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Thread: AX-12A vs standard servos? (plus other beginner questions)

  1. AX-12A vs standard servos? (plus other beginner questions)

    Hi, I'm new to mech warfare and my college robotics club is interested in competing at Shepard university in 2019. In previous years, we have competed in the firefighting robot contest at Shepard, but our faculty adviser recently gave us the go-ahead to order parts to compete in next year's competition.

    When looking into what servos to use, my adviser recommended we use Dynamixel AX-12As. The only problem is that these are not directly compatible with the arduino megas that we currently have. In addition, there are no frame kits to make a quadruped that are readily available to 3d print or buy (which aren't expensive), which means we'd have to fabricate our own (not that there's anything wrong with that, but ordering one would be preferable).

    On the other hand, I originally intended to use standard servos from amazon such as this or this. In addition, they work well with arduino without any additional circuitry and there's plenty of frames to buy and STL files online to print.

    Which brings me to my main question: what's the difference between the AX-12 and the servos I posted? The last one even has a higher stall torque than the AX-12, so why does it cost so much more?

    Also, this might sound like a stupid question, but where the heck do you buy the scoring transponder from? I googled and read forum post for hours but google revealed nothing and all the links on this forum seem to be dead

    Thanks in advance!

  2. #2

    Re: AX-12A vs standard servos? (plus other beginner questions)

    The rteam folks in the Mech Warfare forum are working on the transponder situation I think.
    I have two transponders that I won't be using, so I could sell you one, too.

    I don't know if you've done enough engineering already to learn that there exist about four levels of quality in most fields:
    1. Utter crap, that doesn't actually do what it says.
    2. Cheapest possible crap that does sort-of what it says, for at least a little while.
    3. Mid-level gear that does what it says, at least when you first get it, and that won't die from at least some normal wear.
    4. High-end gear that will last for a long time, won't degrade over that time, and the supplier has tons of help to hand-hold you through solutions.

    The cost goes up a lot for each level -- at least doubled, if not more. Engineering for long-term durability is a very different job from engineering for meeting a given paper spec in the cheapest way possible.

    I'd imagine the servos you link at are somewhere between 1. and 2. on that scale. Probably 2. The AX-12A have proven themselves as about number 3. on the scale. They also have a number of helpful features that hobby servos don't:
    1) spare parts available from the vendor
    2) ability to read back out position, temperature, load, voltage, and other parameters
    3) 300 degree position control (and 360 degree turn control in wheel mode)
    4) built-in overload protection that often will save the servo from death (assuming you let it cool off when it shuts down)

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    Re: AX-12A vs standard servos? (plus other beginner questions)

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    Love jwatte's categories. Have to agree on his assessment of where the different servos fall in quality. Our whole club (maybe 6-7+ mechs) use the AX-12s. We do additional heat sinking to them (instructions available) as well as take care to not overload them (keep your mech weight down as well as the limb length short). We've had very little issues with them. If your advisor is willing to pony up for the AX12s go that way.

    As for the controller, our club either uses the openCM9.04 board or some arduino/compatible board. The openCM board uses an Arduino-like IDE, it has all the buffer circuitry to talk to the AX12s, and costs $10. If you do go the Arduino route, you do need to build the buffer circuitry to talk to the AX12s. There is SW code out there to communicate with the AX12s not using the additional circiutry... but do't go that way. You'll end up burning out the AX12 UARTs and trashing them.

    There are many varieties of transponders/target plates out there. They are all custom designed. When Trossen was sponsoring Mech Warfare they were available from them. Since we took it over, we have been spinning our own. Unfortunately almost every year the transponders/target plates have changed. One again, our target plates for 2018 Mech Warfare will be slightlydifferent with some new cool features. Hopefully this will be the last spin of them we use... unless we find bugs. We don't sell them but will release the files to anyone who wants to make their own. Not all the variants out there will be compatible. You need to find out what ShepFest uses.

    As for frames/brackets/3D prints/guns/breaches, etc... we have a bunch of this posted on our R-Team Robotics Club site. It may not be to cohesive though. I've been working on putting together a reference mech that anyone could use as a starting point and modify as desired (including code for the openCM boards) but its a slow process between work and keeping my own mech function. There's hope that I might have it done in time for 2019...

    Good luck and hope to see you at Robogames 2019 as well!

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    Re: AX-12A vs standard servos? (plus other beginner questions)

    Quote Originally Posted by GhengisDhon View Post
    There is SW code out there to communicate with the AX12s not using the additional circiutry... but do't go that way. You'll end up burning out the AX12 UARTs and trashing them.
    Eh? The arbotix(-M) has used the software-only option for all its revisions so far and I'm pretty sure that KurtE also experimented with a software-only option for the 5V tolerant versions of the Teensy-3.x. The biggest reason for using buffer ICs is that most newer microcontrollers are not 5V tolerant, so they absolutely require logic level translation to protect the controller from the 5V signals sent from the servos on the dynamixel buss not vice-versa. The MX servos have had numerous issues and I have personally experienced a few of them losing their buffer ICs regardless of whether or not the controller used buffer ICs (I actually only ever used the various Robotis CM-510/530/700/730/900 with them). AX servos with dead buffer ICs are much rarer partly because everything in the AX/RX/EX runs off 5V while MX servos are mostly 3.3V with 5V buffers, and I find myself unable to actually recall any posts on the forum where it was attributed directly to use with the software-only UART control of an arbotix.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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