RC Servo Behavior

Using Arduino Motors, Mechanics, Power and CNC
1

I have been reading about Servos + Arduino and am about to purchase some for my project. However - one gating item is a question I have not been able to find an answer to. With a Servo powered (ground and power) with the control line low (no pulse at all) - what is the behavior of the servo? Assume there is no external mechanical 'load' on servo - will it stay in the position it was left at?

I need to control 18 servos - one at a time. So I would like to do something like this:

foreach(servo)
{
attach(servo)
move(servo)
detach(server)
}

I just want to make sure that 'detached' servos stay where I left them!

2

From what I've seen the analog servos stop movement and position holding. You might get one of the servos you intend to use and experiment.

3

bfrye:
With a Servo powered (ground and power) with the control line low (no pulse at all) - what is the behavior of the servo?

I think this may vary from servo to servo. I found that connecting a 4,700 ohm resistor from signal to ground can help to stop stray movement. However it will not make them hold position.

It would be best to attach the servos once and leave them attached. Without a signal they will not not resist any load that causes them to change position. Then they will jump back to position the next time a signal is provided.

...R

4

So called "analogue" servos have a very poor hold torque when they are not continuously updated with pulses.

The more expensive "digital" servos usually short circuit (brake) their internal motor when in target position, thus yielding a much higher hold torque. The IC inside the "analogue" servos does not support such feature and leaves the motor open/unbraked, therefore the lower hold torque of those, virtually only resulting from gearbox losses.

5

Assume there is no external mechanical 'load' on servo - will it stay in the position it was left at?

Well I guess Newton tells us that it will, since things stay at rest if they're not disturbed.

My only test, though, was that after a servo.detach() my (cheap hobby) servo horn gets loose: easy to move by hand and I think any holding force it has then is just incidental internal friction. When detached but powered up, it feels to me exactly the same as when not powered up. With it still attached, and getting its 50Hz reminder, I could feel it acting against my hand when I tried to move it. Not very scientific tests, but that's my take on it. YMMV though, since I only have cheap hobby servos.