Spurious triggering of servos

I have two servos and a small motor on a project a Nano and a dc power supply. The motor has its ground switched by a transistor and runs off unregulated 12V while the Arduino runs off a 5V switching supply which draws from the 12V supply. There's 2F of capacitance on the 12V side and another 1F on the 5V side so power is pretty stable when the motor switches on and off.

The switched ground wire for the motor runs about 3' directly alongside the control wires for the two servos whose control wires are twisted together.

Even with the servo control wires disconnected at the Arduino end, but with power available to the servos, the running of the motor causes the servos to go to zero positions. Has anyone experience with control signals being induced from such an arrangement of wires traveling alongside each other? I put a scope on the disconnected wires (still running alongside the switched ground wire for the motor and still attached to the servos) and could see noise on the order of maybe 2-10 mV on the servo control lines when the motor is running. The code at this point is not referencing the servos at all, they just have power and no signal.

To be further clear, even when the servos are fully wired, attached and given a signal, the powering of the motor causes interference in their positioning.

Sample code below. I really just want to know if anyone has experienced interference in control lines that might explain this. Thanks much - Mark M.

const int propPin = 9;

void setup()
{
Serial.begin(9600); // set up Serial library at 9600 bps
Serial.println("entering setup");
pinMode (propPin, OUTPUT);
}

void loop()
{
//prop run
Serial.println("Entering prop run");
digitalWrite (propPin, HIGH);
delay (2000);
digitalWrite (propPin, LOW);
Serial.println("exiting prop run");
delay(1500);
}

mmmalmberg:
The switched ground wire for the motor runs about 3' directly alongside the control wires for the two servos whose control wires are twisted together.

Even with the servo control wires disconnected at the Arduino end, but with power available to the servos, the running of the motor causes the servos to go to zero positions. Has anyone experience with control signals being induced from such an arrangement of wires traveling alongside each other?

Yes, running wires in parallel like that where some carry large switched current loads and
the others are sensitive signals is bound to lead to problems. That's why we use twisted pairs,
screened cable and don't run power wiring alongside signal wiring.

One thing you don't mention is the +12V supply to the motor - this carries the same
switched current as the ground wire to the motor, but in the opposite direction.

If you use twisted pair for those two wires you will greatly reduce the interference they
put out. Then if you use screened cable and / or twisted pair for the servo signals and
their grounds(*), this will greatly reduce the interference they will pick up.

And keeping the power wiring away from the servo signals will really improve things too.

(*) Each servo signal twisted with its own ground wire, not twisted with each other, and
only common the grounds at one central point.

Great, Mark, thanks for the confirmation. In the interest of time I went ahead and added a transistor to switch servo power off while running the motor (since this is consistent with how I'm using the setup). Glad to know now and that's the last time I'll wire something like that. Should have known better, but at least now I do:) Thanks again.