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At least 991 records · Page 55

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NASA NTRS1988

Dynamo theory prediction of solar activity

The dynamo theory technique to predict decadal time scale solar activity variations is introduced. The technique was developed following puzzling correlations involved with geomagnetic precursors of solar activity. Based upon this, a dynamo theory method was developed to predict solar activity. The method was used successfully in solar cycle 21 by Schatten, Scherrer, Svalgaard, and Wilcox, after testing with 8 prior solar cycles. Schatten and Sofia used the technique to predict an exceptionally large cycle, peaking early (in 1990) with a sunspot value near 170, likely the second largest on record. Sunspot numbers are increasing, suggesting that: (1) a large cycle is developing, and (2) that the cycle may even surpass the largest cycle (19). A Sporer Butterfly method shows that the cycle can now be expected to peak in the latter half of 1989, consistent with an amplitude comparable to the value predicted near the last solar minimum.

Schatten, Kenneth H.↗

NASA NTRS1989

Design of an active helicopter control experiment at the Princeton Rotorcraft Dynamics Laboratory

In an effort to develop an active control technique for reducing helicopter vibrations stemming from the main rotor system, a helicopter model was designed and tested at the Princeton Rotorcraft Dynamics Laboratory (PRDL). A description of this facility, including its latest data acquisition upgrade, are given. The design procedures for the test model and its Froude scaled rotor system are also discussed. The approach for performing active control is based on the idea that rotor states can be identified by instrumenting the rotor blades. Using this knowledge, Individual Blade Control (IBC) or Higher Harmonic Control (HHC) pitch input commands may be used to impact on rotor dynamics in such a way as to reduce rotor vibrations. Discussed here is an instrumentation configuration utilizing miniature accelerometers to measure and estimate first and second out-of-plane bending mode positions and velocities. To verify this technique, the model was tested, and resulting data were used to estimate rotor states as well as flap and bending coefficients, procedures for which are discussed. Overall results show that a cost- and time-effective method for building a useful test model for future active control experiments was developed. With some fine-tuning or slight adjustments in sensor configuration, prospects for obtaining good state estimates look promising.

Marraffa, Andrew M.↗

NASA NTRS1989

On-orbit Metrology and Calibration Requirements for Space Station Activities Definition Study

The Space Station is the focal point for the commercial development of space. The long term routine operation of the Space Station and the conduct of future commercial activities suggests the need for in-space metrology capabilities analogous when possible to those on-Earth. The ability to perform periodic calibrations and measurements with proper traceability is imperative for the routine operation of the Space Station. An initial review, however, indicated a paucity of data related to metrology and calibration requirements for in-space operations. This condition probably exists because of the highly developmental aspect of space activities to date, their short duration, and nonroutine nature. The on-orbit metrology and calibration needs of the Space Station were examined and assessed. In order to achieve this goal, the following tasks were performed: an up-to-date literature review; identification of on-orbit calibration techniques; identification of sensor calibration requirements; identification of calibration equipment requirements; definition of traceability requirements; preparation of technology development plans; and preparation of the final report. Significant information and major highlights pertaining to each task is presented. In addition, some general (generic) conclusions/observations and recommendations that are pertinent to the overall in-space metrology and calibration activities are presented.

Cotty, G. M.↗

NASA NTRS1988

Active region coronal loops - Structural and variability

X-ray images of a pair of active region loops are studied which show significant, short time-scale variability in the line fluxes of O VIII, Ne IX, and Mg XI and in the 3.5-11.5 keV soft X-ray bands. Vector magnetograms and high-resolution UV images were used to model the three-dimensional characteristics of the loops. X-ray light curves were generated spanning four consecutive orbits for both loops individually, and light curves of the loop tops and brightest points were also generated. The largest variations involve flux changes of up to several hundred percent on time scales of 10 minutes. No significant H-alpha flare activity is reported, and loop temperatures remain in the four to six million K range. The decay phases of the light curves indicate radiative cooling, inhibition of conduction, and some type of 'continued heating' due to ongoing, underlying activity at the microflare level.

Haisch, Bernhard M.↗

NASA NTRS1989

Active polar region on the nucleus of Comet Halley

The images of the nucleus of Comet Halley returned by the Giotto spacecraft reveal a number of active regions on the surface, one of which is near the expected location of the rotation pole. This feature is larger and brighter than other source regions, suggesting that the mechanism that drives this source is also different. At this active region near the rotation pole, the sun was circumpolar for a significant portion of the solar encounter. Continuous insolation heated the nucleus to greater depths than in other areas, producing the broad, active polar region.

Reitsema, H. J.↗

NASA NTRS1989

Selection of active member locations in adaptive structures

The effective use of multiple passive and active members in adaptive structures necessitates that these members be optimally distributed throughout the structure. In truss structures, the problem falls into the class of combinatorial optimization for which the solution becomes exceedingly intractable as the problem size increases. This is overcome by using the simulated annealing algorithm to obtain near optimal locations for passive and/or active members. The maximization of the rate of energy dissipation over a finite time period as the measure of optimality is adopted. The selection of optimal locations for both passive and active members is consistently treated through the use of the energy dissipation rate criterion within the simulated annealing algorithm. Numerical examples are used to illustrate the effectiveness of the methodology for large truss structures.

Chen, G.-S.↗

NASA NTRS1989

Active-member control of precision structures

This paper presents the results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure. These experiments are directed toward the development of high performance structural systems as part of the Control/Structure Interaction program at JPL. Order of magnitude reductions in dynamic response are achieved with relatively simple control techniques. The practical implementation of high stiffness, high bandwidth active-members in a precision structure highlights specific issues of importance relating to the modelling and implementation of active-member control.

Fanson, J. L.↗

NASA NTRS1988

Active late-type stars

The basic mechanisms and characteristics of stellar activity in evolved stars are discussed, summarizing and analyzing the results of ground-based and space observations obtained in different regions of the electromagnetic spectrum. The types of active stars, the parameters controlling activity, and the underlying astrophysics are reviewed; the specific applications of optical and IR photometry, optical and UV spectroscopy, X-ray photometry and spectroscopy, and radio observations are considered; and particular attention is given to results on RS CVn systems and M dwarfs. Extensive diagrams, graphs, and tables of numerical data are included.

Linsky, Jeffrey L.↗

NASA NTRS1989

Anomalous quartz from the Roter Kamm impact crater, Namibia - Evidence for post-impact hydrothermal activity?

Quartz pebbles from the Roter Kamm impact crater (the Namib Desert, SWA/Namibia) were examined for evidence of impact-induced hydrothermal activity, using results from microprobe analyses, neutron activation analyses, transmission IR spectroscopy, and X-ray diffractometry. It was found that the pebbles consisted of pure quartz, which contains three different types of fluid inclusions. These were identified as primary inclusions (5-10 microns) that record the formation conditions of the quartz, very small (less than 1 micron) secondary inclusions associated with the grain boundaries, and late inclusions of irregular size. It is concluded that the quartz and the primary inclusions may provide evidence for a postimpact phase of extensive hydrothermal activity, generated by the residual heat from the kinetic energy of the impact.

Koeberl, Christian↗

NASA NTRS1989

Ultraviolet events observed in active regions. I - Observations and scenario

UV line data obtained in solar active regions on and near the limb, taken with the Ultraviolet Spectrometer and Polarimeter experiment on the Solar Maximum Mission are examined. The study provides insight into the physical processes behind sudden localized brightenings (or microflares) that occur within these active regions and their relation to surging activity. Time sequences of rasters and rasters through the line (taken in Ly-alpha and N V lines simultaneously) and C IV dopplergrams are the core of these data. They show the brightening events on the disk and Doppler shifts in dynamic events on the disk and above the limb. The study suggests, for the events, a localized energy deposition in a region of the chromosphere that heats the material and produces a pressure pulse. This mechanism explains the brightenings in transition region lines and also the observed surging behavior and jet-like events.

Fontenla, Juan M.↗

NASA NTRS1990

Experiments On Active Members In Large Space Structures

Report discusses continuing research on structures including active members, which incorporate sensors, actuators, and electronic circuits to monitor and control vibrations. Describes experiments on two structures with active members, progress in design, testing, and simulation of behavior of active members. Objective is to develop systems to enhance performances of large, flexible structures in space. Also applicable to some terrestrial structures and testing equipment involving close tolerances in feedback control of forces and/or positions.

Fanson, James L.↗

NASA NTRS1989

The active flexible wing aeroservoelastic wind-tunnel test program

For a specific application of aeroservoelastic technology, Rockwell International Corporation developed a concept known as the Active Flexible Wing (AFW). The concept incorporates multiple active leading-and trailing-edge control surfaces with a very flexible wing such that wing shape is varied in an optimum manner resulting in improved performance and reduced weight. As a result of a cooperative program between the AFWAL's Flight Dynamics Laboratory, Rockwell, and NASA LaRC, a scaled aeroelastic wind-tunnel model of an advanced fighter was designed, fabricated, and tested in the NASA LaRC Transonic Dynamics Tunnel (TDT) to validate the AFW concept. Besides conducting the wind-tunnel tests NASA provided a design of an Active Roll Control (ARC) System that was implemented and evaluated during the tests. The ARC system used a concept referred to as Control Law Parameterization which involves maintaining constant performance, robustness, and stability while using different combinations of multiple control surface displacements. Since the ARC system used measured control surface stability derivatives during the design, the predicted performance and stability results correlated very well with test measurements.

Noll, Thomas↗

NASA NTRS1989

The onset of the solar active cycle 22

There is a great deal of interest in being able to predict the main characteristics of a solar activity cycle (SAC). One would like to know, for instance, how large the amplitude (R sub m) of a cycle is likely to be, i.e., the annual mean of the sunspot numbers at the maximum of SAC. Also, how long a cycle is likely to last, i.e., its period. It would also be interesting to be able to predict the details, like how steep the ascending phase of a cycle is likely to be. Questions like these are of practical importance to NASA in planning the launch schedule for the low altitude, expensive spacecrafts like the Hubble Space Telescope, the Space Station, etc. Also, one has to choose a proper orbit, so that once launched the threat of an atmospheric drag on the spacecraft is properly taken into account. Cosmic ray data seem to indicate that solar activity cycle 22 will surpass SAC 21 in activity. The value of R sub m for SAC 22 may approach that of SAC 19. It would be interesting to see whether this prediction is borne out. Researchers are greatly encouraged to proceed with the development of a comprehensive prediction model which includes information provided by cosmic ray data.

Ahluwalia, H. S.↗

NASA NTRS1990

Active control of a flexible structure using a modal positive position feedback controller

The feasibility of a new Modal Positive Position Feedback (MPPF) strategy in controlling the vibration of a complex flexible structure using a single piezo-electric active structural member is demonstrated. The control strategy generates its control forces by manipulating only the modal position signals of the structure to provide a damping action to undamped modes. This is in contrast to conventional modal controllers that rely in their operation on negative feedback of both the modal position and velocity. The proposed strategy is very simple to design and implement as it designs the controller at the uncoupled modal level and utilizes simple first order filters to achieve the Positive Position Feedback effect. The performance of the new strategy is enhanced by augmenting it with a time sharing strategy to share a small number of actuators between larger number of modes. The effectiveness of the new strategy is validated experimentally on a flexible box-type structure that has four bays and its first two bending modes are 2.015 and 6.535 Hz respectively. A single piezo-electric actuator is utilized as an active structural member to control several transverse bending modes of the structure. The performance of the active control system is determined in the time and the frequency domains. The results are compared with those obtained when using the Independent Modal Space Control (IMSC) of Meirovitch. The experimental results suggest the potential of the proposed strategy as a viable means for controlling the vibration of large flexible structures in real time.

Poh, S.↗

NASA NTRS1989

Experimental evaluation of active-member control of precision structures

The results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure are described. These experiments are directed toward the development of high-performance structural systems as part of the Control/Structure Interaction (CSI) program at JPL. The focus of CSI activity at JPL is to develop the technology necessary to accurately control both the shape and vibration levels in the precision structures from which proposed large space-based observatories will be built. Structural error budgets for these types of structures will likely be in the sub-micron regime; optical tolerances will be even tighter. In order to achieve system level stability and local positioning at this level, it is generally expected that some form of active control will be required.

Fanson, James↗

NASA NTRS1990

Active control of bearing preload using piezoelectric translators

In many spacecraft applications, mechanisms are required to perform precision pointing operations or to sometimes dither about or track a moving object. These mechanisms perform in a predictable and repeatable manner in benign temperature environments. Severe thermal gradients experienced in actual space applications however, cause assemblies to expand and contract around their bearings. This results in unpredictable changes in bearing preload, and hence bearing friction. This becomes a limitation for servos controlling pointing accuracy. Likewise, uncontrollable vibrations may couple into fixed preload (hence, fixed stiffness) mechanisms and limit pointing accuracy. Consequently, a complex problem faced today is how to design mechanisms that remain insensitive to changing thermal and vibrational spacecraft environments. Research presented involves the simplified modeling and test results of an actuator module that used piezoelectrically preload controlled bearings. The feasibility of actively controlling bearing preload was demonstrated. Because bearing friction is related to preload, a thermally active system designed with aluminum components and a 440 C bearing, was friction tested at temperatures ranging from 0 to 70 C (32 to 158 F). Effectiveness of the translators were demonstrated by mapping a controllable friction range throughout tested temperatures. It was learned that constant preload for this system could be maintained over an approximate 44 C (79 F) temperature span. From testing, it was also discovered that at the more deviate temperatures, expansions were so large that radial clearances were taken up and the duplex bearing became radially preloaded. Thus, active control of bearing preload is feasible but may be limited by inherent geometry constraints and materials used in the system.

Nye, Ted W.↗

NASA NTRS1989

Influence of solar activity on middle atmosphere associated with phases of equatorial quasi-biennial oscillation

Earlier studies on the influence of solar activity variations within a 11-year solar cycle on temperature changes in the middle atmosphere revealed that while the temperature in the mesosphere showed strong responses to changes in solar activity, the stratosphere remained almost unaffected. Recent studies showed that when the temperature data were grouped into east or west phase of the equatorial quasi-biennial oscillation (QBO) in stratospheric zonal wind, significant relationships of temperature in the lower stratosphere and troposphere could be obtained with 10.7 cm solar radio flux. Positive correlations in high latitude regions and negative correlations in mid-latitude and tropical regions were obtained during winter when the QBO was in its west phase. During the east phase, converse relationships were indicated. These results inspired this study on the response of solar activity in 11-year cycle on the temperature structure of the middle atmosphere in the two phases of equatorial QBO of zonal wind at 50 mb, in tropics, mid-latitude and antarctic regions.

Mohanakumar, K.↗

NASA NTRS1989

General overview of the solar activity effects on the lower ionosphere

Solar activity influences the ionospheric D region. That influence manifests itself both in the form of various solar induced disturbances and in the form of the D region dependence on solar activity parameters (UV-flux, interplanetary magnetic field, solar wind etc.) in quiet conditions. Relationship between solar activity and meteorological control of the D region behavior is considered in detail and examples of strong variations of aeronomical parameters due to solar or meteorological events are given.

Danilov, A. D.↗