Using linear actuators on a robot arm

How about using linear actuators to drive a robot arm instead of stepper motors or servos? The big 340+ oz/inch servos cost a fortune
and are incapable of lifting a measly five pounds with an 18 inch arm length.
12V actuators are down to $59 on feeBay and that's less than the price of the large servos.
Of course an actuator would weigh a lot more than a servo but so what - they lift 300 pounds. Heck, you could jack up your car with it.
Caveats I see is actuators are a lot slower and eat current. Full extension on an actuator is maybe 45 secs and they can draw 5 amps but I suspect that lighter loads would not consume so much.
Any thoughts on this infallible idea?
Pokey.
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I see your idea for this. The only problem would be getting the actuator to send some feed back like in a servo. Then the controler could judge the distance the actuator has extended. Another great idea would be to get some feed back on current draw as well then the controler could judge the weight or load on the actuator. At the very least you could put in some bump/stop switches to tell if the actuator has fully extended or fully retracted. Anybody else putting more thought into this...

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I know I just posted, but here's something I found right away. Take a look.
http://www.tritonlinear.com/tritoncomspecificationsenglish.htm
Seems to be just what your looking for.

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Here's something a little more RC/Robot friendly...
http://www.emsjomar.com/linear.aspx
Good Luck.

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slow_mac snipped-for-privacy@yahoo.com wrote:

Aren't the devices you are talking about nothing more than a screw-drive system driven by a motor with a forward/reverse switch? It gets the power simply by using gears and sacrificing speed for force. If the parameters are what you are looking for, then it should work fine. But as others have said, you'll probably want to add position feedback (simplest case just limit switches). There are plenty of options of building your own system of gears and or levers with standard motors as well to get whatever leverage you need - including a screw drive - or pulleys and cables for example.
I think you don't tend to find those types of linear actuators in robots as much because they tend to be too slow for many applications. But if high force and low speed works for you I think they would be a fine option.
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If there are gears in the system wouldn't it be easier and cheaper just to in stall a Gear Tooth Counter? Just a thought.
Casey
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if your into building things here are crude images of a satillite sized actualtor I made
http://eds.dyndns.org:81/~ircjunk/actuator /
as for positional feedback, something like a potentiometer at the joint may siut you fine.
dan
slow_mac snipped-for-privacy@yahoo.com wrote:

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slow_mac snipped-for-privacy@yahoo.com wrote:

I'm dealing with this problem too, in trying to make a robot arm. Servos just aren't strong enough, even the expensive ones. As far as getting position feedback on the arm, I would suggest measuring the actual position of the output arm rather than trying to measure something in the geartrain and calculate the position. If your home-made parts are in the same range of precision mine are (which is to say, poor), then it's hard to get good feedback by calculating where the arm *should* be. A potentiometer on the axis of rotation works, or you might go for a resistive "bend sensor," as seen in those old Nintendo Power Gloves.
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