MG90S Servo.. Ain't No Stoppin' Us Now

ercoerco Posts: 19,601
edited 2019-09-05 - 03:26:07 in Robotics
I just got four metal gear 9 gram MG90S servos from a US seller. They work OK, but there is no mechanical stop. You can just rotate them endlessly by hand. Never seen that before. I thought the internal mechanical potentiometer would get torn up if it went beyond maybe 270.

Ever seen that, or do I have the only four ever made?
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Comments

  • ercoerco Posts: 19,601
    I just received 6 more from China which operate the same way, so I guess the ones I received are the new normal, for this servo anyway.

    That is all.
  • There's always a new way to cut a corner!
  • erco wrote: »
    I just received 6 more from China which operate the same way, so I guess the ones I received are the new normal, for this servo anyway.

    That is all.

    Could it be that the potentiometer resistive element is a full 360 degrees with the input voltage connections 180 degrees apart? That would allow the wiper to rotate continuously but would make it harder to determine the position of the wiper since both halves of the potentiometer would traverse the same voltage range.

    The other explanations that come to mind are multi turn potentiometer, optical/hall encoder, or the potentiometer gear slipping when it reaches the stop. Be interesting to take one apart to find an explanation.
  • 2 minute teardown?
  • ercoerco Posts: 19,601
    xanadu wrote: »
    2 minute teardown?

    Whenever I get 2 minutes!
  • wait a moment. Almost there. Here we are.

    2 minutes for @erco

    Enjoy!

    Mike
  • I have some experience with the LewanSoul LX-16A servos which also have no end stops but in their case, they allow for switching between angular and continuous rotation modes via software. The angle feedback sensor is still a potentiometer though. It is just one which can be rotated through a full 360 degree arc. This particular pot only gives valid readings for about 330 degrees of that rotation. Maybe this servo uses a similar potentiometer.

    Photo of potentiometer in LX-16A servo
    20190822-05.jpg
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