Modal Survey Test and Model Correlation of the CASSIOPE Spacecraft

A comprehensive modal survey test based on multi-input multi-output experimental modal analysis techniques was conducted on the CASSIOPE spacecraft. This paper describes the details of the methodology used to perform the successful experimental modal test to efficiently extract the critical modes of the spacecraft. Results from the modal test have been used to validate the analytical finite element model and to provide confidence in the structural integrity of the spacecraft design. The test was performed on the flight model of the CASSIOPE spacecraft in the final stages of integration, which included all of the payload and bus instruments and electronics boxes. The multiple-input excitation for the spacecraft was generated using two portable electrodynamic modal shakers installed on the top and bottom of the spacecraft to distribute the excitation energy and the response was measured using 81 miniature accelerometers. A digital multi-channel data acquisition system was used to record the time domain data and calculate the frequency domain spectra. Advanced modal analysis software was used to extract modal parameters from the measured data and critical modes were compared with predictions from the finite element model. Most modes identified through the experimental data compared favorably with the predictions. Nevertheless, some differences were large enough to require iterative update of the finite elelement model. The structural dynamics information from the updated finite element model was used to plan the mechanical vibration qualification test and predict the response of the spacecraft to the launch vehicle environmental loads through coupled loads analysis.

In this paper, the ML-MM estimator, a multivariable frequency-domain maximum likelihood estimator based on a modal model formulation, will be represented and improved in terms of the computational speed and the memory requirements. Basically, the design requirements to be met in the ML-MM estimator were to have accurate estimate for both of the modal parameters and their confidence limits and, meanwhile, having a clear stabilization chart which enables the user to easily select the physical modes within the selected frequency band. The ML-MM method estimates the modal parameters by directly identifying the modal model instead of identifying a rational fraction polynomial model. In the ML-MM estimator, the confidence bounds on the estimated modal parameters (i.e., frequency, damping ratios, mode shapes, etc.) are derived directly by inverting the so-called Fisher information matrix and without using many linearization formulas that are normally used when identifying rational fraction polynomial-based models. Another advantage of the ML-MM estimator lies in its potential to overcome the difficulties that the classical modal parameter estimation methods face when fitting an FRF matrix that consists of many (i.e., 4 or more) columns, i.e., in cases where many input excitation locations have to be used in the modal testing. For instance, the high damping level in acoustic modal analysis requires many excitation locations to get sufficient excitation of the modes. In this contribution, the improved ML-MM estimator will be validated and compared with some other classical modal parameter estimation methods using simulated datasets and real industrial applications.

Experimental modal analysis and vibro-acoustic testing at leonardo laboratories

Development of a large modal survey test fixture for measuring constrained-boundary modes and frequencies of space station modules and other shuttle payloads is described. Problems with fixture-to-test article interface mechanisms and the adverse effects on modal data are discussed, as well as redesign of the mechanisms. In view of the difficulties experienced in fixture development and fixed-base testing, alternative freesuspension techniques are discussed for deriving constrained vibration mode shapes for space shuttle payloads using unconstrained or free-boundary dynamic properties. These methods are the residual flexibility method, which requires measurement of a set of freeboundary modes plus the residual flexibility of the payload-to-orbiter interfaces, and the mass-additive approach, which involves the measurement of unconstrained modes of a payload with large masses attached to the interfaces.

Active vibration control of thin-plate structures with partial SCLD treatment

Sandia National Laboratories is developing the Scaled Wind Farm Technology center to enable rapid, cost-efficient testing and development of transformative wind energy technology. ATA Engineering, Inc., was contracted as part of this effort to dynamically characterize a number of components and assemblies to be used at the Scaled Wind Farm Technology center.This paper documents several of the tests that were performed and describes the approaches, results, and challenges encountered during testing. The test articles that were dynamically characterized are the Vestas V27 wind turbine blades, Vestas V27 towers in a free-free boundary condition, and Vestas V27 towers in a bolted condition.KeywordsWind turbinesModal testingModel validationInertia propertiesCorrelation and updating

Modal Test and Model Correction for Double Orbit Rocket Sled

The Topex/Poseidon satellite's structural development approach adhered to more stringent requirements than previous efforts, since this spacecraft must withstand not only launch/flight loads but also those planned for the 'protoflight' vehicle's structural and environmental tests. There requirements encompassed conservatively estimated design loads, high safety factors, and extensive analytical simulations. The structural adequacy of the satellite was verified by means of a ground testing program to comply with both JPL institutional requirements and Arianespace requirements. Modal survey tests of the satellite have been successfully completed.

Experiencies gained with present modal survey test techniques range from excellent results to unrealistic normal mode parameters. Present experimental modal analysis procedures auffer under the absence of a criterion for optimal exciter placement. In order to increase the reliability and robustness of modern identification methods, a technique for optimal exciter point selection was developed based on combinatorical optimization. Applying three force tuning procedures published in the past and a new method, reliable normal mode parameters of an analytical eleven-degree-of-freedom system and of a real structure could be determined, which verifies the performance of the proposed technique.

System identification plays a crucial role in structural dynamics and vibro-acoustic system optimization. A number of industrially- as well as socially relevant applications will be discussed. The most popular modeling approach is based on the modal analysis concept, leading to an interpretation in terms of visualized eigenmodes. Using the modal models, design improvements can be predicted and the structure optimized. The main modal testing procedures and modal parameter identification methods are reviewed, including both input/output and output-only approaches. The current critical elements in system identification for these applications are outlined and discussed in the broader context of the changing role of testing in the product engineering process. New trends in modal analysis that specifically address these problems are reviewed and illustrated with case studies. This includes the issues of instrumentation, test definition, measurement principles, and parameter estimation.

Both Gade & Herlufsen started working for Bruel and Kjaer Company, when the modern era of structural measurements began in the early 1980s. In (Brown and Allemang, Sound Vib Mag 19–25, January 2007), it is even argued that the modern era of Experimental Modal Analysis already started one or maybe two decades earlier. In 1982 we participated at IMAC I, the first international modal analysis conference at Hyatt Orlando, USA—the time considered when modal analysis changed from being a pure scientific concept to be a practical engineering tool. Many tests were at that time limited to Single Input/Single Output, SISO measurements. In 1983, we were involved in the release of Bruel and Kjaer’s first Dual Channel Signal analyzer. Today Dual channel FFT analysis is the established method of performing Frequency Response Function measurements used for what we might call classical mobility based modal testing and analysis. Over the years, we have seen progress and development in all fields of the measurement chain from the input vibration transducers via the signal conditioning to the analog to digital conversion, real time- and post-processing of functions followed by modal parameter estimation and applications of the modal model. This paper will deal with what we see (and we both have been involved with from a Bruel & Kjaer perspective) as some of the most important/biggest steps achieved in this progress. Today it is not unusual that modal analysis is performed using several hundreds of high dynamic range measurement channels connected to dedicated smart modal accelerometers (with build-in Transducer Electronic Data Sheets (TEDS)) doing Multiple Input Multiple Output measurements with automated modal parameters estimation and model correlation with large Finite Element models.

Bladed disks are critical parts of aeronautic turbojet engines requiring numerous dynamic analysis tests to fully achieve their design process. This work presents subspace state-space identification techniques for estimation of modal parameters of bladed disks using realistic although numerically generated test case data. Indeed, modal testing of bladed disks can exhibit high modal density leading to modal estimation issues. This study focuses on subspace state-space identification framework as it is an efficient way to process vector valued time series and thus to estimate modal parameters in a context of high modal density. In order to evaluate the techniques of interest, a versatile modal model has been specifically implemented to simulate data representative of full scale rotating aeronautic fan modal tests. This model is easily adjustable allowing to evaluate the identification methods in a more or less severe estimation context. The performance of the investigated methods are discussed and compared with the prescribed parameters of the model. Moreover, different techniques to estimate the order of the associated state-space model are reviewed and tested over several simulated configurations. Finally, a method to evaluate the uncertainties over the modal parameters using covariance of estimator results and pseudospectrum computation is proposed and discussed over the investigated test cases.

When a vibration test is performed, the dynamic modes of a shaker or shaker table may be excited, changing the response of the parts depending on their location on the shaker and possibly invalidating the test. However, shaker systems are difficult to model because of joints with unknown properties, unknown stiffness and damping effects of the magnetic field and because drawings of the internal components are rarely available. This work explores the use of experimental/analytical substructuring to create a test based model of a shaker. A modal test was performed and a modal model of a shaker was obtained with an adapter plate attached. This was coupled to a finite element model of the adapter plate, and the Transmission Simulator Method was used to assemble the two and to remove one copy of the adapter plate. The model thus obtained can then be used to predict the variation in the environment across the top of the shaker as different components are connected.

sAircrafts with high Mach speeds are exposed to severe thermal load, which affect the performance of aircraft components. Although several experimental studies have been performed on the mechanical behavior of aircraft structures subjected to high temperatures, the experimental research on dynamic characteristics of structures under thermal load is limited. In this work, a series of laboratory modal tests have been carried out on carbon fiber reinforced silicon carbide matrix (C/SiC) composite plates under various radiant-heating conditions. High temperatures are achieved using quartz lamps, and their dynamic responses are measured using a scanning laser vibrometer. The modal parameters are evaluated by PolyMAX at several static temperature distributions. The temperatures are acquired by both full-field noncontacting infrared thermography and contacting thermocouples. The flat plates are with both free and constrained boundary conditions and the high temperature modal survey are performed for two kinds of thickness. The stiffened plate with free boundary condition is further studied as a more practical structure. Both the changes of the modal frequency and modal shape as temperature rises are presented. The effects of the thermal loads on the modal characteristics are investigated by test. The results show that the thermal stresses have a much greater effect on the modal characteristics than the change of the material properties due to heating. The test results could provide technical material for the limited database of high-temperature modal testing results.

The results and lessons learned from a space-flight experiment, the Mir Structural Dynamics Experiment, are summarized. One of the main objectives for the experiment was to demonstrate the feasibility of performing on-orbit modal testing on large space structures to extract modal parameters that will be used to correlate mathematical models. On-orbit tests were performed in the Mir-alone and Shuttle-Mir mated configurations, and test data were recorded with a variety of existing and new instrumentation systems. Modal analysis was performed on the collected test data to extract modal parameters, that is, frequencies, damping factors, and mode shapes. Model refinements were performed on the Mir-alone and Shuttle-Mir mated configurations. The design sensitivity approach was used for refinement, which adjusts structural properties to match analytical and test modal parameters. Test and analytical responses were compared to evaluate the model verification process. The results demonstrated that on-orbit modal testing and model refinement for large space structures are feasible within operational constraints.