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Thread: How would you build a Dynamixel torso joint?

  1. #1
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    How would you build a Dynamixel torso joint?

    Hello Mechanics Gurus,

    I am trying to build a torso turntable joint for Pi Robot that uses a Dyamixel servo as the actuator. I have attached a picture of Pi Robot's base with a 6" lazy susan lying on top in roughly the location I want it to be mounted. The rest of the robot would then be mounted on top of the lazy susan. The lazy susan is sitting on a 1/4" sheet of acrylic and there is only about 1/2" space beneath that sheet and the laptop that you can see below it. So I am thinking the servo would have to be mounted in the robot torso and connected to the top half of the lazy susan. Alternatively, I could add another acrylic layer above the current top layer if it makes more sense to mount the servo to the base.

    I built something like this before using a 4" lazy susan which was too small (allowed the torso to rock too much) and I created some ugly hack for attaching the servo's rotation platter to one half of the lazy susan.

    Anyway, how would you go about building this kind of joint? Would you even use this kind of lazy susan or are there better turntable bearings out there for this purpose? Would you drive the turntable directly using the servo, or use a belt and/or gears?

    BTW, the robot above the turntable weighs about 6-7 lbs.

    Looking forward to some ideas!

    --patrick

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  2. #2
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    Re: How would you build a Dynamixel torso joint?

    How much rotation are you looking for and which dynamixel?

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    Re: How would you build a Dynamixel torso joint?

    Good questions--I would like at least 90 degrees rotation to each side, so 180 in total. I have spare AX-12s lying around but would be willing to upgrade to a higher torque model if required. (Speed is generally not an issue.)

    --patrick
    The Pi Robot Project
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  4. #4

    Re: How would you build a Dynamixel torso joint?

    VEX has just released this turntable


    http://www.vexrobotics.com/276-1810.html

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    Re: How would you build a Dynamixel torso joint?

    Thanks for the link! I actually happened upon that turntable a few days ago when I was ordering some stand-offs from Vex but thought it might not be a large enough to prevent wobbling with 6-7 lbs mounted on top of it. But it might be worth a try at $19.95. I'd have to mount a Vex gear cog to the dynamixel servo somehow and then figure out a way to mount the servo such that the cog meshes with that outer 60-tooth gear...

    --patrick
    The Pi Robot Project
    http://www.pirobot.org

  6. #6

    Re: How would you build a Dynamixel torso joint?

    Hi Patrick
    I use a similar thing for my arm base (200mm dia) I just use a servo in the center and a bracket i made a spacer to connect the servo to the arm base the height from the bottom of the servo to the bottom of the arm base is 70mm when i redo the top half i will mount the servo on the arm base to reduce to height reqiured.
    I do have some issues with the rotary base its not very stable i used blocks as slides to try and reduce the wobble of the structure (ball rollers would be better) the other issue is the rotation is not smooth may be due to the friction of the blocks and the offset weight of the arm.

    The press steel and bearings type lazy Susan will always have a wobble. A 150mm dia thrust bearing running in grooved washers with the balls held in a race and the higher tolerances should reduce the wobble'
    the other drive method is a timing belt that way you can change the gearing to increase torque

    peter

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    Re: How would you build a Dynamixel torso joint?

    Thanks Peter--I had never heard of a thrust bearing before so that is a good lead. I'm looking for something as rock steady as possible since the entire upper body of the robot including arms and head/camera (Kinect) will be riding on it. And when it comes to inverse kinematics of the arms and PCL stuff with the Kinect, I don't want any unnecessary wobble.

    --patrick
    Last edited by Pi Robot; 05-01-2011 at 09:18 AM.
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    Re: How would you build a Dynamixel torso joint?

    Patrick, you may find this implementation by Upgrayd to be a useful reference.

  9. #9

    Re: How would you build a Dynamixel torso joint?

    Patrick
    I think even with a large thrust bearing fast and out of balance loads will still cause a problem. A while ago i tried using a taper bearing (wheel bearing ) with a bolt to provide pre load this was more stable than than my 200mm lazy Susan bearing. I was thinking to combine the 2 lazy susan on top to transfer the load out and the taper bearing on the bottom to provide pre load to stop the structure from trying to lift with out of balance loads the draw back is an increase in friction.
    while dining with some friends i seen a new type of lazy susan bearing its a ring 30cm in dia but 1.5cm *1.5cm bearing enclosure (larger bearings?) what impressed me was its quite(no bearing rattle) and stable very little movement in the bearing assy but heaver that a normal lazy Susan bearing (for some reason you get strange looks when you dismantle someone table before dinner )

    peter

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    Re: How would you build a Dynamixel torso joint?

    Quote Originally Posted by Stobs View Post
    Patrick, you may find this implementation by Upgrayd to be a useful reference.
    Thanks Stobs. I wonder what Upgrayd is using as the bearing itself? Or is that washer attached directly to the servo plate?

    --patrick
    The Pi Robot Project
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