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Thread: Dynamixel position accuracy, calibration

  1. #1

    Dynamixel position accuracy, calibration

    With the AX-12, is it normal to get current position readings that are consistently 1 or 2 units off the goal position? For example, if I set a group of actuators to position 512, then take the current position, I get values ranging from 510 to 514.

    Has anyone been able to calibrate them using the control table? The manual says:

    Calibration.
    Data used for compensating for the differences between the potentiometers used in the Dynamixel units. The user cannot change this data.
    The above statement implies that they are pre-calibrated...

    I considered calibrating each one of them in software based on typical readings for that particular actuator; however, at different positions, it will be plus or minus 1 or 2 units in either direction. It's not consistent, so I can't just say ID X gets +2. Due to this quirk the effective resolution is actually quite lower than 1024.

    It's bothersome because it's throwing off my radian conversion, which effects the accuracy of any calculations. I can live with it though.

    Are the RX servos more precise? I'm planning to order one to test with during the sale, hoping it's spot on...
    Last edited by Adam; 05-18-2010 at 10:36 PM.

  2. #2
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    Re: Dynamixel position accuracy, calibration

    This is the compliance coming in. Notice that your AX-12s probably don't whine and hiss like cheap hobby servos: the PID is a bit loosened up so that the motor/driver is under less load as it doesn't try to maintain the exact perfect position.

    You can tune the compliance slope (I don't think you want to touch the calibration), but you'll get back the whining servo noise (and possibly see more heat issues). The RX servos will likely have the same issue, as they use similar control software.

    -Fergs
    Last edited by lnxfergy; 05-18-2010 at 11:01 PM.

  3. #3

    Re: Dynamixel position accuracy, calibration

    I ran in to a similar situation, where the AX-12s were overshooting the mark a bit. Slowing the motors down helped.

  4. #4

    Re: Dynamixel position accuracy, calibration

    Thanks for the replies.

    I'll play around with the compliance settings. I think I'd like to tighten it up if that is the issue. In a walker that is constantly under load, some compliance is a good thing. I'm prototyping an arm, and I think I prefer precision.

    It's not a speed issue in this case. I'm already running really slow for my prototyping.

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