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Thread: How can avoid the jerking of 6-DOF arm made up of AX-12

  1. How can avoid the jerking of 6-DOF arm made up of AX-12

    Hi,

    We are made a 6DOF robotic arm using AX12A. It is jerky while playing back a recorded motion. (We are controlling the dynamixels using ROS(Robotic Operating System) and ROS-dynamixel library) . What all are the parameters in AX-12A helps to reduce the jerking.

    How the Moving Speed and Punch value parameter helps to avoid jerking ?. I am changed Moving Speed parameter to 60rpm, but reconnecting the dynamixel its value is resets. I am updated the firmware again this happens. How can I update its value.

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    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    How frequently are you sending new positions to the servos? Are you using a 'sync_write' packet, or a single 'write_word' packet for each servo, to update the goal positions?

    I am pretty sure that only changes to the EEPROM range of the control table persist after poweroff. RAM entries in the control table return to default or sensed values at power on.

    I know that I had at least two posts describing, in quite a bit of detail, how the algorithm (as I understood it) controlling the AX-12 used the various parameters, but searching for posts by tician containing 'slope' produces nothing. It will take a while to finish digging through my posts to find it.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
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    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  4. Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    Thanks for the replay.. How changing the Moving Speed and Punch value parameter helps to avoid jerking ?.

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    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    Again, I ask:

    How frequently are you sending new positions to the servos? Are you using a 'sync_write' packet, or a single 'write_word' packet for each servo, to update the goal positions?

    How often you send the goal position to the servos has a very powerful effect on whether or not you can achieve smooth motion.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  6. Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    I am using ROS dynamixel library.I am recording joint positions from ROS node and playing back its positions. I dont know which of these 'sync_write' packet, or a single 'write_word' is using

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    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    There are lots of dynamixel libraries for ROS, which one are you using?

    At what rate are you saving and later replaying the poses? (10Hz, 30Hz, 60Hz. Need a number.)
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
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  8. #8

    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    Also, jerking can be caused by overloading or too much inertia in the arm. Leverage from arm length is a pain!

    There are at least two kinds of jerking:
    - Sending updates too slow, with enough power, so that the servos go to the next position, and stop, and then get the next command. High update rates will usually prevent this. 60 Hz is good. 30 Hz might be enough. Reducing the speed of the servo movements will also help with this.
    - Too much mass/inertia in the system, often coupled with flex (insufficient rigidity) in construction and backlash in gear trains. This causes oscillation and secondary motion in the system that causes the servos to jerk around, trying to get the system to the desired state. Stronger servos, more rigidity, lighter weight, and shorter levers/segments are the solutions for this problem.

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    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    Have you guys ever tried calculating the inverse velocity kinematics and programming the servo Moving Speed accordingly for each time step? That's on my list of things to play with.

  10. #10

    Re: How can avoid the jerking of 6-DOF arm made up of AX-12

    I heard that someone else seemed to remember claims that the servos tend to overheat if you use the speed control instead of interpolation (see here). I haven't actually tried it, just something I thought you might want to be aware of.

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