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I want to make a low thrust linear actuator with a cheap continuous rc servo motor and then control it with a microcontroller. The thing is, I want to find out what the minimum degrees is that a motor can move but it seems to be something that is not documented. I can see the pulse range that you can control the motor with but I also have no idea how a pulse length relates to a given output. Is there some place that I can find some documentation?

PS I am sort of wanting the FS5106R motor at the moment.

Chris Stratton
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Gerhard
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3 Answers3

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The servo you mention is a continuous rotation servo, which is really a bit of a hack - an aftermarket modification now being done for you at the factory or distributor.

These devices do not have a closed positioning loop - to the extent that they have proportional control at all, it's of speed or torque. These are made by taking a normal servo design, and disconnecting the feedback potentiometer from the output shaft (or even substituting fixed resistors), so the internal positioning loop is opened and you get to control the apparent loop error by commanding a position different than the one it's feedback input is "stuck" at.

I've not used this particular model, but those I have used had very poor control - there was very little input range between full-on in one direction and full-on in the other. It's too bad someone doesn't make one with a closed velocity loop, though that would require a different motor technology or a rotation detection scheme (incremental optical encoder? electronically commutated brushless motor?)

Chris Stratton
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If it's a dc servo motor it will have a shaft position sensor built in and you use this in a control loop (with a motor drive amplifier) to set the shaft position to exactly the point where you want it to be. You don't have to worry about micro-positioning the motor because the "control-loop" does that for you.

Andy aka
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  • On a normal R/C servo, yes - but not on these, where the feedback loop has been intentionally broken to make a continuous rotation device. – Chris Stratton Aug 26 '13 at 01:14
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The accuracy of the servo system is mostly dependent on the rot. potentiometer inside the servo, and also on the backlash of the mechanical system.

If you want a good output I strongly suggest that you move your sensor to the linear output (using a linear pot maybe), that way all the mechanical backlash will be hidden by the control loop.

arthur.b
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