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Thread: Need Dynamixel Advice

  1. Re: Need Dynamixel Advice

    It is built, and I am displaying the torques in real time...
    It does float around the 1/4 max torque for periods of time, but is usually lower and sometimes spikes to between 50% and 75%.
    I have had it go into overload for torque a couple times, but not necessarily when I would have expected it or when I could see it very high, but I suppose it must have spiked briefly.
    Disturbing thing is that now that heavy-lifting MX64 is sticky and hard to turn when powered off. I made some adjustments to the joint last night, so it might just be that I have over-tightened something, but I am scared I have done something bad as suggested in some other threads about over-torqueing/over-heating. I thought I was being careful... never had sustained torques above 25% max. But alas... I guessing I'll be ordering an MX106 and redesigning the 3D shell.

  2. #12

    Re: Need Dynamixel Advice

    Disturbing thing is that now that heavy-lifting MX64 is sticky and hard to turn when powered off.
    You're seeing what I, and others on this forum, are seeing.

    Try this: Hook up only the dynamixel to a power supply, and measure the current draw. If the power supply indicates it, then great! If not, use the amp meter on your multimeter.
    If it's around 60 mA, then the servo is fine.
    If it's higher than that, then the motor is probably at least a little bit damaged.

    I've had a very similar experience: Designs that should totally work actually end up with busted servos. I don't know if they have a bad batch of motors, or if the servo has some inherent design flaw, or what, but there seems to be a real problem here.
    Occasional spikes are common, too, even if the typical load is 25% or less. I think that's just par for any mechanical design, and shouldn't be anything to worry about. In general, I think that's totally acceptable and to be expected. I couldn't keep the "overload shutdown" feature on, though, because of those spikes; now I'm just keeping "overtemp shutdown" on.
    Also, I can't keep "overvolt alarm" on, because just by moving some servo, another servo will go into overvolt alarm because of the spikes they generate on the power wiring. Have you seen this same problem?

    In the end, I am now monitoring heat, and I shut everything down (from the controller side) if I get above 60 degrees on the sensor. I also use active cooling fans on the heavily loaded joints. Haven't gotten back to walking around just yet, but maybe tonight -- it'll be interesting to see what comes out of that.

  3. Re: Need Dynamixel Advice

    The servo still moves smoothly and accurately when told to move to a goal position, but moving it by hand with no torque (or powered off) is super sticky and jerky. Unfortunately this completely ruins a key scenario for me which is recording of key-frames by positioning the arm by hand and recording the servo positions. I can't gracefully or accurately position the arm now.
    I took the servo apart and found that if I remove the top gear I can rotate the output shaft easily and I can also rotate the motor head pretty easily but I do feel tiny consistent "bumps" as it rotates... almost like rotating a stepper motor by hand. But when it is assembled it requires great effort to move the horn... can't even budge it by hand unless there is some lever attached to the horn for leverage.
    Is there any chance of repair or exchange? Has anyone diagnosed what the root cause is at the motor level? I'm inclined to take it further apart to see what's up but suspect that would void any warrantee.

    I'm also thinking about adding cooling fans. Did you power them off the same line? Where did you get them?


  4. #14

    Re: Need Dynamixel Advice

    I do feel tiny consistent "bumps" as it rotates
    That means the motor is damaged, unfortunately. I thought I could get by without fixing that when the problem was "small" but that didn't work out for me.

    Is there any chance of repair or exchange?
    Yes, you can have the motor replaced, for a fee (which is less than the cost of the entire servo, at least for the MX series...)

    Personally, I think there's something wrong with how these servos are either put together, or measured, or something. This kind of problem should not be happening with this frequency, especially with people who just keep the things still, monitoring loads and temperatures, etc. I don't think the built-in temperature probe is capable of protecting the motor. At this point, I turn off torque to the servo if it ever shows 60 degrees or more (from the controller side) and I'm hoping this will be enough. I've had the servo-built-in temperature trigger set at 75 degrees (default from factory is 80) and that has not been able to prevent motor damage.

    So, Robotis will service these, although you should go through the RMA process of whatever company you bought them from, and they will set it up with Robotis. If Trossen, email their technical support to get it started.

  5. #15
    Join Date
    Sep 2010
    Rep Power

    Re: Need Dynamixel Advice

    It really sounds like the motor is damaged, and it will probably only be a matter of time before the H-bridge burns out trying to move the motor. There are several possible causes, but not really sure which is most likely (over heating, over current, over loaded, shock damage, manufacturing defect, etc.).

    Only Robotis can replace the servo's motor or pcb, and any modifications to the internals (other than replacing the gearset) will void the warranty. So, time for an RMA.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    more bleh

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