TECHNOLOGY IN SYSTEMS
Computers for Harsh Environments
Ruggedizing Commercial Products to Withstand the Most Demanding Environments
Meeting the challenges for ruggedizing systems designed for the commercial market can enable them to bring the latest technology advances into the harshest environments where they can operate reliably.
JARED FRANCOM AND DAVID KUMMER, PARVUS
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Rugged box-level systems are a vital component for a variety of demanding conditions on land, sea and air. However, how these rugged systems are created can be as different as the applications in which they are deployed. One such distinction among these systems is “rugged” versus “ruggedized.” The term “rugged” refers to systems that were created from the ground up to meet the requirements of specific harsh environments. Conversely, the term “ruggedized” refers to a commercial product that was not originally intended for harsh-use application, but was enhanced to endure airborne, ground vehicle and/or shipboard deployments.
While both “rugged” and “ruggedized” systems have their unique offerings, ruggedized systems are growing in demand as this solution provides a cost-effective method for implementing the latest computing technology that meets stringent environmental standards. Additionally, ruggedized systems benefit from the advances made by OEMs and can mean a faster time-to-deployment than their rugged counterparts.
Commercial networking products, such as those offered by Cisco, have proven to be an attractive platform for ruggedized products. Additionally, Cisco is credited with helping to define many of today’s networking standards and protocols, actively contributing to the standards committees within the Internet Task Force, IEEE, and other groups. As a consequence, with the widespread adoption of Cisco products across many industries, combined with its comprehensive feature set, Cisco-based rugged computing solutions are increasingly being deployed in a variety of applications on board land and air vehicles (Figure 1). Two case studies examine how Parvus engineers ruggedized two Cisco networking products: the IE-3000 and the 4948E.
Rugged conditions encountered by military vehicles such as this Humvee require ruggedized computing systems specifically engineered to withstand harsh conditions.
Cisco Switches: Candidates for Ruggedization
One of Cisco’s latest Ethernet switches, the IE-3000, recently proved to be a suitable ruggedization candidate for use in harsh environments. This switch was designed for industrial Ethernet applications, including factory automation, energy and process control and intelligent transportation systems (ITSs). Its intended commercial use already exceeded traditional commercial environment, since the IE-3000 included extended temperature and enhanced shock, vibration and surge ratings not typically offered by commercial networking gear. Later named the DuraNET 3000, the ruggedized IE-3000 delivers the security, advanced Quality of Service (QoS) and manageability that customers expect from Cisco IOS-based switching technology, but it is designed with mechanical enhancements to support deployment of data, video and voice services in extreme environments (Figure 2).
The DuraNET 3000 from Parvus is a ruggedized version of Cisco Systems’ IE-3000 industrial Ethernet switch, specifically hardened for use in demanding networking technology refresh applications.
Although the IE-3000 is considerably more rugged than the norm for a commercial product, many military customers require specific electromagnetic interference (EMI) compliance standards—specifically MIL-STD-461—for radiated and conducted emissions as well as radiated and conducted susceptibility.
Meeting this rugged EMI standard requires protection against input voltage inversion, voltage surges and over-voltage spikes in accordance with MIL-STD-704 and 1275. This was accomplished through the implementation of a reverse voltage/overvoltage protection circuit. Engineers also implemented several improvements, such as designing a sealed enclosure with good EMI gaskets and creating proper test cables. Moreover, proper grounding techniques and good bonding between chassis surfaces were critical in creating an enclosure that acts as a Faraday cage. Since external power leads are typically unshielded in test and application, they can be the single largest point of noise and susceptibility. By including a well-designed filter at the point where power enters the system, the ruggedized IE-3000 complies with EMI requirements as the filter prevents internal noise from exiting the system and protects sensitive electronics from external noise that otherwise might enter the system.
Like many commercial products, the IE-3000 includes RJ-45 network connectors. Although adequate for its original purposes, these RJ-45 connectors are notoriously prone to failure under extreme vibration and do not provide ingress protection against dust and water. Parvus engineers removed and replaced them with locking headers that ultimately terminated with circular MIL-DTL-38999 style connectors that not only protect against dust, water, vibration and shock, but also bring ports to the outside world.
Although a cableless design is optimal for rugged conditions, when ruggedizing an existing commercial product that includes cables, not all cable may be eliminated, so additional steps need to be taken to ensure stability. Since the IE-3000 contains some cabling, engineers leveraged rigid flex circuits and board-to-board interfaces where possible and implemented cable braiding, tie-downs and other strain relief features to maximize reliability and prevent the cables from disconnecting or being severed in vibration or shock.
Heat issues are often credited as the largest contributor to system failures, so ruggedizing systems to meet these thermal challenges is a critical step. Thermal management for defense applications has always been a challenge due to the high operating temperatures of the latest processors and dense packaging needed for environmental ruggedness. Cisco’s standard IE-3000 switch relies on internal heat sinks and a vented case with passive air flow through the case to cool the unit. However, relying on convection cooling only inside a completely sealed box would have severe limits, so for the DuraNET 3000, the incorporations of conduction-cooling techniques enabled the maximization of heat transfer, while allowing the unit to remain fanless and passively cooled.
To reduce weight and to speed the DuraNET 3000’s heat transfer rate, engineers removed all of Cisco’s standard finned heat sinks and replaced them with heat spreaders, a conduction-cooling mechanism. The inclusion of heat spreaders, a thin sheet of metal incorporated on top of a device to help dissipate heat, significantly reduced thermal issues inside the IE-3000. These heat spreaders route heat through an internal rail/truss system that supports all of the circuit card assemblies against shock and vibration while dissipating the heat to the aluminum enclosure that incorporates finning on the outside to maximize surface area for cooling.
As the example of the IE-3000 demonstrates, ruggedizing a commercial product involves a number of intricate engineering procedures. However, the degree of ruggedization depends on the application for which the system is intended. Since the DuraNET 3000 will be deployed in the harshest of military environments, including tracked vehicles, navy ships and aircrafts, the specific ruggedization techniques implemented will allow the system to endure these environments.
Similarly, a number of ruggedized systems designed for compute-intensive applications are gaining popularity. To accommodate the high port density and power requirements of these systems, ruggedization techniques such as cableless and fanless designs aren’t possible. However, as demonstrated by the following case study, ruggedization techniques can still be implemented that won’t compromise the system’s high-performance capabilities.
High Performance Meets Ruggedization
Cisco Systems’ high-performance Catalyst 4948E data center switch is a desirable device for many applications because of its 48 Gigabit Ethernet downlinks, plus three 10 Gigabit Ethernet uplinks (2 copper/1 fiber) and a Gigabit Fiber uplink. To make this switch accessible to demanding networking environments, Parvus engineers deployed a series of mechanical enhancements that support the deployment of data and multimedia services in wider thermal, shock, vibration, altitude and humidity conditions than offered by the standard commercial Cisco version. Dubbed the DuraNET 4948, this powerful, multilayer switch enables demanding military and civil IP networking technology refresh programs to leverage the best that Cisco switching technology has to offer, but in a ruggedized 19-inch rackmount solution suitable for rugged applications (Figure 3).
The DuraNET 4948 from Parvus is a ruggedized version of Cisco Systems’ high performance Catalyst 4948E data center switch.
As the intended use for this Cisco switch is in data center environments, its original operating temperature was 0° to 40°C. For this system to be deployed in rugged environments and meet Military Standard 810G (the de facto standard for rugged military electronics), the operational temperature range needed to be extended to -40° to 54°C with the ability to power up when the temperature changes rapidly from 71° to 54°C without time for stabilization at 54°C.
With many ruggedized systems, ensuring a wide operating temperature is a top priority. This proved to be the central engineering challenge when ruggedizing the 4948E because powering the system in cold temperatures instantly created two temperature problems. First, none of the components are rated to power on below 0°C. Second, once the system is operational, some components are self-heating and dissipate a substantial amount of heat (more than 275 watts unit wide), while other components can remain well below their minimum operating temperatures. Cold components need to be warmed at the same time that hot components need to stay cool.
To solve this problem, engineers implemented two types of internal heaters to pre-heat the system and protect specific components from damage. The first type of heater provided broad heating across the Catalyst 4948E circuit board. The second type of heater was specifically targeted to the compact flash memory, as this critical component wouldn’t operate, or suffered permanent damage, at the lower temperatures. Due to its limited internal self-heating, the compact flash memory—unlike other components—wasn’t staying warm enough to operate properly, necessitating its own dedicated heater.
Conversely, at the high end of the temperature range, additional techniques needed to be implemented to remove the heat from the system. Since the 4948E requires more than 275 watts, heat dissipation was a critical concern as well. The engineers found the best option for removing heat was to keep Cisco’s original design of using fans to force air out of the system. However, since moving parts are not ideal in rugged systems, additional ruggedization procedures were implemented to mitigate any risks involved in using fans.
Ruggedization Techniques Ensure Reliability
The first step in ruggedizing the fans was upgrading the fans themselves. A fan with a slower RPM yet with a higher air volume was selected. This fan would last longer and thus was selected for the extended temperature range. Secondly, the engineers ruggedized each component of the fan itself by potting the windings and conformal coating the fan PC board. Conformal coating is a process where a coating material is applied to electronic circuitry to protect it from moisture, dust, fungus, corrosive chemicals and temperature extremes.
Conformal coating also proved to be an integral ruggedization technique for the DuraNET 4948. To address the potential hazard of blowing corrosive and conductive salt fog from ocean environments directly over the system’s sensitive electronic components, engineers were diligent in conformal coating all circuit boards and using other corrosion-resistant coatings for all the metal in the system. Plus, to ensure against any possible corrosion, engineers sealed critical electronic components with room temperature vulcanization (RTV) silicone rubber and applied bulb grease to all connector contacts.
The 4948E was redesigned to operate in an extended temperature range, but the existing firmware wasn’t capable of controlling the device given the new thermal envelope. To counteract this problem, Parvus engineers created new firmware for the DuraNET 4948 that controls the fans and the heaters across the extended temperature range, while using Cisco controls for the original operating range of 0° to 40°C.
For many rugged and ruggedized designs, redundancy is a critical element as it significantly increases the MTBF. When ruggedizing the 4948E, the engineers included two power supplies that were redesigned to meet aircraft-grade military standards, and provide for EMI filtering. These supplies were designed to share current, reducing the operational duty cycle to less than 50% per supply, and thereby extending the operational life of the supplies. In addition, each supply was designed to handle the full load of the switch with more than 20% margin. This extends the operational life of the system and keeps the system running if a power supply should fail.
As illustrated by the creation of the DuraNET 3000 and the DuraNET 4948, the process of ruggedizing a commercial product for harsh application use is no small feat. However, ruggedized products have the benefit of capitalizing on the technological advancements made by the world’s leading network manufacturers. When combined with proven ruggedization techniques and innovative engineering efforts, ruggedized systems offer a robust, cost-effective computing choice engineered to meet the world’s most demanding environments.
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