is there a ratcheting servo device ?

Has anyone ever heard of a (hobby sized) servo device that would let you, say rotate a servo arm to close a pair of grippers, then lock it in place mechanically so you wouldn't need to keep
powering the servo to hold an object ? Of course, it would need a way to release it, preferably electrically.
I guess a better alternative would just be a lead screw setup, but I thought I'd ask anyway. I have been wondering about this for a while & figured you guys would know if anyone would !
Thanks ! JCD
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pogo wrote:

Haven't seen one, but as such it doesn't take that much power to hold an object, especially if the gripper uses leverage (most do). Most of the power is consumed when lifting an object, unless the arm or other mechanism is counter-balanced (most aren't).
If you look at the older gripper I have on my site it's actually made from a ratcheting clamp. I remove the ratchet. One could leave the ratchet in place, and release it by some means, such as a solenoid, but I think it's overkill. Now you have the weight of the solenoid and the releasing mechanism on the gripper, and power consumption goes back up again.
Anyway you can get these clamps at many woodworking outlets. Cost is a buck or two.
-- Gordon
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Thanks Gordon. I actually copied your idea about a year ago and made one almost identical to that one. I didn't think about leaving the ratchet part in, though. And yeah I agree about adding the weight of a solenoid - not really desirable.
Here's a photo for those that are interested:
http://www.waycoolgear.com/house/servogripper.jpg
Thanks ! JCD
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wrote:

You might consider facing the gripping surface with something soft and with a high coefficient of friction. This could reduce the amount of gripping force needed to actually hold an object.
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I haven't.

Here are a few ideas to think about...
You could always spring load the gripper so the spring holds it closed. That way, you only need power to hold it open. The spring would bias the action of the servo making it stronger for closing and weaker for opening. If you kept power applied, it could produce a stronger holding force, but would still reduce the load on the servo and I assume reduce the power drain as well.
Also, if you design it right, you should be able to make it hold by friction of the parts alone. It's just a matter of using the correct gear ratio or the equivalent created with a series of levers. But that might end up creating a gripper that opens and closes too slowly for what you want to do. Your lead screw idea is likely to produce enough leverage to hold by friction alone.
If you are designing the gripper to grab one very specific size of object which will allow the gripper to close to the same position every time, then you can use the same trick that is used with vise grip pliers. You just use levers that are the correct length so they lock into place when the gripper is in the closed position. With the help of a spring between the levers and the gripper, you could make it lock on to slightly different sized objects.
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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wrote:

Thanks Curt. The spring-loading idea sounds about as good as anything else I can think of. Thanks again ! JCD
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pogo wrote:

Look up Toggle Clamps, or the more general 'over center mechanism'. Another example is the Mole Grip. These all generate a large force which is self-locking.
You will need some compliance (ie spring) to control the force applied to the object, or it may get damaged ...
Dave
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