In order to display ODS's or mode shapes from a set of response time waveform measurements, they must all be acquired so that each measurement contains the response of the structure at the same moment in time. Another way of saying this is that all structural responses must contain the correct magnitude & phase relative to one another for each time sample
If all of the channels of time response are simultaneously acquired, the responses will contain the correct magnitude & phase relative to one another.
Simultaneous acquisition requires a separate sensor for each point & direction (DOF) of measurement, and a large, multi-channel data acquisition system. Since this is typically too expensive, data is usually acquired a few channels at a time in separate Measurement Sets.
During repeatable acquisition, approximately the same time waveform is obtained in the sampling window of the analyzer or acquisition system, regardless of when it is acquired.
A trigger is usually required to capture a repeatable event in the sampling window.
Repeatable acquisition will yield the same Fourier spectrum of successively sampled time waveforms, as shown below.
Acquisition of a Repeatable Event.