I have a Pi 3 Model B (powered by its ac adapter) sending multiple PWM control signals to up to 5 servos (SG5010). These servos draw usually 300 mA when they move and up to 1 A each when stalled so I plan to use an external power supply for them.
I know that for the servos to work their power and the PWM signal have to share the same ground. What I’m worried about is if it is safe to connect the Pi ground pin directly to the ground from the external power supply (which is powering the servos). I ask this because (as I understand it) servos usually generate noise that can travel through their ground pin and therefore to the Pi and fry it if it’s high enough.
I’ve seen that in other projects this is not a problem but in all of those projects they only use one or two servos or they don’t draw as much current as I’m going to do.
Do I need to isolate the Pi ground from the external power ground? If so, what methods are usually used?
As joan has answered you NEED to share a ground to the control interface of your servos.
How this is done is dependent on the physical layout of the wiring, and is somewhat of an art.
Ideally the control interface and servo power should not share any wiring, as any voltage drop in the common wiring will be introduced into the control interface.
The best practice would be to interconnect the Pi ground directly to the ground of the control interface (ideally routing all connections on the same path).
The power should be connected to the servos by separate wiring, and utilise a separate ground connection point (if available).
I doubt that with small servos using decent wiring and short cables there would be any problems, but following the best practice costs nothing.
You need to connect the Pi ground to the servo power supply ground. You will not be able to control the servos unless you do so.
I have controlled a robot arm with 5 servos. I had to use an external PC power supply to get the amperage needed at 5V.