Pixhawk causing motor sync issues.

I believe I have a problem with my Pixhawk and was hoping for some help.  Also posted this on rcgroups.

The issue is that when my speed controllers are hooked up to the Pixhawk they exhibit a bad sync issue.  However when they are connected to my old APM (3.1) or just a receiver directly they do not exhibit these symptoms.  This has resulted in 4 crashes now until I just tonight narrowed down the issue to the Pixhawk.

Can anyone advise what is going on?  Here are the specs of my setup:

Afro 30A ESC's with SimonK firmware (have tried all versions, and made my own with various parameters)

Tiger MT2216-9 1100kv motors

4S 3700mah battery



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            • Ah, well since it works on the 5V BEC, even through the Pixhawk, I don't think that's the problem.  

              Now I'm no electrical engineer, my day job is as a computer admin.  But my best guess, at least in my specific situation, is that it is a ground loop introducing noise.  One ground goes via the power module to the Pixhawk over the 6 pin connector, and the other ground is via the power module to the power distribution board.  If I remove this alleged loop by either disconnecting the 6 pin connector to the Pixhawk, or by removing the power module entirely, or by not using the Pixhawk at all, the problem goes away.  Or, as is my solution now, replacing my Afro ESC's with the 3DR branded ones.  Maybe the problem combination is that the Afro's can't handle the alleged ground loop noise as well as the 3DR ESC's can.  Or maybe the 3DR ESC's are wired differently to eliminate this noise.  But really I don't have the tools, skills, or experience to know what the problem is for sure.  All I can do is post my observations.

              Back in the day my hobby was car audio.  On my first install I experienced a bad ground loop resulting in hearing the RPM of the engine via the alternator through all of the speakers because the head unit was grounded to the front of the car, while the amps were grounded to the rear of the car (ground loop).  The fix was to run the ground for the head unit all the way to the back of the car, and the problem was solved.  In the Pixhawk, I wonder if I tied the two ground wires from the 6 pin connector to the ground of the power distribution board if that would fix the problem.  But I don't know enough to be sure, and I'm not brave enough to try.

              • I wonder if I tied the two ground wires from the 6 pin connector to the ground of the power distribution board if that would fix the problem.  But I don't know enough to be sure, and I'm not brave enough to try.


                  Did you read this? http://is.gd/y8D7Ny 

                Someone said it was gibberish; so take it for what it's worth. There is disagreement of it helping, but nobody has said it would hurt anything. This issue is not confined to DIY. I've seen it discussed at RCG.

                When I had problems with Maytech ESC's, I wired a UBEC to the 'AUX OUT' connector because there wasn't enough wire to reach to the other side to the 'MAIN OUT' and I thought both rails were on the same ground circuit. Recently IIRC, someone said the ground is not on the same circuit for 'MAIN OUT' and 'AUX OUT', so when installing the Spiders yesterday I rewired the UBEC to #7 on 'MAIN OUT'.

                Because I have since returned the Maytechs, there is no way to prove the rewiring would have fixed the sync problems, but now with the spiders everything seems ok. Had it not rained I would have flown.

                Also note I disconnected all + wires from the ESC's when using the 3DR versions, and only used the HW UBEC, plus I am using OPTO now. 

                Below is my wiring. It does not loop the PM to the rails, but the HW UBEC is on #7 'Main Out' connector. I say try examples in that thread and see what happens. Note the use of a separate battery in a few of the examples.


                • Developer
                  I can assure you all the PWM channels ground and power are connected in parallel on the Pixhawk, however Pixhawk does not provide power out to the servos...

                  There are some ESC's like the KDE, that need 5v to be supplied to the servo rail. These would require that you have a Bec attached to the servo rail.
                  • Developer
                    The RX is on its own rail
                    You should NEVER run anything other than your RX off that rail
                  • It must provide some 5v out at the rail though right? Because it powers your RX off that same rail?

                  • Thanks I thought that was the case.  As it is an RTF build I am not questioning the wiring, I am sure it is fine and it all makes sense to me.

                    Good new is that I finally made the problem move to motor no.5 bad new is that it is doing the same thing.

                    Made the problem move by taking the good (original ESC/Motor 5) and moved it to position 6.  Rewired it to PDB No6 position for good measure.

                    This fixed No6.  This worked perfectly.

                    Then, I installed a brand new 3DR ESC (my last one) as ESC No5. and connected it to the original motor 6 which is now in the No5 position.  I was hoping to see if the problem stayed with the No6 chain or moved.  It Moved!!!  Finally.

                    After three ESC, three motors and several changes of this sort I made the problem move.  Not sure why but when I tried this before the problem did not follow the ESC or Motor.

                    Now the only thing that was original in this chain was one motor and cable (black) connecting it to the new ESC.

                    Installed new motor, including wires.  No5 motor now exhibits the same problem.  It will spin up ok but randomly stall, pause, skip and then keep going.  It happens at different RPM.

                    I am happy that I have finally made the problem move. What confuses me is that this is now the third completely new combo I have put in and I still get exactly the same issue. I did find an original cable with exposed wires, it looked like it has separated the insulation as it was pulled down the arm.  Could this have been destroying the ESC's as I put them in?

                    So now I guess I should buy another 3DR ESC and see if that fixes the issue.

  • Good news, I think, so far, so good.  I changed the Pixhawk for another one I had set aside for a fixed wing build (Both 3DR original units) and my Motor6 issue has gone away.  I am thinking it is a hardware issue with that particular board.

    However, first time arming and spinning up all motors run properly. I have noticed two things, if I drop the throttle to zero and then spin the motors up they jump to about 30%,(Same on previous board) and separately to this if I ramp up to about 30% on the first spin up of the motors they will idle nicely for about 3 minutes and then speed up to about 40% percent.

    At all times, when applying flight power (ie above 30%) all motors seem solid and running at the correct speed.

    I have not noticed this on my test quad running Afro ESC's but only on the Y6B running 3DR ESC's.

    I will keep bench testing until I am confident that the motors won't desync and see how it flys.  Changing the Pixhawk seems to have done the trick although I am still curious why 3.1.x does not produce the same fault on this board.  At least I seem to have identified my issue with my No 6 Motor losing sync by changing boards.

    BTW, to get GPS to work I had to load the Beta version of 3.2 via Mission Planner.  The default 3.2 loaded OK but did not recognise the GPS.

    • Spoke too soon. Didn't fix it, Motor No.6 still having issues.  New FC board, new motor, new ESC and even swapped Motor/ESC 5 for 6 still having issues on No.6!  Any ideas?

      • I didn't get as far as you; the sync issue was so bad on the bench I didn't bother putting it together.

        Tomorrow I will be testing ZTW Spiders and make absolutely certain the wiring is correct. Odd though that 3DR ESC's work, sort of.

  • I'm looking to put together a 680 hex, and I'd prefer to use a Pixhawk.  I've selected the SunnySky V4010 motors (KV375) and 6s batteries.  Each motor will draw less than 20A at peak.  With that low per motor current draw and a 6s system, my ESC options are somewhat limited (if I want to avoid using oversized ESCs).  I had originally thought of using Afro ESCs, but I'm glad I stumbled across this issue before ordering them.  Maybe re-flashing Afros will work, but I'm hoping to do better than "maybe".  I've clearly learned form this thread that the correct method is to attach both the signal and ground wires from the Pixhawk to the ESC.

    I would just suck it up and order the ESCs from 3DR, but I only see one model of ESC listed, and it's only spec'ed for up to 4s systems.  Other than 3DR's ESCs, are there any options that can be used with 6s voltage and can be recommended with a fairly high confidence that there won't be sync issues when used with a Pixhawk FC?

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