Dear all,
I’ve been struggling to deal with vibrations in my quad copter and am trying to diagnose the cause and I hope someone can offer some advice. The problem appears to be intermittent and may be related to a second problem I’ve noticed with a motor/esc combination.
My Pixhawk flight controller is well damped (using moongel) and I’ve ensured as much as possible that wires are not transmitting vibrations to the controller. I have balanced the props and the motors. I feel happy that I have done all the right mechanical things to minimise vibrations because I have had some flights with very low XY and Z vibrations values in the log file. But I’m finding that there is no guarantee that a given flight will be so vibration free.
I’ve also recently experienced an issue with one of the motor/escs and wonder if this could be related. When I arm (or use motor spin test in Mission Planner), one of the motors (channel 3, front left on X quad) fails to spin, but just stutters back and forth. Recalibrating the ESCs fails to resolve it. Reversing the polarity of the motor can make it spin but when returning it to the desired polarity it may spin properly or may continue to stutter. Attaching another motor to the ESC in question at this stage seems to work fine and attaching the motor in question to another ESC works fine. And having done all these checks the original arrangement also works fine so that I am able to proceed and fly. However, that offending motor/esc combination failed in mid flight today with unsurprising results.
The intermittent nature makes it hard for me to decide if it is motor, ESC or Pixhawk, but I suspect ESC.
What I’ve noticed in the log file is that on flights when the vibrations are strong, the signal to RCOU channel 1-4 oscillates. I don’t know if this is in response to the vibrations or the cause. The faulty ESC/motor combination doesn’t appear to behave any differently than the others except just at the point of today’s crash, when it spikes a bit.
I’ve attached two log files from today. The earlier one has low vibrations with all seeming to work well. The second is from a subsequent flight with bad vibrations which also ended in a crash when one motor cut out - presumably because of the same fault I’ve noticed at start up. The faulty combination is in channel 3 of the Pixhawk (front left of the X quad).
So my questions are:
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Is the stuttering motor most likely to be due to motor, ESC or pixhawk (or something else)? Any thoughts as to why it is intermittent and why it can sometimes be resolved by switching polarity?
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Could the oscillation in the channel outputs be caused by vibrations or the cause of vibrations?
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Could a faulty ESC that is causing an intermittent stutter in the motor during start up also be causing vibration/oscillation that I’m seeing? The vibrations by far pre-date the fault with the esc/motor but I wonder if it could be a worsening fault.
Frame: Mikrokopter quad, 4S
Motors: Mikrokopter MK3638
ESCs: JDrones 30A Ultra PWM, BEC 2A/5V
Props: APC 14x4.7
Pixhawk with Arducopter 3.2.1 (36b405fb) and power module
External GPS UBlox Neo6 (I think)
Many thanks
Jeremy
