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

https://www.youtube.com/watch?v=i7_gVoY8q8M

https://www.youtube.com/watch?v=VTK81IANyUE

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Very well said Lorenz



I feel like inserting a rant here.... but won't feed the Troll....

Phil

Fact of the matter is, APM 2.XX has 5 volt PWM output and there were no problems related to motor sync issues. It was the decision to use 3.3 volt PWM and not 5 volt PWM  when it was readily available, evidently causing the motor sync issue only with PixHawk at 3.3 volts.

Let's hear from the users as to what they would prefer....3.3volt PWM or 5 volt PWM  They aren't easily fooled.

While I perfectly agree this is a difference, its not the only one, and we shouldn't conclude this is *the* relevant one (as explained in my last post). Pixhawk's output *changes* at 400 Hz, while APM maxes out at 200 Hz changes (APM has a 490 Hz update rate, but the actual PWM width only changes at the main loop rate - so the ESC sees a step response with 200 Hz max).

Our tests suggested that when lowering the maximum signal update rate some ESCs stopped having sync issues. This however merely means that the ESCs are not doing their input signal conditioning correct (we don't want to make the flight controller slower! that would limit performance.).

One ESC notoriously having issues with signal speed was the AfroESC with its own firmware. Changing to SimonK firmware on the same ESC resolved it. *Interestingly* HobbyKing has since completely halted shipping it with the original firmware and ships with SimonK as default now.

http://www.hobbyking.com/hobbyking/store/__39708__Afro_ESC_30Amp_Mu...

You still have not explained why, you chose not to have 5 volt PWM when it was readily available. Are you saying 5 volt PWM is bad? 

Unlike other ESCs come with an "airplane" firmware - with features like braking, soft-start or voltage cutoff - and can be reprogrammed with open-source code, the Afro series was designed by Mr. Hamasaki (aka TimeCop) and ran SimonK code from birth.

There is now an option to use BLHeli firmware on the Afros, but otherwise they only exist with SimonK. No other "original" firmware.

I can just refer to the user reports we got - it very well might have been an update to the Firmware revision which made the difference.

Hi Kendall,

There are two main reasons we opted for 3.3V: The first reason being that in the complete population of receivers we tested (Spektrum, Futaba, FrSky) the output voltage was 3.0 to 3.3V. So essentially the de-facto standard is 3.3V (also in normal TTL logic) and with the drive capacity Pixhawk has its well within the standard range (outperforming e.g. a Spectrum AR10000 in terms of current drive capability, as one of the authors in this thread or the corresponding one on drones-discuss found out and highlighted). Now you could still ask why not 5V like on APM, even though the rest of the market uses 3.3V. The reason to not do this are the other peripherals like camera triggers people hook onto these pins (the APM Firmware when running on Pixhawk supports the use of these pins as GPIOs). We do not know if the peripherals people connect are 5V tolerant and with most RC receivers and other flight controllers outputting 3.3V levels, we wanted to rather be as standard as possible for maximum compatibility.

"The first reason being that in the complete population of receivers we tested (Spektrum, Futaba, FrSky) the output voltage was 3.0 to 3.3V."

"even though the rest of the market uses 3.3V"....

The market of what? Receivers? Is the Pixhawk a receiver?

Sorry but I think the rest of the market use a 5v output! (APM, Naza, Wkm, Crius, Mk, Rabbit, Naze32, Vrbrain, KK, CC3D, Zero UAV, etc etc etc ....)

@Papi78: Why would you assume that all the other boards are outputting 5V levels? Have you measured them all? It would be interesting data, so please share. Here is the schematic of the CC (the CC3D doesn't seem to have any shifter onboard, too, as you will easily be able to tell from the photos of it):

http://wiki.openpilot.org/download/attachments/12386767/CopterContr...

Its 3.3V straight off the 3.3V MCU (some might argue having protection resistors there would be a good idea). I wouldn't be surprised if that applies to more of the autopilots you listed than you assume.

Please keep in mind though (before now doing a full market research on signal levels) that 3.3V is a perfectly valid PWM pulse with margin and that a cable (or missing GND connection) able to introduce a 1.5V drop easily also introduces a 3.5V drop. You can't build a safe system relying on particular resistance values or floating ground - a decent system setup has to be one to two orders of magnitude (a factor 10-100) away from a badly conditioned signal (so 0.01 to 0.1V drop as absolute maximum, and never anything like 1V).

Just another datapoint to this.  Last night I did some testing of my own.  I decided to pull out an old school Futaba 72MHz receiver, and see what it was doing for PWM voltage levels, to determine what if any "standard" there is.  What I found was that the PWM level coming out of the Rx was simply whatever voltage it was supplied with.  Whether I used a 6V BEC supply, 5V, or even 3.3V from a benchtop power supply, the PWM level was the same as the input.  I even tried to see how low the Rx would operate, and it went down to 2.44V, at which point it happily output a PWM signal at 2.44V level. 

I also tested another old Rx, this time a JR 2-channel unit, and had the same result.

Conclusion:  There is no such thing as a standard PWM voltage level.  Certainly not 5V.  In the distant past, the level was simply whatever the Rx was supplied with.  In the recent past, the level was set by the internal operating voltage level of the Flight Controller, which meant 5V for Atmega-class systems. This, since the PWM was generated internally by the micro-controllers.  Now with Pixhawk it is 3.3V since that is what it's microcontroller operates at.  But this is nothing new, as Lorenz' points out that modern digital receivers also output signals at 3.3V levels.

This latest kerfuffle is just another example of Kendall and Joe's backwards scientific method that goes something like this:

Problem: Something doesn't work right.

Conclusion: 3DR/Arducopter must have screwed something up.

Hypothsis: I found something I don't like about their design or program.

Testing: The first confirmation bias I can find. 

I would like to add this here, not as a criticism of the hardware (lord knows I don't have a clue) but for the information of those who might find it useful.  

Frame: Tricopter

ESC: Afro 20A ESC (each ESC is grounded to the Pixhawk)

Motor: iFlight MT2212 1000kv

Props: 11x4.5 CF (I fly at 1,200m, around 10% humidity and current temps around 90F)

Power System: 3s

This was my second rebuilding of the tricopter.  Due to the weight I had upgraded from 10x4.5 to 11x4.5 carbon fiber props.  I had a couple of test flights on the prop/motor/ESC combination before the rebuild and I thought everything was okay except for some tuning issues.  I tore down and rebuilt (6% weight savings) last week and took it out for a test flight.  After putting a pack through it I decided to try an autotune.  On the first roll, the tricopter simply rolled over and fell to the ground.  Two broken CF booms and some severed wiring will not be a big deal to replace, but I am now concerned about this issue.  

Before this thread, I hadn't heard of sync problems with a high kv, low(?) pole count  motor.  I knew that high pole motors could have issues and that low kv motors could have sync issues, but before I found this thread I didn't know this was an significant issue.  I assume that I have created this sync issue by using an oversized prop on the 2212 motor.  However, looking back on past flights, I can think of two other times that the copter rolled unexpectedly using the 10x4.5 props before this incident.  One time the 'copter was high enough that it recovered, however, the second led to a crash into a nice soft ornamental tree.  However, outside of those two instances I had probably 20 successful flights on those props.  

The way I see it, I have to change something.  I probably have to accept that the 11" props are  too much for these motors.  However, that alone will not solve the issues that I have had with the 10" props.  I just purchased a set of  iPower MT2217's and 18A Kiss ESCs that I could divert from a quad build, but ideally, I would like to find a way to make this setup work, so if anyone has some suggestions, I am willing to learn.  

I have considered changing this build from a AV rig to a more performance based FPV tricopter.  Does anyone have an opinion on whether I would be more or less likely to experience sync issues on a 4s system with 9" props on this build?  

 

Hi Kyle

I'm no expert but I have used much of the same kit you list, so FWIW here are my thoughts :

1) Did you re-flash your AfroESCs? (See my overly length post earlier about that being the root cause of my similar issue with a 3DR Y6 + PixHawk + Afro). This is an absolute must for all AfroESCs. Manual calibration alone is not enough in my opinion as you just don't know what firmware is on it.

2) Save the Kiss ESC for smaller Acro or high performance FPV quads. Ideally, save them for a build where you're going to use a MultiWii and can enable OneShot. Bare in mind these ESC are not reflashable, and bare in mind that while the reports on performance are incredible, they also have their teething issues and a few have come down in smoke. A reflashed Afro (to my knowledge) has never done this and is more of a known quantiy while you're ironing out issues with a new build.

3) Always test on the ground up to full throttle ( I use strong zip ties that are loose enough to see that it is off the ground and in control)

4) If possible measure the current draw at 1/2 and full throttle for each motor. I appreciate this isn't everyones cup of tea but well worth it and easy to do with a cheap HK Turnigy amp meter. 

5) While I've never flown a tri, I would say 3s / 10x4.5 / 1000kv is not going to be sufficient for a AV system. On 3DR style UAVs I always run 4s / 11x4.5 / 880kv which would be significantly more thrust given I have 4 or 6 motors vs your 3.

Hope that's of some use?

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