using linuxcnc for laser fiber marking

OK that means the PID is not the issue.

It means the XY2-100 interface is outputting the numbers
that LinuxCNC commands within the following error limits

Maybe the error is in the interface, are you sure you have the clock and data polarity correct?

I used to use your configuration for testing, but now I have edited that configuration and now I know that my configuration was incorrect
this is my new config :



with :
FERROR =    3
MIN_FERROR = 3

i have following error.

The error is related to my high speed!
What should I do to configure pid for high speed ?! 

My impression is that the squares are good, so G1 moves are working correctly at the feed rate that you are using.
So the issue seems to be with the G0 moves.

As a first step, try reducing the MAX_VELOCITY in the INI to closer to the same speed as the feed rate in the video. Then if that fixes the lossof position problem, increase it until the issue re-appears, Then back off slightly.

Galvos should be very repeatable, you don't need to worry about part mass, so you don't need much spare capability.

First comment out this line

setp hm2_5i25.0.xy2mod.write-timer-number 1

Then set all PID P values to 2000

Then set a reasonable acceleration value
(you cannot set it to an unachievable value,
this guarantees overshoot in the galvanometer)

MAX_ACCELERATION = 9999999999999999999 Will not work

Then when you are getting closer to tuning you need to add a little bit of FF2
(say around 0.0005 but this needs to be tuned)

You tune with halscope (plotting velocity and following error)

Thanks to PCW , your answer was very helpful .
My system is much more stable now.