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Thread: Dynamixel servo- 'Deadzone'

  1. #11
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    Re: Dynamixel servo- 'Deadzone'

    Would slip-rings work in the middle of a chain, or would they mess up the signal?

    I may have to experiment with this. They may add too much noise.
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  2. #12

    Re: Dynamixel servo- 'Deadzone'

    There was a tall bipedal walker at MW2013 that used a slip ring. Worked fine for that. The main concern for me would be power handling, as a large number of servos would draw a lot of power. If noise for the TTL signal is a problem, I think you can improve that by either doubling up circuits (less likely to get noise on two circuits at once) or lowering the TTL signal speed.

  3. Re: Dynamixel servo- 'Deadzone'

    Another doubt regarding the encoder, is the encoder installed before or after the gears? (in MX64)

    Also, there is note in Dynamixel documentation which says:
    "Current load is inferred from the internal torque value, not from Torque sensor etc.
    For that reason, it cannot be used to measure weight or torque; however, it must be used only to detect which direction the force works."

    Then how is the value calculated?

  4. #14
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    Re: Dynamixel servo- 'Deadzone'

    The absolute position encoder is installed on the shaft of the output gear opposite the horn, so it directly matches the rotation of the horn. Putting an absolute position encoder anywhere else renders it largely useless because of gearing ratios and backlash.

    The 'Present Load' value in all dynamixels, except the MX-64/106 and all members of the Dynamixel Pro, is the PWM value used to drive the H-bridge at any given moment. It can roughly approximate current permitted through the motor, but has no fixed relationship to any real-world units of measure.
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  5. #15

    Re: Dynamixel servo- 'Deadzone'

    has no fixed relationship to any real-world units of measure
    Note that the MX-64 is based on the RX-64 manual, and sometimes it still contains RX-64 information.

    Also, the PWM value used can be turned into average-current if you know the current voltage and the impedance of the motor. And if you then know the efficiency of the motor, you can turn that into power, and then if you know the gearing you can turn that into torque. It would be very approximate, though :-)

  6. Re: Dynamixel servo- 'Deadzone'

    thanks tician, jwatte!

    the documentation for dynamixel is a bit confusing at times. In compliance margin/slope section, they say the diagram shows the relation between the position and torque of the motor. If the graph you see that the torque drops to zero (I think) at the compliance margin. So how does the motor hold the load at that point? Or does the motor just drop the load coz torque becomes 0?! Can someone shed some light on how this compliance feature work?

  7. #17
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    Re: Dynamixel servo- 'Deadzone'

    I know I've linked to this thread before about the control algorithm and effects of the various register values, but the advanced search function of the forum still does not find it when searching for keyword: 'compliance' and user: 'tician' (remove the user parameter and it does find it).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
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  8. Re: Dynamixel servo- 'Deadzone'

    Hi tician,

    I read the explanation you gave in that link. In that you say:
    "...Once deltaP is less than or equal to the Margin, the MOSFET driver is turned off"

    Does that mean, the torque becomes zero? In my experience, the servo torque doesn't become 0. It just hold its position at the goal position. Torque zero means the load will just drop.

  9. #19
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    Re: Dynamixel servo- 'Deadzone'

    The algorithm is constantly updating the current position and motor PWM value at >100Hz. If there is a load attached, then the horn is being rotated outside the compliance margin of the goal position causing the algorithm to re-enable torque to maintain position within the compliance margin of the goal position. No load, then no need for movement/torque.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
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  10. #20

    Re: Dynamixel servo- 'Deadzone'

    Is compliance margin still a thing on the MX-64? I thought it did it all with the PID controller.

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