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Thread: Twitch - My 3 Servo Hexapod

  1. #1
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    Twitch - My 3 Servo Hexapod

    I've been working on a new bot for the past couple of months. I finally conquered a software bug last night, and with no anticipated barriers to completion in the way now... I present this new bot, which I dub 'Twitch'! Before I blab on, here is the current level of progress:
    Click image for larger version. 

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    My inspiration is the TicTac6 robot by Amino-San. It uses 3 servos, moves quickly, and was an instant must-build when I saw the video back in January.

    What I'm using:
    -3 AX-12A servos
    -2 Pololu Wixel MCUs with built-in wireless capabilities
    -TriState Buffer board with a Sparkfun Level Converter to interface/power the Wixel and servos
    -1050 mAh 3-S LiPo battery
    -3-S LiPo battery monitor
    -3D printed parts, bioloid brackets, 3/16" Delrin plate, .03" Polycarbonate sheet, etc.
    -Arbotix Commander (modified to interface with a Wixel; documenting thread...)

    This is a prototype. I have no idea if the AX-12A is going to be strong enough, and I will likely end up building an MX-28T version of Twitch eventually. It will be interesting to see how much payload capacity it has. Were it to qualify for Mech Warfare (which it shouldn't), it'd probably need at least MX-64's to handle all of the guns/camera/scoring stuff.

    So that initial picture... shows the three servos and the 4-bar linkage, actuated in the middle of one of the links that makes this configuration work. I still need to (finalize the) design and print the brackets (one of them is done and visible above) that will mount the "feet," as well as actually design the feet. My plan is to get software going, create a stand so the bot is off the ground, attach flimsy cardboard feet cutouts, and then figure out an optimal design from there. Test feet might be done from Delrin initially, and then get some polycarbonate (1/4" thick?) waterjetted/laser cut.

    In addition to the inherent neatness of a capable 3 servo walker, I grant myself awesome points for the custom electronics setup. The Wixel is a cheap TI chip-based board (~45 MHz) with built in wireless communication (not compatible with xBees/zigbee/bluetooth/wifi). The Wixel plus interface board is nice and small. I've not seen a smaller setup that does both wireless and (hardware-based) Dynamixel communication... though I suspect Upgrayd has something like this in the pipeline.

    The Wixel uses the SDCC toolchain for software. I modified Upgrayd's AVR Dynamixel libraries (with a bit of code from WebbotLib thrown in) in order to interface the Wixel with AX-12s. Took me a while to get it right. Work in progress code is on Github.

    With just three servos, I reckon the battery should be good for at least half an hour.

    Other pictures: (click to embiggen)
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  2. #2
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    Re: Twitch - My 3 Servo Hexapod

    Isn't this an "unopod" rather than a "hexapod" then? It looks fun!

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    Re: Twitch - My 3 Servo Hexapod

    What's an unopod? There nothing 'uno' about it :P Multiple of various things . That said, no idea what to refer to this kind of walker as.

    Cire wanted to see the CAD for some reason. If nothing else, the perfect contrast of CAD will clue people into what the picture above is showing.
    Click image for larger version. 

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    Re: Twitch - My 3 Servo Hexapod

    "hexapod" means "six legged." Your things has... zero? one? two? "legs," so calling it a "hexapod" might be inaccurate. Although, from looking at the previous video there's six points of contact with the ground.
    But you're right, it's hard to know what to call it. It looks cool, though!
    Do you have a video of it walking/crawling/locomoting?
    Last edited by jwatte; 08-19-2012 at 12:44 PM.

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    Re: Twitch - My 3 Servo Hexapod

    I'm still working on code, and have yet to make the feet (in case you wondered). Currently learning a bit more about C to get around memory limitations (I guess) on the Wixel. Others have called it a hexapod, so I went with that. (There are other 3 servo hexapods using a different locomotion approach, too)

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    Re: Twitch - My 3 Servo Hexapod

    Finally got within my memory limitations by not including the USB functionality. Only downside there is that the Wixel won't go into bootload mode with such a program. Easy to remedy, though; I added a button to the electronics that puts the Wixel in bootloader mode when pressed. I've now got a random gait that moves the servos. Now I just need to design and make the actual walking/turning gaits. For now I'll just do forward/backward/turn in place.

    I 3D printed a housing for power switch + T connector that bolts onto an AX-12's case. The 0.03" polycarbonate battery holder in turn is attached to this housing. The inside is about 50% filled with epoxy.
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    Last edited by Gertlex; 08-27-2012 at 01:20 PM.

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    Re: Twitch - My 3 Servo Hexapod

    This weekend's progress has been three-fold!

    I made a stand for Twitch. A CD spindle is now reinforced by a 3D printed plug on the underside, has an adapter for Twitch at the top, and has corkboard epoxied to the bottom for weight/friction/strength.

    Click image for larger version. 

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    I prototyped the crazy curvy legs with cardboard. Rather than try to do the feet in CAD, and make sure I have all the clearances right, I opted to use cardboard to prototype the legs. The cardboard is good to use while I develop the walking gaits... If I mess up, nothing important will break. I'll then use the cardboard as a guide in making actual parts in CAD, and get them cut by a waterjet... The first pic below is a good reminder of the clearances needed that necessitate the crazy leg shapes. I haven't done the straight feet on ends of the bot yet, but Twitch can stand on these cardboard legs...

    (Click to embiggen)
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    And software. I've now successfully ported Webbotlib's GaitRunner library to the Wixel, and can now use Gait Designer to make poses/gaits. I figured out the rough sequences for the gaits on paper, and have successfully made an initial walking gait... Video!

    Last edited by Gertlex; 08-27-2012 at 01:20 PM.

  9. Re: Twitch - My 3 Servo Hexapod

    Lookin good, i like how the legs 'cross', looks intriguing

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    Talking Re: Twitch - My 3 Servo Hexapod

    Twitch is functionally complete!

    Video from last night:
    https://www.youtube.com/watch?v=ka1bHdljBA0


    The video features single-speed testing, basically. Works great on both carpet and linoleum. I experimented a short while ago with faster speed, and walking forward results in curved paths. This is easy to adjust; just need to make the gait slightly non-symmetric.

    And details...

    I finally got the code fully debugged yesterday morning. It was mostly an issue of getting all of the transitions between gaits - walking/turning/not moving - to be handled properly. A few bugs were simply forgetting to set a variable when changing gait. And debugging was done using a single LED (the center AX-12's LED, no less...). That was slow and tedious. But victory.

    Once I had the code debugged, I started cutting out the legs from 3/32" black delrin. That took a few hours, but the result was perfect. I never used CAD for the legs' design. I ended up tracing the cardboard legs on graph paper, and then scanning the traces in. Imported the scans into Inkscape and make smooth vector curves, made sure I had the right size, and then printed out the templates. When I get later versions done, I'll just import the vectors into Solidworks.
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    The end legs were stenciled on cardboard to size, traced with a knife on the plastic and cut. I then dremeled the sides of the top end down and tapered (hard to see...) these until they fit snugly in between 3D printed parts (this was not planned, but worked great!).
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    The ridiculous legs mounted:
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    Literally just as I was finishing cleaning up after making the legs, the mail lady arrived with a package containing brass rods and shaft collars with which I replaced the loose bolts I was using. Prices on ebay were way better than McMaster-Carr...
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    The final Wixel-board. I added another button for resetting, which allows me to keep the USB plugged in when testing. The other button gets me into bootloader mode... since I had to not include the USB libraries in my code due to memory limitations (I'm using 96% of whatever the equivalent of RAM is for MCU's; I've got plenty of space for more gaits, though...)
    Click image for larger version. 

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    I was originally going to use this rubber band to hold the battery in place, but that's proved unnecessary. Instead it works as a great way to pick Twitch up.
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    Finally, a size comparison; I've got a definite incentive to make the feet hinged so that Twitch folds up to be more portable:
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  11. #10
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    Re: Twitch - My 3 Servo Hexapod

    Awesome project and fantastic work!


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