Frame oscillations with 4S

I wonder if anyone can explain why my quad should exhibit serious frame oscillations before take off revs with a 4S battery, but flies perfectly with a 3S.

The Minin flickers with the 4S too, but is steady on the 3S (only powered from the 5V anologue end)...?

This is with an APM 2.6, Fw3.1.5. Afro 30A EScs and SunnySky 980kV motors.

Best DP

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  • Have you tuned the PIDs for the higher voltage?

    By increasing voltage you essentially increase the gain of the system.

    So you can go into oscillations due to the gain now being too high.

    If you tune for 4s you will probably be fine.

    This is not a unusual situation.

  • David,

    higher harmonics generated in the process of inversion 

    3S DC >  3-phase AC

    don't match signatures of higher harmonics generated by

    4S DC > 3-phase AC


    so resonant frequencies of airframe, SunnySky motors are affected by the 4S or 3S

    specific inversion signatures (higher harmonics) the way described by you.

    Try to replace inverter by another make or use digital oscilloscope to watch and record

    specific higher harmonics signatures vs. RPM and accelerometer data saved in log file.

    Process recorded accelerometer data with FFT analysis software to discover resonant frequencies in each case ( 3S vs.  4S).

    Test every motor in separate to discover if all motors are affected by resonance and if RPM

    speed curve rise match each other.

  • Easy.

    On 4S the aircraft is overpowered so the PIDs for roll and pitch are more "sensitive."

    Try running with the largest prop (maybe an 1147?) you can.  This will bring the motor RPM down and soften control authority

    To stop the Minim from flickering power it from a separate +5 volt BEC.

    • Agree with the above. I encountered the same problem with my 450 when "upgrading" to 4s lipo. As I read on a similar post on diydrones PIDs react in a percentage to battery voltage. So changed PIDs.

      I personally lowered PIDs to a point that it was flyable and then used AUTOTUNE for the rest. Mission accomplished! flies great.

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