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At least 433 records · Page 24

Spacecraft design project multipurpose satellite bus MPS

The thrust of this project was to design not a single spacecraft, but to design a multimission bus capable of supporting several current payloads and unnamed, unspecified future payloads. Spiraling costs of spacecraft and shrinking defense budgets necessitated a fresh look at the feasibility of a multimission spacecraft bus. The design team chose two very diverse and different payloads, and along with them two vastly different orbits, to show that multimission spacecraft buses are an area where indeed more research and effort needs to be made. Tradeoffs, of course, were made throughout the design, but optimization of subsystem components limited weight and volume penalties, performance degradation, and reliability concerns. Simplicity was chosen over more complex, sophisticated and usually more efficient designs. Cost of individual subsystem components was not a primary concern in the design phase, but every effort was made to chose flight tested and flight proven hardware. Significant cost savings could be realized if a standard spacecraft bus was indeed designed and purchased in finite quantities.

Kellman, Lyle↗

Real-valued composite filters for correlation-based optical pattern recognition

Advances in the technology of optical devices such as spatial light modulators (SLMs) have influenced the research and growth of optical pattern recognition. In the research leading to this report, the design of real-valued composite filters that can be implemented using currently available SLMs for optical pattern recognition and classification was investigated. The design of real-valued minimum average correlation energy (RMACE) filter was investigated. Proper selection of the phase of the output response was shown to reduce the correlation energy. The performance of the filter was evaluated using computer simulations and compared with the complex filters. It was found that the performance degraded only slightly. Continuing the above investigation, the design of a real filter that minimizes the output correlation energy and the output variance due to noise was developed. Simulation studies showed that this filter had better tolerance to distortion and noise compared to that of the RMACE filter. Finally, the space domain design of RMACE filter was developed and implemented on the computer. It was found that the sharpness of the correlation peak was slightly reduced but the filter design was more computationally efficient than the complex filter.

Rajan, P. K.↗

Sensor filter designs for large flexible spacecraft

Traditional control methods may excite flexible modes causing degraded performance or instability of large flexible space structures (LFSS). Control techniques developed for control of LFSS requires a numerical model of the structure and some knowledge of model error. This will be increasingly difficult with the complex space structures planned for the future. If filters could be used to condition the sensor output, control design would be less demanding. This paper is presented in viewgraph format.

Nimmo, Nancy A.↗

Analysis of iced wings

A method for computing ice shapes along the leading edge of a wing and a method for predicting its aerodynamic performance degradation due to icing is described. Ice shapes are computed using an extension of the LEWICE code which was developed for airfoils. The aerodynamic properties of the iced wing are determined with an interactive scheme in which the solutions of the inviscid flow equations are obtained from a panel method and the solutions of the viscous flow equations are obtained from an inverse three-dimensional finite-difference boundary-layer method. A new interaction law is used to couple the inviscid and viscous flow solutions. The application of the LEWICE wing code to the calculation of ice shapes on a MS-317 swept wing shows good agreement with measurements. The interactive boundary-layer method is applied to a tapered ice wing in order to study the effect of icing on the aerodynamic properties of the wing at several angles of attack.

Cebeci, T.↗

Experiments with simplified computed-torque controllers for free-flying robots

The effect of neglecting base accelerations in an inertial space endpoint computed-torque (CT) controller is examined. Numerical simulations predict a rather wide set of conditions under which CT controllers can neglect base accelerations in their dynamic model with insignificant performance degradation. Two predictions are experimentally verified with a laboratory robot. These two experiments show that negligible additional endpoint controller error results with a simplified CT controller for the given robot mass parameters and manipulator articulation.

Koningstein, R.↗

Analysis of iced wings

A method for computing ice shapes along the leading edge of a wing and a method for predicting its aerodynamic performance degradation due to icing is described. Ice shapes are computed using an extension of the LEWICE code which was developed for airfoils. The aerodynamic properties of the iced wing are determined with an interactive scheme in which the solutions of the inviscid flow equations are obtained from a panel method and the solutions of the viscous flow equations are obtained from an inverse three-dimensional finite-difference boundary-layer method. A new interaction law is used to couple the inviscid and viscous flow solutions. The application of the LEWICE wing code to the calculation of ice shapes on a MS-317 swept wing show good agreement with measurements. The interactive boundary layer method is applied to a tapered iced wing in order to study the effect of icing on the aerodynamic properties of the wing at several angles of attack.

Cebeci, Tuncer↗

The behavior of a Costas loop in the presence of space telemetry signals

When the Costas loop is operated in the presence of a residual carrier space telemetry signal, the loop reconstructs its carrier reference from an input signal whose carrier component is not completely suppressed. The telemetry signal investigated in this work is generated by phase shift-keying the data onto a subcarrier and then phase modulating onto the sinusoidal carrier. The telemetry modulation index, telemetry bit rate, subcarrier waveform, and subcarrier frequency are shown to be the key system parameters that contribute to the performance degradation of a Costas loop. Furthermore, the effect of Doppler shift on the loop is also investigated in this work.

Nguyen, Tien M.↗

Portable microwave test packages for beam-waveguide antenna performance evaluations

Portable microwave test packages used to evaluate a new 34-m-diameter beam-waveguide (BWG) antenna are described. The experimental methodology involved transporting test packages to different focal points of the BWG system and making noise temperature, antenna efficiency, and holography measurements. Comparisons of data measured at the different focal points enabled determinations of performance degradations caused by various mirrors in the BWG system. It is shown that, due to remarkable stabilities and accuracies of radiometric data obtained through the use of the microwave test packages, degradations caused by the BWG system were successfully determined.

Otoshi, Tom Y.↗

Technology planning for long range utilization of Space Station Freedom

A system is described with specific examples for identifying and selecting advanced technologies for use with the Space Station Freedom (SSF) that positively impact capabilities and costs. Emerging advanced technologies should be compared with SSF technological needs and selected according to an optimal combination of readiness, development risks/costs, performance requirements, and crew safety. The selected technologies should be planned to the extent that the technology continues advancing to the point of readiness in a manner consistent with the objectives and timeframe of the SSF program. An SSF Advanced Studies component is described that worked to advance critical emergent technologies including advanced heat rejection, closed air-loops, high-efficiency space-power systems, and advanced power storage. The efforts can lead to the identification of crucial parameters such as performance degradation, lifetime, growth, and area that are relevant to the SSF overall objectives.

Ahlf, P.↗

Rapid single-axis torque-bounded slewing of SCOLE configuration

The first general research objective was to address control design challenges of the Spacecraft Control Laboratory Experiment (SCOLE) via the two stage approach: (1) slew the whole as if it were a rigid body about one Space Shuttle body axis each time using the onboard Reaction Control System (RCS) thrusters; and (2) damp out excited vibrations. The second objective was to examine the feasibility of applying the approach to shuttle-attached flexible space structures. The following was accomplished: (1) a standard bang-bang control technique was adapted; (2) a slew rate limit was imposed in the design; and (3) slew acceleration deviation was defined as the index of slew performance degradation.

Lin, Jiguan Gene↗

Extended-Task-Space Control For Robotic Manipulators

Extended-task-space concept provides unified approach for control of both redundant and nonredundant manipulators: theoretical framework for complete description of set of mechanism commands in terms of task-space commands, and singularity-avoidance scheme providing robustness at and near singular points of workspace while not degrading performance in nonsingular regions.

Backes, Paul G.↗

Implementation of a model based fault detection and diagnosis for actuation faults of the Space Shuttle main engine

In a previous study, Guo, Merrill and Duyar, 1990, reported a conceptual development of a fault detection and diagnosis system for actuation faults of the space shuttle main engine. This study, which is a continuation of the previous work, implements the developed fault detection and diagnosis scheme for the real time actuation fault diagnosis of the space shuttle main engine. The scheme will be used as an integral part of an intelligent control system demonstration experiment at NASA Lewis. The diagnosis system utilizes a model based method with real time identification and hypothesis testing for actuation, sensor, and performance degradation faults.

Duyar, A.↗

Scheduling and rescheduling with iterative repair

This paper describes the GERRY scheduling and rescheduling system being applied to coordinate Space Shuttle Ground Processing. The system uses constraint-based iterative repair, a technique that starts with a complete but possibly flawed schedule and iteratively improves it by using constraint knowledge within repair heuristics. In this paper we explore the tradeoff between the informedness and the computational cost of several repair heuristics. We show empirically that some knowledge can greatly improve the convergence speed of a repair-based system, but that too much knowledge, such as the knowledge embodied within the MIN-CONFLICTS lookahead heuristic, can overwhelm a system and result in degraded performance.

Zweben, Monte↗

Towards autonomous fuzzy control

The efficient implementation of on-line adaptation in real time is an important research problem in fuzzy control. The goal is to develop autonomous self-organizing controllers employing system-independent control meta-knowledge which enables them to adjust their control policies depending on the systems they control and the environments in which they operate. An autonomous fuzzy controller would continuously observe system behavior while implementing its control actions and would use the outcomes of these actions to refine its control policy. It could be designed to lie dormant when its control actions give rise to adequate performance characteristics but could rapidly and autonomously initiate real-time adaptation whenever its performance degrades. Such an autonomous fuzzy controller would have immense practical value. It could accommodate individual variations in system characteristics and also compensate for degradations in system characteristics caused by wear and tear. It could also potentially deal with black-box systems and control scenarios. On-going research in autonomous fuzzy control is reported. The ultimate research objective is to develop robust and relatively inexpensive autonomous fuzzy control hardware suitable for use in real time environments.

Shenoi, Sujeet↗

Performance evaluation of land mobile satellite system under fading and interference using multiple TCM by Monte-Carlo simulation

The performance of the land mobile satellite system (LMSS) was evaluated by using two trellis coded modulation (TCM) schemes under multipath fading and interference. The results were also compared with uncoded QPSK. The trellis coding formats included a trellis code designed for optimum performance on the fading channel that typifies satellite to mobile communications, as well as one designed for optimum performance in the additive white Gaussian noise (AWGN) channel. The results show that, in Rayleigh fading, the TCM code designed for optimum performance in such an environment performs 4 dB better than the TCM code designed for optimum performance in AWGN, and 10 dB better than the uncoded format, at a bit error rate (BER) of 10 exp -4. Additional results on the performance degradation due to the nonlinearities in the satellite transponder and the adjacent and cochannel interference show that the TCM system is more sensitive than the uncoded system to the phase distortion caused by these impairments.

Kwa, S. C.↗

Implementation of a model based fault detection and diagnosis technique for actuation faults of the SSME

In a previous study, Guo, Merrill and Duyar, 1990, reported a conceptual development of a fault detection and diagnosis system for actuation faults of the Space Shuttle main engine. This study, which is a continuation of the previous work, implements the developed fault detection and diagnosis scheme for the real time actuation fault diagnosis of the Space Shuttle Main Engine. The scheme will be used as an integral part of an intelligent control system demonstration experiment at NASA Lewis. The diagnosis system utilizes a model based method with real time identification and hypothesis testing for actuation, sensor, and performance degradation faults.

Duyar, A.↗

Vision-based recursive estimation of rotorcraft obstacle locations

The authors address vision-based passive ranging during nap-of-the-earth (NOE) rotorcraft flight. They consider the problem of estimating the relative location of identifiable features on nearby obstacles, assuming a sequence of noisy camera images and imperfect measurements of the camera's translation and rotation. An iterated extended Kalman filter is used to provide recursive range estimation. The correspondence problem is simplified by predicting and tracking each feature's image within the Kalman filter framework. Simulation results are presented which show convergent estimates and generally successful feature point tracking. Estimation performance degrades for features near the optical axis and for accelerating motions. Image tracking is also sensitive to angular rate.

Leblanc, D. J.↗

New definitions of pointing stability - ac and dc effects

For most imaging sensors, a constant (dc) pointing error is unimportant (unless large), but time-dependent (ac) errors degrade performance by either distorting or smearing the image. When properly quantified, the separation of the root-mean-square effects of random line-of-sight motions into dc and ac components can be used to obtain the minimum necessary line-of-sight stability specifications. The relation between stability requirements and sensor resolution is discussed, with a view to improving communication between the data analyst and the control systems engineer.

Lucke, Robert L.↗