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Ground Vibration and Normal Mode Testing

Ground Vibration and Normal Mode Testing

Ground Vibration Tests, or GVTs, are modal tests on aircraft and spacecraft. A ground vibration test has a unique combination of the importance of structural dynamics in aircraft and safety requirements. As such, data quality and accurate analysis results are of utmost importance. The modal characteristics obtained from a GVT are often used for mathematical updating of both modal and flight flutter models, assessment of structural modifications, and gathering auxiliary data for flight flutter tests.

Most aircraft modal tests excite the structure with multiple modal shakers. Data Physics offers modal shakers in high force ranges for large scale modal tests. Learn more about SignalForce Modal Shakers.

A ground vibration test or GVT almost always involves multiple inputs and multiple outputs (MIMO) frequency response testing. Random excitation is sometimes used, but most often stepped sine techniques are used to supply the required energy for the system. Stepped sine testing allows the user to obtain test results by exciting a single frequency at a time and sweeping through frequencies. This application is closed loop and allows the user to specify different measurement bands with different sweep speed and resolution. Stepped sine testing is particularly useful when testing systems that contain composites and display non-linear behavior or require larger force levels for adequate excitation.

Another nice feature of SignalCalc analyzers is the ability to have distributed DSP. The front end hardware chassis can be distributed to various locations under the aircraft and linked with a timing cable for extremely accurate phase matching. Channel to channel phase specifications do not change whether using one chassis or multiple chassis. This distribution of the front end aids with cable management as well. Uniform length cables can be used, ribbon cables or other multi-signal cables do not need to be considered, and errors from cable noise are reduced. Reduced setup time is an additional benefit achieved with simplified cable management.

Normal Mode Testing

Normal mode testing, also called appropriated excitation testing, is often used for aircraft vibration tests and other large scale, highly critical vibration tests. With normal mode testing, several coherent sinusoidal forces are applied to the structure at the resonance frequency. The amplitude and phase of these force inputs are “tuned” to excite only a single mode. By doing this, modal separation is achieved physically rather than mathematically and each mode is viewed in its purest possible form. Once the mode is physically isolated, modal parameters and modal shapes are extracted realtime.

Stepped Sine and Normal Mode Testing are two key applications that make SignalCalc Dynamic Signal Analyzers well suited for large channel count modal acquisition. Learn more about SignalCalc Dynamic Signal Analyzers.