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At least 289 records · Page 16

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NASA NTRS1989

Digital robust control law synthesis using constrained optimization

Development of digital robust control laws for active control of high performance flexible aircraft and large space structures is a research area of significant practical importance. The flexible system is typically modeled by a large order state space system of equations in order to accurately represent the dynamics. The active control law must satisy multiple conflicting design requirements and maintain certain stability margins, yet should be simple enough to be implementable on an onboard digital computer. Described here is an application of a generic digital control law synthesis procedure for such a system, using optimal control theory and constrained optimization technique. A linear quadratic Gaussian type cost function is minimized by updating the free parameters of the digital control law, while trying to satisfy a set of constraints on the design loads, responses and stability margins. Analytical expressions for the gradients of the cost function and the constraints with respect to the control law design variables are used to facilitate rapid numerical convergence. These gradients can be used for sensitivity study and may be integrated into a simultaneous structure and control optimization scheme.

Mukhopadhyay, Vivekananda↗

NASA NTRS1989

Digitally modulated bit error rate measurement system for microwave component evaluation

The NASA Lewis Research Center has developed a unique capability for evaluation of the microwave components of a digital communication system. This digitally modulated bit-error-rate (BER) measurement system (DMBERMS) features a continuous data digital BER test set, a data processor, a serial minimum shift keying (SMSK) modem, noise generation, and computer automation. Application of the DMBERMS has provided useful information for the evaluation of existing microwave components and of design goals for future components. The design and applications of this system for digitally modulated BER measurements are discussed.

Shalkhauser, Mary Jo W.↗

NASA NTRS1989

A geometric comparison of video camera-captured raster data to vector-parented raster data generated by the X-Y digitizing table

The relative accuracy of a georeferenced raster data set captured by the Megavision 1024XM system using the Videk Megaplus CCD cameras is compared to a georeferenced raster data set generated from vector lines manually digitized through the ELAS software package on a Summagraphics X-Y digitizer table. The study also investigates the amount of time necessary to fully complete the rasterization of the two data sets, evaluating individual areas such as time necessary to generate raw data, time necessary to edit raw data, time necessary to georeference raw data, and accuracy of georeferencing against a norm. Preliminary results exhibit a high level of agreement between areas of the vector-parented data and areas of the captured file data where sufficient control points were chosen. Maps of 1:20,000 scale were digitized into raster files of 5 meter resolution per pixel and overall error in RMS was estimated at less than eight meters. Such approaches offer time and labor-saving advantages as well as increasing the efficiency of project scheduling and enabling the digitization of new types of data.

Swalm, C.↗

NASA NTRS1990

Distributed digital signal processors for multi-body structures

Several digital filter designs were investigated which may be used to process sensor data from large space structures and to design digital hardware to implement the distributed signal processing architecture. Several experimental tests articles are available at NASA Langley Research Center to evaluate these designs. A summary of some of the digital filter designs is presented, an evaluation of their characteristics relative to control design is discussed, and candidate hardware microcontroller/microcomputer components are given. Future activities include software evaluation of the digital filter designs and actual hardware inplementation of some of the signal processor algorithms on an experimental testbed at NASA Langley.

Lee, Gordon K.↗

NASA NTRS1990

Comparison of digital controllers used in magnetic suspension and balance systems

Dynamic systems that were once controlled by analog circuits are now controlled by digital computers. Presented is a comparison of the digital controllers presently used with magnetic suspension and balance systems. The overall responses of the systems are compared using a computer simulation of the magnetic suspension and balance system and the digital controllers. The comparisons include responses to both simulated force and position inputs. A preferred digital controller is determined from the simulated responses.

Kilgore, William A.↗

NASA NTRS1991

Development and testing of controller performance evaluation methodology for multi-input/multi-output digital control systems

Described here is the development and implementation of on-line, near real time controller performance evaluation (CPE) methods capability. Briefly discussed are the structure of data flow, the signal processing methods used to process the data, and the software developed to generate the transfer functions. This methodology is generic in nature and can be used in any type of multi-input/multi-output (MIMO) digital controller application, including digital flight control systems, digitally controlled spacecraft structures, and actively controlled wind tunnel models. Results of applying the CPE methodology to evaluate (in near real time) MIMO digital flutter suppression systems being tested on the Rockwell Active Flexible Wing (AFW) wind tunnel model are presented to demonstrate the CPE capability.

Pototzky, Anthony↗

NASA NTRS1990

A class of systolizable IIR digital filters and its design for proper scaling and minimum output roundoff noise

A class of infinite impulse response (IIR) digital filters with a systolizable structure is proposed and its synthesis is investigated. The systolizable structure consists of pipelineable regular modules with local connections and is suitable for VLSI implementation. It is capable of achieving high performance as well as high throughput. This class of filter structure provides certain degrees of freedom that can be used to obtain some desirable properties for the filter. Techniques of evaluating the internal signal powers and the output roundoff noise of the proposed filter structure are developed. Based upon these techniques, a well-scaled IIR digital filter with minimum output roundoff noise is designed using a local optimization approach. The internal signals of all the modes of this filter are scaled to unity in the l2-norm sense. Compared to the Rao-Kailath (1984) orthogonal digital filter and the Gray-Markel (1973) normalized-lattice digital filter, this filter has better scaling properties and lower output roundoff noise.

Lei, Shaw-Min↗

NASA NTRS1991

Integration of airborne Thematic Mapper Simulator (TMS) data and digitized aerial photography via an ISH transformation

A simple method for enhancing the spatial and spectral resolution of disparate data sets is presented. Two data sets, digitized aerial photography at a nominal spatial resolution 3,7 meters and TMS digital data at 24.6 meters, were coregistered through a bilinear interpolation to solve the problem of blocky pixel groups resulting from rectification expansion. The two data sets were then subjected to intensity-saturation-hue (ISH) transformations in order to 'blend' the high-spatial-resolution (3.7 m) digitized RC-10 photography with the high spectral (12-bands) and lower spatial (24.6 m) resolution TMS digital data. The resultant merged products make it possible to perform large-scale mapping, ease photointerpretation, and can be derived for any of the 12 available TMS spectral bands.

Ambrosia, Vincent G.↗

NASA NTRS1992

Computer aided design of digital controller for radial active magnetic bearings

A five degree of freedom Active Magnetic Bearing (AMB) system is developed which is controlled by digital controllers. The model of the radial AMB system is linearized and the state equation is derived. Based on the state variables feedback theory, digital controllers are designed. The performance of the controllers are evaluated according to experimental results. The Computer Aided Design (CAD) method is used to design controllers for magnetic bearings. The controllers are implemented with a digital signal processing (DSP) system. The control algorithms are realized with real-time programs. It is very easy to change the controller by changing or modifying the programs. In order to identify the dynamic parameters of the controlled magnetic system, a special experiment was carried out. Also, the online Recursive Least Squares (RLS) parameter identification method is studied. It can be realized with the digital controllers. Online parameter identification is essential for the realization of an adaptive controller.

Cai, Zhong↗

NASA NTRS1992

Digital control algorithms for microgravity isolation systems

New digital control algorithms were developed to achieve the desired acceleration transmissibility function. The attractive electromagnets have been taken as actuators. The relative displacement and the acceleration of the mass were used as feedback signals. Two approaches were developed to find that controller transfer function in Z-domain, which yields the desired transmissibility at each frequency. In the first approach, the controller transfer function is obtained by assuming that the desired transmissibility is known in Z-domain. Since the desired transmissibility H sub d(S) = 1/(tauS+1)(exp 2) is given in S-domain, the first task is to obtain the desired transmissibility in Z-domain. There are three methods to perform this task: bilinear transformation, and backward and forward rectangular rules. The bilinear transformation and backward rectangular rule lead to improper controller transfer functions, which are physically not realizable. The forward rectangular rule does lead to a physically realizable controller. However, this controller is found to be marginally stable because of a pole at Z=1. In order to eliminate this pole, a hybrid control structure is proposed. Here the control input is composed of two parts: analog and digital. The analog input simply represents the velocity (or the integral of acceleration) feedback; and the digital controller which uses only relative displacement signal, is then obtained to achieve the desired closed-loop transfer function. The stability analysis indicates that the controller transfer function is stable for typical values of sampling period. In the second approach, the aforementioned hybrid control structure is again used. First, an analog controller transfer function corresponding to relative displacement feedback is obtained to achieve the transmissibility as 1/(tauS+1)(exp 2). Then the transfer function for the digital control input is obtained by discretizing this analog controller transfer function via bilinear transformation. The stability of the resulting Z-domain closed loop system is analyzed. Also, the frequency response of the Z-domain closed-loop transfer function is determined to evaluate the performance of the control system.

Sinha, Alok↗

NASA NTRS1993

Analog-To-Digital Converter For Sum-Of-Squares Measurements

Analog-to-digital converter (ADC) circuit designed as part of larger circuit intended to measure root-mean-square current on 20-kHz powerline. Provides digital output of 11 bits of data plus 1-bit overflow signal at sampling rate of 4 MHz. Output processed by multiplying-and-accumulating circuitry to obtain sum of squares and digitized current samples accumulated during preset number of consecutive sampling periods. Used to digitize current samples from ADC directly as alternative or auxiliary output. Notable features include low power and fast conversion.

Osborn, Stephen H.↗

NASA NTRS1993

Digital Electronic Still Camera

Digital electronic still camera part of electronic recording, processing, tansmitting, and displaying system. Removable hard-disk drive in camera serves as digital electronic equivalent of photographic film. Images viewed, analyzed, or transmitted quickly. Camera takes images of nearly photographic quality and stores them in digital form. Portable, hand-held, battery-powered unit designed for scientific use. Camera used in conjunction with playback unit also serving as transmitting unit if images sent to remote station. Remote station equipped to store, process, and display images. Digital image data encoded with error-correcting code at playback/transmitting unit for error-free transmission to remote station.

Holland, Samuel D.↗

NASA NTRS1992

A monitor for the laboratory evaluation of control integrity in digital control systems operating in harsh electromagnetic environments

This paper presents a strategy for dynamically monitoring digital controllers in the laboratory for susceptibility to electromagnetic disturbances that compromise control integrity. The integrity of digital control systems operating in harsh electromagnetic environments can be compromised by upsets caused by induced transient electrical signals. Digital system upset is a functional error mode that involves no component damage, can occur simultaneously in all channels of a redundant control computer, and is software dependent. The motivation for this work is the need to develop tools and techniques that can be used in the laboratory to validate and/or certify critical aircraft controllers operating in electromagnetically adverse environments that result from lightning, high-intensity radiated fields (HIRF), and nuclear electromagnetic pulses (NEMP). The detection strategy presented in this paper provides dynamic monitoring of a given control computer for degraded functional integrity resulting from redundancy management errors, control calculation errors, and control correctness/effectiveness errors. In particular, this paper discusses the use of Kalman filtering, data fusion, and statistical decision theory in monitoring a given digital controller for control calculation errors.

Belcastro, Celeste M.↗

NASA NTRS1991

Stewardship of very large digital data archives

An archive is a permanent store. There are relatively few very large digital data archives in existence. Most business records are expired within five or ten years. Many kinds of business records that do have long lives are embedded in data bases that are continually updated and re-issued cyclically. Also, a great deal of permanent business records are actually archived as microfilm, fiche, or optical disk images - their digital version being an operational convenience rather than an archive. The problems forseen in stewarding the very large digital data archives that will accumulate during the mission of the Earth Observing System (EOS) are addressed. It focuses on the function of shepherding archived digital data into an endless future. Stewardship entails storing and protecting the archive and providing meaningful service to the community of users. The steward will (1) provide against loss due to physical phenomena; (2) assure that data is not lost due to storage technology obsolescence; and (3) maintain data in a current formatting methodology.

Savage, Patric↗

NASA NTRS1993

Real-time transmission of digital video using variable-length coding

Huffman coding is a variable-length lossless compression technique where data with a high probability of occurrence is represented with short codewords, while 'not-so-likely' data is assigned longer codewords. Compression is achieved when the high-probability levels occur so frequently that their benefit outweighs any penalty paid when a less likely input occurs. One instance where Huffman coding is extremely effective occurs when data is highly predictable and differential coding can be applied (as with a digital video signal). For that reason, it is desirable to apply this compression technique to digital video transmission; however, special care must be taken in order to implement a communication protocol utilizing Huffman coding. This paper addresses several of the issues relating to the real-time transmission of Huffman-coded digital video over a constant-rate serial channel. Topics discussed include data rate conversion (from variable to a fixed rate), efficient data buffering, channel coding, recovery from communication errors, decoder synchronization, and decoder architectures. A description of the hardware developed to execute Huffman coding and serial transmission is also included. Although this paper focuses on matters relating to Huffman-coded digital video, the techniques discussed can easily be generalized for a variety of applications which require transmission of variable-length data.

Bizon, Thomas P.↗

NASA NTRS1993

Digital Image Compression Using Artificial Neural Networks

The problem of storing, transmitting, and manipulating digital images is considered. Because of the file sizes involved, large amounts of digitized image information are becoming common in modern projects. Our goal is to described an image compression transform coder based on artificial neural networks techniques (NNCTC). A comparison of the compression results obtained from digital astronomical images by the NNCTC and the method used in the compression of the digitized sky survey from the Space Telescope Science Institute based on the H-transform is performed in order to assess the reliability of the NNCTC.

Serra-Ricart, M.↗

NASA NTRS1991

Digital modelling, ideal state reconstructor, and control for time-delay sampled-data systems

The cascaded discrete-time state-space representation of a cascaded continuous-time system with fractional input delays is established. Based on the time-delay digital modelling, a practically implementable ideal state reconstructor is also established such that system states are exactly reconstructed via the measurement histories of inputs and outputs without a state observer. By utilizing the blockpulse function approximation the digital modelling of cascaded continuous-time systems with fractional input delays can be carried out, and an artificial input design method is proposed to determine the state feedback gain. Thus the practically implementable digital control law can be established for digital control of time-delay sampled-data systems. An illustrative example is shown to demonstrate the effectiveness of the proposed method.

Tsai, J. S. H.↗

NASA NTRS1993

Aircraft digital flight control technical review

The Aircraft Digital Flight Control Technical Review was initiated by two pilot induced oscillation (PIO) incidents in the spring and summer of 1992. Maj. Gen. Franklin (PEO) wondered why the Air Force development process for digital flight control systems was not preventing PIO problems. Consequently, a technical review team was formed to examine the development process and determine why PIO problems continued to occur. The team was also to identify the 'best practices' used in the various programs. The charter of the team was to focus on the PIO problem, assess the current development process, and document the 'best practices.' The team reviewed all major USAF aircraft programs with digital flight controls, specifically, the F-15E, F-16C/D, F-22, F-111, C-17, and B-2. The team interviewed contractor, System Program Office (SPO), and Combined Test Force (CTF) personnel on these programs. The team also went to NAS Patuxent River to interview USN personnel about the F/A-18 program. The team also reviewed experimental USAF and NASA systems with digital flight control systems: X-29, X-31, F-15 STOL and Maneuver Technology Demonstrator (SMTD), and the Variable In-Flight Stability Test Aircraft (VISTA). The team also discussed the problem with other experts in the field including Ralph Smith and personnel from Calspan. The major conclusions and recommendations from the review are presented.

Davenport, Otha B.↗