Spacecraft dynamics characterization and control system failure detection

Two important aspects of the control of large space structures are studied: the modeling of deployed or erected structures including nonlinear joint characteristics; and the detection and isolation of failures of the components of control systems for large space structures. The emphasis in the first task is on efficient representation of the dynamics of large and complex structures having a great many joints. The initial emphasis in the second task is on experimental evaluation of FDI methodologies using ground-based facilities in place at NASA Langley Research Center and Marshall Space Flight Center. The progress to date on both research tasks is summarized

Reliability issues in active control of large flexible space structures

Efforts in this reporting period were centered on four research tasks: design of failure detection filters for robust performance in the presence of modeling errors, design of generalized parity relations for robust performance in the presence of modeling errors, design of failure sensitive observers using the geometric system theory of Wonham, and computational techniques for evaluation of the performance of control systems with fault tolerance and redundancy managemen

A dynamic measure of controllability and observability for the placement of actuators and sensors on large space structures

The degree of controllability of a large space structure is found by a four step procedure: (1) finding the minimum control energy for driving the system from a given initial state to the origin in the prescribed time; (2) finding the region of initial state which can be driven to the origin with constrained control energy and time using optimal control strategy; (3) scaling the axes so that a unit displacement in every direction is equally important to control; and (4) finding the linear measurement of the weighted "volume" of the ellipsoid in the equicontrol space. For observability, the error covariance must be reduced toward zero using measurements optimally, and the criterion must be standardized by the magnitude of tolerable errors. The results obtained using these methods are applied to the vibration modes of a free-free beam

Number and placement of control system components considering possible failures

A decision making methodology is presented which is intended to be useful in the early stages of system design, before a control system is designed in detail. The methodology accounts for the likelihood of failure among the sensors and actuators in a control system. A method to compute the degree of controllability and degree of observability of a system for a given set of actuators and sensors is also presented

Control of large flexible space structures

Progress in robust design of generalized parity relations, design of failure sensitive observers using the geometric system theory of Wonham, computational techniques for evaluation of the performance of control systems with fault tolerance and redundancy management features, and the design and evaluation od control systems for structures having nonlinear joints are described

Reliability analysis of an ultra-reliable fault tolerant control system

This report analyzes the reliability of NASA's Ultra-reliable Fault Tolerant Control System (UFTCS) architecture as it is currently envisioned for helicopter control. The analysis is extended to air transport and spacecraft control using the same computational and voter modules applied within the UFTCS architecture. The system reliability is calculated for several points in the helicopter, air transport, and space flight missions when there are initially 4, 5, and 6 operating channels. Sensitivity analyses are used to explore the effects of sensor failure rates and different system configurations at the 10 hour point of the helicopter mission. These analyses show that the primary limitation to system reliability is the number of flux windings on each flux summer (4 are assumed for the baseline case). Tables of system reliability at the 10 hour point are provided to allow designers to choose a configuration to meet specified reliability goals

Reliability issues in active control of large flexible space structures

Progress in robust failure detection and isolation, control system reconfiguration, and performance evaluation of systems having redundancy management features is briefly summarized

Component number and placement in large space structure control

Studies are carried out to assist the designer of the control system for a large flexible space structure in his choice of how many actuators and sensors to incorporate in the system, and where to locate them on the structure. The degree of controllability and minimum control energy strategy for driving the system are described

Multispectral Quantum Dots-in-a-Well Infrared Detectors Using Plasmon Assisted Cavities

We present the design, fabrication, and characterization, of multi-spectral quantum dots-in-a-well (DWELL) infrared detectors, by the integration of a surface plasmon assisted resonant cavity with the infrared detector. A square lattice and rectangular lattice cavity, formed by modifying the square lattice have been used in this design. By confining the resonant mode of the cavity to detector active region, the detector responsivity and detectivity have been improved by a factor of 5. A spectral tuning of 5.5 to 7.2 μm has been observed in the peak response of the detectors, by tuning the lattice constant of the cavity. Simulations indicate the presence of two modes of absorption, which have been experimentally verified. The use of a rectangular lattice predicts highly polarization sensitive modes in x- and y-direction, which are observed in fabricated detectors. A peak detectivity of 3.1 x 10^9 cm √(Hz)/W was measured at 77 K. This design offers a cost-effective and simple method of encoding spectral and polarization information, in infrared focal plane arrays

A multi-spectral and polarization-selective surface-plasmon resonant mid-infrared detector

We demonstrate a multi-spectral polarization sensitive mid-infrared dots-in-a-well (DWELL) photodetector utilizing surface-plasmonic resonant elements, with tailorable frequency response and polarization selectivity. The resonant responsivity of the surface-plasmon detector shows an enhancement of up to 5 times that of an unpatterned control detector. As the plasmonic resonator involves only surface patterning of the top metal contact, this method is independent of light-absorbing material and can easily be integrated with current focal plane array processing for imaging applications.Comment: 8 pages, 3 figure