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Weird servo behavior

edited December 2013 in Egg-Bot
My original server started acting strangely - it was chattering and moving slowly, haltingly to up and down. So I installed a replacement, but this one is acting weird.

I adjust it before plotting, such that it has 3 to 4 mm of travel on either side of "pen contact." But after a couple minutes of plotting, the servo's range starts to drift upwards. Eventually, the pen doesn't make it to the drawing surface, and the up position maxes out the pen arm travel.

At this point I abandon the plot and do a "pen up, motors off." Over the next few minutes, the servo very slowly returns to the original up position. I'm not an electronics guy, but it seems like there's some capacitance thing going on. If it was software, I don't think the servo would return with no input. Could it be the power supply, or interference? I did get one good, complex (long) plot before it started doing this. I bought a two-pack of servos; I'll put the second new one on there tonight and update if that fixes it...


  • I've never heard of a servo motor doing that, so there may well be something wrong with it.  I cannot imagine a situation in which the software or interference would have an effect on that. I suppose that a power supply malfunction could cause something like this, but you probably would see other signs of that as well. 

    Also, you might consider writing to us (please contact our shop directly by e-mail) to request a replacement for your original servo.  If that doesn't fix it, then let's try to see if it's a problem with the EBB or power supply.  We'd like to help you get to the bottom of it. :)
  • Happened with two different servos?  Might this be a problem with the clock crystal or related circuitry on the EBB board?  The timing of the pulse width controls how far the servo arm moves.  And the timing is controlled by either an external xtal or one internal to the uprocessor.  If it is slewing for some reason, then might there be such behavior?  I suppose if the pin on the PIC generating the PWM is impacted there could also be a problem (e.g., some other problem resulting in high frequency interrupts on the chip, robbing time from the interrupt responsible for managing the PWM duty cycle causing late issue)?
  • That's a good point; I was thinking of a possible problem on the analog side.  You're absolutely right that this could be a "PWM" type problem due to a clocking or interrupt issue.  (And if so, it would be the first time that I've seen it.)

    jgr4: Would you be willing to try (yet another) replacement servo or to send the EBB, power supply, and servo back to us for testing?
  • Ha, you guys are way over my head now :-)  Thanks for the responses.

    I had ordered a pack of two servos - they're the exact same ones that come with the kit. I installed the second new servo this evening to test. I also used a different USB cord, moved the eggbot further from the computer, and plugged it into a different outlet. The issue persists...

    I made a video and uploaded it to youtube - the sound is bad, but there's a running commentary.
    During the video I say that the pen down looks good, but it does trend upward. Pen up and down positions are set to 66 and 47, by the way.

    I would be happy to send any combination of parts to you for testing - just let me know the address. Let me know if there's anything else I might try on this end. I'm dead in the water at this point.

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