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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

The effects of excitation waveforms and shaker moving mass on the measured modal characteristics of a 2- by 5-foot aluminum plate

Ground vibration tests were conducted to compare and to investigate the effects of five excitation waveforms and the shaker moving mass (equipment and armature used to attach the shaker to the structure) on the experimental modal characteristics of a 2- by 5-ft aluminum plate using fast Fourier transform techniques. The five types of excitation waveforms studied were sine dwell, random, impact, sine sweep, and impulsive sine. The results showed that the experimental modal frequencies for all types of excitation were within 3 percent, while the modal damping data exhibited greater scatter. The sets of mode shapes obtained by the five types of excitation were consistent. The results of the shaker moving mass investigation on the 2- by 5-ft aluminum plate showed that modal frequency decreases and modal damping remains relatively constant with an increase in shaker moving mass. The generalized mass of the structure appears to decrease with an increase in shaker moving mass. In addition, it was seen that having a shaker near a node line can reduce some of the effects of the added shaker moving mass on the frequencies and the damping.

Voracek, David F.↗
NASA NTRS2008

Structural Dynamic Analysis of a Spacecraft Multi-DOF Shaker Table

Finite element enforced response analysis was performed on a three axis expander head shaker table to aid in the design of the table structure and vibration control system. The payload for this shaker system is a generic spacecraft with a multitude of flexible modes across a broad frequency band. A Craig-Bampton representation of the spacecraft was used to expedite analysis of multiple shaker table designs. The analysis examines the required forces in the actuators for a constant amplitude base acceleration sine sweep test, the resulting forces in the spacecraft and table attachment restraints, and the resulting accelerations on the spacecraft structure. The results show the spacecraft response is very high at the spacecraft center of gravity (CG) due to the high CG offset and cantilever effect of the low frequency spacecraft bending modes. The high response can be addressed by "notching" the input vibration levels to avoid over-testing the spacecraft. At frequencies above 25 Hz, the spacecraft modal effective masses are very small, and the response of the shaker table dominates the response. Anti-resonances of the shaker table in the frequency range of interest reduce the acceleration output and require much higher actuator forces to achieve the acceleration specification. These effects may require stiffening the shaker structure to move the modes out of the test frequency range or increasing the shaker table damping.

Pray, Carl↗
NASA NTRS2008

Structural Dynamic Analysis of a Spacecraft Multi-DOF Shaker Table

Finite element enforced response analysis was performed on a multiple degree of freedom expander head shaker table to aid in the design of the table structure and vibration control system. The payload for this shaker system is a spacecraft with a multitude of flexible modes across a broad frequency band. A Craig-Bampton representation of the spacecraft was used to expedite analysis of multiple shaker table designs. The analysis examines the required forces in the actuators for a constant amplitude base acceleration sine sweep test, the resulting forces in the spacecraft and table attachment restraints, and the resulting accelerations on the spacecraft structure. The results show the spacecraft response at low frequencies is very high near the tip due to the low order spacecraft bending and axial modes. The high response can be addressed by notching the input vibration levels to avoid overtesting the spacecraft. At frequencies above 25 Hz, the spacecraft modal effective masses are very small, and the response of the shaker table dominates the response. Anti-resonances of the shaker table in the frequency range of interest reduce the acceleration output and require much higher actuator forces to achieve the acceleration specification. These effects may require stiffening the shaker structure to move the modes out of the test frequency range or increasing the shaker table damping.

Pray, Carl↗
NASA NTRS2020

Multiple Shaker Placement for Ground Vibration Test of X-59 Aircraft Using Topology Optimization

A multiple shaker placement methodology is developed and tested using a topology optimization technique. Current multiple shaker placement methodology requires optimum accelerometer placement and optimum single-shaker placement techniques. The proposed methodology is tested using a finite element model of the X-59 Low Boom Flight Demonstrator aircraft. The effective independence and the driving point acceleration transfer function (DPATF) methods are used for the accelerometer placement study. In this study, four shakers are used to excite each mode more effectively during the ground vibration test; all the modes of interest thus are separated into four groups. Each shaker takes care of a separate group of modes. Grouping the modes of interest is performed utilizing topology optimization. The number of modes for each group therefore will be automatically decided during grouping. For each group of modes, perform the following two steps to determine optimal location of four shakers: 1) At each accelerometer location, compare the magnitude of DPATF values at natural frequencies, select the minimum value, and make a vector with these minimum values of the DPATF magnitudes for each group; and 2) Select the degrees of freedom corresponding to the maximum value of this vector. The objective function value is the maximum value of the vector with minimum value of the magnitude of the superposed acceleration transfer function. This objective function value is maximized by changing the modes for each group. Forty accelerometers are enough to have good correlation between mode shapes obtained from the reduced order model and the simulated ground vibration test.

Optimization Analysis↗
NASA NTRS2018

Comparing Free-Free and Shaker Table Model Correlation Methods Using Jim Beam

Finite element model correlation as part of a spacecraft program has always been a challenge. For any NASA mission, the coupled system response of the spacecraft and launch vehicle can be determined analytically through a Coupled Loads Analysis (CLA), as it is not possible to test the spacecraft and launch vehicle coupled system before launch. The value of the CLA is highly dependent on the accuracy of the frequencies and mode shapes extracted from the spacecraft model. NASA standards require the spacecraft model used in the final Verification Loads Cycle to be correlated by either a modal test or by comparison of the model with Frequency Response Functions (FRFs) obtained during the environmental qualification test. Due to budgetary and time constraints, most programs opt to correlate the spacecraft dynamic model during the environmental qualification test, conducted on a large shaker table. For any model correlation effort, the key has always been finding a proper definition of the boundary conditions. This paper is a correlation case study to investigate the difference in responses of a simple structure using a free-free boundary, a fixed boundary on the shaker table, and a base-drive vibration test, all using identical instrumentation. The NAVCON Jim Beam test structure, featured in the IMAC round robin modal test of 2009, was selected as a simple, well recognized and well characterized structure to conduct this investigation. First, a free-free impact modal test of the Jim Beam was done as an experimental control. Second, the Jim Beam was mounted to a large 20,000 lbf shaker, and an impact modal test in this fixed configuration was conducted. Lastly, a vibration test of the Jim Beam was conducted on the shaker table. The free-free impact test, the fixed impact test, and the base-drive test were used to assess the effect of the shaker modes, evaluate the validity of fixed-base modeling assumptions, and compare final model correlation results between these boundary conditions.

Modal Test↗
NASA NTRS2008

Virtual Shaker Testing: Simulation Technology Improves Vibration Test Performance

In the field of vibration testing, the interaction between the structure being tested and the instrumentation hardware used to perform the test is a critical issue. This is particularly true when testing massive structures (e.g. satellites), because due to physical design and manufacturing limits, the dynamics of the testing facility often couples with the test specimen one in the frequency range of interest. A further issue in this field is the standard use of a closed loop real-time vibration control scheme, which could potentially shift poles and change damping of the aforementioned coupled system. Virtual shaker testing is a novel approach to deal with these issues. It means performing a simulation which closely represents the real vibration test on the specific facility by taking into account all parameters which might impact the dynamic behavior of the specimen. In this paper, such a virtual shaker testing approach is developed. It consists of the following components: (1) Either a physical-based or an equation-based coupled electro-mechanical lumped parameter shaker model is created. The model parameters are obtained from manufacturer's specifications or by carrying out some dedicated experiments; (2) Existing real-time vibration control algorithm are ported to the virtual simulation environment; and (3) A structural model of the test object is created and after defining proper interface conditions structural modes are computed by means of the well-established Craig-Bampton CMS technique. At this stage, a virtual shaker test has been run, by coupling the three described models (shaker, control loop, structure) in a co-simulation routine. Numerical results have eventually been correlated with experimental ones in order to assess the robustness of the proposed methodology.

Ricci, Stefano↗
NASA NTRS1992

The design concept of the 6-degree-of-freedom hydraulic shaker at ESTEC

The European Space Agency (ESA) has decided to extend its test facilities at the European Space and Technology Center (ESTEC) at Noordwijk, The Netherlands, by implementing a 6-degree-of-freedom hydraulic shaker. This shaker will permit vibration testing of large payloads in the frequency range from 0.1 Hz to 100 Hz. Conventional single axis sine and random vibration modes can be applied without the need for a configuration change of the test set-up for vertical and lateral excitations. Transients occurring during launch and/or landing of space vehicles can be accurately simulated in 6-degrees-of-freedom. The performance requirements of the shaker are outlined and the results of the various trade-offs, which are investigated during the initial phase of the design and engineering program are provided. Finally, the resulting baseline concept and the anticipated implementation plan of the new test facility are presented.

Brinkman, P. W.↗
NASA NTRS2011

A Procedure for Accurately Measuring the Shaker Overturning Moment During Random Vibration Tests

Motivation: For large system level random vibration tests, there may be some concerns about the shaker's capability for the overturning moment. It is the test conductor's responsibility to predict and monitor the overturning moment during random vibration tests. If the predicted moment is close to the shaker's capability, test conductor must measure the instantaneous moment at low levels and extrapolate to higher levels. That data will be used to decide whether it is safe to proceed to the next test level. Challenge: Kistler analog formulation for computing the real-time moment is only applicable to very limited cases in which we have 3 or 4 load cells installed at shaker interface with hardware. Approach: To overcome that limitation, a simple procedure was developed for computing the overturning moment time histories using the measured time histories of the individual load cells.

predict overturning↗
NASA NTRS1973

Capabilities of electrodynamic shakers when used for mechanical shock testing

The results of a research task to investigate the capabilities of electrodynamic vibrators (shakers) to perform mechanical shock tests are presented. The simulation method employed was that of developing a transient whose shock response spectrum matched the desired shock response spectrum. Areas investigated included the maximum amplitude capabilities of the shaker systems, the ability to control the shape of the resultant shock response spectrum, the response levels induced at frequencies outside the controlled bandwidth, and the nonlinearities in structural response induced by a change in test level.

Keegan, W. B.↗
NASA NTRS1957

A wide-frequency-range air-jet shaker

This paper presents a description of a simple air-jet shaker. Its force can be calibrated statically and appears to be constant with frequency. It is relatively easy to use, and it has essentially massless characteristics. This shaker is applied to define the unstable branch of a frequency-response curve obtained for a nonlinear spring with a single degree of freedom.

VIBRATION AND FLUTTER↗
NASA NTRS2008

Vibration Response Models of a Stiffened Aluminum Plate Excited by a Shaker

Numerical models of structural-acoustic interactions are of interest to aircraft designers and the space program. This paper describes a comparison between two energy finite element codes, a statistical energy analysis code, a structural finite element code, and the experimentally measured response of a stiffened aluminum plate excited by a shaker. Different methods for modeling the stiffeners and the power input from the shaker are discussed. The results show that the energy codes (energy finite element and statistical energy analysis) accurately predicted the measured mean square velocity of the plate. In addition, predictions from an energy finite element code had the best spatial correlation with measured velocities. However, predictions from a considerably simpler, single subsystem, statistical energy analysis model also correlated well with the spatial velocity distribution. The results highlight a need for further work to understand the relationship between modeling assumptions and the prediction results.

Cabell, Randolph H.↗
NASA NTRS1966

Air bearing provides friction-free support for shaker system slip table

Air bearing system supports a shaker system slip table with minimum friction. At each corner of a square of grooves made on the table, a hole is drilled through the table and fitted with air connections. Air pressure is simultaneously fed to the four fittings forming an air bearing.

Skoff, R. W.↗
NASA NTRS1978

A study of multiple-shaker modal survey testing

The principal objective was to examine and to assess the practical value of a method of multiple-shaker sinusoidal modal vibration testing known as Asher's method. Numerical studies which simulate the application of Asher's method and a unique experimental implementation of the method were completed. Another objective of the research was to develop and to demonstrate with numerical simulation a quantitative method for determining from transfer function data the number of dominant modes of vibration in sinusoidal structural response.

Hallauer, W. L., Jr.↗
NASA NTRS1979

Design of a piezoelectric shaker for centrifuge testing

The design of a prototype piezoelectric shaker and its development to date is described. Although certain design problems remain to be solved, the piezoelectric system shows promise for adaptation to a larger payload system, such as the proposed geotechnical centrifuge at the Ames Research Center.

Canclini, J. G.↗