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Thread: ROS Hexapod project Golem: MX-64 4dof

  1. #31
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    Re: Golem: MX-64 4dof Hexapod project.

    This is truly impressive.

    I'm wondering: what does 4dof legs on a hexapod give you that 3 doesn't? I'm curious and more than a bit envious.

    I would like to build a large hexapod shortly, but mechanics is my weak point.

  2. #32
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    Re: Golem: MX-64 4dof Hexapod project.

    There is a bit more stability and strength with 4dof but my honest answer would be I wanted it to look cooler.

  3. #33

    Re: Golem: MX-64 4dof Hexapod project.

    Technically, you get to control the angle of incidence to the ground. With 3 DOF, the angle is lower the further out the leg stretches. With 4 DOF, you can always have 90 degree angle to the ground, which (if you have rotating contact pads) means you can totally avoid slip.

  4. #34
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    Re: Golem: MX-64 4dof Hexapod project.

    I like both the answers above. I do want my next Groucho to have the presence associated with the name.

    I have an interesting idea for a head that raises from the body to look at things on a higher level.

  5. #35
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    Re: Golem: MX-64 4dof Hexapod project.

    Kevin,

    Have you found that any of the motors could have been replaced with MX-28s? Or conversely would some of them use higher torque?

    I've seen some Hexapods use lower torque motors for the places where the motor moves primarily horizontally.

    And thanks for sharing the images of Golem. I'm looking forward to seeing more of this fantastic robot.
    -
    A little learning is a dangerous thing.
    Last edited by DangerousThing; 04-03-2014 at 04:04 AM. Reason: Fix spelling.

  6. #36
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    Re: Golem: MX-64 4dof Hexapod project.

    Hi, just shooting in a comment. Using 4 DOF clearly has both con and pros. Obviously additional DOF add both weight and cost. The extra mass on each leg isn't a good factor if you want a faster hexapod though.

    Having the 4. DOF also gives the leg more flexibility but the IK can be more complex due to many solutions. Keeping the tarsus constantly perpendicular to the ground is the easiest way and wouldn't require much torque either, so using a MX-28 should work just fine.

    And the coolness factor increases for every DOF..
    Kåre Halvorsen aka Zenta
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  7. #37
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    Re: Golem: MX-64 4dof Hexapod project.

    Quote Originally Posted by Zenta View Post
    Hi, just shooting in a comment. Using 4 DOF clearly has both con and pros. Obviously additional DOF add both weight and cost. The extra mass on each leg isn't a good factor if you want a faster hexapod though.

    Having the 4. DOF also gives the leg more flexibility but the IK can be more complex due to many solutions. Keeping the tarsus constantly perpendicular to the ground is the easiest way and wouldn't require much torque either, so using a MX-28 should work just fine.

    And the coolness factor increases for every DOF..
    I'll agree with the coolness factor, but only with fellow travelers. The average person, including my wife, Lee, is only annoyed by the increase in cost.

    Does it matter where the 4th servo is? All the 4DOF legs I've seen have two servos up near the base, and two other servos down the leg. I'm trying to think how that will work out in flexibility.

    And if it wouldn't be too much trouble to teach an old fogey something, can somebody define the bones/joints? I'm not used to thinking in biological terms and want to be on the same page as everybody else.

    Thanks in advance.

  8. #38

    Re: Golem: MX-64 4dof Hexapod project.

    Keeping the tarsus constantly perpendicular to the ground is the easiest way and wouldn't require much torque either
    I'm not sure about the "not much torque" part. Assuming all other servos are locked, won't the joint at the foot will see the entire torque from the ground contact point to the center of the bot?

    define the bones/joints
    The same terminology is used in computer animation and simulation, too, btw.
    The "joint" is the axis or point of rotation (axis for a single-axis servo; point for ball or universal articulation.)
    The "bone" is the rigid distance between one point/axis of rotation and the next point/axis of rotation.

    And, technically, you can have "sliding" or "translating" joints where the movement is linear rather than rotational; the joint is then the sliding interface (a line or plane.)

    And, in math, because each object in 3D has 6 degrees of freedom (expressed for example as XYZ translation, and heading/pitch/roll rotation) you can express the joints as constrained versus drive degrees of freedom. A typical rotational servo constrains five degrees (all of translation, two of three rotation) and allows rotation in one degree of rotation. A linear actuator constrains two position degrees, and all three rotation degrees, and allows movement along one degree of translation freedom.
    You can then express all of this as a set of tensors and do all kinds of cool math on it :-)
    Last edited by jwatte; 04-03-2014 at 11:05 AM.

  9. #39

    Re: Golem: MX-64 4dof Hexapod project.

    Quote Originally Posted by DangerousThing View Post
    And if it wouldn't be too much trouble to teach an old fogey something, can somebody define the bones/joints? I'm not used to thinking in biological terms and want to be on the same page as everybody else.
    Yep - I know the feeling. My mental translations are:
    Coxa - Hip Horizontal
    Femur - Hip Vertical
    Tibia - Knee
    Tars - Ankle

    As for 4dof - I also agree with the Coolness or Badness look of the 4ths DOF. And for a test case, I do have a 4DOF Lynxmotion Round Hexapod with their 4DOF T-Hex legs. Zenta (Kåre) has some nice demos of the T-Hex and 4DOF including:


    Mine use HS-645mg servos which is probably pushing it, but it is fun!
    Kurt

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    Re: Golem: MX-64 4dof Hexapod project.

    Quote Originally Posted by jwatte View Post
    I'm not sure about the "not much torque" part. Assuming all other servos are locked, won't the joint at the foot will see the entire torque from the ground contact point to the center of the bot?
    No. When the tars is perpendicular to ground the bot will kinda balance their tibias on top of the tars part. That doesn't require much torque at all. A simple explanation of the torque required is the horizontal distance from the tip of the feet to the tars joint/servo multiplied with actual vertical load on the joint (the load is determined by many factors, simplified the worst case is 1/3 of bot mass). As you can see the required torque is very little when the horizontal distance is 0.

    There are other factors though, after all the tars is an extension of the tibia. Some torque is needed due to the force of momentum while walking. Not sure if that's the correct words. Tired, going to bed. .
    Last edited by Zenta; 04-03-2014 at 05:43 PM. Reason: replaced tars with tibia.. writting on phone..
    Kåre Halvorsen aka Zenta
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    Zenta's YouTube channel
    Zenta's Blog
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