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RC Servos are great because they are low cost, widely available, easy to control, and pretty accurate. One disadvantage is that they usually have very limited range of motion. This is because they are mostly for use on actuating RC control surfaces that rarely move more than 120 degrees.

various rcservos

Our FTC robotics teams uses them a lot; but, often we need to rotate more than 120 deg and often would like 360 deg or more. Robot design requires the solution to be small and light weight as it will usually be at the end of an extension arm. The game rules (and practicality) requires the solution to use a 3-wire RC servo. Also, space and alignment issues usually make using external gear sets problematic.

Last season we needed a large "grabber" at the end of our arm and resorted to using a continuous rotation servo the rotated until it torque limited. This worked but was far from ideal as it over-stressed the servo and we had minimal control on the "grabber" - we could open it or close it.

Our ideal solution would be small, light-weight, and inexpensive (add less than 50% to the weight, cost, or the length of any dimension of motor).

Given our constraints, how can we rotate an axis more than 360 deg and still maintain positional accuracy?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Chuck ♦
    Commented Jun 28, 2017 at 14:28
  • $\begingroup$ Hey, I know you didn't get an answer, but try a stepper motor! :) $\endgroup$
    – tuskiomi
    Commented Jun 28, 2017 at 14:49
  • $\begingroup$ @tuskiomi Unfortunately "The game rules requires the solution to use a 3-wire RC servo." $\endgroup$
    – markshancock
    Commented Jun 28, 2017 at 15:18
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    $\begingroup$ Ah. Then you're looking for what's called a winch servo $\endgroup$
    – tuskiomi
    Commented Jun 28, 2017 at 15:20
  • $\begingroup$ Thanks for your comment @tuskiomi, if the OP edits this question to fit community guidelines, this would make a good answer (with a little more detail *8'). $\endgroup$
    – Mark Booth
    Commented Jun 28, 2017 at 16:02

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While originally creating this question I ended up finding an RC servo that can do this.

HiTECH makes an RC servo (HS-785HB) that uses a muti-turn potentiometer rather than a standard rotary potentiometer as its position senor. This allow the servo to rotate 3.5 turns (1260 degrees!!!) while still maintaining positional control. It is pretty slow (almost 10 sec to do a full rotation) but it has an immense (1.8A) stall torque. The thing is a beast! HS-785HB

It is also called a Winch Servo (as @tuskiomi mentioned) since it is commonly used as a sheet winch on RC sailboats. Note that from what I have seen, not all Winch Servos are multi-turn. So far this is the only one I have found.

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  • $\begingroup$ Thanks for this, but damn if these things aren't expensive AF compared to other servos. The other winch servos I've found are just free-wheeling 360° servos, so this one is exactly what I need, but I need 5 of them and $250 is a tough nut to crack. It's also enormous! (which probably contributes to the cost) $\endgroup$
    – ipmcc
    Commented Feb 27 at 12:27
  • $\begingroup$ Since I posted this, I have found some cheaper options depending on the torque and number of rotations you need. $\endgroup$
    – markshancock
    Commented Apr 10 at 18:54

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