Pedersen VP2000 810M retrofit, severe servo hunting

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03 Jul 2019 11:47 #138515 by Juckix
Z movement is achieved by lifting the bed rather than the spindle via a central ballscrew, and as far as I am aware there is no counterbalancing.

I tried a few different BIAS value (0.001-100, as well as negative values) without seeing any change in the oscillation. At values above 10 the machine jerks when motion is enabled, which is to be expected I guess.

Further, increased Integral values decreases the period of oscillation but seems to be necessary for a low f-error. I have a tune (something like P=10, D=0.1, FF1=0.01, FF2=0.001) which produces very little hunting (more like 0.002mm rather than 0.015) but has a completely unsatisfactory f-error while moving.

Lastly what I found a bit odd is that the hunting shows up as instant on the DRO, it basically jumps 0.015 up and down repeatedly without any inbetween.

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03 Jul 2019 13:52 #138528 by andypugh
What is the physical size of a single encoder count?
You might need to add a little deadband to the PID.

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11 Jul 2019 14:42 - 11 Jul 2019 16:29 #139106 by Juckix

andypugh wrote: What is the physical size of a single encoder count?
You might need to add a little deadband to the PID.


The slides are graduated to 0.004mm but it's hardware interpolated to 0.001mm. The latter number is the deadband set on X and Y and it works well there. Raising it leads to more severe hunting on those axes.
Last edit: 11 Jul 2019 16:29 by Juckix. Reason: Wrong scale, clarification

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15 Jul 2019 11:42 #139414 by andypugh
I would suggest setting the BIAS to the same voltage as the Z shows steady state. It just helps make the PID tuning a bit more symmetric between up and down.

Did you decide if these were torque-mode or velocity-mode drives?

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