Inconsistent motor performance

[alfa budiman]

I'm having issues with the performance of my drone's motors during flight testing. I confirm that the ESCs have been calibrated properly as per https://docs.modalai.com/flight-core-pwm-esc-calibration/

For testing, I use Q Ground Control and have tried manual mode and stabilized mode.

I arm the drone (with propellers on) and gradually increase the throttle, but not enough to make the drone actually get off the ground, and I keep the throttle there. One of the motors eventually stops spinning or starts slowing down. As for which motor, it's not always the same and can vary, so I don't think the ESC / motor is the problem.

If I throttle it and do try to make it lift off, it starts tilting to one side so I immediately decrease the throttle to level it and keep it on ground.

I have also tested using motor test sliders on vehicle setup and also run into very inconsistent motor performance. When I throttle all the motors at the same time, sometimes one of the motors doesn't spin. Again, I can confirm that I can spin each motor individually. Also, when I arm the drone without propellers, all the motors spin, I put a piece of tape on the motor shaft and I can visibly see there's almost always a motor that isn't spinning properly or it slows down when it shouldn't. It's not always the same motor.

I have done this test using both hexacopter and quadcopter configurations. With the hexacopter configuration, there were 2 motors that didn't perform as expected.

What should I do to address this issue?

I don't think the motors / ESCs are the issue. I don't think it's a battery issue either.

I did another test using the motor_test function on mavlink shell on the drone with quadrotor configuration.
I execute

motor_test test -m n -p N

where n is the motor number (from 0 to 3) and N is the % of PWM output.

The motors spin up and maintain that constant rate until given another PWM output value. They respond exactly as expected without any issues. I've even tested with propellers on but only up to N = 10 any higher is probably unsafe and may risk the drone lifting off.

I should note that the motors can be quite sluggish to spool up when propellers are on, compared to spinning up immediately during testing without propellers.

[Chad Sweet]

Do you have experience building and tuning a custom drone? There is a lot that goes into it and it can be dangerous so please be careful!

Here is the PX4 guide: https://docs.px4.io/v1.12/en/config_mc/pid_tuning_guide_multicopter_basic.html

[alfa budiman]

Honestly, I have zero experience building and tuning a custom drone. This custom drone was handed to me by my research group and it's my job to get it to fly programmatically with MAVROS.

I was under the impression that the VOXL Flight Deck would already have its PID controllers tuned as I didn't see a section on PID tuning on the documentation for the Flight core.

Regarding that tuning procedure, it requires the drone take off and hover. My drone can't take off, it starts tilting to one side and I have to immediately lower the throttle else it will tip over. Not sure how to address this, do I just trial and error different controller values until it is "stable enough" ?

Or will disabling MC_AIRMODE automatically make it able to take off?

My understanding is if its disabled then there is no roll/pitch/yaw so the drone should be only be able to move up or down, but I have never touched that setting.

[Chad Sweet]

it's not possible for a flight controller to have PID parameters for an unknown drone. Every drone has different tuning parameters. We sell the VOXL m500, pre-tuned and flight tested, just for this purpose.