Occasional spikes in the accelerometer graphs

Hi - I have a 3DR quad running 3.0.1 and I have been working on reducing vibration to a reasonable level. My current setup has the APM 2.5 on the top tier of the stack with 2 layers of moon gel under each corner of the APM and I have a rubber band lightly wrapped around it so the APM doesn't fall off.

My graphs of AccX, Y, and Z look good for the most part with X and Y +/- 1 and Z between -9 and -10. Unfortunately there are occasional spikes that appear and every now and then I think they make the copter flip over and crash. What might cause these spikes? The frame and components on the frame seem solid. I attached the log from my most recent flight with a crash at the end.

Any insight is appreciated.


2013-07-14 12-54 15.log

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  • OK, I looked at your graph and although all 3 axes spike, it seems that the Z axis spike is bigger than the other two and always goes negative which is what you would see with a substantial ground strike (hard landing or bounce off the ground), these don't seem like spikes generated by vibration, because you would normally see corresponding high and low spike (which you do see in the X and Y but not the Z.

    It seems like the Z axis is possibly temporarily bottoming against or hitting something occasionally or you are bouncing the copter off the ground. 

    I just graphed it against CTUN Baro Alt, (You might want to give that a try), it seems like a number of the Z Down spikes are coincident with zero altitude, which supports the bouncing off the ground scenario.

    And I notice others are coincident with sudden changes in altitude which suggests a rapidly increased or decresed throttle setting possibly resulting in resonant vibration.

  • Hi Ned, 

    I'll see if I can help I wrote the current Wiki section on Vibration and have done a few blogs about it as well.

    First, it is really important that if you "wiggle" your APM a bit that no corner, edge or surface is actually able to hit anything else.

    If it moves enough in flight to touch, those brief touches cause large spikes and do cause trouble.

    Second, is it one axis or more than one that spikes and if so, which ones and how large are the spikes. (A log file with IMU enabled) of your accels "spiking" would be helpful.

    Third, it is equally important that aside from having a "relief loop in your wiring going to the APM that it also is not able to "bump" into anything during flight, surprising accel deviations can be transferred by the wires.

    Fourth two layers of moon gel might be too much (I'm not saying it is, just might be). 

    When your copter goes through various speed ranges and maneuvers you may be developing a resonant vibration at some point which can allow excess movement to actually amplify vibration temporarily rather than suppressing it.

    Finally, I have found that a Velcro strap with soft foam loosely retained between it and the APM does a more satisfactory and complementary job of retaining the APM and actually helps with vibration suppression better than a rubber band.

    There have been a couple cases where a faulty accelerometer was suspected, but it is not a very convincing scenario.

    If you could stick a graph on here, maybe a picture of your APM and retention setup and a more detailed description of the exact circumstances of when your problem occurs I will try to help you with it.

    (For instance, does it happen while you are just hovering in place or in gusty winds or while maneuvering around.)

    Thank You,


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