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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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  • $\begingroup$ Welcome to Robotics markshancock, but I'm afraid that shopping questions really aren't a good fit for a stack exchange site. We prefer practical, answerable questions based on actual problems that you face. Take a look at How to Ask and tour for more information on how stack exchange works, and the Robotics question checklist for details of how to write a good question. $\endgroup$ – Chuck Jun 26 '17 at 17:53
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Chuck Jun 28 '17 at 14:28
  • $\begingroup$ Hey, I know you didn't get an answer, but try a stepper motor! :) $\endgroup$ – tuskiomi Jun 28 '17 at 14:49
  • $\begingroup$ @tuskiomi Unfortunately "The game rules requires the solution to use a 3-wire RC servo." $\endgroup$ – markshancock Jun 28 '17 at 15:18
  • $\begingroup$ Ah. Then you're looking for what's called a winch servo $\endgroup$ – tuskiomi Jun 28 '17 at 15:20
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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 is 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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  • $\begingroup$ Glad you found an answer! You can mark your own question as solved by accepting your own answer :) $\endgroup$ – Chuck Jul 5 '17 at 14:13

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