40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  Engineering group Lab elog  Not logged in ELOG logo
Message ID: 250     Entry time: Thu Apr 8 17:01:52 2021
Author: Stephen 
Type: How To 
Category: Modal Testing 
Subject: Troubleshooting low vibrometer signal / absence of ring down 

Regina's problem statement:

 I attached examples of two measurements I got and wanted to know if these look reasonable. I took 6 measurements total, and I attached the first and last measurements. The graph on the last page is a picture of the weighting step for reference.

I didn't see significant ring down in all my measurements. Is this to be expected? I thought since the baffle is now rigidly mounted, the vibration should go almost to zero. I was also getting some low amplitude noise throughout the entire frequency domain that didn't show up in the first couple measurements I took. I tried to reset the vibrometer like you mentioned but they were still present. Is this a problem?

Stephen's reply:

1) Vibrometer may need a higher diffusivity surface to improve signal level.

Replying to your absence of ring down in your measurements - I agree that it looks like the vibrometer output is not behaving well, for one reason or another. In the freely suspended case, I was thinking this was due to large yaw and pitch motion causing high signal variation. Given that the symptom occurs when the baffle is fixed, I think the likeliest reason is the low signal, due to the low scatter and highly specular surface finish of this baffle (aka shiny). One way to troubleshoot would be to attach a compliant, diffusely reflecting material to the surface - think a small square of the adhesive-coated part of a Post-It note, for example - then tune the focus of the vibrometer and see if the vibrometer's signal level bar improves. If the signal level improves, take a hammer-excited measurement, and see if you see any ring down. If this behaves as you might expect, you could generate mode shape data with your excitation roving around the surface of the baffle while your response is fixed - just one baffle point would need your diffuse Post-It square (I might go with a central, or near central, location).

2) Test Article is not well understood, so try measuring something that has been characterized before.

If you try playing with the diffusivity and focus but the signal level doesn't improve, or if you don't see any ring down still, try pointing the vibrometerat the suspension cage and exciting the suspension with the hammer - that should give some real signal regardless of the precise setup, and if that gives similarly mystifying results, let me know and we can think a little harder about what might be going on. I feel pretty confident that between these two tests, you will find an answer.

3) Add another transducer (ie. a witness accelerometer) for comparison.

Another way to support your understanding of your setup (and a good practice) would be to mount the accelerometer to the suspension cage, adjacent to the baffle mounting brackets, or even to the mounting brackets themselves. This accelerometer would supply a witness to the low frequency resonances of the cage, which you may excite during your measurements, and might also provide some insights to the baffle panel resonances (rigid coupling with lower modal mass = smaller vibrations, but likely still above the noise floor of the accel) supporting your eventual successful vibrometer measurements.

4) Notes about mounting an accelerometer.

Mounting would involve collecting a ~[1mm x 1mm x 1mm] chunk of beeswax, spreading the beeswax onto the face of the accelerometer opposite the cable (I like to press it with the outer, flat-ish surface of my thumb nail to spread), and pressing the beeswaxed face onto a flat surface - think a 5 second push with all of your arm strength, which should create a thin layer with plenty of tackiness to hold the accel in any orientation. If it doesn't seem like it could hold for a day, then you might need more beeswax, or more force to create that thin layer. Note: the main way that an accel can go from useful to not is to experience a shock event, so I would recommend that you use some kapton tape to affix the accelerometer cable to the suspension cage - this will strain relieve your cabling and provide a fall restraint, and the potential for the cabling to influence the measurement is minimal here because it is remote from the baffle.

Attachment 1: regina_troubleshooting_GraphsBKHammer.pdf  429 kB  Uploaded Thu Apr 8 18:08:22 2021  | Hide | Hide all
regina_troubleshooting_GraphsBKHammer.pdf regina_troubleshooting_GraphsBKHammer.pdf regina_troubleshooting_GraphsBKHammer.pdf
ELOG V3.1.3-