Homebrew servo?

--The application is the angular rotation of a heavy load on a mobile platform.     --The goal is to make something that can be controlled via
Stamp that behaves like an RC servo, but using a *much* larger motor. Anyone got plans for such a beastie?     --TIA,
--
"Steamboat Ed" Haas : You just can't beat
Hacking the Trailing Edge! : cruisin' in a steamboat
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I bought the "guts" of an RC Servo as a replacement part from Lynxmotion. I suppose if you replace the transistors on it with larger ones, you might be able to control a larger motor.
Can you be more specific on just how large you're going for?
--
Randy M. Dumse
www.newmicros.com
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Geez... I haven't posted here in a long time. Anyway...
Since you haven't gotten an answer in a few days, I'll just go ahead and toss this out.
I played with hacking a cheap servo a long time ago. I took the electronics out of the servo and mechanically connected the pot to the shaft of a larger dc motor. I then took the output of the little board and ran it through a couple of large transistors.
It worked, but not as well as I would have liked. Not really having a need for that setup at the time, I didn't try to improve on the design. What actually happened for me was that the motor was rather slow and also had a tendancy to over/under drive the desired position.
Probably could have improved it with a better pot and more attention to the motor drive cirtuit. In fact, I've seen others do it that way - but if I wanted to do something like that again, I would probably forget about the servo electronics and send pwm to the motor through an h-bridge with a brake input and send the feedback from a position pot on the motor back to an A/D converter in the stamp.

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: wanted to do something like that again, I would probably forget about the : servo electronics and send pwm to the motor through an h-bridge with a brake : input and send the feedback from a position pot on the motor back to an A/D : converter in the stamp.     --Yeah, that's the thing: I haven't got a clue how to do the feedback circuit, to prevent motor overrun. I figure the feedback wouldn't be taken directly off of the motor shaft, but off of the output shaft that exits a gearbox. Any links to anything like this would be appreciated.
--
"Steamboat Ed" Haas : You just can't beat
Hacking the Trailing Edge! : cruisin' in a steamboat
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steamer wrote:

I think the accepted method of avoiding over-run is to slow down as much as needed when you get close to the stopping point. The speed and slow down point have to be designed to meet your machinery specifications. (but don't ask me to design it)
Bill K7NOM
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steamer wrote:

Servo City (www.servocity.com) sells a mod kit that takes a standard servo and, though external gears, boosts its output many, many times. I suspect the size and torque is still not what you will need for your project, but it may give you ideas. (The guy says "patent pending," but you have no idea what's being patented, and besides, if it's for your own scientific experimentation, you can build what you want.)
Their unit, with a Hitec 805BB, delivers something like 1,700 oz-in. How much torque will you be needing?
-- Gordon
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: Their unit, with a Hitec 805BB, delivers something like 1,700 oz-in. How : much torque will you be needing?     --Yeah, that sounds plenty beefy; IIRC I'm only using 300 oz-in to drive the x-axis on my Bridgeport mill..
--
"Steamboat Ed" Haas : You just can't beat
Hacking the Trailing Edge! : cruisin' in a steamboat
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A mill? Are you making an x/y table? If so, seems to me you would be better off using stepper motors.
By the way, I think the stamp has examples of how to use the a/d pins.
wrote:

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    --Actually no. What I want to do is convert one of those cheapie windshield wiper motors, the ones going for around $19.- from HSC, the ones with the attached gearbox. What I want to do is have the gearbox output arm behave like a servo, when given "orders" by a Stamp; i.e. rotate one way but just so far, which means I've got to get feedback from the output shaft, the same way a "normal" servo does it.
--
"Steamboat Ed" Haas : A greasy donut, a cup of
Hacking the Trailing Edge! : coffee and thou...
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steamer wrote:

These guys built an RC servo from a windshield wiper motor: http://www.cpg1.freeserve.co.uk/servos/servos.htm
If you want to do less and buy more, These guys sell a big two channel motor controller that will accept either an encoder or a pot for feedback and operates as a closed loop controller: http://www.roboteq.com /
Good Luck, Bob
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: These guys built an RC servo from a windshield wiper motor: : http://www.cpg1.freeserve.co.uk/servos/servos.htm
    --Kewl! That's exactly where I'm headed! :-)
--
"Steamboat Ed" Haas : You just can't beat
Hacking the Trailing Edge! : cruisin' in a steamboat
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http://www.oatleyelectronics.com/kits/k165b.html
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