Why dont we see continuous rotation servos with degree control?

Discussion in 'General Electronics Chat' started by Aklem, Aug 2, 2009.

  1. Aklem

    Thread Starter Active Member

    Jun 18, 2009
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    why cant the pot have the lock taken off so it just rolls back to the original resistance value?? therefore it can have degree control but still continuously turn.

    so it cycles from 0 to 10k then to 0 (0k, 1k, 2k, 3k, 4k, 5k, 6k, 7k, 8k, 9k, 10k, 0k, 1k etc.)

    hiteck or gws would be rich!
     
  2. someonesdad

    Senior Member

    Jul 7, 2009
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    I know at least one was made once -- I have one made by General Radio somewhere...
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    The position indicating pot on a wind vane used to be made with no stop.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can buy continuous rotation pots from Bourns I think, it was in another thread.
     
  5. HarveyH42

    Active Member

    Jul 22, 2007
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    How many different application are there for precise continuous rotation? Most of the time, you don't want a full rotation with a servo. You want it to stop before whatever it's linked to gets tangled and damaged. There are plenty of motors with position encoders, and chips to help deal. Seems like a lot of people modify servos for robot drives, maybe they will start selling them that way from the factory, if they haven't already (have seen them, but got the impression they were modified after manufactured).
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Pots without stops are more for mechanical safety than 360' position measurement.
    Any conventional pot will have a dead spot 'between the terminals' - if there is a gap between the track ends, there will be no output at some point, without a gap the wiper would short the track ends.

    One very rare type is a 'Sine - Cosine' pot. These have a continuous circular resistance track with four equally spaced connections, plus the wiper.
    By using all the terminals (or alternate cross pairs), the wiper position can be calculated anywhere in 360'.


    Most indstrial servos use optical encoders rather than pots.

    These allow unlimited rotation and typically are set up with a motor and gearbox driving a leadscrew (ballscrew) with either a rotary encoder connected to the leadscrew (so the accuracy is not reduced by the backlash of the gearbox) or with a linear scale encoder reading back the position of the part being moved.

    Typical measuring accuracy on this type of setup on (e.g.) a machine tool is at the micron or better level, though that is no guarantee the mechanics can position to that accuracy.
     
    Last edited: Aug 3, 2009
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    For cont. rot. with position control consider the stepper motor. Your odd pot setup would require custom drive electronics to take advantage of the continuous rotation.

    Using a 10 turn pot would give you servo control over 10 turns. Gears can also be used.

    The question is why do you want to do it?
     
  8. Aklem

    Thread Starter Active Member

    Jun 18, 2009
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    i just wondered...
    well if you wanted a winch system that would pull an object to a certain point exactly then you would just count the rotations...
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Are you kidding? Many of the little robots now use continuous rotation servos for wheels (translation). The setup he described would do everything, you could move the robot continuously at a set speed or turn the wheels a specific rotation amount to move an exact amount.

    The only other alternatives are costly messy add on optical encoders for rotating servos, or gearmotors with optical encoders which are also expensive and bulky.

    If the pot had a fairly sharp transition from max to min it would work ok, like rolling a binary counter. It would just turn the direction closest to get to the position you commanded.

    I think it's brilliant. Send an email to HITEC Aklem. :)
     
  10. Radiocrazy

    New Member

    Aug 5, 2009
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    Synchros and servos are used by the Navy for antenna and gun direction of 360 degree. At least in the 70s and 80s. I believe there is a section related to its application and theory in the N.E.E.Ts series of navy pubs online.
     
  11. Radiocrazy

    New Member

    Aug 5, 2009
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