Tuning a tinyG – an alternate approach

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  • April 22, 2014 at 2:32 am #5809
    flux
    Member

    Thanks for describing your method. I have not yet tried it, but:

    I would say at this point I would not use feed rates for the materials I mill much above F1200, so have tentatively concluded that the 12o’clock setting for servo drive current is adequate with headroom.

    As you note, it’s different when moving in free air and then milling something. I had some trouble missing steps when milling aluminum, so I decided to just go full throttle in my current pots. (The motors are rated 3A anyway and TinyG can give 2.5A.) So while this approach can give bounds for doing free-air G0 movements, it doesn’t really answer what kind of settings one needs to actually mill hard materials.

    Sadly we cannot adjust the current limits in software for different situations (or can we? the edge branch gives some new ‘motor power’ setting ranging from 0.0 to 1.0), I bet that could be useful in using different settings for moving in air, milling PCBs and milling hard materials.

    April 22, 2014 at 8:57 am #5811
    cmcgrath5035
    Moderator

    flux wrote

    As you note, it’s different when moving in free air and then milling something. I had some trouble missing steps when milling aluminum, so I decided to just go full throttle in my current pots. (The motors are rated 3A anyway and TinyG can give 2.5A.) So while this approach can give bounds for doing free-air G0 movements, it doesn’t really answer what kind of settings one needs to actually mill hard materials.

    Sadly we cannot adjust the current limits in software for different situations (or can we? the edge branch gives some new ‘motor power’ setting ranging from 0.0 to 1.0), I bet that could be useful in using different settings for moving in air, milling PCBs and milling hard materials.

    It will be interesting to see if Alden describes the motor power parameter as a ‘software potentiometer’. I believe he said somewhere to leave it at = 1 for now, it is a feature that he is working on in the future.

    I have a somewhat different interpretation of drive motor current, that being that the drivers can switch up to 2.5A into an inductive load before they near their dissipation design limit. The pots are available to reduce un-needed torque, which becomes heat, to extend the longevity of the driver and motor components.
    Does your tinyG run hot, particularly near the end of test.7.gcode?
    I keep finger checking mine – no rise really noticeable, but I have a somewhat unique (I think) implementation with the tinyG pwb bolted to a large heatsink, no fans, plus the small individual driver ‘dip-sinks’.

    For jolly’s, I am going to put an ammeter in series with my 24V supply just to see what it says during the various phases of my test case and your test cases.

    If, as I suspect, the torque required from the motors, with the available current, is inadequate to control the spindle in air while running the test, then clearly they won’t have reserve torque to push the spindle along if the force required to push the spindle thru the material is even moderate. That force is a complex combination of the spindle motor torque, material hardness, bit sharpness, diameter, etc.,
    you know all that from dealing with Al.

    An interesting question then becomes: for a given machine mass (spindle plus X gantry), how does increased velocity in an air test case translate into reserve force available to push the machine at lower feed rate (real work) with the same accuracy.
    Sounds like a good Homework and followup lab experiment for ME 305 or perhaps a good Master Thesis!

    Its probably time to revisit the Spindle pages on the ShapeOko forum, to see if other have test results in this space.

    April 23, 2014 at 10:46 am #5818
    cmcgrath5035
    Moderator

    I ran a few pot position tests with more or less expected results.
    Increased current to the motors did improve my machines performance/ reduce drift.

    For example, moving the pots from 12 o’clock to 4 o,clock, my test GCode described at the top of this thread ran without drift at F10000, where it had begun to drift with the pots at 12 o’clock.
    I did not attempt to go higher in Feedrate, so this is just another data point.

    I attempted some instrumentation with two DMMs, low cost items from HFT (free, actually). I have no knowledge of their averaging capabilities/characteristics.

    One DMM in series using the 10A setting, the other measuring Power Supply output Voltage.

    With pots at 4 o’clock, I measured input current = 1.61A while the motors remained ‘locked’ after reset. When released, current dropped to about 0.1A, so roughly 1.5A was feeding the motor windings.
    I don’t have a ‘scope and have not dug enough to know what the tinyG drive waveforms look like.
    Does anyone have a guesstimate as to the average duty cycle for the winding waveform?

    I also observed a fairly consistent input voltage of 24.2V when idle, 23.5V when measuring 1.6A. That is about 3% drop, probably within spec for my less than premium 24V 6A power supply.

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