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Thread: AX12+ how to turn off overload detection

  1. #1
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    AX12+ how to turn off overload detection

    Hello to everyone,

    I'm an italian software developer, I live in Verona, I was born in 1967 and my name is Andrea.
    My hobby is robotics.

    I'm reporting here a question I've asked in private to lnxfergy: and his reply down below:

    QUESTION:
    Dear lnxfergy,
    Please I'd like to ask you a question

    I've seen your post:
    http://forums.trossenrobotics.com/showthread.php?t=4192

    I'm trying AX12+ and I see that they have a running torque of only 8Kg*cm, after that it shuts down.

    I've seen you were talking about turning off overload detection.
    Do you mean clearing bit 5 of 0x12 (Alarm Shutdown register) ?
    Does this way release more torque?

    I've seen that robotis doc says it has an holding torque of 15Kg*cm @12V 1.4A
    I measured current, but it sinks no more than 1A and then it shuts down.

    Could you suggest me other cheap motors or servo motors suitable to build a little robotic arm (arm lenght is 15 cm and forearm length is 15cm )?

    Thank you in advance for your help
    Andrea

    REPLY FROM lnxfergy:
    Holding torque does not equal moving torque. The general consensus is that the AX-12 moving torque is about half the rated holding torque, which is about what you are seeing.

    I would also strongly recommend that you do not turn off overload shutdown. It's there for a reason -- to preserve the geartrain. If you turn off the shutdown, expect to strip gears out

    Unfortunately, torque costs money. The AX-12s are one of the better torque to dollars ratio servos available.

    I'd really suggest you post future questions into the forum -- that way these responses stay around for everyone to benefit from! Seriously, we don't bite, and you've done your research, so you'd get plenty of good responses.

    -Fergs

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    Re: AX12+ how to turn off overload detection

    Thank you lnxfergy

    Please another question, just for testing I'd like to turn off overload shutdown, the method is to clear bit 5 of 0x12 (Alarm Shutdown register) ?

    Will this way AX12 release more torque or will it just stop as before but without shutting down so that it will shutdown later for temperature?

    Thank you in advance
    Andrea

  3. Re: AX12+ how to turn off overload detection

    Quote Originally Posted by ratto1234 View Post
    Thank you lnxfergy

    Please another question, just for testing I'd like to turn off overload shutdown, the method is to clear bit 5 of 0x12 (Alarm Shutdown register) ?

    Will this way AX12 release more torque or will it just stop as before but without shutting down so that it will shutdown later for temperature?

    Thank you in advance
    Andrea
    If you clear bit 5 of entry 0x12, the AX-12 will not stop when it overloads. You will either end up stripping the gears, or you will burn out the motor driver. It might still get too hot and switch off torque, so if you're bound and determined to destroy a servo to see where the limits are, make sure you clear bit 2 of that entry as well.

    - Jon
    "The best way to predict the future is to invent it." - Alan Kay, inventor of Smalltalk

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    Re: AX12+ how to turn off overload detection

    thank you Jon, lnxfergy was right, you don't bite

    of course I don't want to burn out the AX12.

    I'm testing it and powering it with a laboratory power supply which monitors current.
    During the test I'm observing the current drawn and after a few seconds I'd switch it off.

    The fact is that on the robotis web site (http://www.robotis.com/zbxe/dynamixel_en) they say that at max torque (15Kg*cm) it should draw 1.5A @ 12V.

    The problem is that AX12 stops moving when it draws just 1A, then after about 5 seconds it shuts down.

    To explain better my test:
    - I've connected an arm with a weight to the AX12 which produces 10.2Kg*cm when the arm is horizontal.
    - I start with the arm down (vertical) and I command to rise tha arm 90 degrees (to get the arm horizontal).
    - Before reaching the horizontal position AX12+ is no longer able to move (but it only sinks 1A, it does not try to sink 1.5A).
    - I've monitored the "Present Load (0x28)" when it's stopped and it reads 1023, which means maximum torque while it's drawing just 1A, not 1.5A.

    For this reason I suspect there is something strange maybe in the calibration or in the AX12 F/W.

    I've tryed to power it with 13.9V (register 0xD Highest limit voltage is factory set to 14V), it releases a little more torque.

    Do you know if there is the possibilit to change calibration parameters or AX12 F/W?

    Thank you in advance
    Andrea

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    Re: AX12+ how to turn off overload detection

    Again, the issue here is moving vs. holding torque. The 15kg-cm rating is for holding torque, the moving torque has typically been seen to be less. There really isn't more torque "to release". Turning off the alarm shutdowns is a sure way to cook/break the servo.

    -Fergs

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    Re: AX12+ how to turn off overload detection

    Thank you Fergs,

    I understand that, but I was expecting to see the AX12 drawing 1.5A as rated before reaching "Present Load" = 1023 which is the maximum torque perceived by the electronics inside the AX12.
    But I understand that it's not possible.

    Actually indidicating the holding torque alone without telling the real running torque is just a trap for mechanics beginners like me. Robotis is not transparent regarding this.

    I'll try other Servo or Motors or other ways, or may be I'll give up the arm project.
    I'll let you know.

    Thank you very much for your help.
    Andrea

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    Re: AX12+ how to turn off overload detection

    Quote Originally Posted by ratto1234 View Post
    Actually indidicating the holding torque alone without telling the real running torque is just a trap for mechanics beginners like me. Robotis is not transparent regarding this.
    This isn't just a Robotis practice -- it's basically all servo manufacturers. Torque ratings in the servo industry are all fairly wishy-washy, and are typically a bit trumped up denoting the holding torque just before the servo explodes....

    EDIT: I doubt you'll find a $40-50 servo that outputs as much torque as the AX-12 and has similar reliability (yes, we can buy really cheap high torque servos from China, but they tend to die spontaneously or explode into flames). If you need more torque than a $40 servo can do, it quickly jumps into the $100 per servo, there aren't a lot of high torque servos in between....

    -Fergs
    Last edited by lnxfergy; 06-20-2010 at 11:18 AM.

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    Re: AX12+ how to turn off overload detection

    I'm looking at this now:
    http://www.servocity.com/html/hs-805bb_mega_power.html
    price is 40$
    Stall Torque (6.0V): 343.01 oz/in. (24.7kg.cm)
    But it's just analog, and so it does not have all the advantages of AX12.
    Maybe I could substitute the electronics of the Servo with the open servo project one.
    What do you think?

    I'll look deeper into Servo offered on the market.

    Andrea
    Last edited by ratto1234; 06-20-2010 at 11:59 AM.

  9. #9
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    Re: AX12+ how to turn off overload detection

    That's a mega servo and it is quite a bit larger and heavier than an AX-12.



    I also have a handy list of the more powerful servos here..
    http://billyzelsnack-robotics.blogsp...ht-ratios.html
    Last edited by billyzelsnack; 06-20-2010 at 12:28 PM.

  10. #10
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    Re: AX12+ how to turn off overload detection

    You're right, it's really big!

    It's very interesting the list of servos in the link.

    I'll study it.

    Thank you again
    Andrea

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