Tuning for heavier weight?


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    Default Tuning for heavier weight?

    Hi Tom, I had to machine some holes in a big piece of pipe a few days ago. Weight was about 1000 lbs and it really messed up the tuning in my 4th axis. Started to oscillate a little if I moved it at any speed. Didnt alarm out, I guess my following error is not set properly for that axis. I should look into that too I guess.

    It was only an indexing job and managed to get it done by simply moving super slow to each postion. Is there any quick fix I could do to my tuning to make it more stable on larger jobs like this? What would be best guess? Maybe cut gains in half or something? More D gain? What do you think?

    Mark

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    Default Re: Tuning for heavier weight?

    Hi Mark,

    Wow. Adding mass should theoretically reduce gains so you would increase overall gain to compensate. Increase PIDs proportionally. But then if the system has a resonance such as mass-spring-mass such as motor-leadscrew-table then the resonance would shift lower and might cause problems. If the mass isn’t rigidly mounted to table that would add a new resonance.

    Any amplifier saturation could also cause problems.

    Tuning with a Bode plot, without the extra mass, we look at the 0db Mag crossover frequency and check if there is good phase margin. But then we imagine what if we added mass which would shift the Magnitude down and it’s crossover to a lower frequency would there still be good phase margin?

    HTH

    Regards
    TK http://dynomotion.com


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    Default Re: Tuning for heavier weight?

    Ok sounds like theres no easy soloution other then a proper re-tuning. Was afraid of that. There is a possibility of these coming back. Maybe next time I will go into the tuning page and do some tests. These are very expensive parts, half afraid I would move the wrong axis or something and crash into it.

    Seems like some kind of resonance for sure, mechanically the setup would be very rigid in this 4th axis. Im using pole zero filter on this axis as well, and I really dont understand much about them. Was a trial and error thing that worked out awesome for me. Is there any rule of thumb where lower/higher numbers result in more stability or is it impossible to tell without doing more testing or a bode plot?



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    Default Re: Tuning for heavier weight?

    Hi Mark,

    Ok sounds like theres no easy soloution other then a proper re-tuning. Was afraid of that.
    Sorry.

    Seems like some kind of resonance for sure, mechanically the setup would be very rigid in this 4th axis.
    What axis was going unstable? It seems like supported only in the middle (is that plastic?) would not be very rigid.

    For the 4th axis moment of inertia increases with the 4th power of radius so that has to be huge.

    That's related to my favorite question: how many times harder to spin a 2 inch cube compared to a 1 inch cube?

    How does your A Axis work? Is there a motor driving through a gear box? If I recall correctly the moment of inertia reflects from one side of a gear to the other by the square of the gear ratio.


    Is there any rule of thumb where lower/higher numbers result in more stability or is it impossible to tell without doing more testing or a bode plot?
    Not really. Again it depends on the shape of the phase plot. If the crossover is shifted will there still be phase margin at the new crossover? High I gain can often cause less phase margin at lower frequencies.

    Regards
    TK http://dynomotion.com


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Tuning for heavier weight?

Tuning for heavier weight?