Den 2021-05-04 kl. 01:22, skrev Jon Elson:
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.
Agree with my mathematics.
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.
TINY bit of FF2 agree with my mathematics. My mathematics say you need I
to get accurate following. I found manually tuning usually work rather well.
Mathematical modeling and analysis is more useful for slow systems,
instability happen also to slow systems, oscillations may take minutes.
In serial produced product it make sense with more thorough analysis to
make sure marginals are good.
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.
My mathematics say if you feed motor with a voltage in sort of motor's
back EMF work like a velocity feedback so voltage control speed. My
mathematics also say FF1 compensate for the motor's back EMF or velocity
if servo drive have velocity as input signal. So I would say there is
agreement also here in sort of.
Nicklas Karlsson
Jon
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