2000 - Draper Laboratory

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Redundant SensorsActuatorFigure 7. Adding cross connections between network terminals.The trade-offs are:System Without Cross Connections+ No potential for faults to propagate between channels(electrical or data).- Many two-fault conditions can lead to system failure.- Distributed I/O processing is limited to what can be performedwith only one channel of data.System with Cross Connections+ System can be made tolerant of most two-failure conditions.+ Adjacent channel data provides many options for distributedI/O processing.+ Adjacent channels can monitor each other's health, localizenetwork failures.- Adds hardware (size, weight, cost, power) to network I/Osystem.- Requires isolation to avoid fault propagation betweenchannels.It is believed that with careful design, both the added hardwareand the potential for fault propagation can be minimized.The benefits of having the cross connections outweighthese costs.CONCEPT FOR A FAULT-TOLERANT NETWORK I/O SYSTEMIn the previous discussions, it has been shown that many alternativedesign solutions exist for creating I/O networks. No singlesolution will be optimal for all applications. Yet there arecompelling reasons for pursuing a system design based on asingle approach, using interchangeable components with astandard interface. With the understanding that no singleapproach will be optimal and that technological advancescould render some of these conclusions obsolete, this sectiondescribes a concept for a fault-tolerant network I/O system.The following are the design requirements:• The network I/O system must be designed to support functionsthat are critical; that is, malfunction or loss of the functionendangers the crew or the ship. Providing this level ofdependability requires a TMR network I/O architecture.• The primary network media will be copper wire. Fiber-opticand wireless media will be used to a lesser extent to supportspecial installation requirements.• The network topology will be buses rather than rings. Aring topology complicates the installation of wiring andpower distribution, places too much dependence on thesurvival of individual network terminals, and makes eachterminal larger, more costly, and more power consuming.• The network will supply dedicated power to the I/O networkterminals over the same copper wire network used totransmit data.• The ship will use many small I/O networks. The size of thesenetworks will be limited to the maximum of several hundredterminals and a data rate of below 1 Mb/s.• The capability to cross-strap individual sensors and actuationterminals is provided.Figure 8 illustrates an overall system concept that meets theserequirements.CONCLUSIONSThe use of fault-tolerant I/O networks shows great promise forproviding highly automated and completely dependable controlsystems for ships. There appear to be few major technologicalbarriers to the development and introduction of thesenetworks. Many of the components needed are emergingfrom the development of industrial and automotive data multiplexing,data acquisition, and control systems. But theseapplications do not have the requirement for fault and damagetolerance, nor are the environmental requirements asdemanding. There is a need for a substantial engineeringdevelopment effort. Realizing the full benefits of theapproach will require highly integrated, miniaturized, andFault-Tolerant Input/Output (I/O) Networks Applied to Ship Control 33
Network Control ComputersTriple Twisted Copper Wire Bus NetworkSupplying Data and Terminal Power(Routed and Shielded for Survivability)NetworkTerminalPowerSuppliesSimplex Sensor or ActuationNode (Nonessential Function)Triple Redundant Sensor orActuator Node (Critical Function)Dual Redundant Sensor orActuation Node (Essential Function)Separate Source ofActuation Electrical PowerBus “Spurs” into Harsh orVulnerable AreasFigure 8. Concept for a complete network I/O system.harsh environment electronics, produced at moderate cost,that are not currently available "off the shelf." Commercial-offthe-shelfproducts will only appear after visionary developershave proved the concept with a successful "first-of-its-kind"system. This paper is intended to stimulate interest in such adevelopment.REFERENCES[1] Moschopoulos, J., "Advanced Integrated Control andInformation Systems in the U.S. Navy," Eleventh ShipControl Systems Symposium, Southampton, UK, 1997.[2] Hammett, R., "Seawolf Ship Control PerformanceMonitoring Provides Fault Tolerance and Simplified Maintenance,"Intelligent Ships Symposium II, Philadelphia,1996.34Fault-Tolerant Input/Output (I/O) Networks Applied to Ship Control
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2000 - Draper Laboratory

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