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New data capture options.

Though chart recorders remain important, researchers now have various other choices for data recording.

Researchers who need to capture very long data streams can now choose from several technologies. The venerable pen-based chart recorder, once the only available option, has largely given way to digital archiving methods--except in applications where either a permanent record or real-time recording are needed.

Although recordings on magnetic media are theoretically permanent, technical obsolescence of readout equipment can reduce the effective lifetime of the data--just ask anyone with data stored on 8-in. floppy disks.

The options we'll discuss are VHS tape-based recording (which captures very long data streams at low cost), high-speed reel-to-reel tape (which gives high bandwidth and long data-stream records), and thermal array recorders (which provide human-readable output). Of course, for some applications, pen-based chart recorders remain the technology of choice.

Half-inch VHS-type tape cartridges are "among today's highest capacity and most cost-effective mass storage technologies," according to Tom Balue, marketing manager with Metrum Peripheral Products, Littleton, CO. Data transfer rates can reach 4 MB/sec, compared to only 1 MB/sec for optical disks. VHS cartridges also have nearly three times the data-storage capacity (18 GB for ST-160 cartridges) of optical disks as well.

Professional-quality cartridge tape costs only $1.14/megabyte (ST-160), compared to $7/megabyte for the next lowest-cost media (8-mm cartridge videotape) and $96.11/megabyte for 12-inch optical WORM (write once, read many times) disks.

High-quality VHS cartridge tape is so inexpensive because of economies of scale, due to its worldwide acceptance in the quality-conscious broadcast industry.

Landing that data stream. McDonnell Douglas is using an RSR-512 data recorder from Metrum Information Storage, Littleton, CO, to gather data on vibrations experienced by the landing gears in its MD-11 wide-body passenger jets. "We are trying to get enough dynamic data to understand the phenomenon behind the vibration mode," says Ollie Angell, instrumentation engineer at Douglas Aircraft's Commercial Transport Aircraft Div. in Long Beach, CA.

To capture vibration activity, Douglas engineers have installed a host of sensors on the center-line gear and wired their outputs into the data recorder, which is mounted in the plane's center accessory compartment below the cabin floor.

The analog sensor signals actually pass first through signal-conditioning amplifiers. The data recorder then further conditions them, converts them to digital signals, multiplexes them into a serial digital data stream, and records them on high-quality VHS-format magnetic tape at up to 1.28 mega-samples/second.

Pushing the data rate. VHS is a good choice for those who need to capture a lot of data inexpensively. There are applications, however, that generate even more data and push bandwidths well beyond the 6 MHz that videotape is designed to handle.

One of the most demanding applications for data recording is radio astronomy, especially very-long-baseline interferometry (VLBI). Here, high bandwidth combines with data records extending over hours or even days.

At the Massachusetts Institute of Technology's Haystack Observatory in Westford, MA, Hans Hinteregger and Alan Rogers are working on an advanced system that may push linear recording technology beyond that achievable with helical scan techniques, like VHS. They hope the linear recording technology that they're developing will find its way into a new generation of recording equipment that will replace helical scan techniques for cartridge-tape systems.

They start with a commercially made reel-to-reel tape transport system, then soup it up with customized recording and playback heads. Their current equipment uses conventional miniature bulk-ferrite construction. With this technology, they have already pushed data rates into the gigabit/second range.

The Haystack team is looking for funding to develop a new generation of multitrack, high-speed heads. "VLBI operations costs are now dominated by head replacement expenses," says Hinteregger. "To minimize cost, processes and a flexible production line common to high-volume products, like thin-film disc or other format tape heads, should be used."

New life for chart recorders. Capturing data on magnetic media TABULAR DATA OMITTED requires the use of computerized data-analysis equipment to make the information useful. In some applications, a chart record is still best. "For example," says Bill Porter, marketing communications manager for Gould Electronics in Cleveland, "the folks at NASA still collect data on long strip charts. They can find anomalous patterns by eye faster than they could searching with a computer." Strip charts are the best choice for human-readable data archives.

The two options for chart recording are thermal array printers and pen recorders. Putting out charts on a thermal array printer has a number of advantages: annotation can be flexible and automatic, grids can be scaled to engineering units and printed at the same time as the waveforms, and the number and widths of channels can be selected at run time.

Thermal arrays do have their limitations, however: only one color (black) is available, records fade after a time, and there is a delay of a few seconds before the trace appears from under the print head.

That delay can be important in medical situations. "Doctors much prefer pen recorders," says Gould's Porter. "When monitoring drug injections, they have to see the results immediately. Even a few seconds delay can be disastrous."

Advanced thermal array systems like Gould's TA11 and the DMS1000 data management system from Western Graphtec, Irvine, CA, combine a computer having a graphical user interface with either an 11- or 15-in. chart recorder respectively. These units can display the trace in real time on a computer screen while the thermal array prints the chart. The Gould TA11 adds portability to its feature package.
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Author:Masi, C.G.
Publication:R & D
Date:Dec 1, 1993
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