EEVblog - 2021-12-09
A follow-up video to the Electrodynamic Shaker, showing you the critical importance of Coherence measurement. Something you'll only get taught in the school of hard knocks! Using the Ling Electrodynamic shaker, a measurement accelerometer, and a Dynamic Signal Analyser to set up a vibration test system. 00:00 - Electrodynamic Shaker 01:20 - The most inportant thing in vibration measurement 03:56 - What is a Dymanic Signal Analyser? 06:28 - It's all about the system response 08:12 - Your expensive calibrated setup is worthless! 08:44 - What is Coherence? 12:17 - If you've got a really crappy shaker 13:10 - Let's set up a vibration test jig 13:54 - How to power and accelerometer 16:44 - What happens if you leave it flapping around in the breeze? 20:14 - Traps for young players 22:39 - Let's sweep this sucker 26:50 - Tighten your nuts! 28:22 - Show me your coherence plot or GTFO! Previous Video: https://www.youtube.com/watch?v=1Y2L6QLOi-c Forum: https://www.eevblog.com/forum/blog/eevblog-1443-do-this!-why-coherence-measurement-is-important/ Subscribe on Odysee: https://odysee.com/@eevblog:7 EEVblog Web Site: http://www.eevblog.com The 2nd EEVblog Channel: http://www.youtube.com/EEVblog2 EEVdiscover: https://www.youtube.com/eevdiscover Support the EEVblog through Patreon! http://www.patreon.com/eevblog AliExpress Affiliate: http://s.click.aliexpress.com/e/c2LRpe8g Buy anything through that link and Dave gets a commission at no cost to you. Donate With Bitcoin & Other Crypto Currencies! https://www.eevblog.com/crypto-currency/ T-Shirts: http://teespring.com/stores/eevblog #ElectronicsCreators #Vibration #Measurement
I used to do electronics and electrical designs to control hydraulics using by anything from the US coast guard to the Swedish Navy. While we had all kinds of certifications like Bureau Veritas etc. the various nations military always wanted to do their own testing including vibration testing and honestly I was surprised to see our electronics and electrical cabinets passing those tests, they were very violent. The most interesting issue we had was when we did an installation on some new mine sweepers with hulls made of fiber glass to prevent attracting and setting off mines and all the hydraulic gear we delivered was custom made from stainless steel to be non magnetic. However being navy ships they still had very powerful radio equipment on board, so having a fiberglass hull and powerful radio, it would make some of our equipment freak out whenever they used the radio. In the end I had to fly there to solve the issues by making sure all cables were shielded and terminated properly plus adding a handful of nano farad sized capacitors at strategic locations. I used a roll of aluminum foil to narrow down the most sensitive locations. Overall a fun experience and they signed off on the installation after a few days. This is of course part of a much longer story including bar fights, a black eye and being denied access to a foreign naval base.
Cheers,
Jake
Really really cool
Cheers.
Awesome!
thats awesome
wow thats awesome!
This is the best video in a long time! Great job Dave and thank you for all the effort that you put into this channel during the years. I started watching
these videos as a teenager, now I am a Physicist.
I don't think I would be where I am now without your work. Keep it up!
Working as a defense contractor, this is really insightful! You always see the vibrational energy spectra at the bottom of source control documents, and the mech-e’s audibly dread vibe-qualification - but to actually see how it’s done brings a lot of light to both the how and why.
Thanks Dave for the fairly detailed overview on vibration table setup. You are right, being taught this type of information in school would be extremely rare.
Being from the US, I enjoyed your Aussie Techno-jargon.
Having retired from engineering in 2016, I must say, the job of vibe testing setup has gotten easier over the years, as well as the number of pieces of equipment used, and size of the equipment. Probably no one is using a water cooled valve shaker power amp these days.
That little shaker you have is really cute, I have never seen one nearly that small. My warped mind has already come up with an alternate use, such as (as a joke), covertly attach it to the wall of a non-technical department head, and randomly play assorted barnyard noises throughout the day, just barely loud enough to be heard.
We actually DID go over this in school! In an elective barely anyone took.... I almost forgot about this, and definitely didn't know about this application.
I'm surprised it was even an elective!
Very cool. I repaired those very large shakers from MB Dynamics and Ling back in the 80’s working for a certain US defense contractor. The MB Dynamics C210 used these pieces of spring steel in a ring around the armature to suspend it and they would crack and break which in turn messed up the coherence response. It took a couple days of tear down and re-assembly to replace those stupid spring steel parts, what a PITA. Thanks for the cool video, it’s something most probably never come across as EE’s.
Did the same for vibratory feeders, where the springs were easy to get to to change, though of course getting them was the expensive part. Even though the design was based on a 1970's machine, and it was all TTL logic inside, and used a silicon solar cell slice as optical detector, because that gave a large area sensor cheap, and it was them loaded with a resistor, and then AC coupled to a 2 transistor amplifier inside the sensor housing, after which it was robust enough signal wise to be fed to a comparator on the logic board, where a LM311 converted it to TTL level to drive a cascaded set of BCD counters, which was then used with a load of XOR gates feeding into some multi input AND gates to give a "count equal to" signal, that, appropriately delayed, first reset the counter chain, and then operated a 7474 flip flop, that drove a rotary actuator that switched chutes.
Vibrator feeder was all analogue, half wave rectified, and then using phase control to vary the voltage applied to the massive coil that pulled on the steel pole piece.
Another one used variable voltage transformers, and when the one blew up I simply replaced it with the phase control version, using a circuit remarkably similar to a light dimmer, it was actually the guts of one, why reinvent the wheel, plus it came with all the snubbing and RFI suppression in it, with a large MOV and a 10A diode to turn it to pulsing DC. Did add in a second pot to limit amplitude, as it was otherwise capable of running the vibrator hard enough to slam into the end stops. That used a similar counter sensor, but instead used an integrated counter timer DIN mount block, all the logic in a single block, though it did need a separate 5V supply for the 6V illumination lamp, 12V for the sensor power, and a 24VDC power supply, all isolated from each other.
Great to see structural dynamics in electrical engineering!
Accelerometers can also be attached using beeswax, as it is easy to detach compared to glue.
The BIG problem with accelerometers (along with poor low frequency responses and triboelectrics in the cables) is that they influence the dynamics of the PCB with their mass, so you never measure true PCB responses, even though you get good coherence! Mass changes the mode shapes of the PCB and this changes everything.
This is why we use Vision based Modal Analysis, where cameras measure submicron vibrations without adding mass to structures. Also you get to actually see what is happening because you measure a spatial response as well (not just a single point), so you see individual components moving and you can pinpoint problems and even shift mode shapes.
I do have a laser displacement sensor...
i was thinkin the same thing..the accelerometer cables would have some influence on the setup... aswell as the table the rig is sitting on!
We used dental cement to attach accelerometers.
@EEVblog wo
would love to see that, and compare it with the PZT accelerometer. They tend to just measure velocity though. Our HP VSA can do the math to integrate & diff but not sure about your one.
Steve Mould did a video on the visual measurement of vibration, the results end up quite trippy. It was able to show issues in big-arsed machine tools that it's tough to imagine being picked up with accelerometers. He also used it to show the micro-movements in the muscles around his eyes.
Yup, when I was doing vibration testing we had 1 of these HP devices, but we never used it, as it was too simple. We used either a sine or a random closed loop control system. 2 very big boxes of electronics that were driven by a PDP-11 computer (yes this was back in the 1980's and the PDP-11 was a powerful computer back then). We could control the vibration at a selected point on the shaker or fixture. We could also set up multiple points to control the vibration, by 1 of several fancy mathematical functions (usually just an average). We normally just controlled the input to the fixture that was designed to hold the product/device under test (DUT). We would then characterize the fixture to determine if it was suitable for the testing that was planned. As long as the fixture didn't have any resonances at any of the test frequencies, then you can control the vibration input from anywhere on the shaker or fixture and the control system would ensure that the coherence is always 1 across the frequency range of the test.
I also did research into vibration testing. The 2 most interesting things were: a) Impulse driven device characterization, b) mechanical heterodyning of structures. Unfortunately I left to pursue other electronics design goals before really being able to write papers on either of these fascinating topics.
I agree, lot easier to use a shaker controller, but shows you how far the field has advanced.
Watching the fundamentals videos that Dave puts out shows that there is a lot of things about working with electronics that aren't taught in schools.
I had five (four of them core) classes in my ME program where coherence played a major role and was heavily emphasized in our dynamics testing. But to be fair, it was THE go to place for structural dynamics and vibrations research and the faculty throughout the colleges history pretty much wrote the book on how to do this stuff so I don't think it's an average case!
I found this quite interesting in several aspects. The first is that I once participated in a 3-1/2 day customer training occasion offered by Bruel & Kjaer. One of the things they demonstrated was that you had to lock the accelerometer cable to the same plate where the accelerometer was attached. Probably using bees' wax on both the accelerometer and the cable. Another thing was to use the smallest available accelerometer available. The same rule then was also applicable to microphone selection on sound level meter, especially if you wanted it to work beyond some 20 kHz. The other thing is the coherence measuring instrument that I had never seen in use. I do see plenty of similarities to my HP 3563A Control Systems Analyzer, though. It also has plenty of math built in. The most impressive feature it has is a synthesizing of the the PID circuit out of the Bode (& Nyquist / Nichols) results. I once did that and was duly astonished. Otherwise, I mostly have used just the Bode diagram for my control systems tuning.
Wow this is useful. I am currently going through the DO-160 training course, where one of the tests is for vibration. This video really helps to understand a lot of things!
We did do this and I still use it on a daily basis. I work in vibrations/acoustics and our school had amazing curriculum. Now all modern softwares automatically calculate coherence and save it with your measurement data. Amazing stuff thanks for sharing.
Dave, great video! Great timing, too. I supervise a group of test technicians and I have a few of them training on vibration test next week. Good information for them to have.
Fascinating content, thank you so much for sharing this knowledge. Given that material properties are known, would modern digital twin finite simulation of eigenfrequencies (modes) prior the components layout be an alternative to coherence testing, or simply an advantage?
I'm not an electronics engineer by education, so it's the first time I see this parameter talked about in a context other than statistical data or optics / wave physics. Still, makes sense to me.
In the early days before I could find a Ling actuator I made one from a large speaker magnet. I put a linear bearing down the center of the magnet to support the test table. I later used a voice coil driver from a large disk drive. It was about 6 inches in diameter. I would use it horizontally rather than vertically, Over the years I found a few actual Ling drivers. We use one here every day to test each every vibe sensor that we make. The vibe sensor uses a 3 axis accelerometer. We mount the device at 45 degrees to each axis so a single setup excites all channels with similar magnitude. So we check the gain of all channels with one setup too. the engineering phase of the design few if any of these measurement details matter much. Initially the goal is to excite the assembly over the specified range or larger to find anything that resonates and moves. It could be a tall leaded component or the entire PCB itself. A component could flex enough at resonance to fatigue and break off. It the entire pcb flexes traces and solder joints are at risk of failure. If you find any suspicious resonances you might just excite the system at that point for a long time to see if you can induce a failure. If the product is exposed to vibration at some characteristic frequency you might drive it there for a long time too. Modern SMDs are so small and stiff that their resonant frequencies are too high to matter but they suffer when the pcb flexes. So board stiffeners and rigid mounts are important. Once all this real work is done you can perform the test in the spec even though by now it means nothing. We often far beyond spec limits to see how much margin we have.
Sure when it will be done by analog wristwatch for part duration testing and durable?
And now I NEED a dynamic signal analyzer in my life. SEE WHAT YOU HAVE DONE! hehehe.
Now seriously, excellent video! I work at a physics lab at UBA, and we actually teach the concept of coherence in the context of light and sound sources, lasers, interferometry and such. But it had not occurred to me to use the same concept when analyzing the dynamic response of a mechanical system. Thank you!
You can do the same with regular speakers, which will show that different manufacturers of speakers do actually make them sound different, because they are all more or less non linear over the response. They might show a frequency response curve in a standardised fixture, but pretty much you can be sure that that is only approximate, and your unit will be different.
The more expensive ones come with either compensation in the amplifier, generated from a test speaker, that had it's response and coherence measured, or actually have positional feedback, so that at least the measuring point is going to be correct. That is why cones need to be stiff, and why there are titanium cones, and other exotic materials. But most use paper, because it is easy to form, and very cheap.
But to use this your source must be good, and pointless if the recording you play has been mastered in a studio with less accurate speakers, as then you only can get the response the studio had. Even more so if the sound has been compressed with lossy compression, the compression has to be nearly equal to the uncompressed to get a non audible difference. For me recording that turned out to be 320k 48kHz, as the recording did not sound different to the original.
diamond tweeters FTW!.. no (sound) break up until 75Khz!.. almost double what they are ever gonna be driven at...
Is that 320 kb/s compression on an MP3? As a music compression enthusiast (snob?) this is really valuable info
@academic pachyderm Yes, I could not hear the difference, but could just about at the next lower step of 240kb/s, so decided to go the better step and run the encode. Later on I moved to lossless compression, and thus have a lot of audio stored as either ogg or flac now. Just needs a transcode if going to a media player that does not support it, I really need to replace the battery in the LG player, which does do FLAc and OGG natively, though it also has the issue the flash is worn out to a great extent, it has been used so much I ran into flash write limitations.
Paper and plastics are used because they have internal damping. Metal cones will ring at certain frequencies. Whilst the perfect transducer will act like a piston in practice the break up modes on stiff cones make for complex filters to fix the issues.
Glad to see an explanation of what the guys are doing on the other side of the wall at our test laboratory. Before a wall was put up I thought there primary purpose was to drive us deaf and insane with their sine sweeps. Now just product EMI problems are the sanity stressors.
20:02 nice, its similar to cross spectrum, which we use in computing vision to match signals to know the difference between them. Its useful to find drift in the images, for example.
Thank you for the video, bit long for me :) but still thank you for making it! In short, all is about validation so if one does not have coherence you have no way to control your system and be sure what and why it is happening. In process to know your equipment is a must. Been there done that, not sure if I ever called it coherence :). That was term for physics, - "coherent waves"
Can one compensate for the mechanical setup's various resonances and make the coherence better? Like every mechanical system will have some problematic frequencies. Or is it more a case of working around it where your desired measurement range doesn't coincide with the mechanical systems resident frequencies?
I was taught about coherence in my radar courses, it's an important aspect in high-end radar systems and in signal processing of speckle in synthetic aperture radar and optical images. Very cool video.
@EEVblog Would be interesting to see a video about piezo accelerometer charge amplifier signal conditioning and setup and real world examples of measuring displacement etc. You have the test bed :) very interesting series on vibration. Thank you.
FWIW I did get taught this in a final year Machine Dynamics class as part of my Mech Eng degree some 25 years ago... Remember getting a demo with a much older HP DSA that cost an arm and a leg after we'd learned all the maths. Wasn't as clear and useful explanation as this video though. Well done for explaining it so clearly with good examples of potential gotchas.
When you increase the play speed, the speaker applies more vibration to the device Dave is testing.
Sine sweeps: If you are going to do slow sine sweeps, you really, really want to do them at very low power. Else, you will create nonlinearities due to resonances, which will screw with your measurement. It doesn't happen in the random signal because the energy is not so concentrated. For sine sweeps, the rule is to go slow, but also softly...
This same thing applies very strongly to DRC calibration sweeps for loudspeakers: If you use a sine sweep, try playing it back at a nearly inaudible volume: You pick up so much SNR from the convolution (and from lenghening the sweep, even a minute or two is enough), that it just doesn't matter that you aren't using much of the dynamic range of your recording setup. Let it be low level, that will avoid exciting mechanical resonances in your room so much they go nonlinear. It really doesn't take much: Sine waves are much more powerful than they sound.
(Also, if you're doing a stereo set, try swapping the left and right DRC filters, and do a listening test to see if it sounds more transparent/nice or not. You're welcome!)
I’m posting this comment so your can see what a hobbyist who is familiar with signal sources, oscilloscope and spectrum scopes got out of this. This is clearly a mechanical engineering thing. It was very interesting to see this worked through. I really liked seeing that mechanical engineering analysis has to deal with a lot of subtleties. Seeing the electronics used in a mechanical engineering system shows how pure electronics works interfaced to a real world physical system. I didn’t understand what a dynamic signal analyzer was before this. I thought it was just a measurement of a changing electrical signal, but now see it is an analysis system for a transducer output when correlated to a physical input. The coherence demonstration was very interesting, too, since I now get the system integrity testing that it measures to ensure that h you are not reporting artifact in the analysis.
I have a question: what would a mechanical engineer say they were doing when using this system to test a part? Or, a senior engineer wants the data out of this type of testing system: what would they tell the junior engineer they wanted using their technical terms?
As in real estate, there are three important factors: fixture, fixture, and fixture. 30 yrs ago we used a GR 7 kW (IIRC, shaker ~ 1 m dia.) system to measure fiber optic components... after verifying the fixture. The GR shaker/amplifier system did most of the heavy lifting, except for mounting plate resonance.
My thoughts tell me this same system can be used to test the effectiveness of vibration isolation devices such as the rubber mounts on hard disk drives, etc.
I love old school HP instruments. I still have a photocopy of a manual for an HP digitizing oscilloscope I was using in my student years...
curious, if you are searching for things like flex in the PCB, would the lower mount that looks to be made of plastic(?) not flex quite a bit more? acting as a dampener?
Dare I congratulate you on a very coherent explanation?
i love this guy this guy is awesome how is he able to afford all of this .
They do this for buildings! First, the engineer designs the superstructure by coming up with a model of it. This consists of a massless "rod" with masses attached where the floors would be. For complex structures, there are multiple rods/masses, coupled together. The resonant frequencies are calculated. The idea is to avoid resonances in the 2-4 Hz range. That's where the most destructive vibrations from earthquakes occur. After the structure is erected (less the walls, etc, which weigh very little), giant motors, with eccentric weights, are attached. Strain sensors are attached at various places on the beams. The motors are then turned on.....WUMP....WUMP...WUMP..WUMP.WUMPWUMP.... The resonances are adjusted by attaching masses to lower the frequencies, tensioning cables to raise them.
Thank you for this wonderfully instructive video. I'd like to make a few "not-so-important" comments (without 100% accuracy guarantee).
1) I think the low coherence in the measurement where you place the small accelerometer at the tip of the empty plate is probably not due to the natural frequency of the plate, but rather because the plate is placed at a node (not capable of being driven by any frequency and in any of its modes), so the accelerometer picks up more electrical noise. A plate with low damping and uniform material like the one in the video, would tend to give higher coherence near its natural frequencies (it wouldn't behave too non-linearly near resonance).
2) In addition to your emphasis on the need for superglue mounting, I'd like to add that firm mounting of vibration sensors has a tremendous effect on coherence, especially above 2 kHz. I can guarantee that a healthy measurement at 2 kHz and above cannot be made on any sensor that is attached to the surface with tape.
Thanks again for pointing out these very important details & for your beautiful presentation!
Can we replace the speaker coil (the apparatus for generating vibration) with a vibration motor (eccentric mass on shaft) and vary the motor speed to sweep frequencies?
Swept sine with a tracking filter means you do not see parametric amplification. You excite at one frequency and get another. Like driving a spring from the ends - it moves sideways at half the frequency.
Sounds like system self resonance monitoring to me. Interesting. Can you get software to cancel any known errors in the jig.
I know all this stuff and you did a really good job explaining it. Atta boy. :)
I struggle to understand how to use a 555 from my teachers, yet I understood "bigass magnet" and some other things from this vid. Good video
How about strapping a Crystal oscillator to this to show the resulting side bands.
You could also show the effect on ceramic capacitors maybe?
What type of specs are people looking for or requiring in PCB vibrational measurements? I understand not breaking parts and also measuring product signals under vibration... but what help is sticking an accelerometer on the PCB itself?
Do they make vibrational damped PCB layup?
I have done a bit of vibe in my life, never heard of coherence... We only use Transmissibility (ratio of Monitor channel divided by Control channel). We use a multi-channel closed-loop control system. There is one or several Control accelerometers, and one or more Monitor accelerometers all around the DUT. We look for amplitude changes on different spots of DUT from Monitor accelerometers wrt to Control accelerometer(s). I can also plot phase inversion, but I frequently have a situation of phase inversion from one spot to another. In a complex product that I test, I don't see how this, if it is sensitive to even phase inversion, is of great insight. My controller actually does not care about phase, only magnitude of signal from accelerometers. I can flip one of my accelerometers upside down and still the reading is the same. If I flipped 2nd accelerometer and tried coherence, I will get a crap value?
sounds like you have a more modern day view of what Dave was doing. thanks for explanation.
Great topic. While talking about vibration, can someone please explain per root Hertz to me in the context of dynamic response?
From best to worst, methods of attaching accelerometer:
Bolted
Epoxy / Cement
CA adhesive
Magnet
Double-sided tape
Wax
Touch
Cost to calibrate accelerometer seems a bit much... Perhaps nobody wants to deal with a hobbyist? My accelerometers cost $500 (single-axis)-$2000 (triaxial/fancy), and calibration costs us $200 I believe. We calibrate yearly. But we have a calibration lab right in the building (yet they are a different company due to conflict of interest concerns).
But we did teach this in school! Now if the students paid attention or understood a word of it is another subject. 🤣
BobC - 2021-12-08
We did this primarily to ensure accuracy and repeatability as part of our DoE (Design of Experiments) process. The worst part overall was to do reference characterizations on the many, many shake jigs we used. Each had a reference fiducial for attaching the accelerometer. We'd always validate the signal source, the driver amp, the shaker axes, the accelerometers, the signal processing system, and, of course, the shake jigs. Only after that can you mount the DUT (Device Under Test). And then, for reference runs, we'd have to repeat the entire process in reverse as we took down the test setup.
Ennar - 2021-12-09
That well explains why environmental tests are so expensive, thanks!
John McCormick - 2021-12-09
Analysis to paralysis! Enjoyed watching.
安笑生 - 2021-12-09
There are always a lot of factors in an experiment, either controlled or non-controlled. A wrongly setup will eventually cause the inaccuracy to the results which leads to fail of analysis results. It’s really an annoying and repetitive process there.
steelcuts - 2021-12-09
Ii j
Bruce Nitroxpro - 2021-12-18
@Ennar , You are reading my mind!