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Thread: Using AX-12A in zero torque mode.

  1. Using AX-12A in zero torque mode.

    Hi, I'm very new to robotics, I'm planning to make heavy use of the AX-12A capability of reading back current position. However I noticed that (when powered off) the servos are actually quite stiff! This will make manually posing my robot a bit hard!

    In this video they seem softer for some reason, is that because of a special zero torque setting when powered on?



    best regards!

  2. #2

    Re: Using AX-12A in zero torque mode.

    the servos are actually quite stiff! This will make manually posing my robot a bit hard!
    They are stiff when you try to turn the horn directly, because there is no leverage to generate torque.
    When the robot is assembled, the brackets/legs/arms give you a lot of leverage, and suddenly those same horns feel like butter.
    There is no difference between "on with 0 torque" and "off." It is only a difference in the "distance" part of "torque = distance * force."
    Last edited by jwatte; 06-06-2015 at 08:27 PM.

  3. Re: Using AX-12A in zero torque mode.

    Thank you, well that's bad for me then I understand about the leverage but my original design was using some mechanical advantage to gain torque, the problem with this is you also increase the stiffness of the arm and the servo will just not turn unless the general mechanism is really sturdy (and heavy). In essence the reduction system works excellent when it's powered by the servos but becomes unusable when moving it manually. Well you live and learn

    Thanks for the reply
    Last edited by ZanQdo; 06-06-2015 at 02:03 PM.

  4. #4

    Re: Using AX-12A in zero torque mode.

    Can you post pictures, or at least a diagram? The servos aren't *that* hard to turn manually.
    Maybe something else is going on.
    Did you use cycloidal gears, or strain wave gears? Those are not back-drivable and would make the assembly impossible to move without power.

  5. Re: Using AX-12A in zero torque mode.

    Quote Originally Posted by jwatte View Post
    Did you use cycloidal gears, or strain wave gears?
    I'm using herringbone style involute spur gears based on this

    Click image for larger version. 

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    Thanks for the help

    Edit: I see a few problems with this now, for example the pinion may be far too small, providing little leverage to turn the servo. Also there are rigidity (or lack of) problems all over the design. I'm a 3D animator, not used to real world problems
    Last edited by ZanQdo; 06-15-2015 at 11:35 PM.

  6. Re: Using AX-12A in zero torque mode.

    I have a followup question. I have noticed the motors are actually softer to turn when powered off than when powered on with torque set to zero. Is that normal and why is that?

    thanks!

  7. #7
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    Re: Using AX-12A in zero torque mode.

    It is completely normal, and I'm pretty sure it has to do with how the H-bridge is controlled. Thinking that each of the two half-bridge ICs gets only a single pin to control it (one pin connected to one half-bridge input and also controlling a SOT-23-3 MOSFET/BJT inverter connected to other half-bridge input) which results in each half-bridge IC having two states when powered ("high-side on"+"low-side off" or "high-side off"+"low-side on") which then results in four motor states ('rotate clockwise', 'rotate counter-clockwise', 'braking-high', or 'braking-low') without a coasting state (all MOSFETs off and using just the intrinsic diodes to let existing/generated current through motor circulate/dissipate).

    If it is actually using four pins to control each MOSFET in the H-bridge individually, then it should be capable of coasting and/or regenerative braking. If so, it is possible that the firmware never lets the motor enter a coasting state (always powered in one direction or braking). It is also possible that any time the motor is connected to a battery or other power source, then the firmware places the motor in a 'coast' or 'regen' state so that an external torque driving the servo horn will cause the motor to try to push current into the battery (charging a battery would be a significant electrical load to a small generator like the AX-12 motor). When not connected to a battery, back-driving the servo horn fast enough can charge up the capacitor enough to make the servo 'sort of' power on (LED may flash), but because the voltage regulator does not provide sufficient electrical loading and the motor leads are otherwise open-circuit, it does not require any extra torque to back-drive the servo horn.

    My head is not very clear right now. Sorry if this is nonsensical.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  8. Re: Using AX-12A in zero torque mode.

    Quote Originally Posted by tician View Post
    It is completely normal, and I'm pretty sure it has to do with how the H-bridge is controlled.
    Fantastic answer, I'll come back to it when I learn more about electronics

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    Re: Using AX-12A in zero torque mode.

    That is interesting. My AX12s are certainly more firm with power connected, but software not running than with them totally powered off. Even if i disable servo torque, they are not as soft as with no power.

    For example, my hex normally lives on top of a big paint can so its legs can move.

    Power on -> no movement
    Software on, command servos -> move to home position
    Software off (including a torque disable at the end -> servos slightly relax, legs drop a little bit
    Power off -> all legs drop

  10. Re: Using AX-12A in zero torque mode.

    Yep, in my case this is a bit of a hassle to be honest but what can you do

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