Setting up an FRF measurement campaign is a complex and error-prone task. Rotated sensors, swapped or broken cables and wrong calibration values can lead to a completely useless dataset. Even the most experienced test engineers with the best procedures cannot ensure 100% error-free setups.
The operational deflection shapes illustrate the motion of the sensors at a specific frequency for a specific excitation. At lower frequencies, the ODS should mainly show the component moving like a rigid body. If that is not the case, according to the sensor movements you can understand what is going on and fix it right at the beginning of your measurement campaign.
When beginning a new measurement campaign (link), it is recommended to start by measuring only 3 impact positions: one in X, Y and Z-direction and jump directly to Analyze to display the operational deflection shapes. If at low frequencies all sensors move consistently in the 3 directions, the test setup is free of errors and you can go on with all the other measurements. If not, then you need to understand what is wrong and fix it.
The operational deflection shapes can be displayed in Analyze.
After toggling the ODS button (and moving the cursor at low frequencies), if a sensor moves with a similar amplitude but in a different direction than the others, it is probably oriented differently on the test than in DIRAC. You should check the orientation of the sensor on the test object and in DIRAC, if needed, with the help of the photo viewer. Then, you should go to Prepare and rotate the sensor to correct it. When it is done, go back to Analyze and play the ODS: all sensors should now move consistently.
After toggling the ODS button (and moving the cursor at low frequencies), if two sensors move with similar amplitude but in a different direction than the others, their cables might have been swapped when they were connected to the measurement system. You should identify the two sensors in the 3D Viewer, go to the DAQ sensor mapping and compare the connection to the cables in the DAQ settings of the front end. After correcting the mapping, all sensors will move consistently in the ODS.
After toggling the ODS button (and moving the cursor at low frequencies), if a sensor moves in the same direction but with a completely different amplitude than the others, very likely an incorrect calibration value has been set. You have to check the sensor calibration value in the DAQ settings, correct it and then verify with the ODS.
After toggling the ODS button (and moving the cursor at low frequencies), if a sensor moves totally differently or hardly moves at all, its cable might be broken. If that is the case, there will also be a white row in the coherence matrix for the corresponding sensor channels. To fix it, you have to change the broken cable, measure the three impacts one more time and check them with the ODS again.
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