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Unread 02-06-2004, 06:23 AM
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Question Q about potentiometers

this is not nessecarily related to the building of the robot but this has been bugging me for quite a while now...
can i connect a potentiometer to the wires of a servo... obviously throgh a power source and all that but will it work.. i thought about it because i know joysticks are basicaly based on ptentiometers at the base that measure the movement...and does anyone know if there is a NON-programming way to controll power output, i know that potentiometers as well cause thats whats in a speaker with volume control...

really appreaciate help!
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Unread 02-06-2004, 06:37 AM
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Re: Q about potentiometers

pots are only designed to create a small signal, not to control any significant power - so no, you cant control a servo motor by connecting a pot to its power leads

the servos they give us, the hiTech units, are contolled by a digital pulse width modulated input, so putting a pot in series with its leads, or across them, would not create the type of signal the servo unit needs to see.

If you explain what it is you are trying to accompish, Im sure it can be done somehow - but not as simply as you want it to be.
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Unread 02-06-2004, 06:51 AM
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Re: Q about potentiometers

Quote:
Originally Posted by Nick R.
i know that potentiometers as well cause thats whats in a speaker with volume control...

really appreaciate help!
Engineers like to use the volume control analogy to demonstrate the variable nature of a pot. In you stereo though, the pot is contolling a very tiny voltage at the input of a big power amp. As you turn up the volume the power amp output also rises. Don't try hooking up a pot to your servoes, as Ken said the signals are different. There is actually a tiny circuit board inside with a custom integrated circuit and other components that drive the servo motor. It acts as a power amp, but the input signal is PMW so it has to be converted into a signal the motor can use.
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Unread 02-06-2004, 04:53 PM
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Re: Q about potentiometers

Quote:
Originally Posted by KenWittlief
pots are only designed to create a small signal, not to control any significant power - so no, you cant control a servo motor by connecting a pot to its power leads

the servos they give us, the hiTech units, are contolled by a digital pulse width modulated input, so putting a pot in series with its leads, or across them, would not create the type of signal the servo unit needs to see.

If you explain what it is you are trying to accompish, Im sure it can be done somehow - but not as simply as you want it to be.
i want to controll a motor, (forget abotu the robot controller and all that stuf.. its not related) using a pot meter, so as i rotate the pot, the motor turns
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Unread 02-06-2004, 05:00 PM
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Re: Q about potentiometers

do you mean when you turn the pot up and down the motor goes faster and slower, or to you mean the motor tracks the pot postion, like a servo does?

you can get large pots that will handle several watts to control a small DCmotor, like you would have on a train set

if you want the motor to act like a servo, so the motor turns and points like the pot does, then you need a second pot connected to the motor shaft, and you need a linear power amp and an op amp circuit

the control pot would be compaired to the position of the feedback pot (attached to the motor shaft) and the error signal would be what drives the imput of the power amp - the output of the power amp makes the motor move until the two pot positions are matched.

This would be a PID feedback control loop.
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Unread 02-06-2004, 05:03 PM
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Re: Q about potentiometers

yes i want i want the motor to track the pot like a servo and i realized i have to put one on the shaft as well, but what is that circuit you mentioned and how can i make it/buy it cheap
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Unread 02-06-2004, 06:49 PM
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Re: Q about potentiometers

how much power are you talking about? volts and amps?

you could make a linear amp from a couple transistors, but its not really simple, since it has to be able to go both positive and negative to make the motor spin both ways.
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Unread 02-06-2004, 06:54 PM
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Re: Q about potentiometers

let me see.. it should be 5v @ 25A and or 12v @ 10A
basicaly a PC power block..

you seem to know alot about electronics.. where did you learn it all?
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Unread 02-06-2004, 07:20 PM
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Re: Q about potentiometers

Suny Buffalo - thats what happens when you get a BSEE :c)

if you are gonna be pulling that much power you are going to need a servo amp - you can get them commercially, but I dont know if it will meet your definiton of 'cheap' :c)
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Unread 02-06-2004, 07:23 PM
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Re: Q about potentiometers

it doesnt have to be a servo, i can work with motors too its not a prob.. but throw me the figures anyway.. or at least if you could direct me at it, like a store or somth..

really appreciate your help!
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Unread 02-06-2004, 07:42 PM
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Re: Q about potentiometers

a motor that is being driven as you described is a servo motor

Ive used stuff from this company before, its a good place to start - looks like you are interested in servo motors and drivers (not stepper motors...)

if you can explain what your applicaiton is I might be able to help you more.

http://www.applied-motion.com/
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Unread 02-06-2004, 07:55 PM
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Re: Q about potentiometers

the products in the company you gave me are all AC motors..
my application: basicaly i move my hand, a robotic counterpart follows the 'instructions' no i'v got all the mechanical stuff figured out, all i'm left with is how to hook up the pot's so it actualy sends the signal
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Unread 02-06-2004, 08:55 PM
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Re: Q about potentiometers

oops! your right, I did see the 'brushless' part and didnt notice they are all AC motors

if you search yahoo for "servo motor amplifiers" you will get lots of websites showing up, and there should be some that explain how they work better than I can do off the top of my head.

these guys might be closer to what you want:

http://www.copleycontrols.com/motion...iers/dc_b_amp/

Last edited by KenWittlief : 02-06-2004 at 09:01 PM.
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Unread 02-07-2004, 08:13 AM
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Re: Q about potentiometers

Thnx for the help, i'll try and find something there
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