HYBRID COMPUTING FOR LUNAR EXCURSION MODULE STUDIES

Transcription

1 HYBRD COMPUTNG FOR LUNAR EXCURSON MODULE STUDES Arthur Burns Analog Computing Section Grumman Aircraft Engineering Corporation Bethpage, New York NTRODUCTON Hybrid computation plays an important role in our man-in-space effort. Large scale combined analog-digital studies in support of NASA's Project Apollo are now being performed. The computers utilized are an BM digital computer and three consoles of Reeves 500 analog computing equipment. These are linked by an Adage 770 Computer Link that provides an analog-to-digital and digital-to-analog capacity of 48 and 55 channels, respectively. The effort described herein concerns one of the latest applications of a computing technique that has become a "semicontroversial" topic in recent years. Reluctance to use both computing systems concurrently in the solution of one problem has stemmed from their inherent incompatibility-the continuous versus the discrete domain. As a result, most analog and digital computer installations have been physically and philosophically isolated from one another. Professional computer organizations also have been slanted toward one or the other type of machine. The arrival of the space age has changed this picture considerably. Recently, as evidenced by the large number of hybrid facilities being developed around the country, there appears to be an awakening to the fact that modern day problems require the advantages of both computers. Efforts have been made to organize computing teams for hybrid projects. t is apparent that a softening of the old barriers has occurred. n addition, an important byproduct has been produced in the form of "cross education" between the analog and digital programmers. By necessity, each is beginning to appreciate the advantages and disadvantages of both machines. At Grumman Aircraft Engineering Corporation, preliminary investigations into the field of combined analog-digital computation began late in The results of these studies led to the acquisition of a small, flexible linkage system to interconnect an BM 704 digital computer with a large analog computer installation. The initial application of this system was a verification of the theoretical studies that had revealed that, for certain types of problems, a hybrid technique had distinct advantages over all-analog and all-digital simulations. 1 When the BM 704 was replaced by the BM 7090, the linkage system was modified accordingly in order to be compatible with and to utilize the best features of this new digital computer. The next few years (through 1963) saw no major changes in the linkage system as the BM

2 894 PROCEEDNGS - FALL JONT COMPUTER CONFERENCE, 1965 evolved into the BM During this period, relatively small-scale problems were solved using the hybrid technique. These included some simulation work on the stabilization and control system of the Orbiting Astronomical Observatory,2 a system identification study,3 and a missile homing dynamics problem. 4 Active work on the development of the LEM (Lunar Excursion Module) commenced at Grumman in January Although the initial computer studies were of the all-analog and all-digital variety, it was immediately clear that for some of the more complicated programs, then in the planning stage, an elaborate hybrid computing complex would be required. Accordingly, a decision was made to expand the existing linkage equipment to accommodate these large problems. This expansion ultimately proved unfeasible and a completely new, large scale system was ordered and delivered early in This system is now being used in the LEM hybrid studies. The following discussion describes the reasons for the hybrid approach, the equipment utilized, the problem that is solved, and the implementation of the hybrid computing technique. DSCUSSON Why the Hybrid Approach? The choice of the type of computer complex to be used in simulation studies is becoming increasingly difficult. This is due largely to: The continually increasing scope and complexity of simulation programs, The rising popularity of digital, real-time simulation techniques, and The reluctance of many engineers to depart from conventional all-analog and all-digital methods. As part of the LEM project some computer studies are planned that combine many subsystems into an integrated simulation. The result will be a considerably detailed representation of the LEM vehicle. Add to this already ambitious undertaking the facility for including a pilot, external visual displays, and actual flight hardware, and the result is a problem that exceeds the state of the art of allanalog and all-digital techniques. techniques. For real-time operation, these scale Even the high speed digital computers in use today are not fast enough for an all-digital real time solution. Sampled data studies indicate a maximum allowable computation interval that, when exceeded, proc;uces instability and prohibitive inaccuracies. For the large scale studies, the interval required by an BM to solve the appropriate equations exceeds this limit. The only alternative for an alldigital solution is thus a relaxing of the complexity of the mathematical models. This can be a painful process. Rather than compromise the aims of the program, a more desirable solution is to abandon the all-digital idea and investigate other computing approaches. The problems involved in an all-analog solution are of staggering proportions. Equipment requirements quickly exceed a feasible amount. Resolution requirements on some trajectory parameters are on the order of percent. This is impossible to obtain without resorting to multiple amplitude scali~g changes would have to be made automatically, resulting in an even larger equipment load. Finally, a huge amount of logic and memory is necessitated (particularly by the various guidance laws). t is generally well known that these operations are very unwieldy on the analog computer. Having eliminated these approaches, the logical choice is to "go hybrid." Although the utilization of the hybrid technique does present some unique problems, these are mostly of a logistical nature ( computer scheduling, coordination, etc. ). The present state of the art of computer linkage hardware is such that analog and digital equipment may be confidently interconnected. The resulting computing system is one that, unlike the. all-digital and all-analog techniques, can result in a large scale simulation study that meets the required objectives. The LEM Mission The Lunar Excursion Module is the vehicle in which, as part of Project Apollo, two astronauts will land on the moon. The mission starts with the LEM detaching from the orbiting Command Service Module (CSM) and inserting itself into a coasting elliptical orbit (Fig. 1 a). At a point in the vicinity of the pericynthion, the powered descent portion begins. This is accomplished by firing the throttle able descent engine. Execution of the proper

3 HYBRD COMPUtNG FOR LUNAR EXCURSON MODULE STUDES 895 Separation \ ~ ~... ~ ~, ~~ CSM " /)J'--" ~... nsertion nto " '\ / LEM.., Coasting Elliptical '\ / LEM ",Orbit \ ", \ / \ Powered \ e Descent \ J \ / ~ /1 '" /CSM Circular / ~ Orbit / Figure la. LEM mission (descent). attitude profile by the LEM causes sufficient deceleration for the landing maneuver. After an exploration period, and with the CSM in the proper orbit position, the ascent engine is ignited, using the descent stage as a launching pad. The LEM is launched from the lunar surface and then performs the rendezvous and docking maneuvers with the CSM (Fig. 1 b). At any time during the descent the mission may be aborted (Fig. lc) and the LEM will perform an "early rendezvous" with the CSM. A reaction jet control system, including jet select logic and pulse ratio modulators, is used for rotational and translational control. Additional attitude control is provided during powered descent by descent engine gimballing. Guidance is implemented via on-board computer, manual inputs, or combinations thereof. Simulation Facilities The simulation complex shown in Fig. 2 can be thought of as organized into three general categories: 1. The hardware necessary to insert a man in the loop including an instrumented LEM cockpit, external visual displays that present appropriate visual cues to the pilot, and control consoles for monitoring the runs, failure insertions, etc. 2. The computer facility consisting of an BM digital computer, a Reeves 500 analog computer, and special purpose computing hardware, and 3. An Adage 770 analog-digital computer linkage system. Although the features of general-purpose an(llog and digital computers are well known, computer linkage systems are still fairly unique. The Adage is composed of two ADC's (analog-to-digital converters), two 24-channel multiplexers, 35 sample and hold amplifiers, and 55 DAC's (digital-toanalog converters). Three channels of fixed discrete

4 896 PROCEEDNGS - FALL JONT COMPUTER CONFERENCE, :: Docking \ "- /1 CSM "\ \ Rendezvous \\ f 0 /Lift-Off \ \~EM E)-) \ \ ''-.....,/,/ ) \ ---- / "" / " "'- /'/ Figure lb. LEM mission (ascent). data information (switch positions), each consisting of a 14-bit digital word, may also be transmitted to the digital computer. Utilization of the mode control feature enables the digital computer to control the OPERATE, REST, BALANCE CHECK, and HOLD modes of the analog computer. However, the analog consoles, recorders, control console, and the digital computer are "mode slaved" in such a way that the lowest commanded mode always dominates. Regardless of what mode it has commanded, the digital computer can sense the actual mode of the analog. Because the digital computer can write initial conditions into the analog integrators and then throw them into HOLD, it may now use the same DAC for any other problem input during the OP ERATE portion of the run. Thus, the output of a "time-shared" DAC may be patched to the initial condition input of an integrator as well as any other input (except another integrator initial condition). Digital input/ output sequences are initiated by appropriate READ and WRTE commands from the BM The A-to-D, D-to-A, and discrete data channels may be selected at random. The outputs of four flip-flops whose states are controlled by the digital computer are made available at the analog patch panel. These are called Function Outputs and can be used to drive relays on the analog computer. A prominent feature of the Adage 770 is the control panel shown in Fig. 3. This unit contains the switches and indicators necessary to operate and test the linkage equipment. n the manual mode, with the aid of the control panel, all instructions to the Link may be entered and all data and control registers examined. This enables a major portion of checkout and trouble-shooting procedures to be accomplished without using the digital computer. Assignment of Computer Tasks Allocating portions of the problem between the

5 HYBRD COMPUTNG FOR LUNAR EXCURSON MODULE STUDES 897 Figure lc. LEM mission (typical abort). analog and digital computer should not consist merely of having the analog relieve the digital of some of its computational burden. The primary consideration, rather, should be the utilization of the best features of each. Efficient computer usage is also an important factor. With as powerful (and expensive) a machine as the BM being utilized, it is imperative for computer efficiency that its computation interval be as close as possible to (but less than) the maximum allowable for real time. Once. this is achieved, the requirement for a minimum amount of analog equipment should naturally follow. The usual starting point is to assign the high frequency dynamic equations to the analog computer and those involving large dynamic ranges and considerable logic to the digital. For example, the LEM reaction control system modulators provide rapid pulses of thrust to the vehicle, resulting in relatively high-frequency attitude accelerations. Thus the control system, calculation of the body forces and moments, and resulting rotational dynamics are placd on the analog. n addition, descent engine gimballing, and reaction control system fuel computations are also placed on this computer. Translational and trajectory equations, calculation of variable mass and inertias, descent and ascent engine thrusts, and all axis transformations are placed on the BM Appropriately, the on-board guidance computer is also simulated on this dital computer. Reaction control system modulators, jet select logic, thruster shaping circuits, and gimballed engine logic are taken care of by special purpose computing hardware. The resultant computer configuration is shown in Fig. 4. Operation of Problem Real Time: A typical automatic msson run begins with the analog computer in BALANCE

6 ~ 898 PROCEEDNGS - FALL JONT COMPUTER CONFERENCE, 1965 Analog -.. ~ AD ~... Link A ~ A ~ Digital A ~ H,, ~ ~ Extern._ Visual Displays, LEM - -- Monitor Cockpit Console Figure 2. Simulation facilities. CHECK (all amplifier inputs grounded). The digital computer then writes initial conditions into the appropriate DAC's and sends the analog into RE SET. The analog integrators now have the appropriate initial condition output voltages. The analog computer is then sent into HOLD. f time sharing of DAC's between initial condition and variable quantities is used, the values of the.latter at time = 0 are then transferred to the particular DAC's involved. The digital computer then causes the analog to go into OPERATE. The precision interval clock is simultaneously turned on, and the basic computation interval has started. Body attitude rates generated on the analog are sampled by the digital and integrated to produce vehicle attitude. By applying the appropriate guidance law, attitude errors are generated. These are sent back via DAC's to the control system that produces corrective moments to the LEM. The guidance laws also determine when translational commands are required. These are also transmitted to the analog where the translational forces are generated. Because these forces contain high frequencies and cannot be sampled fast enough, they are integrated first and then sampled. The digital computer then takes the derivative using successive values. From the calculated average force, the resultant translational motion is calculated. Descent or ascent engine thrust is calculated and sent to the analog for computing moments due to engine misalignments and center of gravity offsets. Updated values of moments of inertia and center of

7 HYBRD COMPUTNG FOR LUNAR EXCURSON MODULE STUDES 899 AD LNK Digital Computer Analog Computer Attitude Errors Guidance Laws.. Control System Engine Thrus'ts Translational Eqns nertias Body' Forces Axis Transformations C. G. Positions.. Body Moments - Altitude (For "Quick Look") Engine Thrusts.. Rotational Eqns Control J CSM Position Control System Fuel Control System Fuel nertias Attitude Rate~ Engine Gimballing Gimbal Angles...L Mass ntegral of Body Forces Total Fuel.. t! Special Purpose Computing Hardware Jet Select Logic Gimbal Logic Pulse Ratio Modulators Jet Thrust Shaping Figure 3. Adage 770 control panel and manual control instructions. Manual Controls POWER ON/OFF - Power to 770. REMOTE MANUAL - Permits either manual or digital computer control of read/write operations in the 770. READ SCAN UPPER LMT - Sets upper limit of sequential read scan. WRTE SCAN UPPER LMT - Sets upper limit of sequential write scan. REFERENCE VOLTAGE NTERNAL/EXTERNAL - gravity positions from consumed values of reaction control system and engine fuels are transmitted to the analog at each computation interval. Altitude above the lunar surface is also sent to the analog for general monitoring purposes. During the course of a run, the digital computer also selects control system deadband values and feedback gains. This is accomplished by the setting the proper Function Output flip-flops. Function relays on the analog computer are thus energized and the appropriate analog circuitry switched in. Fast Time: Although the studies are primarily run in real time, certain coasting phases of the mission may be accomplished in fast time. For this mode the analog computer is sent into RESET. The LEM is then assumed to remain at a constant atti- Selects either internal or analog computer reference voltage. MANUAL NPUT REGSTER - For entering manual data and instructions into the interface write buffer. MANUAL WRTE - Enters the contents of the manual input register into the 770. MANUAL NTERRUPT COMMAND - Generates a manual interrupt signal. MANUAL READ - Manually duplicates a read request signal from the digital computer. ndicators READ ADDRESS - Displays the read channel address. tude, and the digital computer goes through its computations as fast as it can using larger iteration intervals. The problem is returned to real time by new initial conditions being written by the digital computer and the analog sent once more into OP ERATE. Future Studies At the time of this writing, the LEM hybrid study has been made assuming fully automatic guidance. The capability for both manual and automatic control should be available soon. The simulation will then contain, in addition to the hybrid computer complex, a fixed base, instrumented LEM cockpit, a contrql and monitor console, and external

8 900 PROCEEDNGS - FALL JONT COMPUTER CONFERENCE, 1965 Figure 4. Allocation of computer tasks. visual displays. The displays will present a view of the lunar surface, CSM, and stars as would be observed by the crew members during a major portion of the mission. With the introduction of this additional simulation hardware, computer interface requirements become much more severe. Not only are more DAC's and ADC's required but some digital-to-digital type converters as well. These are necessary since the digital computer must now read both analog (throttle and controller) signals and digital (switch positions) information as well as drive both analog and digital displays in the cockpit. CONCLUSONS The preceding has described an application of a hybrid computing technique that is now making contributions to the Apollo project. t has been shown that, for the large scale LEM computer studies, this is the only feasible method to use. The stringent speed and resolution requirements, and the complexity of the problem eliminate the all-analog and all-digital approaches. The combined analog-digital system, on the other hand, provides the engineer with a computer complex that: ", contains a considerable amount of logic and memory,

9 HYBRD COMPUTNG FOR LUNAR EXCURSON MODULE STUDES 901 provides high resolution where needed, and is capable of real-time and fast-time operation. Discussions of combined analog-digital techniques usually end with a debate. on the probability of the digital computer replacing the analog. Many believe that hybrid computation is only a part of a transitional period and that due to the rapidly increasing speed and input/output flexibility of digital machines, a decade from now they will be performing most if not all simulation-type problems. To those companies that in the past few years have expanded their analog and digital facilities and find both being used to full capacity, these prophecies are only of academic interest at present. This also is true for the personnel involved in landing men on the moon in the beginning of the next decade. They know that many times before 1970, with the aid of computers, they must simulate this feat. They are concerned with the 1965, not 1975, state of the art. The hybrid technique described here is being used today and similar techniques will be used in the next several years. For large scale simulation studies such as are being made for the LEM-for problems of such high order of complexity-the hybrid approach is the only computing method by which the required objectives may be attained. ACKNOWLEDGMENTS Regarding the implementation of the hybrid computing technique described here, the author has merely reported the work of many people. He would especially like to acknowledge the significant contributions made by R. Phagan and H. Ahders of LEM Dynamic Analysis, and J. Sachleben, G. Connelly, R. Alleva, A. Mackenzie, and J. Casey of Computing Sciences. REFERENCES 1. A. J. Burns and R. E. Kopp, "Combined Analog-Digital Simulation," presented at the 1961 Eastern Joint Computer Conference, Washington, D. C. (Dec , 1961). 2. G. Zetkov and R. Fleisig, "Dynamic Analyses of OAO Spacecraft Motion by Analog-Digital Simulation," presented at the Space Electronics Session of the 1962 RE nternational Convention, New York City, (March 29, 1962). 3. R. E. Kopp and R. J. Orford, "Linear Regression Applied to System dentification for Adaptive Control Systems," presented at the 17th Annual Meeting and Space Flight Exposition, Pan Pacific Auditorium, Los Angeles, California (Nov ,1962). 4. W. Valckenaere, R. Helm and H. Ahders, "Dynamics of Homing Guidance," Grumman Aircraft Engineering Corporation Report ADR (March 1963). 5. Adage, ncorporated, Reference Manual for the Adage 770 Hybrid Computer Linkage System) (Jan. 1964). 6. J. H. Sachleben, "Why Hybrid Computing?," Grumman Aircraft Engineering Corporation Research Department Computing Report CR 65-2 (Feb. 1965). 7. G. Connelly and F. Romani, "A Report on the ADAGE 770 Computer Link and Operating Procedures Applicable to Analog Computation," Grumman Aircraft Engineering. Corporation Research Department Computing Report CR 65-3 (March 1965).

10