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  1. First steps for Super Mega Microbot

    I posted a video of our turret a while ago, and now I finally have one of the mech itself!



    It is a quad with 3dof legs based on Dongbu DRS-0101 servos. The onboard computer will be an odroid U3 plus an AVR for I/O multiplexing and other miscellaneous functionality. In this video, both the turret and the odroid are not yet mounted. Instead, I've got an extra lipo wedged in place where the odroid goes to get the center of gravity to a more reasonable place.

  2. #2

    Re: First steps for Super Mega Microbot

    I've been curious about the Dongbu servos. They came out just after I got my current set of Dynamixels. How are they working?

  3. Re: First steps for Super Mega Microbot

    They seem to be working really well. I've done some characterization and they can hold 12 kgf*cm indefinitely without overheating, and can peak up to 17 kgf*cm. The serial protocol is reasonably sane, and the mounting system is convenient. In this mech, all the orange parts (and the one green one), are stock brackets from their humanoid kit.

    The documentation has a few registers misnumbered from reality, which probably just indicates that it isn't that widely used by anyone programming it at the register level yet, but it in any case, it wasn't hard to figure out the correct layout. As far as I can tell, with the caveat that I have not used Dynamixels myself, they appear to be superior to the comparable Dynamixel in every way aside from network effects (i.e. you're unlikely to find anyone at an event you can swap parts with), with the added benefit of being slightly cheaper.

  4. Re: First steps for Super Mega Microbot

    I've started a quad with the Dongbu DRS-0101 servos too. I have all the IK working, starting on the gait code.


  5. Re: First steps for Super Mega Microbot

    Quote Originally Posted by nvrdunn View Post
    I've started a quad with the Dongbu DRS-0101 servos too. I have all the IK working, starting on the gait code.
    Are you starting on gait generation from scratch? If you're interested, I have a hacked up copy of NUKE which is slightly more general than the Dynamixel only one, but I ended up not using it in favor of writing something new.

  6. Re: First steps for Super Mega Microbot

    Quote Originally Posted by jpieper View Post
    Are you starting on gait generation from scratch? If you're interested, I have a hacked up copy of NUKE which is slightly more general than the Dynamixel only one, but I ended up not using it in favor of writing something new.
    Yeah I was starting from scratch, if you have some code the would be great.

  7. Re: First steps for Super Mega Microbot

    Funnily enough, I just got some Herkulex servos 2 days ago, and I'm working on a derivative of the USB2AX for them.

    Which registers are misnumbered? Have you found any other errors in the doc?
    In your demo, how many orders do you use to drive the legs? do you interpolate yourself or do you leave it to the servos?
    ---
    Personal blog: http://xevel.org
    USB2AX documentation: http://xevelabs.com

  8. Re: First steps for Super Mega Microbot

    Quote Originally Posted by Xevel View Post
    Funnily enough, I just got some Herkulex servos 2 days ago, and I'm working on a derivative of the USB2AX for them.
    I just got the RS232 adapter when I ordered mine, although a USB interface would certainly be more convenient than the separate USB-serial adapter I'm using now.

    Quote Originally Posted by Xevel View Post
    Which registers are misnumbered? Have you found any other errors in the doc?
    The PWM value appears to be at RAM register 64 when the docs claim 62.
    The only position value I've found is at RAM register 60, which I'm assuming to be calibrated position because I haven't found (or looked that hard yet) for the absolute or differential position. It did seem that those other positions weren't where they were documented to be.

    I haven't been keeping a detailed list of documentation errors, but perhaps I will start.

    Quote Originally Posted by Xevel View Post
    In your demo, how many orders do you use to drive the legs? do you interpolate yourself or do you leave it to the servos?
    For each update I'm using a single S_JOG command to command all the servos at once.

    In that demo, I'm sending commands every 40ms with a play time of 80ms. That seemed to result in somewhat stable transitions from one command to the next with minimal loss in accuracy. I believe the control loop on the servo is running at 11.2ms; I might try sending commands closer to that period when I'm running from the onboard computer as opposed to a desktop PC as my gait engine is rate agnostic. The desktop isn't able to manage jitter well enough to warrant running at the higher rate.

  9. Re: First steps for Super Mega Microbot

    While we know there still isn't any large competition in the near future, we made up a "qualification" video just to prove to ourselves that we at least had everything working end to end. The long dimension in the room we were shooting in was 13ft, so only a little shorter than a full arena.


  10. Re: First steps for Super Mega Microbot

    And now, some more upgrades in progress... a prototype gyro stabilized turret. This is just a proof of concept video, with a hacked up aluminum gimbal frame and an off the shelf Martinez gimbal controller with its code slightly modified to stabilize about the pitch and yaw axes instead of pitch and roll.

    Click image for larger version. 

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    The upshot is, even while walking, it keeps the video plenty usable and keeps the turret aimed about as well as any of us can aim it against a moving target.

    Now I'm tweaking the controller's firmware to use the Dongbu servo protocol, so that we can aim it instead of just stabilizing. If that works out, I'll probably build a more suitable turret bracket, and possibly switch to a custom controller that has two IMUs for even better stabilization.

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