 |
 |
||||||||||||
|
ATA Engineering is the leading independent company for performing modal tests/modal surveys, which we have been performing for over 25 years. We pioneered the use of random excitation for modal testing and extended the method to include multishaker random testing and polyreference data processing. We have unsurpassed experience in modal testing of aerospace and industrial systems, including large scale testing for aerospace systems with over 400 accelerometers and eight shakers. The aerospace structures we have tested include aircraft, space launch vehicles, rocket motors, and satellites. Other modal tests we have performed include robotic figures, propeller blades, generator sets, fixtures, and countless others. ATA can take responsibility for one or more phases of a typical modal survey—pretest analysis, test planning, test execution, and model correlation/updating. All of our modal testing equipment is portable and is typically transported to the customer's facility to perform on-site modal tests of large systems. In addition, ATA has a laboratory for in-house testing of small components. Additional information:
A typical modal test is shown in the figure above. This figure shows the procedure used for the General Atomic Aeronautical System's Inc. Predator Aerial Vehicle. We begin while the structure is being built and will soon be ready to be test. A full Finite Element Model (FEM) has been developed by the customer or by ATA Engineering. Analytical results and optimization algorithms are used to determine the Test-Analysis Model (TAM), ideal set of measurement locations called the test Degrees of Freedom (DOF). This is where instrumentation will be applied and measurements will be recorded. Electrodynamic shakers or impact hammers are used to apply an input into the structure. Accelerometers are used to measure the response. Frequency Response Functions (FRF) are then calculated and stored. The data quality is evaluated by analyzing signal to noise, reciprocity, coherence, spectral content of input force, FRF phase and the overall FRF appearance. After verifying quality data, the parameter estimation process begins.
Parameter estimation techniques, such as Polyreference, are used to extract the modal parameters. These include the structures natural frequencies, damping estimates, and mode shapes. Mode Indicator Functions (MIF) and stability diagrams are generated to aid in determining the poles (natural frequencies) of the structure. Once the poles are selected, the residues are calculated and then the mode shape coefficients. The mode shape coefficients are then used to animate or display the mode shapes.
MAC Plot
Technical References and Organizations ATA Engineering Technical Papers - Large selection of technical papers published by ATA Engineering and SDRC IMAC (International Modal Analysis Conference) - Conference held anually by the Society of Experimental Mechanics (SEM) Sound and Vibration Magazine - A magazine for noise and vibration. Includes buyers guides Test Magazine - Detailed listing of test products and services Vibration InstituteINCE - Institute of Noise Control EngineeringIES - Institure of Environmental Sciences and Technology SAVIAC - Shock & Vibration Information Analysis Center Modal Testing by D. J. Ewins - A good modal analysis book. The table of contents is listed here with a link to buy online. Overview of Modal Analysis Theory - ATA Presentation Curve Fitting Approach - ATA Presentation Colleges and Universities |
||||||||||||
Different
shaker locations may excite different modes. ATA engineers always
evaluate the validity of the test results and use Modal Assurance
Criteria (MAC) and orthoganality calculations to verify all the modes
are identified. Comparisons are performed between sets of test results
and also analysis results. Additional testing is performed if there are
unidentifiable modes.
