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.
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.
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.
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.
What is the tone of your copter? I'm beginning to wonder if we are missing the obvious.
The tone of my 750 Kv copter is:
o slightly less than hover, 230 Hz (greater than 200Hz)
o hover, 250 Hz
o cruise 280Hz
o max speed 370Hz (less than 400Hz)
Then is that why hard fixing my (and others) APM to the frame is OK? Could this be a KV issue (mine just happens by accident to be in a really good spot)?
If a copter operates at 200Hz or 400Hz, That could be problematic (especially at 400Hz because the that is when motors are are at high throttle and generate the largest amplitudes).
So if you have a higher KV motor that goes 12,000 rpm at full throttle you are generating a huge 400 Hz signal. My copter does operate at 300Hz, but in actual flight is not nearly as problamatic as 200Hz or 400Hz.
It would be interesting to have those that have had problems hard mounting to the frame, test their copters. A link to an easy tone generator is:
http://www.audionotch.com/app/tune/
Try hover, takeoff, and full throttle (you can just disconnect all the motors except one or two; the frequency won't change if your props are balanced).
This is a really simple test that novices can do. For now, I'm parking this insight into my back pocket (and don't bother responding with puns on that) :)
P.S. Caution on converting rpm to Hz. Simply dividing rpm by 60 seconds per minute does not tell the important part of the story. If the props are reasonably balanced, the primary source of vibration is from prop wash. If there are two props, the tone of your copter will be twice that. Also, as the prop wash passes over your frame, it will cause a z-stutter that is also twice that. So don't be surprised of the tone of your copter is 2*rpm/60 (if you are using 2 blade props).
So i will propose one:
-this post was intended as an experience. It is not a recommendation. I will indeed subscribe to this word of caution : " Please note: these are my observation and mileage may vary for others. So pay attention to all the safety concerns and do the bench testing well before attempting a actual flight. I am not responsible for any crash and injury"
-secondly, after reading binary arguments, i will propose a consensus : since best apparent results are obtained in vinrations amplitude with a hard fix method ; since best apparent results are obtained in vibrations frequencies with very soft dampening material; since adding dampening material does not degrade significantly the control of the vehicle, then:
Increase the mass of APM by hard fixing any heavy item to it, add some moon gel dampening between this mass and the high frequency sources. Obviously eliminate vibrations at the source by balancing props,motors.
Can this be a consensus for the community until further scientific results would show otherwise ?
@Forrest
I was having problems with sonar spikes, so i disabled it and i get pretty nice alt holding and gps loitering. My zacc graph was under to -/+ 1 (using moongel) and the spikes i got in that were because the sonar would periodically decide to go crazy for what appears a single sample. Its a shame the code doesnt average samples so spikes dont ruin things.
The answer to sampling the waves as they go by as you sip beer (see earlier post) was an appropriate irregular sampling rate (hence no singularity and aliasing).
http://label2.ist.utl.pt/vilela/Cursos/Sampling_And_SuperSL3.pdf is one explanation.
So obviously this was rejected. Was it because of the hardware?
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