Absolute linear scales

Probably the P term is too high
Note that FF1 is also completely wrong

FF1 can be set to 1 _IF_ you scale the analog velocity command so that its in
machine units per second at 10V in to the servo drive

So for example, if your machine moves 200 mm/s with a analog command of 10V
you would set:

MAX_OUTPUT = 200.0
OUTPUT_MIN_LIMIT = -200.0
OUTPUT_MAX_LIMIT = 200.0
OUTPUT_SCALE = -200.0
and then FF1 would be 1 (or very close)

The advantage of doing this is that its much easier to guess good PID numbers (and the PID number are the same for inch and mm machines)

What would be a good starting value for FF1 with the setup I have now? If I change to this setup:

MAX_OUTPUT = 200.0
OUTPUT_MIN_LIMIT = -200.0
OUTPUT_MAX_LIMIT = 200.0
OUTPUT_SCALE = -200.0

Can I still use this tuning guide:

gnipsel.com/linuxcnc/tuning/servo.html

? I would like to use that because it is simple enough for me to understand

Tero

Lowering P to 1.0 and FF1 to 0.02 made it possible to move axis slowly without too bad vibrations. What is lowest acceptable value for P ? Should I lower internal Gain of servo amplifier so I could increase LinuxCNC P value?

Tero

Normally you tune the drive and LinuxCNC separately

Both the drives and LinuxCNCs gains (P term on LinuxCNC amd P+I on the drives velocity loop)
should be as high as possible (while still stable) to minimize the following error

You should be able to use JTs tuning guide above in any case.

Note that if you change the output scaling from 10 to 200 this will mean you need to increase
P 20 times to get the same result

Also note that I pulled 200 out of a hat (to say it politely)
(you need to calculate this number from drive settings or measure it)

With these values I was able to get minimum f-error and stable axis:


# Second axis
[AXIS_1]

TYPE = LINEAR
HOME = 0.000
MAX_VELOCITY = 134
MAX_ACCELERATION = 268
P = 0.4
I = 0.0
D = 0.0
FF0 = 0.0
FF1 = 0.055
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.001
BACKLASH = 0.000
MAX_OUTPUT = 10.0
OUTPUT_MIN_LIMIT = -10.0
OUTPUT_MAX_LIMIT = 10.0
INPUT_SCALE = 1.000
OUTPUT_SCALE = -10.000
MIN_LIMIT = -254
MAX_LIMIT = 1200
FERROR = 0.8
MIN_FERROR = 0.2

Tero

I would normally expect to see a little bit of I and D in a fully-tuned system.

I followed this guide gnipsel.com/linuxcnc/tuning/servo.html and there I and D are not used. Or there is a mention that I should help a bit when tuning is close. I did not see any over/undershoots with scope so I let I alone. Also it was my first servo tune so I am quite sure it is not perfect... Well maybe some rainy day it will be :laugh:


Tero

Ok. Now everything else works but software limits. Those are not activated if not homed. If I would like to "tweak" this a little where is this done in source?

Tero

Are you running 2.7 with NO_FORCE_HOMING or 2.8 with the Absolute options?

2.7.6 with NO_FORCE_HOMING. I am considering of "tweaking" command.c a little bit:

forum.linuxcnc.org/forum/10-advanced-con...-work-without-homing

Tero