Servo connections to mRo R15

I’m new to Pixhawk and Ardupilot for helis, though I have some experience with a hexcopter. The output pins on the R15 board don’t supply 5v on the middle pins, so the heli’s servos need to get 5v (or whatever) from a separate distribution board.

My question is, what’s the best practice regarding the ground wire(s)? With a multirotor I only need to connect the ESCs’ signal wires to the FC board since the ESCs have a common ground with the rest of the system. With my heli I’ve made provision for the servos to connect direct to the 5v power supply by their + and - wires. The R15 also gets its power from the same power supply, so am I right to assume that I need only connect the servos to the R15 by their signal wires. Or should I splice a second ground wire to each servo so that I can connect their signal and ground direct to the R15?

Allan @abenn1 ,
I don’t make any modifications to my servo wires. I plug them directly into the servo rail of the flight controller. Then I have either a ESC with an internal BEC or a voltage regulator. I connect that into the servo rail of the flight controller to provide power to the servos. I am not familiar with your flight controller so you may want to consult the documentation but that is typically how it is done.

That will work fine on a Pixracer. The servo rail power and ground is completely isolated from the Flight Controller.

Thanks guys. The board is mRo Pixhawk R15. I have the manual, and it’s handy for the pinouts, but nothing else :unamused:

I’ve checked, and the ground pins on the power input socket are common with the ground pins on the 6 ESC/servo outputs though, as you say, the 5v power is separate. I’m using a CC Talon 90 ESC which has a built-in 10A BEC so I understand that by plugging it into one of the outputs it will provide 5v to the other 5 outputs to power the servos without the need for any modification. No problem there.

But, just to be sure, is it okay to provide 5v power to the FC board, through its power socket, direct from the same BEC output?

Usually there is a Power Module powering that FC that supplies 5.3V.

Yes, I use a power module on my hexcopter, but on this heli I don’t really want it, so I’m simply feeding 5v direct, and disabling the power monitoring functions.

Maybe I’m over-thinking this, but with or withoug a power module my concern is whether or not there will be an issue if I feed the 6-pin power socket of the board and the servos + pins from the same 5v source. I’m thinking there won’t be, because the servos + pins are not connected internally to the FC’s 5v.

I feed both the servo rail and the flight controller power from my voltage regulator. Never had a problem

Thanks Bill. That gives me the confidence to proceed. Just installed the hardware on my heli today, so now I can proceed with configuration. A big learning curve for me!

BTW, the existance of these separate supplies is good. For example, if you have a serial sonar it is an error to supply it from the FC (in the end, probably from a tiny regulator inside the FC): when the sonar emits it is going to extract a high current, and that may affect its reliability. So connect TX/RX from a FC serial port, and supply it from elsewhere.

So in this case you can supply the sonar from the BEC on an ESC, doing the appropiate wiring. The best is take ground from a big ground point electrically near the battery ground, such as on a PDB or the frame plates (not the FC, the receiver, etc).

In my case I don’t plan on having sonar, but I can see that separating the output + pins from the rest of the FC is to prevent large current flows through the FC, irrespective of what device is connected to them – potentially power-hungry digital servos in the case of a heli.

Depending on servo type (power/speed/brushless/brushed) and BEC performance you might have significant voltage drops on the servo rail and on the FC supply, if they are both powered by the same BEC/regulator.
Autopilots and FBL units generally don’t like voltage drops.
The safest is to have the FC powered by its own regulator.

Actually my 5v power supply in this heli for the past 5 year or so is the BEC in the Talon 90 ESC plus a stand-alone CC 10A BEC paralleled together via a pair of Schottky diodes, so if one fails the other will take over. But I think I may go for a power module for the FC once I’ve got everything set up.

Thanks to everyone who’s contributed on this issue.