build your own servos using larger motors ?

Wondering if anyone knows of ready-made or plans/kits for building your own servos using larger motors, feedback resistors or encoders, and brackets ? The kind of servos I am talking about might have a motor diameter of 2-3 inches.
Thanks for any help! JCD

pogo wrote:
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Sure. You can get commercial servomotors and motor controllers, and there's more hobbyist grade stuff in that category than there used to be. Trossen and Roboteq have some suitable gear. If you want a more component-oriented approach, here's one:
http://www.uhu-servo.de/servo_en/index.htm
There are many more; it's not hard.
                John Nagle                 Animats

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Hi John. Thanks for the info. Actually I am looking for something more along the lines of Jim Frye's Monster Servo project ( http://www.lynxmotion.com/images/html/ht01.htm ) but where the motors, brackets, etc, come prematched - ready to be assembled.
Something else I would like to see is examples of chaining cheap, low power servos together through mechanical means to end up with a larger lever mechanism that can lift larger weights than the individual servos.
Thanks ! JCDeen

pogo wrote:
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There are large R/C servos, up to about the 190 in/oz level. See:
    http://www.rcplanet.com/High_Torque_Servos_s/1292.htm
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I can think of a number of hokey ways to do this, from pulleys and string to a series of differentials, but it's easier, smaller, and cheaper to get a bigger motor. Unless you really like machine shop work, don't do by mechanical means what can be done by electrical means.
                John Nagle

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That just looks like a typical DC motor/resistor/feedback. I used to make systems using analog controllers.I don't know what type of controller this has. Most things today would use steppers, but I don't know whats available. You could complicate it using a computer for processing. For the system shown, you would basically need a DC power amp and a 12 to 24 volt DC motor. A power supply and rotary resistor pot and a differential amplifier to get the control voltage.
greg

(GregS) wrote:
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One system here made in the 70's had a DC motor over 6 inches in diameter and no gearing, slip rings, and probably a resistor encoder instead of optical. Of course the servo on the 85 foot antenna I used to control had a 150 HP AC motor for the X axis, optical feedback, and various controllers over the years.
On day I pushed the power button on that motor and the whole NASA Apollo tracking station lost power. Blew two breakers and had to drive a few miles to reset everything !
greg
greg

(GregS) wrote:
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OH that AC motor also drove the hydraulic controller.
Things got bad one day with another drive problem. We were tracking an Alsep package on the moon, and they got out a very large wrench to manually servo the antenna. That only lasted so long to keep up.
greg

GregS wrote:
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Steppers are more or less obsolete for applications with variable loads or in battery powered systems. They have to be powered for the worst case at all times, they draw power when stopped, and they don't do force control or force feedback. Modern technology is to use relative encoders and a microcontroller driving an H-bridge.
                    John Nagle

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With feedback there is no need to use a stepper, but by using a stepper, your allready going to the right place from the getgo. A stepper current can be reduced or even shut off after the position is stable. Depending on the motor it may be geared so there is always some stabilization.
I would assume that the control is pulse modulated. I built one system a few years ago using a DC motor and simulating a stepper, just by feeding pulses into it. Pulses provide more accurate torque control. i'm probably just repeating the obvious.
greg