On 05/03/2021 12:03 PM, Nicklas SB Karlsson wrote:
Den 2021-05-03 kl. 17:42, skrev Jon Elson:
Assuming machine is lubricated there shear force in
oil/grease depend on speed so it might actually be rather
close to viscous friction and hence rather close to
linear. Then an oil/grease pump is used for lubrication
there will be a thin oil film even at slow speed and
standstill otherwise stick-slip effect might happen.
In doubt FF2 make any big difference anyway unless you are
able to hit accurate with FF1.
There are many "frictions" in the system. Shafts and
bearings, possibly belts and pulleys, leadscrews and of
course the linear slide itself. I'm lumping them all together.
My current tuning scheme is set P low enough so the machine
moves, but with lots of following error. Set I and D to
zero, and FF0 also, or possibly a little D if required.
Then, adjust FF1 very carefully, until the error on various
velocity jogs is nearly zero. Some systems have a
nonlinearity in the amps, so you have to compromise for
minimal following error at a median speed. Others are quite
linear, and one FF1 value gives very small error over a wide
range of speed.
Then, add just a TINY bit of FF2 to reduce the accel/decel
spikes. Then, find the highest P that you can without
oscillation, add a little D and that about does it.
This tuning scheme works pretty well with analog velocity
servos and my PWM system, where the motor's back EMF works a
lot like velocity feedback.
Jon
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