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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Can you get us logs from the Pixhawk and APM from this testing?  It might be helpful.  This is an interesting discovery.

Sure, will do when I get home from work.  I do have the one here from one of my crashes due to this issue.

Are there any logs you'd like me to enable before doing a bench test?

On the Pixhawk, there should be one called RC Input or something like that, if you're running the latest firmware.  I think it's in RC3.1.

CTUN, and Motors or RC Output it might be called now.

Basically we just want to look at the signal coming in from the Rx, and what is being sent out to the motors.

Are there any other differences between the setups?  How are you feeding signals to the Pixhawk from the RX?  PPM Sum, or SBUS?

Both APM and Pixhawk use PPM Sum, but with the receiver only in my video it's just regular PWM.

And this problem does not happen with APM?  Are there any other differences you can think of?

Bit of a long shot, but could even be some kind of electrical noise?

I've been flying Pixhawk on a quad for a while, and haven't noticed this at all.  F-20A ESC's, SimonK, and 800kV motors.  We've never seen high kV motors like you have, be affected by sync issues.

We might need to scope this unfortunately.

No there is no problem with the APM (as per the second video).  Can't imagine what else could be different, I'm even using the same power module on the APM as I am on Pixhawk. 

The only other clue I can offer is that I can get some sync issues with just the receiver if I remove the ground wire pin from the servo connector on the ESC as per the following diagram and video: https://www.youtube.com/watch?v=H-1j7X_vNw0

However in this video the receiver is powered by the Pixhawk as well (but the receiver is controlling the ESC, not the Pixhawk) so it's possible it's just the same issue manifested differently.  And I did not have the problem with just the signal wire connected to APM.  Maybe there is some difference in the ground path between APM and Pixhawk, and these ESC's are sensitive to that?  I'm bumping up against my limits of electrical knowledge here...

I may try this test again tonight but power the Receiver direct from the ESC instead.  Maybe that will give another clue to the problem.

This sounds like it's already solved.

So you have been trying to run the Pixhawk system without using the signal ground wire to the ESC?  Why?

So the problem goes away if you connect the ground?  And if you don't use a ground, running signal to the Rx, you do get the problem?

I always use a ground wire on the ESC signal cable.  I've never had a problem doing it that way.  If it worked in the past without the ground, I can only imagine that was just luck.  

No, the ground doesn't fix the issue with the Pixhawk.  I have the ground connected now but as you can see I still have the problem.

The reason I only had the signal wire connected originally is because it was recommended by this guide for the APM: http://copter.ardupilot.com/wiki/apm25board_overview/

  • When connecting your ESCs to the OUTPUT connector clip the BEC power leads on each ESC’s 3 wire connector.
    • Or connect only the ESC’s signal wires to the OUTPUT connector.

Also, it is a neater install only having 4 thin wires going to the flight controller.  But regardless the ground is connected on all of them now and I still have the problem.  So in summary, with APM signal only is ok, but for Pixhawk, Ground and Signal or Signal only doesn't matter, still have sync issues.

Ok.  We'll look at it more.

Did I misunderstand that video though?  It seemed to show that connecting the ground solved it?

Hey Randy,

Yes in that video it did fix the problem.  That is basically the situation I am at with my first video in this thread where a separate receiver works fine.

This is basically the test rig I have been using in these examples.  The only reason the Pixhawk is there is because I am using it to power the separate receiver out of convenience.  It is not controlling anything.

That diagram is a working configuration with no sync issues.  If I remove the ground wire in this diagram, or keep the ground wire and connect the ESC to the Pixhawk, that's when I get the problems.

I am not sure if my issues based on the same problem:

Y6B/Pixhawk and Changed to new ESCs (RCTimer 30A Opto) because I had the same issues before with the 3DR 20A ESCs. First I thougt it was a ESC Calibration issue, but the Motors spin visibly at the same speed after calibration and direct test at lowest possible throttle input. Tryed to set Thottle_armed and Throttle min to higher values (140,150) but its still the same. Yesterday it was a very calm day here in Germany and I made a new Autotune process. In the middle Roll process the copter looses suddenly about 3 meters and crashed to the ground before he could correct the roll movement. It sound like one side of the copter (Motor 5 and 6) stopped while the roll movement.

I know that its very hard to follow my explanation without any logs e.g. but it seems to be that Xoltri is not the only one with these issue.

My ESC ground wires where always connected, and I left some time between ESC calibration and repowering the copter.

Try the test in my first video and see if you can recreate sync issues on your motors with the quad strapped down.

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