Expert simulation automates space electric power system

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<ul><li><p>Exoert Simulation Automates Space Electric Power System </p><p>Scott A. Starks1 and David LV Elizandro2 </p><p>ASA has undertaken the task N of designing a manned space station. To decrease airborne and ground-support costs and t o achieve other benefits, subsystem designers have been encouraged to automate activities ordinarily performed by humans. NASA's Advanced Technology Advisory committee and the Automation and Robotics Panel of t h e California Space Institute pub- lished reports clearly identifying the space station electrical power system as a primary candidate for implementing new robotics/ automation techniques for moni- toring, trend identification, fault diagnosis, system control/load management, automated mainte- nance, repair and replacement, and operations. </p><p>Conditions affecting a space </p><p>~ </p><p>Though significant differences </p><p>exist between the operating environment of the </p><p>electric power system on a space station and </p><p>on a terrestrial power system, technology </p><p>innovation developed through space-based projects can and will </p><p>ultimately have an impact on earth-based power systems </p><p>power supply system are substantially different from those affecting typical terrestrial power supply systems. In the United States, practically all power systems are interconnected, so that the loss of a large generating unit is replaced from the entire interconnected system, retained by frequency bias generation control on all sys- tems, until arrangements are made between the deficient system and its neighbors. Each terrestrial system has full- time dispatchers to operate the system and engineers and repair crews who can reach the affected lines or plant in a short time. In the space system, there are no interconnec- tions, and engineers not aboard the space craft would </p><p>An artist's conception of space station Freedom at the permanently manned configuration, which is scheduled to be achieved by late 1999. The configuration at this time consists o f U.S., European, and Japanese laborato- ry modules and the U.S. habitation module and three sets o f solar arrays providing more than 56 kilowatts o f electrical power. Freedom, in its permanently manned configuration, can support a crew o f four. </p><p>University of Texas at El Paso, Electrical Engineering Department </p><p>East Texas State University, Computer Science Department </p><p>require months to come aboard, rather than minutes or hours. The October 1989 issue of Computer Applications in Power featured a reliable power supply system for the space station proposed at that time. Because of budget con- straints, the size of the space sta- tion has been reduced to contain only four people, instead of eight as planned originally, and this means that power supply should not call for activity from any of them, if possible. The new system is much more automated, and makes maximum use of expert sys- tems techniques. </p><p>Many benefits accrue from the utilization of computer-based automation technology in terms of the management, operation, monitoring, and control of com- plex space subsystems. On the </p><p>space station, the electric power system plays a unique role in coordinated operation with other onboard sub- systems in that it supplies a primary resource (electric power) upon which all other subsystems depend. This is a key interdependency because any degradation of the electric power system impacts other subsystems and ultimately the operation of the overall space station. </p><p>In part because of the unique and complex role that the electrical power system will play, there is a great potential for applying automation techniques. Automation can result in increased safety and reliability by providing rapid responses to faults or anomalous conditions in a fail-safe/fail+perational manner. An auto- mated system may provide greater operational flexibility by allowing for the reprogramming of critical operating limits to conform to changes in system or component performing capabilities. </p><p>When effectively applied, automation provides the opportunity for lowering both initial and lifetime costs. Onboard automation can alleviate the large crew and ground support requirements substantially. Onboard analysis of electric power system data results in the reduction of the volume of data required to be down- linked for ground analysis, thus decreasing communica- </p><p>ISSN 0895-0156/91/$1.0001991 IEEE </p><p>_ _ _ _ ~ </p><p>October 1991 11 </p></li><li><p>tion requirements. In addition, increased reliability gained through automated trend and state monitoring techniques can reduce the hardware redundancy requirements. Resupply costs will be reduced by extend- ing the useful operating life of components by continual- ly monitoring and executing appropriate responses. </p><p>Space Station Electric Power System Concepts Space station Freedom has recently undergone a down- sizing and redesign. The redesign was driven by desire t o have a more cost-effective program, to reduce extravehicular activity, and to simplify the design and ultimately the operation of the station. Current plans call for an initial man-tended capability station to be func- tional by December 1996. Whereas the original plans for the station called for an aggregate power generating capacity of 75 kilowatts, the new design calls for a power generating capacity of 56.25 kilowatts in the stations final operational configuration. This will be accom- plished by the building of three solar panels supported on a preintegrated, seven-section, 31 7-foot truss, as opposed to the previous design, which incorporated four panels. </p><p>The initial man-tended operational capability will require six assembly flights using the space shuttle. After these flights, space station Freedom will have a capability of generating 19 kilowatts of electric power. This will be sufficient to support experiments dealing primarily with materials processing in a microgravity environment. The man-tended capability restricts the station to be fully operational only with the space shut- tle attached. </p><p>By the end of 1999, the space station will be in transi- tion to a permanently manned configuration. The transi- tion is to be accomplished in discrete phases and will require eleven additional assembly flights of the space shuttle beyond the six needed to achieve the man-tend- ed configuration. In this final state, space station Freedom will be suited for permanent habitation by a four man crew, instead of the original proposed crew of eight. This places additional demands of increasing the level of automation in the stations various systems, including the electric power system. The crew will be interchanged at periodic intervals of time. In the perma- nently manned configuration, the electric power generat- ing capacity will be 56.25 kilowatts. </p><p>The electric power system for the space station will be complex, requiring a vast amount of sophisticated software. The primary objective of the system will be to ensure a reliable supply of electric power for the various power uses on the space station. The nature of this task is largely shaped by a power distribution network design that will incorporate a high degree of redundancy in con- junction with a critical limitation on the availability of power and the concordant reluctance to waste it. </p><p>An artists conception of space station Freedom at the man-tended configuration. One of two nodes and the U.S. laboratory module provide pressurized work space for visiting astronauts carried to Freedom aboard the shuttle. The man-tended configuration is scheduled to be achieved by late 1996. The two large solar arrays deployed at the top o f the preintegrated truss furnish Freedom with 13.75 kilowatts of electric power. </p><p>To accomplish the task effectively, the following sub- </p><p>m The various demands for power must be sched- uled flexibly. </p><p>m Proper management of the various sources of power must be maintained to assure the continu- ous health and operation of the station. Faults in the system must be identified and isolated. Unscheduled power demands must be anticipated and planned for so as to minimize their disruptive effects on the power available to other power demanding loads. </p><p>The power distribution network that will actually be incorporated into space station Freedom is currently in design, and, consequently, hands-on experience in oper- ating and managing its various subsystems is nonexis- tent. At the present time, there are no firm strategies for dealing with many of the concerns and issues involved in the allocation and management of space station elec- </p><p>tasks must be worked simultaneously: </p><p>12 IEEE ComputerApp/ications in Power </p></li><li><p>tric power. Many of the issues that actual faults, isolating faults, - must be dealt with have, however, been identified in the context of high-level design constraints. facilitates the modeling of fault recovery actions. </p><p>Expert system technology recommending corrective actions, and implementing </p><p>Numerous projects have been initiated involving the develop- ment of knowledgebased systems integrated with actual electric power system breadboards and </p><p>complex reasoning in the </p><p>system operations </p><p>rn The Power-Distribution module is responsible for the recognition of any long term trends that could ulti- mately affect the health of </p><p>context of electric power </p><p>t es tbeds with the purpose of demonstrating in a real-time envi- ronment the viability of advanced automation approach- es for space station onboard and ground-support applications. In addition, NASA Ames Research Center, in cooperation with NASA operation centers, initiated an ambitious long-term research and development pro- gram, the Systems Autonomy Demonstration program. The purpose of this program is to demonstrate in a real- time operational setting the benefits and capabilities achieved by applying advanced automation techniques (primarily those incorporating artificial intelligence) to system monitoring, operation, and control. </p><p>Distributed System Concept The size and complexity of the space station electric power system clearly favors a distributed software approach based upon a mixture of traditional automa- tion with advanced artificial intelligence and simulation techniques. Such systems can effectively utilize con- cepts from discrete event simulation to assist in the monitoring, planning and coordination of the actions of the electric power system with a number of related sub- systems. Several other factors contribute to the move toward distributed software architectures: </p><p>o A general trend toward smaller, localized proces- sors demands the development of distributed sys- tems to support global coordination. A distributed architecture will reduce the software verification burden. Partitioning a system into operationally independent agents facilitates a robust verification effort by allowing agents to be verified independently. </p><p>Given that a distributed architecture is a viable approach, there are many issues that must be addressed for the proper operation of the electric power system. These are best described by using the following distinct </p><p>The Resource Management module must work with temporal data regarding the various qualitative and quantitative states of resources as well as power generation by each resource and power storage. The Demand Management module will work with temporally dynamic data regarding the states, scheduling, and power requirements of subsystems. The Fault Detection, Isolation, and Recovery mod- ule is tasked with identifying anomalies, diagnosing </p><p>the distribution hardware. </p><p>Role of Artificial Intelligence Technical elements from artificial intelligence, specifical- ly knowledgebased systems, present opportunities for assisting the task of automating the space station elec- tric power system. Knowledgebased or expert systems use domain knowledge and inference procedures to solve problems of narrow domain requiring some degree of expertise. In such systems, knowledge is represented explicitly using AI techniques, and exists separate from the parts of the program responsible for inferencing. </p><p>Data and modulespecific knowledge is organized into knowledge areas (KAs), which can be accessed by heuristics. For power system applications, heuristics can be used in concert with knowledge bases and simu- lation models to generate schedules and automate oper- ations. Listed below are sample heuristics that may be used for formulating decisions based on states of vari- ous resources, distribution networks, and subsystems. </p><p>R Power allocation: Determine a distribution path for the next subsystem waiting to receive power where the amount of power is determined by the strategic goals of the mission. </p><p>m Resource selection: Determine which resource, among those available, will be used to provide power to a given subsystem. </p><p>R Interval selection: Determine which time interval is most suitable to provide the necessary power to a subsystem. A trade-off in this situation is stored versus generated power. </p><p>Expert system technology facilitates the modeling of complex reasoning in the context of electric power system operations. This is illustrated with the following examples of heuristics, which may be utilized by the appropriate functional area modules mentioned previously. </p><p>R If there are intervals that will completely contain the power requirements of a subsystem, the inter- val that yields the most synchronous schedule over all primary resources is chosen. </p><p>R If insufficient power is available from primary resources, backup resources are considered. Backup resources are capable of powering a s u b system but are not the resources of choice. An operation is scheduled on a backup only when it cannot be scheduled on a primary resource in a desired time window. Both a complete fit and a </p><p>October1991 13 </p></li><li><p>An artists conception of space station Freedoms US. and international laboratories and U S . habitation module. The U S . laboratory and habitation module are at bottom let?, the Japanese experiment module is top right, and ESA labo- ratory module is top left. Shown mated to the node between the U S . lab and the ESA module is the assured crew return vehicle used for emergency returns to Earth of space station crew members. An airlock is mated to the top of the other node, which also is outfitted with a cupola. </p><p>partial fit are considered on backup resources, sim- ilar to the approach taken for scheduling on prima- ry resources. </p><p>L If a subsystem requires a certain amount of power, stored power may be used for a portion of the experiment until resources are available to gener- ate the necessary power to complete the experi- ment. Here, trade-offs would be considered between stored versus generated power. </p><p>1 If more than one resource is equally well suited to meet power demands of a subsystem, the resource that minimizes setups is selected. </p><p>Tuning parameters will also be available that enable the designer to generate schedules closer to objectives in the strategic and operatio...</p></li></ul>


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