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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Rapid tuning CW laser technique for measurements of gas velocity, temperature, pressure, density, and mass flux using NO

An intracavity-doubled rapid-tuning CW ring dye laser was used to acquire fully resolved absorption profiles of NO line pairs in the A-X band at 225 nm at a rate of 4 kHz. These profiles were utilized for simultaneous measurements of flow parameters in the high-speed 1D flows generated in a shock tube. Velocity was determined from the Doppler shift, measured using a pair of profiles simultaneously acquired at different angles with respect to the flow direction. Temperature was determined from the intensity ratio of the adjacent lines. Pressure and density were found both from the collisional broadening and the fractional absorption. From this information the mass flux was determined. The results compare well to 1D shock calculations.

Chang, Albert Y.↗
NASA NTRS1992

Optimizing tuning masses for helicopter rotor blade vibration reduction including computed airloads and comparison with test data

The development and validation of an optimization procedure to systematically place tuning masses along a rotor blade span to minimize vibratory loads are described. The masses and their corresponding locations are the design variables that are manipulated to reduce the harmonics of hub shear for a four-bladed rotor system without adding a large mass penalty. The procedure incorporates a comprehensive helicopter analysis to calculate the airloads. Predicting changes in airloads due to changes in design variables is an important feature of this research. The procedure was applied to a one-sixth, Mach-scaled rotor blade model to place three masses and then again to place six masses. In both cases the added mass was able to achieve significant reductions in the hub shear. In addition, the procedure was applied to place a single mass of fixed value on a blade model to reduce the hub shear for three flight conditions. The analytical results were compared to experimental data from a wind tunnel test performed in the Langley Transonic Dynamics Tunnel. The correlation of the mass location was good and the trend of the mass location with respect to flight speed was predicted fairly well. However, it was noted that the analysis was not entirely successful at predicting the absolute magnitudes of the fixed system loads.

Pritchard, Jocelyn I.↗
NASA NTRS1990

Solvent-tuned intramolecular charge-recombination rates in a conjugated donor-acceptor molecule

The nonradiative charge-recombination rates from the charge-transfer state of a new conjugated donor-acceptor molecule (p-cyano-p-prime-methylthiodiphenylacetylene) can be tuned over almost an order of magnitude by varying the polarity of the solvent. These measurements of intramolecular recombination show a turnover of rates as a function of emission energy, consistent with the 'normal' and 'inverted' behavior of Marcus theory. Steady-state spectra and time-resolved measurements make it possible to quantitatively compare thermal and optical electron-transfer rates as a function of driving force and demonstrate their correspondence.

Khundkar, Lutfur R.↗
NASA NTRS1992

Self-tuning control of attitude and momentum management for the Space Station

This paper presents a hybrid state-space self-tuning design methodology using dual-rate sampling for suboptimal digital adaptive control of attitude and momentum management for the Space Station. This new hybrid adaptive control scheme combines an on-line recursive estimation algorithm for indirectly identifying the parameters of a continuous-time system from the available fast-rate sampled data of the inputs and states and a controller synthesis algorithm for indirectly finding the slow-rate suboptimal digital controller from the designed optimal analog controller. The proposed method enables the development of digitally implementable control algorithms for the robust control of Space Station Freedom with unknown environmental disturbances and slowly time-varying dynamics.

Shieh, L. S.↗
NASA NTRS1992

Optimization methods for passive damper placement and tuning

The effectiveness of viscous elements in introducing damping in a structure is a function of several variables, including their number, their location in the structure, and their physical properties. In this paper several optimization problems are posed to optimize these variables. The paper investigates various metrics to define the optimization problem, and compares the damping profiles that are obtained. Both discrete and continuous optimization problems are formulated and solved, corresponding, respectively, to the problems of placement of damping elements and to the tuning of their parameters. The paper particularly emphasizes techniques to make feasible the large scale problems resulting from the optimization formulations. Numerical results involving a lightly damped tested structure are presented.

Milman, M. H.↗
NASA NTRS1992

A fixed tuned broadband matching structure for submillimeter SIS receivers

We have designed, fabricated, and tested a quasi-optical spiral antenna mixer with a Nb/AlO(x)/Nb tunnel junction. This design incorporates a hybrid antenna fed by a planar logarithmic spiral antenna to couple to the radiation field, as previously done with Pb based devices, as well as a newly designed matching circuit. This matching circuit is a relatively complex structure requiring several layers of photolithographic processing on top of the actual tunneling device. Computer modeling of the device predicted the measured bandwidth to within 8 percent, making scale model measurements unnecessary. We have obtained a good match from 210 GHz to 460 GHz between the antenna and a relatively large area (1.25 by 1.25 sq microns) tunnel junction with omega R(sub N) C approx. = 2 - 4.4. This compares to simple inductive stubs that attain only a few percent of total bandwidth in the submillimeter band or inductively tuned SIS arrays with an upper limit of operating frequencies well below the submillimeter band. Noise temperatures were measured at 345 GHz, 426 GHz, and 492 GHz yielding double sideband noise temperatures at 200 K, 220 K, and 500 K, respectively.

Buettgenbach, Thomas H.↗
NASA NTRS1992

Evaluation of integrated tuning elements with SIS devices

The resonance of integrated tuning stubs in combination with SIS detectors is measured and modeled. The predicted resonances are compared with measurements of stubs integrated with Nb/Al2O3/Nb junctions in a log-periodic antenna using a Michelson interferometer. Different stub lengths were made on different substrates (on 200 micron thick quartz and on a 7 micron thick silicon membrane) and the results show a fairly good agreement with the model calculations. Quartz substrates showed resonances up to 580 GHz, silicon membrane stub resonances reach as high as 480 GHz. An observed resonance at 560 GHz is probably a substrate effect from the membrane. The gap frequency for all the samples is 650 GHz and no resonances are detected above this frequency. Up to the maximum detected frequency dispersion is found to be negligible.

Dierichs, M. M. T. M.↗
NASA NTRS1992

Tuning a fuzzy controller using quadratic response surfaces

Response surface methodology, an alternative method to traditional tuning of a fuzzy controller, is described. An example based on a simulated inverted pendulum 'plant' shows that with (only) 15 trial runs, the controller can be calibrated using a quadratic form to approximate the response surface.

Schott, Brian↗
NASA NTRS1993

Three point lead screw positioning apparatus for a cavity tuning plate

Three lead screws are provided for adjusting the position of a traversing plate. Each of the three lead screws is threaded through a collar that is press fitted through the center of one of three pinion gears. A sun gear meshes with all three pinion gears and transversely moves the three lead screws upon actuation of a drive gear. The drive gear meshes with the sun gear and is driven by a handle or servomotor. When the handle or servomotor rotates the drive gear, the sun gear rotates causing the three pinion gears to rotate, thus, causing transverse movement of the three lead screws and, accordingly, transverse movement of the transversing plate. When the drive gear rotates, the traversing plate is driven in and out of a microwave cavity. Thus, the length or size of the cavity can be tuned while maintaining the traversing plate in an exact parallel relationship with an opposing plate on another end of the cavity.

Calco, Frank S.↗
NASA NTRS1993

Prediction of pressure drop in fluid tuned mounts using analytical and computational techniques

A simplified model for predicting pressure drop in fluid tuned isolator mounts was developed. The model is based on an exact solution to the Navier-Stokes equations and was made more general through the use of empirical coefficients. The values of these coefficients were determined by numerical simulation of the flow using the commercial computational fluid dynamics (CFD) package FIDAP.

Lasher, William C.↗
NASA NTRS1994

Tuned mass damper for integrally bladed turbine rotor

The invention is directed to a damper ring for damping the natural vibration of the rotor blades of an integrally bladed rocket turbine rotor. The invention consists of an integral damper ring which is fixed to the underside of the rotor blade platform of a turbine rotor. The damper ring includes integral supports which extend radially outwardly therefrom. The supports are located adjacent to the base portion and directly under each blade of the rotor. Vibration damping is accomplished by action of tuned mass damper beams attached at each end to the supports. These beams vibrate at a predetermined frequency during operation. The vibration of the beams enforce a local node of zero vibratory amplitude at the interface between the supports and the beam. The vibration of the beams create forces upon the supports which forces are transmitted through the rotor blade mounting platform to the base of each rotor blade. When these forces attain a predetermined design frequency and magnitude and are directed to the base of the rotor blades, vibration of the rotor blades is effectively counteracted.

Marra, John J.↗
NASA NTRS1995

An automated method of tuning an attitude estimator

Attitude determination is a major element of the operation and maintenance of a spacecraft. There are several existing methods of determining the attitude of a spacecraft. One of the most commonly used methods utilizes the Kalman filter to estimate the attitude of the spacecraft. Given an accurate model of a system and adequate observations, a Kalman filter can produce accurate estimates of the attitude. If the system model, filter parameters, or observations are inaccurate, the attitude estimates may be degraded. Therefore, it is advantageous to develop a method of automatically tuning the Kalman filter to produce the accurate estimates. In this paper, a three-axis attitude determination Kalman filter, which uses only magnetometer measurements, is developed and tested using real data. The appropriate filter parameters are found via the Process Noise Covariance Estimator (PNCE). The PNCE provides an optimal criterion for determining the best filter parameters.

Mason, Paul A. C.↗
NASA NTRS1994

Robustness of reduced-order multivariable state-space self-tuning controller

In this paper, we present a quantitative analysis of the robustness of a reduced-order pole-assignment state-space self-tuning controller for a multivariable adaptive control system whose order of the real process is higher than that of the model used in the controller design. The result of stability analysis shows that, under a specific bounded modelling error, the adaptively controlled closed-loop real system via the reduced-order state-space self-tuner is BIBO stable in the presence of unmodelled dynamics.

Yuan, Zhuzhi↗
NASA NTRS1998

Low-Loss NbTiN Films for THz SIS Mixer Tuning Circuits

Recent results at 1 THz using normal-metal tuning circuits have shown that SIS mixers can work well up to twice the gap frequency of the junction material (niobium). However, the performance at 1 THz is limited by the substantial loss in the normal metal films. For better performance superconducting films with a higher gap frequency than niobium and with low RF loss are needed. Niobium nitride has long been considered a good candidate material, but typical NbN films suffer from high RF loss. To circumvent this problem we are currently investigating the RF loss in NbTiN films, a 15 K Tc compound superconductor, by incorporating them into quasi-optical slot antenna SIS devices.

Kooi, J. W.↗
NASA NTRS1997

Multicriteria Gain Tuning for Rotorcraft Flight Controls (also entitled The Development of the Conduit Advanced Control System Design and Evaluation Interface with a Case Study Application Fly by Wire Helicopter Design)

Handling qualities analysis and control law design would seem to be naturally complimenting components of aircraft flight control system design, however these two closely coupled disciplines are often not well integrated in practice. Handling qualities engineers and control system engineers may work in separate groups within an aircraft company. Flight control system engineers and handling quality specialists may come from different backgrounds and schooling and are often not aware of the other group's research. Thus while the handling qualities specifications represent desired aircraft response characteristics, these are rarely incorporated directly in the control system design process. Instead modem control system design techniques are based on servo-loop robustness specifications, and simple representations of the desired control response. Comprehensive handling qualities analysis is often left until the end of the design cycle and performed as a check of the completed design for satisfactory performance. This can lead to costly redesign or less than satisfactory aircraft handling qualities when the flight testing phase is reached. The desire to integrate the fields of handling qualities and flight,control systems led to the development of the CONDUIT system. This tool facilitates control system designs that achieve desired handling quality requirements and servo-loop specifications in a single design process. With CONDUIT, the control system engineer is now able to directly design and control systems to meet the complete handling specifications. CONDUIT allows the designer to retain a preferred control law structure, but then tunes the system parameters to meet the handling quality requirements.

Biezad, Daniel↗
NASA NTRS1999

Gravity and Macro-Model Tuning for the Geosat Follow-on Spacecraft

The US Navy's GEOSAT Follow-On (GFO) spacecraft was launched on February 10, 1998 and the primary objective of the mission was to map the oceans using a radar altimeter. The spacecraft tracking complement consisted of GPS receivers, a laser retroreflector and Doppler beacons. Since the GPS receivers have not yet returned reliable data, the only means of providing high-quality precise orbits has been though satellite laser ranging (SLR). The spacecraft has been tracked by the international satellite laser ranging network since April 22, 1998, and an average of 7.4 passes per day have been obtained from US and participating foreign stations. Since the predicted radial orbit error due to the gravity field is two to three cm, the largest contributor to the high SLR residuals (7-10 cm RMS for five day arcs) is the mismodelling of the non-conservative forces, not withstanding the development of a three-dimensional eight-panel model and an analytical attitude model for the GFO spacecraft. The SLR residuals show a clear correlation with beta-prime (solar elevation) angle, peaking in mid-August 1998 when the beta-prime angle reached -80 to -90 degrees. In this paper we discuss the tuning of the non-conservative force model, for GFO and report the subsequent addition of the GFO tracking data to the Earth gravity model solutions.

Lemoine, Frank G.↗