Control Operations in Advanced Aerospace Systems

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CONTROL OPERATIONS IN ADVANCED AEROSPACE SYSTEMS

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R. Graham

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This talk is a tour through some of NASA's interests in the control of distributed parameter systems. The discussion is going to be in a number of specific technologies. I am going to say a few words about the control of large structures in space, automation and robotics, guidance and control of advanced aircraft, and, finally, the distributed management of systems. I will start with large space systems. In recent years in Washington, in one of the buildings of the Smithsonian Museum, you can find an exhibit which is a retrospective of an exhibit a hundred years old - the 1876 Centennial Exhibit that was held a hundred years ago to show some of the great advances in technology that had been made in the first hundred years of the country's development as a nation. The real giants of the Centennial, even of the retrospective, were the big steam engines . There were very large engines in the original exhibit, and there are smaller steam engines in the retrospective at the Smithsonian. Over in the corner of the exhibit, in a very inconspicuous place, is the Otto engine, which is an internal combustion engine. It is built to look much like a steam engine. A few of its features that are different from the steam engine are explained rather apologetically in the description that goes with it. I wonder how many people then could have any idea of the dinosaur- like demise of the giant steam engine and the evolution of, first, the Otto internal combustion engine, and then turbines and other engines. I bring this up, because I think it describes fairly well where we are in space structures today, particularly large space structures. We are really psychologically attuned to thinking about structures in a one - g environment. I suspect that a one - g environment dominates our structures much more than we realize. It is going to take some evolution in space structures before we fully begin to understand how to build large objects for space . And probably the best way to understand that is to begin building them, I will show you a couple of examples of space structures that NASA is working on today. figure.l shows a 15 meter antenna that is designed to point at the Earth. The only two elements of it that are in compression are the vertical column and the large hoop . They are made out of composite materials and are designed with a high strength to weight ratio. But they are also designed to fit in a very small container when the antenna is being transported to space. While 15 meters in diameter, the antenna folds into a box about a meter by a meter by about two meters . The hoop folds up like the outer circumference of an umbrella. The reason I show this is because it is a distributed control system problem. The antenna is shaped by some 96 control cables that are attached to various points along the ribs in each section of the antenna. Motor drives allow each one of these cables to be adjusted

Fig . 1. Fifteen Meter Earth Pointing Antenna separately. Of course, this can control the local shape of the antenna, but it is coupled to the rest of the shape, too. You cannot adjust the curvature or the displacement of one section only you adjust the whole antenna by tuning these wires. The way you find 0111. how we ll the shape control system is performing is, first, by near , field mea surement s and, second, with m e a suring devices on the antenna itself. Another test is to get the far fields of the antenna and see what the signal strength is across a rather wide band. This antenna is being designe d to run from 2.3 to 11. 7 GHz. One of the que stions which face s the operators o f this type of antenna is how much information they have to derive locally and how much information they can deriv e in t.he far fi e ld in order to optimize the antenna shape. This is a problem which is worthy of attention and is certainly of interest to NASA. On a larger scale, the United States, in cooperation with se veral foreign countries, plans t o develop, construct and deploy a manned space station a permanent manned presence in space some time around the middle of the next decade. figure 2 shows an early version of the station. This is a dual beam station that is now the ba se line design . As you can see, the two beam structure forms the ba c kbone of the station, and the cross - arm provides the mounting point s for both the photovoltaic and solar dynamic heat engine power- generator. There is a provision for docking a shuttle, facilities for servicing satellites that are brought to the station by the orbital manuvering vehicle, and communications facilities and scientific instrumentation on both ends of the structure. Finally, there are free fliers or platforms

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The space station is going to have to be able to grow on orbit; we are going to have to be able to change it substantially and still maintain its performance characteristics. Figure 3 shows a m o re advanced stage of that evolution. Note the robotic arm that is removing a satellite from a servicing bay. That is a robotic element of the advanced station, which w;ll also be on the near- term station. It is very important to have a robotic capability , particularly for in- space operations or extra - vehicular activity EVA. Figure 4 shows NASA's EVA system. It involves about 400 pounds of machinery which, again, has to be made very, very reliable and self - diagnosing.

Fig. 2 Baseline, Dual Beam, Space Station Design that are part of the space station infrastructure. There are on the order of half a dozen separate flying objects, both in equatorial orbit, which you see here, but also in polar orbit, that would be part of the space station infrastructure. Some of the station control problems have to do with the pointing and the stability of the structure. This is not a gravity gradient stabilized structure. An earlier version was; however, it did not provide enough appropriate operating volume on orbit for doing the things the station has to do, so this station has to be dynamically stabilized. There are five meter elements in the structure, and they are designed so that anyone can be removed without destroying the structural integrity of the system. They are very, very stiff composite structures, but they have very little intrinsic da mping, so it. is likely that the station will have to have active and/or passive damping to control its response to disturbances. It is important that it respond well to disturbances because the microgravity environment that can be produced in the station will be an important part of materials processing and other experimental capabilities that can be done on the station. Various activities will be happening on the station: people will be moving around; attitude control will have to be maintained; and both satellites and space shuttles will have to dock with it from time to time. The station has to be well behaved through all these disturbances.

Fig. 3 Robotic Arm Removing a Satellite from Space Station Servicing Bay

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The characterization of the inherent damping in the system cannot be very well simulated on Earth, so we are going to have to be accommodating and probably adaptive in our control systems that stabilize and damp the structure. Operations in space always carry an element of risk. One of the things that has to be considered is the ability of control systems to determine when they themselves, or when some other aspects of the overall system, are not functioning properly. That means that the information in the control loops of both the forces and the response of the station are going to have to tell you a lot about the health of the control system, as well as what is happening to the station. To say it another way, the control system is going to have to be somewhat self- diagnosing, and be able to report when it (or the station) is experiencing trouble. It will also have to be able to try to localize that problem and, when necessary, respond to it with a minimum amount of human intervention. That seems to me a particular challenge in control engineering. You are trying to make the control system monitor its own state of health and use that information in additional processes that are critical to the safety of the station.

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Fig. 4 NASA's Extra Vehicular Activity (EVA) System

We w ould like to see many of the functions now requiring man in space done instead by robotics _. probably first in a mode that I would call tele · robotics. Tele - robotics is baSically using a man somewhere in a safer environment, either on earth or in the station or in the shuttle, to control things outside. The robotic capability will then go to one of a more skilled assistant that can make some decisions of its own while it is manuevering and working in space and, finally, to a rather autonomous capability. The first generation of tele - robotic systems, like the arm that is attached to the shuttle now, has essentially no time delay between the command and the operation of the arm. On the other hand, systems such as shown in Figure 5, which is a conceptual Mars

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surface rover _. an unmanned geologic sampler on the surface of Mars has a loop delay time from Earth that is somewhere between 25 and 40 minutes, depending on where the earth is and where Mars is in its orbit. [t does not take long to start accumUlating phase shift or time delay, and you cannot very we\1 make a control system that stops working at 10 - 3 Hz.

I!S!cl!:."-.i shows an airplane in a high alpha, or high angle of attack, where the airplane is rea1\y in a stall, and shedding large vortices. This particular high alpha configuration has some military use as we1\ as possible uses in sta1\ recovery, but at the same time it can put aircraft in conditions that the pilot cannot control. NASA has had an extensive program of developing ways of changing the command authority in the aircraft to help it recover from situations such as this. You can also see additional means of controlling the airplane wit.h the thrust vector control vanes that have been placed in the engine nozzle. These are for yaw, but you can imagine placing pitch vanes there as we1\ to provide control when the airflow over t.he aircraft is in such a turbulent state that it can not reasonably drive the control surfaces in the normal aerodynamic fashion. The characterization and control of substantially unstable aircraft is going to be a challenge for the next several decades. To go further into the future type of aircraft , shows a concept described recently. [t is called the aerospace plane. The aerospace plan" is basically an airbreathing aircraft, probably fuded by hydrogen. Most of its volume would be a liquid hydrogen tank. But the aircraft would be designed to fly at hypersonic speeds certainly Mach 7, probably Mach 10 plus, and possibly up to orbital speeds. The engine at these high speeds would basically be a duct _ a supersonic ramjet. The aircraft has to fly in a constrained flight regime. If it gets too low in the at.mosphere, the drag and the heating will cause it to either stop accelerating or, worse yet, to overheat. [f it gets too high, it can not get enough oxygen 1.0 run the engines to provide sufficient thrust. The performance of the aircraft and its control are tightly coupled. F~~~~_ J

Fig. 5 Mars Surface Rover for Unmanned Geologic Sampling So what you have to do is build a great deal more ski\1 into this type of system. At the same time, the early plans to operate this may involve a man on Eart.h using the stereoscopic displays that are generated by T.V. cameras to provide a map of the terrain the rover is supposed to cross. Then the man will steer a course and transmit t.hat from Earth to Mars. The rover will navigate that course by itself, stop, transmit another set of pictures, plot another course, and go again.

KEY AEROSPACE PLANE TECHNOLOGIES

Next, consid e r the control of advanced aircraft, such as shown in .Fi.&..t!.r:"-_ . ~. Aircraft today arc being designed so that. t.heir stability can be almost entirely imposed by active means. Less and less must come from the intrinsic aerodynamic s of the aircraft. This has led to weight savings, to dficiencies in the aerodynamic desiGn and, of course, addilional demands on the control system. NASA has a plane flying now the X · 29A which has forward swept wines, wit.h canards in front of those wings. The X 29A, which is unstable aerodynamica1\y, is designed, in fact, to be so. One benefit. is that t.his unstable spacecraft is very, very maneuverable. This can be very hard on the pilot from time to time and, in fact, on t.he struct.ure of the airplane. Such aircraft can reach flight. regimes that arc br,Y()f;r} t.ho se common in aeronautics. Fig. 7 The Airbreathing Aerospace Plane

HIGH ALPHA TECHNOLOGY

V A LID AT ED PRE DI CTI ON METHO DS COI\ITROllABllIT Y C R ITFA !A DATA BAS E

Finally, consider the distributed management control of systems. figure _..!!. shows the Hubble space telescope which we hope to launch soon after we start flying the space shuttle again. [t is basically an astronomical instrument on orbit and has a very clean environment. for seeing both planetary and also more distant astronomical objects . The figure indicates data coming back to Earth. What it does not show is the use of this resource something highly pricted by astronomers. An astronomical institution, at J ohns Hopkins University, will act as the central point for the astronomical community to make decisions on the use of this scarce resource . The way NASA is likely to proceed with future instruments in orbit is shown more clearly in f.Yl.ur~

Fig. 6 Airplane in a High Alpha Configuration

[t shows a space sta tion as a generic instrument, in fact a very complex one, in orbit, and of course there are a large number of USers on the ground. Given the

for a large community of participants, most of whom will be on the ground. Of course, the first issue is to try to identify and formulate the questions. And that is something that I do not believe has been done yet in these latter problems, and most particularly in t.he distributed management of systems.

Fig. 8 Hubble Space Telescope

Fig. 10 Instruments and Resources in Space Utilized by Participants on the Ground

NASA is trying to do something along that line itself. There is a classic problem in large organizations and particularly in large government organizations. They tend to work in the evolution of ideas but not very well in the revolution of ideas. One of NASA's predecessors did a great deal of work in aeronautics and aerodynamics but, in retrospect, too little in the early days of the jet engine and the liquid fuel rocket engine . I would like to avoid that happening again.

Fig. 9 Space Station Interaction With Users on Earth communications and the data processing capability that exist today, ther~ is no need to pull all these people into one place. They can work in a somewhat independent fashion. Nevertheless, whatever they wish to do has to be brought together in a coherent plan before it gets to the space station or to any instrum~nt in orbit. Constraints imposed on any operation of the station include the constraints on space station power, on communications, on cooling, on propulsion, on stability and control, and on expendables. We also have to control the environment of the instruments, including the acceleration, the location, the direction, and so on. Coupling the users to the space station will have to be well thought out as a control problem in order to make sure that the uses of the station are optimized at least in some sense. The generic version of this problem is shown in Figure 10. It is something that NASA will be facing in the foreseeable future as it puts instruments and resources into space and tries to make best use of them

And so, to try to obtain a broad base of good ideas, revolutionary as well as evolutionary, NASA has instigated a search for innovation in the technical community. So far, NASA ha s made one such announcement of opportunity in materials, and will be putting out a series of such announcements that will include control disciplines. They will be evaluated on their technical merit, and they will provide only a broad area description of what the government wants . The trouble is, again, in asking the questions. If you can make a very precise description of the problem, you are already a long way to solving it. We would like to find new ideas as well. You can find out more by writing to NASA Headquarters, Office of Aeronautics and Space Technology, Washington, DC 20546, and ask them to provide you with information on announcements of opportunity for R&D in an area you are particularly interested in. They will send you information when the announcements are made from time to time. NASA hopes to fund efforts up to a year in duration with some prospect for renewal. At the same time, we are going to make this a high risk technical program, and we do not expect everything to be continued. But we look forward to your help, both in solving the problems that we can identify, and in new opportunities and challenges.