standard file formats via digital communication links.
Recording data locally is simple, reliable and tamper proof. “We have implemented an on-board data logging engine inside our net concentrator I/O system,” reports Scott Saunders, the director of strategic marketing for Moore Industries. “Our net concentrator system allows the user to pick and choose which I/O points are recorded locally, and to specify when to turn the data logger on and off.”
Local data logging can be the easiest way to satisfy regulatory requirements because data is stored before transmission and before human interface or intervention.
Many of these local data recorders also provide data processing. “Our data monitors record process data for later retrieval,” says Charlie Strack, marketing manager for STI Automation Products Group. “Ethernet or a cell phone modem can be used to transfer the data to a host device. This technology offers high resolution, 24-bit data input channels with digital software-based signal processing capability,” adds Strack.
Most local data storage devices comply with open standards such as OPC, XML, SOAP and .NET. Stored data can be transmitted via hard-wired or wireless means.
Once data is collected at the I/O level, many products are available for presentation, storage, and analysis. These products fall into three main areas: PC-based software packages, web-based software packages, and stand-alone digital recorders.
Stand-alone digital recorders are simple to use and work well for operators used to traditional chart recorders. “Our paperless recorders incorporate graphic displays to show trends, bar graphs, digital values or combinations of these basic displays. They can also provide customized displays, store electronic data for local recall, or enable data acquisition over a network,” says Bradway of Honeywell.
Web-based data recorders serve data to the web for access via web browsers. “Our DataNet OPC software is a Web-based presentation and logging software solution,” reports William Glover , the product development manager at BizWareDirect. “DataNet uses OPC technology to display real-time data from industrial devices on a live Web page. Data are then available for viewing and printing reports from any computer connected to the Internet or a company intranet,” concludes Glover.
PC-based data recorders are part and parcel of most Windows-based HMI software. “Information once recorded on paper chart recorders is now primarily gathered, presented and archived electronically via PC-based solutions,” says Steve Ludwig, the manager of commercial programs in the automation control and information Group at Rockwell Automation.
“Depending on the informational requirement, plant floor data may be presented and archived by HMI software for operator use, middleware systems for management use, or moved into business systems for tracking and archiving,” adds Ludwig.
Lower cost data recording can be implemented via CE-based devices. “With Historian Collector functionality added to a CE-based operator interface, a user can record data for a fraction of the cost of a chart recorder; under $800,” according to Paul Daugherty, the manager of operator interfaces at GE Fanuc Automation.
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