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Blackbox moves servo on serial disconnect

Discussion in 'Controller Boards' started by Jackson Seal, Apr 18, 2023.

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  1. Jackson Seal

    Jackson Seal New
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    I have an 2 axis ACRO system w/servo controlled by a Blackbox x4, with a python script utilizing michaelfranzl/gerbil for motion planning.

    The general workflow is:
    • open serial port and initiate connection
    • start polling
    • wait for grbl 'welcome message'
    • unlock alarm
    • move pen up
    • home
    • move between series of points, pen up, sleep, pen down, etc
    • return to home
    • disconnect
    Rarely, as the script is disconnecting from the serial port, the pen will move up farther than I ever command it to, jamming the system mechanically.

    I know you wouldn't support 3rd party libraries, but is this something that's been seen before with BlackBox or grbl? Something I could be missing?
    Could this be something that happens when the PWM output stops suddenly before the power does, and the servo interprets this is a different commanded position? If so, how could I work around that?
     
    #1 Jackson Seal, Apr 18, 2023
  2. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    As servos rely on pulse trains for position, resets etc (times when firmware is not running) stops pulse trains. Design your hardware so m3S0 does not result in a crash
     
    #2 Peter Van Der Walt, Apr 18, 2023
  3. Jackson Seal

    Jackson Seal New
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    Okay, thanks for the response. I suppose I could feed the 5V for the servo through the relay to cut power when the pulse train is not ready, right?
     
    #3 Jackson Seal, Apr 18, 2023
  4. David the swarfer

    David the swarfer OpenBuilds Team
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    with GRBL's servo control it will always move the servo to whichever end is PWM level 0 when an M5 is received. (common at the end of gcode programs)
    M5 is sometimes different position to M3 S0, but close to it.
    This means that your mechanical setup must allow the servo to move to that extreme, AND that end of movement should be 'pen up' so it is safe.

    So, the disconnect is sometimes triggering a 'M5' condition which I believe sets the PWM level to 0. different servos will react differenly to that condition, most will drive to one end of travel, some will just maintain current position (probably the expensive digital ones :) )

    Better to configure the hardware so any movement is safe.
     
    #4 David the swarfer, Apr 19, 2023
    Peter Van Der Walt likes this.

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