I finally did the test of hard fixing the APM case to the frame, thus not having two separate masses with vibration dampening materials in between, as before (i.e. : frame/props on one side and APM on the other side).
This is explain by Forrest Frantz in a post I made earlier about moongel use here:
and in more details here:
I have to say that I felt on my bottom when I saw the results : TWICE less vibrations with this method WITHOUT any moongel nor any other vibration dampening techniques. JUST ONE SINGLE mass (APM fixed on frame directly).
Look at this graph that shows ZACC within plus or minus 0,1g which is twice less than the best +-0,2g I had with moongel combined with rubber bobbins and/or silicon gel. It is measured on the exact same aircraft (except moongel was removed and rubber bobbins were replaced by hard pylons). APM is stcked to the plate with NON foam double sided tape (hard double sided tape, as I did not want to drill screw holes in the APM plastic case).
Not only have I less vibration but I will also get better control on the craft since I get rid of the lag effect of dampening materials.
I confirm thus the theory of Forrest Frantz. It works for my frame.
It would be nice if others could confirm this also on their frame.
I publish this UPDATE after the information about 200hz dangerous vibrations came out thanks to the debate initiated of this post: the APM and its integrated sensor's hardware have a limit (due to low processing power limiting sampling frequency - that should be solved with the more powerful PX4 hardware) in such a manner that all frequencies of 200hz (and multiple of) will impact the APM control (accel) and will go through the 20hz software filter, undetected (thus noise id processed instead of valid signal, possibly creating catastrophies). As there is no way to analyze in a log AccelZ graph if you get these 200 Hz frequencies on your arducopter, it is necessary as a precaution to use dampening technique(s) to eliminate these high frequencies (even if the AccelZ log seems to show low vibrations by direct fixing APM on the frame).
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Greg
Glad that helped.
With the Pixhawk and its two acc. and with different sampling rates, are there reasons not to mount the fc. directly?
thanks
As you know, I've never had problem with hard mounting the FC to the frame with lots of data from my ships and others backing that up (a special program was built that we all have access to to test the hypothesis). The data has been clear. In all cases (except when sacrificing ship control), any dampener or isolation between the frame and FC increased vibration at the FC. But, there were some that swore (no data) that their ships had issues with a firm mount.
With the Pixhawk double sets of accelerometers and gyros, the FC is less likely to be impacted by vibration. So now, it would seem even more so that one can simply worry about detectable vibration levels at the FC without the worry of vibrations in sync with the sampling rate fooling the accelerometers.
The Pixhawk comes with double sided foam tape to mount it. Can you do two 2-minute hovers and post the resulting two log files (IMU, CTUN, GPS, ATT, & CURR must be enabled) to add more hard data to this discussion (do not change any PIDs or anything else between the flights):
- One flight with the foam tape between the frame and FC
- One flight with the FC firmly bonded to the frame
Doing each flight in loiter for two minutes is a great way to ensure similarity in the flights. I'll be glad to run the analysis for you. This is the the results of my latest duration quad using the Pixhawk.
Hi Frantz
Sure here you go. But please note that I have the Dropix, and since it comes without case it is sandwiched between ground and top plate with mm EPP in between. This is more to ensure rigidness than dampening since the fc can not move at all.
I have tested alpha gel and other foam stuff last week, but it gave me very unsatisfying results...
2014-08-21 17-31-27.log
2014-08-21 17-31-27.kmz
Good vibration and good stability. My guess is that if the frame was lighter, then the rotors could work less hard (current mid throttle is at 676) and all the numbers would improve. But it's fine the way it is.
If you want to make the frame lighter, send a photo as a place to start.
On the HoverAnalysis, when the error 76 message occurs, select debug. It will then go to the exact line of code causing the issue and will probably tell you what the problem is. If not, let me know what it said. Sometimes it's an international setting thing.
btw frantz
I would like to run the Hover analysis myself, but I always get run time error 76. And your log analyzer file works just fine, so I assume the inserted path is correct.
it"s on win8.1/office 2010
Well great ....... I originally Had the amp 2.5 secured with thin foam double sided tape ....and was flying without problems .... Interesting to note that since I began using moon-gel etc , and trying to "reduce" vibrations, as per advice .... I have had a series of problems ....I had put this down to 3.0.1 .. and vibration or em interference, but am now beginning to think otherwise .....
I will go back to my original method of securing and see if my problems disappear !
serves me right should have stuck to what was working OK ..!!
Thanks for posting this ...
Thanks all for your input and patience..
- so I'm using 750K/V x 11.1V or 8,325K motors and end up between the evil 200 Hz and 400 Hz frequencies.
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