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Thread: Counter-balance/springs

  1. #1


    Looking at all of the fantastic walkers, I'm surprised that I don't see any counter-balancing springs.

    When a biped stands up, knees slightly bent, it seems as if the ankle and knee servos have to hold up essentially the entire weight of the robot (especially when standing single-legged.) A simple application of linear springs, perhaps from spurs sticking out from the knees and heels, could remove a lot of this tension, basically neutralizing most of the effects of gravity.

    Why is that kind of design not popular? Is there some Achilles' heel that makes it not work? I wouldn't think the weight of the springs would be more than what you save in not needing to keep everything upright with servos and batteries, but maybe I'm wrong in that?

  2. #2
    Join Date
    Jun 2011
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    Re: Counter-balance/springs

    Both Hikari and Tychus use springs. Though cire is lazy and doesn't post pics [/nameandshame]

    I've gotten the impression from darkback and cire that a big part of it is that the springs can be hard to mount properly.

    That's all I'm qualified to say.

  3. #3
    Join Date
    Dec 2007
    Whidbey Island, WA
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    Re: Counter-balance/springs

    So...Yeah...she uses springs in her knees. As her knees bend they bend the springs which "want" to be straight. She can walk around for about 20 minutes before her battery warning goes of or a knee goes into thermal shutdown depending on how much walking around I have her do. That said I still need to work on getting her walking gate just right. For Mech Warfare she has to pick her feet up really high, and the BB's on the ground still pose a bit of a problem. On a metal hardcore arena she does fine.

    I originally used softer springs stretched across the front of the knee, and connecting each side of her ankles to her feet. The ankle ones were damn near impossible to balance, and the knee ones made her tip over forwards.

    Hope this helps.


  4. #4

    Re: Counter-balance/springs

    Yes, that helps! Thanks for the picture.

    Walking around on BBs, huh? I imagine you'd want to use thicker aluminum plates for the feet, and groove them in a grid with 1/4" ball nose mills, to make "spiky" feet, perhaps?

  5. #5
    Join Date
    Sep 2010
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    Re: Counter-balance/springs

    I had used elastic bands (think they were from asparagus or maybe broccoli) in the knees of a Bioloid Comprehensive humanoid a bit over two years ago during a Machine Vision class project. I had to use the wireless camera that came with the Expert kit to navigate around red dowels to reach a blue dowel (or vice versa), but the crappy NiMH battery kept running down too quickly. By the time the bot was finally getting around the row of obstacle dowels, the battery would be so low the bot could not keep from falling over. It was an attempt at a quick fix, but I ended up ditching it and the CM-5+NiMH battery by MacGyvering a CM-510 and LiPo battery into the chest cavity.

    I know the PR2 uses spring counterbalanced arms to great effect and I've often considered using spring counterbalancing, but never really gotten around to making a serious attempt at it. In theory, it is just two complementary low-K springs pre-stretched to keep the joint in a neutral position against the effects of gravity with the servo fighting only the low force of stretching one spring and the force of moving the actual joint and any load attached to it. The design I've been considering for a larger quad actually uses a pair of linear actuators to adjust the spring pre-loading to maintain the same neutral position no matter what payload the bot is carrying (within the pre-stretching capacity of the springs and linear actuators).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    more bleh

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