Vibration Isolating Mounts

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gplracerx
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Vibration Isolating Mounts

Postby gplracerx » Fri May 23, 2014 12:11 am

I've started wondering if some of the noise problem with accelerometer data is high frequency vibration that's aliased by the relatively low sampling rate. Note that using a low sampling rate, like 10 or even 30Hz without a low pass filter adjusted to half the sampling frequency before sampling makes the problem worse. That's especially true if a lower frequency is just fewer samples rather than an average of several samples at the fundamental rate. The intrinsic noise level of iPhone/iPod accelerometer is fairly low. It might not need filtering if the vibration noise could be minimized. You may not be able to eliminate the fundamental resonant frequencies of the vehicle suspension. And you don't want the mount so soft that the video image is compromised.

Since an adjustable LPF isn't an option, I'm thinking that some sort of vibration isolation in the mount would be a good idea. The DIY drone community uses vibration isolation for their inertial motion units and cameras. With cameras, the vibration problem is called the jello effect. Of course the motors and propellers are particularly problematic with planes and helicopters. Other than that, I haven't been able to find much. RAM claims its mounts offer at least some vibration isolation, but they don't have any data in their ads.


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Re: Vibration Isolating Mounts

Postby gplracerx » Sun Jun 01, 2014 2:33 am

After some experimentation, I've decided the problem isn't so much vibration from the car as too much flexibility in the mount. Gluing a ball to the windshield is something of a last resort. I've ordered the RAM Triple Suction Base with B-Ball RAP-B-365-224-1U to see if that helps. The Optrix super sucker mount might be an option also.


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Harry
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Re: Vibration Isolating Mounts

Postby Harry » Fri Aug 15, 2014 12:34 pm

Someone from ACE pointed me to one of their apps specialized to measure vibration. You may have a look into it, it is free.

http://vibrochecker.com

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gplracerx
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Re: Vibration Isolating Mounts

Postby gplracerx » Tue Aug 19, 2014 1:42 am

Thanks, I'll try that. I'm thinking now that the problem isn't so much vibration isolation as a mount that doesn't itself vibrate significantly in the frequency range of interest. I used the small RAM suction cup that comes with the OPTRIX case to mount an iPhone to the windshield and I could see the iPhone vibrating with respect to the car whenever I hit a bump. I definitely don't see that with the three cup mount. But since the mounting balls on the RAM mounts are hard rubber, there will always be some flex. The longer the mounting arm, the more flex there is and the lower the fundamental resonance frequency. High frequencies can be filtered without distorting the data. Low frequencies, not so much.


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Harry
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Re: Vibration Isolating Mounts

Postby Harry » Tue Aug 19, 2014 8:09 am

I measured the dominant vibration frequency when driving on a highway (using the LapTimer standard RAM mount with the arm 45 degree horizontal / vertical) , and the tool reported 40 Hz. No idea what to do with this, just more information. :-)

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gplracerx
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Re: Vibration Isolating Mounts

Postby gplracerx » Tue Aug 19, 2014 4:34 pm

I've found that a low pass filter with a 2Hz cutoff won't significantly distort acceleration data. 40Hz is way above that. The problem comes if the sampling rate is too low. You need to sample at 80Hz to avoid aliasing a 40Hz signal to a lower frequency. That's why I recommend you set the accelerometer frequency to 100Hz, or as high as it will go. Then a low pass filter with a cutoff frequency of ~2Hz or a time constant of ~0.1 sec will remove the 40Hz noise and still not distort the vehicle data.


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Harry
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Re: Vibration Isolating Mounts

Postby Harry » Tue Aug 19, 2014 4:45 pm

The named tool samples at 100 Hz. Talked to them on how to interpret the 40 Hz and the result is simple... It is the engine running at 2400 rpm and I confess it tested using my Land Rover Defender with a Diesel engine ;-) That on the other hand is pretty interesting! One could measure engine rpm without OBD - by measuring vibration :-) But so far it doesn't look like it is possible to do that fast enough to catch rpm movements...

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gplracerx
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Re: Vibration Isolating Mounts

Postby gplracerx » Tue Aug 19, 2014 9:29 pm

Maybe. You could test that by using a longer or shorter arm on the mount. Either will change the fundamental resonance frequency of the mount. If the peak is still at 40Hz, then it's the engine. If the peak shifts to higher frequency with a shorter arm or lower frequency with a longer arm, it's the mount. Or try it in a different gear at the same speed.

As I remember, in Formula One before the TV coverage got all the telemetry, the engine rpm was determined with a real time frequency analyzer on the sound from the on board microphone. With that, though, I think you have to compensate for the number of cylinders and the fact that each cylinder only fires every other revolution. For a V8, you would get four pulses per revolution. So 12,000 rpm would be 200 x 4 = 800Hz. You would have to sample at at least 1600Hz to measure that.

The magnitude of the peak is important as well.


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Re: Vibration Isolating Mounts

Postby bulls23 » Thu Aug 21, 2014 11:09 am

I remember Harry talking about that kind of RPM measurements years ago. I tried this App (http://wpsm.free.fr/en/racingtach.html) and the results are pretty amazing and you can even use an inductive clamp if the engine sound is too masked. Setup works just the way you described it gplracerx. That App seems to be outdated though or at least I can't get it from the App store. Here's another one:

https://itunes.apple.com/de/app/engine- ... 01811?mt=8

This still might be useful for non OBD cars.


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Harry
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Re: Vibration Isolating Mounts

Postby Harry » Thu Aug 21, 2014 2:00 pm

You are right, we discussed it. So this is actually the third approach to get RPMs besides OBD and acceleration - using audio this time :-)

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