Users claim differing ESC sync performance between APM and PIXHAWK

[Jonathan Challinger]

"Replace pixhawk with apm, NO PROBLEMS 5 lipos no problems at all, put the pixhwk back and back to unstable motors."

Is there some difference in the output signal between PIXHAWK and APM? Have they been compared with an oscilloscope?

[Randy Mackay]

 

     The only difference that I’m aware of is that the APM is 490hz while the Pixhawk is 400hz.

 

-Randy

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[john...@gmail.com]

Is there any difference in internal update of the PWM generator? While the hardware outputs PWM at 400/490hz the internal update of the output value is something else.
I am just speculating that if the PX updates more often, this could be seen as a more aggressive motor control by the ESC and lead to sync problems not seen on APM. It could also be something as simple as different gains, leading to more/less aggressive motor behavior.

Regardless, as long as someone can confirm PX pulses are fine, this is probably just yet another motor sync problem and technically not APM related. The PX/APM is just exposing something marginal.

- JAB

[Randy Mackay]

JAB,

 

     Ah, the internal update rate is also faster.   So the motor rate is updated at 400hz on the Pixhawk vs 100hz on the APM2.

 

-Randy

[Ian Ray]

I can't specifically comment on the APM vs Pixhawk comparison, but this might be tangentially related - I noticed yesterday while bringing up a Quad platform using a Pixhawk and T-Motor SimonK ESC's that the motors would occasionally knock or stall at startup when the RC_SPEED = 490. Especially when using the CLI "motors" test.

Changing RC_SPEED = 400 seemed to resolve the issue. I'd figured this out but then forgotten about it on a Y6 platform using the same model motors & ESCs awhile back. 700kv motors, so shouldn't be the notorious sync issues that low KV motors seem to be prone to?

It seems the RC_SPEED default value is 490hz?

[Robert Lefebvre]

Is this from Xoltri?

http://www.diydrones.com/forum/topics/pixhawk-causing-motor-sync-issues?xg_source=activity

Another difference between the APM and PH, is that the PWM level is 3.3V for the PH, and 5.0V for the APM.  Why that would make a difference, I don't know, but just pointing it out for completeness.

[Gary McCray]

Hi All,

From the appearance of these posts, it seems that various ESCs might not be able to handle the faster update rate of the PX's.

If this is true, wouldn't the simple solution be to supply a user parameter that would either let the user deprecate the update rate to the same as the APM or, possibly better set the update rate directly?

I know one more weirdo parameter, but it seems like the simple solution to the problem.

At least worth checking if this might fix the problem anyway.

Best Regards,

Gary

[Robert Lefebvre]

But the thing is, the update rate may be faster on the PX, but at this point, AFAIK, the stabilization loops are still running at 100Hz.  So the Pixhawk will be getting 4 identical updates.  So really no update at all.

And the problem seems to occur most often with step-function changes in output.  So in that case, the update *rate* wouldn't matter at all?

--

[john...@gmail.com]

If the problem is sync loss because of rapid PWM changes (regardless of frequency), then you have a motor sync issue. Meaning wrong esc timing and/or to large propeller. You can mask the problem by degrading the PID/PWM performance, but I how do you know for sure it won't happen in the air if circumstances align just so?

[Robert Lefebvre]

Yeah, I understand that.  But the question, or what is being alleged here, is that the problem is worse for Pixhawk.  So why would that be?

I would like to see what comes out of the user Xoltri's Pixhawk which has been sent back to 3DR.  He demonstrated that it really was worse on the Pixhawk than APM, or direct to the reciever.  All his testing was basically a pass-through, using a throttle-cut switch to make step-function throttle changes.  The Pixhawk was the only system to cause his high kV, small propeller motors lose sync.  And IIRC, he even was able to do it with non-simonk ESC's.  With high KV motors, and small props.

[Jonathan Challinger]

Can we reproduce his test on an oscilloscope and see what's going on?

[Robert Lefebvre]

I suppose it's possible, but we'd probably have to get all his stuff, and I don't think he was willing to do that.

[john...@gmail.com]

I saw the vidoes and the funny thing is that since the ESC is a totally separated system, what's happening is technically "impossible". The PWM input has no direct connection to motor sync timing, it's only a control value.

The only thing I can think of grapsing at straws is that the PX is making signals outside the scope of the PWM 1-2ms range and that the ESC is reacting to those in an undocumented way. It would be interesting so see if other bards of ESC's also have the problem.

Here is what I propose. If he is willing and lacking a scope, have him program the APM with the RCInput - JitterTest program and connect the PX PWM outputs to the APM rc inputs to see what is happening. https://github.com/diydrones/ardupilot/tree/master/libraries/AP_HAL_AVR/examples/RCJitterTest


- JAB

[john...@gmail.com]

Err.. nevermind.. I just remembered that the APM PPM input is hard coded to ignore flawed inputs. The test won't work.....

[christophe divoire]

J'ai évidemment le même problème avec ma configuration. 

Je parle ici: http://diydrones.com/forum/topics/my-first-flight-in-pixhawk-it-ended-badly?commentId=705844%3AComment%3A1611487

ma configuration:

4x 580kv x4110s sunnysky

4X prop 15x5.5 type de dji de carbone

4x esc HK Afro 30A

3DR Pixhawk et dernier firmware

Le Dimanche 23 mars 2014 19:54:02 UTC +1, Challinger l'Écrit de Jonathan:

"Remplacer pixhawk avec apm, AUCUN PROBLÈME 5 lipo aucun problème, mis le pixhwk dos et à l'arrière des moteurs instables."

Y at-il une différence dans le signal de sortie entre PIXHAWK et APM? Ont-ils été comparés avec un oscilloscope?