Octoquad Motors Spinning at different RPMs when arming

ArduCopter Copter 4.0
1

Right after arming, PWM output on all 8 motors go up to 1600 RPM and only 2 settles down after a few seconds to 1250. The others go down to about 1560 but still with 60 to 100 difference between some outputs.
I’ve tried calibrating the ESC again and again, radio throttle range is 1102 to 1927 but that didn’t work
We’re using a herelink, and already set the PILOT_THR_BHV = 7, even setting it to 0 and starting the throttle at minimum position doesn’t resolve the problem.
Tested the motors using motor tester in Mandatory Hardware, least reliable motor spin at 17% so I set MOT_SPIN_ARM to 0.18 and MOT_SPIN_MIN to 0.2.
Calibrated AHRS, Level and compass before flight, baro reading is stable.
Motors are sluggish and unresponsive in Stabilize mode.
I tried to set all motor output as passthru for channel 3, to check if it’s an ESC or motor issue, but all motors are super responsive and spin roughly at the same RPM (measured with an optical tacometer)

Any suggestions to make it arm at the desired idle rpm?

Thanks,
Imad

2

How are you testing this? On the bench with no props?

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3

On the bench with no props on. Props are 22in and i have no way to fix the drone to the ground so i cannot risk testing with the props on as the RPM on all motors on start is too high

4

This is meaningless and a waste of time. You will learn nothing from this. The only way to test motors in that condition is with Mission planners Motor Test.

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5

Using motor tester motors spin fine and as expected, and all at roughly same RPM, even when i pass thru throttle channel to all motors they work fine. It’s just after arming it motors act weird.

6

As I said, meaningless. They are not acting weird they are acting as expected under closed loop control with no feedback from the sensors to commanded action.

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7

OK I will try to get some propellers on and check how it acts.
I’m just confused because I’m doing the same procedures as when building a VTOL, same motors, same ESCs, sensors and cube orange and in QStabilize mode without props on and the motors spin at roughly the same RPM and within the MOT_SPIN_ARM parameter. Though quad rotors not octoquad.

And thank you so much for the quick replies!

8

This is control theory 101. Closed loop controllers do not operate properly in open loop configurations.

You can only test the closed loop behavior in real airborne flight.

If you want to test on the ground, the only meaningful test is a open loop test, the motors test in mission planner.