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Brianmcm
12-22-2009, 04:47 AM
I've been experimenting with controlling a servo using the Propeller and had a question that is toward the servo side. I was interested in getting the ohm value of the wiper off the servo pot to use as a data point. The servo is a Tower Pro and the center line from the pot is connected to the servo board through a 1K resistor. I tapped into that line, between the pot and the resistor, and with an ohm meter was reading 2.7k ohms, without any power to the servo. Interestingly the minute I apply power to the servo the meter reads as though that line is open. However, I can still control the servo. The "grounds" from the power supply board and the Propeller board are·connected together, however power to the Propeller is by a 9V battery, seperate from the servo power. I was hoping to be able to watch the ohm value of the servo pot change as the servo moves to various positions.
What am I doing wrong?
Any guidance would be greatly appreciated.
Thank you,
Brian

Beau Schwabe (Parallax)
12-22-2009, 05:55 AM
Brianmcm,

As you have observed, if you try to measure resistance in a circuit you will obtain false readings (even if the supply is disconnected) and you could damage the multimeter. What you might have better luck with is measuring the voltage across the center post of the pot and ground.

If your interested in detecting the position of a servo without tearing into the servo, you might want to look at this link...

http://forums.parallax.com/showthread.php?p=582447

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Brianmcm
12-23-2009, 05:18 AM
Thank you Beau.

THe information was very helpful. Much appreciated.

Brian

CassLan
12-23-2009, 05:28 AM
Nice Moustache Beau!

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CannibalRobotics
12-23-2009, 05:37 AM
You could also watch the voltage on the wiper just as the servo does.
To see it, attach the negative probe of the VOM to the servo ground and the positive to the wiper on the servo. That voltage will vary proportionally to the servo position.
To use it, add a current limiting resistor and an ADC to input the wiper position into the Prop.
Jim-

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Send $1 to CannibalRobotics.com.

Beau Schwabe (Parallax)
12-23-2009, 06:33 AM
@CassLan - http://forums.parallax.com/images/smilies/smile.gif

@Brianmcm - let us know what your results are and what method you decide to use.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.