How to stop a servo

I have a servo that can't never reach his position and I want some how to stop it trying. Basically I want to know if there is some command that will stop a servo from reaching its programmed position.

By "his position" do you mean the servo can't reach ANY position that you command? If so, what does the servo do?

I'm using it with this robotic claw. And I want to catch an elastic item and I want it to press as much as possible.

So presumably you wish to squeeze the item until the servo stalls and then back off. With a normal servo, you have no position feedback, so you can’t check how far it has got. Servos with a feedback mechanism do exist but they tend (obviously) to be a bit more expensive. You might be able to measure how much current the servo is consuming and get an idea that way of whether it has stalled.

andrei141592: I have a servo that can't never reach his position and I want some how to stop it trying. Basically I want to know if there is some command that will stop a servo from reaching its programmed position.

Servo's aren't designed to be overloaded in this way. Unless you can command it to go to its current position it will try hard to move (and perhaps get too hot).

Maybe you can add a spring or some kind of clutch to the control arm to relieve the strain on the servo. The link won't work, so I can't see how it's connected.

Just thinking that (have never tried it) a resistor inline on the power line into the servo might limit the amount of force exerted.

Mechanically back in the old days and maybe now too would be to use something called a "servo saver".

I have a servo that can't never reach his position and I want some how to stop it trying. Basically I want to know if there is some command that will stop a servo from reaching its programmed position.

I'm using it with this robotic claw. And I want to catch an elastic item and I want it to press as much as possible.

You could make a limit switch setup for the claw such that when the limit switch is actuated the servo could be commanded to hold its current position. would take some mechanical and code work, but most likely doable. Working out the details would be very useful for others wanting similar setups.

You could add a piezo disc to a finger as a pressure sensor. Increment the position and check the pressure. After it reaches 'x' oz then stop incrementing and continue with whatever you want to do with the object.

It's VERY (read: VERRRY!!!) bad practice to hold full pressure on a servo 'forever.' It draws a tremendous amount of current, and will burn out soon enough. Or it may eventually strip or break a gear. For the longest lifespan don't use more than about 3/4 of the -rated- force of the servo.

justone: Just thinking that (have never tried it) a resistor inline on the power line into the servo might limit the amount of force exerted.

That power line also feeds current to the control circuitry. I'd imagine there's a risk you'd cause it to go haywire. It would be better to add a resistance inside the servo to one of the wires that lead to the motor.

Another option would be to tap onto the potentiometer output inside the servo and use it to read the position then adjust the sservo command to be near it so it stops trying to press so hard.

And finally you could add current sense to the servo power lines. When the current is high you know the servo is moving or trying to move; again back off on the position command until current drops.

Old video of a servo being stopped when the paper clip touches the piece of aluminum foil.

http://www.youtube.com/watch?v=3zQalB0bfNE

Well I had to give it a try (resistor inline on the power line) so I loaded up the "Knob" sketch and using a Hitec HS-310 servo I tried some various resistors. It didn't like the 100 ohm (wouldn't budge)but it did OK on the 10 ohm and I could stop it but the pressure was a little too much (I'd say about half of what it would normally be) so I tried 20 ohm and that seemed to do a good job of putting enough pressure and left alone it sweeps like normal. No twitching or signs the servo was not liking it.

But with a micro servo (don't have one) or true digital (ditto) things might be a little different. Just had to find out that's all.