Can anyone give a bit of explanation of the characteristics of the foam blocks that come in the PH2.1 kit? There are 3 types, as shown below.
I’m having issues around 50-60Hz, but any information on the intended applications of the different types of foam would be great - I’d rather not resort to trial and error as a first step after the problems with getting replacement foam for the original Pixhawk…
Thanks very much.
To start with, to determine any issues you are having with resonance, do a quick flight with high speed logging running on IMU 1, then one an IMU 2
Run the FFT on these, and post the result.
The foams are all hard mount and offer very low isolation assistance. They are just for mounting.
If you do need to add external isolation for your frame, ensure to do not allow a rolling moment to be induced… the high CoG of the cumbe on the full carrier has a significant moment, so any isolation would need to consider this in its design.
I assume you are on a gas Heli or super large frame? Some pictures of your setup, logs etc will be important. You need the data to find the right solution
Thanks very much for the information - good to know.
There is nothing exotic about the machine, it is a folding quad in 15" triple blade props, carrying an A6000 on a single axis servo gimbal. Foxtech hover 1 frame. The FC has been mounted in a few different places - the frame has a damped plate from which the battery hangs, where the FC can be mounted, but as expected, with the large mass of battery and camera attached it caused all sorts of issues in flight. So the Cube is now mounted to the top plate.
I have tried hard mounted with VHB tape, various foam isolators, and with the top plate direct mounted to the rest of the frame, and installed on standoffs as in the picture below. I have now added a bit of stiffening to the top plate, and mounted the cube with velcro for convenience for now. This is giving me my best results so far. Please excuse the untidy build, it is not final
Here is an FFT from a 2 minute hover in pos hold this morning, no wind
And here are the accelerometers:
link to the logfile from that flight here:
What I’m aiming for is more like this, from a 550 x8 on 10" props with a 3dr pixhawk, note the peak vibrations are at around 50 mph…
I’ll try further stiffening the FC plate, might try some more isolation as well, but as you say the higher cog of the cube + carrier does bear consideration.
I would really like to get the accelerometer noise down to a minimum, this seems the key to getting good performance in pos hold or auto in windy/turbulent conditions. I guess cutting the vibes in half from where they are now should do it.
I may be barking up the wrong tree though, so very grateful for any thoughts on the above.
Cheers,
the “Vibrations” look very high. But maybe they are not as high as they look like - at least in relation to your 550 x8. In the first case it is the raw ACC values in the second the filtered IMU values, The vibes (Vibrations 3.3) on the 15’’ model look pertty good.
The vibes on the 15" model aren’t too bad now perhaps, but the X8 will position hold in gusting 30mph winds without shifting more than a few cm in any direction. By contrast the larger machine will wander about up to a metre or so in any direction, even in conditions of total calm. I had attributed this to the lower noise floor in the accelerometer data permitting more accurate position estimation?
edit - I admit I am not sure if the vibes graphing option is affected by the raw imu logging below, creating the appearance of a more of a problem? I am more working backwards from not getting the flight characteristics I would like…
Here are vibes from the flight with log linked above
and here are the vibes from the x8 test flight also referenced above:
Any document on how to use FFT?
What is recommended LOG_BITMASK?
@PittRBM set log_bitmask to 655358 (all+full IMU). Prepare for a long wait for the logfiles to download 
ctrl-f in mission planner brings up the FFT button. AFAIK there is no documentation for this feature, though there is stuff in the forums.
Thanks!
Few more questions:
What’s “Bins”?(lower left corner)
What does “MSG” mean? (ACC GYR MSG, lower right corner)
And does it calculate the whole flight or just one specific time frame?
Since the vibes are good on both systems vibration does not seem to be the cause.
It looks more like a GPS issue. The sat count changes frequently between 13 and 17. Regrading your setup this is pretty much. So I blame it on the trees…
Yes satellite signal not great down there! The closest spot I can pop out for a quick test at the moment.
Anyway, I replaced the standoffs for the flight computer plate with some natural rubber isolators and have got the vibes down a bit more, pos hold did seem better. below is Vibe analysis and FFT. Top is with the plate mounted on plastic standoffs, bottom is on the rubber. Max vibes are down, though averages not much changed. FFT shows the ~180Hz peak down and 60-70Hz down a bit, but not by so much. Seems all good on the vibrations front to me now though…
Hi!
I’m also wondering, what the foam actually is, that comes with Pixhawk2.1. I also got the three types of foam @Charlie_R described and used the ones in the middle of his picture. Unfortunately I had to remove the controller and damaged the foam pads. Does anyone know where to get replacement pads? Or something similar?
Thank you very much
Felix
