EEVblog 1443 - DO THIS! - Why Coherence Measurement is IMPORTANT - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: December 09, 2021, 11:44:38 am
content: A follow-up video to the Electrodynamic Shaker, showing you the critical important of Coherence measurement. Using the Ling Electrodynamic shaker, a measurement accelerometer, and a Dynamic Signal Analyser to set up a vibration test system.

author: Old Fart Analog Engineer
author_rank: Contributor
timestamp: December 09, 2021, 04:04:10 pm
content: Loudspeaker engineers are laughing their asses off at this! This is several orders of magnitude worse than the worst speakers ever made. "coherence" is magic-speak for "heavily averaged total system distortion / non-linearity + noise" which include the transducer measurement device and physical environment. To put it simply, the vibration source, test fixture and measurement transducer have to be calibrated before measuring the DUT. Duh! Since the system and DUT can be quite massive, and each DUT requires different mounting fixtures, this needs to be performed before making measurements of the DUT. Two major issues with this sort of test (i.e. "gotchas") are finding the location to mount the transducers(s) on the DUT and since the test system has automatic gain and massive time averaging, it does not show the instantaneous behavior and stresses on the DUT (because they are compensated and averaged out).For reference, back in the 60's, US guided missiles had small fins to steer them. These operated at up to 10, kHz. and were under high mechanical stress during flight of the missile. Very high speed cameras filming the fins were used to see the deformations and resonances.

author: DH7DN
author_rank: Regular Contributor
timestamp: December 10, 2021, 12:43:51 am
content: Very interesting and educational video! I really liked itMy heart skipped a beat from that magnet snap at 7:28 markI would like to do such DIY experiments myself one day, especially the calibration of accelerometers by laser interferometry.

author: EEVblog
author_rank: Unknown
timestamp: December 10, 2021, 08:17:16 am
content: Quote from: Old Fart Analog Engineer on December 09, 2021, 04:04:10 pmThe test in this video was designed to be dramatically bad so you can see the effects. It's might be Duh! to you, but countless engineers have never heard of coherence, and you can't just say "they have to be calibrated", otherwise people will think if they have a calibrated shaker and calibrated accelerometer then that's it, job done. Couldn't be further from the truth.

author: alxpo
author_rank: Contributor
timestamp: December 10, 2021, 05:23:24 pm
content: Hmmm... If coherence is such a great measure, why do we still use crap like frequency response, SNR, THD and IMD?

author: EEVblog
author_rank: Unknown
timestamp: December 14, 2021, 07:29:02 am
content: Quote from: alxpo on December 10, 2021, 05:23:24 pmBecause they aren't remotely the same thing. Horses for courses.Go to a vibration test house and tell them you want a THD calibration test on your setup and you'll get laughed out of the room.

author: Jeff1946
author_rank: Regular Contributor
timestamp: December 15, 2021, 03:29:46 pm
content: I remember visiting a facility that was doing a shake test on a 5000 lbs (2.3 kgs) piece of equipment we had designed. We parked our car on the street outside of the facility. A coworker said as we were getting out of the car, "This must be the right place look at the parking meters vibrating."

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: December 15, 2021, 04:39:03 pm
content: Quote from: Jeff1946 on December 15, 2021, 03:29:46 pm2267,962 kgs

author: rsridhar
author_rank: Contributor
timestamp: January 05, 2022, 01:55:39 am
content: Wow it was so interesting to see such an old shaker and HP DSA in use!I work for Data Physics - the company that currently owns Ling Electronics (not the same shaker used in the video; that is actually Ling Dynamic Systems...long story).I work in the DSP (dynamic signal analyzer and vibration controller) division that was originally founded by two HP engineers that worked on that HP DSA used in this video!Our new DSAs are so much more advanced than those used in this video - custom vibration controllers and very advanced signal analyzers for realtime processing!Im glad to see this content on EEVBlog! Cheers!

author: SilverSolder
author_rank: Super Contributor
timestamp: January 06, 2022, 04:21:19 am
content: Quote from: rsridhar on January 05, 2022, 01:55:39 amNew equipment is extremely advanced and very cool, but with the older stuff it is easier to understand what it does (and how it does it)!