Virtual point methodology is an innovative concept well-suited for all of the source characterization and TPA activities described above. In this context, the concept of the virtual point comes from the need to specify forces as well as moments exactly at an interface.
In a computer-based simulation, these models are easily obtained by calculating the dynamics at any discrete location with six degrees of freedom (DoF) information. Within the testing world, this is not so straightforward. Interfaces usually cannot be directly measured at a discrete location because they consist of bolts and holes, as shown in Figure 3‑3. Also, typical measurement equipment is only capable of measuring translational DoF.
The virtual point provides a solution to both of these issues. With the virtual point, sensors and impacts are placed around the virtual point, and Interface Displacement Modes (IDMs) are used to transform the measured FRFs to collocated 6-DoF virtual point FRFs, assuming rigid behavior of the region.
Figure 3-3: Graphical depiction of the virtual point methodology
This Virtual Point technology is directly integrated into SOURCE’s sister software: DIRAC. Thus, the benefits of using DIRAC in combination with SOURCE are immediately apparent: the blocked or interface forces can be obtained directly at the interfaces including up to 6-DoF. SOURCE can also be used on datasets obtained without the use of DIRAC, but less accurate results will typically be obtained when the Virtual Point technology from DIRAC is not used.
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