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Normally locked rotation? Power-off brake?

Thinking about the mechanicals of this a little more, I've been considering servos and steppers.  I don't need speed or precision, really.  But I do need to be able to move in either direction, which I now understand to be complicated with servos.  So I considered steppers, which at least can move in either direction. 

A particular challenge of this application, though, is once the rotation has happened I'd like it to be locked in place to resist rotational forces from wind and chop, and I'd rather that not draw power, making even a stepper with holding power difficult.  So I'm thinking that I need some type of normally locked mechanism.  Something like a ratchet, but I'd still like to be able to move either direction, without full rotation.

Perhaps something like a stepper motor and a pin that can be dropped into the rotation mechanism and retracted using a solenoid? A little googling suggests that I might be looking for a "power-off brake" but these all look very expensive.

Any ideas? Honestly the holding tension doesn't have to be absolute, just enough that the motor can be turned off when in position and have some expectation of holding in non-extreme conditions. Perhaps the initial pressure can always be in place and just have the stepper push through it to move the thing?  That does seem like it would draw extra power, though.

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What if you use a servo, and try this modification so you can monitor the position? Then you can power off the servo after getting it into position, and just passively monitor the position. If it changes, you can power the servo back on and reposition it.


Thanks 6677. Interesting suggestion, a quick wikipediaing suggests that a worm gear drive is "self locking" under some conditions to do with the  lead angle and friction angle.  I can't quite picture that in this application, but I'm thinking one would have a worm/screw setup on the pole that holds the antenna and direct drive that from the servo.  I think, though, that this would only allow rotation in one direction? Or does it allow movement in both directions, but only initiated by the stepper, not by the pole.

Seems also that worm gearing doesn't neccessarily self-lock under conditions of vibration, but this isn't a safety situation so that might be ok.

I need to draw some pictures, I know!

Back in the old days (100+ years), folks could design for most any kind of mechanical motion you'd want - without the use of fancy motor controllers, micro-processors and the like.  Could you describe your application requirements a bit more: resolution, speed, repeatability?  It sounds like you're positioning an antenna - is that right?  That's a typical application for a worm drive (which can rotate in both directions).


EDIT: I just read through some of your previous blogs and better see your application.  You want the pole stationary and have the antenna element/assembly manuvere itself around the pole?

Thanks ignoblegnome, that's a cool mod.  I think, though, that back-rotation would still occur and one would be constantly running the servo to re-position it.  Worth thinking about though!

this solution won't lock the motor solid, but it resists movement pretty hard. Some how short the motor power together ( that is the 2 terminals at the motor, not the actual power used to drive the motor.) I bet it will be nigh on impossible to move a gear motor very far and hard to move a servo. Obvious it will move slowly as the back emf dissipates. I've seen this design used in tape libraries to drastically slow the drop of a heavy tape loader arm. Not sure if this is enough for your project.