Search NASA⌕ Search

Engineering topics

Lim, Tae W.

Publications and source records attributed to Lim, Tae W..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Interior Noise Reduction by Adaptive Feedback Vibration Control

The objective of this project is to investigate the possible use of adaptive digital filtering techniques in simultaneous, multiple-mode identification of the modal parameters of a vibrating structure in real-time. It is intended that the results obtained from this project will be used for state estimation needed in adaptive structural acoustics control. The work done in this project is basically an extension of the work on real-time single mode identification, which was performed successfully using a digital signal processor (DSP) at NASA, Langley. Initially, in this investigation the single mode identification work was duplicated on a different processor, namely the Texas Instruments TMS32OC40 DSP. The system identification results for the single mode case were very good. Then an algorithm for simultaneous two mode identification was developed and tested using analytical simulation. When it successfully performed the expected tasks, it was implemented in real-time on the DSP system to identify the first two modes of vibration of a cantilever aluminum beam. The results of the simultaneous two mode case were good but some problems were identified related to frequency warping and spurious mode identification. The frequency warping problem was found to be due to the bilinear transformation used in the algorithm to convert the system transfer function from the continuous-time domain to the discrete-time domain. An alternative approach was developed to rectify the problem. The spurious mode identification problem was found to be associated with high sampling rates. Noise in the signal is suspected to be the cause of this problem but further investigation will be needed to clarify the cause. For simultaneous identification of more than two modes, it was found that theoretically an adaptive digital filter can be designed to identify the required number of modes, but the algebra became very complex which made it impossible to implement in the DSP system used in this study. The on-line identification algorithm developed in this research will be useful in constructing a state estimator for feedback vibration control.

Lim, Tae W.↗
NASA NTRS1994

On-line, adaptive state estimator for active noise control

Dynamic characteristics of airframe structures are expected to vary as aircraft flight conditions change. Accurate knowledge of the changing dynamic characteristics is crucial to enhancing the performance of the active noise control system using feedback control. This research investigates the development of an adaptive, on-line state estimator using a neural network concept to conduct active noise control. In this research, an algorithm has been developed that can be used to estimate displacement and velocity responses at any locations on the structure from a limited number of acceleration measurements and input force information. The algorithm employs band-pass filters to extract from the measurement signal the frequency contents corresponding to a desired mode. The filtered signal is then used to train a neural network which consists of a linear neuron with three weights. The structure of the neural network is designed as simple as possible to increase the sampling frequency as much as possible. The weights obtained through neural network training are then used to construct the transfer function of a mode in z-domain and to identify modal properties of each mode. By using the identified transfer function and interpolating the mode shape obtained at sensor locations, the displacement and velocity responses are estimated with reasonable accuracy at any locations on the structure. The accuracy of the response estimates depends on the number of modes incorporated in the estimates and the number of sensors employed to conduct mode shape interpolation. Computer simulation demonstrates that the algorithm is capable of adapting to the varying dynamic characteristics of structural properties. Experimental implementation of the algorithm on a DSP (digital signal processing) board for a plate structure is underway. The algorithm is expected to reach the sampling frequency range of about 10 kHz to 20 kHz which needs to be maintained for a typical active noise control application.

Lim, Tae W.↗
NASA NTRS1994

Structural damage detection of space truss structures using best achievable eigenvectors

A method is presented by which measured modes and frequencies from a modal test can be used to determine the location and magnitude of damage in a space struss structure. The damage is located by computing the Euclidean distances between the measured mode shapes and the best achievable eigenvectors. The best achievable eigenvectors are the projection of the measured mode shapes onto the subspace defined by the refined analytical model of the structure and the measured frequencies. Loss of both stiffness and mass properties can be located and quantified. To examine the performance of the method when experimentally measured modes are employed, various damage detection studies using a laboratory eight-bay truss structure were conducted. The method performs well even though the measurement errors inevitably make the damage location more difficult.

Lim, Tae W.↗
NASA NTRS1992

Structural dynamic interaction with solar tracking control for evolutionary Space Station concepts

The sun tracking control system design of the Solar Alpha Rotary Joint (SARJ) and the interaction of the control system with the flexible structure of Space Station Freedom (SSF) evolutionary concepts are addressed. The significant components of the space station pertaining to the SARJ control are described and the tracking control system design is presented. Finite element models representing two evolutionary concepts, enhanced operations capability (EOC) and extended operations capability (XOC), are employed to evaluate the influence of low frequency flexible structure on the control system design and performance. The design variables of the control system are synthesized using a constrained optimization technique to meet design requirements, to provide a given level of control system stability margin, and to achieve the most responsive tracking performance. The resulting SARJ control system design and performance of the EOC and XOC configurations are presented and compared to those of the SSF configuration. Performance limitations caused by the low frequency of the dominant flexible mode are discussed.

Lim, Tae W.↗
NASA NTRS1992

Underlying modal data issues for detecting damage in truss structures

Independent of the modal identification techniques employed for damage detection, use of measured modal data limits the expectations for damage location. These limitations are examined using the distribution of modal strain energy and the sensitivity of the frequency and mode shapes to structural stiffness changes. For given measured modal information of specific accuracy, this examination reveals the following: (1) damage detection is feasible for members that contribute significantly to the strain energy of the measured modes, (2) the modes which are most effective in detecting damage to certain critical members can be identified, and (3) a relationship can be drawn between the accuracy of the measured modes and frequencies and damage detection feasibility.

Kashangaki, Thomas A-L.↗
NASA NTRS1992

Underlying modal data issues for detecting damage in truss structures

The modal strain energy distribution and frequency and mode shape sensitivities to structural stiffness changes are studied in order to overcome the current limits of modal data limit measurements in damage location. It is established that damage detection is feasible for members significantly contributing to the strain energy of the measured modes; the modes that are most effective in detecting the damage of critical members are identifiable, and a relationship is formulated between the accuracy of the measured modes and frequencies and the probability of damage detection.

Kashangaki, Thomas A.-L.↗
NASA NTRS1992

Structural dynamic interaction with solar tracking control for evolutionary Space Station concepts

The paper addresses the sun tracking control system design of the Solar Alpha Rotary Joint (SARJ) and the interaction of the control system with the flexible structure of Space Station Freedom (SSF) evolutionary concepts. The significant components of the Space Station pertaining to the SARJ control are described, and the tracking control system design is presented. Finite element models representing two evolutionary concepts, Enhanced Operations Capability (EOC) and Extended Operations Capability (XOC), are employed to evaluate the influence of low frequency flexible structure on the control system design and performance. The design variables of the control system are synthesized using a constrained optimization technique to meet design requirements, to provide a given level of control system stability margin, and to achieve the most responsive tracking performance. The resulting SARJ control system design and performance of the EOC and XOC configurations are presented and compared to those of the SSF configuration.

Lim, Tae W.↗
NASA NTRS1991

Sensitivity of Space Station alpha joint robust controller to structural modal parameter variations

The photovoltaic array sun tracking control system of Space Station Freedom is described. A synthesis procedure for determining optimized values of the design variables of the control system is developed using a constrained optimization technique. The synthesis is performed to provide a given level of stability margin, to achieve the most responsive tracking performance, and to meet other design requirements. Performance of the baseline design, which is synthesized using predicted structural characteristics, is discussed and the sensitivity of the stability margin is examined for variations of the frequencies, mode shapes and damping ratios of dominant structural modes. The design provides enough robustness to tolerate a sizeable error in the predicted modal parameters. A study was made of the sensitivity of performance indicators as the modal parameters of the dominant modes vary. The design variables are resynthesized for varying modal parameters in order to achieve the most responsive tracking performance while satisfying the design requirements. This procedure of reoptimization design parameters would be useful in improving the control system performance if accurate model data are provided.

Kumar, Renjith R.↗
NASA NTRS1991

Sensor placement for on-orbit modal testing

A systematic procedure of placing accelerometers for the on-orbit modal identification of large flexible space structures is addressed. Target modes for modal testing are selected by examining the modal cost of each mode. Assuming that a time-domain modal identification algorithm such as Eigensystem Realization Algorithm is employed to identify the modes from measured time response data, the sensors are placed to ensure the recovery of the target modes. As an application example of the procedure, an accelerometer placement study for the Space Station Freedom On-Orbit Modal Identification Experiment is presented.

Lim, Tae W.↗
NASA NTRS1991

Sensitivity of Space Station Alpha Joint robust controller to structural modal parameter variations

The baseline design of the photovoltaic-array sun-tracking control system is presented with special attention given to the optimization of design variables related to stability and performance. The procedures utilized to attenuate control/structure interaction are described, and controller sensitivity to variations in structural modal parameters is examined. Constrained optimization techniques determine the stability margin and tracking capability, which are then tested for sensitivity to variations in natural frequency, mode-shape amplitudes, and modal-damping ratios. The effect of variations in the modal parameters of the dominant modes on performance indicators is also examined to optimize tracking-performance responsiveness and satisfy the design requirements. It is concluded that accurate on-orbit modal data are necessary to make the reoptimization procedure effective in improving the performance of the control system.

Kumar, Renjith R.↗
NASA NTRS1990

Dynamic Characteristics of Space Station Freedom Mars and Lunar Evolution Reference Configurations

One concept for a manned mission to Mars uses an evolutionary version of Space Station Freedom (SSF) as a transportation node. The station is modified by the addition of dual keels, an upper and lower boom, additional laboratory and habitation modules, increased power and an assembly platform. With these modifications the station is called the Mars Evolution Reference Configuration (MERC). The mass of the station is 65 percent greater than the mass of SSF and its moments of inertia through the mass center are greater by approximately a factor of four. Over a period of months, several flights from Earth to low-Earth-orbit carry the components of a manned Mars piloted vehicle (MPV) to the MERC where the vehicle is constructed on the assembly platform. After each flight the station is reboosted to an appropriate altitude, such that the orbit decay due to atmospheric drag forces lowers the spacecraft to the proper altitude at the appropriate time for rendezvous with the next assembly flight. When the assembly process is completed, the MPV, which has a mass of approximately 200,000 lbm, is situated on the evolutionary station. The mass increase of the MERC with MPV system over SSF is 112 percent and the moments of inertia about axes through the mass center increase by up to a factor of 12. When the MPV is assembled, inspected and verified, the mission is ready to proceed and the MPV is moved from the station to a staging area and mated with fueled trans-Mars injection stages for the flight to Mars. This presentation describes a finite element model of the MERC formulated to investigate the expected low frequency modes and its variation with the addition of a large payload. A basic reboost procedure using near-continuous firing of reaction control system jets is proposed with off-modulation of the jets used to control flight attitude. The reboost procedure is described with the closed-loop attitude control dictating jet on/off cycling based on feedback signals which contain both the rigid body rotation information and the elastic rotations local to the attitude sensor. The presentation contains a description of the dynamic response at critical points of the station during the reboost and concludes with results of a brief study of the dynamic characteristics of a Lunar transportation node configuration.

Ayers, J. Kirk↗
NASA NTRS1990

Dynamic characteristics of a Space Station Freedom Mars evolution reference configuration

One concept for a manned mission to Mars uses as a transportation mode an evolutionary version of Space Station Freedom (SSF), termed the Mars Evolution Reference Configuration (MERC). The MERC is configured by adding to SSF dual keels, an upper and lower boom, additional laboratory and habitation modules, increased power, and an assembly platform. A finite-element model of the MERC was formulated to investigate the expected low frequency modes, and its variations with the addition of large payload. A basic reboost procedure using near-continuous firing of reaction control system jets is described with the closed-loop attitude control system. The elastic dynamic behavior at critical points during a reboost of the MERC, both with and without the Mars piloted vehicle installed, is examined.

Ayers, J. Kirk↗
NASA NTRS1990

Control/structure interaction study of a Space Station Freedom first flight concept during reboost

In the current design, the Space Station first-flight configuration will be assembled in a LEO and reboosted to a higher altitude such that the orbital decay due to atmospheric drag forces will lower the spacecraft to the proper altitude at the appropriate time to rendezvous with the next-assembly flight of the orbiter. The reboost maneuver will be performed by firing the on/off reaction control system jets to increase the forward velocity. This paper describes the design of a basic orbital reboost maneuver based on a burn-coast-burn scenario and a closed-loop attitude control system which will control the firing sequences of the jets to maintain the spacecraft attitude within required bounds. The design is then used to investigate the infuence of the elastic component response on the reboost jet firing logic.

Lim, Tae W.↗