Recognising misconfigured homing

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  • August 9, 2019 at 1:18 pm #11455
    JuKu
    Member

    First, TinyG software works, this is not a bug report.

    I find that it is possible to misconfigure parameters, specifically on homing: My application uses G28.4 (homing without resetting values) to probe Z heights. I normally have Zmin on top, and that is where I get a reference 0. On probing, I configure the switches to home down towards Zmax, which triggers the switch.

    Now, with some combination of homing speed, acceleration and latch parameters, it can happen that speed is so high and acceleration is slow, that the machine can’t stop within the given travel. The stop travel is so long that the backup travel does not clear the switch. The machine does what the user asked, but not what the user wanted, and the unexpected error is confusing.

    The questions:
    – Is there a way to find out which combination of values are safe and which are not? (I would think there is no easy way, but I’m asking anyway…)
    – Even if not, can I somehow find out from the log that the error was due to misconfiguration and not a real mishap on the machine?

    August 9, 2019 at 2:05 pm #11456
    cmcgrath5035
    Moderator

    Having not seen reference to G28.4 before, I checked https://github.com/synthetos/TinyG/wiki/Gcode-Support and don’t see it listed, but some quick search and find it defined here for Smoothie http://smoothieware.org/supported-g-codes.

    It could be documented somewhere else for tinyG – did you find it somewhere?

    August 9, 2019 at 3:37 pm #11457
    JuKu
    Member

    I don’t remember where I found it, maybe Alden told me:-). But for the discussion, it doesn’t matter if we are talking about g28.2 or g28.4; the difference is only that the latter doesn’t zero the coordinates after the homing cycle. The point is, that it is possible for the user to set parameters so that it isn’t possible for the homing to work; I would like to detect the condition. (Also perhaps to point out for the other readers that a failed homing might be about their settings, not a TinyG problem.)

    August 11, 2019 at 12:48 pm #11458
    cmcgrath5035
    Moderator

    Understand your goal, have some thoughts but not a working machine to test them on at the moment.

    I am not sure there is a numerical solution for what you describe, if I understand correctly. My understanding is that traverse to Zmin has too much momentum when the switch is first contacted to stop and backoff properly.
    Unknowns to tinyG and likely a typical user:
    Mass of the moving element (in this case, a spindle?)
    Strength of the Z axis motor (NEMA 17, 23, ec.)
    Current setting on tinyG driver.

    I think you’ll understand my list – do I understand the issue correctly?

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