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Thread: Is it normal that even with the same model, some dynamiels generate some sound?

  1. Is it normal that even with the same model, some dynamiels generate some sound?

    Hi, even with the same model, some units generate some sound when they are powered (no load and not being turned). I just plugged them to a controller one by one and assigned ID and Baudrate under Dynamizel Wizard. Is this normal? The sound is like the one we heard when we used RC servo motors made about 20 years ago.

  2. #2

    Re: Is it normal that even with the same model, some dynamiels generate some sound?

    The PID controller may be overshooting and re-correcting back and forth.
    Assuming this only happens when they're on, and torque is enabled.
    If you disable the torque, does the sound go away?

    Also, you will want to tune the PID of your servos once you know how much or little load will be on them, as the defaults are "okay" for "general use" but you can get better response if you know exactly what the situation is.

  3. Re: Is it normal that even with the same model, some dynamiels generate some sound?

    Quote Originally Posted by jwatte View Post
    The PID controller may be overshooting and re-correcting back and forth.
    Assuming this only happens when they're on, and torque is enabled.
    If you disable the torque, does the sound go away?

    Also, you will want to tune the PID of your servos once you know how much or little load will be on them, as the defaults are "okay" for "general use" but you can get better response if you know exactly what the situation is.

    Hi jwatte. Thanks for the suggestion. I did a test on two motors separately. They behaved as following:

    I connected a motor to a PC and scanned it under Dynamixel Wizard. The software found it. I turned the torque on, no sound. I selected Goal Position and click on -90 degrees for example, sound came out. I turned off the torque, sound disappeared. Then, when I turned on the torque again, also no sound. However, when I clicked on a new Goal Position, say 0 degree, then sound appeared again. Sound disappeared when I turned off the torque.

    Also, when I looked at the Goal and Present Position graphs, one some motors, I saw dotted, rather than solid, Present Position curve after the motors turned to some positions without load. Is this normal?

    Why some units are dead silent but others have sound. Are some of them defective? Do I need to worry about the motors having some issues?

  4. #4

    Re: Is it normal that even with the same model, some dynamiels generate some sound?

    Yes, that sounds exactly like an output that's under no load, and thus the PID controller is over-eager.
    Either add some load on the servo, or tune the PID to be sloppier (but then of course the response when it is under load will be mushier.)

    Try screwing a bracket into the horn, and load the bracket lightly with a rubber band. Not a lot of tension, just enough that it can "twang."
    If the servo still makes ticky noises, you could be worried about the sensor being mis-adjusted or the motor or gearbox having sticky spots, but if it settles down with a small, consistent preload, then you have nothing to worry about.

    This is another benefit of adding torsion springs to help the motors at rest in walking robots, by the way; not only do the motors run cooler and battery last longer, but it also provides pre-load when you lift the robot up, so the servos aren't suddenly finding themselves in "free fall" with no load to work against.

  5. Re: Is it normal that even with the same model, some dynamiels generate some sound?

    Quote Originally Posted by jwatte View Post
    Yes, that sounds exactly like an output that's under no load, and thus the PID controller is over-eager.
    Either add some load on the servo, or tune the PID to be sloppier (but then of course the response when it is under load will be mushier.)

    Try screwing a bracket into the horn, and load the bracket lightly with a rubber band. Not a lot of tension, just enough that it can "twang."
    If the servo still makes ticky noises, you could be worried about the sensor being mis-adjusted or the motor or gearbox having sticky spots, but if it settles down with a small, consistent preload, then you have nothing to worry about.

    This is another benefit of adding torsion springs to help the motors at rest in walking robots, by the way; not only do the motors run cooler and battery last longer, but it also provides pre-load when you lift the robot up, so the servos aren't suddenly finding themselves in "free fall" with no load to work against.
    Thanks. What do you mean by "twang"? For this test, since there are so many positions, is testing each motor under 2-5 different positions sufficient?

    As for the torsion springs, don't they require the motors more effort in order to turn to the opposite direction?

  6. #6

    Re: Is it normal that even with the same model, some dynamiels generate some sound?

    The rubber band has enough tension if you can pluck it with your finger and it sounds like "twang."

    Yes, torsion springs add more torque in one direction. But, if your robot is standing against gravity, then compensating with torque equal to half gravity means that the motors only need to work half as hard when standing, and then work equally half as hard the other way when lifting the legs, as opposed to working against full gravity when standing, and then almost not at all when lifting.

    If you believe you will stand more than lift, you might even go all the way and make the legs stand by themselves with springs, and then the motors only work when lifting the legs. Saves on battery and overheating, for most walking designs!
    Last edited by jwatte; 4 Weeks Ago at 10:47 PM.

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