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Thread: torque control for ax 12a

  1. torque control for ax 12a

    Hi all,

    What I want is to let ax-12a to keep certain amount of torque which is less than max torque limit since I am using ax-12a for my gripper to grab something. The way I am using right now is sending one position that motor can't reach and it will try to get there. But it will always pop out the overload error after around several seconds even the torque I got from motor is lower than its limit I set. I searched a lot and found there is no direct torque controller for ax-12a. So I wonder why motor will overload even its torque is less than the limit and whether there is a way to achieve what I what. Besides, I am using ROS to communicate and control dynamixel motors.

    Thanks in advance for any hints.

  2. #2

    Re: torque control for ax 12a

    Might try Robotis forum:

    Not sure what you have tried? Example setting: ?

    Have you tried to query the current load: ?

  3. #3

    Re: torque control for ax 12a

    If the servo cannot get to the desired goal, it will generate an error to tell you this. The practice of "set a goal position that is not achievable" is not generally useful for controlling these servos.

    If you just need a certain amount of torque, perhaps you should be using torque mode instead?

    However, the servo will keep applying the torque, which will overload it, unless the max/goal torque is no more than 20% of max torque or so (varies based on servo specifics and installation/cooling.)

  4. Re: torque control for ax 12a

    Thank you for your reply.

    I understand it will generate torque when it can't reach the goal position which is what I want. My original thought is it will keep outputing torque until it is overheat as long as the output torque is less than its max torque. But the reality is motor just overloads even when its torque is less than half of the max torque. Do you think it is normal case? Or it is because I don't give enough voltage or something else.

    I checked the AX12 and it doesn't support torque control mode. Or is there any way to hack it?

  5. #5

    Re: torque control for ax 12a

    Yes, almost all motors of this kind have a continuous duty rating of 20-25% of the maximum instantaneous rating. It's a fact of physics.

    There is another thread on this board about adding external heat sinks to AX12A motors. You cut out a profile of aluminum sheet, and then open up the servo, bond it with heat compound, and close the servo back with the heat sinks sticking out the sides. That might give you better performance.

    I don't think the AX12 can be hacked into torque mode; that's a MX series thing only. The reason is that, if the motor provides a particular torque, it will often either keep spinning forward, or be back-driven, until it does a full revolution, and the AX12A only has coverage of its position sensor for about 300 degrees of the rotation. (The MX servos have 360 degree coverage.)
    Last edited by jwatte; 02-22-2019 at 11:35 PM.

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