Vit Babuska

Dynamic Modeling and Experimental Validation of a Cable- Loaded Panel.

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 lookedat how to calculate cable properties and the validation of these cablemodels on onedimensional beam structures with uniform cable lengths. This paper looks at how to extend that process to twodimensional spacecraftlike panels with nonuniform cable lengths. A shear beam model is used for cable properties. Twomethods of calculating the tiedown stiffness are compared.Of particular interest is whether or not handbooks of cable properties can be created ahead of time and appliedwith confidence. There are three frequency bands in which cable effects can be described. Before any cables become resonant, the cable effects are dominated bymass 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. Thedynamic cablemodelingmethod is validated as a distinct improvement over the lumped-mass characterization of cables commonly used today.

Modeling and Experimental Validation of Space Structures with Wiring Harnesses

Powerand 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 dynamicmeasurements suggest that amore complete representation of cable effects is needed to improvemodel predictive accuracy. To that end, a studywas 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 behaviorwas developed. Themodelingmethod was validatedwith a considerable amount ofmodel simulation and experimental data for a variety of cables attached to a free–free beam. At low frequencies, the cable effect was dominated bymass 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 beamwere shown to be valid for all cable families studied.As a result, thefinite elementmodelingmethod itself was validated.

Experimental Techniques and Structural Parameter Estimation Studies of Spacecraft Cables

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.

Study of Free-Free Beam Structural Dynamics Perturbations due to Mounted Cable Harnesses

Abstract : Signal and power harnesses on spacecraft buses and payloads can alter structural dynamics, as has been noted in previous flight programs. The community, however, has never undertaken a thorough study to understand the impact of harness dynamics on spacecraft structures. The Air Force Research Laboratory is leading a test and analysis program to develop fundamental knowledge of how spacecraft harnesses impact dynamics and develop tools that structural designers could use to achieve accurate predictions of cable-dressed structures. The work described in this paper involved a beam under simulated free boundary conditions that served as a validation test bed for model development.

Cable Effects on The Dynamics of Large Precision Structures

A top level overview of the effect cables have on the dynamic response of precision structures is presented. The focus of this paper is on precision, low-damping, low-first modal frequency space structures where cables are not implicitly designed to be in the load path. The paper presents the top-level, Phase I results which include pathfinder tests, an industry/government/academia survey, modeling and testing of individual cable bundles, and modeling and testing of cables on a simple structure. The end goal is to discover a set of practical approaches for updating well defined dynamical models of cableless structures. Knowledge of the cable type, position and tie-down method is assumed to be known. Simulation sensitivity analysis of the effect cables have on a precision structure has also been completed. Each section of the paper will focus on the details of each area.

System identification of space structures

Large precision spacecraft present a challenging control problem. They are modally dense, have a very large frequency bandwidth over which disturbances can affect payload performance, and are lightly damped. The performance of passive vibration isolation systems is limited by the constraints of physics and mechanical components, particularly at low frequencies. This motivates the need for an active isolation system. High fidelity system models are required to obtain the high performance control. Developing these high fidelity system models of large space structures is a challenge for experiments, algorithms, and computational resources.

Cable Effects Study: Tangents, Rabbit Holes, Dead Ends, and Valuable Results

Lessons learned during a study on the effects that electrical power and signal wiring harness cables introduce on the dynamic response of precision spacecraft is presented, along with the most significant results. The study was a three year effort to discover a set of practical approaches for updating well-defined dynamic models of harness-free structures where knowledge of the cable type, position, and tie-down method are known. Although cables are found on every satellite, the focus was on precision, low damping, and very flexible structures. Obstacles encountered, classified as tangents, rabbit holes, and dead ends, offer practical lessons for structural dynamics research. The paper traces the historical, experiential progression of the project, describing how the obstacles affected the project. First, methods were developed to estimate cable properties. Problems were encountered because of the flexible, highly damped nature of cables. A beam was used as a test article to validate experimentally derive...

Dynamics of Cable Harnesses on Large Precision Structures

This paper presents experimental results and modeling aspects for electrical power and signal cable harnesses used for space applications. Dynamics of large precision structures can be significantly influenced by subsystems such as electrical cables and harnesses as the structural mass of those structures tends to become smaller, and the quantity of attached cables continues to increase largely due to the ever increasing complexity of such structures. Contributions of cables to structural dynamic responses were observed but never studied, except for a low scale research effort conducted at the Air Force Research Laboratory, Space Vehicles Directorate (AFRL/VSSV). General observations were that at low frequencies cables have a mass loading effect while at higher frequencies they have a dissipative effect. The cables studied here adhere to space industry practices, identified through an extensive industry survey. Experimental procedures for extracting structural properties of the cables were developed. The structural properties of the cables extracted from the extensive experimental database that is being created can be used for numerical modeling of cabled structures. Explicit methods for analytical modeling of electrical cables attached to a structure in general are yet to be developed and the goal of this effort is to advance the state of the art in modeling cable harnesses mounted on lightweight spacecraft structures.

Active Vibration Control of a Deployable Optical Telescope

The U.S. Air Force Research Laboratory developed the deployable optical telescope testbed as part of the large deployable optics research program. The goal of this programwas to investigate the feasibility of a deployable space telescope concept and to mature critical enabling technologies to provide risk reduction for the general mission concept. This paper discusses many of the challenges encountered in laboratory testing of complex, sensitive, deployable systems. Implementation of a white-light interferometer for initialization and calibration, a pencil beam or single-pixel heterodyne interferometer for measuring primary mirror motion, and a Twyman–Green interferometer system for wave-front monitoring are discussed. Development of integrated system models and system identification methods for controller design are discussed. Active structural control was demonstrated to maintain optical alignment of the telescope while subjected to simulated reaction wheel disturbances, ambient vibration and atmospheric beam steering. Wave-front sensing and interferogram analysis were used to quantitatively assess optical performance, but wave-front error was not fed back to the controller.

Sequential multisine excitation signals for system identification of large space structures

In this paper, we present a novel sequence of multisine excitation signals that are more time efficient than sine dwell excitation while eliminating the effects of nonlinearity induced harmonics from the frequency response data. The sequential multisine excitation signal is demonstrated on the deployable optical telescope testbed at Air Force Research Laboratory