18,419 research outputs found
Comparative evaluation of approaches in T.4.1-4.3 and working definition of adaptive module
The goal of this deliverable is two-fold: (1) to present and compare different approaches towards learning and encoding movements us- ing dynamical systems that have been developed by the AMARSi partners (in the past during the first 6 months of the project), and (2) to analyze their suitability to be used as adaptive modules, i.e. as building blocks for the complete architecture that will be devel- oped in the project. The document presents a total of eight approaches, in two groups: modules for discrete movements (i.e. with a clear goal where the movement stops) and for rhythmic movements (i.e. which exhibit periodicity). The basic formulation of each approach is presented together with some illustrative simulation results. Key character- istics such as the type of dynamical behavior, learning algorithm, generalization properties, stability analysis are then discussed for each approach. We then make a comparative analysis of the different approaches by comparing these characteristics and discussing their suitability for the AMARSi project
Middeck Active Control Experiment (MACE), phase A
A rationale to determine which structural experiments are sufficient to verify the design of structures employing Controlled Structures Technology was derived. A survey of proposed NASA missions was undertaken to identify candidate test articles for use in the Middeck Active Control Experiment (MACE). The survey revealed that potential test articles could be classified into one of three roles: development, demonstration, and qualification, depending on the maturity of the technology and the mission the structure must fulfill. A set of criteria was derived that allowed determination of which role a potential test article must fulfill. A review of the capabilities and limitations of the STS middeck was conducted. A reference design for the MACE test article was presented. Computing requirements for running typical closed-loop controllers was determined, and various computer configurations were studied. The various components required to manufacture the structure were identified. A management plan was established for the remainder of the program experiment development, flight and ground systems development, and integration to the carrier. Procedures for configuration control, fiscal control, and safety, reliabilty, and quality assurance were developed
All-propulsion design of the drag-free and attitude control of the European satellite GOCE
This paper concerns the drag-free and attitude control (DFAC) of the European Gravity field and steady-state Ocean Circulation Explorer satellite (GOCE), during the science phase. GOCE aims to determine the Earth's gravity field with high accuracy and spatial resolution, through complementary space techniques such as gravity gradiometry and precise orbit determination. Both techniques rely on accurate attitude and drag-free control, especially in the gradiometer measurement bandwidth (5-100mHz), where non-gravitational forces must be counteracted down to micronewton, and spacecraft attitude must track the local orbital reference frame with micro-radian accuracy. DFAC aims to enable the gravity gradiometer to operate so as to determine the Earth's gravity field especially in the so-called measurement bandwidth (5-100mHz), making use of ion and micro-thruster actuators. The DFAC unit has been designed entirely on a simplified discrete-time model (Embedded Model) derived from the fine dynamics of the spacecraft and its environment; the relevant control algorithms are implemented and tuned around the Embedded Model, which is the core of the control unit. The DFAC has been tested against uncertainties in spacecraft and environment and its code has been the preliminary model for final code development. The DFAC assumes an all-propulsion command authority, partly abandoned by the actual GOCE control system because of electric micro-propulsion not being fully developed. Since all-propulsion authority is expected to be imperative for future scientific and observation missions, design and simulated results are believed to be of interest to the space communit
Technical Challenges Associated with In-Air Wingtip Docking of Aircraft in Forward Flight
Autonomous in-air wingtip docking of aircraft offers significant opportunity for system level performance gains for numerous aircraft applications. Several of the technical challenges facing wingtip docking of fixed-wing aircraft are addressed in this paper, including: close proximity aerodynamic coupling; mechanisms and operations for robust docking; and relative state estimation methods. A simulation framework considering the aerodynamics, rigid-body dynamics, and vehicle controls is developed and used to perform docking sensitivity studies for a system of two 5.5% scale NASA Generic Transport Model aircraft. Additionally, proof of- concept testing of a candidate docking mechanism designed to move the primary wingtip vortex inboard suggests the viability of such an approach for achieving robust docking
Scaled bilateral teleoperation using discrete-time sliding mode controller
In this paper, the design of a discrete-time slidingmode
controller based on Lyapunov theory is presented along
with a robust disturbance observer and is applied to a piezostage
for high-precision motion. A linear model of a piezostage was
used with nominal parameters to compensate the disturbance
acting on the system in order to achieve nanometer accuracy. The
effectiveness of the controller and disturbance observer is validated
in terms of closed-loop position performance for nanometer
references. The control structure has been applied to a scaled
bilateral structure for the custom-built telemicromanipulation
setup. A piezoresistive atomic force microscope cantilever with a
built-in Wheatstone bridge is utilized to achieve the nanonewtonlevel
interaction forces between the piezoresistive probe tip and
the environment. Experimental results are provided for the
nanonewton-range force sensing, and good agreement between
the experimental data and the theoretical estimates has been
demonstrated. Force/position tracking and transparency between
the master and the slave has been clearly demonstrated after
necessary scalin
MIT Space Engineering Research Center
The Space Engineering Research Center (SERC) at MIT, started in Jul. 1988, has completed two years of research. The Center is approaching the operational phase of its first testbed, is midway through the construction of a second testbed, and is in the design phase of a third. We presently have seven participating faculty, four participating staff members, ten graduate students, and numerous undergraduates. This report reviews the testbed programs, individual graduate research, other SERC activities not funded by the Center, interaction with non-MIT organizations, and SERC milestones. Published papers made possible by SERC funding are included at the end of the report
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Development and Demonstration of a TDOA-Based GNSS Interference Signal Localization System
Background theory, a reference design, and demonstration
results are given for a Global Navigation Satellite
System (GNSS) interference localization system comprising a
distributed radio-frequency sensor network that simultaneously
locates multiple interference sources by measuring their signalsâ
time difference of arrival (TDOA) between pairs of nodes in
the network. The end-to-end solution offered here draws from
previous work in single-emitter group delay estimation, very long
baseline interferometry, subspace-based estimation, radar, and
passive geolocation. Synchronization and automatic localization
of sensor nodes is achieved through a tightly-coupled receiver
architecture that enables phase-coherent and synchronous sampling
of the interference signals and so-called reference signals
which carry timing and positioning information. Signal and crosscorrelation
models are developed and implemented in a simulator.
Multiple-emitter subspace-based TDOA estimation techniques
are developed as well as emitter identification and localization
algorithms. Simulator performance is compared to the CramérRao
lower bound for single-emitter TDOA precision. Results are
given for a test exercise in which the system accurately locates
emitters broadcasting in the amateur radio band in Austin, TX.Aerospace Engineering and Engineering Mechanic
High Contrast Imaging and Wavefront Control with a PIAA Coronagraph: Laboratory System Validation
The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high
performance coronagraph concept able to work at small angular separation with
little loss in throughput. We present results obtained with a laboratory PIAA
system including active wavefront control. The system has a 94.3% throughput
(excluding coating losses) and operates in air with monochromatic light.
Our testbed achieved a 2.27e-7 raw contrast between 1.65 lambda/D (inner
working angle of the coronagraph configuration tested) and 4.4 lambda/D (outer
working angle). Through careful calibration, we were able to separate this
residual light into a dynamic coherent component (turbulence, vibrations) at
4.5e-8 contrast and a static incoherent component (ghosts and/or polarization
missmatch) at 1.6e-7 contrast. Pointing errors are controlled at the 1e-3
lambda/D level using a dedicated low order wavefront sensor.
While not sufficient for direct imaging of Earth-like planets from space, the
2.27e-7 raw contrast achieved already exceeds requirements for a ground-based
Extreme Adaptive Optics system aimed at direct detection of more massive
exoplanets. We show that over a 4hr long period, averaged wavefront errors have
been controlled to the 3.5e-9 contrast level. This result is particularly
encouraging for ground based Extreme-AO systems relying on long term stability
and absence of static wavefront errors to recover planets much fainter than the
fast boiling speckle halo.Comment: 18 pages, 12 figures. Accepted for publication in PASP. The pointing
control scheme for this system is described in a separate paper
(Coronagraphic Low-Order Wave-Front Sensor: Principle and Application to a
Phase-Induced Amplitude Coronagraph, The Astrophysical Journal, Volume 693,
Issue 1, pp. 75-84 (2009)
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