7 Checking VP consistency

The Virtual Point transformation is key for next-level vibration engineering. Checking the VP consistency is vital to ensure that the measured data is transformed correctly to the Virtual Points in the model.  The response and force consistency inspect the measured data of the sensors and excitations, respectively, to check their positions and orientations as well as the rigidity requirement of the Virtual Point Transformation.  Since the rigidity around a VP is an assumption to perform a correct Virtual Point Transformation, these quality indicators give you great insights into the quality of your final model.

You should check the consistency of your data after measuring the excitations of one VP. This allows to spot and correct problems before moving to the next VP. Integrating this check into your measurement process ensures high-quality test-based modeling. (link)

How to: Visualize the consistency

1. The matrix viewer gives the possibility to get an overview of all the VPs in the system and allows to quickly spot which VP group has a problem with consistency.
2. It is possible to change between force and response consistency by selecting the desired mode below the matrix.
3. The depicted consistency is dependent on the frequency range defined below the matrix viewer.
4. The Validate card shows all the impacts and sensors belonging to each VP and it is helpful to spot which of them are causing the problem.
5. When the problematic one is identified, it is then possible to analyze the details of each FRF in the Graphing area. Here the pink line is the result of transforming the response of the FRF to the Virtual Point, and then transforming it back to the original sensor or excitation.
6. The consistency plot shows the consistency between the pink curve and the blue one (average).

How to: Check response and force consistency

When checking the consistency of your model, you can follow the next steps to be sure you are not missing anything.

1. Select the Measured preset in the matrix viewer.
2. Set the matrix viewer to response or force consistency mode.
3. Average over the frequency range of interest.
4. Increase the minimum value of the consistency until you see white areas appearing in the matrix.

The white areas correspond to problems in your data.

The white areas correspond to the FRFs with the lowest consistency. If the whole matrix turns white at the same time while zooming in into the consistency axis, you have very similar data quality. If you’re satisfied with the minimum consistency, you can move on to the next quality checks.

How to: Spot inaccurate sensor positions

The first check we recommend doing is verifying whether the sensors are positioned and oriented correctly. This can be done quickly by checking the response consistency of the system in a low-frequency range, where you would expect rigid body movement. To do that, you have to change the matrix mode to Response consistency and type the correct min and max values to check the low frequencies only.

If you see some white rows in the matrix, a sensor is wrongly positioned or oriented. To verify the troublesome one, you can check either the ODS or the response consistency of the VP in the Validate card. Once you find it, return to the Prepare module and position it correctly (the Photo Viewer could help you). After correcting it,  check again the response consistency.

How to: Spot inaccurate excitation position

As for the sensors, if the force consistency is bad at low frequencies (where you expect rigid body movement), one excitation is positioned wrong. To check it, you should set the matrix to the Force consistency mode and set the frequency to a low range.

If you see one white column in the matrix, that excitation is positioned wrongly. You can also check the Validate card after setting it to force consistency. Once you find it, return to the Prepare module and move the excitation to the correct location, with the help of the Photo viewer if needed. After correcting it, check again the response consistency.

How to: Spot local flexibility

By checking the consistency, you can also verify whether the local rigidity assumption around the VP holds. If the (response) consistency is good at low frequencies, but drops at higher ones, very likely the assumption of local rigidity around the VP is violated. You can check this by setting the response consistency mode in the matrix and sliding the maximum frequency slider from low to high frequencies. If the matrix becomes whiter while increasing the maximum frequency, your system presents local flexibility. You can also check this by looking at the consistency plot in the Graphing card.

If that is the case, you need to adapt the test setup by moving the sensors and impacts closer to the VP, and repeat the measurements.

If the VP has been modeled with only the translational DoFs, including the rotational degrees of freedom might be a possible solution to improve the model.