The most common transfer function measurement is the FRF, or frequency response function. Similar to the FFT Analyzer, the FRF is a characteristic of a system that has a measured response resulting from a known applied input. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response. This is most easily done with a multi-channel FFT Analyzer. Single input transfer function evaluation is a standard feature on SignalCalc Dynamic Signal Analyzers.
Frequency response measurements are used extensively in modal analysis of mechanical systems. Like the FFT Analyzer, the frequency response function is actually a complex measurement and includes both a real and imaginary component that can be resolved into magnitude and phase.
Frequency response functions can have various units and meanings associated with them. For instance, when performing modal analysis, it is most common to measure and calculate accelerance frequency response functions. Accelerance is acceleration per unit force. Mobility is velocity per unit force, and compliance is displacement per unit force.
Transmissibility is another common transfer function measurement. Transmissibility is evaluating the response of an input to an output in the same units. Most often phase is not of interest or in lock. Commonly this is done in acceleration output per acceleration input because accelerometers are the most common form of dynamic vibration measurement. Some applications where transmissibility is important are in evaluation of isolation systems or isolators, vibration transmission for human comfort, and vibration fixture design and evaluation.
Performing an FRF calculation with a single input to a system (SIMO) is a basic measurement, however performing calculations with multiple inputs to the system (MIMO) can become more complex and computationally intensive. As the number of references increases, the number of computations multiplies. The SignalCalc MIMO Transfer Function application permits analyzing Multiple-Input / Multiple Output (MIMO) processes and systems such as those encountered in advanced structural modal vibration studies. MIMO analysis measures the transfer functions of a system while it is stimulated by multiple shakers or environmental inputs.
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