Using an iPhone to measure vibration

[edmac]

Hi Everybody

I am currently a student, and we are interested in getting vibration data from in-service aircraft. We are interested in measuring accelarations when an aircraft is taxiing. We were thinking of a way to get this type of data without having to install complicated equipment, and hoped that we can maybe rely on the good will of people that might be interested to help us out.

We thought that it might be possible to use an iPhone to do measurements. We found some apps that can do this for us, and it seems as if the accelerometers will be good enough for what we need. Do any of you think this would be feasible, and if so, would anybody be willing to take part in such a study? There are obviously issues surrounding the use of phones etc, and I am not sure if all phones need to be switched off, or if flight mode is sufficient, but we would first like to see if this might be a viable idea.

I know there are obvious restrictions, and I am assuming the FTI guys will have a heart attack about this idea, but we have thought of several applications that it can be used for if it might be a viable option. We have actually seen some guys in the States that are using it on Light Aircraft for Prop balancing.

Cheers

Ed

[ray cosmic]

Hi, an Iphone works in Flight mode with apps 'ISeismo' and 'Seismo Data':ok:

[jimjim1]

Perhaps consider Wii controller too?

Google [wii controller hack] for ideas to get started with.

How to hack a Wii controller into a personal flight recorder | Video « Wonder How To (http://www.wonderhowto.com/how-to-hack-wii-controller-personal-flight-recorder-272819/)

iPhone sounds a lot easier of course.

Edited to add -
How to use a Wii Controller to play games on your iPhone 3GS | Video « Wonder How To (http://www.wonderhowto.com/how-to-wii-controller-play-games-iphone-3gs-308300/)

[edmac]

Cool, thanks, that is an interesting article. I was thinking that it must be possible with a Wii, but was not sure how you would hack it.

[Apollo30]

I wonder if an iPhone is an appropriate tool for scientific measurement. I suggest taking a more sophisticated tool (sorry I can't suggest something - I'm not an expert in this field) to measure on the one hand and to develope your own measurement software on the other hand.
A Wii controller sounds a little bit more applicable tool, because here you can code your own software.

[P1-FTE]

Might be worth a try, i've played around with a Microsoft mobile version on a PDA before in an aircraft, but not on a Flight Trial. How do you know/ can quantify what the iphone is actually telling you, how accurate is it recoding vibrations, probably as good as the App; and was that made for just a bit of fun or did the developer consider a more serious type of application?

A quick ball park check maybe but nothing more accurate unless you know exactly what the iphone is telling you before you start.

I might think about (sorry if you have already) for the iphone approach

What’s the vibration range (in Acceleration, Displacement, Velocity)
What’s the accuracy ( full scale %)
I don’t have an iphone but does the screen lock (stopping the program) battery life, mounting etc etc.

Hope that helps

[edmac]

Hi Folks

Here is a link of some students that used it for some or other gravity experiment in the Vomit-Comet. I am not actually sure how useful it was, because there was no mention of the iPhone results in their final report.

Accelerometer Data (http://www.wavefrontlabs.com/Wavefront_Labs/Accelerometer_Data.html)

And if you go to the forums for manufacturer of the following app, they do discuss using an iPhone for propeller balancing.

Showcase: Vibration Turns Your iPhone into a Spectrum Analyzer | MacResearch (http://www.macresearch.org/showcase-vibration-turns-your-iphone-spectrum-analyzer)

Software (http://www.dld-llc.com/Diffraction_Limited_Design_LLC/Software.html)

Cheers

Ed

[dangermouse]

a frequency spectrum, the amplitude at a specific frequency, displacement of a component, whole body acceleration?

The type of sensor you need depends on the data requirement.

I dont have an Iphone so can't comment on the applicability of the accelerometer which I know it does have.

The accelerations you are probably looking for may be of too low a magnitude or short a duration to be usefully recorded.

just my 2p

DM

[Double Zero]

One word comes screaming out from this; CALIBRATION !

[DOMO1977]

I have used one of these for simple vib checks on rotorcraft interior parts with some success. GCDC Products (http://www.gcdataconcepts.com/products.html)
We do have a full on vib measuring kit that uses a laptop for proper analysis, but that is quite cumbersome and takes a bit of time to set up.
I find that keeping one of these USB jobs in your pocket is handy for a quick check if you spot something during a flight. It's 3 axis aswell and comes with FFT software and plugs straight into your PC! at about £80 it's a lot cheaper than you iPhone if you drop it in the hangar or on the pan!

[Flyt3est]

I would suggest that the frequency range is inadequate for a kick off, for most aircraft vibe work you are going to want 0-10KHz as a starter. I would also question how the phone is attached to the structure to avoid damping of the signal?

I could probabaly spend most of the day telling you why I think your theory is over-simplifying a fairly serious subject matter.. But I guess it depends on what you are trying to achieve. For example, prop balancing, give me an I phone as a measuring device and I will show you a balanced propeller without ever making an adjustment.. You get the idea.

Why not buy a simple vibe tracker and bolt it to suitable structure, that will give you a rough indication?

Dangermouse and Double Zero are talking baseline sense here fella. :ok:

[FTE Pruner]

Overall, consideration should be given to the requirements for the results.

If there is no intention to use this data for certification and it is a means of utilising a ready packaged and easily available device for a university type project, then as long as the mounting of the device has been assessed for flight safety (EMI, physical integration into aircraft, etc) and all the necessary paperwork is complete (if required) then I see no problem with using it.

The posts above are all correct, you need to make sure that you use the right FTI for the job to get data which can be used for certification evidence. In your case, you need to make sure that the results you get are actually correct and not affected by your measurement technique (to make sure you get a good grade).

Ultimately (as has been asked before), what are you measuring? Is it low frequency acceleration data to measure passenger side-force during turning, or is it high frequency vibration?

I have seen data from an I phone accelometer App compared to INS data from take-off, and to be honest, there wasn't much difference between the results. As I say, you would be silly to try and use it as certification evidence, but easily good enough for a uni project.

The low frequency response is definitely acceptable, but if you are looking at higher frequency then why not put the Iphone (and your proposed mounting bracket) on a vibration table and compare to known data across your intended frequency range. This might give you the answer you need.

FTE Pruner

[Genghis the Engineer]

Adding to FTEP's post, the planning, execution and reporting alone of calibrating the iPhone's suitability as a vibration monitory may, done well, carry the bulk of the marks available for your assignment.

The rest will most likely be in how well you use that information to explain and show the limits on actual test data obtained using your iPhone.

G

[Simon150]

Agree with Genghis. Sounds like a great student project.

The key to success may be identifying the right question - an appropriate subject for the iphone.

In the industry we rarely generate 'random' data sets. The recording of vibration parameters is a costly business and is usually instigated as part of a risk-driven exercise to better understand or confirm the understanding the environment of a certain (usually very specific) piece of equipment or location.

I don't subscribe to flyt3ests general post stating that the iphone is inadequate. 10Hz-10kHz is a fairly standard range for aircraft measurements. However, depending on the subject of interest, and the fact that you don't need to sell the results to the FAA or EASA, I am sure you could find a more suitable frequency range. For example, you wouldn't want to use fltt3ests 0-10kHz setup to measure low frequency undercarriage vibration during touch-down as the system sensitivity needed to capture useful data at 10kHz would inevitably result in saturation at lower frequency (higher G) conditions.

Component suppliers will also be uninterested in any frequency above 2-3kHz as the energy involved is so low there is negligable risk to the equipment. Typically, component testing will not go above 3kHz for this reason (as well as test limitation). So even though data may be available to 10kHz, the upper ranges will never get looked at.

The 3 - 10 kHz range is really only of concern to engine mounted equipment and even then generally only concerned with gearbox or fuel system associated equipment that my be affected by gear meshing or pump pressure pulsations. Structural damping sees that these high frequencies don't propogate far through the structure. HP spool fan passing frequencies will also fall into this range but would not propogate any distance through the structure.

It may be 'ride quality' that would be the best application for the iphone. The human body doesn't respond to these high frequences either and robust data is only needed upto a couple of hundred Hz to be useful. If you are looking at the taxi case in particular, then an assessment of the human impact of aircraft vibration may be an acceptable topic.

Good thread! Let us know how things progess.

[Flyt3est]

Simon -

Concur with your post, should qualify my own statements by saying that Edmac's outline requirement was pretty vague in nature and the reference to prop balancing made me twitch toward engine excited vibes as well as low freq taxiing signals. I agree though, I look forward to hearing a bit more about the study case.

:ok:

FT

[Shawn Coyle]

Having run up several helicopters without rotor blades, I'm wondering if you can run a propeller airplane engine up without the prop, to see what engine vibrations are…

[Genghis the Engineer]

Almost certainly not; unlike a modern helicopter, a piston engined aeroplane has an ungoverned engine - without the propeller load you'd most likely get an overspeed and engine damage.

I did some interesting work umpty years ago on a certain Australian piston engine that was giving us a bit of trouble - switching propellers and fuels, then running on a ground test bed with high rate vibration analysers (sensors + FFT); it told us a lot. With the growing use of MOGAS, led by cost escalation and likely eventual unavailability of AVGAS, similar work on more common engines might of of considerable use.

G

[JimCrawford]

The original post was concerned with accelerations during taxying. I've recorded some measurements using a GCDC device, mentioned earlier, with a sample rate of 200hz just to try and get a handle on the environment experienced by rack mounted scientific equipment. Mediocre taxyway surfaces produced by far the worse shocks, flight being rather boringly benign, despite manouvers. This was on a BAe146. I also sneakily attached the device to a rack in the lab before it was sent to be installed in the aircraft. This involved being trundled across the pan on trolley wheels, lifted by fork lift and set down on seat rails, manouvered by hand. This generated the overheard remark " everytime we shift this rack a little red light in that unit flashes " !! There are no prizes for guessing the most stressful environment for delicate equipment!

Sounds like a great project if, as suggested by others, the requirements, resources and procedures are well defined. Probably the hardest and most worthwhile part.

Jim

[edmac]

Hi Folks

Thanks for your very useful feedback. I have not been on the site for a while, but I am glad to see the constructive comments. We are indeed interested in passenger comfort, hence would like to relate passenger experience to vibrations measured. The problem is that we have no way of getting representative data, hence this idea. The other application we thought of was related to lateral loading on the ground. There is a huge FAA program where they are measuring the lateral loading of various aircraft when taxiing on the ground. The problem with this project is that we are having some difficulty interpreting some of the data. Here is one of the reports www.tc.faa.gov/its/worldpac/techrpt/ar02-129.pdf (http://www.tc.faa.gov/its/worldpac/techrpt/ar02-129.pdf).
I guess we were trying to think of a way of measuring representative data similar to the data mentioned in the report. I will let you know how we get on. We are planning to do some calibration tests and try to figure out what the best type of mounting should be. In the end we will try to come up with some ideas what it could be used for and what not, what the mounting should look like etc. In the end we are not interested in hugely accurate data, but in data that is "good enough". The trick will be to determine what "good enough" means!