structure borne vibration sensing? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: March 07, 2015, 04:59:27 pm
content: I've been asked to look into a relatively low cost solution to measuring the surface borne vibrations in a large planetary gearboxLots of people offer high end vibration analysis kit, but it's all rather an over kill for what is needed here. i don't need to measure the absolute vibration levels, just be able to spot a change (ie, if the system is about to fail etc) so that the system can be shut down if it does.I see there is an FFT library for the STM32 processors, and the dev boards are cheap, so i wondered about using one of those. Read in a single ADC channel, FFT it, and compare each frequency band with some "pass or fail" criteria.The issue is that so far, no one can give me sensible figures for the amplitude of the vibration or it's frequency content! (fundamental frequency will obviously depend on rotational velocity and the harmonics on tooth counts etc)So, i'm left wondering what sort of sensor to use, and how much gain i might need? Anyone done this sort of thing before. Some sort of surface mount microphone might do the job?

author: Tinkerer
author_rank: Frequent Contributor
timestamp: March 07, 2015, 06:41:30 pm
content: Just one path to look into would be MEMS devices(covers a wide range of devices but the basics are the same). They are used to sense changes in motion typically. Very popular things, automotive industry uses them etc. Even used in regards to microphones. I have never delt with them myself however.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 07, 2015, 08:26:06 pm
content: I just couldn't control my OCD, the word you want is "borne".

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2015, 12:05:01 am
content: I worked with a guy that had a patent on a bearing monitor almost 40 years ago. His device looked at the ratio between the RMS and the peak. I actually worked with his device out in the field. In practice industry didn't care about future failures. Industry wanted to wait till the last moment before failure. That was a GROSS RMS grinding away value. You have it so easy today. At the same time we were looking at new compressors to find defects. Rotation divided into sixteen sections, sixteen samples each on expensive crude FFT boards. We learned sample signatures and looked for deviation from that.Epoxy on piezo crystals might be a good sensor.

author: SeanB
author_rank: Super Contributor
timestamp: March 08, 2015, 08:36:26 am
content: I have some vibration sensos that you simply bolt onto the unit in a tapped hole, which use a crystal and a mass to generate a voltage output. VP6 made by Lamerholm in the UK, who still make a similar unit.http://www.instronics.com/Lamerholm.htmlOtherwise you can go to the auto parts store and get some knock sensors for ICE vehicles, and use them, with an appropriate high impedance amplifier for the crystal types or a high gain amplifier for those with a magnetostrictive type.Here is a video I did of one attached to an old hard drive. Audio is in another file I have, but the sound is similar. I did the sensor in another take without doing any video on it, for Dexter.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 08, 2015, 09:01:38 am
content: I don't need to measure the absolute vibration levels, just be able to spot a change (ie, if the system is about to fail etc) so that the system can be shut down if it does.Using a digital approach with FFT seems like huge overkill to me. Do you really need to know the frequency distribution of the vibration?I would be inclined to measure the reference vibration level of a known good gearbox (under load) and then double it (or whatever) and use that as the threshold. If the threshold is exceeded then an alarm is generated.In high value jet engine aircraft the gas turbines are fitted with a vibration sensor that drives a meter in the cockpit. The pilot/flight engineer can monitor the vibration level at all times an immediately knows if the engine is exceeding its normal level.

author: BradC
author_rank: Super Contributor
timestamp: March 08, 2015, 10:31:30 am
content: Quote from: Richard Head on March 08, 2015, 09:01:38 amWhich is perfectly justified when the effect of a blade coming loose can be the loss of a wing or a whacking great hole in the fuselage. You want to know _very_ early in the piece if you are about to grenade so you have the ability to make the judgement call to shut it down. In practice catastrophic disassembly still happens, but it's nice to have the ability to at least prevent some of it.

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2015, 11:18:53 am
content: The equipment is cheap and plentiful. However the OP has no idea what sound signature is normal and what constitutes a pending failure. That testing can eat up a lot of time and assumptions will be made. Wear might be detectable. In the end a fatigue crack may be the most common failure and not be detectable at all.

author: max_torque
author_rank: Super Contributor
timestamp: March 08, 2015, 02:01:46 pm
content: I had thought about using a knock sensor, but i'm not sure if they are sensitive enough (~25mV/g). I suspect the amplitude of the vibration in the surface of the gear case will be rather low, as it's pretty massive. Of course, sensible positioning of the sensor will help.The reason i though FFT was because i don't really know the failure mode, and the effect of that failure on the output waveform.The fundamental frequency will be proportional to the gearboxes rotational speed, and i suspect out-of-balance forces due to that will swamp all the rest of the higher harmonic frequency content. Something like a "chipped tooth" for example will be i suspect impossible to spot doing just a bulk RMS of the signal etc?With an FFT, we characterise the amount of energy in each of the frequency "Bins" (probably have to be vs rotational speed of course) and then look for significant changes etc.it's certainly not a trivial exercise, and to get to a level of sensing robustness that means non gross failures can be detected is even less trivial

author: woodchips
author_rank: Frequent Contributor
timestamp: March 08, 2015, 10:13:55 pm
content: Start simple? Get a loudspeaker, glue a weight to the centre of the cone, solder some wires to the terminals and connect to an oscilloscope, look at waveform.Might not tell you anything, but if there are a few major frequencies then who knows.

author: robrenz
author_rank: Super Contributor
timestamp: March 08, 2015, 11:42:25 pm
content: Analog devices ADXL103 or 203 1V/g analog output can resolve to 1mg filtered to 60hz

author: BradC
author_rank: Super Contributor
timestamp: March 09, 2015, 01:52:54 am
content: Quote from: max_torque on March 08, 2015, 02:01:46 pmKnock sensors are not broadband units either. They are tuned ~5-7khz depending on the vehicle and the bore diameter (ie there are different part numbers for different engines for good reason).

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 09, 2015, 05:36:17 am
content: Aircraft also use a metal chip detector. They are normally used in gearboxes and the like. They have two metal contacts located on a small magnet. The sensor is placed in the oil pan. As the gearbox wears small bits of metal are attracted to the magnet and short the two contacts together.They have been used for years and are very useful for detecting abnormal wear or a collapsing bearing.

author: ivaylo
author_rank: Frequent Contributor
timestamp: March 09, 2015, 05:58:47 am
content: I helped some time ago develop a system for measuring turbine vibrations. The designer used some of thesehttps://www.endevco.com/?search-class=DB_CustomSearch_Widget-db_customsearch_widget&widget_number=preset-1&active-1=1&all-2=Products&cs-all-0=Piezoelectric+Accelerometers&search=SearchWhat I remember is that they weren't as axpesive as they look, some had the preamplifier built in and were absolutely top notch quality. I can ask the exact model number we used.

author: pickle9000
author_rank: Super Contributor
timestamp: March 09, 2015, 06:24:35 am
content: I like the fft idea. Easy enough to hook up a test.The sell would be that it could find multiple failure types and be used as a diagnostic system for other issues (such as wear).No yelling on the following comment please. Things like this are good if you are a contractor because it requires software to be changed for different applications. If your the expert you get the job. My belief is to have long term relationships with customers. That is money in the bank.

author: robrenz
author_rank: Super Contributor
timestamp: March 26, 2015, 12:14:51 pm
content: Inexpensive high sensitivity of 1V/g and cheap signal conditioningsensor linksignal conditioning info