Servo sweep when supplying power to Arduino and Servo at the same time

Other Hardware Motors, Mechanics, Power and CNC
1

I have a robot that uses an Arduino Mega to control two giant scale servos.

The servos have their own separate power supply.

In my sketch I set the the initial position of the servos in the setup function.

 servo1.write(60);    //set servos to back/middle position
  servo2.write(120);

This is a default position the servos resolve to when they are not in use and when the robot shuts down.

If I power up the Arduino first then then servos second there is no initial sweep, and the servos behave as expected.

I added a rf remote on/off switch to the robot that supplies power to the Arduino and servos at the same time.

When the Arduino and servos get power simultaneously, the servos do a rapid sweep then set to the correct position specified in the setup function.

Is there a way to eliminate this initial sweep? If the servos are already at the default position when the robot is off, why does it sweep away and back on startup?

2

Did you write the servos to start position BEFORE attaching?

servo1.write(60);    //set servos to back/middle position
servo2.write(120);
servo1.attach(pin?);
servo2.attach(pin?);

When a servo is attached, it goes to 90 degrees by default unless told otherwise.

3

I attached servos before writing position.

servo1.attach(9);     //head servos
  servo2.attach(10);

  servo1.write(60);    //set servos to back/middle position
  servo2.write(120);

I'll try attaching after setting position and report back.

4

I still get the same sweep if I write position before attaching pins.

I think I need to introduce mosfets to control power sequence. Servos are sweeping because they get power but they don't have a control signal until Arduino comes online a fraction of a second later? So I need to get Arduino online first then use it to control a mosfet which powers servos.

5

Or maybe I need pull-down resistors on the servo line to keep it from getting a signal till the Arduino has a chance to initialize the IO pin?

6

Typical would be to wire common ground between servo power supply and arduino so that you PWM signal has a return path.

Courious what would happen if instead of connecting to arduino ground, you wired the ground into a digital pin.

Once your code is booted and your ready to send a signal, start the signal and then set the digital pin to input.

Just a random thought.