LMS Test.Lab Ground Vibration Testing is a complete solution for performing stepped sine and normal modes testing. Together with LMS Test.Lab Spectral Testing, LMS Test.Lab Structures provides excitation capacities ranging from random excitation and swept sine to stepped sine and normal modes, giving engineers an abundance of open loop and closed loop excitation possibilities.
With the V12 acquisition card, the LMS SCADAS III is a very compact system relative to the high channel count. Up to 180 channels can be built on one master frame and up to 204 channels on a slave frame. The combination of master and slave frames provides a distributed system configuration. This reduces the length of the wires and makes the test setup easier.
LMS Test.Lab Ground Vibration Testing has been set up so that channel configuration can be easily carried over from one solution to another. Once the channel setup is defined, all the acquisition applications can share the same configuration; you only have to load the appropriate acquisition tool and fix the corresponding parameters to start the measurements. This significantly speeds up the use of different excitation signals when performing a ground vibration test on an aircraft.
Random excitation
Combined with LMS Test.Lab Source Control, LMS Test.Lab Spectral Testing provides a productive state-of-the-art modal testing solution supporting structural excitation by up to 16 shakers. Random, burst random, sine, burst sine, periodic chirp excitation can be easily defined. Random excitation provides the phase separation technique which gives a first analysis of the structure. Modal analysis can be performed right after the measurements. Most modes can already be selected during a random excitation measurement. As a result, the test duration is reduced by one third. The LMS Test.Lab Spectral Testing solution provides more information on this topic.
MIMO sine testing
MIMO sine testing provides a multiple input and output swept sine acquisition and a multiple input and output stepped sine acquisition.
By exciting the structure with a sweeping sine over a period longer than one block, LMS Test.Lab MIMO Sine Sweep Testing combines the high sine excitation signal-to-noise ratio with broadband testing measurement speed. During MIMO testing, multiple sine sweeps with different phase conditions between the averages provide correct frequency response functions. Accurate sine extraction results in leakage-free response spectra, while a system identification step prior to the sine sweep helps control excitation levels and further increase the dynamic range of the 24-bit A/D converters. Harmonic distortion spectra and excitation level control following a reference profile provide insights into the structure’s nonlinear behavior.
LMS Test.Lab MIMO Sine Sweep Testing provides more information on this topic. By exciting the structure with a stepped sine, LMS Test.Lab MIMO Stepped Sine Testing combines the high sine excitation signal-to-noise ratio and front-end controlled input force and provides proper excitation on lightly damped structures. Up to 16 exciters with different types of level control are available. Fixed voltage level on exciters can be used for nonlinearity characterization under open loop conditions. Amplitude and phase control of input forces provide high accuracy in phase control closed loop during the stepping phase by a special MIMO FRF based control algorithm, using singular value decomposition for the FRF matrix inversions. Multiple frequency ranges present fine frequency resolution around resonances where necessary, and coarse resolution elsewhere to save time. LMS Test.Lab MIMO Stepped Sine Testing provides more information on this topic.
MIMO normal modes testing
LMS Test.Lab MIMO Normal Modes Testing provides a multiple input and output phase appropriation technique.
It is designed to directly measure resonance frequency, damping and modes shape on a structure. The Test.Lab MIMO Normal Modes Testing automatically adjusts the amplitude and phase of the forces injected into the structure to tune a single normal vibration mode. Manual or automatic resonance tracking and force appropriation techniques are available. The frequency and forcing vector must be identified and applied so that the complete structure vibrates only according to the desired mode and that all acceleration responses are in phase quadrature with the input force vector. LMS Test.Lab MIMO Normal Modes Testing provides more information on this topic.
Using modal data from LMS Test.Lab, researchers at the Marshall Space Flight Center created hybrid models with LMS Virtual.Lab to determine critical loading on giant rocket nozzles of NASA’s next-generation Ares launch vehicle. As part of its responsibility for many of the modal tests within the U.S. space agency, Marshall’s Structural Dynamics Test Branch also will use LMS Test.Lab for ground vibration tests of the new vehicles. In these and other projects, NASA engineers and researchers rely on LMS technologies in designing and testing advanced spacecraft that will travel the solar system in the coming decades.
One of the greatest difficulties in designing aircraft structures is that only a very limited number of prototypes are available and then only late in the development process. When prototypes finally become available, it is essential to perform the required testing in as little time as possible. This application note explains how testing teams can get the greatest possible volume of information from every test while preventing errors that would make it necessary to run tests again.
