Ok, I already know the basics of powering the board, currently I am running a 3S battery through a UBEC set at 5V, and that is plugged into one of the spare servo inputs on the APM, which in turn passes the voltage to the reciever and servos.
Except...
I have a tail servo rated at 3S, direct. That one servo has power and ground directly connected to the battery, and only the signal wire goes to the APM. It worked fine for a while, but on the weekend I had an issue that led to a crash. As best as I can tell, there's some kind of interference related to my Xbee which I just added. Basically, the servo pulses whenever the Xbee is transmitting.
I suspect the issue has to do with a voltage drop on the APM side of things reducing the voltage peak of the PWM signal going to the 3S servo to drop to the point it doesn't see it anymore.
Anyway, I am considering making a few changes:
I want to de-solder the jumper which will allow me to power the APM directly, instead of using power from the servo rail. Question is, can the APM really take 3S power direct? 11.1V? I have seen mixed things about this. BTW, I am talking about a APM 2560 bought 1 month ago, if it matters.
Second thing is, I want to reconfigure my UBEC to output 6V instead of 5V, and pass that power to the servo rail. Is this OK? I know my receiver can take it as I operated like that previously. And the servo will benefit from the added speed and torque. But I have seen nothing saying if the APM can take 6V on the rail? I read the thing saying to only feed it 5V, but I assume that was when the solder was jumpered for external power?
I would then continue to use the 3S servo. Hopefully this will clear up the power problems. If I did this, what would be the voltage peak of the PWM signal going to the tail servo be? 5V (internally regulated APM level) or 6V?
Replies
It turns out another guy is already flying his quad with a setup similar to this, running 5.3V to the board, no problems.
However, I have taken another slightly different route now. I performed the Jason power hack. And I hooked up an L7805CV 5V voltage regulator to power the APM and receiver. It seems to work perfectly fine. This is rated to provide up to 1A. Previously I measured the APM and receiver drawing 0.2A. So it should be fine?
I tested the setup, and all looks great.
Am I missing anything? Any reason why it has to be more complicated than this? Anything wrong with the L7805? (reliability?)
Ok, I think I licked the servo noise problem. I removed my shielded wire, and ran a simple 3-conductor twisted servo wire. I ran the ground to the APM rail, instead of direct to the battery. The signal wire also goes to APM (obviously) in the same connector as the ground. The power wire directly goes to the battery. I also have a 4700uF cap glued to the servo, and a ferrite ring on the power wire only. It appears all noise is gone!
Ok, now back to questions about powering the board.
How much voltage can the board take? Is 5.3V OK?
My idea is this. Do the Jason Power Hack, and split my servo power output from the rest of the board. The servo input power rail will be linked to the board.
Change the jumper on my UBEC to supply 6V. Power the output servo rail directly from the 6V supply. I think this should work in similar fashion to how my tail servo operates. It seems to be OK feeding a higher power to the servos, and leaving the signal wire on 5V. I think good servos only need to see a peak-to-peak V of 3-4V for the PWM signal. Now my other 3 servos are powered by 6V. Then use a diode to drop the APM supply voltage 0.7V to 5.3V. I will also have a large capacitor bank to help prevent brownouts on the APM. I made one up with 5x3300uF caps that can power the board and 3 servos for ~0.1sec. Once I split the servo rail off, it should last much longer. If the servos don't have access to the cap bank, I don't really see any problem since they don't seem to have any meaningful boot time?
The other reason for the diode is to prevent the cap-bank from draining into the servo rail. It will only let power into the board, but not out of the cap-bank and into the servo output rail. That's also why I need to run at 6V, because of the 0.7V drop across the cap.
Anybody have any input?
The biggest question, is can I use a diode to drop the 6V supply to 5.3V, will that be OK for the APM?
I agree that I'm probably confusing two issues. Ultimately I don't know what brought down the heli, all I know is that the first symptom was a loss of tail control. I have two things I'm investigating:
Power supply to the APM and possible brown-out or reset
Noise on the tail servo signal line.
You're wrong about the required PWM voltages. It's not true that "anything more than 0V is a 1". In fact, the specifications require a 3.5-7V peak-to-peak square wave. The APM is outputting , at best, a 5V square wave. I don't even know if it's really that high or not.
It's an interesting idea to run the ground through the APM, but I'm a little unsure about that as it might cause a ground loop problem or similar. I'll have to consider that more. But, I will look into it.
The APM may very well have reset. I don't know. I do know that most of the event was not recorded in the log.
There have been a number of people suffering a brown-out/reboot/lock-out/freeze problem lately.
I think your barking up the wrong tree. It's a noise problem on the signal line to your tail servo. I have a Sky Fun and with the srevos that came with it it would twitch all over when the X-bee was transmitting. I replaced the servos with better one and the problem completely went away. You me be extra suceptable to RF noise on you tail servo because of how the wires are laid out. Servo's ground going one way and signal fallowing another path. If your APM was glitching out it would reboot and you would know it. I know RF shouldn't mess with the PPM signals, but strange things can happen when dealing with noise
You could try running a ground wire from the servo rail along with the signal wire and twist them together and use a ferrite.
Keep you APM powered from the BEC.
Ok, I wanted to get to the bottom of this power problem, so I soldered up some connectors and ran my Power Analyser on the radio system to see what I could get.
Interesting.
Ok, first of all, the results for the APM and the receiver, all servo disconnected. APM was running the Xbee and GPS, but not the mag or sonar at this time. I saw a max power draw of only 0.22Amps. That's less than I thought. It's so low, I think I'm going to bring my Fluke meter from work to double check. But lets go with it for now.
Then I checked the same setup with the 3 swashplate servos plugged in. I madly stirred the sticks, and the highest I saw was only 1.57A That surprised me. So, I pushed down on the swashplate as hard as I dared, all 3 servos buzzing away, and I saw only 2.5A. Wow. So, I then decided to see what I could get out of it if I plugged another 958 servo in to the tail channel. Then, again madly stiring the sticks, this time even faster than before, I saw a peak of 3.5A. Still not even close to my advertised 7.5A limit.
But then I saw, min voltage on the display... 4.92V. That's interesting. I wonder how that happens. Maybe whatever circuit is controlling the FETs in the BEC can't respond fast enough to some voltage spikes?
Is that low enough to cause a brownout?
One of my ideas was to avoid a brownout by powering the APM and receiver with the built-in power regulator on the APM. This would ensure that no matter what, the servos can't take out the APM. Unfortunately, it seems like the power regulator on the APM really isn't big enough for the function it is intended for?
http://www.diydrones.com/forum/topics/powering-the-apm-board-separa...
Well, I made a bit of progress on this issue.
I played around with some capacitors for a while to try and filter any noise on the powerlines. I robbed a couple big ones from an old video card. I put 1500uF on the 11V supply to the servo, and 3300uF on the servo rail of the APM. At first I thought they were making a difference. (do they need inline resistors to really act like a filter?)
What I quickly realized was that the issue is 100% determined by the location of the Xbee antenna. Moving the Xbee around, I can completely eliminate the problem, or if I point the antenna at the servo or servo wire, it goes totally nuts. I may be able to keep using this servo if I am careful.
I'll probably replace the servo wires with twisted triad wire. Move the big cap to the servo end (but honestly, I suspect the issue is coming in the signal wire). And find a really good place for the Xbee. But I'm not sure where that is yet.
The newer APMs can take a 3S directly, but it won't be able to power the Oilpan + peripherals, so it's not really an option. See here:
http://www.diydrones.com/forum/topics/powering-the-apm-board-separa...
The only way you can connect a 6V UBEC to the servo rail is by breaking the solder jumper, but then you'd have no power to the APM. To answer your last question, the voltage peak of the PWM signal would still be the APM voltage, 5V.
My guess at your tail servo issue is a grounding problem. Can you put a multimeter (or better, a scope) between the battery negative and the UBEC output ground? The battery negative and UBEC output ground might only be loosely tied together, in which case a large momentary current draw (like Xbee transmit) might pull the ground around. If this is the problem, then adding a connection between UBEC output ground and battery negative should solve it, and *probably* won't fry anything. :-)