I wanted to open up a discussion about how we are powering our avionics, meaning servos and APM, on helicopters as they present a unique challenge in the APM world, and must be treated carefully. After crashing my Octocopter last weekend because I let the avionics battery drain, this has come front-and-center for me. I want to make sure it doesn't happen to my helicopter.
Helicopters present a particular problem because we don't have to worry only about the battery dying, but we have to worry about the servos doing something which drags down the APM supply voltage. While looking into the situation, I found this really interesting thread:
http://www.helifreak.com/showthread.php?t=398142
Long story short here, these guys are using a new Savox servo which is very fast and powerful, and it turns out it can draw a LOT of amps. A problem occurs when using a BEC powered by the main flight battery as the avionics power supply. If the servos move too fast, they draw a LOT of amps, it's a spike load, and the switching regulator cannot respond fast enough, and the voltage drops momentarily. It's not a big problem for the servos, but it is a problem for their FBL controllers. In some cases it causes a reboot.
Now, my servos are not that fast or powerful, but it's still a concern. Something to be aware of and watch out for.
This is one of the reasons I have decided to use HV servos, and power them directly from a battery. It does help the situation, but I think we should all consider using some capacitance in the system to prevent the problem. That was the solution in the case of these Savox servos.
This weekend I am going to be working on trying to improve the code to be able to monitor a second battery voltage, and also give some warnings about any problems
Replies
Very interesting technical problem !
I saw that a lot of people tried to overcome the problem by adding capacitance or getting a higher amp BEC with more or less success...
My 2cts is that a parameter have been neglected : the serial resistance of the output LC of DC/DC which slow down the current ramp-up when you servo is required to move fast and then the voltage drops.
Unfortunately this is an internal characteristics that u can't read on the box.
It's like trying to empty a big water tank through a tiny hole. Making the tank bigger don't solve anything...
For example for the TPS5450 (DC/DC controller) that I use, the required output caps for getting the minimum ripple is a 35mOhms 330uF. If you look the cost of that caps, you will understand why a 5$ 5amp chinese Bec just can't do the job.
The solution is looking for a high quality bec or designing your own ! Using a separate battery works but it's painfull.
Feel free to correct me if I'm wrong.
I run my 600 heli with the servos directly powered from the battery, even with 1.4F of capacitance on the RX with a 7A BEC, a bunch of rapid head servo movements will drop the voltage enough to cause the DragonLink to brownout and reboot. The APM actually makes this much easier, because I can plug the battery into the output pins (not using the little jumper) and they are then isolated from the rest of the board. The tail servo is only 6V compatible so it has it's own 4.5A bec inline on the servo cable. I have a separate BEC to power the DragonLink and APM.
Wow they did that with a CC BEC pro rated for 20 amps output. I think the CC BEC pro can handle most of our servos uses for any 600 size heli or bigger, and as long as we power the APM from a different pack, so yes that means running a second battery on your electric heli.
At this point in time I personally think most APM heli users are going for flight time and not crazy 3D, which means this won't be a major issue for us right now, but it's a good thing in mind in the future when we can start programming 3D stuff.
On my 500 setup, I run the built in BEC on my CC ICE 50 to power the RX and APM, and use a Turingy 5 amp BEC for the small servos. Also I've never crashed a heli until I put an APM on one!
I must have crashed my helicopter at least 20 times (no joke) when i was starting out because of power issues. Either directly related to voltage drops causing the APM to brown-out or more subtle after effects of not having a solid power supply.
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