Help diagnose a motor speed issue on Pixhawk please!!

Hi All,

Hope I'm putting this in the right forum to start off with.

I'm having a problem with the speed of a motor on a quad using a Pixhawk. It's motor 3, it's slower than the rest. Noticeable particularly at low RPMs. The interesting thing is when I swap around the ESC signal wires from the other motors into Main Out 3 (where motor 3 is usually plugged into) I get the same issue with the other motors.

I'm using a PS4 controller over the telemetry link (as I don't have my usual Tx with me right now). I think it's something to do with yaw control. When I do a radio cal using the PS4 controller it sets the trim for RC3 as 1100; the same as RC3_MIN. The trims for RC1, RC2 and RC4 are all around 1500 where I'd expect them. When I set the trim to 1500 on RC3 I still seem to have the same issue. I think it seems to be linked to the fact the throttle and yaw are on the same stick on the controller.

Does anyone have any thoughts on how to fix this or further diagnose the issue? That would be really appreciated!

Please let me know if you have any questions about my setup; happy to provide whatever info is necessary.

Thanks, Tom

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Hi Crady,

Thank you for the reply.

I'm really struggling to fix this so any help would be really appreciated.

I've gone through the setup for the joystick and re-calibrated the accelerometers. I did notice that the horizon had drifted since I'd first calibrated level.

I've subsequently put the quad in acro mode and am still getting odd speeds with the motors (I think acro mode should be unaffected by a potentially poor accelerometer calibration). Generally, one motor spins a lot slower than the rest (usually motor 3 but it's also been other motors too).

I'm using these ESCs: http://www.hobbywing.com/goods.php?id=338&filter_attr=6481.6480 

which I bought from hobbyking here: https://hobbyking.com/en_us/hobbywing-xrotor-multi-rotor-esc-opto-1...

I'm using it with 4S and T-motor 3510 360KV motors. On the hobbywing website it says it can handle 4S; where I actually bought it from on hobbyking it says it only goes up to 3S; do you think this will be affecting things?

Is there any special setup required for relatively low KV motors?

Cheers, Tom

I assume you are actually hovering when testing ?  - or are you one of those that expect this to work in a dry-test ?

Post a .BIN log of: 

in no or weak little wind:  takeoff,  hover at ~2meter for 10 seconds, land. 

Hi Andre,

Thanks for the reply.

I can't take off due to the motor issue. It's very noticeable on the ground. When trying to take off the quad tilts forward and the props impact the ground.

Can post a bin file of on the ground testing later if that would be useful?

Do you have any thoughts on what could be causing the issue at this stage?

Cheers, Tom

set RC3_MIN == RC3_TRIM

make sure ESC's are calibrated

set MOT_SPIN_ARMED,40 .... 60 ... 80  ... maybe even 100   (some at a time) , and arm (throttle at minimum).    you want to see ok RPM, but be far from takeoff.   

MOT_SPIN_ARMED will just spin motors (no PID/regulation involved)  on arming, you should then see all start equally , working at same RPM  - if they don't , one is slower, - it's most likely defective(or it's ESC not calibrated)  - try to swap motors/ESC's 

No special setup required however, ESC calibration is critical (see Andre K's reply).  These links should help if you haven't performed an ESC calibration or aren't sure if it was done correctly:

http://ardupilot.org/copter/docs/esc-calibration.html

https://youtu.be/gYoknRObfOg

https://youtu.be/Hl-Q7RPOn18

https://youtu.be/TS4OWAcfAQY



Tom Moore said:

Hi Crady,

Thank you for the reply.

I'm really struggling to fix this so any help would be really appreciated.

I've gone through the setup for the joystick and re-calibrated the accelerometers. I did notice that the horizon had drifted since I'd first calibrated level.

I've subsequently put the quad in acro mode and am still getting odd speeds with the motors (I think acro mode should be unaffected by a potentially poor accelerometer calibration). Generally, one motor spins a lot slower than the rest (usually motor 3 but it's also been other motors too).

I'm using these ESCs: http://www.hobbywing.com/goods.php?id=338&filter_attr=6481.6480 

which I bought from hobbyking here: https://hobbyking.com/en_us/hobbywing-xrotor-multi-rotor-esc-opto-1...

I'm using it with 4S and T-motor 3510 360KV motors. On the hobbywing website it says it can handle 4S; where I actually bought it from on hobbyking it says it only goes up to 3S; do you think this will be affecting things?

Is there any special setup required for relatively low KV motors?

Cheers, Tom

I see I wrote set RC3 ....  that was just meant to confirm your setup was normal.

also - "make sure ESC's are calibrated"  first,   if they ARE , and still very uneven RPM when level,  then the rest will give a clue ...

 I did not mean "do it all at once"  :)

Thank you for the replies, your help is really appreciated!

I've calibrated the ESCs using the transmitter method and then, when I wasn't sure that had worked, the automatic method. I'll go through it again and see if there's an issue somewhere. When performing a motor test they all appear to spin at the same speed - would this indicate that the ESCs are calibrated correctly?

I'll go through all the suggestions and see if we can get this thing flying! Thanks again for the help and fingers crossed.

Cheers, Tom

PS: one final question, in regards to setting RC3_MIN and RC3_TRIM; why are these equal? I thought that RC3 controls the yaw of the copter and if they are equal the copter would be constantly yawing? I've probably misunderstood something though so am happy to learn what's actually going on!


Yes, it sounds like all are calibrated.

Be sure that every motor is on the correct output, with correct propeller direction. 

once you arm (and throttle is in the regulated zone - not spinning just due to MOT_SPIN_ARMED)   - then RPM of each will by inflluenced by attitude/PIDs.   Then (no props)  you should be able to see that whichever motor is lowest (when tilting the airframe), will spin faster than the others/oppposite motors.  

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