By Wilson Dizard III
WASHINGTON - Navy tests have shown that a selection of commercial off-the-shelf (COTS) VME single-board computers - even some convection-cooled COTS boards - are capable of withstanding the environmental rigors of helicopter operations at sea.
Yet these same tests, which experts at the U.S. Naval Surface Warfare Center (NSWC)-Crane Division conducted from October 1995 to November 1996, suggest that ruggedized COTS VME board enclosures need substantial improvements before they will be appropriate for helicopters.
The tests at NSWC-Crane in Crane, Ind., indicate that COTS computer enclosures intended to house digital signal processors (DSPs) would fail when exposed to the punishment they would receive when installed in an antisubmarine helicopter.
NSWC-Crane experts concluded that the best means to produce DSPs to meet the Navy`s current shortage of the specialized computers is to use COTS VME boards installed inside a special enclosure that the Navy is developing on a proof-of-concept basis. The resulting unit is intended to serve as a replacement for the Navy`s AN/UYS-2A Airborne Low Frequency Sonar, better known as ALFS.
Four manufacturers tested
The boards and enclosures that were subjected to the military testing were from four manufacturers. Navy officials, for liability reasons, are seeking in test results to separate the vendors from the products they tested.
The manufacturers, in fact, would not have agreed to participate in the testing if they knew their names would be released, says John Homme, project manager for the effort at NSWC-Crane. Homme, himself, would not match the manufacturers with the products his team tested.
Military & Aerospace Electronics, with help from a knowledgeable source at NSWC-Crane, has linked the products with the participating companies.
The vendors that participated in the tests are Matrix Corp. of Raleigh, N.C.; Power Systems Group Ltd. of Irvine, Calif.; American Rugged Enclosures Inc. of Cincinnati; and DY 4 Systems Inc. of Kanata, Ontario.
Matrix officials provided their ENC-55 convection enclosure with heat exchanger, and two boards: the MR-CPU331-N-2 68030 convection-cooled, 6U VME board, equipped for extended temperature and ruggedized; and the MX-CPU331-N-2 68030 convection cooled, 6U VME board, equipped not only for extended temperature operation, but also conformally coated.
Power Systems provided a Number 1 ATR Short conduction enclosure, and a Number 1.5 ATR Tall Long convection enclosure.
American Rugged provided an Omega convection enclosure.
DY 4 supplied a conduction-cooled 6U VME board 68030, conformally coated and ruggedized; and a conduction-cooled 6U VME board 68030 equipped for extended temperature.
"[The] products were chosen to represent a range of robustness from low-end convection cooled to high-end conduction cooled," according to the NSWC-Crane study entitled "COTS Enclosures Environmental Characterization Test Report," Navy experts also selected the units on the basis of product availability and the vendors` willingness to participate in the tests.
Tests mimicked helicopter conditions according to military electronic standards for shock, vibration, heat removal, operating temperature, and altitude extremes, humidity, electromagnetic interference, and airborne noise
Homme says the activity came at the behest of officials at the Naval Sea Systems Command PMS-428 program office which supports Navy sonar DSP programs, including AN/UYS-1, AN/UYS-2, AN/UYS 2A, and others that form components of various Navy sonar systems including AN/BSY-2, AN/SQQ-89, and P-3-Upgrade 3.
"We want to COTSify our signal processors," Homme says. "We were tasked to look at VME enclosures and card sets."
VME bus computers have been a mainstay of military and aerospace signal processor design for more than a decade, due to their design flexibility, reliability, and the wide range of options available.
"Really what we were looking for is if we can meet the environmental requirements for a rotary wing aircraft," Homme explains. The tests were designed to determine whether the equipment could stand up to operational conditions on a Lockheed Martin/Sikorsky SH-60 Seahawk LAMPS carrier-based antisubmarine helicopter.
"We have a signal processor currently targeted for that helicopter. We would like to replace that with a form, fit, and function [replacement] commercial DSP," Homme says. "We did a market survey to see if the needed conduction and convection boards, conformally coated, and extended temperature devices were available. We also wanted to know if the boards needed hand stiffening to survive vibration and shock."
Homme says that his team at Crane "didn`t have a good feeling for what kind of cooling techniques would survive these environments. We ran these enclosures through a series of helicopter requirements and issued a test report."
Boards pass tests
The Matrix Corp. and DY 4 6U VME boards passed all the tests that the Crane Division team used, including some in which the enclosures containing the boards failed catastrophically.
To the surprise of some, the NSWC-Crane test results fly in the face of conventional wisdom that only conduction-cooled mil-spec boards will suffice for demanding applications such as helicopters.
"We believe we can use convection-cooled modules with extended-temperature-screened devices," Homme says. Navy investigators determined that some convection-cooled boards would be able to function in temperatures ranging from -40 to 55 degrees Celsius.
Convection-cooled systems typically remove heat with forced air blown over the entire board, while conduction-cooled systems more often operate in closed boxes that conduct heat away from the center of the boards and toward the sides, where air or circulating liquid removes the heat.
Homme`s team determined that the convection-cooled boards from Matrix meet their stringent environmental requirements for antisubmarine helicopters. Homme points out that convection-cooled boards are cheaper than conduction-cooled modules. "There`s more of a market for convection-cooled boards," he says. "The conduction-cooled units are more of a niche market."
The tests for the two Matrix convection-cooled boards included eight series of vibration tests, three temperature/altitude tests, one humidity test, and six radiated susceptibility tests. "Therefore, it was concluded that the extended-temperature 6U VME modules are acceptable for use in a rotary wing aircraft environment," according to the report.
Similarly, the DY4 conduction-cooled board passed all the tests as well, "therefore, it was concluded that the extended-temperature 6U VME conduction modules are acceptable" for use in helicopters, the report states.
The COTS VME boards from Matrix and DY 4 did not require special stiffening to pass the tests.
Enclosures have problems
The enclosures involved in the test, however, did not fare so well as the boards. "They had numerous failures, including in the vibration isolation subassemblies, and in the power supplies and related subassemblies," the report states.
Homme`s group reached several conclusions about convection-cooled enclosures provided by Matrix Corp., Power Systems Group, and American Rugged:
- the power train assemblies require wider temperature operating ranges, enhanced electromagnetic interference (EMI) power line conducted emissions suppression, and more vibration resistance;
- the enclosures` vibration damping systems are inadequate and need to be reassessed;
- the enclosures require improved shielding to attenuate emissions of electro-magnetic interference;
- the minor corrosion observed during humidity testing did not affect electrical performance and was not considered an area of concern;
- thermal responses were adequate on two of the three enclosures - Homme declined to specify which two - yet adequate air flow should be verified on any enclosure selected; and
- the various deficiencies of the convection-cooled enclosures could be mitigated by careful selection of subassemblies and additional engineering tailored for helicopter conditions.
The NSWC researchers found the conduction-cooled enclosure from Power Systems generally performed better than the convection cooled enclosures. However, its power train assemblies require enhanced power line-conducted emissions suppression; the enclosure required improved shielding to attenuate EMI radiated emissions; humidity testing caused minor corrosion that was not a big problem; and the thermal performance of the enclosure was marginal. The conduction-cooled enclosure`s cold wall reached temperatures about seven to 12 degrees Celsius above the maximum 85 C operating temperature of 6U VME conduction modules, when operating in 55 C.
The equipment was subjected to humidity testing, with the power off, for a period of 10 days. The gear then was powered on and monitored for 15 minutes. According to the test report, substitution of mounting hardware parts with readily available corrosion-resistant parts could eliminate the minor problems. Some battery leads, light-emitting diode leads, switches, hard drives, and other parts showed minor corrosion indications.
"Operating for short periods of time marginally above 85 degrees C would probably be acceptable," the report states. "The reliability reduction would need to be determined by looking at the amount of time the module was operated above the recommended temperature."
The Crane Division team found that careful selection of subassemblies used in a conduction-cooled enclosure could ease the problems, if it were combined with additional engineering needed to prepare the system for conditions experienced in a helicopter.
The completion of testing brought an end to the first phase of the project. In the second phase, NSWC officials will work with Systems Design and Analysis Inc. of Indianapolis to design an enclosure which is a form, fit, and function replacement for the existing signal processor mounted on the SH-60 helicopter.
"We have formed an integrated process team and commenced experiments," Homme says. "We are in the design stage now and expect to begin bending metal in October." The resulting DSP, which will use a 6U module, will be a proof of concept unit.
"The important thing is an underlying understanding of the level of robustness of those cards in a harsh environment," Homme says.
The Crane Division Group used the following tests to evaluate the boards and enclosures:
- shock: MIL-STD-810E;
- vibration: MIL-STD-810E;
- humidity: MIL-STD-5400;
- temperature/altitude: MIL-STD-5400:
- airborne noise: HDWALF0310;
- thermal design: MIL-STD-2036; and
- electromagnetic environmental effects: a subset of MIL-STD-461C/D.
During the tests, the equipment was first subjected to low levels of stress that were gradually increased to military specification levels.
To obtain detailed data sets describing the tests, contact Homme by phone at 812-854-2212. The Navy itself will not release the product-specific detailed test results to anyone except the vendors who participated in the tests.
