EEVblog 1443 - They Don't Teach This in School! (Coherence)

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: • EEVblog 1442 - DON'T D...
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[@flymypg]

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.

[@nrdesign1991]

That well explains why environmental tests are so expensive, thanks!

[@jdmccorful]

Analysis to paralysis! Enjoyed watching.

[@_a_x_s_]

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]

Ii j

[@BruceNitroxpro]

@@nrdesign1991 , You are reading my mind!

[@TheStuffMade]

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

[@gristlevonraben]

Really really cool

[@kartikeyajp8294]

Cheers.

[@gforge9708]

Awesome!

[@kartikeyak3425]

thats awesome

[@yobeh7380]

wow thats awesome!

[@jacobfaseler5311]

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.

[@marco56702]

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!

[@teawizardry]

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.

[@EEVblog]

I'm surprised it was even an elective!

[@humgarllc1737]

@@EEVblog Coherence a tiny part of random signals and systems/processes that's barely 10 minutes of materials (but builds on solid understanding of signals, system and probability so it's not something you can easily explain to people without the background knowledge). It's a class you can technically avoid even if you specialize in signal processing, but you'd be crippled without it if your specialization has anything to do with systems, including quantitative finance or statistics.

[@motion-scope]

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.

[@EEVblog]

I do have a laser displacement sensor...

[@WacKEDmaN]

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!

[@peterldelong]

We used dental cement to attach accelerometers.

[@RealSuperDuperCooper]

@@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.

[@gerryjamesedwards1227]

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.

[@neil2742]

Dare I congratulate you on a very coherent explanation?

[@mcconkeyb]

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.

[@modeler4]

I agree, lot easier to use a shaker controller, but shows you how far the field has advanced.

[@peterldelong]

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.

[@SeanBZA]

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.

[@ujjawaltiwary8151]

They say, Dave is still playing with his vibrator. :D

[@billharris6886]

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.

[@ArtyomGalstyan]

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!

[@coryballiet8260]

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.

[@InssiAjaton]

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.

[@Pablo_El_Mago]

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!

[@theagnihotris9763]

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.

[@jessicav2031]

When a grid's mis-aligned with ano-ther behind, that's a moi........aliasing.

[@nameredacted1242]

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).

[@johnyoungquist6540]

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.

[@petevannuys5642]

Sure when it will be done by analog wristwatch for part duration testing and durable?

[@dentakuweb]

That was way more interesting than I expected.

[@sonic2000gr]

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...

[@tolikechicken]

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!

[@KeritechElectronics]

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.

[@ChargelessElectron]

Very, very good. Thank you Dave.

[@SudaNIm103]

Great Video - Awesome demo!

[@milantrcka121]

Big shakers typically use a feedback accelerometer to linearize the driver itself and to control the acceleration in a closed loop servo. Some 75 -80 kg device was designed to take 30 g crash safety. During a test the feedback accelerometer fell off. This resulted in a 100 g (not grams, earth gravity acceleration). A massive motor tore off, main chassis bent, a pitiful sight. We used to stand in a respectful distance during tests.

[@Rx7man]

guess there wasn't any redundancy!... an extra accelerometer (wouldn't have to be terribly fancy) that would trip at 40g's and shut down the system might have saved some damage.. but I guess they found the breaking point!

[@milantrcka121]

@@Rx7man 30g crash safety limit is just that. In a crash, the device is not supposed to kill the pilot or a ship crew, no ejected material, entire box shall shall not detach from its structural mounts even if the vibration isolators fail. Internally, we do not care at this point. So 40g limit would not help a whole lot.

[@Rx7man]

@@milantrcka121 I mean a safety to shut down the machine so that the test could be re-run.. I didn't mean that the run where an error happened would be valid.. with two sensors you could shut down the test if there's more than X% disagreement between them too

[@arandomguy4478]

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

[@ebenwaterman5858]

I know all this stuff and you did a really good job explaining it. Atta boy. :)

[@ramansridharan4562]

Wow - that is an old shaker and analyzer!! The shaker in this video is actually Ling Dynamic Systems, not Ling Electronics - they are two different brands now! I work for the US company that now owns Ling Electronics. We started out making Dynamic Signal Analyzers (our founders were HP engineers that worked on that HP DSA used in the video!), and acquired the shaker business later on. Our DSAs are are so much more advanced and interesting now! We make "custom DSAs" for vibration control specifically as is normal in the industry; and of course very advanced dynamic signal analyzers that make that HP look like a casio 4 button calculator!

[@kevincozens6837]

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.

[@magnusoksbltherkelsen2453]

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.

[@SeanBZA]

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.

[@WacKEDmaN]

diamond tweeters FTW!.. no (sound) break up until 75Khz!.. almost double what they are ever gonna be driven at...

[@academicpachyderm5155]

Is that 320 kb/s compression on an MP3? As a music compression enthusiast (snob?) this is really valuable info

[@SeanBZA]

@@academicpachyderm5155 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.

[@gr4eme1975]

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.

[@markbrown8097]

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.

[@johnwilliamson467]

Nice delay line there in the accelerometer line . Enjoyed the video

[@0202fabrice]

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.

[@monad_tcp]

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.

[@marcfruchtman9473]

Very Interesting! Thank you.

[@davefiddes]

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.

[@andymouse]

Interesting stuff !....cheers.

[@karama300video]

Very interesting video!

[@Bianchi77]

Nice video clip, keep it up, thank you for sharing it :)

[@WacKEDmaN]

i wanna see that PI shaken to deth!!...hopefully a future vid!..make all them electrons fall out! thanks Dave.. coherence makes sense now!.. and its not resonance!

[@omitsura]

When you increase the play speed, the speaker applies more vibration to the device Dave is testing.

[@jimsmind3894]

I wonder why the first video on a 'vibration generating device' was so popular... 🤔🤣

[@miguelangelsimonfernandez5498]

In my neck of the woods (Spain) they used to teach this but we never got the chance to see it in action. All pen and paper cases. Now with the stupid Bologne reform of higher education it's all a terrible mess and they no longer teach it. May the politicians all burn in Hell in a high flame for what they did to education in Europe.

[@stephenholland6328]

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?

[@daic7274]

@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.

[@markclark787]

I used an air shaker to find rattles in cars at a dealer, I am glad they do not build them the way they used to.

[@bart416]

But we did teach this in school! Now if the students paid attention or understood a word of it is another subject. 🤣

[@mrlithium69]

it was taught. on page 846 of the textbook.

[@Pootycat8359]

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.

[@krnlg]

Very interesting!

[@Antony_Jenner]

At last a Sunday morning adjustable cocktail shaker.🤪🤪

[@Debraj1978]

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?

[@justmemyselfandi5532]

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"

[@darrenwilkinson1742]

Awesome Dave, does the quality, accuracy and frequency response of the amplifier used affect the accuracy of these measurements? Thanks

[@RGD2k]

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!)

[@nameredacted1242]

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?

[@modeler4]

sounds like you have a more modern day view of what Dave was doing. thanks for explanation.

[@demoncloud6147]

What is the value of Coherence at Resonance frequency(mechanical) ? 0?

[@EEVblog]

The resonant point of the shaker is usually the upper limit frequency.

[@linmajon]

This explains why almost all "tests" made by popular youtubers are absolutely worthless. Phone drop tests, scratch tests, battery tests - you name it.

[@kstxevolution9642]

it's worthless in any quantitative sense, tho. they have a qualitative value

[@EEVblog]

@@kstxevolution9642 Correct.

[@Audio_Simon]

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?

[@gtcollection6933]

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?

[@christianedelmann6880]

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?

[@chrispollard6568]

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.

[@ovnibus_cx3cp]

Hello @EEVblog Some inquire, is not necessary the calibration in the desired frequency span for a proper amplitude linearity response ???

[@mcasual7657]

Happy 2022 by the way :)

[@rexbk09]

Love, Love, Love

[@byloyuripka9624]

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?

[@FSXgta]

That's a nice bass shaker

[@techmakerandhacker7867]

i love this guy this guy is awesome how is he able to afford all of this .

[@johnshaw359]

Sounds like system self resonance monitoring to me. Interesting. Can you get software to cancel any known errors in the jig.

[@patnutoris4054]

I should have watched this 10 years ago. :( It seem like I did these tests for 5 years without even understanding any of it.

[@steviecandtheplace2b]

I see similarities with audio frequency coherence, power factor and calibrating the Pat tester before testing a cable, and then it starts to go over my head… still interesting to watch tho.

[@alexscarbro796]

How about strapping a Crystal oscillator to this to show the resulting side bands. You could also show the effect on ceramic capacitors maybe?

[@defenestrated23]

Oh, now we see the nonlinearity inherent in the system! Nonlinearity inherent in the system! Help! Help! I'm being distorted!

[@andrewphi4958]

Shmick response... *taking notes*

[@georganatoly6646]

very interesting, glad the video didn't shy away from 'scary' looking maths, though I'm surprised the display referred to the complex part of the number as 'imaginary,' never liked that term

[@joopterwijn]

Guess that the wooden workbench would impact the measurement (to flexible on a certain frequency)

[@sdp8483]

Correct me if I am wrong but I think a vibration controller like the VR system that was flashed on screen in the previous video removes the need for coherence. At work we use the VR system and the controller takes care of outputting the right signal based on a control accelerometer. I don't run the vibe tables much but the few times I have I never had to do the measurement from this video.

[@EEVblog]

If you've got a fancy pancy rig that handles it automatically, sure. But that's just the shaker platform, you should still test the coherence of your mounting hardware as that forms part of the system.

[@erikdenhouter]

I hope you repaired the skirt of that shaker, dirt will cause a lot of problems falling into the air gap of the coil.

[@msmith2291]

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.

[@13yroldgosu-gosu-sin24]

"You can count those if you really want to" LOL

[@__--JY-Moe--__]

it's shak'n...hopp'n...skipp'n...& a bopp'n!!! make a worm gear, then rotate + & - 45 deg!! or purchase a reciprocating saw!! going 2 sleep now!!

[@ChillBing]

Nice

[@TheMlg556]

golden!

[@dutchy661]

Little question, wouldn't it be preferred to mount the base plate central on the shaker to avoid additional vibration to the product?

[@dutchy661]

Sorry, I was only 3/4 into the video 😅

[@moddquad8362]

Shouldn't the shaker be mounted to something heavy? It moves around on it's own due to vibration at 25:22.

[@williamsquires3070]

If all else fails, attach a large, round plastic disk to it and use as a subwoofer! 😆

[@jeremylister89]

👍 It's intuitive that the plate should be big, solid and much heavier than the pcb.

[@EEVblog]

Yep. Bigger the plate the bigger the shaker needed though.

[@jeremylister89]

@@EEVblog yep 👍

[@ericcameron281]

can you build a speaker out of the shaker to reproduce music? let's what is inside the shaker.

[@milantrcka121]

It is a speaker. You should hear a big one! Like 1 meter in diameter.

[@jesset2550]

cool I always wondered how this was done

[@mscir]

How about shaking something and illuminating it with a strobe at the same time?

[@T2D.SteveArcs]

What about the natural frequency of the shaker and or the plate? This would effect the coherence? Great video as always Dave

[@EEVblog]

The resomant point of the shaer is usually the upper frequency limit. As for the plate and other mounting hardware, this is what the test is designed to find out. You don't want to be operating at any resonant point of your test hardware.

[@T2D.SteveArcs]

@@EEVblog 👍 thanks for answering Dave that's really cool considering the size of your channel AWESOME 😎 love your channel been subbed for years.. If you have time take a look at my channel high power electronics for instance 10kw Vacuum tube Tesla coil. 300kv xray transformer s (real 300kv 30cm plus jump) Xray equipment taredowns, 18kw DIY half-bridge and so on.. I only started uploading 18months ago so quality isn't up there yet but I intend to work at it as my audience grows.. your feedback would mean a lot.. All the best from the old dart Steve 👍👍

[@WacKEDmaN]

cool... been waiting for this.... enjoy ya holidays Dave and family.. where ya headed? north, south, west? .... east?!

[@EEVblog]

Wherever the government lets us go, of course. I'm an obedient citizen, of course.

[@WacKEDmaN]

@@EEVblog ya should be fine for anywhere in NSW by wednesday!.. maybe sooner!

[@EEVblog]

@@WacKEDmaN Thank you for my freedom Sir! I will always be your loyal servant.

[@WacKEDmaN]

@@EEVblog lol wat?... U been on the sauce or somethin mate?! (i know ya dont drink!) im hoping the new one doesnt ruin plans right before xmas :/

[@rowanjones3476]

Attach a cone and you’ve got yourself a makeshift loudspeaker driver.

[@Iamalizard]

So you are giving away the secrets of the Philips Motional Feedback System (MFS) speakers from the seventies, mjum nice, collector items! DIY high-end speakers with EEVblog and learn something

[@jamesttol]

I wish i had a DSA to test audio circuits.

[@PsiQ]

**Scribbling notes:** - Shaker - Big Assmagnet - Micro ferrets - uneven sweeping => Coherence .. Thats what i'm gonna name that Bar :-)

[@szabolcs__]

Dave with a pico scope would be more interesting , bc you can play with the software at home. (Iam saying this , because hopw some kid watching it and wana try it out at home)

[@garrygemmell5676]

Hmm I was wondering if that is one of the motors they use in those "male exercising" equipment devices? I bet 50% of the male "members" on here just thought the exact same thing! >;o)