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Thread: Help with Dynamixel AX-12+, continuous flashing Red LED?

  1. Help with Dynamixel AX-12+, continuous flashing Red LED?

    Hello all,

    I recently acquired an AX-12+ Dynamixel motor and wanted to write some code in order to program it using Arduino.

    I am in a bit of a snag, I seem to be having problems powering it up. For starters, I am using a 12V battery, and no signal, i.e. I am not sending the motor anything. Before adding a 100 ohm resistor the LED on the Dynamixel would flash continuously, now with the resistor it stays red until I disconnect it.

    I do not think that its an over voltage issue because I have done similar attempts with 9 V and 8 V. I think it may be an overheating problem.

    Thanks everyone!

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    Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    What type of 12V battery? Alkaline batteries will not work well unless you are using at least 'D' size and/or lantern batteries because of the current requirements of the servo (anything that cannot sustain at least 1A at 12V will not work reliably for even a single AX-12+). There will be no overheating unless you are actually using the servo under very high loading or you killed something within the servo by wiring it incorrectly. 8V is the absolute minimum voltage where the servo can actually operate, but that is dependent on the current capacity of the battery being sufficiently high to prevent any drops in voltage during movement of the servo (the original batteries shipped with Robotis AX-12 kits was a rather crap 8S 'AA' NiMH).

    How are you connecting the battery to the servo and controller? The three pins of the cable must not be reversed or otherwise incorrectly connected else you will kill the servo. The ground signal of all components (battery, servo, and controller) must be connected together. There should be nothing but wire between the battery terminals and the servo power pins. Resistance on the power lines will cause large voltage drops that cause many problems with the servos.

    If you have an adequate power supply that is correctly connected to the servo, the LED should flash once at initial application of power. The LED should never stay lit unless you send it a command packet to do so. The LED should only flash after power-up if it encounters an error defined by the Alarm LED register (overload and over-temperature are the default), but a crap power supply that repeatedly drops below the minimum voltage of the servo could cause something very similar (supply starts, servo powers on, supply overloads, voltage drops, servo powers off, supply restarts, servo powers on, supply overloads, etc.).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  3. Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    Thanks for the reply, yes I was in fact using alkaline batteries (1 9V and 1 3V in series), I may have wired the servo incorrectly, but I do not think that I have killed the servo as the same problem reoccurs with correct wiring. So I will not use any slight resistance on the line. Tomorrow, I will use the recommended battery.

    Thanks for your advice!

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    Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    A decent 12V power supply (>2A) does not cost very much, especially compared to replacing non-rechargeable batteries, and will work wonders for years to come. The $20 12V5A supplies from Robotis and Trossen can reliably power an entire DARwIn-OP with its 20 MX-28 servos and PC+webcam, and adding a $5 AX/MX power hub with the barrel connector will make wiring much easier and much safer.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  5. Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    I checked again, and as it happens, I am in fact using a 9V D battery, and am simply looping wires around the battery directly to the servo. To secure the wires in place, I am using a 9V hat.
    Last edited by IsaacWiebe; 07-21-2014 at 12:03 PM.

  6. Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    I actually went back to using a 9V D battery from Duracell and still received the same problem. However, this time it is flashing continuously as "normal". Could it possibly be a heat issue? I don't think its an over/under voltage issue.

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    Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    A 9V battery is very unlikely to be able to source enough current. That is why those of us who use batteries for our 'bots with Dynamixels normally use LiPoly batteries. A 9V battery, as far as I've seen, can only source about 800mA before the voltage drops heavily. a single AX-12A w/ ArbotiX needs a minimum of 1A (1000mA) to run correctly. And at that, the lower your voltage goes, the weaker the servos are. At 7.4V (minmum voltage) Lnxfergy found AX-12's to be lethargic to say the least.

    If you have an extra PC power supply sitting around, you can jump the correct wires on the 20/24 pin connector (green wire to any black) and take +12V directly off of any of the Molex (four large pins, 2 black wires in center, 2 colored wires on outside of connector) to run the setup. If that works fine, you're running into an undercurrent situation.
    Last edited by ArduTank; 07-21-2014 at 01:38 PM.

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    Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    While a Duracell D-cell may start at ~1.6V when manufactured, it rapidly decays to 1.3V and lower after no more than 30 minutes at 1A or greater load. A 9V (6S) alkaline D-cell pack will not work reliably, ever. A 12V (8S) alkaline D-cell pack may work for maybe 30~45 minutes before under-voltage resets begin to affect operation. At least NiMH levels out to sustain a voltage of 1.2~1.0V for a significant portion of the discharge cycle, but the voltage of an alkaline battery simply plummets with time.

    The solution is very simple and quite inexpensive compared to even a single AX-12: buy a 12V >2A AC-DC power supply. As ArduTank mentioned, you could also go with the slightly more expensive option of a LiPo charger (itself likely requiring a >2A 12V power supply, unless it has a built-in AC-DC power supply) and 3S LiPo with >1000mAh capacity and at least a 20C rating (20C in a 1Ah pack is 20A maximum current before the battery cells risk catastrophic damage).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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    Re: Help with Dynamixel AX-12+, continuous flashing Red LED?

    Here's a cheap option:

    Battery (2200mAH, 20C, 3s 11.1V LiPo)

    Charger (runs off of any 12V input that goes to a barrel jack, included leads can be used to make a setup to power it; slow charge, does my 2200mAH from 9.6V(minimum charge) to 12.6V(max charge) in about 30-45 minutes)

    Total is less than $20 plus shipping, US warehouse set in the links. Don't use the global warehouse, if you are in US, it'll increase shipping times 3-5 fold.

    That is the setup that I use, and it does fine for 12x AX-12A/+'s with a raspberry pi+webcam added to the mix (about 1hr-1 1/2hr of time before I need to charge when using it with PyPose)

    The charger only needs about a 1-1.5 amp supply (many external hard drives/other household equipment use these with a barrel jack already on the end)

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