Actual motor min pwm does not correspond to PWM_MIN


For a VTOL, I’m trying to get the motors to spin when armed (at very low speeds, just to check if the motors are spinning correctly and in the correct direction before taking off).

I’ve followed the documentation posted here to create a custom mixer (custom.main.mix) and a script in init.d/airframes (created in ROMFS/px4fmu_common/…) that loads the mixer by calling set MIXER custom.

Since it is for testing purposes, only the multirotor part of the VTOL is mixed right now. Below is the multirotor part of the mixer.

R: 4+ 10000 10000 10000 0

In this script in init.d/airframes, I also call param set PWM_MIN1 1100 (also for channel 2, 3 and 4). If I then look in parameters in QGroundControl, min pwm is set to 1100 for those channels. If I then arm, the pwm is set to 900. The other lines in the script are executed (battery cells are set correctly etc).

I can use Idle speed to bump it up to 1120, but that seems weird to me. Why include the PWM_MIN if it doesn’t change anything?

What did I miss?


This should be the default behaviour for the quad motors, actually!

@sfuhrer I don’t understand what’s missing here :thinking:, do you?

Set your trim to middle .

Trims are still default and center on the remote

I don’t think trims should be necessary for this. It should be a combination of PWM_DISARMED and PWM_MIN and the correct ESC calibration.

I don’t think this should depend on ESC calibration. The PWM that is being sent (checked with oscilloscope) is 900 us instead of what is specified with PWM_MIN

Hi @Etruscian,
It’s best to leave the PWM_MIN at default. To specify at what PWM the motors are of a VTOL in MC mode with minimal throttle, you can change VT_IDLE_PWM_MC .
If you still have issues it would be best if you could share a log file, then we can help you better.

@sfuhrer That fixed it. Thank you! For a multicopter, I should use PWM_MIN right? What is the reason that they are not the same?

I think so, and guess it’s because on a VTOL you only want the hover PWM on the MC motors, and not also on the FW actuators.