Replies to This Discussion

Permalink Reply by Jake Stew on May 16, 2012 at 9:01am

You might be best to just get a proper 5V servo.  Messing with all that wiring can certainly increase your chances of frying the APM.  You could also have interference problems and ground loops and diode burnouts or other problems.  I don't think the added weight and complexity of wiring is worth it.  KISS right?

It would be a real heart breaker to crash or burn out your $200 APM over a $10 servo issue.

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Permalink Reply by JD on May 16, 2012 at 11:09am

Looking at other servos was a good idea... but I'm running into some problems.

The APM is restricted to 5v right...  But I also believe it is restricted to 3amps correct?

My servo is a HS-5645MG which stalls at 157oz using 6v.  The servo arm that is attached to the servo is 2.5" long.

The total aircraft weight is about 6lbs... I don't know the calculation but the torque needed is beefy.

I can drop down the voltage on powering my current servo but the torque drops to the 114oz range. So that won't work.

I could get another servo in the same size form factor that outputs 144oz torque at 4.8v.  The stall amps draw is at 3amps.

I could go larger yet but the amp draw goes over 3amps which is bad news.  So that puts me in the same position that I am in now where I would have to separate the power using another BEC correct?

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3D RoboticsPermalink Reply by Joshua Ott on May 16, 2012 at 11:37am

You could power the servo directly, and just run the signal wire to the APM?

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ModeratorPermalink Reply by Dean Wynton on May 16, 2012 at 11:40am

If it is likely to draw that much current? then yes...

But if your aircraft is only 6 pounds then I doubt you need so much torque?, I have about 180oz on a warbird that weighs 30 pounds!

I would suggest that you need a servo of around 70oz torque and even that will be more than enough...

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ModeratorPermalink Reply by Dean Wynton on May 16, 2012 at 12:03pm

I am assuming you are talking about the elevator? if not? and another control surface then even less will be good

And it depends how you will be flying it? 60mph hard loops then 50 - 70oz will be fine

100+ mph and hard loops then you will need a little more, but if the aircraft only weighs 6 pounds it probably couldn`t handle the stresses involved in such maneuvers?

You can always ask advice at your local model shop, on what servo they recommend? as long as they know their job well..

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3D RoboticsPermalink Reply by Joshua Ott on May 16, 2012 at 2:33pm

A mechanical solution would be the easiest way for sure. Balance tabs if it's a control surface, do you need a huge amount of travel, why 2.5" control arms?

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Permalink Reply by Jake Stew on May 16, 2012 at 5:10pm

The 5v and gnd pins are all tied together on the output rail.  You can run lots of current through these rails because there's no components involved.  If you notice any problems just take two lengths of wire and solder them along the rails on the underside.  Or just bridge them with solder.

If the trace melted at some point you can easily just do what I said above to fix it.  No chance of burning out your board.

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Permalink Reply by JD on May 17, 2012 at 10:08am

I think I have a good plan!  I'll use the motor controller to supply 5v to the APM and R/X thru the output rail via the throttle ch.  The separate BEC will power the servo/s directly using the a power and ground via a Y harness.

From the APMs servo output I'll connect the just ground and signal to the servos ground and signal thus maintaining common ground for the whole system.

With this setup I hope to have more flexibility in what I can connect to the APM without limitations on the amps or voltages within reason.   I'm still experimenting with the airframe and how much weight and where the CG needs to be.  So I still require flexibility with the servo and the length of servo arm..

I'm totally thankful for all the responses for help... thanks guys.   I'm be wiring everything up in the next several days I'll keep you all posted.

Cheers.

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Permalink Reply by Karl on May 23, 2012 at 4:39pm

I had a similar problem, which was due to the BEC in the ESC not having any smoothing capacitors, which could drive the servo's fine but the voltage drop when all 4 servo's were under load would cause the APM to lock up and lock the servo's into position (no reboot). I ended up using a spare ESC which had smoothing capacitors to power the APM only. Both BEC's are powered up off a common connection from 3 x 3S2200mAh LiPo that gives me 20 min flight time in a Multiplex Mentor with around 800mAh remaining.

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Permalink Reply by John Grouse on July 4, 2012 at 5:33am

I've recently set up a new camera gimbal to stabilize roll and pitch and the standard size servo's benefit greatly from running at 6v due to the increase in speed in correcting the level of the camera. I was going to set the 5A UBEC to 6v and feed the in/out common rail with that, however after reading the warnings about sticking with 5v I may need to set the APM board to use a separate supply - one for the gimbal servos, fed from the 6v bec to the common rail (motor esc disabled) and the APM fed separately from the battery via the Vin connection. Would the 3cell be too high a voltage to run it this way?

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Permalink Reply by JD on July 4, 2012 at 11:56am

I've seen a lot of guys on in here that have cooked their APM 2.0 due to power issues.

My approach is that I run a UBEC to power just the APM 2.0 and use the aircraft's ESC to power the R/X and servos.

I run just one battery, so I have y harness on my 5000mh 4s lipo one feeds the APM and the other the ESC.

The other thing I do is that I have a switch on the APM between the battery so that any sparking while trying to connect the battery won't effect the APM.  Don't know if that matters but I also like to turn on the APM after the RX has powered up.

Ive been told you'll need to isolate the APM from the ESC if running different voltages, so you don't want the ESC to feed power to the APM or the other way around.  So pull the hot wires that go to and from your APM and be sure you are maintaining a common ground  if you are running a single battery. This will allow for the AMP to signal the servos with out powering them.

The other way I've been told is to power just the servos with the UBEC and have the ESC power the APM and RX.   In this approach you will need to just power the servos with ground and power,  from the APM I've been told to use just ground and signal.  So it would take a Y harness for ground going to the servo, one to the UBEC and one to the APM.

I think that the best option to power the APM is using the output rail, using both or just the input rail seems like it can cause issues with over amperage.

Anyway that's my take on it.  Do come careful reading on maintaining common grounds... because that will really screw up electronics if its not done correctly. 

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