Group Assignment 5: The Quality Assurance Checks
1. Coherence Function Check
1.1 Shaker test coherence check
Figure 1: Driving point FRF at point 5.
Figure 2: node number on the wood plate
The coherence defines the proportion of the response which is completely accounted for by a linear response to
the measured values lie between 0 and l, with a poor coherence [1]. From the above figures, we can find that
the hammer test at point 4 has very poor coherence plot, but the most spikes happen near the resonance region.
Also, the coherence of shaker test is much better than that of hammer test. The poor coherence in our case may
be caused by instrumentation noise on one or both of the force or unmeasured extraneous excitation. Another
possibility is that there are some non-linearities of the wood plate itself.
1.2 Hammer test coherence check
Figure 3: Driving point FRF&Coherence at point 4
Figure 4: Driving point FRF&Coherence at point 6
2. Reciprocity check
Figure 5: Reciprocity between point 7 and point 9
Figure 6: Difference between FRFs
The check is performed by measuring two transfer mobility FRFs at point 7 and point 9, swapping the
locations of the driving point and the accelerometer between the two measurements. The FRFs are then
compared. Also, the change of FRFs between two measurements is plotted as shown in the above figure. Based
on the comparison, even there are some relative high differences at certain frequency range; we can consider
the linearity check acceptable.
3. Symmetric check
Figure 7: FRFs of point 4 and point 6
Figure 8: FRFs at point 7 with two levels of force
The symmetric check was made by comparing the driving point FRFs at point 4 and point 6. The result of
symmetric check is poor because the wood plate is not fully fixed to the table. Using only a couple clamps
may introduce unsymmetrical boundary condition at the base.
4. Homogeneity check
For the homogeneity check, we impacted the plate at point 7 with two different levels of force. It can
obviously be seen that the measurement made using low-level excitation is poorer than that made using
high-level excitation. However, there were no large discrepancies between the location and magnitudes of
the peaks corresponding to modes of vibration of this structure and, therefore, the check were deemed to
be satisfactory.
5. FRF shape check using shaker test data
Figure 9: Shaker test FRF and Phase
Figure 11: Hammer test FRF and Phase at point 7
Figure 10: Zoom in frequency from 200~800 Hz
Figure 12: Repeatability check at point 7
For both hammer test and shaker test, we found the FRF shape check results are satisfactory to meet the
following two statements.
-Anti-resonance should always exist between adjacent resonances.
-There should be a negative phase shift through resonances and a positive phase shift through antiresonances. For point mobility FRFs, the value of phase should always be between 0 and - 180 degrees. [1]
6. FRF repeatability check
From the repeatability check, we found that there is little difference between two FRFs. We can conclude
that the environmental noise to the hammer test won’t have a large effect on the modal properties of the
structure.
Reference
[1] Quality assurance procedures for the modal testing of building floor structure.