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Re: [Paparazzi-devel] crashed, 5V supply overloading issue

From: John Burt
Subject: Re: [Paparazzi-devel] crashed, 5V supply overloading issue
Date: Mon, 10 Aug 2009 08:59:29 -0700
User-agent: Thunderbird (Windows/20090605)

Yes, I am referring to the PPM signal from the RC receiver to PPZ.

It was pretty clear that the fault was due to an overloaded 5V supply. On the ground I was able to faithfully re-create the AP crash conditions simply by rapidly exercising all the servos. I also did fairly extensive testing to debug this problem, including measuring current drain with all servos operating, which is what clarified the issue.

I have suggested in the past that PPZ be updated with a higher capacity 5V converter, and given what you wrote below, it might also be a good thing to source the 3.3v line off the battery, rather than the 5V supply. However, the "temporary" solution is to simply use a separate 5V supply for external devices like servos, modems, etc.

Adam Spence wrote:
John, what are you referring to with the RC RX cable? Is this the PPM cable attaching the receiver to PPZ?

There's been a couple of crashes like this discussed on IRC recently. I'm not sure if the fault was identified but the overloading of the 5v PSU was suggested. This could easily happen when the servos are driven to the limit and stall due to the hardware set up. Where ever possible the currents drawn by the servos need to be measured when all control surfaces are at the limit. If there are any stall conditions the setup needs to be adjusted either physically on the air frame or by reducing the control throws in the config file.

The 5V PSU is not only responsible for supplying the servo power but also the 3v3 regulators power! Therefore if the 5v line drops due to too much current being drawn then it may take out the main CPU. For my particular setup with an 868 modem, the CPU, GPS and Modem can take upto 500mA of current. The limit of the 5V PSU is 2.25A so roughly there may be something in the region of 1.75A left for all of the servos. That only needs a couple of servos to stall and you could be in trouble.

Due to the above, I will be removing the 5V connection to the servos and providing another regulated 5v directly from the battery. This will have a limit of maybe 5-10A which is complete over kill.

I've just noticed the current-limiting switch on the hardware, this means the servos can only draw 1A and then the switch kicks in and notifies the CPU using GPIO. Does anyone know what the firmware does in this instance? That therefore means that there's even less current available for the servos and anything else which you may have connected on the 5v rail... hmmm. I see this can trigger from 0.5A to 1.25A, anyone ever tested this? I'm wondering now what would happen if this were to fire, the servos would be left in their current position (or driven back slightly from mechanical pressure). The AP might (depending on what the GPIO signal does) try and keep driving the servos at their current position which would mean that when the current limiter recovers it then goes straight to the limit again?

Anyway I've rambled enough this morning!

2009/8/9 John Burt <address@hidden <mailto:address@hidden>>


    I crashed an Easystar just like that during early flight testing.
    The AP (Tiny 2.11) went nonresponsive and dived into the ground,
    despite my having immediately flipped the switch to manual. It would
    seem that the AP processor just locked up - truly a worst case
    scenario. The cause of the behavior was an undervoltage spike
    condition due to sourcing the four servos and RC RX off the 5V
    supply. In case you haven't already, you should always power your
    hardware off a separate BEC (which begs the question of why the AP
    has a 5V source anyway).

    AP generated noise on the ground wire was other cause of bad AP
    behavior that I experienced. This causes glitching and reduced RC
    range. It has been covered ad nauseam in other posts, but I'll
    repeat it: it's especially helpful to put an RF choke on the RC RX
    cable. Try to wrap the wire 4+ times around the choke.

    Martin P wrote:

        Hello everyone!
        Last weekend I had a good AUTO1 flight with my Minimag. It
        reacted to pitch and roll commands, and I was able to control
        the throttle. I had an attitude command in my flightplan which
was never left (as I know now). Yesterday I tried again, without the attitude statement and
        without a holding point block.
        In AUTO1, it killed the throttle. Switched to manual and landed
        to change battery because it was not fully charged.
        Took off again. Switched to AUTO1 and it immediately stabilized
        the plane. Strangely, this time it did not react to the roll
        command. I flew around using the rudder. When I used the
        elevator stick, the autopilot controlled the throttle. Good, so
        Suddenly the plane dived down vertically from an altitude of
        roughly 40m. No reaction, just kept going vertical and slowly
        spinning around the longitudinal axis until it crashed.
        There was not as much damage done as I expected. These foamies
        are awesome. All RC components including Tiny and Xbee are OK.
        Except for the Lipo which is badly bent out of shape and will be
        replaced for safety reasons.
        Unfortunately I can't see much from the log as I had lost the
        link probably minutes before. I was in stage "HOME" because I
had left the fly-too-far circle before and came back manually. The switch was on manual so I probably tried to rescue. The Tiny
        was unplugged which can have happened at the crash or before.
        One aileron was ripped off and the horizontal IR was missing (I
        found it in the grass).
        So far, my report. The Minimag is already repaired and I'll keep
        Greetz, Martin

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