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Lampater U.,NASA | Herter T.,Cornell University | Keas P.,CSA Engineering Inc. | Harms F.,Kayser Threde GmbH | And 4 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

During observation flights the telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is subject to disturbance excitations over a wide frequency band. The sources can be separated into two groups: inertial excitation caused by vibration of the airborne platform, and aerodynamic excitation that acts on the telescope assembly (TA) through an open port cavity. These disturbance sources constitute a major difference of SOFIA to other ground based and space observatories and achieving the required pointing accuracy of 1 arcsecond cumulative rms or better below 70 Hz in this environment is driving the design of the TA pointing and control system. In the current design it consists of two parts, the rigid body attitude control system and a feed forward based compensator of flexible TA deformation. This paper discusses the characterization and control system tuning of the as-built system. It is a process that integrates the study of the structural dynamic behavior of the TA, the resulting image motion in the focal plane, and the design and implementation of active control systems. Ground tests, which are performed under controlled experimental conditions, and in-flight characterization tests, both leading up to the early science performance capabilities of the observatory, are addressed. © 2010 SPIE.

Maly J.R.,CSA Engineering Inc. | Erickson D.,National Research Council Canada | Pargett T.J.,CSA Engineering Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

The Gemini Planet Imager (GPi) is an instrument that will mount to either of two nominally identical Telescopes, Gemini North in Hawaii and Gemini South in Chile, to perform direct imaging and spectroscopy of extra-solar planets. This 2,000-kg instrument has stringent mass, center-of-gravity, flexure, and power constraints. The Flexure Sensitive Structure (FSS) supports the main opto-mechanical sub-systems of the GPi which work in series to process and analyse the telescope optical beam. The opto-mechanical sub-systems within the FSS are sensitive to mechanical vibrations, and passive damping strategies were considered to mitigate image jitter. Based on analysis with the system finite element model (FEM) of the GPi, an array of 1-kg tuned mass dampers (TMDs) was identified as an efficient approach to damp the first two FSS flexural modes which are the main sources of jitter. It is estimated that 5% of critical damping can be added to each of these modes with the addition of 23 kg of TMD mass. This estimate is based on installing TMD units on the FSS structural members. TMD mass can be reduced by nearly 50% if the units can be installed on the opto-mechanical sub-systems within the FSS with the highest modal displacements. This paper describes the structural design and vibration response of the FSS, modal test results, and plans for implementation of the TMDs. Modal measurements of the FSS structure were made to validate the FEM and to assess the viability of TMDs for reducing jitter. The test configuration differed from the operational one because some payloads were not present and the structure was mounted to a flexible base. However, this test was valuable for understanding the primary modes that will be addressed with the TMDs and measuring the effective mass of these modes. © 2010 SPIE.

Jeon S.K.,CSA Engineering Inc. | Murphey T.W.,Air Force Research Lab
Collection of Technical Papers - AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference | Year: 2011

The increasing demand for greater CubeSat mission capabilities has led to the need for more complex deployable mechanisms within the limited packaged volume. This paper presents a meter-class deployable boom featuring a single burn wire release mechanism and motor-less deployment actuation by the stored strain energy of bi-stable tape springs. Bi-stable tape springs are rolled about two independently rotating central hubs, where the unique and controlled release of strain energy unrolls the hubs and drives boom deployment linearly outward with a nearly constant torque. At the end of deployment, the tape springs lock-out to remove the deployment degree of freedom from the structure while providing structural stiffness, derived from the two inwardly facing and offset bi-stable tape springs, spanning from end to end. The presented device has stowed dimensions measuring 5.0cm by 3.8cm by 3.8cm, well within the packaging requirements of a 1-U CubeSat. The mechanical design and deployment properties are investigated and presented. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Keas P.,CSA Engineering Inc. | Brewster R.,Orbital Sciences Corp | Guerra J.,CSA Engineering Inc. | Lampater U.,Deutsches SOFIA Institute | And 3 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in a Boeing 747SP. The telescope is housed in an open cavity and will be subjected to aeroacoustic and inertial disturbances. The image stability goal for SOFIA is 0.2 arc-seconds (RMS). Throughout the development phase of the project, analytical models were employed to predict the image stability performance of the telescope, and to evaluate pointing performance improvement measures. These analyses clearly demonstrated that key aspects which determined performance were: 1) Disturbance environment and relevant load-paths 2) Telescope modal behavior 3) Sensor and actuator placement 4) Control algorithm design The SOFIA program is now entering an exciting phase in which the characteristics of the telescope and the cavity environment are being verified through ground and airborne testing. A modal survey test (MST) was conducted in early 2008 to quantify the telescope modal behavior. We will give a brief overview of analytical methods which have been employed to assess/improve the pointing stability performance of the SOFIA telescope. In this context, we will describe the motivation for the MST, and the pre-test analysis which determined the modes of interest and the required MST sensor/shaker placement. A summary will then be given of the FEM-test correlation effort, updated end-to-end simulation results, and actual data coming from telescope activation test flights. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Babuska V.,Sandia National Laboratories | Coombs D.M.,CSA Engineering Inc. | Goodding J.C.,CSA Engineering Inc. | Ardelean E.V.,Schafer Corporation | And 2 more authors.
Journal of Spacecraft and Rockets | Year: 2010

Power- and data-handling cables, which can account for up to 30% of a satellite's dry mass, couple with the spacecraft structure and impact dynamic response. Structural dynamic measurements suggest that a more complete representation of cable effects is needed to improve model predictive accuracy. To that end, a study was performed to characterize cable harness impacts on dynamic response. From this study, a finite element modeling method supported by empirically determined cable properties and structural behavior was developed. The modeling method was validated with a considerable amount of model simulation and experimental data for a variety of cables attached to a free-free beam. At low frequencies, the cable effect was dominated by mass and stiffness, changing the apparent stiffness; damping was a secondary effect. At higher frequencies, where the cables themselves were resonant, the cable effect was dissipative, increasing the apparent damping in addition to affecting the overall frequency response. Tiedown stiffness was found to be an important, but difficult to measure, parameter. Finite element models of a cabled beam were shown to be valid for all cable families studied. As a result, the finite element modeling method itself was validated. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Goodding J.C.,CSA Engineering Inc. | Ardelean E.V.,Schafer Corporation | Babuska V.,Sandia National Laboratories | Robertson L.M.,U.S. Air force | Lane S.A.,U.S. Air force
Journal of Spacecraft and Rockets | Year: 2011

Signal and electrical power cables pose unique challenges to spacecraft structural design and are often poorly modeled or even neglected. The objective of this research was to develop test methods and analysis techniques to accurately model cable-loaded spacecraft, using linear finite element models. Test methods were developed to characterize cable extensional and bending properties when subjected to low-level lateral dynamic loads. Timoshenko beam theory, including shear and bending, was used to model cable lateral dynamics, and the model formulation applicability was validated through experiment. An algorithm was developed to estimate cable area moment of inertia and shear area factor, shear modulus product, from a single driving point mobility function. Test methods and the parameter estimation algorithm were validated, using metallic rod test specimens. Experiments were performed on cables of differing constructions and spans, to develop a database for finite element modeling validation experiments. Copyright © 2011 by the American Institute of Aeronautics and Astronautics.

Coombs D.M.,CSA Engineering Inc. | Goodding J.C.,CSA Engineering Inc. | Babuska V.,Sandia National Laboratories | Ardelean E.V.,Schafer Corporation | And 2 more authors.
Journal of Spacecraft and Rockets | Year: 2011

Power and signal cable harnesses on spacecraft are often at 10% of the total mass and can be as much as 30%. These cable harnesses can impact the structural dynamics of spacecraft significantly, specifically by damping the response. Past efforts have looked at how to calculate cable properties and the validation of these cable models on one-dimensional beam structures with uniform cable lengths. This paper looks at how to extend that process to two-dimensional spacecraftlike panels with nonuniform cable lengths. A shear beam model is used for cable properties. Two methods of calculating the tiedown stiffness are compared. Of particular interestis whetherornot handbooks of cable properties canbe created ahead of time and applied with confidence. There are three frequency bands inwhich cable effects canbe described. Before any cables become resonant, the cable effects are dominated by mass and static stiffness. After all the cables become resonant, the effect is dominated by increased damping in the structure. In between these two frequency cutoff points, there is a transition zone. The dynamic cable modeling methodis validated as a distinct improvement over the lumped-mass characterization of cables commonly used today. Copyright © 2011 by the American Institute of Aeronautics and Astronautics,.

Gibert J.M.,Clemson University | Austin E.M.,CSA Engineering Inc. | Fadel G.,Clemson University
Rapid Prototyping Journal | Year: 2010

Purpose - The purpose of this paper is to focus on the changing dynamics of the ultrasonic consolidation (UC) process due to changes in substrate geometry. Past research points to a limiting height to width ranging from 0.7 to 1.2 on build features. Design/methodology/approach - Resonances of a build feature due to a change in geometry are examined and then a simple non-linear dynamic model of the UC process is constructed that examines how the geometry change may influence the overall dynamics of the process. This simple model is used to provide estimates of how substrate geometry affects the differential motion at the bonding interface and the amount of energy emitted by friction change due to build height. The trends of changes in natural frequency, differential motion, and frictional energy are compared to experimental limits on build height. Findings - The paper shows that, at the nominal build, dimensions of the feature the excitation caused by the UC approach two resonances in the feature. In addition trends in regions of changes of differential motion, force of friction, and frictional energy follow the experimental limit on build height. Originality/value - This paper explores several aspects of the UC process not currently found in the current literature: examining the modal properties of build features, and a lumped parameter dynamic model to account for the changes in of the substrate geometry. © Emerald Group Publishing Limited [ISSN 1355-2546].

Sneed R.C.,CSA Engineering Inc. | Cash M.F.,CSA Engineering Inc. | Chambers T.S.,CSA Engineering Inc. | Janzen P.C.,CSA Engineering Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

Secondary mirrors for large ground-based telescopes often require positioning systems with payload capacities around 1000 kg, relative accuracies within a few micrometers, and resonant frequencies above 15 Hz. A suitable six-legged parallel manipulator, or hexapod, has been developed for sub-micron level positioning of large optical payloads in six degrees of freedom. This 1000 kg class hexapod has tip/tilt rotational ranges of ±1800 arcsec, relative accuracies within 1%, and resolutions of better than ±0.2 arcsec, along with a piston translational range of ±30 mm, relative accuracy within 1%, and resolution of better than ±1 μm. The center of rotation of the system may be placed at an arbitrary location within the overall range limitations. The axial stiffness of each of the six actuators tested greater than 100 N/μm. The actuators use high precision roller screws and employ two degree of freedom universal end-joints. The preload on the joints eliminates backlash due to transitions from tension to compression and maintains friction moment of <10 Nm. An additional rotational degree of freedom is allowed in the body of the actuator to achieve the proper kinematic constraints for the motion platform. The actuators have power-off hold capability to protect against power loss and reduce heat dissipation. Overall heat dissipation has been measured and techniques have been studied to reduce its impact. The paper describes the actuator design and hexapod performance in support of planned use in ground test and validation of the James Webb Space Telescope. © 2010 SPIE.

Pendleton S.C.,CSA Engineering Inc. | Basile J.P.,CSA Engineering Inc. | Fowler E.,CSA Engineering Inc.
Collection of Technical Papers - AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference | Year: 2010

A dual payload flat plate adapter (FPA) supports co-manifested primary payloads allowing one rocket launch to support two satellites. The payloads may be part of a single mission requiring a tandem satellite launch or the payloads may be piggybacked on the same launch vehicle to maximize access to space for small satellites. Multi-payload adapters ensure launch vehicle excess space lift capacity is not wasted, providing small satellite manufacturers lower cost to orbit. Requirements for FPAs include designs that are optimized for both stiffness and weight as well as being low in design and manufacturing cost. This paper details two FPAs through design, manufacture, test and delivery. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

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