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the land of sun+snow
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Maybe zoomkat will tell you which servos he has that don't twitch on power application.
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Maybe zoomkat will tell you which servos he has that don't twitch on power application.

I just use cheap analog servos powered by a 7805 chip with diode to output 5.7v. If the servos in this discussion are digital servos, then that might be where the issue lies. Some of the discription is strange in that no matter where the servo is physically positioned, it always moves 30 deg. This is strange in that the servo should move to some default control signal position when attached.
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the land of sun+snow
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Some of the discription is strange in that no matter where the servo is physically positioned, it always moves 30 deg. This is strange in that the servo should move to some default control signal position when attached.

These are 2 different issues, as I noted in post #2. Re the 2nd half, whenever you start sending pulses to a
servo it will normally jump at maximum speed, because it's unlikely to be at the commanded target position
[except as OP noted, for his particular situation].

Re the 1st half, I have some GWS servos that jump literally 30-deg when power is applied, and NO pulses are
being sent. If I re-apply power several times, I can get the servo to jump a full 180 deg around the clock,
until it gets over to its mechanical stops.
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Re the 1st half, I have some GWS servos that jump literally 30-deg when power is applied, and NO pulses are
being sent. If I re-apply power several times, I can get the servo to jump a full 180 deg around the clock,
until it gets over to its mechanical stops.

Are the gws servos analog or digital? The "and NO pulses are being sent." sounds like a possible floating control pin situation. Corrupt or inappropriate control signals can cause analog servos to erratically turn until they hit the hard stop.

edit: When you code "detach" your servo, does it behave strangely? If you physically remove the control wire, does the servo behave strangely?
« Last Edit: January 28, 2013, 12:11:19 am by zoomkat » Logged

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Both of the servos I have (2 diff brands) do something similar.  They (or at least one) actually give 2 little twitches before a bigger twitch, then go to either 900 or wherever I have them positioned in setup, but sometimes it will be just three little twitches before default.  The only thing that seems to make a difference (maybe) is how much torque is on them at power on.  They also twitch every time I turn on the serial monitor.
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the land of sun+snow
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GWSes are analog. But good thinking. I just tried 2 GWS servos, and it doesn't matter if
the signal pin is grounded or not. One of the GWSes simply twitches noticeably on power
application, and then comes back to the same position. The other one turns 20-30 degrees
or so, even with a moderate torque load on it.
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I got a new servo yesterday, and it does the same thing.  It does twitch in the opposite direction though.

I'm adding a relay to delay the power to the servo.
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New York
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digitalWrite( 5, HIGH);
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When you start up an Arduino, the servo will automatically jump the servo a bit and move.  One way to prevent this could be to always have a voltage applied, but maybe like a "sleep" mode where no power is being drawn other than the LED on the main board.  Does that help at all?
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-Matt-

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