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Re: [Paparazzi-devel] crashed

From: Adam Spence
Subject: Re: [Paparazzi-devel] crashed
Date: Mon, 10 Aug 2009 08:50:30 +0100

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>

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 far.
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 trying.
Greetz, Martin

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